Research Of Long-chain Alkanes Isomerization Over Tungsten Based Catalysts

As one of the main routes to produce high-octane gasoline blending components in the crude oil refining industry, alkanes isomerization especial the long–chain alkanes isomerization has drawn much attention throughout the world because it can greatly improve the gasoline quality.The objective of our work is to investigate the isomerization of n-heptane over tungsten oxide based catalysts. The effects of the preparation and reaction conditions on the conversion and isomerization selectivity of n-heptane were studied carefully. The main contents of the thesis were discussed as follows:(1). It was found that the WO_x catalyst obtained by H₂ reduction at 575℃in a period of 6 hours exhibited a better catalytic activity with the reaction temperature 350℃, WHSV 1.36 h-1 and the catalyst weight 0.4g(the conversion of n-heptane reached 28.6% the isomerization selectivity achieved 89.7%, Data were taken after 6h of run). The physicochemical properties of catalysts were measured by XRD and BET technique, and the characterized result showed that the active phase of WO_x catalyst for n-heptane hydroisomerization could be considered WO₂(WO₂(Hx)ac) and W3O. This catalytic behavior was interpreted using the participation of two mechanisms: a bifunctional one on the WO₂(WO₂(Hx)ac) phase and a monofunctional metallacycle one on the W3O phase. By reduction the surface area obviously increased. The catalyst showed the mesoporous character, and the major isomerization products were 2 and 3-methylhexanes; the main hydrocracking products were C₃ and C₄.(2). It was indicated that catalytic activity of WO_x catalyst for n-heptane isomerization was markedly enhanced and the reduction temperature was reduced by incorporation of Ni in the catalyst. It was found that a high isomerization selectivity of over 80% with an overall conversion of nearly 51.0% could be achieved on Ni-WO_x catalysts, when the calcination temperature was 800℃, the Ni content was 2%, the reduction temperature was 525℃. The conversion of n-heptane on 2%Ni-WO_x catalysts was much high than on the WO_x catalyst. the selectivity of n-heptane on Ni-WO_x catalysts was lower than on the WO_x catalyst. Characterized studies showed that the active phase of 2%Ni-WO_x catalyst for n-heptane hydroisomerization could be considered WO₂. This catalytic behavior was interpreted using the participation of mechanism: a bifunctional one on the WO₂(WO₂(Hx)ac) phase. The catalytic activity of Me-WO_x (Me=Al, Zn, Cr, Cu, Co, Ni) catalyst for n-heptane isomerization was studied. The catalytic activity of WO_x catalyst for n-heptane isomerization was enhanced by incorporation of Ni and Co in the catalyst. It was indicated that isomerization selectivity of 2%Ni-WO_x catalyst for n-heptane isomerization was enhanced by incorporation of Mo in the catalyst.(3) N-Heptane isomerization on the WO_x/SiO₂ catalyst was studied. The WO_x/SiO₂ catalyst shows lower catalytic activity than bulk WO_x catalyst, which can be explained by the interaction between the support and WO3 in WO_x/SiO₂ catalyst, the conversion of n-heptane on the WO_x/SiO₂ catalyst was only about 3%.