Based on Ni(OH) 2 Method for preparing formic acid and hydrogen by redox media conversion of PET waste plastics

Abstract

The present invention provides a method for preparing formic acid and hydrogen by converting PET waste plastics based on Ni(OH)2 redox media. The preparation method of the Ni(OH)2 electrode comprises the following steps: A1, dissolving a nickel salt in water to form a solution A; A2, dissolving sodium carbonate in water to form a solution B; A3, diluting solution A into solution B to form a solution C, immersing an electrodeposited conductive substrate in solution C, and performing an electrodeposition reaction; and preparing a Ni(OH)2 electrode. The present invention also provides a method for preparing formic acid and hydrogen by converting PET waste plastics based on Ni(OH)2 redox media, wherein a Ni(OH)2 electrode is used as an anode and a cathode hydrogen evolution catalytic electrode is assembled into an electrolysis system, water reduction is achieved at the cathode to generate hydrogen, and nickel hydroxide is oxidized to hydroxyl nickel oxide. Subsequently, the NiOOH electrode is placed in an alkaline hydrolysis solution of PET waste plastics, and the hydroxyl nickel oxide is reduced to nickel hydroxide.

Description

Technical Field

[0001] The present invention belongs to the field of electrocatalytic technology, and relates to electrocatalytic upgrading and recycling technology of waste plastics, and specifically to a method for preparing formic acid and hydrogen by converting PET waste plastics based on Ni(OH)2 redox medium. Background Art

[0002] With the continuous development of industrialization, the massive consumption of traditional fossil energy such as coal, oil and natural gas has caused a series of environmental pollution and energy crisis problems. Under this background, the development and development of clean and efficient environmental governance technologies and the exploration of clean and sustainable energy supply are important challenges that countries around the world are currently facing and urgently need to solve. Plastics, as an important class of polymer products, have been widely used in daily production and life since the first report of synthetic phenolic plastics in 1907 due to their light weight, durability and easy processing. However, the unreasonable disposal and accumulation of a large amount of waste plastics not only causes serious pollution to the environment and endangers the health and safety of human beings on earth, but also a large amount of waste plastics cannot be reasonably recycled, which is also a serious waste of carbon resources. In view of this, it is of great significance to develop and utilize clean and efficient environmental energy catalytic technology for upgrading and recycling waste plastics to prepare high-value chemicals.

[0003] At present, scientists from all over the world are actively exploring and developing clean technologies that can recycle and utilize waste plastics. Among them, as a technology that can be carried out at room temperature and pressure, electrocatalytic conversion technology that can be driven by green renewable electricity is widely used in the field of energy storage and conversion. It is worth noting that plastics are generally polymer products made by polycondensation or polyaddition reactions, and such products are rich in elements such as carbon, hydrogen and oxygen. Among them, polyethylene terephthalate (PET) plastics can be converted into terephthalate and ethylene glycol monomers by alkaline hydrolysis because they contain rich ester groups. Recently, researchers have tried to apply electrocatalytic technology to the conversion of PET waste plastics, and successfully achieved the preparation of formate from ethylene glycol, a product of anodic oxidation and hydrolysis, and the co-production of hydrogen at the cathode. Therefore, it is of great significance to apply electrocatalytic technology to the recycling of waste plastics to prepare high-value-added chemicals and fuels.

