Synergic regulation of crystalline phase, composition and morphology of SAPO-34 via metal assistance and in situ etching strategy

SAPO-34 with low silica content and good mass transfer property is highly desired catalytic material for syngas-to-olefins and methanol-to-olefins (MTO) reactions. However, the synthesis of low-silica SAPO-34 by common low-cost template remains a challenge as the product is always bothered by impurity contamination or CHA/AEI intergrowth. Here, a metal assistance (Me = Zn, Mg, Co, Mn) and in situ etching strategy was developed to allow the construction of hollow low-silica SAPO-34, which exhibited long catalytic lifespan and ultra-high ethylene plus propylene selectivity (86.9 wt%) in MTO reaction. The crystallization process was studied by monitoring the evolution of a Zn-containing low-silica gel with triethylamine (TEA) as the template. It was revealed that Zn atoms were more active than Si atoms to participate in the nucleation of ZnAPSO-34, which effectively inhibited the CHA/AEI intergrowth. ZnAPSO-34@SAPO-34 was obtained with further crystallization. The selective etching of Zn-containing components occurred in subsequent cooling stage owing to inhomogeneous distribution of framework T atoms, leading to the hollow low-silica SAPO (Si/(Si + Al + P) ≈ 0.07 in mole) with pure CHA phase. This work provides a facile approach for the synergic regulation of crystal phase, composition and morphology of SAPO materials.