Method for producing carbon nanotubes by low-temperature catalytic cracking of VOCs (Volatile Organic Compounds)

Abstract

The invention belongs to the field of gas treatment, and relates to a method for producing carbon nanotubes by low-temperature catalytic cracking of VOCs (volatile organic compounds), TiO2 is used as a carrier to load metal Ni, alkaline MgO is introduced to regulate the acidity of TiO2, and the metal Ni is reinforced to be fixed to obtain a special catalyst. The catalyst adsorbs a carbon source obtained after VOCs are cracked to the surface within a low-temperature range, carbon atoms and by-products are generated after reaction, the carbon atoms are deposited on the surface of the catalyst and reach a saturated state, and finally CNTs are separated out and formed. The morphology performance of the carbon nanotubes is improved by controlling the retention time of a carbon source on the surface of the catalyst and optimizing the loading capacity of metal Ni, meanwhile, a certain amount of CO is introduced to assist in catalyzing and reducing amorphous carbon deposition on the surface of the catalyst, and CNTs formed on the surface of the catalyst are stripped through an online ultrasonic auxiliary system and other methods.

Description

Technical Field

[0001] The present invention belongs to the field of gas treatment, and particularly relates to a method for low-temperature catalytic cracking of VOCs to produce carbon nanotubes by using a multifunctional low-temperature nickel-based titanium dioxide catalyst. Background Art

[0002] Volatile organic compounds (VOCs) are organic compounds with a relatively high saturated vapor pressure (greater than 13.33 Pa), low boiling point, small molecular weight, and easy to volatilize at room temperature under standard conditions. Although there are many methods for treating VOCs pollution at present, most of them are adsorption methods, combustion methods, biological methods, etc. These methods not only have high costs but also low degradation efficiencies. In addition, some emerging technologies such as plasma technology, photocatalytic oxidation, and membrane separation technology also have disadvantages such as high energy consumption, low catalyst efficiency and easy deactivation, and high investment costs. Chemical vapor deposition (CVD) is a technology that can effectively convert VOCs into high-value-added products (such as nano-carbon products like carbon nanotubes), but due to high-temperature conditions, the catalyst surface is prone to agglomeration, resulting in rapid deactivation, and the conversion rate and purity of the generated nano-carbon products are not high, and they are easily doped with metals and amorphous carbon, etc., which requires further purification, increasing the investment cost. Therefore, it is urgent to develop an efficient and low-temperature-convertible catalytic system to promote the generation of nano-carbon products.

[0003] Titanium dioxide is an inorganic compound. Due to its large specific surface area, rich surface active sites, excellent thermal stability and chemical inertness, it is often used as a catalyst carrier. At the same time, TiO2 is non-toxic, inexpensive and easy to prepare on a large scale, meeting the requirements of green chemistry and having been widely used in industrial waste gas treatment. Its high efficiency and economy provide the feasibility for the resource utilization of VOCs. Summary of the Invention

[0004] Object of the Invention: The technical problem to be solved by the present invention is to provide, in view of the deficiencies of the prior art, a method for low-temperature catalytic cracking of VOCs to produce carbon nanotubes by using a multifunctional low-temperature nickel-based titanium dioxide catalyst, so as to solve the problems existing in the prior art such as high-temperature energy consumption requirements, catalyst deactivation, low product purity, and amorphous carbon deposition, and provide a new way for the low-cost and high-performance preparation of CNTs.

[0005] To achieve the above object, the technical solution adopted by the present invention is as follows: A method for low-temperature catalytic cracking of VOCs to produce carbon nanotubes uses TiO2 as a carrier, loads metal Ni, and introduces MgO to load and fix metal Ni to obtain a multifunctional low-temperature nickel-based titanium dioxide catalyst; under the action of the above catalyst, VOCs are low-temperature catalytically cracked to generate carbon nanotubes.

[0006] Preferably, the loading amount of the metal Ni is 1%-10% of the total mass of the catalyst.

