A sea urchin-like copper-based catalyst and its application in the transfer hydrogenation and deoxygenation reaction of vanillin

Abstract

The present invention discloses a sea urchin-shaped copper-based catalyst and its application in the transfer hydrogenation and deoxygenation reaction of vanillin, belonging to the technical field of transfer hydrogenation reactions. The present invention breaks the technical prejudice that copper-based catalysts are not suitable for transfer hydrogenation and deoxygenation reactions. By using copper silicate with a sea urchin-shaped porous structure as an unconventional copper-based catalyst precursor, a sea urchin-shaped copper-based catalyst is obtained by reduction. It has been verified that it has good conversion rate and selectivity in the transfer hydrogenation and deoxygenation reaction of vanillin. In addition, the sea urchin-shaped copper-based catalyst obtained by reducing copper silicate with a sea urchin-shaped porous structure modified by carbon as a precursor has better conversion rate and selectivity compared with the sea urchin-shaped copper-based catalyst obtained by using copper silicate with an unmodified sea urchin-shaped porous structure as a precursor, and its catalytic performance can be comparable to that of noble metal catalysts.

Description

Technical Field

[0001] The present invention belongs to the technical field of transfer hydrogenation reaction, and particularly relates to a sea urchin-shaped copper-based catalyst and its application in the transfer hydrogenation deoxygenation reaction of vanillin. Background Art

[0002] Lignocellulosic biomass is one of the important renewable resources, belonging to green resources and having advantages such as rich volume and easy availability, fully meeting the world's fuel demand. Biomass pyrolysis oil has problems of low combustion efficiency and insufficient energy conversion rate due to its high oxygen content (20%-50%) and oxygen-containing functional groups such as carboxylic acids and phenols with less heat release during combustion. Hydrodeoxygenation (HDO) is an effective method to reduce the oxygen content in pyrolysis oil.

[0003] Hydrogenation reaction is a general term for a class of reactions in which hydrogen is added to unsaturated compounds (such as alkenes, alkynes, aldehydes, ketones, etc.). This kind of reaction usually proceeds with the help of a catalyst, which can convert unsaturated bonds (such as double bonds or triple bonds) into saturated bonds. For example, the vanillin hydrodeoxygenation reaction is reduced to vanillyl alcohol. Hydrogenation reactions are mainly divided into two categories: direct hydrogenation and transfer hydrogenation.

[0004] Direct hydrogenation usually involves the direct reaction of hydrogen molecules with double bonds or functional groups on the reactants. Under the action of a catalyst, hydrogen molecules are activated and decomposed into hydrogen atoms or hydrogen ions, and then react with organic molecules. Usually, metal catalysts (such as platinum, palladium, nickel, etc.) are involved to promote the cracking of hydrogen. Direct hydrogenation requires direct contact between hydrogen and the substrate for reaction. The catalyst usually needs to be activated at high temperature to effectively adsorb and crack hydrogen. It is usually more selective, and the reaction conditions (such as temperature, pressure, and type of catalyst) can be controlled to selectively hydrogenate specific functional groups or unsaturated bonds.

[0005] Transfer hydrogenation is a hydrogenation reaction carried out by transferring hydrogen atoms or hydrogen ions from a hydrogen source. In such reactions, hydrogen does not directly come from hydrogen gas, but from other chemical substances (such as alcohols, amines, or borohydrides, etc.). Through a catalyst, the hydrogen in these chemical substances is transferred to the reactants. Since the hydrogen source itself is already a stable hydrogen source and does not require additional hydrogen supply, transfer hydrogenation can be carried out at room temperature, and the reaction conditions are relatively mild. However, it also results in relatively low reaction selectivity, multiple hydrogen source transfers may occur, and the nature of the hydrogen source affects the reaction selectivity.

[0006] To improve the efficiency of transfer hydrodeoxygenation reaction, noble metal catalysts are usually selected, such as palladium Pd, platinum Pt, rhodium Rh, gold Au, etc., because noble metals have higher reactivity, can effectively activate hydrogen or hydrogen sources, and promote the transfer process of hydrogen atoms. However, noble metals are scarce and the cost of noble metal catalysts is relatively high. In practical applications, catalysts of transition metals nickel Ni and cobalt Co are used, but they are inferior to noble metal catalysts in terms of both conversion rate and selectivity.

[0007] Copper-based catalysts refer to catalysts with copper as the main component and are widely used in various chemical reactions. Copper-based catalysts have become important industrial catalysts due to their relatively low cost, excellent catalytic performance, and unique role in certain reactions. It has good catalytic activity and cost advantages, but still faces some challenges in terms of stability and selectivity. Therefore, its catalytic effect in the transfer hydrodeoxygenation reaction is also very unsatisfactory and is usually excluded from the selection. Summary of the Invention

[0008] Aiming at the limitations of existing catalysts for transfer hydrodeoxygenation reaction, the present invention proposes a sea urchin-like copper-based catalyst, which not only has a low cost, but also has a conversion rate and selectivity comparable to those of noble metal catalysts in the transfer hydrodeoxygenation reaction of vanillin.

[0009] The present invention protects a sea urchin-like copper-based catalyst, which is obtained by reducing copper silicate with a sea urchin-like porous structure.

[0010] As a preferred embodiment of the present invention, the reduction method is to reduce the copper silicate powder with a sea urchin-like porous structure at 450-650 °C using a 10% hydrogen-argon mixed gas to obtain a Cu / SiO2 catalyst.

[0011] As a preferred embodiment of the present invention, the copper silicate with a sea urchin-like porous structure is carbon-modified copper silicate.

[0012] As a preferred embodiment of the present invention, the preparation method of the copper silicate with a sea urchin-like porous structure includes the following steps:

[0013] Step 1, synthesize monodisperse silica microspheres by the Stöber method, and uniformly disperse the silica microspheres in deionized water to form a silica microsphere suspension.

