Method for preparing lignin oil by continuous hydrogenation depolymerization of lignin through microwave and microfluidics coupling

Abstract

The invention discloses a method for preparing lignin oil by continuous hydrogenation depolymerization of lignin through microwave and microfluidics coupling, and belongs to the technical field of high-value utilization of biomass resources. The method comprises the following steps: adding water, mannitol, n-amyl alcohol and p-toluenesulfonic acid into a wood fiber raw material, performing in-situ extraction on lignin under microwaves, and performing capillary regulation to form a three-phase emulsion and a droplet microreactor; a Pd / carboxylated carbon nanotube-formic acid aqueous solution and n-amyl alcohol-lignin mixed flow form Pickering emulsion, microwave hydrogenation depolymerization is performed in a heart-shaped channel microreactor, a catalyst is cooled, separated and recycled, and an organic phase is distilled to obtain n-amyl alcohol and lignin oil. According to the invention, a series catalytic system for in-situ extraction of lignin and transfer hydrogenolysis is constructed, a micro-fluidic chip design is further combined, a complex one-pot reaction is divided into two characteristic reaction units, efficient preparation of high monophenol lignin oil is realized through continuous flow series connection, and the method is a novel simple, efficient and energy-saving lignin oil preparation strategy.

Description

Technical Field

[0001] This invention belongs to the field of high-value utilization technology of biomass resources, specifically relating to a method for preparing lignin oil by continuous hydrogenation and depolymerization of lignin coupled with microwave and microfluidics. Background Technology

[0002] Lignin, one of the most structurally complex organic components in biomass, is rich in aromatic ring structures and is an important potential raw material for the preparation of high-value-added phenolic chemicals. Existing lignin conversion processes mostly employ batch or traditional solvothermal methods, whose energy input depends on external heating and conduction. Microscale energy transfer is limited, easily leading to significant temperature gradients and inhomogeneous reactions. Especially in multiphase systems, low interfacial mass transfer efficiency and difficulty in precise reaction control restrict process scale-up and product selectivity. Microwave heating offers advantages in high energy efficiency and internal heating, but traditional reactors suffer from uneven field intensity distribution, interfacial instability, and significant local overheating, resulting in complex reaction pathways and unstable products. Continuous flow and microfluidic technologies achieve efficient heat and mass transfer through precise control of flow rate and temperature. However, for complex multiphase systems in biomass refining, microfluidic technology alone cannot be applied, and the energy input relies on heat conduction, leading to complex heat transfer modules that result in high equipment and maintenance costs, and preventing in-situ control of the energy field in micro-regions. Therefore, the coupling of microwave and microfluidics provides an ideal approach to solving the energy and mass transfer bottlenecks in complex lignin systems. Microwaves enable rapid, deep, and selective energy input, while microfluidic systems ensure stable interfaces and uniform flow fields. The two complement each other significantly in terms of energy penetration and thermal management.

[0003] In recent years, lignin catalytic liquefaction conversion has become an important research direction for the preparation of phenolic compounds and liquid fuels. Common technical routes mainly include: 1) Catalytic pyrolysis: under high temperature and high pressure hydrogen conditions, hydrogenation cracking is achieved using a metal catalyst to obtain aromatic hydrocarbons or cycloalkanes (CN202110265005.7; CN202311627358.2); 2) Hydrothermal liquefaction: using a polar solvent as a medium, a hydrogen source (such as hydrogen, formic acid, or the alcohol solvent itself) is introduced to carry out a catalytic reduction reaction (CN202510670713.7; CN202410229659.8); 3) Acid-base catalysis: using acid or base catalysis to hydrolyze lignin to generate oligomers, which are then converted into monophenols and oligomers through catalytic pyrolysis or liquefaction processes (CN202510868314.1); 4) Catalytic oxidation: by oxidation, Cα-OH in the lignin structure is converted into carbonyl groups, thereby weakening the ether bond energy and increasing the phenol monomer formation rate (CN202511107779.1).

[0004] Although the above methods have achieved the conversion of lignin into high-value-added products to a certain extent, there are still obvious shortcomings: (1) The reaction device is mostly a batch reactor, which requires long-term reaction under high pressure hydrogen and high temperature, resulting in high energy consumption and stringent equipment requirements; (2) The active sites of the catalyst are concentrated in the β-aryl ether bond structure, which is difficult to retain after separation and treatment, and the lack of reaction sites leads to limited monophenol yield; (3) Lignin has low solubility in water-containing solvents, uneven product distribution, and easy generation of coke by-products, and its water-sensitive characteristics further reduce monophenol selectivity; (4) Heat transfer is difficult to be uniform in the traditional batch system, residence time is difficult to control precisely, mass transfer in the solid-liquid-gas multiphase system is complex, strong stirring is required, and both continuous and large-scale production are limited.

