CN113774399A

CN113774399A - Method for co-producing hydrogen, formic acid and terephthalic acid from waste PET (polyethylene terephthalate) plastic through electrocatalysis

Info

Publication number: CN113774399A
Application number: CN202010499309.5A
Authority: CN
Inventors: 段昊泓; 周华; 栗振华; 任悦; 孔祥贵
Original assignee: Tsinghua University; Beijing University of Chemical Technology
Current assignee: Tsinghua University; Beijing University of Chemical Technology
Priority date: 2020-06-04
Filing date: 2020-06-04
Publication date: 2021-12-10
Anticipated expiration: 2040-06-04
Also published as: CN113774399B

Abstract

The invention relates to a method for co-producing hydrogen, formic acid and terephthalic acid from waste PET plastic through electrocatalysis, which comprises the following steps: (1) dissolving waste PET plastics in an alkaline aqueous solution to obtain an aqueous electrolyte containing ethylene glycol and terephthalate; (2) assembling an anode, a cathode and the aqueous electrolyte solution containing ethylene glycol and terephthalate obtained in step (1) into an electrolytic cell; and (3) applying a voltage to the electrolytic cell assembled in step (2) to perform electrolysis, wherein water in the aqueous electrolyte undergoes a reduction reaction at the cathode to generate hydrogen gas, and ethylene glycol undergoes an oxidation reaction at the anode to generate formic acid. The invention utilizes the coupling of the PET plastic and the electrolyzed water to improve the hydrogen production rate and the energy conversion efficiency in the electrolysis process and effectively reduce the hydrogen production cost by the electrolyzed water. PET is converted into terephthalate and formate, and the high-purity terephthalic acid is obtained by separation and can be used for PET production, and meanwhile, formic acid is an important chemical raw material.

Description

Method for co-producing hydrogen, formic acid and terephthalic acid from waste PET (polyethylene terephthalate) plastic through electrocatalysis

Technical Field

The invention belongs to the technical field of recycling waste PET (polyethylene terephthalate) plastics and hydrogen production by electrolyzing water, and particularly relates to a method for co-producing hydrogen, formic acid and terephthalic acid from waste PET plastics through electrocatalysis.

Background

Polyethylene terephthalate (PET) is a plastic polymerized from terephthalic acid and ethylene glycol and is widely used in disposable beverage bottles and food packaging. Currently, over 70% of the beverage packaging materials worldwide are PET plastics. According to statistics, the total yield of PET plastics in 2016 worldwide reaches 9472 million tons, and the yield is expected to reach 1 hundred million tons in 2020. Because PET plastics are difficult to degrade in the environment (the degradation period is 200-400 years), most of the waste PET plastics enter the sea or are buried, and serious threat is caused to the ecological environment and the environment of the earth. The recovery rate of PET plastics in China is low, a large number of polyester bottles are discarded after being used once, and are mainly treated in an incineration or landfill mode, so that serious environmental pollution and resource waste are caused. By recycling and utilizing the PET wastes, the environmental pollution can be effectively reduced, and the aim of saving resources is fulfilled. In recent years, the recycling and utilization of waste PET products have attracted extensive attention and research by researchers around the world.

The recovery method of PET waste comprises a physical method and a chemical method. Patent (CN 104708730B, 2015) discloses a recycling process of PET bottles, comprising the following steps: A. feeding, B, screening, C, wet crushing, D, gravity rinsing, E, hot alkali cleaning, F, water washing, G, centrifugal dehydration, H, three rinsing, I, centrifugal dehydration and J, packaging. Although the physical method is simple in operation and low in cost, the product obtained by recovery has low purity and quality, and cannot be used for high-end products. Patent (CN 110229062 a, 2019) discloses a recycling method of waste PET polyester, which comprises the following steps: 1. cutting the waste PET polyester product into PET polyester chips, and then washing and drying the PET polyester chips; 2. stirring and mixing the PET polyester chips obtained in the step 1 with polypropylene glycol, ethylene glycol and a catalyst, then heating to 140-160 ℃ for a first reaction, and then heating to 190-200 ℃ for a second reaction. The products recovered by chemical processes are of higher quality but often require the use of large amounts of catalyst and organic solvents and are carried out at high temperatures. Therefore, the development of a novel waste PET plastic recycling process still has great scientific and economic values.

The water electrolysis process comprises an anodic Oxygen Evolution Reaction (OER) and a cathodic Hydrogen Evolution Reaction (HER), wherein OER is still the rate-controlling step that constrains the overall reaction. This is mainly due to the fact that the oxygen production process needs to go through a complex four-electron reaction process, whereas the O produced₂The added value is not high. In addition, H is produced simultaneously₂And O₂Potential mixing in the reactor presents a safety hazard.

