oxidation of bio-renewable glycerol to value-added

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Applied Energy

journal homepage: www.elsevier.com/locate/apenergy

Oxidation of bio-renewable glycerol to value-added chemicals throughcatalytic and electro-chemical processes

Amin Talebian-Kiakalaieha,b, Nor Aishah Saidina Aminb,⁎, Kourosh Rajaeib, Sara Tarighia

a Faculty of Petrochemicals, Iran Polymer and Petrochemical Institute (IPPI), P.O. Box 14965-115, Tehran, Iranb Chemical Reaction Engineering Group (CREG), Faculty of Chemical and Energy Engineering, Universiti Teknologi Malaysia (UTM), 81310 Skudai, Johor, Malaysia

H I G H L I G H T S

• A review on catalytic and electro-catalytic oxidation of glycerol.

• Various catalytic synthesis methods have been evaluated.

• Different physico-chemical characteristics of catalysts have been evaluated.

A R T I C L E I N F O

Keywords:GlycerolOxidationElectro-chemicalHeterogeneousBio-catalystMetal nanoparticle

A B S T R A C T

Due to its unique structure, characteristics, and bio-availability, glycerol transformation into value-added che-micals has been in the spotlight in recent years. This study provides a comprehensive review and critical analysison catalytic and electro-chemical oxidation of glycerol into commodity chemicals, which have broad applica-tions to the pharmaceutical, polymer, and food industries. Various synthesis methods (e.g. impregnation, sol-immobilization, incipient wetness, and deposition precipitation) for the preparation of the catalysts are dis-cussed. Catalytic performance of mono-, bi-, multi-, and non-metal supported catalysts on carbon black, acti-vated carbon, graphene, single- or multi wall-carbon nano-tubes, layered-double hydroxides, metal oxides, andpolymers are evaluated. Among the methods, sol-immobilization is highly commended since fine metal NPscould be homogeneously distributed on the support, reported as an effective factor for controlling the selectivityof the desired product. In particular, the environmentally benign novel polymeric structures, illustrate sig-nificant breakthroughs in production of commodity chemicals compared to the conventional materials.Homogeneous oxidation of glycerol by enzymes and microorganisms also displayed acceptable performanceparticularly in production of DHA, but at the expense of long reaction time. Unlike the homogenous and het-erogeneous catalytic processes, electro-chemical oxidation could be tuned for high product selectivity by con-trolling the nature, composition and structure of the electro-catalyst as well as the electrode potential. Mostimportantly, combination of electro-chemical oxidation of glycerol with oxygen or water reduction process infull- and electrolysis-cells, respectively could be the ultimate goal in this field. Simultaneous generation of value-added chemicals and electrical energy would have significant economical and environmental merits compared tothe conventional processes. The current state-of-the-art of the glycerol oxidation process and recommendationsfor further research are also included.

1. Introduction

Petroleum exploitation and its cracking to simple hydrocarbons inthe early 20th century is the most important breakthrough to in-dustrialization. Fossil fuel has been our main source of energy for

approximately a century. The current oil production rate reaches toabout 12 Mt/day and it is predicted that its demand will increase sig-nificantly to around 16 Mt/day by 2030 due to the significant increasein the world’s population and rapid industrial development [1,2].Therefore, various types of environmental concerns such as massive

https://doi.org/10.1016/j.apenergy.2018.09.006Received 11 June 2018; Received in revised form 13 August 2018; Accepted 2 September 2018

Abbreviations: GLYLD, glyceraldehyde; GLYCA, glyceric acid; TARA, tartronic acid; TAR, tartronate; GLA, glycolic acid; GA, glyoxylic acid; HPYA, hydroxypyruvicacid; OXALA, oxalic acid; MOXA, mesoxalate; FA, formic acid; LA, lactic acid; PAL, pyruvaldehyde; Mt/day, million tonnes per day; GLY, glycerate; CA, chron-oamperometry; CV, cyclic-voltammeter; ACA, acetic acid⁎ Corresponding author.E-mail address: [email protected] (N.A.S. Amin).

Applied Energy 230 (2018) 1347–1379

0306-2619/ © 2018 Elsevier Ltd. All rights reserved.

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discharge of CO2 and even the depletion of fossil fuel resources havebecome the main agenda in upholding sustainability. Indeed, cheapsupply of fossil fuel (less than 100 USD/barrel) will no longer beavailable by 2040 [3]. Consequently, there have been many attempts toreplace fossil fuels with renewable energy. Biomass and biofuels arealternative energy sources from renewable energy that could meet thehuge energy demand and at the same time reduce environmental pro-blems. The new term “bio-refinery” is used to describe the process forconverting biomass to food, fuel, and value-added chemicals. Indeed,bio-refineries could significantly reduce the energy demand, CO2

emission, and moderately lower the operating costs [4].Biodiesel, is one of the most important and valuable alternative li-

quid biofuels in the transportation sector. Biodiesel is environmentallyfriendly, technically feasible and biodegradable [5]. Plant lipids andanimal fats (biomass) are common sources of biodiesel. Compared withtraditional diesel fuels, the utilization of biodiesel can reduce 70%hydrocarbons and 50% particulate matters on exhaust emissions[United States Environmental Protection Agency (US EPA)], and net78% CO2 emissions [6]. Application of non-edible oils have attractedmuch attention recently. Researches who compared the benefits ofwaste-to-energy technologies, including incineration, anaerobic diges-tion, and biodiesel reported that the production of biodiesel using scumsludge was the most economic scenario, creating an added value of US$491,949 and US $610,624/year. Also, it is reported that approxi-mately 15.784 million USD/year and environmental gains in the formof carbon credits can be saved for a city (Metropolitan Region ofCampinas, São Paulo State, Brazil) if waste frying oil is collected andutilized to produce biodiesel [7]. Life cycle assessment is a widely usedmethodology for quantifying and examining the environmental aspectsof biodiesel. The analysis of well-to-wheel (WTW) carbon footprint ofbiodiesel helps determine to what extent its widespread implementa-tion can enable a country to reach its GHG reduction targets. As a result,the GHG emission per unit energy of the biodiesel at nominal values forthe process parameters was found to be 360 g e-CO2/MJb. If solar en-ergy is utilized to increase the biomass concentration from 20 to 31wt%, the GHG emission of the algae-derived diesel is reduced to 273 g e-CO2/MJb. Indeed, biodiesel production is environmentally viable andpolicies and incentives should ideally support research, development,and commercialization for this process [8].

The EU Energy and Climate Change Package (CCP) became opera-tive on April 6, 2009, which was reported the EU goals for 2020: areduction of 20% in greenhouse gas (GHG) emissions compared to1990; an improvement of 20% in energy efficiency (compared toforecasts for 2020) and a 20% share of renewable energy in the totalEuropean energy mix. Part of the latest 20% share, is represented by a10% minimum target for biofuels in the transport sector to be achievedby all Member States [9]. Thus, due to enormous advantages of bio-diesel, different countries around the world have devised plans to en-hance their biodiesel production. For instance, United State producedmore than 2.7×109 gallons biodiesel in 2009. The U.S. biodiesel in-dustry has reached a key milestone with more than 1 billion gallons ofbiodiesel production in 2011 [10]. In addition, it is estimated thatglobal biodiesel market will significantly expand to 45 billion liter by2020 by an average growth rate of 42% per year [11].

Biodiesel production opens a spectacular opportunity for bio-re-finery since glycerol is the main by-product in the transesterificationprocess. Indeed, 1mol of glycerol is formed for every 3mol of biodieselproduced. Glycerol is listed as one of the top 12 most important bio-based chemicals in the world and is predicted to become the majorchemical for future bio-refineries [12]. It is expected the global surge inbiodiesel production in 2020 could produce over 41.9 billion liter ofcrude glycerol [13]. Application of glycerol for the synthesis of value-added chemicals has directly cut the cost of biodiesel production by halffrom 0.63 to 0.35 USD per liter [14,15]. In addition, ethanol productionfrom glycerol could reduce production costs by almost 40% (no steam-explosion needed) when compared with production from conventional

corn-derived bioethanol. Moreover, if fuels and reduced chemicals aretargeted together, there are many advantages for using glycerol oversugars, (the most commonly used fermentation substrate), which mighttranslate into higher yields and lower capital and operational costs. Infact, converting glycerol to pyruvate generates twice the amount ofreducing equivalents produced from glucose or i.e. xylose [16].

Prior to 1999, glycerol was mostly produced from fatty acids (47%),soaps (24%), fatty alcohols (12%), and biodiesel industry (9%).However, biodiesel industry has become the main glycerol producer(64%) since 2009 [17]. Thus, glycerol consumption is expected to in-crease significantly by 50% in 2018. In fact, its consumption is esti-mated to expand from 2000 kt in 2011 to approximately 3070 kt in2018 with revenues of 2.1 $ billion [18].

Glycerol purity is the key component for its industrial applications.The conventional method to produce biodiesel is from vegetable oilwith homogeneous (acid or base) catalyst. Inevitably, the glycerolproduced during the conventional process will include impurities suchas water, methanol, residual catalyst, free fatty acids, un-reactedmono-, di- and tri-glycerides, methyl ester and various organic andinorganic compounds (matter organic non-glycerol (MONG)) [19,20].Consequently, low quality glycerol requires different purification andtreatment steps to reach the> 95% purity for feedstock in industrialrefineries. However, the purification processes are costly and some in-dustries prefer to burn the impure glycerol as waste material and userefined (> 98.5%) glycerol instead of purified crude glycerol. As a re-sult, more than 150 to 250 thousand tons of crude glycerol was wastedin 2006 and 2007 [21,22].

Majority of researchers have used heterogeneous catalysts and non-edible oils for higher quality biodiesel and glycerol production. Forinstance, Bournay et al., [23] used Zn-Al heterogeneous solid catalyst ina continuous process to produce high quality biodiesel (98.3%) andglycerol (98%). Indeed, their catalytic process could totally avoid allthe costly treatment and purification processes for direct application ofcrude glycerol in industries.

Glycerol is edible, bio-sustainable, non-toxic, easily available andhas a multi-functional structure. Thus, glycerol can be used as a re-newable source of value-added chemicals (more than 2000 products) invarious reaction pathways [20,23,24]. Also, the primary and secondaryhydroxyl groups that consist in glycerol molecules are susceptible tovarious chemical reactions, namely hydrogenolysis, esterification, de-hydration, etherification, oxidation, pyrolysis, transesterification, oli-gomerization, polymerization, and carboxylation. Table 1 lists all thecatalytic processes and products that can be produced from glycerol.

Among these possibilities, the oxidation of glycerol is considered tobe one of the most important chemical routes to produce aldehydes,carboxylic acids, and ketones, frequently used in the preparation of newchemicals for cosmetics, medicine or food industry purposes. The mostimportant valuable oxygenates are glyceraldehyde (GLYLD), dihy-droxyacetone (DHA), glyceric acid (GLYCA), hydroxypyruvic acid(HPYA), tartronic acid (TARA), mes-oxalic acid (OXALA), glycolic acid(GLA), and glyoxylic acid (GA). However, glycerol conversion and se-lectivity for a specific product vary according to the reaction conditions(temperature, concentration of reactants, solvent used, ratio of glyceroland catalyst, mass-transfer conditions) and type of oxidation. In addi-tion, reactor configuration, nature of support, relationship between rateof product desorption and subsequent oxidation to other glycerol de-rivative, and the selection of a specific oxidation method, which impliesthe use of a specific catalyst, microorganisms, chemical reagents orelectro-chemical conditions could also affect the activity.

Fig. 1 depicts the glycerol oxidation mechanism. Oxidation of pri-mary hydroxyl groups of glycerol produces TARA and GLYCA. The DHAor keto-malonic acid is produced by oxidation of the secondary hy-droxyl groups of glycerol. On the other hand, OXALA is produced byoxidation of all three glycerol hydroxyl groups [25,26]. Although someof the glycerol oxidation products are highly valuable and expensivesuch as OXALA and TARA, others like DHA have lower market price.

A. Talebian-Kiakalaieh et al. Applied Energy 230 (2018) 1347–1379

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The majority of these products can be used as intermediate materials tosynthesize novel polymers and fine chemicals. For instance, DHA issignificantly used in the cosmetic industry as a safe skin tanning agentand TARA is used as a drug-delivery agent, particularly to increase theabsorption of tetracycline antibiotic into blood [27,28]. Unfortunately,all these valuable chemicals are produced only in a small scale due totheir costly and contaminated production processes. Indeed, hazardousoxidizing agents such as permanganate, nitric acid, and chromic acidare used during the oxidation process.

However, glycerol characteristics serve great opportunities for ap-plication of different heterogeneous catalysts with clean and cheapoxidizing agents such as oxygen, air, and hydrogen peroxide (H2O2) inoxidation reaction. Hence, glycerol oxidation reaction can be per-formed in an electro-chemical process or in the presence of hetero-geneous supported metal, homogenous and biocatalysts [28]. None-theless, the main concerns of oxidation reaction are selectivity of thedesired products, increasing catalytic activity, stability of catalystagainst poisoning, and minimizing the application of alkaline en-vironment [18]. Some of the most important factors that give sig-nificant impact on the desired products selectivity are reaction

temperature, pH of the solution, size of the metal particles, supportcharacteristics and metal nanoparticles (NPs) (e.g. Au, Pd, Pt). Amongthe reported factors, the textural and physico-chemical characteristicsof the supports seem to have profound influence on catalyst activity andproducts distributions [29].

Based on the importance of glycerol as one of the top 12 materials inthe chemical industry and the significant impact of products that can beproduced from glycerol on human’s life, the number of research studieson glycerol has significantly increased recently. Fig. 2 clearly indicatesthat the number of studies on glycerol has doubled in the last 10 years.The two review papers available in this field were published more than5to 6 years ago; the first one is only based on electro-chemical oxidationof glycerol [24], and the second one is a short review [30]. Un-fortunately, the two review papers could not properly address appli-cation of various types of catalysts, the importance of supporting ma-terials, and application of various catalytic synthesis methods related toglycerol oxidation. Thus, a comprehensive review on glycerol oxidationto valuable chemicals is imminent.

The main aim of this review is to evaluate and examine recent novelbreakthroughs in glycerol oxidation process, particularly with the focus

Table 1Catalytic conversion of glycerol into value-added chemicals by different processes.

Different catalytic conversion of glycerol into value-added chemicals


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on catalytic (heterogeneous and homogenous) and electro-chemicalprocesses in alkaline and base-free environments. In addition, the re-view addresses the critical knowledge gaps for enhancing high con-version and selectivity in glycerol oxidation. Also, the pervasive influ-ence of the physico-chemical characteristics and preparation methodsof various metal-supported catalysts on the selectivity of desired pro-ducts will be discussed comprehensively. As a result, the catalytic gly-cerol oxidation studies will be classified into four subsections of mono-,bi-, multi-, and non-metallic supported catalysts on carbon black (C),activated carbon (AC), graphene (G), carbon nano-tubes (CNTs), Mg-Allayered-double hydroxides (LDHs), metal oxides, and polymers. Also,glycerol oxidation under photo-catalytic process will be evaluated inthe presence of various supported samples. Recent developments incatalytic and bio-refinery industries have provided great opportunitiesfor research and overcome various obstacles for commercialization andindustrialization of a bio-based food, fuel, and chemical productionsfrom glycerol.

2. Catalytic oxidation of glycerol

The heterogeneous catalytic oxidation of glycerol with molecularoxygen is considered to be one of the most attractive methods for

production of glycerol derivatives. Selective oxidation of glycerol intofine chemicals have used different supported mono-, bi-, multi- andeven non-metallic catalysts. Metallic catalysts for the oxidation of theprimary hydroxyl group are usually based on Au, Pt, and Pd NPs[25,31,32]. Pd and Pt are characterized based on the lowest hydro-genolysis activity of Group VIII metals with good hydrogenation anddehydrogenation activities maintained. However, alcohol oxidation inaqueous phase using Au as catalyst is more selective and less prone tometal leaching and deactivation caused by over-oxidation and self-poisoning by strongly adsorbed by-products compared to Pt and Pd[33].

Generally, factors that have been evaluated as the most influentialparameters on the glycerol oxidation reaction conversion, productdistribution, product selectivity, and catalyst long-life stability are (1)NaOH presence in reaction medium, (2) different catalyst preparationmethods such as impregnation, sol-immobilization, incipient wetness,and deposition precipitation, which could control the particle size ofmetal NPs, (3) basic or acidic characteristics of support, (4) influence ofsample structure on modification of metal NPs, (5) presence of cappingagents, and finally (6) application of bi- and multi-metallic NPs insteadof mono-metallic ones. Table 2 summarizes some of the results fromprevious studies on catalytic oxidation of glycerol over different metal

Fig. 1. General reaction pathways to produce oxygenated derivatives of glycerol [27].

Fig. 2. Increasing importance of glycerol research (By searching keyword “glycerol” in Scopus).


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Table 2Catalytic oxidation of glycerol over different supported catalysts in the basic and base-free environments.

No Catalyst Reaction conditions Con (%)a Sel (%)bor Y (%)c Refs.

Basic Environment Mono-metallicbased

Au-based 1 1%Au/charcoal 12mmol GLY/20mL H2O 333 K, 3 h, 12mmolNaOH, O2 (0.3 Mpa)

56% 100 (SGLYAC) [34]

2 1%Au/C 0.3 M GLY, NaOH/GLY=4 (mol/mol), 303 K,20 h, O2 (0.3 Mpa)

100% 92 (SGLYAC) [35]

3 Au/C 1.5 M GLY, NaOH/GLY=2 (mol/mol), 333 K, 3 h,O2 (0.1 Mpa)

30% 75 (SGLYAC) [36]

4 Au/C 1.5 M GLY, NaOH/GLY=2 (mol/mol), 333 K, 3 h,O2 (0.1 Mpa)

50% 26 (YDHA),44(YHYPAC)

[36]

5 Au/C A 50 cm3 batch reactor; 333 K, 0.5MPa O2 50 83 (SGLYCA) [37]6 Au/AC 333 K, NaOH/glycerol = 2mol/mol, 3 bar O2 100 60 (SGLYCA) [38]7 Au/AC 298 K, H2O2/glycerol= 3, H2O2 66.1 66.3 (SGLYCA) [39]8 Au/TiO2 A 50 cm3, batch autoclave, 333 K, 145min, 1MPa

O2

83 61 (SGLYCA) [40]

9 Au/TiO2 A continuous fixed-bed reactor, 333 K, 1MPa,30mLmin−1 O2

32 37 (SGLYCA) [40]

10 Au/TiO2 323 K, glycerol/metal= 1000/1, 300 kpa O2 98 73 (SGLYCA) [41]11 Au/Al2O3 A 300 cm3 batch reactor, 333 K, 5 h, 0.6 MPa O2 77 30 (SGLYCA) [42]12 Au/Nb2O5 67 47 (SGLYCA)13 Au/Nb2O5 363 K, NaOH/Glycerol= 2, 6 bar O2 93 45 (SGLYCA) [43]14 Au/5CXs 333 K, NaOH/glycerol = 2, 3 bar O2 100 50 (SGLYCA) [44]15 Au/CNSd 333 K, NaOH/glycerol = 2mol/mol, 5 bar O2 100 45 (SGLYCA) [45]16 1%Au/G 393 K, NaOH/glycerol = 2/1, H2O2 100 56 (SGLA) [46]17 Au/G-SGTe 298–373 K, 1–8 bar O2, 7 h 100 50 (SGLYCA) [47]18 Au NPs 373 K, 3 h, 5 bar O2 100 36 (SGLA) [48]19 Au/MWCNT 333 K, NaOH/glycerol = 2mol/mol, 3 bar O2 100 60 (SDHA) [49]20 Au/MWCNT1tt900

f 75 50 (SGLYCA) [50]21 AuC/MWCNTg 93 47 (SGLYCA) [51]22 Au/Al2O3 333 K, NaOH/glycerol = 4, 5 bar O2, 2.5 h 99 60 (SGLCYA) [52]23 Au/Al2O3 (5% H2)

h 100 60 (SC3 prod) [53]24 Au/Al2O3 (THPC)

i 51 (SC1 prod)

