Preparation method and application method of catalyst for producing aromatic aldehyde by oxidizing agricultural waste lignin

Abstract

The invention discloses a preparation method and an application method of a catalyst for producing aromatic aldehyde by oxidizing agricultural waste lignin, and belongs to the field of lignin resource utilization. The preparation method of the catalyst for producing aromatic aldehyde through lignin oxidation comprises the following steps: mixing lanthanum salt, B-site metal salt and citric acid in a solvent to obtain a precursor solution; and soaking a polymethyl methacrylate template in the precursor solution, reacting, and calcining to obtain the catalyst for producing aromatic aldehyde by lignin oxidation. The catalyst for producing aromatic aldehyde through lignin oxidation, provided by the invention, has the advantages of low cost, adjustable electronic structure, high oxygen mobility, excellent oxidation-reduction performance and the like, has good catalytic performance, and is suitable for being used as a lignin alkali-free oxidation catalyst. The prepared porous catalytic material with micropores, mesopores and macropores is beneficial to mass transfer of lignin macromolecules and the catalyst, beta-O-4 bonds of lignin can be efficiently broken, and alkali-free oxidation production of value-added aromatic aldehyde chemicals is facilitated.

Description

Technical Field

[0001] The present invention relates to the field of lignin resource utilization, and in particular to a method for preparing a catalyst for producing aromatic aldehydes by oxidizing agricultural waste lignin and an application method thereof. Background Art

[0002] As an important renewable carbon resource, lignocellulosic biomass has the potential to replace traditional fossil resources in the transition to a more sustainable energy structure. Although the industrial technologies for converting cellulose to ethanol and hemicellulose to furfural have matured, the value-added utilization of lignin remains in its infancy, severely restricting the economic feasibility and environmental sustainability of comprehensive biomass utilization. The unique three-dimensional aromatic network structure of lignin molecules provides opportunities for its targeted conversion to produce aromatic chemicals such as aromatic aldehydes, aromatic ketones, aromatic acids, aromatic esters, and phenols. To date, a series of thermochemical methods have been developed to convert lignin into value-added aromatic aldehyde chemicals, including liquefaction and pyrolysis. Among them, the oxidation method during liquefaction can break the CC / CO bonds of lignin under mild conditions while retaining the aromatic ring structure to produce aromatic aldehydes. Aromatic aldehydes not only have important applications in the chemical, pharmaceutical, and food industries, but can also be used as biomass-based building materials. This research direction has great academic significance.

[0003] Currently, research on the catalytic oxidative depolymerization of lignin to produce aromatic aldehyde chemicals primarily focuses on wet alkaline oxidative depolymerization (using oxygen or air as the oxidant). Catalysts that have been developed and utilized primarily include metal oxides, (supported) noble metals, transition metal (salts), organic complexes, and rare earth perovskite oxides. Among them, ABO3-type perovskite composite metal oxides have attracted considerable attention from catalysis researchers due to their low cost, tunable electronic structure, high oxygen mobility, and excellent redox performance. Generally speaking, the A-site (rare earth or alkaline earth element) cations of ABO3 perovskite composite metal oxides stabilize the structure, while catalytic performance depends primarily on the B-site (transition metal element) cations, particularly the number of exposed B-site ions. Because different B-site metal ions can alter their oxidation state or introduce oxygen vacancies, researchers often modify the B-site metal ions to tune catalytic performance. However, alkaline solution catalytic oxidation techniques typically generate large amounts of inorganic salts, making subsequent product separation difficult.

[0004] Therefore, how to develop a catalytic scheme with other solvent systems to replace the alkaline solution system for the catalytic oxidation of lignin is of great research value. Summary of the Invention

[0005] The present invention aims to provide a method for preparing and applying a catalyst for producing aromatic aldehydes by oxidizing agricultural waste lignin, thereby addressing the aforementioned problems in the background art. The catalyst for producing aromatic aldehydes by oxidizing lignin provided by the present invention has the advantages of low cost, adjustable electronic structure, high oxygen mobility, and excellent redox performance. It exhibits good catalytic performance and is suitable for use as an alkali-free lignin oxidation catalyst.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] One of the technical solutions of the present invention is to provide a method for preparing a catalyst for producing aromatic aldehydes by oxidizing lignin, comprising the following steps:

[0008] mixing a lanthanum salt, a B-site metal salt, and citric acid in a solvent to obtain a precursor solution;

[0009] The polymethyl methacrylate template is immersed in the precursor solution, reacted, and then calcined to obtain the catalyst (rare earth lanthanum-based perovskite catalyst) for lignin oxidation to produce aromatic aldehydes.

[0010] Preferably, in the precursor solution, the concentration of the lanthanum salt is 0.04 to 0.06 mol / L, the concentration of the B-site metal salt is 0.04 to 0.06 mol / L, and the concentration of citric acid is 0.08 to 0.12 mol / L.

[0011] Preferably, the metal species of the B-site metal salt is Ni or Mn.

[0012] Preferably, the solvent is ethanol.

[0013] More preferably, the molar ratio of the total molar amount of the lanthanum salt and the B-site metal salt to the citric acid is 1:1.

[0014] More preferably, the lanthanum salt is lanthanum nitrate; and the B-site metal salt is nickel nitrate or manganese nitrate.

[0015] Preferably, the mass ratio of the polymethyl methacrylate template to the precursor solution is 1:8.

[0016] Preferably, the reaction temperature is 30-35° C. and the reaction time is 3-4 days.

[0017] Preferably, after the reaction, a drying step is further included; the drying temperature is 30-35° C. and the drying time is 11-13 hours.

[0018] Preferably, the calcination treatment is: heating to 400-450°C at a heating rate of 1°C / min, pre-calcining for 3-4 hours, then heating to 700-750°C at a heating rate of 3°C / min, and calcining for 4-5 hours.

[0019] Preferably, the method for preparing the polymethyl methacrylate template comprises the following steps:

[0020] Mixing water, acetone and methyl methacrylate, stirring and treating to obtain a precursor solution;

[0021] Mixing 2,2-azobisisobutyronitrile (oil-soluble initiator) and potassium persulfate (water-soluble initiator) in water to obtain an initiator solution;

[0022] The initiator solution is mixed with the precursor solution to undergo polymerization reaction to obtain the polymethyl methacrylate template.

