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DILUTE ACID HYDROLYSIS OF CELLULOSE

AND CELLULOSIC BIO-WASTE USING A

MICROWAVE REACTOR SYSTEM

A. Orozco, M. Ahmad, D. Rooney and G. Walker

School of Chemistry and Chemical Engineering, Queens University Belfast, Belfast,

Northern Ireland, UK.

Abstract: The dilute acid hydrolysis of grass and cellulose with phosphoric acid was undertakenin a microwave reactor system. The experimental data and reaction kinetic analysis indicate thatthis is a potential process for cellulose and hemi-cellulose hydrolysis, due to a rapid hydrolysisreaction at moderate temperatures. The optimum conditions for grass hydrolysis were found tobe 2.5% phosphoric acid at a temperature of 1758C. It was found that sugar degradationoccurred at acid concentrations greater than 2.5% (v/v) and temperatures greater than 1758C.

In a further series of experiments, the kinetics of dilute acid hydrolysis of cellulose was investi-gated varying phosphoric acid concentration and reaction temperatures. The experimental dataindicate that the use of microwave technology can successfully facilitate dilute acid hydrolysis ofcellulose allowing high yields of glucose in short reaction times. The optimum conditions gave ayield of 90% glucose. A pseudo-homogeneous consecutive first order reaction was assumedand the reaction rate constants were calculated as: k1 0.0813 s

21; k2 0.0075 s21, which

compare favourably with reaction rate constants found in conventional non-microwave reactionsystems. The kinetic analysis would indicate that the primary advantages of employing micro-wave heating were to: achieve a high rate constant at moderate temperatures: and to preventhot spot formation within the reactor, which would have cause localised degradation of glucose.

Keywords: dilute acid hydrolysis; cellulose hydrolysis; microwave reactor; reaction kinetics;bio-ethanol.

INTRODUCTION

Bio-ethanol Production

There are several methods to manage solidbiowaste such us landfilling, composting,incineration, recycling (Reiht et al., 2002).These methods normally have adverseeffects in the environment, e.g., landfill putres-cible waste producing methane, leachate andodour. Furthermore, landfill requires a largeamount of land causing detrimental of the

landscape and local amenities. Incinerationhas the advantages that it can generateenergy from waste and reduce the volumeand weight of waste by up to 90% and 70%,respectively. The perceived disadvantage ofincineration is that it produces gas pollutants(carbon dioxide and dioxins) and fly ash.

Biomass contains five different sugars(xylose, glucose, arabinose, mannose andgalactose), and all which require processing(usually fermentation) to form economicallyviable products (Korte et al., 2002). In theUnited States 94% of bio-ethanol is producedfrom corn (Broder et al., 2001), furthermore,

Brazil produces most of its liquid vehicularfuel from sugar cane (Ackerson et al., 1991).

Several approaches have been examinedfor the hydrolysis of waste cellulose to sugarsand the subsequent fermentation into bio-ethanol and other bio-products (Broder et al.,2001). Most research has been focussed onthe production of bio-ethanol using simul-taneous saccharification and fermentation(SSF), but considerable improvements havebeen made in the technologies for the pro-duction of ethanol from lingo-cellulosic ma-terials (Lark et al., 1997).

Cellulose Hydrolysis

The main challenges in producing ethanolfrom renewable lingo-cellulosic biomass(e.g., MSW) have been found in hydrolysisstage of the process. The hydrolysis of cellu-lose to glucose only occurs at economicallyviable yields when a catalyst is used. Thethree main catalyst classifications are: enzy-matic, concentrated acid and dilute acid cata-lysts (Badger, 2000). The main advantages inusing enzymatic catalysts are the highspecific characteristic of enzymes (i.e., no

by-products), enzymes operate under mildconditions, are environmentally friendly and

446 Vol 85 (B5) 446449

Correspondence to:

Dr G. Walker, School of

Chemistry and Chemical

Engineering, Queens

University Belfast, Belfast,

BT9 5AG, Northern Ireland,

UK.

E-mail: g.walker@qub.ac.uk

DOI: 10.1205/psep07003

09575820/07/$30.00 0.00

Process Safety and

Environmental Protection

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a small amount of enzyme results in high yields. In usingenzymatic hydrolysis however, pre-treatment is necessaryto open up the structure and to provide access for theenzyme to the active sites. Pre-treatment is usually pre-formed by energy intensive physical methods, high tempera-ture and pressure or the use of a chemical solvent e.g., diluteacid. Also, the presence of lignin can be inhibitory to the

enzyme hydrolysis. Other disadvantages of enzyme hydroly-sis include the long reaction times, large reactors and thehigh cost of enzymes.

