Preparation method of holocellulose

Abstract

The invention belongs to the technical field of biomass material treatment, and particularly relates to a holocellulose preparation method. Biomass raw materials and pretreatment liquid are mixed for pretreatment, solid-liquid separation is carried out, holocellulose is obtained, and the mass percentage content of lignin in holocellulose is lower than 6%; the pretreatment liquid comprises a hydrogen peroxide aqueous solution and a catalyst; the pretreatment is carried out under the condition that the pH value is 2-7; the catalyst comprises one or more of tungstic acid, molybdic acid, tungsten carbide, molybdenum carbide, tungsten sulfide, molybdenum sulfide, heteropolyacid and heteropolyacid salt; the heteropoly acid comprises phosphotungstic acid and / or phosphomolybdic acid; the heteropoly acid salt comprises phosphotungstate. According to the method, oxygenolysis of the lignin in the biomass raw material can be achieved in an environment-friendly and efficient mode, the high-quality holocellulose is prepared, and the mass percentage content of the lignin is lower than 6%.

Description

Technical Field

[0001] The invention belongs to the technical field of biomass material processing, and in particular relates to a method for preparing holocellulose. Background Art

[0002] Holocellulose refers to the part of papermaking plant fiber raw materials that is left after the extract and lignin are removed (i.e., the general term for hemicellulose and cellulose). Existing methods for preparing holocellulose, such as chlorination, sodium chlorite, chlorine dioxide, and peracetic acid, use toxic and flammable chlorine gas, which poses a safety hazard and pollutes the environment with byproducts; the chlorine dioxide method requires special equipment to prepare and store chlorine dioxide, which is costly. The most widely used methods for preparing holocellulose from existing biomass raw materials are the sodium chlorite method and the hydrogen peroxide-acetic acid method. Among them, the sodium chlorite pretreatment is to use acetic acid to adjust the pH value of 6wt% sodium chlorite solution to 3.6-3.8, which is pretreated at 80°C for 2h to remove 67.7wt% of the biomass raw material lignin. The lignin content in the obtained sample is 10.8%, and it contains chlorine substituents, which is not conducive to biological enzyme degradation. In addition, this method not only produces toxic substances such as chlorine dioxide and chlorine gas, but also the sodium chlorite used is a carcinogen. Although the lignin content in the holocellulose prepared by the hydrogen peroxide-acetic acid method is low, a large amount of hydrogen peroxide and acetic acid is used. The produced peracetic acid has a strong irritating effect on human eyes, skin mucosa and upper respiratory tract. Inhalation can cause inflammation of the larynx and bronchus and other diseases, which is harmful to the human body, and the reagent cost is high. Summary of the invention

[0003] In view of this, the object of the present invention is to provide a method for preparing holocellulose, which can realize the oxidative decomposition of lignin in biomass raw materials in an environmentally friendly and efficient manner to prepare high-quality holocellulose, wherein the mass percentage of lignin is less than 6%.

[0004] In order to achieve the above object, the present invention provides the following technical solutions:

[0005] The present invention provides a method for preparing holocellulose, comprising the following steps:

[0006] The biomass raw material and the pretreatment liquid are mixed for pretreatment, and the solid-liquid separation is performed to obtain holocellulose, wherein the mass percentage of lignin in the holocellulose is less than 6%;

[0007] The pretreatment liquid includes an aqueous hydrogen peroxide solution and a catalyst; the pretreatment is performed at a pH value of 2 to 7;

[0008] The catalyst comprises one or more of tungstic acid, molybdic acid, tungsten carbide, molybdenum carbide, tungsten sulfide, molybdenum sulfide, heteropoly acid and heteropoly acid salt;

[0009] The heteropoly acid includes phosphotungstic acid and / or phosphomolybdic acid; the heteropoly acid salt includes phosphotungstate.

[0010] Preferably, the concentration of hydrogen peroxide in the pretreatment solution is 50-150 g / L.

[0011] Preferably, the concentration of the catalyst in the pretreatment liquid is 5 to 50 mmol / L.

