Preparation method and application of nitrogen-doped graphene-supported noble metal catalyst

Abstract

The invention belongs to the field of catalysis technology, and in particular to a method for preparing a nitrogen-doped graphene-supported noble metal catalyst, and its application in α, β-unsaturated aldehyde selective hydrogenation. The method uses ammonia as plasma gas source, adjusts discharge power under a certain vacuum degree, produces high-energy ammonia plasma, uniformly bombards graphene (GR) surface in a short time, obtains nitrogen-doped graphene (NPGR), and then prepares nitrogen-doped graphene-supported noble metal catalyst by sodium borohydride reduction method. The catalyst is used for the selective hydrogenation reaction of aqueous phase α, β-unsaturated aldehydes. Compared with GR-supported noble metal catalysts, the catalytic activity of nitrogen-doped graphene-supported noble metal catalysts is greatly improved, and shows higher unsaturated alcohol selectivity.

Description

Technical Field

[0001] The present invention belongs to the field of catalysis technology and relates to a preparation method and application of a nitrogen-doped graphene-supported noble metal catalyst, and specifically relates to a method for preparing nitrogen-doped graphene by plasma-modified graphene and using the nitrogen-doped graphene as a carrier to support a noble metal to prepare a catalyst, and the application of the catalyst in the selective hydrogenation of α,β-unsaturated aldehydes. Background Art

[0002] In recent years, a number of novel support materials have been developed for the selective hydrogenation of unsaturated aldehyde / ketone conjugated molecules (such as citral and cinnamaldehyde). Graphene, as a novel support material, offers advantages such as excellent conductivity, high electron mobility, a high specific surface area, and stable chemical properties. The preparation of supported metal nanocatalysts using graphene as a support has become a hot topic of research.

[0003] However, the original graphene surface is hydrophobic, which is conducive to the adsorption of organic compounds thereon, but is unfavorable for the anchoring of metal nanoparticles, resulting in lower catalytic activity. Therefore, in order to improve the anchoring and dispersion of metal nanoparticles on the graphene surface, its surface can be functionalized, such as doping heteroatoms (O, N) etc., to provide a large number of anchoring sites and improve surface hydrophilicity. The traditional treatment method is to first treat the graphene material with acid (such as concentrated H2SO4 and HNO3) to cause partial oxidation of the surface. After the surface is loaded with metal nanoparticles, the hydrophobic surface is restored through chemical reduction or heat treatment to improve the adsorption of organic substrates on the catalyst and promote the generation of the reaction. This treatment process is relatively complicated and also brings certain environmental problems to the treatment of waste acid liquid.

[0004] Plasma is known as the fourth state of matter. It is an aggregation of ions, excited atoms, ionized atoms / molecules, free electrons, and free radicals. Plasma can generally be used for surface modification of materials. On the one hand, it changes the degree of surface defects of carbon-based materials through the bombardment of high-energy particles. On the other hand, it introduces corresponding functional groups onto the surface of the material through the grafting of free radicals at the surface active sites, but does not destroy the overall properties and structure of the material. Chinese patent CN112495374A introduces oxygen-containing functional groups on the surface of graphene through oxygen plasma modification technology to improve its surface hydrophilicity. However, the stronger the hydrophilicity of the material, the better. Hydrophilicity is conducive to the anchoring of metal nanoparticles on it, but is not conducive to the adsorption of organic substrates. In addition, compared with nitrogen atoms, oxygen has a stronger electronegativity, and the electronic conjugation between oxygen and graphene after oxygen doping is relatively weak. Secondly, compared with oxygen, an active element, nitrogen doping is more stable.

