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RSC Advances
dioxide as solvent, microwave, and reactive extrusion have CH3COONa $ 99%) and were obtained commercially without
been paid close attention.18–20
further purication.
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 esterication 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
esterication 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 denite 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 esterication mechanism of
lignin by MASPS could be obtained to give more theoretical
support for chemical modication of lignin.
2.2. Esterication of lignin model compounds
The esterication of lignin model compounds by MASPS was
carried out in the same customized stirring ball mill (reactor) as
the esterication 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 rened by dissolving in
alcohol and precipitated with water. Specically, 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 esteried
samples were carried out by FTIR, NMR and UV/vis spectrom-
eter. The operating conditions of these analyses were same as
the esteried 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
esterication of lignin.31 In order to further learn about the
esterication mechanism of lignin by MASPS, the esterication
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 ied 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
Zhao, Xiaohong
