Stability and catalytic properties of porous acidic (organo)silica materials for conversion of carbohydrates

Article abstract of DOI:10.1016/j.molcata.2013.11.013

Sulfonic acid-modified mesoporous SBA-15-type silica and hybrid organosilica catalysts were prepared using the co-condensation and grafting functionalization methods. The catalytic activity of these materials for fructose dehydration to 5-hydroxymethylfurfural was studied in aqueous medium and in DMSO. The prepared materials were characterized by XRD, SEM, elemental analysis, nitrogen physisorption and ²⁹Si CP MAS NMR. Special focus was on evaluating the influence of the structural modification of the mesoporous catalysts on their stability under the conditions of catalytic conversion of carbohydrates. In aqueous medium all catalysts show very low activity and HMF selectivity. However, when the reaction is carried out in DMSO, HMF yields of up to 88% can be achieved with a minimum amount of sulfonic acid-modified catalyst. Among the materials tested, the hybrid organosilica-based catalyst shows the highest dehydration rate. It is demonstrated that pure silica-based catalysts degrade strongly under the catalytic conditions. The hydrothermal stability of the mesoporous catalysts is improved in the presence of organic moieties in their structure. The co-condensed organic-inorganic catalysts retain much better the mesoporosity and are less susceptible to the loss of catalytic functional groups anchored to their surface compared to their pure silica counterparts. The methodology for the introduction of functional groups has also a profound impact on the catalyst stability. Species incorporated into the catalysts via the direct co-condensation method show much higher stability than those introduced via a post-synthetic grafting procedure.

Full text of DOI:10.1016/j.molcata.2013.11.013

Journal of Molecular Catalysis A: Chemical 388–389 (2014) 81–89
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Stability and catalytic properties of porous acidic (organo)silica
materials for conversion of carbohydrates
William N.P. van der Graaff, Karen Garrido Olvera, Evgeny A. Pidko∗, Emiel J.M. Hensen∗∗
Inorganic Materials Chemistry Group, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, The Netherlands
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 May 2013
Received in revised form 5 October 2013
Accepted 11 November 2013
Available online 19 November 2013
Sulfonic acid-modified mesoporous SBA-15-type silica and hybrid organosilica catalysts were prepared
using the co-condensation and grafting functionalization methods. The catalytic activity of these materi-
als for fructose dehydration to 5-hydroxymethylfurfural was studied in aqueous medium and in DMSO.
The prepared materials were characterized by XRD, SEM, elemental analysis, nitrogen physisorption and
29Si CP MAS NMR. Special focus was on evaluating the influence of the structural modification of the
mesoporous catalysts on their stability under the conditions of catalytic conversion of carbohydrates. In
aqueous medium all catalysts show very low activity and HMF selectivity. However, when the reaction
is carried out in DMSO, HMF yields of up to 88% can be achieved with a minimum amount of sulfonic
acid-modified catalyst. Among the materials tested, the hybrid organosilica-based catalyst shows the
highest dehydration rate. It is demonstrated that pure silica-based catalysts degrade strongly under the
catalytic conditions. The hydrothermal stability of the mesoporous catalysts is improved in the presence
of organic moieties in their structure. The co-condensed organic–inorganic catalysts retain much better
the mesoporosity and are less susceptible to the loss of catalytic functional groups anchored to their
surface compared to their pure silica counterparts. The methodology for the introduction of functional
groups has also a profound impact on the catalyst stability. Species incorporated into the catalysts via the
direct co-condensation method show much higher stability than those introduced via a post-synthetic
grafting procedure.
Keywords:
5-Hydroxymethylfurfural
Hydrothermal stability
Periodic mesoporous organosilica
SBA-15
Sustainable chemistry
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
The conversion of fructose to HMF is an important model reac-
tion in carbohydrate chemistry. It can be promoted by different
Depleting fossil resources, as well as environmental concerns
regarding their extraction and utilization, necessitate the develop-
ment of new technologies based on more sustainable feedstock for
the production of important chemical intermediates and fuel com-
ponents. An important candidate to partly replace fossil feedstock is
carbohydrate biomass because of its abundance, renewable nature
and relatively uniform chemical composition facilitating its utiliza-
tion in the chemical industry [1,2]. The transformation of the main
constituents of carbohydrate biomass, cellulose and hemicellulose,
to useful chemicals proceeds via specific key intermediates that are
usually referred to as platform chemicals. A wide range of value-
added chemicals can be obtained from these compounds. One of the
most promising platform molecules is 5-hydroxymethylfurfural
(HMF), which can serve as a precursor for monomers (e.g., fur-
furyldicarboxylic acid, FDCA) and fuel components [3,4].