[0007] Preferably, the addition amount of the MgO is 0.5%-5% of the total mass of the catalyst. The introduction of basic MgO conducts acidic regulation on TiO2 and strengthens the loading and fixation of the metal Ni.

[0008] Furthermore, the multifunctional low-temperature nickel-based titanium dioxide catalyst in the present invention is specifically prepared through the following steps: a. Mix TiO2 and MgO to form a composite support; b. Immerse the composite support in a nickel salt solution by the impregnation method, take it out after sufficient impregnation, dry it, and then calcine it to obtain the catalyst.

[0009] Preferably, in step a, the mixing method of TiO2 and MgO is mechanical stirring and mixing, and the mixing time is 1-5 hours.

[0010] Preferably, in step b, the nickel salt solution is an aqueous solution of nickel nitrate, and the concentration is 0.1-1 mol / L.

[0011] Preferably, in step b, the drying temperature is 60-120 °C, and the drying time is 6-12 hours; the calcination temperature is 300-600 °C, and the calcination time is 2-5 hours.

[0012] Preferably, a certain amount of the catalyst is added to the reactor, a certain amount of VOCs and CO are introduced in the temperature range of 200-400 °C, the residence time and flow rate are controlled, after the end of one-stage reaction, CNTs are peeled off by an on-line ultrasonic assistance system and collected, and continuous reaction is carried out to realize continuous production of high-purity CNTs.

[0013] Furthermore, the specific steps include: S1. Introduce VOCs into the reactor and carry out a cracking reaction in the presence of the multifunctional low-temperature nickel-based titanium dioxide catalyst; S2. Control the residence time of the carbon source on the catalyst surface to be 10-60 minutes; S3. Introduce CO, control the volume ratio of VOCs to CO to be 4:1-2:1, and the flow rate to be 10-100 ml / min; S4. Peel off the CNTs formed on the catalyst surface.

[0014] Preferably, the VOCs are aromatic hydrocarbon compounds, including but not limited to at least one of benzene, toluene, and xylene; Preferably, the CNTs are peeled off from the catalyst surface by an on-line ultrasonic assistance system, the ultrasonic frequency is 20-100 kHz, and the ultrasonic power is 100-500 W.

[0015] Compared with the prior art, the present invention has the following beneficial effects: (1) The catalyst of the present invention uses a TiO2 support as a carbon source adsorption center, and its large specific surface area ensures the contact space between the carbon source and the active components of the catalyst. In the temperature range of 200 - 400 °C, after the carbon source cracked from VOCs is adsorbed onto the catalyst and undergoes a reaction, carbon atoms and by-products (such as H2, H2O, etc.) are generated. Subsequently, the carbon atoms are deposited on the catalyst surface and reach a saturated state, and finally CNTs are precipitated. By controlling the residence time of the carbon source on the catalyst surface and optimizing the loading amount of metal Ni, the morphological properties of CNTs are improved. Meanwhile, a certain amount of CO is introduced and its flow rate is controlled to assist in reducing the amorphous carbon deposition on the catalyst surface. The CNTs formed on the catalyst surface are peeled off by methods such as an on-line ultrasonic assistance system. Due to the introduction of MgO, the metal Ni on the catalyst surface is fixed, avoiding the mixing of metals in CNTs, effectively improving the purity of CNTs. Finally, by controlling the flow rate and flux of CO introduced, not only the purity of CNTs is further improved, but also the high activity of the catalyst is maintained.

[0016] (2) The present invention uses TiO2 as the catalyst support. The surface acidic sites can adsorb and enrich the carbon source, and at the same time help to maintain the structural stability of the catalyst, having a certain anti-poisoning ability; the introduction of MgO adjusts the surface acidic sites of the TiO2 support, which is beneficial to reducing the catalytic cracking temperature of VOCs. Meanwhile, the metal Ni in the catalyst is fixed, not only improving the purity of CNTs, but also maintaining the catalytic activity of the catalyst and prolonging its service life. The introduction of CO not only effectively reduces the deposition of amorphous carbon on the catalyst surface, but also the generated CO can be reused.

