TRANSITION METAL-CONTAINING ALUMINOSILICATE ZEOLITE

Abstract

A synthetic aluminosilicate zeolite catalyst containing at least one catalytically active transition metal selected from the group consisting of Cu, Fe, Hf, La, Au, In, V, lanthanides and Group VIII transition metals, which aluminosilicate zeolite is a small pore aluminosilicate zeolite having a maximum ring size of eight tetrahedral atoms, wherein the mean crystallite size of the aluminosilicate zeolite determined by scanning electron microscope is >0.50 micrometer.

Claims

1. A catalyst comprising at least one of catalytically active metal selected from copper (Cu), iron (Fe), or vanadium (V) on an aluminosilicate zeolite having a CHA framework and a mean crystallite size, determined by scanning electron microscope, of >0.50 microns.

2. The catalyst of claim 1, wherein the mean crystallite size determined by scanning electron microscope is >1.00 microns.

3. The catalyst of claim 1, wherein the catalytically active metal is copper.

4. The catalyst of claim 3, wherein a majority of the copper is present as copper oxide.

5. The catalyst of claim 1, wherein the catalytically active metal is iron.

6. The catalyst of claim 1, wherein the catalytically active metal is vanadium.

7. The catalyst of claim 1, wherein the catalytically active metal is present from about 0.1 to 10 weight percent based on the total weight of the zeolite.

8. The catalyst of claim 1, wherein the catalytically active metal is present from about 0.5 to 5 weight percent based on the total weight of the zeolite.

9. The catalyst of claim 1, wherein the zeolite has a silica-to-alumina ratio (SAR) of 10 to 28.

10. The catalyst of claim 1, wherein said catalyst is characterized as achieving a greater than 60% NOx conversion at temperature below 200 deg. C. after the catalyst has been hydrothermally aged at a temperature of at least 750 deg. C. for at least 24 hours in at least 10% water vapor.

11. An exhaust system for an engine, which system comprising a catalyst according to claim 1 and a reductant disposed upstream of the catalyst.

12. The exhaust system of claim 11, wherein the reductant is a nitrogenous-based reductant.

Description

EXAMPLE 1

Preparation of Zeolite Samples

Zeolite A

[0026] Small crystallite CHA was prepared according to Example 1 of U.S. Pat. No. 6,709,644 (the entire contents of which is incorporated herein by reference).

Zeolite B

[0027] Large crystallite CHA was prepared according to a method of making SSZ-13 by S.I. Zones and R A. Van Nordstrand, Zeolites 8 (1988) 166 (the entire contents of which is incorporated herein by reference) also published on International Zeolite Association Synthesis Commission web-site http://www.iza-online.org/synthesis/, as follows:

[0028] The source materials were: [0029] sodium hydroxide (1 N), (Baker, reagent grade); [0030] N,N,N, trimethyl-1-adamantanammonium hydroxide (RN—OH)(O.72M); [0031] deionized water; [0032] aluminium hydroxide (Reheis F-2000 dried gel, 50% Al.sub.2O.sub.3); and fumed silica (Cab-Q-Sil, M5 grade, 97% SiO.sub.2).

[0033] The reaction mixture was prepared as follows: [0034] (1) 2.00 g 1N NaOH+2.78 g 0.72 M RN.OH+3.22 g water, add sequentially to a Teflon cup of a Parr 23 mL autoclave; [0035] (2) (1)+0.05 g aluminum hydroxide, mix until solution clears; [0036] (3) (2)+0.60 g fumed silica, mix until uniform.

[0037] The reaction mixture was crystallised: [0038] in a teflon-lined 23 mL autoclave (Parr model 4745) at a temperature of 160° C. for 4 days without agitation;

[0039] After cooling to room temperature the mixture was filtered, washed with de-mineralised water and air-dried overnight.

[0040] The resulting product was characterised by powder x-ray diffraction and identified as: [0041] CHA zeolite with a SiO.sub.2/A1203 ratio of 28 as determined by ICP.

[0042] SEM analysis showed: [0043] cubes of 2-5 micrometers.

