On demand production of ethers or alcohols from furfural and HMF by selecting the composition of a Zr/Si catalyst

Article abstract of DOI:10.1039/d0cy01427c

Zr/Si mixed oxides with different amounts of silica (0-50 wt%) were prepared by a simple sol-gel method. These catalysts were tested in a cascade reaction of furfural and 5-hydroxymethyl furfural (HMF) with 2-butanol to form the corresponding alcohols or ethers through a combination of transfer hydrogenation and etherification processes under mild conditions. The selectivity can be finely tuned by changing the silica content which heavily impacts the acid-base properties. On pure ZrO2, featuring acid-base pairs, only the catalytic transfer hydrogenation occurs leading to alcohol products. In contrast, on ZrSi30 the etherification reaction is strongly favored owing to near one-to-one Lewis and Br?nsted acid sites. Ethers can thus be produced in high yields (≥90%), in particular the highly valuable diether of 5-hydroxymethyl furfural. The catalysts were characterized by N2 and CO2 absorption isotherms, FT-IR of adsorbed pyridine, 2-propanol TPD, and STEM-EDX mapping. For the first time we show that the tuning of acid-base properties by simple silica addition, with the somewhat unexpected rise of Br?nsted acidity, may represent the starting point for the development of cheap but highly active and selective catalysts for furfural and HMF transformation.

Full text of DOI:10.1039/d0cy01427c

Catalysis
Science &
Technology
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On demand production of ethers or alcohols from
furfural and HMF by selecting the composition of
a Zr/Si catalyst†
Cite this: DOI: 10.1039/d0cy01427c
a
a
Nicola Scotti, *a Claudio Evangelisti,b
Federica Zaccheria, Filippo Bossola,
a
a
Vladimiro Dal Santo and Nicoletta Ravasio
Zr/Si mixed oxides with different amounts of silica (0–50 wt%) were prepared by a simple sol–gel method.
These catalysts were tested in a cascade reaction of furfural and 5-hydroxymethyl furfural (HMF) with
2
-butanol to form the corresponding alcohols or ethers through a combination of transfer hydrogenation
and etherification processes under mild conditions. The selectivity can be finely tuned by changing the silica
content which heavily impacts the acid–base properties. On pure ZrO , featuring acid–base pairs, only the
2
catalytic transfer hydrogenation occurs leading to alcohol products. In contrast, on ZrSi30 the etherification
reaction is strongly favored owing to near one-to-one Lewis and Brønsted acid sites. Ethers can thus be
produced in high yields (≥90%), in particular the highly valuable diether of 5-hydroxymethyl furfural. The
Received 16th July 2020,
Accepted 5th October 2020
2 2
catalysts were characterized by N and CO absorption isotherms, FT-IR of adsorbed pyridine, 2-propanol
TPD, and STEM-EDX mapping. For the first time we show that the tuning of acid–base properties by simple
silica addition, with the somewhat unexpected rise of Brønsted acidity, may represent the starting point for
the development of cheap but highly active and selective catalysts for furfural and HMF transformation.
DOI: 10.1039/d0cy01427c
rsc.li/catalysis
from fine chemistry, to pharmaceuticals, polymers, solvents,
lubricants and biofuels. They can be prepared through
Introduction
In the search for a renewable source able to significantly
reduce the anthropogenic carbon footprint on a global scale,
lignocellulose is the most promising feedstock due to its
traditional hydrogenation systems by using molecular
hydrogen (furfuryl alcohol is industrially produced over toxic
copper-chromite catalysts) or by catalytic transfer
7
–9
abundance and non-food character. Lignocellulose is
a
hydrogenation (CTH) processes.
prolific material being the starting point for the synthesis of a
wide range of aliphatic and aromatic compounds that are
commonly derived from the traditional petrol-based
Environmentally acceptable direct hydrogenation catalysts
for the transformation of furfural and HMF into related
alcohols or diols are based on nanoparticles of Cu, Pd, Pt,
1
–5
6,7,10–27
chemistry.
Ru, Au, Co, and Ni.
