WO1991009823A1 - Photocatalytic process for degradation of organic materials in a vaporized or gaseous state - Google Patents

Abstract

A novel photocatalytic process is provided for the controlled degradation of vaporized and gaseous organic materials into environmentally compatible products comprising at least carbon dioxide. The process employs a solid catalyst comprising at least one transition element, water vapor, and molecular oxygen to form a reaction mixture with the organic material which is degraded in the presence of ultraviolet, visible, or near infrared photoenergy. The catalytic process has multiple applications including the destruction of organic solvents; the elimination of pollutants from potable water and industrial waste water; and the elimination of organic hazardous and/or toxic substances from collected wastes.

Description

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PHOTOCATALYTIC PROCESS FOR DEGRADATION OF ORGANIC MATERIALS IN A VAPORIZED OR GASEOUS STATE

FINANCIAL SUPPORT The research for the present invention was supported by a grant from Synlize, Inc.

FIELD OF THE INVENTION The present invention is directed to photo- catalytic processes and reaction methods for the controlled degradation of organic materials present in a gaseous state such that environmentally compatible reaction products alone are yielded.

BACKGROUND OF THE INVENTION

Consumer and industrial waste management has become one of the most serious and urgent problems of modern life. The severity of the problem is staggering and many proposed solutions are in direct conflict with health and safety standards for the public and with laws and policies protecting the environment. In essence, the problem may be summarized as follows: How does one treat or dispose of human and industrial waste products, natural and synthetic organic materials in the main, without further polluting air, water, and soil; and without creating additional risks and hazards caused by the formation and^' release of toxic and/or hazardous substances which often are more dangerous to the public than the original waste materials themselves. The problem becomes even more complex and difficult after recognizing the variety of organic materials found generally in waste products; and after appreciating the fact that proposed solutions effective against pollutants present in solid and/or liquid states as in water and soil are not suitable nor useful for effectively treating organic pollutants in the air and in gaseous environments generally . The most common solutions to date for treating solid and/or liquid pollutants have been and remain either land disposal or burning of the organic waste material in open-air or closed incinerators. The thermal volatilization technique, however, has major drawbacks, which include: often incomplete destruc¬ tion of the organic material; the frequently inadequate disposal of the incinerated products, if they are not released into the ambient air and environment at large; the common release of toxic products of partial combustion which when volatilized become health hazards for the employees and the public; and the general limitations and costs a process dedicated to thermal destruction of the organic material.

Other approaches for degrading or destroying organic materials (also termed "mineralizing" by some workers) also focus primarily upon eliminating waste in the solid and/or liquid states; and have, in the main, followed three divergent approaches. The first is application-oriented and seeks practical means for chemical regeneration of exhausted filter adsorption matter, most often exhausted granular activated carbon or "GAC". These techniques utilize conventional liquid solvents to desorb the organic material retained by the GAC filter and focus primarily on the ability of various solvents, aqueous and organic, to extract organic materials such as substituted benzene and phenol compounds from exhausted GAC [Posey and Kin, J... PCF 59:47-52 (1987); Martin and Ng, Water Res. 18:59-73 (1984); Martin and Ng, Water Res. 1_9_: 1527-1535 (1985); Crittenden et al., Jour. AWWA:74-84 (1987)]. It will be noted and appreciated that the single goal of this general approach is to provide regenerated GAC. There is little or no interest nor attention to the question of how to dispose of the liquid solvent after the solvent has desorbed the organic materials and regenerated the adsorbant filter matter; neither is there any application or benefit for reducing organic pollutants in the air or in a volatilized state.

The second general approach is also applica- tion-oriented and is intended primarily for treating domestic water sources for the removal of hazardous natural and/or synthetic organic compounds which are present in relatively low concentration levels, typically 50-500 parts per billion. A constant requirement and characteristic of this approach is the use of an ultraviolet light initiated reaction in combination with a chemical oxidizing agent for the degradation of the hazardous organic compounds in the water or other aqueous liquid. Either ozone gas or hydrogen peroxide is typically used as the chemical oxidizing agent. However, it will be noted and appreciated that no effective use of this approach has ever been made or contemplated for degrading organic pollutants in the air or in a vaporized state.

If ozone gas is employed as the oxidizing agent, the gas must be generated where it will be used because ozone is an unstable gas; freshly generated ozone will react in the presence of ultraviolet light with organic compounds to yield a wide diversity of oxidized carbon-containing reaction products, most often peroxides and hydroxyl derivatives of the hazardous organic compounds present initially. In addition, the rate of reaction is often dependent on ultraviolet light intensity. Representative publications describing this technique include: William H. Glaze, Enviro . Sci. Technol . 1:224-230 (1987); Masten and Butler, Ozone Science and Engineering 8_:339-353 (1987); Peyton and Glaze, "Mechanism of Photolytic Ozonation," in Photo¬ chemistry of Environmental Aquatic Systems . American Chemical Society, 1987, pages 76-88.

Alternatively, if hydrogen peroxide is employed as the chemical oxidizing agent in combination with ultraviolet light, a variety of smaller molecular weight organic compounds have been degraded partially or completely - if present initially in relatively low concentration. Major differences in the ability to degrade chemically similar compositions have been noted; and complete destruction of hazardous organic materials can occur after reaction with only simple aliphatic compounds. Representative of this technique are the publications of: Sundstrom e_t . , Hazardous Waste and Hazardous Materials 3_:101-110 (1986); Weir e al. , Hazardous Waste and Hazardous Materials _4:165-176 (1987); Koubek, E., Ind. Eng. Chem. Proc. Res. Dev. 14:348 (1975); Malaiyandi e_t al. , Water Research 14:1131 (1980); Clarke and Knowles, Effluent and Water Treatment Journal 22:335 (1982).

The third general approach is far more theoretical and research oriented. It focuses upon the photopromoted catalytic degradation of organic material in aqueous suspensions or solutions and in liquid mixtures of water and organic solvents. All of these investigations utilize molecular oxygen [0„] as an oxidizing agent in the form of oxygen saturated or aerated water in combination with a solid catalyst, most often a semiconductor transition element oxide in powder form. This reaction process, often termed "heterogeneous photocatalysis , " utilizes a continuously illuminated, photoexcitable solid catalyst to convert reactants adsorbed on the photocatalyst surface. These photocatalysts are semi-conductors which are believed to bring the reactants in the fluid into contact with electrons and/or positive holes which are generated within the solid by photons of energy equal to or higher than the band-gap of the solid catalyst [Teichner and Formenti], "Heterogeneous Photocatalysis," Photoelectrochemistry , Photocatalysis, and Photo- reactors (M. Schiavello, editor), D. Reidel Publishing Company, 1985, pages 457-489]. These investigations are particularly concerned with purification of drinking water supplies and the aquatic environment; and seek to degrade organic materials such as organo-chlorine compounds in aqueous suspensions or solutions. Accordingly, these methods do not recognize nor perceive any application for air or gaseous environments; and are generally unconcerned with organic pollutants present in a gaseous state. A representative listing of recent experiments following this third approach is provided by: Matthews, R.W., Wa_t. Res.. 2jD:569-578 (1986); Matthews, R.W., J.. Catal. 97_:565 (1986); Okamoto et al. , Bull. Chem. Soc. Jpn. 58.: 2015-2022 (1985); Pruden and Ollis, Environ. Sci. Technol. 17:628-631 (1983); Ollis e_t al. , J.. Catal. 8_8:89-96 (1984); Ollis, D.F., Environ. Sci. Technol. 19:480-484 (1985); Chang and Savage, Environ. Sci . Technol . 15:201-206 (1981); Tokumaru et. a . , "Semicon¬ ductor-Catalyzed Photoreactions of Organic Compounds," Organic Phototrans-formations in Nonhomogeneous Media (Marye Anne Fox, editor), American Chemical Society, Washington, D.C., 1985, pages 43-55; Pelizzetti e_t al. , La_ Chimica El ' Industria j57:623-625 (1985); R.L. Jolley, "Waste Management Trends: The Interface of Engineering With Chemistry and Toxicological Monitoring," Abstracts of the 193rd National Meeting of the American Chemical Society, April 5-10, 1987.

