JPS6245342A - Catalyst for reducing content of nitrogen oxide in combustion exhaust gas - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

Detailed Description of the Invention [Summary of the Invention] A silicate having a layered structure (layered silicate) in which at least one of the metals Ti, Zr, V, W, Mo or Ce is used as one or more of its oxides. Disclosed is a catalyst for reducing the nitrogen oxide content of a flue gas containing in combination: (a) an acid-activated catalyst which still substantially maintains the structure of layered silicas 1 to 1 or its crystalline layers; (b) BET surface active [layered silica 1 to
(C) the atomic ratio between the silicon contained in the acid-activated kaolin and the metal contained in the oxide is increased by at least 15%, preferably by at least 50910, relative to the BET surface area of . 2-50, preferably O14-25
It is characterized by

[Industrial Application Field 1] The present invention relates to a catalyst that reduces the nitrogen oxide content of combustion exhaust gas.

[Prior art] When fossil fuels are burned, nitrogen oxides (NOx) are formed from both nitrogen-containing components and air nitrogen and reach the atmosphere, causing serious environmental pollution. can be converted into It has been shown that oxidation of ammonia proceeds without loss of ammonia, 7 and therefore only a relatively small amount of reducing agent is required. It is already known that various catalysts are used to reduce NOx using ammonia.

For example, from German Publication No. 27110175, the stoichiometric ratio of vanadium, molybdenum and 7·' or tungsten V12 :\·fOW (where -X-
This type of catalyst is known, which is made of an oxide (V X V where X and y are set to ○ x x 8, O≦y≦5 and 0.3≦(x10y)≦8).

Furthermore, a method for reductive decomposition of nitrogen oxides in exhaust gas is known from German Patent No. 2 458 888, in which a gas mixture containing nitrogen oxides and molecules, nitrogen and ammonia is brought into contact with a catalyst composition. The catalyst composition contains essential component (A) titanium as an oxide (along with iron or vadogium as a boiled product) in an intimate mixture.

These catalysts have the disadvantage that the relatively expensive transition metal compounds they contain as catalytically active components are utilized to only a small extent due to their non-optimal distribution. The active product can also be spread on an inert solid support, clearly improving its economy, but there is a risk that the catalytic activity will be significantly reduced upon dilution with inert substances. These catalysts convert the SO2 often contained in the flue gas into S
It has the disadvantage that O3 can oxidize, resulting in salt deposits, for example, in subsequent equipment.

Furthermore, a catalyst is known from German Publication No. 3438367 that reduces the nitrogen oxide content of combustion exhaust gas by reductive reduction, and this catalyst is particularly suitable for (A) an aqueous oxidized compound of titanium or silicon. (B) 80 to 95% by weight of a catalytic oxide containing sulfur oxide obtained by heat shielding, (B) 0 to 5% by weight of a catalytic oxide containing vanadium oxide, (
C) 1-15% by weight of a catalytic oxide, e.g. of tungsten
It is composed of.

What is essential for this type of catalyst is that from 3iQ2 and TiO2 a solid acid is formed, the acidity of which is modified by sulfuric acid or ammonium sulfuric acid. The distribution of this fixed acid is due to NH3 to the catalyst surface.
It appears to be critical for controlling adsorption and therefore essential for improving catalyst activity.

In this case, Si02 is used as a silicic acid sol.

As is well known, silicic acid gel is produced from S i 02 sol,
It is characterized by its high BET surface area as well as a high degree of porosity, but the macropore fraction is low and, as a result, the effect on mass transfer is also low and the catalytic effect is therefore poor.

Furthermore, from German Publication No. 2748471, a catalyst material is also known which is used in the gas phase reduction of nitrogen oxides with ammonia, in particular to reduce the nitrogen oxide content of combustion exhaust gases. Metallic copper, vanadium, chromium, molybdenum, tungsten, manganese, iron or cerium oxides or sulfates are supported on a shaped carrier containing titanium oxide and clay minerals with an average particle size of 0.1 to 100. It is configured. In particular, clay minerals of the type kaolin can be used. This clay mineral is a necessary interlayer silicate. The use of these clay minerals in amounts up to 15% by weight only slightly increases the strength of the catalyst. Regarding the catalytic effect, these additives in the amounts mentioned play no role. Higher amounts even have a negative effect in this respect and further reduce the resistance of the catalyst to SOX-containing exhaust gases.

