Esterification mechanism of lignin with different catalysts based on lignin model compounds by mechanical activation-assisted solid-phase synthesis

Article abstract of DOI:10.1039/c7ra10482k

In order to learn about the esterification mechanism of lignin by mechanical activation-assisted solid-phase synthesis (MASPS) technology, lignin model compounds, p-hydroxy benzaldehyde (H), vanillin and vanillyl alcohol (G), and syringaldehyde (S), were used in the reaction with acetic anhydride, with 4-dimethyl amino pyridine (DMAP), sodium acetate, and sulfuric acid as catalysts. FTIR, NMR, and UV/vis analyses of the products showed that all of the catalysts could enhance the esterification. Both the phenolic hydroxyl and aliphatic hydroxyl participated in the esterification and the reactivity of the basic structural units of lignin had a descending order of H, G, and S. Oxidations could happen in the presence of unsaturated groups such as aldehyde in the lignin model compounds. The catalytic mechanism of the three kinds of catalyst was different, and the catalytic activity had a descending order of DMAP, sodium acetate, and sulfuric acid. The reactivity of phenolic hydroxyl was higher than that of aliphatic hydroxyl with DAMP as the catalyst, but the reactivity of aliphatic hydroxyl was higher than that of phenolic hydroxyl with sodium acetate or sulfuric acid as the catalyst. With sulfuric acid as the catalyst, some side reactions took place and resulted in the ring cleavage or cross-linking of the benzene ring. Consistency verification indicated that the use of lignin model compounds for studying the esterification mechanism of lignin was reasonable and feasible.

Full text of DOI:10.1039/c7ra10482k

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
RSC Advances
View Article Online
View Journal | View Issue
PAPER
Esterification mechanism of lignin with different
catalysts based on lignin model compounds by
Cite this: RSC Adv., 2017, 7, 52382
mechanical activation-assisted solid-phase
synthesis†
a
a
Xiaohong Zhao,ab Yanjuan Zhang, Liping Wei,a Huayu Hu,a Zuqiang Huang,
*
*
Mei Yang,a Aimin Huang,a Juan Wua and Zhenfei Fenga
In order to learn about the esterification mechanism of lignin by mechanical activation-assisted solid-phase
synthesis (MASPS) technology, lignin model compounds, p-hydroxy benzaldehyde (H), vanillin and vanillyl
alcohol (G), and syringaldehyde (S), were used in the reaction with acetic anhydride, with 4-dimethyl
amino pyridine (DMAP), sodium acetate, and sulfuric acid as catalysts. FTIR, NMR, and UV/vis analyses of
the products showed that all of the catalysts could enhance the esterification. Both the phenolic
hydroxyl and aliphatic hydroxyl participated in the esterification and the reactivity of the basic structural
units of lignin had a descending order of H, G, and S. Oxidations could happen in the presence of
unsaturated groups such as aldehyde in the lignin model compounds. The catalytic mechanism of the
three kinds of catalyst was different, and the catalytic activity had a descending order of DMAP, sodium
acetate, and sulfuric acid. The reactivity of phenolic hydroxyl was higher than that of aliphatic hydroxyl
with DAMP as the catalyst, but the reactivity of aliphatic hydroxyl was higher than that of phenolic
hydroxyl with sodium acetate or sulfuric acid as the catalyst. With sulfuric acid as the catalyst, some side
reactions took place and resulted in the ring cleavage or cross-linking of the benzene ring. Consistency
verification indicated that the use of lignin model compounds for studying the esterification mechanism
of lignin was reasonable and feasible.
Received 21st September 2017
Accepted 7th November 2017
DOI: 10.1039/c7ra10482k
rsc.li/rsc-advances
Chemical modication of lignin can efficiently improve its
performances and expand its applications. More superior
1. Introduction
lignin-based products can be gotten from modied lignin such
as aminated lignin, hydroxymethylated lignin, methyl methac-
rylate (MMA) graed lignin, and urea-formaldehyde modied
lignin.6–8 Furthermore, more excellent characteristic(s) of
modied lignin endow its more applications. For example,
modied lignin can be used as a curing agent or exibilizer of
epoxy resins.9
Esterication of lignin is commonly used for three objec-
tives: the analysis of hydroxyl content, the modication of
lignin, and the gra of lignin with other polymer.10–12 As
a method of modication, it can improve the photothermal
stability, water resistance, oxidation resistance, compatibility
with non-polar polymers, and dissolution in non-polar
solvents of lignin.13–15 The esterication is usually carried out
in liquid phase with acid, acid anhydride, or acyl chloride as
esterifying agent, pyridine, 4-dimethyl amine pyridine
(DMAP), or 1-methyl imidazole as catalyst, and acid anhydride,
pyridine, THF, 1,4-dioxane, or N-methyl pyrrolidone as
solvent.16,17 The reaction usually needs a long time and the
recovery of modied lignin is time-consuming and tedious. So,
some new preparation methods such as supercritical carbon
Lignocellulosic biomass, as a sustainable alternative for energy,
fuel and chemical production, attracts a lot of attention.1,2
Lignin is one of the three main components of lignocellulosic
biomass.3 It is estimated that more than 150 billion tons of
lignin are produced annually by plant growth worldwide.4 More
than 60 billion tons of industrial lignin was obtained as
a byproduct of pulp in paper-making and biofuel industries, but
only approximately 2% was used for producing lignin-based
materials due to the deciency in properties of lignin,
including brittleness, low reactivity, poor compatibility with
polymers, etc., limiting its application scope.5 The remainder
mainly has been burned as an energy source or discarded as
waste, leading to the waste of resources and growing environ-
mental problems.
aSchool of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004,
China. E-mail: zhangyj@gxu.edu.cn; huangzq@gxu.edu.cn
bCollege of Materials and Environmental Engineering, Hezhou University, Hezhou
542899, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra10482k
52382 | RSC Adv., 2017, 7, 52382–52390
This journal is © The Royal Society of Chemistry 2017

