A method for photocatalytically co-producing hydrogen, ethylene and adipic acid from waste PE plastics

Abstract

The present invention discloses a method for photocatalytically co-producing hydrogen, ethylene and adipic acid from waste PE plastics, comprising the following steps: (1) dissolving waste PE plastics in an acidic aqueous solution to obtain an aqueous electrolyte containing succinic acid; (2) assembling a photoanode, a cathode and the aqueous electrolyte containing succinic acid obtained in step (1) into a photoelectrochemical reaction cell; (3) applying light irradiation and a bias voltage to the photoelectrochemical reaction cell assembled in step (2) to carry out a photocatalytic reaction, wherein water in the aqueous electrolyte undergoes a reduction reaction at the cathode to generate hydrogen, and succinic acid undergoes an oxidation reaction at the photoanode to generate ethylene and adipic acid. The present invention utilizes the coupling of PE plastics and photoelectrolysis of water to improve the hydrogen production rate and energy conversion efficiency of the photocatalytic process, and effectively reduces the cost of hydrogen production by photoelectrolysis of water. PE is converted into gaseous ethylene that is easy to separate and can be reused for the production of high-value products such as ethylene oxide and the original PE. At the same time, adipic acid is also an important chemical raw material.

Description

Technical Field

[0001] The present invention belongs to the technical field of recycling of waste PE plastics and photocatalytic water splitting for hydrogen production, and particularly relates to a method for co-producing hydrogen, ethylene and adipic acid by photocatalytic recycling of waste PE plastics. Background Art

[0002] Synthetic polymers are indispensable in modern human life and are widely used in industries such as packaging, construction, transportation, electronics and healthcare. Many of these applications rely on inexpensive disposable materials - plastics, so plastics are often precisely constructed and given the properties required for the target. In the past 70 years, the global production of petroleum-based synthetic plastics has increased sharply, from less than 2 million tons in 1950 to 380 million tons in 2015, and the production is expected to double again in the next 20 years. Approximately 80% of plastics are discarded in landfills or the natural environment, posing a serious threat to the earth's ecology and environment. Among various plastics, polyethylene (PE) is the most produced plastic in the world, with an annual production of over 100 million tons. Compared with the successful recovery of raw materials from polyethylene terephthalate and polystyrene, PE is extremely inert and difficult to degrade without special treatment. This is mainly because PE consists of non-polar saturated high molecular weight hydrocarbon chains. Thermal cracking and catalytic pyrolysis can only depolymerize PE into complex low calorific value gases, liquid hydrocarbons and coke products at high temperatures (>400°C).

[0003] The recycling methods of PE waste include physical methods and chemical methods. Patent (CN 101041725 B, 2007) discloses a recycling process of PE, including one or more combinations of harmless recycling processes such as mixing, crushing, screening, heating desulfurization, swelling, saponification, emulsification, kneading, tablet pressing and granulation. The physical process is complex, and the properties of the products obtained by recycling are poor and cannot be used to prepare high-end products. Patent (CN 102504330B, 2011) discloses a recycling method of waste PE, which includes the following steps: 1) placing reaction raw materials such as polyethylene plastics in a reaction kettle and carrying out thermal cracking reaction under vacuum conditions; 2) cooling the cracking products to 260 - 335°C and discharging the gas mixture; 3) continuing to cool the materials in the reaction kettle to room temperature, adding a solvent for extraction, and separating the liquid and solid to obtain a solid product and a liquid product; 4) distilling the liquid product to obtain a solvent and a target product of polyolefin wax product. The products recycled by chemical methods have higher quality, but often require the use of large amounts of catalysts and organic solvents and need to be carried out at high temperatures. Therefore, developing a new recycling process of waste PE plastics under mild conditions has great scientific and economic value.

[0004] The photoelectrochemical water splitting process involves the oxygen evolution reaction (OER) at the anode and the hydrogen evolution reaction (HER) at the cathode. Among them, the four-electron OER reaction is the rate-determining step that restricts the entire reaction, and the added value of O2 is not high. In addition, H2 and O2 are generated simultaneously in the reactor, and the potential mixing poses a safety hazard. Summary of the Invention

[0005] The object of the present invention is to provide a method for recycling waste PE plastics. Adding PE to the acidic electrolyte will decompose to produce succinic acid, and succinic acid can be oxidized prior to the OER reaction at the anode, consuming the active oxygen in water to obtain ethylene products and effectively promoting the HER reaction. This reaction can recycle waste PE plastics and improve the efficiency of hydrogen production by photoelectrocatalytic water splitting at the same time.

