J. CHEM. SOC. FARADAY TRANS., 1992, 88(2O), 3079-3086 3079

Photochemical Reactions of Biphenyl and its Chlorinated Derivatives on the Surfaces of y-Alumina and Silica-Alumina Yun Mao and John Kerry Thomas* Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556, USA

Photochemical reactions of biphenyl and its chlorine-substituted derivatives on the surfaces of y-alumina and silica-alumina have been investigated. Diffuse reflectance spectra show that biphenyl molecules adsorbed on activated surfaces (T, > 350 "C) form chemical products and charge-transfer complexes. The existence of the latter is also confirmed via observation of an EPR signal on photoirradiation into the CT band. Photoirradiation of the adsorbed organic molecules leads to a redox process on the surface, radical cations are formed, and dechlorination takes place. The coadsorption of persulfate and dinitrobenzene significantly influences the electron-transfer processes. The photochemical products on both activated and non-activated surfaces were separated by HPLC, and the collected fractions were studied spectroscopically. The important reaction of radical cations is hydrolysis resulting in hydroxybiphenyl, the radical BP(0H)' is suggested as a key intermediate in hydrolysis.

There is a growing interest to find eficient and economic methods for treating man-made pollutants in the environ- ment. An important concept is to initiate reactions that lead to transformation or decomposition of the organic contami- nants into less dangerous One class of contami- nants is polychlorinated biphenyls (PCBS) .~ .~ PCBs are insoluble in water, but they are lipophilic and tend to accu- mulate in fats and oils. Consequently, PCBs can be concen- trated from low background concentrations through foodwebs to reach concentrations that are orders of magni- tude higher in living systems. PCBs are relatively stable to UV irradiation at an energy lower than the ionization thresh- old. Hetero-photochemical catalytic reactions on the surfaces of solids, particularly TiO, , ' y 2 v 5 have received considerable attention; these studies include the formation of organic radical cations on insulator Little attention has been paid to the photochemical reactions on the surfaces of insulator oxides, such as silica and alumina. However, hetero- photocatalysis on insulators could be a potential method for waste treatment, because it takes advantage of the interaction between the excited state of PCBs and polar surfaces, leading to chemical reaction.

Reactions in diluted aqueous solution of PCBs on y- irradiation have been reported." PCBs adsorbed on silica gel or montmorillonite are degraded mainly to less chlorinated biphenyl^.^^ Studies on reactions of the upper triplet states of biphenyl, chlorobiphenyl and bromobiphenyl in aqueous micellar system have also been reported." In this report, biphenyl (BP), mono- and di-chlorobiphenyl (CBP and DCBP) are selected as model compounds for PCBs, and their photophysical and photochemical reactions on y-alumina and silica-alumina surfaces are studied. The photochemistry of the organic compounds adsorbed on surfaces was moni- tored by diffuse reflectance, fluorescence and EPR spectros- copy. Separation, collection and assignment of the chemical products was conducted by HPLC and spectral methods after dissolving the sample in aqueous methanol solution. From the results obtained, one can infer the chemical reac- tion mechanism of the photogenerated surface species involves among others, the formation of BP(0H)' radical as a key intermediate. The reaction mechanism of BP, CBP and DCBP on insulator surfaces offers a key to the understanding of degradation processes in the environment.

Experimental Chemicals Biphenyl (BP) from Aldrich (99%) was purified by chromato- graphic separation (silica gel as the stationary phase and cyclohexane as the mobile phase), and recrystallised from cyclohexane. 4-Chlorobiphenyl and 4,4'-dichlorobiphenyl (CBP and DCBP) (Alfa) were used as received. 2,2'-Biphenol and 4,4'-biphenol (Chem Service) were used as HPLC stan- dards as received without further treatment. y-Alumina and silica-alumina were received from LaRoche Chemical Inc. and Aldrich Chemical, respectively. The average surface areas of y-alumina and silica-alumina supplied by the producer, are 200 and 475 m2 g-', respectively. The solid powder was activated by heating before use.

