Eun-Jae Shin

Chlorine–nickel interactions in gas phase catalytic hydrodechlorination: catalyst deactivation and the nature of reactive hydrogen

The gas phase hydrodechlorination of chlorobenzene and 3-chlorophenol (where 473 K⩽T⩽573 K) has been studied using a 1.5% w/w Ni/SiO2 catalyst which was also employed to promote the hydrogenation of benzene, cyclohexene and phenol. In the former two instances the catalyst was 100% selective in removing the chlorine substituent, leaving the aromatic ring intact. While the dechlorination of chlorobenzene readily attained steady state with no appreciable deactivation, the turnover of 3-chlorophenol to phenol was characterised by both a short and a long term loss of activity. Chlorine coverage of the catalyst surface under reaction conditions was probed indirectly by monitoring, via pH changes in an aqueous NaOH trap, HCl desorption after completion of the catalytic step. Contacting the catalyst with the chlorinated reactants was found to severely limit and, depending on the degree of contact, completely inhibit aromatic ring reduction although a high level of hydrodechlorination activity was maintained. Hydrogen temperature programmed desorption (TPD) reveals the existence of three forms of surface hydrogen which are tentatively assigned as: (i) hydrogen bound to the surface nickel; (ii) hydrogen at the nickel/silica interface; (iii) spillover hydrogen on the silica support. The effect of chlorine–nickel interactions on the resultant TPD profiles is presented and discussed. The (assigned) spillover hydrogen appears to be hydrogenolytic in nature and is responsible for promoting hydrodechlorination while the hydrogen that is taken to be chemisorbed on, and remains associated with, the surface nickel metal participates in aromatic hydrogenation. Hydrodechlorination proceeds via an electrophilic mechanism, possibly involving spillover hydronium ions. The experimental catalytic data are adequately represented by a kinetic model involving non-competitive adsorption between hydrogen and the chloroaromatic, where incoming chloroaromatic must displace the HCl that remains on the surface after the dechlorination step. Kinetic parameters extracted from the model reveal that chlorophenol has a higher affinity than chlorobenzene for the catalyst surface but the stronger interaction leads to a greater displacement of electron density at the metal site and this ultimately leads to catalyst deactivation. IntroductionThe reductive dehalogenation of organic halides is not only important as a synthetic rout but is now gaining increasing significance as a potential methodology for treating toxic halogenated waste In the latter application, the organic halide is converted to the corresponding hydrocarbon and the HCl that is produced can be readily separated while the hydrocarbon is recycled as a means of waste minimisation. Thermal hydrodechlorination processes only proceed to an appreciable degree at temperatures in excess of 973 K5 but the presence of a catalyst lowers the operating temperature significantly The catalytic hydrodechlorination of chlorobenzene and chlorophenol(s), known environmental hazards promoted using a solid silica supported nickel catalyst is considered in this paper. The catalytic hydrodechlorination of chlorobenzene(s) has been reported in both the ga–11 and liqui–14 phases using palladium–16 platinum rhodiu and nicke based catalysts. The treatment of chlorophenols has, by comparison, received less attention but catalytic data are available for the liquid phase reaction over carbon supported palladiu and gas phase transformations over nickel system as well as the electrochemical dechlorination on palladized electrodes Liquid phase hydrodehalogenations can proceed in the presence of both molecular hydrogen (at pressures up to 50 atm) and hydrogen donors such as metal hydrides, formic acid and its salts and alcohols The mechanism of C–Cl bond hydrogenolysis in heterogeneous systems is still far from understood and a number of kinetic models have been propose–11,13,15 to account for the observed catalytic trends. Moreover, the reaction has been viewed in terms of both electrophili and nucleophili substitution and attempts have been made to identify the possible reactant–catalyst interactions A marked drop in dechlorination activity with reaction time has been reported for supported palladium rhodiu and bulk nickel catalyst while time invariant reaction profiles have been generated for Ni/Al2O3 (ref. 8) and a Pt/zeolite The drop in dechlorination activity has been linked in one instance to a loss of the supported active phas and also to a surface poisoning due to the formation of stable surface chloride species The study of catalytic dechlorination is still in a formative stage and the published studies are, in essence, a compilation of rate data which characterise individual systems while such issues as the nature of the reactive adsorbed species, the catalytically active site(s) and the source of catalyst deactivation (when it occurs) are still not established. In this paper we provide kinetic data for the gas phase (molecular) hydrogen treatment of chlorobenzene and 3-chlorophenol and, where feasible, compare our results with the above cited reports. The effect of prolonged exposure of the nickel/silica catalyst to concentrated chlorinated aromatic gas streams is examined, particularly in terms of the changes to the nature of the surface reactive hydrogen.

