Preparation method and application of ammonium citrate modified mesoporous SBA-15 supported Ni catalyst

Abstract

The invention discloses a preparation method and application of an ammonium citrate modified mesoporous SBA-15 supported Ni catalyst, and belongs to the field of catalytic materials. By adjusting the molar ratio of ammonium citrate to the nickel precursor, the distribution state of nickel species on the carrier is accurately regulated and controlled, so that the weight percentage of nickel is 6%, and the balance is the SBA-15 molecular sieve; under the optimized Ni loading capacity, CA / Ni < 2 + > molar ratio and 800 DEG C reaction conditions, the prepared catalyst shows excellent methane dry reforming performance, the CH4 conversion rate reaches 95%, the CO2 conversion rate reaches 98%, the hydrogen-carbon ratio reaches 0.85, and the catalyst can stably run within 100 h without inactivation signs. The excellent performance is attributed to good nickel dispersibility, small nickel nanoparticles, strong metal-carrier interaction and moderate alkaline site concentration of the catalyst, so that efficient synergy of CH4 dissociation and CO2 activation processes is realized. The method has the advantages of cheap raw materials, simple operation and large-scale industrial application potential through a one-step impregnation synthesis method.

Description

TECHNICAL FIELD

[0001] The application relates to a preparation method and application of an ammonium citrate modified mesoporous SBA-15 supported Ni catalyst, and belongs to the field of catalytic materials. BACKGROUND

[0002] As one of the most promising reactions for producing syngas, dry reforming of methane (DRM, CH4+CO2→2H2+2CO, ΔH 0 298K =+247 kJ / mol), which can produce high-value-added chemicals by simultaneously using two major greenhouse gases (methane and carbon dioxide) causing environmental problems, has attracted great interest from scholars.

[0003] Although noble metal catalysts (such as Pt, Rh, Ru, Ir, etc.) have excellent catalytic activity, good stability, and strong resistance to carbon deposition and sintering for the DRM reaction, the high cost ultimately limits its large-scale application. Unlike this, as an element of the same main group, Ni is considered to be an ideal catalytic material for large-scale industrial application in the DRM reaction due to its low price and good breaking ability of C-C bond and C-H bond. However, the DRM reaction is usually carried out at high temperature, and according to the Ostwald ripening mechanism, metal nickel is prone to migration and sintering at high temperature to form large nickel particles / clusters, and the larger particle size of nickel has a lower carbon saturation degree, so the driving force for carbon diffusion through the nickel crystal is enhanced, and with the occurrence of CH4 cracking, CO disproportionation and other side reactions, coke is more likely to deposit and wrap the metal, causing catalyst deactivation. Therefore, scholars focus on achieving a breakthrough in stabilizing nickel-based catalysts through the design and development of nanomaterials.

[0004] Loading nickel nanoparticles on inert SiO2 is a common strategy in the art. Mesoporous silica SBA-15 is widely used due to its highly ordered two-dimensional hexagonal structure, narrow pore size distribution, thick pore wall and high specific surface area. Researchers have explored various synthesis routes (such as impregnation method, hydrothermal method, deposition precipitation method, etc.) to develop active and stable nickel-based catalysts, aiming to reduce its sintering and coking tendency. Among them, the impregnation method is widely used due to its simple operation. However, the traditional impregnation method usually causes problems such as uneven dispersion of active sites, large nickel particle size, weak metal-support interaction, etc. The problems brought by the traditional impregnation method are that the catalyst prepared by the traditional impregnation method has low DRM activity and is easy to deactivate, which restricts its industrial application. SUMMARY

[0005] The application aims to provide a preparation method and application of a nickel-based catalyst with simple preparation process, excellent catalytic activity and good stability, by precisely regulating the coordination chemical environment of a nickel precursor and optimizing the distribution state of nickel species on a carrier, so that a catalytic material with high nickel dispersion, small-size nickel nanoparticles, strong metal-carrier interaction and moderate alkalinity is obtained.

