COMPOSITE MICROPOROUS CARBONS FOR FUEL GAS STORAGE

Abstract: A method of making a microporous carbon material which includes providing a precursor selected from the group consisting of crystalline salts of aromatic sulfonates and nonporous polymeric salts. Prior to carbonization, the precursor is treated by ion exchange or other conventional processes to introduce a metal such as iron, nickel and cobalt into the precursor. Heat treatment of such composite precursors in the temperature range of 350 to 850 DEG C in an inert oxygen free atmosphere for a sufficient time promotes thermally induced hydrogen abstraction and rearrangement of BSUs which result in the formation of a composite, carbonaceous microporous material which contains a metal component, and has a pore size distribution in the range of about 4-15 ANGSTROM . The structural and storage characteristics of the microporous carbon materials are also disclosed.

Description: COMPOSITE MICROPOROUS CARBONS FOR FUEL GAS STORAGE Background of the Invention The present invention relates in general to activated carbon materials, and more specifically to carbon adsorbents suitable for use as storage media for light fuel gases. Porous carbons and carbon molecular sieving materials are widely used in adsoφtion applications which include gas separations and other chemical applications based on physical adsoφtion. The following U.S. patents are typical of the prior art and teach a wide variety of materials and processes relating to die current applications for activated carbons: U.S. Patent No. 4,205,055 - Maire et al. U.S. Patent No. 4,261,709 - Itoga et al. U.S. Patent No. 4,263,268 - Knox et al. U.S. Patent No. 4,526,887 - Sutt, Jr. U.S. Patent No. 4,540,678 - Sutt, Jr. U.S. Patent No. 4,594,163 - Sutt, Jr. U.S. Patent No. 4,775,655 - Edwards et al. U.S. Patent No. 4,832,881 - Arnold, Jr. et al. U.S. Patent No. 4,902,312 - Chang U.S. Patent No. 5,071,450 - Cabrera et al. U.S. Patent No. 5,086,033 - Armor et al. U.S. Patent No. 5,098,880 - Gaffney et al. U.S. Patent No. 5,208,003 - Simandl et al. U.S. Patent No. 5,232,772 - Kong U.S. Patent No. 5,298,313 - Noland U.S. Patent No. 5,300,272 - Simandl et al. Although the above prior art teaches porous carbons for a wide variety of usage, the above patents do not teach the use of these materials as a storage medium for light fuel gases at die supercritical conditions required for such applications. Furthermore, die above prior art requires that die activated carbon be foπned by multiple process steps which are bodi tune consuming and costly, and do not provide for a carefully controlled pore size range which is a requirement for optimal gas storage. U.S. Patents 4,839,331 and 4,040,990 are directed to the formation of carbonaceous adsorbents from pyrolized polysulfonated polymers, but do not teach or suggest the use of these materials for gas storage. The '331 and '990 patents include die use of starting materials which are macroporous, and require multiple steps in order to achieve die activated carbon product. Furthermore, the patents teach the foπnation of activated carbons having a multimodal pore size distribution with a pore sizes rangmg between 50-10,000 A. Activated carbons having pore sizes with such a wide size distribution would not be suitable for use as gas storage materials. U.S. Patents 4,716,736 and 5,385,876 to Schwarz et al. teach the use of activated carbon materials suitable for use as an adsorbent for light gases such as hydrogen and methane. These patents however require methods of preparation in which die activated carbon is formed by multiple process steps. In addition, an article entitled Influence of Pore Geometry on the Design of Microporous Materials for Methane Storage, by R. Cracknell, P. Gordon and K.E. Gubbins, which appears in J. Phys. Chem., 1993, 97, 494-499 addresses the advantage of storing methane by adsoφtion in microporous materials, and d e merits of currently available zeolites and porous carbons. The article is dieoretical in nature, and concludes that the prior art fails to teach or provide the technology to economically store methane, and that considering the state of the art, that it would be more economical to store mediane as a bulk fluid. The article observes that key factors which are important in the design of a suitable microporous material are first that the microporous material be such that the amount adsorbed minus the amount retained, when die methane is released, should be a maximum. Second, diat the microporosity (fraction of the micropore volume) should be a maximum; diat