[0012] Preferably, the mass ratio of the dry weight of the biomass raw material to the pretreatment liquid is 1:8-12.

[0013] Preferably, the pretreatment is carried out under static heating conditions; the static heating temperature is 60-90°C.

[0014] Preferably, the static heating time is 1 to 3 hours.

[0015] Preferably, the static heating time is 10 to 20 hours; when the static heating time is 10 to 20 hours, the prepared holocellulose is nano-scale oxidized cellulose.

[0016] Preferably, the biomass raw material is a powder with a particle size of less than 50 mesh.

[0017] Preferably, the catalyst is phosphotungstic acid and / or sodium phosphotungstate.

[0018] Preferably, the yield of the holocellulose is 50-65%.

[0019] The present invention provides a method for preparing holocellulose, comprising the following steps: mixing a biomass raw material and a pretreatment liquid for pretreatment, and separating the solid and the liquid to obtain holocellulose, wherein the mass percentage of lignin in the holocellulose is less than 6%; the pretreatment liquid comprises an aqueous hydrogen peroxide solution and a catalyst; the pretreatment is carried out under a pH value of 2 to 7; the catalyst comprises one or more of tungstic acid, molybdic acid, tungsten carbide, molybdenum carbide, tungsten sulfide, molybdenum sulfide, heteropolyacids and heteropolyacid salts; the heteropolyacid comprises phosphotungstic acid and / or phosphomolybdic acid; the heteropolyacid comprises phosphotungstate. The present invention uses hydrogen peroxide to oxidize lignin, and uses a catalyst to enhance the oxidative removal effect of hydrogen peroxide on lignin. The catalyst used in the present invention takes into account both metal ion activation and acid catalysis, which are two mechanisms for catalyzing hydrogen peroxide to oxidize lignin. While catalyzing the decomposition of hydrogen peroxide to produce free radicals, HO can also be produced. +, both of which can oxidize lignin. In addition, the degradation product of lignin is an organic acid, which can form peroxycarboxylic acid with hydrogen peroxide, and hydrogen peroxide and tungstic acid or molybdic acid can also form peroxyacids, and these peroxyacids strengthen the oxidation performance of the system, and heteropolyacids (phosphotungstic acid, phosphomolybdic acid) and heteropolyacids (phosphotungstates) have strong heteropolyacid oxidation and catalytic performance, and have a stronger catalytic hydrogen peroxide oxidation effect on lignin compared to ordinary tungstic acid and molybdic acid. Therefore, the present invention can efficiently realize the oxidative decomposition of lignin in biomass raw materials, and prepare high-quality holocellulose, the mass percentage of lignin is less than 6%, the mass percentage of carbohydrates is higher than 85%, and the residual rate of lignin in the holocellulose prepared by the present invention is low, and the retention rate of carbohydrates is high.

[0020] In addition, the hydrogen peroxide used in the present invention is a green oxidant, which is non-toxic, harmless, and easy to handle. It decomposes into water and oxygen after heating, significantly reducing pollution to the environment. The present invention achieves the goal of efficiently preparing holocellulose (carbohydrate mass percentage ≥ 90%) from biomass raw materials in 1 hour, and improves production efficiency. The catalyst acidity of the pretreatment liquid in this method is weaker than sulfuric acid, has little corrosion to equipment, can be recycled, and reduces equipment requirements and maintenance costs. DETAILED DESCRIPTION

[0021] The present invention provides a method for preparing holocellulose, comprising the following steps:

[0022] The biomass raw material and the pretreatment liquid are mixed for pretreatment, and the solid-liquid separation is performed to obtain holocellulose, wherein the mass percentage of lignin in the holocellulose is less than 6%;

[0023] The pretreatment liquid includes an aqueous hydrogen peroxide solution and a catalyst; the pretreatment is performed at a pH value of 2 to 7;

[0024] The catalyst comprises one or more of tungstic acid, molybdic acid, tungsten carbide, molybdenum carbide, tungsten sulfide, molybdenum sulfide, heteropoly acid and heteropoly acid salt;

[0025] The heteropoly acid includes phosphotungstic acid and / or phosphomolybdic acid; the heteropoly acid salt includes phosphotungstate.