Instrumentation

Because of the opaque nature of the solid samples of y- alumina and silica-alumina, diffuse reflectance spectrometry was used to measure absorption spectra." A UV-VIS spec- trophotometer (Perkin-Elmer 552) equipped with a specular reflectance accessory was used. Steady-state emission was measured on a spectrofluorometer (SLM Aminco, SPF 5OOC) equipped with a 250 W Xe lamp, a Hamamatsu photomulti- plier (R928P) detector and a computer unit.

EPR spectra were recorded on an electron paramagnetic resonance spectrometer (Varian Associate, E-lines centery series) equipped with an X-band klystron and a rectangular cavity. The g value was determined by comparison with DPPH. The samples are excited by irradiation through a slit in the cavity. Samples for EPR measurements were contained in 5 mm 0.d. quartz tubes, connected to wider glass tubing that was attached directly to a vacuum line.

Separation and quantitative identification of non-ionic photochemical products were performed by a high- performance liquid chromatograph (Waters) which was equipped with a NOVA PAK Cls column and UV- absorption detector. The eluent consisted of an aqueous acetonitrile solution. Identification of chloride ion was accomplished with a Waters Ion Chromatograph, equipped with an IC-anion column and a conductivity detector

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3080 J. CHEM. SOC. FARADAY TRANS., 1992, VOL. 88

(Waters 430). The eluent was borate-gluconate solution at pH 8.5.

Photoirradiation was carried out, using an Xe lamp (150 W) equipped with a 20 cm water filter to remove IR radi- ation, or a chemical photoreactor equipped with mercury lamps 254 or RPR 300 nm lamps (Rayonet, Southern New England Company, USA). The photoflux was determined with the chemical actinometer K,Fe(C204).2

Sample Preparation

Solid samples for diffuse reflectance and luminescence spectra were thermally activated as follows: 1.0 g y-alumina (or silica-alumina) powder was heated for 20 h under air at tem- peratures ranging from 160 to 750°C. After thermal activa- tion, the powder sample was cooled to room temperature in a desiccator. Quartz crucibles were used to minimize contami- nation. Immediately after cooling, the powder was mixed with an aliquot of the solution of the desired compound in cyclohexane. The solid sample was dried in air at ca. 30 "C.

After photoirradiation, the samples were extracted with aqueous methanol solutions under a nitrogen or air atmo- sphere. Samples for HPLC and ion chromatography were first centrifuged to remove most of the powder and then fil- tered through a millipore frit (0.45 pm).

Results Absorption and Emission of Biphenyl and Chlorinated Biphenyls Adsorbed on Surfaces

The measured diffuse reflectance and fluorescence spectra of BP, CBP and DCBP adsorbed on y-alumina and silica- alumina surfaces are summarized in Table 1. The spectra on y-alumina are similar to those on silica-alumina. As an example, the characteristics of BP will be described subse- quently in detail. Fig. l(a) shows the diffuse reflectance spectra of BP on the surface of y-alumina. On y-alumina acti- vated either with high temperature (T, = 750°C) or with low temperature (T, = 130°C), absorption bands centred at 250 nm are observed with a bandwidth of ca. 50 nm. This band is attributed to the BP molecules which are physically adsorbed and weakly interact with the surface sites. The shape of the BP spectrum on y-alumina (T , = 130°C) is identical to that observed in while the absorption spectrum of BP in solution is diffuse, resulting from at least two transitions.13 It is difficult to assign the 0-0 band and estimate the red shift accurately. Stokes shifts were estimated according to Berl- mans definition (see Table l).', Chlorine substitution of BP Table 1 Experimental data of the diffuse reflectance and fluores- cence spectra of BP, CBP and DCBP adsorbed on the surfaces of y-alumina and silica-alumina [loading concentration: (1-3) x mole g-'

~ ~~

compound BP CBP DCBP ~~

diffuse reflectance

peak position/nm peak widt hlnm" structure

fluorescence spectrum peak positionlnm peak bandwidthlnm" structure concentration sensitivity mirror similarity Stokes shift/cm-'

spectrum 250

broad/50 no

3 14 broad145

no no no

3600

253 broad/57

no

317 broad/52

nob no no

3550

258 broad155

no

326 broad154

nob no no

3570

Peak widths are determined as widths at half height of peaks. There were unresolved shoulders. The measurement conditions are

the same as in Fig. 1.

