DESC:TECHNICAL FIELD:

[001] The present disclosure generally relates to a method for synthesizing graphene, and more particularly relates to an inorganic template-based method for synthesizing graphene from biomass derivatives, and especially rom non-graphitic precursors.

BACKGROUND:
[002] Graphene is a one-atom-thick planar sheet of sp2-bonded carbon atoms, where the carbon atoms are densely packed in a honeycomb crystal lattice. The carbon-carbon bond length in graphene is about 0.142 nm. Graphene is the basic structural element of some carbon allotropes including graphite, carbon nanotubes and fullerenes. Due to graphene's exceptional properties, it is considered a “wonder-material” suitable for replacing many other materials in numerous devices and applications. Graphene, a type of carbon nanomaterial, possess extraordinary electrical, thermal and mechanical properties. It is one of the most versatile nanomaterial of the twenty-first century.

[003] Graphite is one of the three naturally occurring allotropes of carbon and occurs naturally in metamorphic rock in different parts of the globe. Synthesis of graphene by mechanical exfoliation of graphite is one effective way of creating a single layered and multi-layered graphene. Existing techniques for growing graphene are chemical vapor deposition, sonication, thermo-engineering, carbon dioxide reduction, cutting open carbon nanotubes, and graphite oxide reduction to name a few. However, the quality of graphene produced does not meet the theoretical potential of the material. Further, the methods are not cost effective on a large scale. Hence the mass adoption of graphene across various industries has not been possible.

[004] Hence there is a need for a method for synthesizing graphene economically from affordable, non-graphitic, and eco-friendly precursors.

SUMMARY:
This summary is provided to introduce a selection of concepts in a simple manner that are further described in the detailed description of the disclosure. This summary is not intended to identify key or essential inventive concepts of the subject matter nor is it intended to determine the scope of the disclosure

[005] In one embodiment, an inorganic template-based method for synthesizing graphene is disclosed. The method includes mixing sodium lignosulfonate and an ionic liquid to form a mixture. The mixture thus formed is subjected to an ionothermal carbonization to form a product. The product is then isolated and dried to form a dried product. A salt solution is added to the dried product to form a salt rich product. The salt rich product is subjected to graphitization and the graphitized product is washed with an acid and water and dried to produce graphene. The method is performed in absence of a catalyst and in presence of an inorganic template.
[006] Further advantages and other details of the present subject matter will be apparent from a reading of the following description and a review of the associated drawings. It is to be understood that the following description is explanatory only and is not restrictive of the present disclosure

BRIEF DESCRIPTION OF THE FIGURES:

[007] The disclosed system and method will be described and explained with additional specificity and detail with the accompanying figures in which:

[008] Figure 1 is a flow chart illustrating an inorganic template-based method for synthesizing graphene;
[009] Figures 2a and 2b are Scanning Electron Microscopy (SEM) photographs of graphene at different magnifications, in accordance with an embodiment of the present disclosure;
[0010] Figure 3 is an X-Ray Diffraction analysis of graphene, in accordance with an embodiment of the present disclosure;
[0011] Figure 4 is Raman Spectroscopy of graphene in accordance with an embodiment of the present disclosure;
[0012] Figure 5 is X-Ray Photoelectron spectroscopy of graphene in accordance with an embodiment of the present disclosure; and

[0013] Figure 6 illustrates graphene as a mechanical property-enhancing additive for polylactic acid (PLA) based sheets.

[0014] Further, persons skilled in the art to which this disclosure belongs will appreciate that elements in the figures are illustrated for simplicity and may not have necessarily been drawn to scale. Furthermore, in terms of the construction of the device, one or more components of the device may have been represented in the figures by conventional symbols, and the figures may show only those specific details that are pertinent to understanding the embodiments of the present disclosure so as not to obscure the figures with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS:

[0015] For the purpose of promoting an understanding of the principles of the disclosure, reference will now be made to the embodiment illustrated in the figures and specific language will be used to describe them. It will nevertheless be understood that no limitation of the scope of the disclosure is thereby intended. Such alterations and further modifications to the disclosure, and such further applications of the principles of the disclosure as described herein being contemplated as would normally occur to one skilled in the art to which the disclosure relates are deemed to be a part of this disclosure.

[0016] It will be understood by those skilled in the art that the foregoing general description and the following detailed description are exemplary and explanatory of the disclosure and are not intended to be restrictive thereof. Throughout the patent specification, a convention employed is that in the appended drawings, like numerals denote like components.

[0017] Reference throughout this specification to “an embodiment”, “another embodiment” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the disclosure. Thus, the appearances of the phrase “in an embodiment”, “in another embodiment” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.

