DE3529060A1 - Catalyst for reducing the nitrogen oxide content of combustion gases - Google Patents

Abstract

Catalyst for reducing the nitrogen oxide content of combustion gases, which catalyst contains at least one of the metals Ti, Zr, V, W or Ce in the form of one or more of their oxides. The active component of the catalyst is a combination of one or more of these oxides with an acid alumosilicate having a sheet structure.

Description

The invention relates to a catalyst for reduction the nitrogen oxide content of combustion gases.

When fossil fuels are burned, nitrogen oxides (NO _x ) are formed both from nitrogenous components and from atmospheric nitrogen, which get into the atmosphere and pose a serious environmental impact. In particular, the formation of the photochemical "smog" is attributed to NO _x . In view of the continuously increasing NO _x emissions in recent years, there is great interest in reducing the NO _x content of combustion exhaust gases.

It is known that nitrogen oxides can be converted by NH ₃ into N ₂ and H ₂ O and that this reaction is quite selective over a wide temperature range, ie in the presence of a high excess of oxygen (as is usually present in combustion exhaust gases) without excessive ammonia losses takes place due to oxidation, so that only relatively small amounts of reducing agent are required. Various catalysts for NO _x reduction using ammonia are also already known.

So z. B. the use of molecular sieves to catalyze the NO _x reduction using NH ₃ known. For example, according to DE-OS 30 00 383, a clinoptilolite is used as the catalyst in the hydrogen form, which has been obtained by ion exchange of a naturally occurring clinoptilolite with an ammonium nitrate solution and subsequent washing with a strong acid.

Furthermore, a catalyst is described in DE-OS 33 28 653, which consists of a ceramic molecular sieve, whose passage cross-sectional diameter in an order of magnitude below the critical molecular diameter from ammonia to above the critical Molecular diameter of nitrogen.  

The catalytic activity of these molecular sieve catalysts is therefore determined by certain pore structures. With these catalysts, there is a risk that the crystal structure will be impaired at high reaction temperatures under the action of the water vapor contained in combustion gases in high concentration, while at low temperatures the willing absorption of the water vapor leads to a reduction in activity. A serious disadvantage appears above all that when using molecular sieves such as H-mordenite and H-clinoptilolite, the pollutant N ₂ O is formed in a considerable concentration, such as. B. from the work of JR Kiovsky, PB Koradla and CT Lin. in Ind.Eng.Chem.Prod.Res. Dev. 19 (1980) 218.

DE-AS 24 10 175 also discloses a catalyst composed of oxides of vanadium, molybdenum and / or tungsten with stoichiometry V _{12- x - y} Mo _x W _y , where x and y are given by the relationships 0 x 8.0 y 5 and 0.3 ( x + y ) 8 are set.

It is also a method from DE-AS 24 58 888 for the reductive destruction of nitrogen oxides in exhaust gases known in which a nitrogen oxide, molecular oxygen and ammonia-containing gas mixture with a catalyst composition is brought into contact as essential components (A) titanium in the form of oxides in an intimate mixture with (B) iron or vanadium contains in the form of oxides.

Such catalysts now obviously have the disadvantage that the contained as catalytically active components relatively expensive transition metal compounds to only one low degree can be exploited because their distribution is not is optimal. Although z. B. the aforementioned DE-AS 24 58 888 before, the active component through inert, solid carriers stretch, which undoubtedly improves the economy  is, but there is a dilution with inert material there is a risk that the catalytic Activity is greatly reduced.

As is well known, properties such as porosity and specific surface area are of great importance for assessing the suitability of a combustion medium. If you use e.g. B. γ-Al ₂ O ₃ or diatomaceous earth with a high surface area, on the one hand an adequate distribution of the active components is to be expected, on the other hand, however, a reduction in the observed reaction rate must be expected, since such surface-rich carriers are predominantly microporous, which means that the diffusion determines the rate becomes. In contrast, predominantly macroporous carriers, such as α-Al ₂ O ₃ , promote mass transport, but their specific surface area is only a few m ² / g, so that the dispersion of the active component is insufficient.

