Field of the invention
The present invention relates to a process for the direct production of monolayer graphene
nanosheets by the liquid phase exfoliation of graphite in biomass derived solvents such as
levulinic acid. More specifically, the present invention discloses the use of biomass derived
levulinic acid as sustainable and green solvent for the production of high quality graphene
directly from graphite. The solvent can be isolated from the red seaweed, Kappaphycus alvaretii
during its hydrolysis. The process is scalable and solvent is recyclable and reusable for
subsequent processes.
Background and prior art of the invention
Due to the unique properties of graphene useful for various high-end applications, it is always
desirable to produce graphene from inexpensive carbon sources such as graphite. Graphite is
widely used for the production of graphene sheets by liquid phase exfoliation. There are various
solvents such as A^-Methyl-2-pyrolidone (NMP), Af Af-dimethylformamide, benzyl benzoate as
well as in binary mixtures of various liquids are reported to exfoliate graphite to single or
multilayer graphene sheets.
Reference may be macje to the article entitled "High-yield production of graphene by liquidphase
exfoliation of graphite" [Nature Nanotechnology, 2008, 3, 563-568], wherein liquid phase
exfoliation of graphite was done in organic solvents such as AMVlethyl-2-pyrolidone (NMP),
A^A^-Dimethylacetamide (DMA), y-Butyrolactone (GBL), benzyl benzoate and l,3-dimethyl-2-
imidazolidinone (DMEU) etc. to produce defect free few layer graphene, which can be dispersed
in organic solvents (with -0.01 mg.mL-1).
Reference may be made to the article entitled "Cationic surfactant mediated exfoliation of;
graphite into graphene flakes"[Carbon, 2009, 47, 3288-3294], wherein a method for the
production of graphene nanoflakes containing cationic surfactant cetyltrimethylammonium
bromide and acetic acid forming a stable colloidal suspension in organic solvent such as N,Ndimethylformamide
(DMF) was developed.
Reference may be made to the article entitled "Gram-scale, production of graphene based on
solvothermal synthesis and sonication" [Nature Nanotechnology, 2009, 4, 30-33], wherein a
bottom-up approach for gram-scale production of carbon nanosheets based on common reagents
such as ethanol and sodium in equimolar ratio to yield graphene nanosheets was demonstrated.
Reference may be made to the article entitled "Graphene sheets from worm-like exfoliated
graphite" [Journal of Materials Chemistry, 2009, 19, 3367-3369], wherein the authors have first
synthesized the worm like exfoliated graphite (WEG) using sulfuric acid and hydrogen peroxide,
which was then dispersed in organic solvent such as'NMP and sonicate to produce high quality,
graphene.
Reference may be made to the article entitled "Liquid Phase Production of Graphene by
Exfoliation of Graphite in Surfactant/Water Solutions" [Journal of The American Chemical
Society, 2009, 131, 3611-3620], wherein graphene was produced through the exfoliation of
graphite using water with the assistance of surfactant such as sodium dodecylbenzene sulfonate
(SDBS) followed by ultra-sonication.
Reference may be made to the article entitled "Liquid-phase exfoliation of graphite towards
solubilized graphenes" [Small, 2009, 5, 1841-1845], wherein single layer solubilized graphene
was prepared by the. liquid phase exfoliation of graphite using solvents such as
hexafluorobenzene, octafluorotoluene, pentafluorobenzonitrile, pentafluoronitrobenzene,
pentafluoropyridine and pyridine etc. followed by sonication. The colloidal dispersion of
graphene (10 |ig.mL-l) in DMF was transparent and was stable up to 1 month.
Reference may be made to the article entitled "One-Pot Synthesis of Fluorescent Carbon
Nanoribbons, Nanoparticles, and Graphene by the Exfoliation of Graphite in Ionic Liquids"
[ACS Nano, 2009, 3, 2367-2375], wherein fluorescent carbon nanoribbons and graphene were
obtained by the exfoliation of graphite in ionic liquids such as l-methyl-3-butylimidazolium
tetrafluoroborate or l-methyl-3-butylimidazolium chloride and their binary mixture with water.
.Reference may be made to the article entitled "Preparation of graphene relying on porphyrin »
exfoliation of graphite" [Chemical Communication, 2010, 46, 5091-5093], wherein graphene
was produced by the exfoliation of graphite using porphyrin (5,10,15,20-tetraphenyl-21H,23Hporphine)
and their derivative in NMP.
V
Reference may be made to the article entitled "Highly concentrated graphene solutions via
polymer enhanced solvent exfoliation and iterative solvent exchange" [Journal of The American
Chemical Society, 2010, 132, 17661 -17663], wherein a rapid, room temperature and
ultracentrifugation free approach to exfoliate graphite using a nontraditional solvent ethanol was
achieved through the addition of stabilizing polymer ethyl cellulose and the graphene solution
was concentrated to a level exceeding 1 mg/mL.
