CN115161806A

CN115161806A - CNFs production process based on double-strategy circulation TEMPO of direct circulation and extraction recovery

Info

Publication number: CN115161806A
Application number: CN202210743002.4A
Authority: CN
Inventors: 苏荣欣; 陈绍煌; 黄仁亮; 樊洁; 丁国杰
Original assignee: Tianjin Rumijiye New Material Co ltd; Tianjin University
Current assignee: Tianjin Rumijiye New Material Co ltd; Tianjin University
Priority date: 2022-06-28
Filing date: 2022-06-28
Publication date: 2022-10-11

Abstract

The invention discloses a CNFs production process based on double-strategy circulation TEMPO of direct circulation and extraction recovery, which comprises the following steps: TEMPO mediated catalytic oxidation, solid-liquid separation, separation and oxidation of cellulose and reaction liquid, and homogeneous nano-fibrosis to obtain CNFs; part of the filtrate is directly recycled, and TEMPO is recovered from the rest filtrate through extraction and is concentrated through distillation treatment for the next batch of cyclic reaction. The invention reduces the adverse effect of sodium salt by directly reusing part of the filtrate, and ensures the quality of CNFs obtained by multiple cyclic reactions. In addition, TEMPO in the filtrate is recovered through extraction and distillation and is used for a new batch of cellulose catalytic oxidation reaction, and the carboxyl content and the fiber morphology of the obtained CNFs product are kept stable. The method simply and efficiently recycles expensive toxic chemicals TEMPO, reduces environmental hazards, avoids further wastewater treatment operation, saves production cost, is beneficial to the actual production process, is a low-cost green sustainable process, and promotes the marketization of CNFs products.

Description

CNFs production process based on direct circulation and extraction recovery double-strategy circulation TEMPO

Technical Field

The invention belongs to the technical field of nano material preparation, and particularly relates to a low-cost CNFs production method for directly recycling and extracting and recovering filtrate.

Background

The 2, 6-tetramethylpiperidine-1-oxygen free radical (TEMPO) mediated oxidation of natural wood cellulose can be carried out under aqueous conditions, has high reaction efficiency and regioselectivity, and has huge potential for bridging biomass resources and high and new nanotechnology. During TEMPO oxidation, carboxyl groups introduced on the surface of cellulose fibres cause them to delaminate and swell due to osmotic effects and electrostatic repulsion between negatively charged fibres. Thus, individual dispersed Cellulose Nanofibrils (CNFs) can be obtained by mechanical treatment with low energy consumption. The large number of carboxyl groups present on the surface of CNFs can impart different functional characteristics, such as hydrophobicity, mechanical strength, thermal stability, uv-shielding properties, and deodorizing properties, by exchanging or cross-linking with counter ions of metal or alkylammonium ions. The unique properties of CNFs bring them into maturity stage in various applications such as mechanical reinforcement, packaging, adsorption and separation, thermal regulation and biomedicine. In addition, CNFs also show great commercial value and are currently commercially used in ballpoint pens, cosmetics, and deodorant adult diapers.

TEMPO has a good effect on the regioselective oxidation of cellulose primary hydroxyl groups, is a necessary step in the production of CNFs, but TEMPO is expensive, is an organic chemical substance toxic to aquatic organisms, is easy to enrich in the environment, and cannot be directly discharged into wastewater after the production process is finished. Therefore, the recycling of TEMPO is an important requirement in the aspects of environment and economy, and is beneficial to the green and low-cost production of CNFs. TEMPO has been covalently immobilized on nanoparticles or polymers for recycling, but oxidized cellulose prepared by this method has a reduced carboxyl content. It was reported that TEMPO was immobilised at the end of a monomethoxy poly (ethylene glycol) of molecular weight 2000 (mPEG 2000-TEMPO) for reaction, and that after the first cycle the cellulose nanowhisker obtained had a carboxyl content of 0.55 mmol/g, much lower than the sample prepared with fresh TEMPO (0.92 mmol/g). With increasing number of cycles, the carboxyl content drops to around 60% of the original value. In addition, the CNFs produced by using immobilized TEMPO have the problem of nonuniform diameter, have larger micron-sized fibers and cannot be completely and mechanically separated into single nano-sized fibers, so that the quality of the CNFs product is greatly reduced.

