Tuning the Basicity of a Metal-Nitrogen-Carbon Catalyst

Tuning the Basicity of a Metal-Nitrogen-Carbon Catalyst

Author: ChemistryViews.org

Cycloadditions of epoxides and CO2 provide an atom-economical path to cyclic carbonates. Metal-organic frameworks (MOFs) can serve as catalysts or catalyst precursors for conversions of CO2. PCN-224, for example, is a porphyrin-based MOF that can coordinate different metals in the porphyrin units. Coordinated cobalt can serve as a Lewis-acidic site that can promote the ring-opening of the epoxy reactants. However, it occupies the basic nitrogen site in the porphyrin, which weakens the absorption of CO2. Thus, balancing the acidic/basic site concentrations in the framework is important for developing effective catalysts.

Weijun Yang, Hunan University, Changsha, China, and colleagues have developed a catalyst for cycloadditions of epoxides and CO2 based on PCN-224 with tunable acidic/basic site concentrations. The team first synthesized PCN-224 without metals in the center of the porphyrin rings and then modified the MOF using ZnCl2 and CoCl26 H2O to obtain bimetal-functionalized PCN-224−ZnCo. Then, the bimetallic MOF was pyrolyzed at 950 °C to volatilize Zn (pictured), which partly frees up the basic nitrogen sites, produces activity-promoting defects, and increases the porosity.

The team found that the catalyst after pyrolysis contains well-dispersed Co2O3 nanoclusters in an N-doped porous carbon matrix. The acidic/basic site ratio of the catalyst can be tuned by varying the Zn/Co ratio in the precursor. The team found that a catalyst based on a PCN-224−ZnCo precursor with a Zn2+/Co2+ ratio of 1 :1 has suitable properties for carbon dioxide absorption and the efficient promotion of cycloadditions of CO2 with epoxides.



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