Bifunctional catalysts for the hydroisomerization of n-alkanes: the effects of metal–acid balance and textural structure

Abstract

The hydroisomerization of n-alkanes over bifunctional catalysts has played an increasingly important role in the modern petroleum industry due to its evident effects on the enhancement of the quality of fossil fuels. However, the preparation of bifunctional catalysts with excellent catalytic performance remains a significant challenge because of the complicated synergistic effects between the metal sites and Brønsted acid sites and the limited diffusion of alkene intermediates in the acid support. To overcome this challenge, the improvement of the metal–acid balance and the synthesis of hierarchical acid supports have become two commonly employed strategies. On the one hand, the favourable metal–acid balance is beneficial for the ideal consecutive hydroisomerization mechanism, which can be achieved by choosing the appropriate ratio of metal sites to Brønsted acid sites, controlling the nanoscale distance between two active sites, reducing the acidity and function of Brønsted acid sites, enhancing the function of metal sites and lowering the cost of preparation. On the other hand, via the “top down” (like desiliconization post-treatment) or “bottom up” (like templating methods) methods, the prepared hierarchical acid supports have more pore mouths or evidently reduced diffusion length. As a result, the alkene intermediates can either undergo isomerization at pore mouth instead of diffusing in the microporous channels or diffuse out fast from the short microporous channels, so that the diffusion limitation of alkene intermediates can be overcome and their cracking is inhibited. In this review, we summarize the recent advances in the developments of bifunctional catalysts for n-alkane hydroisomerization. The novel synthesis methods of the bifunctional catalysts are introduced, as well as the catalytic behaviours and the reaction mechanisms over catalysts. The effects of the metal–acid balance and the textural property on the catalytic performances are also discussed. The summarized studies reveal excellent potential for the design and preparation of novel bifunctional catalysts with excellent catalytic performances for n-alkane hydroisomerization.