POROUS MATERIALS HAVING A DUAL SURFACE

Abstract: Porous materials having a dual surface are disclosed. Also disclosed is a method for their preparation. An example of such a material is a silica gel reacted with a reactive silane intermediate such as (CH₃)₂Si=O and then, with a reactive silane such as to give a dual treated material having -(CH₃)₂SiO) x H on the exterior surface and on the interior surface of the pores.

Description: According to Plueddemann, in the chapter on silylating agents in "Encyclopedia of Chemical Technology", 3rd edition, volume 20, page 962 et seq., silylation is the displacement of active hydrogen from an organic molecule by a silyl group. Plueddemann further states that "The active hydrogen is usually OH, NH, or SH, and the silylating agent is usually a trimethylsilyl halide or a nitrogen-functional compound. A mixture of silylating agents may be used; a mixture of trimethylchlorosilane and hexamethyldisilazane is more reactive than either reagent alone, and the by-products combine to form neutral ammonium chloride." Thus, what Plueddemann has described is what those skilled in the art regard as the "normal way" to silylate organic molecules using reactive silanes. It has been beneficial to industry to have this approach available to alter organic molecules to achieve certain new molecules. Those skilled in the art have extrapolated silylation of organic molecules to silylation of inorganic molecules as well. For example, it is known that silicas, used as fillers for compounded rubbers, could be treated with reactive silanes such as trimethylchlorosilane and/or hexamethyldisilazane to place trimethylsilyl groups on the surface of such silicas. This treatment arises through the reaction of the hydroxyls on the surface of the silica, with the reactive silanes. See, for example, Hertl, W. and Hair, M.L., "Reaction of Hexamethyldisilazane with Silica", J. of Phys. Chem., Volume 75, No. 14, 1971 and Chmieloweic, J. and Marrow, B.A., "Alkylation of Silica Surfaces", J. of Coil. and Inter. Sci., Volume 94, No. 2, August 1983 and Boksanyi, L., Liardon, O. and Kovats, E., Advances in Coil. and Inter. Sci., 6 (1976), pages 95 to 137. Particulate support materials used in liquid chromatography applications also benefit by silylation techniques. It is common in this art to use reactive silanes to treat the particulate materials to cover up reactive hydroxyl groups to improve the chromatographic properties of polar molecules. See, for example, L. R. Snyder and J. J. Kirkland, Introduction to Modern Liquid Chromatography, 2nd edition, Wiley Interscience, N.Y. 1979. A more significant advance in the silylation art came about by the use of reactive silanes, which also contained organofunctional groups, to silylate surfaces. The desired result was to create a material having a novel end-use which was dependent on the type of organofunctional group included in the silylating silane. For example, in U.S. Patent No. 4,379,931, issued on April 12, 1983, Plueddemann used unique reactive silanes, for example to treat various particulate materials which were then used to extract metal ions from solution. For most of the practical applications known in the prior art, the preferred form of silylation is that which is carried out in solution. There are, however, some silylation applications wherein the silylation reaction is carried out in the vapor phase. It can be concluded therefore that it is generally known in the art to use various reactive silanes to react with hydroxyls on the surfaces of various materials. What is disclosed in this invention is the use of reactive silane intermediates to react with the hydroxyls on the exterior surfaces of porous materials and then the use of reactive silanes to react with the hydroxyls on the interior pore surface of the porous materials to provide a dual surface material in the sense that the silicon-containing groups attached to the exterior surface are different thatn the silicon-containing groups on the interior surface of the porous material. Further, it is contemplated that the silicon-containing groups covalently bonded to the exterior surface of the porous material will be predominantly those obtained from the reactive silane intermediate while the silicon-containing groups covalently bonded to the interior surface of the porous material will be predominantly those obtained from the from the reactive silanes. It is believed by the inventors herein that such dual surface materials are unique, having never been disclosed anywhere in the published literature. Such dual surface materials may be prepared by selecting dual groups that independently adjust transport properties to, and chemical activities of, such dual surface materials. According to a first aspect of this invention, there is provided a method of preparing a dual surface, porous material which comprises: (I) contacting a porous