WO2008066275A1

WO2008066275A1 - Hydrotalcite-zeolite composites and catalysts thereof by nox storage method

Info

Publication number: WO2008066275A1
Application number: PCT/KR2007/005810
Authority: WO
Inventors: Hyun-Sik Han; Gon Seo; Young-San Yoo; Gi-Won Park
Original assignee: Heesung Catalysts Corp
Current assignee: Heesung Catalysts Corp
Priority date: 2006-11-29
Filing date: 2007-11-19
Publication date: 2008-06-05
Anticipated expiration: 2009-05-29
Also published as: KR100827788B1

Abstract

Disclosed herein is a hydrotalcite-zeolite composite including zeolite exchanged with alkali metal ions or alkali earth metal ions and hydrotalcite substituted with transition metals, and a catalyst for storing nitrogen oxides produced using the composite. The catalyst for storing nitrogen oxides can store a large amount of nitrogen oxides and has excellent thermal and mechanical stability and resistance to sulfur dioxide poisoning.

Description

[DESCRIPTION]

[Invention Title]

HYDROTALCITE-ZEOLITE COMPOSITES AND CATALYSTS THEREOF BY

NOX STORAGE METHOD

[Technical Field]

The present invention relates to a hydrotalcite-zeolite composite comprising

zeolite exchanged with alkali metal ions or alkali earth metal ions and

hydrotalcite substituted with transition metals, a method of producing the

composite, and a catalyst for storing nitrogen oxide, produced using the

composite, which can store a large amount of nitrogen oxides, and which has

excellent thermal or mechanical stability and resistance to sulfur dioxide

poisoning.

[Background Art]

Nitrogen oxides included in exhaust gases emitted from diesel automobiles

cause serious air pollution, and thus the emission of nitrogen oxides is

becoming more strictly regulated in the interests of environmental protection.

Since nitrogen oxides have strong toxicity and cause secondary air pollution, for

example, ozone generation, in IV and V of EURO emission standards, the i SUBSTITUTE SHEET upper allowable limits of the emission of nitrogen oxides are gradually

decreasing to 0.25 g/km and 0.08 ~ 0.20 g/km, respectively. Since diesel

engines are operated in an excess oxygen atmosphere, the exhaust gases

emitted therefrom contain more oxidizing substances, such as oxygen, nitrogen

oxides, etc., than reducing substances, such as unbumed hydrocarbons,

carbon monoxide, etc. For this reason, even when three-way catalysts are

used, nitrogen oxides cannot be removed through an oxidation-reduction

reaction because the amount of oxidizing substances is not balanced with the

amount of reducing substances. Accordingly, in the purification of exhaust

gases of diesel automobiles, it is very important to develop methods of

efficiently removing nitrogen oxides while minimizing incidental facilities and

additional expenses.

In order to solve the above problem, as a conventional method of removing

nitrogen oxides, a method of removing nitrogen oxides by additionally supplying

urea, serving as a reductant, to exhaust gases to reduce the nitrogen oxides is

known in the art. This method is a method of removing nitrogen oxides by

reducing the nitrogen oxides using ammonia obtained by hydrolyzing urea, and

is referred to as a urea-selective catalytic reduction (Urea-SCR)) method

2

SUBSTITUTE SHEET because harmless urea is used as a reductant instead of strongly toxic

ammonia. Ammonia has strong reducing ability and thus can efficiently reduce

and remove nitrogen oxides, but is problematic in that additional equipment,

such as a urea injection apparatus, a hydrolysis reactor, a storage apparatus,

etc., is required, and a social infrastructure, such as a sales network for a urea

aqueous solution, etc., is required to be established, and thus the introduction of

Urea-SCR method as a diesel automobile exhaust gas purification method is

delayed.

Meanwhile, a method of reducing and removing nitrogen oxides by storing

nitrogen oxides in exhaust gases in a catalyst and then injecting fuel into the

catalyst at regular intervals, thus desorbing the nitrogen oxides stored in the

catalyst in an oxidation atmosphere, called "an NOx storage and reduction

(NSR) method", is being commercially used. In this method, in an oxidation

atmosphere, nitrogen oxides are stored in barium oxide supported in alumina,

and in a reduction atmosphere formed due to the injection of fuel, the nitrogen

oxides are desorbed. The injected fuel is decomposed into reducing

substances by precious metals supported in alumina together with barium oxide,

and the reducing substances reduce and remove the desorbed nitrogen oxides.

