Fractionation of lignocellulosics. Solubilization of corn stalk hemicelluloses by autohydrolysis in aqueous medium

FRACTIONATION OF LIGNOCELLULOSICS.

SOLUBILIZATION OF CORN STALK HEMICELLULOSES

BY AUTOHYDROLYSIS IN AQUEOUS MEDIUM

MANUEL RUBIO*, JUAN F. TORTOSA{, JOAQUIÂ N QUESADA and DEMETRIO GOÂ MEZ

Department of Chemical Engineering, Campus of Espinardo, University of Murcia, P.O. Box 4021,Murcia, Spain

(Received 17 May 1997; accepted 29 April 1998)

AbstractÐThis paper discusses the variation pro®les of the three main polymeric families (hemicellu-loses, cellulose and lignin) of milled corn stalk (Zea mayÈs) after aqueous thermomechanochemical treat-ment. The experimental system used combine explosion and shear e�ects facilitating the solubilizationof hemicelluloses in the aqueous medium.

Two starting materials (ethanol±benzene extracted and ethanol±benzene and water extracted) wasused. Hemicelluloses are totally solubilized at 2208C (10 min) although there is a loss of lignin (up to athird); cellulose is practically una�ected.

A kinetic analysis for the solubilization step was done for calculating the frequency factor and the ac-tivation energy for the main treatment using the analytical characterization data from the obtainedsolid residues. # 1998 Published by Elsevier Science Ltd. All rights reserved

KeywordsÐWood treatment; biomass; hemicelluloses; cellulose; fractionation; autohydrolysis.

1. INTRODUCTION

Since the ®rst energy crisis in 1973, countrieswith extensive forestry reserves, such asUnited States, Finland and Canada, havemade great e�orts to become as self-su�cientas possible in carbon-based fuels and chemicalproducts through developing economicallyviable processes for their production.Although vegetable matter is basically com-posed of three related polymeric families (thecarbohydrate-based hemicelluloses and cellu-lose, and the polyphenol-based lignin), theirheterogeneity makes their use very expensivein the search for the above products since themixtures of the obtained products are complexto separate or treat biologically.

One way to resolve this problem involvesthe fractionation of the lignocellulosic biomassinto its polymeric components to obtain lessheterogeneous starting products with a view tousing processes (catalytic solvolysis, organosol-volysis, pyrolysis, liquefaction, etc.) whichwould increase their individual values andmake the whole process more economically

viable since all the fractions would, in fact, bebeing used.

This work deals with the solubilization ofhemicelluloses by a process of autohydrolysiscombined with sudden decompression. It hasbeen found that the process is e�ective whenthe temperature is higher than the glass tran-sition point of lignin, close to 1758C.1 Thisfractionation method is interesting because itdoes not involve the addition of a chemical re-agent and so cuts the operational costs quitesubstantially. The method is proposed as apossible alternative to steam explosion pro-cesses and it involves a thermomechanicaltreatment in an aqueous medium during whichthe soluble fragments are extracted con-stantly.2

In addition, the hemicellulosic juices derivedfrom the autohydrolysis, which are rich in sol-uble pentoses, can be used to obtain chemicalproducts, the most important of which are fur-fural and xylitol.

2. EXPERIMENTAL

An experimental setup of four stainless-steelautoclaves in cascade was used, the last of

Biomass and Bioenergy Vol. 15, No. 6, pp. 483±491, 1998# 1998 Published by Elsevier Science Ltd. All rights reserved

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*To whom all correspondence should be addressed.{In Memoriam.

483

which acted as sudden cooler. The two middleautoclaves were used to promote the reactionand sudden decompression and were equippedwith external electrical resistances andMagnedrive stirrers (Autoclave Engineers).

The autoclaves were connected by pneu-matic valves which, when opened, permittedthe almost instantaneous passage of the sus-pension due to the great di�erential pressure.The experimental device was completed by gasfeed and compression systems and a centralpanel for controlling and measuring. A generaloutline of the device is shown in Fig. 1.

3. MATERIALS

Corn stalks were dried to ambient tempera-ture and ground in a hammer-blade grinderwith a sieve size of 2 mm. The milled stalks(0.32±1 mm) were extracted with a 1:2 v/v ofethanol±benzene (starting material: EBE) andreextracted with boiling water (starting ma-terial: WEBE).

4. ANALYTICAL METHODS

The analytical protocol followed is shown inFig. 2. The wet determination methods corre-sponded to the following norms: ASTM D110(solubility in boiling water), ASTM D1106±56(Klason lignin), ASTM D1787 (pentosans)

and ASTM D1103±60 (cellulose). The holocel-lulose content was determined by chloritetreatment.3 Furfural was quanti®ed bymeasuring the absorbance of the juice at

277.5 nm and applying an absorptivity coe�-cient of 156 l/g cm.