solid having hydroxyl groups on both its exterior surface and its interior pore surface with a reactive silane intermediate capable of forming covalent bonds with hydroxyl groups on said exterior surface of said porous solid and selected from: (1) silenes, (2) silylenes, and (3) silanones; (II) allowing the mixture from (I) to react thereby treating said exterior surface; (III) thereafter, contacting and mixing the treated porous solid from (II) with a reactive silane, or its hydrolysis product, for a period of time to allow the reactive silane, or its hydrolysis product, to diffuse into the interior of the porous solid and covalently to bond with hydroxyl groups on said interior pore surface; and (IV) subsequently isolating the dual surface porous solid from the mixture of (III). According to a second aspect of this invention, there is provided a method of preparing a dual surface, porous material which comprises (A) contacting a porous solid having hydroxyl groups on both its exterior surface and its interior pore surface with a reactive silane intermediate capable of forming covalent bonds with hydroxyl groups on said exterior surface and selected from: (1) silenes, (2) silylenes, and (3) silanones; (B) allowing the mixture from (A) to react, thereby treating said exterior surface; and (C) subsequently isolating from the mixture of (B) a dual surface porous solid having covalently bonded silicon-containing groups on its exterior surface and unreacted hydroxyl groups on its interior pore surface. This invention also provides a treated porous material comprising a porous material having, covalently bonded to its exterior surface, silicon-containing groups obtainable by the reaction of a reactive silane intermediate with hydroxyl groups originally present on said exterior surface, and, covalently bonded to its interior pore surface, further silicon-containing groups other than said silicon-containing groups, obtainable by the reaction of a reactive silane, or its hydrolysis product, with hydroxyl groups on said interior pore surface, said treated porous material being obtainable by the method defined above according to a first aspect of the invention. The invention also provides a treated porous material having, covalently bonded to its exterior surface, silicon-containing groups obtainable by the reaction of a reactive silane intermediate with hydroxyl groups originally present on said exterior surface, and, having unreacted hydroxyl groups on its interior pore surface, said treated porous material being obtainable by the method defined above according to a second aspect of the invention. Thus, the ultimate goal of this invention is to provide a process which will give a composition that has a dual surface. What is meant by "dual surface" for purposes of this invention, is that the silicon-containing groups reacted to the exterior surface of the porous material are different in nature than the silicon-containing groups reacted to the surface of the porous interior of the porous material. This goal can be achieved by first reacting the porous material with a small amount of a highly reactive silane intermediate to treat the exterior surface of the porous material with little or no diffusion into the pores and therefore, little or no treatment within the pores by the highly reactive silane intermediate, and then, reacting the so-treated porous material with an excess of a reactive silane and allowing the reactive silane, or its hydrolysis product, to diffuse into the pores and covalently bond to the interior surface. The major factor that allows the inventive process to produce the inventive compositions herein is that the reactive silane intermediates react very rapidly with the exterior surface hydroxyls and therefore, their physical entry into the pores is severely limited. Additional assistance to prevention of interior reaction is provided by adsorbed water, when present. Even if some of the reactive silane intermediate should reach the interior of the pore before reacting with the exterior surface hydroxyls, it is thought by the inventors that when some physisorbed water is present in the pores, the reactive silane intermediate immediately contacts water in the pores with which it reacts to form disilanols, e.g. (R)₂Si: + HOH → (R)₂Si(OH)₂ and thus the reactive silane intermediate is prevented from reaching the hydroxyl groups that are situated deep in the interior surface of the pores. Moreover, any newly formed reactive silane intermediate that reaches any pore interior that has previously been the site of the (R)₂Si: + HOH reaction, will immediately encounter the previously formed (R)₂Si(OH)₂ which acts as an additional reaction site to react preferentially with the (R)₂Si: to form dimers and other oligomers. A second benefit of adsorbed water is that when present, it can reduce the amount of the reactive silane intermediate precursor