[0001] The present disclosure relates to the field of nano-sized carbon materials. In particular, it pertains to the process of separation of carbon nanotubes obtained from catalytic decomposition of methane. BACKGROUND
[0002] Catalytic decomposition of methane is a well-known process to obtain hydrogen and carbon nanotubes (CNTs). The catalytic process is mostly carried out in the presence of a transition metal catalyst. The CNTs obtained by the above-mentioned process inevitably contain impurities and the process also results in a spent catalyst. Almost all the processes known till date generate CNTs with impurities and the spent catalyst is also wasted and cannot be reused. The impurities present in the CNTs obtained from the above-mentioned process may be carbonaceous impurities or metal catalyst particles.
[0003] The impurities present in unpurified CNTs severely reduce their mechanical strength or electrical properties and interfere with most of the desired properties of the CNTs. The removal of the above-mentioned impurities from CNTs is desirable but difficult because the CNTs are insoluble and thus inseparable through chromatography. Much effort has therefore been expended in the development of separation and purification techniques to obtain CNTs in pure form, such as, chemical and physical processes.
[0004] US20150225243Al discloses a chemical process for purifying carbon nanotubes by heating the carbon nanotube slurry at an elevated temperature not exceeding 110 °C and further treating the heated carbon nanotube slurry with an acid to dissolve transition metal particles. The addition of acid to separate and purify the CNTs results in removal of the catalytic metal residues and thus the spent catalyst is removed as an impurity.
[0005] US7488875 discloses a method for purifying carbon nanotube material comprising, contacting a carbon nanotube material comprising carbon nanotubes, a magnesia support and a catalyst transition metal with a liquid mixture comprising carbon dioxide and water. It further discloses that the liquid mixture when reacts with at least some of the magnesia support or with at least some of the catalyst transition metal forms water-soluble compounds. However, the carbon nanotube material is subjected to repeated chemical treatments to form water-soluble species, filtered, and rinsed multiple times, as desired, to obtain the desired carbon nanotube purity. This may lead to surface rupture of CNTs and modification in its physiochemical properties.

[0006] Briefly, the chemical process of separation and purification has several disadvantages,
such as, structure distortion of CNT due to oxidation, surface rupture of CNT, modification in
the inherent physical and chemical properties of CNTs, introduction of oxygenated functional
groups (-OH, -C=0 and -COOH) in CNT structure. The disadvantages lead to low yields of
CNTs.
[0007] The physical process of separating CNTs is based on the differences in the physical
size, aspect ratio, gravity, and magnetic properties, etc. of the CNT. In principle, this method
does not require chemical oxidation, and therefore prevents CNTs from severe damage.
However, the physical method is complicated, time-consuming and less effective. Main
disadvantage of the physical processes is the low yields of product CNTs.
[0008] In view of the above, the separation and purification methods of CNTs, i.e. single
walled CNT (SWCNT) and multi walled CNT (MWCNT) is accompanied by disadvantages.
Thus, there is a need for a process that separates the CNTs effectively without compromising
the yield.
SUMMARY OF THE INVNETION
[0009] In an aspect of the present disclosure, there is provided a process for separation and
purification of carbon nanotubes comprising: (a) obtaining a spent catalyst; (b) heating the
spent catalyst under oxidizing conditions to obtain a first mixture; (c) dispersing the first
mixture in at least one solvent followed by sonication to obtain a solution; and (d) processing
the solution to obtain purified carbon nanotubes.
[0010] These and other features, aspects, and advantages of the present subject matter will be
better understood concerning the following description and appended claims. This summary is
provided to introduce a selection of concepts in a simplified form. This summary is not
intended to be used to limit the scope of the claimed subject matter.
BRIEF DESCRIPTION OF DRAWINGS
[0011] The detailed description is described concerning the accompanying figure. In the figure,
the left-most digit(s) of a reference number identifies the figure in which the reference number
first appears. The same numbers are used throughout the drawings to reference like features
and components.
[0012] Figure 1 compares the active Ni content. Figure 2(a) illustrates active Ni content of the
fresh metal catalyst and Figure 2(b) illustrates active Ni content of the recovered metal catalyst,
in accordance with an implementation of the present disclosure.
[0013] Figure 2 illustrates morphology of the metal catalyst obtained after the separation of
CNT, in accordance with an implementation of the present disclosure.