This time, if we use kaolin modified by a specified method, it will bring about a synergistic exchange effect with other oxidation catalyst components, improve activity, and reduce the nitrogen acid and arsenic content of combustion exhaust gas, which is a particularly expensive oxidation catalyst component. It has now been found that catalysts of the type mentioned can be obtained which can be removed satisfactorily by the economical use of a reducing agent. In general, it has been found that a higher degree of SOX resistance can also be obtained.

[Problem to be solved by the invention 1 Therefore, an object of the present invention is to
i, Zr, V, W, Mo or Ce as one or more of their oxides to form a layered silicate
It is an object of the present invention to provide a catalyst that can reduce the nitrogen oxide content of combustion exhaust gas containing a combination of a layered silicate and a layered silicate.

[Means for solving the problem 1 According to the present invention, this catalyst has the following characteristics: (a)
A layered silicate is an acid-activated and not yet X-ray amorphous kaolin that still substantially maintains its crystalline layered structure, and (b) has a BET surface area that is at least as large as the BET surface area of the kaolin before acid activation. 15%, preferably at least 5
(C) [1 The atomic ratio between the silicon contained in the activated kaolin and the metal contained in the oxide is between 0.2 and 50;
Preferably it is 0.4-25.

The term "acid activation" of kaolin means in the present invention the partial removal of aluminum from the crystal lattice of the layered ciwate.

Acid activation presumably erodes the crinkle cate layer from its edges and causes ions to elute from the hemiplane phase. The remaining SiO4 tetrahedral phase leads to a change in the orientation of the layer, especially during drying due to the wedging effect and steric hindrance. However, in this case the crystal layer+R structure is not substantially destroyed. An intermediate state is reached between the crystal structure of untreated kaolin, which is present initially or already exists, and the fully destroyed crystal structure of amorphous silicic acid.

Acid activation generally increases the specific surface area measured by the BET method. The high specific surface areas obtained are also obtained after intimate mixing of the acid-activated kaolin and the oxidizing components of the catalyst, even when these components are used in large amounts. On the other hand, as mentioned above, acid activation must not proceed to the extent that only amorphous silicic acid remains.

This is because in this case it is clear that no exchange effect on the oxidizing components of the catalyst takes place, and a considerable decrease in the catalytic activity is observed with increasing silicic acid content.

The reason why particularly highly active catalysts are obtained using acid-activated kaolin is not yet clear in detail, but it appears to be based on the particularly advantageous crystal structure of this material.

It exhibits a synergistic exchange effect with metal fluorides.

However, although no clear explanation of the mechanism for this exchange action has been provided to date, the existence of a silicate layer structure that has been reoriented through acid activation is considered to be an essential premise. That is, if, for example, a silicic acid gel with an equivalent BET surface area is used instead of the acid-activated bioactivated layered silicate according to the invention for reacting with the oxidized metal component, a clear reduction in the catalytic activity occurs.

The acid-activated kaolin used in the present invention preferably has a specific surface area of about 5 to 100 m2/! ; ] range.

Acid dissolution is generally carried out until the SiO2 content of the acid-activated kaolin is increased by at least 5%, preferably at least 10%, over the starting material. S depending on starting material
i02 content is 45-90% by weight, preferably 50-90%
It becomes 65 weight ω%. In this case, acid dissolution is not performed until only X-ray amorphous S i 02 remains. The acid dissolution is discontinued until a certain degree of crystallinity of the acid-activated layered crinkle-1. The proportion of extractable silicates is determined by triangulating with soda solution the washed and dried filter cake obtained after carrying out the acid dissolution.

Outbo, Journal of the Chemical Society of Japan, Vol. 72 (1951), p. 573].

Preferably, the proportion of the pore volume corresponding to macropores with a diameter of >80 nm is preferably at least 25%. In this case, the pore volume is determined by mercury pomametry.

