Hydrodechlorination Reaction

Pd-Ag bimetallic catalysts supported on carbon xerogels have been used in the hydrodechlorination reaction of 1,2-dichloroethane [103,104], Pd and Ag were deposited by co-impregnation using a solution of palladium and silver nitrates. Metal particle size ranged from 2 to 5 nm in Pd catalysts but had a wider distribution (4 to 20 mn) in Ag catalysts. Bimetallic Pd-Ag catalysts showed small particle alloys of 3 to 4 nm. The bulk Ag content in this alloy was limited to about 50 wt%, which fixed the minimum Pd surface content of the alloy at about 10 wt%. Pd catalysts produced mainly ethane, whereas bimetallic Pd-Ag catalysts were selective for the production of ethylene. The ethylene selectivity increased with silver fraction at the alloy surface. 

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Lindane, a widespread insecticide constituted of technical mixtures of hexa-chlorocyclohexane (HCH), was dechlorinated using Pd/C, Pt/C, or Raney-Ni. The product using the multiphasic system was benzene, obtained within 1 hour. The base initially promoted HCl ehmination from HCH to yield trichlorobenzene, which then underwent the usual hydrodechlorination reaction. 

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. 

When using Pt/a-alumina, the product selectivity did not change with time (ethane was the primary product), although TCA conversion declined rapidly. The difference between the selectivities for the two Pt catalysts as they deactivate suggests that the ri-alumina support may initiate the reaction. Specifically, HCl elimination to form DCE (Reaction 2) appears to be the initial reaction that occurs primarily on the acidic sites of the ri(5)-alumina support. Hydrogenation of this unsaturated chlorocarbon to DCA then occurs on the Pt. This is consistent with the observation that large concentrations of DCE were only observed on the two catalysts that had acidic sites (Ti8-alumina,Pt/Ti-alumina Table 1). However, Pt has activity for the HCl elimination step since the Pt/a-alumina was active for the hydrodechlorination reaction while the a-alumina was not. In addition, because DCE was observed in the effluent when using the Pt/p-alumina before the conversion decreased, deactivation most likely occurred first by the loss of... 

Palladium complexes containing basic, bulky phosphines (dippp, dippe, dippb, i-Pr3P) have been demonstrated to efficiently catalyze the hydrodechlorination reaction of chloroarenes with methanol or sodium formate (Eq. 14) [102]. Of the ligands examined, dippp exhibited the highest catalytic activity. 

Metal-doped carbon gels have been used as catalysts in several reactions, including the removal of pollutants in either gas or aqueous phase, ORR and methanol electrooxidation for fuel cell applications, C=C double-bond hydrogenations, skeletal isomerization and hydrodechlorination reactions, and other organic syntheses. The activity and/or selectivity of metal-doped carbon gels was better than that of other, more widely used supported catalysts in many of the... 

In addition to the reaction pathways of the previously discussed substrates, 4-chlorobenzaldehyde 1c can be hydrodehalogenated. Indeed, the main product obtained using the Pd catalysts on fumed titania was toluene 6c which is the hydrogenolysis product of 4-chlorotoluene 3c. No benzyl alcohol 2c was formed. Apart from about 50-60% of toluene, 4-chlorotoluene 3c, ether 4b and saturated aromatic ring product 5c are present in the range of between 5 and 30%. In this case, the different acidic properties of the Pd catalysts on fumed titania are of minor importance. The release of hydrogen chloride from the hydrodechlorination reaction compensates for the initial differences of the catalyst acidity. 

In order to investigate the modification of catalysts during the hydrodechlorination reaction, we performed an EXAFS analysis of the more significant Pt samples. 

The small oxygen and/or chlorine contribution to the signal is thus covered by the heavy metal scatterers. The PtOO catalyst after reductive activation exhibits a dispersion almost at monoatomic level (CN(Pt)=1.6) and, after catalytic run, a moderate increase in particle size (CN(Pt) = 4.6) and the presence of chlorine (CN(Cl) = 0.9) on the metallic phase. These results cannot be rationalized on the basis of the structure insensitivity of the hydrodechlorination reaction, since the most active catalyst, Pt09, shows a much higher size of Pt particles. Catalytic activity seems, therefore, to be related to the stability of metal phases under reaction conditions. 

The initial chlorine doping of MgO support upon catalyst preparation plays an important role in preventing excessive overheating phenomena at the beginning of the hydrodechlorination reaction, when MgO reacts with forming HCl to form a Sorel-type magnesium oxo-chloride phase, which is stable under reaction conditions. 

Usually, hydrodechlorination is performed in the presence of noble metals (e.g., Pd, Pt, and Rh). Different metals will lead to different hydrodechlorination reaction activity and selectivity. In many instances, Pd is found to have the highest hydrodechlorination reaction activity. 

R158 I. Hannus, Zeolites as Catalyst Support in Hydrodechlorination Reaction , in Silica and Silicates in Modern Catalysis, ed. I. Halasz, Transworld Research Network, Trivandrum, India, 2010, p. 477. 

This result can be explained assuming that the number of chlorine atoms attached to the organic structure has a stronger influence on the adsorption strength than the type of organic structure (TTCE has four chlorine atoms and CBZ only one, while aromatic structures are considered to hold stronger interaction with the metallic surface). The large importance of the chlorine atoms in the metal-catalysed hydrodechlorination reactions and their substantial role in... 

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