Method for catalyzing dehydrogenation of methylcyclohexane by using composite catalyst

Abstract

The invention discloses a method for catalyzing methyl cyclohexane dehydrogenation by using a composite catalyst, belongs to the technical field of catalysis, and aims to solve the problems of low activity, poor stability and insufficient carbon deposition resistance of the existing methyl cyclohexane dehydrogenation catalyst. The method comprises two steps of catalyst pretreatment and dehydrogenation reaction, the composite catalyst is a modified CrN loaded Pt monatomic site catalyst, modified CrN is used as a carrier, and monatomic Pt is used as an active component; the modified CrN carrier is prepared through the four steps of zinc-chromium-cerium hydrotalcite preparation, silicon dioxide coating, nitridation and hydrofluoric acid etching and ascorbic acid modification, the oxygen vacancy of the carrier can be increased through the oxidation-reduction characteristic of the Ce element and the modification effect of ascorbic acid, and the metal-carrier interaction is enhanced. The catalyst realizes high conversion rate of methyl cyclohexane dehydrogenation reaction, high toluene selectivity and high long-term use stability, and is suitable for efficient release of hydrogen energy in a liquid organic hydrogen carrier technology.

Description

Technical Field

[0001] This invention relates to the field of catalysis technology, and more specifically to the application of a composite catalyst in the catalytic dehydrogenation of methylcyclohexane. Background Technology

[0002] With the global energy structure shifting towards cleaner and lower-carbon energy sources, hydrogen energy, as a highly efficient and pollution-free clean energy carrier, has attracted widespread attention for its development and utilization. Liquid organic hydrogen carrier (LOHC) technology, due to its advantages such as high hydrogen storage density, safe transportation, and good reversible recycling, has become an important direction for hydrogen energy storage and transportation. Among them, methylcyclohexane (MCH) is considered an ideal LOHC medium due to its high hydrogen storage density (6.2 wt%), low toxicity, and good compatibility with existing petrochemical infrastructure. The methylcyclohexane dehydrogenation reaction is the core of LOHC technology. This reaction is a strongly endothermic process that requires high temperatures, thus placing stringent requirements on the catalyst's activity, selectivity, stability, and resistance to carbon deposition.

[0003] Existing methylcyclohexane dehydrogenation catalysts mainly use noble metals such as Pt and Pd as active components, and Al2O3, CeO2, and hydrotalcite as supports. For example, patent document CN116417629B discloses a CrN-supported Pt single-atom site catalyst, which uses CrN as a support and loads Pt single atoms through atomic layer deposition, exhibiting excellent activity and resistance to CO poisoning in the electrocatalytic oxidation of hydrogen. However, when this catalyst is directly applied to the methylcyclohexane dehydrogenation reaction, it has the following shortcomings: (1) Poor support compatibility: Although CrN support has excellent conductivity and acid resistance, it has few oxygen vacancies on the surface and weak metal-support interaction (SMSI), which makes Pt single atoms easy to agglomerate in high-temperature dehydrogenation reaction and the catalyst has insufficient stability. (2) Low utilization of active sites: The dehydrogenation reaction of methylcyclohexane requires the support to provide certain acidic / basic sites to assist in the activation of CH bonds. The surface electronic properties of pure CrN support are difficult to meet the reaction requirements, resulting in low catalyst activity. Insufficient resistance to carbon buildup: At high temperatures, methylcyclohexane is prone to side reactions such as cracking and polymerization, which generate carbon deposits that cover the active sites of the catalyst, leading to rapid catalyst deactivation.

[0004] To address these issues, existing technologies often optimize catalyst performance by introducing promoters such as Ce and Mg or by modifying the support. For example, modifying the hydrotalcite support with CeO2 utilizes the redox properties of Ce and the oxygen vacancy effect to enhance the activity and stability of Pt-based catalysts. However, this approach uses a support with a relatively small specific surface area, making Pt particles prone to agglomeration, and the dispersion of Ce is difficult to control, thus limiting further improvements in catalyst performance.

