Cellulose liquefaction in acidified ethylene glycol

Edita Jasiukaityt _e Æ Matjaz Kunaver ÆMatija Strlic

Received: 14 March 2008 / Accepted: 15 February 2009

� Springer Science+Business Media B.V. 2009

Abstract Wood pulp cellulose was used in a study

of its catalyzed liquefaction in the presence of

ethylene glycol, p-toluene sulfonic acid monohydrate

or sulphuric acid being the catalysts. For this study,

microcrystalline cellulose, Whatman filter paper no. 1

and cotton linters with molar masses of 76,000,

699,000 and 1,910,000 g mol-1, respectively were

used. This liquefaction was studied by gravimetric

determinations, by X-ray diffraction analysis of the

residual cellulose and by monitoring of the molar

mass decrease over different time intervals, using

size-exclusion chromatography. The disordered

regions, even of cellulose with the highest molar

mass degraded in the initial minute of liquefaction.

However, the highly ordered cellulose regions

remained relatively stable for a longer time. None

the less, partial degradation of the highly ordered

regions of the cellulose was achieved.

Keywords Cellulose � Catalyzed liquefaction �Size-exclusion chromatography � Ethylene glycol �Crystallinity � X-ray

Introduction

Renewable plant biomass sources, such as lignocell-

ulosics and other polysaccharides, are gaining

importance as a suitable replacement for fossil-fuel

resources.

Wood is one of the more important natural

products. With respect to both its structural and

chemical properties, wood is a complex, non-uniform

material. However, the depolymerization of macro-

molecular wood components in a liquefaction

process, followed by reaction with specific organic

reagents, enables one to convert wood into a potential

feedstock for the synthesis of new, environmentally

friendly polymers (Wei et al. 2004; Kishi et al. 2006).

The depolymerization of wood components can be

achieved with phenol or polyhydric alcohols, under

acid-catalyzed conditions (Lin et al. 1994, 1995;

Alma et al. 1998; Kobayashi et al. 2004).

The liquefaction of wood, as a unit, is not

completely understood. Despite the fact, extensive

studies to elucidate the mechanisms were initiated

more than 10 years ago (Emsley and Stevens 1994).

Cellulose powder, steamed lignin and the mixtures of

these two components have been used by Kobayashi

et al. (2004) as a model compounds. They studied the

reaction process during liquefaction using polyhydric

alcohol. The liquefaction of lignin with phenol has

been studied on the basis of the behaviour of model

substances, such as guaiacylglycerol–b-guaiacyl

ether (Lin et al. 2001a, b). Thus, these authors

E. Jasiukaityt _e (&) � M. Kunaver

National Institute of Chemistry, Hajdrihova 19,

1000 Ljubljana, Slovenia

e-mail: edita.jasiukaityte@ki.si

M. Strlic

Faculty of Chemistry and Chemical Technology,

University of Ljubljana, Askerceva 5, 1000 Ljubljana,

Slovenia

123

Cellulose

DOI 10.1007/s10570-009-9288-y

attempted to clarify the mechanism of the reaction of

cellulose with phenol under the acid-catalyzed con-

ditions, using cellobiose as a model compound.

A mechanism for the degradation of cellulose during

ethylene glycol supported liquefaction was proposed

by Yamada and Ono. These authors demonstrated the

formation of a large quantity of hydroxyethyl gluco-

sides, in the early stage of reaction. These glucosides

subsequently decomposed into the 2-hydroxyethyl

levulinate (Yamada and Ono 2001).

The high crystallinity and/or high degree of

cellulose polymerization (DP) tended to constrain

cellulose depolymerization during the liquefaction

reaction. The disordered, amorphous zones of cellu-

lose fibers were more accessible to the penetration of

certain solvents, while the crystalline regions

remained unaffected. In addition, only a prolonged

treatment with an appropriate diluted acid can reduce

the DP to a certain value, the leveling-off degree of

polymerization (LODP; Fengel and Wegener 1989).

It has been reported that the rate of hydrolysis of the

amorphous cellulose regions is much higher than

the rate of hydrolysis of the crystalline regions. The

products of such hydrolysis can be even removed

from a cellulose fiber, leaving the crystalline regions

almost untouched (Zhao et al. 2006). Thus, the tight

packing of cellulose in the crystalline regions causes

the reaction kinetics to be dependent on the diffusion

rate of the liquefying agents into the highly packed

macromolecular system.

The aim of the current study was to clarify the

influence of the cellulose DP on its liquefaction in

ethylene glycol under acid catalysis. Due to better

affinity of organic sulfonic acids to cellulose (Mun

et al. 2006), p-toluene sulfonic acid monohydrate and

sulphuric acid were chosen as the catalysts in our

study of the liquefaction of cellulose in ethylene

glycol.