[0006] The method for co-producing hydrogen, ethylene, and adipic acid by photoelectrocatalytic waste PE plastics of the present invention includes the following steps:

[0007] (1) Dissolve waste PE plastics in an acidic aqueous solution to obtain an aqueous electrolyte containing succinic acid;

[0008] (2) Assemble a photoanode, a cathode, and the aqueous electrolyte containing succinic acid obtained in step (1) into a photoelectrochemical reaction cell;

[0009] (3) Apply light irradiation and a bias voltage to the photoelectrochemical reaction cell assembled in step (2) to carry out a photoelectrocatalytic reaction. The water in the aqueous electrolyte undergoes a reduction reaction at the cathode to generate hydrogen, and succinic acid undergoes an oxidation reaction at the photoanode to generate ethylene and adipic acid.

[0010] The acidic aqueous solution can be an aqueous solution of nitric acid. The acidic aqueous solution is used to dissolve waste PE plastics to obtain an aqueous electrolyte containing succinic acid.

[0011] The concentration of the acid contained in the acidic aqueous solution can be 1 - 5 mol / L. However, the concentration range of the acid in the acidic aqueous solution is not limiting, as long as it can dissolve waste PE plastics. There are also no specific limitations on the temperature and time of the dissolution process, as long as it can dissolve waste PE plastics.

[0012] The photoanode is selected from one or more of n-type semiconductor oxides and nitrides. Among them, the metal in the n-type semiconductor oxide is selected from one or more of titanium, bismuth, and vanadium. More specifically, the photoanode is one or more of titanium dioxide, bismuth vanadate, and bismuth oxide.

[0013] The light irradiation applied during the photoelectrocatalytic process in step (3) is AM 1.5G (100 - 280 mW cm -2) The bias voltage is in the range of -0.5 to 1.5 V. However, this voltage range is not restrictive. The applied light intensity and the magnitude of the voltage are related to the size of the catalyst and the electrolysis device, the assembly method, etc. As long as water and succinic acid can react at the cathode and photoanode of the photoelectrochemical device respectively.

[0014] The present invention couples the recycling of PE with the process of photocatalytic water splitting for hydrogen production, and replaces the traditional oxygen evolution process at the photoanode with a more thermodynamically favorable organic photoelectrocatalytic oxidation process. This not only improves the hydrogen production rate and energy conversion efficiency of water splitting, obtains high-purity hydrogen, and avoids the generation of oxygen at the photoanode, but also converts PE into gaseous ethylene and adipic acid that are easy to separate, realizes the oxidation of organic matter to generate high-value-added products, and simultaneously improves the hydrogen production efficiency at the cathode.

[0015] The core of the present invention is the process of photoelectrocatalytically producing ethylene, adipic acid and hydrogen from PE in an aqueous electrolyte. Specifically, 1. PE will undergo hydrolysis in acidic aqueous solution to generate dicarboxylic acids (such as succinic acid). Succinic acid is prone to oxidative decarboxylation at the photoanode catalyst to produce ethylene, or decarboxylation coupling to produce adipic acid; 2. If there is no organic matter such as succinic acid in the electrolyte, the anodic reaction is oxygen evolution reaction, and the overpotential required for it is much higher than that of succinic acid oxidation. The result is that at the same voltage, the photocurrent density will increase greatly after adding succinic acid, thus increasing the hydrogen production rate and reducing the energy consumption of water splitting. In the present invention, after combining the recycling of PE with water splitting, the dissolution of PE and water splitting are integrated into one, and the succinic acid produced by the hydrolysis of PE can also promote water splitting to produce hydrogen, so it is more economical in terms of cost. Description of the Drawings

[0016] Figure 1 It is the scanning electron microscope image and X-ray diffraction pattern of the titanium dioxide nanorods prepared in Example 2;

[0017] Figure 2 It is the polarization curve of the titanium dioxide nanorods in Example 2;

[0018] Figure 3 It is the polarization curve of bismuth oxide loaded on the titanium dioxide nanorods in Example 4;

[0019] Figure 4 It is the schematic diagram of the principle of the photoelectrocatalytic decomposition of PE device in Example 4;

[0020] Figure 5 It is the gas chromatogram of the ethylene recovered in Example 5.

[0021] Figure 6 It is the gas chromatogram of the hydrogen in Example 5. Detailed Description of the Invention

[0022] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0023] Example 1 Preparation of Titanium Dioxide Nanorod Photoanode Material

[0024] 1. Cut fluorine-doped tin oxide conductive glass into pieces of 2.0 cm × 3.0 cm, and then ultrasonically wash with acetone, isopropanol, and deionized water for 30 min each, and dry for later use.

