Sorbitol dehydration into isosorbide over a cellulose-derived solid acid catalyst

Article abstract of DOI:10.1039/c6ra05214b

The dehydration of sorbitol to isosorbide over an environmentally benign cellulose-derived solid acid catalyst (CCS) has been investigated in order to develop a sustainable process for isosorbide production. The as-prepared CCS sample contained large amounts of -COOH, phenolic hydroxyl and -SO₃H groups and presented satisfactory catalytic activity, isosorbide selectivity and recyclability under water-free conditions. Various methods such as BET, XPS, TG, SEM, FT-IR and pyridine adsorption were conducted to characterize the CCS catalyst in order to elucidate the possible catalyst structure-performance relationship. The results of FT-IR and pyridine adsorption revealed that most of the acid sites contained in CCS catalyst were assigned as strong Br?nsted sites, which were crucial for catalytic sorbitol dehydration into isosorbide. When further compared to the conventional solid acid catalyst (niobic acid), the investigated CCS catalyst in this study was considered as one of the promising alternatives to the highly efficient catalysts for isosorbide production from sorbitol.

Full text of DOI:10.1039/c6ra05214b

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Sorbitol dehydration into isosorbide over
a cellulose-derived solid acid catalyst
Cite this: RSC Adv., 2016, 6, 49528
Jie Zou,a Dong Cao,a Weitong Tao,a Shaoyin Zhang,a Li Cui,a Fanli Zengb
a
*
and Weijie Cai
The dehydration of sorbitol to isosorbide over an environmentally benign cellulose-derived solid acid
catalyst (CCS) has been investigated in order to develop a sustainable process for isosorbide production.
The as-prepared CCS sample contained large amounts of –COOH, phenolic hydroxyl and –SO3H
groups and presented satisfactory catalytic activity, isosorbide selectivity and recyclability under water-
free conditions. Various methods such as BET, XPS, TG, SEM, FT-IR and pyridine adsorption were
conducted to characterize the CCS catalyst in order to elucidate the possible catalyst structure–
performance relationship. The results of FT-IR and pyridine adsorption revealed that most of the acid
Received 27th February 2016
Accepted 2nd May 2016
sites contained in CCS catalyst were assigned as strong Bronsted sites, which were crucial for catalytic
¨
sorbitol dehydration into isosorbide. When further compared to the conventional solid acid catalyst
(niobic acid), the investigated CCS catalyst in this study was considered as one of the promising
alternatives to the highly efficient catalysts for isosorbide production from sorbitol.
DOI: 10.1039/c6ra05214b
www.rsc.org/advances
considerations, heterogeneous solid acid catalysts have been
regarded as potential alternatives for liquid acids.12,13 These
1. Introduction
types of catalysts could be easily separated and recovered aer
the reaction and be used repeatedly several times. Therefore, it
was remarkably desirable to select solid acids as catalysts to
heterogeneously catalyze the dehydration reaction of sorbitol to
replace conventional mineral acids. To date, a great variety of
solid acids such as metal(IV) phosphates, sulfated tin oxides,
zeolites, sulfated titania, Amberlyst-15, superhydrophobic
mesoporous acid and silicotungstic acid have been investigated
as catalysts for isosorbide production from sorbitol sequential
dehydration.7,8,14–17 Compared to the homogeneous mineral
acid catalysts, heterogeneous solid acid catalysts contributed
several advantages, such as safety in handling, easy separation
from reaction system, good recyclability and less corrosion of
the reactor.18,19 However, numerous developed solid acid cata-
lysts still exhibited a lower activity, which then needs a higher
temperature and longer reaction time to obtain a satisfactory
isosorbide yield. Moreover, some catalysts such as zeolite and
ion exchange resin presented less hydrothermal stability
leading to structure distortion and leaching of acid sites with
water and/or products.20 Thus, it is still a great challenge and
there remains a chance to develop an efficient, low cost solid
acid catalyst with excellent recyclability for the dehydration of
sorbitol at moderate conditions. More recently, a cellulose- and/
or glucose-derived carbonaceous solid acid catalyst, which was
environmentally benign, of metal-free composition, cheaper
and easy to separate, has generated great interest and been
successfully applied on a relatively large scale.21–29 Such a type of
