CN115677863B - Method for preparing cellulose ester by microwave heating method in homogeneous system and cellulose ester - Google Patents
Method for preparing cellulose ester by microwave heating method in homogeneous system and cellulose ester Download PDFInfo
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Abstract
The application provides a method for preparing cellulose ester by a microwave heating method in a homogeneous system, wherein in a homogeneous cellulose solution, anhydride is dropwise added into the cellulose solution at a constant speed by the microwave heating method under the normal pressure condition to carry out esterification reaction to obtain cellulose ester; wherein, the microwave heating method adopts a gradient heating process. By adopting the technical scheme of the application, the application utilizes the homogenization reaction, reduces the side reaction of the esterification reaction by a gradient heating process, improves the substitution degree of cellulose ester, and realizes the rapid and efficient synthesis of the cellulose ester with high and controllable substitution degree under the condition of normal pressure and low temperature.
Description
Technical Field
The application relates to the technical field of cellulose ester chemical synthesis, in particular to a method for rapidly preparing cellulose ester by microwave heating in a homogeneous system and the cellulose ester.
Background
Cellulose is the most abundant biomass material in nature and the most widely available, and hundreds of billions of tons of cellulose are produced by photosynthesis alone each year. The method has the characteristics of reproducibility, biodegradability, biocompatibility and the like, effectively utilizes cellulose, is beneficial to reducing environmental pollution and dependence on fossil resources, and has important significance for realizing carbon neutralization in China. Cellulose is a linear polymer of Anhydroglucose (AGU) linked by beta-1, 4 glycosidic linkages, each AGU being at C 2 、C 3 And C 6 The hydroxyl groups are respectively connected, and various cellulose derivatives can be prepared through etherification and esterification reactions, so that the application range of the cellulose derivatives is widened. Cellulose derivatives have been widely used in medicine, wastewater treatment and textilesAnd food and the like are closely related to human life, and cellulose ester is one of the most important derivatives of cellulose, can be made into various materials such as rubber negative films, protective coatings, liquid crystal displays and the like, and has wide application prospect and economic value.
At present, the industrial production method of cellulose ester mainly comprises the steps of mixing cellulose with excessive corresponding anhydride in a heterogeneous system, and reacting under the action of an acid catalyst. These production methods have the disadvantages of environmental pollution, uncontrollable substitution of products and waste of raw materials, and the esterification reaction time is generally long. These problems can be solved by synthesizing cellulose esters in a homogeneous system. However, because of the large number of hydrogen bonds within and outside the cellulose molecule, the cellulose is insoluble in common organic solvents, limiting its chemical modification. Furthermore, melt processing is also difficult because the cellulose does not undergo any thermal transition before the thermal decomposition temperature is reached. Thus, dissolution of cellulose into a homogeneous solution can greatly reduce the limitations on its chemical modification. With continuous research on cellulose solvents, ionic liquids and CO are currently mainly used 2 Cellulose solvent systems such as reversible solvents. The ionic liquid has good effects on cellulose dissolution and modification, but the ionic liquid has high cost and limits the application of the ionic liquid. CO 2 The reversible solvents have proven to be reliable and inexpensive, and are capable of controlling the degree of substitution of cellulose esters in such systems. Chinese patent No. CN104277121A discloses a method for producing cellulose ester by using cellulose and CO 2 The cellulose ester is homogeneously synthesized after the cellulose is dissolved by the reversible solvent, but the reaction time is longer, the reaction time is more than 1h, a higher substitution degree is required to be obtained, and the substitution degree is required to be 3 under the condition of 0.5MPa depending on the extension reaction time and/or the reaction temperature, and the extension reaction time is required to be 5h, so that the production efficiency is low and the energy consumption is increased. Chinese patent No. CN107722127a discloses a method for preparing cellulose ester, which uses vinyl ester as acylating agent to prepare cellulose ester, but has the problem of producing toxic by-products such as aldehydes. Chinese patent No. CN110283254A discloses a fiberThe preparation method of propylene carbonate adopts microwave heating to prepare cellulose carbonate in a heterogeneous system, but has the problem of overhigh reaction pressure; meanwhile, although microwave irradiation is adopted, carbon dioxide gas is introduced, and the reaction time is 0.1-1.5 h, according to the embodiment provided by the method, only 0.5MPa can be obtained, and the reaction is carried out for 0.5h at 40 ℃ under the microwave irradiation, no specific technical scheme capable of realizing the cellulose acylation reaction rapidly is provided, and no influence of the microwave irradiation on the substitution degree is clear. Chinese patent No. CN101597336a discloses a method for rapidly synthesizing cellulose carbamate by microwave heating, which uses a cellulose/urea mixture to prepare cellulose carbamate in a heterogeneous system, but has the problems of low substitution degree and uncontrollable substitution. Liu Chuanfu et al, paper Science&In Technology,2007 Vol.26 No.6", the reaction of cellulose and phthalic anhydride is carried out by taking an ionic liquid AmimCl as a medium at room temperature, the reaction time is increased from 1h to 5h, when the reaction temperature is increased from 80 ℃ to 100 ℃, the substitution Degree (DS) of the cellulose can only be increased from 0.21 to 0.62, obviously, the substitution degree is improved only in a limited effect on the essence only depending on the use amount of the improved anhydride and the improvement of the reaction conditions (temperature and time).
