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New Photosynthesis: Direct Photoconversion of Biomass to Molecular Oxygen and Volatile Hydrocarbons? Elias Greenbaum,* Carol V. Tevault, and C. Y. Ma Oak Ridge National Laboratory,$ P.O. Box 2008, Oak Ridge, Tennessee 37831-6194 Received April 15, 1994. Revised Manuscript Received November 3, 1994@
The simultaneous photoevolution of molecular oxygen and volatile hydrocarbons was observed when ferric ions and other photosensitizers such as semiconducting oxides were implanted in cellulose and wood under high pressure and irradiated with n e a r - W and visible light. Control experiments with purified microcrystalline cellulose and lignin indicated that both of these components of wood could undergo phototransformation. However, in the case of lignin, only volatile hydrocarbons were observed. Although UV-induced structural degradation of lignocellulosic substrates is well-known, the present studies reveal that in an inert atmosphere a novel photochemistry occurs that is qualitatively different from that which occurs in air. In an inert atmosphere the photochemistry bears a formal analogy to normal photosynthesis in that molecular oxygen is photoevolved and a reduced photoproduct, more reduced than the consumed substrate, is produced. This analogy is discussed in the context of oxidation-reduction levels of the photoproducts, the source of reductant for the photoredox reactions, and elementary theories of the photophysics and photochemistry in reaction cavities of the structured matrices in which the light-induced reactions occur.
Introduction and Background Photosynthesis is the biological process by which atmospheric carbon dioxide is transformed into C 0 2 fixation products. Although plants are capable of synthesizing a broad spectrum of reduced carbon products, a major fraction of global photosynthesis results in carbon dioxide being reduced to the carbohydrate level. The overall equation of photosynthesis is usually summarized as C 0 2 H20 (CH20) 0 2 . This relatively simple equation, however, fails t o suggest the intricate photophysical and photobiochemical processes that are required t o achieve the reduction of CO2. In particular, the vectorial light reactions occurring in a structured matrix separated by the photosynthetic membrane result in permanent charge separation induced by photon ab~orption.l-~ As reviewed by Fiest and H o ~lignocellulosic , ~ substances such as wood undergo W-induced degradative reactions. The first systematic study of the action of ultraviolet light on cellulose was that of Stillings and Van N ~ s t r a n d .In ~ this work it was shown that cotton fibers irradiated in a nitrogen atmosphere underwent an increase in the number of reducing sugars with a corresponding evolution of carbon monoxide and carbon
+
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+
Abstract published in Advance ACS Abstracts, December 15,1994. (1)Deisenhofer, J.;Norris, J. R. The Photosynthetic Reaction Center; Academic Press, Inc.: New York, 1993;Vol. 1 and 2. (2)Bowker, R. R. In The Photosynthetic Reaction Center; Breton, J., Vermeglio, A., Eds.; Plenum: New York, 1993;Vol. 2: Structure, Spectroscopy and Dynamics. (3)Amesz, J.,Hoff, A. J., Van Gorkum, H. J.,Eds. In Current Topics in Photosynthesis; Kluwer Academic Publishers: Amsterdam, 1986. (4)Feist, W. C.; Hon, N.-S. Chemistry of Weathering and Protection In The Chemistry of Solid Wood; Rowell, R., Ed.; American Chemical Society: Washington, DC, 1984. (5)Stillings, R. A.; Van Nostrand, R. J. The Action of Ultraviolet Light upon Cellulose. I. Irradiation Effects. 11. Post-Irradiation Effects J . Am. Chem. SOC.1944,66, 753-760. @
dioxide. Desai and Shields6 studied the photochemical degradation of cellulose filter paper and showed that a spectrum of fully reduced and oxygenated hydrocarbons had been produced. However, unlike Stillings and Van Nostrand, who purged the system with nitrogen, Desai and Shields worked with rubber-stoppered static irradiation tubes that were initially filled with air. It is noteworthy that in the Desai-Shields experiments hydrocarbons were observed only after irradiation periods longer than 1-2 h (ref. 6, p 1431, whereas no delay was observed in an initially oxygen-free atmosphere (vide infra). The photochemical degradation of cellulose falls into two classes: direct photolysis and photosensitized degr a d a t i ~ n .In ~ particular, Hon8demonstrated that metal salts, especially ferric ions, greatly affected free-radical formation. The focus of Hon’s work was an electron paramagnetic resonance study of wavelength and the concentration dependence of a ferric-ion photosensitizer on radical formation in cellulose at 77 K. Inorganic ions have also been used in a protective role for photoinduced graft copolymerization of methyl methacrylate onto cellu1ose.g The key results of the experiments reported in this paper are that in an inert atmosphere photosensitized wood substrates such as poplar and pine as well as highly purified microcrystalline cellulose undergo a biomimetic photosynthetic reaction. For wood and (6)Desai, R. L.; Shields, J. A. Photochemical Degradation of Cellulose Material Makromol. Chem. 1969,122, 134-144. (7)Hon, N.-S. Formation of Free Radicals in Photoirradiated Cellulose. 111. Effect of Photosensitizers J . Polym. Sci.: Polym. Chem. Ed. 1975,13,1933-1941. (8) Hon, N.-S.Formation of Free Radicals of Photo-Irradiated Cellulose. IV. Effect of Ferric Ion J . Appl. Polym. Sei. 1975,19,2789nnnn LlYl.
