Synthesis and characterization of platinum-zirconium catalyst supported on Bicontinous lamellar silica Mordenite for n-hexane hydroisomerization

Abstract

Catalytic hydroisomerization of n-alkane has been one of the vital processes in the petroleum refining industry to improve the quality of gasoline. The existing catalysts have registered low isomerization performance due to the poor accessibility of active sites for the reactant molecules and the strong acid sites tend to be more selective towards cracking, thus affecting the overall efficiency of the process. This study investigated the catalytic performance of modified mordenite zeolite in n-hexane hydroisomerization. A novel protonated mordenite catalyst with bicontinuous lamellar morphology (HM@KCC-1) was successfully prepared via a microemulsion system with a mordenite seed-assisted crystallization technique. Platinum (Pt) was loaded by wet impregnation method and the catalytic performance was compared with Pt supported on commercial mordenite zeolite. The catalysts were characterized with X-ray diffraction, field emission scanning microscopy, transmission electron microscopy (TEM), surface area analyzer and electron spin resonance spectroscopy. The acidity was determined by pyridine and 2,6-lutidine adsorbed Fourier transformation infrared (FTIR) spectroscopy, while the catalytic performance was conducted in a microcatalytic pulse reactor at 150-350 °C under a hydrogen stream. The higher catalytic activity of Pt/HM@KCC-1 was achieved with 75% conversion, 98% isomer selectivity and 74% isomer yield compared to Pt/HM with 60% conversion, 40% isomer selectivity and 24% isomer yield at 300 °C. This was attributed to the well-dispersed Pt nanoparticles on the bicontinuous lamellar structure of HM@KCC-1 evidenced from the TEM images and the moderate acid sites on Pt/HM@KCC-1 as shown by FT-IR which favoured the dehydrogenation/hydrogenation function and the skeletal isomerization, respectively. Furthermore, the effect of zirconium (Zr) incorporation was investigated with different Zr loading (1, 5, 10wt %) on HM@KCC-1. The results showed formation of permanent Lewis acid sites which was selective towards the generation of mono-branched isomers. Hence, Zr has great potential as a promoter with 5Zr/HM@KCC-1 exhibiting the best catalytic performance. Zr as a promoter in Pt/5Zr-HM@KCC-1 was prepared by impregnation with 0.5wt % Pt. The isomer yield followed the order of Pt/5Zr-HM@KCC-1(86) > Pt/HM@KCC-1(74) > 5Zr-HM@KCC-1(60), best catalyst showed remarkable increased strength of Lewis acid sites and selectivity towards the di-branched isomer. Zr evidently enhanced the formation of molecular hydrogen-generated protonic acid sites which plays an important role in the hydroisomerization process. The optimum isomer yield for the n-hexane hydroisomerization obtained by response surface methodology was 85.7% at a reaction temperature of 293 °C, reduction temperature of 474 °C and flow of hydrogen over catalyst weight of 502 ml.g-1min-1. The experiment carried out at these optimum conditions yielded 84.1% isomer with 1.9% error. This study has highlighted the efficient design of Zr promoted Pt on HM@KCC-1 catalyst with appropriate metal/acid sites functions for hydroisomerization. In conclusion, the promising performance of PtZr/HM@KCC-1 catalyst demonstrated the potential to be used for the production of high-quality fuel, particularly for n-alkane hydroisomerization in the refining processes