Violations of Löwenstein's rule in zeolites

Fletcher, Rachel E.; Ling, Sanliang; Slater, Ben

doi:10.1039/c7sc02531a

Cited by 101 publications

(111 citation statements)

References 47 publications

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“…DR-UV-Vis-NIR spectra ( Figure S11) of SSZ-13 with high DCC proved the formation of the typical α-Fe and α-O absorption bands of the active site (Scheme 1), while such bands are absent in samples with very low DCC (Section S.3.1, Supporting Information). These results align with our earlier observations by 57 Fe Mössbauer and electronic spectroscopy, that assigned divalent α-Fe II species -located in 6MRs where two Al atoms are sited opposite of each other -as the active species for MPO in Fe-SSZ-13. 27 The link between MeOH productivity and DCC strengthens these findings and further suggests DCC as a descriptor for Al arrangements that allow Fe based MPO.…”

Section: Methane Partial Oxidation On Ssz-13 From Differentsupporting

confidence: 92%

“…here: as-made zeolites, containing OSDA, in equilibrium with their 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 synthesis liquor). 44,57 Zeotype materials kept within their synthesis liquor are known to be non-rigid: e.g. Si-islanding is a well-documented phenomenon in silicoaluminophoshates.…”

Section: Analysis Of Synthetic Factors Contributing To DCC Variationmentioning

confidence: 99%

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Synthesis–Structure–Activity Relations in Fe-CHA for C–H Activation: Control of Al Distribution by Interzeolite Conversion

et al. 2019

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The search for structurally relevant Alarrangements in zeolites is an important endeavor for single site catalysis. Little is known about the mechanisms and zeolite dynamics during synthesis that are responsible for creating those Al-ensembles. Here, new synthetic strategies for creating Al-hosts in small-pore zeolites suitable for divalent cation catalysis are uncovered, leading to a mechanistic proposal for Al-organization during crystallization. As such, unique synthesis-structure-activity relations are demonstrated for the partial oxidation of methane on Fe-exchanged CHA-zeolites. With modified interzeolite conversions, the divalent cation capacity of the resulting high Si SSZ-13 zeolites (Si/Al ~ 35) can be reproducibly controlled in a range between 0.04 and 0.34 Co 2+ /Al. This capacity is a proxy for the distribution of framework aluminum in pairs and correlates with the methanol production per Al when these zeolites host the -Fe II redox active site. The uncovered IZC synthesis-structure relations paint an Al-distribution hypothesis, where incongruent dissolution of the starting USY zeolite and fast synthesis kinetics with atypical growth modes allow assembling specific Alarrangements, resulting in a high divalent cation capacity. Prolonged synthesis times and high temperatures overcome the energetic barriers for T-atom reshuffling favoring Al-isolation. These mechanisms and the relations uncovered in this work will guide the search for relevant Al-ensembles in a range of zeolite catalysts where controlling the environment for a single active site is crucial.

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Section: Methane Partial Oxidation On Ssz-13 From Differentsupporting

confidence: 92%

Section: Analysis Of Synthetic Factors Contributing To DCC Variationmentioning

confidence: 99%

Synthesis–Structure–Activity Relations in Fe-CHA for C–H Activation: Control of Al Distribution by Interzeolite Conversion

et al. 2019

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“…Despite continuing advances in experimental characterization techniques, determining the precise location of acidic sites in zeolites remains a challenge. It is usually assumed that the Al distribution follows Löwenstein's rule, which empirically states that Al–O–Al triplets are not present in any aluminosilicate framework, but recent calculations suggest possible violations of this rule …”

Section: Computational Detailsmentioning

confidence: 99%

“…It is usually assumed that the Al distribution follows Lçwenstein's rule, [24] which empirically states that Al-O-Al triplets are not present in any aluminosilicate framework, but recent calculations suggest possible violations of this rule. [25] The initial structure of the all-silica form of zeolite BEA is taken from the IZA-SC database. [26] In order to generate three structural variants of the Na-BEA zeolite, 8r andom Si atoms are replaced with Al atoms following Lçwenstein's rule.…”