Disclosure of Invention

Technical problem

If the traditional anode oxygen production process is replaced by the more thermodynamically favorable organic electrocatalytic oxidation process, the method is a new idea for realizing the oxidation of the organic matter so as to generate a high value-added product and simultaneously improve the cathode hydrogen production efficiency. PET is added into the alkaline electrolyte and can be decomposed to generate terephthalate and ethylene glycol, and the ethylene glycol can be oxidized at the anode before OER reaction, active oxygen in water is consumed to obtain a formic acid product, and the HER reaction is effectively promoted.

The invention aims to provide a method for recycling waste PET plastics, which can recycle the waste PET plastics and improve the efficiency of hydrogen production by water electrolysis.

Technical scheme

One aspect of the present invention provides a method for co-producing hydrogen, formic acid and terephthalic acid from waste PET plastic through electrocatalysis, comprising the steps of:

(1) dissolving waste PET plastics in an alkaline aqueous solution to obtain an aqueous electrolyte containing ethylene glycol and terephthalate;

(2) assembling an anode, a cathode and the aqueous electrolyte solution containing ethylene glycol and terephthalate obtained in step (1) into an electrolytic cell; and

(3) and (3) applying voltage to the electrolytic cell assembled in the step (2) to perform electrolysis, wherein water in the aqueous electrolyte undergoes a reduction reaction at a cathode to generate hydrogen gas, and ethylene glycol undergoes an oxidation reaction at an anode to generate formic acid.

The basic aqueous solution may include an aqueous solution of one or more bases selected from the group consisting of sodium hydroxide, potassium hydroxide, and lithium hydroxide. The alkaline aqueous solution is used for dissolving waste PET plastics to obtain an aqueous electrolyte containing ethylene glycol and terephthalate. The concentration of the alkali contained in the alkaline aqueous solution may be 0.1 to 10 mol/L. However, the concentration range of the alkali in the alkaline aqueous solution is not limited as long as the waste PET plastic can be dissolved. The temperature and time of the dissolution process are not particularly limited as long as the waste PET plastic can be dissolved.

The anode may comprise one or more selected from nanocrystals, oxides, hydroxides, oxyhydroxides, phosphides, sulfides, nitrides, borides, carbides of transition metals. Wherein the transition metal comprises: one or more of manganese, iron, cobalt, nickel, copper and zinc. More specifically, the anode may be cobalt phosphide, cobalt-iron phosphide, or cobalt oxyhydroxide.

The voltage applied during the electrolysis of step (3) may be in the range of 0.5 to 50V. However, the voltage range is not limited, and the magnitude of the applied voltage depends on the size of the electrolytic device, the manner of assembly, and the like, as long as water and ethylene glycol can be reacted with each other at the cathode and the anode of the electrolytic device.

Advantageous effects

The invention couples the recycling of PET with the hydrogen production process by alkaline electrolysis of water, thereby not only improving the cathode hydrogen production rate of the electrolysis of water, obtaining high-purity hydrogen and avoiding the production of anode oxygen, but also converting PET to obtain terephthalic acid and formic acid.

The core of the invention is the process of electrolyzing PET in alkaline electrolyte to obtain terephthalic acid, formic acid and hydrogen. Specifically, the core of the application is: 1. PET can be hydrolyzed in alkaline water to generate PET monomers (terephthalic acid and ethylene glycol), the ethylene glycol is easy to be oxidized and cracked in an anode catalyst to produce formic acid, and the terephthalic acid is relatively stable and does not react; 2. if the electrolyte does not contain organic matters such as glycol, the anode has oxygen evolution reaction, and the overpotential required by the anode is much higher than the oxidation of the glycol, so that the current density is greatly improved after the glycol is added under the same voltage, the hydrogen production rate is improved, and the energy consumption of water electrolysis is lower. In the invention, after the recovery of PET is combined with alkaline electrolyzed water, the dissolution of PET and the electrolyzed water are integrated, and meanwhile, the glycol generated by hydrolysis of PET can promote the electrolyzed water to produce hydrogen, so the invention has economical efficiency in cost.

Compared with the pure electrolytic water, the invention utilizes the coupling of the PET plastic and the electrolytic water to improve the hydrogen production rate and the energy conversion efficiency in the electrolytic process and effectively reduce the hydrogen production cost by the electrolytic water. In addition, PET is converted into terephthalate and formate, and high-purity terephthalic acid is obtained by separation and can be used for PET production, and meanwhile, formic acid is an important chemical raw material.

