A process for the preparation of high-grade graphene from subbituminous coal

Abstract

The present invention relates to the process for preparation of high-grade turbostratic graphene from subbituminous tertiary coal from North eastern region of India by ultrasonic-assisted chemical oxidation and ultrafiltration followed by thermal reduction process. More particularly, the present invention relates to the efficient, scalable, low-cost process for preparation of high-quality graphene by ultrasonication supported chemical oxidation, ultrafiltration followed by thermal reduction process from low-grade subbituminous Indian coals. The synthesis of coal-derived graphene material is confirmed by various analytical methods such as High-Resolution Transmission Electron Microscope (HRTEM), Field Emission Scanning Electron Microscopy (FESEM), X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and Fourier Transform Infrared (FTIR) spectroscopy. This process can produce graphene on a large scale without using harsh acids, making it safer and more eco-friendly than existing methods. The present invention offers a sustainable and effective way to produce graphene with potential applications in different domains.

Description

[0001] A PROCESS FOR THE PREPARATION OF HIGH-GRADE GRAPHENE FROM SUBBITUMINOUS COAL

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to the process for preparation of high-grade Turbostratic graphene from subbituminous tertiary coal from North eastern region of India by ultrasonic-assisted chemical oxidation and ultrafiltration followed by thermal reduction process. More particularly, the present invention relates to the efficient, scalable, low-cost process for preparation of high-quality graphene by ultrasonication supported chemical oxidation, ultrafiltration followed by thermal reduction process from low-grade subbituminous Indian coals.

[0004] BACKGROUND OF THE INVENTION

[0005] Graphene, which is also known as the "material of the future", is a two-dimensional graphitic material characterized by hexagonal carbon atom networks. It has garnered escalating interest and research endeavors globally, particularly in industrial applications since its discovery in 2004. Graphene finds extensive application in diverse fields including electronics, energy storage, photovoltaic devices, membrane materials, resource recovery, biological engineering, biocatalysis, original filtration, and the development of lightweight and robust composite materials. This widespread adoption is attributed to graphene's exceptional properties, encompassing remarkable electrical conductivity, high optical transparency, advantageous adsorption capabilities, elevated thermal conductivity, and mechanical strength. The multifaceted attributes of graphene have positioned it as a pivotal material in advancing technological innovations across various sectors.

[0006] Several methods are employed for graphene synthesis, each with its own advantages and challenges. Graphene synthesis methods can be classified into two main categories: top-down and bottom-up. Top-down methods involve the physical or chemical separation of graphene layers from bulk graphite or other sources, such as micromechanical exfoliation, direct sonication, electrochemical exfoliation / functionalization and super acid dissolution. Bottom- up methods involve the chemical or physical assembly of graphene layers from smaller carbon units, such as chemical vapor deposition, confined self-assembly, arc discharge, epitaxial growth on SiC, unzipping of carbon nanotubes, reduction of CO etc. Graphene can be synthesized from various source materials and the selection of source material often depends on the synthesis method and the desired properties. In the process of graphene synthesis, a variety of source materials are commonly utilized. These include natural graphite, graphite oxide, and carbon nanotubes (CNTs). Additionally, polymeric materials, silicon carbide (SiC), and metal substrates have been employed. Gallium-based liquid metals serve as another category of source materials. Furthermore, gases containing carbon are also used in the synthesis of graphene. These diverse source materials provide a broad range of options to produce graphene, each with its own unique properties and potential applications.

[0007] Reference may be made to article published in Energy Fuels, 2012, 26, 5186-5192, wherein synthesis of chemically derived graphene and graphene-noble metal composites by employing coal under catalytic graphitization, chemical oxidation, and dielectric barrier discharge (DBD) plasma-assisted deoxygenation disclosed. The study reveals that the degree of graphitization and high crystallinity of the coal-derived carbon has an important contribution for obtaining high-quality graphene sheets. During the process, the coal was ground and sieved to obtain coal powder and then ball-milled for 1.5 hours. The transformation of coal into graphite-like carbon was conducted by heat treatment in the presence of Fe using Fe2(SO4)3 which was well-mixed by ball milling for 2 min, and then the mixture was subjected to catalytic graphitization at 2400 °C for 2 h under argon environment. In DBD Plasma treatment, the oxidized coal powders are placed into a quartz tube in a DBD reactor. The reactor is purged with H2 gas to remove air. This treatment is carried out at room temperature and atmospheric pressure for 5 minutes. The graphene sheets, found in this process, have layer structures with plenty of ripples and folded regions, indicating their high quality and ultrathin nature.

[0008] Reference may be made to article published in Journal of Nanoscience and Nanotechnology, Vol. 12, 1^4, 2012, wherein new solution phase technique for the preparation of graphene from coal has been studied. First, graphene oxide (GO) was synthesized in the concentrated sulfuric acid and was oxidized by NaNO2 and then thermal treatment was undertaken under argon flow at 1100 °C for 3 h and the graphene film was attained. The precursor graphene oxide got from coal was examined by atomic force microscopy, dynamic light scattering and X-ray diffraction, the results showed the GO was a small and single layer sheet. The AFM results revealed that the thickness of the GO sheets was uniform (~0.46 nm) and had a structure with a diameter of ~50 nm. XRD of GO gave the diffraction peak at 20 = 21.5°, came from the inter-GO diffraction, shows a d-spacing of 0.41 nm corresponding with the AFM result. The Raman spectrum of GO shows two peaks at 1365 and 1590 cm-1, corresponding to D band and G band. For the graphene got after thermal treatment, the G band shifts to higher wave numbers, reaching 1606 cm-1, and the ratio ID / IG increases from 0.63 to 0.87. The XPS analysis of the graphene shows most of the carbon was having sp2bonding. The graphene exhibits an n-type behavior, which is caused by the N atoms came from the ammonia or the coal’s own which were doped into the graphene lattice.

[0009] Reference also may be made to article published in Journal of Materials Chemistry A, 2014, 2, 19633-19640, wherein synthesis of 3D hollow porous graphene balls (HPGBs) from coal tar pitch using nano-MgO as a template and KOH as an activator has been reported. In this study, a simple and direct process was used that involved coating liquefied pitch onto MgO particles, heating and activating the mixture, and subsequent removing the MgO template by acid washing. The HPGBs were found to have a high specific surface area (1947 m2 g-1), a balanced pore structure (total pore volume of 1.16 cm3 g- 1 ), and a thin carbon shell with low ash content and no impurities. FESEM and TEM analysis showed that the HPGBs had a 3D hollow spherical shape with a thin porous shell consisting of macropores, mesopores, and micropores. The pore size distribution showed two sharp peaks for micropores at 0.6-0.7 and 1.2 nm, and a broad peak for macropores centered at 118 nm, with a continuous distribution of mesopores between 35 and 95 nm. The diameter and thickness of the HPGBs were controlled by the size of the nano-MgO template. The TEM images also revealed the graphene-like structure of the HPGBs with a low degree of graphitization and many defects. N2 adsorption-desorption data showed that the HPGBs had a type IV isotherm with a hysteresis loop, indicating the presence of multimodal pores. From the Raman spectroscopy study, the well-known D-band and G-band of carbon materials are found at around 1320 and 1590 cm'1, respectively. The strong D-band indicated the low degree of graphitization and the large amount of disorder and defects in the HPGBs, which may result from the KOH activation.

