Transition-metal oxides methanol synthesis

In this section we describe INS studies of molybdenum trioxide, a precmsor of molybdenum disulfide catalysts ( 7.5), and transition metal oxides which catalyse complete or partial oxidation of hydrocarbons, and copper zinc oxide catalysts, which catalyse methanol synthesis from carbon monoxide and dihydrogen ( 7.3.3). 

The performance of many metal-ion catalysts can be enhanced by doping with cesium compounds. This is a result both of the low ionization potential of cesium and its abiUty to stabilize high oxidation states of transition-metal oxo anions (50). Catalyst doping is one of the principal commercial uses of cesium. Cesium is a more powerflil oxidant than potassium, which it can replace. The amount of replacement is often a matter of economic benefit. Cesium-doped catalysts are used for the production of styrene monomer from ethyl benzene at metal oxide contacts or from toluene and methanol as Cs-exchanged zeofltes ethylene oxide ammonoxidation, acrolein (methacrolein) acryflc acid (methacrylic acid) methyl methacrylate monomer methanol phthahc anhydride anthraquinone various olefins chlorinations in low pressure ammonia synthesis and in the conversion of SO2 to SO in sulfuric acid production. 

Besides supported (transition) metal catalysts, structure sensitivity can also be observed with bare (oxidic) support materials, too. In 2003, Hinrichsen et al. [39] investigated methanol synthesis at 30 bar and 300 °C over differently prepared zinc oxides, namely by precipitation, coprecipitation with alumina, and thermolysis of zinc siloxide precursor. Particle sizes, as determined by N2 physisorpt-ion and XRD, varied from 261 nm for a commercial material to 7.0 nm for the thermolytically obtained material. Plotting the areal rates against BET surface areas (Figure 3) reveals enhanced activity for the low surface area zinc... 

There are several metal-oxide combinations other than the Cu/ZnO that have been reported active in methanol synthesis at low temperatures and pressures. These can be divided into catalysts containing copper and an oxide and catalysts containing a transition metal and an oxide. 

Mankind has produced acetic acid for many thousand years but the traditional and green fermentation methods cannot provide the large amounts of acetic acid that are required by today s society. As early as 1960 a 100% atom efficient cobalt-catalyzed industrial synthesis of acetic acid was introduced by BASF, shortly afterwards followed by the Monsanto rhodium-catalyzed low-pressure acetic acid process (Scheme 5.36) the name explains one of the advantages of the rhodium-catalyzed process over the cobalt-catalyzed one [61, 67]. These processes are rather similar and consist of two catalytic cycles. An activation of methanol as methyl iodide, which is catalytic, since the HI is recaptured by hydrolysis of acetyl iodide to the final product after its release from the transition metal catalyst, starts the process. The transition metal catalyst reacts with methyl iodide in an oxidative addition, then catalyzes the carbonylation via a migration of the methyl group, the "insertion reaction". Subsequent reductive elimination releases the acetyl iodide. While both processes are, on paper, 100%... 

In this context, rare earths on transition metal substrates attracted considerable research attention from two directions i) to understand the overlayer growth mechanisms involved [3] and ii) to prepare oxide-supported metal catalysts from bimetallic alloy precursor compounds grown in situ on the surface of a specific substrate [4,5]. The later studies are especially significant in terms of understanding the chemistry and catalytic properties of rare earth systems which are increasingly used in methanol synthesis, ammonia synthesis etc. In this paper, we shall examine the mechanism of Sm overlayer and alloy formation with Ru and their chemisorption properties using CO as a probe molecule. 

The chemistry of acetate on transition metal surfaces is important for a variety of selective oxidation processes. Methanol and vinyl acetate syntheses are two such important oxidation chemistries where acetate intermediates have been postulated. In VAM synthesis, acetate is a critical intermediate in both VAM formation, as well as in its decomposition to CO2. The latter unselective decarboxylation path becomes important at higher operating temperatures. Understanding the mechanism for decarboxylation and VAM synthesis may ultimately aid in the design of new catalyst formulations on new operating conditions. 

As described here, it is understandable that the activity and selectivity of methanol synthesis over Cu-based catalysts depend upon not only the Cu dispersion but also the starting salts for preparation and the residual ions on the catalysts and their effects on the structure of active sites. As the candidate of new catalyst system for methanol synthesis, the following catalysts have been vigorously studied in recent years (1) Cu-ZnO containing new components (2) precious metal such as Pd, Pt, Au, and multicomponent catalysts containing precious metals (3) transition metal except for Cu and their complex oxides (4) homogeneous catalysts consisting of Ru, Ni, etc. 

Interest in new compositions and new synthetic routes in the context of catalysis is growing, and recent examples of the synthesis and use of non-oxidic, ceramic compositions in catalysis include SiC as a support for Ni or Pt in CO hydrogenation (2), SiC as a support for Co and Mo for thiophene hydrodesulphurisation (3), transition metal (Ti, Ta, Mo or W) carbides for methanol decomposition (4), early transition metal carbides, nitrides or borides for hydrodenitrogenation of quinoline (5), and the synthesis of high surface area molybdenum carbide (6). 

The next question to be asked concerns the composition of the particles. For the case of monometallic particles this could in principal be a trivial question. If the particle is known to contain a single element, the only question which then arises concerns the oxidation states of the metal, and this can be determined by X-ray photoelectron spectroscopy. For example, the colloidal metals described in Section 6.2.2.1 and prepared by Dye and coworkers by alkalide and electride reduction of salts of gold, copper, platinum, nickel, and molybdenum (as well as several main group metals and metalloids) were analyzed by XPS [73] which showed the presence of only zerovalent metal. Oxidized metal was detected only for nickel and molybdenum (among the transition metals) and this only after exposure to an oxidizing solvent such as methanol. These results show that if a sufficiently powerful reducing agent is used in the colloid synthesis, the surface of the particles can be kept in a reduced metallic state. 

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