RHO Zeolites with High Hydrothermal Stability

RHO zeolites were synthesized from aluminosilicate gels with sodium and cesium and, optionally, 18-crown-6 ether as a structure directing agent. Phase-pure RHO zeolite samples with different Si/Al ratios and particle morphologies were obtained. In the presence of crown ether phase, pure RHO zeolite was obtained with Si/Al ratios in the range of 3.4-3.8, which appeared as spherical agglomerates of nanosized crystallites. In the absence of crown ether and high-sodium concentration, nanometer-size RHO zeolite with Si/Al ratio of 1.5 and polyhedral morphology was obtained. Lowering the sodium content in the synthesis mixture resulted in crystallization of phase-pure RHO zeolites with Si/Al ratios of 2.8-3.0 with morphologies of spherical agglomerates of tiny crystals next to larger polyhedrons. RHO zeolite samples with a Si/Al ratio as low as 2.8 in their proton form were found to be hydrothermally stable for 3 h at 750 degrees C in air with 12% water vapor, which is exceptional behavior at such a low Si/Al ratio. Crystallinity was maintained despite substantial dealumination. Samples with higher Si/Al ratio showed similar high stability. Loading RHO zeolite with palladium was found to stabilize the zeolite even more because more aluminum was retained in the framework, and dealumination was less pronounced after steam treatment. Pd-loaded RHO zeolite with a Si/Al ratio of 2.8 retained most of its crystallinity after steam treatment for 3 h at 850 degrees C in air with 12% water vapor. The remarkable stability of RHO zeolites at a low Si/Al ratio can be explained by the double eight-rings the framework is composed of. After dealumination by two or even three Al atoms of a double eight-ring it maintains its integrity by the six or five remaining oxide bridges between the two rings.

Automated summary

RHO zeolites with different Si/Al ratios and particle morphologies were successfully synthesized by adding crown ether and adjusting the sodium content. The zeolites showed exceptional stability even at low Si/Al ratios, which can be attributed to the double eight-ring framework.