Photocatalytic flow method for lignin fragmentation

Abstract

The invention relates to a photocatalytic flow method for the selective fragmentation of lignin, comprising: (i) immobilising a photocatalyst on a support forming a bed, and introducing the support with the immobilised photocatalyst into a column that is transparent to radiation; (ii) and continuously circulating a lignin solution through the column packed with the bed, under illumination, to cause the fragmentation of the lignin, yielding vanillin and syringaldehyde, and such that the method is carried out continuously.

Description

[0001] Photocatalytic flow method for lignin fragmentation

[0002] DESCRIPTION

[0003] FIELD OF INVENTION

[0004] The present invention falls within the general field of photochemistry, and in particular, relates to a device and a process for obtaining high value-added petrochemical compounds (mainly vanillin and singaldehyde) from lignin present in plant biomass.

[0005] STATE OF THE ART

[0006] Currently, around 95% of industrially used organic compounds originate from petroleum. Therefore, discovering alternative and renewable sources of these petrochemical compounds is crucial for ensuring a sustainable economy. Lignocellulosic biomass contained in agricultural and forestry waste has become established as a potential source of petrochemicals for two main reasons: (i) its abundance, as it is considered the largest source of carbon that does not compete with food reserves, and (ii) its chemical composition, consisting of a carbohydrate fraction densely packed with a biopolymer, which constitutes the lignin fraction. Currently, biorefineries are able to process the carbohydrates to obtain biofuels, while the lignin is discarded and used as a lubricant or low-grade fuel.However, lignin, given its composition rich in aromatic units, could become a source of high-value aromatic compounds, provided efficient valorization strategies are available. Therefore, developing new technologies to extract the aromatic compounds contained in lignin is crucial for expanding the family of bio-petrochemical compounds and simultaneously improving the competitiveness of biorefineries in the market by maximizing the utilization of plant biomass.

[0007] Lignin is a very complex material, composed of aromatic subunits with oxygenated groups that link together to form chains. However, there is a wide variety of these bonds that connect the subunits. When lignin valorization methods are used, the aim is to break as many of these bonds as possible to release the monomers. However, no method is 100% effective. The result is the production of monomers, as well as a distribution of small oligomers containing between 2 and 4 linked aromatic units.

[0008] At the end of the process, methodologies can be used to isolate the monomers and oligomers that can be used for different applications.

[0009] Currently, the most widespread technology for lignin depolymerization involves thermocatalysis, although other procedures based on enzymatic catalysis or basic catalysis have been described. Briefly, this technology allows for the chemical reactions necessary to fragment lignin. Its main drawback is its lack of economic competitiveness. The need to operate under high temperature and pressure conditions, in the presence of catalysts based on noble metals and harmful or hazardous compounds, increases investment and operating costs. Furthermore, such extreme reaction conditions favor undesirable reactions, such as hydrogenation, hydrodeoxygenation, and carbon-carbon coupling processes, among others.

[0010] Alternativamente, se ha demostrado que la despolimerización de la lignina se puede llevar a cabo empleando energía solar por medio de fotocatalizadores (Nat. Catal. 2018, 1 , 772-780; Solar energy-driven lignin-first approach to full utilization of lignocellulosic biomass under mild conditions', Xuejiao Wu, Xueting Fan, Shunji Xie, Jinchi Lin, Jun Cheng, Qinghong Zhang, Liangyi Chen, Ye Wang; Sci. Bull. 2019, 64, 1658-1666; Redox-neutral photocatalytic strategy for selective C-C bond cleavage of lignin and lignin models via PCET process; Yinling Wang, Yue Liu, Jianghua He, Yuetao Zhang; Chem. Sci. 2015, 6, 7130-7142; Selective photocatalytic C-C bond cleavage under ambient conditions with earth abundant vanadium complexes', Sarifuddin Gazi, Wilson Kwok Hung Ng, Rakesh Ganguly, Adhitya Mangala Putra Moeljadi, Hajime Hirao and Han Sen Soo).These photocatalytic molecules or nanoparticles are capable of generating, solely through light irradiation, highly reactive species that drive the redox processes involved in the selective fragmentation of lignin. Therefore, this technology allows for lignin valorization under ambient conditions, minimizing costs and unwanted reactions.

[0011] Vanillin and syringaldehyde stand out among the most attractive products of lignin fragmentation due to their wide range of applications. Both compounds are essential components in the formulation of pharmaceuticals, cosmetics, and fragrances. However, to date, no photocatalyst-based methodologies capable of producing these compounds have been reported.

[0012] In the article ACS Catal. 2019, 9, 9, 8443-8451 ; Ligand-Controlled Photocatalysis of CdS Quantum Dots for Lignin Valorization under Visible Light, Xuejiao Wu, Shunji Xie, Chenxi Liu, Cheng Zhou, Jinchi Lin, Jincan Kang, Qinghong Zhang, Zhaohui Wang, Ye Wang; the fragmentation of solubilized lignin by means of a photocatalyst based on CdS semiconductor nanoparticles that are dispersed in the lignin-containing solution itself is described.

[0013] US patent application 20220127217A1 describes a process for fragmenting lignin into its aromatic constituents by means of thermocatalytic oxidation. This process involves the use of noble metals as catalysts and operates at temperatures above 150°C and pressures above 30 bar.

[0014] Patent application WO2016126207A1 describes a vanadium-based photocatalyst for the fragmentation of C-C bonds under illumination. It mentions the possibility of its use in lignin fragmentation, although no data is provided.

[0015] Patent CN113275038B describes the use of a mixture of titanium and iron-containing oxides as a photocatalyst, along with hydrogen peroxide, for the fragmentation of lignin once dispersed in a liquid. Only fragmentation results of single molecules, using lignin as a model, are included.

[0016] Patent CN110002972 describes the procedure by which various photocatalysts based on noble metals such as iridium and ruthenium can be used to selectively fragment lignin under illumination.

[0017] Patent CN116328843 describes the use of a mixture of heterogeneous catalysts based on BiVCU and MIL-53(Fe) to carry out the selective fragmentation of lignin, although there is no detailed information on the results of its application.

