Polyethylene terephthalate (PET) plastic high-value utilization method based on combination of hydrothermal pretreatment and photocatalytic reduction

Abstract

The invention discloses a PET plastic high-value utilization method based on combination of hydrothermal pretreatment and photocatalytic reduction, which comprises the following steps: pretreating PET by adopting a hydrothermal method without adding alkali to obtain hydrolysate containing benzoic acid (PTA) and ethylene glycol (EG), and then treating a hydrothermal pretreatment solution of the PET by adopting a photocatalytic method based on C and S co-doped carbon nitride to obtain the PET plastic high-value utilization method based on combination of C and S co-doped carbon nitride. According to the method, ethylene glycol oxidation and carbon dioxide reduction reactions are coupled, CO gas-phase chemicals with high additional value are obtained, meanwhile, the ethylene glycol recovery process is avoided, the purification cost of liquid-phase products is reduced, and the problems of low efficiency, high cost and low additional value of products in the PET plastic treatment process in the prior art are solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of PET plastic recycling, and in particular relates to a method for high-value utilization of PET plastic based on the combination of hydrothermal pretreatment and photocatalytic reduction. Background Art

[0002] Polyethylene terephthalate (PET) is lightweight, acid- and alkali-resistant, and airtight, making it widely used in beverage bottles, food packaging, and engineering plastics. However, discarded PET products gradually break down into microplastics in the natural environment through sunlight, water flow, and other factors. These microplastics can not only spread over long distances through media such as water, but can also accumulate in organisms and produce toxic effects. Therefore, the recycling of PET has received increasing national attention. However, currently, recycled PET is mostly used in low-end products such as recycled fibers. Physical recycling methods can easily lead to performance degradation, and chemical recycling is costly due to the high alkali content.

[0003] At present, it has become an urgent need in the industry to develop green, low-cost and non-secondary pollution methods to convert PET plastic waste into high-value chemicals.

[0004] The hydrothermal chemical pretreatment of PET without the addition of alkali has the advantages of high treatment efficiency and high product added value (benzoic acid (PTA) and ethylene glycol (EG)), making it an environmentally friendly and economical pretreatment method. More importantly, the hydrothermal pretreatment liquid based on photocatalytic technology for treating PET can couple ethylene glycol oxidation and other reduction reactions (such as hydrogen evolution and oxygen reduction to produce hydrogen peroxide, etc.), which can not only effectively decompose the product but also obtain high-value-added chemical products, which is of practical significance for reducing the cost of PET plastic processing. Photocatalytic CO2 reduction uses photocatalysts to excite free electrons under light, converting CO2 into carbon-based fuels or chemicals such as methanol, methane, and CO. The reaction conditions are mild (room temperature and pressure), and it has the dual value of carbon emission reduction and energy conversion. Constructing a mechanism for the combined use of hydrothermal pretreatment and photocatalytic reduction to remove PET can solve the problems of low efficiency, high cost and low product added value in the existing technology for treating PET plastics, and has important commercial value. Summary of the Invention

[0005] To address the problems of low efficiency, high cost and low product added value in the prior art of processing PET plastics, the present invention provides a method for high-value utilization of PET plastics based on the combination of hydrothermal pretreatment and photocatalytic reduction. PET is pretreated by a hydrothermal method without adding alkali to obtain a hydrolyzed solution containing benzoic acid (PTA) and ethylene glycol (EG). The hydrothermal pretreated solution of PET is then treated by a photocatalytic method based on C and S co-doped carbon nitride. The ethylene glycol oxidation and carbon dioxide reduction reactions are coupled to obtain CO gas-phase chemicals with high added value. At the same time, the ethylene glycol recovery process is avoided, thereby reducing the cost of purifying the liquid product.

