Ind. Eng. Chem. Res. 2007, 46, 705-715
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Aqueous-Phase Hydrodechlorination of 2,4-Dichlorophenol over Pd/Al2O3: Reaction under Controlled pH Guang Yuan† and Mark A. Keane*,‡ Department of Chemical and Materials Engineering, UniVersity of Kentucky, Lexington, Kentucky 40506, and Chemical Engineering, School of Engineering and Physical Sciences, Heriot-Watt UniVersity, Edinburgh EH14 4AS, Scotland
The aqueous-phase hydrodechlorination (HDC) of 2,4-dichlorophenol (2,4-DCP) over a Pd/Al2O3 catalyst has been studied with adjustment of the bulk solution pH either by base (NaOH and NH4OH) addition prereaction or with a continual supply of base during HDC. With a starting pH in the range 7-11.6, the solution pH can be controlled to within (0.1 with an external supply of NaOH during HDC. A preliminary kinetic analysis suggests that 2,4-DCP HDC is predominately stepwise, yielding 2-chlorophenol (2-CP) as the partially dechlorinated product, which is further converted to phenol and ultimately to cyclohexanone. Pd/Al2O3, operating under controlled pH, delivered maximum HDC activity in the pH range 7-8, where the selectivity with respect to 2-CP was lower than that associated with pH ) 11.6. In the absence of mass transport constraints, initial HDC rate as a function of 2,4-DCP concentration exhibits Langmuir-Hinshelwood mechanistic behavior involving competitive adsorption of H2 and 2,4-DCP with the surface addition of two H atoms as rate limiting. The pH effect on initial HDC rate can be represented by a generic correlation that demonstrates a 4-fold increase in 2,4-DCP adsorption, where solution pH was lowered from 13 to 7 but there was a pH insensitivity with respect to the rate constant for surface reaction. 1. Introduction As a nondestructive approach, catalytic hydrodechlorination (HDC) is a promising methodology for the treatment of organochlorinated waste.1-4 Chlorophenols (CPs), which represent a class of significant pollutants/byproducts in industrial effluents and “recalcitrant molecules” in the environment,5 have been examined as model reactants in both gas-6,7 and liquidphase8-12 HDC studies. The aqueous-phase HDC of CPs over supported Ru,9 Pd,10-14 and Ru-Pd15 catalysts, operated under mild conditions, has been demonstrated to be effective, selective, and exhaustive. Phenol as the fully dechlorinated product10,12 can be further hydrogenated to (commercially valuable) cyclohexanone11,13,16 or to the much less toxic cyclohexanol.9,15 Chlorophenols are appreciably more hydrophilic than the majority of chlorinated pollutants17 and can be readily extracted into an alkaline aqueous phase: the solubility (298 K) of 2,4dichlorophenol (2,4-DCP) and pentachlorophenol (PCP) is 8.2 g dm-3 (pH ) 7.72) and 10.9 g dm-3 (pH ) 7.68), respectively.18 As a direct consequence, aqueous-phase HDC of CPs, as a unit process, can be either implemented directly in diluted wastewater detoxification or integrated, after separation units,19-21 in an industrial waste treatment process, serving to selectively recycle end product and limit emissions. Recently, HDC of CPs in the aqueous phase has been studied in fixed-bed reactor22 and membrane23,24 systems, work that represents a significant move toward practical application. While an understanding of reaction pathway and mechanism is essential for the development of a viable industrial process, comprehensive kinetic studies of CP HDC in aqueous media are limited9,10 because of the complexity of the reaction.3,25 Using 4-chlorophenol (4-CP) as the model feed, Felis et al.9 proposed a Langmuir-Hinshelwood (LH) kinetic model for * Correspondingauthor.Tel.: +44(0)1314514719.E-mail: M.A.Keane@ hw.ac.uk. † University of Kentucky. ‡ Heriot-Watt University.
reaction over 2.63% w/w Ru/C (initial [4-CP] e 311 mmol dm-3, Cl/Ru e 478 mol mol-1, T ) 313-353 K, total pressure 0.4 MPa), where a 4-CP f phenol f cyclohexanol consecutive reaction scheme was considered. As a general observation, HCl generated during liquid-phase HDC may serve to leach the catalytically active metal phase8,26 and inhibit HDC.27-29 In order to neutralize both the HCl produced and the surface acidic sites, while maintaining (chloro-)phenolate species in solution, Felis et al. ensured that excess NaOH was added to keep the pH of the reaction mixture g13.9 However, the possibility of HDC inhibition at high [NaOH] and the issue of catalyst stability were not discussed in their report. The HDC of 4-CP (initial [4-CP] ) 10 mmol dm-3, Cl/Pd) 213 mol mol-1, T ) 303-358 K, H2 pressure ) 0.11-0.37 MPa) was also studied over a 1% w/w Pd on an activated carbon cloth,10 where a first-order dependence with respect to both H2 and 4-CP was established and the HDC rate showed little response to the addition of NH4OH (where the starting pH e 8.3). At such a low Cl/Pd ratio (213 mol mol-1) and initial [4-CP] (10 mmol dm-3), the HCl generated did not significantly poison or leach out the supported Pd over a 3 h run. It is clear that any selection/optimization of a catalytic HDC system requires a better understanding of solution pH effects. In our previous work,30,31 the aqueous-phase HDC of 2,4-DCP and 2-CP was studied over a wide pH range (1.5-12.6),30,31 where the performance of Pd/C was compared with Pd/Al2O3. In the case of Pd/C, a HCl inhibitory effect was observed (initial pH ) 1.5-5.0)31 and a significant increase in 2,4-DCP HDC activity was only evident where pH g 9.0 (with the addition of NaOH),30 in agreement with the literature.9,10 However, the addition of excess base (initial pH ) 12.6-10.0) led to an inhibition of 2-CP HDC, resulting in a preferential partial dechlorination of 2,4-DCP.30 While operation at lower pH (e5.0) can facilitate the adsorption of (chloro-)phenol on activated carbon32,33 and enhance complete dechlorination,10,34 the lower HDC rate and possibility of catalyst deactivation due to HCl poisoning31 are decided long-term drawbacks. On the basis of our previous studies, Pd/Al2O3, which is more active
10.1021/ie060802o CCC: $37.00 © 2007 American Chemical Society Published on Web 01/03/2007
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Table 1. Characteristics of the Pd/Al2O3 Catalyst catalyst
Pd loading (w/w %)
ABET (m2 gcat-1)
dhPd (nm)
APd a (m2 gPd-1)
pHpzc
Pd/Al2O3
1.2
160
2.4
227
7.8
a
Specific Pd surface area APd ) 6/(FPddhPd), FPd ) 12.02 g cm-3.
