WO2024007911A1

WO2024007911A1 - Fe-mof/ben@cnts composite conductive material, preparation method therefor, and use thereof

Info

Publication number: WO2024007911A1
Application number: PCT/CN2023/103108
Authority: WO
Inventors: 潘君廷; 马俊怡; 赵立欣
Original assignee: 中国农业科学院农业资源与农业区划研究所; 中国农业科学院农业环境与可持续发展研究所
Priority date: 2022-07-05
Filing date: 2023-06-28
Publication date: 2024-01-11
Also published as: CN115181284B; CN115181284A

Abstract

The present invention relates to the technical field of environmental protection. Disclosed are a Fe-MOF/Ben@CNTs composite conductive material, a preparation method therefor, and the use thereof. The preparation method comprises the following steps: adding a methanol solution of bentonite and carbon nanotubes into mixed methanol solutions of 2-methylimidazole and cobalt nitrate, adding a methanol solution of ferric chloride, and carrying out a hydrothermal reaction; after the hydrothermal reaction is finished, cooling the solution to room temperature, and performing standing, washing and drying to obtain the Fe-MOF/Ben@CNTs composite conductive material. The composite conductive material is used for buffering a high-grease load in an anaerobic digestion system and can solve the difficulties of poor stability, poor conductivity and the like in application of Fe-MOF, thereby achieving efficient β-oxidative decomposition of LCFAs and improving the yield of methane of the anaerobic digestion system.

Description

A Fe-MOF/Ben@CNTs composite conductive material, preparation method and application

Technical field

The invention relates to the field of environmental protection technology, and in particular to a Fe-MOF/Ben@CNTs composite conductive material, a preparation method and its application.

Background technique

Food waste is an important part of urban domestic waste. It has the characteristics of high salt content, high organic matter content, and is highly perishable. If not properly handled, it will cause many environmental problems. Anaerobic digestion technology can use the coordinated metabolism of a variety of anaerobic microorganisms to decompose macromolecular organic compounds such as cellulose, starch, protein and oil rich in food waste to produce biomethane without the need for external oxygen supply. , while achieving energy and resource recovery.

Although a large amount of oils and fats in food waste have high methane-producing potential, a large amount of long-chain fatty acids (LCFAs) produced by hydrolysis in anaerobic reactors will be adsorbed on the surface of microorganisms, hindering the mass transfer process and poisoning microorganisms, resulting in The anaerobic digestion system is inefficient and unstable.

Iron materials have good redox activity. When used in anaerobic digestion systems in the form of Fe(0), Fe(II), Fe(III), etc., they can provide electrons, reduce the redox potential of the reaction system, and have an important impact on the methane production efficiency. Has better lifting effect. However, due to the lack of crystal structure support, iron materials have disadvantages such as easy agglomeration and poor stability in anaerobic digestion systems. In addition, the corrosion products of iron will cover its surface, reducing its electron donating ability and reactivity, limiting its large-scale application. In the existing technology, there are also records of using Fe-MOF materials to catalyze reactions in various stages of anaerobic digestion. However, Fe-MOF has poor conductivity and is prone to volume changes, which also limits its application.

Carbon nanotubes (CNTs) are allotropes of carbon with a tubular structure rolled into cylindrical graphite sheets. Because of their special specific strength and excellent electrochemical properties, they are used as modification materials for Fe-MOF in anaerobic digestion systems. There is huge potential to improve reaction efficiency and electronic selectivity. Although the surfaces of both materials have many active sites, a binder still needs to be added during the modification process to improve the bonding between the two, and the added binder has an inhibitory effect on methane production.

Contents of the invention

The purpose of the present invention is to provide a Fe-MOF/Ben@CNTs composite conductive material with excellent electron selectivity, a preparation method and its application. The composite conductive material is used to buffer high grease loads in anaerobic digestion systems and can solve the problem of Fe - Application problems such as poor stability and poor conductivity of MOF enable efficient β-oxidative decomposition of LCFAs and increase methane yield in anaerobic digestion systems.

