KR102572408B1 - Silver ion-exchanged zeolite catalyst and catalytic reaction system for low-concentration methane combustion using the same - Google Patents

Abstract

The present invention discloses a silver ion-exchanged zeolite catalyst and a catalytic reaction system for low-concentration methane combustion using the same. According to the present invention, a zeolite catalyst having a predetermined Ag/Al molar ratio in which silver is ion-exchanged for low-temperature combustion of low-concentration methane under ozone conditions is provided.

Description

Silver ion-exchanged zeolite catalyst and catalytic reaction system for low-concentration methane combustion using the same}

The present invention relates to a zeolite catalyst in which silver is ion-exchanged and a catalytic reaction system for low-concentration methane combustion using the same.

In the midst of excessive use of fossil fuels due to rapid industrialization, interest in developing eco-friendly energy is increasing in order to reduce pollutants generated thereby.

Among them, natural gas is in the limelight due to its abundant reserves and economic feasibility. Natural gas is a mixed gas containing methane as a main component and light hydrocarbons such as ethane, propane, and butane, and is mined from gas fields on the seabed or on land.

Gas mined from gas fields was burned when there was no means of transportation, but long-distance transportation became possible with the development of piping technology. Compared to liquid fuel, natural gas is difficult to transport and store, so its use and range have been limited. However, with the development of the liquefaction process in 1960, it became possible to liquefy gas from production areas and use it as a means of maritime transportation.

However, environmental problems such as global warming are emerging due to an increase in atmospheric emissions of methane, a main component of natural gas.

It is known that the greenhouse effect per unit molecule of methane is about 70 times higher than that of carbon dioxide, but decomposition through oxidation is the most difficult among hydrocarbons due to the strong C-H single bond.

Methane, which has a huge impact on the greenhouse effect, is emitted from various sources such as fossil fuel (coal, crude oil, gas) mining, landfill waste, livestock manure, and natural gas internal combustion engines (vehicles, ships, etc.).

Among them, in a natural gas internal combustion engine, methane unburned in a combustion chamber is directly discharged into the atmosphere, and generally has a low concentration of 1% or less. Various post-processing technologies are being studied to reduce the emission of low-concentration methane, but it is known that it is difficult to secure high removal efficiency at low temperatures due to kinetic limitations of low-concentration methane combustion.

In particular, unburned methane emitted from natural gas internal combustion engines is generated in a large amount under cold start conditions where the internal temperature of the engine is lower than normal. When the exhaust gas temperature is over 350°C due to the high temperature inside the engine, the combustion reaction of methane by the catalyst can be smoothly induced due to sufficient heat energy supply, but the emission is too low for methane to burn under the cold start condition at the beginning of engine startup. A problem arises in that low-concentration methane that is not reacted due to insufficient gas temperature is discharged as it is.

Pd-based noble metal catalysts are mainly used as effective catalysts for the combustion of low-concentration methane.

However, non-noble metal-based catalysts generally require high temperature conditions of 350 ° C or higher to burn 50% of methane, and high reaction temperature conditions are disadvantageous in terms of energy efficiency and catalyst durability of post-treatment devices. Therefore, in order to efficiently burn low-concentration methane emitted from various moving sources (natural gas vehicles) and stationary sources, it is necessary to develop a new concept of a catalytic reaction system that uses a non-noble metal catalyst and consumes only low energy.

Korean Patent Registration Publication 10-2044604

In order to solve the above problems of the prior art, the present invention is to propose a silver ion-exchanged zeolite catalyst capable of improving low-concentration methane combustion activity at low temperature and a catalytic reaction system for low-concentration methane combustion using the same.

In order to achieve the above object, according to an embodiment of the present invention, silver is ion-exchanged for low-temperature combustion of low-concentration methane under ozone conditions, and a zeolite catalyst having a preset Ag/Al molar ratio is provided.

The low-concentration methane may be low-temperature combustion at a temperature of 75 to 175 °C.

The low-concentration methane may be low-temperature combustion at a temperature of 125 to 150 °C.

The Ag/Al mole ratio may range from 0.001 to 0.08.

A concentration ratio of the low-concentration methane and the ozone may range from 1:3 to 1:11.