[0017] (3) The present invention realizes the resource treatment of VOCs, reduces the pressure of treating VOCs in the large-scale production process of factories, reduces the adverse impact on the environment, and at the same time generates CNTs with high added value, having certain economic benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The following further specifically describes the present invention in conjunction with the drawings and specific embodiments, and the above and / or other advantages of the present invention will become clearer.

[0019] Figure 1 It is a reaction device diagram.

[0020] Figure 2 It is a diagram of the purity of CNTs prepared in each example and control example.

[0021] Figure 3 It is a diagram of the carbon source conversion rate of each example and control example. SPECIFIC EMBODIMENTS

[0022] The present invention can be better understood according to the following embodiments.

[0023] As Figure 1 shown, it is a reactor device diagram for the low-temperature catalytic cracking of VOCs to produce carbon nanotubes according to the present invention. Among them, a VOCs inlet 1 is provided on one side of the reactor, and a CO inlet 2 is provided on the other side, which are respectively used to inject VOCs and CO into the reactor. An on-line ultrasonic assistance system 5 composed of YM-ZDY003 ultrasonic vibration rods is arranged inside the reactor, and a catalyst feeding channel 6 is arranged in the on-line ultrasonic assistance system 5 for feeding the multifunctional low-temperature nickel-based titanium dioxide catalyst of the present invention into the reactor. An exhaust port 3 is provided at the top of the reactor, and a discharge port 4 is provided at the bottom. During specific preparation, a certain amount of catalyst is added to the reactor, a certain amount of VOCs and CO are introduced in the temperature range of 200-400 °C, the residence time and flow rate are controlled, after the end of one-stage reaction, CNTs are peeled off by the on-line ultrasonic assistance system and collected, and continuous reaction is carried out to realize continuous production of high-purity CNTs.

[0024] Comparative Example 1

[0025] In this comparative example, the specific preparation method of the catalyst is as follows:

[0026] (1) Weigh 10 g of TiO2 as the carrier.

[0027] (2) Prepare a 0.5 mol / L nickel nitrate aqueous solution, and add the carrier in step (1) to it by the impregnation method, so that the loading amount of Ni is 5% of the total mass of the catalyst, and stir well for 6 hours.

[0028] (3) Dry the impregnated liquid at 80 °C for 12 hours, and then calcine it in a muffle furnace at 400 °C for 3 hours to obtain a multifunctional low-temperature conversion type Ni-based TiO2 catalyst.

[0029] Apply the obtained catalyst to the resource utilization of VOCs to prepare CNTs:

[0030] (4) Pass aromatic hydrocarbon compounds such as benzene, toluene, and xylene as VOCs into the reactor, and carry out cracking reaction at 300 °C after adding the catalyst.

[0031] (5) Control the residence time of VOCs on the catalyst surface to be 30 minutes.

[0032] (6) Introduce CO, control the volume ratio of VOCs to CO to be 2:1, and the flow rate to be 50 ml / min.

[0033] (7) Use an on-line ultrasonic assistance system (ultrasonic frequency is 50 kHz, ultrasonic power is 300 W) to peel off the CNTs formed on the catalyst surface.

[0034] Collect carbon nanotubes and detect their purity.

[0035] Comparative Example 2

[0036] In this comparative example, the specific preparation method of the catalyst is as follows:

[0037] (1) Mix TiO2 and MgO in a mass ratio of 20:1, and mechanically stir for 3 hours to form a solid mixture.

[0038] (2) Add the solid mixture in step (1) to 200 ml of distilled water and stir well for 6 hours.

[0039] (3) Dry the stirred liquid at 80 °C for 12 hours, and then calcine it in a muffle furnace at 400 °C for 3 hours to obtain the catalyst.

[0040] Apply the obtained catalyst to the resource utilization of VOCs for the preparation of CNTs:

[0041] (4) Pass aromatic hydrocarbon compounds such as benzene, toluene, and xylene as VOCs into the reactor, and carry out a cracking reaction at 300 °C after adding the catalyst.

[0042] (5) Control the residence time of VOCs on the catalyst surface to be 30 minutes.

