G Model
CATTOD-9963; No. of Pages10
ARTICLE IN PRESS
K. Sun et al. / Catalysis Today xxx (2016) xxx–xxx
9
forward rate must be inhibited under these conditions. The increase
in CO2 selectivity with temperature observed over Pd/C (A) also
argues against the equilibrium assumption, since the WGS reac-
(A) and Pd/C (A) under conditions similar to those employed for OA
deoxygenation are discussed below.
The Pd/SiO2 (A) exhibits WGS activity at 260 and 300 ◦C; how-
ever, the measured rates are very low (Table 3). A conservative
estimate of the rate under He at 300 ◦C (5.5 × 10−4 mol/g/h) is com-
parable to that reported for a 1 wt.% Pd/SiO2 catalyst using CO and
H2O partial pressures of 24 and 22 Torr, respectively [27]. The for-
ward WGS reaction over Pd/SiO2 (A) is strongly inhibited by H2.
Inhibition of the WGS reaction by H2 has been reported for Pd sup-
ported on ceria under similar reaction conditions; however, the
underlying reason for this behavior was not elucidated [27].
WGS rates over Pd/C (A) and Pd/SiO2 (A) are equivalent at 260 ◦C
under He (1:1 H2O:CO molar ratio). For Pd/C (A), the turnover fre-
quency (TOF) calculated from the measured rate at 300 ◦C under
He is 2.15 × 10−3 s−1. This value is about one order of magni-
tude lower than reported for a 1 wt.% Pd/Al2O3 catalyst at 300 ◦C
albeit using much higher partial pressures of CO and H2O (24.3
18.7 kcal/mol—within the expected range for supported Pd [13].
Significantly, H2 does not inhibit the forward WGS reaction over
Pd/C (A). In fact, the reaction exhibits a small positive order in H2
at 260 ◦C. The available rate data (Table 3) at 260 ◦C under 10% H2
are consistent with reaction orders of ∼0 and ∼0.7 for CO and H2O,
respectively.The WGS results establish that both Pd/SiO2 (A) and
Pd/C (A) are active under conditions closely approximating those
prevailing during OA deoxygenation; however, under 10% H2, the
WGS activity of Pd/C (A) is ∼5 times higher than Pd/SiO2 (A) at
260 ◦C and ∼9 times higher at 300 ◦C. The higher WGS activity
of Pd/C (A) under 10% H2 may explain, in part, its much higher
CO2 selectivity in OA deoxygenation, but WGS activity alone can-
not account for the observed support effect. The rate of the WGS
reaction over Pd/C (A) at 260 ◦C with a 1:1 H2O:CO ratio is only
2.3 × 10−4 mol/g/h, whereas the average OA deoxygenation rate
(a conservative estimate for typical reaction kinetics) under these
conditions is 2.7 × 10−3 mol/g/h—more than an order of magni-
tude higher. A similar comparison of rates at 300 ◦C supports the
same conclusion: the forward WGS reaction is too slow to account
entirely for the high CO2 selectivities observed in OA deoxygenation
over Pd/C (A).
ity of Pd/C (A), because the forward WGS reaction is too slow. At
higher H2 partial pressures, OA deoxygenation is inhibited and CO2
selectivity is markedly lower. The simultaneous losses of overall
activity and CO2 selectivity are traceable to lower DCX activity.
Higher partial pressures of H2 favor DCN; however, the increase
in DCN activity does not compensate the loss of DCX activity.
These results are consistent with H2 and CO inhibition of the DCX
stearic acid over Pd/C (A) [9]. Under H2 at 260 ◦C, CO2 selectivity
is nearly zero, n-heptane selectivity is lower, and HDO products
(octanal, 1-octanol, n-octane) are formed. Apparently, DCN via an
aldehyde intermediate is favored over Pd/C under a hydrogen-rich
atmosphere, as reported by Rozmyslowicz, et al. [28] for lauric acid
deoxygenation in the liquid phase. Multiple experiments evidence
that CO2 selectivity over Pd/C (A) increases with temperature, con-
sistent with higher activation energy for DCX than for DCN (Fig. 11).
5. Conclusions
Pd-catalyzed DCX and DCN are parallel pathways that are sen-
sitive to the catalyst support, promoters (e.g., Na), and reaction
conditions (temperature, H2 partial pressure). OA deoxygenation
occurs via DCN with 1-heptene and octanal as intermediate prod-
ucts over Pd/SiO2. In contrast, Pd supported on activated carbon
exhibits DCX and DCN activity. Low H2 partial pressures (5–10%)
increase deoxygenation activity and CO2 selectivity over Pd/C (A),
whereas higher H2 partial pressures inhibit DCX and promote DCN.
In the absence of H2, Pd/C (A) initially has very low initial deoxy-
genation activity and deactivates rapidly with TOS. We infer that
the presence of adsorbed H favors hydrogenation of surface alkenyl
and alkyl groups avoiding rapid catalyst deactivation. The much
greater WGS activity of Pd/C (A) than Pd/SiO2 (A) under H2 can
account partially for its higher CO2 selectivity in OA deoxygena-
tion, but WGS activity alone cannot explain the observed support
effects. The higher Na content of Pd/C (A) may explain its higher
CO2 selectivity and lower deactivation rate relative to Pd/C (B).
Pd supported on high-purity acetylene carbon black exhibits only
DCN activity under these conditions, indicating that support polar-
ity (arising from oxygen-containing surface functional groups) also
influences the OA deoxygenation performance of Pd/C catalysts.
4.4. Summary of reaction pathways
Fig. 10 illustrates reaction pathways for OA deoxygenation to n-
heptane over supported Pd. OA deoxygenation occurs via DCN with
1-heptene and octanal as intermediate products over Pd/SiO2. The
indirect DCN pathway proceeding via HDO to octanal is observed
for OA deoxygenation over Pd/SiO2 (A). HDO is enhanced by H2
pressure, and 1octanol and n-octane are observed as minor prod-
ucts under H2 at 260 ◦C. We infer that OA deoxygenation occurs
mainly via direct DCN over Pd/SiO2 (B) and Pd/C (C); n-heptane
selectivity (due to sequential HY of 1-heptene) is high (>98% and
>95%, respectively) and declines only marginally with TOS.
Pd supported on activated carbon has DCX and DCN activity at
235–300 ◦C under 10% H2. Pd/C (A) Exhibits 75% initial CO2 selec-
tivity at 300 ◦C under 10% H2, consistent with the performance of
Pd/C (A) in liquid-phase deoxygenation of fatty acids [8–11]. The
results evidence that direct DCX and DCN + WGS/HY are the main
reaction pathway(s) leading to n-heptane and CO2. The CO2 selec-
tivity (∼33%) of Pd/C (A) under H2 at 300 ◦C is significantly greater
than calculated assuming WGS equilibrium under these conditions
(21.4%), implying the existence of a direct DCX pathway. Moreover,
DCN + WGS/HY cannot account entirely for the high CO2 selectiv-
Acknowledgment
This research was supported by the National Science Foundation
under an EFRI HyBi grant (EFRI-0937721).
Please cite this article in press as: K. Sun, et al., Catalytic deoxygenation of octanoic acid over silica- and carbon-supported palladium: