Hydrogenation with metal oxide catalysts

Komarewsky, V.I. Miller, D. Hydrogenation with metal oxide catalysts. Adv. Catal. 1957, 9,707-715. 

Dehydrogenation is the reverse reaction of hydrogenation. It is preferably carried out with metal oxide catalysts, but metal catalysts are also used at low temperatures since they favor the hydrogenolysis of C-C bonds. 

The hydrogen-bonding interactions within the complexes W2CI4-(y-OR)2(OR)2(ROH)2 and V Cli y-OR)2(ORT)2(Rf0H)2 may provide the molecular analogues with which to model the structure and reactivities of transition metal oxide catalysts that possess surface hydroxyl groups. The thermal treatment which is often carried out in the pretreatment of metal oxides (leading to the loss of -OH... 

As catalysis proceeds at the surface, a catalyst should preferably consist of small particles with a high fraction of surface atoms. This is often achieved by dispersing particles on porous supports such as silica, alumina, titania or carbon (see Fig. 1.2). Unsupported catalysts are also in use. The iron catalysts for ammonia synthesis and CO hydrogenation (the Fischer-Tropsch synthesis) or the mixed metal oxide catalysts for production of acrylonitrile from propylene and ammonia form examples. 

The hydrogenation of 1,3-butadiene over metal oxide catalysts with D2 or H2 gives rise to predominantly 2-butene, whereas conventional metal catalysts give rise to 1-butene as the main product. Explain the differences in the product distribution in light of the reaction mechanisms. 

Furans can be converted into N- alkylpyrroles by heating with primary amines and alumina. Similar thermal conversions of furans and benzo[6]furans to their sulfur analogues in the presence of alumina or other metal oxide catalysts and hydrogen sulfide are also known. l,3-Diphenylbenzo[c]furan is converted into the thiophene by heating with phosphorus pentasulfide. The mechanism of these reactions is obscure. 

Interestingly, the first examples of catalytic oxygen transfer involved the so-called Milas reagents,4 formed by mixing heterogeneous metal oxide catalysts with a solution of hydrogen peroxide in tert-butanol. These reagents were used for the vicinal hydroxylation of olefins (reaction 10). 

In the second step, the synthesis gas and additional steam are passed over a metal oxide catalyst at about 400°C. Under these conditions, the carbon monoxide component of the synthesis gas and the steam are converted to carbon dioxide and more hydrogen. This reaction of CO with H20 is called the water-gas shift reaction because it shifts the composition of synthesis gas by removing the toxic carbon monoxide and producing more of the economically important hydrogen ... 

As discussed in Section 10.1, asymmetric epoxidation of C=C double bonds usually requires electrophilic oxygen donors such as dioxiranes or oxaziridinium ions. The oxidants typically used for enone epoxidation are, on the other hand, nucleophilic in nature. A prominent example is the well-known Weitz-Scheffer epoxidation using alkaline hydrogen peroxide or hydroperoxides in the presence of base. Asymmetric epoxidation of enones and enoates has been achieved both with metal-containing catalysts and with metal-free systems [52-55]. In the (metal-based) approaches of Enders [56, 57], Jackson [58, 59], and Shibasaki [60, 61] enantiomeric excesses > 90% have been achieved for a variety of substrate classes. In this field, however, the same is also true for metal-free catalysts. Chiral dioxiranes will be discussed in Section 10.2.1, peptide catalysts in Section 10.2.2, and phase-transfer catalysts in Section 10.2.3. 

In the steam-reforming process (Fig. 1), the hydrocarbon feedstock is first desulfurized by heating to 370°C in the presence of a metallic oxide catalyst that converts the organosulfur compounds to hydrogen sulfide. Elemental sulfur can also be removed with activated carbon absorption. A caustic soda scrubber removes the hydrogen sulfide by salt formation in the basic aqueous solution. 

Metal and metal oxide catalysts with this capability were reported in the 1980s. Several of these metals occur in widely distributed petroleum samples and are among the above listed as constituents of primordial Earth planetary orbit dust. In the metallic and compounded state some have the ability to catalyze hydrogenation of carbon to kerogen like high viscosity hydrocarbons. Kerogen, a heavier petroleumlike hydrocarbon mixture occurs in tar sands and porous shales (oil shale). Thus the question of conceivable prehistoric or more recent petroleum from methane generation must be considered. 

PBS and PCS have both been shown to be effective reagents for the oxidation of organic species. It is therefore likely that their employment with metal-centred catalysts, in place of hydrogen peroxide, will grow in the future. 

Heat-resistant ABS resins can also be produced by polymerizing styrene with modified nitriles, such as fumaronitrile and maleonitrile. Fumaronitrile can be produced from acrylonitrile (Figure 15.7) in a two-step process involving the addition of hydrogen cyanide followed by oxydehydrogenation over metal oxide catalysts. The compatibility of styrene-co-acrylonitrile-co-fumaronitrile (SANF) terpolymers and SAN has been studied [75]. High-gloss, heat-resistant... 

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