Pd-silica catalysts

The results of the asymmetric hydrogenations on the modified silica gels are given in Table 3.1. 

The mechanism proposed by Padgett and Biemer in 1964 was based on the idea that the catalysts have two adsorption sites which differ in their affinity for the two enantiofaces of the substrate molecules. 

Colloid catalysts with chiral stabilizing agents can be used as asymmetric catalysts in the reactions of prochiral compounds. One such catalyst is the well-known Skita-catalyst (colloidal Pt or Pd with gum-arabicum as an optical active polysaccharide as a protective colloid). These catalysts have been used very often in a number of hydrogenations of unsaturated compounds, including prochiral molecules, but never were their asymmetrizing action noted. Nevertheless, including chiral components as protective colloids in such catalysts allowed for the discovery of asymmetric effects in their action. Indeed, Balandin, Klabimovskii et al. found small 

For a long time this sort of catalysts received no attention as an asymmetric catalyst and only recently were the above mentioned results repeated and significantly improved. 

The Pt-PVP-Cnd catalyst widi a composition of 10 300 50, proved to be active in the hydrogenation of EtPy at 24°C and 4.6 bar hydrogen. Besides, Collier et al. reported the hydrogenation of EtPy with an ee of 40% at 17% conversion using a Pt colloid prepared by metal evaporation and stabilized with MeEtCO + DHCnd . 

---

Supported Cu-Pd and Ni-Pd catalysts were prepared by impregnation of aerosil and alumina supports with solutions of Cu and Ni salts followed by reduction of these precursors in flowing a hydrogen-helium mixture (1 10) at 360°C and modified with a solution of TA + PdCl2 at pH 5. The Ni-Pd-silica catalyst produced an optical yield of 7% from the hydrogenation of EAA. Table 4.15. shows the results... 

The pore diameter limitation also was found for Pd-Cnd catalysts active in the C=C hydrogenation of ( )-2-phenylcinnamic acid. On a 5% Pd-silica catalyst the ee s increased with increasing average pore diameter of silica, and the conclusion was that Pd metal particles should be located in pores large enough to accommodate the bulky alkaloid-modifier molecules and the substrate forming intermediate complex modifier-reactant that have been identified on the surface of Pd-titania catalyst (Nitta et al. ). Palladium metal particles in smaller pores are difficult to modify and they behave therefore as non-selective centers. 

Silica-supported metal (e.g., Pd/Si02) catalysts also have surface silanol groups that can react with the alkoxysilane groups of the complexes. These combination catalysts consist of a tethered complex on a supported metal. A Rh complex was tethered to the surface of a Pd/Si02 catalyst, and the tethered catalyst was more active for the hydrogenation of aromatic compounds than the free complex or the supported catalyst separately.33 It is possible that the H2 is activated on the supported metal and the hydrogen atoms migrate to the silica, where they react with the reactant molecules coordinated by the tethered complex. 

The Pd-KOH/silica, Pd-CsOH/siiica, Ni-KOH/siiica and Ni-CsOH/silica catalysts were tested at various temperatures. The selectivity to MIBK, MIBC, and isophorone as a function of temperature after 24 h on-line is shown in Figure 2. The conversion at the three temperatures is shown in Figure 3. 

Selective reduction of alkynes to cis alkenes using hydrosilane functions immobilized on silica gel in acetic acid in the presence of a Pd(0) catalyst system has been reported.239... 

Although supported Pd catalysts have been the most extensively studied for butadiene hydrogenation, a number of other catalysts have also been the object of research studies. Some examples are Pd film catalysts, molybdenum sulfide, metal catalysts containing Fe, Co, Ni, Ru, Rh, Os, Ir, Pt, Cu, MgO, HCo(CN)3 5 on supports, and LaCoC3 Perovskite. There are many others (79—85). Studies on the well-characterized Mo(II) monomer and Mo(II) dimer on silica carrier catalysts have shown wide variations not only in catalyst performance, but also of activation energies (86). 

Depending on the final application, the catalyst may require activation prior to the reaction. For example, if you prepare Pd/silica by impregnating the silica support with a Pd(N03)2 precursor, and then calcining and burning out the nitrates as nitrogen oxides, the calcination will also oxidize the Pd. Such catalysts are usually activated by treatment with FI2, reducing the oxides at the active site back to Pd°. 

Four kinds of supported Pd complexes are prepared (Scheme 3.13) catalyst I is the PEG-modified silica-supported Pd cluster catalyst II is the PEG-modified silica-supported (Cp)Pd(allyl) complex catalyst III is the silica-supported Pd cluster catalyst IV is the silica embedded in a thin PEG film-supported Pd cluster. 

In this respect it should be said that even open faces such as the (210) and the stepped (001) surface do not dissociatively adsorb CO at 25-125°C (97). This suggests that unsupported Pd is a rather poor methanation catalyst. Under 1 atm total pressure in a CO + H2 mixture, the Pd black catalyst (210-nm crystallites) produces methane but, here again, the activity level is about two times lower than that of Pd/Si02 catalysts (4.6-nm Pd particle size), and about two orders of magnitude less active than Pd/ A1203 catalysts (4.8-nm Pd particle size) (98). It therefore seems that the effect of dispersion here is not pronounced with respect to the support effect. Silica, as an inert support, does not influence the activity of Pd to the same extent as does more the acidic alumina. 

Careful infrared study of CO chemisorbed by Pd/Si02 catalysts in an SMSI versus a non-SMSI state verified that after HTR, silicon species are distributed in the Pd surface layer. For the catalysts reduced at 300°C (LTR), the B/L intensity ratio (B = bridging CO L = linearly bound CO) is a monotonic function of Pd particle size (Fig. 17). On the other hand, the B/L ratios for Pd/Si02 catalysts that experienced HTR show considerable departure from this universal curve (Fig. 17) (208). Of course, a relatively higher proportion of linearly bound CO for Pd/Si02 catalysts in the SMSI state is believed to follow from the existence of silicon, rather uniformly interdispersed in the metal surface, resembling the case of CO adsorption on silica-supported Pd-Ag alloys (209). 

Now we can consider the pressure gap and the structure gap. Concerning the pressure gap, it was concluded from IR spectroscopy that between UHY up to 10 Torr the reaction mechanism was the same for Pd/silica model catalyst [125]. 

The effect of the reverse spillover in the oxidation of CO on supported model catalysts has been observed by several other authors on various systems Pd/mica [133], Pd/alumina [103,131, 132, 144, 163] Pd/MgO [45, 161], Pd/silica [104] it can increase the reaction rate by a factor as large as 10. 

Shore, S. G., Ding, E. R., Park, C., and Keane, M. A., Vapor phase hydrogenation of phenol over silica supported Pd and Pd-Yb catalysts. Catalysis Commun 2002, 3 (2), 77-84. 

Benzodiazepines with reasonably high specific activity have been also tritium-la-belled165 using adsorbed tritium or activated tritium. These methods166,167 are fast and do not require one to synthesize the precursors. In the AcT (activated tritium) method the substrate to be labelled, spread on different catalyst supports (such as silica-alumina 980-25 containing 0.5% Ru, 1% Ni or 1% V alumina catalyst Al-3945, 1% Ni, 1% Pd or 5% Pd silica, 1% Ni and carbon, 4.6% Pd the metal catalysts were activated by heating to 600-700 °C in hydrogen or on untreated silica-alumina), has been reacted... 

---