Process for producing high surface area activated carbon

Abstract: Process for producing activated carbon, preferably high surface area activated carbon, from inexpensive cellulosic precursors, such as paper, by pretreating the cellulosic precursor with an activating agent, such as phosphoric acid, followed by carbonization in an inert atmosphere at an elevated temperature to produce activated carbon. The activated carbon can be further activated to increase its surface area by heating it in an oxidizing atmosphere at an elevated temperature to yield activated carbon having a surface area of at least 1000 m²/g.

Description: Field of the Invention The invention relates to a process for producing high surface area activated carbon from paper such as newspapers by pretreating the paper with an activating agent, such as phosphoric acid, followed by carbonization in an inert atmosphere at 450°C to 1200°C and then preferably activation in an oxidizing atmosphere, such as steam, at a temperature above 700°C. Background of the Invention Activated carbon can be used for adsorption purposes, such as the extraction of gases or vapors from products, for adsorption of liquids and for solvent recovery in various applications. One process used in producing highly active gas adsorbent carbon consists of charring carbonaceous material, such as coconut shells, by application of heat and the further treatment of the carbon so produced with an oxidizing agent such as steam which slowly oxidizes the carbon. U.S. Patent No. 1,694,040 discloses a process for making dense strong activated charcoal by impregnating nut cellulose with a dehydrating agent, such as phosphoric acid or zinc chloride, heating the mixture to a temperature not materially below 350°C, leaching out the soluble components and then reheating the resulting carbon in an oxidizing atmosphere. U.S. Patent No. 1,819,165 discloses a process for producing highly active gas adsorbent carbon by impregnating carbonaceous material with phosphoric acid, then calcining said impregnated material in the absence of oxygen and then submitting the calcined product to controlled partial oxidation at temperatures above 700°C. U.S. Patent No. 3,305,314 discloses a process for producing permanently dehydrated heat treated material from a cellulosic textile material which comprises subjecting the cellulosic material to a controlled partial and selective decomposition of the cellulosic molecule by immersing the cellulosic material in an acid solution to wet the material therewith, removing the wet material from the solution, drying the material to remove the solvent therefrom and heat treating the dried material in an oxidizing atmosphere whereby decomposition involves a rupture of the carbon-oxygen and carbon-hydrogen bonds and the evolution of water, but falling just short of the scission of the main cellulose molecule, through the rupture of the carbon-carbon bonds with the evolution of hydrogen. Although active carbon has been used extensively in gas and vapor adsorption devices, it is also well suited for the storage of gases such as methane. One of the best active carbons for the storage of methane is derived from chemical activation of coke. These carbons have very high surface areas (2500 to 3000 m²/g) and are costly to produce. It is an object of the present invention to provide a process for producing high surface area activated carbons preferably high surface activated carbons, from an inexpensive cellulosic precursor. It is another object of the present invention to produce activated carbons, preferably high surface area activated carbons, that can be utilized in storage containers to more efficiently store gases such as methane, hydrogen and acetylene. It is another object of the present invention to provide a process for producing activated carbons, preferably high surface area activated carbons from paper. It is another object of the present invention to provide a process for producing high surface area activated carbon that is cost effective to produce and easy to make. Summary of the Invention The invention relates to a process for producing activated carbon which comprises the steps: a) treating paper with at least one activating agent selected from the group consisting of phosphoric acid and ammonium phosphate, with the proviso that if phosphoric acid is used the amount of phosphoric acid is from 3 to 20 pph based on the weight of the paper and if ammonium phosphate is used the amount of ammonium phosphate is from 3.5 to 23 pph based on the weight of the paper; b) heating the treated paper of step a) at a temperature from 450°C to 1200°C in an inert atmosphere for a time period sufficient to produce an activated carbon material; and c) heating the carbon material of step b) at a temperature in excess of 700°C in an oxidizing atmosphere for a time period sufficient to produce activated carbon having a surface area of at least 1000 m²/g. The abbrevation "pph" means "parts per hundred" According to the invention the paper, preferably comprises in excess of 90% cellulose. In step a) of the treatment, the paper is contacted with a liquid activating agent, i.e. a solution of phosphoric acid or ammonium phosphate, to stabilize the carbon framework of the cellulose so carbon is not lost during subsequent heat treatment. The phosphoric