JPH08281060A - Nitrogen oxide absorbent and its production - Google Patents

Abstract

PURPOSE: To provide a NOx absorbent capable of selectively and efficiently absorbing low concentration NOx in combustion exhaust gas or mixture gas, simply disorbing it and being reutilized and to develop the use in various shapes such as a filling cylinder and hollow fiber membrane. CONSTITUTION: This NOx (x=1-2) absorbent consists essentially of at least one or more kinds of inorganic compounds selected from among porous alumina, titania and zirconia having average fine pore diameter and fine pore peak diameter in the range of 5-30nm and at least >=0.25cc/g fine pore volume and >=100-<=300m<2> /g specific surface area value by a BET method.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a powder form using an alumina-based, titania-based or zirconia-based porous body that selectively absorbs and removes low-concentration NOx contained in combustion exhaust gas and mixed gas, The present invention relates to a NOx removal device having a granular, pellet-shaped, film-shaped, or honeycomb-shaped structure.

[0002]

2. Description of the Related Art Up to now, as seen in JP-A-5-31357 and JP-A-5-146675, it has been known that activated alumina is present as an NOx adsorbent together with activated carbon and zeolite. All of them have extremely large specific surface area and have fine pores. Therefore, the phenomenon that highly adsorptive substances such as water, NOx, SOx are simultaneously adsorbed on the pore surface by physical adsorption force is used. It was done. Therefore, it is used as a NOx and SOx adsorbent in the combustion exhaust gas, but the selectivity and adsorption amount of NOx adsorption were extremely low. Therefore, in order to improve the adsorption property, a method of uniformly dispersing a basic component such as Mg 2 O and Ca 2 O 3 may be used, but there is a drawback that desorption is irreversible.

On the other hand, recently, as a new NOx absorber,
Yttrium-strontium-cobalt system as described in "Chemical Society of Japan", 1993 No. 6, 703-708, and "Catalyst", Vol. 33, 1991, No. 2, 163, published by The Chemical Society of Japan. Perovskite complex oxides such as complex oxides have become known. These characteristics have a very high selectivity for NOx absorption, and the amount of absorption is generally as large as 0.1 mol or more per mol of the absorber compound. However, although these complex oxides absorb a large amount of NOx, they react with water vapor that absorbs at the same time to form a nitrite group or a nitrate group and destroy the perovskite structure, so that they cannot be easily regenerated, It had the drawback that it could not be reused.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to provide a NOx absorber characterized by reversibly releasing / absorbing NOx and a method for producing the same, in order to solve the above problems. To do.

[0005]

The object of the present invention is basically achieved by the following constitution.

That is, the main component is made of at least one kind of porous inorganic compound selected from alumina, titania, and zirconia, and the average pore diameter and the pore peak diameter are 5 to 30 nm. The volume is at least 0.
25 cc / g and BET specific surface area value of 100 m ²
/ G or more and 300 m ² / g or less N
It is an Ox (x = 1 to 2) absorber.

By the way, in order to increase the amount of NOx absorbed, it is necessary to have an appropriate pore size distribution. The average pore diameter and the pore peak diameter are described as 5 to 30 nm, but NOx
In order to be efficiently absorbed, it is necessary to adjust the pore size within an appropriate range. When the pore diameter is 5 nm or less, usually, when some NOx molecules are aggregated to form a cluster, it is difficult for the NOx molecules to penetrate into the pores and be efficiently absorbed. When it is 30 nm or more, the pores become too wide, and the absorptivity is significantly reduced. For the above reason, the pore size is 6 to
More preferably, it is 20 nm. Also, the larger the pore volume, the more preferable, but at least 0.25c
c / g or more, preferably 0.35 cc / g or more, and more preferably 0.40 cc / g or more. Usually, in the adsorption of a substance, it was considered that an adsorbent having a large pore volume and a high specific surface area has a higher adsorption ability, but this is not always the case for NOx adsorption, and NOx is the most efficient in a certain appropriate range. Moreover, it was revealed by the present invention that a large amount is adsorbed. That is, the specific surface area of the BET method is in the range of 100 m ² / g or more and 300 m ² / g or less, preferably 120 m ² / g or more and 250 m ² / g or less. In this range, NOx is practically efficiently absorbed / released. As described above, by advancing the optimization of the conditions, it becomes possible to improve the water adsorption amount to 10% by weight or more and the NOx absorption amount to 5% by weight or more.