[0023] The present invention improves the preparation process of the polymethyl methacrylate template in order to subsequently prepare a hierarchical perovskite catalyst having macropores, mesopores and micropores.

[0024] Preferably, the volume ratio of water, acetone and methyl methacrylate is 15:5:7.

[0025] Preferably, the stirring speed of the stirring treatment is 330-350 r / min, the time is 1.5-2.0 h, the stirring treatment is carried out in a nitrogen atmosphere and under heating conditions, the heating temperature is 75-85° C., and the nitrogen flow rate is 2-3 mL / min.

[0026] Preferably, the molar ratio of 2,2-azobisisobutyronitrile to potassium persulfate is 3:1.

[0027] Preferably, the mass ratio of the 2,2-azobisisobutyronitrile to methyl methacrylate is 1:430.

[0028] Preferably, the polymerization reaction temperature is 75-85° C., the time is 1.0-2.0 h, and the polymerization reaction is carried out under stirring conditions, and the stirring speed is 330-350 r / min.

[0029] Preferably, after the polymerization reaction, the process further comprises the steps of centrifugal separation and drying; the centrifugal speed of the centrifugal separation is 2500-3500 r / min, and the centrifugal time is 5.0-6.0 h; the drying temperature is 30-40° C., and the drying time is 11-13 h.

[0030] The second technical solution of the present invention is to provide a catalyst for producing aromatic aldehydes by oxidizing lignin obtained according to the above preparation method.

[0031] The third technical solution of the present invention is to provide an application of the above-mentioned catalyst for producing aromatic aldehydes by oxidizing lignin in an alkali-free system to catalyze the oxidation of lignin to prepare aromatic aldehydes.

[0032] Preferably, the lignin is corncob alkali lignin (CAL).

[0033] A fourth technical solution of the present invention provides a method for preparing aromatic aldehydes by catalytic oxidation of lignin in an alkali-free system, comprising the following steps:

[0034] Lignin, the catalyst for producing aromatic aldehyde by oxidizing lignin and an alcohol solvent are mixed, and an oxidation reaction is carried out in an oxidizing atmosphere to produce aromatic aldehyde.

[0035] Preferably, the usage ratio of the lignin, the catalyst for producing aromatic aldehydes by oxidation of lignin, and the alcohol solvent is 1.0 g:0.1 g:30 mL.

[0036] Preferably, the oxidizing atmosphere is an oxygen atmosphere, and the pressure of the oxygen is 1.0 MPa.

[0037] Preferably, the temperature of the oxidation reaction is 180° C., the time is 2.0 h, and the oxidation reaction is carried out under stirring conditions, and the stirring speed is 900 r / min.

[0038] Preferably, the alcohol solvent is a methanol aqueous solution, an ethanol aqueous solution or an isopropanol aqueous solution in a volume ratio of 1:1.

[0039] The invention uses an alcohol-water co-solvent as the solvent system, oxygen as the green oxidant, and alkali lignin from corncobs, an agricultural waste product, as the raw material. Aromatic aldehydes can be produced through a heating reaction. The catalyst exhibits excellent catalytic performance in the alkali-free oxidation of lignin, achieving a yield of up to 6.55% for a 2-hour oxidation reaction at 180°C and 1.0 MPa of oxygen.

[0040] Preferably, the lignin further comprises a drying step before the reaction, wherein the drying temperature is 105° C. and the drying time is 6 to 8 hours.

[0041] Preferably, the oxidation reaction is carried out in a microreactor, and the size of the microreactor is 100 mL.

[0042] The catalyst for producing aromatic aldehydes by oxidizing lignin prepared in this invention is a LaBO3 perovskite-type composite oxide, which effectively breaks lignin's β-O-4 and β-5 bonds, particularly β-O-4 bonds, thereby promoting the production of aromatic aldehydes. Furthermore, the invention develops a catalytic application method that uses an alkali-free system instead of the traditional alkaline system, avoiding the generation of inorganic waste and the cumbersome subsequent product separation steps, in line with the concept of green catalysis. Consequently, efficient alkali-free catalytic oxidation of CAL to produce high-value aromatic aldehydes is achieved.

[0043] The main reason why the catalyst designed by the present invention for producing aromatic aldehydes by oxidation of lignin can catalyze the reaction in an environment without alkaline solution is that the catalyst easily forms oxygen vacancies, which is conducive to oxygen adsorption and activation; at the same time, its acidic sites are more likely to combine with activated oxygen species intermediates, thereby leading to the cleavage of β-O-4 bonds. In addition, the present invention also conducted relevant static adsorption experiments to verify the feasibility of the LaBO3 catalyst catalytic oxidation of CAL and the mass transfer effect of solid-solid phase catalysis ( Figure 8 ).

[0044] The beneficial technical effects of the present invention are as follows:

[0045] The catalyst for producing aromatic aldehydes by oxidizing lignin provided by the present invention has the advantages of low cost, adjustable electronic structure, high oxygen mobility and excellent redox performance, has good catalytic performance, and is suitable as a lignin alkali-free oxidation catalyst.

[0046] The preparation method of the present invention is simple in process and easy to implement; the reaction system is green and alkali-free, the conditions are mild, the controllability is good, the catalytic effect is good, the yield of the aromatic aldehyde is relatively high, the high-value utilization of lignin can be achieved, and industrialization is easy.