acid or ammonium phosphate (mono-basic or dibasic) could be added to the paper in at least three ways. (1) The paper can be dipped into a dilute solution of phosphoric acid or ammonium phosphate, and the slurry can then be evaporated to dryness by heating between 100° and 150°C. (2) The paper can be dipped into a dilute solution of phosphoric acid or ammonium phosphate and the excess solution can be removed by filtering and squeezing the excess water from the newspaper. The phosphate treated newspaper can then be dried by heating between 100° to 150°C. (3) Concentrated phosphoric acid (or ammonium phosphate) can be sprayed uniformly on the newspaper and then the newspaper can be directly carbonized without an evaporation step. The amount of phosphoric acid (85%) added to the newspaper using any of the above three methods is between 3 and 20 pph based on the weight of the newspaper. The most preferred level would be about 7 to 10 pph of 85% phosphoric acid. The same equivalent amount of elemental phosphorus in the form of ammonium phosphate can also be used. For example, if dibasic ammonium phosphate, (NH₄)₂HPO₄, is employed, the preferred amount would be between 3.5 and 23 pph based on the weight of the newspaper. The most preferred level would be about 8.0 to 11.5 pph. The preferred method of addition of the activating agent would be spraying it on the newspaper in a concentrated form. This method is economical since evaporation is not required and the paper can be immediately carbonized. The amount of time that the newspaper can be in contact with the phosphate is small, less than one minute, since the desirable reaction of phosphate with the newspaper is fast. In one preferred method, one can spray between 5 and 34 pph, based on the weight of the paper, of a 50% phosphoric acid solution. The sample can be carbonized directly without an evaporation step. If dibasic ammonium phosphate is used in the spraying technique, the preferred amount is between 7 and 46 pph of a 50% solution of dibasic ammonium phosphate. The treatment of the paper with with the activating agent prior to carbonization has been discovered to substantially increase the yield of the carbon in step b) of the process. In excess of 60%, preferably in excess of 70%, most preferably in excess of 80% of the theoretical carbon yield can be obtained by using this pretreatment step a) of the process. Additionally, the derived carbon from step b) is believed to develop a preponderance of micropores (less than 30Å) which when subjected to an oxidizing treatment as in step c, will produce high surface activated carbon that is ideally suited for storage of gases, such as methane, hydrogen or acetylene. This favorable microporosity is believed to result from the cellulose structure which contains a polymeric backbone with many functional groups. The carbonization of the cellulose leaves a micropore structure through elimination of these groups which can then be accessible to the oxidizing activation treatment in step c) if employed to produce a high surface area activated carbon. The carbonization treatment in step b) is carried out in any inert atmosphere, such as nitrogen or argon, at a temperature from 450°C to 1200°C, preferably from 500°C to 1000°C and more preferably from 600°C to 1000°C. In this step, the paper is carbonized and, as stated above, the pretreatment of the paper as recited in step a) will result in a greater carbon yield and can produce an activated carbon having a surface area of about 500 m²/g. In the carbonization treatment step, the heating should be maintained for a sufficient period of time to assure proper carbonization. Generally a time period of at least 10 minutes could be sufficient with a time period of 10 minutes to 5 hours being preferred and a time period of 15 minutes to 2 hours being most preferred. The time period is dependent on the temperature of the treatment with the higher temperature requiring a shorter time period and thus for optimum results, an appropriate heat treatment temperature and time period can be selected. The carbon material produced in step b) can then be further activated, if desired, in an oxidizing atmosphere for a time period at an elevated temperature sufficient to produce activated carbon having a surface area of at least 1000 m²/g and preferably greater than 1300 m²/g. Preferably, this treatment of the carbon material can be performed by maintaining the carbon at a temperature above 700°C, and preferably above 800°C while in contact with an oxidizing environment, such as steam, to preferably burn off at least 30% of the carbon. Generally, a time period of from 5 minutes to 5 hours would be sufficient. Although the preferred activation step c) would be carried out using steam, it could be carried out with CO₂, air, mixtures of CO₂ with steam and/or air as well as other physical and chemical activation procedures which are known in the art. The high surface area activated carbon produced is ideally suited for use in containers that can be used to store gases such as methane, hydrogen and acetylene. The activated carbon could be mixed with a suitable binder, such as a sugar based material, to form rigid objects which can be utilized to fabricate various shape and size articles. Due to the relatively high surface area of the activated carbon