Basic Environment Mono-metallicbased

Pt-based 25 5%Pt/C 1M GLY, 333 K, 21 h, 30% NaOH, Air (0.1 Mpa) 60% 47.5 (SGLYAC) [54]26 1% Pt/C 0.3 M GLY, NaOH/GLY=4 (mol/mol), 50 uC, 4 h,

O2 (0.3 Mpa)81.6% 50 (SGLYAC) [55]

27 1%Pt(Citrate) 323 K, NaOH/glycerol = 3 64 69 (SGLYCA) [56]28 PtC/MWCNT 333 K, 3 bar O2, 4 h, 3 nm Particle size 81 67 (SGLCYA) [57]29 PtC-AuC/MWCNT 98 47 (SGLCYA)30 1%Pt/CNT 333 K, 7 h, 6 bar O2 88 60 (SGLYCA) [58]31 1%Pt-0.65%Cu/CNT 90 56 (SGLYCA)

Pd-based 32 5% Pd/C 10% GLY 333 K, 30% NaOH, 5 h, air (0.1 Mpa) 100% 70 (SGLYAC) [59]33 1% Pd/G 0.3 M GLY, NaOH/GLY=4 (mol/mol), 323 K, 1 h,

O2 (0.3 Mpa)90% 62.4 (SGLYAC) [60]

34 Pd/AC 333 K, 1 h, 6 bar H2O2 100 20 (SGlYCA) [61]35 Pd-Ag/C 353 K, 5 wt% glycerol, 50mg Cat, 0.3 MPa O2 52 44 (YDHA) [62]36 1%Pd/CNT 333 K, 7 h, 6 bar O2 90 56 (SGLYCA) [58]37 Pd/CTFj 323 K, 0.3 Mpa O2, 3 h 98 81 (SGLYCA) [63]

Others 38 Cu/CoO 363 K, 0.1 Mpa O2, 3 h 80 41 (SGLA) [64]Bi-metallic based 39 1% (Pd+Au)/ G 0.3 M GLY, NaOH/GLY=4 (mol/mol), 323 K, 2 h 100% 39.1 (SGLYAC) [60]

40 1% (Au@Pd)/G 0.3 M GLY, NaOH/GLY=4 (mol/mol), 323 K, 2 h,O2 (0.3 Mpa)

100% 45.5 (SGLYAC) [65]

41 Au-Pd/C A 100mL autoclave reactor; 323 K; 4 h; Gl/M(3400), 0.3 MPa O2

90.4 71.1 (SGLYCA) [66]

42 Au-Pd/TiO2 A 50 cm3 Parr autoclave, 333 K, 4 h, 1MPa O2 100 52.7 (SGLYCA) [67]43 2.2Au-Pd/CZ A 150 cm3 batch reactor, 333 K, 6 h, 3 bar O2 100 60 (SGLYCA) [68]44 1%(Au+Pt)/C 0.3 M GLY, NaOH/GLY=4 (mol/mol), 323 K, 4 h,

O2 (0.3 Mpa)69.3% 58.3 (SGLYAC) [55]

(continued on next page)


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Table 2 (continued)

No Catalyst Reaction conditions Con (%)a Sel (%)bor Y (%)c Refs.

Basic Environment Bi-metallic based 45 Au-Pt/C 1.5 M GLY, NaOH/GLY=2 (mol/mol), 333 K,1.5 h, O2 (0.1 Mpa)

50% 36 (YDHA), 30(YHYPAC)

[36]

46 Au-Pt/TiO2 A 100 cm3 batch reactor, 363 K, 1 atm O2 100 87 (SLA) [69]47 Au-Pt/LaMnO 373 K, 0.3 Mpa O2, 6 h 75 70 (SGLYCA) [70]48 Au-Pt/LaMnO 373 K, 0.3 Mpa O2, 24 h 100 88 (STARA)49 Au-Pt/LaCrO 373 K, 0.3 Mpa O2, 6 h 95 86 (SLA)50 Au-Pt/nCeO2 373 K, NaOH/Glycerol of 4mol/mol, 5 bar O2 99 80 (SLA) [71]51 Au-Pt/Ht 333 K, Glycerol/metal= 1000, 300 kpa O2 64 69 (SGLYCA) [56]52 2%Pt9Sn1/C-Rl 333 K, 100mg catalyst, 8 h 91.1 50 (SGLYCA) [72]53 3%Pd-1%Bi (C3-1B)m 323 K, 0.3 g catalyst, Atmospheric pressure 95 68 (SGLYCA) [73]54 5%Pt-5%Bi-C 333 K, 0.5 g catalyst, 6 h, oxygen flow 150mL/min 91.5 49 (SDHA) [74]55 Au-Cu/ZnO 333 K, NaOH/glycerol molar ratio of 2, 5 h, 6 bar

O2

95 59 (SGLYCA) [75]56 Au-Ag/ZnO 10 77 (SGLYCA)57 Pt–Bi/charcoalk 50% GLY, 323 K, O2/GLY=2 (mol/mol), air (0.1

Mpa)80% 80 (SDHA) [76]

58 1:0.5Au-Rh-C2 323 K, NaOH/Glycerol= 1mol/mol, < 6 nmparticle size, 0.3MPa O2

– 78 (SGLYCA) [77]

59 Au-Cu/CeZrO 373 K, 0.6 Mpa O2, 5 h 80 79 (SGLYCA) [78]60 Ag95− Pt5/CeO2 373 K, 0.5 Mpa O2, 5 h 54 51 (SGLA) [79]

Multi-metallic based 61 Pd-Ce NPs/Fe-MIL-101-NH2

333 K, 5 nm particle size, 20 Psi O2, 353 K – 55 (YDHA) [80]

Others 62 HPW-MCM-48 100mL batch reactor; 373 K; 6 h 30 40.5 (SDHA)33.6(SGLYCA)

[81]

Base-freeEnvironment

Mono-metallicbased

Au-based 63 Au/SiO2 353 K, 24 h, H2O2 100 99 (SACA) [82]64 Au/Fe3O4 373 K, 1MPa O2, 3 h 43 61 (SGLYCA) [83]65 Au-Pt/TEGOn 333 K, 0.3 MPa O2, 4 h 60 53 (SGLYCA) [84]

Pt-based 66 Pt/MWCNT 150mLmin−1 O2, Temperature 333 K; time 6 h 70.1 2.8 (SDHA);69.8(SGLYCA)

[85]

67 Pt/S-MWCNT 150mLmin−1 O2, Temperature 333 K; time 6 h 90.4 13.3 (SDHA); 68.3(SGLYCA)

[86]

68 Pt-Sb/MWCNT 150mLmin−1 O2, Temperature 333 K; time 2 h 90 51.4 (SDHA) [87]69 1%Pt/CNT 333 K, 7 h, 6 bar O2 < 25 10 (SGLYCA) [58]70 1%Pt-0.65%Cu/CNT <40 15 (SGLYCA)71 Pt/ACPO

o 333 K, 6 h, glycerol/Pt molar ratio 890 55 35.3 (SGLYCA)15.7(SDHA)

[88]

72 Pt/ACIWIp 18.8 30.8 (SGLYCA)10.9

(SDHA)73 Pt@C/MWCNT 333 K, 0.5 Mpa O2, 3 h 58 85 (SGLYCA) [89]74 Pt-NPs/HT 333 K, 6 h, catalyst (16mg) 63 68 (SGLCYA) [90]75 Pt/HT 333 K, 5 h, glycerol/metal= 800 (mol/mol) 55 75 (SGLYCA) [91]76 Pt/Fe3O4@PPy 333 K, 0.5 Mpa O2, 24 h 91 55 (SGLYCA) [92]77 Pt/TiO2 423 K, 0.5 Mpa O2, 18 h 78 70 (SLA) [93]78 Pt/N2.5C-XC-72 333 K, 0.6 Mpa O2, 3 h 67 74 (SGLYCA) [94]

Pd-based 79 1%Pd/CNT 333 K, 7 h, 6 bar O2 Not active Not active [58]Others 80 LDH-[Cr(SO3-salen)]q 333 K, 0.2 g catalyst, 4 h 72 42.7 (SDHA) [95]

81 HPMo-V2 423 K, 3 Mpa O2, 3 h, a mmol catalyst 100 60 (SFA) [96]

Base-freeEnvironment

Bi-metallic based 82 PdPb/Al2O3 318 K, 5 vol% H2O2, 1 h 100 59 (SDHA) [97]Multi-metallic based 83 Au-Pd-Pt/TiO2 373 K, 24 h, substrate/metal= 2728/1 36.9 64.1 (SGLYCA) [98]

84 Cu (II)-Mg-Al (LDH) 333 K, 4 h, Ph= 7, 3% H2O2 40.3 40.3 (SDHA)38.3 (SFA)

[99]

a Conversion.b Selectivity.c Yields to the particular product given in parentheses.d Au based carbon nano-spheres catalyst.e Au/Graphen sample which synthesized by gold-sol (SGT) method.f Au/MWCNT thermal treatment at 900 K.g Au/MWCNT synthesized through the “sol immobilization” technique.h 5% H2 as reducing agent in Al2O3 catalyst.i THPC as reducing agent in Au/Al2O3 catalyst.j Pd nanoparticles within a nitrogen-containing covalent triazine framework (CTF).k Fixed bed reactor.l 2%Pt9Sn1/C-R, catalyst supported on the activated carbon (C) and reduced (R) at 300 °C with H2.m Cx-yB, x, y, weight % of Pd and Bi loaded on resin and “B” means reduced by Borohydride (BH4).n TEGO: Thermally Expanded Graphene Oxide.o Pt/ACPO sample synthesized by application of polyols (PO).p Pt/ACIWI sample synthesized by incipient wetness impregnation (IWI) method.q Mg-Al layered-double hydroxides (LDH) intercalated by sulphonato-salen-chromium (III) complex.


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supported catalysts. Research studies are classified in this table similarto this review content. Indeed, all the studies are divided in two maingroups of basic and base-free environments and each section includedmono-metallic, bi-metallic, multi-metallic, and other catalysts sub-sec-tions. This table provides a spectacular opportunity for comparison ofthe researches in various conditions. For instance, Rodrigues et al. [57]findings were totally different over Ptc/MWCNT (SGLCYA=67%) com-pared to the main product (SDHA= 60%) over the Au/MWCNT [38,49].Indeed, the impact of noble metals on the obtained desired productscould be clearly seen from these results. Also, Ribeiro et al. [58] com-pared the catalytic activity of mono- (Pt or Pd) and bi-metallic (Pt-Cu)based catalysts supported on CNT in both basic- and base-free en-vironments. The results are reported in the rows number 30, 31, 36, 71,72, and 79 of Table 2. In detail, mono-metallic Pt/CNT and Pd/CNTsamples showed clear results of high activity in the basic environmentswith about 88–90% conversion compared to the very low activity in thebase-free environment. Particularly, Pd/CNT was totally not active inbase-free environment. Similarly, more than 90% glycerol conversionand 56% SGLYCA were obtained over the bi-metallic (1%Pt-0.65%Cu/CNT) in the basic environment, while GLYCA selectivity dropped to lessthan<15% at<40% reaction conversion in the base-free environ-ment. In fact, the significant impact of basic- and base-free environ-ments are clear in this study, thus we should separate different re-searches based on the environment that they studied. As mentionedbefore, another significant factor in the synthesize method. Lei et al.[88] compared (rows 69 and 70 Table 2) the catalytic activity of Pt/ACmono-metallic sample which synthesized by application of polyols (PO)and incipient wetness impregnation (IWI) techniques. As results, theformer techniques showed significant improvement in selectivity ofdesired products (35% SGLYCA and 15.7% SDHA) at 55% reaction con-version, while the IWI method led to the dramatic fall in reactionconversion (18.8%) and about 40% reduction in products selectivities.Finally, Zhang et al. [96] proposed that the glycerol oxidation to FAcould be the substitute of conventional reforming process for producingH2 due to its lower energy requirement but higher yield and selectivity.

2.1. Glycerol oxidation in alkaline medium

Hosts of homogeneous and heterogeneous catalytic oxidation ofglycerol studies have been performed in an alkaline environment. Thepresence of an alkaline medium was demonstrated to favor glyceroldeprotonation, the first step in the oxidation process, considered es-sential for the oxidation of the primary alcohol [25,31,100]. In fact, theimportance of alkaline environment in the oxidation reaction has beenevaluated by many research groups.

Yuan et al. [34] reported that the activity and selectivity of Aucatalysts in enabling the oxidation reactions particularly for polyoloxidation, depends critically on the co-presence of a base (usuallyNaOH, or in essence OH−) [76]. For instance, glycerol oxidation usingvarious supported Au NP catalysts showed quite high yields of GLYCAin the presence of highly basic (molar NaOH/GL > 1) conditions, butexhibited no activity without NaOH [59]. Basic OH is usually con-sidered to be an initiator or promoter for alcohol/polyol activation byabstracting a proton from the hydroxyl or one of the hydroxyls in thesubstrate alcohol/polyol molecules [100]. The presence of alkalinemedium impacts not only the activity, but also the selectivity of cata-lysts in the aerobic oxidation of alcohols/ polyols. Thus, any furtherunderstanding of the catalytic chemistry would be impossible withoutunravelling the effects of NaOH alone on the reactivity and productselectivity of alcohols/polyols in the aerobic oxidations. The resultsrevealed that glycerol consumption rate employing NaOH alone(without Au catalyst) is 5–70 times lower than the rates in the co-pre-sence of Au and NaOH catalysts. However, the GL consumption rateemploying only Au catalyst in NaOH-free solution (i.e., without co-presence of NaOH) was even several times slower than the rate em-ploying NaOH alone, which highlights importance of the co-presence of

Au and NaOH for the oxidation reaction.As a result, in a catalytic cycle, the base catalyst (OH−) could in-

itiate the reaction by attacking the H of the hydroxyl group to producean anionic complex [R1R2CHOHeOH]−, in which the OeH bond of thehydroxyl group becomes lengthened and weakened (Fig. 3). An O2

molecule would then react with the H of the R1R2CH skeleton via thetransition state TS1, in which the CeH bond of R1R2CH is weakened. Atthis TS1 of 2-PO oxidation, the CeH bond is increased to 1.26 Å, whilethe OOeH bond is 1.41 Å. Via the TS1, the O2 molecule becomes ac-tivated, with the formation of OOH−, and the H atom in the hydroxylgroup transfers to the catalyst OH, yielding a H2O molecule and a ke-tone product. The as-formed OOH− would also react with H2O togenerate a H2O2 and a OH− (the catalyst), entering another catalyticcycle. A second channel leading to alcohol oxidation would be openedby the reaction between H2O2 and [R1R2CHOHeOH]− to form anotherketone product via TS2 as the transition state. Also, it is reported thatO2 closes the catalytic cycle by serving as an electron scavenger andregenerating hydroxide ions that are consumed in the reaction to pro-duce aldehyde intermediates and acid products [100]. The key ele-mentary steps for the two reaction channels are summarized below:

+ → −− −Channel I: R R CHOH OH [R R CHOH OH]1 2 1 2 (1)

− + → + +− −[R R CHOH OH] O R R CO OOH H O1 2 2 1 2 2 (2)

+ → +− −Channel II: OOH H O H O OH2 2 2 (3)

− + → + +− −[R R CHOH OH] H O R R CO OH 2H O1 2 2 2 1 2 2 (4)

Among these elementary steps, the R1R2CeH bond breaking owingto the O2 attack (Eq. (2)) is the rate-determining step that possesses thehighest barrier.

2.1.1. Mono-metallic catalysts2.1.1.1. Au-based catalysts. Undoubtedly, mono-metallic supportedcatalysts on various types of supports are the most popularheterogeneous catalysts reported in the glycerol oxidation reaction.Application of mono-metallic Au supported on three different carbonsnamely graphite (G, 10m2/g), ribbon-type carbon nano-fiber (CNF-R,109m2/g), and carbon nano-spheres (CNS, 3m2/g) was reported foroxidation of purified and crude glycerol in liquid phase [45]. The initialresults revealed that the highest glycerol conversion and GLYCAselectivity were obtained over Au/CNS sample and crude glycerolwas not completely oxidized even after 7 h. Meanwhile thecommercial glycerol reached about 45% GLYCA selectivity at 100%glycerol conversion after 7 h. Au/CNS sample had many open edges atthe surface due to the presence of the “unclosed” graphitic layers, tofacilitate high chemical reactivity, due to the small size of Au particles.In fact, the CNS pore sizes are approximately ≤6 nm, while the size ofthe Au particles (4.2 nm) and the molecular size of glycerol (0.62 nm)are smaller than CNS pore sizes. Thus, the active sites can be located inthe approximately small pores of CNS, generating a strong steric effectcausing the molecules to be more likely confined favoring the oxidationof primary alcohol group of glycerol.

In another attempt, the synthesis of different types of Au supportedcarbon metal (AC, G, and three types of CNFs with different structureswith respect to the arrangement of the graphitic planes of platelet (CNF-P), fishbone (CNF-F), and ribbon (CNF-R)) catalysts, prepared by twodifferent approaches (impregnation (IMP) or gold-sol (SGT)), has beeninvestigated by Gil et al., [101]. The characterization results revealedthe distribution size of Au particle in Au/AC and Au/G samples weresimilar (8–12 nm). However, the Au size was different based on thenature of CNFs. In fact, the Au particle size was in the following orderfor the three different CNFs: Au/CNF-R (12–20 nm) < Au/CNF-F(22–25 nm) < Au/CNF-P (22–27 nm). The preparation method ofcatalyst has a significant effect on the activity. Indeed, samples pre-pared by SGT method exhibited higher glycerol conversion and GLYCAselectivity as compared to the samples prepared by IMP. This is evident


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when the Au/G-SGT sample completed the conversion in 400min, butthe Au/G-IMP managed to perform only 70% glycerol conversionwithin the same time. Approximately 50% of GLYCA selectivity wasobtained from Au/G-SGT catalyst at 30% glycerol conversion. Gen-erally, the catalyst activity is in the following order Au/G-SGT > Au/CNF-R > Au/CNF-F > Au/CNF-P > Au/AC. In fact, G, the mostcrystalline metal, promoted strong anchoring of small, thin, and facetedAu particles, to facilitate the proton abstraction from glycerol and in-creased the catalyst conversion.

The reaction mechanism for catalytic oxidation of glycerol using aquasi-homogenous solution of Au NPs, prepared by the Turkevichmethod, was investigated by Skrzynska et al. [48]. Their main focuswas to determine glycerol oxidation reaction mechanism in the pre-sence of Au NPs. Fig. 4 illustrates the obtained reaction pathway forglycerol oxidation over supported Au NPs catalyst. Red, blue, and greenarrows correspond to simple oxidation, CeC cleavage reactions, andCannizzaro reactions, respectively. Abbreviations CR and NCR stand forcatalytic and non-catalytic reaction, respectively, whereas the “+” and“−” shows the extent favorability. In detail, there are two main pos-sible pathways for the primary glycerol transformation. In the firstpossible route, glycerol can be directly oxidized to GLYCA, due to thehighest observed selectivity of GLYCA at the initial stages and parti-cularly it reached about 100% at lower temperatures. This reactionproceeds favorably in the presence of a catalyst, a base, and an excess ofoxygen (CR++, NCR+). In the second possible reaction route, directoxidative cleavage of glycerol to GLA and FA, probably through theformation of highly reactive radical-type species without intermediateformation of partially oxidized C3 products (e.g. GLYCA). The secondpossible route only requires the presence of a base and oxygen, whilethe presence of a catalyst seems not to be essential. Notably, in theabsence of oxygen glycerol conversion is almost completely suppressed.Indeed, reaction conversion more than 0.5% cannot be obtained in theabsence of oxygen and without the gold catalyst even by a base andhigh reaction temperature (e.g. 80 °C). Then, GLYCA can be split intoGLA and C1 components, mainly formaldehyde (NCR++, CR+). Thelatter is supposedly immediately oxidized to FA by oxygen (CR++) oris involved in the sequence of Cannizzaro-type cross-reactions. Furtheroxidation of GLYCA to TARA seems to proceed through the formation ofHPYA (CR++), which can easily undergo classical hydrolytic CeCcleavage (NCR++). The last possible products (i.e., GA and OXALA)can be obtained by direct oxidation and/or oxidative cleavage reactionsfrom both TARA and GLYCA. It seems that direct oxidation is favored in

the presence of Au NPs, whereas high temperatures and strong basicconditions favor oxidative cleavage and straight hydrolytic transfor-mations, such as: retro-aldol, α- and β-dicarbonyl cleavage, oxidativecleavage with CO2 evolution, and Cannizzaro cross-reactions.

