Preparation method and application of nitrogen-doped Ni hollow carbon catalyst

Abstract

The invention relates to a preparation method and application of a nitrogen-doped Ni-hollow carbon catalyst. According to the method, nitrogen atoms are introduced into a hollow carbon nano catalyst in a doping mode, and the in-situ nitrogen-doped and post-treatment nitrogen-doped hollow carbon nanosphere packaged Ni / NPs catalyst is obtained. The obtained catalyst has a developed pore structure and relatively high metal dispersity, and is beneficial to transmission of reaction substances and exposure of more active centers. The obtained in-situ nitrogen-doped Ni-coated NHCs catalyst has an excellent catalytic effect, and the phenol conversion rate of 99.68% and the cyclohexanol selectivity of 99.79% can be obtained.

Description

Technical Field

[0001] The present invention belongs to the technical field of catalytic chemical engineering, and specifically refers to a preparation method and application of a nitrogen-doped Ni@hollow carbon catalyst. Background Art

[0002] As the most representative depolymerization component in bio-oil, phenol can be synthesized into KA oil (a mixture of cyclohexanol and cyclohexanone) through a specific catalytic hydrogenation reaction, which plays an important role in the petroleum industry. However, the production of cyclohexanol from phenol hydrogenation usually involves harsh reaction conditions such as high temperature and high pressure, resulting in problems such as easy deactivation of the catalyst and low product yield. For example, the Ni-based catalyst can achieve a phenol conversion rate of 99.9% and a cyclohexanol selectivity of 99%, but the reaction conditions are harsh. The Pt / Al2O3 catalyst can efficiently catalyze the hydrogenation of phenol under appropriate reaction conditions. Usually, the reaction temperature is 100-200 °C and the hydrogen pressure is 0.5-2.0 MPa. Under these conditions, the phenol conversion rate can be close to 100%, and the selectivity of cyclohexanol can also reach a relatively high level, about 85%-95%.

[0003] Therefore, constructing a catalyst with high activity and stability under mild conditions is of great significance for the selective hydrogenation of phenol to cyclohexanol. By doping nitrogen into the metal@hollow carbon framework, the catalytic active centers are increased and the catalytic ability is enhanced. Summary of the Invention

[0004] The purpose of the present invention is to provide a preparation method and application of a nitrogen-doped Ni@hollow carbon catalyst for the limitations existing in the current technology. This method introduces nitrogen atoms into the hollow carbon nanocatalyst by in-situ nitrogen doping to obtain a hollow carbon nanosphere encapsulated Ni / NPs catalyst with in-situ nitrogen doping and post-treatment nitrogen doping; the obtained catalyst has a developed pore structure and a high metal dispersion, which is beneficial to the transport of reactants and the exposure of more active centers. The in-situ nitrogen-doped Ni@NHCs catalyst obtained by the present invention has excellent catalytic effects, and can obtain a phenol conversion rate of 99.68% and a cyclohexanol selectivity of 99.79%.

[0005] The technical solution of the present invention is as follows:

[0006] A preparation method of a nitrogen-doped Ni@hollow carbon catalyst, the method comprising the following steps:

[0007] (1) Under magnetic stirring at 20-40 °C, dissolve NiCl2·6H2O in the first mixed solvent, and then sequentially add tetraethyl orthosilicate and ammonia water; after stirring for 2-4 h, centrifuge and wash the obtained substance, and dry it overnight at 90-110 °C to obtain Ni / SiO2;

[0008] Among them, the mixed solvent consists of ethanol and water; the volume ratio of the two is 2-4:1;

[0009] Add 0.2-0.3 g of NiCl2·6H2O, 1-4 mL of tetraethyl orthosilicate, and 1-4 mL of ammonia water to every 80 mL of the mixed solvent;

[0010] The mass percentage concentration of ammonia water is 20%-30%;

[0011] (2) Under magnetic stirring at 20-40 °C, disperse Ni / SiO2 in the second mixed solvent, and then successively add cetyltrimethylammonium bromide, 3-aminophenol, formaldehyde, and ammonia water. After stirring for 10-20 h, centrifuge, wash, and dry the obtained substance to obtain the RF material;

