PETR 10 |
| The role of organic cations (e.g., quaternary ammonium cations) in zeolite synthesis and crystallization remains enigmatic and controversy surrounds aspects of their function. In addition, the role of the solvent(s) has remained elusive. In this work, all-explicit-molecule molecular dynamics simulations have been carried out using established force fields along with ab initio simulations for computing partial charges. We have found a remarkable behavior supporting the external scaffold mechanism, whereby adsorption of the cations on silica precursors expels water selectively from the Si-O-Si bridge sites. This finding accompanied with a kinetic slow down of water mobility result in protecting these bonds from hydrolysis. In addition, we have found that alcohols have an interesting effect on the stability of nanoparticles via hetero-molecule H-bond network causing solvent cavities. Free energies of nanoparticle association rationalize for the first time the enhanced stability of zeolitic precursors in solution arising from the dynamics of cation adsorption that selectively unscreen electrostatic interactions. The implications for zeolite nucleation and growth are discussed. |
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Zeolite Catalysis for Green Chemistry in Synthesis of Chemicals and Fuels
8:20 AM-12:00 PM, Sunday, April 6, 2008 Morial Convention Center -- Rm. 209, Oral
Division of Petroleum Chemistry |