The acid activation of the kaolin-containing starting material can be carried out in a manner known per se, preferably using an aqueous inorganic acid such as, for example, hydrochloric acid or sulfuric acid. However, organic acids such as formic acid and acetic acid can also be used. The acid concentration ranges from 1 to 60% by weight, preferably from 10 to 40% by weight, based on the dry matter content. Preliminary wet sorting of the raw materials has also proven advantageous. The acid treated material is washed with acidified water if necessary and filtered.

The catalyst according to the invention is also characterized by a high degree of resistance to sulfur oxides and also to sulfuric acid, which is due to the acid activation of kaolin. It was confirmed that the catalyst was severely attacked by sulfur oxides or tAu, resulting in mechanical failure and premature catalyst deterioration.

On the other hand, although catalysts prepared using amorphous silicic acid are resistant to sulfur oxides and sulfuric acid, their NOx activity is clearly poor.

Starting compounds for the metal oxide component of the catalyst according to the invention include, on the one hand, the corresponding metal oxides and, on the other hand, substances convertible into metal oxides, such as metals, hydroxides and in particular salts, complex compounds and/or hydroxides. Alcohol or salts derived from these acids are used. these are,
If desired, it can be used together with a reducing agent and/or an additive which acts as a button forming agent.

Examples of cerium include Ce203, CeO2, Ce (
It is possible to use starting from SO4)2 and Ce2 (C204>3. Suitable starting materials for zircon oxide are, in addition to oxidized hydrates, for example zircon salts, zirconyl salts such as Zr (SO4)2.7 rc14, Z r O
(# l and Zr (C204>2.

Starting materials for the tungsten component include, for example, tungsten oxide (e.g. WO3, WIo 029, W401
1, WO2), mono- and poly-tungstic acids, heteropolyacids, tungstates, tungsten halides and tungsten oxyhalides.

Corresponding molybdenum compounds can also be used in place of these tungsten compounds. For vanadium, in particular V205, VO2, V203 and VO are suitable starting compounds, and also ortho- and poly-vanadates or vanadates, vanadium halides and vanadium oxyhalides (e.g. VO(,!3),
Various vanadium or vanadyl salts are also suitable.

Suitable titanium compounds are oxides and oxide hydrates as well as titanium or titanyl salts, especially halides and sulfates. Preferably titanyl sulfate is used. Metal-organic compounds can also be used, such as esters of titanic acid, such as isopropyl titanate.

Metal oxides individually in the following concentration ranges: T i 02 = 10-80 wt% W03 and/or MoO3 = 1-25 wt% V20
It has been found that particularly advantageous results are obtained if 5 = 0.1-25% by weight CeO2 = 1-25% by weight and acid-activated kaolin accounts for the balance of the active ingredient.

In the case of suitable catalysts, the metal oxides are present as binary or especially ternary combinations.

In the case of ternary combinations, the metal oxides are present in the following preferred proportions: (a) (TiO2 + WO3 and/or MOO3 + V205 > = 10780% by weight (b) (TiO2
+CeO2+V205) =10-80 Small hanging% (C) (TiQ2+7r○2+V2O5) -10-8
0 weight 9 also (d) (WO3h yo U/or MoO3+CeO2+2
Q5) = 10-25% by weight (e) (WO3 composition/or MoO3+ZrO2
+\/205 ) = 10-25% by weight, with the acid-activated kaolin accounting for the balance of the active ingredient, respectively.

The weight ratios between the metal oxides in the ternary combination preferably each include the following ranges: (a) WO3 and/or MoO3/T i 02=
0.01-0.25 V205/Ti○2 = 0.01-0.11(b)C
eO2/T i 02 = 0.05-0.23\/2
0s/T i○2 = 0.01-0.11(C)
Z ro2/T i 02 = 0.01-0.
24■205/Ti○2 = 0.01-0.11(d
) CeO2/WO3 and/or MoO3 = 0.
1- 5.0 V20s /WO3 and/or MoO3-0,1-
2,5 (e) V205 /W○3 and/or MoO3=
0.1-2.5 Zr02/WO3 and/or MoO3=0.
1-10° The catalyst according to the invention can be obtained, for example, by impregnating acid-activated kaolin with a solution containing one or more of the above-mentioned metals as salts and/or complexes, followed by calcination.