[0026] Unless otherwise specified, the present invention has no special requirements on the sources of the raw materials used, and commercially available products known to those skilled in the art may be used.

[0027] The invention mixes biomass raw materials and pretreatment liquid for pretreatment, separates solid and liquid, and obtains holocellulose.

[0028] As an embodiment, the biomass raw material is a biomass raw material containing cellulose and lignin components; the biomass raw material includes wood and / or crop straw, and in a specific embodiment, the wood is poplar.

[0029] As an embodiment, the biomass raw material is a powder with a particle size of less than 50 mesh.

[0030] As an embodiment, the pretreatment liquid includes an aqueous hydrogen peroxide solution and a catalyst; the catalyst includes one or more of tungstic acid, molybdic acid, tungsten carbide, molybdenum carbide, tungsten sulfide, molybdenum sulfide, heteropolyacid and heteropolyacid salt, and in specific embodiments, heteropolyacid or heteropolyacid salt.

[0031] As an embodiment, the heteropolyacid includes phosphotungstic acid and / or phosphomolybdic acid, and in a specific embodiment, it is phosphotungstic acid; the heteropolyacid salt includes phosphotungstate, and in a specific embodiment, it is phosphotungstate; the phosphotungstate is sodium phosphotungstate.

[0032] As an embodiment, the pretreatment is carried out under the condition of pH value of 2 to 7, and 3 to 6 in the specific embodiment; the concentration of hydrogen peroxide in the pretreatment liquid is 50 to 150 g / L, and 50 to 100 g / L in the specific embodiment; the concentration of the catalyst in the pretreatment liquid is 5 to 50 mmol / L, and 10 to 20 mmol / L in the specific embodiment; the ratio of the dry weight of the biomass raw material to the mass of the pretreatment liquid is 1:8 to 12, and 1:9 to 11 in the specific embodiment. When the concentration of the catalyst in the pretreatment liquid is higher than 10 mmol / L, the static heating time is ≤ 1 h. When the concentration of the catalyst in the pretreatment liquid is low, the static heating time needs to be extended.

[0033] As an embodiment, in order to prepare high-purity holocellulose, the minimum concentration of the polyacid and polyacid salt in the pretreatment solution is 5 mmol / L, and the pH value of the pretreatment solution is 2-7, and in a specific embodiment, 3-6.

[0034] Hydrogen peroxide is a green oxidant that oxidizes lignin, and its degradation products are pollution-free water and oxygen. One of the functions of the catalyst is that its special spatial structure can catalyze the oxidation performance of hydrogen peroxide on lignin; the second function is that some anions can form peroxides to oxidize lignin. The system of hydrogen peroxide catalyzed by the catalyst used in the present invention can oxidize lignin and retain carbohydrates, thereby preparing high-purity holocellulose.