[0005] In summary, existing technologies for the efficient and selective depolymerization of lignin for oil production still suffer from problems such as poor reaction uniformity, low energy utilization, and complex equipment. There is an urgent need to develop a novel reaction system that combines energy enhancement and process synergy to achieve the green and efficient conversion of lignin. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a method for the continuous hydrogenation and depolymerization of lignin to prepare lignin oil by coupling microwave and microfluidics. The method achieves in-situ extraction and efficient conversion of lignin through the synergistic effect of microwave energy field and microfluidic system, significantly improving reaction rate and selectivity of target product.

[0007] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0008] A method for the continuous hydrogenation and depolymerization of lignin to prepare lignin oil via microwave-microfluidic coupling includes the following steps:

[0009] 1) Water, p-toluenesulfonic acid, and mannitol are added to the wood fiber raw material and mixed. Then, n-pentanol is added through a capillary tube to form a density-layered extraction system. The wood fiber is extracted in situ in a microwave field. The lignin is extracted through the n-pentanol layer and output as a n-pentanol-lignin mixed stream. The size and flow rate of the n-pentanol droplets are restricted by the capillary tube to drive the formation of a stable n-pentanol-water two-phase mass transfer interface driven by density difference. After natural phase separation, the water is separated from the top of the reactor.

[0010] 2) Water, formic acid, and Pd / carboxylated carbon nanotube catalyst are mixed and stirred to form an aqueous suspension. The aqueous suspension is then combined with the n-pentanol-lignin mixed flow obtained in step 1) in a series of microfluidic channels and introduced into a channel-type microreactor. Microwave radiation reaction is carried out in the channel-type microreactor. After the reaction system is cooled, the liquid phase is separated, the catalyst is recovered by filtration, and the organic phase is distilled under reduced pressure to obtain n-pentanol and lignin oil products.

[0011] 3) After the lignin extraction in step 1) is completed, solid-liquid separation is performed. The remaining solid is cellulose. The reaction solution is separated into xylan powder by methanol back precipitation. The mother liquor is evaporated under reduced pressure to recover p-toluenesulfonic acid.

[0012] Furthermore, in step 1), the capillary inner diameter is 0.12~2 mm, and the n-pentanol mass flow rate is 1~5 mL / min.

[0013] Furthermore, in step 1), the mass ratio of wood fiber raw material, water and p-toluenesulfonic acid is 1:5:5.

[0014] Furthermore, in step 1), the mass ratio of lignocellulose raw material to mannitol is 1:0.05.

[0015] Furthermore, in step 1), the microwave heating temperature is 80~120℃, the reaction time is 5~60min, and the back pressure of the reaction system is adjusted to 0.5MPa.

[0016] Furthermore, in step 1), the length-to-diameter ratio of the reaction vessel used for n-pentanol extraction is 3:1.

[0017] Furthermore, in step 2), the channel-type microreactor adopts a heart-shaped channel structure, containing 24 independent reaction units, with a channel diameter of 1 mm and an effective volume of 2 mL. It adopts a dual-inlet and one-outlet feeding design. The chip is clamped and sealed by polytetrafluoroethylene and PEEK plate, and the back pressure of the reaction system is maintained at 0.1~0.5 MPa.

[0018] Furthermore, in step 2), formic acid accounts for 5% of the mass of the aqueous phase, and the amount of Pd / carboxylated carbon nanotube catalyst added is 5% of the mass of the aqueous phase. The emulsified and dispersed liquid through the strong mixing effect of the central channel of the channel-type microreactor forms a stable suspension.

[0019] Furthermore, in step 2), the flow rate of the n-pentanol-lignin mixed stream is 1-2 mL / min, and the flow rate of the aqueous suspension is 0.5-1 mL / min. The two are combined in the microwave reactor to carry out the hydrogenation and depolymerization reaction.

[0020] Further, in step 2), the preparation method of Pd / carboxylated carbon nanotube catalyst is as follows: Na2PdCl4 is dissolved in deionized water to form a precursor solution, dried carboxylated carbon nanotubes are added, and the mixture is aged after stirring at room temperature, dried after vacuum evaporation, and then reduced in a hydrogen-nitrogen mixed gas atmosphere by heating to 300℃ at 5℃ / min and holding for 2h.