0.45

n \

\ v > 0.30 0, -

0.15

0.0' ' ' ' ' ' ' 250 290 330 370 410 450

Ilnm

0.00 200 300 400 500

I / n m 290 330 370

n/n rn

Fig. 1 (a) Diffuse reflectance spectra of BP adsorbed on y-alumina activated at two different temperatures, 750°C (-) and 130°C (. . a) ; loading concentration; 2 x mol g-'. (b) Diffuse reflec- tance spectra of BP adsorbed on non-activated y-alumina after pho- toirradiation at 254 nm ; steady-state irradiation, during irradiation samples were kept at room temperature and without shaking; irra- diation time, from bottom to top: 0, 2.5, 5, 10 and 15 min; loading concentration: 2 x mol g-'. (c) Fluorescence spectrum of BP on y-alumina surface (T, = 750 "C) (-), BP loading concentration: 2 x low6 mol g-'; and fluorescence spectrum of BP in isopentane solution (. . .), BP concentration 5 x mol dm-3. Excitation at 250 f 5 nm at room temperature

induces a bathochromic shift of the absorption band com- pared to non-substituted BP. The adsorbed state of the chlorine-substituted BPs does not exhibit a significant shift compared with that in solution.

The absorption spectrum of BP on y-alumina (T , = 750 "C) shows additional absorption bands, namely a shoulder at 300 nm on the tail of the 250 nm band and a broad band in the range of 310-450 nm. The shoulder at 300 nm may be tenta- tively assigned to chemical products (vide infra). The broad band at 310-450 nm may be attributed to a charge-transfer complex (CT complex), where BP acts as an electron donor (D), and the surface sites with 0, as electron acceptor (A). This assignment will be discussed later. Compared with pyrene,14 the diffuse reflectance spectra of BP adsorbed on y-alumina or silica-alumina show relative weak CT bands. However, the formation of the photoinduced BP'+ shows the existence of CT complexes of BP (vide infra). The weak CT absorption may be due to a smaller yield of the CT complex with BP. Compared to pyrene, also a steric effect could operate in the case of BP which can take up a non-planar configuration.

Photoirradiation of adsorbed BP gives an additional broad absorption band near the red edge of the main BP absorp- tion [Fig. l(b)]. This is reminiscent of CT complexes which usually exhibit such characteristics, i.e. an additional absorp- tion band near the red edge of parent spectrum. The broad band appears only in the presence of oxygen and increases

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with irradiation time. This band may be ascribed to a mixture of CT complexes and products. Later, it will be shown that after extraction of the samples with aqueous methanol solution these species are converted into chemical products.

The fluorescence spectrum of BP adsorbed on y-alumina shows an unstructured broad band with a peak at 314 nm [Fig. l(c)]. In a solution (isopentane) two adsorption peaks at 304 and 314 nm are observed [see dotted line in Fig. l(c)]. In the adsorbed state (BP/silica-alumina) the band at 304 nm disappears. The fluorescence intensity increases with increased loading of BP. Self-quenching and excimer emission are not observed, even at CQ. mol g-'.

Observation of BP, CBP and DCBP Radical Cations on Surfaces

Following activation at 750 "C, the y-alumina or silica- alumina powder was cooled and mixed with a BP solution in cyclohexane. The aerated reactant suspension exhibits an EPR signal due to the thermal formation of BP" [Fig. 2(a)]. In the absence of 0, no noticeable EPR signal is observed. The EPR spectra of aerated BP/silica-alumina samples have stable hyperfine structures that exist for several days, while the EPR spectra on y-alumina exhibit only indistinct struc- ture.

The EPR spectrum of BP on silica-alumina shows asym- metrical hyperfine structure with a g value of 2.0036 and seven lines with a splitting of 0.64 G. The hyperfine structure is usually initiated by an interaction of unpaired electrons with protons. The radical and cation centre delocalizes over both rings of the structure.

