<p>Montmorillonite, a widely available clay mineral known for its high cation-exchange capacity and versatile surface chemistry, was strategically functionalized to serve as a heterogeneous catalyst for the cycloaddition of carbon dioxide to styrene oxide. Two preparation strategies for heterogeneous catalysts were explored: ion-exchange of interlayer cations incorporating pyridinium and imidazolium salts, and surface grafting via halogenated silanes followed by quaternization with pyridine and 1-methylimidazole. The result in both cases was an immobilized quaternary salt, representing a catalytically active species. The grafted materials exhibited superior performance, achieving up to 98% conversion and 100% selectivity toward styrene carbonate under solvent-free conditions. Catalytic activity was strongly influenced by the type of halide used, with iodide-functionalized systems outperforming bromide analogs. Catalysts prepared by ion-exchange exhibited moderate activity, with conversions up to 94%, but suffered from low selectivity due to side reactions promoted by the acidic nature of the clay. The addition of iodide co-catalyst improved selectivity, confirming the importance of halide anions in epoxide activation. Comprehensive characterization (elemental analysis, UV-Vis, FT-IR, XRD, and nitrogen physisorption) confirmed successful functionalization and preservation of the clay structure. The catalysts demonstrated good reusability. Compared to ion-exchanged systems, grafted materials showed enhanced stability and catalytic efficiency without requiring additional co-catalysts. These findings highlight the potential of halogen-functionalized montmorillonite as a sustainable and efficient platform for CO₂ valorization via cyclic carbonate synthesis, contributing to the development of green catalytic technologies.</p>