It will be noted and appreciated that the above-identified publications demonstrate and describe the often conflicting and sometimes contradictory state of knowledge and understanding regarding the capabilities and control of photo- catalytic oxidation of different organic materials in aqueous suspensions using oxygen as an oxidizing agent. Clearly, there are explicit and recurring questions regarding the activity of the metal oxide catalyst employed, for instance between highly active and less active forms of titanium dioxide. In addition, there are continuing and unresolved discrepancies and contradictions as to whether photocatalytic oxidation in water using oxygen as an oxidizing agent can provide total degradation of all classes of organic materials with a complete conversion and destruction to carbon dioxide and other products. Note that there are multiple reports in the literature providing data that only a partial oxidation of organic materials occurs in aqueous liquids, without a complete degradation into carbon dioxide [Carey e_t. a_l. , Bull. Envir . Contam. Toxic. 16:697-701 (1976); Oliver e_t a . , Envir. Sci. Technol. 1_3: 1075-1077 (1979); Hustert ejt al. , Chemosphere l_2:55-58 (1983); Olis et al. , J.. Catal. j38.:89 (1984)]. There are also other articles stating that a few chemically uncomplicated organic con¬ taminants found in liquid water can be completely mineralized under tightly controlled conditions when in the presence of a titanium dioxide catalyst illuminated with near-ultraviolet light [Barbeni et al. , Nouv. J_. d_e Chim. 8.:547 (1984); Barbeni et al. , Chemosphere l_4:195-208 (1985); Matthews, R.W., 1. Catal. 91_: 5 5 (1986) and Water Res. .20:569-578 (1986)]. Clearly, the current state of knowledge and expectations in this area is in flux and presently offers a variety of directly opposing and contrary views yet to be reconciled. A most recent and innovative line of investiga¬ tion, complimenting and substantially expanding upon the previously known general approaches, is described within U.S. Patent No. 4,861,484 issued August 29, 1989. This methodology provides a photocatalytic process for the complete degradation of organic materials into environmentally compatible products such as carbon dioxide. The organic material to be degraded is obtained in a water-containing fluid state; and then combined with a solid catalyst comprising at least one transition element and with a peroxide to form a reaction mixture. Photoenergy absorbable by the catalyst is then added to the reaction mixture in order to yield environmentally compatible degradation . products including at least carbon dioxide. The vitality and effectiveness of this procedure has been independently verified and confirmed by the publications of Tanaka e_t. a . , New. J_. Chem, 13:5-7 (1989); Anderson, J.V., "Proceedings of the Energy and Environment Conference of the Arizona Chapter of the Association of Energy Engineers," Tempe , Arizona, April, 1989; and Magrini et al. , "Proceedings of the 24th Intersociety Energy Conversion Engineering Conference," Washington,- D.C., August, 1989. Insofar as is presently known, therefore, these most recent publications represent and exemplify the current frontiers of innovation and knowledge in this field.

Nevertheless, despite the existence of these various technical and research developments, there has been no attempt to employ or consolidate the existing divergent approaches and investigative efforts for purposes of degrading pollutants in the air; nor has there been any recognition or appreciation that a single catalytic process might be suitable for degrading a wide variety of organic materials in a vapor state. Clearly, therefore, a catalytic process which will effectively degrade gaseous organic materials generally in a reliable manner, without the concurrent creation of toxic intermediates, would be recognized and appreciated today as a major advance and substantive improvement over all the presently known and available techniques.

SUMMARY OF THE INVENTION The present invention provides a photocatalytic μrocess for the degradation of organic materials in a gaseous state, this process comprising the steps of: obtaining the organic material to be degraded in a gaseous state; combining the gaseous organic material with a solid catalyst comprising at least one transition element, with molecular oxygen, and with water vapor to form a reaction mixture; and adding photoenergy absorbable at least in part by said solid catalyst energy to said reaction mixture such that environmentally compatible reaction products comprising at least carbon dioxide are yielded. The process may be utilized with all organic materials in gaseous or vaporized form without regard to actual formulation, structure, or concentration so long as adequate quantities of molecular oxygen and water vapor are present in the reaction mixture. The present invention has a plurality of applications and uses as a photocatalytic process for elimination of a wide variety of hazardous and/or toxic organic matter which can be converted, made, or obtained in a gaseous state.

DETAILED DESCRIPTION OF THE FIGURES The present methodology may be more easily and completely understood when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a view of a closed, single batch reaction apparatus useful in practicing the present invention;

Fig. 2 is a flow diagram of a continuous flow reaction apparatus useful in practicing the present invention;

Fig. 3 is a detailed frontal view of the continuous flow reaction chamber in the apparatus of Fig. 2;

Fig. 4 is a graph illustrating the photo- catalytic degradation of trichloroethylene in a static reactor as a function of time using the present invention;

Fig. 5 is a graph illustrating the effect of photoenergy upon the degradation of trichloroethylene in a static reactor as a function of time;

Fig. 6 is a graph illustrating the photo¬ catalytic degradation of trichloroethylene as a function of flow rate using a continuous flow reaction apparatus; Fig. 7 is a graph illustrating the effect of initial concentration upon the rate of degradation for trichloroethylene in a continuous flow reaction apparatus ; Fig. 8 is a graph illustrating the photo- catalytic degradation of benzene as a function of flow rate in a continuous flow reaction apparatus;

Fig. 9 is a graph illustrating the photo- catalytic degradation of toluene as a function of initial concentration using a continuous flow reaction apparatus;

Fig. 10 is a graph illustrating the photo- catalytic degradation of iso-octane as a function of varying initial concentration in a continuous flow reaction apparatus;

Fig. 11 is a graph illustrating the photo- catalytic degradation of commercial freon (C-F-C1-) as a function of flow rate using a continuous flow reaction apparatus; Fig. 12 is a graph illustrating the effect of temperature upon the degradation of trichloro¬ ethylene in a static, single batch reactor apparatus; and

Fig. 13 is a graph illustrating th-e effect of varying quantities of water vapor upon the degradation of trichloroethylene in a static, single batch reaction apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is a photocatalytic process for the controlled degradation of one or more organic materials in a gaseous state into environmentally compatible reaction products comprising at least carbon dioxide. In its most general form, this catalytic process comprises three steps: obtaining the organic material to be degraded in a gaseous state; combining the gaseous organic material with a solid catalyst comprising at least one transition element, molecular oxygen, and water vapor as a reaction mixture; and adding photoenergy in the ultraviolet and/or visible and/or near infrared regions which is absorbable at least in part by the solid catalyst to the reaction mixture to yield environmentally compatible reaction products comprising at least carbon dioxide.

Despite the superficial simplicity of the present invention in its most general definition, this unique catalytic process provides a number of major advantages and unexpected benefits for its users. These include:

(1) The reaction products produced by the present photocatalytic process provide environ¬ mentally compatible reaction products which can be released directly into the ambient environment; or can be easily controlled and quickly disposed in an environmentally safe manner. In all instances, the final reaction products will be gaseous in form; and will include carbon dioxide; and when a halogen (X) and/or hydrogen (H) and/or nitrogen (N) are present originally, will also typically include HX, water, and/or some nitrogen containing composi¬ tions such as ammonia or various nitrogen oxides. Such reaction products are environmentally compatible themselves; as in the instance of C0_ production, or are easily neutralized as in the instance of HX production by subsequent reaction to give sodium or calcium salts; or are otherwise containable. Such reaction products also avoid and eliminate the real and present dangers of releasing hazardous, toxic, or biologically non-degradable materials into the air, water, and soil as is unfortunately the present practice with waste disposal management methods today.

(2) The photocatalytic process of the present invention is able to provide a complete degradation of both simple and complex organic materials generally. Unlike methods previously known in this art, this general catalytic process will completely degrade arenes, alkanes, alkenes , aryl halides, alkyl halides, haloalkanes, alkyl aryl halides, and their derivatives into environmentally compatible reaction products in which carbon is completely converted to carbon dioxide. Moreoever, the present process is able to completely degrade organic compositions which in the past have been most difficult to eliminate on any scale. As will be empirically demonstrated hereinafter, trichloro¬ ethylene, a freon, benzene, toluene, and iso-octane are completely degraded; these hazardous and often difficult-to-eliminate materials merely exemplify the vitality of the invention.