[0021] Compared with the prior art, the present invention has the following advantages:

[0022] (1) Traditional lignin conversion processes often employ intermittent solvothermal methods, which suffer from low energy transfer efficiency, limited mass transfer, and unstable interfaces, making it difficult to obtain high-purity monophenol products. This invention introduces a microwave enhancement strategy into a microscale continuous flow system, using a liquid-liquid-solid three-phase Pickering emulsion structure to couple extraction and catalytic reactions within the same process. By precisely controlling the reaction micro-regions, energy field distribution, and fluid dynamics, the energy and interface transfer bottlenecks of traditional lignin conversion reactions are overcome.

[0023] (2) In-situ extraction and continuous reaction are integrated to avoid losses from multi-step separation. In-situ extraction of lignin is achieved through a density-layered system of water-n-pentanol; the extract can be directly introduced into the downstream hydrogenation and depolymerization reaction without intermediate separation or solvent replacement; the continuous flow process reduces the risk of secondary condensation and coking of lignin.

[0024] (3) The microwave-microfluidic synergistic system of the present invention significantly improves energy penetration, heat transfer uniformity and interface stability, and realizes efficient and continuous conversion of lignin from solid biomass to lignin oil. Monophenolic substances account for 20-42% of the total product mass and have good purity and controllable distribution.

[0025] (4) The droplet microreactor design achieves uniform and controllable reaction. The droplet size is controlled by the capillary and density difference to form a stable three-phase emulsion; the series droplet microreactor structure ensures no backmixing between droplets and the reaction results are reproducible; at the same time, it is easy to scale up to a multi-channel parallel connection to achieve stable continuous production.

[0026] (5) Green low-pressure hydrogenation system with efficient and recyclable catalyst. Formic acid is used as the hydrogen source to avoid the need for external high-pressure hydrogen; Pd / carboxylated carbon nanotube catalyst has good dispersion performance and is easy to filter and recover; under microfluidic conditions, the catalyst cycle stability is improved and the Pd loss rate is reduced.

[0027] (6) Byproducts and solvents can be recovered and resources are fully utilized. The residue after the reaction can be washed to obtain cellulose, and xylan can be further separated by reverse precipitation to achieve full utilization of the components; p-toluenesulfonic acid and n-pentanol can be recovered and reused under reduced pressure; the product lignin oil is purified by distillation, and the system has a high degree of closed-loop circulation.

[0028] The method of this invention not only realizes the green and efficient utilization of lignin, but also provides a new technological path for the continuous transformation of complex multiphase biomass systems, which can be widely applied in the fields of fine chemicals and renewable energy. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the microwave-heated lignin extraction-microfluidic catalytic device of this application;

[0030] Figure 2 This is a schematic diagram of the microchannel reactor in this application;

[0031] Figure 3 The two-dimensional NMR spectrum and structural diagram of the lignin extracted in this application are shown.

[0032] Figure 4 The results of gas chromatography-mass spectrometry (GC-MS) of the lignin oil in this application and the distribution of major products are shown. Detailed Implementation

[0034] The present invention will be further illustrated below with reference to specific embodiments. These embodiments are implemented based on the technical solutions of the present invention, and it should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention.

[0035] The preparation method of the Pd / carboxylated carbon nanotube catalyst used in the following examples includes: dissolving 69 mg Na2PdCl4 in 25 mL of deionized water and stirring thoroughly to form a homogeneous precursor solution, wherein the mass fraction of Pd is controlled at 2.5%. Adding 1 g of pre-dried carboxylated carbon nanotubes (Xianfeng Nano, 133-86-4) to the solution and stirring continuously at room temperature for 2 h to ensure the solution fully wets and uniformly disperses on the support surface, followed by aging for 12 h to enhance the binding between metal ions and the support. After removing excess liquid by vacuum evaporation, the resulting wet sample is subjected to vacuum rotary evaporation at 60 °C to obtain a solid sample of Pd-loaded carboxylated carbon nanotubes. Subsequently, the dried sample is placed in the center of a quartz tube fixed-bed furnace, and a hydrogen-nitrogen mixed gas with a volume ratio of H2:N2=1:9 is introduced at a total flow rate of 100 mL / min. The temperature is increased to 300 °C at 5 °C / min and held for 2 h to complete the reduction reaction. After the reaction was completed, the sample was naturally cooled to room temperature, taken out and ground evenly under nitrogen protection, and stored in an inert gas or vacuum drying bottle. The resulting catalyst was designated as Pd / carboxylated carbon nanotubes (2.5%).

[0036] In the following examples, the component content of residual bamboo powder was determined by the standard NERL method. Bamboo powder was hydrolyzed with 72% sulfuric acid (25°C, 1 h) and 4% sulfuric acid (121°C, 2 h) to obtain monosaccharides, acid-insoluble lignin, and acid-soluble lignin. The monosaccharide content was determined by high-performance liquid chromatography (HPLC, using a Bio-Rad Aminex HPX-87H column). The chromatographic conditions were: 5 mmol / L sulfuric acid as the mobile phase, flow rate 0.6 mL / min, column temperature 55°C, and a differential detector at 40°C. The acid-soluble lignin content was determined by ultraviolet spectroscopy. The acid-insoluble residue was dried and weighed to obtain lignin. The calculation formula includes:

[0037] Y 纤维素保留 =m 残余竹粉的纤维素 / m 未处理竹粉的纤维素 ×100%;

[0038] Y 半纤维素保留 =m 残余竹粉的半纤维素 / m 未处理竹粉的半纤维素 ×100%;

[0039] Y 木质素脱除 =1−m 残余竹粉的木质素 / m 未处理竹粉的木质素 ×100%.

[0040] The monophenolic compounds (propyl side chain structure) in lignin oil were detected structurally using gas chromatography-mass spectrometry (GC-MS, HP-5 MS column) and quantified using gas chromatography-flame ionization detector (GC-FID, HP-5 column). The chromatographic conditions were: 1 μL sample, split ratio 20:1, injection temperature 250℃, and detection temperature 300℃. The column temperature program was set as follows: 50℃ for 3 min, then increased to 280℃ at a rate of 8℃ / min and held at 280℃ for 5 min. The monophenol content in lignin oil was calculated using the following formula:

[0041] Y 单酚得率 =m 各单酚质量总和 / m 加入竹粉中木质素总质量 .

[0042] Figure 1 and Figure 2 The flowchart of the microwave-heated lignin extraction-microfluidic catalysis process of this application includes the following steps:

[0043] (1) Under microwave heating conditions, a density-layered extraction system was formed using water as the reaction solvent, mannitol as the lignin protectant, n-pentanol as the extractant, and p-toluenesulfonic acid as both an acid catalyst and a surfactant. The lignin fibers underwent in-situ extraction in the microwave field, and the lignin, after extraction through the n-pentanol layer, was output through an independent outlet and entered the downstream catalytic reaction module. Precise matching of mass transfer rate and reaction kinetics was achieved by controlling the capillary inner diameter (0.12~2 mm) and the n-pentanol flow rate; the water-to-lignin mass ratio was 1:10, the p-toluenesulfonic acid concentration was 20%, the mannitol concentration was 5%, and the n-pentanol mass flow rate was 2~5 mL / min. The length-to-diameter ratio of the reaction vessel used for n-pentanol extraction was 3:1.

[0044] By restricting droplet size and flow rate through capillaries, a stable n-pentanol-water-solid three-phase emulsion structure is formed driven by density difference. The upper-stage extract and the lower-stage catalyst are combined in a series of microfluidic channels to construct a droplet-type microreactor, realizing a continuous process from lignin extraction to conversion.

[0045] (2) The catalytic reaction module uses carbon nanotubes loaded with palladium nanoparticles (Pd / carboxylated carbon nanotubes) as both a catalyst and a nano-solid surfactant to achieve microwave radiation reaction in a microchannel reactor. Formic acid aqueous solution and Pd / carboxylated carbon nanotube blended fluid and n-pentanol-lignin mixed flow form Pickering emulsion, and the catalyst is fixed near the phase interface, and the reaction proceeds efficiently at the water-pentanol phase interface.

[0046] The microchannel reactor employs a heart-shaped channel structure containing 24 independent units. Each channel has a diameter of 1 mm and an effective volume of 2 mL. Its dual-inlet, single-outlet feed design achieves strong mixing and stable emulsification. The chip is sealed by clamping it between PTFE and a PEEK plate, and a back pressure system maintains a stable flow rate of 0.1–0.5 MPa, ensuring continuous feeding and smooth multi-stage reactions.

[0047] Example 1

[0048] (1) Add 5g of dried bamboo powder, water, and p-toluenesulfonic acid to the reaction tube at a mass ratio of 1:5:5, and stir thoroughly to ensure that the bamboo powder is completely wetted and in a flowable state. Then add 0.25g of mannitol. Place the reaction tube in the microwave reaction chamber and continuously pump n-pentanol at a flow rate of 2mL / min; connect a 0.12mm inner diameter PEEK tube to the end of the pump outlet as an adjustable outlet channel, and stabilize the pressure of the reaction system at 0.5MPa by adjusting the back pressure valve. n-Pentanol flows out continuously from the liquid outlet above, and the flow rate remains constant. Turn on microwave heating and continuously extract lignin at 100℃. During the reaction, 95% of the lignin in the bamboo powder is extracted and transported to the microchannel reactor with the continuously flowing n-pentanol phase (n-pentanol-lignin mixed flow).

[0049] (2) The microchannel reactor adopts a two-inlet-one-outlet structure (see Figure 2 In addition to the n-pentanol-lignin mixed flow, another flow path was a catalyst-hydrogen source aqueous suspension. The preparation method was as follows: 0.25 g formic acid and 0.25 g Pd / carboxylated carbon nanotubes (2.5%) were added sequentially to 50 mL of deionized water, followed by ultrasonic treatment to form a stable suspension, which served as the aqueous reactant. This suspension was pumped in by a high-pressure syringe pump at a flow rate of 1 mL / min, and combined with the upstream n-pentanol-lignin mixed flow (flow rate 2 mL / min) into the microchannel reactor. Under the condition of a maximum microwave output power of 250 W, a system back pressure maintained at 0.5 MPa, and a reaction temperature maintained at 180 °C, the continuous hydrogenation and depolymerization reaction of lignin was achieved. After the reaction, the system was cooled to room temperature and naturally separated into liquid and liquid phases. The Pd / carboxylated carbon nanotube catalyst was recovered by filtration; the organic phase was distilled under reduced pressure to obtain n-pentanol and lignin oil products. The reactor setup is shown in [link to reactor description]. Figure 1 .

[0050] (3) After 20 min, the lignin in step (1) was completely extracted. The remaining solid was washed to obtain cellulose with a purity of 95%. The reaction solution was back-precipitated with methanol to separate 52% xylan powder. After filtration and collection of xylan, the mother liquor was evaporated under reduced pressure to recover p-toluenesulfonic acid. Before entering the microchannel reactor, the structure of the high molecular weight lignin in the n-pentanol-lignin stream was characterized. Its two-dimensional HSQC NMR spectrum is shown in [reference needed]. Figure 3 The monophenolic compounds in the obtained lignin oil mainly contain propyl side chain structures ( Figure 4 The total yield of monophenolic products was 37%, with propionic acid being the main product: guaiacol unit yield was 4%, and 2,6-dimethoxyphenol unit yield was 32%.

[0051] Example 2

[0052] (1) Add 5g of dried bamboo powder, water, and p-toluenesulfonic acid to the reaction tube at a mass ratio of 1:5:5, and stir thoroughly to ensure that the bamboo powder is completely wetted and in a flowable state. Then add 0.25g of mannitol. Place the reaction tube in the microwave reaction chamber and continuously pump n-pentanol at a flow rate of 2mL / min; connect a 0.12mm inner diameter PEEK tube to the end of the pump outlet as an adjustable outlet channel, and stabilize the pressure of the reaction system at 0.5MPa by adjusting the back pressure valve. n-Pentanol flows out continuously from the liquid outlet above, and the flow rate remains constant. Turn on microwave heating and continuously extract lignin at 80℃. During the reaction, 83% of the lignin in the bamboo powder is extracted and transported to the microchannel reactor with the continuously flowing n-pentanol phase (n-pentanol-lignin mixed flow).

[0053] (2) Another flow path is a catalyst-hydrogen source aqueous suspension. Its preparation method is as follows: 0.25 g formic acid and 0.25 g Pd / carboxylated carbon nanotubes (2.5%) are added sequentially to 50 mL of deionized water. After ultrasonic treatment, a stable suspension is formed, which serves as the aqueous phase reactant. This suspension is pumped in by a high-pressure injection pump at a flow rate of 1 mL / min, and flows together with the upstream n-pentanol-lignin mixed flow (flow rate 2 mL / min) into the microchannel reactor. Under the condition of a maximum microwave output power of 250 W, the system back pressure is maintained at 0.5 MPa, and the reaction temperature is maintained at 180 °C to achieve continuous hydrogenation and depolymerization of lignin. After the reaction, the system is cooled to room temperature and naturally separates into liquid and liquid phases. The catalyst Pd / carboxylated carbon nanotubes can be recovered by filtration. The organic phase is then distilled under reduced pressure to obtain n-pentanol and lignin oil products.

[0054] (3) After 60 min, lignin was completely extracted. The remaining solid was washed to obtain cellulose with a purity of 95%. The reaction solution was back-precipitated with methanol to separate 68% xylan powder. After filtration and collection of xylan, p-toluenesulfonic acid was recovered by vacuum evaporation of the mother liquor. The total yield of monophenol products was 42%, of which propenol was the main product: 4-propylguaiacol yielded 5%, and 4-propyl-2,6-dimethoxyphenol yielded 37%.

[0055] Example 3

[0056] (1) Add 5g of dried bamboo powder, water, and p-toluenesulfonic acid to the reaction tube at a mass ratio of 1:5:5, and stir thoroughly to ensure that the bamboo powder is completely wetted and in a flowable state. Then add 0.25g of mannitol. Place the reaction tube in the microwave reaction chamber and continuously pump n-pentanol at a flow rate of 2mL / min; connect a 0.12mm inner diameter PEEK tube to the end of the pump outlet as an adjustable outlet channel, and stabilize the pressure of the reaction system at 0.5MPa by adjusting the back pressure valve. n-Pentanol flows out continuously from the liquid outlet above, and the flow rate remains constant. Turn on microwave heating and continuously extract lignin at 120℃. During the reaction, 98% of the lignin in the bamboo powder is extracted and transported to the microchannel reactor with the continuously flowing n-pentanol phase (n-pentanol-lignin mixed flow).

[0057] (2) Another flow path is a catalyst-hydrogen source aqueous suspension. Its preparation method is as follows: 0.25 g formic acid and 0.25 g Pd / carboxylated carbon nanotubes (2.5%) are added sequentially to 50 mL of deionized water. After ultrasonic treatment, a stable suspension is formed, which serves as the aqueous phase reactant. This suspension is pumped in by a high-pressure injection pump at a flow rate of 1 mL / min, and flows together with the upstream n-pentanol-lignin mixed flow (flow rate 2 mL / min) into the microchannel reactor. Under the condition of a maximum microwave output power of 250 W, the system back pressure is maintained at 0.5 MPa, and the reaction temperature is maintained at 180 °C to achieve continuous hydrogenation and depolymerization of lignin. After the reaction, the system is cooled to room temperature and naturally separates into liquid and liquid phases. The catalyst Pd / carboxylated carbon nanotubes can be recovered by filtration. The organic phase is then distilled under reduced pressure to obtain n-pentanol and lignin oil products.

[0058] (3) After 5 minutes, lignin was completely extracted. The remaining solid was washed to obtain cellulose with a purity of 95%. The reaction solution was back-precipitated with methanol to separate 18% xylan powder. After filtration and collection of xylan, p-toluenesulfonic acid was recovered by vacuum evaporation of the mother liquor. The total yield of monophenol products was 12%, the yield of 4-propylguaiacol was 2.1%, and the yield of 4-propyl-2,6-dimethoxyphenol was 9.9%.

[0059] Example 4

[0060] (1) Add 5g of dried bamboo powder, water, and p-toluenesulfonic acid to the reaction tube at a mass ratio of 1:5:5, and stir thoroughly to ensure that the bamboo powder is completely wetted and in a flowable state. Then add 0.25g of mannitol. Place the reaction tube in the microwave reaction chamber and continuously pump n-pentanol at a flow rate of 2mL / min; connect a 2mm inner diameter PEEK tube to the end of the pump outlet as an adjustable outlet channel, and stabilize the pressure of the reaction system at 0.5MPa by adjusting the back pressure valve. n-Pentanol flows out continuously from the liquid outlet above, and the flow rate remains constant. Turn on microwave heating and continuously extract lignin at 100℃. During the reaction, 90% of the lignin in the bamboo powder is extracted and transported to the microchannel reactor with the continuously flowing n-pentanol phase (n-pentanol-lignin mixed flow).

[0061] (2) Another flow path is a catalyst-hydrogen source aqueous suspension. Its preparation method is as follows: 0.25 g formic acid and 0.25 g Pd / carboxylated carbon nanotubes (2.5%) are added sequentially to 50 mL of deionized water. After ultrasonic treatment, a stable suspension is formed, which serves as the aqueous phase reactant. This suspension is pumped in by a high-pressure injection pump at a flow rate of 1 mL / min, and flows together with the upstream n-pentanol-lignin mixed flow (flow rate 2 mL / min) into the microchannel reactor. Under the condition of a maximum microwave output power of 250 W, the system back pressure is maintained at 0.5 MPa, and the reaction temperature is maintained at 180 °C to achieve continuous hydrogenation and depolymerization of lignin. After the reaction, the system is cooled to room temperature and naturally separates into liquid and liquid phases. The catalyst Pd / carboxylated carbon nanotubes can be recovered by filtration. The organic phase is then distilled under reduced pressure to obtain n-pentanol and lignin oil products.

[0062] (3) After 20 min, lignin was completely extracted. The remaining solid was washed to obtain cellulose with a purity of 95%. The reaction solution was back-precipitated with methanol to separate 55% xylan powder. After filtration and collection of xylan, p-toluenesulfonic acid was recovered by vacuum evaporation of the mother liquor. The overall conversion rate of monophenol products was 22%, of which propenol was the main product: the yield of 4-propylguaiacol was 3.4%, and the yield of 4-propyl2,6-dimethoxyphenol was 18.2%.

[0063] Example 5

[0064] (1) Add 5g of dried bamboo powder, water, and p-toluenesulfonic acid to the reaction tube at a mass ratio of 1:5:5, and stir thoroughly to ensure that the bamboo powder is completely wetted and in a flowable state. Then add 0.25g of mannitol. Place the reaction tube in the microwave reaction chamber and continuously pump n-pentanol at a flow rate of 1mL / min; connect a 0.12mm inner diameter PEEK tube to the end of the pump outlet as an adjustable outlet channel, and stabilize the pressure of the reaction system at 0.5MPa by adjusting the back pressure valve. n-Pentanol flows out continuously from the liquid outlet above, and the flow rate remains constant. Turn on microwave heating and continuously extract lignin at 100℃. During the reaction, 95% of the lignin in the bamboo powder is extracted and transported to the microchannel reactor with the continuously flowing n-pentanol phase (n-pentanol-lignin mixed flow).

[0065] (2) Another flow path is a catalyst-hydrogen source aqueous suspension. Its preparation method is as follows: 0.25 g formic acid and 0.25 g Pd / carboxylated carbon nanotubes (2.5%) are added sequentially to 50 mL of deionized water. After ultrasonic treatment, a stable suspension is formed, which serves as the aqueous phase reactant. This suspension is pumped in by a high-pressure injection pump at a flow rate of 0.5 mL / min, and flows together with the upstream n-pentanol-lignin mixed flow (flow rate 2 mL / min) into the microchannel reactor. Under the condition of a maximum microwave output power of 250 W and a system back pressure maintained at 0.5 MPa, the reaction temperature is maintained at 180 °C to achieve continuous hydrogenation and depolymerization of lignin. After the reaction, the system is cooled to room temperature and naturally separates into liquid and liquid phases. The catalyst Pd / carboxylated carbon nanotubes can be recovered by filtration. The organic phase is then distilled under reduced pressure to obtain n-pentanol and lignin oil products.

[0066] (3) After 20 min, lignin was completely extracted. The remaining solid was washed to obtain cellulose with a purity of 95%. The reaction solution was back-precipitated with methanol to separate 55% xylan powder. After filtration and collection of xylan, p-toluenesulfonic acid was recovered by vacuum evaporation of the mother liquor. The overall conversion rate of monophenol products was 27%, of which propenol was the main product: the yield of 4-propylguaiacol was 4.2%, and the yield of 4-propyl2,6-dimethoxyphenol was 22.3%.

[0067] Comparative Example 1

[0068] Weigh 5g of bamboo powder (40-60 mesh) and 0.5g of Pd / C (5% Pd loading, purchased from Aladdin), add 45mL of methanol and 5mL of deionized water to a high-pressure reactor. After purging the reactor three times with nitrogen, introduce hydrogen to bring the system pressure to 3MPa (the reaction pressure was 6MPa). Heat to 220℃ with stirring at 700rpm and react for 3 hours. After the reaction, allow to cool naturally to room temperature, remove the reaction solution, and filter to remove lignocellulose residue and catalyst. Evaporate the filtrate under reduced pressure, then perform three liquid-liquid extractions with dichloromethane and water (reaction solution:water:dichloromethane, volume ratio 1:1:2). Combine the resulting organic layers, dry with anhydrous sodium sulfate, and filter. Finally, remove dichloromethane by rotary evaporation under reduced pressure to obtain a brownish-brown lignin oil product. The lignin removal rate was 69.7%, and the lignin monophenol yield was 46.6%. The remaining residue had a cellulose retention rate of 89.4% and a hemicellulose retention rate of 56.1%.

[0069] Comparative Example 2

[0070] Weigh 5g of bamboo powder (40-60 mesh) and 0.5g of Pd / C (5% Pd loading, purchased from Aladdin), add 25mL of ethanol and 25mL of deionized water to a high-pressure reactor. After purging the reactor three times with nitrogen, introduce hydrogen to bring the system pressure to 3MPa (the reaction pressure was 3MPa). Heat to 210℃ with stirring at 700rpm and react for 2 hours. After the reaction, allow to cool naturally to room temperature, remove the reaction solution, and filter to remove lignocellulose residue and catalyst. Evaporate the filtrate under reduced pressure, then perform three liquid-liquid extractions with dichloromethane and water (reaction solution:water:dichloromethane, volume ratio 1:1:2). Combine the resulting organic layers, dry with anhydrous sodium sulfate, and filter. Finally, remove dichloromethane by rotary evaporation under reduced pressure to obtain a brownish-brown lignin oil product. The lignin removal rate was 75.3%, and the lignin monophenol yield was 26.5%. The remaining residue showed a cellulose retention rate of 75.3% and a hemicellulose retention rate of 2%.

