Neutron Diffraction and Computational Study of Zeolite NaX:  Influence of SIII‘ Cations on Its Complex with Benzene

Abstract

The crystal structure of zeolite NaX (Si:Al ) 1.2) and that of its complex with benzene have been determined at 5 K by Rietveld analysis of powder neutron diffraction data in space group Fd3. For NaX, a ) 25.0328-(5) Å, R wp ) 4.25%, and R p ) 3.27%. For NaX + benzene, a ) 25.0496(7) Å, R wp ) 4.60%, and R p ) 3.52%. The sodium ions were located preferentially in sites SI′ and SII, both of which are fully occupied. The remaining cations were found at the SIII′ site in the 12-ring window. These cations are coordinated by two oxygen atoms, O(1) and O(4), in a corner of the 4-ring window where the aluminum is located. Our simulations on cation positioning in NaX (Si:Al ) 1) predict the preferred occupancy of SIII′ sites that are facing AlO 4 tetrahedra and support our crystallographic results. Less than half the benzene molecules were found at the six-ring window in the supercage, where they interact with the SII cations. No benzene was found in the 12-ring window, presumably because of the presence of the SIII′ sodium cations, but there remains a possibility that undetected benzene is adsorbed near these cations.

Accurate Fit and Thermochemical Analysis of the Adsorption Isotherms of Methane in Zeolite 13X

“…Our value is within 1% of the values obtained by Woods and Rowlinson (1989) and Maurin et al (2006) from GCMC simulation, and by Ding et al (1988) from the method of isosteres applied to the low-pressure region of the adsorption isotherms. By taking the limit of equation (14) ideal-gas phase.…”

“…The chemical composition Na 88 (AlO 2 ) 88 (SiO 2 ) 104 has been reported by Chkhaidze et al (1985) for the sample of zeolite 13X that was used to measure the isotherms shown in Figure 1. According to Zhu and Seff (1999), all sites I and II in zeolite 13X are fully occupied by sodium ions; therefore, in the unit cell of the zeolite sample used by Chkhaidze et al (1985), 32 Na + ions would be expected to be located at sites I, 32 Na + ions would be expected to occupy sites II, while the remaining 24 Na + ions should occupy sites III. Methane molecules can only interact with Na(2) or Na(3) ions, because Na(1) ions are located inside the β cages; these have a mean effective aperture of 2.2 Å (Breck 1964) which is much smaller than the molecular collision diameter of 3.8 Å for CH 4 (Mourits and Rummens 1977).…”

“…Methane molecules can only interact with Na(2) or Na(3) ions, because Na(1) ions are located inside the β cages; these have a mean effective aperture of 2.2 Å (Breck 1964) which is much smaller than the molecular collision diameter of 3.8 Å for CH 4 (Mourits and Rummens 1977). Moreover, the manner in which CH 4 molecules interact with Na(2) and Na(3) ions has been elucidated from molecular statistical calculations (Bezus et al 1978;Kiselev and Du 1981) and Monte Carlo simulation studies (Woods and Rowlinson 1989;Maurin et al 2006).…”

“…Zeolite 13X (also known as zeolite NaX) is a synthetic crystalline sodium aluminosilicate that is widely used in industry for the separation of methane (CH 4 ) and carbon dioxide (CO 2 ) from landfill gases (Sircar et al 1988;Kumar and VanSloun 1989;Sircar and Myers 2003), for the separation of ternary mixtures of CH 4 , CO 2 and N 2 for upgrading natural gas (Dong et al 1999;Cavenati et al 2006), and for the catalytic conversion of CH 4 and CO 2 mixtures to synthesis gas and higher hydrocarbons (Eliasson et al 2000;Zhang et al 2001). Since CH 4 has a global-climate warming potential which is 21-times higher than that of CO 2 (EPA 2010), the recovery of CH 4 from landfill gases, coal mines or coal beds (Olajossy et al 2003), and from some other sources, has become of the utmost importance.…”

Adsorption of n-Alkanes in Faujasite Zeolites: Molecular Simulation Study and Experimental Measurements

“…Sites III are reported to be of a higher potential energy than sites I, I' and II (Beauvais et al 2004). At a low occupancy (Si/Al > 1.5), cations are known to occupy sites I, I' and II only (Vitale et al 1997).…”