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 strength of a SAPO-34 silicoaluminophosphate has been studied by FTIR spectrophotometry and by combined calorimetry-gas chromatography of ammonia desorption. On the basis of the in situ FTIR analysis of the surface species generated in the transformation of methanol into olefins and of the evidence of the existence of oxonium ions as intermediates, a mechanism has been proposed for the reaction, in which the oxonium ions are successively methylized by means of propagation reactions in the solid phase and ethyloxonium, propyloxonium, and so forth, are formed. This mechanism is mainly conditioned by shape selectivity.