Reference may be made to the article entitled "Direct exfoliation of graphene in organic solvents
with addition of NaOH" [Chemical Communication, 2011, 47, 6888-6890], wherein graphite
powder was directly exfoliated to produce graphene dispersion in organic solvents such as
benzyleamine (BA), NMP, DMA, cyclohexanone and benzyl benzoate with or without addition
of NaOH.
Reference may be made to the article entitled "Graphene Dispersion and Exfoliation in Low
Boiling Point Solvents" [The Journal of Physical Chemistry C, 2011, 115, 5422-5428], where
defect free graphene was produced by the exfoliation of graphite using low boiling point
solvents.
Reference may be made to the patent entitled "Supercritical fluid process for producing nano
graphene platelets" [US Patent No. 8,696,938 B2], where a process for producing highly
conductive pristine or non-oxidized nano graphene platelets (NGPs) was disclosed.
Reference may be made to the patent entitled "Exfoliation of graphite using ionic liquids" [US
2011/0319554 A1],.where a method to exfoliate graphite in ionic liquids was disclosed.
Reference may be made to the paper entitled "Production of partially reduced graphene oxide
nanosheets using a seaweed sap" [RSC Advances, 2014, 4, 64583-64588] where graphene oxide
was reduced using a seaweed juice obtained by mechanically crushing fresh Kappaphycus
alvarezii.
Reference may be made to the patent entitled "One kind of graphene and graphene preparation"
[CN104386684 A], where a method for the production of graphene using salt catalysts from
agricultural and forestry waste, tea, walnut shell and dry seaweed is disclosed. However there is
no mention for the application of benign deep eutectic solvents for the production.
4
ELHT 3D-12-2016 15 • 53
Reference may be made to the provisional patent application entitled " Process for the production
of graphene sheets with tunable functionalities from seaweed promoted by deep eutectic
solvents" [Green Chemistry, 2016, 18, 2819-2826], where an improve process for the facile
production of functionalized graphene nanosheets from fresh seaweed employing deep eutectic
solvents (DESs) is described. In this invention, direct production of Fe3CVFe graphene, ZnO/Zn
doped graphene and SnCVSn doped graphene nanosheets from granular seaweed biomass
obtained after expelling the liquid (useful as liquid fertilizer) from fresh seaweed, Sargassum
tenerrimum is described.
Table 1 summarizes the different solvent systems used so far for the exfoliation of
graphite towards production of graphene and comparison with the results obtained in the present
invention.
Tablel: List of reported solvents which used for the exfoliation of graphite to graphene using
ultrasonication technique and comparison with the present invention.
Entr
y
l
2
3
4
5
6
7
Solvent used
AA-methyl-2-
pyrrolidone (NMP)
Aqueous NaOH (pH
= 11.0)
Pentafl uorobenzonit
rile (C6F5CN)
Hexafl uorobenzene
(C6F6)
Octafluorotoluene
(C6F5CF3)
Pentafl uoropyridine
(C5F5N)
NMP
Method/
Time (h)
Sonicatio
n/0.5
Sonicatio
n/xx
Sonicatio
n/1.0
Sonicatio
n/ 1.0
Sonicatio
n/ 1.0
Sonicatio
n/1.0
Sonicatio
n/460
Concentrati
on of
exfoliated
graphene
(mg.mL"1)
0.01
0.002-0.02
0.1
0.07-0.08
0.05
0.05
1.2
No. of graphene
layers
Monolayer (—1
wt%)
Multilayer
Multilayer
Multilayer
Multilayer
Multilayer
Monolayer (-4
wt%)
ID/I
G
rati
0
_-
0.5
2
—
—
—
—
—
Reference
Nature
Nanotechnol
2008,3,563.
Chem.
Commun.,
2014,50,2751
Small, 2009, 5,
1841
Small, 2009, 5,
1841
Small, 2009, 5,
1841
Small, 2009, 5,
1841
Small, 2010, 6,
864
5
•1HI 3Q-I2.-2G16 1 5 : 53
8
9
;.10. .
11
12
13 ;
14
15
16
17
18
19
20
21
22
23
NaOH + NMP
NaOH + N,Ndimethylacetamide
(DMA)
NaOH +
Benzylamine (BA)
NaOH +
Cyclohexanone
(CYC)
Chloroform
Isopropanol (IPA)
Acetone
Chloroform
IPA
Acetone
CYC
NMP
DMF
1-Propanol
Water-ethanol
(40%)
Water-IPA (55%)
Sonicatio
n/xx
Sonicatio
n/xx
Sonicatio
n/xx
Sonicatio
n/xx
Sonicatio
n/ 0.5
Sonicatio
n/0.5
Sonicatio
n/0.5
Sonicatio
n/48
Sonicatio
n/48
Sonicatio
n/48
Sonicatio
n/48
Sonicatio
n/48
Sonicatio
n/48
Sonicatio
n/ 0.33
Sonicatio
n/ 1.0
Sonicatio
n/1.0
0.07
0.06
0.06
0.05
0.0034
0.0031
0.0012
0.07
0.07
0.01
0.2
0.2
0.2
0.025
0.01
0.02
Few-layer
Few-layer
Few-layer
Few-layer
< 5 Layers
< 5 Layers
< 5 Layers
-10 Layers
< 5 Layers
-10 Layers
-10 Layers
-10 Layers
-10 Layers
Monolayer
-10 Layers
-10 Layers
—
—
—
Chem.
