Cerium modified copper-based catalysts, methods of making and use in n-methylation reactions

Abstract

This invention discloses a method for preparing a cerium-modified copper-based catalyst, comprising the following steps: (1) simultaneously dissolving water-soluble copper, zinc, aluminum, and cerium salts in deionized water to obtain a metal salt solution; (2) adding an aqueous precipitant solution dropwise to the metal salt solution for co-precipitation, transferring the resulting mixture to a 70°C constant temperature oven for aging, filtration, water washing, drying, and then calcining to obtain a metal oxide precursor; (3) placing the metal oxide precursor in a reaction tube and carrying out a reduction reaction under a hydrogen atmosphere to obtain a cerium-modified copper-based catalyst. This invention introduces a CeO2 promoter, utilizing its interfacial interaction with Cu and ZnO to modulate the electronic structure of the active component Cu, optimizing the adsorption and activation behavior of reactant molecules, resulting in higher catalytic activity in the N-methylation reaction of aniline and methanol, achieving higher aniline conversion and N-methylaniline selectivity under mild reaction conditions.

Description

Technical Field

[0001] This invention relates to the field of catalysts for the synthesis of N-methylaniline, specifically to a cerium-modified copper-based catalyst and its preparation method, and more specifically to the application of this catalyst in the N-methylation reaction of aniline and methanol to prepare N-methylaniline. Background Technology

[0002] The N-methylation reaction of aniline with methanol is a key route for the preparation of important chemical intermediates such as N-methylaniline and N,N-dimethylaniline, and its products have wide applications in pharmaceuticals, dyes, and pesticides. Compared with traditional methylating agents (such as halogenated methanes and dimethyl sulfate), heterogeneous catalytic processes using methanol as a green methylating agent have become an important development direction in this field due to their advantages such as environmental friendliness, high atom economy, and easy catalyst separation and recovery. Among them, Cu / ZnO / Al2O3 catalysts have been widely studied and attempted for application in this reaction system due to their low cost and high initial activity.

[0003] However, applying Cu / ZnO / Al2O3 catalysts to the N-methylation reaction of aniline and methanol faces two technical bottlenecks: First, the active component, copper nanoparticles, has a high surface energy under reaction temperature conditions, making them prone to migration and agglomeration, leading to a significant decrease in the catalytic activity specific surface area and a rapid decline in reaction activity. This makes it difficult for the catalyst's operational stability to meet the requirements of long-term industrial operation. Second, Al2O3, as a structural and support component, possesses Lewis and Brønsted acid sites. These acidic sites on its surface can non-selectively catalyze side reactions such as polymerization and condensation of aniline or reaction products during the reaction, generating tar-like macromolecular compounds that deposit on the catalyst surface. This not only reduces catalyst activity but also affects product quality.

[0004] The aforementioned problems are compounded: the agglomeration of active components leads to a reduction in active sites, while carbon deposition caused by acidic sites further exacerbates pore blockage and hinders mass transfer. Both factors jointly limit the catalyst's lifetime and reaction efficiency. Therefore, how to effectively control the acidity of the support surface while suppressing copper component sintering, and achieve a synergistic effect of high activity, high selectivity, and long-term stability, is a technical challenge that urgently needs to be solved in this field. Summary of the Invention

[0005] Therefore, the purpose of this invention is to provide a cerium-modified copper-based catalyst, its preparation method, and its application in the N-methylation reaction. By introducing CeO2 promoter, which has unique oxygen storage and release capabilities and metal-support interactions, the Cu / ZnO / Al2O3 catalyst is modified. This aims to improve the catalyst's activity while maintaining its original cost advantage and effectively inhibit the sintering of the active copper component, ultimately achieving a synergistic improvement in the catalyst's activity, selectivity, and stability in the N-methylation reaction of aniline and methanol.

