CN113351210B - A Cu-based catalyst and its use in photocatalytic water hydrogen production-5-HMF oxidative coupling reaction - Google Patents

Abstract

The invention provides a Cu-based catalyst and a method for using the Cu-based catalyst to carry out photocatalytic hydrogen production-5-HMF oxidation coupling reaction in water, wherein the chemical expression of the Cu-based catalyst is Cu/CoAlO, and the catalyst is prepared by using Cu ²⁺ 、Co ²⁺ And Al ³⁺ Hydrotalcite of a metal cation as precursor in H ₂ /N ₂ Or H ₂ Roasting at 550-650 ℃ under the mixed atmosphere of/Ar to obtain the metal nano particles with a Co @ CuCo core-shell structure on the surface of the carrier by Cu and Co species, wherein the metal particles are uniformly distributed, and the average particle size of the metal particles is 2-10 nm; the valence band value VB = 1.7-2.0 eV, and the conduction band value CB is-0.8-1.0 eV. The catalyst is applied to the photocatalytic water hydrogen production-5-HMF oxidation coupling reaction under the condition of no additional alkali, not only improves the hydrogen production efficiency, but also realizes the efficient directional conversion from 5-hydroxymethylfurfural to 2, 5-furandicarboxylic acid under the neutral condition. The Cu-based catalyst has outstanding catalytic performance, simple preparation and environmental protection.

Description

A Cu-based catalyst and its use in photocatalytic hydrogen production from water-5-HMF oxidative coupling reaction

technical field

The invention relates to the technical field of catalysis, in particular to a Cu-based catalyst and its use in photocatalytic water hydrogen production-5-HMF oxidative coupling reaction to obtain high value-added chemicals while improving hydrogen production performance.

Background technique

With the rapid development of the economy and the continuous improvement of people's living standards, the demand for energy continues to grow. The development of clean, non-polluting, renewable, and high-energy-density hydrogen energy is crucial to solving current energy and environmental issues. Photocatalytic hydrogen production technology can utilize abundant and clean solar energy, and has the characteristics of environmental friendliness and low energy consumption. Therefore, it is a hydrogen energy production method with broad application prospects. In the traditional photocatalytic water splitting hydrogen production reaction, photoexcited electrons reduce protons to generate H ₂ , but the oxidation ability of excited holes is not fully utilized. Therefore, if abundant and easily available raw materials (such as 5-HMF and its derivatives) are used to capture the excited holes and couple the photocatalytic water hydrogen production reaction with the 5-HMF oxidation reaction, while promoting the hydrogen production efficiency , to obtain high-value-added downstream products, which can not only effectively promote solar energy utilization efficiency, but also realize high-value conversion of renewable resources. In recent years, to alleviate the current energy crisis, the conversion of renewable 5-HMF resources into value-added chemicals has attracted increasing attention. 5-Hydroxymethylfurfural (5-HMF) produced by dehydration of C carbohydrates is listed as one of the 12 sustainable 5-HMF value-added chemicals by the U.S. Department of Energy, and its downstream product 2,5 _- furandicarboxylic acid (FDCA ) is considered an important monomer substitute for synthetic polymers, especially polyethylene furanate (PEF), which is considered a potential substitute for the polymer polyethylene terephthalate .

Document 1 In Visible-Light-Driven Valorization of Biomass Intermediates Integrated with H ₂ Production Catalyzed by Ultrathin Ni/CdS Nanosheets.Journal of the American Chemical Society, 2017,139(44):15584-15587, Sun et al. The cadmium sulfide nanosheets (Ni/CdS NSs) as photocatalysts can realize simultaneous hydrogen evolution and selective oxidation of 5-HMF-derived alcohols under visible light. After 22 h of reaction, the conversion of 5-hydroxymethylfurfural (5-HMF) Among the products, the selectivity of 2,5-diformylfuran (DFF) is close to 100%. When a strong base is added to the system, DFF will quickly convert to FDCA due to the instability of the aldehyde group. However, due to the lower conduction band position of CdS, the hydrogen evolution rate is lower.

Document 2Chen et al. in P-doped Zn _x Cd _1-x S solid solutions as photocatalysts for hydrogen evolution from water splitting coupled with photocatalytic oxidation of 5-hydroxymethylfurfural. Applied Catalysis B: Environmental 233 (2018) 70–79, prepared S-rich The vacant P-doped Zn _x Cd _1-x S (Zn _x Cd _1-x SP) acts as a photocatalyst for both water splitting to produce hydrogen and photocatalytic oxidation of 5-HMF. After 8 hours of reaction, the selectivity of DFF was 65% among the converted products of 5-hydroxymethylfurfural, and the highest hydrogen production rate was 786μmol·g ^-1 ·h ^-1 .