TEMPO is used as catalyst, and nitroxyl radical, hydroxylamine and nitrosonium are used circularly for oxidation and may be reused after the reaction. For example, the related studies directly reuse the entire filtrate after the TEMPO oxidation reaction of cellulose, and the results show that the carboxyl content of the resulting CNFs gradually decreases as the number of cycles increases due to the accumulation of NaCl in the cycling reaction. In order to eliminate the adverse effect of sodium salt, relevant research is carried out to remove salt from filtrate by electrolysis, however, TEMPO is degraded in the electrolytic treatment process, so that TEMPO forms an open-loop structure, and the catalytic oxidation performance of TEMPO is seriously affected. Therefore, a new TEMPO recovery strategy is tried to be developed, TEMPO is recovered more simply and efficiently, and CNFs products with the same quality are obtained, so that the sustainable and low-cost production of the CNFs is realized, and the stable carboxyl content of the products is ensured.

Disclosure of Invention

Aiming at the defects in the prior art, the invention provides a low-cost CNFs production method for directly recycling and extracting and recovering filtrate, aiming at providing a new simple and efficient technical way for recycling toxic chemicals TEMPO so as to realize low-cost and green production of CNFs with stable quality.

The technical solution of the invention is as follows:

a CNFs production process based on direct circulation and extraction recovery double-strategy circulation TEMPO specifically comprises the following steps:

1) TEMPO-mediated catalytic oxidation: dissolving TEMPO and NaBr in deionized water, adding a cellulose raw material, stirring, fully soaking and swelling, and adding a NaClO solution to initiate an oxidation reaction;

2) Solid-liquid separation: separating the oxidized cellulose and the reaction solution by using a solid-liquid separation device; mechanically treating the oxidized cellulose, and realizing nano-fibrosis by adopting high-pressure homogenization to obtain CNFs;

3) Treating a reaction solution: dividing the reaction solution obtained by separation in the step 2) into direct circulating filtrate and extraction recovery filtrate according to a proportion;

4) Directly and circularly reacting the filtrate: adding fresh TEMPO, naBr and NaClO solution into the direct circulating filtrate obtained in the step 3), and repeating the steps 1) to 2) to carry out circulating reaction;

the circulating reaction and the first reaction in the step 1) form a first batch of reaction together;

5) And (3) extracting, recovering and recycling filtrate: extracting and recovering a catalyst TEMPO by using an extracting agent in the extracted and recovered filtrate in the step 3), and distilling to obtain a TEMPO/extracting agent concentrated solution; and (3) adding part of fresh TEMPO, naBr and NaClO solution again, and repeating the steps 1) to 2) to carry out the next batch of circular reaction.

Further, the dosage of TEMPO corresponding to each gram of cellulose raw material in the step 1) is 0.05 to 0.2 mmol, and the dosage of NaBr corresponding to each gram of cellulose raw material is 0.5 to 2.5 mmol; the mass ratio of the cellulose raw material to the NaClO solution in the step 1) is 1: (3 to 7); the mass ratio of the cellulose raw material to the deionized water is 1: (40 to 200).

Preferably, the cellulose material is one or more of bleached softwood pulp, bleached hardwood pulp and cotton.

Further, the time of the oxidation reaction in the step 1) is 1 to 5 hours; and continuously dropwise adding an NaOH solution in the oxidation reaction process, and adjusting the pH to 10.0-10.5.

Further, the solid-liquid separation device in the step 2) is a plate-and-frame filter press or a positive pressure filter.

Further, in the step 2), the high-pressure homogenizing pressure is 70 to 110MPa, and the homogenizing times are 6 to 12.

Further, in the step 3), the directly circulating filtrate accounts for 50wt% -80 wt%, and the extract recovered filtrate accounts for 20wt% -50 wt%.

Further, in the step 4), the amount of supplemented fresh TEMPO is 20-50 wt% of the adding amount of TEMPO in the step 1); the added amount of the added NaBr is 20-50 wt% of the added amount of the NaBr in the step 1); the amount of the added NaClO solution is the same as the amount added in the step 1).

