Patent Publication Number: US-2011052462-A1

Title: Filters for removal of volatile siloxanes and lifetime extension of photocatalytic devices

Description:
BACKGROUND 
     This invention relates generally to the use of ultraviolet photocatalytic oxidation (UV-PCO) technology for the improved decontamination of fluids in fluid purifier systems, especially in air purification systems. More specifically, the present invention relates to removing volatile silicon-containing compounds (VSCCs) by enhancing a surface characteristic of a VSCC filter located upstream of a photocatalyst in a UV-PCO air purifier system. 
     Some buildings utilize air purification systems to remove airborne substances such as benzene, formaldehyde, and other contaminants from the air supply. Some of these purification systems include photocatalytic reactors that utilize a substrate or cartridge containing a photocatalyst, generally an oxide-based semiconductor. When placed under an appropriate light source, typically a UV light source, the photocatalyst oxide interacts with airborne water molecules to form hydroxyl radicals or other active species. The hydroxyl radicals then attack the contaminants and initiate an oxidation reaction that converts the contaminants into less harmful compounds, such as water and carbon dioxide. It is further believed that the combination of water vapor, suitably energetic photons and a photocatalyst also generates an active oxygen agent like hydrogen peroxide as suggested by W. Kubo and T. Tatsuma, 20 Analytical Sciences 591-93 (2004). 
     A commonly used UV photocatalyst is titanium dioxide (TiO 2 ), otherwise referred to as titania. Degussa P25 titania and tungsten oxide grafted titania catalysts (such as tungsten oxide on P25) have been found to be especially effective at removing organic contaminants under UV light sources. See U.S. Pat. No. 7,255,831 “Tungsten Oxide/Titanium Dioxide Photocatalyst for Improving Indoor Air Quality” by Wei et al. 
     A problem with air purification systems using UV-PCO technology has arisen. Currently available systems exhibit a significant loss in catalytic ability over time. This loss of catalytic ability has been at least partially attributed to volatile silicon-containing compounds (VSCCs), such as certain siloxanes, present in the air. 
     The aggregate amount of volatile organic compounds (VOCs) in air is typically on the order of 1 part per million by volume. In contrast, VSCC concentrations are typically two or more orders of magnitude lower. These VSCCs arise primarily from the use of certain personal care products, such as deodorants, shampoos and the like, or certain cleaning products or dry cleaning fluids, although they can also arise from the use of room temperature vulcanizing (RTV) silicone caulks, adhesives, lubricants, and the like. When these silicon-containing compounds are oxidized on the photocatalyst of a UV-PCO system, they form relatively non-volatile compounds containing silicon and oxygen that may deactivate the photocatalyst. Examples of non-volatile compounds of silicon and oxygen include silicon dioxide, silicon oxide hydroxide, silicon hydroxide, high order polysiloxanes, and the like. These compounds may be at least partially hydrated or hydroxylated when water vapor is present. Increasing the catalyst surface area alone does not necessarily slow the rate of deactivation as might be expected if the deactivation occurred by direct physical blockage of the active sites by the resultant non-volatile compound containing silicon and oxygen. 
     There is a need for improved UV-PCO systems that can aid in the elimination of fluid borne contaminants in a fluid purifier and can operate effectively in the presence of typically encountered levels of VSCCs such as siloxanes. 
     SUMMARY 
     An ultraviolet photocatalytic oxidation air purification system includes a VSCC filter upstream of the photocatalyst. An additive is bonded to or specifically associated with the surface of the VSCC filter to enhance removal of VSCCs by enhancing a surface characteristic of the filter. The additive enhances removal by creating an acidic site, by increasing the surface area of the filter, or by facilitating preferential interaction between the surface and the VSCC, thereby promoting the VSCCs to bond with the VSCC filter. During times of non-operation, removed VSCCs may be further immobilized on the VSCC filter by mineralization. The removal of the VSCCs upstream of the photocatalyst increases the useful life of the photocatalyst. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an ultraviolet photocatalytic oxidation air purification system. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic diagram of an ultraviolet photocatalytic oxidation air purification system  10 , which includes inlet  12 , outlet  14 , optional prefilter  16 , VSCC filter  18 , filter surface treatment  20 , photocatalytic reactor  22  (which includes substrate  24 , photocatalytic coating  26 , and UV source  28 ), fan  30 , mineralization unit  32 , and controller  34 . 
