Sulfonic acid functionalized deoxycellulose catalysts for glycerol acetylation to fuel additives

Article abstract of DOI:10.1016/j.apcata.2014.05.018

Various forms of sulfonic acid (SO₃H) functionalized cellulose-based heterogeneous catalysts were prepared, and their properties on glycerol acetylation reactions to mono-, di-, and triacetin products were investigated. Superior catalytic activity and stability for the reactions was observed on SO₃H-NDOC catalyst that is obtained by (i) crosslinking of the cellulose units with epichlorohydrin into a three-dimensionally-networked deoxycellulose-matrix and (ii) sulfonic acid functionalization on the crosslinked deoxycellulose support via CS atomic linkages (CSO₃H). It was found that sulfonic acid functionalization via an ether type OS atomic linkage (COSO₃H) or the direct utilization of the pristine cellulose support without a fortification by crosslinking led to significant deactivation of the catalysts by substantial acidity loss. The intrinsic glycerol conversion turnover rate on SO₃H-NDOC was comparable to that of commercial Amberlyst-15 solid resin, demonstrating the effectiveness of this renewable cellulose-derived heterogeneous catalyst for glycerol acetylation to fuel additives.

Full text of DOI:10.1016/j.apcata.2014.05.018

Applied Catalysis A: General 482 (2014) 31–37
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Sulfonic acid functionalized deoxycellulose catalysts for glycerol
acetylation to fuel additives
1
1
∗
Inbae Kim , Jaesung Kim , Doohwan Lee
Catalysis and Nanomaterials Laboratory, Department of Chemical Engineering, University of Seoul, Siripdae-gil 13, Jeonnong-dong,
Dongdaemun-gu, Seoul 130-743 Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Various forms of sulfonic acid ( SO H) functionalized cellulose-based heterogeneous catalysts were
prepared, and their properties on glycerol acetylation reactions to mono-, di-, and triacetin products
were investigated. Superior catalytic activity and stability for the reactions was observed on SO3H-
NDOC catalyst that is obtained by (i) crosslinking of the cellulose units with epichlorohydrin into a
three-dimensionally-networked deoxycellulose-matrix and (ii) sulfonic acid functionalization on the
3
Received 22 January 2014
Received in revised form 12 May 2014
Accepted 16 May 2014
Available online 27 May 2014
crosslinked deoxycellulose support via C S atomic linkages (
C
SO3H). It was found that sulfonic acid
Keywords:
Cellulose
Glycerol acetylation
Heterogeneous catalyst
Sulfonic acid functionalization
Renewable energy
functionalization via an ether type O S atomic linkage (
C
O SO3H) or the direct utilization of the pris-
tine cellulose support without a fortification by crosslinking led to significant deactivation of the catalysts
by substantial acidity loss. The intrinsic glycerol conversion turnover rate on SO3H-NDOC was compa-
rable to that of commercial Amberlyst-15 solid resin, demonstrating the effectiveness of this renewable
cellulose-derived heterogeneous catalyst for glycerol acetylation to fuel additives.
©
2014 Elsevier B.V. All rights reserved.
1
. Introduction
of ␣-amino nitriles [5], quinolines [6], and amide and amine deriva-
tives [7]. The SO H-Cell catalyst was also effectively applied for the
synthesis of 1,4-dihydropyridine by Safari et al. [8] and of 2,4,5-
triarylimidazoles by Shingare et al. [9]. These previous studies on
3
Cellulose is abundant natural resource that is being utilized as
a raw material for various value-added products. In recent years,
its non-conventional utilization as a feedstock for liquid biofuel and
bio-derived chemicals is being extensively explored due to growing
concern related to sustainability of the current petroleum-based
energy and materials infrastructure [1,2]. In another respect, cel-
lulose is of particular interest as a bio- or eco-friendly alternative
for versatile applications in the fields of medical, pharmaceutical,
and specialty materials [3]. Cellulose is a natural polymer con-
sisting of three hydroxyl groups per anhydroglucose unit (AGU)
that enable functionalization of the structure for use in hetero-
the SO H-Cell catalyst are attractive considering its reasonable sta-
3
bility under reaction conditions that appear to be hostile to the
integrity of the catalysts. Specifically, the presence of acids, water,
and alcohols in reactants/products, or elevated reaction tempera-
tures reported therein may facilitate hydrolysis of the cellulose and
the immobilized sulfonic acid group.