[0005] Therefore, how to improve the stability of graphene-based catalysts and enhance their catalytic performance is an urgent problem to be solved. Summary of the Invention

[0006] The purpose of the present invention is to provide a method for preparing a loaded noble metal catalyst using nitrogen-doped graphene as a carrier, and to use the nitrogen-doped graphene for the selective hydrogenation of α,β-unsaturated aldehydes in an aqueous phase.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] A method for preparing a nitrogen-doped graphene-supported noble metal catalyst comprises the following steps:

[0009] (1) A certain mass of graphene powder was placed in a glass culture dish and placed in a low-temperature plasma device; the plasma discharge parameters (including gas source, discharge power, vacuum degree, and discharge time) were adjusted; first, the graphene surface was activated using oxygen plasma, and the discharge parameters for generating oxygen plasma were: power of 160 W, vacuum degree of 20 Pa, and duration of 3 min;

[0010] (2) Switching to ammonia gas, bombarding the activated graphene surface with ammonia plasma generated by discharge, ultimately doping nitrogen-containing functional groups into the graphene to obtain nitrogen-doped graphene (NPGR);

[0011] (3) Weighing the nitrogen-doped graphene obtained in step (2) and dispersing it in deionized water, adding a certain mass of a noble metal salt aqueous solution according to a certain noble metal loading amount, stirring for a certain time, and then adding a sufficient amount of sodium borohydride (NaBH4) aqueous solution dropwise under vigorous stirring to reduce it at room temperature; after room temperature reduction, separating by suction filtration, washing with deionized water, and vacuum drying at 60°C overnight to obtain a nitrogen-doped graphene-loaded noble metal catalyst.

[0012] The graphene used in the present invention has a thickness of 1 to 3 nm and a sheet diameter of 10 to 20 μm.

[0013] Furthermore, the discharge parameters for generating ammonia plasma in step (2) are: power of 100-180 W, vacuum degree of 100-300 Pa, and duration of 1-10 min; the above process is repeated three times for each sample.

[0014] Furthermore, the loading amount of the noble metal is calculated based on the mass of the nitrogen-doped graphene and is 2.5 to 7.5 wt.%;

[0015] Furthermore, the concentration of the noble metal salt aqueous solution is 10 mg / mL; and the molar ratio of sodium borohydride to the noble metal is 40-45:1.

[0016] Furthermore, the noble metal is palladium (Pd), platinum (Pt) or ruthenium (Ru).

[0017] The nitrogen-doped graphene-supported noble metal catalyst prepared by the present invention is used for the selective hydrogenation of α,β-unsaturated aldehydes in a high-temperature aqueous phase, and more specifically, for the selective hydrogenation of cinnamaldehyde as the α,β-unsaturated aldehyde.

[0018] The advantages of the present invention are:

[0019] (1) The present invention introduces a small amount of nitrogen-containing functional groups onto the graphene surface through oxygen plasma activation and ammonia plasma modification technology, and then prepares the loaded noble metal catalyst through an aqueous phase reduction method. Nitrogen doping not only exhibits moderate hydrophilicity, but also the lone pair electrons on the nitrogen atom provide electrons for the coordination and anchoring of metal nanoparticles. At the same time, the introduction of nitrogen atoms maintains p-π conjugation with graphene, which facilitates the adsorption of the benzene ring in the cinnamaldehyde molecule, thereby causing the C=O in the cinnamaldehyde to adsorb on the metal nanoparticles, thereby hydrogenating the C=O bond.

[0020] (2) Compared with oxygen-doped graphene-supported noble metal catalysts, O has a stronger electronegativity, and the electronic conjugation between O and graphene after doping is relatively weak, so the catalytic performance is not as good as that of N. Secondly, compared with oxygen, a more active element, nitrogen doping is more stable. In other words, nitrogen doping is beneficial to improving the catalytic performance of the catalyst on the one hand, and on the other hand, it is beneficial to improving the stability of the catalyst itself. This is the purpose of studying nitrogen doping.

[0021] (3) The present invention adopts low-temperature plasma technology to modify graphene, which is simple to operate and environmentally friendly.

[0022] (4) The catalyst prepared by the present invention is applicable to the hydrogenation reaction of α,β-unsaturated aldehydes in aqueous phase, and also has high catalytic activity and good selectivity for unsaturated alcohols in aqueous phase. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 Graphs of water contact angles of pristine graphene (GR), oxygen-doped graphene (OPGR) obtained in Comparative Example 2, and nitrogen-doped graphene (NPGR) obtained in Example 1 of the present invention;

[0024] Figure 2 The dispersion diagrams of pristine graphene (GR), oxygen-doped graphene (OPGR) obtained in Comparative Example 2, and nitrogen-doped graphene (NPGR) obtained in Example 1 of the present invention in water and oil phase systems are shown in FIG.