Zeolite C

[0044] A reaction mixture was prepared of molar composition 60 SiO.sub.2-1.5 Al.sub.2O.sub.3-6 Na.sub.2O-12 NNNAnOH-2640 H.sub.2O, where NNNAnOH is the structure directing agent (SDA) or template N,N,N-trimethyladamantanammonium hydroxide

[0045] The reaction was prepared using cab-o-sil M5 (Cabot Corporation) as the source of silica, sodium aluminate (BDH Ltd), sodium hydroxide (Alfa Aesar). The SDA (NNNAnOH) was prepared following the method described in U.S. Pat. No. 4,544,538 (the entire contents of which is incorporated herein by reference). The required amount of the SDA solution was weighed out and the NaOH added and stirred until it dissolved. The sodium aluminate solid was then added with stirring and stirring was continued until it dissolved. The cab-o-sil was then mixed in and the resulting mixture transferred to a 1 L stainless steel autoclave. The autoclave was sealed and the mixture heated to 165 C with stirring (300 rpm) for 4 days.

[0046] The resulting product was identified as a CHA type material by powder x-ray diffraction.

[0047] Visually, the product crystals were approximately 2 microns on edge. The product composition had a silica-alumina ratio (SAR) of 24:1.

EXAMPLE 2

Preparation of 3 wt % Cu/Aluminosilicate Zeolite

[0048] Copper was deposited on zeolites A, B and C prepared according to Example 1 by the standard wet impregnation method using copper acetate as the copper precursor. For 10 g of aluminosilicate zeolite, 0.471 g of copper acetate was dissolved in a sufficient amount of water to wet the aluminosilicate zeolite material. The solution was added to the aluminosilicate zeolite material and stirred. The wet powder was dried at 105° C., before being calcined at 500° C. for 2 hours. Following calcination, a majority of the copper is understood to be present as copper (II) oxide.

[0049] The copper-loaded catalysts prepared according to this Example were designated as Catalysts A, B and C. Catalysts prepared according to Example 2 are referred to as “Fresh Catalysts A-C”.

EXAMPLE 3

Hydrothermal Ageing

[0050] Fresh Catalysts A-C prepared according to Example 2 were hydrothermally aged in an atmosphere containing 10% oxygen, 10% water, balance nitrogen at 750° C. for a period of 24 hours. The hydrothermally aged catalyst is referred to as “Aged Catalysts A-C”.

TABLE-US-00001 TABLE 1 surface area, silica alumina ratio, crystal size and copper loading of the different catalysts (fresh). Average SEM Chabazite BET Silica to Crystal Aluminosilicate surface alumina ratio Dimension Cu loading code area (SAR) (micrometer)* wt % A 784 26 0.15 3 B 634 24 0.5 3 C 616 24 1.4 3 *The samples were dispersed in methanol and subjected to ultrasound for 20 mins and a drop of this liquid was put on a standard carbon padded Scanning Electron Microscope (SEM) stub. Counting and sizing was determined by number averaged digital particle size analysis, based on “thresholding” the intensities from each pixel of an image, and exploiting the differences in intensity between particles and the background. The software assumes that each object detected is circular/spherical.

EXAMPLE 4

Activity Tests

[0051] The NO.sub.x conversion of Catalysts A-C of Examples 2 and 3 at an inlet gas temperature of 200° C. or 400° C. are given in Table 2. The NO.sub.x reduction performance was measured on a powder sample in a laboratory reactor by ramping the catalyst at 5° C. per minute in a gas mixture containing 500 ppm NO and NH.sub.3, 10% O.sub.2, 10% H.sub.2O and N.sub.2.

TABLE-US-00002 TABLE 2 NOx conversion at a catalyst inlet gas temperature of 200° C. and 400° C. for Fresh and 750° C. 24 hour-Aged Conditions Average SEM Cu 500° C. Calcined 750° C. Aged Crystal Dimension Loading % NOx Conversion % NOx Conversion Catalyst SAR (micrometer) † wt % 190° C. 200° C. 400° C. 190° C. 200° C. 400° C. A 26 0.15 3 73 86 99 44 58 96 B 24 0.5 3 85 95 99 51 66 97 C 24 1.4 3 87 97 99 68 83 99 † See notes on Table 1.

[0052] It can be seen from Table 2 that the activity of the catalysts generally follows a trend of increasing activity with crystallite size. Hence we conclude that larger crystallite size aluminosilicate zeolite materials are surprisingly more active either fresh or hydrothermally aged than catalysts prepared from smaller crystals of the same aluminosilicate zeolite material.