Mesoporous carbon-silica solid acid catalysts for producing useful bio-products within the sugar-platform of biorefineries

Article abstract of DOI:10.1039/c4gc01037j

Useful bio-products are obtainable via the catalytic conversion of biomass or derived intermediates as renewable carbon sources. In particular, furanic ethers and levulinate esters (denoted bioEs) have wide application profiles and can be synthesised via acid-catalysed reactions of intermediates such as fructose, 5-hydroxymethyl-2-furaldehyde (HMF) and furfuryl alcohol (FA) with ethanol. Solid acid catalysts are preferred for producing the bioEs with environmental benefits. Furthermore, the versatility of the catalyst in obtaining the bioEs from different intermediates is attractive for process economics, and in the case of porous catalysts, large pore sizes can be beneficial for operating in the kinetic regime. Carbon-based materials are attractive acid catalysts due to their modifiable surface, e.g. with relatively strong sulfonic acid groups (SO₃H). Considering these aspects, here, we report the preparation of mesoporous (SO₃H)-functionalised-carbon/ silica (C/S) composites with large pores and high amounts of acid sites (up to 2.3 mmol g^-1), and their application as versatile solid acid catalysts for producing bioEs from fructose, HMF and FA. The mesoporous composites were prepared by activation of an organic compound deposited on the ordered mesoporous silicas MCF (mesostructured cellular foam) and SBA-15, where the organic compound (p-toluenesulfonic acid) acted simultaneously as the carbon and SO₃H source. The atomic-level characterisation of the acid nature and strengths was performed by ³¹P solid-state NMR studies of an adsorbed base probe, in combination with FT-IR and XPS. Comparative catalytic studies showed that the C/S composites are interesting catalysts for obtaining bioEs in high yields, in comparison with classical solid acid catalysts such as sulfonic acid resin Amberlyst-15 and nanocrystalline (large pore) zeolite H-beta. the Partner Organisations 2014.

Full text of DOI:10.1039/c4gc01037j

Green Chemistry
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Mesoporous carbon–silica solid acid catalysts
for producing useful bio-products within the
sugar-platform of biorefineries†
Cite this: Green Chem., 2014, 16,
292
4
a
a
a
a
Patrícia A. Russo,‡ Margarida M. Antunes,‡ Patrícia Neves, Paul V. Wiper,
a
b
b
a
a
Enza Fazio, Fortunato Neri, Francesco Barreca, Luís Mafra, Martyn Pillinger,
Nicola Pinna* and Anabela A. Valente*
c
a
Useful bio-products are obtainable via the catalytic conversion of biomass or derived intermediates as
renewable carbon sources. In particular, furanic ethers and levulinate esters (denoted bioEs) have wide
application profiles and can be synthesised via acid-catalysed reactions of intermediates such as fructose,
5-hydroxymethyl-2-furaldehyde (HMF) and furfuryl alcohol (FA) with ethanol. Solid acid catalysts are pre-
ferred for producing the bioEs with environmental benefits. Furthermore, the versatility of the catalyst in
obtaining the bioEs from different intermediates is attractive for process economics, and in the case of
porous catalysts, large pore sizes can be beneficial for operating in the kinetic regime. Carbon-based
materials are attractive acid catalysts due to their modifiable surface, e.g. with relatively strong sulfonic
3 3
acid groups (SO H). Considering these aspects, here, we report the preparation of mesoporous (SO H)-
functionalised-carbon/silica (C/S) composites with large pores and high amounts of acid sites (up to
.3 mmol g− ), and their application as versatile solid acid catalysts for producing bioEs from fructose,
1
2
HMF and FA. The mesoporous composites were prepared by activation of an organic compound deposited
on the ordered mesoporous silicas MCF (mesostructured cellular foam) and SBA-15, where the organic com-
3
pound (p-toluenesulfonic acid) acted simultaneously as the carbon and SO H source. The atomic-level
3
1
Received 4th June 2014,
Accepted 2nd July 2014
characterisation of the acid nature and strengths was performed by P solid-state NMR studies of an
adsorbed base probe, in combination with FT-IR and XPS. Comparative catalytic studies showed that the C/S
composites are interesting catalysts for obtaining bioEs in high yields, in comparison with classical solid acid
catalysts such as sulfonic acid resin Amberlyst™-15 and nanocrystalline (large pore) zeolite H-beta.
DOI: 10.1039/c4gc01037j
www.rsc.org/greenchem
is composed of lignin and carbohydrate polymers, the latter
representing the major portion. Carbohydrates can be chemi-
Introduction
Concerns about the diminishing reserves of crude oil and the cally transformed into various useful bio-products (denoted
worldwide socio-economic dependence on fossil fuels, as well bioEs) such as alkyl levulinates (ALs) and the furanic ethers
as the effects of anthropogenic CO , are stimulating the devel- 5-(alkoxymethyl)-2-furfural (5AMF) and 2-(alkoxymethyl)-furan
2
opment of alternative routes to chemicals and fuels. Special (2AMF) (Scheme 1). ALs find applications in different sectors
attention is being paid to biomass as a renewable and abun- of the chemical industry, e.g. as bio-solvents, plasticizing
dant carbon source, particularly non-edible lignocellulosic agents, odorous substances, (bio)fuel additives and as building
1
0,11
biomass which can be obtained from forest, agricultural, blocks for chemical transformations.
In particular, ethyl
1
–9
12–15
16
municipal and industrial wastes.
Lignocellulosic matter levulinate (EL)
and 5-(ethoxymethyl)-furfural (5EMF)
possess interesting properties such as oxygenated fuel exten-
ders for gasoline, diesel and biodiesel (e.g. to improve engine
efficiency, reduce pollutant emissions, etc.).
a
Department of Chemistry, CICECO, University of Aveiro, Campus de Santiago,
3
810-193 Aveiro, Portugal. E-mail: atav@ua.pt
b
The furanic ethers 5AMF and 2AMF can be synthesised via
Università degli Studi di Messina, Dipartimento di Fisica e di Scienze della Terra,
Viale F. Stagno d’Alcontres, 31 98166 Messina, Italy
the acid-catalysed reactions of 5-hydroxymethyl-2-furaldehyde
c
17–28
Institut für Chemie, Humboldt Universität zu Berlin, Brook-Taylor-Straße 2, 12489
(
HMF)
or furfuryl alcohol (FA), respectively, with aliphatic
HMF and FA are derived from the catalytic con-
Berlin, Germany. E-mail: nicola.pinna@hu-berlin.de
29–34
alcohols;
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4gc01037j
These authors contributed similarly to this work.
version of hexose and pentose-based carbohydrates, respecti-
vely (Scheme 1). The HMF and FA routes can additionally lead
‡
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organics have shown very promising catalytic activity in acid-
catalysed reactions of biomass in comparison with commercial
5
4,58,62,65
catalysts such as zeolites or Amberlyst-15.
Carbon-ordered mesoporous silica composites are particu-
larly interesting materials for the production of SO H-function-
3
alised catalysts for biomass conversion. These solids combine
the attractive characteristics of ordered mesoporous silicas for
catalytic applications, namely, high surface areas, pore
volumes and tunable pore sizes, with the attractive properties
of the carbon, specifically the high stability for liquid-phase
5
6,57,61,66
reactions and easily modifiable surface.