[0004] It is worth noting that in the process of electrocatalytic conversion of PET to produce formic acid and hydrogen, in order to avoid the diffusion of products to the counter electrode and their consumption, expensive ion exchange membranes are usually used to divide the electrolytic cell into two chambers, the positive and negative chambers. However, the use of ion exchange membranes will not only increase the cost of electrocatalytic conversion of PET waste plastics, but also greatly increase the internal resistance of the system due to the slow ion transmission capacity of the ion exchange membrane, thereby increasing the energy consumption of the conversion system. Therefore, the development of membrane-free electrolysis technology for the conversion and utilization of PET waste plastics is of great significance for reducing cost input and energy consumption. Nickel hydroxide, due to its excellent charge storage capacity and redox cycle stability, has been applied by researchers to decouple water decomposition reactions (patent application number: 201510799110.3). In the absence of ion exchange membranes, the anode water oxidation and cathode hydrogen evolution reactions are separated in different time and space. However, the anode water oxidation half-reaction not only requires high energy consumption, but also has low product utilization value. The present invention oxidizes the hydroxyl nickel electrode obtained in the electrocatalytic hydrogen evolution process and directly applies it to the conversion of PET plastic hydrolysis products, realizing the high-selectivity conversion of hydrolysis products into formic acid without any external energy input, while avoiding the high-energy water oxidation reaction. The method uses nickel hydroxide redox media to electrocatalytically convert PET to co-produce formic acid and hydrogen, successfully dividing the process in different time and space, and realizing the upgrading and recycling of PET waste plastics in a membrane-free device. Summary of the invention

[0005] The purpose of the present invention is to overcome the defects of the above-mentioned prior art and to provide a method for preparing formic acid and hydrogen by converting PET waste plastics based on Ni(OH)2 redox media.

[0006] The purpose of the present invention can be achieved by the following technical solutions:

[0007] First aspect:

[0008] A method for preparing a Ni(OH)2 electrode comprises the following steps:

[0009] A1. Nickel salt is dissolved in water to form solution A;

[0010] A2, sodium carbonate is dissolved in water to form solution B;

[0011] A3. Dilute solution A into solution B to form solution C, immerse the electrodeposited conductive substrate in solution C, and perform an electrodeposition reaction; obtain a Ni(OH)2 electrode.

[0012] The nickel salt includes at least one of nickel nitrate, nickel sulfate and nickel chloride.

[0013] In the solution A, the concentration of the nickel ions is 0.1-1M.

[0014] In the solution B, the concentration of sodium carbonate is 1-2M.

[0015] In the solution C, the concentration of nickel ions is 0.5-2 mM.

[0016] The electrodeposition voltage is 1.3V-2V, and the electrodeposition time is 15-20min.

[0017] The present invention adopts an integrated electrode prepared by an in-situ anodic electrodeposition method, which does not require the use of a binder and a coating process, and the prepared electrode has good structure and cycle stability; in addition, the present invention can effectively control the loading amount of the electrode material by regulating the electrodeposition voltage and time, which is conducive to industrial production and use.

[0018] The Ni(OH)2 electrode has a high proton-electron storage capacity and excellent redox reversibility. The Ni(OH)2 electrode can be prepared by an electrochemical deposition method, a hydrothermal synthesis method, a coating method, or the like.

[0019] The electrochemical deposition method includes cathode electrodeposition method and anode electrodeposition method.

[0020] Second aspect:

[0021] The present invention provides a method for preparing formic acid and hydrogen by converting PET waste plastics based on Ni(OH)2 redox medium. The method uses Ni(OH)2 electrode as a proton-electron storage medium to convert PET into formic acid and hydrogen in a membraneless electrolytic cell in steps. The method specifically comprises the following steps:

[0022] S1, Ni(OH)2 electrode as anode and cathode hydrogen evolution catalytic electrode are assembled into an electrolysis system, water reduction is achieved at the cathode to generate hydrogen, and nickel hydroxide is oxidized to nickel oxyhydroxyl to obtain NiOOH electrode; wherein the electrolyte is an alkaline solution, and the alkaline solution includes KOH or NaOH; in view of the fact that only Ni(OH)2 is oxidized to NiOOH at the anode in this process, and no oxygen evolution reaction occurs, it can be carried out in a single-tank membraneless electrolysis cell;

[0023] S2. The NiOOH electrode is placed in an alkaline hydrolyzate of PET waste plastics, and the nickel oxide is reduced to nickel hydroxide, while the ethylene glycol in the PET hydrolyzate is oxidized to formic acid.

