Reanalysis of CO₂-silicalite-1 structure as monoclinic twinning

Abstract

Structural analysis of CO2-silicalite-1 crystals using the single-crystal X-ray diffraction method reveals that the crystal system is twinned monoclinic. The structure of silicalite-1 (MFI-type zeolites) loaded with CO2 has previously been solved in orthorhombic system [1], but the R value is still large. Since CO2 molecules are too small to maintain the orthorhombic structure, it is considered that the CO2-silicalite-1 crystals should be in the relaxed monoclinic twin phase. The diffraction data in the previous report is recalculated in P21 /n as the monoclinic twinning.

Entrance and diffusion pathway of CO2and dimethyl ether in silicalite-1 zeolite channels as determined by single-crystal XRD structural analysis

“…The dotted lines are calculated according to the reversible first-order reaction model and the linearized plots are inserted. n eq are 5.62 and 6.74 (molecules/ unit cell) for CO 2 and DME loadings, which are results of the gravimetric adsorption measurement and TG (thermogravimetric) analysis at the respective conditions (Fujiyama et al, 2014b;. The plots are consistent with the model.…”

“…Thus, almost all the reflections are overlapped, and the pseudo-merohedral twinning refinement works successfully. The twinning refinement was performed using the command line 'TWIN' and 'BASF' (Fujiyama et al, 2014a). The refinement of the framework atoms converged with no restraints, and then the refinement with CO 2 and DME was conducted.…”

“…CO 2 and DME are selected as guest molecules for the following reasons. (i) The most stable adsorption sites of CO 2 and DME are the straight channels and sinusoidal channels, respectively (Fujiyama et al, 2014b;. The correlation between the pathway and the stable adsorption site can be determined.…”

Adsorption structure of dimethyl ether on silicalite-1 zeolite determined using single-crystal X-ray diffraction

“…The collected reflections were corrected for Lorentz polarization factors and the absorption effect. Structural analysis was conducted in the monoclinic twin in P2 1 /n.1.1 as described in the report (Fujiyama et al, 2014a). The structure was solved using a direct method, and difference-Fourier synthesis was used for the remaining atoms (SHELXTL; Sheldrick, 2008).…”

“…The mobility of guest molecules in the pore system was studied using NMR spectroscopy (Shen et al, 1990;Kolokolov et al, 2010;Nishchenko et al, 2012), and computational studies were also conducted to reveal the diffusion behavior (Makrodimitris et al, 2001;Krishna et al, 2006). A wide range of data has been reported, but very few actual adsorption structures have been reported except for aromatic molecules (van Koningsveld et al, 1989;van Koningsveld, Jansen & Man, 1996, van Koningsveld, Jansen & van Bekkum, 1996van Koningsveld & Koegler, 1997;Nishi et al, 2005;Kamiya et al, 2011Kamiya et al, , 2013 and CO 2 (Fujiyama et al, 2013(Fujiyama et al, , 2014a. Determining the adsorption structures is important to understand the adsorption properties.…”