Here we report the first synthesis and properties of
a
structurally-fortified sulfonic acid functionalized deoxycellulose-
based catalyst that is constructed by immobilization of SO H
3
geneous catalysts. Choplin et al. immobilized a Pd(OAc) /5-TPPTS
groups onto a three-dimensionally networked deoxycellulose
2
[
TPPTS = P(m-C H SO Na) ] catalytic precursor onto cellulose and
matrix via C S atomic linkages (referred to here as SO H-NDOC).
6
4
3
3
3
reported that the resulting material effectively catalyzed an allylic
alkylation reaction of (E)-cinnamyl ethyl carbonate with morpho-
line without sizeable leaching of Pd from the catalyst [4]. Shaabani
et al. reported a sulfonic acid functionalized cellulose catalyst
The catalyst was applied for glycerol acetylation with acetic acid for
the production of mono-, di-, and triacetin (Scheme 1). This catalytic
transformation is considered to be an effective route for utilization
of the excessive glycerol byproduct obtained from biodiesel syn-
thesis [10,11]. The reaction can be conducted in the liquid phase at
moderate temperatures below 373 K, and the acetylated-glycerol
products can be used as value-added fuel additives [12]. Properties
(
SO H-Cell) and its effective catalytic properties for the synthesis
3
of SO H-NDOC for the reaction are also compared with sulfonic acid
3
∗ Corresponding author. Tel.: +82 2 6490 2371; fax: +82 2 6490 2364.
functionalized deoxycellulose (SO H-DOC) and cellulose (SO H-
3
3
E-mail address: dolee@uos.ac.kr (D. Lee).
1
Cell) catalysts as well as commercial Amberlyst-15 acid resin.
I. Kim and J. Kim equally contributed to this work.
http://dx.doi.org/10.1016/j.apcata.2014.05.018
926-860X/© 2014 Elsevier B.V. All rights reserved.
0

3
2
I. Kim et al. / Applied Catalysis A: General 482 (2014) 31–37
Scheme 1. Glycerol acetylation with acetic acid to mono-, di-, and triacetin products.
2
2
2
. Experimental
2.2. Characterization of the catalysts
.1. Catalysts preparation
Brönsted acidity of the samples was characterized by the neu-
tralization titration method [14]. In the typical procedure, 0.05 g
of catalyst powder was treated with 20 ml of 1.0 M aqueous NaCl
solution for 1 h at room temperature with sonication. A longer
treatment had no effect on acidity of the samples. After centrifu-
gal separation, the supernatant solution was treated with a 0.01 M
aqueous NaOH solution using phenolphthalein as the neutraliza-
tion indicator. The method does not titrate the hydroxyl or thiol
groups as was confirmed from the measurements conducted on
the pristine cellulose and SH–DOC samples. Carbon and sulfur
contents in the dried catalyst samples were obtained by elemental
analysis (Flash 2000 CHNS analyzer, Thermo Scientific). The struc-
tures of pristine cellulose, crosslinked deoxycellulose (NDOC), and
.1.1. Preparation of SO H-Cell catalyst
3
The SO H-Cell catalyst was prepared according to the procedure
3
reported by Safari et al. [8]. Briefly, 5 g of microcrystalline cellu-
lose powder (Alfa, particle size <220 ␮m) was dispersed in 20 ml
of n-hexane (Aldrich, HPLC grade) with vigorous stirring in an ice
bath, and then 9 mmol of chlorosulfonic acid (Aldrich, 99%) was
added slowly in a drop-wise manner for 2 h. The resulting mixture
was stirred for an additional 2 h, filtered, and washed with acetoni-
trile (Aldrich, 99.9%). The SO H-Cell sample was dried at 313 K in a
convection oven for 12 h.