In this study, the liquefaction of microcrystalline

cellulose, of Whatman filter paper no. 1 and of cotton

linters, with molar masses of 76,000, 699,000 and

1,910,000 g mol-1, respectively was carried out. The

objective was to determine the influence of cellulose

DP, structural features of the microfibrils and surface

morphology on the cellulose liquefaction in the

presence of ethylene glycol as a liquefying reagent,

with sulphuric acid or p-toluene sulfonic acid mono-

hydrate as catalysts.

Experimental

Materials

Microcrystalline cellulose (MC, MW 76,000 g mol-1,

Acros Organics), purified cellulose sheets (Whatman

filter paper no. 1, WH, MW 699,000 g mol-1),

cotton linters (CT, MW 1,910,000 g mol-1, Radece,

Slovenia) were used without any pretreatment. Ethyl-

ene glycol (E.G, Merck), 97% sulphuric acid (H2SO4,

Merck), p-toluene sulfonic acid monohydrate (PTSA,

Acros Organics), N,N-dimethylacetamide for HPLC

(DMAc, Fluka) lithium chloride (LiCl, Acros Organ-

ics, dried before use at 180 �C in vacuo for 24 h and

kept in desiccator) and sodium hydroxide (NaOH,

Merck) were of reagent grade and were used without

further purification.

Cellulose liquefaction

Thirty gram of cellulose (dried at 105 �C for 24 h),

150 g of E.G and 4.5 g (3% w/w based on the E.G) of

H2SO4 or PTSA as a catalyst were placed in a

500 cm3 glass reactor (three necks), equipped with a

mechanical stirrer. The liquefaction was carried out at

150 �C over 4 h. Samples from reaction mixture were

taken at different time intervals and immediately

cooled in an ice-bath. The acidity component was

neutralized with dilute NaOH solution to prevent

further cellulose degradation prior to characterization

of the products.

Measurement of residual cellulose content

The yield of liquefaction was evaluated by determin-

ing the residual cellulose content. Each sample was

diluted with an excess of distilled water and vacuum-

filtered through filter paper. Insoluble residues were

rinsed several times with DMAc and acetone. The

samples were dried in vacuum at 50 �C for 24 h. The

residue content was determined as the weight of

the obtained solids relative to the starting amount of

cellulose (Eq. 1).

Residual cellulose %ð Þ ¼ Wt

W0

� 100 ð1Þ

W0 is the weight of starting cellulose and Wt is the

weight of residual cellulose.

Cellulose

123

FT-IR spectroscopic analysis

The residual cellulose samples were analyzed using a

Perkin–Elmer Spectrum-1 FTIR spectrophotometer.

The transmittance measurements were conducted

using the KBr pellet method in frequency range from

4,000 to 500 cm-1.

NMR spectrometry

13C NMR spectra of derivatives were recorded using

a Unity Inova 300 Varian NMR spectrometer oper-

ating at 75 MHz. The measurements were conducted

in DMSO-d6 at 25 �C and tetramethylsilane (TMS)

was used as an internal standard.

X-ray diffraction

XRD measurements were performed on a Siemens

D5000 PANalytical X’Pert PRO system in order to

estimate the crystalline–amorphous ratio of the initial

cellulose samples and of the residues. The diffracted

intensity of CuKa radiation (1.5406 A) was measured

in 2h intervals between 10� and 30�. The X-ray

diffractograms of all samples were analyzed using the

empirical procedure of Segal et al. (Segal et al. 1959).

The calculation of the crystallinity index (Cr�I.)followed the Eq. 2:

Cr:I: %ð Þ ¼ I200 � Iam

I200

� 100 ð2Þ

Here, I200 is the maximum intensity of the diffraction

from the (200) plane at 2h = 22.8� and Iam is the

intensity of the amorphous background scatter mea-

sured at 2h = 18�.

Figure 1 shows the approach taken in the deter-

mination of the crystalline cellulose content in the

microcrystalline cellulose by the Segal method.

The crystallite widths of both the original and the

residual cellulose were estimated and evaluated using

Scherrer equation (Eq. 3; Klug and Alexander 1974)

from the line profile of the (200) reflection at

2h = 22.8� that refers to the width of crystallite

(Andersson et al. 2003).

L ¼ Kk= b cos hð Þ ð3Þ

Here, L is the crystallite width, h is the Bragg angle,

k is the wavelength of the radiation, K is a constant

and b is the corrected width of the line given by the

specimen. A value of K of 0.9 for half-width of the

line profiles was used.

The measured line widths include the effects of

crystallite size and instrumental broadening. The

effect of instrumental broadening on the line widths

was assessed by measuring the width of line of a

large undistorted crystal under identical operating

geometries. The instrumental broadening amounted

to 0.059� for the half-width where the silicon crystal

was used as a sample (Warren and Biscoe 1938). The

line profiles were assumed to follow a Gaussian

distribution.

Size-exclusion chromatography

Samples for size-exclusion chromatography were

prepared as follows: the aliquot of neutralized

reaction mixture was mixed with distilled water and

any suspension of residual cellulose was filtered

through a 0.45 lm polyamide membrane filter

(Milipore). The obtained cellulose residue was rinsed

several times with distilled water and dissolved in a

LiCl/DMAc solvent system.