Concentrated acid processes use relatively mild tempera-tures and the only pressure involved is that created in pump-ing materials from vessel to vessel. These low temperaturesand pressures minimize the degradation of sugars to undesir-able by-products. Concentrated acid disrupts the hydrogenbonding in the cellulose chain converting it to an amorphousstate. The cellulose, once de-crystallized, forms a homo-geneous gel with the acid, which allows hydrolysis reactions.Water can then be added at low temperatures to dilute thesolution, providing conditions to form glucose.

Waste grass clippings have the potential to be a biomassfeedstock for ethanol production due to the high content ofcellulose and hemi-cellulose (approximately 60% and 70%w/w dry mass). Hemi-cellulose can decompose at tempera-tures of approximately 1608C to form xylose and othersugars. However, for the decomposition of crystalline cellu-lose temperatures of 2004008C are usually required, withthis generating problems with sugar degradation. At thesetemperatures, cellulose degrades into hydroxymethyl furfuraland xylose degrades into furfural (Wyman, 1994; Zaldivaret al., 2001).

The aim of this research, is to study dilute acid hydrolysisof cellulose and waste cellulosic biomass (grass clippings)with phosphoric acid (110%v/v), employing a microwave

reactor. Process parameters investigated include variationin reaction temperature (1508C to 2008C) and acid catalystconcentration. The product sugars from the reaction, princi-pally glucose, are intended for fermentation in bio-ethanolproduction.

MATERIALS AND METHODS

Acid Hydrolysis of Grass and Cellulose

Mixtures of 5% (w/w) grass clippings (or cellulose) withdistilled water were prepared. To analyse the effect of acidconcentration phosphoric acid at 1.0, 2.5, 5.0, 7.5 and10.0%(v/v) were used at 1758C. To study of effect of temp-

erature on the system, the reactions were carried out at150, 160, 175 and 2008C with 12%(v/v) phosphoric acid.The acid was added before starting the reaction in order toavoid pre-hydrolysis. The grass and cellulose hydrolysisexperiments were undertaken in a 100 ml microwave reactorsystem (Explorer PLS, Focused Synthesis Instrumentation).

Sugar Analysis

After the hydrolysis reaction the solution was filtered(Whatman filter paper #54), the pH of the liquid wasmeasured and adjusted with calcium hydroxide to pH 4.00,centrifuged at 13 000 rpm for 15 min and then filtered through

0.45 mm syringe filter (after Agblevor et al., 2004). Sugars inthe hydrolysate were measured using a Supelcosil LC-NH2

column with: isocratic elution at a flow rate of 0.8 ml min21;85 : 15 acetonitrile : water as the mobile phase; an evapora-tive light scattering detector (ELSD) Sedex 55. The operationconditions of the ELSD were 408C, 2 bar. Nitrogen was usedas a carrier gas. An ADC 12 single channel Pico logger with12 bits resolution connected to a PC with Picolog data acqui-sition software was used for recording data. Quantification ofthe sugars was carried out using Origin Pro 7.5 SR 5 soft-ware and calibration curves were developed for sugar stan-dards of L-arabinose, D-mannose, D-glucose, D-galactose,D-xylose and D-cellobiose and correlations coefficients foreach standard was r2 . 0.99. The total reducing sugar con-centration in the sample was determined after according toprocedure Determination of Structural Carbohydrates and

Lignin in Biomass described by the NREL (2004). All theexperiments were carried out in triplicate with the averageconcentrations and yields reported.

RESULTS AND DISCUSSION

Material Characterization

Initially the grass cuttings were analysed for suitability inthe ethanol production process, analyses included moisturecontent, chemical composition (CHN analysis), ash content,cellulose content and calorific value (see Table 1).

Dilute Acid Hydrolysis of Waste CellulosicMaterial (Grass) to Sugars

Figure 1 illustrates a chromatogram of the products of thehydrolysis of grass cuttings, indicating that xylose, glucoseand arabinose were the principal sugars formed. Figure 2illustrates a plot of sugar yield versus phosphoric acid con-centration for the hydrolysis of grass cuttings. The maximumsugar yield occurred at 2.5% of phosphoric acid, with a totalsugar yield of approximately 190 mg sugar per g of dry grasscuttings. The maximum yield achieved for xylose was,67 mg g21 dry grass corresponding to 24% of the total theor-etical yield. For arabinose, the maximum yield achieved wasalso at 2.5% phosphoric acid at 24.8 mg g21 of dry grass of72% of the total theoretical yield. Generally, an increase in

the acid concentration decreased the sugar yield due to

Figure 1. Chromatogram of grass hydrolysis at 1758C, 2.5 v/v%phosphoric acid, 15 min reaction time.

