ES2983890A1 - METHOD FOR OBTAINING GRAPHENE MATERIALS FROM A LOW TEMPERATURE COKE (Machine-translation by Google Translate, not legally binding) - Google Patents

Abstract

Method for obtaining graphene materials from low-temperature coke. The invention relates to a method for obtaining graphene materials selected from coke oxide, graphene oxide and graphene from green coke, which comprises a basic treatment and subsequently an acid treatment of the starting material. (Machine-translation by Google Translate, not legally binding)

Description

DESCRIPTION

Method for obtaining graphene materials from a low-temperature coke

This invention is situated within the petrochemical and carbochemical industry sector, in the field of materials processing to prepare graphene materials, both graphene oxides and graphenes. The applications of interest are very varied and include numerous sectors (image, energy, etc.) since the materials prepared from this invention can even be used in emerging applications.

STATE OF THE TECHNIQUE

Graphene is a carbon sheet with sp2 hybridization and monatomic thickness that has extraordinary mechanical and electronic properties, the subject of study in recent years (US8066964B2).

Its application is directed to numerous sectors, such as energy conversion and storage, catalytic organic synthesis, water purification, etc. (Camblor et al. J. Electromag. Waves Appl. 2011,25(14-15), 1921-1929; González et al. Carbon, 2012, 50 (3),828-834, US2010/0176337). For all these reasons, synthesis, particularly that aimed at large-scale production to satisfy this great social demand, is still today a topic of great interest and a challenge yet to be overcome. Today, one of the most accepted methods for preparing graphene on a large scale (industrial scale) is the chemical route, which involves an initial oxidation of the starting material, usually graphite, which generates a graphite oxide with a three-dimensional graphite-type structure, but with its sheets oxidized both at the edges and at the basal planes and the sheets more separated from each other. In this way, the exfoliation of these sheets to give rise to graphene oxide occurs more easily than in the case of graphite and is therefore more applicable at an industrial level.

Methods for graphite oxidation have been known for over a century (Staudenmaier et al. Chem. Ges., 1898, 31, 1481-1487) where the formation of graphite oxide by oxidation with concentrated nitric acid, concentrated sulfuric acid and potassium chlorate was already described. This methodology has been widely studied and, above all, improved, mainly with a view to reducing the danger and handling difficulties of the reagents used. One of the most widely used today is the so-called "Hummers method" which uses sodium nitrate, potassium permanganate and concentrated sulfuric acid for the treatment of graphite (Hummers et al. J. Am. Chem. Soc., 1958, 80 (6), 1339-1339) or some of its many variants in which mainly the proportions of the reagents used are changed (WO2011/016889A2, Botas et al. Carbon, 2012, 50 (1), 275-282).

Nowadays, there is a great awareness towards the use of natural resources. In this respect, it must be taken into account that graphite is a natural resource that is conditioned by its scarce availability (China, India, etc.) and location (geopolitical aspects). There are some examples of its use as starting materials for graphene, other more sophisticated materials such as carbon nanofibres and short segments of graphitic carbon fibres (US2009/0028778A1). However, the decarbonisation of the petrochemical and carbochemical energy industry means that it is widely pushing towards the use of its by-products (mainly cokes) as alternative materials to graphite to prepare graphene materials.