Commitn.,
2011,47,6888
Chem.
Commun.,
2011,47,6888
Chem.
Commitn.,
2011,47,6888
Chem.
Commun.,
2011,47,6888
Langmuir,
2010,2/5,3208
Langmuir,
2010,2(5,3208
Langmuir,
2010,26,3208
J. Phys. Chem.
C, 2011, 115,
5422
J. Phys. Chem.
C, 2011, 7/5,
5422 •
J. Phys. Chem.
C, 2011, 115,
5422
J. Phys. Chem.
C, 2011, 115,
5422
J. Phys. Chem.
C, 2011, 115,
5422
J. Phys. Chem.
C,2011, 115,
5422
Nanotechnolo
gy, 2011,22,
365601
J. Nanopart.
to., 2012, 14,
.1003
J. Nanopart.
to., 2012, 14,
1003
6
:L.HX 3Q- 1 2 - 2 8 1 6 1 5 : 5' 3
24
25
26
Orthodichlorobenzene
(QDCB)
Levulinic acid
Levulinic acid
Sonicatio
n/0.5
Sonicatio
n/2.0
Sonicatio
n/1.5
0.03
0.065
0.049
Fewlayer/monola
yer
Monolayer
Few layer
0.5
.7
0.4
9
Nano Letter,
2009, 9, 3460-
3462
Present
study
Object of the invention
The main objective of the present invention is to prepare graphene nanosheets by the liquid
phase exfoliation of graphite powder in biomass derived solvents.
Another object is to use fresh Kappaphycus alvarezii, red seaweed as a biomass resource to
produce levulinic acid.
Another object is to use cultivated Kappaphycus alvarezii to produce levulinic acid.
Another object is to mechanically expel the juice present in freshly harvested Kappaphycus
alvarezii to yield K-carrageenan rich granules as taught in prior art.
Another object is to produce refined carrageenan from K-carrageenan rich granules through the
process explored in the prior art.
Another object is to use the obtained hydrolyzed mixture for the liquid phase exfoliation of
graphite powder.
Another object is to use the obtained levulinic acid rich mixture of bio-based solvent for the
liquid phase exfoliation of graphite powder.
Another object is to use commercially available graphite powder for the liquid phase exfoliation.
Another object is to exfoliate graphite powder in commercially available levulinic acid.
Another object is to exfoliate graphite powder in biomass derived solvents by ultra-sonication.
7
LHX 3 0 - 1 2 - 2 0 1 6 1
i
Another object is to optimize ultrasonication dose and time duration required for obtaining the
graphene nanosheets in the biomass-derived solvents.
Another object is to prepare graphene nanosheets by the exfoliation of graphite powders in
biomass derived solvents by ultra-sonication followed by centrifugation.
Another object is to prepare graphene nanosheets by the liquid phase exfoliation of graphite
i
powder in levulinic acid by ultra-sonication followed by keeping at room temperature for 24 h to
allow to formation of graphene doped crystal of levulinic acid.
Another object is to prepare graphene sheets by the melting of graphene doped crystal of
levulinic acid followed by centrifugation.
Another object is to characterize the liquid phase exfoliated graphite powder by modern
analytical tools to confirm formation of graphene sheets.
Another object is to characterize the graphene dispersions in the biomass derived solvents by
UV-Vis spectrophotometer.
i
Another object is to wash the graphene nanosheets isolated from the biomass derived solvents
with water several times followed by drying to obtain dry graphene powders.
Another object is to characterize the graphene powder employing powder XRD, Raman, XPS,
FT-1R, TEM and AFM to confirm formation of graphene nanosheets.
Another object is to scale up production of graphene nanosheets in biomass derived solvents.
Another object is to scale up production of graphene nanosheets in commercially available
levulinic acid.
Summary of the invention
The main embodiment of the present invention is the process for the facile production of single
layer graphene sheets by the liquid phase exfoliation of graphite in biomass-derived solvents
comprises the steps of:
8
(i) submerging 0.1 -0.15 g of graphite in 20-30 ml of biomass derived solvents selected from
levulinic acid;
(ii) ultra-sonication of the above dispersion of graphite in levulinic acid obtained in step(i) for a
time period ranging from 60-160 minutes;
(iii) keeping the above ultrasonicated dispersion obtained in step (ii) for 24 h at room
temperature;
(iv) melting the levulinic acid crystal thus formed in step (iii) followed by centrifugation at 2000-
6000 rpm for 20-120 minutes forms graphene sheets; and
(v) centrifiiging graphite dispersion as obtained in step (ii) at 5000 rpm for 1 h to get single
layer graphene sheets.