[0024] In the electrochemical hydrogen production process, a Ni(OH)2 electrode is used as an anode and is assembled with a cathode hydrogen evolution catalytic electrode to form an electrolysis system, water reduction is achieved at the cathode to generate hydrogen, and nickel hydroxide is oxidized to nickel oxyhydroxide. No oxygen evolution reaction occurs in this process, and the use of an ion exchange membrane can be avoided. Subsequently, the NiOOH electrode oxidized in the above electrochemical hydrogen production process is placed in an alkaline hydrolyzate of PET waste plastics, and the nickel oxyhydroxide is reduced to nickel hydroxide, and ethylene glycol in the PET hydrolyzate is oxidized to formic acid. The above two steps can be performed alternately and cyclically by utilizing a Ni(OH)2 / NiOOH redox intermediate as a proton-electron storage medium.

[0025] In the electrochemical hydrogen evolution process, the Ni(OH)2 electrode and the cathode hydrogen evolution electrode can be electrolyzed in an electrolytic cell with a chamber to produce hydrogen without using an ion exchange membrane to isolate the cathode and cathode half reactions. The electrolytic cell pressure should be able to drive the cathode hydrogen evolution reaction and the oxidation reaction of the anode nickel hydroxide electrode, while avoiding the occurrence of water oxidation reaction.

[0026] In a sodium carbonate solution containing nickel ions, a three-electrode system is adopted, with foamed nickel, foamed copper, carbon cloth or carbon paper as the electro-deposition conductive substrate, and a Ni(OH)2 electrode is prepared by a constant potential or constant current anodic oxidation electrodeposition method.

[0027] The catalyst material of the cathode hydrogen evolution catalytic electrode includes a noble metal-based material such as Pt and Pd; or a material based on carbide, phosphide, sulfide, or selenide; or a single substance or compound material based on iron, cobalt, nickel, copper, molybdenum, or tungsten.

[0028] The alkaline solute used in the alkaline hydrolyzate of the PET waste plastics includes at least one of KOH, NaOH, carbonate and bicarbonate.

[0029] The voltage applied to the electrolytic cell is 1.3V-1.8V.

[0030] The method for preparing formic acid and hydrogen by converting PET waste plastics through Ni(OH)2 redox media includes two steps: electrochemical hydrogen production and spontaneous oxidation of PET waste plastics.

[0031] In the electrochemical hydrogen production process of step 1, Ni(OH)2 is used as an anode electrode and assembled with a cathode hydrogen evolution catalytic electrode to form an electrolysis system. Ni(OH)2, as an anode reactant, loses protons and electrons and is oxidized to NiOOH. Correspondingly, protons and electrons are combined on the cathode catalytic electrode to produce hydrogen. In this step, only the oxidation reaction of Ni(OH)2 occurs at the anode, avoiding the occurrence of oxygen evolution reaction. There is no problem of mixing hydrogen and oxygen, and no ion exchange membrane is required, so it can be carried out in a membraneless electrolyzer.

[0032] After the electrochemical hydrogen evolution process in step 1 is completed, the NiOOH electrode obtained by oxidation is moved into the alkaline hydrolyzate of PET waste plastics for step 2 without any external energy input. NiOOH undergoes a redox reaction with ethylene glycol in the PET hydrolyzate, ethylene glycol is oxidized to break the CC bond to generate formic acid, and NiOOH obtains protons and electrons and is reduced to Ni(OH)2. The electrochemical hydrogen evolution process in step 1 and the spontaneous chemical reaction process of converting the PET hydrolyzate in step 2 can be alternately cycled through a nickel-based redox medium. As an embodiment, when the electrochemical hydrogen evolution system in step 1 is electrolyzed, the electrolytic cell voltage applied is in the range of 1.3-1.8V.

[0033] As an embodiment, the electrolyte in the electrochemical hydrogen evolution system in step 1 is a 0.1-10M alkaline solution (KOH, NaOH, Na2CO3, K2CO3, etc.).

[0034] As an embodiment, the concentration of the alkaline hydrolyzate of PET waste plastic in step 2 is 0.01-1 M. The alkaline solution includes KOH, NaOH, Na2CO3 or K2CO3, etc.; preferably, the concentration of the alkaline solution is 1-3M.

[0035] The present invention uses NiOOH and ethylene glycol in PET hydrolyzate to undergo spontaneous redox chemical reaction, ethylene glycol is oxidized to formic acid by NiOOH, and NiOOH is reduced to Ni(OH)2. This process is different from the traditional electrolytic water decoupling method, and this process does not require external power supply and equipment.