3
2
.1.2. Preparation of SO H-DOC catalyst
SO H-NDOC samples were analyzed by Fourier-transform infrared
3
3
The sulfonic acid functionalized deoxycellulose catalyst (SO H-
spectroscopy (Nicolet 6700, Thermo Scientific). The IR spectra were
obtained from thin wafers that were prepared by pelletizing mix-
ture powders of the dried sample and KBr. The porosity of the
samples was indirectly characterized by measuring the swelling
of the samples in the presence of water [15]. For the measurement,
0.15 g of dried catalyst was soaked into 20 ml of deionized water
for 48 h at room temperature and separated by centrifugation. The
degree of swelling was defined as a weight ratio of water absorbed
into the sample to the initial weight of dried catalyst, described as
3
DOC) was prepared as shown in Scheme 2(a). In the first step,
chlorodeoxycellulose (Cl–DOC) was prepared according to the pro-
cedure reported by Takagai et al. [13]. Briefly, 2 g of the cellulose
powder was dispersed in 50 ml of DMF (Junsei, 99.5%) in a reflux
condenser equipped flask at 353 K for 2 h with vigorous stirring.
Then, 1.8 ml of SOCl (Aldrich, 99%) was added and the mixture was
2
stirred for an additional 2 h. The resulting mixture was centrifuged
and washed with acetone and deionized water. In the second step,
the collected sample was dispersed in 180 ml of NMP (Junsei, 99%)
at 343 K for 30 min. The mercaptodeoxycellulose (SH–DOC) was
obtained by adding 4 g of NaHS (Aldrich) with stirring for 3 h. Then,
the mixture was centrifuged and washed with copious amounts
of H O. In the final step, the SO H-DOC was obtained after treat-
W − W
0
Sw =
(1)
W
0
where Sw is the degree of swelling, W is the weight of cellulose
catalyst after water absorption, and W0 is the initial weight of dried
catalyst.
2
3
ing the collected SH–DOC in 100 ml of H O2 (Junsei, 30%) at room
2
temperature for 5 h with stirring. The resulting catalyst powder was
centrifuged, washed with H O, and dried in a vacuum oven at 313 K.
2.3. Glycerol acetylation reaction
2
The acidity on the samples did not increase by an additional acidifi-
cation treatment with 0.1 M HNO3 solution for 12 h indicating that
the resulting sulfonic acid groups were completely protonated.
Glycerol acetylation with acetic acid was conducted in a 500 ml
three-neck round-bottom flask equipped with a reflux condenser
at atmospheric pressure. All the catalysts were utilized as powders.
Amberlyst-15 (Rohm & Hass) beads were crushed into powders
with sizes between 125 and 180 ␮m. The reactor was placed in
a heating mantle, and the inner temperature of the reactor was
isothermally maintained by PID control. In the typical procedure,
78.2 g of acetic acid (Alfa, 99.7%) and 1.0 g of the catalyst powder
were placed into the reactor at a constant temperature of 353 K, and
then 20.0 g of glycerol (Alfa, HPLC grade) at the same temperature
was added with stirring. The products were sampled periodically
during the reaction using a micro-syringe and were analyzed with a
flame ionization detector (FID) equipped gas chromatograph (Agi-
lent 7890A). After each reaction batch, the catalyst was collected by
centrifugation, washed with methanol (Amberlyst-15) or deionized
water (SO3H-Cell, SO3H-DOC, SO3H-NDOC) several times, and dried
in a vacuum oven at 323 K for 20 h for utilization in the subsequent
reaction batch. Glycerol conversion turnover rate (TOR) to com-
bined di- and triacetin products on SO3H-NDOC and Amberlyst-15
was obtained according to the following equation:
2
.1.3. Preparation of SO H-NDOC catalyst
3
The sulfonic acid functionalized three-dimensionally-
networked deoxycellulose catalyst (SO H-NDOC) was synthesized
3
as displayed in Scheme 2(b). The synthesis procedure and method
including the Cl [13], SH, and SO H functionalization were
3
the same to those applied for SO H-DOC, except that the func-
3
tionalization was conducted on the networked cellulose matrix
(
NDOC) which was prepared by crosslinking of cellulose units with
epichlorohydrin (EP) as a linking agent. Briefly, 4 g of microcrys-
talline cellulose particles was dispersed in 160 ml of 2.4 M NaOH
aqueous solution for 10 min. The dispersed cellulose particles were
completely dissolved into a homogeneous solution after a low
temperature treatment in a liquid N bath for 15 min. The resulting
2
cellulose solution was heated to 313 K, and various amounts of EP
(
Alfa, 99%) were added with vigorously stirring for 2 or 12 h. The
product was centrifuged, washed with copious amounts of water,
and dried in a vacuum oven at 323 K.
rate for di- and triacetin (mmol g 1
−
h
−1
)
Glycerol TOR = Production
(2)
Acidity on the catalyst (mmol g 1
−
)

I. Kim et al. / Applied Catalysis A: General 482 (2014) 31–37
33
Scheme 2. Synthesis procedure of (a) sulfonic acid functionalized deoxycellulose (SO3H-DOC) and (b) sulfonic acid functionalized three-dimensionally networked deoxy-
cellulose (SO3H-NDOC) catalyst.
