catalytic behaviour of four different supported noble metals in the crude glicerol oxidation - ok

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Applied CatalysisA: General 499 (2015) 89–100

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Applied Catalysis A: General

j ournal homepage: www.elsevier .com/ locate /apcata

Catalytic behaviour of four different supported noble metals in thecrude glycerol oxidation

Elżbieta Skrzyńska a,b,∗, Soraya Zaid b, Jean-Sébastien Girardon b, Mickaël Capron b,Franck Dumeignil b,c,∗∗

a Faculty of Chemical Engineering and Technology, CracowUniversity of Technology, Ul.Warszawska 24, 31-155Cracow, Polandb CNRS UMR8181, Unité de Catalyse et Chimiedu Solide, UCCS, Université de Lille 1 Sciences et Technologies, F-59655 Villeneuve d’Ascq, Francec Institut Universitaire de France, MaisondesUniversités, 103, Boulevard Saint-Michel, 75005 Paris, France

a r t i c l e i n f o

Article history:

Received 25 January 2015

Received in revised form 30 March 2015

Accepted 7 April 2015

Available online 15 April 2015

Keywords:

Crude glycerol oxidation

Supported noble metal catalysts

Gold

Platinum

Palladium

Silver

a b s t r a c t

The activity of four different noble metals (Ag, Au, Pd and Pt) in the liquid phase oxidation of pure

glycerol was confronted with the results obtained with a crude glycerol fraction, received from a large-

scale biodiesel production plant. The catalysts were characterized by numerous techniques, giving insight

into actual metal loading (elemental analysis by ICP and XRF), surface morphology (nitrogen absorption

methods—BET and porosity), chemical state of both the support andthe metal (XRD and XPS), and, finally,

the metal particle size distribution (TEM microscopy). A good dispersion of totally reduced noble metal

particles of a nanometric size (an average metal diameters were equal 3.5 nm, 4.2 nm, 4.7 nm and 21.2 nm

for respectively Pd, Pt, Au and Ag) was accompanied with a comparable values of total metal loadings

on the alumina support (from 0.95 and 0.96wt.% for Pt and Pd, up to 0.98 and 1.13 wt.% for Au and

Ag supported catalysts, respectively). In terms of initial reaction rate, the most active sample was the

Au/Al2O3 catalyst, both using pure (12976 mol h−1 molAu

−1) or crude glycerol (1230 mol h−1 molAu−1).

However, comparison of the selectivities and conversions after 1–2 h shows that the most robust and

resistant catalyst – toward the impurities present in crude glycerol – isPd/Al2O3 , with a loss of conversion

less than 50% (in respect to analogous reaction using pure glycerol) and almost unchanged high selectivity

to glyceric acid (close to 80–90%). Ag/Al2O3 also showed a relatively high resistance to impurities in termsof glycerol conversion, but with a drastic modification of its selectivity. The activity of the two other

catalysts was dramatically affected with a conversion divided by ca. 4 a nd even 10 for the Pt and the

Au catalysts, respectively, when using crude glycerol instead of pure glycerol. Finally, the effect of each

main impurity (MONG-NM, i.e., matter organic non-glycerol and non-methanol; ash; methanol; sulphur

compounds) was independently studied. In any case, the sulphur compounds and MONG-NM were the

impurities the most detrimental for the performances of catalysts. Thus, they should be removed in

priority from crude glycerol fractions before reaction, while ashes and methanol should not be considered

as completely undesirable.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Nowadays, glycerol is considered as an important bio-derived

platform molecule and a feedstock for biorefineries [1–5]. Its

∗ Corresponding author at: Faculty of Chemical Engineering and Technology, Cra-

cow University of Technology, Ul. Warszawska 24, 31-155 Cracow, Poland.∗ ∗ Corresponding author at: CNRSUMR8181,Unité de Catalyse et Chimiedu Solide,

UCCS, Université de Lille 1 Sciences et Technologies, F-59655 Villeneuve d’Ascq,

France.

E-mail addresses: [email protected] (E. Skrzyńska),

[email protected] (F. Dumeignil).

upgrading by chemical or biochemical transformations has thus

attracted much attention, and especially in the last decade [1–19].

Even if the use of glycerol as a raw material for producing a

large variety of chemicals is well documented in the literature

[1,4,6], only a very few applications have reached industrializa-

tion mainly due to the cost of purified glycerol. To reach viability,

many processes would need to be adapted to the use of cheap

dirty waste by-produced with biodiesel (the so-called “glycerine”

or “crude glycerol”), which currently is the main source of glycerol

on the market [1–6,20]. As purification of glycerine is a relatively

expensive multistep process [1,4,6,20], the possibility of direct

transformation of crude glycerol could be very attractive from an

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90 E. Skrzyńska et al./ Applied Catalysis A: General 499 (2015) 89–100

industrial point of view. Indeed, in the liquid phase glycerol oxi-

dation reaction, in addition to the obvious direct economic benefit

brought when using a cheaper raw material, some other indirect

economic as well as environmental advantages can be listed: (i)

the unreacted base used in the conventional process of biodiesel

production (i.e., basic transesterification) could be directly used in

a downstream oxidation process, which is often also conducted

under basic conditions; (ii) the globalamountof wastemineralsalts

(from base neutralization) shouldthus be reduced; (iii) the number

of purification steps would also be reduced with only one purifica-

tion downstream for recovering the final product. Unfortunately,

as the most of the scientific papers dealing with catalytic conver-

sion of glycerol to value-added products are based on the use of a

purified raw material [1,10–19]. Hence, the effect of the different

impurities present in crude glycerolstreams is still not well known.

