Sol-Gel Synthesis of Nano-Scaled Metal Fluorides ...

1

Sol-Gel Synthesis of Nano-ScaledMetal Fluorides – Mechanism and

Properties

Erhard Kemnitz, Gudrun Scholz and Stephan Rudiger

Humboldt-Universitat zu Berlin, Institut fur Chemie, Brook – Taylor – Str. 2,

D – 12489 Berlin, Germany

1.1 Introduction

Sols are dispersions of nanoscopic solid particles in, for example, liquids – i.e., colloidal

solutions. The particles can agglomerate forming a three-dimensional network in the

presence of large amounts of the liquid thus forming a gel. Inorganic sols are prepared

via the sol-gel process, the investigation of which started in the nineteenth century. This

process received great impetus from the investigations of Stober et al. [1], who studied the

use of pH adjustment on the size of silica particles prepared via sol-gel hydrolysis of tetra-

alkoxysilanes.

1.1.1 Sol-Gel Syntheses of Oxides – An Intensively Studied and Widely Used Process

Hydrolysis of alkoxysilanes and later on of metal alkoxides in organic solutions has

become an intensively studied and widely used process [2]. The most common products

are almost homodispersed nanosized silica or metal oxide particles for, e.g., ceramics or

Functionalized Inorganic Fluorides: Synthesis, Characterization & Properties of Nanostructured Solids Edited by Alain Tressaud

� 2010 John Wiley & Sons, Ltd

COPYRIG

HTED M

ATERIAL

glasses, or the aqueous sols are used to prepare different coatings for, e.g., optical

purposes. Optical applications depend on differences in the respective indices of refraction

of the coated material and the applied layer. The latter has to be of very uniform and

thoroughly adjusted thickness.

The sol-gel hydrolysis of alkoxysilanes, the most intensively explored one, basically

proceeds in two steps. The first step is the hydrolytic replacement of alkoxy groups, OR, by

hydroxyl groups, OH, shown schematically in Equation (1.1) for the first alkoxy group:

SiðORÞ4 þH2O ! ðROÞ3SiOH þ ROH (1:1)

Because of their relatively high hydrolytic stability, hydrolysis of alkoxysilanes (1.1)

has to be catalysed by Brønstedt acids or bases.

In the second step, the primary hydrolysis products undergo condensation reactions

under elimination of water (Equation (1.2)) or alcohol (Equation (1.3)).

X3Si-OH þ HO-SiX3 ! X3Si-O-SiX3 þH2OðX¼OR; OHÞ (1:2)

X3Si-OH þ RO-SiX3 ! X3Si-O-SiX3 þ ROHðX¼OR; OHÞ (1:3)

As a result tiny particles with a very open structure are formed. The overall process can

be controlled by adjusting the reaction conditions. The colloidal solution of these

particles, the sol, can be used as such for, e.g., coating or it can be worked up to yield,

eventually, nanoscopic oxide particles. However, metal oxide sols obtained in this way

always contain a sometimes remarkable organic part. Its separation demands calcination

temperatures of at least 623 K in order to convert the ‘precursors’ into pure metal oxide

materials.

Substituting a certain part of the alkoxidic groups by nonhydrolysable ones, such as

alkyl groups in the case of alkoxysilanes or phosphonic acid in the case of metal alkoxides,

organically modified oxides, i.e. inorganic-organic hybrid materials, have been prepared.

1.1.2 Sol-Gel Syntheses of Metal Fluorides – Overview of Methods

Selected metal fluorides can, in application-relevant fields, outperform metal oxides and

silica. Thus, for instance, magnesium fluoride and aluminium fluoride and, in particular,

alkali hexafluoroaluminates have both a lower index of refraction and a much broader

spectral range of transparency even than silica, making them very interesting for optical

layers.

Consequently, several approaches for the preparation of nanoscopic metal fluorides and

metal fluoride thin layers have been developed and proposed. Besides physical methods

such as milling, laser dispersion or molecular-beam epitaxy, different chemical methods

exist. Basically, three approaches can be distinguished:

(i) Postfluorination of a metal oxide preformed via sol-gel route [3].

This route is shown schematically in Scheme 1.1. The disadvantages of this route are, to

name two, incomplete fluorination of the bulk metal oxides and decrease of surface area in

course of the fluorination.

2 Functionalized Inorganic Fluorides

(ii) Preparation of a precursor containing a metal compound with organically bound

fluorine such as trifluoroacetate, which is calcined to decompose the fluoroorganic

component under formation of metal fluoride [5].

This route, shown in Scheme 1.2, also starts from metal alkoxides, which are reacted in

solution with, e.g., trifluoroacetic acid to form metal trifluoroacetate sol. This can be

M(OR)x + xH2O MOx/2 (sol) + xROH

hydrolysis coating

MOx/2(gel) bulk MOx/2(gel) film

fluorination+

calcination

fluorination

MFx or MOx/2/MFx glass MFx or MOx/2/MFx film

Scheme 1.1 Metal fluoride preparation via post fluorination of sol-gel prepared metal oxides(Reproduced from [4] by permission of Elsevier Publishers)

metal trifluoroacetate (TFA) precursor solution

M(OR)x and/or MOAc + CF3COOH + H2O + org. solvent

drying coating

metal-TFA(gel) bulk metal-TFA(gel) film

heating

metal fluoride powder metal fluoride film

heating at higher temperature

MOx/2/MFx powder MOx/2/MFx film

heating

Scheme 1.2 Metal fluoride preparation via metal fluoroacetate sol-gel formation and followingthermal decomposition. (Reproduced from [4] by permission of Elsevier Publishers)

Sol-Gel Synthesis of Nano-Scaled Metal Fluorides – Mechanism and Properties 3

used for coating experiments. The decisive final step is the thermal decomposition of the

fluoro-organic constituent, because of which thermolabile materials cannot be coated.

Another disadvantage is the probability that oxidic components can be formed as admix-

tures or oxofluorides.

(iii) Fluorolytic sol-gel process as counterpart to the hydrolytic one.

The fluorolytic sol-gel route follows rather strictly the ‘classical’ hydrolytic one by

reacting metal alkoxides in anhydrous solution with hydrogen fluoride instead of the

hydrogen oxide of the ‘classical’ process. Consequently, it results eventually in metal

fluorides instead of metal oxides.

The fluorolytic sol-gel process, its execution, mechanism, scope as well as properties

and possible fields of application of its products are the subjects of this chapter.

1.2 Fluorolytic Sol-Gel Synthesis

Metal alkoxides can be regarded as metal salts of alcohols, where the latter are very weak

Brønstedt acids. Acids that are stronger than the respective alcohol can therefore replace

alkoxy groups attached to the metal ion under liberation of the alcohol and formation of the

metal fluoride according to Equation (1.4).

MðORÞn þ x HF ! MðORÞn�xFx þ x ROH ðM¼metal ionÞ (1:4)

In fact, starting with aluminium isopropoxide [6], a broad range of metal alkoxides have

been subjected to a sol-gel-like liquid-phase fluorination with hydrogen fluoride in organic

solution [4, 7]. Although Equation (1.4) closely resembled Equation (1.1) there is an

important difference in that condensation reactions like those of Equations (1.2) and

(1.3) are not possible in the fluorolysis system. On the other hand, the fluorolysis reactions

typically result in the formation of a sol-gel. The formation of a gel was already mentioned

in the first paper on metal alkoxide fluorolysis, reporting the reaction of aluminium

isopropoxide in alcoholic solution with an ethereal solution of hydrogen fluoride [6].