3 SUBSTITUTE SHEET Unlike the Urea-SCR method, the NSR method is convenient in that nitrogen

oxides are removed by the injection of fuel, and thus additional facilities for

storing and supplying urea or specific chemicals are not required, but is

problematic in that since a large amount of fuel is used in order to convert

exhaust gas to a reduction atmosphere, the air-fuel ratio becomes low, and

since a large amount of nitrogen oxides is stored in a catalyst so that the

regeneration cycle of a catalyst is increased, which means that the volume of

the catalyst must be increased. Considering the above points, the NSR

method is suitable for removing nitrogen oxides from exhaust gases emitted

from middle and small sized automobiles, which are more difficult to be

provided with additional facilities than large sized automobiles.

The performance of an NSR catalyst is primarily evaluated by the amount of the

nitrogen oxides stored in the catalyst. Further, the NSR catalyst must have

high hydrothermal stability because exhaust gases which are to be purified by

an automobile purification catalyst contain a large amount of water and are

exposed to violent temperature variation. Furthermore, the NSR catalyst must

have high resistance to sulfur dioxide as well as mechanical stability. Diesel oil

contains sulfur compounds, even though the amount thereof is small, the sulfur

4

SUBSTITUTE SHEET compounds are formed into sulfur dioxide through a combustion process of fuel,

and the sulfur dioxide strongly reacts with barium oxide, which is a conventional

storage agent, thus being formed into barium sulfate. For this reason, barium

oxide is converted into a sulfate when it is exposed to sulfur dioxide, and thus

the amount of nitrogen oxides stored in the catalyst is greatly decreased.

Therefore, storage materials having large adsorption selectivity reflecting the

difference between nitrogen dioxide adsorptivity and sulfur dioxide adsorptivity

are required. When sulfur dioxide is weakly adsorbed and nitrogen dioxide is

strongly adsorbed, the storage materials are less poisoned by sulfur dioxide,

and can thus store a large amount of nitrogen dioxide.

[Disclosure]

[Technical Problem]

As conventional nitrogen oxide storage materials of the NSR catalyst, barium

oxides supported in alumina have been used. Barium oxides have a large

storage capacity and excellent hydrothermal stability, but do not have strong

adsorptivity. Further, since nitrogen dioxide is formed into a nitrate when it is

stored and is formed into oxides when it is desorbed, the expansion and

contraction of volume must be repeated in the storage and desorption

5 SUBSTITUTE SHEET processes. In catalysts for purifying automobile exhaust gases, which must be used for a long time without being replaced, the repetition of expansion and contraction causes mechanical fatigue, thus worsening the physical properties of catalyst. For this reason, it is preferred that materials that can store a large amount of nitrogen dioxide and have strong adsorptivity, and in which the volume thereof does not repeatedly expand and contract, be used as the NSR catalyst.

[Technical Solution]

Considering that when using one type of basic material, for example, barium oxide only, it is not easy to control the degree of adsorption of acidic gas, such as nitrogen dioxide or sulfur dioxide, in accordance with the intended purpose, the present inventors have proposed a zeolite-hydrotalcite composite which stores nitrogen oxides in pores thereof or between crystal planes thereof in an oxidation atmosphere without forming a nitrate, and desorbs nitrogen dioxide in a reduction atmosphere. Since the zeolite-hydrotalcite composite is used as a material for storing nitrogen dioxide, it is expected that the breaking of the structure of the zeolite-hydrotalcite composite due to the repeated state change will be prevented, and synergistic effect of using the advantages of two kinds of

6 SUBSTITUTE SHEET materials will occur. That is, attempts to increase the amount of stored nitrogen oxides by ion-exchanging zeolite anions with alkali earth metals or substituting hydrotalcite with transition metal ions have been made. The present inventors have also attempted to improve nitrogen oxide storage capacity and hydrothermal stability by forming hydrotalcite, existing in the form of nanoparticles, on the surface of zeolite, and to increase the resistance to sulfur dioxide by adjusting the storage properties of two kinds of storage materials.