(where pH ) 7-9)30 and stable31 than Pd/C, represents a better option in the aqueous-phase HDC of CPs. As a consequence, the main objective of this study is to demonstrate the feasibility of controlling 2,4-DCP HDC performance over Pd/Al2O3 through pH adjustment. A preliminary kinetic analysis and a correlation between pH and initial HDC rate are presented. 2. Experimental Section 2.1. Materials. 2,4-DCP and NaOH (both 99+%) were purchased from Aldrich Chemical Co., and NH4OH (36.5-38%, analytical reagent (AR)) was supplied by Mallinckrodt Baker, Inc.; all the chemicals were used as received. Stock 2,4-DCP solutions were prepared with deionized water (electronic resistance g 15 MΩ). The concentration of the base solutions was determined by standard acid titration. The Pd/Al2O3 catalyst (nominal loading ) 1% w/w Pd), supplied by Aldrich, was sieved (ATM fine test sieves) into batches of particle diameter < 400 mesh (37 µm). The Pd content, Brunauer-EmmettTeller (BET) surface area (ABET), surface-area-weighted average particle size (dhPd), and the pH associated with the point of zero charge (pHpzc) for the catalyst were determined as described elsewhere30,31 and are given in Table 1. 2.2. Catalytic Procedure. All the liquid-phase HDC reactions were carried out in a modified commercial stirred glass reactor (Ken Kimble Reactors Ltd.) equipped with a H2 supply at a constant (Brooks mass flow controller) volumetric flow rate (250 cm3 min-1). There was no measurable conversion in the absence of the H2 supply. Liquid coolant (ca. 278 K) was used to condense all volatiles: loss of the reactor liquid contents in the H2 flow was negligible (3.2 mmolCl min-1 gcat-1). The significantly lower fractional dechlorination at extended reaction time (XCl ) 0.68 at t ) 120 min without base addition) suggests an HCl inhibitory effect, which can be linked to the lower bulk solution pH ( 0, can be attributed to the decrease in solution acidity (Ka) during HDC as 2,4-DCP (pKa ) 7.89 at 298 K) is converted to 2-CP (pKa ) 8.56 at 298 K), PhOH (pKa ) 10.0 at 298 K), and CHN (pKa ) 16.7 at 298 K).37 HDC activity and selectivity are compared in Figure 3, where pH control spanned the range 7-11.6. In accordance with our previous observation, the highest initial HDC rates were recorded at pH between 7 ((RHDC)i ) 5.8 mmolCl min-1 gcat-1) and 8 ((RHDC)i ) 6.5 mmolCl min-1 gcat-1), and the associated XCl vs t profiles overlapped. With the increase of solution pH, the initial HDC rate decreased to 4.7 mmolCl min-1 gcat-1 (pH ) 9.1) and was largely unresponsive (4.1 ( 0.3 mmolCl min-1 gcat-1) to pH increase beyond 10.2. Dechlo-
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Figure 2. NaOH addition (-+-), HCl production (-0-), and pH (s) as a function of reaction time during the HDC of 2,4-DCP: (a) pH controlled at 7; (b) pH controlled at 11.6; [2,4-DCP]i ) 28.5 mmol dm-3; [NaOH] supply ) 2.15 mol dm-3; Wcat ) 0.3 g dm-3; T ) 303 K.
