Propane metathesis by a tandem catalytic system: Dehydrogenation and hydrogenation over PtSn/Al2O3, and metathesis over Re 2O7/Al2O3

Article abstract of DOI:10.1246/cl.2012.254

Metathesis of propane into n-butane and ethane using a tandem catalytic system consisting of two catalysts (PtSn/Al₂O₃ for dehydrogenation of alkane and hydrogenation of alkene, and Re₂O ₇/Al₂O₃ for alkene metathesis) has been investigated. When the temperatures of two catalyst-beds are controlled independently, this system shows high conversion and is selective for the formation of n-butane.

Full text of DOI:10.1246/cl.2012.254

doi:10.1246/cl.2012.254
254
Published on the web February 25, 2012
Propane Metathesis by a Tandem Catalytic System:
Dehydrogenation and Hydrogenation over PtSn/Al2O3,
and Metathesis over Re2O7/Al2O3
Shinya Furukawa, Tetsuya Shishido,* and Tsunehiro Tanaka*
Department of Molecular Engineering, Graduate School of Engineering, Kyoto University,
Nishikyo-ku, Kyoto 615-8510
(Received December 15, 2011; CL-111195; E-mail: shishido@moleng.kyoto-u.ac.jp)
Metathesis of propane into n-butane and ethane using a
essentially inadaptable for light gaseous alkanes. On the other
hand, the heterogeneous alkane metathesis by single-site catalyst
such as alumina-supported tungsten hydride (W(H)3/Al2O3) has
also been reported by Basset and his co-workers.4-8 They
reported that this single-site catalyst operates via the metal
centers with alkanes to give metal-carbene complex (¡-H
elimination) as well as free olefins (¢-H elimination and ¢-alkyl
transfer). This catalyst works under moderate conditions,
however, product yields are low and delicate treatments are
required for the catalyst preparation.
In this study, we investigated a tandem alkane metathesis by
using two kinds of heterogeneous catalysts that can be prepared
easily. PtSn/Al2O3 and Re2O7/Al2O3 have been employed as a
dehydrogenation/hydrogenation catalyst and a metathesis one in
this system. PtSn/Al2O3 is well known as a typical dehydrogen-
ation catalyst and to suppress coking and cracking.9-11 Re2O7/
Al2O3 catalyst shows high catalytic activity for olefin metathesis
even at ambient temperature.12-14 These catalysts were easily
prepared by a conventional impregnation method from £-Al2O3
(JRC-ALO-4), [Pt(NH3)4](NO3)2, SnCl2¢2H2O, and NH4ReO4.15
The loading amount of Pt was 0.6 or 1.0 wt %, with 0.6 of Pt/Sn
molar ratio. The loading amount of Re2O7 was 18 wt % which
showed the highest activity in olefin metathesis.12,14 The alkane
metathesis was carried out in a closed circulation system with
twin reactors as shown in Figure 1a. The twin reactors were
placed in two furnaces, therefore, the temperature of catalyst-
beds can be controlled independently. PtSn/Al2O3 catalyst was
mounted in the first reactor and Re2O7/Al2O3 in the second.
Before the reaction, PtSn/Al2O3 was reduced at 773 K for 2 h
in 6.7 kPa of H2 and Re2O7/Al2O3 was oxidized at 673 K for 1 h
tandem catalytic system consisting of two catalysts (PtSn/Al2O3
for dehydrogenation of alkane and hydrogenation of alkene, and
Re2O7/Al2O3 for alkene metathesis) has been investigated.
When the temperatures of two catalyst-beds are controlled
independently, this system shows high conversion and is
selective for the formation of n-butane.
Alkane metathesis (eq 1) for the transformation of lower
alkanes into higher homologues has attracted attention, and is
still a challenge in chemistry.
2CnH2nþ2 ꢀ C2ðnꢀ1ÞH4ðnꢀ1Þþ2 þ C2H6
ð1Þ
Petroleum contains alkanes as major components. In the
move toward various carbon sources including coal, natural gas,
and biomass to replace petroleum products due to oil production
decline, the Fischer-Tropsch (F-T) process (reductive oligome-
rization of CO and H2) to produce synthetic fuel appears to
become increasingly important. Production of diesel fuel by F-T
process is attractive since major products consist of higher
alkanes with a quite low sulfur content (<1 ppm). However,
F-T process yields an alkane mixture with no molecular weight
control. Moreover, a variety of refinery and petrochemical
streams and biomass conversions give a large amount of light
alkanes. Unfortunately, there is no practical process to convert
a mixture of light alkanes into higher homologues. Thus, the
development of an effective catalytic system for alkane meta-
thesis is promising for converting light alkanes, and provides a
great impact to industrial chemistry.
There have been many discussions on the reaction mech-
anism of alkane metathesis. It is commonly believed that alkane
metathesis proceeds in three steps; alkane dehydrogenated to
alkene and then the resulting alkene is transformed to higher
alkene by metathesis. Finally, the formed higher alkene is
hydrogenated to corresponding alkane.1-8 Based on this mech-
anism, several efforts have been devoted to combine the
functions of catalysts (dehydrogenation/hydrogenation and
metathesis) in alkane metathesis. Burnett and Hughes reported
the first catalytic alkane metathesis, the so-called Chevron
process in the 1970s which was carried out in a continuous flow
system.1 But this system requires extremely high pressure of
alkanes to obtain enough concentration of intermediate olefins.
Recently, Goldman et al. reported a homogeneous catalytic
system of tandem alkane metathesis using Ir-based pincer
complexes for dehydrogenation and a Schrock-type Mo catalyst
