KINETIC AND TRACER STUDIES ON METHANOL CONVERSION OVER SUPPORTED PLATINUM CATALYSTS

Hydrogen-deuterium exchange in methanol over Al₂,O₃, platinum black and alumina supported platinum catalysts have been investigated by means of mass spectrometry. The hydrogen-deuterium exchange proceeds faster over Pt/Al₂O₃ than either platinum black or alumina, and the rate (as well as the rate of dimethyl ether formation) increases with platinum dispersion. From the reactions CH₃OH + D₂, CD₃OH + H₂ and CD₃OH + D₂ the different exchange rates of the CH₃ and OH groups (R₁, and R₂, respectively) were determined. For all catalysts studied the exchange in the OH group was two or three orders of magnitude faster than CH₃ group. Exchange in the methyl group needs higher activation energy: R₁/R₂ ratio increases with temperature. Catalyst pretreatment has an important effect: higher activation temperature results in a higher R₁/R₂ ratio.

A methematical method is suggested for the evaluation of exchange rates of a molecule with two different types of hydrogen and a model is given for the interpretation of measured data: methanol is activated mainly on Al₂,O₃, forming surface methoxide groups on the Lewis acidic sites. This methoxide can react with deuterium atoms having been activated over platinum sites. With increasing dispersion the metal-support interface is enhanced which leads to higher rate of exchange.