Title

STUDY OF PARTIAL OXIDATION OF METHANOL OVER FINELY DIVIDED MOLYBDENUM OXIDE (SELECTIVE OXIDATION)

Date of Completion

January 1984

Keywords

Engineering, Chemical

Degree

Ph.D.

Abstract

High surface area MoO(,3) prepared in a flame reactor has been studied in regards to the partial oxidation of methanol. Observations of surface species, the surface and bulk of solid phase and gas phase by infrared, mass spectrometry and gravimetric method yield information about reaction intermediates, site structure, electronic state, the mechanism of oxygen participation and reaction mechanism.^ The oxidation of methanol on molybdenum oxide can be represented by a mechanism involving oxygen vacancies of both terminal(double-bonded) oxygen and bridged(single-bonded) oxygen, and oxygens on the surface. Methanol chemisorbs dissociatively, and the dehydrogenation of methoxy is strongly dependent on the type of vacancy and its electronic state. Formaldehyde and CO are produced mainly from terminal oxygen vacancy Mo('5+) and Mo('4+) respectively, while dimethyl ether, methylal and methyl formate are made from bridged-bonded oxygen vacancy sites. The latter have to be arranged in a specific configuration that allows interaction between adsorbed species on terminal oxygen vacancy and that on bridged-bonded oxygen vacancy.^ At working condition the most abundant vacancies are terminal oxygen vacancy with one electron trapped (Mo('5+), VT('-)) and bridged-bonded oxygen vacancy with two electrons trapped (Mo-(' )2e(, )-Mo, VB('2-)), and this VT('-)/VB('2-) acts as a redox couple. The existence of space-charge layer which results from ionosorbed oxygen is a key factor to balance the reduction force by methanol by the oxidation force by oxygen so that the catalyst is at a stable state with a certain degree of reduction.^ The reaction rate is controlled by the oxygen diffusion on the surface while the selectivity is governed by the electronic state on the surface. The formations of the higher-order byproducts can be reduced by using high oxygen concentration in reaction mixture or at high temperature above 300(DEGREES)C where the bulk phase becomes mobile and this reduces the bridged-bonded oxygen vacancies by restructuring and the formation of shear domains along bridged-bonded oxygen vacancies.^