(9)Kubota, H.;Ogiwara, Y. Effect of Metallic Ions in Photo-Induced Graft Copolymerization onto Cellulose J.Appl. Polym. Sci. 1972,16, 337-344.
0887-0624/95/2509-0163$09.00/0 0 1995 American Chemical Society
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164 Energy & Fuels, Vol. 9, No. 1, 1995
purified cellulose, this reaction is the simultaneous photoevolution of molecular oxygen a n d volatile hydrocarbons, molecules that, at least in the case of microcrystalline cellulose, are more reduced than the irradia t e d substrate. The only source of oxygen for this
UV LIQHT SOURCE SYSTEM VENTED TO HOOD
HEATED
reaction is the aqueous medium used t o dissolve the sensitizing ions or the “hydrate” of the carbohydrate. Like natural photosynthesis, this W photoreaction is clearly an endergonic process.
t TEMPERATURE MONITOR
Experimental Section
U
MICROCOMPUTER
He CARRIER GAS
Sample Preparation. Three general classes of samples were used for the experiments: (1)wood, usually poplar or pine; (2) Avicel, a highly purified microcrystalline cellulose; and (3) purified lignin. Ferric chloride or nitrate solution was forced into the sample by placing it in a Model FA-030 SLMAminco French pressure cell and subjecting it t o 20 000 psi with an American Instrument Co. power laboratory press. The concentration of the ferric ion solution was usually 3 M. A similar preparative treatment was employed for insertion of semiconducting photosensitizers such as ZnO. The colloidal suspension used for insertion had a concentration of 10 mg/ mL. In the case of microcrystalline cellulose and lignin, the particles were entrapped on a fiberglass filter pad. Apparatus: Hydrocarbon and Oxygen Detection. A continuous-flow system was constructed to measure the photoproduction of molecular oxygen and volatile hydrocarbons. The photoreaction chamber consisted of a quartz cylinder with inlet and outlet ports. Stainless steel tubing was connected to the ports by Cajon O-ring compression seals. Helium carrier gas continuously purged the reaction chamber. Volatile photoproducts were swept out of the chamber and carried downstream through heat-tape wrapped lines to the gas detectors. The oxygen sensor was a Hersch electrogalvanic cell,Iowhich is a two-electrode lead and silver device, separated by a filter paper membrane that is impregnated with 24% KOH. When oxygen flows into the cell it is electrocatalytically reduced. The geometry of the cell causes the electrons generated in the redox reaction to flow in an external circuit. The current was measured with a Keithley Model 485 digital picoammeter. Calibration of the oxygen sensor was achieved with an inline electrolysis cell and Faraday’s law of electrochemical equivalence. Hydrocarbon detection was achieved with a Gow-Mac Model 23-7001702 total-hydrocarbon analyzer. Tentative identification of the hydrocarbons was made with a HewlettPackard Model 5890 Series I1 gas chromatograph equipped with a 5971 Series mass selective detector. Trapping and analysis of the samples are described in the following sections. Light Source and Preparation of Triple-SorbentTraps. The lamp used for these experiments was a Canrad-Hanovia compact xenon-cathode tip lamp. According to the manufacturer’s specifications, about 20% of the emitted light is in the UV ( ~ 4 0 nm) 0 with the balance approximately equally divided between the visible and infra-red. Control experiments with cutoff filters, as well as additional controls with and without sensitizer, clearly indicate that UV photons caused the observed photochemistry. All reactions were performed in a temperature-stabilized water-jacketed cell. Control experiments with heating tape indicated that thermal chemistry was not responsible for the observed reactions. The traps were constructed from 6 mm 0.d. (4 mm i d . ) stainless steel tubing 76 mm in length. The ends of tubes were ground out t o 4 mm, and the end edges were filed so that the ferrules fit over them easily. The tubes were washed in an ultrasonic bath with Micro liquid laboratory cleanser, rinsed thoroughly with distilled water, washed with methanol, and finally rinsed with methylene chloride for degreasing. After (10)Baker, W. J.; Combs, J. F.; Zinn, T. L.; Wotring, A. W.; Wall, R. F. The Galvanic Cell Oxygen Analyzer Ind. Eng. Chem. 1959, 51.
