Biobased and Sustainable Alternative Route to Long-Chain Cellulose Esters

Article abstract of DOI:10.1021/acs.biomac.6b01584

Fatty acid cellulose esters (FACEs), which have been identified recently as sustainable film materials, are conventionally synthesized by the use of the reaction with acyl chloride/anhydride pyridine in the presence of LiCl/N,N-dimethylacetamide. In this study, we have developed a new synthetic route to FACEs using a vinyl ester of long chain fatty acid, which is an excellent biobased and highly reactive reagent, for the functionalization of cellulose. The developed method involves the synthesis of the long aliphatic fatty acid vinyl ester via a transition-metal-catalyzed transvinylation reaction between vinyl acetate and the fatty acid, followed by its subsequent reaction with cellulose to yield FACEs. In this work, we have used vinyl oleate as a model precursor to introduce the fatty acid chain to cellulose. The covalent grafting of the fatty acid chain to the free hydroxyl groups of cellulose was achieved through potassium carbonate (K₂CO₃)-catalyzed transesterification of vinyl oleate in the presence of N-methyl pyrrolidone as solvent with low toxicity. Successful functionalization of cellulose was confirmed by FTIR, ¹³C CP-MAS NMR, X-ray diffraction, and the thermogravimetric analysis. The results obtained showed that the functionalization efficiency of the cellulose increased with higher temperature and prolonged reaction times. The strategy proposed in the present work is an important step onward in terms of sustainability because the long-chain vinyl ester can be synthesized from a renewable and biobased source, and the toxic and corrosive chemicals commonly employed for cellulose esterification are avoided.

Full text of DOI:10.1021/acs.biomac.6b01584

Article
pubs.acs.org/Biomac
Biobased and Sustainable Alternative Route to Long-Chain Cellulose
Esters
,†
†
‡
Mohamed Jebrane,* Nasko Terziev, and Ivo Heinmaa
†Department of Forest Products/Wood Science, Swedish University of Agricultural Sciences, Box 7008, SE-750 07 Uppsala, Sweden
‡National Institute of Chemical Physics and Biophysics, Akadeemia tee 23, 12618 Tallinn, Estonia
ABSTRACT: Fatty acid cellulose esters (FACEs), which have
been identified recently as sustainable film materials, are
conventionally synthesized by the use of the reaction with acyl
chloride/anhydride pyridine in the presence of LiCl/N,N-
dimethylacetamide. In this study, we have developed a new
synthetic route to FACEs using a vinyl ester of long chain fatty
acid, which is an excellent biobased and highly reactive reagent,
for the functionalization of cellulose. The developed method
involves the synthesis of the long aliphatic fatty acid vinyl ester
via a transition-metal-catalyzed transvinylation reaction
between vinyl acetate and the fatty acid, followed by its subsequent reaction with cellulose to yield FACEs. In this work, we
have used vinyl oleate as a model precursor to introduce the fatty acid chain to cellulose. The covalent grafting of the fatty acid
chain to the free hydroxyl groups of cellulose was achieved through potassium carbonate (K CO )-catalyzed transesterification of
2
3
vinyl oleate in the presence of N-methyl pyrrolidone as solvent with low toxicity. Successful functionalization of cellulose was
1
3
confirmed by FTIR, C CP-MAS NMR, X-ray diffraction, and the thermogravimetric analysis. The results obtained showed that
the functionalization efficiency of the cellulose increased with higher temperature and prolonged reaction times. The strategy
proposed in the present work is an important step onward in terms of sustainability because the long-chain vinyl ester can be
synthesized from a renewable and biobased source, and the toxic and corrosive chemicals commonly employed for cellulose
esterification are avoided.
INTRODUCTION
The use of crude fatty acids as esterifying agents is the best
option for synthesis of FACEs, but their extremely low
reactivity toward cellulose’s hydroxyl groups and the steric
effect prevent the grafting reaction. Attempts to convert in situ
■
Polysaccharides are increasingly recognized as promising
materials in biotransformation processes for achieving a
biobased economy. In particular, much attention has been
given to cellulose and its derivatives for their potential
application in numerous areas of modern life, as a result of
the environmental benefits, high versatility, renewability,
the fatty carboxylic acids into more reactive entities have been
17,18
made by using p-toluenesolfonyl chloride,
anhydride
or acetic
19−21
as coreactant. For an homogeneous modifica-
tion, the mixed solvent N,N-dimethylacetamide with lithium
1
nontoxicity and abundance of this natural polymer. In
4−6,8−10,13−15
chloride (DMAc/LiCl) systems is widely used.
addition, there has been a growing interest in developing new
biobased materials, which can compete with fossil-based
materials in performance, and thus developing new technolo-
gies or modifying the existing ones by using environmentally
friendly chemical processes.