Further, in the step 4), after the last circulation, the directly circulated filtrate accounts for 0wt%, and the extracted and recovered filtrate accounts for 100wt%.

Preferably, in the step 4), the cycle number of the direct cycle reaction is 1 to 4.

Further, in the step 5), the mass ratio of the extracting agent to the extracted and recovered filtrate is (1 to 5): 10.

further, after the distillation treatment in the step 5), the TEMPO/extractant concentrated solution is 0.5-5 wt% of the extraction recovery filtrate;

further, in the step 5), the amount of newly added fresh TEMPO is 10-30 wt% of the total addition amount of TEMPO in the first batch of reaction; the newly added NaBr is the same as the total addition of NaBr in the first batch of reaction; the amount of the NaClO solution added again is the same as the total amount of NaClO added in the first batch of reaction.

Further, the extraction agent in the step 5) is one or more of n-propyl acetate, ethyl acetate, n-hexane and methyl tert-butyl ether.

The invention has the beneficial effects that:

1) By directly recycling part of the filtrate and supplementing part of fresh reagents, the adverse effect of sodium salt accumulation in the filtrate is reduced, so that the stable quality (carboxyl content and fiber morphology) of the CNFs obtained by multiple cyclic reactions is ensured.

2) By carrying out extractive distillation operation on part of filtrate which is not directly reused, the obtained concentrated solution can be directly used for a new batch of cellulose catalytic oxidation reaction, and the carboxyl content and the fiber morphology of the obtained CNFs product are kept consistent with those of the original product.

3) The method has the advantages that the two modes are matched for use, so that expensive toxic chemical TEMPO is recycled simply and efficiently, the discharge of the toxic chemical TEMPO into waste water is avoided, and the environmental hazard is reduced; and the further wastewater treatment operation is avoided, the production cost is saved, and the method is favorable for the actual production process.

4) According to the invention, the reaction solution is circulated by direct circulation and extraction distillation double strategies, a low-cost green sustainable process is developed, the obtained CNFs have excellent and stable quality, have the potential of industrial scale-up production, and are expected to promote the commercial products of the CNFs to be pushed to the market.

Drawings

FIG. 1 is a transmission electron microscope image of CNFs prepared in example 3 of the present invention.

FIG. 2 is a comparison of the carboxyl contents of the CNFs prepared in the comparative example and in the different examples.

Detailed Description

The technical solution of the present invention is further explained below with reference to the embodiments and the accompanying drawings.

Example 1

Weighing 1.5 g of TEMPO and 12 g of NaBr, dissolving in 4.54 kg of deionized water, adding 0.1 kg of cotton, fully stirring for swelling, adding 0.35 kg of NaClO, reacting for 4 hours, and dropwise adding NaOH solution during the reaction to adjust the pH of the reaction system to 10.2. After the reaction was completed, the oxidized cotton slurry and the reaction solution were separated by a positive pressure filter, and the filtrate (about 5 kg) was collected. And mechanically treating the oxidized cotton slurry, and homogenizing for 10 times under the pressure of 70 MPa to obtain the CNFs product.

3.5 kg (70 wt%) of the filtrate was weighed out for the first cycle and the remaining 1.5 kg was stored refrigerated for future use. 0.45 g TEMPO and 3.6 g NaBr are added, dissolved in 3.5 kg filtrate, 1.05 kg deionized water is added, 0.1 kg cotton is added, and the flower is fully stirred and swollen. Then 0.35 kg of NaClO was added to the reaction mixture to react for 4 hours, during which NaOH solution was added dropwise to adjust the pH of the reaction system to 10.2. After the first circulation reaction was completed, the oxidized cotton slurry and the reaction solution were separated by using a positive pressure filter, and the filtrate (about 5 kg) was collected. The mechanical treatment is the same as the first reaction.

3.5 kg (70 wt%) of the filtrate was weighed out for the second round of reaction, and the remaining 1.5 kg was stored refrigerated for further use. The raw materials, reagent addition and reaction steps are the same as those of the first cycle reaction. After the second circulation reaction was completed, the oxidized cotton slurry and the reaction solution were separated by a positive pressure filter, and the filtrate (about 5 kg) was collected. The mechanical treatment is the same as the first reaction.