All the solutions of residual cellulose in LiCl/

DMAc were filtered through the PTFE filters

(0.45 lm) prior to injection. The LiCl concentration

in the sample solutions was 1% (0.1 g of LiCl in

10 cm3 of DMAc).

The HP-AGILENT system consisted of an iso-

cratic pump HP 1100, refractive index detector

AGILENT 1100 (detection cell temperature: 40 �C)

and column thermostat (temperature: 80 �C). The

sample injection volume was 100 lL, and the sample

5 10 15 20 25 30

Iam

I200

.u.a ,y tisnetnI

2θ, o

Fig. 1 Diffractogram of microcrystalline cellulose

Cellulose

123

concentration 0.1%. The column used was PLgel

5 lm MIXED C 7.5 9 300 mm. The eluent (1%

LiCl/DMAc), filtered through the 0.45 lm polyamide

membrane filter (Supelco), was pumped into the

system at a flow rate of 0.5 cm3 min-1. The

chromatographic data were processed with PSS

(Polymer Standards Service) WinGPC Unity

software.

Pullulan standards (Polymer Laboratories) with

peak molecular masses of 1,660,000, 788,000,

404,000, 212,000, 22,800, 5,900 and 667 g mol-1

were used for calibration. The standards with molec-

ular masses of 1,660,000, 788,000 and 404,000

g mol-1 were prepared in individual 0.1% solutions,

while pullulan standards with molecular masses of

212,000, 22,800, 5,900 and 667 g mol-1, were pre-

pared as mixed standards in a solution containing

0.025% of each standard. The standards were weighed,

transferred into 2 and 5-cm3 volumetric flasks and

dissolved in DMAc. Finally, appropriate volumes of

8% LiCl in DMAc were added in order to achieve 1%

solution of LiCl (Strlic and Kolar 2003; Strlic et al.

2002).

Results and discussion

FT-IR analysis

FT-IR analyses of the residues that were obtained

after 4 h of MC, WH and CT liquefaction, were

performed to ensure that residual cellulose was being

dealt with and not insoluble cellulose derivatives. All

of the residues that were analyzed exhibited the

characteristic bands of cellulose: 3,419–3,342 cm-1

(OH stretching), 2,901 cm-1 (CH stretching),

1,430 cm-1 (CH2 bending), 1,371–1,373 cm-1 (CH

bending), 1,060–1,031 cm-1 (CO stretching). The

obtained results suggested the presence of residual,

not derivatized, cellulose. Since all of the FT-IR

spectra were very similar, that of the residue from

the WH cellulose is given as an example (Fig. 2).

Cellulose liquefaction reaction in acid-catalyzed

E.G

Cellulose liquefaction in an acidic, non-aqueous E.G

medium seemed to be analogous to hydrolysis. The

effects of H2O cannot be completely excluded from

the system as H2O may be preferentially adsorbed by

the cellulose (Valley 1955). Accordingly, it was taken

into account that approximately 5% (w/w based on

cellulose) of H2O is adsorbed by cellulose and

additional 3% (w/w based on H2SO4) or 9.4% (w/w

based on PTSA) of H2O is provided by catalysts used

for the liquefaction. Therefore, it can be assumed that

the primary reaction is hydrolysis, followed by

glycosidation of the new reducing groups. Cellulose

degradation in acidified E.G begins with glycosidic

oxygen protonation, followed by carbonium ion

formation and scission of the glycosidic bond.

After the glycosidic bond is broken, cellulose

4000 3500 3000 2500 2000 1500 1000 5000,0

0,2

0,4

0,6

0,8

1,0

2

1

%T

(n

orm

aliz

ed)

cm-1

0,0

0,2

0,4

0,6

0,8

1,0

3

1

%T

(n

orm

aliz

ed)

4000 3500 3000 2500 2000 1500 1000 500

cm-1

a b

Fig. 2 a FT-IR spectrum of the WH (1), FT-IR spectrum of the residual WH after 240 min of degradation under catalysis of PTSA

(2). b FT-IR spectrum of the residual WH after 240 min of degradation under catalysis of H2SO4 (3)

Cellulose

123

depolymerization could occur at both the reducing

(O1–H) and the non-reducing (O4–H) chain ends.

Reducing chain ends are especially reactive due to

the stabilized proton in the carbonium ion (I), this

carbonium ion, initially being attacked by E.G to give

the 2-hydroxyethyl-D-glucopyranoside and the refor-

mation of the 2-hydroxyethyl oxonium ion (II)

(Scheme 1).

Due to a higher carbonium ion (I) than the

carbonium ion (Ia) reactivity with E.G, the reaction

between the non-reducing ends of the chain and

E.G is negligible. The structure of 2-hydroxyethyl-D-

glucopyranoside was confirmed by 13C NMR (Fig. 3;

Table 1). Thus, it can be concluded that cellulose

degradation by acid catalyzed glycolysis occurs at the

reducing end of the chains.