Certain industrial cokes have a three-dimensional structure similar to that of graphite (made up of stacked sheets of carbon with sp2 hybridization) but with a greater number of defects, mainly due to the fact that they are obtained at a much lower temperature (500-1000°C). They are considered pre-graphitic materials (if heated to graphitization temperatures of 2500-3000°C they give rise to graphite) and their use as precursors of other types of materials, such as activated carbons, is known. This latter field is widely developed and involves physical or chemical activation (the latter for example with basic impregnation -NaOH, KOH etc. and subsequent treatment at high temperature around 300-400°C which is what produces the objective porous three-dimensional structure in this type of materials (Deng et al., Journal of Inorganic Materials 2014, 29(3): 245-249, Zhu et al. Taiwan Association for Aerosol Research, 2020, , 21, 4, 200540). Another type of materials obtained from cokes, different from activated carbons in their morphology and therefore in their properties, are graphene materials. Usually cokes have to be The graphene materials can be prepared using cokes, but they can be subjected to a prior ordering treatment (graphitization, heat treatment at temperatures around 3000°C), but mainly sustainability issues have led to the development of graphene materials directly from cokes, with some examples already in the literature. In patent application WO 2014/140399, the preparation of graphene materials is done using petrochemical and carbochemical cokes directly. These are subjected to a Hummers-type process in which the quantities of reagents are modified and give rise to graphene materials with properties similar to those of graphene materials obtained from graphites. Other documents delve into the capacity of this methodology to form graphene materials (Sierra et al. Fuel, 2016, 166, 400-403). Other alternative methodologies to the use of chemical oxidation methods (Hummers type) from cokes are, for example, direct electrochemical exfoliation (Saha et al. 2D Mater Appl, 2021, 5, 75) or with surfactants (US20170370009A1), solvent exfoliation (WO/2014/140399) also combining this last method with the use of surfactant agents (US2017369320A1) or with thermal exfoliation methods (Sun, et al. Metall. Res. Technol., 2020, 117, 6, 605).

The use of cokes for the preparation of other different nanometric carbon materials, such as quantum dots, has also been described (Ye et al. Nature Communications, 2013, 4, 2943), which highlights the versatility of this type of materials to be precursors of nanometric carbon materials.

It is however known that the structure and properties of cokes depend on the precursor used (for example, carbochemical or petrochemical origin) and the conditions of production, often determined by the main product to be obtained in its synthesis (many cokes are by-products of the petro and carbochemical industry). In general terms, cokes with less order in their crystalline structure (directly related, as we have seen, to their capacity to generate graphene materials) are obtained at lower temperatures (below 1000°C), and are less ordered the lower the treatment temperature. In this case, we speak of green coke. The material in these unordered cokes is, however, susceptible to reacting with the reagents of the chemical route to form graphene (Hummers type or similar), which poses problems in the preparation of this type of material. The same occurs with those cokes that have a high content of non-carbon elements in their composition, such as sulfur or metallic elements. These types of elements are characteristic of certain types of cokes, for example petroleum coke (fuel coke). This means that on certain occasions the use of green cokes to prepare graphene materials by the chemical route can pose problems, such as deflagrations.

DESCRIPTION OF THE INVENTION

The present invention describes a method of obtaining a product selected from the group consisting of coke oxide, graphene oxide and graphene, characterized in that it comprises transforming (at least) one pre-graphitic material of the low-temperature coke or green coke type, without graphitization treatment and by the chemical route.

A first aspect of the present invention relates to a method of obtaining a green coke oxide from green coke, characterized in that it comprises the following steps:

a. treat the coke in a basic medium at pH above 7;

b. treating the solid obtained in step (a) in an acidic medium at a pH below 5; and

c. chemically oxidize the solid obtained in step (b),

with the proviso that in the method of the invention a graphitization treatment of the green coke is not carried out, that is, without a heat treatment prior to step (a) of about 2500-3000°C.

The term "green coke" in the present invention refers to a pregraphitic material obtained at a temperature below 900, preferably below 700 °C. Green cokes can be obtained in different ways and from various precursors. They can be petroleum cokes, derived from petroleum residues; carbochemical cokes derived from pitches originating from coal tar; or synthetic cokes obtained from highly aromatic derivatives such as, for example, anthracene oil or naphthalene. When obtained at low temperatures (<900 °C), their crystalline order is deteriorated and their content of elements other than carbon increases.

The pregraphitic materials of the present invention, unlike more ordered pregraphitic materials with a more carbonaceous composition, are obtained at lower preparation temperatures, which leads to a less ordered crystalline structure and a composition with a high content of elements other than carbon, and may include metallic elements (ash). Like other pregraphitic materials obtained at higher temperatures (>900°C), they are good precursors of graphite through heat treatments at temperatures around 2500-2800 °C. However, the preparation of graphene materials from them by the chemical route poses problems of high reactivity (see example 2). Therefore, their use is not recommended or, if it could be carried out, it would present additional technical problems that would make their implementation on an industrial scale difficult due to this increase in reactivity during processing.

By "graphenic material" in the present invention is meant a product selected from coke oxide, graphene oxide or graphene.