The gel formation is obviously due to an important consequence of the replacement of

alkoxy groups by fluoride, i.e., the Lewis acidity of the metal ion increases leading to a

strengthening of the interaction between (liberated) alcohol molecules and metal ions. As a

result alcohol molecules that can occupy ligand positions might establish a loose net

between (partly) fluorinated metal ions resulting eventually in metal fluoride sol or even

gels. Surprisingly, attempts to isolate pure AlF3 by drying and calcining the gel were not

successful; the product obtained had an understoichiometric amount of fluorine even when

the primary reaction has been carried out with an overstoichiometric amount of HF [8]. An

additional fluorination of the dried gel under gentle conditions (see below) has proved to

be a suitable way to remove the attached organic components resulting in X-ray amor-

phous, highly Lewis acidic aluminium fluoride with unusual large specific surface area,

named HS-AlF3 [9].


1.2.1 Mechanism and Properties

To gain insight into the fluorolytic sol-gel process, elucidating the mechanism and

optimizing both the experimental procedure and the properties of the products, the

influence of all adjustable synthesis parameters was tested as well as the process. In

particular, the products were analysed by a broad range of analytical, experimental and

theoretical methods. Most of these investigations focussed on the synthesis and properties

of HS-AlF3, followed by HS-MgF2. The results and consequences of these investigations

are presented and discussed below as well as in subsequent chapters.

1.2.1.1 Approach to Mechanism and Resulting Properties

The fluorolytic sol-gel synthesis of metal fluorides is summarized in Scheme 1.3.

The synthesis was investigated in detail, aiming its optimization for HS-AlF3 [8] and

also for HS-MgF2 [10, 11]. In short, the metal alkoxide, the structure of which can be very

complex [12], is dissolved in an alcohol or another suitable organic solvent (step 1 in

Scheme 1.3), so that in the case of aluminium isopropoxide the tetrameric structure is

predominantly preserved. A solution of hydrogen fluoride in, e.g., ether or alcohol is added

in approximately stoichiometric amounts to the alkoxide solution. Ratios of HF:Al from 2

to 4 are well tolerated, resulting in a clear, translucent sol (step 2 in Scheme 1.3), which

more-or-less rapidly becomes a gel, depending on concentration and type of solvent (step 3

in Scheme 1.3). Varying the type of alkoxide, from methoxide to ethoxide, isopropoxide to

butoxide, did not markedly affect the outcome of the fluorolytic reaction.The only mean-

ingful criteria for estimation of these and other synthesis parameters were thus the surface

area and especially Lewis acidity of the final HS-AlF3.

[M(OR)n]x +S HF/S’+

M-F-SS’ wet gel

M-F dry gel

M-F-SS’ sol

HS-MFn

1

2

3

4

5

[M(OR)n]y/S

Scheme 1.3 Fluorolytic sol-gel synthesis of high surface area metal fluorides (for explanationsof the numbers 1 to 5 see text)


There is yet another route to the synthesis of metal fluoride sol-gel, exemplified for

aluminium and magnesium, namely the direct reaction of the metal with an alcoholic HF

solution [13]. Upon drying the sol-gel under mild conditions, at about 343 K under vacuum

or freeze drying, or under microwave irradiation, a solid, X-ray amorphous dry gel is

formed (step 4 in Scheme 1.3), which contains, in case of the Al-F-system, large amounts

of organic material, indicated by a carbon content of about 20 %–30 %. An empirical

formula based on elemental analysis for a dry gel prepared from Al(OiPr)3 in iPrOH is

AlF2.7[OCH(CH3)2]0.3�0.7-0.8(CH3)2CHOH. With metal ions of lower Lewis acidity,

decisive lower carbon contents were found, e.g. 3 %–7 % C in the Mg-F-system.

Obviously, part of the alcohol, which is the predominant constituent of the wet gel, is

very tightly attached to the highly Lewis acidic Al3þ ion, as can also be seen from its

thermo-analysis (Figure 1.1).

The weakly endothermic mass loss of about 24 % up to 473 K can clearly be attributed to

the release of solvating iPrOH and the more pronounced smaller one around 495 K to the

split off of alkoxide groups. Evaluating thermal analysis data of many different

experiments, it became obvious that the mass loss proceeds stepwise. The alcohol content

of the steps corresponds to the respective compositions of about AlF3:1ROH after heating

at 343 K under vacuum, about AlF3:0.45ROH after heating at 573 K in N2, and about

AlF3:0.1ROH after continued heating up to about 600 K [13]. The exothermal peak in

Figure 1.1 at 836 K is due to crystallization, i.e. of �-AlF3 formation. In order to obtain

a still X-ray amorphous aluminium fluoride, the dry gel with its understoichiometric

fluorine content has to be freed from its organic constituents under fluorinating condi-

tions. This can be accomplished in a gas-solid reaction at elevated temperatures up to

573 K with vaporized fluorocarbon compounds such as CHClF2 diluted with an inert

gas. An aluminium fluoride is obtained, named HS-AlF3, which is still amorphous, has a

specific surface area of about 200 m2/g and shows extremely high Lewis acidity (see

below). An unwanted consequence of the extreme Lewis acidity is the readily occurring

coke formation preventing the use of fluorochloroethane compounds as fluorinating

agents. The postfluorination step is essential for HS-AlF3 formation, therefore all

parameters have been comprehensively tested, such as type and concentration of the

fluorinating agent, flow rate, temperature, and also ageing of the Al-F-sol. Even under

optimum conditions with CHClF2 or CH2F2 the formation of black spots or sometimes

of ‘channels’ could be observed indicating that at these spots the formation of HS-AlF3

had started, which then subsequently catalysed coke formation. Such side reactions are,

for obvious reasons, not possible using HF as fluorinating agent. In addition, the

stronger fluorinating HF can be used at lower temperatures and should preserve the

amorphous state with its high surface area even better. The latter criterion could only be

fulfilled using rather diluted HF, obviously to reduce the otherwise high reaction

enthalpy. However, ‘HS-AlF3’ prepared with gaseous HF did not show the expected

Lewis acidity. It turned out that HF had behaved as base, which became attached to the

strongest Lewis acid sites of the solid, thereby blocking them. Only by additional longer

flushing with a stream of inert gas or, even better, of CHClF2 vapour at elevated

temperatures (up to 573 K) did the material become the expected strong solid Lewis

acid, i.e. HS-AlF3. HS-AlF3 could also be prepared with elemental fluorine using a dry

Al-F-gel of low carbon content obtained by microwave heating of an Al-F-sol under

autologous pressure followed by microwave-assisted drying.


The results of experimental investigation are in accordance with the following tentative

mechanism of the sol-gel fluorination. Upon addition of HF to the alkoxide solution a

stepwise replacement of –OR by –F starts, whereby the coordinating alcohol as linking

group prevents the formation of a three dimensional purely F-linked crystal.

The sol-gel state, almost immediately formed, kinetically prevents the stoichiometric

fluorination of the aluminium species. Upon removing the alcohol under gentle condi-

tions the gel structure only partly collapses; alcohol molecules of the first ligand sphere

remain obviously attached to Al. When these molecules are removed under appropriate

mild conditions there is no crystallization taking place but the disordered X-ray

amorphous state connected with an unusual high surface area remains and a part of

the Al atoms becomes co-ordinately unsaturated consequently exhibiting very high

Lewis acidity (see below). For HS-AlF3 a surface area up to 400 m2/g has been

determined by N2 adsorption/desorption experiments. Typical isotherms are shown in

Figure 1.2.

30

40

50

60

70

80

90

100

TG (%)

–3.50–3.00–2.50–2.00–1.50–1.00–0.500

DTG /(%/min)

373 473 573 673 773 873

Temperature/K

0

1.0

2.0

3.0

4.0

5.0

6.0

[2]Ion Current –11∗10/A

TG m41m45

m87(x100)

DTG b

373 473 573 673 773 873

Temperature/K

–15

–10

–5

0

5

10

DTA /μV

30

40

50

60

70

80

90

100

TG (%)

–3.50–3.00–2.50–2.00–1.50–1.00–0.500

DTG /(%/min)TG

DTG a

DTA

472

821495

836415

–23.85%

↑exo

–9.48%–5.65%

Figure 1.1 TA-MS of AlF2.7[OCH(CH3)2]0.3�0.7�0.8(CH3)2CHOH; (a) Thermoanalytical curves inN2 (19.50 mg); (b) Ion current curves for m/z 41 (C3H5

þ), m/z 45 (C2H5Oþ), and m/z 87 (C5H11O

þ),indicating the release of propene, i-propanol, and diisopropyl ether, respectively. (Reprinted withpermission from [9] Copyright (2005) RSC.)


