scholarly journals Perfluorinated Trialkoxysilanol with Dramatically Increased Brønsted Acidity

2021 ◽  
Author(s):  
Felix Feige ◽  
Lorraine A. Malaspina ◽  
Elena Rychagova ◽  
Sergey Ketkov ◽  
Simon Grabowsky ◽  
...  
Keyword(s):  
Brønsted Acidity ◽  
Brönsted Acidity

Proton Acidity and Proton Mobility in ECR-40, a Silicoaluminophosphate That Violates Löwenstein’s Rule

2019 ◽  
Author(s):  
Michael Fischer
Keyword(s):  
Density Functional ◽  
Water Clusters ◽  
Computational Study ◽  
Acid Catalyst ◽  
Ab Initio Molecular Dynamics ◽  
Brønsted Acidity ◽  
Hydrothermal Route ◽  
Proton Mobility ◽  
Brönsted Acidity ◽  
A Minor

<p>The silicoaluminophosphate zeotype ECR-40, which has the MEI topology, contains linkages of AlO<sub>4</sub> tetrahedra via a common oxygen atom, thereby violating the famous “Löwenstein’s rule”. Due to the proven existence of Al-O-Al linkages in this material, it constitutes an ideal model system to study the acidity and mobility of protons associated with such unusual linkages. In addition, their properties can be directly compared to those of protons associated with more common Si-O-Al linkages, which are also present in ECR-40. In this work, static density functional theory (DFT) calculations including a dispersion correction were employed to study the preferred proton sites as well as the Brønsted acidity of the framework protons, followed by DFT-based ab-initio molecular dynamics (AIMD) to investigate the proton mobility in guest-free and hydrated ECR-40. Initially, two different proton arrangements were compared, one containing both H[O6] protons associated with Al-O-Al linkages and H[O10] protons at Si-O-Al linkages, the other one containing only H[O10] protons. The former model was found to be thermodynamically favoured, as a removal of protons from the Al-O-Al linkages causes a local accumulation of negative charge. Calculations of the deprotonation energy showed a moderately higher Brønsted acidity of the H[O10] protons, at variance with previous empirical explanations, which attributed the exceptional performance of ECR-40 as acid catalyst to the presence of Al‑O‑Al linkages. The AIMD simulations (<i>T</i> = 298 K) delivered no appreciable proton mobility for guest-free ECR-40 and for low levels of hydration (one H<sub>2</sub>O per framework proton). Under saturation conditions, framework deprotonation occurred, leading to the formation of protonated water clusters in the pores. Pronounced differences between the two types of framework protons were observed: While the H[O10] protons were always removed from the Si-O-Al linkages, the Al-O-Al linkages remained mostly protonated, but deprotonation did occur to a minor extent. The observation of a degree of framework deprotonation of Al-O-Al linkages differs from the findings reported in a recent computational study of hydrated aluminosilicate zeolites with such linkages (Heard et al., <i>Chem. Sci.</i> <b>2019</b>, <i>10</i>, 5705), pointing to an influence of the overall framework composition. Further inspection of the AIMD results showed that a coordination of water molecules to framework Al atoms occurred in many cases, especially in the vicinity of the Al-O-Al linkages, sometimes resulting in a pronounced modification of the linkages through additional bridging oxygen atoms. Given the changes in the local structure, it can be expected that such modified linkages are especially prone to break upon dehydration. Thus, in addition to elucidating the deprotonation behaviour of protons associated with different types of linkages, the calculations also provide insights into possible reasons for the instability of Al-O-Al linkages, clarifying why Löwenstein’s rule is mostly obeyed in materials that are formed via a hydrothermal route.</p>

scholarly journals Theoretical Study of Zirconium Isomorphous Substitution into Zeolite Frameworks

Molecules ◽  
2019 ◽  
Vol 24 (24) ◽  
pp. 4466
Author(s):  
Duichun Li ◽  
Bin Xing ◽  
Baojun Wang ◽  
Ruifeng Li
Keyword(s):  
Acid Site ◽  
Density Functional ◽  
Theoretical Study ◽  
Atomic Radius ◽  
Acid Sites ◽  
Isomorphous Substitution ◽  
Dispersion Correction ◽  
Brønsted Acidity ◽  
Functional Theory ◽  
Brönsted Acidity