[0014] Step 2: Add the silica microsphere suspension into the mixed solution containing Cu(NO3)2·3H2O and NH3·H2O, and mix under magnetic stirring. In the mixed solution containing Cu(NO3)2·3H2O and NH3·H2O, the mass fraction of Cu(NO3)2·3H2O is 0.16% - 0.3%, the volume fraction of NH3·H2O is 4% - 10%, and the mass ratio of silica microspheres to Cu(NO3)2·3H2O is 13:8 - 10.

[0015] Step 3: Transfer the mixed solution obtained in Step 2 to an autoclave, react at a temperature of 120 - 150 °C for 12 - 24 h, centrifuge and filter, then wash with distilled water and ethanol until the pH value is 7, and dry to obtain a light blue-green solid powder, which is copper silicate with a sea urchin-like porous structure.

[0016] As a preferred embodiment of the present invention, the method for carbon modification of copper silicate with a sea urchin-like porous structure is as follows: Add the copper silicate powder with a sea urchin-like porous structure into an ethanol solution containing resorcinol and formaldehyde, then add NH3·H2O, and react under magnetic stirring for 9 - 48 h, centrifuge and filter, and wash multiple times to obtain the carbon-modified copper silicate powder with a sea urchin-like porous structure.

[0017] In the ethanol solution containing resorcinol and formaldehyde, the mass fraction of resorcinol is 0.11 - 0.24%, the volume fraction of formaldehyde is 0.20 - 1.0%, and the addition amount of NH3·H2O accounts for 1.2 - 1.3% of the volume fraction of the ethanol solution.

[0018] As a preferred embodiment of the present invention, the particle size of the selected silica microspheres is 200 - 500 nm.

[0019] The present invention also protects the application of the above-mentioned sea urchin-like copper-based catalyst in the transfer hydrogenation and deoxygenation reaction of vanillin. The specific operation is as follows: Add isopropanol, the above-mentioned sea urchin-like copper-based catalyst, and vanillin into a high-pressure reactor, then seal the reactor, purge the residual air with N2 and fill it with 2 MPa N2, control the temperature at 150 - 180 °C, react under stirring, and the reaction time is 90 - 360 min, where the rotation speed is controlled at 500 - 900 rpm. If the rotation speed is too fast, there will be a problem of liquid splashing, and if the rotation speed is too slow, some silica spheres will sink to the bottom and cannot be dispersed; after the reaction is completed, separate the product liquid and the catalyst.

[0020] The present invention breaks the technical prejudice that copper-based catalysts are not suitable for transfer hydrodeoxygenation reactions. Using sea urchin-like porous structure copper silicate, an unconventional copper-based catalyst, as a precursor, a sea urchin-like copper-based catalyst is obtained by reduction. It has been verified that it has good conversion and selectivity in the transfer hydrodeoxygenation reaction of vanillin. In addition, the sea urchin-like porous structure copper silicate modified by carbon as a precursor, the obtained sea urchin-like copper-based catalyst has better conversion and selectivity compared with the one using the unmodified sea urchin-like porous structure copper silicate as a precursor, and its catalytic performance can be comparable to that of noble metal catalysts. Description of the Drawings

[0021] Figure 1 Schematic diagram of the synthesis route of Cu / SiO2-C and Cu / SiO2 catalysts;

[0022] Figure 2 Schematic diagram of the morphology and fine structure of the Cu / SiO2-C catalyst;

[0023] Figure 3 Schematic diagram of the morphology and fine structure of the Cu / SiO2 catalyst;

[0024] Figure 4(a) is the N2 adsorption-desorption isotherm curve of four Cu / SiO2-C catalysts with different carbon contents and Cu / SiO2 catalyst;

[0025] Figure 4(b) is the pore size distribution diagram of four Cu / SiO2-C catalysts with different carbon contents and Cu / SiO2 catalyst;

[0026] Figure 5(a) is the thermogravimetric curve of the precursors of five catalysts;

[0027] Figure 5(b) is the thermogravimetric curve of five catalysts;

[0028] Figure 6(a) is the H2-TPR experimental result diagram of copper silicate precursors with different carbon contents;

[0029] Figure 6(b) is the catalytic performance diagram of Cu / SiO2-C catalysts with different carbon contents at 180 °C, N2 2 MPa, reaction for 300 min, and reduction temperature of 600 °C;

[0030] Figure 6(c) is the XRD spectrum of Cu / SiO2-C catalysts with different carbon contents at a reduction temperature of 600 °C;

[0031] Figure 6(d) is the TEM diagram of Cu / SiO2-C catalysts with different carbon contents;

[0032] Figure 6(e) shows the catalytic performance of the Cu / SiO2-C catalyst with a carbon content of 3.5% at 180 °C, N2 2 MPa, reaction for 300 min, and different reduction temperatures;

[0033] Figure 6(f) shows the XRD patterns of the Cu / SiO2-C catalyst with a carbon content of 3.5% at different reduction temperatures;

[0034] Figure 7(a) shows the XRD patterns of the Cu / SiO2-C catalyst and the Cu / SiO2 catalyst;

[0035] Figure 7(b) shows the slow-scan XRD patterns of the Cu / SiO2-C catalyst and the Cu / SiO2 catalyst;

[0036] Figure 8(a) shows the XPS survey spectra of the Cu / SiO2-C catalyst and the Cu / SiO2 catalyst;

[0037] Figure 8(b) shows the Cu 2p spectra of the Cu / SiO2-C catalyst and the Cu / SiO2 catalyst;

[0038] Figure 9(a) shows the Cu LMM Auger spectra of the Cu / SiO2-C catalyst and the Cu / SiO2 catalyst;

[0039] Figure 9(b) shows the performance of the Cu / SiO2-C catalyst and the Cu / SiO2 catalyst in the catalytic transfer hydrogenation of vanillin;