In the published literature reports on photocatalytic water hydrogenation-5-hydroxymethylfurfural oxidative coupling reaction, it can be found that under neutral conditions, the oxidation products of 5-HMF are basically 2,5-diformylfuran ( DFF). Only under strong alkaline conditions, due to the instability of the aldehyde group at high pH, will be rapidly converted to 2,5-furandicarboxylic acid (FDCA). Although adding alkaline solution to the reaction system can improve the selectivity to FDCA, it does not conform to the development trend of green chemistry and easily causes equipment corrosion. Therefore, it is of great significance to design and synthesize environmentally friendly photocatalysts with high selectivity for FDCA in coupling reactions under non-strongly alkaline conditions.

Literature 3Feng et al. In Interfacial Structure-Determined Reaction Pathway and Selectivity for 5-(Hydroxymethyl)furfural Hydrogenation over Cu-Based Catalysts.ACS Catal.2020,10,1353-1365, CuMgAl-LDH, CuCoAl-LDH and other precursors were synthesized by co-precipitation method LDH precursor was reduced under 10% H ₂ /Ar atmosphere at 300°C for 4 h to prepare a Cu-based catalyst for hydrogenation of 5-hydroxymethylfurfural.

Literature 4Zhao et al. explained in Layered Double Hydroxide Nanostructured Photocatalysts for Renewable Energy Production.Advanced Energy Materials, 2016, 6(6): 1501974 that layered double hydroxides (LDHs) as a typical two-dimensional layered material, due to its excellent The excellent electronic properties and the ability of fast transport of photogenerated electron-hole pairs make them show great application prospects in the field of photocatalysis. Based on its compositional tunability, the introduction of specific photosensitive metal ions into the laminate can tune its energy band structure. Coupling reactions have great development prospects in the field of photocatalytic hydrogen production, but in this emerging field, there are few reports on the preparation of photocatalytic materials from LDH. Therefore, LDHs provide a powerful structural platform for the development of highly active photocatalysts suitable for photocatalytic coupling reactions.

Therefore, our idea of invention is: using LDHs as a platform, based on its topological effect and compositional adjustability, introduce photosensitive ions such as Cu ²⁺ into hydrotalcite laminates, design and synthesize a kind of photoresponsive ability, high activity and suitable for It is a photocatalyst for the photocatalytic water hydrogen production-5-HMF oxidative coupling reaction, and realizes the enhancement of catalytic performance under the condition of no external base.

Contents of the invention

The purpose of the present invention is to provide a Cu-based catalyst, which is specially used for the photocatalytic water hydrogen production-5-HMF oxidative coupling reaction.

The Cu-based catalyst of the present invention has a chemical expression of Cu/CoAlO, wherein the molar ratio of Cu to Co is 1:3 to 1:5, and the molar ratio of Co to Al is 1.5 to 2.5; the structural characteristics of the catalyst It is Cu and Co species that form metal nanoparticles with a Co@CuCo core-shell structure on the surface of the carrier, and the metal particles are uniformly distributed, and the average particle size of the metal particles is 2-10 nm; the valence band value of the catalyst is VB=1.7-2.0 eV, leading The band value CB is -0.8 to -1.0 eV.

The Cu/CoAlO catalyst is specially used for photocatalytic water hydrogen production-5-HMF oxidative coupling reaction. Under the condition of no external alkali, the highest hydrogen production rate can reach 796.8～888.3μmolh ^-1 g ^-1 after 6 hours of reaction, FDCA selectivity It can reach 93.7-95.5%, much higher than the performance reported in the literature. It shows that the catalyst not only promotes the improvement of hydrogen production efficiency, but also realizes the efficient and directional conversion of 5-HMF to FDCA under neutral conditions.

The above-mentioned Cu/CoAlO catalyst is obtained by calcining hydrotalcite containing Cu ²⁺ , Co ²⁺ and Al ³⁺ metal ions at a high temperature of 550-650°C in a mixed atmosphere of H ₂ /N ₂ or H ₂ /Ar of.

