Catalytic hydrodehalogenation as a detoxification methodology
Claudia Menini, Colin Park, Eun-Jae Shin1, George Tavoularis, Mark A. Keane∗Department of Chemical Engineering, The University of Leeds, Leeds LS2 9JT, UK
Halogenated aromatic compounds exhibit high tox-icity and have long been regarded as a major sourceof environmental pollution [1,2]. To date, incinera-tion has largely been the preferred methodology forhandling, or rather disposing of, such organic waste
∗ Corresponding address. Department of Chemical and MaterialsEngineering, University of Kentucky, Lexington KY 40506-0046,USA.
1 National Renewable Energy Laboratory, Golden, CO 80401-3393, USA
but the requisite high destruction/removal efficiency(>99.9999%) is difficult to achieve when dealing withhalogenated aromatics [3]. Moreover, incineration cangenerate highly toxic polyhalogenated dibenzofuransand dibenzodioxins [4,5] largely as a result of a re-duction in the temperature of the residual gases in thevent. Incineration costs are based on halogen contentand are ever increasing which, allied to prohibitivelegislation [6], will soon render incineration as anunfeasible option. Non-thermal technologies whichinclude [7–9] direct adsorption on activated car-bon, solvent extraction, use of ionizing radiation andmicrobial degradation do not constitute exhaustive
356 C. Menini et al. / Catalysis Today 62 (2000) 355–366
solutions. Solvent extraction/carbon adsorption onlyoffer means of concentration and if the extractedcomponent is a mixture of chlorinated isomers,these are not, without some difficulty, recovered forreuse. Biological treatment is capable of degradingorganochlorine compounds but the rates of conversionare low necessitating the construction of oversizedand expensive reactors. Catalytic hydrodechlorina-tion represents an alternative and innovative approachwhereby the hazardous material is transformed into re-cyclable products in a closed system with limited toxicemissions. From an economic viewpoint, taking in-cineration as the principal means of treatment, a moveto a catalytic hydrogen treatment represents immedi-ate savings in terms of fuel and/or chemical recovery.Indeed, Kalnes and James [10] in a pilot-scale studyclearly showed the appreciable economic advantagesof hydrodechlorination over incineration. Moreover,mixed isomers arising from “uncontrolled” chlori-nation reactions can be converted back to the singleparent raw material precursor from which they origi-nated or to some target partially dechlorinated isomer,in short a unique process of chemical synthesis.
While catalytic hydroprocessing is well established,catalytic hydrodehalogenation is still in a forma-tive stage. Nevertheless, the catalytic hydrodechlo-rination of chlorobenzene and chlorophenol(s), asmodel reactants, has been reported both in the gas[11–25] and liquid [26–30] phases using palladium[18–20,27–30], platinum [26,27], rhodium [19,20,27]and nickel [11,12–17,21–25] based catalysts. Thetreatment of bromobenzenes has not been studied tothe same extent and few published accounts couldbe found. The significant reports have focused onliquid phase reactions with Wei et al. [31] employingmetallocenyl diphosphine complexes in a homoge-neous catalysis application, Yu et al. [32] using apolymer-anchored Pd catalyst for hydrodebromina-tions in ethanol while Armedia et al. [33] found thatPd supported on SiO2/AlPO4, ZrO2 and MgO wasactive in liquid phase transformations. Gas phase de-bromination has been touched upon as addenda to themain thrust of the studies undertaken by Kraus andBazant [18] and Moreau et al. [34]. Hydroprocessingof polychlorinated aromatics, particularly multiplechlorine removal from a parent phenol has receivedscant attention in the literature. Limited reaction dataare, nonetheless, available for the hydrodechlorination
of dichlorophenols in the gas [23,24,35] and liquid[30] phase. The database for catalytic hydrodechlo-rination is certainly expanding if one compares thecontents of review articles dating from 1980 [36] and1996 [37]. The majority of the published studies haveinvolved a compilation of rate data to characterizeindividual metal/halo-reactant systems and the mech-anism of carbon–halogen bond hydrogenolysis froman aromatic host is still open to question. Catalyst de-activation during hydrodehalogenation has been notedin several instances [14,17,22,26,32,33,38] and at-tributed to either coke formation [14,26] or poisoningby the hydrogen halide that is formed [14,22,32,38].However, the effects of halogen/metal interactionson catalyst structure have still to be investigated ina comprehensive fashion. There is also a need forfundamental kinetic studies and there have been fewattempts [12,14,18,19] to construct kinetic modelsthat are based on mechanistic considerations. In thispaper, we set out to assess the viability of employingsupported nickel catalysts (Ni/SiO2) to hydrodehalo-genate mono-chlorobenzene, mono-chlorotoluene,mono-chlorophenol, mono-bromobenzene and poly-chlorinated phenols. Catalyst durability is considered,the possibility of structure sensitivity is probed,changes to metal dispersion during catalysis are exa-mined and kinetic/mechanistic features are discussed.The nature of catalyst deactivation is examined andpreliminary characterization results for an unexpectedgrowth of highly ordered carbon during hydrodechlo-rination are also provided.