[0012] Among them, the second mixed solvent consists of ethanol and water, and the volume ratio of the two is 10:20-30;

[0013] Add 0.3-0.5 g of Ni / SiO2, 0.6-0.8 g of cetyltrimethylammonium bromide, 0.05-0.6 g of 3-aminophenol, 100-700 μL of formaldehyde, and 700-900 μL of ammonia water to every 300-400 mL of the second mixed solvent;

[0014] (3) Then, under a N2 atmosphere, calcine at 700-900 °C for 1-3 h at a heating rate of 3-5 °C / min to obtain Ni / SiO2@NCs; finally, under magnetic stirring at 40-50 °C, dissolve sodium hydroxide in deionized water, and then add Ni / SiO2@NCs; after stirring for 11-13 h, centrifuge and wash the obtained substance to neutrality, and dry it overnight at 75-85 °C to obtain the Ni@NHCs catalyst;

[0015] Among them, add 3-5 g of sodium hydroxide and 0.3-0.5 g of Ni / SiO2@NCs to every 100 mL of deionized water;

[0016] Application of the nitrogen-doped Ni@hollow carbon catalyst obtained by the said method as a catalyst in the selective hydrogenation reaction of phenol.

[0017] Specifically, it includes the following steps:

[0018] Add phenol, the said nitrogen-doped Ni@hollow carbon catalyst, and n-hexane to the reaction kettle, seal it, and fill it with 0.5-1.5 MPa of H2, and react at 120-140 °C for 0.5-1.5 h to obtain cyclohexanol;

[0019] Among them, add 0.05-0.15 g of phenol and 0.03-0.07 g of catalyst to every 10 mL of n-hexane;

[0020] The beneficial effects of the present invention are as follows:

[0021] The present invention prepares a Ni NPs catalyst encapsulated in hollow carbon nanospheres by using a pre-synthesis strategy. By changing the nitrogen doping method, the number of active centers in the catalyst can be regulated. In-situ nitrogen doping affects the composition structure and electronic properties of the catalyst by participating in the formation of the catalyst precursor, thereby improving the catalytic performance. Ni@NHCs has a developed pore structure and a high metal dispersion, which is conducive to the transport of reactants and the exposure of more active centers. Moreover, the cavity structure of the Ni@NHCs catalyst not only provides an ideal space for the enrichment of reaction molecules, but also can serve as a protective barrier for metal particles, enhancing the catalytic activity and stability of the catalyst.

[0022] The in-situ nitrogen-doped Ni@NHCs catalyst obtained in the present invention has excellent catalytic effects: under the reaction conditions of 130 °C, 1 MPa H2, and 1 h, Ni@NHCs can achieve a phenol conversion rate of 99.68% and a cyclohexanol selectivity of 99.79%. In addition, kinetic analysis shows that Ni@NHCs has the lowest reaction activation energy (Ea = 48.09 KJ·mol-1), which is beneficial to the selective hydrogenation reaction of phenol. Ni@NHCs still has high catalytic activity after being recycled 7 times, and the constructed Ni@hollow carbon catalyst and its catalytic performance for phenol hydrogenation are studied, and the used catalyst still maintains a complete hollow structure. More importantly, Ni@NHCs shows good catalytic activity in the hydrogenation reactions of various lignin derivatives and has general applicability. Description of the Drawings

[0023] Figure 1 XRD patterns of different catalysts in Examples 1, 4 - 7;

[0024] Figure 2 Raman spectra of different catalysts in Examples 1, 4 - 7;

[0025] Figure 3 XPS full spectra of different catalysts in Examples 1, 4 - 7;

[0026] Figure 4 C1s spectra of different catalysts in Examples 1, 4 - 7;

[0027] Figure 5 N1s spectra of different catalysts in Examples 1, 4 - 6;

[0028] Figure 6 Ni 2p spectra of different catalysts in Examples 1, 4 - 7;

[0029] Figure 7TEM spectrum of the Ni@NHCs catalyst after use in Example 1 of the application examples;

[0030] Figure 8 Effect of different solvents on the hydrogenation reaction of phenol in the application examples. Detailed implementation manners

[0031] The above solution is further described below in conjunction with specific embodiments; it should be understood that these embodiments are used to illustrate the basic principles, main features and advantages of the present invention, and the present invention is not limited by the scope of the following embodiments; the implementation conditions adopted in the embodiments can be further adjusted according to specific requirements, and the implementation conditions not specified are usually the conditions in conventional experiments.