As can be seen from the above analysis, in the prior art, the preparation of cellulose ester has the technical problems of long reaction time, uncontrollable substitution degree and high reaction temperature and pressure, meanwhile, the substitution degree is extremely difficult to control when anhydride is adopted as an esterification raw material, especially when a microwave heating process is adopted for reaction, the substitution degree can not meet the expectations although the reaction time can be shortened, especially when anhydride is adopted as the raw material, a large amount of organic base is added in the preparation process of a homogenization system, so that acid-base neutralization causes a large amount of side reaction, thereby reducing the substitution degree of cellulose ester, and the uncontrollable substitution degree appears along with the increase of the reaction temperature, so that the substitution degree in the actual production process is greatly different. Therefore, the adoption of the method for preparing the cellulose ester with high substitution degree and accurate and controllable speed and efficiency has the technical problems of optimizing the reaction conditions, simplifying the production process, reducing the production cost and being capable of realizing the high-speed and high-efficiency preparation of the cellulose ester.
Disclosure of Invention
Aiming at the technical problems existing in the prior art, the application discloses a method for preparing cellulose ester by a microwave heating method in a homogeneous system and the cellulose ester, which realize rapid preparation of the cellulose ester under normal pressure by utilizing microwave heating, greatly improve the synthesis efficiency on the premise of keeping the advantages of controllable substitution degree of a product of the homogeneous system and environmental protection, and the method has the advantages of simple operation, mild condition, rapid reaction, no need of adding a catalyst, high and controllable substitution degree, no toxic byproducts, realization of rapid continuous cellulose esterification and universality on reaction substrates with different activities.
In order to achieve the above purpose, the application provides the following technical scheme, namely, a method for preparing cellulose ester by a microwave heating method in a homogeneous system, wherein in a homogeneous cellulose solution, anhydride is dropwise added into the cellulose solution at a constant speed by the microwave heating method under normal pressure to carry out esterification reaction, so as to obtain cellulose ester; wherein, the microwave heating method adopts a gradient heating process.
Preferably, the gradient heating process comprises a constant-speed heating stage and a rapid heating stage; the uniform temperature rising stage is synchronously carried out with the anhydride dropping process.
Preferably, the dropping speed of the anhydride is 0.125-2 mL/min; the temperature rising rate of the constant temperature rising stage is 0.5-50 ℃/min.
Preferably, in the constant-speed heating stage, the reaction temperature is heated from 20 ℃ to 25-60 ℃; in the rapid heating stage, the reaction temperature is heated to 40-100 ℃.
Preferably, the method comprises the steps of dropwise adding anhydride into a homogeneous cellulose solution at a constant speed under normal pressure, sequentially carrying out a constant speed heating stage-a rapid heating stage, then maintaining for 1-10 min to obtain a reaction mixed solution, and carrying out precipitation, washing, purification and drying on the reaction mixed solution to obtain the cellulose ester.
Preferably, anhydride is added into the homogeneous cellulose solution, microwave heating reaction is carried out, reaction mixture is obtained after the reaction is finished, and the reaction mixture is precipitated, washed, purified and dried to obtain the cellulose ester.
Preferably, the anhydride is one or more of acetic anhydride, propionic anhydride, butyric anhydride, valeric anhydride, caproic anhydride, enanthic anhydride, caprylic anhydride, nonic anhydride and capric anhydride.
Preferably, the washing and purification of the reaction mixture adopts water and/or lower aliphatic alcohol as a solvent; the volume ratio of the water and/or the lower fatty alcohol to the reaction mixed solution is 1-10:1; and/or, when water is mixed with the lower fatty alcohol, the lower fatty alcohol is any one or more of methanol, ethanol, propanol, isopropanol, n-butanol and tert-butanol, and the volume ratio of water to the lower fatty alcohol is 0.5-5:1; and/or when water is mixed with the lower fatty alcohol, the lower fatty alcohol is one or more of methanol, ethanol, propanol, isopropanol, n-butanol and tert-butanol, and the volume ratio of water to the lower fatty alcohol is 1-10:1.
Preferably, in the microwave heating reaction, the frequency of the microwave is 300 MHz-300 GHz electromagnetic wave, and the reaction power is 1-200W.
Preferably, the homogeneous cellulose solution preparation method comprises mixing cellulose, organic base and organic solvent, introducing CO under the conditions of 0.1-2 MPa and 40-80 DEG C 2 Carrying out a homogenization reaction to obtain cellulose-based CO 2 Reversible ionic liquid compound, the reaction system is converted from heterogeneous phase into transparent clear homogeneous cellulose solution.
