(404a) Aqueous-Phase Hydrogenation Of Organic Acids On Monometallic Supported Catalysts: A Combined Experimental And Theoretical Study

Depletion of fossil fuel combined with environmental and
political concerns with fossil fuels have accelerated research on technologies
to convert biomass into liquid fuels, i.e biofuels. Aqueous-phase hydrogenation
(APH) reactions are crucial for biofuel production including: ethanol
production by hydrogenation of fermentation products (organic acids) [1], gasoline production by hydrogenation of pyrolysis-oils [2], and alkane production by
aqueous-phase dehydration/hydrogenation of carbohydrates [3]. APH reactions
involve hydrogenating a range of functionalities, including acids (C-OOH
bonds), sugars (C-O-C bonds), alkenes (C=C bonds) and aldehydes (C=O bonds).
The objective of this project is to develop a fundamental understanding for
aqueous-phase hydrogenations of organic acids by a combined theoretical and
experimental approach.

We are in the process of measuring the catalytic activity
for aqueous-phase hydrogenation of acetic acid with supported monometallic Ru,
Pt, Pd, Ni and Cu based catalysts in a fixed bed reactor.  The Ru catalyst is
significantly more active than Pt. The reaction is first order with respect to
H₂ for Ru/Carbon with an apparent activation energy of 31 kJ/mol. The
reaction is 0.6 order with respect to H₂ for Pt-based catalysts with
an apparent activation energy of 33 kJ/mol.

These
experimental data are combined with density functional theory (DFT) calculations
to offer us insight into why some monometallic catalysts are more active.  We
have calculated the energetics and kinetics of probable elementary steps for
acetic acid hydrogenation on Ru(0001).  The results are compared with previous
DFT calculations for hydrogenation of acetic acid on Pt, Pd and Cu [4-6]. With the additional information afforded by theory we are attempting to identify the
physical origin of the difference in the activities of these monometallic
catalysts.  This study will provide valuable insight for designing a more
versatile APH catalyst in the future, which will probably be bi-metallic.

[1]  T. Eggeman, D. Verser, Recovery of
organic acids from fermentation broths, Appl. Biochem. Biotechnol. 2005,
121-124, 605.

[2]  G. W. Huber, S. Iborra, A. Corma,
Synthesis of transportation fuels from biomass: chemistry, catalysts and
engineering, Chem. Rev. 2006, 106, 4044.

[3]  G. W. Huber, J. A. Dumesic, An overview
of aqueous-phase catalytic processes for production of hydrogen and alkanes in
a biorefinery, Catal. Today 2006, 111, 119.

[4]  R. Alcala, J. W. Shabaker, G. W. Huber,
M. A. Sanchez-Castillo, J. A. Dumesic, Experimental and DFT studies of the
conversion of ethanol and acetic acid on PtSn-based catalysts, J. Phys. Chem. B
2005, 109, 2074.

[5]  V. Pallassana, M. Neurock, Reaction
paths in the hydrogenolysis of acetic acid to ethanol over Pd(111), Re(0001),
and PdRe alloys, J. Catal. 2002, 209, 289.

[6]  M. A. Natal Santiago, M. A.
Sanchez-Castillo, R. D. Cortright, J. A. Dumesic, Catalytic reduction of acetic
acid, methyl acetate, and ethyl acetate over silica-supported copper, J. Catal.
2000, 193, 16.