Low-temperature pregraphitic materials are commercially available, abundant, and much cheaper to manufacture than synthetic graphite.

Furthermore, these types of materials are by-products not only of the energy industry but also for the production of high added value materials. The economic interest of implementing the method of the present invention is therefore very high.

In another preferred embodiment of the invention, the method for obtaining coke oxide comprises a step prior to step (a) of grinding or sieving the green coke. Preferably, the green coke after this treatment has a particle size between 70-100 microns (^m) measured by sieving or Coulter determination. In the method of the present invention, green coke can also be used without controlling the particle size, in which case longer reaction times could be necessary.

In another preferred embodiment, the basic medium of step (a) is an aqueous solution comprising KOH or NaOH, preferably at room temperature (i.e., between about 15-30°C) and with stirring. The aqueous medium may be distilled water, deionized water, milliQ water or any other purity, as well as other polar solvents.

In an even more preferred embodiment, after step (a) a purification of the solid obtained is carried out. The purification of the solid obtained by this basic treatment can be carried out by washing, filtration, centrifugation, ultracentrifugation or dialysis, preferably by filtration and washing with water.

In another preferred embodiment, the present invention comprises as step b) an acid treatment after the basic treatment, where the acid medium of step (b) is an aqueous solution comprising HNO3, H2SO4 or HCl. The aqueous medium may be distilled water, deionized water, milliQ water or any other purity, as well as other polar solvents.

In an even more preferred embodiment, this acid treatment in aqueous medium can be carried out under reflux at a temperature of about 100°C, although alternative methods such as ultrasonic reaction in an autoclave, or other methods known to a person skilled in the art, could be used.

In an even more preferred embodiment, after step (b) a purification of the solid obtained is carried out. The purification of this acid treatment can be carried out by washing, filtration, centrifugation, ultracentrifugation or dialysis, preferably by filtration and washing with water.

In a preferred embodiment, the oxidation of step c) comprises the treatment of the solid obtained in step b) to obtain coke oxide in an aqueous medium, preferably distilled water, deionized water, milliQ water or any other purity, preferably using as reagents the list consisting of sodium nitrate, potassium permanganate, sulfuric acid, hydrogen peroxide or any combination thereof, preferably the oxidation is carried out with sodium permanganate and subsequently with hydrogen peroxide and more preferably at a temperature between 30 and 45°C.

In an even more preferred embodiment, the method of the present invention uses coke/potassium permanganate weight ratios of between 1/6 and 1/3 depending on the coke. The preferred reaction volumes correspond to 20 vol% sulfuric acid and 80 vol% 3% hydrogen peroxide, the volume percentages being expressed with respect to the total reaction volume, or also 20 vol% sulfuric acid and 8 vol% hydrogen peroxide and 72 vol% distilled water, again expressed with respect to the total reaction volume, these proportions being able to be varied preferably depending on the characteristics of the starting material.

In another preferred embodiment, the method for obtaining coke oxide comprises separating the oxide obtained in step (c) from the pregraphitic material or unoxidized coke. Preferably, this separation can be carried out by washing, filtration, centrifugation, ultracentrifugation or dialysis and subsequent decantation of the supernatant. If it is not possible to separate the solid oxidized material by centrifugation, the separation can be carried out only by ultracentrifugation, preferably at high speeds, up to 11,500 rpm, or dialysis.

In an even more preferred embodiment, this purification or separation takes place by repeating the sequences for separating the oxides mentioned above, after adding water, which may be deionized or of another purity. This sequence is repeated repeatedly until the water separated by decantation has a neutral pH.

Another aspect of the present invention relates to a method for obtaining graphene oxide from green coke comprising steps (a) to (c) of the method for obtaining coke oxide described previously; and

d. exfoliate the coke oxide obtained in step (c).

The exfoliation of coke oxide is carried out by laminar separation using any of the procedures known to any expert in the field, such as heat treatment, solvent exfoliation, or ultrasound.

In an even more preferred embodiment, the exfoliation of the green coke oxides takes place by ultrasonic treatment, preferably for periods between 30 minutes and eight hours to produce the corresponding graphene oxide.

