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Nano-sized Metal Fluorides: Novel Approaches to Lewis Acid Catalysts

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A special issue of Molecules (ISSN 1420-3049). This special issue belongs to the section "Organometallic Chemistry".

Deadline for manuscript submissions: closed (31 October 2016) | Viewed by 17340

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Special Issue Editor

Prof. Dr. Erhard Kemnitz

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Guest Editor

Full University Professor, Chemistry Department, Humboldt-Universität zu Berlin, Brook-Taylor-Str. 2, D-12489 Berlin, Germany
Interests: mechanistic aspects of heterogeneously catalysed fluorination reactions, non-aqueous fluorolytic sol–gel synthesis of nanoscopic metal fluorides; nano metal fluorides as solid Lewis acids; nano metal hydroxide fluorides as biacidic Brønsted/Lewis acids and bases; metal fluoride sols for antireflective coating; inorganic–organic hybrid materials based on nano metal fluorides
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Special Issue Information

Dear Colleagues,

Metal fluorides and fluorinated metal oxides have been widely used for over 70 years as heterogeneous fluorination catalysts in, e.g., the industrial productions of chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and hydrofluorocarbons (HFCs). In the past 10 years, many groups worldwide focused their interests on the synthesis of nanoscopic metal fluorides and metal hydroxide fluorides in order to make use of the high potential Lewis and/or Brønsted acidity of these solids. Thus, highly-active, new catalysts for several reactions were recently developed, exhibiting high Lewis acidity, based on the high degree of structural distortion and the high surface areas of the nanoparticular metal fluoride phases. Metal ions at the surface of a particle cannot have a coordination which is as symmetric as that of the ions inside a particle. They are coordinatively under-saturated. This effect, together with the strong electron-withdrawing power of fluorine, brings about the inherent Lewis acidity of the respective metal ions. Thus, aluminium chloride fluoride (ACF) and nanoscopic, high surface area aluminium fluoride, HS-AlF₃, are exceptionally strong Lewis acids; the acidity of which fits fully with their position in the Lewis acidity scale, calculated by K.O. Chrsite and D. Dixson, for isolated gaseous metal fluoride molecules, where AuF₅ and SbF₅ are the strongest Lewis acids, followed by AlF₃, AlFCl₂, AlF₂Cl, and AlCl₃. It is interesting to note that magnesium fluoride, typically regarded as a neutral compound, also shows distinct Lewis acidity, although not nearly as strong as that of nanoscopic AlF₃, when obtained on the nanoscopic scale.

We believe it is an appropriate time to take stock of the efforts and results of the many groups that have made progress in the field of metal fluoride based, novel, solid Lewis acids for a wide variety of reactions. Hence, the planned Special Issue, “Nano-sized Metal Fluorides: Novel Approaches to Lewis Acid Catalysts”, in Molecules, will summarize and update the knowledge gained over recent years.

Prof. Dr. Erhard Kemnitz
Guest Editor

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Keywords

Nanoscopic metal fluorides
Lewis acidity
Lewis-Brnsted biacidic metal hydroxide fluorides
Metal fluorides as support
Metal fluoride supported precious metal catalysts
C-F activation
C-H activation
C-C coupling reactions
Nanomaterials
Nanotechnology
Surface characterization of metal fluorides

Published Papers (3 papers)

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Research

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42147 KiB

Open AccessArticle

On the Morphology of Group II Metal Fluoride Nanocrystals at Finite Temperature and Partial Pressure of HF

by Zeinab Kaawar, Stefan Mahn, Erhard Kemnitz and Beate Paulus

Molecules 2017, 22(4), 663; https://doi.org/10.3390/molecules22040663 - 21 Apr 2017

Cited by 4 | Viewed by 4233

Abstract

We have investigated the bulk and surface properties of the group II metal fluorides CaF

_{2}

, SrF

_{2}

and BaF

_{2}

using periodic density functional theory (DFT) calculations and surface thermodynamics. Our bulk results show that the best agreement with experiment is [...] Read more.