The tentative mechanism is given in more detail and supported by the analytical

investigations discussed in the following section.

1.2.2 Insight into Mechanism by Analytical Methods

Different spectroscopic, microscopic and diffraction methods like IR and Raman spectro-

scopy, TEM or XRD were applied to characterize educts, intermediates or products of the

sol-gel process. For a detailed insight into the mechanism of the fluorolytic sol-gel process,

however, the application of NMR spectroscopy is the method of choice. The NMR

experiments, both in liquid and in solid state, allow direct observations of local structures

and their changes even if the matrices suffer from a loss of lattice periodicity. For the

fluorolytic so-gel process both liquid state NMR experiments were realized for the

alkoxide solutions, sols and thin gels as well as solid state MAS NMR experiments for

the alkoxide, dried alkoxide fluoride gels and high-surface fluorides including 1H, 13C,27Al and 19F as sensitive spin probes. The same spin probes along with the use of 1D and

2D NMR experiments allow local structures in liquids and solids to be addressed

and directly compared, to follow their changes with a progressive degree of fluorination

and finally to derive a possible reaction pathway for this process. Due to the good

experimental accessibility of the mentioned spin probes, a detailed study was conducted

for the reaction steps ending up with HS-AlF3. Results of these studies are presented

briefly in the following section.

250

200

150

100

50

00.0 0.1 0.2 0.3 0.4

Relative Pressure (p/p0)

Vol

ume

adso

rbed

[cm

3 /g]

0.5 0.6 0.7

adsorption

0

0

1

2

100 200 300Pore diameter [Å]

Por

e vo

lum

e dV

/d lo

gD [c

m3 /

g]

desorption

0.8 0.9 1.0

Figure 1.2 N2 adsorption/desorption isotherms and pore size distribution of HS-AlF3 (Reprintedwith permission from [6] Copyright (2003) Wiley-VCH.)


1.2.2.1 Sol-Gel Reaction

1.2.2.1.1 Aluminiumisopropoxide (Al(OiPr)3) as Solid Precursor CompoundThe basis for an understanding of changes during the fluorination process is the knowledge

and assignment of molecular structures existing in solid Al(OiPr)3 and its solution in iPrOH.27Al MAS NMR measurements of solid Al(OiPr)3 give unambiguous indications for the

existence of two distinguishable aluminium sites in the matrix in agreement with XRD

findings [14, 15] (Figure 1.3). A simulation of the spectrum (Figure 1.3) results in typical

chemical shift values for AlO4 (di¼ 61.5 ppm) and AlO6 (di¼ 1.7 ppm) units [16, 17]. Both

the kind and the intensity ratio of AlO4 : AlO6 as 3 : 1 confirm the tetrameric molecular

structure (see also Figure 1.5, 1) existing in the tetragonal crystal structure [14, 15].

In addition to 27Al, the respective 1H-13C CP MAS NMR spectrum allows a distinction

to be made not only between CHO and CH3 groups but also between terminal (Al-O) and

bridging (Al-O-Al) isopropoxide units. Very narrow line widths in part allow the assign-

ment of 18 distinguishable carbon sites in the NMR spectrum in agreement with the crystal

structure (see Figure 1.10 (top)) [14, 15].

According to suggestions made by Abraham [17], terminal (63 ppm) and bridged

(66 ppm) CHO – groups are located in the low-field part of the spectrum; terminal

CH3 groups are detected in the range between 28–30 ppm and bridging CH3 groups

dominate the high-field part of the spectrum (see Figure 1.10 (top)), [16]).

1.2.2.1.2 Aluminiumisopropoxide (Al(OiPr)3) Dissolved in IsopropanolFirst 1H NMR studies on possible structures of Al(OiPr)3 in different solutions range

back to 1963 [18] followed by first 27Al NMR measurements in 1973 [19]. Since that

(ppm)–100 –50050

(kHz)

–400–2000200400

Figure 1.3 27Al MAS NMR spectrum of Al(OiPr)3 (�r ¼ 20 kHz; B0 ¼ 9.4 T) (solid line:experiment, dotted line: simulation; insert: central transitions with quadrupolar splitting)(Reprinted with permission from [16] Copyright (2006) Humboldt University.)


time aluminium isopropoxide solutions and possible species therein, using CCl4, ben-

zene or toluene as solvents, are subjects of current interest applying different NMR

techniques [20–22]. Isopropanol is the standard solvent used for the fluorolytic sol-gel

synthesis [4, 6–8], so results of a careful reinvestigation of possible isopropanolic

solution are presented in summary.27Al NMR spectra of Al(OiPr)3 dissolved in isopropanol are shown in Figure

1.4a,b; the spectrum recorded for the same sample dissolved in diethylether is

depicted in Figure 1.4c.

The spectrum obtained for the etheric solution (Figure 1.4c) indicates the existence of

only tetrameric aluminium isopropoxide species in solution (AlO6 (2.5 ppm, 26.7 %),

AlO4 (61.8 ppm, 73.3 %), 1 in Figure 1.5) but the situation in isopropanolic solution is

much more complex. Applying two different fields (Figure 1.4a,b), both spectra reveal,

beside AlO6 and AlO4, the existence of an additional fivefold coordinated aluminium

species AlO5 at 32 ppm. They support therewith the existence of trimeric Al(OiPr)3 species

(3 in Figure 1.5). Calculated intensities of the spectra support the assumption of further

species. Cyclic trimeric species (2 in Figure 1.5) with possible distorted AlO4 polyhedra

explain the strong low-field shifted resonance observed at 85 ppm [16, 23].

(ppm)–100–50050100150200250

a

c

(ppm)–100–50050100150200250

b

Figure 1.4 27Al NMR spectra of different Al(OiPr)3 – solutions: a and b Al(OiPr)3 in iPrOHrecorded with a 400 MHz – spectrometer (a) and a 600 MHz – spectrometer (b), c Al(OiPr)3 indiethylether (400 MHz – spectrometer). For all: solid: experimental spectrum, dashed: simu-lation and dotted: decomposition. (Reprinted with permission from [23] Copyright (2007)American Chemical Society.)


Moreover, an equilibrium between different trimeric species seems to be feasible, which

involves a trimeric species with central sixfold oxygen-coordinated aluminium. The latter

may result from the interaction of trimeric species 3 (Figure 1.5) and a solvent molecule as

shown in Figure 1.6 [16].

In the appropriate 1H and 13C NMR spectra, no indications are observable that distin-

guish between tetramers 1 and trimers 2 and 3 (Figure 1.5), respectively. The only possible

discrimination between bridging and terminal isopropoxide groups holds for all of the

mentioned Al(OiPr)3 species (Figure 1.5) [16]. Summing up these results it can be

concluded that the isopropanolic solution of Al(OiPr)3 is dominated by the tetrameric

Al(OiPr)3 species 1 (Figure 1.5) accompanied by a smaller amount of trimeric species.

1.2.2.2 Structural Changes at the Fluorination Process in Sols and Thin Gels

On the basis of the 27Al NMR spectra of isopropanolic Al(OiPr)3 solution, structural

changes were followed after adding increasing amounts of HF/iPrOH solution to the

aluminium isopropoxide solution according to Equation (1.4). In dependence on the

molar Al:F ratio the appearance of the reaction mixture ranges from clear sols (4:1) to

opaque gels (1:3). Figure 1.7 represents the 27Al and 19F NMR spectra obtained with rising

content of fluorine (from a to d). It is obvious that in this order the amount of AlO6 species

O AlO

AlO

AlO

OO

O O

OO

AlO

OAl O

Al

O

AlOO O

O

O

O

1 2

O

O

AlO

OAl

O

AlOO O

O

O

O = OiPr

3

Figure 1.5 Possible structural units of Al(OiPr)3 in solution. (Reprinted with permission from[23] Copyright (2007) American Chemical Society.)

