ORIGINAL PAPER

Reactive dissolution of cellulose and pulp through acylationin pyridine

Sara R. Labafzadeh • Jari S. Kavakka •

Katja Sievanen • Janne Asikkala •

Ilkka Kilpelainen

Received: 21 March 2012 / Accepted: 26 April 2012 / Published online: 15 May 2012

� Springer Science+Business Media B.V. 2012

Abstract The direct acylation of cellulose and

different pulps with various acid chlorides was

systematically screened. The syntheses were started

in a heterogeneous solid–liquid reaction medium in

hot pyridine with aliphatic and aromatic acid chlo-

rides. After a few hours, depending on the reagent

used, a homogenous solution was obtained. The

obtained cellulose esters usually show a high degree

of substitution (DS) and polymerization and are

soluble in organic solvents. Esterification of softwood

dissolving pulp, hardwood kraft pulp and hardwood

kraft pulp-hemicellulose poor were also studied. The

results show that almost identical DS were obtained

for pulp derivatives compared to esters of microcrys-

talline cellulose. Thermogravimetric analysis and

differential scanning calorimetry of the synthesized

materials showed an improved thermal stability and

various discrete thermal transitions compared to the

original cellulose. The scanning electron microscopy

images of derivatives showed a relatively flat and

smooth surface with an absence of fibrous structure.

The reactive dissolution of cellulose or pulp in

pyridine is a straightforward and easy route to obtain

long-chain aliphatic and aromatic cellulose esters.

Keywords Pulp � Acid chlorides � Pyridine �Esterification � Reactive dissolution

Introduction

The abundantly available bio-polymer, polysaccharide

cellulose (1), is a virtually inexhaustible renewable raw

material with fascinating properties. Over 187 million

tons of pulp are produced annually in the world and

only ten percent is transformed into cellulose deriva-

tives (Forstall 2002; Vaca-Garcia et al. 1998). This

hydrophilic and biodegradable linear homo-polymer,

consumed heavily by the paper industry, consists of

D-anhydroglucopyranose units (AGU) linked by b-(1-4)

glycosidic bonds. The free hydroxyl functionalities

(C2, C3 and C6, Scheme 1) in each AGU form intra

and inter molecular hydrogen bonds readily. Also, the

linear cellulose chains form an aggregated ‘fringed

fibrillar’ supramolecular structure. This partly crys-

talline and partly amorphous fibrous material is poorly

soluble in any standard organic solvents.

In recent years, interest in homogenous derivatiza-

tion of cellulose has been growing mainly because it

Electronic supplementary material The online version ofthis article (doi:10.1007/s10570-012-9720-6) containssupplementary material, which is available to authorized users.

S. R. Labafzadeh (&) � J. S. Kavakka (&) �K. Sievanen � J. Asikkala � I. Kilpelainen

Laboratory of Organic Chemistry,

Department of Chemistry, University of Helsinki,

P.O. Box 55, 00014 Helsinki, Finland

e-mail: Sara.labaf@helsinki.fi

J. S. Kavakka

e-mail: Jari.Kavakka@helsinki.fi

123

Cellulose (2012) 19:1295–1304

DOI 10.1007/s10570-012-9720-6

has become possible to attempt regioselectively syn-

thesis of cellulose derivatives and obtain products with

greater uniformity than in heterogeneous mixtures

(Edgar et al. 1998). The conventional solvent systems

include 10 % NaOH, NaOH/Urea and widely used

DMA/LiCl, which can be used for dissolving cellu-

lose. However, usually some relatively lengthy (up to

24 h) pre-treatment of cellulose is required to obtain

complete solubility. Also, the reactive nature of the

aqueous solvents limits the scope of the applicable

reactions. Dissolution can be achieved also with ionic

liquids (IL) such as alkylimidazolium-based IL (Swat-

loski et al. 2002). The obtained cellulose solutions can

have concentration as high as 25 % (w/w). However,

with high concentrations the viscosity of cellulose-IL

solution increases rapidly and the solutions become

shear-thinning, which is a problem from the viewpoint

of repeatability of reactions (Granstrom et al. 2008).

