Metal molybdates

In addition to these principal commercial uses of molybdenum catalysts, there is great research interest in molybdenum oxides, often supported on siHca, ie, MoO —Si02, as partial oxidation catalysts for such processes as methane-to-methanol or methane-to-formaldehyde (80). Both O2 and N2O have been used as oxidants, and photochemical activation of the MoO catalyst has been reported (81). The research is driven by the increased use of natural gas as a feedstock for Hquid fuels and chemicals (82). Various heteropolymolybdates (83), MoO.-containing ultrastable Y-zeoHtes (84), and certain mixed metal molybdates, eg, MnMoO Ee2(MoO)2, photoactivated CuMoO, and ZnMoO, have also been studied as partial oxidation catalysts for methane conversion to methanol or formaldehyde (80) and for the oxidation of C-4-hydrocarbons to maleic anhydride (85). Heteropolymolybdates have also been shown to effect ethylene (qv) conversion to acetaldehyde (qv) in a possible replacement for the Wacker process. 

Decomposes on heating or on treatment with alkalies reacts with lead chloride and other metal salts to form their metal molybdates ... 

Normal molybdates are prepared by two methods (1) precipitating the insoluble metal molybdates obtained by adding the salt solution of the desired metal to a solution of sodium or potassium molybdate, and (2) neutrahzing a slurry or solution of molybdenum trioxide with the hydroxide or carbonate of the desired metal. For example, sodium molybdate, Na2Mo04, may be obtained as a dihydrate by evaporating an aqueous solution of molybdenum trioxide and sodium hydroxide. Heating the dihydrate at 100°C converts it to the anhydrous salt. Also, the normal molybdates of certain metals can be prepared by fusion of molybdenum trioxide with the desired metal oxide. 

Characterization of Catalysts. The BET surface areas of the divalent metal molybdate catalysts were practically the same as that of the Mo03/Si02, and they... 

Figure 1. Temperature progiammed reduction of silica supported M0O3 and metal molybdates.

Figure 2. The conversion of ethane and the yields of acetaldehyde and ethylene in the C2H6 + N2O reaction on silica supported M0O3 and metal molybdates at 823 K.

Some characteristic data for the oxidation of ethane on different metal molybdates at 823... 

Rb2Mo04 This catalyst was found to be much more active and selective than the divalent metal molybdates. As shown in Figure 2, very little decay was obsei ved in the conversion of ethane. In contrast with the previous catalysts, acetaldehyde was the main product of partial oxidation at 823 K it was fornied with a selectivity of 23-24%. The selectivity for ethylene was 10-13%. As it appears from Figure 2, the yield of acetaldehyde formation was about 5 times higher than on Mo03/Si02... 

Divalent Metal Molybdates. Except for the first point, the conversion of ethane did not change in the conditioning period at 823 K and it lay in the same range as for the N2O as oxidant. The main product was ethylene the selectivity of its foimation mai kedly exceeded that obtained with N2O as oxidant. Acetaldehyde was formed with 4.8% and 6.8% selectivity on the Mg and Zn salts. As a result, the yields for ethylene and acetaldehyde were much higher than in the case of N2O oxidation (Figure 5). Other hydrocarbons and alcohols were also detected in very small concentrations, with less than 1% selectivity. 

Rb2Mo04 This catalyst was found to be much more active than the divalent metal molybdates. Although the catalyst underwent a significant activity loss, even in the steady state the conversion was about 9.0%. The selectivities for ethylene and acetaldehyde formation in this state was 46% and 7.1%. Some data for the oxidation of ethane on molybdates with O2 are also listed in Table II. 

In a closed circulation system the decomposition of N2O on the divalent metal molybdates (activated in vacuum at 700 K) was measurable above 600 K. The initial... 

The activity sequence for molybdate catalysts was Mg - Mn - Zn. The Rb salt behaved similarly, but it was less active than the above metal molybdates. 

Comparison of the Catalytic Performances of Supported M0O3 and Metal Molybdates... 

Oxidation with N2O. In the evaluation of the catalytic data, we first compare the catalytic behavior of the metal molybdates with that of Mo03/Si02. Data obtained for this catalyst eailier (7) under the present conditions aie also included in Figure 2 and the appropriate tables. The conversion of ethane on Mo03/Si02 at 823 K was... 

