Catalyst – solid sorbent – or support therefor: product or process – Solid sorbent – Free carbon containing
Reexamination Certificate
1998-05-14
2003-02-04
Hendrickson, Stuart L. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Solid sorbent
Free carbon containing
C423S44500R
Reexamination Certificate
active
06514906
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a carbonaceous char having reduced catalytic activity for the decomposition of hydrogen peroxide and other materials susceptible to catalytic chemical conversion in the presence of the char.
BACKGROUND OF THE INVENTION
Carbonaceous chars such as activated carbons, especially those produced at high temperatures (i.e., above 700° C.), often possess catalytic properties which may interfere with their use in certain applications. The origins of this catalytic activity can usually be attributed to two principal factors: (1) the inorganic non-carbon ash constituents of the char, e.g. iron, potassium, and calcium, and (2) the inherent catalytic properties of the carbon itself. If the char is used for the adsorption, removal, and/or recovery of substances from fluid streams, the presence of catalytic reactivity in the char may lead to an unwanted chemical conversion of one or more of the stream components into materials that contaminate the final process stream, interfere with the physical adsorption of the other stream components, or interfere with other intended functions of the char or stream components. Moreover, the reactions which lead to such catalytic chemical conversion are often highly exothermic, increasing the likelihood of ignition of the char itself under some conditions.
Examples of processes in which the catalytic reactivity of the char is a liability include the purification or use of reactive or oxidizable products such as hydrogen peroxide and organic glycols, and the adsorption, recovery, and re-use of reactive or oxidizable solvents such as acetone and methyl ethyl ketone. In these types of applications, the component of the stream with the highest utility is itself a material which can be decomposed or chemically converted if catalyzed by the carbon under certain conditions. In other applications it may be desirable to preserve a reactive constituent of the process stream, e.g. hydrogen peroxide, such that it is available for reaction with species other than the carbon itself. Removal of the catalytic properties of the carbon would allow more of such components to exist for their intended function. Less obvious is the need for a non-catalytic, adsorptive carbon for applications in which the potentially reactive component is an interference, constituting only a small part of the adsorbate loading. For example, in streams containing small concentrations of an oxidizable material such as hydrogen sulfide and much higher concentrations of an adsorbable, recoverable, and re-usable organic, it is possible that the oxidation of the hydrogen sulfide, which is only weakly physically adsorbed per se, can cause a build-up over time of highly adsorbable and polar reaction products, such as sulfuric acid, which can greatly interfere with the adsorption of the organic components of the stream. If the inherently reactive carbon catalytic sites can be deactivated, maximum use can be made of the physical adsorption properties of the carbon for the removal and recovery of the organic stream components.
Prior art methods to reduce the catalytic reactivity of carbonaceous chars have been directed almost exclusively to the removal or deactivation of catalytically-active ash constituents which may be present in the char. For example, it is well known to remove acid-soluble ash constituents by acid-washing of the char. After treatment, the char is invariably rinsed with water and/or an aqueous solution of a base to remove and/or neutralize the acid. Hydrofluoric acid solution, an extremely hazardous material, has been found to be particularly effective for removing such ash components. Ash reductions exceeding an order of magnitude are usually achievable by these methods. Other ash removal processes treat the char first with caustic solution to remove caustic-soluble components, followed by the aforementioned acid extraction treatment to remove the remaining acid-soluble components. In other embodiments of the art, the ash components are treated with agents such as silanes to reduce their catalytic reactivity without removing them from the carbon surface. In all of these prior art methods, however, little regard has been given to the inherent catalytic activity of the carbon itself, which alone may be sufficient to interfere with the intended function of the char.
Where the reactivity of the carbon has been considered, no methods have been identified or reported which can deactivate the carbon effectively and largely irreversibly. For example, it has been observed that oxygen can chemisorb to the carbon surface and cause the carbon to become less catalytically active. However, after use, and upon high-temperature thermal treatment of the char to remove other adsorbed materials, it is found that the oxygen is also lost from the carbon. Upon removal of the oxygen, an increase in the inherent catalytic activity of the carbon once again becomes manifest, requiring additional post- treatment of the thermally-treated char to deactivate the carbon prior to re-use. Therefore, the oxygen appears to have merely masked, and not destroyed, the reactivity of the carbon.
Accordingly, it is the object of the present invention to provide a carbonaceous char having reduced inherent catalytic activity for use in those applications where the inherent catalytic reactivity of the carbon is a concern. It is further the object of the present invention to provide a carbonaceous char having reduced catalytic activity wherein said reduced activity is largely unaffected by exposure to high-temperatures and is, in fact, enabled by such temperatures.
SUMMARY OF THE INVENTION
Generally, the present invention comprises a carbonaceous char having an inherent carbon catalytic activity which is greatly reduced when compared to chars of the prior art derived from wood, nutshell, fruitstone, peat, lignite, sub-bituminous coal, bituminous coal, semi-anthracite coal, anthracite coal, or synthetic polymers. Moreover, the reduced inherent catalytic activity provided by such chars is largely irreversible upon high-temperature thermal treatment of the char.
In a preferred embodiment of the invention, such chars are prepared by first contacting a char with a non-nitrogen-containing Bronsted acid, such as hydrochloric acid, at temperatures at or below the boiling point of the acid or aqueous acid solution. Thereafter, the treated char is heated to a temperature above 750° C. Nitrogen-containing acids such as nitric acid are known to increase the inherent catalytic activity of carbonaceous chars under some treatment conditions and are, therefore, not generally preferred in the preferred embodiment of the present invention. The amounts of acid used in the preferred embodiment are such that the carbon contains preferably 1-10 millimoles of acid hydrogen per mole of carbon prior to raising the temperature of the acid-containing carbon to or above 750° C. It may also be desirable to use such acid to reduce any non-carbon contributions to the overall catalytic activity of the char. This is not necessary for those applications requiring only the deactivation of the inherent catalytic activity of the carbon itself.
If the char is rinsed with water following acid treatment, the rinse is preferably carried out so that the pH of the rinse water is less than 5, preferably less than 1, to ensure that sufficient acid is present on the char to achieve the required deactivation of the catalytic carbon sites upon raising the temperature. Upon exposure of the acid-containing char to temperatures equal to or greater than 750° C., the deactivation of the carbon reactivity is largely effected. Heating the acid-containing carbon is carried out preferably in an inert or oxygen-free atmosphere, while cooling the deactivated char to ambient temperatures is preferably carried out in an oxygen-containing atmosphere to impart additional, reversible deactivation properties to the char according to the methods of known prior art. Other prior art methods for deactivating other reactive elements of the c
Calgon Carbon Corporation
Cohen & Grigsby P.C.
Hendrickson Stuart L.
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