is die space taken by die atoms of the microporous material and die space wasted by poor packing of the crystallites should both be minimized. The audiors found that adsoφtion in a porous material offers the possibility of storing methane at high density while maintaining moderate physical conditions for die bulk phase, and that die search for a suitable material is currently an active area of research. It can therefore be seen from above that d ere is a continuing search for suitable light fuel gas storage materials, and that a key objective in developing such a material is the formation of a geometry that provides for optimum pore size distribution that maximizes die excess adsoφtion, i.e. the density in the pore minus the bulk density, for a given temperature and pressure. The key objective in developing such a material, is to provide a geometry which will provide the maximum storage which is recoverable for use. Summary of the Invention The present invention is directed to microporous carbon adsorbent materials suitable for use as storage media for light fuel gases having a uniform pore size distribution in the range of about 4 to 15 A, and a high surface area in which the micropores comprise the major proportion of the surface area. The precursor for d e preparation of these microporous carbons are crystalline salts of aromatic sulfonates or nonporous polymeric salts with highly organized structure. The micropores are created during heat treatment or pyrolysis within a critical temperature range. In one embodiment, microporous carbons with a pore stmcture tailored for adsoφtion of methane molecules at ambient temperatures and with sufficient bulk densities are prepared by temperature controlled heat treatment of a poly(sodium-4-styrenesulfonate) precursor. The precursor has a M.W. of 70,000 and is in a crystalline powder form. The precursor is carbonized in an oxygen free inert atmosphere such as nitrogen at temperatures in the range of about 450 to 850° C. The carbonized samples are then water extracted until the pH of the extract is in the acid range. By carefully selecting the heating rate, the final heat treatment temperature and time at temperature, the micropore size distribution and bulk densities of the resulting products can be controlled and modified. For example, higher heat treatment temperatures can produce microporous carbon with higher bulk density (~ 0.8 g/cm 3 ), and also higher methane adsoφtion capacity at higher pressures (- 150 V/V at 50 atm.). The most favorable adsorbent for methane at lower pressures and higher temperatures is prepared at the lower end of ie heat treatment temperature range. For an adsorbent for methane storage hi the following working conditions: charging up to 50 atm, discharging down to - 1 atm with a working temperature between 20 and 35° C, carbonization at about 800° C is recommended. Porous carbons formed by the method of the present invention can be prepared with uniform pore size distribution and high surface area, and with a uniform surface structure containing few if any mesopores or macropores. The change in heat treatment or pyrolysis conditions may introduce some level of heterogeneity in the pore sizes and surface functionalities, but the average size of the majority of pores remain around 10 A. These materials are therefore particularly suitable for use as storage media for light fuel gases such as methane or hydrogen, or as catalyst supports. Brief Description of the Drawings For a fuller understanding of the nature and objects of die invention, reference should be made to die following detailed description of a preferred mode of practicing the invention, read hi connection with the accompanying drawings, in which: FIG. 1 are Differential Thermal Analysis thermograms of three precursors selected for use in the present invention. FIG. 2 is a Nitrogen Adsorption Isotherm at -196° C for the carbon sample of Example 1. FIG. 3 A, B, and C are Adsoφtion isotherms for SF 6 , CF 4 and CH 4 at three temperatures for the carbon sample of Example 1. FIG. 4 are pore size distributions for die carbon sample of Example 1, evaluated by SF 6 , CF 4 and CH 4 . Detailed Description of the Invention As will become apparent to those versed in the art of solid state chemistry, all of the following criteria are important in selecting the precursors for use in making the activated carbons of the present invention. We have found that microporous carbons with controlled pore size distributions in the range of 4 to 15 Λ and with high bulk density as materials suitable for storage of