Controlling acidity and external surface morphology of SAPO-34 and its improved performance for methanol to olefins reaction

Hollow SAPO-34 with thin-wall were successfully synthesized by mixed templates and varying the silicon content in the gel. The silicon enrichment at the surface of crystals occurs on both low silicon and high silicon samples. The cubic crystals of the molecular sieve transformed from hollow to dense phase with increase of silicon content. The surface of molecular sieve crystals turned from being abundant of macroporous openings to be integrated without visible macroporous cracks with the increase of silicon content in the gel. The thin-wall SAPO-34 with low acid density and Si(4A1) framework structure, synthesized from the low silicon content (n(SIO2)/ n(Al2O3) = 0.1) of the gel, presented the highest ethylene plus propylene selectivity. The hollow structure SAPO-34 with medium acid density obtained with medium silicon content (n(SIO2)/n(Al2O3) = 0.26) in the gel possessed the longest lifetime.