that resides in the pores. Hence, the reactive silane intermediate is effectively blocked from the interior of the pores. In this inventive method then, the first phase of the process requires two steps. The first step, i.e. step (I), requires contacting a porous material with a highly reactive silane intermediate and step (II) requires allowing the highly reactive silane intermediate to react with the porous material to obtain a "treated" porous material. It should be obvious to those skilled in the art that steps (I) and (II) can be carried out simultaneously. Step (III) is then carried out by contacting the "treated" porous material with a reactive silane and allowing the reactive silane, or its hydrolysis product, to diffuse into the pores and react with the hydroxyl therein. Subsequently, the desired end-product is isolated from the reaction mixture by some means. For purposes of this invention, in steps (I) and (II), the highly reactive silane intermediates are those that are known in the art. Such highly reactive silane intermediates are selected from a group consisting of silenes, silylenes and silanones. The first group of highly reactive silane intermediates, the silenes are highly unstable organosilicon compounds derived from precursor silanes by various means to be described infra. Their general structure is notable owing to the presence of a single silicon to carbon double bond, i.e. Si=CH₂. The existence of the silenes has been postulated for many years on the basis of the products obtained from certain organosilicon reactions but it has only been recently that such materials were actually isolated so that their existence was proven. See Brook, A.G., Abdesaken, F., Gutekunst, B., Gutekunst, G., and Kallury, R.K., Chem. Comm., 191 (1981). The silenes can be generated from precursor silanes by several different methods, for example by pyrolytic decomposition: Guselnikow, L.E., Flowers, M.C., Chem. Comm., 64 (1967); thermal rearrangement: Slutsky, J., Kwart, H., J. Org. Chem., 38, 3659 (1973); photochemical rearrangement: Nahadaira, Y., Kanovchi, S., Sakurai, H., J. Am. Chem. Soc., 96 5621 (1974); elimination from silyl halides or esters: Jones, P.R., Lim, T.F.O., J. Am. Chem. Soc., 99, 2013 (1977); rearrangement of silyl carbenes: Barton, T.J., Holkman, S.K., J. Am. Chem. Soc. 102 1584 (1980); and, disproportionation of trimethylsilyl radicals: Tokach, S.K., Koob, R.D., J. Am. Chem. Soc., 102, 376 (1980). Such silenes include, for example, These highly reactive silane intermediates react with the surface hydroxyls of porous materials upon contact with such hydroxyls. Thus, using (CH₃)₂Si=CH₂ as an example of a silane, and using ≡SiOH as the designation for a surface hydroxyl on silica, the following reaction is believed to take place: ≡SiOH + (CH₃)₂Si=CH₂ → ≡SiOSi(CH₃)₃ . Another group of highly reactive silane intermediates useful in this invention are the silylenes. Their general structure is notable owing to the presence of a radical, i.e. Si:. Silylenes can be formed from disilanes, for example, methoxydisilanes which undergo thermally induced alpha-elimination to produce the organosilylenes. See Atwell, W. H. and Weyenberg, D. R., J. Am. Chem. Soc., 90, 3438 (1968). Such reactive silane intermediates can also be generated via photochemical methods as shown, for example, in Sakurai, H., Kobayashi, Y., and Nahadaira, K., J. Am. Chem. Soc., 93, 5272 (1971). Such silylenes include, for example, These highly reactive silane intermediates react with the surface hydroxyl of porous materials upon contact with hydroxyls. Thus, using (CH₃)₂Si: as an example of a silylene, and ≡SiOH as the designation for a surface hydroxyl on silica, the following reaction is believed to take place: ≡SiOH + (CH₃)₂Si: → ≡SiO{Si(CH₃)₂} x H where x is an integer of less than about 10. Another group of highly reactive silane intermediates useful in this invention are the silanones. Their general structure is notable owing to the presence of a ketonic oxygen, i.e. Si=O in the molecule. Like the silenes, the silanones are suspected to exist but have never been isolated and identified. However, strong evidence exists that they are present in an operational sense in certain reaction mixtures. See, for example, H. Okinoshima and W. P. Weber, J. Organometal. Chem., 155, 165, (1978) and T. J. Barton and W. D. Wulff, J. Am. Chem. Soc., 101, 2735 (1979). These highly reactive silane intermediates react with the surface hydroxyls of porous materials upon contact with such hydroxyls. Thus, using (CH₃)₂Si=O as an example of a silanone, and ≡SiOH as the designation for a surface hydroxyl on silica, the following reaction is believed to take place: ≡SiOH + (CH₃)₂Si=O → ≡SiO{(CH₃)₂SiO} y H where y is an integer of less than about 10. The porous materials found useful in this invention are those materials which are porous solids, having hydroxyl groups on their