carbonaceous material could be readily synthesized by
Nowadays, the efficient conversion of biomass and/or biomass-
derived feedstocks into high value chemicals or liquid fuels is of
great interest due to concern over the depletion of fossil fuels
and to leverage sustainable development.1–3 As one of the top
twelve platform chemicals, sorbitol, which was easily obtained
from glucose hydrogenation, has achieved increasing atten-
tion4,5 and has been transformed into liquid alkanes, polyols,
and isosorbide.6 Moreover, isosorbide, a useful ne chemical
synthesized from intramolecular dehydration of sorbitol, has
been widely applied in various elds such as medicines and
polymers.7,8 In particular, the addition of isosorbide co-
monomer might remarkably enhance the glass transition
temperature of PET polymer for its high temperature
application.9,10
Hence, a great deal of effort has been devoted to investigate
isosorbide production from sorbitol. In recent years, mineral
acids such as H3PO4, H2SO4 and HCl were rst studied under
homogeneous reaction conditions. Fleche et al. indicated that
sorbitol was successfully dehydrated into isosorbide with 77%
yield using H2SO4 as catalyst at 135 C.11 Although a high yield
ꢀ
of isosorbide was achieved, these liquid acids possessed several
drawbacks from the standpoint of environment, safety, corro-
sivity, separation and operating cost. Based on these
aFaculty of Light Industry and Chemical Engineering, Dalian Polytechnic University,
116023 Dalian, China. E-mail: caiwj@dlpu.edu.cn; Fax: +86-411-8632-2228; Tel:
+86-411-8632-4482
bThe College of Life Sciences, Peking-Tsinghua Center for Life Sciences, Peking
University, 100871 Beijing, China
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incomplete carbonization of renewable cellulose followed by was performed. Finally, the ltered black precipitate was
sulfonation. It has been generally accepted that the obtained thoroughly washed with hot deionized water until no sulfate
carbonaceous solid acid that contained exible polycyclic ions were detected in the effluent. The resulting material was
nanosheets with numerous phenolic hydroxyl, –COOH and dried in a vacuum oven at 60 ꢀC for 12 h. The as-prepared
–SO3H functional groups presented better catalytic behaviour cellulose-derived solid acid catalyst is denoted as CCS in
than conventional solid acid catalysts in acid-catalyzed reac- the following discussion.
tions such as hydrolysis and esterication.30,31 Furthermore, the
acidity of the carbonaceous materials was estimated to be
comparable with the acidity strength of concentrated H2SO4
2.3 Catalyst characterization
because some –SO3H groups were linked by strong H-bonds, The specic surface area of the catalyst was measured by the
resulting in strong acidity due to mutual electron-with- BET (Brunauer–Emmett–Teller) method. The measurement was
drawal.22 It has been reported that both sorbitol conversion and conducted on an ASAP 2010 Micromeritics apparatus and con-
isosorbide selectivity were greatly affected by the types and/or sisted of nitrogen adsorption at ꢁ193 ꢀC. Prior to measurement,
ꢀ
concentrations of acid sites on the catalyst surface together the sample was rst outgassed at 300 C for 3 h under N2 ow
with their acidic strength.7 Moreover, the synthesis of (50 mL minꢁ1).
carbonaceous-based solid acids with tunable acid sites is still in
its infancy and has not been investigated in depth.32–35
Raman spectra were obtained using a Jobin-Yvon T64000
spectrometer with an Ar ion laser as the illumination source
Inspired by recent research work, a carbon-based solid acid (532.08 nm) and a CCD detector cooled at ꢁ196 ꢀC. The Raman
catalyst with cellulose as a precursor was synthesized by spectra of the catalysts were recorded with the laser power
a hydrothermal method and selected as an environmentally below 0.7 mV in order to avoid laser heating inuences.
friendly and recyclable catalyst in sorbitol dehydration into
The surface morphology and elemental mapping of the
isosorbide under solvent-free experimental conditions. Char- investigated catalyst were studied by scanning electron
acterizations such as BET, XPS, TG, SEM, FT-IR and pyridine microscopy (SEM) on a JEOL JSM-6460LV equipped with energy-
adsorption were further conducted to construct a relationship dispersive X-ray detection. The operational acceleration voltage
between the catalyst structural properties and its catalytic was 15 kV. The sample was sputter coated with gold for 1 min
performance. The as-prepared solid acid exhibited promising before SEM characterization.
