Synthesis of shaped ZSM-5 zeolites by dry-gel conversion with seed gel as directing agent

Abstract

Shaped binderless ZSM-5 zeolites were prepared via a rapid dry-gel conversion technique from aluminosilicate extrudate. Silica gel and boehmite were well mixed and extruded into cylindrical shaped extrudates with diameter of 2 mm with the aid of silica sol, ZSM-5 zeolite seed gel, and/or tetrapropylammonium hydroxide (TPAOH) solution. The total molar ratio of TPAOH to SiO 2 was fixed at 0.025. With same amounts of structure directing agent (TPAOH), the addition of seed gels accelerated the crystallization of the zeolite. The seed gel directing agent composition was 0.35TPAOH:1SiO 2:20H 2O: 4C 2H 5OH. The seed gel not only provided crystal nuclei for rapid crystallization but also controlled the size of the ZSM-5 crystals. Especially, the morphology of these extrudates was well maintained in the crystallization process. The crystallization processes of zeolite were characterized by X-ray diffraction (XRD), thermogravimetric (TG) analysis, and Fourier transform infrared (FTIR) spectroscopy. The results showed that the growth of the zeolite was accompanied by the occupation of the zeolite channels by TPAOH. When the TPAOH/SiO 2 molar ratio was 0.025, the sample achieved 100% relative crystallinity after 3 h crystallization. The morphology and textural properties were characterized by scanning electron microscopy (SEM) and nitrogen sorption isotherms. The obtained shaped zeolites comprised nanosized crystals (200 nm) and exhibited a hierarchical structure with high mesopore volume (0.28 cm 3·g -1). The temperature- programmed desorption of ammonia (NH 3-TPD) was used to assess the acidity of these shaped zeolites. The results showed that this shaped zeolite possessed an acidity similar to that of a commercial H-ZSM-5 catalyst. © Editorial office of Acta Physico-Chimica Sinica.