[0014] Figure 3 compares the field emission scattering electron microscopy (FESEM) images
of CNTs with and without carrying out the centrifugation step, in accordance with an
implementation of the present disclosure.
[0015] Figure 4 illustrates the evaluation of the magnetically separated MWCNTs obtained
after the magnetic separation, in accordance with an implementation of the present disclosure.
Figure 4(a)
[0016] Figure 5 displays the comparison results through TEM images; Figure 5(a) and (b)
display the TEM images of the spent catalyst obtained from the decomposition of methane;
Figure 5(d) displays the TEM image of the first mixture obtained after oxidation; Figure 5(c)
represents the TEM image after the purification of carbon nanotubes; Figure 5(e) and (f)
represents the TEM image of the purified carbon nanotubes obtained by a process without
carrying the oxidation step, in accordance with an implementation of the present disclosure.
DETAILED DESCRIPTION
[0017] Those skilled in the art will be aware that the present disclosure is subject to variations
and modifications other than those specifically described. It is to be understood that the present
disclosure includes all such variations and modifications. The disclosure also includes all such
steps, features, compositions, and compounds referred to or indicated in this specification,
individually or collectively and any and all combinations of any or more of such steps or
features.
Definitions
[0018] For convenience, before further description of the present disclosure, certain terms
employed in the specification, and examples are collected here. These definitions should be
read in the light of the remainder of the disclosure and understood as by a person of skill in the
art. The terms used herein have the meanings recognized and known to those of skill in the art,
however, for convenience and completeness, particular terms and their meanings are set forth
below.
[0019] The articles "a," "an" and "the" are used to refer to one or more than one (i.e., to at least
one) of the grammatical object of the article.
[0020] The terms "comprise" and "comprising" are used in the inclusive, open sense, meaning
that additional elements may be included. Throughout this specification, unless the context
requires otherwise the word "comprise", and variations, such as "comprises" and "comprising",
will be understood to imply the inclusion of a stated element or step or group of elements or
steps but not the exclusion of any other element or step or group of elements or steps.

[0021] The term "including" is used to mean "including but not limited to". "Including" and "including but not limited to" are used interchangeably.
[0022] The term "oxidizing condition" refers to conditions in general, where gas phase oxidation (in the presence of air) could be carried out. In the oxidation under such condition, the impure CNTs are heated at a controlled rate in air at a temperature of 250 °C for an extended time.
[0023] The term "magnetic separation" refers to a process wherein the as-received CNTs were refined by applying a permanent magnet to a suspension of CNTs to separate the high-magnetic fraction from CNTs; the purified CNTs are collected as dispersion in the solvent which are taken for further processing..
[0024] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the disclosure, the preferred methods, and materials are now described. All publications mentioned herein are incorporated herein by reference. [0025] Several parameters may be presented herein in a range format. It is to be understood that such range format is used merely for convenience and brevity and should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. For example, a temperature range of about 30°C to about 50°C should be interpreted to include not only the explicitly recited limits of about 30°C to about 50°C, but also to include sub-ranges, such as 45°C to 48°C, and so forth, as well as individual amounts, including fractional amounts, within the specified ranges, such as 32.2°C, 40.6°C, and 49.3°C, for example.
[0026] The present disclosure is not to be limited in scope by the specific embodiments described herein, which are intended for the purposes of exemplification only. Functionally equivalent products, compositions, and methods are clearly within the scope of the disclosure, as described herein.
[0027] As stated earlier in the background section, the separation and purification of CNT obtained from catalytic decomposition of methane, along with the recovery of spent catalyst, is a problem due to several reasons. Hence, an efficient process which can separate and purify said CNTs along with recovering the spent catalyst, is required. The present disclosure provides a process for effective separation and purification of carbon nanotubes without using chemicals. The purity of the product CNTs obtained from the process of the present disclosure

