CN113774399B

CN113774399B - Method for co-producing hydrogen, formic acid and terephthalic acid from waste PET plastic through electrocatalytic

Info

Publication number: CN113774399B
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: 2023-05-02
Anticipated expiration: 2040-06-04
Also published as: CN113774399A

Abstract

The invention relates to a method for co-producing hydrogen, formic acid and terephthalic acid from waste PET plastics by electrocatalytic reaction, 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 comprising ethylene glycol and terephthalate salt obtained in step (1) into an electrolytic cell; and (3) applying a 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, and ethylene glycol undergoes an oxidation reaction at an anode to generate formic acid. The invention utilizes the coupling of PET plastic and electrolyzed water to improve the hydrogen production rate and the energy conversion efficiency in the electrolysis process, and effectively reduces the hydrogen production cost of the electrolyzed water. The PET is converted into terephthalate and formate, and high-purity terephthalic acid is obtained through separation, so that the PET can be used for PET production, and simultaneously, formic acid is also an important chemical raw material.

Description

Method for co-producing hydrogen, formic acid and terephthalic acid from waste PET plastic through electrocatalytic

Technical Field

The invention belongs to the technical field of waste PET plastic recycling and water electrolysis hydrogen production, and particularly relates to a method for co-producing hydrogen, formic acid and terephthalic acid from waste PET plastic through electrocatalytic.

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. At present, more than 70% of the world's beverage packaging material is PET plastic. The total production of PET plastic in 2016 is up to 9472 ten thousand tons worldwide, and in 2020 it is expected to be up to 1 hundred million tons. Because PET plastics are difficult to degrade in the environment (the degradation period is 200-400 years), most of waste PET plastics enter the ocean or are buried, and serious threat is caused to the ecology 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 disposable use and are mainly treated in a burning or landfill mode, so that serious environmental pollution and resource waste are caused. Through recycling and utilizing PET waste, environmental pollution can be effectively reduced, and the aim of saving resources is fulfilled. In recent years, the recycling and utilization of waste PET articles has attracted extensive attention and research by global researchers.

The recovery method of PET waste includes physical method and chemical method. Patent (CN 104708730 b, 2015) discloses a recycling process of PET bottles, comprising the following steps: A. feeding, screening, wet crushing, gravity rinsing, hot alkali cleaning, water flushing, centrifugal dewatering, three rinsing, centrifugal dewatering and packing. Although the physical method is simple to operate and low in cost, the purity and quality of the product obtained through recycling are low, and the product cannot be used for high-end products. Patent (CN 110229062 a, 2019) discloses a recycling method of waste PET polyester, comprising the steps of: 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 ℃ to perform a first reaction, and then heating to 190-200 ℃ to perform a second reaction. The quality of the product recovered by chemical methods is higher, but it is often necessary to use a large amount of catalyst and organic solvent, and it is performed at high temperature. Therefore, the development of the novel waste PET plastic recycling process still has great scientific and economic values.

The electrolytic water process comprises an anodic Oxygen Evolution Reaction (OER) and a cathodic Hydrogen Evolution Reaction (HER), wherein OER is still a rate controlling step limiting the overall reaction. This is mainly due to the complex four electron reaction process that the oxygen production process needs to undergo, however, O is produced ₂ The added value is not high. In addition, H is produced simultaneously ₂ And O ₂ Potential mixing in the reactor presents a safety hazard.

Disclosure of Invention

Technical problem

If the thermodynamically more favorable organic matter electrocatalytic oxidation process is used for replacing the traditional anode oxygen production process, the method is a new idea for realizing the oxidation of the organic matter so as to generate high added value products and simultaneously improving the cathode hydrogen production efficiency. PET is added into alkaline electrolyte to decompose and generate terephthalate and ethylene glycol, and the ethylene glycol can be oxidized at the anode before OER reaction, so that active oxygen in water is consumed to obtain formic acid product, and 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 hydrogen production efficiency through water electrolysis.

