Modified Bentonite Catalysts: Structure–Property Relationships, Modification Strategies, and Perspectives for Waste Valorization into Hydrogen-Rich Products

Natural bentonite, dominated by montmorillonite-type layered aluminosilicates, is increasingly considered as an inexpensive and environmentally acceptable support or catalyst for the thermochemical conversion of waste carbon streams. Its relevance to catalysis arises from a combination of cation-exchange capacity, swelling behavior, hierarchical porosity after modification, and the possibility of generating Brønsted, Lewis, redox and metallic active sites. This review critically evaluates bentonite modification strategies—acid activation, alkaline and sodium exchange, inorganic oxide pillaring, transition- and noble-metal impregnation, organosilane or surfactant functionalization, and carbon-based hybridization—with emphasis on their consequences for texture, acidity, thermal stability, metal dispersion and catalytic behavior. The review then connects these structure–property relationships with the emerging application of modified bentonites in plastic-waste thermocatalysis and hydrogen-rich product formation. The recent literature indicates that acid-modified bentonite can substantially improve liquid hydrocarbon formation from polyethylene, binder-free bentonite pellets can operate at kilogram-batch scale for drop-in fuels, and Ni-, Fe- or Ni–Fe-modified bentonite-type catalysts can promote tar cracking, reforming and gas upgrading. However, direct evidence for high hydrogen yields from plastic waste over bentonite remains narrower than the evidence for liquid-fuel production, biomass pyrolysis or model-compound reforming. Therefore, this article distinguishes between direct plastic-waste evidence and transferable evidence from biomass, acetic acid, tar and hydrocarbon reforming studies. The analysis identifies Ni-impregnated acid-activated or pillared bentonite, Ni–Fe/bentonite, and La/Ca-promoted Ni/bentonite as the most promising routes for plastic-derived hydrogen-rich syngas, while acid activation alone is best regarded as a pretreatment rather than a complete catalyst design. Key limitations include catalyst deactivation by coke, metal sintering, chloride poisoning from PVC, inconsistent reporting of gas yields, and insufficient life-cycle and techno-economic analysis. A roadmap is proposed for designing reproducible, scalable bentonite catalysts for circular plastic-waste valorization.