subcriticalwater–aperspectivereactionmedia...

Presented at 3rd International Symposium on Environmental Management, SEM – Towards Sustainable Technologies (SEM2011)

at University of Zagreb, Faculty of Chemical Engineering and Technology, 26–28 September, 2011, Zagreb, Croatia

Subcritical Water – a Perspective Reaction Mediafor Biomass Processing to Chemicals: Study onCellulose Conversion as a Model for Biomass

I. Pavloviè, Ý. Knez, and M. Škerget*

University of Maribor, Faculty of Chemistry and ChemicalEngineering, Smetanova 17, 2000 Maribor, Slovenia

Abstract: Biomass and water are recognized as a key renewable feedstock in sus-tainable production of chemicals, fuels and energy. Subcritical water (SubCW), or com-monly referred as hot compressed water (HCW), is the water above boiling and belowcritical point (CP; 374 °C, 22.1 MPa). It has gained great attention in the last few de-cades as a green, cheap, and nontoxic reagent for conversion of biomass into valuablechemicals. In this paper, hydrothermal reactions of cellulose, as the model biomass sub-stance, with subcritical water at mild temperature and pressure regimes have been stud-ied. The experiments were done in a batch reactor in the temperature range of 220 ° –300 °C. The main products distributed in liquid, gaseous and solid phase were separatedand quantified. The conversions to each group of products were found strongly depend-ent on the temperature and residence time.

Key words:

Biomass, cellulose, subcritical water (SubCW), hydrothermal conversion, bio-oils

Introduction

Due to excessive reliance on nonrenewable re-sources and environmentally unacceptable produc-tion processes, the chemical industry is under in-creasing pressure and continuous improvement inperformance, enabling more sustainable and saferproduction. The main technological challenge liesin “green chemical engineering” which comprisesthe selection of renewable raw materials, integra-tion of material constraints, costs and safety, and in-creasing of energy and material efficiency in pro-duction and services.1 In this context, biomass andwater are recognized as key feedstock for a sustain-able chemical industry, which provide entirelynovel opportunities for production of chemicals,nowadays predominantly based on fossil resources.Integration of energy, fuels and chemicals produc-

tion thorough the concept of biorefineries is themost probable scenario in future.2 Utilization ofby-products and waste biomass for new or alreadyknown purposes leads to maximization of totalvalue of biomass as raw material and increases theprofitability, in comparison to the classical “one-product” approach.

Biomass, commonly referred to as lignocellu-lose, has a complex structure. It is constituted fromthree main structural components: cellulose, hemi-celluloses and lignin. The most abundant compo-nents are cellulose and hemicelluloses, named car-bohydrates, representing almost 75 % of biomassweight.2 Cellulose is a linear polymer of glucosemonomers linked via �-1-4-glycoside bonds in rib-bon-like crystalline structure, resistant to low tem-perature water penetration/dissolution and enzymeattack. Cellulose fibers constitute cell walls, givingstrength to biomass. Hemicellulose is a heteroge-neous polymer composed of C5-sugars or pentoses

I. PAVLOVIÈ et al., Subcritical Water – a Perspective Reaction Media for Biomass …, Chem. Biochem. Eng. Q. 27 (1) 73–82 (2013) 73

*Corresponding author: Mojca Škerget; Phone: ++386 2 22 94 463;Fax: ++386 2 2527 774; E-mail: [email protected]

Original scientific paperReceived: September 13, 2012

Accepted: February 4, 2013

(xylose and arabinose) and C6-sugars or hexoses(mannose, glucose, galactose), in different ratios,depending on feedstock. It is a branched polymerwith amorphous structure and is relatively easilyhydrolysable to its constituent sugars, unlike cellu-lose. Together, cellulose- and hemicellulose-sugarsrepresent the most important green feedstock forconversion into chemicals and fuels.2 The secondmost abundant chemical component is lignin(20–25 % of biomass weight). Lignin is a complex,highly aromatic polyphenolic material available inplants in different composition, molecular weightand amount. It has a complex, branched structure,mostly synthesized in enzyme polymerization ofphenylpropane subunits of trans-p-coumaryl alco-hol, trans-p-coniferyl alcohol and trans-p-synapylalcohol (aromatic monomers).4 It acts as a reinforc-ing agent, giving mechanical strength to biomass.Lignin does not have a regular structure like cellu-lose, but it is chemically and physically hetero-gonous, even the chemical structure is unknown.However, it has great potential as a versatile build-ing block within the biorefinery concept of biomassusage.5 Also minor components of biomass such asproteins, fats and oils or extractives (tannins, lig-nans, stilbens, terpenes, etc.) have great potential asfeedstock for many valuable chemicals.6

