WO2008151823A2 - Zeolite catalyst for removing nitrogen from exhaust gases - Google Patents

Abstract

The invention relates to alternative DeNOx catalyst systems for treating exhaust gases containing nitrogen oxides, said catalyst comprising a mass catalyst which consists of a zeolite and is an extrudate in the form of a honeycomb body. The invention also relates to uses of said catalyst system.

Description

 Zeolite catalyst for the denitrification of exhaust gases

The present invention relates to a DeNOx catalyst system for treating nitrogen oxides-containing exhaust gases comprising a bulk catalyst of a zeolite, wherein the bulk catalyst is an extrudate in the geometry of a honeycomb body.

Nitrogen oxides that are produced during combustion processes are among the main causes of acid rain and the associated environmental damage, and are the cause of the so-called summer smog, which leads to damage to health. Their emission should be prevented by removing them from the exhaust gases before they are released to the environment.

Sources of nitrogen oxide emissions into the environment are mainly motor vehicle traffic and incinerators, in particular power plants with furnaces or stationary internal combustion engines and waste incineration plants.

A reduction in NO _x emissions can, for. B. in automotive engines by engine-side settings or power plants with boiler firing by using very pure fuels or by optimizing the combustion systems are achieved, but these firing measures both technical and economic limits set. The aim is the most complete removal of NO _x and N ₂ O with little technical effort by novel catalyst systems. The denitrification of exhaust gases is also referred to as DeNOx. For exhaust gas treatment of stationary applications, such as in power plant, combustion or combustion processes, as well as in automotive technology, selective catalytic reduction (SCR) is one of the most important DeNOx techniques. Hydrocarbons (HC-SCR) or ammonia (NH ₃ -SCR) or NH ₃ precursors such as urea (AdBlue®) are usually used as reducing agents.

In the selective catalytic reduction, the principle is based on the fact that selected reducing agents selectively reduce nitrogen oxides in the presence of oxygen. Selective here means that the oxidation of the reducing agent is preferred (selective), which takes place with the oxygen nitrogen oxides and not with the molecular oxygen present in the exhaust much more abundant. Ammonia or ammonia precursors have proven themselves as reducing agents with the highest selectivity.

Typically, in automotive applications, urea is used in particular because of its non-toxicity, which likewise has very good solubility in water and can therefore simply be added to the exhaust gas as water solution to be metered.

Before the actual SCR reaction, ammonia must first be formed from urea. This is done in two reaction steps, collectively referred to as the hydrolysis reaction. First, NH ₃ and isocyanic acid are formed in a thermolysis reaction:

(NH ₂ ) CO → NH ₃ + HNCO (thermolysis)

Subsequently, in a hydrolysis reaction, the isocyanic acid is reacted with water to give ammonia and carbon dioxide. HNCO + H ₂ O → NH ₃ + CO ₂ (hydrolysis)

To avoid solid precipitates, it is necessary that the second reaction takes place sufficiently quickly by selecting suitable catalysts and sufficiently high temperatures (from 250 ° C.). At the same time, modern SCR catalysts assume the function of the hydrolysis catalyst.

The ammonia produced by the thermohydrolysis reacts on the SCR catalyst according to the following equations:

4NO + 4NH ₃ + O ₂ → 4N ₂ + 6H ₂ O (1)

NO + NO ₂ + 2NH ₃ → 2N ₂ + 3H ₂ O (2)

6NO ₂ + 8NH ₃ → 7N ₂ + 12H ₂ O (3)

At low temperatures in the exhaust system (<300 °), the conversion proceeds predominantly via the reaction 2. For a good low-temperature conversion, it is therefore necessary to set a NO ₂ : NO ratio of about 1: 1. Under these circumstances, the reaction 2 can already take place at temperatures from 170 ° to 200 °.

In general, catalysts consist either completely of the catalytically active component, which are what are known as bulk catalysts, which are also known to the person skilled in the art as unsupported catalysts, or the actual active component is applied to a carrier material, then it is referred to as coating catalysts. Catalyst systems can be broadly distinguished in powder, shaped body and monolith catalysts, which are optionally coated. The shaped-body catalysts mostly consist of ceramic particles or zeolites which are extruded into shaped bodies. Typical dimensions are here z. B. cylindrical moldings of 1.5 to 2.5 mm in diameter and 1.0 to 5.0 mm in length. The ceramic materials are often alumina or silica. These catalysts are mostly used in fixed bed reactors. Important parameters of fixed bed reactors are, in addition to the catalytic activity, the bulk density, the pressure loss properties and the surface of the catalyst. Furthermore, the design effort that is necessary for the preparation of the catalyst, an important economic parameter.

