The synthesis of ZSM-5 and Theta-1 in the presence of diethanolamine: Theoretical modeling of ZSM-5

Abstract

The synthesis of zeolites from gels comprising silica, alumina, potassium and/or sodium hydroxide or chloride, diethanolamine, and water has been studied previously, and several zeolites including Theta-1 and ZSM-5 were synthesized. Diethanolamine was considered to have both a pore-filling role and charge-compensatory role (if the amine group was protonated), and hence its overall effectiveness as a stabilizer depends upon the type of zeolite structure and the aluminum content. From these considerations, a model has been proposed which has been used to provide information on the stabilizing effect of diethanolamine within Theta-1 and ZSM-5. For a range of framework aluminum contents and distributions, the model was used to calculate the fraction of diethanolamine which was protonated and the ratio of metal cations to aluminum. A good agreement was observed between calculated and experimentally obtained data. The importance of the SiO 2 Al 2O 3 ratio on the formation of ZSM-5 and Theta-1 may be explained in terms of the relative stability afforded by the pore-filling effect of diethanolamine and its charge-compensating role. From these considerations, the model may be used to predict the gel SiO 2 Al 2O 3 ratio required to produce the two zeolites. The SiO 2 Al 2O 3 ratios calculated using the model are in good agreement with the values obtained from experiments.