(744g) Aldehyde Water Shift Reaction: Comparative Investigation of Supported Metal Catalysts

There has been significant interest in the use of H₂O
as a reactant for biomass conversion given its high concentration in typical
feed streams. Steam
reforming and 5-hydroxymethylfurfural oxidation are two important examples.[1], [2] The
aim of research described in this paper was to investigate the use of H₂O as a terminal oxidant for the partial oxidation of
aldehydes. The aldehyde water shift (AWS) reaction oxidizes the aldehyde
and produces H₂ (Equation 1).

CH₃CHO + H₂O -> CH₃ COOH
+ H₂ (Eq. 1)

Prior investigations of the
AWS reaction have focused on homogeneous catalysts.[3] The commercial
implementation of these types of catalysts can be cost-prohibitive. To our
knowledge, no heterogeneous catalyst has been reported for this reaction. We explored
the use of Mo₂C-, CeO₂-, and Al₂O₃-supported
metals (Cu, Pt, Au), the design of which was inspired by that for highly active water
gas shift (WGS) catalysts. In particular, the most active heterogeneous WGS
catalysts are bifunctional and possess distinct sites for H₂O dissociation
and CO oxidation.[4] The target metal
loadings were equivalent to 0.1 monolayer. The catalysts were loaded into a
quartz flow-through reactor and the reactions were then carried out at 200-280^oC.

The AWS activities (Figure.
1) and product distributions (Figure. 2) were strong functions of the support. The
observed AWS activities and activation energies suggest that the rate-limiting
step, which is believed to be H₂O dissociation, occurred on the support surface. Among
all the catalysts studied, the Mo₂C-supported catalysts possessed the highest AWS activities, with activities at least 5 times
higher than those for the other materials. For the Mo₂C-supported catalysts,
the admetal did not significantly affect the activity. The Cu/CeO₂
showed the highest AWS activity among the CeO₂-supported materials. Like Mo₂C, CeO₂ is
active for H₂O dissociation. The
high activity of the Cu/CeO₂ catalyst clearly illustrates the
importance of Cu particles for
this material. In addition
to the AWS reaction, the major side
reactions were the Cannizzaro reaction, which produces ethanol and acetic acid,
and aldol condensation, which forms crotonaldehyde. The Mo₂C-supported
catalysts and Cu/CeO₂ were more than 60% selective
to the AWS reaction. Our results suggest that the cooperation between the sites
for H₂O dissociation and aldehyde oxidation may have contributed
to the high AWS activities and selectivities. In contrast, the other
CeO₂- and Al₂O₃-supported catalysts produced primarily
crotonaldehyde via the aldol condensation
reaction, which appeared to be catalyzed by weak acid sites. These and other results will be presented
in the paper.

References

[1]      Davda, R. R.; Shabaker,
J. W.; Huber, G. W.; Cortright, R. D.; Dumesic, J. A. Appl. Catal. B
Environ. 2005, 56, 171-186

[2]      Zhang, Z.; Deng, K.
ACS Catal. 2015, 5, 6529-6544

[3]      Brewster, T. P.; Goldberg,
J. M.; Tran, J. C.; Heinekey, D. M.; Goldberg, K. I. ACS Catal. 2016,
6, 6302-6305

[4]      Schweitzer, N. M.; Schaidle,
J. A. ; Ezekoye, O. K.; Pan, X. ; Linic, S.; Thompson, L. T. J. Am. Chem.
Soc. 2011, 133, 2378-2381