[0018] All the aforementioned studies share the common characteristic that the photocatalyst is dissolved or dispersed in the liquid containing the lignin. Consequently, these systems operate in batch mode, meaning that after the reaction is complete, the reactor is stopped to isolate and recover the photocatalyst and to isolate and purify the products. Therefore, there is a need to establish a methodology that uses (i) low-cost, abundant, and non-toxic materials as photocatalysts, and (ii) photocatalytic reactors that operate in continuous flow to fragment the lignin.

[0019] The present invention allows the photocatalytic valorization of lignin in a continuous manner and eliminating the additional steps dedicated to the recovery of catalysts.

[0020] The method of the invention can be implemented in the emerging biorefinery sector, where the aim is to valorize lignocellulosic biomass. Currently, although strategies exist to valorize the carbohydrate fraction contained in lignocellulosic biomass, the lignin fraction is discarded due to the lack of a technology that allows for its controlled and economically competitive depolymerization.

[0021] To date, there are no examples in the literature of photocatalytic flow reactors for lignin valorization, and in fact there are only a few examples of photocatalytic systems that have been shown to be capable of fragmenting lignin (Wang, H.; Giardino, GJ; Chen, R.; Yang, C.; Niu, J.; Wang, D..Photocatalytic Depolymerization of Native Lignin toward Chemically Recyclable Polymer Networks. ACS Cent. Sci. 2023, 9, 1 , 48-55; Nat. Catal. 2018, 1 , 772-780; Solar energy-driven lignin-first approach to full utilization of lignocellulosic biomass under mild conditions', Xuejiao Wu, Xueting Fan, Shunji Xie, Jinchi Lin, Jun Cheng, Qinghong Zhang, Liangyi Chen, Ye Wang).Unlike previous studies that use inorganic salts or nanoparticles as photocatalysts, this invention uses photocatalytic molecules that can be easily modified to include specific functional groups to promote their covalent anchoring to a support without affecting their catalytic properties.

[0022] DESCRIPTION OF THE INVENTION

[0023] The present invention solves the problems described in the prior art by providing a continuous lignin fragmentation procedure that does not require system shutdown. The method is carried out in a continuous flow reactor in which a low-cost, non-toxic photocatalyst is immobilized on a support. The present invention relates to a method for the selective fragmentation of lignin by means of light, performed continuously.

[0024] The photocatalytic flow method for the selective fragmentation of lignin comprises:

[0025] (i) Immobilizing a photocatalyst on a support, and introducing the support with the immobilized photocatalyst (bed) into a radiation-transparent column,

[0026] (i) and continuously circulating a lignin solution through the column loaded with the bed, under illumination, to cause fragmentation of the lignin, obtaining vanillin and syringaldehyde, and such that the method is carried out continuously.

[0027] The reactor can be a fluidized bed reactor, or a fixed bed reactor, preferably a fluidized bed reactor.

[0028] The term “bed” refers to the support assembly with the photocatalyst immobilized. Furthermore, due to its characteristics and how this bed behaves when the system is in operation, it is called “fluidized.”

[0029] The expression “molecular photocatalyst” refers to the photocatalyst before it is immobilized on the support.

[0030] The molecular photocatalyst is modified or functionalized in a step prior to its immobilization on the support.

[0031] The photocatalyst is immobilized by first functionalizing the molecular photocatalyst with functional groups that have an affinity for the support. These groups can be, but are not limited to, silanes, halomethyls, amines, esters, amides, anhydrides, alcohols, or carboxylic acids. Other catalyst functionalizations and their subsequent immobilization on other supports can be performed using conventional methods.

[0032] The support can be composed of materials such as metal oxides in the form of nanoparticles, non-metal oxides in the form of nanoparticles, glass wool or quartz, quartz fiber or polymer resins.

[0033] Specific examples of oxides that can be used as a support include, but are not limited to, oxides such as Al₂O₃, ZrO₂, SnO₂, I₂Ch, Ga₂O₃, or mixtures thereof. Similarly, the support can be made of polymeric resins. Some specific examples of resins include, but are not limited to, resins selected from polymers or copolymers of vinyl compounds (e.g., styrene, vinyl acetate, acrylic ester, methacrylic ester, cellulose ester resins, Wang resins, and aminomethyl polystyrene resins).

[0034] In a preferred embodiment, step (i) is carried out using silicon oxide (SiO2) nanoparticles with sizes between 40 and 60 microns, as a support.

[0035] In the case of particular embodiments where oxides are used as a support, the functional groups to be introduced into the photocatalyst are silane groups or derivatives of carboxylic acids, such as amides, esters or anhydrides.

[0036] In the case of particular embodiments where resins are used as a support, the photocatalyst is modified by adding a reactive group, either by introducing a heteroatom such as oxygen (alcohol) or nitrogen (amine) that allows the reaction with an electrophilic resin (such as the brominated Wang resin), or by introducing a halomethyl group, which allows the reaction with a nucleophilic resin, such as the Wang resin.

[0037] The photocatalytic species are irreversibly anchored by covalent bonds to the support particles, achieving a high degree of coating.

[0038] The photocatalyst can be a compound such as an aromatic ketone, a derivative of such an aromatic ketone, sodium benzophenonadisulfonate, tetrabutyl ammonium decatungstate, sodium decatungstate, uranyl nitrate hexahydrate, or uranyl perchlorate.

[0039] The aromatic ketone may be, for example, 5,7,12,14-pentacentetrone, pentacene-6,13-dione, 9-fluorenone, benzophenone, aminobenzophenone, 4,4'-dimethoxybenzophenone, 4,4'-dichlorobenzophenone, acetophenone, dibenzosuberenone, 3,6-dimethoxy-9 / 7-thioxanthen-9-one; 9,10-phenanthrenoquinone, xanthone, thioxanthone, eosin Y, anthraquinone (AQ), 2-chloroanthraquinone, 2-tert-butylanthraquinone, 2-anthraquinonecarboxylic acid or sodium salt of anthraquinone-2-sulfonic acid.

[0040] Other types of photocatalysts can be semiconductor nanoparticles, such as cadmium sulfide, indium zinc sulfide, or cesium lead bromide nanoparticles. According to a preferred embodiment, the photocatalyst is anthraquinone immobilized by a covalent bond on a silica support. This prevents it from dispersing in the lignin-containing solution.