[0010] Reference may be made to article published in Carbon, 153 (2019) 585-591, wherein one-step and facile method to synthesize graphene-based materials from coal under ambient conditions via direct CO2 laser scribing was reported. The resulting laser scribed graphene from coal (C- LSG) was considered and found to have excellent electrical conductivity, electrochemical sensitivity, and ionic storage properties. The authors used many characterization processes such as Raman spectroscopy, X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM) to study the structural and morphological properties of C-LSG. They found that C-LSG had a graphene-like structure with improved crystallinity, as evidenced by the sharp 2D peak in the Raman spectrum and the average lattice spacing of 0.34 nm in the HRTEM image. The XRD results also showed that C-LSG had a higher degree of graphitization compared to raw coal. The mechanism behind the formation of C-LSG involves the laser-induced pyrolysis of coal and the subsequent formation of graphene-like structures. During the laser scribing process, the coal is heated to a high temperature, causing the release of volatile components and the formation of a carbon-rich residue. The high temperature also brings the rearrangement of carbon atoms, indicating the formation of graphene-like structures.

[0011] Reference may be made to article published in Chemical Physics 530 (2020) 110607, wherein micro-sized water-soluble graphene sheets were produced by exfoliation method using acids (sulfuric acid and nitric acid) utilizing coal as a starting material. XRD, FESEM, TEM, FTIR, Raman and UV-vis absorbance measurements were used to explore crystalline features, structural morphology, elemental composition, optical band gap and emission behavior of coal derived graphene. The results confirmed the formation of few layered rGO sheets with flake type structures. HRTEM images confirmed the formation of layered structures with an interplanar distance of 0.257 nm. FTIR measurements confirmed the presence of oxygen based functional groups, while Raman spectra showed two pronounced peaks at -1353 cm-1(D band) and 1617 cm-1(G band). UV-vis absorbance spectra showed a broad absorption edge associated with significant broadening in 200-450 nm and trailing up to IR region, indicating partial restoration of sp2bonded carbons. PL spectroscopy measurements showed a broad emission peak at -510 nm, which was fitted into three peaks corresponding to n-7i*, K- TT* and n-o* electronic transitions.

[0012] Reference also may be made to article published in Carbon, 181 (2021) 408-420, wherein facile one-pot process has been reported using only HNO3 to obtain graphene oxide (GO) from coal. The process involves oxidative scissoring of coal, followed by exfoliation and centrifuging to isolate thin graphene nanosheets. The study showed that the coal derived GO has higher sp2content, larger graphitic domains and more carboxylic groups than the graphite derived GO. The study also suggested an oxidative scissoring mechanism for this one-pot process. Such coal derived GO (coal-GO) mainly contains carboxylic groups at the edges, while the graphitebased GO (graphite-GO) has more epoxide groups on the basal plane. The coal-GO and coal- rGO have thin layered nanostructures with lateral sizes of 0.8-2.0 nm and <200 nm, respectively. The coal-rGO shows a higher degree of graphitization and sp2 / sp3ratio than the graphite-rGO, indicating a better preservation of the graphitic domains in coal. Coal-GO and coal-rGO show lower crystallinity and larger interlayer spacing than graphite-GO and graphite- rGO. Coal-GO has a turbostratic structure with randomly oriented graphene layers. Raman spectroscopy analysis showed that coal-GO and coal-rGO have higher D / G ratio and lower ID / IG ratio than graphite-GO and graphite-rGO, indicating more defects and disorder in the coal-derived samples. Coal-GO has a higher degree of oxidation than graphite-GO. Coal-GO and Coal-rGO have smaller lateral sizes and fewer layers than graphite-GO and graphite-rGO. Coal-rGO shows six-fold symmetry of graphene in the SAED pattern.

[0013] Reference may be cited to the article published in International Journal of Membrane Science and Technology, 2022, 9, 1-12, where the study introduces a methodology for the synthesis of reduced graphene oxide (RGO) from low-grade coal via a modified hydrothermal process. The synthesized RGO was subjected to a series of characterization techniques, and the influence of reaction time and temperature on the RGO synthesis was examined. The Hummers method was employed for graphene synthesis, and a modified hydrothermal process was utilized to convert graphene oxide (GO) into RGO, offering an environmentally friendly, water-based approach. Scanning Electron Microscopy (SEM) images revealed a wrinkled and crumpled morphology of the RGO, with a thickness ranging between 1-2 nm. Energy-Dispersive X-ray Spectroscopy (EDS) study indicated that the RGO was mostly composed of carbon and oxygen, with a carb on-to-oxy gen ratio of 2.5:1. X-ray Diffraction (XRD) analysis displayed that the RGO possessed a crystalline structure, with a (002) peak observed at 29 = 25.5°. Fourier-Transform Infrared Spectroscopy (FTIR) spotted the presence of functional groups such as hydroxyl, carboxyl, and epoxy groups on the surface of the RGO. Raman spectroscopy identified two distinct peaks at 1350 cm'1(D band) and 1590 cm'1(G band), indicative of the presence of sp2- hybridized carbon atoms in the RGO. Overall, the characterization results confirmed and successful synthesis of RGO from low-grade coal using the modified hydrothermal method.

[0014] Reference may be made to article published in Nanomaterials, 2022, 12, 2989, wheerin synthesis of stable graphene oxide (GO) from lignite-based synthetic graphite using the Hummers method has been demonstrated. Besides, the RGO was prepared by utilizing the reducing agent sodium borohydride (NaBH4, 50 mM) at 105 °C. The XRD investigation showed that the RGO got from low-grade coal had a similar crystal structure to that of RGO got from different sources. The field emission scanning electron microscopy (FESEM) images showed that the RGO sheets derived from low-grade coal had a wrinkled and crumpled morphology, which is typical of RGO. The RGO sheets had a lateral size of 1-2 millimeters and a thickness of 2-3 nm, as demonstrated by the transmission electron microscopy (TEM) images. The Raman spectra showed the characteristic of RGO with D band at 1350 cm-1and a G band at 1580 cm-1. The XPS investigation showed that the RGO got from low grade coal had a high carbon content and a low oxygen content, which is desirable for electrochemical applications.

