Hydrodehalogenation of haloarenes over Silica supported Pd and Ni

Applied Catalysis A: General 271 (2004) 109–118

Hydrodehalogenation of haloarenes over Silica supported Pd and Ni A consideration of catalytic activity/selectivity and haloarene reactivity Mark A. Keane∗ Department of Chemical & Materials Engineering, University of Kentucky, Lexington, KY 40506-0046, USA Received in revised form 3 February 2004; accepted 4 February 2004

Abstract The gas phase catalytic hydrodehalogenation of haloarenes (423 K ≤ T ≤ 593 K) is presented as a viable means of treating hazardous concentrated halogenated gas streams for the recovery/reuse of valuable chemical feedstock, i.e. a progressive green processing strategy. The action of a Ni/SiO2 and Pd/SiO2 of comparable metal loading (ca. 5% w/w) was compared where the Pd catalyst delivered specific hydrodehalogenation activities that were up to three orders of magnitude greater. Reduction of Pd/SiO2 was far more facile to generate a supported zero valent phase that was characterized by a narrower distribution of smaller metal particles, delivering a (TEM derived) mean Pd particle diameter = 4.0 nm. Temperature programmed reduction (TPR) analysis of Ni/SiO2 demonstrated higher reduction temperature requirements (723 K as opposed to 523 K for Pd/SiO2 ) yielding an average Ni diameter = 9.3 nm. Hydrodehalogenation activity/selectivity is strongly dependent on the electron withdrawing/donation properties of the ring substituents where electron donation serves to activate the ring for hydrogen scission of the C–X bond. Variations in isomer reactivity can be attributed to steric hindrance limitations. Lumping chloroarene isomers together, the following trend of decreasing hydrodechlorination rates is established: chlorophenol(s) > chlorotoluene(s) > chlorobenzene > dichlorophenol(s), trichlorophenol(s) > dichlorobenzene(s) > trichlorobenzene(s), bromochlorobenzene > pentachlorophenol > hexachlorobenzene. Compensation behavior is established for the dehalogenation of a range of haloarenes over Ni/SiO2 , a response that is used to predict a feasible temperature for the treatment of a multi component haloarene feed. © 2004 Elsevier B.V. All rights reserved. Keywords: Catalytic hydrodehalogenation; Haloarenes; Nickel/silica; Palladium/silica; Dehalogenation kinetics

1. Introduction Haloaromatics in industrial effluents have long been regarded as a major source of environmental pollution due to the high toxicity and persistence of these compounds [1–3]. Such waste typically arises due to the non-selective nature of halogenation processes leading to unwanted halogen-rich isomers [4] and is a concern in the manufacture of herbicides, insecticides, heat transfer agents, dyes and plant growth regulators [5,6]. Moreover, adopting a life cycle stance, the handling of a haloarene product after its productive lifetime has expired is also a decided remediation issue. The mounting evidence of adverse ecological and public health impacts [7] has resulted in increasingly stricter legislation to limit such emissions [8,9], which has lent an added degree of urgency

∗

Tel.: +1 859 257 8028; fax: +1 859 323 1929. E-mail address: [email protected] (M.A. Keane).

0926-860X/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.02.051

to the development of robust decontamination methodologies. Established “end-of-pipe” control strategies draw on adsorption, air/steam stripping, condensation, thermal incineration, catalytic and wet air oxidation [10–15]. Incineration can result in the generation of highly toxic polychlorinated dibenzofurans/dibenzodioxins [16,17] and the primary measures in combustor design and technology to minimize the generation of “products of incomplete combustion” (PICs) cannot guarantee compliance with future legislated emissions [18]. Photocatalysis, ozonation and supercritical water oxidation represent advanced oxidation technologies that are showing promise as remediation technologies [19–21] but are still hampered by practical considerations and have a high-energy demand [22]. Catalytic steam reforming is emerging as a feasible methodology [23] but is again destructive in nature, albeit there is the possibility of generating synthesis gas as product. Ultrasonic degradation [24] and electrocatalytic reduction [25] are other techniques that are now undergoing feasibility studies. The existing non-thermal

110

M.A. Keane / Applied Catalysis A: General 271 (2004) 109–118

approaches do not constitute exhaustive solutions, essentially offering a means of phase transfer/concentration and if the extracted materials are mixtures of halogenated isomers these are not (without some difficulty) recovered for reuse. Biological treatment is capable of degrading halo-organics [26] but conversion is low, necessitating the construction of oversized and expensive reactors. Catalytic hydrodehalogenation represents an alternative and innovative approach whereby the hazardous material is transformed into recyclable products in an enclosed system with no (or limited) toxic emissions. Moreover, mixed isomers that arise from the halogenation processes can be converted back to the single parent raw material precursor from which they originated, in short a unique process of chemical desynthesis that advances “green” processing technology. Incorporation of catalytic HDC units in distillation/separation lines is envisaged with a HCl recovery unit where the HCl can be concentrated downstream to any level desired, the hydrogen gas scrubbed and washed to remove trace contaminants and recycled to the reactor. The author has recently [27] highlighted the appreciable environmental and economic advantages associated with a move to catalytic hydroprocessing as a means of waste minimization/recovery. Catalytic hydrodehalogenation has been reported for homogenous systems [28,29] with both catalyst and reactant in the liquid phase. While high turnovers have been achieved, this approach is not suitable for environmental remediation purposes due to the involvement of additional chemicals and the associated product/solvent/catalyst separation steps. The heterogeneous catalytic hydrodechlorination of chlorobenzene and chlorophenol(s) as model reactants has been studied in both the gas [30–41] and liquid [42–52] phases, the latter involving the use of hydrogen donors as well as molecular hydrogen. The removal of multiple chlorine atoms from an aromatic host has been examined to a lesser extent [36,39,53–60] while hydrodebromination reactions have received scant attention in the literature [61–64]. Based on three comprehensive reviews of the existing hydrodehalogenation literature [65–67], it is clear that Pd is the most active dechlorination metal but Ni systems, as less costly catalysts, also represent a promising option and nano-scale Ni metal supported on SiO2 has been shown to exhibit high resistance to halogen poisoning [40,68]. There is now a definite need for fundamental kinetic data associated with an array of halogenated reactants and catalyst systems, particularly when accompanied by a simple kinetic model to facilitate reactor design. It is for this reason that the reaction data presented in this paper are subjected to pseudo-first order kinetics as a means of evaluating dehalogenation rate constants. The value of these constants will influence reactor dimensions depending on the requisite operation criteria, i.e. complete dehalogenation or removal of one or more halo-substituents to yield a partially dehalogenated (target) high value product [69]. The conversion of a range of haloarenes over Ni/SiO2 and Pd/SiO2 bearing the same (ca. 5% w/w) metal loading is consid-

ered in this paper wherein the intrinsic activity/selectivity of the metal site and differences in haloarene reactivity are addressed.