Five silica-supported (Cab-O-Sil 5M, BET sur-face area= 194 m2 g−1) nickel catalysts of varyingnickel loading (in the range 1.5–20.3% w/w) wereprepared by homogeneous precipitation/depositionas described in detail elsewhere [39]. The hydratedsamples, sieved in the 150–200mm mesh range, werereduced, without a precalcination step, by heating ina 100 cm3 min−1 stream of dry hydrogen (99.9%) at afixed rate of 5 K min−1 (controlled using a Eurotherm91 e temperature programmer) to a final temperature
C. Menini et al. / Catalysis Today 62 (2000) 355–366 357
of 673±1 K which was maintained for 18 h. The acti-vation (and reaction) temperature was monitored andcontrolled by a thermocouple inserted in a thermo-well within the catalyst bed, reactor temperature wasconstant to within±1 K. The nickel content (accurateto within ±2%) was measured by atomic absorptionspectrophotometry (Perkin Elmer 360 spectropho-tometer) and the water contents were determined bythermogravimetry (Pye Unicam thermobalance) [40],water content was less than 5% w/w. Selected catalystsamples were contacted with flowing HCl and HBrgas (in the range 20–150 cm3 min−1 for 0.25–5 min)at 548 K. The HCl/HBr contact was exothermic andgave rise to an increase in catalyst bed temperatureby up to 34 K. The freshly activated and pretreatedcatalyst samples were analyzed by transmission elec-tron microscopy (TEM) using a Philips CM20 TEMoperated at an accelerating voltage of 200 KeV. Thespecimens were prepared by suspending the catalystin isopropyl alcohol with ultrasound, evaporating adrop of the resultant suspension onto a ‘holey’ carbonsupport grid. The particle size distribution profilespresented in this study are based on a measurementof over 500 individual particles. The average particlesizes obtained from TEM analysis did not diverge bymore than±15% from the values obtained from COchemisorption [39]. Scanning electron microscopic(SEM) analysis was performed on a Hitachi S700 fieldemission SEM where the sample was deposited on astandard SEM holder and double coated with gold.
2.2. Catalytic reactor system
All the catalytic reactions were carried out underatmospheric pressure, in situ immediately after theactivation step, in a fixed bed glass reactor (ratio ofcatalyst particle to reactor diameter= 0.12) over thetemperature range 473 K≤ T ≤ 573 K. The catalystwas supported on a glass frit and a layer of glass beadsabove the catalyst bed served as a preheating zone andensured that the reactants reached the reaction tem-perature before contacting the catalyst. The catalyticreactor has been described previously in some detail[41,42] but the pertinent features are given below. AModel 100 (kd Scientific) microprocessor controlledinfusion pump and a Merck–Hitachi LC-6000A pumpwere used to deliver the halogenated feed via aglass/teflon air-tight syringe and teflon line over a
range of volumetric feed rates and the vapor was car-ried through the catalyst bed in a stream of purifiedhydrogen, the flow rate of which was monitored usinga Humonics (Model 520) digital flowmeter. Undi-luted chlorobenzene (CB, Aldrich, >99.9%) chloro-toluene (CT, Aldrich, 99%) and bromobenzene (BB,Aldrich, >99.9%) and methanolic solutions of selectedchlorophenol (CP, Aldrich, >99.9%), dichloroben-zene (DCB, Aldrich, >99.9%) dichlorophenol (DCP,Aldrich, >99.9%) and trichlorophenol (TCP, Aldrich,99%) isomers and pentachlorophenol (PCP, Aldrich,98%) served as feedstock. Passage of each reactantfeed in a stream of hydrogen through the empty re-actor did not result in any detectable conversion. Thecatalytic measurements were made at an overall gasspace velocity of 2250 h−1 where the partial pressureof hydrogen was maintained at 0.9 atm while the CBand BB partial pressure were varied in the range0.02–0.1 atm. The catalytic system has been shown[15] to operate with negligible diffusion retardationof the reaction rate under the stated conditions; effec-tiveness factor (h) >0.9999 at 573 K. Heat transporteffects can also be disregarded when applying thecriteria set down by Mears [43].
