Environ. Sci. Techno/. 1995, 29, 439-445
Reductive Dechlorination of Carlron Tetrachloride in Water Catalyzed by Mineral-Supported Biomimetic Cobalt Macrocycles
Introduction
Halogenated hydrocarbons are common soil, sediment, and groundwater pollutants (1-5). Many halogenated hydrocarbons are toxic, mutagenic materials that are resistant to microbiological degradation and hence have long environmental half-lives. The aliphatic carbon atoms bonded to more than two halogens or the carbon atoms in aromatic rings with high halogen substitution have high formal oxidation states, which render such molecules resistant to LJERKA UKRAINCZYK,*'+ the degradation under aerobic conditions and susceptible M A L A M A CHIBWE,' to a reductive degradation (1). This suggests the possibility THOMAS J . PINNAVAIA,' A N D STEPHEN A . B O Y D t of using the reductive dehalogenation to detoxify halogenated organic compounds. Certain anaerobic microDepartment of Crop and Soil Sciences, and Department of Chemistry and Center for Fundamental Materials Research, organisms reductively degrade such compounds by coMichigan State University, East Lansing, Michigan 48824 metabolism; however, these processes are often slow and require proper environmental conditions for microbial activity and growth (6-8). Furthermore, high contaminant concentrations may be toxic to the pollutant-degrading Reductive dehalogenation, mediated by nonspecific bacteria (6, 7). In principle, the problem of toxicity can be biomimetic Co macrocycles, was studied in aqueous overcome by using biomimetic catalysts rather than live systems using carbon tetrachloride as a model cells. Reductive dehalogenation has been shown to be compound. Two water-soluble macrocycles, cobalt mediated by transition metal macrocycles, such as coenzyme F430, and cobalamin (9-16). These macrocycles t e t r a k i s( N- m e t h y 1-4- p y r i d in iu my1) p or p h y r i n alongwith several other biomimetic complexes (12,13,17(CoTMPyP) cation and cobalt tetrasulfophthalocyanine 22) have been studied in homogeneous abiotic aqueous (CoPcTs) anion, were used as homogeneous and systems as potential remediation catalysts for biomimetic mineral-supported catalysts. The supported catalysts reductive dehalogenation of halogenated hydrocarbons. were prepared by exchanging CoTMPyP on the Reductive dehalogenation catalyzed by biomimetic hectorite, fluorohectorite, and amorphous silica surface macrocycles exhibits broad substrate specificity (10-13, and by exchanging CoPcTs on the layered double 23). Among various metallated macrocycles, those containing Co and Ni were found to be the most effective hydroxide surface. Supported macrocycles were catalysts (11-13). The catalytic activity of Co can be catalytically active in the dechlorination of CC14 and attributed to its low spin electronic configuration in various the initial reaction rates followed Michaelis-Menten macrocycles and to the unique property of cobalt to form kinetics. The value of vmaxwas correlated t o the metal-carbon bonds in water (24). The superior catalytic previously reported orientation of macrocycles in the activity of Ni appears to be limited to coenzyme F430, which interlayers and to the accessibility and electronic was shown to be more active than Co macrocycles (11,131, state of the metal center, following the order: CoTMPyPwhile other Ni porphyrins exhibited lower catalytic activity silica > CoPcTs-layered double hydroxide > CoTthan Co porphyrins (12). The implementation of homogeneous macrocyclic cataMPyP-fluorohectorite > CoTMPyP-hectorite. In both lysts in remediation technologies is impractical because of heterogeneous and homogeneous systems, volatile problems with separating the aqueous catalyst, reactants, reaction products accounted for less than 30% of CC14 and products. This problem could be overcome by degraded. In short-term experiments (2 h), homoimmobilizing macrocyclic complexes on a solid support. geneous CoTMPyP was more active than heterogeneous There are numerous examples of supported macrocycles catalysts, while homogeneous CoPcTs was deactiand their applications in heterogeneous catalysis;however, vated due to aggregation, and degraded less CC14 than their application in aqueous systems has rarely been investigated (22). Recently photoreductive dehalogenation supported CoPcTs. In long-term experiments (3 of aqueous bromoform has been shown to be catalyzed by days), where large CC14/macrocycle ratios were used, the anionic macrocycle (CoPcTs)adsorbed on the positively silica-supported CoTMPyP was more active than charged titania surface (21). homogeneous CoTMPyP, suggesting that adsorption , High surface area minerals, such as layered clay minerals stabilized the catalyst. and colloidal oxides, are attractive potential catalyst supports for environmental remediation in aqueous systems from both an economical and practical point of view. * Address correspondence to this author at his present address: Department of Agronomy, Iowa State University, Arnes, IA 50011; e-mail address: I-ukrain @ iastate.edu. t Department of Crop and Soil Sciences. Department of Chemistry and Center for Fundamental Materials Research.
