Inorganic carbon-supported iron catalyst and its preparation method and application

Abstract

The present invention discloses an inorganic carbon-supported iron catalyst and its preparation method and application. The catalyst is obtained by surface-treating a carbon carrier to form oxygen-containing functional groups, then introducing the carbon carrier into a mixed solution containing an iron-based metal salt and an inorganic ammonium salt to fully disperse the carbon carrier, and then sequentially drying and sintering. The mass ratio of the carbon carrier, the iron-based metal salt and the inorganic ammonium salt is 5 to 8:1 to 5:20 to 25. The catalyst is based on the synergistic effect between the carbon carrier, the inorganic ammonium salt and the iron-based metal salt. The inorganic ammonium salt is used to provide nitrogen elements for the catalyst system, forming a fixed-point nitrogen doping with the carbon carrier, thereby improving the binding between the carrier and the cobalt element, ensuring that the cobalt element is exposed to the catalyst surface, and avoiding the carbon deposition effect of traditional organic molecules during the high-temperature cracking process and clogging the pore structure of the carrier. In addition, the blending of the inorganic ammonium salt and the iron-based metal salt can promote the dispersibility of the two, thereby improving the activity and cyclic stability of the catalyst.

Description

Technical Field

[0001] The present invention relates to an iron-based catalyst, in particular to an inorganic carbon-supported iron-based catalyst and a preparation method and application thereof, belonging to the field of hydrogenation catalyst preparation. Background Art

[0002] Aromatic amine compounds are important fine chemicals and also major bulk chemicals. In 2020, their market demand exceeded 6 million tons and continues to increase annually. They play an irreplaceable role in high-end chemical sectors such as pharmaceuticals, pesticides, dyes, and electronic materials. The reduction of nitro compounds is an important method for preparing amine compounds and their derivatives. Hydrogen is a renewable, green, environmentally friendly, and widely available resource. Catalytic hydrogenation technology using hydrogen as a reducing agent has gradually developed into a key means of nitro reduction, attracting widespread attention from academia and industry. However, in actual production, current catalytic hydrogenation technology still has significant room for improvement in terms of atomic economics and efficient, green production. This is primarily due to the high energy consumption caused by the high-temperature and high-pressure reaction environment, as well as the increased waste, waste gas, and energy consumption caused by by-product separation and product purification. Precious metal (platinum, palladium)-based catalysts remain the fastest-growing and most widely used catalysts. The disadvantages of precious metal catalysts are poor availability, high cost, and poor tolerance to toxicity of molecules such as CO and S. Therefore, in the catalytic hydrogenation reduction reaction of nitro compounds, people urgently need low-cost, high-performance non-precious metal catalysts to replace precious metal catalysts.

[0003] Carbon-supported catalysts with MNC (nitrogen-doped carbon-supported iron-based metal catalyst, M represents iron-based metal) structures exhibit excellent hydrogenation activity during nitro hydrogenation reactions. However, the traditional preparation method is mainly achieved through in-situ cracking, that is, cracking a precursor composed of organic small molecules containing nitrogen atoms and metal ions. However, the active sites of the MNC structure prepared by this method are usually located within the pore structure. Due to the large molecular volume and limited solubility of aromatic nitro compounds, during the gas-solid-liquid three-phase catalytic reaction, the reactants or substrates block the pores and embed the active sites, resulting in a serious decrease in catalytic activity after multiple cycles, which manifests as poor cyclic stability. Therefore. How to change the limitations of the confined distribution of active sites is the key to improving the hydrogenation stability of MNC structure catalysts.