According to another aspect of the present invention, as a method for preparing a zeolite catalyst for low-temperature combustion of low-concentration methane under ozone conditions, ammonium salt-type zeolite is heat-treated for a predetermined time and temperature to form acid-type zeolite in which hydrogen cations are bonded to aluminum anions. converting; Adding and stirring the acid type zeolite to an aqueous solution of silver nitrate; Separating the solid catalyst through a filtration device; washing and drying the separated solid catalyst; and calcining the washed and dried solid catalyst to prepare a catalyst having silver ion exchange and a predetermined Ag/Al molar ratio.

According to another aspect of the present invention, a catalytic reaction system for low-temperature combustion of low-concentration methane includes a dielectric barrier discharge (DBD) reactor for supplying ozone generated through an oxidation reaction of oxygen or air; and a catalytic combustion reactor in which silver is ion-exchanged and filled with a zeolite catalyst having a predetermined Ag/Al molar ratio, and which burns the low-concentration methane under low-temperature conditions using ozone supplied from the DBD reactor. do.

According to the present invention, it is possible to maximize low-concentration methane combustion performance at low temperatures by using ozone, which has stronger oxidizing performance than conventional oxidizing agents, and N ₂ O oxidizing agent showing methane oxidation activity at around 200 to 250 °C Unlike, methane combustion performance at significantly lower temperatures can be improved by using ozone, which generates reactive oxygen species at lower temperatures, as an oxidizing agent.

In addition, by implementing methane combustion under low temperature conditions without introducing additional heat sources, it is possible to increase the energy efficiency of the aftertreatment device and reduce operating costs.

1 is a diagram showing a manufacturing process of a zeolite catalyst in which palladium and silver are ion-exchanged for low-temperature combustion of low-concentration methane according to a preferred embodiment of the present invention.
2 shows an X-ray diffraction (XRD) graph of a zeolite catalyst in which silver is ion-exchanged according to an embodiment of the present invention.
3 is a diagram showing the configuration of a catalytic reaction system for low-concentration methane combustion according to an embodiment of the present invention.
4 is a graph of low concentration methane combustion performance and a selectivity graph of carbon dioxide of a Pd-HBEA (0.55 wt%) catalyst and an Ag-HBEA (0.55 wt%) catalyst.
5 is a graph of the low-concentration methane combustion performance of the catalyst according to the Ag / Al molar ratio (0.001 to 0.08).
6 is a graph of low-concentration methane combustion performance by a catalyst according to the ratio of ozone concentration to methane concentration (2 to 11).

Since the present invention can make various changes and have various embodiments, specific embodiments are illustrated in the drawings and described in detail.

However, this is not intended to limit the present invention to specific embodiments, and should be understood to include all modifications, equivalents, and substitutes included in the spirit and scope of the present invention.

In the present invention, a silver ion-exchanged zeolite catalyst capable of effectively inducing low-concentration methane oxidation by ozone is proposed, and an optimal silver ion exchange amount range is suggested. In addition, optimal reaction conditions such as reaction temperature, ozone concentration relative to methane, and catalytic amount (w/f) relative to the flow rate of supplied gas are suggested at which the catalyst according to this embodiment can show effective activity.

Zeolites are valuable as catalysts, catalyst supports, adsorbents and ion exchange materials in chemical processes. Zeolite means crystalline aluminum silicate oxide, and silicon and aluminum combine with oxygen to form a tetrahedral structure. An aluminum tetrahedron in which four oxygen atoms of a divalent anion are bonded to aluminum of a trivalent cation has a monovalent anion. Hydrogen cations, ammonium ions and Na ions are bound to this anion, but ion exchange with other metal cations is possible.

The metal ion exchanged with the zeolite as an isolated species can induce a high dispersion effect of the active metal and prevent degradation of catalytic performance due to thermal agglomeration. In many studies, zeolites with a Si/Al ratio higher than 10 are mainly used because the higher the Si/Al ratio, the higher the structural stability of the zeolite. If the Si / Al ratio is low, the amount of active metal substitution increases due to the increase in ion exchange active sites, which can cause structural instability.

Recently, catalysts in which metal cations are ion-exchanged in various zeolite structures have been actively applied and studied for methane oxidation. Among the active metals, catalysts in which precious metals are ion-exchanged are very active. The zeolite catalyst in which palladium (Pd) is ion-exchanged has a reaction temperature that can oxidize 50% of methane even though molecular oxygen is used as an oxidizing agent. It shows excellent low-temperature catalytic activity at 250°C. In the case of a zeolite catalyst in which silver (Ag), a noble metal-based metal, is ion-exchanged, when a molecular oxygen oxidizer is applied, the temperature showing 50% methane conversion is 600 ° C, which is lower than that of a palladium ion exchange catalyst. It is known to show activity.