Brønsted and Lewis acidic catalysts [5,6]. For commercial appli-
cations, heterogeneous catalysts are generally preferred in view
of easier separation and regeneration. Silica is among the most
widely used carrier materials for heterogeneous catalysts due to
its high mechanical and thermal stability [7] as well as the possi-
bility to tune its chemical properties by the introduction of different
functional groups [8].
Even without a catalyst, HMF can be obtained from fructose
by high-temperature dehydration in ionic liquids [6] and in sev-
eral other solvents such as DMSO [9–11]. The utilization of an acid
catalyst allows performing the reaction at milder reaction condi-
tions with high efficiency. However, from an environmental point
of view water as a solvent is preferred. Fructose dehydration in
aqueous media suffers from side reactions resulting in low yields
of the desired HMF product [12].
Various solid acid catalysts and in particular organic acid-
functionalized ordered mesoporous silica materials have been
applied in a large number of acid-catalyzed reactions, such as con-
densation and acetalization reactions [13,14]. For the conversion
of biomass-derived compounds, water is considered the preferred
and “green” reaction medium. However, in aqueous environment
∗
Corresponding author. Tel.: +31 40 247 2189.
Corresponding author. Tel.: +31 40 247 5178.
E-mail addresses: e.a.pidko@tue.nl (E.A. Pidko), e.j.m.hensen@tue.nl
∗∗
(E.J.M. Hensen).
1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.11.013

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W.N.P. van der Graaff et al. / Journal of Molecular Catalysis A: Chemical 388–389 (2014) 81–89
the catalyst stability can be compromised due to hydrolysis of the
active sites [15]. In order to increase the stability of these materials
in the presence of water, specific microenvironments can be cre-
ated inside their pores which suppress the decomposition of the
(organo)silica material. Increased hydrophobicity of the pores may
also decrease the local water concentration, thereby decreasing the
tendency to form humins upon the conversion of carbohydrates,
as well as increasing the material’s resistance to hydrolysis and
subsequent loss of functional groups.
Hydrophobicity can be conveniently controlled by introduction
of stable organic bridging groups between the silicon moieties,
creating a hydrophobic microenvironment inside the catalyst
pores. Hybrid organic–inorganic periodic mesoporous organosilica
(PMO) materials have gained significant attention for application in
adsorption and membrane separations [16,17] as well as in cataly-
sis [18]. Its synthesis involves the hydrolysis and template-assisted
condensation of bis-(trialkoxy)silane-functionalized compounds,
by which highly ordered materials can be obtained [19–21]. It
has been suggested that the increased hydrophobicity leads to
increased hydrothermal stability of the materials [19]. By opti-
mizing the hydrophobicity of silane precursors, functionalized
organosilica materials with enhanced hydrothermal stability can
be obtained [19].
Fig. 1. Sulfonic acid groups used for functionalization of (organo)silica mesoporous
materials.
Sigma–Aldrich), HCl (37% aqueous solution, BDH Prolabo),
hydrogen peroxide (30 wt% aqueous solution, Sigma–Aldrich),
chlorosulfonic acid (98%, Fluka) were used as received without
further purification.
2.1. Materials synthesis
Catalytic functionalities can be introduced in (organo)silicas
by either post-synthesis grafting on the porous material
2.1.1. SBA-15 and C C-bridged PMO
SBA-15. Synthesis of SBA-15 was performed according to estab-
lished procedures [24,25]. To a stirred solution of Pluronic P123
(2 g) in aqueous HCl (75 mL, pH = 1.5), TMOS (3.3 g, 22 mmol) was
added at 40 ◦C and the mixture was stirred for overnight. The mix-
ture was then transferred to a Teflon-lined autoclave and aged at
100 ◦C for 24 h. After filtration, the solid was washed thoroughly
with water and ethanol and dried in air. Detemplation of the mate-
rial was performed by calcination at 550 ◦C for 8 h in static air.
or co-condensation of
a functionalized alkoxysilane with an
(organo)silica precursor. [8] During grafting, the functionalized
alkoxysilane reacts with the free surface silanol moieties. The
nature and properties of the resulting grafted species depend on
the concentration and distribution of surface silanol groups in the
materials. Furthermore, one cannot exclude self-condensation of
the organosilane precursors on the catalyst surface resulting in the
formation of a poorly defined amorphous agglomerates inside the
catalyst pores. Co-condensation usually ensures a better anchoring
and more homogeneous distribution of the introduced functional
groups than grafting [22,23].
Recently, Dumesic et al. [15] demonstrated that co-condensed
sulfonic acid-modified SBA-15 and ethene-bridged periodic meso-
porous organosilica C-C-PMO and their admixtures can successfully
be used as catalysts for the conversion of fructose to HMF in a fixed-
bed reactor. However, it was found that under reaction conditions
(THF/H2O 4:1, T = 130 ◦C) functionalized groups were hydrolyzed
and removed from the active catalyst. This raises questions on the
suitability of such materials for application as catalysts for biomass
conversion.
