WO2013092707A1 - Method for modifying the pore size of zeolites - Google Patents

Method for modifying the pore size of zeolites Download PDF

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Publication number
WO2013092707A1
WO2013092707A1 PCT/EP2012/076141 EP2012076141W WO2013092707A1 WO 2013092707 A1 WO2013092707 A1 WO 2013092707A1 EP 2012076141 W EP2012076141 W EP 2012076141W WO 2013092707 A1 WO2013092707 A1 WO 2013092707A1
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zeolite
metal
zeolites
mfi
exchanged
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PCT/EP2012/076141
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French (fr)
Inventor
Arno Tissler
Gerd Maletz
Klaus Wanninger
Andreas Bentele
Martin Schneider
Markus Reichinger
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Clariant Produkte (Deutschland) Gmbh
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Publication of WO2013092707A1 publication Critical patent/WO2013092707A1/en

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J29/00—Catalysts comprising molecular sieves
    • B01J29/04—Catalysts comprising molecular sieves having base-exchange properties, e.g. crystalline zeolites
    • B01J29/06—Crystalline aluminosilicate zeolites; Isomorphous compounds thereof
    • B01J29/40—Crystalline aluminosilicate zeolites; Isomorphous compounds thereof of the pentasil type, e.g. types ZSM-5, ZSM-8 or ZSM-11, as exemplified by patent documents US3702886, GB1334243 and US3709979, respectively
    • B01J29/42—Crystalline aluminosilicate zeolites; Isomorphous compounds thereof of the pentasil type, e.g. types ZSM-5, ZSM-8 or ZSM-11, as exemplified by patent documents US3702886, GB1334243 and US3709979, respectively containing iron group metals, noble metals or copper
    • B01J29/46—Iron group metals or copper
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
    • B01J20/02—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material
    • B01J20/10—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material comprising silica or silicate
    • B01J20/16—Alumino-silicates
    • B01J20/18—Synthetic zeolitic molecular sieves
    • B01J20/186—Chemical treatments in view of modifying the properties of the sieve, e.g. increasing the stability or the activity, also decreasing the activity
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
    • B01J20/28—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof characterised by their form or physical properties
    • B01J20/28054—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof characterised by their form or physical properties characterised by their surface properties or porosity
    • B01J20/28078—Pore diameter
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
    • B01J20/28—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof characterised by their form or physical properties
    • B01J20/28054—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof characterised by their form or physical properties characterised by their surface properties or porosity
    • B01J20/28078—Pore diameter
    • B01J20/2808—Pore diameter being less than 2 nm, i.e. micropores or nanopores
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
    • B01J20/28—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof characterised by their form or physical properties
    • B01J20/28054—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof characterised by their form or physical properties characterised by their surface properties or porosity
    • B01J20/28095—Shape or type of pores, voids, channels, ducts
    • B01J20/28097—Shape or type of pores, voids, channels, ducts being coated, filled or plugged with specific compounds
    • B01J35/30—
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B39/00—Compounds having molecular sieve and base-exchange properties, e.g. crystalline zeolites; Their preparation; After-treatment, e.g. ion-exchange or dealumination
    • C01B39/02—Crystalline aluminosilicate zeolites; Isomorphous compounds thereof; Direct preparation thereof; Preparation thereof starting from a reaction mixture containing a crystalline zeolite of another type, or from preformed reactants; After-treatment thereof
    • C01B39/026—After-treatment
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/50—Zeolites
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/92—Chemical or biological purification of waste gases of engine exhaust gases
    • B01D53/94—Chemical or biological purification of waste gases of engine exhaust gases by catalytic processes
    • B01D53/9404—Removing only nitrogen compounds
    • B01D53/9409—Nitrogen oxides
    • B01D53/9413—Processes characterised by a specific catalyst
    • B01D53/9418—Processes characterised by a specific catalyst for removing nitrogen oxides by selective catalytic reduction [SCR] using a reducing agent in a lean exhaust gas
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J2229/00—Aspects of molecular sieve catalysts not covered by B01J29/00
    • B01J2229/10—After treatment, characterised by the effect to be obtained
    • B01J2229/12—After treatment, characterised by the effect to be obtained to alter the outside of the crystallites, e.g. selectivation
    • B01J2229/126—After treatment, characterised by the effect to be obtained to alter the outside of the crystallites, e.g. selectivation in order to reduce the pore-mouth size
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J2229/00—Aspects of molecular sieve catalysts not covered by B01J29/00
    • B01J2229/10—After treatment, characterised by the effect to be obtained
    • B01J2229/18—After treatment, characterised by the effect to be obtained to introduce other elements into or onto the molecular sieve itself
    • B01J2229/186—After treatment, characterised by the effect to be obtained to introduce other elements into or onto the molecular sieve itself not in framework positions
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J2229/00—Aspects of molecular sieve catalysts not covered by B01J29/00
    • B01J2229/30—After treatment, characterised by the means used
    • B01J2229/32—Reaction with silicon compounds, e.g. TEOS, siliconfluoride
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J2229/00—Aspects of molecular sieve catalysts not covered by B01J29/00
    • B01J2229/30—After treatment, characterised by the means used
    • B01J2229/38—Base treatment

Definitions

  • the present invention relates to a method for modifying the pore size of zeolites as well as to zeolites obtainable by means of the method which are suitable inter alia as catalyst for the selective catalytic reduction (SCR) of hydrocarbons.