The reactor effluent was frozen in a liquid nitrogentrap for subsequent analysis which was made usingan AI Cambridge GC94 chromatograph equippedwith a split/splitless injector and a flame ionizationdetector, employing a DB-1 50 m× 0.20 mm i.d.,0.33mm capillary column (J&W Scientific), data ac-quisition and analysis were performed using the JCL6000 (for Windows) chromatography data systemconverting peak area to mole fraction using detailed(20 point) calibration plots. Quantitative analysis wasbased on relative peak area with acetone as solventwhere reproducibility was better than±0.1% and thedetection limit typically corresponded to a feedstockconversion less than 0.1 mol%. A chlorine (in theform of HCl product) mass balance was performedby passing the effluent gas through an aqueous NaOH(3.5–8.0 × 10−3 mol dm−3, kept under constant agi-tation at≥300 rpm) trap and monitoring continuouslythe pH change by means of a Hanna HI ProgrammablePrinting pH Bench-Meter. The concentration of HClgenerated was also monitored by titrimetric analysisof the NaOH trap solution; chlorine mass balancewas complete to better than±10%. Qualitative anal-ysis for the presence of chlorine gas was made by
358 C. Menini et al. / Catalysis Today 62 (2000) 355–366
analysis of the trap for the formation of hydrates atT > 283 K [44]. The test was negative in every in-stance which confirms that hydrogenolytic cleavageof halogen substituents from an aromatic host yieldsthe corresponding hydrogen halide as the only inor-ganic product. At the end of each catalytic run thecatalyst was normally heated in flowing dry hydrogenat 5 K min−1 to 673 K and maintained at this temper-ature for at least 12 h. In many instances the reactionwas repeated under identical conditions where ratereproducibility was better than±7%.
3. Results and discussion
The gas phase hydrodechlorination of chloroben-zene (CB) over the range of reactant partial pressures,temperatures and nickel metal loadings that wereconsidered generated benzene and HCl as the onlydetected products. The conversion of bromobenzene(BB) likewise yielded only benzene and HBr and aro-matic ring reduction, in both instances, can be posi-tively discounted. The catalytic hydrogen treatment ofeach chlorophenol (CP) isomer generated only phenoland HCl and there was no detectable formation ofcyclohexanone or cyclohexanol as a result of a furtherhydrogenation of phenol. A minor degree of CP iso-merization activity (<1 mol% conversion) was evidentin this study but we did not isolate even trace amountsof benzene or chlorobenzene in contrast to the re-sults reported by Chon and Allen for a Ni/Mo–Al2O3catalyst [12]. A selective hydrodechlorination is to beexpected given the reported [45] bond dissociationenergies of aromatic C–Cl (406 kJ mol−1) and C–OH(469 kJ mol−1) which show that the hydrodeoxygena-tion step is the more energetically demanding.
