COLL 349 |
| Raul F Lobo, Department of Chemical Engineering, Department of Chemical Engineering, University of Delaware, Newark, DE 19716 |
| High-silica zeolites are a prototypical example of an important class of materials that self-assemble from mixtures of inorganic and organic moieties. This complex mixture evolves from an ‘amorphous gel’ into a highly organized hybrid crystalline material, where an inorganic host framework surrounds an organic guest or structure-directing agent. Much effort has been devoted to understanding the nucleation and growth mechanisms of these materials and in particular a model system —the so-called clear gel synthesis of silicalite (siliceous ZSM-5) using tetrapropylammonium hydroxide (TPA OH) as structure director—has been closely examined. The most general finding has been that previous to the observation of any crystals, the silica-TPAOH-water mixture rapidly microsegragates forming two metastable phases: a continuous water-rich phase containing most of the water, a fraction of the TPAOH and a small amount of silica, and a discontinuous silica-rich phase that is observed in the form of nanoparticles of ~3-5 nm. Their presence, before and during crystal growth, has been established by many groups, although their role in the nucleation and growth of zeolites remains unclear and several —mutually exclusive—hypothesis have been put forward as to their shape, composition and internal structure. In this talk I will present the results of recent investigations of the properties and structure of these suspensions of TPA-silica nanoparticles and their relation to the growth of silicalite crystals. We will show that many similarities exist between the properties of the TPA-silica nanoparticles and the properties of surfactant self-assembled systems. We will describe the underlying chemical forces that drive the aggregation of the silica and the TPA and discuss the basis for the high stability of concentrated suspensions of these nanoparticles. The colloidal forces between nanoparticles and between the nanoparticles and the growing zeolite surface will be examined and a model for the growth of the zeolite crystals that incorporate the nanoparticles will be presented. This ‘colloidal’ view of the nanoparticles leads naturally to highlight parallels between this system and the synthesis of ordered mesoporous silicates and the base-catalyzed synthesis of sol-gel materials. |
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Awards Lectures
2:00 PM-4:05 PM, Tuesday, March 30, 2004 Marriott -- Orange County 4, Oral
Division of Colloid and Surface Chemistry |