In this study, sulfonic acid-modified silica and organosil-
ica catalysts were prepared. Preparation of the materials was
performed by either grafting a sulfonic acid trialkoxysilane deriva-
tive onto prepared SBA-15 and C-C-PMO, or by co-condensation
with either silica (TEOS) or ethane-bridged organosilica (bis-
triethoxysilylethane, BTESE) precursors. The functional groups
used in this study are shown in Fig. 1. The hydrothermal stability
of the synthesized materials and their reactivity for fructose dehy-
dration in water and DMSO were investigated. Special focus was on
the analysis of the influence of the functionalization method and
the nature of the support material on the catalyst stability.
C
C-Bridged PMO (C-C-PMO). For C-C-PMO, BTESE was added
to a surfactant solution at 35 ◦C with a final synthesis ratio of 1
BTESE: 0.03 P123: 8.3 HCl: 318 H2O: 5.33 n-BuOH. After stirring
overnight at the same temperature, the mixture was aged for 48 h
at 100 ◦C. The resulting material was washed thoroughly with water
and ethanol and dried in air. Detemplation of the material was
performed by Soxhlet-extraction with acetone overnight.
2.1.2. Co-condensation
General. In a typical synthesis P123 (2 g) was dissolved in 2 M
aqueous HCl (62.5 g) at 40 ◦C. Then silica (TEOS) or organosil-
ica (BTESE) precursor was added (18 mmol) and the mixture was
allowed to prehydrolyze. Upon formation of a slightly blurry sus-
pension, functionalized organosilane (PTES, MPTMS or CSPTMS,
2 mmol) was introduced to the mixture and stirred overnight at
40 ◦C. The mixture was then transferred to either a Teflon-lined
autoclave or an autoclavable PP-bottle and kept statically in an oven
at 100 ◦C for 48 h. After drying in air, the materials were detem-
plated by Soxhlet-extraction with acetone and dried in air.
Propylsulfonic acid-modified SBA-15 (SBA-Pr-coc) and -PMO
(PMO-Pr-coc). Co-condensation was performed using MPTMS as the
functionalized precursor. Oxidation of the surface mercaptopropyl
groups to the corresponding propylsulfonic acid moiety was car-
ried out in situ by adding 0.7 g of 30% aqueous H2O2 to the synthesis
mixture.
2. Experimental
Phenylsulfonic acid-modified SBA-15 (SBA-Ph-coc) and -PMO
(PMO-Ph-coc). Co-condensation was performed using PTES as
the functionalized precursor. Before sulfonation, the phenyl-
functionalized material was dried in vacuo for 3 h at 100 ◦C.
Sulfonation was carried out by suspending 1 g of dried material
in dry CH2Cl2 (15 mL) at room temperature followed by the intro-
duction of 5.2 mL of chlorosulfonic acid. The mixture was refluxed
for 1 h and after cooling down quenched with glacial acetic acid.
Fructose (99%, Sigma–Aldrich), DMSO (Sigma–Aldrich),
TEOS (98%, Merck), bis(triethoxysilyl)ethane (BTESE, 99%,
Sigma–Aldrich), Mercaptopropyltrimethoxysilane (MPTMS, 95%,
Sigma–Aldrich), Phenyltriethoxysilane (PTES, 98%, Sigma–Aldrich),
2-(4-chlorosulfonylphenyl)-ethyltrimethoxysilane
50% solution in CH2Cl2, ABCR), Pluronic P123 (Mn ≈ 5800,
(CSPTMS,

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83
After filtration, the residue was washed thoroughly with water,
THF/water 1:1 and THF consecutively and dried in air.
Ethylphenylsulfonic acid-modified SBA-15 (SBA-C2Ph-coc) and
-PMO (PMO-C2Ph-coc). Co-condensation was performed using
CSPTMS as the functionalized precursor.
the adsorption data obtained. The mesopore volume and mesopore
size distribution were calculated using the Barrett–Joyner–Halenda
(BJH) method using the adsorption branch of the isotherm
(2–50 nm). Unit cell parameters were calculated using the formula
√
a 2d100
/
3, with d100 being the d-spacing distance represented
by the (1 0 0) diffraction peak. Wall thickness was determined by
substracting the BJH pore diameter from the unit cell parameter a0
[26]. The samples for X-ray photoelectron spectroscopy (XPS) mea-
surements were prepared by pressing the solids onto carbon film,
and then transferred to a Thermo Scientific K-Alpha XPS apparatus
equipped with a monochromatic Al K␣ X-ray. The background pres-
sure was 2 × 10−9 mbar. Fitting was carried out with the CasaXPS
program. The reference line for calibration was the C 1s line at
284.5 eV. Solid-state 29Si CP MAS NMR spectra were recorded on
a Bruker DMX-500 NMR spectrometer using a 4 mm zirconia rotor
with a spinning rate of 5 kHz. The chemical shifts were referenced
to SiMe4. The sulfur, hydrogen and carbon content of selected cat-
alysts was determined by Mikroanalytisches Laboratorium Kolbe,
Mülheim a/d Ruhr, Germany. Prior to the measurements the sam-
ples were dried in vacuo at 90 ◦C overnight. Scanning electron
microscopy (SEM) was performed using a Philips environmental
scanning electron microscope FEIXL-30 ESEM FEG in high-vacuum
mode at low voltage. FTIR spectra were recorded on a Bruker Vertex
V70v system. The spectra were acquired as an average of 20 scans
at a resolution of 2 cm−1. All samples were dried in vacuum for 3 h
at 150 ◦C prior to measurement.
