Positions of cations and molecules in zeolites with the mordenite framework IX dehydrated H-mordenite via acid exchange

Abstract

Nearly complete removal of cations was accomplished for natural mordenite by preliminary Na-exchange followed by exposure to 2M HCl for 1 month at 90°C. The electron microprobe analysis (atomic Si/Al 4.7) indicates no detectable extraction of Al. However the cell dimensions after dehydration at 300°C followed by cooling to room temperature ( a 18.178(7) b 20.394(6) c 7.488(4)Å) are lower than for “hydrogen-mordenite” prepared via an ammonium-exchanged intermediary ( a 18.223(7) b 20.465(9) c 7.531(4)Å). Furthermore the tetrahedral distances are smaller (T4 down 0.017Ȧ, T3 0.010, T1 0.008, T2 0.002). Diffractions from the HCl-treated mordenite are sharp, and the positional and population parameters are close to those for the de-ammoniated mordenite. The simplest interpretation is that treatment with HCl results in extraction of Al from tetrahedral sites into other positions in the crystal coupled with migration of Si so that a crystalline structure is retained: however, many subtle complications require cautious interpretation.