[0004] By introducing pre-prepared carbon materials into the precursor, the precursor cracking process occurs on the surface of the carrier, which can improve the distribution of active sites to a certain extent and make more active sites distributed on the surface of the carrier carbon. However, in order to promote better dispersion of metal centers and increase the density of single-atom sites, ligands usually need to be introduced. However, previous organic ligands are mostly carbon-rich organic ligands containing nitrogen atoms. During the cracking process, the active centers are mostly nanoscale and coated with carbon. At the same time, the ligands can produce a carbon deposition effect and block the pores of the porous carrier. While increasing the internal diffusion resistance, some of the active sites inside the pores are inactivated due to entrapment. Therefore, the catalysts often show low catalytic activity. Therefore, how to overcome the confined distribution of active sites while improving the intrinsic performance of the sites is the key to constructing a nitro hydrogenation catalyst with an MNC structure and structure-effect efficiency. Summary of the Invention

[0005] In response to the problems existing in the prior art, the first object of the present invention is to provide an inorganic carbon-supported iron-based catalyst. The catalyst is based on the synergistic effect between a carbon support, an inorganic ammonium salt and an iron-based metal salt. The inorganic ammonium salt is used to provide nitrogen to the catalyst system, forming a fixed-point nitrogen doping with the carbon support, thereby improving the binding between the support and the cobalt element, ensuring that the cobalt element is exposed to the catalyst surface, and avoiding the carbon deposition effect of traditional organic molecules during high-temperature cracking and clogging the pore structure of the support. In addition, the blending of the inorganic ammonium salt and the iron-based metal salt can promote the dispersion of the two, thereby improving the activity and cyclic stability of the catalyst.

[0006] The second object of the present invention is to provide a method for preparing an inorganic carbon-supported iron-based catalyst, which has the advantages of simple process and easy operation, and uses non-precious metal iron-based catalysts, which greatly reduces the cost of the catalyst, does not require large and complex devices, is more environmentally friendly, and can meet the requirements of industrial production.

[0007] The third object of the present invention is to provide an application of an inorganic carbon-supported iron-based catalyst for the catalytic hydrogenation of nitro compounds. Using a carbon-supported iron-based catalyst for the catalytic hydrogenation of nitro compounds not only results in milder reaction conditions but also exhibits excellent selectivity and cyclic stability. Testing has shown that the catalyst provided by the present invention exhibits a selectivity of >99% and a conversion rate of >99% for the catalytic hydrogenation of nitro compounds to corresponding amine compounds. Furthermore, after 16 cycles of use, the catalyst still exhibits a conversion rate of >90% and a selectivity of >99%.

[0008] In order to achieve the above technical objectives, the present invention provides a method for preparing an inorganic carbon-supported iron-based catalyst, comprising: performing surface treatment on a carbon support to form oxygen-containing functional groups; then introducing the carbon support into a mixed solution containing an iron-based metal salt and an inorganic ammonium salt and fully dispersing the mixture; and then sequentially drying and sintering the mixture to obtain the catalyst;

[0009] The mass ratio of the carbon carrier, the iron-based metal salt and the inorganic ammonium salt is 5-8:1-5:20-25.

[0010] As a preferred solution, the surface treatment process of the carbon carrier is as follows: placing the carbon carrier in an inorganic acid for surface activation treatment, and then washing and drying to neutrality by extraction.

[0011] As a preferred solution, the inorganic acid is at least one of sulfuric acid, nitric acid, hydrochloric acid and perchloric acid; and the pH of the inorganic acid is 1-3.

[0012] As a preferred solution, the mass volume ratio of the carbon support to the inorganic acid is 0.05 to 0.15 g / ml. More preferably, the mass volume ratio of the carbon support to the inorganic acid is 0.1 g / ml.

[0013] As a preferred solution, the iron-based metal salt is at least one of an iron salt, a cobalt salt and a nickel salt.

[0014] As a preferred solution, the inorganic ammonium salt is at least one of ammonium chloride, ammonium sulfate and ammonium bicarbonate.

[0015] As a preferred solution, the carbon carrier is one of activated carbon, coconut shell charcoal, microsphere carbon and carbon nanotubes.

[0016] As a preferred solution, the fully dispersing method is one of ultrasonic dispersion, stirring dispersion and oscillation dispersion.

[0017] As a preferred solution, the sufficient dispersion time is 1 to 10 hours.

[0018] As a preferred solution, the drying method is oven drying, and the conditions are: temperature of 50 to 120° C., and drying time of 1 to 10 hours.