[0005] Therefore, developing a methylcyclohexane dehydrogenation catalyst that combines high activity, high selectivity, high stability, and resistance to carbon deposition is key to promoting the industrial application of LOHC technology. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of existing methylcyclohexane dehydrogenation catalysts, such as low activity, poor stability, and insufficient resistance to carbon deposition, and to provide a method for catalyzing the dehydrogenation of methylcyclohexane using a composite catalyst.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A method for dehydrogenating methylcyclohexane using a composite catalyst includes the following steps: (1) Catalyst pretreatment: The composite catalyst is packed into a quartz tube in a fixed-bed reactor and introduced at a flow rate of 50-100 mL / min. The temperature was increased to 300-350℃ at a rate of 5℃ / min, and the reduction was carried out for 2-4 hours to ensure that the Pt species were completely reduced to the metallic state; then the solution was introduced at a flow rate of 50-100 mL / min. Cool to room temperature; (2) Maintain The carrier gas flow rate is 50-100 mL / min, and the reactor is adjusted to 320-380℃. Methylcyclohexane is pumped into the reactor at a rate of 0.08-0.15 mL / min using a high-pressure constant flow pump. After vaporization, it comes into contact with the catalyst to carry out a dehydrogenation reaction.

[0008] The purity of methylcyclohexane is ≥99.0%.

[0009] Among them, the composite catalyst is a modified CrN supported Pt single-atom site catalyst, with modified CrN as the support and single-atom Pt as the active component. The single-atom Pt is deposited on the modified CrN support and is atomically dispersed.

[0010] The preparation steps of the modified CrN support are as follows: S1. Preparation of Zinc-Chromium-Cerium Layered Double Hydrocarbons: Zinc, chromium, and cerium sources were dissolved in deionized water to form a mixed metal salt solution; sodium hydroxide and anhydrous sodium carbonate were dissolved in deionized water to prepare an alkaline solution; under stirring conditions of 60-80℃ and 300-500 r / min, the alkaline solution was added dropwise to the metal salt solution at a rate of 1-3 mL / min, and the mixture was stirred for 18-24 h to obtain zinc-chromium-cerium layered double hydrocarbons; wherein, , , The molar ratio is (2~4):1:(0.1~0.3).

[0011] S2. Silica Coating: Disperse zinc chromium cerium hydrotalcite in a 0.05-0.1 mol / L tetraethyl orthosilicate aqueous solution at a solid-liquid ratio of 1:(20-50) g / mL. After sonication for 5-10 min, stir for 1-12 h to hydrolyze tetraethyl orthosilicate to generate silica, which then coats the surface of the hydrotalcite. After centrifugation, dry at 60-80℃ for 4-8 h to obtain silica-coated zinc chromium cerium hydrotalcite.

[0012] S3. Nitriding and Etching: Zinc-chromium-cerium hydrotalcite coated with silica is placed in a quartz tube reactor and introduced at a flow rate of 100-200 mL / min. The temperature is increased to 1000-1100℃ at a rate of 5℃ / min and held for 8-10 hours to convert the metallic elements in the hydrotalcite into CrN-Ce composite nitrides; then the process is switched to... The atmosphere was allowed to cool naturally to room temperature to obtain a CrN-Ce composite support coated with silica. The composite support was then soaked in a 5-8% hydrofluoric acid aqueous solution for 2-3 hours. It was then washed 3-5 times alternately with distilled water and ethanol to remove residual etching agent. Finally, it was dried at a vacuum of -0.08 to -0.1 MPa and a temperature of 40-80°C for 2-24 hours to obtain the CrN-Ce composite support.

[0013] S4. Ascorbic acid modification: The CrN-Ce composite carrier is dispersed in an aqueous solution containing ascorbic acid, with a mass ratio of ascorbic acid to CrN-Ce composite carrier of 0.05-0.1:1. The mixture is stirred at 30-50℃ for 2-4 hours to partially... Restore to Meanwhile, defect sites are formed on the surface of the carrier; after centrifugation, the modified CrN carrier is dried at 60-80℃ for 4-8 hours.

[0014] Furthermore, the zinc source is selected from one or both of zinc chloride and zinc nitrate; the chromium source is selected from one or both of chromium chloride and chromium nitrate; and the cerium source is selected from one or both of cerium nitrate and cerium chloride.

[0015] Among them, atomic layer deposition was used to deposit single-atom Pt on a modified CrN support.

[0016] Preferably, the atomic layer deposition (ALD) method involves the following steps: using trimethyl(methylcyclopentadienyl)platinum as the Pt source, ozone as the oxidant, and high-purity nitrogen as the carrier gas; loading the modified CrN support into the ALD reactor, evacuating it, and then purging with high-purity nitrogen for 25-35 min; maintaining the reactor temperature at 60-70℃, the Pt source temperature at 60-70℃, and the carrier gas flow rate at 50-100 mL / min; sequentially performing Pt source pulses of 0.7-1 s, nitrogen purging for 10-20 s, ozone pulses of 5-10 s, and nitrogen purging for 10-20 s to complete one deposition cycle; and performing a total of 6-8 complete cycles. By controlling the number of cycles, the Pt loading can be adjusted to ensure that the Pt is atomically dispersed.

