Figure 3. Effect of aging the Raney Ni in ethanol for 18 h
prior to hydrogenation of 8. Reaction conditions: 0.16 M 8,
100 wt % Raney Ni, 25 °C, 30 psig. The black line represents
hydrogenation without aging the catalyst in ethanol, and the
red line represents hydrogenation after aging the catalyst in
ethanol for 20 h under nitrogen
by brief exposure of the catalyst to air to oxidize the adsorbed
CO to CO2.2d,6 In accord with this precedent, the ethanol-
induced catalyst deactivation of Pd/C in the present study
was also reversed by brief oxygen exposure of the catalyst
to air as shown in Figure 4A for hydrogenation of 1. The
H2 uptake profile for a reaction with a catalyst that was aged
in ethanol for 20 h prior to the reaction exhibited a suppressed
reaction rate; the activity was restored after a brief air
exposure. A similar effect of air exposure in restoring
catalytic activity was also noted for the case of Raney Ni
catalyzed hydrogenation of 8 (Figure 4B).
In a series of control experiments, direct exposure of
hydrogenation reaction mixtures utilizing Pd/C or Raney Ni
as catalyst to gaseous CO led to partial or complete
suppression of the reaction, as expected (Figure 5). In accord
with literature observations, brief exposure of CO-poisoned
reaction mixtures to air restored some or all of the catalyst
activity.
The behavior reported in Figures 1-4 was also observed
with other alcoholic solvents including methanol, 1-propanol,
and 2-propanol. While the details of the deactivation pathway
may differ among the different alcohols, the poisoning of
the catalyst surface and subsequent regeneration with air
exposure are common to all of the alcohols studied. It should
be noted that, in contrast to the catalyst poisoning in the
presence of alcoholic solvents, no deactivation was observed
with ethyl acetate as solvent.
Figure 2. Fractional hydrogen uptake profiles for hydrogena-
tion of (a) 0.33 M 3 using 7 wt % of 5% Pd/C at 25 °C and 20
psig and (b) 0.11 M 6 using 5 wt % of 5% Pd/C at 25 °C, 20
psig. The black line represents hydrogenation without aging
the catalyst in ethanol, and the red line represents hydrogena-
tion after aging the catalyst in ethanol for 20 h under nitrogen.
induced deactivation phenomenon was also observed during
debenzylation of 3 and nitro reduction of 6 on Pd/C as shown
in Figure 2.3 The generality of this behavior is further
demonstrated in the Raney Ni catalyzed reduction of ni-
trobenzene as shown in Figure 3.4
Surface science studies on single-crystal group 8 metals
indicate that ethanol readily undergoes dissociative adsorption
on the metal surface to form a metal alkoxide which then
decomposes to a [RCHxO] precursor followed by formation
of CO and carbonaceous species. These transformations are
known to occur on Pd surfaces at temperatures less than 25
°C.5 A similar decomposition of allylic alcohols and R,â-
unsaturated aldehydes under synthetically relevant hydro-
genation conditions over supported metal catalysts was
shown to form adsorbed CO that poisoned the catalyst
surface. The catalytic activity in that case was regenerated
Different supports, i.e., SiO2-Al2O3 and various carbon
sources, metal precursors, and the starting oxidation state7
of the catalyst were tested, and the ethanol poisoning effect
was shown to be general over a wide range of supported Pd
catalysts as reported in Table 1. There were a subset of
(3) Figure 2b only shows the results for the first 2 h of reaction to magnify the
initial reaction rate differences. The experiment with no ethanol age went
to complete conversion in 10 h.
(4) After 30 min, both hydrogenations in Figure 3 were stopped, the headspace
was purged with nitrogen, and 20 psig air was injected. There was no effect
of air exposure on the reaction rate in the absence of the ethanol age. The
complete data set, in the form of a reaction rate profile, including the effect
of air exposure is shown in Figure 4B with an explanation in the text.
(5) (a) Shekhar, R.; Barteau, M. Catal. Lett. 1995, 31, 221-237. (b) Davis, J.
L.; Barteau, M. Surf. Sci. 1987, 187, 387-406. (c) Davis, J. L.; Barteau,
M. Surf. Sci. 1990, 235-248. (d) Shekhar, R.; Barteau, M.; Plank, R.; Vohs,
J. J. Phys. Chem. 1997, 101, 7939-7951. (e) Gates, S. M.; Russell, J. N.;
Yates, J. T., Jr. Surf. Sci. 1984, 146, 199-210.
(6) (a) English, M.; Ranade, V. S.; Lercher, J. A. Appl. Catal. A: Gen. 1997,
163, 111-122. (b) Singh, U. K.; Vannice, M. A. J. Catal. 2000, 199, 73-
84.
(7) Oxidized supported Pd catalysts are presumably present as Pd(OH)2 which
may be reduced in situ with alcoholic solvents. See: Kanie, O.; Grotenberg,
G.; Wong, C.-H. Angew. Chem., Int. Ed. 2000, 39, 4545-4547.
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