Tailoring ZIF-8-based catalysts through template-assisted synthesis and metal modification for organic carbonate production

Introduction. Metal-organic frameworks (MOFs) are promising heterogeneous catalysts owing to their high surface area, tunable porosity, and accessible active sites. ZIF-8 has attracted particular interest for CO₂ utilization, including cycloaddition of CO₂ to epoxides or methanol synthesis. This study investigates the effect of polystyrene templating and bimetallic modification on the physicochemical and catalytic properties of ZIF-8-based materials.

Methods. ZIF-8-based materials were synthesized using unmodified PS and Zn-functionalized polystyrene PS(Zn) nanospheres as structure-directing templates. Following template removal, Ti and Cu were incorporated, yielding Cu/ZIF-8(Zn/Ti) catalysts characterized by SEM, XRD, FTIR, TGA and N₂ sorption. Catalytic performance was evaluated in two reactions: (i) cycloaddition of CO₂ to propylene oxide at 80°C and 2 bar in the presence of co-catalyst, and (ii) methanol synthesis at 200°C and 18 bars.

Results. The type of template strongly influenced the morphology and textural properties of obtained materials. ZIF-8 prepared using PS(Zn) exhibited a homogeneous core-shell morphology and higher specific surface area (943 m²/g) than the material synthesized with unmodified PS (722 m²/g). Template removal further increased the surface area to 1009 m²/g while preserving ZIF-8 crystal structure. Ti and Cu incorporation generated additional catalytic sites while maintaining high porosity. The obtained Cu/ZIF-8(Zn/Ti) achieved a propylene carbonate yield of 43%, 98% methanol selectivity and methanol yield of 83 g_MeOH/kg_cat/h. By comparison, untemplated catalysts achieved 39% of PC yield, 88% of methanol selectivity and 4.7 g_MeOH/kg_cat/h, highlighting the beneficial effect of template-assisted catalyst design. The Cu/ZIF-8(Zn/Ti) crystallinity was preserved after metal incorporation and catalytic testing.

Conclusions. Template-assisted synthesis combined with Ti and Cu modification effectively tailors ZIF-8 catalysts for CO₂ valorization. Zn-functionalized PS templates promote the formation of homogeneous porous structures with enhanced catalytic performance, demonstrating the importance of morphology control in the design of advanced MOF-based catalysts.