The NOx absorber of the present invention has a NOx concentration of 10
0ppm or more and 10000ppm or less, absolute humidity 1.5
%, At least 3 wt% NOx in air at a temperature of 25 ° C., preferably 4%, more preferably 6%. More preferably, it can absorb 8%.

Further, the absorbed NOx is 300-60.
It is released by heating at 0 ° C., but from the viewpoint of energy efficiency, it is preferable that at least 80% or more of NOx is released up to 500 ° C.

On the other hand, in the usual method, when the firing temperature is raised in order to increase the pore diameter to some extent, the pore volume becomes small. Regarding the method for producing an inorganic compound of the present invention, the pore size is large to some extent, and the pore volume is also required to be sufficiently large. Therefore, in the powder production process using a metal alkoxide solution and an inorganic salt solution as a starting material. , A compound containing nitrogen is used as a raw material or an additive to the raw material. This nitrogen-containing compound plays an important role in obtaining the desired NOx absorbent by adjusting the pore diameter and pore volume of the inorganic compound in a well-balanced manner in the firing process.

In the manufacturing process, various methods of adding a compound containing nitrogen can be considered. For example, the general formula M (-OR ¹ ) x (-OH) y (A) M is Al, Ti, Zr. Any one of these metal elements x + y = 3 (Al) or 4 (Ti, Zr) x ≧ 1 R ¹ is a substituted or unsubstituted aliphatic or alicyclic group having 1 to 6 C atoms and the general formula NHx (−R ² —OH) y (B) x + y = 3 y ≧ 1 R ² hydrolyzes a lower aliphatic alcohol solution containing a substituted or unsubstituted C 1-6 substituted or unsubstituted aliphatic or alicyclic group. The NOx absorber of the present invention can be obtained through the steps and the step of firing. In this case, amines were used as an additive for the purpose of controlling the hydrolysis into the metal alkoxide raw material, but a method using a metal inorganic salt is also conceivable in addition to this method. For example, when a nitrate is used as the raw material itself, a case where nitric acid is added as a dispersant by a sol-gel method using a metal inorganic salt such as chloride or sulfate as a solution can be considered.

As described above, the NOx of the present invention is characterized in that the compound containing nitrogen is used in the manufacturing process.
It is a manufacturing method of an absorber. This method provides a NOx absorber that can selectively and efficiently absorb / release low-concentration NOx in combustion exhaust gas or a mixed gas to reuse it.

The method for producing the NOx absorber of the present invention when the starting material is an organic metal such as a metal alkoxide will be described below.

[0014] (Manufacturing method) formula ^{M (-OR 1) x (-OH} ) y (A) M is, Al, Ti, 1 kind of metal element one of Zr x + y = 3 (Al ) or 4 ( Ti, Zr) x ≧ 1 R ¹ is a compound represented by the general formula NHx (—R ² —OH) y (B 1) to 1 mol of a compound represented by a substituted or unsubstituted aliphatic or alicyclic group having 1 to 6 C atoms. ) X + y = 3 y ≧ 1 R ² is each a predetermined amount of a compound represented by a substituted or unsubstituted aliphatic or alicyclic group having a C number of 1 to 6 in an amount of 0.1 to 2.0 mol. , Mix in an inert gas atmosphere,
Dissolve completely. The compound of B exerts a hydrolysis-inhibiting effect on the compound of A, and extremely fine and controlled particles can be obtained by adjusting the concentration appropriately. If the amount of the hydrolysis inhibitor is less than 0.1 mol, the precipitated particles agglomerate and become coarse due to rapid hydrolysis, and the solution becomes cloudy. On the other hand, when the amount is 2.0 mol or more, the effect of the hydrolysis inhibitor is too strong and particles do not precipitate even after several tens of hours. For the above reasons, it is more preferable to adjust the addition amount of the compound B to 0.5 mol to 1.0 mol with respect to 1 mol of the compound A.

