Methanol-to-Olefins Catalysis with Hydrothermally Treated Zeolite SSZ-39

Zeolite SSZ-39 is evaluated for catalyzing the methanol-to-olefins (MTO) reaction. By steaming NH₄–SSZ-39, Al can be removed from framework positions, resulting in an increase in framework-Si/Al_T and thus a lowered active acid site density. The Si/Al_T ratios can be controlled by the steaming temperatures. SSZ-39 steamed at 750 °C, with preserved pore volume and morphology, is an excellent MTO catalyst, as high, stable olefin selectivities, long time-on-stream activity, and low alkane production are observed. Moreover, interesting propylene/ethylene/butylene ratios of 2.8/1/1.1 are obtained, likely related to the shape of the AEI cage. By Cu²⁺-exchanging SSZ-39, evidence is provided to show that Al_T sites in close proximity (high Al_T density) produce the unwanted effects (higher alkane-make and carbonaceous deposits) in nonsteamed materials during MTO.