     Ambient air is drawn into system  10  through inlet  12  by fan  30 . Airstream A passes through prefilter  16  and VSCC filter  18  and then through photocatalyst reactor  22  and fan  30  to outlet  14 . Prefilter  16  removes dust and particles by trapping the dust and particles. VSCC filter  18  removes volatile silicon-containing compounds (VSCCs) so they do not reach photocatalytic coating  26  and degrade performance of photocatalytic reactor  22 . Other volatile organic compounds may also be removed by adsorption on either filter. Although  FIG. 1  depicts prefilter  16  and VSCC filter  18  as separate structures, they may be incorporated into a single filter that performs the functions of prefilter  16  and VSCC filter  18 . 
     Within photocatalyst reactor  22 , ultraviolet radiation from UV source  28  is absorbed by photocatalyst coating  26 , which causes photocatalyst coating  26  to interact with airborne water molecules to produce reactive species such as hydroxyl radicals, hydrogen peroxide, hydrogen peroxide radicals and superoxide ions. These reactive species interact with VOCs in the air to transform the VOCs into byproducts such as carbon dioxide and water. Therefore, airstream A contains less contaminates as it exits system  10  through outlet  14  than it contained when it entered system  10  through inlet  12 . 
     In  FIG. 1 , substrate  24  is depicted schematically as a flat plate. In practice, substrate  24  can take a number of different forms, which may be configured to maximize the surface area on which photocatalytic coating  26  is located or to maximize the extent of non-laminar (e.g. turbulent) flow through the substrate. One example is a honeycomb structure on which photocatalyic coating  26  is deposited and through which airstream A passes. 
     Experimental evidence has highlighted the need to protect the photocatalyst coating  26  from contamination and deactivation by VSCCs. The major source of these VSCCs is believed to be the family of compounds containing volatile methyl siloxanes (VMS) commonly found in cleaners, personal deodorants, shampoos, and a variety of other personal and commercial products. Common members of the VMS family include hexamethyl cyclotrisiloxane and octamethyl cyclotetrasiloxane, often referred to D 3  and D 4 , respectively. Larger molecules D 5 , D 6 , D 7  and so on are also known. Related VSCCs, including linear siloxanes, are also of concern. Cyclic siloxanes can generally be described as D x , where D defines the difunctional unit [—(CH 3 ) 2 Si—O—] and the subscript x represents the number of such units in the ring. For linear siloxanes, the descriptor M is used to represent the monofunctional unit [(CH 3 ) 3 Si—O—], and the linear structures are represented as MD y M, with the subscript y as an integer equal to or greater than 1, and the smallest linear structure shown as M 2 . 
     It is most desirable to remove VSCCs from airstream A upstream of photocatalytic reactor  22  through VSCC filter  18 . VSCC filter  18  can be constructed to optimally purify the air based on the characteristics of the incoming airstream. VSCC filter  18  can be constructed of woven material, non-woven material, particulate matter, monolithic structures, mesh, porous supports, foams, arrays of cells, honeycombs or combinations thereof. The filter material can be fibrous (e.g. nanofibers, microfibers), granular, homogeneous, crystalline, multimaterial, layered structures or composite structures, and may contain crystalline or glassy or both types of materials. The fibrous materials used may be comprised of similar or different fiber diameters, chemistries, and functionalities. Fibers may be organic, inorganic, glass or combinations thereof. High or low surface area materials, or combination thereof, may be used. Examples of high surface area materials include carbons, organics, inorganics, metals and alloys, zeolites, polymer membranes, oxides, non-oxides, aerogels, and combinations thereof. VSCC filter  18  may contain uniformly or nonuniformly sized and distributed voids or pores. 
     A VSCC filter can be used in combination with other filters. For example, a plurality of VSCC filters with the same or different functionalities may be used in series. Additionally, a VSCC filter or a series of VSCC filters can be used in combination with carbon adsorbents (integrated into the filter structure or separate) or alternate filter types such as HEPA filters. 
     In the present invention, a surface characteristic of VSCC filter  18  is altered or enhanced by filter surface treatment  20 . Filter surface treatment  20  incorporates an additive onto the surface of VSCC filter  18  that enhances a surface characteristic of VSCC filter  18  and is chemically attractive to the VSCCs. Filter surface treatment  20  enhances removal of the VSCCs from airstream A. Filter surface treatment  20  may be applied to the entire surface or to a selected surface or surfaces of VSCC filter  18 . A surface of VSCC filter  18  includes any part of VSCC filter  18  that can come in contact with airstream A. This includes the interior cell walls of a honeycomb structure and the exposed surfaces of porous supports. The additive in surface treatment  20  alters a surface characteristic of VSCC filter  18  by bonding to the filter. The additive may alter the filter&#39;s surface by creating an acidic site, by increasing the surface area, or by providing a preferential reactive site, including a catalyst. 