[0018] The terms "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a process or method that comprises a list of steps does not include only those steps but may include other steps not expressly listed or inherent to such process or method. Similarly, one or more devices or sub-systems or elements or structures proceeded by "comprises... a" does not, without more constraints, preclude the existence of other devices or other sub-systems.

[0019] It may be noted that to the extent possible like reference numerals have been used to represent like elements in the drawings. Further, those of ordinary skilled in the art will appreciate that elements in the drawings are illustrated for simplicity and may not have been necessarily drawn to scale. For example, the dimensions of some of the elements in the drawings may be exaggerated relative to other elements to help to improve understanding of aspects of the disclosure. Furthermore, the one or more elements may have been represented in the drawings by conventional symbols, and the drawings may show only those specific details that are pertinent to understanding the embodiments of the disclosure so as not to obscure the drawings with details that will be readily apparent to those of ordinary skilled in the art having the benefits of the description herein.

[0020] Figure 1 is a flow chart of an inorganic template-based method 100 for synthesizing graphene. The method 100 includes mixing sodium lignosulfonate and an ionic liquid to form a mixture at 102. The mixture is subjected to an ionothermal carbonization to form a product at 104. Ionothermal carbonization is using ionic liquid as a solvent in hydrothermal process, instead of using water. The product is isolated at 106. The isolated product is dried to form a dried product at 108. A salt solution is added to the dried product to form a salt rich product at 110. The salt rich product is subjected to graphitization at 112. Graphitization is the process of converting amorphous carbon into crystalline or graphitic carbon. The graphitized product is washed with an acid and water at 114 and dried at 116 to produce graphene. The method is performed in the absence of a catalyst and in the presence of an inorganic template.
[0021] Lignin belongs to a class of complex organic polymers that form key structural materials in the support tissues of plants. Chemically, lignins are cross-linked phenolic polymers. Lignosulfonates, or sulfonated lignins are water-soluble anionic polyelectrolyte polymers. They are by-products from the production of wood pulp using sulphite pulping. Lignosulfonate is a waste by-product of the paper industry, where this is referred as “Black Liquor”. It has been used as a fuel for burners in industries, due to its high calorific value. Sodium lignosulfonate also acts as a chemical precursor for graphene production.

[0022] In some embodiments, the method includes adding a salt solution to the dried product to form a salt rich product. The salt rich product is subjected to graphitization. The graphitized product is washed with an acid and water. The washed product is dried to produce graphene. Graphitization is the process of converting amorphous carbon into crystalline or graphitic carbon. In one embodiment, alkali halides were used as catalyst at high temperature to convert amorphous carbon into two-dimensional graphene.

[0023] In some embodiments, the ionic liquid is 1-(4-sulfobutyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, butyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium chloride, or bis(trifluoromethylsulfonyl)imide. The ionothermal carbonization is carried out at a temperature in a range of 150-200oC for a period of 2-6 hours using a pressurized autoclave. The graphitization is carried out at a temperature in a range of 600-1000oC for a period of 2-8 hours in an inert atmosphere.

[0024] The salt solution comprises chloride salts, nitrate salts or sulfates of calcium, sodium, potassium or lithium. In some embodiments, salts of oxalic acid including sodium oxalate, potassium oxalate and calcium oxalate are used. In some embodiments, salts of citric acid such as zinc citrate, monosodium citrate, Trisodium citrate, ferric citrate, potassium citrate and magnesium citrate are used. In some embodiments, simple chemicals from the saccharide family including xylose, dextrose, starch and maltose are used. It can also include organic acids such as ascorbic acid, tannic acid, tartaric acid and succinic acid. It can also include sodium gluconate, ferrous gluconate, zinc gluconate, potassium gluconate, calcium gluconate or magnesium gluconate. In some embodiments, the salt solution comprises eutectic mixtures of chloride salts, nitrate salts and sulfates of calcium, sodium, potassium or lithium with varying proportions of 20:80 to 90:10. The inorganic template is sodium chloride.

[0025] The sodium lignosulfonate has a molecular weight in a range of 5000-20000. The isolation is carried out using a membrane filter with a pore size of 0.45 micron. Pre-requisites of sodium lignosulfonate precursor used for graphene production include an average molecular weight of 5000 to 20,000 and lignosulfonate content of 45-60%.