[0015] Reference may be made to article published in ACS Omega, 2023, 8, 25142-25154, wherein advantages of preparing graphene from tectonically deformed coal (TDC) compared to normal structural coal (NSC) discussed. The researchers prepared coal-based graphene from both TDC and NSC and analyzed their structural parameters. In the synthesis process, the coal-based graphite samples are oxidized using the modified Hummers method, resulting in graphene oxide (GO) samples. The GO samples are then exfoliated to obtain coal -based graphene oxides. GO samples were placed into a muffle furnace for heat treatment at 900 °C for 5 min, and the cooled product was subjected to heat treatment at 900 °C for 5 min again. These obtained products are further processed by soaking in ethanol, sonicating, and drying to obtain coal- based graphene samples. The entire process involves the conversion of coal into graphite and then into graphene. The HRTEM results showed that the graphene prepared from TDC had perfectly ordered crystal planes and abundant nanochannels, indicating a higher graphitizable carbon structure compared to NSC. XRD patterns revealed that the interlayer spacing, crystallite height, and graphitization degree were similar in both TDC-based and NSC-based samples. Raman analysis focused on the in-plane vibration of graphite sp2carbon atoms (G peak), graphite lattice defects and edge disorder (D peak), and multi -stacking modes of carbon atoms (G peak). The results indicated that the graphitization degree and aromatic ring condensation degree were comparable in both TDC-based and NSC-based samples.

[0016] Reference may be made to Asian J Chem 29, No. 11 (2017): 2425-2428, entitled “Extraction and Characterization of Preformed Mixed Phase Graphene Sheets from Graphitized Sub- Bituminous Coal” where a facile method is reported to extract mixed phase nanometer-sized carbon sheets from sub-bituminous coal. It was determined that the lattice constants (Laand Lc) of sub-bituminous coal were 4.82 and 1.41 nm, respectively. It was estimated that there are five aromatic layers and eight carbon atoms on average in the aromatic lamellae. The resulting graphene sheets show a very broad 2D bump along with broadened D and G bands. The ratio of defect to graphitic is found to be 0.54, suggesting that the formed graphene nanomaterial has less disorder and the ID / ID was found to be 3.40. In the synthesis procedure, centrifugation technique was employed to separate the residue and supernatant, with no reduction step included. Literature only provides few characteristics of the material, without any substantiation of the quantity, quality or purity of synthesized material.

[0017] Reference may be made to the Chinese Patent, CN103771403A, where a method has been disclosed of directly preparing multi 4 aver graphene powder by using lignite slag as a raw material and microwave heating it with a strong base. The method involves mixing lignite slag and a strong base (such as potassium hydroxide, sodium hydroxide, or calcium hydroxide) in a certain mass ratio, putting the mixture into a microwave-absorbing container, heating it in a microwave reactor for 5-50 minutes, and obtaining multi-layer graphene. The product is then washed with water, pickled with acid, and purified to remove inorganic impurities. Repeated deionization washing is employed to eliminate soluble inorganic salts from the product. Subsequently, a sequential cleaning process is implemented, utilizing IM hydrochloric acid and 0.5M hydrofluoric acid successively, to remove indissoluble inorganic salt impurities. The product has the distinctive peaks of graphene in Raman spectroscopy and X-ray diffraction and has a high specific surface area of 1800 m2g-1.

[0018] Reference may be made to the European patent, EP3299337A1, where it describes a method for preparing porous graphene from coal as a raw material, which is low-cost and easy for large-scale production. The method involves four steps: refining the coal to fine powder, immersing the powder in an activation agent solution, carbonizing the mixture in an inert or hydrogen atmosphere, and washing and drying the carbonized product. For the activation process the coal powder is mixed with an activation agent solution, such as KOH, NaOH, or a mixture of alkali metal or alkaline earth metal hydroxides. The mixture is stirred at room temperature for several hours and then dried to obtain a molten mixture. The molten mixture is heated in an inert gas or a mixture of hydrogen and inert gas at a high temperature (500-1200 °C) for several hours. This process converts the coal structure into graphene-like structures and creates micropores on the graphene sheets. The carbonized product is then cooled naturally. The carbonized product is washed with dilute acid and water to remove the residual activation agents and minerals. The product is then dried at a moderate temperature (60-200 °C) to obtain porous graphene. The porous graphene fabricated through this innovative method exhibits multiple structural merits including a substantial abundance of monolayer graphene sheet structures having micropores of less than 2nm, large surface area reaching up to 3345 m2g-1, and a controllable pore structure. The control of the graphene structure is attainable through the introduction of specific atmospheric conditions, adjustment of the coal-to-activation agent ratio, and modulation of the carbonization temperature.

[0019] Reference may be made to United States Patent, US10703634B2, which recites a method has been described for preparing porous graphene from lignite coal as a raw material. In this process, the powdered coal was immersed with an activation agent solution, such as KOH, and dried to get a molten mixture.The molten mixture was carbonized in an inert or hydrogen atmosphere at a high temperature to obtain a carbonized product. The carbonized product was washed with acid and water and dried to obtain the porous graphene. The patent claims that the porous graphene has a high surface area, low heteroatom content, and excellent performance in electrochemical energy storage and gas adsorption. The patent also discloses the effects of various parameters, such as carbonization temperature, activation agent concentration, and hydrogen fraction, on the structure and properties of the porous graphene.

[0020] Reference may be made to United States Patent, US20210284539A1, wherein a novel method for the manufacturing of graphene layers and graphene oxide layers from naturally occurring carbon-rich solid materials such as graphite, coal slags, asphalt, and their combinations has been mentioned. The method involves mixing the carbon-rich materials with a solvent, a surfactant, and optionally some additives, and then irradiating the mixture with microwaves for a short time.Microwaves can cause the carbon-rich materials to exfoliate into thin layers of graphene and graphene oxide, which can be separated from the solvent by filtration or other methods. Remarkably, this process yields graphene layers and graphene oxides within a brief time frame ranging from one second to 300 seconds. The resulting graphene-based materials exhibit applicability in the fabrication of conductive films for touch screens, the production of graphene carbon fibers and three-dimensional porous graphene nanomaterials, and the preparation of graphene-based intelligent nanocomposites for super-lightweight machines and vehicles.

