(58c) Controlling Selectivity of Bio-Oil Model Compound Upgrading with Metal Promoters on Molybdenum Carbide

The upgrading of
thermochemical biofuels could lead to cleaner, more homogeneous fuels[1],
however, the processes and reactions are not very selective. For research
described in this paper, we examine the use of various promoters on molybdenum
carbide to improve the reaction selectivities. Crotonaldehyde is used as a
model compound because it possesses multiple functionalities commonly found in pyrolysis
bio-oils, while also allowing us to probe how changes to the catalyst affect
selectivity to several different potential products[2].

In previous work, we have shown that potassium addition to Mo₂C
catalyst increased the base site density and decreased the acid site density,
and consequently shifted the crotonaldehyde and acetic acid conversion selectivities
in the presence of H₂. The results suggested that the increase in
base sites was responsible for the increase in selectivities to 3-butenal and
acetone, respectively. In this paper, we report on how other metal promoters,
including Fe, shift the crotonaldehyde upgrading selectivity relative to Mo₂C.

The Mo₂C catalysts were synthesized by a
temperature-programmed reaction method as described previously[3].
Metal salt was added to the native (unpassivated) Mo₂C catalyst by
incipient wetness in an oxygen-free, inert atmosphere. The catalysts were dried
in H₂ for 2 hrs at 110‹C, reduced in H₂ for 4 hrs at 450‹C,
and then passivated in 1% O₂/He for at least 6 hrs at room temperature.
All of the catalysts were characterized using x-ray powder diffraction, N₂
physisorption, and inductively coupled plasma optical emission spectrometry.
Base and acid site concentrations were measured via temperature programmed
desorption of CO₂ and NH₃. The activities and selectivities
were measured at atmospheric pressure and 275-350‹C in a flow reactor with a H₂/crotonaldehyde
ratio of 6. The ratio was selected because it is twice the stoichiometric
amount of H₂ required for complete saturation and deoxygenation of
crotonaldehyde to butane.

While K shifted the
selectivity to 3-butenal, Fe on Mo₂C favored the formation of butene
and suppressed formation of butyraldehyde at high temperatures. The model
compound results presented in this paper will demonstrate the ability to tune
the product selectivity of molybdenum carbide catalysts for specific desired
reaction pathways by adding various metal promoters. Combining these results to
promote Mo₂C with a mixture of these promoters could offer a
powerful way to tune the selectivity of Mo₂C for bio-oil upgrading,
depending on the desired character of the upgraded product.