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

<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 (SiO₂, GeO₂, AlPO₄, GaPO₄) and two different OSDA cations (tetramethylammonium [TMA] and quinuclidinium [QNU]). All systems except SiO₂-AST show significant deformations, with a pyritohedron-like distortion of the <i>d4r</i> cages occurring in GeO₂- and GaPO₄-AST, and a displacement of the fluoride anions towards one of the corners of the cage in AlPO₄- and GaPO₄-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>

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

<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 (SiO₂, GeO₂, AlPO₄, GaPO₄) and two different OSDA cations (tetramethylammonium [TMA] and quinuclidinium [QNU]). All systems except SiO₂-AST show significant deformations, with a pyritohedron-like distortion of the <i>d4r</i> cages occurring in GeO₂- and GaPO₄-AST, and a displacement of the fluoride anions towards one of the corners of the cage in AlPO₄- and GaPO₄-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>

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

<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 (SiO₂, GeO₂, AlPO₄, GaPO₄) and two different OSDA cations (tetramethylammonium [TMA] and quinuclidinium [QNU]). All systems except SiO₂-AST show significant deformations, with a pyritohedron-like distortion of the <i>d4r</i> cages occurring in GeO₂- and GaPO₄-AST, and a displacement of the fluoride anions towards one of the corners of the cage in AlPO₄- and GaPO₄-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>

Tailoring the framework composition of carbon nitride to improve the catalytic efficiency of the stabilised palladium atoms

The C/N ratio of the carbon nitride lattice is tailored by doping with carbon to assess the impact on the stabilisation of palladium atoms and their catalytic efficiency in the selective hydrogenation of 2-methyl-3-butyn-2-ol.