Modified dendritic carbon nanotube-based monolithic catalyst and preparation method and application thereof

Abstract

The application relates to a modified dendritic carbon nanotube-based monolithic catalyst and a preparation method and application thereof, and belongs to the field of industrial catalysis. Nickel nanoparticles are prepared by wet chemical reduction of a nickel salt through a reducing agent, a foamed carbon silicon carbide skeleton matrix is soaked in a pre-prepared nickel nanoparticle colloidal aqueous solution, and is taken out and dried; then carbon nanotubes are grown in a tube furnace. The obtained product is subjected to acid washing, water washing to neutral, and drying; the dried solid is soaked in a nickel salt aqueous solution, taken out and dried, the obtained solid is placed in a quartz boat, and the carbon nanotubes are grown for the second time after pre-reduction in the tube furnace; the product is subjected to acid washing, water washing to neutral, and drying; the dried dendritic carbon nanotube-based monolithic catalyst is soaked in a modifier solution, taken out and dried, and calcined in nitrogen to obtain the modified dendritic carbon nanotube-based monolithic catalyst. The catalyst prepared by the application has excellent catalytic performance in an ethylbenzene direct dehydrogenation reaction and has a wide application prospect.

Description

Technical Field

[0001] The present invention relates to the field of industrial catalysis, and in particular to a modified branched carbon nanotube-based monolithic catalyst, a preparation method and an application thereof. Background Art

[0002] Nanocarbon materials, such as nanodiamonds, carbon nanotubes, graphene, carbon nanofibers, etc., have the advantages of high efficiency, environmental protection, low energy consumption, and corrosion resistance compared to traditional metal catalysts. At the same time, because they have a higher specific surface area, more structural defects and functional groups, they can themselves be used as catalysts to efficiently catalyze heterogeneous catalytic processes such as alkane conversion, chemical synthesis, and energy catalysis (Adv.Mater.2019,31,1803762). Therefore, nanocarbon catalysts are one of the most active research directions in the field of nanomaterials and catalysis. However, nanocarbon catalysts are generally in powder form, which can easily cause reactor blockage and generate a large pressure drop in industrial applications. One feasible way to solve this problem is to load it on a macroporous carrier to prepare a monolithic catalyst. The macroscopic shaping of the carrier is conducive to the escape of intermediate products, thereby reducing the formation of coke and maintaining the activity and stability of the catalyst.

[0003] In the direct dehydrogenation of ethylbenzene, nanodiamonds exhibit superior reactivity compared to industrial catalysts under the same reaction conditions (Angew. Chem. Int. Ed. 2010, 49, 8640–864). Currently, there have been many reports on the preparation of powdered nanodiamonds into monolithic catalysts (Chem. Mater. 2014, 26, 10, 3151–3161; Chem. Commun., 2014, 50, 7810—7812; ACS Appl. Mater. Interfaces 2022, 14, 17, 19315–19323). In our previous work, to make nanodiamonds more robust, we prepared a nanodiamond-based monolithic catalyst by loading nanodiamonds on a silicon carbide support coated with carbon nanotubes (Chinese Chemical Letters 34 2023 107808), which showed excellent catalytic performance in the direct dehydrogenation of ethylbenzene. However, the preparation process of nanodiamonds is complex and costly, which limits their widespread application on an industrial scale. This makes the search for low-cost and efficient nanocarbon material catalysts a research focus.

[0004] Carbon nanotubes (CNTs), with their unique structure and high surface area, have broad application prospects, including as composite material reinforcements, energy storage and conversion, biomaterials, and sensors, as well as various catalysts (Chem. Rev. 2019, 119, 1, 599–663). Previous research has shown that nitrogen and oxygen doping (ChemCatChem 2015, 7, 1135–1144; RSC Adv. 2015, 5, 53095–53099) can effectively enhance the catalytic performance of CNTs. Therefore, modified CNTs are an effective alternative to nanodiamonds. Monolithic nanocarbon catalysts can be prepared by growing CNTs on macroporous supports via chemical vapor deposition and subsequently modifying their surface chemical properties to produce them, showing promising applications. Among macroporous supports, silicon carbide (SiC) foam has been used as a monolithic support due to its open-pore structure, high thermal conductivity, and mechanical stability. Our previous work has investigated monolithic catalysts prepared by directly growing CNTs on SiC. However, due to the low surface area and chemically inert surface of silicon carbide, it is not conducive to the loading and dispersion of metals for growing carbon nanotubes, which results in less grown carbon nanotubes and poor catalytic activity compared to nanodiamond-based monolithic catalysts (Chinese Chemical Letters 34 2023 107808).

