Molybdate scheelite structure

With 4o/44Ca- and 92/ioojio- data the differentiation of the translational and librational modes was obtained for the molybdates and tungstates with scheelite structure 98, 100, 101). Tables 29 and 30 reproduce these results. In contrast to early findings, the lower frequency bands were found to be librations and not translations. Furthermore, the band associated with Vi for CaMo04 and CaW04 cannot be expressed as a pure deformational vibration of the MOl group, but this mode is coupled with the translational vibrations, as indicated in Table 29. The relationship between the translational vibration, E, and the square root of the mass of the cation for compounds of the type AMO 4 (A = Ca, Sr, Ba, Pb M = Mo, W) was determined to be linear (100). 

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-... 

Multicomponent Bismuth Molybdate Catalysts with Scheelite Structure... 

Investigations into the scheelite-type catalyst gave much valuable information on the reaction mechanisms of the allylic oxidations of olefin and catalyst design. However, in spite of their high specific activity and selectivity, catalyst systems with scheelite structure have disappeared from the commercial plants for the oxidation and ammoxidation of propylene. This may be attributable to their moderate catalytic activity owing to lower specific surface area compared to the multicomponent bismuth molybdate catalyst having multiphase structure. 

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,... 

When the added metal cation, M(II) and/or M(III), has an ionic radius larger than 0.9 A and forms a molybdate with a scheelite structure, the catalyst system shows a performance quite different than that mentioned above for the scheelite-type catalysts. 

Bi2Mo3Oi2. The proposed model seems to be applicable for all kinds of multicomponent bismuth molybdate catalysts except those having a scheelite structure thus far. 

Improved catalytic performance, selectivity and resistance to fusion, over bismuth molybdate catalysts was reported by McClellan (90) for catalysts obtained by chemically combining bismuth, molybdenum, phosphorus, and silica. After calcination at 450°C, the bismuth phosphomolybdate-on-silica catalyst showed an X-ray pattern of mainly crystalline Bi2(Mo04)3 which subsequently was converted to a new, substantially amorphous, phase after calcination at 800°C. Substantially morphous meant that the X-ray diffraction lines were broad diffuse bands of low intensity. The pattern of lines for this novel phase indicated a scheelite structure. A special interaction of silica with bismuth molybdate was also suggested by Callahan et al. (91). 

Point defects in the form of cation vacancies () were introduced by Aykan et al. (93-95) into molybdates, tungstates, and vanadates with scheelite-type crystal structures. The authors studied the catalytic properties of more than 30 scheelite-structure phases represented by the formula A1 x< xM04 (M = molybdenum, tungsten, and/or vanadium and A may include Li, Na, K, Ag, Ca, Sr, Ba, Cd, Pb, Bi, and/or arare earth element in quantities appropriate to achieve charge balance for the normal oxidation states). It was found that the defects can be introduced... 

Alkhazov et al. (99-101) compared the physicochemical properties and catalytic activities of Group II element molybdates. The calcium, strontium, cadmium, and barium molybdates have a scheelite structure... 

LoJacono et al. (108) also utilized X-ray diffraction methods to study the structural and phase transformations which occurred in the Bi-Fe-Mo oxide system. They detected two ternary compounds containing bismuth, molybdenum, and iron. One of the compounds formed when the atomic ratio Bi/Fe/Mo = 1 1 1 the other formed when the atomic ratio Bi/Fe/Mo = 3 1 2. The X-ray data indicated a close structural relationship of the bismuth iron molybdate compounds with the scheelite structure of a-phase bismuth molybdate. Moreover, their structures were similar to compound X. The structure of the Bi/Fe/Mo = 3 1 2 compound was identical to the compound reported by Sleight and Jeitschko (107). The authors proposed that the structures of both of the compounds could be viewed as resulting from the substitution of Fe3+ in the a-phase lattice. In the Bi/Fe/Mo = 1 1 1 compound, 1 Mo6+ ion is replaced by 2 Fe3+ ions one Fe3+ ion occupies a Mo6+ site the other Fe3+ ion occupies one of the vacant bismuth sites. In the Bi/Fe/Mo = 3 1 2 compound, the Fe3+ ion replaces one Mo6+ ion while the additional Bi3+ ion occupies one of the vacant bismuth sites. 

The molybdenum-oxygen bond strength has been shown to weaken in direct proportion to an increased ionization potential of the A cations in several scheelite structured molybdates. Thus, it would be reasonable to assume that the nature of the A cation will have a profound effect on the molybdenum-oxygen bond strength which, in turn, will influence the activity and the selectivity of the catalysts. 

A different deformed version of the scheelite structure is found for KCrOaCl. One of several structures found for molybdates and tungstates of trivalent metals is the defect scheelite structure, adopted by Eu2(W04)3 and one polymorph of Nd2(Mo04)3. In this structure M ions occupy two-thirds of the sites in the scheelite structure. 

In oxides the Mo ion is usually four-coordinated, as, for example, in the most well-known luminescent molybdate CaMo04 with scheelite structure. This, by the way, is the only luminescent molybdate whose luminescence has been investigated in some detail. 

As far as we know the tetrahedral niobate group occurs only in the fergusonite structure of YNbO4 which is a distorted version of the scheelite structure of CaMo04 This luminescence is bright blue and has a relatively high Tq, viz. 500 jfg decay time is 15 MS at 11 which is short in comparison with the plateau value for the molybdate tetrahedron. It is very unfortunate that no more data exist on this complex with its efficient luminescence. 

Lead molybdate, PbMo04, has the scheelite structure typified by calcium tungstate. Discussions of the growth and physical properties of PbMo04 are included in papers on CaW04 (Ranon and Volterra, 1964). The ion in PbMo04 replaces the Pb ion, and some method of charge compensation must... 

Phases with this structure are found as high-temperature polymorphs of many of the rare earth ortho-niobates and ortho-tantalates, and in many of the pseudoternary systems involving alkali-metal oxides and rare earth tungstates or molybdates. The niobates and tantalates are discussed below but it is not intended to discuss the pseudo-ternary systems here, despite the fact that with the smaller alkali metals (Li and Na) many M R (W04)2 phases have been reported to have the scheelite structure. (See for example Mokhosov et al., 1967 Klevtsov and Kozeeva, 1970). 

Normal isopoly- and peroxymolydates of ammonium and several metals are known. The normal or orthomolybdates may be considered as salts of molybdic acid having formulas H2Mo04 xH20 or M20 Mo03 xH20. They are either of monoclinic or scheelite type crystal structure and obtained as hydrated salts. 

The effect of the additional components has been illustrated by Sleight et al. [117], who prepared a series of Scheelite type (derived from the mineral CaW04) bismuth molybdate phases to which Pb was added to give a series of solid solutions of composition Pbi 3xBi2xx(Mo04). For every two bismuth ions in the structure a cation vacancy (())) was generated. When this series of catalysts was tested a relationship was observed between vacancy concentration and the rate of propene oxidation, as shown in Fig. 5.26. 

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