Further
advantages may also arise from the combination of both
materials, such as improved hydrothermal and mechanical
6
7
stability with respect to carbon materials. Moreover, it has
been found that stronger acid sites or a higher proportion of
stronger acid sites can be created when the carbon is deposited
5
6,65
on silica, i.e., stronger solid acids are produced.
Neverthe-
less, mesoporous carbon–silica composites have been poorly
explored as catalysts for biomass reactions, particularly for the
conversion of saccharides, HMF and FA to bioEs.
We have recently reported a simple method for preparing
3
stable SO H-functionalised carbon-based materials with high
acid site content and strong acidity, which involves the low temp-
erature activation of a carbon precursor that also contains the
65
3
SO H functionality (p-toluenesulfonic acid). It was found that a
stronger solid acid with improved catalytic activity compared to
the pure carbon was obtained when the carbon was deposited
on non-porous silica nanoparticles. Herein, we explore this
approach and the large surface areas and pore volumes of
Scheme 1 Envisioned bio-based products of carbohydrates platform SBA-15 and mesostructured cellular foam (MCF) silicas to
biorefineries.
produce mesoporous carbon–silica acid catalysts with relatively
high carbon, sulfur and acid sites content, in addition to large
to ALs. Different types of acid catalysts have been investigated pores and strong acid sites. The atomic-level characterisation of
31
for producing bioEs. Homogeneous catalysts, such as mineral acid sites and strengths was achieved by the P solid-state NMR
and organic acids, inorganic salts, ionic liquids and hetero- studies of adsorbed triethylphosphine oxide (TEPO) in combi-
polyacids,
saccharides,
effectively
catalyse
the
and FA
reactions
of nation with FT-IR and XPS. The composites were effective cata-
to lysts in the reactions of HMF and FA with ethanol to give bioEs,
1
9,24,35–41
18–20,22–24,36,42
20,29,32,43,44
HMF
5
AMF, 2AMF and ALs. However, heterogeneous acid catalysts as well as in the cascade reaction of fructose–HMF–bioEs; their
have several advantages over homogeneous ones, such as catalytic performances were compared with those of the com-
facilitated separation from the reaction mixture and adequacy mercial catalysts Amberlyst™-15 and nanocrystalline zeolite beta.
for continuous processes.
Commercial ion-exchange resins such as Amberlyst™-15
are amongst the most active solid acid catalysts for
1
9,25,37,45–49
19,21,28,50
producing bioEs from saccharides,
FA.
HMF
These types of acid resins possess strong sulfo-
nic acid groups, although their relatively low thermal stabi-
or
Results and discussion
Characterisation
1
9,29,31–33,51
lities can limit catalytic applications. Sulfonated carbon-based Mesoporous SO H-functionalised carbon–silica composites
3
materials are expected to be more stable and economical than with varied carbon content were prepared by activation of
acid resins. For these reasons, several carbon-based materials p-toluenesulfonic acid deposited into the pores of SBA-15 and
5
2,53
including ordered mesoporous carbons,
graphene- MCF silicas (Table 1). The SBA-15 has cylindrical pores of
5
4,55
53,54
related materials,
composites
carbon nanotubes,
carbon–silica 9.1 nm diameter whereas the MCF consists of 31.5 nm spheri-
5
6–61
or carbons prepared by incomplete carboni- cal pores accessible through 19.9 nm windows. Therefore,
6
2–64
zation of organic compounds
have been modified with composites with cylindrical mesopores (C/SBA(14), C/SBA(45))
SO H functionalities and tested as acid catalysts. Indeed, or large spherical mesopores (C/MCF(40), C/MCF(63)) were
3
graphene-related materials, carbon–silica composites and produced (cf. TEM images in Fig. S1†), where the number in
carbons produced by sulfonation of incompletely carbonized parentheses is the wt% of functionalised carbon. Moreover,
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Table 1 Chemical and textural characteristics of the mesoporous carbon/silica composites
b
d
Coating
(wt%)
Acid sites
−1
a
Sc (mmol g−1)
BET (m g−1)
e
2
f
(cm g−1)
3
h
Sample
R
(mmol g
)
S
V
p
D
p
(nm)
g
SBA-15
MCF
C/SBA(14)
C/SBA(45)
C/MCF(40)
C/MCF(63)
—
—
1.6
1.5
1.0
1.6
—
—
14
45
40
63
—
—
0.8
2.1
2.0
2.2
—
—
1.0
1.9
1.9
2.3
793
668
602
238
279
198
1.10 (0.05)g
2.30 (0.03)
0.95
0.27
0.87
9.1
31.5 (19.9)
9.1
7.6
30.4 (17.3)
22.9 (10.9)
i
i
i
0.39
a
b
c
2 4
H SO /TsOH (w/w) ratio. Weight% of the functionalised carbon component assessed by TGA. Sulfur content determined by elemental
d e f g h
analysis. Amount of acid sites measured by acid–base titration. BET surface area. Pore volume. Micropore volume in parentheses. Pore
diameter. Window width in parentheses.
i
the large pore sizes and volumes of the parent silicas enabled hexagonal arrangement of pores, typical of SBA-15 (Fig. 1). The
the deposition of large quantities of carbon inside the pores.
patterns exhibit the same number of peaks as that of the
The Raman spectra of the materials exhibit the D and G uncoated silica, which correspond to identical values of the
2
−1
bands associated with sp carbon, at ca. 1360 and 1580 cm
G
respectively (Fig. S2†). The ratio of the peak intensities (I /I )
unit cell parameter. Hence, the incorporation of the carbon
occurred without significant modification of the pore struc-
D
is ca. 0.68 for all samples and indicates that the carbon has tural order.
6
8
1
13
very small domains of aromatic rings. The H– C CP MAS
The nitrogen sorption isotherms of both the composites
NMR spectrum of C/MCF(63) (Fig. S3†) is similar to those and parental silicas are type IV, with condensation steps and
reported previously for the materials synthesised using non- hysteresis cycles at high pressures that reflect the presence of
6
5
porous silica as a support. A main resonance appearing at large mesopores in the materials (Fig. 2). The textural pro-
29 ppm is assigned to polycyclic aromatic carbons, and two perties of the composites depend on their carbon content and
weaker resonances at 20 and 139 ppm are due to methyl on the starting silica (Table 1). Those with the highest carbon
groups and carbon bonded to sulfur atoms, respectively. contents have the lowest SBET, V and D compared to the
1
p
p
The wide angle X-ray diffractograms show a single broad corresponding uncoated silica. None of the composites
reflection at ca. 22° 2θ that is typical of the amorphous carbon, contain micropores accessible to N , which contrasts with the
2
overlapped with the contribution from the amorphous silica at silicas, indicating that the micropores located on the mesopore
similar angles (Fig. S4†). The small angle XRD patterns of walls of the silica were filled with carbon. The results suggest
C/SBA(14) and C/SBA(45) show reflections associated with the that the carbon was successfully deposited inside the meso-
pores instead of being exclusively deposited on the external
surface, which would have completely blocked the porosity of
the silica and resulted in non-porous composites with a very
low surface area. This can be attributed to the ability of the
TsOH molecules to adsorb on the pores, which has been
exploited by other authors for the synthesis of ordered meso-
6
9
porous carbons. Moreover, the incorporation of high quan-
tities of functionalised carbon did not lead to mesopore
blocking, therefore the carbon must be fairly well dispersed on
the silica pore walls of C/SBA(45), C/MCF(40) and C/MCF(63).
