Langmuir 1992,8, 2501-2508
2501
Photochemical Reactions of Pyrene on Surfaces of y-Alumina and Silica-Alumina Yun Mao and John Kerry Thomas* Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556 Received February 24, 1992. In Final Form: June 18, 1992
Photochemicalreactions of pyrene on surfacesof y-alumina and silica-alumina were investigated. Pyrene molecules adsorbon these surfaces and form charge-transfer(CT)complexesat activated sites of y-alumina and silica-alumina which are generatedby preheatingthe solidsto T,> 350 OC. The CT complexes exhibit absorption bands near the red edge of the characteristic absorption band of pyrene molecules. Pyrene excimer formation becomes significant on activated y-alumina (T,< 300 "C). Excited states of pyrene are quenched by surface states, leading to the formation of CT complexes. The radical cations formed on photoexcitationadsorb strongly on the surfaces. These species exhibit a multilined EPR signal, while diffuse reflectance studies show that the radical cations also interact with parent molecules forming monopositive dimericradical cations. The most interesting reaction of the radical cations on the surfaces is hydrolysis. The photochemical products on both activated and nonactivated surfaces were separated by HPLC,and the collected fractionswere studied spectroscopically. The pH dependenceof the absorption and fluorescencespectra taken together with results of mass spectrometryindicatesthat the main products are hydroxypyrenes. Water reacts with the radical cation forming the radical Pyr(OH)*,which is a key intermediate in subsequent reactions.
Introduction Photochemistry of species adsorbed on surfaces has received considerable attention over the past decade, and in particular several studies of photoactive organic compounds adsorbed on silica and alumina have been An important application of the studies is an understanding of heterogeneous catalysis at the molecular level, and in the design of potential catalysts to aid in the treatment of pollutants adsorbed on natural materials. Photochemcial properties and subsequent reactions of organic molecules adsorbed on surfaces are influenced by the interaction between the adsorbent and the adsorbate. The nature of the adsorbed states of the molecules can be physical or chemical. Physisorption is similar to that in nonpolar solvents, while chemisorption may involve charge-transfer complexes, proton-addition complexes, or free radicals. Diffuse reflectance and luminescence spectroscopies provide valuable information regarding the phyai- and/or chemisorption of the adsorbates,and electron paramagnetic spectrometry (EPR) identifies the paramagnetic property of the species. Identification of chemical products by HPLC and mass spectrometry combined with spectroscopy makes it possible to infer chemical intermediates and reaction mechanism on surfaces. Of prime interest is the transformation of polycyclic aromatic hydrocarbons (PAH),which are relatively stable on exposure to UV radiation at energies lower than their ionization thresholds. However, photocatalysis of adsorbed PAH occurs relatively efficiently and can be a potential method for the destruction of these waste materials. Pyrene, a typical PAH, is widely used as a probe moleculeto investigate surface properties due to its unique
* Author to whom correspondence should be addressed.
(1) de Mayo, P.; Natarajan, L. V.; Waren, W. R. In Organic Phototransformations in Nonhomogeneous Media; Fox, M. A., Ed.; ACS Symp. Ser. 278; American Chemical Society: Washington, DC, 1984; p 1.
(2) Damme, H. Van; Bergaya, F.;Challal, D. In Homogeneous and Heterogeneous Photocatalysis; NATO SA1Ser. C, Pelizzetti,E., Serpone, N., Eds.; D. Reidel Publishing Co.: Dordrecht, 1985; Vol 174, p 479. (3) Krasnansky, R.; Thomas, J. K. J. Photochem. Photobrol. A; Chem. 1991,57, 81.
photophysical and photochemicalproper tie^.^ It can also be used as a model compound to study photochemical reaction of PAH on surfaces. The EPR observation of radical cations of several PAH has been reported,"' while studies on surface charge-transfer (CT) complexes on y-aluminahave been reported by Oelkrug and Wilkineon.aS Direct observation of CT complexes on surfaces is difficult, and so far, little is known concerning the mechanism of the surface photochemistry of these species. In previous work, photophysical and photochemical properties of pyrene on silica, y-alumina, and laponite surfaceswere reported,3J&l3studies included the formation of the singlet and the triplet states of pyrene, quenching of the excited states by various compounds, and the formation of radical cations. Here, we report chemical reactions of pyrene radical cations on solid surfaces of y-alumina and silica-alumina, identification of the photochemical products, and suggested mechanisms for the observed photochemical reactions.