[0006] The first aspect of this invention is to provide a method for preparing a cerium-modified copper-based catalyst, comprising the following steps: (1) Dissolve water-soluble copper, zinc, aluminum and cerium salts in deionized water to obtain a metal salt solution; (2) The aqueous solution of the precipitant is added dropwise to the metal salt solution for co-precipitation. The resulting mixture is then aged, filtered, washed with water, dried, and calcined to obtain the metal oxide precursor. (3) The metal oxide precursor is placed in a reaction tube and a reduction reaction is carried out under a hydrogen atmosphere to obtain a cerium-modified copper-based catalyst.

[0007] In an optional embodiment, in step (1), the mass ratio of copper salt, zinc salt, aluminum salt and cerium salt is (10-20):(4-10):(1-5):(2-7).

[0008] In one alternative embodiment, in step (2), the precipitant is sodium carbonate, the concentration of the sodium carbonate aqueous solution is 0.09 g / mL, and the co-precipitation temperature is 60°C.

[0009] In one alternative embodiment, in step (2), the drying temperature is 100-120°C and the time is 8-12 hours.

[0010] In one alternative embodiment, in step (2), the calcination temperature is 400°C, the time is 4 h, and the heating rate is 5°C / min.

[0011] In an optional embodiment, in step (3), the metal oxide precursor is first treated at 200°C for 1 h before the reduction reaction, and then heated to 300-400°C for 3 h. The heating rate is 3-5°C / min, and the hydrogen flow rate is 20-100 ml / min.

[0012] A second aspect of the present invention is to provide a cerium-modified copper-based catalyst.

[0013] A third aspect of the present invention is to provide the application of a cerium-modified copper-based catalyst in the catalytic reaction of aniline with methanol to produce N-methylaniline.

[0014] In one alternative embodiment, the reaction temperature of aniline and methanol is 220–260°C, the pressure is 0.1–4 MPa, and the molar ratio of aniline to methanol is 1:(1–2).

[0015] Compared with the prior art, the technical solution of the present invention has the following advantages: (1) By introducing CeO2 promoter with unique oxygen storage and release capabilities, the present invention modulates the electronic structure of active component Cu through its interfacial interaction with Cu and ZnO, optimizes the adsorption and activation behavior of reactant molecules, and makes the catalyst exhibit higher catalytic activity in the N-methylation reaction of aniline and methanol. Under mild reaction conditions, a higher aniline conversion rate and N-methylaniline selectivity can be obtained.

[0016] (2) This invention forms a physical confinement barrier by wrapping highly dispersed cerium oxide particles around Cu grains, which effectively inhibits the migration and sintering of the active component Cu at the reaction temperature. At the same time, it utilizes the excellent oxygen storage and release capacity of CeO2 to oxidize and eliminate the carbon precursor generated during the reaction process, thereby inhibiting the side reactions initiated by the acidic sites of Al2O3 from the source. This achieves a dual synergistic mechanism of "physical barrier sintering" and "chemical elimination of carbon deposits", which is beneficial to the synergistic improvement of catalyst activity, selectivity and stability.

[0017] (3) This invention utilizes CeO2's Ce 4+ / Ce 3+ Reversible transformation properties and their interaction with the Cu interface (Ce 3+ + Cu 2+ Ce 4+ + Cu + The Cu in the catalyst was stabilized. + A quaternary synergistic catalytic system with both hydrogenation / dehydrogenation and redox modulation functions was constructed.

[0018] (4) The present invention is prepared by conventional coprecipitation method, which has a simple process flow, mild operating conditions, and is easy to realize industrial scale-up production. The raw materials are all cheap and readily available copper, zinc, aluminum and cerium salts, without using precious metals or toxic components, which has significant raw material cost advantages and good environmental compatibility.

[0019] (5) Compared with precious metal catalysts, the present invention uses non-precious metal copper as the active center, and achieves an exponential reduction in catalyst cost while obtaining catalytic performance close to that of precious metal catalysts. Compared with unmodified conventional Cu / ZnO / Al2O3 catalysts, the present invention significantly improves the activity of the catalyst through CeO2 modification, increases the utilization rate of aniline, and reduces the overall production cost of industrial applications.