[0005] Therefore, it is necessary to find practical carbon nanotube-based monolithic catalysts and preparation methods to solve the problems of powdered nanocarbon materials being unable to be used as industrial catalysts and nanodiamonds being too expensive to be applied on a large scale. Summary of the Invention

[0006] In order to overcome the shortcomings of the prior art, the present invention discloses a modified branched carbon nanotube-based monolithic catalyst and a preparation method thereof. Inspired by the idea of ​​branches growing from a tree trunk, carbon nanotubes are grown twice on a foamed silicon carbide skeleton matrix by chemical vapor deposition technology, with thin tubes grown on thick tubes to form a branched carbon nanotube structure. Among them, the carbon nanotubes grown once, in addition to being active components, also play a role similar to a "coating", increasing the specific surface area of ​​the skeleton matrix, thereby promoting the dispersion of the nickel catalyst for the secondary growth of carbon nanotubes and growing more and thinner carbon nanotubes. Compared with the monolithic catalyst prepared by growing carbon nanotubes once on foamed silicon carbide, more active sites are exposed, which is beneficial to improving the catalytic performance of the nanocarbon-based monolithic catalyst. The modified branched carbon nanotube-based monolithic catalyst is prepared by further doping and modification, and the catalytic performance of the catalyst is further improved. The prepared monolithic catalyst exhibits excellent catalytic performance in the reaction of direct dehydrogenation of ethylbenzene to produce styrene.

[0007] The technical solutions of the present invention are as follows:

[0008] A method for preparing a modified dendritic carbon nanotube-based monolithic catalyst, comprising the following steps:

[0009] Step 1: Nickel nanoparticles are prepared by wet chemical reduction of a nickel salt using a reducing agent. A foamed silicon carbide skeleton matrix is ​​immersed in a pre-prepared aqueous colloidal solution of nickel nanoparticles, removed, and dried, resulting in Solid A. Step 2: Solid A is placed in a quartz boat and carbon nanotubes are grown in a tube furnace. The resulting product is first acid-washed, then washed with water until neutral, and dried, resulting in Solid B. Step 3: Solid B is immersed in an aqueous nickel salt solution, removed, and dried to obtain Solid C. Step 4: Solid C is placed in a quartz boat and pre-reduced in a tube furnace, where it undergoes secondary carbon nanotube growth. The product is then acid-washed, washed with water until neutral, and dried to obtain a dendritic carbon nanotube-based monolithic catalyst. Step 5: The dendritic carbon nanotube-based monolithic catalyst is immersed in a modifier solution, removed, dried, and calcined in nitrogen to obtain a modified dendritic carbon nanotube-based monolithic catalyst.

[0010] Furthermore, the wet chemical reduction process described in step 1 is carried out at a temperature of 20 to 200° C. and for a time of 10 to 360 minutes; and the drying process described in steps 1 to 5 is carried out at a temperature of 20 to 150° C. and for a time of 1 to 24 hours.

[0011] Furthermore, in the pre-reduction process described in step 4, the pre-reduction atmosphere is hydrogen / nitrogen gas, the flow rate is 5-250 ml / min, the volume concentration of hydrogen is 2-100%, the pre-reduction temperature is 20-600° C., and the pre-reduction time is 20-480 min;

[0012] Furthermore, in the process of growing carbon nanotubes described in steps 2 and 4, the temperature is 400-1000°C, the time is 5-720 min, the reaction gas is a mixture of hydrogen / carbon source (methane or ethylene) / inert gas (nitrogen or argon) in a volume ratio of 1:(0.5-10):(0.5-20), and the flow rate is 10-300 ml / min; the calcination temperature in step 5 is 300-900°C, the calcination time is 20-240 min, and the nitrogen flow rate is 10-200 ml / min.

[0013] Furthermore, the mass ratio of the reducing agent in step 1 to the nickel salt is 1:(0.25-5), and the mass ratio of silicon carbide in the foamed silicon carbide skeleton matrix to nickel nanoparticles to water is 1:(0.005-0.1):(2-4).

[0014] Furthermore, the mass ratio of silicon carbide: nickel salt: water contained in the solid B described in step 3 is 1: (0.0025-0.3): (2-4); and the soaking time described in steps 1 and 3 is 10-360 minutes.

[0015] Furthermore, the reducing agent used in step 1 is selected from hydrazine hydrate, sodium citrate, sodium borohydride, oxalic acid, ethylene glycol, propionic acid alcohol, tannic acid, citric acid, and glucose; and the nickel salt used in steps 1 and 3 is selected from nickel nitrate, nickel acetate, nickel chloride, and nickel sulfate.

