RESUMO
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 [LnIII(NH3-Glu)(ox)]·2H2O, where LnIII = LaIII (I), PrIII (II), NdIII (III), SmIII (IV), EuIII (V), GdIII (VI), TbIII (VII), and DyIII (VIII), NH3-Glu- = NH3+ containing glutamate, and ox2- = 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.
RESUMO
Inspired by the catalytic potential of lanthanide coordination polymers of 3,3',5,5'-azobenzenetetracarboxylic acid (H4abtc), two new isostructural [Ln2III(Habtc)2(DMSO)4]·DMSO·H2O (LnIII = SmIII (I), EuIII = (II), DMSO = dimethyl sulfoxide) were synthesized and characterized. Their single-crystal structures were elucidated and described. Structural transformations of II in the solid state prompted by ligand substitution and thermal treatment were studied, from which genuine reversible transformation of II to [EuIII(Habtc)(H2O)4]·3H2O (II') and [EuIII(Habtc)(H2O)2]·2H2O (IIâ³) was revealed. This illustrates the rare case of reversible transformation in lanthanide coordination polymers. The transformation between II' and IIâ³ was also investigated. Structural transformations among these frameworks are discussed with regard to the coordination environment of EuIII, coordination modes of Habtc3-, and similarities and disparities in framework architecture and registration. In addition, the catalytic performance of II with and without the prior activation in CO2 cycloaddition reaction with epichlorohydrin was studied in comparison with II' and IIâ³. The excellent performance of II disregarding the activation process has been demonstrated with the maximum turnover number and turnover frequency of 7682 and 1921 h-1, respectively, for the activated II and 7142 and 1786 h-1, respectively, for the nonactivated II. The maintenance of the catalytic efficiency over 10 cycles of the catalysis and the regeneration process is illustrated and discussed with respect to structural transformation.