[0035] Although hydrogen peroxide is a strong oxidant, its oxidizing ability for lignin is very weak, and the lignin removal rate for biomass raw materials is less than 30%. In existing reports, in order to improve the oxidative removal of lignin by hydrogen peroxide, three methods are used, namely, base catalysis, acid catalysis, and metal ion activation. Among them, hydrogen peroxide releases hydroxyl radicals and superoxide radicals under base catalysis conditions. This type of method does not completely remove lignin, and the loss of xylan is serious. In the acidic hydrogen peroxide catalytic system, sulfuric acid is used as a catalyst, and more than 50% of acetic acid or formic acid and phosphoric acid are used to synthesize peroxyacid. The peroxyacid and HO + It is the main reason for the oxidation of lignin. In the metal ion activation system, hydrogen peroxide is activated by metal ions to produce free radicals to undergo advanced oxidation reactions, which degrade lignin. In this method, a large amount of hemicellulose is removed, and the lignin removal rate is not high. The present invention takes into account both metal ion activation and acid catalysis. Tungstic acid and molybdic acid can produce HO while catalyzing the decomposition of hydrogen peroxide to produce free radicals. + , both of which can oxidize lignin. In addition, the degradation product of lignin is an organic acid, which can form peroxycarboxylic acid with hydrogen peroxide, and hydrogen peroxide and tungstic acid or molybdic acid can also form peroxyacids, and these peroxyacids further strengthen the oxidation performance of the system. After calculation, phosphotungstic acid is a classic heteropolyacid, and the special spatial configurations such as Keggin and phosphotungstate Silverton give some heteropolyacids strong heteropolyacid oxidation and catalytic properties. In the heteropolyacid provided by the present invention, phosphotungstic acid, phosphotungstate, and phosphomolybdic acid have a stronger catalytic effect than ordinary tungstic acid and molybdic acid, which is more beneficial for preparing holocellulose. The present invention takes into account the characteristics of acid catalysis and metal ion activation mechanism at the same time, further reduces the introduction of acetic acid and formic acid in the acid catalysis system, avoids the introduction of a large number of strong acid catalysts, improves the retention effect of hemicellulose (xylan), and then realizes the selective removal of lignin in the three major components of biomass raw materials, and prepares high-purity holocellulose.

[0036] As an embodiment, the pretreatment is carried out under static heating conditions; the static heating temperature is 60 to 90° C., and in a specific embodiment, it is 70 to 80° C.; the static heating time is 1 to 3 hours, and in a specific embodiment, it is 1 hour or 3 hours.

[0037] As another embodiment, the static heating time is 10 to 20 hours; when the static heating time is 10 to 20 hours, the prepared holocellulose is nano-scale oxidized cellulose.

[0038] As another implementation, the pretreatment is carried out in a sealed reaction container, a sealed reaction kettle in a specific embodiment; the liquid volume of the reaction container does not exceed 50% of the volume of the reaction container.

[0039] As an embodiment, before the solid-liquid separation, the method further includes: cooling the pretreated mixed liquid to room temperature; the cooling is natural cooling.

[0040] As an implementation mode, the solid-liquid separation is filtration or centrifugation, and in a specific embodiment, it is filtration; the filtration is suction filtration.

[0041] As an embodiment, the mass percentage of carbohydrates in the holocellulose is ≥ 90%, and in a specific embodiment, it is 90-96%, and the mass percentage of lignin is less than 6%, and in a specific embodiment, it is 1.7-5.9%.

[0042] As an embodiment, the lignin removal rate of the method for preparing holocellulose provided by the present invention is 88-97%, and in a specific embodiment, it is 90-95%, and the holocellulose yield is 50-65%, and in a specific embodiment, it is 55-60%.

[0043] The present invention retains a large amount of carbohydrates, i.e., holocellulose components, while oxidatively degrading lignin. In the traditional sulfuric acid-catalyzed hydrogen peroxide-acetic acid system (peracetic acid method), sulfuric acid is a catalyst. The more sulfuric acid is added, the stronger the acidity (solution pH value <0), and the more serious the hemicellulose loss. The pH value of the pretreatment solution used in the present invention is 2-7, and the acidity is weaker than 10mM concentration of sulfuric acid. Therefore, among the three major components of cellulose, hemicellulose and lignin in the biomass raw material, only more than 90% of the lignin is oxidized and removed, and the carbohydrate loss is less than 15%, with a high retention rate. Moreover, compared with the sodium chlorite pretreatment method, the present invention is non-toxic and harmless, has a short reaction time, and has less lignin residue. Compared with the hydrogen peroxide-acetic acid pretreatment method, the present invention uses less hydrogen peroxide, and does not need to add a large amount of organic acid, and no peracetic acid gas that irritates the respiratory tract is produced during the process. Therefore, the present invention not only greatly improves the preparation efficiency and quality of holocellulose, but also highlights its significant advantages of low-temperature operation, green environmental protection and efficient output. The preparation method provided by the present invention is very suitable for the large-scale preparation of holocellulose and the raw material demand of biomass refining, and opens up a new path for the diversified and efficient utilization of biomass resources.