The effect of thermal and chemical treatments on catalytic activitywas evaluated by Rodrigues et al. [102,103]. In detail, they synthesizeddifferent types of mono-metallic (Au, Pt, Pd, Ir, and Rh) supportedcatalysts by application of thermal treatment in preparation of differenttypes of ACs (AC0: NOTRIX ROX 0.8 activated carbon, AC1: oxidizedAC0, AC1tt400: AC1 which is thermal treated at 400 °C, AC1tt900: AC1

which is thermal treated at 900 °C). In the first stage, some of thesamples such as Pd/AC0, Rh/AC0, and even Auc/AC0 (Auc: Au catalystprepared by sol-immobilization method with NaBH4 as reducing agent)reached>90% glycerol conversion after 3 h at 60 °C. However, thecatalytic activity of Rh/AC0 exhibited a downward trend by increasingthe reaction temperature and pressure above 60 °C and 3 bar. As results,the Au mono-metallic sample was selected for further investigations inthis study. In the second step, the ACs were prepared with differentlevels of acidity and basicity (various levels of oxygenated functionalgroups on the surface) by chemical treatment. Also, thermal treatmentwas used to improve the textural characteristics of the synthesizedsamples. As results, the Au/AC0, Au/AC1tt900 and Au/AC1tt900+H2

samples with basic supports (limited concentration of oxygenatedgroups) exhibited higher activity. Indeed, the thermal and chemicaltreatments significantly impacted the catalytic activity and stability.

Carbon materials have been proven to be a good catalytic support inliquid phase reactions due to their specific characteristics (e.g. acid andbase resistance, porosity, surface chemistry control and the possibilityof metal support) [81,104,105]. However, potential catalytic applica-tions of carbon nano-structures such as carbon nano-tubes (CNT), CNF,and CNS is trending [106,107]. Some of the CNTs specifications such ashigh surface area, good physical and chemical stability, and excellentelectronic properties have attracted much attention recently. In fact,external surface area of CNTs caused metal NPs to be highly exposedand accessible to reactants, which improves the efficiency of the syn-thesized catalysts. Modification of CNT properties by controllably pla-cing defects or foreign atoms such as nitrogen (N) brings along tre-mendous technological implications [108,109]. As in other crystallinesolid structures, the position of N in CNT network must not exclusivelybe substitutional to affect the properties of the structures. The proper-ties of nano-scopic objects depend crucially on the position of eachatom [110–112]. Bearing in mind that N contains one additional

Fig. 3. Possible mechanisms for aerobic oxidation of alcohols in the presence of NaOH-containing water [34].


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electron as compared to C, novel electronic properties can be expectedif N atoms directly substitute C atoms in the graphitic lattice and onecould anticipate that they would generate an n-type material [113].

Gil et al. [114] synthesized three types of nitrogen-dopped carbonnano-spheres (CNSB (thermal pyrolysis of benzene), CNSAN (thermalpyrolysis of aniline), CNSNB (thermal pyrolysis of nitrobenzene)) as asupport for Au NPs for selective oxidation of glycerol to GLYCA. Thesynthesized catalysts had the following particle size of Au/CNSB

(4.2 nm) < Au/CNSAN (10.5 nm) < Au/CNSNB (13.1 nm). The Au/CNSB with the smallest and well dispersed Au NPs reached to a highmetal-support interaction due to its high electrochemically accessiblesurface and high electron density. As a result, the Au/CNSB reached thehighest activity with 68% GLYCA selectivity at 35% glycerol conversionafter 1 h. Finally GLYCA selectivity reached to about 50% at 100%glycerol conversion after 400min at 60 °C.

Fig. 5 clearly illustrates that textural and chemical characteristics of

Fig. 4. Possible reaction pathways for glycerol oxidation with and without Au NPs (CR: Catalytic reaction; NCR: Non-catalytic reaction) [48].


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support could influence the catalytic activity and selectivity. Indeed,based on the glycerol molecular size (0.620 nm), it is evident that astrong steric effect could be generated resulting in a well-confined formof the glycerol molecules that favored the oxidation of the primaryalcohol group of glycerol (Fig. 5a). Nevertheless, in the case of Au/CNSAN and Au/CNSNB the metal active sites were located on the surfaceof the supports (Fig. 5b). Consequently, the binding of glycerol over theAu surface is sufficient for oxidation of the secondary alcohol group ofglycerol, requiring more space to react.

Rodrigues et al. [38,49–51] compared the catalytic activity of Aumono-metallic samples supported on multi wall carbon nano tubes(MWCNT) and AC. The findings revealed that the distribution of pro-ducts was totally different over Au/AC and Au/MWCNTs catalysts. TheAu/AC led to the oxidation of primary hydroxyl group which causemore selectivity of GLYCA (60%), while Au/MWCNTs favored theoxidation of secondary hydroxyl group of glycerol and increased DHAselectivity (60%). High DHA selectivity over Au/MWCNTs sample couldbe due to the large mesoporous sample (MWCNTs). Results of thermaland chemical treatments of samples in this study were in a very goodagreement with their previous findings, which is reported in Table 2,about the effectiveness of thermal treatment on improvement of cata-lytic activity [102,103].

Metal oxides are the second common supports after carbons, whichare using in the preparation of heterogeneous catalyst for glyceroloxidation reaction. In the selection of particular metal oxide as a sup-port, two utmost characteristics to be considered are strong metalsupport interaction (SMSI) and unique reversible interaction with sev-eral reagents.

TiO2 is one of the favorite metal oxides used as a support in catalyticoxidation of glycerol. Dimitratos et al. [115] evaluated the effect ofweight ratio of PVA/Au, Au concentration, heat treatment, and selec-tion of support (TiO2 or C) on the catalyst activity. Sodium borohydrideand polyvinyl alcohol (PVA)) were used for improving the pore size anddistribution of Au NPs. The characterization results revealed that bycontrolling Au concentration and PVA/Au ratio, well-defined NPs withan average diameter of 3–5 nm could be fabricated. In fact, the increaseof PVA/Au ratio reduced the average Au particle distribution in the Au/C sample. Also, higher Au concentration could reduce the effect of PVA/Au ratio on the Au particle size. However, the Au average particlediameter surged in the Au/TiO2 supported sample when increasing thePVA/Au ratio at high Au concentration. The Au/C catalyst was thehighest active sample with 73% selectivity to GLYCA and 79% glycerolconversion in the presence of the lowest (0.65 g) PVA/Au ratio. How-ever, the Au/TiO2 sample reached to 73.7% GLCYA selectivity at just

50% glycerol conversion with the narrowest particle size distribution(PVA/Au=2).

Another metal oxide used as a support in catalytic oxidation ofglycerol is SiO2. Kapkowski et al. [82] evaluated the efficiency of SiO2,Cu, and Ni and the use of Au NPs support in deep glycerol oxidation in adiluted and viscous liquid H2O2. Generally, the Au/SiO2 samples ap-peared to be more reactive than Au/C sample while the Au/Cu and Au/Ni indicated good activity with 85.7% and 80.2% selectivities of aceticacid at 41.2% and 39.6% glycerol conversions. However, the twosamples were destroyed due to leaching effect by Cu or Ni. However,the Au/SiO2 sample reached to 90–99% acetic acid yield of at completeglycerol conversion in the diluted aqueous glycerol solution. Althoughin a relatively viscose liquid phase these values decreased to 40% and20%, the addition of acetonitrile could improve the acetic acid yield to40%. Also, the 0.1%Au/SiO2 sample exhibited the highest stabilityamong synthesized samples. Complete conversion was attained in thefirst and second runs, however, the conversion dramatically dropped by30% in the third run.

Lakshmanan et al. [116] compared two types of ceria-supported AuNPs catalyst preparation methods, which were chemical reduction withglycerol or conventional hydrogen-reduced method in selective oxida-tion of glycerol to LA. The initial finding revealed that the glycerolbased chemical reduction approach led to synthesizing larger Au par-ticles compared to the H2 reduction method. The catalyst activity test ofthe 1%Au/CeO2 reported larger Au average particle size, decreased theselectivity of GLYCA and surged the LA selectivity. Indeed, the 1% Au/CeO2 sample, prepared by chemical reduction method, and the Auparticle average size of 6 nm conferred the highest LA selectivity of 83%at 98% glycerol conversion.

The increasing demand for high quality products seem to contradictwith the needs to reduce all types of pollution during different manu-facturing processes [117]. Environment friendly chemistry plays animportant role in the design of novel catalysts [118]. In this regard,micro-encapsulation method has been recently recognized as a usefultechnique to immobilize metal catalysts onto polymers. In addition, it isan alternative strategy to enable safe handling, easy recovery, reuse anddisposal at an acceptable economic cost [119,120]. The micro-en-capsulation refers to the incorporation of an active substance in a shellor a matrix of a carrier component. The catalysts could be separatedfrom a reaction mixture by simple filtration and recycled, making themsuitable for green chemistry processes [121]. Despite all the recent ef-forts in this field, the described methods to prepare hierarchicallycomplex Au micro/nano-structures constantly suffered from variousissues (e.g. complex systems, tedious processes, difficult control of

Fig. 5. Surface Morphology of (a) CNS and (b) N-Doped CNS: Effect of Au NPs size on product distribution [113].


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particles’ size and shape, and lower reactivity of immobilized metalcatalysts compared to the corresponding original catalysts) [118,119].

Gil et al. [121] for the first time studied the synthesis conditions ofmicro-encapsulated (Au-PUF), which is Au NPs by in-situ polymeriza-tion of urea and formaldehyde (PUF) in the presence of an organicsolvent (dimethyl sulfide (DMS)), supported on carbon black catalyst.Increasing amount of DMS from 0 to 35mL reduced the Au en-capsulation efficiency from 75 to 40%, while the Au particle size in-creased from about 120 nm to about 300 nm. The lower DMS amountcaused smaller and better dispersion of Au particles with more homo-genous particle size distribution. The catalytic activity tests revealedthat GLYCA selectivity increased by reducing the Au particle size,which showed the importance of catalyst preparation method toachieve better catalytic activity. In detail, the catalyst prepared using30mL DMS gave 12% GLYCA selectivity while the one with just 2.5 mLDMS attained 42.2% GLYCA selectivity.

Furthermore, Gil et al., [117] evaluated the effect of different kindsof stabilizers or stabilization agents (cetyltrimethylammonium bromide(CTAB), sodium dodecyl sulfate (SDS), polyvinyl pyrrolidine (PVP),polyvinyl alcohol (PVA), and tetrakis (hydroxyl methyl) phosphoniumchloride (THPC)) on the morphology, particle size of encapsulated Au,the Au particle size distribution and the encapsulation efficiency.

As results, the Au particle size and Au encapsulation efficiency in-creased in the following order MP.Au-CTAB (131 nm, 0.3 wt%) < MP.Au-SDS (135 nm, 0.7 wt%) < MP.Au-THPC (287 nm, 0.8 wt%) < MP.Au-PVP (339 nm, 1.1 wt%) < MP.Au-PVA (487 nm, 1.3 wt%). The catalytic tests results indicated that catalyst activity increased

with decreased encapsulated Au particle size as follows:CTAB≈DS > THPC > PVP > PVA. Therefore, catalyst with smalland well dispersed metal particles and high metal surface expositionperformed well. The MP.Au-CTAB and MP.Au-SDS reached quite si-milar glycerol conversion of 53% and 52% with 52% selectivity to GLAand about 40% selectivity to GLYCA. Fig. 6 shows a schematic diagramof the PUF micro particles formation process containing Au as a func-tion of the stabilizer. When the stabilizer introduced into the systemwas cationic in nature (CTAB and THPC) an electrostatic attractionbetween AuCl4− ions and the positive charged head groups of thestabilizer took place. However, the anionic stabilizer (SDS) forcedAuCl4− ions away from around their head groups, while, the non-ionicstabilizers (PVP and PVA) formed micelles with no apparent ioniccharges, leading to a high incorporation of AuCl4− ions. In addition, thestrong hydrophobicity of their long hydrocarbon chains, favored thestability of the micelles. Therefore, the concentration of AuCl4− ionsinto the micelle could be promoted. After the addition of the stabilizer,AuCl4− ions were reduced to Au metal using DMS (water-insolublesolvent). Au nuclei stabilized by stabilizer molecules began theirgrowth stage. Subsequently, the solution of elemental Au was thenadded to the polymeric suspension, leading to the formation of PUFmicro-particles containing Au.

Schunemann et al. [122] reported quite different and novel route ofvisible-light-assisted process of glycerol oxidation using direct plas-monic photo-catalysis (DPP) process over two series of mono- and bi-metallic Au supported catalysts. In particular, DPP is an appealing in-tegrated system where plasmonic NPs act as both visible-light absorber

Fig. 6. Schematic procedure for the formation of Au-containing PUF micro-particles using different stabilizing agents [116].


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and active site for the chemical reaction [123,124]. An inherent benefitof DPP is the broader choice of applicable supports, as there is nolimitation to light absorbing semiconductors, as in conventional plas-monic photo-catalysis. This allows the application of supports withdesirable properties such as efficient pore confinement of metal NPs,high surface area, facile morphology control, abundance and cost effi-ciency. Initially a series of mono-metallic sample of Au supported ondifferent types of SiO2 (Mono-dispersed mesoporous silica (MS), SBA-15, KIT-6, and MCM-41) tested in glycerol oxidation reaction and the6%Au/MS sample showed the highest glycerol conversion of 33% in thepresence of visible-light illumination compared to the other mono-metallic samples. Secondly, the increase of Au content from 6wt% to12.5 wt% surged glycerol conversion from 33% to almost 60%. Themain reason for better activity of sample was SiO2 domains whichcaused homogenous distribution of Au NPs on the support (MS). Fi-nally, the activity of 6%Au/MS sample was compared with a synthe-sized bimetallic AueCu sample. The activity test under visible-light il-lumination demonstrated an increase in Cu content from 0wt% in the6%Au/MS sample to 1 wt% in the 5%Au-1%Cu/MS catalyst increasedthe glycerol conversion from 18% to almost 45%, respectively. TheDHA selectivity also increased from 80% to 90% in the presence of6%Au/MS and 5%Au-1%Cu/MS catalysts, respectively.

2.1.1.2. Pt-based catalysts. Pt is the second noble metal which has beenused in various chemical reactions such as glycerol oxidation. However,it is very expensive and strategic metal owing to its localized productionregions (mainly in South Africa, Russia, Canada, and USA) [29]. Thus,many researchers try to reduce the Pt applications and improve itscharacteristics. Zhang et al. [124] investigated the application of fourdifferent bases (LiOH, NaOH, KOH, and Ba(OH)2) in selective oxidationof glycerol to LA over Pt/AC monometallic catalyst. Initialinvestigations in the presence of Pt/AC catalyst and base-freeconditions confirmed the huge influence of basicity on the Pt/ACcatalyst activity. The Pt/AC could reach 44.9% selectivity of LA at58.5% glycerol conversion in the presence of NaOH at 90 °C. However,further investigation exhibited that application of LiOH significantlyenhanced the LA selectivity to 69.3% at 100% glycerol conversion(T= 90 °C, LiOH/glycerol ratio= 1.5). The effect of various alkalinesolutions on Pt/AC catalyst activity was as follows: Ba(OH)2 < KOH < NaOH < LiOH. The main reason for higheractivity of Pt/AC in the presence of LiOH was due to the oxygensolubility in solution. In fact, when more oxygen is dissolved, thehydroxide ions regenerate faster and promote LA production.

The first heterogeneous monometallic Pt-NP based catalyst(Pt1@CK), stabilized by ethylene in glycerol oxidation to LA, was syn-thesized by Oberhauser et al. [125]. This approach curtailed the for-mation of oxidation products and ethylene acted as the hydrogen ac-ceptor. Small Pt-NPs, generated by the metal vapor synthesis (MVS)technique, allowed the synthesis of size-tailored Pt NPs, regardless ofthe support. Indeed, the 1.5 nm sized Pt-NPs, generated by MVS andsupported onto a high surface area carbon Ketjenblack (i.e., Pt1@CK),was easily stabilized against aggregation by high ethylene partialpressure (875 psi) during the reaction. Pt1@CK maintained its catalyticperformance for a maximum duration of 780 h and LA chemo-se-lectivity of 95% in three consecutive catalytic reactions.

The application of two types of Pt supported catalyst on both TiO2

nano-fiber (Pt/NF-TiO2) and TiO2 nano-powder (Pt/NP-TiO2) wasevaluated by Chornaja et al. [126]. Higher glycerol/Pt ratio sig-nificantly reduced the glycerol conversion, while the increase in oxygenpressure from 1 to 6 atm enhanced the activity of the two catalysts. The4.8%Pt/NF-TiO2 reached 63% GLYCA selectivity with a glycerol con-version of 100%. Meanwhile, the 4.8%Pt/NP-TiO2 reached 68% GLYCAselectivity with a glycerol conversion of 95% at the same optimal re-action conditions (0.3 mol/l glycerol feed concentration, 1.5mol/lNaOH, glycerol/Pt ratio was 300mol/mol, P(O2)= 6 atm at 60 °C).

In another study, Li and Zaera, [127] investigated the activity and

selectivity of Pt/SiO2 catalyst as a function of Pt NPs size and shape.Three sets of samples were prepared. Two sets were functions of metalloading and prepared by impregnation using water (W) and ethanol (E)as solvent (Pt/SiO2-W and Pt/SiO2-E) while the third set used pre-madePt colloidal NPs with specific size and shape (cub-octahedral (C) andtetrahedral (T)). For the first two groups of catalysts, samples whichwere made with ethanol displayed an overall better quality in terms ofsize and dispersion. The average Pt NPs size increased with metalloading. The Pt NPs size for the Pt/SiO2-W sample was in the range of3.9–5.7 nm for 0.2 to 10 wt% Pt loading and for the Pt/SiO2-E samplesit was in the range of 1.9–3.8 nm for the same amount of Pt loading.Also, the size dispersion was much narrower for the Pt/SiO2-E (0.3 nm)compared to Pt/SiO2-W (3 nm) sample.

On the other hand, the average NPs size of T-Pt/SiO2 and C-Pt/SiO2

NPs were 5.6 and 9 nm, respectively for the third series of synthesizedsamples. The activity tests revealed that the increase of metal loadingdid not only surged the metal NPs size but also enhanced both activityand chemical selectivity. In fact, larger NPs appeared to exhibit higherintrinsic activity and to favor oxidation at the terminal alcohols groupsof the glycerol. Also, higher activity was observed with cub-octahedralPt NPs compared to tetrahedral samples.

2.1.1.3. Pd-based catalysts. Pd is the third common noble metal whichhas been used widely in various chemical reactions such as glyceroloxidation to value-added chemicals. Hamid et al. [128] synthesizedmonometallic Pd catalyst supported on AC, hydrotalcite (HTc), and AC-HTc. The TEM characterization results revealed that the Pd NPs had anaverage size of 9.0, 9.1, and 11.2 nm on the HTc, AC, and AC-HTcsupports, respectively. Also, CO2-TPD analysis exhibited that Pd/HTcsample possessed the highest concentration of basic sites compared tothe other two samples. Meanwhile, the glycerol conversion and GLYCAselectivity increased in the following order: Pd/AC < Pd/HTc-AC < Pd/HTc and Pd/HTc-AC < Pd/AC < Pd/HTc, respectively.Among all the tested catalysts, the Pd/HTc sample exhibited thehighest GLYCA selectivity (80.4%) at 70.4% glycerol conversion.Higher basic sites and dispersion of Pd NPs are identified as the keyfactors for higher catalytic activity of Pd/HTc.

