Se incorporated into zeolite mordenite-Na: a single-crystal X-ray study

Abstract

Single crystals of self-synthesized mordenite-Na were used for incorporation of elemental selenium. The mordenite sample was first dehydrated at 280 °C and selenium was subsequently incorporated as gas phase at 450 °C for 72 h. Bright orange-colored Se-loaded mordenite was quantitatively analyzed by an electron microprobe yielding Na 6Al 6Si 42O 96 · [Se 7.9]. X-ray data collection of mordenite-Na and Se-loaded mordenite-Na single-crystals were performed at 120 K with synchrotron radiation ( λ=0.79946 Å) using the single-crystal diffraction line at SNBL (ESRF, Grenoble), where diffracted intensities were registered with a MAR image plate. The structures of mordenite-Na and Se-mordenite-Na were both refined in the monoclinic space group Cc converging at R1=5.25% (mordenite-Na), and R1=6.65% (Se-mordenite-Na). A strongly broadened Raman band at approximately 254 cm −1 confirmed the existence of Se chains in the 12-membered channels along the c-axis. Several, low-populated, disordered Se chains with a length up to 10 Å and seven Se atoms were located in the large mordenite channels. During structure refinement nearest and next nearest neighbor Se–Se distances were fixed at 2.34 and 3.62 Å, respectively. Other distances and angles remained unconstrained. Because of electrostatic interaction with the framework and influence of extraframework occupants such as Na + and H 2O molecules, the chains show different geometrical Se arrangement with highly variable dihedral angles. Any other Se species such as Se 6 or Se 8 rings were neither confirmed by structure refinement nor by Raman spectroscopy. There was no indication of a trigonal Se chain geometry within the 12-membered ring channel.