A method for preparing levoglucosan by co-pyrolysis of cellulose and urea

Abstract

The present invention belongs to the technical field of levoglucosan preparation, and specifically relates to a method for preparing levoglucosan by co-pyrolysis of cellulose and urea. The method comprises the following steps: urea and cellulose are mixed and then pyrolyzed, and volatile matter is collected to obtain bio-oil containing levoglucosan. The method provided by the present invention has simple process, convenient operation, does not involve subsequent operations such as catalyst recovery, and has low requirements for pyrolysis reactors; the yield of levoglucosan prepared by using cellulose or lignocellulose biomass and urea as raw materials is high.

Description

Technical Field

[0001] The invention belongs to the technical field of levoglucosan preparation, and particularly relates to a method for preparing levoglucosan by co-pyrolysis of cellulose and urea. Background Art

[0002] Levoglucosan is an important monomer for synthesizing stereochemical compounds. It can be used as a chiral synthon to synthesize oligosaccharides, polymers, resins, drugs and related products. Microorganisms can directly or indirectly use levoglucosan as a substrate to produce itaconic acid, citric acid, lipids and ethanol. Cellulose is the most abundant component in lignocellulosic biomass, accounting for 35%-50% of biomass. Cellulose undergoes rapid pyrolysis at about 500°C in a very short time (about 2 seconds). The main products of pyrolysis include dehydrated sugars mainly composed of levoglucosan, as well as a certain amount of small molecules such as furans and alcohols, aldehydes and ketones.

[0003] Agricultural and forestry wastes belong to lignocellulosic biomass, which can theoretically be converted into high-value levoglucosan. Lignocellulosic biomass contains a lot of ash and alkali metals, and the pyrolysis processes of its three main components: cellulose, hemicellulose, and lignin are not independent, the pyrolysis temperatures are staggered, and the components influence each other. Lignin will significantly inhibit the formation of macromolecular products during the thermal cracking of cellulose, but will promote the decomposition of cellulose into small molecules. The low selectivity of rapid pyrolysis of lignocellulosic biomass (such as corn cobs, sugarcane bagasse, straw, etc.) to levoglucosan is a key issue restricting the development of this process.

[0004] In order to improve the yield of levoglucosan, domestic and foreign scholars have tried to use different methods to promote the conversion efficiency of cellulose to levoglucosan, and optimized the levoglucosan preparation process by pretreating lignocellulosic biomass, pretreating cellulose, changing the pressure of the reaction system, optimizing the reactor, and introducing catalysts during the pyrolysis process. Improving the yield of levoglucosan by pretreatment means is not only complex but also produces a large amount of waste liquid, which will pollute the environment if not properly treated. The method of changing the pressure of the reaction system and optimizing the reactor has high requirements for the reactor and operating conditions. A large number of studies have adopted the method of introducing a catalyst during the reaction process to improve the yield of levoglucosan, such as patent CN102020722A, a method for preparing levoglucosan by catalytic pyrolysis of cellulose, which uses metallic copper as a catalyst and undergoes pyrolysis reaction under anaerobic conditions after mixing with cellulose. Patent CN115212894A discloses a method for preparing levoglucosan from lignocellulose, which uses nano-ferroferric oxide to prepare a magnetic solid acid catalyst, and then the catalyst is mixed with lignocellulose for pyrolysis to prepare levoglucosan. Patent CN114560960B discloses a method for preparing levoglucosan by catalytic pyrolysis of cellulose, which is to use sodium carboxymethyl cellulose, zinc oxide and ferric sulfate to make a magnetic solid acid-base amphoteric catalyst, which is mixed with cellulose and then pyrolyzed. However, it faces the problems of high catalyst cost, easy coking and deactivation, and difficulty in recovery. How to provide a method for preparing levoglucosan with a simple operating process, low dependence on the reactor, and high conversion rate is still a technical problem that those skilled in the art need to solve. Summary of the invention

[0005] In order to solve the above technical problems existing in the prior art, the main purpose of the present invention is to provide a method for preparing L-glucose by co-pyrolysis of cellulose and urea.

[0006] To achieve the above-mentioned object of the invention, the present invention provides a method for preparing L-glucose by co-pyrolysis of cellulose and urea, comprising the following steps:

[0007] The urea and cellulose are mixed and then pyrolyzed, and the condensable volatiles are collected to obtain bio-oil containing levoglucosan. As a preferred technical solution, the pyrolysis temperature is 500-550°C.

[0008] As a preferred technical solution, the pyrolysis is carried out for 5-60 min; more preferably, it is carried out for 5-30 min or 5-15 min.

