Tannic acid-like compound based on microwave catalytic reforming of lignin and preparation method thereof

Abstract

The invention provides a tannic acid-like compound based on microwave catalytic reforming of lignin and a preparation method thereof, belonging to the technical field of biomass conversion and utilization. The invention comprises the following steps: pre-treating lignin to prepare gallic acid esterified lignin; subjecting the gallic acid esterified lignin and a demethylation reduction catalyst to microwave pyrolysis reaction to obtain a microwave pyrolysis product; separating the microwave pyrolysis product to obtain a crude tannic acid-like compound solution and an unactivated demethylation reduction catalyst; calcining and activating the unactivated demethylation reduction catalyst to obtain the demethylation reduction catalyst, which is recycled to step two; purifying the crude tannic acid-like compound solution to obtain a tannic acid-like compound. The invention can efficiently convert the tannic acid-like compound into a tannic acid-like compound through modification treatment combined with microwave pyrolysis technology, and can effectively solve the problems faced by traditional natural resources in extracting tannic acid compounds, such as difficulty in large-scale extraction and uncontrollable product purity and activity.

Description

Technical Field

[0001] The present invention relates to the technical field of biomass conversion and utilization, and in particular to a tannic acid-like compound based on microwave catalytic reforming of lignin and a preparation method thereof. Background Art

[0002] Tannic acid compounds have a polyphenolic structure and are highly reactive. They also possess significant antioxidant, antibacterial, and anti-inflammatory properties, making them widely used in food, medicine, feed, polymer resins, and material synthesis. However, currently, commonly used tannic acid compounds rely primarily on natural extraction or simple subsequent processing, resulting in insufficient natural product supply, low purity, and low activity when used in large quantities. This has limited their potential for application in daily chemicals, biomedicine, scientific research, and industrial engineering. Therefore, the preparation of tannic acid-like compounds from abundant biomass resources is of great significance.

[0003] Lignin is a type of phenolic hydroxyl-rich compound with a phenylpropane structure. Its structure is somewhat similar to that of tannic acid compounds. However, lignin has problems such as large and uneven molecular weight and low active phenolic hydroxyl content, which restrict its further conversion and utilization. Currently, there are some reports on the preparation of tannic acid compounds and the extraction of tannic acid from biomass resources. For example, in the publication number CN105418693B "A method for extracting tannins from walnut shells and the application of the resulting tannins", walnut shells are used as raw materials, and a tannin extract is obtained through a multi-step process. The tannin extract is then purified to obtain pure tannin. In the publication number CN117624262B "A method for extracting and preparing tannic acid from tannin germanium residue", a method for extracting tannic acid from solid waste resources is also proposed.

[0004] However, there are currently few reports on methods for directly converting biomass resources or lignin into tannic acid-like compounds. However, in the publication number CN111945462B "A Lignin / Tannin Composite Polyurethane Coating and Preparation Method Thereof", a coating was prepared by constructing an isocyanate prepolymer of lignin and tannin, which proved that there are certain structural and performance similarities between lignin and tannin or tannic acid. However, there has not yet been a case of preparing tannic acid-like compounds based on the characteristics of lignin raw materials. Summary of the Invention

[0005] The purpose of the present invention is to provide a tannic acid-like compound based on microwave catalytic reforming of lignin and a preparation method thereof, so as to overcome the problems existing in the prior art. The present invention can efficiently convert it into a tannic acid-like compound through modification treatment combined with microwave pyrolysis technology, and can effectively solve the problems faced by the extraction of tannic acid compounds from traditional natural resources, such as the difficulty of large-scale extraction, uncontrollable product purity and activity, etc., and provide new ideas and methods for the preparation of tannic acid-like compounds, and also provide a new path for the high-value utilization of lignin resources.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0007] A method for preparing tannic acid-like compounds based on microwave-catalyzed reforming of lignin comprises the following steps:

[0008] Step 1: pretreating lignin to prepare gallic acid esterified lignin;