In order to achieve the above objects, the present invention provides the following solutions:

A method for preparing Fe-MOF/Ben@CNTs composite conductive material, including the following steps:

Add the methanol solution of bentonite and carbon nanotubes to the methanol mixed solution of 2-methylimidazole and cobalt nitrate, and then add the methanol solution of ferric chloride to perform a hydrothermal reaction; after the hydrothermal reaction is completed, lower to room temperature and let stand. , washed and dried to obtain Fe-MOF/Ben@CNTs composite conductive material.

Further, the preparation method of the Fe-MOF/Ben@CNTs composite conductive material specifically includes the following steps:

(1) Add bentonite (Ben) and carbon nanotubes (CNTs) to methanol and sonicate to form solution A; add ferric chloride to methanol and sonicate to form solution B; add nitric acid Cobalt is dissolved in methanol and ultrasonic to form solution C; 2-methylimidazole is dissolved in methanol and ultrasonic to form solution D;

(2) During the stirring process, pour the solution D into the solution C to obtain a mixed solution A, and then pour the solution A into the mixed solution A to obtain a mixed solution B;

(3) Leave the mixed solution B to stand, and then add the solution B to the mixed solution B to perform a hydrothermal reaction;

(4) After the hydrothermal reaction is completed, lower to room temperature, let stand, wash, and dry to obtain Fe-MOF/Ben@CNTs composite conductive material.

Carbon materials have developed pore structures, considerable specific surface areas, stable physical and chemical properties, and multi-active site surfaces. Using them to modify Fe-MOF can improve mechanical strength, resist oxygen and corrosion, maintain durability, and enhance conductivity. . The efficiency of collaboration between mutually beneficial bacterial groups depends on the efficiency of electron transfer between species. Metal elements with good conductivity can serve as electron carriers to significantly accelerate this process. Iron is the most important metal element in the methanogenesis metabolism process and an essential growth factor for microorganisms. It can directly participate in material and energy metabolism and thus affect the activity of microorganisms, and positively promotes the operating efficiency and stability of the anaerobic digestion system. effect. During the anaerobic digestion process, LCFAs need to undergo multiple β-oxidation cycles to be degraded into acetic acid and hydrogen that can be directly utilized by methanogens. This process is the main rate-limiting step for oils. β-oxidation is a non-spontaneous endothermic reaction. Accelerating the consumption of acetic acid and hydrogen by methanogens is beneficial to overcoming thermodynamic barriers and promoting the forward progress of the β-oxidation reaction. Therefore, the mutual cooperation between LCFAs-oxidizing bacteria and methanogens is a necessary condition for efficient conversion of LCFAs into CH ₄ and CO _2. Strengthening the mutual cooperation will help improve the processing capacity of anaerobic digestion of food waste. Iron-based metal-organic framework (Fe-MOF) is a crystal material formed by coordination polymerization of iron ions and organic ligands. The material has the characteristics of strong stability, high porosity, large specific surface area, and many unsaturated sites, and has great catalytic potential for reactions in various stages of anaerobic digestion. Bentonite (Ben) is a natural clay mineral that has a good alleviation effect on the inhibition of anaerobic digestion of LCFAs. In addition, bentonite has a small hydrophilic particle size and shows great adhesion when mixed with water. It can be used as a binder for CNTs modified Fe-MOF to avoid the introduction of chemical binders that may have inhibitory effects. In order to solve the problem of LCFAs inhibition, the present invention prepares a Fe-MOF/Ben@CNTs composite conductive material to optimize the addition ratio (2.6% VS _substrate ) to improve the buffering capacity of the food waste anaerobic digestion system. Tolerable organic loading rate and enhanced methanation efficiency of LCFAs are of great significance.

Further, in step (1), the mass ratio of bentonite and carbon nanotubes is 2:5;

The mass ratio of bentonite, ferric chloride, cobalt nitrate and 2-methylimidazole is 2: (4-5): (1-2): (5-6), and the preferred mass ratio is 2:4.492:1.17:5.248.

Further, the mixed liquid B described in step (3) is allowed to stand in a nitrogen environment, and the nitrogen is used as a protective gas to isolate oxygen and prevent oxidation reactions from occurring.