[0016] Furthermore, the mass concentration of the modifier described in step 5 is 0.1-1 g / ml, the mass ratio of the dendritic carbon nanotube-based monolithic catalyst to the modifier is 1:(1-50), and the soaking time is 20-600 min. The modifier used in step 5 is selected from urea, ammonia water, melamine, nitric acid, ammonium nitrate, urotropine nitrate, melamine nitrate, phosphoric acid, phosphorous acid, hypophosphorous acid, ammonium phosphate, ammonium hypophosphite, ammonium dihydrogen phosphate, ammonium hydrogen phosphate, triethyl phosphate, phytic acid, and boric acid.

[0017] The modified branched carbon nanotube-based integral catalyst prepared by the present invention is used for direct dehydrogenation of ethylbenzene to produce styrene, and has excellent catalytic activity, selectivity and stability.

[0018] The present invention has the following beneficial effects:

[0019] (1) The present invention adopts a monolithic catalyst design and uses macroporous silicon carbide foam as the skeleton matrix, successfully overcoming the common clogging and pressure drop problems of traditional nano-carbon powder catalysts in industrial applications;

[0020] (2) The present invention forms a branched carbon nanotube structure by growing carbon nanotubes twice. Compared with carbon nanotubes grown once, the prepared catalyst has a branched carbon nanotube structure similar to branches growing on a tree trunk, which can expose more active sites and significantly enhance its catalytic activity.

[0021] (3) In the preparation of the present invention, the raw materials are abundant and cheap, and the preparation route is simple, which solves the problems of complex and high cost in the preparation process of nanodiamonds and finds an effective substitute, which is conducive to large-scale industrial application. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for describing the embodiments or the prior art.

[0023] Figure 1 This is a scanning electron microscope image of the modified dendritic carbon nanotube-based monolithic catalyst prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0024] The present invention is described below with reference to specific embodiments. It will be appreciated by those skilled in the art that these embodiments are intended to illustrate the present invention and are not intended to limit the scope of the present invention in any way.

[0025] Unless otherwise specified, the experimental methods used in the following examples are all conventional methods. Unless otherwise specified, the materials, reagents, etc. used in the following examples are all commercially available.

[0026] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0027] Example 1

[0028] Using hydrazine hydrate as a reducing agent, 1g of hydrazine hydrate and 0.6g of nickel nitrate were wet-chemically reduced at 70°C for 30 minutes to prepare nickel nanoparticles. 0.007g of nickel nanoparticles was then added to 1.4g of water to prepare a colloidal nickel nanoparticle aqueous solution. 0.7g of silicon carbide was immersed in the pre-prepared colloidal nickel nanoparticle aqueous solution for 10 minutes, removed, and dried at 80°C for 12 hours to obtain solid A. Solid A was placed in a quartz boat and carbon nanotubes were grown in a tube furnace at 500°C for 360 minutes using a mixture of hydrogen / methane / nitrogen in a volume ratio of 1:0.5:0.5 at a flow rate of 10ml / min. After removal, the sample was washed with concentrated nitric acid, then washed with water until neutral, and dried at 100°C for 8 hours to obtain solid B. 0.09g of nickel nitrate was added to 1.4g of water to prepare a nickel nitrate aqueous solution. Solid B was immersed in the nickel nitrate aqueous solution for 30 minutes, removed, and dried at 80°C for 12 hours to obtain solid C. Solid C was placed in a quartz boat and pre-reduced in a tube furnace using a hydrogen / nitrogen reducing atmosphere at a flow rate of 50ml / min, a hydrogen concentration of 5% by volume, a temperature of 200°C, and a reduction time of 20 minutes. Carbon nanotubes were then grown at 500°C for 360 minutes using a mixture of hydrogen / methane / nitrogen at a volume ratio of 1:0.5:0.5 at a flow rate of 10ml / min. After removal, the solid was first washed with concentrated nitric acid and then with water until neutral. The resulting solid was then dried at 100°C for 8 hours to obtain a dendritic carbon nanotube-based monolithic catalyst. The mass of the grown carbon nanotubes was weighed to be 0.01 g, and the mass of urea was 0.2 g. The branched carbon nanotube-based monolithic catalyst was immersed in a 0.1 g / ml melamine nitrate solution for 90 min, taken out, dried at 80°C for 12 h, and calcined in a tubular furnace under a nitrogen atmosphere for 120 min, wherein the calcination temperature was 600°C and the nitrogen flow rate was 10 ml / min, to obtain a modified branched carbon nanotube-based monolithic catalyst, denoted as S1.

