WO2014203280A2

WO2014203280A2 - Process of preparation of carbon nanotubes

Info

Publication number: WO2014203280A2
Application number: PCT/IN2014/000410
Authority: WO
Inventors: Rupesh Arun KHARE; Uday Shankar Agarwal; Veedu Sreekumar Thaliyil
Original assignee: Reliance Industries Limited
Priority date: 2013-06-20
Filing date: 2014-06-20
Publication date: 2014-12-24
Also published as: WO2014203280A3; IN2013MU02089A

Abstract

The present disclosure relates to a process for the preparation of carbon nanotubes (CNTs) that consists of introducing a precursor for metal catalyst in an isothermal zone (102) of a heated furnace (104) of an apparatus (100) for the preparation of carbon nanotubes to generate nano-sized catalyst particles; feeding, at least once, a hydrocarbon through at least one inlet port (106) into the isothermal zone of the furnace to yield CNTs. The present disclosure also provides an apparatus (100) for the preparation of the CNTs.

Description

PROCESS OF PREPARATION OF CARBON

NANOTUBES

FIELD

The present disclosure relates to a process for the preparation of carbon nanotubes. BACKGROUND

Carbon nanotubes (CNTs) are cylindrical carbon nanostructures having the length-to- diameter ratio of up to 132,000,000: 1. Such characteristic dimensions facilitate these entities to have unusual properties that enable their application in various fields such as electronic, optics, materials science and pharmaceuticals.

Chemical vapor deposition (CVD) is one of the methods utilized for the production of CNTs. The floating catalyst method and the supported catalyst method are the two variations of _sthe CVD. In the floating catalyst method, a metal precursor and a hydrocarbon are vaporized continuously and passed together with carrier gases into a pre-heated tubular furnace. In the proximal part of the tube, the vapor feed undergoes heating (non-isothermal) to the desired temperature due to heat from the tube walls and the metal precursor is converted into a metallic nanoparticle catalyst, on which the hydrocarbon vapor undergoes pyrolysis to grow CNTs. However, in this process, both compounds are made to pass through a temperature gradient that generates certain undesirable side products of both these chemicals. The yield is also reduced. In the supported catalyst method, the supported catalyst is placed inside the hot zone of the furnace and hydrocarbon vapors are passed over, typically, for about 1 hour to obtain CNTs. Alternatively, the supported catalyst is maintained in a fluidized state in a reactor to which the supported catalyst and hydrocarbon are continuously fed, while withdrawing the grown CNTs along with unconverted and other hydrocarbons. This requires an advance step for catalyst preparation on the support, and the grown CNTs generally have an agglomerated morphology due to CNT growth restricted to the space available in/on/around the support. Also the support material is retained in the product as an additional impurity.

Although information pertaining to the production of carbon nanotubes through the CVD technique is available, a need is still felt to develop a process and/ or an apparatus for the production of carbon nanotubes and other carbonized products which along with being high yielding, obviates the formation of undesirable side products.

DEFINITIONS:

As used in the present disclosure, the following words and phrases are generally intended to have the meanings as set forth below, except to the extent that the context in which they are used indicates otherwise.

The term "Carbon Nano Tubes" (CNTs), in the context of the present disclosure refers to CNTs as well as the carbonized products such as CNT fibers, strands, mats and composites made therefrom.

OBJECTS

Some of the objects of the present disclosure, which at least one embodiment is able to achieve, are discussed herein below.

It is an object of the present disclosure to provide a process for the preparation of carbon nanotubes. It is still another object of the present disclosure to provide processes for the preparation of carbon nanotubes, which obviates generation of undesirable side products.

It is yet another object of the present disclosure to provide an apparatus for the preparation of carbon nanotubes.

It is still another object of the present disclosure to ameliorate one or more problems of the prior art or to at least provide a useful alternative.

Other objects and advantages of the present disclosure will be more apparent from the following description and accompanying drawing which is not intended to limit the scope of the present disclosure.

SUMMARY

The present disclosure provides a process for the preparation of carbon nanotubes (CNTs), said process comprising the following steps: i. introducing a precursor for a metal catalyst into a heated furnace to permit said precursor to reside in an isothermal zofie of said furnace, to obtain nano-sized metal catalyst particles; ii. controlled feeding, at least once, through at least one inlet port leading to said isothermal zone a hydrocarbon; iii. pyrolyzing said hydrocarbon to obtain pyrolyzed hydrocarbon; iv. feeding a carrier gas into said furnace at, at least one step selected from the group of steps consisting of:

• feeding the carrier gas before feeding the precursor;

• feeding the carrier gas along with the precursor;

• feeding the carrier gas after introducing the precursor;

• feeding the carrier gas after formation of the nano-sized metal catalyst particles;

• feeding the carrier gas along with the hydrocarbon;

• feeding the carrier gas after feeding the hydrocarbon; and v. depositing said pyrolyzed hydrocarbon on the surface of said nano-sized metal catalyst to yield CNTs.

