Supported palladium silica

Silyl enol ethers are other ketone or aldehyde enolate equivalents and react with allyl carbonate to give allyl ketones or aldehydes 13,300. The transme-tallation of the 7r-allylpalladium methoxide, formed from allyl alkyl carbonate, with the silyl enol ether 464 forms the palladium enolate 465, which undergoes reductive elimination to afford the allyl ketone or aldehyde 466. For this reaction, neither fluoride anion nor a Lewis acid is necessary for the activation of silyl enol ethers. The reaction also proceed.s with metallic Pd supported on silica by a special method[301j. The ketene silyl acetal 467 derived from esters or lactones also reacts with allyl carbonates, affording allylated esters or lactones by using dppe as a ligand[302]... 

A silica-supported palladium reagent has been used to convert iodobenzene to butyl benzoate, in the presence of CO and butanol. Diaryl ketones can also be... 

The separation factors are relatively low and consequently the MR is not able to approach full conversion. With a molecular sieve silica (MSS) or a supported palladium film membrane, an (almost) absolute separation can be obtained (Table 10.1). The MSS membranes however, suffer from a flux/selectivity trade-off meaning that a high separation factor is combined with a relative low flux. Pd membranes do not suffer from this trade-off and can combine an absolute separation factor with very high fluxes. A favorable aspect for zeoHte membranes is their thermal and chemical stability. Pd membranes can become unstable due to impurities like CO, H2S, and carbonaceous deposits, and for the MSS membrane, hydrothermal stability is a major concern [62]. But the performance of the currently used zeolite membranes is insufficient to compete with other inorganic membranes, as was also concluded by Caro et al. [63] for the use of zeolite membranes for hydrogen purification. 

Figure 4.5. XRD pattern showing the and (200) reflections of Pd in two silica supported palladium catalysts and of a c reference sample. The reader may use th ...

Figure 6.3 XRD pattern showing the (111) and (200) reflections of Pd in two silica-supported palladium catalysts and of a Pd reference sample. The reader may use the Bragg equation (6-1) to verify that the Pd (111) and (200) reflections are expected at angles 20 of 40.2° and 46.8° with Cu Ka radiation (lattice constant of Pd is 0.389 nm, d ] =0.225 nm, d2oa=0.194 nm, 2=0.154 nm from Fagherazzi et al. [8]).

Another way to produce acetic acid is based on a carbonylation of methanol in the so called Monsanto process, which is the dominant technology for the production of acetic acid today [15]. Acetic acid then is converted to VAM by addition of ethylene to acetic acid in the gas phase using heterogeneous catalysts usually based on palladium, cadmium, gold and its alloys (vinylation reaction 3 in Fig. 2) [16] supported on silica structures. 

The co-existence of at least two modes of ethylene adsorption has been clearly demonstrated in studies of 14C-ethylene adsorption on nickel films [62] and various alumina- and silica-supported metals [53,63—65] at ambient temperature and above. When 14C-ethylene is adsorbed on to alumina-supported palladium, platinum, ruthenium, rhodium, nickel and iridium catalysts [63], it is observed that only a fraction of the initially adsorbed ethylene can be removed by molecular exchange with non-radioactive ethylene, by evacuation or during the subsequent hydrogenation of ethylene—hydrogen mixtures (Fig. 6). While the adsorptive capacity of the catalysts decreases in the order Ni > Rh > Ru > Ir > Pt > Pd, the percentage of the initially adsorbed ethylene retained by the surface which was the same for each of the processes, decreased in the order... 

With alumina-supported palladium, platinum and rhodium and silica-supported platinum [65,66] in the temperature range 20—200°C, no molecular exchange between adsorbed 14C-ethylene and gaseous ethylene is observed, whilst with hydrogen, small quantities of methane are formed at 100°C and above with platinum and rhodium and at 200° C withpallad-... 

Acetylene, when adsorbed on active nickel catalysts, undergoes self-hy-drogenation with the production of ethylene [91], although the extent of this process is less than with ethylene. Similar behaviour has been observed with alumina- and silica-supported palladium and rhodium [53], although with both of these metals ethane is the sole self-hydrogenation product some typical results for rhodium—silica are shown in Fig. 21. 

Adsorption and Catalytic Properties of Palladium Supported by Silica, Alumina, Magnesia, and Amorphous and Crystalline Silica-Aluminas... 

Several supports were studied, including so-called poly-alumazane, which is prepared by subsequent treatment of silanol rich silica with aluminum trichloride and ammonia. With the resulting support palladium catalysts with very high dispersion were obtained. 

