Stability of MOFs

MOFs can be considered as organic zeolite analogs, as their pore architectures are often reminiscent of those of zeolites a comparison of the physical properties of a series of MOFs and of zeolite NaY has been provided in Table 4.1. Although such coordinative bonds are obviously weaker than the strong covalent Si-O and Al-O bonds in zeolites, the stability of MOF lattices is remarkable, especially when their mainly organic composition is taken into account. Thermal decomposition generally does not start at temperatures below 300 °C [3, 21], and, in some cases. 

This indicates that post-synthesis modification of MOFs by coordinating hydrophobic ligands to unsaturated metal sites may be a powerful method to generate new sorbents for gas separation under humid conditions. Amine functionalization to target the water stability of MOFs will be further discussed in the next section. 

By considering the results of previous experiment, they synthesized cobalt-, nickel-, copper-, and zinc-based, new pillared MOFs of similar topologies which exhibited good water stability [233]. The grafted methyl group on the benzene dicarboxlate (BDC) ligand introduced steric factors around the metal centers consequently, water stability of MOF drastically improved. 

All these studies demonstrated that water stability of MOFs can be improved by incorporating specific factors (e.g., metal-ligand strength, thermodynamic and kinetic factors, etc.) which govern the structural stability of the framework. 

From natural zeolites to the recently discovered meso- and macro-porous materials, the ordered porous frameworks are all constructed by inorganic species. However, in the past ten years, a new family of porous compounds composed of metal-organic frameworks (MOFs) has attracted enormous attention. The main reason is that the poor thermal and chemical stability of MOFs has been somewhat improved. In addition, the discovery of some advantages of MOFs that are lacking in molecular sieves and mesoporous materials has also stimulated the research on MOFs. 

Here, BDC is 1,4-benzene-dicarboxylate (terephthalic acid) and Zn40(BDC)3 represents the MOF-5 imit composition. The reaction equilibrium can be shifted toward formation of the MOF product by tuning the concentration profiles of the solvent, the water released, or the nitrate ions produced. Since esterification reactions can be driven in both directions without difficulty, it appears evident that the stability of MOF materials in applications could depend on polar protic environments and pH values (128). 

The previous comments combined with the relatively low thermal stability of MOFs with respect to zeolites and other porous inorganic materials used as catalysts have led to the general assumption that MOFs are instable catalysts, this instability limiting the real applicability of these materials in catalysis. 

In the following sections, we discuss a number of important achievements in catalysis and separation using MOFs, without trying to be exhaustive. Wherever possible, stability and reusability of MOFs are assessed as well, and their performance is compared to that of homogeneous or heterogeneous reference systems. 

Chiral MOFs have to cope with two kinds of challenges when they are used as catalysts. First, in those cases where chirality is induced via sp carbon centers, the incorporation of such centers may confer a degree of flexibility to the framework that eventually impairs the stability after desolvation. Such problems can be circumvented, for instance, using mixed ligand systems where a rigid nonchiral backbone ensures the stability of the structure [82], MOFs with large and flexible chiral ligands... 

For future studies on MOF-based slurry systems, there is basic selection of criteria that needs to be satisfied by both MOF and the liquid solution. The selection of the MOF possessing the appropriate pore size for the preparation of the slurry system is very important to guarantee that the size of the liquid is large enough and does not occupy the pores which leaves no space for C02 to adsorb. Moreover, the structural stability of the MOF in the aqueous solution is essential so that it does not lose its porous framework nor its surface area. The selection of the liquid candidate is crucial, as it should not provide any extra mass transfer resistance for C02 molecules. Further, experimental and computational investigations are still required to understand the separation mechanism in slurry mixtures and to have insight into the different types of interactions between the gas, liquid, and solid materials. 

Water stability is a major challenge that has to be overcome before metal organic framework can be used in removing carbon dioxide from flue gas. The core structure of MOF reacts with water vapor content in the flue gas leading to severe distortion of the structure and even failure. As a consequence, the physical structure of MOF is changed, e.g., reduction of porosity and surface area, etc. that decreases the capacity and selectivity for C02. Complete dehydration of flue gas increases the cost of separation. It is therefore essential for MOFs to exhibit stability in the presence of water up to certain extent [91]. 

A second area that will be important in the future is the continued development of MOFs and ZlFs [152]. Much as the discovery of AlP04-based materials revolutionized the catalyhc use of zeolites when only aluminosilicates were known, MOFs and ZlFs have the potential to revolutionize low temperature processes such as oxidations and organic reachons [153]. Newly discovered materials along these same lines are covalent organic frameworks, the so-called COFs [154]. These materials have similar channels to those known for MOFs and ZlFs but tend to have higher thermal stability. 

Molecular mechanics calculations (MM2(85)) were employed to rationalize the relationship between stmcture and the equilibrium constants of the thiol-disulfide interconversion (Scheme 6) <1990JA6296>. An excellent correlation (r=0.93) between experimental AG values and calculated differences in strain energy A3 if was obtained, AG = 0.41 kj moF and /. SE = 0.5 kj moF thereby supporting the facile formation and stability of 1,2-dithianes. 

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