rination efficiency, in terms of fractional HDC, declined at higher solution pH where XCl ) 0.5 was recorded at the end of a 45 min run with pH control at 11.6 (Figure 3a), while HDC approached completion (XCl > 0.9) for pH ) 7-8. This inhibitory effect can be attributed to the electrostatic repulsion between chlorophenolate anions in solution (dominant reactive species at pH >10) and the negatively charged Pd/Al2O3 surface at solution pH > pHpzc ()7.8). The 2-CP selectivity (S2-CP) dependence on X2,4-DCP coincided where pH ) 7.0-10.2 (Figure 3b), but significantly higher S2-CP values were generated where pH ) 11 as the stronger electrostatic repulsion inhibited complete HDC to phenol. Comparing HDC performance under controlled pH (7-10.2) with that in the absence of NaOH (see Figure 1c), all of the S2-CP values associated with controlled pH are higher at X2,4-DCP > 0.25. The selectivity with respect to cyclohexanone (SCHN) was less than 10% with no obvious dependence on solution pH; cyclohexanol formation was not significant (1000 rpm, H2 flow rate >150 cm3 min-1, and H2 consumption rate (at T ) 303 K) < 8.6 mmol min-1 dm-3. Under the same conditions, the maximum H2 consumption rate in this study ) 1.2 mmol min-1 dm-3, and any significant G/L interfacial mass transfer constraints can be discounted. Moreover, 2,4-DCP HDC over Pd/Al2O3 was identified as a fast reaction where H2 transport at the liquid/solid (L/S) interface was found to be the predominant physical constraint.12,13 With catalyst particle size < 45 µm, the highest HDC initial rate ((RHDC)i) recorded (T ) 303 K) in the same reactor system under an established kinetic regime was 3.5 mmol gcat-1 min-1.12,33 With the application of catalyst particle size < 37 µm, the (RHDC)i values recorded in this study fall within the range 1.76.7 mmol gcat-1 min-1, i.e., the same order of magnitude as in the previous work, and L/S interfacial mass transfer contributions can be taken to be negligible. Taking [2,4-DCP]i ) 28.5 mmol dm-3, Wcat ) 0.3 g dm-3, and pH ) 7 ( 0.1, 2,4-DCP HDC was monitored at T ) 303 and 283 K, where (RHDC)i was found to be equal to 5.9 and 1.7 mmol gcat-1 min-1, respectively. The associated apparent activation energy (∆Ea) is 45 kJ mol-1, exceeding the typical lower limit for physical control38 and providing additional evidence of HDC operation free from appreciable transport effects. The stability of Pd/Al2O3 during 2,4-DCP HDC was considered in our previous studies,31,35 where catalyst reuse following
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a washing with deionized water, drying (383 K in air), storage, and reactivation in H2 (room temperature) resulted in (RHDC)i values that were 15% less than that generated for the “fresh” catalyst. The recycled catalyst was found to have suffered a 5% loss of Pd content after the first run where pH ) 12.5-7.8. In addition, a direct reuse of Pd/Al2O3 without removal from the reactor, operated under the same conditions,35 revealed that the reused catalyst retained ca. 94% HDC activity. These HDC runs were conducted in the presence of NaOH, but catalyst recycle following HDC in the absence of base delivered (RHDC)i that was ca. 30% less than the original value with an associated 11% loss of starting Pd. The HCl produced during HDC (pH ) 5.0-1.5) not only inhibited HDC rate (as is also shown in Figure 1b) but also induced Pd leaching/poisoning.31 The HDC experiments associated with the kinetic study presented in this paper were conducted under conditions where reaction time, [organo-Cl]i/[Pd], and solution pH were such that any loss of HDC activity can be considered to be inconsequential. 3.2.2. Kinetic Analysis: Initial HDC Rates. Suzdorf et al.39 reported that the gas-phase conversion of DCP over Pd/Al2O3 follows a series of consecutive steps with cyclohexanol as the ultimate product, i.e., DCP f CP f PhOH f CHN f cyclohexanol. In the case of gas-phase 2,4-DCP HDC over Ni/ SiO2,40 both 2-CP and 4-CP were identified as products of partial HDC, PhOH was the sole product of complete dechlorination, and stepwise rather than concerted HDC to PhOH was found to prevail. In our previous work,12 the dominant 2,4-DCP reaction pathway over Pd/C was identified as the stepwise 2,4DCP f 2-CP f phenol. In this study, the relative importance of a stepwise HDC of 2,4-DCP to 2-CP (eq 1) and the possible parallel reactions listed below (eqs 2 and 3), k1
2,4-DCP + H2 98 2-CP + HCl k2
2,4-DCP + 2H2 98 PhOH + 2HCl k3
2,4-DCP + 4H2 98 CHN + 2HCl
(1) (2) (3)
are assessed for HDC under controlled pH (7 ( 0.1). The initial reaction rates can serve as quantifiable means of determining the predominant pathway according to
(R2-CP)i -R1 ) (Y2-CP)i | ) -(R1 + R2 + R3) t)0 -(R2,4-DCP)i
(4)
where -R1, -R2, and -R3 are rates of reactions 1, 2, and 3 based on experimentally determined 2,4-DCP consumption; (R2-CP)i is the initial production rate of 2-CP; -(R2,4-DCP)i is the initial consumption rate of 2,4-DCP; and (Y2-CP)i is the initial differential yield of 2-CP. The differential yield of 2-CP (Y2-CP) is plotted as a function of [2-CP]/[2,4-DCP] in Figure 4, where R2-CP and -R2,4-DCP at time tj are calculated from a secondorder finite differential
R)
Figure 4. Differential 2-CP yield (Y2-CP) as a function of the concentration ratio ([2-CP]/[2,4-DCP]) with linear fitting: [organo-Cl]i/[Pd] ) 2000; pH ) 7 ( 0.1; [NaOH] supply ) 0.4-2.15 mol dm-3; [2,4-DCP]i ) 7.628.5 mmol dm-3; T ) 303 K.
(Cj+1 - Cj) (Cj+1 - Cj) dC |t)tj ) + dt (tj+1 - tj) (tj - tj-1) (Cj+1 - Cj-1) (tj+1 - tj) ‚ (5) (tj+1 - tj-1) (tj - tj-1)
The parameters Cj+1, Cj, and Cj-1 represent concentrations (of 2-CP or 2,4-DCP) at time tj+1, tj, and tj-1, respectively. It can be seen (Figure 4) that the 2-CP differential yield exhibits a
Figure 5. Initial 2,4-DCP consumption rate (-(R2,4-DCP)i) as a function of initial 2,4-DCP concentration ([organo-Cl]i/[Pd] ) 2000; pH ) 7 ( 0.1; [NaOH] supply ) 0.4-2.15 mol dm-3; [2,4-DCP]i ) 2.8-31.9 mmol dm-3. Note: dashed line represents LH fitting using eq 6 where n ) 1; dotted line represents LH fitting using eq 6 where n ) 2; solid line represents LH fitting using eq 6 where n ) 3; see Table 3 for regression results.
