SAPO-34 with a low acidity outer layer by epitaxial growth and its improved MTO performance

Although it is known that a low-silica SAPO-34 zeolite gives good MTO performance, the direct synthesis of a SAPO-34 zeolite with a low acid site density suffers from the problem of low yield and impure crystallinity. In this work, SAPO-34 zeolite samples with an appropriate low acid site density were synthesized as a low acid site outer layer on a high acid site core by epitaxial growth using a two-step hydrothermal synthesis. The samples were characterized by SEM, TEM, EDS-lining, XRD, ICP, XPS, TPD-NH3, TGA-Py and Ar adsorption. EDS-lining confirmed a silica distribution of a high-silica core and low-silica outer layer in the two-step synthesized SAPO-34 particles, and determined the thickness of low-silica outer layer to be 200 nm. The two-step synthesized samples have the CHA structure and gave good molecular sieve selectivity. The second-step growth used a low concentration of silica in the source to decrease the Si/Al ratio of the bulk solid phase, leading to the reduction of acid site density. Catalytic activity evaluation in a microreactor revealed that the two-step synthesized SAPO-34 zeolite gave both better MTO performance and hydrothermal stability compared to a zeolite produced by the one-step synthesis. The low acid site outer layer of the two-step synthesized samples had reduced coking and diffusion limitation, resulting in increased lifetime. This work provides a practical catalyst synthesis strategy of SAPO-34 catalysts for the MTO industry, by showing that a small high-silica core coated by a low-silica outer layer of appropriate thickness is a good structure for improved MTO performance.