When preparing a powder, the above-prepared aluminum alkoxide solution is placed in a container and left in a conditioned atmosphere as a simple method. After standing, hydrolysis proceeds due to moisture in the air, and gelation is completed within 1 to several days. In addition, gelation can be completed within several hours by forcibly hydrolyzing while dropping a 5 to 30% HNO ₃ solution. After the gel is dried in a dryer until it is apparently free of volatile components, the gel is dried at 500 to 1000 ° C., preferably 6
00-900 ° C, more preferably 700-800 ° C,
Firing is performed in the temperature range of 10 minutes to 3 hours.

In order to coat a honeycomb or a monolith, a base material made of an inorganic material is first prepared. As the material of the base material, it is usually preferable to use a material having excellent heat resistance, corrosion resistance, and weather resistance such as alumina, mullite, zirconia, titania, silica, and cordierite. In addition, even if the surface of the base material is dense and smooth,
It may be either porous and undulating. next,
The above-mentioned alkoxide solution is applied to the surface of the base material. As a coating method, it is simple and suitable to immerse the porous substrate in the coating solution for a certain period of time and then slowly pull it up. At this stage, the solution applied to the surface of the base material is hydrolyzed by the moisture (humidity) contained in the air to become an aluminum hydroxide gel.

Further, by utilizing this absorber, it becomes possible to design the NOx absorber and the NOx removing device for various purposes. That is, granular or powdered NOx
NOx adsorption cylinder filled with absorber, hollow fiber membrane-like, flat membrane-like and honeycomb-like structure in which the adsorbent is laminated or applied in a film form on the surface of a base material to produce a NOx removing device for combustion exhaust gas, denitration device Can be used as

As described above, according to the method of the present invention, granular, powdery, membranous or honeycomb-shaped porosity can be obtained by performing a simple operation such as drying from an alkoxide solution and then firing without performing any complicated treatment. Alumina NOx absorber can be obtained.

(Evaluation method) NOx obtained by the present invention
By using the absorber, it becomes possible to stably separate gas and liquid in a high temperature and high pressure region for a long period of time.

The pore size of the obtained inorganic membrane structure is evaluated by a gas adsorption method or a differential scanning calorimetry method.

In the case of the gas adsorption method, the water content of the sample and N
When measuring mesopores of 2 nm or more after sufficiently releasing Ox, nitrogen gas is adsorbed, and the Dolimore-
The pore size is determined by the Heal method. When measuring micropores of 2 nm or less, adsorb argon gas and
The pore size distribution and the pore volume are determined by the rvath-Kawazoe method.

The crystal structure of the NOx absorber is determined by the X-ray diffraction method or the electron beam diffraction method. Particularly, in the case of a film sample, a method of increasing the diffraction intensity from the film sample by X-ray diffractometry, focusing the X-ray source as a line focus and squeezing it with an entrance slit, and making it incident at 1 to 10 ° near the sample surface. Take

When the diffraction peak is broad due to the X-ray diffraction method and the crystallinity is difficult to specify, that is, the so-called amorphous form, the following electron beam diffraction is used to determine whether it is an amorphous structure or an ultrafine particle structure having high crystallinity. To do.