surfaces. Such materials for example are silicas, silica gels, stannia, alumina, titania, zirconia, and the like. Also, these materials can be porous glass, controlled pore glass, controlled pore ceramics or plastic as long as the material has, or will form, hydroxyl groups on its surface. The form of the porous material is not overly critical. Particulate porous materials, as well as filaments, slabs, discs, blocks, spheres, films and other such forms can be used in this invention. Also contemplated within the scope of this invention is the treatment of particulate materials by the process of this invention, and the subsequent forming of the treated particulate materials into filaments, slabs, discs, blocks, spheres, films, membranes, sheets, and the like. Preferred for this invention are the porous metalloid and metallic oxides such as silica, alumina, zirconia and titania in all of their related forms. Most preferred are the silicas. Also contemplated within the scope of this invention are porous mixed metallic oxides such as Na₂O: Al₂O₃: 5SiO₂: nH₂O, wherein n is the moles of water of hydration, and the like. The first phase of this process is the contacting of the porous material with the reactive silane intermediate. As indicated above, the reactive silane intermediates are formed from silane precursors and one method is to have the porous material be intimately contacted with a solution of the precursor silane so that when the silane precursor generates the reactive silane intermediate, the reactive silane intermediate can immediately contact the exterior surface of the porous material and react with it. Failure to intimately contact the porous material with the silane precursor while the reactive silane intermediate is being formed, results in a wasteful use of the reactants, since the reactive silane intermediate tends to react with itself or with newly formed surface bonds, or reaction container walls. For this method it is preferred that adsorbed water be present in the pores to reduce entry of the silane precursor. This water also reduces the opportunity for the reactive silane intermediate to reach deep into the pores. Thus, in one method the silane precursor in a solvent solution such as benzene or toluene is intimately contacted with the porous material. If particulate, the porous material is used as a suspension. The reactive silane intermediate is then generated in-situ , for example, by the application of ultraviolet light to the solution, and the reactive silane intermediate makes immediate contact with the exterior surface of the porous material and decreases the chances of ineffective use of the reactive silane intermediate. In another method, the reactive silane intermediate can be generated in a chamber independent of the liquid dispersion of the porous material and transported by some means to the liquid dispersion of the porous material. For example, the precursor silane in solvent solution can be passed through an irradiation zone where the reactive intermediates are generated and the solvent solution flow carries the generated reactive silane intermediates to the porous material. Flow recycling of unexpended precursor silane allows effective utilization of the precursor silane to treat the exterior surface of the porous material. It is also contemplated within the scope of this invention to carry out the first phase of the process in a vapor phase. Thus, in a third method the precursor silane is placed in a chamber and contacted with the equilibrium vapor of the reactive silane intermediate. The chamber has two regions, one, an irradiation region and, two, a reaction region. The silane precursor is irradiated in the irradiation region to generate the reactive silane intermediate which moves toward the porous material in a random fashion. The porous material which is also located in the reaction region of the vacuum chamber, and some short distance away from the zone of irradiation, is thereby randomly hit on the exterior surface with the reactive silane intermediate thereby causing a reaction between the reactive silane intermediate and the exterior surface hydroxyls of the porous materials. In a fourth method, the precursor silane vapor chamber can be equipped to allow a flow of gas, such as helium, to remove the reactive silane intermediate as well as unreacted precursor silane vapor. This helium flow then carries the reactive silane intermediate to the porous material for reaction therewith. These methods and their variations allow for more selective exterior surface treatment, as the excitation used to generate the reactive silane intermediate is not allowed to contact the porous material and, thus, the precursor silane need not be kept out of the pores to prevent reaction with the interior hydroxyls. It is obvious then that the resulting porous material is not generally reacted on the interior surfaces in this phase