[0009] Step 2, subjecting the gallic acid esterified lignin and the demethylation reduction catalyst to microwave pyrolysis reaction to obtain a microwave pyrolysis product;

[0010] Step 3: Separating the microwave pyrolysis product to obtain a crude tannic acid-like compound solution and an unactivated demethylation reduction catalyst; calcining and activating the unactivated demethylation reduction catalyst to obtain a demethylation reduction catalyst, which is recycled to step 2; purifying the crude tannic acid-like compound solution to obtain a tannic acid-like compound;

[0011] Furthermore, the step 1 is specifically as follows:

[0012] Add lignin to deionized water, then add catechol and mix well, then add concentrated sulfuric acid to carry out phenolization reaction, then wash with deionized water, and perform the first vacuum drying to obtain phenolized lignin;

[0013] Phenolated lignin is added to dimethyl sulfoxide, followed by gallic acid, and the mixture is heated and stirred to mix uniformly, followed by the addition of trifluoromethanesulfonic acid for esterification, followed by repeated washing with dimethyl sulfoxide and a second vacuum drying to obtain gallic acid esterified lignin;

[0014] Furthermore, the mass ratio of the lignin, deionized water, catechol, and concentrated sulfuric acid is 1:(15-25):(1-3):(0.01-0.1); the mass concentration of the concentrated sulfuric acid is 98%; the temperature of the phenolization reaction is 40-60°C, and the time is 1-2 hours; washing with deionized water is specifically: repeatedly washing with deionized water until the pH is 5-6; the temperature of the first vacuum drying is 45°C, and the time is 24 hours;

[0015] Furthermore, the mass ratio of the phenolic lignin, dimethyl sulfoxide, gallic acid and trifluoromethanesulfonic acid is 1: (10-15): (2-4): (0.1-0.4); the temperature is raised to 60°C; the esterification reaction temperature is 60-80°C and the time is 3-5 hours; the second vacuum drying temperature is 40°C and the time is 24 hours;

[0016] Furthermore, the step 2 is specifically as follows:

[0017] Before microwave pyrolysis begins, a silicon carbide mesh is placed at the bottom of the lower section of the reactor, on which a demethylation reduction catalyst is placed. A nitrogen atmosphere is then introduced into the reactor to isolate oxygen, and the temperature is then controlled in three sections from top to bottom.

[0018] Gallic acid esterified lignin is added into the reactor. Gallic acid esterified lignin is added from the upper end of the reactor and then subjected to microwave pyrolysis reaction from top to bottom under the action of gravity according to the three-stage temperature control setting. After the reaction is completed, a mixture of tar-like substance and catalyst is obtained on the silicon carbide mesh;

[0019] Furthermore, the three-stage temperature control is specifically as follows:

[0020] Set the power of the upper microwave to 1500-2000 W, the temperature to 600-1000 °C, and the reaction residence time to 0.05-0.1 s; set the power of the middle microwave to 600-800 W, the temperature to 400-600 °C, and the reaction residence time to 0.05-0.1 s; set the power of the lower microwave to 200-300 W, the temperature to 150-250 °C, and the reaction residence time to 20-60 min;

[0021] Furthermore, the mass ratio of the gallated lignin to the demethylation reduction catalyst is 1:(5-10); the demethylation reduction catalyst is a metal-modified SBA-15 zeolite catalyst; preferably a transition metal composite cerium oxide (CeO2), including one of Co-CeO2 / SBA-15, Fe-CeO2 / SBA-15, and Ni-CeO2 / SBA-15;

[0022] Furthermore, the step three is specifically as follows:

[0023] adding a mixed solvent of water and ethanol to the microwave pyrolysis product, performing ultrasonic stirring, and then performing solid-liquid separation to obtain a crude tannic acid-like compound solution and an unactivated demethylation reduction catalyst; calcining the unactivated demethylation reduction catalyst in an air atmosphere, and then activating it in a hydrogen atmosphere to obtain the demethylation reduction catalyst, which is recycled to step 2; adding the crude tannic acid-like compound solution to diethyl ether, stirring it thoroughly, and then allowing it to stand to obtain diethyl ether insoluble matter; performing reduced pressure distillation on the diethyl ether insoluble matter until no distillate is produced, and collecting the undistilled fraction to obtain the tannic acid-like compound;

[0024] Furthermore, the mass ratio of the microwave pyrolysis product, water and ethanol mixed solvent is 1: (10-30); the volume ratio of water and ethanol is 1:1; the power of ultrasonic stirring is 100-300 W, and the time is 30 min; the calcination temperature is 700 ° C, and the time is 15-30 min; the activation temperature is 600 ° C, and the time is 1-2 h; the mass ratio of the crude tannic acid compound solution and ether is 1: (3-5); the standing time is 2-3 h; and the temperature of the reduced pressure distillation is 50 ° C.

[0025] A tannic acid-like compound based on microwave-catalyzed reforming of lignin is obtained based on the above-mentioned method for preparing a tannic acid-like compound based on microwave-catalyzed reforming of lignin.

[0026] The above technical solution has the following advantages or beneficial effects:

[0027] The present invention provides a tannic acid-like compound based on microwave catalytic reforming of lignin and a preparation method thereof. The method utilizes abundant lignin resources (conservatively estimated, the annual output of kraft lignin, a by-product of the pulp and paper industry alone, exceeds 50 million tons) and, after pretreatment, combines the lignin with microwave pyrolysis technology to efficiently convert it into a tannic acid-like compound. This method can effectively solve the problems faced in extracting tannic acid compounds from traditional natural resources, such as the difficulty of large-scale extraction and the uncontrollable purity and activity of the product. It provides new ideas and methods for the preparation of tannic acid-like compounds, and also provides a new approach to the high-value utilization of lignin resources.

[0028] Furthermore, the present invention pre-treats lignin to prepare gallic acid esterified lignin, and then reacts the lignin with catechol under acidic conditions for phenolization, and then esterifies the lignin with gallic acid under acidic conditions to significantly increase the content of phenolic hydroxyl groups in the lignin. The innovation lies in that the catechol phenolization treatment reacts with the active sites 5 or 6 of the benzene ring on the phenylpropanoid structural unit of the lignin (ortho-para active sites), thereby increasing the content and activity of the phenolic hydroxyl groups on the lignin. The esterification is then carried out with gallic acid, and the carboxyl groups on the gallic acid can be esterified with the phenolic hydroxyl groups on the lignin, thereby significantly increasing the overall phenolic hydroxyl content of the lignin. At the same time, the gallic acid on the esterified graft contains a typical pyrogallol structure. Although its overall molecular weight is large, its structure is similar to that of tannic acid compounds, and it has the potential to prepare tannic acid-like compounds.

[0029] Furthermore, the present invention processes gallic acid esterified lignin in a downtube microwave reactor. By regulating the conditions of the microwave reactor in three sections (upper, middle, and lower), the gallic acid esterified lignin undergoes a rapid pyrolysis reaction from top to bottom under the action of gravity. The key to converting the gallic acid esterified lignin into tannic acid-like compounds lies in limited and reasonable bond breaking, so that neither secondary reactions to generate a large number of small molecular compounds or permanent gases nor polymerization reactions to form carbon can occur. Therefore, the present invention utilizes the ingenious design of this gravity-down tube reaction to, on the one hand, prevent the gallic acid esterified lignin from undergoing secondary cracking to generate small molecular gas products, and, on the other hand, prevent the gallic acid esterified lignin from coking and carbonization through rapid reaction. As a result, the gallic acid esterified lignin can undergo limited bond breaking to obtain a tar-like substance with a suitable molecular weight distribution, which falls onto the silicon carbide mesh at the bottom and undergoes further demethylation reduction reaction with the catalyst.