Fig. 1. Photographs of the synthesis intermediates for SO3H-NDOC and the final forms of cellulose-based catalysts; (a) cellulose solution, (b) crosslinked cellulose matrix, (c)
SO3H-NDOC hydrogel, (d) SO3H-NDOC after drying, (e) SO3H-DOC after drying, (f) SO3H-Cell after drying.

3
4
I. Kim et al. / Applied Catalysis A: General 482 (2014) 31–37
antisymmetric bridge stretching (1164), and anhydroglucose ring
stretching (1059, 667, 559) vibration were identified [16–18]. The
overall spectral features of NDOC, and SO H-NDOC samples were
3
similar to that of pristine cellulose, but the absorption bands of
OH inplane deformation, C O antisymmetric bridge stretching,
and anhydroglucose ring stretching vibrations were not observed.
Differently, these IR bands were clearly observed on SO H-Cell
3
and SO H-DOC samples. The results were reasonable and support
3
the formation of three dimensionally networked cellulose chains
in NDOC and SO H-NDOC, because the crosslinking of cellulose
3
units would substantially hinder the vibration of anhydroglucose
ring structures. Notice that the characteristic absorption bands of
epoxy ring (1267, 854) and C Cl (761) stretching vibration pre-
sented in EP were not observed in NDOC [19]. The substitution
of Cl functional groups on Cl-NDOC was confirmed with the
−
1
characteristic C Cl stretching vibration (749, 719 cm ) [20], and
this did not appear on SH-NDOC and SO H-NDOC samples (the
3
IR spectra of Cl-NDOC and SH-NDOC are shown in Fig. S2). The
formation of sulfonic acid functional group on DOC and NDOC
via C S atomic linkage (
C SO H) was confirmed in SO H-DOC
3
3
−
1
and SO H-NDOC catalyst, on which C SO H (620 cm ) and S
O
3
3
(
C
1035) stretching vibrations were observed [14,21]. In addition, the
O stretching vibration (1724 cm ) band appeared on the sam-
−
1
ple indicated that some parts of unsubstituted hydroxyl groups on
SO3H-DOC and SO3H-NDOC samples oxidized further into ketone
groups [22].
Fig. 2. FT-IR spectra of SO3H-NDOC, NDOC, SO3H-DOC, SO3H-Cell, pristine cellulose,
and epichlorohydrin (EP).
Properties of the resulting SO H-NDOC are different from those
3
. Results and discussion
3
of SO H-DOC and SO H-Cell, as compared in Table 1, in which three
3
3
SO H-NDOC samples prepared at varied EP/AGU mole ratios and
3.1. Characteristics of the catalysts
3
crosslinking reaction time are displayed. The degree of swelling,
which is indicative of the porosity of the sample, increased by
approximately 140% with an increase in crosslinking time from
Photographs of homogenous cellulose solution, crosslinked cel-
lulose matrix, and the final forms of SO H-NDOC catalyst (before
3
2
h to 12 h at a EP/AGU ratio of 10. Noticed that EP does not
and after drying) described in Scheme 2 are presented in Fig. 1
photographs of the Cl-NDOC and SH-NDOC can be found in Sup-
homogeneously crosslink and was used in excess amounts in the
preparation. An increase in the EP/AGU ratio from 10 to 15 did not
(
plementary information, Fig. S1). The final dried form of SO H-DOC
3
affect the degree of swelling. The acidities of SO H-NDOC sam-
and SO H-Cell catalysts are also displayed for comparison. The
3
3
ples were lower than that of SO H-DOC, which was reasonable
results clearly demonstrate the formation of a cellulose hydrogel
by networking of the dispersed cellulose units, where the result-
ing matrix was stable under the subsequent deoxy-chlorination,
3
considering that some parts of the hydroxyl groups presented in
the pristine cellulose units were consumed by crosslinking with
EP, resulting in a decrease in the number of sites available for sul-
thiolation, and oxidation steps for the preparation of SO H-NDOC
3
fonic acid functionalization. The acidity of the SO H-Cell was low
catalyst. In contrast, the deoxychlorination of homogenously dis-
3
because the–SO H substitution on this catalyst could occur only on
persed cellulose for SO H-DOC that was conducted without prior
3
3
the near surface of the microcrystalline cellulose support. It also
crosslinking of the cellulose chains resulted in precipitation of
the chlorinated deoxycellulose powders with completely different
should be noted that the sulfonic acid functional groups on SO H-
3
NDOC and SO H-DOC were linked to the cellulose matrix via C
S
morphological features. The FT-IR absorption spectra of SO H-
3
3
atomic bonding, whereas those on the SO H-Cell were linked to the
NDOC, NDOC, SO H-DOC, SO H-Cell, pristine cellulose, and EP
3
3
3
matrix via C
O S atomic bonding. As a consequence, their stabil-
shown in Fig. 2 evidently indicate the crosslinking of cellulose
units (NDOC) and the formation of sulfonic acid functional group
ity against hydrolysis is significantly different, as will be discussed.
