Catalytic dehydration of fructose to HMF over sulfonic acid functionalized periodic mesoporous organosilicas: Role of the acid density

Article abstract of DOI:10.1039/c4cy00137k

Here, we reported the catalytic efficiency of sulfonic acid functionalized periodic mesoporous organosilica (PMO) materials with crystal-like pore walls in the dehydration of fructose to HMF in water. We found that the proton loading, the localization of acid sites and the surface hydrophobicity played a crucial role in the PMO activity.

Full text of DOI:10.1039/c4cy00137k

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Catalytic dehydration of fructose to HMF over
sulfonic acid functionalized periodic mesoporous
organosilicas: role of the acid density†
Cite this: DOI: 10.1039/c4cy00137k
Received 3rd February 2014,
Accepted 28th February 2014
ab
b
c
b
Cláudia Bispo, Karine De Oliveira Vigier, Mariana Sardo, Nicolas Bion,
c
a
b
Luís Mafra, Paula Ferreira* and François Jérôme*
DOI: 10.1039/c4cy00137k
www.rsc.org/catalysis
Here, we reported the catalytic efficiency of sulfonic acid
functionalized periodic mesoporous organosilica (PMO) materials
with crystal-like pore walls in the dehydration of fructose to HMF
in water. We found that the proton loading, the localization of acid
sites and the surface hydrophobicity played a crucial role in the
PMO activity.
development of such processes. Clearly, innovative strategies
need to be investigated to decrease the cost and ecological
footprint of these processes. In this context, the rational
design of solid acid catalysts capable of promoting the dehy-
dration of hexoses to HMF directly in water remains a chal-
lenging issue.
Although a large amount of hexoses can be dissolved in
water, its use as a solvent for the production of HMF requires
two scientific issues to be tackled: (1) the possible acid-
catalyzed rehydration of HMF yielding formic and levulinic
acids as two side products and (2) the deactivation of cata-
lytic acid sites either by solvation with water or leaching. To
date, heterogeneously-catalyzed dehydration of hexose to
HMF has been successfully performed in water in the pres-
Introduction
During the last two decades, much effort has been given to
the catalytic conversion of biomass to chemical platforms,
bioproducts and transportation fuels. Within this context,
carbohydrates have attracted considerable attention mainly
because they represent a huge reservoir of renewable carbon
with a production estimated to about 140 billion tons per year.
Among all possible transformations, the acid-catalyzed
dehydration of hexoses to 5-hydroxymethylfurfural (HMF) is
of prime importance since HMF is now considered as a
chemical platform with strong market potential especially for
5–7
ence of metal phosphates (mainly based on Nb, Fe, V, Zr or Ti),
8
,9
metal oxides (ZrO2 or TiO ) and zeolites. In most of the
2
reported processes, HMF was extracted (sometimes in a
continuous mode) from the aqueous phase by liquid–liquid
phase extraction thus limiting the side rehydration of
HMF to levulinic and formic acids and affording HMF
with 20–60% yield. Recently, SBA-type mesoporous silica-
1
–3
the fabrication of safer and renewably sourced polymers.
In this reaction, the choice of the solvent is crucial and gov-
erns the selectivity of the process. In the current literature,
ionic liquids (ILs) are often preferred for the dehydration of
hexoses since ILs are capable of dissolving large amounts of
hexoses while allowing convenient isolation of HMF by liq-
supported sulfonic sites (SBA-SO H) have been found to be
3
capable of catalyzing the dehydration of fructose in a water–
methylisobutylketone-2-butanol biphasic system affording
1
0
HMF with a yield of 60%. In 2013, Karimi hypothesized that
the surface hydrophilicity of sulfonated SBA materials facili-
tated the desorption of HMF from the catalytic surface, thus
4
uid–liquid phase extraction. However, the best ILs are also
prohibitively expensive for use on a large scale. In addition,
utilization of ILs as a reaction medium is mostly restricted to
the use of Lewis acids since cation metathesis readily occurs
in the presence of a Brønsted acid, thus limiting the
1
1
contributing to limit the side rehydration of HMF. Although
high yield of HMF was obtained in aqueous solution in the
presence of a sulfonated SBA material, one should mention
that such a catalyst is unfortunately deactivated in water due
a
12,13
Department of Materials and Ceramic Engineering, CICECO, University of Aveiro,
to a strong solvation and leaching of the active sites.
