Fluorolytic Sol Gel Research Articles

Sol–gel synthesis and characterisation of nanoscopic strontium fluoride

Strontium fluoride nanoparticles have been successfully synthesized via the fluorolytic sol–gel synthesis. The investigation by DLS, XRD, TEM and N2 adsorption/desorption measurements (BET) revealed the existence of sol particles with a hydrodynamic diameter of approximately 6 nm, crystallite sizes below 10 nm and high surface area of 180 m2 g−1. The sols are highly transparent and stable for a few months, however, with time all sols tend to undergo gelation. The ageing process of these sols was investigated using WAXS. As a result, Ostwald ripening as a reason for gel formation can be excluded. By following the reaction progress fluorine-containing crystalline species were detected by XRD and solid state 19F MAS NMR spectroscopy indicating the formation of an intermediate phase during the fluorolysis reaction.

Metal fluoride-based transparent nanocomposites with low refractive indices

The utilisation of magnesium and aluminium fluoride nanoparticles in the preparation of transparent composites leading to materials with superior properties was investigated. Nanoscopic magnesium and aluminium fluoride has been prepared by the fluorolytic sol-gel route from the alkoxides and was surface modified by the reaction with trifluoroacetic acid or perfluorobutyric acid. IR spectroscopic experiments of the xerogels and crystal structure analysis of a trinuclear [Mg3(μ3F)(μ-TFA)6(OCH3)2(py)](3-) cluster unit indicate that the carboxylate group is bound to the particle surface in a monodentate or bidentate bridging fashion. These particles were successfully incorporated into acrylate polymers with up to 40 wt% content to give fully transparent material. Ellipsometry and m-line measurements of thin films show the reduction of the refractive index of composite films with increasing metal fluoride filler content.

Magnesium Hydroxide Fluorides – New Materials with Adjustable Composition and Properties

AbstractFluorolytic sol–gel synthesis with aqueous HF results in highly disordered, nanoscopic magnesium hydroxide fluorides with very large BET surface areas, low carbon contents, and adjustable stoichiometries. MgFx(OH)(2–x) materials that can withstand high temperatures can be synthesized in a very wide stoichiometric range by fluorolytic sol–gel synthesis (0 ≤ x ≤ 2). Large surface areas and a very wide range of tunable acid–base properties are characteristic for these compounds, which opens an exciting possibility of their application not only as new catalysts but also as support materials.

Novel sol–gel prepared zinc fluoride: synthesis, characterisation and acid–base sites analysis

The fluorolytic sol–gel route sets a milestone in the development of synthesis methods for nanoscopic fluoride materials. They exhibit fundamentally distinct properties in comparison to classically prepared metal fluorides. To broaden this area, we report in this paper the first fluorolytic sol–gel synthesis of ZnF2. The obtained sol was studied with dynamic light scattering (DLS). The dried ZnF2 xerogel was investigated with elemental analysis, thermal analysis, powder X-ray diffraction (XRD), solid-state MAS NMR, and N2 adsorption–desorption measurements. The characterisations revealed a remarkably high surface area of the sol–gel prepared ZnF2. To determine key parameters deciding its prospects in future catalytic applications, we studied the surface acidity–basicity by using in situ FTIR with different probe molecules. Compared to the previously established MgF2, weaker Lewis acid sites are predominant on the surface of ZnF2 with some base sites, indicating its potential as a heterogeneous catalyst component. In short, we believe that the successful synthesis and detailed characterisation of nanoscopic ZnF2 allow follow-up work exploring its applications, and will lead to studies of more metal fluorides with similar methods.

Mechanistic insight into formation and changes of nanoparticles in MgF2 sols evidenced by liquid and solid state NMR

The fluorolytic sol-gel reaction of magnesium methoxide with HF in methanol was studied by (19)F, (1)H and (13)C liquid and solid state NMR. In (19)F NMR five different species were identified, three of which belong to magnesium fluoride nanoparticles, i.e. NMR gave access to local structures of solid particles in suspensions. The long-term evolution of (19)F signals was followed and along with (19)F MAS NMR experiments of sols rotating at 13 kHz mechanistic insights into the ageing processes were obtained.

Preparation and stabilization of aluminium trifluoroacetate fluoride sols for optical coatings

For the first time, aluminium fluorides in liquid phase are available for optical applications. By modifying the conditions of the fluorolytic sol-gel synthesis of aluminium fluorides transparent sols with low viscosities were obtained. These sols consist mainly of small oligomeric or cluster units of aluminium fluoride which are not measurable by DLS, WAXS, SAXS and show unusual narrow signals in solid state NMR. Isolated particles with diameters up to five nanometers can be identified by TEM measurements and allow the use of their sols in optical and anti reflecting coatings. The sol particles were modified by trifluoroacetic acid to prevent agglomeration, and as a result, the obtained xerogels can be re-dispersed transparently in organic solvents.