2. Experimental 2.1. Catalyst preparation and characterization A ca. 5% w/w Pd and Ni loading on silica (Cab-O-Sil 5 M, surface area = 194 m2 g−1 ) was achieved by impregnation with aqueous solutions of Pd(NO3 )2 or Ni(NO3 )2 . The metal content (accurate to within ±2%) was measured by inductively coupled plasma-optical emission spectrometery (ICP-OES, Vista-PRO, Varian Inc.) from the diluted extract in HF. The catalyst precursor samples, sieved in the 75–150 ␮m mesh range, were contacted with a 100 cm3 min−1 stream of dry H2 (99.9%) for 0.5 h at room temperature followed by a temperature ramp (10 K min−1 , controlled using a Eurotherm 91e temperature programmer) to a final temperature of 723 K and 523 K for Ni/SiO2 and Pd/SiO2 , respectively. The temperature was maintained for 18 h where the catalyst bed temperature was independently monitored using an on-line data logging system (Pico Technology, model TC-08) and found to be constant to within ±1 K. A secondary ion mass spectrometric analysis (SIMS, VG ESCALAB) of each activated catalyst (pressed into indium foil) revealed only the presence of Si, O, Ni, and Pd on the surface. BET surface areas, temperature programmed reduction (TPR) and hydrogen chemisorption behavior were determined using the commercial CHEM-BET 3000 (Quantachrome) unit. After outgasing at 523 K for 30 min, at least two cycles of nitrogen adsorption–desorption in the flow mode were employed to determine total surface area using the standard single point BET method. Directly after BET measurement, the samples (ca. 0.2 g) were loaded in a U-shaped Pyrex glass cell (10 cm × 3.76 mm i.d.) and reduced in 5% (v/v) H2 /N2 (controlled using a Brooks mass flow controller) under the conditions given above. The effluent gas passed through a liquid N2 trap and H2 consumption was monitored by a thermal conductivity detector (TCD) with data acquisition/manipulation using the TPR WinTM software. The samples were swept with 20 cm3 min−1 dry N2 for 1 h, cooled to room temperature and subjected to H2 chemisorption using a pulse (50–100 ␮l) titration procedure. The BET surface areas before and after TPR/H2 chemisorption were recorded and found to be unchanged: BET surface area and hydrogen uptake values were reproducible to within ±5% and the values quoted in this paper are the mean. Powder X-ray diffractograms (XRD) were recorded with a Philips X’Pert instrument using nickel filtered Cu K␣ radiation. The samples were mounted in a low background sample holder and scanned at a rate of 0.02◦ step−1 over the 20 ≤ 2θ ≤ 85◦ range with a scan time of 5 s step−1 . The diffractograms were compared with the JCPDS-ICDD [70] references for identification purposes. High-resolution

M.A. Keane / Applied Catalysis A: General 271 (2004) 109–118

transmission electron microscopy (HRTEM) analysis was carried out using a Philips CM20 TEM microscope operated at an accelerating voltage of 200 kV. The passivated (in a 2% (v/v) O2 /He) specimens were prepared for analysis by ultrasonic dispersion in butan-2-ol, evaporating a drop of the resultant suspension onto a holey carbon support grid. The metal size detection limit was found to be ca. 0.1–0.2 nm, as noted elsewhere [71]. The mean particle sizes and size distributions presented in this study are based on a measurement of over 600 individual particles. 2.2. Catalytic reactor system All the catalytic reactions were carried out under atmospheric pressure, in situ immediately after the activation step, in a fixed-bed continuous flow glass reactor (i.d. = 15 mm) over the temperature range 423 K ≤ T ≤ 593 K. Isothermal operation was maintained by diluting the catalyst bed with ground glass (75–150 ␮m); the ground glass was mixed thoroughly with catalyst before insertion into the reactor. It is generally true that laboratory scale fixed-bed reactors approximate plug flow pattern [72]. In this case, the ratio of reactor diameter to catalyst particle (=90) exceeded the lower limit of 10 set by Froment and Bischoff [73], satisfying the application of plug-flow conditions. The catalyst was supported on a glass frit and a layer of glass beads above the catalyst bed served as a heating zone, ensuring that the reactants reached the reaction temperature before contacting the catalyst. A Model 100 (kd Scientific) microprocessor controlled infusion pump was used to deliver the halogenated feed via a glass/PTFE air-tight syringe and PTFE line at a fixed, calibrated, rate and the vapor was carried through the catalyst bed in a stream of purified H2 , the flow rate of which was monitored using a Humonics (Model 520) digital flowmeter. The H2 partial pressure was in the range 0.89–0.99 atm where H2 was at least ten times in excess relative to stoichiometric quantities. The range of halogenated feedstock included chlorobenzene (CB), bromobenzene (BB), 2-, 3-, and 4-chlorotoluene (CT), 2-, 3-, and 4-chlorophenol (CP), 4-bromophenol (BP), 1,2-, 1,3-, and 1,4-dichlorobenzene (DCB), 1,2,3-, 1,2,4-, and 1,3,5-trichlorobenzene (TCB), hexachlorobenzene (HCB), 2,3-, 2,4-, 2,5-, 2,6-, 3,4-, and 3,5-dichlorophenol (DCP), 2,3,5-, 2,3,6-, 2,4,5-, and 2,4,6-trichlorophenol (TCP), pentachlorophenol (PCP), and 1,2-, 1,3-, and 1,4-bromochlorobenzene (BCB); all the halogenated reactants (Aldrich, 98–99.9%) were used without further purification. The halogenated feedstock was either feed undiluted to the reactor or diluted in n-hexane or methanol where the catalyst weight (W) to inlet molar haloarene flow rate (FX ) ratio spanned the range 0.4–2762 g h molx −1 ; methanol and hexane served solely as diluent and did not impact on hydrodehlogenation activity/selectivity. Mass diffusion contributions under reaction conditions were evaluated taking the approach

