Post-synthesis deposition of mesoporous niobic acid with improved thermal stability by kinetically controlled sol-gel overcoating

Article abstract of DOI:10.1039/c9ta01459d

Niobia is a well-known solid acid catalyst owing to its intrinsic Br?nsted and Lewis acidity. However, catalyst development with this oxide has been limited by our ability to control its pore structure and thermal stability. Here we report a novel post-synthetic approach for preparing mesoporous niobia catalysts. This method relies on controlling the kinetics of niobium(v) ethoxide to deposit conformal Nb₂O₅ overcoats on SBA-15 in a typical St?ber solution. Full Nb₂O₅ coverage over the mesopores of SBA-15 was achieved by adding 4 monolayer equivalents of precursor (4Nb₂O₅?SBA-15), which was verified by X-ray photoelectron spectroscopy. This overcoated SBA-15 had a high surface area and retained a narrow as well as ordered pore size distribution. Importantly, the typical structural transition from the amorphous to pseudo-hexagonal Nb₂O₅ phase did not occur with the overcoat after calcination at 773 K. Limiting this crystallization imparts an unprecedented thermal stability to our niobia overcoat, which enables the acid sites to be well preserved after catalyst regeneration. Furthermore, 4Nb₂O₅?SBA-15 showed higher yields than commercial niobia (HY-340) and lab-synthesized bulk niobia in two probe reactions: xylose dehydration to furfural and Friedel-Crafts alkylation. In both cases, the improvement could be explained by the unique structural features of the niobia overcoat, including a favorable ratio of Br?nsted and Lewis acid sites in the case of xylose dehydration and a high proportion of isolated Nb-OH groups for the alkylation reaction. Such structural features and unprecedented thermal stability provide additional tools for synthetizing unique solid acid catalysts using a simple post-synthesis deposition method.

Full text of DOI:10.1039/c9ta01459d

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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DOI: 10.1039/C9TA01459D
ARTICLE
Post-synthesis deposition of mesoporous niobic acid with improved
thermal stability by kinetically controlled sol-gel overcoating
Yuan-Peng Dua, Florent Héroguela, Xuan Trung Nguyena and Jeremy S. Luterbachera*
Received 00th January 20xx,
Accepted 00th January 20xx
Niobia is a well-known solid acid catalyst owing to its intrinsic Brønsted and Lewis acidity. However, catalyst development
with this oxide has been limited by our ability to control its pore structure and thermal stability. Here we report a novel post-
synthetic approach for preparing mesoporous niobia catalysts. This method relies on controlling the kinetics of niobium (V)
ethoxide to deposit conformal Nb2O5 overcoats on SBA-15 in a typical Stöber solution. Full Nb2O5 coverage over the
mesopores of SBA-15 was achieved by adding 4 monolayer equivalents of precursor (4Nb2O5@SBA-15), which was verified
by X-Ray Photoelectron Spectroscopy. This overcoated SBA-15 had a high surface area and retained a narrow as well as
ordered pore size distribution. Importantly, the typical structural transition from the amorphous to pseudo-hexagonal Nb2O5
phase did not occur with the overcoat after calcination at 773 K. Limiting this crystallization imparts an unprecedented thermal
stability to our niobia overcoat, which enables the acid sites to be well preserved after catalyst regeneration. Furthermore,
4Nb2O5@SBA-15 showed higher yields than commercial niobia (HY-340) and lab-synthesized bulk niobia in two probe
reactions: xylose dehydration to furfural and Friedel-Crafts alkylation. In both cases, the improvement could be explained by
the unique structural features of the niobia overcoat, including a favorable ratio of Brønsted and Lewis acid sites in the case
of xylose dehydration and a high proportion of isolated Nb-OH groups for the alkylation reaction. Such structural features and
unprecedented thermal stability provide additional tools for synthetizing unique solid acid catalysts using a simple post-
synthesis deposition method.
DOI: 10.1039/x0xx00000x
for producing niobia catalysts have been reported with a special focus
on maximizing acidity and surface area including through template
Introduction
assisted approaches,18–20 exfoliation21 and hydrothermal processes.11
The catalysts prepared by exfoliation and hydrothermal processes
usually show strong acidities but lower surface areas (less than 200
m2⋅g−1). Mesoporous Nb2O5 with high surface areas can be obtained
using a template assisted synthesis but its pore size distributions are
typically broader (± 2 nm) than that of mesoporous silica (e.g. MCM-
41 and SBA-15). Recently, a novel post-synthesis method was
reported where Nb2O5 was overcoated on SBA-15 by atomic layer
deposition (ALD) and this overcoated SBA-15 had both a narrow pore
size distribution (± 1 nm) as well as a high surface area.22
Nevertheless, the high cost of ALD instrumentation and coating
processes motivates the development of simpler and more affordable
liquid-phase strategies that are much more compatible with large scale
catalyst synthesis. Additionally, the activity for acid catalysis using
overcoated SBA-15 was not fully investigated. We recently reported
a simple and scalable sol-gel based approach for overcoating Al2O3
on SBA-15. This approach relied on slowing down the condensation
rate of Al(sec-BuO)3 by adding ethyl acetoacetate (EAA) and
Niobium pentoxide (niobia) is a popular material in several catalytic
and chemical applications including for use as sensing materials,1–3
catalysts in biomass conversion,4,5 electrochromic devices,6
photocatalysts in oxidation,7 and acid catalysis.8–14 One of the reason
for its popularity in catalysis is the acidic properties of niobium oxide,
which have a high structural dependency.15–17 Hydrated niobium
pentoxide, usually denoted as Nb2O5·H2O, is an amorphous solid and
possesses strong Brønsted and Lewis acidity, while other more
thermodynamically stable phases, including its pseudo-hexagonal
(TT-phase) and monoclinic form, are less acidic and non-acidic,
respectively.15,16 Because solid acid catalysts are considered less
corrosive and more environmentally benign than traditionally used
homogeneous acids, interest in niobic acid-based catalysts has been
increasing in the past decade. In this context, many synthesis methods
a. Laboratory of Sustainable and Catalytic Processing, Institute of Chemical Sciences
and Engineering, École Polytechnique Fédérale de Lausanne (EPFL), CH-1015
Lausanne, Switzerland.
