Environ. Sci. Technol. 1993, 27,2104-21 11
Photoreductive Dehalogenation of Bromoform with Titanium Dioxide-Cobalt Macrocycle Hybrid Catalysts Ronald J. Kuhler,t Gregory A. Santo,? Thomas R. Caudill,* Erlc A. Betterton,’#*and Robert G. Arnold?
Department of Civil Engineering and Engineering Mechanics, and Department of Atmospheric Sciences, University of Arizona, Tucson, Arizona 85721 The hybrid semiconductor-macrocycle catalyst, Ti02-Cotetrasulfophthalocyanine, (Ti02-CoTSP) effectively enhances the solar-promoted reductive dehalogenation of bromoform (CHBr3)under anaerobic conditions. Reaction rates are 4-10 times faster than those obtained using silanized TiO2, unmodified TiO2, or direct (uncatalyzed) solar photolysis under the same conditions. CHBr3 is reduced to its lower homologues, dibromomethane (CH2Brz), and bromomethane (CH3Br). HBr is also produced. No other major dehalogenation products are observed, although methane is found in trace amounts after prolonged photolysis. 2-Propanol, the sacrificial electron donor, is oxidized stoichiometrically to acetone. The catalyst is stable in sunlight for at least 30 h without loss of activity. The reaction mechanism is postulated to involve nucleophilic attack of Co(I)TSP,generated by the semiconductor, on CHBr3. The organometallic complex that is formed, TiO2-CoTSP-CHBr2, is postulated to subsequently undergo photolysis via homolytic Co-C bond cleavage to regenerate the catalyst. 2-Propanol is oxidized by valence band holes.
Introduction The widespread industrial and agricultural use of halogenated aliphatic compounds as solvents, degreasing agents, pesticides, and fumigants has led to the contamination of many ground water systems. Strong incentives exist for improving methods of remediation and developing new treatment technologies. One approach is to dehalogenate the organic halides to yield less harmful products. It has been observed (1) that reduction is effective in (partially) dehalogenating the more heavily substituted homologues, implying that a sequential treatment scheme in which the target compound is first partially dehalogenated under reducing conditions may promote subsequent oxidative transformations. Although much effort has focused on abiotic oxidation of organic halides (H202,03,TiOz/O2,hv/02,e.g., refs 1-81, comparatively little work has been directed toward abiotic reductive dehalogenation. Here, we describe the use of a hybrid semiconductor-cobalt macrocycle catalyst for the reductive dehalogenation of bromoform in aqueous %propanol. The use of macrocycles for catalysis of reductive dehalogenation bridges the gap between abiotic and biotic systems. Bacterial transition-metal coenzymes,including corrinoids, coenzyme F.430, hematin, and cytochrome P450 have been implicated in the biologically mediated reductive dehalogenation of halogenated organics although the mechanisms by which these macrocycles operate remain t Department of 1 Department
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Civil Engineering and Engineering Mechanics.
of Atmospheric Sciences.
Envlron. Sci. Technol., Vol. 27, No. 10, 1993
unknown (9-15). For example, bromoform is reduced to carbon monoxide in rat liver microsomes under anoxic conditions in the presence of cytochrome P450 (16). Partly because of the complexity of biological systems, attention has been given to detailed studies of abiotic model systems. Gantzer and Wackett (12)recently reported that vitamin B12, coenzyme F430, and hematin catalyze the reductive dechlorination of polychlorinated ethylenes and benzenes by strong reductants such as Ti(II1) citrate or dithiothreitol. Krone et al. (14)have shown that, in the dark, corrinoids catalyze the anaerobic reductive dehalogenation of C C 4 and its lower homologues, CHC13, CHzCl2, and CH3C1, by Ti(II1) citrate. Methane production was observed in each case. The reaction mechanism is not well understood, but the authors suggest that it could involve the further reduction of Co(III)B12-R to yield an unstable radical anion intermediate, Co(III)B12-R-, that subsequently decomposes via cobalt-carbon bond cleavage. However, it should be noted that the one-electron reduction potential of Co(III)B12-CH3 is very negative (approximately -1.3 V in dimethylformamide/l-propanol(17)). Co(1) macrocycles, including phthalocyanines, corrinoids, and cobaloximes, are known to be exceptionally powerful nucleophiles that undergo rapid S Nreactions ~ with many halogenated hydrocarbons yielding organometallic complexes containing stable Co-carbon u-bonds. Co(I)B,,
+ RX -.+ Co(III)B12-R + X-
(1)
Cyanocobalamin (vitamin B12) and cobalt 4,4’,4”””’tetrasulfophthalocyanine display many structural similarities. The octahedral cobalt ion is coordinated at the equatorial plane for four nitrogen atoms of the macrocycle, and the two remaining axial coordination sites are occupied by a variety of inorganic or organic ligands (or remain vacant). Here we abbreviate cobalt corrinoid as CoB12 while cobalt tetrasulfophthalocyanine is abbreviated as CoTSP. The oxidation state of the metal and the nature of the axial ligands are specified when necessary. Pratt (18)has summarized much of the earlier work for B12. The organometallic chemistry of CoTSP is not so well developed, but Bl2 may be relied upon for insight into its behavior. In addition to the simple alkyl halides (RX), olefins (e.g.,BrCH=CH2), acetylenes (e.g., BrCECH), and cyclic compounds (e.g., CsHllI and C,jH&H&l) are also subject tonucleophilic attack by Co(I)B12. Co(1)corrinoids also react with nonhalogenated electrophiles such as acetylene and ethylene oxide, and Co(II1) corrinoids are known to react with acetylene, malononitrile, CH2(CN)2, and nitromethane, CH3NO2, to give stable organometallic complexes (19). Alkyl fluorides and halogenated aromatics are unreactive. 0013-936X/93/0927-2104$04.00/0
0 1993 American Chemical Soclety
I
-3 I
I
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u
=5.7i0.3
t
v
C0lll)loTSP
HCOO/HCOOH
I
ClCHiliCOHllCHillCHOH
3 - hYB Ti02
c1
c2
Ca
L I
Cobalt
20
15
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5 0 -5 ChargeQ [105rno1 g I ]
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Flgure 2. Determinationof pHz%of P25 Ti02 in 50 % (v/v) 2-propanol
(24).