[0030] Furthermore, the microwave pyrolysis of gallic acid esterified lignin proposed in the present invention is carried out in a downtube microwave reactor, and the bond breaking and conversion of the raw materials are cleverly achieved through the innovative design of three-stage temperature control. Specifically, the microwave power of the upper section is set to 1500-2000 W, the temperature is 600-1000 ° C, and the reaction residence time is 0.05s-0.1s. The gallic acid esterified lignin raw material of the upper section passes through first, so the rapid heating characteristic of microwaves is mainly utilized to achieve rapid heating and preliminary dehydration, decarboxylation and other decomposition of the raw materials at high temperature and extremely short gas phase residence time; the microwave power of the middle section is set to 600-800 W, the temperature is 400-600 ° C, and the reaction residence time is 0.05-0.1s, mainly to achieve limited bond breaking of the raw materials of the upper section reaction in a medium temperature environment and an extremely short gas phase residence time, and to prevent the generation of small molecular gases generated by secondary cracking; the microwave power of the lower section is set to 200-300 W, the temperature is 150-250 ℃, the reaction residence time is 20 to 60 minutes, and the reaction is mainly carried out on a silicon carbide mesh with a demethylation reduction catalyst (metal-modified CeO2 / SBA-15 catalyst). The reaction in the lower stage at a medium-low temperature is to prevent the product obtained in the middle stage from coking into carbon in the lower stage. On the other hand, at such a reaction temperature, the tar-like substance produced in the middle stage can react with the demethylation reduction catalyst to effectively remove the guaiacyl structure and the ortho-methoxy group in the syringyl structure generated by the β-O-4 breakage of the lignin itself in the tar-like substance, thereby The steric hindrance of the lignin structural unit is reduced, its reaction activity is improved, and a tannic acid-like compound with a molecular weight distribution of about 1700 is generated; at the same time, the metal-modified CeO2 / SBA-15 catalyst used in the present invention is used as a demethylation reduction catalyst, which makes good use of the excellent pore structure and acidic reaction sites of SBA-15, and loads CeO2, utilizing CeO2's good role in replacing precious metals, while loading Co, Ni, Fe, etc. as co-catalysts, which is conducive to the effective removal of methoxy groups 3 and 5 on the benzene ring in the lignin structural unit.

[0031] Furthermore, the present invention, while separating and purifying the product, performs air atmosphere calcination and hydrogen atmosphere activation treatment on the separated solid, removes coke on the catalyst by air atmosphere calcination, and reduces the loaded co-metal by hydrogen activation to obtain a fresh demethylation reduction catalyst, which can be recycled in the microwave pyrolysis process, thereby achieving sustainable utilization of resources. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 Schematic diagram of the process for preparing tannic acid-like compounds based on microwave catalytic reforming of lignin according to the present invention;

[0033] Figure 2 This is a diagram of the raw material of kraft lignin, a by-product of the pulp and paper industry, in Example 1 of the present invention;

[0034] Figure 3 This is a schematic diagram of the tannic acid-like compound product obtained in Example 1 of the present invention;

[0035] Figure 4 This is the GPC molecular weight chart of the pulp and paper industry by-product kraft lignin raw material in Example 1 of the present invention;

[0036] Figure 5 This is the GPC molecular weight chart of the tannic acid-like compound product obtained in Example 1 of the present invention. DETAILED DESCRIPTION

[0037] The present invention will be further described in detail below with reference to specific embodiments, which are intended to explain the present invention rather than to limit it.

[0038] In order to enable those skilled in the art to better understand the solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of the present invention.