CBP and DCBP adsorbed on y-alumina and silica- alumina (T , = 750 "C) all exhibit EPR signals, but show only minimal hyperfine structure. Fig. 2(b) and 2(c) present the EPR spectra of CBP and DCBP adsorbed on silica-alumina (T , = 750 "C), respectively. These spectra show that chlorine

t 5G 3380G

t v 1 2 y G 3380 G

U t 12.5 G 3380 G

Fig. 2 First-derivative EPR spectra on the silica-alumina (T , = 750 "C); sample preparation see text. EPR spectrum recording condi- tions: microwave power 10 mW, microwave frequency 9.5 GHz, modulation frequency 100 kHz, modulation amplitude 1 G ; receiver gain factor 5 x lo3. (a) BP, (b) CBP, (c) DCBP

30 1

4-

.- .! ,,I O L

0 5 10 15 20 25 irradiation time/min

Fig. 3 Relative intensities of the EPR absorption increase with cavity photoirradiation. Photon energy was adjusted by sharp cut-off filter, Kopp 0-52,A > 350 nm. Photon source: Xe lamp, 150 W

30 I I

0 10 20 30 40 50 60 1/1 0-4 einstein min-'

Fig. 4 Dependence of the EPR absorption intensities on the photo- flux intensities. EPR spectra recording conditions : 77 K, microwave power 10 mW, microwave frequency 9.5 GHz, modulation frequency 100 kHz, modulation amplitude 10 G, gain factor: 5 x lo3. Photo- irradiation was performed in photoreactor with RPR 3000 8, lamps

substitution of BP blurs the hyperfine structures of the EPR signal.

Alternatively, BP'+ can be generated by photoirradiation of the BP/solid samples. For silica-alumina (T , = 750 "C) samples, the photoirradiation produces additional radical cations. The photoionization of BP/silica-alumina (T, = 75OOC) is irreversible, and the EPR signal remains after irra- diation stopped. It is noteworthy that photoirradiation with an energy lower than that of the BP adsorption band also generates radical cations. Fig. 3 shows the increase in the relative intensities of the EPR signal with irradiation time.

For unactivated silica-alumina, photoirradiation at room temperature with UV light did not produce any observable radical cations. However, at 77 K UV irradiation produces a significant cation radical signal. If the sample temperature is increased to 120 K, the EPR signal rapidly disappears. Fig. 4 shows the influence of the irradiation intensity on the cation radical yield at 77 K, where a linear relationship between the irradiation intensity and the cation radical yield is observed. This is consistent with a monophotonic process.

Coadsorption of Electrophiles

Persulfate ion (S,Og-), a strong electron acceptor,' does not directly oxidize aromatic hydrocarbons at room tem- perature. Aliquots of an ammonium persulfate saturated cyclohexane solution (ca. lo-' mol dm-3) were mixed with BP solutions and then loaded on to activated silica-alumina. Fig. 5 shows that the thermal chemical yield of BP'+ on silica-alumina increases with increasing coadsorption of

The enhancement of reactivity with S,Oi- may be under- stood on the base of formation of SO;- radicals which are very strong oxidants (1.96 V us. SHE) S,Og- may undergo an electron-transfer process on the surfaces, resulting in the

(NH,),S,O*.

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J. CHEM. SOC. FARADAY TRANS., 1992, VOL. 88

A A - A

A A

' " " ' ' ' ' -

60 I I

coadsorption of persulfate/l O - ' O mol g-'

Fig. 5 Chemical yields of BP radical cations on the silica-alumina surface increase with the coadsorption concentration of ammonium persulfate. BP loading concentration: 3 x mol g-'. EPR spec- trum recording conditions were same as in Fig. 2

SO;- radical. The latter can further react with BP molecules uia direct electron transfer.17

(1)

(2) The coadsorption of persulfate has a twofold effect, i.e.

elimination of recombination between electrons and radical cations and enhancement of the chemical yield uia reaction (2). Accordingly, the enhanced reactivity with coadsorption of persulfate is indicative of the electron-transfer process on the surface.

