O-methylation of dihydroxybenzenes with methanol in the vapour phase over alkali-loaded SiO2 catalysts:: A kinetic analysis

The vapour-phase O-methylation of the dihydroxybenzenes (DHBs; o-dihydroxybenzene (catechol), m-dihydroxybenzene (resorcinol), and p-dihydroxybenzene (hydroquinone)) with excess methanol has been investigated over alkali (Li, Na, K, and Cs) oxide loaded-SiO2 as catalysts. The reaction takes place in two consecutive steps, the formation of the monomethoxyphenol in the first step (rate constant, k(1)) and the dimethoxybenzene in the second step (rate constant, k(2)). The two steps have been assumed to follow a pseudo-first-order kinetics with respect to the substrates (DHB in the first step and the monomethoxy phenol in the second step) and the kinetic parameters, k(1), k(2), t(max), and R-max have been calculated. The trends in the values of the above kinetic parameters have been explained on the basis of the reactivity and electronic properties of the three DHBs, the surface basicity of the catalysts, and the mode of adsorption of the molecules on the support.