Silicon redistribution, acid site loss and the formation of a core–shell texture upon steaming SAPO-34 and their impact on catalytic performance in the Methanol-to-Olefins (MTO) reaction

SAPO-34 is a commercially-implemented silicoaluminophosphate catalyst for selective high yield production of ethene and propene from methanol, but temperature regeneration in the presence steam leads to its deactivation. A comprehensive investigation effect prolonged hydrothermal treatment on structure properties explains changes catalytic methanol-to-olefins (MTO) performance. Microcrystalline powdered (ca. 3 µm crystals, Al17.1P15.6Si3.3O72) two batches larger single crystals different Si concentration (20–100 µm; Al17.3P14.7Si4.0O72 Al17.7P12.3Si5.9O72) were steamed (pH2O = 0.95 atm) at 873–1023 K up 240 h. The acidity (NH3-TPD), crystallinity (PXRD), framework cation environment (solid-state 27Al, 29Si 31P MAS NMR) porosity followed all materials; amenable crystal X-ray diffraction, FIB-SEM synchrotron IR microspectroscopy, including operando study during methanol dimethyl ether conversions. Some level steaming improved lifetime materials MTO catalysis without affecting their olefin selectivity, although more severe conditions led formation core–shell structures, microporosity loss eventually 1023 K, recrystallization dense phase. All these irreversible occurred faster with higher contents. initial increase results an activated reduction acid site density (Eact 146(18) kJ mol?1), result redistribution within SAPO loss. Operando online product analysis conversion suggests similar reaction pathways calcined greatly reduced methoxy group densities latter. gradual development optically dark cores upon progressive was shown by be due regions meso- macropores, mapping possess low hydroxyl densities.