Role of Strontium Cations in ZSM-5 Zeolite in the Methanol-to-Hydrocarbons Reaction

The selectivity of the methanol-to-hydrocarbons (MTH)reactioncan be tuned by modifying zeolite catalysts with alkaline earth metals,which typically increase propylene selectivity and catalyst stability.Here we employed Sr2+ as its higher atomic number in comparisonto the zeolite T atoms facilitates characterization by scanning transmissionelectron microscopy and operando X-ray absorption spectroscopy. Sr2+ dispersed in the ZSM-5 micropores coordinates water, methanol,and dimethyl ether during the MTH reaction. Complementary characterizationwith nuclear magnetic resonance spectroscopy, thermogravimetric analysiscombined with mass spectrometry, operando infrared spectroscopy, andX-ray diffraction points to the retention of substantially more adsorbatesduring the MTH reaction in comparison to Sr-free zeolites. Our findingssupport the notion that alkaline earth metals modify the porous reactionenvironment such that the olefin cycle is favored over the aromaticcycle in the MTH, explaining the increased propylene yield and lowerdeactivation rate.