Currently, the use of ILs in a larger (industrial) scale

gives rise to challenges such as an easy and inexpen-

sive preparation of suitable IL-solvent, recycling of

the IL, and handling of the (often) toxic IL-waste. In

addition, the dissolution of cellulose with all of the

abovementioned solvent systems can often require a

prolonged dissolving period under heating.

One major pathway to obtain cellulose derivatives

is acylation reactions, for which a comprehensive

review has been provided by Heinze et al. (2006).

Simple and mixed cellulose esters of short-chain

carboxylic acids (C2 to C4) are produced in industrial

scale to obtain synthetic fibers, photographic and

X-ray films, coatings, textile and cigarette filter

industries (Crepy et al. 2009). Commercial cellulose

acetate is typically prepared using acetic acid and

acetic anhydride in the presence of a strong acid

catalyst (Cheng et al. 2010). On the contrary, the

preparation of cellulose esters of long-chain aliphatic

or aromatic acids via the anhydride strategy is not

economically feasible. Instead, these cellulose esters

can be derived from reaction of cellulose with acid

chlorides in the DMA/LiCl solvent system or in ILs

(Heinze et al. 2006; Edgar et al. 1998). However, ionic

liquids or DMA/LiCl mixture are not yet feasible

solvents for large-scale production, which has made

the higher esters of cellulose largely unattainable.

Acylation of cellulose with long fatty-acid derivatives

is troublesome since the products are poorly soluble in

ILs and usually products with low DS-values are

obtained (Barthel and Heinze 2006). These constraints

can be circumvented with the use of the DMA/LiCl

system if the restrictions of the DMA/LiCl solvent

system are accepted.

The ideology ‘dissolve first–react later’ has limited

the use of cellulose as a cheap starting material in

organic synthesis. However, direct acylation of cellu-

lose can also be carried out under heterogeneous

conditions to obtain high DS derivatives. When cellu-

lose or pulp is suspended in dry pyridine and stirred

under heating with aliphatic or aromatic acid chlo-

rides, cellulose esters with high DS-values are

obtained through reactive dissolution (Scheme 1)

(Malm et al. 1951; Bras et al. 2007). The poor solubility

of unmodified cellulose in pyridine is accepted as such

and the reaction is started as a heterogeneous mixture.

Progress of the reaction makes the product soluble and

thus drives the reaction forward. Depending on the

reactant, after a certain DS the cellulose derivative

becomes soluble in hot pyridine and finally a homog-

enous solution is obtained. Here, pyridine acts as a

cellulose-swelling organic compound that can partially

break the intermolecular hydrogen bonds and conse-

quently increase the reactivity of cellulose, while acid

chlorides will act both as reagent and as solvent. Several

studies have shown the capability of the acid chloride-

pyridine procedure to prepare cellulose esters effi-

ciently without prior dissolution of cellulose and this

Scheme 1 Acylation of

cellulose in pyridine

1296 Cellulose (2012) 19:1295–1304

123

approach is considered to be the classical approach to

obtain cellulose esters (Bras et al. 2007; Malm et al.

1951). These reports have involved modification of

cellulose with a full series of long- chain acids in

optimized reaction conditions. However, the majority

of the literature is focused on aliphatic esters of

cellulose, but examples to obtain aromatic esters are

only few (Stampfli et al. 1990; Garces et al. 2003).

Further, there are no systematic studies of the suitability

of the acid chloride-pyridine method for various pulps.

In the current study, we have systematically

screened the suitability of the acid chloride-pyridine

method to obtain various cellulose derivatives from

different cellulose preparations (pulps). The objective

of the present work was to examine whether acid

chloride-pyridine method is feasible for the produc-

tion of aromatic cellulose esters from pulps and to

explore the properties of the prepared polymers.

