274
SATO, KOIZUMI, AND NOZAKI
become effective to release the products at temperatures
as high as 550ꢀC. In contrast, the phenol conversion passes
through a maximum at 500ꢀC over the pure CeO2 (Fig. 4),
because methanol decomposes at 550ꢀC.
8. Santacesaria, E., Grasso, D., Gelosa, D., and Carra, S., Appl. Catal. 64,
83 (1990).
9. Santacesaria, E., Diserio, M., Ciambelli, P., Gelosa, D., and Carra, S.,
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Zuretti, G., Appl. Catal. 13, 161 (1984).
At the methylation temperature of 600ꢀC, however, the
catalysts are steeply deactivated, and coke formation is ob- 11. Kotanigawa, T., Yamamoto, M., Shimokawa, K., and Yoshida, Y., Bull.
Chem. Soc. Jpn. 44, 1961 (1971).
12. Kotanigawa, T., and Shimokawa, K., Bull. Chem. Soc. Jpn. 47, 950
served. The methanol decomposition without phenol is not
deactivated at 600ꢀC regardless of the basic strength of the
(1974).
catalysts. Thus, the deactivation in the methylation at 600ꢀC
13. Kotanigawa, T., and Shimokawa, K., Bull. Chem. Soc. Jpn. 47, 1535
is said to be attributed to the polymerization of phenol fol-
lowed by coking.
(1974).
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351 (1989).
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165 (1989).
CONCLUSION
Selective ortho-methylation of phenol with methanol
was performed over CeO2-MgO catalysts prepared using
a molten mixture of cerium (III) nitrate, magnesium ni-
trate, and citric acid. The CeO2-MgO was found to have
attractive catalytic performance without decay of activities
and had excellent selectivities to the sum of o-cresol and
2,6-xylenol higher than 98% at a temperature range be-
tween 450 and 550ꢀC. The pure CeO2 exhibited an efficient
ortho-selectivity and the optimum activity in the methyla-
tion at 500ꢀC, whereas the pure MgO showed the optimum
activity at 550ꢀC with a decay in the catalytic activity. This
is concerned with the strength of basic sites of catalyst; the
pure CeO2 had only weak basic sites in contrast to strong
basicity of the pure MgO.
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Appl. Catal. 61, 89 (1990).
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In the XRD measurement, the CeO2-MgO catalyst was
a mixture of MgO and an interstitial solid solution of CeO2
with MgO. It is confirmed that citric acid used in the prepa-
ration heightens the dispersion of the interstitial solid so-
lution of Mgx Ce1ꢁx/2O2 in the MgO matrix. In the results
of methanol decomposition without phenol, methanol was
decomposed into CO, CO2, and CH4 over the CeO2-MgO
catalysts, without forming dimethyl ether.
The turnover frequency based on the weak basic sites is
constant at CeO2 content higher than 6.7 mol% regardless
of preparation methods. The reaction mechanism of the
ortho-methylation over CeO2-MgO catalyst is speculated:
the ortho position of phenol adsorbed perpendicularly on a
weak basic site on CeO2 species is selectively alkylated by
methanol which is possibly activated in the form of formyl
or hydroxy methyl group rather than methyl cation.
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