CN113186561B

CN113186561B - Method for promoting electrocatalytic oxidation of methane to generate methyl chloride by using chloride ions

Info

Publication number: CN113186561B
Application number: CN202110456330.1A
Authority: CN
Inventors: 郑耿锋; 王启豪
Original assignee: Fudan University
Current assignee: Fudan University
Priority date: 2021-04-26
Filing date: 2021-04-26
Publication date: 2022-05-20
Anticipated expiration: 2041-04-26
Also published as: CN113186561A

Abstract

The invention belongs to the technical field of methane electrocatalytic oxidation, and particularly relates to a method for promoting the electrocatalytic oxidation of methane to generate methyl chloride by using chloride ions. The invention firstly prepares electro-catalyst-cobalt-nickel composite spinel oxide nano-particles for electro-catalytically oxidizing methane to generate methyl chloride. Then the electrocatalyst is used as a catalytic reaction anode, and the electrolyte containing chloride ions is adopted, and the cobalt-nickel composite oxide nano particles are used for electrocatalysis of methane oxidation under the condition of the existence of chloride ions in the electrolyte, so that methyl chloride is selectively generated, and side reactions such as carbon dioxide, oxygen and the like are inhibited. The invention improves the traditional gas phase conversion method of chlorine and methane, and develops the method into the reaction of gas-liquid phase interface by the method of electrochemically generating active chlorine in situ. The method has the advantages of wide raw material source, simple preparation method, environmental protection and low price, and is beneficial to popularization and application.

Description

Method for promoting electrocatalytic oxidation of methane to generate methyl chloride by using chloride ions

Technical Field

The invention belongs to the technical field of methane electrocatalytic oxidation, and particularly relates to a method for producing methyl chloride by electrocatalytic oxidation of methane.

Background

Methane is a byproduct of oil production and is widely found in natural gas, shale gas, and combustible ice. Due to their abundant and low cost, methane is becoming an important precursor for chemical and fuel alternatives to coal and petroleum. Therefore, the reasonable utilization of methane provides conditions for the decarburization in the current petroleum industry. Although methane is emitted to a lesser extent than carbon dioxide, it has thirty times the ability to absorb and emit infrared radiation than carbon dioxide. In the last two decades, methane has contributed eighty-six times more to global warming than carbon dioxide. The tetrahedral configuration of methane shows high symmetry and low polarity. At the same time, the dissociation of carbon-hydrogen bonds can be combined with valuable products (such as CO, CH)₃OH，C₂H₄Etc.) are comparatively extremely high. Thus, selective oxidation of methane remains a challenge. The industrial methane direct conversion reaction requires a large energy and capital cost and is subject to severe reaction conditions (temperature)>900K and pressure>2.5 bar). At lower temperatures in order to reduce the maintenance costs of the plant and to reduce carbon dioxide emissions<500 K) Processes have been developed for the direct conversion of methane to oxidation products. However, this type of process requires the use of expensive oxidizing agents, such as H₂O₂、NO_xAnd oleum. The in situ generation of active oxides in electrochemical processes is a clean alternative solutionThe solution is decided.

Disclosure of Invention

The first purpose of the invention is to provide a high-activity and low-cost catalyst for electrocatalytic oxidation of methane to generate methyl chloride;

the second purpose of the invention is to provide a method for producing methyl chloride by using the catalyst to carry out electrocatalytic oxidation on methane, and the principle is shown in figure 1.

The invention provides an electrocatalyst for electrocatalytic oxidation of methane to generate methyl chloride, which is a cobalt-nickel composite spinel oxide nanoparticle and is prepared by the following steps:

(1) dissolving cobalt and nickel salt in deionized water, wherein the total amount of cobalt and nickel is 2-3mmol, and adding 2.5-3.5mL of concentrated ammonia water;

(2) stirring the obtained mixed solution at room temperature for more than 10min, and blowing air into the solution;

(3) putting the mixed solution into a reaction kettle, and reacting for 1-6 h at 100-200 ℃; naturally cooling after the reaction is finished, performing centrifugal separation, washing for a plurality of times by using deionized water, and drying to obtain a metal hydroxide precursor;

(4) and (4) heating the metal hydroxide precursor prepared in the step (3) to 200-300 ℃ at a heating rate of 0.5-50 ℃/min, and carrying out heat treatment for 1-5 h in an inert gas atmosphere.

In the invention, the cobalt salt is one or more of cobalt chloride, cobalt acetate, cobalt sulfate and cobalt nitrate; the nickel salt is one or more of nickel chloride, nickel acetate, nickel sulfate and nickel nitrate.