Jn another embodiment of the present invention, the obtained graphene entrap in the levulinic
acid crystal formed during exfoliation.
In another embodiment of the present invention, the ultrasonication time for exfoliation of single
layer graphene in levulinic acid is preferably 120 minutes and centrifugation speed is preferably
5000 rpm for a time period preferably ranging between 60-90 minutes.
In another embodiment of the present invention, the exfoliation of graphite in levulinic acid has
high selectivity towards single layer graphene sheets (84%) with dispersion of 0.049-0.65
mg/mL of graphene in levulinic acid.
In yet another embodiment of the present invention, the process is scalable and solvent is
recyclable and reusable for subsequent processes.
Brief description of drawing
Figure 1: Optimization of exfoliation parameters such as ultra-sonication time and centrifugation
speed for the exfoliation of graphite using levulinic acid done by ultra-sonication technique.
Figure 2: (A-B) TEM images of graphene (G-120) obtained after ultrasonication of graphite in
LA for 120 min (C) HR-TEM image of a section of a graphene monolayer (inset of this image
showed profile diagram) and (D) filtered image of part of the region in the yellow square.
9
ELH.I 3Q- 12- 2Q16 IS : 54
Figure 3: AFM images of graphene (G-120) obtained after sonication of graphite in LA for 12.0
min (top images) and their height profile diagram (bottom images).
Figure 4: (A) Large scale exfoliation of graphite to graphene in LA (photo taken after
ultrasonication for 2 h followed by standing at room temperature for 24 h) (B) Raman spectra
and (C) PXRD spectra of graphene obtained from large scale batch.
Detailed description of the invention
The present invention provides a scalable process for the synthesis of contamination free
few/single layered graphene nanosheets with high electrical conductivity directly from graphite
powder using biomass derived solvents through liquid-phase exfoliation of graphite.
In an embodiment of the present invention the seaweed as mentioned in step (i) is red seaweed
such as Kappaphycus alvarezii. Crushing and centrifugation of the red seaweed has been done to
separate the seaweed sap and K-carrageenan rich granular biomass.
In another embodiment of the present invention the yields of the seaweed sap and dry granular
biomass from freshly harvested seaweed were in the range of -70% (w/v) and ~4% (w/w)9
respectively.
In yet another embodiment of the present invention the refined carrageenan as mentioned in step
(ii) was produced from dry granular biomass through the process explored in the prior art.
In yet another embodiment of the present invention the hydrolyzed mixture as mentioned in step
(iii) was prepared by acid hydrolysis of refined carrageenan through the process explored in the
prior art.
In yet another embodiment of the present invention the levulinic acid rich mixture of solvent as
mentioned in step (iv) was extracted by ethyl acetate from the hydrolyzed mixture.
In yet another embodiment of the present invention the hydrolyzed mixture as mentioned in step
(iii), solvent extracted levulinic acid as mentioned in step (iv) and commercially available
10
EL.H.I 58 - 1 Z - 2 0 1 6 1 5 : 5'4
biomass derived solvent such as levulinic acidwas used for the liquid phase exfoliation of
graphite to obtain single or few-layered graphene nanosheets.
In yet another embodiment of the present invention the graphite powder was ultrasonicated
in commercially available levulinic acid as mentioned in step (vi) followed by
centrifugation to obtain few layered graphene nanosheets.
In yet another embodiment of the present invention the graphite powder was ultra-sonicated in
commercially available formic acid as mentioned in step (vi) followed by centrifugation. The
graphite did not exfoliate in formic acid.
In yet another embodiment of the present invention the graphite powder was ultra-sonicated in
commercially available ethyl lactate as mentioned in step (vi) followed by centrifugation. The
graphite did not exfoliate in ethyl lactate.
In yet another embodiment of the present invention the graphite powder was ultra-sonicated in
commercially available y-valerolactone as mentioned in step (vi) followed by centrifugation. The
graphite did not exfoliate in y-valerolactone.
In yet another embodiment of the present invention the graphite powder was ultra-sonicated in
hydroly^ed mixture as mentioned in step (v) followed by centrifugation. The graphite did not
exfoliate in hydrolyzed mixture.
In yet another embodiment of the present invention the graphite powder was ultra-sonicated in
ethyl acetate extracted levulinic acid rich solvent mixture as mentioned in step (vi) followed by
. centrifugation to obtain few layered graphene nanosheets. The graphite exfoliated in ethyl
acetate solvent extracted levulinic acid rich solvent mixture.
In yet another embodiment of the present invention the ultra-sonicated mixture as obtain in step
(vi) allowed to keep at room temperature for 24 hours to allow the formation of graphene doped
crystal of levulinic acid followed by melting and centrifuging to obtain single layered graphene
nanosheets.
In yet another embodiment of the present invention is to scale up production of single layered
graphene by exfoliation of graphite in pure levulinic acid.
11
DELHI 38-12-2016 IS:54
In yet another embodiment of the present invention is to scale up production of single layered
graphene by exfoliation of graphite in levulinic acid rich solvent mixture obtained by the
hydrolysis of K-carrageenan.
