Chemically Impregnated Zeolite and Method for Chemically Impregnating and Coating Zeolite

The invention relates to processes for producing chemically impregnated zeolite and coated, chemically impregnated zeolite, and to the products of such processes. A preferred embodiment of the invention is a process for chemically impregnating zeolite crystals with potassium hydroxide, and then coating the impregnated zeolite crystals with a quaternary ammonium cation.

Zeolites are hydrated metal aluminosilicate compounds with well-defined (tetrahedral) crystalline structures. Because zeolite crystals (both natural and synthetic) have a porous structure with connected channels extending through them, they have been employed as molecular sieves for selectively absorbing molecules on the basis of size, shape, and polarity.

Volumes packed with zeolite crystals (for example, small zeolite crystals chosen to have size in the range from 0.2 mm to several millimeters) have been employed in water and air (or other gas) filtration systems to selectively absorb contaminants from a flowing stream of water or gas.

It has been proposed to treat zeolite crystals by impregnating them with quaternary ammonium cations (such as tetramethylammonium, tetraethylammonium, hexadecyltrimethylammonium, dibenzyldimethylammonium, benzyltriethylammonium, and cetyltrimethylammonium), to enhance the zeolite's capacity to absorb heavy metal, benzene, toluene, and xylene contaminants from water. See, for example, Cadena, et al., "Treatment of Waters Contaminated with BTX and Heavy Metals Using Tailored Zeolites," New Mexico Waste-management Education and Research Consortium Technical Completion Report for Project No. WERC-91-41 (Feb. 1992). If not impregnated with a quaternary ammonium cation (QAC), zeolite does not function adequately as a molecular sieve for organic chemicals such as benzene, toluene, and xylene.

It has also been proposed to impregnate an aqueous solution of permanganate (such as permanganate of potassium, sodium, magnesium, calcium, barium, or lithium) into pores of substrates such as silica gel, alumina, silica-alumina, activated bauxite, and activated clay. The resulting impregnated porous substrates have been employed for filtering and deodorizing air. See, for example, U.S. Pat. No. 3,049,399, issued Aug. 14, 1962, to Gamson, et al.

However, zeolite crystals have not been impregnated (throughout their volume) with permanganate.

Further, those skilled in the art have avoided exposing quaternary ammonium cations or salts to oxidizing agents, and especially to strong oxidizing agents such as permanganates (to avoid violent reactions of the type predicted in the literature).

The inventor has found that zeolite crystals can readily be impregnated with a usefully high concentration of potassium hydroxide, and that, under certain conditions, such impregnated zeolite reacts too rapidly to be practically useful for some air filtration applications.

For air (and other gas) filtration, water filtration, and animal feed applications, it would be desirable to reduce the rate at which potassium hydroxide-impregnated zeolite absorbs selected contaminants, and to control such absorption rate. Similarly, for animal feed applications, it would be desirable to impregnate zeolite with metal compounds, amino acids, vitamins, and other substances, and to reduce the release rate of the impregnating substance from the zeolite. However, until the present invention, it was not known how to achieve these objectives.

In one class of embodiments, the invention is a process for impregnating zeolite crystals with potassium hydroxide (KOH). The product of such process is an embodiment of the invention. A preferred embodiment of the inventive process results in zeolite crystals uniformly impregnated with potassium hydroxide (with potassium hydroxide content in the range from about 5% to about 15% by weight). This embodiment includes the steps of dehydrating the zeolite crystals until they have moisture content substantially below their initial moisture content (and preferably until they have a moisture content of about 5%), then immersing the dehydrated zeolite crystals in (or spraying the dehydrated crystals with) aqueous potassium hydroxide (having a potassium hydroxide concentration of about 39% by weight) at 190.degree. F. and thoroughly mixing the resulting slurry, and finally air drying the mixed slurry to produce KOH-impregnated zeolite crystals. The zeolite crystals (after they are dried) are uniformly impregnated with a concentration of potassium hydroxide in the range from about 5% to about 15% of the total product weight.

In other preferred embodiments, the method of the invention includes the steps of impregnating zeolite crystals with potassium hydroxide (preferably by immersing them in aqueous potassium hydroxide) and coating the impregnated zeolite with a quaternary ammonium cation (QAC).