linear dependence on the ratio [2-CP]/[2,4-DCP]. A linear regression was employed to extrapolate to the initial value ((Y2-CP)i), i.e., [2-CP]/[2,4-DCP] f 0. The intercept of the regression equation gave a (Y2-CP)i ) 0.94, suggesting that 2,4DCP is initially converted principally to 2-CP, i.e., stepwise dechlorination predominates. With a continuous supply of base ([NaOH] ) 0.4-2.15 mol dm-3) to control pH (7 ( 0.1), a series of 2,4-DCP HDC reactions were carried out where [2,4-DCP]i ) 2.81-31.91 mmol dm-3. The resultant initial 2,4-DCP consumption rate (-(R2,4-DCP)i) is plotted as a function of [2,4-DCP]i in Figure 5, where -(R2,4-DCP)i can be taken as a measure of the rate of the 2,4-DCP f 2-CP step. The observed -(R2,4-DCP)i passed through a maximum ([2,4-DCP]i ) 15-20 mmol dm-3) as [2,4DCP]i is increased, which is typical of a Langmuir-Hinshelwood (LH) dependence of rate on concentration. In terms of an applicable LH model, nine possible mechanisms with surface reaction as the rate-limiting step are given in Table 2. A competitive adsorption of H2 and 2,4-DCP on the same sites forms the basis for mechanisms I/II/III, mechanisms IV/V/VI consider noncompetitive adsorption, and mechanisms VII/VIII/
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Table 2. Possible LH Mechanisms Associated with 2,4-DCP HDC mechanism: adsorption/reaction/desorption steps and kinetic equations I
competitive adsorption of H2 (nondissociative) and 2,4-DCP (nondissociative), surface reaction controlled. K1
(1) H2 + δ 798 H2‚δ
K2
(2) 2,4-DCP + δ 798 (2,4-DCP)‚δ k1
(3) (2,4-DCP)‚δ + H2‚δ 98 (2-CP)‚δ + HCl + δ k1K1K2[H2][2,4-DCP]
-R2,4-DCP )
II
K3
(4) (2-CP)‚δ798 2-CP + δ
(1 + K1[H2] + K2[2,4-DCP] + [2-CP]/K3)2 competitive adsorption of H2 (dissociative) and 2,4-DCP (nondissociative), surface reaction controlled (addition of one H atom). K1
(1) H2 + 2δ 798 2H‚δ
K2
(2) 2,4-DCP + δ 798 (2,4-DCP)‚δ k1
(3) (2,4-DCP)‚δ + H‚δ 98 (2-CP)‚δ + Cl‚δ K3
(4) (2-CP)‚δ 798 2-CP + δ
K4
(5) Cl‚δ + H‚δ 798 HCl + 2δ 1/2
k1K1 K2[H2] [2,4-DCP]
-R2,4-DCP )
III
(K4 . 1)
1/2
(1 + K11/2[H2]1/2 + K2[2,4-DCP] + [2-CP]/K3)2 competitive adsorption of H2 (dissociative) and 2,4-DCP (nondissociative), surface reaction controlled (addition of two H atoms). K1
(1) H2 + 2δ 798 2H‚δ
K2
(2) 2,4-DCP + δ 798 (2,4-DCP)‚δ k1
(3) (2,4-DCP)‚δ + 2H‚δ 98 (2-CP)‚δ + HCl + 2δ K3
(4) (2-CP)‚δ798 2-CP + δ k1K1K2[H2][2,4-DCP]
-R2,4-DCP )
IV/V/VI
(1 + K11/2[H2]1/2 + K2[2,4-DCP] + [2-CP]/K3)3 noncompetitive adsorption of H2 (dissociative and nondissociative) and 2,4-DCP (nondissociative), surface reaction controlled. K1
(1) H2 + δ′ 798 H2‚δ′
(m ) 1)
K1
or H2 + 2δ′ 798 2H‚δ′(m ) 1/2)
K2
(2) 2,4-DCP + δ 9 7 8 (2,4-DCP)‚δ k1
(3) (2,4-DCP)‚δ + H2‚δ′ 98 (2-CP)‚δ + HCl + δ′(model IV) k1
or (2,4-DCP)‚δ + 2H‚δ′ 98 (2-CP)‚δ + HCl + 2δ′ k1
or (2,4-DCP)‚δ + H‚δ′ 98 (2-CP)‚δ + Cl‚δ′
(model V)
(model VI)
K3
(4) (2-CP)‚δ 798 2-CP + δ k1(K1m[H2]m)nK2[2,4-DCP]
-R2,4-DCP ) VII/VIII/IX
(1 + K1m[H2]m)n(1 + K2[2,4-DCP] + [2-CP]/K3) noncompetitive adsorption of H2 (dissociative and nondissociative) and 2,4-DCP (nondissociative), substrate (2,4-DCP) inhibition at high concentrations, surface reaction controlled. K1
(1) H2 + δ′ 798 H2‚δ′
(m ) 1)
K2
(2) 2,4-DCP + δ 9 7 8 (2,4-DCP)‚δ
K1
or H2 + 2δ′ 798 2H‚δ′
KSI
(3) (2,4-DCP)‚δ + 2,4-DCP 798 (2,4-DCP)2‚δ
k1
(4) (2,4-DCP)‚δ + H2‚δ′ 98 (2-CP)‚δ + HCl + δ′ k1
or (2,4-DCP)‚δ + 2H‚δ′ 98 (2-CP)‚δ + HCl + 2δ′ k1
(m ) 1/2)
or (2,4-DCP)‚δ + H‚δ′ 98 (2-CP)‚δ + Cl‚δ′
(model VII) (model VIII)
(model IX)
K3
(5) (2-CP)‚δ 798 2-CP + δ -R2,4-DCP )
k1(K1m[H2]m)nK2[2,4-DCP] (1 + K1m[H2]m)n(1 + K2[2,4-DCP] + K2KSI[2,4-DCP]2 + [2-CP]/K3)
IX are consistent with a noncompetitive adsorption of H2 and 2,4-DCP but with substrate (2,4-DCP) inhibition. The rate expressions for models I/IV/VII reflect a nondissociative adsorption of H2, while the other models presume dissociative H2 adsorption with different surface reaction steps assigned as rate limiting. Models III/V/VIII assume a surface HDC with addition of two H atoms, while models II/VI/IX propose a ratelimiting dechlorination involving the addition of one H atom and a fast consecutive HCl formation. For HDC operated under constant H2 concentration in bulk solution (saturation), the
applicable rate expression associated with models I-VI takes the form
-(R2,4-DCP)i )
kK[2,4-DCP]i (1 + K[2,4-DCP]i)n
(6)
The k and K terms are fitting parameters that are directly proportional to the rate constant of the limiting surface reaction (k1, see Table 2) and the adsorption equilibrium constant for 2,4-DCP (K2), respectively; n can equal 1, 2, or 3, representing
Ind. Eng. Chem. Res., Vol. 46, No. 3, 2007 711 Table 3. Regression Results Associated with the Proposed LH Models (See Table 2) for Initial 2,4-DCP Consumption Rate (-(R2,4-DCP)i) Dependence on [2,4-DCP]i (eqs 6 and 7) with 95% Confidence Limitsa k (mmol gcat-1 min-1)
