Effects of Site Occupancy, Cation Relocation, and Pore Geometry on Adsorption Kinetics in ETS-4

Abstract

Strontium-exchanged titanium silicate, Sr-ETS-4, is a new molecular sieve with promises for exciting applications in gas separation. The literature mentions poor thermal stability and low adsorption capacity as the drawbacks of the as-synthesized Na-ETS-4 and therefore the need for cation exchange. Upon ion exchange, Sr-ETS-4 shows appreciable kinetic selectivity between methane and nitrogen, which can be improved further by controlled dehydration. In this study, we trace the changes at the molecular level behind this improvement. By combining distributed information in the literature, it is shown that the governing factors are changes in pore geometry with progressive dehydration, changes in ion occupancy, and relocation of cations due to Sr exchange.

Molecular Simulation of the Adsorption of Methane in Engelhard Titanosilicate Frameworks

“…In the case of ETS-4, we also included structural water molecules. Indeed, according to the thermal gravimetry analysis (TGA) experiments of Nair et al 11 and Marathe et al, 17 ETS-4 loses all nonstructural water molecules on activation up to 373 K, but the structural molecules are only lost between 373 and 443 K, after which the ETS-4 material collapses entirely. We are comparing our simulations to adsorption experiments preceded by activation at 373 K, 24 therefore our simulations should take into account the structural water molecules.…”

“…The pore openings of titanosilicate ETS-4 can be thermally controlled, with the crystal lattice contracting at elevated temperatures in what is called the “molecular gate effect”. − This can improve its adsorption properties and enhance its potential for natural gas purification. In fact, these materials were already tested experimentally for the separation of methane from landfill gas by changing the extra-framework cations and also controlling the pore opening by heat treatment. − ETS-10 and ETS-4 have also been applied as potential heterogeneous base catalysts, , and in the stabilization of radical cations produced by photoirradiation of organic molecules adsorbed in the pores . Very recently, the properties of ETS-10 and ETS-4 for storage and release of gaseous NO have been studied by several experimental approaches and by density functional theory (DFT) …”

“…Although there have been numerous experimental studies of adsorption of gases like H 2 , N 2 , CO 2 , O 2 , Ar, Xe, NO, CH 4 , C 2 H 6 , and C 2 H 4 in ETS materials, ,,− ,− the detailed mechanism of adsorption in these frameworks is not yet fully understood, particularly with regard to the interactions between adsorbates and mobile extra-framework species (not only cations, but also strongly adsorbed water molecules that may be present to stabilize the framework). Molecular simulations are important tools for revealing the microscopic interactions that have a significant impact on macroscopic properties of adsorbents like the adsorption capacity, selectivity, diffusion, and molecular sieving.…”

Sr(ii)-UPRM-5 titanium silicate framework thermally induced contraction: in situ high temperature XRD and 29Si MAS NMR

“…8-14 Such structures contract upon thermal dehydration, allowing for tailoring of the material pore dimensions. 13,15, 16 As-synthesized ETS-4 features extra-framework sodium cations that act as counterbalancing species. Replacement of these by strontium(II) not only enhances the material stability, 15, 16 it also allows for better control of the flexibility of the framework.…”

“…13,15, 16 As-synthesized ETS-4 features extra-framework sodium cations that act as counterbalancing species. Replacement of these by strontium(II) not only enhances the material stability, 15, 16 it also allows for better control of the flexibility of the framework. 17 a Department of Chemical Engineering, University of Puerto Rico-Mayagüez Campus, Mayagüez, Puerto Rico, 00681-9000.…”