[0032] In the following, unless otherwise specified, all raw materials are obtained through commercial purchase or prepared by conventional methods in this field.

[0033] Preparation of the catalyst Ni@NHCs in Example 1

[0034] First, under magnetic stirring at 30 °C, NiCl2·6H2O (0.2468 g) was dissolved in ethanol / water (60 / 20 mL), and then tetraethyl orthosilicate (2.8 mL) and ammonia water (2.5 mL - 28 wt%) were added successively. After stirring for 3 h, the obtained substance was centrifuged and washed 3 times, and dried overnight at 100 °C to obtain Ni / SiO2. Secondly, under magnetic stirring at 30 °C, Ni / SiO2 (0.4 g) was dispersed in ethanol / water (100 / 240 mL), and then cetyltrimethylammonium bromide (0.75 g), 3-aminophenol (0.3 g), formaldehyde (375 μL), and ammonia water (800 μL - 28 wt%) were added successively. After stirring for 16 h, the obtained substance was centrifuged and washed 3 times, and dried overnight at 80 °C to obtain Ni / SiO2@RF. Then, under a N2 atmosphere, it was calcined at 800 °C for 2 h at a heating rate of 4 °C / min to obtain Ni / SiO2@NCs. Finally, under magnetic stirring at 45 °C, sodium hydroxide (4 g) was dissolved in deionized water (100 mL), and then Ni / SiO2@NCs (0.4 g) was added. After stirring for 12 h, the obtained substance was centrifuged and washed until neutral, and dried overnight at 80 °C to obtain the Ni@NHCs catalyst, that is, an in-situ nitrogen-doped Ni@NHCs catalyst with a carbon layer thickness of 18 nm.

[0035] Preparation of in-situ nitrogen-doped catalysts with different carbon layer thicknesses

[0036] Preparation of the catalyst Ni@NHCs-0.1 in Example 2

[0037] Other steps are the same as those in Example 1, except that when the addition amount of 3-aminophenol is changed from 0.3 g to 0.1 g and the addition amount of formaldehyde is changed from 375 μL to 125 μL. The obtained catalyst is named Ni@NHCs-0.1, and the carbon layer thickness is 8 nm;

[0038] Preparation of the catalyst Ni@NHCs-0.5 in Example 3

[0039] Other steps are the same as those in Example 1, except that when the addition amount of 3-aminophenol is changed from 0.3 g to 0.5 g and the addition amount of formaldehyde is changed from 375 μL to 625 μL. The obtained catalyst is named Ni@NHCs-0.5, and the carbon layer thickness is 47 nm.

[0040] Preparation of the catalyst Ni@NHCs-U in Example 4

[0041] The previous steps are the same as those in Example 1, except that resorcinol is used instead of 3-aminophenol. After adding 0.3 g and stirring for 16 h, the obtained substance is centrifuged and washed 3 times, and dried overnight at 80 °C to obtain Ni / SiO2@RF-J. Then, the post-nitrogen dopant urea is added to Ni / SiO2@RF-J and ground evenly. Under a N2 atmosphere, it is calcined at 800 °C for 2 h at a heating rate of 4 °C / min to obtain Ni / SiO2@NCs. Finally, under magnetic stirring at 45 °C, sodium hydroxide (4 g) is dissolved in deionized water (100 mL), and then Ni / SiO2@NCs (0.4 g) is added. After stirring for 12 h, the obtained substance is centrifuged and washed until neutral, and dried overnight at 80 °C to obtain the post-treated nitrogen-doped Ni@NHCs-U catalyst.

[0042] Preparation of the catalyst Ni@NHCs-M in Example 5

[0043] Other steps are the same as those in Example 4, except that the post-dopant is melamine. The obtained catalyst is named Ni@NHCs-M.

[0044] Preparation of the catalyst Ni@NHCs-D in Example 6

[0045] Other steps are the same as those in Example 4, except that the post-dopant is dicyandiamide. The obtained catalyst is named Ni@NHCs-D.