[0039] It should be noted that the terms "first", "second", etc. in the description and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that the numbers used in this way can be interchanged where appropriate, so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions. For example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0040] Example 1:

[0041] like Figure 1 、 Figure 2 Figure 4 and Figure 5 As shown, the present invention provides a method for preparing tannic acid-like compounds based on microwave catalytic reforming of lignin, comprising the following steps:

[0042] Step 1: add 100 g of kraft lignin, a by-product of the pulp and paper industry, to 1500 g of deionized water, then add 100 g of catechol and mix and stir evenly, then add 1 g of concentrated sulfuric acid with a mass concentration of 98%, carry out phenolization reaction at 40 ° C for 2 h, then repeatedly wash with deionized water until the pH is 5-6, and vacuum dry at 45 ° C for 24 h to obtain phenolized lignin; add 100 g of phenolized lignin to 1000 g of dimethyl sulfoxide, then add 200 g of gallic acid, heat to 60 ° C and stir and mix evenly, add 10 g of trifluoromethanesulfonic acid, carry out esterification reaction at 60 ° C for 5 h, then repeatedly wash with dimethyl sulfoxide, and then vacuum dry at 40 ° C for 24 h to obtain gallic acid esterified lignin;

[0043] Step 2: 100 g of gallic acid esterified lignin was subjected to microwave pyrolysis in a downtube microwave reactor. Before the pyrolysis reaction began, a nitrogen atmosphere was introduced into the downtube microwave reactor to keep it oxygen-isolated. The temperature control was then divided into three sections from top to bottom, namely: the microwave power of the upper section was set to 1500 W, the temperature was 600 ° C, and the reaction residence time was 0.1 s; the microwave power of the middle section was set to 600 W, the temperature was 400 ° C, and the reaction residence time was 0.1 s; the microwave power of the lower section was set to 200 W, the temperature was 150 ° C, and the reaction residence time was 60 min. A silicon carbide mesh was provided at the lowest end of the lower section of the reactor, and 500 g of Co-CeO2 / SBA-15 demethylation reduction catalyst; starting a pyrolysis reaction, adding gallic acid esterified lignin into a downtube microwave reactor, after the gallic acid esterified lignin is added from the upper end of the reactor, the reaction is carried out from top to bottom under the action of gravity according to the three-stage setting conditions, and after the reaction is completed, a mixture of tar-like substance and catalyst is obtained on a silicon carbide mesh;

[0044] Step 3, placing a mixture of 50 g of tar-like substance and catalyst in an ultrasonic stirrer, adding 500 g of a mixed solvent of water / ethanol with a volume ratio of 1:1, stirring at an ultrasonic power of 100 W for 30 min, and then performing solid-liquid separation. The separated liquid is a crude tannic acid compound solution, and the separated solid is an unactivated demethylation reduction catalyst. The unactivated demethylation reduction catalyst is calcined in an air atmosphere at 700 ° C for 15 min to remove coke, and then activated in a hydrogen atmosphere at 600 ° C for 1 h to obtain a demethylation reduction catalyst that can be recycled to step 2; 10 g of the crude tannic acid compound solution is added to 30 g of ether, stirred thoroughly and allowed to stand for 2 h, and the ether insoluble matter is subjected to reduced pressure distillation at 50 ° C until no distillate is produced, and the undistilled portion is collected as the tannic acid compound.

[0045] like Figure 3As shown, a tannic acid-like compound based on microwave-catalyzed reforming of lignin is obtained based on the above-mentioned method for preparing a tannic acid-like compound based on microwave-catalyzed reforming of lignin.

[0046] Specifically, the molecular weight and phenolic hydroxyl content of kraft lignin and tannic acid-like compounds, which are by-products of the pulping and papermaking industry, were analyzed, as shown in Table 1.

