Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
2001-07-18
2004-08-17
Bell, Mark L. (Department: 1755)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C423S598000, C423S610000, C423S611000, C423S612000, C423S613000, C427S453000, C427S255190, C427S255310, C428S620000, C428S623000, C428S632000, C428S660000, C428S663000, C428S323000, C428S457000, C428S469000, C428S472000
Reexamination Certificate
active
06777374
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to partial oxidation of organic compounds in the gas phase to produce valuable compounds on supported and immobilized photocatalysts which have been deposited using a flame aerosol coating method.
BACKGROUND OF THE INVENTION
The cost of handling, treating and disposing of wastes generated annually in the United States has reached as much as 2.2% of the gross domestic product, and continues to rise. The chemical manufacturing industry generates more than 1.5 billion tons of hazardous waste and 9 billion tons of non-hazardous waste annually. Roughly half of the releases and transfers of chemicals reported through the Toxic Release Inventory and 80-90% of hazardous waste generation reported through the Resource Conservation and Recovery Act (RCRA) are due to chemical manufacturing. Organic chemicals comprise the largest amount of toxic release. Many of these releases can be minimized by using better management of materials and energy, more efficient process control, optimizing process conditions, and recycling and reusing waste and byproducts. However, cleaner products methods can be achieved by adopting “green synthetic” methods.
Oxidation reactions are used in industry for producing aliphatic and aromatic aldehydes, alcohols, ketones and carboxylic acids. Generally, oxidation involves splitting of C—C or C—H bonds with concomitant formation of C—O bonds. For example, the partial oxidation of hydrocarbons by molecular oxygen to form oxygenates, which are further used as building blocks in manufacturing plastic and synthetic fibers, is an important process in the chemical industry. Oxidation reactions are usually catalyzed and are carried out in liquid or gas phase. The current processes are energy intensive, have low conversion coefficients, and generate environmentally hazardous waste and byproducts.
Current processes for producing these highly desired oxygenates require stringent operating conditions such as high temperatures and pressure, strong acids, free radicals (halogenated starting materials) and corrosive oxidants. Although these processes are currently being used, they have low energy efficiencies and generate environmentally hazardous waste and byproducts. One major reason selectivities are low is that the desired products, such as carbonyls and alcohols, are more easily oxidizable by oxygen than is the parent hydrocarbon. Over-oxidation must be minimized by maintaining conversions low, which is a serious disadvantage from the viewpoint of chemical processing and economics. Therefore, a major challenge is to find reaction pathways that produce the primary product with high selectivity and at high conversion rates for the hydrocarbons.
Most oxidations are highly exothermic and may generate high localized temperatures and hot spots on the catalyst surfaces, decreasing the service lifetime of the catalyst and resulting in over oxidation of desired products. Over oxidation can be minimized only by keeping conversions low. Therefore, a major challenge in this field is to find a reaction pathway that affords the primary product with high selectivity and a high conversion rate of the hydrocarbons. There is a strong research effort underway to meet this challenge. Unfortunately, several commonly used catalysts for oxidation reactions are toxic heavy metals such as chromium and vanadium, or strong acids such as sulfuric acid or nitric acid. Pollution is inevitable in loading, recovering, and regeneration of these catalysts. A cleaner alternative is needed.
Photocatalytic or photoactivated reactions are applicable to a wide range of valuable industrial processes, including organic synthesis, photodestruction of toxic compounds, and purification of drinking water. The anatase form of TiO
2
has been the most extensively used in photocatalytic reactions because of its high activity and chemical stability. For example, the anatase phase of titania can be used as a photocatalyst for several problems of environmental interest, as a catalyst for sulfur removal, for toxic metals capture, and as an additive in cosmetics because of its effective sunscreen properties.
The electronic structure of titania is characterized by a filled valance band and an empty conduction band. When a photon with energy exceeding the band gap energy is incident, an electron is readily excited to the conduction band, leaving a hole in the valence band. If surrounding and surface conditions are correct, the excited electron and hole pair can participate in reduction-oxidation reactions. The quantum efficiency of the semiconductor photocatalyst depends on the recombination lifetime of holes and electrons, and the rate of interfacial charge transfer. Therefore, crystal structure, grain size, and attendant surface morphologies can affect quantum efficiencies.
Recently, titania has been used in an atmospheric pressure process for coating steel substrates to provide for stainless and corrosion resistance characteristics. Several researchers have been studying different applications for titania films, relying on its self-cleaning and superhydrophilic properties. The feasability of this technology on a commercial scale has also been demonstrated by the implementation of numerous small-scale applications for treating contaminated air and water streams.
Photocatalytic oxidation of many organic molecules, including saturated hydrocarbons, by optically excited semiconductor oxides is thermodynamically feasible in the presence of oxygen at room temperature. UV light-assisted oxidation has been shown to be promising for oxidizing cyclohexane and epoxidizing small olefins. Selectivities different from those obtained by other oxidation routes have been reported, illustrating the potential of the method for syntheses, provided that that the expected product can be obtained with an acceptable yield.
The rate of photooxidation is affected by the solvent type, colloidal preparation, the oxygen concentration, catalyst surface area, and light intensity. Aromatic aldehydes have been synthesized by oxidating the methyl groups of toluene and derivatives by routes which can also yield the corresponding alcohols and carboxylic acids. Various factors controlling the yield and specificity of the reaction products have been investigated, as the photophysics of the various excited states reactions occurring at the titanium dioxide
on-aqueous solution interfaces. Such additional thermodynamic control or redox chemistry for systems containing the semiconductor catalysts has been previously ascribed to differences in that the adsorption of the various oxidizable species to be adsorbed on semiconductor surfaces.
Most studies related to synthesizing compounds using titania catalysts have been performed in the liquid phase. Sahle-Demiessie et al., in
Ind. Eng. Chem. Res.
1999, 38, 3276-3284, describe the activation and oxygenation of hydrocarbons in the gas phase by irradiating supported titania films with ultraviolet irradiation. The oxidation in the gas phase eliminates an additional separation step, as the catalyst is supported. Product adsorption can also be minimized by using slightly elevated temperatures. Furthermore, operating conditions such as feed rates, humidity, mixing conditions, and residence times can be readily controlled. One concern, especially in gas-phase reactors for complete oxidation to carbon dioxide, has been the formation of byproducts that lead to catalyst deactivation. For wide-scale applicability and commercialization of this technology, it is essential to develop reactors with well-controlled thin films that are not readily deactivated.
Designing the photocatalytic reactor with films of controlled characteristics is thus an important consideration for practical application. Several different methodologies have been used to produce titania powders and films, including wet processes, sol-gel processes, and gas phase processes. Gas phase coating processes have the advantage that no toxic liquid waste byproducts are produced, and, moreover, these properties can
Biswas Pratim
Gonzalez Michale A.
Sahle-Demessie Endalkachew
Sikdar Subhas K.
Wang Zhong-Min
Bell Mark L.
Browdy & Neimark, P.L.L.C.
Hailey Patricia L.
Kornbau Anne
The United States of America as represented by the Environmental
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