Mo-V mixed oxides

Solid heteropoly compounds are suitable oxidation catalysts for various reactions such as dehydrogenation of O- and N-containing compounds (aldehydes, carboxylic acids, ketones, nitriles, and alcohols) as well as oxidation of aldehydes. Heteropoly catalysts are inferior to Mo-Bi oxide-based catalysts for the allylic oxidation of olefins, but they are much better than these for oxidation of methacrolein (5). Mo-V mixed-oxide catalysts used commercially for the oxidation of acrolein are not good catalysts for methacrolein oxidation. The presence of an a-methyl group in methacrolein makes the oxidation difficult (12). The oxidation of lower paraffins such as propane, butanes, and pentanes has been attempted (324). Typical oxidation reactions are listed in Table XXXI and described in more detail in the following sections. 

Sol-Gel synthesis and characterization of Nb-Mo and Nb-Mo-V mixed oxides as potential catalysts for the selective oxidation of propane... 

Fig. 2.1-8 TP reduction spectrum of an Mo-V mixed-oxide catalyst with hydrogen as probe molecule (Bohling 1997]. In spite of equal BET surface areas (ca. 16m g ) the amorphous catalyst is considerably more active.

Acrylic acid catalyst, based on Mo/V mixed oxide copper lowers reaction temperature... 

Primary screening can be done on wafer-based ternary mixed metal oxide libraries. For the oxidative dehydrogenation of ethane, two interesting libraries consist of Ni-Ce-Nb and Ni-Ce-Ta oxides. The maximum amount of ethylene produced is 1800 ppm at 400 °C in nickel-rich regions of the catalyst mixture (Figure 3.35b) compared with inactive Mo-V-Nb oxide catalysts (Figure 3.35a) [7]. 

FIGURE 7 (A) Raman spectra of a solution containing molybdenum, tungsten, and vanadium asafunction of pH (B) spectra showing the main band of the wet mixed-metal polyoxo paste. Raman spectra of the wet paste obtained from Mo-V-W oxide solutions with a Mo V W ratio 6 2 1 during drying at 100 °C (a), and of the solid after drying at 100 °C (b). 

In the ternary systems Mo-Nb-V mixed oxide phases containing Mo-Nb and Mo-V have been detected in addition to molybdenum oxide, as reveled in Table 2 for the samples Mo/Nb/V=3/l/l and 3/0.5/1.5, respectively. 

The reduction of molybdate salts in acidic solutions leads to the formation of the molybdenum blues (9). Reductants include dithionite, staimous ion, hydrazine, and ascorbate. The molybdenum blues are mixed-valence compounds where the blue color presumably arises from the intervalence Mo(V) — Mo(VI) electronic transition. These can be viewed as intermediate members of the class of mixed oxy hydroxides the end members of which are Mo(VI)02 and Mo(V)0(OH)2 [27845-91-6]. MoO and Mo(VI) solutions have been used as effective detectors of reductants because formation of the blue color can be monitored spectrophotometrically. The nonprotonic oxides of average oxidation state between V and VI are the molybdenum bronzes, known for their metallic luster and used in the formulation of bronze paints (see Paint). 

Al-Saeedi, J.N. and Guliants, V.V. (2002) High-throughput experimentation in multicomponent bulk mixed metal oxides Mo-V-Sb-Nb-O system for selective oxidation of propane to acrylic acid. Appl. Catal. A Gen, 237, 111. 

Figure 1.18 Survey and expanded V 2p and Mo 3d XPS spectra form a Mo-V-Sb-Nb mixed oxide catalyst after calcination in nitrogen (a) and air (b) atmospheres [145]. The data indicate a lesser degree of oxidation in nitrogen, a result that was correlated with the promotion of reactions leading to the production of propene and acrylic acid rather than acetic acid, the main product obtained with the fully oxidized sample. (Reproduced with permission from Elsevier.)...

Pulse radiolysis studies on [Mo204(edta)]2 , [Mo202S2(edta)]2, [Mo204(cys)2]2- and [Mo204(C204)2(H20)2]2- at pH 6, in which the Mo2 is reduced with eeq and Zn+ to the MoIV,v mixed-valence form have been reported.145 The latter absorbs in the visible range (e 300-500 M-1 cm-1 per dimer). In the absence of 02 the decay is relatively slow (f1/2 —5-10 s), and does not result in Mo production. With 02 present there is rapid ( 108M-1s-1) oxidation back to Mo. ... 

The state-of-the-art catalyst system is a Mo-V-Nb-Te mixed oxide [52], This catalyst is quite sensitive to its synthesis and process parameters and the automated catalyst synthesis tools described above were capable of synthesizing these and other challenging mixed metal oxides successfully. The workflow was validated by synthesizing a region of the known Mo-V-Nb-Te catalyst system phase space in the primary scale and secondary scale (Fig. 3.19a and b). Very good agreement between primary, secondary, and literature optima were obtained. One of the pri-... 

Thorsteinson etal.21 investigated mixed Mo-V-Nb catalysts containing 20 to 30 at% Vfor the oxidative dehydrogenation of ethane. Extensive reduction by ethane at 400 °C showed an increase of the intensity of the Nb peaks relative to those of Mo. Comparison of catalysts with and without V showed that this strongly facilitates the reoxidation of Mo4+ to Mo6+ at the surface. 

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

Beyond providing bulk structural information about 3-D metal oxide phases, Raman spectroscopy can also provide information about the terminating (and thus 2-D) surface layers of bulk metal oxides. For example, surface Nb=O, V = O, and Mo=O functionalities were detected by Raman spectroscopy for bulk Nb2Os, and for vanadium-niobium, molybdenum-vanadium, molybdenum-niobium, and vanadium-antimony mixed oxide phases (Guerrero-Perez and Banares, 2004 Jehng and Wachs, 1991 Zhao et al., 2003). 

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