[0006] To achieve the above technical purposes, the application discloses a preparation method of an ammonium citrate modified mesoporous SBA-15 supported Ni catalyst, and the steps are as follows: S1, preparing a nickel nitrate hexahydrate solution and an ammonium citrate solution with a molar ratio of ammonium citrate to nickel ions being (0.1-2):1; S2, adding the nickel nitrate hexahydrate solution drop by drop into the continuously stirred ammonium citrate solution to slow down the change of the local concentration gradient, so that the complexing process is more uniform and controllable; after the addition is completed, stirring for 7h to obtain a mixed solution; S3, adding the mixed solution into SBA-15 molecular sieves, adding 8.8ml of the mixed solution per gram of SBA-15 molecular sieves, then forming a uniform suspension by ultrasonic mixing, and obtaining a white powder by rotary evaporation and drying of the suspension; S4, placing the obtained white powder in a muffle furnace in an air atmosphere for calcination treatment to cover a carbon layer on the nickel precursor to derive fine nickel nanoparticles; S5, grinding, tabletting and sieving the product obtained by calcination, so as to obtain the ammonium citrate modified mesoporous SBA-15 supported Ni catalyst.

[0007] Further, in the preparation of the nickel nitrate hexahydrate solution in S1, the concentration of nickel is 15mol / L, and the concentration of ammonium citrate used in the preparation of the ammonium citrate solution is 0.03mol / L-0.52mol / L.

[0008] Further, the stirring speed is 300rpm during the process of slowly adding the nickel nitrate hexahydrate solution drop by drop into the continuously stirred ammonium citrate solution.

[0009] Further, before adding the mixed solution into the SBA-15 molecular sieves, the SBA-15 needs to be dried in a 120℃ air-blast drying oven for 6-12h to eliminate the adsorbed water in the SBA-15 carrier; and an appropriate amount of liquid is sucked to ensure that the weight percentage of Ni is 6%.

[0010] Further, the rotary evaporation conditions are a rotation speed of 100rpm and a temperature of 70℃; and the drying is performed in an air-blast drying oven with a temperature of 60℃ for 6-12h.

[0011] Furthermore, the dried white powder was placed in a muffle furnace under an air atmosphere for calcination treatment to coat the nickel precursor with a carbon layer to derive fine nickel nanoparticles; the calcination treatment temperature was 600℃, the heating rate was 5℃ / min, and it was held at 600℃ for 4h.

[0012] Furthermore, the catalyst is sieved to 40-60 mesh to ensure full exposure of active sites and improve mass transfer efficiency.

[0013] An ammonium citrate-modified mesoporous SBA-15 supported Ni catalyst, comprising a support and an active component supported on the support, wherein the support is an SBA-15 molecular sieve, and the active component is nickel, wherein the nickel accounts for 6% by weight and the balance is SBA-15 molecular sieve, and the weight of the organic carbon layer generated after calcination of the support and ammonium citrate is negligible.

[0014] An application using ammonium citrate-modified mesoporous SBA-15 supported Ni catalyst involves placing 50 mg of the ammonium citrate-modified mesoporous SBA-15 supported Ni catalyst in a quartz-lined tube. A layer of quartz wool is placed at both ends of the catalyst, and a methane dry reforming reaction is carried out in a fixed-bed reactor. The catalyst is first reduced in situ at 600 °C for 1 h under a high-purity H2 atmosphere at a rate of 10 °C / min. After reduction, argon gas is introduced at a rate of 50 ml / min to purge the pipeline for 1 h. After purging, a methane-carbon dioxide mixture is introduced at a gas hourly space velocity (GHSV) of 30000 ml·g⁻¹. cat -1 ·h -1 The reaction temperature is 650-800℃; the ratio of methane to carbon dioxide mixed gas is 1:1.

[0015] Compared with the prior art, the present invention has the following beneficial effects: 1) This invention utilizes a one-step impregnation method modified with ammonium citrate to prepare a supported catalyst. By precisely controlling the coordination chemical environment of the nickel precursor, the distribution of nickel species on the surface and inside the pores of the SBA-15 support is optimized, and a catalytic material with high nickel dispersion, small-sized nickel nanoparticles, strong metal-support interaction, and moderate alkalinity is successfully obtained.

