Hydrogenation over metal oxides

Some trends in the study of carbon monoxide and carbon dioxide hydrogenations over metal oxides and metal-metal oxide systems were examined in this chapter. These reactions usually take place over multicomponent and often multiphase catalyst systems with bifunctional sites, and produce HC and alcohols. The selectivity in these reactions is basically determined by the nature of the catalytic function, which determines largely the interaction of carbon oxides and hydrogen on the catalyst surface. CO binds to coordinatively unsaturated Zn + sites and acts as a... 

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. 

Pyridazino[l,2-pyridazine derivatives are easily reduced with either hydrogen over a catalyst or with metal hydrides. Reaction of the pyridazino[l,2- ]pyridazine-l,4,6,9-tetrone (44) with hydrogen over platinum oxide gives the hydrogenated product (45) (66JOC1311). This product can be further reduced with lithium aluminum hydride to give the octahy-dropyridazino[l,2- ]pyridazine (24) (67JA4875). 

Chemoselective reduction of conjugated enones to allylic alcohols via hydrogen transfer from propan-2-ol over metal oxides is investigated in vapour phase conditions. The unique ability of Mgo to reduce exclusively carbonyl group is observed. However, because of the high basicity of MgO side reactions are present. It is shown that by doping the Mgo catalyst with HC1 a significant decrease of its basicity occurs and consequently side reactions are minimized. 

Fatty alcohols are obtained by direct hydrogenation of fatty acids or by hydrogenation of fatty acid esters. Typically, this is performed over copper catalysts at elevated temperature (170°C-270°C) and pressure (40-300 bar hydrogen) [26], By this route, completely saturated fatty alcohols are produced. In the past, unsaturated fatty alcohols were produced via hydrolysis of whale oil (a natural wax occurring in whale blubber) or by reduction of waxes with sodium (Bouveault-Blanc reduction). Today, they can be obtained by selective hydrogenation at even higher temperatures (250°C-280°C), but lower pressure up to 25 bar over metal oxides (zinc oxide, chromium oxide, iron oxide, or cadmium oxide) or partially deactivated copper chromite catalysts [26],... 

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... 

The partial oxidation of methanol to formaldehyde over metal oxides is important industrially, and the results of experiments done in the transient regime are summarized for M0O3 (249). The results in Fig. 41 show that the conversion of methanol and the selectivity toward formaldehyde in a Mars-van Krevelen process are both favored by a relatively oxidized surface. Weber (250) has shown by theoretical calculations that the surface methoxy intermediate should lose one of its hydrogens as a hydride ion attracted to Mo and not as a hydrogen atom or proton attracted to a doublely bonded or bridging oxygen. 

BD undergoes over metal oxide catalysts [9]. A characteristic feature of this type of hydrogenation is 1,4-addition of two H atoms, i.e. addition of two H at terminal position of 1,3-BD molecule. 1,2-Addition was found when the hydrogenation was carried out over ZrOz catalyst by using a hydrogen donor molecule, cyclohexadiene, instead of Hz [9]. Unlike to the hydrogenation by Hz over oxide catalysts, Si-H added to the 1,2-position of 1,3-BD to yield (1) and no product from... 

For the last two decades, attention has been focused on redressing the ozone depletion in the earth s protective layer. It is believed that chlorine radicals dissociated from chlorofluorocarbons (CFCs), upon irradiation of sun s UV in the stratosphere, promotes the ozone depletion. Hence, in addition to development of CFC alternatives there is an urgent need for the safe disposal of CFCs. Several processes such as pyrolysis, incineration, photocatalysis, oxidative destruction over metal oxide or zeolite catalysts and destruction at very high temperatures ( by plasma technique ) are reported in the literature for the disposal of CFCs[ 1-5]. But all these processes yield harmful products like CO, HF/F2 etc. Catalytic conversion of chlorinated organics in presence of hydrogen seems to be a better technique as it yields either hydrofluorocarbons(HFCs) or hydrochlorofluorocarbons(HCFCs) whose ozone depletion potential is either zero or very low and yet most of these products act as CFC alternatives. 

