RU2480405C1 - Nanostructured carbon material and method for production thereof - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: present invention relates to a method of producing carbon nanostructures. The method of producing nanostructured carbon material involves pyrolysis of hydrocarbons on a catalyst in form of magnesium oxide in continuous mode at temperature ranging from 800 to 950°C and removing magnesium oxide from the nanostructured carbon material by dissolving in a solution of an inorganic acid and then washing the obtained graphene flakes in ammonia solution.

EFFECT: simple method, high specific surface area of the material and output thereof, as well as low content of catalyst in the end product.

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Description

The present invention relates to a method for the production of carbon nanostructures.

There are many ways to produce carbon nanostructures. They can be divided into two main categories: high temperature methods and low temperature methods. Most high-temperature methods are based on sublimation of carbon in an inert atmosphere, such as an electric arc discharge process, laser ablation, and solar technology. Low temperature methods are, for example, chemical vapor deposition (CVD) using catalytic decomposition of hydrocarbons, gas phase catalytic growth from carbon monoxide, production by electrolysis, heat treatment of a polymer, local low temperature pyrolysis or local catalysis.

CVD (chemical vapor deposition) is a method in which carbon nanostructures are obtained from the gas phase by growing them on a substrate using large amounts of catalyst at a low temperature (600-1000 ° C).

The resulting nanostructures contain defects, which leads to bending in the structures. Also, the catalyst in the obtained nanostructures is present in various amounts, from 50% to as low as 1-10%.

The closest analogue to the claimed group of inventions are technical inventions described in the patent of the Russian Federation No. 2258031, publ. 08/10/2005, class СВВ 31/02.

Powdered catalyst is continuously fed into the tubular reactor, moving it along the axis of the reactor. As a catalyst, you can use, wt.%: Ni 70 ÷ 90, MgO 10 ÷ 30 or Co 40 ÷ 60, Al ₂ O ₃ 40 ÷ 60; or Mo: Co: Mg with a molar ratio of 1: 5: 94, respectively. The process is conducted continuously with countercurrent contacting of the catalyst and hydrocarbon. In the first zone or zones, the catalyst is activated by gases leaving the pyrolysis of hydrocarbons at 450 ÷ 600 ° C. The residence time of the catalyst 5 ÷ 180 minutes The activated catalyst is fed into the zone or zone of pyrolysis with a temperature of 550 ÷ 1000 ° C. A hydrocarbon, for example methane, is supplied there in countercurrent flow. The residence time of the catalyst in the pyrolysis zone is 0.5 ÷ 180 minutes. The carbon material formed together with the catalyst is continuously withdrawn from the reactor and separated from the catalyst by dissolving the catalyst in dilute nitric acid. The invention allows to obtain hollow curved fibers. The resulting product is a curved hollow fiber with a diameter of 20-60 nm with conical walls and a specific surface area of about 100 m ² / g.

The disadvantages of the known technical solutions are that the production method is complicated using hard-to-reach catalysts, in addition, the resulting product has a low specific surface area, many defects, contains a catalyst. Due to the treatment of the material with nitric acid, its surface contains a significant amount of oxygen, which for some applications significantly degrades the characteristics of the material.

The objective of the invention is to remedy the above disadvantages.

The general technical result consists in simplifying the method, increasing the specific surface of the material and reducing the content of catalyst and oxygen in the finished product. The technical result is ensured by the fact that the method of producing a carbon nanostructured material in the form of layered graphene flakes involves the pyrolysis of hydrocarbons when heated on a catalyst in the form of pure magnesium oxide in a continuous or batch mode. In this case, the synthesis is carried out mainly at a synthesis temperature of from 800 to 950 ° C. In addition, carry out the cleaning of layered graphene flakes from magnesium oxide by dissolving in a dilute solution of hydrochloric and (or) nitric acid, followed by washing the obtained graphene flakes in a solution of ammonia. The catalyst for methane pyrolysis is magnesium oxide (MgO) obtained by decomposition of magnesium hydroxide (Mg (OH) ₂ ). The crystalline particles of the catalyst - magnesium oxide, as well as the starting magnesium hydroxide are in the form of hexagons with a diameter of 100 to 1000 nm and a thickness of 50 to 300 nm.