In developing an aftertreatment device that suppresses low-concentration methane emissions, a methane oxidation reaction at a lower temperature must be implemented in order to secure high energy efficiency. To oxidize methane at low temperatures, a fundamentally more powerful oxidizing agent is required. Ozone is the second most powerful oxidizing agent after hydroxyl free radicals, and is one of the most powerful low-temperature oxidizing agents that can exist in nature. Ozone can oxidize various hydrocarbons even under very low temperature conditions ranging from room temperature to 300°C.

However, ozone is thermally unstable and decomposes into molecular oxygen at high temperatures above 300°C. Therefore, in order to utilize the strong oxidizing power of ozone, it is reasonable to apply it to the low-temperature oxidation reaction of 300 °C or less, more preferably 200 °C.

The present invention relates to a silver-substituted zeolite catalyst showing high low-temperature catalytic activity for a low-concentration methane combustion reaction using an ozone oxidizer and a reaction system thereof.

As described above, silver-substituted zeolite catalysts are known to be inferior in activity to palladium-substituted zeolite catalysts in the combustion reaction by molecular oxygen. However, as shown in this Example, in the methane combustion reaction by ozone, the silver substitution catalyst was far superior in activity to the palladium substitution catalyst.

In this specification, the activation temperature range of the silver ion exchange zeolite catalyst is presented, and the conditions for the silver content and methane to ozone concentration ratio conditions that can secure the optimal methane combustion activity of the silver-introduced catalyst at a low temperature of 200 ° C or less are proposed. .

Hereinafter, with reference to the drawings, a manufacturing process of a silver ion-exchanged zeolite catalyst and a low-concentration methane oxidation reaction will be described in detail.

1 is a diagram showing a manufacturing process of a zeolite catalyst in which palladium and silver are ion-exchanged for low-temperature combustion of low-concentration methane according to a preferred embodiment of the present invention.

In this embodiment, low-concentration methane is defined as a concentration of 1 vol% or less in the atmosphere, and may be unburned methane generated in a cold start condition in an internal combustion engine.

First, heat treatment is performed under dry air flow conditions for a predetermined temperature and time to convert ammonium salt type zeolite into acid type zeolite in which hydrogen cations are bonded to aluminum anions (step 100).

Here, the heat treatment temperature and time may be 6 hours at 500 °C.

The acid type zeolite is added to an aqueous solution of palladium chloride or an aqueous solution of silver nitrate having an appropriate concentration and stirred at room temperature for 24 hours (step 102).

After the stirring is completed, the aqueous solution and the solid catalyst are separated through a filter (step 104), and then the catalyst is washed with an excess of deionized water (step 106).

Thereafter, drying in air at 110 ° C for 12 hours (step 108), followed by calcination at 550 ° C for 4 hours to prepare a palladium ion-exchanged zeolite catalyst and a silver ion-exchanged zeolite catalyst (step 110).

2 shows an X-ray diffraction (XRD) graph of a zeolite catalyst in which silver is ion-exchanged according to an embodiment of the present invention.

Referring to FIG. 2 , since only the rotation patterns of the zeolite crystal plane were observed, it was confirmed that silver ions were effectively highly dispersed in the zeolite structural lattice and ion exchange was performed.

The catalyst according to this embodiment is applied to a catalytic reaction system for low-temperature combustion of low-concentration methane, and the catalytic reaction system is a dielectric barrier discharge (DBD) supplying ozone generated through an oxidation reaction of oxygen or air. A reactor and a catalytic combustion reactor in which a zeolite catalyst in which silver is ion-exchanged are charged, and which burns the low-concentration methane under low-temperature conditions using ozone supplied from the DBD reactor may be included.

In this embodiment, the DBD (Dielectric Barrier Discharge) plasma reactor is composed of a SUS mesh surrounding the outside of a dielectric quartz tube and a copper rod in the center of the cylinder. Using a power supplier (Jungwoo System), an alternating current (AC) voltage input of 16.0 kV with a frequency corresponding to 409 Hz and a size of 2nd cutoff was supplied to the DBD plasma reactor, and an oscilloscope (Tektronix TDS 220) was used to supply the DBD plasma reactor. Waveforms and magnitudes of voltage and current were observed.

O ₂ (99.999%) corresponding to 10% of the total flow rate supplied to the reaction was flowed into the plasma reactor, and the ozone concentration was controlled by controlling the secondary voltage.