The thickness of the carbon coating can be estimated from
the difference between the pore size of the composite and
uncoated silica. The carbon content of C/SBA(14) is insuffi-
cient to cause a measurable change of pore size. Considering
that part of the carbon in this sample is filling the micropores
of the silica, most probably a significant portion of the meso-
pore surface is not covered with carbon. The estimated thick-
nesses of the coatings of C/SBA(45) and C/MCF(63) are 1.5 nm
and 8.6 nm, respectively. For C/MCF(40), values of 1.1 and
2.6 nm are obtained from the difference between the pore
sizes and window sizes, respectively. Hence, thicker carbon
layers were formed near the windows during the synthesis of
C/MCF(40). The coating thicknesses in C/SBA(45) and C/MCF(63)
are also not entirely uniform, as indicated by the less
Fig. 1 Small angle X-ray diffractograms of SBA-15 (a), C/SBA(14) (b) and
C/SBA(45) (c).
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Fig. 3 FT-IR spectra of (a) C/SBA(14), (b) C/SBA(45), (c) C/MCF(40) and
(d) C/MCF(63).
mesopore range and thus are not expected to hinder diffusion
through the pores.
The FT-IR spectra of the composites (Fig. 3) exhibit bands
at 1090, 960, 804 and 464 cm− arising from the silica com-
ponent of the materials (the spectra of the parent silicas are
shown in Fig. S5 of ESI†). Additionally, the spectra of C/SBA(45),
C/MCF(40) and C/MCF(63) show bands associated with
1
the carbon and its functional groups. Specifically, the bands at
777, 1719 and 1390 cm− indicate the presence of carboxylic
1
1
acid, ketone and hydroxyl functional groups, respectively,
whereas those at 1183 and 625 cm− are associated with the
1
3
SO H groups bonded to the carbon. The COOH, CvO and
C–OH functional groups are produced by oxidation of the
carbon by the small amounts of sulfuric acid used for the
synthesis. The band at 1600 cm− is ascribed to the skeletal
vibrations of the C–C bonds. The carbon-related bands are not
clearly visible in the spectrum of C/SBA(14), due to the low
carbon content of this sample and low intensity of its bands
compared to those of silica.
1
Fig. 2 Nitrogen adsorption–desorption isotherms at −196 °C of the
C/S composites and corresponding uncoated silicas, plotted as amount
adsorbed per unit of surface area (open symbols – adsorption; closed
symbols – desorption).
X-ray photoelectron spectroscopy (XPS) analysis was per-
formed to gain additional information on the surface compo-
sition of the composites (Fig. 4). The S 2p spectra have two
contributions at 164 and 169 eV associated with sulfur in SH
7
0
3
steep condensation steps on their isotherms compared to and SO H groups, respectively, with most of the sulfur
those on the corresponding uncoated silicas (Fig. 2). Further- belonging to the latter. C/MCF(63) has the highest relative
more, desorption from the mesopores of C/SBA(45) occurs over amount of SO H (82.8%), followed by C/MCF(40) and C/SBA(45),
a wide range of p/p°, and the desorption branch of the C/MCF(63) both containing similar relative amounts (>71%). These
isotherm comprises two steps. The step at higher p/p° is results contrast with the complete absence of SO H found for
3
3
6
9
associated with desorption from the mesopores accessible TsOH carbonized at higher temperature, and can be attribu-
through 10.9 nm windows, whereas the small step at lower ted to the low temperature activation process used here. The
p/p° corresponds to mesoporosity accessible through narrower C 1s regions are composed of four contributions at 284.6, 286.3,
regions (ca. 4–5 nm), which however only accounts for less 287.7 and 289.1 eV ascribed to C–C, C–O (as in C–OH), CvO
7
1,72
than 10% of the total pore volume of the sample. This means and COOH, respectively.
The percentage of C–O bonding
that the non-uniformity of the carbon layer creates narrowed decreases in the following order C/MCF(40) > C/SBA(14) >
regions inside the mesopores of C/SBA(45) and C/MCF(63). C/MCF(63) > C/SBA(45) (Table S1†). Neither the C–O bonding
Nevertheless, most of these narrower regions have sizes in the percentage nor the SH/SO
3
H ratio is directly correlated with
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1
31
Fig. 5 H-decoupled P MAS NMR of the mesoporous C/S composites.
Fig. 4 (a) S 2p and (b) C 1s X-ray photoelectron spectra of the meso-
porous C/S composites.
(
TEPO); the higher the chemical shift value, the stronger the
7
3
31
acid site. The P MAS NMR spectra of the composites
exhibit broad line-shapes indicating a distribution of acid sites
(Fig. 5). In order to facilitate comparisons between the
samples the spectra were deconvoluted and fitted using five
Gaussian components centered at ca. 98, 87, 74, 61 and
the H SO /TsOH mass ratio used for the synthesis (Tables 1
2
4
and S1†), in contrast to what was found when non-porous
6
5
silica particles were used as a support. Hence, other factors,
such as the amount, location and dispersion of the carbon
inside the pores, seem to play a role on the final surface com-
position of the materials. The relatively high surface Si/C and
O/C ratios of C/SBA(14) confirm that a significant part of the
silica surface is not covered with carbon (Table S2†).
The FT-IR and XPS results discussed above show that the
materials have several types of surface acidic functionalities
3
such as SO H, COOH and C–OH, which means that the acid
sites quantified by titration correspond to the total amount of
acidic groups (Table 1). The composites with the highest
carbon contents exhibit higher amounts of sulfur and acid
sites. The amount of acid sites decreases in the following
order C/MCF(40) > C/MCF(63) ≈ C/SBA(45) ≫ C/SBA(14). Since
part of the pore surface of C/SBA(14) is not coated with
carbon, the total acid sites of this sample, measured by titra-
tion, possibly includes weak silanol groups. Comparison of the
acid sites and S contents of each sample, together with the
fact that the acid sites content include acid groups other than
5
2 ppm (Table S3†).