[0017] This invention uses CrN as the basic support and constructs a CrN-Ce composite support by introducing Ce element and modifying it with ascorbic acid. The introduction of Ce element can utilize... / The reversible redox properties increase the number of oxygen vacancies on the support surface, enhance the strong metal-support interaction, and inhibit Pt single-atom aggregation; simultaneously, ascorbic acid modification can increase the surface activity of CrN. The ratio enhances the electronic interaction with Pt, inhibits Pt migration, and thus enhances long-term stability. The reducing effect of ascorbic acid can further improve... and The ratio of [specific parameters] optimizes the electronic properties of the carrier surface, while increasing the specific surface area and pore volume of the carrier, thus promoting the adsorption and diffusion of methylcyclohexane molecules.

[0018] This invention selects a 5-8% hydrofluoric acid solution and sets the etching time to 2-3 hours. While etching away the silica coating layer, mesopores are formed on the CrN surface, exposing more CrN surface active sites and enhancing the adsorption capacity for reactants.

[0019] Compared with the prior art, the advantages of this invention are: applying the modified CrN-supported Pt single-atom site catalyst to the dehydrogenation reaction of methylcyclohexane, and simultaneously coordinating reaction conditions such as temperature and feed rate, to achieve efficient conversion of methylcyclohexane and highly selective generation of toluene. At the same time, the anti-carbon deposition properties of the modified CrN support effectively extend the catalyst's service life. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention are further described below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely for the purpose of helping to understand the technical content and effects of this invention and should not be considered as limitations on this invention. Example 1

[0021] A method for dehydrogenating methylcyclohexane using a composite catalyst, comprising the following steps: (1) Catalyst pretreatment: 0.5 g of composite catalyst is packed into a quartz tube in a fixed-bed reactor and introduced at a flow rate of 80 mL / min. The temperature was increased to 320℃ at 5℃ / min and reduced for 3 hours to ensure complete reduction of Pt species to the metallic state; then the flow rate was switched to 80 mL / min. Cool to room temperature; (2) Dehydrogenation reaction: maintain The carrier gas flow rate was 80 mL / min, and the reactor was adjusted to 350 °C. Methylcyclohexane with a purity of 99.5% was pumped into the reactor at a rate of 0.12 mL / min using a high-pressure constant flow pump. After vaporization, it came into contact with the catalyst to carry out a dehydrogenation reaction.

[0022] The preparation of the composite catalyst includes two steps: preparation of modified CrN support and atomic layer deposition of Pt.

[0023] The method for preparing the modified CrN support is as follows: Preparation of S1 zinc chromium cerium hydrotalcite: Weigh 11.88 g zinc nitrate hexahydrate (0.04 mol, zinc source), 4.00 g chromium nitrate nonahydrate (0.01 mol, chromium source), and 0.86 g cerium nitrate hexahydrate (0.002 mol, cerium source), dissolve in 100 mL deionized water to form a mixed metal salt solution. Weigh out sodium hydroxide and anhydrous sodium carbonate, dissolve them in deionized water to prepare a mixed alkaline solution (2 mol / L NaOH and 0.5 mol / L NaOH). (Volume ratio 2:1); The reaction system was heated to 70℃, the stirring speed was adjusted to 400r / min, and the alkaline solution was added dropwise to the metal salt solution at a rate of 2mL / min. The mixture was stirred continuously for 20h, and the zinc chromium cerium hydrotalcite was obtained by centrifugation. S2 Silica Coating: The above-mentioned hydrotalcite was dispersed in a 0.08 mol / L tetraethyl orthosilicate aqueous solution (solid-liquid ratio 1:30 g / mL), ultrasonicated for 8 min, and stirred at room temperature for 6 h; after centrifugation, it was dried in an oven at 60 ℃ for 6 h to obtain zinc chromium cerium hydrotalcite coated with silica. S3 Nitriding and Etching: A silica-coated hydrotalcite is loaded into a quartz tube reactor, and air is introduced... (Flow rate 150 mL / min), heat to 1050 °C at a rate of 5 °C / min, hold for 9 h; then switch to The atmosphere was naturally cooled to room temperature to obtain a CrN-Ce composite support coated with silica; the support was soaked in a 6% hydrofluoric acid aqueous solution for 2.5 h, washed 4 times alternately with distilled water and ethanol, and dried at -0.09 MPa vacuum and 60 °C for 12 h to obtain the CrN-Ce composite support. S4 Ascorbic acid modification: The CrN-Ce composite carrier was dispersed in an aqueous solution of ascorbic acid (mass ratio of ascorbic acid to carrier 0.08:1), stirred at 40℃ for 3h; after centrifugation, it was dried in an oven at 70℃ for 6h to obtain the modified CrN carrier.