The studies where the crude glycerol fractions were used as

a raw material are rare [21], especially in the field of glycerol

oxidation in the liquid phase. Recently, we compared different

grades of glycerine and investigated in details the effect of each

major impurity on theperformances of platinum supportedon alu-

mina in both basic and neutral reaction media [20]. The organic

matter non-glycerol and non-methanol (called MONG-NM), com-

prising mainly of various fatty acid derivatives, was identified as

themost problematic contaminantof crude glycerolfraction, show-ing the highest detrimental effect on the glycerol conversion. The

same conclusion was reported later by Chan-Thaw et al. [22], who

worked on oxidation of raw glycerol (from edible rapeseed veg-

etable oil transesterification) usingsupported Au-Pd nanoparticles.

The highest initial activity at 50◦C using non-treated raw glyc-

erol (1672 mol/mol h) was achieved over a 1% Au6Pd4/AC catalyst,

whichalso showedthe highest activity in a process using commer-

cial pure glycerol (3205mol/molh). Gil et al. [23] attempted the

oxidation of partially purified glycerol fractions (minimum purity

not less than 95.5%) at 60◦C under 5 bar of O2 over carbonaceous

materials-supported gold catalysts. They reported almost 50% of

glycerol conversion after 10h of test with 95.5% glycerol. The per-

formances were much better when using almost pure, neutralized

97.1% glycerol with60% of conversion after4 h. Essentially the sameconversion was observed using a high-purity 99.5% commercial

anhydrous product, but with a higher selectivity to glyceric acid

(65% vs. 45% when using the neutralized fraction). Sullivan and

Burnham [24] used model titanium-supported gold catalysts. They

proved that such catalytic system is very sensitive to impurities

present in a crude (68.5%) glycerol fraction. The authors obtained

less than 20% conversion after 24h at 60◦C under air flow atatmo-

spheric pressure; a remarkably enhanced production of formic

acid from crude glycerol fraction in comparison to analogous test

with pure glycerol and purified fractions (after partial removal of

potassium, phosphorous and fatty acid derivatives) was observed

without any tentative explanation [24]. Finally, Kondamudi et al.

[25] studied photooxidation over a titanium di-silicide catalyst

(TiSi2)at65 ◦C under atmospheric pressure, where almost 64% con-version with 100% selectivity to glyceric acid were achieved after

6 h of reaction, using crude glycerol of an unspecified composition.

In the present paper, we evaluated the impact of glycerol

purity on the performances of four different noble metals-based

catalysts, namely: Ag/Al2O3, Au/Al2O3, Pd/Al2O3 and Pt/Al2O3.

Three of them, i.e., gold, palladium and platinum supported

catalysts, are commercially available and well known for their

high activity in the partial oxidation of pure glycerol in the liquid

phase. In numerous studies, various research groups investigated

the effect of such parameters as: the reaction temperature, the

oxygen pressure, the glycerol concentration, the presence of base,

the catalyst synthesis method, etc . [1,5,10,12–20]. Nevertheless,

the effect of glycerol purity was investigated only over gold and

platinum catalysts [20,22,24]. As each of the aforementioned

research groups used different glycerol fractions, further using dif-

ferent reaction conditions, it seems quite difficult to draw reliable

comparative conclusions on the role of each impurity on catalytic

performances modulation over each type of catalyst. From the

best of our knowledge, there are no articles concerning oxidation

of crude glycerol in the liquid phase using other monometallic

catalytic systems, especially those based on palladium and silver.

Herein, the abovementionedcatalysts were tested underidenti-

cal reactionconditions (0.3M glycerolconcentration in the reaction

mixture, 60◦C, 5 bars of oxygen, NaOH/glycerol molar ratio equal

4 and glycerol/catalyst weigh ratio of 11), using both commercial

anhydrous glycerol and a crude glycerol fraction received from

a biodiesel plant. Then, in order to decouple the effect of each

impurity and to avoid misinterpretation due to possible interac-

tions and synergism, eachof identified andquantified impurity was

separately added to pure glycerol solution and the results were

compared in terms of glycerol conversion and selectivity to main

products. We believe that the outcomes of this study will give

elements to decide which type of catalyst should be used for the

oxidation of crude glycerol fractions.

2. Experimental

2.1. Materials

Anhydrous glycerol 99% from Sigma-Aldrich and crude glycerol

from Orlen Południe S.A. were used for the catalytic tests. Themain

composition of the crude glycerol fraction is given elsewhere [20]

and consists of 47.4 wt.% of glycerol, 29.1 wt.% of methanol (HPLC),

8.6 wt.%of water (Karl Fischer titration),1.3 wt.%of ash(gravimetric

method) and 13.6wt.% of matter organic, non-glycerol and non-

methanol (MONG-NM, calculated according to IUPAC guidelines

[26]). The sulphur concentrationin thecrude fraction(0.1 wt.%) was

further determined by portable XOS Sinide OTG analyzer based on

Monochromatic Wavelength Dispersive X-RayFluorescence(MWD

XRF).

Sodium sulfate (≥

99.0%, ACS reagent from Sigma-Aldrich),methanol (≥99.9% HPLC grade from Aldrich) and thioglycolic acid

(≥98.0%, pure from Fluka) were used as received without any fur-

ther purification. The MONG-NM used for supplementary tests was

obtained by physical separation from the crude glycerol fraction

(hydrophobic top layer of the fraction).

Silvernitrate (≥99.0%, ACS reagent), methanol (≥99.9%CH3OH),

formaldehyde (37 wt.% in H2O) and sodium hydroxide (purum)

were all purchased from Sigma Aldrich, and alumina oxide powder

(activated basic Al2O3, Merck) was used for preparation of silver-

supported catalyst.

Commercial 1 wt.% Au/Al2O3 catalyst (AUROliteTM from Sterm

Chemicals) and1 wt.%Pt/Al2O3 fromSigma-Aldrich weregrounded

and sieved to obtain fraction 50–125m, while 1 wt.% Pd/Al2O3(powder from Sigma-Aldrich) was used as received. No additional

pretreatment procedure was applied to these catalysts before

testing.

