Local Distortions in a Prototypical Zeolite Framework Containing Double Four-Ring Cages: The Role of Framework Composition and Organic Guests

Abstract

<p>Cube-like double four-ring (<i>d4r</i>) cages are among the most frequent building units of zeolites and zeotypes. In materials synthesised in fluoride-containing media, the fluoride anions are preferentially incorporated in these cages. In order to study the impact of framework composition and organic structure-directing agents (OSDAs) on the possible occurrence of local distortions of fluoride-containing <i>d4r</i> cages, density functional theory (DFT) calculations and DFT-based molecular dynamics simulations were performed for AST-type zeotypes, considering four different compositions (SiO<sub>2</sub>, GeO<sub>2</sub>, AlPO<sub>4</sub>, GaPO<sub>4</sub>) and two different OSDA cations (tetramethylammonium [TMA] and quinuclidinium [QNU]). All systems except SiO<sub>2</sub>-AST show significant deformations, with a pyritohedron-like distortion of the <i>d4r</i> cages occurring in GeO<sub>2</sub>- and GaPO<sub>4</sub>-AST, and a displacement of the fluoride anions towards one of the corners of the cage in AlPO<sub>4</sub>- and GaPO<sub>4</sub>-AST. While the distortions occur at random in TMA-containing zeotypes, they exhibit a preferential orientation in systems that incorporate QNU cations. </p><p>In addition to providing detailed understanding of the local structure of a complex host-guest system on the picosecond timescale, this work indicates the possibility to stabilise ordered distortions through a judicious choice of the OSDA, which might enable a tuning of the material’s properties.</p>

Reverse ADOR: reconstruction of UTL zeolite from layered IPC-1P

“…This is an interesting result because D4R-containing zeolites or any extra-large pore zeolite with such a high Si content are seldom prepared without any hetero-element (such as Ge) or fluorides. 43 Fig. 8 H-K micropore size distributions of samples after reconstruction with different Si:Ge compositions, with and without NH 4 F.…”