The kinetics of the formation of methacrolein and carbon dioxide during the oxidation of isobutylene over a multicomponent catalyst$(M_a^{1+}M_b^{2+}M_c^{3+}Bi_xMo_yO_z$; $M_a^{1+}$ = alkali metal, $M_b^{2+}$=Co, Ni, Pb, $M_c^{3+}=Fe)$ has been studied by the steady-state reaction technique.
The catalyst has been investigated in its response to reduction by isobutylene and reoxidation by gaseous oxygen using the pulse microreactor method.
From the kinetic and energetic date, it was determined that the selective oxidation of isobutylene to methacrolein and complete oxidation to carbon dioxide occur via a redox mechanism.
But the rate of methacrolein formation cannot be satisfactorily described by the rate equation of redox model at low temperatures and high partial pressures of isobutylene.
The apparent activation energies and reaction orders vary with reaction conditions. At high temperatures (＞410℃) and with oxygen: isobutylene ratio higher than 6:1, the rate of methacrolein formation is limited by catalyst reduction, the apparent activation energy being approximately 20kcal/mole. When oxygen: isobutylene is lower than 4.3:1, the rate of methacrolein formation is limited by catalyst reoxidation and the apparent activation energy is 42.5kcal/mole.
Carbon dioxide is produced both from the oxidation of hydrocarbon deposits present on the surface of the catalyst under steady-state conditions and from the consecutive oxidation of methacrolein; the former pathway predominates at low temperatures (＜400℃) and high partial pressures of isobutylene, while the latter pathway predominates at high temperatures and low partial pressures of isobutylene.
The activation energies for methacrolein formation are greater than those of the carbon dioxide formation.
By the pulse method, it was found that the reduction rate of fully oxidized catalyst was first order in isobutylene, and the reoxidation rate of partially reduced catalyst was half order in oxygen.
The activation energy for reduction of the catalyst that has been partially reduced was 21kcal/mole in good agreement with 20kcal/mole obtained by the steady-state method.
The activity and selectivity of the catalyst are higher when it is partially reduced and than when it is fully oxidized.