Notwithstanding cellulose acetate and other short-chain
cellulose derivatives, recently, long fatty acid cellulose esters
FACEs) have drawn much attention and have been identified
as sustainable and biodegradable plastics.
However, the ecological impact and the price of such
compounds limit their utilization to a laboratory scale.
In this study, we present a new acylation process based on
the transesterification reaction between cellulose hydroxyl
groups and vinyl ester of fatty acid. Compared with the
traditional esterification methods using either acyl chlorides or
anhydrides, the novel proposed transesterification method
using vinyl ester of fatty acid obtained from innocuous acyl
donor is indeed a mild acylation method. This new approach
generates acetaldehyde as a byproduct that is not acidic and can
be easily removed from the reaction medium because of its low
boiling point. It has been reported that polysaccharides
including cellulose can be successfully esterified by this
(
2
−7
Traditionally,
FACEs are synthesized mainly by the pyridine−acyl chloride or
2
,3,6−12
anhydride reactions.
These reaction systems produce
aggressive hydrochloric acid as a byproduct, resulting in
cellulose degradation. To limit acid degradation of cellulose,
4
,13,14
bases such as pyridine, its derivative DMAP,
or trimethyl-
9
amine, are usually used to neutralize the acid generated.
Received: October 27, 2016
Revised: December 29, 2016
1
6
2
Nitrogen stream or vacuum are also applied to remove
gaseous HCl as it is formed.
©
XXXX American Chemical Society
A
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2
2−26
method.
Recently, it has been shown that cellulose
obtained residue using a mixture of petroleum ether/dichloromethane
(
4:1; v/v), and the reaction yielded 98% VO.
nanowhiskers can be easily esterified by vinyl acetate and
vinyl cinnamate under microwave activation.
27,28
The obtained VO was characterized by FTIR-ATR and NMR
To our
−
1
spectroscopy. IR-ATR (Figure 1) (ν, cm ): 3091 (vinyl C−H
knowledge, the use of vinyl ester of long-chain fatty acids as
a reagent to esterify cellulose has not yet been exploited.
Accordingly, in this study, we explored the sustainable and
nontoxic functionalization of microcrystalline cellulose (MCC)
using vinyl ester of fatty acid under mild conditions. This
method first called upon the synthesis of vinyl ester of fatty acid
by the transvinylation of oleic acid (OA) with vinyl acetate. The
synthesized vinyl oleate (VO) was then used to functionalize
cellulose through a transesterification reaction between a
synthesized precursor and the hydroxyl groups of cellulose,
using K CO as mild catalyst and NMP as low toxic solvent.
2
3
The supramolecular/crystal structure and thermal properties of
functionalized MCC were characterized using FTIR, 13C CP-
MAS nuclear magnetic resonance (NMR) spectroscopy, X-ray
diffraction (XRD), and thermogravimetry.
EXPERIMENTAL SECTION
■
Figure 1. FTIR absorbance spectra of oleic acid (OA) and vinyl oleate
VO).
Materials. Microcrystalline cellulose (powder, 20 μm) prepared
from cotton linters, sulfuric acid (95%), chloro(1,5-cyclooctadiene)
(
iridium-(I) dimer (Ir 57.2%) ([Ir(cod)Cl] ), OA (97%), vinyl acetate
2
stretching), 3006 (chain unsaturation C−H stretching), 2923 and
(
VAc), N-methyl-2-pyrrolidone (NMP), potassium carbonate
2
854 (saturated CH stretching modes), 1758 (CO stretching of
(
K CO ), sodium acetate, and sodium sulfate were purchased from
2
2
3
vinyl ester), 1646 (nonconjugated CC stretching), 1464 (H bending
Sigma-Aldrich. All other solvents were ACS-grade and used as
received.