1,4-Dinitrobenzene (DNB) was studied as another electro- philic coadsorbate. Fig. 6 shows a first-derivative EPR spec- trum obtained from a sample with DNB/BP coadsorbed on silica-alumina (T, = 750 "C). The loading concentrations of BP and DNB are and mol g-', respectively. The hyperfine structure of BP cation radical is not well developed. On coadsorption of DNB on the sample the hyperfine struc- ture of BP" is obscured due to the superposition of bands from BP" and DNB'-. DNB alone on the surface exhibits an unstructured EPR absorption peak (see dotted line in Fig. 5), which has been ascribed to the DNB anion and overlaps with the peak from BP'. The adsorption of DNB on the silica-alumina surface does not block the adsorption of BP molecules, despite the higher loading of DNB (ca. 2 x mol g-'). This indicates that DNB mol- ecules are absorbed on the reduction sites, while BP mol- ecules are absorbed on the oxidation sites on the surfaces.

S,Og- + e- -+ SO:- + SO;-

SO;- + BP +SO:- + BP'+

A

- t v 10 G 3380G

Fig. 6 First-derivative EPR spectra on the silica-alumina surfaces (T, = 750°C); spectrum of coadsorption of BP and DNB: (-); spectrum of BP: (---); spectrum of DNB: (. . .). EPR-spectrum recording conditions : room temperature, microwave power 10 mW, microwave frequency 9.5 GHz, modulation frequency 100 kHz, gain factor 5 x lo3, modulation amplitude 10 G

0.75

h

% 0.50 % Y

0, - 0.25

0.00 ' ' ' I 200 300 400 50 0

A/nm

Fig. 7 Diffuse reflectance spectra of BP/DNB on silica-alumina after photoirradiation (A > 280 nm), irradiation time, from bottom to top: 0,10,20 and 30 min

From Fig. 6 it can be shown that coadsorption of DNB does not significantly enhance the thermal redox process. However, the coadsorption of DNB induces significant changes during photoirradiation. Fig. 7 presents the absorp- tion spectra of BP/DNB sample after photoirradiation (2 > 280 nm). A new absorption band appears a t 440 nm and increases with irradiation time, while the photochemical experiment with BP and DNB is deaerated cyclohexane solu- tion gives a quite different absorption band.Ig

Separation of Chemical Products of BP

Fig. 8 shows HPLC spectra of chemical products after extrac- tion of photoirradiated samples with aqueous methanol solu- tion (10 vol.% water). A group of peaks (Fl , F2 and F3) which is eluted before the BP peak is ascribed to products. The yield of F2 on unactivated surfaces is markedly higher than that of F1 and F3, while on activated surfaces (T, = 75OoC), the yields of F1 and F3 are significantly increased. Photoirradiated samples were dissolved in aqueous methanol containing Fe(CN);- (5 x mol dm-3) results in higher product yields. This indicates Fe(CN):- takes part in product formation and helps to formulate a mechanism (vide infra).

5.0 1 4.0 -

3.0 -

2.0 -

1.0 -

0 2 4 6 7 8110 t/min

Fig. 8 Chromatographic spectra of HPLC analysis for the photoin- duced chemical products. BP was adsorbed on silica-alumina acti- vated at 20°C (-) and 750°C (. . a) from cyclohexane solutions. The reactant suspensions were irradiated in the photoreactor with 254 nm, then the reactant mixture was added by aqueous methanol (90 vol.% methanol), see text. Column: C,, , eluent: 33% acetonitrile, isochratically, flow rate 0.5 an3 min-

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12 1 I

10

L 8 ' 6 9

cv r

.$ 4

2 0

nitrogen air oxygen CC14/N2

Fig. 9 Chemical yield of F2 obtained from BP/silica-alumina ( K = 20 "C) samples under various experimental conditions, see text

According to previous work with pyrene, these products may be referred to as hydroxybiphenyl~.'~ The assignment will be discussed later.

Fig. 9 shows the yield of F2 under various reaction condi- tions. If the photoirradiation is carried out under N,, the yield of F2 is negligible. If CCl, is coadsorbed on the deaer- ated sample, the yield of F2 is low. As 0, is necessary for formation of F2, it is consistent with the formation condition of BP'+.