†
Electronic Supplementary Information (ESI) available: Including additional
characterization data and evaluation of mass transfer effects. See
DOI: 10.1039/x0xx00000x
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maintaining a low precursor concentration in the reaction media using a calibrated mass spectrometer using the mass 16 Da (tVhieewmAratsicsleoOfntlihnee
DOI: 10.1039/C9TA01459D
a syringe pump setup.23 Here, we extend this kinetic control strategy molecular ion of ammonia).
to the niobia sol-gel system and develop a new post-synthesis method
Diffuse-reflectance Fourier transform infrared spectroscopy with
to prepare a SBA-15 type mesoporous niobia as a solid acid catalyst. pyridine probe (Pyridine-DRIFTS) was performed on a PerkinElmer
Thanks to its unique structural features and acidity, this overcoated Frontier IR instrument. The physisorbed water was removed in situ by
catalyst is highly active for both xylose dehydration and Friedel-crafts heating the samples to 423 K under reduced pressure (<10-2 mbar) for
alkylation and has an unprecedented thermal stability against acidity 3 h. A pyridine-helium mixture was then fed into sample cell at 323
loss, which commonly occurs due to structural reorganization.
K by flowing pure He (~50 mL⋅min−1) into a round bottom flask
containing cold pyridine and 4Å molecular sieve. After purging with
He for 15 min, the DRIFTS spectra were then recorded at elevated
temperatures under a continuous flow of pure He.
Experimental section
Chemicals and Materials
All chemicals used in this study were analytical grade and
obtained from commercial suppliers. They were used without further
purification unless stated otherwise. Air and moisture-sensitive
reagents were handled using a nitrogen filled glove box or a standard
Schlenk line apparatus. Xylose, anisole, and benzyl alcohol, Pluronic
123 (P123), ethyl acetoacetate and decane were purchased from
Sigma Aldrich. Tetraethyl orthosilicate (TEOS), 2-furaldehyde
(furfural) and tetramethyl orthosilicate (TMOS) were purchased from
Acros. HCl(aq) (ca. 37%) was purchased from Merck. Ethanol was
purchased from Fisher Scientific. Niobium (V) ethoxide (Nb(OEt)5)
was purchased from ABCR. NH3(aq) (ca. 25%) and toluene were
purchased from VWR Chemicals. Dioxane was purchased from Roth.
Hydrated niobium oxide (Nb2O5·nH2O, HY-340) was provided free
of charge by Companhia Brasileira de Metalurgia e Mineração
(CBMM). Furfural and EAA were purified by distillation under
reduced pressure prior to being used. The water used in this study was
purified by a Millipore Milli-Q Advantage A10 water purification
system resulting in a resistivity higher than 18 MΩ·cm. All gases were
purchased from Carbagas.
Powder x-ray diffraction (XRD) measurements were performed
in a PANalytical Empyrean system (Theta-Theta, 240 mm) using Cu
K-α as a radiation source. X-Ray Photoelectron Spectroscopy (XPS)
measurements were carried out using a PHI VersaProbe II scanning
XPS microprobe (Physical Instruments AG, Germany). Analysis was
performed by using a monochromatic Al Kα X-ray source at a power
of 24.8 W with a beam size of 100 µm. The spherical capacitor
analyzer was set at a 45° take-off angle with respect to the sample
surface. The pass energy was 46.95 eV yielding a full width at half
maximum of 0.91 eV for the Ag 3d 5/2 peak.
Transmission electron microscope (TEM) images were obtained
on a FEI Talos instrument with a 200 kV acceleration voltage. The
samples were loaded on Lacey carbon grids. High-angle annular dark
field scanning transmission electron microscopy (HAADF-STEM)
images were obtained using an annular dark field detector to acquire
images with atomic number contrast. Energy-Dispersive X-ray
spectroscopy (EDX) was performed in the HAADF-STEM mode and
analyzed using Bruker Esprit software.
1H magic-angle spinning solid-state nuclear magnetic resonance
(ssNMR) experiments were performed on a Bruker 400MHz
spectrometer equipped with a 2.5 mm probe. A regular spin echo
program with a spinning rate of 35 kHz was used to acquire the
spectra. The samples were dried at 393 K under reduced pressure for
3 h and packed in the rotors in a glove box under inert atmosphere.
