Influences of catalyst formulation on the catalytic activity of modified HZSM-5 in the aromatization of light paraffins

Abstract

The aromatization of light paraffins was studied over extruded modified HZSM-5 catalysis. The HZSM-5 zeolite (SiO₂/AI₂/O₃ = 23) catalysts were sequentially treated by dealumination, Ga ion-exchange, and silylation via chemical liquid deposition (CLD) using tetraethyl orthosilicate (TEOS). The modified HZSM-5 catalysts in powder form were shaped in a cylindrical form in order to meet the mechanical properties. The physical and chemical properties of catalysts were characterized by several techniques including BET, TPD of IPA, TPO, TPR, XRD, radial crushing strength, bulk density, and attrition loss. Catalytic activity was tested in a continuous flow fixed-bed reactor at 500 ℃, I atm, and WHSV of 5 h⁻¹. The optimized steaming condition was at 400 ℃ and partial pressure of steam at 5 kPa (CLD/Ga/Ac/ZP5). The results showed that the CLD/Ga/Ac/ZP5 catalyst exhibited 89% conversion, aromatic selectivity 92% with 64% p-xylene selectivity in xylenes. The higher Catalytic performance could be due to the stronger Br0nsted acid strength generated during mild steaming, thus enhancing n-pentane aromatization. Therefore, powdery CLD/Ga/ Ac/ZP5 catalyst was selected to be shaped with pseudoboehmite binder. The extrudates exhibited lower in p-xylene selectivity in xylenes due to binder acid sites. Results showed that the strength of extruded CLD/Ga/Ac/ZP5 catalyst increased with pseudoboehmite binder contents but the activity decreased due to lower its active sites. The optimum mass ratio of zeolite to pseudoboehmite was 4:1. The extruded HZSM-5 catalysts prepared with 3 vol% acetic acid solution and 550 ℃ calcination temperature exhibited the highest in radial crushing strength.