Softwood dissolving pulp (SWDP), Hardwood kraft

pulp (HWK) and Hardwood kraft pulp-hemicellulose

poor (HWKHP) were used as the sources of starting

material and microcrystalline cellulose as a reference.

Experimental

Materials

Decanoyl and palmitoyl chloride were obtained from

TCI. Pyridine and chloroform were provided by VWR

international, BDH Prolabo. All other reagents used in

cellulose modification were purchased from Aldrich

and used as such: Avicel�; 4-nitrobenzoyl chloride;

4-methoxybenzoyl chloride; toluoyl chloride; trimeth-

ylacetyl chloride; acetyl chloride; propionyl chloride;

valeryl chloride; hexanoyl chloride; N,N-dimethyl-

acetamide; methanol; anhydrous lithium chloride;

endo-N-hydroxy-5-norbornene-2,3-dicarboxilic acid

imide (e-HNDI); 2-chloro-4,4,5,5-tetramethyl-1,3,2-

dioxaphospholane (2-Cl-TMDP); chromium(III) ace-

tylacetonate (cr(acac)3); betaine hydrochloride;

benzoyl formic acid; and dichloromethyl methyl ether.

SYLFAT� 2LT (tall oil fatty acid) was provided by

Arizona chemical (Rauma, Finland), mainly contain-

ing 59 % linoleic acid and 28 % oleic acid. The tall oil

fatty acid chloride (TOFA-Cl) was prepared by gently

refluxing a mixture of TOFA and tionyl chloride.

Deuterated chloroform 99.8 % containing 0.03 %

TMS (Eurisotop) was stored at 4 �C. SWDP was

purchased from Domsjo (Sweden) containing 3.5 %

hemicelluloses and less than 0.5 % lignin with

viscosity of 520 ml/g. HWK was provided by Kaski-

nen (Finland) including 24.7 % hemicelluloses with

viscosity of 870 ml/g. HWKHP was prepared from

HWK by alkaline extraction in order to reduce the

hemicelluloses to 13.7 %. SWDP and HWK samples

were fibrillated in ethanol under reflux and dried in

vacuum to provide a better possibility for reagents to

penetrate the fibrous material, while the HWKHP

samples were used as such.

Typical procedure for cellulose acylation

in pyridine

Avicel� (0.25 g, 1.54 mmol) and acyl chloride (5

equiv per AGU-unit) were added into 10 ml of

pyridine (anhydrous). The sample (2.5 % w/v) was

allowed to react at 90 �C for 3 h. The reaction was

quenched by deionised water (50 ml). The polymer

was isolated by filtration, washed several times with

300 ml of deionised water and vacuum-dried. For

further purification, the filtrate was dissolved in

chloroform (25 ml) and precipitated by methanol

(100 ml), filtered and washed with 300 ml methanol

and finally vacuum-dried overnight. Some samples

were purified two or three times by solubilization/

precipitation process with chloroform and methanol,

respectively. The final white product was obtained

with a weight increase from 140 to 280 % depending

on the acid chloride used. In case of cellulose acetate,

further purification with chloroform was not done.

Instead, the product was purified through severe

washing by methanol.

Typical procedure for cellulose acylation

in DMA/LiCl

Esterification of cellulose has been intensively studied

in the presence of dimethylacetamide/lithium chloride

(DMA/LiCl) as a solvent system (Vaca-Garcia et al.

1998). For comparison, acylation of cellulose was

therefore carried out in homogenous media of DMA/

LiCl in ambient temperature for 3 days according to a

method described elsewhere (King et al. 2010). In this

approach, a homogenous solution of cellulose in

DMA/LiCl was prepared prior to acylation with

different equivalents of decanoyl chloride (2).