[0029] Example 2

[0030] Using propionic acid alcohol as a reducing agent, 2g of propionic acid alcohol and 0.5g of nickel chloride were wet-chemically reduced at 20°C for 360 minutes to prepare nickel nanoparticles. 0.0035g of nickel nanoparticles were then added to 1.7g of water to prepare a colloidal nickel nanoparticle aqueous solution. 0.7g of silicon carbide was immersed in this pre-prepared colloidal nickel nanoparticle aqueous solution for 30 minutes and then dried at 20°C for 24 hours to obtain solid A. Solid A was placed in a quartz boat and carbon nanotubes were grown in a tube furnace at 700°C for 360 minutes using a mixture of hydrogen / methane / nitrogen in a volume ratio of 1:10:20 at a flow rate of 300ml / min. After removal, the sample was washed with concentrated nitric acid, then washed with water until neutral, and dried at 150°C for 60 minutes to obtain solid B. 0.1g of nickel chloride was added to 1.7g of water to prepare a nickel chloride aqueous solution. Solid B was immersed in the nickel chloride aqueous solution for 10 minutes, removed, and dried at 60°C for 5 hours to obtain solid C. Solid C was placed in a quartz boat and pre-reduced in a tube furnace using a hydrogen / nitrogen reducing atmosphere at a flow rate of 5ml / min, a hydrogen concentration of 5% by volume, a temperature of 600°C, and a reduction time of 20 minutes. Carbon nanotubes were then grown at 700°C for 360 minutes using a mixture of hydrogen / methane / nitrogen in a volume ratio of 1:10:20 at a flow rate of 300ml / min. After removal, the solid was washed with concentrated nitric acid, then washed with water until neutral, and dried at 100°C for 8 hours to obtain a dendritic carbon nanotube-based monolithic catalyst. The mass of the grown carbon nanotubes was weighed to be 0.012 g, and the mass of urotropine nitrate was weighed to be 0.5 g. The branched carbon nanotube-based monolithic catalyst was immersed in a 0.2 g / ml urotropine nitrate solution for 240 min, taken out, dried at 100° C. for 12 h, and calcined in nitrogen for 60 min, wherein the calcination temperature was 800° C. and the nitrogen flow rate was 50 ml / min, to obtain a modified branched carbon nanotube-based monolithic catalyst, recorded as S2.

[0031] Example 3

[0032] Using oxalic acid as a reducing agent, 0.5g of oxalic acid and 2.5g of nickel chloride were wet-chemically reduced at 200°C for 10 minutes to prepare nickel nanoparticles. A colloidal solution of nickel nanoparticles was prepared by adding 0.07g of nickel nanoparticles to 2.8g of water. 0.7g of silicon carbide was immersed in the prepared colloidal solution for 360 minutes and then dried at 50°C for 12 hours to obtain solid A. Solid A was placed in a quartz boat and carbon nanotubes were grown in a tube furnace at 800°C for 600 minutes using a mixture of hydrogen / methane / nitrogen (1:7:15 by volume) at a flow rate of 150ml / min. After removal, the sample was washed with concentrated nitric acid, then washed with water until neutral, and dried at 120°C for 3 hours to obtain solid B. 0.1g of nickel chloride was added to 2.8g of water to prepare a nickel chloride aqueous solution. Solid B was immersed in the nickel chloride aqueous solution for 360 minutes, removed, and dried at 120°C for 2 hours to obtain solid C. Solid C was placed in a quartz boat and pre-reduced in a tube furnace using a hydrogen / nitrogen reducing atmosphere at a flow rate of 20ml / min, a hydrogen concentration of 15% by volume, a temperature of 500°C, and a reduction time of 200 minutes. Carbon nanotubes were then grown at 800°C for 600 minutes using a mixture of hydrogen / methane / nitrogen in a volume ratio of 1:7:15 at a flow rate of 150ml / min. After removal, the solid was washed with concentrated nitric acid, then washed with water until neutral, and dried at 120°C for 3 hours to obtain a dendritic carbon nanotube-based monolithic catalyst. The mass of the grown carbon nanotubes was weighed to be 0.01 g, and the mass of ammonium nitrate was 0.2 g. The branched carbon nanotube-based integral catalyst was immersed in a 0.5 g / ml ammonium nitrate solution for 60 min, taken out, dried at 100°C for 12 h, and calcined in a tubular furnace under a nitrogen atmosphere for 240 min, wherein the calcination temperature was 600°C and the nitrogen flow rate was 130 ml / min, to obtain a modified branched carbon nanotube-based integral catalyst, recorded as S3.