Preferably, the organic solvent is a polar aprotic organic solvent with a boiling point of more than or equal to 150 ℃; and/or the organic solvent is selected from one or more of dimethyl sulfoxide (DMSO), N-Dimethylformamide (DMF) and N, N-dimethylacetamide (DMAc); and/or, the organic base has a pKa >20; and/or the organic base is selected from one or more of 1, 8-diazabicyclo [5,4,0] -7-undecene (DBU), 1,5, 7-triazabicyclo [4, 0] dec-5-ene (TBD), 1, 5-diazabicyclo [4,3,0] non-5-ene (DBN), phosphazene base (P2-Et) and 1, 4-diazabicyclo- [2, 2] octane (DABCO).
Preferably, the cellulose is selected from microcrystalline cellulose, alpha-cellulose, and one or more of cellulose isolated from corncob, cotton, pulp, wood pulp, bamboo pulp, or from agroforestry straw; the cellulose has the structural formula:
wherein n is the Degree of Polymerization (DP), and 100 < n < 2000; wherein the molar ratio of the anhydride to glucose units (AGU) in the cellulose is 1-10:1.
The cellulose ester prepared by adopting the technical scheme has the substitution degree ranging from 0.1 to 3. According to the need, cellulose esters having a specific degree of substitution can be produced by controlling the reaction conditions.
According to the application, on the basis of the prior art, the preparation of the cellulose ester with specific substitution degree can be carried out at normal temperature and normal pressure by optimizing the process conditions and simultaneously meeting the principles of saving cost and simplifying the process, the concentration of the solution of the cellulose ester can be reduced to 1wt%, the acid anhydride is used as a reactant with high chemical activity, the chemical structure and activity are still stable under the action of microwaves, the cellulose ester with high and controllable substitution degree is obtained in a very short time, and the prepared cellulose ester can be dissolved in one or more of dimethyl sulfoxide, methanol, acetone and chloroform serving as organic solvents.
On the other hand, the microwave heating method is applied to the esterification reaction of anhydride and cellulose, the anhydride is used as a reactant with high chemical activity, and a large amount of organic alkali exists in a homogeneous reaction system, so that the acid and the alkali undergo a violent neutralization reaction, the reaction is fast under the catalysis of microwaves, the substitution degree of cellulose ester is greatly influenced, the cellulose ester prepared by directly adopting the one-time feeding method in the prior art cannot obtain the expected substitution degree, and the substitution degree of the obtained cellulose ester cannot be accurately controlled due to uncontrollable side reaction. The application combines the gradient heating method and the process of uniformly dropping the anhydride, so that the slowly dropped anhydride takes precedence over the cellulose in a homogeneous system to carry out esterification reaction, thereby greatly reducing side reaction, having mild and rapid reaction, short reaction time, being capable of being realized under normal pressure and low temperature, being beneficial to large-scale production, reducing energy consumption and saving cost.
The technical scheme of the application has the technical effects that:
1. by adopting the technical scheme of the application, the cellulose ester with high and controllable substitution degree is rapidly and efficiently synthesized under the condition of normal pressure and low temperature by utilizing the homogeneous reaction system under the condition of microwave heating.
2. By adopting the technical scheme of the application, the method of combining a gradient heating mode with uniform-speed dropwise addition of the anhydride is adopted, so that side reaction in the esterification reaction process is reduced, and the substitution degree of cellulose ester is improved.
3. By adopting the technical scheme of the application, the substitution degree of the product can be controlled by adjusting the reaction conditions, the method is simple to operate, mild in condition, rapid in reaction, free from adding catalyst, free from generating toxic byproducts, universal to reaction substrates with different activities, capable of effectively overcoming the problems of long reaction time and low efficiency in the traditional heating method for preparing cellulose ester, and capable of realizing rapid continuous cellulose esterification.
4. By adopting the technical scheme of the application, the substitution degree of the cellulose ester prepared according to the specific reaction condition is controllable, and the control of the production process is facilitated by constructing the relation diagram of the reaction condition and the substitution degree in production, thereby being beneficial to the large-scale promotion of industrial production.
Drawings
FIG. 1 is a nuclear magnetic resonance spectrum of a cellulose ester product prepared in example 1 of the present application.
FIG. 2 is a nuclear magnetic resonance spectrum of the cellulose ester product prepared in example 1 of the present application.
FIG. 3 is an infrared spectrum of the cellulose ester product prepared in example 1 of the present application.
FIG. 4 is an XRD spectrum of the cellulose ester product prepared in example 1 of the present application.
FIG. 5 is a TGA curve of the cellulose ester product prepared in example 1 of the present application.
FIG. 6 is a DSC curve of the cellulose ester product prepared in example 1 of the present application.
Detailed Description
The objects, technical solutions and advantages of the embodiments of the present application will be more apparent, and the technical solutions in the embodiments of the present application will be clearly and completely described, and it is apparent that the described embodiments are some embodiments of the present application, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the application without making any inventive effort, are intended to fall within the scope of the application.
The application provides a method for preparing cellulose ester by a microwave heating method in a homogeneous system, wherein in a homogeneous cellulose solution, anhydride is dropwise added into the cellulose solution at a constant speed by the microwave heating method under the normal pressure condition to carry out esterification reaction to obtain cellulose ester; wherein, the microwave heating method adopts a gradient heating process.
Preferably, the gradient heating process comprises a constant-speed heating stage and a rapid heating stage; the constant-speed heating stage is synchronously carried out with the anhydride dropping process.