Detoxification of dichlorophenols by catalytic hydrodechlorination using a nickel/silica catalyst

Heterogeneous catalytic dechlorination is presented as a viable means of treating/detoxifying concentrated chlorinated gas streams. The gas-phase hydrodechlorination of the six individual dichlorophenol (DCP) isomers was studied over the temperature range 473K⩽T⩽573K using a 1.5% w/w Ni/SiO2 catalyst. The variation of catalyst activity and selectivity with time on stream and temperature is illustrated while the possible role of thermodynamic limitations is addressed. The catalytic conversion of the three chlorophenol (CP) isomers is also considered for comparative purposes where, in every instance, the catalyst is 100% selective in promoting dechlorination, leaving both the benzene ring and hydroxyl substituent intact. A sequence of increasing chlorine removal rate constants (at 573 K) is established, i.e. 2,3-DCP<2-CP<4-CP<3-CP⩽2,5-DCP<2,4-DCP⩽2,6-DCP<3,4-DCP<3,5-DCP, and discussed in terms of steric, inductive and resonance stabilisation effects. Detoxification efficiency is quantified by phenol selectivity and the ultimate partitioning of chlorine in the parent organic or product inorganic host. Hydrodechlorination is shown to be an electrophilic reaction where, in the absence of appreciable steric constraints, chlorine removal is more energetically demanding from DCP than CP. The reaction pathway, with associated pseudo-first-order rate constants, for the conversion of each DCP isomer is presented.

Gas phase catalytic hydroprocessing of trichlorophenols

The catalytic hydrodechlorination of four trichlorophenol (TCP) isomers (2,3,5-TCP, 2,3,6-TCP, 2,4,5-TCP and 2,4,6-TCP) was studied in the gas phase using an Ni/SiO2 catalyst over the temperature range 473 K ≤ T ≤ 573 K. The catalyst was 100% selective in removing chlorine(s), leaving the hydroxyl group and benzene ring intact. Dechlorination proceeds via stepwise and concerted routes and the relative importance of each is dependent on the nature of the isomer where steric rather than resonance effects appear to determine the ultimate product distribution. Dechlorination efficiency is quantified in terms of phenol yield, chlorine removal rate and the ultimate partitioning of chlorine in the parent organic or product inorganic host. The reaction pathways, with associated pseudo-first order rate constants, for the conversion of 2,3,6-TCP and 2,4,6-TCP are presented. The effect of time and temperature on process selectivity is discussed and the nature of catalyst deactivation is considered. © 2000 Society of Chemical Industry

Gas-phase hydrodechlorination of pentachlorophenol over supported nickel catalysts

The gas-phase hydrodechlorination of pentachlorophenol (PCP) over nickel/silica and nickel/Y zeolite catalysts at 573 K has been studied. Each catalyst was 100% selective in cleaving the C–Cl bonds, leaving the hydroxyl substituent and benzene ring intact. The variation of catalytic activity and selectivity (in terms of partial and full dechlorination) with time-on-stream is illustrated and catalyst deactivation is addressed. Dechlorination efficiency is quantified in terms of dechlorination rate constants, phenol selectivity/yield and the ultimate partitioning of chlorine in the parent organic and product inorganic host. Increasing the nickel loading on silica was found to raise the overall level of dechlorination while the use of a zeolite support introduced spatial constraints that severely limited the extent of dechlorination. Product composition was largely determined by steric effects where resonance stabilisation had little effect. The reaction pathways, with associated pseudo-first-order rate constants, are also presented.

Detoxifying chlorine rich gas streams using solid supported nickel catalysts

Catalytic hydrogen treatment is presented as a viable low energy means of treating/detoxifying concentrated chlorinated gas streams to generate recyclable raw materials. Nickel (1.5% w/w and 15.2%) loaded silica and nickel (2.2% w/w) exchanged Y zeolite catalysts have been used to hydrotreat a range of chlorophenols (CPs), dichlorophenols (DCPs), trichlorophenols (TCPs) and pentachlorophenol (PCP) over the temperature interval 473 K</=T</=573 K. In every instance the nickel catalysts were 100% selective in cleaving the chlorine component from the ring, leaving the aromatic nucleus and hydroxyl substituent intact. The effects of varying process time and temperature are considered in terms of phenol yield and the ultimate partitioning of chlorine in the parent organic and product inorganic hosts. Chlorine removal rates, hydrodechlorination selectivity and apparent activation energies are also provided. Prolonged exposure of the catalysts to the concentrated chlorine gas streams resulted in an irreversible loss of activity which is related to the total concentration of chlorine that had been hydroprocessed. Hydrodechlorination proceeds via irreversible stepwise and/or concerted routes as is illustrated for the treatment of 2,3,5-TCP. Increasing the nickel content was found to raise the overall detoxification efficiency while the use of a zeolite support introduced spatial constraints which had a strong bearing on process selectivity.

Structure Sensitivity in the Hydrodechlorination of Chlorophenols

The gas phase hydrodechlorination of methanolic and mixed methanol/water solutions of 2-chlorophenol, 2,6-dichlorophenol, 2,4,5-trichlorophenol and pentachlorophenol has been studied at 573 K over nickel/silica catalysts of varying (1.5–20.3 wt.% Ni) nickel loading. Each catalyst was 100% selective in promoting hydrodechlorination: the variation of catalytic activity and selectivity with time-on-stream is illustrated and catalyst deactivation is addressed. Dechlorination is quantified in terms of specific rate constants, phenol selectivity/yield and chlorine removal efficiencies. Increasing the nickel loading resulted in a marked increase in dechlorination efficiency while the introduction of water into the feed lowered the activity.