The method described in this patent application makes it possible to obtain graphene oxides from mainly monolayer/bilayer green coke with a height of around 1.4-3 nm and with perimeter sizes varying mainly between 200 and 800 nanometers, determined by analysis of the materials obtained by atomic force microscopy (AFM). These values may vary depending on the starting green coke. The number of oxygen atoms per 100 carbon atoms in each material is preferably around 20-25 atoms, a value that corresponds to that of graphene oxides prepared from graphite.

In those cases where green cokes have a high content of metallic elements (analysis for example in ashes), many of them of high added value (vanadium, nickel, iron, etc.), these elements could be easily obtained from the by-products of the treatments (a) and fundamentally from the treatment (b). Furthermore, since these reactions (treatments) are carried out with pure reagents and there are no large quantities of oxidation by-products of the green coke, this method can be used to easily recover this type of high added value elements and in a simpler way than using the supernatants of the oxidation step (step (c) of the present invention). This undoubtedly constitutes a significant advance for the sustainable development of this type of materials.

Therefore, in another preferred embodiment, metals from green coke are recovered from the supernatant obtained in the purification of the solid from steps (a) and/or (b). Preferably, from the purification of step (b) and even more preferably, high added value metals such as vanadium, nickel or iron are recovered.

The coke oxides and graphene oxides obtained can also be used directly in a multitude of applications, such as in catalysis, energy storage devices, etc.

Another aspect of the present invention relates to a method of obtaining graphene comprising the steps:

a), b), c) and d) of the method for obtaining graphene oxide described previously, and e) thermally reducing the graphene oxide obtained in step d) to graphene.

The reduction of coke oxide or graphene oxide in step (e) to give rise to the corresponding graphene can be carried out by any reduction methodology, such as chemical reduction, with hydrogen, electrochemical, etc. and even combinations thereof. Processes known to any expert in the field (for example see WO2011/016889A2).

In step e) of the method of the present invention, temperatures in the range of 800 to 2800 °C can be used, with different residence times of more than 10 h, with heating ramps of 0.1 °C minutes-1 to 100 °C minutes-1, in an inert atmosphere, for example nitrogen, argon or combinations of inert gases with reducing gases such as hydrogen.

In a preferred embodiment, in step e) of the method of the present invention, temperatures in the range of 800-1000 °C are used, with heating ramps around 1-30 °C minute-1, preferably with residence times at the final temperature of between 5 min and 2 hours and the use of inert atmospheres such as, for example, nitrogen.

The method described in the present invention allows obtaining, preferably, graphenes in the form of monolayers by thermal reduction of green coke oxides (or graphene oxides) at different temperatures.

Throughout the description and claims the word "comprises" and its variants are not intended to exclude other technical features, additives, components or steps. For those skilled in the art, other objects, advantages and features of the invention will become apparent partly from the description and partly from the practice of the invention. The following examples and figures are provided by way of illustration, and are not intended to be limiting of the present invention.

BRIEF DESCRIPTION OF THE FIGURES

Fig. 1: Shows the TG (solid line) and DTG (dashed line) curves of the green coke used as a starting product.

Fig. 2: Shows the thermogravimetric analysis of treated coke CK-B+A.

Fig. 3: (a) AFM measurements and (b) atomic force microscopy height profile of the CKrO-B+A material.

Fig. 4: Shows (a) AFM measurements and (b) height profile of CKrO-B+A-Low.

Fig. 5: Sample (a) AFM image and (b) height profile of the materials obtained with only acid pre-oxidation treatment and after purification.

Fig. 6: SEM images show (a) the coke treated with base+acid (CK-B+A), (b) the CK-B+A coke subjected to 8h of ultrasound and (c) the CK-B+A coke subjected to a flash pyrolysis process at 350°C.

Fig. 7: Shows the SEM image of a graphene obtained by thermal reduction up to 1000°C of graphene oxide.

Fig. 8: Sample (a) AFM image and (b) height profile of the materials (above the limit height to be considered graphenic) obtained with only basic pre-oxidation treatment and after purification.

EXAMPLES

Below, a series of tests carried out by the inventors are described, which are representative of the effectiveness of the method of the invention for the use of low-temperature pregraphitic materials for the preparation of graphene, graphene oxide or oxide of pregraphitic materials.

