Other Metal Oxide Catalysts

All of the above studies have focused on oxides containing vanadium. While vanadium catalysts are very widely used, and studied, the use of soHd-state NMR is not 

Molybdenum oxide catalysts are widely employed in oxidation, hydrodesulfur-izahon and hydrodenitrogenation reachons. The achve phase is most commonly supported on alumina, although mixed-metal oxides are also used. As for supported vanadium catalysts, both stahc and MAS NMR techniques were widely employed in early studies of such systems [125-129]. Like and Al, Mo is a 

These sites have similar chemical shifts, hence are difficult to resolve by MAS NMR, but have different Cq, allowing them to be distinguished by MQMAS NMR. 

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Of little use commercially except as a route to anthraquinone. For this purpose it is oxidized with acid potassium dichromate solution, or better, by a catalytic air oxidation at 180-280 C, using vanadates or other metal oxide catalysts. 

Other metal oxide catalysts studied for the SCR-NH3 reaction include iron, copper, chromium and manganese oxides supported on various oxides, introduced into zeolite cavities or added to pillared-type clays. Copper catalysts and copper-nickel catalysts, in particular, show some advantages when NO—N02 mixtures are present in the feed and S02 is absent [31b], such as in the case of nitric acid plant tail emissions. The mechanism of NO reduction over copper- and manganese-based catalysts is different from that over vanadia—titania based catalysts. Scheme 1.1 reports the proposed mechanism of SCR-NH3 over Cu-alumina catalysts [31b],... 

Iron molybdate and other metal oxide catalysts... 

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. 

Photo-initiated AOPs are subdivided into VUV and UV oxidation that are operated in a homogeneous phase, and in photocatalysis (Fig. 5-15). The latter can be conducted in a homogeneous aqueous phase (photo-enhanced Fenton reaction) or in a heterogeneous aqueous or gaseous phase (titanium dioxide and certain other metal oxide catalysts). These techniques apply UV-A lamps or solar UV/VIS radiation and they are in pre-pilot or pilot status. According to Mukhetjee and Ray (1999) the development of a viable and practical reactor system for water treatment with heterogeneous photocatalysis on industrial scales has not yet been successfully achieved. This is mainly related to difficulties with the efficient distribution of electromagnetic radiation (UV/VIS) to the phase of the nominal catalyst. 

The most active catalyst is chromium oxide [7]. Silica (Si02) or aluminosilicates (mixed Si02/Al203) are used as the support material. The support is sometimes modified with titania (Ti02). The chromium oxide (Cr Os) catalyst was originally developed by Phillips Petroleum Company and is referred to as Phillips catalyst. Other metal oxide catalysts were developed primarily at Standard Oil of Indiana, the best known among them being the molybdenum oxide (Mo Os) catalyst. 

By far the largest outlet for benzene (approx. 60%) is styrene (phenyl-ethene), produced by the reaction of benzene with ethylene a variety of liquid and gas phase processes, with mineral or Lewis acid catalysts, are used. The ethylbenzene is then dehydrogenated to styrene at 600-650°C over iron or other metal oxide catalysts in over 90% selectivity. Co-production with propylene oxide (section 12.8.2) also requires ethylbenzene, but a route involving the cyclodimerization of 1,3-butadiene to 4-vinyl-(ethenyl-) cyclohexene, for (oxidative) dehydrogenation to styrene, is being developed by both DSM (in Holland) and Dow. 60-70% of all styrene is used for homopolymers, the remainder for co-polymer resins. Other major uses of benzene are cumene (20%, see phenol), cyclohexane (13%) and nitrobenzene (5%). Major outlets for toluene (over 2 5 Mt per annum) are for solvent use and conversion to dinitrotoluene. 

The monomer ethylbenzene is made from petroleum-derived benzene and ethylene. Free radical polymerization is usually carried out in the gas phase with an iron oxide or other metal oxide catalyst yielding PS resin ... 