[0071] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing lignin oil by continuous hydrogenation and depolymerization of lignin coupled with microwave and microfluidics, characterized in that: Includes the following steps: 1) Water, p-toluenesulfonic acid, and mannitol are added to the wood fiber raw material and mixed. Then, n-pentanol is added through a capillary tube to form a density-layered extraction system. The wood fiber is extracted in situ in a microwave field. The lignin is extracted through the n-pentanol layer and output as a n-pentanol-lignin mixed stream. The size and flow rate of the n-pentanol droplets are restricted by the capillary tube to drive the formation of a stable n-pentanol-water two-phase mass transfer interface driven by density difference. After natural phase separation, the water is separated from the top of the reactor. 2) Water, formic acid, and Pd / carboxylated carbon nanotube catalyst are mixed and stirred to form an aqueous suspension. The aqueous suspension is then combined with the n-pentanol-lignin mixed flow obtained in step 1) in a series of microfluidic channels and introduced into a channel-type microreactor. Microwave radiation reaction is carried out in the channel-type microreactor. After the reaction system is cooled, the liquid phase is separated, the catalyst is recovered by filtration, and the organic phase is distilled under reduced pressure to obtain n-pentanol and lignin oil products. 3) After the lignin extraction in step 1) is completed, solid-liquid separation is performed. The remaining solid is cellulose. The reaction solution is separated into xylan powder by methanol back precipitation. The mother liquor is evaporated under reduced pressure to recover p-toluenesulfonic acid.

2. The method for preparing lignin oil by continuous hydrogenation and depolymerization of lignin coupled with microwave and microfluidics according to claim 1, characterized in that: In step 1), the capillary inner diameter is 0.12~2mm, and the mass flow rate of n-pentanol is 1~5mL / min.

3. The method for preparing lignin oil by continuous hydrogenation and depolymerization of lignin coupled with microwave and microfluidics according to claim 1, characterized in that: In step 1), the mass ratio of wood fiber raw material, water and p-toluenesulfonic acid is 1:5:

5.

4. The method for preparing lignin oil by continuous hydrogenation and depolymerization of lignin coupled with microwave and microfluidics according to claim 1, characterized in that: In step 1), the mass ratio of lignocellulose raw material to mannitol is 1:0.

05.

5. The method for preparing lignin oil by continuous hydrogenation and depolymerization of lignin coupled with microwave and microfluidics according to claim 1, characterized in that: In step 1), the microwave heating temperature is 80~120℃, the reaction time is 5~60min, and the back pressure of the reaction system is adjusted to 0.5MPa.

6. The method for preparing lignin oil by continuous hydrogenation and depolymerization of lignin coupled with microwave and microfluidics according to claim 1, characterized in that: In step 1), the length-to-diameter ratio of the reaction vessel used for n-pentanol extraction is 3:

1.

7. The method for preparing lignin oil by continuous hydrogenation and depolymerization of lignin coupled with microwave and microfluidics according to claim 1, characterized in that: In step 2), the channel-type microreactor adopts a heart-shaped channel structure, containing 24 independent reaction units. The channel diameter is 1 mm and the effective volume is 2 mL. It adopts a dual-inlet and one-outlet feeding design. The chip is clamped and sealed by polytetrafluoroethylene and PEEK plate, and the back pressure of the reaction system is maintained at 0.1~0.5 MPa.

8. The method for preparing lignin oil by continuous hydrogenation and depolymerization of lignin coupled with microwave and microfluidics according to claim 1, characterized in that: In step 2), formic acid accounts for 5% of the mass of the aqueous phase, and the amount of Pd / carboxylated carbon nanotube catalyst added is 5% of the mass of the aqueous phase. The emulsified and dispersed liquid through the strong mixing effect of the central channel of the channel-type microreactor forms a stable suspension.

9. The method for preparing lignin oil by continuous hydrogenation and depolymerization of lignin coupled with microwave and microfluidics according to claim 1, characterized in that: In step 2), the flow rate of the n-pentanol-lignin mixed stream is 1-2 mL / min, and the flow rate of the aqueous suspension is 0.5-1 mL / min. The two are combined in the microwave reactor to carry out the hydrogenation and depolymerization reaction.

10. The method for preparing lignin oil by continuous hydrogenation and depolymerization of lignin coupled with microwave and microfluidics according to claim 1, characterized in that: In step 2), the preparation method of Pd / carboxylated carbon nanotube catalyst is as follows: Na2PdCl4 is dissolved in deionized water to form a precursor solution, dried carboxylated carbon nanotubes are added, and the mixture is aged after stirring at room temperature, dried after vacuum evaporation, and then reduced in a hydrogen-nitrogen mixed gas atmosphere by heating to 300℃ at 5℃ / min and holding for 2h.

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