In the electron beam diffraction method, in the case of a film sample, the cross section is cut to about 200 μm and mechanically polished to reduce the film thickness to about 50 μm. After that, the film is further thinned to about 0.1 μm with an ion milling apparatus using argon ions, and then it is examined with a transmission electron microscope. In the case of a powder sample, the sample can be prepared by ion milling, or can be directly observed with a transmission electron microscope.
Now, after focusing on the target site to evaluate the crystalline state,
A selected-area aperture stop having an appropriate size is inserted, and the selected-area electron diffraction mode is selected to obtain an electron diffraction image. In the case of electron diffraction, in the case of crystallinity, distinct diffraction spots or diffraction rings appear. In the case of amorphous, the above-mentioned spots and diffraction ring do not appear, and a so-called halo pattern is obtained. That is, although the atomic scattering corresponding to the statistical interatomic distance based on the amorphous disordered atomic arrangement strengthens each other, it is very broad, but it has an intensity distribution and decays to a high angle side. It is a peculiar halo figure, and it is concluded that this is an amorphous structure.

Structural analysis was carried out by the above two methods, and when no diffraction peak was obtained by the X-ray diffraction method and no diffraction line was obtained by the electron beam diffraction method, the amorphous structure was adopted.

[0026]

【Example】

Examples 1 to 3 (Production method) Aluminum isopropoxide (Al (O
For 1 mol of C ₃ H ₇ ) ₃ ), diethanolamine (N
H (C ₂ H ₄ OH) ₂ ) 0.5 (Example 1), 1.0
(Example 2) and 2.0 mol (Example 3) were weighed and mixed in a pure nitrogen gas atmosphere, and after completely dissolving, 0.5 mol / liter of ethyl alcohol was used. Prepare so that the solution has a concentration.

The prepared aluminum isopropoxide solution is placed in a glass petri dish and left in the atmosphere adjusted to a relative humidity of 80% for 24 hours. After standing, hydrolysis is promoted by moisture in the air, and gelation is completed. The gel was dried in a dryer set at 60 ° C. until it had no apparent water content, and then calcined at 600 ° C. for 30 minutes in the air to obtain an alumina absorber.

Comparative Example 1 (Production Method) In Example 1, 1 mole of aluminum isopropoxide (Al (OC ₃ H ₇ ) ₃ ) was added without addition of diethanolamine (NH (C ₂ H ₄ OH) ₂ ). Other than the following, after preparing the sample under the same conditions as in Example 1,
The NOx adsorption characteristics were evaluated.

(Evaluation Method) Next, the NOx absorption characteristics of each sample were examined by using a temperature rising type adsorption / desorption device. First, the sample is enclosed in a glass sample cell and heated while the line is evacuated to desorb previously adsorbed impurities. After holding at 550 ° C for 20 minutes, it was cooled to room temperature while flowing pure helium gas, and then NOx was increased to 2000
After switching to a helium mixed gas containing ppm, NOx was adsorbed to the sample until saturation was reached while observing the weight change. Since water also has an absorbing capacity for the material and may cause an error in weight measurement, the water concentration is 1000 p
The inflow of outside air containing water was prevented so that it became pm or less.

(Evaluation Results) NO produced by the above method
The crystal structure of the x absorber was identified. First, the X-ray diffraction method was performed by a powder X-ray diffraction method using a Cukα tube. Power supply voltage 40 kV, current 20 mA, scanning speed 2 ° /
It was a continuous scan of minutes. No specific X-ray peak appeared in any of the samples in the range of 20 to 70 °, and low broad undulation was observed. Furthermore, a sample thinned to 0.1 μm with an ion milling device using argon ions was examined with a transmission electron microscope, and electron diffraction was performed with the field of view restricted. A nice halo figure was obtained.

Further, for sample A, the BET specific surface area, average pore diameter and pore volume were determined by the gas adsorption method.

BET specific surface area 320 m ² / g average pore peak diameter 38 nm (main peak) and 1
2 nm (sub peak) Pore volume 0.35 cc / g Table 1 shows the results evaluated in the above Examples 1 to 3 and Comparative Example 1.