Table 1. Characterisation of grass cuttings.

Sample % Carbon % Hydrogen % Nitrogen % Ash % Cellulose

Grass 43.93 6.34 0.76 13.01 21.6

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possible sugar degradation, with xylose most affected by anincrease in acid concentration.

The acid concentration did not have a significant effect onglucose formation (as opposed to xylose or arabinose), with ayield of 61 mg g21 dry grass corresponding to 20% of thetotal theoretical yield at phosphoric acid concentration of2.5%. Analysing the relationship between sugar degradationand acid concentration, suggests that pentoses (arabinoseand xylose) are prone to degradation with increased acidconcentration, which may indicate that the acid also acts asa catalyst for sugar decomposition.

Figure 3 illustrates the effect of temperature on grasshydrolysis with dilute phosphoric acid. The maximum sugaryield was achieved at a temperature of 1758C for each ofthe sugars analysed. Reaction temperature is an important

factor for sugar degradation. The data in Figure 3 indicatethat with a reaction temperature of 2008C, the total sugaryield decreases by approximately 30%. The sugar yielddata for this higher temperature indicate that pentosesugars are more susceptible to degradation than hexosesugars.

Dilute Acid Hydrolysis of Cellulose to Glucose

The kinetics of the hydrolysis reaction using 2%, 4% and7.5% phosphoric acid as the catalyst at a temperature of

1608C was investigated, with the experimental dataillustrated in Figure 4. The data indicate that the reactionproceeded quite rapidly to give a maximum yield ofapproximately 50% after 5 min, however, the yield decreasedwith further increase in reaction time. This may indicate thatthe reaction follows a sequential scheme. For extended reac-tion times, it was also noted that when the samples were

removed from the reactor, a black solid had formed, whichwas attributed to charring or carmelisation of glucose andother sugars, which would evidently decrease the recordedglucose yield. These data also indicate that acid concen-tration affects the reaction and product yield, with morerapid glucose formation found with increased acid concen-tration. However, the highest glucose yield and thus optimumreaction conditions was found to be 35 min with 7.5% acidconcentration.

In order to describe the reaction kinetics, a pseudo-homo-geneous consecutive first order reaction was used, thatemploys a two-step irreversible reaction mechanism todescribe the formation and degradation of sugars. The

model, represented in equation (1), has been successfullyused by other researchers for the hydrolysis of lignocellulosicmaterials (Gamez et al., 2006; Tellez-Luis et al., 2002).

Lignocellulosic material!k1

Reducing sugars!k2

Degradation products (1)

where k1 is the rate constant for sugar formation; and k2 is therate constant for sugar degradation.

Based on this reaction model the differential equations thatdefine the reaction rate equations are as follows:

dP

dt k1P (2)

dS

dt k1P k2S (3)

Equation (2) describes the monomerization reaction rate andequation (3) describes the sugar formation rate, where P isthe polymer concentration and S is sugar concentration.Based on this model, an Excelw solver was then used to mini-mize the error value by altering the values of k1 and k2 withthe constraints that k1 and k2 must be greater than zero.

Figure 3. Grass hydrolysis with 1 v/v% phosphoric acid at differenttemperatures, glucose, xylose and arabinose yield in mg g21 drygrass (microwave reactor).

Figure 4. Cellulose hydrolysis kineticsglucose formation in micro-

wave reactor: effect of catalyst concentration (phosphoric acid cata-lyst, temperature 1608C).

Figure 2. Grass hydrolysis at different phosphoric acid concentration,glucose, xylose and arabinose yield in mg g21 dry grass (microwavereactor).

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The optimum solution was found to be: k1 0.0813 s21 and

k2 0.0075 s21 (see Figure 5). Malester and Green (1992)

working on the kinetics of dilute acid hydrolysis of celluloseoriginating from MSW with sulphuric acid at 2258C and pHof 0.42 quoted the rate constants as k1 0.0773 s

21 andk2 0.0445 s

21. The value of k1 in this study is higher butthat the value of k2 is much lower, which indicates thatsugar/glucose degradation is less predominant under thereaction conditions in this study. This difference could be pri-marily attributed to the catalyst used and the effect of micro-waves on the rate of glucose degradation (temperaturedifference between the two systems was 2258C versus1608C in this study).

