CN116161659A - Method for preparing porous carbon nano-sheet by utilizing waste polylactic acid - Google Patents
Method for preparing porous carbon nano-sheet by utilizing waste polylactic acid Download PDFInfo
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- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B32/00—Carbon; Compounds thereof
- C01B32/30—Active carbon
- C01B32/312—Preparation
- C01B32/318—Preparation characterised by the starting materials
- C01B32/324—Preparation characterised by the starting materials from waste materials, e.g. tyres or spent sulfite pulp liquor
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- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B32/00—Carbon; Compounds thereof
- C01B32/15—Nano-sized carbon materials
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- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B32/00—Carbon; Compounds thereof
- C01B32/30—Active carbon
- C01B32/312—Preparation
- C01B32/342—Preparation characterised by non-gaseous activating agents
- C01B32/348—Metallic compounds
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/13—Energy storage using capacitors
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- Materials Engineering (AREA)
- Environmental & Geological Engineering (AREA)
- Nanotechnology (AREA)
- Carbon And Carbon Compounds (AREA)
Abstract
The invention belongs to the technical field of carbonization of waste polylactic acid (PLA), and discloses a method for preparing a porous carbon nano sheet by using waste polylactic acid, which specifically comprises the following steps: (1) Uniformly mixing waste PLA and a solid strong alkali compound, and performing dry ball milling to obtain PLA degradation product lactate; (2) And heating lactate at 500-900 ℃ under the condition of protective gas atmosphere for 0.5-2 h for carbonization, and then pickling, washing and drying carbonized products to obtain the porous carbon nano-sheet. According to the invention, the overall process flow design of the method is improved, the mechanochemical effect and the 2-step method of carbonization treatment are utilized to cooperate, waste PLA is degraded into organic micromolecular salt-lactate under the mechanochemical effect, and then the lactate is carbonized, and the waste PLA can be converted into the porous carbon nano-sheet with high specific surface area under the assistance of the organic micromolecular salt, so that the porous carbon nano-sheet rich in micropores, mesopores and macropores is prepared.
Description
Technical Field
The invention belongs to the technical field of carbonization of waste polylactic acid (PLA), and in particular relates to a method for preparing a porous carbon nano sheet by using waste polylactic acid.
Background
The porous carbon nano sheet is a two-dimensional carbon nano structure formed by stacking graphene sheets with nanoscale thickness, has the advantages of large specific surface area, developed pore structure, low density, abundant functional groups, high stability and the like, and is widely applied to the fields of adsorption of organic/inorganic pollutants, energy storage and conversion, oxidation-reduction reaction and the like. The traditional method for preparing the porous carbon nano-sheet comprises solid dechlorination, pyrolysis, chemical vapor deposition and the like. However, these methods typically require expensive or toxic precursors, thus limiting the large-scale preparation and application of porous carbon nanoflakes.
The plastic mainly consists of carbon elements, and the conversion of the plastic into the carbon nanomaterial with high added value is attracting a great deal of attention from researchers. Plastics can be classified into petroleum-based plastics and bio-based plastics according to the source. Currently, most reports are about the carbonization of petroleum-based plastics (e.g., polypropylene, polyethylene, polystyrene, and polyethylene terephthalate) to produce nanocarbon materials (e.g., hollow carbon spheres, carbon nanotubes, carbon nanofibers, and porous carbon nanoflakes). For waste petroleum-based plastics, a common strategy for preparing porous carbon nanoflakes from waste plastics is a template and KOH activation combined method. For example, patent ZL 201410219177.0 reports that plastics such as waste polypropylene and polyethylene are used asThe precursor takes organic modified montmorillonite as a template to prepare a carbon nano-sheet, and then the carbon nano-sheet with high specific surface area is generated by KOH activation (the activation effect generated by KOH at high temperature). The disadvantages of this method are the need to use highly corrosive hydrofluoric acid to remove the template (only hydrofluoric acid can remove organically modified montmorillonite