Post synthesis of hierarchical SAPO-34 via citric acid etching: Mechanism of selective desilication

Realizing the controllable adjustment on textural and acid properties of silicoaluminophosphates (SAPOs) is crucial for obtaining excellent-performance catalyst via acid-etching method. However, there are difficulties due to the lack of understanding of the mechanism of selective desilication for acid etching. In this paper, parent SAPO-34 were exposed to a series citric acid solutions aimed to investigate the influence of acid concentration on the physicochemical properties of the resulted SAPO-34 and elucidate the mechanism of selective desilication. The results showed that citric acid concentration plays a decisive role in efficiently manufacturing hierarchical pore structure and selectively extracting specific Si species. The diverse acid resistance of Si–O–Si and Si–O–Al bonds near or in different Si species (Si islands and isolated Si(4Al)) is responsible for the selective desilication of citric acid. The coupling effects of the differential distribution of Si species in parent SAPO-34 and the selective desilication influence the generation of the secondary pore and the strength and amount of acid sites of the resulted SAPO-34. Compared to the parent SAPO-34, the acid-treated SAPO-34 exhibited a longer lifetime and a higher selectivity for light olefins (ethylene and propylene) when used in methanol-to-olefins (MTO) reaction.