[0033] Example 4

[0034] Using sodium borohydride as a reducing agent, 1g of sodium borohydride and 0.8g of nickel acetate were wet-chemically reduced at 150°C for 120 minutes to prepare nickel nanoparticles. 0.006g of nickel nanoparticles were then added to 1.4g of water to prepare a colloidal nickel nanoparticle aqueous solution. 0.7g of silicon carbide was immersed in this pre-prepared colloidal nickel nanoparticle aqueous solution for 80 minutes, removed, and dried at 120°C for 6 hours to obtain solid A. Solid A was placed in a quartz boat and carbon nanotubes were grown at 850°C in a tube furnace for 360 minutes using a mixture of hydrogen / methane / nitrogen in a volume ratio of 1:3:12 at a flow rate of 250ml / min. After removal, the sample was washed with concentrated nitric acid, then washed with water until neutral, and dried at 130°C for 2 hours to obtain solid B. 0.2g of nickel acetate was added to 1ml of water to prepare a nickel acetate aqueous solution. Solid B was immersed in the nickel acetate aqueous solution for 120 minutes, removed, and dried at 130°C for 1 hour to obtain solid C. Solid C was placed in a quartz boat and pre-reduced in a tube furnace using a hydrogen / nitrogen atmosphere at a flow rate of 30ml / min, a hydrogen concentration of 30% by volume, a temperature of 300°C, and a reduction time of 300 minutes. Carbon nanotubes were then grown at 850°C for 360 minutes using a mixture of hydrogen / methane / nitrogen in a volume ratio of 1:3:12 at a flow rate of 250ml / min. After removal, the solid was washed with concentrated nitric acid, then rinsed with water until neutral, and dried at 110°C for 4 hours to obtain a dendritic carbon nanotube-based monolithic catalyst. The mass of the grown carbon nanotubes was weighed to be 0.013 g, and the mass of melamine nitrate was 0.26 g. The branched carbon nanotube-based monolithic catalyst was immersed in a 1 g / ml melamine nitrate solution for 350 min, taken out, dried at 130°C for 6 h, and calcined in a tubular furnace under a nitrogen atmosphere for 20 min, wherein the calcination temperature was 700°C and the nitrogen flow rate was 70 ml / min, to obtain a modified branched carbon nanotube-based monolithic catalyst, recorded as S4.

[0035] Example 5

[0036] Using tannic acid as a reducing agent, 1g of tannic acid and 2g of nickel sulfate were wet-chemically reduced at 50°C for 150 minutes to prepare nickel nanoparticles. 0.008g of nickel nanoparticles were then added to 2.0g of water to prepare a colloidal nickel nanoparticle aqueous solution. 0.7g of silicon carbide was immersed in this pre-prepared colloidal nickel nanoparticle aqueous solution for 150 minutes, removed, and dried at 150°C for 1 hour to obtain solid A. Solid A was placed in a quartz boat and carbon nanotubes were grown in a tube furnace at 1000°C for 5 minutes using a mixture of hydrogen / methane / nitrogen in a volume ratio of 1:3:5 at a flow rate of 250ml / min. After removal, the sample was washed with concentrated nitric acid, then washed with water until neutral, and dried at 130°C for 2 hours to obtain solid B. 0.15g of nickel sulfate was added to 1ml of water to prepare a nickel sulfate aqueous solution. Solid B was immersed in the nickel sulfate aqueous solution for 240 minutes, removed, and dried at 130°C for 1 hour to obtain solid C. Solid C was placed in a quartz boat and pre-reduced in a tube furnace using a hydrogen / nitrogen reducing atmosphere at a flow rate of 200ml / min, a hydrogen concentration of 50% by volume, a temperature of 400°C, and a reduction time of 180 minutes. Carbon nanotubes were then grown at 1000°C for 5 minutes using a mixture of hydrogen / methane / nitrogen in a volume ratio of 1:3:5 at a flow rate of 250ml / min. After removal, the solid was washed with concentrated nitric acid, then rinsed with water until neutral, and dried at 110°C for 4 hours to obtain a dendritic carbon nanotube-based monolithic catalyst. The mass of the grown carbon nanotubes was weighed to be 0.01 g, and the mass of melamine nitrate was weighed to be 0.2 g. The branched carbon nanotube-based monolithic catalyst was immersed in a 0.3 g / ml melamine nitrate solution for 160 min, taken out, dried at 110°C for 6 h, and calcined in a tubular furnace under a nitrogen atmosphere for 60 min, wherein the calcination temperature was 500°C and the nitrogen flow rate was 200 ml / min, to obtain a modified branched carbon nanotube-based monolithic catalyst, denoted as S5.