Here we report on the use of a semiconductor-macrocycle hybrid catalyst (Ti02-CoTSP) to catalyze the solarpromoted dehalogenation of bromoform under anaerobic conditions. Experimental Procedures
Materials. Unless otherwise stated, all chemicals were reagent grade and used without further purification. Water, with a resistivity of 118 Ma cm, was obtained from a Millipore Milli-Q system, and nitrogen gas was >99.995 % . Monosodium 4-sulphophthalate was prepared by titration of the free acid with 15 M sodium hydroxide and recrystallized from absolute ethanol. The Ti02 (Degussa P25 anatase) has an average primary particle size of 30 nm and a BET surface area of 50 f 15 m2 g1 (23). The pHzpc of P25 Ti02 was determined by titration and found to be 5.7 f 0.3 in 50% 2-propanol (see Figure 2) (24). This is close to the pHzpc reported by others for Ti02 in water (20,25). Preparation of Ti02-CoTSP. CoTSP was synthesized by the method of Day et al. (26),which is based on that of Weber and Busch (27). The CoTSP hybrid catalyst, Ti02-CoTSP, was synthesized according to the method of Hong et al. (20) using 6 g of Ti02 with 5 mL of 3-(aminopropyl)triethoxysilane (Aldrich, 98 % ) to modify the semiconductor surface. CoTSP (0.023 g) was then added to the modified semiconductor (3 g) in pyridine. Atomic absorption analysis showed that Ti02-CoTSP contained approximately 335 ppm Co by weight. This is equivalent to 5.7 X lo4 M CoTSP for the standard hybrid catalyst concentration of 1 mg/mL used here. The calculated surface coverage is 27 % , assuming an area of -400 A2 for each CoTSP molecule and a surface area of 50 m2g-l for the TiOz. This is close to the 30% calculated by Hong et al. (20). FTIR analysis of hybrid Ti02-CoTSP powder confirmed the presence of sulfonyl and silylgroups. In order to effectively retard recombination of the ecB- hfB pair produced by irradiation it is necessary for the electron- and hole-sinks to be physically adsorbed or chemically attached to the surface of the semiconductor. The strength of adsorption can play a major role in determining the overall rate of photocatalysis (25, 28). Alcohols are fairly strongly bound to the surface of Ti02 (291,and since 2-propanol is also a good radical scavenger, it was selected as the sacrificial donor for the h b in this work. Anaerobic aqueous Ti02-CoTSP suspensions were bleached when exposed to sunlight for several hours but Envlron. Scl. Technol., Vol. 27, No.
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were stable in the presence of added 2-propanol, which presumably prevented radical attack on the chromophore. The CoTSP macrocycle remained bound to the surface of the Ti02 after treatment with dilute acids or bases. Co(1I)TSP and Co(1)TSP remained attached to the Ti02 for more than 2 months when stored as aqueous suspensions in sealed glass flasks on the laboratory bench. Exposure of the stored Co(1) hybrid catalyst to air immediately returned the blue color of Co(I1). Co(1I)TSP can be rapidly reduced to Co(1)TSP in solution by a variety of reducing agents including NaBH4, NaZS, N2H4, and HOC2H4SH (18, 26, 30, 31). The associated color changes are distinctive and can be easily monitored by UV-visible spectrophotometry since the extinction coefficients are very high (-lo4 M-l cm-l).
-
N
-0.05 V
Co(1I)TSP Co(1)TSP (3) blue "E grey/yellow The attachment of the macrocycle to the surface of a semiconductor appears to have a relatively small effect on the redox properties of the cobalt ion (see later). Flameless atomic absorption analysis was performed with a Perkin-Elmer 360 equipped with a Perkin-Elmer HGA-400 programmer and graphite furnace attachment. A mixed element lamp (PE no. 303-6103) was used with the wavelength set at 246.7 nm. Atomization temperature was 2500 "C and sample volume was 20 pL. All samples and standards were diluted with a matrix modifier (1% v/v "03 and 1%NHdN03). Standards were prepared from CoC1~6H20(Malinckrodt). Solar Photolysis Experiments. A major goal of this work was to investigate the feasibility of using solar energy to facilitate the degradation of bromoform. Experiments were conducted over a perod of several months and so, due to seasonal and daily variability in the solar flux, it is difficult to compare absolute rates of experiments performed at different times. For Tucson (latitude 32" 13'), the calculated solar fluxes over the 300-500 nm region at solar noon are 6.9 X 10l6and 5.2 X 10l6photons cm-2 s-1 at summer and winter solstice, respectively. All experiments were conducted on clear days between 1O:OO h and 14:OO h. The solar flux at these limits is within 9 % of the noon maximum. Actinometric measurements (32) with 5 mL of ferrioxalate solutions, which absorb from