[0047] Example 2:

[0048] like Figure 1 As shown, the present invention provides a method for preparing tannic acid-like compounds based on microwave catalytic reforming of lignin, comprising the following steps:

[0049] Step 1: add 100 g of enzymatic hydrolysis lignin, a by-product of cellulose ethanol, to 2500 g of deionized water, then add 300 g of catechol and mix well, add 10 g of concentrated sulfuric acid with a mass concentration of 98%, carry out phenolization reaction at 60 ° C for 1 h, then wash repeatedly with deionized water until the pH is 5-6, and vacuum dry at 45 ° C for 24 h to obtain phenolized lignin; add 100 g of phenolized lignin to 1500 g of dimethyl sulfoxide, then add 400 g of gallic acid, heat to 60 ° C and stir to mix well, add 40 g of trifluoromethanesulfonic acid, carry out esterification reaction at 80 ° C for 3 h, then wash repeatedly with dimethyl sulfoxide, and then vacuum dry at 40 ° C for 24 h to obtain gallic acid esterified lignin;

[0050] Step 2: 100 g of gallic acid esterified lignin was subjected to microwave pyrolysis in a downtube microwave reactor. Before the pyrolysis reaction began, a nitrogen atmosphere was introduced into the downtube microwave reactor to keep it oxygen-isolated. The temperature control was then divided into three sections from top to bottom, namely: the microwave power of the upper section was set to 2000 W, the temperature was 1000 ° C, and the reaction residence time was 0.05 s; the microwave power of the middle section was set to 800 W, the temperature was 600 ° C, and the reaction residence time was 0.05 s; the microwave power of the lower section was set to 300 W, the temperature was 250 ° C, and the reaction residence time was 20 min. A silicon carbide mesh was placed at the lowest end of the lower section of the reactor, and 1000 g of Fe-CeO2 / SBA-15 demethylation reduction catalyst; starting the pyrolysis reaction, adding the gallic acid esterified lignin into the downtube microwave reactor, after the gallic acid esterified lignin is added from the upper end of the reactor, the reaction is carried out from top to bottom under the action of gravity according to the three-stage setting conditions, and after the reaction is completed, a mixture of tar-like substance and catalyst is obtained on the silicon carbide mesh;

[0051] Step 3, placing a mixture of 50 g of tar-like substance and catalyst in an ultrasonic stirrer, adding 1500 g of a mixed solvent of water / ethanol with a volume ratio of 1:1, stirring at an ultrasonic power of 300 W for 30 min, and then performing solid-liquid separation. The separated liquid is a crude tannic acid compound solution, and the separated solid is an unactivated demethylation reduction catalyst. The unactivated demethylation reduction catalyst is calcined in an air atmosphere at 700 ° C for 30 min to remove coke, and then activated in a hydrogen atmosphere at 600 ° C for 2 h to obtain a demethylation reduction catalyst that can be recycled to step 2; 10 g of the crude tannic acid compound solution is added to 50 g of ether, stirred thoroughly and allowed to stand for 3 h, and the ether insoluble matter is subjected to reduced pressure distillation at 50 ° C until no distillate is produced, and the undistilled portion is collected as the tannic acid compound.

[0052] Specifically, the molecular weight and phenolic hydroxyl content of the enzymatically hydrolyzed lignin and tannic acid-like compounds, which are byproducts of the raw material cellulose ethanol, were analyzed, as shown in Table 1.

[0053] Example 3:

[0054] like Figure 1 As shown, the present invention provides a method for preparing tannic acid-like compounds based on microwave catalytic reforming of lignin, comprising the following steps:

[0055] Step 1: add 100 g of kraft lignin, a by-product of the pulp and paper industry, to 1800 g of deionized water, then add 150 g of catechol and mix well, add 3 g of concentrated sulfuric acid with a mass concentration of 98%, carry out phenolization reaction at 50 ° C for 1.5 h, then repeatedly wash with deionized water until the pH is 5-6, and vacuum dry at 45 ° C for 24 h to obtain phenolized lignin; add 100 g of phenolized lignin to 1200 g of dimethyl sulfoxide, then add 250 g of gallic acid, heat to 60 ° C and stir to mix well, add 30 g of trifluoromethanesulfonic acid, carry out esterification reaction at 65 ° C for 4 h, then repeatedly wash with dimethyl sulfoxide, and then vacuum dry at 40 ° C for 24 h to obtain gallic acid esterified lignin;

