| Popis: |
The kinetics of the isomerization of n-butenes was studied in a static reaction system over synthetic, crystalline, sodium silica-alumina, known as Na13X, and over a series of copper 13X zeolites prepared by partial exchange of the sodium. It was shown that all samples were active for cis-trans isomerization and for double-bond shift. The results conformed to a triangular reaction scheme in which all possible reaction paths between the three-component mixture of isomers were parallel, reversible and first order. The relative rate constants were determined for Na13X at 305 °C, and for all the Cu13X samples at 105 °C; in all cases the relative amounts of products were kinetically controlled. Na13X was active for n-butene isomerization, without skeletal rearrangement, over a temperature range 200–300 °C. The course of reaction was in marked contrast to that reported for amorphous silica-alumina, in particular the Na13X exhibited a marked preference for double bond shift rather than cis-trans isomerization. The partial replacement of sodium cations with cupric cations had a profound effect upon the reaction rate, activation energy, and stereoselective nature of the process; but this effect was not linear with percentage exchange. When the extent of exchange was below approximately 30% the Cu13X zeolites exhibited a very strong preference for the formation of trans-but-2-ene from but-1-ene, and an induction period was a characteristic feature. Such behavior was attributed to a radical type mechanism in which the active sites were the copper cations contained in the relatively inaccessible type I cation sites, and where reduction of cupric to cuprous ions was possible. These cuprous ions would be capable of forming unstable complexes with olefins. When the extent of exchange was higher the reaction followed an entirely different course. There was preferential formation of the thermodynamically less stable cis-but-2-ene, and the reaction exhibited many of the characteristics typical of a carbonium ion type of mechanism. The production of the active sites responsible for such an ionic type of activity was explained in terms of the presence of cupric cations in surface cation sites of the lattice although the precise nature of such sites was uncertain. |
