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

Abstract

The diastereomers of 4,4′-trimethylenebis(1-methyl-1-benzylpiperidinium) are synthesized and used as organic agents in the synthesis of pure-silica molecular sieves in order to investigate further the nature of structure-direction. The diastereomer A that has two trans-1-benzyl- cis-1-methyl-4-(trimethylene)piperidinium moieties adopts a predominant conformation in solution phase at room temperature with N-benzyl groups in the equatorial positions. This molecule strongly directs the formation of the pure-silica analogue of zeolite beta. On the contrary, the diastereomer B that has two cis-1-benzyl- 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 topology. This is somewhat surprising since B is less cylindrical than A and appears more likely to form a multidimensional pore system. These results suggest that the true organic agent that structure-directs the BEA topology is the 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 topology with a rather long crystallization time suggests that cis-1-benzyl-4-(trimethylene)piperidinium manages to fit in the one-dimensional 12-membered ring pore possibly by changing conformation by piperidinium ring flipping.