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
View Article Online
Paper
RSC Advances
dioxide as solvent, microwave, and reactive extrusion have CH3COONa $ 99%) and were obtained commercially without
been paid close attention.18–20
further purication.
In order to strengthen the activity of lignin and avoid the
solvent pollution in liquid synthesis and tedious collection of
the product, an environmentally friendly and effective
mechanical activation-assisted solid-phase synthesis (MASPS)
technology was adopted to prepare acetylated lignin in our
previous work,21 and the esterication mechanism of lignin was
discussed. However, it is difficult to get comprehensive infor-
mation of the mechanism due to its complicated structure, and
more feasible ways are required to further investigate the
esterication mechanism of lignin.
Lignin is composed of three phenyl propane units of p-
hydroxyphenyl (H), guaiacyl (G), and syringyl (S), which con-
nected with C–O–C and C–C.22 Lignin model compounds, such
as the monomers of these three phenyl propane units or their
dimer, trimer, tetramer and hexamer, are usually used to study
the correlated theory of lignin for their denite structure.23 For
example, lignin model compounds have been used to study the
pyrolysis chemistry, bond dissociation enthalpies, laccase
promoted oxidation, peroxidative oxidation, aerobic oxidation,
and reductive degradation of lignin.24–27 The degree of reaction
and related chemical metrology data are easily monitored and
the products are simple with the use of lignin model
compounds. The physical and chemical properties of lignin can
be speculated based on those of its model compounds, leading
to further promote the research on application performances of
lignin and broaden the application elds.
In this paper, four lignin model compounds, p-hydroxy
benzaldehyde (H), vanillin and vanillyl alcohol (G), and syrin-
galdehyde (S) (their structures are shown in Fig. S1†), were
acetylated by MASPS with DMAP, sodium acetate, and sulfuric
acid as catalysts. The resulting samples were analyzed by
Fourier transform infrared spectroscopy (FTIR), nuclear
magnetic resonance (NMR) and Ultraviolet/visible (UV/vis)
spectrometer. The reactivity of basic structure units (H, G, and
S) was discussed by comparing the three kinds of aldehyde, and
the reactivity of hydroxyl groups was discussed by comparing
two representative compounds of G type. For systematic inves-
tigation on the acetylation of typical lignin model compounds,
the comprehensive information of esterication mechanism of
lignin by MASPS could be obtained to give more theoretical
support for chemical modication of lignin.
2.2. Esterication of lignin model compounds
The esterication of lignin model compounds by MASPS was
carried out in the same customized stirring ball mill (reactor) as
the esterication of lignin.28 In order to learn about the catalytic
action of DMAP, two feeding ways were used. In one way, the
reactants and DMAP were added in the reactor at the same time.
In the other way, one reactant and DMAP were pre-reacted for
5 min and then the other reactant was added to the reactor.
When other two catalysts (sodium acetate and sulfuric acid)
were used, the reactants and catalyst were simultaneously
added to the reactor. A xed amount of milling balls (300 mL,
the ratio of milling balls to material was 6 mL gꢀ1) was rst
added to a jacketed stainless steel tank (1200 mL), and then
a mixture of lignin model compound, acetic anhydride (n(acetic
anhydride) : n(lignin model compounds) ¼ 3), and catalyst
(2 wt% of the reactants) was added to the tank and subjectedꢁto
milling at the speed of 300 rpm with the temperature of 80 C
for 1.5 h. The crude products were rened by dissolving in
alcohol and precipitated with water. Specically, the milling
balls were washed with 50 mL anhydrous ethanol, then 50 mL
water was added into the wash solution and le to stand until
the product were precipitated out. The suspension was ltered
by vacuum ltration and the lter cake was washed with
deionized water until the ltrate was neutral. In order to avoid
being oxidized, the lter cake was vacuum dried at 35 ꢁC for
24 h, and then the product was sealed with a sealing bag and
stored in a silica-gel desiccator.
2.3. Characterization
Chemical structure and quantitative analysis of the esteried
samples were carried out by FTIR, NMR and UV/vis spectrom-
eter. The operating conditions of these analyses were same as
the esteried lignin and provided in ESI.†13,28
3. Results and discussion
There are a lot of catalysts for biomass conversion, including
metal catalysts, base catalysts, acid catalysts, and so on.1,2,29,30
Either acid catalyst or alkaline catalyst is effective for the
esterication of lignin.31 In order to further learn about the
esterication mechanism of lignin by MASPS, the esterication
of lignin model compounds were systematically studied with
DMAP (usual base) and sodium acetate (salt of strong alkali
weak acid) as representatives of alkaline catalyst and sulfuric
acid as a representative of acidic catalyst.
2. Experimental
2.1. Materials
The p-hydroxy benzaldehyde (98%), vanillin ($99%), vanillyl
alcohol (98%), and syringaldehyde (98%) were purchased from
Aladdin Industrial Corporation. Enzymatic hydrolysis lignin
(95.92% of lignin (10.60% of acid soluble and 85.32% of klason
lignin), 1.05% of ash, and 3.02% of polysaccharide, with pH of
3.1. FTIR analysis
7) and alkali lignin (90.45% of lignin (13.43% of acid soluble The FTIR spectra of lignin model compounds and their esters
and 77.02% of klason lignin), 8.085% of ash, and 8.11% of catalyzed by DMAP are shown in Fig. 1, and those of the ester-
polysaccharide, with pH of 7) were kindly supplied by Ji'nan ied samples catalyzed by sodium acetate and sulphuric acid
Yang Hai Chemical Co., Ltd. (China). All other chemical are shown in ESI (Fig. S2 and S3).† The main assignments are
reagents were of analytical grade (DMAP >99.5%, H2SO4 >98%, presented in Table 1.
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 52382–52390 | 52383