The remaining filtrate after the first reaction and the first cycle reaction and all the filtrate after the second cycle reaction (8 kg in total) were stirred and mixed uniformly, then 3.2 kg of n-propyl acetate was added, and TEMPO in the filtrate was extracted by stirring sufficiently. The distillation operation was further carried out on the obtained extract phase until 99wt% of the extractant was volatilized and 1wt% of TEMPO/n-propyl acetate concentrate remained.

The concentrate can be used for the next batch of TEMPO catalyzed cellulose oxidation reaction. 80wt% TEMPO (1.92 g) was recovered and 1.5 g was taken for the first reaction and the remainder was used for the next recycle. The other raw materials, the reagent dosage and the reaction steps are the same as the first reaction.

Example 2

1.7 g of TEMPO and 10.5 g of NaBr are weighed and dissolved in 4.39 kg of deionized water, 0.1 kg of softwood pulp is added, the swelling cellulose is fully stirred, 0.5 kg of NaClO is added for reaction for 3 hours, and NaOH solution is dropwise added during the reaction to adjust the pH of the reaction system to 10.2. After the completion of the reaction, the oxidized cellulose pulp and the reaction solution were separated by a positive pressure filter, and the filtrate (about 5 kg) was collected. And mechanically treating the oxidized cellulose pulp, and homogenizing for 8 times under the pressure of 80 MPa to obtain the CNFs product.

3 kg (60 wt%) of the filtrate was weighed for the first cycle reaction, and the remaining 2 kg was stored under refrigeration for future use. 0.68 g of TEMPO is added, 4.2 g of NaBr is dissolved in 3 kg of filtrate, 1.40 kg of deionized water is added, the swelling cellulose pulp is fully stirred, 0.5 kg of NaClO is added for reaction for 3 hours, and NaOH solution is added dropwise during the reaction to adjust the pH of the reaction system to 10.2. After the first-cycle reaction was completed, the oxidized cellulose pulp and the reaction solution were separated by a positive pressure filter, and the filtrate (about 5 kg) was collected. The mechanical treatment is the same as the first reaction.

3 kg (60 wt%) of the filtrate was weighed for the second cycle reaction, and the remaining 2 kg was stored refrigerated for further use. The reagent addition was the same as in the reaction step and the first cycle reaction. After the second circulation reaction was completed, the oxidized cellulose pulp and the reaction solution were separated by a positive pressure filter, and the filtrate (about 5 kg) was collected. The mechanical treatment is the same as the first reaction.

The remaining filtrate after the first reaction and the first circulation reaction and all the filtrate (9 kg in total) after the second circulation reaction were stirred and mixed uniformly, 4.5 kg of n-propyl acetate was then added, and TEMPO in the filtrate was extracted by thorough stirring. The distillation operation was further carried out on the obtained extract phase until 98wt% of the extractant was volatilized, leaving 2wt% of TEMPO/n-propyl acetate concentrate.

The concentrate can be used for the next batch of TEMPO catalyzed cellulose oxidation reaction. 80wt% TEMPO (2.448 g) was recovered and 1.7 g was taken for the first reaction and the remainder was used for the next recycle reaction. The rest raw materials, the reagent dosage and the reaction steps are the same as the first reaction.

Example 3

Weighing 1.7 g of TEMPO and 10.5 g of NaBr, dissolving in 3.89 kg of deionized water, adding 0.1 kg of hardwood pulp, fully stirring for swelling, adding 5 kg of NaClO, reacting for 2 hours, and dropwise adding NaOH solution during the reaction to adjust the pH of the reaction system to 10.2. After the completion of the reaction, the oxidized cellulose pulp and the reaction solution were separated by a positive pressure filter, and the filtrate (about 5 kg) was collected. And (4) mechanically treating the oxidized cellulose pulp for 6 times under the pressure of 80 MPa to obtain the CNFs product.