Catalyst durability in the continuous single pass hy-drodechlorination of chlorobenzene over two Ni/SiO2catalysts bearing a low and high nickel loading is as-sessed in Fig. 1 wherein the ratio of the fractionalconversion at a particular time-on-stream (xCl) to theinitial conversion over a freshly activated catalyst (x0)is related to process time. The data plotted in Fig. 1were obtained from periodic sampling of the effluentstream for up to 800 h of continual operation whichrepresents, for the lower nickel loaded sample, a to-tal chlorine to nickel mole ratio of 2× 104:1. Thenickel-dilute catalyst, albeit there is a decided scatter
Fig. 1. Variation of the ratio of time dependent (xCl) to ini-tial (x0) fractional conversion of CB at 573 K as a function oftime-on-stream over 1.5% w/w (j) and 15.2% w/w (m) Ni/SiO2.
of experimental data, largely retained its initial activitywhile the higher loaded catalyst exhibited a discernibledecline in conversion with prolonged use. The lattercatalyst still delivered an appreciable degree of con-version and remained fully selective in terms of solelypromoting benzene formation but the initial activityultimately fell by a factor of 0.7. The possible involve-ment of a nickel particle size effect is probed in Fig. 2where the specific hydrodechlorination rate (per ex-posed nickel surface area) in the conversion of CB and2-CP is plotted as a function of nickel particle size. Anincrease in the supported nickel particle size consis-tently generated, for both feedstock, a higher specificchlorine removal rate. The nickel particle sizes consi-
Fig. 2. Specific hydrodechlorination rate as a function of nickelparticle size for the conversion of CB (m) and 2-CP (j) at 573 K.
C. Menini et al. / Catalysis Today 62 (2000) 355–366 359
Table 1Phenol yield (Y) from the hydroprocessing of a range of polychlo-rinated phenolic feedstock over three nickel loaded silica catalystsat 573 Ka
Reactant % Ni w/w Y (%)
2,6-DCP 1.5 52,6-DCP 11.9 212,6-DCP 20.3 29
2,4,5-TCP 1.5 52,4,5-TCP 11.9 162,4,5-TCP 20.3 24
PCP 1.5 4PCP 11.9 13PCP 20.3 19
a x = 0.30±0.06.The concentration of surface nickel sites waskept constant in the three catalyst beds.
dered in this study fall within the so-called mitohedri-cal [46] region wherein catalytic reactivity can show acritical dependence on morphology. However, Estelleet al. [17] also noted evidence of structure sensitivityin hydrodechlorination over unsupported nickel cat-alysts. The observed antipathetic structure sensitivitywith larger supported particles having a higher activitythan smaller ones supports the existence of some formof ensemble effect. Coq et al. [19], on the other hand,have reported that CB dechlorination is enhanced oversmaller palladium and rhodium particles. The yieldof phenol from a polychlorinated feedstock was alsofound, in this study, to be particle size dependent asshown in Table 1 where complete removal of the chlo-rine complement to produce phenol was favored inevery case at higher nickel loadings.
Hydodehalogenation has been viewed in termsof both a nucleophilic [18,47] and electrophilic[11,12,22,23] attack of hydrogen on the adsorbedhalo-aromatic. There is general agreement in theliterature that the reactive hydrogen is adsorbed dis-sociatively [14,15,17–20,48] while the involvementof spillover species has also been proposed [14,48].The effect of aromatic ring substitution, with varyingassociated electron-donor/acceptor properties, on therate of single hydrodechlorination steps under iden-tical reaction conditions is illustrated in Table 2. Theobserved increase in dechlorination rate in movingfrom a strongly electron-withdrawing substituent (–Clin 1,4-DCB and 2,3-DCP) to the elctron donating–OH (in 3-CP) and –CH3 (in 3-CT) is diagnostic of
Table 2Hydrodehalogenation rates at 548 K over 1.5% w/w Ni/SiO2 forselected single halogen removal stepsa
a Inlet partial pressure of halogenated reactant= 0.04 atm.