produced, a container using this carbon material can adsorb a high volume of gas, i.e. methane, per volume of carbon. Although this type of high surface area activated carbon is ideally suited for use in gas storage containers, it also will be ideally suited for use in devices for adsorbing gases and vapors such as purifiers, deodorizers, dechlorinators, detoxicators and the like. Example 1 (Comparative Example) 100g of shredded newspaper was treated in a tray with a solution of 1.9g of ammonium phosphate in 1000ml of water. The tray was placed on a hot plate at 110°C to remove the excess water and dry the shredded newspaper. The dried newspaper was then heated in a nitrogen atmosphere, from room temperature to 1000°C over a period of 4 hours and held at 1000°C for about 0.5 hours. A carbon yield of 21.5% was obtained having a surface area of 500 m²/g. This Example shows that unless sufficient carbonising agent is employed, the desired surface area is not attained. Example 2 A solution of 1.5g of 85% phosphoric acid in 300ml of water was prepared. 30g of shredded newspaper were added and mixed for 1.0 hour. The paper was filtered to remove the excess liquid (150ml of liquid were retained on the paper). The paper was then dried on a hot plate and carbonized at 500°C for 2 hours in a nitrogen atmosphere. A carbon with a yield of 31.2% was obtained. The material was activated in a rotating furnace with a 16 cubic inch (0,26 dm³) activating chamber at 850°C for 5.0 hours with steam. The steam environment was generated by passing 2.5 cubic feet (0,071 m³) of argon per hour through water at 22°C. An activated carbon with a yield of 39.3%, a phosphorus content of 2.30% and a surface area of 1516 m²/g was obtained. The overall yield of activated carbon was 12.0% as based on the starting weight of the newspapers. Example 3 30.1g of shredded newspapers were placed in a tray. A solution of 1.5g of ammonium phosphate dissolved in 300ml of water was added. The tray was placed on a hot plate to dry. The dried paper was heated to 270°C in air for 10 minutes and then the carrier gas was switched to nitrogen. The material was carbonized at 500°C for 2 hours. A carbon with a yield of 35.8% was obtained. The carbon was activated in a rotating furnace with a 16 cubic inch (0,26 dm³) activating chamber at 850°C for 4.5 hours with steam. The steam environment was generated by passing 2.5 cubic feet (0,071 m³) of argon per hour through water at 22°C. An activated carbon with a yield of 32.9%, a phosphorus content of 2.89% and a surface area of 1661 m²/g was obtained. The overall yield of activated carbon was 11.8% as based on the starting weight of the newspapers. Example 4 (Comparative Example) 30g of shredded newspapers were placed in a furnace and carbonized at 500°C for 2 hours in a nitrogen atmosphere. The carbonization yield was only 23.9%. The material was activated in steam at 850°C for 4.0 hours in a rotating furnace with a 16 cubic inch (0,26 dm³) activating chamber. The steam environment was generated by passing 2.5 cubic feet 0,071 m³ of argon per hour through water at 22°C. An activated carbon with a yield of 19.9% and a surface area of 1260 m²/g was obtained. The overall yield of activated carbon was 4.8% as based on the starting weight of the newspapers. The results of this test show that when the paper was not pretreated with phosphoric acid, the yield of carbon and the activated carbon surface area were lower. Example 5 A solution of 1.4g of 85% phosphoric acid and 300ml of water was added to 30g of a conventional paper towel material in a dish and the slurry was mixed for 1 hour. The slurry was evaporated to dryness on a hot plate. The dried material was carbonized at 500°C for 3 hours in a nitrogen atmosphere. A carbonization yield of 32.7% was obtained. The carbonized material was steam activated in a rotating furnace with a 16 cubic inch (0,26 dm³) activating chamber at 850°C for 5.0 hours. The steam environment was generated by passing 2.5 cubic feet (0,071 m³) of argon per hour through water at 0°C. An activated carbon with a yield of 44.1%, a phosphorus content of 0.97%, and a surface area of 1597 m²/g was obtained. The overall yield of activated carbon was 14.4% as based on the starting weight of the paper. Example 6 A solution of approximately 50% phosphoric acid was prepared by mixing 180g of phosphoric acid (85%) and 120ml of water. The solution was placed in an atomizer bottle. 30.0g of shredded newspapers were placed in a tray and the acid solution was sprayed over the paper from the atomizer while mixing until 6.0g [3.5g of phosphoric acid (85%)] had been added. The paper, mixed for another 5 minutes, appeared to be dry and no visible wet spots were evident. The sample was carbonized at 600°C in a nitrogen atmosphere for 2 hours. The carbonization yield was 36.3%. The carbon was activated in a rotating furnace with a 16 cubic inch (0,26 dm³) activating chamber at 850°C for 6.0 hours with steam. The steam environment was generated by passing 2.5 cubic feet (0,071 m³) of argon per hour through water at 22°C. An activated carbon with a yield of 30.6%, a phosphorus content of 2.56% and a surface area of 1797 m²/g was obtained. The overall yield of the activated carbon was 11.1% as based on the starting weight of the newspapers. Example 7 100g of shredded newspapers were