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The overall goal of this research is to examine the adsorption and reaction mechanisms of lactic acid, a biorenewable molecule, with magnesium oxide using theoretical methods.  Previous experimental work showed that polylactic acid polymers grown on various MgO nanomaterials have different morphologies, which motivates this study of initial surface reactions.  In the first stage of this research, the adsorption modes of lactic acid (LA) on model (MgO)_n (n = 2, 4, 6, 8, 9, 12) clusters have been examined.  Methods employed include density functional theory (DFT) - specifically the PBE functional - and second-order perturbation theory (MP2); single-point calculations using coupled cluster theory including CCSD(T) are also in progress.

As originally hypothesized, dissociative adsorption of the carboxylic acid group leads to the most stable LA-(MgO)₂ complexes, followed by dissociative adsorption of the hydroxy group. Molecular adsorption of the carboxy or hydroxy groups lies about 50-70 kcal/mol higher in energy. In the lowest energy complexes, two oxygen atoms from LA commonly interact with the (MgO)₂ cluster.  These two oxygen atoms are not necessarily the two from the carboxylic acid group.  We find that the dissociative adsorption of the carboxy and/or hydroxy groups are spontaneous (barrierless) at low-coordinated sites.  The most favorable carboxy dissociation product is predicted to have a binding energy of 92.6 (111.2) kcal/mol at the PBE(MP2)/TZV(d,p) level of theory.

Examination of basis sets up to the triple-zeta level of theory show that the binding energies and relative isomer energies of lactic acid-MgO complex structures are mostly insensitive to basis set.  However, MP2 calculations predict binding energies that are approximately 20% stronger than those predicted by PBE.  CCSD(T) single point calculations are expected to indicate which level of theory is more reliable for prediction of absolute binding energies.  Even so, the relative isomer ordering is consistent regardless of level of theory.

As expected, the size of the MgO cluster greatly affects the binding energies.  Lactic acid binds to (MgO)_n (n = 4, 6, 8) clusters with binding energies that are approximately 15-20 kcal/mol lower than (MgO)₂.  Binding energies to the larger n = 9 and 12 clusters are approximately 30 kcal/mol lower than (MgO)₂. Depending on the coordination environment of the lactic acid molecule, full optimization of the (MgO)₉  and (MgO)₁₂ systems leads to hexagonal “nanotube” structures such as those observed previously for formaldehyde on (MgO)₁₂, which are significantly (~10 kcal/mol) lower in energy than rectangular structures.  (It should be noted that MgO nanomaterials can have a hexagonal morphology.)  We have found that addition of a third layer eliminates this structural distortion.

The initial LA-(MgO)_n complex formation stage of this research (performed by graduate student Lila Pandey) is nearing completion.  Current undergraduate Christian Montes is examining how water affects the predicted LA-(MgO)_n complexes. Postdoctoral fellow Choongkeun Lee is examining secondary reactions that can occur on the surface of MgO after LA binds; this will show whether polymerization reactions aside from the standard dehydration reaction can occur.  Overall, this project is providing necessary information about how biorenewable molecules that have multiple functional groups such as lactic acid interact with metal oxide surfaces.

The ACS-PRF DNI grant has enabled my group to initiate a new area of research.  Graduate student Lila Pandey has benefitted from the opportunity to apply not only DFT but also MP2 and CCSD(T) calculations to a system of experimental relevance.  He has learned how to read scientific papers and extract important details.  Undergraduate Christian Montes is just starting his scientific career, so this project is enabling him to learn the basics of building initial structures, optimizing geometries, and analyzing binding energies and structures from various levels of theory.  Postdoctoral fellow Choongkeun Lee is learning how to perform transition state searches on complex systems as well as developing a stronger view of how theoretical calculations can be employed to elucidate experimental reactions.