Oxidation of methanol to formaldehyde with vanadium pentoxide catalyst was first patented in 1921 (90), followed in 1933 by a patent for an iron oxide—molybdenum oxide catalyst (91), which is stiU the choice in the 1990s. Catalysts are improved by modification with small amounts of other metal oxides (92), support on inert carriers (93), and methods of preparation (94,95) and activation (96). In 1952, the first commercial plant using an iron—molybdenum oxide catalyst was put into operation (97). It is estimated that 70% of the new formaldehyde installed capacity is the metal oxide process (98). 

High Density Polyethylene. High density polyethylene (HDPE), 0.94—0.97 g/cm, is a thermoplastic prepared commercially by two catalytic methods. In one, coordination catalysts are prepared from an aluminum alkyl and titanium tetrachloride in heptane. The other method uses metal oxide catalysts supported on a carrier (see Catalysis). 

The most successful class of active ingredient for both oxidation and reduction is that of the noble metals silver, gold, ruthenium, rhodium, palladium, osmium, iridium, and platinum. Platinum and palladium readily oxidize carbon monoxide, all the hydrocarbons except methane, and the partially oxygenated organic compounds such as aldehydes and alcohols. Under reducing conditions, platinum can convert NO to N2 and to NH3. Platinum and palladium are used in small quantities as promoters for less active base metal oxide catalysts. Platinum is also a candidate for simultaneous oxidation and reduction when the oxidant/re-ductant ratio is within 1% of stoichiometry. The other four elements of the platinum family are in short supply. Ruthenium produces the least NH3 concentration in NO reduction in comparison with other catalysts, but it forms volatile toxic oxides. 

A 5 wt.% CoOx/Ti02 catalyst was prepared via an incipient wetness technique in which an aqueous solution of Co(N03)2 6H20 (Aldrich, 99.999%) was impregnated onto a shaped Ti02 (Milleimium Chemicals, commercially designated as DT51D, 30/40 mesh), as described in detail elsewhere [6]. Other supported metal oxide catalysts, such as FeOx, CuO, and NiOx, were obtained in a fashion similar to that used for preparing the CoO, catalyst. 

In the following scheme, an oxidation pathway for propane and propene is proposed. This mechanism, that could be generalized to different hansition metal oxide catalysts, implies that propene oxidation can follow the allylic oxidation way, or alternatively, the oxidation way at C2, through acetone. The latter easily gives rise to combustion, because it can give rise to enolization and C-C bond oxidative breaking. This is believed to be the main combustion way for propene over some catalysts, while for other catalysts acrolein overoxidation could... 

Thermal treatment—Processes in which vapor-phase contaminants are destroyed via high-temperature oxidation the primary categories of thermal treatment used to treat MTBE and other oxygenates include thermal oxidation, which employs a flame to generate the high temperatures needed to oxidize contaminants, and catalytic oxidation, which employs lower temperatures in the presence of a catalyst (typically platinum, palladium, or other metal oxides) to destroy contaminants. 

With certain types of catalysts it is easy to postulate that more than one type of chemisorption site may exist on the solid surface. For example, in the case of metal oxide catalysts, one might speculate that certain species could chemisorb by interaction with metal atoms at the surface, while other species could interact with surface oxygpn atoms. Consider the possibility that species A adsorbed on one type of site will react with species B adsorbed on a second type of site according to the following reaction. 

Hitachi Cable Ltd. (35) has claimed that dehydrogenation catalysts, exemplified by chromium oxide—zinc oxide, iron oxide, zinc oxide, and aluminum oxide—manganese oxide inhibit drip and reduce flammability of a polyolefin mainly flame retarded with ATH or magnesium hydroxide. Proprietary grades of ATH and Mg(OH)2 are on the market which contain small amounts of other metal oxides to increase char, possibly by this mechanism. 

Metal oxides possess multiple functional properties, such as acid-base, redox, electron transfer and transport, chemisorption by a and 71-bonding of hydrocarbons, O-insertion and H-abstract, etc. which make them very suitable in heterogeneous catalysis, particularly in allowing multistep transformations of hydrocarbons1-8 and other catalytic applications (NO, conversion, for example9,10). They are also widely used as supports for other active components (metal particles or other metal oxides), but it is known that they do not act often as a simple supports. Rather, they participate as co-catalysts in the reaction mechanism (in bifunctional catalysts, for example).11,12... 

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