of the process nor do the pores contain the dihydroxy monomers or oligomers. Thus, when the reactive silane of the second phase, i.e. step (III) of this inventive process, is contacted by the surface treated porous material of steps (I) and (II), only the moiety resulting from the reactive silane will be found on the interior surface of the pores, resulting in a "purer" dual surface material. Steps (I) and (II) can be carried out for a period of time of from 1 minute to 24 hours. Generally, for purposes of this invention it is preferred to carry out steps (I) or (II) over about a 1 to 8 hour time period to ensure that the exterior surface of the porous material is well treated. The temperature at which steps (I) and (II) are carried out is not narrowly critical and can range from 0°C to 400°C. Preferred is a room temperature to 200°C reaction temperature. The amount of reactive silane intermediate useful in this invention depends on the number of exterior surface hydroxyls to be reacted. Typically, a stoichiometric amount equivalent to the exterior surface hydroxyls plus some excess of the reactive silane intermediate is required to cover all surface hydroxyls because of the potential side reactions involved. Typically, 50 to 1000% excess is used. This excess is with respect to the exterior hydroxyl groups. With respect to the total hydroxyls on both the exterior and interior surfaces, the amount of the reactive silane intermediate amounts to about .001 to 1%. When adsorbed water is present in the pores to consume excess amounts of the reactive silane intermediate, much larger amounts of the intermediate are tolerable, up to 100% of the total hydroxyls present. If it is desirable to achieve less than stoichiometric coverage of the surface hydroxyls, then obviously, less reactive silane intermediate should be used. In the second phase of the process, i.e. step (III), the material obtained by steps (I) and (II) is contacted with a reactive silane. It should be noted that steps (I) and (II) deal with a "reactive silane intermediate" while step (III) deals with a "reactive silane". For purposes of this invention, what is meant by "reactive silane" is that the silane is sluggish in its reactivity such that it will not react with nor displace the newly formed groups on the exterior surface of the porous material but will react with the silica silanols. This reaction is preferably carried out in bulk solution as the reactive silanes are generally neither reactive enough nor volatile enough to use as vapors. Thus, the reactants are mixed together in a solvent solution, with or without, heating. The object of step (III) is to allow the hydrolysis and diffusion, or the diffusion and hydrolysis of the reactive silane, in the pores of the porous material. Since the exterior surface hydroxyls of the porous material have been effectively covered by the reactive silane intermediate in steps (I) and (II), the reactive silane does not have available to it the reactive sites on the exterior surface of the porous material. Thus, the reactive silane has only the hydroxyls remaining on the interior surfaces of the porous material available for reaction. Therefore, step (III) must be carried out for a sufficient period of time to allow for the diffusion of the reactive silane into the pores, and hydrolysis of the silanes therein (or hydrolysis of the silane and then diffusion of the hydrolyzate into the pore to react). Some of the more highly reactive silanes which do not require hydrolysis prior to reaction with the interior hydroxyl groups may also be used as vapors. Step (III) can be carried out for a period of time of from several minutes to several hours. As indicated above, this phase of the process depends on the rate of hydrolysis of the reactive silane and the rate of diffusion of the silane, or its hydrolysis product, into the pores of the porous material. Preferred for this invention is a reaction time for step (III) of from 10 minutes to 24 hours. Most preferred is a time of 1 to 6 hours. The temperature at which step (III) is carried out is more critical than steps (I) and (II), yet it is not narrowly critical. As one would expect, increased temperatures enhance the rate of reaction but, increasing the temperature does not appear to promote undesirable side reactions. Thus, the temperature used in step (III) can range from 0°C to 300°C. Most preferred is the reflux temperature of the reaction mixture at about 70°C to 120°C. The amount of reactive silane useful in this invention depends on how many of the surface hydroxyls of the pore interior one wishes to cover. An excess of the reactive silane is not critical as this reactive silane does not displace any of the exterior surface groups obtained by steps (I) and (II). Typically, a stoichiometric quantity, based on reactive hydroxyls of the interior pore surfaces, is used herein. If the reactive