     First, the additive in surface treatment  20  can make the surface of VSCC filter  18  acidic. For example, the additive may be sulfuric acid, trifluoromethane sulfonic acid, or a compound containing at least one sulfonic acid group. While SiO 2  is essentially entirely inert to sulfuric acid, the Si—O bonds of D x  molecules may provide sufficient strain to render these species sufficiently reactive to sulfuric acid to form sulfate esters. These sulfate esters are partially or fully soluble and dissolve in sulfuric acid. In the presence of sufficient water, the sulfate esters can hydrolyze to form polymeric, relatively non-volatile polysiloxanes (silicones). Subsequent dehydration can cause further condensation and polymer chain growth, eventually forming non-volatile solids. 
     In one embodiment, sulfuric acid is incorporated onto an activated carbon or charcoal filter. The activated carbon filter is impregnated, infiltrated, or saturated with sulfuric acid by soaking an activated carbon cloth in sulfuric acid, and removing and rinsing the cloth. The sulfuric acid does not chemically change the carbon, instead, it bonds to the carbon or is otherwise retained in residual voids or porosity in the carbonaceous material (e.g. mesoporosity). When airstream A passes through VSCC filter  18 , the VSCCs are attracted to and at least partially dissolve in the sulfuric acid, delaying or preventing the VSCCs from reaching photocatalyst reactor  22 . In this embodiment, the sulfuric acid supported on the carbon functions as a heterogeneous catalyst. Alternate heterogeneous catalyst examples include acid clays, ion exchange resins, and acidic zeolites. 
     In another embodiment, an acidic surface is created by incorporating a polyelectrolyte containing sulfonic acid groups onto an active carbon filter. The sulfonic acid groups (R—SO 2 OH) may be part of larger aliphatic or aromatic hydrocarbons, or part of carbon-, silicon-, or phosphorus-based polymeric species. Nafion® is one commercially available example of a polyelectrolyte containing sulfonic acid groups. Other commercial examples exist and compounds or polymers can be readily synthesized with sulfonic acid functionality. The H atom makes the functional group highly acidic and stable as the salt form. In the presence of water (e.g. moisture or humidity), sulfonic groups can serve as a reactive medium for VSCCs. These highly acidic environments at least partially solubilize and polymerize volatile and semi-volatile VSCCs by either trapping them irreversibly or creating less volatile silicon-containing species. 
     Additionally, a catalyst may be selected and incorporated onto the acidic surface to promote further hydrolysis, oxidation, or polymerization of the trapped VSCCs. For example, catalytic particles comprising or containing noble metals and alloys may be used. Nanoparticles of platinum, rhodium, rhenium, palladium, gold, silver, osmium, ruthenium, or iridium metals or their oxides may be used. In one example, platinum (possibly containing platinum oxide), which is commonly used to promote reactions in fuel cells, is incorporated onto the filter as a catalyst. The filter may be comprised of materials other than, or in addition to, active carbon, such as charcoal, acid clay, ion exchange resins, acidic zeolites or any combination thereof. 
     Second, the additive in surface treatment  20  can enhance removal of VSCCs by increasing the surface area of filter  18 . The increased surface area increases the adsorptive properties of the filter. Examples of surface area increasing additives include oxide gels, metal oxide gels and mixed metal oxide gels. In a specific example, silica gel (an oxide gel) is used. Silica gel is a highly porous structure with a surface area around 800 m 2 /gram of material (highly tailorable based on processing) that would increase the surface area of the filter through its open porosity. Silica gel is an excellent adsorbent material and contains a continuous network of silicon and oxygen. However, it adsorbs both VSCCs and water, and water adsorption can compete with and prevent subsequent adsorption by contaminant species such as VSCCs by occupying active sites on the silica gel. To mitigate or control water adsorption, the high surface oxide systems are rendered with a bound organic functionality (e.g. alkyl or hydrocarbon chains such as octyl-modified). The resulting silica gel would be relatively hydrophobic compared to the untreated gel. 