[0024] The specific steps of atomic layer deposition of Pt are as follows: using trimethyl(methylcyclopentadienyl)platinum as the Pt source, ozone as the oxidant, and high-purity nitrogen as the carrier gas (flow rate 80 mL / min); the modified CrN support is loaded into the atomic layer deposition reactor, and after evacuation, high-purity nitrogen is purged for 30 min. The reactor and Pt source temperatures are set to 65℃, the Pt source pulse time is 0.8 s, nitrogen purging is 15 s, ozone pulse is 8 s, nitrogen purging is 15 s, and the deposition cycle is repeated 7 times to obtain the composite catalyst.

[0025] The catalyst pretreatment and dehydrogenation reaction conditions in Examples 2-3 and Comparative Examples 1-3 are the same, the only difference being that the catalyst preparation steps are slightly different. Example 2

[0026] Compared to Example 1, only step S1 was adjusted: the amount of cerium source was changed to 0.43 g of cerium nitrate hexahydrate (0.001 mol), and the mixed metal salt solution was added... The number of atomic layer deposition (Pt) cycles was changed to 6; the remaining steps (S2-S4, atomic layer deposition parameters) were completely consistent with Example 1. Example 3

[0027] Compared with Example 1, only step S4 was adjusted: the mass ratio of ascorbic acid to CrN-Ce composite carrier was changed to 0.1:1; the number of atomic layer deposition Pt cycles was changed to 8; the remaining steps (S1-S3, atomic layer deposition parameters) were completely consistent with Example 1.

[0028] Comparative Example 1 Step S4 (ascorbic acid modification) in Example 1 was omitted, and Pt was directly deposited on the CrN-Ce composite support prepared in S3 (7 cycles); the remaining steps (S1-S3, atomic layer deposition parameters) were the same as in Example 1.

[0029] Comparative Example 2 Compared to Example 1, only step S1 was adjusted: no cerium source was added, and the mixed metal salt solution was... (0.04 mol zinc nitrate hexahydrate, 0.01 mol chromium nitrate nonahydrate); retain the S4 ascorbic acid modification step (mass ratio 0.08:1); atomic layer deposition Pt cycle 7 times; the remaining steps (S2-S4, atomic layer deposition parameters) are the same as in Example 1.

[0030] Comparative Example 3 Compared with Example 1, only the etching conditions of step S3 were adjusted: the CrN-Ce composite carrier coated with silica was immersed in a 10% (mass fraction) hydrofluoric acid aqueous solution for 1 hour; the remaining steps (S1-S2, S4, atomic layer deposition parameters) were the same as in Example 1.

[0031] Comparative Example 4 Compared to Example 1, only the atomic layer deposition of Pt was performed 4 times, while all other steps were the same as in Example 1.

[0032] The conversion rate and selectivity were tested by gas chromatography analysis of the raw material and product concentrations, and the conversion rate of methylcyclohexane and the selectivity of toluene were calculated. The method for testing long-term stability is as follows: after a continuous reaction of 168 h, samples are taken and the conversion rate is analyzed by gas chromatography, and the decrease in conversion rate is calculated.

[0033] The test method for anti-carbon deposit ability is as follows: after the reaction is 168h, the amount of carbon deposit is determined by thermogravimetric analysis (TGA, air atmosphere, heating to 800℃ at 5℃ / min).

[0034]

[0035] Compared to the examples, Comparative Example 1, due to the lack of ascorbic acid modification, resulted in... The proportion decreased and the specific surface area decreased significantly, indicating a decline in the surface area of ​​CrN. As the proportion decreases, Pt tends to agglomerate, leading to insufficient conversion and long-term stability, and increased carbon deposition. Comparative Example 2, lacking Ce doping, suffers from a lack of available oxygen vacancies, resulting in insufficient active sites and the worst conversion and stability. Comparative Example 3, due to excessive etching of the support with high-concentration HF and short-time etching of residual silica, experiences a decrease in specific surface area and pore volume, leading to localized Pt aggregation and insufficient conversion and stability. In Comparative Example 4, the fewer atomic layer deposition cycles result in lower Pt loading, insufficient catalytic active centers, and a tendency for active sites to be lost, ultimately leading to poor conversion and stability.

