n-Heptane isomerization over platinum and phosphorus supported on modified molybdenum oxide-mesoporous silica

Abstract

Catalytic isomerization of n-alkanes into the corresponding branched isomers is an important reaction to produce clean fuel with high quality. Therefore, continuing studies on efficient catalysts for isomerization have been conducted in recent years. In this study, mesostructured silica nanoparticles (MSN) were mixed physically with platinum (Pt) and molybdenum oxide (MoO3) to prepare Pt/MSN and MoO3/MSN for n-heptane isomerization. Besides, the effect of support was studied by employing the bicontinuous concentric lamellar silica (KCC-1) which was prepared by microwaveassisted microemulsion, as MoO3 support (MoO3/KCC-1). In order to improve the catalytic activity, the effect of phosphorus (P) loading was carried out by impregnation of MoO3/KCC-1 with phosphoric acid to form P/MoO3/KCC-1. The catalysts were characterized using x-ray diffraction, surface area analysis, scanning electron microscopy, hydrogen-temperature programmed reduction, nuclear magnetic resonance, ultraviolet-visible, Fourier transform infrared (FTIR) and electron spin resonance (ESR) spectroscopies. High activity of n-heptane isomerization was observed on MoO3/MSN compared to the Pt/MSN in the presence of hydrogen at 350 °C, with yield of mono- and di-branched iso-heptane reaching 36.6% and 6.8%, respectively. ESR and FTIR studies indicated that the high activity and stability of MoO3/MSN could be attributed to the dissociative-adsorption of molecular hydrogen to form atomic hydrogen, which subsequently formed active (MoOx)-(Hy)+. The interaction of Pt/MSN and molecular hydrogen formed Pt-H, which was not active in n-heptane isomerization. In comparison, the MoO3/KCC-1 possessed low activation energy (28.1 kJ/mol), as well as gave higher yield of isomers (42.2%) compared to MoO3/MSN (35.8%). The result was related to the unique morphology of silica KCC- 1, which allowed high accessibility of bulky mass reactant to the active sites. The P/MoO3/KCC-1 showed a decrease in the Brønsted acid while new Lewis acidic centers were formed at 1624 cm-1 and 1587 cm-1, as observed by 2,6-lutidine adsorbed infrared. High yield of isomers obtained by P/MoO3/KCC-1 was related to the participation of the acidic centers at 1624 cm-1 and 1587 cm-1, in the formation of protons by trapping electrons, as well as high accessibility to active (MoOx)-(Hy)+. The ANOVA analysis indicated that the reaction temperature was the prominent significant variable in the production of isomers. Based on the optimization experiment, 44.9% yield of isomers was obtained at the optimum condition of 311 °C, treated at 464 °C for 6 h. This study highlighted the potential of modified mesoporous silica in the catalysis research, especially for linear alkane isomerization.