111

outlined previously [40] where the molecular diffusion coefficients were calculated using Satterfield’s [74] method. The catalytic system was found to operate with negligible diffusion retardation of the reaction rate; effectiveness factor (η) > 0.99 at 573 K. Interparticle and intraparticle heat transport effects can also be disregarded when applying established criteria [75]; the temperature differential between the catalyst particle and bulk fluid phase was <1 K. In a series of blank tests, passage of each halogenated feed in a stream of hydrogen through the empty reactor, i.e. in the absence of catalyst, did not result in any detectable conversion. All the data presented here have been generated in the absence of any significant catalyst deactivation where each catalytic run was repeated (up to six times) using different samples from the same batch of catalyst: the measured rates did not deviate by more than ±8%. The reactor effluent was frozen in a liquid nitrogen trap for subsequent analysis by capillary GC as described elsewhere [40]; the detection limit corresponded to a feedstock conversion <0.1 mol%. The system of linear differential equations that describe the reaction kinetics for multiple dehalogenation was solved using Maple V, the commercial mathematical package. The degree of dehalogenation (xX ) is given by xX =

[Xorg ]in − [Xinorg ]out [Xorg ]in

(1)

where [Xorg ] represents the concentration (mol dm−3 ) of halogen associated with the aromatic feed and [Xinorg ] is the concentration of inorganic halide (HX) produced; in and out refers to the inlet and outlet reactor streams, respectively. The selectivity (as a percentage) with which (say) CB (SCB )is produced from DCB can be calculated from SCB (%) =

[CB]out × 100 [DCB]in − [DCB]out

(2)

while the yield of CB (YCB )is given by YCB (%) =

[CB]out × 100 [DCB]in

(3)

A halogen (in the form of HX product) mass balance was performed by passing the effluent gas through an aqueous NaOH (3.5–8.0 × 10−3 mol dm−3 , kept under constant agitation at ≥300 rpm) trap and monitoring continuously the pH change by means of a Hanna HI Programmable Printing pH Bench-Meter. The concentration of hydrogen halide generated was also measured by titrimetric analysis of the NaOH trap solution using a Metrohm (Model 728) Autotitrator (AgNO3 , combined Ag electrode); halogen mass balance was complete to better than ±10%. Qualitative analysis for the presence of Cl2 or Br2 was made by analy-

112

M.A. Keane / Applied Catalysis A: General 271 (2004) 109–118

Table 1 Metal content, BET surface area and metal phase characteristics associated with the two activated catalysts Ni/SiO2 Percent metal (w/w) BET surface area (m2 g−1 ) TPR H2 consumption Tmax (K)a Dispersion (%)c Average Metal particle size (nm)c Metal surface area (m2 gmetal −1 )c Average Metal particle size (nm)d Metal particle size range (nm)d a b c d

4.6 178 642a , 673 9 11.0 48 9.3 <1 to 30

Pd/SiO2 5.3 172 381b , 445 21 4.9 96 4.0 <1 to 20

Number in bold denotes the major TPR peak. H2 release. From H2 chemisorption analysis. From HRTEM analysis.

sis of the trap for the formation of hydrates at T > 283 K [4].

3. Results and discussion 3.1. Characterization of Ni/SiO2 and Pd/SiO2 The metal loading, BET surface area, metal dispersion/particle diameter/surface area in the activated catalysts, obtained from H2 chemisorption and HRTEM measurements, are given in Table 1. The agreement in terms of the average metal particle diameter obtained from both analytical techniques is reasonable, i.e. within ±20%. The temperature programmed reduction (TPR) profiles can be compared in Fig. 1 and the characteristic temperatures (Tmax ) for maximum H2 consumption are recorded in Table 1; the TPR conditions match those used for actual catalyst activation prior to hydrodehalogenation. The direct reduction of Ni/SiO2 generated a sharp H2 consumption peak at ca. 640 K with a secondary peak (at ca. 670 K) and a gradual return to baseline, a reduction pattern that is in good agreement with previous reports of Ni/SiO2 TPR [76–78]. The

main peak has been assigned [79] to the decomposition of nickel nitrate (the precursor salt) to NiO with a subsequent reduction to Ni metal. The Pd/SiO2 TPR profile is characterized by a sharp negative peak (at ca. 380 K) with an ill-defined (positive) H2 consumption at ca. 445 K. There is a general consensus that Pd can absorb H2 to form “Pd hydrides” at ambient temperature [80,81]. The decomposition of the Pd hydride phase has been linked to a negative peak (hydrogen release) in the TPR profile where T < 393 K [80–83]. The absence of any obvious H2 consumption (during TPR) in advance of H2 release presupposes the existence of the metallic phase prior to TPR [80]. There is an apparent discrepancy in the literature concerning the TPR response of supported Pd that is compounded by incomplete procedural descriptions and a mismatch in terms of sample pretreatment prior to TPR. While it has been reported that PdO reduction on SiO2 and Al2 O3 supports takes place at room temperature [84–86], H2 consumption peaks up to 523 K have also been reported [80,87,88]. The appearance of a positive TPR peak in the Pd/SiO2 profile shown in Fig. 1(ii) is indicative of a temperature induced reduction step and is in the line with the latter reports. Based on the TPR analysis, the final reduction temperatures for Ni/SiO2 and Pd/SiO2 were set at 723 K and 523 K, respectively; subsequent selected area electron diffraction (SAED) confirmed that the supported Ni and Pd distributed was present in the metallic form and not as an oxide. Moreover, there was no evidence of bulk NiO or PdO from XRD analysis: the XRD patterns for the two activated samples are given in Fig. 2. The Ni/SiO2 diffractogram is characterized by three peaks (at 44.5◦ , 51.8◦ , and 76.3◦ , corresponding to (1 1 1), (2 0 0), and (2 2 0) planes of metallic nickel) that are consistent with an exclusive cubic symmetry. The XRD analysis likewise reveals that cubic Pd is present on SiO2 with four peaks at 40.1◦ , 46.7◦ , 68.1◦ , and 82.15◦ corresponding, respectively, to (1 1 1), (2 0 0) (2 2 0), and (3 1 1) Pd planes. The markers included in Fig. 2 illustrate the position and relative intensity of the XRD peaks for cubic Ni and Pd taken from the JCPDS standards [70]. The representative TEM images provided in Fig. 3 serve to illustrate the nature of the metal dispersion in both catalysts where the spherical particle morphology is suggestive of relatively weak metal/support interaction. It is immediately evident from the TEM derived particle size distribution given in Fig. 4 that Pd on silica is characterized by a narrower distribution of smaller particles. The latter is a result of the greater ease of reducibility and consequent lower requisite reduction temperature to obtain a metallic phase. The elevated reduction temperature in the case of Ni/SiO2 translates into increased particle mobility and agglomeration to generate a larger mean particle diameter. 3.2. Ni/SiO2 and Pd/SiO2 activity/selectivity

