Synthesis of 4,4 '-trimethylenebis (1-benzyl-1-methylpiperidinium) diastereomers and their use as structure-directing agents in pure-silica molecularsieve syntheses

The diastereomers of 4,4'-trimethylenebis(1-methyl-1-benzylpiperidinium) ar e synthesized and used as organic agents in the synthesis of pure-silica mo lecular sieves in order to investigate further the nature of structure-dire ction. The diastereomer A that has two trans-1-benzyl-cis-1-methyl-4-(trime thylene) piperidinium moieties adopts a predominant conformation in solutio n phase at room temperature with N-benzyl groups in the equatorial position s. This molecule strongly directs the formation of the pun-silica analogue of zeolite beta. On the contrary, the diastereomer B that has two cis-1-ben zyl-trans-1-methyl-4-(trimethylene) piperidinium moieties whose most stable conformer has N-benzyl groups in the axial positions appears not to direct the formation of materials with the BEA topology but rather the MTW topolo gy. This is somewhat surprising since B is less cylindrical than A and appe ars more likely to form a multidimensional pore system. These results sugge st that the true organic agent that structure-directs the BEA topology is t he diastereomer that has N-benzyl(eq)-N-methyl (ax)-piperidinium moieties ( diastereomer A). The fact that the diastereomer B is able to give the MTW t opology with a rather long crystallization time suggests that cis-1-benzyl- 4-(trimethylene) piperidinium manages to fit in the one-dimensional 12-memb ered ring pore possibly by changing conformation by piperidinium ring flipp ing. (C) 1999 Elsevier Science B.V. All rights reserved.