In yet another embodiment of the present invention the few/single layered graphene nanosheets
as obtained in step (vii) and (ix) allowed to repeated washings with milli-Q water.
In yet another embodiment of the present invention the optimized time for ultra-sonication is 120
minutes.
In yet another embodiment of the present invention the optimized centrifugation speed is 5000
rpm for 60 minutes to remove unexfoliated graphite powder.
In yet another embodiment of the present invention the optimized centrifugation speed is > 12000
rpm for 60 minutes to remove exfoliated few/single layered graphene nanosheets.
In yet another embodiment of the present invention the sheet like morphology was visible in the
microscopic images such as TEM and AF.M images.
In yet another embodiment of the present invention the formation of few layered graphene
nanosheets was confirmed by TEM.
In yet another embodiment of the present invention the formation of single layered graphene
nanosheets was confirmed by microscopic images (TEM and AFM) and Raman spectroscopy.
The Raman spectra of single layered graphene nanosheets showed sifting of the 2D band to a
lower frequency and this shifting in 2D band indicates the formation of single layered graphene
nanosheets.
In yet another embodiment of the present invention the preparation of contamination free
(levulinic acid free) graphene nanosheets was confirmed by FT-IR spectroscopy. The exfoliated
graphene nanosheets samples were washed several times with water and no trace of levulinic
acid was found in the graphene nanosheets as evident from the FT-IR spectra indicating the
formation of contamination free graphene nanosheets.
12
DELHI 5Q-1.2-2Q16 15 : 5 4
In yet another embodiment of the present invention the graphene paper was prepared by vacuum
filtration of a homogenous dispersion of graphene nanosheets in ethanol through a
polyvinylidene fluoride membrane.
In yet another embodiment of the present invention the single layered graphene nanosheets paper
showed electrical conductivity 23.36 S m"1.
>
In yet another embodiment of the present invention the single layered graphene nanosheets
showed dispersion stability in different organic solvents such as ethanol, isopropyl alcohol,
dimethyl sulphoxide, chloroform and N-methyl-2-pyrrolidinone etc.
In yet another embodiment of the present invention the single layered graphene nanosheets did
not show dispersion in different organic solvents such as water, acetone, dimethyl formamide,
toluene and dichloromethane etc.
The process comprises the following steps (Flowchart 1):
(i) Crushing and centrifugation of freshly harvested seaweed (Kappaphycus alvarezii) to
separate sap and solid residue (dried K-carrageenan rich granular biomass)
(ii) Produce refined carrageenan from K-carrageenan rich granular biomass
(iii) Preparing levulinic acid and formic acid mixture (Hydrolyzed mixture) by acid
hydrolysis of refined carrageenan through the process explored in the prior art
(iv) Extracting levulinic acid from the hydrolyzed mixture by ethyl acetate solvent
extraction
(v) Ultra-sonicating the mixture of graphite and hydrolyzed mixture as obtain in step (iii)
(vi) Ultra-sonicating the mixture of graphite and commercially available biomass
levulinic acid as obtained in step (iv)
(vii) Centrifugation of ultra-sonicated mixture as obtain in step (v) and step (vi) to
obtained few layered graphene nanosheets
(viii) The ultra-sonicated mixture as obtain in step (vi) staying at room temperature for 24
hours to allow to formation of graphene doped crystal of levulinic acid
(ix) Melting and centrifugation of graphene doped crystal of levulinic acid as obtain in
step (viii) to obtained single layered graphene nanosheets
13
DELHI 50 - 12 - 2.QI6 1 S : 54:
(x) Collecting and repeated washing the blackish product as obtained in step (vii) and
(ix) and ensuring the formation of single or few layered graphene nanosheets
To produce superior quality graphene by the exfoliation of graphite in the levulinic acid, various
parameters such as ultra-sonication duration of sonication and centrifugation speed were
optimized. To carry out the experiment, the graphite powder-levulinic acid dispersion was ultrasonicated
for 30-120 min to exfoliate graphite to graphene nanosheets followed by centrifugation
with varying rotating speed (2000 rpm to 5000 rpm) for 1 hour duration. After that, the
exfoliated graphite dispersion was allowed to stand at room temperature (25 °C) for up to 10
days. Successive measurement of UV-vis absorbance at 500 nm after every 48 hours interval was
recorded to optimize the stability of graphene dispersion in levulinic acid (Figure 1).
The following examples serve to provide the best mode of practice for the present invention, and
should not be constructed as limiting the scope of the invention.
Example-1
In a typical experimental procedure, 100 mg of graphite powder was added to a glass vial
containing 20 mL of levulinic acid followed by ultra-sonication for 30 min to exfoliate graphite
to graphene nanosheets. The supernatant obtained from the solution upon centrifugation at 2000-
5000 rpm did not show any dispersion of graphene.
This example teaches that the exfoliation of graphite is not observed in levulinic acid when
30 min ultra-sonication time is employed in the present invention.
