Interlayer spacing in pillared and grafted MCM-22 type silicas: density functional theory analysis <i>versus</i> experiment

Han, Yong; Chatterjee, Puranjan; Alam, Sardar Bilal; Prozorov, Tanya; Slowing, Igor I.; Evans, James W.

doi:10.1039/d2cp03391g

Cited by 6 publications

(6 citation statements)

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“…In describing the construction of our models for MCM-22, it is instructive to recall the experimental synthesis procedure which is illustrated in Figure for the pure silica version, ITQ-1, of MCM-22 . This process involves a precursor MCM-22P which consists of an array of weakly interacting layers (with a period of around 27.9 Å orthogonal to the layers), the surfaces of which are fully hydroxylated (Figure a).…”

Section: Mcm-22 Models and Dft Methodologymentioning

confidence: 99%

“…The structure within this unit cell has been well characterized in previous experiments and by DFT theory. [7][8][9]12 Figure 1a also illustrates our selection of a structure model for the external surface, which simply corresponds to that of the fully hydroxylated surfaces of the layers of the precursor and thus supports two silanol groups for each lateral external surface unit cell.…”

Section: Periodic Mcm-22 Models and Plane-wave Dft Analysismentioning

confidence: 99%

“…In describing the construction of our models for MCM-22, it is instructive to recall the experimental synthesis procedure which is illustrated in Figure 1 for the pure silica version, ITQ-1, of MCM-22. 12 This process involves a precursor MCM-22P which consists of an array of weakly interacting layers (with a period of around 27.9 Å orthogonal to the layers), the surfaces of which are fully hydroxylated (Figure 1a). Formation of MCM-22 occurs via calcination during which the internal surfaces are completely dehydroxylated, and the periodicity orthogonal to the layers decreases by about 2.7 Å, yielding a final periodicity of 25.2 Å (Figure 1b).…”

Section: Periodic Mcm-22 Models and Plane-wave Dft Analysismentioning

confidence: 99%

“…In our plane-wave DFT calculations, the k mesh is always taken to be 2 × 2 × 1 for both our calculations of binding at internal pore surfaces using a bulk MCM-22 unit cell, and at an external surface of MCM-22. 12 For the latter surface calculations, we set the vacuum thickness between slabs of MCM-22 to be no smaller than 20 Å along the direction perpendicular to the slab surfaces. The slab thickness equals the dimension of a single unit cell orthogonal to the layers, i.e., effectively the slab consists of a single layer of the precursor MCM-22P.…”

Section: Periodic Mcm-22 Models and Plane-wave Dft Analysismentioning

confidence: 99%

“…7−9 These materials are also tunable with respect to Al content 10 and also with respect to pillaring. 11,12 A recent experimental study demonstrated substantial uptake of REE Yb and Nd in MCM-22, well above that for natural zeolites. 13 Furthermore, this study revealed some enhancement of REE uptake with increased Al content of the MCM-22 and also demonstrated recyclability, both suggesting that Brønsted acid sites (BAS) play a significant role in REE binding, which should be mediated by ionic interactions in this system.…”

Section: Introductionmentioning

confidence: 97%

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DFT Analysis of the Binding of Rare Earth Nitrates at Internal and External Surfaces of MCM-22

Han,

Ash,

Chatterjee

et al. 2024

J. Phys. Chem. C

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MCM-22-type zeolites constitute a well-characterized tunable class of aluminosilicates suitable for elucidating the fundamental aspects of the binding of rare earth elements (REEs) in layered materials. Starting from the pure silica version ITQ-1 with a unit cell of Si 72 O 144 , a model for periodic bulk crystalline MCM-22 with a finite Al concentration is provided by replacing a Si atom with an Al atom in the unit cell at suitable tetrahedral sites near an internal pore surface. Then, a H atom is added to an O atom bridging Si and Al atoms to create a Brønsted acid site (BAS). There are no internal silanol groups in this bulk model. To generate a model for an external surface, we adopt the fully hydroxylated surface structure of a layer within the ITQ-1 precursor with two silanols per lateral unit cell. A BAS on the external surface can be generated by replacing a near-surface Si atom with an Al atom and adding a H atom, as above. The strength of binding at a BAS of REE, X, taken to be present in the solution phase as nitrates, is determined from the energy change in the reaction X(NO 3 ) 3 + �Si−{OH}−Al� → �Si−{OX(NO 3 ) 2 }−Al�+ HNO 3 . The strength of binding at the silanols is determined similarly. Binding energies are determined from two approaches. The first performs periodic plane-wave density functional theory (DFT) total energy analysis for an entire unit cell of MCM-22. The second utilizes cluster models capturing the local environment of REE binding sites and performs DFT analysis with localized basis sets. The two approaches yield consistent results for Nd, revealing similarly strong binding at either an external or internal BAS, but much weaker binding at a silanol site. This is consistent with the picture deduced from recent experiments. We also comment on binding at Al-bridged siloxane sites, which have been suggested as alternative binding sites to BAS.

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Section: Mcm-22 Models and Dft Methodologymentioning

confidence: 99%