(3) The photocatalytic process of the present invention avoids the unintended formation of hazardous and/or toxic reaction intermediates and reaction by-products. In order for a chemical process used for the destruction of toxic/hazardous organic pollutants to be truly viable and practically employed, the process intermediates and products must be of known, acceptable chemical identify and be non-toxic. In comparison, a degradation process which is capable under operating conditions of producing an exhaust containing unknown or toxic intermediate products and/or hazardous reaction by-products is neither viable nor practical. A hypothetical example of a non-viable and unacceptable process is one in which the decomposition of trichloroethylene [CC1--CHC1] yields as an inter¬ mediate or product the lethal chemical warfare agent, phosgene [CC1_=C=0]. This possible result should be directly compared with the empirically demonstrated results of degrading trichloroethylene using the present invention in which gaseous C0- and HC1 are the only reaction products.

A second, real-world example for comparison purposes is provided by the report from the Air and Energy Engineering Research Laboratory of the

Environmental Protection Agency's Office of Research and Development, EPA/600/2-86/079. This report, dated September, 1986, is authorized by M.A. Palazolo, CL. Jamogchian, J.I. Steinmetz, and D.L.

Lewis, and is entitled "Destruction of Chorinated

Hydrocarbons by Catalytic Oxidation." This report describes an investigation of the performance of an Ultrox International pilot-plant scale vapor phase reactor using an ultraviolet-light promoted oxidative process employing 0- and 0- with a proprietary catalyst for the destruction of trichloroethylene (hereinafter "TCE") and dichloro- ethylene (hereinafter "DCE") at 88°F. Although no TCE or DCE was detectable in the discharged effluent, three different reaction products totalling about

25 mole percent of the input were present. Two of these products were identified by gas chromato- graphy/mass spectrum analysis as methyl formate and methyl acetate. This operative consequence is directly comparable with but opposite in result to the empirically documented reaction products yielded by the present invention for the destruction of TCE - namely, gaseous CO- and HC1 exclusively. (4) The novel method provided herein is unique in its ability to degrade large quantities and high concentrations of organic pollutants in a vaporized or gaseous state into carbon dioxide and other environmentally compatible products regardless of the formulation or structure of these organic materials. Unlike conventionally known liquid phase irradiation techniques employing photoenergy and molecular oxygen, the present method is not limited in its applicability to only very dilute solutions (e.g., parts per million levels) of pollutants. This capability is empirically documented hereinafter and satisfies a generally recognized and long-standing need in this art.

(5) The present invention also allows for the purposeful concentration of reactants and degradation of hazardous and/or toxic organic materials in vaporized form at concentration levels never before possible. It is clear that supplying the reactants in the gas phase makes conveniently available an enormous range of reactant concentra- tions. This, in turn, provides a much broader range of options for the concentration of reactants than is generally available when the reaction involves a catalyst in contact with a liquid phase substance. For example, the concentration of molecular oxygen [0_] in the gas phase at any partial pressure of oxygen is about 100 times greater than its concentra¬ tion in a saturated aqueous solution in equilibrium with 0- at that pressure. Accordingly, the user may employ either high, moderate, or low concentra- tions of 0„ as seems best under .the actual use conditions. Similarly, the organic material to be degraded, being in a vaporized state, can be concentrated or diluted, if desired or required.

(6) The photocatalytic process of the present invention intends and expects that the gaseous organic material to be degraded be combined with molecular oxygen and water vapor. The amount of molecular oxygen required for the photocatalytic degradation mechanism to proceed must be sufficient for both a complete conversion of such carbon as is present initially into carbon dioxide; and for a conversion into H-0 of such hydrogen as is not converted into HX if a halogen is present, or into hydrides of elements other than X or C if such elements are present. The introductory gaseous stream containing the organic material to be degraded may contain a wide range of proportions of organic matter, dioxygen, water vapor, and an inert gaseous carrier such as nitrogen. Alternatively, air may be used directly as both a carrier gas and as a source for dioxygen.

(7) The photocatalytic process of the present invention is able to be utilized at ambient environ¬ ment temperatures. It is expected that the ambient temperature will vary within the extremes considered normal in the temperate zone, that is, substantially in the range from 0-40°C. In certain applications, however, the catalytic process will be employed at elevated temperatures, up to and exceeding 125°C. As will be empirically demonstrated hereinafter, the present invention may be affected by temperature in a most unusual and unexpected way; increases in temperature may directly reduce and diminish the rapidity and efficacy of the degradation process. This characteristic of the methodology, when encountered, should be considered carefully when practicing the invention; and is deemed to be a distinguishing phenomenon which differentiates the present methodology from its predecessors.

(8) By virtue of being present in vaporized or gaseous form when introduced as a reactant, there is no loss or dissipation of the organic material to be degraded, or of the molecular oxygen, or of the water vapor, individually or after combination into a mixed gaseous stream. Moreover, the internal pressure of an introductory gaseous stream carrying the mixed reactants can be easily adjusted to meet the various temperature and other reaction conditions without difficulty.

(9) The present photocatalytic process is extraordinarily and uniquely rapid in its degradation effects. As will be empirically evidenced herein¬ after, the bulk of the gaseous organic matter can be destroyed in a matter of seconds once photoenergy has been added to the prepared reaction mixture. Often the entire effective concentration of the organic material (at least 99%) is degraded in less than 3 minutes' duration. Moreover, the user can optimize^' and meaningfully reduce the total real operating time required for destroying concentrated, bulk quantities of gaseous organic matter by employing a continuous flow reactor apparatus rather than a single batch, closed volume reaction chamber.

(10) No chemical pretreatment whatsoever of the gaseous organic material to be degraded is required. This absence of pretreatment applies equally to both the gaseous organic materials themselves; and to any other vaporized adjunct materials concurrently or concomittantly resulting from the act of converting the organic material into a gaseous state. Neither the polluting organic material nor the adjunct organic matter need be pretreated or chemically broken down in order to utilize the present invention so long as each is present in a gaseous state. The broadly defined catalytic process of the present invention may be usefully employed in a minimum of steps directly; or by optional elaboration upon the minimal manipulative steps of the general process in a variety of different settings and applications. These include the destruction of organic wastes from: the chemical industry; paper and pulp mills; the petrochemical industry; the polymer and plastics industry; degreasing agents used in manufacturing and engine maintenance; spent solvents used in cleaning fabrics, metals, and machinery; spent propellants used in spray painting, and aerosols generally; organic pollutants of ground and surface waters; toxic and carcinogenic organic dielectrics; and spills of liquid hydrocarbon fuels generally . In order to more easily understand and better appreciate the environmentally compatible photo¬ catalytic process comprising the subject matter as a whole of the present invention, it is useful to describe first the details of the individual reactants and then the characteristic reaction conditions with appropriate representative empirical examples which further illustrate the variety of advantages and different use applications which the present invention provides.

A. THE INDIVIDUAL REACTANTS

The Gaseous or Vaporized Organic . Material To Be Completely Degraded

The organic material able to be degraded by the present invention is at least one, and typically a mixture of different organic compositions in admixture - all of which are present in a gaseous or- vaporized state. The source of the organic material is expected to be waste including: commercial and industrial waste products; toxic and non-toxic chemical compositions; environmentally hazardous and non-hazardous substances; volatile organic solvents; monomers or other volatile organic compounds used in industrial production of polymers; petrochemicals; fine or heavy chemicals; liquid fuels; lubricants; propellants; refrigerants; cleaning agents; and gaseous mixtures of organic matter and non-reactive carrier gases (such as molecular nitrogen or argon) in combination. A large, diverse, and varied range of chemical compositions are included within the general class of vaporized or gaseous organic material to be completely degraded. These include relatively low molecular weight saturated organic substances such as alkanes, substituted alkanes without limitation; haloalkanes and perhaloalkanes; vaporized higher molecular weight unsaturated compounds including arenes, aryl alkanes and their derivatives, aryl halides, aryl alkyl halides, olefins and haloolefins; oxygen-containing organic compounds; and nitrocompounds, amines, and other nitrogen containing classes. A representative, but non-exhaustive listing of different kinds and types of vaporized or gaseous organic material able to be degraded is provided by Table I below,

Table I

Alkanes

Straight Chain Alkanes (such as octane, decane, and hexadecane) Branched Chain Alkanes (such as isooctanes) Cycloalkanes (such as cyclohexane)

Arenes and Their Derivatives

Benzene

Alkylbenzenes (such as toluene and xylenes)