properties with high isosorbide selectivity and stability. This
Thermogravimetric analysis (TGA) measurements were con-
study might shed light on the development of more efficient ducted on a TDA Q50 instrument in a N2 ow (50 mL minꢁ1).
solid acid catalysts for the production of high value chemicals The catalyst (10 mg) was placed in a quartz crucible and the
from biomass and/or biomass-derived feedstocks.
temperature was increased from room temperature to 600 ꢀC at
ꢁ1
ꢀ
a heating rate of 10 C min
.
X-ray photoelectron spectroscopy (XPS) was conducted using
an ESCALAB 250 spectrometer equipped with an Mg-Ka X-ray
source operating at 100 W. The charging effect was corrected
by referencing the binding energy of C1s at 284.8 eV.
The functional groups over the surface of the CCS catalyst
were detected by FT-IR spectra using the standard KBr disc
method in the range from 400 to 4000 cmꢁ1 at 4 cmꢁ1 resolution
on a Digilab FTS 3100 FTIR spectrometer. A mixture of catalyst
and KBr (catalyst/KBr ¼ 1/100, mass ratio) was pressed into self-
supporting wafers (15 mm) for measurement.
2. Experimental
2.1 Chemicals and reagents
Both microcrystalline cellulose and commercial niobic acid
were purchased from Shanghai Aladdin Industrial Co. Ltd. All
other chemicals were supplied by Tianjin Kermel Chemical
Reagent Co. Ltd. and used without further purication.
2.2 Catalyst preparation
The cellulose-derived carbonaceous solid acid catalyst was
The types of acid sites dispersed on the catalyst surface were
synthesized by hydrothermal carbonization and sulfonation further investigated by pyridine adsorption. Prior to the
of microcrystalline cellulose. Typically, 3.0 g microcrystalline measurement, the catalyst (20 mg) was pressed into self-
cellulose (50 mm) was rst added into 60 mL deionized water supporting wafers (15 mm) and outgassed at 300 ꢀC for 1 h
under magnetic stirring. The resulting liquid mixture was under vacuum in order to remove the adsorbed impurities over
then transferred to a Teon-lined stainless steel autoclave the catalyst surface. Aer quenching to room temperature, the
(100 mL), heated to 230 ꢀC and kept at this temperature for 2 spectra were acquired as the background reference. Subse-
h at autogenous pressure. Aer quickly cooling the autoclave quently, pyridine vapor was introduced into the cell for 0.5 h
to room temperature, the collected precipitate was repeatedly followed by further evacuation for 0.5 h so as to remove the
washed with hot water and dried overnight at 90 ꢀC. The physically adsorbed pyridine. The catalyst was heated to 150 ꢀC
obtained black solid was then dispersed in 150 mL of for 10 min and then another spectrum was acquired. The pre-
concentrated sulfuric acid (>96%), heated to 150 ꢀC and sented spectra were achieved by subtracting the spectra before
maintained at this temperature for 6 h with a continuous N2 and aer pyridine adsorption. The ratio of the concentration of
ow (30 mL minꢁ1) to introduce –SO3H functional groups. Bronsted acid sites to Lewis acid sites was calculated based on
¨
Aer quenching to room temperature, 1000 mL deionized the ratio of the peak areas centered at ca. 1540 cmꢁ1 and 1450
water was slowly added into the liquid mixture and ltration cmꢁ1, respectively, as reported in the literature.36,37
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The acidity of the CCS catalyst was studied by the NH3-TPD sample. Sorbitol conversion and isosorbide selectivity were
technique. Prior to NH3 adsorption, the sample was also out- calculated as follows:
gassed at 300 ꢀC for 1 h in order to remove the impurities over
the catalyst surface. Aer cooling down to 100 ꢀC, NH3
adsorption was carried out under a ow of NH3/He (10 vol%, 50
mL minꢁ1) and maintained for 30 min. The ow was then
switched to He (50 mL minꢁ1) so as to remove the physically
adsorbed NH3. NH3-TPD was conducted in the He ow (50 mL
Csorbitol% ¼ (moles of reacted sorbitol/moles of initial sorbitol)
ꢂ 100%
(2)
Sisosorbide% ¼ (moles of carbon in the produced isosorbide/
moles of carbon in the reacted sorbitol) ꢂ 100%
(3)
minꢁ1) by increasꢁin1g the temperature to 850 C with a heating
ꢀ
rate of 5 ꢀC min when the desorbed NH3 molecules were
detected using online mass spectrometry (Incon quadrupole).
