Synthesis of core-shell SAPO-34@SAPO-18 composites by the epitaxial growth method and their catalytic properties for the MTO reaction

Core-shell SAPO-34@SAPO-18 composites possessing SAPO-34 single-crystal cores and SAPO-18 polycrystalline shells were synthesized by the epitaxial growth method. The core-shell SAPO-34@SAPO18 composites have been characterized by XRD, SEM, TEM, N-2 physisorption, solid state NMR, and NH3-TPD. Finally, their methanol-to-olefin (MTO) catalytic activity was evaluated at 425 degrees C, and coking behavior was investigated using a Thermax700 thermo-gravimetric analysis (TGA) instrument. The formation of SAPO-18 shell layers was affected by the crystal size, the surface roughness of core SAPO-34, and the silica content of SAPO-18 colloidal solutions. The core-shell SAPO-34@SAPO-18 composites exhibit better catalytic performance for the MTO reaction with over two times prolonged catalytic lifetime and nearly 9% improvement of selectivity to ethylene and propylene compared to the original core SAPO-34. The coke content of the core-shell catalyst distinctly increases to 22% from 19%, showing better utilization of the internal cage after decoration of the shell layer. The enhanced catalytic performance may originate from the passivation of strong acid sites on the external surface of core SAPO-34 by SAPO-18 shell layers.