2.2. Catalyst preparationmethod

The silver supported alumina oxide catalyst was prepared by

chemical reduction in the liquid phase. The alumina support pow-

der (fraction 50–125m) was suspended in a methanol solutionof silver nitrate and, after having adjusted the pH of the reaction

mixture to 8 with sodium hydroxide, a 2M aqueous solution of

formaldehyde was used as a reducing agent. The suspension was

mixed and heated under the reflux for 90min. Then, the solid was

separated by filtration, washed with distilled water and dried at

110 ◦C for 24h prior testing.


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E. Skrzyńska et al./ Applied Catalysis A: General 499 (2015) 89–100 91

2.3. Catalysts characterization

The specific surface area(BET method)and poresize distribution

(BJHmethod)were measured by the nitrogenadsorption technique

using a TriStar II 3020 apparatus from Micromeritics.

XRD analysis was performed at ambient temperature on a D8

Advanced apparatus from Bruker AXS instrument. The samples

werescanned ata rateof 0.02◦ per stepoverthe5◦≤2 ≤90◦ range(scan time= 3 s step−1). The diffractograms were indexed using the

JCPDS database.

The oxidation state of the metals on the surface of fresh

and used catalysts was determined by XPS analysis (VG ESCALab

220XL from Thermo Fisher Scientific) using a monochromatized

aluminium source (AlK˛= 1486.6eV). The high-resolution spectrawere recorded with a 40eV pass energy, and the value of the C1s

core level (284.6eV) was used for calibration of the energy scale.

Curve fitting was performedusing the CasaXPS software taking into

account a Shirley-type background subtraction, symmetric peak

shape of silver, gold and palladium, and an asymmetric peak shape

of platinum LA(1.2,85,70) [27].

For transmission electron microscopy analysis two apparatus

were used. The first one, a “TEM FEI Tecnai G2-20 twin” electronic

microscope, working with an accelerating potential of 200kV,

enabledobservationof samples witha veryhigh resolution(2–5 nmscale), while the second one, a Philips CM 30 Transmission elec-

tron microscope, working with an accelerating potential of 300 kV

and equipped withan energy dispersive spectrometer (EDS) for the

local chemical analysis (spatial resolution 10−5 mm3), enabled also

operating in a transmission scan mode (STEM mode), which is use-

fulfor mapping of the catalyst surface composition.The samples for

TEMwere preparedfrom a diluted suspension of catalystin ethanol.

A drop of suspension was placedon a Lacey carbon-coated grid and

allowed to dry in air. The particle size distribution was calculated

by countingover 400 particles over multiple areas using the Visilog

6.5 software.

The metal loading in fresh platinum, palladium and gold cat-

alysts was determined by inductively coupled plasma-atomic

emission spectroscopy (ICP-AES, Vista Pro Varian). The sampleswere prepared by Hot Block/HCl/HNO3 digestion [28]. For the

silver-supported catalyst, due to low sensitivity and accuracy to

Ag+ by ICP, X fluorescence spectroscopy was applied using a S2

Ranger Bruker spectrometer equipped with Pd X-ray tube.

2.4. Catalytic performances evaluation

A typical experiment of the liquid phase glycerol oxidation

was carried out in a 300 cm3 semi-batch stainless steel reactor

equipped with a gas-induced turbine, 4 baffles, a thermocouple,

and a thermo-regulated oxygen supply system. In eachexperiment,

200cm3 of a pure or crude glycerol solution in water were heated

to 60◦C, and the reaction was started when the calculated amount

of NaOH with selected catalyst (0.5g) were flushed into the reac-tor and pressurized with oxygen (5bar). In all the experiments,

the initial concentration of glycerol was 0.3 M, and the amount of

base was adjusted to give a NaOH/glycerol molar ratio of 4. For

the supplementary studies, well-controlled quantities of selected

and desired amount of additive (Na2SO4, methanol, MONG-NM,

thioglycolic acid) were introduced in the reactor together with

the reagents. The products were periodically sampled (1cm3 sam-

ples directly acidified to quench the reaction) and analyzed with

an Agilent 1200 HPLC equipped with a Rezex ROA-Organic Acid

H+ column (300×7.8 mm) and a reflective index detector (RID). A

solution of H2SO4 (0.0025 M) in deionized water (0.5cm3 min−1)

was used as an eluent. The identification and quantification of the

obtainedproducts wasdone by comparison withthe corresponding

calibration curves.

3. Results and discussion

3.1. Catalysts characterization

First, the metal loading in the fresh catalysts was determined.

Results presented in Table 1 show that the real metal loading in all

the tested catalysts was close to the 1w t.%, for the Au, Pd and Pt

supported materials (metal loading guaranteed by producer), and

also for the Ag/Al2

O3

material prepared by the chemical reduction

method.

Due to the relatively low metal loadings used, the surface prop-

erties of each catalyst, i.e., the specific surface area and the pore

size distribution (Table 1, Fig. 1), undoubtedly reflected the char-

acter of the corresponding alumina support. While information on

the aluminas used for the preparation of commercial catalysts is

unavailable, this can be verified for the lab-prepared silver sup-

ported catalyst (Table 1, Figs.1 and 2).

Combining the results of nitrogen adsorption–desorption anal-

ysis with XRD analysis (Fig. 2), we can see that the Au/Al2O3and Pt/Al2O3 catalysts were prepared using gamma alumina

supports with respectively high (>200m2 g−1) and moderate

(


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Fig. 1. Nitrogen adsorption/desorption profiles (insets) and pore diameter distribution calculated by the BJH method of freshalumina-supported catalysts: (a) Ag/Al2O3 and

(b) corresponding bare alumina support; (c) Au/Al2 O3; (d) Pd/Al2O3; (e) Pt/Al2O3.

metallic particles very well dispersed on the support surface. The

TEM images (Fig. 3) are on line with this assumption.