Fig. 1. TPR profiles generated for (i) Ni/SiO2 and (ii) Pd/SiO2 .

The basic premise of this study was to apply a simple kinetic methodology that is appropriate for comparing the ac-

M.A. Keane / Applied Catalysis A: General 271 (2004) 109–118

113

Fig. 2. XRD patterns for the activated (a) Ni/SiO2 and (b) Pd/SiO2 . Note: the solid lines indicate peak position (with relative intensity) for cubic Ni and Pd.

tivity of silica supported Ni and Pd while serving as a means of assessing the reactivity/feasibility of hydroprocessing a range of halogenated aromatics. Plug-flow operation, under steady state where hydrogen was maintained far in excess, yields the following applicable reactor/kinetic expression

 1 W ln =k (4) 1 − xX FX where FX is the inlet molar halogen feed rate (mol h−1 ), xX is the fractional dehalogenation over a given weight (W) of catalyst; the parameter W/FX (units, g h molx−1 ) has the physical significance of contact time. From a consideration of gas phase reaction equilibrium constants [39,58], dehalogenation under the stated reaction conditions can be taken to be irreversible. Representative linear plots of ln(1-xCl )−1 versus W/FCl , generated for the HDC of representative chlorobenzenes are given in Fig. 5(a). Each line can be seen to pass through the origin, diagnostic of adherence to pseudo-first order kinetics. The specific rate constants extracted from this pseudo first order treatment are given in Table 2, wherein it can be seen that HDC is promoted to a far greater degree over the Pd catalyst. It is instructive to note that the specific HDC activity decreased, for both catalysts, in the order CB > DCB > TCB. In sub-

Fig. 3. Representative TEM images of (a) Ni/SiO2 and (b) Pd/SiO2 .

stituted aromatic systems, reactivity is typically related to localized (inductive) and delocalized (resonance) effects [89]. The lower dechlorination rates associated with di- and tri-Cl substitution is indicative of an electrophilic mechanism, as noted previously [33,38], where the presence of a second and third Cl substituent (as opposed to hydrogen) serves to deactivate the ring for electrophilic attack via an inductive effect that decreases the electron density of

114

M.A. Keane / Applied Catalysis A: General 271 (2004) 109–118 Table 2 Specific pseudo first order HDC rate constants (see Fig. 5) for the conversion of CB, 1,2-DCB, and 1,2,3-TCB over Ni/SiO2 and Pd/SiO2 at T = 423 K Reactant

k (molCl h−1 (m2 (gmetal )−1 )−1 ) Pd/SiO2

CB 1,2-DCB 1,2,3-TCB

Fig. 4. Metal particle size distributions for the activated Ni/SiO2 (open bars) and Pd/SiO2 (solid bars).

10−3

1.5 × 2.3 × 10−4 1.1 × 10−4

Ni/SiO2 3.5 × 10−6 1.4 × 10−7 7.5 × 10−8

the aromatic ring, i.e. destabilizes the cationic transition state. Conversion of the DCB and TCB reactants generated partially as well as fully dechlorinated products, in agreement with earlier studies dealing with Cl removal from polychlorinated aromatics [39,50,53,55,58,90]. The difference in catalyst performance is immediately apparent in Fig. 5(b) where the fractional conversion of 1,2-DCB (xDCB )is markedly greater over Pd/SiO2 while the associated benzene selectivity is so much higher that the two sets of data are far removed with no possible overlap. 3.3. Haloarene reactivity

Fig. 5. (a) Pseudo-first order kinetic plots for the HDC of CB (䊏, 䊐), 1,2-DCB (䉱, ), and 1,2,3-TCB (䊉, 䊊) over Ni/SiO2 (solid symbols) and Pd/SiO2 (open symbols) at 423 K. (b) Benzene selectivity (Sbenzene ) as a function of the fractional conversion of 1,2-DCB (xDCB )over Ni/SiO2 (䉱) and Pd/SiO2 ( ) at 423 K.

