Catalytic hydrodechlorination at low hydrogen partial pressures: Activity and selectivity response

The gas phase hydrodechlorination (HDC) of chlorobenzene (CB) and 1,3-dichlorobenzene (DCB) (373 K = T = 423 K, P_total = 1 atm) was performed over Pd/SiO₂ (1.4-8.3%, w/w Pd) where the hydrogen partial pressure (P_H2) was varied from 0.046 to 0.925 atm. The catalysts have been characterized in terms of BET area, temperature programmed reduction (TPR), hydrogen chemisorption/temperature programmed desorption (TPD) and TEM analyses. The HDC response to changes in contact time, chloroarene feed linear velocity and catalyst particle size were evaluated to establish reaction conditions wherein transport constraints were minimized, i.e. contact time = 2.33 s, feed rate = 0.014 mol_Cl h^-1 and particle size = 45-75 µm. HDC selectivity was found to be influenced by transport constraints, which resulted in a preferential partial HDC of 1,3-DCB. Under conditions of chemical control, larger Pd particles (mean values in the range 3-9 nm) delivered higher specific HDC rates. The dependence of HDC rate on P_H2 can be accounted for in terms of a Langmuir-Hinshelwood type model involving dissociative adsorption of H₂ and associative adsorption of chloroarene with competition for surface sites. Applicability of this Model is demonstrated in terms of the ?² (chi-square) test where the kinetic Models were discriminated on the basis of the standard F-test (% confidence). A dependence of HDC selectivity on P_H2 is demonstrated where the conversion of 1,3-DCB at a molar H₂/DCB inlet = 1.25 (P_H2 = 0.092 atm) resulted in a higher selectivity to benzene as a result of a lesser competition for surface sites. © 2009 Elsevier B.V. All rights reserved.