Cellulose (2012) 19:1295–1304 1297

123

DS determination

The degree of substitution was determined by 31P-

NMR method for those samples which were soluble in

DS determination media (King et al. 2010). 150 ll of

pyridine and 1 ml of CDCl3 in two portions were

added to 25 mg of cellulose esters and the mixture was

agitated until completely soluble. To the solution,

2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane

(2-Cl-TMDP, 200 ll, 1.26 mmol) was added in two

portions, whereby internal standard endo-N-hydroxy-

5-norbornene-2,3-dicarboxilic acid imide solution

(e-HNDI, 125 ll, 121.5 mM in Pyr:CDCl3/3:2,

0.0152 mmol) was added in between and vortex

mixed until clear solution was obtained. Finally,

cr(acac)3 (1 ml, 0.08 M in CDCl3, mmol) was added

and the 31P-NMR spectra were recorded. The DS

calculations are described previously (King et al. 2010).

In the case of insoluble derivatives, DS was deter-

mined by elemental analysis according to an equation

described previously (Vaca-Garcia et al. 2001).

Analytical methods

FTIR spectra from solid samples were recorded with a

Bruker alpha-P FTIR spectrometer with a diamond

ATR.

All 1H- and 13C-NMR spectra were measured on a

Varian Unity INOVA 500 NMR spectrometer (500 MHz

proton frequency) equipped with 5-mm triple-

resonance (1H, 13C and 15N) gradient probe-head at

27 �C in CDCl3. Quantitative 31P-NMR were per-

formed using a Varian Unity INOVA 600 spectrom-

eter (600-MHz proton frequency) equipped with a

5-mm direct detection broadband probe-head at 27 �C.

Molar mass determination was performed using an

Agilent system including degasser, pump, autosampler,

column oven (1100 series), diode array UV detector

(1050 series) and refractive index detector (1200

series). The gel permeation columns were Waters

Styragel guard, HR-5E and HR-1 (7.8 9 300 mm)

connected in series. THF was used as the mobile phase

at a flow rate of 0.5 ml/min. The GPC system was

calibrated with polystyrene standards (890, 1,000,

4,000, 9,000, 42,300, 177,000, 434,000, 1,270,000 Da)

using UV detection at multiple wavelengths and

refractive index. The Agilent Chemstation (rev. A.

10.02) with Agilent GPC addon (rev. A 02.02) was used

to calculate the molar mass distributions.

Thermal properties of cellulose esters were analyzed

by means of thermogravimetric analysis (TGA) and

differential scanning calorimetry (DSC). TGA was

performed by a Mettler TGA/SDTA 851e using a

temperature range from 50 to 600 �C with a heating rate

of 10 �C/min in 50 ml/min N2. The thermal decompo-

sition temperature (Td) was taken as the onset of

significant (C0.5 %) weight loss. Differential scanning

calorimetry (DSC) was carried out by DSC Q200. The

temperature was programmed from 40 to 250 �C at a

heating rate of 10 �C/min in an atmosphere of nitrogen.

The heating/cooling/heating method was used and after

cooling the data from second run were recorded.

Scanning electron microscopy (SEM) imaging of

unmodified and modified derivatives was carried out

with a Hitachi S-4800 FESEM.

Results and discussion

Direct acylation of cellulose in pyridine offers an

efficient way to prepare high DS products. This

approach works very well with long-chain fatty acids

and also with aromatic acid chlorides. However,

attempts to obtain short-chain (C\ 6) and also unsat-

urated cellulose esters were failed. After 3 h reaction

with short acid chlorides including propionyl (C3),

valeryl (C5) and hexanoyl (C6) chloride, a dark black

solution was obtained and there were problems with

purification. Syntheses of charged derivatives through

reactive dissolution in pyridine were also unsuccessful.

Optimization of reaction conditions

Reaction conditions were first optimized by varying the

reaction time, the substrate-reactant ratio and the

substrate-pyridine ratio in acylation of Avicel� with

decanoyl chloride (2). It was found that the heteroge-

neous mixture of 0.5 g of cellulose and 5 equivalents of

2 in 20 ml of pyridine gave a homogenous solution after

stirring for 1 h at 90 �C. After 3 h, the reaction was

quenched with water and the precipitate was filtered.