Cellulose weight loss

Figure 4 shows the percentage of residual celluloses

as a function of the reaction time. With the cellulose

samples, catalyzed by H2SO4, *50% of the initial

celluloses was liquefied and converted into the

soluble products during the initial 30 min of

treatment.

Cellulose liquefaction that was catalyzed by PTSA

proceeded significantly slower. Only 22–40% of the

initial cellulose was liquefied in the first 30 min. The

celluloses with molar masses of 76,000, 699,000 and

1,910,000 g mol-1 under H2SO4 catalysis, were

converted to soluble products in extents up to 98.9,

95 and 85%, respectively. In the case of PTSA

catalysis, the cellulose series representatives were

O

H

HO

H

HO

H

H

OHH

OH

O

H

O

H

HO

H

OH

OHHH

OH

n

+ H3O

O

H

HO

HHO

H

H

OHH

OH

O

H

O

H

HO

H

OH

OHHH

OH

n

H

+

OH

O

H

HO

H

HO

H

O

OHH

H

OH

OH

O

H

HO

H

HO

H

OHH

H

OH

+ +

O

H

HO

H

HO

H

OHH

OH2

OH

+

+

HOOH

HOOH

n-2

O

H

HO

HHO

H

HOH

H

OH

O

H

O

H

HO

H

OHOH

H

H

OH

H

+

OH2

OH

II

I

OH

H

HO

H

OHH

H

OH

HOOH

Ia

O

H

HO

H

OHH

OH

+

HOOH

OH2

OH

II

+

OH

H

OH

H2O+

O

H

HO

H

OHH

OH

H

OH

O

H

HO

HO

+

+

?

+ +

Scheme 1 Cellulose liquefaction reaction scheme in acid-catalyzed E.G

Cellulose

123

liquefied in extents up to 98.7, 85 and 72%. A greater

rate of cellulose weight loss is observed under

catalysis of strong mineral acid such as in the

E.G-H2SO4 system. Figure 4 shows that the weight

loss of MC under H2SO4 catalysis follows a pseudo

first-order kinetics, while both WH and CT under

H2SO4 catalysis appears to follow a bi-exponential

model, which consists of two parallel reactions: the

fast one corresponding to the depolymerization of

disordered inter-crystalline regions and the slow one

corresponding to the degradation of crystalline

regions. In contrast to the WH and CT weight losses

under H2SO4 catalysis, for MC, WH and CT the

weight loss under PTSA catalysis follows pseudo-

first-order kinetics. The bi-exponential model in the

case of WH and CT depolymerization under H2SO4

catalysis is a result of the greater hydronium ion

concentration. The dissociation of 1 mol of PTSA

provides 1 mol of hydronium ion that is involved in

glycosidic oxygen protonation followed by carbonium

ion formation (Scheme 1), while after dissociation of

1 mol of H2SO4, consequently 2 mol of hydronium ion

are provided for the further reaction. Accordingly, the

dissociation of 0.045 mol of H2SO4 present in the

liquefaction mixture depolymerized WH and CT

amorphous regions with the higher rate than the

dissociation of 0.023 mol of PTSA (Table 2).

Consequently, the amorphous cellulose regions

present in WH and CT samples, were depolymerized

with the twice higher rate under H2SO4 catalysis than

in the case of PTSA. When the hydronium ions

started to interfere with the accessible reducing ends

in the crystalline WH and CT cellulose regions, the

rate of the weight loss became dependent only on

the amount of the accessible reducing ends in the

crystalline cellulose regions and not on the hydro-

nium ion concentration. The weight loss of the MC

O

H

HO

H

HO

H

O

OHH

H2C

H

OH

CH2

H2C

OH2'

1'

1

5

6

4

3 2

Fig. 3 Structure of 2-hydroxyethyl-D-glucopyranoside

Table 1 13C NMR shifts of 2-hydroxyethyl-D-glucopyrano-

side in dimethylsulfoxide–d6

d (ppm)

C10 C20 C1 C2 C3 C4 C5 C6

69.09 60.11 98.73 72.18 72.54 70.24 73.45 60.89

Carbon atom positions as indicated in Fig. 3

0 50 100 150 200 250

1

10

100

Time, min

Res

idua

l cel

lulo

se (

%)

MC + H2SO

4

WH + H2SO

4

CT + H2SO

4

1

10

100

Res

idua

l cel

lulo

se (

%)

MC + PTSAWH + PTSACT + PTSA

0 50 100 150 200 250

Time, min

Fig. 4 Percentage of weight loss (Eq. 1) during the acid-catalyzed glycolysis of cellulose, with time

Table 2 Amounts of H2SO4 and PTSA and equivalent

hydronium ion concentrations used in liquefaction of MC, WH

and CT

Catalyst Amounta (mol) [H?] (mol/L)

H2SO4 0.045 0.09

PTSA 0.023 0.023

a 4.5 g of each catalyst used for the liquefaction, that made 3%

w/w based on E.G

Cellulose

123

sample differed from the WH and CT samples.

