Alkali-promoted silica-supported

Unpromoted Cu/Si02 is found to have a low activity for methanol synthesis from H2/CO mixtures, whereas an increased activity from H2/CO2. Alkali metal promotion increases the activity for methanol synthesis from the H2/CO mixtures, probably due to the increase in surface OH groups engaged in the formation of the formate species which are the precursors to the methanol. Cu/Si02 powder catalysts (with 5 wt% Cu) can be prepared by ion exchange of silica with Cu(NOs)2 in aqueous solution, followed by calcination and reduction. Such preparations contain very fine Cu particles ( 0.5 nm) on a powdered silica support as revealed by HRTEM. 

Various Mo-containing catalysts were also tested in the dehydrogenation of propane354-358 and butane.359,360 Alkali promotion was shown to have a beneficial effect, specifically to increase selectivity due to lowering the acidity of the samples.355,360 With silica-supported binary and ternary molybdates, the best results were achieved with NiMo04 and Coo.5Ni0.5Mo04 (16% yield at 27% conversion).354... 

Duma and Honicke were the first to report the successful use of N2O in propene epoxidation. A PO yield of 5% was obtained over silica-supported iron oxide catalysts promoted with Na ions [43bj. The pore shape and diameter of the support as well as iron oxide dispersion are crucial parameters in the reaction [43b,cj. Doping vdth alkali metal may also considerably affect the Fe dispersion, and favor epoxidation over allylic oxidation [43fj. Further modification by boron can also significantly enhance the catalytic performance of the K-doped FeO /SBA-lS catalyst [43gj. 

To date Sasol has used only iron-based catalysts. Not only is iron much cheaper than the alternative metals, but it also produces more olefins. For the fixed-bed reactors the silica-supported and alkali-promoted catalyst is prepared by precipitation techniques (ref. 2). A recent improvement in catalyst formulation has resulted in a more reactive catalyst as well as a lower cost per reactor charge. The catalyst used in the fluidized-bed reactors is prepared by fusing suitable iron oxides together with the... 

Pearson also reported that the reaction over silica-supported alkali metal hydroxide catalysts is promoted markedly by the presence of water in the range of water/HCHO molar ratio = 1 to 5. With an acetic acid/HCHO/water molar ratio of 4.9/1/2.7, an SV of 750 h and a temperature of 405 °C, the yield of acrylic acid reaches 41.4 mol% based on the charged HCHO (8.5 mol% based on acetic acid) at the conversion of 53% the selectivity to acrylic acid is 78 mol% based on HCHO. 

Reaction over Base Catalysts. - Pearson reported that the reaction with propionic acid over silica-supported alkali metal catalysts is promoted by the presence of water, in the same way as the reaction with acetic acid. 

It is found that only alkali and alkaline earth hydroxides can promote the reaction that is, acidic and amphoteric oxides are inactive for the production of acrylonitrile. The best performances are obtained with silica-supported hydroxides of Cs, Rb, and K. The activity for the formation of acrylonitrile decreases in the order of Cs = Rb = K > Na > Li, and Ba > Ca Mg. This indicates that the activity is related to the electronegativity (basic property) of metal ions corresponding to the hydroxides supported on silica. 

A study of the product selectivites of variously supported Co catalysts (kieselguhr, silica, alumina, bentonite, Y-zeolite, mordenite, and ZSM-5) was carried out by Bessel (37). AAdiereas the lower acidity supports such as silica and alumina produced mainly linear hydrocarbons, the acidic supports produced more branched products. At higher temperatures, the latter produced aromatics as well. The isomerization and aromatization are secondary, acid-promoted reactions of the FT olefins. This is then equivalent to a combination of the FT and the Mobil olefins to gasoline process. (With iron-based catalysts, this approach is unlikely to be successful because alkali promotion is essential and the alkali would neutralize the required acid sites on the zeolite support.) Calleja and coworkers (38) studied the FT performance of Co/HZSM-5 prepared by incipient wetness impregnation. Promotion with thorium, being basic, decreased the acidity of the zeolite and so less aromatics were formed and consequently more of the heavier hydrocarbons emerged from the reactor because of the depressed level of secondary reactions. 

Iron Catalysts. Alkali promotion is vital for the activity and selectivity of Fe catalysts (20). In general, the selectivity shifts to the longer chain products as the alkali content is increased. It is debatable whether other compounds or supports have any strong, direct influence. They can, however, have a marked influence on the effectiveness of the alkaline promoter. For instance, any silica or alumina present can interact with the alkali to form silicates or aluminates that will be less basic than the free alkali and hence the basicity of the iron surface would be lowered. Even if the supports do not chemically interact with the alkali, the alkali will be distributed not only on the iron surface but also on the surface of the support and thus the basicity of the iron surface will be lowered. Overall, this means that the amount of alkali that has to be added needs to be adjusted to take into account the chemical nature and surface areas of the other components present. 

Promoter Location. It might be expected that promotion of supported catalysts would be difficult because the promoter localizes on the support. However, ESCA studies have shown that, in the case of silica-supported ruthenium promoted with alkali carbonates, the promoter preferentially associates with the ruthenium. Conversely, the high heat of adsorption of the alkali on carbon supports (up to 450 kJ moP leads to preferential association of the alkali with the carbon. 

Conventionally, a fixed bed catalyst containing palladium, a promoter metal, and an alkali metal acetate is used. The fixed bed catalyst components are supported on a porous carrier such as silica, zirconia or alumina. 

Many commercially available support materials contain small amounts of contaminations which can have promoting effects on syngas reactions, as for example, alkali compounds or iron oxide in Si02 materials, or TiC>2 in AI2O3, etc. Acidic precursors can leach out these contaminations and actually concentrate them on the metal surface. It has been proved that contaminated commercial silicas can in this way (often unnoticed by the authors) induce methanol synthesis by Cu, or the synthesis of higher alcohols (ethanol, mainly) on rhodium [34]. 

The hypothesis supporting the decisive role of Ca(OH)2 was advanced by Hansen [134], as early as in 1944. According to Hansen the Ca + ions can replace alkalis in a reaction product and hence they promote their further reaction with silica, with the new batch of expansive gel formation. This problem was then developed by Chattegi [106], as it has been mentioned earlier. Recently, Thomas returned to the Hansen s hypothesis (see Fig. 6.36) [135]. Studying experimentally gel in the concrete samples from an old dam, he found the proves of potassium ions by calcium ions replacement. This process occurs when the gel is migrating to the areas occupied by rich in calcium hydroxide cement matrix. 

Group 1 alkali metals (including potassium) are poisons for cobalt catalysts but are promoters for iron catalysts. Catalysts are snpported on high-surface-area binders/supports such as silica, alumina, and zeolites (Spath and Dayton, 2003). Cobalt catalysts are more active for FTS when the feedstock is natnral gas. Natnral gas has a high H2 to carbon ratio, so the water-gas-shift is not needed for cobalt catalysts. Iron catalysts are preferred for lower quality feedstocks such as coal or biomass. 

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