[0006] The purpose of the present invention is achieved through the following technical solutions:

[0007] A method for high-value utilization of PET plastic based on a combination of hydrothermal pretreatment and photocatalytic reduction, comprising the following steps:

[0008] S1. Pretreating PET by hydrothermal method without adding alkali to obtain hydrolyzate:

[0009] Add PET plastic to water, with a mass ratio of PET plastic to water of (1-30):(1-40), transfer the material to a hydrothermal reactor, control the reaction atmosphere, heat to 130°C-230°C, and react for 10h-40h to obtain a hydrolysis pretreatment liquid containing benzoic acid (PTA) and ethylene glycol (EG);

[0010] The reaction atmosphere is nitrogen or carbon dioxide;

[0011] S2. Preparation of C and S co-doped carbon nitride using supramolecular preassembly method:

[0012] Using N-rich organic matter as raw material, under the induction of organic acid, a calcined precursor is obtained by reaction at room temperature; the precursor is placed in a tube furnace, the reaction atmosphere is controlled, and it is directly calcined at a reaction temperature of 350°C-650°C and a reaction time of 2h-48h to obtain C and S co-doped carbon nitride;

[0013] The N-rich organic matter is one or more of dicyandiamide, melamine, and urea, which can be mixed in any proportion;

[0014] The organic acid is one or both of methanesulfonic acid and 1,2-ethanedisulfonic acid, and the mixture can be in any ratio; the reaction atmosphere is nitrogen or air;

[0015] S3, coupled hydrothermal pretreatment-photocatalytic CO2 reduction reaction:

[0016] The C and S co-doped carbon nitride obtained in S2 is added to the hydrolysis pretreatment liquid containing benzoic acid and ethylene glycol obtained in S1, and the mass ratio of the C and S co-doped carbon nitride to the hydrolysis pretreatment liquid containing benzoic acid and ethylene glycol is (1-10):(20-80). Carbon dioxide gas is introduced at room temperature. After the adsorption is completed, the light is controlled and the reaction is carried out for 4h-36h to obtain high-value-added CO gas. Finally, the reaction liquid is subjected to reduced pressure distillation and acidification to obtain benzoic acid solid.

[0017] Furthermore, the reaction atmosphere described in S1 is carbon dioxide, heated to 200° C., and reacted for 16 hours.

[0018] Furthermore, the reaction atmosphere described in S2 is air, the reaction temperature is 550° C., and the reaction time is 24 h.

[0019] Furthermore, the light irradiation in S3 is xenon lamp or ultraviolet light irradiation; the acid used in the acidification treatment is one of hydrochloric acid, sulfuric acid and nitric acid.

[0020] The present invention discloses a method for high-value utilization of PET plastics based on a combination of hydrothermal pretreatment and photocatalytic reduction. The PET hydrolysis rate is 83%-100%, the molar fractions of benzoic acid and ethylene glycol in the hydrolyzate are 36%-38% and 28%-29%, respectively, the ethylene glycol conversion rate is ≥98%, and the photocatalytic carbon dioxide conversion CO yield is 175.2 μmol g -1 h -1 The hydrogen yield was 54.8 μmol g -1 h -1 .

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] 1. The method for high-value utilization of PET plastic based on the combination of hydrothermal pretreatment and photocatalytic reduction described in the present invention adopts a hydrothermal method without adding alkali to pretreat PET to obtain a hydrolyzate; the process is simple and highly operable.

[0023] 2. The present invention discloses a method for high-value utilization of PET plastics based on a combination of hydrothermal pretreatment and photocatalytic reduction, which regulates the co-doping of C and S in carbon nitride to improve the efficiency of photogenerated charge separation and the selectivity of carbon dioxide reduction. These changes are of great significance for achieving efficient photocatalytic CO2 reduction to produce CO.

[0024] 3. The method for high-value utilization of PET plastic based on the combination of hydrothermal pretreatment and photocatalytic reduction described in the present invention adopts coupled hydrothermal pretreatment and photocatalytic CO2 reduction reaction, which has the dual value of carbon emission reduction and high-value utilization of waste. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The accompanying drawings are used to provide further understanding of the present invention and constitute a part of the specification. They are used to explain the present invention together with the embodiments of the present invention and do not constitute a limitation of the present invention.