O

AlO

OAl

O

AlO

O OO

O

4

O

AlO O

Al

O

AlO

O O

OO

4'

HOiPr

O : OiPr

HOiPr

Figure 1.6 Possible equilibria between different trimeric Al(OiPr)3 species in isopropanolicsolution. (Reprinted with permission from [16] Copyright (2006) Humboldt University.)


(narrow line in Figure 1.4a,b; central units of 1 (Figure 1.5) is decreased. In the same

manner a decreasing proportion of the sum over all fourfold Al species is found and,

contrary to that, an increasing amount of fivefold coordinated aluminium species AlO5

(signals at about 35 ppm) [23].

All 19F spectra are characterized by a group of three sharp signals (–161 ppm, –163 ppm,

–165 ppm) with different intensities. All these signals are in a typical region for fluorine

bounded on aluminium centres in a mixed oxygen-fluorine coordination with different

fluorine ratios [13, 24–28]. With higher fluorine content, the intensity of the 19F NMR

spectrum is more and more dominated by a broad peak at about –160 ppm (Figure 1.7,19F,d). These line-broadening effects result mainly from 19F-19F homonuclear dipolar

couplings ending up in one broad peak in the static 19F NMR spectrum for the gel with

molar ratio Al:F as 1:3. 1H and 13C NMR spectra of sols and gels show two main effects

(ppm)–200–180–160–140–120–100

a

b

c

d

–171 ppm

–165 ppm

–163 ppm

–161 ppm

–147 ppm–156 ppm

a

(ppm)–400–300–200–1000100200300400500

b

c

d

27Al 19F

Figure 1.7 27Al NMR and 19F NMR spectra of different sols and wet gels(B0¼9.4 T). For all:solid line: experimental spectrum, dashed: simulation, dotted: decomposition. From a to dincreasing content of fluorine. Molar ratios Al: F: (a) 4 : 1, (b) 2 : 1, (c) 1 : 1, (d) 1 : 2. (Reprintedwith permission from [23] Copyright (2007) American Chemical Society.)


with increasing fluorine supply: the portion of bridging isopropoxide groups is decreas-

ing while the portion of terminal isopropoxide groups is affected only with higher

fluorine supply [16, 23]. Obviously, the fluorination starts by protonation of bridging

isopropoxide groups with the consequence of a line broadening of the corresponding

signals, while the intensities of signals of terminal groups first remain constant. This

first step is also supported by recently accomplished DFT calculations [23, 29].

A subsequent fluorination step may involve an attack of fluorine ions or HF on the

central Al atoms of 1 (Figure 1.5) and the substitution of the protonated isopropoxide

group by fluorine.

1.2.2.2.1 Isolated Single Crystals as IntermediatesEvidence for a stepwise introduction of fluorine into the coordination sphere of Al is also

given by the successful isolation of single crystals of partly fluorinated aluminium as well

as magnesium alkoxide fluorides, Al3(OiPr)8F�DMSO (Figure 1.8a) [9], Al3(OiPr)8F�Py

(Figure 1.8b) [30] and Mg6F2(OMe) 10(MeOH)14 (Figure 1.11) [31, 32]. In each case the

isolation of single crystals works only if the fluorine supply is very low, i.e. Al:F or

Mg:F > 1.

Three distinguishable aluminium sites and one fluorine site are expected for

Al3(OiPr)8F�Py in the 27Al and 19F MAS NMR spectra, which are given in Figure 1.9.

Simulation of the 27Al NMR spectrum supports this assumption and the decomposition

obtained is given in Figure 1.9. The chemical shift values are close to those of crystalline

Al(OiPr)3 emphasizing their structural similarity. As a consequence of the coordination of

fluorine and a solvent molecule the originally higher symmetric AlO6 unit, now AlO4FPy,

has considerably larger quadrupolar parameters [30].

The closeness to the Al(OiPr)3 structure may also be seen by comparing the two 1H-13C

CP MAS NMR spectra (Figure 1.10).

a

O(7)

O(8)

Al(3)O(3)

O(4)O(2)

O(9)

FS

O(1)

O(5)

O(6)

Al(2)

Al(1)

b

O7

O8 O3 O1

O2

O5

O6

Al2

Al1

N1F1

O4Al3

Figure 1.8 Crystal structure of (a) Al3(OiPr)8F�DMSO (Reprinted with permission from [9]Copyright (2005) RSC); (b) Al3(OiPr)8F�Py (Reprinted with permission from [30] Copyright(2008) Humboldt University of Berlin.)


Those CHO and CH3 groups located on bridged Al-O-Al positions (s. 1, 3 in Figure 1.5),

i.e. at the positions> 65 ppm and< 25 ppm, respectively, are especially strongly affected

by the new substituents on the central aluminium site. In contrast, the terminal groups are,

as expected, less influenced.

(ppm)1020304050607080

(ppm)10203040506070

Figure 1.10 1H-13C CP MAS spectrum of Al3(OiPr)8F�Py (bottom) in comparison withanalogous spectrum of its precursor Al(OiPr)3 (top) (�rot ¼ 10kHz, B0¼ 9.4 T) (Reprinted withpermission from [16] Copyright (2006) Humboldt University, Reprinted with permission from[30] Copyright (2008) Humboldt University of Berlin.)

–160 ppm

(ppm)

–320–300–280–260–240–220–200–180–160–140–120–100–80–60–40–200(ppm)–1000100

Figure 1.9 27Al and 19F MAS NMR spectra of Al3(OiPr)8F�Py; (�rot ¼ 25 kHz, B0 ¼ 9.4 T)(Reprinted with permission from [30] Copyright (2008) Humboldt University of Berlin.)


Likewise, the polymeric nature of magnesium methylate results in the bulky

Mg6F2(OMe)10(MeOH)14 (Figure 1.11), the synthesis of which can easily be reproduced.

In this case the structure is also very similar to that of the fluorine-free compound

Mg(OCH3)2 [33]. To preserve the cube-like shape, two fluorine ligands have to be

introduced. The high symmetry of the crystal, however, results in one narrow 19F signal

at �174 ppm, as shown in [32, Figure 5 therein].

1.2.2.3 Changes in Dry Gels with Progressive Fluorination

Isolated single crystals and their structures only give an idea of very early steps of

fluorination, producible only with an understoichiometric fluorine supply. Changes in

the local aluminium and fluorine coordination in solutions, sols and thin gels are

012

011

07

0604

010

08

09

Mg3Mg2

Mg1 02

0503

F

01

Figure 1.11 Crystal structure of Mg6F2(OMe)10(MeOH)14 (Reprinted with permission from [31]Copyright (2008) Wiley-VCH.)


highlighted above (Figure 1.7). Drying of such thin and thick wet gels with different Al:F

molar ratios leads exclusively to X-ray amorphous materials. However, based on the

relevant liquid state NMR spectra (Figure 1.7), fundamental modifications can also be

expected for the local coordinations in these solids. Together with the data already

presented, a consistent mechanism of the fluorination process is then deducible.