is in the range of 80% - 90%. The process of the present disclosure also recovers the spent catalyst which can be reused for the catalytic decomposition of methane. [0028] In an embodiment of the present disclosure, there is provided a process for separation and purification of carbon nanotubes comprising: (a) obtaining a spent catalyst; (b) heating the spent catalyst under oxidizing conditions to obtain a first mixture; (c) dispersing the first mixture in at least one solvent followed by sonication to obtain a solution; and (d) processing the solution to obtain purified carbon nanotubes.
[0029] In an embodiment of the present disclosure, there is provided a process for separation and purification of carbon nanotubes as described herein, wherein the spent catalyst has a metal content in a range of 65-70%) and carbon nanotube content is in the range of 30 % - 35 %. In another embodiment of the present disclosure, the spent catalyst has a metal content in a range of 68-70%) and carbon nanotube content is in the range of 31 - 34%. In yet another embodiment of the present disclosure, the spent catalyst has a metal content in a range of 69-70%> the spent catalyst has a carbon nanotube content of 31 - 32 %>.
[0030] In an embodiment of the present disclosure, there is provided a process for separation and purification of carbon nanotubes as described herein, wherein the process recovers the spent catalyst.
[0031] In an embodiment of the present disclosure, there is provided a process for separation and purification of carbon nanotubes as described herein, wherein the purified carbon nanotubes have purity percentage in a range of 80- 95 % and the purified carbon nanotubes have a CNT weight percentage in the range of 80 - 95 %>. In another embodiment of the present disclosure, the purified carbon nanotubes have purity percentage in a range of 80 - 90 % and the purified carbon nanotubes have a CNT weight percentage in the range of 80-90 %>. [0032] In an embodiment of the present disclosure, there is provided a process for separation and purification of carbon nanotubes as described herein, wherein heating the spent catalyst under oxidizing conditions is carried out at a temperature in the range of 250 °C - 350 °C for a period in the range of 30 minutes - 60 minutes to obtain a first mixture. In another embodiment of the present disclosure, heating the spent catalyst under oxidizing conditions is carried out at a temperature in the range of 250 °C - 300 °C for a period in the range of 30 minutes - 60 minutes to obtain a first mixture. In yet another embodiment of the present disclosure, heating the spent catalyst under oxidizing conditions is carried out at a temperature of 250 °C for a period in the range of 30 minutes - 60 minutes to obtain a first mixture. [0033] In an embodiment of the present disclosure, there is provided a process for separation and purification of carbon nanotubes, as described herein, wherein dispersing the first mixture

in at least one solvent is carried out at a temperature in a range of 30 °C - 50 °C, followed by sonication at a frequency in the range of 40 - 50 Hz to obtain a solution. In another embodiment of the present disclosure, dispersing the first mixture in at least one solvent is carried out at a temperature in a range of 30 °C - 50 °C, followed by sonication at a frequency of 45 Hz to obtain a solution.
[0034] In an embodiment of the present disclosure, there is provided a process for separation and purification of carbon nanotubes as described herein, wherein the at least one solvent is selected from the group consisting of water, ethanol, iso-propanol, methanol, and combinations thereof. In another embodiment of the present disclosure, the at least one solvent is water. [0035] In an embodiment of the present disclosure, there is provided a process for separation and purification of carbon nanotubes as described herein, wherein processing the solution to obtain purified carbon nanotubes comprises step selected from centrifugation, magnetic separation, drying, or combinations thereof.
[0036] In an embodiment of the present disclosure, there is provided a process for separation and purification of carbon nanotubes as described herein, wherein processing the solution to obtain purified carbon nanotubes comprises step selected from magnetic separation, drying, or combinations thereof.
[0037] In an embodiment of the present disclosure, there is provided a process for separation and purification of carbon nanotubes as described herein, wherein processing the solution to obtain purified carbon nanotubes comprises step selected from centrifugation, drying, or combinations thereof.
[0038] In an embodiment of the present disclosure, there is provided a process for separation and purification of carbon nanotubes, as described herein, wherein the centrifugation is carried out at a rotation speed in a range of 500-1000 rpm for a period in the range of 5-10 minutes. In another embodiment of the present disclosure, the centrifugation is carried out at a rotation speed in a range of 600 - 800 rpm for a period in the range of 5-10 minutes.
EXAMPLES
[0039] The following examples are given by way of illustration of the present invention and should not be construed to limit the scope of the present disclosure. It is to be understood that both the preceding general description and the following detailed description are exemplary and explanatory only and are intended to provide further explanation of the claimed subject matter.