>
Example-2
In a typical experimental procedure, 100 mg of graphite powder was added to a glass vial
containing 20 mL of levulinic acid followed by ultra-sonication for 60 min to exfoliate graphite
to graphene nanosheets. The supernatant obtained from the solution upon centrifugation at 2000-
5000 rpm gave a dispersion of graphene nanosheets.
This example teaches the exfoliation of graphite in levulinic acid when 60 min ultrasonication
time is employed in the present invention.
14
DELHI SQ- 12 - 2816 15.: 54:
ExampIe-3
In a typical experimental procedure, 100 mg of graphite powder was added to a glass vial
containing 20 m.L of levulinic acid followed by ultra-sonication for 90 min to exfoliate graphite
to graphene nanosheets. The supernatant obtained from the solution upon centrifugation at 2000-
5000 rpm gave a dispersion containing graphene nanosheets.
This example teaches the production of graphene nanosheets in levulinic acid upon 90 min
ultra-sonication time and exfoliated graphene showed better dispersion stability when compared
to the graphene dispersions obtained using 30 and 60 min of ultrasonication.
Example-4
In a typical experimental procedure, 100 mg of graphite powder was added to a glass vial
containing 20 m.L of levulinic acid followed by ultra-sonication for 120 min to exfoliate graphite
to graphene nanosheets. The supernatant obtained from the solution upon centrifugation at 2000-
5000 rpm gave a dispersion containing few-layered graphene nanosheets as observed under
TEM.
This example teaches the production of few-layered graphene nanosheets in levulinic acid
upon employment of 120 min of ultra-sonication time.
Example-5
In a typical experimental procedure, 100 mg of graphite powder was added to a glass vial
containing 20 mL of levulinic acid followed by ultra-sonication for optimized 120 min to
exfoliate graphite to graphene nanosheets. The supernatant obtained from the solution upon
centrifugation at 2000 rpm to give a dispersion containing graphene nanosheets with 6 h
stability.
This example teaches the exfoliation of graphite in levulinic acid upon optimized ultrasonication
time (120 min) and centrifugation at 2000 rpm gave a dispersion of graphene
nanosheets with less dispersion stability at room temperature (25 °C).
15
DELHI 3 O: - 1 2 - 2.016 1 5 : 5 4
Example-6
In a typical experimental procedure, 100 mg of graphite powder was added to a glass vial
containing 20 mL of levulinic acid followed by ultra-sonication for optimized 120 min to
exfoliate graphite to graphene nanosheets. The supernatant obtained from the solution upon
centrifugation at 2500 rpm gave a dispersion containing few-layered graphene nanosheets with
dispersion stability of about 8 h.
This example teaches the exfoliation of graphite in levulinic acid upon optimized ultrasonication
time (120 min) and centrifugation at 2500 rpm gave a dispersion of graphene
nanosheets with less dispersion stability at room temperature (25 °C) but slightly more in
comparison to the one discussed in example 5.
Example-7
In a typical experimental procedure, 100 mg of graphite powder was added to a glass vial
containing 20 mL of levulinic acid followed by ultra-sonication for optimized 120 min to
exfoliate graphite to graphene nanosheets. The supernatant obtained from the solution upon
centrifugation at 5000 rpm gave a dispersion containing graphene nanosheets with stability of 24
h.
This example teaches the exfoliation of graphite in levulinic acid upon optimized ultrasonication
time (120 min) and centrifugation at 5000 rpm gave a well dispers.ible few layered
graphene nanosheet at, room temperature (25 °C) and the sheet like morphology can be observed
in high resolution TEM images.
Example-8
In a typical experimental procedure, 100 mg of graphite powder was added to a glass vial
containing 20 mL of ethyl lactate followed by ultra-sonication for optimized 120 min to exfoliate
graphite to graphene nanosheets. The supernatant obtained from the solution upon optimized
centrifugation speed at 5000 rpm did not show any dispersion.
This example teaches that the exfoliation of graphite is not observed in ethyl lactate upon the
optimized ultra-sonication time and centrifugation speed.
16
Example-9
In a typical experimental procedure, 100 mg of graphite powder was added to a glass via)
containing 20 m.L of y-valeroiactone followed by ultra-sonication for optimized 120 min to
exfoliate graphite to graphene nanosheets. The supernatant obtained from the solution upon
optimized centrifugation speed at 5000 rpm did not show any dispersion.
This example teaches that the exfoliation of graphite is not observed in y-valero lactone upon
employment of optimized ultra-sonication time and centrifugation speed.
Example-10
In a typical experimental procedure, 100 mg of graphite powder was added to a glass vial
containing 20 mL of formic acid followed by ultra-sonication for optimized 120 min to exfoliate
graphite to graphene nanosheets. The supernatant obtained from the solution upon optimized
centrifugation speed at 5000 rpm did not show any dispersion.
This example teaches that the exfoliation of graphite is not observed in formic acid upon
employment of optimized ultra-sonication time and centrifugation speed.