[0025] 2. Use the fluorine-doped tin oxide conductive glass obtained in step 1 as the substrate, and place it in a uniform mixed solution of 15 mL of deionized water and hydrochloric acid each, and sequentially add 0.3 mL of tetrabutyl titanate and 0.1 M BiCl3. After reacting at 150 °C for 20 h, take it out and rinse with deionized water to obtain a titanium dioxide nanorod array.

[0026] 3. Put the material obtained in step 2 above into a porcelain boat, place it in a muffle furnace, and the heating program is: heat from room temperature to 450 °C at a rate of 5 °C / min, keep warm for 2 h, and then naturally cool to room temperature to obtain a titanium dioxide nanorod photoanode material, denoted as TiO2.

[0027] Experimental result analysis: As Figure 1 shown in the scanning electron microscope results of (a), TiO2 nanorods grow uniformly on the conductive glass. Figure 1 The X-ray diffraction pattern of (b) confirmed that the synthesized material is titanium dioxide.

[0028] Example 2 Photoelectrochemical Activity Test of TiO2 Photoanode

[0029] 1. Dissolve 0.3 g of succinic acid in 30 mL of 0.5 mol / L Na2SO4 solution, and adjust the pH = 7 with 5 mol / L sodium hydroxide solution.

[0030] 2. Use TiO2 as the working electrode, silver / silver chloride electrode as the reference electrode, and platinum sheet as the counter electrode to assemble a three-electrode system. Test the polarization curves in 0.5 mol / L Na2SO4 solution (as a blank comparison) and the solution obtained in step 1 respectively, with light illumination of AM 1.5G (280 mW cm -2 ), voltage range -0.6 - 1.2 V versus Ag / AgCl, and scanning rate 0.05 V / s.

[0031] Experimental result analysis: AsFigure 2 As shown, after adding succinic acid to the electrolyte, the initial potential of oxidation decreases and the photocurrent density increases. Specifically, as shown in Table 1, at a voltage of 0.6V versus RHE, the current density in a pure 0.5mol / L Na2SO4 solution (blank control) is only 0.6mA / cm 2 , while the photocurrent density in the electrolyte solution after adding succinic acid increases to 2.4mA / cm 2 , indicating that the amount of hydrogen production is faster and the conversion efficiency of current to hydrogen is higher.

[0032] Example 3 Preparation of Bismuth Oxide Photocathode Material Loaded with Titanium Dioxide

[0033] 1. Cut the fluorine-doped tin oxide conductive glass into pieces of 2.0cm×3.0cm, and then ultrasonically wash them with acetone, isopropyl alcohol, and deionized water for 30 minutes each, and dry for later use.

[0034] 2. Use the fluorine-doped tin oxide conductive glass obtained in step 1 as the substrate, place it in a homogeneous mixed solution of 15mL of deionized water and hydrochloric acid each, and sequentially add 0.3mL of tetrabutyl titanate and 0.1M BiCl3. After reacting at 150°C for 20h, take it out and rinse with deionized water to obtain a titanium dioxide nanorod array.

[0035] 3. Put the material obtained in step 2 above into a porcelain boat, place it in a muffle furnace, and the heating program is: heat from room temperature to 450°C at a rate of 5°C / min, keep it warm for 2h, and then cool it naturally to room temperature to obtain a titanium dioxide nanorod photocathode material, denoted as TiO2.

[0036] 4. Use the TiO2 obtained in step 3 as the working electrode, the silver / silver chloride electrode as the reference electrode, and the platinum sheet as the counter electrode. Electroplate in a mixed solution of 50mL of 0.04M Bi(NO3)3 and 0.4M KI aqueous solution (adjust the pH to 1.7 by adding HNO3) and 20mL of 0.23M p-benzoquinone ethanol solution, with a voltage of -0.1V and a time of 300s to obtain a bismuth oxide electrode material loaded with titanium dioxide.

[0037] 5. Put the material obtained in step 2 above into a porcelain boat, place it in the middle of a muffle furnace, and the heating program is: heat from room temperature to 450°C at a rate of 5°C / min, keep it warm for 2h, and then cool it naturally to room temperature. After taking it out, place it in a tube furnace (at the upstream position of the gas flow), introduce 10% hydrogen-argon gas, with a flow rate of 20mL / min, and the heating program is: heat from room temperature to 450°C at a rate of 1°C / min, keep it warm for 2h, and then cool it naturally to room temperature to obtain a bismuth oxide electrode material loaded with titanium dioxide, denoted as BiO x / TiO2.

