Side-chain alkylation of toluene with propene on caesium/nanoporous carbon
catalysts
Mark G. Stevens, Melony R. Anderson and Henry C. Foley*
Center for Catalytic Sciences and Technology, Department of Chemical Engineering, University of Delaware,
Colburn Laboratory, Academy Street, Newark, Delaware 19716, USA. E-mail: foley@che.udel.edu
Received (in Bloomington, IN, USA) 13th November 1998, Accepted 11th January 1999
Caesium/nanoporous carbon materials are powerful solid-
base catalysts, promoting the side-chain alkylation of
toluene with propene in a continuous flow reactor at
conditions as mild as 150 °C and 50 psig.
experiments. Once the propene was consumed, secondary
reactions began to become important. As was the case in the
reaction of benzene over Cs/NPC,16 aromatic-ring coupling
produced species such as bibenzyl, dimethylbiphenyls, and
methyldiphenylmethanes. Additionally, cyclization of iso-
butylbenzene produced 2-methylindan. If 1-methylindan and
1,2,3,4-tetrahydronaphthalene were produced, they remained
below our detection limits. Both side reactions should produce
When a strong base, such as Na metal, is employed as a catalyst,
alkylation occurs at the benzylic hydrogen of the side chain.
1
–5
During the later 1950s, many researchers
explored this
chemistry, most notably, Pines et al.1
,2,4
Recently, there has
H
2
and GC measurements of the vapor phase over the products
confirmed that H indeed had been produced. Control experi-
ments using carbon without C produced no detectable reaction.
been much interest in examining novel solid-base catalysts for
larger scale reactions such as side-chain alkylation of toluene
2
2
with methanol6 and olefins.
–9
10,11
To date, however, there are
no industrial processes that take advantage of this chemistry to
At higher conversions, the catalyst achieved over nine turnovers
based on the total moles of Cs, a lower limit value that confirms
the catalytic nature of the reaction.
12
produce alkylbenzenes from lower-cost toluene. As early as
1
964 Foster10 had shown that graphite intercalation compounds
In a second set of experiments, to avoid the possible
complication of Cs leaching into the liquid phase, toluene was
converted to butylbenzenes in the vapor phase using a tubular
flow reactor. A mixture of toluene in propene (5 mol %) was
circulated over the catalyst at temperatures from 150–400 °C
and 4 bar,‡ (Fig. 1, Table 2). The increase in the availability of
propene reduced the toluene coupling to an undetectable level.
Propene coupling, however, did occur with the excess propene
of alkali metals readily promoted the side-chain alkylation of
toluene with ethylene, but reaction was slow, required high
pressure and it was not clear if the alkali metal had remained
intercalated in the graphite.
In contrast to the graphite intercalation compounds of alkali
metals, which exfoliate readily,13 we have shown that Cs
entrapped in nanoporous carbon is well dispersed and very
strongly bound.14 Preparing nanoporous carbon (NPC) with
macropores provides for facile molecular ingress and egress to
the catalytic sites.15
in the system to form C
cyclohexane, hex-1-ene and dimethylbutenes. Above 150 °C,
ca. 10% of the propene that reacted went to the C products,
6
compounds such as 4-methylpentene,
6
We have shown that Cs/NPC is active enough to break the C–
indicating the potential utility of Cs/NPC as a catalyst for
producing higher-molecular-weight olefin monomers. This
propene coupling may also account for the decrease in activity
at > 250 °C. At 350 and 400 °C the catalysts gained mass (3.6
and 6.0% of their initial mass, respectively). This suggests that
deactivation arose due to the formation of propene oligomers in
the pores, resulting in a loss of diffusive transport to the
catalytically active sites. The sample of catalyst used at 400 °C
not only gained mass, but also was coated with a hexane-
soluble, waxy film.
2
1
H bond in benzene (110 kcal mol ) and to promote its
condensation to biphenyl.16 Given this result, we expected the
catalyst to remove the more facile benzylic hydrogen from
toluene readily, and if an olefin such as propene were present, to
produce n-butylbenzene and isobutylbenzene. At the same time
in the case of propene, cyclization and release of dihydrogen
could lead to the dicyclic products 1-methylindan, 1,2,3,4-tetra-
hydronaphthalene and 2-methylindan.
Batch, liquid phase reactions† of toluene and propene over
this nanoporous carbon catalyst containing ca. 10 wt% Cs
produced n-butylbenzene, isobutylbenzene and 2-methylindan.
Table 1 displays the results of several experiments carried out at
We find that Cs/NPC yields high iso/normal butylbenzene
ratios (!10). This indicates that the predominant mechanism
proceeds via surface anions and through the formation of the
1
7
1
50 °C and at various conversions. The major product was
benzyl anion from the toluene substrate. Pines and Stalick
isobutylbenzene. Propene was the limiting reactant in all these
Table 1 Results of batch studya
Conversion (%)
Toluene selectivity (%)
Turn-
over
Toluene Propylene Coupled Isobutyl n-Butyl Indan
0
0
1
1
3
6
7
a
.03
.12
.03
.06
.49
.82
.12
0.40
1.41
18.84
18.35
38.19
0.00
0.00
0.44
0.55
1.29
2.49
3.19
100.00
82.99
70.44
72.47
70.23
71.66
73.11
0.00
0.00
9.17
0.00
17.01
19.94
17.14
14.72
12.69
11.75
0.039
0.137
2.367
2.452
3.934
10.657
9.274
9.83
13.76
13.16
11.95
≈ 100.00
≈ 100.00
Conversion of toluene and propylene and selectivity were calculated
Fig. 1 Vapor-phase conversion and catalyst turnovers versus time on stream
at 4 bar gas pressure (5% toluene in propylene) and temperature. Conversion
of toluene, was calculated from GC of the liquid phase. Turnovers were
from GC of the liquid phase. Turnovers were calculated by titrating the used
catalyst for Cs content with H
GCMS.
2 4
SO . All products were identified by
2 4
calculated by titrating the used catalyst for Cs content with H SO .
Chem. Commun., 1999, 413–414
413