[0019] As a preferred solution, the sintering process is carried out under a protective atmosphere, and the process is: heating to 400-1000° C. at a heating rate of 4-20° C. / min, and keeping the temperature for 1.5-10 hours.

[0020] As a preferred solution, the protective atmosphere is nitrogen and / or helium.

[0021] The present invention also provides an inorganic carbon-supported iron catalyst, which is prepared by any one of the preparation methods described above, wherein the specific surface area of ​​the iron catalyst is 850 to 1000 m 2 g -1 .

[0022] The present invention also provides an application of an inorganic carbon-supported iron-based catalyst for catalytic hydrogenation of nitro compounds.

[0023] As a preferred embodiment, the reaction conditions of the catalytic hydrogenation are: H2 pressure ≤ 10 bar, temperature ≤ 200°C, time 1 to 10 hours, and a mass ratio of catalyst to nitro compound of 1:2 to 4. More preferably, the mass ratio of catalyst to nitro compound is 1:3.

[0024] Compared with the prior art, the beneficial technical effects of the technical solution of the present invention are:

[0025] 1) The carbon-supported iron-based catalyst provided by the present invention is based on the synergistic effect between the carbon support, inorganic ammonium salt and iron-based metal salt. The inorganic ammonium salt is used to provide nitrogen to the catalyst system, forming a fixed-point nitrogen doping with the carbon support, thereby improving the binding between the support and the cobalt element, ensuring that the cobalt element is exposed to the catalyst surface, and avoiding the carbon deposition effect of traditional organic molecules during high-temperature cracking and clogging the pore structure of the support. In addition, the blending of the inorganic ammonium salt and the iron-based metal salt can promote the dispersion of the two, thereby improving the activity and cyclic stability of the catalyst.

[0026] 2) The preparation method provided by the present invention has the advantages of simple process and easy operation, and adopts non-precious metal iron-based raw materials, which greatly reduces the cost of the catalyst, does not require large and complex devices, is more environmentally friendly, and can meet the requirements of industrial production.

[0027] 3) In the technical solution provided by the present invention, the carbon-supported iron catalyst is used to catalyze the hydrogenation reaction of nitro compounds. Not only are the reaction conditions milder, but also the catalyst has excellent selectivity and cyclic stability. According to tests, the selectivity of the catalyst provided by the present invention for the catalytic hydrogenation of nitro compounds to the corresponding amine compounds is greater than 99%, and the conversion rate is greater than 99%. After 16 cycles of use, the conversion rate of the catalyst is still greater than 90%, and the selectivity is greater than 99%. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 X-ray diffraction patterns of the Co / AC-AP, Co / AC and AC-AP catalysts provided by the present invention;

[0029] Figure 2 This is a high-resolution transmission electron microscope image of the Co / AC-AP catalyst prepared in Example 1 of the present invention;

[0030] Figure 3 Transmission electron microscope image of the Co / AC-AP catalyst prepared in Example 1 of the present invention and its particle size distribution image;

[0031] Figure 4 This is the XPS spectrum of the Co / AC-AP catalyst prepared in Example 1 of the present invention;

[0032] Figure 5 This is a graph showing the recyclability of the Co / AC-AP catalyst prepared in Example 1 of the present invention;

[0033] Figure 6 Schematic diagram of the conversion rate and selectivity of the Co / AC-AP catalyst prepared in Example 1 of the present invention in catalytic hydrogenation of nitro compounds to corresponding amine compounds. DETAILED DESCRIPTION

[0034] The technical solutions of the present invention are further illustrated below through specific embodiments. It should be understood that the protection scope of the present invention is not limited to the following embodiments; it should be understood that the multiple method steps mentioned in the present invention do not exclude the addition of other method steps before and after the combination step or the insertion of other method steps between the combination steps; it should be understood that the enumerated embodiments are only used to illustrate the present invention and are not used to limit the scope of the present invention; it should be understood that the numbering of each method step involved in the present invention is only an effective tool for facilitating the identification of each step, and does not limit the order of arrangement of each method step or limit the scope of implementation of the present invention; it should be understood that in the absence of substantial changes in the technical content, it should also be regarded as the scope of implementation of the present invention.