Presumably, this was the reason why the rate of the

weight loss of the highly crystalline MC sample in

both cases, under H2SO4 or PTSA catalysis, did not

depend on the hydronium ion concentration and

followed the first-order kinetics. Thus, the greater

hydronium ion concentration is a determinant factor

for WH and CT weight loss to follow bi-exponential

model under H2SO4 catalysis.

Molecular weight distribution and crystallinity

change

The consequences of the cellulose liquefaction, in

E.G under catalysis by H2SO4 or PTSA were

monitored using size-exclusion chromatography

(SEC). Molar mass averages (MW) of the residual

cellulose samples, taken at different time intervals,

were determined by SEC, relative to pullulan

standards.

Molecular weight distributions obtained by size-

exclusion chromatography enable one to monitor the

cellulose degradation process that occurs by acid-

catalyzed glycolysis. The SEC chromatograms of the

residual cellulose samples, demonstrated the presence

of an analogous degradation pathway for WH (MW

699,000 g mol-1) and for CT (MW 1,910,000

g mol-1), while MC (MW 76,000 g mol-1) behaves

differently. As an example, chromatograms (normal-

ized to the sample weight) of WH (MW

699,000 g mol-1) and of residual celluloses from this

sample, taken at 1 min and 240 min of liquefaction in

the E.G-H2SO4 system and in the E.G-PTSA system,

are shown in the Fig. 5.

Compared with the chromatogram of the initial

WH sample, the chromatogram of the WH, taken

after 1 min of liquefaction, shows that the main peak

with narrow weight distribution is shifted to a lower

molecular weight. The increased intensity of the main

peak and the appearance of a low-molecular weight

shoulder can be interpreted in the terms of the

simultaneous hydrolysis and crystallization of the

amorphous fraction in the microfibrils (Wood et al.

1989). After 1 min of the liquefaction of WH, the

newly formed cellulose crystallites were composed

from approximately 12 glucose units. About 240 min

of the treatment resulted in a loss of the low molar

mass shoulder. This loss was due to the fast

accumulation and slow degradation of the crystallites,

as indicated by the increase of the main peak

intensity, observed in the chromatogram of the WH

sample, taken after 240 min.

During the first minute of cellulose liquefaction in

E.G, the initial polymerization degree of WH and CT,

decreased to 261 and 308 under catalysis by H2SO4

and to 336 and 342 under catalysis by PTSA,

respectively (Table 3).

Due to the sharp initial drop of DPw, the assump-

tion could be made that cellulose degradation occurs

predominantly at chain centers. However, due to

3 4 5 6 7 8

0

5

10

15

20

25

log M

RI d

etec

tor

resp

once

, a.u

.

0 min1 min

240 min

a

3 4 5 6 7 8

0

5

10

15

20

25

0 min

1 min

240 min

RI d

etec

tor

resp

once

, a.u

.

log M

b

Fig. 5 SEC chromatograms of residual cellulose (MW 699,000 g mol-1) from the samples, taken at 0, 1 and 240 min of liquefaction

catalyzed by H2SO4 (a), PTSA (b)

Cellulose

123

increased crystallinity of WH and CT after 5 min of

liquefaction (Table 3), the initial drop in the cellulose

molar mass has to be considered in terms of the

primary breakdown of covalent bonds and glycosidic

linkages in the amorphous cellulose regions together

with breakdown of inter-chain hydrogen bonds and

intra-chain hydrogen bonds. The glycosidic linkages

in the less ordered cellulose regions, at 150 �C in the

acid-catalyzed E.G system, undergo rapid scission,

forming new reducing end-groups that immediately

give rise to the end-wise degradation at the ends of

the remaining inaccessible (crystalline) regions

(Sharples 1957; Table 4).

During the first minute of MC liquefaction, the

initial DP of 470, decreased to 264 under catalysis

with H2SO4 and to 330 under catalysis with PTSA

(Table 2). The degradation of the MC in acidified

E.G proceeds differently. This can be deduced from

the SEC chromatograms of the MC initial sample

where it can be seen that the shoulder at the low

molecular-weight is already present, meaning that

WH and CT behave similar to MC once the less

ordered regions have been hydrolyzed (Fig. 6). After

1 min of the MC liquefaction, the cellulose crystal-

lites that gave rise to the low molecular-weight

shoulder, tended rapidly to accumulate and subse-

quently to increase the intensity of the main peak in

the chromatogram of MC sample that had been

subjected to 240 min of treatment.