3
810-193 Aveiro, Portugal. E-mail: pcferreira@ua.pt
In 2002, Inagaki and co-workers reported the synthesis of
b
Université de Poitiers, CNRS UMR 7285, Institut de Chimie des Milieux et
mesoporous benzene–silica with crystal-like pore walls
Matériaux de Poitiers (IC2MP), ENSIP, 1 rue Marcel Doré, 86022 Poitiers, France
1
4
12,13,15
16
c
(Ph-PMO). We
and others have shown that their
Department of Chemistry, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal
functionalization with propylsulfonic acid sites yields solid
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cy00137k
acid catalysts (Ph-PMO-PrSO H) that are much more active
3
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and stable in water than the traditionally used sulfonated
periodic mesoporous silicas such as SBA-SO H, MCM-SO H
or HMS-SO H. Although sulfonated PMO cannot be consid-
3
Results and discussion
Synthesis and characterization of acid-functionalized PMOs
3
3
ered as a hydrophobic material, the high activity of Ph-PMO-
Different kinds of sulfonated PMO with varying sulfonic acid
loadings and molecular structures were synthesized using
PrSO H was attributed to the presence of phenyl rings in the
3
pore wall that creates a hydrophobic environment capable of
partly preventing the solvation of sulfonic sites by water.
literature procedures or modification of existing ones
1
3,15
24,25
(Scheme 1).
PMO-1a–d were prepared by co-condensation
Owing to its high stability in water, Ph-PMO-PrSO H is
of different molar ratios of 1,4-bis(triethoxysilyl)benzene
(BTEB) and 3-mercaptopropyltrimethoxysilane (MPTMS) in
the presence of a surfactant, followed by oxidation of
the thiol to sulfonic acid groups. The acid loadings in the
PMO-1 from a to d were, respectively, 1.11, 0.56, 0.36 and
3
now receiving more and more attention, especially in the
field of water-generating reactions. Recently, Dumesic and
1
7
co-workers have reported the use of ethane-bridged PMO
materials for the continuous production (in tubular reactors)
−
1
of HMF in a single phase solution of THF and H O (4 : 1 w/w).
0.25 mmol g . A propylsulfonic acid biphenyl PMO material
named PMO-2 was obtained using a similar methodology but
substituting the BTEB for 4,4′-bis-(triethoxysilyl)biphenyl
(BTEBP). The acid concentration in the final material was
2
Although this PMO is amorphous, authors have clearly dem-
onstrated the superior stability of ethane-bridged PMO mate-
rials as compared to that of SBA-SO H further confirming the
3
−
1
contribution of such materials for biomass processing.
0.56 mmol g . A phenyl-bridged PMO sulfonated in the
aromatic ring, called PMO-3, was prepared by sulfonation of
the phenyl PMO (Ph-PMO) obtained by polycondensation of
Here, we report that the chemical nature of organic brid-
ges within the pore wall, the location of the sulfonic sites, as
well as the proton density, play a key role in the activity of
sulfonated PMO in the aqueous catalytic synthesis of HMF
from fructose. In particular, we show that sulfonated meso-
porous benzene and biphenylene bridged PMOs with crystal-
like pore walls are capable of promoting the dehydration of
fructose in neat water with high turn-over frequencies (TOF).
In an attempt to understand the difference of catalytic behav-
2
6
BTEB. The description of the protocol adopted to prepare
the studied materials is given in the ESI.† The acid content in
−
1
PMO-3 was 0.64 mmol g . All materials were characterized
by the usual solid state analytical techniques (see the ESI†).
