2,798,878 Jnited States Patent Office Patented July 9, 1957

2 weeks; and, finally, the method must be safe from the hazard of-explosion.

One of the objects of the invention is to provide a 2,798,878 method of oxidizing graphite to graphitic oxide with the PREPARATION OF GRAPHITIC ACID production of a high quality product of low carbon-to Williaria S. Hummers, Jr., Detroit, Mich., assignor to Na oxygen ratio. tional Lead Company, New York, N. Y., a corporation Another object of the invention is to provide a method of New Jersey of preparing graphitic oxide in a hazard-free manner with a processing time of one or two hours, or less. No Drawing. Application July 19, 1954, 0. Another object of the invention is to provide a method Seria No. 444,400 of preparing graphitic oxide in high yield, with a favor ably low consumption of oxidizing agents. 16 Claims. (C. 260-348) Another object of the invention is to provide a method of preparing graphitic oxide with materials which are

This invention relates to the preparation of graphitic 5 relatively cheap, available, and readily handled. oxide from graphite, and more particularly is concerned Other objects of the invention will appear as the de with an improved method of carrying out the oxidation scription thereof proceeds.

In accordance with the invention, the starting material, step involved. which is a graphitic substance which may be graphite, Graphitic oxide, sometimes known as graphitic acid, 20 has been known for almost a century, and is an important and is preferably a high grade of synthetic or natural material for experimental studies in electrode processes graphite, such as Ceylon graphite, is treated with a water of colloidal properties of materials related to humic acids, free mixture of concentrated sulphuric acid and an an in inorganic structural chemistry, and the like. Graphitic hydrous nitrate compound of the class of nitric acid, so

dium nitrate, potassium nitrate, barium nitrate, and other oxide has also been suggested for use as a lubricant. It 25 inorganic anhydrous nitrate salts, together with an an was first prepared by Brodie in 1859 (Phil. Trans. 149, hydrous permanganate such as potassium permanganate. 249 (1859)) by repeated treatment of Ceylon graphite with an oxidation mixture consisting of potassium chlo Sodium permanganate may be used if anhydrous but this rate and fuming nitric acid. Since then, many other salt is hygroscopic so that potassium permanganate is

preferred. The temperature for the reaction is preferably procedures have been devised for forming graphitic 30 Oxide, nearly all dependent upon strong oxidizing mix about 35 C., but may be as high as 45° C. and as low as tures containing one or more concentrated acids and ox 25 C. to 30° C. Above 45 C. the overall yields tend idizing materials. The methods most commonly used at to be low because of oxidation to carbon dioxide, and the present time are the original Brodie synthesis and below about 30° C. the reaction proceeds unduly slowly. one described by Staudenmaier, in which the graphite is However, the temperature may rise considerably higher oxidized in concentrated sulphuric and nitric acids with 35 than 45° C. upon the initial addition of permanganate, potassium chlorate. Even recent workers in the field but if heat transfer in the system is good, and the quantity Such as Hofmann (Z. anorg. allg. Chem. 234 4, 311 of permanganate initially added is small, then such tran

sient temperatures of even as high as 70° C. do no (1937)), and Hamdi (Koll.-Beih. 54 563 (1943)), used harm, the system rapidly coming back to the preferred the Staudenmaier process, in which concentrated sul 40 phuric acid and concentrated (63%) nitric acid are range.

mixed together with graphite to be oxidized and over a The nature of the product obtained can be quickly period of a week the requisite amount of potassium chlo checked by pouring some of the acid oxidizing mixture rate is added slowly and carefully with cooling, and with containing the graphitic oxide into a large excess of water removal of chlorine dioxide evolved with an inert gas and observing the color produced. Conversions too such as carbon dioxide or nitrogen. This Staudenmair 45 low to be of any practical use, as well as no conversion Hofmann-Hamdi method uses more than 10 grams of at all, give a black suspension. The suspension becomes potassium chlorate for each gram of graphite treated and "r dark green at carbon-to-oxygen ratios of about 3.0 to explosion is a constant hazard. 3.4, and these are usable products for some uses. When There are all stages of oxidation from pure graphite the oxidation has proceeded to a carbon-to-oxygen ratio to pure graphitic oxide, and the efficacy of an oxidation 50 of about 2.8 to 3.0 the color is light green. At lower method may be judged by the proportion of graphitic carbon-to-oxygen ratios the color is yellowish green, be oxide in the product, or more accurately, of the carbon coming bright yellow at the lowest carbon-to-oxygen to-oxygen ratio of the product. Even with a method ratios obtainable of about 2.05 to 2.4. which is efficacious in producing a product of low car A number of oxidizing treatments illustrative of prior bon-to-oxygen ratio (which for good samples of graphitic 55 and other methods, and a number of oxidizing treatments oxide should lie between 2.1 and 2.9), if such a method illustrative of the oxidation process of the present inven is to be useful, it must result in a minimum of wastage tion are given in the tabulation which follows. Where of graphite by oxidation to carbon dioxide. (Overall the carbon-to-oxygen ratio of the product obtained was yields in many preparative methods are very low.) determined by analytical procedures the ratio is given. Moreover, the method must not be wasteful of oxidizing 60 agents, particularly when commercial use of the graphitic Where this was not done, the color of the product deter oxide produced is contemplated; the method must permit mined as noted above gives a clear indication of the ex preparation in a matter of hours rather than days or even tent and therefore the success of the oxidizing process. - 2,798,878 3 4. TABLE Basis : gram graphite