During the next 4 h, cellulose liquefaction in the

acid-catalyzed E.G systems proceeds at a signifi-

cantly slower rate as the ordered (crystalline) regions

began to degrade. This period represents the random

glycolysis of the glycosidic bonds in the accessible

regions of the polysaccharide (Nelson and Tripp

1953). Only small amounts of the residual celluloses,

with relatively high DP values from 272 to 143 were

obtained. Similar behaviour of crystallite degradation

during strong acid hydrolysis has been demonstrated

by several authors (Nelson and Tripp 1953; Bouchard

et al. 1992), where some chains in the crystalline

regions degrade completely and become soluble

causing great weight loss, while others remain quite

untouched, maintaining the high DP values of the

insoluble residue, where the LODP does not signif-

icantly change over longer times. In addition, after

the LODP is reached, due to the slow E.G penetration

into the crystalline cellulose regions, small amounts

of soluble fragments could be produced, causing a

continuous weight loss in residual cellulose.

Cellulose degradation during acid-catalyzed

liquefaction

A plot of the number of scissions per cellulose chain

(DP0/DP - 1) against time (Fig. 7) was made in

order (Calvini et al. 2008) to minimize errors due to

the polydispersity ratios. It indicates that MC cellu-

lose suffered approximately two scissions per chain,

while WH and CT experienced 16 and 45 scissions,

respectively. MC degraded more slowly, as expected.

Microcrystalline cellulose (MC) powder is pro-

duced by the acid reflux hydrolysis of wood, where

amorphous regions of the cellulose fibers are hydro-

lyzed, leaving a very highly crystalline residue with

its leveling-off degree of polymerization (Battista

1971). Thus, the depolymerization of MC in acidified

E.G should represent the breakdown pathway of

Table 3 Average polymerization degree DPw of residual

cellulose samples with time of reaction with H2SO4 and PTSA

as catalysts

Time

(min)

H2SO4 PTSA

MC WH CT MC WH CT

0 470 4,320 11,790 470 4,320 11,790

1 264 261 308 330 336 342

3 256 267 305 275 279 306

5 230 264 291 254 278 299

15 186 253 285 211 278 283

60 174 250 274 167 271 270

120 162 242 265 146 276 262

240 149 239 246 143 272 264

Table 4 Crystallinity (%) of residual MC, WH and CT cel-

lulose with time

Time

(min)

H2SO4 PTSA

MC WH CT MC WH CT

0 83.26 78.19 79.77 83.26 78.19 79.77

5 87.76 90.23 92.74 87.52 92.64 91.91

60 86.54 92.28 91.88 86.58 92.28 91.96

120 83.93 89.30 91.00 86.53 90.87 91.96

240 79.54 74.23 90.16 85.05 71.30 90.50

Cellulose

123

crystalline regions only. In the current study, after

initial 1 min treatment a slightly increased crystal-

linity, crystallite width and significant decrease in DP

were observed. Taking into account that in MC some

of the surface cellulose material may be disordered,

such MC could be used as a reference to study

degradation of crystalline cellulose regions.

Zou et al. (1994) have suggested that the ratio

DPz/DPw is a good guide to the homogeneity of the

degradation process of Whatman filter paper and

cotton linters. The DPz/DPw ratios and DPw associ-

ated with the current study are presented in Fig. 8.

The ratios, after the initial 1 min treatment, remain

fairly constant, indicating that the degradation pro-

ceeded gradually by attack by the E.G at the

accessible cellulose reducing ends, presumably on

the crystallite surface.

Rate of cellulose depolymerization

The rate of cellulose depolymerization was calculated

using the Ekenstam equation (Ekenstam 1936):

1=DPt � 1=DP0 ¼ kt ð4Þ

Here, k reaction rate constant, DPt and DP0 are the

polymerization degrees at times t and 0, respectively.

Very high rates of the depolymerization and strong

deviations from linearity in plots were observed in

the current study. The rates of glucoside bond

breakage during the first 5 min of MC depolymer-

ization were determined to be 7.90 9 10-4 and

9.67 9 10-4 min-1 in E.G-H2SO4 and E.G-PTSA

systems, respectively. The beginning of the WH and

CT depolymerization was especially fast. Therefore,

due to the limitation of the Ekenstam equation, the

3 4 5 6

0

5

10

15

20

25

RI d

etec

tor

resp

once

, a.u

.

log M

0 min

1 min240 min

a

3 4 5 6

0

5

10

15

20

25

240 min 1 min

0 min

RI d

etec

tor

resp

once

, a.u

.

log M

b

Fig. 6 SEC chromatograms of the residual cellulose (MW 76,000 g mol-1) from the samples, taken at 0, 1 and 240 min of

liquefaction, catalyzed by H2SO4 (a) and PTSA (b)

0 50 100 150 200 250

0,0

0,5

1,0

1,5

2,0

2,5a

MC + H2SO

4

MC + PTSA

DP

0 /DP

- 1

Time, min

0

4

8

12

16

b

WH + H2SO

4

WH + PTSA

DP

0 /DP

- 1

0

10

20

30

40

50c

CT + H2SO

4

CT + PTSAD

P0 /D

P -

1

0 50 100 150 200 250

Time, min0 50 100 150 200 250

Time, min

Fig. 7 Scissions per chain (DP0/DP - 1), with time during depolymerization in acidified E.G; MC (a), WH (b) and CT (c)