Phenol Oxygen Substituted and Carbon Substituted Alkylphenols

Aniline

Nitrogen Substituted and Carbon Substituted Alkylanilines

Catechol

Oxygen Substituted and Carbon Substituted Alkylcatechols Resorcinol

Oxygen Substituted and Carbon Substituted Alkyl- resorcinols

Hydroquinone

Oxygen Substituted and Carbon Substituted Alkylhydro- quinones

Benzyl Chloride

Alkylbenzyl Chlorides

Chlorobenzenes

Alkylchlorobenzenes Dichlorobenzenes

Alkyldichlorobenzenes

Polychlorobenzenes

Polychloroalkylbenzenes

Nitrobenzene Alkylnitrobenzenes

Dinitrotoluenes

Chlorophenols Table I ( Cont ' d )

Oxygen Substituted and Carbon Substituted Alkyl- chlorophenols

Polychlorophenols Oxygen Substituted and Carbon Substituted Alkylpoly- chlorophenols

Diphenylethylene

Stilbenes

Napththalene , Anthracene, and Phenanthrene Chloronaphthalenes

Alkylnaphthalenes

Naphthols

Oxygen Substituted and Carbon Substituted Alkylnaphthols

Chloronaphthols Benzoic Acid

Oxygen Substituted and Carbon Substituted Alkyl- benzoic Acids

Salicylic Acid

Oxygen Substituted and Carbon Substituted Alkyl- salicyclic Acids

Chlorobiphenyls

Dichlorobiphenyls

Polychlorobiphenyls

Olefins and Other Unsaturated Compounds Simple Alkenes

Alkadienes

Vinyl Chloride

Vinyl Bromide

Dichloroethylenes Trichloroethylene

Tetrachloroeth lene

Acrylonitrile

Chloroprene

Styrene Acrylamide Tab le I ( Cont ' d )

Alkylhalides

Dichloroethanes and Dibromoethanes Trichloroethanes and Tribromoethanes Tetrachloroethanes and Tetrabromoethanes

Bromofluoroalkanes and Bromochlorofluoroalkanes CFC1-, CF_C1₂, and Other Chlorofluorocarbons Methyl Chloride and Methyl Bromide Methylene Dichloride and Methylene Dibromide Chloroform and Bromoform

Carbon Tetrachloride and Carbon Tetrabromide

Examples of Other Classes

Trichloroacetic Acid

Alkyl and Aryl Thiocarbamates Alkyl and Aryl Amines

Alkyl and Aryl Mercaptans

Alkyl and Aryl Thioethers

Alkyl and Aryl Nitriles

Nitroalkanes Alcohols

Aldehydes

Ketones

Even a cursory reading of Table I will reveal many different classes of organic substances whose members have been classified as toxic or environ¬ mentally hazardous compositions by federal and state agencies such as the EPA, OSHA, and NIOSH. In addition, many of these substances are known or believed to be carcinogenic or carcinogen-promoters whose use is carefully controlled by various health and safety agencies. All of these comprise the general membership of organic material obtainable in the vapor phase and able to be completely degraded into environmentally safe reaction products comprising at least carbon dioxide; and in some cases, reaction products which are easily convertable into environmentally safe materials.

The Purity and Sources of the Gaseous or Vaporized Organic Matter

Via the very range and variety of the different kinds of toxic/hazardous organic matter able to be degraded using the present photocatalytic process, it is expected and intended that the sources of these organic pollutants will be equally diverse. In most instances, the organic pollutants will not exist in gaseous form initially; rather, the organic matter will likely appear as solids, liquids, and/or mixtures of these. In such instances, it is required that these substances be vaporized, preferably under controlled conditions, to yield gaseous organic materials of mixed or pure chemical content. For example, the state and form of the organic material to be degraded initially may appear as a liquid containing varying amounts of water. Typically, such an organic material is a solid or a mixture of solids and liquids in either pre- dominantly organic or aqueous form. Examples of predominately aqueous mixtures are ground water; polluted potable water; and industrial waste water. The common predominately organic liquid mixtures typically include industrial solvent residues; organic solvents used for dry cleaning and water proofing; organic cleansing agents and abstraction liquids in the petroleum industry and refining processes; and organic liquids used as degreasers and solvents for metals and metal deposition in the high technology industries. It will be recognized and appreciated also that many of the organic consumer and industrial waste liquids typically contain small amounts of water present inherently or obtained concomitantly as a result of their earlier uses.

It is also intended and expected that the act of conversion from the initial condensed state to a vaporized one will in many instances increase the concentration of organic materials to be degraded as a concomitant side-effect, particularly when the material is supplied in dilute aqueous form. In this manner, a mixed gaseous state reactant containing increased .concentrations of. various vaporized organic materials to be degraded can be purposely prepared which are between ten and one thousand times the concentration originally presented.

It is also expected that a number of different organic solvents individually or in combination may be present alone or with water as the initial liquid or semi-liquid carriers for the organic material to be degraded. It is recognized that these- organic solvents may also be toxic and/or hazardous in themselves and should therefore be concomitantly degraded as much as possible. Such would be the case if a particular pollutant is most effectively dissolved in a highly toxic or carcinogenic organic solvent. Under these circumstances, it is most desirable that the organic solvent itself be consumed during the degradation process in addition to destruction of the intended organic pollutant. A representative listing of organic solvents generally desired to be concomitantly degraded is provided by Table II below.

Ta bl e II

Degradable Organic Solvents

Acetonitrile

N,N-dimethylacetamide 2,2-dimethylpropionitrile

Dimethylsulfoxide

Propionitrile

Sulfolane

Acetone 1-butanol

2-butanol tert-butyl alcohol

N,N-dimethylacetamide

N,N-dimethyIformamide Ethanol

Ethanolamine

Ethyl acetate

Methanol

Methyl acetate Methyl ethyl ketone

1-propanol

2-propanol

Triethylamine

Tetrahydrofuran

Molecular Oxygen as a Co-Reactant

An essential and required co-reactant for the photocatalytic degradation process of the present invention is molecular oxygen [0-] or dioxygen as it is sometimes called. Quantitatively, the amount of molecular oxygen to be made available should be stoichiometrically sufficient for complete conversion of carbon to CO-; and for conversion of hydrogen into H-0 - if the hydrogen is not converted to HX or another compound not comprising either carbon or halogen.

The source, origin, or point of supply for the molecular oxygen is not of important or relevance. Ambient air is an excellent source of molecular oxygen for purposes of practicing the present methodology; the other gaseous components of clean, ambient air (N₂» CO-, Ar, H-0) need not be separated from the molecular oxygen prior to its use as a reactant. Alternatively, sealed tanks (large or small) containing compressed air, or mixed gases including molecular oxygen, or pure 0- can be utilized. If desired, the molecular oxygen can be generated physically or chemically in the field or on-site; and employed as a reactant immediately after its generation.

Water Vapor as a Co-Reactant

It is also an essential requirement that water vapor, H-0 in gaseous form, be present as a part of the prepared reaction mixture for degradation purposes. Quantitatively, the water vapor need be present only in a measurable amount; or in an amount sufficient to supply a stoichiometric requirement, for - hydrogen. However, it is generally useful for water vapor to be present in the range from 0.1-50% by volume in the prepared reaction mixture. Moreover, the actual purity of the water vapor employed and its source of origin are neither important nor relevant factors. Environmental air is an excellent source of water vapor without regard to the percentage of water vapor in the air, the true amount or quality* of the other gaseous components of the air, or its status (ambient, compressed, treated, filtered, etc.). If desired or required under specific use circumstances, the percentage content by volume of the water vapor in a gaseous carrying stream can be increased by purposeful evaporation of liquid water, production of steam, and any other conventionally known means for intentionally increasing water vapor content and concentration. Similarly, if the organic material to be degraded is initially in a water-containing liquid state, the act of converting the organic material into a gaseous state will typically also provide abundant quantities of water vapor as a concomitant result.

The Solid Catalyst Comprising at Least One Transition Element

The catalysts used in the present invention are solids preferably used in a powdered or subdivided form in order to expose a large surface area for reaction. Each solid catalyst contains at least one transition element ' able to absorb photoenergy of a specified type and wavelength range. Transition elements are those which, as elements or in any of their commonly occurring oxidation states, have partly filled "d" or "f" shells. The reader is presumed to have both knowledge and familiarity of the properties and characteristics of transition elements generally as these are described in F.A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry , 5th edition, John Wiley and Sons, New York, 1988 - the text of which is expressly incorporated by reference herein. Within this broad class of catalysts, solids containing oxides of transition elements are highly preferred. A repre¬ sentative, but not-inclusive listing of preferred metal oxide catalysts includes those listed in Table III below.

