Catalyst – solid sorbent – or support therefor: product or process – Solid sorbent – Free carbon containing
Reexamination Certificate
1999-07-07
2001-06-26
Hendrickson, Stuart L. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Solid sorbent
Free carbon containing
C502S427000
Reexamination Certificate
active
06251822
ABSTRACT:
This invention relates to a method of making various pore size distributions in the micropore range by a method that involves oxidizing a carbon precursor in the form of pitch, followed by activating. Mesoporous carbon is made from pitch by combining the pitch with selected metal catalysts.
BACKGROUND OF THE INVENTION
Activated carbon has found use in various applications such as air and water purification, hydrocarbon adsorption in automotive evaporative emission control and cold start hydrocarbon adsorption, etc. Microporous structure carbon (pore diameter less than 20 angstroms and BET surface area of 1000-3000 m
2
/g) are suitable for many applications such as gas phase adsorption e.g. light hydrocarbons and H
2
S, while other applications require larger size of pores in the carbon for optimum adsorption and/or catalytic activity. For example, removal of larger molecular size pollutants such as humine, protein, etc. in liquid phase, in addition to conventional gaseous pollutants, such as hydrocarbons, or certain kinds of pesticides require specific surface properties and pore size distributions. When catalytic or chemical reaction is limited by mass and heat transfer, larger size of pores in the carbon is preferred. Mesoporosity in the carbon is sometimes also required for adequate catalyst loading and dispersion.
Activated carbon monoliths, whether in the form of a coating on a substrate, or a shaped structure of activated carbon, have found use in various applications especially where durability and low pressure drop is required, such as in some chemical reactions using strongly acidic or basic solvents or other corrosive media.
Therefore it would be an advancement in the art to have a method of making activated carbon, especially in the form of honeycombs with various pore sizes to match the particular applications.
The present invention discloses such a method in which activated carbon having microporosity or mesoporosity is produced from pitch.
SUMMARY OF THE INVENTION
In accordance with one aspect of the invention, there is provided a method of making activated carbon of various pore size distributions that involves providing an oxidized pitch, carbonizing the pitch when the softening point of the pitch is less than about 250° C., and activating the pitch to produce activated carbon. A catalyst metal can be included with the pitch to produce mesoporosity in the activated carbon.
DETAILED DESCRIPTION OF THE INVENTION
This invention relates to making microporous and mesoporous activated carbon using pitch as a carbon precursor. The resultant microporous carbon is useful for many adsorption applications in both gas and liquid phases. The resultant mesporous carbon is useful for carbon catalyst support for chemical processing, and in liquid phase applications such as filtration of dyes, proteins, vitamins, humic acid, etc.
By microporous carbon is meant that at least about 80% of the pore volume has a pore diameter less than 20 angstroms and BET surface area of 1000-3000 m
2
/g.
By mesoporous carbon according to this invention is meant that at least 50% of the total pore volume is in the range of 20 to 500 angstroms and no more than 25 percent pore volume is in the range of large pores (>500 angstroms).
The starting carbon sources of this invention are a group of thermoplastic carbon forming materials, defined in this invention as pitch, including carbonaceous residues that are derived from processing petroleum, coal, or other fossil hydrocarbon sources, e.g petroleum pitches, and coal tar pitches; and synthetic pitches such as those derived from pyrolysis of naphthalene and polyvinychloride. Pitches are composed of condensed aromatic structures with varying degrees of substitution. They are thermoplastic, meaning that they have glass transition temperatures at either below or above room temperatures.
It is preferred that pitches of lower molecular weights and low softening points, such as viscous pitch or pitches having softening point less than 40° C. be used for coating applications. It is preferred that pitches with higher softening points (such as extruded solid pellets) having softening points higher than about 40° C., or preferably higher than about 100° C., be used for shaping applications.
The advantages of using pitch to make activated carbon versus other precursors such as thermosetting resins as phenolic resins are: (1) Carbon yields from pitches are much higher than from other sources such as thermosetting resins as phenolic resins. Typical carbon yields are 90% or greater. (2) The product is strong and experiences considerably less shrinkage during processing. (3) The raw materials and processing are both more cost effective than that with resins.
At some point before carbonization or activation, the pitch is oxidized to render it infusible. The oxidation can be done at about 200-300° C. in air for 1 to 5 hours. There is normally about 6-12% weight gain (based on the amount of carbon forming precursor) during oxidation because of addition of oxygen to the structure. The oxidation should be carried out as completely as possible to form a highly crosslinked structure.
One useful method of making the activated carbon derived from pitch is to coat an inorganic substrate such as a honeycomb with a coating suspension or solution of the pitch. The pitch is pre-dissolved in a suitable solvent such as hexane, toluene, tetrahydrofuran, etc. Choice of a solvent depends essentially on the nature of the pitch. Also it must be compatible with other components in the process.
The substrate has an outer surface from which pores extend into the substrate. The coating penetrates into and is distributed throughout these pores as a coating thereon.
In its most useful form the monolithic substrate has means for passage of a fluid stream therethrough, e.g., a network of pores communicating from the outside to the inside, and/or through channels extending from one end of the monolith to the other for passage of the fluid stream into one end and out through the other end.
The substrate must have enough strength to function in the application and be capable of withstanding the heat-treating temperature experienced in forming the activated carbon coating.
It is desirable that the overall open porosity of the substrate be at least about 10%, preferably greater than about 25% and most preferably greater than about 40%. For most purposes, the desirable range of porosity is about 45% to about 55%. Preferably the pores of the substrate material create “interconnecting porosity” which is characterized by pores which connect into and/or intersect other pores to create a tortuous network of porosity within the substrate.
Suitable porous substrate materials include ceramic, glass ceramic, glass, metal, clays, and combinations thereof. By combinations is meant physical or chemical combinations, e.g., mixtures, compounds, or composites.
Some materials that are especially suited to the practice of the present invention, although it is to be understood that the invention is not limited to such, are those made of cordierite, mullite, clay, magnesia, and metal oxides, talc, zircon, zirconia, zirconates, zirconia-spinel, magnesium alumino-silicates, spinel, alumina, silica, silicates, borides, alumino-silicates, eg., porcelains, lithium aluminosilicates, alumina silica, feldspar, titania, fused silica, nitrides, borides, carbides, eg., silicon carbide, silicon nitride or mixtures of these. Cordierite is preferred because its coefficient of thermal expansion is comparable to that of carbon, increasing the stability of the activated carbon body. Some typical ceramic substrates are disclosed in U.S. Pat. Nos. 4,127,691 and 3,885,977. Those patents are herein incorporated by reference as filed.
Suitable metallic materials are any metal or alloy or intermetallic compound that provides durable structural service, and does not soften below about 600° C. Particularly useful are alloys which are predominantly of iron group metal (i.e. Fe, Ni, and Co), either with carbon (e.g. steels, especially stainles
Peng Y. Lisa
Williams Jimmie L.
Corning Incorporated
Gheorghiu Anca C.
Hendrickson Stuart L.
Herzfeld L. Rita
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