Further, the temperature of the hydrothermal reaction in step (3) is 150-170°C, the residence time is 1 h, the preferred reaction temperature is 160°C, and the heating rate is 3°C/min.

Further, the resting time in step (4) is 10-15h, and the preferred resting time is 12h.

Further, in step (4), the drying temperature is 60-80°C and the drying time is 10-14h. The preferred drying temperature is 70°C and the drying time is 12h.

A Fe-MOF/Ben@CNTs composite conductive material prepared by the preparation method. The electron-accepting ability and electron-donating ability of the Fe-MOF/Ben@CNTs obtained by the present invention reach more than 0.552 μmol e ^- /g and 0.685 μmol e ^- /g respectively.

The Fe-MOF/Ben@CNTs composite conductive material is used to promote anaerobic digestion and increase the methane yield of the anaerobic digestion system.

The invention discloses the following technical effects:

The present invention is compounded of materials with crystal structure and carbon skeleton structure. It does not need to carry out enzymatic pretreatment process during anaerobic digestion, and does not need to control the sludge TS within 10%. It can process high-concentration raw materials and has a relatively high cumulative methane production. The control group can improve by 75%.

The Fe-MOF/Ben@CNTs composite conductive material prepared by the present invention has an electron-accepting ability and an electron-contributing ability reaching more than 0.552 μmol e ^- /g and 0.685 μmol e ^- /g respectively, and can be used to buffer high oil loads in anaerobic digestion systems. It can solve application problems such as poor stability and poor conductivity of Fe-MOF, achieve efficient β-oxidative decomposition of LCFAs, and improve methane yield in anaerobic digestion systems.

Description of the drawings

In order to explain the embodiments of the present invention or the technical solutions in the prior art more clearly, the drawings needed to be used in the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some of the drawings of the present invention. Embodiments, for those of ordinary skill in the art, other drawings can also be obtained based on these drawings without exerting creative efforts.

Figure 1 shows the cyclic voltammetry curve of Fe-MOF/Ben@CNTs;

Figure 2 shows the daily methane production of each treatment group during the anaerobic fermentation process;

Figure 3 shows the cumulative methane production of each treatment group during the anaerobic fermentation process;

Figure 4 shows the changes in acetic acid concentration during anaerobic fermentation;

Figure 5 shows the changes in propionic acid concentration in each treatment group during the anaerobic fermentation process;

Figure 6 shows the changes in pH value of each treatment group during the anaerobic fermentation process.

Detailed ways

Various exemplary embodiments of the invention will now be described in detail. This detailed description should not be construed as limitations of the invention, but rather as a more detailed description of certain aspects, features and embodiments of the invention.

It should be understood that the terms used in the present invention are only used to describe particular embodiments and are not intended to limit the present invention. In addition, for numerical ranges in the present invention, it should be understood that every intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or value intermediate within a stated range, and any other stated value or value intermediate within a stated range, is also included within the invention. The upper and lower limits of these smaller ranges may independently be included or excluded from the range.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although only the preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and/or materials in connection with which the documents relate. In the event of conflict with any incorporated document, the contents of this specification shall prevail.

It will be apparent to those skilled in the art that various modifications and changes can be made to the specific embodiments described herein without departing from the scope or spirit of the invention. Other embodiments will be apparent to the skilled person from the description of the invention. The specification and examples of the present invention are exemplary only.

Regarding the terms "includes", "includes", "has", "contains", etc. used in this article, They are all open terms, meaning they include but are not limited to.

Example 1

Preparation method of Fe-MOF/Ben@CNTs composite conductive material:

(1) Weigh 2g of calcium bentonite and 5g of CNTs that have passed through a 325 mesh sieve, add it to 100mL of methanol, and use ultrasonic treatment for 30 minutes to completely disperse it to form solution A. At this time, the bentonite exhibits a nanostructure in the solution; weigh 4.492g of FeCl ₃ ·6H ₂ O was added to 100mL of methanol and ultrasonicated to form yellow solution B; 1.17g Co(NO) ₃ · _6H2O was dissolved in 100mL of methanol under ultrasonic in a beaker to obtain solution C; 5.248g of 2-methylimidazole was ultrasonicated Dissolve 100mL in methanol to obtain solution D;