2.5 kg (50 wt%) of the filtrate was weighed for the first cycle reaction, and the remaining 2.5 kg was stored under refrigeration for future use. 0.85 g TEMPO and 5.25 g NaBr were added, dissolved in 2.5 kg filtrate, 0.89 kg deionized water was added, and 0.1 kg hardwood pulp was added, stirred well and swollen. Then, 1 kg of NaClO was added thereto to react for 2 hours, during which time a NaOH solution was added dropwise to adjust the pH of the reaction system to 10.2. After the first-cycle reaction was completed, the oxidized cellulose pulp and the reaction solution were separated by a positive pressure filter, and the filtrate (about 5 kg) was collected. The mechanical treatment is the same as the first reaction.

2.5 kg (50 wt%) of the filtrate was weighed for the second round of reaction, and the remaining 2.5 kg was stored under refrigeration for future use. The addition amounts of raw materials and reagents are the same as the reaction steps and the first circulation reaction. After the second cycle reaction was completed, the oxidized cellulose pulp and the reaction solution were separated by a positive pressure filter, and the filtrate (about 5 kg) was collected. The mechanical treatment is the same as the first reaction.

The remaining filtrate after the first reaction and the first circulation reaction and all the filtrate (9 kg in total) after the second circulation reaction were stirred and mixed uniformly, 4.5 kg of n-propyl acetate was then added, and TEMPO in the filtrate was extracted by thorough stirring. The extract phase obtained was subjected to a further distillation operation until 95% by weight of the extractant had evaporated and 5% by weight of TEMPO/n-propyl acetate concentrate remained.

The concentrated solution can be used for the next batch of TEMPO catalytic oxidation cellulose reaction. 80wt% TEMPO (2.72 g) was recovered and 1.7 g was taken for the first reaction and the remainder was used for the next recycle reaction. The other raw materials, the reagent dosage and the reaction steps are the same as the first reaction.

Comparative example

Weighing 1.5 g of TEMPO and 12 g of NaBr, dissolving in 4.54 kg of deionized water, adding 0.1 kg of softwood pulp, fully stirring swelling cellulose, adding 0.35 kg of NaClO, reacting for 4 hours, and dropwise adding NaOH solution during the reaction to adjust the pH of the reaction system to 10.2. After the completion of the reaction, the oxidized cellulose pulp and the reaction solution were separated by a positive pressure filter, and the filtrate (about 5 kg) was collected. And (3) mechanically treating the oxidized cellulose pulp, and homogenizing for 10 times under the pressure of 70 MPa to obtain the CNFs product.

All the filtrate was used for the first recycle reaction. And adding 0.1 kg of cellulose pulp without adding TEMPO and NaBr, fully stirring and swelling, then adding 0.35 kg of NaClO, reacting for 4 hours, and dropwise adding NaOH solution to adjust the pH value of a reaction system to 10.2. After the first-cycle reaction was completed, the oxidized cellulose pulp and the reaction solution were separated by a positive pressure filter, and the filtrate (about 5 kg) was collected. The mechanical treatment is the same as the first reaction.

All the filtrate is used for the second circulation reaction, TEMPO and NaBr are not supplemented, and 0.1 kg of cellulose pulp is added to be fully stirred and swelled. The reagent addition was the same as in the reaction step and the first cycle reaction. After the second circulation reaction was completed, the oxidized cellulose pulp and the reaction solution were separated by a positive pressure filter, and the filtrate (about 5 kg) was collected. The mechanical treatment is the same as the first reaction. Collecting the three reaction filtrates for further wastewater treatment.

Carboxyl content test method:

the experiment used a conductometric titration method to test the charge density of the CNFs prepared in the above example. Before the experiment, the CNFs were dispersed in deionized water at a concentration of 0.1wt%, and stirred well for 60 min to obtain a transparent suspension. Then hydrochloric acid is added dropwise to adjust the pH value to 2 to 3, and the mixture is stirred for 30 min. Then 0.1M NaOH solution was added at a rate of 0.1 ml/min. A set of data was recorded every 1 min during the test using a conductivity meter. The test results are shown in fig. 2.

Practice of the invention proves that 50-80 wt% of TEMPO and NaBr can be directly reused in the cyclic reaction; in the extraction distillation process, the recovery rate of TEMPO can reach 70-90 wt%; the material cost is reduced by 30-50%; the obtained CNFs product has stable quality, the carboxyl content is 1.3 to 1.6 mmol/g, and the fiber diameter is 3 to 4 nm.

