Photocatalyst, preparation method and application of photocatalyst in conversion of toluene into benzaldehyde

Abstract

The invention discloses a photocatalyst, a preparation method and application of the photocatalyst in conversion of methylbenzene into benzaldehyde. The invention belongs to synthesis of a zinc-based MOF compound constructed by a crystal engineering strategy and application of the zinc-based MOF compound in photocatalytic efficient selective oxidation of toluene into benzaldehyde. Specifically, the invention relates to a method for high-efficiency and high-selectivity catalytic oxidation of toluene into benzaldehyde under visible light irradiation and room temperature conditions, and preparation and application of the zinc-based MOF composite material with anthraquinone and Zn-O double-photoactivity units.

Description

Technical Field

[0001] This invention relates to the synthesis of a zinc-based MOF compound, Zn-AQ, constructed using a crystal engineering strategy, and its application in the efficient and highly selective photocatalytic conversion of toluene into benzaldehyde. The invention also relates to methods for directly converting toluene into high-value-added benzaldehyde under visible light irradiation and room temperature, as well as the preparation and application of zinc-based MOF crystalline materials with three-photon catalytic activity. Background Art

[0002] The paper Journal of the American Chemical Society, Vol. 144, No. 33, 15295-15302, points out that the selective oxidation of toluene to benzaldehyde remains an industrial challenge, mainly due to side reactions or overoxidation. The paper Nature Communications, Vol. 10, 2425, points out that the current industrial process for producing benzaldehyde mainly involves liquid-phase chlorination of toluene followed by saponification. Obviously, these industrial processes have many steps and low selectivity. The paper Angewandte Chemie International Edition, Vol. 54, No. 31, 8928-8933, points out that in recent years, oxygen vacancy catalysis generated by metal-organic complex catalysts has shown high selectivity for organic transformations. However, slow catalytic efficiency and structural instability during surface reconstruction limit their application. The paper Journal of the American Chemical Society, Vol. 145, No. 23, 12737–1274, points out that multiphoton excitation tandem catalytic platform, as an emerging development strategy, has shown high efficiency and synergistic effects in the field of organic catalysis. At present, the multiphoton excitation work reported in the papers Angewandte Chemie International Edition, Vol.61, No.2, e202114490 and Journal of the American Chemical Society, Vol.145, No.4, 2170–2182 has achieved inert C (sp) by integrating ligand-to-metal charge transfer (LMCT), photoinduced electron transfer (PET) and photoinduced hydrogen atom transfer (HAT) processes. 3 )–H bond functionalization. However, Advanced Science, Vol.11, No.36, 2404293 pointed out that the activity of photoinduced HAT reagents is too high, which can quickly activate C(sp 3 )–H bonds, but also leads to a decrease in selectivity. Therefore, there is an urgent need to develop a catalytic oxidation method that is both efficient and highly selective for C(sp3 )–H bond method.

[0003] The paper Chemical Reviews, Vol.122, No.2, 2017–2291 pointed out that natural oxidases with PCET catalytic activity can activate C(sp) in a non-HAT manner through concerted or stepwise electron transfer and proton transfer processes. 3 )–H bond, generating a carbon radical intermediate. For example, quinol oxidase (QOs) is a natural enzyme with oxidative proton-coupled electron transfer (PCET) activity. QOs can oxidize ubiquinol to ubiquinone with high selectivity and efficiency under mild conditions. Despite this, there are few reports on oxidative PCET catalytic systems. This is because the oxidation site in the system is closely related to the The compatibility and appropriate spatial distance of base sites are difficult to regulate and control. The modular structural characteristics of metal-organic frameworks enable the active sites to be periodically arranged on the framework, which helps prevent mutual interference between catalytic sites and improves the stability of the structure. The structural designability of MOFs allows multiple functional units to be integrated into the framework, which is conducive to the construction of a multi-photon excited catalytic platform. The active sites inside the MOFs pores can adsorb and fix substrates, which is more conducive to the synergistic catalytic effect of multifunctional sites.

[0004] The purpose of this invention is to overcome the shortcomings of the existing technology and integrate two photoactive units, anthraquinone and Zn–O, into a MOF crystalline material. Through this integration, the high selectivity of lattice oxygen in the oxidation process and the high catalytic efficiency of the multi-photon excitation process are combined, and finally a zinc-based MOF crystalline material with three-photon catalytic activity, namely Zn-AQ compound, is synthesized. With Zn-AQ as the photocatalyst, under parallel light tandem excitation, C(sp 3 )–H bonds and O2, achieving efficient synergistic effects of each functional unit and catalyzing the efficient and selective oxidation of toluene to benzaldehyde. Summary of the Invention