[0020] (6) By introducing CeO2, this invention achieves a synergistic improvement in activity and selectivity while maintaining the cost advantage of copper-based catalysts, and significantly reduces the amount of methanol used, providing a new, efficient and economical catalyst solution for the N-methylation reaction of aniline and methanol. Attached Figure Description

[0021] Figure 1 The XRD patterns of the Cu / ZnO / Al2O3 / CeO2 catalyst of Example 1 and the Cu / ZnO / Al2O3 catalyst of Comparative Example 1 are shown below. Figure 2 XPS spectra of the Cu / ZnO / Al2O3 / CeO2 catalyst of Example 1 and the Cu / ZnO / Al2O3 catalyst of Comparative Example 1 of this invention; Figure 3 SEM images of the Cu / ZnO / Al2O3 / CeO2 catalyst of Example 1 and the Cu / ZnO / Al2O3 catalyst of Comparative Example 1 of the present invention; Figure 4 These are TEM images of the Cu / ZnO / Al2O3 / CeO2 catalyst of Example 1 and the Cu / ZnO / Al2O3 catalyst of Comparative Example 1 of the present invention. Detailed Implementation

[0022] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present application.

[0023] Example 1 A cerium-modified copper-based catalyst, the preparation method of which is as follows: Weigh out 15.70 g Cu(NO3)2·3H2O, 7.44 g Zn(NO3)2·6H2O, 3.75 g Al(NO3)3·9H2O, and 4.34 g Ce(NO3)3·6H2O respectively, and add them to a 500 mL Erlenmeyer flask. Then add 220 mL of deionized water and place the flask in a constant temperature water bath at 60℃. Stir and dissolve for 30 min to prepare a nitrate mixed solution with a total metal ion concentration of 0.5 mol / L. Separately, weigh out 15.26 g of anhydrous sodium carbonate and place it in a beaker. Add 170 mL of deionized water and sonicate to dissolve it. After complete dissolution, transfer the solution to a 500 mL dropping funnel and add it dropwise to the nitrate mixed solution at a rate of 1 drop / s. After the addition is complete, seal the Erlenmeyer flask and continue stirring at 60℃ for 4 h. After stirring, transfer the mixture to a constant temperature incubator at 70℃ for 12 hours of aging. After aging, the precipitate was filtered and washed with deionized water until the filtrate was neutral. The filter cake was collected and dried in a 100°C oven for 12 hours. The dried filter cake was ground into a uniform powder, placed in a crucible, and calcined in a muffle furnace at 400°C with a heating rate of 5°C / min for 4 hours to obtain the metal oxide precursor. Finally, the precursor was subjected to H2... The temperature-programmed reduction was carried out in a reaction tube under a specific atmosphere. The specific operation of the temperature-programmed reduction was as follows: first, the temperature was increased to 200℃ for 1 h, and then increased to 350℃ for 3 h. The heating rate was 4℃ / min, and the gas flow rate was 20 ml / min. After the reduction was completed, the modified catalyst Cu / ZnO / Al2O3 / CeO2 was obtained.

[0024] Example 2 A cerium-modified copper-based catalyst, the preparation method of which is as follows: 10.56 g Cu(NO3)2·3H2O, 5.23 g Zn(NO3)2·6H2O, 2.35 g Al(NO3)3·9H2O, and 6.52 g Ce(NO3)3·6H2O were weighed out and added to a 500 mL Erlenmeyer flask. Then, 220 mL of deionized water was added, and the flask was placed in a 60°C water bath and stirred for 30 min to dissolve. Separately, 10.25 g of anhydrous sodium carbonate was weighed out and placed in a beaker. 170 mL of deionized water was added and sonicated to dissolve the sodium carbonate. After complete dissolution, the solution was transferred to a 500 mL dropping funnel and added dropwise to the nitrate mixture at a rate of 1 drop / s. After the addition was complete, the Erlenmeyer flask was sealed and stirred at 60°C for 4 h. After stirring, the mixture was transferred to a 70°C incubator for 12 hours of aging. After aging, the precipitate was filtered and washed with deionized water until the filtrate was neutral. The filter cake was collected and dried in an oven at 110°C for 10 hours. The dried filter cake was ground into a uniform powder, placed in a crucible, and calcined in a muffle furnace at 400°C for 4 hours at a heating rate of 5°C / min to obtain a metal oxide precursor. Finally, the precursor was reduced in a reaction tube under H2 atmosphere by a programmed temperature rise. The specific operation of the temperature rise reduction was as follows: first, it was treated at 200°C for 1 hour, and then reduced at 350°C for 3 hours. The heating rate was 4°C / min, and the gas flow rate was 40 ml / min. After the reduction was completed, the modified catalyst was obtained.