Finally, from DE-OS 34 38 367 is a catalyst to reduce the nitrogen oxide content of combustion gases known by selective reduction, which from (A) 80 to 95 wt .-% of a sulfur oxide-containing catalytic Oxides that u. a. by thermal treatment of an aqueous Oxide compound of titanium or silicon available , (B) 0 to 5% by weight of a vanadium oxide-containing catalytic oxide and (C) 1 to 15 wt .-% of a catalytic Oxides, e.g. B. of tungsten exists.

The formation of a solid acid composed of SiO ₂ and TiO ₂ , the acidity of which is modified by the treatment with sulfuric acid or ammonium sulfate, is regarded as essential for this catalyst. The solid acid distribution is considered essential for controlling the NH ₃ adsorption on the catalyst surface and thus for improving the catalytic activity.

SiO ₂ is used in the form of silica sol. As is known, silica gels are obtained from SiO ₂ sols, which are characterized by high BET surfaces and high porosity, but the proportion of macropores is low, which has the negative effects on mass transport described above. A typical commercial silica gel shows the following pore distribution

Pore diameter (nm) Pore volume (%) 7.5 - 1454.314 - 8024.380 - 175020.6

≦ λτ 17500.8

It is also known from the literature that the characteristic properties of binary or ternary solid acids, especially their acidity, are strongly dependent on the atomic ratio of the metals involved. (M. Itoh, M. Mattori and K. Tanabe, J. Catal. 35 (1974) 225). It must therefore be assumed that a high catalytic activity is limited to a narrow concentration range of the SiO ₂ .

The invention is therefore based on the object Catalyst for the reduction of the nitrogen oxide content of combustion exhaust gases to provide the optimal catalytic effectiveness with simultaneous special economic use of the active ingredients guaranteed.

Surprisingly, this task can be accomplished by one Solved catalyst, at least one of the Metals Ti, Zr, V, W or Ce in the form of one or contains several of their oxides and characterized by is that the effective component of the catalyst a combination of one or more of these oxides an acidic aluminosilicate with a layer structure.  

Preferably, the atomic ratio between the im acidic aluminosilicate containing silicon and the (the) in the metal (s) contained in the oxide (s) 0.2 to 50, preferably 0.4 to 25.

The subject of the older German patent application P 35 24 160.8 is a catalyst for reducing the nitrogen oxide content of combustion exhaust gases, which contains an acidic aluminosilicate with a layer structure as an effective component. Such a catalyst, which can be obtained, inter alia, by treating suitable starting materials containing phyllosilicate with aqueous mineral acid, has a high activity for the reduction of nitrogen oxides by means of ammonia to H ₂ O and N ₂ . It has now surprisingly been found that the combination of an acidic aluminosilicate with a layer structure and one or more oxides of at least one of the metals mentioned above has a higher catalytic activity than either the acidic aluminosilicate or the corresponding metal oxides separately.

At the present time, no clear mechanistic explanation for this synergistic interaction can be given, but the presence of the silicate layer structure seems to be a necessary prerequisite. The steric and electrostatic conditions of the interlayer activities are apparently modified by the presence of the metal oxides introduced in such a way that the adsorption of the NH _{3 takes place} with its binding strength which is favorable for the further catalytic process. If, instead of the acidic sheet silicate according to the invention, either a BET surface corresponding to silica gel or a sheet silicate whose crystal structure has been destroyed by high-temperature treatment is used for the reaction with the metal components, the catalytic activity will decrease significantly.

Another aspect of the synergistic interaction the bimodal pore structure must be considered. The The catalyst of the invention has a pronounced one Maximum pore diameter distribution in the area the macropores, so that a rapid mass transfer is guaranteed is. These are referred to as macropores a pore diameter of ≦ λτ80 nm. The proportion of the pore volume which on pores with a diameter of ≦ λτ80 nm is absent, is preferably at least 55%.

On the other hand, there are sufficient quantities Mesopores have a high specific surface area for the catalytic conversion available. The Mesopores generally have a pore diameter of about 14 to 80 nm.

Acid alumosilicates with a layer structure are understood for example those of the smectite type, in particular of the montmorillonite type. This includes the natural ones acidic alumosilicates or the artificial ones, by acid treatment of natural neutral aluminum silicates with Layer structure obtained products. In the first category for example, the Fuller earths and the Japanese acidic clays. The products in the second category can by treating a neutral layered silicate Starting material with aqueous acids, especially mineral acids, while lowering the cations of the Main groups I to III of the periodic table can be obtained.