According to another variant, the catalyst is prepared by mechanically mixing acid-activated kaolin with one or more oxides or salts of the above-mentioned metals (e.g., by milling in a ball mill), and then adding this mixture optionally to the above-mentioned metals. It is obtained by impregnation with a solution containing one or more metals as salts and/or complex compounds, followed by calcination.

Furthermore, the catalyst according to the invention can be prepared by precipitating or re-precipitating 1 tsp. You can also get it.

Precipitation or re-precipitation of compounds containing one or more of the metals mentioned above is carried out in the presence of a mixed suspension of acid-activated kaolin and oxides or salts of one or more of the metals mentioned above. You can also do it with After washing and removing foreign ions, it is fired again.

In this way, near-optimal intimate mixing of the oxidized metal component and acid-activated kaolin is achieved.

When the metal oxide component is composed of multiple metal oxides,
Each starting compound can be precipitated together or sequentially in several steps; the order of the precipitation steps generally influences the catalyst activity and must then be optimized in each individual case.

Of course, it is also reasonable to impregnate the acid-activated kaolin with a solution of the corresponding transition metal compound, if necessary after one or more precipitation steps. In this case, impregnation can be carried out either before or after shaping and calcination of the catalyst.

The catalyst according to the invention can also contain inert support substances. Generally, the catalyst is present as a shaped body;
In particular spheres, tablets, extrudates, elongated or squamous honeycombs (also called "channel lattices"), rods,
Exists as a tube, ring, wheel or saddle.

These molded bodies can be obtained, for example, by tabletting or extrusion pressing the catalyst material, and additives for improving moldability can be mixed therein as required. Additives of this type are, for example, graphite or aluminum stearate.

In addition, additives that improve the surface structure can also be mixed. In this case, for example, an organic material can be used that burns while leaving a pore structure during subsequent firing.

The use of additives to improve moldability is not absolutely necessary. This is because the starting materials can also be bonded with metal components or added with neutral bentonite or other binders such as cement. Shaping is generally carried out with the addition of water or an organic solvent, such as a dihydric or pre-alcohol.

The catalyst according to the invention is generally dried after shaping and heated to about 200-700°C, preferably 300-55°C.
It is fired at a temperature of 0 °C. Additionally, inorganic fiber materials can be added before molding to increase strength. Calcination activates the catalyst and achieves its favorable properties, especially when obtained at the specified temperature ranges mentioned above.

Typical operating procedures for preparing catalysts according to the invention are described in the Examples below.

The invention furthermore provides the use of a catalyst according to the invention using 1NH3 as a reducing agent for reducing the nitrogen oxide content of flue gases which, in addition to the common exhaust gas components, also contain sulfur oxides (SOx). It also relates to

In the case of reduction of NH3, the nitrogen oxide content of the flue gas is reduced by forming N2 and N20. Nitrogen oxides (NOx) include various nitrogen compounds, such as No, N2O3, NO2, N2
05, but mainly No and NO2, especially NO are important.7 The NOx j seismic intensity of the flue gas to be purified can vary over a wide range and is generally
It ranges from pm to 5 volumes@06. The molar ratio of NH3:NOX is generally between 03 and 3, preferably between 0.6 and 1.5, and can be set by means of control technology so that maximum NOx conversion is achieved with as low a N1-1a quenching as possible. . NH3 can be administered either in gaseous form or as an aqueous solution.

The catalyst according to the invention has a nitrogen oxide content of 1) compared to known catalysts.
It is characterized by selective conversion of ammonia, which is preferably used as a base. In the case of the known process, even at high operating temperatures, a considerable part of the ammonia is not consumed for the desired NOx removal, but is oxidized by the oxygen present in the exhaust gas. This results in additional nitrogen formation or lowers the observed NOx conversion between the reactor inlet and outlet, as well as unnecessary N83 consumption.