Application of various solvents (acetonitrile (CH3CN), dimethylsulfoxide (DMSO), and CH3CN/H2O) and terminal oxidants (benzo-quinone (BQ), air, O2) over Pd supported catalyst was investigated inselective oxidation of glycerol to DHA [129]. The catalytic oxidation ofglycerol with 5mol%Pd/C catalyst and in the presence of BQ andCH3CN reached to>96% selectivity of DHA at 97% glycerol conver-sion at room temperature. However, by only a simple change in solventfrom CH3CN to the mixture of CH3CNn/H2O (7:1), 100% conversion canbe obtained in just 3 h. Furthermore, the reaction was completed inonly 15min when DMSO was used as solvent. All these results revealedthat solvent has a significant effect on the rate of oxidation reaction.Meanwhile, DHA yields of 69% and 73% were obtained in the presenceof CH3CN/H2O solvent and 10mol% Pd/C catalyst, respectively withthe aid of O2 and air as the terminal oxidants.

The second most common metal oxide considered as a support incatalytic oxidation of glycerol is Al2O3. Chornaja et al. [130] in-vestigated the use of monometallic Pd NPs supported on Al2O3, Y2O3, C,and Pyrex in the selective oxidation of glycerol to GLYCA. The initialactivity revealed that 1.25%Pd/Al2O3 and 2.5%Pd/Al2O3 samplesreached up to the highest selectivities of 66% and 74% for GLYCA at 70and 74% glycerol conversions, respectively. However, other samplesexhibited less than 53% glycerol conversion at 60 °C in 420min.

2.1.1.4. Other mono-metallic catalysts. As mentioned earlier, themajority of reported studies in this field have used noble metals (e.g.Au, Pt, and Pd) in their catalyst synthesis processes. However, someresearch groups tried to reduce the catalyst synthesis cost by utilizingcheaper metals (e.g. Ni, Rh, Ir) as active sites. Xu et al. [131] used Ru(OH)4 NPs supported on reduced graphite oxide (r-GO) in the presence


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of various Lewis acids (AlCl3, CrCl3, ZrCl2, and FeCl3) at differentreaction temperatures (120 – 180 °C), O2 pressures (0.5–1.5 Pa) forglycerol oxidation to FA. Initially, application of solely Ru(OH)4/r-GOor FeCl3 samples could not achieve more than 20% conversion.However, combination of Ru(OH)4/r-GO and other four Lewis acidsand combinations of Ru(OH)4r-GO with FeCl3 recorded the best resultswith 53.5% yield of FA at 95.5% glycerol conversion.

Another metal oxide examined in glycerol oxidation was iron oxide.The first successful attempt was performed by Crotti end Farnetti,[132]. They investigated application of various iron complexes with thepolydentate ligand bis (2-pyridinylmethyl) amine (BPA) samples forproduction of DHA in the presence of H2O2 as oxidant. The initial ac-tivity tests indicated that Fe(OTf)2+ BPA sample gave the highestglycerol conversion of 47% with 49% selectivity to DHA among all thetested Fe supported samples. However, further investigations revealedthat only two products were produced (DHA and FA) in all the ex-perimental runs. However, by tuning the experimental conditions, thehighest glycerol conversion of 50% was achieved with complete elim-ination of FA and 100% selectivity to DHA. Further investigation re-cently by Farnetti and Crotti, [133] on iron salt (Fe(OTf)2 revealed thatabout 98% FA selectivity and just traces of DHA and GLYCA obtained atcomplete glycerol conversion in the presence of MeCN:H2O (2:1), and4.4 of H2O2/glycerol ratio in 106min.

Jedsukontorn et al. [134] investigated the TiO2-induced photo-catalytic oxidation of glycerol in the presence of H2O2. They evaluatedthe effect of TiO2 dosage (1–3 g/L), H2O2 concentration (0.3–1.5M),light UV intensity (1.1 4.7 mW/cm2) and irradiation time (4–8 h). Theactivity tests revealed that TiO2 and H2O2 had significant effect onglycerol conversion in the presence of UV-light. All experimental runswithout UV-light did not perform. Also the absence of one, either TiO2

or H2O2 in the presence of UV-light gave very low glycerol conversion.However, the presence of all three components (TiO2, H2O2, and UV-light) is essential to reach a good glycerol conversion and product se-lectivity. 71.25% GLA and 19.61% GLYCA selectivities at 71.4% gly-cerol conversion were obtained at 3 g/L TiO2, 1.5 mol/LH2O2, and4.68mW/cm2 light intensity after 8 h. Other efforts in the photo-cata-lytic partial oxidation of glycerol to DHA in water were performed byMaurino et al. [135] and Minero et al., [136] using Degussa P25 andMerck TiO2 catalysts and their fluorinated derivatives. In fact, by usingfluorinated Degussa P25 and very low concentration of glycerol (ca.1 mM), almost 100% selectivity to DHA was obtained for conversionslower than 30% [134].

2.1.2. Bi-metallic based catalystsAs reported earlier, applications of bi-metallic catalysts in oxidation

of glycerol to value-added chemicals have attracted much attention inthe last decade. In fact, combinations of two metals, which one or bothof them are noble metals could reduce the catalyst synthesis cost andimprove the catalytic activity in the majority of cases. Also, bi-metallicsamples could improve catalytic activity compared to the mono-me-tallic ones due to ensemble electronic bi-functional effects.

Improvement of catalytic activity in the presence of bimetallic cat-alysts conferred by Rodriguez et al. [137]. Using a series of Au-Pd/Cbimetallic catalysts prepared by electro-less deposition (ED) in liquidphase and basic environment. In fact, Au was systematically and con-trollably deposited on the Pd surface of a monometallic Pd/C sample toform a series of catalyst with various surface composition of Au. Thecatalyst activity increased significantly by increasing the θAu value,which is the actual coverage of Au on Pd, to θAu = 0.5 where TOF wasabout 20 times larger than Pd/C and 4 times larger than Au/C mono-metallic samples. The higher activity of bimetallic sample could beattributed to ensemble electronic bi-functional effects or combination ofthese reasons. The highest catalytic activity (TOF=2.3 s−1) at θAu =0.5 inferred both Pd and Au sites were involved in the rate determiningstep. Concerning product distribution, GLYCA selectivity remainedapproximately constant (80%) at 50% glycerol conversion for Au-Pd/C

bimetallic catalyst even by high Au coverage up to the θAu< 0.98. Infact, the result of this study was slightly better compared to the previousfindings over mono- and bi-metallic catalysts of Pt and Au supports[38–50,107,125]. Also, Zhao et al. [138] reported, supporting Pd on Ausurface eliminate undesired growth of Pd NPs and significantly reducedcatalyst deactivation process during the glycerol oxidation reaction.

Mimura et al. [139] was one of the first groups to report synthe-sizing supported bimetallic samples on metal oxide (Au0.83-Pd/TiO2,Au0.91-Pd/TiO2, and Au/TiO2). The Au/TiO2 reached over 87% of theproduction yield (GLYCA, TARA, GLA), while Au0.83-Pd/TiO2 reachedthe highest yield (65%) of GLYCA among all samples tested. Evaluationof Pd effect on bimetallic samples activity exhibited that Au has anaverage diameter of 3.4 nm in Au monometallic sample. However, Ausize increased to 7.1 nm in the Au-Pd bimetallic sample. The preciseSTEM characterizations of NPs provide interesting results and the re-searchers concluded that the presence of Pd at the edge corner of theparticles reduced the side reactions.

Application of bismuth (Bi) as promoter in the bimetallic catalyticoxidation of glycerol was evaluated by various researchers. Hu et al.[140] did a research on the sequential impregnation of Pt and then Bi,followed by NaBH4 reduction for high glycerol oxidation rate. In ad-dition, three types of activated carbons (AC1 (SBET= 600m2/g,DP=4 nm); AC2 (SBET= 1200m2/g, DP=2.2 nm); and AC3(SBET= 1600m2/g, DP=1.6 nm)) were used as support. Catalystssupported on AC2 and AC3 had higher Pt dispersion due to the largersurface area. However, the catalyst activity decreased when the supportsurface area increased. This may be explained from the average supportpore size and molecular sizes of reactants glycerol (0.620 nm), H2O(0.027 nm), and O2 (0.074 nm). In fact, the molecular size of glycerol,comparable to the average pore size of AC2 and AC3 supports, led topore diffusion resistance. The optimum catalyst composition was de-termined to be 3 wt% Pt, 0.6 wt% Bi, supported on NoritDarco 20–40mesh AC1 to reach the maximum DHA yield of 48% at 80% glycerolconversion corresponding to 80 °C, 30 psig, and initial pH=2.

Zhang et al. [141] evaluated the application of monometallic (Pt/AC) and bimetallic (Cu-Pt/AC) supported catalyst in selective oxidationof glycerol to lactic acid (LA). The bimetallic 0.5%Cu-1%Pt/AC sampleexhibited the highest activity with 69.3% selectivity to LA at 80%glycerol conversion and 437 h−1 TOF value, while the 1%Pt/AC onlyreached 56.6% LA selectivity at 40.6% glycerol conversion and 239 h−1

TOF value at 90 °C in 4 h. The characterization results revealed that Cucould improve the dispersion of Pt on the support. The bimetallicsamples possessed 2.3–3.5 nm average particle size compared to the4.2 nm conferred by the monometallic catalyst. It was also found Cucould prevent the aggregation of some Pt-Cu interfaces. In addition,proper surface Pt/Cu molar ratio (0.62) and the presence of Cu+ andCu0 species also had vital roles in glycerol conversion and LA selectivityenhancement. However, the existence of Cu2+ species would decreaseglycerol conversion and LA selectivity.

One of the recent studies, which reported promising results was theuse of flower-like Bi2WO6 as a highly selective visible-light photo-cat-alyst toward aerobic selective oxidation of glycerol to DHA usingoxygen as oxidant in water at room temperature and atmosphericpressure [142]. Consequently, the highest DHA yield of 87% and se-lectivity of 91% were achieved by Bi2WO6 (B) sample at 96% glycerolconversion in 5 h. Fig. 7 demonstrates the proposed photo-catalyticreaction mechanism over Bi2WO6 (B) catalyst. Under visible-light ir-radiation, electron–hole pairs are generated by the semiconductorBi2WO6 in water. The adsorbed glycerol on the surface of Bi2WO6 wasoxidized by the positive holes to form the corresponding intermediate,which further reacted with oxygen or activated oxygen (e.g. O2

−) toproduce the target product DHA. In addition, the adsorption capacity ofDHA was weaker than glycerol over Bi2WO6, suggesting DHA can beeasily desorbed from the surface of Bi2WO6, which was beneficial forobtaining high selectivity to DHA. Indeed, the photo-activity test onDHA in water over Bi2WO6 under visible-light irradiation suggested


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that DHA was rather stable in the present photo-catalytic reactionsystem, which could also be one of the reasons contributing to highDHA selectivity.

2.1.3. Multi-metallic based catalystsCurrent research efforts have been primarily focused in two direc-

tions: (1) designing well-defined bimetallic noble metal catalysts and(2) incorporating earth-abundant metals to noble metal systems [143].The high costs of noble metal catalysts and limited availability oftenpose significant economic challenges in practical applications of thesecatalysts. Replacing expensive noble metals with cost-effective non-noble metal catalysts without loss of performance will clearly make theaqueous phase oxidation (APO) of biomass economically sounds moreattractive. Non-noble metals are believed to be able to tune the bindingstrength between functional groups and noble metals [74,144].Therefore, research efforts have been made to incorporate traceamounts of non-noble metal promoters such as Bi, Cu, Ni to noble metalsystems [145]. These noble/non-noble metal combinations are found toeffectively enhance Au, Pt, and Pd catalyst activity and selectivity, bypreventing CeC cleavage of glycerol molecules [146].

The synthetic hydrotalcite-like solids, also called as layered doublehydroxides (LDHs) generally qualify as clay [147]. However, whencompared with cationic clay minerals, LDHs have wider range of che-mical compositions of layers based on a different choice of metals in theframework and interlayer anions. The ratio of metal atoms within thelayers can be finely tuned in a wide range. The calcination of LDHs alsoyielded metal oxide structures with well-dispersed active species [148].The doping of additional transition metals into MgAl-based LDHs canfurther tailor the catalytic performance in the oxidation.

In fact, LDHs high melting temperature (2135 °C) and thermal sta-bility combined with all other characteristics makes it to be a promisingcatalyst support for applications such as environmental catalysis andfine chemical production [149,150]. For catalytic purposes, high sur-face areas are often desirable. Fortunately, over the last few years, LDHshave been successfully prepared by different methods, allowing one toobtain materials with tunable surface areas and chemical compositions.

For the first time, Zhou et al. [146] explored the possibility for anactivation of molecular oxygen using a series of synthetic LDHs withhydrotalcite (HT)-like layered structures with the incorporation of athird metal (Ni, Zn, Cu, Co) as catalysts for the selective oxidation ofglycerol. Cu/Al-Mg LDH catalyst showed the highest activity among allthe tested samples at various conditions. FA was obtained as a mainproduct with 51.27% selectivity at 86% glycerol conversion. However,the calcination of CueAleMg LDH sample at 650 °C for 6 h totallytransformed the catalyst activity and product distribution. As a result, itreached to just 70.0% selectivity of FA at 97.3% glycerol conversion. Ina similar study, Wang et al. [151] also used a series of Mg-Al LDHssupported by different transition metals of chromium (Cr), Manganese(Mg), Iron (Fe), Cobalt (Ca), and Copper (Cu). The activity test showedthat only LDH-[Cu(SO3-Sole)] sample reached the highest GLYCA

selectivity of 86.1% at 82% glycerol conversion. The main reason forthe high activity of Cu supported sample was because Cu species en-hanced the oxidation ability of O2 to primary alcohol in glycerol byreacting readily with eCHO group in intermediate of GLYLD.

Wu et al. [152] evaluated the addition of two amino modifiers insynthesizing a series of CuNiAl/HTs catalysts (CuNiAl-HT-C3H6NH2 andCuNiAl-HT-C5H10N2H3) for selective oxidation of glycerol to GLYCA.The CuNiAl-HT-C5H10N2H3 sample with the highest basicity reachedthe highest GLYCA selectivity of 76% and 68.1% glycerol conversion at60 °C in 4 h. The latter catalytic activity was similar to the mono-metallic Au/C commercial sample. This proves that the addition ofamino groups led to the improvement in surface Lewis basicity of cat-alyst which enhanced the catalytic activity by the activation of the C-Hbond of alkoxy intermediate to form GLYLD. Similarly, the surfacehydrophobicity of catalyst which led to the increase in GLYCA se-lectivity was enhanced by addition of amine groups.

Jin et al. [143] investigated two types of MgOeAl2O3 preparationmethods namely co-precipitation (c) and modified sol-gel (s) on theactivity of cobalt-based catalysts (Co0.15/Mg3Al-c and Co0.15/Mg3Al-s)in glycerol oxidation to di-carboxylic acids (TARA and OXALA). Co0.15/Mg3Al-c catalyst, prepared by co-precipitation method, indicated a re-latively uniform distribution of Co, Mg, and Al elements on the catalystsince Co species were incorporated into the Mg3Al(OH)y(CO3)z frame-work during precipitation. Thus, double-layered structures with uni-formly incorporated eOeCoeOe groups were obtained. In contrast,the Co0.15/Mg3Al-s sample, prepared by modified sol-gel method,showed that Co tends to predominantly dispersed on the surface of thesupport. Authors believed that Co2+ species replaced the surface eOHgroups of Mg3Al(OH)y(CO3)z structure and formed surface MeOe-

CoeOeM (M: Mg or Al) framework during the modified sol-gel process.Consequently, CO0.3/Mg3Al-S catalyst reached about 90% product se-lectivity (64.4% TARA and 24.4% OXALA) at 100% glycerol conversionunder mild conditions of 0.1 MP O2, at 70 °C and in 24 h.

After a few attempts in application of LDHs supported by varioustransition metals in oxidation of glycerol to GLYCA, Villa et al. [149]synthesized Au NPs supported on three spinel-type MgAl2O4 (Com-mercial MgAl2O4, co-precipitation MgAl2O4, and flame pyrolysisMgAl2O4) support using three different techniques, which are tetrakis(hydroxymethyl) phosphonium chloride (THPD)-protected sol, deposi-tion-precipitation with urea (DP calcined), and magnetron sputtering(MS). The characterization results revealed that all the samples uni-formly coated with Au NPs except the THPC one, which indicated largeareas of support material with no Au NPs coverage. The activity testsconfirmed that all the tested samples exhibited typical trends of higheractivity for smaller particle size. The DP-prepared catalyst on co-pre-cipitated MgAl2O4 with the smallest particle size of 2.2 nm was found asthe most active catalyst (1390 (Aumol)−1 h−1). In fact, this catalystreached to more than 80% glycerol conversion with almost 50% GLYselectivity in 40min. In addition, the co-precipitated THPC and FP-THPC samples with a quite similar particle size of 3.9 and 4.2 nm anddifferent Al/Mg ratios of 6 and 4.2, respectively reached differentproducts (C3) selectivity (GLY and TAR). Surprisingly, the Au/Mg ratiowas the main determining factor due to Al-rich surface promoting theCeC bond cleavage. Besides that, Au particle size also has been re-ported to have a high influence on product selectivity.

2.1.4. Non-metallic based catalystsThe recently developed “self-supporting” strategy [153] constitutes

an attractive approach for immobilization of homogeneous catalysts.Different from the conventional approaches using various solid sup-ports for homogeneous catalyst immobilization, the self-supportedcatalysts were constructed via supra-molecular assembly of a multi-topic bridging ligand with an appropriate metal salt. A coordinationpolymer is generated, with catalytic active sites that preserve thestructural features of its homogeneous catalyst counterpart, but is in-soluble in common solvents and thus obviates the need for an external

Fig. 7. Reaction mechanism for selective oxidation of glycerol to DHA usingBi2WO6 sample under visible-light irradiation [142].


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support [154,155].Following the recent interest in exploring the applications of novel

transition metal complexes in catalysis, supra-molecular chemistry, andmaterial sciences [156,157] the preparation of main-chain iridium co-ordination polymers from rigid bis-benzimidazolium salt precursorsand their application as self-supported catalysts in the oxidative con-version of glycerol to LA is carried out. Sun et al. [153] prepared aseries of very interesting catalysis of NHC-Ir coordination polymers. Theresults of this study attracted much attention due to extremely highactivity and selectivity of the prepared samples at mild reaction con-ditions. Also, the NHC-Ir sample exhibited an amazing reusability withup to 31 times just by a simple filtration process. The highest potassiumlactate yield of 98% was obtained over NHC-Ir sample in air with gly-cerol (1.1 mL) catalyst volume 2.3 mg, KOH (2.5mg), and H2O (8.3 mL)at 115 °C in 24 h.

One of the environmentally friendly alternative routes for glyceroloxidation is dehydrogenation via a hydrogen transfer reaction. In thelast few decades, transfer hydrogenation has been developed as an ef-ficient reduction method for organic substrates such as ketones, alde-hydes and imines. These reactions are homogeneously catalyzed bytransition metal derivatives, with the hydrogen donors generally em-ployed being FA and 2-propanol. However, glycerol can be envisaged asa di-substituted 2-propanol, to perform a transfer dehydrogenation ofglycerol in the presence of a suitable catalyst and an appropriate hy-drogen acceptor. In principle, both primary and secondary hydroxylgroups of glycerol can be dehydrogenated, while, according to the re-lative oxidation potentials of secondary vs. primary alcohols, the hy-drogen transfer is expected to selectively occur from the secondaryhydroxyl group, yielding DHA as the main product. Table 3 illustrates afew early studies on novel homogeneous catalytic systems for chemo-selective dehydrogenation of glycerol to DHA. As summarized; theobtained reaction conversions were below 37%. However, the DHAselectivities were in acceptable range of 23–93%. All these results in-ferred that glycerol oxidation over multi-components homogenouscatalysts still require more research and investigations.

2.2. Glycerol oxidation in base-free environment

As reported in previous sections, changing reaction conditions,support, and even structure of base material could significantly changedthe catalyst activity and selectivity to the desired product. However, theconventional glycerol oxidation reaction in alkaline environment led tothe formation of salts of the acids instead of free carboxylic acids whichis desirable in industrial products. Thus, researchers are considering abase-free environment for oxidation of glycerol. The preliminary in-vestigations revealed that the supported Pt catalysts are effective in thisfield. However, the catalysts enhance the CeC band cleavage andproduce CO2 and FA, while leaching of Pt deactivated sample fast [18].Various research groups are working intensely in search of varioussupport materials, conditions, and metals for improvement of glyceroloxidation reaction to the desired products in a base-free environment.Thus, we report recent attempts on base-free glycerol oxidation reactionin the following section.