Technical proposal

In one aspect, the invention provides a method for co-producing hydrogen, formic acid and terephthalic acid from waste PET plastics by electrocatalytic reaction, comprising 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 comprising ethylene glycol and terephthalate salt obtained in step (1) into an electrolytic cell; and

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

The alkaline aqueous solution may include an aqueous solution of one or more bases selected from 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 10mol/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 include one or more selected from nanocrystals, oxides, hydroxides, oxyhydroxides, phosphides, sulfides, nitrides, borides, carbides of transition metals. Wherein the transition metal comprises: a mixture of one or more of manganese, iron, cobalt, nickel, copper, 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 0.5-50V. However, the voltage range is not limited, and the magnitude of the applied voltage may be related to the size of the electrolyzer, the manner of assembly, etc., as long as water and ethylene glycol can be allowed to react at the cathode and anode of the electrolyzer, respectively.

Advantageous effects

The invention couples the recycling of PET with the hydrogen production process of alkaline electrolyzed water, not only improves the cathode hydrogen production rate of electrolyzed water to obtain high-purity hydrogen, avoids the generation of anode oxygen, but also converts 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 present application is: 1. PET can be hydrolyzed in alkaline water to generate monomers (terephthalic acid and ethylene glycol) of PET, ethylene glycol is easy to be oxidized and cracked in an anode catalyst to produce formic acid, and terephthalic acid is relatively stable and does not react; 2. if the electrolyte is free of organic matters such as ethylene glycol, oxygen evolution reaction occurs at the anode, the required overpotential is much higher than that of ethylene glycol oxidation, and the current density is greatly improved after ethylene glycol is added under the same voltage, so that the hydrogen production rate is improved, and the energy consumption of electrolyzed water is lower. In the invention, after PET recovery and alkaline electrolyzed water are combined, PET dissolution and electrolyzed water are integrated, and simultaneously ethylene glycol generated by PET hydrolysis can promote electrolyzed water to produce hydrogen, so that the invention has economical cost.

Compared with pure electrolyzed water, the invention utilizes the coupling of PET plastic and electrolyzed water to improve the hydrogen production rate and the energy conversion efficiency in the electrolysis process, and effectively reduces the hydrogen production cost of the electrolyzed water. In addition, PET is converted into terephthalate and formate, and high-purity terephthalic acid is obtained through separation, so that the method can be used for PET production, and simultaneously, formic acid is also an important chemical raw material.

Drawings

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

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

FIG. 3 is a polarization curve of nickel foam supported cobalt-iron phosphide of example 4;

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

FIG. 5 is a schematic diagram 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 more apparent, the technical solutions of the embodiments of the present invention will be clearly and completely described in the following in conjunction with specific embodiments, and it is apparent that the described embodiments are some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.

Example 1 preparation of foam Nickel-supported cobalt phosphide electrode Material

1. Cutting foam nickel into pieces with the size of 2.5cm multiplied by 3.0cm, then ultrasonically washing for 30min by using 36.5mg/mL of dilute hydrochloric acid, washing by using acetone, ethanol and deionized water respectively, and drying for later use.

2. And (3) electrodepositing the foam nickel obtained in the step (1) in a cobalt nitrate hexahydrate aqueous solution with the voltage of-1V for 300s by taking the calomel electrode as a working electrode and a platinum sheet as a counter electrode to obtain the cobalt hydroxide loaded by the foam nickel.

3. Placing the material obtained in the step 2 into a porcelain boat, placing the porcelain boat in the middle of a tube furnace, placing 1g of sodium hypophosphite into the tube furnace (at the position of the upstream of the air flow), introducing argon, and heating the material at the flow rate of 40 mL/min: heating to 300 ℃ from room temperature at 5 ℃/min, preserving heat for 2 hours, and naturally cooling to room temperature to obtain the foamed nickel-loaded cobalt phosphide electrode material, which is denoted as CoP/NF.

Analysis of experimental results: as shown in the scanning electron microscope result of FIG. 1 (A), cobalt phosphide nano-sheets were uniformly grown on a foam nickel skeleton. The X-ray diffraction pattern of fig. 1 (B) determines 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. CoP/NF is used as a working electrode, a calomel electrode is used as a reference electrode, and a platinum sheet is used as a counter electrode, so that a three-electrode system is assembled. Polarization curves were tested in 112.2mg/mL KOH solution (as a blank comparison) and the solution obtained in step 1, respectively, with a voltage range of-0.1-0.8V vs Ag/AgCl, and a scan rate of 0.01V/s.