We have investigated the bulk and surface properties of the group II metal fluorides CaF

_{2}

, SrF

_{2}

and BaF

_{2}

using periodic density functional theory (DFT) calculations and surface thermodynamics. Our bulk results show that the best agreement with experiment is achieved with the B3LYP and PBE functionals. We determined the relative importance of the low index surfaces in vacuum and found that an fluoride microcrystal exposes only the (111) surface in which the undercoordinated cations are sevenfold coordinated. With methods of ab initio surface thermodynamics, we analyzed the stability of different surfaces under hydrogen fluoride (HF) pressure and determined the presumable shape of the crystals with respect to different HF concentrations and temperatures. In the case of CaF

_{2}

and SrF

_{2}

, the calculated shapes of the crystals agree well with TEM images of fluorolytic sol-gel synthesized nanocrystals at room temperature and high HF concentration. Full article

(This article belongs to the Special Issue Nano-sized Metal Fluorides: Novel Approaches to Lewis Acid Catalysts)

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Figure 1

4453 KiB

Open AccessArticle

Hydrodechlorination of Tetrachloromethane over Palladium Catalysts Supported on Mixed MgF₂-MgO Carriers

by Magdalena Bonarowska, Maria Wojciechowska, Maciej Zieliński, Angelika Kiderys, Michał Zieliński, Piotr Winiarek and Zbigniew Karpiński

Molecules 2016, 21(12), 1620; https://doi.org/10.3390/molecules21121620 - 25 Nov 2016

Cited by 5 | Viewed by 5803

Abstract

Pd/MgO, Pd/MgF₂ and Pd/MgO-MgF₂ catalysts were investigated in the reaction of CCl₄ hydrodechlorination. All the catalysts deactivated in time on stream, but the degree of deactivation varied from catalyst to catalyst. The MgF₂-supported palladium with relatively large metal particles appeared the best catalyst, characterized by good activity and selectivity to C₂-C₅ hydrocarbons. Investigation of post-reaction catalyst samples allowed to find several details associated with the working state of hydrodechlorination catalysts. The role of support acidity was quite complex. On the one hand, a definite, although not very high Lewis acidity of MgF₂ is beneficial for shaping high activity of palladium catalysts. The MgO-MgF₂ support characterized by stronger Lewis acidity than MgF₂ contributes to very good catalytic activity for a relatively long reaction period (~5 h) but subsequent neutralization of stronger acid centers (by coking) eliminates them from the catalyst. On the other hand, the role of acidity evolution, which takes place when basic supports (like MgO) are chlorided during HdCl reactions, is difficult to assess because different events associated with distribution of chlorided support species, leading to partial or even full blocking of the surface of palladium, which plays the role of active component in HdCl reactions. Full article

(This article belongs to the Special Issue Nano-sized Metal Fluorides: Novel Approaches to Lewis Acid Catalysts)

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Review

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1500 KiB

Open AccessReview

Metal Fluorides, Metal Chlorides and Halogenated Metal Oxides as Lewis Acidic Heterogeneous Catalysts. Providing Some Context for Nanostructured Metal Fluorides

by David Lennon and John M. Winfield

Molecules 2017, 22(2), 201; https://doi.org/10.3390/molecules22020201 - 28 Jan 2017

Cited by 7 | Viewed by 6749

Abstract

Aspects of the chemistry of selected metal fluorides, which are pertinent to their real or potential use as Lewis acidic, heterogeneous catalysts, are reviewed. Particular attention is paid to β-aluminum trifluoride, aluminum chlorofluoride and aluminas γ and η, whose surfaces become partially fluorinated or chlorinated, through pre-treatment with halogenating reagents or during a catalytic reaction. In these cases, direct comparisons with nanostructured metal fluorides are possible. In the second part of the review, attention is directed to iron(III) and copper(II) metal chlorides, whose Lewis acidity and potential redox function have had important catalytic implications in large-scale chlorohydrocarbons chemistry. Recent work, which highlights the complexity of reactions that can occur in the presence of supported copper(II) chloride as an oxychlorination catalyst, is featured. Although direct comparisons with nanostructured fluorides are not currently possible, the work could be relevant to possible future catalytic developments in nanostructured materials. Full article

(This article belongs to the Special Issue Nano-sized Metal Fluorides: Novel Approaches to Lewis Acid Catalysts)

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