High-silica, heulandite-type zeolites prepared by direct synthesis and topotactic condensation
There are both natural minerals and synthetic zeolites that possess the HEU framework topology. These materials have a limited compositional range (Si/Al < 6), and the natural zeolites often contain a large amount of impurities such as Fe^(3+). The preparation of impurity-free HEU-type zeolites with higher Si/Al ratio could open many areas of application, particularly in catalysis. Here, we report the first high-silica HEU-type zeolite that can be prepared via two different procedures. In the first method high-silica HEU (denoted CIT-8) is prepared using a topotactic condensation mechanism (layered precursor denoted CIT-8P); CIT-8P is obtained from a low-water synthesis in fluoride media. CIT-8 prepared in this manner has a product Si/Al ratio of 9.8 ± 0.7 and a micropore volume of 0.10 cm^3 g^(−1) (measured by nitrogen adsorption). The variable temperature powder X-ray diffraction shows that CIT-8 forms via topotactic condensation from CIT-8P along the b axis. Additionally, high-silica heulandite can be synthesized directly from a hydroxide-mediated reaction mixture (denoted CIT-8H), and has a Si/Al ratio of 6.4 ± 0.3 and a micropore volume of 0.10 cm^3 g^(−1). Both synthesis methods produce zeolites that expand the compositional range of HEU-type zeolites. These synthetic methods allow for the addition of other heteroatoms, and titanium-containing CIT-8 is prepared as an illustrative example.
© 2015 The Royal Society of Chemistry. Received 31st March 2015; Accepted 19th May 2015; First published online 27 May 2015. We would like to thank Thomas Rea (Chevron Energy Technology Company) for collecting the RED data and Dr Stacey Zones (Chevron Energy Technology Company) for insightful discussions. Also, we would like to thank Dr Sonjong Hwang (Caltech) for assistance with solid-state NMR collection and interpretation. Chevron Energy Technology Company provided funding for this work. J.E.S. would like to thank the NDSEG for their support through a fellowship. PQ Corporation is thanked for providing us with sodium silicate.
Supplemental Material - c5ta02354h1_sup.pdf