2.2.1. Mono-metallic supported catalysts2.2.1.1. Au-based catalysts. As mentioned in Section 2.2, among noblemetals, Pt is the most common metal that has been used in glyceroloxidation in base-free environment. Hence, applications of Ausupported samples are rarely reported in this field. The studyconducted by Liu et al. [161] investigated the glycerol oxidation toDHA in a base-free medium using different synthesized Au/Al2O3, Au/TiO2, Au/ZrO2, Au/NiO, and Au/CuO catalysts. The effects of variousreaction temperatures, pressures, and catalyst loading evaluated todetermine the optimal reaction conditions. The 2 wt%Au/CuO catalystexhibited the highest activity among the synthesized sample with DHAyields up to 80% at 40–50 °C. In fact, for the first time, this studyobtained DHA as the only primary product of glycerol oxidationreaction using supported Au NPs in the base-free conditions.

Yuan et al. [162] investigated the effect of acid-base property of theAu/Mg-Al catalyst on the catalytic activity. The variation in Mg/Al ratioof support led to significant changes in catalysts textural characteristics.Increasing the Mg/Al ratio from 0 to 4.8 reduced the surface area from222 to 197m2/g, while the pore volume and pore diameter increasedfrom 0.28 to 0.71 cm3/g and 3.6 to 11.7 nm, respectively. Various Mg/Al ratios also led to the significant changes in product distribution ofglycerol oxidation. The Au/Mg-Al catalyst with the strongest acidity(without Mg) reached the highest DHA selectivity (81.7%) and just4.4% selectivity to GLYCA. Meanwhile, increasing the basicity to thehighest amount (Mg/Al= 4.8) gave 49% and just 10.9% GLCYA andDHA selectivity, respectively.

2.2.1.2. Pt-based catalysts. One of the earlier attempts which confirmssuccessful glycerol oxidation to a desired product (e.g. GLCYA) in thebase-free condition performed by Mengyuan et al. [163]. Theyevaluated the application of different AC size (253.2–9.3 μm) assupport for Pt mono-metallic catalyst. Initial findings revealed that PtNPs were highly dispersed on support surface with average particle sizeof 2.8–5 nm. The S50 (5 wt% Pt supported on activated carbon whichwas ball milled for 50min) sample reached to 46.7% yield of GLYCA at70.3% glycerol conversion at 60 °C in 6 h. In fact, the Pt/C catalystactivity increased from 50 to 70.3% by decreasing the AC supportparticle size from 253.3 to 9.3 μm. The main reason for the latter surgeof catalytic activity was higher accessibility of reaction to the Pt NPs.

Three types of Pt supported carbon nano-fibers (CNF) catalyst (Pt/CNFs, Pt/S-CNFs, and Pt/HNO3-CNFs) were synthesized by Zhang et al.[164]. The Pt/S-CNFs sample possessed the smallest Pt particle size(1.5–1.8 nm) among the three prepared catalysts. It conferred thehighest-glycerol conversion (89.9%), almost two times higher thanglycerol conversion over Pt/CNFs (30.1%) and Pt/HNO3-CNFs (47.3%)with 83.2% selectivity to GLYCA in 6 h reaction time at 60 °C. In ad-dition, Wang et al. [165] synthesized a series of Pt supported on me-soporous carbon nitride (MCN) samples. The Pt particle size decreasedfrom 3.4 nm to 2 nm in Pt/MCN-1 to Pt/MCN-6, respectively by in-creasing the N content in the support. However, the surface area (SBET),pore volume (VP), and pore diameter (DP) increased significantly. Theactivity test exhibited that the support basicity enhanced the glyceroloxide, GLYLD and GLYCA. In particular, 58.3% GLYCA and 24.3%GLYLD yields at 63.1% glycerol conversion were obtained at 60 °C in4 h over the Pt/MCN-5 sample. The Pt/MCN-6 sample with the smallest

Table 3Catalytic oxidation of glycerol over supported polymer samples.

Catalysts Oxidants Other reaction conditions Con (%) Sel (%) or Y(%)

Refs.

Ir (1,5- hexadiene) (4,7-dimethyl- 1,10-phenanthroline) Cl

Acetophenone A Schlenk tube; 373 K; 0.5 h; [Ir] = 3.0× 10−3M;[K2CO3]/[Ir] = 2

28 47 (SDHA) [158]

[HIr(cod)Prn- N(CH2CH2PPh2)2] Benzaldehyde A Schlenk tube; 373 K; 1 h; [Ir] = 3.0× 10−3 M 37 23 (YDHA) [159][O⊂ Cu4{N(CH2CH2O)3}4

(BOH)4[BF4]25.0 mmol H2O2 (35% inH2O)

A round-bottom flasks or Pyrexcylindricalvessels; 298 K;30 h

7.5 93 (SDHA) [160]


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Pt particle size reached the lowest GLYCA selectivity with higher C1products selectivity.

Initial studies on application of CNTs as a support for catalyticoxidation of glycerol in a base free environment were reported about adecade ago. Generally, there are limited numbers of studies on theapplication of CNTs in base-free condition. Zhang et al. [164] hadconfirmed that N-dopped MWCNT could enhance the dispersion of Pton support and reduced the Pt particle size to 1.8 nm. Indeed, the Pt/N-MWCNT-973 could reach 83% GLYCA selectivity at 54.9% glycerolconversion; almost double the glycerol conversion over the Pt/MWCNTat 60 °C. Lei et al., [88], for the first time, evaluated the effect of metalparticle size on catalytic activity in the base-free environment. Indeed,increasing Pt loading from 2 to 8 wt% increased the Pt NPs size from 1.5to 2.9 nm in the Pt NPs/CNT samples. The catalytic activity reached itspeak (TOF=1000 h−1) for Pt size up to 2.5 nm. In addition, larger PtNPs size also enhanced GLYLD selectivity (8 wt% Pt/CNTs – 43.5%),while smaller size Pt NPs (2 wt% Pt/CNTs) favors GLYLD transforma-tion to GLCYA by 54% selectivity. Indeed, smaller Pt NPs with moreatoms at vertices and edges, strongly adsorb hydroxide, and favored theformation of terminal diol.

Ricahrd et al. [166] reported interesting findings on application ofpolymer-supported Pt catalyst (Pt/PDVB-dep, Pt/SPDVB-dep, Pt/PDVB-imp, Pt/SPDVB-imp) in glycerol oxidation reaction in a base-free con-dition. In this study, the activity of the polymer supported catalysts wascompared with the commercial Pt/C and Pt/Al2O3 samples. Pt NPs weredeposited in defined size, content, and distribution on the polymer. Allof the prepared samples have similar Pt size of 1.5–6 nm. The activitytest revealed that polymer supported samples were as active or evenmore active than carbon catalyst with almost 95% glycerol conversioncompared to 90%. These samples also exhibited 60% GLYCA selectivitycompared to 40% over Pt/C sample during a 24 h reaction time at 60 °C.Furthermore, the polymer supported sample product yield was twicemore (60%) than Pt/Al2O3 with just 30% GLYCA yield in the first 8 h ofreaction. In fact, polymers could change the polarity of the supportfrom hydrophobic, which is a major factor influencing catalytic per-formance, to the hydrophilic. The results of this study affirmed thatpolymeric support is highly potential to be used as a support in glyceroloxidation reaction instead of commercial oxides and carbons.

2.2.2. Bi-metallic based catalystsNing et al. [167] was the first group to demonstrate soluble Bi

species in reaction solution could enhance the catalytic performance ofPt/NCNT catalyst for selective oxidation of glycerol to DHA in a base-free environment. Initially, they used Pt/NCNT as catalysts with anaverage diameter of 3.2 nm. It reached 60.5% selectivity of GLYCA at31.5% glycerol conversion and 11% DHA selectivity. Then pre-loadedBi sample (Pt-Bi5/NCNT) significantly increased the DHA selectivity to55.5%. The initial reaction rate also improved from 66.2 mmol/m2 h forPt/NCNT to 107.4 (mmol/m2 h) for Pt-Bi5/NCNT sample. Furthermore,concurrent application of 100mg Pt/NCNT and 50mg Bi/NCNT in-creased the DHA selectivity to 53.5% at 34.3% glycerol conversion,which was almost similar to Pt-Bi5/NCNT activity. The experimentalstudies demonstrated Bi leaching from Bi/NCNT and Bi adsorption onPt/NCNT performance. If similar performance can be achieved by usinga simple mechanism mixture, the preparation of catalyst might besimplified. Thus, a direct application of Bi salts or oxides into the re-action solution was performed in order to confirm the hypothesis. Theaddition of Bi(NO3)3·5H2O, BiCl3 and Bi2O3 to the reaction medium ledto a significant improvement of DHA selectivity from 11.1% to 64.1%,57.5%, and 59.2%, respectively. Thus, regardless of the way how Bi wasintroduced to the reaction system, similar enhancement of DHA se-lectivities could be reached. The optimal amount of Bi(NO3)3-5H2O wasabout 1 wt% for achieving 78% DHA selectivity. The Bi1-Pt/NCNT (in-situ) system exhibited 10% higher conversion compared to Pt-Bi1/NCNT sample. Further investigation with antimony (Sb) instead of Bialso confirmed the earlier findings in this study and DHA selectivity

increased to 80.5% using Sbx-Pt/NCNT (in-situ) system. Therefore, theresearchers concluded that the promotion mechanism might be uni-versal across the group 5 metals.

Fig. 8 illustrates the mechanism of Bi1-Pt/NCNT (in-situ) system forselective oxidation of glycerol to DHA. Pt/NCNT sample is active andselective for the production of GLYLD from glycerol (Fig. 8a). However,Bi3+ was adsorbed onto Pt NPs to firstly occupy the high-energy stepsites, and then the terrace sites, in the presence of soluble Bi3+ in thereaction solution. As a consequence, the selectivity was tuned byblocking the high-energy Pt sites where the adsorption and activation ofprimary hydroxyl groups or over-oxidation and transformation of DHAwere preferred. Finally, the Bi–Pt sites were formed on the terracesurfaces of Pt NPs through the geometrical blocking effect (Fig. 8b). TheDHA selectivity improved on these sites due to the chelation effect ofBi–Pt catalysts with the secondary hydroxyl group of glycerol.

Brett et al. [168] evaluated the application of two bimetallic cata-lysts in oxidation of glycerol to C3 products (GLYA and TARA) at am-bient temperature (23 °C). The Au-Pt/MgO and Au-Pd/MgO were syn-thesized by immobilizing colloidal metal particles on MgO support. Theinitial experimental results revealed that the metal molar ratios andsupport material had strong effect on the catalytic activity. The Au-Pt/MgO catalyst with 1:1 mol fraction reached about 80% selectivity of C3products at 60 °C. However, the selectivity of C3 products obtained wasabout 90% at just 40 °C by increasing the metal ratio of Au/Pt (1/3).The catalyst still exhibited high conversion (42.5%) with morethan>88% selectivity of C3 products when extending the reactiontime to 24 h and simultaneously decreased the reaction temperature tojust 23 °C. All the experimental runs on the Au-Pt/MgO catalyst in-dicated that it was more active and selective to C3 products comparedto the Au-Pd/MgO sample. In addition, Shen et al. [169] reportedsynthesizing the Au-Pt/TiO2 with Au/Pt atomic ratio of 3/1, whichexhibited high activity with 50%, 30%, and 20% selectivities to GLYLD,DHA, and GLYCA. The Au-Pt/TiO2 catalyst with higher Pt content ac-tually led to higher selectivity to GLYLD and GLYCA while with higherAu content the DHA formation was enhanced. These effects indicatedthe role of electronic properties of the catalyst in tuning, not only ac-tivity but also selectivity to desired products in the glycerol oxidation.

Villa et al. [170] conducted a study on the application of supportedAu catalysts for oxidation of glycerol in acidic environment. A series ofmonometallic clusters deposited on AC, TiO2, MgAl2O4, and H-morde-nite zeolite were synthesized by sol immobilization technique. Initialfindings exhibited that 1%Au/AC, 1%Pd/AC, and 1%Pt/AC samplesachieved very low catalyst activity with C1 molecules (e.g. FA) as themain product. Meanwhile, 1%Au/H-mordenite and 1%Pt/H-mordenitesamples indicated C1 products reduced, but C3 products increasedsignificantly. Thus, they concluded that the support material was cru-cial in products selectivity. Furthermore, they continued their study andprepared a range of supported bimetallic Au-Pt catalysts by sol im-mobilization method using acidic (H-mordenite, SiO2, MCM-41, andsulfated ZrO2) and basic (NiO and MgO) oxides as supports [171]. Interms of activity, the presence of a basic support such as MgO (iso-electric point of 10.4) increased the activity (657mol molAuPt−1 h−1).The Au-Pt/MgO was able to maintain high selectivity toward GLYCAeven at high conversion as long as the temperature was maintainedbelow 25 °C. Au-Pt/MgO was a very active catalyst, exhibiting an initialactivity (657 converted moles of glycerol molAuPt−1 h−1) superior tothose observed when the same Au-Pt NPs were immobilized on NiO(283mol of glycerol molAuPt−1 h−1) and the acidic supports (113, 105,156, and 228mol of glycerol molAuPt−1 h−1 for S-ZrO2, H-mordenite,SiO2 and MCM41, respectively). Indeed, a higher selectivity to C3compounds (GLYCA+GLYLD+TARA+DHA) (∼95%) was obtainedusing acidic supports whereas basic supports, in particular MgO, pro-moted CeC bond cleavage, forming C2 and C1 products in highamounts (59%).

Hirasawa et al. [172] studied the performance and reaction me-chanism over a bimetallic Pd-Ag/C catalyst in selective oxidation of


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glycerol to DHA. Initially, the performance of various Pd/C sampleswith Ti, Mn, Ni, Re, Ir, Au, and Bi was compared with Pd-Ag/C catalystand the highest DHA selectivity of 81.9% was achieved over Pd-Ag/Csample. In addition, application of various supports was tested (SiO2,TiO2, Al2O3, ZrO2, and CeO2) for Pd-Ag catalyst. All the supports exceptCeO2 reached more than 80% DHA selectivity. However, the glycerolconversion reported was less than 20% for all the tested samples. Theaddition of Ag to Pd improved the interaction between glycerol and thecatalyst surface. The adsorption of carboxylic acids with β-hydroxylgroup, 1,3-dicarboxylic acids, and the ketones with β-dicarbonylstructure on Pd blocked the sites for oxygen activation causing thedeactivation of the Pd-Ag/C sample.

2.2.3. Non-metallic based catalystsIn the study conducted by Sharninghausen et al. [173], a series of

iridium (Ir) complexes samples were synthesized as the first homo-geneous catalysts for the conversion of glycerol to LA in the base-freeenvironment. These samples had higher activity and selectivity than thepreviously reported heterogeneous systems. In addition, H2 was gen-erated as a useful by-product. The advantage of their method fulfils themain principles of green chemistry since the process required mildconditions, solvent free, renewable feedstock, degradable product andcatalyst loadings as low as 0.003mol%. The highest LA selectivity of96.6% was achieved at 94% glycerol conversion over the spectacularC12H16IrN4O2.BF4 catalyst with 15:1 ratio of glycerol/water after 24 hat 130 °C. The synthesized sample also had been applied to crude-gly-cerol without prior purification. Same level of high LA selectivity ob-tained albeit with lower activity. Conventionally, the metal-catalyzeddehydrogenation step yields either DHA or GLYLD, which then can beconverted into LA (in the form of lactate) under alkaline conditions.However, the synthesized iridium sample gave selectivities higher than95% for LA-the highest as reported to date for any chemo-catalyzedprocesses and more typical of fermentative processes. The by-productH2 could also be employed elsewhere in a commercial plant reducingthe overall cost of the process. Fig. 9 depicts reaction pathways ofglycerol to LA and other common by-products in green and red, re-spectively. In detail, the first step was Ir-catalyzed dehydrogenation ofglycerol to DHA or GLYLD, which are known to interconvert in alkalinemedia. Based on the literature, authors believed that the Ir precursors

synthesized in this study might be more selective for the dehy-drogenation of the secondary alcohol to yield DHA over GLYLD, con-sistent with our observation that DHA led to higher yields of LA thanGLYLD when used as starting material. Subsequent base-catalyzed de-hydration of GLYLD would lead to PAL and its enol form, which reactsby an intra-molecular Cannizaro reaction to yield LA. The major sideproduct observed was EG, which is probably produced from GLYLD by aretro-aldol reaction, with concomitant formation of FA.

Despite the enormous applications of zeolite and heteropoly acid(HPA) catalysts as active components in various heterogeneous cata-lytic processes, application of these types of catalysts are rarely re-ported in glycerol oxidation reaction. For the first time, application ofzeolite in a continuous oxidation reaction investigated by Lari et al.[174] using iron containing zeolite catalyst in the presence of molecularoxygen. They synthesized a series of iron containing catalysts (FeA, Na-FeA, FeZ-s873, FeZ-s1173, FeS, FeS-s873, FeSA-s1173, Fe/Z4O, Fe/Z1000, and Fe/S) using different preparation methods of various cat-alysts with different Bronsted or Lewis acidic site concentrations andstrengths. However, among the prepared samples, the FeS-s873 catalystwhich prepared by isomorphous substitution of iron in the all-silicaframework followed by activation by steam (873 K) reached DHA yieldof 90% due to its mild activity and high dispersion of iron species in theform of isolated cations or small FeOx clusters in extra framework po-sitions.

2.3. Bio-catalysis

Glycerol can also be oxidized with biocatalysts, such as enzymes andmicroorganisms to produce DHA [28]. Initially, Bertrand observed theproduction of DHA from glycerol through a bacterial route in 1898. TheSorbose bacillus was used to oxidize certain secondary alcohols to theircorresponding ketone. The Sorbose bacillus was found to be identicalwith the acetic acid bacterium Bacillus xylinum. DHA was producedfrom the enzymatic activity of DHA-synthase (DHAS) in-carboxydobacterium, Acinetobacter sp. strains jc1 Dsm3803, when itwas grown in methanol [175]. Intact cells of the following micro-organisms, Acetobacter suboxydans ATCC 621, Acetobacter xylinum a-9,Gluconobacter melanogenus IFO 3293, and G. melanogenus IFO3294,were studied for their glycerol oxidizing activity [176]. As a result, the

Fig. 8. Schematic reaction mechanism for the selective oxidation of glycerol (a) to GLYLD over Pt/NCNT and (b) to DHA in the BiX–Pt/NCNT (in-situ) system [168].


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xylinum A-9 reached the highest activity.In the last decade, Gluconobacter oxidans formerly known as A.

suboxydans was one of the most extensively used microorganism inresearch for DHA production [177]. It was a Gramnegative bacteriumobligate aerobe belonging to Acetobacteraceae (found only on flowersand fruits) [178] family. It produced DHA via incomplete oxidation ofglycerol with the activity of glycerol dehydrogenase [176,179–181].Gatgens and Degner had genetically manipulated the DHA-producing G.oxidans strains to overproduce the glycerol dehydrogenase and im-proved DHA production in the conventional process for large scale DHAproduction [182].