Methods for conversion of biomass into chemi-cals and fuels are divided into biochemical methods(e.g. enzymatic fermentation to ethanol,7 anaerobicdigestion to methane,8 etc.) and thermochemicalmethods (pyrolysis, gasification and liquefaction).9

Hydrothermal (HT) processes are defined asthermochemical reactions in water media at hightemperatures (above 200 °C) and high pressures(high enough to keep water in liquid phase orsupercritical state). They have gained increased at-tention in the last decades due to their several ad-vantages over other biomass conversion methods,including the use of water as a cheap, environmen-tally friendly reaction medium with unique proper-ties, and the ability to use wet heterogeneous bio-mass feedstock (waste, sewage sludge, etc.) withoutprior dewatering (drying) or other pretreatment pro-cess. Furthermore, HT reactions of complex bio-mass materials offer the possibility to produce ver-satile products, from solids, liquids to gases,through different HT processes (e.g carbonization,10

aqueous phase reforming,11 liquefaction12 and gasi-fication13). The real advantage of HT processes liesin the unique properties of subcritical (SubCW) andsupercritical water (SCW), significantly differentcompared to water at ambient conditions. The prop-erties of SubCW and SCW as a solvent and reactantin reactions with biomass have been reviewed by G.Brunner14 and A. Kruse and E. Dinjus15. The pres-ent article is focused on the properties and behavior

of subcritical water (SubCW) in reactions with cel-lulose as a model biomass compound.

Subcritical water (SubCW) is pressurized waterat temperatures above its boiling point at ambientpressure and below critical point (Tc=374 °C,pc=22.1 MPa, �c=320 kgm–3). Within a region closeto critical conditions (near-critical or subcritical),water properties become more sensitive to pressureand temperature changes. Above the critical point,water is called supercritical water (SCW) and itsproperties vary between liquid-like and gas-likewith changing temperatures and pressures withoutany phase transition. The most relevant propertiesof SubCW as a reaction media are miscibility, di-electric constant, ionic product, electrolytic solventpower, transport properties (viscosity, diffusion co-efficients and ion mobility), hydrogen bonding, etc.These properties are strongly influenced by thetemperature and water density so they could be ma-nipulated as parameters to enhance reaction selec-tivity to desired products. The dielectric constantdecreases by temperature due to lowering of hydro-gen bonds number, making SubCW more similar tohydrocarbon solvents. For this reason, the solubilityof hydrophobic organic compounds and light gasesin subcritical water is increased and thus they canbe extracted and subsequently easily separated fromreaction mixture upon cooling. The ionic product ofsubcritical water (Kw = [H+][OH–]) increases withtemperature and is greater by 1 to 2 orders of mag-nitude than at ambient temperatures, so SubCW hasan important role in acid- and base-catalyzed reac-tions (e.g. biomass hydrolysis). Furthermore, goodtransport properties (high diffusion coefficient andthermal conductivity, and low viscosity) makeSubCW more similar to a gas than a liquid, and en-able high reaction rates.16 All these properties ofSubCW greatly impact biomass decomposition, ma-inly through reactions of solvolysis, depolymer-isation and further decomposition of monomers bycleavage, dehydration, decarboxylation, deammina-tion and rearrangements of reactive fragments.12,16

The present work is focused on uncatalyzedhydrothermal transformations of microcrystallinecellulose as a representative model biomass, in sub-critical water at mild temperatures and autogenouspressure. Biomass conversion rate and a yield ofeach group of products are strongly influenced bymany factors such as reaction temperatures, waterproperties, reactor system, catalysts implied, heat-ing rate, biomass properties, etc.17 The influence ofreaction temperature (220 °C – 300 °C) and resi-dence time (3–60 minutes) in batch reactor were ex-amined in relation to overall conversion and con-version to various products (solid, liquid and gas-eous).

74 I. PAVLOVIÈ et al., Subcritical Water – a Perspective Reaction Media for Biomass …, Chem. Biochem. Eng. Q. 27 (1) 73–82 (2013)

Materials and methods

Materials

Microcrystalline cellulose powder was ob-tained from Merck (Germany). Deionized waterhaving resistivity of 18 M� cm–1 was obtained bywater purification apparatus (ELGA, Veolia). Sugarstandards (glucose, cellobiose, fructose, galactose,ribose, etc.) and other high-purity reagents forHPLC analysis were purchased from Sigma-Ald-rich (Germany).

