196
Y. Xi, R.J. Davis / Journal of Catalysis 254 (2008) 190–197
Table 3
Turnover frequency of transesterification of tributyrin with methanol based on
CO uptake
2
tributyrin with excess methanol quantitatively described the
catalytic activity of the activated Mg–Al hydrotalcite. The
decomposed-rehydrated sample, having Brønsted base sites re-
siding on and between the brucite-like layers was an order of
magnitude more active on a surface area basis than a decom-
posed sample which exposed Lewis base sites. However, the
TOF values of the HT-d-r and HT-d samples do not show a
a
−1
b
−1
(s )
Catalyst
Obs. TOF (s
)
TOF
k1
HT-d
HT-d-r
0.065
0.057
0.074
0.15
0.032
0.0037
0.018
0.051
0.081
0.12
HT-d-r-333
HT-d-r-373
HT-d-r-423
HT-d-r-473
HT-d-r-723
0.12
significant difference when normalized by the CO adsorption
2
0.031
0.0031
0.027
capacity. Heating a rehydrated sample to 473 K caused a loss
in both activity and CO2 adsorption capacity, thus suggesting
water management on HT-d-r is crucial for effective catalysis.
Brønsted base sites were active in the presence of water, but
high levels of hydration caused rapid deactivation of the cata-
lyst, presumably by ester hydrolysis to butyric acid that reacted
with the base sites.
a
Based on initial conversion of tributyrin, normalized by CO uptake.
2
b
Based on the rate constant k in Table 2, normalized by CO uptake.
1
2
brucite-like layers and coordinate tetrahedrally to both the layer
and the interlayer oxygens [39]. The in situ DRIFTS study by
Yang et al. also indicates an increasing interaction between in-
terlayer carbonate anion and the Mg–Al hydrotalcite layer dur-
ing interlayer water removal [36]. As HT-d-r was decomposed
between 473 and 723 K, the CO2 adsorption capacity remained
fairly constant as shown in Fig. 6, and was similar to the CO2
Acknowledgments
This work was supported by the Chemical Sciences, Geo-
sciences and Biosciences Division, Office of Basic Energy Sci-
ences, Office of Science, U.S. Department of Energy, grant No.
DE-FG02-95ER14549.
−
6
−2
adsorption capacity of HT-d (2.2 × 10 molm ).
Based on CO2 uptake, the turnover frequencies (TOF) of
transesterification of tributyrin with methanol over activated
HT were calculated and listed in Table 3. The observed TOF
values (obs. TOF) based on initial rate of consumption of trib-
utyrin are similar to those derived from the fitted values of k1
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(
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