[0026] Figure 1 The XRD patterns of the material prepared by the method for high-value utilization of PET plastic based on the combination of hydrothermal pretreatment and photocatalytic reduction in Example 1 of the present invention and the XRD patterns of the material prepared in the comparative example;

[0027] Figure 2 The C1s XPS graph of a material prepared by a method for high-value utilization of PET plastic based on a combination of hydrothermal pretreatment and photocatalytic reduction according to Example 1 of the present invention and the C1s XPS graph of a material prepared in a comparative example;

[0028] Figure 3 The S2p XPS graph of a material prepared by a method for high-value utilization of PET plastic based on a combination of hydrothermal pretreatment and photocatalytic reduction according to Example 1 of the present invention and the S2p XPS graph of a material prepared in a comparative example;

[0029] Figure 4 This is a graph showing the cyclic performance of the material in the coupled hydrothermal pretreatment-photocatalytic reduction reaction in a method for high-value utilization of PET plastic based on a combined hydrothermal pretreatment-photocatalytic reduction method according to Example 1 of the present invention;

[0030] Figure 5 This is a reaction schematic diagram of a method for high-value utilization of PET plastic based on the combination of hydrothermal pretreatment and photocatalytic reduction described in the present invention. DETAILED DESCRIPTION

[0031] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0032] Figure 5 This is a reaction diagram of a method for high-value utilization of PET plastic based on a combination of hydrothermal pretreatment and photocatalytic reduction as described in an embodiment;

[0033] Example 1:

[0034] A method for high-value utilization of PET plastic based on a combination of hydrothermal pretreatment and photocatalytic reduction, comprising the following steps:

[0035] S1. Pretreating PET by hydrothermal method without adding alkali to obtain hydrolyzed solution: 1 g of PET plastic was added to 100 mL of water, which was transferred to a hydrothermal reactor. Carbon dioxide gas was continuously introduced and stirred at room temperature for 0.5 h. The reaction temperature was controlled at 200° C. and maintained for 16 h to obtain a hydrolysis pretreatment solution containing benzoic acid (PTA) and ethylene glycol (EG), as shown in Table 1.

[0036] S2. Preparation of C and S co-doped carbon nitride by supramolecular preassembly method: 1 g of melamine was added to 100 mL of water, mixed with 1,2-ethanedisulfonic acid aqueous solution (3 g / L), and reacted at room temperature with uniform stirring for 2 h; then, the precursor was collected, washed and dried; finally, it was transferred to a tube furnace, heated to 550°C in air, and maintained for 4 h; after cooling to room temperature, C and S co-doped carbon nitride was obtained;

[0037] S3, coupled hydrothermal pretreatment-photocatalytic CO2 reduction reaction: 30 mg of the obtained C, S co-doped carbon nitride was added to the hydrolysis pretreatment liquid containing ethylene glycol (EG), carbon dioxide gas was introduced at room temperature, and stirred at a constant speed for 0.5 h; after the adsorption-desorption equilibrium was reached, the xenon lamp was controlled to irradiate at room temperature, and the reaction was carried out for 5 h to obtain CO and H2 gas (the performance was evaluated by gas chromatograph, and the results are shown in Table 2); then, the reaction liquid was collected, filtered, and concentrated to 5 mL by pressure distillation; finally, concentrated hydrochloric acid was added until the solution was obviously acidic, and benzoic acid was obtained after all the crystals were precipitated. At the same time, the catalyst after the reaction was collected, washed and dried, and its recycling performance was tested. The results are as follows Figure 4 shown.

[0038] As shown in Table 1, after reacting at 200 °C for 16 h, the PET plastic was completely decomposed with a decomposition rate of up to 100%, and the molar fractions of benzoic acid and ethylene glycol in the hydrolyzate were close to 36% and 28%, respectively.