[0025] Example 3 A cerium-modified copper-based catalyst, the preparation method of which is as follows: 19.42 g Cu(NO3)2·3H2O, 9.55 g Zn(NO3)2·6H2O, 2.87 g Al(NO3)3·9H2O, and 6.34 g Ce(NO3)3·6H2O were weighed out and added to a 500 mL Erlenmeyer flask. Then, 220 mL of deionized water was added, and the flask was placed in a 60°C water bath and stirred for 30 min to dissolve, preparing a nitrate mixed solution with a total metal ion concentration of 0.5 mol / L. Separately, 17.31 g of anhydrous sodium carbonate was weighed out and placed in a beaker, then 170 mL of deionized water was added and sonicated to dissolve it. After complete dissolution, the solution was transferred to a 500 mL dropping funnel and added dropwise to the nitrate mixed solution at a rate of 1 drop / s. After the addition was complete, the Erlenmeyer flask was sealed and stirred at 60°C for 4 h. After stirring, the mixture was transferred to a 70°C incubator for 12 hours of aging. After aging, the precipitate was filtered and washed with deionized water until the filtrate was neutral. The filter cake was collected and dried in a 120°C oven for 10 hours. The dried filter cake was ground into a uniform powder, placed in a crucible, and calcined in a muffle furnace at 400°C with a heating rate of 5°C / min for 4 hours to obtain a metal oxide precursor. Finally, the precursor was reduced in a reaction tube under H2 atmosphere by a programmed temperature rise. The specific operation of the temperature rise reduction was as follows: first, it was treated at 200°C for 1 hour, and then reduced at 350°C for 3 hours. The heating rate was 4°C / min, and the gas flow rate was 20 ml / min. After the reduction was completed, the modified catalyst was obtained.

[0026] Comparative Example 1 The difference from Example 1 is that Ce(NO3)3·6H2O is not added, but otherwise the same as in Example 1, and a Cu / ZnO / Al2O3 catalyst is prepared.

[0027] Comparative Example 2 The difference from Example 1 is that Ce(NO3)3·6H2O is replaced with Mg(NO3)2·6H2O, while the rest is the same as in Example 1, and a Cu / ZnO / Al2O3 / MgO catalyst is prepared.

[0028] Test Example 1: Characterization of the Catalyst I. XRD Characterization of Catalysts To investigate the phase composition and the state of the active components of the catalysts, X-ray diffraction (XRD) was performed on the Cu / ZnO / Al2O3 catalyst samples of Comparative Example 1 and the Cu / ZnO / Al2O3 / CeO2 catalyst samples of Example 1. The results are as follows: Figure 1 As shown.

[0029] Figure 1Both the Cu / ZnO / Al2O3 and Cu / ZnO / Al2O3 / CeO2 catalysts exhibited distinct diffraction peaks, indicating their good crystallinity. The first set of diffraction peaks appeared at 2θ angles of approximately 43.4°, 50.5°, and 74.1°, corresponding to the (111), (200), and (220) crystal planes of the face-centered cubic structure of metallic copper (Cu), respectively, which is in high agreement with the standard PDF card (PDF#04-0836). This result indicates that during the preparation and reduction activation of the catalysts, the oxidized copper species in the precursor were successfully reduced to metallic Cu. 0 Copper metal, as the main active component of this catalyst system, provides the necessary hydrogenation / dehydrogenation active centers for the N-methylation reaction of aniline and methanol.