an electrophilic mechanism. This would presume thatthe aromatic ring in the transition state is cationicwith respect to the initial state and electropositivesubstituents lower the activation barrier by stabilizingthe transition state. The lower rate of hydrodebromi-nation (of BB) relative to that of hydrodechlorination(of CB) suggests a less effective activation of theC–Br bond for attack by hydrogen. This may be ex-plained on the basis of the lower electron affinity ofbromine compared with chlorine [49] that translatesinto a reduced strength of adsorption and poorer ac-tivation. In terms of the prevalent surface steps, asa first approach we assume that adsorption of bothreactants is non-competitive. The latter does find sup-port in the fact that the reaction order with respect toCB has been shown elsewhere [15] to be independentof the partial pressure of hydrogen. Taking hydrogenadsorption to be dissociative on sitesZ′
H2(g) + Z′ 2H-Z′
where adsorption is in equilibrium (K1) and the sur-face hydrogen reacts with chlorobenzene (as a rep-resentative case) adsorbed on separate (Z) sites in anirreversible step (k)
CB-Z + 2H -Z′ → B(g) + HCl-Z + 2Z′
Under these reaction conditions each dehalogenationstep is thermodynamically very favorable [15,23]and the recorded conversions are far removed fromequilibrium. The chloroaromatic adsorption involvesa direct “catalytically significant” interaction of thechlorine component with the surface and the HCl thatremains on the catalyst is displaced by incoming CBin an equilibrated step (K2)
CB(g) + HCl-Z CB-Z + HCl(g)
360 C. Menini et al. / Catalysis Today 62 (2000) 355–366
The rate expression that follows from the above stepsis given by
r = kK1PCBPH2/PHCl
K−12 + PCB/PHCl
The predicted (using the above expression) and exper-imentally determined steady state hydrodechlorinationand hydrodebromination rates as a function aromaticpartial pressure are compared in Fig. 3, the rates ofhydrodebromination at 473 K were too low to be mea-sured accurately. In every instance the dehalogenationrates were elevated with increasing partial pressure(over the range considered in this study) and thecatalyst delivered a consistently greater degree of hy-drodechlorination. The value of theK2 parameter wassignificantly lower for BB (3× 10−3 at 523 K) whencompared with CB (0.16) under the same conditionswhich is indicative of a weaker interaction with thesurface. The percentage deviation predicted from ex-perimental rates is presented in Fig. 4 where the fit tothe BB results is inferior; percentage deviation is notgreater than±18%. The model does provide a good
Fig. 3. Variation of the experimentally determined and calculatedCB (solid lines) hydrodechlorination (j) and BB (dotted lines) hy-drodebromination (m) rates with varying CB (and BB) partial pres-sure at (a) 473 K; (b) 523 K; (c) 573 K.P HCl = 0.001− 0.03 atm;P HBr = 0.0007− 0.009 atm.
Fig. 4. Deviation (%) of predicted from calculated CB hydrodechlo-rination (j) and BB hydrodebromination (m) rates as a functionof aromatic partial pressure.
representation of the experimental data over the statedprocess conditions. While the kinetic expression is re-liable it alone can not accredit the associated reactionmechanism. It is recognized that a significant devia-tion may result over a wider range of aromatic partialpressures or at lower initial hydrogen pressures. Workis ongoing to consider other mechanistic based kineticmodels and to extend the input database of experi-mental rates over a wider range of reaction conditions.
While the supported nickel systems employed inthis study did not exhibit any short term decline inhydrodehalogenation activity, there was an evidentdrop in rate (depicted in Fig. 1) for the higher metalloadings with prolonged and continuous use. Loss ofdechlorination activity has been attributed elsewhere[14,22,32,38] to a poisoning or site-blocking effect bythe HCl that is produced. It is worth noting that Coqet al. [19] observed a greater resistance to poison-ing, during CB hydrodechlorination, with increasingparticle size in the case of supported palladium cat-alysts. The chlorine and bromine coverage of a low(1.5% w/w) and high (15.2% w/w) loaded Ni/SiO2was probed by monitoring, via pH changes in theNaOH scrubbing solution, the desorption of HCl andHBr after completion of the catalysis step. The num-ber of moles of HCl and HBr desorbed after a seriesof catalytic reactions at different aromatic partialpressures and reaction temperatures are recorded inTable 3. In every instance the number of moles of hy-drogen halide desorbed from the catalyst exceeded thenumber of exposed surface nickel sites regardless of
C. Menini et al. / Catalysis Today 62 (2000) 355–366 361
Table 3Moles of HCl and HBr desorbed from 1.5 and 15.2% w/w Ni/SiO2 after a series of CB and BB hydrodehalogenation reactions
the nickel loading. Surface coverage by HCl and HBralone cannot account for any loss of initial activityand under reaction conditions the catalyst surface is,in effect, saturated with hydrogen halide. The nickelparticle size distribution was however shifted to highervalues after extended catalyst use as is illustratedby the histograms presented in Fig. 5. An agglom-eration of palladium metal particles (supported onTiO2, SiO2, ZrO2 and Al2O3) was also observed by
Fig. 5. Nickel particle size distribution in (a) freshly activated catalyst sample (open bars); (b) after extended use in the hydrodechlorinationof CB (cross-hatched bars); (c) after contact with HCl gas (solid bars). 15.2% w/w Ni/SiO2.