[0046] Preparation of the nitrogen-free doped catalyst in Example 7

[0047] Other steps are the same as those in Example 1, except that resorcinol is used instead of 3-aminophenol as the carbon source, and the addition amount is 0.3 g. The obtained catalyst is named Ni@HCs.

[0048] Characterization of the catalyst results in Example 8

[0049] The XRD patterns show the crystal phase structures of different catalysts. As Figure 1 shown, all catalysts exhibit a broad and weak diffraction peak at around 22°. This peak corresponds to the graphite diffraction plane, indicating the presence of an amorphous carbon structure with low crystallinity in all catalysts. In addition, all catalysts exhibit distinct diffraction peaks at approximately 44.35°, 51.78°, and 76.52°, which correspond to the (111), (200), and (220) crystal planes of Ni, respectively, suggesting that Ni has been successfully loaded onto the support. Additionally, by comparison, it can be seen that the incorporation of N does not change the crystal structure of the catalyst. And in-situ nitrogen-doped Ni@NHCs (Example 1) exhibits relatively weak metal diffraction peaks, indicating that the metal particle size is the smallest and the metal dispersion is the highest in the in-situ nitrogen-doped catalyst.

[0050] Raman spectroscopy was further used to study the degree of defects on different catalysts. As Figure 2 shown, the Raman spectrum is divided into four carbon bond configurations. The calculated ID1 / IG values of Ni@NHCs (Example 1), Ni@NHCs-U (Example 4), Ni@NHCs-M (Example 5), Ni@NHCs-D (Example 6), and Ni@HCs (Example 7) are 2.02, 1.91, 1.97, 1.95, and 1.86, respectively, indicating that the Ni@NHCs (Example 1) catalyst has the highest degree of defects. In-situ nitrogen doping affects the crosslinking degree by participating in the formation of phenolic resin, and thus more defects are formed during the subsequent carbonization process. The N atoms in the post-nitrogen dopants obtained in Examples 4-6 do not affect the formation of phenolic resin and are prone to uneven distribution during the subsequent carbonization process. Therefore, compared with post-nitrogen doping, in-situ nitrogen doping can create more excellent structural characteristics for the catalyst.

[0051] XPS was further used to study the elemental composition and valence bond states of the catalysts. Figure 3 As shown, C, O, N, and Ni elements are present in Ni@NHCs (Example 1), Ni@NHCs-U (Example 4), Ni@NHCs-M (Example 5), and Ni@NHCs-D (Example 6), indicating that the N element has been successfully introduced into the catalyst. While the nitrogen-free Ni@HCs (Example 7) only exhibits C, O, and Ni elements. As Figure 4 shown, the C-C / C═C characteristic peak with the largest peak intensity indicates that the phenolic resin precursor has been completely carbonized and has been converted into an sp2 C structure. As Figure 5As shown, the XPS spectrum of N 1s can be divided into four peaks. The peaks centered at approximately 398.3 eV, 399.7 eV, 401.2 eV, and 402.7 eV correspond to Pyridinic-N, Pyrrolic-N, Graphitic-N, and Oxidized-N, respectively. Since Pyridinic-N is only connected to two C atoms, it retains a pair of lone pair electrons outside the valence electrons, which will induce defects in the carbon-based formation and is beneficial to the loading of metals. As Figure 6 shown, six peaks are fitted in the XPS spectrum of Ni 2p. The peaks with binding energies at approximately 852.3 eV and 870.6 eV correspond to Ni 0 2p 3 / 2 and Ni 0 2p 1 / 2 , and the peaks with binding energies at approximately 855.6 eV and 873.5 eV correspond to Ni 2+ 2p 3 / 2 and Ni 2+ 2p 1 / 2 . The peaks with binding energies at approximately 861.4 eV and 879.8 eV correspond to satellite peaks. It can be seen from Figure 3 that the binding energies of Ni 0 in the nitrogen-doped catalyst are all shifted to lower energies relative to the nitrogen-free doped catalyst Ni@HCs (Example 7), and are in the opposite direction to the shift direction of the N binding energy, which further confirms the metal-support interaction. It should be noted that both N and Ni 0 in the in-situ nitrogen-doped Ni@NHCs (Example 1) show the largest degree of shift. This difference indicates that electron transfer is more likely to occur between the support and the metal in in-situ nitrogen doping. This also means that in-situ nitrogen doping can make the catalyst have a more sufficient electron-rich Ni surface, thereby increasing the number of Ni 0 active centers to a higher level, which also provides sufficient active H for the phenol hydrogenation reaction.

