Sulfated mesoporous tantalum oxides in the shape selective synthesis of linear alkyl benzene

Article abstract of DOI:10.1002/anie.200800721

(Figure Presented) Tantal(izing) catalysis: Mesoporous tantalum oxides were treated with 1.0 M sulfuric acid and evaluated for their catalytic activity and selectivity to 2-phenyl isomers in the alkylation of benzene with bulky olefins (see scheme). The s

Full text of DOI:10.1002/anie.200800721

Communications
DOI: 10.1002/anie.200800721
Alkylation
Sulfated Mesoporous Tantalum Oxides in the Shape Selective Synthesis
of Linear Alkyl Benzene**
Junjie Kang, Yuxiang Rao, Michel Trudeau, and David Antonelli*
Linear alkylbenzenes (LAB), the primary intermediates in
detergent industry, are commercially manufactured by the
alkylation of benzene with C10–14 n-alkenes.[1–3] Among LAB
isomers, 2-phenyl isomers are the most favorable starting
materials for the production of ecofriendly domestic and
industrial detergents because of their high solubility and
biodegradability.[4] The development of catalysts with a high
selectivity to 2-phenyl isomers in benzene alkylations is an
area of great interest. The alkylation of benzene with olefins
proceeds through a carbonium ion mechanism.[5] The relative
stabilities of the formed carbonium ions increase with the
carbon number towards the center of olefin chains. When
homogeneous catalysts such as HF or AlCl3 are used, a
thermodynamic mixture of LAB isomers is always obtained.
In addition to their low selectivity to the desired 2-phenyl
isomers, the use of highly corrosive and toxic HF or AlCl3
poses disposal problems. Considerable efforts have been
made to carry out alkylation reactions over environmentally
friendly solid acid catalysts.[6–11] Strength, distribution and
number of acid sites, surface area, pore size, geometry and
pore size distribution, and hydrogenation/dehydrogenation
ability of solid acid catalysts are the key factors that
determine their activity and selectivity in alkylation reactions.
Studies on the alkylation of benzene with 1-dodecene over
FAU, BEA and EMT zeolites showed that the selectivity
towards the least bulky 2-phenyldodecane increased with an
increase of porous constraints, while activity decreased due to
diffusion limitation in the channels.[12] The very low activity of
H-ZSM5 indicated that its channels did not provide enough
space for the formation of the potential bulky LAB isomers.
The larger pore openings of H-USY improved the diffusion of
reactants and products, leading to 100% conversion, but
lower 2-phenyldodecane selectivity (25.5%).[13]
diffusion rates. Furthermore, the selectivity to desired prod-
ucts can be tuned by optimizing the pore size of mesoporous
catalysts. The monoalkylation selectivity for benzene alkyla-
tions was significantly enhanced to 89.9% when AlCl3 was
grafted onto mesoporous molecular sieves.[14] Shape selective
synthesis of LAB has been carried out over AlMCM-41/Beta
zeolite composites, which combined the advantages of both
microporous and mesoporous materials.[15] Though a high 2-
phenyldodecane selectivity (76%) was achieved, the upper
conversion limit was only 48% even at 1208C after 2 h.
Nb and Ta oxides exhibit special properties such as high
stability, variable oxidation states useful in tailoring catalytic
properties, as well as variable acidic properties crucial to acid-
catalyzed reactions. For example, niobic acid is an active
catalyst for the alkylation of benzene with methanol and the
catalytic activity is markedly enhanced when the catalyst was
treated with a dilute phosphoric acid solution.[16,17] Because of
their high surface area (400–900 m2 gÀ1) and controlled pore
sizes (20–100 Š),[18–20] mesoporous Nb and Ta oxide could be
potential catalysts for alkylation reactions and rival those of
non-porous Nb and Ta oxides. In recent studies from our
group, sulfated mesoporous Nb oxide showed extremely high
catalytic activity, almost 200 times greater than sulfated bulk
oxide in the benzylation of anisole with benzyl alcohol.[21]
Mesoporous Ta oxide is more thermally stable[22] and thus
shows even greater promise as a catalyst than its Nb
counterpart. Herein, we report the catalytic properties of
sulfated mesoporous Ta oxides for the alkylation of benzene
with bulky olefins.