[0016] 2) Under the thermodynamically significant carbon deposition tendency at 650℃, the invented catalyst achieves a CH4 conversion of 74%, a CO2 conversion of 84%, and a hydrogen-to-carbon ratio of 0.74. Under the high-temperature reaction conditions of 800℃, the CH4 conversion reaches 95%, the CO2 conversion reaches 98%, and the hydrogen-to-carbon ratio reaches 0.85. It operates stably for 100 hours without showing signs of deactivation. The average particle size of metallic nickel on the freshly reduced catalyst is 3.65 nm, and after 100 hours of reaction, the nickel particle size on the catalyst only increases to 5.76 nm.

[0017] 3) The raw materials used in the preparation process of the catalyst are low in price, and the synthesis process only needs one-step impregnation, which is simple to operate. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 X-ray diffraction patterns of the catalysts prepared in Example 1 and Comparative Examples 1-3 of the present application.

[0019] Figure 2 H2 temperature programmed reduction patterns of the catalysts prepared in Example 1 and Comparative Examples 1-3 of the present application.

[0020] Figure 3 Transmission electron microscopy (TEM) images and particle size distribution patterns of the catalysts prepared in Example 1 and Comparative Examples 1-3 of the present application: a is a TEM image and particle size distribution pattern of 6Ni2 / S before reaction; b is a TEM image and particle size distribution pattern of 6Ni2 / S after reaction; c is a TEM image and particle size distribution pattern of 6Ni2 / S before reaction; d is a TEM image and particle size distribution pattern of 6Ni2 / S after reaction. 0.1 TEM images and particle size distribution patterns of 6Ni2 / S before reaction; b is a TEM image and particle size distribution pattern of 6Ni2 / S after reaction; c is a TEM image and particle size distribution pattern of 6Ni2 / S before reaction; d is a TEM image and particle size distribution pattern of 6Ni2 / S after reaction. 0.5 TEM images and particle size distribution patterns of 6Ni2 / S before reaction; b is a TEM image and particle size distribution pattern of 6Ni2 / S after reaction; c is a TEM image and particle size distribution pattern of 6Ni2 / S before reaction; d is a TEM image and particle size distribution pattern of 6Ni2 / S after reaction. IMP TEM images and particle size distribution patterns of 6Ni2 / S before reaction.

[0021] Figure 4 CO2-temperature programmed desorption patterns of the catalysts prepared in Example 1 and Comparative Examples 1-3 of the present application: a is a CO2-temperature programmed desorption pattern of 6Ni2 / S; b is a CO2-temperature programmed desorption pattern of 6Ni2 / S; c is a CO2-temperature programmed desorption pattern of 6Ni2 / S; d is a CO2-temperature programmed desorption pattern of 6Ni2 / S. 0.1 CO2-temperature programmed desorption patterns of 6Ni2 / S; b is a CO2-temperature programmed desorption pattern of 6Ni2 / S; c is a CO2-temperature programmed desorption pattern of 6Ni2 / S; d is a CO2-temperature programmed desorption pattern of 6Ni2 / S. 0.5 CO2-temperature programmed desorption patterns of 6Ni2 / S; b is a CO2-temperature programmed desorption pattern of 6Ni2 / S; c is a CO2-temperature programmed desorption pattern of 6Ni2 / S; d is a CO2-temperature programmed desorption pattern of 6Ni2 / S. IMP CO2-temperature programmed desorption patterns of 6Ni2 / S.

[0022] Figure 5 20h activity evaluation of the catalysts prepared in Example 1 and Comparative Examples 1-3 of the present application, the reaction conditions are as follows: temperature 650℃, CH4 / CO2 ratio 1:1, gas flow rate 25ml / min, catalyst 0.05g, in-situ reduction at 600℃ for 1h at a temperature rising rate of 10℃ / min under a H2 flow rate of 80ml / min.

[0023] Figure 6 100h stability evaluation of the catalyst prepared in Example 1 of the present application, the reaction conditions are as follows: temperature 800℃, CH4 / CO2 ratio 1:1, gas flow rate 25ml / min, catalyst 0.05g, in-situ reduction at 600℃ for 1h at a temperature rising rate of 10℃ / min under a H2 flow rate of 80ml / min.

