Catalytic Longevity of Hierarchical SAPO-34/AlMCM-41 Nanocomposite Molecular Sieve In Methanol-to-Olefins Process.

Abstract

SAPO-34/AlMCM-41, as a hierarchical nanocomposite molecular sieve was prepared by sequential hydrothermal and dry-gel methods studied for catalytic conversion of methanol to light olefins. Pure AlMCM-41, SAPO-34, and their physical mixture were also produced and catalytically compared. Physicochemical properties of materials were mainly investigated using XRD, N2 isothermal adsorption-desorption, FESEM, FT-IR, NH3-TPD, and TG/DTG/DTA techniques. Micro-meso hierarchy of prepared composite was demonstrated by XRD and BET analyses. Catalytic performance of materials illustrated that the methanol conversion of the prepared composite was about 98% for 120 min, showing a higher activity than the other catalysts. The initial reaction selectivity to light olefins of the composite was also comparable with those for the other catalysts. Furthermore, the results revealed that SAPO-34/AlMCM-41 preparation decreased the concentration and strength of active acid sites of the catalyst which could beneficially affect the deposition of heavy molecular products on the catalyst. However, as observed, the prepared composite was deactivated in olefins production faster than pure SAPO-34. The small mean pore diameter of composite could be mainly responsible for its pore blockage and higher deactivation rate. Meanwhile, since the SAPO-34 prepared by dry-gel method had inherently high mesoporosity, the AlMCM-41 introduction did not promote the molecular diffusion in the composite structure. The coke content was found 15.5% for deactivated composite smaller than that for the SAPO- 34 catalyst which could be due to the pore blockage and deactivation of the composite in a shorter period.