In all examples, a green petroleum coke called CK is used as the starting material. The characterizations by elemental analysis of this coke are shown in Table 1, where the carbon content of 90% stands out, as well as the content of N (2.6%) and S (0.7%).

Table 1: Elemental analysis and carbon/oxygen weight ratio of the original coke (CK) and the treated coke according to the different treatments followed in the examples

The thermogravimetric analysis of coke (CK) stands out (TG and DTG curve, Fig. 1) with an initial weight loss of around 400°C, indicating that this coke has not been subjected to temperatures much higher than that (which corresponds to a green coke).

Example 1.- Preparation of graphene oxides from green petroleum coke using the chemical route and by the process of the invention that includes initial basic treatment and then acid treatment before oxidative treatment.

The green coke used as starting material (CK) was initially subjected to a basic process consisting of 1 g of coke in 10 mL of 1 M KOH being stirred for 1 h, and the mixture being filtered over a paper ream to discard the liquid. The solid is washed several times with Milli-Q water. The solid obtained from the basic treatment is refluxed with 10 mL of 40% HNO3 for 30 minutes. The solid is vacuum filtered through a paper ream and washed with Milli-Q water. The solid obtained is called CK-B+A. Its elemental characterization (Table 1) shows that this solid has a higher content of O (24.7%) and N (6.49) than the original coke CK, suggesting that these treatments partially oxidize the original coke. This is also suggested by the thermogravimetric analysis of the CK-B+A residue (Fig. 2) where a thermal decomposition peak appears at lower temperatures (over 300°C) that may correspond to these new, more thermally unstable oxidized compounds. On the other hand, the higher yield at 1000°C of this treated sample (over 60%) and the movement of the second weight loss peak up to 600°C suggest that these treatments contribute to removing part of the carbon material.

The CK-B+A material is subjected to a Hummers-type oxidation process (chemical route) similar to that used to prepare graphite and graphene oxides from graphite. The adjusted amounts of reagents are a 1:1:6 ratio of Coke/NaNO3/KMnO4, at 35°C, for 3h, followed by treatment with 3% H2O2 for 1h more at 35°C. From the obtained mixture, the green coke oxide (obtained in 20% yield) is separated by centrifugation at 11500 rpm for 3h. The solid obtained is subjected to ultrasonic treatment for 2h to form the graphene oxide CKrO-B+A. The AFM analysis of this sample (Fig. 3a and 3b) shows that it is mainly a single/double layer graphene oxide (sheet height up to 3 nm) and sheet size up to 500 microns.

Example 2.- (Not part of the invention) Preparation of graphene oxides from green petroleum coke using the chemical route and by the standard procedure described in previous works including only oxidative treatment.

The green coke used as starting material (CK) was subjected to the same oxidative treatment process as in Example 1 (Hummer's type), that is, without prior basic or acid treatments. 1 g of green coke was reacted with NaNO3 and KMnO4 in proportions of 1:1:6, at 35°C, for 3 h, followed by treatment with 3% H2O2 for a further 1 h at 35°C. The reaction was initially very violent, with deflagrations being observed that led to the abort of the reaction.

This example demonstrates the viability of the treatment described in this work compared to that described in the literature for cokes. This is due to the excessive reactivity of the starting green coke as a result of having been obtained at low temperature.

Example 3.- Preparation of graphene oxides from green petroleum coke using the chemical route and by the procedure described in this patent that includes initial basic treatment and then acid treatment before oxidative treatment under more adjusted reagent conditions.

The green coke used as starting material (CK) was subjected to a process analogous to that of Example 1 but in which the proportions of the oxidation reagents were varied. Initially, 1 g of coke was subjected to a basic process in 10 mL of 1 M KOH and stirred for 1 h, and the mixture was filtered through a ream of paper to discard the liquid. The solid was washed several times with Milli-Q water. The solid obtained from the basic treatment was refluxed with 10 mL of 40% HNO3 for 30 minutes. The solid was vacuum filtered through a ream of paper and washed with Milli-Q water. The resulting solid CK-B+A, similar to that obtained in Example 1, was subjected to a Hummers-type oxidation process (chemical route) similar to that used to prepare graphite and graphene oxides from graphite. The adjusted amounts of reagents are a 1:3 ratio of Coke/KMnO4, at 35°C, for 1 h, followed by treatment with 3% H2O2 for a further 1 h at 35°C. From the mixture obtained, the green coke oxide (obtained in 60% yield) is separated by centrifugation at 11500 rpm for 3 h. The solid obtained is subjected to ultrasonic treatment for 2 h to form the graphene oxide CKrO-B+A-Low.