[0024] Figure 7 TEM image and metal particle size distribution pattern of the catalyst prepared in Example 1 of the present application after 100h DRM reaction. DETAILED DESCRIPTION

[0025] The embodiments of the present application will be further described below with reference to the accompanying drawings: The raw materials, reagents or devices used in the embodiments of the present application can be obtained from conventional commercial channels or can be obtained by existing known methods, unless otherwise specified.

[0026] Example 1: 6Ni 0.5 Preparation of 6Ni 6Ni 0.5 CA / Ni in 6Ni 2+ The molar ratio is 0.5, the active metal Ni content is 6wt%, and the carrier is SBA-15 molecular sieve. The preparation method is as follows: (1) 3.8g of nickel nitrate hexahydrate and 1.59g of ammonium citrate were weighed respectively and dissolved in 50ml of deionized water, respectively, and stirred at room temperature until completely dissolved to obtain two uniform solutions. The nickel nitrate hexahydrate solution was slowly added dropwise to the continuously stirred ammonium citrate solution, and after the addition was completed, the stirring was continued for 7h at a stirring speed of 300rpm; (2) 4.4ml of the above solution was measured and added dropwise into 0.5g of SBA-15, and ultrasonic mixing was carried out for 30min at room temperature, and then the suspension was rotary evaporated at 70℃ and dried in a 60℃ air drying oven for 8-12h; (3) The white powder obtained after drying was placed in a muffle furnace in an air atmosphere, and calcined at a temperature increasing rate of 5℃ / min to 600℃ for 4h. After the furnace temperature dropped to room temperature, the catalyst sample was taken out, the calcined sample was ground, tabletted, and the particles were sieved to 40-60mesh, to obtain a catalyst with a nickel content of 6wt% and a molar ratio of CA / Ni 2+ of 0.5, denoted as 6Ni 0.5 / S.

[0027] Example 1: 6Ni IMP Preparation of 6Ni IMP / S.

[0028] Example 1: 6Ni 0.1 Preparation of 6Ni Prepared similarly to the preparation method of Example 1, except that the mass of ammonium citrate added was 0.32 g, denoted as 6Ni 0.1 / S.

[0029] Preparation of Comparative Example 3: 6Ni2 / S catalyst Prepared similarly to the preparation method of Example 1, except that the mass of ammonium citrate added was 6.35 g, denoted as 6Ni2 / S.

[0030] Figure 1 X-ray diffraction patterns of the catalysts prepared in Example 1 and Comparative Examples 1-3, 6NiO / S prepared by traditional impregnation method IMP / S at 2θ = 37.2, 43.2, 62.8° corresponding to the (111), (200), (220) crystal planes of the cubic crystal structure of NiO were clearly observed, indicating that the nickel species was aggregated on the surface of the support in large size particles. In contrast, no obvious diffraction peaks of NiO were detected on the ammonium citrate modified catalysts, confirming that the nickel nanoparticles were small in size and uniformly distributed on the support in a highly dispersed state.

[0031] Figure 2 H2 temperature programmed reduction (H2-TPR) patterns of the catalysts prepared in Example 1 and Comparative Examples 1-3, it can be seen that the hydrogen reduction peak temperature of the catalyst prepared by ammonium citrate modified impregnation method shifted to high temperature compared with the catalyst prepared by traditional impregnation method, and the hydrogen consumption was greater, indicating that there was a stronger interaction between the active metal nickel and the support in the catalyst prepared by modified impregnation method.

[0032] Figure 3 wherein a, b, c, d are respectively 6Ni 0.1 / S, 6Ni 0.5 / S, 6Ni2 / S, 6Ni IMP Transmission electron microscopy (TEM) images and particle size distribution diagrams of 6Ni 0.5 / S, 6Ni 0.5 / S, 6Ni

[0033] Figure 4 wherein a, b, c, d are respectively 6Ni 0.1 / S, 6Ni 0.5 / S, 6Ni2 / S, 6Ni IMP The CO2-TPD characterization graph of / S, and the calculated basic site concentration of each catalyst is 0.136 mmol / g, 0.220 mmol / g, 0.327 mmol / g, and 0.177 mmol / g, respectively. Increasing ammonium citrate can improve the basicity of the catalyst. Moderate basicity is conducive to the adsorption and activation of CO2, maintains the reaction balance, and promotes the gasification of carbon deposition, thereby improving the activity and stability of the catalyst.