For denitrification reactions, reactors used for heterogeneous catalytic gas phase reactions are generally suitable in principle, as long as the structural characteristics of the reactor as a whole are suitable for side panels corresponding to high flue gas volumes. ing. The permissible space-time velocity (scale G) is 500 to 20,000 gas per hour per catalyst 11, preferably 100
In the range 0 to 1500 J+ gas, this space-time velocity is calculated for gas at O'C and 1 bar. Hereinafter, space-time velocities will be expressed with a dimension of 11-1 for the sake of simplicity. Suitable reaction temperature is about 200-600℃, preferably 250℃
~43oC. At significantly higher temperatures, oxidation of ammonia by the monooxygen present in the exhaust gas occurs, whereby the ammonia undergoes a reaction with nitrogen oxides, resulting in a lower degree of denitrification. However, this undesirable effect is not as great with the catalysts of the invention as with the known catalysts.

[Examples] Typical examples for the production and use of catalysts according to the present invention will now be described.

In particular, the effectiveness of a catalyst with respect to the removal of nitrogen oxides from gas mixtures also containing oxygen and sulfur oxides is determined by contacting the catalyst with a gas stream, the gas stream being used as a bulk material for removing nitrogen oxides from a gas mixture containing this catalyst. passed through an electrically heated tube. The gas mixture used has the following composition: 02 3% by volume N20 10% by volume N,O 750% by volume f4ppm NO250 ppm by volume NH3800% by volume fTtppm S○2950 ppm by volume 5○3 50% by volume N2 balance to 100% by volume.

The concentrations of the components NO and NO2 in the gas mixture were measured continuously by a certified analyzer before and after passing through the catalyst bulk layer (chemiluminescence method). As a measure of the effectiveness of the catalyst with respect to the reduction of nitrogen oxides, the conversion rate of the component NOx after reaching the stationary state is used, defined by the following formula: NO conversion rate (UNQx) × state of the gas mixture .

Example 1 (a) 2k (SPSI #Soil) having a BET surface area of 11 butts/g and the chemical composition shown in Table 1 was stirred with an aqueous HCf2 solution at 80° C. for 6 hours. The HCi content was approximately 22% by weight, based on the dry substance.The filter cake was filtered with suction and the filter cake was washed with acidified water (HCi).
The pH value was adjusted to 3.5 with 100 ml of water). The acid-activated kaolin (hereinafter referred to as SAK) obtained has 2
It had a BET surface area of 24 butts/g after drying at 00°C. Its chemical composition is also shown in Table 1.

(b) Add 720jJ of T! to 400g of S A K suspension obtained in step (a). 03O4 (titanyl sulfate) was immersed with stirring and neutralized by adding ammonia.

The solids were filtered with suction and washed free of sulfate, 12
It was dried for 15 hours at 0° C. and kneaded with a solution of 5.2 g of ammonium vanadate obtained by addition of ammonium tungstate with a 1.6-fold excess of oxalic acid dihydrate. The amount of solvent was chosen so that a paste-like material that would easily stick to the staff was obtained.

1−( corresponding to a macropore with a diameter of >8() nm
The percentage of pore volume measured by Q-stomatametry is 87.
It was 4%.

Example 2 400 g of kaolin used as starting material in Example 1 was exposed to ammonium tungstate and ammonium vanadate without acid treatment corresponding to the procedure of Example 1(b).

The composition of all catalysts, the reaction temperature and the NOx conversion at a space-time velocity of 5000 h-1 are shown in Table 2.

The above gas mixture was used to carry out the reaction and the NO3 conversion was calculated according to the above formula.

[Effect of the invention 1] According to the present invention, the use of acid-activated kaolin has sufficiently high activity, and the nitrogen oxide content in the combustion exhaust gas can be more effectively reduced without loss of ammonia used as a reducing agent. A reducing catalyst is obtained.

