Unexpected Phase Selectivity in Germanosilicate Zeolite Synthesis: Discovery and Structure of HPM-18, a Novel d4r-Containing Structure

For pure silica zeolite structures containing double 4-rings (d4r) units, such as ITW, one could expect that it should be possible to prepare zeolite solid solutions in the whole range of Ge fractions (Gef = Ge/(Ge+Si) from 0 to 1) by the fluoride route. This is because d4r units, which are strained in pure silica compositions, are known to be favored by both fluoride ions inside the unit and Ge atoms in (some of) its corners. In this work we show that using 1,3,4-trimethylimidazolium and fluoride anions as structure-directing agents, the phase selectivity of the crystallization changes from ITW at pure silica compositions to a new zeolite structure, called HPM-18, at a Gef in the gel as small as 0.15. HPM-18 contains the same density of d4r as ITW (one d4r every 12 tetrahedral atoms, T) and the same fraction of T sites belonging to d4r (2/3 of T sites). Hence, the reasons for this change in phase selectivity of the crystallization is intriguing, specially considering that HPM-18 is significantly more porous and less dense than ITW. We also found that while the structure of HPM-18 is ordered for low Gef materials, when the Gef increases two dimensional correlated disorder occurs and up to five different ordered polymorphs can be identified in the resulting intergrowth materials. Here we report synthesis, structural characterization and energy minimization calculations that help explain the observations. An energy penalty for the Si and Ge substitution in ITW helps understand the observed phase selectivity change, while the reduced energy difference between HPM-18 polymorphs in GeO2 compositions explain the increased disorder as the Ge fraction increases.