Acid-catalyzed dehydration of fructose to HMF
3
1
ior observed between all tested sulfonated PMO, P magic-
angle spinning (MAS) NMR spectroscopy of chemisorbed
triethylphosphine oxide (TEPO) was employed as a local
phosphorous-based probe molecule is highly sensitive to the
In our experiments, fructose was chosen as a model feed for
HMF production from sugars. Note that glucose is a far more
relevant feed but, to date, Bronsted acids are not capable of
converting glucose to HMF. Therefore, the activity of different
PMOs was evaluated in neat water in the acid-catalyzed dehy-
dration of fructose to HMF. Typically, 0.23 g of fructose was
dissolved in 0.52 g of water (i.e. 44 wt% of fructose) and
heated to 160 °C in the presence of 16 wt% (related to the
amount of fructose, i.e. 37 mg) of solid acid catalyst. To limit
1
8–23
nature and strength of acid sites.
To the best of our
knowledge this is the first time that the acid sites contained
in a sulfonic acid functionalized PMO material are character-
3
1
ized at a molecular level by P MAS NMR using adsorbed
TEPO probe molecules.
Scheme 1 Schematic representation of sulfonic acid functionalized PMO materials prepared in this work.
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the side rehydration of HMF to levulinic and formic acids,
the reaction was conducted in a biphasic system composed
of H O as the catalytic phase and a mixture MIBK (methyl-
2
isobutylketone)–2-butanol (70/30) as an extraction phase.
Such a biphasic system aimed at separating HMF from the
water phase to limit its rehydration to levulinic and formic
acids. The reaction progress (fructose conversion and yield of
HMF) was monitored by HPLC. Prior to evaluating the perfor-
mance of sulfonated PMO, the reaction was first performed
obtained after 180 min of reaction. In this case, the thermal
degradation of HMF was faster than its formation explaining
the low selectivity observed at 130 °C. Reversely, an increase
of the reaction temperature from 160 °C to 180 °C had obvi-
ously a positive effect on the reaction rate but with no signifi-
cant improvement in the selectivity to HMF. Under these
conditions, a maximum yield of HMF of 59% was obtained
after 45 min of reaction (vs. 58% at 160 °C). Altogether, these
results showed that, over PMO-1a, 160 °C is an acceptable
compromise between reaction rate and HMF selectivity.
In an attempt to identify more active catalysts, different
sulfonated PMO materials were tested in the dehydration of
fructose to HMF in water (Table 2). We applied the optimized
conditions described in Table 1 to all PMOs.
in the presence of SBA-SO H which was used here as the ref-
3
erence catalyst. Results are compiled in Table 1. Please note
that in all cases, the yield of HMF reached a maximum and
then drops for a longer reaction time due to the acid cata-
lyzed degradation of HMF. Hence, otherwise noted, in
Table 1, only the maximum yield of HMF is provided.
We observed that the catalyst activity of propylsulfonic
benzene-bridged PMO was related to the proton loading. As a
general trend, decreasing the proton loading from 1.11 mmol
From the results presented in Table 1, it appeared that the
−
1
reference SBA-SO H was highly active (TOF = 860 h )
3
−
1
−1
affording a maximum yield of HMF of 55% after 70 min of
g
(PMO-1a) to 0.56 mmol g
(PMO-1b) and then to
−
1
reaction. Under similar conditions, PMO-1a also exhibited a
0.36 mmol g (PMO-1c), gradually increased the TOF from
520 h to 660 h and finally to 945 h , respectively. On the
other hand, a decrease of the proton loading from 0.36 to
−
1
−1
−1
−1
high TOF (520 h ) but lower than that of SBA-SO
3
H. The
maximum yield of HMF remained, however, in a similar
range (58%). The lower activity of PMO-1a compared to that
of SBA-SO H might be attributed, in the first approximation,
3
to hydrophobic interactions. Fructose is indeed a highly polar
molecule and one may suspect that adsorption of fructose on
the organosiliceous PMO-1a may be less favourable than over
a material containing a highly hydrophilic surface like
−
1
0.25 mmol g has nearly no effect on the TOF. These results
highlighted that isolation/dispersion of sulfonic acid sites on
the PMO surface plays a key role in its activity. In addition,
we checked the influence of remaining surface silanol groups
on the PMO activity. To this end, silanol groups of PMO-1c
were silylated according to a conventional method involving
reaction with trimethylchlorosilane yielding Si-PMO-1c (see
the ESI†). When comparing PMO-1c and Si-PMO-1c (with
similar proton loading, entries 3 and 5, Table 2), it clearly
showed that the silylation of the silanol groups caused a dra-
SBA-SO H. Note that other parameters (acid strength, acid
3
site location, hydrophobicity, silanol concentration) may also
impact the activity of PMO-1a. This aspect will be discussed
later in the manuscript.
