1-Hexene isomerization over sulfated mesoporous Ta oxide: The effects of active site and confinement

Article abstract of DOI:10.1021/ja076584n

The isomerization of 1-hexene at 343 K was conducted over a series of sulfated mesoporous Ta oxides with pore sizes ranging from 12 to 30 A. The results were compared to the popular industrial catalysts HY-zeolite, H-ZSM5, and Amberlyst 15. The conversion of 1-hexene to trans/cis-2-isomers can reach 95.89% in 4 h, which is roughly 2 times faster than Amberlyst 15 and 20 times faster than HY-zeolite and H-ZSM5. GC analysis confirmed that trans- and cis-2-hexene isomers formed through a double bond shift process as the only two principal products. The molar ratio of trans/cis-2-hexene isomers varies with pore size, clearly showing confinement effects in this process. The selectivity of our best material toward the trans-isomer is at least 3 times greater than that of either zeolite or Amberlyst 15. The best catalytic results in this studywere achieved when using C₁₂ H₂SO₄ mesoporous Ta oxide, which possesses a pK_a of -8.2 and 19.8 mmol/g acid sites. The X-ray diffraction pattern showed complete retention of the mesostructure throughout catalysis as evidenced by a single broad reflection. Pyridine adsorption and infrared spectroscopy (IR) suggest that both Bronsted sites (1538 cm^-1) and Lewis sites (～1443 cm^-1) coexist on the surface of these materials. The large amount of Bronsted acid sites, high BET surface areas, high acid strength Ho, and superior diffusion properties of C₁₂ H₂SO₄ mesoporous Ta oxide are probably responsible for its extremely high catalytic activity and selectivity. Copyright

