Electronic Activity Relationship for Methacrolein Formation Over 4th Period Transition Metal Phosphomolybdates

Abstract

Phosphomolybdate compounds have been investigated for their structural characteristics and oxidation activity toward isobutane. The phosphomolybdates were synthesized from phosphomolybdic acid and the fourth period transition metal cations Cr³⁺, Mn²⁺, Fe³⁺, Fe²⁺, Co²⁺, Ni²⁺ Cu²⁺, Cu⁺ and Zn²⁺. Two compounds were synthesized per transition metal: where (i) all the protons had been replaced by the particular transition metal, and (ii) only partial proton replacement leaving a single proton per phosphomolybdate. X-ray diffraction analysis has shown that a primitive cubic phase is apparent with some of the transition metal phosphomolybdates. Each solid was exposed to isobutane using the anaerobic low-pressure steady-state technique. Category 1 exponential-like distributions of methacrolein were observed with all the transition metal phosphomolybdates, except the lower oxidation state iron and copper salts, Fe_1.5[PMo₁₂O₄₀] and Cu₃[PMo₁₂O₄₀] respectively. Activation energies ranged from 51.31 ± 0.27 kJ mol⁻¹ (Cr[PMo₁₂O₄₀]) to over 200 kJ mol⁻¹ (Zn_1.5[PMo₁₂O₄₀]). Phosphomolybdates with counter cations which are one or two electrons deficient from either a 3d ⁵ or 3d ¹⁰ configuration (in this case 3d ³, 3d ⁸ or 3d ⁹) had the lowest activation barriers for methacrolein formation. A computational investigation into HOMO–LUMO band gap energies agrees with the association. The presence of protons also enhances Category 1 product formation and may be attributed to migration of H⁺ through the bulk of the solid.

Graphical Abstract