[0047] The present invention adopts the colloidal crystal template method to prepare a porous catalytic material with micropores, mesopores and macropores, which is beneficial to the mass transfer between lignin macromolecules and the catalyst, can efficiently break the β-O-4 bond of lignin, and is beneficial to the alkali-free oxidation production of value-added aromatic aldehyde chemicals. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0049] Figure 1 The crystal structure and chemical structure spectrum of the catalyst of Example 1; wherein a is an XRD spectrum, b is a Raman spectrum, and c is an FTIR spectrum;

[0050] Figure 2 N2 adsorption-desorption isotherm (a) and pore size distribution curve (b) of the catalyst of Example 1;

[0051] Figure 3 This is a 50,000-fold magnified SEM photograph of the catalyst of Example 1; wherein a is LaNiO3, and b is LaMnO3;

[0052] Figure 4This is the EDS graph of the catalyst of Example 1; wherein a is LaNiO3, and b is LaMnO3;

[0053] Figure 5 XPS spectra of La 3d (a), Ni 2p (b), Mn 2p (c) and O1s (d) of the catalyst of Example 1;

[0054] Figure 6 The O2-TPD (a), EPR (b), H2-TPR (c), CO2-TPD (d) and NH3-TPD (e) curves of the catalyst of Example 1;

[0055] Figure 7 The activity diagram of bio-oil formed by catalytic oxidation of lignin by the catalyst of Example 1 and its performance comparison with that of Comparative Example 1; wherein a is the CAL conversion rate, b is the yield of liquid oil, c is the residue rate of the solid product, and d is the yield of aromatic aldehydes;

[0056] Figure 8 (a) The UV-visible spectrum and adsorption capacity of the solution after the catalyst of Example 1 adsorbed CAL (b);

[0057] Figure 9 2D HSQC NMR spectra of CAL and 2D HSQC NMR spectra of liquid products obtained by catalytic oxidation of CAL with and without the LaBO3 catalyst of Example 1. DETAILED DESCRIPTION

[0058] Various exemplary embodiments of the present invention are now described in detail. This detailed description should not be considered as a limitation of the present invention, but should be understood as a more detailed description of certain aspects, features, and embodiments of the present invention. It should be understood that the terms used in the present invention are only for describing specific embodiments and are not intended to limit the present invention.

[0059] In addition, for numerical ranges in the present invention, it is understood that each intervening value between the upper and lower limits of the range is also specifically disclosed. Each smaller range between any stated value or stated range, and any other stated value or intervening value in the stated range, is also included in the present invention. The upper and lower limits of these smaller ranges may independently be included or excluded in the range.

[0060] Unless otherwise specified, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art to which the present invention pertains. Although preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present invention. It should be noted that any details not described herein are conventional procedures in the art and are not the focus of the present invention.

[0061] The terms “include,” “including,” “have,” “contain,” etc. used in the present invention are open-ended terms, meaning including but not limited to.

[0062] The main reagents used in the following examples are:

[0063] The corncob alkali lignin (CAL) used was prepared by hydrothermally pretreating corncobs to degrade hemicellulose into xylooligosaccharides. The hydrothermal residue was then treated with a dilute alkaline solution to obtain a cellulose-rich residue and lignin. The effluent from the alkaline treatment was adjusted to acidic conditions (pH = 2) to precipitate the lignin. The resulting corncob alkali lignin had a purity of 94.42%, a weight-average molecular weight of 3258 g / mol, a number-average molecular weight of 2260 g / mol, and a polydispersity index of 1.44 and was used directly without further purification.

[0064] Methanol (99.5%), isopropyl alcohol (99.7%), ethyl acetate (99.5%), hydrochloric acid (37%), acetone (99.5%), anhydrous ethanol (99.5%), nickel nitrate hexahydrate (Ni(NO3)2·6H2O, ≥98%) were purchased from Xilong Science Co., Ltd.; lanthanum nitrate hexahydrate (La(NO3)3·6H2O, ≥99%), cerium nitrate hexahydrate (Ce(NO3)3·6H2O, 99.95%), citric acid monohydrate (9 9%) and vanillin (99%) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.; manganese nitrate hexahydrate (Mn(NO3)3·6H2O, 98%), methyl methacrylate (99%), syringaldehyde (≥98%), p-hydroxybenzaldehyde (≥98%), 2,2-azobisisobutyronitrile (98%), potassium persulfate (>99%), dodecane (>99%), and deuterated dimethyl sulfoxide (DMSO-d6, 99.9%) were purchased from Shanghai MacLean Biochemical Co., Ltd.

[0065] Unless otherwise specified, the "room temperature" in the present invention is 10-30°C.

[0066] Unless otherwise specified, all raw materials used in the following examples of the present invention are commercially available products.

[0067] Example 1

[0068] A method for preparing a catalyst for producing aromatic aldehydes by oxidizing lignin, comprising the following steps:

[0069] (1) Water, acetone, and methyl methacrylate were mixed in a volume ratio of 15:5:7, and heated in an 80°C water bath under nitrogen protection (flow rate of 2 mL / min) with a stirring speed of 340 r / min for 1.5 h to obtain a precursor solution;

[0070] (2) dissolving an oil-soluble initiator 2,2-azobisisobutyronitrile and a water-soluble initiator potassium persulfate in a molar ratio of 3:1 in 150 mL of deionized water, and preheating the mixture at 80° C. with a stirring speed of 340 r / min to obtain an initiator solution;

[0071] (3) The initiator solution of step (2) was poured into the precursor solution of step (1) (so that the mass ratio of 2,2-azobisisobutyronitrile to methyl methacrylate was 1:430), and the reaction was carried out at 80°C for 1.5 hours (the stirring speed during the reaction was 340 r / min), and then naturally cooled to room temperature. The sample was centrifuged at a centrifugal speed of 3000 r / min for 5.5 hours and then dried at 35°C for 12 hours to obtain a polymethyl methacrylate colloidal crystal template;

[0072] (4) Lanthanum nitrate, B-site metal salt (prepared accordingly when the B-site metal salt is nickel nitrate or manganese nitrate) and citric acid were dissolved in 80 mL of anhydrous ethanol at room temperature and stirred until a uniform and transparent precursor solution (La 3+ and B n+ The concentration of citric acid was 0.04 mol / L and 0.08 mol / L);

[0073] (5) The polymethyl methacrylate template was immersed in the precursor solution at room temperature (the mass ratio of the polymethyl methacrylate template to the precursor solution was 1:8), reacted at room temperature for 3 days, vacuum filtered, and dried at 30°C for 12 hours. Subsequently, the sample was transferred to a muffle furnace and heated from room temperature to 400°C at a heating rate of 1°C / min, kept constant at this temperature for 3.0 hours, and then heated to 750°C at a heating rate of 3°C / min and kept at this temperature for 4.0 hours to obtain rare earth lanthanum-based perovskite catalysts with different B-sites (according to the different B-site metal salts added, the prepared rare earth lanthanum-based perovskite catalysts were recorded as LaNiO3 and LaMnO3, respectively).

