A low-temperature approach to synthesize low-silica SAPO-34 nanocrystals and their application in the methanol-to-olefins (MTO) reaction

Abstract

A low-temperature strategy is developed for the synthesis of low-silica SAPO-34 with a tunable Si content and relatively uniform Si distribution in the crystals, which is hitherto difficult to achieve. It is demonstrated that a lower crystallization temperature and a silicon source with relatively low reactivity are important factors leading to successful synthesis. The crystal size of SAPO-34 could be effectively decreased to 200 nm through a seed-assisted approach. The local atom environments of low-silica samples are investigated by solid state MAS NMR, which confirms the unique existence of Si(4Al) species in the framework. The obtained low-silica SAPO-34 exhibits excellent catalytic performance in the MTO reaction and the occurrence of catalyst deactivation varies with the acid properties. Through optimizing the Si content of the samples, a long catalyst life and a high initial/maximum selectivity to ethylene plus propylene could be achieved simultaneously over SAPO-34 with a Si content of Si/(Si + Al + P) = 0.047. This result would be valuable for the improvement of the catalytic properties of SAPO-34 used for a commercial MTO fluidized-bed reactor. However, SAPO-34 with a very low Si content (e.g. Si/(Si + Al + P) = 0.039) exhibits a shortened catalyst life due to the insufficient Brønsted acid sites in spite of the high selectivity to light olefins.