model I and II (eq 6, n ) 2) III (eq 6, n ) 3) IV, V, and VI (eq 6, n ) 1) VII, VIII, and IX (eq 7) a
19.9 ( 1.6 30.2 ( 2.1 5.5 ( 0.4 11.5 ( 3.6
K (dm3 mmol-1) 5.09 ( 0.09 × 10-2 2.97 ( 0.05 × 10-2 0.32 ( 0.13 8.0 ( 3.8 × 10-2
KSI (dm3 mmol-1)
residual sum of squares
standard deviation
3.6 ( 2.1 × 10-2
0.621 0.537 2.677 0.581
0.18 0.17 0.38 0.18
[organo-Cl]i/[Pd] ) 2000; pH ) 7 ( 0.1; [NaOH] supply ) 0.4-2.15 mol dm-3; [2,4-DCP]i ) 2.8-31.9 mmol dm-3.
mechanisms IV/V/VI, I/II, and III, respectively. The initial rate equations associated with models VII/VIII/IX are similar to that for mechanisms I and II but with one additional fitting parameter KSI (eq 7), which corrects for the possibility of substrate inhibition (SI) at higher concentrations (Table 2).
-(R2,4-DCP)i )
kK[2,4-DCP]i (1 + K[2,4-DCP]i + KKSI[2,4-DCP]i ) 2
(7)
The nonlinear fitting program that is part of the statistical toolbox of Matlab (The MathWorks, Inc.) was used to discriminate the rival models, and the results are given in Table 3. On the basis of the residual sum of squares and standard deviations, models IV/V/VI provided a rather poor fit where the calculated variation of -(R2,4-DCP)i as a function of [2,4-DCP]i could not capture the rate maximum that was observed in the experimental data (see Figure 5), which suggests the models associated with noncompetitive adsorption are not applicable to this system. Considering models I/II/III, a better fit resulted where n ) 3 in eq 6 (model III), albeit the alternative mechanistic models I/II (n ) 2 in eq 6) also provided an adequate representation of the experimental data (see Figure 5). Taking an overview of the literature on liquid-phase hydrogen mediated reactions, kinetic models involving dissociative H2 adsorption and surface reaction with the addition of 2 H atoms are well-established41-43 and confirmed in both gas-phase44 and liquid-phase HDC reactions.45,46 For the regression equation associated with models VII/VIII/IX (eq 7), the additional parameter KSI did not result in a significantly improved fit when compared with the standard deviation associated with models I/II/III (Table 3). Moreover, the confidence intervals of the fitting parameters k, K, and KSI were (31%, (48%, and (58%, respectively, which are much broader than those associated with models I/II/III, indicating that these fitting parameters are less sensitive in capturing the trend of experimental data, i.e., correction due to substrate inhibition may not be applicable in this instance. On the basis of the foregoing, the experimentally observed initial HDC rate dependence on [2,4-DCP]i is consistent with competitive H2 and 2,4-DCP adsorption with surface reaction as rate limiting; the applicable rate expression is given in eq 6. This represents an adequate starting point for a meaningful analysis of the initial HDC rate dependence on solution pH, which is the major concern of this study. It should be noted that the linear dependence of Y2-CP on [2-CP]/[2,4-DCP] (see Figure 4) implies that the further HDC of 2-CP may follow the same LH mechanism(s) as that associated with 2,4-DCP HDC (see Table 2). Taking 2-CP conversion as a dechlorination to phenol, k4
2-CP + H2 98 PhOH + HCl
(8)
where 2,4-DCP is converted predominantly to 2-CP (rather than phenol), the following dependence of Y2-CP on [2-CP]/[2,4-
DCP] results,
Y2-CP )
(
R2-CP k4 [2-CP] )1-R2,4-DCP k1 [2,4-DCP]
)
(9)
which is diagnostic of a consecutive HDC of 2,4-DCP to 2-CP and then to phenol. The slope of the linear fit in Figure 4 (intercept set at 1) gives the k4/k1 ratio, which equals 1.3. As the ratio exceeds unity, the presence of the second Cl substituent has an overall deactivating effect with the result that the entire Cl complement is less susceptible to H2 cleavage. This response finds support in previous work,6,44,47,48 where it was demonstrated that increasing Cl substitution serves to reduce the electron density associated with the ring carbons, lowering haloarene reactivity. 3.3. Correlation between Initial HDC Rate and pH. In this study, HDC was operated in neutral or basic media (pH g 7.0) where HCl desorption from the catalyst can be considered as fast and irreversible because of neutralization in solution by NaOH. The dependence of initial DCP consumption rate (-(R2,4-DCP)i) on solution pH was monitored over the pH range 7.0-12.9 (controlled to within (0.1) where any HCl inhibitory effects are negligible (see Section 3.2.1). From the preceding analysis, it is evident that -(R2,4-DCP)i dependence on pH reflects the response of the intrinsic surface reaction activity (k1, see Table 2) and chemisorption (K2, see Table 2) behavior to variations in solution pH. The latter, in turn, impacts on the degree of 2,4-DCP dissociation in solution. Consequently, the development of a correlation between -(R2,4-DCP)i and reaction pH ([H+]) can draw on changes to k1 and K2. Equilibria involving the nondissociated or chlorophenolic form of 2,4DCP (HB) and the dissociated or chlorophenolate form (B) in solution and interaction with the catalyst surface sites (δ) can be written as Ka