The amount of loaded catalyst, the duration of synthesis and the volumetric gas flow rate are determined using the dimensionless parameter K, which characterizes the allowable load on the catalyst and is calculated by the formula:

Where

Q - volumetric gas flow, m ³ / h;

τ is the duration of the synthesis, h;

m is the mass of the catalyst, g;

Sa is the specific surface area of the catalyst, m ² / g;

l is the catalyst loading length in the direction of gas flow, m

The optimal value of the parameter K, at which a CNM with a surface of more than 1800 m ² / g is obtained, is in the range from 5 · 10 ^-6 to 15 · 10 ^-6 .

Carbon nanostructured material (CNM) in the form of layered graphene flakes with a specific surface area of more than 1800 m ² / g is obtained as described above. They have the number of graphene layers in the flakes from 1 to 3. In the free state after drying, the material is a black powder with a bulk density of 0.2-0.5 g / cm ³ and a specific electrical resistance of 0.03-0.08 Ohm · cm .

The purification of the CNM from the catalyst is carried out in two stages.

At the first stage, magnesium oxide is dissolved in a solution of dilute hydrochloric and (or) nitric acid when heated to a temperature of from 50 to 100 ° C. Duration of cleaning varies from 0.5 to 4 hours. The concentration of acid is from 5 to 20 wt.%.

At the second stage, chlorides and (or) nitrates are washed in an ammonia solution when heated to a temperature of 50 to 100 ° C. The cleaning time is from 0.5 to 2 hours. The concentration of the ammonia solution is from 10 to 50 wt.%. After purification in ammonia, the CNM is washed with distilled water to a pH of 6 to 8 and dried in air at a temperature of 80 to 140 ° C. Drying time from 5 to 24 hours.

The above method for producing CNMs can be produced both in a continuous and batch process. The use of a periodic synthesis process can significantly simplify the synthesis apparatus, reduce capital costs for the development and manufacture of equipment, increase the reliability and safety of the synthesis process. For the synthesis of CNMs in a batch process without loss of product quality, it is necessary to provide a certain load of the source gas on the reactor volume, the load is determined from the ratio

Where

Q - volumetric gas flow, m ³ / h;

τ is the duration of the synthesis, h;

V is the volume of the reactor, m ³ .

The value of K1 should be more than 2.5.

Physically-chemically declared result is achieved due to the low catalytic activity of magnesium oxide. Carbon covers magnesium oxide in a uniform layer. But the thickness of this layer is limited by the activity of the catalyst and does not exceed several graphene layers. To obtain a substance with a high specific surface, it is necessary to ensure that the carbon layer as a result of the pyrolysis reaction covers the catalyst surface in one or two graphene layers. In this case, the carbon layer must not be continuous in order to ensure complete removal of the catalyst. The minimum value of the thickness of the carbon layer is achieved by the choice of temperature and the optimal parameter of the load on the catalyst - K for the selected temperature. The correct choice of the catalyst morphology ensures the presence of holes in the carbon layer through which the catalyst is removed during chemical washing. If the process is carried out in a batch mode, it is necessary to supply a certain gas flow rate - K1, which would ensure the release of the reactor from neutral gas in the presence of which the catalyst preheats. A material that does not contain oxygen compounds on the surface is achieved by using hydrochloric acid, which does not contain oxygen compounds.

Removal from the surface of chlorine after washing in hydrochloric acid is achieved by washing in an aqueous solution of ammonia, followed by washing in distilled water to pH 6-8.

On the contrary, the achievement of the maximum number of oxygen groups on the surface of the material is achieved by treatment only with nitric acid, followed by washing in distilled water. In this case, the material will not contain chlorine, therefore, additional washing with ammonia will not be necessary.

The following examples of implementation reveals the essence of the claimed method and characterizes the resulting material.

Example 1

CNMs were obtained by pyrolysis of natural gas at a temperature of 900 ° С on a catalyst in the form of magnesium oxide, Fig. 1.