SUBSTITUTE SHEET [Advantageous Effects]

The surface area of the zeolite-hydrotalcite composite, measured using a

nitrogen adsorption method, is 78 m²/g, which is very small. The reason is

assumed to be that, although the zeolite-hydrotalcite composite was formed, the

crystallinity of mordenite zeolite is very low, so that micropores are insufficiently

grown, thereby decreasing the surface area thereof.

[Description of Drawings]

FIG. 1 is a schematic block diagram showing a process of preparing a zeolite-

hydrotalcite composite;

FIG. 2 is a graph showing an X-ray diffraction pattern of zeolite Y, synthesized

in Example 1 ;

FIG. 3 is a graph showing an X-ray diffraction pattern of hydrotalcite,

synthesized in Example 2;

FIG. 4 is a graph showing X-ray diffraction patterns of a zeolite Y-hydrotalcite

composite synthesized in Example 3;

FIG. 5 is scanning electron micrographs of the zeolite Y-hydrotalcite composite

synthesized in Example 3; and

FIG. 6 is a graph showing nitrogen dioxide temperature-desorption curves of a

8

SUBSTITUTE SHEET physically mixed zeolite Y-hydrotalcite composite in Example 5.

[Best Mode]

The present invention provides a method of producing a zeolite-hydrotalcite

composite, including: preparing a zeolite stock solution and a hydrotalcite stock

solution and mixing the stock solutions to conduct a hydrothermal reaction, and

provides a zeolite-hydrotalcite composite prepared using the method.

Further, the present invention provides the method of producing a zeolite-

hydrotalcite composite, further including: supporting one or more platinum group

metals on the zeolite-hydrotalcite composite, and a catalyst for storing nitrogen

oxides produced using the method.

In the present invention, the zeolite is ANA zeolite, BEA zeolite, MFI zeolite,

MOR zeolite, or zeolite Y.

In the present invention, the hydrotalcite is substituted with a transition metal,

such as cobalt, cerium, copper, iron, or nickel.

In the present invention, the hydrothermal reaction is conducted at a

temperature of 90 - 120^°C for 4 -12 hours.

In the present invention, the platinum group metal is supported on the zeolite-

hydrotalcite composite such that the amount thereof is 0.1 ~ 4.0%.

9 SUBSTITUTE SHEET [Mode for Invention]

The composite of the present invention includes zeolite and hydrotalcite.

However, since the composition, crystal structure and physical and chemical

properties of zeolite are different from those of hydrotalcite, first, a zeolite stock

solution and a hydrotalcite stock solution are separately prepared and aged.

The zeolite stock solution and the hydrotalcite stock solution are mixed and then

hydrothermally reacted to prepare a zeolite-hydrotalcite composite.

Hereinafter, a process of preparing the zeolite-hydrotalcite composite will be

described in detail. Although the crystal structure of hydrotalcite is different

from that of zeolite, some of the preparation conditions thereof, such as

synthesis temperature, stock solution pH, etc., are the same as each other.

Therefore, a zeolite precursor solution and a hydrotalcite precursor solution are

mixed and then reacted under specific conditions, thus preparing a zeolite-

hydrotalcite composite including zeolite and hydrotalcite. FIG. 1 schematically

shows a process of preparing the zeolite-hydrotalcite composite. Here, the

previously-aged zeolite precursor solution and hydrotalcite precursor solution

are mixed, stirred and then hydrothermally reacted, thus synthesizing a zeolite-

hydrotalcite composite in which zeolite and hydrotalcite are simultaneously

10 SUBSTITUTE SHEET formed. In this case, hydrotalcite is supported on zeolite, and thus dispersity and mechanical stability are increased. The synthesized zeolite-hydrotalcite composite is put into an alkali earth metal anion-dissolved solution and thus anion-exchanged. When the ion exchange reaction is slowly conducted or is not easily conducted due to the large size of the anions and the large amount of water coordinated with the anions, the ion exchange reaction is accelerated by refluxing and heating the mixed solution of the zeolite-hydrotalcite composite and the alkali earth metal anion-dissolved solution at high temperatures. The zeolite-hydrotalcite composite is sufficiently ion-exchanged and then calcined, thus fixing anions on zeolite. Meanwhile, in order to reduce nitrogen dioxide, desorbed in a reducing atmosphere, an NSR catalyst must be supported with precious metals, such as platinum, palladium, rhodium and the like. The NSR catalyst is supported with the precious metals thorough impregnation, coprecipitation or evaporation, calcined, and then reduced. Through this process, the precious metals are supported on the zeolite-hydrotalcite composite, and thus act as active spots for reducing and removing nitrogen dioxide. When nitrogen dioxide is stored in barium oxide, which is chiefly used as a