Example 1: Process of separation of carbon nanotubes (CNTs) from spent catalyst obtained from the decomposition of methane:
[0040] When the process of methane decomposition is carried out using a catalyst, such as, nickel-alumina (M-AI2O3) based catalyst, the formation of CNTs were observed on/in the catalyst. The spent catalyst shows a ID/IG ratio of 1.12 measured through Raman spectroscopy. This suggests that the spent catalyst obtained after the decomposition of methane contained CNTs, as ID/IG ratio reflects both the purity and the defect density of CNTs. The ratio between the intensities of the disorder-induced D band and the first-order graphite G band (ID/IG) provides a parameter that can be used for quantifying disorder. Moreover, the presence of the CNTs was also confirmed by the Field Emission Scanning Electron Microscopy (FESEM) of the spent catalyst, which showed that the spent catalyst contained 30-35 % CNTs by weight. It was further observed through High Resolution Transmission Electron Microscopy (HRTEM) analysis that the CNTs growth on the spent metal catalyst followed both tip and base growth mechanism.
[0041] The spent catalyst (5-10 gm) obtained from the process of methane decomposition was subjected to oxidation in the presence of air at a temperature of 250 °C for a period of 30 minutes to 1 hour. The air oxidation can be carried out in any furnace, like muffle furnace, tube furnace, etc. The oxidized product (first mixture) obtained was then subjected to sonication. The oxidized product (first mixture) obtained from the oxidation step was dispersed in water (at least one solvent) at a temperature of 30 °C and sonicated. Normally, the chemical inertness of CNTs hamper their processability, i.e., the solubility of CNTs in water is also limited and thus the CNTs cannot be separated easily using solvents such as water. Due to less solubility of CNTs in any of the solvents, it is also very difficult to separate one CNT with another as they tend to aggregate together.
[0042] However, with the sonication step of the process of the present disclosure, the problem of insolubility of CNTs was eventually solved, as sonication of CNTs resulted in formation of dangling bonds. In order to separate CNTs efficiently from the spent catalyst according to the present disclosure, both probe sonication and sonication bath were applied. Moreover, the amplitude of probe sonication was varied by keeping in mind the destruction of CNTs as the cavitation phenomenon may break the graphitic layers of MWCNTs. Thus, the sonication time was varied from 0.5 hrs to 5 hrs by maintaining both high and low amplitude for probe sonicator and bath sonicator, and in each 30 min gap, samples were taken out for metal analysis. The sonication was performed at a temperature of 30 °C and resulted in a solution.

[0043] The solution obtained from the sonication step was further processed, i.e., the solution
was centrifuged using a centrifuge machine at a rotation speed of 500 rpm for 5-10 mins. The
centrifugation step resulted in settling (sinking) of the denser metal catalyst as sediment, while
the CNTs remained dissolved in the solvent. Further to this, a step of magnetic separation was
carried out in a magnetic arrangement The CNTs obtained after the magnetic separation were
dried to obtain the purified carbon nanotubes. To evaluate the morphology of the purified
carbon nanotubes Transmission electron microscopy (TEM) analysis was done and the results
have been displayed in Figure 4.
[0044] The separation and purification process of the present disclosure resulted in the
separation of 70% CNTs having a purity of 80-90%.
Example 2: Process of recovery of spent catalyst (recovered catalyst)
[0045] As discussed in the Example 1 after the centrifugation step metal catalyst was obtained
as a sediment. The catalyst recovered after centrifugation was found to contain 57.61% active
Ni content (Figure lb), as compared to the active Ni content of fresh catalyst being 59.7 (Figure
la).
[0046] The morphology of the metal catalyst obtained after the separation of CNT was also
studied (Figure 2). It was observed that the spent catalyst exhibited a wide size variation with
irregular shapes, while the separated or recovered catalyst showed uniform spherical shapes
with a narrower particle size distribution. Thus, the recovered catalyst was as good as the fresh
catalyst and can be further reused for carrying out the decomposition of methane.
Example 3:
[0047] To establish the criticality of the process of the present disclosure a process similar to
the process described in Example 1 was repeated, however the step of centrifugation was not
performed. The CNTs obtained from this process were analyzed through field emission
scanning electron microscopy (FESEM) technique. The FESEM images of CNTs with and
without carrying out the centrifugation step is displayed in Figure 3a (without centrifugation)
and Figure 3b (with centrifugation). It can be clearly seen from Figure 3 that the CNT obtained
after centrifugation has a better purity and uniform morphology than the one obtained without
carrying out the centrifugation step.
Example 4:
[0048] A similar process, as provided above in Example 1, was carried out, wherein the spent
catalyst was not heated under the oxidizing conditions, i.e., without the oxidation step. Further,
the characterization of the spent catalyst under different conditions was done through URTEM.