The quantities of desorbed NH3 at different temperatures were
calculated according to the relative areas of the deconvolution
peaks from the spectra of desorbed NH3, as follows:
2.5 Procedure for recycling the catalyst
The recyclability of the CCS catalyst was investigated for the
dehydration of sorbitol to estimate the catalyst stability. The
reactions were repeatedly conducted under identical experi-
mental conditions as mentioned above. Aer each run, the aged
catalyst was ltered and thoroughly washed with hot deionized
D ¼ DTotal ꢂ (A/ATotal) ꢂ 100%
(1)
where DTotal (mmol gꢁ1) is the total amount of desorbed NH3; A
is the area of a deconvolution peak and ATotal is the total areas of
all the deconvolution peaks of the desorbed NH3.
ꢀ
water and dried at 90 C overnight for the next reaction.
3. Results and discussion
The content of –SO3H groups was also determined by acid–
base titration following the procedures in previous reports.25,38
Typically, the catalyst (0.2 g) was added to an aqueous solution
of NaCl (0.1 mol Lꢁ1, 20 mL). The mixture was sonicated for 60
min and then magnetically stirred for 24 h at room temperature.
Aer centrifugal separation, the obtained ltrate (5 mL) was
titrated with aqueous NaOH (0.05 mol Lꢁ1) with phenolphtha-
lein as the indicator. The titration procedure was repeated three
times and the average value was reported.
To identify whether sulfonate groups were successfully
graed onto the prepared carbonaceous solid acid catalyst, the
sulfur content together with C and H concentration was deter-
mined by elemental analysis operated on an EURO EA 300 series
elemental analyzer with ush combustion at 950 ꢀC (O2 ow of
20 mL minꢁ1). The catalyst (1.0 mg) was rst placed in a tin can
and automatically injected into the instrument. L-Cystine
(C6H12N2S2O4) was selected as the standard substance. The
amount of –SO3H groups was estimated based on the S content.
The physical and chemical properties of the as-prepared CCS
catalyst were investigated by a series of characterization
methods. The surface area and pore volume were 4.11 m2 gꢁ1
and 0.055 cm3 gꢁ1, respectively, calculated from BET measure-
ment. A slight decrease was observed when compared to the
values of untreated microcrystalline cellulose (6.93 m2 gꢁ1
,
0.072 cm3 gꢁ1). The decrease of pore diameter might be because
of the existence of a large amount of surface functional groups
over the catalyst surface aer the hydrothermal and sulfonation
processes.31 The hydrothermal carbonization process acceler-
ated the cellulose structure to undergo dehydration, decompo-
sition, hydrolysis and a rearrangement reaction to form
a polycyclic aromatic structure embedded with active –COOH
and –OH groups, which supplied sufficient active sites for
sulfonation. Hence, the following sulfonation treatment resul-
ted in the incorporation of –SO3H sites into the carbonaceous
material. These surface sites could occupy the pores leading to
the decline of surface area and pore volume.
2.4 Experimental procedure
The catalytic dehydration of sorbitol was conducted in a stain-
less steel batch reactor (60 mL). Briey, a mixture of 1.0 g
sorbitol and 0.1 g powdered catalyst was poured into the reactor
and purged with N2 for several times. Subsequently, the reactor
was heated to the investigated temperature under magnetic
stirring at 800 rpm. The reaction time was calculated when the
reactor temperature reached the target one. Aer the reaction,
the reactor was quickly cooled to room temperature in an ice-
water bath and an appreciable amount of water was added.
The resulting mixture was ltered in order to separate the
catalyst. Then, the obtained liquid solution was analyzed by
HPLC (Wates e2695) equipped with UV and refractive index (RI)
detectors and a Shodex SUGAR SC1011 column (8 ꢂ 300 mm).