[0041] The solvent for obtaining the lignin solution can be selected from non-aqueous organic solvents. For example, it can be a polar organic solvent such as ethyl acetate, dichloromethane, chloroform, methanol, isopropanol, 1,1,1,3,3,3-hexafluoro-2-propanol, acetone, acetonitrile, α,α-dimethylformamide, N,N-dimethyllacetamide, N-methyl-2-pyrrolidone, tetrahydrofuran, α-terpineol, bromoform, methyl iodide, dichloroethane, trichloroethane, trichloroethylene, chlorobenzene, o-dichlorobenzene, fluorobenzene, iodobenzene, or chloronaphthalene, among others. The solvent can be a nonpolar organic solvent such as n-pentane, n-hexane, n-octane, cyclohexane, methylcyclohexane, cyclohexadiene, toluene, o-xylene, m-xylene, p-xylene, ethylbenzene or eumene, among others.

[0042] According to particular embodiments, in step (i), the support modified with the photocatalyst is introduced and packed into a radiation-transparent column, which can be, for example, a straight or spiral column.

[0043] Step (ii) may be preceded by a lignin extraction and solubilization step, which can be carried out using conventional methods, such as the organosolv, Kraft, lignosulfate, or any other method that allows the solubilization of lignin in organic solvents, among those mentioned above. The organosolv method is preferred for obtaining the lignin.

[0044] The selective lignin fragmentation method can take place in a single step of dissolving the fluid containing the lignin solubilized by the reactor, or with multiple steps (recirculation), which would involve connecting the product reservoir elements (4) and the lignin reservoir (1) in Figure 1.

[0045] The residence time of the lignin in the reactor is adjusted to achieve optimal fragmentation in a single step or in multiple steps of lignin dissolution. The residence time can range from 1 to 75 hours, for example, from 20 to 26 hours. In one particular embodiment, the residence time is between 23 and 25 hours.

[0046] The lignin solution flows through the reactor bed where the support with the immobilized photocatalyst is located, and under illumination, the photogenerated charges on the photocatalyst carry out the selective fragmentation of the lignin. The method of the invention results in low molecular weight oxygenated aromatic molecules, namely monomers (mainly vanillin and syringaldehyde) and oligomers of 2, 3, or 4 linked aromatic units with oxygenated groups.

[0047] In a preferred embodiment, the selective fragmentation of lignin is achieved in a single column step - the column being part of the reactor - using non-aqueous base solvents.

[0048] The invention further relates to a system comprising a fluidized bed photocatalytic reactor configured to operate continuously, wherein the photocatalyst units are immobilized on a bed through which a lignin solution is flowed.

[0049] The immobilization of photocatalytic species makes it possible to avoid the recovery and recycling processes of the photocatalyst after the lignin valorization process.

[0050] Lignin fragmentation occurs in the column that is part of the reactor, which contains the bed made up of the support with the immobilized photocatalyst, when the lignin solution circulates through the bed and the column is under illumination.

[0051] The photocatalytic system for the selective fragmentation of lignin according to the method defined above comprises:

[0052] - a fluidized bed photocatalytic reactor comprising a support on which a photocatalyst is immobilized, forming a bed,

[0053] - a lignin solution,

[0054] - a flow system to control the flow rate of the lignin solution,

[0055] - a lighting system that allows homogeneous irradiation of the reactor.

[0056] In a preferred embodiment, the photocatalytic system for the selective fragmentation of lignin according to the method defined above comprises:

[0057] - a reservoir containing the starting solubilized lignin;

[0058] - a flow system comprising: a hydraulic element for pumping the lignin and a conduit for transferring the lignin solution to and from the reactor; - a photocatalytic fluidized bed reactor comprising: a column of light-transparent material filled with the bed, filters such as porous plates, at the inlet and outlet of the reactor;

[0059] - a lighting system that allows homogeneous irradiation of the reactor;

[0060] - a reservoir where the reaction product is collected.

[0061] Porous plates are an alternative for performing the function of filters, which prevent the loss of the bed by entrainment, but allow the entry and exit of solubilized lignin.

[0062] The photocatalytic system for the selective fragmentation of lignin according to the method defined above comprises, according to a particular embodiment shown in Figure 1:

[0063] - a reservoir (1) containing the starting solubilized lignin;

[0064] - a flow system (2) comprising a hydraulic-type element for the displacement of the lignin solution and a conduit that transfers the lignin solution to, and from, the reactor;

[0065] - a fluidized bed photocatalytic reactor (3) comprising: a column (3.a) of material transparent to the working light that is filled with the bed (3.b), porous plates (3.c) at the inlet and outlet of the reactor,

[0066] - a lighting system (3.d) that allows homogeneous irradiation of the reactor;

[0067] - a reservoir (4) where the reaction product is collected.

[0068] The flow system (2 in Figure 1) comprises a hydraulic element that allows fluid to move through the reactor. In specific embodiments, the flow system consists of a peristaltic pump that controls the flow rate of the lignin solution and its recirculation. Other options for the hydraulic element include the use of syringe or diaphragm pumps to propel the lignin-containing fluid.

[0069] The column and bed (3.ay and 3.b in the example implementation of Figure 1) can have varying lengths and diameters. The length and cross-sectional diameter of the photocatalytic reactor are adjusted to maximize irradiation of the photocatalyst.

[0070] The fluidized bed (3.b in Figure 1) can:

[0071] (1) consist solely of a support with an immobilized photocatalyst; or (2) consist of the support with the immobilized photocatalyst and transparent spheres (glass or quartz) to minimize the internal volume without affecting the transmission of irradiation.

[0072] In the case referred to as (2) (previous paragraph) the transparent spheres have the function of creating a space (devoid of catalyst and support) in the column to minimize the expenditure of material.

[0073] The porous plates (3.c in the example in Figure 1) are made of conventional ceramic or polymeric material for these components, and have a pore size smaller than the size of the particles that make up the support.

[0074] The lighting system (3.d in the embodiment shown in Figure 1) may comprise a monochromatic or polychromatic light source. The lighting is provided at an angle, preferably perpendicular, to the walls of the column, which are transparent to radiation and the flow of lignin solution. The lighting system uses a wavelength chosen to maximize absorption by the photocatalyst.

[0075] According to particular designs, the peristaltic pump is arranged in a way that connects it to the transparent column.

[0076] According to a particular embodiment, the system comprises:

[0077] - a fluidized bed photocatalytic reactor comprising a support on which a photocatalyst is immobilized, forming a bed,

[0078] - a lignin solution,

[0079] - a flow system to control the flow rate of the lignin solution,

[0080] - a lighting system that allows homogeneous irradiation of the reactor.