Tab le III

Preferred Transition Element Catalysts TiO- SrTiO- BaTiO- NiTiO- CoTiO, 3

RuO-/ 'TT:iO-

Pt/SrTiO-

Pt/BaTiO-

ZnO

^Fe2°3-^Fe3°4

^Sb2°5

W0₃ Sn-Sb-oxide

^La2°3 ^Nd2°5 ^Pr6°ll

Co -Zeolite Fe³⁺-Zeolite

Ti ⁺-Zeolite

Montmorillonite Clay Monazite Sand

Solid solutions of Bi-O- and other metal oxides having the formula MO wherein M is Nb, Ta, Mo, W, V, or Si; and wherein the ratios of Bi 2-03-:M0xare from 61:1 or 1:3 Of these, titanium dioxide and, in particular, Degussa P25 grade TiO- is most preferred. Degussa prepared titanium dioxide is primarily in the anatase form (rather than the rutile form) and has been demonstrated to be highly active in the complete degradation of alkanes, arenes, haloalkanes, haloalkenes, haloaromatic phenolics, halophenolics , and other classes of organic compounds into carbon dioxide and other products. The catalysts used in the present process may be selected and used individually or employed in combination as a mixture of two or more catalysts. The preparation of solid catalysts suitable for use in the present catalytic process are conventionally known in the art and may be prepared in accordance with their published methods of preparation or can be commercially purchased from a variety of different suppliers.

Photoenergy

The present invention is unique in its ability to photocatalytically degrade gaseous or vaporized organic material in a controlled manner into environ¬ mentally compatible reaction products comprising at least carbon dioxide. As is empirically demonstrated hereinafter, the gaseous organic materials are degraded at very rapid rates in the presence of photoenergy by combining them with molecular oxygen, water vapor, and a suitable solid catalyst. As used herein, photoenergy is defined as electromagnetic radiation of any wavelength. It will be appreciated also that the entirety of the catalytic reaction process may be conducted on demand for preset time periods, continuously or cyclically as desired or required by the user.

When photoenergy is added to the reaction mixture, it is the ultraviolet, visible, and near infrared wavelengths (200-1,000 nm) which are most effective in enhancing the activity of the catalysts. In the examples which follow herein, it will be recognized that when photoenergy was added to the reaction mixture, such photoenergy was obtained using laboratory scale lamps and illumination devices having a limited and identifiable range of light wavelengths. In larger scale industrial or commercial applications, it is expected that sunlight or artificial light from a variety of different sources may be employed to provide wavelengths in the 200-1,000 nm range. When such incident photoenergy is added, there is substantial enhancement of gaseous organic material degradation per Einstein of incident energy above those obtainable by known processes which do not utilize transition element catalysts. In addition, the use of photoenergy provides substantially increased yields of organic material degradation in comparison to those yields obtained by conventional processes.

Preferred Reaction Conditions

It is generally preferred that the catalytic process of the present invention be conducted at ambient environmental temperature, that is within the temperature range of the surrounding environment generally presumed to be between 0-40°C. Higher temperatures may be required to maintain desired partial pressures of pollutants and/or to keep pollutants in the vapor phase. Such increases in temperature will not substantially change the resulting reaction products.

Environmentally Compatible Reaction Products

The photocatalytic process of the present inven¬ tion provides complete degradation of gaseous organic material into environmentally compatible reaction products which include at least carbon dioxide in every instance. It is expected that the reaction products will evolve in a gaseous state; and will then be removed or effectively neutralized using conventional chemical means. Clearly, the exact identity of the reaction products will vary and depend upon the particular chemical formulation and structure of the gaseous organic material being degraded; and will, at least in some instances, also vary with the composition of the other vaporized organic compounds (solvents, etc.) which initially carried the particular organic material and were concomitantly converted into a gaseous state. Nevertheless, a variety of typical reactions and reaction products which are merely illustrative of the present invention as a whole are provided by Reactions A-F of Table IV below.

Table IV

CO, H₂0 + HC1

C0₂ + H-0 + HN0₃

For illustrative purposes only, the preferred catalyst, Degussa P25 titanium dioxide, is employed in each example. Because of the variety of organic materials illustratively represented by Table IV, it is useful to briefly summarize each reaction individually. Reaction A represents the complete degradation of vaporized organic solvents which are not resistant to decomposition by the present catalytic process. While carbon tetrachloride serves as the immediate example, it is clear that other vaporized chloro derivatives of saturated carbon (such as chloroform) and other halogen derivatives of carbon in a gaseous state will react in a similar fashion to yield carbon dioxide and HX (wherein X is any halogen). Reaction B is representative of saturated hydrocarbons and saturated alkyl derivatives generally which are obtained or have been converted into gaseous form. Linear, branched, and cyclic alkanes and alkyl derivatives in a gaseous state will also be completely decomposed. Reaction C represents vaporized linear and branched halo-olefins in both substituted and non-substituted form. Reaction D illustrates the complete degradation of an aromatic hydrocarbon and is intended to represent all arenes generally obtainable or convertable in gaseous form. Reaction E illustrates the complete degradation of a halo-substituted aromatic compound. Reaction F exemplifies the degradation of a class of compounds encountered as industrial waste by-products for which there is presently no safe method of decomposition and elimination; the nitrogen containing reaction products may also include other oxygen derivatives of nitrogen in addition to or instead of that specifically identified. Table IV is merely representative of the variety of photocatalytic reactions provided by the present invention. The gaseous reaction products comprise at least carbon dioxide in each and every instance; and include other environmentally compatible products which are either non-toxic and non-hazardous in themselves, or can be chemically neutralized or otherwise converted quickly and easily into environ¬ mentally compatible products.

REACTION PARAMETERS, FACTORS, CHARACTERISTICS,

AND SUPPORTING EMPIRICAL DATA To further describe the subject matter as a whole comprising the photocatalytic process of the present invention, a variety of different reaction parameters, influential factors, and identifying characteristics with supporting experiments performed in the laboratory will be described. These parameters, factors, characteristics, and laboratory scale experiments will serve to merely illustrate the diversity of applications, formats, and gaseous organic materials which can be catalytically degraded by the present process. While the equipment, experimental design, and empirical data are solely in laboratory scale terms, it is clear that the described reactions can be expanded at will to meet the diversity and the scale of industrial and commercial operations. In addition, it will be expressly understood that while a limited number of different gaseous organic materials are degraded using the preferred transition element catalyst, water vapor, molecular oxygen, and ultraviolet light energy - these empirical details do not either restrict or limit the present invention in any way. To the contrary, the described empirical experiments are merely representative of the number, variety, and diversity of gaseous organic materials and reactive conditions which can be advantageously employed using the present photocatalytic process.

Single Batch Reaction and Continuous Flow Reaction Capabilities

One characteristic feature of the present methodology is its ability to be performed either as a single batch reaction or as a continuous flow reaction process. In the laboratory, two different kinds of apparatus were employed to demonstrate each capability. The single batch reaction system is illustrated using the apparatus shown in Fig. 1. As seen therein, the apparatus includes a pyrex flask 10, typically of one liter volume, having a plurality of ports or openings 12a-12c. The internal surface of the reaction flask 10, has been coated with a solid catalyst 14 comprising at least one transition element; and experimentally was always Degussa P25 titanium oxide (TiO-) which was prepared and introduced onto the internal surface of the reaction flask as will be described hereinafter. Through one port 12a, a thermometer 16 has been inserted into the interior volume of the reaction flask 10. Through the port 12b, an ultraviolet lamp 18^' has been inserted into the interior of the pyrex flask 10 such that ultraviolet photoenergy will be radiated throughout the interior of the reaction flask. The third port 12c supports the inlet 24 for the introduction of a gaseous stream comprising the organic material in a gaseous state to be degraded, the water vapor, and the molecular oxygen. After the gaseous reactants have been intro¬ duced via the inlet 24 into the interior of the reaction flask 10, the port 12c is sealed using a valve or other conventional means effective to achieve this purpose. The reaction flask 10 is itself disposed within a heating mantle 20 which, in turn, is electrically connected to a transformer 22 a shown. The heating mantle 20 and transformer 22 serve as the heating means to provide and maintain a prechosen temperature at which the degradation reaction is to proceed. Upon completion of the intended experiment, the resulting gaseous reaction products are withdrawn for analysis by gas chromato- graphy, ion-selective potentiometric analysis, or other analytical assays as will be described hereinafter . In comparison, the continuous flow reaction apparatus employed experimentally is illustrated by Figs. 2 and 3 respectively. Fig. 2 is a flow diagram of the continuous reactor apparatus employed for the experiments to be described hereinafter. As shown, an introductory (or eluent) gaseous stream comprising air or a carrier gas mixed with molecular oxygen is introduced through a piping and tubing system 31 including a pressure regulator 40, a plurality of flow controller valves 42, and a plurality of flow meters 44. The gaseous organic material to be degraded and water vapor are contained within thermostatically controlled chambers 46 and are intermixed with the gaseous stream containing molecular oxygen by piping 33 and the three-way valve 50. The eluent gaseous stream now containing the organic material to be degraded, the water vapor, and molecular oxygen are introduced via flow tubing 35 to the interior of the reaction chamber 52. After an appropriate reaction time, the resulting reaction products are discharged as an effluent via tubing 37 into a water trap 54 for any gaseous acids in the effluent stream. The gaseous effluent then proceeds via tubing 39 through another three-way valve 42 directly to a gas chromatograph 56 for analysis of the reaction products in the effluent gaseous stream. When valves 42 and 50 are appropriately set, the inlet gas composition can also be analyzed by gas chromatography.

