Palladium on silica

The use of appropriate ligands for a given metal can permit one to obtain high purity deposits. The Tjballyl and T -cyclopentadienyl ligands coordinated to palladium seem to be especially attractive for the deposition of pure palladium on silica surfaces. It has been proposed that [Pd(T hC3H5)(T -C5H5)] under a helium atmosphere could easily react with a silica surface to produce a surface allyl palladium species and cyclopentadiene [57] (9.9) ... 

In this work, when the support is changed, the shift of the pCO band above 2000 cm-1 may be connected to a variation of the electronic density at the metal atom. This density is much lower for palladium on silica-alumina (pCO = 2100 cm-1) than for palladium on MgO (vCO = 2065). 

The obvious decrease in the number of electron-acceptor sites with palladium deposition on silica-alumina strongly suggests an interaction between the metal and these sites. Turkevich (28) first demonstrated that palladium behaves like an electron-donor toward tetracyanoethylene we suppose that it can be the same toward an electron-acceptor site of a solid support. In that hypothesis, palladium should have a partial positive charge on the second class of supports. This is actually observed by the adsorption of CO. This adsorbate can be considered as a detector of the electronic state of palladium. The shift toward higher frequencies of the CO band reflects a decrease in the back donation of electrons from palladium to CO. Thus, palladium on silica-alumina or HY is electron-deficient compared with the silica- or magnesia-supported metal. Moreover, the shift of CO vibration frequency is roughly parallel to the increase of activity thus, these two phenomena are connected. We propose that the high activity of palladium on acidic oxides is related to its partial electron deficiency. 

Figure 5. X-ray of pure silica gel (top), 10.86% palladium on silica gel under vacuum (middle), and same sample under 30-mm. hydrogen pressure...

Figure 8. Magnetic susceptibility of palladium on silica gel as function of adsorbed hydrogen and NO at 0° C.

NMR STUDIES of ADSORBED CARBON MONOXIDE. In the n.m.r. investigation of molecules adsorbed on solid supported metals, such as palladium on silica or platinum on alumina, both the chemical shift and line width of the observed nucleus in the adsorbate are drastically affected by the physical properties of the sample. The chemical shift anisotropy of a nucleus in a molecule... 

In hexachlorocyclopentadiene, all the chlorines are hydrogenolyzed and both double bonds saturated in vapor phase hydrogenation over palladium on silica at 330 "C, giving cyclopentane in 82% yield. ... 

In a different type of reaction, alkenes are photooxygenated (with singlet O2, see 14-7) in the presence of a Ti, V, or Mo complex to give epoxy alcohols, such as 180, formally derived from allylic hydroxylation followed by epoxidation. " In other cases, modification of the procedure gives simple epoxidation. " Alkenes react with aldehydes and oxygen, with palladium-on-silica " or a ruthenium catalyst, " " ... 

Palladium on silica gel or charcoal has been found to promote tire decarbonization of carbon monoxide,00 whereas a nickel catalyst consisting... 

Isobutylidenediurea is used as a slow-release fertilizer. If there is a surplus of isobutyraldehyde, recent work has shown that it can be decarbonylated to propylene over a palladium on silica catalyst.220 There is also the possibility of dehydrogenation to methacrolein for conversion to methaciylic acid, then to methyl methacrylate. Dehydration of isobutyl alcohol would produce isobutylene for conversion to methyl-ferf-butyl ether, although this would probably be uneconomical. 

A supported palladium catalyst for the Heck reaction (5.13) could be reused several times with little loss in activity.39 Heterogeneous catalysts for the Heck reaction, such as palladium on silica and platinum on alumina can perform just as well in the Heck reaction as homogeneous catalysts.40... 

Ethylene. During World War II the Germans manufactured more than 60,000 t/yr of ethylene [74-85-1], C2H4, by hydrogenation of acetylene, using palladium on silica gel as catalyst. Subsequendy, cracking of hydrocarbons displaced this process. However, it is still utilized for purification of ethylene containing small amounts of acetylene as contaminant (5) (see Ethylene). 

Noble Metal Catalysts. The most frequently mentioned noble metal hydrocracking catalysts are platinum and palladium on silica-alumina. The unsulfided noble metal catalysts have high hydrogenation activity and lower acidity than the supported metal sulfides discussed previously. 

Eisen and Ivanov [54] converted hydrocarbons of various types in a stream of hydrogen on a 0.5% palladium on silica gel catalyst. For carrying out reaction chromatographic analysis of hydrocarbon mixtures, optimum temperatures for the hydrogenation of aromatic hydrocarbons are 315—325°C. Simultaneously at the specified temperatures hydrogenation of olefins, diolefins, cyclopentenes, etc., occurs. Conversions of the hydrocarbons are presented in Table 4.1. 

Eisen and Ivanov [60] elaborated a pulse, micro-catalytic gas chromatographic method of selective hydrogenation of alkenes and cyclohexenes at 90°C on 5% palladium on silica gel as the catalyst. Under these conditions aromatic hydrocarbons are not hydrogenated, and cyclopentenes are hydrogenated to an insignificant extent. 

The nickel-on-kieselguhr catalyst was from the same lot as used by H. S. Taylor for his catalytic researches. The 0.3 % palladium on silica gel and... 

The reactions between one volume of ethylene and two volumes of deuterium were studied with ethylene in the liquid phase at —78° and 400-800 psi over nickel on kieselguhr, palladium on charcoal, and palladium on silica-gel catalysts. The results as shown in Table I indicate an extensive redistribution reaction taking place among the deuteroethanes and no... 

Padgett R.E. and Beamer R.L. (1964) Stereospecific hydrogenation using palladium-on-silica gel catalysts, J. Pharm. Sci. 53,689-690. 

Other studies have illustrated that alkaline ILs such as l-butyl-3-methyU-midazolium dicyanamide [C4mim][DCA] or benzalkonirun methoxide [BA][MeO] as well as benzalkonium tert-butoxide ([BA][t-BuO]) enhance the selectivity toward citronellal [12, 13]. When the catalyst contained a layer of [C4mim][DCA] and palladium on silica, the selectivity toward citronellal approached 100%. However, the catalytic activity of the [C4mim][DCA] was very low although the citronellal yield was high (Figure 12.3) [12]. 

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