Molecular hydrogen originated protonic acid sites over protonated fibrous silica Y zeolite for N-hexane isomerization

Abstract

Nowadays, isomerization of linear alkanes to their branched isomers has become the most demanded process in increasing the octane rating of gasoline. Fibrous silica zeolite Y (FY) catalyst has been prepared via the microemulsion system with zeolite Y seed assisted crystallization. The platinum loaded on protonated FY (Pt/HFY) was prepared by protonation of FY followed by the impregnation of platinum on HFY. The catalysts were characterized by X-ray Diffraction (XRD), Field Emission Scanning Electron Microscope (FESEM), nitrogen physisorption (BET), Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR) and Fourier Transform Infrared (FTIR) spectroscopies. The XRD results of the FY catalyst exhibited diffraction peaks in the range of 4° to 40° 2θ, which has been attributed to the faujasite-type zeolite Y. FESEM images showed a spherical morphology with bicontinuous lamellar in the range of 500 - 700 nm in diameter, while the EDX result showed that silica was a dominant material in Pt/HFY. NMR spectra confirmed the presence of both tetrahedral and octahedral Al species with dominant Q4 sites in HFY. Nitrogen physisorption results showed that the presence of fibrous silica generated mesoporosity in the range of 3 - 6 nm and increased the BET surface area from 473 to 550 m2/g, while the addition of Pt slightly decreased the BET surface area to 483 m2/g which might be due to pore blockage by the Pt particles. At 573 K, n-hexane isomerization over Pt/HFY resulted in 72.3% n-hexane conversion with 91.4% selectivity of mono-branched, 7.7% selectivity of di-branched and 0.9% selectivity of cracking products (C3-C5). The result obtained for Pt/HFY is higher than that of Pt/HY with only 27.2% conversion of n-hexane and selectivity of 70.7% mono-branched and 1.0% di-branched isomers. In the absence of Pt, HY exhibited a very low activity with less than 5% of n-hexane conversion and 51.5% selectivity of mono-branched isomer without di-branched isomers, whereas HFY achieved higher conversion and isomers selectivity compared to HY. The acidity of the catalysts was determined by pyridine preadsorption IR spectroscopy which showed that the presence of fibrous silica increased the number of Lewis acid sites and generated more protonic acid sites. Hydrogen adsorbed IR study revealed that the protonic acid sites which acted as active sites in the isomerization were formed via the dissociative-adsorption of molecular hydrogen releasing electrons close to the Lewis acid sites. The presence of fibrous silica which possessed strong Lewis acid sites increased and stabilized the formation of protonic acid sites by trapping of electrons. Thus, it is suggested that the presence of Pt and fibrous silica improved the activity and stability of zeolite Y in the n-hexane isomerization via the hydrogen spillover mechanism.