Hydrodechlorination catalysts

E-J Shin, A Spiller, G Tavoularis, MA Keane. Chlorine-nickel interactions in gas phase catalytic hydrodechlorination catalyst deactivation and the nature of reactive hydrogen. Phys. Chem., Chem. Phys. 1 3173-3181, 1999. 

In the preparation of hydrodechlorination catalyst the following surface reactions are involved ... 

Hydrogenation of Carbon Tetrachloride. Carbon tetrachloride can be hydrogenated, ie, hydrodechlorinated, to chloroform over a catalyst (25,26) or thermally (27). Although there are no industrial examples of this process at this time, it will receive more attention as more carbon tetrachloride becomes available as the CFC-11 and -12 markets decline (see, Chlorocarbons and chlorohydrocarbons, carbon tetrachloride). Chloroform can be further hydrodechlorinated to methylene chloride (28,29). 

Very recently, efforts have turned towards the use of [PdCii -CjHjjCKIPr)] 5 for the hydrodechlorination of polychlorinated phenyl substrates [14], In these cases, [PdCri -CjHjjCKIPr)] 5 proved to be less active than the non-allyl containing dimer [Pd( J,-Cl)Cl(IPr)]2 7 for the complete hydrodechlorination of 1,2,4,5-tetrachlo-robenzene to benzene, with conversions of 40% and 95% respectively found at 80°C and 0.02 mol% catalyst loading in the presence of KO Bu/ PrOH. 

An apparent particle size effect for the hydrodechlorination of 2-chlorophenol and 2,4-dichlorophenol was observed by Keane et al. [147], Investigating silica supported Ni catalysts (derived from either nickel nitrate or nickel ethane-diamine) with particles in the size range between 1.4 and 16.8 nm, enhanced rates for both reactions were observed with increased size over the full range (Figure 13). As electronic factors can be ruled out in this dimension, the observed behavior is traced back to some sort of ensemble effect, known from CFC transformations over Pd/Al203... 

Figure 13. Specific 2-CP (open symbols) and 2,4-DCP (solid symbols) hydrodechlorination rate constant K) as a function of the average Ni particle diameter ( nO for reaction over Ni catalysts prepared via impregnation with nitrate (0,0), deposition-precipitation (A,A) and impregnation with nickel ethanediamine ( , ) r= 423K reaction data refer to aqueous solutions. (Reprinted from Reference [147], 2003, with permission from Royal Society of Chemistry).

The next example originates from our own laboratory Two potential intermediates for the angiotensin-converting enzyme inhibitor benazepril can be synthesized using cinchona modified noble metal catalysts (3). While the hydrogenation of the a-ketoester has been developed and scaled-up into a production process (10-200 kg scale, chemical yield >98%, ee 79-82%), the novel enantioselective hydrodechlorination reaction (see Section in) could be a potential alternative to the established synthesis where the racemic a-bromobenzazepinon is used [75]. At the moment both selectivity and productivity of the catalyst are too low and substitution reactions occur less readily with the chloro analog. 

Reactions involved in the formation of OPDA are given in Scheme I. The most important issue in the preparation of DPDA from CNA is to find a specific catalyst, which can catalyze both the hydrodechlorination and the reduction steps. In this case the conversion of... 

The basis for the design of catalysts for Scheme I was the need to obtain higher rates for the hydrodechlorination step than for the reduction of the nitro group of CNA. If rate of conversion of CNA to 4-ch1oro- 1,2-orthophenylenediamine (CPDA1 is high in this case the formation of DPDA is strongly hindred as. due to the substituent effect, the rate of hydrodechlorination of CPUA is low. 

As shown in Fig. 4.,the rates of DPDA and CP0A formation and thus yields were strongly influenced by the amount of catalyst used. Under optimized reaction condition and at 100 % of conversion OPDA selectivity around 96 % could be obtained. Heat treatment is used to stabilize the ionic palladium formed in surface reaction (2). All of the catalysts were treated in nitrogen at 100°C for 3 hours. The increase of the temperature of treatment resulted in significant increase in the hydrodechlorination activity as shown in Fig. 5. 

Similarly, a Pd/Al203 catalyst system for the hydrodechlorination of CCl2F2 to CH2F2 was developed. Microwave irradiation produced the catalyst in a much shorter reaction times, and a twofold increase in catalytic activity was observed76. 

Berry, F.J., Smart, L.E., Prasad, P.S.S., Lingaiah, N. and Rao, P.K., Microwave heating during catalyst preparation influence on the hydrodechlorination activity of alumina-supported palladium-iron bimetallic catalysts,... 

Catalyst particles generally consist of a metal deposited onto the surface of a support and are denoted by metal/support, e.g. Pd/C indicates palladium metal on a carbon support. Among the metals used for catalysis, Pd is often found to be the most active metal. (Augustine 1965) For example, in the aqueous hydrodechlorination of 1,1,2-trichloroethane, Pd catalysts achieved significantly more conversion than Pt or Rh catalysts. (Kovenklioglu et al. 1992) Catalyst supports can vary in shape, size, porosity and surface area typical materials include carbon, alumina, silica and zeolites. 

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