Nannic acid-like compound based on lignin microwave catalytic reforming and preparation method of tannic acid-like compound

Abstract

The invention provides a tannic acid-like compound based on lignin microwave catalytic reforming and a preparation method thereof, and belongs to the technical field of biomass conversion and utilization, and the preparation method comprises the following steps: pretreating lignin to prepare gallic acid esterified lignin, carrying out a microwave pyrolysis reaction on the gallic acid esterified lignin and a demethylation reduction catalyst, and obtaining the tannic acid-like compound based on lignin microwave catalytic reforming. 2, microwave pyrolysis is carried out, a microwave pyrolysis product is obtained, the microwave pyrolysis product is separated to obtain a crude tannic acid compound solution and an unactivated demethylation reduction catalyst, the unactivated demethylation reduction catalyst is calcined and activated to obtain the demethylation reduction catalyst, and the demethylation reduction catalyst is recycled in the step 2; purifying the crude tannic acid-like compound solution to obtain a tannic acid-like compound; according to the method disclosed by the invention, the tannic acid can be efficiently converted into the tannic acid-like compound through modification treatment in combination with a microwave pyrolysis technology, and the problems of difficulty in large-scale extraction, uncontrollable product purity and activity and the like in extraction of the tannic acid compound from traditional natural resources can be effectively solved.

Description

Technical Field

[0001] The 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, strong reactivity, and significant antioxidant, antibacterial and anti-inflammatory properties. They are widely used in food, medicine, feed, polymer resins or material synthesis. However, the commonly used tannic acid compounds currently rely mainly on natural extraction or simple subsequent processing, which leads to problems such as insufficient supply of natural products, low purity and low activity when used in large quantities, thus affecting their application potential in daily chemical products, biomedical fields, scientific research and analysis fields, and industrial engineering. Based on this, it is of great significance to prepare tannic acid-like compounds from abundant biomass resources.

[0003] Lignin is a type of phenolic hydroxyl-rich compound with a phenylpropane structure, and its structure has certain similarities with tannic acid compounds. However, lignin has problems such as large and uneven molecular weight and low active phenolic hydroxyl content, which restricts its further transformation and utilization. At present, there are certain 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 obtained tannins", walnut shells are used as raw materials, and tannin extracts are obtained through a multi-step treatment process, and then the tannin extracts are purified to obtain pure tannins; the publication number CN117624262B "A method for extracting and preparing tannic acid from tannin germanium slag" also proposes a method for extracting tannic acid from solid waste resources.

[0004] However, there are few reports on the direct conversion of biomass resources or lignin to prepare tannic acid-like compounds. However, in the publication number CN111945462B "A lignin / tannin composite polyurethane coating and its preparation method", the coating is prepared by constructing an isocyanate prepolymer of lignin and tannin, which proves that there are certain structural and performance similarities between lignin and tannin or tannic acid. However, there has not 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 of large-scale extraction difficulties, uncontrollable product purity and activity faced in extracting tannic acid compounds from traditional natural resources, and provide new ideas and methods for the preparation of tannic acid-like compounds, and also provide a new way 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: A method for preparing tannic acid-like compounds based on microwave catalytic reforming of lignin comprises the following steps: Step 1, pretreating lignin to prepare gallic acid esterified lignin; Step 2, subjecting gallic acid esterified lignin and a demethylation reduction catalyst to microwave pyrolysis reaction to obtain a microwave pyrolysis product; 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, and recycling it to step 2; purifying the crude tannic acid-like compound solution to obtain a tannic acid-like compound; Furthermore, the step 1 is specifically as follows: Add lignin to deionized water, then add catechol, mix and stir evenly, add concentrated sulfuric acid to carry out phenolization reaction, then wash with deionized water, and perform the first vacuum drying to obtain phenolized lignin; Phenolated lignin is added to dimethyl sulfoxide, and then gallic acid is added, the mixture is heated and stirred to be uniformly mixed, and then trifluoromethanesulfonic acid is added to carry out esterification reaction, and then the mixture is repeatedly washed with dimethyl sulfoxide, and then vacuum dried for a second time to obtain gallic acid esterified lignin; 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 concentrated sulfuric acid is 98%; the temperature of the phenolization reaction is 40-60°C, and the time is 1-2h; 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 24h; 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 temperature of the esterification reaction is 60-80°C, and the time is 3-5h; the temperature of the second vacuum drying is 40°C; and the time is 24h; Furthermore, the step 2 is specifically as follows: Before microwave pyrolysis begins, a silicon carbide net is set at the bottom of the lower section of the reactor, a demethylation reduction catalyst is placed on the silicon carbide net, and then a nitrogen atmosphere is introduced into the reactor to keep oxygen away, and then the temperature is controlled in three sections from top to bottom; 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 conditions of 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; Furthermore, the three-stage temperature control is specifically as follows: 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; Furthermore, 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; preferably a transition metal composite cerium oxide (CeO2), including one of Co-CeO2 / SBA-15, Fe-CeO2 / SBA-15, and Ni-CeO2 / SBA-15; Furthermore, the step three is specifically as follows: 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 a demethylation reduction catalyst, which is recycled to step 2; adding the crude tannic acid-like compound solution to ether, stirring it fully and then standing it to obtain ether insoluble matter, performing reduced pressure distillation on the ether insoluble matter until no distillate is produced, and collecting the undistilled portion to obtain a tannic acid-like compound; 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.