Catalyst characterization
Physisorption was conducted on a Micromeritics 3 Flex
instrument. The samples were dried under reduced pressure at 423 K
prior to N2 adsorption. The specific surface area (SBET) and pore size
distribution were determined using Brunauer–Emmett–Teller (BET)
theory and the Barrett—Joyner—Halenda (BJH) model, respectively.
Ammonia temperature-programmed desorption (NH3-TPD) was
performed on a Micromeritics Autochem 2920 instrument. In both
experiments, the samples were dried for 0.5 h with He flow (50
mL·min−1) at 573 K. For NH3-TPD, an excess amount of NH3(g) was
fed into the sample cell after the sample was cooled down to 393 K.
The weakly adsorbed NH3(g) was then removed by continuously
flowing He for 30 min. Subsequently, the temperature was ramped to
773 K (10 K·min−1) and the desorption of ammonia was quantified by
Material synthesis
HY-340 was pretreated by calcination at two different
temperatures for 3 h. HY-340 calcined at 573 K is simply denoted as
HY-340 whereas HY-340 calcined at 773 K is denoted as HY-340T.
Silica spheres were synthesized by the Stöber method.24 Typically, 3.8
mL of TEOS, 7.7 mL NH3 (ca. 25%) and 18 mL of deionized water
were mixed with 120 mL ethanol. The mixture was stirred and reacted
at room temperature for 24 h. The product was centrifuged and
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washed with ethanol and then water. SBA-15 was synthesized based
12Nb2O5@SiO2 was synthesized using a similar protocol: 5 g
View Article Online
DOI: 10.1039/C9TA01459D
on a previously published method that was slightly modified.25 Stöber silica was dispersed in a solution containing 50 mL ethanol,
Specifically, 2.8 g P-123 was dissolved in 104 g of 1.6 M HCl(aq) by 0.04 mL NH3 and 0.75 mL H2O. A mixture of 1.6 mmol Nb(OEt)5,
vigorous stirring. 4 mL of TMOS was then slowly added and the 4.8 mmol EAA and 9 mL ethanol was then injected into the
resulting mixture was stirred for 24 h. Subsequently, the suspension suspension at the rate of 1.5 mL⋅min−1. The subsequent experimental
was transferred into a Teflon lined stainless steel autoclave and heated
at 110 °C for 24 h. After the hydrothermal treatment, the surfactant
was removed by ethanol extraction using a standard Soxhlet apparatus
for 24 h. Both silica substrates were calcined at 773 K for 5 h under a
flow of synthetic air (ramping rate: 2 K⋅min−1). Mesoporous niobia
(m-Nb2O5) was synthesized following a reported method.18 Briefly, 1
g P-123 was mixed with 10 g of ethanol. This solution was then mixed
with 2 g of NbCl5 while vigorous stirring was applied. The solvent of
the resulting mixture was then slowly evaporated and the resulting
solid was aged in air at 313 K for 2 days. After aging, the gel was
calcined in a muffle furnace at 673 K for 5 h.
conditions are the same as the procedure for xNb2O5@SBA-15. All
the overcoated samples were thoroughly washed with ethanol and
water and calcined at 773 K for 3 h to remove the remaining organic
impurities.
Catalytic testing
Xylose dehydration was performed in an Alltech 10 mL thick-
walled glass reactor. Typically, 75 mg xylose was dissolved in 2 mL
H2O and mixed with 3 mL toluene. 100 mg of catalyst was then loaded
into the reactor and mechanically stirred (550 rpm) at 393 K for 5 h.
The organic phase was analyzed by an Agilent Technologies 7890 A
gas chromatography (GC) equipped with a flame ionization detector
(FID) and a HP-5 column (50 m, 0.32 mm). The aqueous phase was
analyzed by an Agilent Technologies 1260 infinity high performance
liquid chromatography (HPLC) equipped with a refractive index
detector, a UV-Vis detector and a Bio-Rad Aminex HPX-87H
column. The chromatography was carried out with a flow rate of 0.6
mL·min−1 at 333 K using 5 mM H2SO4(aq) as the mobile phase. The
products were quantified using calibration curves produced by
diluting analytical grade standards.