[0009] As a preferred technical solution, the heating rate of the cellulose to its pyrolysis reaction temperature is 500-1000°C / s. As a preferred technical solution, the urea and cellulose are dried to constant weight before the pyrolysis reaction, and the drying temperature is generally 12-36 hours at normal pressure and 95-115°C. This step mainly removes moisture from cellulose and urea.

[0010] As a preferred technical solution, the mass ratio of cellulose to urea is 1-10:1; preferably 3-6:1; for example 3:1; 3.3:1; 4:1; 5:1; 6:1.

[0011] The present invention uses cellulose as a raw material. The cellulose can be purchased directly from a manufacturer or a cellulose-enriched substrate obtained by pre-treating lignocellulosic biomass by different means. For the directly purchased cellulose, such as microcrystalline cellulose, α-cellulose, cotton wool, filter paper;

[0012] The cellulose-enriched substrate obtained by pre-treating lignocellulosic biomass by different means refers to agricultural and forestry wastes and other wastes with cellulose, hemicellulose and lignin as the main components, including corn cobs, bagasse, straw, branches, furniture processing waste sawdust, etc.; the methods of obtaining cellulose-enriched substrates by pre-treatment means include biological methods and chemical methods. Among them, the chemical method mainly reacts lignocellulosic biomass with corresponding solvents through chemical reaction methods to obtain cellulose-enriched substrates. The biological method is to depolymerize lignocellulosic biomass through the metabolic process of microorganisms to obtain cellulose-enriched substrates.

[0013] For the step of mixing urea and cellulose for pyrolysis, the cellulose raw material and urea can be dry mixed, or the cellulose can be placed in a urea solution for immersion by an equal volume impregnation method, dried, and then ground. For example, the cellulose is first immersed in deionized water until adsorption is saturated, and the free water on the surface of the cellulose is removed after filtration. The volume of liquid that the cellulose can absorb is determined, and the same volume of urea solution is prepared according to the value. The dry cellulose is added to the above urea solution for full absorption, and dried and ground to obtain a cellulose urea mixture, wherein the mass ratio of cellulose to urea is 1-10:1; preferably 3-6:1. As a preferred technical solution, the operation of collecting volatiles includes passing the pyrolysis product through a condensation device, and the condensation temperature is -10°C to 0°C.

[0014] The method for preparing levoglucosan provided by the present invention has the following principle: urea is decomposed by heat to generate a large amount of CO 2 and NH 3, producing a gas explosion effect, which accompanies the entire process of thermal decomposition of cellulose. When cellulose is heated in the reactor, the cellulose on the surface is condensed to form a barrier, which makes it difficult for the volatile matter inside to evaporate, thereby generating more charcoal. The gas explosion effect can break the barrier formed by the condensation of functional groups on the surface of the reactants, allowing the volatile matter generated by the internal cellulose to evaporate, and is beneficial to the heat and mass transfer inside the material. The occurrence of condensation reaction to form charcoal during the pyrolysis of cellulose is inhibited, cellulose is decomposed by heat, the conversion rate of cellulose to charcoal is reduced, and more volatile matter is generated, so the conversion rate of cellulose to levoglucosan is improved.

[0015] The main beneficial effects of the present invention are as follows:

[0016] (1) Compared with complex pretreatment methods, the process of the present invention is simple, easy to operate, does not involve subsequent catalyst recovery and other operations, and has lower requirements for the pyrolysis reactor;

[0017] (2) The raw materials are widely applicable, and high value-added levoglucosan can be prepared using cellulose or lignocellulosic biomass and urea as raw materials;

[0018] (3) Low production cost. Compared with catalysts, urea is cheap.

[0019] (4) The yield of levoglucosan is high, which can improve the utilization value of lignocellulosic biomass. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 : Thermogravimetric curve of urea.

[0021] Figure 2 : Comparison of the relative contents of levoglucosan in the bio-oils obtained in each embodiment and the control group.

[0022] Figure 3 : Comparison of the relative contents of levoglucosan in the bio-oils obtained in each comparative example and in Example 2. DETAILED DESCRIPTION

[0023] Those skilled in the art can refer to the content of this article and appropriately improve the process parameters. It should be particularly noted that all similar replacements and modifications are obvious to those skilled in the art, and they are all considered to be within the scope of protection of the present invention. The methods and applications of the present invention have been described through preferred embodiments, and relevant personnel can obviously modify or appropriately change and combine the methods and applications described herein without departing from the content, spirit and scope of the present invention to implement and apply the technology of the present invention.

[0024] Unless otherwise specified, the raw materials or auxiliary materials used in the present invention can be obtained from the market.