[0040] Figure 10(a) shows a schematic diagram of the process and products of the transfer hydrogenation of vanillin;

[0041] Figure 10(b) shows a schematic diagram of the conversion rate and selectivity of the Cu / SiO2-C catalyst over time;

[0042] Figure 10(c) shows a schematic diagram of the conversion rate and selectivity of the Cu / SiO2 catalyst over time;

[0043] Figure 10(d) shows a schematic diagram of the conversion rate of the Cu / SiO2 and Cu / SiO2-C catalysts from 90 to 360 min;

[0044] Figure 10(e) shows a schematic diagram of the cycling performance of the Cu / SiO2 and Cu / SiO2-C catalysts;

[0045] Figure 11(a) shows the XPS spectra of Cu / SiO2-C with different Cu 0 / + :Cu 2+ contents;

[0046] Figure 11(b) shows the Cu LMM spectra of Cu / SiO2-C with different Cu 0 / + :Cu 2+ contents;

[0047] Figure 11(c) shows XRD patterns of Cu / SiO2-C with different Cu 0 / + :Cu 2+ contents;

[0048] Figure 11(d) shows the conversion rate and MMP selectivity of vanillin transfer hydrogenation with different Cu 0 :Cu + :Cu 2+ contents and at 180 °C, N2 2 MPa, isopropanol solvent, and reaction for 5 h;

[0049] Figure 11(e) shows XPS spectra of Cu / SiO2 with different Cu 0 / + :Cu 2+ contents;

[0050] Figure 12(a) shows the experimental results of H2-TPD for Cu / SiO2 and Cu / SiO2-C catalysts;

[0051] Figure 12(b) shows the kinetics of vanillin transfer hydrogenation over Cu / SiO2-C catalyst at different temperatures;

[0052] Figure 12(c) shows the kinetics of vanillin transfer hydrogenation over Cu / SiO2 catalyst at different temperatures;

[0053] Figure 12(d) shows the Arrhenius curves of vanillin transfer hydrogenation for Cu / SiO2-C and Cu / SiO2;

[0054] Figure 12(e) shows the in-situ Fourier transform infrared spectroscopy (in-suit FTIR) of vanillin on Cu / SiO2-C catalyst. Detailed implementation manners

[0055] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific implementation manners. The embodiments of the present invention are given for the purpose of illustration and description, and are not exhaustive or limit the present invention to the disclosed form. Many modifications and variations are obvious to those of ordinary skill in the art. The embodiments are selected and described to better illustrate the principles and practical applications of the present invention, and enable those of ordinary skill in the art to understand the present invention and thus design various embodiments with various modifications suitable for specific purposes.

[0056] Copper silicate (CuSiO3) is a multifunctional inorganic compound, and its unique physical and chemical properties (such as high specific surface area, adjustable pore size, excellent catalytic activity) make it widely used in multiple fields. As a catalyst, it is mainly applied to pollutants such as NO x , CO, and VOCs in motor vehicle exhaust and industrial waste gas, or Pb in industrial wastewater (such as electroplating, mining) 2+, Cd 2+ , As 2+ and other toxic heavy metals. Given the challenges that copper-based catalysts still face in terms of stability and selectivity as mentioned in the background art, their catalytic effect in the transfer hydrodeoxygenation reaction is also very unsatisfactory and is usually excluded from the selection. Even when copper-based catalysts are applied to the transfer hydrodeoxygenation reaction, they usually appear in the form of composite catalysts, and there is still a large gap in catalytic effect compared with noble metal catalysts.

[0057] Copper silicate has various morphologies, including crystal structures, amorphous forms, and porous nanostructures. The research of the present invention has found that copper silicate with a sea urchin-like porous structure (hereinafter referred to as the Cu / SiO2 catalyst) has good catalytic performance in the transfer hydrodeoxygenation reaction of vanillin. After further carbon modification analysis experiments on it (hereinafter referred to as the Cu / SiO2-C catalyst), it is found that it has comparable conversion and selectivity to noble metal catalysts in the transfer hydrodeoxygenation reaction of vanillin.

[0058] The preparation methods of the Cu / SiO2 catalyst and Cu / SiO2-C of the present invention are given below. Figure 1 The schematic diagram of the synthesis routes of Cu / SiO2-C and Cu / SiO2 catalysts is shown.

[0059] 1. Monodisperse silica microspheres were synthesized by the Stöber method. 0.13 g of silica microspheres were uniformly dispersed in 20 mL of deionized water to form a silica microsphere suspension.

[0060] 2. The silica microsphere suspension was added to a mixed solution containing 0.1 g of Cu(NO3)2·3H2O and 3 mL of NH3·H2O (the solvent was 30 mL of deionized water), and mixed under magnetic stirring.

[0061] 3. The mixed solution obtained in step 2 was transferred to an autoclave (100 mL) and reacted at 150 °C for 12 h. After centrifugation and filtration, it was washed with distilled water and ethanol multiple times until the pH value was 7. After drying, light blue-green solid powder copper silicate was obtained, and part of the copper silicate was reserved for use.

[0062] 4. 0.2 g of copper silicate powder was added to an ethanol solution containing 0.0725 g of resorcinol and 0.08 mL of formaldehyde (80 mL of ethanol solvent), and then 1 mL of NH3·H2O was added. After reacting under magnetic stirring for 9 - 48 h, it was centrifuged, filtered, and washed multiple times to obtain brown powder, which was carbon-modified copper silicate.

[0063] 5. The light blue-green solid powder and the brown powder were respectively reduced with 10% hydrogen-argon mixture at 450-650 °C for 2 h to obtain the Cu / SiO2 catalyst and the Cu / SiO2-C catalyst.