[0039] As shown in Table 2, C and S co-doped carbon nitride exhibited good photocatalytic performance. After 5 h of light irradiation, ethylene glycol was almost completely oxidized with a conversion rate of up to 98%. The photocatalytic carbon dioxide conversion yield was 175.2 μmol g -1 h -1 The hydrogen yield was 54.8 μmol g -1 h -1 .

[0040] Figure 1 The XRD patterns of the material prepared by the method for high-value utilization of PET plastic based on the combination of hydrothermal pretreatment and photocatalytic reduction in Example 1 and the XRD patterns of the material prepared in the comparative example;

[0041] Figure 2 The C1s XPS graph of a material prepared by a method for high-value utilization of PET plastic based on a combination of hydrothermal pretreatment and photocatalytic reduction in Example 1 and the C1s XPS graph of a material prepared in a comparative example;

[0042] from Figure 1 and Figure 2 It can be seen that the crystal structure and element state diagram of carbon nitride co-doped with C and S prepared in Example 1. Figure 1 As shown, the material exhibits an obvious characteristic peak at 2θ = 27.4°, indicating that the crystal structure of the material has not collapsed.

[0043] Figure 3 The S2p XPS graph of a material prepared by a method for high-value utilization of PET plastic based on a combination of hydrothermal pretreatment and photocatalytic reduction in Example 1 and the S2p XPS graph of a material prepared in a comparative example;

[0044] like Figure 2 、 Figure 3 As shown, compared with the g-C3N4 prepared in the comparative example, the C-C bond in the C1s spectrum of the material prepared in Example 1 is enhanced, and an S signal appears in the S2p spectrum, indicating that C and S are co-doped into the material structure.

[0045] Figure 4 This is a graph showing the cycling performance of the material in the coupled hydrothermal pretreatment-photocatalytic reduction reaction in a method for high-value utilization of PET plastic based on a combined hydrothermal pretreatment-photocatalytic reduction method in Example 1;

[0046] like Figure 4 As shown, the material prepared in Example 1 exhibits good stability, and in the cycle experiment, the CO yield of the photocatalytic carbon dioxide conversion remains basically unchanged.

[0047] like Figure 5 As shown, the present invention provides a reaction schematic diagram of a method for high-value utilization of PET plastic based on the combination of hydrothermal pretreatment and photocatalytic reduction, which has the dual value of carbon emission reduction and high-value utilization of waste.

[0048] Example 2:

[0049] A method for high-value utilization of PET plastic based on a combination of hydrothermal pretreatment and photocatalytic reduction, comprising the following steps:

[0050] S1. Pretreating PET by hydrothermal method without adding alkali to obtain hydrolyzed solution: 2 g of PET plastic was added to 150 mL of water, which was transferred to a hydrothermal reactor. Nitrogen was continuously introduced and stirred at room temperature for 0.5 h. The reaction temperature was controlled at 170° C. and maintained for 20 h to obtain a hydrolysis pretreatment solution containing benzoic acid (PTA) and ethylene glycol (EG), as shown in Table 1.

[0051] S2. Preparation of C and S co-doped carbon nitride by supramolecular preassembly method: 1 g of a mixture of dicyandiamide and melamine (0.2:0.8) was added to 100 mL of water, mixed with an aqueous solution of methanesulfonic acid (8 g / L), and reacted at room temperature with uniform stirring for 1 h; then, the precursor was collected, washed and dried; finally, it was transferred to a tube furnace, heated to 500°C in air, and maintained for 6 h; after cooling to room temperature, C and S co-doped carbon nitride was obtained;

[0052] S3. Coupled hydrothermal pretreatment-photocatalytic CO2 reduction reaction: Referring to Example 1, the photocatalytic ethylene glycol oxidation and photocatalytic carbon dioxide reduction to CO performance of the prepared C, S co-doped carbon nitride were investigated.

[0053] The results are shown in Table 2. The material prepared in Example 2 exhibits good photocatalytic ethylene glycol oxidation performance and good performance in the photocatalytic carbon dioxide reduction reaction to CO.