of CH and CH groups), 1141 (ester C−O stretching), 949 (CH out-
2
3
of-plane deformation of CHCH ), 868 (CH out-of-plane
Instrumentation. IR-ATR Instrument. Spectrum Two FTIR
Perkin-Elmer) was equipped with an UATR Diamond accessory,
2
2
deformation of −CHCH ), and 722 (skeletal vibration of
(
2
1
−
(CH ) −). H NMR (700 MHz, CDCl , δ) (Figure 2): 0.89 (t,
which allows collection of FTIR spectra directly on a sample without
any special preparation. The “pressure arm” of the instrument was
used to apply a constant pressure (monitored by software) to the
sample positioned on top of the Diamond crystal to ensure a good
contact between the sample and the incident IR beam. All FTIR
2
n
3
−1
spectra were collected at a spectrum resolution of 4 cm , with 32
−1
scans over the range from 4000 to 450 cm .
Nuclear Magnetic Resonance. H and 13C NMR spectra in CDCl3
were recorded in Bruker BioSpin 700 MHz using TMS as reference.
Multiplicities are abbreviated as follows: s = singlet, d = doublet, dd =
doublet of doublets, t = triplet, q = quartet, and m = multiplet.
Solid-state 13C CP MAS NMR spectra of modified and unmodified
cellulose were obtained at room temperature on a Bruker Avance-II
spectrometer operating at 150.9 MHz using a custom-built MAS NMR
probe. The samples were packed into 4 mm zirconia rotors. The
sample spinning speed was regulated to 12.0 Hz, the contact time was
1
1
ms, and the relaxation delay between the acquisitions was 5 s.
Chemical shifts were relative to TMS used as an external standard. For
each sample, a total of 20 000 scans were accumulated.
Thermogravimetric Analysis. Thermogravimetric analysis (TGA)
1
Figure 2. H NMR spectra of oleic acid (OA) and vinyl oleate (VO)
in CDCl and peak assignments.
3
plots were carried out using a Mettler-Toledo TGA/SDTA 851e,
−1
under nitrogen with a flow rate of 40 mL min , using alumina pans.
The samples were ramparted from room temperature to 600 °C at a
3
H, −CH ), 1.28−1.32 (m, 20H, aliphatic −CH −), 1.65 (m, 2H,
3
2
−1
scanning rate of 10 °C min .
−
2
CH −CH −C(O)−), 2.02 (m, 4H, −CH −CHCH−CH −),
2
2
2
2
X-Ray Diffraction Analysis. XRD patterns were recorded on
PANanalytical B.V. X’Pert3 powder diffractometer (Netherland) with
Cu Kα radiation in the angular range of 2θ = 5−70° at 25 °C.
Synthetic Strategies. Synthesis of Vinyl Oleate. Vinyl ester of
OA was synthesized by mixing 10.56 g (37.40 mmol) OA with a 10
equiv excess of vinyl acetate in a three-necked round-bottomed glass
flask equipped with a reflux condenser and thermometer and thereafter
.35−2.40 (t, 2H, −CH −C(O)−), 4.57 and 4.88 (dd, 2H, COO−
2
CHCH ), 5.35 (m, 2H, −CHCH−), and 7.27−7.30 (dd, 1H,
2
13
COO−CHCH ). C NMR (700 MHz, CDCl , δ) (Figure 3): 14.2
2
3
(
C , −CH ), 22.8 (C , CH −CH −), 24.7 (C , −O−C(O)−CH −
r
3
q
3
2
c
2
CH −), 27.3 (C , −O−C(O)−CH −), 29.2 (C and C , −CH −
2 b 2 h k 2
CHCH−CH −), 29.4−29.6 (C and C , −O−(CO)−(CH ) −
2
d,e
o
2
2
CH −CH −, CH −(CH ) −CH −), 29.8−29.9 (C and C
,
2
2
3
2
2
2
f,g
l−n
degassed with argon. The catalyst [Ir(cod)Cl] (0.01 equiv), along
−CH −CH −CH −CHCH−CH −CH −CH −CH −), 32.0 (C ,
2
2
2
2
2
2
2
2
p
with sodium acetate (0.03 equiv), was then added, and the reaction
mixture was kept under magnetic stirring in a dry argon atmosphere at
CH −CH −CH −), 97.0 (C , −(CO)−O−CHCH ), 130 (C ,
3 2 2 t 2 i,j
−CH −CHCH−CH −), 141.4 (C , −(CO)−O−CHCH ),
2 2 s 2
1
00 °C for 16 h. After reaction, the mixture was poured in water and
170.9 (C , −O−C(O)−).