The influence of temperature and fructose content on the
HMF yield, collected with PMO-1a, was also investigated. A
decrease of the fructose content from 44 wt% to 10 wt% did
not result in a significant change of the HMF yield (58% at
−
1
−1
matic decrease in the PMO activity (TOF = 945 h and 380 h
for PMO-1c and Si-PMO-1c, respectively). A similar trend was
observed when phenyl bridges were replaced by a more
−
1
hydrophobic moiety such as biphenyl (PMO-2, TOF = 350 h ).
It should be noted that, for the most active PMO (1a–d), the
maximum yield of HMF remained in a similar range
(50–58%). Selected kinetic profiles are provided in the ESI†
(Fig. S9 and S10).
4
4 wt% vs. 53% at 10 wt%) (Table 1). A decrease of the reac-
tion temperature from 160 °C to 130 °C, however, dropped
the reaction rate and the yield of HMF at an unacceptable
level. At this temperature, only 24% yield of HMF was
3
Table 1 Acid-catalyzed dehydration of fructose over A15, SBA-SO H and PMO-1a
+
a
−1
b
c
−1
Catalyst
SBA-SO
H loading (mmol g
)
Time (min)
Conv. (%)
Yield (%)
TOF (h
)
d
3
H
0.89
1.11
1.11
1.11
1.11
70
75
90
180
45
96
95
96
82
96
55
58
53
24
59
860
520
244
nd
d
PMO-1ae
PMO-1a
f
PMO-1a
PMO-1a
g
nd
a
b
c
Determined by titration (see Experimental section for more details). Determined by HPLC. The TOF was measured at a conversion <20%.
d
e
f
4
4 wt% of fructose in water (0.5 mL), 1.5 mL of MIBK/2-butanol (70/30), 160 °C. From an aqueous solution of 10 wt% of fructose. Reaction
g
performed at 130 °C, 10 wt% of fructose. Reaction performed at 180 °C, 10 wt% of fructose.
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Table 2 Acid-catalyzed dehydration of fructose over different PMOa
probe molecule were carried out. This technique has been
previously applied to characterize the acidity of solid acids
+
b
−1
c
−1
Entry
Catalyst
H loading (mmol g
)
TOF (h
)
1
9,21
19
such as zeolites,
Al-MCM-41 with different aluminum contents,
12-tunstophosphoric acid materials,
1
2
3
4
5
6
7
PMO-1a
PMO-1b
PMO-1c
PMO-1d
Si-PMO-1c
PMO-2
1.11
0.56
0.36
0.25
0.34
0.56
0.64
520
660
945
910
380
350
450
21
sulfonic
2
1
resin (Amberlyst 15), functionalized SBA-15 with different
1
7,20,21
acid loadings,
and arenesulfonic acid functionalized
2
2
SBA-15. TEPO acts as a base that binds to the acid sites
located on the material surface. Depending on the strength
and nature of the TEPO-surface binding, the phosphorous
nucleus (of TEPO) will sense differences in its chemical envi-
PMO-3
a
4
4 wt% of fructose in water (0.5 mL), 1.5 mL of MIBK/2-butanol
b
(70/30), 160 °C. Determined by titration (see Experimental section
31
c
ronment due to changes of the local P electron density,
for more details). The TOF was measured at conversion <20%.
3
1
thus resulting in distinct P chemical shifts. Overall, moni-
3
1
toring P chemical shifts of TEPO will give us a scale for
3
1
Altogether, these results tend to support that the rein-
forcement of the hydrophobicity of PMO has also a detrimen-
tal effect on its activity. In particular, we suspect that an
optimal hydrophilic/lipophilic balance is required to assure
an efficient dispersion of sulfonated PMO in the aqueous
phase while favouring an optimal adsorption of fructose on
the PMO surface.
acidity strength. P resonances located at high frequencies
(left-shifted) reflect the presence of strong acid sites, such as
the TEPO⋯HO S hydrogen bond interactions. In opposite,
3
interactions involving TEPO⋯HOSi groups yield, in general,
the most right-shifted resonances as they are the less acidic
functional groups in the studied PMOs.