  • SCR selective catalytic reduction
  • DeNO x The denitrification of combustion exhaust gases is also called DeNO x .
  • SCR selective catalytic reduction
  • Customary methods for doping zeolites with metals comprise for example methods such as liquid ion exchange, solid-phase ion exchange, vapour-phase ion exchange, mechanical-chemical processes, impregnation processes and the so-called extra-framework processes.
  • a high desorption temperature is therefore advantageous in particular because thus the desorption step simultaneously serves to oxidize the hydrocarbons and this takes place more completely at higher temperatures.
  • the quantity of the metal-containing zeolite used is firmly predetermined (DE 102007024125.0)
  • the SCR reaction is temperature-dependent.
  • these catalysts are used preferably in combustion engines in automobiles or heavy goods vehicles for denitrifying the exhaust-gas stream, a high proportion of hydrocarbons is adsorbed in the SCR zeolite as the SCR reaction takes place mostly at below 300°C, depending on the driving style of the vehicle driver.
  • the risk of sudden ignition increases with the strong adsorption of the hydrocarbons in the zeolite if short-term temperature peaks occur. This leads to the destruction of the zeolite material, to damage to the catalyst and in the worst case to the vehicle igniting and exploding.
  • narrow-pored zeolites are used as catalysts, for example SAPO-34 with a CHA topology.
  • a disadvantage of narrow-pored zeolites is that an iron loading, in particular by liquid exchange methods, is almost
  • the object of the present invention was to provide a method with which zeolites can be obtained which have a high SCR activity and simultaneously a low adsorptivity for
  • hydrocarbons in particular for aromatic hydrocarbons.
  • a method for modifying the adsorptivity of zeolites comprising the steps of a) providing a metal-exchanged, silicon-rich zeolite, b) treating the zeolite with an aqueous solution of an alkali silicate, c) filtering, drying and calcining the treated zeolite d) reacting the calcined zeolite with an ammonium
  • the method according to the invention surprisingly makes it possible for the size of the entrance pores of zeolites to be reduced, meaning that already no or few hydrocarbons, in particular aromatic hydrocarbons, can diffuse or penetrate into the inside of the zeolite, but leaves the inner structure of the zeolite, in particular the diameter of its inner channels, unchanged.
  • zeolite is meant, within the framework of the present invention as defined by the International Mineralogical
  • the zeolite structure contains voids and channels which are characteristic of each zeolite.
  • the zeolites are divided into different structural types (see above) according to their topology.
  • the zeolite framework contains open voids in the form of channels and cages which are normally occupied by water molecules and extra-framework cations which can be exchanged. An aluminium atom attracts an excess negative charge which is compensated for by these cations.
  • the inside of the pore system is represented by the catalyt ically active surface. The more aluminium and the less silicon a zeolite contains, the denser is the negative charge in its lattice and the more polar its inner surface.
  • the pore size and structure are determined, in addition to the parameters during
  • the Si/Al ratio of a zeolite according to the invention lies in the range from 10 to 500 (corresponds to an S1O 2 /AI 2 O 3 ratio (module) of 20 - 1000), preferably from 10 to 300.
  • Zeolites are differentiated mainly according to the geometry of the voids which are formed by the rigid network of the S1O 4 /AIO 4 tetrahedra.
  • the entrances to the voids are formed by 8, 10 or 12 "rings" (narrow-, average- and wide-pored zeolites).
  • Specific zeolites show a uniform structure (e.g. ZSM-5 with MFI topology) with linear or zig-zag channels, while in others larger voids attach themselves behind the pore openings, e.g. in the case of the Y and A zeolites with the topologies FAU and LTA.
  • 10 and 12 "ring" zeolites are preferred according to the invention.
  • any zeolite in particular any 10- and 12-"ring" zeolite, can be used within the framework of the present invention.
  • Zeolites with the topologies AEL, BEA, CHA, EUO, FAO, FER, KFI, LTA, LTL, MAZ, MOR, MEL, MTW, LEV, OFF, TON and MFI are preferred according to the invention.