Run ml. Other Ingredients . . . . Mesh of Temp., Color HSO Graphite C.

15 1g. Na2Cr2O7------325 30 Black. 15 4 g. Na2Cr2O7------325 ... 30. D0. 15 3.g. KMnO4, 15 ml. 70% HNO3------325 30 O0, 20 1.g. KClO3, 10 ml, 70% HNO3------200-325 0-60 Dark Green. 30 3.g. KMnO4, 3g. NaNO3------200-325 30 Do. 30, 3 g. KMnO, 1. g. NaNO3------...--- 200-325 45 Green. 22-33g. KMnO, kg. NaNO3- 325 45 Bright Yellow 22-23.g. KMnO4, 1g. NaNO3- 200-325 45 Yellow. 22-33 g. KMnO4, 0.5 g. NaNO3 --325 45 Do. 22-23.g. KMnO, 0.5 g. NaNO3. -325 35 Bright Yellow. 22, 3 g. KMnO, 0.5 g. NaNO3 ... -325 35 D0. 22-33 g. KMnO, 1.0g. furning HNO3------325 45 Light Green. 22% 3g. KMnO, 1.0 g. BaNO3------325 45 Yellow, 203 g. KMnO, 3.0 g, furning FINO3-...------325 45 Do. In Table I, run D, represents the conventional Stau denmaier-Hofmann-Hamdi procedure. It will be noted about one-twentieth to one-fiftieth should be anhydrous that while a product of satisfactory carbon-to-oxygen 20 nitrate for optimum results. : ratio was obtained, two weeks or 336 hours were taken. The graphitic oxide is recovered from the acid oxidizing The temperature range shown indicates the fluctuation bath in known fashions, the most commonly used method occurring, the oxidizing liquid heating up when chlorate being that of diluting the reaction product mixture with is added bit being cooled down whenever the temperature several volumes of water and recovering the graphitic 25 oxide by decantation, centrifuging, dialyzing out the acids, gets too high. Run C shows an attempt at making gra and salts, and the like. Such methods will not be de phitic oxide, using potassium permanganate and ordinary scribed in detail herein since they are adequately covered 70% nitric acid. Even a twenty-four hour oxidation at in the chemical literature, 30° C. failed to produce anything more than a black The present process is particularly favorably adapted color. Runs A and B show the use of sodium chromate in a sulphuric acid bath where even a seventy-two hour 30 for continuous oxidation, in view of the short oxidation oxidation failed to proceed beyond the black stage. time of some thirty minutes. Thus, a continuous flow of in complete contrast to runs A, B, C, and D, the re the acid oxidizing mixture together with the graphite can mainder of the runs, E through N inclusive, show pro be maintained in coils immersed in a bath at the selected cedures in accordance with the invention wherein con temperature, for example 35° C., the rate of flow being centrated sulphuric acid was used together with potassium 35 such that the transit time is approximately thirty minutes. permanganate and an anhydrous nitrate which in most The oxidized product is recovered from the mixture at the cases was sodium nitrate, but barium nitrate and funning end of this time, and the sulphuric acid can be recovered nitric acid, were used as well. In all cases, green to in accordance with conventional methods, concentrated yellow products were obtained of excellent carbon-to and reused. This makes for marked economy of opera oxygen ratio, and at processing times varying from one 40 tion, and such economy is further increased by the quite half to two hours, which is quite remarkable in this art. modest amounts of consumed reactants, particularly po An optimum sulphuric acid concentration has been found tassium permanganate, which need be supplied. to be about 22 ml. of concentrated sulphuric acid per The following nitrates among others can be used, gram of graphite treated. The reaction will work well although it will be understood that in the case of calcium with as little as 20 ml. of concentrated sulphuric acid 45 nitrate and magnesium nitrate these are deliquescent salts per gram of graphite, and indeed less sulphuric acid can and generally occur with water of crystallization, so that be used, but then some unreacted graphite may be left if they are used, it will be necessary to provide them in an over. Naturally, more sulphuric acid may be used per anhydrous form, and where nitrogen pentoxide is used it gram of graphite than 22 ml., and this is not objectionable may be necessary to add a minute amount of water, but except from a standpoint of cost as it means a greater 50 no more than enough to react to form anhydrous nitric waste of sulphuric acid, if the latter is rejected, or greater acid: recovery costs if the sulphuric acid is recovered. Sodium nitrate For complete oxidation of the graphite, the most favor Potassium nitrate able ratio of permanganate salt, preferably potassium Barium nitrate permanganate, is between 2% and 3 grams of