The crude product was dissolved in chloroform and

precipitated by methanol. Purified white powder-like

cellulose esters were dried in high vacuum.

The initial heterogeneous reaction condition is

clearly seen as a non-linear increase of DS when the

reaction between decanoyl chloride (2) and Avicel�

was studied varying the equivalence of 2 (Fig. 1).

1298 Cellulose (2012) 19:1295–1304

123

Figure 2 also shows a marked non-linear reduction

of the glass transition temperature (Tg) when the molar

ratio of 2 used per anhydroglucose units increased.

This indicates that the processability of modified

polymers is improved. The non-linear decrease of Tg

most probably originates from the heterogeneous

conditions at the beginning of the reaction.

The SEM micrographs of modified Avicel� with 2

presented in the supplementary material illustrated a

decrease in the fibrous structure of the polymer when

the equivalence of 2 was increased. Such morpholog-

ical observation agrees with the results of thermal

analysis and DS measurement (Figs. 1, 2).

Analysis of the products

Several aliphatic and aromatic cellulose esters were

prepared using the optimized conditions (Table 1) and

the synthesis of charged and other derivatives were also

attempted. However, attempts to synthesize charged

and other derivatives through reactive dissolution

failed. It was also tested whether various pulp samples

behave similarly as the Avicel�. Softwood dissolving

pulp (SWDP), hardwood kraft pulp (HWK) and

hardwood kraft pulp-hemicellulose poor (HWKHP)

were therefore modified by toluoyl chloride (3) under

optimized conditions.

Spectroscopic analysis of cellulose esters

The resulting cellulose esters were further character-

ized by means of ATR-FTIR, 13C- and 1H-NMR in

order to confirm their purity and structure (data is

shown in the supplementary material).

Figure 3 illustrates the FTIR spectrum of the

unmodified and modified SWDP with toluoyl chloride

(3). A significant decrease in the intensity of hydroxyl

groups (3,345 cm-1) and appearance of strong absorp-

tion band at 1,717 cm-1 relating to carbonyl ester

groups (C=O) represent the efficiency of the acylation

reaction. Identical results were achieved for HWK and

HWKHP and the data are presented in the supple-

mentary material.

Modified HWK (entry 13, Table 1) and HWKHP

(entry 14, Table 1) with toluoyl chloride (3) in the

optimized conditions did not dissolve in organic

solvents, while modified SWDP (entry 12, Table 1)

had a good solubility in CHCl3 and therefore it also

became possible to characterize the product of treated

SWDP by 1H- and 13C-NMR spectroscopy. Figure 4

supplies strong evidence for successful acylation of

the SWDP pulp with 3. The appearance of a large

phenyl group compared to anhydroglucose units in 1H-

NMR of esterified pulp (Fig. 4a) proves that cellulose

esters with high DS have been obtained.

DS determination of cellulose derivatives

by quantitative 31P-NMR and elemental analysis

The DS of the soluble products in chloroform was

studied using the 31P-NMR method described earlier

(vide supra) (King et al. 2010), while the DS of

insoluble samples were measured by elemental anal-

ysis (Vaca-Garcia et al. 2001). The DS-data is

presented in Table 1 and in the supplementary

material.

0

1

2

3

0 1 2 3 4 5

DS

Equivalences

Fig. 1 Degree of substitution versus n(decanoyl chloride)/

n(AGU)

0

40

80

120

160

200

1 2 3 4 5

Gla

ss T

rans

ition

Tem

pera

ture

(°C

)

Equivalents

Fig. 2 Glass transition temperature variation in function of

decanoyl chloride quantity (mole per AGU)

Cellulose (2012) 19:1295–1304 1299

123

DS values close to 3 were obtained after 3 h

reaction time with all derivatives tested with one

exception. The sterically hindered pivaloyl chloride

(4) gave a DS of 2.5 with prolonged heating (entry 6,

Table 1). Since 3 h modification of cellulose by 4

resulted in only moderate DS (0.5), the reaction was

carried out with various reaction times. Prolonged

heating up to 22 h increased the DS to 2.0, while the

best result was obtained after 72 h (DS=2.5).

