(281b) Deactivation Of La-X Zeolites In isobutane/2-Butene Alkylation

Premature catalyst deactivation is preventing the realization of solid acid catalyzed processes for isobutane/n-butene alkylation in refineries. A detailed insight in the reaction leading to deactivation is necessary to determine appropriate process condition and improve the design of the catalysts. For this purpose a series of reactions was conducted, in which the catalyst was removed and characterized after different times on stream. Four distinct stages were identified:

1. Stable alkylation: In the beginning of the reaction carbenium ions are formed on Brønsted acid sites. This leads to a decrease of the concentration of free Brønsted acid sites and the pore volume as well as an increase of the carbon content of the catalyst. After 1.9 h on stream these parameters become constant. During this stage quantitative conversion of 2-butene was observed.

2. Transformation of the deposits: After 5.7 h a decrease in the number of free Brønsted acid sites occurs followed by a decrease of the pore volume. However, the carbon content remained unchanged. This is explained by redistribution of the carbonaceous deposits into smaller entities. Some of these are migrating to the pore mouths where they are alkylated.

3. Slow deactivation: 2-butene is not transformed quantitatively anymore. We suggest that alkylation occurs at a small number of active sites close to the pore openings. The carbon content increases strongly due to the formation of polymerization products on Lewis acid sites outside the zeolite pores.

4. Rapid deactivation: The conversion of 2-butene starts decreasing rapidly after 12.9 h on stream rapidly. The integral production distribution shows that octane isomers are not formed any more. However, the formation of polymeric deposits continues.

Analysis of the carbonaceous deposits showed that bicyclic compounds with alkyl side chains are the dominant species formed on the catalyst. It is interesting to note that the nature of the deposits hardly changed while the catalyst was still active.