light gases can be prepared from crystalline salts of aromatic sulfonates, in particular, naphthalene sulfonates and disulfonates, andiraquinone sulfonates and polystryrene sulfonates. These precursors are characterized with high theπnal stability and melting pomts higher than 300° C. Heat treatment of such compounds at sufficiently high temperatures and in die absence of oxygen can promote thermally mduced hydrogen abstraction and formation of macrocyclic structural units hi the solid phase before melting. Thus, for these precursors the well-known phenomenon of "aromatic growth" can be assumed to occur i the solid state. For instance, SEM results of a CNDS, a carbon derived from naphthalene disulfonates (NDS), clearly confirm that carbonization of this compound occurs predominantly from the solid state. However, it cannot be excluded that carbonization from d e vapor phase may also have taken place to a minor extent. Solid state carbonization togetiier with the presence of alkali metal cations and the released sulfur containing intermediates, which also form melted phases during carbonization are factors controlling the size and thickness of the formed basic structural units (BSUs). Carbonaceous materials obtained from NDS for instance consist of black plate-like crystallites having a residual hydrogen content < lwt%. High resolution TEM clearly reveals that bodi high temperature and low temperature materials obtained from this precursor are composed from BSUs with sizes about 10 A and composed from two or three graphite-like layers. Small-angle X-ray scattering (SAXS) results also demonstrate mat the basic structural units in these samples have graphitic structure but show that there is considerable distortion of lattice planes due to micropores. The carbonization process and the microstructural characteristics of the carbonaceous materials obtamed by the present mvention have been studied by Differential Thermal Analysis (DTA), and analyses of nitrogen, methane, CF 4 , and SF 6 adsoφtion isotherms by conventional procedures well known to the art. Thermograms from Differential Thermal Analysis of three selected precursors are presented in FIG. 1. These results show that die formation of carbonaceous material from these precursors takes place in a relatively narrow temperature range characterized with simultaneous exotiiermic and endothermic transformation. These transformations are related to decomposition, aromatic growdi and structural rearrangement occurring within a close temperature reghne. This fact suggests that the heating rate has significant influence on the microstructure of the foπned carbonaceous material. High temperature endotherms are related to additional rearrangement within the formed carbon framework, thus suggesting temperature ranges where different microtextures may form. All soφtion isotiieπns in die low pressure range were carried out on a GEMINI 2370 Soφtion Analyzer (Micromeritics). Before each experiment, the samples were heated for 10 hours at 200 °C, and tiien outgassed at this temperature under a vacuum of IO "5 atmospheres. Soφtion of nitrogen was measured at -196° C, and the data obtained were used to evaluate specific surface areas S N2 , total pore volumes and micropore volumes (Vmic) using the Dubinin-Radushkewich equation. Evaluation of die surface area which comprises the micropore as a percentage of the total surface area of die material is described in die article Micropore Structure Development in Poly(Sodium-4- Sty renesulf onate) Derived Carbons, by K. Putyera, J. Jagiello, T.J. Bandosz, and J.A. Schwarz which appears in Carbon, 1995, Vol. No. page which is incoφorated herem by reference. Typical values should range from about 50 to 90% and preferably from about 70 to 85%. A typical nitrogen adsoφtion isotherm is shown i FIG. 2. The rapid increase in the adsorbed volume at low pressure indicates die presence of micropores in this material. An additional increase close to atmospheric pressure is due to larger pores, probably related to itercrystalline spaces. Soφtion of methane, tetrafluorocarbon and sulfur hexafluoride was measured at three different temperatures near ambient using the same apparatus, but equipped with a thermostatic system. FIG. 3 presents adsoφtion isotherms of CH 4 (a), CF 4 (b) and SF 6 (c). These isotiierms were used for evaluation of micropore size distributions accorduig to Relationship Between Energetic and Structural Heterogeneity of Microporous Carbons Determined on the Basis of Adsorption Potentials in Model Micropores, by J. Jagiello and J.A. Schwarz; which