[0035] Example 1

[0036] The carbon-supported iron catalyst provided in this embodiment is a cobalt catalyst, and its preparation process is as follows:

[0037] 1. Activate 10.0 g of activated carbon in 100 ml of nitric acid at pH 1. After washing and drying, a carbon support with a surface rich in oxygen-containing functional groups was obtained, which was denoted as NAC.

[0038] 2. Add 0.5g of ammonium persulfate and 0.02g of cobalt salt to a 10ml flask, dissolve evenly, then add 1.0g of NAC, disperse for 2h, and then dry at 100℃ for 6h to obtain a black precursor;

[0039] 3. The precursor was transferred to a tube furnace, heated to 800°C at a heating rate of 5°C / min, and kept at this temperature for 3 h to obtain a carbon-supported cobalt catalyst, which was recorded as Co / AC-AP.

[0040] Comparative Example 1

[0041] The preparation process of this comparative example is exactly the same as that of Example 1, except that no ammonium salt is added, and the obtained catalyst is recorded as Co / AP.

[0042] Comparative Example 2

[0043] The preparation process of this comparative example is exactly the same as that of Example 1, except that no cobalt salt is added. The obtained catalyst is recorded as AC-AP.

[0044] The above three catalysts were subjected to XRD test, and the results are as follows Figure 1 As shown in the figure, no identifiable peaks of elemental metal cobalt nanocrystals were found in the peak spectra of the three catalysts, while obvious broad peaks could be observed at 25° and 44°, which corresponded to 002 and 001 of the carbon support respectively; peaks of cobalt elements could be observed at 44° and 51° on both Co / AC and Co / AP-AC, but it was obvious that the cobalt peak of Co / AC was sharper. Preliminary analysis showed that this might be due to the agglomeration of some cobalt elements.

[0045] In order to observe the morphology and microstructure of the catalyst more clearly, the present invention also conducted a high-resolution transmission electron microscope test on the catalyst. The test results are as follows: Figure 2 As shown in the figure, all statistically analyzed Co / AC-AP catalysts exhibited very small cobalt nanoclusters, approximately 10 nanometers. Compared to the Co / AC catalyst without the addition of inorganic heteroatom salts, the active metal particles were 10 times smaller and highly dispersed. The Co atoms, highlighted by white circles, were atomically dispersed within the N / C matrix, with no aggregated particles. These characterizations indicate the presence of a CN-bound matrix within the Co / AC-AP catalyst. Further XPS spectroscopy analysis of the Co / AC-AP catalyst revealed the presence of Co-NC centers.

[0046] Furthermore, the present invention also conducted a catalytic hydrogenation experiment on the above three catalysts, and the process was as follows:

[0047] The catalyst was loaded into a tank reactor, and the substrate p-chloronitrobenzene, hydrogen, and a mixed solvent were introduced. The mass ratio of the catalyst to the substrate was 1:33, the hydrogen partial pressure was 0.5 MPa, and the mixed solvent was a mixture of water and ethanol in a volume ratio of 1:1. The conditions for the catalytic hydrogenation reaction were: reaction temperature 120°C, reaction time 3 h. After the reaction, the conversion rate of the substrate and the selectivity of aniline were recorded. The results are shown in Table 1.

[0048] Table 1

[0049] Case Conversion rate Selectivity Example 1 ＞99.9％ ＞99.9％ Comparative Example 1 20％ 99％ Comparative Example 2 / /

[0050] As can be seen from Table 1, the catalyst provided in Comparative Example 1 does not contain ammonium salt, and its conversion rate is only 20%. The catalyst provided in Example 1 of the present invention has a conversion rate and selectivity of ≥99.9% for the catalytic hydrogenation of p-chloronitrobenzene to aniline, which shows an excellent catalytic effect. This is because the ammonium salt can provide nitrogen on the one hand, forming a fixed-point nitrogen doping with the carbon support, thereby improving the binding between the support and the cobalt element, ensuring that the cobalt element is exposed to the catalyst surface, and avoiding the carbon deposition effect of traditional organic molecules during the high-temperature cracking process and clogging the pore structure of the support. On the other hand, the inorganic ammonium salt can also play a good dispersing effect on the cobalt element, thereby improving the activity and cycle stability of the catalyst, thereby greatly improving the conversion rate and selectivity of the catalyst. The catalyst obtained in Comparative Example 2 does not contain cobalt salt, and the product yield is too small to accurately test the reaction conversion rate and selectivity. This proves that the catalyst provided by the present invention uses an iron-based metal salt as a catalytically active component, and the excellent catalytic performance of the catalyst can only be achieved under the synergistic effect of the iron-based metal salt, the inorganic ammonium salt and the carbon support.