In the case of monolith catalysts, a honeycomb body is produced, which is optionally coated with a so-called washcoat. The main body consists in this case mostly of a mineral ceramic, e.g. Cordierite, or a metal. The washcoat is a powder suspension which i.a. Contains ceramic powder to obtain a large surface area. This powder suspension is applied to the honeycomb, dried and then impregnated with an active component and then activated by calcination. Important parameters here, in addition to the catalytic activity, are the cell density, i. the number of channels per inflow unit and pressure loss at the catalyst. Other parameters are the surface of the catalyst and the design effort for the preparation of the catalyst.

Essentially, two catalyst systems are used to remove nitrogen oxides and nitrous oxide from exhaust gases. In one system, the catalytically active component consists of the oxides of vanadium or titanium (V / Ti oxides), in a zeolite system. In the SCR catalysts, metal-exchanged zeolites (also called metal-doped zeolites) have proven to be active SCR catalysts employable over a wide temperature range. They are mostly non-toxic and produce less N ₂ O and SO ₃ than the usual catalysts based on V ₂ Os.

Both catalyst systems, d. H. The system based on zeolites and that based on V / Ti oxides are characterized in practice by, among other things, different catalyst geometries. In the zeolite catalysts, the catalyst packages are made from cylindrical extrudates. For V / Ti oxide catalysts, honeycomb extrudates of various dimensions are typically used in which the channels have a square footprint.

Depending on the catalyst geometry, two different reactor concepts are realized. Cylindrical extrudates are commonly used in fixed bed reactors with one or more catalyst beds. Fixed bed reactors are characterized by a low geometric surface and because of the high flow resistance by a high pressure drop and by low design effort. Honeycomb catalysts are commonly used in so-called frame reactors. Frame reactors are characterized by a high surface area, by low pressure loss and by a high design effort in the production.

The object of the present invention was to provide a zeolite-based catalyst system for removing nitrogen oxides and nitrous oxide from an exhaust gas stream having a high surface area and exhibiting a low pressure drop, wherein the catalyst system is to be produced with a low design cost. The object is achieved by a catalyst system for the treatment of nitrogen oxides-containing exhaust gases, which comprises a mass catalyst of a zeolite, wherein the mass catalyst is an extrudate which assumes the geometry of a honeycomb body and wherein the weight fraction of zeolite in the extrusion mass of the extrudate between 20 and 95%, preferably between 30 and 60%, and the zeolite is a catalytically active zeolite.

In the context of this application, a honeycomb body is to be understood as meaning a shaped body which has parallel channels which are separated from one another by walls. The channels may have an arbitrary polygonal base, e.g. a square, rectangular or hexagonal. Particularly preferred are rectangular base areas, as these simplify the production process and allow a space-saving arrangement of individual moldings in frame reactors. With the base of the channels, the area is meant, which is flowed through vertically.

Compared to zeolitic catalysts prepared by a coating process, the zeolite mass catalyst is characterized by a higher specific catalyst mass in the reaction space, by a significantly lower loss of catalytic activity due to attrition of the catalyst surface, as in the case of coating catalysts the case would be, and by a significantly simplified manufacturing process.

Compared with the fixed bed reactors, the reactors with catalyst system according to the invention have the advantage that the pressure loss, ie the pressure difference between the reaction gas at the reactor outlet and at the reactor inlet, is significantly lower is. Furthermore, the catalyst system according to the invention is characterized by an increased geometric surface on the reaction gas side, by less "fouling" of the reactor on the reaction gas side and by improved process gas flow.

Compared to conventional moldings, the monolithic catalysts are additionally characterized by greatly reduced dust formation and thus triggered catalyst losses during loading and unloading of the reactor as well as the possibility of replacing parts of the catalyst bed in the operating time of the catalyst.

The term "zeolite" in the context of the present invention as defined by the International Mineralogical Association (D.S. Coombs et al., Can. Mineralogist, 35, 1997, 1571) is a crystalline substance from the group of aluminum silicates with a spatial network structure of the general formula

M ^{n +} n [(AlO ₂₎ x (SiO ₂₎ yh-tHzO

understood that consist of SiO ₄ / AlO ₄ tetrahedra, which are linked by common oxygen atoms to a regular three-dimensional network. The ratio of Si / Al = y / x is always> 1 according to the so-called "Lowenstein rule", which prohibits the adjacent occurrence of two adjacent negatively charged A10 ₄ tetrahedra. Although, at a low Si / Al ratio, more exchange sites for metals are available, the zeolite is becoming increasingly thermally unstable.