[0064] For the Cu / SiO2 catalyst and the Cu / SiO2-C catalyst obtained under the above ratios and reaction conditions, the copper loading is 15 wt.%. In addition to the above ratios and reaction conditions, the present invention also tried other ratios and reaction conditions, which are recorded in Table 1. Different formulations and reaction conditions lead to different lengths, thicknesses of the "spines" of the grown sea urchins, and the thickness of the core-shell. The clusters shown in Table 1 also belong to the sea urchin shape, but they are stuck together and are non-standard sea urchin shapes, which will affect the catalytic performance to a certain extent.

[0065] Next, the Cu / SiO2 catalyst and the Cu / SiO2-C catalyst will be discussed respectively from aspects such as morphological structure, geometric structure, electronic structure, and the performance of catalytic transfer hydrogenation of vanillin.

[0066] Table 1

[0067]

[0068] I. Morphological structures of the Cu / SiO2-C and Cu / SiO2 catalysts

[0069] The morphologies and fine structures of the Cu / SiO2-C catalyst and the Cu / SiO2 catalyst were studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), as shown in Figure 2 、 3 respectively. It can be seen from the SEM images and TEM images that the Cu / SiO2-C catalyst is in the shape of a sea urchin, and the spines on the silica shell are assembled by nanotubes. Elemental mapping shows that Si, O, C, and Cu atoms are evenly dispersed throughout the structure. On the surface of the Cu / SiO2 catalyst, the C element is evenly distributed; for the Cu / SiO2 catalyst and the Cu / SiO2-C catalyst, the Cu element is relatively evenly distributed on the surface.

[0070] By comparing Figure 2 and Figure 3, it can be clearly seen that the sea urchin-like morphological structure of the catalyst after carbon modification remains basically unchanged, while the thickness of the doped amorphous carbon is only about 0.198 nm. In the morphological characterization of the Cu / SiO2-C catalyst, high-resolution transmission electron microscopy (HRTEM) and the corresponding Fourier transform (FFT) images show that the lattice spacing is 0.208 nm, corresponding to the (111) plane of metallic copper; at the same time, it can be observed that the particles are not pure metallic copper, but a composite of metallic copper (crystalline) and the Cu-O-Si interface (amorphous or low crystallinity). Through the statistical analysis of the particle size distribution, the average particle size of the Cu / SiO2-C catalyst is 11.97 ± 3.18 nm.

[0071] For the unmodified Cu / SiO2 catalyst, obvious metal particles can be seen from its SEM and TEM images. By calculating the lattice spacing from the HRTEM image, it can be seen that there is obvious Cu2O present. Through the calculation of the particle size distribution, the average particle size of the Cu / SiO2 catalyst is 17.66 ± 4.58 nm, which is significantly larger than that of the Cu / SiO2-C catalyst. From the HRTEM and the corresponding EDS elemental distribution map of the Cu / SiO2 catalyst, it can be known that its main structure is the Si element.

[0072] Through the analysis of the morphological structure, it can be known that carbon modification improves the dispersion degree of metal particles to a certain extent. And a small amount of carbon modification does not destroy the basic sea urchin-like structure of the catalyst, and improves the surface microenvironment of the catalyst, thus enhancing the performance of the catalyst.

[0073] II. Geometric Structure Characterization of Cu / SiO2-C and Cu / SiO2 Catalysts

[0074] To better explore the influence of carbon modification on the performance of the Cu / SiO2-C catalyst, four Cu / SiO2-C catalysts with different carbon contents were prepared, and the carbon contents were 2.5%, 2.8%, 3.5%, and 4.5% respectively. The corresponding catalysts are denoted as Cu / SiO2-C-2.5, Cu / SiO2-C-2.8, Cu / SiO2-C-3.5, and Cu / SiO2-C-4.5.

[0075] Figure 4(a) shows the N2 adsorption-desorption isotherm curves of four Cu / SiO2-C catalysts with different carbon contents and the Cu / SiO2 catalyst, and Figure 4(b) shows the pore size distributions of four Cu / SiO2-C catalysts with different carbon contents and the Cu / SiO2 catalyst.

[0076] Combined with Table 2 (specific surface area and pore volume data of Cu / SiO2-C catalysts and Cu / SiO2 catalysts with four different carbon contents), it can be seen that the adsorption behaviors of the catalysts before and after carbon modification both have obvious H3-type hysteresis loops generated by mesoporous solids. The curve behaviors of these five catalysts are similar. With the rise of monolayer adsorption, there are micropores on the surface. Then, as the pressure increases, the adsorption amount further increases with multilayer adsorption. Through pore size analysis, most of the materials are micropores (<2 nm), with a small part of mesopores (2 - 50 nm).

[0077] Table 2

[0078]

[0079] Combined with the specific surface area and pore volume data, it can be known that the specific surface area of the Cu / SiO2-C catalyst with a carbon content of 2.5 - 4.5% decreases with the increase of carbon content, from 230.31 m 2 / g to 141.77 m 2 / g; the pore volume first increases with the increase of carbon content (from 0.25 cc / g to 0.35 cc / g), but will decrease significantly after exceeding a certain amount (from 0.35 cc / g to 0.17 cc / g).

[0080] Compared with the Cu / SiO2 catalyst without carbon modification, the specific surface area and pore volume of the first three groups of Cu / SiO2-C catalysts are improved to a certain extent. However, the fourth group of Cu / SiO2-C catalysts has too much carbon content, which will instead decrease slightly. This may be because the carbon doped later uses resorcinol and formaldehyde to polymerize to form larger carbon particles, resulting in the blockage of more mesopores and the decrease of pore volume.

[0081] Combined with organic elemental analysis (EA) and thermogravimetric analysis (TG), referring to the curves shown in Figure 5(a) 、 5(b) It is obvious that with the increase of carbon doping amount, the weight loss rate increases. Among them, Figure 5(a) is the thermogravimetric curve of the precursors of the five catalysts, and Figure 5(b) is the thermogravimetric curve of the five catalysts.