(2) During the stirring process, quickly pour solution D into solution C to obtain mixed solution A, then quickly pour solution A into mixed solution A, and stir for 12 minutes to obtain mixed solution B;

(3) After leaving mixed liquid B in a nitrogen environment for 0.5h, add solution B into mixed liquid B, stir for 0.5h, and then transfer to a hydrothermal reactor for hydrothermal reaction at a heating rate of 3°C/min. The final temperature of the reaction reaches 160°C and stays for 1 hour;

(4) After the reaction, place the mixed system into an anaerobic workstation at an ambient temperature of 25°C and let it stand for aging for 12 hours. After aging, it was centrifuged and washed three times with methanol, placed in a vacuum drying oven, and dried at 70°C for 12 hours to obtain Fe-MOF/Ben@CNTs.

The electron-accepting ability and electron-donating ability of the Fe-MOF/Ben@CNTs prepared in this example reached 0.552 μmol e ^- /g and 0.685 μmol e ^- /g respectively. The cyclic voltammetry curves of Fe-MOF/Ben@CNTs and each monomer material are shown in Figure 1. Fe-MOF/Ben@CNTs has the largest curve area, indicating that it has the highest specific capacitance and enhanced charge and discharge capabilities.

The Fe-MOF/Ben@CNTs prepared in Example 1 is applied to the anaerobic digestion of food waste:

The substrate used for anaerobic fermentation is food waste, and the TS and VS contents of the raw materials are 27% and 25% respectively. The inoculum was taken from a continuously stirred reactor operating normally in the laboratory under medium temperature conditions (36±1°C), with a solid content of 6%.

The sequential batch anaerobic fermentation reactor is equipped with a 500mL reagent bottle with a special feeding adapter for the fermentation tank with good sealing. The organic load of food waste is 50g VS/L. The inoculation volume is 120mL. Use tap water to adjust the volume to the effective volume of the reactor. 400mL. In order to test the regulatory effect of Fe-MOF/Ben@CNTs composite on the accumulation of LCFAs, 2 g of glyceryl trioleate (GTO) was added to the reactor to construct LCFAs inhibition. Add 4g Fe-MOF/Ben@CNTs composite material, 4g CNTs, and 4g Fe-MOF to the three reactors respectively (the preparation method of Fe-MOF is: combine Co(NO) ₃ ·6H ₂ O and 2-methylimidazole Dissolve in methanol by ultrasonic, mix quickly during the stirring process, seal and let stand, age for 24 hours, centrifuge and wash with methanol three times, and vacuum dry at 70°C for 12 hours to obtain (marked as T1, T2, T3) without adding any conductive materials. The reactor is used as a control (marked as CK). The biogas produced by fermentation flows from the air outlet above the reactor through a silica gel tube into an aluminum foil air bag for storage. The fermentation cycle is 90 days. The gas volume is measured every day, and the gas composition is analyzed every 3 days and every 5 days. Collect fermentation liquid samples. Mix the fermentation liquid thoroughly before sampling, and collect about 10 mL of samples each time.

The daily methane production of each treatment group during the anaerobic fermentation process is shown in Figure 2. Since the fermentation substrate contains a large amount of oil components with low hydrolysis rates, and the LCFAs produced by hydrolysis need to undergo multiple β-oxidation cycles before they can be utilized by methanogens, each treatment group showed a long stagnation period for methanogenesis. The daily methane production of the T1, T2, T3 and CK groups reached more than 50 mL on the 21st, 26th, 19th and 25th days respectively and entered the peak of methane production successively. T1 group produces nails The peak of methane appeared slightly later than that of the T3 group, but the difference between the two peaks of methane production in the T1 group was small, 515mL and 482mL respectively; while the two peaks of methane production in the T3 group were 531mL and 269mL respectively, and the second The peak is significantly lower than the first one. This may be because Fe-MOF has high reactivity but poor stability, and its promotion effect on methanogenesis by anaerobic fermentation is difficult to maintain and stabilize. The use of CNTs modification can solve the problem of Fe-MOF's poor stability and easy deactivation. Compared with the T2 group, the methanogenesis stagnation period of the T1 group was significantly shortened and the methanogenesis peak value was significantly increased, indicating that Fe-MOF can effectively improve the reactivity of CNTs in the anaerobic fermentation system.

