Current Research

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Heterogeneous Catalysis
Zeolite Synthesis
Solid-State Nuclear Magnetic Resonance

Heterogeneous Catalysis
Catalysis is the study of key additives that enable low-energy reaction pathways for desirable chemical reactions.  We are interested in a particular class of catalysts known as zeolites.  Zeolites are very stable non-toxic and easily made into solid pellets preventing contamination of valuable chemical feedstocks.  Mature commercial processes that use zeolite catalysts include paraffin reformulation, butene isomerization, methanol to gasoline (MTG) and fluid-catalytic cracking (FCC, the core technology in gasoline refining).

Research in our group is focused on gaining additional insight into zeolite catalytic processes in order to improve efficiency.  We are particularly interested in demonstrating catalytic routes to new chemicals like pharmaceutical intermediates, specialty plastics and other consumer products like fragrances and artificial sweeteners.  In order for zeolite catalysts to make an impact in these chemical markets the catalytic process must be very selective, i.e., high conversion to a single target molecule.

Green Chemistry describes the popular growing trend to reduce the use of harmful solvents, eliminate the use of dangerous reagents and replace traditional multi-step syntheses with cleaner more efficient reactions.  This is the major motivation for targeting the specialty chemical markets is to reduce the use of dangerous chemicals like chlorofluorocarbons (CFCs) and deadly reagents like HF or POCl2.  For example, the pharmaceutical industry is estimated to produce 75 mole equivalents of byproducts for every mole of therapeutic compound.  We hope to demonstrate zeolitic routes to a few key intermediates with potential to improve the environmental impact of the drug market.
 

Zeolite Synthesis
Since the earliest reports of zeolite synthesis by Barrer in the 1950s this field has resembled a scavenger hunt for new materials. By varying the "template" molecule added to an aqueous solution of mineralized silica (and co metal) nearly 100 different zeolites have been synthesized. Other synthetic variables including the source of the inorganic precursors, the mineralizing agent (OH- or F-) and the reactant concentrations have also resulted in new crystalline materials. In spite of the aggressive research efforts for nearly three decades few if any hard empirical rules have been established that explain the wealth of synthetic results. In fact recent experimental results leave us to question what little we thought we knew about zeolite nucleation and growth, e.g., possible at room temperature, at atmospheric pressure, in non-aqueous environments, and accelerated by acidic anions.

Clearly zeolite synthesis is not a mature field despite the variety of zeolite topologies and compositions. The primary structural feature of the materials that interests us is the low symmetry of the internal surface, e.g., undulating one-dimensional channels (IRF), intersecting channels with different diameters (CON). Such surfaces possess the unrealized potential to catalyze very selective, possibly asymmetric, reactions. The two examples cited were just recently discovered and their syntheses are far from optimized; the majority of the reported information is cited in patent literature. Synthetic goals in our research include the optimization of reported syntheses, variation of the composition of zeolites with known topologies and understanding the influence of synthetic conditions that favor the formation of these low-symmetry crystalline phases.

Our zeolite synthesis investigations are vital and enable three key aspects of every research project in the group. First, we are able to investigate the newest, most promising classes of zeolite materials where there is no tenable source. Most of the synthetic zeolite phases are heavily protected industrial property. Second, the combination of high-resolution spectroscopy and catalytic investigations demands the highest level of sample purity, in reference to both crystallinity and composition. Even small amounts of paramagnetic metal impurities in the mineral sources used in the zeolite preparation will destroy local magnetic field homogeneity. Third, our expertise in materials synthesis enables us to design a material's composition in order to match the catalytic demands for a particular reaction study. For example, acid strength is highly dependent on both Si/M+ ratio and M+ distribution within the crystallite.

Solid-State NMR
Nuclear Magnetic Resonance (NMR) is the most commonly used spectroscopic analysis technique in chemistry.  The greatest utility of NMR is identifying organic compounds in solution based on chemical shift and peak multiplicity.  We are extending the applicability of NMR as a characterization tool for the analysis of chemical processes in condensed media.  The surface mediated reactions of zeolite catalysts are the primary focus.

Structures of adsorbates and reaction intermediates inside the porous cavities of the zeolite catalyst can be obtained using a number of high-resolution NMR techniques.  Accurate distances can be obtained with state-of-the-art pulse sequences like REDOR, TRAPDOR, DRAWS and probe internuclear dipolar interactions.  Information about the reactive, intermediate species provides details about the heterogeneous catalytic mechanism.

Dynamic systems like chemical reactions are challenging systems to study, i.e., like hitting a moving target.  NMR is uniquely capable of probing dynamics occurring from 10-6 to 1 s time-scale that covers many processes of interest.  The NMR hardware in our lab is specialized to monitor chemical reactions under temperatures and pressures that simulate real reaction conditions.  By monitoring the catalytic process in real-time we observe the sequence of events that occur as adsorbates are transformed to important products.