Experimental methods

Experiments of the microcrystalline celluloseconversions were carried out in stirred batch reactormade from HT/HP 1.4980 steel tube (volume ca-pacity = 60 ml) in temperature range from 220° –300 °C and equivalent autogenous pressure. Thesuspensions of 3.0 g cellulose in 30 ml deionizedwater were prepared, pre-mixed on magnetic stirrerand charged in the reactor. The reactor was heatedby electrical wire-heater to the desired temperature.The heating rate was 8–10 °C min–1. Reaction mix-ture was mixed by magnetic stirrer at 600 rpm. Thereaction temperature and pressure were measuredby K-type thermocouple and pressure sensor con-nected to reactor interior, respectively. The reactiontime was measured from the moment the set tem-perature was reached. After a defined reaction time,the reactor was quickly immersed in an ice bath tostop the reaction immediately. The reaction mixturewas cooled to room temperature and the final tem-perature and pressure were recorded. In case gasformation was detected (elevated final pressure atroom temperature), the gaseous phase was collectedin sampling bags. The liquid and solid contents inthe reactor were separately collected as water-solu-ble (WS), acetone-soluble (AS) and acetone-insolu-ble or solid residue (SR) fractions.

The reactor content was filtered using apre-weighted standard filter-paper (Whatman, USA,Grade one >11 µm). The total volume of filtrate,i.e. WS fraction, was measured and stored infreezer until analysis on HPLC. The aliquot of WSfraction was evaporated under reduced pressure at60 °C in rotary evaporator and the content of drymatter in WS phase was determined gravimetri-cally, and the conversion to WS products was cal-culated. The water-insoluble fraction as well as re-actor walls were washed three-times with acetone,filtered through pre-weighed filter-paper, and ace-tone soluble (AS) and acetone insoluble or solidresidue (SR) fractions were obtained. The aliquot ofAS fraction was evaporated under reduced pressurein rotary evaporator at 40 °C and the dry matter ofAS fraction was determined. Solid residue (SR) was

dried overnight in oven at 105 °C and weighed. Thetotal conversions and yield of WS, AS and gas frac-tions, all expressed in %, were calculated relative toinitial amount of dry cellulose according to follow-ing equations:

overall conversion wt

m SR

m initial cellulose

( %)

( )

�

� �100 �100(1)

yield of products wt

m WS or AS

m initial cellulose

( %)

( )

( )

�

� �100(2)

yield of gases and loses wt

m m m

m

WS AS SR

initial

( %) �

� ��

100cellulose

�

�

�

��100

(3)

Similarly, the weight fraction of SR was calcu-lated as:

w SR wtm SR

m initial cellulose( ) ( %)

( )

( )� �100 (4)

where m denotes measured weight of dry matter ofSR, WS or AS fraction. The experiments at eachcondition were done in duplicate and average valuewas calculated.

Analysis

The sugars were determined by HPLC system(Agilent Technologies 1200 Series) equipped withELSD detector (Agilent Technologies 1200 Series),according to modified method reported by F. A.Agblevor et al.18 An Agilent Carbohydrate column(250 × 4.6 mm) packed with 5 µm spherical poly-mer beads coated with proprietary bonding materialwas used together with Eclipse XDB-C18 pre-col-umn (Agilent, 150 × 4.6 mm cartridge, 5 µm) at 30°C. The mobile phase consisted of A: water, and B:acetonitrile. To analyze the mixture of monomersand dimmers, the chromatograph was initially runwith flow rate of 1.2 ml min–1 in the isocratic mode(acetonitrile:water, 85/15 v/v) for 25 min, and thenit was shifted to gradient mode at the flow rate of 1ml min-1. The water content was increased from 15% to 35 % in 10 minutes, and then reduced to 15 %in 10 minutes. The total run time was 45 minutes,with 10 minutes of post run. The ELSD was oper-ated at 40 °C, 0.35 MPa and nitrogen was used asthe nebulizing gas.