The molar ratio of the cobalt salt to the nickel salt is as follows: 1 (0.1-10).

The invention provides a method for generating methyl chloride by electrocatalytic oxidation of methane by using the catalyst, which adopts electrolyte containing chloride ions and promotes the electrocatalytic oxidation of methane by using the chloride ions to generate the methyl chloride, and comprises the following specific steps:

(1) ultrasonically dispersing a cobalt-nickel composite spinel oxide nano electro-catalyst in a solvent, spraying polytetrafluoroethylene dispersion liquid, electro-catalyst dispersion liquid and conductive carbon black on carbon paper layer by layer, drying, and controlling the load amountPrepared at 0.1-100 mg/cm²(ii) a As an electrocatalytic reaction electrode;

(2) contacting methane with the side of the electrode not loaded with the electrocatalyst, and contacting an electrolyte containing chloride ions with the side of the electrode loaded with the electrocatalyst; the electrode is subjected to hydrophobic treatment, methane can contact with the catalyst through the electrode, and the electrolyte cannot diffuse to the other side;

(3) applying positive voltage to the electrode, and controlling the current to be 0.02-2A/cm²Methane is selectively oxidized to methyl chloride.

In the invention, the electrolyte is LiCl, NaCl, KCl and CaCl₂、MgCl₂、BeCl₂One or more of them, the concentration of chloride ion is between 0.01 and 5.4 mol/L.

In the invention, the methane electrooxidation product chloromethane is specifically CH₃Cl、CH₂Cl₂、CHCl₃、CCl₄One or more of (a).

In the invention, the anode catalyst can generate an active chlorine intermediate in situ and activate the carbon-hydrogen bond of methane under mild conditions. The chloride ions compete with water molecules in the electrolyte and are adsorbed on the surface of the catalyst, so that the generation of active oxygen is inhibited, and the selectivity of a methane partial oxidation product, namely methyl chloride, is improved. The cobalt-nickel composite spinel oxide nano electro-catalyst synthesized by the invention has good chemical stability and electro-catalytic activity, and the proportion of cobalt and nickel double components and the applied potential influence the partial current density and the conversion rate of methane oxidation.

Drawings

FIG. 1 is a schematic representation of the method of the present invention. Where a is a methane activation pathway, activated by a conventional oxygen intermediate or a chlorine intermediate as used herein. b is to introduce nickel into the cobalt-nickel composite spinel oxide to adjust the stability of the chlorine intermediate.

FIG. 2 shows the morphology of the cobalt-nickel composite spinel oxide nano electrocatalyst. Wherein a-b is an electron microscope photo of the cobalt-nickel composite spinel oxide nanoparticles; c is the distribution of cobalt nickel and oxygen elements.

FIG. 3 shows a diagram of a cobalt-nickel oxide electrolyte containing saturated NaClAs an electrocatalyst, performance testing for methane oxidation. The histogram corresponds to the left axis and is CH from top to bottom₃Cl、O₂And CO₂The fractional current density. The line graph corresponds to the right axis, and the total current density and Cl are sequentially arranged from top to bottom₂The fractional current density.

FIG. 4 shows the single-pass conversion rate of monochloromethane in a performance test of methane oxidation with saturated NaCl as the electrolyte and cobalt-nickel oxide as the electrocatalyst. The abscissa represents the flow rate of methane and the ordinate represents the per pass conversion of monochloromethane.

Detailed Description

The invention is further illustrated by the following examples, which will aid the understanding of the invention, but do not limit the content of the invention.

Example 1, preparation of a cobalt nickel oxide electrocatalyst, the specific steps were as follows:

cobalt-nickel mixed oxide nanoparticles with a ratio of 1:1 were synthesized by a hydrothermal method, and the morphology of the nanoparticles is shown in FIG. 2. 1mmol Co (OAc)₂·4H₂O and 1 mmoleNi (OAc)₂·4H₂O was dissolved in 25 mL of deionized water. 2.5 mL of 30% aqueous ammonia was added to the solution and stirred vigorously with oxygen for at least 10 min. The solution was then transferred to a 50 mL autoclave at 150 deg.F^oAnd C, reacting for 3 hours. The precipitate was separated by centrifugation and washed several times with deionized water. At 80^oAfter drying, the catalyst was dried at 350 deg.C^oAnd C, calcining in air for 2 h.

