The catalytic activity for the oxidation of isobutane catalyzed by
12-molybdophosphoric acid was much changed
by the V5+- and Cs+-substitution for
Mo6+ and H+, respectively. The best
yield of methacrylic acid was
9.0% and was obtained for the heteropoly catalyst with V and Cs
contents of 1 and 2.5, respectively. The
high catalytic activity obtained for
Cs2.5Ni0.08H1.34PVMo11O40
is presumably due to the high surface area.
The activities of 12-molybdovanadophosphoric acids as well as
those of 12-molybdophosphoric acid partially
salified with Cs+ were controlled by the oxidizing
ability of catalysts. The conversion vs selectivity
relationships, the simulation of the data, and kinetic results for
Cs2.5Ni0.08H1.34PVMo11O40
and
Cs2.5Ni0.08H0.34PMo12O40 catalysts show that the first steps,
i.e., selective and complete oxidation reactions of isobutane,
are
rate-determining and mainly catalyzed by molybdenum ion and that
vanadium ion efficiently accelerates the
selective oxidation of methacrolein to methacrylic acid.
Cs+-substitution for H+ in
H3.84Ni0.08PVMo11O40
greatly change the selectivity up to a Cs content of 2.5 and suppressed
the elimination of the vanadium ion
from the Keggin anion.