Systematic periodic density functional theory computations including dispersion correction (DFT-D) were carried out to determine the preferred location site of Zr atoms in sodalite (SOD) and CHA-type topology frameworks, including alumino-phosphate-34 (AlPO-34) and silico-alumino-phosphate-34 (SAPO-34), and to determine the relative stability and Brönsted acidity of Zr-substituted forms of SOD, AlPO-34, and SAPO-34. Mono and multiple Zr atom substitutions were considered. The Zr substitution causes obvious structural distortion because of the larger atomic radius of Zr than that of Si, however, Zr-substituted forms of zeolites are found to be more stable than pristine zeolites. Our results demonstrate that in the most stable configurations, the preferred favorable substitutions of Zr in substituted SOD have Zr located at the neighboring sites of the Al-substituted site. However, in the AlPO-34 and SAPO-34 frameworks, the Zr atoms are more easily distributed in a dispersed form, rather than being centralized. Brönsted acidity of substituted zeolites strongly depends on Zr content. For SOD, substitution of Zr atoms reduces Brönsted acidity. However, for Zr-substituted forms of AlPO-34 and SAPO-34, Brönsted acidity of the Zr-O(H)-Al acid sites are, at first, reduced and, then, the presence of Zr atoms substantially increased Brönsted acidity of the Zr-O(H)-Al acid site. The results in the SAPO-34-Zr indicate that more Zr atoms substantially increase Brönsted acidity of the Si-O(H)-Al acid site. It is suggested that substituted heteroatoms play an important role in regulating and controlling structural stability and Brönsted acidity of zeolites.

Ordered mesoporous zirconium oxophosphate supported tungsten oxide solid acid catalysts: the improved Brønsted acidity for benzylation of anisole

RSC Advances ◽  
2014 ◽  
Vol 4 (43) ◽  
pp. 22509-22519 ◽  
Author(s):  
Zhichao Miao ◽  
Huahua Zhao ◽  
Huanling Song ◽  
Lingjun Chou
Keyword(s):  
Tungsten Oxide ◽  
Solid Acid ◽  
Solid Acid Catalyst ◽  
Acid Catalyst ◽  
Solid Acid Catalysts ◽  
Acid Catalysts ◽  
Brønsted Acidity ◽  
Brönsted Acidity ◽  
Ordered Mesoporous

A series of WO3 supported on ordered mesoporous zirconium oxophosphate solid acid catalyst was employed in benzylation reaction.

Tuning Effect of the Zeolite Brønsted Acidity on the FeZn Bimetallic Hydrodesulfurization Catalyst

2021 ◽  
Author(s):  
Wenkai Wei ◽  
Xinyue Zhang ◽  
Xuandong Liu ◽  
Rong Guo ◽  
Bo Meng ◽  
...  
Keyword(s):  
Brønsted Acidity ◽  
Brönsted Acidity

Ammonia IRMS-TPD measurements on Brønsted acidity of proton-formed SAPO-34

2011 ◽  
Vol 13 (8) ◽  
pp. 3311-3318 ◽  
Author(s):  
Katsuki Suzuki ◽  
Takuma Nishio ◽  
Naonobu Katada ◽  
German Sastre ◽  
Miki Niwa
Keyword(s):  
Brønsted Acidity ◽  
Brönsted Acidity

scholarly journals TiO2/Zeolite Bifunctional (Photo)Catalysts for a Selective Conversion of Methanol to Dimethoxymethane: On the Role of Brønsted Acidity

2018 ◽  
Vol 122 (51) ◽  
pp. 29359-29367 ◽  
Author(s):  
Kawthar Ftouni ◽  
Louwanda Lakiss ◽  
Sebastien Thomas ◽  
Marco Daturi ◽  
Christian Fernandez ◽  
...  
Keyword(s):  
Brønsted Acidity ◽  
Selective Conversion ◽  
Brönsted Acidity
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