Aluminum speciation identification reveals water interactions in silicoaluminophosphate zeolites

Abstract

Water plays a crucial role in material development. As it is ubiquitous throughout zeolite generation and application, host–guest interaction between zeolite and water attracts broad interest, but mechanistic understanding remains fragmented. Here, advanced solid-state NMR techniques (2D 17 O SPAM-MQ, 27 Al{ 31 P} J -HMQC, 27 Al{ 29 Si} REDOR, and 1 H TQ-SQ NMR) combined with isotopic tracing and theoretical calculations determine water-induced octahedrally coordinated aluminum in silicoaluminophosphate molecular sieves (SAPOs) as an exclusive product of Al(OP) 4 units coordinated with two water molecules—a structure distinct from that in aluminosilicates. Based on the knowledge of aluminum speciation, we elucidate four water interaction mechanisms in SAPOs, including Brønsted-acid interaction, coordination, reversible/irreversible hydrolysis, and capillary condensation. Contrary to conventional wisdom attributing SAPO degradation to Al-O-P hydrolysis, we clarify that desilication dominates structural collapse, establishing Si environments as catalyst durability descriptors. These mechanistic insights decipher the nature of SAPO interacting with water and its fundamental differences from aluminosilicate zeolite.