Preparation method of Pd / TiO2 low-supported catalyst and method for preparing cyclohexanone by efficiently catalyzing selective hydrogenation of phenol by using Pd / TiO2 low-supported catalyst

Abstract

The invention relates to a preparation method of a Pd / TiO2 low-supported catalyst and cyclohexanone prepared by efficiently catalyzing selective hydrogenation of phenol by using the Pd / TiO2 low-supported catalyst, and belongs to the technical field of catalytic hydrogenation. According to the catalyst, P25 serves as a precursor, NaM (M = F, Cl, Br, I, S or P) serves as a modifier, a modified material is obtained through stirring, acid pickling and drying, ultra-low amount of Pd is loaded on modified TiO2 through a coprecipitation method, Pd nanoparticles are reduced and loaded through hydrazine hydrate, and the catalyst is prepared. The preparation method has the advantages that the OH group on TiO2 is replaced by the introduction of anions, an M-Ti < 4 + > bond is formed, the electron density of Ti is enhanced, the electron-rich capability of PdNPs and the stability of PdNPs in a low oxidation state are enhanced, the interaction between the carrier and active components is promoted, the catalytic performance of the catalyst for preparing cyclohexanone through phenol hydrogenation is remarkably enhanced, and the catalyst is suitable for industrial production. And the prepared catalyst has good catalytic stability.

Description

Technical Field

[0001] The invention relates to a preparation method of a Pd / TiO2 low-load catalyst and the method for efficiently catalyzing the selective hydrogenation of phenol to prepare cyclohexanone, belonging to the technical field of catalytic hydrogenation. Background Art

[0002] Cyclohexanone is an important chemical solvent widely used in the paint and printing industries. It is also used in the manufacture of herbicides, antihistamines, polyesters, and synthetic resins. Cyclohexanone (C=O) is used as a raw material to produce caprolactam and adipic acid, which are then converted into nylon 6 and nylon 66, respectively. Industrial production of cyclohexanone can be achieved through three pathways: cyclohexane oxidation, benzene conversion, and phenol hydrogenation. The first pathway is lengthy, energy-intensive, and uses large amounts of raw materials, producing undesirable secondary products, reducing C=O selectivity, and requiring additional separation stages for the main product. The second route is quite lengthy, requiring numerous steps and involving numerous reactions and products. The third pathway, phenol hydrogenation to cyclohexanone, can be divided into a one-step process and a two-step process. The one-step process is the most favored of all the reactions discussed because it shortens the number of steps and time, improves hydrogen utilization efficiency, reduces byproducts, and eliminates the need to separate phenol from other byproducts. However, cyclohexanone is readily further hydrogenated to cyclohexanol after generation, thus exploring suitable reaction conditions to achieve high selectivity for cyclohexanone and high conversion of phenol is a challenge.

[0003] Supported metal nanoparticles (NPs) are widely used in hydrogenation reactions because the interaction between them and the support ensures excellent stability and improves the catalytic performance of the metal. It is reported that noble metals such as Pd, Pt, Rh, and Ru have high hydrogenation activity even at low temperatures, which is due to their good ability to absorb and activate hydrogen. Among them, Pd is the most active metal in hydrogenation reactions, and Pd-based catalysts have attracted much attention due to their high activity and close relationship with the properties of the active support. Studies have shown that Zhou et al. prepared alkali metal K + / Na + The alkali metal-induced change in the electronic structure of Pd NPs enhanced the phenol conversion rate, achieving a 99% phenol conversion rate and 99% cyclohexanone selectivity under mild conditions (Green Chemistry, 2017, 19: 3585-3594). Zhang et al. conducted a ZIF-derived N-doped carbon (CN) material with oxygen vacancies (O V ) was modified with TiO2 nano-islands to prepare Pd@CN@TiO2 (2wt%) catalyst with O VThe TiO2 nanoislands can improve the dispersion of Pd, generate more Pd elements on the support surface, enhance Pd's antioxidant properties, and induce electron-rich Pd NPs, which exhibit excellent catalytic performance in the catalytic hydrogenation of phenol (Applied Surface Science, 2019, 488:555-564). Low-loaded catalysts significantly reduce the cost of precious metal-supported catalyst research and are particularly important. However, designing a catalyst with extremely low loading (0.2 wt%) and high catalytic performance remains a huge challenge.