silane is very sluggish (e.g., organosilanol), then the reaction should be catalyzed with acid or base. In the case of the treated porous material having the groups on the exterior surface derived fron silanones, the reaction cannot be catalyzed with acid and must be catalyzed with base, preferably weak base. In the case of the treated porous material having the groups on the exterior surface derived from silenes and silylenes, both acid and base catalysis can be used, with the proviso that the minimum catalysis required for completion of the reaction be used. The time and temperature of such catalyzed silylations must be carefully adjusted by trial and error to attain adequate interior coverage before the displacement of the covalently bound external group begins. It will be recalled from the foregoing discussion that dihydroxy compounds and oligomers can reside in the interiors of the pores. This is true as long as there is an absence of catalysts. It is known by the inventors herein that the introduction of the reactive silane in this third step, tends to displace these dihydroxy compounds and oligomers from the pore interior and that such displacement did not occur on the exterior surface of the porous material. The process therefore appears to give dual surface compositions. The reactive silanes useful in step (III) of this invention are those silanes which are recognized as conventional silylation reagents. This group includes such silanes as alkoxysilanes, chlorosilanes, acetoxysilanes, alkyldisiloxanes, silylamines, silylamides, silylthioethers and many others. Preferred for this invention are alkoxysilanes, silylamides, and silylamines. Specific silanes which are useful herein include: trimethylchlorosilane; dimethyldichlorosilane; hexamethyldisilazane; N,N'-bis(trimethylsilyl)urea; N-trimethylsilyldiethylamine; N-trimethylsilylimidazole; N,O-bis(trimethylsilyl)acetamide; N,O-bis(trimethylsilyl)trifluoroacetamide; N-methyl-N-trimethylsilyltrifluoroacetamide; t-butyldimethylsilylimidazole; N-trimethylsilylacetamide; N-trimethylsilylpiperidine; hexamethyldisilthiane; O-trimethylsilylacetate; O-trimethylsilyltrifluoroacetate; N-trimethylsilyldimethylamine; N-trimethylsilylmorpholine; N-trimethylsilylpyrrolidine; and N-trimethylsilylacetanilide. In addition, silanes having desirable functional groups may also be used. For example, such a silane as N-(vinyldimethylsilyl)N-methylacetamide is an excellent silane to use in this invention because it leaves the vinyldimethylsilyl group on the interior surface of the porous material. The vinyl group is valuable because once the vinyl group is attached to the interior surface of the porous material, conventional reactions can be used to add to the vinyl group thus creating yet another type of group on the interior surface of the porous material. For example, when the porous material is treated with N-(vinyldimethylsilyl)N-methylacetamide, one can add HSCH₂COOH to the resulting vinyl dimethyl silyl to give a surface group such as HOOCCH₂S(CH₂)₂(CH₃)₂SiOSi≡. Using bis(dimethylsilyl)N-methylacetamide gives a surface treatment such as H(CH₃)₂SiOSi≡ which can also be further reacted with unsaturated molecules to provide further modifications to the interior surface of the porous material. Yet another aspect of this invention is the use in step (III) of organosilanes to silylate the porous material. Such silanes useful in this invention include, for example, (alpha-methacryloxypropyl)trimethoxysilane; (4-aminopropyl)triethoxysilane; {gamma(beta-aminoethylamino)-propyl}trimethoxysilane; (gamma-glycidoxypropyl)trimethoxysilane; [beta-(3,4-epoxycyclohexyl)-ethyl}trimethoxysilane; (beta-mercaptoethyl)trimethoxysilane; (gamma-mercaptopropyl)trimethoxysilane; (gamma-chloropropyl)trimethoxysilane; CH₂=CHC₆H₄CH₂NH(CH₂)₂NH(CH₂)₃-Si(OCH₃)₃·HCl; (CH₃O)₃Si(CH₂)₉CH₃; (CH₃O)₃Si(CH₂)₁₇CH₃ and the like. When it is determined that the reaction in step (III) is essentially finished, the product is typically isolated from the reaction mixture. Thus, step (IV) of this process is the isolation of such products from the reaction mixture. This can be accomplished in a number of ways. For example, the liquid can be decanted, the porous material washed and the liquid decanted successively or, the reaction mixture can be filtered to remove the liquid from the solid product. The final product can be used in this form or it can be dried. If the material is in a particulate form it can be used as is or it can be compressed, sintered, or otherwise formed. It is also contemplated within the scope of this invention to prepare a dual-surface porous material by eliminating step (III) of the defined process. The elimination of step (III) of the process results in an "intermediate" product wherein the surface of the porous material is first treated by steps (I) and (II) to provide a treated porous material having (CH₃)₃SiO, H{(CH₃)₂Si} x