[0038] Example 4 BiOx Photoelectrochemical Activity Test of BiO

[0039] 1. Dissolve 0.3 g of succinic acid in 30 mL of 0.5 mol / L Na2SO4 solution, and adjust the pH to 7 with 5 mol / L sodium hydroxide solution.

[0040] 2. Use BiO x / TiO2 as the photoanode, silver / silver chloride electrode as the reference electrode, and platinum sheet as the counter electrode to assemble a three-electrode system. Test the polarization curves in 0.5 mol / L Na2SO4 solution (as a blank comparison) and the solution obtained in step 1 respectively. The light illumination is AM 1.5G (280 mW cm -2 ), the voltage range is -0.6 - 1.2 V versus Ag / AgCl, and the scanning rate is 0.05 V / s.

[0041] Analysis of experimental results: As Figure 2 shown, after adding succinic acid to the electrolyte, the initial potential of oxidation decreases and the photocurrent density increases. Specifically, as shown in Table 1, at a voltage of 0.6 V versus RHE, the current density in pure 0.5 mol / L Na2SO4 solution (blank comparison) is only 1.3 mA / cm 2 , while the photocurrent density in the electrolyte solution after adding succinic acid increases to 3.7 mA / cm 2 , indicating that the amount of hydrogen production is faster and the conversion efficiency of current to hydrogen is higher.

[0042] [Table 1]

[0043]

[0044] Example 5 Photocatalytic Electrolysis of PE Plastic to Co-Produce Hydrogen, Ethylene and Adipic Acid

[0045] 1. Dissolve 0.9 g of PE in 1.5 mol / L nitric acid solution and react at 180 °C for 4 h.

[0046] 2. Adjust the pH of the solution obtained in step 1 to 7 with 5 mol / L NaOH and use it as the reaction solution. Use the bismuth oxide material supported on titanium dioxide as the photoanode, silver / silver chloride electrode as the reference electrode, and platinum sheet as the counter electrode to assemble a photoelectrocatalytic reaction cell, and then react at a voltage of 0.6 V versus RHE for 6 h. As Figure 4 shown, PE first decomposes into succinic acid in the acidic solution. During the photoelectrocatalytic process, succinic acid is oxidized at the photoanode, deprotonates and carboxyl groups to generate ethylene (see Figure 5 ), while protons are reduced at the cathode to generate hydrogen (see Figure 6 ).

Claims

1. A method for photocatalytically co-producing hydrogen, ethylene and adipic acid from waste PE plastics, characterized in that, The method comprises the following steps: (1) Dissolve waste PE plastics in an acidic aqueous solution, adjust the pH to 7 with an alkali to obtain an aqueous electrolyte containing succinic acid; (2) Assemble a photo-electrochemical reaction cell with a photoanode, a cathode and the aqueous electrolyte containing succinic acid obtained in step (1); (3) Apply light irradiation and a bias voltage to the photo-electrochemical reaction cell assembled in step (2) to carry out a photo-electrocatalytic reaction, water in the aqueous electrolyte undergoes a reduction reaction at the cathode to generate hydrogen, and succinic acid undergoes an oxidation reaction at the photoanode to generate ethylene and adipic acid.

2. The method for photocatalytic co-production of hydrogen, ethylene and adipic acid from waste PE plastics according to claim 1, wherein, The acidic aqueous solution is a nitric acid aqueous solution.

3. The method for photocatalytic co-production of hydrogen, ethylene and adipic acid from waste PE plastics according to claim 1, wherein, The concentration of the acid contained in the acidic aqueous solution is 1-5 mol / L.

4. The method for photocatalytic co-production of hydrogen, ethylene and adipic acid from waste PE plastics according to claim 1, wherein The photoanode is selected from one or more of n-type semiconductor oxides and nitrides.

5. The method for co-producing hydrogen, ethylene and adipic acid by photocatalytic degradation of waste PE plastics according to claim 4, wherein The metal in the n-type semiconductor oxide is selected from one or more of titanium, bismuth and vanadium.

6. The method for photocatalytic co-production of hydrogen, ethylene and adipic acid from waste PE plastics according to claim 1, characterized in that, The photoanode is one or more of titanium dioxide, bismuth vanadate and bismuth oxide.

7. The method for photocatalytic co-production of hydrogen, ethylene and adipic acid from waste PE plastics according to claim 1, characterized in that, The light illumination applied in step (3) is AM 1.5G, 100 - 280 mW cm −2 , and the bias voltage is in the range of -0.5 - 1.5 V.

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