Microwave irradiation also shows a beneficial effect in the preparation of solid-supported palladium catalysts for hydrogenation reactions. Thus, alumina- and silica-supported palladium catalysts were synthesised by conventional and microwave heating, and their physical properties and catalytic activity in the hydrogenation of benzene were compared. The alumina-based system prepared under microwave conditions showed turnover numbers an order of magnitude higher than the conventionally prepared catalysts28. 

Prasad, P.S.S., Lingaiah, N., Rao, P.K., Berry, F.J. and Smart, L.E., The influence of microwave-heating on the morphology and benzene hydrogenation activity of alumina-supported and silica-supported palladium catalysts, Catal. Lett., 1995, 35, 345-351. 

The gas-phase process, successfully commercialized independently by Bayer and USI,417 involves passing a mixture of ethylene, acetic acid and oxygen over a supported palladium catalyst contained in a multitubular reactor at 150 °C and about 5-10 atm pressure. The overall yield in vinyl acetate is about 92%, and the major by-product is C02. The catalyst consists of a palladium salt (e.g. Na2PdCl4) deposited on silica (or alumina) in the presence of a cocatalyst (e.g. HAuC14), reduced and impregnated with potassium acetate before use.384,418 The lifetime of the catalyst is about 2... 

A large number of heterogeneous catalysts have been tested under screening conditions (reaction parameters 60 °C, linoleic acid ethyl ester at an LHSV of 30 L/h, and a fixed carbon dioxide and hydrogen flow) to identify a suitable fixed-bed catalyst. We investigated a number of catalyst parameters such as palladium and platinum as precious metal (both in the form of supported metal and as immobilized metal complex catalysts), precious-metal content, precious-metal distribution (egg shell vs. uniform distribution), catalyst particle size, and different supports (activated carbon, alumina, Deloxan , silica, and titania). We found that Deloxan-supported precious-metal catalysts are at least two times more active than traditional supported precious-metal fixed-bed catalysts at a comparable particle size and precious-metal content. Experimental results are shown in Table 14.1 for supported palladium catalysts. The Deloxan-supported catalysts also led to superior linoleate selectivity and a lower cis/trans isomerization rate was found. The explanation for the superior behavior of Deloxan-supported precious-metal catalysts can be found in their unique chemical and physical properties—for example, high pore volume and specific surface area in combination with a meso- and macro-pore-size distribution, which is especially attractive for catalytic reactions (Wieland and Panster, 1995). The majority of our work has therefore focused on Deloxan-supported precious-metal catalysts. 

The industrially important acetoxylation consists of the aerobic oxidation of ethylene into vinyl acetate in the presence of acetic acid and acetate. The catalytic cycle can be closed in the same way as with the homogeneous Wacker acetaldehyde catalyst, at least in the older liquid-phase processes (320). Current gas-phase processes invariably use promoted supported palladium particles. Related fundamental work describes the use of palladium with additional activators on a wide variety of supports, such as silica, alumina, aluminosilicates, or activated carbon (321-324). In the presence of promotors, the catalysts are stable for several years (320), but they deactivate when the palladium particles sinter and gradually lose their metal surface area. To compensate for the loss of acetate, it is continuously added to the feed. The commercially used catalysts are Pd/Cd on acid-treated bentonite (montmorillonite) and Pd/Au on silica (320). 

Among the transition metals, Pd, Pt, Ir, and Rh in various forms have been reported active in methanol synthesis (32-34), as noted in Section II. Palladium, platinum, and iridium metals were supported on silica, and it has recently been suggested that palladium is present in its valence state Pd(II) which is the active form of the catalyst (70). Under the synthesis conditions the Pd(II) ions could not survive the highly reducing atmosphere of the CO/H2 synthesis gas, and so this valence state would have to be induced by the presence of silicon dioxide. Should this be a general case, silica would not act merely as an inert support, and the silica-supported transition metals would have to be considered binary catalysts whose active state is formed by a support-metal interaction. ... 

Several heterogeneous catalysts have been developed for the hydroxylation of alkanes under mild conditions.68,69 One of them is the bi-catalytic system, which combines the ability of palladium to convert hydrogen and oxygen to hydrogen peroxide, with the capability of the iron ions to activate the hydrogen peroxide to hydroxylate hydrocarbons.70 Iron oxide and palladium supported on silica have been used as efficient catalysts for the oxidation of cyclohexane to the alcohol and ketone, via the in situ generation of hydrogen peroxide in an acetone solvent.71... 

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