The obtained graphene oxide CKrO-B+A-Low was characterized by AFM (Fig. 4), showing that it is mainly a single-layer graphene oxide with a sheet size around 500 nm.

This shows that the process can be used in different proportions of reagents, which will be optimized according to the characteristics of the starting coke.

Example 4- (Not part of the invention). Preparation of graphene oxides from green petroleum coke using the chemical route and including only an acid treatment before the standard oxidative treatment.

The green coke CK is refluxed with 10 mL of 40% HNO3 for 30 minutes. The solid is vacuum filtered through a paper ream and washed with Milli-Q water. The resulting solid CK-A is subjected to a Hummers type oxidation process (chemical route) similar to that used to prepare graphite and graphene oxides from graphite. The adjusted quantities of reagents are a 1:3 ratio of Coke/KMnO4, at 35°C, for 1 h, followed by treatment with 3% H2O2 for 1 more h at 35°C. These are similar to those used in example 3 of this invention. As a difference with this one, at this point the appearance of a large amount of foam is observed in the reaction medium, which requires the mixture to be purified repeatedly, considerably lowering the yield.

From the mixture obtained, the green coke oxide is separated, which cannot be separated by centrifugation at 11,500 rpm for 3 h (same conditions as in example 1 and 3) and which is composed of high-height material corresponding to more than 10 layers (Fig. 5a and b). Sheet heights above what is allowed to be considered graphene material.

This experiment demonstrates the need to perform an initial basic treatment that substantially improves the reactivity of the material in the oxidative reaction for preparing graphene materials.

Example 5- (Not part of the invention). Sonication and reduction/exfoliation heat treatment of a green coke subjected exclusively to basic treatment followed by acid treatment.

The material obtained after the basic and acid treatments described in Example 1, CK-B+A, as already discussed, has a high oxygen content (Table 1). This could suggest that a coke oxide has been formed which could be susceptible to direct exfoliation without the need for oxidative treatment by standard exfoliation procedures.

To verify this, the CK-B+A material, with a coke-like stacked structure as deduced from the SEM photos (Fig. 6a), is subjected to an ultrasound process for 8h. The result is a stable suspension similar to the starting one, suggesting that there has been no exfoliation using this methodology. This is demonstrated in the SEM analysis of the obtained sample (Fig. 6b), with a three-dimensional structure similar to the starting one.

On the other hand, the CK-B+A material was subjected to a standard exfoliation/reduction process by flash pyrolysis at 350°C, with a nitrogen flow of 200 mL/min and a residence time of 10 min. The result is still a non-exfoliated sample of three-dimensional nature similar to the starting CK-B+A, as observed by SEM (Fig. 6c).

These experiments demonstrate the need for the treatment sequence described here to separate the graphene sheets from each other sufficiently with these cokes to induce their exfoliation using these standard methods.

Example 6.- Preparation of graphene from graphene oxide derived from green petroleum coke using the chemical route and by the procedure described in this patent, which includes initial basic treatment and then acid treatment before oxidative treatment under more adjusted reagent conditions (example 3)

The graphene oxide from example 3 was subjected, once dried by freeze-drying, to a thermal treatment in an inert atmosphere up to 1000°C, with a ramp of 2°C/min. The resulting solid (graphene) was characterized by SEM, observing that it is formed by monolayers of graphene (Fig. 7).

Example 7.- Clean extraction of metals contained in the starting green coke, after the acid treatment stage comprising this invention.