*
0013-936W95/0929-0439$09.00/0
@ 1995 American Chemical Society
VOL. 29. NO. 2. 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
439
H3C-
-CH3
CH3 COTMPyP
CoPcTs
FIGURE 1. Structure of the cobalt tetrakis(l-metfiyl-4-pyridiniumyl)po~hyrin(CoTMPyP) cation and cobalt tetrasulfophthalocyanine(CoPcTs) anion.
Investigation of these systems is of interest for remediation of industrial effluents and groundwater and for in situ remediation of soils and sediments. The macrocycle must carry a permanent charge to be adsorbed and retained on the charged surfaces of these supports. Synthetic cationic and anionic porphyrins and phthalocyanines have been intercalated into aluminosilicate clays and layered double hydroxides (25-35). Unlike most neutral porphyrins (36381, cationic porphyrins are stable against demetallation on the acidic clay surfaces (28). Two macrocyclic catalysts, cationic CoTMPyP and anionic CoPcTs (Figure l),were used in this study. These macrocycles have been previously intercalated in layered minerals and characterized (35)byX-ray diffraction (XRD), electron spin resonance (ESR), and ultravioletlvisible ( U V / vis) spectroscopy. Their arrangements in the galleries of layered supports are presented in Figure 2. Carbon tetrachloride (CT) was chosen as a model compound to study dehalogenation reactions by the supported macrocyclic catalysts because its biomimetic dechlorination in homogeneous systems has been studied extensively. Although there are a number of studies on reductive dechlorination of CT in water catalyzed by cobalt macrocycles (10-1 4, la,the mechanismsof these reactions are still not fuuy understood (16). Reaction schemes involving Co(I1) (16) and Co(1) (14) as a catalytically active species and the alkyl-cobalt intermediates have been proposed. The volatile reaction products have been identified, usually accounting from 20-30% (10,16)up to 50% (10,141of CT degraded. The major volatile degradation products are chloroform (CF) and dichloromethane (DM), while usually only trace amounts of chloromethane, methane, and carbon monoxide have been detected (10, 11, 14, 16). The homogeneous dechlorination reactions have usually been performed in buffered solutions using dithiothreitol (DTT; 12,15, l a , titanium(II1) citrate (10,11, 13, 141, cysteine (191, sodium dithionate (201, or sodium disulfide (19) as a reductant. Sodium dithionate was not used in this study because it produces free radicals that destroy CoTMPyP (39). Sodium disulfide was used in the preliminary experiments; however, difficulties were encountered in buffering the pH. The experiments reported here were conducted using DTT as a reductant. 440
1
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 2,1995
a
CoTMPyP-hectorite
II co-
p
-co-
I
f
14.OA
P b
CoTMPyP-fluorohectorite
J 1
I
///// o c o o c o OCO 0 c o 0 c o o
/ / / / A m
19.6A
270
C
CoPcTs-layered double hydroxide
I
1 l l l I I l
I
o c o o c o o c o o c o o coo c o o c o o
23.7A
FIGURE 2 Arrangement of CoTMPyP in the interlayers of (a) hectorite and (b)fluorohectorite and of (c) CoPcTs in the interlayersof layered double hydroxide (according to ref 35).
The objectives of this study were to investigate whether cationic and anionic Co macrocycles supported on high surface area minerals maintain their ability to catalyze reductive dechlorination, to compare the rates of biomimetic reductive dehalogenation by homogeneous and heterogeneous macrocycles, and to investigate the longterm stability of supported catalysts.