To synthetize overcoated samples, we applied a methodology
similar to our previous work where we first calculated the quantity of
Nb(OEt)5 that was necessary to achieve single atomic monolayer
coverage by dividing the SBET of substrate by the Van Der Waals
projection area of Nb(OEt)5 (0.765 nm2, estimated by
MarvinSketch)23. For 0.5 g SBA-15 (SBET = 750 m2⋅g−1), the
monolayer amount of Nb(OEt)5 (i.e. for synthesizing 1Nb2O5@SBA-
15) was 0.8 mmol. As previously shown, we used a bidentate ligand
(EAA) to chelate aluminum sec-butoxide in order to slow down the
gelation rate of alumina for sol-gel overcoating.23 Similarly, the
precursor in this work was prepared by mixing Nb(OEt)5 with 3
equivalent amount EAA under N2 atmosphere. This led to a slower
condensation rate and prevented uncontrolled Nb2O5 precipitation
during sol-gel coating.26 Subsequently, the mixture was diluted with
ethanol and the final volume of the precursor was fixed to 10 mL. The
resulting precursor solution was stirred at ambient temperature for 18
h. The general synthesis method for xNb2O5@SBA-15 was as
follows: 0.5 g SBA-15 was dispersed in a mixture containing ethanol,
water and NH3(aq). The quantities of water and ethanol in the
suspension were proportional to the number of moles of injected
Nb(OEt)5. Specifically, the molar ratio of Nb(OEt)5, water and ethanol
was kept to 1:5:535. The volume of 25 wt% NH3(sq) in the suspension
was fixed at 0.02 mL. The coating was initiated by injecting the
aforementioned precursor solution into the suspension using an
automatic syringe pump (KDS 100 legacy syringe pump) and the
injection rate was set to 1.5 mL⋅min−1. The suspension was vigorously
stirred, soaked in an ice bath during the injection process and then kept
at room temperature for 18 h after the injection was completed.
Friedel-Crafts alkylation was performed in an Alltech 10 mL
thick-walled glass reactor. Typically, 50 mg catalyst was mixed with
1 mmol benzyl alcohol and 20 mmol anisole. The reaction proceeded
by stirring the mixture (550 rpm) at 403 K for 3 h. The products were
diluted with dioxane and analyzed by GC-FID. Benzyl alcohol was
quantified using a calibration curve obtained produced by diluting a
commercial standard while the three other products were quantified
by using the effective carbon number method with decane as an
internal standard.27,28 The absence of significant mass transfer
limitations for both reactions was confirmed by using the experiments
with different stirring rates and by applying the Weisz–Prater
criterion, which is shown in the supporting information.
Results and Discussion
Sol-gel based overcoating of Nb2O5 on silica substrate
An amount of Nb(OEt)5 sufficient to produce 12 monolayers of
the niobia overcoat was used to overcoat SiO2 spheres in order to
image the morphology of the deposited Nb2O5. A uniform Nb2O5 shell
with a thickness of ~2 nm was formed on the sphere (Fig. 1a and b).
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The SBET of SiO2 slightly increased after coating (from 12 to 15 of layers indicating that a regular pore structure was cVoienwseArrvticeled.OTnlihnee
DOI: 10.1039/C9TA01459D
m2⋅g−1), implying that the Nb2O5 shell was slightly porous. The image disappearance of the smaller diffraction peaks with increasing layers
could be attributed to the scattering of X-ray by the overcoat rather
than pore degradation, which was further supported by physisorption
(Fig. 2b) using the BJH model.30 The overcoated samples still had a
narrow pore size distributions and the pore volumes consistently
decreased with increasing niobia loading. More importantly, the mean
pore diameter reduced linearly with the number of Nb2O5 layers,
with higher magnification (Fig. 1b) shows that some nano-sized
Nb2O5 clusters sporadically formed on the silica surface. This
observation is in agreement with our previous simulation and
experimental work which supported that the formation of metal oxide
overcoats via solution sol-gel chemistry route proceed by an
oligomer-deposition mechanism.23,29 Such a mechanism could result
in some porosity when niobia nanoparticles with different sizes
aggregate and cover the surface and the larger niobia clusters might
evolve from the larger sol particles.
implying
a
precise thickness control could be achieved
(approximately 0.8 nm per layer, inset in Fig. 2b). Overcoating Nb2O5
onto SBA-15 was previously reported by either using ALD or
sequential sol-gel grafting.22,31 Comparing to these reported methods,
our new wet-chemistry based approach has the advantages over both
the ALD and grafting method. The presented coating procedure can
be easily performed without a complicated ALD equipment or
multiple grafting steps, yet is capable of achieving a reasonable degree
of thickness control with nano-scale precision that is similar to ALD.
Fig. 1 TEM and STEM images of (a) 12Nb2O5@SiO2 with EDX mapping (inset),
(b) 12Nb2O5@SiO2 with higher magnification, (c) 1Nb2O5@SBA-15 under
HAADF mode and (d) 1Nb2O5@SBA-15 under HAADF mode with EDX
mapping.
We further tried to overcoat SBA-15 as a post-modification
strategy to synthesize mesoporous niobia with high surface area and
an ordered pore structure. Due to the slower condensation kinetics of
the chelated Nb(OEt)5, a niobia overcoat can grow conformally inside
the mesopores of SBA-15, as confirmed by STEM microscopy and
EDX mapping (Fig. 1c and d). The morphology of SBA-15 did not
significantly change with the increasing number of deposited Nb2O5
layers (Fig. S1 to S3). Small-angle XRD was then used to characterize
the porous structures of overcoated SBA-15. Pristine SBA-15 displays
three typical diffraction peaks that are the characteristics of P6mm
hexagonal symmetry. The intensity of the diffraction peaks was
reduced after overcoating but still fully present after 1 layer (Fig. 2a).
