Further Investigations of Racemic and Chiral Molecular Sieves of the STW Topology

Large
single crystals of pure-silica STW-type molecular sieves
are prepared using 1,2,3,4,5-pentamethylimidazolium (12345PMI) and
2-ethyl-1,3,4-trimethylimidazolium (2E134TMI) as organic structure-directing
agents (OSDAs) and α-amino acid additives. The effects of the
amount and type of amino acid on the crystallization of pure-silica
STW materials are investigated. Room-temperature single-crystal X-ray
diffractometry shows that 12345PMI better fits the STW cage than 2E134TMI
and is fully consistent with computed simulations. The synthesis of
an enantioenriched R-STW-type molecular sieve having a Si/Ge ratio
of 2.2 is achieved using a chiral, dicationic OSDA that contains two
stereocenters. The conformation of the chiral OSDA in the chiral framework
has been obtained from a Rietveld refinement using synchrotron powder
diffraction data and is the first structure refinement of an enantioenriched,
inorganic molecular sieve prepared from a pure, asymmetric OSDA.