The cumulative methane production of each treatment group is shown in Figure 3. Although the anaerobic fermentation test lasted for 90 days, the cumulative methane production of the T1 group reached 96% of the total on the 50th day, which shows that the Fe-MOF/Ben The application of @CNTs to actual continuous anaerobic fermentation processes may effectively shorten the hydraulic retention period. The cumulative methane production of the T1, T2, T3 and CK groups were 343, 296, 281 and 201mL/g VS respectively. The cumulative methane production of the T1 group was 16%, 22% and 71 higher than those of the T2, T3 and CK groups respectively. %. Compared with the unregulated control group, Fe-MOF/Ben@CNTs significantly enhanced the cumulative methane production of anaerobic digestion of oil, and its enhancement effect was significantly better than that mediated by CNTs or Fe-MOF alone. This may be because the Fe-MOF/Ben@CNTs composite has good electron selectivity and can strengthen the mutual cooperation between acetic acid bacteria and methanogens by establishing an efficient interspecies electron transfer (IET) pathway, thus accelerating the It consumes acetic acid and promotes the forward progress of the oxidative decomposition reaction of LCFAs.

The changes in acetic acid concentration during the anaerobic fermentation process are shown in Figure 4. The acetic acid concentration in each treatment showed a trend of first increasing and then decreasing. 15 days before fermentation, the acetic acid concentration of T1 group was lower than that of the other 3 groups, but Its concentration increased to 15.5g/L on the 20th day, which was the highest value in the entire fermentation cycle among the four treatment groups. Subsequently, the acetic acid concentration in the T1 group dropped sharply and continued to remain at a low level after 45 days. The above results indicate that Fe-MOF/Ben@CNTs promotes the β-oxidation of LCFAs during the hydrolysis and acidification stage and provides sufficient acetic acid for methanogens; in addition, Fe-MOF/Ben@CNTs can also improve the methanation efficiency of acetic acid, showing A higher acetic acid degradation rate was obtained.

The concentration of propionic acid in each treatment group during the anaerobic fermentation process is shown in Figure 5. Propionic acid is an important intermediate metabolite. It needs to be oxidized and degraded into acetic acid before it can be used by methanogens. Therefore, it is easy to accumulate in the fermentation system at high concentrations. Inhibits the activity of methanogenic bacteria. The propionic acid concentration in the T1 group fluctuated more frequently than other treatment groups, and showed higher concentration peaks on the 20th and 60th days, which were 4.9g/L and 4.3g/L respectively. Despite this, the methane production in the T1 group The situation is less affected by it. It may be that Fe-MOF/Ben@CNTs strengthens the mutual cooperation between propionic acid mutual oxidizing bacteria and methanogenic bacteria, thereby accelerating the conversion rate of propionic acid to methane and increasing methane production.

The changes in pH value of each treatment group are shown in Figure 6. Except for the T1 group, the pH values of the other three treatments showed an overall upward trend, while the T1 group showed an obvious first decrease and then increase trend from the 10th day to the 40th day. As shown in Figures 3 and 4, the concentration of VFAs in the fermentation system increased and accumulated at this stage, but the pH values of the T2, T3 and CK groups were still on an upward trend. This may be a common "suppressed steady state" phenomenon during high-load operation of biogas projects. Ammonium ions, the hydrolyzate of nitrogen-containing organic matter such as proteins in food waste, can buffer the drop in pH caused by the accumulation of VFAs and keep the fermentation liquid alkaline. However, at this time, the activity of methanogens has been inhibited by the large accumulation of VFAs, which results in the anaerobic digestion system operating stably but with low efficiency. Adding Fe-MOF/Ben@CNTs can effectively alleviate this phenomenon, which may be due to electron selection Fe-MOF/Ben@CNTs with better properties improves the activity of mutually beneficial bacteria by promoting IET, keeping their growth and metabolism in a vigorous state, thereby accelerating the consumption of ammonia nitrogen by microbial metabolism and avoiding the accumulation of redundant ammonium ions in the system. It works synergistically with VFAs and pH to form an "inhibited stable state".