[0030] The second set of significant diffraction peaks appears at a 2θ angle of approximately 36.2°, corresponding to the (002) crystal plane of the hexagonal wurtzite structure of zinc oxide (ZnO), consistent with the standard PDF card (PDF#36-1451). ZnO acts as a structural and electronic catalytic agent in the catalyst. By forming a strong metal-support interaction with Cu particles, it modulates the electron cloud density on the Cu surface, thereby affecting the adsorption and activation behavior of reactant molecules and playing a crucial role in regulating catalytic activity and selectivity.

[0031] Notably, no sharp diffraction peaks belonging to crystalline Al₂O₃ were detected in the XRD patterns of either catalyst. This indicates that the Al₂O₃ component exists primarily in an amorphous form within the catalyst. During the co-precipitation preparation process, Al… 3+ The ions are highly dispersed in the precursor framework and form amorphous alumina after calcination. This amorphous structure has a large specific surface area and abundant surface hydroxyl groups, which not only improves the mechanical strength and thermal stability of the catalyst, but also provides a good dispersion support for the active components Cu and ZnO.

[0032] Furthermore, no characteristic diffraction peaks belonging to the cerium oxide crystalline phase were observed in the catalyst with the Ce promoter. This phenomenon indicates that the cerium component is highly dispersed in the catalyst, without significant agglomeration or the formation of large crystalline particles. The highly dispersed cerium oxide species are beneficial for fully exerting their role as a structure promoter, effectively inhibiting the migration and sintering of active components during the reaction process by forming a confined environment around the Cu particles.

[0033] II. XPS Characterization of Catalysts The Cu / ZnO / Al2O3 / CeO2 catalyst prepared in Example 1 and the Cu / ZnO / Al2O3 catalyst prepared in Comparative Example 1 were subjected to full-spectrum and fine-spectrum analysis using X-ray photoelectron spectroscopy (Thermo Scientific K-Alpha, USA). The results are as follows: Figure 2 As shown. Figure 2 Figure (a) shows the Cu2p XPS spectra of Cu / ZnO / Al2O3 and Cu / ZnO / Al2O3 / CeO2; Figure 2 Figure (b) shows the Ce 3d XPS spectrum of Cu / ZnO / Al2O3 / CeO2.

[0034] from Figure 2 As can be observed in Figure (a), two distinct main peaks appear at binding energies of approximately 932.2 eV and 952.1 eV, corresponding to the spin-orbit splitting peaks of Cu 2p3 / 2 and Cu 2p1 / 2, respectively. The binding energy near 932.2 eV is the Cu... 0 Therefore, it can be concluded that the main copper species on the catalyst surface is zero-valent copper (Cu). 0 Compared to the reference sample, Figure 2 XPS spectral analysis of Cu / ZnO / Al2O3 / CeO2 (Cu6.5Zn2.5Al1Ce1) in Figure (a) shows that the introduction of Ce significantly alters the copper valence state distribution on the surface of the Cu / ZnO / Al2O3 catalyst. Compared to the undoped catalyst, it mainly shows metallic Cu. 0 Unlike other catalysts, the Ce-doped catalyst spectrum exhibits distinct shoulder peaks and strong satellite peaks at higher binding energies, directly confirming the presence of high-valence copper species (Cu) on the catalyst surface. + or Cu 2+ The increase in the proportion of ) while zero-valent copper (Cu) 0 The proportion of ) will decrease accordingly.

[0035] Combination Figure 2 (b) Ce valence distribution in Ce 3d spectrum (Ce 4+ / Ce 3+ This suggests that there is a significant electron transfer between Ce and Cu (Ce 3+ + Cu 2+ Ce 4+ + Cu + This electron transfer stabilizes the Cu on the catalyst surface. + Species. Cu in N-methylation reaction 0 It is responsible for the dissociation and recombination of hydrogen, while Cu + As a Lewis acid site, the C=N double bond of the polarized imine intermediate makes it more susceptible to attack by hydrogen atoms, thereby significantly increasing the hydrogenation rate.