The starting materials and the solid wastesobtained were also characterized by Elemental

Fig. 1. Experimental device.

Fig. 2. Analytical schema.

M. RUBIO et al.484

Analysis in a Perkin Elmer 240 C (C, H andN) and by thermal analysis in a Mettler TA300, with air as carrier (200 l/min).

The monomeric sugars in the juices weredetermined by gas chromatography of theirsilylated derivative dissolved in pyridine in aKONIK KNK 3000 apparatus equipped withan FID detector and a 25 m column of 3%OV-101, using a temperature ramp of 160±2508C and sorbitol as internal standard. Thesilylating agent was TSIM, N-(trimethylsilyl)-imidazol. To measure the quantity of solublecarbohydrate polymers, a given volume ofjuice was hydrolysed with 3% H2SO4 for 2 hat boiling point and neutralized by passing itthrough a weak anionic exchange column(Lewatit MP 64). It was then vacuum-dried ina rotary evaporator, dissolved in pyridine and®nally derivated before injection. A similarprocedure was used for analyzing the juicesobtained by total sacchari®cation of the solids,but applying the recovery coe�cients cited inthe literature.4,5

5. OPERATION

An aqueous suspension of milled corn stalk(4% w/w) was prepared 30 min before eachexperiment, and then introduced into auto-clave A3. The installation was purged for5 min with nitrogen and the pressure was thenadjusted so that after heating it was close to11.8 MPa. Once the desired temperature hadbeen reached, the pressure was again adjusted.

After discharge into the collector autoclaveand when the temperature is below 808C, thesystem was slowly decompressed and the sus-pension was ®ltered to give a solid residue anda juice. The juice was frozen and the solid was

dried in a stove at 408C, weighed and itsmoisture determined.

6. EVALUATION OF THE SEVERITY OF THETREATMENT

For a comparison of the experimentalresults obtained in di�erent conditions, theseverity parameter Ro, according to Overendand Chornet,6 was used. This permitted,within a given range of conditions, the inte-gration of the combined e�ects of temperatureand time:

Ro �Zt0

expT�t� ÿ 100

14:75dt

where t is the time in min and T(t) is the tem-perature±time function for gradually heatedprocesses (8C).

A typical temperature±time curve for the ex-perimental device used is shown in Fig. 3. Theexperiments ``at time zero'' correspond to thedischarge of the material when it reaches thedesired temperature.

7. RESULTS AND DISCUSSION

7.1. Components

The characteristics of the starting materialsare shown in Table 1. The contents areexpressed in dry weight percentages.

For the ®rst experiments, the EBE fraction(extracted with 1:2 ethanol±benzene) was cho-sen and two experimental series of 0 and30 min of treatment for each of the tempera-tures. The results obtained by the wet methodscan be seen in Fig. 4 with the help ofthe severity parameter Ro which, as alreadyFig. 3. Temperature pro®l of a typical run.

Table 1. Compositions (% dry basis) of the two startingmaterials from corn stalk

EBE WEBE

Pentosans 28.8 31Holocellulose 74.6 86.2Cellulose 36.8 42.9Klason lignin 13.2 14.8Boiling watersolubility

15.4

Arabinose 3.9 4.2Xylose 24.9 26.5Glucose 32.7 37.6Mannose <3 <3Galactose <2.5 <2.5

Fractionation of lignocellulosics 485

mentioned, permits the results obtained indi�erent conditions to be compared.

As the severity of the treatment rose, thetotal quantity of pentosans and, therefore,holocellulose fell, while the values for the cel-lulose content, despite increased dispersion,showed little variation. However, as indicatedby other authors,7±9 at high severity values thecellulose determination method used was in-adequate for these residues. This fact wasprobably due to the labile character of the cel-lulose in the basic medium used for its deter-mination, leading to lower values thanpredicted. This is seen, furthermore, by thehigher holocellulose content observed in re-lation to that of cellulose (with an almost neg-ligible pentoses content).

As regards the Klason lignin, this showed acertain initial tendency to solubilization fol-lowed by a stabilizing of its value until a logRo of 4.5 was reached, after which it rose,even surpassing its initial value, a phenom-enon usually put down to repolymerization orcondensation with hemicellulose degradationderivates.9±12

In the experiments carried out with theWEBE material (extracted, in addition, withboiling water) almost the same e�ect isobserved (Fig. 5), although in this case theKlason lignin diminishes by 30±40% its orig-inal value until a log Ro of 3.5 is reached. Itthen remains constant and only rises at highlevels of severity. Once again, the ASTMmethod for cellulose determination led tounsatisfactory results at these high Ro levels.