[0081] According to a preferred embodiment, the system comprises:

[0082] - a solution of lignin (1) in acetonitrile,

[0083] - a peristaltic pump (2) to move the lignin solution through the reactor and control the flow rate,

[0084] - a photocatalytic reactor (3) comprising a radiation-transparent column containing a bed of silicon oxide particles with the anthraquinone photocatalyst supported, such that when the lignin solution is flowed through it, the bed becomes fluidized and

[0085] - a reservoir (4) for the lignin solution at the reactor outlet. Fragmentation studies using lignin models demonstrated that the fluidized bed reactor outperformed conventional reactors in conversion and fragmentation efficiency. Studies on natural lignin revealed vanillin and syringaldehyde as the major products, in varying quantities depending on the lignin type, achieving vanillin extraction yields close to 8.0% w / w.

[0086] BRIEF DESCRIPTION OF THE FIGURES

[0087] Figure 1.- (a) Schematic of a preferred embodiment of the photocatalytic bed system including its components: reservoir (1) containing the solubilized lignin solution, (2) the peristaltic pump (defining the flow system), photocatalytic reactor (3) comprising a column (3.a) with a radiation-transparent wall filled with a bed (3.b) of SiO2 nanoparticles on which the anthraquinone photocatalyst is immobilized, and porous plates (3.c) through which the lignin solution can pass but not the bed. The reactor also includes light sources (3.d) (constituting the lighting system). Finally, the solution exiting the reactor is collected in the product reservoir (4). The arrows placed on the conduit connecting (1) and (4) indicate the flow of the liquid containing lignin or derivatives of its photocatalytic fragmentation.The zigzag arrows next to the light source (3.d) indicate the directionality of the lighting provided by the lighting system.

[0088] EXAMPLES

[0089] Materials—KOH (99%) was purchased from Sigma-Aldrich. Ethyl acetate (>99.8%) was purchased from VWR Chemicals. Nitrobenzene (99.5+%) was purchased from TCI. 3-Ethoxy-4-hydroxybenzaldehyde (>98.0%) was purchased from Sigma-Aldrich. N,O-bis(trimethylsilyl)trifluoroacetamide (>99%) was purchased from Sigma-Aldrich. Millipore MQ water. Absolute ethanol (99.8+) was purchased from Fisher Chemicals. 1,4-Dioxane (>99.5%) was purchased from Thermo Scientific Chemicals. Acetone (>99.5%) was purchased from VWR Chemicals. HCl (37%, extra pure) was purchased from Thermo Scientific Chemicals. H₂SO₄ (95–98%) was purchased from Sigma. Brominated Wang resin was purchased from Iris Biotech. Tetrabutylammonium bromide (>98.0%) was purchased from Sigma-Aldrich. 1-Aminoanthraquinone (97%) was purchased from Thermo Scientific Chemicals. Tetrahydrofuran (>99.0%) was purchased from Sigma-Aldrich. Aminomethylated polystyrene (10-200 mesh) was purchased from Sigma-Aldrich. 2-Methyl-9,10-anthraquinone (>99.0%) was supplied by TCI EUROPE.N-Bromosuccinimide (99%) was purchased from Thermo Scientific Chemicals. Benzoyl peroxide (with 25% water) was purchased from Sigma-Aldrich. Benzene (>99.5%) was purchased from VWR Chemicals. Anhydrous magnesium sulfate (97%) was purchased from Thermo Scientific Chemicals. (3-Aminopropyl)trimethoxysilane (97%) was purchased from Sigma-Aldrich. Anthraquinone-2-carboxylic acid (98%) was purchased from Thermo Scientific Chemicals. Dichloromethane (99.8%) was purchased from Thermo Scientific Chemicals. N,N-dimethylformamide (99%) was purchased from Thermo Scientific Chemicals. Oxalyl chloride (98%) was purchased from Thermo Scientific Chemicals. Triethylamine (99%) was purchased from Thermo Scientific Chemicals.

[0090] Experimental methods

[0091] 1. Functionalization and immobilization of AQ

[0092] A preferred photocatalyst is AQ. Scheme 1 shows the AQ molecule, as well as the successive reactions (ac) used for the functionalization of anthraquinone with silane groups that react with the support (SÍÜ2) to produce immobilization.

[0093] Scheme 1 As shown in Scheme 1, the immobilization of AQ on the SiO2 support is carried out via silane groups introduced into the AQ molecule through a sequence of reactions. First, anthraquinone-2-carboxylic acid is treated with thionyl chloride under reflux conditions to produce the corresponding acid chloride (a). Subsequently, a nucleophilic substitution is performed using 3-(trimethoxysilyl)propylamine in toluene as a solvent and in the presence of triethylamine as a base to generate compound (b). Finally, the silane groups incorporated into the anthraquinone react with the hydroxyl groups present on the silica surface, resulting in the immobilization of the photocatalyst on the inorganic support (c).

[0094] As a specific example, starting with 2-methylanthraquinone, it was treated with β-bromo succinimide in the presence of benzoyl peroxide to perform bromination at the benzylic carbon. This compound was then reacted with aminomethyl polystyrene in the presence of a base, yielding 2-methylanthraquinone anchored to the resin.

[0095] According to further embodiments, 1-aminoanthraquinone can be reacted with brominated Wang resin by known methods, allowing anchoring through a position other than the anthraquinone ring.

[0096] 2. Immobilization of anthraquinone on polystyrene-derived resins

[0097] Procedure adapted from reference [1]: To a mixture of the commercial polymer (brominated Wang resin, 100 mg), tetrabutylammonium bromide (11 mg), KOH (8 mg), and 1-aminoanthraquinone (100 mg), 5 mL of tetrahydrofuran are added to form a dispersion under an argon atmosphere. The resulting mixture is heated at 70 °C for 72 h. After cooling to room temperature, the resulting powder is washed by centrifugation at 4500 rpm in methanol, ultrapure water, and methanol again. Finally, the powder is dried under vacuum. Scheme 2 shows the immobilization of 1-aminoanthraquinone on brominated Wang resin.