Haloarene reactivity was assessed using Ni/SiO2 as the less active catalyst where differences in reactant susceptibility to hydrodehalogenation should be more readily apparent. The specific hydrodehalogenation activity of Pd/SiO2 for the range of haloarene reactants considered in this study was up to three orders of magnitude greater than that delivered by Ni/SiO2 . Reaction over Ni/SiO2 was 100% selective in terms of hydrodehalogenation to yield a partially or the fully dehalogenated aromatic and HX with no evidence of halogen gas formation, aromatic ring reduction or scission of the non-halogen ring substituents. No dioxins are formed in the reducing environment and the recovery of valuable chemical feedstock is facilitated. Moreover, the temperature for effective catalytic hydroprocessing (<650 K) is significantly lower than that associated with catalytic combustion. This hydroprocessing strategy promotes an efficient use of resources, greatly reducing both direct and indirect waste/emissions costs and fosters sustainable development. Pseudo-first order rate constants (k) for the dehalogenation (under identical reaction conditions) of each reactant considered in this study are included in Table 3. The magnitude of the rate constants span a wide range where the highest k value (for 3-CP) was greater by over two orders of magnitude than the lowest (HCB). Variations in the hydrodehalogenation rate are dependent on the halogen content and the nature of the co-substituent. Such effects can be assessed from a consideration of the k/kCB ratio, i.e. rate constant for the hydrodehalogenation of each haloarene relative to that for chlorobenzene. Taking an overview of the tabulated data and lumping the isomers together, the families of chloro-based haloarenes exhibited the following trend of decreasing hydrodehalogenation rates: CP > CT > CB >

M.A. Keane / Applied Catalysis A: General 271 (2004) 109–118 Table 3 Pseudo-first order hydrodehalogenation rate constants (T = 553 K) for the conversion of a range of haloarenes over Ni/SiO2 and rate constant ratios relative to the hydrodrodechlorination of chlorobenzene Reactant

103 k (molCl h−1 g−1 )

k/kCB

CB BB 2-CT 3-CT 4-CT 2-CP 3-CP 4-CP 4-BP 1,2-DCB 1,3-DCB 1,4-DCB 2,3-DCP 2,4-DCP 2,5-DCP 2,6-DCP 3,4-DCP 3,5-DCP 1,2-BCB 1,3-BCB 1,4-BCB 1,2,3-TCB 1,2,4-TCB 1,3,5-TCB 2,3,5-TCP 2,3,6-TCP 2,4,5-TCP 2,4,6-TCP PCP HCB

17.1 3.0 18.1 23.2 31.7 17.5 36.7 25.4 23.0 2.5 5.2 4.4 9.3 7.8 4.9 7.2 10.7 12.4 0.3 0.6 1.5 1.6 1.4 1.2 7.2 8.1 9.8 10.2 0.7 0.1

1 0.174 1.059 1.357 1.854 1.025 2.146 1.485 1.353 0.143 0.301 0.259 0.545 0.456 0.287 0.418 0.625 0.727 0.019 0.037 0.090 0.092 0.082 0.068 0.419 0.470 0.571 0.597 0.040 0.007

DCP (TCP) > DCB > TCB ≈ BCB > PCP > HCB. The higher dechlorination rates associated with the chlorophenols and chlorotoluenes can be attributed to electron donation from the hydroxyl or methyl group that activates the ring for electrophilic attack. As a direct corollary and as noted above in Table 2, the additional presence of a second (in DCP and DCB), third (in TCP and TCB), fifth (in PCP), and sixth (in HCB) Cl on the ring has a deactivating effect and renders the entire halogen component less susceptible to hydrogen attack. There is persuasive evidence in the literature [54,90–92] that the haloarene is adsorbed dissociatively with the formation of a surface σ-complex via the aromatic ring carbon with the highest electron density: increasing Cl substitution serves to reduce the electron density associated with the ring carbons, lowering haloarene reactivity. The rate constants for the conversion of BB and BP are lower than those recorded for the chloro-counterparts, although the difference is not as great in the case of BP due to the activating effect of the electron-donating hydroxyl group. The lower reactivity of the bromoaromatics can be attributed to the lower electron affinity of Br (3.364 eV) compared with Cl (3.615 eV) [93] that translates into a less effective activation of the bromo-reactant through surface σ-complex formation. There is however, a considerable spread of k values asso-

115

Table 4 Ratio of parent arene mol fraction (αarene ) to the mol fraction of all the partially dechlorinated arenes (αX-arene ) in the steady state hydrodehalogenation of eight representative polyhalogenated arenes over Ni/SiO2 and Pd/SiO2 at 473 K Reactant

1,3-DCB 2,6-DCP 1,3-BCB 1,3,5-TCB 2,4,5-TCP PCP HCB

αarene /αX-arene Pd/SiO2

Ni/SiO2

3.1 4.4 2.4 1.7 3.0 1.4 0.8

0.3 0.4 0.1 0.2 0.2 0.2 0.1

ciated with the different isomers. The pattern that emerges points to steric hindrance (in the haloarene/catalyst interaction step) as being rate limiting where a close proximity of substituents on the ring limits the degree of dehalogenation. In contrast, resonance effects appear to have a negligible role to play in determining reaction rate in that ortho/para isomers cannot be linked in terms of reactivity when compared with the meta-form. This is particularly marked in the case of the chloro-phenols and toluenes where, if reactivity alone was governed by resonance effects, then the dechlorination rate constants for 2-CP/2-CT and 4-CP/4-CT should be similar and greater than that for 3-CP/3-CT. It has been shown elsewhere [58] that the product composition resulting from the catalytic hydrodechlorination of polychlorinated aromatics is quite different from that predicted on the basis of resonance effects. The reactivity of each BCB isomer is lower than either CB or BB, also in keeping with the electrophilic mechanism where steric hindrance again lowers reactivity. The reactivity of seven representative haloarenes can be assessed from the entries in Table 4 on the basis of the ratio of full to partial hydrodechlorination (in terms of mol fraction ratios). The degree of dehalogenation is clearly inhibited with increasing Cl substitution and, in every instance, the extent of dechlorination is significantly greater over Pd/SiO2 . Hydrodehalogenation selectivity is nonetheless dependent on the nature of the isomer, as shown in Table 5 for the family of TCP isomers. The lower phenol selectivities, for both catalysts, associated with the 2,4,6-TCP isomer can be attributed to the more severe geometric constraints involved in the activation of all three C–Cl bonds where each Table 5 Phenol selectivity in the hydrodechlorination of the four TCP isomers at 473 K over Ni/SiO2 and Pd/SiO2 TCP Reactant

2,3,5-TCP 2,3,6-TCP 2,4,5-TCP 2,4,6-TCP

Sphenol (%) Pd/SiO2

Ni/SiO2

72 60 75 33

18 11 16 2

116

M.A. Keane / Applied Catalysis A: General 271 (2004) 109–118 Table 6 Apparent activation energies (Eapp ) for the hydrodehalogenation of a range of haloarenes over Ni/SiO2 and the associated temperature required for a dehalogenation of 50% of the inlet halogen component (T50 ) where W/FX = 300 g h molx −1

Fig. 6. Relationship between phenol selectivity (Sphenol ) and overall fractional conversion (x) for the hydrodechlorination of 2,4,5-TCP (䊉) and PCP (䊏) over Ni/SiO2 at 548 K.