[0025] Figure 3 X-ray powder diffraction analysis patterns (XRD) of pristine graphene (GR), nitrogen-doped graphene (NPGR) obtained in Example 1 of the present invention, and nitrogen-doped graphene-supported Pt catalyst (Pt / NPGR);

[0026] Figure 4Transmission electron microscopy (TEM) and particle size distribution diagram of the original graphene (GR) in comparative example 1 and the nitrogen-doped graphene (NPGR) loaded with metal Pt catalyst obtained in example 1 of the present invention.

[0027] Figure 5 This is a catalytic performance diagram of the catalyst in Example 1 after being recycled five times. DETAILED DESCRIPTION

[0028] The present invention will be further described below with reference to specific embodiments.

[0029] Example 1:

[0030] 0.5g of graphene was weighed and placed in a plasma treatment device. First, the graphene surface was activated using oxygen plasma. The discharge parameters for generating oxygen plasma were: power of 160W, vacuum of 20Pa, and duration of 3min. Then, ammonia was switched to adjust the discharge time to 6min, the discharge power to 140W, and the vacuum to 200Pa. The above process was repeated three times for each sample to obtain a nitrogen-doped graphene sample (NPGR).

[0031] 0.2 g of the nitrogen-doped graphene sample (NPGR) was weighed and dispersed in 200 mL of deionized water. The mixture was stirred thoroughly, followed by the addition of 2.65 mL of a 10 mg / mL aqueous solution of HPtCl·6H2O and stirring for 30 minutes. 40 mL of a 2 mg / mL aqueous solution of NaBH4 was added dropwise to the dispersion while stirring at 1000 rpm. After the reaction was complete, the resulting product was filtered, washed with deionized water, and dried in a vacuum oven at 60°C overnight to obtain the dried nitrogen-doped graphene (NPGR)-supported Pt catalyst (Pt / NPGR).

[0032] Example 2:

[0033] The specific steps of this embodiment are the same as those of embodiment 1, except that the vacuum degree of the ammonia plasma is changed to 100 Pa.

[0034] Example 3:

[0035] The specific steps of this embodiment are the same as those of embodiment 1, except that the vacuum degree of the ammonia plasma is changed to 250 Pa.

[0036] Example 4:

[0037] The specific steps of this embodiment are the same as those of embodiment 1, except that the vacuum degree of the ammonia plasma is changed to 300 Pa.

[0038] Example 5:

[0039] The specific steps of this embodiment are the same as those of embodiment 1, except that the vacuum degree of the ammonia plasma is changed to 150 Pa.

[0040] Example 6:

[0041] The specific steps of this embodiment are the same as those of embodiment 5, except that the ammonia plasma discharge power is changed to 100W.

[0042] Example 7:

[0043] The specific steps of this embodiment are the same as those of embodiment 5, except that the ammonia plasma discharge power is changed to 160W.

[0044] Example 8:

[0045] The specific steps of this comparative example are the same as those of Example 5, except that the ammonia plasma discharge power is changed to 180W.

[0046] Example 9:

[0047] The specific steps of this embodiment are the same as those of embodiment 5, except that the ammonia plasma discharge power is changed to 120W.

[0048] Example 10:

[0049] The specific steps of this embodiment are the same as those of embodiment 9, except that the ammonia plasma discharge time is changed to 2 minutes.

[0050] Example 11:

[0051] The specific steps of this embodiment are the same as those of embodiment 9, except that the ammonia plasma discharge time is changed to 4 minutes.

[0052] Example 12:

[0053] The specific steps of this embodiment are the same as those of embodiment 9, except that the ammonia plasma discharge time is changed to 8 minutes.

[0054] Example 13:

[0055] The specific steps of this embodiment are the same as those of embodiment 9, except that the ammonia plasma discharge time is changed to 10 minutes.

