Molybdenum-oxide catalysts

Many low molecular weight aldehydes and ketones are important industrial chem icals Formaldehyde a starting material for a number of plastics is prepared by oxida tion of methanol over a silver or iron oxide/molybdenum oxide catalyst at elevated temperature... 

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). 

Methanol undergoes reactions that are typical of alcohols as a chemical class (3). Dehydrogenation and oxidative dehydrogenation to formaldehyde over silver or molybdenum oxide catalysts are of particular industrial importance. 

Oxidation Catalysis. The multiple oxidation states available in molybdenum oxide species make these exceUent catalysts in oxidation reactions. The oxidation of methanol (qv) to formaldehyde (qv) is generally carried out commercially on mixed ferric molybdate—molybdenum trioxide catalysts. The oxidation of propylene (qv) to acrolein (77) and the ammoxidation of propylene to acrylonitrile (qv) (78) are each carried out over bismuth—molybdenum oxide catalyst systems. The latter (Sohio) process produces in excess of 3.6 x 10 t/yr of acrylonitrile, which finds use in the production of fibers (qv), elastomers (qv), and water-soluble polymers. 

Manufacture and Processing. Until World War II, phthaUc acid and, later, phthaUc anhydride, were manufactured primarily by Hquid-phase oxidation of suitable feedstocks. The favored method was BASF s oxidation of naphthalene [91-20-3] by sulfuric acid ia the presence of mercury salts to form the anhydride. This process was patented ia 1896. During World War I, a process to make phthaUc anhydride by the oxidation of naphthalene ia the vapor phase over a vanadium and molybdenum oxide catalyst was developed ia the United States (5). Essentially the same process was developed iadependendy ia Germany, with U.S. patents being granted ia 1930 and 1934 (6,7). 

K. Chen, S. Xie, A.T. Bell, and E. Iglesia, Alkali effects of molybdenum oxide catalysts for the oxidative dehydrogenation of propane, J. Catal. 195, 244-252 (2000). 

Photoreduction of Si02 doped by six valance molybdenum results in decrease of the surface concentration of Mo ions and, as a consequence, reduces the capability of the sample to emit 2. However, such a treatment does not lead to a complete suppression of emission capability because, according to [96], illumination reduces only 30% of Mo ions in molybdenum oxide catalysts deposited on Si02. 

Similar to molybdenum oxide catalyst the capability to emit singlet oxygen is inherent to Si02 doped by Cr ions as well. Similar to the case of vanadium oxide catalysts in this system the photogeneration occurs due to the triplet-triplet electron excitation transfer from a charge transfer complex to adsorbed oxygen. 

Benzoraffln A hydrofining process for treating naphtha fractions derived from coal. It is a fixed-bed, gas-phase process using a cobalt/molybdenum oxide catalyst. Developed jointly by BASF, Veba-Chemie, and Lurgi, Ground 1960. 

Trimm and Irshad 154) have used the influence of the substituents upon the rate of the oxidation of toluene and its derivatives to corresponding aldehydes on a molybdenum oxide catalyst at 460°C for obtaining insight into the mechanism. The rate constants could be correlated by the Hammett equation (series 139, five reactants, 450°C, slope -1). 

Shen, Q Zhu, X., and Dong, J. (2009) Hydrodealkylation of C9-t aromatics to BTX over zeolite-supported nickel oxide and molybdenum oxide catalysts. Catal. Lett., 129, 170-180. 

A. Christodoulakis, E. Heracleous, A.A. Lemonidou and S. Boghosian, An operando Raman study of structure and reactivity of alumina-supported molybdenum oxide catalysts for the oxidative dehydrogenation of ethane, J. 

The iron molybdenum oxide catalyst was structurally characterized by XRD and vibrational spectroscopy (IR and Raman). 

The oxidation of ethanol to acetic acid was among the first heterogeneous catalyzed reactions to be reported, but it has not attracted continued interest. During the 1990ies, however, 100% conversion of ethanol coupled with 100% selectivity to acetic acid was reported in a gas-phase reaction using molybdenum oxide catalytic systems on various supports, at temperatures below 250 Similarly, a tin oxide and molybdenum oxide catalyst was... 