The largest diffraction peak was maintained regardless of the number
Fig. 2 (a) Small-angle XRD results of uncoated and overcoated SBA-15. The
peaks in SBA-15 from small to large angle represent the 100, 110 and 200
diffractions (2D P6mm hexagonal symmetry).32 (b) Pore size distributions of
uncoated and overcoated SBA-15. The inset is the plot of the mean pore
diameter versus the number of deposited niobia layers.
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presence of these strong bonds. A similar observaVtiieownArwticalesOanllisnoe
DOI: 10.1039/C9TA01459D
previously reported for ZrO2 overcoats on SBA-15.30
Structural and acid properties of Nb2O5@SBA-15 and bulk niobia
Because of the porous nature of our niobia overcoat, the silica
functionality (e.g. silanol group) might remain accessible at the
surface. Therefore, we performed XPS to investigate the surface
compositions of Nb2O5@SBA-15 (Fig. 3). The O 1s XPS results of
SBA-15 and four overcoated samples showed a clear transition from
a pure Si-O signal to a Nb-O signal. Although a very weak Si signal
could still be detected (Nb/Si = 9.4, Table 1), we concluded that
4Nb2O5@SBA-15 showed nearly full Nb2O5 coverage on the surface
according to the O 1s data. The signal of Nb 3d, on the other hand,
provided information of the niobium environment. Both bulk niobia
and the overcoated samples showed typical doublet peaks, which is
known to be due to the spin-orbit coupling of the Nb 3d orbitals. The
binding energy of the doublet increases with a higher oxidation state
of the niobium.33,34 The Nb 3d signals of the 4Nb2O5@SBA-15 and
HY-340 almost overlap while the signals of m-Nb2O5 have lower
binding energies, indicating there was an increasing Nb4+ character in
m-Nb2O5. According to the model proposed by Kreissl et. al, Nb4+
exists as an oxygen-deficient five-coordinated center in the Nb2O5
network, which is structurally distinct from the typical octahedral
NbO6 unit. Accordingly, the Nb2O5 overcoat and HY-340 were likely
comprised of mainly NbO6 units but m-Nb2O5 had more pyramidal
NbO5 units on the surface.
Acid catalysis is an important application for niobia. To
evaluate this aspect of the overcoated material, the total acid sites of
Nb2O5@SBA-15 and bulk niobia catalysts were quantified by NH3-
TPD (Table 1). 4Nb2O5@SBA-15 had the largest number of total acid
sites per weight among all samples since its SBET outnumbered all bulk
niobia samples. Overall, HY-340 had the largest number of total acid
sites per surface area among all materials. However, the number of
acid sites were drastically reduced when being calcined at 773 K (HY-
340T). In this case, HY-340 underwent structural transformation from
an amorphous to pseudo-hexagonal phase during the thermal
treatment (Fig. S4), which caused the loss of acid sites as previously
reported.15,16 The as-synthesized m-Nb2O5 had an intrinsic pseudo-
hexagonal structure and a similarly low number of acid sites as the
thermally treated HY-340T. Interestingly, all overcoated
Nb2O5@SBA-15 retained their amorphous structure and large number
of acid sites even after a high temperature thermal treatment (773 K),
which was required to effectively remove the remaining precursor
impurities during the synthesis. We attributed the limited
crystallization to the presence of Si-O-Nb bonds between the overcoat
and the support. The amorphous phase was likely stabilized by the
Fig. 3 (a) O 1s XPS results for SBA-15, overcoated SBA-15 and HY-340 (b) Nb
3d results for 4Nb2O5@SBA-15, m-Nb2O5 and HY-340.
The total number of acid sites per surface area of Nb2O5@SBA-
15 was considerably lower than both types of bulk niobia (Table 1),
which could be explained by incomplete Nb2O5 coverage in the
micropores of SBA-15. Indeed, silanol groups could still be observed
by XPS, DRIFTS (Fig. S5a) and 1H ssNMR (Fig. S6) for
4Nb2O5@SBA-15. We hypothesize that Nb(OEt)5 is probably too
bulky to effectively diffuse into the micropores of SBA-15 and react,
thus leaving some unreacted Si-OH at the surface. This phenomenon
was also observed when ALD was used to overcoat Nb2O5 onto SBA-
15.22 Since the micropores were not completely filled by Nb2O5, they
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contributed to the surface area without adding acid sites, thus lowering dehydration and Friedel-Crafts alkylation. DehydratViniegw Axrtyiclloe sOenlintoe
DOI: 10.1039/C9TA01459D
the acid site density for overcoated SBA-15. Furthermore, we form furfural catalyzed by Nb2O5 has been widely investigated.8,10,13
quantified the number of Brønsted acid sites of bulk and overcoated Xylose is believed to first isomerize into xylulose over a Lewis acid
niobia catalysts using NH3-TPD to quantify the total number of sites site and further dehydrate to furfural, which is catalyzed by Brønsted
present and DRIFTS-pyridine (Fig. S7) to measure the ratio of acid sites.39,40 The selectivity of furfural is highly dependent on the
Brønsted to Lewis acid sites using a previously reported ratio of ratio of Brønsted/Lewis acid sites in the catalyst.13,41 We performed
extinction coefficients. 35 Specifically, the ratio of the two acid types xylose dehydration in a standard biphasic system (water/toluene) in
were estimated using the integrated band area of the peaks at 1445 cm- order to continuously extract furfural and avoid its degradation by
1
and 1535 cm-1, which were assigned to the pyridine adsorbed on further rehydration and condensation reactions (e.g. levulinic acid and
Lewis and Brønsted acid sites, respectively. The catalysts with the humin formation). 4Nb2O5@SBA-15 showed the highest conversion
highest Brønsted/Lewis acid site (B/L) ratio were, in order, HY-340 > (91%) among all overcoated samples (Fig. 4). However, the
m-Nb2O5
> 4Nb2O5@SBA-15. Lewis acid sites have been conversion of HY-340 was higher (96%) than that of 4Nb2O5@SBA-
hypothesized to originate from niobium centers with lower 15 (91%), even though it had slightly fewer acid sites (Table 1).