[0037] Example 6

[0038] Using citric acid as a reducing agent, 2g of citric acid and 2g of nickel sulfate were wet-chemically reduced at 70°C for 60 minutes to prepare nickel nanoparticles. 0.012g of nickel nanoparticles was then added to 2.1g of water to prepare a colloidal nickel nanoparticle aqueous solution. 0.7g of silicon carbide was immersed in this pre-prepared colloidal nickel nanoparticle aqueous solution for 110 minutes, removed, and dried at 110°C for 3 hours to obtain solid A. Solid A was placed in a quartz boat and carbon nanotubes were grown in a tube furnace at 600°C for 180 minutes using a mixture of hydrogen / methane / nitrogen in a volume ratio of 1:10:1 at a flow rate of 250ml / min. After removal, the sample was washed with concentrated nitric acid, then washed with water until neutral, and dried at 130°C for 2 hours to obtain solid B. 0.18g of nickel sulfate was added to 1ml of water to prepare a nickel sulfate aqueous solution. Solid B was immersed in the nickel sulfate aqueous solution for 120 minutes, removed, and dried at 130°C for 1 hour to obtain solid C. Solid C was placed in a quartz boat and pre-reduced in a tube furnace using a hydrogen / nitrogen reducing atmosphere at a flow rate of 200ml / min, a hydrogen concentration of 25% by volume, a temperature of 350°C, and a reduction time of 240 minutes. Carbon nanotubes were then grown at 600°C for 180 minutes using a mixture of hydrogen / methane / nitrogen in a 1:10:1 volume ratio at a flow rate of 250ml / min. After removal, the solid was washed with concentrated nitric acid, then washed with water until neutral, and dried at 110°C for 4 hours to obtain a dendritic carbon nanotube-based monolithic catalyst. The mass of the grown carbon nanotubes was weighed to be 0.01 g, and the mass of phytic acid was weighed to be 0.15 g. The branched carbon nanotube-based integral catalyst was immersed in a 0.4 g / ml phytic acid solution for 400 min, taken out, dried at 110°C for 6 h, and calcined in a tubular furnace under a nitrogen atmosphere for 60 min, wherein the calcination temperature was 900°C and the nitrogen flow rate was 30 ml / min, to obtain a modified branched carbon nanotube-based integral catalyst, recorded as S6.

[0039] Example 7

[0040] Using glucose as a reducing agent, 1g of glucose and 0.6g of nickel nitrate were wet-chemically reduced at 50°C for 30 minutes to prepare nickel nanoparticles. 0.024g of nickel nanoparticles were then added to 1.0g of water to prepare a colloidal nickel nanoparticle aqueous solution. 0.7g of silicon carbide was immersed in the pre-prepared colloidal nickel nanoparticle aqueous solution for 10 minutes, removed, and dried at 90°C for 7 hours to obtain solid A. Solid A was placed in a quartz boat and carbon nanotubes were grown at 650°C in a tube furnace for 240 minutes using a mixture of hydrogen / methane / nitrogen in a volume ratio of 1:10:10 at a flow rate of 100ml / min. After removal, the sample was washed with concentrated nitric acid, then washed with water until neutral, and dried at 130°C for 2 hours to obtain solid B. 0.009g of nickel nitrate was added to 1ml of water to prepare a nickel nitrate aqueous solution. Solid B was immersed in the nickel nitrate aqueous solution for 60 minutes, removed, and dried at 130°C for 1 hour to obtain solid C. Solid C was placed in a quartz boat and pre-reduced in a tube furnace using a hydrogen / nitrogen reducing atmosphere at a flow rate of 250ml / min, a hydrogen concentration of 45% by volume, a temperature of 400°C, and a reduction time of 60 minutes. Carbon nanotubes were then grown at 650°C for 240 minutes using a mixture of hydrogen / methane / nitrogen in a 1:10:10 volume ratio at a flow rate of 100ml / min. After removal, the solid was washed with concentrated nitric acid, then washed with water until neutral, and dried at 90°C for 6 hours to obtain a dendritic carbon nanotube-based monolithic catalyst. The mass of the grown carbon nanotubes was weighed to be 0.015 g, and the mass of ammonium dihydrogen phosphate was weighed to be 0.45 g. The branched carbon nanotube-based monolithic catalyst was immersed in a 0.7 g / ml ammonium dihydrogen phosphate solution for 150 min, taken out, dried at 110°C for 6 h, and calcined in a tubular furnace under a nitrogen atmosphere for 100 min, wherein the calcination temperature was 500°C and the nitrogen flow rate was 30 ml / min, to obtain a modified branched carbon nanotube-based monolithic catalyst, recorded as S7.

