Cationic methylene blue incorporated into zeolite mordenite-Na: a single crystal X-ray study

Single crystals of self-synthesized mordenite-Na were used for incorporation of the cationic dye molecule methylene blue (MB+). Because the molecular size of MB+ based on van der Waals radii (7.0 × 16 Å) is close to the opening of the large 12-membered ring channel of mordenite (6.5 × 7 Å), the molecule fits tightly into this channel. MB+ was incorporated by ion-exchange in aqueous dye solution. The chemical composition of violet MB+-loaded mordenite crystals was determined by an energy dispersive (EDS) element analysis yielding Na5.52MB0.28Si42.02Al5.80O96 × nH2O. X-ray data collection of MB+-loaded mordenite single-crystals (MB-mordenite-Na) was performed at 120 K with synchrotron radiation (λ = 0.79945 Å) using the single-crystal diffraction line at the Swiss Norwegian Beamline, SNBL (ESRF, Grenoble) where diffracted intensities were registered with a MAR image plate. The structure of MB-mordenite-Na was refined in the monoclinic space group Cc converging at R1 = 6.09%. Structural data of MB-mordenite-Na were compared with a structure refinement of pure mordenite-Na also measured at 120 K. Furthermore, successful incorporation of MB+ was confirmed with optical and fluorescence microscopy. Two partly occupied molecule sites were found in the large 12-membered ring channel; one oriented upright, the other slightly inclined within the channel cross section. The tight fit of the molecule in the mordenite channel results in relatively short C ⋯ O distances from the molecule to the zeolite framework, indicating hydrogen bonding and molecule-framework interaction. The incorporation of MB+ also influences the extra framework Na and H2O distribution, resulting in a strongly disordered arrangement.