Among the compounds derived from the upgrading of the
polysaccharide-based fractions of lignocellulose, furfural and
Catalytic systems that don't require the use of molecular
hydrogen are considered even more sustainable alternatives
for the hydroupgrading of biomass-derived molecules, such
as furfural and HMF, allowing substantial reduction of the
5
-hydroxymethyl furfural (HMF) stand out as they are
versatile platform molecules that could be further
transformed to obtain promising biofuels, chemicals, and
9
complexity of production systems and therefore the costs.
6
,7
polymers precursors.
Also the reaction selectivity can be improved if CTH
Two of the main derivatives of furfural and HMF are
respectively furfuryl alcohol and 2,5-bis(hydroxymethyl)furan
occurs through
a
concerted Meerwein–Ponndorf–Verley
(MPV) mechanism, that is without the evolution of molecular
hydrogen, by which the reduction has an exclusive selectivity
towards the transformation of a carbonyl group into a
(BHMF). Both of them find a wide application area, ranging
2
8
hydroxyl group.
Different catalytic systems active in the CTH processes of
furfural and HMF for the synthesis of furfuryl alcohol and
BHMF are reported in the literature. Alongside precious metal
a
CNR, Istituto di Scienze e Tecnologie Chimiche “Giulio Natta” (SCITEC), via Golgi
1
b
9, 20133 Milano, Italy. E-mail: nicola.scotti@scitec.cnr.it
CNR, Istituto di Chimica dei Composti Organometallici (ICCOM), via G. Moruzzi
, 56124 Pisa, Italy
2
9–40
and/or expensive complicated materials,
cheaper catalysts
1
based on Cu, Co, Fe, Hf, Mn, Zr, or zeolites (e.g. Sn- and Zr-
beta) are also reported.
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
2
5,29,32,41–60
d0cy01427c
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Alcohols are attractive compounds for CTH reactions Experimental
being at the same time a solvent and a hydrogen-donor
combined with their low cost, renewable nature and unique
ability to participate in intra- and intermolecular hydrogen
Materials
All chemicals and materials used as supports were obtained
from Sigma-Aldrich, except for HMF (≥95%) that was
purchased from AVA Biochem.
9
transfers. The presence of an alcohol in the reaction
medium also favors the etherification reaction which leads to
a different class of interesting molecules. Indeed, the ethers
of furfural and HMF could be used as biofuels in diesel
Catalyst preparation
7
,8,61–65
blends as cetane boosters.
The zirconia–silica supports were prepared by a sol–gel
procedure. First, zirconyl chloride (ZrOCl ), 30% solution in
2
In the case of HMF, the direct acid-catalyzed etherification
of the hydroxyl group forms a monoether, but the remaining
carbonyl group negatively impacts the fuel grade of the
resulting molecules. To improve the stability and the
miscibility with diesel, the aldehyde group of HMF must be
functionalized as an ether, making the diether the most
desirable product. Whereas the production of monoethers,
8
6
HCl (13 mL), was diluted to 100 mL with deionized water;
such solution was then added dropwise at room
temperature to 200 mL of an aqueous solution of ammonia
at pH of 12 to obtain hydrous zirconia. A solution of
tetraethylorthosilicate : 2-propanol : water (1 : 6 : 2 molar ratio,
respectively) was added to hydrous zirconia and the
suspension was vigorously stirred for 3 days at 75 °C. The so-
obtained solid was thoroughly washed with deionized water
until pH neutral, followed by drying overnight at 110 °C in
air. Finally, the solid was calcined at 350 °C for 4 h in air.
Five supports were prepared with different nominal amounts
6
6–73
which has been studied quite extensively, is far easier,
the diether synthesis is troublesome, involving a reduction
5
1,74–76
step prior to etherification.
The same applies for furfural, where the preparation of
ethers requires the combination of a reduction step followed
7
7–79
by the etherification of the resulting alcohol.
Some examples of the synthesis of furfuryl ethers and
2
of silica (0, 3, 10, 30, and 50 wt% –hereafter named ZrO ,
ZrSi3, ZrSi10, ZrSi30 and ZrSi50). In order to avoid any silicon
contamination from the glass containers, a Teflon container
was used to prepare the pure zirconia support.
