Mark A. Keane

The removal of cadmium and lead from aqueous solution by ion exchange with NaY zeolite

Lead and cadmium removal from aqueous solution by batch ion exchange with a solid Naue5f8Y zeolite has been studied under competitive and non-competitive conditions. The extent of heavy metal (HM) removal is found to be inlependent of the nature of the anion, and equilibrium exchange isotherms are presented for Naue5f8Y treatment of lead and cadmium nitrate and chloride solutions at 293 K. An increase in solution phase HM concentration lowers the affinity of the zeolite for the in-going HM ion, but lead was preferred to the indigenous sodium ion over the entire range of initial metal concentration to zeolite weight ratios (0.3–13 × 10−2 mol dm−3 gz−1) that were studied. Lead removal was much greater than that of cadmium under identical experimental conditions and Naue5f8Y exchange efficiency is shown to increase in the order Ni2+ < Cu2+ < Cd2+ < Pb2+. Exchange selectivity is discussed in terms of metal ion hydration and siting within the zeolite framework. A Pb/Cd/Naue5f8Y ternary exchange isotherm was constructed from 38 pairs of experimental points, and is treated quantitatively in terms of ternary and pseudo-binary separation factors. Treatment of the lead/cadmium solutions resulted in a greater depletion (by a factor of 2) of the lead component.

Gas phase catalytic dehydrochlorination and hydrodechlorination of aliphatic and aromatic systems

The gas phase dechlorination of cyclohexyl chloride and chlorobenzene, over the temperature range 423 K≤T≤573 K, promoted using a silica supported nickel catalyst in the presence of hydrogen has been studied. Chlorine removal from the chloroalkane is shown to occur by dehydrochlorination via an E1 type elimination mechanism to yield cyclohexene and HCl as the products. The reaction was found to exhibit zero order behaviour with respect to hydrogen partial pressure, a temperature dependent reaction order (varying from 0.4 to 0.7) with respect to the chloroalkane and an apparent activation energy equal to 115 kJ mol−1. Turnover of the cyclohexyl chloride reactant was subject to a short term loss of catalytic activity due to a surface poisoning by the HCl that was produced where the presence of hydrogen served to displace the inorganic halide and extend the productive lifetime of the catalyst. A steady state conversion of chlorobenzene was however readily achieved where dechlorination in hydrogen occurs via an electrophilic hydrodechlorination mechanism. Bromine removal from cyclohexyl bromide and bromobenzene is also considered for comparative purposes. The ease of halogen removal and process selectivity are discussed in terms of thermodynamic limitations and reactant/catalyst interactions.

Gas phase catalytic hydrodechlorination of chlorophenols using a supported nickel catalyst

Abstract The gas phase hydrodechlorination of single component and mixtures of o-, m- and p-chlorophenol was studied over the temperature range 423xa0K≤T≤573xa0K using a 1.5%xa0(w/w) Ni/SiO2 catalyst. The variation of catalyst activity with time-on-stream for each isomer is illustrated and the role of reaction temperature and thermodynamic limitations are addressed. The catalyst exhibits both short term and irreversible long term deactivation which is accounted for in terms of competitive adsorption and electronic effects. The hydrogen treatment of both phenol and chlorobenzene under the same reaction conditions are also considered for comparative purposes. The presence of the hydroxyl function enhances the rate of hydrodechlorination via an inductive effect. The relationship between rate and isomer structure is discussed on the basis of reactant adsorption where steric hindrance, in the case of the ortho-form, appreciably restricts dechlorination to such an extent that o-chlorophenol remains unreacted in equimolar o-/p- and o-/m-chlorophenol mixtures. Catalytic hydrodechlorination is viewed as a non-destructive low energy methodology for handling concentrated chlorine gas streams and relationships that describe the dependence of dechlorination rate on chlorine concentration are provided and can be used to evaluate the productive lifetime of the catalyst.

The removal of copper and nickel from aqueous solution using Y zeolite ion exchangers

Abstract Nickel and copper removal from aqueous solution by batch ion exchange with solid lithium-, sodium-, potassium-, rubidium- and caesium-based Y zeolites have been studied under competitive and non-competitive conditions. The extent of transition metal (TM) removal is dependent strongly on the nature of the out-going alkali metal (AM) cation with the overall preference of the zeolite for exchange with both metals increasing in the order CsY