Cl substituent is spaced as far apart as possible on the benzene ring. Product composition in the catalytic treatment of polychlorinated aromatics has been shown to depend on the nature of the catalyst and process conditions, i.e. temperature, concentration, residence time etc. [39,94]. In a sequential hydrodehalogenation, the selectivity with which the ultimate dehalogenated product is produced should be enhanced at higher overall conversions. This is indeed the case with the dechlorination of PCP and a representative TCP isomer over Ni/SiO2 , as shown in Fig. 6. An increase in reaction temperature (from 473 K to 593 K) increased hydrodehalogenation rate and the temperature dependence for the reaction of selected haloarenes over Ni/SiO2 is shown in Fig. 7 as apparent Arrhenius plots. The extracted apparent activation energies that are given in Table 6 reveal that the presence of electron donating substituents on the ring lower the hydrodehalogenation energetics. While the tabulated activation energies span a wide range of values, the eleven

Fig. 7. Apparent Arrhenius plots for the hydrodehalogenation of CB (䉬), BB (䊏), 4-CT (䊊), 3-CP (䉱), 1,3-DCB (夹), 2,6-DCP (䉲), 1,2,4-TCB ( ), 2,4,5-TCP ( ), 2-BCB (䉫), and PCP (䊐) over Ni/SiO2 .

Reactant

Eapp (kJ molx−1 )

T50 (K)

CB BB 4-CT 3-CP 1,3-DCB 1,2-BCB 2,6-DCP 1,2,4-TCB 2,4,5-TCP PCP

59 92 49 40 96 121 58 144 136 151

479 551 445 417 534 594 500 561 526 573

haloarene reactants are linked by a common Compensation Plot as shown in Fig. 8. Compensation phenomenon in heterogeneous catalysis, which has been the subject of a recent comprehensive review [95], takes the form of a sympathetic linear correlation between the observed parameters of the Arrhenius equation (Eapp and ln Aapp ) where ln Aapp = bEapp + c

(5)

While the precise source of compensation behavior has yet to be conclusively established, plausible explanations that have been advanced invoke active site energy distributions and/or enthalpy/entropy relationships. Nevertheless, it is clear that the compensation effect arises in the case of experimental measurements where apparent rather than the true Arrhenius parameters (energetics of the surface reaction(s)) are measured. More importantly, the Compensation phenomenon is typically found for a series of related reactions or catalysts and is often associated with structure sensitive reactions. Couté and Richardson [96] have likewise reported Compensation behavior for the steam reforming of

Fig. 8. Compensation plot for the hydrodehalogenation of CB (䉬), BB (䊏), 4-CT (䊊), 3-CP (䉱), 1,3-DCB (夹), 2,6-DCP (䉲), 3,4-DCP (夽), 1,2,4-TCB ( ), 2,4,5-TCP ( ), 2-BCB (䉫), and PCP (䊐) over Ni/SiO2 , the error bars signify 95% confidence limits.

M.A. Keane / Applied Catalysis A: General 271 (2004) 109–118

CB, DCB and TCB over Ni/CaAl2 O4 and Pt/␥-Al2 O3 . The appearance of a common Compensation line in the case of catalytic hydrodehalogenation is significant in that it is diagnostic of individual reactions proceeding at comparable rates over similar temperature ranges [97], suggesting the feasibility of effectively treating haloarene mixtures. In terms of practicalities, the temperatures needed to dehalogenate 50% of the inlet halogen component (at a fixed W/FX ,) over Ni/SiO2 are also included in Table 6. For this arbitrary level of dehalogenation, there is a temperature differential of almost 180 K associated with the array of haloarene reactants considered in this study. Strict adherence to the Compensation Effect requires the existence of a single temperature, the isokinetic temperature (Tiso ) at which all the Arrhenius plots intersect where 1 (6) Tiso = bR The Tiso extracted from the Compensation plot in Fig. 8 is 633 K, which represents a possible working temperature at which to hydrotreat haloarene mixtures over Ni/SiO2 . Work is now on-going, from this starting point, to assess the viability of effective detoxification/recycle of a multicomponent chloroarene gas stream over Ni/SiO2 at 633 K and will be the subject of a future report.

radical reactions leading to toxic intermediates; (c) possibility of selective halogen removal to generate a reusable/recyclable product; (d) can be employed as a pre-treatment step to detoxify concentrated halogenated streams prior to biodegradation.

Acknowledgements The author is grateful to Dr. P.M. Patterson, E.J. Shin, G. Pina, K.V. Murthy, S. Jujjuri, G. Tavoularis and C. Menini for their contribution to this work. This work was supported in part by the National Science Foundation through Grant CTS-0218591.

References [1] [2] [3] [4] [5] [6] [7]

4. Conclusions [8]

The findings generated in this study support the following conclusions: (i) Reduction of Pd/SiO2 is far more facile than that of Ni/SiO2 at a comparable metal loading (ca. 5% w/w) to generate a narrower distribution of smaller Pd particles that exhibit significantly (up to three orders of magnitude) higher specific hydrodehalogenation activities. The latter is manifested in a predominant complete dehalogenation of polyhalogenated aromatics; (ii) Haloarene reactivity is determined by inductive and steric effects, the former evident in the enhancement of hydrodehalogenation by electron donating (−OH and −CH3 ) substituents, the latter in the inhibition of C-X cleavage by substituents at the ortho-position. (iii) A pseudo-first order kinetic treatment is applicable to generate apparent energy/Arrhenius pre-exponential parameter values that exhibit Compensation behavior. The latter can be used as an indication of an applicable process temperature for the effective treatment of a multi-component haloarene feed. (iv) The advantages of catalytic hydrodehalogenation as a means of haloarene waste minimization/environmental pollution control are: (a) moderate temperature (<650 K) non-oxidative and non-destructive process with lower energy requirements than combustion/incineration and no directly associated NOx/SOx emissions; (b) absence of thermally-induced free

