Hydrodechlorination and hydrogenation over Raney-Ni under multiphase conditions: Role of multiphase environment in reaction kinetics and selectivity

The hydrodechlorination (HDC) and hydrogenation (HG) of 1,3,5-trichlorobenzene (TCB) and of p-chloroacetophenone with hydrogen at atmospheric pressure and at 50degreesC over Raney-Ni in a biphasic organic-aqueous system and in the presence of a phase-transfer agent (Aliquat 336) were studied from a kinetic standpoint. The nature and the concentration of the aqueous base, as well as the concentration of Aliquat 336, were investigated. HDC of 1,3,5-TCB proceeds only in the presence of base and Aliquat 336, and the dechlorination rate of each subsequent chlorine removal increases by one order of magnitude. The highest rates are observed at around 3% KOH or 10% K2CO3. Carbonates and bicarbonates of potassium and sodium are shown to be better bases than KOH. Concentrated aqueous KOH inhibits the reaction, especially if used without Aliquat 336. The dependence of dechlorination rates on the concentration of Aliquat 336 suggests that the phase-transfer agent is adsorbed on the catalytic surface. For p-chloroacetophenone the dechlorination step always precedes reduction of carbonyl, even if is slower that the latter. The roles of base and of Aliquat 336 are not critical in this case, although higher KOH concentrations enhance dechlorination and inhibit reduction of hydroxyl in 1-phenyl ethanol. (C) 2002 Elsevier Science (USA).