Tailor‐Made MgF2‐Based Catalysts by Sol–Gel Synthesis

AbstractIn this paper, recent developments in the field of catalysts based on nanosized metal fluorides are presented with special focus on the exciting and promising magnesium fluoride systems. The majority of these new catalyst systems originate from the work of the authors based on the fluorolytic sol–gel synthesis of metal fluorides, which was developed by this group. They are selected in a way as to highlight the principles and prospects of this new approach to high‐surface‐area, strongly distorted metal fluorides. This new fluorolytic sol–gel route to metal fluorides opens, in the opinion of the authors, a very broad range of scientific and technical applications of the accessible high‐surface‐area metal fluorides. By combining fluorolytic and hydrolytic sol–gel synthesis, this approach can be further extended, also providing access to metal hydroxide fluorides or metal oxide fluorides. The different synthesis strategies as well as the catalytic applications of various MgF2‐based materials are described in detail in this review.

Highly selective metal fluoride catalysts for the dehydrohalogenation of 3-chloro-1,1,1,3-tetrafluorobutane

For the first time, dehydrochlorination and dehydrofluorination reactions are studied on the same substrate, 3-chloro-1,1,1,3-tetrafluorobutane, employing nanoscopic metal fluorides AlF 3, MgF 2, CaF 2, SrF 2, and BaF 2 as catalysts that are prepared according the fluorolytic sol–gel synthesis. AlF 3 is exclusively selective toward dehydrofluorination, whereas BaF 2 is 100% selective toward dehydrochlorination. The acid–base character of the catalysts is investigated and, as a result, mechanistic proposals for the dehydrofluorination and the dehydrochlorination are given. Thus, at high conversion level, selective catalysts for both dehydrofluorination and dehydrochlorination on the same substrate have been developed.

Inorganic–organic nanocomposites based on sol–gel derived magnesium fluoride

Monodispersed magnesium fluoride nanoparticles are utilized for the first time to prepare transparent inorganic-organic nanocomposite materials with improved mechanical properties. The fluorolytic sol-gel synthesis route has been modified for the preparation of monodispersed magnesium fluoride nanoparticles with a size of 2-3 nm. MgF(2) particles are effectively stabilised against agglomeration by phosphonic acids, which strongly bind to the particles and lead to an increased compatibility of the inorganic particles with the organic polymers. This way, highly transparent nanocomposite materials with up to 20 wt% magnesium fluoride in different acrylates are obtained, featuring high dispersion of MgF(2) particles in the polymer matrix and an increased hardness by the factor of 2. The nature of interaction between phosphonic acids and magnesium fluoride is thoroughly investigated by IR and NMR showing a monodentate coordination of phosphonates to the particle's surface.

Sols of nanosized magnesium fluoride: formation and stabilisation of nanoparticles

The formation of magnesium fluoride sols and xerogels according to the fluorolytic sol–gel synthesis based on the reaction of Mg(OMe)2 with non-aqueous HF has been thoroughly investigated by X-ray scattering (WAXS/XRD), TEM, SAXS, DLS and 19F MAS NMR spectroscopy. Mechanistic insights were gained by following the reaction progress and formation of intermediate phases of the fluorination of magnesium alkoxides. For F : Mg ratios of 0.3 and 0.4 the formation of two crystalline phases was observed containing the recently obtained compound [Mg6F2(OCH3)10(CH3OH)14] hexanuclear dicubane units. The stoichiometric reaction yields magnesium fluoride nanoparticles with crystallite sizes below 5 nm, which show broad reflections in the X-ray diffraction pattern. Metal fluoride sols prepared by this way undergo tremendous changes over the first several weeks after synthesis. Immediately after the fluorination, particles of about 120 nm—probably agglomerates—are formed, which break apart in the course of about one month of ageing and low-viscous, transparent sols with particles of about 12 nm are obtained. At the same time structural re-organisation processes within the magnesium fluoride particles are observed by an increase of the (110) reflection in WAXS.

New crystalline aluminum alkoxide oxide fluorides: Evidence of the mechanism of the fluorolytic sol–gel reaction

This study reports three new crystalline aluminum isopropoxide oxide fluorides with molar ratios of Al:F equal to 1:1 and 1:1.25. These are the first three representatives isolated without the incorporation of external donor molecules. Compound 1 Al(4)F(4)(μ(4)-O)(μ-O(i)Pr)(5)[H(O(i)Pr)(2)] contains a tetranuclear unit consisting of two different five fold coordinated AlFO(4)-units, with F exclusively in the terminal position. Compound 2, Al(4)F(4)(μ(4)-O)(μ-O(i)Pr)(5)[H(O(i)Pr)(2)]·Al(5)F(5)(μ(5)-O)(μ-O(i)Pr)(8), contains both a tetranuclear unit (as in 1) and a pentanuclear Al-unit. Al-atoms in the latter are five- and six fold coordinated. Compound 3, Al(16)F(20)(μ(4)-O)(4)(μ-O(i)Pr)(20)·2((i)PrOH), exhibits a slightly higher fluorination degree and contains an oligomeric chain of four F-linked tetranuclear Al-units. In addition to X-ray structure analysis, compound 1 was characterized by different solid state MAS NMR techniques, including (27)Al triple quantum MAS NMR and (1)H, (1)H→(13)C CP, (19)F and (27)Al MAS NMR. On the basis of the collected data, a reliable decomposition of (27)Al single pulse MAS NMR spectra and an unambiguous assignment of the resonances to the respective structural AlFO(4)-units are given. The new crystalline aluminum isopropoxide oxide fluorides are direct evidence of the fluorolytic sol-gel mechanism previously discussed.