[0033]

[Table 1] Examples 4 to 5 (Production method) Aluminum isopropoxide (Al (O
For 1 mol of C ₃ H ₇ ) ₃ ), diethanolamine (N
H (C ₂ H ₄ OH) ₂ ) was weighed so as to be 0.5 mol, mixed in a pure nitrogen gas atmosphere, and completely dissolved while adding water to force hydrolysis. , Using ethyl alcohol to prepare a solution having a concentration of 1.0 mol / liter.

The prepared aluminum isopropoxide solution is placed in a glass petri dish and left in the atmosphere adjusted to a relative humidity of 60% for 48 hours. After standing, hydrolysis is promoted by moisture in the air, and gelation is completed. The gel is dried in a dryer set at 60 ° C. until it is apparently free of water, and then calcined at 700 ° C. (Example 4) and 800 ° C. (Example 5) for 30 minutes in the atmosphere. The sample after firing is placed in air at an absolute humidity of 1.5% and a temperature of 25 ° C.
It was left for 2 hours.

Comparative Example 2 (Manufacturing Method) In Example 2, the firing condition was set to 5
What was made into 30 minutes at 00 degreeC was produced.

(Evaluation result) N obtained by the above method
For the Ox absorber, measurement of specific surface area by BET method,
The average pore peak diameter and the pore volume were determined by the gas adsorption method, and the NO ₂ adsorption experiment was performed by the method described in Example 1. The results are shown in Table 2.

[0037]

[Table 2] Example 6 (Production method) Aluminum isopropoxide (Al (O
100 mol of ion-exchanged water adjusted to pH 2 was added to 1 mol of C ₃ H ₇ ) ₃ ) and hydrolyzed at room temperature for 1 hour so that nitric acid became 0.1 mol per 1 mol of aluminum alkoxide. 1N nitric acid is added to. This solution was heated at 80 ° C. for 48 hours to generate boehmite sol. This was put in a glass petri dish, and water was evaporated in the dryer to form a gel, followed by firing at 700 ° C.

Aluminum isopropoxide (Al (O
For 1 mol of C ₃ H ₇ ) ₃ ), diethanolamine (N
H (C ₂ H ₄ OH) ₂ ) was weighed so as to be 0.5 mol, mixed in a pure nitrogen gas atmosphere, and completely dissolved while adding water to force hydrolysis. , Ethyl alcohol was used to prepare a solution having a concentration of 1.0 mol / liter.

The prepared aluminum isopropoxide solution was placed in a glass dish and mixed with alumina flakes prepared by firing boehmite sol, and the relative humidity was 60.
It is left for 48 hours in an atmosphere whose humidity is adjusted to%. After standing, hydrolysis is promoted by moisture in the air, and gelation is completed. The gel was dried in a dryer set at 60 ° C. until apparently free of water, and then baked at 600 ° C. for 30 minutes in the atmosphere. The sample after firing has an absolute humidity of 1.5% and a temperature of 25.
It was left in the air at ℃ for 72 hours.

(Evaluation result) N obtained by the above method
For the Ox absorber, measurement of specific surface area by BET method,
The average fine diameter peak diameter and the fine pore volume were determined by the gas adsorption method, and the NOx adsorption performance was evaluated by the method shown in Example 1. As a result, BET specific surface area 159 m ² / g average pore diameter 6.4 nm pore volume 0.377 cc / g NO ₂ adsorption capacity 23.6 wt% Example 7 (Production method) 0.1 mol of titanium methoxide in monoethanol 0.1 mol of amine (NH ₂ (C ₂ H ₄ OH)) was weighed, mixed in a pure nitrogen gas atmosphere, completely dissolved, and then a solution having a concentration of 1.0 mol / liter was prepared using methyl alcohol. To be prepared.

The prepared titanium methoxide solution is put in a glass petri dish and left in a constant temperature bath previously adjusted to 60 ° C. and 1% absolute humidity for 48 hours for gelation. next,
Firing was performed in the air at 700 ° C. for 30 minutes to obtain porous titania.