3
1
Fig. 1 shows the P MAS NMR spectra of chemisorbed
TEPO on propylsulfonic acid PMO materials containing dif-
In the last set of experiments, we investigated the effect of
the localization of sulfonic sites on the PMO activity. The
3
1
ferent acid loadings (PMO-1a–c). The P spectra of the
silylated PMO-1c sample (Si-PMO-1c) and sulfonic acid free
Ph-PMO are also shown in Fig. 1 for comparison. The latter
is used as a reference and is characterized by a single
−
1
PMO activity was lowered from 660 to 450 h when the pro-
pyl sulfonic acid sites (in PMO-1b) were replaced by the direct
anchoring of sulfonic acid groups on the phenyl ring
3
1
(PMO-3). This result suggests that the interaction of fructose
P resonance at ca. 58.5 ppm typical of TEPO adsorbed at
2
0,27
with the active sites is more favourable when the sulfonic
acid groups are attached to the surface through a propyl
chain which is more flexible and away from the PMO surface.
In order to assess the stability of the materials in water, the
silanol groups.
For propylsulfonic acid PMOs functionalized with distinct
acid loadings (PMO-1a–c), a large distribution of P chemical
3
1
PMO-1a and SBA-15-SO H have been recycled (see Experimental
3
section for more details on the procedure). The results are
presented in Table 3. In agreement with previous reports from
1
2,13,15
16,17
us
and others,
PMO-1a was found to be more robust
H. In particular, while the PMO-1a
in water than SBA-SO
3
maintained its activity during the second run, the TOF of SBA-
−1
SO H was decreased by a factor of 2 (450 h ) during the second
3
cycle further confirming the higher tolerance of propylsulfonic
acid functionalized PMO to water than SBA-SO H. However, it
3
is worth mentioning that at 160 °C the activity of PMO-1a
started to irreversibly drop after the second catalytic cycle.
3
1
Characterization of PMO-1a–d using P solid-state NMR
To obtain a deeper structural insight on PMO materials,
3
1
P MAS NMR experiments using TEPO as a Brønsted acidity
Table 3 Recycling experimentsa
b
−1
Catalyst
Catalytic cycle
TOF (h
)
PMO-1a
PMO-1a
1
2
1
2
520c
650
860
410
SBA-SO
SBA-SO
3
H
H
3
a
4
4 wt% of fructose in water (0.5 mL), 1.5 mL of MIBK/2-butanol
b
c
(70/30), 160 °C. The TOF was measured at conversion <20%. The
3
1
increase of TOF between the first and second cycles was due to the
presence of remaining HMF in the PMO material after the first cycle.
Fig. 1
P MAS NMR spectra of TEPO probe molecules adsorbed on
PMO-1a, PMO-1b, PMO-1c, Si-PMO-1c and Ph-PMO.
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shifts covering the 35 ppm region (from 60 to 95 ppm) was
observed. This indicates a broad distribution of Brønsted
acid sites associated with the co-existence of various sulfonic
sites in distinct local environments. Based on the literature,
sites with different local environments at the PMO-1a–c
surfaces and thus with different acid strengths.
Generally, the catalytic activity of the PMOs closely relies
on: 1) the sulfonic site density; 2) the localization of sulfonic
sites and 3) the surface hydrophobicity. Ideally, a PMO
exhibiting: 1) phenylene in the pore wall; 2) an anchorage of
sulfonic sites on a propyl chain and 3) a sulfonic site loading
3
1
the P peak observed at 94.2 ppm for PMO-1a–c was attrib-
uted to very strong Brønsted acid sites while the peak at ca.
2
1
6
0.4 ppm is the signature of weaker acid sites. Compared
3
1
−1
to the Ph-PMO, the P resonances associated with the silanol
of 0.36 mmol g is the most active one. Although, at 160 °C,
acid sites in PMO-1a–c are now slightly left-shifted by ~
the long term stability of these PMOs is far from satisfying,
we nevertheless confirmed here that these materials are more
stable in water than Amberlyst 15 or purely siliceous acid-
+
2 ppm. This small discrepancy is likely to be associated with
the introduction of the SO H groups during the preparation
3
of PMO-1a–c, which influences the local chemical environ-
3
functionalized materials such as SBA-SO H opening an inter-
esting route for biomass processing in water.