[0074] The above catalyst is used to catalyze the oxidation of CAL to prepare aromatic aldehydes in an alkali-free system:

[0075] (1) CAL was dried at 105°C for 6 h. 1.0 g of CAL, 0.1 g of catalyst, and 30 mL of alcohol solvent (prepared accordingly when the alcohol solvent was a methanol-water solution, ethanol-water solution, or isopropanol-water solution with a volume ratio of 1:1) were prepared and added to a 100 mL microreactor.

[0076] (2) The reactor was purged with nitrogen to remove air, and then vacuumed, and then O2 at a pressure of 1.0 MPa was injected. The reactor was heated from room temperature to 180°C at a reaction speed of 900 r / min, and the reaction was continued for 2.0 h.

[0077] Example 2

[0078] A method for preparing a catalyst for producing aromatic aldehydes by oxidizing lignin, comprising the following steps:

[0079] (1) Water, acetone, and methyl methacrylate were mixed in a volume ratio of 15:5:7, and heated in a 75°C water bath under nitrogen protection (flow rate of 2.5 mL / min) with a stirring speed of 340 r / min for 2.0 h to obtain a precursor solution;

[0080] (2) dissolving an oil-soluble initiator 2,2-azobisisobutyronitrile and a water-soluble initiator potassium persulfate in a molar ratio of 3:1 in 150 mL of deionized water, and preheating the mixture at 75° C. with a stirring speed of 330 r / min to obtain an initiator solution;

[0081] (3) The initiator solution of step (2) was poured into the precursor solution of step (1) (so that the mass ratio of 2,2-azobisisobutyronitrile to methyl methacrylate was 1:430), and the reaction was carried out at 75°C for 2.0 hours (the stirring speed during the reaction was 330 r / min), and then naturally cooled to room temperature. The sample was centrifuged at a centrifugal speed of 2500 r / min for 5.0 hours and then dried at 30°C for 11 hours to obtain a polymethyl methacrylate colloidal crystal template;

[0082] (4) Lanthanum nitrate, B-site metal salt (prepared accordingly when the B-site metal salt is nickel nitrate or manganese nitrate) and citric acid were dissolved in 100 mL of anhydrous ethanol at room temperature and stirred until a uniform and transparent precursor solution (La 3+ and B n+ The concentration of citric acid was 0.10 mol / L).

[0083] (5) The polymethyl methacrylate template was immersed in the precursor solution at room temperature (the mass ratio of the polymethyl methacrylate template to the precursor solution was 1:8), reacted at room temperature for 4 days, vacuum filtered, and dried at 35°C for 11 hours. Subsequently, the sample was transferred to a muffle furnace and heated from room temperature to 450°C at a heating rate of 1°C / min, kept at this temperature for 3.5 hours, and then heated to 750°C at a heating rate of 3°C / min and kept at this temperature for 4.5 hours to obtain rare earth lanthanum-based perovskite catalysts with different B-sites (according to the different B-site metal salts added, the prepared rare earth lanthanum-based perovskite catalysts were respectively recorded as LaNiO3 and LaMnO3).

[0084] The above catalyst is used to catalyze the oxidation of CAL to prepare aromatic aldehydes in an alkali-free system:

[0085] (1) CAL was dried at 105°C for 6 h. 1.0 g of CAL, 0.1 g of catalyst, and 30 mL of alcohol solvent (prepared accordingly when the alcohol solvent was a methanol-water solution, ethanol-water solution, or isopropanol-water solution with a volume ratio of 1:1) were prepared and added to a 100 mL microreactor.

[0086] (2) The reactor was purged with nitrogen to remove air, and then vacuumed, and then O2 at a pressure of 1.0 MPa was injected. The reactor was heated from room temperature to 180°C at a reaction speed of 900 r / min, and the reaction was continued for 2.0 h.

[0087] Example 3

[0088] A method for preparing a catalyst for producing aromatic aldehydes by oxidizing lignin, comprising the following steps:

[0089] (1) Water, acetone, and methyl methacrylate were mixed in a volume ratio of 15:5:7, and heated in an 85°C water bath under nitrogen protection (flow rate of 3.0 mL / min) with a stirring speed of 350 r / min for 2.0 h to obtain a precursor solution;

[0090] (2) dissolving an oil-soluble initiator 2,2-azobisisobutyronitrile and a water-soluble initiator potassium persulfate in a molar ratio of 3:1 in 150 mL of deionized water, and preheating the mixture at 85° C. with a stirring speed of 350 r / min to obtain an initiator solution;

[0091] (3) The initiator solution of step (2) was poured into the precursor solution of step (1) (so that the mass ratio of 2,2-azobisisobutyronitrile to methyl methacrylate was 1:430), and the reaction was carried out at 85°C for 2.0 hours (the stirring speed during the reaction was 350 r / min), and then naturally cooled to room temperature. The sample was centrifuged at a centrifugal speed of 3500 r / min for 6.0 hours and then dried at 40°C for 13 hours to obtain a polymethyl methacrylate colloidal crystal template;

[0092] (4) Lanthanum nitrate, B-site metal salt (prepared accordingly when the B-site metal salt is nickel nitrate or manganese nitrate) and citric acid were dissolved in 120 mL of anhydrous ethanol at room temperature and stirred until a uniform and transparent precursor solution (La 3+ and B n+ The concentration of citric acid was 0.12 mol / L).

[0093] (5) The polymethyl methacrylate template was immersed in the precursor solution at room temperature (the mass ratio of the polymethyl methacrylate template to the precursor solution was 1:8), reacted at room temperature for 3 days, vacuum filtered, and dried at 30°C for 13 hours. Subsequently, the sample was transferred to a muffle furnace and heated from room temperature to 450°C at a heating rate of 1°C / min, held at this temperature for 3.0 hours, and then heated to 700°C at a heating rate of 3°C / min and held for 5.0 hours to obtain rare earth lanthanum-based perovskite catalysts with different B-sites (according to the different B-site metal salts added, the prepared rare earth lanthanum-based perovskite catalysts were recorded as LaNiO3 and LaMnO3, respectively).

[0094] The above catalyst is used to catalyze the oxidation of CAL to prepare aromatic aldehydes in an alkali-free system:

[0095] (1) CAL was dried at 105°C for 6 h. 1.0 g of CAL, 0.1 g of catalyst, and 30 mL of alcohol solvent (prepared accordingly when the alcohol solvent was a methanol-water solution, ethanol-water solution, or isopropanol-water solution with a volume ratio of 1:1) were prepared and added to a 100 mL microreactor.

