Isotopic labeling studies of the mechanism of dehydrogenation of methanol to methyl formate over copper-based catalysts

Deuterium labeling has been used to study the processes occurring during the conversion of methanol to methyl formate over copper catalysts at 180-210 °C and pressures of 0.3 to 1 atm. Deuterium substitution has a dramatic effect on rate, which decreases in the ratio 8:4:2:1 in the series CH₃OH, CH₃OD, CD₃OH, CD₃OD. This can be interpreted as the product of a primary kinetic isotope effect of four for replacement of CH₃ by CD₃ and a separate thermodynamic isotope effect of two on the concentration of a surface methoxy intermediate when OH is replaced by OD. The isotope effect provides strong support for a mechanism in which the slow step is the conversion of the methoxy group to formaldehyde. Reversibility of the conversion of methanol to methoxy is reflected by hydroxyl group exchange with D₂ at a rate much in excess of methyl formate production. H₂/HD/D₂ equilibration rates are still faster even though methanol coverages are high. The product distribution from CD₃OD CH₃OH mixtures shows that methyl formate formation involves transfer of an H or a D with a discrimination isotope effect of two. This rules out coupling by a methoxy plus CHO step but leaves unresolved the possibility that the formate is produced by a hemiacetal intermediate or by formaldehyde dimerization. This lack of resolution results from isotopic scrambling caused by concurrent transesterification reactions as demonstrated using CD₃OH CH₃OCHO mixtures.