Unexpected phase selectivity in germanosilicate zeolite synthesis: discovery and structure of HPM-18, a novel stable d4r-containing structure

Abstract

For pure-silica zeolites containing double 4-ring (d4r) units, it is generally expected that zeolite solid solutions can form across the full range of Si–Ge compositions. This is because d4r units, although strained in pure-silica compositions, are stabilized by Ge atoms at their corners. In this work, we show that using 1,3,4-trimethylimidazolium and fluoride as structure-directing agents, the phase selectivity of crystallization shifts from zeolite ITW at pure-silica compositions to a new zeolite, HPM-18, at a low Ge fraction (0.15). HPM-18 contains the same d4r density as ITW and the same T-site fraction belonging to d4r, but is significantly more porous and less dense, making this phase selectivity change unexpected. Interestingly, while the structure of HPM-18 is ordered at a low Ge fraction, increasing the Ge fraction introduces two-dimensional correlated disorder, and up to five different ordered polymorphs can be identified in the resulting intergrowth materials. We report synthesis, structural characterization and energy minimization calculations that explain these findings. An energy penalty for Si/Ge substitution in ITW explains the phase selectivity change, while reduced energy differences between HPM-18 polymorphs in GeO₂ compositions account for the increased disorder as the Ge fraction increases.