[0054] Comparative Example:

[0055] Referring to Example 1, the difference is that S2 uses a direct calcination method to prepare carbon nitride;

[0056] S2 is: 7 g of melamine is directly transferred to a tube furnace, heated to 520° C. in air, and maintained for 4 hours; and cooled to room temperature to obtain carbon nitride.

[0057] S1 and S2 are consistent with those in Example 1.

[0058] The results are shown in Table 2. It can be seen from Table 2 that compared with the carbon nitride co-doped with C and S in the embodiment, the photocatalytic performance of the carbon nitride in the comparative example is poor. After 5 hours of light reaction, the ethylene glycol conversion rate is only 40%, and the photocatalytic carbon dioxide conversion rate is 15.1 μmol g -1 h -1 The photocatalytic hydrogen production rate was 23.4 μmol g -1 h -1 .

[0059] Results and Discussion

[0060] Table 1: Hydrolysis performance of PET pretreated by hydrothermal method without adding alkali

[0061]

[0062] Table 2: Performance of coupled hydrothermal pretreatment-photocatalytic reduction reaction

[0063]

[0064] The experimental results obtained by analyzing Example 1, Example 2, and the comparative example are shown in Table 2. The results show that the present invention regulates the co-doping of C and S in carbon nitride to improve the photocatalytic ethylene glycol oxidation efficiency and carbon dioxide reduction performance. The material also exhibits good cycling performance in the coupled hydrothermal pretreatment-photocatalytic reduction reaction, which has practical significance for solving the problems of low efficiency, high cost, and low product added value in the prior art process of treating PET plastics.

[0065] In addition, it should be understood that although this specification is described in terms of implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

Claims

1. A method for high-value utilization of PET plastics based on a combination of hydrothermal pretreatment and photocatalytic reduction, characterized by: The following steps are involved: S1. Pretreating PET by hydrothermal method without adding alkali to obtain hydrolyzate: Add PET plastic to water at a mass ratio of PET plastic to water of (1-30):(1-40), transfer the material to a hydrothermal reactor, control the reaction atmosphere, heat to 130°C-230°C, and react for 10h-40h to obtain a hydrolysis pretreatment solution containing benzoic acid and ethylene glycol; The reaction atmosphere is nitrogen or carbon dioxide; S2. Preparation of C and S co-doped carbon nitride using supramolecular preassembly method: Using N-rich organic matter as raw material, under the induction of organic acid, a calcined precursor is obtained by reaction at room temperature; the precursor is placed in a tube furnace, the reaction atmosphere is controlled, and it is directly calcined at a reaction temperature of 350°C-650°C and a reaction time of 2h-48h to obtain C and S co-doped carbon nitride; The N-rich organic matter is one or more of dicyandiamide, melamine, and urea, which can be mixed in any proportion; The organic acid is one or both of methanesulfonic acid and 1,2-ethanedisulfonic acid, and the mixture can be in any ratio; the reaction atmosphere is nitrogen or air; S3, coupled hydrothermal pretreatment-photocatalytic CO2 reduction reaction: The C and S co-doped carbon nitride obtained in S2 is added to the hydrolysis pretreatment liquid containing benzoic acid and ethylene glycol obtained in S1, and the mass ratio of the C and S co-doped carbon nitride to the hydrolysis pretreatment liquid containing benzoic acid and ethylene glycol is (1-10):(20-80). Carbon dioxide gas is introduced at room temperature. After the adsorption is completed, the light is controlled and the reaction is carried out for 4h-36h to obtain high-value-added CO gas. Finally, the reaction liquid is subjected to reduced pressure distillation and acidification to obtain benzoic acid solid.

2. The method for high-value utilization of PET plastics based on the combination of hydrothermal pretreatment and photocatalytic reduction according to claim 1, characterized in that: The light irradiation in S3 is xenon lamp or ultraviolet light irradiation; the acid used in the acidification treatment is one of hydrochloric acid, sulfuric acid and nitric acid.

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