a
extracted with dichloromethane, and the organic fraction was dried
over anhydrous sodium sulfate before removing the solvent in a rotary
evaporator. Silica-gel column chromatography was used to purify the
General Procedure for the Chemical Functionalization of
Microcrystalline Cellulose. Cellulose Activation. Microcrystalline
cellulose (MCC) was first activated by mercerisation to make its
B
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the vibration at 1060 cm− associated with C−O stretching of the
1
cellulose backbone, which is used as an internal standard.
RESULTS AND DISCUSSION
Chemical Synthesis and Spectroscopic Character-
ization of Cellulose Precursor. For comparison with the
■
Figure 3. 13C NMR spectra of oleic acid (OA) and vinyl oleate (VO)
in CDCl and peak assignments.
3
hydroxyl groups accessible for reaction. A 20 g MCC was mercerised
in 1 L of sodium hydroxide solution (4 M) and then filtered and
washed by a series of dehydrating solvents, methanol, acetone. and
2
hexane. The sample was then oven-dried at 80 °C for 24 h and was
stored under vacuum in a desiccator.
Synthesis of FACEs. Several experiments were carried out, varying
the reaction time and temperatures. Activated MCC (1 g) was placed
in a 100 mL round-bottomed flask, containing a solution of 7 mmol of
VO per 1 g of MCC, 0.15 g K CO (catalyst), and 20 mL of NMP.
2
3
The mixture was then heated under reflux with vigorous magnetic
agitation for different reaction times ranged from 1 to 24 h and
temperatures from 90 to 125 °C. At the end of reaction, the flask
content was filtered and excessively washed with water and ethanol. To
remove any residual traces of unreacted VO and solvent, the modified
MCC was Soxhlet-extracted with ethanol for 6 h and then dried at 80
Figure 4. FTIR absorbance spectra of unmodified and modified MCC
with vinyl oleate (VO-modified) for 3, 6, 12, and 24 h at 90 °C.
classical acylation of cellulose with fatty acids by means of acyl
chloride, the corresponding long-chain vinyl ester was
synthesized. The commercially available OA was functionalized
by transesterification with vinyl acetate to obtain VO according
to the procedure presented in Scheme 1A. The structure of the
new biobased long-chain vinyl ester was confirmed by the
combined spectroscopic analyses reported above.
°
C for 24 h.
Kinetics of the Reaction and the Effect of Reaction Temperature.
To evaluate the synthesis efficiency to afford FACEs, the esterification
reaction was conducted for 3 h at either series stepwise temperature
increases (90−125 °C) or at 90 °C at varying reaction times. After
reaction, the modified MCC was analyzed by FTIR spectroscopy, and
the characteristic vibrations of the grafted acyl groups were used to
estimate the extent of grafting over reaction times and temperatures.
The influence of increasing reaction time or temperature was
Compared with the FTIR spectra of the starting OA, the
spectra of the synthesized VO showed the emergence of new
characteristic absorption bands (Figure 1). These new
quantified by calculating the peak height ratio of I
and
CO/C−O
−1
absorption bands were observed around 3091 cm (vinyl
plotting this ratio as a function of reaction time and reaction
temperature (Figure 6). This method was adopted as a valid means of
quantifying the extent of esterification and investigating reaction
−
1
C−H stretching), 3006 cm (chain unsaturation C−H
−
1
stretching), 2923 and 2854 cm (saturated CH stretching
2
29
−1
kinetics of cellulose modification. The method consists of comparing
modes), 1758 cm (CO stretching of vinyl ester), 1646
−1
−1
the intensity of the CO vibration of the grafted acyl moieties with
cm (non−conjugated CC stretching), 1464 cm (H
Scheme 1. Synthesis of Vinyl Ester of Oleic Acid (A) Followed by the Cellulose Functionalization Procedure (B)
C
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Figure 7. 13C CP-MAS NMR spectra of unmodified and modified
MCC with vinyl oleate for 1, 3, 6, 12, and 24 h at 90 °C.