The commercial catalysts exhibited a relatively narrow particle

size distribution, and almost 30% of all the particles had a diameter

of 3nm. For the Pt and Pd catalysts, the biggest measured particle

had a diameter of 16nm and 12nm, respectively, while for the Au

material, single particles with a diameter of 18–26 nm were also

found. In the case of the lab-made Ag catalyst, the particle size dis-

tribution was less uniform, but 30% of all the particles were below

10nm and almost40% had a diameter between16 and 26nm. Thus,

the main particle diameter calculated as an arithmetic average of

all the particles, increased in the order:

Pd/Al2O3 (3.5±1.5nm)


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Fig. 2. X-ray diffractograms of the fresh catalysts with the position of the main

diffraction lines expected for the metallic particles: Ag (JCPDS card 01-089-3722),

Au (JCPDS card 00-004-0784), Pd (JCPDS card 01-087-0637) and Pt (JCPDS card

00-001-1194). Symbols correspond to: () gamma-Al2O3 (JCPDS 00-010-045); ()

theta-Al2O3 (JCPDS 01-086-1410); (#)Boehmite,gamma-Al2O3·H2O (JCPDS 00-001-

1283); and (*)Gibbsite, Al(OH)3 (JCPDS 00-033-0018).

and 71.2eV for Pt 4f 7/2 [27,30–33]. In each case, the value of the C

1s core level (284.6eV) was used for calibration of the energy scale

and the binding energy of Al 2p level was accordingly essentially

the same, equal 74.6±0.2eV.

The XPS analysis repeated for the used catalysts after oxidation

tests with pure andcrudeglycerol confirmed a good stabilityof the

oxidationstate of the metallic particlesunder operating conditions.

Irrespectiveof thetypeof glycerolusedin theoxidationtests, theBE

of each metallic core level remained almost unchanged with very

small variations at max of +0.1eV or−0.15eV (Table 2). Moreover,

theratio between themetal and alumina measured beforeand after

oxidation tests remained practically the same for each type of the

catalyst.

3.2. Catalytic tests–oxidation of pure and crude glycerol

The performance of the catalysts was assessed in the liquid

phase glycerol oxidation reaction with pure and crude streams.

Eachof the catalystsexhibited differentbehaviours (different activ-

ities and selectivities),but a clear relation between thepurity of the

reactant feed and the reaction rate was observed (Table 3).

For the oxidation of pure glycerol, under our conditions the

most active catalyst was the Au catalyst, which enabled obtain-ing full conversion after only 30min of reaction (Fig. 5). The main

products observed at the beginning of the reaction were: glyc-

eric acid (60% selectivity), glycolic acid (about 20% selectivity)

and formic acid (10–12% selectivity). The selectivity to these two

latter compounds remained constant during the progress of the

experiment, while the glyceric acid selectivity decreased due to

further oxidation to tartronic acid. That stays in agreement with

Fig. 3. TEM imagesand histograms of themetallic particle size distributions observed over thecatalysts: (a) Ag/Al2O3; (b) Au/Al2O3 ; (c) Pd/Al2O3; and (d) for Pt/Al2O3.


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Fig. 4. XPS spectra of the Ag 3d, Au 4f, Pd 3d and Pt 4f levels with corresponding BE values observed for fresh catalysts: (a) Ag/Al2O3; (b) Au/Al2O3; (c) Pd/Al2O3; and ( d)

Pt/Al2O3.

Table 2

Comparison of theBEs estimatedfor each metallic core levels(Ag 3d, Au 4f,Pd 3d andPt 4f)and selected relations between surface atomicconcentrationsfrom XPSanalysis

performedfor fresh and spent catalysts.

Catalyst* Corresponding supported

metal core level BE (eV)

Relative atomic concentration

metal (×100)/Al Na/Al C/Al

Ag/Al2O3—fresh 368.2 1.5 0 0.34

Ag/Al2O3—after test with pure glycerol 368.1 1.5


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Table 3

Comparison of theinitial reaction rates andthe selectivities to main products at 10% and 30% isoconversion forthe oxidationof pure and crude glycerol over Ag, Au, Pd and

Pt supported catalysts. Thereaction conditions are thesame as those in Fig. 5.

Type o f c atalyst Type o f g lycerol Initial reaction

rate*

(molh−1 molMe−1 )

Selectivity to main products (%) at:

10% of glycero lisoconver sion 30% o f gly cer ol isoconvers ion

GA GLYCA FA GA GLYCA FA TARTA OXALA

Ag/Al2O3 Pure 866 27.8 35.9 35.3 27.2 44.8 28.0 0 0Crude 384 49.5 27.0 17.6 n.a. n.a. n.a. n.a. n.a.

Au/Al2O3 Pure 12976 59.8 19.7 10.2 60.4 20.7 12.5 0.9 0.2

Crude 1230 43.4 13.2 39.6 n.a. n.a. n.a. n.a. n.a.

Pd/Al2O3 Pure 1091 91.7 2.8 0 85.8 2.6 1.0 5.7 0.3


Pt/Al2O3 Pure 2898 75.6 10.7 11.0 74.0 9.9 8.1 5.1 2.6


* For the calculation of the specific initial reaction rate, we used the metal loadings estimated by elemental analysis; n.a.—not analyzed/not applicable as the calculation

of selectivities at 30% glycerol isoconversion would require too extensive extrapolation of the experimental results; Abbreviations: GA—glyceric acid, GLYCA—glycolic acid,

FA—formic acid, TARTA—tartronic acid, OXALA—oxalic acid.

achievedwithsimilarselectivitiesto glycolicacid (42%) andglyceric

acid (23%).