HB 798 H+ + B KHB 2
HB + δ 798 HB‚δ KB2
B + δ 798 B‚δ
(10) (11) (12)
Since the apparent adsorption of 2,4-DCP is the sum of both nondissociated and dissociated forms, the total or composite equilibrium adsorption constant (K2) can be presented as
K2 )
+ KB2 + (KHB 2 ([H ]/Ka)) +
1 + ([H ]/Ka)
B ) KB2 + (KHB 2 - K2 )xHB
(13)
where xHB is the molar fraction of the nondissociated form and is defined as xHB ) ([H+]/Ka)/(1 + [H+]/Ka). Similarly, the
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Ind. Eng. Chem. Res., Vol. 46, No. 3, 2007
“total” HDC rate constant (k1) can be developed as kHB 1
HB + H2 98 2-CP (phenolic form) + HCl kB1
B + H2 98 2-CP (phenolate form) + HCl k1 )
+ kB1 + (kHB 1 ([H ]/Ka)) +
1 + ([H ]/Ka)
(14) (15)
B ) kB1 + (kHB 1 - k1 )xHB (16)
The amphoteric nature of the Al2O3 support can also contribute to the dependence of HDC activity on pH. At a solution pH < pHpzc, the catalyst surface favors interaction with anionic species. Conversely, where pH > pHpzc, the surface will exhibit a dynamic electrostatic attraction for any cationic species in solution.49,50 Although the importance of the amphoteric nature of Al2O3 on the adsorption of ionic and partially dissociated organics has been established,50-52 the possible impact on interactions between catalyst surface and chemisorbed reactive species still remains unclear. We could find only one pertinent report53 that linked the effect of surface charge density to catalytic reaction rate, where Pd/C catalytic activity for NO3denitrification was found to be pH dependent and the pHpzc of the catalyst was used to correct the corresponding first-order reaction rate constant. The development of a surface charge influences adsorption/reaction involving ionic species and a correction to kB1 and KB2 must be applied where
kB1
[H+]
) f1 + + k′1 [H ]pzc [H+]
+ K′2 KB2 ) f2 + [H ]pzc
) )
Ka k 1 ) f1 + + kHB 1 - k′1 xHB + k′1 [H ]pzc K 2 ) f2
Ka +
[H ]pzc
+ KHB 2 - K′2 xHB + K′2
(
K)
)
K1[H2]i
(1 + K11/2[H2]i1/2)2
k1
1 K2 (1 + K11/2[H2]i1/2)
(a1xHB + 1)
(23)
(a1xpH)7 HB + 1)
K ) KpH)7
(a2xHB + 1)
(24)
(a2xpH)7 HB + 1)
where a1 and a2 are new fitting parameters and xpH)7 is the HB mole fraction of nondissociated 2,4-DCP (at pH ) 7). The modified form of eq 6 is a rate expression that incorporates dissociation/surface charge parameters and takes the form
-(R2,4-DCP)i ) (a1xHB + 1) (a2xHB + 1) ‚ [2,4-DCP]i kpH)7KpH)7 pH)7 (a1xpH)7 HB + 1) (a2xHB + 1)
(
1+K
pH)7
(a2xHB + 1) (a2xpH)7 HB + 1)
)
n
(25)
[2,4-DCP]i
(17)
(18)
kHB (kB1 )pzc Ka 1 -1 + -1 a1 ) + k′1 k′ [H ]pzc 1
(26)
KHB (KB2 )pzc Ka 2 -1 + -1 + K′2 K′ [H ]pzc 2
(27)
(19)
(20)
Comparing the fitting parameters k and K in eq 6 with the rate expressions in Table 2, we can express these parameters as functions of k1 and K2. Using the rate equation associated with model III as an example, k and K can be represented as
k)
k ) kpH)7
where n can equal 2 or 3 representing mechanisms I/II and III, respectively, and a1 and a2 are dimensionless numbers related to the relevant rate and equilibrium constants according to
and K2′ and k1′ are the adsorption and reaction rate constants of chlorophenolate ions, respectively, at high pH, i.e., [H+] , [H+]pzc. The parameters f1 and f2 (g0) are constants related to saturated charge density that take account of charge effects on reaction/chemisorption of (chloro-)phenolate ions, i.e., f1 ) (kB1 )pzc - k1′ and f2 ) (KB2 )pzc - K2′. Rearranging eqs 13 and 16 with eqs 17 and 18, the correlations between k1 (and K2) and xHB can be expressed as
( (
independent, k and K are both pH dependent with k1 and K2 being functions of xHB. By substituting eqs 19 and 20 into eqs 21 and 22 and evaluating the pH independent parameters in terms of the parameters k and K associated with HDC at pH ) 7 (denoted kpH)7 and KpH)7, see Table 3 for values), the generalized correlation between k/K and xHB for models I, II, and III can be represented as
(21)
(22)
While the initial H2 concentration in solution ([H2]i) and H2 adsorption equilibrium constant (K1) can be taken to be pH
a2 )
( (
) )