[0096] (2) The reactor was purged with nitrogen to remove air, and then vacuumed, and then O2 at a pressure of 1.0 MPa was injected. The reactor was heated from room temperature to 180°C at a reaction speed of 900 r / min, and the reaction was continued for 2.0 h.

[0097] Comparative Example 1

[0098] A method for preparing aromatic aldehydes by oxidizing CAL in an alkali-free system, which differs from Example 1 only in that the addition of a catalyst in the catalytic oxidation process is omitted, specifically:

[0099] (1) CAL was dried at 105°C for 6 h. 1.0 g of CAL and 30 mL of alcohol solvent (prepared accordingly when the alcohol solvent was a methanol-water solution, ethanol-water solution, or isopropanol-water solution with a volume ratio of 1:1) were prepared and added to a 100 mL microreactor.

[0100] (2) The reactor was purged with nitrogen to remove air, and then vacuumed, and then O2 at a pressure of 1.0 MPa was injected. The reactor was heated from room temperature to 180°C at a reaction speed of 900 r / min, and the reaction was continued for 2.0 h.

[0101] Effect verification

[0102] The rare earth lanthanum-based perovskite catalysts with different B positions prepared in Example 1 were characterized and tested for their performance, as follows:

[0103] 1. Intrinsic properties of the catalyst of Example 1

[0104] X-ray diffraction (XRD, PuXi XRD-3X) patterns were recorded in the 2θ range of 10–80°, at a scan rate of 4° / min, and with a counting time of 40 kV and 30 mA using a Cu Kα The radiation was carried out on a Bruker AXSD8Focus diffractometer.

[0105] A LabRAM HR Evolution instrument with an excitation wavelength of 532 nm was used for the spectral range of 50 to 1800 cm -1 Raman spectra of the rare earth lanthanum-based perovskite catalysts prepared in Example 1 with different B positions were collected within the Raman shift range. The scattered light was analyzed using a triple-grating monochromator with a charge-coupled detector. The sample temperature was controlled within 0.1 K.

[0106] The catalyst prepared in Example 1 was analyzed using a FTIR spectrometer (PerkinElmer). The sample and KBr were ground at a mass ratio of 1:100. -1 The spectrum was recorded in the wavenumber range with a resolution of 4 cm -1 , 16 scans were performed for each sample.

[0107] Figure 1 The crystal structure and chemical structure spectra of the catalyst of Example 1; wherein a is the XRD spectrum, b is the Raman spectrum, and c is the FTIR spectrum.

[0108] like Figure 1 As shown in Figure 1, both LaNiO3 and LaMnO3 prepared in Example 1 exhibit pure perovskite structures, and their crystal forms vary due to the different B-site elements. Specifically, LaMnO3 exhibits a cubic crystal form (Pm-3m(221)), while LaNiO3 exhibits a hexagonal crystal form (R-3m(166)). Therefore, in the LaBO3 catalyst of the present invention, the change of the B-site element affects the crystal structure and space group diversity of the catalyst.

[0109] Figure 1 b, at 200cm -1 The following peaks are caused by the displacement of lanthanum ions, 417 cm -1 It is caused by the vibration of oxygen octahedron. For LaNiO3, ~390cm -1 and ~430cm -1 There are weak peaks at 647cm, which belong to the characteristic shoulder peak of Ni-O and the interaction peak of Ni-La. -1 Corresponding to the stretching vibration of MnO6 octahedron. Figure 1 c is the FTIR spectrum of the catalyst of Example 1 (4000~400cm-1 ), it can be seen that ~560cm -1 The peak observed at is caused by metal-oxygen, which is due to the antisymmetric stretching vibration of the BO bond of the BO6 octahedron of ABO3 or is related to the La-O stretching vibration.

[0110] 2. The surface area and pore size distribution of the rare earth lanthanum-based perovskite catalysts with different B positions prepared in Example 1 were measured using a BSD-PM1 device. The samples were degassed at 300°C for 6 h, with a pressure between 5 μm Hg and 10 μm Hg, and an equilibrium time of 10 seconds, and then subjected to N2 adsorption-desorption at 77 K. The BET specific surface area was obtained using an advanced automatic BET analyzer, and the pore size distribution was obtained by the Barrett Joyner Halenda (BJH) method. The test results are shown in Tables 1 and Figure 2 shown.

[0111] Figure 2 (a) N2 adsorption-desorption isotherm and (b) pore size distribution curve of the catalyst of Example 1.

[0112] from Figure 2 It can be seen from a that the two LaBO3 catalysts of Example 1 both show type IV adsorption isotherms in the range of P / P0=0.6~1.0. The isotherms in the low-pressure part are almost linear, indicating that the constructed catalyst has a macroporous structure. As the relative pressure increases, an H3-type hysteresis loop appears, indicating that mesopores exist in the constructed catalyst. No adsorption saturation phenomenon occurs in the isotherms in the high-pressure region, indicating that the pore structure is irregular. The results show that both LaBO3 catalysts have mixed porous structures, that is, they have macropores, mesopores and micropores. In addition, the average pore size of LaNiO3 is the highest, which is 28.3nm (see Table 1, Figure 2 b), the specific surface area is also larger, which means that more active sites can be effectively exposed.

[0113] Table 1 Specific surface area and structural parameters of LaBO3 perovskite catalyst

[0114]

[0115] Note: a Obtained through BET results.

[0116] b Calculation of micropore surface area and volume using the t-plot method.

[0117] c External surface area calculated by subtracting the micropore surface area from the total specific surface area.

[0118] d is obtained from the cumulative pore volume of BJH desorption.

[0119] eThe mesopore volume is obtained by subtracting the micropore volume from the total pore volume.

[0120] f is obtained from the average pore size of BJH adsorption (4V / A).

[0121] g is obtained from the EDS results.

[0122] The surface micromorphology of the LaBO3 perovskite catalysts with different B positions prepared in the examples was analyzed using a Thermo Fisher Scientific (Apreo 2) scanning electron microscope (SEM).

[0123] Figure 3 This is a 50,000-fold magnified SEM photograph of the catalyst of Example 1. In which, a is LaNiO3 and b is LaMnO3.

