Hollow nanocrystals of silicoaluminophosphate molecular sieves synthesized by an aminothermal co-templating strategy

An aminothermal co-templating strategy has been developed to explore the synthesis of SAPO molecular sieves. Nanosized SAPO-56 (AFX) with a hollow morphology is, for the first time, obtained by utilizing triethylamine (TEA) and trimethylamine (TMA) as both the solvent and the template in the presence of HF. The prepared material is well characterized, and the coexistence of TEA and TMA in the crystals is proved by 13C NMR. Molecular modelling indicates that the most stable status for the as-made SAPO-56 is 2TEA per aft cage and 1TMA per gme cage in the AFX structure, which is consistent with the TG results and verifies the co-templating behaviour of the two amines. In addition, the aminothermal crystallization process of SAPO-56 is examined, revealing an interesting in situ phase transformation from SAPO-34 (only containing TEA) to nano SAPO-56 followed by a further in situ post-synthetic leaching to achieve the hollow structure. Both F− anions and organic amines are proposed to be responsible for the dissolution of SAPO-34 and the etching of SAPO-56 nanocrystals. The Si-rich outer layer with abundant Si coordination environments (more Si–O–Al bonds and less Al–O–P bonds) helps the shell of SAPO-56 survive the leaching. This work offers an effective way to prepare SAPO-56 nanocrystals and demonstrates the huge potential of the aminothermal co-templating route for new material synthesis.