ꢀ
The column was operated at 70 C by a column heater and the
operating temperature of detectors was 35 ꢀC. Distilled water
was used as the mobile phase at a ow rate of 0.6 mL minꢁ1. The
injection volume was 10 mL of ltered (25 mm), undiluted Fig. 1 Raman spectrum of the CCS catalyst.
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Fig. 1 shows the Raman spectra of the CCS catalyst. It was method was around 2.05 mmol gꢁ1, which was in good agree-
clearly observed that a distinct pair of characteristic peaks ment with the result of elemental analysis. The slight deviation
centred at ca. 1347 (D band) 1596 cmꢁ1 (G band) was observed. (6.8%) indicated that a small amount of the –SO3H sites might
Normally, the D band is assigned to amorphous structured be located within the catalyst matrix.
carbon because of carbon sheet stacking or incomplete
The morphologies of the untreated microcrystalline cellu-
carbonized catalyst structure,39 whereas the G band is attributed lose and CCS catalyst were also characterized by the SEM
to the sp2 C–C stretching vibration (E2g mode) of well-structured technique. As shown in Fig. 3, fresh microcrystalline cellulose
carbon.40–42 The intensity ratio of the peaks (ID/IG) might be was brous with bre diameters of around 25 mm and its
associated with the degree of crystallinity of the carbonaceous- surface was smooth. Aer hydrothermal treatment at 230 ꢀC,
based solid acid.43 Notably, the larger intensity of the D band the carbonaceous material exhibited a relatively rough surface
revealed that the as-prepared material possessed more amor- and a looser structure with more nanopores because of the
phous structure, which was favourable for the graing of swelling inuence of water.48 Interestingly, the brous structure
sulfonic acid groups to achieve a large concentration of –SO3H
groups on the catalyst surface.44 Furthermore, the large ID/IG
value of CCS catalyst might be attributed to the decline of the
average size of sp2 carbon caused by the addition of –SO3H
groups.45 The abovementioned results indirectly supported the
successful graing of –SO3 groups aer sulfonation treatment
of incompletely carbonized cellulose.
Elemental analysis also revealed that the composition of the
CCS catalyst was CH0.709O0.404S0.0455, suggesting the successful
incorporation of S into the carbonaceous material derived from
the incomplete hydrothermal carbonization of microcrystalline
cellulose. As compared to the untreated microcrystalline cellu-
lose (CH1.704O0.821), the decline of H and O contents might be
due to the combined results of dehydration and carbonization.
In the XPS spectrum of the CCS catalyst (Fig. 2), a single S 2p
peak was observed at around 168 eV suggesting that all S atoms
of the CCS sample were presented in the form of –SO3H groups,
which was in agreement with previous reports.46 This revealed
that sulfonation of incompletely carbonized carbon successively
occurred in concentrated sulfuric acid to form sulfonic acid
groups. Thus, the probable density of –SO3H groups in the
cellulose-derived catalyst could be calculated based on the S
content in the catalyst composition. The value was ca. 2.2 mmol
gꢁ1, which led to strong acidity in the CCS catalyst. The acid
density was also comparable with the previous work.47 This
might be a positive factor for the dehydration of sorbitol. The
density of the –SO3H acid sites estimated by the titration
Fig. 3 SEM images of (A) microcrystalline cellulose, (B) cellulose after
hydrothermal treatment, (C and E) CCS catalyst and (D) mixed, (F) S, (G)
Fig. 2 XPS spectrum of the CCS catalyst.
C, (H) O elemental mappings of CCS catalyst.
