Methanol-to-Olefin Conversion on Silicoaluminophosphate Catalysts: Effect of Brønsted Acid Sites and Framework Structures

Abstract

Silicoaluminophosphates (SAPO) with different framework structures and amounts of Brønsted acid sites, that is, SAPO-34, SAPO-41, SAPO-11, and SAPO-46, were synthesized and studied as catalysts for the methanol-to-olefin (MTO) conversion. Besides the well-known SAPO-34 catalyst, also SAPO-41 exhibits a good MTO performance with a methanol conversion of 100% and a selectivity to light olefins of about 70% at the reaction temperature of 723 K, which maintains up to a time-on-stream of 10 h. The Brønsted acid sites of silicoaluminophosphate catalysts employed in this study were characterized by means of 1 H MAS NMR spectroscopy. These studies were performed with unloaded samples for studying the type and number of OH groups and after loading of ammonia for determining the amount of accessible Brønsted acid sites. The occluded organic species formed on the catalysts during the MTO reaction were analyzed by in situ UV/vis spectroscopy, thermogravimetry-differential thermal analysis (TG-DTA), and gas chromatography-mass spectrometry (GC-MS). On the basis of the characterization and catalytic results, we discuss the effects of Brønsted acid sites and framework structures on the catalytic performance of silicoaluminophosphates in the MTO conversion. The amount of Brønsted acid sites strongly affects the adsorption of methanol and the formation of hydrocarbon pool compounds as well as the catalyst deactivation. On the other hand, the framework structures of the silicoaluminophosphates under study influence the diffusion of products and thus control the product selectivity of the MTO reaction.