Figure 1. Schematic illustration of the photoreaction system used to irradiate and trap the photoproduction of volatile hydrocarbons from cellulose, lignin, or wood. The UV light source is a 1000-W xenon lamp. The heated tape located downstream from the reaction vessel ensured that no hydrocarbons condensed on the tubing between the reactor and triple-sorbent trap. The carrier gas was helium, which vented to the atmosphere after flowing through the trap. drying, the sample flow direction and tube ID number were etched on the tube with a vibrating tool. The traps were plugged at the downstream end with approximately 15 mm of silanized glass wool and then filled with a 14-mm length of each of three carbonaceous adsorbents from Supelco. The most adsorptive one, 60-80 mesh Carbosieve SI11 (Cat. No. l-0184), was packed first, tapping the side of the tube after adding the adsorbent to settle it in the tube. Carbotrap, 20-40 mesh (Cat. No. 2-0287), was added next, again settling the adsorbent by tapping the tube. The least adsorptive material, Carbotrap C 20-40 mesh (Cat. No. 2-0309), was added last with similar treatment. Another 15mm plug of silanized glass wool was then inserted in the upstream end. Swagelok stainless caps and nuts, VespeU graphite ferrules W d n . o.d., Supeltex M-2A), which had been washed using the above cleansing procedure, were used to seal the traps. The traps were conditioned by thermal desorption in batches of 15 on a manifold, which was placed in a gas chromatograph oven. With helium (high purity, passed through a n oxygen and molecular sieve trap) flowing opposite to the sampling direction at a flow rate of 60 mumin, the traps were heated a t 300 “C for 3 h. Trap blanks were analyzed by thermal desorption and combined gas chromatography and mass spectrometry (TDIGCNS), as described in the following section. A schematic illustration of the experimental arrangement used for trapping the samples is indicated in Figure 1. Analysis of Organic Constituents Collected on TripleSorbent Traps. Volatile organic constituents collected on the triple-sorbent traps were analyzed by TD/GC/MS. Mass spectral analysis was performed on a Hewlett-Packard 5971 gas chromatograph and mass-selective detector (GCNSD) operating in electron impact mode (EI). A schematic illustration of the experimental arrangement is illustrated in Figure 2. The following modifications on a Hewlett-Packard 5890 Series I1 gas chromatograph were made in order to introduce the headspace sample into the GC system. A Swagelok V 4 - h . nut was welded on the septum retainer nut for direct connection of traps. A section of aluminum-clad fused silica capillary tubing (0.53 mm id., 5 cm in length) was inserted through the septum to serve as a transfer line to the splithplitless injection port. A capillary inlet adaptor fitting (Restek, Cat. No. 20633) with a V16-in. Swagelok was installed at the base of the injection port. A cryogenic loop, constructed with a 15cm stainless steel tubing (0.04 in. i.d., in. o.d.1, was connected to the inlet adaptor with a V d n . Vespellgraphite ferrule. A fused silica DB-624 column (J&W, 60 m, 0.25 mm id., 1.4pm film thickness) was connected t o the cryogenic loop via a lI16-in. Valco low-dead volume union.