[0082] Referring to Figure 5(a), through the observation of the thermogravimetric curves of the four carbon-modified copper silicates and the unmodified copper silicate, the total weight loss rate of the unmodified copper silicate is 7%, while that of the copper silicate with the most carbon doping is 17%. It can be concluded that with the increase of the carbon modification degree of copper silicate, the increase of carbon content makes the curve decline rate increase and the weight loss rate increase.

[0083] Referring to Figure 5(b), in the thermogravimetric curves of the reduced catalysts, there are no obvious trapezoidal curves for uncarbonated Cu / SiO2, Cu / SiO2-C-2.5, and Cu / SiO2-C-2.8, which is attributed to their low carbon content. When the carbon content increases to more than 3.5%, a weight loss step interval appears; at the same time, with the increase in carbon content, the weight loss rate increases accordingly.

[0084] The catalytic performance of the copper silicate precursors of Cu / SiO2-C-2.5, Cu / SiO2-C-2.8, Cu / SiO2-C-3.5, and Cu / SiO2-C-4.5 catalysts is analyzed below, and the analysis results are referred to Figure 6(a) to 6(e) as shown.

[0085] Referring to Figure 5(a), an obvious small shoulder peak appears after the carbon content increases to 2.8%, which can be attributed to the reduction of a small amount of Cu 2+ and Cu + that interact with the carrier and amorphous carbon. Referring to the H2-TPR experimental result diagram of copper silicate precursors with different carbon contents shown in Figure 6(a), when the carbon content reaches 4.5%, an obvious small peak splits after 300 °C, which can be attributed to the enhanced interaction between carbon and metal.

[0086] The catalytic performance of four Cu / SiO2-C catalysts with different carbon contents was tested at 180 °C, N2 2 MPa, reaction for 300 min, and reduction temperature 600 °C, as shown in Figure 6(b). The results show that there are differences in catalytic performance among catalysts with different carbon contents. At a reduction temperature of 600 °C, the conversion rate and MMP selectivity of Cu / SiO2-C-3.5 are the best. The comparison of the conversion rate and selectivity of various vanillin transfer hydrogenation catalysts in recent years is listed in Table 3. From the comparison of the harshness of reaction conditions, the conversion rate of vanillin, and MMP selectivity, it can be seen that the Cu / SiO2-C catalyst has excellent performance and also has high conversion rate and high selectivity under mild conditions.

[0087] At the same time, it can be seen that the conversion rates of Cu / SiO2-C-2.8 and Cu / SiO2-C-3.5 also reach 100%, which is the same as those of the noble metal catalysts 200-Pt / Mo2TiC2 a , Au / Co3O4 b ; although the transition metal catalyst Co(H4)@C-HPW also has a 100% conversion rate, the MMP selectivity is only 90%, while Cu / SiO2-C-2.8 can also reach 90.24%, and Cu / SiO2-C-3.5 can be as high as 97.78%. However, in terms of catalyst cost, Cu / SiO2-C is much lower than noble metal catalysts and transition metal catalysts.

[0088] Table 3

[0089]

[0090] Figure 6(c) shows the XRD patterns of Cu / SiO2-C catalysts with different carbon contents at a reduction temperature of 600 °C. It can be seen that as the carbon content increases, the peak width corresponding to Cu(111) gradually increases. Combining with the TEM characterization of Cu / SiO2-C catalysts with four carbon contents shown in Figure 6(d), it can be known that carbon modification improves the metal dispersion.

[0091] In view of the optimal catalytic performance of Cu / SiO2-C-3.5 at a reduction temperature of 600 °C, the catalytic performance of Cu / SiO2-C-3.5 catalyst with a carbon content of 3.5% was further tested at different reduction temperatures (450 - 650 °C). The test results are shown in Figure 6 (e), and the corresponding XRD pattern is shown in Figure 6 (f).

[0092] As the reduction temperature increases, the selectivity of MMP also further increases. It can be seen from Figure 6(f) that the increase in the calcination reduction temperature gradually reduces the surface CuO and Cu2O, and makes the peak of Cu(111) gradually strengthen, among which the content of Cu 0 gradually increases. Combining with the previous experiments and literature, it can be known that the increase in the content of Cu 0 is beneficial to the process of catalytic transfer hydrogenation of vanillin. It can be seen from the XRD pattern shown in Figure 6(f) that at reduction temperatures of 600 °C and 650 °C, the surface Cu + is basically reduced, and the diffraction peak of Cu2O(111) basically disappears. However, the MMP selectivity performance of the catalyst at a reduction temperature of 650 °C is slightly lower than that of the catalyst at a reduction temperature of 600 °C, which can be attributed to the possible sintering and agglomeration of Cu at too high a temperature, resulting in a decrease in its performance. At the same time, it can also be known that different Cu 0 and Cu + contents lead to different conversion rates and selectivities, among which the selectivity has a greater impact.

[0093] Through the above data analysis, a carbon content of 3.5% is the optimal carbon doping amount for carbon modification in the present invention. As the carbon doping amount increases, the hydrogen consumption peak temperature range of the Cu / SiO2-C catalyst becomes wider, and the peak area increases significantly, indicating that the interaction between the metal and the carrier of the Cu / SiO2-C catalyst becomes stronger.

[0094] Electronic structure characterization of Cu / SiO2-C and Cu / SiO2 catalysts

[0095] The Cu / SiO2-C-3.5 catalyst and Cu / SiO2 catalyst were characterized by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), etc. First, the surfaces of the Cu / SiO2-C-3.5 catalyst and Cu / SiO2 catalyst were scanned through the XRD pattern to obtain the XRD pattern shown in Fig. 7(a) and the slow-scan XRD pattern shown in Fig. 7(b).