Analysis of experimental results:as shown in fig. 2, when PET plastic is added to the alkaline electrolyte, the initial potential for electrolysis decreases and the current density increases. As shown in Table 1, the current density in the pure KOH electrolyte (blank comparison) was only 10.3mA/cm at 0.5V vs. Ag/AgCl ² And the current density in the electrolytic solution after PET was added was increased to 67.6mA/cm ² The method shows that the hydrogen production amount is faster, and the conversion efficiency from the current to the hydrogen is higher.

Example 3 preparation of foam Nickel-supported cobalt phosphide-iron electrode Material

2. And (3) electrodepositing the foam nickel obtained in the step (1) in a solution of cobalt nitrate hexahydrate and ferric nitrate nonahydrate of 26.2mg/mL and 4mg/mL for 300s with a voltage of-1V as a working electrode, a calomel electrode as a reference electrode and a platinum sheet as a counter electrode to obtain the cobalt hydroxide-iron electrode material loaded by the foam nickel.

3. Placing the material obtained in the step 2 into a porcelain boat, placing the porcelain boat in the middle of a tube furnace, placing 1g of sodium hypophosphite into the tube furnace (at the position of the upstream of the air flow), introducing argon, and heating the material at the flow rate of 40 mL/min: heating to 300 ℃ from room temperature at 5 ℃/min, preserving heat for 2 hours, and naturally cooling to room temperature to obtain the foamed nickel-loaded cobalt phosphide-ferroelectric electrode material, which is denoted as Co _0.9 Fe _0.1 P/NF。

Example 4 Co _0.9 Fe _0.1 P/NF electrochemical Activity test

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

2. Co is to be _0.9 Fe _0.1 P/NF is used as a working electrode, a calomel electrode is used as a reference electrode, and a 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 a blank comparison) and the solution obtained in step 1, respectively, with a voltage range of-0.1-0.8V vs Ag/AgCl, and a scan rate of 0.01V/s.

Analysis of experimental results: as shown in FIG. 3, waste and old alkaline electrolyte is addedAfter PET plastic, the initial potential for electrolysis decreases and the current density increases. As shown in Table 1, the current density in the pure KOH electrolyte (blank comparison) was only 41.1mA/cm at 0.5V vs. Ag/AgCl ² While the current density in the electrolytic solution after PET addition was increased to 114mA/cm ² The method shows that the hydrogen production amount is faster, and the conversion efficiency from the current to the hydrogen is higher.

EXAMPLE 5 preparation of foam Nickel-supported cobalt oxyhydroxide electrode Material

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

Example 6 CoOOH/NF electrochemical Activity test

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

2. CoOOH/NF is used as a working electrode, a calomel electrode is used as a reference electrode, and a platinum sheet is used as a counter electrode, so that a three-electrode system is assembled. Polarization curves were tested in 112.2mg/mL KOH solution (as a blank comparison) and the solution obtained in step 1, respectively, with a voltage range of-0.1-0.8V vs Ag/AgCl, and a scan rate of 0.01V/s.

Analysis of experimental results: as shown in fig. 4, after waste PET plastic is added into the alkaline electrolyte, the initial potential of electrolysis decreases and the current density increases. As shown in Table 1, the current density in the pure KOH electrolyte (blank comparison) was only 11mA/cm at 0.5V vs. Ag/AgCl ² And electrolytic dissolution after PET is addedThe current density in the liquid increased to 74.8mA/cm ² The method shows that the hydrogen production amount is faster, and the conversion efficiency from the current to the hydrogen is higher.

TABLE 1

Example 7 electrocatalytic PET Plastic Co-production of Hydrogen, formic acid and terephthalic acid

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

2. And (3) taking the solution obtained in the step (1) as electrolyte, respectively taking a foamed nickel-loaded cobalt phosphide material as an anode and a cathode to assemble an electrolytic cell, and then electrolyzing for 24 hours under 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 terephthalic acid was precipitated, and a white solid of terephthalic acid was obtained by filtration, and the purity was >95% as measured by liquid chromatography (see fig. 6).

Claims

1. The method for co-producing hydrogen, formic acid and terephthalic acid from waste PET plastics through electrocatalytic comprises the following steps:

(3) Applying a voltage to the electrolytic cell assembled in the step (2) to electrolyze, wherein water in the aqueous electrolyte undergoes a reduction reaction at a cathode to generate hydrogen, and ethylene glycol undergoes an oxidation reaction at an anode to generate formic acid; wherein the anode is cobalt phosphide, cobalt phosphide-iron, or cobalt oxyhydroxide.

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

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

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