Gaseous samples were analyzed by gas chro-matography (Shimadzu) to measure vol% of CO,CO2, CH4 and H2. GC was equipped with 2 m longx 1 mm diameter Restec ShinCarbon 100/120 col-


umn and thermal conductivity detector (T=100 °C).Helium as the carrier gas was used. The columnoven temperature program: 40 °C (hold 3 minutes),raised to 160 °C (8.0 °C min–1, hold for 30 min-utes), split ratio = 1:3.

The AS fraction and solid residue spectra wererecorded using a Shimadzu Instrument IR-8000spectrophotometer within the range of 4000–400cm–1 by standard KBr pelletization method.19

Results and discussion

Effect of reaction temperature and residencetime on cellulose conversion

Fig.1. shows the influence of residence time(0–60 minutes) and temperature (220°, 250° and300 °C) on overall conversion determined by Eq. 1.It is evident, roughly, that decomposition of cellu-lose increases with increasing reaction temperaturefrom 220 °C to 300 °C. At low-temperature HTconversion at 220 °C, conversion increases almostlinearly with increasing of residence time andachieves a value of 63.7 % at 60 minutes, with evi-dent tendency to continue to rise with further in-creasing of residence time. At middle – temperatureHT reactions at 250 °C cellulose starts to decom-pose very quickly, achieving overall conversion of87.7 % in only 5 minutes, but after that, it drasti-cally decreases (to 53.2 % at 60 minutes). At tem-perature 300 °C, maximal overall conversion 78.5% has been achieved in only 3 minutes. Further in-crease in reaction time has no effect on it.

The phenomenon for HT conversion of cellu-lose, observed at temperatures of 250 °C and higher,is a result of competition of the two general groupsof reactions: dissolution and hydrolysis of celluloseto primary hydrolysis products on the one side, and

their further repolymerization, condensation andpyrolysis to form char, gases and oils on the otherside, occurring in parallel in the reaction system ofcellulose with subcritical water.20,21 In general, thecellulose in subcritical water is subjected firstly todepolymerization into oligosaccharides (DPs = 2–6)and monosaccharides (glucose) and then to hydro-lysis and rearrangement/decomposition of glucoseand other monomer sugars to various products:fructose, 5-hydroxymethylfurfural (5-HMF), furfu-ral, pyruvaldehyde, dihydroxyacetone, glyceralde-hydes, glycolaldehyde, erythrose. At higher temper-atures and/or longer reaction time, occurs isomeri-zation, fragmentation and repolymerization of thesesmall compounds into liquid oils, gases andchar.21,22 Similar results were presented by T.Minowa et al.23 for cellulose conversions in cata-lyst-free hot-compressed water at temperature range200 °C –350 °C in batch reactor. They demon-strated that cellulose undergoes too quick decompo-sition at T > 240 °C and almost immediate decom-position at T > 300 °C. According to product distri-bution analysis, they reported that cellulose decom-poses at temperatures below 240 °C only to wa-ter-soluble products (glucose, cellobiose). By in-creasing the temperature above 240 °C, bio-oils andtar were obtained, indicating secondary decomposi-tion and rearrangement of water-soluble sugars tooils and further to gases and char. To discuss moredetailed reaction mechanism of biomass conversionin subcritical water, in this work the product distri-bution in samples obtained from cellulose reactionswith subcritical water at 220 °C, 250 °C and 300 °Cat residence time 3–60 min has been determined.

Effect of reaction temperature and timeon water-soluble (WS) products yield

Fig 2. shows the yield of WS fraction at 220°C, 250 °C and 300 °C, as a function of residencetime. The yield of WS fraction at 220 °C rangesfrom 4.3 % at 3 minutes to 33.6 % at 60 minutes,indicating slow conversion of cellulose to WS prod-ucts at this temperature. By increasing the tempera-ture to 250 °C, yield of WS product increases sig-nificantly. The yield reaches maximum at 5 minutes(51.4 %) and after that it constantly decreases toonly 11.0 % at 60 minutes. By further increasingthe temperature, WS yield drastically decreases. At300 °C, the yield reaches maximal value of only10.0 % at in 3 minutes, and after that it starts to de-crease slightly to 3 % at 60 minutes.

