Catalytic hydrodechlorination of 2,4-dichlorophenol over nanoscale Pd/Fe: Reaction pathway and some experimental parameters
Introduction
In recent years, considerable attention has been given to the use of zero-valent iron for hydrodechlorination of chlorinated hydrocarbons containing one to two carbon atoms (Burris et al., 1995; Burrow et al., 2000; Deng et al., 1999; Farrell et al., 2000a, Farrell et al., 2000b; Gotpagar et al., 1998; Matheson and Tratnyek, 1994; Su and Puls, 1999; Weber, 1996; Wüst et al., 1999). Although the kinetics in batch and column experiments, sorption of the reactants to the iron surface, reaction mechanism and the correlation of rate constants with vertical attachment energies (VAE) were studied, the transformation of chlorinated organic compounds with iron is much slow generally and chlorinated byproducts may be produced and accumulated (Muftikian et al., 1995).
Matheson and Tratnyek (1994) have proposed that the reaction pathways of reductive dechlorination in anoxic Fe–H2O system involve either direct electron transfer from zero-valent iron at the surface of the iron metal or reaction with dissolved Fe2+ or H2, which are products of iron corrosion. Activation of H2 by catalyst could facilitate the reductive dechlorination. It has been found that the application of palladium as a catalyst to iron surface can speed up the hydrodechlorination reaction. Pd/Fe bimetallic complex has shown high efficiency in hydrodechlorinating chloroethenes and chloromethanes, PCBs (Aroclor 1260 and Aroclor 1254) and chlorophenols (Grittini et al., 1995; Liu et al., 2001; Muftikian et al., 1995).
Nanoscale Pd/Fe bimetallic particles, compared with the conventional large particles, have some advantages, including a high specific surface area, a high surface reactivity owing to the presence of palladium and a much lower loading required in the hydrodechlorination process comparing with common iron and common Pd/Fe. Furthermore, the nanoscale particles could remain suspended under very gentle agitation. So it may be possible to inject them into contaminated soils, sediments and aquifers for in-situ remediation of chlorinated organic pollutants (Lien and Zhang, 1999). The application of nanoscale Pd/Fe particles to the hydrodechlorination of chlorinated aliphatics (Lien and Zhang, 1999, Lien and Zhang, 2001), chlorinated aromatics (Zhang et al., 1998) and PCBs (Aroclor 1254) (Wang and Zhang, 1997) has been reported. Faster hydrodechlorination reaction was achieved with nanoscale Pd/Fe bimetallic particles than with common iron and common Pd/Fe.
In this study, nanoscale Pd/Fe bimetallic particles were synthesized and used to hydrodechlorinate 2,4-dichlorophenol. 2,4-dichlorophenol is usually used as the intermediate to synthesize herbicides, pesticides, preservatives, disinfectants and medicines. It is listed as a priority pollutant by the US Environmental Protection Agency (Liu et al., 2001). The objectives of this research were to (1) determine the reaction pathway and reaction rate constants for each step; (2) investigate the effect of various experimental parameters on the reduction of 2,4-dichlorophenol and the formation of products and intermediates over nanoscale Pd/Fe.
Section snippets
Preparation of nanoscale Pd/Fe bimetallic particles
Nanoscale iron particles were produced by adding 0.54 mol/L NaBH4 (96%, Shanghai Chemical Reagent Company) aqueous solution dropwisely to a 1000 mL three-necked flask containing equal volume of 0.27 mol/L FeSO4·7H2O (99.0–101.0%, Juhua Chemical Reagent Company) aqueous solution with mechanical stirring at ambient temperature. Ferrous iron was reduced to zero-valent iron according to the following reaction:The iron particles were washed three times with
Characterization of the nanoscale Pd/Fe particles
Nanoscale Pd/Fe bimetallic particles with different palladium loadings were prepared and their Pd bulk loadings (wt%) are 0.0233%, 0.0541%, 0.0672% and 0.0932%. BET specific surface area (as) of the nanoscale Pd/Fe particles (Pd bulk loading of 0.0541%) is 12.4 m2/g, while a commercially available reduced iron powder (Jinshan metallurgical factory, >98.0%, <200 mesh) just has a BET specific surface area of 0.49 m2/g. Most of the particles are in the size range of 20–100 nm. XRD pattern indicates
Conclusions
In this study we have established that the nanoscale Pd/Fe bimetallic complex is efficient for hydrodechlorination of 2,4-dichlorophenol.
- (1)
2,4-dichlorophenol is transformed to 2-chlorophenol or 4-chlorophenol then to phenol, or to phenol directly. The pseudo-first-order rate constants for each step have been calculated: , , , , .
- (2)
Higher Pd bulk loading is favorable for the catalytic dechlorination of 2,4-dichlorophenol and the intermediates,
Acknowledgments
The authors greatly appreciate the financial support provided by Returnee Foundation of Ministry of Education, PR China (No. [2002]-247).
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