Correlating the Adsorption Preference and Mass Transfer of Xenon in RHO-Type Molecular Sieves

Abstract

The adsorption and diffusion of adsorbates or reactants are the most essential steps in zeolite-based processes for gas separation and catalysis. An explicit description of the relationship between adsorption and mass transport in confined environments of microporous materials is a prerequisite to catalytic reaction and gas separation. The dimensions, shapes, and types of pores are the most essential properties that govern the adsorption and diffusion behaviors of guest molecules. Here, a xenon atom was used as a sensitive probe to provide information about the dynamic adsorption process between the lta cavity and double eight-membered rings (D8R) of DNL-6 molecular sieves with RHO topology by 129Xe NMR and pulsed field gradient (PFG) NMR. Loading-dependent 129Xe NMR presents the two types adsorption environments with different probabilities of xenon distribution, and D8R is the preferential adsorption site. Furthermore, 129Xe PFG NMR exhibits the mass-transport limitations of the xenon atom in DNL-6 as the loading increases. On the basis of molecular dynamics (MD) and Monte Carlo (MC) simulations, the interaction energies between RHO framework and xenon were predicted and the preferred adsorption character of D8R was displayed visually, which further contribute to the understanding of adsorption and diffusion behavior, especially for the loading dependence of intracrystalline diffusion. The diffusion limitation caused by the preferential adsorption of D8R can depress the mass transport in RHO-type molecular sieves. A direct relationship between the adsorption preference and diffusion was established at the molecular level.

Cavity-controlled methanol conversion over zeolite catalysts

“…These cavity-controlled diffusion behaviors, combined with previous studies of the cavity-controlled reaction intermediates and catalytic route, together explain the product distribution differences: RHO-SAPO favors butene production, ethene is formed over LEV-SAPO, and propene is the main product over CHA-SAPO under reaction conditions. Subsequently, 129 Xe NMR spectroscopy using xenon atom as sensitive probe molecular and PFG NMR spectroscopy, reveal the function of the lta cage and double 8-membered rings (D8R) of DNL-6 molecular sieves (RHO) on dynamic adsorption and diffusion behavior [ 92 ]. Simultaneously, direct tracking MTO reaction [ 32 ] and the assessment of diffusivity has been realized by a home-made pseudo-gas chromatography method under actual MTO reaction conditions and the extracted intracrystalline and intercrystalline diffusion behavior provided the quantitative and dynamic evolution of the reactant and product shape selectivity.…”

The Journal of Physical Chemistry C Virtual Special Issue on Advanced Characterization by Solid-State NMR and In Situ Technology and in Recognition of Michael Hunger’s 65th Birthday

“…The Gläser group used in situ flow 1 H and 13 C MAS NMR to monitor CO 2 capture and hydrogenation on nanoporous solids . The Xu group investigated the relationship between adsorption and mass transport in confined pores of molecular sieves by 129 Xe NMR . The effect of the Cu state in Cu/zeolite on methane activation at acid sites using the H/D exchange method was interrogated by the Stepanov group by in situ 1 H MAS NMR monitoring .…”

“…29 The Xu group investigated the relationship between adsorption and mass transport in confined pores of molecular sieves by 129 Xe NMR. 30 The effect of the Cu state in Cu/zeolite on methane activation at acid sites using the H/ D exchange method was interrogated by the Stepanov group by in situ 1 H MAS NMR monitoring. 31 Hensen and coworkers described the nature of enhanced acidity via nearby extraframework aluminum on ultrastabilized zeolite Y by H/D exchange.…”