The acid treatment for transferring the neutral layered silicate Starting materials in the "acidic" Aluminum silicates can be carried out in a manner known per se are, preferably aqueous mineral acids such as hydrochloric acid or sulfuric acid can be used. But you can also organic acids such as formic acid and acetic acid, use. The acid concentration is in the range of 1 to 60% by weight, based on the dry matter content, preferably in the range of 10 to 40% by weight. A previous one Reclassification of the raw material can turn out to be prove advantageous. The acid treated mass is optionally  washed with acidified water, its pH is between 1 and 7, preferably between 2 and 4, and filtered off.

The acid treatment is generally carried out for as long as until the aluminosilicate has an ion exchange capacity from about 30 to 150 meq / g. Because this is easily determinable is, you have a good way of using the catalyst to adapt to the respective conditions.

As starting compounds for the metal oxide component of the catalyst of the invention is used once the corresponding metal oxides and the other in the Metal oxide convertible substances, e.g. B. the metals, the hydroxides and especially the salts, complex compounds and / or the oxygen acids or those of these acids derived salts. These can if necessary together with one as a reducing agent and / or complexing agent acting additive can be used.

Cer can z. B. starting from Ce ₂ O ₃ , CeO ₂ , Ce (SO ₄ ) ₂ , Ce (CO ₃ ) ₂ and Ce ₂ (C ₂ O ₄ ) ₃ can be used. Suitable starting materials for zirconium oxide are in addition to the oxide hydrates such. B. the zirconium and zirconyl salts such as Zr (SO ₄ ) ₂ , ZrCl ₄ , ZrOCl ₂ and Zr (C ₂ O ₄ ) ₂ .

As starting substances for the tungsten component are, for. B. tungsten oxides such as WO ₃ , W ₁₀ O ₂₉ , W ₄ O ₁₁ , WO ₂ , mono- and poly-tungstic acids, heteropolyacids, tungstates, tungsten halides and oxyhalides.

In the case of vanadium, V ₂ O ₅ , VO ₂ , V ₂ O ₃ and VO are suitable starting compounds, as well as ortho- and polyvanadic acid or vanadates, vanadium halides and oxyhalides, such as, for. B. VOCl ₃ , various vanadium or vanadyl salts.

Suitable titanium compounds are in addition to the oxides and Oxide hydrates, the titanium or titanyl salts, in particular the halides and the sulfates. Titanyl sulfate is preferred used. Organometallic compounds can also be used are used, e.g. B. esters of titanium acid, such as isopropyl titanate.

The catalysts of the invention are, for example by soaking the acidic aluminosilicate with a solution, which one or more of the metals mentioned in the form of Contains salts and / or complex compounds, and subsequent Calcining available.

Furthermore, the catalysts of the invention can Failure or overthrow of at least one connection, one contains or more of the metals mentioned, in the presence a suspension of the acidic aluminosilicate, washing out the Foreign ions and subsequent calcination obtained will.

In this way, an almost optimal dispersion is achieved the metal oxide component in the crystal lattice of the acidic Aluminum silicate with a layered structure.

If the metal oxide component consists of several metal oxides, the respective starting compounds can either together or in several steps in succession be felled, the order of the precipitation steps usually affects the catalytic activity and must be optimized in individual cases. It can however, also prove to be useful, the layered silicate, if necessary after one or more precipitation steps, with the solution of a corresponding transition connection to impregnate. The impregnation can both before and after shaping and calcining of the catalyst. According to another variant become the metal compounds or a part thereof by intimate mechanical mixing (e.g. by grinding in a ball mill) with the layered silicate contacted.  

The catalyst of the invention is usually as a solid material, d. H. it does not become an inert Carrier used. The catalyst is usually located as a shaped body, in particular in the form of balls, Tablets, extrusions, elongated or flat honeycomb bodies (the latter are referred to as "channel grids"), Rings, car edges or saddle bodies.

These moldings can, for. B. by tableting or Extrusion of the catalyst mass can be obtained, wherein any additives that improve formability, can be added. Such additives are, for example Graphite or aluminum stearate. Additives, which improve the surface structure, are added. This is e.g. B. organic substances, that left behind in the subsequent calcination burn a porous structure.