(Evaluation Results) The titania prepared by the above method was subjected to structural analysis, pore distribution evaluation, and NOx adsorption performance evaluation.

Crystal structure Anatase type BET specific surface area 271 m ² / g Average pore diameter 5.4 nm Pore volume 0.36 cc / g NO adsorption capacity 10.1 wt% NO2 adsorption capacity 12.6 wt% Example 9 (Production method) ) 0.1 mol of zirconium ethoxide was weighed with 0.1 mol of diethanolamine (NH (C ₂ H ₄ OH) ₂ ), mixed in a pure nitrogen gas atmosphere, completely dissolved, and then mixed with ethyl alcohol Prepare so that the solution has a concentration of 0.0 mol / liter.

The prepared zirconium ethoxide solution is placed in a glass petri dish and left in a thermostat having a humidity of 60 ° C. and an absolute humidity of 1% for 48 hours for gelation. Next, it was fired at 700 ° C. for 30 minutes in the air to obtain porous zirconia.

(Evaluation Results) The zirconia prepared by the above method was subjected to structural analysis, pore distribution evaluation, and NOx adsorption performance evaluation.

Crystal structure Cubic BET specific surface area 134 m ² / g average pore diameter 5.4 nm pore volume 0.28 cc / g NO adsorption capacity 8.7% by weight NO 2 adsorption capacity 13.8% by weight

[0047]

The present invention provides a NOx absorber which selectively and efficiently absorbs low-concentration NOx in combustion exhaust gas or mixed gas, and easily releases and reuses it.
It is possible to develop applications in various shapes such as filling cylinders and hollow fiber membranes.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location B01J 20/08 B01J 20/28 A B01D 53/34 ZAB 20/28

Claims (13)

[Claims] 1. An average pore size and a pore peak size in the range of 5 to 30 nm and a pore volume of at least 0.25 c.
c / g or more, BET specific surface area value of 100 m ² /
at least 1 selected from the group consisting of porous alumina, titania and zirconia, which is not less than 300 g / g and not more than 300 m ² / g
N containing at least one kind of inorganic compound as a main component
Ox (x = 1 to 2) absorber. 2. A NOx concentration of 100 ppm or more and 10,000
The NOx absorber according to claim 1, which is capable of absorbing at least 3% by weight of NOx in air having a ppm or less, an absolute humidity of 1.5% and a temperature of 25 ° C. 3. The NOx absorber according to claim 1, wherein 80% or more of absorbed NOx is released by heating at 350 ° C. or higher in vacuum. 4. The NOx absorber according to claim 1, which reversibly releases / absorbs NOx at a temperature rise or fall of 300 ° C. or lower or 500 ° C. or higher. 5. The NOx according to claim 1, wherein a nitrogen-containing compound is used as a raw material or an additive to the raw material and fired in a powder manufacturing process using a metal alkoxide solution or an inorganic salt solution as a starting material. Absorber manufacturing method. 6. The method for producing a NOx absorber according to claim 6, wherein the firing is performed in the atmosphere at 500 to 1000 ° C. 7. The NOx absorber according to claim 1, which is produced by the method according to claim 5, and is filled with the substance in a granular or powder form. 8. The NOx absorber according to claim 1, which is produced by the method according to claim 5, wherein the substance in powder form is formed into pellets, and the pellets are filled with the substance. Cylinder. 9. The NOx absorbent according to claim 1, which is produced by the method according to claim 5, wherein the substance is laminated or coated in a film form on the surface of a base material. A NOx absorption device comprising a flat film-shaped and a honeycomb-shaped structure. 10. A NOx removing device for a combustion exhaust gas, which has a shape according to claim 7, claim 8 and / or claim 9. 11. An NOx removal device for air purification, which has a shape according to claim 7 and / or 8 and / or 9. 12. A denitration device having the shape according to claim 7 and / or claim 8 and / or claim 9. 13. Claim 7 and / or claim 8.
And / or having a shape according to claim 9 for a catalyst carrier.