11 SUBSTITUTE SHEET storage material of an NSR catalyst, the barium oxide is formed into a nitrate,

and then the nitrate is converted into barium oxide again through a reduction

process. For this reason, barium oxide must be repeatedly expanded and

constricted in storage and reproduction processes. In contrast, since nitrogen

oxides are stored in the zeolite-hydrotalcite composite in a state in which the

nitrogen oxides are coordinated with alkali earth metal ions or transition metal

ions existing in micropores and lattices of the zeolite-hydrotalcite composite, the

volume of the zeolite-hydrotalcite composite is not changed, and thus the

zeolite-hydrotalcite composite has excellent durability.

In the subsequent description, the hydrotalcite-zeolite composite is defined in

the following way. The kinds of anion and zeolite are written after the hyphen

drawn after the abbreviated form (HT) of hydrotalcite, and the mixing ratio of

zeolite to hydrotalcite is subsequently written in parentheses, thus naming a

hydrotalcite-zeolite composite catalyst. For example, HT-NaY(2.0) is a

hydrotalcite-zeolite composite in which Na+-containing zeolite Y is mixed with

hydrotalcite at a ratio of 2:1. If the hydrotalcite-zeolite composite is substituted

with a transition metal, the element symbol of the transition metal is additionally

written before the abbreviated form (HT) of hydrotalcite, for example, Fe-HT-

12

SUBSTITUTE SHEET NaY(2.0).

[Example 1] Process of synthesizing zeolite Y

A zeolite Y stock solution for storing and removing nitrogen dioxide, present in

exhaust gases discharged from diesel automobiles, was prepared. The zeolite

Y stock solution, prepared by mixing 8.2 g of silica, 9.4 g of sodium hydroxide,

3.8 g of sodium aluminate and 36.3 g of water, was aged while being stirred at a

temperature of 25⁰C for 24 hours. Subsequently, 74.5 g of silica, 25.7 g of

sodium hydroxide, 33.8 g of sodium aluminate and 340.6 g of water were added

to the prepared zeolite Y stock solution to form a synthesized stock solution.

Thereafter, the synthesized stock solution was put into a high-pressure reactor,

heated to a temperature of 100 ⁰C , and then reacted for 12 hours to form a

solid product. The solid product was washed, filtered, and then dried at a

temperature of 100^°C to synthesize 70 g of zeolite Y. The yield of the

synthesized zeolite Y was 85% based on the amount of silica. FIG. 2 shows

an X-ray diffraction pattern of the synthesized zeolite Y. The location and

intensity of X-ray diffraction peaks in the X-ray diffraction pattern of the

synthesized zeolite Y coincided strongly with those of X-ray diffraction peaks in

13 SUBSTITUTE SHEET the X-ray diffraction pattern of the zeolite Y reported in the document [M. M. J.

Treacy, J. B. Higgins, "Collection of simulated XRD powder patterns for

zeolites", Elsevier, New York, 1996.]. The average particle size of the

synthesized zeolite y, measured using a scanning electron microscope, was

about 0.5 μm.

[Example 2] Process of preparing hydrotalcite (HT)

Hydrotalcite for storing and removing nitrogen dioxide, present in exhaust gases

discharged from diesel automobiles, was prepared through hydrothermal

synthesis. 36.0 g of aluminum nitrate and 72.5 g of magnesium nitrate were

put into 250 mi of water and then sufficiently stirred to form a solution. Further,

31.8 g of sodium hydroxide and 26.8 g of sodium carbonate were also dissolved

in 250 mi of water to form a solution. These two solutions were mixed at room

temperature, stirred for 24 hours, and then aged to form a mixed solution.

Thereafter, the mixed solution was put into a high-pressure reactor, heated to a

temperature of 100 ^°C, and then reacted for 4 hours to form a solid product.