[0056] Step 2: 100 g of gallic acid esterified lignin was subjected to microwave pyrolysis in a downtube microwave reactor. Before the pyrolysis reaction began, a nitrogen atmosphere was introduced into the downtube microwave reactor to keep it oxygen-isolated. The temperature control was then divided into three sections from top to bottom, namely: the microwave power of the upper section was set to 1800 W, the temperature was 800 ° C, and the reaction residence time was 0.08 s; the microwave power of the middle section was set to 700 W, the temperature was 500 ° C, and the reaction residence time was 0.07 s; the microwave power of the lower section was set to 260 W, the temperature was 200 ° C, and the reaction residence time was 30 min. A silicon carbide mesh was placed at the lowest end of the lower section of the reactor, and 600 g of Ni-CeO2 / SBA-15 demethylation reduction catalyst; starting the pyrolysis reaction, adding the gallic acid esterified lignin into the downtube microwave reactor, after the gallic acid esterified lignin is added from the upper end of the reactor, the reaction is carried out from top to bottom under the action of gravity according to the three-stage setting conditions, and after the reaction is completed, a mixture of tar-like substance and catalyst is obtained on the silicon carbide mesh;

[0057] Step 3, placing a mixture of 50 g of tar-like substance and catalyst in an ultrasonic stirrer, adding 1000 g of a mixed solvent of water / ethanol with a volume ratio of 1:1, stirring at an ultrasonic power of 120 W for 30 min, and then performing solid-liquid separation, the separated liquid is a crude tannic acid compound solution, and the separated solid is an unactivated demethylation reduction catalyst, the unactivated demethylation reduction catalyst is calcined in an air atmosphere at 700 ° C for 20 min to remove coke, and then activated in a hydrogen atmosphere at 600 ° C for 1.5 h to obtain a demethylation reduction catalyst that can be recycled to step 2; 10 g of the crude tannic acid compound solution is added to 40 g of ether, stirred thoroughly and allowed to stand for 2.5 h, and the ether insoluble matter is subjected to reduced pressure distillation at 50 ° C until no distillate is produced, and the undistilled portion is collected as the tannic acid compound.

[0058] Specifically, the molecular weight and phenolic hydroxyl content of kraft lignin and tannic acid-like compounds, which are by-products of the pulping and papermaking industry, were analyzed, as shown in Table 1.

[0059] Table 1 Physical and chemical analysis of lignin raw materials and tannin-like compounds

[0060]

[0061] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing tannic acid-like compounds based on microwave catalytic reforming of lignin, characterized in that: The following steps are involved: S1, pre-treating lignin to prepare gallic acid esterified lignin, specifically: The lignin is added to deionized water, and then catechol is added and mixed and stirred uniformly, and concentrated sulfuric acid is added to carry out a phenolization reaction, and then washed with deionized water and vacuum dried for the first time to obtain phenolized lignin; the phenolized lignin is added to dimethyl sulfoxide, and then gallic acid is added, and the temperature is increased and stirred and mixed uniformly, and then trifluoromethanesulfonic acid is added to carry out an esterification reaction, and then repeatedly washed with dimethyl sulfoxide and vacuum dried for a second time to obtain gallic acid esterified lignin; S2, subjecting gallic acid esterified lignin and a demethylation reduction catalyst to microwave pyrolysis reaction to obtain a microwave pyrolysis product, specifically: Before microwave pyrolysis begins, a silicon carbide mesh is placed at the bottom of the lower section of the reactor, and a demethylation reduction catalyst is placed on the silicon carbide mesh. A nitrogen atmosphere is then introduced into the reactor to isolate oxygen, and the temperature is then controlled in three stages from top to bottom. Gallic acid esterified lignin is added to the reactor, and after the gallic acid esterified lignin is added from the upper end of the reactor, microwave pyrolysis is performed from top to bottom under the action of gravity according to the three-stage temperature control conditions. After the reaction is completed, a mixture of tar-like substance and catalyst is obtained on the silicon carbide mesh. The three-stage temperature control is specifically as follows: The power of the upper microwave was set to 1500-2000 W, the temperature to 600-1000 °C, and the reaction residence time to 0.05-0.1 s; The power of the middle microwave was set to 600-800 W, the temperature to 400-600 °C, and the reaction residence time to 0.05-0.1 s; Set the power of the lower microwave to 200-300 W, the temperature to 150-250 °C, and the reaction residence time to 20-60 min; S3, separating the microwave pyrolysis product to obtain a crude tannic acid-like compound solution and an unactivated demethylation reduction catalyst, calcining and activating the unactivated demethylation reduction catalyst to obtain a demethylation reduction catalyst, and recycling it to S2; The crude tannic acid-like compound solution is purified to obtain the tannic acid-like compound.