The hydrocarbon may be fed iteratively through the at least one inlet port.

Preferably, the precursor for forming the metal nano-sized catalyst and the hydrocarbon to be contacted with said catalyst are directly introduced at the isothermal zone; thereby avoiding the formation of side products of the metal precursor and the hydrocarbon.

The present disclosure also provides an apparatus for the preparation of carbon nanotubes (CNTs). BRIEF DESCRIPTION OF THE DRAWING

The disclosure will now be described with reference to accompanying non-limiting drawings, wherein:

Figure 1 illustrates a schematic of an apparatus (100) for the preparation of carbon nanotubes in accordance of the present disclosure, wherein:

102 represents the isothermal zone of a furnace;

104 represents the furnace with a split part;

106 represents the plurality of inlet ports;

108 represents the main inlet;

1 10 represents the outlet; x axis represents the temperature; and y axis represents the length of the furnace.

Figure 2 (A), (B) and (C) illustrate the transmission electron microscopic images of the CNTs.

DETAILED DESCRIPTION

In accordance with one aspect of the present disclosure, there is provided a process for the preparation of carbon nanotubes (CNTs), including other carbonized products such as CNT fibers, strands, mats and composites. In accordance with the present disclosure it is found that for high yield of CNTS, it is desirable that the catalyst nanoparticles are already formed /present when the hydrocarbon achieves the pyrolysis temperature. Thus, catalytic pyrolysis needs to be achieved rather than thermal pyrolysis. In the known process CNT yield-diminishing pyrolysis of all the hydrocarbons takes place in the entry zone of the tube when the catalyst nanoparticles are yet to be formed and also in the later zones of the tube as the hydrocarbon: nanoparticle ratio is high. The present disclosure focuses on reducing the undesirable conversion of hydrocarbons into products other than CNTs. In one embodiment the entire feeding of the hydrocarbon is at the isothermal zone of the furnace tube. The controlled split/gradual addition of hydrocarbon through several ports along the length of tube or even along the entire tube length is carried out, reducing the undesirable conversion of hydrocarbons into products other than CNTs. The heating by mixing with hot mixture flowing through the tube is found to be faster than the heating from tube walls in the entry zone of the tube. Thus the distributed hydrocarbon feed avoids unnecessary pyrolysis of the hydrocarbon and its conversion to products other than CNTs. It also enables the utilization of the catalyst effectively increasing the overall conversion, yield and reducing impurity levels in the final product.

The catalyst is also tested for deactivation after conversion of hydrocarbon in the tube. To check this, only hydrocarbon is fed to the initial catalyst- CNTs formed and deposited in-situ on the tube walls in the hot zone of the furnace and checked for the conversion of every new shot of hydrocarbon vapors into the tube. It is found that the in-situ formed nanoparticle catalyst is active. Further, the absence of support material results in less agglomerated CNT products as well as elimination of the inorganic impurities in the product. Also a separate step and associated hardware for producing supported catalyst is not required. The process of the present disclosure thus helps to reduce the impurity content in the obtained CNTs.

The process initially includes introducing a precursor for a metal catalyst in an isothermal zone (102) of a heated furnace of an apparatus (as shown in Figure 1) for the preparation of carbon nanotubes (CNTs). The apparatus (100) for carrying out the present process will be described subsequently in the specification. In the isothermal zone, the precursor gets reduced to a nano-sized metal catalyst. The metal nano-sized catalyst of the present disclosure includes but is not limited to non-supported nano- sized metal catalyst. Typically, the metal component of the precursor for the metal catalyst and the nano-sized metal catalyst is at least one selected from the group consisting of iron, cobalt, nickel, magnesium, rhenium, ruthenium, rhodium, palladium, silver, osmium, iridium, platinum and gold.

In accordance with the present disclosure the precursor for the metal catalyst is introduced in the isothermal zone of the furnace. The precursor may be introduced through a side entry inlet (106) or through the main inlet (108) of the furnace.

In the next step, a hydrocarbon is fed in a controlled manner, at least once, through at least one of a plurality of inlet port (106) into the isothermal zone (102) of the heated furnace (104) to pyrolyze the hydrocarbon. The hydrocarbon may be fed in a controlled manner iteratively through the inlet port.