The solid product was washed, filtered, and then dried at a temperature of

100^°C . Subsequently, the solid product was calcined in an electrical furnace at

14

SUBSTITUTE SHEET a temperature of 550⁰C to prepare 32 g of hydrotalcite (HT). The addition ratio

of magnesium and aluminum, constituting the hydrotalcite, was adjusted to 3 by

mole. The yield of the synthesized hydrotalcite was 87% based on alumina,

magnesium oxide and sodium carbonate. FIG. 3 shows the X-ray diffraction

pattern of the synthesized hydrotalcite. The X-ray diffraction pattern of the

synthesized zeolite Y coincided strongly with those of the hydrotalcite reported

in the document [E. Kanezaki, "A thermally induced metastable solid phase of

Mg/AI-layered double hydroxides by means of in situ high temperature powder

X-ray diffraction", J. Mater. Sci. Lett., 17, 371 (1998).].

[Example 3] Preparation of HT-NaY(LO), HT-NaY(0.5) and HT-NaY(2.0)

composites

Zeolite Y-hydrotalcite composites were prepared by mixing the zeolite Y stock

solution prepared in Example with the hydrotalcite stock solution prepared in

Example 2, and then hydrothermally reacting the mixed solution. Specifically,

the zeolite Y stock solution and hydrotalcite stock solution, which were

separately prepared, were mixed and then stirred at room temperature for 2

hours to form a mixed solution. Subsequently, the mixed solution was put into

15 SUBSTITUTE SHEET a high-pressure reactor, heated to a temperature of 100^°C , and then reacted for

12 hours to form a solid product. The solid product was washed, filtered, and

then dried at a temperature of 100^°C . Subsequently, the solid product was

calcined in an electrical furnace at a temperature of 55O⁰C to prepare 75g of a

zeolite Y-hydrotalcite composite (HT-NaY(LO) composite). The yield of the

prepared HT-NaY(LO) composite was 63%.

Meanwhile, zeolite Y-hydrotalcite composites (HT-NaY(0.5) and HT-NaY(2.0)

composites), each having zeolite Y and hydrotalcite content different from those

of the HT-NaY(LO) composite, were prepared by adjusting the mixing ratios of

the zeolite Y stock solution and hydrotalcite stock solution. FIG. 4 shows X-ray

diffraction patterns of the prepared HT-NaY(0.5), HT-NaY(LO) and HT-NaY(2.0)

composites. As shown in FIG. 4, it can be seen that diffraction peaks of

hydrotalcite appeared at diffraction angles of 1 L7°and 23.2°and diffraction

peaks of zeolite Y appeared at other diffraction angles, and that as the amount

of zeolite Y increased, the diffraction peaks of hydrotalcite decreased.

FIG. 5 shows the shapes of the prepared zeolite Y-hydrotalcite composites,

photographed using a scanning electron microscope. Pure zeolite Y had a

particle size of 0.5 - 1.0 μm, and in contrast, hydrotalcite had a small particle

16 SUBSTITUTE SHEET size of 0.05 - 0.1 μm. Hydrotalcite was well dispersed on the surface of zeolite

Y.

[Experimental Example 1] Measurement of surface area

The surface areas of the zeolite Y-hydrotalcite composites, measured using a

nitrogen adsorption method, are given in Table 1. The surface area of the

hydrotalcite (Example 2) is much smaller than that of the zeolite Y (Example 1 ).

Since the surface area of each of the zeolite Y-hydrotalcite composites

(Example 3), calculated from nitrogen adsorption isothermal curves using the

Brunauer-Emmett-Telle (BET) equation, ranges from the surface area of the

hydrotalcite to the surface area of the zeolite Y, it is inferred that each of the

zeolite Y-hydrotalcite composites is a composite of zeolite Y and hydrotalcite.

[Table 1 ]

Surface areas of zeolite Y-hydrotalcite composites depending on the mixing

ratio of zeolite Y and hydrotalcite.

17

SUBSTITUTE SHEET [Example 4] Preparation of HT-KY(LO), HT-MgY(LO), HT-CaY(LO), HT-

SrY(LO) and HT-BaY(LO) composites

HT-KY(LO), HT-MgY(LO), HT-CaY(LO), HT-SrY(LO) and HT-BaY(LO)

composites were prepared by exchanging the anions of the HT-NaY(LO)

composite, prepared in Example 3. First, an aqueous solution, having a

concentration of 0.5 N, including potassium chloride (manufactured by Duksan

Industrial Co., Ltd., 99%), magnesium nitrate (manufactured by Daijung Co.,

Ltd., 98%), calcium nitrate (manufactured by Junsei Corp., 98%), strontium

nitrate (manufactured by Daijung Co., Ltd., 98%) and barium nitrate

(manufactured by Daijung Co., Ltd., 98.5%) was prepared. Thereafter, 10 g of

an HT-NaY(1.0) composite was added to 200 g of the aqueous solution, reflux-

heated at a temperature of 60 ⁰C for 12 hours, and then ion-exchanged.

