Calix[4]arene-Functionalized Titanium-Oxo Compounds for Perceiving Differences in Catalytic Reactivity Between Mono- and Multimetallic Sites

While the difference in catalyticreactivity betweenmono- andmultimetallic sites is often attributed to more than just the numberof active sites, still few catalyst model systems have been developedto explore more underlying causal factors. In this work, we have elaboratelydesigned and constructed three stable calix[4]arene (C4A)-functionalizedtitanium-oxo compounds, Ti-C4A, Ti ( 4 ) -C4A, and Ti ( 16 ) -C4A, with well-defined crystal structures, increasingnuclearity, and tunable light absorption capacity and energy levels.Among them, Ti-C4A and Ti ( 16 ) -C4A can be taken as model catalysts to comparethe differences in reactivity between mono- and multimetallic sites.Taking CO2 photoreduction as the basic catalytic reaction,both compounds can achieve CO2-to-HCOO- conversion with high selectivity (close to 100%). Moreover, thecatalytic activity of multimetallic Ti ( 16 ) -C4A is up to 2265.5 & mu;mol g(-1) h(-1), which is at least 12 times higher than thatof monometallic Ti-C4A (180.0 & mu;mol g(-1) h(-1)), and is the best-performing crystalline cluster-basedphotocatalyst known to date. Catalytic characterization combined withdensity functional theory calculations shows that in addition to theadvantage of having more metal active sites (for adsorption and activationof more CO2 molecules), Ti ( 16 ) -C4A can effectively reduce the activation energyrequired for the CO2 reduction reaction by completing themultiple electron-proton transfer process rapidly with synergisticmetal-ligand catalysis, thus exhibiting superior catalyticperformance to that of monometallic Ti-C4A. This workprovides a crystalline catalyst model system to explore the potentialfactors underlying the difference in catalytic reactivity betweenmono- and multimetallic sites.

Automated summary

In this study, three stable calix[4]arene (C4A)-functionalized titanium-oxo compounds, Ti-C4A, Ti ( 4 ) -C4A, and Ti ( 16 ) -C4A, were designed and compared as model catalysts for CO2 photoreduction. The multimetallic Ti ( 16 ) -C4A showed significantly higher catalytic activity than the monometallic Ti-C4A, achieving a CO2-to-HCOO- conversion with high selectivity and a rate 12 times higher than that of Ti-C4A. The superior catalytic performance of Ti ( 16 ) -C4A was attributed to the rapid multiple electron-proton transfer process facilitated by synergistic metal-ligand catalysis, reducing the activation energy required for the CO2 reduction reaction.