[0056] Example 14:

[0057] The specific steps of this example are the same as those of Example 1, except that the amount of 10 mg / mL H2PtCl6·6H2O aqueous solution added is 1.325 mL, and the corresponding amount of 2 mg / mL NaBH4 aqueous solution added is 20 mL.

[0058] Example 15:

[0059] The specific steps of this example are the same as those of Example 1, except that the amount of 10 mg / mL H2PtCl6·6H2O aqueous solution added is 3.975 mL, and the corresponding amount of 2 mg / mL NaBH4 aqueous solution added is 60 mL.

[0060] Comparative Example 1:

[0061] 0.2g of pristine graphene was weighed and dispersed in 200mL of deionized water with thorough stirring. 2.65mL of a 10mg / mL H2PtCl6·6H2O aqueous solution was then added and stirred for 30 minutes. 40mL of a 2mg / mL NaBH4 aqueous solution was then added dropwise to the dispersion while stirring at 1000 rpm. After the reaction was complete, the resulting product was filtered, washed with deionized water, and finally dried in a vacuum oven at 60°C overnight to obtain a dry, low-temperature plasma-modified graphene-loaded Pt material (Pt / GR).

[0062] Comparative Example 2:

[0063] 0.5 g of graphene was weighed and placed in a plasma treatment device. The graphene surface was activated using oxygen plasma. The discharge parameters for generating oxygen plasma were: power of 160 W, vacuum of 20 Pa, and duration of 3 min. The above process was repeated three times for each sample to obtain an oxygen-doped graphene sample (OPGR).

[0064] 0.2 g of the oxygen-doped graphene sample (OPGR) was weighed and dispersed in 200 mL of deionized water. The mixture was stirred thoroughly, followed by the addition of 2.65 mL of a 10 mg / mL aqueous solution of HPtCl6·6H2O and stirring for 30 minutes. 40 mL of a 2 mg / mL aqueous solution of NaBH4 was added dropwise to the dispersion while stirring at 1000 rpm. After the reaction was complete, the resulting product was filtered, washed with deionized water, and dried in a vacuum oven at 60°C overnight to obtain the dried nitrogen-doped graphene (OPGR)-supported Pt catalyst (Pt / OPGR).

[0065] Comparative Example 3:

[0066] 0.5 g of graphene was weighed and placed in a plasma treatment device. N2 plasma was used to activate and graft the graphene surface. The discharge parameters for generating N2 plasma were: power of 140 W, vacuum of 20 Pa, and duration of 6 min. The above process was repeated three times for each sample to obtain N2 plasma-treated graphene samples.

[0067] Weigh 0.2 g of the N2 plasma-treated graphene sample and disperse it in 200 mL of deionized water. Stir thoroughly, then add 2.65 mL of a 10 mg / mL H2PtCl6·6H2O aqueous solution and stir for 30 minutes. Then, add 40 mL of a 2 mg / mL NaBH4 aqueous solution dropwise to the dispersion while stirring at 1000 rpm. After the reaction is complete, filter the resulting product, rinse with deionized water, and dry it in a vacuum oven at 60°C overnight to obtain the N2 plasma-treated graphene-supported Pt catalyst.

[0068] Comparative Example 4:

[0069] 0.5 g of graphene was weighed and placed in a plasma treatment device. NH3 plasma was used to graft the graphene surface. The discharge parameters for generating NH3 plasma were: power of 140 W, vacuum of 200 Pa, and duration of 6 min. The above process was repeated three times for each sample to obtain NH3 plasma-treated graphene samples.

[0070] Weigh 0.2 g of the NH plasma-treated graphene sample and disperse it in 200 mL of deionized water. Stir thoroughly, then add 2.65 mL of a 10 mg / mL solution of HPtCl·6H2O and stir for 30 minutes. Then, add 40 mL of a 2 mg / mL solution of NaBH4 dropwise to the dispersion while stirring at 1000 rpm. After the reaction is complete, filter the resulting product, rinse with deionized water, and dry it in a vacuum oven at 60°C overnight to obtain the NH plasma-treated graphene-supported Pt catalyst.