Gampine and Eyman [38] after a series of gas phasehydrodechlorination reactions. The freshly activated(15.2% w/w Ni) catalyst is characterized by a narrowparticle size distribution to give a surface weightedaverage diameter equal to 1.7 nm. The catalysts usedin this study have been shown [39] to exhibit strongmetal/support interactions which result in a narrowdistribution of small particles (<4 nm) after activation.A direct contact of the freshly activated catalyst with
362 C. Menini et al. / Catalysis Today 62 (2000) 355–366
HCl gas served to shift the particle size distribution toan appreciably wider range of larger crystallites (seeFig. 5), the surface weighted average is calculated toequal 22.7 nm. Moreover, the HCl treatment resultedin a complete change in nickel particle morphologyand an array of geometrically shaped crystallites ofvarying size was observed by TEM and is illustratedby the micrographs given in Fig. 6. An agglomera-tion of nickel particles (supported on carbon) aftera HCl treatment has been noted elsewhere [50] andwas attributed to a surface mobility of Ni–Cl species.Moreover, larger nickel particles are typically gener-ated with a nickel chloride precursor, regardless of thenature of the support or reduction temperature, dueto crystallite growth involving volatile nickel chloridein the presence of hydrogen and hydrogen chloride[51]. There was no evidence of any significant metalparticle growth in the nickel dilute samples. The HCltreatment also served to lower the dechlorinationrate by a factor of 20. This loss of conversion wasirreversible in that an attempted reactivation of thecatalyst in flowing hydrogen at 673 K over extended
Fig. 6. TEM of the 15.2% w/w Ni/SiO2 after contact with HCl gas and 12 h-on-stream CB hydrodechlorination at 523 K. Note: filamentouscarbon growth is evident in the right hand micrograph.
periods did not restore the activity to any appreciabledegree.
The shift in nickel particle size after HCl treatmentwas accompanied by the unexpected growth of fila-mentous carbon during the ensuing catalysis step asshown in Fig. 6. The nature of this carbon growth isperhaps better illustrated by the low and high magni-fication SEM micrographs given in Fig. 7 that showa multi-point growth of carbon filaments from a cat-alyst surface that had, in this case, been modified bycontact with HBr. These carbon filaments are highlyordered and exhibit little or no curvature, the latticestructure of an isolated carbon filament grown fromthe HBr treated surface is evident in the TEM shownin Fig. 8. The diameter (and distribution of diametersizes) of the filaments grown on a range of catalyst sur-faces correspond, in every instance, to the dimensionsof the surface nickel metal suggesting that the carbonis growing directly from the metal sites. As a con-sequence of the strong metal/support interaction(s) inthese catalysts the metal crystallites remain anchoredto the silica and the carbon filaments grow from the
C. Menini et al. / Catalysis Today 62 (2000) 355–366 363
Fig. 7. Low and high magnification SEMs showing the carbon filament growth during the hydrodechlorination of CB at 553 K over a HBrtreated 15.2% w/w Ni/SiO2 catalyst.