Taking all results into account, three stages of the sol-gel fluorination process can be

identified, which are shown in the following. The first stage is primarily represented by

samples with molar ratios Al to F higher than 1 (meaning low F-contents). Samples with

molar ratios 1:1 and 2:3 (stage 2) mark for the solids the changeover to the third stage, the

aluminium isopropoxide fluoride xerogels, samples with Al:F ratios lower than 0.5.1H-13C CP MAS, 27Al and 19F MAS spectra of dried gels with varying composition, as

depicted in Figures 1.12 and 1.13, illustrate these findings. Beside a general shift of the 13C

1020304050607080

CH3

OAl

CH

OAl Al

CH

OAl

OAlH

F

CH

OAlH

F

CH3

OH

CH

OH

δ13C/ppm

Al(OiPr)3

Al : F

4:1

2:1

1:1

2:3

1:2

1:3

CH3

Figure 1.12 1H-13C CP MAS NMR spectra (central transitions) of Al(OiPr)3 and aluminiumisopropoxide fluoride solids prepared with different molar ratios Al: F as given in the figure (�rot¼10 kHz, B0¼ 9.4 T) (Reprinted with permission from [34] Copyright (2009) American ChemicalSociety.)


signals to higher fields (CH3 groups) and lower fields (CHO groups), a considerable

broadening effect is observable for all bridging CHO – and CH3 groups, and after that,

with a higher proportion of fluorine, also a broadening and finally disappearing of the

signals of the terminal groups (Figure 1.12) [34]. The spectral changes (peak maxima, line

forms) deducible only from the 1H MAS NMR spectra are only minor and the super-

imposition of the several signals makes it difficult to discuss them separately [34]. 27Al and19F MAS NMR spectra however address local changes in the solids during continued

fluorination very clearly.

As mentioned above, the greatest change can be observed by introducing more and more

fluorine and passing the molar ratio Al to F equal to one (see also Figure 1.13). Whereas the

spectral features of the initial Al(OiPr)3 are still present in the 27Al MAS NMR spectra of

samples with low fluorine content, a rising signal at 38 ppm is detected, which is provoked

by a tetrahedrally coordinated aluminium site in the proximity to fluorine as evidenced by19F 27Al CP MAS NMR experiments [34]. Their 19F MAS NMR spectra (Figure 1.13) are

dominated by very sharp signals (FWHM less than 1 kHz), which indicate ordered

(‘crystal-like’) local structures. Most of the species in these solids are more-or-less

4:1

2:1

1:1

2:3

1:2

Al(OiPr)3

1:3

–100–75–50–250255075100 –250–225–200–175–150–125–100–75–50

–156

–162

–171–148

–138 –182

δ27Al/ppm δ19F/ppm

Figure 1.13 27Al and 19F MAS NMR spectra of Al(OiPr)3 and aluminium isopropoxide fluoridesolids prepared with different molar ratios Al: F as given in the figure (�rot ¼ 25 kHz, B0¼ 9.4 T)(Reprinted with permission from [34] Copyright (2009) American Chemical Society.)


isolated; no proximity of the certain F-sites to each other can be stated from 19F-19F spin

exchange experiments [34]. The 3QMAS NMR spectra of samples d and e (Figure 1.13)

indicate the existence of a set of different AlFx(OiPr)4�x – AlFx(OiPr)5�x and

AlFx(OiPr)6�x – species (for the latter x ¼ 3–5) [34]. They are also responsible for the

remarkable line-broadening effects in the corresponding fluorine spectra. The existence of

certain fourfold and fivefold coordinated AlFx(OiPr)CN-x species as intermediate structures

in aluminium isopropoxide fluorides was also unambiguously shown utilizing, for the first

time, ultra high-field MAS NMR at magnetic fields B0 up to 21.1 T [35].

Moving to the third stage, a more and more stable network is formed with Al:F ratios

equal to 1:2 and 1:3. The amount and spread of fourfold and fivefold coordinated

Al-species decreases, ending up with sixfold AlFx(OiPr)6–x species (x ¼ 4 and 5) as

deduced from the chemical shift correlation graphs [26�28, 36, 37].

The comparison of the development of the intensities of single species with rising

fluorination degree with the general development of the certain contributions of the appro-

priate 19F MAS NMR spectra allows a simple correlation of Al and corresponding F-species.

Besides, a variety of possibly terminal fluorine-sites are evident for the highly disordered and

amorphous aluminium isopropoxide fluorides in the up-field part of the spectra [35].

1.2.2.4 Structure of Wet and Dry Aluminium Alkoxide Fluoride Gels

AlF2.3OiPr0.7.�xiPrOH – A Comparison

An attribution of local structures in wet gelatinous and air-sensitive fluoride gels implies

many difficulties, which made the development and testing of inserts for MAS experi-

ments necessary. Alternatively, low-temperature MAS NMR experiments at temperatures

below the melting point of the solvent used allow the gel to be filled directly into the

rotor [38]. 27Al and 19F MAS NMR spectra of a wet gel recorded at a spinning speed of 10

kHz both in a quartz insert and at low temperature are shown in Figure 1.14 together with

the corresponding static spectra.

A broad signal around 0 ppm was obtained for the 27Al static NMR spectrum

(Figure 1.14a), completely covering the region for AlO4, AlO5, AlO6, AlOxFy or AlF6

species. The very broad static 19F signal is superimposed by narrow lines at –150 ppm and

–171 ppm. The latter can be assigned to ‘free’ and mobile F- ions. Rotation at 10 kHz

discloses a substantial line narrowing with a 27Al line at –16 ppm with a shoulder and an

asymmetric decay in the high-field part. The 19F MAS NMR spectra of the wet gel

observed using the quartz insert and applying low temperature are also comparable

(Figure 1.14). A broad main signal is visible at –163 ppm together with spinning side

bands superimposed by narrow peaks as contributions from the solvent.

Comparing the MAS spectra of wet and dry gels, it becomes obvious that most of the

structural features are already preformed in the jelly-like gel and in principle conserved

and strengthened in the dry gel. This holds for all studied nuclei (19F, 1H, 13C, 27Al [38]).

For 27Al this comparison is given in Figure 1.15 [38].

The shoulder and the position of the 27Al central lines as well as the wide spread of

spinning side bands are characteristic features for both wet and dry gels. Sixfold coordi-

nated Al-species (AlFxO6-x) are the dominating units in their structure. Second-order

quadrupolar broadening is the main line-width factor of the Al signals. Four different

structural units could be assigned by additional MQMAS experiments (Figure 1.16, [40]).


The reconstruction of the 27Al MAS NMR spectrum of the xerogel was possible with the

four sites (A-D, Figure 1.16) obtained by analysis of the 3QMAS spectrum. [40]. These

sites are attributed to AlF0-2O6-4 units (A), AlF4-5O2-1 units (38 %, B and C) and AlF5O1

units (53 % D) in the network [27, 40].

The existence of already immobilized –OiPr-groups, incorporated into or associated on

the network, can be unambiguously proven for the wet gel by 13C CP MAS experiments

[38]. In addition, the more rigid structure of the dry gel allows distinctions to be made

between matrix groups and associated immobilized iPrOH molecules [38, 41]. The main

structural features of the dried gel are already built up in the jelly-like gel and change only

little in aging and drying processes.

1.2.2.5 Network Formation with Increasing Fluorine Content

Bearing in mind all NMR spectra recorded for sols, thin wet gels and dried gels

(Figures 1.7, 1.12–1.16) in comparison with those of solid and liquid educts, the following

mechanism can be derived for the fluorolytic sol-gel reaction. In the first step of the

synthesis route of HS-AlF3, which is usually applied, the tetrameric structure of Al(OiPr)3

(ppm)

–1000–800–600–400–20002004006008001000

static, rotor

–16 ppm

27Ala

10 kHz, quartz

(ppm)

–400–350–300–250–200–150–100–50050100

static, glass

–171 ppm–150 ppm

10 kHz, quartz

19Fb

*

*

**

**

*

*

**

**10 kHz, 155 K

**

10 kHz, 150 K

c

Figure 1.14 MAS NMR spectra (B0 ¼ 9.4 T) of the wet gel using glass and quartz as insertmaterials in comparison with the respective static spectra (na: number of accumulations):a) 27Al:�rot ¼ 10 kHz, quartz insert (na: 150000); �rot ¼ 10 kHz, frozen wet gel in the rotor at150 K (na: 1000); (b) 19F: �rot ¼ 10 kHz, quartz insert (na: 192); frozen wet gel in the rotor at155 K (na: 48);*: spinning side bands; (c) picture of a wet gel after rotation at 10 kHz in a quartzinsert. (Reprinted with permission from [38] Copyright (2007) Elsevier Ltd.)


as dominating species (Figure 1.5, 1) represents the starting point for the further reaction

pathway and finally for the formation of the gel network. The fluorination begins with a

protonation of bridging isopropoxide groups, which makes a subsequent attack of fluorine

ions or HF to central Al atoms easy. As result, a substitution of protonated isopropoxide

groups by fluorine occurs.

