Template effects on the pressure-dependent behavior of chabazite-type fluoroaluminophosphates: a computational approach

Abstract

Experimental in-situ studies have shown that the compressional behavior of zeolite frameworks depends not only on the framework topology, but also on the extra-framework content, i.e. the species residing in the pores (cations, water molecules, templates). However, systematic experimental studies of isotypic frameworks with different extra-framework content often face several challenges, such as limited quality of the diffraction data and non-ideal behavior of the pressure-transmitting medium. In order to provide an alternative computational perspective, the compressional behavior of four fluoroaluminophosphate zeolite analogues with a chabazite (CHA) topology was investigated using electronic structure calculations in the framework of dispersion-corrected density functional theory (DFT). The AlPO-CHA systems differ in the nature of the organic template molecule occupying the chabazite cage, with the template species being morpholinium (morph), pyridinium (pyr), dimethylimidazolium (DMI), and trimethylimidazolium (TMI), respectively. For the case of AlPO-CHA_morph, the evolution of the lattice parameters with pressure as obtained from the computations was found to be in agreement with experimental data from an earlier in-situ diffraction investigation. In particular, the occurrence of a structural transition at pressures above 3 GPa could be reproduced. An analysis of the DFT-optimized structures showed that a pronounced elongation of the chabazite cage occurs upon the transition, which is associated with a rearrangement of the morpholinium molecules residing in the cage. Moreover, the double six-ring building units are strongly deformed during the transition. Analogous computations for the other three AlPO-CHA systems showed no indications for the occurrence of a P-induced transition, with only minor discontinuities in the evolution of the structural parameters with pressure. The compressibility in the low-pressure range varied widely depending on the template, with bulk moduli ranging from 19 GPa for AlPO-CHA_pyr to 51 GPA for AlPO-CHA_TMI. Despite the identical composition of the framework, different deformations of the double six-ring units with pressure were observed. The present study highlights the important influence of the template on the compressional behavior of porous framework compounds. Firstly, the molecular volume of the template and the nature of the framework-template interactions impact upon the compressibility in the low-pressure regime. Secondly, the ability of the template to accommodate pressure through continuous rotation and shortening of template-template and framework-template distances determines whether a discontinuous structural transition occurs or not.

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

“…A reoptimisation of AIMD average structures obtained at 150 K, and comparison to the initial structures with fluoride at the cage centre delivers energy differences of -2.2/-3.8 kJ mol -1 per AlPO4 formula unit (-11/-19 kJ mol -1 per fluoride anion) for AlPO4-AST_TMA/AlPO4-AST_QNU, corroborating that the formation of Al-F bonds is energetically favoured over a centre-of-cage position of fluoride. The optimised Al-F bond lengths of 1.90 Å agree with values observed in alumino-phosphates where fluoride anions bridge between two Al atoms 50,52,54.…”

“…8,49 Such bonding environments are found, for example, in AlPO4 and GaPO4 zeotypes with "ringopened" d4r (sti cage) units (SSZ-51, 50 GaPO4-ZON 51 ) and in CHA-and GIS-type fluoroaluminophosphates. [52][53][54] With typical Al-F distances of ~1.85 to 2.0 Å, and Ga-F distances of ~1.9 to 2.2 Å, the distances observed in these zeotypes are shorter than those found in d4r-containing systems, 8 pointing to a different bonding character. The isotropic displacement parameters of F atoms located inside d4r cages are often considerably larger than those of the framework atoms.…”

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

“…fluoride anions bridge between two Al atoms. [53,55,57] This contrasts with the behaviour observed for SiO2-AST and GeO2-AST, where only oscillations around the cage centre occur. It is worth noting that the particular situation in AlPO4-AST also explains the evolution of the F-RMSDs with temperature: When most of the fluoride anions are bonded to a single Al atom during the whole AIMD simulation, their RMSD will be similar to that of the framework oxygen atoms due to the similar bond strength, as is indeed observed for 150 K.…”

Differential penetration of ethanol and water in Si-chabazite: High pressure dehydration of azeotrope solution

“…Such a "PBE-D2" combination of density functional approximation /dispersion correction has been widely used in silicate modeling, ensuring a good accuracy/cost compromise. Indeed, whereas recent benchmark studies [33] indicate slightly better results for the (dispersion-corrected) PBE-sol approach in the computation of zero-K structural parameters of neutral zeotypes, (dispersioncorrected) PBE works slightly better in the case of aluminophosphates [34]. Our choice of selecting PBE-D2 for the present investigation is justified by the fact that, in the case of the pressure-induced water-ethanol incorporation in Si-FER [9] [35], this theoretical approach provided an average roomtemperature framework structure in very good agreement with the X-ray refinements (see [9]).…”