Figure 5. FTIR absorbance spectra of unmodified− and modified
MCC with vinyl oleate (VO-modified) at 90, 105, 115, and 125 °C for
3
h.
Figure 6. Peak heights ratio (I
and temperature for MCC modified with vinyl oleate.
) as a function of reaction time
CO/C−O
−
1
bending of CH and CH groups), 1141 cm (ester C−O
2
3
−1
stretching), 949 cm (CH out-of-plane deformation of CH
−1
CH ), 868 cm (CH out-of-plane deformation of −CH
2
2
−1
30,31
CH ), and 722 cm (skeletal vibration of −(CH ) −).
In
2
2 n
addition to the appearance of new characteristic bands, a
−
1
disappearance of carboxyl groups at ∼2672 cm (OH
−1
stretching) and at 1708 cm (CO stretching) was observed,
confirming the complete transformation of OA into VO.
1
The transvinylation reaction was further confirmed by H
NMR and 13C NMR. The H NMR spectra of the synthesized
VO showed the emergence of new resonances typical for the
vinyl ester protons (δ 4.7, 5.1, and 7.5), in addition to those of
the starting fatty acid precursor (Figure 2).
1
Figure 8. 13C CP-MAS NMR spectra of unmodified and modified
MCC with vinyl oleate at 90, 105, 115, and 125 °C for 3 h.
Esterification of Cellulose by Vinyl Oleate and
Spectroscopic Characterization. A biobased and more
sustainable route to FACEs was investigated by using a
transesterification process under heterogeneous conditions.
The VO was chosen as long acyl chain donor for its known
capacity to increase the hydrophobicity of polar biomolecules.
Reactions were performed according to Scheme 1B, with
potassium carbonate as a mild catalyst and NMP as low toxic
13
The C NMR analysis of the synthesized VO (Figure 3) was
in agreement with the evidence shown by H NMR (Figure 2).
1
In addition to the unchanged resonances related to the fatty
acid, spectra of the synthesized VO showed a new group of
signals resonating at δ 97.0 and 141.4, which were identified as
methylenic and methinic carbons of the vinyl ester group
32
(Figure 3).
D
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Figure 9. TGA thermograms of unmodified (MCC) and modified MCC with vinyl oleate for 1, 3, 6, 12, and 24 h at 90 °C (A) and for 3 h at 90,
05, 115, and 125 °C (B).
1
Figure 10. DTG thermograms of unmodified (MCC) and modified MCC with vinyl oleate for 1, 3, 6, 12, and 24 h at 90 °C (A) and for 3 h at 90,
05, 115, and 125 °C (B).
1
functionalized MCC with vinyl oleate (VO-modified). In
addition to the prominent carbonyl stretching vibration at
−
1
1
1
735−1743 cm (ν
) and the C−O stretching vibrations at
CO
C−O
157−1230 cm− (ν ), some new characteristic vibrations of
the grafted aliphatic acyl chain moieties were identified, namely,
1
the strong skeletal bands of long-chain moieties at 2922 and
853 cm− (νC−H), 1370 cm (γC−H), and 720 cm (CH
1
−1
−1
2
2
−
1
rocking). Additionally, a new vibration arose at 3010 cm ,
which is a characteristic band of unsaturated function (H−C
5
,31,33
C) of oleic chain.
These bands confirm the esterification
reaction in VO-modified MCC. Similar FTIR features were
observed when fatty acid moieties were introduced into the
5
−18,21
cellulose using acyl chlorides as a precursor.
Figure 11. X-ray diffraction patterns of unmodified (MCC) and
modified MCC with vinyl oleate for 1, 3, 6, 12, and 24 h at 90 °C (A)
and for 3 h at 90, 105, 115, and 125 °C (B).