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of cellulose was disrupted and many carbon spheres with that the as-prepared CCS catalyst contained a large amount of
diameters in the range of 100–200 nm appeared, indicating the various surface functional groups such as –OH, –COOH and
incomplete carbonization of cellulose in the hydrothermal –SO3H groups, which is different from conventional solid acids
conditions. With the further inuence of sulfonation, a signi- with single functional groups.
cant morphological change of carbonaceous material was
TG analysis was selected to determine the thermal stability of
observed. The catalyst surface became more rough and the the investigated CCS catalyst. As shown in Fig. 5, the as-
average diameter of carbon spheres decreased to 20–40 nm. prepared material was thermally stable below 270 ꢀC with
Fig. 3 also shows the elemental mappings of the whole region only a slight weight loss (ca. 4.3%). This loss was probably due
(Fig. 3E) for the various elements, which were investigated: C, O to the evaporation of adsorbed water from the catalyst surface.
and S. It could be clearly seen that S was uniformly dispersed on Furthermore, a large weight loss occurred in the higher
the surface of the CCS catalyst meaning highly dispersed –SO3H temperature region ranging from 270 ꢀC to 350 ꢀC, assigned to
groups, which was favourable for the interaction of reactant and the thermal decomposition of the surface functional groups
acid sites.
and the destruction of the carbonaceous material's structure.53
In order to identify the functional groups bonded to the Hence, it could be elucidated that the CCS catalyst exhibited the
ꢀ
surface of the investigated solid acid catalyst, FT-IR character- satisfactory thermal stability and might be used below 270 C.
ization was conducted and the result is presented in Fig. 4. The In this study, experimental conditions for the catalytic dehy-
presence of –SO3H groups in the CCS catalyst was directly evi- dration of sorbitol were operated below 220 ꢀC. Thus, the as-
denced by the FT-IR result. The vibration bands observed at ca. prepared carbon material was suitable for this reaction.
1049 and 1164 cmꢁ1 were assigned to SO3ꢁ stretching vibrations
To further elucidate the acid types on the CCS catalyst and
¨
and O]S]O stretching vibrations in –SO3H groups, respec- the ratio of strong Bronsted and Lewis acid sites, pyridine
tively, suggesting that the resulting carbonaceous material adsorption was carried out and the collected spectra are pre-
contained the –SO3H groups in good agreement with the result sented in Fig. 6. The result clearly depicts the co-existence of
of the abovementioned element analysis.21 Moreover, the peaks Bronsted and Lewis acid sites in the CCS catalyst. Briey, an
¨
centered at ca. 1632 and 1709 cmꢁ1 were attributed to the C]C intense peak at ca. 1541 cmꢁ1 due to the N–H bending of the
and COO– stretching vibrations, respectively, suggesting the pyridinium ion was the characteristic adsorption peak of pyri-
¨
existence of polycyclic aromatic rings, which were formed from dine on Bronsted acid sites. Another comparatively less intense
the incomplete carbonization of the microcrystalline cellulose band at ca. 1450 cmꢁ1 was assigned to pyridine adsorption on
substance.49,50 The adsorption peaks in the range of 1400–1600 Lewis acid sites. A sharp peak located at ca. 1490 cmꢁ1 was
cmꢁ1 were assigned to the stretching vibration of the imidazole attributed to the combined adsorption of a pyridine molecule
ring embedded in the polymer matrix. On the other hand, on Bronsted and Lewis acid sites.54 The commercially available
¨
another clear band at 3431 cmꢁ1 corresponded to the –OH niobic acid, a typical industrial inorganic solid Bronsted acid,
¨
stretching vibration in phenolic hydroxyl (–OH) and –COOH was also investigated for comparison purposes. The DRIFT
groups formed during the process of carbonization and sulfo- spectra of the commercial niobic acid exhibited almost iden-
nation.51 A broad band at 2200–2520 cmꢁ1 (shown in the inset tical peaks except that the intensities of the bands of the CCS
in Fig. 4) could be attributed to an overlap of the bending mode catalyst were much larger than the values of the commercial
(Fermi resonance) of –OH–O] connected via a strong H bond, niobic acid, i.e., the amount of the acid sites in CCS catalyst was
¨
as shown in the strong liquid Bronsted acid CF3SO3H, indi- considerable higher than those in niobic acid. In addition, the
¨
cating that a number of the –SO3H groups were in close prox- ratio of Bronsted acid sites to Lewis acid ones calculated based
imity to each other.52 All the abovementioned results elucidated on the peak areas centred at ca. 1541 cmꢁ1 and ca. 1450 cmꢁ1
,
respectively, resulted in the following order: CCS (7.8) >
Fig. 4 FT-IR spectrum of the CCS sample.