Materials and Methods AU chemicals were reagent grade unless otherwise indicated
and were used without further purification. CoTMPyP was
obtained as the tetrachloride salt from Midcentury Co, (Posen, IL). CoPcTs was synthesized as the tetrasodium salt by a published procedure (30,40). Natural hectorite [M(Uo,dMg5,31Li0,1d(S$Ozo(OH,F)zl from Hector, CA, was obtained in a spray-dried form from the Baroid Division of NL Industries, The CEC of this clay was 0,7 mmok g-1(41), Synthetic fluorohectorite [Li1.&4&1.&1.6) (Sh)O2oF41 was prepared by crystallization of a fluorohectorite glass of equivalent composition according to the general procedure by B e d (42). The CEC of this material was 1,9 mmolc g-l (43). Synthetic hydrotalcite-like LDH of the type [Mgdz(OH)14][CO3]x yHz0 was prepared bythe method ofReichle etal. (44). Silica gel (chromatographic grade, Aldrich)with a surface area of 500 m2 g-' was used without further modifications. Preparation of Supported Catalysts. Preparation of layered catalystshas been described previously (35). Silicasupported CoTMPyP was prepared by reacting 1 g of silica gel suspended in 50 mL of water with 1 x mmol of CoTMPyP for 24 h at pH 7.5 (adjusted with NaOH) under nitrogen. The suspension was then centrifuged giving colorless supernatant, indicating that all CoTMPyPhas been adsorbed. The product was washed three times with water and freeze-dried. The Soret band of suspended CoTMPyPSG was at 438 nm, and the complex was ESR silent, indicating that Co was oxidized to Co(II1). DechlorinationExperiments. Dechlorinationreactions were studied using a modified method of Krone etal. ( 1I). The reaction was carried out in a 2-mL anaerobic mixture placed in a 10-mL glass vial crimp-sealed with a Tefloncoated rubber septum (Hewlett-Packard)and wrapped with aluminum foil. The vials were filled and sealed in an anaerobic glovebox (Plas-Labs) under 10% H2/90% N2 atmosphere. In short-term experiments (reaction time is 2 h), the catalyst/reductant molar ratio was 1:lOOO (0.1mM CoTMPyP or CoPcTslO.1 M DTT), and in the long-term experiments (3 days) this ratio was 1:25000 (0.002 mM CoTMPyP or CoPcTsl0.05 M DTT). In short-term experiments, the homogeneous catalyst was prepared as a 0.01 M stock solution and added by a microsyringe. The heterogeneous catalyst was added by a 100-pL pipet as an aqueous suspension (0.55 wt % suspension of CoTMPyP-FH, 1.03wt % suspension of CoTMPyPH, 0.34 wt % suspension of CoPcTs-LDH). CoTMPyP-SG was added as a solid (0.020@. Dithiothreitiol(Mallinckrodt) was added as a solid. Freshly prepared, deoxygenerated, and autoclaved 0.1 M sodium phosphate buffer (pH 7.5) was added by a pipet, and the vials were sealed. Reaction was initiated by injecting freshly prepared CT (99.9%, Burdick & Jackson) stock solution (0.062MI in 2-propanol through a septum 5 min after assembling the reductant/ catalystlbuffer mixture. The vials were shaken on a controlled temperature rotary shaker at 25 "C for a desired period of time. In long-term experiments, the catalyst was added by a microsyringe (homogeneous) or pipet (heterogeneous) to 75 mL of the phosphate buffer, and 2 mL of this mixture was pipetted into 10-mL glass vials. The other reactants were added as in short-term experiments. The experiments were performed in duplicate, and each vial was analyzed only once. One control, containing CT and DTT in phosphate buffer, was run for each time point. Two random controls were runas well: one containing CT and supported catalyst in phosphate buffer, and the other containing CT and the phosphate buffer. The loss of CT from the controls
was less than 1%in all short-term experiments (after 2 h) and less than 4% in all long-term experiments (after3 days). To test whether the dechlorination is catalyzed by the complex at the interface, the reacting suspension in shortterm experiments was filtered after approximately 30 min in the glovebox through a 0.1-pm Teflon filter into another vial containing 0.015 g of solid DTT. The vials were sealed, and the degradation of CT was followed for 2 h. Although the filtrate was weakly colored (indicating some of the macrocycle was desorbed from the support and/or some of the colloidal particles passed through the filter),less then 2% of CT was dechlorinated in a 2-h time period, implying that the macrocycle did not leach into liquid phase in a significant amount. The kinetics of CT degradation and the appearance of CF and DM was followed by headspace analysis using a Varian gas chromatograph with a DB-625 (0.25 mm i.d. x 30 m, J&W Scientific) column and a flame-ionization detector. The column temperature was 35 "C, injector temperature was 200 "C, and detector temperature was 250 "C. Headspace was sampled manually using a 0.1-mL gas-tight syringe. The peak identification and quantification were done using external standards made in 0.1 M phosphate buffer. In the preliminary experiments, it was determined that the peak areas of the standards were not affected by the presence of the solid catalyst in the amount used in this study.