[0004] TiO2 is an inorganic compound that is a white solid or powdered amphoteric oxide. TiO2 exists in three mineral crystal structures: rutile, anatase, and brookite. Rutile is more stable, and anatase and brookite easily convert to rutile when heated. TiO2 has been introduced as an alternative carrier for heterogeneous catalysts due to its variable phase structure and redox chemical properties, as well as the high activity of its nanoparticles in various reactions at low pressures and low temperatures. Compared to metal oxides in other semiconductor and semimetal materials, the Ti-O bond in TiO2 is more polar, and water adsorbed on the surface dissociates due to polarization, easily forming hydroxyl groups. These surface hydroxyl groups can improve the performance of TiO2 as an adsorbent and various monomers, facilitating surface modification.

[0005] The surface area, acidity, pore structure and other characteristics of the catalyst support have a significant impact on the performance of the hydrogenation catalyst. To solve the problem of poor activity at low loading, modifying the support is a feasible way to improve the catalytic activity. The addition of some anionic modifiers can improve the structure and chemical properties of the support. Jiang et al. adjusted the surface morphology, hydrophobicity, acidity and electronic properties of the Pd / HZSM-5 catalyst by controlling the F doping amount. They found that the highly electronegative F can replace the OH group on the HZSM-5 to form a special F-Al structure, which inhibits the adsorption of water on the active sites of the catalyst metal, thereby effectively improving the catalytic performance of the selective hydrodeoxygenation of biomass-derived ketones (Renewable Energy, 2021, 179: 1262-1270). Summary of the Invention

[0006] The present invention provides an anion-modified low-load catalyst and a preparation method thereof, which mainly solves the problem of high cost of current noble metal-loaded catalysts in the phenol hydrogenation reaction process.

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

[0008] A method for preparing a catalyst for hydrogenating phenol to cyclohexanone, comprising the following steps:

[0009] Step 1. Add 2 g / L P25, NaM (M = F, Cl, Br, I, S, P), and HM (M = F, Cl, Br, I, S, P) in a molar ratio of 2:1 to deionized water, stir at room temperature for 1-2 hours, centrifuge the reaction suspension, wash three times with deionized water and anhydrous ethanol, and dry at 60-80°C for 10-12 hours;

[0010] Step 2. Disperse the P25-M (M = F, Cl, Br, I, S, P) obtained in step 1 in deionized water to prepare a suspension;

[0011] Step 3. Add PdCl2·2H2O aqueous solution to the suspension obtained in step 2, with the volume ratio of PdCl2·2H2O aqueous solution to the suspension being 1:30, and stir at room temperature for 1 h (denoted as Pd / P25-M);

[0012] Step 4. Add 1 mol / L NaOH to step 3 and stir for 30 min-1 h;

[0013] Step 5. Place the solution obtained in step 4 in a 75°C water bath, add the hydrazine hydrate solution dropwise to the suspension, and stir vigorously for 20 min-1 h. Cool the reduced suspension, centrifuge and wash 3 times, and dry it in a vacuum drying oven. The hydrazine hydrate content is 80%, and the mass ratio of the added hydrazine hydrate to the loaded Pd is 25:1.

[0014] The mass fraction of the PdCl2·2H2O aqueous solution in step 6 is 0.2-5wt%.

[0015] The amount of NaOH added in step 7 must make the solution pH>11.

[0016] The reduced suspension in step 8 needs to be washed with deionized water until it becomes neutral, and vacuum drying refers to vacuum drying at 80° C. for 12 h.