49532 | RSC Adv., 2016, 6, 49528–49536
Fig. 5 TG curve of the CCS catalyst.
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As shown in Fig. 7, the NH3-desorption prole of the CCS
ꢀ
catalyst started from ca. 150 C and then rapidly increased to
maximum peak (ca. 210 ꢀC) together with another big hump at
around 350 ꢀC. Then, it progressively declined until 800 ꢀC. The
peak in the NH3-TPD prole might be resolved into several
deconvolution peaks, indicating the existence of acid sites with
various strengths. Notably, the peak at ca. 201 ꢀC was associated
with the hydrogen-bound ammonia (weak); moreover, the
partial contribution of physically adsorbed NH3 could not be
ruled out. Another two peaks (ca. 340 and 478 ꢀC) corresponded
to the medium strong and very strong acid sites,56 respectively.
The presented very strong acid sites were attributed to the
–SO3H groups, whereas the weak acid sites were assigned to the
–OH groups on the catalyst surface. The quantity of desorbed
ammonia at distinct temperatures was calculated according to
the areas of the deconvolution peaks (eqn (1)) and resulted in
the following order: weak sites (0.8 mmol gꢁ1) < medium strong
sites (1.8 mmol gꢁ1) < very strong sites (1.5 mmol gꢁ1). The total
concentration of strong sites was ca. 80%, highly in line with the
previous result of pyridine adsorption, revealing that most of
the acid sites on the surface of the CCS catalyst belonged to
strong acid sites.
Fig. 6 Py-FT-IR spectra of the CCS catalyst and commercial niobic
acid.
commercial niobic acid (1.4). Hence, the abovementioned FT-IR
data revealed that most of the acid sites (ꢃ84%) in the CCS
¨
sample belonged to strong Bronsted acid sites, which was
a positive factor for the dehydration reaction. Upon further
Considering the results of the previous reports,57–59 the
possible reaction mechanism of sorbitol dehydration into
isosorbide mainly consists of two sequential steps: sorbitol
was rst dehydrated to form monocyclic sorbitans such as 1,4-
sorbitan, 2,5-sorbitan and 1,5-sorbitan; then, isosorbide was
produced from another dehydration reaction occurring on 1,4-
sorbitan instead of other sorbitans. The effect of reaction
parameters on isosorbide yield, including catalyst dosage and
reaction temperature, was investigated to assess the catalytic
performance of the CCS catalyst. First, a blank test was carried
out to elucidate the catalyst role in isosorbide production from
sorbitol. In the absence of catalyst, only a slight sorbitol
conversion (ꢃ7%) without any detectable isosorbide was
observed at 160 ꢀC. In contrast, with the addition of CCS
catalyst, both sorbitol conversion and isosorbide selectivity
remarkably increased to ca. 63% and 35%, respectively, at 160
ꢀC. The main observed products were isosorbide, 1,4-sorbitan
together with trace amount of 1,5-sorbitan by-product. Other
non-identied compounds are denoted here as ‘unknown’. As
shown in Fig. 8, 100% sorbitol conversion was achieved when
the reaction temperature was above 190 ꢀC. As to the iso-
sorbide selectivity, it was still affected by further increasing
the reaction temperature. Isosorbide selectivity steadily
increased from 160 ꢀC to 200 ꢀC and then slightly decreased at
220 ꢀC. The maximum value of isosorbide selectivity (67%) was
ꢁ1
ꢀ
heating to 450 C, the band at ca. 1540 cm was still clearly
observed for the CCS catalyst, indicating its strong acidity.
Based on the previous literature,7 both sorbitol conversion and
isosorbide yield were greatly affected by the quantity of
¨
Bronsted acid sites. Therefore, the catalyst with higher amount
¨
of Bronsted acid sites might give rise to better catalytic perfor-
mance in mild experimental conditions.