Such a big difference between the activities of the catalysts canbe first explained in the light of the metallic particles distribution

on the catalyst’s surface (Table 1). That would follow the concept

of the “structure-sensitive” behaviour introduced for this reaction

by Demirel et al. [35], and further confirmed by the works of Prati

et al.[36] andCarretinet al.[37], andalsofurther discussed byDim-

itratos et al. [16]. It is generally accepted that small particles with

narrow size distribution usually means better metal dispersion on

the catalyst’s surface and higher activity. Indeed, as it can be seen

from Figs. 3 and 5, silver supported catalyst with the highest aver-

age diameter and widest particle size distribution (21.2±18.6nm)

was the least active in the pure glycerol oxidation. It also pro-

moted an oxidative cleavage of glycerol molecule leading directly

to glycolic and formic acids (respectively 35.9% and 35.3% selectiv-

ity at 10% of glycerol conversion—Table 3), while the other metalsfavoureda non-destructive oxidationof hydroxylgroup to tartronic

acid via glyceric acid. Further transformation of primary products

should proceed by deep oxidation reactions, leading to mesoxalic

acid, oxalic acid, formic acid and finally to carbon dioxide in the

formof carbonate under basicreaction conditions. Possible reaction

pathways were discussed in details in a previous paper [34].

Although a complex and thoroughgoing studies are required to

explain the nature of silver active sites in the glycerol oxidation

process at liquid phase, very high selectivity to glycolic acid seems

to be characteristic for this metal. Analogous behaviour was pre-

viously reported by Ketchie et al. [38], who tested gold and silver

powders in the glycerol oxidationreaction. Theyconcluded thatnot

only particle size distributionplays a significant role in the glycerol

oxidation process and other parameters must also be considered.That also stay in agreement with our experiences with the blank

testsand quasi-homogenous glycerol oxidationover goldnanopar-

ticles [34], as well as on the results of catalysts screening from

samples prepared by different methods, where the mean diame-

ter of the particles (platinum, gold andsilver) varied between 2 nm

and 260 nm. As it can be seen from Fig. 5, the purity of glycerol is

another parameter which greatly affects the process.

Application of the crude glycerol fraction as a raw material for

theoxidation process evidently blockedthe catalystsactivity.In any

case, the decrease in the initial reaction rate was significant, from a

factor 2 in the case of Ag/Al2O3 to a 10 fold decrease over Au/Al2O3(Table 3). The highest resistance exhibited the catalysts with both

the lowestand the highest averagediameterof the particles(3.5nm

for Pd and 21.2nm for Ag, respectively). Except for the palladium

catalyst, which enabled obtaining 24% of conversion after 2h at

60 ◦C, the conversions were much below 20%. The selectivities to

the main products were then compared at 10% of glycerol isocon-version (Table 3). Note that while the conversion of pure glycerol

over the gold catalyst was 27% after 3 min of reaction, comparison

with data at 10% conversion in the case of using crude glycerol was

reliable due to the relatively stable selectivities to main products

(i.e., glyceric, glycolic and formic acids) observed during the first

20min of reaction.

A decrease in selectivity to glyceric acid in favour of formic

acid was observed over the Pt and Au catalysts when shifting to

the crude glycerol feed. As methanol was present in the crude

glycerol fraction (29.1 wt.%), its conversion to formic acid should

be expected. However, the contribution of this side reaction was

subtracted when calculating glycerol conversion and selectivities

(method explained in details elsewhere [20]). Thus, overproduc-

tion of formic acid could only be explained by a change in glycerolreactivity over the solids placed under crude glycerol conditions.

The dominating reaction pathways (i.e., selective oxidation of

hydroxyl groups leading successively to glyceric and tartronic

acids, and oxidative cleavage directly to glycolic and formic acids)

can compete with destructive, deep oxidation of glycerol molecule

to C1 derivatives. Possibility of bothreactionpathways was verified

previously by kinetic calculations and proposed for both the non-

catalytic oxidation at basic medium and the quasi-homogenous

oxidation of pure glycerol using unsupported gold nanoparticles

[34]. What is meaningful is that the production of formic acid is

only marginally increased from 0% to 2.8%, over the Pd catalyst

(Table 3), with a similar trend for the selectivity to glycolic acid

(small increase from 2.8% to 4.9%). In the case of the Ag/Al2O3

catalyst, a completely opposite situation was observed, as theselectivities to C1 and C2 derivatives decreased in favour of that of

glyceric acid. Thus, it seems reasonable that the impurities present

in the crude glycerol fraction blocked the sites responsible for

oxidative cleavage of glycerol, keeping active the sites responsible

for the non-destructive oxidation of hydroxyl groups. Thus, in

spite of poisoning, due to limited concentration of impurities in

the crude glycerol fraction, an increase of the overall number of

available metal particles (i.e. increase of Ag active sites responsible

for unique high selectivity to glycolic acid) should improve the

contribution of oxidative cleavage reactions in the crude glycerol

oxidationprocess and make the decrease of glycolic acid selectivity

less important. Additional experiments using the silver catalyst

with a substantially larger loading (2.3 wt.% Ag/Al2O3) and a parti-

cle size distribution comparable to that of the 1.13wt.% Ag/Al2

O3


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Fig. 5. Glycerol conversion() andselectivityto main products:glycericacid (), glycolic acid (),oxalicacid(), tartronicacid (),formicacid(×) intheoxidation ofpure

glycerol (a–d) and crudeglycerol (e–h). Catalyst: Ag/Al2O3 (a and f); Au/Al2O3 (b and f); Pd/Al2O3 (c and g); Pt/Al2O3 (d andh). Reaction conditions: 200cm3 ofpureor crude

glycerol solution with nominal 0.3M concentration of glycerol; NaOH/glycerol molar ratio of 4; 0.5g of catalyst;60 ◦C; 5 bars o f O2; and a stirring speed of 1500rpm.


