Au-Pd Catalyst

The total amount of vinyl acetate produced per pulse agreed with the VAM STY data obtained from the fixed bed reactor with the exception that the Pd-Au catalyst produced more vinyl acetate in the TAP reactor than the Pd-Au w/KOAc catalyst. This can be attributed to the Pd-Au catalyst s VAM production rate being limited by the desoiption of VAM when operated at elevated pressures and with a... 

The points for Ag and Pd-Ag alloys lie on the same straight line, a compensation effect, but the pure Pd point lies above the Pd-Ag line. In fact, the point for pure Pd lies on the line for Pd-Rh alloys, whereas the other pure metal in this series, i.e., rhodium is anomalous, falling well below the Pd-Rh line. Examination of the many compensation effect plots given in Bond s Catalysis by Metals (155) shows that often one or other of the pure metals in a series of catalysts consisting of two metals and their alloys falls off the plot. Examples include CO oxidation and formic acid decomposition over Pd-Au catalysts, parahydrogen conversion (Pt-Cu) and the hydrogenation of acetylene (Cu-Ni, Co-Ni), ethylene (Pt-Cu), and benzene (Cu-Ni). In some cases, where alloy catalysts containing only a small addition of the second component have been studied, then such catalysts are also found to be anomalous, like the pure metal which they approximate in composition. 

The hydrogenation of benzene over supported Pd-Au catalysts initially exhibits a rise in activity as gold is added to the catalyst, but further addition brings about a pronounced activity decrease (772). The same authors find a marked increase in catalytic hydrogenation activity for Pd-Au alloy microspheres containing up to 60 at.% gold as compared with that measured for palladium. 

Vinyl acetate reactors that have a supported Pd/Au catalyst packed in 25-mm (0.082-ft) ID tubes and exothermic heat of reaction removed by generating steam on the shell side. The feed contains ethylene, oxygen, and acetic acid in the vapor phase at 150 to 175°C (302 to 347°F). 

Also, Marsh and co-workers [145] showed that gold on cobalt oxide particles, supported on a mechanical mixture of zirconia-stabilised ceria, zirconia and titania remains active in a gas stream containing 15 ppm SO2. Haruta and co-workers [207] found that although the low-temperature CO oxidation activity of Ti02-supported Au can be inhibited by exposure to SO2, the effect on the activity for the oxidation of H2 or propane is quite small. Venezia and co-workers [208] reported that bimetallic Pd-Au catalysts supported on silica/alumina are resistant to sulphur poisoning (up to 113 ppm S in the form of dibenzothiophene) in the simultaneous hydrogenation of toluene and naphthalene at 523 K. 

Venezia and co-workers also used bimetallic Pd-Au catalysts supported on ASA for the hydrogenation of aromatic compounds [208]. The simultaneous hydrogenation of toluene and naphthalene in the presence of dibenzothiophene was studied. This type of catalyst was shown to be resistant to sulphur poisoning (at 523 K, until 113 ppm S in the form of dibenzothiophene). 

Vinyl acetate is being produced industrially from acetic acid, ethene and oxygen using Pd-Au catalysts and the reaction proceeds with selectivities as high as 96% [103,237-239,243,244] (see Sect. 6.4.2) ... 

Still several problems remain unsolved to make the DSHP-HPPO process economically viable (i) safety the reaction of H2 with O2 in the presence of a flammable solvent (methanol) puts high hurdles on safety (ii) removal of acid and bromide the Bronsted acid and the bromide needed to produce HP from the elements have to be removed before the generated H P solution can be used for epoxidation (iii) solvent recycle after the generated H P solution has been used for epoxidation, the methanol has to be separated and recycled. During work-up some additional by-products are formed formaldehyde, acetaldehyde, propionaldehyde, methyl formate, dimethoxymethane, 1,1-dimethoxyethane and 1,1-dimethoxypro-pane. These compounds are difflcult to separate (many make azeotropes with methanol), so the recovered methanol will be contaminated. However, even small amounts of aldehydes or formates can poison the Pd or Pd/Au catalyst. Additional equipment needed to solve these problems will increase the investment costs. 

Influence of preparation procedure on physical and catalytic properties of carbon supported Pd-Au catalysts. 

It should be noted that despite the attention given to the liquid HO Ac layer, more traditional surface-mediated mechanisms remain the conventional model in the literature. For example, recent kinetic studies on vinyl acetate synthesis over Pd-only [9] and Pd/Au [10] catalysts suggest that the enhanced activity of the Pd/Au catalyst originates from its ability to provide more binding sites for O2 and thus increase the O2 mobility on the catalyst surface. 

The process, first disclosed in 1968, was commercialized in 1973. Yields in this process were very high (99%) and ease of operation was excellent. The major difficulty encountered was with catalyst precipitation during product removal. To minimize the problematic catalyst precipitation and to stabilize the catalyst, 10-15% water was included in the reaction mixture and the catalyst -product separation was conducted as an adiabatic flash. The inclusion of large amounts of water and the restriction to an adiabatic flash meant that the conversion was limited by product removal, not the reaction rate, and that there were large recycle streams of acetic acid and water. Additional minor difficulties were the cogeneration of traces of acetaldehyde which ultimately lead to propionic acid and iodine containing impurities. While the propionic acid was removable by distillation (with a dedicated unit of operation), the iodine has proven more problematic. It was important to remove essentially all the iodine (to < 40 ppb) during purification since iodine is a poison for the Pd/Au catalyst used in vinyl acetate production. 

R. C. Tiruvalam et al. Aberration corrected anal)tical electron microscopy studies of sol-immobilized Au -I- Pd, Au Pd and Pd Au catalysts used for benzyl alcohol oxidation and hydrogen peroxide production, Faraday Discuss., 2011, 152(0), 63-86. 

Other bimetallics synthesized via dendrimers include Ag-Au and Pd-Au sys-tems. " In the latter system, it was shown that the Pd-Au catalysts prepared via cocomplexation and sequential complexation routes exhibited different catalytic... 

The selective oxidation of arabinose to arabinonic acid by molecular oxygen has been carried out at 60°C in water on monometallic and bimetallic Pd-Au catalysts supported on nanosized ceria. Arabinose is extracted from a hemicellulose called arabi-nogalactan found in larch species, with galactose and D-glucuronic... 

Pd-Au catalysts for the production of vinyl acetate - review of the patent and scientific literature, H. Lansink-Rotgerink, Proc. GOLD 2006, Limerick, Ireland, Sept. 2006, 34. 

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