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w
VEGTRA DATA SY8lEM
r I
fi
THERMAL DEWRPTON OVEN I
I II
I
I
I
I
fT(r-300 200
HYDROCARBONS
I
Figure 2. Schematic illustration of the thermal desorptiongas chromatograph-mass spectrometer (GC-MS) analysis system. Following the collection of the sample as illustrated in Figure 1, the triple-sorption trap is placed at the inlet of the GC-MS system. By use of the indicated thermal desorption oven, all desorbed hydrocarbons were cryogenically focused and subsequently released into the capillary column of the system. Tentative identification of the photoproduced hydrocarbons was made with a Hewlett-Packard Model HP 5971 mass-selective detector. In a typical analysis, a triple-sorbent trap was placed in a tube furnace (Watlow Cable Heater, in. x 24 in., 120 V, 240 W, 2 A) held at 260 "C and purged with helium at a flow rate of 40 mumin for 5 min in the opposite direction of sampling flow. The desorbed material was transferred directly through the GC injector port to a cryogenic loop at the head of a capillary column. The cryogenic loop was immersed in a liquid nitrogen bath during the desorbing process. The GC oven temperature program was initiated when the liquid nitrogen bath was removed from the cryogenic loop. The GC oven was held a t 30 "C for 5 min, then increased to 220 "C at a rate of 8 "C/min. The flow rate of carrier gas (helium) was held a t 1.24mumin throughout the entire oven temperature program. Both the injector temperature and GC/MSD transfer line temperature were held a t 280 "C. Electron impact spectra were obtained with an electron energy of 70 eV, emission current of 300 PA, and a source temperature of 180 "C.Mass spectral data were acquired at a rate of 1.4scads over a mass range of 30-300 amu. Blank traps (Figure 2) were analyzed prior to sample collection. If any contaminant component was detected in the blank trap, the trap was reconditioned and reanalyzed t o ensure blank trap integrity. A mixture of gas standard, consisting of isoprene, benzene, trichloroethylene, tetrachloroethylene, toluene, and m-xylene a t a concentration of approximately 100 ng/mL, was spiked onto the trap and analyzed by TD/GC/MS t o check desorption efficiency. Sample traps were analyzed in the same manner as were the blank traps. Performance. Results obtained from the GC/EI analysis were used to tentatively identify those components that were present in the irradiated wood sample but not in the unirradiated wood sample. Identification was based on a computer match of the mass spectrum generated from the unknown component with the mass spectrum of a standard in the Wiley Library. Identities for the major components (Table 1)were scrutinized by comparing their mass spectral data with those listed in the Eight Peak Index of Mass Spectra (3rd edition; The Royal Society of Chemistry, Nottington, U.K., 1986). Volatile oxygenated hydrocarbons were found in abundance normalized to the most abundant component.
Results and Discussion Substrates and Photoproducts. Figure 3 is a plot of the simultaneous photoevolution of oxygen and
volatile hydrocarbons from ferric (chloride) ion-sensitized microcrystalline cellulose. The complex time courses of hydrocarbon and oxygen evolution, which qualitatively track each other, are probably indicative of the fact that the gases are escaping from a hetero-
4
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Figure 3. Simultaneous photoproduction of hydrocarbons and molecular oxygen from UV-irradiated photosensitized cellulose. Avicel, a highly purified microcrystalline cellulose, was entrapped on a fiberglass filter pad after being subjected to the high-pressure treatment described in the Experimental Section. The characteristic complex time course of the simultaneous photoevolution of molecular oxygen and volatile hydrocarbons is probably a reflection of the escape of the gases from the entrapped, unstirred heterogeneous matrix. Further details of the time profile of these data are presented in the discussion section. Table 1. Distribution and Relative Abundance of Hydrocarbons Obtained from Ferric-Sensitized Crystalline Cellulose compound identified formula chloromethane CH3CI 2-propanol C3H70H Cc4 tetrachloromethane trichloroethane C2HCl3 acetic acid CH3COOH 5-methyl-2-hexanone C7H140 3-chloro-2-(chloromethyl)-l-propanone C4HsCk tetramethylfuran CsH120
geneous substructure. This heterogeneous matrix is entrapped on the fiberglass filter pad. All the photoreactions studied in these experiments, cellulose, lignin, and lignocellulose(wood),had similar patterns: initially high rates of oxygen and hydrocarbons production, followed by a peak, and then declining to a lower steady state. The simultaneous appearance of oxygen and hydrocarbons is an unambiguous demonstration of the endergonic nature of this photoreaction, since the hydrocarbon photoproduct is more reduced than cellulose and no external reductant was supplied t o this system. Simultaneous oxygen and hydrocarbon data were also recorded for cut sections of wood such as poplar and pine. However, in the case of lignin, only volatile hydrocarbons were produced; no oxygen was observed. Tables 1 and 2 contain a listing of the compounds that have been tentatively identified with cellulose and poplar substrates using ferric ions as the photosensitizer. When no photosensitizers were added to the system, the photoreactions were not observed. The sensitizer used for the ferric sensitized data was ferric chloride. As indicated by the presence of chlorinated photoproducts, ferric chloride is a reactant as well as
Greenbaum et al.