In the two previous graphs the intersectionpoint between cellulose and holocellulose (cor-responding to the total solubilization of the

hemicelluloses) occurs in the range 4.3±4.6, co-

inciding with the 4±4.5 mentioned by Overend

and Chornet6 for similar materials.

Figure 6 shows a Ross diagram, widely used

in the paper industry, to present the results of

the treatment on the di�erent starting ma-

terials.

Fig. 4. Runs with EBE material.

Fig. 5. Runs with WEBE material.

Fig. 6. Ross diagrams.

M. RUBIO et al.486

These diagrams show how the hemicellu-loses are solubilized as the severity of thetreatment increases (points on the left hardline) although there is a loss, at low severitylevels, of a fraction of the lignin present (up toa third) and even of a cellulose fraction (outerpoints on the diagram) at high levels althoughnot so pronounced.

7.2. Process kinetic

It is di�cult to formulate a kinetic modelbased on the successive series of reactionswhich the di�erent polymeric fractions mightundergo. Any attempts to do so have tendedto concentrate on an overall consideration ofthe hemicelluloses and on the formulation ofkinetic models which only take their solubil-ization into account.

Some authors13±16 have either ®tted theirdata to simple two-stage kinetics (slow andfast) or to one pseudo-®rst-order kinetic tocover the whole range of hemicellulose solubil-ization.15

However, in general, these simple formu-lations have only been approximate as theyhave ignored the initial step during which thetemperature is raised, and so the values havedepended on the kinetic parameters obtainedfrom the operational characteristics of the ex-perimental setup, especially from its speci®cpossibilities of heat transmission.

In an attempt to ®nd the values of thecharacteristic kinetic parameters (Ea and K0)of Arrhenius equation and to compare thenwith those found by other authors for similarreaction systems, we ®tted the data from theexperimental series with two autoclaves usingthe WEBE material. As a representative vari-able of the hemicellulose concentration in thesolid, we used the pentosan/holocelluloseratio, which is the equivalent of the xylose/(xylose + glucose) ratio used by otherauthors.15

For a pseudo-®rst-order kinetic:

ln�P=H�i�P=H�o �

Zt0

Kdt

where:K= K0 exp [ÿEa/RT(t)](P/H)i = pentosan/holocellulose ratio of the

solid residue(P/H)o = idem of the starting materialt= time (minutes)

K= Arrhenius type rate constantK0=frequency factor (minÿ1)Ea=activation energy (J/mol)R= gas constant 8.032 J/K molT(t) = absolute temperature as a function

of time (K).For data ®tting, a modi®ed direct search

method of Hooke and Jeeves was used. Thisminimizes the sum of the squared errors, as afunction of two variables (activation energyand frequency factor). In this way, a value of108,400 J/mol was obtained for the activationenergy and 1.088 E 11 minÿ1 for the frequencyfactor. These values agree with those of thebibliography13±16 for the one-step kinetic or ofthe rapid stage of the two-step kinetic.

7.3. Elemental analysis

The carbohydrates from the hemicellulosesand cellulose contain more oxygen than thelignin aromatic structures so that it is to beexpected that if there is a selective solubil-ization of the polysaccharides then these willbe an increase in the C/O ratio in the residueobtained. Figure 7, which shows the (residueC/O)/(initial C/O) ratio for both materials ver-sus Ro, clearly demonstrates this tendency.For conditions of high severity (log Ro>6),the absolute values of the C/O ratio reach1.67, near to those of indulin AT lignin (1.82)and iotech lignin (1.76). At low severity valuesthe C/O ratio practically remains unchargedfrom the initial ratio, two phenomena of

Fig. 7. Elemental analysis.


contrary e�ects overlapping: the solubilizationof a certain quantity of lignin and the pro-gressive solubilization of the hemicelluloses. Inthe case of EBE, the C/O ratio begins to risesteeply at higher severity values than in thecase of WEBE, which is in accordance withthe ``native lignin'' character which Browning3

describes for a portion of the water extractiblesubstances from wood.

7.4. Thermal analysis

Using thermal analysis in a dynamic atmos-phere of air, Chauvette et al.17 calculated thedecomposition ranges of lignocellulosic poly-mers as 200±3168C for hemicelluloses, up to3608C for cellulose (maximum at 3478C) andabove 3608C for lignin (maximum at 4578C).Figure 8 shows results of the thermal analysisapplied to some solids from the treatment.Note the progressive solubilization of thehemicelluloses as the severity parameterincreases and the diminution of the ligninpeak height as the fraction of least molecular

weight is solubilized and its increase (at high-est log Ro values) during structural repolymer-ization or condensation.

7.5. Characterization of the autohydrolyticjuices

Once solubilized, the polymeric fragmentsmay be transformed into monomers, which, intheir turn, are transformed into other productsthrough the action of temperature and auto-generated acid catalysts.

