Accurate low-pressure kinetics for isobutane oxidation over phosphomolybdic acid and copper(II) phosphomolybdates

A low-pressure steady-state technique has been used to investigate the rates and mechanisms of the oxidation of isobutane over H3[PMo12O40], CuH4[PMo12O40]2, Cu2H2[PMo12O40]2, Cu2.5H[PMo12O40]2, and Cu3[PMo12O40]2. Observed oxidation products over all catalysts are methacrolein, 3-methyl-2-oxetanone, acetic acid, carbon dioxide and water. The most selective catalyst for methacrolein formation at low temperatures (<496 °C) is Cu2.5H[PMo12O40]2, where both Cu(II) reduction and acid sites play a role. The least active catalyst at low temperatures is phosphomolybdic acid followed by Cu3[PMo12O40]2. This activity is reversed at higher temperatures. The 3-methyl-2-oxetanone is a unique product and is likely to be the precursor to methacrylic acid. Acetic acid is also probably a precursor to complete oxidation. Catalyst deactivation or restructuring is significant only over H3[PMo12O40].