Divalent metal molybdates

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... 

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... 

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... 

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) ... 

From our study reported here, from our earlier studies of divalent metal molybdates [10-11], and oxidation literature in general [1], we can postulate a reaction mechanism (Scheme 2) which takes all of these factors into account and is consistent with them. Accordingly,... 

The addition of divalent metal cation, M(II) with ionic radius smaller than 0.8 A (Ni2+, Co2+, Fe2+, Mg2+, Mn2+, etc.), to the pure bismuth molybdate increases the specific surface area of the catalyst system, but the specific activity of the tricomponent system, Mo-Bi-M(II)-0 never exceeds that of pure bismuth molybdate. 

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... 

Fven though the literature on this topic has been mainly focussed on the structural and chemical-physical properties of Bi molybdates, and on the reachvity of its various polymorphs (the a, P and y structures), the industrial catalyst consists of several divalent and trivalent metal molybdates. Indeed, Bi is present in minor amounts in catalyst formulahons. The two classes of molybdate contribute differently to catalyhc performance (1) trivalent Bi/Fe/Cr molybdates, having the Schee-lite-type shucture, contain the catalytically active elements while (2) divalent Ni/Go/Fe/Mg molybdates, having the Wolframite-type structure, mainly enhance the catalyst re-oxidahon rate. 

If the acidified molybdate solution that forms the Mogg cluster is then reduced in the presence of some divalent metal ions, a three fragment cluster of the form (M057M6 (50) type, for example, [ VO(H20) g Mo2(M-H20)2(/x-OH))3l Oi7(NO)2058(H20)2 3]" -=[ VO(H20) 6 Mo2 3 ( Mog 2 Moi )3] (Figure 6) is produced. The Mo57M6 -type cluster with three M017 ... 

In conclusion, to make an excellent catalyst system, it is important to activate bismuth molybdate by both the divalent and trivalent metal cations with ionic radii smaller than 0.8 A at the same time. A part of the increasing activity of the Mo-Bi-M(II)-M(III)-0 system compared to the pure bismuth molybdate comes from the increase in surface area and the remains arise from the increase in specific activity. Semiquantitative evaluations of tri- and tetracomponent bismuth molybdates are listed in Table V in comparison with simple bismuth molybdate catalyst. 

Metal hydroxides in general are anion-selective in acid solution and turn to be cation-selective beyond a certain pH, called the point of the iso-selectivity, pHpjS it is pHpjS = 10.3 for ferric oxide and pHpis = 5.8 for ferric-ferrous oxide [72]. Adsorption of multivalent ions may also control the ion selectivity of hydrous metal oxides because of its effect on the fixed charge in the oxides. For instance, hydrous ferric oxide, which is anion-selective in neutral sodium chloride solution, turns to be cation-selective by the adsorption of such ions as divalent sulfate ions, divalent molybdate ions, and trivalent phosphate ions [70,73]. It is worth emphasizing that such an ion-selectivity change due to the adsorption of multivalent ions frequently plays a decisive role in the corrosion of metals. 

The next major advance was the introduction of cobalt and nickel, which form divalent molybdates having the formulas C0M0O4 and NiMo04. The first disclosure of Co-Fe-Bi-Mo-0 catalysts was made by Nippon Kayaku Co., Japan for use in the selective oxidation of propylene to acrolein (16). The presence of the divalent transition-metal cation along with iron and bismuth molybdate produced a catalyst with significantly enhanced activity and selectivity. This discovery was... 

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