[0052] Influence of different catalysts on the performance of phenol hydrogenation reaction in application examples

[0053] The phenol hydrogenation reaction was carried out in a 50-mL stainless steel autoclave. Phenol (0.1 g), catalyst (0.05 g), and solvent (10 mL of n-hexane) were added to the autoclave. Subsequently, the autoclave was purged with H2 five times repeatedly and filled to 1 MPa. Under the set temperature conditions, the catalytic reaction was carried out at a stirring rate of 800 rpm for 1 h. After the reaction was completed, the liquid components were analyzed using a Shimadzu (QP2010SE) gas chromatograph.

[0054] There are significant differences in the catalytic effects of in-situ nitrogen-doped catalysts with different carbon layer thicknesses on the hydrogenation of phenol to cyclohexanol. The conversion rate of phenol catalyzed by Ni@NHCs-0.1 with the thinnest carbon layer thickness (Example 2) is 30.84%, and the selectivity for cyclohexanol is 63.56%. When the carbon layer thickness is increased to a medium thickness, Ni@NHCs (Example 1) shows an increasing conversion rate of phenol and selectivity for cyclohexanol. However, when the carbon layer thickness is further increased, Ni@NHCs-0.5 (Example 3) exhibits a decreasing conversion rate of phenol and selectivity for cyclohexanol, which are 49.30% and 80.49% respectively. By comparison, it can be found that as the thickness of the hollow carbon layer of the catalyst increases, the catalytic performance shows a trend of first increasing and then decreasing, and Ni@NHCs (Example 1) with a moderate carbon layer thickness shows the best catalytic effect. Through TEM images ( Figure 7 ) it can be found that a thinner carbon layer thickness cannot keep the catalyst in a complete hollow structure, which inevitably has an adverse effect on the catalytic performance. However, a thicker carbon layer thickness will reduce the mass transfer efficiency and weaken the accessibility of the reactants to the active centers. Therefore, Ni@NHCs (Example 1) with a medium carbon layer thickness exhibits the best catalytic activity.

[0055] The mass transfer and diffusion of reactants and the contact situation between the substrate and the catalyst have an important impact on the hydrogenation reaction. In the Ni@NHCs (Example 1) catalytic phenol hydrogenation reaction system with phenol (0.1 g), Ni@NHCs (0.05 g), solvent (10 mL), 110 °C, 1 MPa H2, and 1 h, the effects of different reaction solvents on this reaction were investigated. As Figure 8 shown, in three alcohol solvents, methanol, ethanol, and n-butanol, the conversion rates of phenol hydrogenation catalyzed by Ni@NHCs (Example 1) are relatively low, only 19.83%, 27.09%, and 22.21% respectively, and the selectivities for cyclohexanol do not exceed 75%. When n-hexane is selected as the reaction solvent, the contact between phenol and the catalyst is not inhibited, so that Ni@NHCs (Example 1) can exert its maximum catalytic effect.

[0056] Nitrogen-doped catalysts can improve the conversion rate and selectivity of the phenol hydrogenation reaction. However, different nitrogen doping methods also have an impact on the catalytic performance. By comparison, it can be seen that the in-situ nitrogen-doped catalyst has the best catalytic activity in the reaction of phenol hydrogenation to cyclohexanol. Subsequently, although the post-nitrogen-doped catalysts Ni@NHCs-U (Example 4), Ni@NHCs-M (Example 5) and Ni@NHCs-D (Example 6) have better catalytic effects than Ni@HCs (Example 7), with the conversion rate of phenol increased to 58.45%, 63.98% and 60.69% respectively, and the selectivity of cyclohexanol increased to 86.57%, 91.35% and 82.77% respectively, there is still some gap between the catalytic effects of the post-nitrogen-doped catalysts and the in-situ nitrogen-doped catalyst. This is because in-situ nitrogen doping participates in the formation of the carbon precursor, resulting in a greater degree of defect in the catalyst, which can better stabilize metal particles and provide more active centers for the reaction. In addition, there is a greater degree of electron transfer in the in-situ nitrogen-doped catalyst, which keeps the content of reduced Ni at a higher level, thereby increasing the number of active H in the reaction system. In addition, Ni@NHCs (Example 1) obtained a phenol conversion rate of 99.68% and a cyclohexanol selectivity of 99.79% under the reaction conditions of 130 °C, 1 MPa H2 and 1 h.