Taking a series of experimentally determined initial 2,4-DCP consumption rates associated with HDC operated in the pH range 7-12.9 ((0.1) and fixed [2,4-DCP]i (28.5 mmol dm-3), the reaction pH was correlated to the -(R2,4-DCP)i using eq 25; the fit can be seen in Figure 6a, and the regression results are listed in Table 4. It is apparent that both LH models reproduced the experimentally determined trend (Figure 6a), albeit the rate expression associated with model III (n ) 3) delivered the smaller residual sum of squares and standard deviation. It must be stressed that this correlation captured the experimentally observed initial HDC rate maximum in the pH range 7-8. Both equations (n ) 2 and 3) generated small values for a1, which suggests that pH has little effect on the “apparent” reaction rate constant (k), as is evident from the results presented in Figure 6b. In terms of a possible electrophilic HDC mechanism,40,54 Felis et al.9 proposed that the dissociated (chlorophenolate) form should be more reactive, i.e., k1′ > kHB 1 . However, the negatively charged catalyst surface at high pH ([H+] , [H+]pzc) may also modify the interaction between the chlorophenolate ions in solution and the surface active sites, i.e., (kB1 )pzc > k1′, which serves to compensate for the former effect with the result that k is pH insensitive. The value of a2 is such that 2,4-DCP chemisorption is less favored at higher pH, a response that results from application of models I/II/III and is illustrated in Figure 6b where K increases 4-fold with a pH decrease from 13 to 7. Although pH effects in determining 2,4-DCP adsorption
Ind. Eng. Chem. Res., Vol. 46, No. 3, 2007 713
4. Conclusions With the addition of NaOH and NH4OH, it was confirmed that an external adjustment of solution pH facilitates some control over catalytic performance in the aqueous-phase batch HDC of 2,4-DCP over a 1.2% w/w Pd/Al2O3 catalyst characterized by dhPd ) 2.3 nm and pHpzc ) 7.8. Operating over the pH range 7-11.6, the pH of the reaction mixture can be controlled to within (0.1 with a continuous external supply of aqueous NaOH solution during reaction. HDC operated under controlled pH exhibited a maximum catalytic activity in the pH range of 7-8, where the selectivity with respect to 2-CP was lower than that delivered at higher pH (g11.6) (X2,4-DCP g 0.4). A preliminary kinetic analysis suggests that 2,4-DCP is predominantly converted to 2-CP and displays a [2,4-DCP]i dependence that is consistent with Langmuir-Hinshelwood (LH) behavior. A nonlinear regression of the experimentally determined initial dechlorination rates as a function of [2,4-DCP]i has demonstrated that three LH models involving competitive adsorption of H2 and 2,4-DCP fit the data well. A mechanism consistent with dissociative adsorption of H2 and surface reaction (the addition of two H atoms) as rate-limiting provides the best overall representation of the experimental results. The effect of pH on the initial HDC activity was analyzed by a generic correlation, which reveals that the adsorption of 2,4-DCP increased by a factor of 4 when the pH was lowered from 13 to 7, while the apparent surface reaction constant remained unaffected. Acknowledgment This work was supported in part by the National Science Foundation through Grant CTS-0218591. Nomenclature Figure 6. (a) Initial 2,4-DCP consumption rate (-(R2,4-DCP)i) as a function of solution pH: dotted line represents correlation using LH rate eq 25 where n ) 2; solid line represents correlation using LH rate eq 25 where n ) 3. Note: the MS Excel spreadsheet was used to fit the experimental data with a nonlinear least-square method by iteration. (b) Fitting parameters k (apparent rate constant for 2,4-DCP conversion) and K (apparent adsorption constant for 2,4-DCP) as a function of pH: [2,4-DCP]i ) 28.5 mmol dm-3; [NaOH] supply ) 2.15 mol dm-3; Wcat ) 0.3 g dm-3; T ) 303 K. Table 4. Results of the Correlation between pH and the Initial 2,4-DCP Consumption Rates (-(R2,4-DCP)i) Using the Proposed LH Models (eq 25) with Associated 95% Confidence Limitsa model
a1
a2
I and II (n ) 2) 1.9 ( 0.7 × 10-2 3.70 ( 0.07 III (n ) 3) 2.2 ( 0.6 × 10-2 2.97 (0.06
residual sum standard of squares deviation 0.0645 0.0548
0.10 0.09
a [2,4-DCP] ) 28.5 mmol dm-3; [NaOH] supply ) 2.15 mol dm-3; i Wcat ) 0.3 g dm-3; T ) 303 K.
from solution on Pd/Al2O3 or Al2O3 have not been reported in the literature, Liu and Huang52 have recorded an enhanced monochlorophenol uptake on ZnS with a decrease in pH from 10 to 8, which indirectly supports our correlation results. In a recent critical review,55 a lower uptake of phenolic compounds on activated carbon at pH > pKa was noted and attributed principally to electrostatic repulsion between a negatively charged surface and the anionic phenolate, a proposal which matches our contention. The analysis that we have provided has proved adequate in quantitatively describing the relationship between 2,4-DCP HDC rate and solution pH. As a generic approach, we believe that it is applicable to all aqueous HDC reactions over solid catalysts.