[0017] The modified catalyst is applied to phenol hydrogenation reaction, and the modified catalyst is added to a 2wt% phenol aqueous solution; the reaction temperature is controlled at 80-100°C, the reaction pressure is 0.5MPa, and the reaction time is 1-5h. After the reaction is completed, the supernatant is centrifuged, extracted, dried, and then the product composition is analyzed by gas chromatography.

[0018] Beneficial effects of the present invention:

[0019] 1. The co-precipitated loaded Pd nanoparticles reduced the deposition on the outer surface and increased the dispersion of Pd, thereby effectively improving the catalytic activity of the phenol hydrogenation reaction to cyclohexanone.

[0020] 2. The modification of anions enhances the electron density of Ti, affecting the adsorption and activation of the catalyst to the reactants. It also enhances the electron-rich ability of PdNPs and their stability in low oxidation states, promotes the interaction between the support and the active components, and significantly enhances the catalytic performance of phenol hydrogenation to cyclohexanone. The resulting catalyst has good catalytic stability.

[0021] 3. Under the premise of a low loading of 0.2 wt%, the conversion rate of phenol can reach about 66% and the selectivity of cyclohexanone can reach 99.9% through the modification of anions.

[0022] 4. The anion-modified Pd / TiO2 low-load catalyst of the present invention is used for the selective hydrogenation of phenol to prepare cyclohexanone. It has significantly improved catalytic activity and selectivity compared with the unmodified Pd / TiO2 catalyst and has potential application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is the XRD spectrum of Pd / P25-M prepared in Example 1.

[0024] Figure 2 TEM characterization of Pd / P25-M prepared in Example 1. DETAILED DESCRIPTION

[0025] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0026] Example 1

[0027] (1) Preparation of P25-M (M = F, Cl, Br, I, S, P)

[0028] 2 g / LP25, NaM (M = F, Cl, Br, I, S, P) and HM (M = F, Cl, Br, I, S, P) were added to deionized water in a molar ratio of 2:1, stirred at room temperature for 1-2 hours, the suspension after reaction was centrifuged, washed three times with deionized water and anhydrous ethanol, and dried at 60-80 ° C for 10-12 hours to obtain an anion modified catalyst.

[0029] (2) Preparation of Pd / P25-M (M=F, Cl, Br, I, S, P)

[0030] 0.1 g of P25-M (M = F, Cl, Br, I, S, P) was dispersed in deionized water, a PdCl2·2H2O aqueous solution with a mass fraction of 0.2-5 wt% was added, and the mixture was stirred at room temperature for 1 h. 1 mol / L NaOH was added dropwise until the solution pH was > 11, and the mixture was stirred for 30 min-1 h. The resulting solution was placed in a 75 ° C water bath, and 3-8 mL of 80% hydrazine hydrate solution was added dropwise. The mixture was stirred vigorously for 20 min-1 h. The reduced suspension was cooled, centrifuged and washed three times, and dried in a vacuum drying oven at 80 ° C for 12 h.

[0031] Example 2

[0032] Effects of different loading amounts of Pd / P25 before modification on the hydrogenation of phenol to cyclohexanone

[0033] 0.05g of unmodified Pd / P25 catalyst with varying loadings was added to a polytetrafluoroethylene (PTFE) inner sleeve, along with 10mL of a 2wt% aqueous phenol solution. The catalyst was then packaged in a 50mL stainless steel autoclave. The reaction temperature was controlled at 80-100°C, the reaction pressure was 0.5MPa, and the reaction time was 1-5h. After the reaction, the supernatant was centrifuged, extracted, dried, and the product composition analyzed by gas chromatography. It can be seen that, up to 2.5wt%, the phenol conversion rate remained unchanged as the Pd loading decreased due to its high activity. Subsequently, the phenol conversion rate decreased, while the cyclohexanone selectivity increased slightly. When the Pd loading was reduced to 1-0.2wt%, neither the phenol conversion rate nor the cyclohexanone selectivity showed significant changes. Extending the reaction time to 5h revealed a significant improvement in the phenol conversion rates at both 1wt% and 0.6wt%, while the phenol conversion rate at 0.2wt% only slightly increased. Therefore, the present invention selected 0.2wt% as the loading standard to investigate the changes in phenol hydrogenation performance before and after modification.