According to the procedure described in this patent, once the green coke has been subjected to a basic and purified treatment, it must be subjected to a subsequent acid treatment. Once this second treatment is finished, the solid is recovered by filtration for further treatment. In this example, the acid water is collected after this filtration and subsequent washing of the solid and is analysed by ICP-mass to determine its composition. The solution is completely transparent and only has a slight yellow hue, indicating that there are no carbonaceous residues as occurs after the procedure of the subsequent oxidation stage, which represents an improvement when its recovery is required. The ICP results (Table 2) confirm that it is in this solution where the metallic elements are collected. In this particular example, the presence of iron (Fe), nickel (Ni) and especially Vanadium (V) stands out.

Table 2: Results of analysis of metallic elements of interest by ICP-mass of the acidic waters recovered after the acid treatment of a green coke previously treated with base.

Example 8: (Not part of the invention). Preparation of graphene oxides from green petroleum coke using the chemical route and including only a basic treatment before the standard oxidative treatment.

The green coke used as starting material (CK) was subjected to the initial basic process consisting of stirring 1 g of Coke in 10 mL of 1 M KOH for 1 h, and filtering the mixture over a ream of paper to discard the liquid. The solid is washed several times with Milli-Q water. The resulting solid is subjected to a Hummers-type oxidation process (chemical route) similar to that used to prepare graphite and graphene oxides from graphite. The adjusted amounts of reagents are a 1:3 ratio of Coke/KMnO4, the same as those used in example 3 of this invention. The temperature was 35°C and the time 3h. At this point, and as a difference from example 3, a more intense appearance of a pink vapor is observed. Next and in a manner analogous to example 3, the treatment with 3% H2O2 is carried out for 1 more hour at 35°C.

From the mixture obtained, green coke oxide is separated, which is in this case more difficult to separate by centrifugation at 11,500 rpm for 3 h than the material under comparison obtained in examples 3 and even 1 (higher oxidant concentration). The AFM analysis of this material (Fig. 8) confirms that it basically consists of groupings of sheets of more than 10 layers (limit to be considered as low-layer graphene), which highlights the need to carry out the acid treatment after the basic treatment and before the oxidation treatment.

Claims (12)

1. A method of obtaining a green coke oxide from green coke, characterized in that it comprises the following steps: a. treat the coke in a basic medium at pH above 7; b. treating the solid obtained in step (a) in an acidic medium at a pH below 5; and c. chemically oxidize the solid obtained in step (b). 2. The method according to claim 1, wherein the basic medium of step (a) is an aqueous solution comprising KOH or NaOH, preferably at room temperature and with stirring. 3. The method according to any of claims 1 or 2, wherein the acidic medium of step (b) is an aqueous solution comprising HNO3, H2SO4 or HCl, preferably at reflux at a temperature of 100°C or by ultrasound in an autoclave. 4. The method according to any of claims 1 to 3, wherein after step (a) and/or (b) a purification of the solid obtained is carried out. 5. The method according to claim 4, wherein the purification is carried out by filtration and washing with water. 6. The method according to claim 4 or 5, wherein in the purification of the solid a supernatant is obtained in which metals from the green coke are recovered. 7. The method according to any of claims 1 to 6, wherein the oxidation of step (c) comprises the treatment of the solid obtained in step (b) with sodium nitrate, potassium permanganate, sulfuric acid, hydrogen peroxide or any combination thereof, preferably with sodium permanganate and hydrogen peroxide and more preferably at a temperature between 30 and 45°C. 8. The method according to any of claims 1 to 7, wherein after step (c) the coke oxide obtained is separated from the non-oxidized coke, preferably by washing, filtration, centrifugation, ultracentrifugation or dialysis. 9. The method according to any of claims 1 to 8, wherein prior to step (a) the green coke is ground or sieved, preferably to a particle size between 70-100 ^m measured by Coulter. 10. A method for obtaining graphene oxide from green coke comprising steps (a) to (c) of the method for obtaining coke oxide described according to any of claims 1 to 9; and d. exfoliate the coke oxide obtained in step (c). 11. The method according to claim 10, wherein the exfoliation is carried out by ultrasound. 12. A method for obtaining graphene from green coke comprising steps (a) to (d) of the method described according to any of claims 10 or 11; and e. thermally reduce the graphene oxide obtained in step (d), preferably at a temperature between 800°C and 1000 °C, with heating ramps of between 1-30 °C/minute, in an inert atmosphere.