Table 1. Hydrogenation of benzoic acid over metal oxide catalysts.

The kinetics of methanol oxidation over metal oxide catalysts were elegantly derived by Holstein and Machiels [16], The kinetic analysis demonstrated that the dissociative adsorption of water must be included to obtain an accurate kinetic model. The reaction mechanism can be represented by three kinetic steps equilibrated dissociative adsorption of methanol to a surface methoxy and surface hydroxyl (represented by K,), equilibrated dissociative adsorption of water to two surface hydroxyls (represented by K ), and the irreversible hydrogen abstraction of the surface methoxy intermediate to the formaldehyde product and a surface hydroxyl (the rate determining step, represented by kj). For the case of a fully oxidized surface, the following kinetic expression was derived ... 

Liljenroth (1918) was the first to describe a nonlinear phenomenon in heterogeneous catalysis. He studied the platinum-catalyzed oxidation of ammonia and analyzed the stabdify of the steady state of this process. Apparently, Davies (1934) was the first to systematically observe nonlinear phenomena and to point out the chemical nature of the rate hysteresis. In the 1950s, Boreskov and colleagues (Boreskov et al., 1953 Kharkovskaya et al., 1959) studied the oxidation of hydrogen over metal catalysts and found that in a certain range of reaction parameters, very different values of the steady-state reaction rate corresponded with the same gas composition. These data were obtained at isothermal conditions. For more historical information on multiplicity of steady states, see Yablonskii et al. (1991) and Marin and Yablonsky (2011). 

Improvements in acrylonitrile yield are also reported with other vapor phase promoters. A patent assigned to Monsanto Co. (125) describes the use of sulfur and sulfur-containing compounds in the feed gas mixture for production of acrylonitrile or methacrylonitrile from propane or isobutane over metal oxide catalysts. Examples of effective sulfur-containing compounds include alkyl or dialkyl sulfides, mercaptans, hydrogen sulfide, ammonium sulfide, and sulfiir dioxide. Best results are apparently achieved using a molar ratio of sulfur (or sulfur compound) to hydrocarbon of 0.0005 1 to 0.01 1. Nitric oxide has also been examined as a gas-phase promoter for propane and isobutane ammoxidation (126). However, it does not appear to be as effective as halogen or sulfur. Selectivities to acrylonitrile from propane are only about 30% over an alumina-supported chromium-nickel oxide catalyst. 

Ammonia is produced by the Haber-Bosch process. A hydrogen/nitrogen mixture, obtained from steam treatment of natural gas, is purified and passed over metallic oxide catalyst under pressure. The gases react at temperatures above 450°C forming ammonia gas which is absorbed in water to form ammonium hydroxide. 

Chia, M., Dumesic, J. A., 2011. Liquid-phase catalytic transfer hydrogenation and cycUzation of levulinic acid and its esters to y-valerolactone over metal oxide catalysts. Chemical Communications 47, 12233—12235. 

Formaldehyde is readily reduced to methanol by hydrogen over many metal and metal oxide catalysts. It is oxidized to formic acid or carbon dioxide and water. The Cannizzaro reaction gives formic acid and methanol. Similarly, a vapor-phase Tischenko reaction is catalyzed by copper (34) and boric acid (38) to produce methyl formate ... 

It is not affected by halogens or acids, except for phosphoric and hydrofluoric acids. Phosphoric acid attacks fused silica at temperatures of 300-400°C, and hydrofluoric acid attacks it at room temperature, forming silicon tetrafluoride and water. At high temperatures silica reacts with caustic alkalis, certain metallic oxides, and some basic salts, and cannot be used for incinerating these materials. Over 1600°C, fused silica is reduced to silicon by carbon. It can also be reduced at high temperature by hydrogen. It is unaffected by water under normal conditions but is attacked by strong solutions of alkalis. 

According to the Ellingham diagram of oxides, water or steam (H20) is stabler than many metal oxides over a wide and useful range of temperatures, and hydrogen can reduce many metal oxides by reactions of the type... 

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