Alcohol oxidation, MOFs

The attractive (80) features of MOFs and similar materials noted above for catalytic applications have led to a few reports of catalysis by these systems (81-89), but to date the great majority of MOF applications have addressed selective sorption and separation of gases (54-57,59,80,90-94). Most of the MOF catalytic applications have involved hydrolytic processes and several have involved enantioselec-tive processes. Prior to our work, there were only two or three reports of selective oxidation processes catalyzed by MOFs. Nguyen and Hupp reported an MOF with chiral covalently incorporated (salen)Mn units that catalyzes asymmetric epoxidation by iodosylarenes (95), and in a very recent study, Corma and co-workers reported aerobic alcohol oxidation, but no mechanistic studies or discussion was provided (89). 

Scheme 8 Schematic for the formation of Ru MOF-5 and its applications on alcohol oxidation...

A Pd-MOF reported by Corma [60] was also found to be active in the partial oxidation of alcohols using air to oxidize 3-phenyl-2-propen-l-ol (cinnamyl alcohol). Ciimamyl alcohol is a suitable substrate to probe the activity and chemoselectivity of a catalyst for the aerobic alcohol oxidation. With Pd-MOF as catalyst and ambient pressure air as the oxidant, total conversion of ciimamyl alcohol was observed after... 

Mixed-valence Ru"-Ru" paddlewheel carboxylate complexes also have potential for oxidation reactions after incorporation in a microporous lattice with porphyrinic ligands. This MOF can be used for oxidation of alcohols and for hydrogenation of ethylene. Both the porosity of the lattice and the abihty of the diruthenium centers to chemisorb dioxygen are essential for the performance of the catalyst [62, 64]. 

T.T. Tidwell, Oxidation of alcohols to carbonyl compounds via alkoxysulfonium ylides The Mof-fatt, Swern and related reactions, Org. React 1990, 39, 297-572. 

To account for these observations, it has been proposed that hydroperoxy radicals are produced during the oxidation of alcohols containing an a-hydrogen [22-26]. These radicals have a high termination rate constant (>10 L moF s the reaction is said to be diffusion-controlled [14, 27, 28]) they remove radicals from the system rapidly (eq. (9)) ... 

Nonactivated secondary alcohols were oxidized to the corresponding ketones with initial TOFs up to 100 mol mol h . Even less water-soluble and less reactive alcohols like 2-octanol could be oxidized with rates up to 20 mol moF h . Primary alcohols were oxidized to the corresponding acids. By adding TEMPO (2,2,6,6-tetramethylpiperdinyl-l-oxyl) the intermediate aldehyde could be trapped. As catalyst, the Pd complex of bathophenanthroline disulfonate (Structure 1) was used (bathophenanthroline is commercially available at approx. US 300/5 g). 

Hexavalent. Uranium hexafluoride, UFe, is one of the best-studied uranium compounds in existence due to its importance for uranium isotope separation and large-scale production ( 70 000 tons per year). All of the actinide hexafluorides are extremely corrosive white (U), orange (Np), or dark brown (Pu) crystalline solids, which sublime with ease at room temperature and atmospheric pressure. The synthetic routes into the hexafluorides are given in equation (13). The volatility of the hexafluorides increases in the order Pu < Np < U in the liquid state and Pu < U < Np in the solid state. UFg is soluble in H2O, CCI4, and other chlorinated hydrocarbons, is insoluble in CS2, and decomposes in alcohols and ethers. The oxidative power of the actinide hexafluorides are in line with the transition metal hexafluorides and the order of reactivity is as follows PuFs > NpFg > UFs > MoFs > WFg. The UFs molecule can also react with metal fluorides to form UFy and UFg. The same reactivity is not observed for the Np and Pu analogs. 

With a similar approach, Fischer and coworkers entrapped Ru nanoparticles inside porous [Zn40(BDC)3] (MOF-5) by hydrogenolysis of the adsorbed volatile ruthenium species [Ru(COD)(COT)] (COD — 1,5-cyclooctadiene, COT = 1,3,5-cyclooctatriene) [17]. The included Ru nanoparticles had a size range of 1.5-1.7 nm, and the intact framework of MOF-5 was cmifirmed by different spectroscopic methods. The resulting solid Ru MOF-5 was tested for oxidation of benzyl alcohol however, only a modest conversion of 25% to benzyl aldehyde was obtained, and the XRD revealed the breakdown of the structure of MOF-5 as well as the loss of framework porosity. In contrast, the crystallinity of Ru(S>MOF-5 remained when it was used to catalyze the hydrogenation of benzene to cyclohexane with 25% conversion under 3 bar H2 at 75°C (Scheme 8). 

Postsynthetically modified frameworks were also used as heterogeneous catalyst for the same reaction. Proch et al. [104] loaded MOF-177 with platinum nanoparticles via gas-phase infiltration of volatile [MejPtCp ] precursor under reduced pressure followed by reduction under hydrogen pressure. While this material efficiently catalyzes the oxidation of benzylic and allylic alcohols under solvent and base-free conditions at room temperature, it could not be recycled because of stability issues. 

Magnetically separable catalysts synthesised by Saikia that show effective utilisation of Fe304 impregnated chromium-based MOF have been used for the solvent-free oxidation of benzyl alcohol in the presence of TBHP. Sulfonic acid functionalised chromium MILlOl can be used as an efficient catalyst for the vapour-phase dehydration of butanol by oleic acid. Also, bimetallic MOF catalysts of iron and chromium (Fe(Cr)-MIL-lOl) have received attention for epoxidation of styrene to styrene oxide with excellent selectivity and catalytic recyclability. ... 

The introduction of Ru nanoparticles with retaining the MOF of MOF-5 was carried out in Ref. [120]. After the introduction of Ru in the form of [Ru(cod)(cot)]3j MOF-5 into the framework, hydrogenolysis is carried out, which results in the formation of Ru nanoparticles inside the cavities with the formation of the Ru MOF-5 material. This catalytic system shows a moderate activity in the oxidation of benzyl alcohol to benzaldehyde (Reaction 5) ... 

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