[0005] To solve the above problems, the present invention provides an effective crystal engineering construction strategy. Anthraquinone and Zn-O photofunctional units are introduced into the metal-organic framework through a coordination self-assembly process and a solvothermal method to obtain a crystalline material Zn-AQ with three-photon catalytic activity. Subsequently, using Zn-AQ as a photocatalyst, the conversion of toluene to benzaldehyde was achieved efficiently and selectively under 30W, 405nm LED light. The method of the present invention is simple and easy to implement, and the reaction conditions are mild. The prepared Zn-AQ crystalline material has the characteristics of rapid and simultaneous activation of C(sp 3)–H bond and O2. Its pores adsorb toluene through π–π stacking, which not only shortens the distance between the substrate and the catalytic site, but also forms the oxidation sites and The optimal spatial distance between the base site and the substrate. Na ions stabilize the Zn–O surface during its reconstruction. This invention achieves a true "artificial oxidase-like" reaction. The catalyst is recyclable and efficiently and selectively photocatalytically converts toluene to benzaldehyde in a heterogeneous catalytic system.

[0006] The technical solution of the present invention:

[0007] A photocatalyst is a zinc-based MOF photocatalyst that can be used to catalyze the oxidation of toluene to benzaldehyde. The photocatalyst adopts a coordination self-assembly strategy, uses an organic dye 2,7-dicarboxyanthraquinone (H2L-AQ) as a ligand, and Zn 2+ and Na + Using bimetallic ions as metal nodes, a three-dimensional porous metal-organic framework (Zn-AQ) was synthesized. This method combines the high selectivity of lattice oxygen in the oxidation process with the high catalytic efficiency of multiphoton excitation, achieving highly selective (93.7%) oxidation of toluene to benzaldehyde under visible light conditions. The specific steps are as follows:

[0008] Zinc salt, sodium salt, H2L-AQ, tetrafluoroboric acid (HBF4), and an organic solvent were mixed and stirred. The resulting solution was transferred to a high-pressure hydrothermal reactor for a hydrothermal reaction. After the reaction, the solution was cooled to room temperature and filtered to remove the yellow block crystals of Zn-AQ. The yellow block crystals of Zn-AQ were washed, vacuum-dried, and finally used to obtain Zn-AQ.

[0009] Furthermore, the mass ratio of zinc salt (such as zinc nitrate, zinc chloride, zinc sulfate, etc.), sodium salt (such as sodium perchlorate, sodium chloride, sodium nitrate, etc.), H2L-AQ, tetrafluoroboric acid and organic solvent (such as DMAC, DMF, DMSO, etc.) is 1-50:1-50:1-20:1-200:1-500;

[0010] Furthermore, the hydrothermal temperature is 80-180°C, the hydrothermal time is 1-6 days, and the cooling rate is 5-40°C / h;

[0011] Furthermore, the yellow block crystals of Zn-AQ were washed with two or more of DMAC, acetonitrile, and ethanol;

[0012] Furthermore, the vacuum drying temperature is 20-100° C. and the time is 2-24 h.

[0013] A photocatalyst is used in the conversion of toluene to benzaldehyde, specifically: toluene is used as a substrate, acetonitrile is used as a solvent, and oxygen is used as an oxidant. During the three-photon excitation process, the first photon induces the ligand H2L-AQ to activate the C–H bond of toluene through a proton-coupled electron transfer process to generate a carbon radical intermediate. At the same time, the ligand H2L-AQ absorbs the second photon and activates O2 to form a carbon radical intermediate through an energy transfer (EnT) process. 1 O2. In addition, the Zn–O–Na structure absorbs the third photon to generate oxygen vacancies, which are rapidly activated. 1 O2 generates lattice oxygen for efficient selective oxidation of toluene to benzaldehyde. The specific process is:

[0014] Zn-AQ was dispersed in acetonitrile, and toluene was then added to a quartz photoreaction tube, along with a rotor. The reaction was carried out in an O2 atmosphere under illumination from an LED light source (wavelength 360-500 nm) and magnetic stirring.

[0015] Furthermore, the mass ratio of Zn-AQ to substrate toluene is 1-5:1-300.

[0016] Beneficial effects of the present invention:

[0017] (1) In order to solve the problems of overoxidation and poor selectivity in the process of oxidizing toluene to produce benzaldehyde in the chemical industry, this chapter adopts the coordination self-assembly strategy to 2+ and Na + A three-dimensional porous Zn-AQ was constructed using anthraquinone with dual photofunctionality as a metal node and used for the catalytic oxidation of toluene to produce benzaldehyde with a selectivity of up to 93.7%.

[0018] (2) The study of the cocrystal structure of toluene and Zn-AQ showed that the pores adsorbed toluene through π-π stacking, which not only shortened the distance between the substrate and the catalytic site, but also formed the oxidation sites and The appropriate spatial distance between the basic site and the substrate.

[0019] (3) Under multi-photon excitation, anthraquinone molecules activate the C–H bond of toluene and O2 through the PCET process and EnT process, respectively, to generate carbon radical intermediates and 1 O2. The third photon induces the formation of oxygen vacancies in the Zn–O–Na secondary structural unit, which is rapidly activated. 1 O2 generates lattice oxygen. By precisely controlling the spatial distance of the photoactive units in Zn-AQ, the overoxidation caused by the active intermediates generated during the electron transfer process is effectively avoided, thereby improving the selectivity of the oxidation process. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1This is a structural diagram of the single crystal X-ray diffraction test of the Zn-AQ crystalline material prepared in the step of Example 1, including (a) one-dimensional Zn···Na polyacid chain; (b) double-layer ligand H2L-AQ and its coordination mode; (c) two-dimensional layered structure; (d) three-dimensional framework structure; (e) eutectic of toluene and Zn-AQ framework; (f) interaction between toluene molecules and ligands.

[0021] Figure 2 Schematic diagram of Zn-AQ and three-photon excitation building blocks.

[0022] Figure 3 This is the X-ray powder diffraction pattern of the Zn-AQ crystalline material prepared in the steps of Example 1.

[0023] Figure 4 Spectra of the Zn-AQ crystalline material prepared in the step of Example 1, including (a) X-ray photoelectron spectrum; (b) Raman spectrum; and (c) oxygen vacancy-electron paramagnetic resonance spectrum.

[0024] Figure 5 The diagrams show the results of the Zn-AQ crystalline material prepared in the steps of Example 1, including (a) singlet oxygen-electron paramagnetic resonance spectrum and (b) selective oxidation experiment of α-terpinene.

[0025] Figure 6 These are spectra of Zn-MOF material and toluene under different conditions in the step of Example 2, including: (a) electron paramagnetic resonance spectrum; (b) mass spectrum of TEMPO-captured benzyl carbon radicals.

[0026] Figure 7 This is a light intensity dependence kinetic experiment of benzaldehyde prepared by photocatalytic oxidation of toluene in the step of Example 2.

[0027] Figure 8 Spectra of the benzaldehyde product prepared by photocatalytic oxidation of toluene in the step of Example 2, including: (a) H NMR spectrum; (b) C NMR spectrum. DETAILED DESCRIPTION

[0028] The specific implementation of the present invention is further described below in conjunction with the accompanying drawings and technical solutions.

[0029] Example 1:

[0030] Take Zn(NO3)2 (20 mg), NaClO4 (20 mg), H2L-AQ (10 mg), HBF4 (0.1 mL) and DMAC (3 mL) and stir for 10 minutes. The obtained solution was transferred to a high-pressure hydrothermal autoclave and reacted at 140 ° C for 3 days. After the reaction, it was slowly cooled to room temperature at a rate of 10 ° C / h, and then the yellow block crystals of Zn-AQ were filtered out. The crystals were washed with DMAC and ethanol and dried in a vacuum drying oven at 70 ° C for 12 hours to finally obtain 5 mg of Zn-AQ. The crystal structure was measured by single crystal X-ray diffractometer, as shown Figure 1 As shown, the metal center forms a one-dimensional Zn-Na heterometallic chain through oxygen bridges, in which the Zn-O bond bridges the adjacent Na+ nodes. The ligands form a two-dimensional layered network through π-π stacking (building a double-layer structure and interweaving with the metal chain, which is further stacked to form a three-dimensional framework. Toluene molecules can be specifically embedded in the pores, and the toluene aromatic ring and the ligand plane realize electron transfer through π-π interaction. The benzyl C-H bond forms a bond with the adjacent carbonyl oxygen. The C–H···O hydrogen bond network synergistically promotes proton transfer. The results of powder X-ray diffraction analysis show that the Zn-AQ sample is pure. Figure 3 As shown. Figure 4 It can be seen that the oxygen vacancies of Zn-AQ can adsorb and activate O2. Zn-AQ was tested for active oxygen, such as Figure 5 It can be seen that 1 O2 is the main reactive oxygen species in the oxidation process.

[0031] Example 2:

[0032] 5.4 mg of Zn-AQ (3 mol%) was dispersed in 3 mL of acetonitrile. 21 μL of toluene was then added to a 15 mL quartz photoreactor tube, and a rotor was added. The reaction was carried out in an O2 atmosphere under 405 nm LED illumination and magnetic stirring. The products were qualitatively and quantitatively analyzed by TRACE1300 gas chromatography or H-NMR spectroscopy. Figure 6 As shown in the figure, when an acetonitrile suspension containing DMPO and Zn-AQ is illuminated, the EPR spectrum shows the characteristic peak of alkoxy radicals. After adding toluene to the suspension, the intensity of the characteristic peak of carbon radicals increases with the extension of illumination time. This result shows that Zn-AQ can activate toluene to generate a carbon radical intermediate. In addition, when the radical scavenger TEMPO is added to the photocatalytic oxidation of toluene system, the mass spectrum of toluene benzyl radicals can be detected by mass spectrometry. By comparing the slopes of the reaction kinetics of the two within 3 hours of 405nm LED light full power and half power irradiation, as shown in the figure, Figure 7 As shown in the figure, the full power: ideal half power = 8:1, indicating that the entire photocatalytic process involves a nonlinear three-photon excitation pathway. Figure 8As shown, the H NMR and C NMR spectra of the product benzaldehyde were detected, proving that benzaldehyde was successfully prepared.

[0033] Example 3:

[0034] The steps were the same as in Example 1, except that Zn(NO3)2 was replaced by ZnCl2, NaClO4 was replaced by NaCl, DMAC was replaced by DMF, and the reaction temperature and time were changed to 80°C and six days.

[0035] Example 4:

[0036] The steps were the same as in Example 1, except that Zn(NO3)2 was replaced with ZnSO4, NaClO4 was replaced with NaNO3, and DMAC was replaced with DMSO. The reaction temperature and time were changed to 180°C and one day.

[0037] Example 5:

[0038] The steps are the same as those in Example 2, except that the 405nm LED is replaced with a 395nm LED light source.

Claims

1. A photocatalyst, characterized in that The photocatalyst is a zinc-based MOF photocatalyst that can be used to catalyze the oxidation of toluene to benzaldehyde. The photocatalyst adopts a coordination self-assembly strategy, uses an organic dye 2,7-dicarboxyanthraquinone H2L-AQ as a ligand, and Zn 2+ and Na + Synthesis of a three-dimensional porous metal-organic framework Zn-AQ using double metal ions as metal nodes.

2. The method for preparing a photocatalyst according to claim 1, characterized in that: The specific steps are as follows: zinc salt, sodium salt, H2L-AQ, tetrafluoroboric acid HBF4 and organic solvent are mixed and stirred; the obtained solution is transferred to a high-pressure hydrothermal kettle for hydrothermal reaction; after the reaction is completed, it is cooled to room temperature and then filtered out yellow block crystals of Zn-AQ; the yellow block crystals of Zn-AQ are washed and vacuum-dried and finally Zn-AQ is obtained.

3. The method for preparing a photocatalyst according to claim 1, wherein: The mass ratio of the zinc salt, the sodium salt, H2L-AQ, the tetrafluoroboric acid and the organic solvent is 1-50:1-50:1-20:1-200:1-500.

4. The method for preparing a photocatalyst according to claim 1, wherein: The zinc salt is zinc nitrate, zinc chloride or zinc sulfate; the sodium salt is sodium perchlorate, sodium chloride or sodium nitrate; and the organic solvent is DMAC, DMF or DMSO.

5. The method for preparing a photocatalyst according to claim 1, wherein: The hydrothermal temperature is 80-180°C, the hydrothermal time is 1-6 days, and the cooling rate is 5-40°C / h.

6. The method for preparing a photocatalyst according to claim 1, wherein: The yellow block crystals of Zn-AQ were washed with two or more of DMAC, acetonitrile and ethanol.

7. The method for preparing a photocatalyst according to claim 1, characterized in that: The vacuum drying temperature is 20-100°C and the time is 2-24 hours.

8. Use of a photocatalyst according to claim 1, or a photocatalyst prepared by the preparation method according to any one of claims 2 to 7, in converting toluene into benzaldehyde, characterized in that: Toluene is used as substrate, acetonitrile as solvent and oxygen as oxidant. In the three-photon excitation process, the first photon induces the ligand H2L-AQ to activate the C–H bond of toluene through the proton-coupled electron transfer process to generate a carbon radical intermediate. At the same time, the ligand H2L-AQ absorbs the second photon and activates O2 to form 1 O2; In addition, the Zn–O–Na structure absorbs the third photon to generate oxygen vacancies, which are rapidly activated 1 O2 generates lattice oxygen for efficient selective oxidation of toluene to benzaldehyde.

9. The use according to claim 8, characterized in that The specific application process is: Zn-AQ is dispersed in acetonitrile, and then toluene is added to a quartz photoreaction tube, and a rotor is added; the reaction is carried out in an O2 atmosphere under the conditions of LED light source illumination and magnetic stirring.

10. The use according to claim 9, characterized in that The wavelength of the LED light source is 360-500nm; the mass ratio of Zn-AQ to substrate toluene is 1-5:1-300.

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