Liquid phase catalytic hydrodechlorination of 2,4-dichlorophenol over carbon supported palladium: an evaluation of transport limitations

The catalytic hydrodechlorination (HDC) of aqueous 2,4-dichlorophenol (2,4-DCP) solutions over Pd/C catalysts (1-10% w/w Pd) has been investigated at 303 K in a stirred slurry reactor. The experimental results have shown that 2,4-DCP is converted to phenol quantitatively and 2-chlorophenol (2-CP) is the only intermediate product within detect limitations (greater than or equal to 0.2 mM). The system is 100% selective in terms of dechlorination and phenol hydrogenation only proceeds once complete dechlorination has been attained. The reaction pathway is illustrated and HDC progress is related to pH changes in solution. The mass-transfer limitations have been evaluated experimentally using the diagnostic criteria associated with varying hydrogen flow rate, stirring speed, catalyst concentration and particle size. Experimental results combined with parameter estimation have revealed the influence of mass transfer at the liquid/solid interface and intraparticle diffusion in limiting HDC rate. These effects can be minimized for the less active 1% w/w Pd/C catalysts where the stirring speed > 1000 rpm, hydrogen flow > 150 cm(3) min(-1), catalyst concentration < 0.5 g dm(-3) and particle sizes < 45 mum. The selectivity trends associated with 1% w/w Pd/C were the same whether the system operated under physical transport or chemical control. The selectivity with respect to 2-CP was however limited by mass-transfer processes in the HDC reaction using higher Pd loadings. (C) 2002 Elsevier Science Ltd. All rights reserved.