A1 ) fitting parameter, dimensionless A2 ) fitting parameter, dimensionless ABET ) BET surface area (m2 gcat-1) APd ) specific Pd surface area (m2 gPd-1) B ) dissociated (chlorophenolate) form of 2,4-dichlorophenol C ) concentration of reactant/product (mmol dm-3) CHN ) cyclohexanone CP ) chlorophenol [2-CP] ) concentration of 2-CP in bulk solution (mmol dm-3) DCP ) dichlorophenol [2,4-DCP] ) concentration of 2,4-DCP in bulk solution (mmol dm-3) dhPd ) surface area weighted average Pd particle size (nm) F1 ) constant, defined as f1 ) (kB1 )pzc - k1′ (mmol gcat-1 min-1) F2 ) constant, defined as f2 ) (KB2 )pzc - K2′ (dm3 mmol-1) G ) gas phase (H2) HB ) nondissociated (chlorophenolic) form of 2,4-dichlorophenol k ) fitting parameter in eqs 6 and 7 (mmol gcat-1 min-1) k1 ) rate constant for 2,4-DCP conversion to 2-CP (mmol gcat-1 min-1) B k1 ) rate constant for 2,4-DCP (chlorophenolate) conversion to 2-CP (mmol gcat-1 min-1) kHB 1 ) rate constant for 2,4-DCP (chlorophenolic) conversion to 2-CP (mmol gcat-1 min-1)
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k1′ ) rate constant for 2,4-DCP (chlorophenolate) conversion to 2-CP at higher solution pH, i.e., pH . pHpzc (mmol gcat-1 min-1) k2 ) rate constant for 2,4-DCP conversion to PhOH (mmol gcat-1 min-1) k3 ) rate constant for 2,4-DCP conversion to CHN (mmol gcat-1 min-1) k4 ) rate constant for 2-CP conversion to PhOH (mmol gcat-1 min-1) K ) fitting parameter in eqs 6 and 7 (dm3 mmol-1) K1 ) H2 adsorption equilibrium constant (dm3 mmol-1) K2 ) 2,4-DCP adsorption equilibrium constant (dm3 mmol-1) KB2 ) 2,4-DCP (chlorophenolate) adsorption equilibrium constant (dm3 mmol-1) K2HB ) 2,4-DCP (chlorophenolic) adsorption equilibrium constant (dm3 mmol-1) K2′ ) 2,4-DCP (chlorophenolate) adsorption equilibrium constant at higher solution pH, i.e., pH . pHpzc (dm3 mmol-1) K3 ) 2-CP desorption equilibrium constant (dm3 mmol-1) K4 ) HCl desorption equilibrium constant (dm3 mmol-1) KSI ) substrate (2,4-DCP) inhibition equilibrium constant (dm3 mmol-1) L ) liquid phase (reaction mixture) organo-Cl ) chlorine atoms associated with organic compound PhOH ) phenol pzc ) point of zero charge of the catalyst R ) reaction rate (mmol gcat-1 min-1) R2-CP ) reaction rate for 2-CP production (mmol gcat-1 min-1) -R2,4-DCP ) reaction rate for 2,4-DCP consumption (mmol gcat-1 min-1) S2-CP ) selectivity with respect to 2-CP (%) SCHN ) selectivity with respect to CHN (%) t ) reaction time (min) T ) temperature (K) Wcat ) catalyst concentration (gcat dm-3) XCl ) fractional dechlorination, dimensionless X2,4-DCP ) fractional conversion of 2,4-DCP, dimensionless xHB ) molar fraction of the nondissociated chlorophenol, dimensionless Y2-CP ) differential yield of 2-CP, dimensionless Subscripts i ) initial j ) time interval index for the experimental data Greek Letters δ ) surface active site F ) density (g cm-3) Literature Cited (1) Kovalchuk, V. I.; d’Itri, J. L. Catalytic Chemistry of Chloro- and Chlorofluorocarbon Dehalogenation: From Macroscopic Observations to Molecular Level Understanding. Appl. Catal., A 2004, 271, 13. (2) Coq, B.; Figueras, F. Bimetallic Palladium Catalysts: Influence of the Co-metal on the Catalyst Performance. J. Mol. Catal., A 2001, 173, 117. (3) Urbano, F. J.; Marinas, J. M. Hydrogenolysis of Organohalogen Compounds over Palladium Supported Catalysts. J. Mol. Catal., A 2001, 173, 329. (4) MacKenzie, K.; Frenzel, H.; Kopinke, F.-D. Hydrodehalogenation of Halogenated Hydrocarbons in Water with Pd catalysts: Reaction Rates and Surface Competition. Appl. Catal., B 2006, 63, 161. (5) Meunier, B. Catalytic Degradation of Chlorinated Phenols. Science 2002, 296, 270. (6) Keane, M. A. Hydrodehalogenation of Haloarenes over Silica Supported Pd and Ni. A Consideration of Catalytic Activity/Selectivity and Haloarene Reactivity. Appl. Catal., A 2004, 271, 109.
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ReceiVed for reView June 23, 2006 ReVised manuscript receiVed November 7, 2006 Accepted November 20, 2006 IE060802O
