OXIDATION OF o-XYLENE TO PHTHALIC ANHYDRIDE
293
interaction with aromatic intermediates than on oxidized other processes, such as selective and total oxidation, are
VOx surface; this will make desorption difficult and will en- enhanced at higher temperatures. The presence ofuncoated
hance formation of dimers by reaction between adjacent TiO2, exhibiting Lewis acid sites, can lead to a stronger in-
adsorbed species. Third, the presence crystalline V2O5 can teraction between aromatic intermediates and the catalyst
also have some influence in residue formation, for its char- surface, making desorption difficult and enhancing the re-
acteristics, as shown by FTlR spectra of used samples, vary action between adsorbed species. The Brønsted acid sites
when the V2O5 loading increases from 1 to 5% .
of the crystalline V2O5 can also influence the residue’s char-
A possible mechanism for dimerization has been sug- acteristics, FTIR spectra of used samples showing weaker
gested before (11); a modified version is
C–O, >CH2, and –CH3 bands for higher V2O5 loading. Lat-
eral groups of dimers which stay on the surface can undergo
partial oxidation, thereby causing the structure of the ad-
sorbed residue to vary with the time that samples stayed
under reaction conditions.
ACKNOWLEDGMENTS
The authors are grateful to the British Council and Junta Nacional
de Investiga c¸ a˜ o Cient ´ı fica e Tecnol o´ gica (JNICT) for financial support.
The assistance of Mr. H. Dee in obtaining the mass spectrometry data is
gratefully acknowledged.
REFERENCES
1
2
3
. Nikolov, V., Klissurski, D., and Anastasov, A., Catal. Rev.-Sci. Eng. 33,
19 (1991).
. Wainwright, M. S., and Foster, N. R., Catal. Rev.-Sci. Eng. 19, 211
1979).
. Wainwright, M. S., and Hoffman, T. W., Canad. J. Chem. Eng. 55, 557
1977).
3
(
(
4. Centi, G., Pinelli, D., and Trifir o` , F., J. Mol. Catal. 59, 221 (1990).
5. G a¸ sior, M., G a¸ sior, I., and Grzybowska, B., Appl. Catal. 10, 87 (1984).
6. Grabowski, R., Grzybowska, B., Haber, J., and S �l oczynski, J., React.
Kinet. Catal. Lett. 2, 81 (1975).
7
. Inomata, M., Miyamoto, A., Ui, T., Kobayashi, K., and Murakami, Y.,
lnd. Eng. Chem. Prod. R&D 21, 424 (1982).
together with a corresponding process, applying to o-
tolualdehyde. Its main feature is the simultaneous bonding
8
9
. Wachs, I., Chem. Eng. Sci. 45, 2561 (1990).
of both alkyl groups to reduced V or to Ti4 sites before ei-
ther has begun to be oxidized; it is unlikely that such species
will revert to those from which selective oxidation products
may arise, so under low conversion conditions their prin-
cipal fate will be to form dimers, or higher polymers. Ulti-
mately they can either only desorb or become oxidized to
+
. G a¸ sior, M., Grzybowska, B., and Czerwenka, M., in “Proceedings
Vth Int. Symp. Het. Catal., Varna, 1983,” Part I, p. 75 (D. Shopov,
A. Andreev, A. Palazov, and L. Petrov, Eds.), Bulgarian Acad. of Sci.
Varna, 1983.
1
1
1
0. G a¸ sior, M., Haber, J., and Machej, T., Appl. Catal. 33, 1 (1987).
1. Bond, G. C., and K o¨ nig, P., J. Catal. 77, 309 (1982).
2. Vanhove, D., and Blanchard, M., Bull. Soc. Chim. Fr. 3291 (1971).
CO2. A further factor which may contribute to the strong 13. Vanhove, D., and Blanchard, M., Bull. Soc. Chim. Fr. 4134 (1971).
retention of these species is �-bonding of the aromatic ring 14. Cavani, F., Centi, G., Foresti, E., Trifir o` , F., and Busca, G., J. Chem.
Soc. Faraday Trans. I 84, 237 (1988).
to an adjacent site (this effect being either additional to or
a substitute for the �-bonded adsorption), as is also shown
in the diagram above. This bond has the same character as
1
5. Wainwright, M. S., and Hoffman, T. W., Canad. J. Chem. Eng. 55, 552
1977).
6. Saleh, R. Y., and Wachs, I. E., Appl. Catal. 31, 87 (1987).
(
1
that formed, for example, between benzene and AlCl3. Par- 17. Kopinke, F., Creten, G., and Froment, G. F., “New Developments in
tial oxidation of strongly-retained species may occur before
desorption, since changes in the FTIR spectra are observed
with increasing reaction time.
Selective Oxidation by Heterogeneous Catalysis,” Studies in Surface
Science and Catalysis,Vol. 72, p. 317 (P. Ruiz and B. Delmon, Eds.),
Elsevier Sci., Amsterdam, 1992.
8. Bal’zhinimaev, B. S., and Pinaeva, L. G., Kinet. Katal. 36, 59 (1995).
9. L o´ pez-Isunza, F., and Kershenbaum, L. S., Chem. Eng. Sci. 47, 2817
1
1
(
1992).
CONCLUSIONS
2
0. Cheng, Y.-S., L o´ pez-Isunza, F., Mongkhonsi, T., and Kershenbaum, L.,
Appl. Catal. A:General 106, 193 (1993).
The characteristics and the amount of the adsorbed
residue are highly dependent on the catalyst composition
and operating conditions used. The formation of these
2
2
2
1. Dias, C. R., Portela, M. F., and Bond, G. C., J. Catal. 157, 353 (1995).
2. Dias, C. R., Portela, M. F., and Bond, G. C., J. Catal. 157, 334 (1995).
3. Bond, G. C., Dias, C. R., and Portela, M. F., J. Catal. 156, 295 (1995).
products is favored at lower W/F and lower temperatures; 24. Niwa, M., Ando, H., and Murakami, Y., J. Catal. 70, 1 (1981).