exposed faces. The growth of ordered carbonaceousmaterial via catalysis is well established but typicallyinvolves an unsaturated hydrocarbon feedstock at tem-peratures in excess 723 K [52,53]. Carbon growth isinitiated by a concomitant adsorption and decomposi-tion of the hydrocarbon on specific faces of the metalfollowed by a dissolution of the carbon atoms into themetal particle and diffusion at other faces as graphiticlayers. The growth and characteristics of the carbon isvery much dependent on catalyst composition and thereaction conditions. There was no evidence of such or-dered carbon growth on the used catalyst samples thathad not been contacted directly with HCl or HBr. Thespent (% w/w Ni> 1.5) catalysts did however exhibitan appreciable lay down of amorphous carbon or cokeand preliminary XPS analysis has revealed the pres-ence of a surface “nickel carbide” species. Tempera-ture programmed oxidation, in a 5% v/v O2/He flow,of the used and demineralized catalyst samples re-vealed an amorphous carbon content of up to 8% w/w.Carbon deposition may in fact be responsible for thegradual loss of activity in the higher nickel loaded cat-alysts where the effect is cumulative as more of theactive phase is occluded by coke deposits. Indeed, thedispersion and morphology of supported metal parti-
cles can have a large effect on coke deposition duringcatalysis [54] and the larger nickel particles may bemore prone to coking during hydrodehalogenation.In any case, the generation of ordered carbon fila-ments from these catalysts was only induced after adirect contact with gaseous hydrogen halide. Chlorineis known to act as an electron acceptor with respectto transition metals [19,55] and the presence of resid-ual chlorine has been shown to disrupt the hydrogenchemisorption characteristics of nickel [14,56], thesource of this low temperature carbon growth must beelectronic in nature. The direct interaction of the hy-drogen halide with the catalyst results in a completereconstruction of the metal crystallites which may ex-pose orientations that promote the growth of orderedcarbon. The presence of the hydrogen halide on thesurface must modify the electron density at the metalsites with the result that a dissociative (or destructive)chemisorption of CB is favored. This proposal findssupport in the work of Chambers and Baker [57] whoreported that trace amounts of chlorine in an ethy-lene feed promoted the carbon deposition activity (at673 K) of cobalt and iron powders.
Once the Ni/SiO2 catalysts had been contactedwith HCl and HBr gas, hydrodechlorination activity
364 C. Menini et al. / Catalysis Today 62 (2000) 355–366
Fig. 8. TEM of an isolated carbon filament grown during the hydrodechlorination of CB at 553 K over a HBr treated 15.2% w/w Ni/SiO2
catalyst.
was almost completely suppressed and the catalystspromoted predominantly the formation of carbonfilaments. It is true that this shift in selectivity isdetrimental in terms of our initial intention of de-veloping stable and selective hydrodehalogenationcatalysts. Furthermore, the changes that are wroughtdue to a direct interaction of the hydrogen halide withthe catalyst are so extreme that they do not mimic,in a meaningful way, any surface effects during thecatalytic step. Nevertheless, this serendipitous lowtemperature growth of highly ordered carbon can beturned to good effect in terms of adsorption applica-tions. We are now attempting to optimize the growthof this ordered carbon that will then be assessed in
terms of adsorption capacity in the uptake of halo-genated aromatics. The application of catalysis to theabatement of pollution by haloarenes can now followtwo routes, a two-pronged approach: (i) a direct con-version of “halogenated waste” to reusable feedstockor some target isomeric product; (ii) a transformationof the same waste into a potential adsorbent mate-rial that can then be used, in turn, to concentratehalogenated waste in a “separations application”.
4. Conclusions
Nickel supported on silica promotes the hydrode-halogenation of a range of halogenated aromatics in
C. Menini et al. / Catalysis Today 62 (2000) 355–366 365
temperature range 473–573 K where ring hydrogena-tion and non-halogen substituent hydrogenolysis stepsare not observed. Hydrodehalogenation proceeds viaan electrophilic mechanism where the presence ofelectron donating ring substituents raises the overallreaction rate. The reaction is structure sensitive in thatthe specific dechlorination rate and complete dechlo-rination to the parent aromatic are favored by largernickel metal particles. Under reaction conditions thecatalyst surface is saturated with hydrogen halide thatis displaced by incoming reactant. While the nickeldilute samples (1.5% w/w) retain the initial activityover prolonged (up to 800 h) periods, the higherloaded catalysts exhibit a gradual decline in activity.The latter can be attributed to halogen/nickel inter-actions that lead to a growth of the supported metalphase and may influence a more prevalent coking inthe case of the larger nickel particles. A direct contactof the freshly activated catalysts with HCl or HBrgas serves to diminish the ensuing hydrodechlorina-tion activity but promote the growth of filamentouscarbon from the catalyst surface. Such a pretreatmentresults in a complete restructuring of the supportedmetal phase leading to a destructive chemisorption ofthe halogenated reactant and the formation of highlyordered carbon at an abnormally low temperature.
