Confinement Effects on Organic Reactions
There is emerging interest in understanding the effects of confinement of organic molecules in nanoporous solids on their chemical properties and reactivity. Possible applications include the use of these materials as nanoreactors for organic synthesis, as chemical sensors, for drug and gene delivery, and for design of asymmetric catalysts for the synthesis of chiral organic molecules. In our research, we have been examining the effect of pore confinement on the thermochemical reactions of fuel model compounds that involve free-radical intermediates. For nanoreactors, we use hexagonal mesoporous silicas such as MCM-41 and SBA-15, which allows for confinement in pores of varying sizes ranging from ca. 2-7 nm in diameter. To immobilize the probe molecules onto the pore walls, we use covalent attachment via condensation of phenols or alcohols with the surface hydroxyl groups. This has the advantage over typical silane coupling chemistry in that surface bound products are easily retrieved for analysis by simple aqueous base hydrolysis of the silica.
Attachment of Diphenylpropane into a Mesoporous Silica via Phenol Condensation
This research involves the synthesis and characterization of the mesoporous silicas and organic-inorganic hybrid materials. Characterization tools such as FTIR, multinuclear NMR, nitrogen physisorption (BET analysis), and TGA-MS reside in our group. Additional techniques such as XRD, SEM, and TEM are available through collaboration with other research groups at ORNL. Pyrolysis reactions results in both gas-phase and surface-attached products which can be independently analyzed. To understand the influence of pore confinement, the pyrolysis chemistry is compared with that of the same molecule in the gas phase and tethered to the surface of a non-porous silica such as Cabosil.
From Pyrolysis of Phenethyl Phenyl Ether Confined in MCM-41
Depending on the molecule being investigated, and the exact nature of the free-radical decay mechanism, we have seen both modest rate increases and decreases. We have recently discovered that the product selectivity in the pyrolysis of phenethyl phenyl ether (PPE) in mesoporous silicas can be dramatically altered compared with typical gas-phase behavior. The alpha-/beta- product selectivity shown above, which is 3:1 in the gas phase, increases to 45:1 in an SBA-15 silica that also contains tethered biphenyl molecules as separating molecules on the surface. Research continues to explore the origin of these effects and our ability to control and manipulate this product selectivity.
We are also interested in understanding how pore nanoconfinement influences the molecular dynamics, and how this may impact chemical reactivity. Molecular modeling and simulations are providing considerable insights in this area. These studies are conducted in collaboration with Professor Alan Chaffee of Monash University in Australia.
MCM-41 2.9 nm pore with
Our experimental studies of pore confinement on dynamics use fluorescent probe molecules such as pyrene, which can be probed by steady-state and time-resolved fluorescence spectroscopy techniques. A complementary experimental approach we have recently begun to use is Quasi-Elastic Neutron Scattering (QENS), which is conducted in collaboration with Dr. Ken Herwig at the Spallation Neutron Source (SNS) at ORNL. The QENS experiments are conducted on the BASIS backscattering spectrometer at SNS, which can examine diffusive and relaxation dynamics of molecules on the atomic length scale on the pico- to nanosecond time scale. Our studies of PPE confined in MCM-41, when compared with the structurally related diphenylpropane, provide evidence for hydrogen bonding of the ether oxygen in PPE with surface silanols as suggested from the molecular dynamics simulations. We also observe that hydrogen bonding becomes increasingly significant at lower PPE grafting densities.
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Physical Organic Chemistry R & D Projects