Isomerization of 1-butene on supported and unsupported metal-oxygen cluster compounds (heteropoly oxometalates)

On 12-tungstophosphoric acid (HPW) supported on silica, 1-butene is isomerized to cis- and trans-2-butene at 100 degrees C and higher while a temperature of 150 degrees C is required to form isobutene. While the conversion remains at 80-90% regardless of the loading of HPW on the support and the reaction temperature, the selectivity to isobutene reaches a maximum of 33% at a loading of 23% HPW/SiO2 and 300-350 degrees C. Small quantities of C-n (n = 3, ..., 8) species are also formed. These results together with those from ammonia poisoning, temperature-programmed desorption of ammonia, H-1 MAS NMR and infrared spectroscopy show that HPW/SiO2 possesses sites of strength as high as or higher than those on HZSM-5, that such sites are required for the skeletal isomerization process and that both mono- and bimolecular processes are occurring on the catalyst. The absence of dependence of conversion of 1-butene and selectivities to cis- and trans-2-butene on loading and reaction temperature can best be rationalized on the basis of a rapid attainment of an equilibrium between the aforementioned three species and sec-butyl carbenium ions from which pool the energy barrier to the primary butyl carbenium and hence to isobutene is surmounted, given appropriate conditions.