Optimize the usage amount of Fe-MOF/Ben@CNTs prepared in Example 1 for batch anaerobic digestion of food waste:

The same as Example 1, the food waste load in the 500mL fermentation tank was 20g VS, and 2g glyceryl trioleate (GTO) was added to establish LCFAs inhibition. Food waste VS Fe-MOF/Ben@CNTs with a mass of 1.6%, 2.2%, 2.6%, 3.0%, and 3.4% were added to 5 groups of fermentation tanks respectively, and a control group without Fe-MOF/Ben@CNTs was set up. The cumulative methane production of each treatment group is shown in Table 1:

Table 1 Effect of different Fe-MOF/Ben@CNTs addition amounts on methane production from anaerobic digestion

The results show that the optimal addition amount of Fe-MOF/Ben@CNTs is 2.6% of the VS mass of food waste, which can increase the cumulative methane production by 75%.

The Fe-MOF/Ben@CNTs prepared in Example 1 is applied to the continuous anaerobic digestion of food waste:

The hydraulic retention time of continuous anaerobic digestion is set to 15d, the organic loading rate (OLR) when the device is started is set to 0.8g VS/L/d, and the added mass of Fe-MOF/Ben@CNTs is 2.6% of the daily feed VS mass. After 30 days of startup, the OLR is gradually increased, up to a maximum of 12g VS/L/d.

The above-described embodiments only describe the preferred modes of the present invention and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, those of ordinary skill in the art can make various modifications to the technical solutions of the present invention. All deformations and improvements shall fall within the protection scope determined by the claims of the present invention.

Claims

A method for preparing Fe-MOF/Ben@CNTs composite conductive material, which is characterized by including the following steps:

Add the methanol solution of bentonite and carbon nanotubes to the methanol mixed solution of 2-methylimidazole and cobalt nitrate, and then add the methanol solution of ferric chloride to perform a hydrothermal reaction; after the hydrothermal reaction is completed, lower to room temperature and let stand. , washed and dried to obtain Fe-MOF/Ben@CNTs composite conductive material.
The preparation method according to claim 1, characterized in that it specifically includes the following steps:

(1) Add bentonite and carbon nanotubes to methanol and sonicate to form solution A; add ferric chloride to methanol and sonicate to form solution B; dissolve cobalt nitrate in methanol and sonicate to form solution C; add 2-methyl Dissolve imidazole in methanol and ultrasonicate to form solution D;

(2) During the stirring process, pour the solution D into the solution C to obtain a mixed solution A, and then pour the solution A into the mixed solution A to obtain a mixed solution B;

(3) Leave the mixed solution B to stand, and then add the solution B to the mixed solution B to perform a hydrothermal reaction;

(4) After the hydrothermal reaction is completed, lower to room temperature, let stand, wash, and dry to obtain Fe-MOF/Ben@CNTs composite conductive material.
The preparation method according to claim 2, characterized in that the mass ratio of bentonite and carbon nanotubes in step (1) is 2:5;

The mass ratio of bentonite, ferric chloride, cobalt nitrate and 2-methylimidazole is 2: (4-5): (1-2): (5-6).
The preparation method according to claim 2, characterized in that the mixed liquid B in step (3) is allowed to stand in a nitrogen environment.
The preparation method according to claim 2, characterized in that the temperature of the hydrothermal reaction in step (3) is 150-170°C, and the residence time is 1 hour.
The preparation method according to claim 2, characterized in that the standing time of step (4) is 10-15h.
The preparation method according to claim 2, characterized in that the drying temperature in step (4) is 60-80°C, and the drying time is 10-14h.
A Fe-MOF/Ben@CNTs composite conductive material prepared by the preparation method according to any one of claims 1 to 7.
The application of the Fe-MOF/Ben@CNTs composite conductive material described in claim 8 in promoting anaerobic digestion.