Bizzini et al. [183] reported the formation of DHA phosphate byEnterococcus faecalis when using glycerol as a carbon source. In parti-cular, they proposed two different pathways of glycerol metabolismaccording to the phosphorylation–oxidation pattern under aerobic andanaerobic conditions. Under anaerobic conditions, glycerol is oxidizedby glycerol dehydrogenase and then phosphorylated by DHA kinase,whereas under aerobic fermentation, the majority of the E. faecalisstrains metabolize glycerol by phosphorylation through the action of aglycerol kinase and then oxidation byglycerol-3-phosphate oxidase.Similarly, Joseph et al. [184] reported that a gram-positive bacteriumlike Listeria monocytogenes essentially needs a glycerol kinase (GlpK1)for glycerol uptake and metabolism. A nanoparticle-supported enzymesystem was described [185] for the bio-transformation of glycerol intoDHA (yield of 160 g per g immobilized enzyme) in the presence of thecofactor NAD+ using glycerol dehydrogenase from Cellulomonas sp.This has revealed the possibility of producing value-added chemicalsfrom bio-renewable resources with in-situ co-factor regenerationthrough designed reaction pathways from not native to living organ-isms. When the redox potential of the microorganism is a limitation forthe substrate range, the use of mediators has been recommended topromote electron transfer from the substrate to the enzyme [186].During the oxidation process, the mediator reduces oxidized electronshuttles. Mediators act as catalysts by reducing the activation energy ofthe reaction [187]. Liebminger et al. [186] used 2,2,6,6-tetra-methylpiperidine-1-oxyl (TEMPO) at low concentrations (< 6 mM) topromote the conversion of glycerol in the presence of laccase fromTrameteshirsuta to GLYLD. Higher concentrations increased the

conversion rate of glycerol but decreased the selectivity, and com-pounds like GLYCA and TARA are formed. The pH of the mediumplayed an important role because TEMPO decomposed and the enzymewas inactivated under acid conditions (pH 4). A patent by Wolf [188]reported the production of GLYLD via the reaction of glycerol with amethanol dehydrogenase purified from Methylobacterium organophilum.The culture medium for the bioconversion contained up to 20% glycerolwith pH in the range of 5–11, and the temperature ranging from 5 to50 °C under aerobic conditions. Maximum conversion of 35%of theglycerol was reported in a period of 24 h. The enzymatic production ofGLYCA from glycerol has also been achieved through oxidative fer-mentation by the action of acetic acid bacteria such as Acetobacter andGluconobacter strains in a jar fermenter. A bioconversion of 136.5 g L−1

was obtained when using G. frateurii and 101.8 g L−1 with A. Fropicallis[189].

As the glycerol molecule is a small uncharged molecule, it can crossthe inner cell membrane through an integral membrane protein (gly-cerol uptake facilitator, GlpF, in the case of Escherichia coli) which en-ables facilitated diffusion. Glycerol can be metabolized to glycerol-3-phosphateglycerol kinase (GlpK). Therefore, it has the potential to besubstituted for common and bigger carbohydrates used in the fermen-tation process, such as glucose, starch or sucrose [190]. Table 4 de-scribes some examples of strains which can grow in the presence ofglycerol as a carbon source.

Rocha-Martin et al. [191] has investigated glycerol dehydrogenase(GlyDH) catalyzes the region-selective oxidation of glycerol to yieldDHA. Three recombinant GlyDHs from Geobacillusstearothermophilus,from Citrobacterbraakii and from Cellulomonas sp. were stabilized bycovalent immobilization. The highest activity recoveries (40–50%) ofthe insoluble preparations were obtained by immobilizing these en-zymes with the presence of polyethylene glycol (PEG). Noteworthy,these immobilized preparations were more stable and less inhibited byDHA than their soluble counterparts. In particular, GlyDH from G.stearothermophilus immobilized on agarose activated with both amineand glyoxyl groups and cross linked with dextran aldehyde was 3.7-foldless inhibited by DHA than its soluble form. It also retained 100% of itsinitial activity after 18 h of incubation at 65 °C and pH 7. This is one ofthe few examples where the same immobilization protocol has

Fig. 9. Reaction pathways of glycerol to lactic acid (green) and other common by-products (red) [173].


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minimized enzyme product inhibition and maximized thermal stability.

2.4. Summary

In summary, different types of supports (carbon supports, carbonnano-tubes (CNTs), Mg-Al layered-double hydroxides (LDHs), metal-oxides, and polymeric supports) with combination of various noblemetals (e.g. Au, Pt, Pd) have been widely used in glycerol oxidationreaction to synthesize a novel catalyst with high stability and activity.The majority of research studies reported that the presence of particularnoble metal or support as the most influential factor on catalyst activity.Besides, the number of basic sites, synthesize method, pore size, andapplication of bi- or tri-metals instead of mono-, have been reported asother influential factors to achieve higher activity of the synthesizedcatalysts.

In particular, application of sol-immobilization approach for cata-lyst synthesis exhibits significant effect on NPs pore and particle sizesand activity of the final sample. However, conventional synthesizingmethods such as impregnation suffer from poisoning and rapid deac-tivation. In addition, photo-catalytic oxidation of glycerol is a very in-teresting approach for transformation of glycerol to value-added che-micals due to its environmentally friendly characteristics and moderatereaction parameters. Unfortunately, besides acceptable results that areobtained in this method, there are still limited numbers of studies in thisarea.

Also, it is recommended that application of noble metals should berestricted due to their non-economical nature as well as their limitedavailability in future. Application of other types of catalysts such assupported hierarchical zeolites, heteropoly acids, and even metal or-ganic frameworks (MOFs) in this field is rarely reported. These types ofcatalysts have showed acceptable results in other research areas and itis recommended the application should start or accelerate the glyceroloxidation process. In fact, application of in-effective synthesis ap-proaches (e.g. impregnation) and materials (e.g. carbon and noblemetals) should be replaced with novel ones to reach the ultimate goals,which are production of value-added chemicals through bio-refineriesinstead of petrochemical industry.

3. Electro-chemical oxidation of glycerol

The electro-chemical oxidation of glycerol has been a subject ofgrowing interest among researchers in the last few years [208–210].

Glycerol oxidation to CO2 generates 14 Fmol−1, which is more thantwice them ethanol energy content. Furthermore, glycerol is producedin large amounts as a by-product (10% w/w) in biodiesel manu-facturing and can be considered as a potential candidate for fuel cells.In this context, the knowledge about the mechanism of electro-oxida-tion of glycerol must be improved before practical systems with ap-propriate catalysts can be designed. Concerning this issue, recent stu-dies have provided important information about the electro-oxidationpathways by using high performance liquid chromatography [211] andin-situ FTIR spectroscopy [210,212,213].

The main advantage of the electro-chemical process when comparedto other oxidation processes is better control of the desired productsselectivity when the reaction is performed in aqueous media with lowreaction temperature and pressure. The electro-chemical oxidation ofglycerol can be performed in fuel cell and electrolysis cell through areversed ionic diffusion migration process which is anion-exchangemembrane (AEM) or a direct proton diffusion migration process alsoknown as proton-exchange membrane (PEM). The fuel cell leads to co-generation of value-added chemicals and electrical energy through di-rect oxidation reaction of glycerol at the anode and reduction of oxygenat the cathode (Fig. 10a). However, the electrolysis cell leads to co-generation of hydrogen and value-added chemicals as well as waterreduction at the anode and cathode, respectively (Fig. 10b). In a PEM,instead of just water, the anodic side contains an aqueous solution ofthe organic compound. The organic compound and water are oxidisedto CO2, H+ and electrons at the anode. The H+ travels through themembrane where it recombines with the electrons, which have passedthrough the external circuit to form pure H2 gas at the cathode. Theexternal circuit provides the electrical potential to drive the reaction(Fig. 10c). In an AEM, the fuel is supplied at the anode and oxygenthrough air, and water are supplied at cathode. Fuel is oxidized atanode and oxygen is reduced at cathode. At cathode, oxygen reductionproduces hydroxides ions (OH−) that migrate through the electrolytetowards the anode. At anode, hydroxide ions react with the fuel toproduce water and electrons. Electrons go through the circuit producingcurrent (Fig. 10d). The overall electro-chemical oxidation of glycerolinto hydrogen presented in Eq. (5):

+ → +C H O 3H O 3CO 7H3 8 3 2 2 2 (5)

with the anode and cathode reactions given by Eqs. (6) and (7):

+ ↔ + ++ −C H O 3H O 3CO 14H 14e3 8 3 2 2 (6)

Table 4Glycerol oxidation using bio-catalysts.

Microorganism Products obtained Yield Refs.

Clostridium pasteurianum Butanol1,3-propanediolLactate

17 g L−1, 30mol%18mol% under phosphate limitation and 34mol% under iron limitation25mol%

[192][193]

Klebsiella pneumoniae Ethanol1,3-propanediol

Yield: 31 g L−1; productivity: 1.2 g L−1 hYield: 53 g L−1; productivity: 1.7 g L−1 h

[194][195]

Escherichia coli EthanolSuccinic acid

86%7%

[196]

Clostridium butyricum 1,3-propanediol 35–48 g L−1, 5.5 g L−1 h [197]Citrobacter freundii 1,3-propanediol

Lactic acid25 g L−1

4.6 g L−1[198]

Klebsiella planticola EthanolFormate

30mmol L−1

32mmol L−1[199]

Schizochytrium limacinum Docosahexaenoic acid Yield: 1.68 g L−1; productivity: 0.28 g L−1 day [200]Enterobacter aerogenes Ethanol

Hydrogen0.96 g L−1 at 0.85mol.mol glycerol−1

1.12 g L−1[201]

Propionibacterium freudenreichii sp. shermanii Vitamin B12 4.01mgmL−1 in batch culture [202]Propionibacterium acidipropionici Propionic acid 44.62−1.12 g L−1 and 0.20 ± 0.0075 g L−1 h

106 g L−1[203][204]

Anaerobiospirillum succiniciproducens Succinic acid 19 g L−1 in feed batch fermentation [205]Gluconobacte roxydans Dihydroxyacetone 385 g in 34 h from 400 g glycerol

∼30 g L−1[206][207]

Acetobacter tropicalis D-glyceric acid 22.7 g L−1 from 200 g L−1 glycerol in 4 days [188]


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http://mg

http://mL

+ ↔+ −14H 14e 7H2 (7)

In order to produce hydrogen by electrochemical reforming at aviable rate it is necessary to have fast reaction kinetics. However, theglycerol oxidation reaction is complex, thus it is predicted there may bea large over-potential required in order for the reaction to proceed at areasonable rate. Therefore, choosing a suitable electro-catalyst to

minimize this over-potential will be critical. Based on the types ofproducts which can be obtained in a PEM reactor, there are three mainreaction pathways: partial oxidation, dissociative oxidation, and de-hydration.

Fig. 11 exhibits a summary of possible reaction pathways occurringin this system. First, glycerol is oxidized to GLYLD, isomerized to LA,and further oxidized to GLYCA [214]. GLA and CO2 are formed by CeC

Fig. 10. Working principle of (a) fuel cell, (b) electrolysis cell, (c) proton exchange membrane (PEM), and (d) anion exchange membrane (AEM) [29].

Fig. 11. Glycerol oxidation pathways in a PEM reactor [214].


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bond cleavage. The third pathway is the formation of acetol, which canbe produced via acid-catalyzed dehydration in contact with a Nafion®membrane. Nafion® can be the source for the protons for this reactionas it is highly acidic when hydrated.

The first report related to the successful co-generation of chemicalsand energy by electro-chemical oxidation of glycerol was accounted bySimoes et al. [29] in 2012, followed by Zhong et al. [215] who eval-uated electro-catalytic oxidation of glycerol for co-generation of value-added chemicals and electricity over a Pt/C anode catalyst in an anionexchange membrane fuel cell. The results revealed that the pH of thereaction media, glycerol concentration, and electrode over potentialhad significant impact on the desired products (C3 acids) selectivities.

The first successful attempt for electro-catalytic oxidation of gly-cerol to only value-added chemicals was reported by Ciriminna et al.[216]. In their work, OXALA with 98% selectivity at 100% reactionconversion in the selective oxidation of glycerol using a Br/TEMPOcatalyst was reported. They used 2,2,6,6-tetramethyl piperidine-1-oxyl(TEMPO) in the presence of NaOCl as a regenerating oxidant at pH 10and 275 K. They observed amount of DHA and TARA increased as in-termediate by-products in the presence of micro-porous sol-gel silicaglasses dopped with TEMPO. To explain the improvement of catalyticactivity, it should be considered that in contrast to traditional surfacederivatisation methods that leave the anchored nitroxyl radicals un-protected at the material surface, the sol-gel homogeneous encapsula-tion isolates the radicals within nano-porous sol-gel cages, that is withinthe hundreds of square meters per gram of internal surface area(430m2/g for SG-TEMPO) compared to the negligible amount of mo-lecules entrapped at the outer reaches (1 ± 2m2/g) of the particulatematerial. And it is precisely this unique structural feature of sol-gelsthat protects the radicals from the intra-molecular quenching of themolecules bound in proximity at the silica surface, which was recentlyfound to be responsible for the continuous decrease in the activity ofsilica-anchored TEMPO in consecutive alcohol oxidation runs withOCl-/Br- as internal oxidant.

Criminna et al. [217] obtained 25% yield of DHA during a waste-free glycerol electro-chemical oxidation reaction at the anode throughthe application of a small electron potential (1.1 V vs Ag/AgCl) to aglycerol/water solution (0.05M) with bio-carbonate (0.2M) at pH 9.1using 15mol% TEMPO after 20 h. The results revealed that long reac-tion time up to 200 h did not cause over-oxidation to carboxylic acid.

Even though there are limited numbers of research studies onelectro-chemical oxidation of glycerol reported in literature [27,28],the knowledge related to the electro-catalytic oxidation of glycerol haveexpanded significantly during the last five years which reflected theimportance of value-added chemicals such as DHA, GLYCA, MA, andTARA from a bio-renewable source such as glycerol. Similar to thecatalytic oxidation of glycerol, the elector-catalytic reaction could alsobe performed in both alkaline and base-free environments. However,the majority of electro-catalytic oxidation of glycerol studies has beenperformed in the alkaline environment but only few in acidic or neutral(base-free) environments.

The application of noble metals NPs such as Au, Pt as electrodes inacidic or alkaline medias or supported metal NPs for electro-catalyticoxidation of glycerol in simultaneous generation of value-added che-micals and energy has attracted much attention recently [218,219].Grace and Pandian [88] reported the first novel process of noble metalsNPs (Pt, Pt-Pd, and Pt-Pd/Ru) dispersion in nanometer scale ontoconducting polymer materials without using bulk Pt electrodes forglycerol oxidation. Pt-Ru supported poly-aniline sample registered thehighest activity in glycerol oxidation reaction. Initial studies carried outin the presence of supported mono-metallic Pt/C and Pd/C samples.However, Pt is very expensive and Pt and Pd deactivation duringelectro-catalytic oxidation reaction in prolonged reaction time aremajor disadvantages of Pt and Pd noble metals [220]. Thus, applicationof Au NPs on various support materials was proposed as the most ef-fective method for oxidation reaction. In addition, during the last fewyears some techniques have been developed for clearer or higherquality detection of products during the electro-catalytic reaction. Forinstance, the in-situ FTIR and iso-topically labeled glycerol was pro-posed to study the CeC band cleavages of glycerol and the 13CO2 and12CO2 behaviors in the electro oxidation of glycerol over polycrystallinePt [221]. The second method is [211] the combination of cyclic- vol-tammeter (CV) and online chromatographic techniques for faster de-tection of products in glycerol oxidation reaction. These detectionmethods provided a detailed insight to the different mechanisms ofglycerol oxidation on Pt and Au electrodes. Table 5 summarizes some ofthe previous studies in electro-catalytic oxidation of glycerol to value-added chemicals.

3.1. Mono-metallic supported catalysts

3.1.1. Au-based catalystsRecent studies revolve on application of Au NPs with or without

metal NPs supported on different types of carbons. Electro-oxidation ofglycerol over a new series of Au NPs on various support catalysts (ex-tended poly (4-vinylpyridine) functionalized graphene (Au-P4P/G),carbon black (Au-CB), P4P functionalized reduced graphene oxide (Au-PmAP/G), and poly (m-aminophenol) (Au-P4P/rGO)) revealed that Au-P4P/G possessed the highest activity toward C3 products [224]. Highactivity of catalyst could be attributed to the weakest interaction be-tween Au NPs and GLYCA. The lowest d-bond center of Au-P4P/G fa-cilitated desorption of GLYCA to desorbs from Au surface and hinderedfurther decomposition of GLYCA into other products. The Au-P4P/Gcatalyst could achieve 68.6% GLYCA production at 0.2 V and the ratiobetween C3 products and others (C1+C2) was 4.92 compared to theAu-CB with just 0.96.

The main focus of some research is merely on improving the electro-oxidation reaction conditions and finding the optimum amount ofparameters in this field to enhance the catalytic activity. The evaluationrelated to the effect of 1–20wt% Au loading on the Au/C electrodeactivity indicated that high amount of Au loading inhibited formationof Au oxide reducing glycerol oxidation due to poisoning caused by

Table 5Electro-chemical oxidation of glycerol using different supported samples.

Catalysts Reaction conditions Con (%)a Sel (%) or Y (%)b Ref

Pt/C 1.0M glycerol, 0.5 V fuel cell voltage, 6 h 20.4 83 (SC3(GLYCA & TARA)) [215]Pt/C 150–500mV electrode potential, 70–20 kJ/mol activation energy, 508–533 K – 80 (SCa (CO2, Co, CH4)) [218]Pd60-Ni40/C

Pd80-Ag20/CA three-electrode cell, onset potential of 0.4 V, 4 h 30

2445 (SGLA)50(SGLA)

[219]

Au/C-AQAu/C-NC

<0.45 V potential, 12 h 95.682.2

61.8 (YTAR) [222]

Au/C 1.5 V onset potential, 50 °C, 6 h 80 26 (STAR)57(SMOXA)

[223]

a Conversion.b Selectivity or yield.


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accumulation of glycerol oxidation products within the layers [225]. Inanother study, the effect of three different particle sizes (≤4.7 nm,14.7 nm, and≥ 43 nm) of Au/C catalyst was investigated. The catalystwith the smallest particle size unexpectedly registered the highest ac-tivity. The Au/C with particle size less than 4.7 nm had twice the ac-tivity of Au/C with particle size larger than 43 nm. Fundamentally,small Au particle could provide the main requirement of a fuel cellcatalyst namely high mass activity, low over potentials and high sta-bility [226].de Souza et al. [227] further compared the application ofdifferent 3-carbon-atom chain alcohols with glycerol. 1-propanol, 2-propane-1,2-diol and propane-1,3-diol were used in the electro-oxida-tion process on Au electrode in an alkaline medium. The results in-dicated that the reactivity of alcohols decreased in the following order:glycerol > propane-1,2–1,2-diol ≫ propane-1,3-diol > 2-pro-panol≈ 1-propanol. Interestingly, glycerol and propane-1,2-diol ex-hibited high CeC bond breaking rate due to the presence of productswith C1 and C2 atom chain. However, the electro-oxidation of otheralcohols led to products with C3 atom chain. Thus, the presence ofvicinal OH groups in the alcohol molecule is a key component for theCeC bond cleavage in alkaline environment. In fact, more electrons peralcohol molecule and a higher current density could be produced. Fi-nally, Zhang et al., [228] reported that the addition of CeO2 in Au/Ccatalyst could significantly improve catalytic performance for glycerolelectro-oxidation. More negative shift was obtained after addition ofCeO2 to the Au/C sample compared to the Au/C catalyst. The peakcurrent densities (jP) over Au-CeO2/C and Au/C samples were 75.4 and20.4 mA cm−2 and the current densities at −0.3 V(j-0.3V) were 7.1 and1.8 mA cm-2, respectively. The results indicated that AueCeO2/C hadgreat anti-poisoning effect on CO and was a more stable catalyst than E-TEK Pt/C for glycerol oxidation in the presence of CO.

In addition, Siqueira et al. [229] was the first group to report theapplication of polyvinyl pyrrolidone (PVP) as reducing and cappingagents for Au NPs synthesis in electro-oxidation process. They alsocombined PVP-Au NPs with CNT to create thin films on to the indiumtin oxide (ITO) using layer-by-layer (LbL) method. The result revealedthat PVP-Au NPs/CNTs LbL films covered the entire ITO surfacehomogenously, and positively enhanced the magnitude of the anodic

currents in glycerol electro-oxidation. The prepared sample demon-strated great potential for application in practical fuel cells. Fig. 12describes the preparation method and LbL structure of the Au NPs/CNTs.