[0124] Figure 4 This is the EDS graph of the catalyst of Example 1, where a is LaNiO3 and b is LaMnO3.

[0125] Depend on Figure 3 It can be observed that the surface of the prepared LaBO3 catalyst is composed of irregular smooth particles, belonging to a sparse porous material with different pore structures, which is consistent with the results of the N2 adsorption isotherm. In addition, the EDS results ( Figure 4 The atomic ratio of La to B-site metal ions in LaMnO3 is basically the same as that in Table 1). The atomic ratio of La to B-site metal ions in LaMnO3 is less than 1, while that in LaNiO3 is greater than 1, indicating that there is a loss of B-site metal cations in LaNiO3, resulting in anion defects. For LaMnO3, La is partially missing, and in order to maintain electrical neutrality, Mn may be oxidized to a higher valence state (such as Mn 4+ ), resulting in a higher stoichiometric ratio of Mn than La on the surface. 3. X-ray photoelectron spectroscopy (XPS) was obtained using a Thermo Fisher Scientific Escalab 250Xi system with Al Kα (1486.6 eV) radiation as the excitation source. Spectra were acquired at ambient temperature and under ultrahigh vacuum. The binding energy of C 1s (284.8 eV) was used as an internal reference. The core levels of La 3d, Ni 2p, Mn 2p, and O 1s species were recorded, and their relative intensities were determined by integrating the Gaussian deconvoluted signals using curve fitting.

[0126] Figure 5 XPS spectra of La 3d (a), Ni 2p (b), Mn 2p (c) and O 1s (d) of the catalyst of Example 1.

[0127] from Figure 5As can be seen from a, the double peak state of La 3d splits into 3d at 832-834eV and 849-851eV respectively. 5 / 2 and 3D 3 / 2 The spin-orbit splitting of La 3d in the two catalysts is around 16.70 eV (Table 2), indicating that La ions are mainly in the +3 valence state. Figure 5 b) confirmed that the peaks at 872.00 eV and 880.00 eV correspond to Ni 2p 1 / 2 and Ni 2p 1 / 2 The vibration satellite peaks of Ni 2p 1 / 2 The peak can be convolved into two peaks centered at 871.04 and 872.84 eV, corresponding to Ni 2+ and Ni 3+ The Mn 2p spectrum can be convoluted into two peaks at 641.00eV and 642.17eV, which are respectively attributed to Mn 3+ and Mn 4+ , which is consistent with the EDS results. Since the catalysts of the present invention are all obtained by high-temperature calcination in air atmosphere, the valence state of the B element usually tends to show a higher oxidation valence state, that is, B (n+1)+ However, there is B n+ The existence of B n+ The emergence of oxygen atoms is due to the escape of oxygen atoms from LaBO3, thus forming oxygen vacancies, and the electrons released by oxygen vacancies cause B (n+1)+ Restore to B n+ Therefore, a high proportion of B n+ / B (n+1)+ This indicates that more active oxygen vacancies are generated, which can accelerate the transfer of oxygen atoms from the gas phase to the catalyst surface. n+ / B (n+1)+ The ratio is the lowest, which is because the Ni-O covalency of LaNiO3 is strong, which inhibits the formation of oxygen vacancies. The B 2p peak intensity gradually decreases with the increase of the nuclear charge number of the B-site element (Mn, Ni), while the B n+ and B (n+1)+ The binding energy of B increases (Table 3), indicating that the electron cloud density around the B element decreases, that is, the positive charge of the B ion increases, which improves the metal's ability to accept electron pairs, and thus may increase the surface Lewis acidity, making it easier to combine with molecular oxygen. And when B=Ni, it is easiest to combine with molecular oxygen. Figure 5 d and Table 4 show that the O ads / O lat The ratio is the highest, which is 2.05, indicating that the oxygen vacancy concentration of LaNiO3 is high. ads / O lat It increases with the increase of the nuclear charge number of the B-site element.

[0128] Table 2 Binding energy of La 3d orbital in LaBO3 perovskite catalyst of Example 1 (eV)

[0129]

[0130] Table 3 Binding energy of B 2p orbitals in the LaBO3 perovskite catalyst of Example 1 (eV)

[0131]

[0132] Table 4 Binding energy of O1s orbital in LaBO3 perovskite catalyst of Example 1 (eV)

[0133]

[0134] 4. Temperature-programmed desorption (O2 / CO2 / NH3-TPD) and temperature-programmed reduction (H2-TPR) were both performed using a Microtrac BEL BELCAT chemical adsorption instrument. O2-TPD test conditions: 150°C nitrogen pretreatment for 30 minutes, O2 adsorption at room temperature for 30 minutes, N2 purge for 60 minutes, and then heating to 800°C at a rate of 10°C / min. CO2 / NH3-TPD test procedure: The sample was pretreated in 99.9% pure helium (50 mL / min) at 250°C for 60 minutes. After cooling to 50°C, the sample was switched to a CO2 / He or NH3 / He mixture for adsorption for 60 minutes. After a helium purge for 30 minutes, the temperature was increased at 10°C / min to 700°C, and the signal was recorded. H2-TPR was also performed using a Microtrac BEL BELCAT. After pretreatment with Ar at 250°C for 60 minutes, the sample was heated from 50°C to 700°C at a rate of 10°C / min. Electron spin resonance (EPR) tests of the catalysts were performed using a BRUKE EMXPLUS instrument with a sweep width of 2000 G, a power of 2 mW, a scan time of 60 s and a scan time of 5.12 ms.

[0135] Figure 6 These are the O2-TPD (a), EPR (b), H2-TPR (c), CO2-TPD (d) and NH3-TPD (e) curves of the catalyst of Example 1.