[0021] Reference may be made to United States Patent, US 11, 535, 518B1, in which a method has been disclosed for making graphene-structured products directly from coal using molten salts.lt involves the process of contacting coal with a molten salt at a conversion temperature ranging from about 850 °C to about 1100 °C in an inert atmosphere. The molten salt opens the graphite layers in coal and exfoliates them into graphene sheets. The process also generates tailing gases that can be captured and condensed into petroleum fluids. It also describes the methods for separating and recoveringgraphene-structured products, rare earth elements and the crude oil liquids from the molten salt and coal mixture. The methods include acid washing, filtration, leaching, solvent extraction, and adsorption.

[0022] Reference may be made to the United States Patent Application Publication, US20220024771, where the patent elucidates a method of producing graphene oxide and reduced graphene oxide from purified coal powder using a mild oxidizing acid such as nitric acid. It involves the process of pulverizing, heating, and acid-washing the raw coal to remove organic, metal, and silica impurities. The purified coal powder has an average particle size of about 50 p or less and is free of trace metals such as Fe, Mg, and Al. The purified coal powder is placed in a mild oxidation atmosphere at an elevated temperature of about 65 °C to 200 °C for a period of time of about 1 min to 24 hours to produce a mixture containing graphene oxide. Graphene oxide can be separated from unreacted or unexfoliated coal impurities by centrifugation, filtration, or sedimentation. The process of reduction includes heating the graphene oxide to a second elevated temperature of about 200 °C to 2000 °C for a second period of time of about 10 minutes to 300 minutes in an inert atmosphere. The reduced graphene oxide has smaller nanosheets than standard graphite-derived graphene oxide and shows few-layered graphene of 3-5 layers.

[0023] While graphene exhibits exceptional properties, but the synthesis process is not without limitations and challenges. An industrial scale up of several synthesis techniques that are good for generating high-quality graphene is frequently difficult. One major obstacle still stands in the way of producing high-quality graphene on a large scale. Again, ensuring uniformity in the size, structure, and properties of synthesized graphene sheets is challenging because this may contain structural defects, impurities or functional groups introduced during the synthesis process. These defects can influence the material's electronic, mechanical, and thermal properties. Achieving precise control over the number of graphene layers (monolayer, bilayer or multilayer) is also challenging. But, certain synthesis methods, particularly those involving precise control and high-quality substrates, can be expensive.

[0024] The fallowing discussion presents a review of the existing literature pertaining to the production of graphene from coal, rendering an overview of the prior art references in this field. These references serve as a testament to the extensive research conducted in this area and provide valuable insights into the techniques and methodologies employed in the production process. The above information disclosed is only for the enhancement of understanding of the background of the invention.

[0025] Thus, keeping in view the drawbacks of the hitherto reported prior arts and at least the aforementioned issues, developing cost-effective methods for large-scale production is still a critical challenge for commercial applications. On the other hand, some synthesis methods involve harsh chemicals or high-energy processes thereby contributing to environmental concerns. Developing more environmentally friendly and sustainable synthesis approaches is a decisive focus in this graphene research.

[0026] In view of the above, present invention relates to an efficient, scalable, low-cost and at the same time high quality process for the preparation of graphene, by ultrasonication supported chemical oxidation, ultrafiltration followed by thermal reduction. In this process, the Tertiary subbituminous Northeast Indian (NER) subbituminous coal used as abundant raw materials.

[0027] OBJECTIVES OF THE INVENTION

[0028] The primary object of the present invention is to provide a process for preparation of Turbostratic graphene from low-grade subbituminous Indian coals by ultrasonic-assisted chemical oxidation and ultrafiltration followed by thermal reduction process.

[0029] Another objective of the present invention is to provide a process for preparation of Turbostratic graphene from carbon sources selected and collected from the NER locations containing low-grade sub-bituminous coals that have different carbon percentages.

[0030] In another objective, of the present invention is to provide a process for preparation of Turbostratic graphene by using eco-friendly oxidant.

[0031] Yet another objective of the present invention is to provide a process for preparation of Turbostratic graphene by incorporation of ultrasonication and dialysis / ultrafiltration steps underscores a commitment to efficient processing. This approach potentially leads to a higher yield of high-quality graphene.

[0032] Still another objective of the present invention is to provide a process for preparation of Turbostratic graphene by using minimal amount of water and ammonia solution for the neutralization step enhances the efficiency of graphene. A further object of the present invention is to provide a process for preparation of Turbostratic graphene in which entire process is easily scalable, indicating its potential for large-scale production.

[0033] These objectives of the present invention, as well as other objectives related thereto, will be readily apparent post consideration of the description of the invention, together with reference to the contents of the Figures of the drawings. Additional objects, advantages and other novel features of the invention will appear as the description proceeds and in part will become apparent to those skilled in the art upon examination of the following.

[0034] SUMMARY OF THE INVENTION:

[0035] Additional features and embodiments of the present disclosure will be better understood through the techniques and other aspects of the disclosure. Other embodiments of the invention are described in detail herein and are considered a part of the claimed disclosure.

[0036] The present invention, and in accordance with main aspect of the present invention, achieves a process for preparation of Turbostratic graphene from low-grade subbituminous Indian coals by ultrasonic-assisted chemical oxidation and ultrafiltration followed by thermal reduction process.

[0037] In another aspect of the present invention, the process preparation of high grade graphene from low-quality carbon source in 25-30 % yield having specific surface area (SSA) 642 m2g with a pore volume of 0.298 cm3g'att low-temperature thermal reduction.

[0038] In another aspect of the present invention, the process for preparation of graphene from low- quality subbituminous Indian coals comprising the step of:-

[0039] (a) Pulverizing the carbon source having the particle size 53-70 micron.

[0040] (b) Acid leaching the pulverized subbituminous coals as obtained in step (a) with hydrofluoric acid (30%).

[0041] (c) Washing the acid leaching pulverized subbituminous coals with deionized water till neutralization. (d) Drying the neutralized acid washed pulverized subbituminous coals as obtained in step (c) using hot air oven at the temperature in range of 600-800°c.

[0042] (e) Mixing the dry acid washed coal powder as obtained in step (d) with 50-60% formic acid (HCOOH) to form a slurry and 20-30% H2O2 in ice cold condition with continuously stirring the mixture for next 24-28 hours.

[0043] (f) reaction mixture as obtained in step (e) under ultrasonication at frequency in range of 20- 40 KHz at temperature in range of 20-3 °C for period in range of 18-24 hours.

[0044] (g) resulting dark brown solution as obtained in step (f) was transferred to stirred cell for dialysis / ultrafdtration process using 100-120 kDa membrane under N2 pressure.