Preferably, the dropping speed of the anhydride is 0.125-2.5 mL/min; the temperature rising rate of the constant temperature rising stage is 0.5-50 ℃/min.
Preferably, in the constant-speed heating stage, the reaction temperature is heated from 20 ℃ to 25-60 ℃; the temperature is raised to 40-100 ℃ in the rapid temperature raising stage.
Preferably, the method comprises the steps of dripping anhydride into a homogeneous cellulose solution at a constant speed under normal pressure, sequentially carrying out a constant speed heating stage-a rapid heating stage, maintaining for 1-10 min to obtain a reaction mixed solution, and carrying out precipitation, washing, purification and drying on the reaction mixed solution to obtain cellulose ester.
Preferably, in the microwave heating reaction, the frequency of the microwave is 300 MHz-300 GHz electromagnetic wave, and the reaction power is 1-200W.
Preferably, the homogeneous cellulose solution is prepared by mixing cellulose, organic base and organic solvent, and then introducing under the conditions of 0.1-2 MPa and 40-80 DEG CCO in 2 Carrying out a homogenization reaction to obtain cellulose-based CO 2 Reversible ionic liquid compound, the reaction system is converted from heterogeneous phase into transparent clear homogeneous cellulose solution. Wherein the organic solvent is a polar aprotic organic solvent with a boiling point of more than or equal to 150 ℃; and/or the organic solvent is selected from one or more of dimethyl sulfoxide (DMSO), N-Dimethylformamide (DMF) and N, N-dimethylacetamide (DMAc); the pKa of the organic base is more than 20; and/or the organic base is selected from 1, 8-diazabicyclo-bis [5,4,0]]-7-undecene (DBU), 1,5, 7-triazabicyclo [4, 0]]Dec-5-ene (TBD), 1, 5-diazabicyclo [ 4.3.0 ]]Non-5-ene (DBN), phosphazene base (P2-Et), 1, 4-diazabicyclo- [2,2]One or more of octane (DABCO).
In the application, the cellulose raw material is selected from microcrystalline cellulose, alpha-cellulose and one or more of cellulose separated from corncob, cotton, paper pulp, wood pulp, bamboo pulp or straw of agriculture and forestry matters; the structural formula is as follows:
wherein n is the Degree of Polymerization (DP), and 100 < n < 2000.
As a preferred example, the acid anhydride is one or more of acetic anhydride, propionic anhydride, butyric anhydride, valeric anhydride, caproic anhydride, enanthic anhydride, caprylic anhydride, pelargonic anhydride, and capric anhydride.
As a preferred embodiment, the molar ratio of anhydride to glucose units (AGU) in the cellulose is 1-10:1.
The cellulose ester prepared by adopting the technical scheme has the substitution degree ranging from 0.1 to 3. According to the need, cellulose esters having a specific degree of substitution can be produced by controlling the reaction conditions.
The technical scheme of the application is further described in detail through specific examples. The experimental procedures, which do not address the specific conditions in the examples below, are generally carried out under conventional conditions or under conditions recommended by the manufacturer.
Example 1
The embodiment provides a preparation method of cellulose acetate, which comprises the following specific steps:
(1) 4.72g of corncob cellulose (dp=1440), 100g of dimethyl sulfoxide (DMSO), 13.28g of 1, 8-diazabicyclo-bis [5,4,0] -7-undecene (DBU) were mixed to obtain a mixture; wherein the molar ratio of DBU to glucose units (AGU) in the cellulose is 3:1;
(2) Introducing CO into the mixture obtained in the step (1) under the pressure condition of 0.5MPa 2 Carrying out system homogenization reaction for 2h at 55 ℃ to obtain cellulose-based CO 2 Reversible ionic liquid compound to form transparent clear homogeneous cellulose solution with concentration of 4wt%; cooling the homogeneous cellulose solution to 20 ℃;
(3) Placing 17.70g of the cellulose solution obtained in the step (2) in a microwave reactor, dropwise adding 2.5mL of acetic anhydride into the cellulose solution at a speed of 0.25mL/min under normal pressure (0.1 MPa), wherein the dropwise adding time is 10min (first reaction time), and stirring while dropwise adding, wherein the molar ratio of acetic anhydride/AGU is=6/1;
heating acetic anhydride to 40 ℃ at constant speed by microwaves at the same time of dropwise adding, heating the acetic anhydride to 80 ℃ quickly by microwaves after dropwise adding, and keeping for 5min (second reaction time) to obtain a reaction mixed solution; wherein, the dripping of acetic anhydride and the heating up of microwave heating are synchronously carried out;
(4) Precipitating the reaction mixed solution, washing with 10 times of water, and finally drying to obtain the cellulose acetate.
Example 2
This example differs from example 1 in that in step (3), 2.5mL of acetic anhydride was added dropwise to the cellulose solution at a rate of 0.25mL/min for a period of 10min (first reaction time);
and (3) heating the acetic anhydride dropwise while heating the acetic anhydride at a uniform speed by microwaves to increase the first reaction temperature to 40 ℃, and after the acetic anhydride dropwise is finished, rapidly heating the acetic anhydride by microwaves to increase the second reaction temperature to 80 ℃ and keeping the first reaction time for 1min to obtain a reaction mixed solution.
