Scheelite-type

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. 

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-... 

The most important point in designing a scheelite-type catalyst is making lattice vacancies in the structure (89-96). Both molybdenum and bismuth are essential elements, and several types of scheelite having lattice vacancies were reported as excellent catalysts for the allylic oxidation. 

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. 

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. 

A very attractive theory concerning the oxidation mechanism for scheelite-type molybdates containing bismuth is presented by Sleight and Linn [297] and is described below. 

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... 

In scheelite-type systems containing divalent ions, the tolerance for vacancies was more limited than in systems containing substantial amounts of monovalent ions. A vacancy limit of about 7.5% appeared to prevail at calcination temperatures between 550° and 800°C (97). The results for the oxidation of propylene over A2i3xBi J(f>xMo0.i, (A2+ = Pb, Cd, or Ca), compositions showed that when x = 0, the activity was very low but increased rapidly with increasing defect concentration. When bismuth was absent, the activity and selectivity were very poor on comparison with bismuth containing defect scheelites. 

Additionally, Aykan et al. (98) reported the results for scheelite-type systems in which A sites are occupied by divalent elements and bismuth, and M sites contain vanadium and molybdenum. The tolerance for vacancies in this system was reported to be 15% of the A cation sites. Good yields of acrolein were obained when bismuth and defects were present in the scheelite-structured catalysts. 

Although it was not suggested that defects are required for selective oxidation over other catalysts, the results indicated that defects and bismuth must be present for high activity and selectivity over scheelite-type catalysts. The authors concluded that the defects which were introduced into the bulk of these phases must manifest themselves in some manner at the surface. The question of how the introduction of defects into these phases affected their catalytic properties was not resolved. However, the active site for catalysis was suggested as a cation vacancy which could abstract a proton from an olefin to form the well-established allyl intermediate and should offer considerable stabilization to a surface hydroxyl group. 

Figure 3.11 FTIR skeletal spectra of a-Zr(HPOi)2 H2O, CdMo04 (scheelite type) and the Keggin-type heteropolyacid salt K3PM012O40.

Some phosphates of the AM2(P04)3 family form other modifications as well neptunium phosphate NaNp2(P04)3 with monazite type structure, Pl ln (this type was obtained by heating the trigonal form in Ar + 5% H2 at 1100 °C [5,35]) neptunium and plutonium phosphates AgNp2(P04)3 and AgPu2(P04)3 with scheelite type structure, 74i/a [46], thorium phosphate NaTh2(P04)3 with... 

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. 

Fig. 17. Crystal structure of LiGdCl (anti-scheelite type) and its relationship to the Cap2 type of structure.

Lacomba-Paralles R, Errandonea D, Segura A, Ruiz-Fuertes J, Rodriguez-Hemandez P, Radescu S, Lopez-Solano J, Mujica A, Munoz A (2011) A combined high-pressure experimental and theoretical study of the electronic band-structure of scheelite-type AW04 (A = Ca, Sr, Ba, Pb) compounds. J Appl Phys 110 043703... 

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