Surface area molybdate catalysts

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... 

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. 

The specific activity of pure bismuth molybdate, a-phase Bi2(Mo04)3 or y-phase Bi2Mo06, is fairly high. However, owing to its low surface area, the activity per unit weight of the catalyst is not so prominent. 

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. 

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. 

Fig. 13. The catalytic activity forming acrolein per unit surface area of the supported bismuth molybdate catalysts (52). (Q) BbMojCWCoMoO, ( ) Bi2Mo3On/Co]i izFei.ijMoO -...

The lower activation energy of the multicomponent bismuth molybdate catalyst suggests that some structural or electronic modification of the active component, Bi2(Mo04)3, is given by M(II) and M(III) molybdates, which also contribute to the enlargement of the surface area of bismuth molybdate. 

Cobalt molybdate, Girdler No. G35A, received as pellets, then crushed and sieved to 35 to 65 mesh. 1.3% Co + 6.1% Mo on alumina BET nitrogen surface area 241 sq. meters per gram stabilized with thiophene in a flow reaction at 400° C. before use. Weight of catalyst used is noted with each set of results. 

Bulk Mixed Oxide Catalysts. - Raman spectroscopy of bulk transition metal oxides encompasses a vast and well-established area of knowledge. Hie fundamental vibrational modes for many of the transitional metal oxide complexes have already been assigned and tabulated for systems in the solid and solution phases. Perhaps the most well-known and established of the metal oxides are the tungsten and molybdenum oxides because of their excellent Raman signals and applications in hydrotreating and oxidation catalysis. Examples of these two very important metal-oxide systems are presented below for bulk bismuth molybdate catalysts, in this section, and surface (two-dimensional) tungstate species in a later section. 

Supported aqueous phase catalyst (SAPC) involves generating a thin water film on a high surface area support surrounded by a bulk organic solvent.20 SAPC suffers from mass transfer problems despite the sharp increase in interfacial surface area and leaching problems of the water-soluble catalyst. The SAPC method has been effectively applied to cyclohexene oxidations using a simple water-soluble ammonium molybdate catalyst with the aid of supported surfactant molecules.21... 

Characterization data evidenced that the prepared NiMo04 is stoichiometric and that Cs is deposited only on the catalyst surface (atomic ratio Cs/Mo = 0.03) not affecting the molybdate structure. However, Cs doping causes a decrease of the catalyst surface area Sbet (NiMo04) = 44.1 m /g and Sbet (3% Cs-NiMo04) = 28.7 mVg. Moreover, the promoted sample exhibits a higher surface basicity, electrical conductivity and also a larger resistance to reduction [4,5,12]. 

Iron molybdates, with atomic ratio Mo/Fe=3, were prepared by coprecipitation and sol-gel techniques in acid medium. Characterisation results show that sol-gel catalysts have much higher surface areas than coprecipitated catalysts. Study of catalytic activity shows that Fe-defective catalysts, prepared by sol-gel technique, perform better than an industrial catalyst in the same conditions. In fact these catalysts achieve higher performances at SOK lower than industrial catalysts. 

A detailed description of a chromia-on-alumina catalyst prepared by impregnation has been given elsewhere . Another supported nonmetallic catalyst widely used commercially is cobalt molybdate-on-alumina. The preparation of this catalyst using an alumina support with controlled pore-size distribution is as follows. Silica-stabilized alumina, with greater than 50% of its surface area in 3-8 nm pores and at least 3% of the total pore volume in pores greater than 200 nm in diameter, is impregnated with an aqueous solution of cobalt and molybdenum. The finished oxysulfide catalyst was tested for hydrodesulfurization of petroleum residuum at 370°C and 100 atm for 28 days and compared with a convential cobalt-molybdate catalyst having a major portion of the surface area in 3-7 nm pores. The latter catalyst and controlled pore catalyst maintained 57 and 80% activity, respectively. 

The nickel molybdate catalyst used in this study, prepared by the precipitation method described by Mazzocchia [4], has been supplied by Elf Atochem (France). The bulk Mo / Ni ratio in this solid is practically equal to 1 it can exist as phases a and P, differing by the coordination of Ni ions. Only the results for the a form will be reported here. The specific surface area as measured by N2 adsorption is about 40 m / g. 

With the exception of the lead oxide the metal oxides used are very interesting materials with respect to catalytic applications. Tungstates and molybdates are widely used in partial oxidation reaction [6], iron oxides are the basis of many important industrial cat ysts, which are for instance, used for the dehydrogenatitm of ethyl benzene to styrene. If the surfactant template could be removed from the structures, very high surface area catalysts could be accessible. 

The detailed method of preparation of the catalyst is also important [31,32]. Among various factors, the preparation method affects the pore size, surface area, and the distribution of iron molybdate and molybdenum oxide. These factors affect the behavior of the catalyst [33]. 

Hydrotreating catalysts are usually produced by impregnating preformed y-alumina supports with aqueous solutions of ammonium molybdate. Particles are then dried and calcined. Molybdenum oxide forms a uniform surface layer on the alumina by reaction with the sirrface hydroxyl groups. It is important that the surface area of the alumina is consistent with the amount of molybdenum oxide required for the hydrotreating catalyst specification (typically 1 wt% M0O3 = 12... 

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