The fluorolytic sol–gel reaction of aluminium alkoxides: a multinuclear MAS NMR study of structural influences of the synthesis parameters

The fluorolytic sol–gel reaction between aluminium alkoxides and non-aqueous HF leads, after drying in vacuum, to xerogels. These can be seen as intermediates for the preparation of aluminium fluorides with large surface areas having a great potential for application in diverse fields of heterogeneous catalysis. This work reports about a comprehensive systematic study of the involved local structures of the aluminium alkoxide fluorides and their changes provoked by (a) the variation of sol gel synthesis parameters, (b) a subsequent drying and (c) the exposure of the materials to different atmospheres. The main analytical tools are solid state MAS NMR investigations of the naturally abundant nuclei in these kinds of materials, namely 27Al, 19F for probing the Al- and F-environments and 1H and 13C for probing the organic moieties. The results of previous studies become more comprehensive and were underlined by the findings reported here.

Aluminum Alkoxide Fluorides Involved in the Sol–Gel Synthesis of Nanoscopic AlF3

AbstractIn the search for intermediates during the sol–gel synthesis of nanoscopic AlF3, the fluorolysis of aluminum isopropoxide by HF in pyridine has been investigated. By varying the Al to HF ratio from 0.5 to 2, three aluminum alkoxide fluorides, Al3F(Py)(OiPr)8, Al7F10(μ4‐O)(OiPr)9(Py)3·1.34Py, and Al10F16(μ4‐O)2(OiPr)10(Py)4·4.17Py, were isolated and structurally characterized. The structures of these compounds are discussed as possible snapshots in the consecutive fluorolytic sol–gel formation of nanoscopic AlF3 and give deeper insight into the probable reaction pathway. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Sol–gel prepared nanoscopic metal fluorides – a new class of tunable acid–base catalysts

In this article, the high potential of the fluorolytic sol–gel process to synthesise nanoscopic metal fluorides with different acid–base properties is shown. These nanoscopic materials exhibit high potential to be used as heterogeneous catalysts due to their high surface areas and their tunable surface properties. Thus, for each specific reaction the required surface properties of the catalysts can be “adjusted” to achieve a high yield and selectivity of the desired product. As a consequence, a greener method of chemical production can be accomplished. Moreover the cheap and easy synthesis of the catalysts using basic chemicals not only makes them interesting for fundamental research but also provides an easy transformation to industrial applications.

Cyclisation of citronellal over heterogeneous inorganic fluorides—highly chemo- and diastereoselective catalysts for (±)-isopulegol

Based on a fluorolytic sol-gel synthesis, nanoscopic metal fluorides and partly hydroxylated metal fluorides were synthesized; varying the F : OH ratio inside these solids yielded catalysts with different combinations and variable strength Lewis and Brønsted acid sites, which demonstrated unexpected catalytic properties for the diastereoselective synthesis of (+/-)-isopulegol.

Novel Sol–Gel Synthesis of Acidic MgF2−x(OH)x Materials

Novel magnesium fluorides have been prepared by a new fluorolytic sol-gel synthesis for fluoride materials based on aqueous HF. By changing the amount of water at constant stoichiometric amount of HF, it is possible to tune the surface acidity of the resulting partly hydroxylated magnesium fluorides. These materials possess medium-strength Lewis acid sites and, by increasing the amount of water, Brønsted acid sites as well. Magnesium hydroxyl groups normally have a basic nature and only with this new synthetic route is it possible to create Brønsted acidic magnesium hydroxyl groups. XRD, MAS NMR, TEM, thermal analysis, and elemental analysis have been applied to study the structure, composition, and thermal behaviour of the bulk materials. XPS measurements, FTIR with probe molecules, and the determination of N(2)/Ar adsorption-desorption isotherms have been carried out to investigate the surface properties. Furthermore, activity data have indicated that the tuning of the acidic properties makes these materials versatile catalysts for different classes of reactions, such as the synthesis of (all-rac)-[alpha]-tocopherol through the condensation of 2,3,6-trimethylhydroquinone (TMHQ) with isophytol (IP).

The fluorolytic sol–gel route to metal fluorides—a versatile process opening a variety of application fields

The recently developed fluorolytic sol-gel route to metal fluorides opens a very broad range of both scientific and technical applications of the accessible high surface area metal fluorides, many of them have already been applied or tested. Specific chemical properties such as high Lewis acidity and physical properties such as high surface area, meso-porosity and nano-size as well as the possibility to apply metal fluorides on surfaces via a non-aqueous sol make the fluorolytic synthesis route a very versatile one. The scope of its scientific and technical use and the state of the art are presented.