[0096] As can be seen from the XRD spectrum shown in Fig. 7(a), the surfaces of the two catalysts were reduced in a H2 / Ar atmosphere at 600 °C. It can be seen that the broad peak at 2θ = 20~30° can be attributed to the mixed phase of SiO2 and C. Peaks were shown at 2θ = 43°, 51°, and 74°, corresponding to the Cu(111), Cu(200), and Cu(220) crystal planes. Therefore, it can be known that there is no obvious copper oxide on the surface of the catalyst after reduction in hydrogen-argon at 600 °C.

[0097] After refined slow-scan analysis, as shown in Fig. 7(b), it can be seen that there is a slight shift in the peak position of the carbon-modified catalyst, with a small-angle shift from 43.51° to 43.37°; at the same time, the peak intensity corresponding to the Cu(111) crystal plane becomes weaker. Thus, it can be known that a small amount of carbon doping is beneficial to the dispersion of metallic copper on the catalyst surface, thereby stabilizing the active sites of the catalyst and increasing the effective area of the contact active sites. This is consistent with the previous TEM images and EDS element distribution maps.

[0098] The surface chemical states of the Cu / SiO2-C-3.5 catalyst and Cu / SiO2 catalyst were further characterized by XPS. Fig. 8(a) is the XPS survey spectrum of the Cu / SiO2-C-3.5 catalyst and Cu / SiO2 catalyst, showing that the catalyst contains Cu, O, C, and Si elements, corresponding to the EDS element distribution map. The results show that no other active metal centers were introduced into the catalyst after carbon modification by means of the polymerization of resorcinol and formaldehyde; at the same time, it can be seen from the spectrum that the C characteristic peak of the Cu / SiO2-C-3.5 catalyst is significantly stronger than that of the Cu / SiO2 catalyst. Through further Cu 2p spectral analysis, it can be known that the characteristic peaks at 932.6 eV and 934.8 eV belong to Cu 0 / + and Cu 2+ , as shown in Fig. 8(b).

[0099] Through the elemental and valence state analysis of the catalyst by XPS, combined with the Cu LMM Auger spectrum shown in Fig. 9(a) and the corresponding catalytic transfer hydrogenation performance graph of vanillin shown in Fig. 9(b), after fitting and peak deconvolution, it can be analyzed that the Cu valence state content ratio of the Cu / SiO2 catalyst is Cu 0 :Cu + :Cu2+ = 0.11 : 0.46 : 0.43, while the Cu valence state content ratio of the Cu / SiO2-C-3.5 catalyst is Cu 0 :Cu + :Cu 2+ = 0.24 : 0.36 : 0.4. The content of the active center Cu that affects the transfer hydrogenation reaction process of vanillin 0 / + is significantly increased, indicating that the carbon modification method can affect the components of different valence electrons.

[0100] Performance Study on the Catalytic Transfer Hydrogenation of Vanillin over Cu / SiO2-C and Cu / SiO2 Catalysts

[0101] The transfer hydrogenation deoxygenation performance tests of Cu / SiO2-C-3.5 and Cu / SiO2 catalysts for vanillin were carried out for comparison. In the transfer hydrogenation reaction with isopropanol as the solvent, various products were detected. See Figure 10(a). In addition to the target product 2-methoxy-4-methylphenol (MMP), there are also two by-products, vanillyl alcohol (HMP) and the condensation product 3-methoxy-4-hydroxybenzyl alcohol isopropyl ether (IMP). In the current research on the hydrogenation of vanillin, both HMP and MMP products have been reported. However, it is relatively easy to obtain the product vanillyl alcohol by directly hydrogenating the aldehyde group. Therefore, in this invention, MMP was selected as the target product to study the performance differences between Cu / SiO2-C and Cu / SiO2 catalysts.

[0102] General catalytic reactions and cyclic experiments were carried out in a 25 mL stainless steel high-pressure reactor (Anhui Kemi Machinery Technology Co., Ltd.). 30 mg of the catalyst and 1 mmol of the reactant were added to 10 mL of the solvent (isopropanol) and mixed. After the mixture was evenly dispersed by ultrasonic, it was transferred to the stainless steel high-pressure reactor. Then, N2 was introduced to purge the reactor several times to remove air, and then N2 was filled to a certain pressure. The heating thermocouple was turned on to the target temperature, and the time to maintain this temperature was set. After the reaction ended, when the reactor cooled to room temperature, the catalyst was separated by filtration using a 0.22 um filter, and the reaction liquid was collected. In the general cyclic experiment, the catalyst was separated from the reaction system by centrifugation, washed several times with an isopropanol solution, and then used for repeated performance tests.

[0103] The products were identified by gas chromatography-mass spectrometry (GC-MS, Thermo Fisher Scientific-TXQ QuntumXLS), and quantitative analysis was carried out using gas chromatography (Shimadzu, GC-2010 Plus) equipped with an FID detector and a KB-WAX capillary column (Kromat, USA). Chromatographic analysis method: Using N2 as the carrier gas, controlling the pressure to 0.1 MPa, setting the initial temperature to 120 °C, raising the temperature to 240 °C at a heating rate of 10 °C / min, and holding for 3 min.

[0104] The performance of the catalyst before and after carbon modification was detected through basic condition experiments. Figures 10(b) and 10(c) show the changes in conversion and selectivity of the Cu / SiO2-C-3.5 catalyst and the Cu / SiO2 catalyst at 180 °C, N2 2 MPa, stirring speed 600 rpm, and reaction time of 90 - 360 min, respectively. It can be clearly seen from the comparison that both the conversion and selectivity of the catalyst after carbon modification are improved.

[0105] After the reaction for 5 - 6 h, the reaction basically reached equilibrium. The unmodified Cu / SiO2 catalyst had a conversion of 87% and an MMP selectivity of 62% for vanillin. For the carbon-modified Cu / SiO2-C-3.5 catalyst, the conversion of vanillin reached 99% under the same conditions, and the MMP selectivity was further increased to 89%.

