Leaching in Specific Facets of ZIF-67 and ZIF-L Zeolitic Imidazolate Frameworks During the CO2 Cycloaddition with Epichlorohydrin.

Zeolitic imidazolate frameworks (ZIFs) have been profusely used as catalysts for inserting CO2 into organic epoxides (i.e., epichlorohydrin) through cycloaddition. Here, we demonstrate that these materials suffer from irreversible degradation by leaching. To prove this, we performed the reactions and analyzed the final reaction mixtures by elemental analysis and the resulting materials by different microscopies. We found that the difference in catalytic activity between three ZIF-67 and one ZIF-L catalysts was related to the rate at which the materials degraded. Particularly, the {100} facet leaches faster than the others, regardless of the material used. The catalytic activity strongly depended on the amount of leached elements in the liquid phase since these species are extremely active. Our work points to the instability of these materials under relevant reaction conditions and the necessity of additional treatments to improve their stability.

Post-Synthetic Surface Modification of Metal-Organic Frameworks and Their Potential Applications.

Metal-organic frameworks (MOFs) are porous hybrid materials with countless potential applications. Most of these rely on their porous structure, tunable composition, and the possibility of incorporating and expanding their functions. Although functionalization of the inner surface of MOF crystals has received considerable attention in recent years, methods to functionalize selectively the outer crystal surface of MOFs are developed to a lesser extent, despite their importance. This article summarizes different types of post-synthetic modifications and possible applications of modified materials such as: catalysis, adsorption, drug delivery, mixed matrix membranes, and stabilization of porous liquids.

Valorization of CO2 through the Synthesis of Cyclic Carbonates Catalyzed by ZIFs.

One way to exploit CO2 is to use it as a feedstock for the production of cyclic carbonates via its reaction with organic epoxides. As far as we know, there is still no heterogeneous catalyst that accelerates the reaction in a selective, efficient and industrially usable way. Cobalt and zinc-based zeolitic imidazole frameworks (ZIFs) have been explored as heterogeneous catalysts for this reaction. In particular, we have prepared ZIF-8 and ZIF-67 catalysts, which have been modified by partial replacement of 2-methylimidazole by 1,2,4-triazole, in order to introduce uncoordinated nitrogen groups with the metal. The catalysts have shown very good catalytic performance, within the best of the heterogeneous catalysts tested in the cycloaddition of CO2 with epichlorohydrin. The catalytic activity is due ultimately to defects on the outer surface of the crystal, and varies in the order of ZIF-67-m > ZIF-67 > ZiF-8-m = ZIF-8. Notably, reactions take place under mild reaction conditions and without the use of co-catalysts.

Effect of the Synthesis Conditions of MIL-100(Fe) on Its Catalytic Properties and Stability under Reaction Conditions.

MIL-100(Fe) is a metal-organic framework (MOF) characterized by the presence of Lewis acid and Fe(II/III) redox sites. In this work, different synthesis methods for the preparation of MIL-100(Fe) are studied. Depending on the source of fluorine, different phases can be obtained: MIL-100(Fe) and an Fe trimesate with unknown structure which we call Fe(BTC). These materials were characterized using numerous techniques and applied in the reaction of CO2 cycloaddition with epichlorohydrin, a reaction catalyzed by Lewis acid sites. It was observed that samples with more Fe(BTC) phase were more active in the reaction. However, all samples, under reaction conditions, transformed into a less active phase.

New Generation of MOF-Monoliths Based on Metal Foams.

Herein, it has been developed a method to prepare metallic foams starting from Zamak5 (ZnAlCu alloy) with different pore sizes. The Zamak5 metallic foam is designed to serve as a support and metallic precursor of ZIF-8. In this way, composite materials MOF-metal can be prepared, these composites have a large number of application in energy exchange processe such as: adsorption or chemical reactions. Additionally, this method of sythesizing MOFs is environmentally friendly thanks to absence of solvents. Hanerssing the low melting point of the linker, the linker is infiltrated into the foam where the foam and the linker react to form the ZIF-8. In this way we have managed to transform part of the foam into ZIF-8 crystals that remain adhered to the foam. The foams have been characterized and modeled studying the mechanical and electrical properties, finding that both can be predected by various models. Among these, Ashby and Mortensen models for mechanical properties and Ashby and Percolation model for electrical properties stand.