[0041] Example 8

[0042] Nickel nitrate was wet-chemically reduced with ethylene glycol as a reducing agent (2.5g ethylene glycol and 3g nickel acetate) at 80°C for 70 minutes to prepare nickel nanoparticles. A colloidal solution of nickel nanoparticles was prepared by adding 0.036g of nickel nanoparticles to 1.4g of water. 0.7g of silicon carbide was immersed in the pre-prepared colloidal solution for 10 minutes, removed, and dried at 90°C for 7 hours to obtain solid A. Solid A was placed in a quartz boat and carbon nanotubes were grown in a tube furnace at 900°C for 120 minutes using a mixture of hydrogen / methane / nitrogen (1:10:5 by volume) at a flow rate of 300ml / min. After removal, the sample was washed with concentrated nitric acid, then washed with water until neutral, and dried at 110°C for 3 hours to obtain solid B. 0.2g of nickel acetate was added to 1ml of water to prepare a nickel acetate aqueous solution. Solid B was immersed in the nickel acetate aqueous solution for 150 minutes, removed, and dried at 130°C for 1 hour to obtain solid C. Solid C was placed in a quartz boat and pre-reduced in a tube furnace using a hydrogen / nitrogen reducing atmosphere at a flow rate of 250ml / min, a hydrogen concentration of 20% by volume, a temperature of 600°C, and a reduction time of 450 minutes. Carbon nanotubes were then grown at 900°C for 120 minutes using a mixture of hydrogen / methane / nitrogen in a volume ratio of 1:10:5 at a flow rate of 300ml / min. After removal, the solid was washed with concentrated nitric acid, then washed with water until neutral, and dried at 60°C for 12 hours to obtain a dendritic carbon nanotube-based monolithic catalyst. The mass of the grown carbon nanotubes was weighed to be 0.01 g, and the mass of phosphoric acid was 0.5 g. The branched carbon nanotube-based monolithic catalyst was immersed in a 0.1 g / ml phosphoric acid solution for 600 min, taken out, dried at 120°C for 4 h, and calcined in a tubular furnace under a nitrogen atmosphere for 180 min, wherein the calcination temperature was 300°C and the nitrogen flow rate was 30 ml / min, to obtain a modified branched carbon nanotube-based monolithic catalyst, which was recorded as S8.

[0043] Comparative Example 1

[0044] Using hydrazine hydrate as a reducing agent, 1g of hydrazine hydrate and 0.6g of nickel nitrate were wet-chemically reduced at 70°C for 30 minutes to prepare nickel nanoparticles. 0.007g of nickel nanoparticles was then added to 1.4g of water to prepare a colloidal solution of nickel nanoparticles. 0.7g of silicon carbide was immersed in the pre-prepared colloidal solution for 10 minutes, removed, and dried at 80°C for 12 hours to obtain solid A. Solid A was placed in a quartz boat and carbon nanotubes were grown in a tube furnace at 500°C for 360 minutes using a mixture of hydrogen / methane / nitrogen in a volume ratio of 1:0.5:0.5 at a flow rate of 10ml / min. After removal, the solid was first washed with concentrated nitric acid, then washed with water until neutral, and dried at 100°C for 8 hours to obtain a carbon nanotube-based monolithic catalyst, designated D1.

[0045] Comparative Example 2

[0046] 0.09g of nickel nitrate was added to 1.4g of water to prepare a nickel nitrate aqueous solution. 0.7g of silicon carbide was immersed in the nickel nitrate aqueous solution for 30 minutes, removed, and dried at 80°C for 12 hours to obtain solid C. Solid C was placed in a quartz boat and pre-reduced in a tube furnace using a hydrogen / nitrogen reducing atmosphere at a flow rate of 50ml / min, a hydrogen concentration of 5%, a temperature of 200°C, and a reduction time of 20 minutes. Carbon nanotubes were then grown at 500°C for 360 minutes using a mixture of hydrogen / methane / nitrogen at a volume ratio of 1:0.5:0.5 at a flow rate of 10ml / min. After removal, the product was washed with concentrated nitric acid, then washed with water until neutral, and dried at 100°C for 8 hours to obtain a carbon nanotube-based monolithic catalyst, designated D2.

[0047] Example 9

[0048] The prepared monolithic catalyst (S1-8, D1-2) was placed in a reaction tube of a fixed-bed reactor, and a 2.8% ethylbenzene / nitrogen mixture was introduced at 550°C with a gas flow rate of 10 ml / min. The performance test of the direct dehydrogenation of ethylbenzene was carried out, and the steady-state reaction results of the reaction for 20 hours are listed in Table 1 (from GC analysis results). As can be seen from the table, the prepared modified branched carbon nanotube-based monolithic catalysts showed a higher styrene yield and very high selectivity, and have a very good prospect. Comparing S1-S8 with D1 and D2, it can be seen that, relative to the once-long tube and unmodified carbon nanotube-based monolithic catalysts, the modified branched carbon nanotube-based monolithic catalysts obtained by growing carbon nanotubes twice and modified have a significantly improved styrene yield.