[0007] A tannic acid-like compound based on microwave catalytic reforming of lignin is obtained based on the above-mentioned method for preparing a tannic acid-like compound based on microwave catalytic reforming of lignin.

[0008] The above technical solution has the following advantages or beneficial effects: The present invention provides a tannic acid-like compound based on microwave catalytic reforming of lignin and a preparation method thereof. The abundant lignin resources are utilized (it is conservatively estimated that the annual output of kraft lignin, a by-product of the pulp and paper industry alone, exceeds 50 million tons), which are efficiently converted into tannic acid-like compounds after pretreatment in combination with microwave pyrolysis technology. The problems of difficulty in large-scale extraction, uncontrollable product purity and activity, etc. faced in extracting tannic acid compounds from traditional natural resources can be effectively solved, and new ideas and methods are provided for the preparation of tannic acid-like compounds, and a new approach is also provided for the high-value utilization of lignin resources.

[0009] Furthermore, the present invention pre-treats lignin to prepare gallic acid esterified lignin, and the content of phenolic hydroxyl groups in lignin is greatly increased by reacting lignin with catechol under acidic conditions for phenolation, and then reacting with gallic acid under acidic conditions for esterification. The innovation lies in that the catechol phenolation treatment reacts with active sites 5 or 6 of the benzene ring on the phenylpropanoid structural unit of lignin (ortho-para active sites), thereby increasing the content and activity of phenolic hydroxyl groups on lignin. Then, the esterification is carried out with gallic acid on the basis of phenolization, so that the carboxyl groups on the gallic acid can be esterified with the phenolic hydroxyl groups on the lignin, thereby greatly increasing the overall phenolic hydroxyl content of lignin. At the same time, the gallic acid on the esterification 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.

[0010] Furthermore, the present invention processes the gallic acid esterified lignin in a descending tube microwave reactor, and regulates the conditions of the microwave reactor in three sections, namely, upper, middle and lower sections, so that the gallic acid esterified lignin undergoes a rapid pyrolysis reaction from top to bottom under the action of gravity. The key to converting and preparing tannic acid-like compounds from the gallic acid esterified lignin is to limit and reasonably break bonds, so that neither secondary reactions can occur to generate more small molecular compounds or permanent gases, nor polymerization reactions can occur to form carbon. Therefore, the present invention utilizes the ingenious design of the gravity descending tube reaction, which can prevent the gallic acid esterified lignin from undergoing secondary cracking to generate small molecular gas products, and can also prevent the gallic acid esterified lignin from coking and carbonization through rapid reactions, so that 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 net at the bottom and undergoes further demethylation reduction reaction with the catalyst.