Cellulose

123

rates of glucoside bond breakage were determined

only for the first minute of depolymerization. The

WH depolymerization during the first minute pro-

ceeded with the rates of 11.46 9 10-3 and

9.23 9 10-3 min-1 in the E.G-H2SO4 and E.G-

PTSA systems, respectively. The initial rates of

glucosidic bond breakage in CT were determined to

be 10.41 9 10-3 and 9.68 9 10-3 min-1 and in

E.G-H2SO4 and E.G-PTSA systems, respectively.

From these results, it is not possible to determine

100

150

200

250

300

350

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

DP

z/DP

w

MC + H2SO

4

DP

wa

100

150

200

250

300

350

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0b

DP

z/DP

w

MC + PTSA

DP

w

240

260

280

300

320

340

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0c

DP

z/DP

w

WH + H2SO

4

DP

w

240

260

280

300

320

340

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0d

DP

z/DP

w

WH + PTSA

DP

w

001011240

260

280

300

320

340

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0e

DP

z/DP

w

CT + H2SO

4

DP

w

Time, min

240

260

280

300

320

340

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0f

DP

z/DP

w

CT + PTSA

DP

w

001011

Time, min

100011

Time, min100011

Time, min

100011

Time, min100011

Time, min

Fig. 8 The change of the DPw (filled squares) and DPz/DPw

(squares) of MC, WH and CT residual cellulose samples with

time of reaction with H2SO4 and PTSA as catalysts;

MC ? H2SO4 (a), MC ? PTSA (b), WH ? H2SO4 (c),

WH ? PTSA (d), CT ? H2SO4 (e) and CT ? PTSA (f)

Cellulose

123

weather or not the initial cellulose depolymerization

is random or organised. After the amorphous cellu-

lose regions have been hydrolyzed, one can analyse

the slower depolymerization of cellulose crystallites.

Cellulose crystallite degradation during

acid-catalyzed liquefaction

In order better to understand the degradation that

occurs in the MC, WH and CT crystalline regions and

to determine the influence of simultaneous hydrolysis

and crystallization of amorphous fraction in the

microfibrils (Wood et al. 1989) the average widths

of crystallites were evaluated using the Scherrer

equation (Klug and Alexander 1974).

MC crystallites during the initial 5 min of treat-

ment tended to accumulate (crystallized) by slightly

increasing crystallite widths from 5.1 to 5.2 nm and

from 5.1 to 5.3 nm in E.G-H2SO4 and E.G-PTSA

system, respectively (Table 5). The accumulation of

MC crystallites can be also interpreted from the SEC

chromatograms in Fig. 6a. Here, the low-molecular

weight shoulder, which is observed for the beginning

of the reaction, disappears with time.

The crystallization resulted either from the forma-

tion of new crystallites or by crystallization on the

surface of existing crystallites. Since in MC there is

only a small amount of the disordered surface

material, it is rapidly hydrolyzed and consequently,

due to crystallization a slight increase in crystallites

width is observed. After the crystallization of amor-

phous regions is over, the crystallites are exposed to

the degradation, thereafter at the end of the treat-

ment the width of MC crystallites was reduced to

4.6–5.0 nm in the presence of H2SO4 and PTSA as

catalysts, respectively. Considering the results

obtained, one may claim that due to the greater

hydronium ion concentration, H2SO4 degrades MC

crystallites more extensively than PTSA. The more

extensive degradation of MC crystalline regions was

confirmed by obtained reduced degrees of crystallin-

ity from 83.26 to 79.54% and from 83.26 to 85.05%

in the presence of H2SO4 and PTSA as catalysts,

respectively.

The WH crystallite degradation during the initial

minute of the liquefaction in both of the E.G-H2SO4

and E.G-PTSA systems, suffered simultaneous

hydrolysis and crystallization of the amorphous

fraction in the microfibrils in the same manner as

the MC. In the first 60 min, the width of the residual

WH crystallites increased from 6.8 to 6.9 nm,

simultaneously increasing the crystallinity of WH

residual sample to 92.28%. Therefore, one may claim

that the accumulation of the crystallites during the

first hour proceeded faster, than the crystallite

degradation. On completion of the accumulation of

new WH crystallites, the degradation of remaining

crystallites was induced. During the final 2 h of WH

liquefaction, the crystallinity decreased by about

15–20% analogously in both E.G-H2SO4 and in

E.G-PTSA systems. In the mean time, due to induced

crystallite degradation, the width of the crystallites

decreased from 6.9 to 5.9 nm and from 6.9 to 6.1 nm,

respectively. According to the obtained results, the

more extensive degradation of WH crystallites using

H2SO4 as a catalyst due to the greater hydronium ion

concentration was observed.