  • Zeolites of the topological structures BEA, MFI, FER, MOR, MTW and TUN are quite particularly preferred.
  • zeolite-like materials can likewise be used, such as are described for example in US 5,250,282, to the full disclosure content of which reference is made here.
  • the metal content or the degree of exchange of a zeolite is decisively determined by the metal species present in the zeolite.
  • the zeolite can be doped either with only a single metal or with different metals.
  • -, ⁇ - and ⁇ -positions which define the position of the exchange spaces (also called “exchangeable positions or sites") . All these three positions are available to reactants during the NH 3 -SCR reaction, in particular when using MFI, BEA, FER, MOR, MTW and TRI zeolites.
  • the quantity of metal calculated as corresponding metal oxide is 1 to 5 wt.-%, relative to the weight of the metal-doped zeolite.
  • more than 50% of the exchangeable sites i.e. -, ⁇ - and ⁇ -sites
  • more than 70% of the exchangeable sites are exchanged.
  • free sites should always still remain which are preferably Bronstedt acid centres. This is because NO is strongly absorbed both on the exchanged metal centres and also in ion-exchange positions or at Bronstedt centres of the zeolite framework.
  • the doping metals if at all possible do not form a stable compound with
  • alkali silicate aqueous base solutions of S1O 2 which can be
  • a zeolite which has an average pore size of from 0.5 to 0.6 nm.
  • Si-Al 2 Si0 2 /Al 2 C>3 ratio (module) of between 20 and 1000, in developments of the invention from 20 to 200.
  • Example 1 Production of an RPS (reduced pore-size) MFI zeolite

Abstract

The present invention relates to a method for modifying the adsorptivity of zeolites, comprising the steps of a) providing a metal-exchanged, silicon-rich zeolite, b) treating the zeolite with an aqueous solution of an alkali silicate, c) filtering, drying and calcining the treated zeolite d) reacting the calcined zeolite with an ammonium compound, followed by calcining. Furthermore, the present invention comprises a modified zeolite obtainable by means of the method according to the invention and its use as catalyst for the selective reduction of hydrocarbons.

Description

Method for modifying the pore size of zeolites
The present invention relates to a method for modifying the pore size of zeolites as well as to zeolites obtainable by means of the method which are suitable inter alia as catalyst for the selective catalytic reduction (SCR) of hydrocarbons.
Metal-doped zeolites are known from the state of the art and are for example used as catalysts in stationary and mobile exhaust-gas purification. In particular they are often used in exhaust-gas catalysts for purifying diesel exhaust gases in combustion engines.
Because of the harmful effects of nitrogen oxide emissions on the environment, it is important to further reduce these emissions. Clearly lower N0X emission limits for stationary and motor vehicle exhaust gases than are customary today are planned in the United States for the near future and are also being discussed in the European Union.
In order to observe these limits, in the case of mobile combustion engines (diesel engines) this can no longer
exclusively be achieved by measures inside the engine, but only by an exhaust-gas post-treatment, for example with suitable catalysts.
The denitrification of combustion exhaust gases is also called DeNOx. In automobile engineering, selective catalytic reduction (SCR) is one of the most important DeNOx techniques.
Hydrocarbons (HC-SCR) or ammonia (NH3-SCR) or NH3 precursors such as urea (Ad-Blue®) usually serve as reducing agents.
Metal-exchanged zeolites (also called "metal-doped zeolites") have proved to be very active SCR catalysts that can be used in a broad temperature range. They are in most cases non-toxic and produce less N20 and SO3 than the customary catalysts based on V205. In particular iron-doped zeolites represent good alternatives to the normally used vanadium catalysts, because of their high activity and resistance to sulphur under
hydrothermal conditions. Customary methods for doping zeolites with metals comprise for example methods such as liquid ion exchange, solid-phase ion exchange, vapour-phase ion exchange, mechanical-chemical processes, impregnation processes and the so-called extra-framework processes.
The SCR method involves achieving two different objects as on the one hand hydrocarbons are to be separated off from the exhaust-gas stream and on the other hand a catalytic
denitrification using iron- or copper-doped zeolites is required. Zeolite material in the H form is frequently used as hydrocarbon trap (so-called cold start trap) .
Zeolites which are used as hydrocarbon trap have specific characteristic properties. Thus the hydrocarbon absorption capacity is to be as high as possible and the zeolites are simultaneously to have a high hydrocarbon desorption
temperature. A high desorption temperature is therefore advantageous in particular because thus the desorption step simultaneously serves to oxidize the hydrocarbons and this takes place more completely at higher temperatures.
If, on the other hand, zeolites are used in SCR catalysis, a high hydrocarbon adsorption capacity is disadvantageous.
