Thermal decomposition of methanol adsorbed on alumina

The kinetic behavior of methanol adsorbed on alumina powder was studied by thermal desorption and isotopic tracer techniques using an ultrahigh vacuum system. Several products were desorbed; CH3OH is predominant at low temperatures, H2CO, H2O, and CH3OCH3 become significant at 5000K and higher, CO and CH4 are observed above 7000K (CO predominates), H2 is desorbed at both low and high temperatures peaking with H2CO and CO, and finally CO2 is observed in small amounts at all temperatures. The isotopic composition of ethers desorbed after coadsorption of CH3OH and CD3OD was investigated. Below 5000K, only CH3OCH3, CD3OCH3, and CD3OCD3 appeared, while above this temperature the deuterium in ethers approached a random distribution. From this result it is concluded that below 5000K, adsorbed methanol (methoxide) is stable with respect to hydrogen exchange whereas at higher temperatures the H for D exchange is facile. In the presence of gas phase CD3OD, thermal desorption was examined from CH3OH-preadsorbed substrate. The ether produced was primarily CH3OCH3 suggesting that ether is formed through the interaction of two adsorbed methoxides. The reactivity of adsorbed methoxides is discussed and it is suggested that heterogeneity is induced by adsorption and/or desorption