[0043] (6) Pass in CO, control the volume ratio of VOCs to CO to be 2:1, and the flow rate to be 50 ml / min.

[0044] (7) Use an online ultrasonic assistance system (ultrasonic frequency is 50 kHz, ultrasonic power is 300 W) to strip the carbon nanotubes formed on the catalyst surface.

[0045] Collect carbon nanotubes and detect their purity.

[0046] Comparative Example 3

[0047] In this comparative example, the specific preparation method of the multifunctional low-temperature conversion type Ni-based TiO2 catalyst is as follows:

[0048] (1) Mix TiO2 and MgO in a mass ratio of 20:1, and mechanically stir for 3 hours to form a support.

[0049] (2) Prepare a 0.5 mol / L nickel nitrate aqueous solution, and use the impregnation method to add the support in step (1) to it, so that the loading amount of Ni is 5% of the total mass of the catalyst, and stir well for 6 hours.

[0050] (3) The impregnated liquid was dried at 80 °C for 12 hours and then calcined in a muffle furnace at 400 °C for 3 hours to obtain a multifunctional low-temperature conversion type Ni-based TiO2 catalyst.

[0051] The obtained catalyst was applied to the resource utilization of VOCs for the preparation of TiO2:

[0052] (4) Aromatic hydrocarbon compounds such as benzene, toluene, and xylene were used as VOCs and introduced into the reactor. After adding the catalyst, a cracking reaction was carried out at 300 °C.

[0053] (5) The residence time of VOCs on the catalyst surface was controlled to be 30 minutes.

[0054] (6) An online ultrasonic-assisted system (ultrasonic frequency of 50 kHz and ultrasonic power of 300 W) was used to strip the CNTs formed on the catalyst surface.

[0055] The carbon nanotubes were collected and their purity was detected.

[0056] Example 1

[0057] In this example, the specific preparation method of the multifunctional low-temperature conversion type Ni-based TiO2 catalyst was as follows:

[0058] (1) TiO2 and MgO were mixed at a mass ratio of 20:1 and mechanically stirred for 3 hours to form a support.

[0059] (2) An aqueous solution of nickel nitrate with a concentration of 0.5 mol / L was prepared. The support in step (1) was added thereto by the impregnation method so that the loading amount of Ni was 5% of the total mass of the catalyst, and it was fully stirred for 6 hours.

[0060] (3) The impregnated liquid was dried at 80 °C for 12 hours and then calcined in a muffle furnace at 400 °C for 3 hours to obtain a multifunctional low-temperature conversion type Ni-based TiO2 catalyst.

[0061] The obtained catalyst was applied to the resource utilization of VOCs for the preparation of CNTs:

[0062] (4) Aromatic hydrocarbon compounds such as benzene, toluene, and xylene were used as VOCs and introduced into the reactor. After adding the catalyst, a cracking reaction was carried out at 300 °C.

[0063] (5) The residence time of VOCs on the catalyst surface was controlled to be 30 minutes.

[0064] (6) CO was introduced, and the volume ratio of VOCs to CO was controlled to be 2:1, and the flow rate was 50 ml / min.

[0065] (7) Use an on-line ultrasonic-assisted system (ultrasonic frequency: 50 kHz, ultrasonic power: 300 W) to exfoliate the carbon nanotubes formed on the catalyst surface.

[0066] Collect the carbon nanotubes and detect their purity.

[0067] Example 2

[0068] In this example, the specific preparation method of the multifunctional low-temperature conversion type Ni-based TiO2 catalyst is as follows:

[0069] (1) Mix TiO2 and MgO in a mass ratio of 25:1, and mechanically stir for 3 hours to form a support.

[0070] (2) Prepare an aqueous solution of nickel nitrate with a concentration of 0.5 mol / L. Add the support obtained in step (1) thereto by the impregnation method so that the loading amount of Ni is 5% of the total mass of the catalyst, and stir thoroughly for 6 hours.

