Hydrothermal stability of SAPO-34 in the methanol-to-olefins process

A new methanol-to-olefins (MTO) process for the production of ethylene, propylene, and butenes has been jointly developed by UOP and Norsk Hydro. The process converts methanol to 49 mol-% ethylene, 32 mol-% propylene, and 10 mol-% butenes (based on carbon) at near 100% conversion. The SAPO-34 catalyst used in this process requires frequent carbon-burn regenerations to remove nonvolatile coke deposits from within the micropores of the molecular sieve. The paper describes the results of accelerated hydrothermal aging of SAPO-34 powder to understand what structural changes might occur during the multiple reaction-regeneration cycles of the MTO process. No degradation of the SAPO-34 molecular sieve structure is seen after either lower temperature hydrothermal aging (450 degrees C) or 205 cycles of reaction and regeneration in the MTO process. Partial SAPO-34 structural collapse to a dense-phase crystalline aluminophosphate and condensed silica has been found to occur at severe hydrothermal conditions (650 degrees C). Both the pure SAPO-34 powder and spray-dried catalyst undergo this transformation to a similar extent. These findings indicate that the SAPO-34 structure collapse does not appear to be a major concern for the long-term stability of the MTO catalyst.