Polyfunctional Zeolite-Based Catalysts for Hydrogen-Free Upgrading of Petroleum Fractions

The growing demand for high-quality motor fuels, combined with increasingly stringent environmental regulations, has stimulated research aimed at developing advanced catalytic methods for upgrading petroleum feedstocks. Conventional technologies for the deep purification and upgrading of petroleum fractions are based on hydrotreating processes that require an external H2 supply, elevated pressure, and dedicated infrastructure, resulting in high capital and energy costs. In this context, hydrogen-free catalytic upgrading is considered a promising alternative approach. This review analyzes current approaches to the hydrogen-free catalytic upgrading of hydrocarbon feedstocks, with particular emphasis on zeolite-based catalysts. The main reaction pathways, including dehydrogenation, isomerization, dehydrocyclization, and intermolecular hydrogen transfer, are discussed in relation to the structural characteristics of the catalysts and their acidic and redox properties. Particular attention is given to microporous ZSM-5, Y, and BEA zeolites, as well as MCM-41-type mesoporous materials, hierarchical micro–mesoporous composites, and nanocatalysts. The effects of the distribution of Brønsted and Lewis acid sites, pore architecture, and modification with metals (Ni, Co, Zn, Mo, rare-earth elements, etc.) on the catalytic activity, selectivity, and operational performance of catalytic systems are examined in detail. The analysis demonstrates that the efficiency of hydrogen-free upgrading processes is governed by the synergistic interaction between the acidic and metallic functions of the catalyst surface, while also depending on the accessibility of active sites and mass transfer. The development of hierarchical porosity can reduce diffusion limitations; however, excessive formation of secondary porosity may lead to a partial loss of the zeolite crystalline structure and acid sites. Similarly, metal incorporation enhances catalyst activity along key reaction pathways, but its effects on selectivity, coke formation, and stability are determined by the nature, loading, and location of the metal sites. Therefore, the prospects for hydrogen-free upgrading are primarily associated with optimizing these interrelated characteristics. Analysis of pilot- and industrial-scale experience with early hydrogen-free processes indicates that current restrictions on sulfur, benzene, and total aromatic contents determine the target selectivity requirements for such catalytic systems. Overall, hydrogen-free catalytic upgrading represents a promising complement to conventional hydroprocessing technologies, offering potential advantages in terms of improved energy efficiency, reduced dependence on hydrogen, and lower environmental impact. Further advances in this field are expected to rely on the coordinated control of acidic and metallic functions, pore structure, and mass transfer, together with the integration of physics-based modeling, systematic experimental screening, and data-driven analytical methods.