template, hydrochloric acid cannot), and the highly corrosive KOH-high temperature activated carbon nanoflakes. Meanwhile, the process requires an additional activator, so that the preparation process is toxic to the environment, the time is prolonged, and the cost is high. On the other hand, in recent years, the disadvantages of shortage of raw materials, difficult degradation, environment unfriendly and the like of petroleum-based plastics are increasingly remarkable, and the development of bio-based plastics is advanced. Common biobased plastics include PLA and polyhydroxyalkanoates. PLA is the highest yielding bio-based plastic, yielding 39.46 ten thousand tons worldwide in 2020, and is expected to reach 59.58 ten thousand tons in 2022. The use of large amounts of PLA articles undoubtedly generates large amounts of waste PLA, thus posing a great burden and hazard to the environment. Typical disposal methods for waste PLA include biodegradation, incineration, mechanical recovery, and chemical recovery. The biodegradation process is slow, and even under the environment of high temperature and high humidity, the biodegradation process still takes 90 days to be completely degraded into CO 2 And H 2 O. Incineration can extract energy from plastic waste, but releases a large amount of CO in the process 2 And harmful substances, which cause serious pollution to the environment. The mechanical recovery process causes a reduction in PLA performance with limited recovery times. Common chemical recovery such as hydrolysis, alcoholysis and the like can obtain products with high added value such as lactic acid monomers, alkyl lactic acid and the like. There are also disadvantages such as randomness of the hydrolysis process, the need for organic solvents (e.g., chloroform, acetone, etc.), catalysts (e.g., metal salts, lewis acid base pairs, ionic liquids, etc.), resulting in high cost, environmental toxicity. Therefore, there is a need to develop innovative technologies to maximize the utility of waste PLA, reducing environmental hazards. In addition, the conversion of waste PLA into porous carbon nanoflakes has not been reported so far.
Disclosure of Invention
In view of the above-mentioned drawbacks or improvement needs of the prior art, an object of the present invention is to provide a method for preparing porous carbon nanoflakes by using waste polylactic acid, wherein the overall process flow design of the method is improved, and by utilizing the combination of mechanochemical action and 2-step method of carbonization treatment, waste PLA is degraded into organic small molecular salt-lactate under mechanochemical action, and then the lactate is carbonized, and the waste PLA can be converted into porous carbon nanoflakes with high specific surface area with the aid of the organic small molecular salt, so as to prepare porous carbon nanoflakes rich in micropores, mesopores and macropores.
In order to achieve the above object, according to the present invention, there is provided a method for preparing porous carbon nanoflakes using waste polylactic acid, comprising the steps of:
(1) Uniformly mixing waste PLA and a solid strong alkali compound, and performing dry ball milling to obtain PLA degradation product lactate; wherein the dry ball milling time is 0.5-5 h;
(2) And (3) heating the lactate obtained in the step (1) for 0.5-2 hours at 500-900 ℃ under the protective gas atmosphere condition to carbonize, and then pickling, washing and drying the cooled carbonized product to obtain the porous carbon nano-sheet.
In a further preferred aspect of the present invention, in the step (1), the solid strong alkali compound is an alkali metal hydroxide, preferably sodium hydroxide or potassium hydroxide.
As a further preferred aspect of the present invention, in step (1), the mass ratio of the waste PLA to the solid alkali compound is 3:1 to 1:3.
As a further preferred aspect of the present invention, in the step (1), the ball milling speed used for the dry ball milling is 50r/min to 600r/min.
As a further preferred aspect of the present invention, in the step (1), the waste PLA is selected from the group consisting of waste PLA powder, waste PLA fiber, and waste PLA sheet.
As a further preferred aspect of the present invention, in step (2), the protective gas is nitrogen or an inert gas.
As a further preferred aspect of the present invention, in the step (2), the acid washing is specifically hydrochloric acid washing.
Compared with the prior art, the method of the invention adopts 2-step treatment and utilizes the integral cooperation of mechanochemical action and carbonization treatment to prepare the porous carbon nano sheet with high specific surface area from waste PLA, thereby effectively realizing the upgrading chemical recycling of the waste PLA.