Recently, much interest has been given to cellulose

esters with very low DS, mainly due to their simple

synthesis procedures and also their interesting prop-

erties such as water and gas permeability as well as

selective adsorption ability (Peydecastaing et al.

2006). Cellulose esters of 4 with low DS (0.5) retain

the fibrous structure compared to those obtained after

22 h and 72 h as it is apparent from the SEM images

shown in the supplementary material and could be

useful for specialty applications, e.g. fabrication of

water-repellent particleboards.

Modified Avicel� with p-nitrobenzoyl chloride

(entry 9, Table 1) and treated HWK (entry 13, Table 1)

and HWKHP (entry 14, Table 1) with toluoyl chloride

(3) were not soluble in organic solvents, thus prohib-

iting NMR analysis. However, according to IR

hydroxyl-free products were obtained showing achieve-

ment of high DS materials. Therefore, elemental

analysis was carried out to calculate the DS (Table 1).

Reaction of pulp samples with 3 gave almost

identical DS when compared to Avicel� (entry 11, 12,

13, 14, Table 1), demonstrating the efficiency of

reactive dissolution also for esterification of various

types of pulp. In addition, comparing the DS of

400140024003400

Tra

nsm

ittan

ce (%

)

Wavenumber cm-1

a

b

Fig. 3 IR spectra of a unmodified SWDP and b modified

SWDP with toluoyl chloride

Table 1 Synthesized cellulose derivatives

Entry Substrate Acid chloride Reaction time (h) Yield (%) DS Tg (�C) Td(onset) (�C)

1 Avicel Acetyl chloride 3 83 2.7 175 313

2 Avicel Decanoyl chloride 3 63 2.9 39 353

3 Avicel Palmitoyl chloride 3 60 2.9 45 358

4 Avicel Pivaloyl chloride 3 –d 0.5 182 305

5 Avicel Pivaloyl chloride 22 –d 2.0 174 345

6 Avicel Pivaloyl chloride 72 –d 2.5 157 354

7 Avicel Biphenyl-4-carbonyl chloride 3 91 2.8 96 290

8 Avicel Methoxybenzoyl chloride 3 79 2.7 148 333

9 Avicel p-nitrobenzoyl chloride 3 98 2.6e 209 332

10 Avicel 4-tert-butylbenzyl chloride 3 92 2.9 181 355

11 Avicel Toluoyl chloride 3 81 2.9 159 354

12 SWDPa Toluoyl chloride 3 82 2.9 162 343

13 HWKb Toluoyl chloride 3 99 2.4e 165 345

14 HWKHPc Toluoyl chloride 3 93 2.5e 164 354

a Softwood dissolving pulpb Hardwood kraft pulpc Hardwood kraft pulp-hemicellulose poord Great majority of the product was lost using the purification procedure described abovee Products are insoluble in DS determination media using 31P-NMR. Consequently, DS was determined by elemental analysis for

insoluble samples

1300 Cellulose (2012) 19:1295–1304

123

different pulps, it can be concluded that raising the

content of hemicelluloses reduces the DS.

The efficiency of cellulose acylation was also

investigated through reactive dissolution procedure in

comparison to homogenous functionalization of cel-

lulose in DMA/LiCl (data shown in supplementary

material). A lower DS (2.6) was obtained for modified

cellulose with 5 equivalents of decanoyl chloride (2) in

DMA/LiCl compared to DS 2.9 achieved for cellulose

esters of 2 acylated in pyridine.

Gel permeation chromatography of cellulose esters

Gel permeation chromatography (GPC) analysis of the

cellulose derivatives showed that no hydrolysis of

cellulose backbone occurs under the optimized reac-

tion conditions (see supplementary material). For

comparative purposes, the molar mass distribution of

treated SWDP and Avicel� with toluoyl chloride (3) is

shown in Fig. 5 and confirms the broader molar mass

distribution and higher molar mass of treated SWDP

compared to modified Avicel�.