In other preferred embodiments, the method of the invention includes the step of impregnating zeolite crystals with metal sulfates or soluble salts (such as Fe2SO4, FeCl3, CuSO4, CuCl2, ZnSO4, ZnCl2, CoSO4, CoCl, MnSO4, MnCl, sodium selenate, or MgSO4), or other substances such as amino acids (e.g., methionine or lysine), urea, free amines, or vitamins (such as A, D, E, K, Biotin, Choline, Niacin, Riboflavin, and Pantothenic acid). Preferably, the impregnating vitamins are water soluble vitamins, but fat soluble vitamins can also be employed. Preferably, the impregnation step is performed by immersing the zeolite in (or spraying the zeolite with) aqueous sodium selenate, aqueous urea, an aqueous amino acid, an aqueous free amine, an aqueous vitamin, an aqueous metal sulfate, or an aqueous salt of iron, copper, zinc, cobalt, manganese or magnesium. Preferably, the impregnated zeolite is coated with a QAC.

In each embodiment, the presence of the QAC coating allows regulated time release control of the impregnating substance, and thus permits a controlled release diffusion (or absorption) rate in applications in which the coated, impregnated zeolite is employed to absorb contaminants from a fluid such as air or water.

An important aspect of the invention is that the characteristics of the QAC coating on the impregnated zeolite crystal can be varied to control the reaction rate of the impregnating substance within the zeolite. Such characteristics can be varied by changing the concentration of the coating solution employed to coat the impregnated zeolite crystal.

The coated, impregnated zeolite resulting from all embodiments of the inventive method, or a mixture of the coated, impregnated zeolite resulting from two or more embodiments, or a mixture of uncoated, impregnated zeolite and coated, impregnated zeolite resulting from any embodiment of the inventive method, can be used for a variety of molecular sieving applications, such as for animal feed, or filtration of contaminants from fluid (such as air or water). The number of combinations of coated and uncoated crystals can match specific environmental circumstances which can be calculated by analysis of the air or water to be treated. Mixtures of coated and uncoated KOH-impregnated zeolite can be used to react with hydrogen sulfide, acetone, ethylene glycols, formaldehyde, and other contaminants.

FIG. 1 is a cross-sectional view of a zeolite crystal impregnated with potassium hydroxide.

FIG. 2 is a cross-sectional view of the impregnated zeolite crystal of FIG. 1, after it has been coated with a QAC in accordance with the invention.

FIG. 3 is a cross-sectional view of a zeolite crystal impregnated with a substance selected from the set of metal surfates, soluble salts, amino acids, urea, free amines, and vitamins.

FIG. 4 is a cross-sectional view of the impregnated zeolite crystal of FIG. 3, after it has been coated with a QAC in accordance with the invention.

In one class of embodiments, the invention is a process for impregnating zeolite crystals (for example, crystals having size 0.125 inch.times.0.10 inch, 0.25 inch.times.0.125 inch, 0.125 inch.times.0.50 inch, or 0.50 inch.times.0.75 inch) with potassium hydroxide, and the product of such process. A preferred embodiment of such process, for producing zeolite crystals uniformly impregnated with potassium hydroxide, includes the steps of: dehydrating the zeolite crystals until they have moisture content substantially below their initial moisture content (and preferably until their moisture content is about 5%), then immersing the dehydrated zeolite crystals in (or spraying the dehydrated crystals with) aqueous potassium hydroxide (having a potassium hydroxide concentration of about 39% by weight) at 190.degree. F. and thoroughly mixing the resulting slurry, and finally air drying the mixed slurry to produce potassium hydroxide-impregnated ("KOH-impregnated") zeolite crystals. The zeolite crystals (after they are dried) are uniformly impregnated with a concentration of potassium hydroxide in the range from about 5% to about 15% of the final product of the process. FIG. 1 shows one such impregnated zeolite crystal whose channels have been impregnated with potassium hydroxide 2.

The inventor's experiments have demonstrated that unimpregnated zeolite has a pH of 6.1, that zeolite impregnated with 5% potassium hydroxide (in accordance with the invention) has a pH of 10.6, and that zeolite impregnated with 10% potassium hydroxide (in accordance with the invention) has a pH of 11.5.