NH3-TPD characterization was also conducted to assess the
total acidity of the CCS catalyst. In general, the various NH3-
desorption temperatures elucidate the different strengths of the
acid sites. Normally, the higher NH3-desorption temperature
corresponds to the stronger acidity. The acid sites might be
denoted as weak, medium strong and very strongꢀsites at NH3-
ꢀ
desorption temperatures of 150–300 C, 300–400 C and above
400 ꢀC, respectively, and the amount of acid sites was associated
with the peak areas of the NH3-TPD curves.55,56
ꢀ
obtained at 200 C. In addition, the gradual decrease of sor-
bitan selectivity upon heating elucidated the occurrence of the
consecutive dehydration of sorbitol to isosorbide, suggesting
that sorbitan acted as an intermediate, which was in good
agreement with the previous result.57 At 220 C, the unknown
ꢀ
humin by-products, which might be formed from oligomeri-
zation and/or self-polymerization of isosorbide or cross-
polymerization between sorbitan and isosorbide,57 were
observed to increase leading to the decrease of isosorbide
selectivity.
Fig. 7 NH3-TPD profile of the CCS catalyst.
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Fig. 10 Comparison of catalytic performance of CCS and commercial
niobic acid in dehydration of sorbitol (blank: CCS; full: niobic acid.
Sorbitol: 1 g; catalyst: 0.1 g; reaction temperature: 200 ꢀC; reaction
time: 1 h).
Fig. 8 Effect of reaction temperature on sorbitol conversion and
isosorbide selectivity using CCS catalyst (sorbitol: 1 g; catalyst: 0.1 g;
reaction time: 1 h).
Catalyst dosage greatly controlled the reaction kinetics and it
was necessary to optimize this factor in order to achieve the
satised selectivity to the targeted product.60 The effect of
catalyst amount on the dehydration of sorbitol was investigated
(Fig. 9). When the weight ratio of CCS catalyst to sorbitol was
0.04, both 85% sorbitol conversion and 46% isosorbide selec-
¨
Bronsted acid sites was more efficient for sorbitol dehydration
to isosorbide. As discussed in the results of pyridine adsorption,
the amount of Bronsted acid sites contained in the CCS catalyst
was almost eight times higher than the value of niobic acid. All
the above results clearly point out that the investigated CCS
catalyst could efficiently catalyze sorbitol dehydration.
¨
ꢀ
tivity could be obtained at 200 C aer 1 h reaction. Then the
The catalyst recyclability is regarded as one of the important
factors of an efficient solid acid catalyst to reduce the opera-
tional cost. In the case of carbonaceous solid acid catalysts with
–SO3H groups, the catalyst deactivation was mainly caused by
the leaching of sulfur species during the reaction process.
Hence, the stability of the CCS catalyst for the dehydration of
sorbitol has also been tested with several runs at 200 ꢀC. As
shown in Fig. 11, isosorbide yield was observed to be almost
similar aer being repeatedly used for four times. Therefore, it
could be concluded that the CCS catalyst was easily reused
without any remarkable loss of performance. Elemental anal-
ysis of the product aqueous solution revealed that no remark-
able leaching of S occurred during the recycle test. The
elemental analysis result of the aged catalyst suggested similar
amounts of –SO3H groups for the fresh and used catalysts (2.2
isosorbide yield progressively increased upon increasing the
ratio from 0.04 to 0.12 that a high value of 68% could be ach-
ieved for 1 h. With adding more catalyst, increase of isosorbide
yield was limited. In contrast, a slight decline of isosorbide
selectivity was observed, which might be due to the excess acid
sites resulting in the formation of humins by-product.
A commercial solid acid (niobic acid) was also investigated
for comparison purpose. From the results presented in Fig. 10,
it could be clearly observed that the niobic acid sample only
exhibited 81% sorbitol conversion with much less isosorbide
selectivity (ꢃ43%) at 200 ꢀC. However, the formation of sorbitan
by-product was much higher compared to the value for the CCS
catalyst, which was in accordance with the previous reports that
the catalyst with weak acidity produced a high concentration of
sorbitan.7 It is also worth mentioning that beyond the amount
of acid sites, catalytic performance was also greatly affected by
the types of acid sites. The catalyst with a high quantity of
Fig.
9
Effect of catalyst dosage (reaction temperature: 200 ꢀC, Fig. 11 Recycle usage of the CCS catalyst. Sorbitol: 1 g; catalyst: 0.1 g;
reaction time: 1 h).
reaction temperature: 200 ꢀC; reaction time: 1 h.
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Renewable Energy and Gas Hydrate, Chinese Academy of
Sciences (y407k21001).
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