At early fluorination states (state 1, Scheme 1.4) unconverted Al(OiPr)3-molecules exist

in the sols and gels to some degree – either in its tetrameric or in its linear trimeric form. In

the following the latter may be partly fluorinated and, if stabilizing donors (D) are

(ppm)–150–125–100–75–50–250255075100

wet gel160 K

(ppm)

–150–125–100–75–50–250255075100

Xerogel

a

bνrot = 25 kHz

Figure 1.15 27Al MAS NMR spectra of the wet and dried gel (xerogel) with focus on the centralregion (400 MHz spectrometer; na: number of accumulations): (a) comparison of 27Al MASNMR spectra of the frozen wet gel with (solid line, 160 K) and without (dotted line, 155 K) 19F(cw) decoupling at cryo-temperatures, for both: �rot ¼ 10 kHz, na: 1000; (b) possible differentcontributions to the spectrum of the xerogel including the n ¼ 0 spinning sideband of thesatellite transition: (- - -). (s. also Neuville, Massiot [39]); experimental spectrum (——);simulated spectrum (��) and decomposition (� - � - � -) (see also Table 2 in [38]). (Reprintedwith permission from [38] Copyright (2007) Elsevier Ltd.)


accessible, even crystals of Al3(OiPr)8F•D can be isolated (Scheme 1.4, 3). Nevertheless,

for aluminium isopropoxide fluorides resulting from sols without donor molecules the

formation of ordered similar species Al3(OiPr)8F•iPrOH is presumable as evidenced, for

example, by the corresponding sharp signals centred at –160 ppm and –171 ppm (19F).

The early formation of tetrahedrally coordinated species AlF4 can be demonstrated for

solids prepared with low fluorine supply. This species does not seem to exist in the

corresponding sols. Instead, it is plausible that these species are stabilized as solvated

species AlFx(HOiPr)6�x3�x (x¼ 4) in the sols. Drying results in a partial loss of the solvent

molecules and the formation of ‘incorporated’ AlF4-species. Additionally, further linking

processes of the AlF4(HOiPr)2� species lead to the formation of bigger units (beginning of

a gelous network), which, as a consequence, are predominantly built of AlFx((HOiPr)6-x-

octahedra (x ¼ 3 to 5).

The rise of the F-content (molar ratio Al: F< 1) leads to an irregularly strongly distorted

and disordered solid. The corresponding sol consists of a loose network, imaginable in

solution as presented earlier in [23]. Vacuum drying leads to cleavage of iPrOH molecules

coordinating to Al-species existing in these sols and gels, forming a variety of fourfold,

fivefold and sixfold coordinated Al-species like AlFx(H)OiPr)4�x, AlFx((H)OiPr)5�x and

AlFx((H)OiPr)6�x as identified by 3QMAS and ultra high-field MAS NMR measurements

[34, 35]. The units are sterically separated; as in the 19F-19F EXSY NMR experiments,

nearly no spin exchange could be observed ([34], state 2, s. Scheme 1.5).

Finally this gelous network is strengthened and stabilized by cross-linking with higher

fluorine supply. Vacuum drying leads then to xerogels that consist predominantly of

sixfold coordinated AlFx((H)OiPr)6�x species (x: 3�5)[38, 40]. Coordinating solvent

molecules are stabilized by the formation of H-bridges. The local structures of the xerogel

are preformed in the wet gel. Nevertheless, ordered local structures are also observable for

the xerogels.

The derived mechanism is schematically depicted in Schemes 1.4 and 1.5.

20 0 –20

40

20

0

–20

–40csqc

δMAS(27Al) in ppm

δ MQ

MA

S(27

Al)

in p

pm CD

B

A

–40 –60

Figure 1.16 3QMAS spectrum of the aluminium alkoxide fluoride gel (Al:F¼ 1:3) acquired at14.1 T (Reprinted with permission from [40] Copyright (2009) American Chemical Society.)


State 1

O

AlO

OAl

O

AlOO O

O

OO

AlO

OAl O

OAl

O

AlOO O

O

O

O

FAl

tF

F F

FAl

F

FF

O*

O* F

AlFAl

O

O

Al*O

AlF

F

O*

F

FF

F

FF

FF

F

F AlF

O*Al

O

F

AlO

AlO*

O*

F

O

F

FF

F

F

F

F

FF

F

*O

O

AlO

OAl

O

AlOO O

D

O

F

sidereaction

if donor present isolable

21

3

– O*

species present in isopropoxide fluoridesols with a ratio F: Al lower 1…

…and in thecorresponding solids.

The loss of solvent leads toformation of AlF4 speciesalong with un convertedAl(OiPr)3 1 and cross-linkedAlFx(OiPr)6–x species.

Scheme 1.4 Derived mechanism for the beginning sol-gel fluorination process with lowfluorine supply (O*: iPrOH; O: -O-iPr). (Reproduced from [34] by permission of theAmerican Chemical Society.)

State 2

solid:wet sol:

F

AlF FAl

O

O

Al*O

AlF

F

O*

F

FF

F

FF

FF

F

F AlF

O*Al

O

F

AlO

AlO*

O*

F

O

F

FF

F

F

F

F

FF

F

*O

F

Al FAl

O

Al

AlF

F

*O

F

FF

F

FF

F

F

FAl Al

O

F

AlO

AlO*

O*

F

O

F

FF

F

FF

FF

F

*OFAl

O

O

Al*O

AlF

F

O*

F

FF

F

FF

F

F AlF

Al

F

Al

AlO*

O*O

F

FF

F

F

F

F

FF

F

O

AlO

OAl

O

AlOO O

O*

O

F

minor ?

3` (D = O*)

– O*

indications forformationof a similarcompound like 3

irregularsolid

and further

Scheme 1.5 Derived mechanism for a progressive fluorination degree (reproduced from [34]by permission of the American Chemical Society)


1.2.2.6 Structural Characterization of Sol-Gel Derived High-surface Fluorides

1.2.2.6.1 Vibrational Spectroscopy of HS-AlF3

The IR transmission spectra of HS-AlF3 were recorded and compared with those of

aluminium isopropylate and of dry aluminium fluoride gel – i.e. the Al moieties involved

in HS-AlF3 synthesis (Figure 1.17) [8], as well as of crystalline AlF3 phases (Figures 1.18

and 1.19) [42]. In Figure 1.17 the CH3 and CH as well OH absorption bands of isopropoxide

and/or isopropanol can be seen at wave numbers higher than 2800 cm-1 in the spectra of

aluminium isopropylate and of dry aluminium fluoride gel, whereas the broad band in the

HS-AlF3 spectrum can be attributed (i) to water adsorbed in course of sample preparation

due to the high Lewis acidity, and (ii) to water residues present in the bulk as a consequence

of the formation of diisopropylether at postfluorination. At low wave numbers there are the

absorption bands of Al-O and Al-F valence vibrations (360–700 cm-1) and of C-C frame

vibrations of the organic constituents. These spectra show the disappearance of organic

components converting the dry gel, the ‘precursor’, into HS-AlF3.

The amorphous, highly disordered state of HS-AlF3 follows convincingly from a

comparison of its IR spectra with those of �-AlF3 shown in Figure 1.18. Since crystalline

�-AlF3 also gives a well resolved Raman spectrum (Figure 1.19) all the distorted X-ray

amorphous phases of HS-AlF3 do not yield any useful information.

