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 by with counter-ions such as ammonium or transition metal cations (NH4
+, Fe3+,K+), or
by replacing Mo6+ with (Ni3+, Co3+, Mn3+) in the polyoxometalate 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 200-450°C.
The Keggin structure of these heteropolycompounds was ascertained by XRD, UV and IR
measurements. 31P NMR measurements and thermal behaviour 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 cyclohexadiene. The
catalysts obtained by substituting the acidic proton or coordination atom exhibited higher selectivity
and stability than the mother parent compound H3PMo12O40. 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 (>400oC) 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.
Keywords: dehydrocyclization; heteropolyacid; catalyst; n-hexane; Keggin ion .
Advances in Chemical Engineering and Science, 2013, 3, 82-92