An imidazolium dication affords a stable UWY-like zeolite and enables a subtle structure direction towards EMM-17

Abstract

Zeolites with interconnected medium (10Ring) and large (12R) pores, such as the UWY framework type, hold significant potential for catalysis but may suffer from instability due to a high germanium content required for their synthesis. Addressing this limitation, we report the synthesis of HPM-9, a novel UWY-like germanosilicate zeolite, utilizing 1,1′-(1,8-octanediyl)bis(3-methylimidazolium) (8BMI) as the organic structure-directing agent (OSDA) in fluoride media. HPM-9 achieves significantly lower germanium content (Ge_f = Ge/(Ge + Si) down to 0.14) compared to the original IM-20 (Ge_f = 0.3), resulting in markedly improved stability upon calcination and exposure to water. Structural analysis using synchrotron powder X-ray diffraction (SPXRD) suggested disorder and DIFFaX simulations revealed that HPM-9 exhibits minimal stacking faults along the a-axis, corresponding to an intergrowth with a polymorph where double four-membered rings (d4r) are partially replaced by single four-membered rings (s4r), likely driven by the reduced Ge content. Furthermore, decreasing the Ge content further (Ge_f ≤ 0.05) subtly shifts the structure direction towards EMM-17, a zeolite featuring interconnected 11R and 10R pores. ¹⁹F MAS NMR suggests the presence of d4r units in the synthesized EMM-17, and we propose a mechanism where the less stable d4r-containing polymorph C nucleates first, templating the subsequent growth of d4r-free polymorphs A and B by epitaxial intergrowth. Comparative studies with analogous OSDAs (7BMI and 9BMI) confirmed the optimal linker length of 8BMI for directing towards stable UWY structures. The enhanced stability of HPM-9 opens avenues for the practical application of UWY-type zeolites in catalysis.