     A chemically similar substance with extensive porosity is an aerogel. Silica gel and other metal oxide and mixed metal oxide compounds can be in the form of aerogels. An aerogel can be created by processing sol-gel precursors or colloidal silica under conditions that render an aerogel. For example, a sol-gel precursor can be coated onto either a honeycomb or porous filter and then processed under conditions that render the sol-gel an aerogel. Aerogels are also commercially available. Cabot Corporation offers Nanogel®, a silica-based aerogel which is hydrophobic, in a powder, bead or blanket form. Commercially available aerogel materials typically have surface areas nominally 600-800 m 2 /g, which is one order of magnitude higher than the Degussa P25 TiO 2  typically used as a UV photocatalyst. Aerogels are generally hydrophilic as prepared, but can be rendered more hydrophobic by chemical treatment. Metal oxide gel and aerogel processing techniques are known to one skilled in the art. 
     Any suitable oxide or mixed-oxide gel or aerogel that is derivatized to render the surface hydrophobic may be used as the additive in filter surface treatment  20 . For example, oxide gels, metal oxide gels and mixed metal oxide gels having hydrophobic surfaces may be used. Si—O based aerogels may also be used. The Si—O chemistry of the silica increases the likelihood that the VSCCs, which also have Si—O type functionality, associate and react with the surface of the filter. Further, a mixture of surface area increasing additives can be used to control site competition between purely organic VOC and silicon-containing VMS or related VSCCs. 
     Third, the additive in surface treatment  20  can be a reaction inducing material which facilitates a preferential interaction or reaction with VSCCs. This additive renders the surface of filter  18  more reactive to VSCCs so that the VSCCs bond to filter surface treatment  20  on VSCC filter  18  before reaching photocatalyst reactor  22 . For example, the presence of iron (Fe) in oxide-based ceramic materials renders the ceramic phase more reactive towards silica-type networks. For example, Fe—O—Si networks are easy to form (e.g. through sol-gel chemical processing). Fe—O linkages with Ti—O (as found in TiO 2 ) and Si—O (as found in SiO 2 ) are also easy to form to create mixed oxide networks (e.g. Fe—O—Si— and Fe—O—Ti—). It follows that the presence of iron (Fe) on a titania (TiO 2 ) substrate reacts easily with species containing Si—O— bonding, such as VSCCs. Iron-doped titania is available commercially or may be created using a process known to one skilled in the art. Commercial iron-doped TiO 2  (2 wt % Fe) is available from Degussa. Other oxides or mixed oxides (e.g. titanium oxide plus tungsten oxide) may also be used as the material to be doped. 
     Iron-doped silica networks are also beneficial because of their increased surface areas. The benefits of both silica and titania iron-doped networks can be achieved by combining the networks. Such combinations of titania and silica iron-doped networks can be created via mechanical mixing of the constituents or through chemical mixing or co-synthesis via solution or aerosol techniques. Other transition metal dopants may be substituted for, or used in combination with iron. 
     Any of the above filter and additive materials can be used alone or in combination to remove VSCCs. For example, hydrophobic aerogels can be used with activated carbon to remove both VSCCs and VOCs. The additives may be used as particulate additives to a filter or may be coated onto a separate active or passive support. For example, an activated carbon cloth (available from Calgon Carbon) can be impregnated with acidic polyelectrolyte or sulfuric acid to make an active support for volatile or semi-volatile siloxane removal. A passive support for siloxane removal is created by coating an aluminum honeycomb with an additive. 
     Coating or impregnation methods, if necessary, can be applied externally to the VSCC filter  18  through vapor, solution, slurry-based, dipping or similar deposition methods, or can be grown in-situ from selected starting compositions or architectures. Coating, impregnation, or modification can be single or multi-step with the same or different compositions and/or distributions. 
     During times of non-operation, contaminants may be immobilized further on VSCC filter  18  by mineralization. Mineralization causes additional oxidation and converts the partially oxidized siloxane molecules to silica, further rendering it unavailable for volatilization. Mineralization may also be caused by exposure to one or more of the following: microwave radiation, inductive heating, thermal treatment, air pulsing, ultrasonics, UV-PCO, plasma treatment, mechanical agitation, chemical washing, or other suitable methods. The VSCC filter  18  and surface treatment  20  may be constructed to be easily maintained and economical so as to minimize material and ownership costs. 
     Controller  34  coordinates operation of mineralization unit  32  with the operation of fan  30  and UV source  28 . For example, mineralization unit  32  may be operated when UV source  28  is not in operation. In some cases, fan  30  may be operated in conjunction with mineralization unit  32 ; for example, to draw moist or heated air into VSCC filter  18  to further the mineralization process. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.