As noted previously herein, ' this apparatus and design is merely of laboratory scale and convenience; many other different configurations and designs for a continuous flow apparatus and reactors are deemded possible and expected. Neverthe¬ less, the system illustrated by Fig. 2 is intended to simulate practical use conditions. The initial gaseous stream of air from the compressed tank source is split into three portions, one of which goes through a vessel to pick up the desired amount of organic substance to be degraded at a precisely controlled temperature. By controlling the tempera¬ ture and the ratio of flow rates for each gaseous stream, the composition of the gaseous stream can be altered over a wide range from a few percent to a few parts per million (ppm) allowing a huge variety of test conditions to be performed at will. As noted, the gaseous stream carrying mixtures of organic pollutant, water vapor, and molecular oxygen is introduced into the photoreactor chamber; the reaction allowed to proceed; and the effluent gaseous stream is discharged through a trap and directed subsequently through analytical apparatus to detect and evaluate the totality of resulting reaction products accurately. A detailed view of the photoreactor chamber 52 is provided by Fig. 3. The photoreactor 52 is formed of two quartz shells or vessels 60, 62 wherein one vessel 60 has been- inserted into the spatial interior of the other vessel 62. An inlet 64 and an outlet 66 are mounted at the two ends of the quartz shells. A solid catalyst 68 comprising at least one transition element is disposed as a coating onto the inner surface of the quartz vessel 62. In actual practice, the quartz shells have outer dimensions of thirty two centimeters length and 6 centimeters/5 centimeters outer/inner diameters. A 13 watt, 254 nm ultraviolet lamp 70 is introduced into the interior volume of the quartz vessel 60. The exterior of the quartz vessel 62 is wrapped by electrical heating tape 76 and a layer of insula¬ tion 78. The reaction temperature is thus controlled by the heat released by the heating tape 76 and the cooling effect introduced by the air jet 72 within the inner quartz vessel.

In practice, an eluent gaseous stream containing a mixture of gaseous reactants is introduced into the confined space 80 formed by the individual walls of the two quartz vessels 60, 62. The eluent gaseous stream is introduced via the inlet 64 at the bottom and then passes through the confined space 80 where reactive contact is made with the solid catalyst coating 68. As the gaseous reactive stream continues over the length of the confined space 80, the degradation reaction proceeds at a controlled flow rate and temperature. Upon completing the entire circuit distance provided by the confined space 80 between the two quartz vessel walls 62, 60, the gaseous effluent stream is discharged and removed via the outlet 66 for immediate analysis. In this manner, new reactants are continuously introduced in a gaseous stream via the inlet 64, are processed continuously within the confined space 80 in a controlled manner; and are subsequently discharged and removed via the outlet 66 upon completion of the desired reaction duration.

All the experiments described hereinafter employ the preferred solid catalyst, Degussa P25 TiO- , as an aqueous slurry which was coated fully or partially onto a solid support material such as the internal surface of the reaction flask. Usually, about 1.0 gram of Degussa P25 TiO- powdered catalyst (average size of particles being about 30 μm was slurried into about 20.0 ml of water; and this prepared slurry was coated onto the interior surfaces of the glass or quartz reaction vessel. The reaction vessel was rotated by hand to uniformly cover the interior surfaces, followed by air evaporation of the slurry water. The coated catalyst was completely dried and conditioned at 125°C under vacuum for 12 hours before being exposed to the reactants.

The reaction chamber was protected from extraneous light by careful masking of the^' exterior surfaces. Photoenergy was supplied to the prepared gaseous reaction mixture internally using a low pressure mercury lamp. The progress of the degrada¬ tion reactions was monitored by routinely measuring the amounts of gaseous organic pollutants in the introductory eluent and remaining within the discharged effluent gaseous stream. The organic pollutants, effluent, CO-., and any other carbon-con¬ taining products were detected, identified, and quantified by gas chromatography routinely. The GOW-MAC series 55C gas chromatography apparatus used a 6 foot by 1/8 inch Porapak Q column and thermal conductivity detection. Chloride ion yielded from mineralization of chloro-organic pollutants within the reaction cell and collected in the water trap was detected and quantified using an Orion chloride-ion-selective electrode in conjunction with a Radiometer PHM-85 meter. The individual procedures and conditions of assay for each of these analytical methods and apparatus is known and conven- tional in the art; accordingly, it is deemed that none of their assay specifics or details need be recited herein.

Experimental Series 1: Decomposition of TCE Vapor in a Static Reactor; Dependence on Time of Reaction, Concentration of TCE Vapor, and

UV Irradiation

The single batch reactor apparatus illustrated within Fig. 1 was employed. An introductory gaseous stream comprising 50% air (including molecular oxygen), and varying amounts of trichloroethylene (hereinafter "TCE") vapor and water vapor was introduced into the interior of a 1,050 ml reaction flask; 60% of the interior surface of the flask had been previously coated with 1.00 gram of TiO- catalyst. The internal reaction temperature within the flask was maintained uniformly at 125°C and a 13 watt, 254 nm lamp was employed to irradiate the interior of the coated flask. The concentration of TCE vapor in the introductory (eluent) gaseous stream was intentionally varied from about 1,000 ppm_. to about 5.0%. The results over varying reaction times ranging from 0-3 minutes are graphically illustrated by Fig. 4 for TCE at an introductory concentration of 1,000 parts per million.

As noted by Fig. 4, a three minute duration of reaction reduces the total TCE content to only 50 ppm. When the experiment as repeated using the higher introductory concentration values of 5% TCE, 45% water vapor, and 50% air, 90% of the TCE was converted in only one minute's reaction time. This observed rate corresponds to the capability of decomposing 11-12 grams of TCE per hour using a 13 watt, 254 nm lamp. In addition, analysis of the resulting reaction, products within the effluent gaseous stream revealed HC1 and CO- as the only compositions present.

Another set of experiments was conducted to demonstrate the necessity of photoenergy and the presence of a photocatalytic reaction as the mechanism by which the TCE is degraded. In this instance also, the closed, single batch reactor system and apparatus was employed; and a reaction performed at 100°C with an initial TCE concentration of 146 ppm was utilized. The introductory gaseous stream contained 5% water vapor and 95% air. Identical experiments were conducted in the presence of and in the absence, of 254 nm radiation over a 1.0 g film of TiO- . The results are graphically illustrated by Fig. 5.

As noted within Fig. 5, conversion of TCE was measured after 60 seconds or sooner. Within this reaction time, the identical reaction mixtures maintained in darkness showed very little degradation of TCE. In comparison, upon the addition of 254 nm photoenergy to the prepared reaction mixtures, the TCE was degraded at an extremely rapid rate; and resulted in a greater than 90% conversion rate and degradation of TCE at the end of 60 seconds reaction time. The effect and necessity of adding photoenergy light to the prepared reaction mixture is thus deemed to be unequivocally proven.