Sulfur contents on the SO H functionalized deoxycellulose-based
on NDOC support via C S atomic linkages (
C
SO H). On pristine
3
3
catalysts were higher than the acidity on the samples, indicating
that a sizable portion of the substituted SH species remained in
cellulose, the characteristic IR absorption bands of cellulose includ-
−1
ing OH (3346 cm ) and C H (2899) stretching, absorbed water
the matrix without conversion to the final SO H group.
in cellulose (1644), OH inplane deformation (1431–1319), C
O
3
Table 1
Properties and synthesis conditions of the sulfonic acid functionalized cellulose-based catalysts.
Catalyst
Aciditya
mmol gcat−1)
Sulfur contentb
Sulfur/carbon
atomic ratio (%)
Degree of swelling
EP/AGU molar ratio
Crosslinking
reaction time (h)
−1
b
(
(mmol-S gcat
)
SO3H-NDOC-1
SO3H-NDOC-2
SO3H-NDOC-3
SO3H-DOC
SO3H-Cell
Amberlyst-15
0.37
0.97
0.84
1.84
0.62
4.77
1.8
2.4
3.0
2.9
0.6
–
5.3
7.6
9.7
9.9
1.1
–
2.4
4.4
3.4
3.3
2.1
–
10
10
15
–
–
–
2
12
12
–
–
–
EP: epicholorohydrin, AGU: anhydroglucose unit in cellulose.
a
Estimated by neutralization titration method.
Estimated by elemental analysis.
b

I. Kim et al. / Applied Catalysis A: General 482 (2014) 31–37
35
Table 2
Glycerol conversion and product selectivity on the catalysts at 353 K for 3 h of batch reaction.
Catalyst
Glycerol conversion (%)
Glycerol conversion to DA and TA (%)
Product selectivity (%)
DA
MA
TA
SO3H-NDOC
SO3H-DOC
SO3H-Cell
Amberlyst-15
Blank
98
99
93
100
49
51
56
26
60
3
48
44
72
40
94
45
48
26
54
6
7
8
2
6
0
Reactant feed condition: C2H4O2/C3H8O3 = 6, catalyst loading amount = 5 wt% based on the glycerol feed, MA = monoacetin, DA = diacetin, TA = triacetin.
Table 3
3.2. Catalytic properties for glycerol acetylation reaction
Acidity (mmol-H gcat ) and sulfur contentsa (mmol-S gcat ) on the catalysts mea-
+
−1
−1
sured prior to each reaction batch.
