The development of a catalytic process for the hydrotreatment of haloarenes

Abstract

The abatement of halogenated compounds released from various processes into the environment is now recognized as a pressing environmental issue. Catalytic hydrodehalogenation (HDH) has emerged as a potential technology that can facilitate waste transformation and reuse. In this thesis, a comprehensive study of the critical reaction/catalyst variables is considered for the HDH of a range of haloarenes (chloroand bromo-phenols and chlorobenzenes) using (commercial) Pd/Al2O3 and (laboratory synthesised) Au supported on -Fe2O3 and Fe3O4. Aqueous phase (T = 303 K) HDH is demonstrated to be structure sensitive where smaller Pd particles are intrinsically more active and metal/support interactions have a major impact on reaction selectivity (notably at pH = 3). HDH follows an electrophilic aromatic substitution mechanism: the rate of C–Br bond(s) scission is an order of magnitude greater relative to C–Cl (due to the lower bond dissociation energy). The presence of a second halogen substituent has a deactivating effect (by lowering electron density in the aromatic ring). Solvent (water, organic and water/organic mixtures) effects have been established where, in absence of mass transport limitations, reaction in water delivers significantly higher HDH rates and selectivity is unaffected by solvent composition. A mathematical analysis demonstrates that these effects are principally (ca. 80 % contribution) the result of the variations in solvent dielectric constant where the molar volume represents a secondary consideration. The advantages of a shift from batch to continuous HDH are established in terms of: (i) more efficient gas to liquid H2 transfer; (ii) enhanced HDH rates; (iii) prolongued catalyst lifetime. The potential of HDH as a means of waste transformation to a commercial product is demonstrated in the gas phase (T = 423 K) hydroconversion of 2,4-dichlorophenol into cyclohexanone (over Pd) and 4- chlorophenol (over iron-oxide supported Au). In both cases, a contribution due to spillover hydrogen is established. A comprehensive programme of (Au) catalysts synthesis and characterization, in terms of TPR, H2 chemisorption/TPD, XRD, DRS UV-Vis, SEM/TEM and BET/pore volume measurements is provided and correlated with HDH kinetics. The results presented in this thesis demonstrate the feasibility and flexibility (in terms of operation, rate/selectivity control and product reuse) of catalytic HDH as a progressive means of haloarene waste treatment.