In both fresh and regenerated catalysts, the MCM-41 supported catalysts are better than amorphous silica supported ones. Physically mixed molybdenum oxide catalysts with Def-MCM41 support are particularly active. The steady state activity decreases in the order Mo/DM > Mo/M > Mo/Si02. Interestingly, the rate of deactivation also seems to depend on... 

In the epoxidation of alkenes with tert-butyl hydroperoxide and a molybdenum oxide catalyst, addition of an aliphatic amine first accelerates the formation of the intermediate 229 and also favours the production of epoxide in favour of the alternative fragmentation to carbonyl compounds (Scheme 5)356. 

Vanadium-Molybdenum Oxide Catalysts. - The mixed oxides of V and Mo... 

Although metals or even promoted metals have very low sulfur tolerances in synthesis reactions, other materials, such as metal oxides, nitrides, borides, and sulfides, may have greater tolerance to sulfur poisoning because of their potential ability to resist sulfidation (18). The extremely low steady-state activities of Co, Ni, and Ru metals in a sulfur-contaminated stream actually correspond to the activities of the sulfided metal surfaces. However, if more active sulfides could be found, their activity/selectivity properties would be presumably quite stable in a reducing, H2S-containing environment. This is, in fact, the basis for the recent development of sulfur active synthesis catalysts (211-215), which are reported to maintain stable activity/ selectivity properties in methanation and Fischer-Tropsch synthesis at H2S levels of 1% or greater. Happel and Hnatow (214), for example, reported in a recent patent that rare-earth and actinide-metal-promoted molybdenum oxide catalysts are reasonably active for methanation in the presence of 1-3% H2S. None of these patents, however, have reported intrinsic activities... 

One of the first cells that allowed exposure of the catalyst sample to reaction conditions was developed by Schrader and coworkers (Cheng et al., 1980) key feature of the design is the placement of the catalyst, which is pressed into a wafer, on a rotor. The authors investigated the structural evolution of alumina-supported molybdenum oxide catalysts during calcination. Chan and Bell (1984) reported on the formation of... 

Hu et al. (1995) reported extensive Raman experiments characterizing supported molybdenum oxide catalysts during methanol oxidation. In contrast to the stable oxidation state of vanadia, the valence of surface molybdenum species decreased during catalysis. The original band... 

Ethanol oxidation on a supported molybdenum oxide catalyst was investigated by Zhang et al. (1995) by application of Raman spectroscopy. After activation in 02, the Raman spectrum of a 1 wt % Mo03/Si02 catalyst showed the presence of highly distorted octahedral monooxo species, as well as of small amounts of Mo03. Introduction of ethanol at room temperature resulted in a drastic decrease in the intensity of the terminal Mo = O band, accompanied by a shift of this band to 960 cm-1 and a blue coloration of the sample. An increase in reaction temperature to 250 °C restored the white color and produced a shift of the Mo = 0 band to a frequency of 972 cm-1, which is indicative of a dioxo site. Only oxidation at 500 °C for 2 h shifted the band to its original position. 

Molybdenum trioxide constitutes an active model catalyst for the oxidation of propene in the presence of gas-phase 02 at temperatures above approximately 600 K (Grzybowska-Swierkosz, 2000). Reduction of M0O3 in propene and oxidation of Mo02 in 02 were investigated by time-resolved XAFS spectroscopy combined with mass spectrometry (Ressler et al., 2002). Reduction and reoxidation of M0O3 x are of particular interest because they constitute the two fundamental transformations of the so-called redox mechanism for partial oxidation of alkenes on molybdenum oxide catalysts. 

Molybdenum Oxides. Molybdenum oxide catalysts are prepared by the addition of hydrochloric acid to an ammoniacal solution of molybdic acid or ammonium molybdate. By heating to 400-500°C the molybdate is decomposed to the oxide.216 M0O3 is reduced to Mo02 in a stream of hydrogen at 300-400°C. 

Alumina supported molybdenum oxide catalysts are extensively used in hydrotreating reactors. Under reaction conditions the oxides are not stable because they are transformed into molybdenum sulphides. In practice this step is carried out in a so-called sulphiding step. In this step the catalyst is pretreated with a mixture containing a sulphur compound. The sulphiding step was investigated by carrying out a TPS study. TPS is strictly analogous to TPR, except for the gas mixture which is here a H2/H2S mixture. 

---