for olefin metathesis.2,3 Although their system exhibits good
performance for liquid alkanes, the homogeneous system is
Figure 1. The outline of the catalytic system of (a) tandem
type alkane metathesis and single bed type; the two catalysts
were (b) physically mixed or (c) layered.
Chem. Lett. 2012, 41, 254-256
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255
Figure 3. Product distribution after 7 h of propane metathesis
catalyzed by 0.6 wt % PtSn/Al2O3 (Pt/Sn = 0.6) and 18 wt %
Re2O7/Al2O3; tandem type (a, PtSn/Al2O3; 623 K, Re2O7/
Al2O3; 373 K), physically mixed (b, 623 K) and layered
(c, 623 K). C2, C3, and C4 olefins are abbreviated to C2// ,
C3// , and C4// , respectively.
Figure 2. Time course of product yields of propane metathesis
catalyzed by tandem system of 0.6 wt % PtSn/Al2O3 (Pt/Sn =
0.6) and 18 wt % Re2O7/Al2O3. PtSn/Al2O3; 623 K, Re2O7/
Al2O3; 373 K.
in 6.7 kPa of O2, respectively. The platinum dispersion of the
pretreated PtSn/Al2O3 catalyst was determined to be 19% by CO
pulse chemisorption.15 The reactant, 13.3 kPa (1070 ¯mol) of
propane was introduced (C3H8/Pt = 210, C3H8/Re = 14.4). The
reaction temperature was kept at 623 K for PtSn/Al2O3 and
373 K for Re2O7/Al2O3, respectively. The products were
analyzed by online TCD gas chromatograph. The time course
of product yields of propane metathesis is shown in Figure 2. At
the initial stage, C2, C3, and C4 olefins were formed in the ratio
of 1:2:1, and then the yields plateaued. Hydrogen was also
evolved in nearly equimolar amount to the olefins. After that, the
yields of ethane (C2) and n-butane (C4) linearly increased as the
reaction time increased, suggesting that propylene metathesis
and subsequent hydrogenation proceeded. The ratio of cis/trans-
2-butene was about 1/3 at any time. These changes in the
product distribution suggest that a set of following reactions
proceed sequentially (eqs 2-5). The reason why the formation of
the olefins plateaus may be promotion of subsequent hydro-
genation due to accumulation of hydrogen.
Figure 4. Time course of product distribution of propane
metathesis catalyzed by 1.0 wt % PtSn/Al2O3 (Pt/Sn = 0.6)
and 18 wt % Re2O7/Al2O3. PtSn/Al2O3; 623 K, Re2O7/Al2O3;
373 K. C2, C3, and C4 olefins are abbreviated to C2// , C3// , and
C4// , respectively.
ethane was obtained, suggesting that undesired cracking of
propane occurred. On the other hand, the tandem system gave
n-butane and ethane as main products. It appears that controlling
the temperature of each catalyst bed independently is necessary
in order to undergo propylene metathesis. The yields of ethane
and n-butane increased as the reaction time increased as shown
in Figure 4. The selectivity to n-butane in C4 products (C4/
(C4 + C4// )), total yield of C4 and C5 alkanes and propane
conversion reached 92%, 7.5%, and 21%, respectively after
120 h. Moreover, turnover number (mol of converted C3H8 per
mol of surface Pt atom estimated by CO pulse chemisorption)15
was up to 233 after 120 h, which is much higher than that of
W(H)3/SiO2-Al2O3 catalyst reported by Basset et al. (120 after
120 h).7 Although the yield of 1-butene was negligible, a small
amount of pentane (0.3%) was observed. In addition, the tandem
system gave no branched alkane such as isobutane and
isopentane.
C3H8 ꢀ C3H6 þ H2
2C3H6 ꢀ C2H4 þ 2-C4H8
C2H4 þ H2 ꢀ C2H6
2-C4H8 þ H2 ꢀ n-C4H10
ð2Þ
ð3Þ
ð4Þ
ð5Þ
The yield of ethane was larger than that of n-butane. In
contrast, the yield of ethylene became smaller than that of 2-
butene after 3 h of reaction where alkanes were dominant. These
may be because hydrogenation of ethylene is kinetically more
favorable than that of 2-butene due to steric hindrance. Figure 3
shows the product distributions of propane metathesis over
tandem- and single-bed catalytic systems after 7 h of reaction.
Two kinds of single-bed systems in which the two catalysts were
physically mixed or layered (Figures 1b and 1c) were compared
with the tandem catalytic system. The reaction temperature
of the single bed was kept at 623 K in each case. Although
dehydrogenation of propane to propylene proceeded, no product
of propylene metathesis (2-butene and ethylene) was obtained by
using two kinds of single-bed systems. A trace of methane and
In conclusion, the results show the great effectiveness of the
tandem catalytic system for alkane metathesis. Although the
optimization of some conditions such as reaction scale, reaction
temperature, catalyst amount, and Pt/Sn ratio are yet to be
investigated, the tandem system has very flexible reaction
Chem. Lett. 2012, 41, 254-256
© 2012 The Chemical Society of Japan
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256
conditions and many possible combinations of dehydrogen-
ation/hydrogenation and metathesis catalysts, depending on the
nature of alkanes. It was found that controlling temperature of
the two catalyst-beds independently is important for alkane
metathesis. These findings have opened up an opportunity for
tandem alkane metathesis in a continuous flow system.
Angew. Chem., Int. Ed. 2005, 44, 6755.
6
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This work was supported by JSPS Research Fellowships for
Young Scientists and the Research Seeds Quest Program (Lower
Carbon Society) (2010) under Japan Science and Technology
Agency.
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Chem. Lett. 2012, 41, 254-256
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/