Reaction mechanism of methanol dehydrodenation on sodium carbonate catalyst
The aim of this study is to identify the reaction mechanism of methanol dehydrogenation on sodium carbonate catalyst. Quantitative analyses of the products of methanol dehydrogenation on sodium carbonate catalyst at 963 K indicate that methane is formed in parallel with formaldehyde, while carbon monoxide is mainly produced from further decomposition of formaldehyde. In a specially designed fixed-bed reactor, more than half of the methanol conversion takes place in the post-catalytic space, where the selectivity for formaldehyde is in the same range as for the reaction in the catalyst bed. It is therefore suggested that free radicals produced on the catalyst surface play an important role in methanol dehydrogenation. Temperature-programmed desorption of methanol on sodium carbonate and transient isotope experiments show that a hydrogen species is strongly adsorbed on the catalyst, but carbon-containing species are weakly adsorbed. Temperature-programmed reaction experiments indicate that noncatalytic thermal decomposition of formaldehyde is more significant than the surface reaction at high temperatures. Based on these facts, it is proposed that chemisorbed methanol is dissociated on the catalyst surface into adsorbed hydrogen and a gas-phase . CH2OH radical. Recombination and desorption of the former is rate-determining, and the latter initiates a series of homogeneous reactions that result in the final reaction products. The proposed mechanism is useful for further improving the catalyst.