[0026] In some embodiments, other ionic liquids that may be used with this process include Butyl-3-methylimidazolium tetrafluoroborate ([Bmim] BF 4), 1-butyl-3-methylimidazolium chloride and bis(trifluoromethylsulfonyl)imide [OMIM][NTf2]). The molarity of the sodium chloride solution used in this process, ranges from 1 to 5M. In some embodiments, the molarity of sodium chloride is 1M. The temperature used for pyrolysis range from 700 to 1000oC. However, at temperatures exceeding 800oC, the sodium chloride template starts melting, as the melting point of NaCl is 800oC. The process of melting further leads to breaking down of NaCl template, which is responsible for the production of layered graphene. Hence, in some embodiments, the temperature for pyrolysis is around 800oC.

[0027] The graphene thus obtained was analysed with Scanning Electron Microscopy, X-Ray Diffraction, Raman Spectroscopy and X-Ray Photoelectron spectroscopy.

[0028] Figures 2a and 2b are Scanning Electron Microscopy photographs of graphene at different magnifications, in accordance with an embodiment of the present disclosure. The formation of layers of graphene with lateral size ranging from 5-7 microns is evident from Figure 2b. In addition to being a template for synthesizing graphene, the inorganic salt used in the current invention acts as a porogen, aiding in the development of a network of pores, as is evident from figure 2a.

[0029] Figure 3 is an X-Ray Diffraction analysis of graphene, in accordance with an embodiment of the present disclosure. The presence of a strong peak at 22.3 degrees, corresponding to (002) crystallographic plane, indicates a slight increase in the d-spacing, when compared to that of pristine graphite. (002) corresponds to the orientation of atomic planes in the crystal structure of graphene. This increase in d-spacing could be attributed due to the residual oxygen functional groups, present in the sample. The peak present at 42 degrees, corresponding to (100) indicate the lane reflections.

[0030] Figure 4 is Raman Spectroscopy of graphene in accordance with an embodiment of the present disclosure. The presence of a peak centred at 1335 cm-1 denotes the presence of defects in the sample. The other at 1590 cm-1 on the other hand denotes the presence of sp2 carbon in the sample. The ratio of intensity of defect peak with respect to graphitic peak remains high, indicating the presence of structural disorders in the form of sp3 carbon, in the as-prepared samples.

[0031] Figure 5 is X-Ray Photoelectron spectroscopy (XPS) of graphene in accordance with an embodiment of the present disclosure. To understand the chemical composition of samples in high resolution, XPS measurements were performed. The presence of strong peaks corresponding to oxygen and carbon at 531 eV and 284 eV, confirm the purity of graphene, produced by the process. The absence of XPS peaks relevant to Na 1s and Cl 2p confirms the complete removal of template.

[0032] Figure 6 illustrates graphene as a mechanical property-enhancing additive for polylactic acid (PLA) based sheets. Sample 1 indicates virgin PLA sheets and sample 2 and 3 indicates PLA sheets prepared with the addition of graphene in different dosages (0.5 grams per 10 kg and 1 gram per kg respectively). As shown in Figure 6, the sheets fabricated with 1 gram of graphene added to 10 kg of PLA, 22% improvement in flexibility was achieved. All these test results were performed at TU Delft, Netherlands as per ASTM standards.

Example 1

[0033] The following examples are intended as illustrative and non-limiting and represent specific embodiments of the present disclosure. The examples show a method to prepare graphene using non-graphitic precursors.

[0034] 5 grams of sodium lignosulfonate, having a molecular weight in a range of 5000 to 20000, was dispersed in 25 ml of 1-(4-sulfobutyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)imide. The solution was stirred for adequate mixing. The solution was subjected to ionothermal carbonization at 180oC for 4 hours using a pressurized autoclave. The product, a char like substance, was isolated and washed using double distilled water for about 5 times. The resultant product was dried at 100oC for 12 hours. The ionothermal treatment helps in hydrolyzing lignin, subsequently helping in the formation of porous graphene. The dried product was mixed with 25 ml of 5M NaCl, to substitute few of the aromatic groups of lignin with chlorine. This sample was dried to obtain a NaCl-rich sodium lignosulfonate. The above sample was subjected to graphitization. The graphitization process occurs at 800 oC for 5 hours under an argon gas atmosphere in an annealing furnace. Under heating, sodium lignosulfonate decomposes steadily, yielding a partially graphitized carbon, which transforms into multi-layered graphene, under the influence of NaCl as an inorganic template. The resultant product was washed with one molar solution of HCl, followed by water. Following the process of washing, the sample was dried to obtain pure graphene. The conversion efficiency of sodium lignosulfonate to graphene using this process is 25 to 35%, based on the temperature used for pyrolysis. The graphene produced consists of interconnected sheets of graphene that helps in preventing restacking of the individual graphene sheets. The properties of the graphene obtained are presented in Table 1.
Example 2
Graphene as a barrier additive for Polylactic acid (PLA) based food packaging

[0035] Polylactic acid (PLA) is a biopolymer, which is poised to replace petrochemical derived polymers due to its biodegradable characteristics. However, due to its poor barrier properties, the applications of PLA in food packaging have been very limited until today.