It is evident that WS compounds are the majorliquid products of cellulose liquefaction at 220 °Cbecause yield of AS compounds at this temperatureis very low (Figure 3). This result suggests thatconversion of cellulose at low temperature (T 220 °C) occurs only through reactions of dissolu-


F i g . 1 – Effect of residence time on overall conversion ofmicrocrystalline cellulose at temperatures 220 °C,250 °C and 300 °C

tion and hydrolysis to water-soluble sugars (oligo-mers and monomers) and further products of theirdehydration to carboxylic acids, aldehydes and ke-tones. Secondary reactions to bio-oil (AS phase),gases or char could be neglected at this tempera-ture, even at high residence time. Secondary reac-tions of WS primary products start at 250 °C andare more obvious at 300 °C. Even at higher temper-atures or/and long residence time they undergo ac-celerated decomposition/conversion to water-insol-uble, long-chain carboxylic acids/esters and hydro-carbons, referred to as bio-oils (here AS fraction).The similar conclusions were presented by T. Mino-wa et al.23 and E. Kamio et al.24

To gain better insight into the reaction mecha-nism of cellulose transformation in subcritical wa-ter, a more detailed analysis of WS products wasdone. Even changes in color of WS fractions ob-tained at different temperatures and reaction timesindicated different composition (Figure 4).

Considering that cellulose undergoes decompo-sition to oligomers and monomers sugars, the deter-mination of most relevant carbohydrate sugars (cel-lobiose, sucrose, glucose, fructose, ribose, xylose,arabinose, mannose, galactose, maltose and lactose)in WS fraction was done on HPLC. Fig. 5a-b pres-ents yields of mainly detected sugars as a functionof residence time at 220 °C and 250 °C. Only threecompounds were detected in all samples: glucose,cellobiose and lactose. At 220 °C the glucose con-centration slowly increased with reaction time from11.2 % at 3 minutes to maximal 57.2 % at 30 min-utes.

This is a result of slow hydrolysis of cellulose.Small quantities of cellobiose were obtained only atshort reaction time (up to 10 minutes), after that, byprolongation of reaction all cellobiose depolyme-rized to glucose. At 250 °C, content of glucosereached maximum 63.3 % in only 3 minutes, afterthat it decreased rapidly and at 15 minutes only 2.8% of glucose remained. Cellobiose was detectedonly in samples obtained at 3 and 5 minutes of re-action time, indicating rapid cellobiose conversionto glucose at these temperatures. In all samples ob-tained at 300 °C, no sugars were detected, confirm-ing that at these temperatures depolymerization ofcellulose and further glucose conversion to otherproducts occurs in a few seconds. This confirmsthat conversions at this temperature are very ra-pid.23,26 A similar effect was detected by S. Kumarand R. B Gupta during the research of hydrolysis ofmicrocrystalline cellulose in subcritical and super-critical water (temperature range of 302–405 °C,pressure of 27.6 MPa, residence time 2.5–8.1s),showing that 66.8 % of crystalline cellulose can beconverted to hydrolysis products (oligomers andglucose) at 335 °C and 27.6 MPa in only 4.7 s.20


F i g . 2 – Effect of temperature (220 °C, 250 °C and 300 °C)and residence time on WS yield

F i g . 4 – Color changes of WS products obtained at varioustemperature (220 °C, 250 °C and 300 °C) and re-action times (3 to 60 minutes)

F i g . 3 – Effect of temperature (220 °C, 250 °C and 300 °C)and residence time on AS yield

According to these results and considering that glu-cose is the most important bio-based platformfeedstock for production of commodity chemicals,such as sugar alcohols, levulinic acid, succinic acidand others,26 it is very important to optimize pro-cess conditions to maximize glucose yield as inter-mediate precursor in chemicals production.

Besides the temperature (and water properties),the results suggest that residence time also affectproduct distribution (and reaction mechanism). Inbatch reactor, residence time of mixture is long, en-abling the secondary reactions of the hydrolysisproducts and increased yield of oil and char, andlow yield of sugars. The comparative study of reac-tions of cellulose in batch and flow-type of reactorsystems showed that in flow-type reactor systems,the cellulose hydrolyzes to minimal contents ofcondensation/pyrolysis products, whereas in batch-type higher yields of these products were detect-ed.12,22 Therefore, it is very important to investigatethe fundamental reactions of biomass degradationin appropriate reactor systems and conditions, toavoid secondary reactions of primary products.