The zeolite structure contains cavities and channels characteristic of each zeolite. The zeolites are classified according to their topology into different structures (see above). Splits. The zeolite framework contains open cavities in the form of channels and cages which are normally occupied by water molecules and extra framework cations that can be exchanged. An aluminum atom has an excess negative charge which is compensated by these cations. The interior of the pore system represents the catalytically active surface. The more aluminum and the less silicon a zeolite contains, the denser the negative charge in its lattice and the more polar its inner surface. The pore size and structure are determined by the Si / Al ratio, which determines most of the catalytic character of a zeolite, in addition to the parameters of preparation (use or type of template, pH, pressure, temperature, presence of seed crystals) ,

The presence of 2- or 3-valent cations as a tetrahedral center in Zeolithgerust the zeolite receives a negative charge in the form of so-called anion sites in the vicinity of the corresponding cation positions are. The negative charge is compensated by the incorporation of cations in the pores of the zeolite material. The zeolites are mainly distinguished by the geometry of the cavities formed by the rigid network of SiO4 / AlO4 tetrahedra. The entrances to the cavities are formed by 8, 10 or 12 "rings" (narrow, medium and large pore zeolites). Certain zeolites show a uniform structure structure (eg ZSM-5 with MFI topology) with linear or zigzagging channels, in others close behind the Porenoffnungen larger cavities, z. B. in the Y and A zeolites, with the topologies FAU and LTA.

In principle, any zeolite, in particular any 10 and 12 "ring" zeolite, can be used in the context of the present invention. According to the invention, preference is given to zeolites with the topologies AEL, BEA, CHA, EUO, FAO, FER, KFI, LTA, LTL, MAZ, MOR, MEL, MTW, LEV, OFF, TONE and MFI. Very particular preference is given to zeolites of the topological structures BEA, MFI, FER, MOR, MTW and TRI.

Typically, the metal content or degree of exchange of a zeolite is significantly determined by the metal species present in the zeolite. This allows the zeolite to be doped with only a single metal or with different metals. The preferred metals for exchange and doping are catalytically active metals such as Fe, Ce, Co, Ni, Ag, V, Rh, Pd, Pt, Ir. According to the invention, very particular preference is given to zeolites containing iron or cobalt species.

There are usually three different centers in zeolites, referred to as α, β, and γ positions, which are the locations of exchange sites (also referred to as "interchangeable."

All these three positions are for reactants during the NH3-SCR process.

Reaction accessible, especially when using MFI, BEA, FER, MOR, MTW and TRI zeolites.

In a preferred embodiment of the catalyst system, the free passage area flowing through the exhaust gas flow flowing perpendicularly is between 20 and 80%, preferably between 60 and 80% and most preferably about 70%. This passage area is calculated from the quotient of the area freely flowed through by the exhaust gas flow divided by the inflow surface of the catalyst body. In the context of the application, the catalyst body is to be understood as the object which arises after extrusion and drying and immediately before insertion into a frame. Too large a passage area would make the catalyst body unstable because the walls would be too narrow and therefore unstable. at Too small a passage area, the pressure loss of the process or combustion gas would be too large over the Formkorper.

In a further preferred embodiment, the cell density of the honeycomb body is between 1 and 200 per cm ² , preferably between 2 and 50 per cm ² . The cell density is calculated from the number of channels per start-up unit. There are limits to the production of honeycomb cartons with a high cell density. Generally, a high cell density is desirable, as the surface area increases with increasing cell density.

The honeycomb body contains a mixture of zeolite, binder, filler and additive. The zeolite represents the catalytically active component. The proportion by weight of zeolite in the honeycomb body (ie of the extrudate) is, as stated above, between 20 and 95%, preferably between 30 and 60%. The binder is made of an inorganic material, e.g. Boehmite, which allows the shaping process. The weight fraction of the binder is between 5 and 60%, preferably between 20 and 40%. The filling material consists of a porous carrier material, e.g. Alumina, with a weight fraction of the honeycomb body of 0 to 40%, preferably between 10 and 30%. The additive consists of a material which affects the physical properties of the honeycomb or the mixture of components e.g. a Tixotropiermittel with a weight proportion between 0 and 10%. The proportion by weight is calculated from the quotient of the mass of zeolite divided by the total mass of the honeycomb body.

An extrudable mass is produced by mixing zeolite with water, binders and possibly inert filler material and additives. Binders and additives are chosen so that they influence the properties of the mixture, eg the flow properties, so that the extrusion process and the drying process tion and hardening process during production is made possible.