[0098] Scheme 2

[0099] THF: tetrahydrofuran TBAB: tetrabutylammonium bromide

[0100] PS: polystyrene

[0101] 3. Immobilization procedure using aminomethylpolystyrene

[0102] Alternatively, the procedure described in the previous section can be carried out using aminomethyl polystyrene instead of Wang's resin and 2-bromomethylanthraquinone instead of 1-aminoanthraquinone, using a procedure adapted from reference [2]. For this purpose, the 2-bromomethylanthraquinone molecule was prepared beforehand according to the following procedure: a mixture of 2-methylanthraquinone, N-bromosuccinimide (1.25 equivalents), and benzoyl peroxide (0.1 equivalents) was dissolved in benzene and heated under reflux for 16 h. Once the reaction was complete, the mixture was cooled to room temperature and washed twice with water. The organic phase was dried over magnesium sulfate, filtered, and concentrated under vacuum to obtain the desired compound. Scheme 3 shows the immobilization of 2-bromomethylanthraquinone on aminomethyl polystyrene.

[0103] AQCH2NH@PS

[0104] BPO: benzoyl peroxide

[0105] NBS: N-bromosuccinimide

[0106] Scheme 3

[0107] 4. Immobilization of AQ on glass wool:

[0108] Procedure adapted from reference [3]. To immobilize the catalyst on glass wool, the glass wool was initially pre-activated. This was done by immersing the glass wool in a piranha solution (3 parts concentrated sulfuric acid and 1 part 30% hydrogen peroxide) for 10 min. The material was then washed with distilled water for 5 min and dried at room temperature. A 3% solution of (3-aminopropyl)trimethoxysilane in ethanol was then added to the glass wool and stirred at room temperature for 2 h. The resulting material was washed twice, sonicated for 15 min in ethanol, and heated to 80 °C for 1 h to dry.Once the functionalized glass wool was obtained, the photocatalyst derivative was anchored by reacting it with 2-bromoethylanthraquinone as described in the previous section. Scheme 4 shows the immobilization of 2-bromomethylanthraquinone on glass wool:.

[0109] AQ@LV

[0110] Scheme 4

[0111] 5. Immobilization of AQ on SiO2 nanoparticles

[0112] Another catalyst support material was silica gel. For this, 2-anthraquinonecarboxylic acid (1 g, 3.97 mmol) was added to a Schlenk flask along with a magnetic stirrer. The flask was evacuated and refilled with argon three times. Then, dichloromethane (20 mL) and N,N-dimethylformamide (80 mL) were added. The resulting suspension was cooled to 0 °C, and oxalyl chloride (504 mL, 5.95 mmol) was added dropwise. After 10 minutes, the reaction mixture was allowed to warm to room temperature and stirred for 2 hours. At this point, all volatile components were removed from the mixture, leaving the corresponding 2-anthraquinonecarboxylic acid chloride as a residue. This compound was redissolved in 20 mL of dry dichloromethane under an argon atmosphere and (3-aminopropylthmethoxysilane (1.12 mL, 4.77 mmol) was added dropwise at 0 °C. After 10 minutes with magnetic stirring, triethylamine (1.1 mL, 7.94 mmol) and the mixture was allowed to warm to room temperature, maintaining magnetic stirring for 16 h. A saturated aqueous solution of sodium bicarbonate (20 mL) was then added and the mixture was extracted twice with 20 mL portions of dichloromethane. The combined organic phases were dried over magnesium sulfate, filtered, and concentrated under vacuum, yielding the desired compound without further purification.

[0113] Once the functionalized AQ was obtained, it was immobilized on silica using a procedure adapted from reference [4]. For this, the silica gel was first activated by suspending 20 g of it in 100 mL of distilled water and carefully adding 20 mL of concentrated hydrochloric acid. The resulting suspension was heated to 90 °C under vigorous stirring for 5 h. After cooling the mixture to room temperature, the activated silica was recovered by filtration, washing it with water and acetone, and drying it under vacuum at 80 °C. Finally, the photocatalyst was anchored to the silica as follows: 10 g of the activated silica were placed in a flask under argon, followed by the functionalized AQ (300 mg) and toluene (30 mL). The resulting suspension was heated to 100 °C for 72 h. After cooling to room temperature, the resulting material was filtered and washed with dichloromethane, ethyl acetate, methanol, and dichloromethane again.The supported catalyst was dried under vacuum at 50 °C. Scheme 1 shows the immobilization of 2-anthraquinonecarboxylic acid on silica gel.

[0114] 6. Immobilization of AQ on Al2O3 nanoparticles:

[0115] Following a procedure adapted from reference [5], 80 mL of a 0.25 M solution of 2-anthraquinonecarboxylic acid in acetone was shaken in the presence of 2 g of basic alumina in the dark for 16 h. The supported catalyst was recovered by centrifugation at 4500 rpm and washed with acetone three times. Finally, the resulting material was dried under vacuum for 16 h. Scheme 5 shows the immobilization of 2-anthraquinonecarboxylic acid on aluminum oxide:

[0116] AQ@AI 2O3

[0117] Scheme 5

[0118] 7. Lignin extraction from wood samples (dioxanosolv)

[0119] Two types of wood are used: beech and pine. Generally, 5 g of wood are added to 150 mL of a mixture containing HCl and dioxane in water (0.28 M HCl and 2% v / v water in dioxane) in a 250 mL round-bottom flask. The mixture is magnetically stirred and maintained at 80°C using a reflux system to facilitate solid-liquid extraction for 6 h. Afterward, the mixture is filtered through a 0.45 µm nylon filter and washed with dioxane. The solid, primarily composed of the carbohydrate fraction, is discarded, while the lignin-rich filtrate is concentrated by vacuum evaporation to a final volume of 10 mL, yielding lignin black liquor. The liquor is then dissolved in 10 mL of acetone, and the lignin in the black liquor is precipitated by adding this mixture to 200 mL of cold water.Finally, after keeping it under agitation for 1 h, the precipitate is filtered, washed and left to dry under vacuum for 24 h to obtain lignin dioxanosolv.

[0120] 8. Monomer yield by nitrobenzene oxidation method

[0121] The total amount of monomers present in lignin is determined by the alkaline oxidation method in nitrobenzene as reported in the article Chem, 2019, 5, 6, 1521-1536; Breaking the Limit of Lignin Monomer Production via Cleavage of Interunit Carbon-Carbon Linkages; L., L. Lin, X. Han, X. Si, X. Liu, Y. Guo, F. Lu, S. Rudic, S. F. Parker, S. Yang, Y. Wang DOI: 10.1016 / j.chempr.2019.03.007. For this, the material to be analyzed, i.e., the dioxanesolv lignin described in the previous section (0.040 g), is mixed with nitrobenzene (0.4 mL) and 7 mL of a 2 M aqueous NaOH solution. The mixture is heated to 170 °C in a sealed pressure tube for 2 h. The mixture is then cooled using an ice-water bath, and 1 mL of a freshly prepared solution of 3-ethoxy-4-hydroxybenzaldehyde (5 mM) in 0.1 M NaOH is added as an internal standard. The mixture is transferred to a separatory funnel and washed three times with 15 mL of dichloromethane.The resulting aqueous phase is acidified with 2 M HCl to a pH below 3.0 and then extracted twice with 20 mL of dichloromethane and once with 20 mL of diethyl ether. The combined organic phases are washed with water (20 mL), dried over magnesium sulfate, and filtered before concentrating the sample under reduced pressure. The residue is recovered using three 0.2 mL pyridine fractions and transferred to a gas chromatography vial. O-bis(trimethylsilyl)trifluoroacetamide (0.15 mL) is then added, the vial is sealed, and heated at 50 °C for 30 min to carry out the derivatization of the monomers. Finally, the sample is analyzed by GC-MS using equipment previously calibrated with an internal standard and authentic samples of the monomers to be analyzed.

[0122] 9. Operating conditions of photocatalytic reactors

[0123] Three types of configurations were used for the photocatalytic reactors. In the first, the photocatalyst (anthraquinone) is dispersed in the lignin solution and is designated AQ(batch). In the second configuration, silica support particles with the immobilized photocatalyst are dispersed in the lignin solution and are designated AQ@SiO2(batch). In the third configuration, the support (silica), with the immobilized photocatalyst, is used as the bed in a fluidized bed flow reactor and is designated AQ@SiO2(flow). Alternatively, the catalyst can be immobilized on alumina, AQ@Al2O3(flow), on aminomethyl polystyrene resin, AQCH2NH@PS(flow), on Wang resin, AQNH@PS(flow), or on glass wool, AQ@LV(flow).

[0124] Experiment AQ(batch). The lignin sample (50 mg) and the photocatalyst (5.0 mg) were suspended in acetonitrile in a 10 mL quartz vial transparent to radiation and illuminated with Everbeam LED lamps (50 W) emitting at a wavelength of 365 nm.

[0125] AQ@SiO2(batch) experiment: The lignin sample (50 mg) and supported photocatalyst (100 mg) were suspended in acetonitrile in a 10 mL quartz vial transparent to radiation and illuminated with Everbeam LED lamps (50 W) emitting at a wavelength of 365 nm.

[0126] Experiments AQ@SiO2(flow), AQ@Al2O3(flow), AQCH2NH@PS(flow), AQNH@PS(flow), AQ@LV(flow): For the design of the flow reactor, a Büchi peristaltic pump was used to transport the lignin solution (4 g L' 1 ) or lignin models (0.3 g L' 1The solution was dissolved in acetonitrile, flowed from the feed vessel to the photocatalytic reactor, and finally collected in the outlet vessel as depolymerized lignin or as fragments of the lignin models. The flow rate of the lignin solution or lignin models was maintained at 5 mL / min. -1 The reactor column (3.a in Figure 1) is composed of a Diba Omnifit™ EZ SolventPIus™ column with an internal diameter of 10 mm and a height of 15 cm. The column was filled with the bed (3.b in Figure 1), which consisted of the support containing the immobilized photocatalyst (1 g per 0.5 g of lignin to be depolymerized). The lighting system (3.d in Figure 1) consisted of Everbeam LED lamps (50 W) emitting at a wavelength of 365 nm. Porous porcelain plates with an average pore size of 2 microns were used (3.c in Figure 1).

[0127] In all cases, AQ was used as the photocatalyst. The reaction time was 24 h when lignin was used as the substrate, and irradiation was carried out with 365 nm LED lamps (50 W). Product quantification was performed by gas chromatography-mass spectrometry (GC-MS). Note that the products were derivatized (silylated) with A / ,O-bis(th-methylsilyl)th-fluoroacetamide before quantification to facilitate identification. When lignin models were used as the substrate, the reaction took place over 16 h, using 200 mg of catalyst support per 0.1 mmol of lignin model.

[0128] 10. Analysis of reaction products

[0129] Once the photochemical fragmentation of lignin was complete in all experiments, the peristaltic pump was fed with pure acetonitrile to flush all the starting material and products out of the catalytic bed, leaving only the supported catalyst inside the reactor. The resulting solution in acetonitrile was concentrated under vacuum to produce the reaction stock. This mixture was weighed, and 40 mg samples were taken for analysis by single quantum heteronuclear coherence (HSQC) magnetic resonance. Another 40 mg sample was also taken and a known quantity of ethylvanillin was added as an internal standard. This sample was dissolved in 1 mL of pyridine, and 0.15 mL of N,O-bis(trimethylsilyl)trifluoroacetamide was added. The mixture was heated to 50 °C for 30 min and analyzed by gas chromatography-mass spectrometry (Agilent HP-5MS capillary column (60 m x 0.25 mm x 0.25 mm, USA) and the Agilent HP5975A mass selective detector (MSD, USA)) calculating the yield of vanillin and singaldehyde using a previously prepared calibration curve. Finally, another sample of the reaction crude was dissolved in tetrahydrofuran and analyzed by gel permeation chromatography.

[0130] Example 1

[0131] Photocatalytic fragmentation of 2-phenoxy-1-phenylethanol using AQ under batch conditions

[0132] Photocatalytic fragmentation experiments were carried out using 2-phenoxy-1-phenylethanol as a model molecule and AQ under batch conditions.