The degradation of CT in acidified E.G proceeded

relatively in the same manner as that of the WH

degradation. The amorphous regions in CT were

hydrolyzed during the first 5 min of CT liquefaction

and, consequently, the greatest degrees of crystallin-

ity were achieved. The increased widths of

crystallites, confirmed the initial crystallization and

accumulation of the amorphous fractions as well as

during the degradation of both the MC and the WH.

After the accumulation of crystallized amorphous

fraction was over, the degradation of remaining

crystallites was induced, as confirmed by the reduc-

tion of crystallite width from 6.9 to 6.7 nm and from

6.9 to 6.6 nm, in the E.G-H2SO4 system and in the

E.G-PTSA system, respectively.

By comparing MC, WH and CT crystallites width

at the end of the liquefaction, the MC crystallites

were observed to be the smallest in size with the

Table 5 Width changes of the crystallites (nm) of residual

MC, WH and CT cellulose with treatment time, evaluated

using Scherrer equation

Time

(min)

H2SO4 PTSA

MC WH CT MC WH CT

0 5.1 6.8 6.4 5.1 6.8 6.4

5 5.2 6.6 6.9 5.3 7.1 6.7

60 5.1 6.9 6.7 5.1 6.9 6.7

120 4.8 6.7 6.7 5.2 6.9 6.6

240 4.6 5.9 6.6 5.0 6.1 6.6

Cellulose

123

largest reduction of width (4.6–5.0 nm) after degra-

dation in E.G-H2SO4 and in E.G-PTSA systems,

respectively. The more extensive MC crystallites

degradation is a result of more reducing ends

available for the reaction than in the case of WH

and CT. Thereof, the assumption could be made that

the crystallite degradation occurs on the surface of

the crystallites by attacking accessible MC, WH and

CT reducing ends by E.G.

The cellulose degradation during acid-catalyzed

liquefaction was evaluated by plotting the number of

scissions per cellulose chain (DP0/DP - 1) against

time (Fig. 7). Figure 7 shows that MC cellulose

suffered approximately 2 scissions per chain, while

WH and CT suffered 16 and 45 scissions per chain,

respectively. Considering that the CT suffered much

more scissions per chain than WH and MC, it would

be logical to expect the lowest residual amounts. The

residual MC, WH and CT amounts as well as the

width of crystallites differed also due to the number

of reducing ends available for the acid-catalyzed

glycolysis. Since the number of reducing ends is

proportional to reciprocal DP, the residual WH and

CT consequently contains less reducing ends than

MC, that can also explain their slower weight loss

and formation of wider crystallites. However, due to

the smallest number of reducing ends, the greatest

amounts of residual cellulose and the largest crystal-

lites were obtained after CT liquefaction in both the

E.G-H2SO4 and in the E.G-PTSA systems. In addi-

tion, the amorphous cellulose in CT was hydrolyzed

during the initial period, whereas some was imme-

diately converted into the soluble fragments, some

crystallized on the surface of the crystallites, conse-

quently increasing their average width. In comparison

to the MC and WH crystallites, due to the most

increased average width of the CT crystallites, the

smallest total surface area was exposed to the E.G

attack.

Conclusions

Microcrystalline cellulose, Whatman filter paper no.

1 and cotton linters with molar masses of 76,000,

699,000 and 1,910,000 g mol-1, respectively were

liquefied at 150 �C in the presence of ethylene glycol

and under catalysis of sulphuric acid or p-toluene

sulfonic acid monohydrate. After four hours the

achieved yield of cellulose liquefaction was

72–98.9%. The lower molar mass cellulose chains,

with higher number of reducing ends, were liquefied

and consequently lesser amounts of residual cellulose

were obtained.

From the gravimetric determination of cellulose

residue, we have concluded that the molar mass of the

initial cellulose and the physical structure are impor-

tant parameters in cellulose liquefaction, under acid-

catalyzed glycolysis. It was determined that the rate

of cellulose weight loss depends on hydronium ion

concentration and on the number of reducing ends.

The cellulose degradation was monitored using

size-exclusion chromatography. The observed

changes in the molecular weight distribution of the

cellulose samples (Microcrystalline cellulose,

Whatman filter paper no. 1, cotton linters) followed

the same degradation trend, independent of the

starting molar mass and physical structure. The

combination of the high temperature, glycol concen-

tration and the amount of catalysts caused

particularly rapid depolymerization of the less

ordered cellulose regions. In addition, partial degra-

dation of crystalline cellulose regions was achieved.

The molecular weight distributions and the change of

crystallites width with time showed that the degra-

dation of cellulose crystallites proceeded at the

accessible cellulose reducing ends on the surface of

the cellulose crystallites.

Acknowledgments The authors gratefully acknowledge

financial support of the Ministry of Higher Education,

Science and Technology of the Republic of Slovenia and

Slovenian Research Agency (programme P2-0145).

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