The specific application steps of Cu/CoAlO catalyst are as follows:

Add reactant 5-HMF, Cu/CoAlO and deionized water in the sealable reactor, wherein the mass ratio of 5-HMF and Cu/CoAlO is 2～6, the mass concentration of 5-HMF is 1.0～3.3g/ L: Under magnetic stirring, fully replace the air in the reaction bottle with high-purity Ar, irradiate the reaction solution with a 300W Xe lamp (λ>380nm), and react for 1-8 hours under sufficient stirring to obtain the FDCA product.

The photocatalytic water hydrogen production-5-HMF oxidative coupling reaction pathway is as follows:

Since the Cu-based catalyst used has suitable valence and conduction band positions, its conduction band position meets the thermodynamic requirements for reducing H protons to hydrogen, and the valence band position not only meets the VB value required for oxygen production, but also reaches the 5-HMF VB value required for oxidation. Therefore, under the condition of no external base, the excited holes oxidize 5-HMF to DFF, and at the same time oxidize water to generate O ₂ , and the oxygen generated by water oxidation further rapidly oxidizes DFF to FDCA.

Fig. 1 is the HRTEM photo and particle size distribution diagram of the Cu/CoAlO catalyst whose Cu/Co molar ratio is 1/5 prepared in Example 1, as can be seen from the figure, the metal particles are evenly distributed on the carrier, and the nanoparticle size range It is 2.0-10.0nm, and the average particle size is 5.8nm.

Fig. 2 is the X-ray energy spectrum (EDS) line scan result of metal Co and Cu in the Cu/CoAlO catalyst in the Cu/CoAlO catalyst of 1/5 that Cu/Co molar ratio prepared in embodiment 1, contrasts the distribution of element Co and Cu It can be seen that Co is distributed throughout the metal particles and Cu is concentrated on the surface of the metal particles, so it can be concluded that the catalyst active component metal nanoparticles present a core-shell structure of Co@CuCo.

Fig. 3 is the XPS valence band spectrogram and the Tauc figure of the Cu/CoAlO catalyst of 1/5 for the Cu/Co molar ratio prepared in embodiment 1, can find out the valence band (VB) of catalyst by figure (a) valence band spectrogram ) value is 1.73eV, and the energy bandgap (Eg) of the catalyst can be drawn as 2.67eV from Figure (b) Tauc diagram.

Fig. 4 is the energy band structure diagram of the Cu/CoAlO catalyst that the Cu/Co molar ratio prepared in Example 1 is 1/5, as can be seen from the figure, the CB value of the catalyst is lower than the reduction potential (0eV) of the H proton, at Under the irradiation of a light source with an appropriate wavelength, the thermodynamic requirements for reducing H protons to hydrogen can be met. In addition, because CuCo/Al ₂ O ₃ has a suitable valence band position, it not only meets the VB value (1.23eV) required for oxygen generation, but also achieves the VB value (0.82eV) required for 5-HMF oxidation. Therefore, holes can not only oxidize 5-HMF to DFF, but also oxidize water to generate O ₂ , and the oxygen produced by water oxidation can further rapidly oxidize DFF to FDCA.

Figure 5 is a line graph of the hydrogen production over time in the photocatalytic water hydrogen production-5-HMF oxidative coupling reaction of the Cu/CoAlO catalyst with a Cu/Co molar ratio of 1/5 prepared in Example 1, within 6h , the amount of hydrogen produced by the catalyst tended to increase with the prolongation of the reaction time, the highest hydrogen production rate reached 888.3μmolh ^-1 g ^-1 , and the hydrogen production after 6h was 9.78μmol.

Fig. 6 is the HPLC spectrogram of the reaction solution in the photocatalytic water hydrogen production-5-HMF oxidative coupling reaction of the Cu/CoAlO catalyst with a Cu/Co molar ratio of 1/5 prepared in Example 1, as can be seen from the figure, The peak position of 5-HMF was 2.8 min, and the peak position of 2,5-furandicarboxylic acid (FDCA) was 2 min. Only FDCA, 5-HMF and a small amount of DFF were detected in the liquid phase product after 6 hours of reaction. No other products were detected, indicating that the coupling reaction is highly selective for FDCA.

Fig. 7 is that the Cu/Co molar ratio prepared in Example 1 is the concentration curve of the product FDCA of 1/5 Cu/CoAlO catalyst in photocatalytic water hydrogenation-5-HMF oxidative coupling reaction, the concentration of 5-HMF Conversion and FDCA selectivity curves. When reacted for 6h, the conversion rate of 5-HMF was 11.2%, and the selectivity of FDCA was 95.5%.