However, in order that the catalytic activity for the SCR reaction is guaranteed, the quantity of the metal-containing zeolite used is firmly predetermined (DE 102007024125.0) The SCR reaction is temperature-dependent. As these catalysts are used preferably in combustion engines in automobiles or heavy goods vehicles for denitrifying the exhaust-gas stream, a high proportion of hydrocarbons is adsorbed in the SCR zeolite as the SCR reaction takes place mostly at below 300°C, depending on the driving style of the vehicle driver. The risk of sudden ignition increases with the strong adsorption of the hydrocarbons in the zeolite if short-term temperature peaks occur. This leads to the destruction of the zeolite material, to damage to the catalyst and in the worst case to the vehicle igniting and exploding.
This problem can be avoided if narrow-pored zeolites are used as catalysts, for example SAPO-34 with a CHA topology. A disadvantage of narrow-pored zeolites is that an iron loading, in particular by liquid exchange methods, is almost
impossible. For example, the use of copper-exchanged SAPO-34 is known which, however, in the SCR reaction, at higher temperatures leads to an increased formation of nitrous oxide and a lower stability vis-a-vis steam and water. It is also possible that copper-exchanged zeolites may tend to dioxin formation in the SCR reaction.
Therefore, the object of the present invention was to provide a method with which zeolites can be obtained which have a high SCR activity and simultaneously a low adsorptivity for
hydrocarbons, in particular for aromatic hydrocarbons.
This object is achieved according to the invention by a method for modifying the adsorptivity of zeolites, comprising the steps of a) providing a metal-exchanged, silicon-rich zeolite, b) treating the zeolite with an aqueous solution of an alkali silicate, c) filtering, drying and calcining the treated zeolite d) reacting the calcined zeolite with an ammonium
compound, followed by renewed calcining
The method according to the invention surprisingly makes it possible for the size of the entrance pores of zeolites to be reduced, meaning that already no or few hydrocarbons, in particular aromatic hydrocarbons, can diffuse or penetrate into the inside of the zeolite, but leaves the inner structure of the zeolite, in particular the diameter of its inner channels, unchanged.
By "zeolite" is meant, within the framework of the present invention as defined by the International Mineralogical
Association (D.S. Coombs et al . , Can. Mineralogist, 35, 1997, 1571), a crystalline substance from the group of the
aluminosilicates with a spatial network structure of the general formula
Mn+ n[ (A102 ) x(Si02)y] ί¾20 which consist of Si04/A104 tetrahedra which are linked by common oxygen atoms to form a regular three-dimensional network. The Si/Al=y/x ratio is always ≥1 according to the so- called "Lowenstein Rule", which states that two adjacent negatively-charged A104 tetrahedra must not occur next to each other. Thus, although more exchange sites are available for metals with a low Si/Al ratio, the zeolite becomes
increasingly thermally unstable. The zeolite structure contains voids and channels which are characteristic of each zeolite. The zeolites are divided into different structural types (see above) according to their topology. The zeolite framework contains open voids in the form of channels and cages which are normally occupied by water molecules and extra-framework cations which can be exchanged. An aluminium atom attracts an excess negative charge which is compensated for by these cations. The inside of the pore system is represented by the catalyt ically active surface. The more aluminium and the less silicon a zeolite contains, the denser is the negative charge in its lattice and the more polar its inner surface. The pore size and structure are determined, in addition to the parameters during
production (use or type of templates, pH, pressure,
temperature, presence of seed crystals), by the Si/Al ratio which influences the greatest part of the catalytic character of a zeolite. In the present case it is particularly preferred if the Si/Al ratio of a zeolite according to the invention lies in the range from 10 to 500 (corresponds to an S1O2/AI2O3 ratio (module) of 20 - 1000), preferably from 10 to 300.
In liquid-exchange methods for producing metal-exchanged
(doped) zeolites there is a strong affinity for the
replacement of polyvalent and heavy metal cations with lighter cations and in particular with hydrogen and/or NH4 +.
In hydrated zeolites, dehydration takes place mostly at temperatures below approximately 400 °C and is very largely reversible .
Because of the presence of 2- or 3-valent cations as
tetrahedron centre in the zeolite framework the zeolite receives a negative charge in the form of so-called anion sites in the vicinity of which the corresponding cation positions are located. The negative charge is compensated for by incorporating cations into the pores of the zeolite
material. Zeolites are differentiated mainly according to the geometry of the voids which are formed by the rigid network of the S1O4/AIO4 tetrahedra. The entrances to the voids are formed by 8, 10 or 12 "rings" (narrow-, average- and wide-pored zeolites). Specific zeolites show a uniform structure (e.g. ZSM-5 with MFI topology) with linear or zig-zag channels, while in others larger voids attach themselves behind the pore openings, e.g. in the case of the Y and A zeolites with the topologies FAU and LTA. Generally, 10 and 12 "ring" zeolites are preferred according to the invention.