[0071] Table 1 shows the organic element (OEA) analysis of pristine graphene (GR), oxygen-doped graphene (OPGR) obtained in Comparative Example 2, and nitrogen-doped graphene (NPGR) obtained in Example 1 of the present invention. After plasma treatment, the nitrogen content in the carrier increased from 0.05% (GR) to 1.81% (NPGR). Similarly, compared to GR (approximately 0.71 wt.%), the oxygen content of OPGR also increased slightly after oxygen plasma treatment (approximately 4.04 wt.%), demonstrating that the cold plasma treatment successfully doped the graphene surface with small amounts of nitrogen and oxygen.

[0072] Table 1: Organic element (OEA) analysis of GR and NPGR

[0073]

[0074] The water contact angles of oxygen plasma treated graphene (OPGR), ammonia plasma treated graphene (NPGR) and control graphene (GR) are shown in Figure 2. Figure 1 As shown. Figure 1 The contact angle shows that after graphene is treated with oxygen plasma and ammonia plasma, it shows amphiphilicity, but the degree of hydrophilicity is different: OPGR is more hydrophilic, which is not conducive to the adsorption of non-polar substrates (α, β-unsaturated aldehydes) on its surface during the catalytic process, while the appropriate hydrophilicity of NPGR is conducive to the anchoring of metal nanoparticles on its surface during the preparation of metal catalysts, and is also conducive to the adsorption of non-polar substrates (α, β-unsaturated aldehydes) on its surface during catalytic hydrogenation.

[0075] like Figure 2 The figure shows the dispersion of pristine graphene (GR), oxygen-doped graphene (OPGR) obtained in comparative example 2, and nitrogen-doped graphene (NPGR) obtained in example 1 of the present invention in water and oil phase systems. It can be seen from the figure that pristine graphene shows lipophilicity. In contrast, OPGR and NPGR can be well dispersed in water and CAL ( Figure 2 c-2d, 2e-2f), showing their amphiphilic surface. It can be seen that doping some heteroatoms (including nitrogen and oxygen) in graphene improves its hydrophilicity to a certain extent. This helps to anchor metal nanoparticles on its surface, thereby increasing the reaction rate. Note that although OPGR is uniformly dispersed in water ( Figure 2 c), but because its surface contains slightly more oxygen-containing groups, OPGR has a certain aggregation in the oil ( Figure 2 d), which is not conducive to the adsorption of CAL thereon.

[0076] In addition, the calculated dispersion of Pt particles on GR, OPGR, and NPGR is shown in Table 2: It can be seen that nitrogen atoms are introduced into graphene after ammonia plasma treatment, and this nitrogen doping improves the dispersion of Pt on graphene.

[0077] Table 2: Pt dispersion on GR, OPGR and NPGR

[0078]

[0079] like Figure 3 The figure shows the XRD pattern of the nitrogen-doped graphene-supported Pt catalyst obtained in Example 1 of the present invention. As can be seen from the figure, the prepared material is consistent with the standard cards of graphene and Pt (JCPDS NO.26-1079 and JCPDS NO.04-0802), indicating that the physical phase of the sample is a graphene-supported noble metal Pt catalyst. Figure 4 TEM images also confirmed the successful loading of Pt nanoparticles.

[0080] The catalyst prepared in the above example was applied to the selective hydrogenation reaction of cinnamaldehyde. The reaction conditions were as follows:

[0081] Solvent: 38g deionized water; 2g cinnamaldehyde; 0.04g catalyst; hydrogen pressure: 3.0MPa; reaction temperature: 80°C; reaction time: 4h. The catalytic performance is shown in Table 3.

[0082] The catalyst in Example 1 was recovered by centrifugal filtration after the first reaction and recycled 5 times. The experimental results are shown in Figure 5 The results of the recycling experiment show that after five cycles, the catalytic performance of the catalyst has not declined significantly, the conversion rate of cinnamaldehyde remains at 80%, and the selectivity for cinnamyl alcohol is 84%, indicating that the prepared catalyst has good stability.

