A chemical approach for ultrafast synthesis of SAPO-n molecular sieves

Silicoaluminophosphate (SAPO-n) molecular sieves are important microporous materials for chemical processes like MTO. Classical synthesis route for SAPO-n, i.e. hydrothermal method, generally requires dozens of hours (>24 h). In this work, we reported a chemical approach to significantly reduce required crystallization time to 40–120 min by adding citric acid (CA) in synthesis gel. The time consumed with the proposed process is comparable with that by special heating intensification method while it is simpler, cheaper, and greener than latter. The ultrafast crystallization was attributed to the non-classical nucleation theory inspired by kinetic analysis, in which chelation interaction between CA and Al3+ favored metastable phase generation evidenced by XRD and 27Al NMR. Other chelate agents (tartaric acid, ethylenediaminetetraacetic acid) with multi carbonic group also showed promising interaction. Moreover, the superior crystallization promotion effect could be applicable in the synthesis of other SAPO-n molecular sieves like SAPO-11, which indicates its great potentials in the future. With further optimization by structure promoter, the obtained SAPO-34 demonstrated sub-micron particle size, high ratio isolated Q0 state Si specie, and hierarchical structure, characterized by the SEM, XPS, 29Si NMR, TEM, and N2-isotherms. The promoted physiochemical properties favored superior performance during MTO test. This work opens up a new perspective to synthesize SAPO-n molecular sieves by a simple chemical approach in a highly efficient and environment friendly manner.