coordination numbers and Brønsted acid sites are contributed by the
Despite the high conversion, the furfural selectivity of HY-340 was
Nb-OH groups associated to NbO6 units.36 Therefore, the acid
much lower (44%) than that of the overcoated catalyst, which was
characterization results of HY-340 and m-Nb2O5 agreed with the
likely due to the aforementioned side reactions that occurred at nearly
structural features that were suggested by the XPS experiments.
full conversion.42 A higher selectivity could be obtained (50%) with a
Namely that the niobium in HY-340 had a higher coordination number
shorter reaction time (3 h), but HY-340 was still less selective to
than the niobium in m-Nb2O5. However, XPS data could have led us
furfural than other catalysts. Moreover, HY-340 calcined at 773 K
to believe that 4Nb2O5@SBA-15 and HY-340 would have had similar
also showed low selectivity towards furfural. Therefore, we suggested
B/L ratio because these analyses suggested that their surfaces were
that the predominant Brønsted acidity (B/L ratio > 5) in HY-340 could
mainly composed of NbO6 units. Instead, the much smaller B/L ratio
favour rehydration and condensation reactions thus lowering the
(1/3) of 4Nb2O5@SBA-15 could have resulted from the self-
selectivity towards furfural.
condensation of Nb-OH group during the calcination at 773 K, which
could have proceeded by a similar mechanism as what occurs on
amorphous silica surfaces.37,38
Catalytic behavior of mesoporous Nb2O5 and bulk catalysts
To investigate the catalytic activity of overcoated SBA-15 as a solid
acid, we used two typical acid-catalyzed reactions as probes: xylose
Table 1. Characterization data of overcoated and bulk niobia catalysts
SBET
Nb/Si
ratioa
Total acid sitesb
Brønsted acid sitesc Lewis acid sitesd
Structuree
(m2/g)
µmol g–1
µmol m–2
-
µmol g–1
-
µmol g–1
-
SBA-15
750
-
-
amorphous
1Nb2O5@SBA15
2Nb2O5@SBA15
558
521
0.12
0.63
228
210
0.41
0.40
-
-
-
-
amorphous
amorphous
3Nb2O5@SBA15
419
2.3
237
0.56
-
-
amorphous
4Nb2O5@SBA15
m-Nb2O5
471
91
9.4
-
296
139
0.63
1.53
76
79
220
60
amorphous
TT-phase
HY-340
HY-340T
72
67
-
-
278
97
3.86
1.45
238
-
40
-
amorphous
TT-phase
a Determined by XPS.
b Determined by NH3-TPD.
c Determined by pyridine-DRIFTS
e Determined by XRD
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benzyl anisole) and benzyl ether (side product) were oVbietawinAretidcleinOnthlinies
DOI: 10.1039/C9TA01459D
reaction. The four tested catalysts showed identical product
distributions and similar selectivity to benzylated products (Fig. 5a).
Fig. 4 Xylose dehydration using different niobia catalysts. Points represent
conversion and bars represent furfural selectivity.
The increasing conversion with each deposition cycle can likely be
explained by the increasing extent of niobia coverage on the surface,
which led to an increasing number of acid sites (especially Brønsted
acid sites) and increased number of adsorption sites for xylose. At the
same time, selectivity did not increase as rapidly because it likely
depended more on the ratio of Brønsted acid and Lewis acid sites
rather than the total number of acid sites. This ratio likely remained
fairly constant with the increasing coverage, so the selectivity did not
vary as much as the conversion increased. Interestingly, we observed
that 3Nb2O5@SBA-15 and 4Nb2O5@SBA-15 had slightly increased
selectivities to furfural. This result might be related to limited xylose
degradation reactions that occur in the solution. Increasing the
conversion of xylose might lower the extents of those side reactions
Fig. 5 (a) Yield and distribution of products in the Friedel-Crafts alkylation
catalyzed by different forms of niobia. (b) Catalyst stability test of
and hence slightly increase the selectivity. The highest yield of 4Nb2O5@SBA-15 and HY-340 after regenerations. The yield is the sum of the
two benzylated products.
furfural (56% at 3h) was obtained with 4Nb2O5@SBA-15, and we
attributed the high yield to its balanced B/L ratio, which maximized
conversion while limiting byproduct formation.