3
1
The TEPO P chemical shifts indicate that all of the com-
posites possess acid sites ranging from very strong
(
(
96–98 ppm), strong (88 ppm), medium (74 ppm) to weak
61 ppm) acidity. The resonance at ca. 52 ppm is due to physi-
7
4
sorbed TEPO species. The XPS and FT-IR results revealed
that the composites have several types of acidic functionalities
with different acid strengths. Hence, combining the results
from XPS and FT-IR with the P chemical shift ranges of
adsorbed TEPO, we assign the resonances at 61 and 74 ppm to
TEPO interacting with the relatively weak OH and COOH
groups, whereas the higher chemical shift at 88 ppm is associ-
3
1
ated with stronger SO
resonance at ca. 98 ppm to sulfuric acid ester groups, which
are expected to be stronger than SO H. The resonance at
1 ppm dominates the spectrum of C/SBA(14), which is
3
H groups. We also tentatively assign the
3
6
explained by a high portion of the silica being uncoated by
carbon and consequently the surface of this sample contains a
significant amount of weakly acidic silanol groups. C/MCF(63)
contains the highest relative amount of the strongest acid sites
SO H, suggest that a portion of the sulfur of the samples is not
3
included in surface acidic groups. Some of the sulfur belongs
to SH groups and may also be in the bulk. It is worth mention-
ing that using silicas with large pores and pore volumes it was
possible to produce materials with higher amounts of S and
acid sites than those prepared by coating silica nano-
(
resonances at 98 and 88 ppm), followed by C/MCF(40) and
C/SBA(45), which have acid sites of similar strength, consistent
with the results obtained from XPS. The spectrum of the
benchmark acid catalyst Amberlyst™-15 displays a single reso-
nance at 90.5 ppm. This means that our catalysts have acid
sites of weaker, comparable strength and also a small amount
of stronger acid sites than the acid resin.
6
5
particles. Interestingly, most of the composites also contain
significantly higher S and/or acid sites content than similar
5
4
5
6,58,59,61
materials reported in the literature.
This is because
the carbon precursor molecule has SO H groups in its compo-
3
sition. In contrast, the common methods for synthesising this
type of material first involves the carbonization of a carbon
precursor deposited inside the pores followed by a sulfonation
Catalytic studies
5
6,58,59,61
procedure (e.g. with concentrated H
2
SO
4
).
Reaction of HMF to bioEs. The reaction of HMF in the pres-
The acid strength was qualitatively assessed by observing ence of the carbon (C/S) composites gave 5EMF and EL (bioEs
3
1
the P chemical shifts of adsorbed triethylphosphine oxide denotes 5EMF plus EL) as the main products in total yields of
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which was the main product formed initially (80–83% yield at
2
–6 h). The conversion of 5EMF to EL is favoured by strong
2
1,26
Brønsted acidity.
Hence, our strongest solid acid catalyst
C/MCF(63) was the most effective for producing EL; 42% EL
yield compared to 23–29% for the remaining catalysts, at 100%
conversion and 24 h (Fig. 7 and S6†).
For each pair of C/S composites with the same silica
support, the catalyst with the highest total amount and
strength of acid sites (AcS) led to faster initial reaction of HMF
and higher yield of bioEs (i.e. C/MCF(63) and C/SBA(45) in
comparison with C/MCF(40) and C/SBA(14), respectively,
Fig. S7†). The differences in catalytic results were more pro-
nounced for the C/SBA-15 composites than for the C/MCF
ones, most likely due to the larger differences of acid pro-
perties in the former case. On the other hand, for each pair of
C/S composites with the same silica support, the more active
catalyst (Fig. 6 and S7†) possessed lower specific surface area,
pore volume and sizes (Table 1) than the less active one (i.e.
C/SBA(45) and C/MCF(63) in comparison with C/SBA(14) and
C/MCF(40), respectively). Hence, the acid properties of the C/S
catalysts seem to play a major role in the catalytic reaction,
and, on the other hand, suggest good active site accessibility
with the texture properties not causing significant constraints
on the catalytic reaction (i.e. the catalytic reaction systems are
likely operating under the kinetic regime). This hypothesis is
further supported by a comparison of the catalytic perform-
ances of C/S materials with similar acid properties, but
different structural/textural properties, namely, C/MCF(40) and
C/SBA(45). The C/MCF(40) material has a much higher meso-
porous volume (ca. three times greater) and larger pores
Fig. 6 Kinetic profiles of the reaction of HMF with ethanol in the pres-
ence of composites (a) C/SBA-15 (C/SBA(45) (Δ), C/SBA(14) (O)) or (b)
C/MCF (C/MCF(63) (Δ), C/MCF(40) (O)). Reaction conditions: 0.33 M
HMF, catalyst loading = 10 gcat dm , 110 °C. The dashed lines are visual
guides.
−3
(
ca. six times greater) than C/SBA(45). Despite the differences
in textural/structural properties, the two composites led to
similar catalytic results, which correlate with their similar acid
properties (Fig. S8†).
The catalytic performances of C/MCF(63) and C/SBA(45)
compare favourably to various carbon-based materials pre-
viously tested as catalysts in the same reaction under similar
conditions, namely, sulfonated partially reduced graphene
5
4
54
oxide, sulfonated carbon nanotubes, sulfonated carbon
5
4
black, and non-porous silica nanospheres coated with sulfo-
6
5
nated carbon (Table 2). The same applies when comparing
the C/S catalysts to microporous crystalline or mesoporous
amorphous aluminosilicates, such as nanocrystalline zeolite
H-beta (as determined by catalytic tests carried out under
similar reaction conditions, Fig. S9†) and mesoporous
3
3
Al-TUD-1.
The catalytic performances of the C/S composites were
Fig. 7 Dependency of the yields of 5EMF (black symbols) and EL (white
symbols) on the time of reaction of HMF in the presence of the compo- further compared to that of the classical catalyst Amberlyst™-
sites (a) C/SBA-15 ((C/SBA(45) (triangles), C/SBA(14) (circles)) or (b)
1
5 which possesses a macroreticulated polymer matrix
C/MCF ((C/MCF(63) (triangles), C/MCF(40) (circles)). Reaction con-
functionalised with sulfonic acid groups. These types of resins
are very active catalysts for the conversion of furanic com-
pounds (HMF, FA) to bioEs, and are thus good benchmark
−3
ditions: 0.33 M HMF, catalyst loading = 10 gcat dm , 110 °C. The dashed
lines are visual guides.
1
8,19,21,25,28,29,31–33,50
catalysts.
The texture properties and the
9
5–99% within 2–6 h reaction at 110 °C (conversion >99%; acid sites accessibility of the acid resins depend on their swel-
Fig. 6 and 7). The reaction mechanism of HMF to EL is ling ability in the liquid media. In order to minimise the swel-
complex and involves the intermediate formation of 5EMF ling effects, Amberlyst™-15 was ground into a very fine powder
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Table 2 Comparison of the catalytic results for the C/S catalysts with those of various other catalysts tested in the reaction of HMF with ethanol
b
Reaction conditions
a
Cat. load (gcat dm−3
c
Catalyst
T (°C)
[HMF]
0
(M)
)
t (h)
Conv. (%)
bioEs yield (%)
Ref.
C/SBA(45)
C/MCF(63)
CST-1
S-RGO
S-GO
S-CNT
S-CB
Amberlyst-15
H-Beta
Al-TUD-1(21)
Al-MCM-41(25)
Al-MCM-41(50)
110
110
110
110
100
140
140
110
110
110
140
140
140
140
140
100
140
140
75
0.33
0.33
0.33
0.33
0.5
0.33
0.33
0.33
0.33
0.3
0.7
0.7
0.7
0.7
0.12
0.20
0.12
0.12
0.39
0.2
10
10
10
10
10
10
10
10
2/4
2/4
2/4
4
12
24
24
2/4
6
4
5
5
5
98/99
99/100
92/99
98
85
99
99
95/99
73
89/96
95/99
84/97
96
83
86
85
75/85
78
96
84
78
99
97
∼85
85
∼87
∼85
68
—
—
65
54
50
54
54
65
—
33
21
21
21
21
26
83
26
26
19
22
18
10
10
98
n.m.
n.m.
n.m.
n.m.