166 Energy & Fuels, Vol. 9, No. 1, 1995 Table 2. Distribution and Relative Abundance of Hydrocarbons Obtained from Poplar photoproduct formula chloromethane methanol propanol furan methylacetate furfural Table 3. Distribution of Hydrocarbons Obtained from ZnO-Sensitized Poplar photoproduct formula 2-butanol hexanal 2-pentylfuran C3-cyclopentane 2-dodecen-1-01 decanal 2-octyldodecan-1-01 heptadecane 1-heptadeconol pentadecane 1-heptadecanol
photosensitizer. However, as also indicated in Table 1, the production of 2-propanol, acetic acid, 5-methyl-2hexanone, and tetramethylfuran demonstrates that a significant number of compounds are produced that are not derived from ferric chloride. The data of Table 3 which use semiconductor sensitizers clearly indicate that photoproducts can be produced using sensitizers which do not participate in the reaction. The lignin used was Indulin (Westvaco);the avicel used was Avicel PH105. Poplar and pine were obtained from commercial sources and used as received. For the data of Figure 3, the only possible source of oxygen is either the water in which the ferric ions are dissolved and inserted into the cellulose fibers or the “water”of the CHzO monomers comprising the cellulose substrate. Experiments with isotopic labels should, in principle, be capable of resolving this question. The simultaneous onset of photoproduced hydrocarbons and molecular oxygen indicates a formal analogy to higher plant photosynthesis where light-driven molecular oxygen and COz fixation compounds are produced. In this case cellulose is driven to a more reduced state and oxygen is evolved. In both cases oxidized and reduced products are produced simultaneously. The product distribution for photosensitized poplar is presented in Table 2. The sample of poplar used for this experiment is a single-cut section. The ferric ions were impressed into the wood block as a whole, which was then inserted into the reaction chamber for irradiation and product identification. The major fractions of the poplar sample are, of course, cellulose, hemicellulose, and lignin. Semiconducting Photosensitizers. Certain semiconducting oxides, such as ZnO, WOs, TiOn, etc., are an obvious group t o consider in the context of these reactions. Ohnishi et al.ll studied the bleaching of lignin by a photocatalyzed reaction on semiconducting photocatalysts. The mechanism of action of the semiconducting photocatalysts can be understood in terms of the schematic illustration presented in Figure 4. ~
(11)Ohnishi, H.; Matsumura, M.; Tsubomura, H.; Iwasaki, M. Bleaching of Lignin Solution by a Photocatalyzed Reaction on Semiconductor Photocatalysts Znd. Eng. Chem. Res. 1989,28, 719-724.
REDUCED BIOMASS
I/-----. BIOMA
IDIZED BIOMASS
SEMICONDUCTOR
hv
Figure 4. Schematic illustration of a photosensitizing semiconductor particle. Upon absorption of a W photon, an electron is elevated from the valence band to the conduction band. This charge separation gives rise to mobile holes and electrons that can undergo redox chemistry as illustrated. An example of some of the products obtained using ZnO as the photosensitizer is presented in Table 3.