Glycerol conversion and product selectivity on the sulfonic acid
functionalized cellulose based catalysts obtained at 353 K for 3 h in
Catalyst
Reaction batch
batch reactions are displayed in Table 2. The SO H-NDOC catalyst
3
1st
2nd
3rd
4th
that was utilized for the reaction was the SO H-NDOC-2 sample
3
SO3H-NDOC
SO3H-DOC
0.97 (2.4)
1.84 (2.9)
0.62 (0.6)
4.77
0.71
0.63 (1.1)
0.11
0.74 (1.8)
0.63 (1.3)
0.04
0.70 (1.6)
0.56 (1.1)
0.02
shown in Table 1, and it will be used and discussed hereafter. The
results on the commercial Amberlyst-15 acid resin catalyst are also
shown for comparison. A blank batch reaction conducted without
catalyst exhibited a considerable glycerol conversion with exclu-
sive product selectivity toward monoacetin, reflecting the facile
reaction kinetics toward monoacetin and the thermodynamic char-
acteristic that the Gibbs free energy for formation of 1-monoacetin
is substantially lower than those of 1,3-diacetin and triacetin [23].
Therefore, assessment of catalytic activity of the samples was con-
ducted based on the level of glycerol conversion toward combined
SO H-Cell
3
Amberlyst-15
4.65
4.46
4.50
aSulfur contents are shown in parenthesis.
reaction batch, demonstrating that the–SO H group immobilized
on the NDOC matrix via a C S atomic linkage (
3
C SO H) was sta-
3
ble against the hydrolysis reaction environment. The slight acidity
loss at the first reaction batch appears to be due to the removal
of some sulfonic acid species that were unstably anchored on the
deoxycellulose matrix. It was also confirmed by the occurrence
of homogeneous reactions (353 K) in the recovered reactants by
filtration of the solid catalysts (after 1 h of the 1st reaction batch) as
shown in Fig. 5. The results indicated that glycerol conversion to di-
and triacetin on SO H-NDOC, SO H-DOC, and SO H-Cell catalysts
di- and triacetin products. The results indicated that SO H-NDOC
3
and SO H-DOC catalysts demonstrated nearly equivalent catalytic
3
activity and product selectivity to the commercial Amberlyst-15
benchmarking catalyst. The comparatively low glycerol conversion
toward di- and triacetin products on SO H-Cell reflected its low
3
3
3
3
catalytic activity, mainly due to the relatively low acidity on the
increased without these solid acids due to the presence of the acid
species hydrolyzed or defragmented from the catalysts during the
first reaction batch. The constructive effects of the deoxycellulose
sample. The high glycerol conversion observed on SO H-DOC was
3
due to its high acidity and occurrence of homogenous reaction as
will be discussed with the structure stability of these catalysts.
crosslinking on SO H-NDOC were clear when the evolution of glyc-
3
erol conversion through the consecutive batch runs on this catalyst
3.3. Stability of the catalysts
was compared to that on SO H-DOC, as shown in Fig. 4. Although
3
the stable C S atomic linkage between the deoxycellulose support
and SO H functional group also formed on SO H-DOC, substantial
Stability of the catalysts was investigated by conducting
3
3
consecutive batch reactions utilizing the spent catalysts. Fig. 3
deactivation of the catalyst occurred as evidenced by the decrease
in glycerol conversion throughout consecutive reaction batches.
displays glycerol conversion and product selectivity on SO H-DOC,
3
SO H-NDOC, SO H-Cell, and Amberlyst-15 catalysts as a function
Concomitantly, it was found that acidity on SO H-DOC declined
3
3
3
of time for the consecutive reaction batches at 353 K (catalyst load-
ing = 5 wt% based on the glycerol feed amount, C H O /C H O
sharply after the first reaction batch by approximately 67% and
showed only a slight decrease with the subsequent batch runs. The
results suggest that, when the structure was not fortified by an
extensive crosslinking of the deoxycellulose matrix, a substantial
2
4
2
3
8
3
molar ratio = 6). Fig. 4 shows the results in which glycerol conver-
sion to di- and triacetin products on the catalysts are displayed for
each consecutive batch run. These results collectively indicate the
superior stability of SO H-NDOC catalyst for the reaction to SO H-
loss of the SO H immobilized deoxycellulose occurs as frag-
3
ments from the catalyst surface resulting in catalyst deactivation.