2.1.3. Grafting
General. Functionalization of the parent material (SBA-15 or C-C-
PMO) was carried out by refluxing thoroughly dried material (3 h,
100 ◦C, vacuum) in a solution of an excess of the functionalized
organosilane (MPTMS, PTES or CSPTMS) in dry toluene overnight,
followed by filtration and thorough washing with toluene.
Propylsulfonic acid-modified SBA-15 (SBA-Pr-graft) and -PMO
(PMO-Pr-graft). The functionalization was performed using
MPTMS as grafting agent. The resulting mercaptopropyl-modified
(organo)silane (ca. 1 g) was dispersed in an aqueous solution of
H2O2 (30 wt%, 10 mL) and stirred for 6 h at 60 ◦C. After cooling the
mixture to room temperature, a 10 volume% aqueous solution of
sulfuric acid was added. Then the solid was filtered off and washed
thoroughly with water and dried in air.
Phenylsulfonic acid-modified SBA-15 (SBA-Ph-graft) and PMO
(PMO-Ph-graft). The functionalization was performed using the
PTES as grafting agent. Sulfonation was carried out by suspending
1 g of dried phenyl-functionalized material in dry CH2Cl2 (15 mL)
at room temperature and adding 5.2 mL of chlorosulfonic acid. The
mixture was refluxed for 1 h and after cooling down quenched with
glacial acetic acid. After filtration, the residue was washed thor-
oughly with water, THF/water 1:1 and THF consecutively and dried
in air.
3. Results and discussion
Ethylphenylsulfonic acid-modified SBA-15 (SBA-C2Ph-graft) and
PMO (PMO-C2Ph-graft). The functionalization was performed using
the CSPTMS as grafting agent. After washing with toluene, the
material was washed thoroughly with water and dried in air.
3.1. Characterization of materials
Fig. 2 a) and (b) shows X-ray diffraction patters of the par-
ent as well as functionalized SBA-15 and PMO-type materials,
respectively. For most samples, the (1 0 0) reflection characteris-
tic for p6mm symmetry was observed, evidencing the formation
of uniform hexagonal porous structures. In addition, the pres-
ence of the (1 1 0) and (2 0 0) reflections for SBA-15, SBA-Pr-coc
and SBA-Ph-coc samples evidences a long-range order in the
materials. However, the intensity of these reflections diminishes
with increasing bulkiness of the introduced functional group. For
materials prepared by co-condensation, structural properties and
ordering strongly depend on the size and nature of the functional
groups introduced in the silica matrix. The decreasing intensity of
the (1 1 0) and (2 0 0) reflections reveal that the co-condensation
method results in lower ordered materials compared to the non-
functionalized ones and modified by grafting [27]. Especially the
incorporation of the very bulky ethylphenylsulfonic acid species,
combined with the relatively harsh synthesis conditions (2 M HCl),
results in the materials showing only a low ordering. Attempts
to use milder synthesis conditions gave unsatisfactory results
regarding the distribution of the sulfonate moieties because of
the much higher rates of hydrolysis and condensation of the
respective functionalized precursor compared to those for TEOS
or TMOS. As a result, formation of separate polymeric structures
of SiC2PhSO3H and silica was observed in the synthesis mixture.
The lower structural ordering was less apparent in the case of
co-condensation of TEOS with the PrSO3H and Ph-functionalized
alkoxysilanes, although in the latter case the material was dam-
aged and evidence of sulfonation of the silanol moieties was
observed by elemental analysis before and after hydrothermal
treatment.
2.2. Material stability tests
The stability of the synthesized materials was assessed by a
hydrothermal treatment. In a typical procedure, an appreciable
amount (ca. 100–200 mg) of material was suspended in water
(2.5 mL) and kept at 120 ◦C for seven days. After cooling down,
the materials were thoroughly washed with water, collected by
centrifugation and dried in air.
2.3. Catalytic tests
To an appropriate amount of catalyst (either 6 or 60 mg, vide
infra), 2 mL of fructose stock solution of 60 mg/mL solvent was
added in a 4 mL thick-walled glass reaction vial, which was heated
in an oil bath at 120 ◦C for 3 h. The reaction mixtures were then
quenched in cold water, filtered and diluted for HPLC analysis.