Example 2, preparation of a cobalt oxide electrocatalyst, the specific steps were as follows:

the cobalt oxide nanoparticles were synthesized by a hydrothermal method. 2 mmol of Co (OAc)₂·4H₂O was dissolved in 25 mL of deionized water. 3.0 mL of 30% aqueous ammonia was added to the solution and stirred vigorously with oxygen for at least 10 min. The solution was then transferred to a 50 mL autoclave at 120 deg.F^oAnd reacting for 4 hours under the condition of C. The precipitate was separated by centrifugation and washed several times with deionized water. At 100^oC after drying, the catalyst is dried at 300 deg.C^oAnd calcining the mixture C in air for 2 hours.

Example 3, the method for promoting the electrocatalytic oxidation of methane to methyl chloride by using chloride ions in the electrolyte comprises the following specific steps:

the above catalyst was dispersed ultrasonically in isopropanol solution. Uniformly spraying polytetrafluoroethylene emulsion, catalyst dispersion liquid and conductive carbon black on carbon paper layer by layer, and controlling the load of the catalyst to be 1mg/cm²And drying the electrode under an infrared lamp to serve as a catalytic reaction electrode. The electrodes were then mounted in an electrolytic cell and the tests were performed at room temperature and atmospheric pressure. The electrode was used as an anode, and the flow rate of methane gas was controlled to 20 mL/min. Saturated NaCl is used as electrolyte, and the flow rate of the electrolyte is controlled to be 10 mL/min. And (3) taking silver/silver chloride as a reference electrode, and controlling the voltage of 1.8-2.5V. Methane is selectively oxidized to monochloromethane and small amounts of carbon dioxide. The resulting product partial current densities and total currents are shown in fig. 3. For example, at a potential of 2.5V, the partial current density of monochloromethane can reach 14 mA/cm²While the partial current density of carbon dioxide is only 0.1 mA/cm²。

Example 4, the method for promoting the electrocatalytic oxidation of methane to methyl chloride by using chloride ions in the electrolyte comprises the following specific steps:

a catalytic reaction electrode was prepared as described in example 3 above by synthesizing cobalt nickel mixed oxide nanoparticles in a 1:2 ratio by a hydrothermal method. The electrodes were mounted in an electrolytic cell and the tests were performed at room temperature and atmospheric pressure. The electrode is used as an anode, the flow rate of methane gas is controlled to be 5-20 mL/min, saturated NaCl is used as electrolyte, and the flow rate of the electrolyte is controlled to be 5 mL/min. The voltage of 2.5V is controlled and applied by taking silver/silver chloride as a reference electrode. The conversion of monochloromethane increases with decreasing methane flow rate. The conversion of monochloromethane is shown in figure 4. For example, when the flow rate of methane was controlled to be 17.9 mL/min, the single pass conversion of monochloromethane was 0.8%. After the flow rate of methane was adjusted to 5.8 mL/min, the single pass conversion of monochloromethane was increased to 2.8%.

Claims

1. A method for electrocatalytic oxidation of methane to generate methyl chloride by utilizing an electrocatalyst is characterized in that a chloride ion-containing electrolyte is adopted, and the chloride ions are utilized to promote the electrocatalytic oxidation of methane to generate the methyl chloride; the method comprises the following specific steps:

(1) ultrasonically dispersing a cobalt-nickel composite spinel oxide nano electro-catalyst in a solvent, spraying polytetrafluoroethylene dispersion liquid, electro-catalyst dispersion liquid and conductive carbon black on carbon paper layer by layer, and drying, wherein the loading amount is controlled to be 0.1-100 mg/cm²(ii) a As an electrocatalytic reaction electrode;

(3) applying positive voltage to the electrode, and controlling the current to be 0.02-2A/cm²Methane is selectively oxidized to methyl chloride;

the electrocatalyst is cobalt-nickel composite spinel oxide nanoparticles, and is prepared by the following steps:

(1) dissolving cobalt salt and nickel salt into deionized water, wherein the total amount of cobalt and nickel is 2-3mmol, and adding 2.5-3.5mL of concentrated ammonia water;

2. The method according to claim 1, wherein the cobalt salt is one or more of cobalt chloride, cobalt acetate, cobalt sulfate and cobalt nitrate; the nickel salt is one or more of nickel chloride, nickel acetate, nickel sulfate and nickel nitrate; the molar ratio of the cobalt salt to the nickel salt is as follows: 1 (0.1-10).

3. The method of claim 1, wherein the electrolyte is LiCl, NaCl, KCl, CaCl₂、MgCl₂、BeCl₂One or more of them, the concentration of chloride ion is 0.01-5.4 mol/L.

4. The process according to claim 1, characterized in that the product of the electro-oxidation of methane is methyl chloride, in particular CH₃Cl、CH₂Cl₂、CHCl₃、CCl₄One or more of (a).