In principle, any zeolite, in particular any 10- and 12-"ring" zeolite, can be used within the framework of the present invention. Zeolites with the topologies AEL, BEA, CHA, EUO, FAO, FER, KFI, LTA, LTL, MAZ, MOR, MEL, MTW, LEV, OFF, TON and MFI are preferred according to the invention. Zeolites of the topological structures BEA, MFI, FER, MOR, MTW and TUN are quite particularly preferred. According to the invention zeolite-like materials can likewise be used, such as are described for example in US 5,250,282, to the full disclosure content of which reference is made here. Further zeolite materials preferred according to the invention are mesoporous zeolite materials of silicates or aluminosilicates which are known under the name M41S and are described in detail in US 5,098,684 and US 5,102,643, to the full disclosure content of which reference is likewise made.
Further, so-called silico-aluminophosphates (SAPOs) which can be produced from isomorphically exchanged aluminophosphates can be used according to the invention. With all these above-named materials, of sole importance is that they have at most a "10-ring topology".
Typically, the metal content or the degree of exchange of a zeolite is decisively determined by the metal species present in the zeolite. The zeolite can be doped either with only a single metal or with different metals.
There are usually three different centres in zeolites,
designated the so-called -, β- and γ-positions, which define the position of the exchange spaces (also called "exchangeable positions or sites") . All these three positions are available to reactants during the NH3-SCR reaction, in particular when using MFI, BEA, FER, MOR, MTW and TRI zeolites.
The so-called -type cations show the weakest bond to the zeolite framework and are the last to be filled in a liquid ion exchange. From a degree of exchange of around 10% the degree of occupancy increases markedly as the metal content increases and amounts to around 10 to 50% in total at a degree of exchange of up to M/A1=0.5. Cations at this site represent very active redox catalysts.
On the other hand, the β-type cations which represent the most-occupied position and catalyze the HC-SCR reaction most effectively during liquid ion exchange, in particular with small degrees of exchange, display an average bonding strength to the zeolite framework. This position is filled immediately after the γ-position and, from a degree of exchange of around 10%, its degree of occupancy falls as the metal content increases and amounts to around 50 to 90% for a degree of exchange of up to M/A1=0.5. In the state of the art it is known that from a degree of exchange of M/A1>0.56 typically only polynuclear metal oxides are still inserted or deposited. The γ-type cations are those with the strongest bond to the zeolite framework and thermally the most stable. They are the least-occupied position during liquid ion exchange, but are filled first. Cations, in particular iron and cobalt, in these positions are highly active and are the most catalytically active cations.
The preferred metals for the exchange and the doping are catalytically active metals such as Fe, Co, Ni, Cu, Ag, Co, V, Rh, Pd, Pt, Ir, quite particularly preferably Fe, Co, Ni and Cu, in quite particularly preferred embodiments Fe or Cu, which can also form bridged dimeric species such as are mainly present in particular at high degrees of exchange.
Overall, the quantity of metal calculated as corresponding metal oxide is 1 to 5 wt.-%, relative to the weight of the metal-doped zeolite. In particular it is preferred that more than 50% of the exchangeable sites (i.e. -, β- and γ-sites) are exchanged. Quite particularly preferably, more than 70% of the exchangeable sites are exchanged. However, free sites should always still remain which are preferably Bronstedt acid centres. This is because NO is strongly absorbed both on the exchanged metal centres and also in ion-exchange positions or at Bronstedt centres of the zeolite framework. Moreover, NH3 preferably reacts with the strongly acid Bronstedt centres, the presence of which is thus very important for a successful NH3-SCR reaction. The simultaneous presence of free radical- exchange spaces and/or Bronstedt acid centres and the metal- exchanged lattice spaces is thus quite particularly preferred according to the invention. Therefore, a degree of exchange of 70-90% is most preferred. At a degree of exchange of more than 90%, a reduction in activity was observed during the reduction of NO to N2 and the SCR-NH3 reaction. Because of the danger of the hydrothermal deactivation of metal-exchanged zeolites, which is preceded by a
dealuminization and migration of metal from the ion-exchange centres of the zeolite, it is preferred that the doping metals if at all possible do not form a stable compound with
aluminium, as a dealuminization is thereby promoted.
The treatment with an alkali silicate under the conditions according to the invention surprisingly leads to a reduction in the pore size of the entrance pores which leads in turn to a clearly smaller hydrocarbon loading due to the thus-reduced accessibility of the inner zeolite surface for larger organic molecules .