In the invention, the strong alkali compound and the waste PLA are mixed and ball-milled, and the PLA and the strong alkali compound fully react under the mechanochemical action to promote the PLA to depolymerize to generate lactate, and the step does not involve any organic solvent. Then heating lactate at 500-900 ℃ under the atmosphere of protective gas (such as nitrogen), and carrying out further crosslinking, cyclization, aromatization and other reactions to construct a carbon material skeleton, so as to generate the porous carbon nano-sheet with high specific surface area. The strong electrostatic force in the ball milling product, namely lactate, enables lactic acid ions and metal ions to be tightly combined together, reduces the volatility of the lactate under the high temperature condition, and is favorable for forming porous carbon nano-sheets with high yield under the high temperature pyrolysis. In addition, in the pyrolysis carbonization process, metal ions in lactate and a compound generated by the metal ions serve as a physical template and an activating agent, so that porous carbon nano-sheets with rich nano-pore structures are formed.
The invention converts the waste PLA into the porous carbon nano-sheet with high specific surface area, provides a new way for upgrading chemical recovery of the waste PLA, and provides a new method for converting a large amount of cheap urban and industrial waste PLA into high added value products.
In particular, the invention can achieve the following beneficial effects:
(1) The invention adopts a mechanochemical method to degrade the waste PLA into the lactate, the reaction is carried out at normal temperature, the reaction time is short, no organic solvent is used, and the method is environment-friendly and the reaction cost is low. The high temperature required by the hydrolysis method, various organic solvents required by the alcoholysis method and carbon emission of the incineration method are effectively avoided. The degradation efficiency of the waste PLA is obviously improved, and conditions are provided for preparing the porous carbon nano-sheet by using the organic micromolecular salt to assist the PLA carbonization.
(2) The invention adopts a method of auxiliary carbonization of organic micromolecular salt, and firstly, waste PLA is degraded into lactate. In the pyrolytic carbonization process, the metal of lactate and the metal compound generated in situ thereof serve as physical templates and activators, thereby controllably forming porous carbon nanoflakes with abundant nanopore structures. The reaction process is simple and convenient, only one acidification treatment step is needed, and the method is environment-friendly. Compared with the reported method for preparing the porous carbon nano sheet by using templates such as organically modified montmorillonite, the method does not need to use strong corrosive reagents such as hydrofluoric acid to remove the templates and metal hydroxide to perform high-temperature activation, so that the reaction time is obviously reduced, the corrosion to equipment is avoided, and the economic, environmental and social benefits of PLA upgrading chemical circulation are increased.
Unlike direct carbonization, the carbonization process of the invention is essentially assisted carbonization by small organic molecule salts (small organic molecule salts, i.e., lactate obtained by degrading polylactic acid by ball milling). In addition to the carbon source, the method only needs to use strong alkali compounds (such as alkali metal hydroxide compounds) and does not need to participate in metal oxides or other organic solvents. The ball-milling product is directly carbonized without any treatment, and the whole process is environment-friendly. In addition, in the pyrolysis carbonization process, the metal of the lactate and the metal compound generated in situ serve as a physical template and an activating agent, so that the porous carbon nano-sheet with rich nano-pore structure is controllably formed. Taking the following examples as an example, the obtained porous carbon nano-sheet has the shape of nano-sheet, the size of 300-800 nm and the thickness of 2-3 nm, and the whole carbon nano-sheet is petal-shaped and has special shape.
(3) The invention selects a two-step method to carbonize PLA to prepare a porous carbon nano-sheet, firstly, the waste PLA is degraded into lactate by a mechanochemical method, and then the lactate is pyrolyzed and reacted in a series of crosslinking, cyclization, aromatization and the like under the high temperature and nitrogen atmosphere to construct a carbon material framework. Compared with PLA direct carbonization, the small molecular salt auxiliary technology effectively reduces volatilization of small organic molecules, obviously improves carbonization yield and increases specific surface area of the carbon material. The invention provides a new method for carbonization of waste plastics, effectively solves the difficult problem of recycling of urban and industrial waste PLA, and provides a green approach for upgrading chemical recycling of a large amount of waste PLA.