Subsequently, the ion-exchanged HT-NaY(LO) composite was washed, filtered,

and then dried at a temperature of 100^°C for 12 hours. Finally, the HT-

NaY(LO) composite was calcined in an electric furnace at a temperature of

55O⁰C , thus preparing the HT-KY(LO), HT-MgY(LO), HT-CaY(LO), HT-SrY(LO)

and HT-BaY(LO) composites, in which the anions of the HT-NaY(LO) is

18 SUBSTITUTE SHEET exchanged with alkali metal ions and alkali earth metal ions.

[Comparative Example 1] Preparation of HT-BaY(LO)-PM

A physically mixed sample [HT-BaY(I .O)-PM] was prepared in order to compare

the amount of nitrogen dioxide stored in the zeolite-hydrotalcite composite

according to the present invention with the amount of nitrogen dioxide stored in

the physically mixed sample [HT-BaY(LO)-PM]. The term "PM" means

physical mixing. Here, 10 g of zeolite (BaY) was uniformly mixed with 10 g of

hydrotalcite.

[Experimental Example 2] Measurement of amount of nitrogen dioxide adsorbed

on composite

The amounts of nitrogen dioxide adsorbed in these composites were measured

using a weight type adsorber. A weight type adsorber was exhausted at a

temperature of 200⁰C for 1 hour, and then nitrogen dioxide was put into the

weight type adsorber. Here, the term "adsorption amount" means the amount

of nitrogen dioxide adsorbed or stored in the composite at a nitrogen dioxide

pressure of 20 Torr, and the term "storage amount" means the amount of

19

SUBSTITUTE SHEET nitrogen dioxide remaining in the composite after nitrogen dioxide was adsorbed

in the composite and then the weight type adsorber was exhausted for 1 hour.

As given in Table 2, the adsorption amount of nitrogen dioxide and storage

amount of nitrogen dioxide in the composites are very different depending on

the kind of anion. When sodium ions (Na⁺) are replaced by potassium ions

(K⁺) or strontium ions (Sr²⁺), the storage amount of nitrogen dioxide in the

composite is greatly increased. In particular, the storage amount of nitrogen

dioxide in the composite, in which sodium ions (Na⁺) are replaced by strontium

ions (Sr²⁺), was 103 mg/g_cat, which is very high. Meanwhile, the storage

amount of nitrogen dioxide in the physically-mixed sample [HT-BaY( 1.O)-PM]

was 74 mg/gcat, which is low. Therefore, it can be seen that the formation of

the composite contributed to the increase in the storage amount of nitrogen

dioxide.

[Table 2]

Adsorption amount and storage amount of nitrogen dioxide in hydrotalcite-

zeolite composites having different kind of anions at 200 ^°C

20 SUBSTITUTE SHEET

[Experimental Example 3] Measurement of adsorptivity of nitrogen dioxide

The storage amount and adsorptivity of nitrogen dioxide in the composites and

mixture were evaluated through a nitrogen dioxide heating and desorption test,

21

SUBSTITUTE SHEET and the results thereof are shown in FIG. 6. Samples were left in air at a

temperature of 550 ^°C for 1 hour, and were then saturated and adsorbed with

nitrogen dioxide at a temperature of 200^°C . The samples adsorbed with

nitrogen dioxide were left for 1 hour, and then the desorption procedure of

nitrogen dioxide was examined while the samples were heated to a temperature

of 800 ^°C at a heating rate of 10 ^°C/min. The area of nitrogen dioxide

desorbed from an HT-BaY(LO) composite, which was prepared through

hydrothermal synthesis, was larger than the area of nitrogen dioxide desorbed

from a physically mixed HT-BaY(I .O)-PM mixture. Further, the adsorptivity of

nitrogen dioxide in the HT-BaY(LO) composite was also stronger than the

adsorptivity of nitrogen dioxide in the HT-BaY(LO)-PM mixture. Therefore, it

can be seen that the storage amount of nitrogen dioxide is increased and the

adsorptivity of nitrogen dioxide is also strengthened due to the formation of the

composite, even though the desorption curve of the composite was compared

with the desorption curves of other constituents.

