(81e) Selective Activation and Conversion of Simple Carboxylic Acids and Esters

Selective activation and conversion of simple carboxylic
acids and esters

Ye Xu,^* and Lijun Xu

Center for Nanophase Materials Sciences, Oak Ridge
National Laboratory, Oak Ridge, Tennessee 37831, USA

^* xuy2@ornl.gov

The hydrogenation of carboxylic acids and esters to the
corresponding alcohols are industrially important processes, and will likely be
an integral pathway in biorefineries for the production of valuable chemicals
and fuels from biomass feedstock because carboxylic acids and esters are common
components of such feedstock.  Acids, in particular, are more difficult to
reduce than other classes of organic oxygenates.  Ester hydrogenation is traditionally
catalyzed by Cu and Cu compounds, which requires high temperature and
pressure.  Pt group metals can decompose acids and esters at milder conditions but
offer very limited selectivity for alcohols in gas-phase experiments.

Recently Olcay et al. demonstrated that significant
selectivity for ethanol can be obtained via the aqueous-phase hydrogenation
(APH) of acetic acid on a series of metals, with Ru being the most active and
selective at low temperatures.¹  Ru has also been used in the APH of other organic acids with good activity and selectivity.²  We have performed density functional theory calculations to understand the role of different metal species and surface structure in the activation and conversion of simple carboxylic acids and esters on metal surfaces, using acetic acid and methyl acetate as representative compounds.

Our calculations for the activation and subsequent
hydrogenation of acetic acid on several metals suggest that the activity of acetic
acid conversion is dictated largely by the intrinsic reactivity of the metals.¹  In agreement with experimental results, the selectivity to ethanol is found to depend on reaction conditions.  Dehydrogenation and decomposition are the dominant reaction pathway for acetic acid and methyl acetate at vacuum/low pressure conditions on Ru and Pd, which fundamentally limits the selectivity to ethanol.³  The activity of C-H and C-O bond dissociations is different on close-packed, open, and under-coordinated Pd sites:  Pd step edges and kinks promote both C-H and C-O bond dissociation steps primarily via electronic effects, while (100)-type square surface symmetry is more effective at stabilizing C-O bond dissociation transition states than (111)-type hexagonal symmetry does, thereby differentially promoting C-O relative to C-H bond dissociation.⁴

References

1      H. Olcay, L. Xu, Y. Xu, and G. W. Huber, ChemCatChem
2, 1420 (2010).

2      Y. Q. Chen, D. J. Miller, and J. E. Jackson, Ind.
Eng. Chem. Res. 46, 3334 (2007).

3      L. Xu and Y. Xu, Surf. Sci. 604,
887 (2010).

4      L. Xu and Y. Xu, Catal. Today
doi:10.1016/j.cattod.2010.12.021.