[0036] The method for preparing formic acid and hydrogen by converting PET waste plastics based on Ni(OH)2 redox media provided by the present invention can not only simplify the device for electrocatalytic recovery of PET waste plastics and avoid the use of expensive ion exchange membranes, but also reduce the energy consumption of electrocatalytic reduction of water to produce hydrogen, while obtaining high value-added chemicals.

[0037] Compared with the prior art, the present invention has the following beneficial effects:

[0038] The present invention realizes the preparation of high-purity hydrogen in a single-tank electrolytic cell without an ion exchange membrane by using a Ni(OH)2 electrode with proton-electron storage capacity as an auxiliary electrode, and at the same time, without applying any additional energy consumption, the NiOOH obtained by oxidation in the electrochemical hydrogen evolution process is used to realize the conversion of ethylene glycol, which is difficult to separate, in the PET hydrolyzate into formic acid. The present invention organically combines the electrochemical hydrogen evolution process with the chemical oxidation conversion of PET waste plastics by using Ni(OH)2 redox media, avoids the use of expensive and consumed ion exchange membranes, is conducive to reducing the economic cost and improving the economic benefits in the process of recycling PET waste plastics, and is more conducive to industrial application. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings:

[0040] Figure 1 Schematic diagram of the system for converting PET waste plastics to produce formic acid and hydrogen with the assistance of Ni(OH)2 electrode;

[0041] Figure 2 is the SEM image of Ni(OH)2 electrode;

[0042] Figure 3 Optical images of Ni(OH)2 and NiOOH electrodes, where the right image is the NiOOH electrode and the left image is the Ni(OH)2 electrode;

[0043] Figure 4 It is the cyclic voltammetry curve of Ni(OH)2 in 1M KOH solution;

[0044] Figure 5 It is the constant current charge-discharge curve of Ni(OH)2 in 1M KOH solution;

[0045] Figure 6 This is the NMR spectrum of PET hydrolyzate before and after adding NiOOH electrode. DETAILED DESCRIPTION

[0046] The present invention is described in detail below in conjunction with embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, some adjustments and improvements can also be made without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0047] The present invention is a method for preparing formic acid and hydrogen by converting PET waste plastics into formic acid and hydrogen through Ni(OH)2 redox media. Figure 1 The process is divided into two steps: electrochemical hydrogen production and spontaneous oxidation of PET waste plastics:

[0048] In the electrochemical hydrogen production process of step 1, Ni(OH)2 is used as an anode electrode and assembled with a cathode hydrogen evolution catalytic electrode to form an electrolysis system. Ni(OH)2, as an anode reactant, loses protons and electrons and is oxidized to NiOOH. Correspondingly, protons and electrons are compounded on the cathode catalytic electrode to produce hydrogen. When the electrochemical hydrogen evolution process of step 1 is completed, the NiOOH electrode obtained by oxidation is moved into the PET hydrolyzate for step 2. NiOOH undergoes a redox reaction with ethylene glycol in the PET hydrolyzate. Ethylene glycol is oxidized to break the CC bond to generate formic acid, and NiOOH obtains protons and electrons and is reduced to Ni(OH)2. The electrochemical hydrogen evolution process of step 1 and the spontaneous chemical reaction process of converting PET hydrolyzate in step 2 can be carried out alternately and cyclically through a nickel-based redox medium. The chemical reaction equations in steps 1 and 2 are as follows:

[0049] Step 1:

[0050] Anode:Ni(OH)2-e - +OH - →NiOOH+H2O(1)

[0051] cathode: Step 2:

[0052] 6NiOOH + C2H6O2 + 2H2O → 6Ni(OH)2 + 2HCOOH (3)

[0053] Example 1

[0054] Preparation of Ni(OH)2 Electrode by Anodic Oxidation Electrodeposition as a Redox Mediator

[0055] 1. Add 20 ml of deionized water to a 50 ml beaker and add sodium carbonate to make the solution concentration 2 mol / L to form solution A;