The resultant pyrolyzed hydrocarbon gets deposited on the surface of the already formed nano-sized metal catalyst and generates CNTs. The CNTs so generated may be multiple-wall CNTs, single-wall CNTs or double wall CNTs. Other compounds like thiophene may also be used in the synthesis of CNTs (particularly required to make single wall carbon nanotubes) along with the hydrocarbon, catalyst precursor and carrier gas. The diameter of multiple- wall CNTs may range between 20 to 40nm (as shown in Figure 2(A), 2(B) and 2 (C). The temperature in the furnace (104) of the present disclosure can be maintained by different methods such as electrical heating or introducing a pre-heated carrier gas in the furnace.

Carrier gas may be introduced into the furnace at, at least one step selected from the group of steps consisting of: feeding the carrier gas before feeding the precursor; feeding the carrier gas along with the precursor; feeding the carrier gas after introducing the precursor; feeding the carrier gas after formation of the CNTs; feeding the carrier gas along with the hydrocarbon; and feeding the carrier gas after feeding the hydrocarbon.

Typically, the pre-heated carrier gas includes but is not limited to helium, nitrogen, argon and the like. In one embodiment, the carrier gas is nitrogen. The hydrocarbon of the present disclosure is in the form of vapor as per an embodiment of the present disclosure.

A typical embodiment of the process of the present disclosure entails iteratively passing the hydrocarbon through the plurality of inlet ports (106) into the isothermal zone (102) of said furnace (104) in a controlled manner. This is because, gradual and iterative exposure of the hydrocarbon completely utilizes the active stock of nano- sized metal catalyst present in the furnace; thereby achieving a higher yield.

In accordance with the present disclosure the concentration of the precursor for the metal catalyst is in the range of 0.1 wt % to 4 wt % of the hydrocarbon concentration. In one embodiment the furnace is maintained at a temperature ranging from 700 to 1300°C and the flow rate of hydrocarbon is maintained in the range of 20 to lOOsccm. (* seem: standard cubic centimeters per minute) In another embodiment the flow rate of the carrier gas is maintained in the range of 100 to 2000sccm.

Furthermore, the metal precursor for forming the nano-sized metal catalyst and the hydrocarbon to be contacted with the catalyst, are directly introduced at the isothermal zone (102) to avoid the formation of side products of the metal precursor and the hydrocarbon, which is a common occurrence with the conventional processes where the chemicals are exposed to a temperature gradient.

Carbonized products such as CNT fibers, strands and mats can also be prepared in accordance with the present disclosure by processing the CNTs further, in the same apparatus (100), to obtain the desired products. The processing may be done using conventional techniques.

In accordance with yet another aspect of the present disclosure, there is provided an apparatus (100) for the preparation of carbon nanotubes (CNTs) and carbonized products, as shown in Figure 1. The apparatus consists of a furnace (104), typically having one or more non-isothermal zones and an isothermal zone (102), adapted to receive a metal precursor and a hydrocarbon. The apparatus further includes a main inlet (108) and a plurality of inlet ports (106) located in the isothermal zone of the furnace which are adapted to convey the metal precursor and the hydrocarbon to the furnace for the preparation of CNTs.

The apparatus (100) of the present disclosure may further include at least one means (such as a side port or main inlet port) for introducing the precursor for the metal catalyst particles in the isothermal zone of the furnace. The apparatus also includes at least one outlet (1 10) for releasing the CNTs and carbonized products formed in the apparatus.

The present disclosure will now be discussed in the light of the following non-limiting embodiments:

Example 1: Process for the preparation of CNTs in accordance with the present disclosure

A] 2 mg of ferrocene (metal catalyst precursor) was introduced in the isothermal zone of the furnace of the present disclosure maintained at 875 °C to generate iron nanoparticles. 100 mg of camphor (hydrocarbon) was fed at the isothermal zone of the furnace through at least one inlet port. Nitrogen gas (carrier gas) was introduced in the furnace along with the hydrocarbon. The amount of CNT obtained was 3.8 mg (Case A).

After an interval of two minutes, 100 more milligrams of camphor was introduced in the same way but without any ferrocene (Case B). One more dosage of camphor (100 mg) was added subsequently without any ferrocene (Case C). The results of these experiments are summarized in Table 1 below. It is observed that subsequent additions of camphor after the first deposition of ferrocene also contribute to the CNT yield indicating that catalyst is still active.

Table 1 : CNT yield for Example

B] In this example, the hydrocarbon (camphor) dosage was reduced to 25 mg and the catalyst content was increased to 4 mg.

4 mg of ferrocene (metal catalyst precursor) was introduced in the isothermal zone of the furnace of the present disclosure, maintained at 875 °C. 25 mg of camphor (hydrocarbon) were fed at the isothermal zone of the furnace through at least one inlet port. No CNTs yield could be observed at this low content of hydrocarbon (Case A).

