(338c) Hydrodeoxygenation of Biomass Derivatives on Metal-Modified Molybdenum Carbides

Hydrodeoxygenation of Biomass Derivatives on Metal-Modified
Molybdenum
Carbides

Weiming
Wan, Jingguang G. Chen

Department
of Chemical Engineering, Columbia University, New York, NY, 10027

Biomass has attracted significant attentions
as an alternative resource for fuel and chemicals. However, biomass derivatives have a high
O/C ratio and low energy density that make them unsuitable for use in the fuel
and chemical sectors.^[1] Thus, the hydrodeoxygen
(HDO) reaction is necessary to upgrade these biomass derivatives to value-added
products. For example, furfural and glycerol are two typical biomass derivatives.
Through HDO reactions, furfural can be converted to 2-methylfuran, a promising
fuel additive, and glycerol can be converted to propylene or propanediol, an
important monomer.

Transition
metal carbides are novella class of interesting materials that often
demonstrate unique catalytic properties. Due to the high oxygen binding energy,
which helps to break the C-O bond, many metal carbides have shown activities
towards HDO reactions. Mo₂C is a typical transition metal carbide and
it shows high HDO activity for oxygenates such as furfural and propanol.^[2],[3] However, the high oxygen
binding energy makes it hard to remove the surface oxygen to complete the
catalytic cycle. Metal modifiers can be used to adjust the oxygen binding
energy. The strong oxophilic metal modifiers, such as Co and Fe, have slightly
lower oxygen binding energies than Mo₂C, and the modified Mo₂C
surface could have a similar HDO selectivity with better stability. In contrast,
the weak oxophilic metal modifiers, such as Cu and Pt, have much lower oxygen
binding energies than Mo₂C, and the modified Mo₂C surface
should have a different reaction pathway and selectivity.

The
current work combines density functional theory (DFT) calculations and surface
science experiments using temperature programmed desorption (TPD) and
high-resolution electron energy loss spectroscopy (HREELS) under ultrahigh
vacuum (UHV) conditions. The main objective is to study the effect of metal modifiers on
the adsorption geometry and selective HDO of furfural and glycerol on the Mo₂C
surface. The Mo₂C surface was prepared by cycles of ethylene treatment on the Mo(110) single crystal,
and the metal layer was prepared by physical vapor deposition. 

In
the furfural HDO reaction, compared with the Mo₂C surface, furfural
has a similar adsorption geometry on Co-modified Mo₂C, suggesting the
same reaction pathway on both surfaces. The lower oxygen binding energy of the
Co/Mo₂C surface improves the catalytic stability in the TPD
experiments. On the other hand, Pt/Mo₂C has lower oxygen binding
energy than Co/Mo₂C and the furan ring is flat on the Pt/Mo₂C
surface. The flat furan ring and the low oxygen binding energy help produce
more furan through decarbonylation.

In
the glycerol HDO reaction, the Mo₂C surface can break all the C-O
bonds to make propylene. Similarly, the Fe/Mo₂C surface can produce
propylene from glycerol with a better stability. On the other hand, the Cu/Mo₂C
surface can selectively break the C-O bonds. With a high Cu coverage, the Cu/Mo₂C
surface can break one C-O bond to form acetol; with a
low Cu coverage, the Cu/Mo₂C surface can break two C-O bonds to form
ally alcohol.

This
work provides guidance for designing metal-modified carbide HDO catalysts and a
detailed discussion of the DFT calculations and the
surface science experiments will be presented.

References:

[1]    K. Xiong, W.
Yu, D. G. Vlachos, J. G. Chen. ChemCatChem 2015, 7 (9),
1402.

[2]    K. Xiong, W.
Lee, A. Bhan, J. G. Chen. ChemSusChem 2014, 7 (8), 2146.

[3]    H. Ren, W.
Yu, M. Salciccioli, Y. Chen, Y. Huang, K. Xiong, D. G. Vlachos, J. G. Chen. ChemSusChem
2013, 6 (5), 798.