Catalysis by heteropoly compounds .34. Skeletal isomerization of n-butane over Pt- or Pd-promoted cesium hydrogen salts of 12-tungstophosphoric acid

Skeletal isomerization of n-butane has been studied over bifunctional catalysts consisting of noble metals and an acidic cesium salt of 12-tungstophosphoric acid, Cs2.5H0.5PW12O40. At 473 K in the presence of 0.05 atm of H-2, 1 wt% Pt-Cs2.5H0.5PW12O40 (1% in weight of Pt impregnated to Cs2.5H0.5PW12O40) showed activity and selectivity comparable to I wt% Pt-sulfated zirconia, while at 573 K and 0.5 atm of H-2, 1 wt% Pt-and Pd-Cs2.5H0.5PW12O40 became much more active and selective for the formation of isobutane than 1 wt% Pt-sulfated zirconia and 1 wt% Pt-exchanged ZSM-5. Negative dependence of the initial reaction rate on H-2 pressure and little dependence on Pt loading level for Pt-Cs2.5H0.5PW12O40 indicate that the reaction proceeds through a bifunctional mechanism in which n-butane is rapidly dehydrogenated to n-butenes on Pt, n-butenes isomerize to isobutylene on acid sites, and then isobutylene is hydrogenated to isobutane on Pt. A remarkable effect of the presence of hydrogen was the repression of the catalyst deactivation. This is probably caused by the hydrogenation of coke and/or its precursor and a decrease in the butene partial pressure. Very interesting was the unique function of protons; protons present in proximity of Pt or Pd particles suppress the hydrogenolysis that takes place on Pt or Pd and greatly increase the selectivity to isobutane. (C) 1997 Academic Press.