[0036] The use of barrier additives such as graphene with PLA substantially improved the barrier as well as the mechanical properties. Oxygen Transmittance Rate (OTR) tests of polylactic acid based food packaging, modified in-situ with graphene as prepared in Example 1 are presented in Table 2. The tests were prepared in accordance with ASTM F1307-14.

[0037] 1 gram of graphene as prepared in Example 1 was blended with 10 kg of polylactic acid using monolayer blown film extrusion process. The monolayer films were subjected to oxygen transmission rate (OTR) test with a setup having a coffee pod, which was used to evaluate the change in amount of oxygen passing through the polymer film, before and after the addition of graphene. The tests were performed using Mocon Oxtran 2/21 equipment with coulometric sensor. As-prepared membrane was mounted between a sample with oxygen on one side. The permeation of oxygen was noted over a 24-hour period in cc/m2/day. The sample used in this case was coffee pods. Four replicate samples, Rep 1, Rep 2, Rep 3, Rep 4 were prepared. The difference in oxygen permeation was calculated and tabulated in Table 2. It was found that there was a reduction of oxygen transmission rate by more than 30% after the addition of 1 gram of graphene of Example 1 into polylactic acid-based packaging films.
Example 3
Graphene as a mechanical property-enhancing additive for PLA based sheets
[0038] PLA based sheets were prepared by adding 1 gram of graphene to 10 kg of PLA using injection moulding. Three samples were prepared. Sample 1 was virgin PLA sheets. Sample 2 was prepared by adding 0.5 grams of graphene per 10 kg of PLA. Sample 3 was prepared by adding 1 gram of graphene of Example 1 per 10 kg of PLA. The flexibility of the sheet thus prepared increased by 22% when compared to the PLA sheet prepared without the addition of graphene of Example 1. All these test results were performed at TU Delft, Netherlands as per ASTM standards.
Table 1

Parameter Value/specifications
Appearance Greyish black powder
Bulk density 0.00108 g/cm3
Carbon content 89-92%
Oxygen content 8-11%
Dispersibility Soluble in polar aprotic solvents
Impurities 0.001%
Lateral size 5-20 microns

Table 2

[0039] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. The system, methods, and examples provided herein are illustrative only and not intended to be limiting.

,CLAIMS:We Claim

1. An inorganic template-based method (100) for synthesizing graphene, the method (100) comprising:
mixing (102) sodium lignosulfonate and an ionic liquid to form a mixture;
subjecting (104) the mixture to an ionothermal carbonization to form a product;
isolating (106) the product;
drying (108) the isolated product to form a dried product;
adding (110) a salt solution to the dried product to form a salt rich product;
subjecting (112) the salt rich product to graphitization;
washing (114) the graphitized product with an acid and water; and
drying (116) the washed product to produce graphene, wherein the method is performed in absence of a catalyst and in presence of an inorganic template.
2. The method (100) of claim 1, wherein the ionic liquid is 1-(4-sulfobutyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, butyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium chloride, or bis(trifluoromethylsulfonyl)imide.
3. The method (100) of claim 1, wherein ionothermal carbonization is carried out at a temperature in a range of 150-200oC for a period of 2-6 hours using a pressurized autoclave.
4. The method (100) of claim 1, wherein the graphitization is carried out at a temperature in a range of 600-1000 C for a period of 2-8 hours in an inert atmosphere.
5. The method (100) of claim 1, wherein the salt solution comprises chloride salts, nitrate salts or sulfates of calcium, sodium, potassium, or lithium.
6. The method (100) of claim 1, wherein the salt solution comprises eutectic mixtures of chloride salts, nitrate salts and sulfates of calcium, sodium, potassium, or lithium with varying proportions of 20:80 to 90:10.
7. The method (100) of claim 1, wherein the inorganic template is sodium chloride.
8. The method (100) of claim 1, wherein the sodium lignosulfonate has a molecular weight in a range of 5000-20000.
9. The method (100) of claim 1, wherein the isolation is carried out using a membrane filter with a pore size of 0.45 micron.
10. The method (100) of claim 1, wherein the graphene added to polylactic acid using monolayer blown film extrusion process reduces oxygen transmission rate of a monolayer film by about 30% and increases flexibility by about 22%.