Mesoporous C12-Ta oxide was prepared using the ligand-
assisted templating approach with 1-dodecylamine surfactant.
The sulfated samples were produced by treating the template-
free Ta oxides with sulfuric acid. The (100) reflection in the
XRD pattern of sulfated mesoporous C12-Ta oxide demon-
strates the retention of the mesoporous structure after sulfuric
acid treatment. The type IV N2 adsorption/desorption iso-
therms further confirmed the confined mesoporous structure
in the sulfated sample. The BET surface area was found to be
reduced from 582.7 m2 gÀ1 to 292.2 m2 gÀ1 by sulfuric acid
treatment.
The Hammett acidity and acid amounts of Ta catalysts are
summarized in Table 1. Sulfuric acid treatment leads to the
increase in both acid strength and acid amount of mesoporous
C12-Ta oxide, which are essential to alkylation reactions. The
FT-IR spectrum in Figure 1 shows that the sulfated meso-
porous C12-Ta oxide possesses mainly Brønsted-acid sites
(1538 cmÀ1) with a smaller number of Lewis-acid sites
(1448 cmÀ1), while Lewis-acid sites are dominant on H-Y
and H-ZSM5 zeolites.
Mesoporous materials exhibiting surface areas from 300–
2000 m2 gÀ1 and controlled pore sizes offer high reaction rates
and overall efficiency because of a greater number of
available surface sites per gram of catalyst and better
[*] J. Kang, Y. Rao, Prof. D. Antonelli
Department of Chemistry and Biochemistry, University of Windsor
401 Sunset Avenue, Windsor, Ontario N9B 3P4 (Canada)
Fax: (+1) 519-973-7098
E-mail: danton@uwindsor.ca
M. Trudeau
Emerging Technologies, Hydro-QuØbec Research Institute
1800 Boul. Lionel-Boulet, Varennes, Quebec J3X 1S1 (Canada)
[**] The authors wish to acknowledge NSERC for the financial support of
this research.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Table 1: Hammett acidity and acid amount of solid acid catalysts.
1.2% over sulfated mesoporous C12-Nb at 0.5 h, despite of
larger number of relatively strong acid sites. The reason for
this is not understood, and cannot be rationalized by the acid
Catalyst
H0
Acid amount [mmolgÀ1
]
Meso C12-Ta
À6.6
À8.2
À6.6
À8.2
À6.6
À4.4
0.40
19.8
2.48
31.78
1.55
16.1
strength data in Table 1. Recent work in our group on the 17
O
Meso SO42À/C12-Ta
Meso C12-Nb
Meso SO42À/C12-Nb
H-Y zeolite
NMR of mesoporous Nb oxide showed a much more highly
ordered crystal structure in the walls of this material than its
Ta counterpart, which may account for the observed differ-
ence in catalytic behavior.[23] The alkylation reaction is
dependent on the chemisorption of reactants, surface reaction
and desorption of products on the internal pore surface of
solid acid catalysts, which are determined by their pore
structures and sizes. GC results showed that all possible
phenyldodecane isomers were formed. In contrast to homo-
geneous catalysts, the non-attainment of thermodynamic
equilibrium over solid acid catalysts leads to a more favorable
isomer distribution. As shown in Figure 3, the selectivity
H-ZSM5
Figure 1. FT-IR spectra of pyridine adsorbed on a) sulfated mesopo-
rous C12-Ta oxide, b) H-Y and c) H-ZSM5 zeolites.