The extent of grafting of fatty chain into cellulose’s hydroxyl
groups over reaction time and temperature was quantified
through the calculation of the I
peak heights ratio
CO/C−O
(
Figure 6). By increasing both reaction time and temperature,
solvent. Various reaction times (1, 3, 6, 12, or 24 h) and
temperatures (90, 105, 115, or 125 °C) were investigated. The
selected esterification method offers the possibility to graft a
large variety of biobased saturated and unsaturated long-chain
moieties into lignocellulosic materials. Unlike classical methods
involving acyl chlorides, the current method generates vinyl
alcohol as a nonacidic byproduct, which tautomerizes to the
highly volatile acetaldehyde, and thus the equilibrium is
naturally shifted toward the ester formation (Scheme 1B).
The modified cellulose samples were first characterized by
FTIR and 13C CP-MAS spectroscopy. Figures 4 and 5 depict
the FTIR spectra of unmodified MCC (unmodified) and
the I
ratio gradually increased, indicating that the
CO/C−O
MCCs were increasingly esterified. However, the effect of
reaction temperature was more prominent than that of reaction
time; that is, the diffusion of the VO into the cellulose hydroxyl
groups is more favorable by increasing reaction temperature.
This difference may result from the diffusion mechanism that
governs the accessibility of hydroxyl groups as well as the size
effect of the reagent. This is possible as the three hydroxyls on
an anhydroglucose unit of cellulose are not equally accessible.
The primary hydroxyl is more reactive than the two secondary
hydroxyls due to the stereochemical hindrance.
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VO-modified MCC samples were further characterized by
C CP-MAS NMR spectroscopy (Figures 7 and 8). In addition
a variety of plant polyunsaturated fatty acids can be
functionalized and utilized to produce multifunctional
cellulose-based materials for optical, electronic, and selective
separation applications.
1
3
to the dominant pattern corresponding to the carbons of
cellulose, signals for carbons of the newly introduced long chain
oleic moieties appeared clearly in the VO-modified MCC
spectra. These signals were assigned directly on the spectra of
VO-modified MCC according to the nomenclature given on
the associated structure.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: mohamed.jebrane@slu.se.
Thermal Properties of the Synthesized Fatty Acid
Cellulose Esters. The TGAs of native MCC and VO-modified
MCC at different reaction times and temperatures were
performed under continuous nitrogen to study the decom-
position properties of the FACEs. The reported weight loss was
normalized against the initial weight of the analyzed samples
ORCID
Mohamed Jebrane: 0000-0003-0173-9467
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(Figure 9). The analyses showed that thermal degradation of
We gratefully acknowledge Gulaim Seisenbaeva from the
Department of Chemistry and Biotechnology, Swedish
University of Agricultural Sciences (Sweden) for her technical
assistance with the TGA experiments. Furthermore, we thank
Dmitri Panov from Faculty of Science and Technology,
Institute of Chemistry, Tartu University (Estonia) for liquid
NMR analyses. The work in Tallinn was supported by the
Estonian Research Council Grant IUT23-7 and the European
Regional Development Fund project TK134.
native MCC starts at 348 °C following a single weight-loss step.
The VO-modified MCCs were, however, less stable becaue they
started to decompose at temperatures substantially lower than
that of unmodified MCC. These results were expected because
the grafting of long-chain acyl groups into cellulose’s hydroxyl
groups resulted in significant decrease in crystallinity.
In addition to the lower degradation temperatures, modified
MCC showed two main separate degradation steps, attributed
to degradation of grafted fatty acid chain and cellulose
backbone (Figure 10). Similar results were reported in the
literature when cellulose is functionalized with fatty acid
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1
(
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CONCLUSIONS
Functionalization of cellulose with VO in the presence of low
toxic solvent (NMP) and mild catalyst (K CO ) was found to
■
(
14) Crep
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́
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2
3
be highly effective, avoiding the problems of cellulose
degradation and complex methods. Esterification of cellulose
via the vinyl ester of fatty acid means appeared to be an efficient
method for grafting a variety of biobased long chain moieties
into the cellulose, resulting in FACEs with relatively high
grafting rates. The concept comprises a synthesis of vinyl ester
of fatty acid from cheaply available vinyl acetate and a long-
chain fatty acid, followed by a covalent incorporation of the
fatty chain onto the cellulose via transesterification reaction
between cellulose’s hydroxyl groups and the synthesized vinyl
ester. The efficiency of the reaction was found to increase with
increasing reaction time and temperature. However, the effect
of increasing reaction temperature was found to have a
significant effect on the grafting rates. Using this new approach,
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