Dehydrocyclization of n-Hexane over Heteropolyoxometalates Catalysts

Abstract

The catalytic dehydrocyclization of n-hexane was studied here for the first time using a number of compounds based on H 3 PMo 12 O 40 . The described catalysts were prepared by either replacing the acidic proton with counter-ions such as ammonium or transition metal cations (NH 4 + , Fe 3+ , K + ), or by replacing Mo 6+ with (Ni 3+ , Co 3+ , Mn 3+ ) in the polyoxometalate framework, as reported earlier. For comparison purposes, the known (TBA) 7 PW 11 O 39 catalyst system was used. All reactions were conducted at different temperatures in the range 200 - 450 . The Keggin structure of these heteropolycompounds was ascertained by XRD, UV and IR measurements. 31 P NMR measurements and thermal behaviour of the prepared catalysts were also studied. These modified polyoxometalates exhibited heterogeneous superacidic catalytic activities in dehydrocyclization of n-hexane into benzene, cyclohexane, cyclohexene and cyclohexadiene. The catalysts obtained by substituting the acidic proton or coordination atom exhibited higher selectivity and stability than the parent compound H 3 PMo 12 O 40 . Catalytic activity and selectivity were heavily dependent on the composition of the catalyst and on the reaction conditions. At higher temperatures, the catalyst exhibited higher conversion efficiency at the expense of selectivity. Using higher temperatures (>400 ) in the presence of hydrogen carrier gas, selectivity towards dehydrocyclization ceased and methane dominated. To explain the results, a plausible mechanism is presented, based on super-acidic nature of the catalyst systems .