Example-11
In a typical experimental procedure, 100 mg of graphite powder was added to a glass vial
containing 20 mL of levulinic acid followed by ultra-sonication for optimized 120 min to
exfoliate graphite powder to graphene nanosheets. Further upon standing at room temperature
(25 °C) for 24 hours, the levulinic acid present in the solution converted to crystal and upon
melting of the graphene doped levulinic acid crystal at 70 °C followed by centrifugation at 5000
rpm gave a dispersion containing single-layered graphene nanosheets.
This example teaches the production of single-layered graphene nanosheets in levulinic acid
under crystal induced liquid-phase exfoliation of graphite is employed in the present invention.
The production of single layered graphene nanosheets can be confirmed by raman, powder XRD
j
and XPS spectroscopic techniques and single layer sheet can be seen in TEM and AFM images
(Figures 2 & 3).
17
DELHI 3.e-i>-2Q1.6. 15 : 5 4:^
I
Example-12
In a typical experimental procedure, 100 mg of graphite powder was added to a glass vial
containing 20 mL of 20% v/v aqueous solution of levulinic acid (20 mL levulinic acid diluted up
to 100 mL with milli-Q water) followed by ultra-sonication for optimized 120 min to exfoliate
graphite powder to graphene nanosheets. The supernatant obtained from the solution upon
centrifiigation at 5000 rpm did not show any dispersion.
This example teaches that the exfolition of graphite is not observed in aqueous solution of
levulinic acid (20% v/v) upon upon employment of optimized ultra-sonication time and
centrifiigation speed.
Example-13
In a typical experimental procedure, 100 mg of graphite powder was added to a glass vial
containing 20 mL of 40% v/v aqueous solution of levulinic acid (40 mL levulinic acid diluted up
to 100 mL with milli-Q water) followed by ultra-sonication for optimized 120 min to exfoliate
graphite powder to graphene nanosheets. The supernatant obtained from the solution upon
centrifiigation at 5000 rpm did not show any dispersion.
This example teaches that the exfoliation of graphite is not observed in aqueous solution of
levulinic acid (40% v/v) upon employment of optimized ultra-sonication time and centrifugation
speed.
Example-14
In a typical experimental procedure, 100 mg of graphite powder was added to a glass vial
containing 20 mL of 60% v/v aqueous solution of levulinic acid (60 mL levulinic acid diluted up
to 100 mL with milli-Q water) followed by ultra-sonication for optimized 120 min to exfoliate
graphite powder to graphene nanosheets. The supernatant obtained from the solution upon
centrifugation at 5000 rpm did not show any dispersion.
This example teaches that the exfolition of graphite is not observed in aqueous solution of
levulinic acid (60% v/v) upon employment of optimized ultra-sonication time and centrifiigation
speed.
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D E L H I 3 Q - I Z - 2 . Q 1 6 15':"54
Example-15
In a typical experimental procedure, 100 mg of graphite powder was added to a glass vial
containing 20 mL of 80% v/v aqueous solution of levulinic acid (80 mL levulinic acid diluted up
to 100 mL with milli-Q water) followed by ultra-sonication for optimized 120 min to exfoliate
graphite powder to graphene nanosheets. The supernatant obtained from the solution upon
centrifugation at 5000 rpm did not show any dispersion.
This example teaches that the exfolition of graphite is not observed in aqueous solution of
levulinic acid (80% v/v) upon employment of optimized ultra-sonication time and centrifugation
speed.
Example-16
In a typical experimental procedure for scaling-up of exfoliation of graphite, 1.25 g of graphite
powder was added to a glass vial containing 250 mL of levulinic acid followed by placing the
mixture in ultra-sonicator water bath and ultra-sonication for optimized 120 min. Further upon
standing afcroom temperature (25 °C) for 24 hours, the unexfoliated graphite powder was found
to settle down and the upper layer containing graphene doped levulinic acid crystal was
separated out. Upon melting of the graphene doped levulinic acid crystal at 70 °C followed by
optimized centrifugation speed at 5000 rpm gave a dispersion containing single-layered graphene
nanosheets.
This example teaches the large-scale exfoliation of graphite in levulinic acid upon optimized
ultra-sonication time and centrifugation speed is employed in the present invention. The
production of graphene nanosheets can be confirmed by powder XRD and Raman spectroscopy
(Figure 4).
Example-17
In a typical experimental procedure, 100 g of Kappaphycus alvarezii granules (10-15 wt%
moisture content) obtained from the freshly collected seaweed after separating the seaweed
liquid by mechanical crushing was taken in 2 L tap water (Granule: water ratio = 1: 20 w/v) and
autoclaved at 107 °C for 90 min. After autoclaving, the content were centrifuged (5000 rpm for
10 min) in hot condition and the supernatent was separated from the residue. K-carrageenan (-50
19
DELHI- '30-12-20X6 15:54
wt%) was obtained from the hot supernatent through precipitation with isopropyl alcohol
(Supernatent: IP A ratio = 1 : 2 v/v). The moisture content of refined K-carrageenan were found
-14%.
