Ind. Eng. Chem. Res. 1 9 9 4 , 3 3 , 3277
3277
Response to Comments on “Alkylation of Isobutane by Ethylene: A Thermodynamic Study” D. Cornet. and J. M. Goupil Catalyse et Spectrochimie, I.S.M.Ra, University of Caen, F14032 Caen Cedex, France
Sir: Our calculation dealing with the system isobutane ethylene (Goupil et al., 1994) intended to establish the composition at equilibrium, and particularly the relative amounts of alkane fractions with even carbon number. Such data seemed useful t o interpret the results obtained in some reactions performed with chlorinated alumina catalysts. There, the pattern of products steadily changed as the catalyst deactivated. Obviously, the products from actual reactions are not necessarily in equilibrium, neither mutually nor with the reagents. Terms such as “thermodynamic prediction” or “expectation” should not prove ambiguous. As Professor Albright emphasizes, mechanisms involving carbenium ions leads to product distributions far away from equilibrium, particularly between isomers. We stressed the same fact when writing in concluding our paper “the high selectivity for 2,3-DMB is likely due to a kinetic control”. No particular mechanism was assumed in deriving the equilibrium amount of the octane fraction, although the system was restricted to C6-Cg and reagents. However, the equations involve equilibrium constants which ought to be independent. The following Rs were considered:
+
...
primary alkylation:
+
C2H4 i-C,H,, = 2,3-DMB (1) ethylene dimerization:
2C2H4= C4H8linear
alkylation with butene:
(2)
CZH4 + i-C4H1, = TMP (3)
Instead of K3, the constant for the second alkylation could be used as well: C2H4
+ 2,3-DMB = TMP
(4)
and the same mathematical results would follow. These appear in Table 4a of our paper, and the thermodynamic ratio CdCg increases with increasing temperature. However, in reactions carried out with an &03-c1 catalyst, this ratio actually decreased, and this behavior
0888-5885l94I2633-3277$04.50l0
was ascribed to the “selectivity of the reaction”, i.e., t o the reaction mechanism. Therefore the chemical steps leading to TMP should be examined. The distribution of the CS isomers does not give any clue. The observation of an amount of Cg exceeding thermodynamics in some reactions over an A1203-Cl catalyst does not prove that dimerization followed by alkylation (reaction 3) prevails over successive alkylation (reaction 41, and the corresponding sentence in our conclusion is in error. However, addition of ethylene upon a 2,3-dimethyl2-butyl cation yields a primary carbenium ion, while addition of butene on a tert-butyl cation would give a secondary ion, and be therefore more economical. But does oligomerization occur at all? Polymerization indeed occurs in the latest stage of alkylation using liquid acids, transforming light carbenium ions into heavier and heavier ions (Albright, 1988). The situation may be somewhat different with solid catalysts. Over zeolites, dimerization is thought to occur between the free and the protonated olefin (Coma, 1993), and alkene dimers are detected at high catalyst age (Weitkamp, 1980). Thus, successive alkylations certainly occur over any type of catalyst. In the reaction of isobutane with ethylene, the probabilities for an ethylene molecule to react with either a 2,3-dimethylbutyl or a tert-butyl cation should be similar. As very high amounts of Cg and heavier products are found with some 4203-Cl catalyst, another path is suggested, namely dimerization-alkylation. Its importance likely depends on the surface state of the solid.
Literature Cited Albright, L. F.; Spalding, M. A.; Faunce, J.;Eckert, R. E. Alkylation of Isobutane with C4 Olefins: Two Step Process Using Sulfuric Acid as Catalyst. Znd. Eng. Chem.Res. 1988,27,391-397. Coma, A.; Martinez, A. Paraffin-Olefin Alkylation. Catal. Rev. Sci. Eng. 1993,35,484-570. Goupil, J. M.; Poirier, J. L.; Cornet, D. Alkylation of Isobutane by Ethylene: A Thermodynamic Study. Znd. Eng. Chem. Res. 1994,33,712-717. Weitkamp, J. IsoalkandAlkene Alkylation and Alkene Oligomerization on Zeolites. Time on Stream Effects with Isobutand Cis-2-Butene on CeY. Proceedings of the Fifth International Conferenceon Zeolites; Rees, L. V., Ed.; Heyden: London, 1980; pp 858-865.
0 1994 American Chemical Society