HMF
literature.
diethers
have
been
reported
in
the
1
7,31,51,58,61,62,74–76,80–85
One strategy consists in
using a mechanical mixture of a reduction catalyst with an
acid one.
1
6,31,62,83
In order to further improve the productivity
and the selectivity of the desired ethers, the hydrogenation
and etherification activity can be conveniently combined
Catalyst characterization
using
material.
a
unique
bifunctional
heterogeneous
The FT-IR studies of pyridine adsorption and desorption were
carried out with an FTS-60 spectrophotometer equipped with
a mid-IR MCT detector purchased from BioRad. The
1
7,74–76,80–82,84,85
This is particularly promising when
7
4–76,84,85
associated with a H
ether yields (>90%) are rarely reported.
2
-free CTH process,
even if high
8
4
experiments were performed on a sample disk (15–20 mg)
after simple dehydration treatment at 270 °C (20 min in air +
20 min under vacuum). A background spectrum prior to
adsorption was obtained and subtracted from the
subsequently recorded spectra. The adsorption of the probe
molecules was carried out at room temperature, and
desorption steps were performed at different temperatures
from RT to 300 °C. All the spectra were reported after being
normalized by the disk weight.
Transmission electron microscopy (TEM) analysis was
performed using a ZEISS LIBRA200FE microscope equipped
with a 200 kV FEG source. Energy-dispersive X-ray elemental
maps and spectra (EDS – Oxford INCA Energy TEM 200) were
collected along with HAADF-STEM (high angle annular dark
field scanning transmission electron microscopy) images.
The specimens were finely smashed in an agate mortar,
suspended in isopropanol and sonicated, then each
suspension was dropped onto a lacey carbon-coated copper
grid (300 mesh) and the solvent was evaporated.
The full control of the selectivity of the process is
hence mandatory to completely exploit the potentiality of
CTH vs. etherification reactions. Depending on the desired
product that we want to obtain, the catalyst has to be
designed to selectively produce one product or another,
while keeping the activity as high as possible. Some of us
already studied different commercial catalysts for the CTH
5
1
of furfural to furfuryl alcohol. We observed that solids
bearing acid–base pairs on the surface, such as ZrO
2
,
promote only the CTH of the carbonyl group by the MPV
mechanism forming furfuryl alcohol in high yields. In
contrast, catalysts with a genuine Lewis acid character,
such as a 5% zirconia–silica mixed oxide, features fairly
good selectivity towards ethers but with a moderate 64%
yield.
In this work we prepared different Zr/Si oxides by a simple
sol–gel method, and we changed the relative silica content
(0–50 wt%) with the aim of having precise control over
reaction selectivity to obtain “on demand” production of
furfuryl alcohol/BHMF or ether/diether starting from furfural/
HMF under mild conditions (120 °C). 2-Butanol was used as
the solvent, the hydrogen donor and the reactant, being the
ether moiety precursor.
The specific surface area (SSA, BET method) of the
samples was measured after evacuation of the sample (ca. 0.1
g) at 200 °C for 2 h, by collecting N adsorption/desorption
2
isotherms at −196 °C using a Micromeritics ASAP2020 surface
area analyzer.
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Scheme 1 Reaction pathway of furfural.
The adsorption isotherms of CO2 were collected at RT Results
using a Micromeritics ASAP2020 instrument, after evacuation
General mechanism of furfural/HMF transformation
of the sample (ca. 0.1 g) at 200 °C for 2 h.
-Propanol temperature programmed desorption (IPA-
TPD) experiments were carried out with a home-made
2
Under our experimental conditions the process of furfural/
HMF transformation involves mainly the reactions of CTH
and etherification, while the opening of the furan ring has
8
7
apparatus described elsewhere and connected downstream
to a mass spectrometer (Hiden Analytical, HPR20). The
been observed only to
a
limited extent. 2-Butanol
−
1
samples (200 mg) were treated in a He flow (30 mL min ) at
70 °C for 40 minutes. After cooling down to 40 °C, the
simultaneously acts as the solvent, the hydrogen donor and
the co-reactant in the etherification processes, being the
precursor of the ether moiety.