Acknowledgements
This work has been supported by a grant (GR/M5-1420) from the Engineering and Physical SciencesResearch Council (EPSRC). CM also acknowledgespartial support from EPSRC (Quota Studentship No.97302706). The authors wish to thank Dr. R. Brydsonfor his assistance with the TEM/SEM analyses.
[2] B.J. Alloway, D.C. Ayres, Chemical Principles of Environ-mental Pollution, Blackie, Glasgow, 1993.
[3] A. Converti, M. Zilli, D.M. De Favari, G. Ferraiola, J. Hazard.Mater. 27 (1991) 127.
[4] H. R Bauser, Chemosphere 7 (1979) 415.
[5] D.R. van der Vaart, E.G. Marchand, A. Bagely-Pride, Crit.Rev. Environ. Sci. Technol. 24 (1994) 203.
[6] K. Marr, Environmental Impact Assessment in the UnitedKingdom and Germany, Ashgate Publ. Ltd., Brookfield, VT,1997.
[7] R.E. Hester, R.M. Harrison, Volatile Organic Compounds inthe Environment, Issues in Environmental Science and Tech-nology, The Royal Society of Chemistry, Cambridge, 1995.
[8] N. Suprenant, T. Nunno, M. Kravett, M. Breton, HalogenatedOrganic Containing Waste: Treatment Technologies, NoyesData Corp., Park Ridge, NJ, 1988.
A.G. Anshits, Catal. Lett. 29 (1994) 49.[12] S. Chon, D.T. Allen, AIChE J. 37 (1991) 1730.[13] J. Frimmel, M. Zdražil, J. Catal. 167 (1997) 286.[14] E.-J. Shin, A. Spiller, G. Tavoularis, M.A. Keane, Phys.
Chem., Chem. Phys. 1 (1999) 3173.[15] G. Tavoularis, M.A. Keane, J. Chem. Technol. Biotechnol.
74 (1999) 60.[16] G. Tavoularis, M.A. Keane, J. Mol. Catal. A: Chemical 142
(1999) 187.[17] J. Estelle, J. Ruz, Y. Cesteros, R. Fernadez, P. Salagre, F.
Medina, J.-E. Sueiras, J. Chem. Soc., Faraday Trans. 92(1996) 2811.
[18] M. Kraus, V. Bazant, in: J. W. Hightower (Ed.), Proceedingsof the 5th International Congress on Catalysis, North Holland,Amsterdam, 1969, p. 1073.
[19] B. Coq, G. Ferrat, F. Figueras, J. Catal. 101 (1986) 434.[20] P. Bodnariuk, B. Coq, G. Ferrat, F. Figueras, J. Catal. 116
[38] A. Gampine, D.P. Eyman, J. Catal. 179 (1998) 315.[39] M.A. Keane, Can. J. Chem. 72 (1994) 272.[40] B. Coughlan, M.A. Keane, J. Catal. 123 (1990) 364.[41] M.A. Keane, P.M. Patterson, J. Chem. Soc., Faraday Trans.
92 (1996) 1413.[42] M.A. Keane, J. Catal. 166 (1997) 347.[43] D.E. Mears, Ind. Eng. Chem. Proc. Des. Dev. 10 (1971)
543.[44] M. Howe-Grant (Ed.), Kirk-Othmer Encyclopedia of Chemi-
cal Technology, Vol.1, 4th Edition, Wiley, New York, 1991,p. 996.
[45] R.T. Sanderson, Chemical Bonds in Organic Compounds, Sunand Sand Publ., Scottsdale, AZ, 1976.
[46] M. Che, C.O. Bennett, Adv. Catal. 36 (1989) 55.
