Synthesis of hierarchical nanocrystalline zeolites with controlled particle size and mesoporosity
A one step synthesis of nanocrystalline zeolites ZSM-5 and Naβ from a single template system in high yield has been discovered. The size of individual nanocrystals, as well as mesopore surface area and pore volume can be controlled by adjusting the pH of the reaction mixture, as well as the hydrothermal treatment temperature and duration. The mesopore volume and size distribution show a dependence on particle size such that smaller particles lead to higher mesopore volumes and narrower pore size distributions.
1. A method of preparing mesoporous nanocrystalline zeolite comprising:
(a) contacting a silicon source, a single mono-ammonium organic template, and an aluminum source, in a solvent, for a period of time sufficient to produce a zeolite gel by hydrolysis;
(b) adjusting the concentration and pH of the zeolite gel by evaporating 50% to 100% of the solvent;
(c) heating the zeolite gel, at a temperature and pressure sufficient to produce mesoporous nanocrystalline zeolite crystals;
(d) separating the mesoporous nanocrystalline zeolite crystals;
(e) washing and drying the mesoporous nanocrystalline zeolite crystals; and
(f) calcining the mesoporous nanocrystalline zeolite crystals to remove the template from the mesoporous nanocrystalline zeolite crystals.
2. The method of claim 1 , wherein the zeolite is zeolite ZSM-5.
3. The method of claim 2 , wherein the size of the zeolite ZSM-5 nanocrystals is between about 6 to about 40 nm.
4. The method of claim 2 , wherein the total surface area (S tot ) of the zeolite ZSM-5 is from about 320 to about 440 m 2 /g.
5. The method of claim 2 , wherein the internal surface area (S int ) of the zeolite ZSM-5 is from about 264 to about 313 m 2 /g.
6. The method of claim 2 , wherein the external surface area (S ext ) of the zeolite ZSM-5 is from about 20 to about 200 m 2 /g.
7. The method of claim 2 , wherein the mesopore volume (V meso ) of the zeolite ZSM-5 is from about 0.085 cc/g to about 0.43 cc/g.
8. The method of claim 2 , wherein the heating is carried out at a temperature of from about 140 to about 165° C. for about 12 to 72 hours.
9. The method of claim 2 , wherein the zeolite ZSM-5 zeolites are formed at a pH of from about 10.93 to about 11.52.
10. The method of claim 2 , wherein the diameter of the zeolite ZSM-5 mesopores is between about 4 to about 12 nm.
11. The method of claim 2 , wherein the micropore volume (V micro ) of the zeolite ZSM-5 is from about 0.102 to about 0.151 cc/g.
12. The method of claim 1 , wherein the zeolite is zeolite Naβ.
13. The method of claim 12 , wherein the size of the zeolite Naβ nanocrystals are between about 15 to about 40 nm.
14. The method of claim 12 , wherein the diameter of the zeolite Naβ mesopores are between about 10 to about 15 nm.
15. The method of claim 12 , wherein the total surface area (S tot ) of the zeolite Naβ is from about 638 to about 664 m 2 /g.
16. The method of claim 12 , wherein the external surface area (S ext ) of the zeolite Naβ is from about 20 to about 200 m 2 /g.
17. The method of claim 12 , wherein the zeolite Naβ is present as aggregates of from about 120 to about 140 nm.
18. The method of claim 12 , wherein the micropore volume (V micro ) of the zeolite Naβ is from about 0.144 to about 0.233 cc/g.
19. The method of claim 12 , wherein the mesopore volume (V meso ) of the Naβ is from about 0.400 cc/g to about 0.838 cc/g.
20. The method of claim 12 , wherein the heating is carried out at a temperature of from about 120 to about 130° C. for about 30 to about 34 hours.
21. The method of claim 12 , wherein the Naβ zeolites are formed at a pH of from about 12.03 to about 12.19.
22. The method of claim 1 , wherein the solvent comprises water.
23. The method of claim 1 , wherein the silicon source comprises tetraethylorthosilane (TEOS) (C 8 H 20 O 4 Si).
24. The method of claim 1 , wherein the mono-ammonium organic template comprises a tetraalkylammonium cation.
25. The method of claim 24 , wherein the tetraalkylammonium cation comprises tetraethyl ammonium cation or tetrapropyl ammonium cation.
26. The method of claim 1 , wherein the aluminum source comprises sodium aluminate (NaAlO 2 ).