[0036] III. Morphological Characterization of Catalysts To investigate the microstructure characteristics of the catalysts, scanning electron microscopy (SEM) was used to observe the morphology of the Cu / ZnO / Al2O3 reference sample of Comparative Example 1 and the Cu / ZnO / Al2O3 / CeO2 modified catalyst of Example 1 with Ce introduced at different magnifications. The results are as follows: Figure 3 As shown, Figure 3 Figure a shows the microstructure of Cu / ZnO / Al2O3; Figure 3 Figure b shows the microstructure of Cu / ZnO / Al2O3 / CeO2.

[0037] At low magnification (10 μm), from Figure 3 Figure a shows that Cu / ZnO / Al2O3 exhibits agglomerates composed of numerous tightly packed microparticles. These agglomerates have uneven size distribution and irregular shapes, exhibiting an overall "fragmented" or "clustered" aggregation morphology. This phenomenon indicates that strong interaction forces exist between primary nanoparticles during catalyst preparation or calcination, leading to their spontaneous aggregation into secondary particles. This highly agglomerated structure limits the specific surface area of ​​the catalyst to some extent and may hinder the diffusion and mass transfer of reactant molecules. Further increasing the magnification to 1 μm allows for a clearer observation of the morphology of the primary particles constituting the agglomerates. The results show that the interfaces between primary particles are relatively blurred, exhibiting a tightly sintered or connected microstructure, and the particle size distribution is relatively wide. This morphological characteristic is an inherent structural feature of Cu-Zn-Al oxide catalysts prepared by typical co-precipitation methods. The tight packing and severe agglomeration of primary particles not only reduce the effective external surface area and the number of accessible active sites of the catalyst but may also lead to complex pore structures, forming mass transfer bottlenecks within the particles, thus adversely affecting the catalytic reaction efficiency.

[0038] from Figure 3 As observed in Figure b, the introduction of Ce significantly optimized the catalyst morphology, inhibiting particle growth and agglomeration. Compared to the baseline sample Cu / ZnO / Al2O3, the introduction of Ce resulted in the formation of irregular, lamellar particles. This may be due to the different grain growth habits of CeO2. Furthermore, the introduction of Ce created a porous and rough surface, which may be related to the effects of Ce under reducing and oxidizing atmospheres. 4+ / Ce 3+ The interconversion of these metals is closely related. This conversion process is often accompanied by the release and absorption of lattice oxygen, leading to the generation of more defects and pores on the catalyst surface or in the bulk phase. In addition, EDS characterization results of Cu / ZnO / Al2O3 (Cu6.5Zn2.5Al1) and Cu / ZnO / Al2O3 / CeO2 show that the various metals are well dispersed.

[0039] The two samples were further scanned using a transmission electron microscope (TEM), and the results are as follows: Figure 4 As shown. Among them, in Figure 4 a is a Cu / ZnO / Al2O3 catalyst. Figure 4 b is a Cu / ZnO / Al2O3 / CeO2 catalyst.

[0040] from Figure 4 The TEM images showed that the two catalysts had uniformly distributed crystal particles and clearly visible lattice stripes, indicating that the Cu / ZnO / Al2O3 and Cu / ZnO / Al2O3 / CeO2 catalysts had good dispersibility and crystallization performance. Comparing the two (observed at 20 nm), the Ce-modified Cu / ZnO / Al2O3 / CeO2 catalyst showed better particle dispersibility.

[0041] IV. Determination of specific surface area and pore size The specific surface area and pore volume of the samples were determined using a BET specific surface area and pore size analyzer (ASAP2460, Micromeriti SA, USA) at 77 K using the N2 adsorption-desorption isotherm method. The specific surface areas of Cu / ZnO / Al2O3 and Cu / ZnO / Al2O3 / CeO2 catalysts are shown in Table 1.