Such advantageous catalytic properties were not exhibited by the divalent metal molybdates the ethane conversion was low and was not connected with higher selectivity. Ethylene was the main product of oxidative dehydrogenation, but its selectivity and yield were much less than on the above catalysts. Acetaldehyde was produced in only small amounts, with low selectivity. A decrease was observed in the selectivities for C2H4 and CH3CHO with increasing conversion, which is a... 

Oxidation with O2. In this case there was a dramatic difference in the catalytic behavior of supported M0O3 and metal molybdate catalysts. Whereas in harmony with previous studies (16), only the complete oxidation of ethane occuiTed on Mo03/Si02 in the temperature range 500-550 K, on metal molybdates, ethylene was... 

The selectivity of the catalyst may be correlated with the numbers of acidic and basic sites. There are no significant differences in the acidities of the catalysts, but a mai ked vaiiation may be observed in the numbers of basic sites (Table I). It may be assumed that the high number of basic sites on Rb2Mo04 as compai ed with M0O3, and partiCLilai ly divalent metal molybdates, may be responsible for the foimation of acetaldehyde from surface ethoxide species (step 3). 

TABLE 111 Crystal Structure of Divalent-Metal Molybdate (81) ... 

One typical way to improve the catalyst system was directed at the multi-component bismuth molybdate catalyst having scheelite structure (85), where metal cations other than molybdenum and bismuth usually have ionic radii larger than 0.9 A. It is important that the a-phase of bismuth molybdate has a distorted scheelite structure. Thus, metal molybdates of third and fourth metal elements having scheelite structure easily form mixed-metal scheelite crystals or solid solution with the a-phase of bismuth molybdates. Thus, the catalyst structure of the scheelite-type multicomponent bismuth molybdate is rather simple and composed of a single phase or double phases including many lattice vacancies. On the other hand, another type of multi-component bismuth molybdate is composed mainly of the metal cation additives having ionic radii smaller than 0.8 A. Different from the scheelite-type multicomponent bismuth molybdates, the latter catalyst system is never composed of a simple phase but is made up of many kinds of different crys-... 

Bismuth molybdate catalysts activated by the metal cations with ionic radii smaller than 0.8 A (Ni2+, Co2+, Fe2+, Mg2+, and/or Mn2+ with Fe3+) are never composed of a single phase, such as scheelite structure, and many kinds of metal molybdate, including various phases of bismuth molybdate,... 

Molybdenum comprises usually 50% or a little more of the total metallic elements. Most of molybdenum atoms form (Mo04)2 anion and make metal molybdates with other metallic elements. Sometimes a little more than the stoichiometric amount of molybdenum to form metal molybdate is included, forming free molybdenum trioxide. Since small amounts of molybdenum are sublimed continuously from the catalyst system under the working conditions, free molybdenum trioxide is important in supplying the molybdenum element to the active catalyst system, especially in the industrial catalyst system. In contrast, bismuth occupies a smaller proportion, forming bismuth molybdates for the active site of the reaction, and too much bismuth decreases catalytic activity somewhat. The roles of alkali metal and two other additives are very complicated. Unfortunately, few reports refer to these elements, except patents. In this article, discussion is directed only at the fundamental structure of the multicomponent bismuth molybdate catalyst system with multiphase in the following paragraphs. 

Surface analyses were investigated mainly by using XPS (Fig. 7). It was clearly indicated that many composite oxides found by XRD are located un-homogeneously in the catalyst particle. Molybdenum and bismuth are undoubtedly concentrated in the surface layer of the catalyst particle and divalent and trivalent metal cations are found in the bulk of the catalyst. As a result, it is clear that bismuth molybdates, especially its a-phase, is located on the surface of each particle, and metal molybdates of divalent and trivalent cations are situated in the bulk of the catalyst. 

Strictly speaking, it is difficult to conclude which model is most reasonable. However, summing up the results obtained by the surface analyses, it is sure at least that bismuth molybdates are concentrated on the surface of the catalyst particle. Our investigations for Mo-Bi-Co2+-Fe3+-0 also support the conclusion mentioned above, and the core-shell structure proposed by Wolfs et al. may be essentially reasonable. However, since small amounts of divalent and trivalent metal cations are observed in the surface layers, the shell structure may be incompletely constructed. The epitaxial effect has been assumed on the condensation of bismuth molybdates on the divalent and trivalent metal molybdates on the basis of the fact that the y-phase of bismuth molybdate is mainly formed on NiMo04 but the a-phase is predominant on other divalent and trivalent metal molybdates (46). The... 

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