Fructose was detected by an ELSD LTII detector using a Prevail Car-
bohydrate ES (Grace) column (MeCN:H2O 3:1 (v/v), 1 mL min−1
,
50 ◦C), HMF was detected by a UV-detector at 300 nm over a
Pathfinder PS C18 reverse phase column (MeOH/H2O 1:4 (v/v),
0.4 mL min−1, 30 ◦C).
2.4. Characterization
XRD diffraction patterns were recorded on a Bruker D4 Endeavor
powder diffraction system using Cu K␣ radiation with a scan-
ning speed of 0.0049◦ s−1 in the range of 0.5◦ ≤ 2ꢀ ≤ 5◦. Nitrogen
adsorption and desorption isotherms were measured at −196 ◦C
on a Micromeritics ASAP3020 Tristar system in static measure-
ment mode. The samples were pretreated at 150 ◦C for several
hours prior to the measurements. The Brunauer–Emmett–Teller
(BET) equation was used to calculate the specific surface area from
The condensation of sterically demanding
C C-bridged
alkoxysilanes resulted in moderately ordered PMO materials with
a very pronounced decrease in mesoscopic ordering upon the
introduction of propyl or phenylsulfonic acid moieties. Thus, the

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W.N.P. van der Graaff et al. / Journal of Molecular Catalysis A: Chemical 388–389 (2014) 81–89
Fig. 2. XRD patterns of sulfonic acid-modified SBA (A) and C-C-PMOs (B).
bulkiness and flexibility of the organic bridging moieties determine
the structural properties of the mesoporous materials. The long-
range order decreases when co-condensation of bulkier substrate
is used for the synthesis of the PMO materials. The introduction
of functional groups by grafting does not affect significantly the
ordering.
Nitrogen physisorption was used to evaluate porosity of the syn-
thesized materials. The main results are summarized in Table 1, the
corresponding N2 isotherms and further details are presented in the
supplementary information (S1).
The nitrogen physisorption data confirm the main conclusions
made on the basis of the XRD analysis. The decreasing pore order-
ing along with increasing bulkiness of the introduced organic
functionality is evidenced by the deviation from the type-IV adsorp-
tion isotherm (see Figure S1.1) [28]. This effect is particularly
pronounced in the case of C-C-PMOs, because of the increased dis-
tortion of the micelles during synthesis.
Morphology of the synthesized catalysts was further analyzed
by scanning electron microscopy (SEM). The representative SEM
images are shown in Fig. 3. The pure silica SBA-15 is represented
by bead-shaped particle clusters. With the increasing bulkiness of
the organic moieties introduced via the co-condensation method
(materials SBA-Pr-coc, SBA-Ph-coc, SBA-C2Ph-coc: Fig. 3 B, C and
D respectively), the particles become less uniform and tend to
become more disk-shaped. Functionalized PMOs tend to form large
clusters upon introduction of increasingly bulky groups. Grafting
results in more oval particles. Similar trends were observed for all
SBA-type materials prepared by this method (e.g. SBA-Pr-graft and
SBA-C2Ph-graft, see S2).
The propylsulfonic acid groups were introduced by either
simultaneous incorporation and oxidation during synthesis or by
grafting MPTMS and subsequent oxidation by H2O2 and acidifica-
tion with H2SO4. Previous studies [29,30] demonstrated that the
sequential oxidation method can lead to incomplete oxidation of
thiol groups. Therefore, we monitored the oxidation state of the
sulfur-containing moieties in the materials by X-ray photoelectron
spectroscopy (XPS). The S 2p XPS spectra (Fig. 4) show two distinct
features corresponding to sulfur in different oxidation states. The
peak at binding energies between 167 and 171 eV is associated with
SO3H sulfonic acid moieties, and the peak appearing between 161
and 165 eV corresponds to SH thiol groups (Fig. 4) [31].
Full oxidation of thiol moieties was observed for most of the
materials as is evidenced by the occurrence of a clear peak around
169 eV, which is also present in the sulfonated SBA-C2Ph-coc. Only
for SBA-Pr-coc some thiol-containing functionality remained after
the oxidation treatment. This is probably associated with the low
accessibility of some of the SH moieties encapsulated in the silica
matrix during the co-condensation.
The introduction of the sulfonate groups was confirmed by FTIR
spectroscopy (see Figure S3.1). The respective materials show a
characteristic band at 1365 cm−1 due the asymmetric S O stretch-
ing vibration in SO3H moieties [32]. This band was not observed
in the FTIR spectra of the catalysts that do not possess the sulfonate
functionality.
