Mesopore-Modified SAPO-18 with Potential Use as Catalyst for the MTO Reaction

Abstract

SAPO-34 silicoaluminophosphates are well known as catalysts for the synthesis of light olefins–ethylene and propylene–from methanol by using the methanol to olefins (MTO) process, first described by Mobil. SAPO-18, which has a microporous framework structure related to but crystallographically distinct from SAPO-34, has been much less studied but promises to be a potential catalyst in the MTO process. The main drawback of these catalysts in this reaction is their rapid deactivation, due to the deposition of heavy carbonaceous products–coke–on the surface of the solid avoiding the access of methanol molecules to the active centres located inside the pores of the SAPO catalysts. We have used different mesoporogen agents–nanoparticulate carbons and chitosan–aiming to generate mesoporosity in the SAPO-18 crystals, in an attempt to improve the accessibility of the reagent to the active centres of SAPO-18 and, in that way, inhibit catalyst deactivation. All the materials prepared in this work present similar framework composition and silicon distribution and the main difference among them is the hierarchical porosity generated by the mesoporogen additives use in the synthesis, as determined by STEM. Using chitosan polymer as a secondary template results in an increase of the external surface, which improved significantly the internal diffusivity enhancing the life time of the catalyst in the MTO process.