Produced ozone and unreacted oxygen were injected into the catalytic combustion reactor along with methane gas and nitrogen gas streams.

3 is a diagram showing the configuration of a catalytic reaction system for low-concentration methane combustion according to an embodiment of the present invention.

Referring to FIG. 3, a catalyst was placed on a glass wool support layer of a 1/2 inch quartz tube to form a fixed layer. The reactive gas was composed of CH ₄ (500 ppm) + O ₃ (2,500 ppm) + O ₂ (10%) + N ₂ (Balance), the flow rate was fixed at 53.3 ml/min, and the catalyst weight was set at 80 mg. did The particle size of the catalyst used is 250 to 500 was set to The combustion reaction was carried out in the form of an isothermal experiment by changing the shear temperature of the catalyst fixed bed at 75, 100, 125, 150, and 175 °C.

After removing unreacted ozone by connecting an empty tube reactor heated to 225°C to the rear end of the catalytic reactor, methane conversion and product selectivity were measured using gas chromatography (YL6500GC, Youngin Chromas) equipped with a methanizer. measured. Methane conversion rate and selectivity of carbon dioxide (CO ₂ ) were calculated using the following equations.

The separation column used for gas chromatography was Carboxen 1000 (Packed Column, 60/80 mesh, 1/8 in *15 ft, SUS tube), and the column temperature was kept constant at 150 °C. At this time, 10 ml/min of N ₂ was used as the mobile phase gas, and the concentrations of products and unreacted methane were quantified using a flame ionization detector (FID).

Hereinafter, the type of zeolite used in the catalyst, the Ag/Al molar ratio, the concentration of ozone, and the amount of catalyst compared to the flow rate of the supplied gas The low-concentration methane combustion performance is explained in detail.

In the experiments below, 500 ppm of low-concentration methane is mixed at low temperatures between 75 and 175 °C at 25 °C intervals using zeolite catalysts and ozone oxidizers with various silver weight percent (or silver ion exchange rates, Ag/Al). Most of the catalysts with extremely low Ag content showed a methane conversion rate of more than 90% at around 125 to 150 °C. In addition, when the concentration ratio of ozone, an oxidizing agent, to methane concentration was changed, methane conversion rate and carbon dioxide selectivity tended to improve as the ratio of ozone concentration increased.

(Example 1) Comparison of low-concentration methane combustion performance according to the type of ion-exchanged metal in BEA-structured zeolite

4 is a graph showing low-concentration methane combustion performance and selectivity to carbon dioxide of a palladium ion exchange zeolite catalyst (Pd-HBEA) and a silver ion exchange zeolite catalyst (Ag-HBEA).

The experimental conditions were 80 mg of catalyst and the concentration of supplied methane was set to 500 ppm. The concentration of ozone, an oxidizing agent, was adjusted to 2,500 ppm, and the w/f was 1.5 mg·min/ml, all the same. The horizontal axis of all figures is the temperature value set in the isothermal reaction experiment. In FIG. 4, the left vertical axis is the methane conversion rate, and the right vertical axis is the selectivity to carbon dioxide.

The gray line in FIG. 4 shows the low-concentration methane combustion performance of the palladium ion exchange catalyst (Pd-HBEA) prepared using acid-type BEA zeolite having a SiO ₂ /Al ₂ O ₃ molar ratio of 25. It was confirmed that the methane conversion rate improved as the temperature increased, and after 50.5% of methane was converted at 175 ° C, it was confirmed that the methane conversion rate decreased as the temperature increased at a temperature higher than that.

Carbon dioxide selectivity showed more than 90% in all temperature ranges. In the same figure, the black line shows the low-concentration methane combustion performance of the silver ion exchange catalyst (Ag-HBEA) prepared using the same BEA zeolite. The catalyst showed maximum performance at 150 °C, showing that 100% of methane was converted. In the case of the silver ion exchange catalyst, it also shows that most of the converted methane is converted to carbon dioxide.

Compared to the results of previous studies using an oxygen oxidizing agent, it is possible to secure the same level of methane conversion at about 50°C lower temperature conditions even though the palladium content in the catalyst is reduced by 1/6, and the silver content is reduced. This means that the same level of methane oxidation performance can be secured at a temperature between 75 and 100 °C, which is about 500 °C lower, even though it is greatly reduced by 1/11 times. Through this, it was confirmed that high methane combustion performance and energy efficiency could be secured at the same time by applying a catalyst and ozone to the methane combustion reaction.