[0025] The following is combined with the embodiments and the attached Figure 1-2 , further illustrating the present invention:

[0026] Example 1

[0027] (1) Dry the purchased α-cellulose and urea in a drying oven at 105°C for 24 h to constant weight to remove the water absorbed by the cellulose;

[0028] (2) dry-mixing the dried cellulose and urea in a ratio of cellulose to urea of ​​9:1;

[0029] (3) Put the mixed materials into a quartz boat. After the pyrolysis device is heated to 500°C, push the quartz boat to the center of the pyrolysis reactor where the temperature is uniform and pyrolyze for 5 minutes.

[0030] (4) Collect the bio-oil with a cold trap, and place the cold trap in a circulating water cooling box at a temperature of -5°C. The relative content of levoglucosan in the collected bio-oil is 54.76%. In this embodiment, the yield of levoglucosan is 31.88%.

[0031] Compared with the pyrolysis of cellulose alone under the same pyrolysis conditions, the content of levoglucosan in the obtained bio-oil is 48.08%, and the yield is 25.24%, which are significantly improved.

[0032] Example 2

[0033] Compared with Example 1, the only difference between this example and Example 1 is that the ratio of cellulose to urea is 4:1, and the relative content of levoglucosan in the obtained bio-oil is 61.86%. In this example, the yield of levoglucosan is 39.98%.

[0034] Example 3

[0035] Compared with Example 1, the difference between this example and Example 1 is that the ratio of cellulose to urea is 10:3, and the relative content of levoglucosan in the obtained bio-oil is 60.23%. In this example, the yield of levoglucosan is 48.01%.

[0036] The content of levoglucosan in the bio-oil obtained was 25.27% higher than that of cellulose pyrolyzed under the same pyrolysis conditions (48.08%).

[0037] Example 4

[0038] (1) The purchased α-cellulose and urea were dried in a drying oven at 105°C for 24 h to constant weight to remove the water absorbed by the cellulose.

[0039] (2) Use the wet impregnation method to uniformly mix cellulose and urea in a ratio of 9:1. Take 9g of cellulose and soak it in deionized water for one hour. Take it out and drain the water and wipe off the free water on the surface. The weight is 16.84g. It is found that 9g of cellulose can absorb 7.84mL of liquid volume. Then, after 1g of urea is prepared into 7.84mL of solution, another 9g of dried cellulose is taken and added to the solution to fully absorb the solution. After the mixture is dried, it is ground to obtain a material in which cellulose and urea are uniformly mixed in a ratio of 9:1.

[0040] (3) Take 3 g of the uniformly mixed material and put it into a quartz boat. After the pyrolysis device is heated to 500°C, push the quartz boat to the center of the pyrolysis reactor where the temperature is uniform and pyrolyze for 5 minutes.

[0041] (4) Collect the bio-oil with a cold trap, and place the cold trap in a circulating water cooling box at a temperature of -5°C. The relative content of levoglucosan in the collected bio-oil is 57.76%. In this embodiment, the yield of levoglucosan is 33.63%.

[0042] Comparative Example 1

[0043] The difference between this comparative example and Example 2 is that the pyrolysis temperature is 600° C. The relative content of levoglucosan in the bio-oil prepared in this comparative example is 10.72%, and the yield of levoglucosan is 5.63%.

[0044] Comparative Example 2

[0045] The difference between this comparative example and Example 2 is that the pyrolysis temperature is 450° C. The relative content of levoglucosan in the bio-oil prepared in this comparative example is 40.64%, and the yield of levoglucosan is 26.15%.

[0046] Comparative Example 3

[0047] The difference between this comparative example and Example 2 is that step (1) is omitted.

[0048] The relative content of levoglucosan in the bio-oil prepared in this comparative example is 57.85%, and the yield of levoglucosan is 32.43%.

[0049] Comparative Example 4

[0050] The difference between this comparative example and Example 2 is that the operation of step (4) is: collecting the bio-oil with a cold trap, and placing the cold trap in air at a room temperature of 25° C. The relative content of levoglucosan in the bio-oil obtained in this comparative example is 34.51%, and the collection rate of levoglucosan is 18.82%.

[0051] The above description is only a preferred embodiment of the present invention. It should be pointed out that a person skilled in the art can make several improvements without departing from the principle of the present invention. These improvements should also be regarded as within the protection scope of the present invention.

Claims

1. A method for preparing L-glucose by co-pyrolysis of cellulose and urea, characterized in that: Here are the steps: (1) Dry the purchased α-cellulose and urea in a drying oven at 105°C for 24 hours to constant weight to remove the water absorbed by the cellulose; (2) dry-mixing the dried cellulose and urea in a ratio of 10:3; (3) Put the evenly mixed materials into a quartz boat. After the pyrolysis device is heated to 500°C, push the quartz boat to the center of the pyrolysis reactor where the temperature is uniform and pyrolyze for 5 minutes. (4) Collect the bio-oil using a cold trap and place the cold trap in a circulating water cooling box at -5°C.

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