Computer modeling of apparently straight bond angles: The intriguing case of all-silica ferrierite

Abstract

The relationships between synthetic zeolites and their natural counterparts that have been unveiled by theoretical studies have contributed to improving the properties and applications of zeolite-based materials in strategic areas such as industrial catalysis, environmental protection, and solar energy harvesting. To pinpoint the role of modeling in zeolite science, we discuss an example of computationally driven problem solving: can tetrahedral frameworks sustain straight (i.e., 180°) Si-O-Si bond angles? The true crystal symmetry of zeolite ferrierite (FER), especially in its all-silica form, had been intensely debated for 30 years before being solved in the Pmnn space group. Yet there are indications that an Immm structure with energetically unfavorable linear Si-O-Si linkages could be formed at high temperature. To gather insight, we perform density functional theory (DFT) optimizations and frequency calculations of all-silica ferrierite in both the Pmnn and Immm space groups. Our results indicate that Pmnn is more stable than Immm, in line with experiments. While the Pmnn structure is a true minimum in the energy profile of ferrierite, the Immm structure has four imaginary frequency vibrations, three of which are localized on the 180° Si-O-Si angles. This suggests that ferrierites with Immm symmetry may be classified as metastable phases. Such a designation is also supported by first-principles molecular dynamics on Immm FER, showing that the average value of 180° actually results from Si-O-Si angle inversion. An implication of this study with interesting geological and technological consequences is the association of straight Si-O-Si angles experimentally detected in open-framework or low-density silicates to an angle-inversion process occurring at the femtosecond scale. Such flexibility of the apparently flat Si-O-Si linkages might play an important role in sorption phenomena, which are ubiquitous in geological processes and industrial applications alike.

Steering polymer growth by molding nanochannels: 1,5-hexadiene polymerization in high silica mordenite

“…[11] Indeed, figures 10a-e clearly show that the computed positions of the framework atoms are symmetrically distributed around the lines denoting the refined structure. In line with results obtained on other zeolite frameworks, [42,97,98] the "averaging effect" is especially evident for the O positions due to the inherent flexibility of the Si-O-Si angles. Overall, such a comparison underlines quite a good agreement between calculated and refined framework structures.…”

“…[104] Although the DFT machinery surely has limitations due to self-interactions [105,106] for most purposes such scheme delivers a satisfactory accuracy-at a viable cost, as evidenced by benchmark calculations on various zeolitic frameworks. [107][108][109][110][111][112] The selection of PBE-D2 for the present case was prompted by its reliability in reproducing the average framework structure of LTL, [113] high-silica ferrierite, [42,98] and chabazite [97] obtained from in situ diffraction experiments. Such protocol delivers an adequate description of physico-chemical properties of molecules at material interfaces.…”

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

“…The T1−O−T2 angle in the cubic aristotype is fixed to 180° for symmetry reasons [68] . Due to the instability of such straight linkages, [82] a reduction in symmetry occurs in real AST‐type materials: Whereas materials with T=Si and Ge possess symmetry, [11,42,83] those with two different species at the T sites, like AlPO 4 ‐16, crystallise in space group [50,84] . When performing DFT calculations, a further reduction in symmetry is often necessary because the disorder of the OSDA molecules needs to be removed, as discussed above.…”

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

“…The T1-O-T2 angle in the cubic aristotype is fixed to 180° for symmetry reasons. 66 Due to the instability of such straight linkages, 80 a reduction in symmetry occurs in real AST-type materials: Whereas materials with T = Si and Ge possess 4/ symmetry, 9,39,81 those with two different species at the T sites, like AlPO4-16, crystallise in space group 4 ̅ . 47,82 When performing DFT calculations, a further reduction in symmetry is often necessary because the disorder of the OSDA molecules needs to be removed, as discussed above.…”