appears in Langmuir, 1993, 9, 2513-2517. A Cahn 1000 microbalance is used for storage measurements. Data are reported on a weight/weight basis as well as a volume/volume basis. The latter was calculated based on the mercury density of the adsorbent. A more complete description of die storage determination is set forth in die article Hydrogen Storage Systems, by J.A. Schwarz and K.A.G. Amankwah which appears in the U.S. Geological Survey Professional, Paper 1570, which is incoφorated herem by reference. The following examples illustrate various embodiments of the present invention in which carbon samples were prepared at different temperatures in the range of 500° C to 850° C. Example 1 A carbon sample made according to the present invention is prepared by temperature controlled heat treatment of naphthalene- 1,5-disulfonate, disodium salt (NDS), available from Aldrich under Catalog No. 25,089-9. A 10 gm sample of NDS is placed in a sealed quartz tube in an electrically heated oven. The carbonization is carried out in a nitrogen flow (20 ml/min) witii a heating rate of 10 deg/min up to 650° C and then kept at this temperature for 3 hours. After cooling under nitrogen, the material is washed with deionized water to dissolve and remove the sodium salt and soluble intermediates. The porous carbon product is tiien washed with ethanol. The carbon particles produced by tlie above process have an average size of about 10 μm and exhibit a uniform micropore size distribution in the range of about 4 to 15 A. The particles of tiie carbon product have die same general shape as the original precursor, that is tiiey are roughly hexagonal plate-like crystallites. However, SEM results show that d e majority of the particles are deeply cracked and fractured due to high porosity. Under high resolution TEM the porous carbon product shows microtexture composed from randomly distributed small basic structural units of die size of about 10 A, correspondmg to stackups of 2 or 3 polyaromatic layers. Inside the pore walls d e orientation of BSU are almost parallel. The surface area, total pore volume and micropore volume (DR method) for this sample are determined from N 2 adsoφtion data and die results are given in Table IA. H 2 and CH 4 storage capacity for these materials were determined and the results are given in Table IB. Table IA V τ total V * m •ic [m 2 /gm] [ml/gm] [ml/gm] 795 0.48 0.18 Table IB Gas gm/gm v/v p (atm) T(°C) CH 4 0.093 123.73 51.42 27 H 2 0.024 254.294 5.04 -196 Example 2 The process of Example 1 is repeated except diat die sample is carbonized with a 5 deg/min heating rate up to 650° C and tiien kept at this temperature for 3 hours. The resulting carbon was then evaluated as in Example 1. The results are given in Tables 2A and 2B. Table 2A * total V * mi ■c [m 2 /gm] [ml/gm] [ml/gm] 740 0.33 0.16 Table 2B Gas gm gm v/v p (atm) T(° C) CH 4 0.081 107.52 51.17 27 H 2 - - - - Example 3 The process of Example 1 is repeated except that die sample is carbonized with a 10 deg min heathig rate up to 550° C and tiien kept at this temperature for 2 hours followed by a 10 deg/min heating rate to 650° C and held at this temperature for 1 hour. The resulting carbon was then evaluated as in Example 1. The results are given hi Tables 3A and 3B. Table 3A ^N 2 v γ total V ' m ■ic [m 2 /gm] [ml/gm] [ml/gm] 650 0.25 0.1 Table 3B Gas gm/gm v/v p (atm) T(°C) CH 4 0.050 66.87 30.81 27 H 2 - - - - Example 4 The process of Example 1 is repeated except diat the sample is carbonized with a 10 deg/min heating rate up to 850° C and then kept at this temperature for 3 hours. The resulting carbon was then evaluated as in Example 1. The results are given in Table 4 A. No storage data were recorded. Table 4A ^N 2 " total V ' mi •c [m 2 /gm] [iid/gm] [ml/gm] 950 0.63 0.19 Example 5 The process of Example 1 is repeated except that the sample is carbonized with a 5 deg/min heating rate up to 650° C and then kept at tiiis temperature for 1 hour followed by 10 deg/min heating rate up to 850° C and tiien kept at this temperature for 2 hours. The resulting carbon was then evaluated as i Example 1. The results are given hi Tables 5 A and 5B. Table 5A SN 2 " total V " m •ic [m 2 /gm] [ml/gm] [ml/gm] 1030 0.43 0.23 Table 5B Gas gm/gm v/v p (atm) T(° C) CH 4 0.104 137.82 51.29 27 H 2 - - - - Example 6 The process of Example 1 is repeated except that die precursor used is poly(sodium-4-styrenesulfonate) available from Aldrich under Catalog No. 24,305-1. The sample is carbonized at 500° C and then kept at this temperature for 3 hours. The sample was then evaluated