Example 3
This example differs from example 1 in that in step (3), 2.5mL of acetic anhydride was added dropwise to the cellulose solution at a rate of 0.25mL/min for a period of 10min
And heating the acetic anhydride to 40 ℃ at a constant speed by microwaves while dropwise adding the acetic anhydride, heating the acetic anhydride to 80 ℃ by microwaves after the dropwise adding of the acetic anhydride is finished, and keeping the temperature for 3min to obtain a reaction mixed solution.
Example 4
This example differs from example 1 in that in step (3), 2.5mL of acetic anhydride was added dropwise to the cellulose solution at a rate of 0.25mL/min for a period of 10min
And heating the acetic anhydride to 40 ℃ at a constant speed by microwaves while dropwise adding the acetic anhydride, heating the acetic anhydride to 80 ℃ by microwaves after the dropwise adding of the acetic anhydride is finished, and keeping the temperature for 10min to obtain a reaction mixed solution.
Example 5
This example differs from example 1 in that in step (3), 2.5mL of acetic anhydride was added dropwise to the cellulose solution at a rate of 0.5mL/min for a period of 5min;
and heating the acetic anhydride to 40 ℃ at a constant speed by microwaves while dropwise adding the acetic anhydride, and after the dropwise adding of the acetic anhydride is finished, rapidly heating the acetic anhydride to 80 ℃ by microwaves, and keeping the temperature for 5min to obtain a reaction mixed solution.
Example 6
This example differs from example 1 in that in step (3), 2.5mL of acetic anhydride was added dropwise to the cellulose solution at a rate of 0.5mL/min for a period of 5min;
and heating the acetic anhydride to 40 ℃ at a constant speed by microwaves while dropwise adding the acetic anhydride, and after the dropwise adding of the acetic anhydride is finished, rapidly heating the acetic anhydride to 80 ℃ by microwaves, and keeping the temperature for 1min to obtain a reaction mixed solution.
Example 7
This example differs from example 1 in that in step (3), 2.5mL of acetic anhydride was added dropwise to the cellulose solution at a rate of 1mL/min for a period of 2.5min;
and heating the acetic anhydride to 40 ℃ at a constant speed by microwaves while dropwise adding the acetic anhydride, and after the dropwise adding of the acetic anhydride is finished, rapidly heating the acetic anhydride to 80 ℃ by microwaves, and keeping the temperature for 5min to obtain a reaction mixed solution.
Example 8
This example differs from example 1 in that in step (3), 2.5mL of acetic anhydride was added dropwise to the cellulose solution at a rate of 0.125mL/min for 20min;
and heating the acetic anhydride to 40 ℃ at a constant speed by microwaves while dropwise adding the acetic anhydride, and after the dropwise adding of the acetic anhydride is finished, rapidly heating the acetic anhydride to 80 ℃ by microwaves, and keeping the temperature for 5min to obtain a reaction mixed solution.
Example 9
This example differs from example 1 in that in step (3), 2.5mL of acetic anhydride was added dropwise to the cellulose solution at a rate of 0.125mL/min for 20min;
and heating the acetic anhydride to 40 ℃ at a constant speed by microwaves while dropwise adding the acetic anhydride, and after the dropwise adding of the acetic anhydride is finished, rapidly heating the acetic anhydride to 80 ℃ by microwaves, and keeping the temperature for 1min to obtain a reaction mixed solution.
Example 10
This example differs from example 1 in that in step (3), 2.5mL of acetic anhydride was added dropwise to the cellulose solution at a rate of 0.25mL/min for a period of 10min
And heating the acetic anhydride to 40 ℃ at a constant speed by microwaves while dropwise adding the acetic anhydride, and after the dropwise adding of the acetic anhydride is finished, rapidly heating the acetic anhydride to 60 ℃ by microwaves, and keeping the temperature for 5min to obtain a reaction mixed solution.
Example 11
This example differs from example 1 in that in step (3), 2.5mL of acetic anhydride was added dropwise to the cellulose solution at a rate of 0.25mL/min for a period of 10min
And heating the acetic anhydride to 40 ℃ at a constant speed by microwaves while dropwise adding the acetic anhydride, and after the dropwise adding of the acetic anhydride is finished, rapidly heating the acetic anhydride to 100 ℃ by microwaves, and keeping the temperature for 5min to obtain a reaction mixed solution.
Example 12
This example differs from example 1 in that in step (3), 2.5mL of acetic anhydride was added dropwise to the cellulose solution at a rate of 0.25mL/min for a period of 10min
And heating the acetic anhydride to 25 ℃ at a constant speed by microwaves while dropwise adding the acetic anhydride, and after the dropwise adding of the acetic anhydride is finished, rapidly heating the acetic anhydride to 60 ℃ by microwaves, and keeping the temperature for 5min to obtain a reaction mixed solution.