The inventive KOH-impregnated zeolite is useful for a variety of applications such as air filtration and animal feed. For example, KOH-impregnated zeolite absorbs, adsorbs, or reacts with certain air contaminants. For another example, when KOH-impregnated zeolite is fed to ruminant animals (such as cows), it provides buffering (antacid activity) in several digestive organs, including the rumen and small intestine (although KOH is much too caustic to feed alone to such animals). However, for uncoated KOH-impregnated zeolite, the rate of absorption of air contaminants, and the activity rate, may be too high or too low for some air filtration and animal feed applications.

The rate at which KOH-impregnated zeolite absorbs (or reacts with, or both absorbs and reacts with) selected air contaminants or ruminant digestive tract chemicals can be controlled (reduced or increased to a desired level) by applying a quaternary ammonium cation (QAC) coating to the impregnated zeolite. The advantages of the inventive QAC-coated, KOH-impregnated zeolite (over conventional air filtration chemicals) include lower manufacturing cost (including lower drying cost) and a reduced level of dusting during processing and handling, as well as improved contaminant absorption characteristics.

In a first class of preferred embodiments, the inventive method includes the steps of impregnating zeolite with potassium hydroxide and then coating the KOH-impregnated zeolite with a QAC (preferably, cetyltrimethylammonium, although other QACs are suitable for certain applications). FIG. 2 shows one such KOH-impregnated crystal, whose channels contain QAC 4 in the region near the crystal's surface, and whose channels are impregnated with potassium hydroxide 2a throughout the volume of the crystal inside the surface region containing QAC 4.

The QAC coating operation is preferably accomplished by immersing KOH-impregnated zeolite in (or spraying KOH-impregnated zeolite with) liquid cetyltrimethylammonium chloride (with a weight ratio Q/T in the range from 0.1% to 5%, where Q is the cetyltrimethylammonium chloride weight and T is the total weight of the final product of the process). As a result of such immersion (or spraying), a QAC coating is applied to each KOH-impregnated zeolite crystal in the sense that the QAC (cetyltrimethylammonium) enters the channels near each crystal's outer surface but the QAC does not penetrate far into the interior of each crystal. From a practical point of view, the inventor has found that the weight ratio of liquid cetyltrimethylammonium chloride employed for coating KOH-impregnated zeolite crystals should preferably (at least for many air filtration applications) satisfy the following relation: 0.1%<Q/T<0.5%, where Q is the cetyltrimethylammonium chloride weight and T is the total weight of the final product of the process.

The activity rate of QAC-coated, KOH-impregnated zeolite depends on the concentration of the QAC solution with which the KOH-impregnated zeolite is coated. Increasing the QAC concentration will decrease the activity rate. The inventor has found that the leaching rate of KOH from within QAC-coated, KOH-impregnated zeolite (and hence the expected activity rate) is negligible if the weight ratio of the QAC coating is in the range from 1% to 2% (i.e., if the weight of liquid cetyltrimethylammonium chloride employed for coating KOH-impregnated zeolite crystals satisfies the relation 1%<Q/T<2%, where Q is the cetyltrimethylammonium chloride weight and T is the total weight of the final product of the process). To produce QAC-coated, KOH-impregnated zeolite for most air filtration applications, the optimum QAC coating weight ratio is in the range from 0.1% to 0.5% (i.e., the weight of liquid cetyltrimethylammonium chloride employed for coating the hydroxide-impregnated zeolite crystals satisfies the relation 0.1%<Q/T<0.5%, where Q is the cetyltrimethylammonium chloride weight and T is the total weight of the final product of the process). However, for KOH-impregnated zeolite crystals with a very high KOH concentration, it may be desirable to employ a greater amount of QAC for the coating (i.e., the weight of liquid cetyltrimethylammonium chloride employed for the coating should satisfy the relation 0.5%<Q/T, where Q is the cetyltrimethylammonium chloride weight and T is the total weight of the final product of the process).

An optimal KOH-impregnated zeolite product for absorbing (and/or reacting with) any of a wide variety of contaminants (or contaminant groups) from a fluid, or for producing desired buffering activity as an animal feed, can be determined experimentally in the following manner. Uncoated, KOH-impregnated zeolite crystals (preferably produced in the manner described below) are mixed in various ratios with QAC-coated, KOH-impregnated zeolite crystals, and the contaminant absorption and/or reaction characteristics of each mixture studied. The mixture producing the best absorption and/or reaction characteristics is identified as the optimal mixture.