[0133] A table is shown summarizing the optimization studies of the reaction conditions for the photocatalytic fragmentation of 2-phenoxy-1-phenylethanol using AQ under batch conditions. The fragmentation of this molecule is shown in Scheme 6. 365 nm, 16 h Argon Ri=H / OH R2=H / Me

[0134] P1 P2

[0135] Scheme 6

[0136] The conversion shown in Table 1 represents the percentage of substrate consumed, while P1 and P2 represent the percentage of product obtained relative to the theoretical maximum. Input Solvent Additive Conversion (%) P1 (%) P2

[0137] (%)

[0138] 1 Acetone - 41 32 29

[0139] 2 Acetone Trifluoroacetic acid 65 54 40

[0140] 3 1,4-Dioxane Trifluoroacetic acid 16 14 11

[0141] 4 Dichloromethane Trifluoroacetic acid 89 25 18

[0142] 5 methanol Trifluoroacetic acid 42 5 3

[0143] 6 Acetonitrile Trifluoroacetic acid 99 64 12

[0144] 7 acetonitrile - 58 41 27

[0145] 8 a Acetonitrile Trifluoroacetic acid 99 77 56

[0146] 9 a acetonitrile - 99 99 88

[0147] 10 bacetonitrile - 60 52 22

[0148] 11 c acetonitrile - 24 4 - a Reaction carried out in air. b Reaction using 5,7,12,14-pentacenotetrone instead of anthraquinone. c Reaction using 9-fluorenone instead of anthraquinone.

[0149] Table 1

[0150] The optimal reaction conditions (solvent, additives) for the photocatalytic fragmentation of lignin by means of AQ were defined using 2-phenoxy-1-phenylethanol as a model molecule.

[0151] The conditions were: batch reaction, without supported photocatalyst, 16 h reaction in acetonitrile, 20 mol % photocatalyst load with respect to the substrate and irradiation with a lamp emitting at 365 nm (50 W).

[0152] Table 1 shows that the use of acetonitrile as a solvent, in the absence of additives such as trifluoroacetic acid, allowed a conversion close to 99% and especially a selectivity towards P1 and P2 much higher than the rest of the conditions.

[0153] Example 2

[0154] Fragmentation of lignin models with different types of photocatalytic reactors

[0155] Fragmentation experiments were performed on model lignin molecules and with different types of photocatalytic reactors.

[0156] Table 2 shows the conversion values, as well as the yield in products for representative lignin models using different photocatalytic reactors: a batch reactor with the AQ dispersed in the reaction medium, a batch reactor with the anthraquinone immobilized on the support nanoparticles dispersed in the reaction medium, and a fluidized bed reactor.

[0157] Table 2. As shown in Table 2, the reactivity of anthraquinone, that is, the conversion values ​​of the substrates (lignin models) and the yields of the corresponding fragmentation products, depends on the type of photocatalytic reactor used. The results demonstrate that the flow reactor offers similar or superior reactivity to that obtained with batch systems. The reaction conditions were: 16 h reaction time in acetonitrile under an air atmosphere, 20 mol% catalyst loading relative to the corresponding substrate, and irradiation was carried out with a lamp emitting at 365 nm (50 W). In the case of flow reactors, 200 mg of catalyst support were used per 0.1 mmol of lignin model, a concentration of 0.3 g L⁻¹. -1 dissolution of lignin models in acetonitrile and a flow rate of 5 mL / min' 1 EXAMPLE 3

[0158] Photocatalytic fragmentation of lignin

[0159] Lignin fragmentation tests were performed on beech and pine wood using batch and fluidized bed flow reactors, with modifications to the bed composition. As shown in Table 3, the photocatalytic fragmentation efficiency was determined by calculating the weight percent (%w / w) of extracted monomers—vanillin and syringaldehyde—relative to the initial lignin weight. The monomer extraction yield is referenced to values ​​obtained using the nitrobenzene oxidation method (Chem, 2019, 5, 1521–1536), which is considered the gold standard because it offers the highest monomer extraction yield. For pine lignin, the vanillin extraction yield reached approximately 93% (based on the nitrobenzene oxidation method).It is worth noting that to date there are no examples of photocatalytic systems that demonstrate the production of vanillin from natural lignin. The vanillin extraction yields (>90%) rival those obtained by conventional methodologies such as alkaline oxidation (Int. J. Mol. Sci. 2017, 18, 2421; Tarabanko, VE; Tarabanko, N. Catalytic Oxidation of Lignins into the Aromatic Aldehydes: General Process Trends and Development Prospects', doi.org / 10.3390 / ijms18112421), surpass those of emerging methodologies such as electrochemical methods (ACS Sustainable Chem. Eng. 2020, 8, 7300-7307), and significantly exceed the fragmentation yields of other photocatalytic routes (Wang, H.; Giardino, GJ; Chen, R.; Yang, C.; Niu, J.; Wang, D. Photocatalytic Depolymerization of Native Lignin toward Chemically Recyclable Polymer). Networks. ACS Cent. Sci. 2023, 9, 1, 48-55).

[0160] Table 3 aTotal vanillin and syringaldehyde content of the samples expressed as a percentage by weight with respect to the initial weight of lignin determined by the standard nitrobenzene oxidation method. b Vanillin and syringaldehyde extraction yields expressed as a percentage by weight, relative to the initial weight of lignin, and referenced according to the photocatalytic reactor used, namely: (A) batch reactor with unsupported anthraquinone (AQ(batch)), (B) batch reactor with silica-supported anthraquinone (AQ@SiO2(batch)), (C) flow reactor with silica-supported anthraquinone (AQ@SiO2(flow)), (D) flow reactor with alumina-supported anthraquinone (AQ@Al2O3(flow)), and (E) flow reactor with resin-supported anthraquinone (AQCH2NH@PS(flow)).

[0161] (F) flow reactor with resin-supported anthraquinone (AQNH@PS(flow)). (G) flow reactor with glass wool-supported anthraquinone (AQ@LV(flow)). cVanillin and syringaldehyde extraction efficiency expressed with respect to total content and as a function of the photocatalytic reactor used. The reaction conditions were: 24 h reaction in acetonitrile in an air atmosphere and illumination with a lamp emitting at 365 nm (50 W). In batch experiments, 50 mg of lignin were used with 5 mg of AQ (AQ (batch)) or 100 mg of AQ-modified support (AQ@SiO2 (batch)). In flow reactor experiments, 1 g of photocatalyst-modified support was used for every 0.5 g of lignin to be depolymerized. A lignin solution with a concentration of 4 g L⁻¹ was used. 1 which is circulated at 5 mL / min -1 .