[0034] Example 2: Application of the catalyst, 0.05 g of the catalyst prepared in Example 1 and Comparative Examples 1-3 was weighed into a quartz tube, and a layer of quartz wool was placed on the upper and lower ends of the tube. The methane dry reforming reaction was carried out on a fixed bed reactor. The catalyst was first reduced in situ at 80 ml / min H2atmosphere, with a temperature increase of 10 ℃ / min to 600 ℃. After reduction, 50 ml / min argon was introduced, and the pipeline was purged for 1 h. After purging, the temperature was increased to 650 ℃ at a rate of 10 ℃ / min, and then a mixture of methane and carbon dioxide was introduced. The gas space velocity was 30000 ml·g cat -1 ·h -1 The reaction temperature was 650 ℃, and the activity test was carried out for 20 h. The catalyst performance was evaluated by online gas chromatography analysis.

[0035] Figure 5 The DRM reaction activity evaluation results of Example 1 and Comparative Examples 1-3 showed that the activity of all catalysts prepared by ammonium citrate modification impregnation method was better than that of catalysts prepared by traditional impregnation method. Among them, the 6Ni 0.5 / S catalyst had the best activity, with initial conversion rates of CH4: 74%, CO2: 84%, and H2 / CO: 0.74, and conversion rates of CH4: 73%, CO2: 83%, and H2 / CO: 0.73 after 20 h, showing high activity and good stability. In contrast, the initial conversion rates of the 6Ni IMP / S catalyst of Comparative Example 1 were CH4: 68%, CO2: 79%, and H2 / CO: 0.73, and the conversion rates after 20 h were CH4: 62%, CO2: 72%, and H2 / CO: 0.69, respectively. The initial activity was low, and the activity decreased significantly after 20 h, indicating obvious deactivation.

[0036] Example 3: Application of the catalyst, Take 0.05 g of the catalyst prepared in Example 1 into the quartz tube lining, which is padded with a layer of quartz wool at both ends, and perform the methane dry reforming reaction on the fixed bed reactor. The catalyst is first reduced in situ at 80 ml / min H2 atmosphere, heated to 600℃ at 10℃ / min, and reduced for 1 h. After reduction, 50 ml / min argon is introduced, and the pipeline is purged for 1 h. After purging, heat to 800℃ at 10℃ / min, and then introduce the methane carbon dioxide mixed gas, with a gas space velocity of 30000 ml·g cat -1 ·h -1 The reaction temperature is 800℃, and the stability test is performed for 100 h. The catalyst performance is evaluated by online gas chromatography analysis.

[0037] Figure 6 The DRM reaction stability evaluation results of Example 1 are as follows: the CH4 conversion rate is maintained at about 95%, the CO2 conversion rate is maintained at about 98%, and the H2 / CO is maintained at about 0.85. The deactivation rate constant of CH4 conversion rate after 100 h is only 0.0008h -1 , and the deactivation rate constant of CO2 conversion rate is only 0.00009h -1 , which exhibits excellent activity and long-term stability, thanks to the good dispersion, small metal particle size, strong metal-support interaction and moderate basicity of the catalyst.

[0038] Figure 7 The transmission electron microscopy image and metal particle size distribution graph of Example 1 of the present application after 100 h of DRM reaction are as follows: according to Ostwald ripening, small size nanoparticles inevitably migrate and sinter at high temperature due to high surface energy, resulting in an increase in metal particle size. However, through the one-step impregnation method of ammonium citrate modification, the nickel nanoparticles are effectively anchored on the support, and the average particle size after the reaction only increases from the initial 3.65 nm to 5.76 nm, which fully proves that the catalyst prepared by the method has excellent anti-sintering and anti-coking performance.