After HPLC separation of the product mixture, the col- lected fractions were spectroscopically investigated. The absorption and pH dependence of F2 are shown in Fig. lqu) of which both are similar to that of 4,4-biphenol. The emis- sion spectra in the range pH 6-8 exhibit significant change [Fig. lqb)]. The pH dependence is usually a characteristic of phenol-like compounds.20

A quantitative material balance must take into account the starting compound, possible intermediates and stable pro-

0.00 ' 200 300 400 500

A/n m

I . - .

290 330 370 410 450 A/nm

Fig. 10 pH dependence of the absorption (a) and fluorescence (b) obtained from the chromatographic fraction 2 (F2), the sample was evacuated with pump for preconcentration and redissolved in meth- anol

ducts. This is illustrated for only one compound and for one condition, namely photolysis of BP on the silica-alumina surface. For unactivated silica-alumina cu. 60% of BP is adsorbed on the surface from cyclohexane. The product con- centrations were estimated after HPLC separation using the calibration curve of 4,4-biphenol. Under photoirradiation at 254 nm with cu. 8 x lo-, einstein for an aerated BP/silica- alumina sample at a loading concentration of 5 x 10- mole g-', cu. 1% of BP is converted to hydroxylated products, while more than 90% of BP is recovered.

Photochemical Reactions of CBP andDCBP on the Surfaces

Fig. l l (u) and (b) show the chromatographic spectra of the photoirradiated spectra of the photoirradiated CBP and DCBP samples after extraction with aqueous methanol solu- tion. Both give peaks F1, F2 and F3 in the spectra [Fig. l l (u) and (b)], which are assigned to the photochemical products of CBP and DCBP, respectively. As with BP, in the absence of 0, the yield of the oxidative products are negligible. However, dechlorination takes place, leading to formation of some other products. Fig. 12 shows an HPLC spectrum of the chemical products of CBP under an N,-saturated system. It was confirmed that BP and chloride ion are photochemical

0 2 4 6 7 8;lO 0 2 4 6 7 8 1 0 tlmin t/min

Fig. 11 Chromatographic spectra for the chemical products of CBP (a) and DCBP (b). F1, F2 and F3 are corresponding photochemical products, respectively. Sample preparation and analysis were the same as in Fig. 8

1. CBP 5.0 .

>. U .- 2 4.0 - 0) +- .-

3.0 - P 0) z 2.0 - - e

1.0 -

BP

CBP( OH) 2 d Fig. 12 Chromatographic spectrum for the chemical products of CBP under nitrogen-saturated silica-alumina suspension, irradiated at 254 nm in the photoreactor for 20 min. Sample preparation was the same as in Fig. 8. Column: C,, , eluent: 67 vol.% aqueous aceto- nitrile, isochratically, flow rate: 0.5 cm3 min-

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products of CBP; and BP, CBP and C1- are photochemical products of DCBP. C1- was determined by ion chromatog- raphy. Under similar irradiation conditions the dechlori- nation in the heterogeneous system is comparable with that in homogeneous solution.

Discussion Molecular Conformation of the Adsorbed BP

The spectral position, width and structure may present infor- mation about the molecular geometry of BP on surface^.'^.^^ BP has a special character involving twisting about the inter- annular bond. Because of the steric interaction between the hydrogen atoms at the 2,2'- and 6,6'-positions, in non-polar solution the twist angle (0) is determined from solution absorption data to be between 19 and 23".,' According to Berlman's classification, considering the BP structured fluo- rescence spectra, the diffuse absorption spectra and the large Stokes shift, BP in solution belongs to class-3 molecules which have non-planar ground states and planar excited states.'2b BP molecules adsorbed on solid surface could also have a planar form. However, the red shifts in the absorption and fluorescence spectra are not so distinct, suggesting that BP adsorbed on surfaces also maintains a non-planar nuclear conformation. It has been observed that increased planarity of a chromophore gives rise to a large red shift in the absorp- tion and fluorescence spectra. ' The absorption and emission spectrum bandwidths of the adsorbed molecules are broadened compared to that in solution.