[0045] (h) After completion of 1 cycle of dialysis / ultrafiltration, the reaction mixture was neutralized by adding ammonia solution dropwise followed by washing with distilled water 10-15 times until the neutralization point. The residue part was dried in an oven at the temperature in range of 110-140°C to get the oxidized product.

[0046] (i) dried oxidized coals as obtained in step (h) were thermally reduced at temperature in range of 1000-1200°C in the muffle furnace for period in range of 1-2 hour in the N2 atmosphere to obtained required reduced turbostratic graphene.

[0047] In an embodiment of the present invention, the process for preparation of graphene from carbon sources selected from the NER locations containing low-grade sub-bituminous coals that have different carbon percentages. The process encompasses exposing the carbon source to an eco- friendly oxidant.

[0048] In an embodiment of the present invention, the eco-friendly oxidants hydrogen peroxide (30%) and formic acid (60%) which form performic acid in the mixture and carbon source is subjected to ultrasonication in the presence of an eco-friendly oxidant

[0049] In yet another embodiment of the present invention comprises a step of separating the oxidized product from the oxidant.

[0050] In yet another embodiment of the present invention, wherein process restoring the sp2- hybridized carbon network and reducing the oxygen-containing groups of the oxidized coal by heating it at 1000-1200° C for 1-2 hour in a nitrogen atmosphere. In yet another embodiment of the present invention, the separation or purification occurs by the dialysis / ultrafiltration process which has utilized selective membranes to remove unwanted ionic impurities. This technique uses a semi-permeable membrane with pores small enough to hold macromolecules like GO while allowing smaller solutes such as salts and unreacted precursors to diffuse through the membrane. By continuously restoring the dialysate, the concentration of these impurities within the GO suspension progressively decreases, achieving purification.

[0051] In yet another embodiment of the present invention comprises thermal reduction which is essential for the removal of oxygen functional groups including hydroxides, carboxylic acids and epoxides. The removal of these oxygen functionalities leads to the partial conversion of sp3hybridized carbon atoms in GO to their original sp2hybridization state. This rearrangement facilitates for the formation of a more extended arrangement of conjugated double bonds, which is the characteristic of turbostratic graphene. This restored network is responsible for the improved electrical conductivity and other graphene-like properties observed in the final product.

[0052] BRIEF DESCRIPTIONS OF THE DRAWING

[0053] To complete the description and in order to provide for a better understanding of the present invention, a set of drawings is provided. The drawings form an integral part of the description and illustrate an embodiment of the present invention, which should not be interpreted as restricting the scope of the invention, but just as an example of how the invention can be carried out. The drawings comprise the following figures:

[0054] Figure 1 represents the Process flow sheet for the optimized synthesis of graphene / reduced graphene oxide from coal.

[0055] Figure 2 represents the Schematic diagram for each step in the optimized synthesis process of graphene / reduced graphene oxide from coal.

[0056] Figure 3 represents the FTIR spectra of standard reduced graphene oxide (rGO), standard graphene oxide (GO), raw coal (L), acid-washed coal (LA), oxidized coal (LAOD), and reduced coal [LR2T-10], Figure 4 represents the (a) XPS survey spectra, (b) Cis, and (c) 01s spectra ofLR2T-10 (rGO) sample.

[0057] Figure 5 represents the XRD analysis of coal-derived rGO (LR2T-10).

[0058] Figure 6 represents the Raman spectra of coal-derived graphene (LR2T-10) sample.

[0059] Figure 7 represents the (a, b) FESEM image of the graphene derivative at a 1pm and 200 nm scale showing the presence of the graphene sheet-like structure and (c) EDS of the carbon material showing a high carbon percentage at the surface.

[0060] Figure 8 represents the (a) HRTEM images of the coal-derived graphene (LR2T-10) at 100 nm and 5 nm magnifications showing the presence of graphene-like carbon sheets; (b) FFT pattern shows graphitic layers at a 5 nm scale (c); (d) SAED pattern of the graphene derivative showing its crystallinity with the presence of the graphitic planes (002) and (100) [doo2 = 0.329 nm].

[0061] Figure 9 represents the Thermal stability (TGA) of coal-derived graphene (LR2T-10) and standard reduced graphene oxide (rGO) at a heating rate of 10 °C min'1from ambient temperature to 1000 °C.

[0062] Figure 10 represents the a) N2 adsorption-desorption isotherms; (b) DFT pore size distribution; (c) BJH pore size distribution of coal-derived graphene (LR2T-10).

[0063] DETAILED DESCRIPTIONS OF THE INVENTION:

[0064] The following description is not to be taken in a limiting sense but is given solely for the purpose of describing the broad principles of the invention. Embodiments of the invention will be described by way of example, with reference to the above-mentioned drawings showing elements and results according to the present invention.

[0065] The foregoing detailed description of the disclosure is elaborated to provide a clear understanding to the person who is skilled in the art. Additional features, embodiments and advantages of the invention will be described hereinafter which form the subject of the claims of the disclosure, However, the set forth disclosure provide in the specification will best be understood in conjunction with the appended claims and figures as provide heretofore. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present disclosure. It should also be realized by those skilled in the art that such equivalent processes do not depart from the spirit and scope of the disclosure as set forth in the appended claims. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, and designed in various configurations, all of which are explicitly contemplated and make part of this disclosure.

[0066] While the invention has been disclosed with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made, and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from its scope.

[0067] Throughout the specification and claims, the following terms take the meanings explicitly associated herein unless the context clearly dictates otherwise. The meaning of "a", "an", and "the" include plural references. The meaning of "in" includes "in" and "on." Referring to the figures, like numbers indicate like parts throughout the views. Additionally, a reference to the singular includes a reference to the plural unless otherwise stated or inconsistent with the disclosure herein.

[0068] The present invention relates to process for preparation of graphene from low grade Indian Tertiary coal by using ultrasonic-assisted activation followed by ultrafiltration and thermal reduction. The utilization of abundant and cost-effective low-grade coal feedstock to synthesize graphene is attempted for utilizing this resource in an environmentally sustainable manner, while also creating such high-value products in large-scale.