[0044] The technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention, but they should not be construed as limiting the protection scope of the present invention.

[0045] Example 1

[0046] 200 mL of hydrogen peroxide (30 wt% hydrogen peroxide solution) and phosphotungstic acid are added to 800 mL of water and stirred evenly to obtain a pretreatment solution, wherein the concentration of hydrogen peroxide is 67 g / L, the concentration of phosphotungstic acid is 20 mmol / L, and the pH value is 2.4; 100 g of poplar wood powder (particle size <50 mesh) is mixed evenly with 1 L of the pretreatment solution, and then transferred to a closed reactor with a liquid volume not exceeding 50% of the volume of the reactor, and heated at 80°C for 1 hour, and filtered after naturally cooling to room temperature, and the residue obtained is holocellulose.

[0047] Example 2

[0048] The difference from Example 1 is that the phosphotungstic acid in Example 1 is replaced by sodium phosphotungstate, and the remaining steps are the same as Example 1.

[0049] Example 3

[0050] The difference from Example 1 is that the concentration of phosphotungstic acid in the pretreatment solution is 10 mmol / L, the solution is heated for 3 hours, and the remaining steps are the same as Example 1.

[0051] Example 4

[0052] The difference from Example 2 is that the concentration of sodium phosphotungstate in the pretreatment solution is 10 mmol / L, the solution is heated for 3 hours, and the remaining steps are the same as Example 2.

[0053] Example 5

[0054] The difference from Example 1 is that the static heating temperature is 90° C., and the remaining steps are the same as Example 1.

[0055] Example 6

[0056] The difference from Example 1 is that the static heating temperature is 70° C., and the remaining steps are the same as Example 1.

[0057] Example 7

[0058] The difference from Example 1 is that the phosphotungstic acid in Example 1 is replaced by phosphomolybdic acid, and the remaining steps are the same as Example 1.

[0059] Example 8

[0060] The difference from Example 3 is that the phosphotungstic acid in Example 3 is replaced by molybdic acid, the concentration of molybdic acid in the pretreatment solution is 50 mmol / L, and the remaining steps are the same as Example 3.

[0061] Example 9

[0062] The difference from Example 3 is that the phosphotungstic acid in Example 3 is replaced by tungstic acid, the concentration of molybdic acid in the pretreatment solution is 50 mmol / L, and the remaining steps are the same as Example 3.

[0063] Example 10

[0064] The difference from Example 3 is that the phosphotungstic acid in Example 3 is replaced by tungsten carbide, the concentration of molybdic acid in the pretreatment solution is 50 mmol / L, and the remaining steps are the same as Example 3.

[0065] Embodiment 11

[0066] The difference from Example 3 is that the phosphotungstic acid in Example 3 is replaced by molybdenum carbide, the concentration of molybdic acid in the pretreatment solution is 50 mmol / L, and the remaining steps are the same as Example 3.

[0067] Example 12

[0068] The difference from Example 3 is that the phosphotungstic acid in Example 3 is replaced by tungsten sulfide, the concentration of molybdic acid in the pretreatment solution is 50 mmol / L, and the remaining steps are the same as Example 3.

[0069] Embodiment 13

[0070] The difference from Example 3 is that the phosphotungstic acid in Example 3 is replaced by molybdenum sulfide, the concentration of molybdic acid in the pretreatment solution is 50 mmol / L, and the remaining steps are the same as Example 3.

[0071] Comparative Example 1

[0072] 200 mL of hydrogen peroxide (30 wt% hydrogen peroxide solution) was added to 800 mL of water and stirred to obtain a hydrogen peroxide aqueous solution; 100 g of poplar wood powder (particle size <50 mesh) was mixed evenly with the hydrogen peroxide aqueous solution and then transferred to a closed reactor with a liquid volume not exceeding 50% of the reactor volume, and heated at 80° C. for 1 hour, cooled naturally to room temperature and filtered. The obtained residue had a lignin content of more than 25%, which could not be called holocellulose.