Drawings

FIG. 1 is a scanning electron micrograph and an X-ray diffraction pattern of a nickel foam-supported cobalt phosphide prepared in example 1;

FIG. 2 is a polarization curve of nickel foam loaded cobalt phosphide of example 2;

FIG. 3 is a polarization curve of cobalt-iron phosphide loaded with foamed nickel of example 4;

FIG. 4 is a polarization curve of the foamed nickel-supported cobalt oxyhydroxide of example 6;

FIG. 5 is a schematic view of an electrolytic PET apparatus of example 7;

FIG. 6 is a liquid phase diagram of terephthalic acid recovered in example 7.

Detailed Description

Hereinafter, the present specification will be described in more detail.

In order to make the objects, 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 with reference to specific embodiments, and it is obvious that the described embodiments are some embodiments of the present invention, but not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

EXAMPLE 1 preparation of foamed Nickel Supported cobalt phosphide electrode Material

1. Cutting foamed nickel into pieces of 2.5cm × 3.0cm, ultrasonically washing with 36.5mg/mL diluted hydrochloric acid for 30min, washing with acetone, ethanol and deionized water respectively, and drying for later use.

2. And (3) taking the foamed nickel obtained in the step (1) as a working electrode, taking a calomel electrode as a reference electrode and taking a platinum sheet as a counter electrode, and electrodepositing in 29.1mg/mL cobalt nitrate hexahydrate aqueous solution at the voltage of-1V for 300s to obtain the foamed nickel loaded cobalt hydroxide.

3. Putting the material obtained in the step 2 into a porcelain boat, placing the porcelain boat in the middle of a tube furnace, taking 1g of sodium hypophosphite, placing the porcelain boat in the tube furnace (the upstream position of airflow), introducing argon, wherein the flow rate is 40mL/min, and the temperature rise program is as follows: and (3) heating the material from the room temperature to 300 ℃ at the speed of 5 ℃/min, preserving the heat for 2h, and naturally cooling the material to the room temperature to obtain the cobalt phosphide electrode material loaded with the foamed nickel, wherein the cobalt phosphide electrode material is marked as CoP/NF.

And (3) analyzing an experimental result: as shown in the scanning electron microscope result of fig. 1(a), cobalt phosphide nanosheets uniformly grow on the foamed nickel skeleton. The X-ray diffraction pattern of FIG. 1(B) confirmed that cobalt phosphide was synthesized. EXAMPLE 2 CoP/NF electrochemical Activity test

1. 2g of PET was dissolved in 50mL of 112.2mg/mL KOH solution.

2. And assembling a three-electrode system by taking the CoP/NF as a working electrode, taking the calomel electrode as a reference electrode and taking a platinum sheet as a counter electrode. Polarization curves were tested in 112.2mg/mL KOH solution (as blank control) and the solution obtained in step 1, respectively, over a voltage range of-0.1-0.8V vs Ag/AgCl, at a scan rate of 0.01V/s.

And (3) analyzing an experimental result: as shown in FIG. 2, when PET plastic was added to the alkaline electrolyte, the initial potential for electrolysis decreased and the current density increased. Specifically, as shown in Table 1, the current density in pure KOH electrolyte (blank comparison) was only 10.3mA/cm at 0.5V vs Ag/AgCl voltage²While the current density in the electrolytic solution after the addition of PET was increased to 67.6mA/cm²It is shown that the hydrogen production quantity is faster, and the conversion efficiency of the current to the hydrogen is higher.

Example 3 preparation of a foamed nickel-loaded cobalt phosphide-iron electrode material

2. And (2) taking the foamed nickel obtained in the step (1) as a working electrode, taking a calomel electrode as a reference electrode, taking a platinum sheet as a counter electrode, and electrodepositing in 26.2mg/mL cobalt nitrate hexahydrate and 4mg/mL ferric nitrate nonahydrate aqueous solution at the voltage of-1V for 300s to obtain the foamed nickel loaded cobalt hydroxide-iron electrode material.

3. Will be at the topPutting the material obtained in the step 2 into a porcelain boat, placing the porcelain boat in the middle of a tube furnace, taking 1g of sodium hypophosphite, placing the porcelain boat in the tube furnace (the upstream position of airflow), introducing argon gas, wherein the flow rate is 40mL/min, and the temperature rise program is as follows: raising the temperature from room temperature to 300 ℃ at a speed of 5 ℃/min, preserving the heat for 2 hours, and naturally cooling to room temperature to obtain the cobalt phosphide-iron electrode material loaded with the foamed nickel, which is marked as Co_0.9Fe_0.1P/NF。

Example 4 Co_0.9Fe_0.1P/NF electrochemical Activity test

1. 2g of PET was dissolved in 50mL of 112.2mg/mL KOH solution.

