RU2064828C1 - Catalyst for reduction of nitrogen oxides in industrial waste gasses - Google Patents

Abstract

FIELD: nitrogen oxides neutralization in waste gasses exhaust of industrial facilities - heat and power plants, compression stations, gas processing works etc. SUBSTANCE: catalyst has active phase based on platinum group metal and carrier. As active phase in the case it has 11-tungsten silicates of potassium of general formula K_x[SiW₁₁MO₃₉]_yH₂O, with M = Pd,Rn or Pt. x = 8, y = 4; x = 4, y = 6; x = 5, y = 8; or molybdenum silicates of potassium of general formula K_x[SiMo₁₁MO₃₉]_yH₂O, with M=Pd, Rn or Ft.x = 6, y = 14; x = 4, y = 9; x = 5, y = 12 with following shares of components (mass %): 11-tungsten silicates of potassium or 11-molybdenum silicates of potassium in terms of active metal - 0.05 - 0.35; carrier - balance (up to 100%). In preferable versions as active phase catalyst may have mixture of 11-tungsten silicates of potassium taken in equimolar ratios or mixture of 11-molybdenum silicates of potassium taken in equimolar ratios. Catalyst provides 70 - 80 5 purification of exhausted waste gasses from nitrogen oxides under working temperatures of 400 - 450 C and industrial speeds of gas streams. Catalyst demonstrates high selectivity in process of transformation of nitrogen oxides in exhaust molecular nitrogen - 85 - 90 %. EFFECT: catalyst allows to increase degree of nitrogen oxides neutralization in industrial waste exhausted gasses. 3 cl, 1 tbl

Description

The invention relates to a method for preparing a catalyst for neutralizing nitrogen oxides in gas emissions of a number of industrial facilities - thermal power plants, compressor stations, gas processing plants, etc. It is known that the neutralization of other toxic components, such as CO and hydrocarbons, is currently being carried out quite successfully in the USA, Japan, and a number of Western European countries using catalytic methods. However, the implementation of these methods for the effective neutralization of nitrogen oxides in industrial emissions has proved to be a much more difficult scientific problem in terms of practical resolution. True, in a number of works it was proposed to use the NO reaction with ammonia on platinum catalysts to solve the above problem (see 1. France. Patent N 1205311, 1960 France. Patent N 1233712, 1960. 2. US Patent N 3053613, 1962 3. Industrial palladium catalyst APK-2 (TU-113-03-312-91) in Russia). The catalyst according to the method (1) contains 0.5% wt. Pt and leads the process at relatively low space velocities (up to 1000 h ^-1 ). The catalyst according to method (2) contains close amounts of Pt, Rh, Ru, and the process in this case is carried out in two stages. The first stage at a temperature of 150-400 ^o With Pt-catalyst, the second at 150-900 ^o With Pt, Pd, Rh / Al ₂ O ₃ . High platinum content 2% of the mass. Pd is different and the industrial catalyst APK-2.

 In the presence of serious shortcomings noted above (the problem of efficient operation at high volumetric rates of industrial exhausts, the relatively high cost of the catalyst, and the high sensitivity of the catalytic process to the composition of the reaction mixture), there is a major main obstacle to the widespread introduction of the method of neutralizing nitrogen oxides with ammonia in industrial practice .

The main disadvantage of the above method is the high toxicity of ammonia itself, comparable with the toxicity of NO, which makes it practically unsuitable for most industrial facilities. Indeed, the implementation of the NO + NH ₃ process at industrial facilities where gas emissions amount to mln m ³ / year require the storage of a huge number of tanks with toxic ammonia on the territory of the facilities, as well as providing stringent conditions for their safe operation (i.e., essentially a new environmental problem arises).

In this regard, to solve the industrial task of neutralizing NO, it seems most appropriate to develop a catalytic process for the reaction NO + CH ₄ . Indeed, most industrial facilities of thermal power plants, gas processing plants, compressor stations, etc. operate on natural gas. In addition, the use of natural gas eliminates environmental factors associated with the use of NH ₃ .