As regards the distribution of sugars fromthe hemicelluloses, both in their monomericand polymeric forms, Figs 9 and 10 illustratethe percentages of soluble sugars comparedwith their initial values (Table 1) for bothEBE and WEBE starting materials. The quan-tity of monomeric sugars in the solutionincreases to a certain level, beyond which theseverity of the treatment provokes their degra-dation, which is not compensated by theFig. 8. Thermal analysis of solid samples.

Fig. 9. Sugars in the juices from EBE runs.

M. RUBIO et al.488

production of monomers from the soluble

polymers.

The values for the glucose found in the juice

in no case exceeded 8%, which shows that

severity of the treatment does not appreciably

a�ect the cellulose content, showing the good

degree of selectivity of the autohydrolytic frac-

tionation method.

Moreover, the results show that extraction

with boiling water helps solubilize the hemicel-

luloses since the proportion of dissolved sugar,

both in the original and the hydrolyzed juice,

is highest in the least severe conditions, before

the degradation processes have an important

e�ect.

The maximum value for arabinose dissol-

ution is reached at lows degrees of severity

than for xylose, indicating the greatest suscep-

tibility of arabinose to the autohydrolytic pro-

cess, as observed by other authors.18

The evolution of furfural, the predictableimmediate degradation product of the hemicel-lulosic monomers, was followed by measuringthe absorbance of the juice at 277.5 nm, theclearly de®ned band of maximum absorptionfor furfural and, to a lesser extent for hydro-xymethylfurfural (although this is produced bythe acid hydrolysis of hexoses and great quan-tities were not to be expected).

Figure 11 shows the evolution of furfural asinitial (xylan)/(xylan + araban) versus log Ro.There was an overall rise to 40% at a severityof log Ro = 5, which then declined as theseverity rose, due to decomposition of the fur-fural. Note that at low degrees of severity theabsorbance values observed are very low,which suggests that the hemicelluloses and lig-nin fragments in the juice do not interferemuch in the measurement and this may be putdown to the presence of furfural.

The furfural values obtained show a certainsimilarity until log Ro reaches 4, after whichthe proportion of this compound increasesmore rapidly for the WEBE material, perhapsbecause of the greatest propensity of the hemi-celluloses to dissolve and the higher prob-ability of pentosans occurring at low values ofseverity.

In an attempt to represent the carbohydratedegradation processes, Fig. 12 shows log Ro

Fig. 10. Sugars in the juices from WEBE runs.

Fig. 11. Equivalent furfural.


versus the percentages of solubilized pentosans

as deduced from di�erent measurements: from

the solid (dissolved pentosans, curve A), pen-

tosans in the original juice (curve B), pento-

sans in the post hydrolyzed juice (curve C)

and these last added to an amount of xylose

equivalent to the furfural measured (curve D).

The di�erences between the pentosans in the

posthydrolyzed juice and the dissolved pento-

sans may have been because the degradation

products were not detected and/or the posthy-

drolysis time was not su�cient to convert all

the soluble polymers into monomers. Over the

range of severities occurring before the appari-

tion of the maximum in curve C the second

cause seems to be more likely since the quan-

tity of furfural, the precursor of these degra-

dation products, is very small. Once this

maximum has been passed, and even when the

furfural is borne in mind, it is di�cult to

explain the high quantity of dissolved pento-

sans unless the degradation process plays an

important role.

Lastly, even in the most favorable case, the

juices produced by these processes hardly con-

tain 20% of the initial pentosans in the form

of monomers, although if the dissolutions are

submitted to a controlled posthydrolysis, this

percentage can reach 50±60%, as observed by

other authors also using autohydrolysis pro-cesses in aqueous suspension.2

8. CONCLUSIONS

The thermomechanical treatment used get thesolubilization of the most of the hemicellulosesfrom corn stalk into the aqueous medium,obtaining a juice with monomeric and polymericsugars and its degradation products and acellulose±lignin solid product. An acid post-hydrolysis of the juice shows the susceptibility ofthe main present carbohydrate polymers can beordered as: araban>xylan>glucan.

The results are compared using a severityparameter Ro, able to bring together thee�ects of temperature and time of the runsfrom the same experimental apparatus or fromseveral systems of well-known thermal his-tories.

The solubilization data of the corn stalkhemicelluloses are ®tted following a one-stepkinetic model by a direct search optimizationbased in the Hooke±Jeeves method that mini-mizes the sum of squared errors. In this way,the frequency factor and the activation energyvalues are obtained; these values agree withthose of the literature.

AcknowledgementsÐThe authors are indebted to theDGICYT of Ministry of Education and Science (Spain)for ®nancial support.

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Fractionation of lignocellulosics. Solubilization of corn stalk hemicelluloses by autohydrolysis in aqueous medium