permanga 55 Calcium nitrate nate per gram of graphite. Lesser amounts than about 2. Magnesium nitrate grams of permanganate do not give complete oxidation; Fuming nitric acid and amounts in excess of 3 grams may be used, but are Nitrogen pentoxide naturally wasteful. A quite small amount of nitrate is needed, and it is 80 The following permanganates among others may be believed that the effect of the nitrate is possibly more used, but again, in the case of calcium permanganate and catalytic than oxidative. One-half gram of sodium ni magnesium permanganate as well as sodium permanga trate, or its equivalent in barium nitrate or potassium nate, caution must be used to provide and maintain an nitrate or fuming nitric acid or equivalent other anhy hydrous salts since these are also deliquescent: drous nitrate is sufficient. Substantially less than one 65 Potassium permanganate half gram, for example one-third gram of sodium nitrate Barium permanganate per gram of graphite, begins to show a decrease in effect Sodium permanganate compared to one-half gram. More than one-half gram Calcium permanganate may be used, and indeed one gram may be used with good Magnesium permanganate effects, but naturally larger amounts than one-half to one 70 The discussion herein has been in terms of grams, for gram of nitrate are wasteful. , the sake of simplicity, but it will be understood that the Considering the amounts of nitrate and permanganate product can be made in batches of pounds and even more based on the sulphuric acid used, it will be clear that be in which case the proportions will be the same as has tween about one-tenth and one-seventh of the weight of been stated hereinabove. sulphuric acid should be anhydrous permanganate, and 75 It will be understood that the invention is a broad one, 2,798,878 5 6 and variation can be made within the spirit of the inven nitrate, and fuming nitric acid and an anhydrous per tion and the scope of the appended claims. Freedom manganate chosen from the class consisting of potassium from added water, however, is a determining factor for permanganate, barium permanganate, sodium permanga Success, and the oxidizing baths of my invention are char nate, calcium permanganate, and magnesium permanga acterized by their initial freedom from water, except 5 nate to form a reaction mixture, permitting said reaction the relatively minute amounts occurring in commercial mixture to stand for a time sufficient for substantial oxida Sulphuric acid, commercial fuming nitric acid, and for tion of graphite to take place, and subsequently recover that matter in graphite, all of which together will amount ing the graphitic oxide so produced from the reaction to less than about 1% of the total reaction mixture. mixture. In the claims which follow wherein reference is made 10 10. The process of producing graphite oxide which to a temperature range having an upper linnit of about consists of admixing comminuted graphite with an oxidiz 45 C., it will be understood that such range is meant ing liquid consisting essentially of concentrated sulphuric to include transient rises in temperature considerably acid, together with between about one-tenth and one above 45 C., provided that the temperature returns to seventh of its weight of an anhydrous permanganate and the specified range with reasonable promptness and main 5 about one-twentieth to one-fiftieth of its weight of an tenance is for the most part within such temperature anhydrous nitrate to form a reaction mixture, permitting range. said reaction mixture to stand for a time sufficient for What I claim is: substantial oxidation of the graphite to take place, and 1. An acid oxidizing mixture for the oxidative treat subsequently recovering the graphitic oxide so produced ment of graphite consisting essentially of concentrated 20 from the reaction mixture. sulphuric acid, an anhydrous nitrate, and an anhydrous 11. The process of producing graphitic oxide which -permanganate. consists of admixing comminuted graphite with an oxidiz -- 2. An acid oxidizing mixture for the oxidative treat ing liquid consisting essentially of concentrated sulphuric ment of graphite consisting essentially of concentrated acid, together with between about one-tenth and one sulphuric acid, an anhydrous nitrate chosen from the 25 seventh of its weight of an anhydrous permanganate class consisting of sodium nitrate, potassium nitrate, chosen from the class consisting of potassium permanga barium nitrate, calcium nitrate, magnesium nitrate, and nate, barium permanganate, sodium permanganate, cal fuming nitric acid, and