Generally, electronic transitions originating from more planar electronic states produce a more highly structured spectrum, while transitions originating from non-planar states will be distributed among a greater variety of vibronic transitions, thus broadening the spectrum. The lack of struc- ture in the spectra indicates that both ground state and excited state of the adsorbed BP have non-planar geometries. The characteristic of the absorption and fluorescence spectra exhibit changes in molecular conformation due to the inter- action of BP with the surfaces.

Formation of Radical Cations on the Solid Surfaces

The formation of the BP'+, produced either thermally or photochemically, confirms the electron-transfer processes on y-alumina and silica-alumina. The Lewis-acid sites, which are produced by dehydroxylation of the surfaces, are generally recognized as electron acceptors, while PAHs are electron donors. However, in the absence of 0, photoinduced CT bands and BP" are not observed, indicating that 'BP . . . surface'-type CT-complexes are not important in the present system. This result agrees with previous works on other aro- matic where the coadsorption of 0, plays an important role in the formation of organic radical cations. It is also consistent with a recent report that naph- thalene may form an exciplex with oxygen.22b The analysis of chemical products is also consistent with this result (see Fig. 9). However, in the present experiments, superoxide 0;- was not observed either directly by EPR or with the spin-trapping agent 5,5-dimethyl-l-pyrroline N-oxide (DMPO); 0;- is expected to be a primary product following electron transfer. The fate of radical cations is established but that of the elec- tron or negative species remains unclear.23

Irradiation with photons of lower energy than the BP absorption and, but within the CT band, generates BP'+. This indicates that the adsorbed BP form C P complexes with surface sites and 0, [eqn. (311. Ionization is a predominant process of the excited CT complex on the surface and emis-

sion corresponding to the CT band is not observed. Similar results have been observed using pyrene.'

(3) BP + O,/surface f (BP - - - O,/surface) hv

(BP * - * O,/surface) - (BP * * - O,/surface)* BP" + O,/surface'-

(4) * * - O,/surface) + hv'

The non-activated surfaces are not sufficiently polar to thermally generate CT complexes at room temperature. However, at low temperature the adsorbed BP molecules may be ionized on UV irradiation. The dependence of BP" yield on the photoirradiation intensity at 77 K indicates that the photoionization is a monophotonic process (Fig. 4). This is a result of the interaction of BP with the surface at 77 K. Increasing the temperature removes the EPR signal, possibly resulting from the recombination of radical cations with electrons/counterions, or react to form products. The pro- ducts analysis indicates that the last process is an important reaction channel.

Product Assignment and Proposed Reaction Mechanism of BP" with Water

The mode of separation in reverse-phase column (C18) pre- dicts that more polar samples exhibit shorter retention times. The retention times (Fig. 8) show that the products are more polar than the parent compound, and close to that of 2,2'- biphenol and 4,4'-biphenol, which are commercially available standard compounds. F1, F2 and F3 are suggested to be isomeric compounds, where the hydroxy groups are located at various positions on BP (vide infra).

Considering the nucleophilic nature of water molecules the reaction of BP'+ with water may be rationalized as a nucleo- philic addition at the cation centre to form an a d d ~ c t . , ~ In polar solution, i.e. aqueous methanol solution, the adduct formed is involved in a rapid deprotonation, leading to a conjugated BP(0H)' radical. This process is similar to redox inversion reactions, where the oxidizing species is converted into a reducing species.24

(5)

(6) The BP(0H)' radical, a mild reductant, can react with elec-

trophiles to form either hydroxybiphenyl via electron transfer and subsequent deprotonation, or BP(OH)+ cation, which further reacts with water, finally resulting in a dihydroxy compound. The exact mechanism is not yet clear; however, both reactions have been proposed in electrochemistry and radiation c h e m i ~ t r y . ~ ~ . ~ ~

BP( - H)OH + electrophi1e'- (7)

BP( OH) + + electrophile' - (8) BP(0H)' + electrophile +

BP'+ + H,O e BP(OH2)+

BP(OH2)+ e BP(0H)' + H +

I-!% BP(OH), (9) In the present system BP(0H)' radicals may react with

BP'+ as well as other oxidants, such as hexacyanoferrate anion [Fe(CN)z-] or 0,. The presence of Fe(CN);- signifi- cantly enhances the chemical yield, while the presence of 0, reduces the chemical yield. These results can be understood on the basis of the above mechanism. Fe(CN)z- can react with BP(0H)' radicals instead of BP+ ,26 whereas 0, adds to the BP(0H)' radical, leading to peroxy radicals.

