Metal molybdates, structural

Modification of the metal itself, by alloying for corrosion resistance, or substitution of a more corrosion-resistant metal, is often worth the increased capital cost. Titanium has excellent corrosion resistance, even when not alloyed, because of its tough natural oxide film, but it is presently rather expensive for routine use (e.g., in chemical process equipment), unless the increased capital cost is a secondary consideration. Iron is almost twice as dense as titanium, which may influence the choice of metal on structural grounds, but it can be alloyed with 11% or more chromium for corrosion resistance (stainless steels, Section 16.8) or, for resistance to acid attack, with an element such as silicon or molybdenum that will give a film of an acidic oxide (SiC>2 and M0O3, the anhydrides of silicic and molybdic acids) on the metal surface. Silicon, however, tends to make steel brittle. Nevertheless, the proprietary alloys Duriron (14.5% Si, 0.95% C) and Durichlor (14.5% Si, 3% Mo) are very serviceable for chemical engineering operations involving acids. Molybdenum also confers special acid and chloride resistant properties on type 316 stainless steel. Metals that rely on oxide films for corrosion resistance should, of course, be used only in Eh conditions under which passivity can be maintained. 

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. 

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... 

The multifunctionality is achieved through either the combination of two different compounds (phase-cooperation) or the presence of different elements inside a single crystalline structure. In antimonates-based systems, cooperation between the metal antimonate (having a rutile crystalline structure), employed for propane oxidative dehydrogenation and propene activation, and the dispersed antimony oxide, active in allylic ammoxidation, is made more efficient through the dispersion of the latter compound over the former. In metal molybdates, one single crystalline structure contains both the element active in the oxidative dehydrogenation of the hydrocarbon (vanadium) and those active in the transformation of the olefin and in the allylic insertion of the N H2 species (tellurium and molybdenum). 

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. 

We have illustrated the room-temperature synthesis of a crystalline structure prepared by the simultaneous modification of both the structure and chemical composition of the host and guest species domains of a layered precursor. This novel room-temperature chimie douce synthesis technique produced a new class of layered transition-metal molybdate (LTM) materials using calcined LDHs as precursors. These materials, being ionic lamellar solids themselves, may be suitable hosts for the synthesis of intercalated derivatives, thereby creating the potential for the preparation of a whole new class of molecularly designed materials. 

Analytical electron microscopy permits structural and chemical analyses of catalyst areas nearly 1000 times smaller than those studied by conventional bulk analysis techniques. Quantitative x-ray analyses of bismuth molybdates are shown from lOnm diameter regions to better than 5% relative accuracy for the elements 61 and Mo. Digital x-ray images show qualitative 2-dimensional distributions of elements with a lateral spatial resolution of lOnm in supported Pd catalysts and ZSM-5 zeolites. Fine structure in CuLj 2 edges from electron energy loss spectroscopy indicate d>ether the copper is in the form of Cu metal or Cu oxide. These techniques should prove to be of great utility for the analysis of active phases, promoters, and poisons. 

Partially Crystalline Transition Metal Sulphide Catalysts. Chiannelli and coworkers (6, 7, 8) have shown how, by precipitation of metal thio-molybdates from solution and subsequent mild heat-treatment many selective and active hydrodesulphurization catalysts may be produced. We have shown (18) recently that molybdenum sulphide formed in this way is both structurally and compositionally heterogeneous. XRES, which yields directly the variation in Mo/S ratio shows up the compositional nonuniformity of typical preparations and HREM images coupled to SAED (see Figure 2) exhibit considerable spatial variation, there being amorphous regions at one extreme and highly crystalline (18, 19) MoS at the other. 

The ions with six metal atoms have the same structure as the hexa-molybdate ion [Mo6Oi9]2 . The [Mo5VOi9]3 ion, previously identified in acetonitrile medium (166) can be obtained in the solid state by precipitation from an aqueous solution with tetramethyl ammonium as cation (165). The vanadium atoms in both [Mo4V2Oi9]4 and its pro-tonated form [HMo4V2Oi9]3 (pKa = 3.8) are in cis positions. Protonation seems to take place at the oxygen, which bridges the two vanadium atoms. 

The research also revealed new complexities and some questions are still to be answered. The molybdate(VI) system in particular needs further clarification regarding the existence of some polyions. More kinetic and thermodynamic data would also help to improve our understanding of these systems and perhaps lead to a general inclusive explanation of the mechanism of polyoxoanion formation. In this respect the new information about some structural preferences of the different metals in mixed polyoxoanions is of interest and a useful addition to known facts regarding polyoxometalate structures (181). 

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