After an interval of two minutes, 25 more milligrams of camphor was introduced in the same way but without any ferrocene (Case B). One more dosage of camphor (25 mg) was added subsequently without any ferrocene (Case C). The results of these experiments are summarized in Table 2 below. It is observed that subsequent additions of camphor after the first deposition of ferrocene also contribute to the CNT yield indicating that catalyst is still active.

Table 2: CNT yield for Example

These experiments confirm that the catalyst remains active over a period of at least 6 minutes.

From the afore-stated examples it was concluded that to take the advantage of the motion of the catalyst particles, it was essential to add the hydrocarbon along the length of the furnace. Furthermore, addition of hydrocarbon along the length of the furnace decreases the effective residence time of the hydrocarbon inside the process tube (furnace) which reduces the probability of undesired thermal decomposition and helps in improving the yield of CNTs.

The embodiments herein and the various features and advantageous details thereof are explained with reference to the non-limiting embodiments in the description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein.

The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not^' of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.

TECHNICAL ADVANTAGES AND ECONOMIC SIGNIFICANCE

The process and apparatus for the preparation of CNTs according to the present disclosure have the following advantages: - Yield of the CNTs increases

- Generation of side products is avoided

Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

The use of the expression "at least" or "at least one" suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the invention to achieve one or more of the desired objects or results.

The numerical values given for various physical parameters, dimensions and quantities are only approximate values and it is envisaged that the values higher than the numerical value assigned to the physical parameters, dimensions and quantities fall within the scope of the invention and the claims unless there is a statement in the specification to the contrary.

While certain embodiments of the inventions have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Variations or modifications in the process or compound or formulation or combination of this invention, within the scope of the invention, may occur to those skilled in the art upon reviewing the disclosure herein. Such variations or modifications are well within the spirit of this invention. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the invention.

Claims

CLAIMS:

1. A process for the preparation of carbon nanotubes (CNTs), said process comprising the following steps: i. introducing a precursor for a metal catalyst into a heated furnace to permit said precursor to reside in an isothermal zone of said furnace, to obtain nano-sized metal catalyst particles; ii. controlled feeding, at least once, through at least one inlet port (106), leading to said isothermal zone (102), a hydrocarbon; iii. pyrolyzing said hydrocarbon to obtain pyrolyzed hydrocarbon; iv. feeding a carrier gas into said furnace at, at least one step selected from the group of steps consisting of:

• feeding the carrier gas before feeding the precursor;

• feeding the carrier gas along with the precursor;

• feeding the carrier gas after introducing the precursor;

• feeding the carrier gas along with the hydrocarbon;

2. The process as claimed in claim 1, wherein the metal component of said precursor for the metal catalyst and said nano-sized metal catalyst is at least one selected from the group consisting of iron, cobalt, nickel, magnesium, rhenium, ruthenium, rhodium, palladium, silver, osmium, iridium, platinum gold and combinations thereof.

3. The process as claimed in claim 1, wherein said nano-sized metal catalyst is a non-supported nano-sized metal catalyst.

4. The process as claimed in claim 1, further includes a step of introducing at least one pre-heated carrier gas before the step of feeding the hydrocarbon.

5. The process as claimed in claim 1, which includes the step of preheating the carrier gas before introducing the carrier gas in the furnace.

6. The process as claimed in claim 1, wherein the temperature inside said furnace (104) is maintained by at least one method selected from the group consisting of electrical heating and introducing a pre-heated carrier gas in the furnace (104).

7. The process as claimed in claims 4 to 6, wherein said pre-heated carrier gas is at least one selected from the group consisting of helium, argon and nitrogen.

8. The process as claimed in claim 1, wherein said hydrocarbon is in the form of vapor.

9. The process as claimed in claim 1, characterized in that the hydrocarbon is iteratively passed through a plurality of inlet ports (106) into the isothermal zone (102) of said furnace (104).

10. The process as claimed in claim 1, characterized in that the metal precursor for forming the metal nano-sized catalyst and the hydrocarbon to be contacted with said catalyst are directly introduced at the isothermal zone (102); thereby avoiding the formation of side products of the metal precursor and the hydrocarbon.

1 1. An apparatus (100) for the preparation of carbon nanotubes (CNTs) , said apparatus comprising: i. a furnace (104) adapted to receive a metal precursor and a hydrocarbon, wherein said furnace comprises an isothermal zone (102) and one or more non-isothermal zones; and ii. a plurality of inlet ports (106) located in said isothermal zone (102) of said furnace (104) adapted to convey said precursor for metal catalyst and said hydrocarbon to said furnace (104).

12. The apparatus as claimed in claim 1 1, further includes at least one means for introducing said precursor for metal in said isothermal zone (102) of the furnace (104).

13. The apparatus as claimed in claim 1 1, further includes at least one main inlet (108) for introducing the precursor and at least one outlet (1 10) for releasing the CNTs formed in the apparatus (100).