Following the startling discovery of mesostructured M41S metal oxides by the Mobil research group in 1992, I realized that the supramolecular charge-matching pathway used to obtain such structures inherently compromised the crosslinking of the subunits comprising the framework walls. For every uncompensated electric charge on the structure-directing surfactant micelles, and equal charge of opposite sign had to be carried by the oxide framework. In the case of silicate frameworks, for example, this meant that a substantial fraction of the framework SiO4 units had to bear a negative charge by terminating in the form of dangling Si-O bonds. Thus, electrostatic charge-matching assembly pathways limited the crosslinking of the units and compromised the hydrothermal stability of the framework. In an effort to circumvent this limitation, we developed electrically neutral assembly pathways using non-ionic amine and polyethylene oxide surfactants, along with nonionic inorganic precursors such as hydrolyzable metal alkoxides, to assemble electrically neutral framework structures. This approach depended on hydrogen-bond interactions between the structure-directing surfactant micelles and the inorganic precursors for the assembly of the final mesostructure. This strategy also allowed for greater framework crosslinking and improved hydrothermal stability of the framework. Furthermore, it meant that the surfactant could be separated from the as-made mesostructure by simple solvent extraction and recycled, thus avoiding the need to remove the surfactant by cumbersome ion exchange or destructive combustion processes.

  What were the greatest challenges in performing and presenting your work?

One of the anticipated consequences of the electrically neutral self-assembly of metal oxides was a loss in long-range ordering. Metal oxide mesotructures formed through hydrogen-bonding interactions, for example, were characterized as being "wormhole structures." That is, the framework pore pattern resembled randomly interpenetrating arrays of cylindrical channels, as expected for weakly interacting forces that fall off very rapidly with increasing distance between the self-assembling units. In comparison, electrostatically assembled structures exhibited much more elegant hexagonal, cubic, or lamellar long-range pore patterns. However, we were able to show that the framework disorder inherent in the mesostructures assembled through H-bonded pathways was advantageous in improving the performance characteristics of these materials in heterogeneous catalysis applications. The wormhole framework disorder, together with the sponge-like nature of the resulting particles, greatly reduced the diffusion length for reaction in comparison to the diffusion length of the more ordered and monolithic particles formed through electrostatic assembly pathways. Thus, far more efficient condensed phase catalytic reactions could be achieved using wormhole framework structures than ever could be realized with electrostatically assembled structures, simply because the active sites of the disordered frameworks were much more accessible for reaction and less likely to be under diffusion control.

  What unexpected or serendipitous events arose in the course of your research?

Several timely events facilitated our entry into the field of molecular self-assembly in 1992. In the late 1980s Michigan State University had established a Center for Fundamental Materials Research that provided the X-ray diffraction, electron optics, NMR, and adsorption equipment needed to study mesostructured molecular sieves. Equally important, the CFMR had brought me into collaborative interactions with colleagues in condensed matter physics. These people embraced problems related to disorder in the solid state and provided me with invaluable new insights. Thus, the newly acquired physical facilities and the ongoing interdisciplinary collaborations with solid state physicists greatly accelerated our contributions to this emerging new area of research.

Mesostructured metal oxide catalysts hold great promise for the catalytic conversion of molecules that are too large for efficient processing over conventional oxides.

For instance, large-pore mesostructured aluminosilicates are next-generation candidates for the catalytic alkylation of aromatic substrates, particularly those that are unstable in the gaseous state and require the presence of a solvent for reaction. The Lewis acid catalysts in current use (e.g., aluminum chloride) are too inefficient and environmentally problematic for sustainable long-term use as alkylation catalysts. Pharmaceutical chemistry, in particular, is expected to benefit from advances in mesostructured alkylation catalysts. Also, the "heavy end" molecular weight components of petroleum distillates, which have kinetic diameters too large to be accessed by the acid sites of zeolites for cracking to transportation fuels, should be efficiently processed by the new mesostructured catalyst systems. To realize these and related materials applications, however, we need to further improve the acidity and stability of the framework walls. Thus, the next 10 years of research effort on the molecular self-assembly of mesostructured metal oxides will see increased emphasis on controlling the molecular and atomic order in the framework walls of these materials.

  Which of your professional achievements brings you the most satisfaction?

I derive great satisfaction out of the creative processes provided by chemical research. Rearranging atoms into a new form of matter is as exciting to me now as it was in high school when I carried out my first synthesis. For me, making, characterizing, and reporting new composition of matter is as good as it gets!

  What lessons would you draw from your work to pass on to the next generation of researchers?

Scientific research can be viewed and characterized in many different ways. The ways that I have come to appreciate it relate to the special opportunities it provides for individual discovery and the expression of creativity. These two aspects of have been enough to keep me fully committed throughout my career.

Dr. Thomas J. Pinnavaia
Michigan State University
Department of Chemistry
East Lansing, Michigan, USA