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
View Article Online
RSC Advances
Paper
Fig. 1 FTIR spectra of lignin model compounds and their esters catalyzed by DMAP: (a) vanillin, (b) acetylated vanillin, (c) vanillyl alcohol, (d)
acetylated vanillyl alcohol, (e) p-hydroxy benzaldehyde, (f) acetylated p-hydroxy benzaldehyde, (g) syringaldehyde, and (h) acetylated
syringaldehyde.
Table 1 Main FTIR bands assignment of lignin model compounds and
their esters
The peaks of the products catalyzed by sodium acetate were
nearly the same as those catalyzed by DMAP (Fig. S2†), illus-
trating the same functional groups in these products. But some
differences in the FTIR spectra of the products with different
catalysts could be observed. Different extent of esterication
and oxidation led to different intensity of the absorbance of
ester groups and carboxyl groups, and the peak of hydroxyl in
the products catalyzed by sodium acetate was still obvious. The
absorption peaks of aliphatic ester link and phenolic ester link
in acetylated vanillyl alcohol catalyzed by DMAP nearly showed
the same height, but the absorbance of aliphatic ester link was
stronger than that of phenolic ester link in the products cata-
lyzed by sodium acetate. So the catalytic activity and selectivity
of these two kinds of alkaline catalysts for esterication of
lignin model compounds were different. DMAP possessed
higher activity and selectivity than sodium acetate.
The difference between the products catalyzed by alkaline
catalysts and acidic catalyst was also obvious. For the products
catalyzed by sulphuric acid (Fig. S3†), there was nearly no
absorption peak of anhydride in acetylated vanillin and p-
hydroxy benzaldehyde, and the characteristic peaks of carbonyl
became wide. The absorbance of aliphatic ester link was
stronger than that of phenolic ester link in acetylated vanillyl
alcohol. The characteristic absorption peaks of acetylated
syringaldehyde catalyzed by sulphuric acid were similar with
those catalyzed by alkaline catalysts. The peak intensity of
hydroxyl was also very weak because most of the hydroxyl
groups participated in the reaction. The characteristic peaks of
alkyl became complex for the presence of part ring-opening or
coupling reaction. New aldehyde or ketone might be generated
Band position
(cmꢀ1
)
Assignment
3400
2939
1760
1740
1800
1597
1510
1459
1222
O–H stretching of aromatic and aliphatic OH groups
Alkyl group
C]O stretch of phenolic ester
C]O stretch of aliphatic ester
Characteristic band of anhydride
Aromatic skeletal vibration
Aromatic skeletal vibration
O–CH3 deformation
C–O–C of aromatic acetyl groups
As shown in Fig. 1, three regions assigning to O–H, C]O and
C–O–C had obvious changes for the products catalyzed by DMAP.
The C]O stretch of phenolic ester appeared in all the esteried
lignin model compounds for the esterication of phenolic
hydroxyl, and a C]O stretch of aliphatic ester peak appeared in
esteried vanillyl alcohol due to the reaction of aliphatic
hydroxyl. Moreover, the esteried vanillin and p-hydroxy benz-
aldehyde exhibited the characteristic peak of anhydride. It should
not be the residual acetic anhydride since it did not appear in
other esters as the same process was carried out. This may due to
that the oxidation of aldehyde generated COOH groups, which
could form anhydride by dehydration during the following drying
process. For all the esteried lignin model compounds, the
intensity of hydroxyl decreased greatly for the esterication of
hydroxyl groups. Esterication also led to the enhancement in
peak intensity of C–O–C of aromatic acetyl groups.
52384 | RSC Adv., 2017, 7, 52382–52390
This journal is © The Royal Society of Chemistry 2017

