KAIST researchers extend zeolite-like materials into the mesoporous range; highly active catalysts for bulky molecules
Zeolites—nanoporous molecular sieves that can be used to separate or selectively adsorb guest molecules according to size and shape and are excellent acid catalysts—are workhorses of catalysis, representing more than 40% of the entire solid catalysts currently used in the chemical industry. However, because zeolites are nanoporous, with pore diameters <2 nm, their applications have been limited to small molecules that can diffuse through the narrow porous frameworks. There has been much research activity focused on developing zeolite-like materials that can catalyze reactions with larger molecules.
Researchers at the Korea Advanced Institute of Science and Technology (KAIST), led by Ryong Ryoo, report on a new approach for creating new zeolite-like materials—mesoporous molecular sieves (MMSs)—featuring larger pores that can catalyze reactions with larger molecules. Their paper is published in the journal Science.
In the dual-porogenic surfactant-driven synthesis mechanism, mesopores are generated by surfactant aggregates, whereas crystalline microporous zeolite frameworks are generated by multiple quaternary ammonium groups. The wall thickness and framework topology can be adjusted by using surfactants with different gemini-like head groups.
The mesopore diameters are tailorable according to the surfactant tail length or by the addition of hydrophobic swelling agents. The mesoporous structure and strong zeolitic framework acidity result in substantially improved catalytic activities for various organic reactions involving bulky molecules compared with conventional zeolites or amorphous MMSs.
It is also possible to use crystalline MMSs as a selective adsorbent for separation of proteins according to the molecular sizes. Bulky enzyme species can be immobilized via covalent bonding, van der Waals forces, or electrostatic interactions with the zeolite frameworks. Furthermore, the synthesis of MMSs can be extended to other inorganic compositions, such as aluminophosphates.—Na et al.
Kyungsu Na, Changbum Jo, Jeongnam Kim, Kanghee Cho, Jinhwan Jung, Yongbeom Seo, Robert J. Messinger, Bradley F. Chmelka, and Ryong Ryoo (2011) Directing Zeolite Structures into Hierarchically Nanoporous Architectures. Science 333 (6040), 328-332 doi: 10.1126/science.1204452
Karin Möller and Thomas Bein (2011) Pores Within Pores—How to Craft Ordered Hierarchical Zeolites. Science 333 (6040), 297-298. doi: 10.1126/science.1208528