By the term "alkali silicate" is meant according to the invention aqueous base solutions of S1O2 which can be
represented by the general formula M2O x S1O2, wherein M is one or more alkali metals, thus Li, Na or K. Such compounds are often called water glass or alkali salts of silicic acid.
It is preferred that the average pore sizes (determined according to DIN 66135 according to the Horvath-Kawazoe method) of the zeolites used according to the invention lie in the range of from 0.4 to 1.5 nm.
In an embodiment of the method according to the invention a zeolite is used which has an average pore size of from 0.5 to 0.6 nm.
In developments of the present invention the zeolite is selected from the group consisting of ABW, ACO, AEI, AEL, AEN, AET, AFG, AFI, AFN, AFO, AFR, AFS, AFT, AFX, AFY, AHT, ANA, APC, APD, AST, ASV, ATN, ATO, ATS, ATT, ATV, AWO, AWW, BCT, BEA, BEC, BIK, BOF, BOG, BPH, BRE, BSV, CAN, CAS, CDO, CFI,
CGF , CGS, CHA, CHI, -CLO, CON, CZP, DAC, DDR, DFO, DFT , DOH
DON, EAB, EDI, EMT, EON, EPI, ERI, ESV, ETR, EUO, EZT, FAR,
FAU, FER, FRA, GIS, GIU, GME, GON, GOO, HEU, IFR, IHW, IMF,
ISV, ITE, ITH, ITR, ITW, IWR, IWS, IWV, IWW, JBW, JRY, KFI,
LAU, LEV, LIO, LIT, LOS, LOV, LTA, LTF, LTL, LTN, MAR, MAZ,
MEI, MEL, MEP, MER, MFI, MFS, MON, MOR, MOZ, MRE, MSE, MSO,
MTF , MTN, MTT, MTW, MVY, MWW, NAB, NAT, NES, NON, NPO, NSI,
OBW, OFF, OSI, OSO, OWE, PAR, PAU, PHI, PON, PUN, RHO, RON,
RRO, RSN, RTE, RTH, RUT, RWR, RWY, SAF, SAO, SAS, SAT, SAV,
SBE , SBN, SBS, SBT, SFE, SFF, SFG, SFH, SFN, SFO, SFS, SGT,
SIV, SOD, SOF, SOS, SSF, SSY, STF, STI, STO, STT, STW, SVR,
SZR, TER, THO, TOL, TON, TSC, TUN, UEI, UFI, UOS, UOZ, USI,
UTL , VET, VFI, VNI, VSV, WEI, WEN, YUG and ZON, the crystal structures of which are described according to IZA at http : / /www . iza-online . org/ .
In specific embodiments of the invention zeolites are used which have at most a "10-ring topology", such as e.g. MFI, MEL and TUN. In an embodiment of the present invention MFI is used. Likewise a zeolite with MOR topology can also be used.
The zeolites can be used in either their H or their NH4 form, but mostly the H form is preferred.
The metal-exchanged zeolite can be produced and continue to be used in situ directly by means of customary methods known to a person skilled in the art, or else a commercially available metal-exchanged zeolite can also be used in the method
according to the invention. According to the invention the silicon-rich zeolite is
selected such that it has an Si-Al2 (Si02/Al2C>3) ratio (module) of between 20 and 1000, in developments of the invention from 20 to 200.
The preferred application for zeolites obtained according to the invention is exhaust-gas catalysis. The zeolites must therefore be hydrothermally stable. Below an S1O2/AI2O3 ratio of 20 the hydrothermal stability is too low. At too high a module the ion-exchange capacity, i.e. the quantity of
exchangeable metal, is too low and the metal-exchanged zeolite is thus less active in the SCR reaction.
In embodiments of the method according to the invention the metal of the metal-exchanged zeolite is selected from the group consisting of Fe, Cu, Co, Mn, Au, Ag, Ru, Ce, Rh, Pt, Pd, Zr, Ag, W, La, as well as mixtures thereof, quite
particularly preferably Fe, Cu, Co, Ni . In special embodiments of the present invention, the metal is iron or copper, as already mentioned above.
In preferred embodiments of the method the calcining in step d) and f) is carried out at a temperature of 450°C to 750°C. The zeolite structure is damaged at higher temperatures.
Preferably, ammonium nitrate or ammonium sulphate is used as ammonium compound in step e) , in particular for reasons of cost. However, it is possible also to use other ammonium compounds .
The object of the present invention is further achieved by the provision of a zeolite with modified, preferably reduced pore size of the entrance pores obtainable according to the method described in detail above according to the invention. In this context it is important to point out again that the structure and diameter of the inner channels remain unchanged.
The zeolite preferably contains metal and the metal is
selected from the group consisting of Fe, Cu, Mn, Au, Ag, Ru, Ce, Rh, Pt, Pd, Zr, Ag, W, La as well as mixtures thereof. In preferred embodiments the metal is selected from Fe, Cu and Mn or mixtures thereof.
