Controllable fabrication of uniform core–shell structured zeolite@SBA-15 composites

Abstract

Mesoporous core–shell composites with large-pore silica shells are highly desired for a broad spectrum of applications. We report an ultra-dilute liquid-phase coating strategy in an acidic medium for controllable synthesis of uniform micro/mesoporous core–shell composites zeolite@SBA-15 comprising zeolite cores and mesoporous silica SBA-15 shells using triblock compolymer Plunoric P123 as a template. Structural characterizations show that the core–shell composites possess tunable specific surface areas (115–228 m² g⁻¹), large pores (∼7.0 nm in diameter) with plenty of mesotunnels (∼3.0 nm) from silica shells, original crystalline zeolite frameworks, and opened junctions between micropores and mesopores. The silica shells have ordered 2-D hexagonal mesopore channels, most of which are annularly parallel (fingerprint-like arrangement) to the anisotropic zeolite faces. The shell-thickness is crystal face-dependent, which could be facilely tuned in the range of 30–45 and 40–120 nm on a pinacoids/dome faces and b pinacoids of a zeolite single-crystal, respectively. Moreover, the synthesis parameters such as MgSO₄ additive, stirring rate, acidity, temperature and reaction time show great influences on the formation of uniform core–shell composites. Post-hydrothermal treatment at 100 °C has been for the first time adopted to improve mesostructural regularity of the core–shell composites. A scheme regarding surface-induced micellization and hydrothermal rearrangement of mesostructured silica shells in the coating process is proposed to illustrate the formation of core–shell composites. The core–shell composite HZSM-5@SBA-15 (HZ@S15) was employed as a catalyst for methanol to propylene (MTP) conversion, and shows excellent catalytic performance with high methanol conversion (∼98%) and propylene to ethylene (P/E) ratio (∼10.7) as well as propylene selectivity (∼39%).