Experimental Series 2: Decomposition of TCE in a Continuous Flow Reactor

Influence of Reactor Residence Time on Degradation of TCE With and Without Photoenergy

The continuous flow reaction system represented by Figs. 2 and 3 was used with a reactor volume of 210 ml. The dependence of TCE degradation on flow rate (i.e., reactor residence time) was evaluated by introducing an initial concentration of TCE of 650 ppm in air as the eluent gaseous stream at varying flow rates from 0-800 ml//min respec¬ tively. The reaction conditions maintained were: water vapor at a concentration of 2,400 ppm; a reaction temperature of 100°C; and 254 nm photoenergy from a 13 watt lamp. The weight of the Ti0_ catalytic coating was 0.4 g. The introductory gaseous stream content and the discharged effluent gaseous stream were analyzed for carbon dioxide and TCE content via gas chromatography; chloride content of water trap 54 was analyzed by ion-selective electrode measurement and confirmed by pH measurement. The results are provided by Fig. 6.

As seen within Fig. 6, the faster the flow rate, the less the percentage degradation of TCE. It will be appreciated that better than 90% conversion rates for TCE were obtained when the flow rate was approximately 300 ml/min or less, providing a reactor residence time of 42 seconds or more. In addition, regardless of the flow rate and the rate of photocatalytic degradation, the reaction products found always were HC1 and C0_ exclusively.

When the experiment illustrated by Fig. 6 was repeated in the absence of photoenergy, a very different result was obtained. The conditions and measurements were identical except that the lamp was turned off and the flow of reactants proceeded in the dark. No degradation of TCE was detected in all the experiments conducted in the dark. Clearly, therefore, photoenergy absorbable by the catalyst is an essential part of the reaction in order that a continuous-flow degradation system be operative.

Influence of Initial Concentration Upon TCE Degradation

Another experimental series was conducted under similar continuous-flow reaction conditions during which the initial concentration of TCE in the introductory gaseous stream was varied. In this experimental series, the flow rate was maintained consistently at 115 ml/min (residence time 110 seconds) while all other reaction conditions including irradiation at 254 nm were maintained as before. The results are graphically illustrated by Fig. 7.

As noted within Fig. 7, greater than 80% degradation was obtained when the concentration of TCE was approximately 145 ppm or more. As the initial concentration of TCE was increased, the percentage degradation over the identical reactor residence time also increased significantly. Analysis of the reaction products in the discharged effluent, stream revealed that only HC1 and CO- were present. Effect of Hydrogen Peroxide as a Co-Reactant in Degradation of TCE

A limited experimental series was conducted using the continuous flow apparatus illustrated by Figs. 2 and 3 in which air remained the carrier gas for the introductory gaseous stream, the reactor volume was 210 ml, and irradiation was at 254 nm using a 13 watt lamp. The water was replaced by 30% hydrogen peroxide within the evaporator apparatus and approximately 1,000 ppm of mixed water vapor and hydrogen peroxide vapor was incorporated as part of the introductory gaseous stream. Experiments were then conducted using 140 ppm of TCE as the organic material to be degraded at 100°C with a flow rate of 86 ml per minute (corresponding to a reactor residence time of 2.4 minutes). The introduction of hydrogen peroxide vapor into the reaction mixture was found to slightly reduce the percent of TCE degraded as compared to the results obtained in the absence of hydrogen peroxide under otherwise identical conditions (e.g., 90-92% vs. 93-94%). Accordingly, the use of hydrogen peroxide as a co-reactant serves no useful purpose in this reaction system. H-0- vapor should not be- employed as a viable substitute in place of the requisite water vapor component within the reaction mixture defined herein.

Experimental Series 3: Comparison of TCE Degradation in the Vapor State and Liquid States Under Similar Conditions

To empirically demonstrate the major, substan¬ tive differences between the present vapor state methodology and superficially similar liquid state degradation reactions, a series of experiments comparing the decomposition of TCE was undertaken using the static reaction apparatus illustrated by Fig. 1 for vapor phase decomposition. Three series of experiments were performed under substantially similar test conditions. Series A represented the vapor state degradation reaction comprising the present invention. The eluent gaseous stream introduced into the reactor contained 89 mole % air, 5 mole % water vapor, and 6 mole % (2.22 millimoles) TCE in a reactor volume of 1.05 liters. Series B comprised a liquid state reaction mixture in 0.10 liter of liquid water containing 0.1 mole % TCE, (5.5 millimoles) 0.006 ml of liquid "Tide" detergent, and 0.1 gram of suspended Ti0₂ catalyst saturated with air at one atmosphere. Series C was a liquid state reaction mixture in 0.10 liter of 3% aqueous H-0- containing 0.1 mole % TCE (5.5 millimoles) 0.006 ml of liquid "Tide" detergent, and 0.1 gram of suspended TiO- catalyst saturated with air at one atmosphere. Each reaction mixture was irradiated at 254 nm; the indicated difference in initial TCE concentration and reaction tempera¬ tures among Series A and Series B and C are deemed to be minor, non-controlling, and non-decisive factors. The results are given by Table V below.

Ta b le V

A^a B^b C^C

Vapor Liquid Liquid

State State State (Absence (Presence of H-Q-) 3% H₂0₂

Amount of TCE in 2.22 5.5 5.5

Reactor, millimole

Cone, of TCE, millimole/L 2.11 55 55 Input Power of 254 nm 13 12 12

Lamp, W

Wt. of Ti0₂/Reaction, 1.0 1.0 1.0

__

Volume, g/L Temp, °C Rate of Decomp. of TCE, millimole/min Initial Fractional Rate of of Decomp. of TCE, %/min (Vapor/Liquid) Initial Rate Ratio

(Vapor/Liquid) Initial

Fractional Rate Ratio

Vapor composition 89 mole % air, 5 mole % water vapor, 6 mole % TCE; reaction volume, 1.05 L.

Saturated with air at 1 atm.; reaction in 0.10 L of water containing 0.006 mL of liquid "Tide" and 0.1 g of suspended TiO,-.

Saturated with air at 1 atm.; reaction in 0.10L of

3% aqueous H-O- containing 0.006 ml of liquid "Tide" and 0.1 g of suspended TiO-. d Dipping lamp, immersed in reaction mixture. e DeGussa P-25 TiO-; mostly anatase. f From data summarized in Experimental Series 5, "Flow Reactor" section, it can be inferred that the rate of decomposition of TCE in the vapor phase under condition A is somewhat higher at 25°C than at 100°C. Experimentally, it will be noted that a general difference between vapor phase and liquid reactions of organic pollutants is that in the liquid degrada¬ tion (with 0-/H-0-) TiO- becomes discolored in 1 hour of irradiation while in the vapor degradation no change in color (or activity) of TiO- occurs in at least 100 hrs of irradiation.

A close reading and careful evaluation of the data provided by Table V reveals that the vapor phase reaction system degrades TCE at a rate approximately 10,000 times greater than the liquid state systems. This overwhelming difference in degradation activity is a consistent and uniform characteristic of the vapor phase degradation reaction as a whole; and is not limited or restricted to any individual organic pollutant (such as TCE) or any single organic chemical class. To the contrary, the data of Table V are empirical evidence that the vapor phase degradation reaction system provides major, unexpected degradation power, speed, and advantage over other superficially similar systems limited to liquid phase catalytic reactions.

Experimental Series 4: Decomposition of Benzene, Toluene, Iso-Octane, and Trichlorofluoroethane in a Continuous-Flow Reactor

This experimental series utilized the continuous flow reaction apparatus of Figs. 2 and 3 respectively for each experiment with a reactor volume of 210 ml. For this series, the introductory gaseous stream comprised tank air as the carrier gas and source of dioxygen, vaporized organic material to be degraded, and water vapor. Four different organic pollutants were evaluated using the continuous flow apparatus. These were: benzene, toluene, iso-octane, and a commercially sold freon, trichlorofluoroethane . The reaction temperature was 25°C in all instances.

The photocatalytic destruction of benzene is illustrated by Fig. 8. The reaction conditions were: an initial benzene concentration of 390 ppm; water vapor at 2,410 ppm; 99.7% air; 0.4 g Ti0_ ; a reaction temperature maintained at 25°C; a reaction chamber volume of 210 ml; and a 13 watt, 254 nm radiation source.

Fig. 8 illustrates the photocatalytic destruc¬ tion of benzene at different flow rates between 5 and 120 ml/min, which corresponds to a reactor residence time between 42 minutes and 105 seconds respectively. The experimental data presented therein demonstrate the generic principle that benzene in major quantities can be removed effectively using a continuous flow system at the laboratory scale.