The gas phase hydrodechlorination of chlorobenzene over nickel/silica

The gas phase hydrodechlorination of chlorobenzene over the temperature range 473 K⩽T⩽573 K has been studied using a 1.5% (w/w) Ni/SiO2 catalyst. Reproducible turnover frequencies are quoted and the effects of varying such process variables as reaction time and temperature, contact time, chlorobenzene and hydrogen partial pressures are presented. The catalyst was 100% selective in promoting hydrodechlorination and the aromatic ring remained intact in every instance. Under reaction conditions far removed from equilibrium conversions, the catalyst exhibited no appreciable short term deactivation while the maintenance of long term activity was also established. 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. The hydrogenolysis of bromobenzene, 2-chlorophenol and 3-chlorotoluene under the same reaction conditions were considered for comparative purposes where the turnover frequencies decrease (at 573 K) in the order 2-chlorophenol>3-chlorotoluene>chlorobenzene>bromobenzene; reactivity is discussed in terms of thermodynamic limitations and reactant/catalyst interactions. Reaction orders with respect to hydrogen and chlorobenzene partial pressures were obtained at different reaction temperatures and the experimental rate data are adequately represented by an extended power rate expression that approximates the Langmuir–Hinshelwood model for non-competitive adsorption. n n n n© 1999 Society of Chemical Industry

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.

A kinetic treatment of the gas phase hydrodechlorination of chlorobenzene over nickel/silica : beyond conventional kinetics

Abstract The gas phase hydrodechlorination of chlorobenzene over the temperature range 473 K ⩽T⩽573 K has been studied using a 1.5% w / w Ni / SiO 2 catalyst. The catalyst was fully selective in promoting dechlorination where benzene and HCl were the only detected products and there was no evidence of any catalyst deactivation. Reproducible dechlorination rates are provided and the effects of varying both chlorobenzene (0.02– 0.1 atm ) and hydrogen (0.35– 0.95 atm ) partial pressures at various reaction temperatures are presented to generate a database of 108 experimental entry values. The suitability of several kinetic expressions, based on reaction mechanisms that have been advanced in the literature, to represent the experimental data was assessed. On the whole, each mechanistic model provided an adequate fit to the entire experimental dataset but appreciable deviations of the predicted from the actual dechlorination rates were observed under certain reaction conditions. These deviations have allowed us to discriminate between the various mechanistic models and have facilitated a degree of model refinement. The best description was obtained for models featuring surface reaction between non-competitively and dissociatively adsorbed chlorobenzene and spillover hydrogen where the latter was viewed as adsorbed on a non-uniform surface.

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 473u2009Ku2009≤u2009Tu2009≤u2009573u2009K. 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. n n n n© 2000 Society of Chemical Industry

Gas phase hydrogenation/hydrogenolysis of benzaldehyde and o-tolualdehyde over Ni/SiO2

Abstract The gas phase hydrogen treatment of benzaldehyde and o -tolualdehyde was studied over a Ni/SiO 2 catalyst prepared by homogeneous precipitation/deposition. The reactions were conducted in the absence of diffusion limitations and reproducible turnover frequencies are presented. The products generated resulted from the hydrogenation and hydrogenolysis of the substituent Cue5fbO bond and from the hydrogenolysis of the aryl-carbonyl Cue5f8C bond, the aromatic ring remaining intact. The temperature dependencies of the rate constant and product selectivity are illustrated and apparent activation energies for hydrogenation to the aromatic alcohol are given. The influence of the ortho -substituted methyl group on the reactivity of the carbonyl function is discussed. The reaction of benzyl alcohol and 2-methylbenzyl alcohol over the same catalyst was investigated and the overall reaction pathway is identified.

Direct formation of cyclohexene via the gas phase catalytic dehydrohalogenation of cyclohexyl halides

Abstract The gas phase dehalogenation of cyclohexyl chloride and cyclohexyl bromide (where 423xa0K≤ T ≤523xa0K) promoted using silica and zeolite supported nickel catalysts in the presence of hydrogen is presented as a viable one step route for the production of cyclohexene. Cyclohexene is generated via the internal elimination of the corresponding hydrogen halide where the process is 100% selective at T ≤473xa0K. At higher temperatures, cyclohexane and benzene were isolated in the product mixture as a result of the combination of catalytic hydrogenolysis, hydrogenation and dehydrogenation steps. Cyclohexene yield was subject to a short term reversible decline with time-on-stream due to a surface poisoning by the hydrogen halide that was produced but the presence of hydrogen served to displace the inorganic halide and extend the productive lifetime of the catalyst. Bromine removal from cyclohexyl bromide was found to be more facile while the use of a higher loaded (15.2% (w/w) as opposed to 1.5% (w/w)) nickel silica is shown to result in appreciably higher dehydrohalogenation rates. Both Na/Y and Ni–Na/Y zeolites promoted cyclohexene formation but exhibited an irreversible deactivation which is attributed to pore blockage by occluded coke. With a view to optimising catalyst efficiency, the effect on cyclohexene yield of varying such process parameters as reaction time and temperature and reactant(s) partial pressures were studied and the catalytic data are compared with thermodynamic predictions.