Process for making porous ceramic composites with a bimodal pore size distribution

A process for making a porous ceramic composite with a bimodal pore size distribution includes the steps of mixing an organosilicon precursor, an alcohol, water, a catalyst, granules, particles, whiskers or powders of a fumed silica and granules, particles, whiskers or powders of a ceramic material and a combustible material having a diameter in a range of 500 angstroms to 500 microns to form a mixture, pouring the mixture into a mold, allowing the mixture to gel form a ceramic composite and drying the ceramic composite. The process also includes the step of heating the ceramic composite in either air or oxygen to burn away the combustible material. The organosilicon precursor is selected from a group consisting of tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane and tetrabutoxysilane. The alcohol is selected from the group consisting of methanol, ethanol, propanol and butanol.

BACKGROUND OF THE INVENTION
 The field of the invention is processes for making porous ceramic
 composites and their use in forming ultrafilters and catalyst supports.
 Porous ceramic composites have been used to form ultrafilters and catalyst
 supports. The ultrafilters separate gases, liquids and particles. The
 catalyst supports are used for gas and liquid phase reactions.
 The porous ceramic material must have a high ratio of surface area to mass
 and a a high flow rate for a filtered fluid. The high ratio of surface
 area to mass is achieved through either a thin layer of a ceramic
 composite having fine interconnected pores. The high flow rate is achieved
 through a substrate of a ceramic composite having interconnected course
 pores. The thin layer of a fine porous composite is deposited on top of
 the courseporous substrate. A high flow rate is achieved by making a fine
 pore membrane as thin as possible and by applying a high pressure across
 the membrane.
 In a process using a catalyst the catalyst support is formed out a ceramic
 composite having course pores and fine pores. The catalyst support should
 be chemically and physcially stable. This requirement of physical and
 chemical stability makes porous ceramic composites desirable as a catalyst
 support. The fine pores provide a high ratio of surface area to mass in
 order to maximize both the contact and the interaction between a fluid,
 either a gas or a liquid, and the catalyst, such as particles of platinum.
 The course pores permit the fluid to flow through the catalyst support at
 a high flow rate.
 U.S. Pat. No. 4,981,590 teaches a microfilter which includes a support
 layer which is formed out of a ceramic composite having course pores and a
 thin layer which is formed out of a ceramic composite having course pores.
 The support layer and the thin layer are firmly bound to each other. There
 is a sharp geometric transition between the support layer and the thin
 layer. If the thin layer has any pin-holes then the microfilter is ruined.
 U.S. Pat. No. 4,689,150 teaches a separation membrane which includes a
 glassy microporous porous support. The separation membrane has excellent
 heat resistance, corrosion resistance, durability, gas-separability and
 high mechanical strength. The separation membrane is preferably provided
 with a metallic or ceramic microporous membrane vapor-deposited on the
 surface of the glassy microporous membrane. The separation membrane can be
 utilized with high efficiency in such diversified fields as either
 microfiltration or ultrafiltration of fluids, either gases or liquids.
 U.S. Pat. No. 4,562,021 teaches a microfilter which includes a support
 layer of a ceramic composite having interconnected course pores and a thin
 layer of a ceramic composite having interconnected fine pores. Hydrolysis
 is performed on an alkoxide, an organo-metallic compound in order to
 obtain a sol of particles of the oxide. A thickening agent is added to the
 sol. The resulting sol is slip casted to form the thin layer which is
 deposited onto the support layer. The thin layer deposited on the support
 layer is then dried and heat treated to eliminate the thickening agent and
 to sinter the particles of the deposited thin layer. In Chapter 14,
 entitled "Ultrafilters by the Sol-Gel Process," of Ultrastructure
 Processing of Advanced Ceramics, published by John Wiley & Sons of New
 York in 1988, Louis Cot, Andre Larbot amd Christian Guizard have discussed
 the use of membranes in operations requiring separation.
 U.S. Pat. No. 4,874,516 teaches a microfilter which includes a support
 layer of a ceramic composite having interconnected course pores and being
 of a high strength. The support layer is covered by and supports a
 microporous membrane of a polymer, such as a fluorocarbon polymer, which
 partly permeates the surface of the support layer and which acts as a
 microfilter for fine particles. The microfilter exhibits excellent
 corrosion-resistance, durableness and heat-resistance.
 U.S. Pat. No. 4,581,126 teaches a catalyst support which includes a porous
 gel of an inorganic substance, for example a refractory inorganic oxide,
 and has a surface area in the range 125 to 150 square meters per gram, a
 mean pore diameter in the range of 140 to 190 angstroms with at least 80%
 of the pore volume contained in pores having a pore size range of 50 to 90
 angstroms.
