Selective Desilication, Mesopores Formation, and MTO Reaction Enhancement via Citric Acid Treatment of Zeolite SAPO-34

Adjusting the silica composition/structure and thus tuning the acidity besides introducing hierarchical pore structure is the key factor in zeolite SAPO-34 research. In this work, hierarchical zeolite SAPO-34 with well-organized mesopores was prepared via citric acid treatment. The mesopores presented a novel slit shape, which starts from the crystal surfaces then ends at the center of the crystal. It was found that, after citric acid treatment, silicon species of Si­(OSi)1(OAl)3 in the SAPO-34 framework were extracted selectively, at the same time that both the total Lewis acid amount (by NH3–IR) and the Brønsted acid amount on the external surface (by Pyridine-IR) of SAPO-34 zeolite obviously increased. Consequently, the hierarchical SAPO-34 sample exhibited superior catalytic performance in the MTO reaction with about 2.5 times prolonged catalytic lifetime and a nearly 8% improvement of selectivity for ethylene and propylene; this is very important for industrial processes.