Example 13
This example differs from example 1 in that in step (3), 2.5mL of acetic anhydride was added dropwise to the cellulose solution at a rate of 0.25mL/min for a period of 10min
And heating the acetic anhydride to 25 ℃ at a constant speed by microwaves while dropwise adding the acetic anhydride, and after the dropwise adding of the acetic anhydride is finished, rapidly heating the acetic anhydride to 80 ℃ by microwaves and keeping the temperature for 5min to obtain a reaction mixed solution.
Example 14
This example differs from example 1 in that in step (3), 2.5mL of acetic anhydride was added dropwise to the cellulose solution at a rate of 0.25mL/min for a period of 10min
And heating the acetic anhydride to 60 ℃ at a constant speed by microwaves while dropwise adding the acetic anhydride, and after the dropwise adding of the acetic anhydride is finished, rapidly heating the acetic anhydride to 80 ℃ by microwaves and keeping the temperature for 5min to obtain a reaction mixed solution.
Example 15
This example differs from example 1 in that in step (3), 2.5mL of acetic anhydride was added dropwise to the cellulose solution at a rate of 0.25mL/min for a period of 10min
And heating the acetic anhydride to 25 ℃ at a constant speed by microwaves while dropwise adding the acetic anhydride, and after the dropwise adding of the acetic anhydride is finished, rapidly heating the acetic anhydride to 40 ℃ by microwaves and keeping the temperature for 5min to obtain a reaction mixed solution.
Example 16
This example differs from example 1 in that in step (3), 2.5mL of acetic anhydride was added dropwise to the cellulose solution at a rate of 2mL/min;
and heating the acetic anhydride to 40 ℃ at a constant speed by microwaves while dropwise adding the acetic anhydride, and after the dropwise adding of the acetic anhydride is finished, rapidly heating the acetic anhydride to 80 ℃ by microwaves, and keeping the temperature for 5min to obtain a reaction mixed solution.
Example 17
This example differs from example 1 in that the molar ratio acetic anhydride/AGU = 10/1.
Example 18
This example differs from example 1 in that the molar ratio acetic anhydride/AGU = 1/1.
Example 19
This example differs from example 1 in that the molar ratio acetic anhydride/AGU = 2/1.
Example 20
This example differs from example 1 in that the concentration of the cellulose solution is 1wt%.
Example 21
This example differs from example 1 in that the concentration of the cellulose solution is 1.5% by weight.
Example 22
This example differs from example 1 in that the concentration of the cellulose solution is 2% by weight.
Example 23
This example differs from example 1 in that the concentration of the cellulose solution is 8% by weight.
Comparative example 1
The comparative example differs from example 1 in that in step (3), the pressure condition of the reaction was 0.5MPa, and the other conditions were the same.
Comparative example 2
The comparative example differs from example 1 in that in step (3), the pressure condition of the reaction was 2MPa, and the other conditions were the same.
Comparative example 3
The difference between this comparative example and example 1 is that in step (3), 2.5mL of acetic anhydride was added to the fiber solution at one time, and the temperature was raised to 40℃at a constant speed by microwave for 10 minutes, and the temperature was raised to 80℃rapidly by microwave for 5 minutes, to obtain a reaction mixed solution.
Comparative example 4
The difference between this comparative example and example 1 is that in step (3), 2.5mL of acetic anhydride was added to a cellulose solution at once, and the mixture was heated to 40℃at a constant temperature and a heating time of 10 minutes, to obtain a reaction mixture solution.
Comparative example 5
The difference between this comparative example and example 1 is that in step (3), 2.5mL of acetic anhydride was added to a cellulose solution at once, and the mixture was heated to 80℃at a constant temperature for 10 minutes to obtain a reaction mixture.
Comparative example 6
The difference between this comparative example and example 1 is that in step (3), acetic anhydride is added dropwise while heating to 40℃at a constant temperature by microwave, and the reaction is completed to obtain a reaction mixture.
Comparative example 7
The comparative example is different from example 1 in that in step (3), acetic anhydride is added dropwise while heating to 80 ℃ with microwave at constant speed, and the reaction is completed after the acetic anhydride is added dropwise, to obtain a reaction mixed solution.
In the above examples and comparative examples, the time for rapid heating by microwave heating was limited by the microwave power, and it was generally completed within 1 min.
The calculation formula of the substitution degree is DS= (7×I) CH3 )/(3×I H，AGU ) Wherein I CH3 Represents the integral of the esterified methyl proton peak, I AGU Representing the integral of the proton peak on the cellulose.
Referring to fig. 1 and 2, a nuclear magnetic hydrogen spectrogram and a nuclear magnetic carbon spectrogram of example 1 are respectively obtained by 13 C NMR 1 The Degree of Substitution (DS) of the cellulose acetate in example 1 was 2.89 as calculated by H NMR curve. The calculation methods of the other examples and the comparative examples are the same.
Referring to FIG. 3, an infrared spectrum of example 1 is shown at 1752cm -1 The signal peak of carbonyl appears at this point, and the product is cellulose acetate in example 1.
Referring to fig. 4, an XRD spectrum of example 1 is shown to obtain the product of example 1, which has both crystalline and amorphous regions, typically cellulose triacetate.