In addition, in the methane combustion reaction using an ozone oxidizer, the silver ion exchange zeolite catalyst exhibited more powerful low-concentration methane oxidation activity than the palladium ion exchange zeolite catalyst. , means that the ion exchange catalyst is a more suitable catalyst in terms of activity and economy.

(Example 2) Comparison of Low Concentration Methane Combustion Performance of Ag-HBEA Catalysts According to Ag/Al Molar Ratio of Catalysts

Figure 5 shows the performance of low-concentration methane combustion according to the Ag / Al molar ratio of the silver ion exchange catalyst prepared with acid type BEA zeolite.

A catalyst was prepared by changing the Ag/Al molar ratio of the silver ion exchange zeolite catalyst between 0.001 and 0.08. The experimental conditions were 80 mg of catalyst and the concentration of supplied methane was set to 500 ppm. The concentration of ozone, an oxidizing agent, was adjusted to 2,500 ppm, and w/f was the same as 1.5 mg·min/ml. When the combustion performance of the catalysts was compared under the same conditions, the activity increased as the Ag/Al molar ratio increased, showing the highest activity at a molar ratio of 0.04. At higher molar ratios, the performance tended to decrease. It is expected that this is because the oxidation state of isolated Ag species changes as the amount of Ag introduced increases or the amount of Ag clusters supported on the catalyst surface without ion exchange increases.

On the other hand, despite the extremely small amount of silver with Ag/Al molar ratio of less than 0.001 in the catalyst, it showed a high methane conversion rate of more than 60% in the temperature range of 125 ~ 150 °C, which is a high TOF (Turnover) of silver as a zeolite active metal for this reaction frequency) performance.

(Example 3) Comparison of low-concentration methane combustion performance of Ag-HBEA catalysts according to the ratio of ozone concentration to methane concentration

6 is a graph of methane combustion performance measured while changing the concentration of methane from 220 to 1,250 ppm according to 2,500 ppm of ozone. The catalyst used was an Ag-HBEA catalyst with an Ag/Al molar ratio of 0.04, which showed the highest methane conversion rate around 150 ° C in <Example 2>. The amount of catalyst used in the experiment was 80 mg, w/f was 1.5 mg was set to min/ml. As the ratio of ozone concentration to methane concentration increased, the methane conversion rate tended to increase. When 11 times more ozone was introduced than the methane concentration, the methane conversion rate was 100% at 175°C, and the temperature of 175°C It can be seen that in order to convert 50% of methane under the conditions, it is necessary to introduce ozone three times more than the methane concentration.

The embodiments of the present invention described above have been disclosed for illustrative purposes, and those skilled in the art having ordinary knowledge of the present invention will be able to make various modifications, changes, and additions within the spirit and scope of the present invention, and such modifications, changes, and additions will be considered to fall within the scope of the following claims.

Claims (10)

A zeolite catalyst having an Ag/Al molar ratio in the range of 0.02 to 0.08, in which silver is ion-exchanged to burn the methane at a temperature of 125 to 150 ° C under the condition that the concentration ratio of methane and ozone is in the range of 1:3 to 1:11 . delete delete delete delete As a method for producing a zeolite catalyst for burning methane under ozone conditions,
heat-treating the ammonium salt-type zeolite at a preset time and temperature to convert acid-type zeolite in which hydrogen cations are bonded to aluminum anions;
Adding and stirring the acid type zeolite to an aqueous solution of silver nitrate;
Separating the solid catalyst through a filtration device;
washing and drying the separated solid catalyst; and
Calcining the washed and dried solid catalyst to prepare a catalyst in which silver is ion-exchanged,
In order to burn the methane at a temperature of 125 to 150 ° C under the condition that the concentration ratio of methane and ozone is in the range of 1: 3 to 1: 11, the Ag / Al molar ratio of the catalyst is of a zeolite catalyst having a range of 0.02 to 0.08 manufacturing method. As a catalytic reaction system for the combustion of methane,
A dielectric barrier discharge (DBD) reactor for supplying ozone generated through an oxidation reaction of oxygen or air; and
Silver is ion exchanged, a zeolite catalyst having an Ag/Al molar ratio in the range of 0.02 to 0.08 is charged, and the concentration ratio of methane and ozone is in the range of 1:3 to 1:11 using ozone supplied from the DBD reactor. Catalytic reaction system comprising a catalytic combustion reactor for combusting the methane at a temperature of 125 to 150 ° C under.

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