Synthesis Of Bis(indolyl)methanes Over Zeolites

Bis(indolyl)methanes, have attracted much interest in recent years due to their novelstructures and important biological activities. They are usually obtained from Friedel-Craftsalkylation of indole with various aldehydes in the presence of either Lewis or protic acids.However, these traditional catalysts have limitations such as environmental pollution hazardsarising from the disposal of potential toxic wastes and difficulties in their handling. Hence,there is still a need for an efficient catalyst for the synthesis of bis(indolyl)methanes. Zeolites,with the advantages of selective adsorption, high strength and thermal stability, are regardedas an optimum choice among all the solid acid catalysts in recent years.In our research, the synthesis of bis(indolyl)phenylmethane was chosen as the modelreaction and a lactic acid modified Hβ (LA-Hβ) zeolite was found to have the best activityamong the zeolites studied due to its larger surface area, smaller amount of weak acid sitesand higher amount of moderately-strong acid sites. The conversion of benzaldehyde was99.4%, and the selectivity to bis(indolyl)phenylmethane was97.2%under the optimum reactionconditions. It was also found that the regenerated catalyst kept excellent catalytic activity infive subsequent cycles. Moreover, both electronic and steric effects of substituents in thearomatic ring of aldehydes were found to play important roles in this reaction.In addition, we report our studies on the continuous synthesis of bis(indolyl)phenyl-methane. A citric acid modified Hβ (CA-Hβ) zeolite was found to have the best activity andselectivity. Research indicates that more surface Al species were leached during under thisacid treatment, and in turn, offered more active centers on the catalyst surface. And citric acidmodification should favor a decrease of the amounts of Br nsted and Lewis acid, especiallythe latter. Therefore, lower amounts of Br nsted and Lewis acid appear to be advantageous tothis reaction. Moreover, the conversion of benzaldehyde was99.0%, the selectivity tobis(indolyl)phenylmethane was90.5%under the optimum reaction conditions and then keptstable in the100h test.