Results and Discussion Short-TermExperiments. Kinetic data for CT degradation in six representative systems (homogeneous CoTMPyP, CoTMPyP-FH, CoTMPyP-H,CoTMPyP-SG, homogeneous CoPcTs, CoPcTs-LDH) are shown in Figure 3. All of the supported macrocycles were catalytically active. CF and DM were detected by the headspace analysis as the degradation products in homogeneous systems,while only CF was detected in the heterogeneos systems even at reaction times greater than 2 h. The volatile products accounted for less than 30% of CT degraded, which is consistent with previous studies of CT degradation mediated by c o macrocycles (10, 1 1 , 13, 16, 19). The heterogeneous degradation of CT catalyzed by Co macrocycles illustrated in Figure 3 can be described by the first-order kinetics:
The pseudo-first-order rate constants (kobs)were calculated (Table 1) by fitting the integrated form of the eq 1 to the data in Figure 3. The observed pseudo-first-order rate constant (kobs) may conceptually be separated into two constants: kl, a pseudo-first-order rate constant for the formation of CF, and k2, a pseudo-first-order rate constant for the formation of unidentified products (includinglongk2. Then the rate of lived intermediates): kobs = kl appearance of CF is
+
where k3 is a pseudo-first-order rate constant for CF degradation. The rate of formation of DM is VOL. 29, NO. 2. 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY I 4 4 1
0.6
1\
1
0.4
u
0.2
0.0 O.* 0
0.0 0
30
60
90
120
1.o
30
60
90
120
1 .o
I
C.
0.8 0.6 0.4
0.2
0"E
0.2
0.0
0.0
0
30
60
90
120
0
30
60
90
120
0
30
60
90
(20
I
0.2
0'4 0.0
w 0 30 60 90 120
Time (min) FIGURE 3. Degradation of 1.0 mM CT (0)and production of CF (0) and DM (v) at pH 7.5 catalyzed by 0.1 mM Co macrocycle: (a) homogeneous CoTMPyP, (b) CoTMPyP-FH, (c) CoTMPyP-H, (d) ColMPyP-SG, (e) homogeneous CoPcTs, and fl) CoPcTs-U)H. Cutves represent simulation of the CT dechlorination and CF and DM formation, based on the kinetic model presented in eqs 1-3. TABLE 1
Pseude=First=OrderRate Constants ( x 103)for Degradation of CT and Product Formation Catalyzed by Hoatogeneous and Heterogeneous Macrocycles ki
hbs
catalyst
CoTMPyP CoTMPyP-FH CoTMPyP-AS CoTMPyP-H CoPcTs CoPcTs-LDH
(min-l)O SEb (inin-')' 52.2 8.0 31.4 6.4 45.8d 28.9
4.8 0.2 2.3 1.0 7.5 1.3
26.5 1.5 9.9 0.6 nfe 12.8
k2
4
k,
25.7 6.5 21.5 5.8
7.9 1.1 1.2 1.0
5.8 ndc nd nd nd nd
(min-')' (min-l)' ( m i d ) '
nf
16.1
nf 4.3
a Rate constants kbs, k,,k2,and k, obtained by fitting eqs 1-3 to the data in Figure 3. Standard error for k b r . CH2C12was not detected. Initial rate constant. Not fitted because the reaction could not be described by the first-order kinetics.