[Experimental Example 4] Measurement of poison effect of sulfur dioxide

A hydrotalcite-zeolite Y composite, prepared in Example 4, was poisoned with

22 SUBSTITUTE SHEET sulfur dioxide, and the storage amount of nitrogen dioxide in the hydrotalcite-

zeolite Y composite was measured using a weight type adsorber. Composite

samples were left at a temperature of 300 ^°C for 1 hour, and sulfur dioxide vapor

of 10 Torr was applied to the composite samples at a temperature of 200 °C to

adsorb sulfur dioxide in the composite samples for 1 hour. After the sulfur

dioxide was emitted, nitrogen dioxide of 20 Torr was applied to the composite

samples, and then the storage amount of nitrogen oxide was measured. The

storage amounts of nitrogen dioxide in hydrotalcite-zeolite Y composites, which

were measured after the composite samples were poisoned with sulfur dioxide,

are given in Table 3.

Since a large amount of sulfur dioxide was also stored in the hydrotalcite-zeolite

Y composite, in which a large amount of nitrogen dioxide was stored, the

storage amount of nitrogen dioxide in the hydrotalcite-zeolite Y composite was

greatly influenced by the poisoning effect of sulfur dioxide. Hydrotalcite-zeolite

Y composites containing potassium and barium anions having strong alkalinity

stored a large amount of sulfur dioxide. In particular, an HT-KY(LO) catalyst

containing potassium anions stored a large amount of sulfur dioxide, specifically,

166 mg/gcat. After the catalyst was poisoned by sulfur dioxide, the storage

23

SUBSTITUTE SHEET amount of nitrogen dioxide in the catalyst differed greatly depending on the kind

of anion. In particular, the storage amounts of nitrogen dioxide in the HT-

NaY(LO) catalyst and HT-KY(LO) catalyst, containing alkali metal anions, were

much larger that the storage amount of nitrogen oxide in the K2O-BaO/AI2O3

catalyst, which is a catalyst for comparison. In particular, in the HT-NaY(LO)

catalyst containing sodium anions, even after the HT-NaY(LO) catalyst was

poisoned by sulfur dioxide, the storage amount of nitrogen dioxide thereof was

58 mg/gcat, which was about four times that of the HT catalyst, of 15 mg/gcat,

or that of the K2O-BaO/AI2O3 catalyst, which is a catalyst for comparison, of 16

mg/gcat. Therefore, it can be seen that the storage performance of nitrogen

dioxide in the HT-NaY(LO) catalyst was largely maintained.

[Table 3]

Storage amount of sulfur dioxide and storage amount of nitrogen dioxide in

anion-exchanged composites at a temperature of 200⁰C

Catalyst Storage amount of nitrogen dioxide Storage amount

(mg/gcat) of sulfur dioxide

24

SUBSTITUTE SHEET

[Example 7] Preparation of analcime zeolite-hydrotalcite composite

An analcime zeolite-hydrotalcite composite was prepared using an analcime

zeolite stock solution. First, a hydrotalcite stock solution was prepared using

the same method as in Example 2. Next, an analcime zeolite stock solution

was prepared by mixing 61.1 g of silica, 11.3 g of sodium hydroxide, 9.2 g of

sodium aluminate, and 419.6 g of water such that the composition ratio was

6Na₂O:1AI₂O₃:30Siθ₂:780H₂O. The hydrotalcite stock solution and analcime

25

SUBSTITUTE SHEET zeolite stock solution were mixed, and then stirred at a temperature of 25^°C for

2 hours. Subsequently, the mixed solution was put into a high-pressure

reactor, heated to a temperature of 170^°C , and then reacted for 24 hours to

form a solid product. The solid product was washed, filtered, dried at a

temperature of 100 ⁰C , and calcined in an electric furnace at a temperature of

550 ^°C to prepare 58 g of an analcime zeolite-hydrotalcite composite. The

yield of the prepared analcime zeolite-hydrotalcite composite was 60%. In the

X-ray diffraction pattern of the prepared analcime zeolite-hydrotalcite composite,

the characteristic peaks of analcime zeolite appeared strong at diffraction

angles of 15.8^° , 18.2°, 25.9°, 30.5°and 31.9°, and the characteristic peaks of

hydrotalcite appeared weak at diffraction angles of 11.7°, 23.2°, 34.8°and

60.5°[M. M. J. Treacy, J. B. Higgins, "Collection of simulated XRD powder

patterns for zeolites", Elsevier, New York, 1996.].