[0106] After that, the yields of MMP of the two catalysts at the same reaction time are shown in Figure 10(d), and the cyclic stability test chart is shown in Figure 10(e). It can be seen from Figure 10(d) that when comparing the conversion rates at the same reaction time with the efficiency of converting vanillin to MMP at the same reaction time, the performance of the catalyst after carbon modification is significantly higher than that before carbon modification. The turnover frequency and cyclic stability of the catalyst after carbon modification are significantly higher than those of the unmodified catalyst.

[0107] From the perspective of cyclic stability, the Cu / SiO2-C-3.5 catalyst still had an MMP yield of 80% after the fifth cycle, showing a significant improvement in stability compared to the Cu / SiO2 catalyst.

[0108] Next, electronic structure analysis was used to verify and analyze the influence of catalysts with different Cu 0 : Cu + : Cu 2+ contents on their performance laws. Through verification by a large number of reported literatures, the main functions of Cu 0 are hydrogen activation and surface overflow. Cu 0The surface of the nanoparticles interacts with the O-H or C-H bonds of hydrogen donors (such as isopropanol and formic acid) through d-orbital electrons, promoting dehydrogenation to generate active hydrogen species (H*). The dehydrogenation of isopropanol reported in Nature Catalysis, Cu 0 The isopropanol adsorbed on the surface is oxidized to acetone, while releasing hydride anions and protons. The reaction follows the MvK (Mars-van Krevelen) mechanism; meanwhile, Cu 0 The weak adsorption characteristics (through π-bond interaction) of unsaturated bonds (C=O, C=C) reduce the hydrogenation energy barrier and promote selective hydrogenation. In a report in the Journal of Catalysis on DFT simulation calculations, the adsorption energy of the C=O bond (-1.2 eV) on the Cu(111) surface is significantly lower than that of the C=C bond (-0.7 eV), explaining its selective reduction of the carbonyl group. Cu + plays an important role in coordinating the activation and deoxygenation of oxygen species. There is an explanation in Chemical Engineering Journal for confirming its participation in deoxygenation. Therefore, the regulation of the valence state ratio is an important influencing factor for promoting multi-step transfer hydrogenation and deoxygenation reactions. While the carbon content of the catalyst changes through carbon modification, it is accompanied by changes in the Cu 0 : Cu + : Cu 2+ composition structure.

[0109] To further study the influence of the Cu 0 : Cu + : Cu 2+ composition change, by carrying out controllable micro-oxidation of the catalyst in a low-oxygen environment, the composition of Cu 0 : Cu + : Cu 2+ was regulated to obtain Cu / SiO2-C-3.5 catalysts with different contents for further experiments, see Figure 11(a) to Figure 11(e) .

[0110] First, through XPS spectrum analysis, such as Figure 11(a) , 11(e) , it can be visually seen that under the same trace oxidation, the content of Cu 2+ in the Cu / SiO2 catalyst increases significantly faster than that in the Cu / SiO2-C-3.5 catalyst, indicating that carbon modification helps to improve the interaction between the metal and the support.

[0111] After that, combined with the Auger spectrum of Cu shown in Fig. 9(a), the analysis of the contents of Cu 0 and Cu + was carried out. The content of Cu 0 gradually decreases with the prolongation of the oxidation time, and Cu+ and the content of Cu 2+ gradually increases. When the content of Cu 0 / + : Cu 2+ is in the range of 1.49 - 0.55, the catalytic performance of Cu / SiO2-C-3.5 remains with a conversion rate > 99% and an MMP selectivity higher than 95%. When the ratio exceeds 0.55 and the content of Cu 2+ is excessive, the performance deteriorates. At the same time, it can be observed that when only Cu 0 and Cu 2+ or Cu + and Cu 2+ exist, the selectivity of the catalyst does not exceed 90%. This means that Cu 0 and Cu + have a synergistic effect in the catalytic transfer hydrogenation reaction. This rule is consistent with the verified synergistic effect of Cu 0 -Cu + reported in the literature.

[0112] Through data analysis, when the content of Cu 0 : Cu + : Cu 2+ is 22.53:34.47:43, the reaction is carried out at 180 °C, N2 2 MPa, 10 mL of isopropanol solvent, and 1 mmol of vanillin. The catalytic performance of Cu / SiO2-C-3.5 reaches a conversion rate of 100% and an MMP selectivity of 97.78%. At this time, the content of Cu 0 : Cu + is nearly 1:1. This verifies that changing the electronic structure of Cu / SiO2-C-3.5 can change the performance of the catalytic transfer hydrogenation reaction, and by carbon modification, the content composition of Cu 0 : Cu + : Cu 2+ can be regulated to further refine and improve the performance of the catalyst Cu / SiO2-C-3.5 and promote the CTH process.

[0113] To further explore the promoting effect of the Cu / SiO2-C-3.5 catalyst on the transfer hydrogenation reaction of vanillin, an experiment on the temperature-programmed desorption of hydrogen from the catalyst was carried out. Figure 12(a) shows the H2-TPD experimental results of Cu / SiO2 and Cu / SiO2-C-3.5. It can be clearly seen that both the Cu / SiO2 catalyst and the Cu / SiO2-C-3.5 catalyst exhibit two desorption peaks. The desorption peak appearing near 300 °C is attributed to the desorption of hydrogen adsorbed on Cu. At the same time, it is obvious that the peak area of Cu / SiO2-C-3.5 after carbon modification increases, which may be due to the increase in surface defects, resulting in an increase in its H2 adsorption capacity and an increase in the density of active sites. For the latter peak, the Cu / SiO2 catalyst appears at about 450 °C, while the Cu / SiO2-C-3.5 catalyst appears at about 500 °C. The peak position shifts towards a higher temperature, indicating that the Cu / SiO2-C-3.5 catalyst has enhanced adsorption capacity and high metal dispersion, which is beneficial to the transfer hydrogenation reaction.