[0049] Table 1 Catalytic performance of the catalysts prepared in the above examples in the direct dehydrogenation of ethylbenzene

[0050]

[0051]

Claims

1. A method for preparing a modified dendritic carbon nanotube-based monolithic catalyst, characterized in that: The preparation comprises the following steps: Step 1: nickel nanoparticles are prepared by wet chemical reduction of nickel salt using a reducing agent, and the foamed silicon carbide skeleton matrix is ​​immersed in a pre-prepared nickel nanoparticle colloidal aqueous solution, taken out and dried, and recorded as solid A; Step 2: solid A is placed in a quartz boat, and carbon nanotubes are grown in a tube furnace; the obtained product is first acid-washed, then washed with water to neutrality, and dried, and recorded as solid B; Step 3: solid B is immersed in a nickel salt aqueous solution, taken out, and dried to obtain solid C; Step 4: solid C is placed in a quartz boat, and in a tube furnace, after pre-reduction, secondary carbon nanotubes are grown. The tube is acid-washed, then washed with water until neutral, and dried to obtain a branched carbon nanotube-based monolithic catalyst; step 5, immersing the branched carbon nanotube-based monolithic catalyst in a modifier solution, taking it out, drying it, and calcining it in nitrogen to obtain a modified branched carbon nanotube-based monolithic catalyst; the process of growing carbon nanotubes described in steps 2 and 4, the temperature is 400~1000℃, the time is 5~720min, the reaction gas is a mixture of hydrogen / carbon source / inert gas with a volume ratio of 1:(0.5~10):(0.5~20), and the flow rate is 10~300 ml / min; the modifier used in step 5 is urea, ammonia water, melamine, nitric acid, ammonium nitrate, urotropine nitrate, melamine nitrate, phosphoric acid, phosphorous acid, hypophosphorous acid, ammonium phosphate, ammonium hypophosphite, ammonium dihydrogen phosphate, ammonium hydrogen phosphate, triethyl phosphate, phytic acid, or boric acid.

2. The method for preparing a modified dendritic carbon nanotube-based monolithic catalyst according to claim 1, characterized in that: The wet chemical reduction process described in step 1 is carried out at a temperature of 20-200° C. and for a time of 10-360 min. The drying process described in steps 1 to 5 is carried out at a temperature of 20-150° C. and for a time of 1-24 h.

3. The method for preparing a modified dendritic carbon nanotube-based monolithic catalyst according to claim 1, characterized in that: In the pre-reduction process described in step 4, the pre-reduction atmosphere is hydrogen / nitrogen gas, the flow rate is 5~250 ml / min, the volume concentration of hydrogen is 2~100%, and is not 100%, the pre-reduction temperature is 20~600°C, and the pre-reduction time is 20~480min.

4. The method for preparing a modified dendritic carbon nanotube-based monolithic catalyst according to claim 1, wherein: The calcination temperature in step 5 is 300-900° C., the calcination time is 20-240 min, and the nitrogen flow rate is 10-200 ml / min.

5. The method for preparing a modified dendritic carbon nanotube-based monolithic catalyst according to claim 1, characterized in that: The mass ratio of the reducing agent in step 1 to the nickel salt is 1:(0.25-5), and the mass ratio of silicon carbide in the foamed silicon carbide skeleton matrix to nickel nanoparticles to water is 1:(0.005-0.1):(2-4).

6. The method for preparing a modified dendritic carbon nanotube-based monolithic catalyst according to claim 1, characterized in that: The mass ratio of silicon carbide: nickel salt: water in the solid B described in step 3 is 1: (0.0025-0.3): (2-4); the immersion time described in steps 1 and 3 is 10-360 minutes.

7. The method for preparing a modified dendritic carbon nanotube-based monolithic catalyst according to claim 1, characterized in that: The reducing agent used in step 1 is selected from hydrazine hydrate, sodium citrate, sodium borohydride, oxalic acid, ethylene glycol, tannic acid, citric acid, and glucose; the nickel salt used in steps 1 and 3 is selected from nickel nitrate, nickel acetate, nickel chloride, and nickel sulfate.

8. The method for preparing a modified dendritic carbon nanotube-based monolithic catalyst according to claim 1, characterized in that: The mass concentration of the modifier described in step 5 is 0.1~1g / ml, the mass ratio of the dendritic carbon nanotube-based monolithic catalyst:modifier is 1:(1~50), and the immersion time is 20~600min.

9. A modified dendritic carbon nanotube-based monolithic catalyst, characterized in that: The modified branched carbon nanotube-based monolithic catalyst is prepared by the preparation method described in any one of claims 1-8.

10. Use of a modified dendritic carbon nanotube-based monolithic catalyst prepared by the preparation method according to any one of claims 1 to 8, characterized in that: Used to catalyze the direct dehydrogenation of ethylbenzene to produce styrene.

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