In other preferred embodiments, the inventive method includes the step of impregnating zeolite crystals with substances useful for animal feed applications. Examples of such substances include metal sulfates and soluble salts (such as Fe2SO4, FeCl3, CuSO4, CuCl2, ZnSO4, ZnCl2, CoSO4, CoCl, MnSO4, MnCl, sodium selenate, or MgSO4), and other substances such as amino acids (e.g., methionine or lysine), urea, free amines, and vitamins (such as A, D, E, K, Biotin, Choline, Niacin, Riboflavin, and Pantothenic acid). Preferably, the impregnating vitamins are water soluble vitamins, but fat soluble vitamins can also be employed. Preferably, the impregnation step is performed by immersing the zeolite in (or spraying the zeolite with) aqueous sodium selenate, aqueous urea, an aqueous amino acid, an aqueous free amine, an aqueous vitamin, an aqueous metal sulfate, or an aqueous salt of iron, copper, zinc, cobalt, manganese or magnesium.

In these embodiments, the inventive method preferably includes the steps of: dehydrating the zeolite crystals until they have moisture content substantially below their initial moisture content (preferably until their moisture content is about 5%), then immersing the dehydrated zeolite crystals in (or spraying the dehydrated crystals with) an aqueous solution of the impregnating substance at high temperature (e.g., 190.degree. F.) and thoroughly mixing the resulting slurry, and finally air drying the mixed slurry to produce impregnated zeolite crystals. FIG. 3 is a cross-sectional view of one of the resulting zeolite crystals, whose channels have been impregnated with a substance 12 (other than potassium hydroxide).

The impregnated zeolite can be employed as an animal feed supplement, particularly for ruminant animals (such as cows). However, the activity rate of the uncoated, impregnated zeolite may be too high or too low for some animal feed applications. The rate at which the impregnated zeolite releases the impregnating substance to an animal's digestive tract chemicals can be controlled (reduced or increased to a desired level) by applying a quaternary ammonium cation (QAC) coating to the impregnated zeolite.

It is within the scope of the invention to impregnate zeolite with a substance (such as any of those listed above) potassium hydroxide and then coat the impregnated zeolite with a QAC (Preferably, cetyltrimethylammonium, although other QACs are suitable for certain applications). FIG. 4 is a cross-sectional view of one such impregnated crystal, whose channels contain QAC 14 in the region near the crystal's surface, and whose channels are impregnated with a substance 12a (other than potassium hydroxide) throughout the volume of the crystal inside the surface region containing QAC 14.

The QAC coating operation is preferably accomplished by immersing impregnated zeolite in (or spraying impregnated zeolite with) liquid cetyltrimethylammonium chloride (with a weight ratio Q/T in the range from 0.1% to 5%, where Q is the cetyltrimethylammonium chloride weight and T is the total weight of the final product of the process). As a result of such immersion (or spraying), a QAC coating is applied to each impregnated zeolite crystal in the sense that the QAC (cetyltrimethylammonium) enters the channels near each crystal's outer surface but the QAC does not penetrate far into the interior of each crystal. From a practical point of view, the inventor has found that the weight ratio of liquid cetyltrimethylammonium chloride employed for coating KOH-impregnated zeolite crystals should preferably (at least for many applications) satisfy the following relation: 0.1%<Q/T<0.5%, where Q is the cetyltrimethylammonium chloride weight and T is the total weight of the final produce of the process.

The activity rate of QAC-coated, impregnated zeolite depends on the concentration of the QAC solution with which the impregnated zeolite is coated. Increasing the QAC concentration will decrease the activity rate.

Although the QAC in preferred embodiments of the invention is cetyltrimethylammonium, other QACs can be substituted for cetyltrimethylammonium in alternative embodiments.

It may be useful to mix the inventive QAC-coated, KOH-impregnated zeolite crystals with other embodiments of the inventive QAC-coated, impregnated zeolite crystals.

Various modifications and variations of the described method of the invention will be apparent to those skilled in the art without departing from the scope and spirit of the invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments.