Nickel particle size effects in catalytic hydrogenation and hydrodechlorination: phenolic transformations over nickel/silica

The gas phase hydrodechlorination of 2,4-dichlorophenol (2,4-DCP) and 2-chlorophenol (2-CP) has been studied over three series of Ni/SiO2 catalysts prepared by three different methods, i.e. (i) impregnation with nickel nitrate, (ii) impregnation with nickel ethanediamine and (iii) deposition-precipitation. This has led to catalysts with a wide range of Ni loadings (0.7-20.9% w/w Ni) and an average Ni particle size from 1.4 to 16.8 nm. The chloroarene was continuously fed to the reactor as a methanolic or aqueous solution (with subsequent vaporisation) where consistently higher hydrodechlorination activities were associated with the aqueous feed due to methanol/support interactions that served to inhibit the surface hydrogenolytic reaction. Hydrodechlorination of 2-CP yielded the fully dechlorinated aromatic product (phenol) and HCl while 2,4-DCP also yielded the partially dechlorinated (2-CP) aromatic product. There was no detectable aromatic ring reduction and the hydrodechlorination kinetics can be represented by a pseudo-first order approximation. Structure sensitivity has been conclusively established where the specific Cl removal was consistently greater with an increase in the average Ni particle diameter. The specific hydrodechlorination rate for 2,4-DCP was lower than that for 2-CP, diagnostic of a deactivating electronic effect associated with the Cl substituent(s) and this was manifested by higher apparent activation energies (DeltaE(a)) for the 2,4-DCP reaction; the DEa values were unaffected by catalyst preparation/activation/Ni loading. The direct hydrogenation of phenol (as feed) was largely insensitive to Ni particle size albeit there was a discernible decrease in specific phenol consumption over average Ni diameters less than ca. 3 nm. Hydrogenation selectivity remained unchanged and cyclohexanol was the predominant product irrespective of Ni loading or synthesis route.