16
200
16
16
4.3
6
100
100
100
100
∼100
95
∼100
∼100
100
10
ZrO
2
/SBA-15
2
−
SO
SO
4
/ZrO
2
/SBA-15
5
3
H-SBA-15
24
10
24
24
24
24
4
3
HMS-SO H
H-ZSM-5 (11.5)
H-Mordenite(10)
Silica sulfuric acid
H-Y
70
90
9
82
4
H SiW12O40/MCM-41
1.7
42
92
a
b
Value in parenthesis (when applied) is the Si/Al molar ratio. Reaction conditions: T = reaction temperature (°C), [HMF]
0
= initial molar
c
concentration of HMF, Cat. load = catalyst loading, t = time of reaction (h), n.m. = not mentioned. HMF conversion.
with particle sizes of a few hundreds of nanometers, and in maximum yields of 39% (at 83–85% conversion and 4–6 h
tested in the reaction of HMF under similar conditions. The reaction) and 44–48% (at 100% conversion and 24 h reaction),
resin catalyst led to slower conversion of HMF to bioEs than respectively. The C/MCF(63) catalyst led to faster reaction of
our strongest acid C/S catalysts; conversion at 30 min was 53% HMF than C/SBA(45), which correlates with the higher total
6
5
for the resin catalyst, compared to 74% and 83% for C/SBA- amount of acid sites for the former. The higher initial reaction
(
3
7
45) and C/MCF(63), respectively (Fig. 6), and the bioEs yield at rate for C/MCF(63) was accompanied by higher initial yield of
0 min was 46% for the resin catalyst, compared to 71% and the intermediate product HMF, and higher bioEs yields were
8% for C/SBA(45) and C/MCF(63), respectively (Fig. S7†). In reached with time in relation to C/SBA(45). The C/MCF(63)
6
5
this case, the catalytic activity does not correlate with the catalyst led to higher EL yield than C/SBA(45) (21% and 15%,
amount of acid sites which is higher for the acid resin catalyst respectively, at 48 h reaction), and this trend parallels that
−
1 65
(
4.3 mmol
g ). Possibly, the resin catalyst possesses observed for HMF as a substrate using the two catalysts (Fig. 7
SO H
3
some acid sites which are inaccessible and/or subject to and S6†), and may be partly due to somewhat stronger acidity
important steric hindrance effects in their vicinity. The good of C/MCF(63).
catalytic performances of the strongest acid C/S catalysts may
The catalytic performances of C/MCF(63) and C/SBA(45)
be partly due to their favourable acid properties and good were compared to those of various other solid acid catalysts
active site accessibility. Using a greater initial amount of HMF tested in the same reaction. The two composites led to
(
ca. 3.9 times greater than the typical conditions) and less faster conversion of fructose to bioEs yields than powdered
solvent (half the amount), the C/MCF(63) catalyst, for example, Amberlyst-15 tested at 110 °C (the maximum operation temp-
still led to fairly good catalytic results (Fig. S10†). For the more erature recommended is 120 °C): 44% conversion at 4 h reac-
concentrated HMF reaction conditions, the composite catalyst tion for the resin catalyst compared to 73–83% for the
led to faster initial conversion of HMF to bioEs (61% yield at composites, and 9% bioEs yield at 24 h for the resin catalyst
7
4% conversion and 30 min reaction) than Amberlyst™-15 compared to 44–48% yield for the composites. Furthermore,
using less concentrated HMF reaction conditions (46% yield at the composites led to much faster reaction of fructose than
6
5
5
3% conversion).
nanocrystalline zeolite H-beta, as determined by catalytic tests
Reaction of fructose to bioEs. The mesoporous composites carried out under similar reaction conditions (Table 3); 57%
C/MCF(63) and C/SBA(45) were further explored as catalysts for conversion at 24 h compared to 100% for the composites.
the cascade reaction of fructose to bioEs in a single reactor, at Table 3 summarises literature data for various other solid acid
1
40 °C (Fig. 8). The acid-catalysed dehydration of fructose catalysts tested in the one-pot conversion of fructose to BioEs.
gives HMF, which is subsequently converted to bioEs. For the The C/MCF(63) and C/SBA(45) catalysts led to faster reaction
two catalysts, the main products were HMF and bioEs, formed of fructose than non-porous silica nanoparticles coated with
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alternative route to that of HMF (hexose route), specifically
from FA which is industrially produced from furfural (pentose
7
7,78
based route).
FA has been proven to be an interesting feed-
stock for levulinate esters production via atom-economic
methods under relatively mild reaction conditions. Our C/S
composites were very active in the reaction of FA, leading to
1
00% conversion within 30 min, at 110 °C. Initially, FA was
converted to 2-(ethoxymethyl)-furan (2EMF) (36–56% yield at
0 min) which was subsequently converted to EL with yields of
3
up to 79% (Fig. 9). Besides EL, the intermediate product 2EMF
can find interesting applications such as a pharmaceutical and
3
4
food additive, and as a blending component of gasoline.
Nevertheless, our catalysts favour essentially the formation of
EL in the FA reaction system. A comparison of the catalytic
results for the two substrates FA and HMF indicates a much
higher reactivity of FA in comparison with HMF, and that the
FA route leads to higher EL yields. Similar results have been
reported in the literature for different materials tested as cata-
3
3,65
lysts in the two reactions.
The relationships between the acid properties of the C/S
catalysts and the EL yields are similar for the three substrates:
HMF (Fig. 7), fructose (Fig. 8) and FA (Fig. 9). For each pair of
composites possessing the same ordered mesoporous silica
support, a higher total amount of acid sites and stronger
acidity favours the formation of EL. Furthermore, the
C/MCF(40) and C/SBA(45) catalysts which possess similar
acid properties led to similar catalytic results (Fig. S8 and
S11†).
Catalyst stability and reusability. The stability of the
materials in the reaction media can be assessed by a pre-treat-
ment of the catalysts in the solvent, at the reaction temperature
and in the absence of the substrate. Therefore, contact tests in
ethanol (ET) were carried out for C/SBA(45) and C/MCF(63),
giving the treated solids C/SBA(45)-ET and C/MCF(63)-ET,
respectively (for details see the Experimental section). The
Fig. 8 (a) Kinetic profiles and (b) dependency of the yields of HMF (grey amounts of sulfur and acid sites of the treated solids did not
symbols), 5EMF (black symbols) and EL (white symbols) on the time of
reaction of fructose in the presence of C/SBA(45) (triangles) or C/MCF
decrease, suggesting that C/SBA(45) and C/MCF(63) are stable
towards leaching of the surface active species. Moreover, FT-IR
(
63) (circles). Reaction conditions: 0.33 M fructose, water–ethanol (3 : 7
−3
spectra of the treated solids were similar to those of the corres-
ponding original solids (Fig. S5†). The treated solids were
tested as catalysts in the reaction of HMF with ethanol under
typical conditions; the catalytic results were similar to those
of the corresponding original materials (Fig. 10). Further-
v/v ratio) solvent mixture, catalyst loading = 10 gcat dm , 140 °C. The
dashed lines are visual guides.