Typically the zeolite is present in the H form or ammonium form, such that optionally a further exchange can take place.
The zeolite obtainable according to the invention is used as catalyst or adsorber. In preferred embodiments of the present invention the zeolite obtainable according to the invention is used in the selective reduction of nitrogen oxides (SCR) in the presence of hydrocarbons, in particular in motor vehicles.
The invention is described in more detail below with reference to figures and embodiment examples which are not, however, to be considered limiting.
Figure 1 shows a diffraction diagram of an Fe-MFI zeolite of the state of the art and an Fe-MFI zeolite according to the invention with reduced pore size
Figure 2 shows a toluene TPD diagram of Fe-MFI of the state of the art
Figure 3 shows a toluene TPD diagram of Fe-MFI according to the invention
Figure 4 shows a benzene TPD diagram of Fe-MFI of the state of the art Figure 5 shows a benzene TPD diagram of Fe-MFI according to the invention
Method part :
The methods and appliances used are listed below, but are not to be considered limiting.
Determination of the BET surface area:
The BET surface area was determined according to DIN 66131 (multi-point determination) , as well as according to DIN ISO 9277 (European standard 2003-05), in accordance with the determination of the specific surface area of solids by gas adsorption according to the BET method (according to Brunauer, S.; Emett, P.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309.) with a Gemini apparatus from Micromeritics : 0.100 g pulverized sample was heated for 30 min. at 350°C, then connected to the vacuum and heated in the vacuum at 350 °C until it reached a pressure of < 0.1 mbar . After cooling under vacuum and
addition of N2 the sample was re-weighed while heated.
Measurement: 5-point BET measurement with N2 at the temperature of liquid nitrogen in the p/p0 range 0.004 - 0.14.
Determination of the pore-size distribution
The pore-size distribution was determined according to DIN 66135 in accordance with the Horvath-Kawazoe method.
TPD measurement The TPD measurement took place by means of an Autochemll apparatus from Micromeritics , benzene/toluene TPD, and was carried out as follows: Pre-treatment : He flow, [heat at] 5°C/min. from room temperature to 350°C, maintain for 2 h, cool to 40°C;
Charge: with benzene/toluene (vapour pressure at 40°C) guide concentrated He pulses through the sample until constant peak areas are obtained (maximum 20 pulses); Desorption: He flow, heat at 10°C/min. from 40°C to 500°C, maintain for 2 h;
Detection: with mass spectrometer (benzene: amu=78, toluene: amu=91 )
XRD measurement
The XRD measurements took place by means of a D4 Endeavor apparatus from Bruker; measurement parameters: Scan type:
locked coupled;
Scan mode: continuous; 2 theta range: 5 - 50; Step size: 0.030 deg 2 theta; Time/step: 3.0 sec; Divergence slit: V12 mm; Sample rotation: 30 rpm; X-ray tube: Cu 40 kV, 40 mA.
Embodiment examples
Example 1: Production of an RPS (reduced pore-size) MFI zeolite
100 g of an Fe-MFI zeolite (commercially available as Fe-TZP- 302 from Sud-Chemie Zeolites GmbH Bitterfeld) is dispersed in 400g distilled water. Then 140 g of an Na silicate solution (S1O2 content 26.7 wt.-%) is added. The mixture is heated to 55°C and stirred for approx. 30 min. The zeolite is then filtered off and calcined for 3h at 600°C.
Example 2: XRD/BET measurement
Figure 1 shows the comparison of a diffraction diagram of the original Fe-MFI and of the RPS-Fe-MFI. It can be seen clearly that the framework structure of the RPS-Fe-MFI is still completely intact and shows the typical diffraction pattern of a zeolite with MFI structure. Table 1 shows the measured BET surface areas for Fe-MFI and RPS-Fe-MFI. A slight reduction in the BET surface area is seen. However, the remaining surface area of almost 300 m2/g is still sufficient for the use of this material as catalyst.
Table 1 : Comparison of the BET surface areas
Figure imgf000016_0001
Example 3: toluene TPD and benzene TPD
The narrowing of the pores of the zeolite according to the invention can be demonstrated easily with the help of
adsorption tests. With a significant pore narrowing the adsorption of larger (e.g. cyclical or branched) hydrocarbons will reduce clearly. Therefore, toluene and benzene TPD measurements are carried out. The sample to be measured is heated for 2 hours at 350°C in the helium stream. Charging with toluene or benzene then took place at 40°C and then heating at a heating rate of 10 K/min to 500°C. The toluene or benzene desorption was then measured as a function of the temperature by means of a mass spectrometer.
