Sulfated and phosphated mesoporous Nb oxide in the benzylation of anisole and toluene by benzyl alcohol

Article abstract of DOI:10.1021/ja0647147

Materials possessing the high acidities of sulfated zirconia and the diffusion properties of mesoporous oxides are predicted to have numerous applications in the petrochemical industry. Because of surface deactivation and loss of structure under highly acidic conditions, there are few examples of materials which meet these specifications. In this work, mesoporous Nb oxide was treated with 1 M sulfuric acid or phosphoric acid and evaluated for their catalytic activities in the benzylation of toluene or anisole with benzyl alcohol. Characterization by XRD, nitrogen adsorption/desorption, and TEM demonstrated that the mesostructure was surprisingly stable to acid treatment. Pyridine adsorption and infrared spectroscopy (IR) showed a mixture of Lewis and Bronsted sites before and after acid treatment. Titration with a series of indicators demonstrated that sulfated mesoporous Nb oxide possesses a pK_a of -8.2 and 31.784 mmol/g acid sites, roughly 100 times stronger than either bulk phosphated or sulfated niobia, which both possess pK_a values in the range of -3.0. The best catalytic results in this study were achieved when using mesoporous Nb oxide treated with sulfuric acid; the conversion of benzyl alcohol with anisole to the benzylation product was 100% in 30 min, which is 200 times faster than the bulk catalyst. The extremely high activity was rationalized by the high number of strong Bronsted sites on the surface coupled with the superior diffusion properties of the mesoporous system. Copyright