Thermal properties of cellulose derivatives

Thermogravimetric analysis (TGA) and differential

scanning calorimetry (DSC) were used to determine

thermal properties such as thermal stability and glass

transition temperature of the cellulose preparations

and derivatives. As can be seen from Table 1,

degradation temperature (Td) higher than intact Avi-

cel� and pulp (around 320 and 328 �C, respectively)

was achieved with almost all derivatives. It means that

acylation of cellulose induced an increase in thermal

stability. Interestingly, modified cellulose with biphe-

nyl-4-carbonyl chloride (entry 7, Table 1) is an excep-

tion with thermal decomposition temperature of

290 �C, which is lower than intact Avicel� (320 �C).

DSC thermograms of the cellulose esters showed a

phase change, which may be attributed to the glass

transition temperature (Tg) (Table 1, Fig. 6 and sup-

plementary material), while unmodified Avicel� and

pulp, as expected, do not show any transition.

Decanoyl ester of cellulose (entry 2, Table 1) also

presents an endotherm around 76 �C due to the

melting of the material in addition to the glass

transition point at 39 �C, while decanoyl ester of

cellulose modified in DMA/LiCl shows just one

transition around 74 �C related to glass transition

temperature, which is much higher than that of treated

cellulose in pyridine (39 �C) (data shown in supple-

mentary material). It can be concluded that products

with lower glass transition temperature were obtained

012345678

12

Cellulose backbone

9,10,CHCl3

020406080100120140160180

117

89,10

4

1 6

2,3,512

CHCl3

O

OR

O

OR

R =

123

4

5

6

or H

O

78

9 10

109

11

12

RO

n

a bFig. 4 a 1H NMR spectrum

and b 13C NMR spectrum of

treated SWDP with toluoyl

chloride (entry 12, Table 1)

1.E+031.E+051.E+071.E+09

Inte

nsity

Molar mass

a b

Fig. 5 Molar mass distribution of modified a SWDP and

b Avicel� by toluoyl chloride

Cellulose (2012) 19:1295–1304 1301

123

through acylation in pyridine compared to the

DMA/LiCl procedure.

As it was expected, the glass transition temperature

drops off as the extension of reaction time of cellulose

with pivaloyl chloride (4), while the decomposition

temperature grows larger (entry 4, 5 & 6, Table 1).

Morphological analysis of cellulose derivatives

Surface morphological investigation of the cellulose

esters was carried out using scanning electron micros-

copy (SEM). Figure 7 compares SEM microphoto-

graphs of treated and untreated pulps. Microfibrils in

untreated pulps are well separated, while dramatic

morphology changes are observed for treated pulps.

The fibrous morphology in treated pulps was destruc-

tured and considerably porous and more uniform

material was obtained through acylation of pulp in

pyridine.

Unsuccessful attempts

Conversion of cellulose with short-chain aliphatic acid

chlorides using the reactive dissolution method was

also investigated for comparative studies. Attempts to

obtain shorter chain derivatives (Propionyl (C3),

valeryl (C5) and hexanoyl chloride (C6)) were unsuc-

cessful and led to the formation of a black precipitate,

which could not be purified. Obviously, the higher

reactivity of short-chain acid chlorides leads to

saccharification, which jeopardizes these reactions.

Also, esterification of cellulose with tall oil fatty

acid chloride (TOFA-Cl) was conducted with different

reaction times. Unlike in the case of other longer chain

acid chlorides, a homogenous solution was not

obtained after 3, 22 h or even 72 h reaction times of

TOFA-Cl with Avicel� in pyridine. However, the IR

spectra of the TOFA cellulose esters (Fig. 8) indicates

that the esterification reaction has occurred by exis-

tence of the carbonyl peak at 1,744 cm-1 and a drastic

decrease of the hydroxyl peak at 3,461 cm-1. Quite

surprisingly, the products were not soluble in any

organic solvents and hence, their structure could not

be verified by NMR. A probable explanation for this

behavior is the existence of multiple double bonds in

the TOFA chains, which may lead to cross-linking via

Diels–Alder type cycloaddition reactions.