Referring to fig. 5, a TGA profile of example 1, it is possible to obtain a thermal decomposition temperature of 284.6 ℃ for the product of example 1.
Referring to FIG. 6, a DSC plot of example 1 may be obtained for the product of example 1 having a glass transition temperature.
According to the results of the substitution degrees of examples and comparative examples, they were analyzed in combination with the production process, see tables 1 and 2, respectively.
TABLE 1 influence of different anhydride dropping speeds and microwave heating conditions on substitution degree
TABLE 2 cellulose concentration, acetic anhydride/AGU ratio and influence on substitution degree (under atmospheric pressure conditions)
| Cellulose concentration/wt% | anhydride/AGU molar ratio | Degree of substitution | |
| Example 1 | 4 | 6/1 | 2.89 |
| Example 17 | 4 | 10/1 | 3.00 |
| Example 18 | 4 | 1/1 | 0.75 |
| Example 19 | 4 | 2/1 | 1.57 |
| Example 20 | 1 | 6/1 | 2.75 |
| Example 21 | 1.5 | 6/1 | 2.81 |
| Example 22 | 2 | 6/1 | 2.86 |
| Example 23 | 8 | 6/1 | 2.94 |
Referring to table 1, cellulose acetate with different degrees of substitution can be obtained using the anhydride dropping rate, the microwave heating temperature condition, and the second reaction time under different conditions.
Preferably, examples 1 to 8 show that the substitution degree increases rapidly with the increase of the second reaction time, but the substitution degree increases slowly after the increase of the second reaction time to 5 minutes. Specifically, when the anhydride dropping speed is consistent with the microwave heating condition, the substitution value of the product is increased along with the extension of the second reaction time and the law that the substitution value is increased rapidly and then slowed down is presented; when the microwave heating condition is consistent with the second reaction time, the dropping speed of the anhydride is reduced, and the substitution value of the obtained product is higher; when the anhydride dropping speed is the same as the second reaction time, the microwave heating temperature is increased, and the substitution value of the obtained product is increased.
As can be seen from the comparison of the results of example 1 and comparative example 3, the substitution degree of the product obtained by the single-use addition of the acid anhydride was greatly reduced as compared with that obtained by the dropping of the acid anhydride. Further, as a result of examples 1, 7, 9 and 16, and comparative examples 4 to 7, it was found that the slower the dropping speed, the higher the substitution degree was under the same conditions; when the second reaction time is zero, the substitution degree of the cellulose ester thereof rapidly decreases. Referring to example 14, comparative example 5 and comparative example 7, when the first reaction temperature is raised to 80 ℃, the substitution degree is rapidly lowered, because it is possible that one-time addition of acid anhydride or addition of acid anhydride under high temperature conditions may cause more side reactions, resulting in lowering of the substitution degree of the product.
Analyzing the reason: in the preparation process of the homogeneous system of cellulose, a large amount of organic alkali is required to be added, and acid anhydride is added into the homogeneous system, if the adding speed is too high or the material is fed at one time, the acid-base neutralization reaction of the acid anhydride and the organic alkali can be caused, so that the conversion rate of the esterification reaction of the acid anhydride and the cellulose is reduced.
Further, referring to Table 2, comparing the reaction conditions and the substitution results of examples 1 and examples 20 to 23, it can be seen that the cellulose concentration increases, and the substitution value of the obtained product increases accordingly, with the cellulose concentration being as low as 1wt% at the lowest, under the same other reaction conditions. Under the condition of the same reaction conditions, the ratio of the anhydride to the AGU is increased, and the substitution value of the obtained product is increased.
And it can be seen from examples 12, 13 and 15 that the first reaction temperature can be carried out at a low temperature (anhydride dropping stage), and the temperature is rapidly raised after the completion of the anhydride dropping. In the actual production process, the dropping speed of the anhydride can be reduced at a low temperature, and the temperature is raised after the dropping is finished, so that the higher substitution degree is obtained, the energy consumption is reduced at a low temperature, and the cellulose ester with the required substitution degree can be obtained.
In addition, the substitution degrees of comparative examples 1 and 2 were 2.91 and 2.94, respectively, which were slightly improved as compared with example 1, but example 1 was more suitable for mass popularization of practical production from the viewpoints of simplification of process conditions and reduction of production costs.
From the above results, it can be seen from the results in tables 1 and 2 that the influence of the dropping speed of the acid anhydride and the microwave heating conditions (including the first reaction time, the second reaction time, the first reaction temperature, and the second reaction temperature) on the substitution degree is regular, and the process conditions can be formulated according to the cellulose ester with the required substitution degree and the actual production conditions, so that the cellulose ester with the controllable substitution degree can be prepared, that is, the energy can be saved, the operation conditions can be simplified, and the large-scale popularization can be realized.
Obviously, the method adopted by the application can controllably and rapidly synthesize cellulose esters with different substitution values at lower temperature and normal pressure without adding catalyst.
The above is only a preferred embodiment of the present application, which is not to be construed as limiting the scope of the present application, and various modifications and variations of the present application will be apparent to those skilled in the art. Variations, modifications, substitutions, integration and parameter changes may be made to these embodiments by conventional means or may be made to achieve the same functionality within the spirit and principles of the present application without departing from such principles and spirit of the application.