placed in a dish and then a solution of 10g of phosphoric acid (85%) and 100ml of water was poured over the newspaper. The slurry was stirred until the newspapers were saturated. The slurry was then dried on a hot plate. The dried treated paper was carbonized at 600°C for several hours in a nitrogen atmosphere. The carbonized yield was 39.9%. The carbon was activated in a rotating furnace with a 16 cubic inch (0,26 dm³) activating chamber at 950°C for 45 minutes with steam. The steam environment was generated by passing 2.5 cubic feet 0,071 m³ of argon per hour of nitrogen through water at 75°C. An activated carbon with a yield of 25.2%, a phosphorus content of 2.88%, and a surface area of 1902 m²/g was obtained. The overall yield of the activated carbon was 10.1% as based on the initial weight of the newspapers. To evaluate the effectiveness of high surface active carbon for gas storage applications, the effective storage was calculated using the definition of effective storage as the storage of gas at high pressure of 500 psig 34,2 at minus the storage of gas at a low pressure of one atmosphere, psig. This value would represent the amount of deliverable gas to an engine or other receiving means. The effective storage is calculated using the loading ratio correlation equation assuming that the active carbon can be compacted to a density of 0.85 g/cc or about 53 lb/ft³. The loading ratio correlation (LRC) equation was used to evaluate methane loading of the active carbon of several of the above examples. The LRC adsorption isotherm equation is: where L is the loading (milli-moles/g) at a given pressure, P, of methane, Lo is the maximum capacity of the active carbon adsorbent, K is the interaction coefficient and is a constant for a given temperature and carbon, n is a coefficient that measures the deviation from Langmuir behavior (LRC is based on the Langmuir adsorption isotherm in which n=1). The coefficient is a constant for a given carbon at a given temperature. The LRC equation is for a single adsorbate, such as methane, at a constant temperature of 25°C. The loading of methane at 500 psig 34,2 at on the carbon was measured gravimetrically using the conventional McBain sorption balance technique. For Example 2 in which the surface area of the activated carbon was 1516 m²/g, the methane loading at 500 psi was found to be 6.75 milli-moles/g. For Example 3 in which the surface area of the activated carbon was 1661 m²/g, the methane loading at 500 psi 34,2 at was found to be 7.15 milli-moles/g. Although the methane was fed into the container at 500 psig, the methane could be stored in the container at a lower pressure of from 1 psig (0,7 at) up to and exceeding 500 psig (34,2 at). Although the present invention has been described with reference to particular details, it is not intended that these details shall be construed as limiting the scope of the invention.

Claims: A process for producing activated carbon which comprises the steps: a) treating paper with at least one activating agent selected from the group consisting of phosphoric acid and ammonium phosphate, with the proviso that if phosphoric acid is used the amount of phosphoric acid is from 3 to 20 pph based on the weight of the paper and if ammonium phosphate is used the amount of ammonium phosphate is from 3.5 to 23 pph based on the weight of the paper; b) heating the treated paper of step a) at a temperature from 450°C to 1200°C in an inert atmosphere for a time period sufficient to produce an activated carbon material; and c) heating the carbon material of step b) at a temperature in excess of 700°C in an oxidizing atmosphere for a time period sufficient to produce activated carbon having a surface area of at least 1000 m²/g. The process of claim 1 wherein in step a) the activating agent is phosphoric acid. The process of claim 2 wherein the phosphoric acid is sprayed onto the paper. The process of claim 2 wherein the phosphoric acid is in an aqueous solution and before step b) is performed, the paper is dried to substantially remove the water. The process of claim 1 wherein in step a) the activating agent is phosphoric acid; in step b) the temperature is from 450°C to 1200°C for a period of from 10 minutes to 5 hours; and in step c) the oxidizing atmosphere is steam. The process of claim 5 wherein in step b) the temperature is from 500°C to 1000°C for a period of from 15 minutes to 2 hours; and in step c) the temperature is at least 700°C for a period of time sufficient to form activated carbon. The process of claim 1 wherein in step c) the oxidizing atmosphere contains at least one oxygen containing component selected from the group consisting of CO₂, air, steam and mixtures thereof. The process of claim 1 wherein the activating agent is phosphoric acid; in step b) the temperature is from 450°C to 1000°C for a period of time from 15 minutes to 5 hours; and in step c) the oxidizing atmosphere is steam. The process of any of the preceding claims wherein in step c) the activated carbon produced has a surface area of at least 1300 m²/g.

Inventors: BALL DAVID RALPH, GREINKE RONALD ALFRED, LEWIS IRWIN CHARLES, BALL, DAVID RALPH, GREINKE, RONALD ALFRED, LEWIS, IRWIN CHARLES
Applicants: UCAR CARBON TECH, UCAR CARBON TECHNOLOGY CORP.
Application year: 1991
Main classification: C01B31
IPC codes: C01B31, F17C11