Molybdenum-based catalyst systems oxide

Temperature Programmed Reaction. Examination of another redox system, propylene oxidation on M0O3, provides further insight. It is well accepted that propylene oxidation on molybdenum-based catalysts proceeds through formation of allylic intermediates. From isotopic studies it has been demonstrated that formation of the allylic intermediate is rate-determining (H/D effect), and that a symmetric allylic species is formed ( C labelling). 

Molybdenum-based catalysts are highly active initiators, however, monomers with functionalities with acid hydrogen, such as alcohols, acids, or thiols jeopardize the activity. In contrast, ruthenium-based systems exhibit a higher stability towards these functionalities (19). An example for a molybdenum-based catalyst is (20) MoOCl2(t-BuO)2, where t-BuO is the tert-butyl oxide radical. The complex can be prepared by reacting M0OCI4 with potassium tert-butoxide, i.e., the potassium salt of terf-butanol. 

Commercial SCR catalyst used in connection with coal-based power stations are generally composed of base metals, since platinum-group metal catalysts are too readily poisoned and have too narrow an operating temperature window for this application. Favored compositions are titania-based together with active components, normally oxides of vanadium, tungsten, or molybdenum. For these systems the optimum reaction temperature is usually in the range 3(XM00°C. 

Up to now, only a few catalyst systems based on organic polymers such as molybdenum compounds supported on benzimidazole, polystyrene, or poly(gly-cidyl methacrylate) [9] as well as micelle-incorporated manganese-porphyrin catalysts [66] have been tested in the epoxidation of propene. Molybdenum-doped epoxy resins were also employed in the epoxidation of propene with TBHP and propene oxide yields of up to 88% were obtained [65]. The catalysts were employed repeatedly in up to 10 reactions without significant loss of activity and metal leaching proved to be very low. 

The most studied systems for oxidative propane upgrading are vanadium [2], vanadium-antimony [3], vanadium-molybdenum [4], and vanadium-phosphorus [5] based catalysts. Another family of light paraffin oxidation catalysts are molybdenum based systems, e.g. nickel-molybdates [6], cobalt-molybdates [7] and various metal-molybdates [8-9]. Recently, we investigated binary molybdates of the formula AM0O4 where A = Ni, Co, Mg, Mn, and/or Zn and some ternary Ni-Co-molybdates promoted with P, Bi, Fe, Cr, V, Ce, K or Cs [10-11]. A good representative of these systems is the composition Nio.5Coo.5Mo04 which was recently selected for an in depth kinetic study [12] and whose mechanistic aspects are now further illuminated here. 

Partial oxidation of propane was investigated in the presence of molybdenum oxide based catalysts. We have shown the existence of a synergetic effect between the two phases aNiMo04 and aMoOs. Indeed activity and selectivity towards acetic acid and acrylic acid were maximal with a ratio aMo03 / (aNiMo04 + aMoOj) close to 0.25. These results could be explained by an interaction and a mutual covering of the two phases. The addition of bismuth to these mixed systems led to a total or a partial inhibition in the production of acetic acid and an increase in the formation of acrolein and acrylic acid. 

The catalyst systems employed are based on molybdenum and phosphorus. They also contain Various additives (oxides of bismuth, antimony, thorium, chromium, copper, zirconium, etc.) and occur in the form of complex phosphomolybdates, or preferably heteropolyacids deposited on an inert support (silicon carbide, a-alumina, diatomaceous earths, titanium dioxide, etc.). This makes them quite different from the catalysts used to produce acrylic acid, which do not offer sufficient activity in this case. With residence times of 2 to 5 s, once-through conversion is better than 90 to 95 per cent, and the molar yield of methacrylic acid is up to 85 to 90 per cent The main by-products formed are acetic add, acetone, acrylic add, CO, C02, etc. The major developments in this area were conducted by Asahi Glass, Daicel, Japan Catalytic Chemical, Japanese Gem, Mitsubishi Rayon, Nippon Kayaku, Standard Oil, Sumitomo Chemical, Toyo Soda, Ube, etc. A number of liquid phase processes, operating at about 30°C, in die presence of a catalyst based on silver or cobalt in alkaline medium, have been developed by ARCO (Atlantic Richfield Co,), Asahi, Sumitomo, Union Carbide, etc. 

In summary, oxidation of C4 and higher olefins to maleic anhydride is complex, with many intermediates and by-products. Plausible reaction schemes can be formulated, based upon allylie oxidation of the olefin combined with known oxidations of the proposed intermediates. Known catalysts are mainly vanadium or molybdenum oxides, usually without powerful moderators. In view of the complex reaction systems, much careful experimental work will be necessary to elucidate the details of the strong oxidation of olefins. 

The traditional method of using Cr(IV)-based oxidants is obviously unacceptable today. A number of new environmentally benign catalyst systems have recently been developed which can be used with H2O2 or TBHP as oxidant. These promote chemoselective oxidation of secondary alcohols and diols to ketones, an important class of reactions in organic synthesis. The metals used are molybdenum (Kurusu and Masuyama, 1986) and titanium. 

Antimonate-Based Catalysts. In addition to the bismuth-molybdenum oxide catalyst system, several other mixed metal oxides have been identified as effective catalysts for propylene ammoxidation to acrylonitrile. Several were used commercially at various times. In particular, the iron-antimony oxide catalyst is currently used commercially by Nitto Chemical (now Dia-Nitrix Co. Ltd., Japan) and its licensees around the world, although the catalyst was originally discovered and patented by SOHIO (20,21) and by UCB (22). Nitto Chemical improved the basic iron-antimony oxide catalyst with the addition of several elements that promote activity and selectivity to acrylonitrile. Key among these additives are tellurium, copper, molybdenum, vanadium, and tvmgsten (23-25). 

The most effective molybdenum-based oxide catalyst for propane ammoxidation is the Mo-V-Nb-Te-0 catalyst system discovered and patented by Mitsubishi Chemical Corp., Japan, U.S.A. (140). Under single-pass process conditions, acrylonitrile yields of up to 59% are reported, whereas under recycle process feed conditions, the acrylonitrile selectivity is 62% at 25% propane conversion (141). Although the latter results show that the catalyst operates effectively under recycle feed conditions, the catalyst system was originally disclosed for propane ammoxidation under single-pass process conditions. The catalyst was derived from the Mo-V-Nb-0 catalyst developed by Union Carbide Corp. for the selective oxidation of ethane to ethylene and acetic acid (142). The early work by Mitsubishi Chemical Corp. used tellurium as an additive to the Union Carbide catalyst. The yields of acrylonitrile from propane using this catalyst were around 25% with a selectivity to acrylonitrile of 44% (143). The catalyst was also tested for use in a regenerative process mode much like that developed earlier by Monsanto (144) (see above and Fig. 8). Operation under cyclic reduction/reoxidation conditions revealed that the performance of the catalyst improved when it was partially reduced in the reduction cycle of the process. Selectivity to acrylonitrile reached 67%, albeit with propane conversions of less than 10%, since activity in... 

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