The above description is only a preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention disclosed herein should be covered within the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.

Claims

1. A CNFs production process based on direct circulation and extraction recovery double-strategy circulation TEMPO is characterized by comprising the following steps:

3) Treating the reaction solution: dividing the reaction solution obtained by separation in the step 2) into direct circulating filtrate and extraction recovery filtrate according to a proportion;

4) Directly carrying out a circulating reaction on the filtrate: adding part of fresh TEMPO, naBr, naClO solution and deionized water into the direct circulating filtrate obtained in the step 3), and repeating the steps 1) to 2) to carry out circulating reaction;

the cyclic reaction and the first reaction in the step 1) form a first batch of reaction together;

5) Filtrate extraction, recovery and recycling: extracting and recovering a catalyst TEMPO by using an extracting agent in the extracted and recovered filtrate obtained in the step 3), and distilling to obtain a TEMPO/extracting agent concentrated solution; and (3) adding part of fresh TEMPO, naBr, naClO solution and deionized water again, and repeating the steps 1) to 2) to perform the next batch of cyclic reaction.

2. The CNFs production process based on direct circulation and extraction recovery dual-strategy circulation TEMPO as claimed in claim 1, wherein the corresponding TEMPO dosage per gram of cellulose raw material in step 1) is 0.05 to 0.2 mmol, and NaBr dosage is 0.5 to 2.5 mmol; the mass ratio of the cellulose raw material to the NaClO solution in the step 1) is 1: (3 to 7); the mass ratio of the cellulose raw material to the deionized water in the step 1) is 1: (40 to 200).

3. The process of claim 2, wherein the cellulosic material is one or more of bleached softwood pulp, bleached hardwood pulp, and cotton.

4. The CNFs production process based on direct circulation and extraction recovery double-strategy circulation TEMPO according to claim 1, wherein the time of the oxidation reaction in the step 1) is 1-5 h; and continuously dropwise adding an NaOH solution in the oxidation reaction process, and controlling the pH to be 10.0-10.5.

5. The CNFs production process based on direct circulation and extraction recovery double strategy circulation TEMPO according to claim 1, wherein in step 2), the solid-liquid separation device is a plate-and-frame filter press or a positive pressure filter; the high-pressure homogenizing pressure is 70 to 110MPa, and the homogenizing times are 6 to 12.

6. The CNFs production process based on direct recycling and extractive recovery dual-strategy recycling TEMPO according to claim 1, wherein in step 3), the direct recycling filtrate accounts for 50wt% -80 wt%, and the extractive recovery filtrate accounts for 20wt% -50 wt%.

7. The CNFs production process based on direct circulation and extraction recovery dual strategy circulation TEMPO according to claim 6, wherein after the last circulation in step 4), the direct circulation filtrate accounts for 0wt% and the extraction recovery filtrate accounts for 100wt%; the cycle number of the direct cycle reaction is 1 to 4.

8. The CNFs production process based on direct circulation and extraction recovery dual-strategy circulation TEMPO, according to claim 1, characterized in that in the step 4), the amount of supplemented fresh TEMPO is 20wt% -50 wt% of the amount of TEMPO added in the step 1); the amount of supplemented NaBr is 20-50 wt% of the amount of added NaBr in the step 1); the amount of the NaClO solution added is the same as the amount added in the step 1).

9. The CNFs production process based on TEMPO (direct recycle and extractive recycle) dual-strategy recycle of claim 1, wherein in the step 5), the mass ratio of an extracting agent to the extractive recycle filtrate is (1-5): 10; after distillation treatment, the TEMPO/extractant concentrated solution accounts for 0.5-5 wt% of the extracted and recovered filtrate;

the amount of newly added fresh TEMPO is 10-30 wt% of the total addition amount of the first batch of reaction TEMPO; the newly added amount of NaBr is the same as the total addition amount of NaBr in the first batch of reaction; the amount of the NaClO solution added again is the same as the total amount of NaClO added in the first batch reaction.

10. The process of claim 9, wherein the extractant is one or more of n-propyl acetate, ethyl acetate, n-hexane, and methyl t-butyl ether.