2. The method for preparing tannic acid-like compounds based on microwave catalytic reforming of lignin according to claim 1, characterized in that: The mass ratio of the lignin, deionized water, catechol and concentrated sulfuric acid is 1:(15-25):(1-3):(0.01-0.1); the mass concentration of the concentrated sulfuric acid is 98%; the temperature of the phenolization reaction is 40-60°C and the time is 1-2 hours; washing with deionized water is specifically: repeatedly washing with deionized water until the pH is 5-6; the temperature of the first vacuum drying is 45°C and the time is 24 hours.

3. The method for preparing tannic acid-like compounds based on microwave catalytic reforming of lignin according to claim 1, characterized in that: The mass ratio of the phenolic lignin, dimethyl sulfoxide, gallic acid and trifluoromethanesulfonic acid is 1: (10-15): (2-4): (0.1-0.4); the temperature is raised to 60°C; the temperature of the esterification reaction is 60-80°C and the time is 3-5 hours; the temperature of the second vacuum drying is 40°C and the time is 24 hours.

4. The method for preparing tannic acid-like compounds based on microwave catalytic reforming of lignin according to claim 1, characterized in that: The mass ratio of the gallic acid esterified lignin to the demethylation reduction catalyst is 1:(5-10); the demethylation reduction catalyst is a metal-modified SBA-15 zeolite catalyst.

5. The method for preparing tannic acid-like compounds based on microwave catalytic reforming of lignin according to claim 1, characterized in that: The S3 is specifically: A mixed solvent of water and ethanol is added to the microwave pyrolysis product and ultrasonically stirred, followed by solid-liquid separation to obtain a crude tannic acid-like compound solution and an unactivated demethylation reduction catalyst. The unactivated demethylation reduction catalyst is calcined in an air atmosphere and then activated in a hydrogen atmosphere to obtain a demethylation reduction catalyst, which is recycled to S2. The crude tannic acid-like compound solution is added to ether, stirred thoroughly, and then allowed to stand to obtain ether-insoluble matter. The ether-insoluble matter is subjected to reduced pressure distillation until no distillate is produced, and the undistilled matter is collected to obtain a tannic acid-like compound.

6. The method for preparing tannic acid-like compounds based on microwave catalytic reforming of lignin according to claim 5, characterized in that: The mass ratio of the microwave pyrolysis product, water and ethanol mixed solvent is 1:(10-30); the volume ratio of water and ethanol is 1:1; the power of ultrasonic stirring is 100-300 W, and the time is 30 minutes; the temperature of calcination is 700°C, and the time is 15-30 minutes; the temperature of activation is 600°C, and the time is 1-2 hours; the mass ratio of the crude tannic acid compound solution and ether is 1:(3-5); the standing time is 2-3 hours; and the temperature of reduced pressure distillation is 50°C.

7. A tannic acid-like compound based on microwave catalytic reforming of lignin, characterized in that: The method for preparing a tannic acid-like compound based on microwave catalytic reforming of lignin is based on any one of claims 1 to 6.

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