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Fig. 6. Glycerol conversion () and selectivity to main products: glyceric acid (),

glycolic acid () and formicacid (×) in the oxidation of pure glycerol (a) and crude

glycerol (b)over 2.3wt.% Ag/Al2 O3 . Reaction conditions the same as at Fig. 5.

sample (mean particle diameters of 20.9±16.2 nm and

21.2±18.6nm, respectively) proved that a higher concentra-

tion of active sites on the catalyst surface enables maintaining

relatively high selectivity to glycolic acid (40–47%) during the

whole experiment with crude glycerol fraction (Fig. 6).

While the conversion of the crude fraction was again lower in

comparison to tests with pure glycerol (initial reaction rate equal,

respectively to 305 and 492molh−1 molAg−1), a significant out-

come is that at comparable 15% isoconversion the selectivity to

formic acid decreased from 36% to 10% in favour of glyceric acid

(growth from 14% to 35% using pure and crude glycerol, respec-

tively), and essentially the same selectivity to glycolic acid was

maintained, i.e., 41% for oxidation of pure glycerol and 42% while

using the crude fraction. Thus, the ratio between the impuritiesand the silver active sites present on the catalyst surface has to

be controlled and kept at adequate low level, and/or higher metal

loading has to be applied, in order to maintain high conversion

rates and achieve profitable selectivity to more valuable C2 and C3

derivatives.

3.3. Catalytic tests with addition of single impurities

The reasonable questions which arose from tests with crude

glycerol fraction were: (i) what is the impact of separate impu-

rities?; (ii) Which of them should be limited to the minimum?; and

(iii) is there any key enabling proper choice of the catalyst forcrude

glycerol oxidation? In our previous work, we identified the answer

to the first question applied for the platinum supported catalyst

[20]. Namely, MONG-NM, which in majority consists of various

fatty acid derivatives (i.e., fatty acid methyl esters left after incom-

plete separation of biodiesel, as well as mono-,di- and triglycerides

coming from unconverted fatty raw material used for biodiesel

production—more detailed discussion on this topic can be found

elsewhere [20]), was found to be the most problematic impurity,

as its deposition on the catalyst surface physically blocked the

access of glycerol to the active sites. While the performances of the

catalysts exhibiting a high activity might be restored by a simple

washing procedure with an organic solvent, the presence of small

quantities of MONG-NM in the reaction mixture strongly hindered

the glycerol conversion. The mineral salts had only a small detri-

mental effect on the process,while a lowconcentration of methanol

promoted the oxidation by improved oxygensolubility in the reac-

tion mixture. Only the effectof the organic sulphur derivatives was

not assessed. The presence of such components was recently evi-

denced in the crude glycerol fraction, using sulphur sensitive GC

chromatography. Their content was estimated by MWD XRF mea-

surements. The origin of such impurities in crude glycerol fraction

is not obvious.They canbe formed during biodieselproduction pro-

cess, i.e., during neutralization of the basic catalyst conventionally

used for transesterification, as well as during acidic esterification

of low quality vegetable oils and other free fatty acids rich-raw

materials. However, they canalso be present in the initial fatty acidraw material, especially when non-conventional raw materials are

used, like: discards from vegetable oils industry, waste fats from

food processing plants, gastronomy or leather manufacture, etc .

Also, a non-refined oil obtained from crops demanding high levels

of sulphur fertilization (including rapeseed oil) can be responsible

for the presence of organic sulphur derivatives in crude glycerol

fractions [39–41]. Our experiences with Au/Al2O3 suggested that

strong poisoning of the active sites by organic sulphur derivatives

might be expected also in the processes with other noble metals

supported catalysts [42]. Thus, a series of catalytic experiments

with pure glycerol and small amounts of single additives, repre-

senting the impuritiespresent in crude glycerol fractions,including

organic sulphur derivatives, were carried out. On the basis of our

previous experiments with crude glycerol using supported plat-inum [20] and also gold catalysts [42], we decided to add into the

mixture comparable amount of methanol, higher concentration of

sodium sulphate (due to its tinny effect on the conversion), and as

small as possible amount of MONG-NM and thioglycolic acid addi-

tives, as they were expected to be the most powerful and the most

problematic at thesame time (both components shouldbe avoided

in the mixtures injected on the HPLC column, and also possible

strongpoisoning of both thecatalystand the walls of reactor might

appear). Composition of each prepared and tested reaction mixture

can be found in Table 4.

Clear similarities between the behaviour of the catalysts can be

observed (Fig. 7). The presence of methanol or sodium sulphate

affected the glycerol conversion in the lowest extent. A small pro-

motional effect of CH3OH, previously attributed to an increase inthe oxygen solubility in the reaction mixture [20], was evidenced

only when platinum and palladium catalysts were used, although

their selectivity was affected in different ways (Table 5).

The palladium catalyst practically did not exhibit any change

in performances, while the destructive oxidation pathway was

enhanced over the supported platinum catalyst. At 10% isoconver-

sion, the selectivity to glyceric acid dropped from 75.6% to 48% in

favourof formicacid–from11%to almost40%.Forthe other two cat-

alysts, a small detrimental effect of methanol was observed, which

indicated that a purely physical phenomenon of change in oxygen

dissolution in the reaction composition is not the most impor-

tant factor affecting process with methanol addition over these

two catalysts. Indeed, the results of pure glycerol oxidation under

oxygen pressure varying from 3 to 10bars [34] showed that the


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Table 4

Composition of each reaction mixture used to assess the effect of impurities in crude glycerol fractions. Solutions of nominal 0.3M glycerol concentration were prepared

from crude glycerol (11.656 g of crude fraction in 200cm3) or pureglycerol (5.53g of pure, anhydrous glycerol in 200cm3 ) with single, selectedadditive.

