(544e) Understanding Trends in Hydrodeoxygenation of Biomass-Derived Phenolics from C-C Versus C-O Scission Reaction in Ethanol on Stepped Surfaces

Hydrodeoxygenation
(HDO) reactions constitute an essential class of biomass processing reactions
employed for the catalytic deoxygenation of platform molecules. The C-C versus
C-O bond cleavage plays an important role in these reactions. To gain insights
into the HDO of biomass derived molecules, ethanol has been used as a model
molecule. Density
Functional Theory and microkinetic modelling using
CatMAP¹ have been utilized to determine the catalytic activity and
selectivity trends of HDO reaction over stepped sites of transition state
metals. For ethanol hydrogenation, various routes for decomposition have been
studied: α-H abstraction, β-H abstraction, C-C, C-O, and O-H bond
scission. Methane, carbon monoxide and ethane have been found to be the major
products. Methane and carbon monoxide are formed upon the C-C scission and
ethane upon the C-O scission in the ethanol HDO reaction. The activity trend for ethanol HDO
has been observed to be: Rh > Ru > Ni > Ir
> Co > Pd > Re > Pt > Fe > Cu >
Ag > Au for methane production and Co > Ir >
Ni > Rh > Ru > Fe > Pt > Pd > Cu
> Ag > Re > Au for ethane production (Figure 1(a)). Lee et al. have
experimentally determined the trend for HDO product of guaiacol
to be Rh > Pt ~ Ru > Pd which is similar to the
activity trend observed for ethane formation². Rh and Ru cleave both
C-C as well as C-O bonds, however,
the C-C scission products have a
higher turnover than the ones produced upon C-O bond breaking.  At the reaction conditions, Co, Ir, Pt and noble metals are the metals selective for ethane
formation. To better understand the HDO reactions, the change in catalytic
activity has been studied as a function of temperature across the length of the
reactor. The activity trend for the metal catalysts was observed to be the same
throughout the reactor; however, at inlet conditions (373K), the activity is
much smaller than that of the reaction conditions (523K). At 373K, most of the
transition metal catalysts viz. Pd, Pt, Rh, Ir, Ru, Fe, Ni, Co have been found selective
towards ethane production. In order to explore alloys that give a high turnover
and selectivity towards desired products, a number of bimetallic have been
studied (Figure 1(b)).  Co₃Ni
and Ni₃Fe have been found be the metals with the highest turnover of
ethane (10^-3s^-1) and selectivity greater than 0.98
towards ethane. Ni₃Rh has shown the highest production rate of
methane, followed by Ni₃Pt, Ni₃Cu, and Co₃Ru.
The selectivity for methane on these alloys is greater than 0.9.

1.      Medford, A. J.,  Shi, C., Hoffmann, M. J., Lausche,
A. Fitzgibbon, S., Bligaard, T., Nørskov,
J. K. Catalysis
Letters, 145(3), 2015, 794-807

2.     
Lee, C., Sun, J., Suh,
Y., Choi, J. & Ha, J. Catalysis Communication,17 (2012), 54-58