The surface area of the prepared analcime zeolite-hydrotalcite composite was

33 m²/g, which is very small. The reason is that the analcime zeolite-

hydrotalcite composite has micropores, each of which is formed of a ring of

eight oxygen atoms, and thus nitrogen does not pass through the micropores.

2 6

SUBSTITUTE SHEET [Example 8] Preparation of mordenite zeolite-hydrotalcite composite

A mordenite zeolite-hydrotalcite composite was prepared by mixing a mordenite

zeolite stock solution with a hydrotalcite stock solution and then hydrothermally

reacting them. First, a hydrotalcite stock solution was prepared using the

same method as in Example 2. Next, a mordenite zeolite stock solution was

prepared by mixing 216.3 g of colloidal silica (SiO₂ content: 40 wt%), 20.6 g of

sodium aluminate, 149.0 g of tetraethylammonium (TEA) hydroxide and 290.0

g of water such that the composition ratio was

2.46(TEA)2O:1 Na₂θ:1AI₂O₃:20Siθ2:416H₂O. The hydrotalcite stock solution

and mordenite zeolite stock solution were mixed, and then stirred at a

temperature of 25⁰C for 2 hours. Subsequently, the mixed solution was put

into a high-pressure reactor, heated to a temperature of 15O⁰C, and then

reacted for 48 hours to form a solid product. The solid product was washed,

filtered, dried at a temperature of 100 ^°C , and calcined in an electrical furnace

at a temperature of 550 ^°C to prepare 50 g of a mordenite zeolite-hydrotalcite

composite. The yield of the prepared mordenite zeolite-hydrotalcite composite

was 40%. In the X-ray diffraction pattern of the prepared mordenite zeolite-

hydrotalcite composite, the diffraction peaks of mordenite zeolite appeared

27

SUBSTITUTE SHEET strong at diffraction angles of 6.5°, 8.6°, 9.7° , 14.5°, 15.3°, 19.4°, 26.0°and

26.2°, and the characteristic peaks of hydrotalcite appeared weak at diffraction

angles of 34.8°and 60.8°[M. M. J. Treacy, J. B. Higgins, "Collection of simulated

XRD powder patterns for zeolites", Elsevier, New York, 1996.].

(Reference Documents)

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[3] G. SEO, JW. Park, YS. Yoo, HS. Han, Korean Patent No. 10-2006-56659,

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[4] J. H. Kwak, J. Szanyi, C. H. F. Peden, "Non-thermal plasma-assisted NOx

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28

SUBSTITUTE SHEET storage-reduction catalysts based on hydrotalcite effect of Cu in promoting

resistance to deactivation", Catal. Today, 73, 287(2002).

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2 9

SUBSTITUTE SHEET

Claims

[CLAIMS]

[Claim 1 ]

A method of producing a zeolite-hydrotalcite composite, comprising:

preparing a zeolite stock solution and a hydrotalcite stock solution; and

mixing the stock solutions to conduct a hydrothermal reaction.

[Claim 2]

The method according to claim 1 , further comprising:

supporting one or more platinum group metals on the zeolite-hydrotalcite

composite.

[Claim 3]

The method according to claim 1 or 2, wherein the zeolite is ANA zeolite,

BEA zeolite, MFI zeolite, MOR zeolite, or zeolite Y.

[Claim 4]

The method according to claim 1 or 2, wherein the hydrotalcite is

substituted with a transition metal, such as cobalt, cerium, copper, iron, or nickel.

[Claim 5]

The method according to claim 1 , wherein the hydrothermal reaction is

conducted at a temperature of 90 ~ 120^°C for 4 -12 hours.

30 SUBSTITUTE SHEET [Claim 6]

A zeolite-hydrotalcite composite catalyst for storing nitrogen oxides, produced using the method according to claim 1. [Claim 7]

A zeolite-hydrotalcite composite catalyst for storing nitrogen oxides, produced using the method according to claim 2.

3 1 SUBSTITUTE SHEET