In recent years, a number of studies have addressed the problems of creating effective catalysts for neutralizing NO with hydrocarbons (German Patent No. 3642018, 1987; US Patent No. 5149512, 1992). It was also proposed in a number of studies to use for this reaction some industrial samples of catalysts (in particular, palladium APK-2), previously used for the reaction of NO with ammonia. The zeolite-containing catalyst proposed in the FRG Patent for the reaction of NO with propylene operates at temperatures up to 500 ^o C. However, even the use of propylene (more reactive) instead of methane does not allow the authors to achieve high efficiency of the process, the NO conversion on the proposed catalyst does not exceed 50%
In US Patent No. 5149512 (1992), which can be considered the prototype of the present invention, Rh, Cr, Fe, Ag metals in the zeolite matrix are used as the active phase of the catalyst. The reaction of NO with hydrocarbons CH ₄ , C ₃ H _{6 is} carried out at temperatures of 250-600 ^o C. However, despite the very low space velocity of the process 100 h ^-1 , the authors of the patent were not able to achieve high rates of NO conversion, which does not exceed 50- 53% In this case, the efficiency of the NO neutralization process noticeably decreased in the presence of oxygen in the reaction mixture.

The inability to effectively use the proposed catalysts at space velocities of approximately 50 thousand h ^-1 and reaction compositions that meet real conditions, as well as the low selectivity of the process for nitrogen (in the reaction products, along with nitrogen, there are comparable amounts of very toxic nitrous oxide N ₂ O), due to low rates of NO conversion do not allow talking about the industrial effect when using these catalysts.

The objective of the present invention is to develop a catalyst with increased activity (at the level of 70-80% purification) and selectivity (in nitrogen yield), as well as stability at volumetric speeds of the purified gas stream of 50-70 thousand h ^-1 corresponding to industrial conditions.

The problem is solved by the proposed catalyst for the reduction of nitrogen oxides with a methane-containing gas in industrial exhaust gases, including an active phase based on a platinum group metal and a carrier, the distinctive feature of which is that it contains 11 potassium tungstosilicates of the general formula K _x [as an active phase] SiW ₁₁ MO ₃₉ ] • yH ₂ O, where M = Pd, Rh or Pt, x = 6, y = 11; x = 4, y = 6; x = 5, y = 8 or potassium molybdosilicates of the general formula: K _x [SiMo ₁₁ MO ₃₉ ] • yH ₂ O, where M = Pd, Rh or Pt, x = 6, y = 14; x = 4, y = 9; x = 5, y = 12, with the following content of components (% wt.): 11-potassium tungsten silicates or 11-potassium molybdosilicates calculated on the active metal 0.05-0.25; carrier rest (up to 100%)
In preferred embodiments, the catalyst may comprise, as an active phase, a mixture of potassium 11-tungstosilicates taken in equimolar ratios or a mixture of potassium 11-molybdosilicates taken in equimolar ratios.

The catalyst is prepared by impregnating a carrier (substrate), as which may be used alumina, silica-alumina and others. Aqueous solution 11 volframosilikata K _x [SiW ₁₁ MO _39] • yH ₂ O or 11 molibdosilikatov potassium K _x [SiMo ₁₁ MO ₃₉ ] • yH ₂ O, where M, x, y have the above meanings, and subsequent drying at a temperature of 20-100 ^o C.

 The new compounds used as the active phase are oxocomplexes, which are fine crystalline powders of brown color (palladium complex), red color (rhodium complex), orange color (platinum complex), are readily soluble in water and easily applied to the porous base by impregnation.

 An additional property of the invention is due to the fact that the obtained oxo complexes Pd, Rh and Pt contain a large amount of hydrated water, and the presence of OH groups ensures effective interaction of the complexes with the surface of the carrier and high strength of the applied coating.

Another important important property of the invention is related to the fact that the obtained oxo complexes do not contain chlorine at all. The main lattice framework of the complex consists of oxygen atoms included in the coordination polyhedron of the platinoid, which is an unfinished octahedron. The nature of the coordination environment and the coordination unsaturation of platinoid atoms in the structure of the oxocomplex allow the whole process of catalyst preparation to be reduced to single-stage impregnation and subsequent drying at temperatures from room temperature to 70-100 ^o C.

Another property of the invention lies in the fact that the use of new oxo complexes Pd, Rh and Pt for deposition on a porous substrate allows to obtain essentially atomic distribution (dispersion) of platinoid on the surface of the carrier. In this case, it is fundamentally important that such a maximum degree of dispersion of the active metal, close to 100%, is maintained at high temperatures of the catalyst (up to 500 ^o C and above). The obtained result is associated with high stability of the structure of the synthesized crystalline oxo complexes and the preservation of a strong oxygen lattice framework in a wide temperature range (up to 500-800 ^o C).