Claims (5)
1. A method for preparing cellulose ester by a microwave heating method in a homogeneous system is characterized in that in a homogeneous cellulose solution, under the condition of normal pressure, anhydride is dropwise added into the cellulose solution at a constant speed by the microwave heating method for esterification reaction to obtain cellulose ester;
the homogeneous cellulose solution preparation method comprises the steps of mixing cellulose and organic alkaliMixing with organic solvent, and introducing CO under the conditions of 0.1-2 MPa and 40-80 DEG C 2 Carrying out a homogenization reaction to obtain cellulose-based CO 2 Reversible ionic liquid compound, the reaction system is converted from heterogeneous phase into transparent clear homogeneous cellulose solution;
the organic solvent is a polar aprotic organic solvent with the boiling point of more than or equal to 150 ℃;
the organic solvent is selected from one or more of dimethyl sulfoxide (DMSO), N-Dimethylformamide (DMF) and N, N-dimethylacetamide (DMAc);
the organic base has a pKa >20;
the organic base is selected from one or more of 1, 8-diazabicyclo-bis [5,4,0] -7-undecene (DBU), 1,5, 7-triazabicyclo [4, 0] dec-5-ene (TBD), 1, 5-diazabicyclo [4,3,0] non-5-ene (DBN), phosphazene base (P2-Et), and 1, 4-diazabicyclo- [2, 2] octane (DABCO);
wherein, the microwave heating method adopts a gradient heating process;
the gradient heating process comprises a constant-speed heating stage and a rapid heating stage; the uniform temperature rising stage is synchronously carried out with the anhydride dropping process;
the method for preparing cellulose ester comprises the steps of dropwise adding anhydride into a homogeneous cellulose solution at a constant speed under normal pressure, sequentially carrying out a constant speed heating stage-a rapid heating stage, and then keeping for 1-10 min to obtain a reaction mixed solution, and carrying out precipitation, washing, purification and drying on the reaction mixed solution to obtain the cellulose ester;
the dropping speed of the anhydride is 0.125-2 mL/min;
the temperature rising rate of the constant temperature rising stage is 0.5-50 ℃/min;
the reaction temperature is raised from 20 ℃ to 25-60 ℃ in the uniform temperature raising stage;
and in the rapid heating stage, the reaction temperature is heated to 40-100 ℃.
2. The method for preparing cellulose ester by microwave heating in homogeneous system according to claim 1, wherein the acid anhydride is one or more of acetic anhydride, propionic anhydride, butyric anhydride, valeric anhydride, caproic anhydride, heptanoic anhydride, caprylic anhydride, nonanoic anhydride and capric anhydride.
3. The method for preparing cellulose ester by using the microwave heating method in a homogeneous system according to claim 1, wherein in the microwave heating reaction, the frequency of microwaves is 300 MHz-300 GHz electromagnetic waves, and the reaction power is 1-200W.
4. A method for preparing cellulose ester by microwave heating in a homogeneous system according to any one of claims 1 to 3, wherein the cellulose is selected from microcrystalline cellulose, alpha-cellulose, and one or more of cellulose isolated from corncob, cotton, pulp, wood pulp, bamboo pulp or from crop straw; the cellulose has the structural formula:
wherein n is the Degree of Polymerization (DP), and 100<n<2000。
5. The method for preparing cellulose ester by microwave heating in a homogeneous system according to claim 4, wherein the molar ratio of the acid anhydride to glucose units (AGU) in the cellulose is 1-10:1.
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| CN104277121A (en) * | 2013-07-03 | 2015-01-14 | 中国科学院大连化学物理研究所 | Method for preparing cellulose ester by using cellulose |
| CN105175760A (en) * | 2015-07-16 | 2015-12-23 | 广西科开成林科技有限公司 | Preparation method of cellulose acetate butyrate membrane |
| CN107722127A (en) * | 2017-09-27 | 2018-02-23 | 贵州大学 | The preparation method of cellulose esters |
| CN109134670A (en) * | 2018-08-30 | 2019-01-04 | 贵州大学 | A kind of preparation method of acetyl cellulose propionate and its mixed ester |
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| CN104277121A (en) * | 2013-07-03 | 2015-01-14 | 中国科学院大连化学物理研究所 | Method for preparing cellulose ester by using cellulose |
| CN105175760A (en) * | 2015-07-16 | 2015-12-23 | 广西科开成林科技有限公司 | Preparation method of cellulose acetate butyrate membrane |
| CN107722127A (en) * | 2017-09-27 | 2018-02-23 | 贵州大学 | The preparation method of cellulose esters |
| CN109134670A (en) * | 2018-08-30 | 2019-01-04 | 贵州大学 | A kind of preparation method of acetyl cellulose propionate and its mixed ester |
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| Synthesis of cellulose acetate propionate and cellulose acetate butyrate in a CO2/DBU/DMSO system;Qinqin Xu等;CELLULOSE;第25卷(第01期);第205-216页 * |
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