2
samples were saturated with 2-propanol, followed by 1 h
under a He flow to remove the excess and physisorbed
propanol. The temperature was then ramped up to 350 °C at
On solid catalysts like ZrO
CTH reaction follows a concerted MPV mechanism.
2
, it is well-accepted that the
9,38,51
−
1
1
0 °C min under the same carrier gas flow.
In the case of furfural (Scheme 1) the carbonyl group
undergoes CTH to give the corresponding alcohol 2, as the
first step, then the etherification reaction with 2-butanol
eventually takes place to form 2-butyl furfuryl ether 3. Finally,
if the hydrolytic opening of the furan ring occurs, the
Catalytic tests
Before catalytic testing, 100 mg of catalyst were dehydrated at
2
70 °C (20 min in air + 20 min under vacuum) in a glass
88
reactor, then a solution containing 100 mg of furfural or
HMF and 6.2 g of 2-butanol was added at RT. The reactor
formation of the levulinate ester 4 is also observed.
On the other hand, HMF bears CO and –OH functional
groups where the reaction may start (Scheme 2). The carbonyl
group is transformed into a hydroxyl group through a CTH
reaction, while the –OH group undergoes etherification by
reacting with 2-butanol. Thus, by a combination of H-transfer
and etherification, it is possible to obtain the monoether 8 or
the diether, 2,5-bis(isobutoxymethyl)furan 9, while the sole
direct etherification gives the monoether 7. When the
was purged with vacuum, filled with 1 atm of N
2
and then
placed into an oil bath at 120 °C (temperature of the bath =
1
25 °C) under stirring (1000 rpm).
The reaction products were analyzed by GC-FID and GC-
MS (using
a
5%-phenyl-methyl polysiloxane capillary
column), with and without BSTFA (N,O-bis(trimethylsilyl)
trifluoroacetamide) functionalization.
Scheme 2 Reaction pathway of HMF.
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H-transfer is the only active process, the diol 6 (BHMF) is
6
1
formed.
Furfural
The reaction with furfural gives two main products, namely
furfuryl alcohol and 2-butyl furfuryl ether (Scheme 1). Pure
ZrO
2
(Table 1) produces the alcohol in a quantitative yield in
4
h at 120 °C, in agreement with the results that we
5
1
previously reported on commercial zirconia.
With the introduction of silica in the catalysts, the selectivity
to the alcohol decreases in favor of the ether. In the case of
ZrSi3 and ZrSi10 catalysts a mixture of alcohol and ether is
formed (Table 1). The presence of silica favors also the
formation of 2-butyl levulinate 4 as a side-product, especially
with ZrSi3, where it reaches 7.5%. These two catalysts are not
strongly selective towards the ether, but the presence of silica
has a clear effect on the product distribution.
The best performing catalyst is ZrSi30. In this case the
reaction goes to completion within 4 h with a remarkable
selectivity towards the ether of 95%. Interestingly, with ZrSi30
the furfuryl alcohol 2 has not been observed in the reaction
mixture, different from what happens with ZrSi3 and ZrSi10
Fig. 1 Reaction profiles for furfural transformation over ZrSi3 and
ZrSi30.
7
4,89
(Fig. 1).
The very different activity between ZrSi30 and ZrO could
2
BHMF), the monoether of the diol (5-isobutoxymethylfurfuryl
alcohol, 7) and the diether 9. It is significant to note that, under
our experimental conditions, the product of direct etherification
of HMF, 5-(isobutoxymethyl)furfural 7, has not been observed in
the reaction mixture, suggesting that HMF is less prone to
etherification if compared to BHMF. A similar behavior was
already described in the literature, but in that case a small
be further brought to light by performing the reaction using
directly furfuryl alcohol as a reactant. Indeed the former
catalyzes the formation of the ether, while we already
5
1
reported that zirconia is inactive.
A physical mixture of ZrO and SiO leads to the formation
2
2
of furfuryl alcohol 2 as the only product, highlighting the
importance of the structure of mixed ZrSi oxides (Table S1†)
in addressing the reaction towards the formation of the ether.
