Hydrocarbon conversion of long-chain paraffins : Role of metal(Pt, Ru, Ni) &influence of acid sites, structures and textural properties of solid catalysts

Abstract

Hydroisomerization of long-chain hydrocarbons for the production of high quality branched fuel was studied due to their vast availability and product versatility. Most of these long-paraffins are cracked to obtain lower molecular weights fuel fraction, substantial efforts are made globally to transform these long-chain hydrocarbons to branched hydrocarbons and improve their fuel properties. The main objectives in this work were to explore suitable catalysts for hydroisomerization of hexadecane, a model compound for long-chain hydrocarbons. Transformation of hexadecane were carried over bifunctional platinum, ruthenium and nickel catalysts comprising also beta catalysts. As a comparison with ruthenium beta zeolites, ruthenium-Y-zeolite was also studied. The experiments were performed at 210 °C and 40 bar of overall pressure. The gas phase analysis of the products in the hydroisomerization of hexadecane indicated the presence of low molecular weight hydrocarbons, mainly methane. Each catalyst formed methyl-pentadecane and cracking products, which included lower molecular weight and long-chain hydrocarbons, and alkylated hexadecane. The fresh and spent catalysts were characterized using several physico-chemical methods. Fresh catalysts were characterized for specific area and pore size, Brønsted and Lewis acid sites, surface morphology and active metal cluster size. All catalysts exhibited deactivation by severe coking except platinum beta zeolites. The catalyst coking was confirmed by thermogravimetric analysis, organic elemental analysis and size exclusion chromatography. Among all catalysts, nickel beta zeolites formed the most gas-phase hydrocarbons with abundant cracked hydrocarbons. Ruthenium beta zeolites allowed higher isomerization selectivity compared to the parent zeolite possessing however low conversion. Ruthenium Y-zeolite produced the lowest amount of gas-phase hydrocarbons and exhibited a relatively higher isomerization selectivity, even if large amounts of cracking products were also formed. All catalysts exhibited higher than 80 % mass balance except TR-10 (H-Beta-300). The best results were shown by platinum beta zeolite (2 wt.% Pt/H-Beta-25), which formed lower amounts of the gas-phase and cracked hydrocarbons and a high yield of methyl-pentadecane. Temperature and pressure dependence were elucidated in kinetic experiments with a bifunctional ruthenium beta zeolite, which indicated an increase in the yield of methylpentadecane and improved selectivity towards hydroisomerization.