29Si CP MAS NMR spectroscopy was employed to analyze the
condensation degree of the sulfonated propylsilane moieties. The
spectra are shown in Fig. 5. Although cross-polarization NMR spec-
troscopy does not allow the quantification of different Si species,
Table 1
Physiochemical data of the synthesized materials.
Catalyst type
Mesopore volume (cm3 g−1 STP)
BET surface area (m2 g−1
Before HT
)
Wall thickness (nm)
Before HT
Before HT
After HT
After HT
After HT
SBA-Pr-graft
SBA-Pr-coc
SBA-Ph-graft
SBA-Ph-coc
SBA-C2Ph-graft
SBA-C2Ph-coc
PMO-Pr-graft
PMO-Pr-coc
PMO-Ph-graft
PMO-PhSO3H-coc
PMO-C2PhSO3H-graft
PMO-C2PhSO3H-coc
SBA-15
0.7
2.0
0.6
0.4
0.5
0.4
1.3
1.0
1.0
1.1
0.5
1.0
1.2
1.1
0.8
0.9
0.5
0.9
0.6
0.5
1.1
1.5
1.0
0.7
0.8
1.1
0.4
1.4
479
625
542
602
374
571
1044
624
828
714
356
699
1022
1073
465
496
369
520
374
572
928
836
744
642
571
560
159
980
6.9
5.9
7.4
6.0
5.5
n.a.a
5.4
5.4
6.1
n.a.a
6.6
2.7
6.0
6.3
3.9
3.2
4.5
4.1
4.9
n.a.a
4.5
3.2
5.4
n.a.a
6.0
2.5
n.a.a
5.7
C-C-PMO
a
The material was not sufficiently ordered for determination of wall thickness by the applied method.

W.N.P. van der Graaff et al. / Journal of Molecular Catalysis A: Chemical 388–389 (2014) 81–89
85
Fig. 3. SEM images of (A) SBA-15, (B) SBA-Pr-coc, (C) SBA-Ph-coc, (D) SBA-C2Ph-coc, (E) C-C-PMO, (F) PMO-Pr-coc, (G) PMO-Ph-coc, (H) PMO-C2Ph-coc. Scale bar 5 ␮m.
intensity of the T-signals occurs around −67 ppm. This position is
characteristic for T3-bound alkylsilanes.
The introduction of arylsulfonic acid moieties is important for
increasing the Brønsted acidity of the materials. Arylsulfonic acids
are considerably more acidic than alkylsulfonic acids [33,34]. In
order to introduce arylsulfonic acid functionalities, chlorosulfonic
acid (HSO3Cl) can be used as an agent for the electrophilic aromatic
substitution [35]. Its high reactivity is key to the facile introduc-
tion of acid functionality onto the surface aryl moieties. However,
it can also lead to unselective introduction of sulfonates onto other
sites as well as other side reactions. Alternatively, ethylphenyl-
sulfonic acid moieties can be introduced by co-condensation of
(MeO)3Si(CH2)2Ph-p-SO2Cl with (organo)silica precursors. Each
method has its advantages and disadvantages: introduction of the
bulky, but rotationally less restrained (MeO)3Si(CH2)2Ph-p-SO2Cl
causes a severe decline in structural ordering (vide supra), whereas
HSO3Cl reacts unselectively and could possibly destroy the mate-
rial.
Fig. 4. S 2p XPS spectra of mercaptopropyl ( SH) and propylsulfonic acid-modified
materials. As reference, SBA-C2Ph-coc is added as reference for sulfur in sulfonate
form.
Indeed, 29Si MAS spectroscopy evidences a pronounced change
in the surface properties of the materials treated with HSO3Cl
(Fig. 6). The Q3 signal in the chlorosulfonic acid-treated SBA-Ph
strongly decreases compared to the untreated material. The reac-
tion with HSO3Cl leads to an exclusive formation of T3 sites on the
silica surface (Func-SBA-Ph-coc, Fig. 6), which are characterized
by a peak at around 80 ppm for phenylsilanes [36]. This suggests
that HSO3Cl promotes the full condensation of T2-mode bound
organosilanes to the surface. It is possible that chlorosulfonic acid
reacts with water from this condensation, thereby providing a driv-
ing force for this reaction.
indications can be obtained on the binding mode of the organosi-
lane moieties. In accordance with literature [22,23], functionalities
introduced by co-condensation are more strongly anchored in the
(organo)silica matrix than in the case of grafting. No T1-species
could be observed above noise level. On the contrary, the materi-
als functionalized by grafting contained substantially more T1 and
T2 species. Nevertheless, in all functionalized materials the major-
ity of the organosilane moieties seem to be T3-bound, as the main
Fig. 5. 29Si CP MAS NMR spectra of SBA-Pr-graft and SBA-Pr-coc.

86
W.N.P. van der Graaff et al. / Journal of Molecular Catalysis A: Chemical 388–389 (2014) 81–89
Fig. 6. 29Si CP MAS NMR spectra of SBA-PhSO3H before and after sulfonation and of
SBA-C2Ph-coc.