as hi Example 1. The results are given in Tables 6 A and 6B. Table 6A ^N2 Molal V ' m •ic [m 2 /gm] [ml/gm] [ml/gm] 600 0.19 0.14 Table 6B Gas gm/gm v/v p (atm) T(°C) CH 4 0.072 66.76 50.21 27 H 2 - - - - Example 7 The process of Example 6 was repeated except that carbonization was carried out at 600° C. The results are given in Table 7 A. No storage data were recorded. Table 7A $N2 " total V " mi ■c [m 2 /gm] [ml/gm] [ml/gm] 645 0.21 0.15 Example 8 The process of Example 6 was repeated except diat carbonization was carried out at 650° C. The results are given in Table 8 A. No storage data were recorded. Table 8A N 2 * total V m •ic [m 2 /gm] [ml/gm] [ml/gm] 700 0.25 0.16 Example 9 The process of Example 6 was repeated except that carbonization was carried out at 850° C. The results are given hi Tables 9A and 9B. Table 9A ^N2 * total V * mic [m 2 /gm] [ml/gm] [ml/gm] 850 0.38 0.20 Table 9B Gas gm/gm v/v p (atm) T(° C) CH 4 0.109 120.08 53.88 27 H 2 0.036 322.61 41.54 -196 The results presented above provide die evidence of micropore structure development in carbons obtained by carbonization of carefully selected precursors. The choice of heating rate, carbonization temperature and time at temperature are very important factors in that the carbonization of organic precursors hi different temperature regimes is accompanied by chemical transformations within the materials. A suitable heating rate ranges from about 1 to 40 deg/min, and preferably from about 5 to 15 deg/min. The carbonization temperature may range from about 300 to 1000° C, and preferably from about 350 to 850° C. The time at the carbonization temperature may vary from about 1 to 10 hours, and preferably is in the range of about 1 to 4 hours. For example, the carbonization of poly(sodium-4-styrenesulfonate) results in possible structural rearrangement of the polycondensed units in the carbonaceous material, and the release of gaseous intermediates during heat treatment at higher temperatures, which are probably the factors that determine the properties of the microporous carbons. This indicates that by changing the carbonization temperature of poly(sodium-4-styrenesulfonate), one can obtain microporous carbon with desired pore size distributions. The optimum in materials' properties depends upon its final application. A more thorough understanding of the present invention can be obtained by a reading of the 1995 technical paper in Carbon referenced to above which is incoφorated herein by reference. The only known microporous carbon material available in the art, which is relevant to the present invention, is manufactured and sold under the tradename Maxsorb by Kansai Coke and Chemicals Co. Ltd., Amagasaki City, Japan. This material is believed to be made by a multiple step, complex process. The material is very expensive. Maxsorb has a micropore size distribution of about 7 to 20 A, with considerable amount of pores in the mesopore range. A further disadvantage of Maxsorb is tiiat it exhibits unfavorable packing between its BSUs which results in a mercury density of about 0.485 gm cm 3 . The materials of the present invention described above are manufactured directly in a one step carbonization process from carefully selected crystalline precursors. These materials exhibit a narrowly controlled pore size range of about 4 - 15 A and a mercury density of about 0.5 to 1.0 gm cm 3 , with a preferable density of about 0.8 to 1.0 gm/cm 3 , which make them uniquely suitable for light gas storage. In a further embodiment of the present invention, it has been discovered that the introduction of a metal or metal components hito the precursor enhances the storage capacity of the resulting carbonaceous material for hydrogen and methane. The chemical properties of the precursors described above allows one to introduce metals via ion exchange or other processes well-known to the skilled in d e art of preparation of supported metal catalysts. The introduction of metal containing phases either in their elemental form or as clusters does not effect the development of microporosity of the precursor during carbonization. The carbon precursors as described herem, and metals such as tiiose elements included in die family iron, nickel and cobalt; elements which form stable carbides; and combinations of these metals which fonn alloys or intermetallic compounds; and those metals which are known to activate methane, such as magnesium are applicable to tiiis embodiment of the invention. In one specific embodhnent, it has been found tiiat nickel introduced by ion exchange into 1-5 