In conclusion, the method has the advantages of simplicity, convenience, environment friendliness, high reaction efficiency and low cost, and can realize the upgrading, chemical recycling of waste PLA and the efficient preparation of the porous carbon nano-sheet with high added value. The invention can solve the difficult problem of recycling and reusing urban and industrial waste PLA, provides a new green approach for recycling and reusing a large amount of waste PLA, has the characteristics of less required chemical reagent, simple carbonization step, short carbonization time, greenness, simplicity and high efficiency.
Drawings
FIG. 1 is an infrared spectrum of waste PLA and ball-milling degradation products (sodium lactate) of PLA used in example 1; wherein a in fig. 1 corresponds to an infrared spectrogram of raw material waste PLA; b in fig. 1 corresponds to the infrared spectrum of PLA ball-milled degradation product (sodium lactate).
FIG. 2 is a scanning electron microscope image of the waste PLA degradation product (sodium lactate) of example 2 at various magnifications and a comparison of its X-ray diffraction pattern with that of commercial sodium hydroxide; wherein a and b in fig. 2 correspond to scanning electron microscope images of waste PLA degradation products at different magnifications, respectively; c in fig. 2 corresponds to the X-ray diffraction pattern of the waste PLA degradation products; d in fig. 2 corresponds to the X-ray diffraction pattern of commercial sodium hydroxide.
FIG. 3 is a scanning electron microscope image and an X-ray diffraction pattern of the porous carbon nanoflakes prepared in example 3 at different magnifications; wherein a, b and c in fig. 3 correspond to scanning electron microscope images at different magnifications, respectively, and d in fig. 3 corresponds to an X-ray diffraction pattern.
FIG. 4 is a scanning electron microscope image and an X-ray diffraction pattern of the porous carbon nanoflakes prepared in example 4 at different magnifications; wherein a, b and c in fig. 4 correspond to scanning electron microscope images at different magnifications, respectively, and d in fig. 4 corresponds to an X-ray diffraction pattern.
FIG. 5 is a scanning electron microscope image of the porous carbon nanoflakes prepared in example 5 at different magnifications, and a nitrogen adsorption-desorption graph and a pore size distribution diagram of the carbon nanoflakes before and after pickling; the a and b in fig. 5 correspond to scanning electron microscope images (the sample is a carbon nano sheet after pickling) with different magnifications, the c in fig. 5 corresponds to a nitrogen adsorption-desorption graph of the carbon nano sheet before and after pickling, and the d in fig. 5 corresponds to a pore size distribution graph of the carbon nano sheet before and after pickling.
Fig. 6 is a scanning electron microscope image of the porous carbon nanoflakes prepared in example 6 at different magnifications.
Fig. 7 is a high resolution transmission electron microscope image of the porous carbon nanoflakes prepared in example 7 at different magnifications.
Fig. 8 is an atomic force microscope image of the porous carbon nanoflakes prepared in example 8.
FIG. 9 is a scanning electron microscope image of the product obtained in comparative example 1 at different magnifications.
FIG. 10 is an X-ray diffraction pattern of the product obtained in comparative example 2.
Detailed Description
The present invention will be described in further detail with reference to the drawings and examples, in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention. In addition, the technical features of the embodiments of the present invention described below may be combined with each other as long as they do not collide with each other.
Based on the invention, in actual operation, the method can:
firstly, uniformly mixing PLA and a strong alkali compound, then placing the mixture in a tank, and ball milling for 0.5-5 h to obtain PLA degradation product lactate without any post-treatment. And then heating lactate at 500-900 ℃ under nitrogen atmosphere for 0.5-2 h (naturally, under inert gas such as argon), and performing subsequent treatments such as acidification to obtain the porous carbon nano-sheet.
The following are specific examples:
example 1
(1) Mixing 12.00g of waste PLA powder and 6.65g of sodium hydroxide (solid powder, the same applies below) uniformly, loading into a ball milling tank, adding steel balls (with the diameter of 2cm: 7; the diameter of 1.5cm: 5; the diameter of 1.3cm: 7; the diameter of 1.2cm: 10; the diameter of 1cm: 12; the diameter of 0.7cm: 30), ball milling for 2 hours at the rotating speed of 500r/min, and collecting ball milling products of products.