Effect of reaction temperatureand time on acetone-soluble (AS)products yield

The effect of temperature and reaction time onthe conversion of cellulose to acetone-soluble (AS)compounds is demonstrated in Fig.3. It is clear thatAS fraction yield at 220 °C is very low. At 3 min-utes, the AS fraction yield was only 0.4 % andreached maximal value of 4.7 % after 60 minutes.As temperature increases to 250 °C, bio-oils (AS)formation increases from 2.2 % at 3minutes to maximal 21.1 % at 60minutes. With the further increase intemperature to 300 °C, the yield ofAS products was in the range of10.1 % to 14.9 %, and stayed almostconstant at all reaction times. At thesame time, yield of gases and chardrastically increased (Figs. 7 and 8).This implies that at higher tempera-tures (300 °C and higher), conden-sation and cracking reactions of AScompounds (resulting in formationof carbonaceous solid product, re-ferred to as hydrochar, and gases)are obviously dominant over the de-hydration of WS products and for-mation of bio-oils. So, to further in-crease bio-oil yield, this side-reac-tion must be depressed. This couldbe done by using various catalysts,mostly alkaline salts, such asNa2CO3, KOH, K2CO3, etc.26,27,28


F i g . 5 – Mass percentage of detected sugars in WS prod-ucts (on the base of dry WS matter) in dependenceon residence time at (a) 220 °C and (b) 250 °C

F i g . 6 – FTIR spectra of AS sample (experimental conditions: 250 °C, residencetime 15 min)

The compositional structure of AS fraction wasanalyzed using FTIR spectroscopy in the mid-infra-red region (200–4000 cm–1). In Fig. 6, the FTIRspectra of typical AS sample can be seen. It was ob-tained at 250 °C and 15 minutes. All other sampleshave similar functional groups. The spectra showsthat the main absorbance bands were attributed toalcohols (3500 – 3450 cm–1), carboxylic acids (O–Hstretch at 3000 cm–1 and 1430–1370 cm–1), ketones(very strong C=O vibration at 1705 cm–1), primary,secondary and tertiary alcohols, phenols, esters(C–O stretching at 1250–900 cm–1). Therefore, itcould be concluded that the AS fraction consistsmostly of a mixture of carboxylic acids, alcohols,phenols, esters, etc., commonly referred to in litera-ture as biocrude or bio-oil. Similar observations forbio oil produced from model and real biomasscould be found elsewhere.29,30

Effect of reaction temperature and timeon gaseous and solid product formation

Although gaseous and solid products (hydro-char/tar) could be considered primarily as side-products in the HT liquefaction process, highlight-ing the mechanism of their formation is a very im-portant step for further optimization of the liquefac-tion process. During the research, at 220 °C the gasproduction was not evident (no elevated pressurewas detected in reactor after cooling down). There-fore, it can be concluded that the yield of gas andlosses expressed according to Eq. 3, at residencetime 3–30 minutes, represents only experimentallosses of volatile compounds during separation ofwater and acetone by rotary evaporator, eventualuncompleted extraction of acetone-soluble productsfrom solid residue (which could evaporate duringoven-drying at 105 °C), as same as losses due to thedeposition of insoluble char/tar on the reactor wall.At residence time of 3–30 minutes, where theamount of gas phase is negligible (Fig. 7) and nodeposition of insoluble char on reactor wall was ob-served, total dry mass recovery regarding initialcellulose weight is 89.5 %, which indicates thatlosses during experiments were generally approxi-mately 10 %.

As can be seen from Fig. 7, at 250 °C the yieldof gases reached maximum 55.7 % at 15 minutesand after that slightly decreased to 31.6 % at 60minutes. By further increasing temperature to 300°C, significant gas formation was evident throughelevated final pressure in reactor after cooling downto room temperature. The yield reached the value of52.3 % very quickly (in only 3 minutes) and in-creased slightly to 68 % at 60 minutes, indicatingenhanced secondary reactions of AS compounds togaseous and solids at this temperature. A more sig-nificant presence of gas phase has been evidenced

at 250 °C at 10 minutes of residence time. A gas-phase analysis of samples at 250 °C and 300 °Cshows that main gases at 250 °C were CO2 CO andH2 (max. 29,7 vol% at 5 min). This is a result of re-versible water-gas shift reaction at lower tempera-ture in which H2 is formed. With increasing thetemperature to 300 °C, vol% of CO2 is decreasedand at the same time, vol% of CO and CH4 is in-creased, due to another reversible reaction of me-thanation.31

Fig. 8 presents the weight fraction of SR, re-maining after reactions, vs. reaction time at 220 °C,250 °C and 300 °C. From declining curve of SRwith residence time at 220 °C and from light colorof residual cellulose observed after reaction, it isevident that SR represents remained cellulose, notchar. Char formation could be detected at 250 °Cand residence times of 10 minutes and higher. Byincreasing the reaction time, the amount of SRstarted to increase. WS and AS fractions started todecrease at this moment, indicating char formationby condensation and re-polymerization of these liq-


F i g . 7 – Effect of temperature (220 °C, 250 °C and 300 °C)and residence time on “gases and losses” yield

F i g . 8 – Effect of temperature (220 °C, 250 °C and 300 °C)and residence time on solid residue (SR) weightfraction

uid products. At 300 °C, also one part of SR wasgasified, resulting in lower and almost constantweight fraction of SR at this temperature.

