Chemical Engineering Department Seminar Series

"The Self-organization of Organic-inorganic Crystalline Materials"

Raul F. Lobo
Center for Catalytic Science and Technology
Department of Chemical Engineering, University of Delaware

Zeolites are interesting materials from a practical, scientific and aesthetic point of view.  They are microporous silicates containing ordered cavities of the same dimensions as many useful molecules, and their pores are effective nanoreactors where it is possible to control chemical reactivity and selectivity with no parallel among industrial heterogeneous catalysts. They are in fact the most important class of solid acids used in the chemical industry. Their structures are also very appealing due to their complexity and presence of periodic void spaces, properties that make them distinct from many other crystalline solids.  Our group has been investigating how these materials form starting from amorphous silica gels, motivated by the preparation of better and novel materials for catalysis and other applications.  These studies have revealed that the process of zeolite formation can be definitely different from classical ideas of nucleation, as well as different from more recent theories of crystal nucleation of small molecules and proteins. This talk will discuss our findings on zeolite systems and will discuss whether other processes of self-organization of hybrid organic-inorganic materials could proceed through similar mechanisms.

We have investigated the microstructure of silica in basic aqueous solutions containing organic cations and monomeric silica precursors within the context of classical ideas of self-assembly of molecular aggregates. The solution properties can be understood by using the pseudo-phase separation approach coupled to the acid-base chemistry of silanol groups and the Poisson-Boltzmann equation. The silica nanoparticles frequently observed in these systems have a core-shell structure with silica in the core and the organic cations at the shell. Individual particles are observed when the forces between particles are repulsive-as is the case for small cations such as tetramethylammonium or tetrapropylammonium-and extended structures such as ordered mesoporous silicas are formed when the forces are attractive--as is the case for surfactants such as cetyltrimethylammonium. These ideas are useful to understand the evolution of zeolite synthesis gels from nucleation to crystal growth. Although at room temperature the silica and the organic cations are spatially segregated, upon heating the organic cations are embedded within the particles. This transformation signals the onset of structure direction whereby the size and geometry of the organic moiety induce changes in the structure of silica that may lead to zeolite nuclei.  We will show that zeolite nuclei can in fact “identify” each other in a population containing mostly amorphous particles, and that single crystals can arise from reorganization of aggregates of crystalline nanoparticles.