(2) Placing the ball-milling product in an alumina crucible, covering a cover, placing the crucible in a tube furnace, introducing nitrogen into the tube furnace, setting the tube furnace to heat at a heating rate of 10 ℃/min, raising the temperature to a carbonization temperature of 500 ℃, and preserving the temperature for 1.5 hours.
(3) And (3) after the tube furnace is naturally cooled, obtaining a carbonized crude product, and performing post-treatment such as acidification by dilute hydrochloric acid to obtain the porous carbon nano-sheet with the yield of 35wt%.
A in fig. 1 is an infrared spectrum of waste PLA. B in fig. 1 is an infrared spectrum of the PLA ball-milled degradation product obtained in step (1). As can be seen from the figure, compared with the infrared spectrogram of the waste PLA, the infrared spectrogram OH stretching vibration peak and the ester carboxyl vibration peak of the PLA ball milling degradation product show that the waste PLA is successfully degraded to generate sodium lactate.
Example 2
The carbonization temperature in the step (2) in the above example 1 was changed to 550 ℃, the carbonization time was changed to 1h, and the other steps were unchanged, to obtain porous carbon nanoflakes with a yield of 43wt%.
FIG. 2 is a scanning electron microscope image and an X-ray diffraction pattern of the waste PLA degradation product sodium lactate obtained in step (1). The morphology of the waste PLA degradation product sodium lactate is an irregular structure and the size is 500 nm-1000 nm as can be seen from a scanning electron microscope image. From the X-ray powder diffraction pattern, waste PLA was successfully degraded into sodium lactate. In addition, as can be seen by comparing with the X-ray powder diffraction pattern of commercial sodium hydroxide, no sodium hydroxide residue exists in the degradation products.
Example 3
(1) Mixing 10.00g PLA flake and 7.77g potassium hydroxide (solid powder, the same applies below) uniformly, loading into a ball milling tank, adding steel balls (diameter 2cm: 6; diameter 1.5cm: 4; diameter 1.3cm: 6; diameter 1.2cm: 8; diameter 1cm: 10; diameter 0.7cm: 25), ball milling for 4 hours at a rotating speed of 200r/min, and collecting the product potassium lactate.
(2) Placing potassium lactate in an alumina crucible, covering a cover, placing the crucible in a tube furnace, introducing nitrogen into the tube furnace, setting the tube furnace to heat at a heating rate of 5 ℃/min, raising the temperature to a carbonization temperature of 500 ℃, and preserving the temperature for 70min.
(3) And (3) after the tube furnace is naturally cooled, obtaining a carbonized crude product, and performing post-treatment such as acidification by dilute hydrochloric acid to obtain the porous carbon nano-sheet with the yield of 28wt%.
Fig. 3 is a scanning electron microscope image and an X-ray diffraction pattern of a porous carbon nanoflakes prepared by carbonization of organic small molecular salt-assisted waste PLA at 500 ℃. The scanning electron microscope image shows that the appearance of the obtained porous carbon nano-sheet is nano-sheet, the size is 300 nm-800 nm, and the whole porous carbon nano-sheet is petal-shaped. From the X-ray powder diffraction pattern, the porous carbon nanoflakes are characterized by amorphous carbon and partially graphitized.
Example 4
The carbonization temperature in the step (2) in the above example 3 was changed to 600 ℃, the carbonization time was changed to 0.5h, and the other steps were unchanged, to obtain porous carbon nanoflakes with a yield of 45wt%.
Fig. 4 is a scanning electron microscope image and an X-ray diffraction pattern of a porous carbon nanoflakes prepared by carbonization of organic small molecule salt-assisted waste PLA at 600 ℃. The morphology of the obtained porous carbon nano-sheet is nano-sheet, and the size is 500 nm-700 nm. From the X-ray powder diffraction pattern, the porous carbon nanoflakes are characterized by amorphous carbon.
