Polymorphism and Its Influence on Catalytic Activities of Lanthanide-Glutamate-Oxalate Coordination Polymers

To study the relationship between polymorphism and catalytic activities of lanthanide coordination polymers in the cycloaddition reactions of CO₂ with epoxides, the monoclinic and triclinic polymorphs of [Ln^III(NH₃-Glu)(ox)]·2H₂O, where Ln^III = La^III (I), Pr^III (II), Nd^III (III), Sm^III (IV), Eu^III (V), Gd^III (VI), Tb^III (VII), and Dy^III (VIII), NH₃-Glu^- = NH₃⁺ containing glutamate, and ox^2- = oxalate, were synthesized and characterized. Factors determining polymorphic preference, the discrepancy between the two polymorphic framework structures, potential acidic and basic sites, thermal and chemical stabilities, active surface areas, void volumes, CO₂ sorption/desorption isotherms, and temperature-programmed desorption of NH₃ and CO₂ are comparatively presented. Based on the cycloaddition of CO₂ with epichlorohydrin in the presence of tetrabutylammonium bromide under solvent-free conditions and ambient pressure, catalytic activities of the two polymorphs were evaluated, and the relationship between polymorphism and catalytic performances has been established. Better performances of the monoclinic catalysts have been revealed and rationalized. In addition, the scope of monosubstituted epoxides was experimented and the outstanding performance of the monoclinic catalyst in the cycloaddition reaction of CO₂ with allyl glycidyl ether under ambient pressure has been disclosed.