The beneficial effects of the present invention are: for the first time, the application of the supported Cu-based catalyst obtained based on the topological reduction of LDHs precursors in the photocatalytic water hydrogen production-5-HMF oxidative coupling reaction can not only promote the improvement of hydrogen production efficiency, but also It can realize high-efficiency directional conversion from 5-HMF to FDCA under neutral conditions. The Cu-based catalyst has outstanding catalytic performance, simple preparation and environmental friendliness.

Description of drawings:

Figure 1 is the HRTEM photo and particle size distribution diagram of the catalyst prepared in Example 1, wherein Figure a is the HRTEM photo of the catalyst, and Figure b is the size distribution diagram of the metal nanoparticles.

Fig. 2 is the line-scan spectrogram of metal particle X-ray Co and Cu of the catalyst prepared in Example 1.

Figure 3 is the XPS valence band spectrum and Tauc diagram of the catalyst prepared in Example 1, wherein Figure a is the valence band spectrum and Figure b is the Tauc diagram.

Figure 4 is a diagram of the energy band structure of the catalyst prepared in Example 1.

Fig. 5 is a line graph showing the change of the amount of hydrogen produced by the catalyst prepared in Example 1 in the photocatalytic water hydrogen production-5-HMF oxidative coupling reaction with time.

Figure 6 is the HPLC spectrogram of the reaction solution in the photocatalytic water hydrogen production-5-HMF oxidative coupling reaction of the catalyst prepared in Example 1, wherein Figure a is the HPLC spectrum before the reaction, and Figure b is the HPLC spectrum after the reaction for 6h picture.

Fig. 7 is the time-varying curve of the concentration of FDCA, the conversion rate of 5-HMF and the FDCA selectivity curve of the catalyst prepared in Example 1 in the photocatalytic water hydrogen production-5-HMF oxidative coupling reaction. Wherein, figure a is the concentration curve of the product FDCA with time, and figure b is the conversion rate of 5-HMF and the selectivity curve of FDCA.

Detailed ways:

Example 1

_{Cu 2+ , Co 2+ _ _ _ _} ^{_ _} Mixed salt solution with Al ³⁺ metal molar ratio of 1:5:2, adding alkali solution to keep the pH range of the system constant at 10±0.1, co-precipitate to obtain CuCoAl-CO ₃ ^2- -LDHs-like hydrotalcite precursor. The obtained precursor was placed in a tube furnace, and the gas flow rate was kept at 30mL/min under an _H2 / _N2 atmosphere with a hydrogen fraction of 10%, and the temperature of the tube furnace was raised at a rate of 2°C/min. to 600°C and kept at constant temperature for 4h. After the reaction is over, in order to avoid catalyst oxidation, the furnace temperature is naturally cooled to room temperature to obtain a Cu/CoAlO catalyst, wherein the Cu/Co molar ratio is 1/5, and the mass percentages of Cu, Co and Al elements are 8.3%, 42.7% respectively and 7.8%. The obtained samples were characterized, and the results are shown in Fig. 1-Fig. 4. It can be seen from the characterization results that the catalyst metal nanoparticles present a core-shell structure of Co@CuCo alloy, and the metal nanoparticles are uniformly dispersed in the amorphous Al ₂ O ₃ On the carrier, the average size of nanoparticles is 5.8nm. The valence band (VB) value of the catalyst is 1.73eV, and the conduction band (CB) value is -0.94eV. The conduction band value meets the thermodynamic requirements for reducing H protons to hydrogen, and the valence band value meets the VB value required for oxygen production. (1.23eV), also reached the VB value (0.82eV) required for the oxidation of 5-HMF.

The catalyst obtained above is used in the photocatalytic water hydrogen production-5-HMF oxidative coupling reaction. The specific method is: in a sealable reactor with a volume of 20mL, first add 5mg of catalyst and 13mg of 5-hydroxymethylfurfural, Then add 10mL of pure water and seal it with a capper. Under magnetic stirring, replace the air in the reaction bottle with high-purity Ar for 30 minutes, then turn on the 300WXe lamp (>380nm), keep the distance between the lamp and the reaction bottle about 5cm, the lamp current intensity is 16A, irradiate the reaction solution, and the magnetic stirring speed is set to 600rpm, the reaction time is 6h. Samples were taken at 1, 2, 3, 4, 5, and 6 hours of reaction time, the concentration and content of gas phase products were detected by gas chromatography, and the distribution and content of liquid phase products were analyzed by high performance liquid chromatography. The results are shown in Table 1. It can be seen from Table 1 that after 6 hours of reaction, the FDCA selectivity of the catalyst was 95.5% without adding alkali, the hydrogen production amount after 6 hours was 9.78 μmol, and the highest hydrogen production rate could reach 888.3 μmolh ^-1 g ^-1 .