BP(0H)' + 0, + BP(0H)OO' (10)

It was proposed that the hydroxycyclohexadienyl peroxy radical undergoes a concentration-dependent competition,

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favouring peroxide formation at higher concentrations and phenol formation at lower concentration^.^'

The product analysis shows that isomeric hydroxy- biphenyls are the main products in the present system; no phenylphenol was observed. This indicates that reaction (8) proceeds mainly through the formation of BP(OH)+ cations.

The BP(0H)' radical is proposed as the key intermediate for the hydroxylation of BP'+, although it was not directly observed. However, BP(0H)' radicals have been spectro- scopically studied in radiation chemistry" as hydroxyl radical adducts which decay via second-order kinetics. Fur- thermore, it was proposed that BP(0H)' radicals are precur- sors of BP'+ via protonation and subsequent water elimination. '

The resonance structures of BP'+ which can lead to iso- meric product are shown in Scheme 1.

isomeric hydroxy- biphenyls

Scheme 1

The para position could be the most probable conforma- tion for BP". As the yield of F2 is relatively higher than that of other fractions (F1 and F3), F2 may be ascribed to 4,4'- hydro-4,4-dihydroxybiphenyl.

Reactions of Chlorinated BPs on Solid Surfaces

The formation of CBP and DCBP radical cations has been confirmed by EPR measurements (vide supra). The reactions of CBP and DCBP on surfaces can be divided into two types, namely, oxidation degradation, and dechlorination. The former is similar to the reaction pattern for non- chlorosubstituted BP, resulting in chloro-substituted hydroxybiphenyls.

The BP formed in the product mixture of CBP (Fig. 11) is ascribed to the dechlorination product at photoinduced dechlorination, i.e. homolysis of chlorinated biphenyls is well doc~mented. '~ Dechlorination can result from direct chlorine-carbon bond breakage or via a reduction process. Reductants such as dienes and amines are commonly used in photo~hemis t ry .~~

CBP on surfaces may be reduced by intermediate CBP(0H)' radicals, which are weak reductants leading to dechlorination. However, this is contrary to the fact that the presence of CCl, ( lop3 mol dm-3) does not depress the yield of BP. If CBP(0H)' reduces CBP to form the chlorine anion and BP, then CCl, should effectively compete with CBP because of its higher degree of chlorination. It is possible that in the present system the formation of chloride ion is through bond cleavage, as the photon energy is comparable with the (Ph)C-C1 bonding energy, ca. 94.5 kcal mol-'.31 The photo- chemical reactions of DCBP are similar to those of CBP.

Conclusion The photochemical reactions of PCB model compounds, BP, CBP and DCBP on y-alumina and silica-alumina surfaces

have been examined. The adsorbed states of BP, CBP and DCBP experience physical and chemical interaction with the solid surfaces. Diffuse reflectance spectra exhibit the forma- tion of radical cations and charge-transfer complexes on the surfaces, while EPR absorption spectra serve as collaborative evidence for radical cations. The emission spectra indicates BP molecular reconformation on the surface. The radical cations of BP, CBP and DCBP may be generated thermally and photochemically on the surfaces. The coadsorption of electrophiles, namely persulfate and 1,4-dinitrobenzene, sig- nificantly influences the electron-transfer processes. The important reaction of radical cations is hydrolysis resulting in hydroxybiphenyl, while the radical BP(0H)' was suggested as a key intermediate in hydrolysis. The present study reports on several photochemical and chemical reactions of biphenyl surface species, and provides the basis for the potential treat- ment of solids contaminated by chlorinated biphenyls.

We acknowledge research support by the Environmental Protection Agency (EPA-R-815953-01-0).

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Paper 2/02341E; Received 6th May, 1992

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