Dry-gel synthesis of shaped transition-metal-doped M-MFI (M = Ti, Fe, Cr, Ni) zeolites by using metal-occluded zeolite seed sol as a directing agent

Abstract

Shaped transition-metal-doped M-MFI zeolites were prepared using a rapid dry-gel conversion technique from shaped extrudates by using a metal-occluded zeolite seed sol as a directing agent. The metal (Ti, Fe, Cr, Ni)-occluded zeolites seed sols were prepared at a high tetrapropylammonium hydroxide (TPAOH)/SiO2 molar ratio or with the addition of triethanolamine to keep a good dispersion of the transition metal in the seed sols. The silica gel and zeolites seed sols were mixed well and extruded into cylindrical-shaped extrudates with a diameter of 2 mm with the aid of polydimethylsiloxane (PDMS). The PDMS not only facilities the extrusion of the precursor but also increases the compressive strength of the obtained shaped zeolites. The metal-occluded seed sol promoted the distribution of the metal and provided crystal nuclei for rapid crystallization. In this method, the total molar ratio of TPAOH/SiO2 decreased to 0.1–0.175, which lowers the cost of preparing the zeolites. The morphology and textural properties were characterized by scanning electron microscopy and nitrogen sorption isotherms. The obtained shaped M-MFI zeolites comprised nanosized crystals (50–200 nm) and exhibited a good dispersion of the metals in the framework of the zeolite.