naphtiialene disulfonate at levels in die range of 1-50% (w/w) in the resulting carbon enhances bom methane and hydrogen storage when compared to the microporous carbon without the presence of nickel. The foUowing example illustrates tiiis embodiment of the present invention. Example 10 A nickel-containing carbon sample made according to the present mvention is prepared by heat treatment of naphthalene- 1, 5-disulfonate, disodium salt (NDS) previously exchanged using nickel chloride. In carrying out the ion exchange process, 10 gm of NDS is dissolved i 100 ml of deionized water and tiiis solution is then mixed with 100 ml of a water solution of 2.1 gm of NiCl 2 .6H 2 O. This mixture is kept at pH 4 for 12 hours and then the precipitate filtrated and dried at 100° C. The dried sample is men placed in a sealed quartz tube in an electrically heated oven. The carbonization is carried out in a nitrogen flow (20 ml/min) with a heathig rate of 10 deg/min up to 650° C and then kept at this temperature for 3 hours. After cooling under nitrogen, die material is washed with deionized water to dissolve and remove soluble intermediates. Methane and hydrogen storage were measured, and the results are discussed below. The results showed that for methane at P = 28 atm. and T = 25° C, die weight of gas stored on die carbon-nickel composite was 1.5 times greater than the weight showed on the carbon alone if die basis was tlie total weight of carbon in tlie test sample. For hydrogen at P = 5 atm. and T = -196° C, this value was 4. The invention has been described here with reference to several illustrative examples. However, the invention is not limited to tiiose examples. Rather, many modifications and variations thereof would present themselves to those of skill hi the art without departure from the principles of this invention, as defined hi die appended clahns.

Claims: We Claim: 1. A microporous carbon absorbent material which contams a metal component, and which is suitable for use as a storage media for light gases having a uniform pore size distribution in die range of about 4 to 15 A, and a high surface area in which die micropores comprise at least about 50% of said surface area. 2. The material of claim 1 in which the metal component is selected from the group consistmg of iron, nickel and cobalt. 3. The material of claim 1 in which the metal component is selected from the group consisting of elements which form stable carbides. 4. The material of claim 1 in which the metal is selected from elements which form alloys or mtermetallic compounds. 5. The material of clahn 1 in which the metal is known to activate methane, such as magnesium. 6. A microporous carbon absorbent material which is the product of the pyrolysis of a crystalline aromatic sulfonate or nonporous polymeric salt which contains a metal component, said material being suitable for use as a storage media for light gases, and having a uniform pore size distribution in die range of about 4 to 15 A, and a high surface area. 7. The material of claim 3 hi which the micropores comprise at least about 50% of surface area of the material. 8. A method of making a composite microporous carbon material which comprises; (a) providing a polymeric precursor selected from the group consisting of a crystalline aromatic sulfonate and a nonporous polymeric salt which contain a metal component; and (b) carbonizing said precursor in an oxygen free inert atmosphere at a temperature in the range of about 350 to 1000°C for a time sufficient to promote thermally induced hydrogen abstraction and rearrangement of BSUs which result in the formation of a carbonaceous microporous material. 9. The method of claim 8 hi which the carbonizing time ranges from about 1 to 4 hours. 10. The method of claim 8 in which the precursor is heated to the carbonizing temperature at the rate of about 5-15 deg/min. 11. The method of claim 8 in which the pore size distribution of the carbonaceous material is in the range of about 4-15 A. 12. The method of claim 8 in which the metal component of the precursor is selected from the group consisting of iron, nickel, cobalt; elements which form stable carbides, metal alloys and hitermetaUic compounds; and metals known to activate methane, such as magnesium.

Inventors: AMANKWAH KWABENA A G, BANDOSZ TERESA J, JAGIELLO JACEK, PUTYERA KAROL, SCHWARZ JAMES, AMANKWAH, KWABENA, A., G., BANDOSZ, TERESA, J., JAGIELLO, JACEK, PUTYERA, KAROL, SCHWARZ, JAMES
Applicants: UNIV SYRACUSE, SYRACUSE UNIVERSITY
Application year: 1996
Main classification: B01J21
IPC codes: B01J20, B01J21, B01J37