Claims

1. A method for preparing a Ni catalyst supported on ammonium citrate-modified mesoporous SBA-15, characterized in that, The steps are as follows: S1. Prepare nickel nitrate hexahydrate solution and ammonium citrate solution with a molar ratio of ammonium citrate to nickel ions of (0.1-2):1; S2. Add nickel nitrate hexahydrate solution dropwise to ammonium citrate solution under continuous stirring to reduce changes in local concentration gradient and make the complexation process more uniform and controllable; stir for 7 hours after the addition is complete to obtain a mixed solution. S3. Add the mixed solution to the SBA-15 molecular sieve, adding 8.8 ml of mixed solution per gram of SBA-15 molecular sieve. Then, mix by ultrasonication to form a uniform suspension. Perform rotary evaporation and drying on the suspension to obtain a white powder. S4. The dried white powder is placed in a muffle furnace in an air atmosphere for calcination treatment to cover the nickel precursor with a carbon layer to derive fine nickel nanoparticles. S5. Grind, press, and sieve the product obtained from calcination to obtain ammonium citrate modified mesoporous SBA-15 supported Ni catalyst.

2. The method for preparing the ammonium citrate-modified mesoporous SBA-15 supported Ni catalyst according to claim 1, characterized in that, In S1, the concentration of nickel in the preparation of nickel nitrate hexahydrate solution is 15 mol / L, and the concentration of ammonium citrate used in the preparation of ammonium citrate solution is 0.03 mol / L - 0.52 mol / L.

3. The method for preparing the ammonium citrate-modified mesoporous SBA-15 supported Ni catalyst according to claim 1, characterized in that, The nickel nitrate hexahydrate solution was slowly added dropwise to the continuously stirred ammonium citrate solution at a stirring speed of 300 rpm.

4. The method for preparing the ammonium citrate-modified mesoporous SBA-15 supported Ni catalyst according to claim 1, characterized in that, Before adding the mixed solution to the SBA-15 molecular sieve, the SBA-15 needs to be dried in a 120℃ forced-air drying oven for 6 to 12 hours to remove the moisture adsorbed in the SBA-15 support; by taking an appropriate amount of liquid, ensure that the weight percentage of Ni is 6%.

5. The method for preparing the ammonium citrate-modified mesoporous SBA-15 supported Ni catalyst according to claim 1, characterized in that, The rotary evaporation conditions were 100 rpm and 70°C; the drying was carried out in a forced-air drying oven at 60°C for 6–12 hours.

6. The method for preparing the ammonium citrate-modified mesoporous SBA-15 supported Ni catalyst according to claim 1, characterized in that, The dried white powder was placed in a muffle furnace under air atmosphere for calcination treatment to coat the nickel precursor with a carbon layer to derive fine nickel nanoparticles; the calcination temperature was 600℃, the heating rate was 5℃ / min, and it was held at 600℃ for 4h.

7. The method for preparing the ammonium citrate-modified mesoporous SBA-15 supported Ni catalyst according to claim 1, characterized in that, The catalyst is sieved to 40-60 mesh to ensure full exposure of active sites and improve mass transfer efficiency.

8. An ammonium citrate-modified mesoporous SBA-15 supported Ni catalyst prepared using the preparation method according to any one of claims 1-7, characterized in that, The carrier and the active component loaded on the carrier, wherein the carrier is SBA-15 molecular sieve, and the active component is nickel, wherein the weight percentage of nickel is 6% and the balance is SBA-15 molecular sieve, and the weight of the organic carbon layer generated after calcination of the carrier and ammonium citrate is negligible.

9. An application of the ammonium citrate-modified mesoporous SBA-15 supported Ni catalyst as described in claim 8, characterized in that: 50 mg of ammonium citrate-modified mesoporous SBA-15 supported Ni catalyst was weighed and placed in a quartz-lined tube. A layer of quartz wool was placed at both ends of the catalyst, and a dry reforming reaction of methane was carried out in a fixed-bed reactor. The catalyst was first reduced in situ at 600 °C for 1 h under a high-purity H2 atmosphere at a rate of 10 °C / min. After reduction, argon gas was introduced at a rate of 50 ml / min to purge the pipeline for 1 h. After purging, a methane-carbon dioxide mixture was introduced at a gas hourly space velocity (GHSV) of 30000 ml·g⁻¹. cat -1 ·h -1 The reaction temperature is 650-800℃; the ratio of methane to carbon dioxide mixed gas is 1:1.

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