Using a different secondary alcohol, such as 2-octanol,
ZrSi30 forms the corresponding ether (Y = 90% in 4 h), while
over a primary alcohol (1-butanol) the acetal is observed as
the major product, alongside a low amount of ether and an
unidentified compound (Table S1†). As expected, with a
tertiary alcohol (tert-butanol) the reaction does not proceed
since it cannot undergo the CTH process (Table S1†).
74
amount of 7 forms in the reaction mixture. Only by carrying
out the reaction with a tertiary alcohol (tert-butanol), instead of
2-butanol, we observed the formation of 5-(tert-butoxymethyl)
furfural as a unique product. This might be related to the
absence of a H-donor molecule that prevents CTH from
occurring, therefore providing indirect confirmation of the
61
accepted mechanism for the formation of products 8 and 9.
Once again the selectivity of the process strongly depends
on the silica amount. ZrO has a selectivity of 100% towards
2
BHMF 6 and in 8 h the diol yield is slightly lower than 90%.
As observed with furfural, the introduction of silica promotes
the etherification reactions, and the products 8 and 9
increase with the silica content. The main product with ZrSi3
is BHMF 6, even though a small amount of monoether 8
forms. Differently, ZrSi10 shows a quite good selectivity to
the monoether 8 (76.4%), and the diether 9 remains below
5
-Hydroxymethyl furfural
Starting from HMF (Table 2) we observed the formation of three
main products: the diol (2,5-bis(hydroxymethyl) furan 6 –
Table 1 Furfural transformation over Zr-based catalysts after 4 h
10%. The monoether 8 is an interesting intermediate for the
synthesis of anionic surfactants derived from carbohydrates
since the residual hydroxyl group can be sulfonated.
However, the direct etherification of BHMF with 1-dodecanol
in the presence of several Brønsted acids gave very poor
results, the best yield of 10% being obtained in the presence
of Amberlyst 15.
ZrSi30 is confirmed to be the most appealing catalyst, since
in 7 h it forms the diether 9 in high yield, namely 96.2% (C =
2
S (%)
3
S (%)
Catalyst
ZrO
ZrSi3
ZrSi10
ZrSi30
C (%)
2
100
100
100
100
100
0
38
26
95
90
54
73
0
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Table 2 HMF transformation over Zr-based catalysts
S
6
(%)
S8 (%)
S9 (%)
Catalyst
ZrO
t (h)
C (%)
2
8
7
7
3
7
3
89.5
94.7
86.3
74.6
97.9
11.5
100
0
8.8
0
0
ZrSi3
91.2
14.5
0.4
0
ZrSi10
ZrSi30
ZrSi30
ZrSi50
76.4
8.0
1.5
5.2
7.4
89.1
98.5
81.7
0
97.9%, S = 98.5%). To the best of our knowledge this is the
The HAADF-STEM/EDX mapping of ZrSi3, ZrSi10 and
highest yield reported for the diether of HMF obtained in the
ZrSi30 (Fig. 2) revealed no segregation of both zirconium and
silicon in the catalyst grains, but rather highly homogeneous
dispersion of two oxide phases, forming uniform high-
61,74–76,84,85
absence of molecular H , with a single catalyst.
Only
Wei et al. reported an 80% yield in the same ether over an
SBA-15 supported ZrO catalyst using 2-butanol. As already
2
84
2
2
surface area materials. While in the ZrO catalyst no elements
9
1
mentioned, the diether is a higher value-added product
compared to the monoether, due to its better miscibility with
diesel and its higher stability, although it is more difficult to
other than Zr and O were found, it is very interesting to
note that on ZrSi30 EDX semiquantitative analysis revealed
that the Zr/Si molar ratio is almost one-to-one, in agreement
with the nominal Zr/Si weight composition of this sample
(Fig. S4†). A complete XPS investigation of the Zr/Si ratios
has been carried out on the very same materials by some of
61,74–76
produce.
