Tungstated aluminas

Needless to say, solid acid and base catalysts play a key role in the transformation of biomass-derived materials to value-added compounds such as carbonyl compounds [21-33]. For example, lactic acid could be obtained from cellulose using tungstated alumina as a Lewis add catalyst [137], and from glucose using HT as a solid base catalyst [138]. In this parL selective conversions of biomass-sourced materials using solid acid and base catalysts are surveyed alongside mechanistic considerations. 

Potassium tungstate-alumina Mercaptans from alcohols... 

Catalyst (sulfided) Early units nickel molybdate/alumina high nitt ogen conversion little cracking. Later units nickel tungstate/alumina High nitt ogen conversion up to 40% cracking. 

Effect of Catalyst The catalysts used in hydrotreating are molybdena on alumina, cobalt molybdate on alumina, nickel molybdate on alumina or nickel tungstate. Which catalyst is used depends on the particular application. Cobalt molybdate catalyst is generally used when sulfur removal is the primary interest. The nickel catalysts find application in the treating of cracked stocks for olefin or aromatic saturation. One preferred application for molybdena catalyst is sweetening, (removal of mercaptans). The molybdena on alumina catalyst is also preferred for reducing the carbon residue of heating oils. 

Asare, O. K. and Khan, A., "Chemical-Mechanical Polishing of Tungsten An Electrophoretic Mobility Investigation of Alumina-Tungstate Interactions, Electrochemical Society Proceedings,Vol.1,1998,pp. 138-144. 

The rate of y -alumina island formation essentially depends on the nature of the electrolyte used. If outwards migrating (in the terms of Xu et al.102) anions, such as tungstates and molybdates, are used in the anodization process, y- alumina seed crystals are surrounded by pure alumina and crystallization occurs easily. In the case of inwards migrating anions (e.g., citrates, phosphates, tartrates), the oxide material surrounding the y-nuclei is enriched... 

Also for this reaction, namely esterification plus transesterification, mixed oxides, this time acidic in nature, appear to be the most promising alternative. Tungstated zirconia-alumina (WZA), sulfated zirconia-alumina and sulfated tin oxide were shown to be active in the transesterification of soybean oil with methanol at 200-300 °C and in the esterification of n-octanoic acid with methanol at 175-200 °C. Although the order of activities is different for the two reactions, WZA gives high conversions in both readions and it is stable under the reaction conditions [31]. Titania on zirconia, alumina on zirconia and zirconia on alumina also showed good performances [32, 6]. 

Furuta et al. tested a series of strong solid acids (alumina promoted sulfated zirconia, alumina promoted tungstated zirconia and sulfated tin oxide) for the transesterification of soybean oil with methanol at 200-300°C. Reaction yields over 90% were obtained for the alumina promoted tungstated zirconia at reaction times of 20 h using a flow reactor T = 250°C). The activity of the same catalyst was maintained for up to 100 h. 

H2SO4/sodium tungstate extn, alumina column cleanup, SPE cleanup, on-line trace enrichment on LiChrosorb RP-18, 5 m, preconcn column and switching to analytical column... 

Hydrotreating catalysts are composed of cobalt or nickel molybdate or nickel tungstate on an alumina or zeolite support. The materials are sulfided with hydrogen sulfide (H2S) before use, but the final catalysts may retain some oxide and be of complex composition. 

Tkachenko et al reported that compacts in iron, copper, nickel or cobalt matrices had operating temperature limits between 200 and 600 C, but compacts in molybdenum gave satisfactory friction and wear to 900 C in vacuum. A satisfactory composite for high-temperature aircraft brakes was described as containing 25% molybdenum disulphide, alumina and lead tungstate in a nickel matrix . The composites were pressed at 880 to 1080 MPa and sintered at 1010 C for two hours in vacuum. 

Studies related to alumina-supported tungsten oxide metathesis catalysts also continue.Unsupported alumina tungstate, Al2(W04)3, has metathesis activity between 100 and 200° C it has been proposed that this difficult-to-reduce compound is a reasonable precursor to the active metathesis sites for tungsten oxide supported on 7-alumina. However, a controversy exists as to the presence of Al2(W04)3 as a major component on the surface of these catalysts Raman spectra do not show bands that can be attributed to Al2(W04)3. Evidence reported in the literature for the formation of aluminium tungstate on the surface is not valid because of impurities in the reference compounds used. The conclusion that this compound is not a major surface component does not completely rule out the possibility that it is involved in the catalytic active phase,especially since the number of active sites is extremely small, e.g., 10 sites per gram of... 

When this sample was heated in an oxygen atmosphere, a larger exotherm occurred at 1050 C as a fraction of the AloOo support reacted with WO3 to form Al2(W04)3. The formation of Al2(W0 )3 was confirmed by X-ray diffraction measurements. Alumina not utilized in tungstate formation transformed predominantly to 0-AI2O3 only a trace of orAl203 was produced. Thus, the presence of the tungsten oxide surface phase inhibits the transition of 0-AI2O3 to orAl 2O3. 

For example, at a calcination temperature of 100 °C, the yield of epoxide was found to be only 10.3 %, whereas at 500 °C. the yield dramatically increased to over 88 %. However, at a calcincination temperature of 600 °C, the yield drops off suddenly. Work is still on-going as to the nature of the active species. However, solid state NMR, IR, and Raman all show that changes take place upon calcination which is not just the formation of layered tungstates. For example, a dimeric species appears to be stabilized by the alumina support when the phosphotungstic acid salt was immobilized. 

Alumina catalysts activated by additions of dehydrogenating catalysts, e.g., nickel oxide, copper oxide or sulfide, zinc oxide or sulfide, cobalt selenide, zinc phosphate, cadmium tungstate, mixtures of the oxides of zinc and tungsten, of cadmium and molybdenum, etc., are claimed to be superior in the formation of acetaldehyde from mixtures of steam and acetylene at 350° to 400° C.l-la Zinc oxide catalysts may be activated in a similar way by the addition of small amounts of molybdates or molybdic acid, and are effective at 300° to 350° C.121b... 

The active constituent of the catalyst is an oxide of tungsten prepared by partial reduction of WO - Unsupported WOj was found to have activity but better specific activity is obtained by depositing the oxide on a support. A number of grades of alumina and silica were tested. Gamma alumina was found to react with WOj at 400°C to a considerable extent to form aluminium tungstate. This was identified by XPS and tests on AljfWO ) showed it to be inactive as a catalyst for the isomerization reaction. 

A catalyst was prepared by impregnating a sample of alumina with a solution of sodium tungstate and mixing with the aid of ultrasonic agitation. The sample was dried in a vacuum dessicator ( 4 hour) and in air at 90°C (1 hour). Concentrated nitric acid (10 mL) was added and the beaker warmed on a hot plate for 5 minutes and the solid was then washed with 1M nitric acid by decantation. To remove sodium nitrate, 1M nitric acid (250 mL) was added, the catalyst digested for 1 hour on a hot plate and the nitric acid decanted. This washing was repeated three times and the catalyst dried at 150-160°C. 

The activity for skeletal isomerization exhibited by these tungsten catalysts is developed only under specific conditions of treatment and operation. The source of tungsten oxide and the method of support are not critical. The HT-alumina favoured in these tests had the advantage of good dispersion of WO and limited loss by reaction with the support to form aluminium tungstate. 

Finally, scheme D is probably more realistic than either of the previous ones. It is hard to determine without further study which effect will predominate in this case, but in any case tungstate adsorption may be expected to exert an influence on alumina dissolution rates. 

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