[0036] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A method for dehydrogenating methylcyclohexane using a composite catalyst, characterized in that, The steps include: (1) Catalyst pretreatment: The composite catalyst is packed into the quartz tube of the fixed-bed reactor and introduced at a flow rate of 50-100 mL / min. The temperature was increased to 300-350℃ at a rate of 5℃ / min, and the reduction was carried out for 2-4 hours to ensure that the Pt species were completely reduced to the metallic state; then the solution was introduced at a flow rate of 50-100 mL / min. Cool to room temperature; (2) Maintain The carrier gas flow rate is 50-100 mL / min, and the reactor is adjusted to 320-380℃. Methylcyclohexane is pumped into the reactor at a rate of 0.08-0.15 mL / min using a high-pressure constant flow pump. After vaporization, it comes into contact with the catalyst to carry out a dehydrogenation reaction. Among them, the composite catalyst is a modified CrN supported Pt single-atom site catalyst, with modified CrN as the support and single-atom Pt as the active component. The single-atom Pt is deposited on the modified CrN support and is atomically dispersed.

2. The method according to claim 1, characterized in that, The purity of methylcyclohexane is ≥99.0%.

3. The method according to claim 1, characterized in that, The preparation steps of the modified CrN support are as follows: S1. Preparation of Zinc-Chromium-Cerium Layered Double Hydrocarbons: Zinc, chromium, and cerium sources were dissolved in deionized water to form a mixed metal salt solution; sodium hydroxide and anhydrous sodium carbonate were dissolved in deionized water to prepare an alkaline solution; under stirring conditions of 60-80℃ and 300-500 r / min, the alkaline solution was added dropwise to the metal salt solution at a rate of 1-3 mL / min, and the mixture was stirred for 18-24 h to obtain zinc-chromium-cerium layered double hydrocarbons; wherein, , , The molar ratio is (2~4):1:(0.1~0.3); S2. Silica Coating: Zinc chromium cerium hydrotalcite is dispersed in a 0.05-0.1 mol / L tetraethyl orthosilicate aqueous solution at a solid-liquid ratio of 1:(20-50) g / mL. After sonication for 5-10 min, the mixture is stirred for 1-12 h to hydrolyze the tetraethyl orthosilicate to generate silica, which then coats the surface of the hydrotalcite. After centrifugation, the mixture is dried at 60-80℃ for 4-8 h to obtain silica-coated zinc chromium cerium hydrotalcite. S3. Nitriding and Etching: Zinc-chromium-cerium hydrotalcite coated with silica is placed in a quartz tube reactor and introduced at a flow rate of 100-200 mL / min. The temperature is increased to 1000-1100℃ at a rate of 5℃ / min and held for 8-10 hours to convert the metallic elements in the hydrotalcite into CrN-Ce composite nitrides; then the process is switched to... The atmosphere was allowed to cool naturally to room temperature to obtain a CrN-Ce composite support coated with silica. The composite support was then soaked in a 5-8% hydrofluoric acid aqueous solution for 2-3 hours. It was then washed with distilled water and ethanol alternately 3-5 times to remove residual etching agent. Finally, it was dried at a vacuum of -0.08 to -0.1 MPa and a temperature of 40-80°C for 2-24 hours to obtain the CrN-Ce composite support. S4. Ascorbic acid modification: The CrN-Ce composite carrier is dispersed in an aqueous solution containing ascorbic acid, with a mass ratio of ascorbic acid to CrN-Ce composite carrier of 0.05-0.1:

1. The mixture is stirred at 30-50℃ for 2-4 hours to partially... Restore to Meanwhile, defect sites are formed on the surface of the carrier; after centrifugation, the modified CrN carrier is dried at 60-80℃ for 4-8 hours.

4. The method according to claim 3, characterized in that, The zinc source is selected from one or both of zinc chloride and zinc nitrate; the chromium source is selected from one or both of chromium chloride and chromium nitrate; and the cerium source is selected from one or both of cerium nitrate and cerium chloride.

5. The method according to claim 1, characterized in that, Single-atom Pt was deposited on a modified CrN support using atomic layer deposition.

6. The method according to claim 4, characterized in that, The steps of atomic layer deposition (ALD) are as follows: using trimethyl(methylcyclopentadienyl)platinum as the Pt source, ozone as the oxidant, and high-purity nitrogen as the carrier gas; loading the modified CrN support into the ALD reactor, evacuating it, and then purging with high-purity nitrogen for 25-35 min; maintaining the reactor temperature at 60-70℃, the Pt source temperature at 60-70℃, and the carrier gas flow rate at 50-100 mL / min; sequentially performing Pt source pulses of 0.7-1 s, nitrogen purging for 10-20 s, ozone pulses of 5-10 s, and nitrogen purging for 10-20 s to complete one deposition cycle; and performing a total of 6-8 complete deposition cycles.

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