an anhydrous permanganate chosen cium permanganate and magnesium permanganate, to from the class consisting of potassium permanganate, gether with between about one-twentieth and one-fiftieth barium permanganate, sodium permanganate, calcium 30 of its weight of an anhydrous nitrate, chosen from the permanganate, and magnesium permanganate. class consisting of sodium nitrate, potassium nitrate, bar 3. An acid oxidizing mixture for the oxidative treat ium nitrate, calcium nitrate, magnesium nitrate and fum ment of graphite consisting essentially of concentrated ing nitric acid, to form a reaction mixture, permitting said sulphuric acid, together with between about one-tenth reaction mixture to stand for a time sufficient for sub and one-seventh of its weight of an anhydrous perman 35 stantial oxidation of the graphite to take place, and sub ganate and about one-twentieth to one-fiftieth of its weight sequently recovering the graphitic oxide so produced from of an anhydrous nitrate. the reaction mixture. 4. An acid oxidizing mixture for the oxidative treat 12. The process of producing graphitic oxide which ment of graphite consisting essentially of concentrated consists of admixing comminuted graphite with an oxidiz 40 ing liquid consisting essentially of concentrated sulphuric sulphuric acid, together with between about one-tenth acid, together with a minor proportion of a permanganate and one-seventh of its weight of an anhydrous perman and a minor proportion of a nitrate to form a reaction ganate chosen from the class consisting of potassium mixture, permitting said reaction mixture to stand for a permanganate, barium permanganate, sodium permanga time sufficient for substantial oxidation of the graphite nate, calcium permanganate, and magnesium permanga 45 to take place, and subsequently recovering the graphitic nate, together with between about one-twentieth and one oxide so produced from the reaction mixture. fiftieth of its weight of an anhydrous nitrate chosen from 13. The process of producing graphitic oxide which the class of sodium nitrate, potassium nitrate, barium consists of admixing comminuted graphite to an oxidiz nitrate, calcium nitrate, magesium nitrate, and fuming ing liquid consisting essentially of concentrated sulphuric nitric acid. acid, together with a minor proportion of potassium per 5. An acid oxidizing mixture for the oxidative treat manganate and a minor proportion of sodium nitrate to ment of graphite consisting essentially of concentrated form a reaction mixture, permitting said reaction mixture sulphuric acid, together with a minor proportion of a to stand for a time sufficient for substantial oxidation of permanganate and a minor proportion of a nitrate. the graphite to take place, and subsequently recovering 6. An acid oxidizing mixture for the oxidative treat 5 5 the graphitic oxide so produced from the reaction mixture. ment of graphite consisting essentially of concentrated 14. The process according to claim 8 wherein the re sulphuric acid, together with a minor proportion of potas action mixture is maintained at a temperature between sium permanganate and a minor proportion of sodium about 25 C. and about 45 C. during the oxidation step. nitrate. 15. The process according to claim 12 wherein the re 7. An acid oxidizing mixture according to claim 5 in 60 which the total water present is less than about 1%. action mixture is maintained at a temperature between 8. The process of producing graphitic oxide which con about 25 C. and about 45° C. during the oxidation step. sists of admixing comminuted graphite with an oxidizing 16. The process according to claim 12 in which the liquid consisting essentially of concentrated sulphuric acid, temperature is maintained during the oxidation step at an anhydrous nitrate, and an anhydrous permanganate about 35° C. to about 45° C. for about one-half to one to form a reaction mixture, permitting said reaction mix hour. ture to stand for a time sufficient for substantial oxidation of the graphite to take place, and subsequently recovering References Cited in the file of this patent the graphite oxide so produced from the reaction mixture. UNITED STATES PATENTS 9. The process of producing graphitic oxide which con 1,137,373 Aylsworth ------Apr. 27, 1915 sists of admixing comminuted graphite with an oxidizing 70 liquid consisting essentially of concentrated sulphuric acid, OTHER REFERENCES an anhydrous nitrate chosen from the class consisting of Mellor: Inorganic and Theoretical Chemistry, vol. 5 sodium nitrate, potassium nitrate, barium nitrate, calcium (1924), page 828.