Example 5
(1) And cleaning and drying the waste PLA plastic bag, and then placing the waste PLA plastic bag into a pulverizer to obtain waste PLA fragments with the size of 0.5-6 mm.
(2) Mixing 12.00g PLA chips and 9.98g sodium hydroxide uniformly, loading into a ball milling tank, adding steel balls (diameter 2cm: 10; diameter 1.5cm: 6; diameter 1.3cm: 10; diameter 1.2cm: 15; diameter 1cm: 20; diameter 0.7cm: 40), ball milling for 2.5h at a rotating speed of 100r/min, and collecting the product sodium lactate.
(3) Sodium lactate was placed in an alumina crucible and covered with a lid. The crucible is placed into a tube furnace, nitrogen is introduced into the tube furnace, the tube furnace is set to heat at a heating rate of 10 ℃/min, the carbonization temperature is raised to 700 ℃, and the heat is preserved for 1.5 hours at the temperature.
(4) And (3) after the tube furnace is naturally cooled, obtaining a carbonized crude product, and performing post-treatment such as acidification by dilute hydrochloric acid to obtain the porous carbon nano-sheet with the yield of 35wt%.
Fig. 5 is a scanning electron microscope image, nitrogen adsorption and desorption curves and pore size distribution diagram of porous carbon nanoflakes prepared from organic small molecule salt-assisted waste PLA at 700 ℃. The morphology of the obtained porous carbon nano-sheet is nano-sheet, and the size is 500 nm-1000 nm. From the graph of nitrogen adsorption and desorption, the porous carbon nano-sheet has obvious adsorption and desorption hysteresis loop, and the specific surface area is 185.5m 2 And/g, and the porous carbon nano-sheet has a large number of micropores<2 nm), mesopores (2-50 nm) and macropores>50nm)。
Example 6
The carbonization temperature in the step (3) in the above example 5 was changed to 800 ℃, the carbonization time was changed to 1h, and the other steps were unchanged, to obtain a carbon material with a yield of 41wt%.
Fig. 6 is a scanning electron microscope image of porous carbon nanoflakes prepared by carbonization of organic small molecule salt-assisted waste PLA at 800 ℃. The morphology of the obtained carbon material is nano-flake, and the size is 500-600 nm.
Example 7
(1) Mixing 12.00g PLA slices and 36g potassium hydroxide uniformly, loading into a ball milling tank, adding steel balls (with the diameter of 2cm: 7; the diameter of 1.5cm: 5; the diameter of 1.3cm: 7; the diameter of 1.2cm: 10; the diameter of 1cm: 12; the diameter of 0.7cm: 30), ball milling for 0.5h at the rotating speed of 600r/min, and collecting the product potassium lactate.
(2) The potassium lactate is placed in an alumina crucible and covered with a cover, the crucible is placed in a tube furnace, nitrogen is introduced into the tube furnace, the tube furnace is set to heat at a heating rate of 5 ℃/min, the carbonization temperature is increased to 900 ℃, and the temperature is kept for 0.5h.
(3) And (3) after the tube furnace is naturally cooled, obtaining a carbonized crude product, and performing post-treatment such as acidification by dilute hydrochloric acid to obtain the porous carbon nano-sheet with the yield of 30wt%.
FIG. 7 is a high resolution transmission electron microscope image of porous carbon nanoflakes prepared by carbonization of organic small molecule salt-assisted waste PLA at 900 ℃. A in fig. 7 and b in fig. 7 show that the porous carbon nanoflakes consist of a large number of flakes, c in fig. 7 shows that there are a large number of mesopores (2 to 50 nm) in the flakes, and d-surface porous carbon nanoflakes in fig. 7 have a interplanar spacing of 0.4nm.
Example 8
(1) Mixing 12.00g PLA slices and 4.43g sodium hydroxide uniformly, loading into a ball milling tank, adding steel balls (with the diameter of 2cm: 7; the diameter of 1.5cm: 5; the diameter of 1.3cm: 7; the diameter of 1.2cm: 10; the diameter of 1cm: 12; the diameter of 0.7cm: 30), ball milling for 5 hours at the rotating speed of 50r/min, and collecting the product sodium lactate.
