Monolithic activated carbon

Details Monolithic activated carbon Monolithic activated carbon

1998-12-07

2001-03-27

Cole, Elizabeth M. (Department: 1771)

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

Composite having voids in a component

C428S315500, C428S315700, C428S317100, C428S317700, C428S317900

Reexamination Certificate

active

06207264

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The present invention relates to activated carbon articles of manufacture. More particularly, this invention relates to activated carbon monoliths having a relatively high surface area.
BACKGROUND OF THE INVENTION
Activated carbon is an excellent adsorbent. Activated carbon is commercially available in various forms, including granules and powders. In some applications of activated carbon, void spaces between carbon particles are important both for ensuring sufficient adsorbate contact and for allowing fluid to pass through the carbon adsorbent without encountering an excessive pressure drop. In other applications, however, the void space is not a principal concern. In the storage of hydrogen, ammonia, or natural gas, for example, carbon adsorption is most efficient on a volume basis when the carbon is formed into a high-density block with most of the void volume between the individual particles greatly reduced or even eliminated.
Efforts at creating such high-density solid structures are reflected in techniques developed for compaction and binding of activated carbon particles. For example, U.S. Pat. No. 4,000,236 to Redfarn et al. discloses a method for making a conglomerated activated carbon mass by means of a polymer rendered adhesive by a solvent. U.S. Pat. No. 4,717,595 to Watanabe et al. describes a method for producing carbonaceous material from carbon particles covered with a binder. U.S. Pat. No. 5,306,675 to Wu is directed to a method for producing activated carbon structures using methyl cellulose binders and microwave radiation curing.
These and other conventional techniques suffer from one or more serious drawbacks as follows: loss of surface area, corrupted pore distribution, limited temperature resistance, overly fragile green state, and high manufacturing costs. The carbon structures produced utilizing such methods also tend to have reduced surface area, lower adsorption capacity, and an undesirable pore size distribution. Furthermore, these carbon structures are not temperature-resistant, but tend to disintegrate when subjected to elevated temperatures.
Conventional binding techniques in particular cause a significant loss in available surface area for the activated carbon particles. With heretofore available techniques, binding and related agents are known to plug pores of the activated carbon particles whereby the favorable pore-size distribution of the original carbon particles is corrupted in favor of undesirably larger pore sizes.
This plugging phenomena is especially problematic for activated carbons with very high surface areas (>2000 m
2
/g). Efforts at using polymeric resins to bind very high surface area (>2000 g/m
2
) carbons have generally failed heretofore because the resulting carbon structures had either surface areas greatly reduced from those of the original carbon particles or inadequate mechanical strength. In addition to reduced surface area, the thermal stability of carbon structures made by conventional techniques is inadequate for many otherwise appropriate applications.
Inorganic binders also have been used as binders in carbon mixtures to impart strength and thermal stability. For example, U.S. Pat. No. 4,518,704 to Okabayashi et al. describes a process for making activated carbon bodies using a clay binder. Unfortunately, a very expensive sintering step is required for such inorganic binders, e.g. firing at 900° C. in an inert atmosphere. Furthermore, the mechanical strength of such bodies is inadequate for many applications.
Many uses for activated carbon require that the adsorbent fit into canisters and other devices of varying shapes and sizes. Such potential applications for activated carbon articles of manufacture thus far have gone unrealized because the required shapes and sizes for the solid articles could not be obtained. Standard binding methods for creating activated carbon structures often do not permit molding into unique shapes and sizes because the uncured, or green state, of the structure is either too fragile or too inflexible, thereby limiting workability.
Thus, there continues to be a need for improved very high surface area activated carbon structures as well as for methods for making such structures. The need also exists for methods of making activated carbon structures from activated carbon particles generally, without a substantial loss in carbon surface area.
SUMMARY OF THE INVENTION
A highly adsorbent monolithic activated carbon article is obtained by wetting activated carbon particles preferably having a surface area in excess of about 2000 m
2
/g, forming an aqueous emulsion of a polymeric binder, combining the resulting aqueous emulsion with the wetted carbon to produce a slip mixture, forming the obtained slip mixture into a shaped structure, and thereafter curing the structure.
The monolithic activated carbon article produced in the foregoing manner exhibits substantially no change in compressive strength upon heating to 275° C. and consists essentially of the activated carbon and a polymeric binder present in an amount up to 13 percent, based on the weight of the composition. The monolithic activated carbon article has an open pore structure, a surface area in excess of 2000 m
2
/g of article, a pore size distribution such that at least 50 percent of total pore volume is constituted by pores less than about 2 nanometers in diameter and at least about 75 percent of total pore volume by pores less than about 4 nanometers diameter. The monolithic activated carbon article has a bulk density of at least about 0.25 grams/cubic centimeter.
The present invention also provides microporous, monolithic carbonaceous articles having a formation efficiency, as defined below, in excess of about 75 percent and exhibiting substantially no loss in compressive strength upon heating to 200° C. For this aspect of the present invention, the carbonaceous articles consist essentially of activated carbon having a surface area in excess of about 1100 m
2
/g and a polymeric binder present in an amount up to 13 percent based on the weight of the article.
Another aspect the present invention provides microporous, monolithic carbonaceous articles that have a butane activity in excess of about 10 grams per deciliter, consist essentially of activated carbon and a polymeric binder present in an amount up to 13 percent (based on the weight of the article) but exhibit substantially no loss in compressive strength upon heating to 200° C.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
While this invention is susceptible of embodiment in many different forms, there are described herein in detail specific embodiments thereof with the understanding that the present disclosure is to be considered as an exemplification of the principles of the claimed invention and is not to be limited to the specific embodiments illustrated.
As used herein the term “formation efficiency” refers to the surface area of unformed (i.e., unbonded) activated carbon per unit mass divided by the surface area of the carbonaceous article per unit mass of article, and is expressed as a percentage.
The monolithic article of the present invention consists essentially of activated carbon particles bonded together with a polymeric binder while maintaining an open pore structure. The monolithic article is formed from free flowing activated carbon particles having a surface area greater than about 2000 m
2
/g and pores preloaded with water, which particles are combined with a polymeric binder in an amount up to about 13 percent by weight of the resulting composition to form a slip mixture which is then cured. The preloading of pores is effected by prewetting the carbon particles with water.
The resulting mass is a slip mixture which is then formed to a desirable shape by manipulation such as molding or extrusion. The article is then cured to a rigid shape by heating. Suitable emulsifying and rheology modulating agents may be added to the slip mixture as necessary to achieve a desired consistency.
The solid monolithic articles of the present in

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