Type of impurity identified in crude

glycerol/additive used

Tests with pure glycerol Tests with crude glycerola

concentration of impurities (wt.%)

Amount of additive

used

Concentration in the

reaction mixture (wt.%)

Methanol 4.28 cm3 1.7 1.7

Ash/Na2SO4 2.432 g 1.2 0.1

MONG-NMb 0.271 g 0.1 0.8

OSDc/thioglycolic acid 1l 7 ppm 66 ppm

a No additives, impurities originated from crude fraction.b MONG-NM–matter organic non-glycerol and non-methanol [20].c OSD—organic sulphur derivatives.

Fig. 7. Changes in conversion during theoxidation glycerol in thepresenceof various additives: 2.432g of sodium sulphate ();4.28ml of methanol ();0.271g ofMONG-

NM (); 1l of thioglycolic acid (). Conversion of pure glycerol (, full line) and crude glycerol (, dotted line) without additives are given for comparison. Catalysts:

Ag/Al2O3—(a); Au/Al2O3—(b); Pd/Al2O3—(c); Pt/Al2O3—(d). Thereactionconditions are thesame as those in Fig. 5.

performance of the gold nanoparticles practically did not change,

which was also the case for the process in the absence of cata-

lyst. Thus, the effect of a small increase in the oxygen dissolution

caused by the presence of small amounts of methanol, i.e., physi-

cal aspect, might be less important than that of the chemical one,

especially if we remark that selectivities of both catalysts changed

in comparison to the process performed without methanol addi-

tion (Tables 4 and 5). For the gold catalyst, cleavage to formic was

muchmore important (selectivity to formic acid shifted from 10.2%

to 31.5%), while for the silver catalyst, the high selectivity to gly-

colicacid dropped from 35.9% to 14.5%in favour of non-destructive

oxidation to glyceric acid (of which the selectivity increased from

27.8% to 46.5%).

Similarly, the effect caused by addition of sodium sulphate was

more important over silver and gold catalysts than over platinum

and palladium catalysts. In the case of the two latter ones, the

selectivity remained almost unchanged. In contrast, over the gold

material, again a small increase in selectivity to formic acid was

observed (10.2–27.7%). Overthe silver catalyst,a drastic increase in

glyceric acid selectivity was evidenced(from 27.8% to 46.8%), while

the selectivity to glycolic acid remained substantially unchanged

(36.3% vs.35.9%), and finally, the presence of sodium sulphate erad-

icatedthe formicacid formation route (decrease in selectivity from

35.3% to 0%). The difference in the initial reaction rate observed

upon addition of sodium sulphate was the less pronounced com-

paredto thatobservedusing other additives. Moreover,the amount


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Table 5

Comparison of theinitial reaction rates over Ag, Au, Pd and Pt catalystsand selectivities to main products at 10% isoconversion in thepresence of various additives: 2.432g

Na2SO4, 4.28ml CH3OH, 0.271 g of MONG-NM and 1l of thioglycolic acid. Reaction conditions are thesame as those in Fig. 7.

Type of catalyst Type of additive Initial reaction rate

(molh−1 molMe−1 )

Selectivity to main products (%) at 10% of

glycerol isoconversiona

Glyceric acid Glycolic acid Formic acid

Ag/Al2O3 Na2SO4 673 46.8 36.3 0

CH3OH 550 46.5 14.6 21.2

MONG-NM 385 44.3 25.0 16.4

HSCH2COOH 234 59.5a 7.1a 0a

Au/Al2O3 Na2SO4 9551 58.3 12.7 27.7

CH3OH 8176 57.1 9.1 31.5

MONG-NM 1230 65.7 6.8 13.3

HSCH2COOH 492 60.5a 8.7a 4.1a

Pd/Al2O3 Na2SO4 1149 84.1 3.2 0.2

CH3OH 1389 81.9 1.8 1.2

MONG-NM 718 76.9 2.3 0

HSCH2COOH 722 87.7 1.3 0

Pt/Al2O3 Na2SO4 1449 75.3 7.4 14.5

CH3OH 2957 48.0 10.1 39.5

MONG-NM 468 75.0 4.3 16.1

HSCH2COOH 483 72.2a 2.9a 0a

a Forthe tests with thioglycolic acid addition using silver, gold andplatinum,the catalystsselectivities presentedin this table aregiven forthe highest observed values of

glycerol conversion, namely4.1% forAg/Al2 O3, 6.8% for Au/Al2O3 and 7.3% for Pt/Al2O3.

of Na2SO4 used in the additional tests was 10-fold larger than the

amount of ash actually present in the crude glycerol streams used

for the tests. Hence, it is obvious that this component was not

responsible for any important decrease in the glycerol conversion,

especially when the process was carried out over the palladium-

based catalyst.

The third type of tested impurity was a hydrophobic, dark

brown, complex mixture of fatty acids derivatives, present in

crude glycerol fractions after biodiesel separation process [20]. We

previously showed thatthis mixture, recognized as “matter organic

non-glycerol and non-methanol” (MONG-NM), blocks the access

to active sites of Pt/Al2O3 catalysts by reversible adsorption on

the catalyst surface. Regeneration of such deactivated catalysts ishowever possible, but it requires extensive washing with a solvent

capable of dissolving the adsorbed hydrophobic fatty acid deriva-

tives [20]. Concerning silver and palladium catalysts, the glycerol

conversion observed when adding MONG-NM to pure glycerol

was very similar to that observed when directly using the crude

glycerol fraction (Fig.7). Interestingly, the same catalysts were also

the least sensible ones to organic sulphur poisoning among the

tested samples. Indeed, the ratio between the glycerol conversion

in the presence of thioglycolic acid to the glycerol conversion with

pure glycerolafter2 h was 0.35 forPd/Al2O3, 0.15for Ag/Al2O3,and

only 0.10 and 0.07 for Pt/Al2O3 and Au/Al2O3, respectively. Exactly

the same sequence can be found when using the initial reaction

rates ratiosas a criterion(calculated from results in Tables 3 and 5):

0.66Pd/Al2O3 > 0.27Ag/Al2O3 > 0.17Pt/Al2O3 > 0.03Au/Al2O3

As almost 66ppm of sulphur derivatives were detected in

the reaction mixture prepared from the crude glycerol fraction

(Table 2), it seems reasonable to conclude that it was the main

reason why the results of crude glycerol oxidation over gold and

platinum catalysts were worse from that arising from test with

single MONG-NM addition (Fig. 7). All these observations clearly

indicate that MONG-NM and organic sulphur derivatives have the

most detrimental effect on the glycerol conversion in the oxida-

tion process. Although removal of hydrophobic MONG-NM might

be realized by simple two-phase separation (less expensive grav-

itational or induced by centrifugation, tangential flow separation,

etc .), the removal (or deactivation) of organic sulphur derivatives

would require more sophisticated and expensive methods. Thus,

an improvement of the catalyst’s resistance to sulphur should be

considered.