O or H{OSi(CH₂)₂} y O on the exterior surface and hydroxyl groups on the interior pore surface. Thus, this invention also consists of a method of preparing a dual surface, porous material which comprises: (A) contacting a porous material with a highly reactive silane intermediate capable of forming covalent bonds with hydroxyl groups on the exterior surface of the porous material; (B) allowing the mixture from (A) to react thereby treating the porous material exterior surface; (C) subsequently isolating the dual surface porous material from the mixture of (B). These intermediates are useful products for the method disclosed supra for providing dual surface materials using reactive silanes to treat the interior pore surfaces. It is a further object of this invention to provide a composition of matter consisting of a porous material having reacted to its exterior surface, silanol-containing groups which are not silica-silanols and whose interior pore surface contains silica-silanol or other hydroxyl groups different from the exterior groups. The dual-surface porous materials of this invention are useful for chelating metals from solution and high performance liquid chromatography methods (see Instrumental Methods of Analysis, Sixth edition, Willard, H. H., Merritt, Lynne L. Jr., Dean, John A. and Settle, F. A., Jr. D. Van Nostrand Co., N.Y. 1981, pages 529-564). The materials, whose surfaces bear groups capable of further modification, such as H(CH₃)₂SiO- and CH₂=CH(CH₃)₂SiO-, are useful intermediates in the preparation of other surface-modified porous materials. Such modifiable groups are particularly valuable when on the exterior surface. Now, so that those skilled in the art may appreciate and understand the invention described herein, the following examples are offered for illustration purposes only. The examples should not be construed as limiting the invention as defined in the claims. Example 1 A porous particulate SiO₂ with a mesh size of 60-200 on U. S. standard sieves, having a surface area of approximately 300 m²/g and an average of 60 angstrom units pore size, purchased from J. T. Baker Co., Phillipsburg, N.J., as Cat. No. 3405-1 was dried for 2 hours at 110°C in an air convection oven to remove excess water, but not completely remove adsorbed water from the material. Into a two-necked, 50 ml. round bottomed glass flask, there was placed 1.0181 gms. of the dried SiO₂, 0.5541 gms. of dodecamethylcyclohexasilane and 14 mols of dry benzene, all under an argon blanket. This solution was stirred and irradiated at room temperature using a Rayonet® ultraviolet light source at 2537 angstrom units (Rayonet photochemical reactor manufactured by Southern New England Ultraviolet Company, Hamden, Connecticut, USA). After about 5 hours, a gas-liquid chromatograph analysis showed that about 50% of the cyclic silane had been consumed. At this point, apparently no higher molecular weight oligomers had formed as evidenced by the lack of elution of the same on the chromatograph. After about 20 hours of total irradiation, the cyclic silane had been totally used up. The product mixture was filtered and washed with dry reagent toluene. A yellow-colored polymeric substance coated on the surface of the silica was removed by washing with dry heptane followed by a wash with pure ethanol. The solid product was then dried in an air-circulating oven for 2 hours at 60°C. An untreated sample of the SiO₂ used in this example was analyzed for carbon content and it was found to contain 0.12 weight percent carbon while the treated SiO₂ of this example showed 4.76 weight percent carbon. Bulk analysis thus indicates that there is present 1.98 m mole of H(CH₃)₂Si/gm of SiO₂. The larger portion of H(CH₃)₂Si was subsequently displaced as discussed infra. The precursor silane is excluded from the pore regions of the particles by virtue of such region's high polarity and preferential adsorption of water rather than non-polar compounds such as, for example, toluene. Example 2 The treated product from example 1 was subjected to a second treatment using (CH₃O)₃Si(CH₂)₃NH(CH₂)₂NH₂ by mixing 0.1 gms. of the treated material from example 1 with a 5 weight percent dry toluene solution of the silane (0.08 ml in 1.42 ml dry toluene). After about one and one-half hours of refluxing under nitrogen, the reaction was cooled and the gel separated by filtration and washed with reagent grade toluene and dried in an air-circulating oven at 60°C for two hours. This material when analyzed by a comparison of bulk elemental analysis (%C and %N) with surface elemental analysis by ESCA (Electron Spectroscopy for Chemical Analysis) showed the following for the treated SiO₂: The ratio of O 3/2 Si(CH₂)₃NH(CH₂)₂NH₂ groups to groups equalled 0.88 for the interior surface and 0.29 for the exterior surface. Thus, this comparison shows that the interior surface of the silica has predominantly more O 