117

[9]

[10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]

USEPA, Advisory Document No. 8EHQ-14302, 2000. F. Murena, V. Famiglietti, F. Gioia, Environ. Prog. 12 (1993) 233. E.N. Ruddy, L.A. Carroll, Chem. Eng. Prog. 89 (1983) 28. M. Howe-Grant (Ed.), Kirk-Othmer Encyclopedia of Chemical Technology, vol. 6, fourth ed., John Wiley: New York, 1991. P. Dabo, A. Cyr, F. Laplante, F. Jean, H. Menard, J. Lessard, Environ. Sci. Technol. 34 (2000) 1265. R.K. Juhler, S. R Sorensen, L. Larsen, Wat. Res. 35 (2001) 1371. C. Rolf, Kyoto Protocol to the United Nations Framework Convention on Climate Change: a Guide to the Protocol and Analysis of its Effectiveness. West Coast Environmental Law Association, 1998. A.J. Buonicore, W. Davis (Eds.), Air Pollution Engineering Manual. Van Nostrand Reinhold, New York, 1992. R.E. Hester, R.M. Harrison, volatile organic compounds in the environment, Issues in Environmental Science and Technology. Royal Chemical Society, London, 1995. C.S. Fang, S.L. Khor, Environ. Prog. 8 (1989) 270. B. Okolo, C. Park, M.A. Keane, J. Colloid Interf. Sci. 226 (2000) 308. J.J. Spivey, Ind. Eng. Chem. Res. 26 (1987) 2165. D.R. van der Vaart, E.G. Marchand, A. Bagely-Pride, Crit. Rev. Environ. Sci. Technol. 24 (1994) 203. G.R. Lester, Catal. Today 53 (1999) 407. R.M. Heck, R.J. Farrauto, catalytic air pollution control, Commercial Technology. Van Nostrand Reinhold, New York, 1995. B.P. Carpenter, D.L. Wilson, J. Hazard. Mater. 17 (1988) 125. A. Converti, M. Zilli, D.M. De Faveri, G. Ferraiola, J. Hazard. Mater. 27 (1991) 127. R. Weber, T. Sakurai, H. Hagenmaier, Appl. Catal. B: Environ. 20 (1999) 249. D.D. Dionysios, M. Suidan, E. Bekou, I. Baudin, J.-M. Laˆıné, Appl. Catal. B: Environ. 26 (2000) 153. K.-H. Wang, Y.-H. Hsieh, M.-Y. Chou, C.-Y. Chang, Appl. Catal. B: Environ. 21 (1999) 1. K.-S. Lin, H.P. Wang, Appl. Catal. B: Environ. 22 (1999) 261. E. Piera, J.C. Calpe, E. Brillas, X. Domènech, J. Peral, Appl. Catal. B: Environ. 27 (2000) 169. N. Couté, J.T. Richardson, Appl. Catal. B: Environ. 26 (2000) 265. G. Zhang, I. Hua, Advan. Environ. Res. 4 (2000) 29. A.I. Tsyganok, K. Otsuka, Appl. Catal. B: Environ. 22 (1999) 15. S. Pallerla, R.P. Chambers, Catal. Today 40 (1998) 103. M.A. Keane, in M.A. Keane (Ed.), Interfacial Applications in Environmental Engineering, Marcel Dekker, New York, 2002, p. 231. C.M. King, R.B. King, N.K. Bhattacharyya, M.G. Newton, J. Organomet. Chem. 600 (2000) 63.

118

M.A. Keane / Applied Catalysis A: General 271 (2004) 109–118

[29] S.M.H. Tabaei, C.U. Pitman Jr., Tetrahedron Lett. 34 (1993) 3263. [30] M. Kraus, V. Bazant, in J.W. Hightower (Ed.), Proceedings of the 5th International Congress on Catalysis. North-Holland, New York, 1973, p. 1073. [31] B. Coq, G. Ferrat, F. Figueras, J. Catal. 101 (1986) 434. [32] P. Bodnariuk, B. Coq, F. Figueras, J. Catal. 116 (1989) 459. [33] A.R. Suzdorf, S.V. Morozov, N.N. Anshits, S.I. Tsiganova, A.G. Anshits, Catal. Lett. 29 (1994) 49. [34] E.J. Creyghton, H.M.W. Burgers, J.C. Jansen, H. van Bekkum, Appl. Catal. A: General 128 (1995) 275. [35] D.I. Kim, D.T. Allen, Ind. Eng. Chem. Res. 36 (1997) 3019. [36] J. Frimmel, M. Zdražil, J. Catal. 167 (1997) 286. [37] A. Gampine, D.P. Eyman, J. Catal. 179 (1998) 315. [38] E.-J. Shin, M.A. Keane, Appl. Catal. B: Environ. 18 (1998) 241. [39] E.-J. Shin, M.A. Keane, Chem. Eng. Sci. 54 (1999) 1109. [40] G. Tavoularis, M.A. Keane, J. Chem. Technol. Biotechnol. 74 (1999) 60. [41] C. Menini, G. Tavoularis, C. Park, M.A. Keane, Catal. Today 62 (2000) 355. [42] F. Gioia, J. Hazard. Mater. 26 (1991) 243. [43] J.B. Hoke, G. Gramiccioni, E.N. Balko, Appl. Catal. B: Environ. 1 (1992) 285. [44] S. Kovenklioglu, Z. Cao, D. Shah, R.J. Farrauto, E.N. Balko, AIChE J. 38 (1992) 1003. [45] V.I. Simiagina, V.M. Mastikhin, V.A. Yakovlev, I.V. Stoyanova, V.A. Likholobov, J. Mol. Catal. A: Chemical 101 (1995) 237. [46] Y. Ukisu, T. Miyadera, J. Mol. Catal. A: Chemical 125 (1997) 135. [47] C. Sch˝ uth, M. Reinhard, Appl. Catal. B: Environ. 18 (1998) 215. [48] V. Fellis, C. De Bellefon, P. Fouilloux, D. Schweich, Appl. Catal. B: Environ. 20 (1999) 91. [49] J.L. Benitez, G. Del Angel, Catal. Lett. 66 (1999) 13. [50] V.A. Yakovlev, V.V. Terskikh, V.I. Simagina, V.A. Likholobov, J. Mol. Catal. A: Chemical 153 (2000) 231. [51] G. Del Angel, J.L. Benitez, J. Mol. Catal. A: Chemical 165 (2001) 9. [52] G. Yuan, M.A. Keane, Chem. Eng. Sci. 58 (2003) 257. [53] B. Hagh, D. Allen, AIChE J. 36 (1990) 773. [54] B. Hagh, D. Allen, Chem. Eng. Sci. 45 (1990) 2695. [55] F. Gioia, V. Famiglietti, F. Murena, J. Hazard. Mater. 33 (1993) 63. [56] F. Gioia, E.J. Gallagher, V. Famiglietti, J. Hazard. Mater. 38 (1994) 277. [57] C.A. Marques, M. Selva, P. Tundo, J.C.S. Perkin, Trans. 1 (1993) 529. [58] E.-J. Shin, M.A. Keane, J. Chem. Technol. Biotechnol. 75 (2000) 159. [59] Y. Cesteros, P. Salagre, F. Medina, J.-E. Sueiras, Appl. Catal. B: Environ. 25 (2000) 213. [60] Y. Cesteros, P. Salagre, F. Medina, J.-E. Sueiras, Catal. Lett. 67 (2000) 147. [61] Z. Yu, S. Liao, Y. Xu, React. Funct. Polymers 29 (1996) 151. [62] M.A. Aramendia, V. Boráu, I.M. Garcia, J.M. Jiménez, J.M. Marinas, F.J. Urbano, Appl. Catal. B: Environ. 20 (1999) 101. [63] G. Tavoularis, M.A. Keane, J. Mol. Catal. A: Chemical 142 (1999) 187.