[0042] Table 1. Specific surface area (BET), pore volume, and average pore size of Cu / ZnO / Al2O3 and Cu / ZnO / Al2O3 / CeO2

[0043] The results indicate that Ce modification significantly optimizes the texture properties of the Cu / ZnO / Al2O3 catalyst, making it more suitable for the N-methylation of aniline. The specific surface area of ​​the modified catalyst significantly increased from 22.3522 m² / g to 48.8459 m² / g, meaning more active sites are exposed, which is beneficial for the adsorption of reactants (aniline and methanol), thereby improving the reaction rate and conversion. Simultaneously, the pore volume slightly increased from 0.2363 cm³ / g to 0.2680 cm³ / g, indicating that the introduction of Ce did not block the pores but rather promoted the maintenance of the mesoporous structure, facilitating reactant diffusion and product desorption, and avoiding carbon deposition. The average pore size decreased from 26.38 nm to 20.60 nm, but this may be due to particle refinement, forming more small-sized mesopores, which helps improve selectivity and reduce macromolecular side reactions. The data in the table above demonstrate that Ce modification, by optimizing the specific surface area, pore volume, and pore size, provides strong structural support for the efficient catalytic N-methylation of aniline.

[0044] Test Example 2: Catalytic Performance Evaluation I. Catalytic activity and selectivity of the catalyst for the target product A continuous synthesis of N-methylaniline was carried out in a fixed-bed reactor. Aniline and methanol were used as raw materials, with two parallel-flow pumps for injection. The aniline injection flow rate was 0.01 mL / min, and the methanol injection flow rate was 0.006 mL / min (the molar ratio of aniline to methanol was 1:1.35 under these injection conditions); the reaction pressure was atmospheric pressure; the reaction temperature was 220℃; the gas flow rate (N2) was 50 mL / min; and the weight hourly space velocity (WHSV) was 0.45 h⁻¹. -1 Based on the experimental results, the activity, selectivity and stability of the catalysts were compared and analyzed.

[0045] The fixed-bed reactor is installed as follows: First, 0.3 g of quartz wool is placed at the catalyst support plate in the reaction tube as a base and properly compressed to prevent excessive gas flow from blowing the catalyst off. Then, 2 g of catalyst is added. Another 0.3 g of quartz wool is added to stabilize the catalyst layer. Finally, 2 g of quartz sand is placed on top of the bed as a top packing to prevent the initial gas or liquid flow from dispersing the catalyst. Simultaneously, the interparticle gaps in the sand rectify and distribute the carrier gas and feed liquid, eliminating channeling and ensuring uniform fluid flow through the catalyst bed. Furthermore, because quartz sand has excellent thermal conductivity, it can preheat the incoming cold feed, reducing and buffering bed temperature fluctuations caused by liquid introduction, thereby maintaining a stable reaction temperature and space velocity, ensuring the accuracy and repeatability of catalyst performance evaluation.

[0046] After the reaction tube was installed, an airtightness check was performed to confirm that the system was well sealed. Then, under a hydrogen flow rate of 20 mL / min, the temperature was increased to 310 °C at a rate of 5 °C / min for in-situ reduction of the catalyst for 3 hours. After reduction, nitrogen was switched at a flow rate of 50 mL / min to cool the reaction tube to 220 °C. Simultaneously, two horizontal flow pumps were started, delivering the reaction solution to the pipeline at set flow rates and set space velocity, respectively, to officially begin the continuous reaction. Considering that the system had not yet reached a stable state in the initial stage of the reaction, the product from the first 7 hours was not used; the reaction solution from 7 to 9 hours was collected for analysis to ensure the accuracy and representativeness of the evaluation results. Qualitative analysis was performed using gas chromatography-mass spectrometry (GC-MS), and quantitative analysis was performed using gas chromatography (GC), to determine product selectivity and reactant conversion rate to systematically evaluate the catalyst performance. Samples were taken and analyzed every 6 hours, and the average conversion rate and selectivity were calculated. The catalytic performance of the catalyst samples prepared in Examples 1-3 and Comparative Examples 1-2 is shown in Table 2 below.

[0047] Table 2 Catalytic performance of catalysts in Examples 1-3 and Comparative Examples 1-2

[0048] As shown in the table above, compared with Comparative Example 1, the aniline conversion rate of Examples 1-3 is above 90%, while the selectivity of N-methylaniline is slightly improved. This indicates that the modified catalyst significantly improves the aniline conversion rate while maintaining the high selectivity of N-methylaniline. In Example 1, the aniline conversion rate is as high as 94.8%, the selectivity of N-methylaniline is as high as 98.5%, and the selectivity of N,N-dimethylaniline is 1.2%. From the total selectivity of N-methylaniline and N,N-dimethylaniline in Examples 1-3, it is above 99%, and can even approach 100%, showing excellent ability to suppress other side reactions.