The very broad peaks in the IR-spectrum of HS-AlF3 are obviously the consequence of

its amorphous state and can be interpreted as superposition of many unresolved peaks, also

covering the range of vibration bands of the �- and �-AlF3 phases.

1.2.2.6.2 X-ray Diffraction and TEM Investigations of HS-AlF3

In contrast to vibrational spectroscopy, the X-ray diffraction patterns of HS-AlF3 and of its

precursor, the AlF2.3OiPr0.7.�xiPrOH dry gel, show no peak and therefore only indicate the

4000 3500 3000 2500 2000 1500 1000 500

Al(OiPr)3

precursor

HS-AlF3

Tra

nsm

issi

on

Wave number (cm–1)

Figure 1.17 IR spectra of Al(OiPr)3, the AlF2.3OiPr0.7.�xiPrOH gel, and HS-AlF3 (Reprinted

with permission from [8] Copyright (2007) Springer Science þ Business Media.)


X-ray amorphous state of these samples. Only after heating above the crystallization tem-

perature of about 836 K do peaks corresponding to �-AlF3 become apparent (Figure 1.20).

In the TEM micrograph (Figure 1.21) both the agglomeration of nano-particles to micro-

particles as well as the partial crystalinity of the nano-particles can be seen.

1.2.2.6.3 Solid State NMR of HS-AlF3

Subsequent drying of the xerogel in vacuum results in a noticeable loss of –OiPr-groups

and associated iPrOH molecules, which can easily be followed by 1H-13C CP MAS

0 200 400 600 800 1000 1200

α-AlF3

β-AlF3

HS-AlF3

Tra

nsm

issi

on

Wavenumber/cm–1

Figure 1.18 IR-transmission spectra of a-AlF3, �-AlF3, and HS-AlF3 (Reprinted with permis-sion from [42] Copyright (2007) American Chemical Society.)

100

96

383

478

15730 000

25 000

20 000

15 000

10 000

5000

Inte

nsity

[arb

.uni

ts]

200 300 400Wavenumber [cm–1]

500 600

x25

700 800

Figure 1.19 Raman spectrum of a-AlF3 (rhombohedral phase) (Reprinted with permissionfrom [42] Copyright (2007) American Chemical Society.)


experiments [44]. Both 19F and 27Al MAS NMR spectra exhibit after removal of the

organic components the typical shape as recorded for HS-AlF3. The maximum broad 19F

MAS NMR signal lies typically at –165 ppm [41]. The effect of such a drying process is

shown exemplarily for 27Al in Figure 1.22. The spectrum on the bottom (Figure 1.22) is

almost identical with the central line usually measured for HS-AlF3. In contrast to the

xerogel (AlF2.3OiPr0.7.�xiPrOH) the 27Al MAS NMR signal line widths do not narrow

10 20 30 40 50 60

(a)-precursor(b)-precursor in N2 at 350 °C(c)-HS-AlF3(d)-precursor in N2 at 700 °C (α-AlF3)

Inte

nsity

(au

)

2θ

(c)

(d)

(b)

(a)

Figure 1.20 X-ray powder difractograms of (a) AlF2.3OiPr0.7.�xiPrOH dry gel, (b)

AlF2.3OiPr0.7.�xiPrOH dry gel heated to 623K, (c) HS-AlF3, and (d) AlF2.3OiPr0.7

.�xiPrOH drygel heated to 973 K (Reprinted with permission from [8] Copyright (2007) Springer Science þBusiness Media.)

Figure 1.21 TEM micrograph of HS-AlF3 (Reprinted with permission from [6] Copyright(2003) Wiley-VCH.)


much upon increase of the applied magnetic field. Therefore the comparison of MQMAS

experiments at different magnetic field strength is most important for an assignment of

various species. Although the data analysis is very difficult, the presence of various signals

was revealed [40].

A 3QMAS spectrum of HS-AlF3 is given in Figure 1.23.

(ppm)–150–100–50050100

70 °C, 2 h

170 °C, 4 h

-–15 ppm

150 °C, 2 h

100 °C, 2 h

Figure 1.22 27Al MAS NMR spectra (central lines) of a AlF2.3OiPr0.7.�xiPrOH gel thermally

dried in a vacuum at different temperatures (�rot ¼ 25 kHz, B0 ¼ 9,4 T) (Reprinted withpermission from [44] Copyright (2009) Humboldt University of Berlin.)

B'

A'

qccs

20 0 –20δMAS(27Al) in ppm

–40 –60

40

20

0

–20

–40

δ MQ

MA

S(27

Al)

in p

pm

Figure 1.23 3QMAS NMR spectrum of HS-AlF3 acquired at 19.9 T (Reprinted with permissionfrom [40] Copyright (2009) American Chemical Society.)


The line shape analysis of the 27Al MAS NMR spectrum of HS-AlF3 required more than

the two Al-sites extracted from the 3QMAS spectrum (Figure 1.23). A successful fit was

only possible by implementation of two further Al-sites. For all of them the simulation

required the Czjzek-model, i.e. distributions of quadrupolar parameters were used [40].

Now 65 % of the integral intensity of the Al signal belongs to AlF5-6O1-0 and AlF6 units,

respectively. Residues of AlF4-5O2-1 units (25 %) and AlF0-2O6-4 units (11 %) are still

present [40].

1.2.3 Exploring Properties

The most remarkable property of HS-AlF3 is its outstanding Lewis acidity [6, 8, 9, 45],

which is far higher than that of AlCl3, as will be shown later. It is interesting to note that

on a theoretical basis for an isolated ‘AlF3-molecule’ already a Lewis acidity was

predicted ranking among the highest ones at all [46]. In the following a variety of

investigations showing the very high Lewis acidity of HS-AlF3 will be presented and

discussed.

1.2.3.1 Adsorption of Probe Molecules

The strength and/or nature and/or amount of acid sites accessible at the surface of a solid

can principally be determined measuring the interaction with a basic probe molecule. The

higher the acidity of the solid the lower can be the basicity of the probe and vice versa. For

investigations of HS-AlF3, pyridine and its derivatives, ammonia and carbon monoxide,

were employed. Their interactions with the solid have been analysed studying in case of

ammonia desorption with increasing temperature, Temperature Programmed Desorption

TPD, and in case of the other probes via IR spectroscopy.

NH3-TPD: HS-AlF3 was saturated with gaseous NH3 followed by flushing with N2.

Upon gradually heating the sample up to 773 K, i.e. below the crystallization tem-

perature, the adsorbed NH3 is gradually released as shown in Figure 1.24. Compared

to the well-known solid Lewis acid �-AlF3 the desorption from HS-AlF3 occurs up to

much higher temperatures corresponding to a much higher Lewis acidity, and the total

amount of NH3 is also much higher, indicating a higher number of acidic sites per

gram.

Pyridine adsorption: The chemical nature of the acid sites can be seen from photo-

acoustic infrared spectra (PAS) of adsorbed pyridine (Figure 1.25). Frequencies and

intensities of the IR bands show for HS-AlF3 (almost) only Lewis acid sites, whereas a

HS-AlF3 sample showed after treatment with HF predominantly Brønsted acid sites

(Figure 1.25b).

Carbon monoxide adsorption: Carbon monoxide behaves towards a strong acid as a

weak base the interaction of which can be investigated monitoring the CO stretching

region of the IR absorption spectrum of the absorbed CO. The stronger the acid the more is

the CO IR frequency blue-shifted. HS-AlF3 shows the strongest blue shift ever reported for

a solid acid (for details see Chapter 3) indicating that it is, next to ACF (aluminium

chlorofluoride), the strongest solid acid of all [47].