(2) Placing sodium lactate in an alumina crucible, covering a cover, placing the crucible in a tube furnace, introducing nitrogen into the tube furnace, setting the tube furnace to heat at a heating rate of 1 ℃/min, raising the temperature to a carbonization temperature of 500 ℃, and preserving the temperature for 2 hours.
(3) And (3) after the tube furnace is naturally cooled, obtaining a carbonized crude product, and performing post-treatment such as acidification by dilute hydrochloric acid to obtain the porous carbon nano-sheet with the yield of 40wt%.
Fig. 8 is an atomic force microscope image of porous carbon nanoflakes prepared by carbonization of organic small molecule salt-assisted waste PLA at 500 ℃. The thickness of the porous carbon nano-sheet is 2-3 nm as can be seen from an atomic force microscope spectrum.
Comparative example 1
(1) 5.00g of waste PLA was weighed out using an alumina crucible, and the lid was closed. The crucible is placed into a tube furnace, nitrogen is introduced into the tube furnace, the tube furnace is set to heat at a heating rate of 5 ℃/min, the carbonization temperature is raised to 700 ℃, and the heat is preserved for 1h at the temperature.
(2) And (3) after the tube furnace is naturally cooled, obtaining a carbonized crude product, weighing the mass of the carbon material, and the yield is 1wt%.
Fig. 9 is a scanning electron microscope image of a carbonized product of direct carbonization of waste PLA at 700 ℃. The morphology of the obtained carbon material is an irregular block structure and the size is 4-5 mu m as can be seen from a scanning electron microscope image.
Comparative example 2
(1) 5.00g of waste PLA was weighed out using an alumina crucible, and the lid was closed. The crucible is placed into a tube furnace, nitrogen is introduced into the tube furnace, the tube furnace is set to heat at a heating rate of 10 ℃/min, the carbonization temperature is increased to 500 ℃, and the heat is preserved for 1.5 hours at the temperature.
(2) And (3) after the tube furnace is naturally cooled, obtaining a carbonized crude product, weighing the mass of the carbon material, and the yield is 1wt%.
Fig. 10 is an X-ray diffraction pattern of a carbonized product of direct carbonization of waste PLA at 500 ℃. From the X-ray powder diffraction pattern, the resulting carbon material still had characteristic diffraction peaks for some PLA, indicating that the waste PLA was not completely carbonized at 500 ℃.
It will be readily appreciated by those skilled in the art that the foregoing description is merely a preferred embodiment of the invention and is not intended to limit the invention, but any modifications, equivalents, improvements or alternatives falling within the spirit and principles of the invention are intended to be included within the scope of the invention.
Claims (7)
1. The method for preparing the porous carbon nano-sheet by utilizing the waste polylactic acid is characterized by comprising the following steps of:
(1) Uniformly mixing waste PLA and a solid strong alkali compound, and performing dry ball milling to obtain PLA degradation product lactate; wherein the dry ball milling time is 0.5-5 h;
(2) And (3) heating the lactate obtained in the step (1) for 0.5-2 hours at 500-900 ℃ under the protective gas atmosphere condition to carbonize, and then pickling, washing and drying the cooled carbonized product to obtain the porous carbon nano-sheet.
2. The method of claim 1, wherein in step (1), the solid strong base compound is an alkali hydroxide, preferably sodium hydroxide, potassium hydroxide.
3. The method of claim 1, wherein in step (1), the mass ratio of the waste PLA to the solid alkali compound is from 3:1 to 1:3.
4. The method according to claim 1, wherein in the step (1), the ball milling speed used for the dry ball milling is 50r/min to 600r/min.
5. The method of claim 1, wherein in step (1), the waste PLA is selected from the group consisting of waste PLA powder, waste PLA fiber, waste PLA flake.
6. The method of claim 1, wherein in step (2), the protective gas is nitrogen or an inert gas.
7. The method according to claim 1, wherein in step (2), the acid washing is performed with hydrochloric acid.
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