ACIDITY, SURFACE SPECIES, AND MECHANISM OF METHANOL TRANSFORMATION INTO OLEFINS ON A SAPO-34

The nature of the active sites and the distribution of acidity strengt h of a SAPO-34 silicoaluminophosphate has been studied by FTIR spectro photometry and by combined calorimetry-gas chromatography of ammonia d esorption. On the basis of the in situ FTIR analysis of the surface sp ecies generated in the transformation of methanol into olefins and of the evidence of the existence of oxonium ions as intermediates, a mech anism has been proposed for the reaction, in which the oxonium ions ar e successively methylized by means of propagation reactions in the sol id phase and ethyloxonium, propyloxonium, and so forth, are formed. Th is mechanism is mainly conditioned by shape selectivity.