6
5
sulfonated carbon, tested under similar reaction conditions.
For various cases, it is difficult to make clear and fair compari- more, the catalytic performances remained similar after
sons due to the different reaction conditions used, which can subjecting C/SBA(45) and C/MCF(63) to two consecutive treat-
facilitate or not the conversion of HMF to bioEs: in some cases ments in ethanol (C/SBA(45)-ET(2) and C/MCF(63)-ET(2),
higher EL yields were reported using (i) lower temperature and respectively).
catalyst loading, despite a lower initial concentration of fruc-
The recovered C/SBA(45) catalyst was reused in a consecu-
5
3
tose (CNT-PSSA, BSA and CMK-5-PSSA), or (ii) higher temp- tive 6 h batch run, giving high bioEs yield (93%) at high con-
2
6,75
erature and catalyst loading (e.g. zeolites),
or (iii) higher version (99%), with higher selectivity to 5EMF than EL (85%
2
6
catalyst loading (SBA-15-SO
3
H). On the other hand, for some 5EMF plus 8% EL yield), similar to that observed for run
catalysts higher 5EMF yields were reported using lower temp- 1 (79% 5EMF and 16% EL yield at 100% conversion). Similar
erature, despite lower initial fructose concentration and higher trends were observed for the original and reused C/MCF(63)
7
6
catalyst loading (silica-SO
3
H, Fe
3
O
4
@SiO
2
–SO
3
H).
catalysts, i.e. the recovered catalyst led to high bioEs yield,
Reaction of FA to bioEs. The C/S composites were further especially of 5EMF (84% 5EMF and 8% EL yield) at high con-
explored as acid catalysts for the production of EL via an version (99%).
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Table 3 Comparison of the catalytic results for the C/S catalysts with those for various catalysts tested in the reaction of fructose using ethanol as
a
a solvent
Cat. load
(gcat dm−
3
)
Co-solvent
T (°C)
t (h)
Conv. (%)
YHMF (%)
YEL (%)
Y5EMF (%)
Ref.
Catalyst
[Fru]
0
(M)
C/SBA(45)
C/MCF(63)
CST-1
Amberlyst-15
H-Beta (12)
0.33
0.33
0.33
0.33
0.33
0.5
10
10
10
10
10
20
30
5
5
5
5
0.13
0.13
14.2
10
40
40
15.7
15.7
150
150
15.7
15.7
15.7
H
H
H
H
H
—
DMSO
—
2
2
2
2
2
O
O
O
O
O
140
140
140
110
140
100
130
120
120
120
120
175
∼78
110
100
100
100
140
140
160
160
140
140
140
6/24
4/24
24
4/24
4/24
24
24
24
24
24
24
1.3
24
0.75
12
16
24
24
24
20
89/100
83/100
95
44/72
31/57
95
100
>99
>99
>99
>99
100
95
100
95
97
100
>99
92
>99
>99
93
39/17
39/11
28
9/33
3/9
9
9
—
—
—
—
0
—
—
nm
3
11
<1
<1
—
4/11
2/15
7
0/2
—/<1
—
13/33
9/33
27
0/7
—/6
18
71
—
—
—
—
nm
62
44
73
72
63
12
26
—
—
65
—
—
50
50
53
53
53
53
49
48
48
79
76
80
26
26
75
75
26
26
26
b
GO
GO
CNT-PSSA
0.5
—
c
0.07
0.07
0.07
0.07
0.63
0.74
63.6
0.2
0.2
0.2
84
69
60
45
38
—
21
13
—
—
57
7
48
40
8
d
CNF-PSSA
—
—
—
e
CMK-5-PSSA
f
CNT-BSA
Amberlyst-70
Amberlyst-131
Amberlyst-131
H
2
O
—
—
—
—
—
—
—
—
—
—
—
—
Cellulose H
Fe @SiOh–SO
Silica-SO
SBA-15-SO
2
SO
4
g
O
3 4
2
3
H
3
H
3
H
0.29
0.29
0.1
b
H-beta (12.5)
b
H-Beta (19)
—
—
28
42
b
H-Y (6)
0.1
20
24
24
24
—
b
H-Y (2.6)
0.29
0.29
0.29
<1
13
15
b
H-MOR (10)
H-ZSM-5 (11.5)
92
94
—
—
b
17
a
0
Reaction conditions: [Fru] =initial concentration of fructose, co-solvent (when applied), Cat. Load = catalyst loading, T = reaction temperature,
b
t = reaction time. The results are indicated for fructose conversion (Conv.) and product yield (Y); nm = not mentioned. Values in parenthesis
correspond to the Si/Al molar ratio. CNT-PSSA – poly (p-styrenesulfonic acid)-grafted carbon nanotubes. CNF-PSSA – poly(p-styrenesulfonic
acid)-grafted carbon nanofibers. CMK-5-PSSA – benzenesulfonic acid-grafted CMK-5. CNT-BSA – benzenesulfonic acid-grafted carbon
nanotubes. Fe
c
d
e
f
g
h
3 4 2 3 3 4 3
O @SiO –SO H – sulfonic acid immobilised on the surface of silica-encapsulated Fe O nanoparticles. Silica-SO H – silica
supported sulfonic acid.
In order to confirm the absence of soluble active species,
the liquid phase obtained from the contact test of C/SBA(45)
Conclusions
with ethanol (denoted C/SBA(45)-ET(liq)) was tested for the Composites (C/S) consisting of mesoporous silicas SBA-15 or
homogeneous phase reaction of HMF. The substrate was MCF coated with carbon functionalised with acidic groups of
added to C/SBA(45)-ET(liq) to give 0.33 M HMF, and the result- different strengths (such as SO H and COOH) are versatile
3
ing solution was left to react at 110 °C for 6 h. The homo- solid acid catalysts for synthesising useful bio-products. The
geneous phase reaction was sluggish, giving similar HMF composites promoted the conversion of different intermedi-
conversion (20%) to the reaction of HMF without the catalyst ates derived from biomass, namely 5-hydroxymethyl-2-furalde-
(17%). Hence, the catalytic reaction seems to take place in the hyde (HMF), fructose and furfuryl alcohol (FA) to the bio-
heterogeneous phase. In the case of C/MCF(63) it was not products 5-(ethoxymethyl)-2-furfural (5EMF), 2-(ethoxymethyl)-
possible to confirm the heterogeneous nature by the contact furan (2EMF) and ethyl levulinate (EL). The composites, which
−1
test because the filter used (0.2 μm PTFE membrane) could have high total amount of acid sites (up to 2.3 mmol g ), in
not completely separate the catalyst particles from the liquid addition to large mesopores and some very strong acid sites,
phase. Nevertheless, the conversion was much lower than that were synthesised by the activation of varied amounts of
observed for the original catalyst (49% at 6 h reaction, com- p-toluenesulfonic acid deposited on the silicas. The atomic-level
pared to 99% at 2 h reaction for C/MCF(63)). On the other characterisation of the acid nature and strengths was per-
3
1
hand, as mentioned above no significant changes in the formed by P solid-state NMR studies of the adsorbed base
amount of acid sites and S content were observed for C/MCF- probe, in combination with FT-IR and XPS. The C/S catalysts
−1
(
63)-ET, and thus C/MCF(63) seems stable towards leaching. with higher acid site contents (1.9–2.3 mmol g ) and
Furthermore, the catalytic performances of C/SBA(45) and strengths led to higher yields of bioEs in the reactions of HMF
C/MCF(63) remained similar after hydrothermal treatment (95–99% 5EMF + EL yield within 2–6 h reaction), fructose
at 140 °C for 24 h (C/SBA(45)-WT and C/MCF(63)-WT, (44–48% yield of 5EMF + EL, and 11–17% HMF yield at 24 h
respectively; details in the Experimental section), Fig. 10. The reaction) and FA (up to 78% yield at 100% conversion). The
IR spectral features remained similar for all treated solids catalysts were stable towards S leaching and reusable. The per-
(Fig. S5†).