The catalytic properties of sulfated mesoporous C12-Ta
oxide were evaluated in terms of olefin conversion and the
selectivity towards 2-phenyl isomers. At 808C and a benzene/
olefin molar ratio of 10:1, no oligomerization of 1-dodecene
was observed and only monoalkylated phenyldodecanes were
detected. As shown in Figure 2, 100% 1-dodecene conversion
was achieved within 0.5 h. However, the conversion was only
Figure 3. Distribution of phenyldodecane isomers over sulfated meso-
porous C12-Ta oxide as a function of reaction time. a) 2-phenyldode-
cane, b) 3-phenyldodecane, c) 5-phenyldodecane, d) 4-phenyldodecane,
and e) 6-phenyldodecane. Reaction conditions: 808C, catalyst load-
ing=4.0 wt.%.
towards phenyldodecane isomers remains unchanged even
after 6.0 h when sulfated mesoporous Ta oxide was employed
as catalyst. This suggests that it is difficult for the reaction
system to reach thermodynamic equilibrium due to the
limited diffusion of bulky phenyldodecane isomers in the
mesoporous channels.
Figure 4 shows the effect of catalyst loading on 1-
dodecene conversion and 2-phenyldodecane selectivity over
sulfated mesoporous C12-Ta oxide. When catalyst loading was
increased from 0.5 to 4.0 wt.%, 1-dodecene conversion
increased from 28.8% to 100%, whereas the selectivity to
2-phenyldodecane decreased from 52.9 to 41.6% at the
reaction time of 0.5 h. With increased catalyst loading, the
greater number of acid sites leads to higher activity, while the
kinetic availability of external surface acid sites is increased
relative to internal sites resulting in a decrease in the 2-
phenyldodecane selectivity. Further increase in catalyst load-
Figure 2. Olefin conversion in the alkylation of benzene with a) 1-
dodecene and b) 1-tetradecene over sulfated mesoporous C12-Ta oxide.
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trend in pore size versus activity was observed in the
isomerization of 1-hexene over sulfated mesoporous Ta
oxide.[25]
As one of the best solid acid catalysts for benzene
alkylations,[5] H-Y zeolite (Zeolyst, Si/Al = 80) was tested for
comparison. This material gave 100% 1-dodecene conversion
at 808C within 0.5 h. Though the acid amount on H-ZSM5
zeolite is comparable to that on sulfated mesoporous C12-Ta
oxide, no alkylation reaction was detected over H-ZSM5 at
808C. This can be rationalized by the diffusion limitation in
the channels with smaller openings.[12] An alkylation reaction
was conducted over Amberlyst 15 ion exchange resin
(Aldrich). Despite the presence of strong acid sites, Amber-
lyst resin showed lower catalytic activity for the alkylation
reaction, due to its lower surface area (55.1 m2 gÀ1) and thus
smaller amount of active sites on the internal surface of pores.
The selectivities towards the 2-phenyldodecane isomer
were compared at the same conversion and summarized in
Table 3. The selectivity over sulfated mesoporous C12-Ta
Figure 4. 1-Dodecene conversion and 2-phenyldodecane selectivity as
a function of catalyst loading. Reaction conditions: 808C, 0.5 h.
ing to 6.0 wt.% had no obvious effect on 2-phenyldodecane
selectivity.
Table 3: 2-Phenyldodecane selectivity over solid acid catalysts.
Catalyst
Conversion [%]
Selectivity [%]
The effect of the bulkiness of the olefin on the conversion
over mesoporous C12-Ta oxide was studied (Figure 2). When
bulkier 1-tetradecene was used, olefin conversions at 0.5 h
decreased greatly to 17.5%. The longer the olefin chain, the
more difficult for the olefin molecules to access to the active
sites in mesopores. Therefore, activity decreased with an
increase in olefin chain length. In order to further clarify the
effect of pore structure on activity, the alkylation reaction was
carried out over sulfated mesoporous C6-Ta and C18-Ta oxide
prepared by using 1-hexylamine and 1-octadecylamine as
templates. The catalytic activity data is given in Table 2. The
SO42À/C12-Ta
H-Y zeolite
Amberlyst
60.0
61.1
60.4
49.19
25.31
38.31
oxide, H-Y zeolite, and Amberlyst resin at 1-dodecene
conversion of ca. 60% were 49.19%, 25.31%, and 38.31%,
respectively. The high selectivity of sulfated mesoporous C12-
Ta oxide can be attributed to “confinement effects”, which
indicates that selectivity as well as conversion rate are pore
size dependent.[26] Jaenicke et al. reported the shape-selective
catalysts prepared by immobilizing AlCl3 onto a series of
MCM-41 mesoporous silica with different pore sizes.[27] They
found that the selectivity towards monoalkylation product in
the synthesis of linear alkyl benzenes can be controlled by
changing the pore size of the MCM-41 supports. The
mesoporous structure in our sulfated Ta oxide may offer a
similar confined space for the establishment of shape selective
reactions.