[0056] 2. Add 20 ml of deionized water to a 50 ml beaker and add nickel nitrate so that the concentration of the nickel nitrate solution is 0.1 mol / L to form solution B;

[0057] 3. Take solution B and add it to solution A so that the concentration of nickel nitrate in the solution is 0.001 mol / L, forming solution C as an electrodeposition solution;

[0058] 4. A three-electrode system with nickel foam as the working electrode, platinum mesh as the counter electrode, and saturated calomel electrode as the reference electrode was placed in solution C for electrodeposition to prepare a Ni(OH)2 electrode. The applied voltage was 1.4 V for 15 min. After the electrodeposition was completed, the electrode was taken out, rinsed, and dried at room temperature to serve as an auxiliary electrode for electrochemical hydrogen evolution and PET oxidation conversion.

[0059] Figure 2This is the SEM image of the Ni(OH)2 electrode. It can be seen from the figure that the prepared Ni(OH)2 electrode exhibits a three-dimensional multi-level nanosheet structure, which is evenly distributed on the nickel foam conductive substrate.

[0060] Figure 3 Optical images of Ni(OH)2 and NiOOH electrodes. It can be seen from the figure that the NiOOH electrode obtained by oxidation is shown on the right, and the Ni(OH)2 electrode is shown on the left.

[0061] Combined with the above data analysis, it is proved that Ni(OH)2 material has been successfully prepared on the nickel foam conductive substrate. The above Ni(OH)2 electrode with three-dimensional multi-level nanosheet structure will be conducive to fully exposing more reactive sites and increasing the transfer channel and rate of substances.

[0062] Example 2

[0063] The Ni(OH)2 electrode prepared in Example 1 was used as a proton-electron storage medium to convert PET waste plastics to produce formic acid and hydrogen, and an electrochemical hydrogen evolution test and a PET oxidation product selectivity test were performed. The test system was as follows: Figure 1 As shown, the specific steps are as follows:

[0064] The electrochemical hydrogen evolution step was tested using a two-electrode system, with the anode electrode being a Ni(OH)2 electrode and the cathode electrode being a platinum catalytic electrode. The effective areas of the catalytic electrodes were 1 cm 2 The electrolyte was a solution containing 1M KOH. The scan rate of the LSV test was 10mV s –1 The experimental test temperature was 25±2℃. The performance test of NiOOH oxidized PET hydrolysis product was qualitatively analyzed by nuclear magnetic resonance hydrogen spectroscopy.

[0065] In the Ni(OH)2 electrode-assisted electrocatalytic hydrogen evolution step, the redox activity and reversibility of the Ni(OH)2 electrode were tested by CV. Figure 4 As shown, the prepared Ni(OH)2 electrode exhibits a pair of reversible redox peaks, indicating that the electrode prepared by the anodic electrodeposition method can have high redox activity and reversibility. When the voltage exceeds 1.35V, the system current increases greatly, indicating that the oxidation reaction of the anode Ni(OH)2 electrode and the cathode hydrogen evolution reaction can occur under low potential conditions. During this process, no bubbles are generated on the anode Ni(OH)2 electrode, indicating that no oxygen evolution reaction occurs. In addition, the Ni(OH)2 electrode was subjected to charge and discharge tests, as shown in Figure 2. Figure 5 As shown, the electrode exhibits a high charge storage capacity of about 5 C / cm 2 .

[0066] The NiOOH electrode obtained by oxidation during the electrochemical hydrogen evolution process was immersed in the PET hydrolyzate. The electrode color quickly changed from dark brown to light yellow brown, indicating that the NiOOH electrode was reduced to Ni(OH)2 electrode by the ethylene glycol in the PET hydrolyzate. The analysis of the PET hydrolyzate product after the reaction by nuclear magnetic resonance hydrogen spectroscopy showed that formic acid was generated in the solution, while the corresponding ethylene glycol was reduced. Figure 6 The results show that the NiOOH electrode can oxidize the ethylene glycol, which is difficult to separate in the PET hydrolysate, into formic acid product.