[0049] Compared with Comparative Example 1, Comparative Example 2 showed a decrease in aniline conversion rate, indicating that Mg metal element not only failed to play a synergistic role with the copper-based catalyst, but also exhibited a certain antagonistic effect, greatly reducing the catalytic activity of the catalyst. This shows that not any metal element can modify the copper-based catalyst.

[0050] II. Catalytic stability test of the catalyst The stability of the Cu / ZnO / Al2O3 / CeO2 catalyst prepared in Example 1 was further tested. The catalyst of Example 1 was reacted continuously under the above conditions for 500 h, and the test results of its aniline conversion and N-methylaniline selectivity are shown in Table 3 below.

[0051] Table 3. Stability test results of the Cu / ZnO / Al2O3 / CeO2 catalyst in Example 1

[0052] The above results show that the catalyst Cu / ZnO / Al2O3 / CeO2 obtained in Example 1 still has good catalytic activity for aniline during a continuous reaction of 500 h, with the conversion rate of aniline remaining almost above 90%. At the same time, the N-methylaniline selectivity is also above 98%, indicating that the modified catalyst has good hydrothermal stability, long service life, and is not easily deactivated. It can still maintain good catalytic activity and high N-methylaniline selectivity for a long time under high temperature conditions.

[0053] Although the present invention has been described using the above preferred embodiments, it is not intended to limit the scope of protection of the present invention. Any changes and modifications made by those skilled in the art to the above embodiments without departing from the spirit and scope of the present invention shall still fall within the scope of protection of the present invention.

Claims

1. A method for preparing a cerium-modified copper-based catalyst, characterized in that, Includes the following steps: (1) Dissolve water-soluble copper, zinc, aluminum and cerium salts in deionized water to obtain a metal salt solution; (2) The aqueous solution of the precipitant is added dropwise to the metal salt solution for co-precipitation. The resulting mixture is then aged, filtered, washed with water, dried, and calcined to obtain the metal oxide precursor. (3) The metal oxide precursor is placed in a reaction tube and a reduction reaction is carried out under a hydrogen atmosphere to obtain a cerium-modified copper-based catalyst.

2. The method for preparing the cerium-modified copper-based catalyst according to claim 1, characterized in that, In step (1), the mass ratio of copper salt, zinc salt, aluminum salt and cerium salt is (10-20): (4-10): (1-5): (2-7).

3. The method for preparing the cerium-modified copper-based catalyst according to claim 1, characterized in that, In step (2), the precipitant is sodium carbonate, the concentration of the sodium carbonate aqueous solution is 0.09 g / mL, and the co-precipitation temperature is 60℃.

4. The method for preparing the cerium-modified copper-based catalyst according to claim 1, characterized in that, In step (2), the drying temperature is 100-120℃ and the time is 8-12h.

5. The method for preparing the cerium-modified copper-based catalyst according to claim 1, characterized in that, In step (2), the calcination temperature is 400℃, the time is 4 h, and the heating rate is 5℃ / min.

6. The method for preparing the cerium-modified copper-based catalyst according to claim 1, characterized in that, In step (3), the metal oxide precursor is first treated at 200°C for 1 h before the reduction reaction, and then heated to 300-400°C for 3 h. The heating rate is 3-5°C / min, and the hydrogen flow rate is 20-100 ml / min.

7. The cerium-modified copper-based catalyst prepared by the method according to any one of claims 1-6.

8. The application of the cerium-modified copper-based catalyst according to claim 7 in the catalytic reaction of aniline and methanol to produce N-methylaniline.

9. The application according to claim 8, characterized in that, In the reaction of aniline and methanol, the reaction temperature is 220-260℃, the pressure is 0.1-4MPa, and the molar ratio of aniline to methanol is 1:(1-2).

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