A structural feature of the oxocomplexes-polyvungstates and polymolybdates of palladium, rhodium and platinum is the presence of structural elements of the central tetrahedron SiO ₄ and 12 octahedra. Oxocomplexes of the composition K ₄ [SiW ₁₂ O ₄₀ ] • yH ₂ O and K ₄ [SiMo ₁₂ O ₄₀ ] • yH ₂ O are known. Replacing one of the W (or Mo) atoms in the octahedron with a platinoid results in the position of the terminal atom the oxygen in the octahedron MO ₆ (where M = Pd, Rh, or Pt), unlike the octahedra WO ₆ (MoO ₆ ), is occupied by a water molecule, which plays the role of an intraspheric ligand. It is quite mobile and in the course of catalytic reactions occurring at elevated temperatures of approximately> 200 ^° C, it easily gives way to reagent molecules in the coordination polyhedron, for example, NO and CH ₄ , which are chemisorbed on platinoid atoms. At the same time, the structural framework of new compounds, consisting of interconnected MO ₆ octahedra, is very strong. Obviously, the polymer nature of these compounds (M> 2500) also plays an important role in the stability of the synthesized oxo complexes at catalysis temperatures of up to 500 ^o C and higher.

To prepare supported catalysts according to a new technological scheme, new oxo complexes of platinum metals of the following composition were synthesized:
K ₆ [SiW ₁₁ PdO ₃₉ ] 11H ₂ O K ₆ [SiMo ₁₁ PdO ₃₉ ] 14H ₂ O
K ₄ [SiW ₁₁ PtO ₃₉ ] 6H ₂ O K ₄ [SiMo ₁₁ PtO ₃₉ ] 9H ₂ O
K ₅ [SiW ₁₁ RhO ₃₉ ] 8H ₂ O K ₅ [SiMo ₁₁ RhO ₃₉ ] 12H ₂ O
The preparation of new oxo complexes is a common complexation reaction in solutions. The synthesis of polytungstates (and polymolybdates) was carried out according to the following scheme. To a 0.01 M solution of, for example, potassium 11-tungstosilicate, PdCl _{2 was} added as a saturated solution in the ratio Pd: SiW = 1: 1. The reaction mixture was heated at a temperature of 80-90 ^° C for 2 hours with stirring. The pH of the solution of the reaction mixture is maintained by adding HCl (1: 5) of about 4. Brown crystals of K ₆ [SiW ₁₁ PdO ₃₉ ] 11H ₂ O are isolated from the solution upon evaporation. Oxocomplexes of platinum and rhodium are synthesized in the same way (in this case, they use the complexation reaction, respectively salts of PtCl ₄ and RhCl ₃ ), see R.A. Gazarov et al. (A.S. USSR N 1043876, 1983)
The synthesized heteropoly complexes of platinoids were used to prepare the catalysts according to the new simple one-stage scheme. The process of preparation of the catalyst is reduced to the impregnation of the granules (or block) of the carrier, for example, alumina with an aqueous solution of polyvungstate (polymolybdate) palladium (or platinum, or rhodium). The concentration of the impregnating solution of 0.5-10% wt. Granules (or block) of the carrier are kept in a solution of polytungstate (or polymolybdate), for example, for 2 hours at a temperature of 70-100 ^o With subsequent drying at 100-110 ^o C. The concentration of the active metal Pd (Pt, R) or ΣPd, Rh, Pt in the catalyst may be from 0.05 to 0.25% wt.

Example 1. Preparation of a catalyst for the reduction of NO with methane. 0.45 g of salt K ₆ [SiW ₁₁ PdO ₃₉ ] 11H ₂ O is dissolved in 10 ml of water. The granules of the g-alumina support are immersed in the prepared solution and kept at a temperature of 100 ^{° C.} for 2 hours, then dried. The resulting catalyst composition of 7.55% set. salts of K ₆ [SiW ₁₁ PdO ₃₉ ] and 92.45% wt. Al ₂ O ₃ is tested for activity (concentration of the active metal Pd in the sample 0.25% wt.).

Example 2. 0.09 g of salt K ₆ [SiW ₁₁ PdO ₃₉ ] 11H ₂ O and 0.09 g of salt K ₅ [SiW ₁₁ RhO ₃₉ ] 8H ₂ O are dissolved in 10 ml of water. A block carrier (cordierite with a sublayer of γ-alumina) is immersed in the prepared solution and kept at room temperature for one day, then dried. After that, the resulting catalyst composition of 1.5% wt. salts of K ₆ [SiW ₁₁ PdO ₃₉ ] 11H ₂ O, 1.5% salts of K ₅ [SiW ₁₁ RhO ₃₉ ] 8H ₂ O and 97% wt. Al ₂ O ₃ is tested for activity (the concentration of active metals ΣPd, Rh in the sample is 0.05% wt.).