To confirm the proposed reaction pathway, SRsamples were analyzed also by a FTIR spectro-photometer. Characteristic cellulose absorption peaks

are at 3400 cm–1 (O-H stretching vibration), 2900cm–1 (saturated aliphatic C-H stretching vibration),1370 cm–1 (-CH3 bending vibration) and 1100 cm–1

(O-H bending vibration).29 More significant struc-tural changes, comparing it with raw cellulose,started at 250 °C and 60 minutes and became more

evident at 300 °C. As can be seenfrom Fig. 9, sharp absorption at3348 cm–1, 2924 cm–1 and 1423cm–1 of the SR started to decline at250 °C and 60 minutes, indicatingmore significant cellulose decompo-sition. Peaks from 1033 cm–1 to1203 cm–1 totally disappeared at 250°C and 60 minutes, indicating strongdehydration. Furthermore, SR at thisand higher temperatures, has car-bonyl (C=O) absorption peaksaround 1700 cm–1, which suggestsformation of ketones, aldehydes andother carbonyl products. Also, theappearance of the peak at 798 cm–1

in SR, suggests aromatization (aro-matic C–H out-of-plane bending vi-brations).29 All these observationscorrespond to predicted reactions atthree different temperature regimesmentioned above. In Table 1, themain reaction pathways at differenttemperature regimes are proposed.


T a b l e 1 – Reaction regimes and proposed mechanisms for cellulose HT conversion to valuable products insubcritical water (SubCW)

F i g . 9 – Differences in FTIR spectra of raw cellulose and SR samples obtained atvarious experimental conditions

Conclusions

Conversion of cellulose, a biomass model sub-stance, in subcritical water at mild temperature con-ditions (220 °C – 300 °C) and autogenous pressurewas studied, and the results presented. The aim wasto highlight the potential of subcritical water as aunique media for biomass transformation to valu-able products. The results show that various prod-ucts could be obtained. The liquid products wereseparated regarding their solubility in water and ac-etone. A water-soluble phase (WS) represents most-ly cellulose decomposition products, such as sugarmonomers and monomer degradation products (or-ganic acids, 5-HMF, aldehydes etc.). Acetone-solu-ble phase, referred to also as bio-oil, mainly con-sists of hydrophobic phenols and its derivatives, ke-tones, carboxylic acids, long-chain alkanes, etc.Significant gaseous phase was observed only at 300°C, indicating that non-catalytic gasification of bio-mass occurs only at temperatures above 300 °C.

In summary, at the three investigated tempera-tures, different product distribution has been de-tected. The maximal yield (51.4 %) of WS productswas obtained at 250 °C and 5 minutes. Maximalyields of bio-oils were obtained at 250 °C and 60minutes (21.1 %). Gas yield was maximal (68 %) at300 °C and 60 minutes, and solid residue (SR) orchar formation was the most intensive at 250 °Cand 60 minutes. This confirms that targeted controlof process conditions (and properties of SubCW),allows for manipulation with selectivity of reac-tions to desired products. All four main types ofproducts obtained, possess great potential for fur-ther synthesis of commodity chemicals, so selectionof proper reaction conditions is necessary to opti-mize selectivity to desired products and minimizeundesirable products. More detailed insight into thereaction mechanism to products from real biomasssamples is planned in future work.

L i s t o f s y m b o l s a n d a b b r e v i a t i o n s

Tc � critical temperature, °C

pc � critical pressure, MPa

KW � ion product, mol2 L–2

Cp � heat capacity, J g–1 K–1

� � viscosity, µPa s

SubCW � subcritical water

HCW � hot compressed water

CP � critical point

SCW � supercritical water

HT � hydrothermal

DP � degree of polymerization

WS � water-soluble fraction of products

AS � acetone-soluble fraction of products

SR � solid residue

FTIR � Fourier transforms infraredspectrophotometry

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