Dehydrocyclization of n-Hexane over Heteropolyoxometalates Catalysts

The catalytic dehydrocyclization of n-hexane was studied here for the first time using a number of compounds based on H3PMo12O40. The described catalysts were prepared by either replacing the acidic proton with counter-ions such as ammonium or transition metal cations ( , Fe3+, K+), or by replacing Mo6+ with (Ni3+, Co3+, Mn3+) in the poly-oxometalate framework, as reported earlier. For comparison purposes, the known (TBA)7PW11O39 catalyst system was used. All reactions were conducted at different temperatures in the range 200˚C- 450˚C. The Keggin structure of these heteropolycompounds was ascertained by XRD, UV and IR measurements. 31P NMR measurements and thermal be-haviour of the prepared catalysts were also studied. These modified polyoxometalates exhibited heterogeneous super-acidic catalytic activities in dehydrocyclization of n-hexane into benzene, cyclohexane, cyclohexene and cyclohexadi-ene. The catalysts obtained by substituting the acidic proton or coordination atom exhibited higher selectivity and sta-bility than the parent compound H3PMo12O40. Catalytic activity and selectivity were heavily dependent on the composi-tion of the catalyst and on the reaction conditions. At higher temperatures, the catalyst exhibited higher conversion effi-ciency at the expense of selectivity. Using higher temperatures (>400˚C) in the presence of hydrogen carrier gas, selec-tivity towards dehydrocyclization ceased and methane dominated. To explain the results, a plausible mechanism is pre-sented, based on super-acidic nature of the catalyst systems