[0071] (3) Dry the impregnated liquid at 80 °C for 12 hours, and then calcine it in a muffle furnace at 400 °C for 3 hours to obtain the multifunctional low-temperature conversion type Ni-based TiO2 catalyst.

[0072] Apply the obtained catalyst to the resource utilization of VOCs for the preparation of CNTs:

[0073] (4) Introduce aromatic hydrocarbon compounds such as benzene, toluene, and xylene as VOCs into the reactor. After adding the catalyst, carry out a cracking reaction at 300 °C.

[0074] (5) Control the residence time of benzene on the catalyst surface to be 30 minutes.

[0075] (6) Introduce CO, and control the volume ratio of VOCs to CO to be 2:1, with a flow rate of 50 ml / min.

[0076] (7) Use an on-line ultrasonic-assisted system (ultrasonic frequency: 50 kHz, ultrasonic power: 300 W) to exfoliate the carbon nanotubes formed on the catalyst surface.

[0077] Collect the carbon nanotubes and detect their purity.

[0078] Example 3

[0079] In this example, the specific preparation method of the multifunctional low-temperature conversion type Ni-based TiO2 catalyst is as follows:

[0080] (1) Mix TiO2 and MgO in a mass ratio of 30:1, and mechanically stir for 3 hours to form a support.

[0081] (2) Prepare an aqueous solution of nickel nitrate with a concentration of 0.5 mol / L. Add the carrier obtained in step (1) to it by the impregnation method, such that the loading amount of Ni is 5% of the total mass of the catalyst, and stir well for 6 hours.

[0082] (3) Dry the impregnated liquid at 80 °C for 12 hours, and then calcine it in a muffle furnace at 400 °C for 3 hours to obtain a multifunctional low-temperature conversion type Ni-based TiO₂ catalyst.

[0083] Apply the obtained catalyst to the resource utilization of VOCs for the preparation of CNTs:

[0084] (4) Pass aromatic hydrocarbon compounds such as benzene, toluene, and xylene as VOCs into the reactor, and carry out a cracking reaction at 300 °C after adding the catalyst.

[0085] (5) Control the residence time of VOCs on the catalyst surface to be 30 minutes.

[0086] (6) Pass in CO, control the volume ratio of VOCs to CO to be 2:1, and the flow rate to be 50 ml / min.

[0087] (7) Use an online ultrasonic assistance system (ultrasonic frequency is 50 kHz, ultrasonic power is 300 W) to strip the carbon nanotubes formed on the catalyst surface.

[0088] Collect the carbon nanotubes and detect their purity.

[0089] Example 4

[0090] In this example, the specific preparation method of the multifunctional low-temperature conversion type Ni-based titanium dioxide catalyst is as follows:

[0091] (1) Mix TiO₂ and MgO in a mass ratio of 25:1, and mechanically stir for 3 hours to form a carrier.

[0092] (2) Prepare an aqueous solution of nickel nitrate with a concentration of 1.0 mol / L. Add the carrier obtained in step (1) to it by the impregnation method, such that the loading amount of nickel is 10% of the total mass of the catalyst, and stir well for 6 hours.

[0093] (3) Dry the impregnated liquid at 80 °C for 12 hours, and then calcine it in a muffle furnace at 400 °C for 3 hours to obtain a multifunctional low-temperature conversion type Ni-based titanium dioxide catalyst.

[0094] Apply the obtained catalyst to the resource utilization of VOCs for the preparation of CNTs:

[0095] (4) Pass aromatic hydrocarbon compounds such as benzene, toluene, and xylene as VOCs into the reactor, and carry out a cracking reaction at 200 °C after adding the catalyst.

[0096] (5) Control the residence time of benzene on the catalyst surface to be 30 minutes.

[0097] (6) Introduce CO, control the volume ratio of VOCs to CO to be 2:1, and control the flow rate of CO to be 50 ml / min.

[0098] (7) Use an on-line ultrasonic-assisted system (ultrasonic frequency is 50 kHz, ultrasonic power is 300 W) to strip the carbon nanotubes formed on the catalyst surface.

[0099] Collect the carbon nanotubes and detect their purity.

