Hydrogenation benzaldehyde

Laine and co-workers have studied the mechanism involved in rhodium-catalysed benzaldehyde hydrogenation, using [Rh6(CO)i6] as catalyst precursor. Following kinetic arguments, the authors proposed cluster catalysis with a limiting step corresponding to the break of metal-metal bond and/or isomerisation of the cluster formation [22]. 

Because of the complexity of the rhodium-catalyzed reduction of benzaldehyde to benzyl alcohol with CO and H20, it is not possible to fully elucidate the mechanism of catalytic reduction given the extent of the kinetic studies performed to date. However, the results do allow us to draw several important conclusions about the reaction mechanism for benzaldehyde hydrogenation and several related reactions. 

Vidal and Walker (17) have observed that Rh(CO)A is in equilibria with Rh5(C0)i5". It is likely that Rh(CO) is also in equilibria with the cluster species in equations (5)-(8) thus, it must also be a part of the benzaldehyde hydrogenation catalyst solution. 

The effects of benzaldehyde concentrations on turnover frequency are anomalous. Our results indicate that benzaldehyde hydrogenation turnover frequency is independent of benzaldehyde concentration (an apparent zero-order dependence). However, the data in Table 2 indicate otherwise. If the reaction were independent of aldehyde concentration, the rate data should be independent of the type of aldehyde used. This is especially true with p-tolualdehyde and p-anisaldehyde where the structural changes to the aldehyde (addition of p-methyl or p-methoxy) should influence the reactivity of the aldehyde functionality only through electronic effects. Thus, we are forced to conclude that the aldehyde is involved in the rate determining step even though the concentration study does not support its presence. 

Schreifels et al.(Ref. 2) measured the rate of benzaldehyde hydrogenation at 70°C and 6 atm., using Raney nickel. They found that the rate of reaction... 

Benzaldehyde hydrogenation was carried out in a glass batch reactor fitted with a reflux condenser, a mechanical stirrer and an external thermostating jacket [9]. The catalyst (corresponding to 4 mg Pd) was suspended in the solvent (100 ml ethanol) and pretreated in H2 flow (30 ml/min) at 80°C for 1 h. After cooling to the reaction temperature (20°C) benzaldehyde (1.0 ml) and n-octane (0.3 ml), used as internal standard, were added through a serum cap fitted to one arm of the reactor. The reactor was stirred at 1500 rpm and operated at atmospheric pressure in H2 flow. The progress of the reaction was... 

The first-order rate constant of benzaldehyde hydrogenation to benzyl alcohol [9], referred to Pd mass, is reported in Table 1. 

TPR and WAXS data showed that, when Pd and Au have been coimpregnated on active carbon, almost complete alloying occurs. When impregnation of the two metals is consecutive, partial alloying, with the formation of different Au-rich alloys, occurs only after suitable thermal treatment. CO chemisorption data showed that Pd dispersion does not change after the addition of Au, whose surface enrichment is therefore to be excluded. Au proved to have no effect on the Pd activity (per gram) in benzaldehyde hydrogenation, but the increase of turnover frequency with Pd particle size has been confirmed. 

We have prepared the supported Rh(DiPFc) on alumina using PTA as the anchoring agent and have used this catalyst for the hydrogenation of benzaldehyde and several substituted benzaldehydes. Table 2 lists the rate data for the benzaldehyde hydrogenations run in both ethanol and i-propanol. While... 

Thus, benzaldehyde hydrogenation was tested imder practice-relevant conditions in a catalyst test reactor of simple design, and parameter smdies were carried out. The construction of the laboratory plant is shown schematically in Figure 13-17. Since we are dealing with an integral reactor, in spite of the relatively small amount of catalyst in the trickle-bed reactor, only comparitive measurements were carried out. Continuous hydrogenation of benzaldehyde in the solvents hexane and isopropanol ... 

Benzaldehyde hydrogenation reaction was carried out in a fixed-bed Pyrex reactor tube at atmospheric pressure with 200mg of samples and a total flow of 50ml/min. Before testing, the catalysts were in-situ reduced for 16 h at 350" C in a current of H2 (20ml/min). The gaseous reactant and products were analyzed on line by flame ionisation (FID) detector (Delsi ICG 121 Ml). 

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