Experimental Section y-Alumina and silica-alumina were received from -he Chemical, Inc., and Aldrich, respectively. The average surface areas of y-aluminaand silica-alumina supplied by producers are 200 and 415 m2/g, respectively. Before we, the solids were activated by preheating over a temperature range of 130-750 "C. Pyrene (Aldrich,99%) was purified by chromatographicseparation (using silica gel as the stationary phase and cyclohexane as the mobile phase) and recrystallization from cyclohexane;the purity was checkedby UV-visible spectrophotometerand HPLC. (4) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. SOC.1977,99, 2039. (5) Muha, G. M. J. Phys. Chem. 1967,71 (3), 633; 1970,74 (15), 2939. (6) Rooney, J. J.; Pink, R. C. Trans Faraday SOC. 1962,58, 1632. (7) Hall, W. K.; Dollish, F. R. J. Colloid Interface Sci. 1968,26, 261. (8) (e) Oelkrug, D.; Erbse, H.;Plauechinat,M. 2.Phys. Chem. (Munich) 1975,96,283. (b) Oelkrug, D.; Plauschinat, M.; Kwler, R. W. d. humin. 1975,18119,434. (9) Oelkrug, D.; Krabichler, G.; Honnen, W.; Wilkinaon, F.; Willsher, C. J. J. Phys. Chem. 1988,92, 3589. (10) Liu, X.; Iu, K.-K.;Thomas, J. K. Langmuir 1992,8, 539. (11) Pankasem, S.; Thomas, J. K. J. Phys. Chem. 1991,95,6990. (12) Pankasem, S.; Thomas, J. K. J. Phys. Chem. 1991,96,7385. (13) Beck, G.; Thomas, J. K. Chem. Phys. Lett. 1983,94,553.
0 1992 American Chemical Society
Ma0 and Thomas
2502 Langmuir, Vol. 8, No. 10,1992 1-Hydroxypyrenefrom Moleular Probes was used without further pretreatment. The remaining compounds were of the highest purity available. Diffuse reflectance spectra of solid samples were measured on a UV-visible spectrophotometer (Perkin-Elmer 552) equipped with an integrating sphere accessory. Steady-state emission spectra were measured on a spectrofluorometer (SLM Aminco, SPF 500C), equipped with a 250-W Xe lamp and Hamamatsu photomultiplier R928P, and connected to a computer. EPR spectra were recorded on an electron paramagnetic resonance spectrometer (Varian Associate,E-line century series) equipped with an X-band klystron and a rectangular cavity; g values were determined by comparisonto diphenylpicrylhydrazyl (DPPH). Samples for EPR measurements were contained in 5 mm 0.d. quartz tubes, which were attached directly to a vacuum line. For photoionizationmeasurements, samples were irradiated via a shutter in the EPR cavity. Photoirradiation was carried out with either an Xe lamp (150 W) equipped with a 20-cm water filter to remove IR radiation or a chemicalreactor (Rayonet, Southern New England Co.) with RPR 3000-A lamps. The chemical actinometer (K3Fe(CzO& was used to determine the incident photon intensities on the samples. For activation of solid samples, 0.5-1.0g of y-alumina (or silicaalumina) powder was heated for 20 h in quartz crucibles under air at a given temperature. Following thermal activation the powder samples were allowed to cool to room temperature in a desiccator. Immediately after cooling the powder was mixed with an aliquot of pyrene solution in dry cyclohexane. Samples for product analysis were prepared through an extraction procedure, Le., the photoirradiated suspensions were dissolved in aerated or deaerated aqueous methanol, when pyrene and chemical products were transferred into the aqueous methanol phase. Samples for HPLC were first centrifuged (5 X 109 rpm) and filtered through a Millipore filter (0.45 pm) to remove any suspended solid. Separation and quantitative identification of the photochemical products were performed by high-performance liquid chromatography (Waters), equipped with a NOVA-PAK CIScolumn and UV absorption and fluorescence detectors; methanol was used as an eluent. Identification of aldehyde was carried out by reversed-phase HPLC after derivatization of the aldehyde with (2,a-dinitropheny1)hydrazine (2,4-DNPH).14 Mase spectra were recorded on a GC/MS (Finnigan-MAT8430) spectrometer equipped with an electron impact source, fast atom bombardment source, and a data acquisition system. Fractions separated by HPLC were collected,concentrated (100-300 times) under vacuum, and used directly in the mass spectrometer.
