(560an) Coverage-Dependent First Principles Microkinetic Models for Copper-Catalyzed Hydrogenation of Carbonyl Groups: Role of Water and Sacrificial Alcohols

Promoted
copper catalysts are emerging as catalysts for the hydrogenation of aldehyde
and ketone compounds, a key step in biomass conversion. Their superior
selectivity and low material cost make them an attractive alternative to the
conventional platinum and palladium catalysts in the hydrogenation of carbonyl
functionalities. However, the reaction mechanism and nature of the active
site(s) remain debated. Both a single site mechanism in which only metallic
sites contributes, and a multiple-site mechanism in which multiple copper
species and/or support active sites are involved, have been proposed. Catalyst
design and optimization is severely hampered by this lack of fundamental
insight. We applied coverage-dependent first principles microkinetics to elucidate
the hydrogenation mechanism under different process conditions.

In
the hydrogenation of carbonyl functionalities, two pathways are generally
available, depending on which atom is hydrogenated in the first step (Figure 1,
blue). When the carbon atom is hydrogenated first, a stable alkoxy intermediate
is formed, i.e., the alkoxy pathway. Alternatively, if the oxygen atom is
hydrogenated first, an unstable alkyl intermediate is formed, i.e., the alkyl
pathway. In a first study,¹
we showed that the alkoxy pathway is dominant over copper under typical process
conditions. Hydrogenation of alkoxy intermediates is often rate-limiting in the
hydrogenation of carbonyl groups over copper catalysts, leading to high alkoxy
coverages. We showed, however, that adding a small amount of water facilitates
hydrogenation of these stable alkoxy intermediates via proton transfer from
adsorbed water molecules and surface hydroxyl species. As a result, the
rate-limiting step shifts from hydrogenation of the alkoxy intermediate, to
water regeneration. These findings also provide an alternative interpretation
for the role of promoters in Cu-catalysis. Adding ZnO as a promoter drastically
reduces the barriers for hydrogenation of surface oxygen and surface hydroxyl
species, from 119 to 37 kJmol^-1 and from 131 to 35 kJmol^-1 respectively,
and, as a result, greatly increases the global reaction rate.

page-break-after:avoid">

10.0pt;margin-left:14.2pt;text-align:justify">Figure 1. Pathways for the hydrogenation of acetol: direct
pathways (blue) and via keto-enol tautomerism (dotted lines):
2-hydroxypropanaldehyde (orange), 1,2-dihydroxypropylene (green) and
2,3-hydroxypropylene (red). Hydrogenation of a carbon atom (solid lines) occurs
via direct hydrogenation, hydrogenation of an oxygen atom (dashed lines) can
occur via direct hydrogenation or via proton transfer from surface hydroxyl
species or from adsorbed water.²

Alternatively,
when keto-enol tautomerism is considered, 3 additional hydrogenation pathways
open up (Figure 1). We showed that for alpha-hydroxy ketones, such as the
industrially important acetol, the difference in stability between the adsorbed
ketone and enol tautomer is sufficiently low (<30 kJmol^-1) for
the enol pathways to become kinetically relevant.²
Under dry conditions (Figure 2A), the majority (84%) of acetol is hydrogenated
via the 1-enol pathway, consistent with recent isotope labeling studies, while
the remaining 16% is hydrogenated through the acetol pathway. Both 1-enol and
alkoxy hydrogenation are rate-controlling steps under these conditions. The
superior activity for C=C over C=O hydrogenation is rather surprising due to
the high selectivity of copper in the hydrogenation of unsaturated and aromatic
ketones.

The
presence of water again drastically impacts the hydrogenation reaction
mechanism and three regimes can be identified as a function of the inlet H₂O:ketone
molar ratio (Figure 2B). At low water partial pressures, close to dry
conditions, the 1-enol pathway dominates. By adding water however, proton
transfer pathways open up to hydrogenate the stable alkoxy intermediates. As a
result, the contribution of both the acetol and aldehyde pathways increase, up
to a H₂O:ketone molar ratio of 0.5. When more water is added, the
1-enol becomes kinetically irrelevant and the contribution of the aldehyde
pathway starts to go down again, and the mechanism is dominated by the acetol
pathway. Under those conditions, the rate is again controlled by regeneration
of the hydroxyl species and the adsorbed water molecules.

10.0pt;margin-left:14.2pt;text-align:justify">Figure 2. font-family:" times new roman> A. Reaction
path analysis for the hydrogenation of acetol to propylene glycol under dry
conditions. The numbers indicate the fraction of each components consumed in a
particular reaction step. Dotted arrows indicate reactions with a fraction
below 0.5% while green arrows indicate reaction steps with a fraction above
50%. B. Effect of the water to acetol feed flow ratio on the dominant
reaction pathway for the hydrogenation of acetol. The contribution indicates
the fraction of acetol consumed in each particular pathway. Conditions: 500 kg_cat
s mol_acetol^-1, 200 °C , p_H2 = 5 MPa, p_acetol
= 2.5 MPa.

Finally,
the hydrogenation of the alkoxy intermediate can be facilitated by the introduction
of a sacrificial alcohol.³
Rather than following a dehydrogenation-hydrogenation mechanism, which was generally
hypothesized to be the dominant pathway, the proton of the alcohol is directly
transferred to the ketone. This proton transfer between the sacrificial alcohol
and the alkoxy intermediate enhances the reaction rate, similar to water.
Proton transfer from a sacrificial alcohol also results in an alkoxy
intermediate, (sac-alkoxy), which rather than being regenerated back to
the alcohol, is dehydrogenated to its corresponding ketone. As a result, the
rate-limiting step shifts from hydrogenation of the alkoxy intermediate, under
dry conditions or regeneration of water, under wet conditions, to hydrogenation
of the ketone to its alkoxide and dehydrogenation of the sac-alkoxy
species. Because of the higher degree of rate control for ketone hydrogenation,
we find that the addition of gas phase hydrogen has a surprising catalytic
effect – it speeds up the overall reaction without being consumed.

The
detailed microkinetic simulations illustrate the complexity of this seemingly
simple reaction, and show the importance of self-consistent, coverage-dependent
microkinetic simulations to discover the different kinetic regimes.

normal;text-autospace:none">(1)         De
Vrieze, J. E.; Thybaut, J. W.; Saeys, M. Role of Surface Hydroxyl Species in
Copper-Catalyzed Hydrogenation of Ketones. ACS Catal. 2018, 8
(8), 7539–7548.

normal;text-autospace:none"> font-family:" times new roman>(2)         De Vrieze, J. E.; Thybaut, J.
W.; Saeys, M. Role of Keto-Enol Tautomerization in the Copper-Catalyzed
Hydrogenation of Ketones. ACS Catal. 2019, 9, 3831–3839.

normal;text-autospace:none"> font-family:" times new roman>(3)         De Vrieze, J. E.; Urbina
Blanco, C. A.; Thybaut, J.; Saeys, M. Autocatalytic Role of Molecular Hydrogen
in Copper-Catalyzed Transfer Hydrogenation of Ketones. Submit.