Claims (13)

[Claims] (1) Nitrogen oxidation of combustion exhaust gas containing at least one metal Ti, Zr, V, W, Mo or Ce as one or more of its oxides in combination with a silicate having a layered structure (layered silicate) (b) the BET surface area is is at least 15%, preferably at least 5%, of the BET surface area of the kaolin before acid activation.
(c) the atomic ratio between the silicon contained in the acid-activated kaolin and the metal contained in the oxide is between 0.2 and 50, preferably between 0.4 and 25; A catalyst for reducing the nitrogen oxide content of combustion exhaust gas, characterized in that: (2) the SiO_2 content of the acid-activated kaolin is at least 5%, preferably at least 10%, based on the starting material;
Catalyst according to claim 1, characterized in that it is increased in size. (3) Catalyst according to claim 1 or 2, characterized in that the proportion of the pore volume corresponding to macropores with a diameter >80 nm is at least 25%. (4) The metal oxides individually have the following concentration ranges: TiO_2 = 10-80 wt% WO_3 and/or MoO_3 = 1-25 wt% V_2O_5 = 0.1-25 wt% CeO_2 = 1-25 wt% The catalyst according to any one of claims 1 to 3, characterized in that acid-activated kaolin occupies the balance of the active ingredients. (5) The metal oxide has the following Ξ component combination: (a) (TiO_2+WO_3 and/or MoO_
3+V_2O_5)=10-80% by weight (b)(TiO_2+CeO_2+V_2O_5)=1
0-80% by weight (c) (TiO_2+ZrO_2+V_2O_5)=1
0-80% by weight (d) (WO_3 and/or MoO_3+CeO_
2+V_2O_5)=10-25% by weight (e) (WO_3 and/or MoO_3+ZrO_
2+V_2O_5)=10-25% by weight,
Catalyst according to any one of claims 1 to 4, characterized in that acid-activated kaolin each accounts for the balance of the active ingredient. (6) The weight ratio between the metal oxides of the ternary combination is in the following ranges, respectively: (a) WO_3 and/or MoO_3/TiO_2
=0.01-0.25 V_2O_5/TiO_2=0.01-0.11 (b) CeO_2/TiO_2=0.05-0.23V
_2O_5/TiO_2=0.01-0.11 (c) ZrO_2/TiO_2=0.01-0.24V
_2O_5/TiO_2=0.01-0.11 (d) CeO_2/WO_3 and/or MoO_3
=0.1-5.0 V_2O_5/WO_3 and/or MoO_3=0
．． 1-2.5 (e) V_2O_5/WO_3 and/or MoO_
3=0.1-2.5 ZrO_2/WO_3 and/or MoO_3=0.
The catalyst according to claim 5, characterized in that it contains 1-10. (7) The shaped bodies are present in particular in the form of spheres, tablets, extrudates, elongated or flattened honeycombs, plates, rods, tubes, rings, wheels or saddles. The catalyst according to any one of items 1 to 6. (8) Obtained by impregnating acid-activated kaolin with a solution containing one or more of the metals as salts and/or complex compounds, and then calcining the product. The catalyst according to any one of items 1 to 7. (9) mechanically mixing acid-activated kaolin with oxides or salts of one or more of said metals and then impregnating with a solution containing one or more of said metals as salts and/or complex compounds; The catalyst according to any one of claims 1 to 7, which is obtained by subsequent calcination. (10) Precipitating or reprecipitating at least one compound containing one or more of the metals in the presence of a suspension of acid-activated kaolin, washing away foreign ions, and then calcining. The catalyst according to any one of claims 1 to 7, characterized in that the catalyst is obtained by. (11) precipitating at least one compound containing one or more of the metals in the presence of a mixed suspension of acid-activated kaolin and oxides or salts of one or more of the metals; 8. The catalyst according to any one of claims 1 to 7, which is obtained by subjecting the catalyst to precipitation or reprecipitation, washing and removing foreign ions, and then calcination. (12) Claim 1, characterized in that NH_3 is used as a reducing agent in order to reduce the nitrogen oxide content of combustion exhaust gas that also contains sulfur oxides in addition to general exhaust gas components. The catalyst according to any one of items 1 to 11. (13) 250-600°C, preferably 250-430°C
Temperature range of ℃ and 500 to 20,000 per hour per liter of catalyst
13. The catalyst according to claim 12, which is used at a space-time velocity in the range of 1 combustion exhaust gas.