Fig. 7. 29Si CP MAS NMR spectrum of SBA-Pr-graft and SBA-Pr-coc before and after
hydrothermal treatment (HT).
The incorporation of (MeO)3Si(CH2)2Ph-p-SO2Cl however leads
to spectra comparable with the propylsulfonic acid-functionalized
materials with mainly T3 sites, although in this case also T1 signals
could be observed, which could originate from the very disordered
nature of the material.
was observed for SBA-Ph-coc. When comparing the relative loss
of carbon (from 3.79% to 2.57%) to the loss of sulfur in this sam-
ple, it turns out that twice as much sulfur leaches than carbon. The
decrease of the carbon content can be partially attributed to the
loss of residual template. However, when considering initial sul-
fur and carbon contents, every phenyl moiety should already be
sulfonated twice under the assumption that all carbon in the mate-
rial is aromatic. Because this is highly unlikely we suppose that the
chlorosulfonic acid treatment results in the formation of a wide
range of surface sulfate species (e.g. silanol-bound SO3H groups)
that are readily hydrolyzed upon contact with water at elevated
temperatures.
3.2. Hydrothermal stability of the materials
Hydrothermal stability of a representative selection of the syn-
thesized materials was assessed by elemental analysis before and
after hydrothermal treatment at 120 ◦C for seven days. In Table 2,
the sulfur content of the as synthesized and hydrothermally treated
materials is presented. In the case of the materials prepared by
grafting, sulfur content decreased dramatically. For SBA-Pr-graft,
60% of the sulfur was eliminated, whereas for PMO-C2Ph-graft
the sulfur loss was 45%, which could indicate an increased hydro-
thermal stability due to the higher hydrophobicity of the pores. For
the co-condensed materials the decrease in the sulfur content after
the hydrothermal treatment was significantly lower.
The stronger decline in sulfur content observed for the mate-
rials prepared by grafting compared to those compared by
co-condensation points to the higher stability of the functionali-
ties when directly incorporated in the (organo)silica matrix. When
comparing the sulfur content in the hydrothermally treated SBA-
type materials with that of the functionalized PMOs, it seems that
the loss of silica prevails over loss of embedded organic function-
alities, which would be in agreement with the results shown in
Table 1, where wall thickness decreases significantly for hydrother-
mally treated materials. As a result the materials containing the
condensed dipodal organosilane show superior stability over the
other materials considered here. In such materials, the structural
degradation upon the hydrothermal treatment is mainly associ-
ated with the leaching of the monopodal functionality from the
organosilica matrix. Interestingly, a sharp decline in sulfur content
The 29Si CP MAS spectra of the SBA-Pr materials (Fig. 7) show
that the loss of functionalities is mainly due to the loss of groups
attached by one (T1) bond to the carrier surface or matrix. After
hydrothermal treatment, no T1 sites were detected in the materials
prepared by grafting. The population of doubly (T2) or triply bound
(T3) sites is affected only slightly by the hydrothermal treatment.
X-Ray diffraction patterns of the samples before and after hydro-
thermal treatment (Fig. 8) evidence a very strong decline of the
mesoscopic ordering for the co-condensed materials and espe-
cially for pure silica SBA-15. The functionalized PMOs retain their
structural integrity much better. This is also confirmed by nitrogen
sorption experiments (see S1). These observations are in line with
the above proposal on the dominant role of desilication rather than
loss of the largely apolar functional groups for the hydrothermal
decomposition of the materials. The presence of the apolar func-
tionalities within the structure of PMOs enhances stability of the
mesoporous silica. One should note that in the light of the ele-
mental analysis results, the apparent structure-stabilizing effect
of grafting seems to be temporary. Despite the structure of such
materials was largely preserved during the hydrothermal treat-
ment, extensive leaching of the catalytic groups was observed.
Grafted species merely form a layer on the carrier protecting it from
extensive hydrolysis. However this functionalized outer layer is
much more exposed and therefore more easily removed than func-
tionalities embedded in the silica framework [20,21]. This renders
co-condensation the preferred preparation method for functional-
ized organosilica catalysts.
Table 2
Sulfur content of selected materials before and after hydrothermal treatment.
Material
Sulfur content (wt.%)
Before hydrothermal
treatment
After hydrothermal
treatment
3.3. Catalytic activity measurements
SBA-Pr-coc
SBA-Pr-graft
SBA-Ph-coc
SBA-C2Ph-coc
PMO-Pr-coc
PMO-C2Ph-coc
PMO-C2Ph-graft
3.05
1.69
3.32
2.88
2.51
2.85
4.70
4.42
0.68
1.16
3.54
1.75
2.21
2.60
A selection of materials with co-condensed functionalities was
employed in the dehydration of fructose in water and DMSO. The
reactions were carried out using different catalyst loadings (6 and
60 mg/2 mL). The catalytic results in both water and DMSO are sum-
marized in Table 3.

