Species and impacts of metal sites over bifunctional catalyst on long chain n-alkane hydroisomerization: A review

Abstract

The Fischer–Tropsch process is a technical route for the indirect liquefaction of coal in the modern coal chemical industry and also an important means of producing high-value-added products in the coal chemical industry. The market demand for the main product Fischer–Tropsch wax (n-alkane content up to 95 %) is limited, resulting in an overcapacity. The hydroisomerization process can transform Fischer–Tropsch waxes into a low-freezing-point oil-phase mixtures, which can be used as a high-end lubricating base oils to solve the imbalance between supply and demand in the market and expand the comprehensive utilization of coal chemical resources. However, the complex synergistic effects between metal and acid sites and the diffusion limitations of olefin intermediates are major challenges in the design of the bifunctional catalysts. Unlike the majority of studies that focused on analyzing the isomerization functions of acidic carriers, in this review, we focused on the types of metal sites and the factors affecting the metal function of bifunctional catalysts used in n-alkane hydroisomerization in recent years. We introduce bifunctional catalysts supporting different metal types, including noble metals, transition metals and their compounds, bimetallic sites, and other catalyst types. In particular, the “doping effect” of the second metal is, for the first time, proposed to explain its role on the catalyst body, and the effects of the metal particle size and dispersion, crystal plane orientation, metal/acid site ratio, and the distance between the metal and acid sites on the hydrogenation function are also discussed. The summary of previous research shows that the types and structural control of metal sites have greatly improved the performance of n-alkane hydroisomerization over bifunctional catalysts. This provides an important reference for the structural design of new bifunctional catalysts in the future and for catalyst application in the hydroisomerization of Fischer–Tropsch waxes.