Synthesis and characterization of phosphate and nitrate groups supported on Protonated Fibrous Silica Beta-Zeolite for n-Hexane and Cyclohexane Hydroisomerization

Abstract

Research octane number (RON) is used as a reference in petrochemical refining industries to indicate the quality of fuel. A higher RON can be achieved through hydroisomerization. In this study, protonated fibrous silica BEA (HSi@BEA) catalyst with unique bicontinuous concentric lamellar structure morphology was successfully prepared by microemulsion technique coupled with zeolite BEA seed. The HSi@BEA catalyst was compared with protonated commercial BEA zeolite (HBEA) on the hydroisomerization of n-hexane and cyclohexane. The catalysts were characterized using X-ray diffraction (XRD), surface area analysis, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Ultraviolet-Diffuse Reflectance Spectroscopy (UV-DRS), Fourier transform infrared spectroscopy (FTIR), pyridine adsorption FTIR, 2,6-lutidine adsorption FTIR, nuclear magnetic resonance (NMR), and electron spin resonance (ESR). The catalytic performance was conducted in a microcatalytic pulse reactor at 423-623 K under atmospheric pressure. The surface area analysis showed that HSi@BEA catalyst exhibited higher surface area and bigger average pore size compared to the commercial HBEA catalyst. 27Al Magic angle spinning NMR (MAS NMR) results displayed that the additional silica lamellar structure of the HSi@BEA catalyst increased the extra-framework aluminium (EFAl). During hydroisomerization, the additional Lewis acid sites in the HSi@BEA generated high amount of protonic acid sites by playing a role as electron acceptors after the dissociation of H2 or C6 alkanes. The high amount of protonic acid sites in HSi@BEA catalyst enhanced catalytic activity at 523 K with isomers yield of 19.8% and 13.2% for n-hexane and cyclohexane respectively, compared to 2.50% and 6.64% over commercial HBEA catalyst. Further modification of HSi@BEA catalyst with phosphoric acid (P/HSi@BEA) and nitric acid (N/HSi@BEA) by wet impregnation further enhanced the catalytic activity which is attributed to the different behaviour of the fibrous silica BEA support. FTIR analysis showed that the phosphate group favoured to form P-OH species in the catalyst framework which resulted in a higher number of weak acid sites. Additionally, the nitrate group interacted with EFAl species on HSi@BEA catalyst and increased the formation of Brønsted acid sites of the catalyst. In n-hexane and cyclohexane hydroisomerization, the P/HSi@BEA catalyst favoured the production of the n-hexane isomers, while the N/HSi@BEA were selectively towards production of cyclohexane isomers, with isomers yield 50.3% and 48.4%, respectively. This fundamental study exhibits that significant interactions given by such phosphate and nitrate groups with the unique silica fibrous BEA support could enhanced hydroisomerization which contribute to the high RON of fuel.