The absorbed microwave power within the system is afunction of the dielectric loss factor and relative dielectric con-stant of the materials within the reactor. However, these par-

ameters essentially remain constant under the experimentalconditions in this study. More significantly, microwave fieldsusually affect hydrogen-bonding networks which may leadto an increase in the rate of breakage of the cellulosic struc-ture, additionally microwaves are known to increase the rateof reactant and product diffusion. These factors may accountfor the increase in reaction rate constant, k1, in the micro-wave reactor in comparison with conventional systems.Moreover, the kinetic analysis would indicate that the primaryadvantages of employing microwave heating were to:achieve a high rate constant at moderate temperatures:and to prevent hot spot formation within the reactor, whichwould have caused localized degradation of glucose.

CONCLUSIONS

The optimum conditions for grass hydrolysis in the micro-wave reactors were 2.5% phosphoric acid at a temperatureof 1758C, which resulted in a maximum yield for xylose67 mg g21, arabinose 25 mg g21 and glucose of 61 mg g21

(dry grass). It was found that sugar degradation occurred atacid concentrations greater than 2.5%(v/v) and temperatures

greater than 1758C and that pentosas were more susceptibleto degradation than glucose.

Kinetic analysis of the dilute acid hydrolysis of cellulose (toglucose) indicated that the use of microwave technology cansuccessfully facilitate acid hydrolysis allowing high yields ofglucose in short reaction times. The optimum conditionsgave a yield of 90% (w/w) glucose. A pseudo-homogeneous

consecutive first order reaction was assumed and the reac-tion rate constants were calculated as: k1 0.0813 s

21;k2 0.0075 s

21, which compare favourably with reactionrate constants found in conventional non-microwave reactionsystems. The kinetic analysis would indicate that the primaryadvantages of employing microwave heating were to:achieve a high rate constant at moderate temperatures:and to prevent hot spot formation within the reactor, whichwould have cause localised degradation of glucose.

REFERENCES

Ackerson, M.D., Clausen, E.C. and Gaddy, J.L., 1991, Production ofethanol from MSW, IECEC-91. Proceedings of the 26th Interso-

ciety Energy Conversion Engineering Conference, 49 August1991, ANS, Boston, MA, USA, pp. 61.

Agblevor, F.A., Murde, A. and Hames, B.R., 2004, Improved methodof analysis of biomass sugars using high-performance liquidchromatography, Biotechnology Letters, 26: 12071210.

Badger, P.C., 2000, Ethanol from cellulose: A general review. P.1721. Trends in Crops and New Uses (ASHS Press, Alexandria, VA).

Broder, J.D., Harris, R.A. and Ranney, J.T., 2001, Using MSW andindustrial residues as ethanol feedstocks, Biocycle, 42(10): 2326.

Gamez, G., 2006, Study of the hydrolysis of sugar cane bagasseusing phosphoric acid, Journal of Food Engineering, 74(1): 78 88.

Korte, N.E., West, O.R., Liang, L., Gu, B., Zutman, J.L. and Fer-nando, Q., 2002, The effect of solvent concentration on the useof palladized-iron for the step-wise dechlorination of polychlori-nated biphenyls in soil extracts, Waste Management, 22(3):343349.

Lark, N., Xia, Y., Qin, C., Gong, C.S. and Tsao, G.T., 1997, Pro-duction of ethanol from recycled paper sludge using cellulaseand yeast, Kluveromyces marxianus, Biomass and Bioenergy,12(2): 135143.

Malester, I.A., Green, M. and Shelef, G. 1992, Kinetics of dilute acidhydrolysis of cellulose originating from municipal solid-wastes,Industrial & Engineering Chemistry Research, 31(8): 1998 2003.

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Reiht, J.H., Uil, H., Veen, H., Laat, W.T.A.M., Niessen, J.J., Jong,H.W. and Elbersen, H.W.B., 2002, Co-production of bio-ethanol,electricity and heat from biomass wastes: potential and R&Dissues. First European Conference on Agriculture and RenewableEnergy, 1721 July.

Tellez-Luis, S.J., Ramrez, J.A. and Vazquez, M., 2002, Mathematicalmodelling of hemicellulosic sugar production from sorghum straw,J Food Eng, 52(3): 285 291.

Wyman, C.E., 1994, Alternative transportation fuels from biomass,Proceedings of 29th Intersociety Energy Conversion EngineeringConferenceIECEC94, 711 August AIAA, Monterey, CA, USA,pp. 1090.

Zaldivar, J., Nielsen, J. and Olsson, L., 2001, Fuel ethanol productionfrom lignocellulose: a challenge for metabolic engineering andprocess integration, Applied Microbiology and Biotechnology,56(12): 1734.

The manuscript was received 10 January 2007 and accepted forpublication after revision 5 June 2007.

Figure 5. Cellulose hydrolysisglucose formation kinetics in micro-wave reactor (catalyst 4% phosphoric acid, temperature 1608C).

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