[0162] References

[0163]

[0001] R. Liu, S.-C. Cheng, Y. Xiao, K.-C. Chan, K.-M. Tong, C.-C. Ko, Journal of Catalysis 2022, 407, 206-212.

[0164] [2] Y. Hou, P. Wan, Photochemical & Photobiological Sciences 2008, 7, 588-596.

[0165] [3] Do PQT, Huong VT, Phuong NTT, T.-H. Nguyen, HKT Ta, H. Ju, TB Phan, V.-D. Phung, KTL Trinh, NHT Tran, RSC Advances 2020, 10, 30858-3

[0166] [4] GA Price, A. Hassan, N. Chandrasoma, AR Bogdan, SW Djuhc, MG Organ, Regulatory Chemistry International Edition 2017, 56, 13347-13350.

[0167] [5] a) R. Kitakami, K. Inui, Y. Nakagawa, Y. Sawai, W. Katayama, T. Yokoyama, T. Okada, K. Kanamitsu, S. Nakagawa, N. Toyooka, M. Mizuguchi, Bioorganic & Medicinal Chemistry 2021 , 44, 116292; (b) Lindroth R, Maternal KL, Hammarstrom L, C-J. Wallentin, ACS Organic & Inorganic Au 2022, 2, 427-432.

Claims

CLAIMS 1. A photocatalytic flow method for the selective fragmentation of lignin comprising: (i) immobilizing a photocatalyst on a support forming a bed, and introducing the support with the immobilized photocatalyst into a column transparent to radiation, (i) and continuously circulating a lignin solution through the column loaded with the bed, under illumination, to cause fragmentation of the lignin, obtaining vanillin and syringaldehyde, and such that the method is carried out continuously.

2. Method according to claim 1, wherein the support is selected from metal oxides in the form of nanoparticles, non-metal oxides in the form of nanoparticles, polymeric resins, glass wool or quartz and quartz fiber.

3. Method according to claim 1 or 2, wherein the support is a resin selected from polymers or copolymers of vinyl compounds, cellulose ester resins, Wang resins and aminomethyl polystyrene resins.

4. Method according to any one of the preceding claims 1 to 3, wherein the photocatalyst is irreversibly anchored to the support by covalent bonds.

5. Method according to claim 1, wherein the photocatalyst is selected from an aromatic ketone, a derivative of said aromatic ketone, sodium benzophenonadisulfonate, tetrabutyl ammonium decatungstate, sodium decatungstate, uranyl nitrate hexahydrate, and uranyl perchlorate.

6. Method according to claim 5, wherein the photocatalyst is selected from 5,7,12,14-pentacenetrone, pentacene-6,13-dione, 9-fluorenone, benzophenone, aminobenzophenone, 4,4'-dimethoxybenzophenone, 4,4'-dichlorobenzophenone, acetophenone, dibenzosuberenone, 3,6-dimethoxy-9 / 7-thioxanthen-9-one; 9,10-phenanthrenoquinone, xanthone, thioxanthone, eosin Y, anthraquinone (AQ), 2-chloroanthraquinone, 2-tert-butylanthraquinone, 2-anthraquinonecarboxylic acid or sodium salt of anthraquinone-2-sulfonic acid.

7. Method according to claim 1, wherein the photocatalyst is semiconductor nanoparticles, preferably cadmium sulfide nanoparticles, indium zinc sulfide nanoparticles, or cesium lead bromide nanoparticles.

8. Method according to claim 1, wherein the photocatalyst is anthraquinone immobilized by a covalent bond on a silica support.

9. Method according to claim 1, wherein the photocatalyst is modified in a step prior to its immobilization on the support^ 10. Method according to claim 1, wherein the immobilization of the photocatalyst is carried out by previously functionalizing the molecular photocatalyst with groups selected from silane, halomethyl, amine, ester, amide, anhydride, alcohol or carboxylic acids.

11. Method according to claim 1, wherein the lignin is solubilized in a non-aqueous, organic solvent, preferably selected from dichloromethane, ethyl acetate, methanol, isopropanol, 1,1,1,3,3,3-hexafluoro-2-propanol, acetone, acetonitrile, A / ,A / - dimethylformamide, chloroform, N,N-dimethylacetamide; N-methyl-2-pyrrolidone, tetrahydrofuran, a-terpineol, bromoform, methyl iodide, dichloroethane, trichloroethane, trichloroethylene, chlorobenzene, o-dichlorobenzene, fluorobenzene, iodobenzene, or chloronaphthalene, n-pentane, n-hexane, n-octane, cyclohexane, methylcyclohexane, cyclohexadiene, toluene, o-xylene, m-xylene, p-xylene, ethylbenzene and eumene.

12. Method according to claim 1, wherein in step (i), the support modified with the photocatalyst is introduced and packed into a radiation-transparent column.

13. Method according to claim 1, wherein step (i) is preceded by a lignin extraction and solubilization step.

14. Method according to claim 1, wherein the residence time of the lignin in the reactor is between 1 and 75 hours, preferably between 5 and 50 hours, and more preferably between 20 and 26 hours.

15. Method according to any one of the preceding claims, wherein a mixture of monomers and oligomers, preferably vanillin and syringaldehyde, and oligomers of 2, 3 or 4 linked aromatic units and with oxygenated groups, is obtained.

16. A photocatalytic system for carrying out the method of any one of claims 1 to 15, comprising: - a fluidized bed photocatalytic reactor comprising a support on which a photocatalyst is immobilized, forming a bed, - a lignin solution, - a flow system to control the flow rate of the lignin solution, - a lighting system that allows homogeneous irradiation of the reactor.

17. A photocatalytic system according to claim 16, wherein the fluidized bed is selected from: (1) a simple bed formed by a support with an immobilized photocatalyst or (2) a bed with transparent spheres (glass) to minimize the internal volume without affecting the transmission of irradiation.

18. A photocatalytic system according to claim 17, wherein the lighting system comprises a monochromatic or polychromatic light source.

19. A photocatalytic system according to claim 16, comprising - a reservoir containing the starting solubilized lignin; - a flow system comprising: a hydraulic element for pumping the lignin and a conduit that carries the lignin solution to, and from, the reactor; - a fluidized bed photocatalytic reactor comprising: a column of light-transparent material filled with the bed, filters at the reactor inlet and outlet, - a lighting system that allows homogeneous irradiation of the reactor; - a reservoir where the reaction product is collected.

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