In a further preferred embodiment of the catalyst system, the honeycomb body is in the form of a cylinder. In particular, the honeycomb body has a diameter of 10 to 500 mm, preferably of 200 to 400 mm, and / or a height of 10 to 500 mm, preferably of 200 to 400 mm. The height of the honeycomb body is here and below equivalent to the passage of the exhaust gas flow through the honeycomb body. While the diameter of the honeycomb body or more generally the Anstromflache the honeycomb body has an influence on the flow rate of exhaust gases, the height of the honeycomb body is crucial for the conversion of nitrogen oxides contained in the exhaust gas or nitrous oxide contained in the exhaust gas, since the contact time increases with the height of the honeycomb body , Too small highs mean too little turnover and too high altitudes are on the one hand not economical and on the other hand associated with a higher pressure loss. In particular, the preferred dimensions of the honeycomb bodies are particularly suitable for the use of the inventive catalyst systems for the treatment of exhaust gases from internal combustion engines.

In a further preferred embodiment, the honeycomb body of the catalyst system according to the invention has a cuboid shape. In particular, the cuboid honeycomb body has a side length of 10 to 500 mm, preferably 50 to 200 mm, and / or a height of 10 to 500 mm, preferably 150 up to 300 mm. The advantage of cuboid honeycomb bodies lies in special installation requirements and in a space saving. The advantages of the dimensions are the same as the pre ^¬ parts of the dimensions of the cylindrical honeycomb body. The catalyst systems according to the invention are used for the catalytic treatment of exhaust gases from combustion or incineration plants as well as plants for the production of nitric acid, adipic acid or caprolactam.

Further, the catalyst systems of the present invention find use for the catalytic removal of nitrogen oxides from exhaust gases from gasification and combustion processes, e.g. Waste incineration plants or for denitrification of motor vehicle exhaust gases.

Claims

claims A catalyst system for treating exhaust gases containing nitrogen oxides, which comprises a mass catalyst of a zeolite, wherein the mass catalyst is an extrudate having the geometry of a honeycomb body, characterized in that the weight fraction of zeolite in the extruded mass of the extrudate between 20 and 95%, preferably between 30 and 60%, and the zeolite is a catalytically active zeolite. 2. Catalyst system according to claim 1, characterized in that the free and of the exhaust stream to be cleaned vertically through-flow passage area of the honeycomb body between see 20 and 80%, preferably between 60 and 80% and most preferably is 70%. 3. Catalyst system according to claim 1 or claim 2, characterized in that the cell density of the honeycomb body see between 1 and 200 per cm ² , preferably between 2 and 50 per cm ² . 4. Catalyst system according to one of the preceding claims, characterized in that the honeycomb body is present in cylindrical form. 5. Catalyst system according to claim 4, characterized in that the honeycomb body has a diameter of 10 to 500 mm, preferably from 200 to 400 mm, and / or a height of 10 to 500 mm, preferably from 200 to 400 mm. 6. Catalyst system according to one of claims 1 to 5, characterized in that the honeycomb body is in cuboid form. 7. A catalyst system according to claim 6, characterized in that the honeycomb body has a side length of 10 to 500 mm, preferably from 50 to 200 mm, and / or a height of 10 to 500 mm, preferably from 150 to 300 mm. 8. Catalyst system according to one of the preceding claims, characterized in that the honeycomb body has channels with a rectangular base area. 9. Catalyst system according to one of the preceding claims, characterized in that the zeolite is a 10 or 12 "Rmg" zeolite. 10. Catalyst system according to one of the preceding claims, characterized in that the zeolite Fe, Ce, Co, Ni, Ag, V, Rh, Pd, Pt, Ir, particularly preferably Fe or Co contains. 11. A catalyst system according to claim 10, characterized in that the zeolite contains different metals. 12. Catalyst system according to one of the preceding claims, characterized in that the zeolite is selected from a group of zeolites with the topologies AEL, BEA, CHA, EUO, FAO, FER, KFI, LTA, LTL, MAZ, MOR, MEL, MTW , LEV, OFF, TONE and MFI, most preferably zeolites with the topologies BEA, MFI, FER, MOR, MTW and TRI. 13. Use of a catalyst according to any one of claims 1 to 12 for the treatment of exhaust gases from plants for the production of nitric acid, adipic acid or caprolactam. 14. Use of a catalyst according to one of claims 1 to 12 for the catalytic removal of nitrogen oxides from exhaust gases of gasification and combustion processes, in particular waste incineration plants, or for denitrification of motor vehicle exhaust gases.