W.N.P. van der Graaff et al. / Journal of Molecular Catalysis A: Chemical 388–389 (2014) 81–89
87
Fig. 8. XRD patterns of grafted and co-condensed SBA-type (A1-3) and PMO (B1-3) materials before and after hydrothermal treatment. Hydrothermally treated materials
are indicated with HT.
Despite increased hydrothermal stability of the materials con-
taining hydrophobic moieties embedded in their structure, the
nature of embedded functionality does not have ostensible effect on
the catalytic performance. In water, both fructose conversion and
selectivity to HMF are very low. In DMSO, however, all materials
showed considerable activity and selectivity toward HMF. All mate-
rials, except the non-functionalized SBA-15 and C-C-PMO, proved
to display catalytic activity, resulting in higher HMF yields than in
the blank experiment. A clear increase in HMF yield is observed
for the acid-modified organosilica materials, which are similar to
values reported for comparable catalytic systems [36]. An increase
of the catalyst loading does not lead to a significant increase in
HMF yield, and even decreases HMF selectivity when the reaction
is carried out in the presence of the non-functionalized SBA-15 and
C-C-PMO.
conversion and HMF yield in time was followed. Fig. 9 summa-
rizes the respective results for the bare support materials and the
sulfonic acid functionalized catalysts.
The promoting effect of the surface acid functionalities can be
clearly seen from the results in Fig. 9. This effect is particularly pro-
nounced at the early stages of the reaction. A much higher fructose
dehydration rate is observed for the sulfonic acid-modified materi-
als. The catalytic activity of the PMO-C2Ph-coc hybrid material is the
highest among the catalysts considered including the commercially
available Amberlyst-15. An HMF yield of 39% was already achieved
after 20 min in this case, compared to 19% for Amberlyst-15 and
only trace amounts of HMF for the non-functionalized materials.
To investigate the effect of hydrothermal treatment on the cat-
alytic activity of the materials, catalytic tests in DMSO were carried
out with the samples after hydrothermal treatment (see Table 4).
The reactivity of the modified materials was largely unchanged.
This is surprising taking into account the detrimental effect of
the hydrothermal treatment on the structural properties of the
To provide further insight into the relationship between the
chemical composition of the investigated catalysts and their activ-
ity toward fructose dehydration, the evolution of the fructose
Table 3
Results for the dehydration of fructose over acid-modified materials in water and DMSOa.
Catalyst
X in water (%)
Catalyst amount
6 mg
Y in water (%)
Catalyst amount
6 mg
X in DMSO (%)
Catalyst amount
6 mg
Y in DMSO (%)
Catalyst amount
6 mg
60 mg
60 mg
60 mg
60 mg
SBA-Pr-coc
SBA-Ph-coc
SBA-C2Ph-coc
C-C-Pr-coc
C-C-Ph-coc
C-C-C2Ph-coc
Amberlyst-15
SBA-15
6
2
1
6
6
<1
8
5
17
18
18
16
26
16
14
10
2
2
1
1
2
1
2
2
3
1
4
98
98
>99
>99
>99
>99
>99
>99
>99
66
82
83
83
81
83
81
80
67
66
66b
87
86
88
87
86
87
86
42
50
–
>99
>99
>99
>99
>99
89
<1
6
5
11
<1
<1
–
<1
<1
C-C PMO
Blank
6
92
72
–
<1b
–
<1a
89b
a
Conditions: 2 mL 60 mg/mL solution of fructose, T = 120 ◦C, 3 h. X = conversion, Y = HMF yield.
No catalyst used.
b

88
W.N.P. van der Graaff et al. / Journal of Molecular Catalysis A: Chemical 388–389 (2014) 81–89
Fig. 9. Evolution of the relative fructose concentration (left) and HMF formation in time during the fructose dehydration reaction. Conditions: T = 120 ◦C, 2 mL 60 mg/mL
fructose in DMSO, 6 mg catalyst.
Table 4
Technology. The authors of this work would like to thank Brahim
Mezari and Dr. Pieter Magusin for their support with the NMR spec-
troscopy measurements.
Results for the dehydration of fructose over hydrothermally treated acid-modified
materials in DMSOa.
Catalyst
X (%)
Y (%)
Appendix A. Supplementary data
SBA-Pr-c-HT
SBA-Ph-c-HT
SBA-C2Ph-c-HT
C-C-Pr-c-HT
C-C-Ph-c-HT
C-C-C2Ph-c-HT
Amberlyst-15-HT
PMO-HT
98
97
>99
>99
>99
99
>99
79
97
82
79
85
79
80
84
85
56
73
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.molcata.
2013.11.013.
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