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. When a strong base, such as Na metal, is employed as a catalyst, alkylation occurs at the benzylic hydrogen of the side chain. During the later 1950s, many researchers 1–5 explored this chemistry, most notably, Pines et al. 1,2,4 Recently, there has been much interest in examining novel solid-base catalysts for larger scale reactions such as side-chain alkylation of toluene with methanol 6–9 and olefins. 10,11 To date, however, there are no industrial processes that take advantage of this chemistry to produce alkylbenzenes from lower-cost toluene. 12 As early as 1964 Foster 10 had shown that graphite intercalation compounds 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 We have shown that Cs/NPC is active enough to break the C– H bond in benzene (110 kcal mol 21 ) 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 150 °C and at various conversions. The major product was isobutylbenzene. Propene was the limiting reactant in all these 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 H 2 and GC measurements of the vapor phase over the products confirmed that H 2 indeed had been produced. Control experi- ments using carbon without C 2 produced no detectable reaction. 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. 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 in the system to form C 6 compounds such as 4-methylpentene, cyclohexane, hex-1-ene and dimethylbutenes. Above 150 °C, ca. 10% of the propene that reacted went to the C 6 products, 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. 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 benzyl anion from the toluene substrate. Pines and Stalick 17 Table 1 Results of batch study a Conversion (%) Toluene selectivity (%) Turn- Toluene Propylene Coupled Isobutyl n-Butyl Indan over 0.03 0.40 0.00 100.00 0.00 0.00 0.039 0.12 1.41 0.00 82.99 0.00 17.01 0.137 1.03 18.84 0.44 70.44 9.17 19.94 2.367 1.06 18.35 0.55 72.47 9.83 17.14 2.452 3.49 38.19 1.29 70.23 13.76 14.72 3.934 6.82 ≈ 100.00 2.49 71.66 13.16 12.69 10.657 7.12 ≈ 100.00 3.19 73.11 11.95 11.75 9.274 a Conversion of toluene and propylene and selectivity were calculated from GC of the liquid phase. Turnovers were calculated by titrating the used catalyst for Cs content with H 2 SO 4 . All products were identified by GCMS. 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 calculated by titrating the used catalyst for Cs content with H 2 SO 4 . Chem. Commun., 1999, 413–414 413