[0067] The above results show that the Ni(OH)2 electrode can be used to combine the electrochemical hydrogen evolution reaction with the chemical oxidation conversion of PET hydrolysis products, which can not only produce high-purity hydrogen in a membraneless electrolyzer, but also achieve the selective conversion of PET waste plastics at room temperature and pressure without any additional energy input.

[0068] In summary, the present invention (1) utilizes an anodic oxidation electrodeposition method to prepare a Ni(OH)2 electrode with a multi-level nanosheet structure as a redox medium to selectively convert PET waste plastics to produce formic acid and hydrogen; (2) utilizes the method to convert PET waste plastics to produce formic acid and hydrogen and avoids the use of expensive and consumable ion exchange membranes; (3) utilizes the method to convert PET waste plastics to produce formic acid without any external energy input; (4) the method organically combines electrochemical hydrogen evolution and chemical oxidation of PET waste plastics. The present invention realizes the selective conversion of PET waste plastics into high-value-added formic acid and high-purity hydrogen fuel in a membrane-free reactor under mild conditions by utilizing a Ni(OH)2 electrode.

[0069] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art may make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A method for preparing formic acid and hydrogen by converting PET waste plastics using Ni(OH)2 redox media, characterized in that: Using Ni(OH)2 electrode as proton-electron storage medium, PET is converted into formic acid and hydrogen in a membraneless electrolyzer in steps; the method specifically comprises the following steps: S1, Ni(OH)2 electrode as anode and cathode hydrogen evolution catalytic electrode are assembled into an electrolysis system, water reduction is realized at the cathode to generate hydrogen, and nickel hydroxide is oxidized to nickel oxyhydroxyl to obtain NiOOH electrode; The electrolyte is an alkaline solution, and the alkaline solution includes KOH or NaOH; S2, placing the NiOOH electrode in an alkaline hydrolyzate of PET waste plastics, the nickel oxyhydroxide is reduced to nickel hydroxide, and the ethylene glycol in the PET hydrolyzate is oxidized to formic acid; The preparation method of the Ni(OH)2 electrode comprises the following steps: A1. Nickel salt is dissolved in water to form solution A; A2, sodium carbonate is dissolved in water to form solution B; A3, diluting solution A into solution B to form solution C, immersing the electrodeposited conductive substrate in solution C, and performing an electrodeposition reaction; obtaining a Ni(OH)2 electrode; In solution C, the concentration of nickel ions is 0.5-2 mM; The electrodeposition voltage was 1.3 V-2 V, and the electrodeposition time was 15-20 min.

2. The method for preparing formic acid and hydrogen by converting PET waste plastics using Ni(OH)2 redox media according to claim 1, characterized in that: In step A1, the nickel salt includes at least one of nickel nitrate, nickel sulfate and nickel chloride.

3. The method for preparing formic acid and hydrogen by converting PET waste plastics using Ni(OH)2 redox media according to claim 1, characterized in that: The catalyst material of the cathode hydrogen evolution catalytic electrode includes precious metal-based materials based on Pt and Pd; or materials based on carbides, phosphides, sulfides, and selenides; or materials based on single substances or compounds of iron, cobalt, nickel, copper, molybdenum, and tungsten.

4. The method for preparing formic acid and hydrogen based on Ni(OH)2 redox-mediated conversion of PET waste plastics according to claim 1, characterized in that: The solute used in the alkaline hydrolyzate of the PET waste plastics includes at least one of KOH, NaOH, carbonate and bicarbonate.

5. The method for preparing formic acid and hydrogen by converting PET waste plastics using Ni(OH)2 redox media according to claim 1, characterized in that: The voltage applied to the electrolytic cell was 1.3 V-1.8 V.

6. The method for preparing formic acid and hydrogen based on Ni(OH)2 redox-mediated conversion of PET waste plastics according to claim 1, characterized in that: In solution A, the concentration of nickel ions is 0.1-1 M.

7. The method for preparing formic acid and hydrogen by converting PET waste plastics using Ni(OH)2 redox media according to claim 1, characterized in that: In solution B, the concentration of sodium carbonate is 1-2 M.

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