[0083] Table 3: Catalytic performance of catalysts

[0084]

[0085] As can be seen from Table 3, the nitrogen-doped graphene-supported Pt catalyst Pt / NPGR of the present invention exhibits higher catalytic activity and selectivity for unsaturated alcohols than Pt / GR. The catalysts prepared by O doping in Comparative Example 2 and by N grafting without activation in Comparative Examples 3 / 4 exhibit better catalytic effects than those of the individual examples. This is because, in addition to the influence of the plasma treatment gas source, the plasma treatment process parameters (vacuum, time, power) also have a significant impact on the catalytic effect of the prepared catalysts. If the plasma treatment time is too short or the power is too low, the degree of attachment of the active groups to the surface is low, the metal nanoparticles have fewer anchoring sites, and the fewer catalytically active sites will cause the reaction rate to decrease, failing to achieve the desired treatment effect. If the time is too long or the power is too high, the defects on the graphene surface will deepen, and the increase in hydrophilic groups will be detrimental to its adsorption of organic substrates during the reaction, which will also reduce the reaction rate. When the vacuum degree of plasma treatment is high, the number of particles in the cavity is small, the number of particles acting on the surface of the material is small, and the active groups introduced on the surface of the material are small. On the contrary, when the vacuum degree is too low, the number of particles in the cavity is large, the particles collide with each other, offsetting the kinetic energy of the particles, and the number of particles actually acting on the surface of the material is small, which makes the plasma treatment effect unsatisfactory. Therefore, suitable plasma treatment process parameters have a certain effect on the catalytic performance of the catalyst. Examples 2-15 are process parameter screenings for ammonia plasma treatment. They are not catalysts obtained under the optimal plasma parameters (Example 1 is the optimal parameter). Therefore, their catalytic effects may not be as good as Example 1 under the optimal conditions, and the catalytic effects of individual examples therein are not as good as Comparative Examples 2, 3, and 4 (the process treatment parameters of Comparative Examples 2, 3, and 4 are the optimal conditions for screening). In short, the nitrogen-doped graphene-supported Pt catalyst Pt / NPGR of the present invention obtained by screening the plasma gas source and the treatment parameters exhibits excellent catalytic performance.

[0086] The above content is only a specific implementation case of the present invention and does not limit the implementation methods of the present invention. The present invention is not limited to this. Within the relevant field, other different forms of changes or modifications can be made on the basis of the above description. It is not necessary and impossible to list all implementation methods here. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the claims of the present invention.

Claims

1. Application of a nitrogen-doped graphene-supported noble metal catalyst in the selective hydrogenation of cinnamaldehyde to cinnamyl alcohol in a high-temperature aqueous phase, characterized in that: The preparation method of the nitrogen-doped graphene-supported noble metal catalyst comprises the following steps: (1) placing graphene in a low-temperature plasma treatment device; activating the graphene surface using oxygen plasma; wherein the discharge parameters for generating the oxygen plasma are: power of 160 W, vacuum of 20 Pa, and duration of 3 min; (2) Switching the discharge atmosphere to ammonia, bombarding the activated graphene surface with ammonia plasma, the discharge parameters are: power 140W, vacuum degree 200Pa, and duration 6min; obtaining nitrogen-doped graphene; (3) Weighing the nitrogen-doped graphene obtained in step (2) and uniformly dispersing it in deionized water, adding a noble metal salt aqueous solution according to the loading amount of the noble metal catalyst, stirring evenly, adding a sodium borohydride aqueous solution dropwise under stirring, and reducing it at room temperature; filtering and separating, washing with deionized water, and drying to obtain a nitrogen-doped graphene-loaded noble metal catalyst; the loading amount of the noble metal is calculated as 5wt% based on the mass of the nitrogen-doped graphene; The high temperature is 80° C., and the noble metal is platinum.

2. The use according to claim 1, characterized in that The concentration of the noble metal salt aqueous solution was 10 mg / mL.

3. The use according to claim 1, characterized in that The molar ratio of sodium borohydride to the noble metal is 40-45:1.

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