Dealumination of RHO zeolite by acid treatment and recrystallization with organic pore filler

Abstract

Zeolites are key materials in chemical industry exploited for their microporous structure. The control of the Si/Al ratio is one of the primary topics in zeolite science, especially for zeolites with 8-rings as their largest pores (small-pore zeolites), when considering their uses in catalysis and other applications. Although the direct synthesis method often yielded small-pore zeolites with a limited Si/Al ratio, our group successfully demonstrated post-synthetic dealumination in the presence of organic pore fillers. So far several types of small-pore zeolites, including AEI and AFX zeolites, could be dealuminated by acid treatment, still RHO zeolite loses its crystallinity after acid treatment, requiring another strategy for successful dealumination. In this work, successful dealumination of RHO zeolite with preserved crystallinity is demonstrated by acid treatment followed by post-synthesis recrystallization. The decrease in crystallinity during acid treatment is recovered by crystallization in the presence of a saturated silicate solution, and the final Si/Al ratio exceeded 7. When 18-crown-6 ether is added to the treatment solution as a pore-filling agent to avoid the occlusion of excess inorganic cations into the zeolite pores, the final bulk Si/Al ratio is 7.5. The relative crystallinity and micropore volume are 83 % and 0.27 cm³ g⁻¹, respectively, showing its high crystallinity. The resulting RHO zeolite is expected to show a superior hydrothermal stability during catalytic application such as selective catalytic reduction of nitrogen oxides.