2. Mixing Co_0.9Fe_0.1And the P/NF is used as a working electrode, the calomel electrode is used as a reference electrode, and the platinum sheet is used as a counter electrode to assemble a three-electrode system. Polarization curves were tested in 112.2mg/mL KOH solution (as blank control) and the solution obtained in step 1, respectively, over a voltage range of-0.1-0.8V vs Ag/AgCl, at a scan rate of 0.01V/s.

And (3) analyzing an experimental result: as shown in fig. 3, when the waste PET plastic is added to the alkaline electrolyte, the initial potential of electrolysis is lowered and the current density is increased. Specifically, as shown in Table 1, the current density in pure KOH electrolyte (blank comparison) was only 41.1mA/cm at 0.5V vs Ag/AgCl voltage²While the current density in the electrolytic solution after adding PET is increased to 114mA/cm²It is shown that the hydrogen production quantity is faster, and the conversion efficiency of the current to the hydrogen is higher.

Example 5 preparation of a foamed nickel-loaded cobalt oxyhydroxide electrode material

3. And (3) taking the material obtained in the step (2) as an anode, taking an Ag/AgCl electrode as a reference electrode, taking a platinum sheet as a counter electrode, activating by cyclic voltammetry in 56mg/mL potassium hydroxide aqueous solution at the voltage of 0-0.8V vs Ag/AgCl, and circulating for 40 circles to obtain the cobalt oxyhydroxide loaded by the foamed nickel, which is recorded as CoOOH/NF.

Example 6 CoOOH/NF electrochemical Activity assay

1. 2g of PET was dissolved in 50mL of 112.2mg/mL KOH solution.

2. And (3) assembling a three-electrode system by using the CoOOH/NF as a working electrode, a calomel electrode as a reference electrode and a platinum sheet as a counter electrode. Polarization curves were tested in 112.2mg/mL KOH solution (as blank control) and the solution obtained in step 1, respectively, over a voltage range of-0.1-0.8V vs Ag/AgCl, at a scan rate of 0.01V/s.

And (3) analyzing an experimental result: as shown in fig. 4, when the waste PET plastic is added to the alkaline electrolyte, the initial potential of electrolysis is lowered and the current density is increased. Specifically, as shown in Table 1, the current density in pure KOH electrolyte (blank comparison) was only 11mA/cm at 0.5V vs Ag/AgCl²While the current density in the electrolytic solution after the addition of PET was increased to 74.8mA/cm²It is shown that the hydrogen production quantity is faster, and the conversion efficiency of the current to the hydrogen is higher.

[ Table 1]

EXAMPLE 7 Cogeneration of Hydrogen, formic acid and terephthalic acid by electrocatalytic PET Plastic

1. 2g of PET was dissolved in 50mL of 112.2mg/mL KOH solution.

2. And (3) assembling the solution obtained in the step (1) as an electrolyte and a cobalt phosphide material loaded by foamed nickel as an anode and a cathode respectively to form an electrolytic cell, and then electrolyzing for 24 hours at the voltage of 1.8V. As shown in fig. 5, PET is first decomposed into terephthalate and ethylene glycol in an alkaline electrolyte, and during electrolysis, ethylene glycol is oxidized at the anode to produce formic acid and water, and water is reduced at the cathode to produce hydrogen.

3. After the reaction was completed, the pH of the electrolyte was adjusted to 4, and then terephthalic acid was precipitated, and a white solid of terephthalic acid was obtained by filtration, and the purity thereof was > 95% by liquid chromatography (see fig. 6).

Claims

1. A method for co-producing hydrogen, formic acid and terephthalic acid from waste PET plastic by electrocatalysis, comprising the following steps:

2. The method of claim 1, wherein the basic aqueous solution comprises an aqueous solution of one or more bases selected from the group consisting of sodium hydroxide, potassium hydroxide, and lithium hydroxide.

3. The method according to claim 1, wherein the concentration of the alkali contained in the alkaline aqueous solution is 0.1 to 10 mol/L.

4. The method of claim 1, wherein the anode comprises one or more selected from nanocrystals, oxides, hydroxides, oxyhydroxides, phosphides, sulfides, nitrides, borides, carbides of transition metals.

5. The method of claim 4, wherein the transition metal comprises: one or more of manganese, iron, cobalt, nickel, copper and zinc.

6. The method of claim 1, wherein the anode is cobalt phosphide, cobalt-iron phosphide, or cobalt oxyhydroxide.

7. The method of claim 1, wherein the voltage applied in step (3) is in the range of 0.5-50V.