Decoupling of the effects of the impurities allowed us to deduce

some specific trends concerning selectivity modifications. How-

ever, these conclusions cannot be straightforwardly transposed to

real streams, as the complexity of crude glycerol composition is

such that some synergistic or antagonistic effects can be obtained

in the simultaneous presence of various impurities families. More-

over, the differences in the catalyst morphology can also play a

role in the response of the catalytic system to separate impurities,

even if the present data do not allow us to conclude onthat specificaspect. However, under the conditions used in the present paper,

the Pd-based catalyst suffered the least of selectivity modification.

Gold and platinum catalysts should not be used in the oxidation

of crude glycerol in the presence of large amounts of methanol, as

undesirable conversion of glycerol to formic acid was increased in

such a case. Analogous effect was also reported by Sullivan [24] f or

crude glycerol oxidation over Au/TiO2 catalyst. On the other hand,

the presence of organic sulphur derivatives efficiently blocked this

undesirable reaction, and selectivity to formic acid was then neg-

ligible (or very low) for all the tested catalysts. This additive had

also the most marked influence in the change of selectivity over

the Ag catalyst from glycolic to glycericacid, i.e., at 4.1% conversion

these selectivities were 7.1% and 59.5%, respectively. For compari-

son, duringthe test without any additivesthe selectivity to glycolic

acid at initial stage of the process (the lowest analyzed conversion

of 6.3%) was 26.9%, while selectivity to glyceric acidwas only33.7%,

and it further stabilized at the level of 25–27% (Fig. 5a). Unique

high selectivity to glycolic acid was maintained only in the pres-

ence of mineral salt and MONG-NM (respectively 36.3% and 25.0%

at 10% isoconversion). Interestingly, the general performances of

the Ag catalyst were almost identical when using pure or MONG-

NM-added glycerol.

4. Conclusions

The results presented in this paper highlight the impact of

impurities present in crude glycerol fractions in the reaction of

liquid phase glycerol partial oxidation over four different types of


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monometallic noble catalysts supported on alumina. The complex-

ity of crude glycerol composition and its variety depending on its

origin (biodiesel production conditions from which it is issued),

as well as possible synergetic/antagonistic effects between the

impurities, makes it difficult to strictly draw any definitive conclu-

sion. Nevertheless, based on the present results, some rationales

in the choice of proper catalysts for crude glycerol oxidation can

be given. Hence, monometallic gold catalysts, known from their

excellent activity in pure glycerol oxidation, should be avoided

when using unpurified waste fractions from biodiesel production.

This catalyst was the most sensitive to all the tested impurities.

Among the tested monometallic catalysts, palladium seems to be

the most resistant to both methanol, mineral salts, MONG-NM

and even to organic sulphur derivatives, which easily poison gold

and platinum catalysts. Pt/Al2O3 can be used for oxidation of frac-

tions rich in ash (mineral salts) and methanol, while an increase

in formic acid production might be expected in the presence

of this latter. Finally, the silver catalyst, with a high selectiv-

ity to glycolic acid (close to 50% at higher reaction rates) can

be used for processes with elevated MONG-NM concentrations.

Other impurities can drastically change the selectivity (glyceric

acid then becomes the main product), which can be an inter-

esting and non-conventional method of controlling the products

distribution.The results presented in this paper clearly show that the issue

of crude glycerol impurities is very complex and that the effect

of each impurity on the performance of various noble metal cata-

lysts can be substantially different in terms of both the activity and

the selectivity. This opens interesting perspectives in the further

studies on bimetallic catalytic systems. One should also consider

catalytic systems with enhanced sulphur resistance, as this impu-

rity – together with MONG-NM of which a sticky amalgam can be

physically separated before reaction – had the most detrimental

effect on the overall glycerol conversion performances.

Acknowledgments

This work was performed, in partnership with the SAS PIVERT,

within the frame of the French Institute for the Energy Tran-

sition (Institut pour la Transition Energétique (ITE) P.I.V.E.R.T.

(www.institut-pivert.com) selected asan Investmentfor theFuture

(“Investissements d’Avenir”). This work was supported, as part of

the Investments for the Future, by the French Government under

the reference ANR-001.

The authors would like to acknowledge Orlen Południe S.A. for

the crude glycerol samples kindly provided for testing.

Maxence Vandewalle (Université de Lille1 Scienceset Technolo-

gies) is acknowledged for XRF analysis.

The “Fonds Européen de Développement Régional (FEDER)”,

“CNRS”, “Région Nord-Pas-de-Calais” and “Ministère de l’Education

Nationale de l’Enseignement Supérieur et de la Recherche” are

acknowledged for fundings of X-ray diffractometers and XPS/LEIS/

ToF-SIMS spectrometers within the Pôle Régional d’Analyses de

Surface.

The authors would like to thank theEuropeanUnionthroughthe

Małopolska Regional Operational Programme (MROP) 2007-2013,

for funding of portable XOS Sinide OTG analyser.

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catalytic behaviour of four different supported noble metals in the crude glicerol oxidation - ok

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