 U.S. Pat. No. 4,969,990 teaches a catalyst which is useful for
 hydroprocessing a hydrocarbon-containing oil and which contains at least
 one hydrogenation component on an amorphous, porous refractory oxide. The
 catalyst is prepared by impregnating support particles having a narrow
 pore size distribution and a mode pore diameter from about 70 to 80
 angstroms with a solution containing a precursor of the hydrogenation
 components, followed by drying and calcining. The catalyst is useful for
 promoting a number of hydrocarbon hydroprocessing reactions, hydrogenative
 desulfurization, demetallization and denitrogenation, and
 hydrodesulfurization of residuum-containing oils.
 SUMMARY OF INVENTION
 The present invention is directed to a process for making a porous ceramic
 composite which includes the step of mixing an organosilicon precursor, an
 alcohol, water and a catalyst to form a mixture. The process also includes
 the step of pouring the mixture into a mold, allowing the mixture to gel
 form a ceramic composite and drying the ceramic composite.
 In a first separate aspect of the invention the process also includes the
 steps of mixing particles of a combustible material having a diameter in a
 range of 500 angstroms to 500 microns into the mixture and heating the
 porous ceramic composite in either air or oxygen to burn away the
 particles of combustible material to produce a porous ceramic composite
 with a bimodal pore size distribution.
 In a second separate aspect of the invention the process also includes the
 step of mixing granules, particles, whiskers or powders of a fumed silica
 into the mixture to increase the viscosity of the mixture.
 In a third separate aspect of the invention the process also includes the
 step of mixing granules, particles, whiskers or powders of a ceramic
 material into the mixture to increase the structural strength of the
 ceramic composite.
 In a fourth separate aspect of the invention a microfilter is formed out of
 a porous composite with a bimodal pore size distribution.
 In a fifth separate aspect of the invention a catalyst support is formed
 out of a porous composite with a bimodal pore size distribution.
 Other aspects and many of the attendant advantages will be more readily
 appreciated as the same becomes better understood by reference to the
 following detailed description and considered in connection with the
 accompanying drawing in which like reference symbols designate like parts
 throughout the figures.
 The features of the present invention which are believed to be novel are
 set forth with particularity in the appended claims.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 Referring to FIG. 1 in conjunction with FIG. 2 a process for making a
 microfilter 10 which is formed out of a porous ceramic composite with a
 monomodal pore size distribution includes the steps of mixing an
 organosilicon precursor, an alcohol, water, a catalyst, granules,
 particles, whiskers or powders of a fumed silica and granules, particles,
 whiskers or powders of a ceramic material to form a mixture, pouring the
 mixture into a mold, allowing the mixture to gel to form the microfilter
 10 and drying the microfilter 10. The microfilter 10 has a plurality of
 fine pores 11. The porous ceramic composite which is used to form the
 microfilter 10 can have a surface area as high as 2000 square meters per
 gram. In their paper, entitled "Novel Composite Materials for Space
 Structures and Systems," published in Proceedings of the 32nd
 International SAMPE Symposium, Volume 32, pages 760-771, the inventors
 have described the process for making the porous ceramic composite with a
 monomodal pore size distribution. The inventors hereby incorporate their
 above cited article herein by reference.
 The organosilicon precursor is selected from a group consisting of
 tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane and
 tetrabutoxysilane. The alcohol is selected from the group consisting of
 methanol, ethanol, propanol and butanol.
 The catalyst is selected from a group consisting of HF, HCl, HBr, HI, KF,
 KCl, KBr, KI, HNO.sub.3, H.sub.2 SO.sub.4, HOAce, NH.sub.3 OH, NH.sub.4 OH
 and NH.sub.4 Cl. In their above cited article, entitled "Sol-Gel
 Processing of Silica II, The Role of the Catalyst," the inventors have
 discussed all of these catalysts and hereby incorporate their above cited
 article herein by reference.
 The addition of the granules, particles, whiskers or powders of fumed
 silica increases the viscosity of the mixture. The organosilicon precursor
 and the water are miscible in the alcohol. The water in the mixture
 hydrolizes the organosilicon precursor to form the microfilter 10.
 In their article, entitled Sol-Gel Processing of Silica II, The Role of the
 Catalyst, published in the Journal of Non-Crystaline Solids, Volume 87
 (1986), pages 185-198, the inventors discussed their utilization of a
 mixture of an organosilicon precursor, an alcohol, water and a catalyst in
 the sol-gel processing of silica. When they varied the catalysts they
 observed dramatic effects on gelation time, porosity, bulk and apparent
 denisity and volume shrinkage on drying. They obtained porosities in the
 range from two to sixty eight percent for dried and fired gels. The
 inventors hereby incorporate their above cited article herein by
 reference.