The other failed attempt was the preparation of

cationic cellulose ester by treating cellulose with

N-chlorobetainyl chloride in pyridine. N-chlorobetai-

nyl chloride was prepared from reaction of betaine

hydrochloride with thionyl chloride according to a

procedure described elsewhere (Vassel and Skelly

1963). A black solution was achieved after heating

15 min at 90 �C and there were difficulties in precip-

itating the material.

Preparation of the benzoyl formoyl ester of cellu-

lose was also attempted under similar conditions.

Benzoyl formoyl chloride was synthesized by reaction

of benzoyl formic acid and dichloromethyl methyl

ether in dichloromethane using the previously

reported procedure (Ayitou et al. 2009). Similar to

other failed attempts, a black mixture was obtained

after a certain time.

Conclusions

Highly substituted cellulose esters can be achieved

without any prior dissolution of cellulose. The heter-

ogeneous mixture of acid chlorides and cellulose in

pyridine yields a homogenous reaction mixture after

heating. This method leads to highly substituted

cellulose esters within a relatively short reaction time

(3 h). The limiting DS for the solubility of the products

in reaction media varies greatly depending on the

reagent used. For instance, long-chain cellulose esters,

i.e. decanoyl ester are soluble in hot pyridine at low DS

while even highly substituted esters of polar reagents,

i.e. p-nitrobenzoyl chloride are not soluble. The

advantage of this method is that the partial degradation

of cellulosic fibers did not occur according to GPC

-50 0 50 100 150 200 250

Hea

t Flo

w (

W/g

)

Temperature (°C)

a

b

c

d

162 (°C)

164 (°C)

165 (°C)

Fig. 6 DSC analysis of pulp derivatives with toluoyl chloride

a unmodified SWDP b treated SWDP c treated HWK-HP and

d treated HWK

1302 Cellulose (2012) 19:1295–1304

123

results. It was also considerably faster than homoge-

nous esterification in DMA/LiCl or ILs. The study of

cellulose esters thermal properties and degree of

substitution leads to the conclusion that the reactive

dissolution procedure is more efficient than homoge-

nous acylation of cellulose in DMA/LiCl, as far as

thermal properties and degree of substitution are

concerned. In addition, the ecological impacts and

Fig. 7 SEM images of a unmodified SWDP b modified SWDP c unmodified HWK-HP d modified HWK-HP e unmodified HWK and

f modified HWK with toluoyl chloride

Cellulose (2012) 19:1295–1304 1303

123

high price of common solvents such as dimethylace-

tamide/lithium chloride (DMA/LiCl) and ionic liquids

confine their use to a laboratory scale, while pyridine

is rather cheap and truly recyclable via a simple

distillation. However, the reactive dissolution

approach suffers from the limitation that only high

DS products are usually prepared and hence low DS

products are not directly accessible.

This method also works very well with various

types of pulp and makes pulps practical starting

materials when highly reactive reactants, such as acid

chlorides, are used. Combining the result of SEM and

thermal analysis, we can arrive at the conclusion that

the reactive dissolution of pulp through acylation in

pyridine improves the processability and compatibil-

ity of the product with synthetic polymers. We are

currently applying the reactive dissolution strategy to

produce other cellulose derivatives.

Acknowledgments This work was supported by Forest

Cluster Ltd. as a part of the Future Biorefinery (FuBio) project

and by the Academy of Finland (grants 122534 and 132150).

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400140024003400

Tra

nsm

ittan

ce (

%)

Wavenumber Cm-1

a

b

Fig. 8 IR spectra of a unmodified and b modified Avicel� with

TOFA-Cl

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