[0011] 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 stage 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 stage reaction 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 stage 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 stage reaction in a medium temperature environment and an extremely short gas phase residence time, and to prevent the generation of small molecular gases by secondary cracking; the microwave power of the lower stage is set to 200-300 W, the temperature is 150-250 ℃, the reaction residence time is 20-60min, the reaction is mainly carried out on a silicon carbide mesh with a demethylation reduction catalyst (metal modified CeO2 / SBA-15 catalyst), and the reaction of the lower stage at a medium-low temperature condition is to prevent the product obtained in the middle stage from coking into carbon in the lower stage, and 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 of the syringyl structure in the tar-like substance derived from the β-O-4 break of the lignin itself, 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 the good role of CeO2 in replacing precious metals, and at the same time loads 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.

[0012] Furthermore, the present invention, while separating and purifying the product, performs air atmosphere calcination + hydrogen atmosphere activation treatment on the separated solid, removes coke on the catalyst by air atmosphere calcination, reduces the loaded co-metal by hydrogen activation, and obtains fresh demethylation reduction catalyst, which can be recycled in the microwave pyrolysis process, thereby realizing sustainable utilization of resources. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 It is a schematic flow chart of the method for preparing tannic acid-like compounds based on microwave catalytic reforming of lignin of the present invention; Figure 2 This is a raw material diagram of kraft lignin, a by-product of the pulp and paper industry in Example 1 of the present invention; Figure 3 This is a schematic diagram of the tannic acid-like compound product obtained in Example 1 of the present invention; Figure 4 This is the GPC molecular weight diagram of the kraft lignin raw material, a by-product of the pulp and paper industry in Example 1 of the present invention; 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

[0014] The present invention is further described in detail below in conjunction with specific embodiments, which are intended to explain the present invention rather than to limit it. In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme 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 described embodiments 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 creative work should fall within the scope of protection of the present invention. It should be noted that the terms "first", "second", etc. in the specification 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 data 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 that are clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0015] Embodiment 1: 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: 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 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 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 wash repeatedly with dimethyl sulfoxide, and then vacuum dry at 40 ° C for 24 h to obtain gallic acid esterified lignin; 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-free. Then, the temperature was controlled in three sections from top to bottom, namely: the microwave power was set to 1500 W, the temperature was 600 °C, and the reaction residence time was 0.1 s in the upper section; the microwave power was set to 600 W, the temperature was 400 °C, and the reaction residence time was 0.1 s in the middle section; the microwave power was set to 200 W, the temperature was 150 °C, and the reaction residence time was 60 min in the lower section. A silicon carbide mesh was set 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, it reacts 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 substances and catalysts is obtained on a silicon carbide mesh; Step 3: Place 50 g of a mixture of tar-like substance and catalyst in an ultrasonic agitator, add 500 g of a mixed solvent of water / ethanol with a volume ratio of 1:1, stir at an ultrasonic power of 100 W for 30 min, and then perform 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.

[0016] 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.

[0017] Specifically, the molecular weight and phenolic hydroxyl content of kraft lignin and tannic acid-like compounds, which are byproducts of the pulping and papermaking industry, were analyzed, as shown in Table 1.

[0018] Embodiment 2: 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: Step 1: add 100 g of enzymatic lignin obtained as a byproduct of cellulose ethanol to 2500 g of deionized water, then add 300 g of catechol and mix and stir evenly, then 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 and mix evenly, 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; 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-free. Then, the temperature was controlled in three sections from top to bottom, namely: the microwave power was set to 2000 W, the temperature was 1000 °C, and the reaction residence time was 0.05 s in the upper section; the microwave power was set to 800 W, the temperature was 600 °C, and the reaction residence time was 0.05 s in the middle section; the microwave power was set to 300 W, the temperature was 250 °C, and the reaction residence time was 20 min in the lower section. A silicon carbide mesh was set at the lowest end of the lower section of the reactor, and 1000 g of Fe-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, it reacts 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 substances and catalysts is obtained on a silicon carbide mesh; Step 3: Place 50 g of a mixture of tar-like substance and catalyst in an ultrasonic agitator, add 1500 g of a mixed solvent of water / ethanol with a volume ratio of 1:1, stir at an ultrasonic power of 300 W for 30 min, and then perform 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.

[0019] Specifically, the molecular weight and phenolic hydroxyl content of enzymatically hydrolyzed lignin and tannic acid-like compounds of the raw material cellulose ethanol byproducts were analyzed, as shown in Table 1.