It is noted that the amount of BHMF 6 detected
with ZrSi30 during the entire reaction time is very low (0.7%
maximum at 1 h) (Fig. S2†). A further increase in the silica
content was detrimental to the activity of the resulting catalyst
9
1
us. The results confirm the nominal Zr/Si composition and
specifically the one-to-one ratio in the ZrSi30 catalyst.
(Table 2, ZrSi30 vs. ZrSi50, 3 h).
The role of Lewis and Brønsted acid sites is crucial for the
product distribution, and it is often debated in the literature.
It is reported that Lewis acid sites promote CTH by the MPV
mechanism, while it is well-known that Brønsted acidity
favors the –OH etherification reaction. However, strong
Brønsted acid sites may cause side-reactions, such as the
hydrolytic ring-opening to form levulinate products,
Catalyst recycling
In light of the good performance of the ZrSi30 catalyst, we
evaluated its stability through recyclability tests.
In a typical test the catalyst has been separated from the
product mixture, washed with 2-butanol and dehydrated at
2
70 °C before the next run (20 min air + 20 min vacuum).
2
8,61,65,71,72,74,75,92
especially at high reaction temperature.
Fig. S3† shows that ZrSi30 can be used at least four times
without observing any significant loss in activity.
Hence, a material constituted only by Brønsted acid sites
should be effective for direct etherification to form product 7,
being unable to activate the hydrogen transfer process. In
contrast, hydrogen transfer by the MPV mechanism is favored
over solid catalysts with a Lewis acid site and a neighboring
Influence of the catalyst features on activity and selectivity
The distinct Zr/Si composition is the reason behind the
different ability of the catalysts in promoting CTH and
etherification reactions. The silica content reflects on the
surface area of the materials, and in particular on the
structural and acid–base properties. The introduction of a
simple, cheap, available and non-toxic element, such as Si,
allows one to drastically change the nature of both the
catalytic surface and the active site. By selecting the
appropriate Zr/Si ratio the output of the reaction can be
selectively shifted from the alcohol to the ether, under the
same experimental conditions. As previously mentioned, a
simple physical mixture of ZrO2 and SiO2 does not lead to
the formation of the ether.
9
base site. In a previous study, some of us reported that pure
Lewis acid solids, such as Cu/SiO
2
prepared by
a
chemisorption–hydrolysis method, express limited activity in
the CTH of furfural and low selectivity, due to ether
2
formation, but catalysts with acid–base pairs, such as ZrO ,
5
1
are definitely more active and selective. In some cases, pure
2
8,72,74
Lewis acidity accounts also for ether formation,
though the mechanism is still poorly understood.
even
7
5,93
Table 3 Surface area of the Zr-based materials
All the materials show high SSA (Table 3) that positively
impacts their catalytic activity, although the preparation of
high surface area zirconia is not an easy task because of its
low thermal stability. The introduction of Si clearly leads to a
2
−1
Catalyst
SSA (m g )
ZrO
2
222
281
309
353
ZrSi3
ZrSi10
ZrSi30
8
6,91
gradual increase in SSA as already observed.
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Fig. 3 FT-IR pyridine desorption spectra at 120 °C normalized by the
disk weight; inset: quantitative analysis at 120 °C.
The analysis of the –OH region in the FT-IR spectra
(
Fig. 4) of the catalysts offers further insights into their
structure. On pure ZrO , the less-pronounced bands at 3669
and 3777 cm are due to hydroxyl groups over a monoclinic
2
−
1
−
1
zirconia phase, while the one at 3728 cm is ascribed to
–
1
04,105
OH groups over tetragonal zirconia,
in agreement with
9
1
the XRD analysis already reported by some of us. Different
is the case of ZrSi30, where we can find only an intense band
at 3740 cm corresponding to isolated –OH groups over Si,
−
1
1
06
like those typical of pure silica.
This means that the –OH
groups on the surface of ZrSi30 resemble those of silica. A
similar pattern is observed on ZrSi50 (not reported).
Silica is generally known to be a non-acidic material and
needs to be modified to exhibit acidic behavior. Indeed, in our
case, already with the addition of few silica, that is ZrSi3, the
Fig. 2 HAADF-STEM image of the representative catalyst grain of
ZeSi3 (top), ZeSi10 (medium) and ZrSi30 (bottom) with EDX mapping
area (yellow dashed lines) and the corresponding Zr map (red) and Si
map (blue).