[0136] from Figure 6It can be seen from a that with the increase of nuclear charge number, the surface oxygen and lattice oxygen peaks shift significantly to low temperature, indicating that the adsorption capacity of surface oxygen weakens and the migration capacity of lattice oxygen strengthens from LaMnO3 to LaNiO3. In addition, the proportion of adsorbed oxygen area increases from Mn to Ni at B site (Table 5), which once again confirms that the adsorption capacity of surface oxygen weakens and the desorption of oxygen chemically and / or physically adsorbed on the active sites of the catalyst surface is enhanced at low temperature, that is, the number of weakly adsorbed oxygen species increases. When B = Mn, due to Mn 3+ The Jahn-Teller effect can expose more low-coordinated Mn sites, promoting the chemical and / or physical adsorption of oxygen at low temperatures. Figure 6 As can be seen in b, there is a characteristic symmetrical peak at g = 2.003, which is caused by oxygen vacancies. It is worth noting that the intensity of the perovskite axial signal is: LaNiO3>LaMnO3. This shows that when B = Ni, the oxygen vacancy content is the highest, which is conducive to the adsorption and activation of molecular oxygen, thereby promoting redox to a certain extent. Figure 6 As can be seen from the figure c, all catalysts have two signal peaks. The two reduction processes of LaNiO3 are Ni 3+ →Ni 2+ (372℃) and Ni 2+ →Ni 0 (515℃). For LaMnO3, 362℃ is Mn 4+ Reduced to Mn 3+ , the peak above 600℃ is mainly due to Mn 3+ Reduced to Mn 2+ When the nuclear charge number increases, the intensities of the two peaks increase from 70.97 μmol / g to 121.91 μmol / g and from 9.43 μmol / g to 232.70 μmol / g, respectively (Table 6), indicating that the reducibility of the catalyst increases with the nuclear charge number, and LaNiO3 has the strongest reducibility.

[0137] Table 5 O2-TPD semi-quantitative results of LaBO3 perovskite catalyst of Example 1

[0138]

[0139] Table 6 H2-TPR semi-quantitative results of LaBO3 perovskite catalyst of Example 1

[0140]

[0141] Table 7 CO2-TPD semi-quantitative results of LaBO3 perovskite catalyst of Example 1

[0142]

[0143] Table 8 NH3-TPD semi-quantitative results of LaBO3 perovskite catalyst of Example 1

[0144]

[0145] Figure 6 d and e are the CO2-TPD and NH3-TPD curves, respectively, obtained by Gaussian function fitting. Comparison reveals that as the nuclear charge increases, the number of weak base sites decreases from Mn to Ni at the B site (Table 7). Furthermore, the total number and density of acidic sites in the perovskite gradually increase from Mn to Ni at the B site. These phenomena are consistent with the XPS results showing increased surface Lewis acidity, making it more susceptible to molecular oxygen binding.

[0146] 5. Gas chromatography-mass spectrometry analysis was performed on the bio-oil obtained by CAL oxidation catalyzed by the rare earth lanthanum-based perovskite catalysts with different B positions according to Example 1 of the present invention.

[0147] The liquid product obtained by catalytic oxidation of CAL in the example was first dissolved in ethyl acetate, and then its components were analyzed by GC-MS. GC-MS analysis was performed using an Agilent 5977A gas chromatograph coupled with a 5975C mass spectrometer. The aromatic aldehyde analysis column was an HP-5MS (30 m × 0.25 mm × 0.25 μm) with a 1 μL injection volume. The injection port, detector, and interface temperatures were set at 280°C. The heating program was as follows: 60°C, hold for 3 minutes; increase the temperature at 5°C / min to 105°C; increase the temperature at 0.5°C / min to 105.5°C; and increase the temperature at 0.1°C / min to 106°C, hold for 2 minutes. The temperature was then increased at 10°C / min to 115°C; at 0.2°C / min to 117°C and held for 1 minute; at 5°C / min to 127°C; at 10°C / min to 152°C; at 0.4°C / min to 156°C and held for 1 minute; and at 40°C / min to 280°C and held for 2 minutes. The mass spectrometer was operated at a 70 eV ionization voltage and within the m / z range of 30 to 700. The yields of the three aromatic aldehydes, p-hydroxybenzaldehyde, vanillin, and syringaldehyde, after catalytic oxidative depolymerization were calculated.

[0148] Figure 7 The activity diagram of bio-oil formed by catalyzing lignin oxidation by the catalyst of Example 1 and its performance comparison with that of Comparative Example 1; wherein a is the CAL conversion rate, b is the yield of liquid oil, c is the residue rate of the solid phase product, and d is the yield of aromatic aldehydes.

[0149] from Figure 7It can be seen that in the alcohol-water solvent system without catalysis (Comparative Example 1), the conversion rate of CAL (C CAL ) is the highest, reaching 66.41%, the liquid oil yield is 18.07%, and the solid residue yield is 3.7%. When LaBO3 catalyst is used, the generation of liquid oil is promoted ( Figure 7 b) and inhibited the formation of solid residues ( Figure 7 c). At the same time, the two LaBO3 catalysts also promoted the formation of aromatic aldehydes ( Figure 7 d), indicating that LaBO3 perovskite is suitable as a catalyst for the oxidation of CAL to aromatic aldehydes in an alcohol-water co-solvent system. Furthermore, in the iPrOH-H2O co-solvent system, LaNiO3 catalyzed the oxidation of CAL to give the highest yield of aromatic aldehydes (6.55%), demonstrating that iPrOH-H2O is a more suitable solvent system for the catalytic oxidation of CAL than MeOH-H2O or EtOH-H2O.

[0150] 6. The static adsorption properties of CAL by the rare earth lanthanum-based perovskite catalysts with different B positions according to Example 1 of the present invention were measured at a wavelength of 280 nm using a Shimadzu UV-2450 spectrophotometer to verify the feasibility of catalytic oxidation of CAL by two LaBO3 catalysts in an iPrOH-H2O (v / v = 1:1) co-solvent system and the mass transfer effect of solid-solid phase catalysis.

[0151] A 0.1 g sample of the catalyst from Example 1 was completely immersed in 20 mL of a 25 g / L CAL solution and stirred at room temperature for 24 h. The mixture was then centrifuged at 6000 rpm for 5 min. The separated solid fraction was washed three times with freshly prepared iPrOH-H₂O (v / v = 1:1). The resulting liquids were combined and referred to as the adsorption solution. The adsorption solution was then diluted with freshly prepared iPrOH-H₂O (v / v = 1:1). Finally, the adsorption amount was quantified using a standard curve generated from solutions of varying concentrations.

[0152] Figure 8 (a) The UV-visible spectrum and (b) the adsorption capacity of the solution of the catalyst in Example 1 after adsorption of CAL.