(3)
where k4 is a pseudo-first-order rate constant for the formation of DM. Equations 1-3 were solved by the Runge-Kutta method using the k&s from the Table 1 and fitting k2, k3 and k4 values to the data (Table 1). The calculated curves are shown in Figure 3 for all the systems studied, except homogeneous CoPcTs. In the homogeneous CoPcTs system,the fast initial rate rapidly decreases (Figure3e),indicatingthat the catalyst is beinginactivated. The fast decrease in the catalytic activity of this macrocycle has been attributed to the aggregation of CoPcTs in aqueous solutions (45). Intercalation stabilizes CoPcTs against 442
aggregation (30, 331, and thus the rate of heterogeneous degradation does not show the rapid decrease in activity. At the same total catalystlCT molar ratio (l:lO), the degradation rates in CoTMPyP systemswere slower for the heterogeneous systems than the homogeneous system, following the order: CoTMF'yP CoTMPyP-SG CoTMPyP-FH > CoTMPyP-H. Adsorption on clay surfaces has often been shown to alter the reactivity, mobility, and redox properties of transition metal complexes and enzymes (41, 43, 46, 4 7) Reactions catalyzed by macrocycles at the clay or silica surface can differ from homogeneous reactions as a result of changes in the redox potential of the supported macrocycles as well as a result of the geometricalarrangements ofthe macrocycles at the interface, which may yield different product distribution. The lack of formation of DM in the heterogeneous systems in this study is most likelythe result of a change in the reduction potential of the supported macrocycles to a more positive value relative to the aqueous macrocycles. Since the dechlorination of CF to DM has lower reduction potential [B&FIDM) = f0.439 V at pH 7, calculated from free energies of formation in liquid state, 1 M C1- aqueous solution; 11,481 than the dechlorination ofCT to CF [Bo(CTICF)= +0.516Vl, thereduction potential of a supported Co macrocycle may be too high to drive the reduction of CF to DM. Furthermore, in solution both axial positions on Co are available for reaction while in the supported system the reactants may only have access to one of the Co axial positions. The quantitative information on the changes in product distribution can be obtained from the ratio of the rate constant (Table 1) for the formation of CF (kl) to the overall rate constant for CT degradation (kobs): the smaller this ratio, the lower the tendency for the formal two-electron transfer from CT to Co and for CF formation. Alkyl radicals ('CC13 and 'CHCl3) or carbanions formed from CT and CF by one electron reduction (10)instead of donating another electron to Co and reacting with the hydrogen ion to form CF and DM may be reacting with water to give CO and formic acid as well as reactingwith each other and with the organic reductant to give nonvolatile products (10,11, 14,
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 2,1995
16).
Plot of the initial rate (& = ko[CC&lo)of CT degradation by the supported macrocycles as a function of the initial concentration of CT ([CC&l0)is shown in Figure 4. The initial rate constants (b) for heterogeneousCT degradation, calculated by fitting the first-order rate equation to the data for the first 30 min of the experiment, decrease in the order COTMQP-SG> COPCTS-LDH > COTMQP-FH> COTMQPH. It can be assumed that in the initial stages of the reaction, the concentration of CT is much higher than the concentration of any of the reaction products, and thus the product competition for the reactive sites can be neglected. Attempts to measure the amount of CT adsorbed in the suspensions containingonly the supported catalyst without the reductant were unsuccessful because the amount adsorbed was too small. Apparently the decrease in CT concentration in the heterogeneous systems can be attributed solely to its chemical degradation. The reductant is then necessary in order to generate catalytically active Co species. Spectroscopicstudies have shown that Co is present as Co(I1) in the intercalated macrocycles under both 0 2 and NZatmosphere (35). Since the supported catalyst exhibits
61
I
I
I
1
I
1
I
coTMP i COPcTs-LDH
R (mMh')
0
0.5
1
1.5
2
2.5
3
[CCI~IO (mM) FIGURE 4. Plot of the initial rate of degradation of CT in heterogeneous systems as a function of the initial mM catalyst, 0.1 M DTr). Curves represent the Michaelis-Menten model (eq 4). TABLE 2
Michaelis-Menten Parameters for CT Degradation by Heterogeneous Catalysts catalyst
