Y. Wei et al. / Catalysis Communications 12 (2011) 1333–1338
1337
modification of La2O3–MgO with KOH would cause a decline in pore
volume as well. The pore volume of La2O3–MgO is 0.34 cm3 g−1. It
becomes 0.17 and 0.02 cm3 g−1 at KOH loading of 10 and 25 wt.%,
respectively. It was reported by Cosimo and coworkers that the
modification on MgO with alkali metals would result in a decline in
specific surface area and pore volume [25,26]. The drop in specific
surface area and pore volume as well as the morphological change
upon KOH introduction observed over La2O3–MgO and KOH/La2O3–
MgO catalysts are in line with the results of XRD and SEM
characterization. With rise of crystal size due to particle agglomer-
ation, there would be a drop in specific surface area. The loading of
KOH on La2O3–MgO would result in blocking of active sites as well as
blocking of pore openings, and hence a decline in pore volume. The
results demonstrate that the decline in catalytic activity above KOH
dispersion capacity is a combined result of site and pore blockage and
crystal agglomeration caused by the excess presence of KOH.
is at 628 °C, and there are minor peaks at 125 and 235 °C. The
introduction of KOH results in enhancement in the amount of CO2
desorption and rise in desorption temperature. There is CO2 desorption
at ca. 667 and 710 °C detected over the 10wt%KOH/La2O3–MgO catalyst.
The desorption of CO2 at 710 °C is a clear indication of superbasicity [8];
maximum CO2 desorption at higher temperature indicates stronger
basic sites. The CO2-TPD results are in good agreement with the results
of Hammett indicator method (Table 2), providing additional evidence
for the superbasicity of the 10wt%KOH/La2O3–MgO catalyst.
4. Conclusions
A novel solid superbase material has been obtained by loading KOH
onto lanthanum–magnesium composite oxide, followed by thermal
treatment at 500 °C under a flow of high-purity N2. The material shows
high catalytic activity and selectivity in Knoevenagel condensation
reaction. The catalyst with 10 wt.% KOH loading is the best, showing the
highest superbasicity and catalytic activity. It was found that the
catalytic efficiency of the KOH/La2O3–MgO superbase is dependent on
both the strength and amount of superbasic sites. Moreover, the
superbase can be generated by simple thermal treatment of precursor
under an atmosphere of high-purity nitrogen. It is envisaged that it will
find wide applications in the sectors of catalysis and fine chemical
industry. The present results open up a new route for the design and
synthesis of superbase materials of new functionalities using composite
oxides as supports. Further investigation is under way: (i) to disclose the
formation mechanism of superbasic sites, (ii) to confirm the nature of
the catalytically active sites, and (iii) to widen the catalytic applications
of this class of superbase-type materials.
3.2.4. Basicity measurement
3.2.4.1. Hammett indicator method. The basicities of KOH/La2O3–MgO
samples with different KOH loadings are listed in Table 2. The base
strength of the samples are in the 26.5≤H– b33.0 range (please refer
to Supplementary data for detail of determination), demonstrating
that the as-generated KOH/La2O3–MgO catalysts are solid superbases
[27]. The amounts of superbasic sites on KOH/La2O3–MgO are 0, 0.34,
0.57, 0.28, 0.20 and 0.17 mmol/g for KOH loadings of 0, 5, 10, 15, 20
and 25 wt.%, respectively. Obviously, with variation in KOH loading
from 5 to 10 wt.%, there is a rise in the amount of superbasic sites.
With further rise of KOH loading, the amount of superbasic sites
decreases while the base strength of the superbasic sites remains in
the 26.5≤H– b33.0 range. Therefore, for the generation of superbasic
sites, a KOH loading of 10 wt.% is the most suitable, in consistent with
the results shown in Figs. 1 and 2. Combining the catalytic result of
Fig. 1 and superbasicity of Table 2 (see Supplementary data), it is
obvious that the change of catalytic activity for the Knoevenagel
condensation reaction parallels to that of superbasicity. It is rational to
infer that the catalytic performance of the KOH/La2O3–MgO superbase
is dependent not only on the strength of superbasic sites but also on
their amount.
Acknowledgments
The financial supports of the National Natural Science Foundation of
China (Grant Nos. 20873038 and J0830415), Hunan Provincial Natural
Science Foundation of China (10JJ1003), Doctoral Program Foundation
of Institutions of Higher Education of China (Grant No. 200805320001),
and the National 863 Program of China (2009AA05Z319) are gratefully
acknowledged. C.T. Au thanks the Hunan University for an adjunct
professorship.
Appendix A. Supplementary data
3.2.4.2. CO2-TPD. The surface basicity was also estimated by the CO2-
TPD technique. Fig. 4 shows the CO2-TPD profiles of La2O3–MgO and
10 wt.%KOH/La2O3–MgO. Over La2O3–MgO, the main desorption peak
Supplementary data to this article can be found online at doi:10.
1016/j.catcom.2011.05.010.
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