3.1.2. Pt-based catalystsIn one of the early studies in this field, Gomes et al. [230] in-

vestigated the effect of single crystals of Pt on the electrochemicaloxidation of glycerol in low potentials (0.05 V). Pt (1 0 0) and Pt (1 1 0)surfaces favored the breaking of the CeCeC bond while these surfaceswere poisoned at high potentials. However, Pt (1 1 1) surface did notfavor the breaking of CeCeC band and it was more active in oxidationof glycerol due to less poisoning by glycerol dissociation residues. Inanother study, Zuo et al. [231] constructed a rational design and syn-thesized a dendtritic Pt (1 1 1) facet based on nano-materials with en-riched edge and corner atoms as a catalyst for electro-oxidation ofglycerol. The Pt (1 1 1) facets are considered as the most active catalystsfor glycerol oxidation. Thus, the final catalyst was a highly crystallizedPt nano-flowers on the Tellurium (Te) nano-wires as a template.

The procedure for the galvanic replacement reaction with H2PtCl6 isdescribed in Fig. 13. In detail, 1 mM H2PtCl6 (pH=7) was added intothe mixture solution of as-prepared Te nano-materials and CTAB [232](Fig. 13a (step 1)). Consequently, PtCl62− was reduced to Pt atoms,forming an uneven discrete distribution of Pt@Te nano-shell on thesurface of the Te nano-materials [233]. Then, Te atoms in the Te@Pthetero-structure was dissolved and the Pt source around Te nano-ma-terials could quickly capture the electrons from the oxidation process ofTe→ TeO3 [232,234]. The newly formed Pt atoms were deposited in aspecial manner in the presence of CTAB, resulting in a hollow hetero-structure (Fig. 13b) [235,236]. As the reaction continued, the drivingforce for the reaction dropped and un-reacted Te atoms would be dis-sociated, owing to the Kirkend all effect and generated porous Pt nano-tubes (steps 2). However, if another 700 μL of H2PtCl6 was injected intothe solution, the aggravation of the galvanic replacement reactionwould stimulate the decomposition of the hollow hetero-structure, andthe newly formed Pt atoms would adhere to the previous ones with Teatoms, which preferred an island growth mode [237,238] (Fig. 13 (step

Fig. 12. (Top) Schematic procedure of the LbL production using PVP-Au NPs and single-walled CNTs and (Down) AFM images of 15-bilayer PVP-Au NPs/CNTs LbLfilm and its magnified region [231].


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3), Fig. 13c). Consequently, Pt nano-flowers were generated in thepresence of CTAB (step 4). The electro-oxidation results depicted thatthe Pt nano-flowers conferred 2.6 times more activity (0.18 Amg−1)than the commercial Pt/C catalyst (0.07 Amg−1).

After a few initial studies on application of supported monometallicPt/C sample in electro-catalytic oxidation of glycerol, researchers cameout with a novel idea related to the application of bimetallic samples toenhance catalytic activity and stability. Consequently, Siomes et al.[212] synthesized various supported Pd/C, Pt/C, Au/C and bimetallicPd-Au/C, Pd-Ni/C nano-catalysts. The activities of bimetallic sampleswere as follows: Pd0.3-Au0.7/C > Pd0.5-Au0.5/C > Pd0.5Ni0.5/C. Eventhe activities of non-Pt based catalysts (Pdx-Au1-x/C and Pd0.5-Ni0.5/C)for glycerol electro-oxidation were observed to be similar to the Pt/Ccatalyst. The substitute of 50% of Pd with Ni atoms could enhance thecatalytic activity and decrease the catalyst preparation cost sig-nificantly. Simoes et al. [210] extended their study by utilizing bismuth(Bi) instead of Au and Ni for modification of Pd and Pt bimetallic cat-alysts. They synthesized five different catalysts (Pd/C, Pd0.9-Bi0.1/C, Pt/C, Pt0.9-Bi0.1/C, and Pd0.45-Pt0.45Bi0.1/C) by a colloidal route. Thecharacterization results revealed that Bi did not alloy with Pt and Pd.Instead, Bi interacted with the Pd and Pt surfaces. However, the catalystactivity of Bi modified samples was enhanced considerably. Samplesactivity were in the order of Pd/C < Pt/C=Pd0.9-Bi0.1/C < Pt0.9-Bi0.1/C= Pd0.45-Pt0.45Bi0.1/C. The main products were GLY, DHA, andTARA with low potentials but surging the electrode potential led toformation of Mesoxalate (MOXA).

In a study by Garcia et al. [239], two electrodes Pt (1 1 1) and Pt(1 0 0) used for electro-oxidation of glycerol in an acidic media. Theresults revealed that three products were produced (GLYLD, GLYCA,and DHA) on the Pt (1 1 1) electrode, while only GLYLD was detectedthrough the application of Pt (1 0 0) electrode. The reason for the dif-ference in the electro-chemical oxidation of glycerol was due to thedifferent binding modes of dehydrogenated glycerol to the two surfaces.Dehydrogenated glycerol binded to the surface through two single Pt-Cand one double Pt]C bonds by application of Pt (1 1 1) and Pt (1 0 0),respectively. Proposed reaction mechanism (Fig. 14) suggests two dif-ferent active intermediates of initial glycerol dehydrogenation on the Ptsurface: an enediol-type intermediate binding with two adjacent Catoms to the Pt(1 1 1) surface, and an intermediate binding through asingle primary C to the Pt(1 0 0) surface. Different bonding modes to thePt(1 1 1) and Pt(1 0 0) surface have been attributed to the differentcoordination numbers of their Pt surface atoms, allowing only singlePteC bonds on Pt(1 1 1) but favoring a double PteC bond on Pt(1 0 0).The enediol-type intermediate is similar to the intermediate of theGLYLD-DHA isomerization reaction and serves as a precursor to theformation of both GLYLD and DHA on Pt(1 1 1). However, the inter-mediate favored on Pt(1 0 0) yields only GLYLD as soluble product. Inaddition to these two active intermediates, stripping experiments was

suggested the existence of a third “inactive” intermediate, which wasformed from glycerol, GLYLD as well as DHA. The only oxidationproduct observed from the stripping of this adsorbate is CO2. DFT cal-culations suggest that this species is the most stable adsorbate on Pt(1 1 1), but it does not have a soluble counterpart, in contrast to the two“active” intermediates. GLYCA is oxidized by cleavage of CeC bond toFA and GLA. The production of FA is considered on the basis of theonline electrochemical mass spectroscopy (OLEMS) results, whichshowed that CO2 is a product of the glycerol electro-oxidation, and onthe basis of the HPLC results, in which FA was identified as a product ofGLYLD oxidation on both electrodes. In the pathway of the secondaryalcohol oxidation, DHA is further oxidized to HPYA via a four-electrontransfer reaction. DHA is very difficult to oxidize to CO2, explaining thevery small amount of CO2 observed in the OLEMS experiments of DHAoxidation.

One of the initial successful studies in this field has been carried outby Dector et al. [240]. They prepared Pd/C and Pd/MWCNT basedelectro-catalysts by impregnation for glycerol electro-oxidation in amicro-fluidic fuel cell. The average particle size and lattice parametersof the catalysts were 7.5 and 3.5 nm for Pd/C and Pd/MWCNT, re-spectively. The electro-catalytic activity of Pd/C and Pd/MWCNT, in-vestigated in 0.1M glycerol showed that the onset potential for glyceroloxidation on Pd/MWCNT had a negative shift ca. 40 mV compared toPd/C. The maximum power density obtained was 0.51 and 0.7 mWcm−2 for Pd/C and Pd/MWCNT, respectively. The results of this workdid not only infer that glycerol could be used as fuel in a micro-fluidicfuel cell but also proved its performance was similar to that obtainedwith other fuels (glucose).

The impact of different catalyst synthesis strategies (e.g. ethyleneglycol (EG) reduction, sodium borohydride reduction and impregna-tion-H2-reduction) on the metal-support-interaction of Pt/NCNTs in theelectro-oxidation reactions of glycerol, formic acid and CO, and thehydrolysis of ammonia borane (AB) was systematically evaluated byNing et al. [241]. The different characterization results revealed thatthe synthesis methods drastically affected the electronic property of PtNPs. Indeed, Pt NPs preferentially interacted with graphitic nitrogen inEG reduction method due to the electron donating property of graphiticnitrogen, while preferentially interacted with pyridinic nitrogen anddefects in the impregnation-H2-reduction method due to the vacanciescontaining NP favoring the charged metal ion adsorption. The catalyticactivity strongly depended on the electronic property of Pt NPs, whichcan be ascribed by the binding energy of Pt4f7/2(0) from XPS. Thepivotal influence of synthesis method on the Pt NPs electronic propertywas because of the different formation mechanisms of Pt NPs during thesynthesis as well as the metal support interaction (MSI) between Pt andnitrogen groups finely tuned the electronic property through the dif-ferent electron donation-acceptance between Pt and graphitic/pyridinicnitrogen. In an earlier study, the same group reported the MSI between

Fig. 13. (a)Schematic synthesizes procedure of galvanic replacement from Te NWs to Pt nano-flowers with the equivalent charge reaction process, (b) Pt and Tenano-shell on the surface of the Te nano-wires, and (c) Pt nano-flowers [234].


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Pt NPs and NCNTs is higher than between Pt NPs and pristine CNTs. Infact, by changing the nitrogen amount of NCNTs and introducingoxygen groups, the electron enrichment of Pt NPs can be tuned. As aresult, superior catalytic activity was achieved over Pt/NCNTs in theoxidation of glycerol and electro-oxidation of CO, compared withconventional CNTs as support [242].

Xie et al. [243] also fabricated different Pt supported catalystnamely Pt/porous poly-aniline (PANI), Pt/PANI and Pt/C. The Pt/porous PANI hybrid nano-structure, fabricated by attapulgite (ATP) asthe sacrificial template, registered the best activity. When the ratio ofmAn/mATP was 1:3 and reaction time of removing template was 11 h,the Pt/porous PANI catalyst exhibited excellent electro-catalytic ac-tivity (4.6-fold of Pt/PANI) toward glycerol oxidation.

3.1.3. Pd-based catalystsPreviously, Pd is mentioned as the second noble metal widely used

in electro-catalytic oxidation of glycerol. The study by Wang et al.[238] reported application of carbon nitride and graphen (CNx/G) hy-brid support instead of the conventional bulk carbon for Pd NPs inelectro-oxidation of glycerol in 0.5 M NaOH aqueous solution. Fig. 15clearly exhibits the GLYCA selectivity significantly surged by 10–15%while decreasing the FA selectivity by 15% over the Pd-CNx/G com-pared to the Pd-CB catalyst. The enhancement in the C3 products to-ward the Pd-CNx/G catalyst was attributed to the smaller Pds NPscompared to the conventional Pd/CB sample. The interaction betweenPd NPs and nitrogen atoms played an important role in the enhance-ment of selectivity of C3 products. Furthermore, in the study conductedby Kang et al. [244], co-existence of phosphorous and nickel oxideimproved the Pd performance in a composite catalyst (Pd-NiOx-P/C).

Fig. 14. Reaction mechanism of glycerol oxidation over Pt(1 1 1) and Pt(1 0 0) electrodes in acidic solution [239].


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High electro-catalytic activity of the prepared catalyst was dependenton the high electrochemically active surface area (EASA) value(576.3 cm2/mgPd), fast charge transfer kinetic (Tafel slopevalue=68.0 mV/dec) and relatively low activation energy value(Ea=21 kJ/mol).

Next, a Pd core-shell nano catalyst (FeCO@Fe@Pd/C) was tested forelectro-catalytic oxidation of ethanol, EG, and glycerol [245]. The bestresults were reported for oxidation of EG due to its faster electro-cat-alytic rate constant (Kcat) when compared to the glycerol (2 times) andethanol (4 times). The onset potential for ethanol was 110mV and230mV higher than EG and glycerol, respectively. The ratios of forwardto reverse peak current densities (jf/jb) values were 1.28, 2.81 and 2.33for ethanol, EG, and glycerol, respectively. This suggests that ethanoloxidation had poisoned the catalyst. The peak current density (jP) of thecore-shell was between 3.9 and 4.7 higher than the Pd/C monometalliccatalyst. Finally, the Pd core-shell nano-catalyst was very stable againstcarbonaceous poisoning generated during the alcohol oxidation pro-cesses. This behavior is attributed to its smaller NPs size, higher EASAand the positive strain effect on its active surface due to the in-corporation of the core alloy in its Pd lattice.

One of the rarely reported studies was carried out at the actual in-dustrial conditions [246]. The catalytic activity and performance of bis(dibenzylidene acetone) Pd(0) catalyst, Pd(DBA)2, toward glyceroloxidation reaction (GOR) in alkaline half-cell and alkaline direct gly-cerol fuel cell (DGFC) was reported. The electro-oxidation of glycerolon Pd(DBA)2 was characterized in half cell by CV, electrochemicalimpedance spectroscopy (EIS) and CA techniques. The obtained resultshighlighted the excellent electro-catalyst activity of Pd(DBA)2 with thespecific peak current density reported as Sap= 0.48mA/μgPd and onsetpotential was Es=−0.33 V compared to the conventional Pd basecatalysts. CVs results also demonstrated that Pd(DBA)2 was still activeeven after 200 cycles. In order to determine the performance of Pd(DBA)2 in alkaline DGFC, membrane-electrode assembly was fabricatedby employing the Pd(DBA)2 catalyst in anode electrode. Acta com-mercial cathode and Tokuyama anion-exchange membrane was suc-cessfully evaluated in a single passive DGFC. The cell open circuitvoltage and maximum power density of 800mV and 24 mW cm−2 wereachieved, respectively by using static aqueous fuel containing 5 wt% ofglycerol.

3.1.4. Other mono-metallic catalystsThe objectives of these studies were to synthesize and prepare a

catalyst without application of noble metals for reducing environmentaland economical impacts significantly. For example, Oliveira et al. [247]

used free-noble metal Ni/C and NiCO/C nano-catalysts as electrodes inglycerol electro-oxidation study. The reaction over both catalysts wascarried out at potential higher than 1.2 V before the Ni and Co oxy-hydroxides (NiOOH and CoOOH) formed, which have significant effecton product distribution because of the CeC bond cleavage of the ori-ginal molecule. GLY, glycolate, and formate were the main productsand a little amount of TAR and oxalates were detected. The in-situ FTIRmeasurement also revealed the presence of hydroxypyruvate ion andCO2 as the reaction products. Large amount of CO2 was formed withNiCO/C nano-catalyst as the electrode material since the cobalt in thenickel based catalyst enhanced the CeC bond cleavage of the glycerolmolecules.

Oliveira et al. [248] extended their study in the synthesis of acarbon-supported nickel NPs by impregnation method for serving as theanode catalysts. Parameters such as scan rate, temperature, and con-centration of supporting electrolyte and electro-reactive molecule werevaried to investigate the electrochemical effect on the glycerol con-version at the surface of the electrode covered by catalytic oxyhydr-oxide species (β-NiOOH). In general, the oxidative transformation ofglycerol was a limited mass-transport reaction process. Thus, the peakpotential and the peak current of glycerol oxidation on Ni/C electrodesshifted with the scan rate and temperature variation, which are thecharacteristic of an irreversible diffusion-controlled phenomenon. Thisreaction starts in the beginning of the transformation of β-Ni(OH)2species to β-NiOOH. The glycerol oxidation on β-NiOOH layer isstrongly dependent on the OHe concentration. Formate was the mainproduct at higher glycerol concentrations, while at lower glycerolconcentrations, carbonate and oxalate prevailed.

Another novel concept recently reported in electro-oxidation ofglycerol is the simultaneous in-situ formation of a homogenous pre-cursor solution Ni-based catalysts using sodium tartrate and PVP asdispersant or protector at the anode and cathode for glycerol oxidationand hydrogen evolution, respectively [249]. The formation of electro-catalysts and their immobilization on the electrode surface occurredsimultaneously, avoiding the tedious and laborious procedures. Ni saltin the homogeneous solution was deposited to repair the Ni-basedelectro-catalysts on the electrode surface, accompanied by the electro-lysis of glycerol at the anode and the hydrogen evolution reaction at thecathode, exhibiting good working stability. This technique may findpotential applications in the conversion of solar energy into storablefuels via the electrolysis of H2O or small molecules to produce hy-drogen.

3.2. Bi-metallic based catalysts

Recently, various research groups reported synthesizing some stateof the art and novel bimetallic samples for electro-oxidation of glycerol.Dash and Munichandraiah, [250] prepared a series of poly (3,4-ethy-lenedioxythiophene) (PEDOT) supported Pd-Ru nano-flower samples.The Pd5-Ru-PEDOT/C electrode was highly sensitive toward glyceroldetection with sensitivity of 99.8 MA/Cm2 μM−1 and LOD of 0.1 μM.Zalineera et al. [251,252] synthesized two series of catalysts including aself-supported Pd4Bi and Pd1Snx samples with porous nano-structure ofSBET= 75–100m2/g and SBET= 25–80m2/g, respectively. Results re-vealed that aldehyde, ketone, and carboxylate species were produced atlow potentials, while hydroxypyruvate at moderate and CO2 and CO3

-2

at high potentials. Also, the catalyst experimental runs inferred Sncould significantly reduce the required potential from 0.6 V to 0.45 V inelectro-oxidation of glycerol.

Wang et al. [224] and Gomes at el. [253] tested the Au-Ag/C bi-metallic catalyst in their study and reported that Ag addition has led tothe performance of glycerol oxidation reaction at lower potentials. Agcould also change the reaction pathway of glycerol electro-oxidation bybreaking the CeCeC bands and formation of FA. The main reasons forsignificant impact of Ag on the bimetallic catalyst (Au-Ag/C) activityare electronic modification and Ag segregation on the catalyst surface.

Fig. 15. Product distribution and selectivity over Pd NPs on different supports(CB, CNx, CNx/G) at various potentials [240].


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Jin et al. [255] further investigated the surface modification of Ag NPssupported on reduced graphene oxide (RGO) with a small amount of Auand the activity of Au-modified Ag/RGO (Aux-Ag/RGO) catalysts. CVmeasurements indicated that the Aux-Ag/RGO catalyst registered al-most the same activity as Au/RGO in terms of the geometric surfacearea of glassy carbon substrates for glycerol oxidation in alkaline so-lution, while Ag/RGO exhibited no catalytic activity. The Aux-Ag/RGOcatalysts have great advantages over Au/RGO in terms of the Au massspecific activity.

In a different study by Holade et al. [255], various experimentalparameters influencing the straight forward NPs synthesis by BromideAnion Exchange (BAE) method was investigated. They synthesized twobimetallic Au-Pt/C and Au-Pd/C samples as advanced surfactant-freeNPs for anode electrodes design in abiotic or hybrid glucose biofuel cell.Most importantly, this method drastically changed the structure of Au-Pd nanostructures to alloyed system when Au atomic content washigher than 50%. The activity test results revealed that GLY and gly-colate ions were the main products of glycerol conversion in alkalinemedium on Pd NPs when bromide ion to metal (s) molar ratio was 1.5,with a cumulative selectivity higher than 80%. The BAE constitutes apromising and suitable chemical route for fuel cells electrode materialspreparation with the combination of high chemical synthesis yield(minimizing metal loss), good active surface areas and unexpectedcatalytic electro-activities.

A series of bimetallic Pd-Au/C electro-catalysts were synthesized indifferent ratios (100:0, 75:25, 50:50, 25:75, and 0:100), using the boro-hydride reduction method [256]. CV which was assisted by ATR-FTIRin-situ and chronoamperometry (CA) experiments revealed that theaddition of Au remarkably enhanced the electro-catalytic activity. Thehighest current density (≈4mAmgmetal

−1) was achieved for the Pd50-Au50/C and Pd75-Au25/C catalysts, twice more than that achieved byPd/C (2mAmgmetal

−1) demonstrating the beneficial effect of Pd-Aualloy. In addition, Mougenot et al. [257] used plasma supportingmethod for the first time in preparation of different Pt free bimetalliccatalysts (Pd0.7-Au0.3/C, Pd0.35-Au0.3-Pd0.3/C and Au0.15-Pd0.7-Au0.15/C). They reported that plasma supporting method improved the bime-tallic samples activity compared to the conventional wet chemistrymethod because of electrode structure and surface composition.