[0020] Embodiment 3: 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: 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 and stir evenly, then 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 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 1200 g of dimethyl sulfoxide, then add 250 g of gallic acid, heat to 60 ° C and stir and mix evenly, add 30 g of trifluoromethanesulfonic acid, carry out esterification reaction at 65 ° C for 4 h, then wash repeatedly with dimethyl sulfoxide, and then vacuum dry at 40 ° C for 24 h to obtain gallic acid esterified lignin; 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-free. Then, the temperature was controlled in three sections from top to bottom, namely: the microwave power was set to 1800 W, the temperature was 800 °C, and the reaction residence time was 0.08 s in the upper section; the microwave power was set to 700 W, the temperature was 500 °C, and the reaction residence time was 0.07 s in the middle section; the microwave power was set to 260 W, the temperature was 200 °C, and the reaction residence time was 30 min in the lower section. A silicon carbide mesh was set at the lowest end of the lower section of the reactor, and 600 g of Ni-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, it reacts 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 substances and catalysts is obtained on a silicon carbide mesh; Step 3: Place 50 g of a mixture of tar-like substance and catalyst in an ultrasonic agitator, add 1000 g of a mixed solvent of water / ethanol with a volume ratio of 1:1, stir at an ultrasonic power of 120 W for 30 min, and then perform 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.

[0021] Specifically, the molecular weight and phenolic hydroxyl content of kraft lignin and tannic acid-like compounds, which are byproducts of the pulping and papermaking industry, were analyzed, as shown in Table 1.

[0022] Table 1 Physical and chemical analysis of lignin raw materials and tannin-like compounds

[0023] 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, pretreating lignin to prepare gallic acid esterified lignin; S2, subjecting the gallic acid esterified lignin and the demethylation reduction catalyst to microwave pyrolysis reaction to obtain a microwave pyrolysis product; S3, separating the microwave pyrolysis product to obtain a crude tannic acid 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 S1 is specifically: Add lignin to deionized water, then add catechol, mix and stir evenly, add concentrated sulfuric acid to carry out phenolization reaction, then wash with deionized water, and perform the first vacuum drying to obtain phenolized lignin; Phenolated lignin is added to dimethyl sulfoxide, and then gallic acid is added. After heating and stirring to mix evenly, trifluoromethanesulfonic acid is added to carry out esterification reaction, and then the mixture is repeatedly washed with dimethyl sulfoxide and vacuum dried for a second time to obtain gallic acid esterified lignin.

3. The method for preparing tannic acid-like compounds based on microwave catalytic reforming of lignin according to claim 2, 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 h; 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 h.

4. The method for preparing tannic acid-like compounds based on microwave catalytic reforming of lignin according to claim 2, 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.

5. The method for preparing tannic acid-like compounds based on microwave catalytic reforming of lignin according to claim 1, characterized in that: The S2 is specifically: Before microwave pyrolysis begins, a silicon carbide net is set at the bottom of the lower section of the reactor, a demethylation reduction catalyst is placed on the silicon carbide net, and then a nitrogen atmosphere is introduced into the reactor to keep oxygen away, and then the temperature is controlled in three sections from top to bottom; Gallic acid esterified lignin is added into the reactor. After being added from the upper end of the reactor, the gallic acid esterified lignin is subjected to microwave pyrolysis reaction from top to bottom under the action of gravity according to the conditions of three-stage temperature control settings. After the reaction is completed, a mixture of tar-like substance and catalyst is obtained on the silicon carbide mesh.

6. The method for preparing tannic acid-like compounds based on microwave catalytic reforming of lignin according to claim 5, characterized in that: The three-stage temperature control is specifically as follows: 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.

7. 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 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.

8. 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 for ultrasonic stirring, and then solid-liquid separation is performed 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, the mixture is 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 portion is collected to obtain a tannic acid-like compound.

9. The method for preparing tannic acid-like compounds based on microwave catalytic reforming of lignin according to claim 8, 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 min; the temperature of calcination is 700 ° C, and the time is 15-30 min; the temperature of activation 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 reduced pressure distillation is 50 ° C.

10. A tannic acid-like compound based on microwave catalytic reforming of lignin, characterized in that: The method for preparing tannic acid-like compounds based on microwave catalytic reforming of lignin is obtained based on any one of claims 1 to 9.

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