Synthesis of microporous ceramics

Details Synthesis of microporous ceramics Synthesis of microporous ceramics

1996-06-21

2003-09-23

Szekely, Peter (Department: 1714)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

At least one aryl ring which is part of a fused or bridged...

C524S449000, C524S450000, C524S451000

Reexamination Certificate

active

06624228

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to open pore, microporous ceramic materials and their method of manufacture.
2. Description of Related Art
Porous materials play a particularly important role in a number of chemical processing industries and applications. Separations based on membranes are critical in such fields as chemical recovery, purification and dehumidification. Porous oxides (e.g., clays, silica, alumina and zeolite) are the materials of choice as catalysts or catalyst supports in chemical and petroleum processing reactions such as hydrocracking, hydrodesulfurization, reforming, and polymerization.
With respect to membrane technology, inorganic membranes offer a number of advantages over polymeric membranes which are typically limited to uses at temperatures below about 250° C. These include: i) higher operating temperatures, ii) greater structural integrity and hence the ability to withstand higher pressure differentials and backflushing and iii) improved resistance to corrosion. Porous oxide, (e.g., aluminum oxide) and carbon membranes offer some of these characteristics, but advanced materials are still required for improved strength, toughness, structural integrity, temperature stability, water and oxygen resistance, thermal shock resistance, molecular selectivity to small molecules and gases, and high flux.
Similar considerations apply to clay and metal oxide type catalysts or catalyst supports, particularly as relates to stability and thermal shock resistance at temperatures above about 500° C.
Ceramic materials of the Si—C, Si—N, Si—C—N, Si—B—C, Si—BN, Al—N, Si—Al—N, B—Al—N and related types appear to offer many of the properties set forth above. However, the solgel synthesis methods typically used to prepare porous oxide membranes or catalyst supports are incompatible with the preparation of ceramics of the type described above because of the need to use water in their preparation. Sintering or reactive sintering of these ceramics likewise produces materials with pore sizes of from about 0.1 to about 1000 microns which are non-uniform and generally too large for effective molecular separation and other uses described above. Chemical vapor deposition can produce microporous ceramic layers, but this tends to be an expensive, high temperature process with limited ability to tailor complex ceramic compositions.
Recently, researchers have discovered improved methods for preparing ceramics using ceramic precursors as starting materials. A ceramic precursor is a material, a chemical compound, oligomer or polymer, which upon pyrolysis in an inert atmosphere and at high temperatures, e.g., above about 700-1000° C., preferably above 1000° C., will undergo cleavage of chemical bonds liberating such species as hydrogen, organic compounds and the like, depending upon the maximum pyrolysis temperature. The resulting decomposition product is typically an amorphous ceramic containing Si—C bonds (silicon carbide), Si—N bonds (silicon nitride) or other bond structures which will vary as a function of the identity of the ceramic precursor, e.g., Si—C—N, Si—N—B, B—N, Al—N and other bond structures, as well as combinations of these structures. Crystallization of these amorphous ceramic products usually requires even higher temperatures in the range of 1200-1600° C.
The pyrolysis of various ceramic precursors, e.g., polycarbosilanes, polysilanes, polycarbosiloxanes, polysilazanes, and like materials at temperatures of 1200° C. and higher to produce ceramic products, e.g., silicon carbide and/or silicon nitride, is disclosed, for example, in M. Peuckert et al., “Ceramics from Organometallic Polymers”, Adv. Mater. 2, 398-404 (1990).
During pyrolysis, preceramic precursors such as described above liberate various gaseous decomposition species such as hydrogen and organic compounds, including methane, higher molecular weight hydrocarbon molecules, and lower molecular weight precursor fragments. These gases tend to coalesce within the preceramic matrix as they form, resulting in a bulking or swelling to form a voluminous mass of low bulk density. These entrained gases can also lead to the formation of smaller gas bubbles within the developing ceramic mass as the preceramic precursor crosslinks and hardens, resulting in a reduced density ceramic having a mesoporous or macroporous closed-cell structure, without development of a significant amount of open celled micropores.
Where dense, non-porous ceramic materials are sought using ceramic precursors yielding high gas volumes, it is often necessary to conduct the pyrolysis over a very long period of time with very gradual incremental temperature increases and/or under vacuum to assist in removal of these gaseous species at temperatures where they are formed.
SUMMARY OF THE INVENTION
The present invention provides for amorphous, microporous, ceramic materials having a surface area in excess of 50 m
2
/gm, preferably in excess of 100 m
2
/gm, and an open-pore microporous cell structure wherein the micropores have a mean width (diameter) of less than 20 Angstroms and wherein said microporous structure comprises a volume of greater than about 0.015 cm
3
/gm, preferably greater than 0.05 cm
3
/gm, of the ceramic. The invention also provides for a preceramic composite intermediate composition comprising a mixture of a ceramic precursor and finely divided particulate material selected from the group consisting of non-silicon containing ceramics, carbon, inorganic compounds having a decomposition temperature greater than 1000° C. and mixtures thereof, whose pyrolysis product in inert atmosphere or in an ammonia atmosphere at temperatures of up to less than about 1100° C. gives rise to the microporous ceramics of the invention. As used in this application, the term “non-silicon containing ceramics” is defined to exclude oxide phases. Also provided is a process for the preparation of the microporous ceramics of the invention comprising: a) forming an intimate mixture comprising from greater than 30 up to about 99 parts by weight of a ceramic precursor oligomer or polymer having a number average molecular weight in the range of from about 200 to about 100,000 g/mole and from about 1 to less than 70 parts by weight of particulate material selected from the group consisting of non-silicon containing ceramics, carbon, inorganic compounds having a decomposition temperature greater than 1000° C. and mixtures thereof, said particles having a mean particle size of less than about 10 microns, b) gradually heating said mixture in the presence of an inert gas or ammonia gas and in sequential stages with hold times at intermediate temperatures to a maximum temperature in the range of from about 400° C. up to less than about 1100° C. and over a period of total heating and hold time of from about 5 to about 50 hours to form a microporous ceramic product, and c) cooling said microporous ceramic product.
The microporous ceramics prepared in accordance with this invention generally exhibit a surface area within the range of from about 50 to about 400 m
2
/gm based on the combined weight of amorphous phase and particles, and amorphous phase micropore volumes of greater than 0.015 up to about 0.17 cm
3
/g, wherein the volume fraction of micropores in the ceramic product ranges from about 5% to about 32%.
Ceramics produced in accordance with this invention are particularly useful in bulk sorbent applications, as active layers in membrane separation structures and as catalyst supports.
DETAILED DESCRIPTION OF THE INVENTION
As used herein, the term microporous ceramic refers to a ceramic having a porous structure wherein the pores have a mean width (diameter) of less than 20 Angstroms. This definition and the physical and chemical adsorption behavior of microporous materials are disclosed in S. J. Gregg and K. S. W. Sing, “Adsorption, Surface Area and Porosity”, Academic Press, New York, 1982; and S. Lowell and J. F. Shields, “Powder Surface Area and Porosity”, Chapman and Hall, New York, 3rd Edition, 1984. This term is

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