[0057] Although the present invention has been described in detail with general descriptions and specific embodiments above, based on the present invention, some modifications or improvements can be made, which are obvious to those skilled in the art. Therefore, these modifications or improvements made without departing from the spirit of the present invention all fall within the scope of protection required by the present invention.

[0058] Matters not covered by the present invention are well-known technologies.

Claims

1. A preparation method of a nitrogen-doped Ni@hollow carbon catalyst, characterized in that, The method comprises the following steps: (1) Under magnetic stirring at 20 - 40 °C, NiCl₂·6H₂O is dissolved in the first mixed solvent, and then tetraethyl orthosilicate and ammonia water are added in sequence; after stirring for 2 - 4 h, the obtained substance is centrifuged, washed, and dried to obtain Ni / SiO₂; Among them, the mixed solvent consists of ethanol and water; the volume ratio of the two is 2 - 4:1; 0.2 - 0.3 g of NiCl₂·6H₂O, 1 - 4 mL of tetraethyl orthosilicate, and 1 - 4 mL of ammonia water are added to every 80 mL of the mixed solvent; (2) Under magnetic stirring at 20 - 40 °C, Ni / SiO₂ is dispersed in the second mixed solvent, and then cetyltrimethylammonium bromide, 3 - aminophenol, formaldehyde, and ammonia water are added in sequence; after stirring for 10 - 20 h, the obtained substance is centrifuged, washed, and dried to obtain the RF material; Among them, the second mixed solvent consists of ethanol and water, and the volume ratio of the two is 10:20 - 30; 0.3 - 0.5 g of Ni / SiO₂, 0.6 - 0.8 g of cetyltrimethylammonium bromide, 0.05 - 0.6 g of 3 - aminophenol, 100 - 700 μL of formaldehyde, and 700 - 900 μL of ammonia water are added to every 300 - 400 mL of the second mixed solvent; (3) Then, under a N₂ atmosphere, the obtained RF material is heated to 700 - 900 °C and calcined for 1 - 3 h to obtain Ni / SiO₂@NCs; finally, under magnetic stirring at 40 - 50 °C, sodium hydroxide is dissolved in deionized water, and then Ni / SiO₂@NCs is added; after stirring for 11 - 13 h, the obtained substance is centrifuged, washed until neutral, and dried to obtain a nitrogen - doped Ni@hollow carbon catalyst; Among them, 3 - 5 g of sodium hydroxide and 0.3 - 0.5 g of Ni / SiO₂@NCs are added to every 100 mL of deionized water.

2. The preparation method of the nitrogen-doped Ni@hollow carbon catalyst according to claim 1, characterized in that, The mass percentage concentration of ammonia water is 20% - 30%.

3. The preparation method of the nitrogen-doped Ni@hollow carbon catalyst according to claim 1, characterized in that, The heating rate in step (3) is 3 - 5 °C / min.

4. Use of the nitrogen-doped Ni@hollow carbon catalyst obtained by the method according to claim 1, characterized in that, It is used as a catalyst in the selective hydrogenation reaction of phenol.

5. The application according to claim 4, characterized in that, Comprises the following steps: Phenol, the nitrogen - doped Ni@hollow carbon catalyst, and n - hexane are added to a reaction kettle, sealed and filled with 0.5 - 1.5 MPa of H₂, and reacted at 120 - 140 °C for 0.5 - 1.5 h to obtain cyclohexanol; Among them, 0.05 - 0.15 g of phenol and 0.03 - 0.07 g of the catalyst are added to every 10 mL of n - hexane.

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