The methane pyrolysis with the formation of CNM is carried out in a horizontal tubular reactor (1). The tubular reactor has a bore diameter of 160 mm and a length of 1600 mm. The heating zone is 1100 mm. The reactor is placed in a three-zone electric furnace (2), which with the help of heaters (3) and thermocouples (4) ensures that the temperature is maintained with an accuracy of 1 ° C.

A pan with catalyst backfill (5) with a mass of 210.8 grams and a length of backfill of 750 mm was placed in the heated zone. The catalyst was heated in a neutral medium (nitrogen) for 2 hours to 900 ° C. Then for 15 minutes, natural gas was supplied with an hourly flow rate of 407.3 l / h. After cooling, 5.5 g of CNMs were obtained, representing graphene flakes of Fig. 2 with a specific BET area of 2150 m ² / g. The type of catalyst is shown in Fig. 3.

Example 2

Under the conditions described in example 1, methane was pyrolyzed on magnesium oxide. 138 g of magnesium oxide were loaded, the process was conducted at a temperature of 900 ° C for 15 minutes. 310 l / h of methane were fed. After washing, 3.9 g of CNM with a specific area of 1800 m ² / g were obtained.

Example 3

Under the conditions described in example 1, methane was pyrolyzed on magnesium oxide. Loaded 192.6 g of magnesium oxide, conducted the process at a temperature of 900 ° C for 15 minutes. 407.3 L / h of methane were fed. After washing, 4.1 g of CNM with a specific area of 1930 m ² / g were obtained.

Claims (12)

1. A method of producing carbon nanostructured material (CNM), including the pyrolysis of hydrocarbons when heated on a catalyst in the form of crystalline magnesium oxide at an elevated temperature and purification of CNM from magnesium oxide by dissolving inorganic acid in a solution, wherein the CNM is obtained in the form of layered graphene flakes, consumption gas, the amount of catalyst and the duration of the synthesis is determined from the dimensionless parameter of the load on the catalyst K, the removal of the catalyst is carried out in an acid solution, followed by washing the obtained graphene flakes is carried out in an ammonia solution with washing in distilled water. 2. The method according to claim 1, characterized in that the synthesis is carried out at a temperature in the reactor from 800 to 950 ° C. 3. The method according to claim 1, characterized in that the CNM is obtained in the form of layered graphene flakes with a specific area of more than 1800 m ² / g and having a number of graphene layers from 1 to 3. 4. The method according to claim 1, characterized in that the catalyst has a crystalline structure in the form of hexagons with a diameter of the circumscribed circle from 100 to 1000 nm and a thickness of from 50 to 300 nm. 5. The method according to claim 1, characterized in that the dimensionless load parameter on the catalyst K is selected in the range from 5 · 10 ^-6 to 15 · 10 ^-6 . 6. The method according to claim 1, characterized in that the removal of the catalyst to obtain a minimum amount of oxygen compounds on the surface of the CNM is carried out using hydrochloric acid, which does not contain oxygen compounds. 7. The method according to claim 1, characterized in that the removal of the catalyst to obtain a minimum amount of chlorine compounds on the surface of the CNM is carried out using nitric acid, which does not contain chlorine compounds. 8. The method according to claim 1, characterized in that the washing of the obtained graphene flakes in a solution of ammonia with washing in distilled water is carried out to a pH of 6-8. 9. The method according to claim 1, characterized in that the synthesis is carried out in a batch process with a dimensionless load on the volume of the reactor K1 in excess of 2.5. 10. Carbon nanostructured material in the form of layered graphene flakes with a specific surface area of more than 1800 m ² / g, obtained by the method according to claim 1 and having the number of graphene layers in the flakes from 1 to 3. 11. The carbon nanostructured material of claim 10, characterized in that as a result of the removal of the catalyst with hydrochloric acid, it does not contain oxygen compounds. 12. The carbon nanostructured material of claim 10, characterized in that as a result of washing in an ammonia solution it does not contain chlorine compounds.

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