Fig. 9 illustrates the photocatalytic decomposi- tion of toluene at different initial concentrations. The flow rate of the reaction apparatus was 15 ml/min (corresponding to a reactor residence time of 14 minutes). The eluent gaseous stream comprised 4,000 ppm water vapor, 99.6% air. The gaseous stream was passed through a 210 ml reactor volume at 25°C and irradiated at 254 nm by a 13 watt lamp. 17% of the toluene was destroyed under the conditions of Fig. 8 except that the flow rate was 1.9 ml/min (a reaction residence time of 1 minute, 56 seconds) with an initial concentration of toluene of 310 ppm. The only detectable carbon-containing degradation product was CO- which was formed in amounts stoichio- metrically equivalent to the quantities of toluene decomposed . The photocatalytic decomposition of iso-octane is illustrated by Fig. 10. The reaction conditions were: a continuous flow rate of 120 ml/min; a water vapor content of 2,410 ppm; tank air 99.7%; a Ti0_ catalyst coating of 0.4 grams; a reaction volume of 210 ml; and a 254 nm radiation source of 13 watts. The reaction temperature was maintained uniformly at 25°C throughout each experiment. A complete conversion of iso-octane to CO- ws observed.

In addition, a variety of different flow rates and their consequences were used to further characterize the degradation reaction. This empirical data is provided by Table VI below.

Tabl e VI

CONVERSION OF ISO-OCTANE AT DIFFERENT FLOW RATES

Reaction Conditions: initial concentration of iso-octane 670 ppm H-0 content 2,410 ppm temperature 25°C

254 nm radiation source 13 w TiO. 0.4 g reactor volume 210 ml

The data shows both surprising and intriguing results. It will be noted that a complete conversion of 100% was obtained when the rate of flow was held at 9.2 ml/min (reactor residence time 23 minutes) or at flow rates lower than this speed. The reaction products detected showed that only carbon dioxide was yielded. No other composition whatsoever was detected in the effluent gaseous stream; and the measured yields of CO- were equivalent within experimental error to the consumed iso-octane.

Similarly, the photocatalytic destruction of trichlorofluoroethane , a commercial freon, is illustrated by Fig. 11. The reaction conditions included: an intial freon concentration of 2,200 ppm; water vapor at 2,410 ppm; tank air at 99.5%; Ti0_ catalyst at 0.400 grams; a reactor volume of 210 ml; and a uniform temperature for reaction of 25°C.

The data obtained reveals that 60% degradation of the freon was obtained when the flow rate was 14 ml/min (a reactor residence time of 15 minutes). The higher the flow rate, the smaller the percentage conversion, and the higher the freon content in the discharged effluent stream. Analysis of the final reaction products demonstrated amounts of HC1 corresponding to the quantities of freon destroyed.

Experimental Series 5: Effect of Temperature Variation on Decomposition of TCE

The effect and influence of temperature upon the efficacy of the photocatalytic degradation process described herein is both unusual and drastic. This has been documented in several ways. An initial experimental series was conducted using the single batch reaction apparatus of Fig. 1.

Initially, it should be noted and appreciated that the single static system apparatus was employed experimentally for the degradation of TCE as illustrated by Fig. 4 in which the experiment was conducted at 125°C and by Fig. 5 which represents results obtained at 100°C. Subsequent experiments utilizing the continuous flow apparatus and degrada- tion process were evaluated in experiments illustrated by Figs. 6 and 7 respectively during which the reaction temperature was uniformly maintained at 100°C. Meaningful discrepancies in the degradation rates for TCE were recognized by comparing the empirical data of these experiments which suggested that increases in temperature actually hindered and reduced the efficacy of the photocatalytic degradation process. Accordingly, another experimental series was conducted in which the effect of temperature was directly evaluated.

In this experimental series using a single batch static reactor system, the reaction conditions maintained uniformly were: a 200 _^ul amount of liquid TCE (corresponding to a vapor phase concentration of 5.9-7.4% over the temperature range employed); a reaction chamber volume of 1,050 ml; a ratio of combined TCE vapor and water vapor to air of 1:1 by volume; and a reaction time period of 3 minutes. The reaction temperature was purposefully varied from 100-190°C. The results are graphically illustrated by Fig. 12.

It is unmistakably clear that, for the temperature range empirically evaluated, the 100°.C value yielded a far greater rate of degradation than at 125°C. When the reaction temperature was increased to 150°C, there was an additional small decrease in reaction rate. No further meaningful decrease in the rate of the degradation reaction was seen at 190°C. The empirical data also reveal that at the lowest temperature evaluated within this experimental series, 100°C, the initial rate of degradation was about 380 micromoles per minute.

Another series of experiments was conducted using the continuous-flow reaction apparatus under identical conditions - except for the reaction temperature. The introductory gaseous stream had a flow rate of 115 ml/min with 115 ppm of TCE, 2,410 ppm water vapor, and 99.7% air. The resulting data revealed a conversion of greater than 97-93% at 25°C compared with a conversion of 93-94% at 100°C. The data thus clearly demonstrates that the reaction rate at a temperature of 25°C is meaningfully greater to the rate of elevated reaction temperatures of 100°C or greater.

Experimental Series 6: Effect of Varying Water

Vapor Content

The ratio of water vapor to air or molecular oxygen in the introductory gaseous stream has been found to be a very important factor. This was demonstrated empirically.

For part of this experimental series, the single batch, static reaction apparatus of Fig. 1 was employed for the destruction of TCE. The reaction conditions were: an initial 200 ul amount of TCE corresponding to a vapor phase concentration of 5.9% at the reaction temperature of 100°C; and .a reaction chamber volume of 1,050 ml. The percentage of water vapor in the introductory gaseous stream

- 55 - was the variable element. The balance of the vapor phase was air. The results are illustrated by Fig. 13.

As the graphic data shows, it is far more 5 desirable to have the lower concentration of water vapor (5%) in the introductory gaseous stream rather than the larger concentrations (50%-92%). Fig. 13 reveals that 5% water vapor in the introductory gaseous stream yielded decomposition of 1.5 micro-

10 moles of TCE during the one minute's duration of the experiment. This result corresponds to decomposing 68% of the TCE quantity initially present.

In another part of this experimental series,

15 iso-octane was decomposed using the continuous-flow apparatus previously described herein. The test conditions and resulting data are summarized within Table VII below.

Table VII

Reaction conditions temperature 25°C lamp 254 nm, 13 W reactor volume 210 mL TiO- 0.4 g

Air 99.7 - 99.995%

The data of Table VII show that a relatively small amount of water vapor (2,410 ppm) greatly accelerates the photocatalytic degradation of iso-octane in comparison to a complete absence of water vapor in the reaction mixture. It will be noted also that when comparable experiments sub¬ stituting toluene in place of TCE were conducted, very similar degradation results were obtained.

The present invention is not to be restricted in form nor limited in scope except by the claims appended hereto.

Claims

What we claim is:

1. A photocatalytic process for the degradation of an organic material into environmentally compatible products, said process comprising the steps of: obtaining the organic material to be degraded in a gaseous state; combining said gaseous organic material with a solid catalyst comprising at least one transition element, water vapor, and molecular oxygen as a reaction mixture; and adding photoenergy absorbable at least in part by said solid catalyst to said reaction mixture to yield environmentally compatible reaction products comprising at least carbon dioxide.

2. The photocatalytic process as recited in claim 1 wherein said gaseous organic mixture is obtained by vaporization.

3. The photocatalytic process as recited in claim 1 wherein said gaseous organic material is obtained by vaporizing a solvent-containing organic material.

4. The photocatalytic process as recited in claim 1 wherein' said gaseous organic material is carried by a non-reactive carrier gas.

5. The photocatalytic process as recited in claim 1 wherein said gaseous organic material is combined with said water vapor and said molecular oxygen via a non-reactive carrier gas.

6. The photocatalytic process as recited in claim 1 wherein said gaseous organic material to be degraded comprises a mixture of different compositions.

7. The photocatalytic process as recited in claim 1 wherein said gaseous organic material to be degraded comprises at least one toxic substance.

8. The photocatalytic process as recited in claim 1 wherein said gaseous organic material to be degraded comprises pollutants of water.

9. the photocatalytic process as recited in claim 1 wherein said photoenergy is selected from the group consisting of ultraviolet, visible, and near infrared light wavelengths.

10. The photocatalytic process as recited in claim 1 wherein said solid catalyst comprises at least one oxide of a transition element.

11. The photocatalytic process as recited in claim 1 wherein said solid catalyst is titanium dioxide.