A distinctive feature of the synthesized new catalysts based on Pd, Pt, and Rh oxo complexes is the presence of a rigid oxygen crystal lattice framework (in the complete absence of chlorine atoms in the compounds), coordination unsaturation of platinoid atoms in the structure, which makes it possible to directly use these compounds as a catalytically active components after the stage of impregnation of the carrier without any operations on the high-temperature location of the applied complex or subsequent reconstitution the formation of complexes with hydrogen (or another reducing agent) until the appearance of a phase of metallic palladium (platinum, rhodium). When using synthesized oxo complexes, atomic dispersion of platinoids on the surface of the support is also provided. Moreover, the high stability of the oxygen skeleton in the polytungstates (polymolybdates) of palladium, platinum, and rhodium deposited on the substrate, in turn, ensures high stability of atomic dispersion of platinoids even at temperatures of 500-800 ^o C. It is characteristic that, according to the results of our studies, conducting rigid oxidation -vosstanovitelnyh cycles of heat treatment at 500 ^o C (alternating cycles.. oxygen treatment for 2 hours, and then reduction with hydrogen for 2 hours, blowing an inert gas, etc.) does not lead d-marked sintered metal that remains on the carrier surface preferably in the form of isolated atoms. The atomic dispersion of d-metal, achieved in new catalysts, gives an obvious advantage over the known methods for the preparation of platinum catalysts, which today provide a degree of dispersion of not higher than 20-40%
A very important feature of the invention is also the high thermal stability of the highly dispersed state of the d-metal, manifested in the constant presence of water vapor in the gas stream fed to the catalyst. This advantage of new catalysts is fundamentally important for practice, since gas streams processed on industrial catalysts always contain a large amount of water vapor (up to 10% vol. And more). At the same time, it is known that even for the most stable highly dispersed catalysts developed today that contain d-metals, for example, iron-containing zeolites, short-term steam treatment at temperatures of 400-500 ^o С after a short time (about 1 h) leads to the destruction of the active phase and its sintering with the formation on the surface of the carrier of the inactive phase of massive iron oxide.

The results of comparative tests of prepared samples of palladium and palladium-rhodium heteropolyacid catalysts, as well as industrial catalyst APK-2 are presented in the table. The tests were carried out on a flow-through installation in an oxide-recovery. the reaction of CH ₄ + NO at a space velocity of 40 thousand h _-1 and the following composition of the reaction mixture: 2.5% vol. O ₂ ; 0.1% vol. NO; 1.0% vol. CH ₄ ; 7.0% vol. H ₂ O (pairs); rest N ₂ .

The test results shown in the table indicate a significant superiority of the new catalysts. Samples of heteropolyacid catalysts show significantly higher activity at lower process temperatures and provide an effective 70-80% purification of nitrogen oxides at operating temperatures of 400-450 ^o C. In addition, new catalysts exhibit high selectivity in the process of NO conversion (molecular nitrogen yield) in comparison with known catalysts, which are characterized by the presence in the reaction products along with molecular nitrogen of approximately equivalent amounts of very toxic nitrous oxide N ₂ O.

Claims (2)

1. A catalyst for the reduction of nitrogen oxides in industrial exhaust gases with a methane-containing gas, comprising an active phase based on a platinum group metal and a carrier, characterized in that it contains potassium II-tungstosilicates of the general formula as an active phase
K _x [SiW _{1 1} MO _{3 9} ] • уН ₂ О,
where: M Pd, Rh or Pt, x 6, y 11; x 4; y6; x 5, at 8 or II potassium molybdosilicates of the general formula
K _x [SiMO _{1 1} MO _{3 9} ] • уН ₂ О,
where M Pd, Ph or Pt, x 6, y 14; x = 4, y 9; x 5, y 12 at the following content of components, in wt. II potassium tungsten silicate or potassium II molybdosilicate based on the active metal 0.05 0.25
Media Else
2. The catalyst according to claim 1, characterized in that it contains a mixture of potassium II-tungstosilicates, taken in equimolar ratios.  3. The potassium catalyst according to claims 1 and 2, characterized in that it contains a mixture of potassium II-molybdosilicates, taken in equimolar ratios.

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