[0073] Comparative Example 2

[0074] The difference from Example 3 is that the phosphotungstic acid in Example 3 is replaced by silicotungstic acid, the concentration of silicotungstic acid in the pretreatment solution is 20 mmol / L, and the remaining steps are the same as Example 3.

[0075] Comparative Example 3

[0076] The difference from Example 3 is that the phosphotungstic acid in Example 3 is replaced by sodium tungstate, the concentration of sodium tungstate in the pretreatment solution is 50 mmol / L, and the remaining steps are the same as Example 3.

[0077] Comparative Example 4

[0078] The difference from Example 3 is that the phosphotungstic acid in Example 3 is replaced by sodium phosphomolybdate, the concentration of sodium tungstate in the pretreatment solution is 20 mmol / L, and the remaining steps are the same as Example 3.

[0079] Performance Testing

[0080] The yield and lignin removal rate of the holocellulose prepared in Examples 1 to 13 and Comparative Examples 1 to 4 were tested, and the results are shown in Table 1.

[0081] The lignin content was calculated according to the US Energy Laboratory method.

[0082]

[0083] Table 1 Properties of holocellulose prepared in Examples 1 to 13 and Comparative Examples 1 to 4

[0084]

[0085]

[0086] As shown in Table 1, the lignin removal rate of the preparation method of the holocellulose provided by the present invention is 88-97%, and the holocellulose yield is 50-65%, while the lignin removal rate of the method using only aqueous hydrogen peroxide solution without adding a catalyst in Comparative Example 1 is 45%, and the mass percentage of lignin exceeds 25%, and no holocellulose is obtained. In Comparative Examples 2, 3, and 4, in which silicotungstic acid, sodium tungstate, and sodium phosphomolybdate are used as catalysts, the lignin content of the obtained samples is higher than 20%, which cannot be called holocellulose. It can be seen that the present invention can efficiently prepare high-quality holocellulose.

[0087] Although the above embodiment describes the present invention in detail, it is only a part of the embodiments of the present invention rather than all the embodiments. People can also obtain other embodiments based on this embodiment without creativity, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A method for preparing holocellulose, characterized in that: The following steps are involved: The biomass raw material and the pretreatment liquid are mixed for pretreatment, and the solid-liquid separation is performed to obtain holocellulose, wherein the mass percentage of lignin in the holocellulose is less than 6%; The pretreatment liquid includes an aqueous hydrogen peroxide solution and a catalyst; the pretreatment is performed at a pH value of 2 to 7; The catalyst comprises one or more of tungstic acid, molybdic acid, tungsten carbide, molybdenum carbide, tungsten sulfide, molybdenum sulfide, heteropoly acid and heteropoly acid salt; The heteropoly acid includes phosphotungstic acid and / or phosphomolybdic acid; the heteropoly acid salt includes phosphotungstate.

2. The preparation method according to claim 1, characterized in that: The concentration of hydrogen peroxide in the pretreatment solution is 50-150 g / L.

3. The preparation method according to claim 1, characterized in that: The concentration of the catalyst in the pretreatment liquid is 5 to 50 mmol / L.

4. The preparation method according to any one of claims 1 to 3, characterized in that: The mass ratio of the dry weight of the biomass raw material to the pretreatment liquid is 1:8-12.

5. The preparation method according to claim 1, characterized in that: The pretreatment is carried out under static heating conditions; the static heating temperature is 60-90°C.

6. The preparation method according to claim 5, characterized in that: The static heating time is 1 to 3 hours.

7. The preparation method according to claim 5, characterized in that: The static heating time is 10 to 20 hours. When the static heating time is 10 to 20 hours, the prepared holocellulose is nano-scale oxidized cellulose.

8. The preparation method according to claim 1, characterized in that: The biomass raw material is a powder with a particle size of less than 50 meshes.

9. The preparation method according to claim 1, characterized in that: The catalyst is phosphotungstic acid and / or sodium phosphotungstate.

10. The preparation method according to claim 1, characterized in that: The yield of the holocellulose is 50-65%.

Images