Full text of DOI:10.1021/ja0647147

Published on Web 10/11/2006
Sulfated and Phosphated Mesoporous Nb Oxide in the Benzylation of Anisole
and Toluene by Benzyl Alcohol
Yuxiang Rao,† Michel Trudeau,‡ and David Antonelli*,†
Department of Chemistry and Biochemistry, UniVersity of Windsor, 401 Sunset AVenue, Windsor, Ontario N9B 3P4,
Canada, and Emerging Technologies, Hydro-Que´bec Research Institute, 1800 BouleVard Lionel-Boulet, Varennes,
Quebec, Canada J3X 1S1
Received July 3, 2006; E-mail: danton@uwindsor.ca
Acid catalyzed conversions of hydrocarbons have been studied
for several decades. Because of the environmental hazards associ-
ated with the use of liquid acids, much effort has been put into
developing more environmentally friendly solid acids to perform
the same function. Solid acids are mineral oxides such as sulfated
zirconia that contain both Lewis and Brønsted acid sites. These
materials are used in the isomerization of alkanes and other
petrochemical processes such as alkylation of aromatics and
hydrocarbon cracking. Isomerization of linear hydrocarbons is
performed commercially using solid Pt/chlorided Al2O3 catalysts
in the case of n-butane (UOP Butamer process or BP process), and
Pt/mordenite zeolite (Shell Hysomer) in the case of C5/C6 alkanes.1
Trace amounts of transition metals such as Fe, Mn, or Pt2 can
promote isomerization by sulfated zirconia to the extent that the
Figure 1. TEM image of sulfated mesoporous Nb oxide synthesized with
a dodecylamine template.
catalyst performs at room temperature or lower, rivaling the
the activity of Nb and Ta oxides in many hydrocarbon reactions
performance of liquid acid agents2. Despite this high activity,
while maintaining their high selectivity.13 Herein we provide a report
sulfated zirconia has two major problems. These are (a) the low
of the synthesis, characterization, and catalytic properties of
surface area and porosity of the material and (b) rapid deactivation
mesoporous sulfated and phosphated Nb oxides. The catalytic
owing to build up of carbonaceous deposits on the oxide surface.
activity in the benzylation of anisole is studied and compared to
Thus, a higher surface area transition metal oxide catalyst with
standard Nb phosphate and sulfate acid catalysts.
larger pores, resistant to blockage by carbonaceous deposits, is
The synthesis of niobium oxide molecular sieve (Nb-TMS1) was
required. While the tailoring of superacidity into mesoporous silica
successfully achieved by using the ligand-assisted liquid crystal
has been thwarted by deactivation and instability to acids, MCM-
templating approach; the optimized ratio of dodecylamine-to-Nb-
41 has been used as a support for sulfated zirconia,3 although
(OEt)5 used was 0.3:1.6 The as-synthesized samples were further
extension to more active Fe/Mn doped systems has not yet been
treated with 1 M sulfuric acid or phosphoric acid (in methanol
reported. Al-Daous et al.4 described the synthesis 3-d ordered
solution) separately to obtain mesoporous sulfated and phosphated
Nb oxides. Elemental analyses showed that the samples were 1.15
wt % P and 2.03 wt % S for the phosphated and sulfated samples,
respectively.
macroporous sulfated zirconia with ∼120 m2/g surface area that
shows catalytic activity for n-butane isomerization. Mesoporous
phosphated zirconias with high surface acidity have also been
studied by Wong and Ying, although competing layered phases
Figure S1 shows the XRD patterns of pure Nb oxide and
were observed under the conditions of study.5 Because of the ease
sulphated and phosphated Nb oxides. The strong reflection at d )
32 Å in all samples demonstrates the retention of the mesoporous
structures after acid treatment. Nitrogen adsorption/desorption
of preparation, enormous surface areas (400-900 m2/g), controlled
pore sizes, and accessible variable oxidation states, amine-templated
mesoporous Nb (Nb-TMS1) and Ta oxides (Ta-TMS1)6 are ideal
measurements (Figure S2) show a typical type IV isotherm for the
catalyst materials.6 Nb and Ta oxides exhibit special properties such
sulfated and phosphated materials, further confirming that the
as high stability, strong metal support interaction (SMSI), and
mesoporous structure was retained. This stability to acid is
potential for the formation of unique mixed metal oxides, as well
surprising considering that mesoporous silica and alumina lose their
as variable acidic properties, crucial to alkane isomerization and
pore structure under these conditions. Figure 1 shows a transmission
alkylation reactions.7 Hydrated Nb and Ta oxides are highly acidic
electron micrograph (TEM) of mesoporous Nb oxide treated with
in the amorphous form,8 while partially hydrated Nb oxide, calcined
sulfuric acid, clearly illustrating the wormhole pore structure of
at moderate temperatures, is an effective solid acid catalyst. These
this material. It can be estimated from the image that the average
properties can be maintained even in the presence of water.9 Nb
pore size of H2SO4/Meso Nb is around 20 Å.
oxides pretreated with phosphoric or sulfuric acid are solid acid
The FTIR (Fourier transform infrared, Figure S3) analysis of
catalysts with a high ratio of Lewis acidity to Brønsted acidity.10,11
the spectra of pyridine adsorption show that Brønsted (1540 cm-1
)
Generally, Lewis acidity is found in all of the supported Nb oxide
systems, while Brønsted acid sites are only detected in niobia
supported on alumina and silica.12 Precious metal dopants improve
and Lewis (1450 cm-1) acid sites coexist in a roughly 50:50 mixture
on the surfaces of the parent material and that the sulfated and
phosphated materials exhibit a strong dominance of Brønsted sites.
This distribution of acid sites is unusual from the standpoint of
bulk niobia (see above),12 and demonstrates the unique nature of
† University of Windsor.
‡ Hydro-Que´bec Research Institute.
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13996
J. AM. CHEM. SOC. 2006, 128, 13996-13997
10.1021/ja0647147 CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
C could not be washed out. The S content droped in the used
catalyst to ca. 0.5%, and the catalyst could not be reused. To offset
the effects of sulfate leaching and formation of polymers, further
optimization of reaction conditions is obviously necessary.
Amberlyst 15 and MCM-41 were chosen as standards to compare
the catalytic activities in this study. The results showed that
Amberlyst 15 possesses roughly 1.5 times higher activity than
sulfated mesoporous Nb oxide, while MCM-41 has no activity at
all. Our catalyst is also three times more active in this reaction
than SiO2-Si-SCF3.16 Since our material has tunable pores in the
15-30 Å range, it is anticipated that it may find applications in
size and shape selective acid-catalyzed reactions for molecules too
large to fit in zeolite pores, yet small enough that the macroporous
(200-300 Å) structure of Amberlyst 15 would have no effect on
the steric profile of the reaction.
In summary, a series of high surface area mesoporous niobium
oxide materials with a range of acidity of Ho value from -6.6 to
-8.2 were prepared and screened for activity in the acid-catalyzed
benzylation of anisole. The sulfated material showed the highest
activity, almost 200 times greater than bulk sulfated standard. The
high activities were attributed to the mesoporosity and increased
concentration of Brønsted sites on the surface of the mesoporous
channels.
Figure 2. Percent conversion of benzyl alcohol in benzylation of anisole
catalyzed by different mesoporous Nb oxides.
the inner surface of the Nb oxide mesostructure. To further probe
the surface acidity of these mesoporous Nb oxides, the Hammett
acidity and n-butylamine titration methods14 were employed.
Commercially available bulk niobium pentoxides were used here
as standards to gauge the effect of the mesoporous structure on
acidity (Table S1, S2). From these data it is clear that bulk niobia
possesses only very weak acidic sites on the surface (pKa ) +3.3),
even for the sulfated and phosphated samples (pKa ) -3.0).
In contrast, pure mesoporous Nb oxide has a surprising Ho<
-6.6, and after being treated with 1 M sulfuric or phosphoric acid,
its Ho value can reach as low as -8.2, equal to 90% sulfuric acid.14
Table S2 shows the n-butylamine titration data for these materials.
The sulfated mesoporous material possesses 10 times more total
Lewis and Brønsted acid sites (31.78 mmol/g) than the parent (2.478
mmol/g) or phosphated (3.086 mmol/g) oxides, and almost 100
times more acid sites than the bulk samples (0.024-0.338 mmol/
g). The higher BET surface areas (Table S3) of the mesoporous
materials can account for some of this difference, although it is
also clear that sulfate is more effective than phosphate in generating
surface acid sites.
Acknowledgment. The authors wish to acknowledge NSERC
for the financial support of this research. We are also grateful to
Haining Liu for the artwork of reaction and Ahmad Hamaed for
the MCM-41 sample.
Supporting Information Available: Detailed experimental pro-
cedure, power X-ray diffraction patterns, N2 adsorption/desorption
isotherm and FTIR spectra. This material is available free of charge
via the Internet at http://pubs.acs.org.
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The catalytic activities of these mesoporous Nb oxide materials
and bulk Nb oxides were evaluated in the benzylation of anisole
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rapid polymerization of the neat reactants may have occurred over
the course of the reaction, resulting in pore blockage. This excess
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