rU0 (mMl
CoTMPyP-FH CoTMPyP-SG CoTMPyP-H COPCTS-LDH
4.48 5.87 2.34 5.41
v,
(mM h-? 6.60 17.28 1.OB 10.41
no activity in the absence of the reductant, the reduction of Co(I1) to Co(1) by DTT may have to take place in order to produce a catalytically active Co species. Adsorption of CT onto the catalyst and the formation of the Co-CC13 intermediate in the heterogeneous systems depends on the number of reduced Co sites and their accessibility to the reactants. The decrease in k~ with increased CT concentration (Figure 4) indicates that there is a limited number of easily accessible active sites. The concentration of the intermediate complex at the initial stages of the reaction is small compared to the concentrations of Co macrocycle and CT. Thus, the dechlorination reaction in heterogeneous systems is very similar to enzyme-catalyzed reactions, and the initial rate of CT degradation (Figure 4) can be fitted by the Michaelis-Menten kinetic model: (4)
where ,v is the maximum reaction rate for a specified initial Co macrocycle (enzyme) concentration and Km is Michaelis constant. The value of 'K (Table 2) greater than 1implies that the rate of formation of the intermediate complex is slower than the rate of desorption of the products in all the supported systems (49, 50). At longer reaction times, CT concentration decreases, while CF and other reaction products compete for the active surface sites, which increases K,. When K, >->[CCL],eq 4 reduces to the first-
CT concentration (pH 7.5,O.l
order rate equation. The value of ,,v (Table 2) is proportional to the surface density of catalytically active sites, following the order CoTMPyP-SG > CoPcTs-LDH > CoTMPyP-FH > CoTMPyP-H. For layered supports, this order is in good agreementwith the results from the previous spectroscopic studies on the orientation and hydration of the macrocycles in the interlayers (Figure 2; 35). The lowest reaction rates were observed for the CoTMPyP supported on the low charge density clay hectorite, where the macrocycle is lying parallel to the clay layers. The ESR spectra of Co(I1)TMPyP intercalated in hectorite clearly show that Co has no water in the axial coordination sites and its d orbitals, interactingwith electron-deficient siloxane oxygens, are contracted (33, indicating that it is likely to be kinetically inert in electron-transferreactions (51). Water does not diffuse into the CoTMPyP-H interlayers when the clay is fullywetted, and apparently reactants cannot access the interlayers either. Only a small portion of the Co(I1)TMPyP, adsorbed on the clay edges and outer surfaces, has water in axial coordination sites, and those are most likely the only catalytically active sites in CT dechlorination. Intercalated CoTMPyP in a high-charge density clay fluorohectorite is lying at a 27" angle to the clay layers with water in the axial coordination sites. The electronic configuration of aqueous CoTMPyP and CoTMPyP-FH are very similar ( 3 3 ,and the ESR study shows that water diffuses easily in and out of the clay interlayers. Therefore, the decrease in activity for CoTMPyP-FH compared to the aqueous CoTMPyP can be attributed to the slower rates of adsorption of CT on active metal centers in the interlayers. This may be due to the restricted mobility of reactants in the clay interlayers as well as to the smaller number of reduced metal centers because of the restricted access of DTT to the macrocycles in the interlayers. Silica-supported CoTMPyP exhibits the highest activity among supported macrocycles. Metal centers of the macrocycles supported on the spherical silica particles may be more active than on the other supports because they are more accessible to the reductant and to the reactants and possibly because VOL. 29, NO. 2, 1995 I ENVIRONMENTAL SCIENCE & TECHNOLOGY
448
TABLE 3
::b j 0.8
0.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0
-e
8
0.0 0.5 1.0 1.5 2.0 2.5 3.0
16
0.8
lLE!%zd
0.4 0.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0
1.6 12
0.0 0.4
catalyst
ICCl410 (mM)
CoTMPyP CoTMPyP-FH COTMPyP-SG CoTMPyP-H CoPcTs COPCTS-LDH CoTMPyP CoTMPyP-SG
2.3 2.3 2.3 2.3 2.3 2.3 4.6 4.6
(day-')*
SEb
turnover frequency (day-')c
0.49 0.47
0.06
570
0.08
0.57
0.07 0.01 0.09
540 655 105 860 335 640
k
0.09 0.75 0.29 0.28 0.39
0.67 0.06 0.04
895
*
*Calculated forthefirst24 h ofexperioments. Standarderror. 0 Mol of CCl,mol of catalyst-' day-'calculated for the first 24 h of experiment.