formances of the C/S catalysts have been compared to various
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other solid acid catalysts. The acid strengths of the C/Ss
covered those of the benchmark strong acid catalyst
Amberlyst™-15 (a bulk macroreticular sulfonic acid resin),
although the distribution of acid strengths was wider and the
acid site contents were lower than for the acid resin (4.3 mmol
−
1
g
). However, with the C/S catalysts, HMF and fructose were
converted to bioEs at higher rates than with Amberlyst™-15
on the same catalyst mass basis), which may be partly due to
(
favourable texture properties and enhanced active site accessi-
bility of our materials. The catalytic results for the C/S
materials compared favourably to those of various carbon-
based and aluminosilicate catalysts, such as non-porous silica
nanoparticles coated with sulfonated carbon and nanocrystal-
line zeolite H-beta.
The synthesis of C/S composites from biomass derived com-
ponents for paving the way towards greener production of bio-
products can be envisaged. Silica and carbon precursors are
obtainable from waste products with the increasing use of
biomass. For example, biomass fly ash has been used as a
8
1
silica source for the green synthesis of a nanosilicate. On the
other hand, the pulp and paper industry generates ligno-
sulfonate by-products which can be synthetic precursors to the
sulfonic acid carbon component; it has been demonstrated
that these types of compounds possess catalytic activity in the
5
4
conversion of HMF to bioEs.
Experimental
Fig. 9 Dependency of the yields of 2EMF (black symbols) and EL (white Synthesis of the materials
symbols) on the time of reaction of FA (until 24 h) in the presence of
The SBA-15 and MCF silicas were synthesised following pro-
composites (a) C/SBA-15 (C/SBA(45) (triangles), C/SBA(14) (circles)) or
b) C/MCF (C/MCF(63) (triangles), C/MCF(40) (circles)); FA conversion
was always 100%. Reaction conditions: 0.33 M FA, catalyst loading = SBA-15, 4.3 g of tetraethylorthosilicate (TEOS; 98%, Aldrich)
8
2–84
cedures reported in the literature.
Briefly, to synthesise the
(
8
2
−3
1
0 gcat dm , 110 °C. The dashed lines are visual guides.
was added to 2 g of Pluronic P123 (Aldrich) dissolved in 75 mL
of 1.6 M aqueous HCl solution. The mixture was stirred at
4
1
0 °C for 20 h, transferred to an autoclave and heated at
00 °C for 2 days. The solid was collected by filtration, washed
with distilled water, dried at 65 °C and calcined at 550 °C for
8
h. The MCF silica was synthesised through a similar pro-
8
2,83
cedure with some modifications:
4.0 g of mesitylene (98%,
Aldrich) followed by 23 mg of ammonium fluoride (≥98%,
Aldrich) were added to the surfactant solution and the
mixture was stirred for 1 h at 40 °C prior to the addition of
TEOS (4.3 g). The suspension was aged in an autoclave at
100 °C for 24 h. The solid was recovered by filtration, washed
with water, dried at room temperature and calcined at 500 °C
for 8 h.
The carbon–silica composites were synthesised by acti-
vation of various amounts of p-toluenesulfonic acid (TsOH,
Panreac) impregnated on 1 g of mesoporous silica. TsOH was
dissolved in acetone (99.9%, Aldrich) and added to the silica.
The suspension was sonicated for 15 min, stirred for 24 h at
room temperature, and then heated at 100 °C for 6 h and for
Fig. 10 Catalytic performances of C/MCF(63), C/SBA(45) and the
corresponding pre-treated solids (with ethanol once (ET) or twice
(
ET(2)), or with water (WT)) in the reaction of HMF with ethanol; HMF
6
h at 160 °C. The TsOH–silica solid was suspended in 10 mL
conversion was at least 98%. Reaction conditions: 0.33 M HMF, catalyst
loading = 10 gcat dm , 110 °C, 6 h.
−3
of aqueous H SO solution and stirred for 24 h at room temp-
2
4
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2 4
erature. The concentration of the H SO solution was changed Catalytic tests
in order to obtain the desired H SO /TsOH mass ratio R
2
4
(
Table 1). After evaporation of water at 110 °C, the acid impreg- The batch catalytic experiments were performed in tubular
nated solid was heated at 250 °C in a tubular furnace under a glass reactors with pear-shaped bottoms and equipped with an
N flow for 1 h. The resulting solid was washed with distilled appropriate PTFE-coated magnetic stirring bar and a valve. In
2
water (until neutral pH) followed by acetone, and dried at a typical procedure, 0.33 M 5-(hydroxymethyl)-2-furfural (HMF,
6
5 °C. The samples are denoted C/SBA(x) or C/MCF(x), where x Aldrich, 99%) or furfuryl alcohol (FA, Aldrich, 99%), powdered
−3
is the wt% of the functionalised carbon.
catalyst (loading of 10 gcat dm ), and 1 mL of ethanol (Scharlau,
9.9%) were added to the reactor at 110 °C. The reaction
of fructose (0.33 M) in the presence of the catalyst (10 gcat
9
−3
dm ) was carried out using a water–ethanol (3 : 7 v/v ratio)
Characterisation
solvent mixture at 140 °C. The reaction mixtures were heated
The carbon and sulfur contents of the samples were deter- with a thermostatically controlled oil bath, under continuous
mined by elemental analysis with a TruSpec 630 elemental magnetic stirring at 1000 rpm. Zero time (the instant the reac-
analyser. Powder X-ray diffraction (XRD) patterns were tion began) was taken to be the instant when the micro-reactor
measured on a PANalytical Empyrean diffractometer at 45 kV was immersed in the oil bath. The heating time to reach
and 40 mA using Cu Kα radiation (λ = 0.1541 nm). FT-IR 110–140 °C was 3–4 min. The initial reaction rates are based
spectra were recorded on a Bruker Tensor 27 spectrometer on conversion at 30 min reaction. The catalysts were separated
−
1
using pellets of the sample mixed with KBr (400–4000 cm
2
,
after a 6 h batch run by centrifugation, washed with ethanol
56 scans, 4 cm− resolution). Raman measurements were and then water. Prior to reuse the catalyst was separated from
carried out on a JobinYvon T64000 spectrometer (laser λ: the reaction mixture by centrifugation and treated with
32 nm). X-ray photoelectron spectroscopy (XPS) analysis was aqueous H SO (0.2 M) for 4 h at 30 °C. The catalysts were sub-
1
5
2
4
performed on a K-Alpha system from Thermo Scientific, sequently washed with water until the pH was neutral and
equipped with a monochromatic Al Kα source (1486.6 eV), and dried at 85 °C overnight.