Upon absorption of W photons, valence-band electrons are elevated to the conduction band. This gives rise to the creation of mobile electrons and holes. Once again, the formal analogy to green plant photosynthesis is obvious. The energetics of the W band gap ensure that the mobile hole is a strong oxidant while the mobile electron is a strong reductant. Fine-particle semiconducting oxides can be impressed into the lignocellulosic substrate in much the same manner as the ferric ions. The product distribution for ZnO-semiconductor photosensitized conversion is presented in Table 3. Formal Analogy with Photosynthesis. Although the photophysical and photochemical reactions occurring in the sensitized cellulose fibers may not at present be capable of a simple representation, general features of this system clearly present themselves. Perhaps the key feature of green plant photosynthesis is the fact that the photoreactions occur in an organized matrix. The presence of donor and acceptor molecules on spatially separated sides of the photosynthetic membrane enables the initial charge separation t o be stabilized by secondary, tertiary, etc., reactions. These reactions permit the permanent separation of charge created on opposite sides of the membrane in the reaction center during the first several picoseconds following the primary photosynthetic light reaction. Reasoning by analogy with natural photosynthesis, the photoreactions occurring in the sensitized matrices occur in a structured organized matrix. To be sure, the formal analogy with green plant photosynthesis should not be taken too far, since the highly ordered membrane/ donorlacceptor arrangement is not present by biosynthetic design. However, the confined nature of the photoredox ferric ions in the reactive host structure clearly suggests that photoreactions that might not normally occur in homogeneous phase reaction may occur in the unusual structured environment of the photosensitized cellulose. The data of this paper qualitatively support this interpretation. Mass Balance. Mass balance calculations indicate that a t most 0.01% of the substrate material is converted to volatile hydrocarbons and oxygen. This low yield is not surprising based on the restricted movement of the photosensitizing ions in the confined substrate and the reaction cavity that is created during the time course of irradiation. These ideas are further developed in the next section, Theoretical Model. The key point
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of this paper is not to optimize a photoconversion process, but to report the discovery of a new photoreaction. Theoretical Model. A model for photochemistry in organized and confining media has recently been discussed by Weiss et al.12J3 On the basis of a model originally developed by Cohen to describe reactions in crystal^,'^ Weiss et al. further discuss and develop the idea of “reaction cavity”. According to this model, the volume of a reaction cavity can have a broad distribution of volumes, depending on the host structure: crystals or cyclodextrins (small), micelles (large), surfaces (undefined in one or two dimensions.) Weiss et al. also separate cavities into three temporal groups: (1)an initial reaction cavity defined by the space in which the excited states of the reacting molecules are generated; (2) a total reaction cavity, which encompasses the space and molecular environment which the excited molecules and their intermediates explore from the time of their inception to the moment of their final product-determining steps; and (3) a final reaction cavity which includes only the sites in which the product-determining steps occur. The present experiments are in harmony with this model. In particular, the “reaction cavity” and temporal nature of the biopolymer host matrix can be thought of as changing with irradiation time. At the onset of gas production, the unirradiated samples contain the full complement of ferric ions in close proximity to the ordered cellulose matrix. The high initial rates reflect relatively high quantum yields associated with the fully charged system. However, as irradiation proceeds, the immediate reaction sites become depleted of substrate and the rate of hydrocarbon and oxygen production declines. This decline is to be expected since the ferric ions are immobilized in the cellulose substrate; local substrate utilization is not (12) Weiss, R. G.; Ramamurthy, V.; Hammond, G . S. Acc. Chem. Res. 1993,26,530-536. (13) Ramamurthy, V.; Weiss, R. G.; Hammond, G. S. In Advances in Photochemistry; Volman, D. H., Neckers, D. H., Hammond, G. S., Eds.: Wilev-Interscience: New York. 1993: Vol. 18, DD 67-234. (14) Cohen, M. D.Angew. Chem.,int. Ed.Engl.lf#5,14,386-393.
replaced by new material moving in for further reaction. It is here that the formal analogy with green plant photosynthesis reveals an important difference. The basic substrate for photosynthesis is water and carbon dioxide. These molecules freely diffuse into the plant for additional conversion. The depletion in the sensitized cellulose matrix is composed of two sources: local depletion of the surrounding carbon structure and the fact that the chloride ions of the ferric chloride solution used to sensitize the cellulose participate in the reaction. This participation is demonstrated in Tables 1-3, which provide a tentative identification of the volatile hydrocarbons produced in these photoreactions. It can be seen in Table 1that chlorinated hydrocarbons, such as chloromethane, are among the photoproducts.
Conclusions Photosensitized lignocellulosic substrates can undergo direct photoconversion to molecular oxygen and volatile hydrocarbons. This photoconversion bears a formal analogy to green plant photosynthesis in that the reaction is unambiguously endergonic. The qualitative features of the time profiles of the course of oxygen and hydrocarbon evolution can be understood in terms of a simple model employing the concept of reaction cavities. The participation of the biopolymer host matrix in the photoreaction causes the cavity volume to increase with time, causing a decline in the rate of product formation.
Acknowledgment. The authors thank P. F. Britt, J. W. Lee, and J. Woodward for comments and suggestions on the manuscript and D. J. Weaver for secretarial support. This work was supported by the ORNL Laboratory Directed Research and Development Program and the U.S. Department of Energy. Oak Ridge National Laboratory is managed by Martin Marietta Energy Systems, Inc., for the U S . Department of Energy under contract DE-AC05-840R21400. EF940056C