Figure 1. (a,top) Diffuse reflectance spectra of pyrene adsorbed on y-alumina activated at different temperatures, from bottom to top: 130 "C, 450 "C,and 750 O C ; pyrene loading concentration, 1 X 10" mol/g. The ordinate -1ogR is relative diffuse reflectance. (b, bottom) Diffuse reflectance spectra of pyrene adsorbed on y-alumina (T.= 130 "C),theintensityof the broad bandincreases with irradiation time, from bottom to top: 0, 2.5, 5.0, 7.5, and 10 min. The irradiation was carried out with Pyrex glass filter (A 350 OC) form pyrene(33) Ristagno, C. V.; Shine, H. J. J. Org. Chem. 1971,36, 4050. (34) Sioda, R. E. J. Phys. Chem. 1968, 72, 2322. (35) Kanodia, S.; Madhaven, V.; Schuler, R. H. Radiot. Phya. Chem. 1988,32 (5), 661.
adsorbate CT complexes and Pyr'+. The CT complexes are confirmed by the additional absorption bands near the red edge of the characteristicabsorptionband of pyrene molecules and by the fact that irradiation into the CT band yields cation radicals. Monitoring the relative emission yield indicates that excited states of pyrene molecules are quenched by the active surfaces. On the y-alumina (Ta< 300 OC) excited states are also quenched by pyrene resulting in excimer formation. On silica-alumina surfaces excimer emission is not observed. The enhanced excimer formation is related to the decrease in the number of relative sites for pyrene adsorption. This leads to an enhanced formation of ground-state complexes of pyrene or local regions of higher pyrene concentration. This in turn leads to enhanced excimer formation. The radical cations, formed through either thermal or photoinduced processes, are observed by diffuse reflectance and EPR spectroscopy. The multilined EPR absorption and diffuse reflectance spectra also show that Pyr*+dimerizes with the parent molecules to form Pyrz'+. These species strongly adsorb on y-alumina and silicaalumina surfaces and are only removed with aqueous methanol solution. The most interesting reaction of Pyr'+ on the surfaces is hydrolysis. After HPLC separation, spectroscopic analysis of the collected fractions shows a pH dependence of the absorption and fluorescence spectra. The pH dependence and mass spectroscopicanalysis establishthat hydroxypyrenes are important products. The photochemical reaction of pyrene on nonactivated surfaces was studied. The result of the spin trapping suggeststhat the reaction on the nonactivated surfacecould be also a radical process.
Acknowledgment. We acknowledge research support by the EnvironmentalProtection Agency(EPA-R-81595301-0). Dr.S.Pankasem is thanked for helpful discussion. %&try NO. Py,129-00-0;Pya+,34506-93-9; Al203,1344-
28-1;SiO2, 7631-86-9.