0.0 0.5 1.0 1.5 2.0 2.5 3.0
!im 12
0.8
0.4 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0
Pserdo=First=OrderRate Constants for CT Degradation and Turnover Frequencies in Long=Term Dechlorination Experiments
i_,vr""i
0.0 0.5 .1.0 1.5 2.02.5 3.0
Time (days)
FIGURE 5. Degradation of 23 mM CT (0)and production of CF (0) at pH 7.5 catalyzed by 0.002 mM 60 macrocycle: (a) homogeneous CoTMPyP, (b) CoTMPyP-FH, (c) CoTMPyP-H, (d) CoTMPyP-SG, (e) homogeneous CoPcTs, and (f) CoPcTs-LDH.
the reduction potential of Co is less altered on this surface as compared to highly charged layered mineral surfaces. Long-Term Experiments. The effect of immobilization of the macrocycles on the solid support on the loss of their catalytic activity with time was investigated in long-term experiments (Figure5). The experiments were performed with low catalyst amounts (0.002 mM) and high CT concentrations (2.3 and 4.6 mM). The homogeneous CoPcTs catalystwas not deactivatedunder those conditions, because the aggregation of CoPcTs does not occur in very dilute solutions. After 1 day, the reaction did not follow the first-order kinetics because the catalysts had lost some of their activity. Thus, the first-order rate constants and turnover frequencies (mole of CT degraded per mole of macrocycle per day) were calculated for the reaction time of 1 day (Table 3). Among the supported catalysts, CoTMPyP-SG exhibited the highest activity, and CoTMPyP-H exhibited the lowest activity. The frequency turnover was greater for CoTMPyPSG than for the homogeneous CoTMPyP (Table 31, indicating that adsorption on silica surface has significantly increased catalyst longevity. For other supports (H, FH, and LDH), turnover frequencies were less than those of the corresponding homogeneous catalyst. With the exception of hectorite, the loss of catalytic activity of the Co macrocycles does not seem to be promoted by adsorption on the solid. In fact, the homogenous CoTMPyP has lost its activity after 2.5 days while the CoTMPyP-FH and CoTMFyP-SG systems were still active. The smaller turnover frequency of CoTMPyP-FH than that of homogeneous catalyst over the first 24 h of experiment is consistent with the decreased rates observed in short-term experiments for supported macrocycle. Thus, 444 rn ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 2, 1995
the slower rates can be attributed to the slow rates of adsorption of CT on reactive porphyrin metal centers in the interlayers rather than to the loss of catalyst longevity of the macrocycle upon intercalation. Low turnover frequency exhibited by hectorite-supported CoTMPyP may be attributed in the inaccessibility of the intercalated porphyrin and to the alteration of the electronic state of the metal center (35). The hydration and electronic state of Co(1I)PcTs intercalated in LDH are similar to that of CoTMPyP-FH. This macrocycle is perpendicular to the layers of LDH, providing a more open structure and thus more easily accessible metal center as compared to CoTMPyP-FH, which may partly explain the higher dechlorination rates. In addition, the difference in dechlorination rates between CoTMPyP-FH and CoPcTs-LDH may partly arise from the difference in the catalytic activity of two macrocycles in CT dechlorination exhibited in the homogeneous systems (Figure 5a and d; Table 3).
Corclusions The study demonstrates that the supported macrocycles are catalytically active under ambient conditions. The heterogeneous dechlorination reactions can be described by the first-order kinetics with the pseudo-first-order rate constants being correlated to the accessibility and the hydration of the cobalt in supported macrocycles, following the order: CoTMPyP-SG > CoPcTs-LDH > CoTMPyP-FH > CoTMPyP-H. The reaction mechanism may be similar to enzyme-catalyzedreactions where the rate-determining step is adsorption of CT on the active center with the formation of the intermediate complex. Although initial heterogeneous dechlorination rates were less than the corresponding homogeneous rates, increased longevity of supported catalysts led to greater dechlorination in reactions catalyzed by CoTMPyP-SG and CoTMPyP-FH than by homogeneous CoTMPyP at longer reaction times. The charged, mineral-supported macrocycles reductively dechlorinate carbon tetrachloride in water, even at high concentrations that would inhibit microbial activity. Since the reduction potential of the macrocycles can be altered by choosing the appropriate substituents on the periphery of the porphyrin or phthalocyanine ligand, efficient supported catalysts could be designed for reductive dehalogenation by synthesizing macrocycles with tailored redox properties.
Acknowledgments This research was supported by the National Institute for EnvironmentalHealth Sciences Grant P42EF0491, Michigan State University Institute for Environmental Toxicology, and Michigan Agricultural Experiment Station. We thank Dr. Max M. Mortland for helpful discussions.
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Received for review May 10, 1994. Revised manuscript received November 3, 1994. Accepted November 9, 1994.@
ES9402821 @
Abstract publishedin AdvanceACSAbstructs, December 15,1994.
VOL. 29, NO. 2, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 1445
