Kinetic models of vinyl acetate synthesis on new-generation zinc acetate catalysts
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Kinetic Models of Vinyl Acetate Synthesis on New-Generation Zinc Acetate Catalysts O. N. Temkin, H. I. Abanto-Chavez, and Ko B. Hoang Lomonosov State Academy of Fine Chemical Technology, Moscow, 117571 Russia ReceivedJuly4, ! 999
Abstract--A kinetic study of vinyl acetate synthesis on a new KO45 zinc acetate catalyst and a Montecatini MAVC catalyst is carried out. The catalyst is prepared by a new method: zinc acetate is supported on oxidized AGN-2 activated carbon. Mechanistic hypotheses corresponding to 92 different kinetic models are advanced and discriminated. The form of the rate law is shown to depend on the catalyst preparation procedure. INTRODUCTION Researchers who develop commercial catalysts and reaction mechanisms still give considerable attention to a well-known commercial process of vinyl acetate (VA) synthesis [1-7], C2H 2 + CH3COOH " . CH2=CHOCOCH 3 . (I) A B P An increase in catalyst activity and its service life can make this process competitive with the oxidative esterification of ethylene (Moiseev's reaction) [8]. Our experience in improving the zinc acetate catalyst for VA synthesis [6, 7] shows that there is still substantial room for the improvement of Zn(OAc)2/C catalyst technology to prolong its service life and activity while maintaining selectivity. A change in the procedure of activated carbon impregnation with a zinc acetate solution (a switch to circulation of the solution through a bed of the support dispersed in the liquid phase), the use of oxidized activated carbons, and the addition of organic solvents to the aqueous solution of zinc acetate during impregnation [6, 7] made it possible to obtain a catalyst with a specific surface area of 95-100 m2/g. The performance of this catalyst which is better than that of the best new-generation MAVC catalyst from Montecatini, all conditions being the same [7]. We have found [6] that, when zinc acetate is supported on the surface of oxidized activated carbon (AC), pH of the solution increases. The final value of pH (4.9 + 0.1) is independent of the initial value, which is between 2 and 5. The surface of AC serves as a buffer under the conditions of intensive circulation of the solution through a suspended bed of oxidized AC. The adsorption rate and the amount of zinc acetate sup.ported on AC (C) pass through maxima with an increase m temperature (T,r~x= 50~ Our findings suggest that a complex process takes place when zinc acetate is supported on oxidized AC. This process involves the adsorption of salt and ion exchange, as well as the adsorption of acetic acid formed during the process. Therefore, the cata-
lyst activity depends on the conditions of salt deposition on the support (temperature, solution circulation rate, and others). In this work, we study how the procedure of salt deposition affects the form of a kinetic model (rather than merely catalyst activity) provided that the salt, its concentration on the support, and AC are the same. We answer the question of whether the kinetic model for a new catalyst differs from those derived f
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