[0069] Carbon sources selected from the NER locations containing low-grade sub-bituminous coals that have different carbon percentages undergo acid washed using 30% hydrogen fluoride (HF) solution in the first step and further for the oxidation using 1:5 ratio mixture of formic acid (HCOOH) and hydrogen peroxides (H2O2) as an oxidizing agent. After employing ultrasonic irradiation, dialysis / ultrafiltration method has been used to get the pure and neutral oxidized solution. The oxidized sample has been reduced thermally and the final product collected is the reduced graphene derived from coal. The present invention having several advantages that contribute to its potential effectiveness and efficiency in graphene synthesis. Firstly, it utilizes raw materials that are both abundant and cost-effective. Secondly, the incorporation of ultrasonication and dialysis / ultrafiltration steps underscores a commitment to efficient processing. This approach potentially leads to a higher yield of high-quality graphene. Depending on the chemicals used, this process may have a reduced environmental impact compared to other methods, demonstrating a consideration for sustainability. The incorporation of multiple steps allows the versatility in tailoring the graphene properties to specific applications, thereby providing flexibility in the synthesis process. Furthermore, the requirement of a minimal amount of water and ammonia solution for the neutralization step enhances the efficiency. Lastly, the method is easily scalable, indicating its potential for large-scale production. These advantages collectively highlight the potential of this novel method in advancing the field of graphene synthesis. With the aid of analytical characterization methods such as High-Resolution Transmission Electron Microscope (HRTEM), Field Emission Scanning Electron Microscopy (FESEM), X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Fourier Transform Infrared (FTIR) spectroscopy analysis, the formation and quality of coal-derived graphene material was confirmed.

[0070] Figure 1 represents the process flow sheet for the optimized synthesis of graphene / reduced graphene oxide from coal. Figure 2 provides the schematic diagram for each step in the optimized synthesis process of graphene / reduced graphene oxide from coal. In view of the above, present invention relates to an efficient, scalable, low-cost and at the same time high quality process for the preparation of graphene, by ultrasonication supported chemical oxidation, ultrafiltration followed by thermal reduction. In this process, the Tertiary subbituminous Northeast Indian (NER) subbituminous coal used as abundant raw materials.

[0071] SOURCE MATERIAL:

[0072] India has many reserves of low-grade coal, making it a promptly accessible and affordable resource compared to traditional graphene precursors like graphite. Using this abundant asset could essentially bring down the production costs of graphene. The high carbon content means a more noteworthy possible yield in the graphene synthesis process. Initially, two coal samples have been selected for the study based on the ash and fixed carbon content (Table-1). These coal samples are coded as T and L. The coal samples are first air dried, crushed to below 2 mm size, and finally sieved to a <0.53 mm size using standard methods (ASTM 2010). This study involves the use of L coal for all the experiments and further characterizations.

[0073] Table 1: Results of this proximate analysis of raw and acid-treated coal samples (wt%)

[0074] Figure 1 depicted the Process flow sheet for the optimized synthesis of graphene / reduced graphene oxide from carbon source wherein NER locations containing low-grade sub- bituminous coals used as carbon source.

[0075] Figure 2 represented the Schematic diagram for each step in the optimized synthesis process of graphene / reduced graphene oxide from coal .

[0076] The process for synthesizing graphene from low-quality subbituminous Indian coals comprising the step of:-

[0077] (a) Pulverizing the carbon source having the particle size 53-70 micron.

[0078] (b) Acid leaching the pulverized subbituminous coals as obtained in step (a) with hydrofluoric acid (30%).

[0079] (c) Washing the acid leaching pulverized subbituminous coals with deionized water till neutralization.

[0080] (d) Drying the neutralized acid washed pulverized subbituminous coals as obtained in step (c) using hot air oven at the temperature in range of 600-800°c. (e) Mixing the dry acid washed coal powder as obtained in step (d) with 50-60% formic acid (HCOOH) to form a slurry and 20-30% H2O2 in ice cold condition with continuously stirring the mixture for next 24-28 hours.

[0081] (f) reaction mixture as obtained in step (e) under ultrasonication at frequency in range of 20- 40 KHz at temperature in range of 20-3 °C for period in range of 18-24 hours.

[0082] (g) resulting dark brown solution as obtained in step (f) was transferred to stirred cell for dialysis / ultrafiltration process using 100-120 kDa membrane under N2 pressure.

[0083] (h) After completion of 1 cycle of dialysis / ultrafiltration, the reaction mixture was neutralized by adding ammonia solution dropwise followed by washing with distilled water 10-15 times until the neutralization point. The residue part was dried in an oven at the temperature in range of 110-140°C to get the oxidized product.

[0084] (i) dried oxidized coals as obtained in step (h) were thermally reduced at temperature in range of 1000-1200°C in the muffle furnace for period in range of 1-2 hour in the N2 atmosphere to obtained required reduced graphene.

[0085] EXAMPLES:

[0086] The following examples, which include preferred embodiments, will serve to illustrate the practice of this invention, it being understood that the particulars shown are by way of example and for purpose of illustrative discussion of preferred embodiments of the invention and therefore should not be construed to limit the scope of the present invention.

[0087] Example 1:- Synthesis of coal-derived graphene:

[0088] The present embodiment is a 4-step synthesis method of rGO from coal has been optimized which consists of: 1) acid washing, 2) oxidation, 3) dialysis / ultrafiltration and 4) reduction. The detailed study and presentation of each step are mentioned in the following stages.

[0089] STEP 1: ACID WASHING:

[0090] A raw coal sample (L) weighing 100 grams was washed by using a 30% hydrofluoric acid (HF) solution. A slurry was prepared with 1000 ml 30% HF in a beaker and stirred at room temperature for one hour. Following the specified treatment duration, the slurry was allowed to settle and filtered using filter paper (1 A). The residue was washed multiple times with warm distilled water until the pH of the filtrate reached a neutral state. The residue (LA) was then dried at 80°C for 6 hours and cooled gradually for subsequent analysis. The acid treatment resulted in an increase in carbon content and a significant reduction in moisture and ash percentages (decreased by ~ 80%).

[0091] STEP 2: OXIDATION:

[0092] In this experimental procedure, a total of 100 grams of acid-washed coal sample was placed in a beaker. Subsequently, formic acid (60% HCOOH) was added to the beaker. Furthermore, 30% hydrogen peroxide (H2O2) was carefully added drop by drop to the mixture. The solution was stirred with the help of a mechanical stirrer for about 24 hours in an ice bath followed by ultrasonication for another 24 hours.

[0093] STEP 3: DIALYSIS / ULTRAFILTRATION:

[0094] The dialysis process was performed in the next step with the help of a stirred cell using a 100 kDa molecular cut-off membrane. The process was continued until achieving a neutral pH solution. The residue (oxidized sample, LAOD) was collected and dried in the hot air oven for 6 hours at 80°C.

[0095] STEP 4: REDUCTION:

[0096] The thermal reduction of the coal oxide (CO)Zgraphene oxide (GO) (LAOD) sample was optimized at a temperature of 1000°C under a nitrogen (N2) environment. The temperature was initially increased at a rate of 10°C per minute until reaching 800°C. After holding at 800°C for 10 minutes, the temperature was further increased to 1000°C at a rate of 5°C per minute. The sample was kept at 1000°C for a period of 1 hour, followed by cooling to room temperature also in the nitrogen environment. The resulting sample, referred to as LR2T-10, is considered reduced turbostratic graphene oxide (rGO).

[0097] In this process of Turbostratic graphene synthesis from sub -bituminous coal, a yield of 25-28% has been achieved. This significant yield underlines the major efficacy of this synthesis process and the potential of sub-bituminous coal as a viable precursor for quality graphene production.

[0098] EXAMPLE 2:-PROXIMATE AND ULTIMATE ANALYSIS: The evaluation of coal quality and rank can be executed by proximate and ultimate analysis. The results of this proximate analysis (ASTM D7582-15) are shown in Table-1. The ash content of the coal samples gets reduced after acid treatment accompanied by an increase of fixed carbon. The coal under investigation has high carbon content while the nitrogen and hydrogen content of the coal samples are also considerably low. On the other hand, the sulfur content in the treated coal sample is relatively low as compared to raw coal. C, H & N analysis (ASTM-D5373-21) and sulfur analysis (ASTM-4239-18el) also show increased carbon content (Table 2).

[0099] Table 2: Results of this ultimate analysis of raw and acid-treated coal samples (wt%)

[0100] Table 3: Summary of physico-chemical properties of optimized coal sample at different steps of treatment: raw coal (L), acid-washed (LA), oxidized coal (LAOD), thermally reduced coal (rGO)

[0101] EXAMPLE 3 : -OBSERVATIONS FROM ANALYTICAL CHARACTERIZATIONS:

[0102] Fourier transform infrared (FTIR) spectroscopic analysis was conducted to identify and characterize the surface functional groups present on the various coal samples at each stage of the reaction process and shown in Figure 3. Figure 3 depicted FTIR spectra of standard reduced graphene oxide (rGO), standard graphene oxide (GO), raw coal (L), acid-washed coal (LA), oxidized coal (LAOD), and reduced coal [LR2T-10], FTIR spectrum of LA (acid-washed coal) clearly shows the absence of inorganic impurities and unwanted minerals. The FTIR analysis of the synthesized coal oxide (CO) / graphene oxide (GO) exhibits the presence of absorption bands attributed to C-O stretching (vc-o= 1264 cm'1). The peaks near 1685 cm-1indicate the presence of carbonyl (C=O) contents. The main features of FTIR are the strong aliphatic absorptions at 2920-2850 cm-1. The intensity of peaks at 2920 cm-1is greater than the peaks at 2850 cm-1for all the coal samples and indicates the presence of long aliphatic chains in all the coals. The oxygen-containing functional groups like phenols, alcohols, ethers, carboxylic acid, and carbonyls are also predominantly present in northeastern coals. FTIR spectrum of LR2T-10ob served a weak C-H bending (2000-1650 cm'1) which also has the resemblance with the standard reduced graphene oxide (rGO) curve.

[0103] X-ray photoelectron spectroscopy (XPS) was employed to investigate the chemical states of various elements and the presence of functional groups LR2T-10 sample (Figure 4). The survey scan spectra (Figure 4(a)) show that carbon and oxygen are the most prominent elements along with 2 more peaks corresponding to N1 s and S2p. The high-resolution Cis region spectra (Figure 4(b)) of the LR2T-10 sample demonstrates 4 types of carbon bonds at binding energies of 284.8, 286.8, 287.8, and 288.9 eV, corresponding to C-C, C-O, C=O, and O-C=O respectively. Figure 4(c) shows the fitted Ols region of the LR2T-10 sample. There are three 01s peaks centering at 531.5 eV (C=0), 533.0 eV (C-0), and 534.9 eV (0-C=0) and the changes in the intensity of these peaks are per what has been observed in Cis spectra.

[0104] The XRD analysis of coal-derived reduced rGO (LR2T-10) has been carried out and the presence of (002) and (100) planes have been found and shown in Figure 5. Corresponding d- spacing values calculated by Bragg’s law with lattice planes and crystallite size are reported in Table 4. The crystallite size has been determined by using Scherrer’s equation.

[0105] Table 4: d-spacing value and crystallite size of 002 plane obtained from XRD analysis:

[0106] The Raman spectrum (Figure 6) of LR2T-10 exhibited a distinct D band at 1024 cm-1, which displayed comparable intensity to the G band at 1112 cm-1. The ID / IG ratio obtained is 0.92 revealing minimum defects in graphene formation. This observation suggests the presence of significant structural disorders attributed to the severe oxidation conditions. The intensity ratio of the D and G bands, ID / IG gives an insight into the reduction process by removing oxygen functional groups. The 2D peak (-2790 cm’1) consists of two components 2Di and 2D2, roughly at 2690 cm'1and 2920 cm'1respectively. As the number of graphene layers decreases to less than 5, there is a gradual increase in the relative intensity of the lower frequency peak of 2Di. In monolayer graphene, this peak generally becomes the predominant component. The shape of 2D band indicates that the graphene consists of multilayers.

[0107] The morphology of the coal-derived graphene (LR2T-10) was characterized by field emission scanning electron microscopy (FE-SEM) and energy-dispersive X-ray spectroscopy (EDS) (Figure 7). The LR2T-10 sheets messily stack on each other turbostratically. Along with it, an EDS technique was also employed to investigate the elemental composition of the material. It is observed that the graphitic carbon material possesses a high atomic carbon percentage (91.32%) with a slight amount of oxygen (7.70%), and a very negligible amount of sulfur (0.98%). Figure 7 represent the (a, b) FESEM image of the graphene derivative at a 1pm and 200 nm scale showing the presence of the graphene sheet-like structure and (c) EDS of the carbon material showing a high carbon percentage at the surface.

[0108] High-resolution transmission electron microscopy (HRTEM) analysis is essential in characterizing graphene and its derivative materials. TEM analyses have revealed the formation of coal-derived graphene (LR2T-10) and evaluated the crystallinity and the nature of the graphitic material. Figure 8 shows (a) HRTEM images of the coal-derived graphene (LR2T-10) at 100 nm and 5 nm magnifications showing the presence of graphene-like carbon sheets; (b) FFT pattern shows graphitic layers at a 5 nm scale (c); (d) SAED pattern of the graphene derivative showing its crystallinity with the presence of the graphitic planes (002) and (100) [doo2 = 0.329 nm].