2
1,22,27
ment of the neighbour –SiOH groups at the surface.
In
order to unambiguously assign the silanol sites of the reso-
nance at 60.4 ppm, we have silylated the sample PMO-1c
resulting in the spectrum labeled as Si-PMO-1c (Fig. 1). As
expected, the resonance at 60.4 ppm decreases with respect
to the resonances at the left side of the spectrum (the inten-
sity of the Si-PMO-1c spectrum was vertically expanded inten-
tionally, with respect to the others, to emphasize this aspect).
Experimental section
Typical procedure for the acid-catalyzed dehydration of
fructose to HMF
To a series of thick-walled glass reactors, an aqueous layer
consisting of 44 wt% fructose in deionized water (0.5 g) and
1.5 g of an organic layer consisting of 7 : 3 (w : w) MIBK :
2-butanol were added with 16 wt% (related to fructose) of
solid catalyst and a triangular magnetic stirring bar. The
temperature was maintained at 160 °C. At the end of the
reaction, the MIBK : 2-butanol organic phase was decanted
and the aqueous phase was further extracted with MIBK
(3 × 10 mL) and analyzed by HPLC. Please note that fresh
sulfonated PMO and SBA were used. It is worth mentioning
that we observed a decrease of sulfonated PMO and SBA
activities after aging these solid catalysts under air for a
few weeks.
2
7
Similar effects were reported in the literature.
Regardless of the acid loading, all functionalized propyl-
3
1
sulfonic acid PMO materials show the P resonance corre-
sponding to the strong Brønsted acid sites (ca. 94 ppm),
which seems to maintain its relative peak intensity, com-
pared to other acid sites. Interestingly, increasing the proton
−
1
loading of PMOs (from 0.36 to 1.11 mmol g ) mainly
increases the intensity of the resonance at 72.9 ppm, which
3
is associated to –SO H sites having an intermediate Brønsted
acidity strength. The material containing the highest acid
loading (PMO-1a) shows clearly a distribution of Brønsted
acid sites, with the maximum intensity centered at ca.
7
2.9 ppm that corresponds to the main –SO
tion. PMO-1a exhibits the lowest TOF on the dehydration of
fructose supporting that the local environment of the –SO
3
H acid popula-
Recycling experiments
At the end of each reaction, the organic phase containing the
HMF was separated by decantation. The aqueous phase was
washed again with MIBK and then analyzed by HPLC. The
PMO was removed from the aqueous phase by centrifugation
and then reused as collected without any intermediate
purification.
3
H
groups plays a pivotal role in the PMO activity. In particular,
one may suspect that Bronsted acid sites located at 72.9 ppm
are less effective in the acid-catalyzed dehydration of fructose
to HMF presumably due to lower acid strength. It should be
however noted that at a loading ≥0.56 mmol, other parame-
ters may also influence the PMO activity. In particular, solva-
tion of –SO
3
H in a larger extent (phenyl ring being incapable
Acknowledgements
to protect all sulfonic sites from solvation at such a high
1
3
loading) or the problem of acid sites accessibility cannot be
ruled out. In addition, change of the sulfonic loading may
also affect the catalyst surface hydrophilicity and thus the
Authors are grateful to the CNRS and the French Ministry of
Research for financial support. We are grateful to the
Fundação para a Ciência e a Tecnologia (FCT), Fundo Europeu
de Desenvolvimento Regional (FEDER), QREN-COMPETE
(PTDC/QUI-QUI/113678/2009), the European Union, and the
Associate Laboratory CICECO (Pest-C/CTM/LA0011/2013) for
continued support and funding.
1
5
selectivity of the reaction. At this stage, extra analysis is def-
initely required to ascertain the role of the proton loading on
the PMO activity.
Conclusion
Notes and references
We report here that sulfonated phenylene- and biphenylene-
bridged PMOs with crystal-like pore walls are promising acid
solid catalysts for the dehydration of fructose to HMF in
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