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
View Article Online
Paper
RSC Advances
Fig. 2 1H-NMR spectra of acetylated lignin model compounds catalyzed by DMAP: (a) acetylated vanillin, (b) acetylated vanillyl alcohol, (c)
acetylated p-hydroxy benzaldehyde, and (d) acetylated syringaldehyde.
and resulted in the shi of the characteristic peak of carbonyl to characteristic peaks were visible. The peaks at 1.89 ppm
lower wavenumber. Based on the above comparative analysis of (CH3COO), 3.80 ppm (CH3O), 7.18 ppm (m- and p-aromatic
different FTIR spectra, it could be concluded that some side hydrogen in benzene of CH3O), 7.60 ppm (o-aromatic hydrogen
reactions happened during the esterication of lignin model in benzene of CH3O), and 12.00 ppm (COOH) appeared in the
compounds with sulphuric acid as catalyst.
acetylated vanillin. The peaks at 2.08 ppm (CH3COO in alcohol
ester), 2.40 ppm (CH3COO in phenol ester), 3.80 ppm (CH3O),
7.15, 7.0 and 6.95 ppm (m-, p- and o-aromatic hydrogen of CH3O
in benzene) appeared in the acetylated vanillyl alcohol. The
3.2. NMR analysis
NMR spectra of acetylated lignin model compounds are shown peaks at 1.90 ppm (CH3COO), 8.00 and 7.20 ppm (m- and
in Fig. 2 and 3 and Fig. S4–S7.† As shown in Fig. 2, all the o-aromatic hydrogen of ester group in benzene), and 12.00 ppm
Fig. 3 13C-NMR spectra of acetylated lignin-related model compounds catalyzed by DMAP: (a) acetylated vanillin, (b) acetylated vanillyl alcohol,
(c) acetylated p-hydroxy benzaldehyde, and (d) acetylated syringaldehyde.
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 52382–52390 | 52385