operating in constant analyser energy (CAE) mode with a pass
Comparisons of the catalytic results were made on the basis
energy of 200 and 50 eV for survey and high resolution spectra, of similar mass of the catalyst, which is important in terms of
respectively. A spot size diameter of about 400 µm was practical application. The catalytic performances of the pre-
adopted. Transmission electron microscopy (TEM) images pared composites were compared to those of a classical ion-
were recorded with a JEOL 2200FS microscope at 200 kV. Nitro- exchange resin (Amberlyst™-15) and a large-pore zeolite
gen adsorption isotherms at −196 °C were measured with a (H-beta). The commercial cation-exchange resin Amberlyst™-
Micromeritics Gemini 2380, after degassing of the samples at 15 (a macroreticular styrene–divinylbenzene copolymer
1
20 °C overnight. The surface areas were calculated with the bearing benzenesulfonic acid groups; FlukaChemika) was
Brunauer–Emmett–Teller (BET) equation; pore volumes were manually ground using an agate pestle and mortar and sub-
calculated with the α
method; pore sizes were calculated with sequently sieved to give a very fine powder with particle sizes
the DFT method. Thermogravimetric analyses (TGA) were per- of a few hundreds of nanometers (ascertained by SEM). Zeolite
formed under air flow from room temperature to 700 °C, with H-beta was prepared by calcination of commercial NH -form
BEA, Zeolyst, CP814; crystallites with
s
4
−
1
a heating rate of 5 °C min , on a Shimadzu TGA-50. The total zeolite beta powder (NH
4
acid sites content was measured by acid–base titration: the a size of ca. 20–30 nm) at 550 °C for 10 h with a ramp rate of
sample (0.1–0.2 g) was stirred at room temperature for 24 h in 1 °C min− in static air.
1
2
0 mL of 0.1 M NaCl, and then titrated with 0.01 M NaOH.
The evolution of the catalytic reactions was monitored by
3
1
The acid strength of the solids was evaluated by P MAS NMR GC (for quantification of bioEs and FA) and HPLC (for quanti-
of chemically adsorbed triethylphosphine oxide (TEPO). The fication of HMF and fructose). Prior to sampling, the reactors
adsorption of TEPO was performed as follows: 0.1 g of solid were cooled to ambient temperature before opening and work-
was dehydrated at 110–120 °C under vacuum. 0.015 g of TEPO up procedures. The GC analyses were carried out using a
dissolved in 5 mL of anhydrous n-pentane was added to the Varian 3800 equipped with a capillary column (Chrompack,
solid, and the mixture was stirred for 30 min under nitrogen, CP-SIL 5CB, 50 m × 0.32 mm × 0.5 μm) and a flame ionisation
2
and then dried at 50 °C under vacuum. Solid-state NMR experi- detector, using H as the carrier gas. Authentic samples of the
ments were acquired on a Bruker Avance III spectrometer with substrates were used as standards, and calibration curves were
a magnetic field of 9.4 T using a 4 mm double resonance measured for quantification. The HPLC analyses were carried
probe operating at Larmor frequencies of 400.1 MHz and out using a Knauer Smartline HPLC Pump 100 and a Shodex
1
31
31
1
+
1
61.9 MHz for H and P spins, respectively. P { H} MAS SH1011 H 300 mm × 8 mm (i.d.) ion exchange column
NMR spectra were recorded using a rotation speed of 12 kHz, a (Showa Denko America, Inc., New York), coupled to a Knauer
single excitation pulse width of 1.9 µs, employing a radio- Smartline UV detector 2520 (254 nm for HMF), and a Knauer
frequency field strength of 56 kHz (60° flip angle) and 15 s Smartline 2300 differential refractive index detector (for fruc-
1
recycle delay. TPPM-15 scheme was used for H heteronuclear tose); the mobile phase was 0.005 M aq. H
decoupling.
2
SO
4
at a flow rate of
0.8 mL min− , and the column temperature was 50 °C. The
1
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identification of the reaction products was accomplished by
GCMS using a Trace GC 2000 Series (Thermo Quest CE Instru-
ments) – DSQ II (Thermo Scientific), equipped with a capillary
column (DB-5 MS, 30 m × 0.25 mm × 0.25 μm), using He as
the carrier gas. Individual experiments were performed for a
given reaction time and the presented results are the mean
values of at least two replicates. The substrate (Sub) conversion
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%) at reaction time t was calculated using the formula: 100 ×
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The bioEs products were EL (ethyl levulinate) and 5EMF
5-(ethoxymethyl)-furfural) for fructose and HMF as substrates,
[
(
[
(
and EL and 2EMF (2-(ethoxymethyl)-furan) for FA as the
substrate.
Contact tests were carried out for C/SBA(45) and C/MCF(63)
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2
Acknowledgements
This work was partly financed by FEDER (Fundo Europeu de
Desenvolvimento Regional) through COMPETE (Programa Oper-
acional Factores de Competitividade) and by national funds
through the FCT (Fundação para a Ciência e a Tecnologia)
within the project CICECO - FCOMP-01-0124-FEDER-037271
9 M. Balakrishnan, E. R. Sacia and A. T. Bell, Green Chem.,
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012, 14, 1626–1634.
0 M. I. Alam, S. De, S. Dutta and B. Saha, RSC Adv., 2012, 2,
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1 P. Lanzafame, D. M. Temi, S. Perathoner, G. Centi,
A. Macario, A. Aloise and G. Giordano, Catal. Today, 2011,
(FCT ref. PEst-C/CTM/LA0011/2013). The FCT and the European
Union are acknowledged for grants to P.A.R. (SFRH/BPD/79910/
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75, 435–441.
2 L. Bing, Z. Zhang and K. Deng, Ind. Eng. Chem. Res., 2012,
1, 15331–15336.
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011), M.M.A. (SFRH/BPD/89068/2012) and P.N. (SFRH/BPD/
3540/2010) co-funded by MCTES and the ESF through the
2
2
2
2
5
program POPH of QREN. We acknowledge the Portuguese
National NMR Network (RNRMN), supported with funds from
the FCT. BP Amoco Chemical Company is acknowledged for
sponsoring P.V.W.’s postdoc grant. L.M. thanks the FCT for the
FCT Investigator consolidation grant IF/01401/2013.
3 K. S. Arias, S. I. Al-Resayes, M. J. Climent, A. Corma and
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