In order to examine the reusability of sulfated mesopo-
rous C12-Ta oxide, the catalyst was separated by filtration after
the first run and then dried at 1208C for 12 h. The reaction
was repeated under the same conditions using the recovered
catalyst. The XRD pattern and N2 adsorption/desorption
isotherm after the first run confirmed the retention of
mesoporous structure. However, the catalyst lost 17.4% of
its initial surface area after the first run. The catalyst used in
the second run gave a 1-dodecene conversion of 26.8% at
0.5 h. The deactivation of the catalyst could be due to the
blockage of the mesoporous pore by the accumulation of
bulky phenyldodecane isomers during previous runs, or a
small loss of sulfate by leaching as observed for the sulfated
mesoporous Nb oxide in the benzylation of anisole and
toluene.[21] Elemental analysis showed that the sulfur content
in the sulfated mesoporous C12-Ta oxide was 4.18%, while the
sulfur content was slightly decreased to 4.06% after alkyla-
Table 2: Catalytic properties of solid acid catalysts in alkylation reac-
tions.
Catalyst
Olefin
Conversion [%][a]
SO42À/C6-Ta
SO42À/C12-Ta
SO42À/C18-Ta
SO42À/C12-Nb
H-Y zeolite
H-ZSM5
1-dodecene
1-dodecene
1-dodecene
1-dodecene
1-dodecene
1-dodecene
1-dodecene
46.9
100
2.3
1.2
100
0
Amberlyst
13.5
[a] 0.5 h at 808C, catalyst loading=4.0 wt.%.
lower activity of C6-Ta oxide is attributed to the strong
diffusion resistance offered by the pore blocking in smaller
pores.[24] Despite its larger pore size, sulfated C18-Ta oxide
showed very low activity. The low 1-dodecene conversion
offered by sulfated C18-Ta oxide may be caused by its
relatively low surface area and extremely low pore volume,
but may also be related to relative optimization of the
reaction transition state by the two pore sizes. The large C18
pores are less effective at bringing the benzene and alkyl
cation together than the intermediate C12 pores, while the
small C6 pores cause diffusion hindrances. Although more
study may be necessary to fully elucidate this effect, a similar
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Chemie
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with bulky olefins. The catalysts with optimal pore size
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Experimental Section
Mesoporous Ta and Nb oxides were prepared as described previ-
ously.[18] The sulfated oxides were produced by stirring the template-
free oxides with 1.0m sulfuric acid for 12 h.
Powder X-ray diffraction (XRD) patterns were recorded on a
Siemens D5000-2 diffractometer using CuKa radiation. Nitrogen
adsorption/desorption data were collected on Micromeritics ASAP
2010. FT-IR experiments were performed on Bruker Vector 22 FT-IR
spectrometer. The Hammett acidity and acid amounts were measured
according to literatures.[28,29] Elemental analysis was conducted in
Galbraith Laboratories, Inc.
Before reaction, H-Y and H-ZSM5 zeolites were activated at
5008C for 3 h in air, whereas sulfated mesoporous Ta and Nb oxides
were dried at 1208C for 12 h. The liquid-phase alkylation reactions
were carried out in a 100 mL 3-neck round-bottom flask with a reflux
condenser, a nitrogen inlet and a septum. The reactant mixture was
refluxed at atmospheric pressure in an oil bath. Samples were
periodically withdrawn and analyzed using a Varian CP-3800 gas
chromatograph.
Received: February 13, 2008
Published online: May 21, 2008
Keywords: alkylation · heterogeneous catalysis ·
.
mesoporous materials · tantalum
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