[0109] The TEM image of the LR2T-10 (Figure 8) shows the presence of graphitic carbon sheet-like structures. At the 5 nm scale, the regularity of the carbon layers in the sample observed is a characteristic of the graphitic carbon materials. The nature of the graphitic material and its interplanar separation (d-spacing) can be determined from the selected area electron diffraction (SAED) technique. The presence of graphitic planes (002) and (100) in the SAED picture indicates the crystalline character of the carbon material. The interplanar spacing for the typical (002) plane is 0.329 nm, which is comparable to graphite (0.334 nm). The SAED pattern shows that the synthesized coal -derived graphene has a few-layered structure with some amorphous character.

[0110] Thermal stability of coal-derived graphene (LR2T-10) and standard reduced graphene oxide (rGO) were performed in a nitrogen environment at a heating rate of 10 °C per min from ambient temperature to 1000 °C. Figure 9 shows Thermal stability (TGA) of coal-derived graphene (LR2T- 10) and standard reduced graphene oxide (rGO) at a heating rate of 10°C min"1from ambient temperature to 1000 °C.Thermogravimetric curves of coal-derived graphene (LR4T-10) and standard rGO (Figure 9) show significant weight loss variation at the higher temperature (>631.18 °C). LR2T-10 initially shows a sharp decrease in the weight percentage from primary to 80.08% at 115.31 °C due to the trapping of water molecules between the layers and may be moisture present in the samples. Further, the graph also shows a decreasing weight percentage of LR2T-10 from 79.32% at 537.88°C to 76.23% at 631.18°C, and standard rGO simultaneously decreases their weight from 84.81% at 537.88°C to the same temperature and weight percentage as LR2T-10. After that, the LR2T-10 and standard rGO graph shows a 20.98% weight percentage variation at 954°C. Therefore, coal-derived graphene (LR2T-10) at higher temperatures has less weight loss (37.16%) as compared to standard rGO (58.14%).

[0111] The specific surface area of the coal-derived graphene (LR2T-10) was studied by using BET analysis. The coal-derived graphene exhibited a type-IV nitrogen adsorption isotherm as depicted in Figure 10(a). The maximum specific surface area (SSA) obtained from BET measurement was 642 m2g-1with a pore volume of 0.298 cm ’g'1. DFT pore size distribution (Figure 10 (b)) of the material indicates the presence of a large number of micropores along with mesopores. The BJH pore size distribution profile (Figure 10 (c)) of the LR2T-10 sample confirms the presence of mesopores in them.

[0112] MAIN ADVANTAGES OF THE PRESENT INVENTION ARE:

[0113] 1. The process employs low-grade Indian Tertiary subbituminous coal, a cost-effective and readily available raw material, showcasing an innovative approach to utilize abundant resources for graphene synthesis.

[0114] 2. The present invention represents a distinctive 4-step process for synthesizing reduced graphene oxide (rGO) from coal. Process includes acid washing, oxidation, dialysis / ultrafiltration, and reduction, providing an optimized and systematic approach to graphene production.

[0115] 3. The inclusion of ultrasonication-assisted activation and ultrafiltration in the synthesis process adds a unique dimension to the improved process for preparation of graphene. This demonstrates a commitment to employ advanced techniques for enhanced efficiency.

[0116] 4. The improved process for preparation of graphene is designed with its potential for large- scale graphene production. This scalability is crucial for meeting the demands of commercial applications and industrial use.

[0117] 5. Unlike many existing methods, improved process for preparation of graphene avoids the use of harsh acids in the reaction step. Hence, not only enhances safety during the synthesis process but also aligns with environmentally friendly practices, addressing concerns related to chemical use. 6. improved process for preparation of graphene has the capability to produce high-quality graphene from coal feedstock, a pivotal factor for ensuring optimal performance in diverse applications.

[0118] These combined features highlight the innovation and uniqueness of this invention, positioning it as a valuable contribution to the field of graphene synthesis with the potential to impact various industries and applications.

[0119] Finally, when used herein, the term "comprises” and its derivations (such as "comprising”, etc.) should not be understood in an excluding sense; that is, these terms should not be interpreted as excluding the possibility that what is described and defined may include further elements, steps, etc.

Claims

We Claim:

1. A process for preparation of high grade graphene from low-quality carbon source in 25-28 % yield having specific surface area (SSA) 642 m2g with a pore volume of 0.298 cm3g'atat low-temperature thermal reduction comprising the step of:- a) Pulverizing the carbon source having the particle size 53-70 micron. b) Acid leaching the pulverized subbituminous coals as obtained in step (a) with hydrofluoric acid (30%). c) Washing the acid leaching pulverized subbituminous coals with deionized water till neutralization. d) Drying the neutralized acid washed pulverized subbituminous coals as obtained in step (c) using hot air oven at the temperature in range of 600-800°c. e) Mixing the dry acid washed coal powder as obtained in step (d) with 50-60% formic acid (HCOOH) to form a slurry and 20-30% H2O2 in ice cold condition with continuously stirring the mixture for next 24-28 hours. f) reaction mixture as obtained in step (e) under ultrasonication at frequency in range of 20-40 KHz at temperature in range of 20-35°C for period in range of 18-24 hours. g) resulting dark brown solution as obtained in step (f) was transferred to stirred cell for dialysis / ultrafiltration process using membrane under N2 pressure. h) After completion of 1 cycle of dialysis / ultrafiltration, the reaction mixture was neutralized by adding ammonia solution dropwise followed by washing with distilled water 10-15 times until the neutralization point. The residue part was dried in an oven at the temperature in range of 110-140°C to get the oxidized product. i) dried oxidized coals as obtained in step (h) were thermally reduced at temperature in range of 1000-1200°C in the muffle furnace for period in range of 1-2 hour in the N2 atmosphere to obtained required reduced turbostratic graphene.

2. The process for preparation of turbostratic graphene as claimed in claims 1, wherein carbon sources selected from the NER locations containing low-grade sub-bituminous coals having different carbon percentages.

3. The process for preparation of turbostratic graphene as claimed in claims 1, wherein activating and exfoliating the coal structure into graphene-like structures by using a specific mixture [ 1 :5] of formic acid and hydrogen peroxide as an oxidizing agent and applying ultrasonic irradiation at frequency in range of 20-40 KHz.

4. The process for preparation of turbostratic graphene as claimed in claims 1, wherein dialysis / ultrafiltration process occurs by using a 100-120 kDa molecular cut-off membrane.

5. The process for preparation of turbostratic graphene as claimed in claims 1, wherein process restoring the sp2-hybridized carbon network and reducing the oxygen-containing groups of the oxidized coal by heating it at 1000-1200°C for 1-2 hour in a nitrogen atmosphere.

6. The process for preparation of graphene as claimed in claims 1, wherein turbostratic graphene obtained utilizing low-grade subbituminous coals.

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