The sharp decline in acidity during the first reaction batch was
reasonable considering that the concentration of sulfonic acid
functional groups was expected to be higher on the near surface
3
3
DOC and SO H-Cell counterparts, wherein limited deactivation
3
occurred only in the first reaction batch. Conversely, continuous
and substantial deactivation occurred on SO H-DOC and SO H-Cell
3
3
catalysts throughout the consecutive batch runs. The main reasons
for deactivation of these catalysts are different, as evidenced by
the evolution of catalyst acidity. Table 3 shows the acidity and
sulfur contents on the samples that were measured prior to each
consecutive batch reaction. The results clearly suggest that the
of SO H-DOC. The continuous decrease in glycerol conversion
after the second reaction batches without a sizeable acidity loss
3
on SO H-DOC suggested that the structure densification affected
3
the reaction kinetics by limiting the mass transfer. We observed
that a significant densification and solidification of SO H-DOC
3
deactivation on SO H-Cell occurred due to significant acidity loss
throughout the consecutive batch runs. This is reasonable consid-
occurred after washing and drying of the spent catalyst throughout
the consecutive reaction batches (SEM micrographs are shown in
Supplementary information, Fig. S3). Agglomeration and structure
densification of crystalline celluloses by drying due to enhanced
molecular contacts of cellulose chains has been reported [26],
which agree well with the results obtained in this work.
3
ering that the ether type atomic bonding (
C O SO H) between
3
the SO H group and the cellulose matrix is prone to be weak under
3
hydrolysis conditions in the presence of acid, which led to the
removal of the acid groups from the cellulose matrix into molecular
sulfuric acid species [24,25]. In contrast, limited loss of acidity
The extensive study on sulfonic acid functionalized cellulose-
based catalysts in this work collectively suggests that (i) the
and sulfur content occurred on SO H-NDOC only through the first
3

3
6
I. Kim et al. / Applied Catalysis A: General 482 (2014) 31–37
Fig. 4. Glycerol conversion to diacetin (DA) and triacetin (TA) products on the cat-
alyst at 353 K for 3 h of each consecutive batch run (catalyst loading = 5 wt% based
on the glycerol feed, C2H4O2/C3H8O3 molar ratio = 6).
Fig. 5. Glycerol conversion to di- (DA) and triacetin (TA) at 353 K as a func-
tion of time. The reactions were initially conducted with the catalysts (5 wt%,
C2H4O2/C3H8O3 molar ratio = 6) for 1 h, and then the solid catalysts was removed
from the reactants by filtration).
glycerol acetylation reactions. The intrinsic glycerol conversion
turnover rates (TOR) to combined di- and triacetin products
on SO H-NDOC and Amberlyst-15, obtained for the results on
3
the 4th batch shown in Fig. 4, were similar with values of
−
1
2
9.5 and 14.5 h , respectively. These results demonstrated that
the sulfonic acid functionalized three-dimensionally-networked
deoxycellulose (SO H-NDOC) effectively catalyzed glycerol acety-
3
lation reactions with high stability and can be applied to similar acid
catalyzed reactions. The effects of structure and physicochemical
property of crosslinking molecules, degree of crosslinking, and den-
sity of the acid sites in SO H-NDOC catalyst have not investigated
3
in detail in this work, although they can have substantial effects on
the reaction rates and internal mass transport through the catalysts.
On acid resin catalysts, it was reported that extensive crosslink-
ing negatively affected to the glycerol acetylation reaction due to
the mass transfer limitation [27]. However, some extents of cross-
linkages between the cellulose chains appear to be necessary for the
cellulose-based catalysts that can provide free spaces accessible for
the reactant species and also effectively prevent agglomeration and
densification of the overall structure.
Fig. 3. Glycerol conversion and product selectivity on SO3H-DOC, SO3H-NDOC,
SO3H-Cell, and Amberlyst-15 catalysts as a function of time for the consecutive
reaction batches at 353 K (red: 1st, green: 2nd, blue: 3rd, black: 4th batch, cata-
lyst loading = 5 wt% based on the glycerol feed, C2H4O2/C3H8O3 molar ratio = 6). (For
interpretation of the references to color in this figure legend, the reader is referred
to the web version of this article.)
C
S atomic linkage between the deoxycellulose matrix and
SO H functional group and (ii) the extensive crosslinking
4. Conclusions
3
among the deoxycellulose units on SO H-NDOC catalyst effec-
The sulfonic acid ( SO H) functionalized three-dimensionally-
3
3
tively delivered superior catalytic activity and stability for
networked deoxycellulose catalyst (SO H-NDOC) prepared in this
3

I. Kim et al. / Applied Catalysis A: General 482 (2014) 31–37
37
work demonstrated that (i) the reinforcement of deoxycellulose
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