Example 2

The Cu/CoAlO catalyst is prepared according to the method of Example 1, the difference is that the metal molar ratio of Cu, Co and Al in the mixed salt solution is 1:4:2, and the mass percentage of Cu, Co and Al elements in the obtained catalyst 9.6%, 37.6% and 8.5% respectively.

According to the method of Example 1, the obtained catalyst was used in the photocatalytic water hydrogen production-5-HMF oxidative coupling reaction, the results are shown in Table 1, and it can be seen from Table 1 that after 6 hours of reaction, the catalyst was under the condition of no additional alkali The selectivity of FDCA is 94.3%, the hydrogen production amount after 6h is 8.03μmol, and the highest hydrogen production rate can reach 798.0μmolh ^-1 g ^-1 .

Example 3

The Cu/CoAlO catalyst was prepared according to the method of Example 1, except that the metal molar ratio of Cu, Co and Al in the mixed salt solution was 1:3:2, and the calcination reduction temperature was 550°C; the obtained catalyst contained Cu, Co The mass percent contents of Al and Al elements are 10.8%, 33.7% and 10.4%, respectively.

According to the method of Example 1, the obtained catalyst was used in the photocatalytic water hydrogen production-5-HMF oxidative coupling reaction, the results are shown in Table 1, and it can be seen from Table 1 that after 6 hours of reaction, the catalyst was under the condition of no additional alkali The selectivity of FDCA is 94.5%, the hydrogen production amount after 6h is 7.4μmol, and the highest hydrogen production rate can reach 796.8μmolh ^-1 g ^-1 .

Table 1

Among them, Comparative Example 1 and Comparative Example 2 are the literature values of the photocatalytic water hydrogen production-5-HMF oxidative coupling reaction. Comparative example 1 is the hydrogen production of the Ni/CdS NSs catalyst in Document 1 after 6 hours of reaction, and the DFF selectivity after 22 hours of reaction, but the catalyst will quickly convert DFF to FDCA after adding a strong base to the reaction system. Comparative Example 2 is the performance of the Zn _x Cd _1-x SP catalyst in Document 2 after 8 hours of reaction.

As can be seen from Table 1, the catalyst of the present invention has higher selectivity for 2,5-furandicarboxylic acid (FDCA) under the condition of no external base, which is higher than that of the comparative example, and the 5-hydroxymethyl Efficient and targeted conversion of furfural (5-HMF) to 2,5-furandicarboxylic acid (FDCA).

Claims (2)

1. The application of the Cu-based catalyst is characterized in that the catalyst is used for photocatalytic hydrogen production with water-5-HMF oxidation coupling reaction; wherein 5-HMF represents 5-hydroxymethylfurfural; the chemical expression of the Cu-based catalyst is Cu/CoAlO, wherein the molar ratio of Cu to Co is 1-3 to 1; the catalyst has structural characteristics of Cu andthe Co species form metal nano particles with a Co @ CuCo core-shell structure on the surface of the carrier, the particles are uniformly distributed, and the average particle size of the metal nano particles is 2-10 nm; the valence band value VB = 1.7-2.0 eV, and the conduction band value CB is-0.8-1.0 eV; the Cu/CoAlO catalyst is prepared from Cu ²⁺ 、Co ²⁺ And Al ³⁺ Hydrotalcite of metal ions as precursor in H ₂ /N ₂ Or H ₂ The catalyst is obtained by roasting at 550-650 ℃ in the mixed atmosphere of/Ar.

2. The use of the Cu-based catalyst according to claim 1, comprising the steps of: adding reactants of 5-HMF, cu/CoAlO and deionized water into a sealable reactor, wherein the mass ratio of the 5-HMF to the Cu/CoAlO is 2-6, and the mass concentration of the 5-HMF is 1.0-3.3 g/L; under the magnetic stirring, fully replacing the air in the reactor with high-purity Ar, irradiating the reaction solution with a 300W Xe lamp, and reacting for 1-8 h under the full stirring to obtain a 2, 5-furandicarboxylic acid product; the reaction is characterized by being carried out under the condition of no additional alkali.

Images