The use of additives that improve ductility, is not absolutely necessary, as that as Base material used aluminosilicate, even if it was in an intimate mixture with the metal component is present, has plastic deformability. It neutral bentonites or other binders, such as kaolin or cements. The Shaping is generally done with the addition of water or organic solvents, e.g. B. of one or polyhydric alcohols.

The catalysts of the invention are usually dried after shaping and at temperatures from about 200 to 700 ° C, preferably from 300 to 550 ° C calcined.

The catalyst is activated by the calcination and achieves its beneficial properties in this way, especially if the temperature ranges given above be respected.  

Typical procedures in the manufacture of the invention Catalysts are in the examples described.

The invention also relates to the use of the catalysts according to the invention for the reductive reduction of the nitrogen oxide content of combustion exhaust gases which, in addition to the usual exhaust gas components, also contain sulfur oxides (SO _x ), NH _{3 being} used as the reducing agent.

When reducing with NH ₃ , the nitrogen oxide content of the combustion exhaust gases is reduced by the fact that N ₂ and H ₂ O are formed. As nitrogen oxides (NO _x ) are the various oxygen compounds of nitrogen, such as. B. NO, N ₂ O ₃ , NO ₂ , N ₂ O ₅ ; however, they are predominantly NO and NO ₂ , with NO predominating.

The NO _x concentration of the exhaust gases to be cleaned can vary within wide limits; it is generally in the range from 100 vol ppm to 5 vol%. The molar ratio NH ₃ : NO _x is generally 0.3 to 3, preferably 0.6 to 1.5, and can be adjusted by means of control measures in such a way that maximum NO _x conversions with the lowest possible NH ₃ slip be achieved. The NH ₃ can be metered in both in gaseous form and in the form of an aqueous solution.

In principle, all reactors usually used for heterogeneously catalyzed gas phase reactions are suitable for the denitrification reaction, provided the design features of the reactor permit the throughput of correspondingly high flue gas volume flows. Permissible space velocities ( RG ) are in the range from 500 to 20,000, preferably between 1000 and 15,000 liters of gas per hour and liter of catalyst, these space velocities relating to a gas at 0 ° C. and 1 bar. For the sake of simplicity, the space velocity is referred to below with the dimension h ^-1 . Suitable reaction temperatures are in the range from about 200 to 600 ° C, preferably from 270 to 430 ° C. At considerably higher temperatures, the ammonia can be oxidized by the oxygen contained in the exhaust gas, which removes the ammonia from the reaction with the nitrogen oxides, so that the degree of denitrification can decrease.

The following are typical examples of manufacturing and application of the catalysts of the invention.

The effectiveness of the catalysts with regard to the removal of nitrogen oxides from gas mixtures which also contain oxygen and sulfur oxides, among other things, is determined by contacting the catalyst with a gas stream which is passed through an externally electrically heated pipe containing this catalyst as bulk material. The gas mixture used has the following composition:
O ₂ 2% by volume
H ₂ O 10 vol.%
NO 750 vol ppm
NO ₂ 50 vol ppm
NH ₃ 800 vol ppm
SO ₂ 950 vol ppm
SO ₃ 50 vol ppm
N ₂ difference with respect to 100 vol .-%.

The concentration of the components NO and NO ₂ in the gas mixture was continuously measured before and after passing through the catalyst bulk layer through a calibrated analyzer (chemiluminescence method). The degree of conversion of the components NO and NO ₂ after setting the steady state serves as a measure of the effectiveness of the catalysts with regard to the reduction of the nitrogen oxides, defined by the following relationships:

The symbols C _NO and C _{NO2 denote} the concentrations of NO and NO ₂ , the superscripts E and A relating to the state of the gas mixture before and after passing through the catalyst.

example 1

320 g (2 mol) of TiOSO _{4 are introduced} into a suspension of 172 g of H-montmorillonite in 5 l of water and neutralized with half-concentrated ammonia while stirring. The solid is filtered off with suction, washed free from sulfate and with a solution of 2.4 g (20.5 m mol) of NH ₄ VO ₃ , 4 g of oxalic acid dihydrate and 16.5 g (5.6 m mol) of ammonium metatungstate in 50 ml of H ₂ O. and 10 ml of glycerol are milled in a ball mill for 1 h. The mass obtained is extruded into extrusions of 3 mm in diameter, dried at 150 ° C. for 2 hours and then calcined at 450 ° C. for 4 hours. The catalyst produced in this way contains the metals Si, Ti, W and V in an atomic ratio of 108: 106: 3.5: 1. The pore volume determined by the mercury penetration method is 0.68 ml / g and has the following distribution:

Diameter (nm) Volume (%) 17503.880 - 175057.014 - 8020.3
7.5 - 1419.0

The extrusions (3 mm in diameter), which have been shredded to a length of approx. 5 mm, are subjected to an effectiveness test under the conditions described ( RG : 5000 h ^-1 )

Example 2

380 g of TiCl ₄ are hydrolyzed with 2 l of ice water. 170 g of H-montmorillonite are introduced and neutralized with semi-concentrated ammonia. The solid is filtered off with suction, washed free of chloride at 60 ° C. and processed further in the manner described in Example 1. The following NO _x conversions are obtained:

Comparative example

5 l of a sulfuric acid solution of 320 g TiOSO ₄ are neutralized without the addition of the acidic montmorillonite using semi-concentrated ammonia. The titanium oxide hydrate is filtered off and washed free of sulfate; then the preparation is continued as described in Example 1. The catalyst not produced according to the invention produced in this way contains the metals titanium, tungsten and vanadium in an atomic ratio of 106: 3.5: 1 and has the following pore distribution: The following sales are determined in the activity test:

The H-montmorillonite used in Examples 1 and 2 was obtained as follows:

2 kg of one by hydroclassification in aqueous suspension obtained raw bentonite fraction of the grain size = 50 microns with 8 liters of an aqueous HCl solution for 6 hours 80 ° C stirred. The HCl content was approx. 21% by weight, based on the dry matter. You suck off and washes the filter cake abundantly with acidified Water (adjusted to pH 3.5 using HCl).

Claims (11)

1. Catalyst for reducing the nitrogen oxide content of Combustion gases containing at least one of the Metals Ti, Zr, V, W or Ce in the form of one or more their oxides, characterized in that the effective Component of the catalyst a combination of one or using several of these oxides with an acidic aluminosilicate Represents layer structure. 2. Catalyst according to claim 1, characterized in that that the atomic ratio between that in acidic aluminosilicate contained silicon and the (the) in the (the) oxide (s) contained metal (s) 0.2 to 50, preferably 0.4 to 25. 3. Catalyst according to claim 1 or 2, characterized in that that an acidic aluminosilicate from smectite, in particular of the montmorillonite type is used.   4. Catalyst according to one of claims 1 to 3, characterized characterized in that the acidic aluminosilicate by treatment of a layered silicate-containing starting material with aqueous Mineral acid while lowering the cation content Main groups I to III of the periodic table have been obtained is. 5. Catalyst according to one of claims 1 to 4, characterized characterized in that the proportion of the pore volume which the pores with a diameter of ≦ λτ 80 nm, is at least 55%. 6. Catalyst according to one of claims 1 to 5, characterized characterized in that it is used as a shaped body, in particular in Form of balls, tablets, extrusions, elongated or flat honeycomb bodies, rings, wagon wheels or Saddle bodies are present. 7. A catalyst according to claim 6, characterized in that that the shaped bodies by tableting or extrusion, optionally with the addition of the deformability, the mechanical strength and / or the surface structure improving additives are available. 8. Catalyst according to one of claims 1 to 7, characterized characterized by soaking the acidic aluminosilicate with a solution that one or more of the above Metals in the form of salts and / or complex compounds contains, and subsequent calcination is available. 9. Catalyst according to one of claims 1 to 7, characterized characterized that he at least by failing or falling over a compound that includes one or more of the above Contains metals in the presence of a suspension of the acid Aluminum silicate with a layered structure, washing out the foreign ions and subsequent calcination is available.   10. Use of the catalyst according to one of claims 1 to 9, for the reductive reduction of the peroxide content of combustion exhaust gases, which contain sulfur oxides in addition to the usual exhaust gas components, NH _{3 being} used as the reducing agent. 11. Application according to claim 10 in the temperature range of 200 to 600 ° C, preferably from 270 to 430 ° C, and at a space velocity in the range of 500 to 20,000 Liters of combustion exhaust gas per hour and liter of catalyst.