Low-silica chabazite zeolites with high acidity
A microporous crystalline material having a molar silica to alumina ratio (SAR) ranging from 10 to 15 and a fraction of Al in the zeolite framework of 0.63 or greater is disclosed. A method of selective catalytic reduction of nitrogen oxides in exhaust gas that comprises contacting exhaust gases, typically in the presence of ammonia, urea, an ammonia generating compound, or a hydrocarbon compound, with an article comprising the disclosed microporous crystalline is also disclosed. Further, a method of making the disclosed microporous crystalline material is disclosed.
1. A microporous crystalline material comprising a crystal structure having a CHA framework, a molar silica to alumina ratio (SAR) ranging from 10 to 15 and an acidity ranging from 1.35 mmol/g to 2.0 mmol/g.
2. The microporous crystalline material of claim 1 , where the microporous crystalline material has a fraction of Al in the zeolite framework ranges from 0.63 to 1.00 as determined by n-propylamine adsorption.
3. The microporous crystalline material of claim 1 , where the SAR ranges from 12-14.
4. The microporous crystalline material of claim 1 , further comprising at least one catalytically active metal.
5. The microporous crystalline material of claim 4 , where the at least one catalytically active metal comprises copper or iron.
6. The microporous crystalline material of claim 5 , wherein the catalytically active metal comprises copper (Cu), which is present in a Cu:Al atomic ratio of at least 0.25.
7. The microporous crystalline material of claim 6 , wherein said material has at least 65% XRD retention after steaming at 775-800° C. for up to 16 hours.
8. The microporous crystalline material of claim 1 , where said material comprises a mean crystal size ranging from 0.5 to 5 microns.
9. A method of selective catalytic reduction of nitrogen oxides in exhaust gas, said method comprising:
at least partially contacting said exhaust gases with an article comprising a microporous crystalline material comprising a crystal structure having a CHA framework, a molar silica to alumina ratio (SAR) ranging from 10 to 15 and an acidity ranging from 1.35 mmol/g to 2.0 mmol/g.
10. The method of claim 9 , where said contacting step is performed in the presence of ammonia, urea, an ammonia generating compound, or a hydrocarbon compound.
11. The method of claim 9 , where the microporous crystalline material has a fraction of Al in the zeolite framework ranges from 0.63 to 1.00 as determined by n-propylamine adsorption.
12. The method of claim 9 , where the SAR ranges from 12-14.
13. The method of claim 9 , further comprising at least one catalytically active metal chosen from copper or iron.
14. The method of claim 13 , wherein the catalytically active metal comprises copper Cu, which is present in a Cu:Al atomic ratio of at least 0.25.
15. The method of claim 13 , wherein the material has at least 65% XRD retention after steaming at 775-800° C. for up to 16 hours.
16. The method of claim 9 , where said material comprises a mean crystal size ranging from 0.5 to 5 microns.
17. A method of making microporous crystalline material having a molar silica to alumina ratio (SAR) ranging from 10 to 15 and an acidity ranging from 1.35 mmol/g to 2.0 mmol/g, said method comprising:
mixing sources of alumina, silica, alkali containing additive, an organic structure directing agent, water and optionally a seed material to form a gel;
heating the gel in an autoclave to form a crystalline CHA product;
calcining said CHA product; and
ammonium-exchanging said CHA product.
18. The method of claim 17 , further comprising introducing at least one catalytically active metal into the microporous crystalline material by liquid-phase or solid-phase ion exchange, impregnation, direct synthesis or combinations thereof.
19. The method of claim 18 , where the at least one catalytically active metal comprises copper or iron.
20. The method of claim 19 , wherein the catalytically active metal comprises copper Cu, which is present in a Cu:Al atomic ratio of at least 0.25.
21. The method of claim 20 , wherein the material has at least 65% XRD retention after steaming at 775-800° C. for up to 16 hours.
22. The method of claim 17 , wherein the organic structure directing agent comprises N,N,N-Trimethyl-1-adamantylammonium hydroxide.
23. The method of claim 17 , where the alkali containing additive comprises a source of potassium or sodium, or a mixture of thereof.
24. The method of claim 17 , where said material comprises a mean crystal size ranging from 0.5 to 5 microns.