−1
total population of Lewis sites drops from 135 to 75 μmolPy g .
This would make ZrSi30 and ZrSi50 less acidic materials. This is
true for ZrSi50, while it turns out that ZrSi30 has remarkable
acidic properties in the form of both Lewis and Brønsted sites.
It is generally accepted that Brønsted acidity favors alcohol
The properties of binary oxides differ significantly, often
in an unexpected way, from those of the bulk single
components. The acid–base properties depend on the nature
107,108
dehydration reactions,
2 2 3
but a DFT study on a SiO –Al O
9
4–97
and the ratio between the two mother oxides.
By FT-IR of adsorbed pyridine it is possible to distinguish
between Lewis and Brønsted acid sites, while a weak interaction
between the catalyst and the probe molecule (physisorption or
9
8–102
hydrogen bonding) can be excluded above 100 °C.
results, collected at the reaction temperature, namely 120 °C,
The
1
03
are reported in Fig. 3, whilst the quantitative analysis,
determine the amount of adsorbed pyridine (mmolPy gcat ), is
to
−
1
reported in the inset of Fig. 3. Data show a very interesting
trend: while ZrO shows only Lewis sites (bands at 1448 and
2
−1
1
(
610 cm ), the introduction of silica generates Brønsted acidity
bands at 1550 and 1640 cm ) on ZrSi10, but especially on
−1
ZrSi30. It is worth noting that the ZrSi30 catalyst possesses a
near one-to-one ratio between Lewis and Brønsted acid sites,
which well matches with the surface molar Zr/Si composition of
the material, calculated by EDX mapping. On ZrSi50 the
population of both Lewis and Brønsted acid sites drops.
Fig. 4 FT-IR of the ZrSi catalysts, –OH region, normalized by the disk
weight.
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catalyst in 2-propanol dehydration (a common reaction used to
probe the surface acidity of solid materials) suggests that the
process is extremely favored on terminal silanols that interact
with neighboring, coordinatively unsaturated aluminum atoms,
109
which may exhibit Lewis acidic properties. This is particularly
interesting because these pseudo-bridging silanol groups enable
the formation of propene through synergy between Lewis and
Brønsted catalysis.
Therefore, the 2-propanol TPD represents an informative
analysis to better describe our catalyst (Fig. 5). On ZrSi30
propene is formed just from 180 °C, with the maximum at
2
2
20 °C. On the other hand, the profile of ZrO shows that the
propene production is shifted by more than 100 °C above,
with the maximum at 340 °C, indicating very different acidity.
The easier formation of propene on ZrSi30 confirms the
presence of Brønsted acid sites (in agreement with pyridine),
but the cooperative role of Brønsted and Lewis acidity in
olefin formation is also suggested from the work of Larmier
−
1
Fig. 6 Acid and basic sites (μmol g ) of all the materials.
sites (Fig. 6). The combined effect of acid–base sites drives
the selectivity of the reaction from a simple CTH process to
a combined CTH + etherification one. This behavior is
109
et al.
As described before, it is possible to effectively recycle the
catalyst after regeneration treatment at 270 °C (20 min air +
2
particularly clear on HMF: ZrO and ZrSi3 form BHMF 6 in
2
0 min vacuum). The FT-IR analysis of pyridine on the
high yields as the Lewis acid–base pairs are predominant;
differently, in ZrSi10 and ZrSi30, the concurrent and gradual
effect of the decrease of basic sites and the increase of
Brønsted acid sites, pushes towards the formation of ethers.
The catalyst with a moderate amount of both basic sites and
Brønsted acid sites, namely ZrSi10, is selective for the
monoether 8, whereas ZrSi30, which combines Lewis and
Brønsted acid sites without the basic ones, produces the
diether 9 in a 96.4% yield.
With furfural ZrSi3 and ZrSi10 behave in a similar way,
2
whereas ZrO and ZrSi30 confirm their complementary role
in the upgrade of this compound, giving selectively furfuryl
alcohol 2 and 2-butyl furfuryl ether 3, respectively.
