Framework Modifications and Dehydration Path of a Ag⁺-Modified Zeolite with STI Framework Type

Abstract

The effect of Ag + incorporation into stellerite, a natural zeolite with STI framework type, was investigated by means of Single Crystal X-ray Diffraction (SC-XRD), Molecular Dynamics (MD) simulations and X-ray Absorption Fine Structure Spectroscopy (XAFS). At room temperature the complete exchange of the original extraframework ions with Ag + provoked a distortion of the framework accompanied by symmetry reduction from orthorhombic Fmmm to monoclinic F2/m space group. Ag + ions were strongly disordered, with occupancies ranging from 0.02 to 0.24, at partially-occupied sites within zeolitic cages. The combination of ab initio molecular dynamic 2 simulations and XAFS spectroscopy suggested that Ag + is coordinated by three H2O oxygen at 2.37 Å and by two framework oxygen atoms between 2.55 and 3.00 Å. The thermal stability was monitored in situ by SC-XRD (from 25 to 400°C) and by XAFS (from 25 to 650°C). Upon heating the structure transformed to three different topologies: B phase, D' phase, observed here for the first time, and D phase. The unit-cell volume contracted from 4392.85( 14) at room temperature to 3644.4(4) Å 3 , measured at 400°C. Possible Ag + to Ag 0 reduction could be excluded although the formation of Ag + -Ag + clusters could not be unambiguously ruled out.

Pbx(OH)y cluster formation in STI framework-type zeolites: anomalous thermal behaviour and increased thermal stability

“…This change agrees with the common trend observed for STI framework type zeolites (Fig. 1) and can be associated with an initial dehydration process [9,10,11]. The structural transformation in this temperature range (Fig2a-c) is characterized by the shrinking and deformation of the ten-membered ring channels parallel to [100] that become more elliptical.…”

“…After an equilibration time of 15 days under high humidity conditions (relative humidity > 90%), new data were collected at RT on a previously dehydrated crystal. The rehydrated structure has Fmmm symmetry and lattice parameters a = 13.6119 (7), b = 18.1834(9), c = 17.8460 (10) Å and V = 4417.1(4) Å 3 . The latter are very similar to those of the RT structure.…”

“…In previous studies, we reported the effect of heavy-metals incorporation on the thermal stability of a microporous zeolite with STI framework type [9,10,11]. The latter has symmetry Fmmm that can be lowered depending on the type and distribution of the extraframework occupants.…”

“…The interconnection of tetrahedra originates the t-sti-1* cavity [13], where the EF cations and H 2 O are hosted. Zeolites with STI framework type experience upon heating severe framework modi cations, which lead to: i) the rupture of speci c T-O-T connections of the framework; ii) the formation of new structural topologies (namely phase B [14,15,16,17], B' [11], D [18], D' [10]) and, iii) the occlusion of the ten-membered ring channels with subsequent loss of porosity [9,10,18]. The aforementioned structural changes lead to signi cant contraction of the pores accompanied by decrease of the unit-cell volume.…”

Crystal structure of Ag-exchanged levyne intergrown with erionite: Single-crystal X-ray diffraction and Molecular Dynamics simulations

“…A "shortrange" double-valence polarized basis set MOLOPT was used for each atomic kind (VandeVondele and Hutter 2007). Similar setup was successfully used in our previous simulations of zeolites (Cametti et al 2019a(Cametti et al , 2019b.…”