[0051] The catalyst provided in Example 1 of the present invention also has excellent lifespan and cycle stability. Figure 5 As shown, after 16 cycles, the conversion rate of the Co / AC-AP catalyst for the catalytic hydrogenation reaction of p-chloronitrobenzene is still greater than 90%, and the selectivity is greater than 99%.

[0052] In order to verify the applicability of the catalyst provided by the present invention for the hydrogenation reaction of nitro compounds, the present invention also used Co / AC-AP catalyst to carry out catalytic hydrogenation reaction of different nitro compounds. The reaction conditions and process were exactly the same as those of the hydrogenation reaction of p-chloronitrobenzene. The results are as follows: Figure 6 As shown, the tests show that the conversion rate and selectivity of the Co / AC-AP catalyst for the catalytic hydrogenation of other nitro compounds with sensitive groups to corresponding amine compounds are both greater than 99%, and it has universal applicability in the hydrogenation reaction of nitroaromatics.

Claims

1. A method for preparing an inorganic carbon-supported iron-based catalyst, characterized in that: The carbon support is surface treated to form oxygen-containing functional groups, and then introduced into a mixed solution containing an iron-based metal salt and an inorganic ammonium salt and fully dispersed, and then dried and sintered in sequence to obtain; The mass ratio of the carbon support, the iron-based metal salt and the inorganic ammonium salt is 5-8:1-5:20-25; The surface treatment process of the carbon carrier is as follows: placing the carbon carrier in an inorganic acid for surface activation treatment, and then washing and drying to neutrality by extraction; The inorganic acid is at least one of sulfuric acid, nitric acid and perchloric acid; the pH of the inorganic acid is 1 to 3; the mass volume ratio of the carbon support to the inorganic acid is 0.05 to 0.15 g / ml; The iron-based metal salt is at least one of an iron salt, a cobalt salt, and a nickel salt; the inorganic ammonium salt is at least one of ammonium chloride, ammonium sulfate, and ammonium bicarbonate; and the carbon carrier is one of activated carbon, coconut shell carbon, microsphere carbon, and carbon nanotubes; The sintering process is carried out under a protective atmosphere, and the process is: heating to 400-1000° C. at a heating rate of 4-20° C. / min and keeping the temperature for 1.5-10 hours; the protective atmosphere is nitrogen and / or helium.

2. The method for preparing an inorganic carbon-supported iron-based catalyst according to claim 1, wherein: The method of fully dispersing is one of ultrasonic dispersion, stirring dispersion and oscillation dispersion; and the time of fully dispersing is 1 to 10 hours.

3. The method for preparing an inorganic carbon-supported iron-based catalyst according to claim 1, wherein: The drying method is oven drying, and the conditions are: temperature of 50-120° C., and drying time of 1-10 hours.

4. An inorganic carbon-supported iron catalyst, characterized in that: Prepared by the preparation method according to any one of claims 1 to 3, the specific surface area of ​​the iron-based catalyst is 850 to 1000 m 2 g -1 .

5. The use of an inorganic carbon-supported iron-based catalyst according to claim 4, characterized in that: Used for catalytic hydrogenation of nitro compounds.

6. The use of an inorganic carbon-supported iron-based catalyst according to claim 5, characterized in that: The reaction conditions of the catalytic hydrogenation are: H2 pressure ≤ 10 bar, temperature ≤ 200°C, time 1-10 hours, and a mass ratio of catalyst to nitro compound of 1:2-4.

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