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 Bronsted 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.