Figures 2 and 3 show the toluene TPD measurements of Fe-MFI and RPS-Fe-MFI. Table 2 shows the desorbed total quantities of toluene .
Table 2 : Comparison of the desorbed quantity of toluene
Figure imgf000017_0001
On the one hand it is seen that with RPS-Fe-MFI the desorbed quantity of toluene is 3.3 times lower than with Fe-MFI. And on the other hand, with RPS-Fe-MFI the maximum of the
desorption curve is shifted by approx. 40°C to higher
temperatures. Both results show that the treatment method according to the invention has led to a reduction in size of the pores.
Differences between RPS-Fe-MFI and Fe-MFI are also seen in the adsorption of the smaller benzene molecule. This can be seen in Figures 4 and 5 and Table 3.
Table 3 : Comparison of the desorbed quantity of benzene Zeolite Benzene desorption [μιηοΐ/g
sample ]
Fe-MFI 471
RPS-Fe-MFI 254
With RPS-Fe-MFI, the desorbed quantity of toluene is 1.85 times lower than with Fe-MFI. Although the difference is smaller than with the toluene adsorption, it is still significant.

Claims

Claims
Method for modifying the adsorptivity of zeolites, comprising the steps of a) providing a metal-exchanged, silicon-rich zeolite, b) treating the zeolite with an aqueous solution of an alkali silicate, c) filtering, drying and calcining the treated zeolite d) reacting the calcined zeolite with an ammonium compound, followed by calcining.
Method according to claim 1, wherein a zeolite is used which has a pore size of from 0.4 to 0.8 nm.
Method according to claim 2, wherein the zeolite is selected from the group consisting of ABW, ACO, AEI,
AEL, AEN, AET, AFG, AFI, AFN, AFO, AFR, AFS, AFT, AFX,
AFY, AHT, ANA, APC, APD, AST, ASV, ATN, ATO, ATS, ATT,
ATV, AWO, AWW, BCT, BEA, BEC, BIK, BOF, BOG, BPH, BRE,
BSV, CAN, CAS, CDO, CFI, CGF, CGS, CHA, CHI, CLO, CON,
CZP, DAC, DDR, DFO, DFT, DOH, DON, EAB, EDI, EMT, EON,
EPI, ERI, ESV, ETR, EUO, EZT, FAR, FAU, FER, FRA, GIS,
GIU, GME, GON, GOO, HEU, IFR, IHW, IMF, ISV, ITE, ITH,
ITR, ITW, IWR, IWS, IWV, IWW, JBW, JRY, KFI, LAU, LEV,
LIO, -LIT , LOS, LOV, LTA, LTF , LTL, LTN, MAR, MAZ, MEI
MEL, MEP, MER, MFI, MFS, MON, MOR, MOZ, MRE, MSE, MSO,
MTF, MTN, MTT, MTW, MVY, MWW, NAB, NAT, NES, NON, NPO,
NSI, OBW, OFF, OSI, OSO, OWE, -PAR, PAU, PHI, PON, PUN
RHO, RON, RRO, RSN, RTE, RTH, RUT, RWR, RWY, SAF, SAO, SAS, SAT, SAV, SBE , SBN, SBS, SBT , SFE , SFF , SFG, SFH,
SFN, SFO, SFS, SGT , SIV, SOD, SOF, SOS, SSF, SSY, STF,
STI, STO, STT, STW, SVR, SZR, TER, THO, TOL, TON, TSC,
TUN, UEI, UFI, UOS, UOZ, USI, UTL, VET, VFI, VNI, VSV,
WEI, WEN, YUG and ZON.
4. Method according to claim 3, wherein the silicon-rich zeolite is selected such that it has an Si-Al ratio of between 20 and 1000.
5. Method according to claim 4, wherein the metal of the metal-exchanged zeolite is selected from the group consisting of Fe, Cu, Mn, Au, Ag, Ru, Ce, Rh, Pt, Pd, Zr, Ag, W, La as well as mixtures thereof.
6. Method according to claim 5, wherein the calcining in step c) and d) is carried out at a temperature of from 450°C to 750°C.
7. Method according to claim 6, wherein ammonium nitrate or ammonium sulphate is used as ammonium compound in step d) .
8. Zeolite with reduced pore size of the entrance pores obtainable according to the method according to one of claims 1 to 7.
9. Zeolite according to claim 8, wherein the zeolite
contains metal and the metal is selected from the group consisting of Fe, Cu, Mn, Au, Ag, Ru, Ce, Rh, Pt, Pd, Zr, Ag, W, La as well as mixtures thereof.
10. Use of the zeolite according to claim 8 or 9 as catalyst or adsorber. Use according to claim 10 for the selective reduction nitrogen oxides in the presence of hydrocarbons.
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