Full text of DOI:10.1021/ja076584n

Published on Web 12/21/2007
1
-Hexene Isomerization over Sulfated Mesoporous Ta Oxide: The Effects of
Active Site and Confinement
Yuxiang Rao, Junjie Kang, and David Antonelli*
Department of Chemistry and Biochemistry, UniVersity of Windsor, 401 Sunset AVenue,
Windsor, Ontario N9B 3P4, Canada
Received September 7, 2007; E-mail: danton@uwindsor.ca
Template-synthesized meso- and microporous materials com-
involved in isomerization reactions.12 The Hammett acidity and
n-butylamine titration methods were employed to measure acid
strength and acid amount of all the samples. The results are
summarized in Table 1. As is seen from this table, both acid strength
and acid amount of the C12 mesoporous Ta oxide increased after
acid treatment, consistent with previous work on mesoporous Nb
oxide.5
posed of transition metal oxides are attractive solid catalysts.1 Since
,2
the first synthesis of group V mesoporous metal oxides (Nb, Ta-
3
TMS1) were described in 1996, only a few papers have been
reported on their catalytic properties.4 These materials have
potential applications in petrochemical processes because of facile
tuning of pore size, moderate thermal stability, high surface area,
and high surface acidity, all of which are crucial properties in
industrial catalysis. The relatively large pores (12-30 Å) overcome
the size constraint of most zeolites, allowing more facile diffusion
of bulky substrates, while the very high surface areas (200-1200
-6
a
C
12 2 4
H SO mesoporous Ta oxide was evaluated in 1-hexene
isomerization at reflux temperature (343 K) and compared to
Amberlyst 15, HY-zeolite, and H-ZSM5 (Figure 1a). This new
material showed much higher activity than HY-zeolite and H-ZSM5
(roughly 10 times greater) and ca. 50% higher activity than
Amberlyst 15. In order to investigate the effect of pore size on this
2
m /g) create a high concentration of active sites per mass of material.
The transition metal centers in the inorganic framework of the
mesoporous structure also possess variable oxidation states and
empty d-orbitals, properties which allow electron transfer to occur
reaction, C
and tested. The results are shown in Figure S4. Among these three
samples, C12 SO mesoporous Ta showed the highest activity
6
and C18 mesoporous Ta oxides were also synthesized
4a
between the reactants and active site during any catalytic process.
H
2
4
Recent work in our group showed that sulfated mesoporous Nb
and Ta oxides were effective catalysts for benzylation reaction5a
due to their combination of strong acidity and large pore size. In
this report, we investigate the catalytic properties of mesoporous
Ta oxide in the isomerization of 1-hexene and compare the catalytic
(∼95% in 4 h), which can be attributed to a combination of pore
2
size and its higher BET surface area (292.19 m /g, Table S1). The
2
C
6
sample has the second highest surface area (206.40 m /g), but
its smaller pore size leads to the lowest observed activity. The C18
sample has the largest pore size (22.5 Å) but the lowest surface
7,8
2
activities and selectivity with commercially available zeolites
area (188.79 m /g) and thus displays an activity that falls between
9
(
HY-zeolite and H-ZSM5) and the ion exchange resin Amberlyst
that of its two congeners. GC analysis confirmed that trans- and
cis-2-hexene isomers were formed as the only two principal
products. Skeletal isomerization, which requires more energy and
15. The former are currently the most effective catalysts in catalytic
reforming, a process which is important because branched-chain
hydrocarbons are better automotive fuels than their straight-chain
isomers. This reaction is also widely used by many researchers in
both homogeneous10 and heterogeneous processes as a test of a
material’s effectiveness as an acid catalyst.
1
3
can only be performed at elevated temperature (>523 K), was
not observed at 343 K. C and C18 SO mesoporous Ta oxides
6
H
2
4
11
showed very similar selectivity, the ratio of trans/cis-2-hexene is
less than 1, which is close to the ratios observed from HY-zeolite,
H-ZSM5, and Amberlyst 15. However, for the C12 sample, the ratio
of trans:cis isomers can reach up to 3.7 after 6 h, which is very
The synthesis of a tantalum oxide molecular sieve (Ta-TSM1)
was successfully achieved by using the ligand-assisted templating
3
14
15
approach with different chain-length amine surfactants (C
6
∼C18).
close to thermodynamic equilibrium (3.37). Tanchoux et al.
The as-synthesized samples were further treated with 1 M sulfuric
acid in methanol solution to obtain sulfated mesoporous Ta oxides.
The BJH pore sizes slightly decreased after acid treatment, possibly
studied the impact of confinement and nesting effects in the
isomerization of 1-hexene by MCM-41, demonstrating that selectiv-
ity as well as conversion rate were pore size dependent. This is
consistent with the results observed in the present study. The pore
2-
due to the deposit of SO
4
ions inside their wormhole channels
(Table S1). The strong reflections (100) at low angle in the XRD
2 4
size of the C12 H SO mesoporous Ta oxide is 18.2 Å (Table S1),
pattern showed the retention of the mesoporous structure after acid
treatment (Figure S1). This was further confirmed by the type IV
isotherm obtained in the nitrogen adsorption/desorption measure-
ments (Figure S2), although materials synthesized with the smaller
which allows linear trans-2-hexene to pass more easily though its
channels than nonlinear cis-2-hexene. Because of the large pore
size (22.5 Å) of C18
can be formed inside its channel structure and diffuse out freely,
leading to poor selectivity. As for C SO mesoporous Ta oxide,
2 4
H SO mesoporous Ta oxide, both products
hexylamine displayed a type I isotherm. The pore size of the C
6
6
H
2
4
material was estimated as 12 Å on the basis of previous work on
microporous Nb oxide5b and Ti oxide. These studies used TEM
and XRD as a gauge of the pore size, which falls below that of the
BJH method using N as an adsorbent.
2
The FTIR spectrum for C12 mesoporous Ta oxide (Figure S3)
the limitation of its small pore size (12 Å) blocks the penetration
of the reactant, allowing most of the reaction to occur on the
ektexine of the wall, not inside the pores. This leads to low
selectivity. The increase of selectivity over time to the thermody-
namic mixture in the case of the C12 catalyst can be attributed to
the more efficient conversion of cis-2-hexene to trans-2-hexene
by this material relative to the other catalysts in this study. This
faster rate of conversion is consistent with Tanchoux’s observations
on the higher activity of moderate pore size MCM-41 materials in
this reaction. The isomerization of 1-hexene to 2-hexene is a proton
5c
showed a strong peak corresponding to Brønsted acid sites (∼1538
-
1
cm ) which was enhanced after acid treatment. In contrast, HY-
-1
zeolite and H-ZSM5 possess mainly Lewis acid sites (∼1448 cm )
with only a weak absorbance observed for Brønsted acid sites. The
majority of research indicates that Brønsted acid sites are chiefly
394
9
J. AM. CHEM. SOC. 2008, 130, 394-395
10.1021/ja076584n CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Acid Strength and Acid Amount of Solid Acid Catalysts
some of the active sites on the surface and cause the BET surface
area to decrease from 292 to 133.25 m /g (Figure S6). The loss of
(measured by Hammett Indicators and n-Butylamine Titration)
2
acid amount
catalytic activity can also be ascribed to sulfate leaching during
each catalytic run. This is demonstrated by the fact that the used
catalyst can be partially regenerated to 60% its original activity by
treatment with new sulfuric acid. Elemental analysis shows no
increase in %C, demonstrating that loss of surface area is not due
to build up of polymers.
In summary, this study provides a straightforward demonstration
of the influence of acid sites and confinement effect in 1-hexene
isomerization catalyzed over sulfated mesoporous Ta oxides. These
catalysts showed higher activities than Amberlyst 15, HY-zeolite,
and H-ZSM5. Among the three different pore sizes of sulfated
sample
Ho
(mmol/g)
C12 meso Ta
C12 H2SO4 meso Ta
HY-zeolite
H-ZSM5
Amberlyst 15
-6.6
-8.2
-6.6
-4.4
0.40
19.8
1.55
16.1
N/A
N/A
mesoporous Ta oxides studied, C12
2 4
H SO mesoporous Ta showed
both the highest activity and selectivity, which can be attributed to
its high BET surface area, increased concentration of Brønsted sites
on the surface of the mesoporous channels, and optimal pore size
for this particular reaction.
Acknowledgment. The authors acknowledge NSERC for the
financial support of this research.
Supporting Information Available: Detailed experimental pro-
cedure, powder X-ray diffraction patterns, N adsorption/desorption
2
isotherm, and FTIR spectra. This material is available free of charge
via the Internet at http://pubs.acs.org.
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