 In their article, entitled "Porous and Dense Composites from Sol-Gel,"
 published in Tailoring Multiphas and Composite Ceramics, Material Science
 Research, Volume 20, pages 187-194, the inventors described that the
 addition of the granules, particles, whiskers or powders of a ceramic
 material which act as an insert filler increases the structural strength
 of the ceramic filter. In processing silica the ceramic material which may
 be a silicon carbide powder is dispersed in silica gel thereby rendering a
 porous silicon carbide-silica composite. The porous ceramic composites may
 also be prepared by using one of the other systems which follow, namely,
 SiC--SiO.sub.2, SiO.sub.2 --Al.sub.2 O.sub.3, SiO.sub.2 --Si.sub.3
 N.sub.4, SiO.sub.2 --TiC, SiO.sub.2 --Al, SiO.sub.2 --SiO.sub.2
 microspheres, SiO.sub.2 -fumed SiO.sub.2. The inventors hereby incorporate
 their above cited article herein by reference.
 In their article, entitled "Oxide-Nonoxide Composites by Sol-Gel,"
 published in Better Ceramics through Chemistry II, Material Research
 Society Symposium Proceedings, Volume 73, pages 809--814, the inventors
 described the process of making a light-weight, triphasic composite which
 includes the step of impregnating a polymer into of a porous gel
 composite. The light-weight, triphasic composite possesses good abrasion
 resistence and high fracture ductility. The inventors hereby incorporate
 their above cited article herein by reference.
 Referring to FIG. 1 in conjunction with FIG. 3 a process for making a
 microfilter 20 which is formed out of a porous ceramic composite with a
 bimodal pore size distribution includes the steps of mixing an
 organosilicon precursor, an alcohol, water, a catalyst, granules,
 particles, whiskers or powders of a fumed silica, granules, particles,
 whiskers or powders of a ceramic material and particles of a combustible
 material having a diameter in a range of 500 angstroms to 500 microns to
 form a mixture, pouring the mixture into a mold, allowing the mixture to
 gel to form the microfilter 20 and drying the microfilter 20. The
 combustible material may be either large organic molecules, such as ethyl
 cellulose, or large organic or carbon containing particles, such as
 acrylic polymer beads, fine sawdust or graphite.
 The addition of the granules, particles, whiskers or powders of fumed
 silica increases the viscosity of the mixture. The organosilicon precursor
 and the water are miscible in the alcohol. The water in the mixture
 hydrolizes the organosilicon precursor to form the microfilter 20.
 Still referring to FIG. 1 in conjunction with FIG. 3 the process also
 includes the step of heating the catalyst support in either air or oxygen
 to burn away the particles of the combustible material. The microfilter 20
 has a plurality of fine pores 21 with a diameter in a range of 20 to 500
 angstroms and a plurality of course pores 22 with a diameter in a range of
 500 angstroms to 500 microns. The fine pores 21 are interconnected to each
 other and the course pores 22 are isolated from each other, but are
 interconnected by fine pores 21. The microfilter 20 has a surface area of
 approximately 300 square meters per gram.
 Referring to FIG. 4 a microfilter 30 is an analog to the microfilter 20.
 The microfilter 30 includes a plurality of support layers 31 of a ceramic
 composite having interconnected course pores 32 and a pluralty of thin
 layers 33 of a ceramic composite having interconnected fine pores 34. Each
 thin layer 33 is deposited on one of the support layers 31 according to
 U.S. Pat. No. 4,562,021.
 Referring to FIG. 5 a process for making a catalyst support with a bimodal
 pore size distribution includes the steps of mixing an organosilicon
 precursor, an alcohol, water, a catalyst, particles of a fumed silica,
 granules of a ceramic material and particles of a combustible material to
 form a mixture, allowing the mixture to gel form a catalyst support and
 drying the ceramic filter. The process also includes the step of heating
 the catalyst support in either air or oxygen to burn away the particles of
 the combustible material. The process for introducing a catalyst into the
 catalyst support with a bimodal pore size distribution further includes
 the steps of impregnating the catalyst support with a dilute metal salt
 solution, drying the impregnated catalyst support and heating the
 impregnated catalyst support in a vacuum at an elevated temperature
 thereby reducing the metal salt.
 Still referring to FIG. 5 the catalyst support is formed out of a porous
 ceramic composite which has a plurality of fine pores with a diameter in a
 range of 20 to 500 angstroms and a plurality of course pores 32 with a
 diameter in a range of 500 angstroms to 500 microns. The fine and course
 pores are interconnected to each other.
 From the foregoing it can be seen that a process for making porous ceramic
 composites with a bimodal pore size distribution has been described. It
 should be noted that the sketches are not drawn to scale and that distance
 of and between the figures are not to be considered significant.
 Accordingly it is intended that the foregoing disclosure and showing made
 in the drawing shall be considered only as an illustration of the
 principle of the present invention.