(a) Spent catalyst as such: Figure 5(a) and 5(b) display the HRTEM images of the spent catalyst obtained from the decomposition of methane. It is clear from the Figure 5(a) and Figure 5(b) that the metal particles were clearly capped within the carbon layers.
(b) Spent catalyst after the oxidation step: Figure 5(c) displays the TEM image of the first mixture obtained after oxidation.
(c) Spent catalyst without the oxidation step: Figure 5(d)- 5(f) represents the TEM image of the carbon nanotubes obtained by a process in which the oxidation step was not carried out. It is clear from the Figure 5(e) and 5(f) that without oxidation step the metal particles were covered within the carbon layers.
[0049] Moreover, the morphology of the carbon nanotubes was studied with the TEM images under two circumstances: (i) when the process of purification and separation of carbon nanotubes was carried out without the step of oxidation; and (ii) when the process of purification and separation of carbon nanotubes was carried out with the oxidation step. The TEM images for the same are captured in Figure 5. From the Figure 5(a), it is clear that the metal catalyst is still present within the CNT structure, wherein oxidation step was not performed. In contrast, Figure 5(c) represents the purified CNT (without the presence of any metal catalyst) obtained from the process of the present disclosure. The TEM image displayed in Figure 5(c) resembles the TEM image of any other purified CNT. Thus, it is inferred that the process of the present disclosure provides a purified CNT having a purity in the range of 80-90%. Example 5
[0050] Table 1 provided below evaluates the essential parameters of the purified CNT obtained from the process carried out with and without the oxidation step.

[0051] It is clear from the table 1 above that the purity of the CNTs was enhanced by incorporating the oxidation step. The oxidation step mainly involves removal of amorphous carbon which leads to the easier removal of metal nanoparticles and micro particles entrapped inside the amorphous layers. Additionally, removal of amorphous layer also reduce the phonon vibrations associated with the defects and enhance the lattice sp2 vibration. [0052] Thus, the process of the present disclosure establishes that the process not only yields a purified CNT but also recovers the metal catalyst which can be reused, thereby making said process an energy efficient one.
[0053] Although the subject matter has been described in considerable details with reference to certain examples and embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternate embodiments of the subject matter, will become apparent to persons skilled in the art upon reference to the description of the subject matter. It is therefore contemplated that such modifications can be made without departing from the present subject matter as defined.
Advantages of the process of the present disclosure.
[0054] The present disclosure provides a process for separation and purification of CNTs
obtained in the decomposition of methane.
[0055] The process of the present disclosure is simple, robust, highly effective with respect to
the large-scale separation, environmental benign, which provides high yield and purity of the
product CNTs. The time taken for separation and purification of CNTs obtained in the
decomposition of methane is in the range of 2-5 hrs. The process of the present disclosure also
recovers the spent metal catalyst (present as impurity in the CNTs) obtained along with CNTs
in the decomposition reaction of methane.
[0056] The recovered metal catalyst obtained from the process of the present disclosure is
renewable and can further be reused in the decomposition reaction again which reduces the
cost of fresh metal catalyst being employed.

A process for separation and purification of carbon nanotubes comprising:
(a) obtaining a spent catalyst;
(b) heating the spent catalyst under oxidizing conditions to obtain a first mixture;
(c) dispersing the first mixture in at least one solvent followed by sonication to obtain a solution; and
(d) processing the solution to obtain purified carbon nanotubes.
The process as claimed in claim 1, wherein the spent catalyst has a metal content in a
range of 65-70% and carbon nanotube content is in the range of 30 % - 35 %.
The process as claimed in claim 1, wherein the process recovers the spent catalyst.
The process as claimed in claim 1, wherein the purified carbon nanotubes have purity
percentage in a range of 80- 90 % and the purified carbon nanotubes have a CNT weight
percentage in the range of 80 - 90 %.
The process as claimed in claim 1, wherein heating the spent catalyst under oxidizing
conditions is carried out at a temperature in the range of 250 °C - 350 °C for a period in
the range of 30 minutes - 60 minutes to obtain a first mixture.
The process as claimed in claim 1, wherein dispersing the first mixture in at least one
solventis carried out at a temperature in arange of 30 °C - 50 °C, followed by sonication
at a frequency in the range of 40 - 50 Hz at a temperature in the range of 30-40 °C to
obtain a solution.
The process as claimed in claim 1, wherein the at least one solvent is selected from the
group consisting of water, ethanol, iso-propanol, methanol, and combinations thereof.
The process as claimed in claim 1, wherein processing the solution to obtain purified
carbon nanotubes comprises step selected from centrifugation, magnetic separation,
drying, or combinations thereof.
The process as claimed in claim 8, wherein the centrifugation is carried out at a rotation
speed in a range of 500-1000 rpm for a period in the range of 5-10 minutes.