From the correlation between the characterization and the
catalytic data, we can speculate on the reaction mechanism.
On ZrSi10 and ZrSi30 HMF is first adsorbed on a Lewis acid
site where the reaction proceeds through the MPV
mechanism to form BHMF 6. Then the etherification reaction
takes place, promoted in particular by a Brønsted acid site, to
form the product 8 and eventually the diether 9. This
mechanism explains the reaction profiles and especially the
absence of monoether 7 in the product mixture for each
reaction time.
recovered and dehydrated ZrSi30 catalyst (Fig. S5†) shows
that both Lewis and Brønsted acid sites are regenerated,
granting the possibility to reuse the material.
In order to have a full picture of the acid–base features of
the catalysts, we measured the adsorption isotherms of CO
2
,
from which we calculated the number of basic sites (μmol
−
1
g ). In Fig. 6 we plotted the composition in terms of basic
and acid sites of the investigated catalysts. It is evident that
the dramatic effect of silica extends also to the basic
character of the materials. In particular, as the silica content
increases, the number of basic sites quickly drops. It is
interesting to note that ZrSi30 is free of basic sites and as
such it can be considered as a pure solid acid. The poor
catalytic activity of ZrSi50 is the result of the decrease of both
acidity and basicity.
From these characterizations, it turns out that the addition
of silica impacts both acid (quantity and type) and basic
We propose that ZrSi30 is a material where the Lewis acid
4
+
site, as Zr , accounts for the first adsorption of the reactant
furfural or HMF) which is consequently transformed into the
(
corresponding alcohol/diol by a CTH reaction. The so-formed
alcohol is now prone to etherification that it is strongly
favored by the Brønsted acidity. The CTH process over ZrO is
2
enhanced by the presence of acid–base pairs, while that over
ZrSi30 is favored by the quick transformation of BHMF into
the monoether, hence shifting the equilibrium of the reaction
and also preventing the formation of side-products.
Our hypothesis is that the high surface area and the
homogeneity of the materials in terms of Zr and Si inter-
2
Fig. 5 2-Propanol TPD of ZrO and ZrSi30.
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dispersion lead to the formation of a Zr–O–Si network that is
the basis of the peculiar features of ZrSi30, with a near one-
to-one ratio between Lewis and Brønsted acidity. Pyen
is simply achieved by the addition of silica but, however
critical, remains elusive and is worthy of deeper investigation.
It is also worth noting that, from HMF, ZrSi10 gives the
monoether 8 as the main product. This is an interesting
intermediate for the synthesis of anionic surfactants due to
its residual hydroxyl group, while in the literature it is
usually reported the preparation of monoether 7,
which can be used as biofuel, but having worse properties
compared to the diether.
1
08
et al.
had investigated similar materials finding
comparable acidic properties and concluded on the basis of
XPS characterization that Brønsted sites are constituted of
Zr–OH groups. FT-IR investigations, however, did not reveal
any bands ascribable to such groups in our catalyst featuring
Brønsted acidity (Fig. 4). Furthermore, in previous studies on
66–73
1
08,110
zirconia–silica mixed oxides
the electron density on
oxygen, which is related to the acidic properties, has been Conflicts of interest
reported to increase upon addition of silica. But, in another
91
There are no conflicts to declare.
work by some of us, we found a lower electron density on
the oxygen in similar materials, resembling very much what
1
11
Acknowledgements
was described by Hong et al.
They argued that such
oxygens are part of surface –OH groups with slight Brønsted
F. B. and V. D. S. have received funding from the European
Institute of Innovation and Technology (EIT). This body of the
European Union receives support from the European Union's
Horizon 2020 research and innovation program.
character and bond to oxygen vacancies as revealed by the Zr
9
1
XPS peaks. As described in our aforementioned work
,
though, we did not find any evidence of such vacancies and,
in fact, the Zr sites were progressively depleted of electron
density with increasing silica content. All this to say that the
search for the actual Brønsted site is still ongoing and not so
trivial.
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