3/2 Si(CH₂)₃NH(CH₂)₂NH₂ moieties than (CH₃)₂HSi- moieties, while the reverse is true on the exterior surface. Hence, a dual-natured porous material had been achieved. Conditions were not optimized for obtaining complete exclusion of the interior silane moiety, O 3/2 Si(CH₂)₃NH(CH₂)₂NH₂, from the exterior surface, but as the analysis indicates, the (CH₃)₂HSi- moiety predominates. The bulk elemental analysis (%C and %N) before and after the reactive silane treatment shows that silylation of the particles was accompanied by displacement of about half of the (CH₃)₂HSi- moiety from the interior pore regions. Such major displacement did not occur on the exterior surface. This suggests, but the inventors are deemed not to be held to such a theory, that easily displaced, hydrogen-bonded (CH₃)₂Si(OH)₂ and its oligomers were formed and are present in the pores. Such species form easily by the reaction of (CH₃)₂Si= with pore water and are not themselves very reactive with silica silanols in the absence of catalysts. Thus, this theory explains in part why the surface of the particulate material is preferentially treated over the interior surface, when contacted with the reactive silane intermediate. Example 3 The treated material from example 2 was shown to be able to chelate copper ions from a copper sulfate solution. Upon contacting the copper solution, the white powder changed to a deep blue color characteristic of copper fully bound to the entire porous surface of silica whereas ESCA detected only minor amounts of exterior surface copper.

Claims: A method of preparing a dual surface, porous material which comprises: (I) contacting a porous solid having hydroxyl groups on both its exterior surface and its interior pore surface with a reactive silane intermediate capable of forming covalent bonds with hydroxyl groups on said exterior surface of said porous solid and selected from: (1) silenes, (2) silylenes, and (3) silanones; (II) allowing the mixture from (I) to react thereby treating said exterior surface; (III) thereafter, contacting and mixing the treated porous solid from (II) with a reactive silane, or its hydrolysis product, for a period of time to allow the reactive silane, or its hydrolysis product, to diffuse into the interior of the porous solid and covalently to bond with hydroxyl groups on said interior pore surface; and (IV) subsequently isolating the dual surface porous solid from the mixture of (III). A method as claimed in Claim 1, wherein the reactive silane intermediate of Step (I) is a silene. A method as claimed in Claim 1, wherein the reactive silane intermediate of Step (I) is a silylene. A method as claimed in Claim 1, wherein the reactive silane intermediate of Step (I) is a silanone. A method of preparing a dual surface, porous material which comprises (A) contacting a porous solid having hydroxyl groups on both its exterior surface and its interior pore surface with a reactive silane intermediate capable of forming covalent bonds with hydroxyl groups on said exterior surface and selected from: (1) silenes, (2) silylenes, and (3) silanones; (B) allowing the mixture from (A) to react, thereby treating said exterior surface; and (C) subsequently isolating from the mixture of (B) a dual surface porous solid having covalently bonded silicon-containing groups on its exterior surface and unreacted hydroxyl groups on its interior pore surface. A method as claimed in Claim 5, wherein the reactive silane intermediate of Step (A) is a silene. A method as claimed in Claim 5 wherein the reactive silane intermediate of Step (A) is a silylene. A method as claimed in Claim 5, wherein the reactive silane intermediate of Step (A) is a silanone. A treated porous material, comprising a porous solid having, covalently bonded to its exterior surface, silicon-containing groups obtainable by the reaction of a reactive silane intermediate with hydroxyl groups originally present on said exterior surface, and having unreacted hydroxyl groups on its interior pore surface, said treated porous material being obtainable by a method as claimed in any one of Claims 5 to 8. A treated porous material comprising a porous solid having, covalently bonded to its exterior surface, silicon-containing groups obtainable by the reaction of a reactive silane intermediate with hydroxyl groups originally present on said exterior surface, and, covalently bonded to its interior pore surface, further silicon-containing groups other than said silicon-containing groups, obtainable by the reaction of a reactive silane, or its hydrolysis product, with hydroxyl groups on said interior pore surface, said treated porous material being obtainable by a method as claimed in any one of Claims 1 to 4.

Inventors: REVIS, ANTHONY, WILLIAMS, DWIGHT EDWARD
Applicants: DOW CORNING CORPORATION
Application year: 1985
Main classification: B01J20
IPC codes: C04B41, B01J20, G01N30, C08J7, C08J9