[0114] To further study the evolution of the bonding structure of vanillin on the Cu / SiO2-C-3.5 catalyst, an in-situ Fourier transform infrared (in-suit FTIR) experiment was carried out. The in-situ Fourier transform infrared spectrum is shown in Figure 12(b). As the reaction proceeds, the peak position of the C=O stretching vibration at 1680 cm -1 shows a gradually upward bending trend and the peak flattens out and basically disappears at 15 min. This is attributed to the consumption of vanillin and the cleavage of the aldehyde group of vanillin; afterwards, the broad peak at 3500 cm -1 bends gradually upward from downward, indicating the formation of the intermediate vanillyl alcohol intermediate; the bending vibration of -CH3 at 2880 cm -1 gradually bends upward from flat and the bending vibration of C-O at 1280 cm -1 strengthens downward, which is attributed to the generation of MMP, and the changes in other signals represent the processes of adsorption and hydrodeoxygenation of vanillin on the Cu / SiO2-C-3.5 catalyst.

[0115] To further explore the transfer hydrogenation deoxygenation activity of the Cu / SiO2-C-3.5 catalyst and evaluate the kinetics of the catalyst, Figures 12(c) and (d) show the temperature performance tests of the Cu / SiO2 catalyst and the Cu / SiO2-C-3.5 catalyst, respectively. The test results show that in the range of 150-180 °C, the logarithm of the conversion of vanillin, In(1 - conversion of reactant), shows a linear correlation with the reaction time after linear fitting, conforming to the characteristics of first-order reaction kinetics. By fitting the data, it can be seen that the reaction rate constant increases with the increase in temperature, indicating that increasing the temperature can accelerate the reaction. Both the Cu / SiO2 catalyst and the Cu / SiO2-C-3.5 catalyst conform to the first-order reaction kinetics law. At the same time, through the Arrhenius curve and its calculation, the activation energy of the transfer hydrogenation of vanillin by the Cu / SiO2 catalyst ( E a ) is 45.10 kJ / mol, and the activation energy of the Cu / SiO2-C-3.5 catalyst is reduced to 38.92 kJ / mol, indicating that the catalyst Cu / SiO2-C-3.5 modified by carbon has excellent catalytic activity in the aldehyde group transfer hydrogenation reaction. At the same time, combined with the experimental results of H2-TPD, the decrease in activation energy may be attributed to the fact that carbon modification regulates the electronic composition (Cu 0 : Cu + : Cu 2+ ), provides electrons to the vicinity of Cu 0 , increases the content of Cu + , reduces the hydrogen adsorption energy, resulting in a decrease in activation energy and an improvement in the catalytic transfer hydrogenation process.

[0116] Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. All other embodiments obtained by those of ordinary skill in the art and related fields based on the embodiments of the present invention without creative efforts shall fall within the scope of protection of the present invention.

Claims

1. A sea urchin-shaped copper-based catalyst, which is a Cu / SiO2 catalyst obtained by reducing copper silicate powder with a sea urchin-shaped porous structure at 450-650 °C using a 10% hydrogen-argon mixed gas, characterized in that, The copper silicate with a sea urchin-like porous structure is copper silicate after carbon modification. The method for carbon modification of copper silicate is as follows: Add the copper silicate powder with a sea urchin-like porous structure into an ethanol solution containing resorcinol and formaldehyde, then add NH3·H2O, and react under magnetic stirring for 9 - 48 h. Centrifuge and filter, and after washing multiple times, obtain the copper silicate powder with a sea urchin-like porous structure after carbon modification; In the ethanol solution containing resorcinol and formaldehyde, the mass fraction of resorcinol is 0.11 - 0.24%, the volume fraction of formaldehyde is 0.20 - 1.0%, and the addition amount of NH3·H2O accounts for 1.2 - 1.3% of the volume fraction of the ethanol solution.

2. The sea urchin-shaped copper-based catalyst according to claim 1, wherein The preparation method of the copper silicate with a sea urchin-like porous structure includes the following steps: Step 1, synthesize monodisperse silica microspheres by the Stöber method, and uniformly disperse the silica microspheres in deionized water to form a silica microsphere suspension; Step 2, add the silica microsphere suspension into a mixed solution containing Cu(NO3)2·3H2O and NH3·H2O, and mix under magnetic stirring; in the mixed solution containing Cu(NO3)2·3H2O and NH3·H2O, the mass fraction of Cu(NO3)2·3H2O is 0.16 - 0.3%, the volume fraction of NH3·H2O is 4% - 10%, and the mass ratio of the silica microspheres to Cu(NO3)2·3H2O is 13:8 - 10; Step 3, transfer the mixed solution obtained in Step 2 to an autoclave, react at a temperature of 120 - 150 °C for 12 - 24 h, centrifuge and filter, and then wash with distilled water and ethanol until the pH value is 7. After drying, obtain a light blue-green solid powder, which is the copper silicate with a sea urchin-like porous structure.

3. The sea urchin-shaped copper-based catalyst according to claim 2, characterized in that, The selected particle size of the silica microspheres is 200 - 500 nm.

4. Application of the sea urchin-like copper-based catalyst according to any one of claims 1 - 3 in the transfer hydrogenation and deoxygenation reaction of vanillin.

5. The application according to claim 4, wherein Take isopropanol, the catalyst according to any one of claims 1 - 3, and vanillin and add them into a high-pressure reactor. Then seal the reactor, purge the residual air with N2 and fill it with 2 MPa N2. Control the temperature at 150 - 180 °C, the stirring speed at 500 rpm, and the reaction time at 90 - 360 min; after the reaction is completed, separate the product liquid and the catalyst.

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