[64] C. Park, C. Menini, J.L. Valverde, M.A. Keane, J. Catal. 211 (2002) 451. [65] E.V. Lunin, E.S. Lokteva, Russ. Chem. Bull. 45 (1996) 1519. [66] F.J. Urbano, J.M. Marinas, J. Mol. Catal. A: Chemical 173 (2001) 329. [67] F. Alonso, I.P. Beletskaya, M. Yus, Chem. Rev. 102 (2002) 4009. [68] E.-J. Shin, A. Spiller, G. Tavoularis, M.A. Keane, Phys. Chem. Chem. Phys. 1 (1999) 3173. [69] A.R. Pinder, Synthesis (1980) 425. [70] JCPDS-ICDD, PCPDFWIN, Version 2.2, June 2001. [71] P. Burattin, M. Che, C. Louis, J. Phys. Chem. B 45 (2000) 10482. [72] O. Levenspeil, Chemical Reaction Engineering. John Wiley, New York, 1972. [73] G. Froment, K.B. Bischoff, Chemical Reactor Analysis and Design. John Wiley, New York, 1990. [74] C.N. Satterfield, Heat Transfer in Heterogeneous Catalysis. M.I.T. Press, Cambridge, MA, 1970. [75] J.R. Welty, C.E. Wicks, R.E. Wilson, Fundamentals of Momentum, Heat and Mass Transfer. John Wiley, New York, 1984. [76] T. Mizushima, K. Nishida, H. Ohkita, N. Kakuta, Bull. Chem. Soc. Jpn. 75 (2002) 2283. [77] J. Zielinski, Catal. Lett. 31 (1995) 47. [78] M.A. Keane, Can. J. Chem. 72 (1994) 372. [79] C. Louis, Z.X. Cheng, M. Che, J. Phys. Chem. 97 (1993) 5703. [80] N. Nag, J. Phys. Chem. 105 (2001) 5945. [81] G. Neri, M. Musolino, C. Milone, D. Pietropaolo, S. Galvagno, Appl. Catal. A: General 208 (2001) 307. [82] M. Bonarowska, J. Pielaszek, V. Semikolenov, Z. Karpinski, J. Catal. 209 (2002) 528. [83] M. Bonarowska, B. Burda, W. Juszczyk, J. Pielaszek, Z. Kowalczyk, Z. Karpinski, Appl. Catal. B: Environ. 35 (2001) 13. [84] T. Fujitani, E. Echigoya, Nippon Kagaku Kaishi 4 (1991) 261. [85] F. Pinna, F. Menegazzo, M. Signoretto, P. Canton, G. Fagherazzi, N. Pernicone, Appl. Catal. A: General 219 (2001) 195. [86] G. Garcia, J. Vargas, M. Valenzuela, M. Rebollar, D. Acosta, Mater. Res. Soc. Symp. 549 (1999) 237. [87] L.M. Gomez-Sainero, A. Cortes, X.L. Seoane, A. Arcoya, Ind. Eng. Chem. Res. 39 (2000) 2849. [88] J. Murthy, S. Shekar, V. Kumar, K. Rao, Catal. Commun. 3 (2002) 145. [89] W.H. Brown, Organic Chemistry, Saunders College Publ., New York, 1995. [90] Y.A. Serguchev, Y.V. Belokopytov, Kinet. Catal. 42 (2001) 174. [91] M.A. Keane, D.Y. Murzin, Chem. Eng. Sci. 56 (2001) 3185. [92] M.A. Keane, D.Y. Murzin, in Catalysis of Organic Reactions, D.W. Morrell (Ed.), Marcel Dekker. New York, 2002, p. 595. [93] J.A. Dean, Handbook of Organic Chemistry. McGraw-Hill, New York, 1987. [94] E.-J. Shin, M.A. Keane, Catal. Lett. 58 (1999) 141. [95] G.C. Bond, M.A. Keane, H. Kral, J.A. Lercher, Catal. Rev. Sci. Eng. 42 (2000) 323. [96] N. Couté, J.T. Richardson, Appl. Catal. B: Environ. 26 (2000) 217. [97] M.A. Keane, P.M. Patterson, J. Chem. Soc. Faraday Trans. 92 (1996) 1413.