[0100] Example 5

[0101] In this example, the specific preparation method of the multifunctional low-temperature conversion type Ni-based TiO2 catalyst is as follows:

[0102] (1) Mix TiO2 and MgO according to a mass ratio of 20:1, and mechanically stir for 3 hours to form a support.

[0103] (2) Prepare an aqueous solution of nickel nitrate with a concentration of 0.5 mol / L, and add the support in step (1) to it by the impregnation method, so that the loading amount of Ni is 5% of the total mass of the catalyst, and stir well for 6 hours.

[0104] (3) Dry the impregnated liquid at 80 °C for 12 hours, and then calcine it in a muffle furnace at 400 °C for 3 hours to obtain the multifunctional low-temperature conversion type Ni-based TiO2 catalyst.

[0105] Apply the obtained catalyst to the resource utilization of VOCs to prepare CNTs:

[0106] (4) Introduce aromatic hydrocarbon compounds such as benzene, toluene, and xylene as VOCs into the reactor, and carry out a cracking reaction at 300 °C after adding the catalyst.

[0107] (5) Control the residence time of VOCs on the catalyst surface to be 30 minutes.

[0108] (6) Introduce CO, control the volume ratio of VOCs to CO to be 3:1, and control the flow rate of CO to be 50 ml / min.

[0109] (7) Use an on-line ultrasonic-assisted system (ultrasonic frequency is 50 kHz, ultrasonic power is 300 W) to strip the carbon nanotubes formed on the catalyst surface.

[0110] Collect the carbon nanotubes and detect their purity.

[0111] Example 6

[0112] In this embodiment, the specific preparation method of the multifunctional low-temperature conversion type Ni-based TiO2 catalyst is as follows:

[0113] (1) Mix TiO2 and MgO in a mass ratio of 20:1, and mechanically stir for 3 hours to form a support.

[0114] (2) Prepare an aqueous solution of nickel nitrate with a concentration of 0.5 mol / L, and add the support in step (1) thereto by the impregnation method so that the loading amount of Ni is 5% of the total mass of the catalyst, and stir well for 6 hours.

[0115] (3) Dry the impregnated liquid at 80 °C for 12 hours, and then calcine it in a muffle furnace at 400 °C for 3 hours to obtain the multifunctional low-temperature conversion type Ni-based TiO2 catalyst.

[0116] Apply the obtained catalyst to the resource utilization of VOCs to prepare CNTs:

[0117] (4) Introduce aromatic hydrocarbon compounds such as benzene, toluene, and xylene as VOCs into the reactor, and add the catalyst and carry out a cracking reaction at 300 °C.

[0118] (5) Control the residence time of VOCs on the catalyst surface to be 30 minutes.

[0119] (6) Introduce CO, control the volume ratio of VOCs to CO to be 4:1, and control the flow rate of CO to be 50 ml / min.

[0120] (7) Use an on-line ultrasonic-assisted system (ultrasonic frequency is 50 kHz, ultrasonic power is 300 W) to peel off the carbon nanotubes formed on the catalyst surface.

[0121] Collect the carbon nanotubes and detect their purity.

[0122] Figure 2 The purity diagrams of CNTs under different control examples and embodiments are given. It can be seen that: by comparing Control Example 1 and Example 1, it can be known that adding MgO during the catalyst preparation process can effectively improve the purity of CNTs. It is speculated that this is because MgO has a fixing effect on the metal Ni on the surface of the catalyst, preventing the metal from falling off into the CNTs at high temperatures; by comparing Control Example 2 and Example 1, it can be known that the presence of metal Ni can effectively convert the carbon source into CNTs, increasing the stability of the catalyst; by comparing Control Example 3 and Example 1, it can be known that introducing a certain amount of CO can effectively improve the purity of CNTs. This is because CO can remove the amorphous carbon on the catalyst surface during the reaction, preventing it from falling off into the CNTs.