This example teaches the production of refined K-carrageenan from the Kappaphycus
alvarezii seaweed granules.
Example-18
In a typical experimental procedure, 100 g of refined K-carrageenan was taken in 2 L of 2.5 M
HC1 aqueous solution and autoclaved at 105 °C for 60 min followed by extraction with ethyl
acetate. Ethyl acetate was removed under reduced pressure using a rotary evaporator to obtain
concentrated biomass derived solvent.
This example teaches the production of levulinic acid based solvent system directly from
biomass. The production of biomass derived solvent is confirmed by HPLC analysis and
presence of levulinic acid, formic acid, acetic acid with majority of levulinic acid was observed.
Example-19
In a typical experimental procedure, 100 mg of graphite powder was added to a glass vial
containing 20 mL of hydrolyzed mixture obtained by the acid hydrolysis of the seaweed granules
followed by ultra-sonication for optimized 120 min to exfoliate graphite to graphene nanosheets.
The supernatant obtained from the solution upon optimized centrifugation speed at 5000 rpm did
not show any dispersion.
This example teaches that the exfoliation of graphite is not observed in hydrolyzed mixture
upon employment of optimized ultra-sonication time and centrifugation speed.
Example-20
In a typical experimental procedure, 100 mg of graphite powder was added to a glass vial
containing 20 mL of solvent extracted biomass derived solvent from K-carrageenan (levulinic
acid present as major part) followed by ultra-sonication for optimized 120 min to exfoliate
20
LH.X SO- 12 - 2 8 1 6 15 : 5.4 V.
graphite to graphene nanosheets. The supernatant obtained from the solution upon optimized
centrifugation speed at 5000 rpm showed good dispersion.
This example teaches the exfoliation of graphite in biomass derived solvent upon optimized
ultra-sonication time (120 min) and centrifugation at 5000 rpm gave well dispersible graphene
nanosheets dispersion at room temperature (25 °C).
Thus from the above examples 1-8, 120 min sonication time and 5000 rpm centrifugation
speed was found to be optimized conditions for the exfoliation of graphite to few-layered
graphene nanosheets using levulinic acid as media. Further, mixture of solvents extracted from
K-carrageenan was also capable to exfoliate graphite to graphene sheets under these optimized
conditions. Flowchart for demonstration of of exfoliationo f graphite in levunilic acid done by
ultfasonication technique is given below.
Flowchart 1: Photographic demonstration of exfoliation of graphite in levulinic acid done by
ultra-spnjcation technique.
Advantage of the invention
The main advantage of the present invention is the use of inexpensive naturally abundant
biomass-derived solvents for the exfoliation of low value graphite to produce high value single
layer graphene sheets. The use of seaweeds such as Kappaphycus alvarezii to produce levulinic
21
acid by hydrolysis is another advantage of the present invention. The seaweed is cultivable
which does not need fresh water irrigation unlike terrestrial plants making them abundant for
large scale exploitation. Further, the seaweeds are bestowed with very good growth rate and can
be harvested multiple times unlike other terrestrial plants. Production of levulinic acid from Kcarrageenan
extracted from the seaweed is another advantage of the present invention. Among
the various bio-based solvents, levulinic acid was found to exfoliate graphite to single layer
graphene under optimized ultrasonication (120 minutes) and centrifugation (5000 rpm for 1.5 h).
The exfoliation was scalable and also can be achieved by using the mixture of solvents produced
by the hydrolysis of polysaccharides such as K-carrageenan.

We claim
1. A process for the facile production of single layer graphene sheets by the liquid phase
exfoliation of graphite in biomass-derived solvents comprises the steps of:
(i) submerging 0.1 -0.15 g of graphite in 20-30 ml of biomass derived solvents selected from
levulinic acid;
(ii) ultra-sonication of the above dispersion of graphite in levulinic acid obtained in step (i) for a
time period ranging from 60-160 minutes;
(iii) keeping the above ultrasonicated dispersion obtained in step (ii) for 24 h at room
temperature;
(iv) melting the levulinic acid crystal thus formed in step (iii) followed by centrifugation at 2000-
6000 rpm for 20-120 minutes forms graphene sheets; and
(v) centrifuging graphite dispersion as obtained in step (ii) at 5000 rpm for 1 h to get single
layer graphene sheets.
2. The process as claimed in claim 1, wherein obtained graphene entrap in the levulinic acid
crystal formed during exfoliation.
3. The process as claimed in claim 1, wherein the ultrasonication time for exfoliation of single
layer graphene in levulinic acid is preferably 120 minutes and centrifugation speed is preferably
5000 rpm for a time period preferably ranging between 60-90 minutes.
4. The process as claimed in claim 1, wherein the exfoliation of graphite in levulinic acid has
high selectivity towards single layer graphene sheets (84%) with dispersion of 0.049-0.65
mg/mL of graphene in levulinic acid.
5. The process as claimed in claim 1, wherein the process is scalable and solvent is recyclable
and reusable for subsequent processes.