Unfortunately, there are limited numbers of studies on electro-oxi-dation of glycerol in the base-free or acidic environments with thestudies performed using carbon supported catalysts only. One of theinitial attempts in base-free environment was carried out by Kwon et al.[258] who for the first time reported almost 100% selectivity to DHAusing Pt/C electrode in a Bi saturated solution. The main purpose of thisstudy was to evaluating Bi presence in the solution and compared theperformance with Pt-Bi/C electrode, previously conducted by Simoeset al. [210]. The results confirmed the researcher’s hypothesis that the

highest activity could be achieved by constant and full coverage of Bion Pt/C surface. Bi in acidic medium has led to glycerol oxidation toDHA on a Pt/C electrode. In fact, Bismuth blocks the active sites forprimary glycerol oxidation, reduce the onset potential, and increase theactivity by forming a Bi-related active site on the surface ready forsecondary oxidation. The obtained result (100% DHA selectivity) sur-passes the 25–30% selectivity of DHA in alkaline environment [238].

As Fig. 16 illustrates, primary alcohol oxidation leads to the for-mation of GLYLD and GLYCA, the latter species being subsequentlyoxidized to GLA and FA. FA has been shown to be the species leading tothe formation of CO2. However, FA may also easily dehydrate on Pt togive rise to adsorbed CO, and this is the most plausible pathway for theformation of CO during glycerol oxidation. In the presence of Bi on thesurface, GLYLD and GLYCA formation is much reduced, and adsorbedCO is not observed, which indicates that the pathway for primary al-cohol oxidation is blocked at several stages. However, there is a clearlyenhanced formation of DHA, not only relatively, but also in an absolutesense. It is well-known that Bi blocks the formation of poisonous COfrom small organic molecules, such as FA, through a third-body effect.

Pupo et al. [259] synthesized Snx@Pty and Rhx@Pty core–shellelectro-catalysts by chemical reduction method. They studied the cat-alytic activity toward glycerol oxidation in acidic medium. CV analysessuggested the successful synthesis of core–shell structures for Rh@Pt3and Sn@Pt3 catalysts, observed from their similar profiles to poly-crystalline Pt. Higher electro-oxidation currents, as well as lowest onsetpotential for glycerol oxidation were seen for the catalysts with greateramounts of Pt. Characterization results confirmed that core–shell NPswere homogeneously distributed on the carbon black surface. The Sn@Pt3 and Rh@Pt3 catalysts presented better features when taking intoaccount the CV, polarization curves, CA, XRD, and TEM data for fuelcells applications. In fact, with high Pt content, the lower onset po-tential was achieved (130mV for Sn@Pt3 and 120mV for Rh@Pt3).Thus, the proportion of Pt in the precursor metals presented a positiveinfluence on the catalytic activity. The enhanced catalytic activity couldbe attributed to the interactions of Pt electronic structure with the coremetal.

Hong et al. [260] demonstrated a facile method to synthesize aseries of bimetallic Pd-Pt nano-wire networks (NWN) with tuneablecompositions. The modified electronic states of Pd and Pt, improvedelectro-oxidation kinetics of the Pd-Pt nano-crystals, and the synergisticeffects between Pd and Pt account for the enhanced electro-catalyticactivity of the prepared Pd-Pt nano-crystals. All the prepared Pd-PtNWNs obviously possessed higher current density than the commercialPt/C, revealing their superior catalytic activity. The Pd55Pt30NWNssample displayed the highest activity with the peak current value of0.046 A cm−2, which was higher than commercial Pt/C (0.023 Acm−2). The catalytic activity follows the order:

Fig. 16. Schematic diagram of the selective oxidation of glycerol [258].


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Pd55Pt30 > Pd57Pt61 > Pd22Pt41 > Pt/C. Furthermore, the Pd55Pt30mass activity of 1.8 Amgmetal

−1 was better than the reported self-sup-ported PdxBi catalysts reported in literature [251]. The synthesizedNWNs displayed great potential as substitutes for commercial Pt/Ccatalysts for effective catalysis of EG and glycerol electro-oxidation inan alkaline solution.

Similarly, Qi et al. [261] reported electro-oxidation of four differentalcohols namely methanol, ethanol, EG, and glycerol using Pd-Ag/CNTbimetallic catalyst at 80 °C and ambient pressure. In the half-cell systemwith three electrodes, the glycerol oxidation reaction reached to thehighest mass activity of 8.53% mAμgPd-1. The activity for EG, ethanol,and methanol were in the following order:2.105 > 0.305 > 0.105mAμgPd−1, respectively. Direct alcohol fuelcell in the presence of Pd-Ag/CNT (0.5 mgPd.perMEA-1) as an anode cat-alyst exhibited similar order for peak activity with half-cell method. Inparticular, glycerol with 552.4 mW.mgPd.perMEA

−1 conferred the highestpeak mass activity followed by EG 490.4 mW.mgPdperMEA

−1, ethanol404.6 mWmgPd.perMEA

−1 and methanol 270.2 mWmgPd.perMEA−1. The

high activity of Pd-Ag/CNT bimetallic catalyst in electro-oxidation ofalcohols could be attributed to the following reasons: (1) Ag helped Pdto accelerate the reaction rate of aldehyde oxidation reaction (ADOR)and alcohol oxidation reaction (ADR) network, (2) Ag-Pd alloy led tosignificant reduction of Ag particle size from 17.7 nm to 2.7 nm withoutcovering the sample surface by surfactant which led to the high electrochemically active surface area (ECSA) of Pd-Ag alloy, (3) Pd-Ag/CNTcould cleave the CeC band of long chain polyols which provides highfuel efficiency, and finally (4) CNT with high electrical conductivity,high mechanical and thermal stability formed 3D electrode structure onmembrane electrode assembly (MEA) and improve the mass transfer ofalcohol and OH−.

Another successful recent study on application of CNTs was re-ported by Yuan et al. [262]. Their group synthesized a non-Pt basedelectro-catalyst for glycerol electro-oxidation. The CeO2 nano-clusters(1.5 nm) supported on 1-Pyrenecarboxylic acid functionalized MWNTs(PC-MWNTs) which were prepared by two approaches of sequentialpolyelectrolyte functionalization and microwave-assisted self-assemblyused as support for Au NPs. The Au/CeO2/PC-MWNT hybrid electro-catalyst could achieve tremendous activity and stability with a peakcurrent density 28 times the commercial Pt/C sample. The Au/CeO2/PC-MWNT also registered a current density 1.6 times the Pt/C in apractical fuel cell operation. The researchers claimed that their syn-thesized sample was the first to report a higher catalytic performance ofAu-based catalyst compared to the commercial Pt/C catalyst for fuelcell.

In a study done by Li et al. [263], the application of two mono-metallic (Pt/C and Pd/C) and bimetallic (Pt-Au/C and Pd-Au/C) sam-ples for glycerol electro-oxidation was investigated. When Au wasadded to Pt/C and Pd/C, the values of onset potential (Eonset) were 133and 139mV more negative on Pt-Au(wt 4:1)/C and Pd-Au(wt 4:1)/Celectrodes than that on the Pt/C and Pd/C electrodes. Also, the value ofEonset for glycerol electro-oxidation on Pt-Au(wt 4:1)/C electrode was160mV more negative than that on Pd-Au(wt 4:1)/C electrode. Thevalues of current densities (j-0.3V) on Pt-Au(wt 4:1)/C and Pd-Au(wt4:1)/C electrodes were 4.6 and 6.2 times larger than Pt/C and Pd/Celectrodes.

Prasanna and Selvaraj [264] synthesized a novel ternary polymer-CNT composite by application of three monomers (amine terminatedcyclophosphazane (ATCP), hexachlorocyclotri-phosphazene (CP), and2.2′-benzidinedisulfonic acid (BZD)) as support for Pt and Pt-Sn NPs forelectro-oxidation of glycerol. The results revealed that Pt/ATCP-CP-BZD-CNT and Pt-Sn/ATCP-CP-BZD-CNT samples depicted considerablyhigher oxidation current (26.59mA/mg and 38.89mA/mg) at loweronset potential (−0.64 V and −0.74 V) than the commercial Pt/C. Theprepared novel samples also were stable even for more than 100 cycles.Fig. 17 describes the schematic representations for the preparation ofATCP–CP–BZD–CNT composite and Pt and Pt-Sn NPs deposited

ATCP–CP–BZD–CNT composites.Rezaei et al. [265] decorated the Cu filled nano-porous stainless

steel (NPSS) electrode with Pt and Pd (Pt-Pd/Cu/NPSS). The preparedsample gave 4.2 times higher current density (J) at a lower onset po-tential (Eonset =−0.54) compared to Pd/Cu/NPSS and Pt/Cu/NPSSsamples. Pt-Pd/Cu/NPSS also showed 1.3 times more EASA and morepoisoning tolerance than the other two single decorated samples. Thisgroup expanded their investigations and synthesized another bimetallic(Pd-Au/NPSS) catalyst [266]. The results over this sample also weresimilar to the previous bi-metallic sample. In fact, the bimetallic Pd-Aunanostructure exhibited higher current density and mass activity (morethan 2 times) than monometallic Pd/NPSS and Au/NPSS. The onsetpotential for Pd-Au/NPSS was lower than that for Au/NPSS and Pd/NPSS.

3.3. Multi-metallic based catalysts

Recently Martines and his coworkers, [237] tested fast polyolsmethod assisted with microwave to synthesize a tri-metallic sample ofrhodium (Rh)-decorated Pt NPs with iridium oxides (IrOx) (Rh/Pt.IrOx/C) for electro-catalytic oxidation of glycerol. The results confirmed thatRh could improve the electro-chemical stability of catalyst by pre-venting agglomeration effects. These NPs were possible to decorate aposterior in a controllable way by a simple electro-chemical protocol.This method provides great opportunities in synthesizing a multi me-tallic catalyst in a controlled manner.

The application of tri-metallic or multi component catalyst is rarelyreported in this field. However, Do Valle et al. [267] prepared a tri-metallic Pt(1-2x).Rux.Snx.Oy/Ti catalyst for electro-oxidation of glycerolby thermal decomposition of polymeric precursors. The oxidation ofglycerol enhanced significantly in the presence of RuO2 and SnO2 dueto the larger surface area and ability to donate large amount of OHspecies. The Pt0.8.Ru0.1.Sn0.1.Oy/Ti sample depicted approximately170mV less positive than Pt.Oy/Ti samples. Rostami et al. [268] alsosynthesized the electro deposited Pd-Co layers on Au electrode. Themaximum current density of Pd-Co(3)/Au sample was approximatelytwice larger than the Pd/Au. The difference confirmed the positive roleof Co in improving the catalytic activity of Pd on Au surface. Pd-Co(3)/Au also inferred 8mV lower onset potential compared to Pd/Au and Cocould enhance the poisoning tolerance of Pd during the electro-oxida-tion reaction. Pd-Co(3)/Au exhibited high activity even after 200 cy-cles.

3.4. Non-metallic based catalysts

Hickey et al. [269] used a combination of an enzymatic oxalateoxidase and an organic oxidation catalyst (4-amino-TEMPO) for com-plete oxidation of glycerol to CO2. Initially, various commerciallyavailable derivatives of TEMPO were evaluated using CV at pH 7.TEMPO-NH2 indicated the highest catalytic rate of 1453 μA cm−2 8times more than catalyst rate over unmodified TEMPO (282 μA cm−2).The complete electro-chemical oxidation of glycerol by TEMPO-NH2/oxalate oxidase (OxO) was out performed in bulk electrolysis of a so-lution coating both enzymatic and organic catalyst at pH 5.2. Conse-quently, the current density was as high as 1.2mA cm−2 and the re-action products in 22 h reaction time were GLYCA, TARA, Mesoxalicacid, and glyoxalic acid. C NMR analysis also confirmed the presence ofa peak corresponding to C-enriched CO3

−2 at ca. 165 ppm with Cou-lombic yield of 90.6C.

3.5. Summary

Even though there are limited numbers of research studies onelectro-chemical oxidation of glycerol reported in literature [26,27],the knowledge related to the electro-catalytic oxidation of glycerol haveexpanded significantly during the last five years which reflected the


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importance of value-added chemicals such as DHA, GLYCA, MA, andTARA from a bio-renewable source such as glycerol. Similar to thecatalytic oxidation of glycerol, the electro-chemical reaction could alsobe performed in both alkaline and base-free environments. The majorityof electro-catalytic oxidation of glycerol studies has been performed inthe alkaline environment but only few in acidic or neutral (base-free)environments. The electro-chemical approach showed great advantagecompared to the conventional catalytic process. With the new approachone has better control of the desired products selectivity when the re-action is performed in aqueous media with low reaction temperatureand pressure. The application of polymer supports or the combinationof polymers and other supports such as carbon have attracted moreattention recently. The main reasons are the utilization of more en-vironmental friendly materials and reduction of catalyst preparationcosts by utilizing cheaper materials in synthesizing an active and se-lective catalyst for electro-chemical oxidation.

4. Concluding remarks and recommendations

One of the ways for achieving low carbon living is to produce value-added chemicals by the catalytic transformation of bio-sustainable re-sources employed as substitutes for fossil fuels. Value-added chemicalsfrom glycerol have raised great interests due to its huge availability as awaste by-product from the biodiesel industry. Its effective utilizationwill be one of the key factors in future bio-refineries and could sig-nificantly promote biodiesel commercialization and development.Indeed, producing biomass-derived chemicals from glycerol is a globalchallenge. The high cost of industrial grade glycerol limited the eco-nomic viability of its transformations, except for products used in thehigh value niche markets; i.e. cosmetic additives, tanning agent com-ponents, amino acid precursors, or selective metal chelants.Consequently, as the price of glycerol decreases, new products, espe-cially polymers, will be increasingly derived from glycerol.

This review presents various catalytic and electro-chemical pro-cesses for oxidation of glycerol into useful chemicals and materials.Special emphasis was given to the effect of different noble-metals,supports and parameters influencing the selectivity and yield for spe-cific glycerol derivatives. Indeed, various factors that have been

evaluated as the most influential parameters on the glycerol oxidationreaction conversion, product distribution, product selectivity, and cat-alyst long-life stability are (1) the presence of NaOH in reactionmedium, (2) application of different catalyst preparation methods suchas impregnation, sol-immobilization, incipient wetness, and depositionprecipitation which could control the particle size of metal NPs, (3)basic or acidic characteristics of support, (4) influence of samplestructure on modification of metal NPs, (5) application of variouscapping agents, and finally (6) application of bi- or multi-metallic NPsinstead of mono-metallic samples.

Recent breakthroughs in catalyst synthesis and characterization ledto unprecedented achievements in this field. Despite advances in thisfield, there are still many challenges for increasing selectivity and yield.Thus, a clear direction for utilization of recent novel ideas, methods,and materials could significantly improve knowledge in this field.Consequently, researchers are presented with excellent opportunities incatalysis and nano-materials to design highly active catalysts for gly-cerol oxidation to specific useful products. The methods of catalystpreparation (e.g. impregnation, sol-immobilization, incipient wetness,and deposition precipitation) greatly influenced the catalyst activityand selectivity. Indeed, all the catalysts that were synthesized by solimmobilization produced fine metal NPs with highly homogeneousdistribution on the support and effectively controlled the selectivity ofthe desired products.

Oxidation-assisted polymerization of glycerol presented a new fieldof polymerization in aqueous media assisted by oxidation and could beapplied to general polyhydroxy compounds. The process is very newand yet it has achieved acceptable results in the recent few years. A newsystem of chemistry in developing modern oleo-chemistry using gly-cerol as renewable resources is inevitable. The search for hybridhomogeneous–heterogeneous systems that are stable at both acidic andbasic pH is actively investigated. The use of different supports for baseand acid catalyst encapsulation could increase diffusion and catalystactivity. Water-resistant homogeneous–heterogeneous catalysts areneeded as glycerol normally contains some water. Synthesis of newhybrid materials will improve the distribution of the active sites toenhance catalytic activity. Oxygen species present on the catalyst sur-face has high value and could lead to higher glycerol conversion rates.

Fig. 17. Schematic synthesis procedure of ATCP–CP–BZD–CNT composite and Pt and Pt–tin NPs deposited on ATCP–CP–BZD–CNT composites [266].


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Various products for energy or H2 production such as TAR, DHA, HPYand MOXAL can be obtained from the glycerol oxidation reaction. Forexample, TAR involves the oxidation of primary alcohol groups, a re-latively straight forward to achieve on Pd based-catalysts. The DHA andHPY products involve the oxidation of the secondary alcohol group intoa ketonic group. Although all the challenges, it has been successfullyreported in alkaline media on Au-based catalysts. Moreover, productseparation and purification steps are also important aspects on whichstudies should be focussed.

The photo-catalytic process is a novel and promising strategy inglycerol oxidation reaction under milder reaction conditions and withaltered selectivities, compared with the conventional thermo-catalyticroute. As reported in this review limited numbers of research groups areworking on the photo-catalytic oxidation of glycerol and definitely itrequires more efforts and attention in the future particularly applicationof novel heterogeneous catalysts not only TiO2 but also other semi-conductor materials with more suitable band edge energies, as well asvisible light assistance process of glycerol oxidation using direct plas-monic photo-catalysts (DPS) which showed promising results andshould be investigated more intensively in the future. Nevertheless,further research efforts have to be devoted in this field in order tounderstand the reaction mechanisms and optimize the activity/se-lectivity parameters. The combination of selective glycerol oxidationwith hydrogen generation via photo-catalysis evidence that practicalapplications can be achieved ensuring a more sustainable future.

Application of different catalysts in electro-chemical oxidation ofglycerol in a solid polymer fuel cell or in a solid polymer electrolysiscell was discussed for the cogeneration of energy and value-addedchemicals. In-situ infrared spectroscopy measurements and HPLC ana-lyses of oxidation products have shown that the product distributioncan be controlled through catalyst composition and structure andthrough the applied electrode potential. All these insights foresee thepossibility to enhance the activity of the catalysts and to control theselectivity of desired products in glycerol electro-oxidation reaction.Studies on synthesizing an electro-catalyst to control glycerol oxidationto desired product, without cleavage of the CeC bond, is new, andtherefore should be developed. One of the main goals is then to con-siderably increase the selectivity towards a given product by both cat-alyst and potential effects.

Inevitably, cost-efficient production approaches of the glyceroloxidation products would open new markets for the products in thepetrochemical industry, which were restricted due to currently highproduction costs. One of the most important factors in this context issynthesizing non-precious-metal-containing catalysts which exhibitedpromising results and could be suitable alternatives to noble-metalbased catalysts, which are still in the very early stages of investigation.Indeed, some of the reported novel catalysts hold promise to be valu-able catalysts if the selectivities toward single products can be furthertuned. The economic aspect of glycerol oxidation should in general bekept in mind when exploring new active catalysts. However, funda-mental research always allows investigating new reaction pathways toensure a gain in knowledge and give the chance to discover un-precedented reaction pathways.

Indeed, the ideal goal in future bio-refineries would be transfor-mation of crude-glycerol, which contains impurities like methanol, re-sidue homogeneous or heterogeneous catalysts, and other salts, to va-luable products. Finding active and stable catalysts which can catalyzethe reaction in crude glycerol will be a huge step forward in applyingthese catalysts in industry. However, this issue is rarely investigatedand only a few reports can be found in the literature. It will significantlyreduce the biodiesel production costs as well as different environmentalconcerns. Hence, researchers should focus more on utilization of crude-glycerol to determine how to control the effect of various impurities incrude glycerol on the desired products selectivity and catalytic activityin oxidation process. Besides the fundamental aspects, where greatprogress has been achieved over the last years, this last challenge

should not be neglected by the researchers in this field.

Acknowledgement

The authors would like to express their sincere gratitude to theUniversiti Teknologi Malaysia (UTM), for supporting the project underPDRU grant no. 03E49. Also, authors would like to express their sinceregratitude to Iran’s National Elites Foundation and Iran Polymer andPetrochemical Institute (IPPI) for their supports.

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Further reading

[254] Jin C, Zhu J, Dong R, Chen Z, Huo Q. Modification of Ag nanoparticles/reducedgraphene oxide nanocomposites with a small amount of Au for glycerol oxidation.Int J Hydrogen Energy 2016;41:16851–7.


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