Synthesis, Structure, and Optical Activity of HPM-1, a Pure Silica Chiral Zeolite

2-Ethyl-1,3,4-trimethylimidazolium
is a poor organic structure-directing
agent in the synthesis of pure silica zeolites using fluoride as a
mineralizer at 150 °C. Under these conditions only ill-crystallized
solids are obtained after long hydrothermal treatments (several weeks).
It disappoints despite its relatively large size, conformational rigidity,
and intermediate hydrophilic/hydrophobic character, attributes which
would qualify it as a promising structure-directing agent, according
to prior investigations. By raising the crystallization temperature
to 175 °C under otherwise identical conditions, crystallization
is dramatically accelerated. Depending on the water/silica ratio and
crystallization time, two different materials are obtained: the recently
reported pure silica polymorph of the chiral STW-type zeolite, HPM-1,
and the new layered organosilicate, HPM-2. Prolonged heating transforms
these phases into the small-pore ITW-type zeolite, while no signs
of the SOF-type zeolite (formally built from the same layers as STW)
was found. A complete physicochemical and structural characterization
of the as-made chiral HPM-1 zeolite is provided, and the proposed
stabilization of this zeolite by polarization of the Si–O bond
is supported by the observed deviation from tetrahedrality. HPM-1
is optically active, and a study of several crystallites by Mueller
matrix microscopy shows that their optical activity can be individually
measured and that this technique could be useful for the assessment
of the enantiomeric purity of a microcrystalline powder.