Activation of MOFs

Although the structure of the first coordination polymer was reported in 1959, it was 1989 when the structure of another similar material was reported by Hoskins and Robson. The first report of gas adsorption studies on a porous MOF was published in 1997. The stability,purity and activation procedures for MOFs are sometimes issues resulting in difficulties in obtaining repeatable adsorption results. Comparison of X-ray powder diffraction data with single crystal data allows the purity of MOF samples prior to activation to be confirmed. Ideally, the crystalline structure of the porous MOF material formed by the activation procedure is needed to establish that the framework is intact. 

It was proposed that supercritical carbon dioxide extraction inhibits mesopore collapse leaving micropores accessible to gases. It is apparent that supercritical carbon dioxide drying has advantages in activating the porous structures of MOFs. 

The removal of coordinated solvent may lead to the formation of unsaturated or open metal centres, which are high energy sites for adsorption. Therefore, thermal and chemical stability of desolvated MOF porous material structures and activation procedures are important considerations when comparing materials. 

Pores are classified by the International Union of Pure and Applied Chemistry (lUPAC) by pore size as micropores ( 2 nm), mesopores (2-50 nm) and macropores ( 50 nm). Micropores are sometimes divided into ultramicropores ( 0.7 nm) and supermicropores (1.4—2.0 nm). The terms nanopore and nanoporosity are not defined precisely but refer to nanometre-sized pores. Characterisation of the porous structures of materials is difficult because some MOF materials are flexible. A variety of isotherm equations and adsorptives have been used to characterise porous structures using gas adsorption techniques. Porous structures are characterised by surface areas [determined using Langmuir, Bmnauer-Emmett-Teller (BET), Dubinin-Radushkevich (DR), etc., equations], pore volumes [total, micropore (DR), etc.] and pore size distributions. 

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Studies of the catalytic activity of MOFs are in their infancy with some encouraging results emerging in enantioselective catalysis. By contrast, meso-porous solids have already been studied extensively as catalytic supports, particularly of complexes too large to be encapsulated in zeolites. One of the most significant developments in this area is the observation that the constrained encapsulation of chiral catalysts in mesopores can raise the enantioselectivities of reactions well above those observed when the reaction is performed homogeneously. 

It should be noted that one important issue during MOFs preparing is the activation of MOFs after synthesis. Solvents used during synthesis usually remain in the pores of the materials, and activation by heating is usually required to remove these solvent molecules. Studies have shown that activation at elevated temperatures can cause sample decomposition, whereas activation at lower temperatures greatly minimizes the danger of redncing metal ions (Liu et al. 2007). 

From an environmental point of view, MOFs could be considered as improved catalytic systems in comparison with the homogeneous analogues, the transition metal catalysts. In this context, Dhakshinamoorthy et al. reviewed the catalytic activity of MOFs involved in the most relevant acid-base-catalyzed transformations useful in fine chemistry [25-27]. 

It was shown that there was the true heterogeneous catalysis in this case, since no leaching of palladium into the reaction mixture took place. A decrease in the activity of MOF-5 supported palladium in repeated cataljdic cycles was not observed, but a complete loss of the pore volume was marked. Consequently, it may be assumed that palladium is not localized inside the micropores and, probably, is coimected with the outer surface of the crystals. 

AcetaUzation of benzaldehyde with trimethyl orthoformate can be carried out with a series of MOFs constructed from In and BDC or BTC ligands with open In sites. The catalysts are even stable in aqueous medium and can be reused without loss of activity. Owing to the small pores of these MOFs, the reaction only takes place at the outer surface of the crystals [54]. In another MOF constructed from In and 4,4 -(hexafluoroisopropylidene)bis(benzoic acid), the same reaction takes place inside the pores [55]. 

Another SBU with open metal sites is the tri-p-oxo carboxylate cluster (see Section 4.2.2 and Figure 4.2). The tri-p-oxo Fe " clusters in MIL-100 are able to catalyze Friedel-Crafts benzylation reactions [44]. The tri-p-oxo Cr " clusters of MIL-101 are active for the cyanosilylation of benzaldehyde. This reaction is a popular test reaction in the MOF Hterature as a probe for catalytic activity an example has already been given above for [Cu3(BTC)2] [15]. In fact, the very first demonstration of the catalytic potential of MOFs had aheady been given in 1994 for a two-dimensional Cd bipyridine lattice that catalyzes the cyanosilylation of aldehydes [56]. A continuation of this work in 2004 for reactions with imines showed that the hydrophobic surroundings of the framework enhance the reaction in comparison with homogeneous Cd(pyridine) complexes [57]. The activity of MIL-lOl(Cr) is much higher than that of the Cd lattices, but in subsequent reaction rans the activity decreases [58]. A MOF with two different types of open Mn sites with pores of 7 and 10 A catalyzes the cyanosilylation of aromatic aldehydes and ketones with a remarkable reactant shape selectivity. This MOF also catalyzes the more demanding Mukaiyama-aldol reaction [59]. 

For more efficient utilization of MOFs sorbents, several hybrid systems based on MOFs with other solid sorbents have been investigated in the literature. The objective of having hybrid materials is to utilize the synergism between the two sorbents and therefore ultimately improve the overall performance in C02 separation. Moreover, sorbents such as activated carbons, graphenes, and CNTs provide the added feature of high surface area and easily functionalized sites which contribute to the tuning of the final properties of the composite... 

Akhtar, A and Becker, P.B. (2000) Activation of transcription through histone H4 acetylation by MOF, an acetyltransferase essential for dosage compensation. Mol. Cell 5, 367-375. 

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