[0123] Figure 3The carbon source conversion rate diagrams under the control examples and the examples are given. It can be seen that when the mass ratio of TiO2 to MgO is 20:1, the concentration of nickel nitrate aqueous solution is 0.5 mol / L, and the volume ratio of VOCs to CO is 2:1, the carbon source conversion rate reaches the highest at 52%. This indicates that the addition of MgO, the loading of metal Ni, and the introduction of an appropriate amount of CO can effectively improve the carbon source conversion rate, thereby increasing the yield of CNTs.

[0124] The present invention provides an idea and method for a method of catalytically cracking VOCs at low temperature to produce carbon nanotubes. There are many methods and ways to specifically implement this technical solution. The above description is only the preferred embodiment of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention. Each component not clearly defined in this embodiment can be realized by existing technologies.

Claims

1. A method for producing carbon nanotubes by catalytic cracking of VOCs at low temperature, characterized in that, Using TiO2 as a carrier, metal Ni is loaded, and MgO is introduced to fix and load metal Ni to obtain a multifunctional low-temperature nickel-based titanium dioxide catalyst; under the action of the above catalyst, VOCs are catalytically cracked at low temperature to generate carbon nanotubes.

2. The method for producing carbon nanotubes by low-temperature catalytic cracking of VOCs according to claim 1, wherein The loading amount of the metal Ni is 1%-10% of the total mass of the catalyst.

3. The method for producing carbon nanotubes by low-temperature catalytic cracking of VOCs according to claim 1, characterized in that, The addition amount of the MgO is 0.5%-5% of the total mass of the catalyst.

4. The method for producing carbon nanotubes by catalytic cracking of VOCs at low temperature according to claim 1, characterized in that, The catalyst is prepared by the following steps: a. Mix TiO2 and MgO to form a composite carrier; b. Impregnate the composite carrier in a nickel salt solution by the impregnation method, take it out after sufficient impregnation, dry it, and then calcine it to obtain the catalyst.

5. The method for producing carbon nanotubes by catalytic cracking of VOCs at low temperature according to claim 4, characterized in that, In step a, the mixing method of TiO2 and MgO is mechanical stirring and mixing, and the mixing time is 1-5 hours.

6. The method for producing carbon nanotubes by low-temperature catalytic cracking of VOCs according to claim 4, wherein In step b, the nickel salt solution is an aqueous solution of nickel nitrate, and the concentration is 0.1-1 mol / L.

7. The method for producing carbon nanotubes by catalytic cracking of VOCs at low temperature according to claim 4, wherein, In step b, the drying temperature is 60-120 °C, and the drying time is 6-12 hours; the calcination temperature is 300-600 °C, and the calcination time is 2-5 hours.

8. The method for producing carbon nanotubes by low-temperature catalytic cracking of VOCs according to claim 1, characterized in that, In the temperature range of 200-400 °C, the carbon source after cracking VOCs is adsorbed on the surface of the multifunctional low-temperature nickel-based titanium dioxide catalyst, and carbon nanotubes CNTs are generated through a catalytic reaction. After the reaction, the carbon nanotubes CNTs are peeled off by an on-line ultrasonic assistance system.

9. The method for producing carbon nanotubes by catalytic cracking of VOCs at low temperature according to claim 8, characterized in that, The specific steps include: S1. Introduce VOCs into a reactor and carry out a cracking reaction in the presence of a multifunctional low-temperature nickel-based titanium dioxide catalyst; S2. Control the residence time of the carbon source on the catalyst surface to be 10-60 minutes; S3. Introduce CO, control the volume ratio of VOCs to CO to be 4:1-2:1, and the flow rate to be 10-100 ml / min; S4. Peel off the carbon nanotubes CNTs formed on the catalyst surface.

10. The method for producing carbon nanotubes by catalytic cracking of VOCs at low temperature according to claim 8, characterized in that The VOCs are aromatic hydrocarbon compounds, including at least one of benzene, toluene, and xylene; The carbon nanotubes CNTs are peeled off from the catalyst surface by an on-line ultrasonic assistance system, and the ultrasonic frequency is 20-100 kHz, and the ultrasonic power is 100-500 W.

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