[0034] Table 1. Phenol hydrogenation results of different loading amounts of Pd / P25

[0035] Example 3

[0036] Effect of anion-modified P25 catalyst with low 0.2 wt% Pd loading on phenol hydrogenation to cyclohexanone

[0037] 0.05g of anion-modified P25 catalyst with a low Pd loading of 0.2wt% was added to a polytetrafluoroethylene (PTFE) inner sleeve, along with 10mL of a 2wt% aqueous phenol solution. The mixture was then placed in a 50mL stainless steel autoclave. The reaction temperature was controlled at 80-100°C, the pressure at 0.5MPa, and the reaction time was 5h. After the reaction, the supernatant was centrifuged, extracted, dried, and the product composition analyzed by gas chromatography. It can be seen that all four anion-modified P25 catalysts improve the performance of the low Pd loading in the hydrogenation of phenol to cyclohexanone. While the performance results for each anion modification were similar, all significantly outperformed the unmodified P25 catalyst with a low Pd loading in the hydrogenation of phenol to cyclohexanone.

[0038] Table 2. Effects of P25 catalysts with low 0.2 wt% Pd loading after modification with different anions on the hydrogenation of phenol to cyclohexanone

[0039] In addition, it should be understood that although this specification describes hydrogenation catalysis according to the embodiment, not every embodiment contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for preparing a low-load Pd / TiO2 catalyst and its efficient catalytic selective hydrogenation of phenol to produce cyclohexanone, characterized in that: Here are the steps: Step 1. Add 2 g / L P25, NaM (M = F, Cl, Br, I, S, P), and HM (M = F, Cl, Br, I, S, P) in a molar ratio of 2:1 to deionized water, stir at room temperature for 1-2 hours, centrifuge the reaction suspension, wash three times with deionized water and anhydrous ethanol, and dry at 60-80°C for 10-12 hours; Step 2. Disperse the P25-M (M = F, Cl, Br, I, S, P) obtained in step 1 in deionized water to prepare a suspension; Step 3. Add PdCl2·2H2O aqueous solution to the suspension obtained in step 2, with the volume ratio of PdCl2·2H2O aqueous solution to the suspension being 1:30, and stir at room temperature for 1 h (denoted as Pd / P25-M); Step 4. Add 1 mol / L NaOH to step 3 and stir for 30 min-1 h; Step 5. Place the solution obtained in step 4 in a 75°C water bath, add the hydrazine hydrate solution dropwise to the suspension, and stir vigorously for 20 min-1 h. Cool the reduced suspension, centrifuge and wash 3 times, and dry it in a vacuum drying oven. The hydrazine hydrate content is 80%, and the mass ratio of the added hydrazine hydrate to the loaded Pd is 25:

1.

2. The preparation method according to claim 1, characterized in that The mass fraction of the PdCl2·2H2O aqueous solution in step 3 is 0.2-5wt%.

3. The preparation method according to claim 1, characterized in that The amount of NaOH added in step 4 must make the solution pH>11.

4. The preparation method according to claim 1, characterized in that The suspension after reduction in step 5 needs to be washed with deionized water until it becomes neutral.

5. The preparation method according to claim 1, characterized in that The vacuum drying in step 5 refers to vacuum drying at a temperature of 80° C. for 12 h.

6. The modified catalyst was applied to the phenol hydrogenation reaction by adding the modified catalyst to a 2 wt% aqueous phenol solution; the reaction temperature was controlled at 80-100°C, the reaction pressure was 0.5 MPa, and the reaction time was 1-5 h. After the reaction was completed, the supernatant was extracted by centrifugation, dried, and the product composition was analyzed by gas chromatography.

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