However, the differences in activity were distinct from that of
xylose dehydration. Using 4Nb2O5@SBA-15 led to the highest
conversion (84%) and yield of benzyl anisole (63%, including two
isomers) at 3 h. In contrast, HY-340, the catalyst with the largest
number of Brønsted acid sites, led to only 57% conversion. The lowest
conversion was again obtained over m-Nb2O5. HY-340T showed a
reduced activity (37% of conversion) compared to HY-340,
presumably because of the loss of its acid sites during phase
transformation. We also noticed that the catalysts’ color changed
when the conversion neared 100% (Fig. S8). We repeated the reaction
using spent 4Nb2O5@SBA-15 and a lower conversion was obtained
(45%), which can be ascribed to the formation of some condensed
byproducts that blocked acid sites. Therefore, the spent
We further studied the activities of 4Nb2O5@SBA-15 and other
bulk niobia materials during Friedel-Crafts alkylation, which is an
important reaction for arene substitution. Friedel-Crafts alkylation is
typically catalyzed by Lewis acid sites but requires Brønsted acid sites
when the alkylating reagent is an alcohol.21,43,44 The use of a solid acid
and an alcohol alkylation agent is considered more environmentally
friendly than traditional homogeneous Lewis acids (e.g. AlCl3). We
studied the benzylation of anisole using benzyl alcohol because such
benzyl benzene derived molecules are considered industrially
valuable.45,46 Specifically, two structural isomers (para- and ortho-
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4Nb2O5@SBA-15 and HY-340 were regenerated by calcination at
773 K for 1 h to remove deposits and the activities of regenerated
catalysts were studied (Fig. 5b). The conversion of 4Nb2O5@SBA-15
at 3 h slightly decreased after first regeneration but it remained stable
in the second and third catalytic runs. In contrast, the activity of HY-
340 dropped after the second regeneration. Further XRD analyses
showed that the structure of the regenerated HY-340 became pseudo-
hexagonal while 4Nb2O5@SBA-15 remained amorphous (Fig. S4b).
These results further suggested that the acid sites of 4Nb2O5@SBA-
15 had an exceptional resistance against thermal treatment, which
could be advantageous in industrial processes where catalyst
regenerations are required.
View Article Online
DOI: 10.1039/C9TA01459D
As previously mentioned, the Brønsted acid sites are
considered to be the active site in this Friedel-Crafts alkylation.
Therefore, we determined the turnover frequency (TOF) of
4Nb2O5@SBA-15, HY-340 and m-Nb2O5 by dividing their
extrapolated initial reaction rates by the numbers of Brønsted acid
sites (Fig. 6). Interestingly, the TOF of 4Nb2O5@SBA-15 (0.06 s−1)
was 6 times higher than the TOF of HY-340 (0.009 s−1) while m-
Nb2O5 displayed a TOF that was nearly one order of magnitude lower
than these two catalysts (TOF = 0.001 s−1). We hypothesized that the
variation in activity may result from the difference in acidity strength
between these various catalysts. DRIFTS-pyridine was then
conducted and the spectra were collected at 573 K (Fig. 6d) to study
the presence of strongly chemisorbed pyridine. The adsorbed pyridine
species and their corresponding acid type were assigned according to
previous reports in the literature.47,48 We observed only two peaks that
corresponded to Lewis acid sites on m-Nb2O5, whereas pyridines
adsorbing on Brønsted acid sites were found on both 4Nb2O5@SBA-
15 and HY-340. This result substantiated that the acidity of m-Nb2O5
was weaker, thus leading to a significantly lower TOF for Friedel-
Crafts alkylation. In contrast, the peak at 1520 cm−1, which was
assigned to Brønsted acid sites, was measured even at this higher
temperature for both HY-340 and 4Nb2O5@SBA-15, which was
indicative of strong Brønsted acidity . We further examined NH3-TPD
data (Fig. S9) and found that HY-340 had an extra desorption band at
653 K, which was not detected for 4Nb2O5@SBA-15. This result
implied that HY-340 was more acidic, which led us to conclude that
acid strength alone could not account for the higher activity of
4Nb2O5@SBA-15.
Fig. 6 Kinetic study of Friedel-Crafts alkylation. Initial rate determination and
TOF calculation for (a) 4Nb2O5@SBA-15, (b) HY-340 and (c) m-Nb2O5. The
initial reaction rates were determined by the extrapolation to time zero of the
least squares fit of reactant concentration versus reaction time at conversions
below 20%. The reaction conditions were the same as what is shown in the
experimental section except the amount of catalyst added for 4Nb2O5@SBA-
15 and HY-340, which were reduced to 25 mg to maintain low conversion. (d)
DRIFTS-Pyridine results. BPy, LPy and HPy denote pyridine adsorbing on
Brønsted acid sites, pyridine adsorbing on Lewis acid site and H-bonded
pyridine, respectively. The presented data were smoothed using the
adjacent-averaging method with a 15 point window.
In order to understand the discrepancy in acid strength and
catalytic activity between HY-340 and 4Nb2O5@SBA-15, we ran 1H
ssNMR measurements to characterize the surface hydroxyl groups at
the atomic level (Fig. 7). HY-340 showed a broad peak centered at 5.7
ppm and another narrower peak with much lower intensity at 2.7 ppm.
In comparison, 4Nb2O5@SBA-15 showed a broad peak centered at
4.9 ppm and two partially overlapping peaks at 2.4 ppm and 2.2 ppm.