Section: Preparation Of Initial Structuresmentioning

confidence: 99%

Understanding the Reactive Adsorption of H₂S and CO₂ in Sodium‐Exchanged Zeolites

et al. 2018

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Purifying sour natural gas streams containing hydrogen sulfide and carbon dioxide has been a long-standing environmental and economic challenge. In the presence of cation-exchanged zeolites, these two acid gases can react to form carbonyl sulfide and water (H S+CO ⇌H O+COS), but this reaction is rarely accounted for. In this work, we carry out reactive first-principles Monte Carlo (RxFPMC) simulations for mixtures of H S and CO in all-silica and Na-exchanged forms of zeolite beta to understand the governing principles driving the enhanced conversion. The RxFPMC simulations show that the presence of Na cations can change the equilibrium constant by several orders of magnitude compared to the gas phase or in all-silica beta. The shift in the reaction equilibrium is caused by very strong interactions of H O with Na that reduce the reaction enthalpy by about 20 kJ mol . The simulations also demonstrate that the siting of Al atoms in the framework plays an important role. The RxFPMC method presented here is applicable to any chemical conversion in any confined environment, where strong interactions of guest molecules with the host framework and high activation energies limit the use of other computational approaches to study reaction equilibria.

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“…Employing density functional theory (DFT) calculations, Fletcher et al screened variousa rrangements of Si and Al in the unit cell of chabazite at different Si/Al ratios (Si/Al = 17, 11,8 ), as well as some other zeolite frameworks (ABW,L TA,R HO, MOR) at fixed Si/Al ratio. [10] Surprisingly,t hey found many cases where configurations with one or several non-Lçwenstein linkages gave the lowest DFT energy,c ontrary to the common assumption that Al-O-Al linkages are thermodynamically unstable. Because zeolitesa re formed in aqueous media, as ubsequent study by Heard et al investigated the influence of water on the stability of Al-O-Al linkagesi np rotonated chabazite (H-CHA), using ac ombination of DFT structure optimisations and DFT-based ab initio molecular dynamics (AIMD) calculations.…”

Section: Introductionmentioning

confidence: 99%

Proton Acidity and Proton Mobility in ECR‐40, a Silicoaluminophosphate that Violates Löwenstein's Rule

Fischer

2019

Chemistry A European J

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The silicoaluminophosphate zeotype ECR-40 contains linkages of AlO 4 tetrahedra via ac ommon oxygen atom, therebyv iolating the famous" Lçwenstein'sr ule". In this work, ac ombinationo fs tatic density functional theory (DFT) calculations and DFT-based ab-initio molecular dynamics (AIMD) simulations were employed to study the acidity and mobility of protons associated with such unusual linkages. It was found that the Al-O-Al linkages are preferentially protonated, as deprotonationc auses al ocal accumulationo f negative charge. The protons at these linkages possess a somewhat lower Brønsted acidity than those at Si-O-All inks. AIMD simulations for fully hydrated ECR-40 predicted ap artial deprotonation of the Al-O-Al linkages, whereas Si-O-Al linkagesw ere fully deprotonated. Frequently,acoordination of water molecules to framework Al atomsw as observed in the vicinity of the Al-O-Al links. Hence, these linkages appear prone to break upon dehydration, potentially explaining why Lçwenstein's rule is mostly obeyed in materials formed in aqueous media.[a] Dr.

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Violations of Löwenstein's rule in zeolites

Abstract: We predict that in aluminosilicate zeolites, non-Löwenstein Al–O–Al sequences are favoured in the presence of protons and disfavoured when Na+ ions are the charge compensating species.

Cited by 101 publications

References 47 publications

Synthesis–Structure–Activity Relations in Fe-CHA for C–H Activation: Control of Al Distribution by Interzeolite Conversion

Synthesis–Structure–Activity Relations in Fe-CHA for C–H Activation: Control of Al Distribution by Interzeolite Conversion

Understanding the Reactive Adsorption of H₂S and CO₂ in Sodium‐Exchanged Zeolites

Proton Acidity and Proton Mobility in ECR‐40, a Silicoaluminophosphate that Violates Löwenstein's Rule

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Violations of Löwenstein's rule in zeolites

Abstract: We predict that in aluminosilicate zeolites, non-Löwenstein Al–O–Al sequences are favoured in the presence of protons and disfavoured when Na+ ions are the charge compensating species.

Cited by 101 publications

References 47 publications

Synthesis–Structure–Activity Relations in Fe-CHA for C–H Activation: Control of Al Distribution by Interzeolite Conversion

Synthesis–Structure–Activity Relations in Fe-CHA for C–H Activation: Control of Al Distribution by Interzeolite Conversion

Understanding the Reactive Adsorption of H2S and CO2 in Sodium‐Exchanged Zeolites

Proton Acidity and Proton Mobility in ECR‐40, a Silicoaluminophosphate that Violates Löwenstein's Rule

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Understanding the Reactive Adsorption of H₂S and CO₂ in Sodium‐Exchanged Zeolites