[0153] from Figure 8 It can be seen that the LaMnO3 catalyst (14.48 mg·g -1 ) is higher than that of LaNiO3 (8.90 mg·g -1), which is due to the relatively larger specific surface area of ​​the former, but the catalytic activity of LaMnO3 is weaker than that of LaNiO3, indicating that the activity of LaBO3 catalyst in catalyzing the oxidation of CAL to aromatic aldehydes does not depend on the specific surface area of ​​the catalyst.

[0154] 7. Two-dimensional (2D) spectrometry on a Bruker Advance 600 MHz spectrometer 1 H- 13 C heteronuclear single quantum coherence (HSQC)-related NMR spectroscopy test: First, 100 mg of CAL was dissolved in 0.5 mL of DMSO-d6 and then placed in an NMR tube. Test conditions: 1 The relaxation time in the H dimension is 1.5 s, the number of accumulations is 64 times, the number of sampling points is 1024, and the spectral width is 5000 Hz. 13 The number of C sampling points is 256, the spectrum width is 18000 Hz, and the carbon-hydrogen coupling constant is 145 Hz. The solvent DMSO peak (δ C / δ H =39.5 / 2.49) was used for chemical shift calibration. Data processing and plotting were performed using standard MestReNova.

[0155] Table 9 Ratio of CAL basic structural units and the content of β-O-4 linkage in the oxidized liquid oil (based on every 100 C9 units)

[0156]

[0157] a S / G=0.5(S 2,6 +S' 2,6 ) / (G2+G'2).

[0158] b β-O-4 bond cleavage rate = (β-O-4 bond content in CAL - β-O-4 bond content in liquid product) / β-O-4 bond content in CAL.

[0159] Figure 9 2D HSQC NMR spectra of CAL and 2D HSQC NMR spectra of liquid products obtained by catalytic oxidation of CAL with and without the LaBO3 catalyst of Example 1.

[0160] from Figure 9As can be seen in the 2D HSQC NMR results, the catalytic oxidation of LaBO3 primarily occurs on the CAL side chain, rather than on the aromatic ring. Table 9 shows that in the catalyst-free system, only 45.28% of the β-O-4 bonds were broken. In contrast, the use of LaMnO3 and LaNiO3 catalysts significantly promoted the cleavage of β-O-4 bonds, with LaNiO3 in particular achieving the highest β-O-4 bond cleavage rate, reaching 97.80%.

[0161] In summary, the porous LaBO3 catalyst with micropores, mesopores and macropores prepared in the present invention is conducive to the mass transfer between lignin macromolecules and the catalyst, can efficiently break the β-O-4 bond of lignin, and is suitable for catalyzing CAL oxidation in an alkali-free system to produce value-added aromatic aldehydes.

[0162] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by persons skilled in the art should fall within the scope of protection defined by the claims of the present invention.

Claims

1. A method for preparing a catalyst for producing aromatic aldehydes by oxidizing lignin, characterized in that: The following steps are involved: mixing a lanthanum salt, a B-site metal salt, and citric acid in a solvent to obtain a precursor solution; The polymethyl methacrylate template is immersed in the precursor solution, reacted, and then calcined to obtain the catalyst for lignin oxidation to produce aromatic aldehyde.

2. The preparation method according to claim 1, characterized in that In the precursor solution, the concentration of the lanthanum salt is 0.04-0.06 mol / L, the concentration of the B-site metal salt is 0.04-0.06 mol / L, and the concentration of citric acid is 0.08-0.12 mol / L; and / or the metal species of the B-site metal salt is Ni or Mn; and / or the solvent is ethanol.

3. The preparation method according to claim 1, characterized in that The mass ratio of the polymethyl methacrylate template to the precursor solution is 1:8; and / or the reaction temperature is 30-35° C. and the reaction time is 3-4 days.

4. The preparation method according to claim 1, characterized in that The calcination treatment comprises: heating to 400-450° C. at a heating rate of 1° C. / min, pre-calcining for 3-4 hours, then heating to 700-750° C. at a heating rate of 3° C. / min, and calcining for 4-5 hours.

5. The preparation method according to claim 1, characterized in that The preparation method of the polymethyl methacrylate template comprises the following steps: Mixing water, acetone and methyl methacrylate, stirring and treating to obtain a precursor solution; Mixing 2,2-azobisisobutyronitrile and potassium persulfate in water to obtain an initiator solution; The initiator solution is mixed with the precursor solution to undergo polymerization reaction to obtain the polymethyl methacrylate template.

6. The preparation method according to claim 5, characterized in that The volume ratio of water, acetone and methyl methacrylate is 15:5:7; and / or the stirring speed of the stirring treatment is 330-350 r / min, and the time is 1.5-2.0 h. The stirring treatment is carried out under a nitrogen atmosphere and heating conditions, the heating temperature is 75-85° C., and the nitrogen flow rate is 2-3 mL / min; and / or the molar ratio of 2,2-azobisisobutyronitrile and potassium persulfate is 3:1; and / or the mass ratio of 2,2-azobisisobutyronitrile and methyl methacrylate is 1:430; and / or the temperature of the polymerization reaction is 75-85° C., and the time is 1.0-2.0 h.

7. A catalyst for producing aromatic aldehydes by oxidation of lignin obtained by the preparation method according to any one of claims 1 to 6.

8. Use of the catalyst for producing aromatic aldehydes by oxidizing lignin according to claim 7 in catalytic oxidation of lignin to prepare aromatic aldehydes in an alkali-free system.

9. A method for preparing aromatic aldehydes by catalytic oxidation of lignin in an alkali-free system, characterized in that: The following steps are involved: Lignin, the catalyst for producing aromatic aldehydes by oxidation of lignin as claimed in claim 7 and an alcohol solvent are mixed and subjected to oxidation reaction in an oxidizing atmosphere to produce aromatic aldehydes.

10. The method according to claim 9, characterized in that The amount ratio of the lignin, the catalyst for lignin oxidation to produce aromatic aldehyde, and the alcohol solvent is 1.0g:0.1g:30mL; and / or the oxidizing atmosphere is an oxygen atmosphere, and the pressure of the oxygen is 1.0MPa; and / or the temperature of the oxidation reaction is 180°C and the time is 2.0h; and / or the alcohol solvent is a methanol aqueous solution, an ethanol aqueous solution, or an isopropanol aqueous solution in a volume ratio of 1:1.

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