The peak at 2.4 ppm was assigned to Nb–OH and the peak 2.2 ppm
was assigned to the remaining silanol groups from the micropores of
SBA-15 support (1H ssNMR spectra of pure SBA-15 is shown in Fig.
S6, confirming the silanol group assignment). Consequently, both
niobia catalysts showed two Nb–OH species but the ratio of these two
hydroxyl groups seemed to be different. Takagaki et al.21,49,50 have
performed detailed studies on niobia and have assigned the broader
peak to a hydrogen bonded proton and the narrower peak to a terminal
Nb–OH. Takagaki et al. also suggested that the Nb–OH groups with
hydrogen bonding, which lead to the broad peak at higher ppms, had
a stronger acidity. The high proportion of hydrogen bonded Nb–OH
on HY-340 could have led to its stronger acidity, which is in a good
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agreement with our NH3-TPD analyses. Compared to HY-340,
4Nb2O5@SBA-15 had more isolated Nb–OH groups, which were less
acidic than the hydrogen-bonded hydroxyl groups. We attributed this
result to the structural difference between 4Nb2O5@SBA-15 and HY-
340 and the hypothesized models are schematized (Fig. 7). HY-340 is
known to be a mixture of nanoparticles with different sizes and
hydrogen bonds can form within their interlayers. Furthermore, IPA
chemisorption indicated that HY-340 had the highest density of
Brønsted acid sites, which originated from the Nb–OH groups on the
surface. This high density of the surface Nb–OH groups likely further
promoted the hydrogen bond formation. In contrast, the niobia on
4Nb2O5@SBA-15 has a two dimensional structure because it was
deposited as a thin film inside the pores of SBA-15. This
nanostructure likely led the niobia to have a longer distance between
Nb2O5 surfaces, which disfavoured hydrogen bond formation and
increases the presence of exposed isolated Nb-OH groups. We
propose that the high activity of 4Nb2O5@SBA-15 in Friedel-Crafts
alkylation was due to its unique nanostructure and surface
characteristics, which involves these isolated Nb-OH groups. The
relatively sparse Nb-OH groups in the absence of hydrogen bonding
could have lead anisole and benzyl alcohol to adsorb and react more
easily compared to the case where a hydrogen bond network is
formed, which could be unfavorable to the adsorption of such
hydrophobic substrates. Our findings reveal that strong acidity does
not guarantee activity during acid catalysis. An acid catalyst with the
Conclusions
View Article Online
DOI: 10.1039/C9TA01459D
By properly controlling the sol-gel kinetics of Nb(OEt)5, a post
synthesis strategy was developed for synthesizing a SBA-15 type
mesoporous niobia with high surface area and narrow pore size
distribution. This niobic acid exhibits superior catalytic activity in two
probe reactions, namely xylose dehydration and Friedel-Crafts
alkylation. The balanced ratio of Brønsted and Lewis acid sites on
Nb2O5@SBA-15 resulted in a higher selectivity towards furfural
compared to bulk niobia catalysts. On the other hand, the tailored
nanostructure of overcoated catalyst restrained the formation of
interlayer hydrogen bonds, which appeared to be favourable for
Friedel-Crafts alkylation with hydrophobic reactants. This structural
advantage led to a higher activity despite its weaker acidity.
Importantly, the overcoating strategy also imparted a higher thermal
stability to Nb2O5. The acid sites could resist high temperature thermal
treatments (773 K) which led to its activity being retained after
catalyst regenerations. In contrast, these high temperatures caused
phase transformations in HY-340 and the mesoporous niobia
synthesized using traditional template assisted methods, which
irreversibly reduced the number of acid sites. Overall, not only did
kinetically controlled overcoating provide an easy method for
producing high surface area niobia catalysts with a controlled
mesoporous structure, but it also led to unique structural stability and
properties. Both phenomena, which were observed during different
probe reactions, emphasize the importance of localized material
structure in catalysis and could provide new insights and routes for
the rational design of solid acid catalysts.
well-tailored nanostructure could lead to
a higher catalytic
performance even though it had slightly fewer and weaker acid sites.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the European Research Council
(ERC) under the European Union’s Horizon 2020 research and
innovation program (Starting grant: CATACOAT, No 758653), as
well as the Swiss National Science Foundation through grant
PYAPP2_154281 and by EPFL. This work was also accomplished
within the framework of the Swiss Competence Center for Bioenergy
Research (SCCER-BIOSWEET). We thank the EPFL
interdisciplinary center for electron microscopy (CIME) for their
support during electron microscopy measurements. We thank Emilie
Baudat and Pierre Mettraux for their help in performing
measurements with ssNMR and XPS, respectively.
Fig. 7 1H ssNMR spectra and associated proposed schematic structure of HY-
340 and niobia overcoated on silica (4Nb2O5@SBA-15).
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28 L. Shuai, M. T. Amiri, Y. M. Questell-Santiago, F.VHieéwrAorgticuleelO,nYlin.e
Li, H. Kim, R. Meilan, C. Chapple, J. Ralph and J. S.
Notes and references
DOI: 10.1039/C9TA01459D
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