1.2.3.2 Catalytic Test Reactions

Reactions which have to be catalysed by a Lewis acid to proceed can be used as test-

reaction for assessment of the acidity of a material under study. Equations (1.5) to (1.8)

show four reactions, the use of which has been reported [8, 45]:

5 CCl2F2 ! CCl3F þ 3 CCl3F þ CCl4 (1:5)

35 000

30 000

25 000

20 000

15 000

10 000

5000

–5000

0

0 100 200

Temperature/°C

300 400 500

HS-AIF3beta-AIF3

Figure 1.24 NH3-TPD of HS-AlF3 in comparison to �-AlF3 (Reprinted with permission from [7]Copyright (2007) Royal Society of Chemistry.)

Figure 1.25 PAS of pyridine adsorbed on (a) HS-AlF3, (b) HS-AlF3 exposed to HF, (c) HS-AlF3

prepared by post-fluorination with HF, and (d)sample as in(c) additionally heated in CHClF2/N2

flow; peaks at (B) are indicative for Brønsted acidity, and peaks at (L) for Lewis acidity (Reprintedwith permission from [8], Figure, Copyright (2007) Springer Science þ Business Media.)


3 CHClF2 ! CHCl3 þ 2 CHF3 (1:6)

CBrF2CBrFCF3 ! CF3CBr2CF3 (1:7)

CCl2FCClF2 ! CCl3CF3 (1:8)

The educts of the reactions in Equations (1.5) and (1.6) were used as postfluorination

agents in the course of high surface metal fluoride preparation, especially that of HS-AlF3.

As soon as first parts of the dry Al-F-gel are converted into HS-AlF3, a dismutation

reaction according to Equations (1.5) or (1.6) starts, which can be detected by GC. Thus,

these fluorinating agents act conveniently as detectors for Lewis acidity. However, the

reactions in Equations (1.5) and (1.6) already proceed in the presence of comparable weak

Lewis acids; they are therefore no measure of high acidity. The isomerization of

CBrF2CBrFCF3 (Equation 1.7), on the other hand, proceeds only under the catalytic action

of the strongest known Lewis acids, i.e. antimony pentafluoride or aluminium chloro-

fluoride, and is also catalysed by HS-AlF3 but not by AlCl3 [8, 45]. Thus, the isomerization

reaction gives convincing evidence for the exceptional high Lewis acidity of HS-AlF3. As

AlCl3 is widely used in organic synthesis as a Lewis acidic catalyst a further comparison of

its Lewis acidity with that of HS-AlF3 was of interest. Studying the isomerization of

CCl2FCClF2 (Equation 1.8) as test reaction, AlCl3, HS-AlF3, ACF, �-AlF3 and �-AlF3

have been compared regarding their respective catalytic activity. It was found that ACF

and HS-AlF3 were catalytically very active, and �-AlF3 and �-AlF3 not at all, as expected.

Surprisingly, AlCl3 was also not primarily active but became active only under conditions

and after some time as was needed to convert AlCl3 into ACF. The experimental result was

interpreted based on theoretical investigations assuming that under-coordinated Al atoms,

which are a result of the high degree of disorder, are responsible for the Lewis acidity of

HS-AlF3 [45].

Radiotracer investigations: Radiotracer experiments, which also gave evidence for the

exceptional Lewis acidity of HS-AlF3, are discussed in Chapter 3.

1.2.4 Possible Fields of Application

1.2.4.1 Range of Metal Fluorides Obtainable via Sol-Gel Fluorination

The fluorolytic sol-gel synthesis of metal fluorides was originally developed and explored

for aluminium fluoride, which was a piece of luck since both the stepwise synthesis and the

properties of HS-AlF3 showed the influence of the new synthesis process. Thus, almost

immediately after exploration of HS-AlF3 other binary metal fluorides have been similarly

prepared and attempted syntheses of more complex systems started.

Binary metal fluorides: Many binary metal fluorides have been prepared via sol-gel

fluorolysis [4, 7]. The applicability of the synthesis process is primarily limited by the

ready availability of soluble metal alkoxides. However, the synthesis of metal fluorides,

of which the metal ions are very weak Lewis acids, typically does not result in sol

formation but finely dispersed solid fluorides, the XRD of which reflect the aimed-for

compounds.


Thus, upon fluorolysis of the tert-butoxides of Li, Na, K and Cs in THF solution, only

with LiOtBu and NaOtBu did gel formation occur whereas with K- and Cs- alkoxide

immediate precipitation was observed. The dried products of all these alkali metal ions

reflected the corresponding fluorides in XRD and also gave some indications of the

respective hydrogenfluorides due to their higher thermodynamic stability [48]. The most

thoroughly investigated example besides HS-AlF3 is HS-MgF2 [11]. Other high surface

area alkali earth fluorides prepared are HS-CaF2 and HS-BaF2 [49].

Mixed metal fluorides: Prompted by a hypothetical model by Tanabe explaining the Lewis

acidity of guest-host metal oxide systems [50] and its adaptation for fluoride systems [51],

guest-host mixed metal fluoride systems with HS-MgF2 as host have been prepared (see

also Chapter 3). If the radii are comparable, the metal ions guest ions will probably occupy

places of Mg2þ ions. If the guest ion has a higher positive charge than Mg2þ, Lewis acidity

should be created in accordance with the model. This way, solid potential metal fluoride

catalysts with tunable Lewis acidity are accessible. Thus, with up to 20 mol % of Fe3þ,

Ga3þ, V3þ, In3þ, and Cr3þ as guest components, solid solutions with HS-MgF2 as host

have been prepared [52–54]. The synthesis of such systems basically followed the fluor-

olytic sol-gel route described above, however, in some experiments compounds other than

alkoxides have been employed as guest components to be added to the Mg alkoxide

solution. Analyses of these mixed systems by XRD, 19F MAS NMR and photoelectron

spectroscopy showed no evidence of the guest compounds but gave only of the typical

spectral data of MgF2, which however, were not identical with those of crystalline MgF2.

Obviously the guest metal ions were incorporated into the MgF2 lattice as expected.

ESCA investigations in the Cr3þ/MgF2 system (Figure 1.26) showed, for both Mg 1s and F 1s,

binding energies near those of pure MgF2 [54]. A shift that is only small to lower binding

energies gives evidence that the chemical environment of Mg and F in the mixed systems is

influenced by Cr3þ.

Complex metal fluorides: Complex metal fluorides are especially interesting compounds

because of their physical properties, which are suitable for applications such as lasers.

Conventional syntheses typically employ thermal methods starting from the respective

metal fluorides. Complex metal fluorides can also be conveniently prepared via sol-gel

fluorination. The sol-gel syntheses start similar to that of host-guest systems from mixtures

of the respective metal alkoxides but in the respective stoichiometric ratio, which are

subjected to fluorolysis. Contrary to the host-guest systems, where the host lattice is

preserved, complex systems, i.e. fluorometallates, have their specific crystal structure

different from those of the respective binary metal fluorides they are composed of.

Examples of complex metal fluorides prepared via sol-gel fluorolysis are Li3AlF6,

Na3AlF6, K3AlF6, KAlF4, CsAlF4, LiNa2AlF6 [55] and BaAlF5, K2MgF4 and BaMgF4

[49]. Since Al in AlF6 units is ideally shielded, such compounds do not show any Lewis

acidity and are chemically very stable making them suited especially for nonchemical

applications such as protective coating (see below).

1.2.4.2 Application Consequences of the Sol-Gel Synthesis

Specific chemical and physical properties of intermediate states of the sol-gel fluorolysis

can be utilized for quite different modifications and applications. Upon reaction of metal


alkoxides with understoichiometric amounts of hydrogen fluoride a defined part of the

alkoxide groups remain and may be used for chemical modifications of the high surface

metal fluorides. Such modifications include partial substitution of F by OH, chemical

immobilization of metal oxide oxidation catalyst onto the metal fluoride and binding

organic groups to the inorganic metal fluoride. Introduction of highly dispersed noble

metals on metal fluoride is also possible. The sol state, i.e. the colloidal solution of a metal

fluoride in a nonaqueous solvent, opens up the possibility of many interesting applications.

Most important is the ability to prepare different coatings for very different applications.

Thus, catalytic active metal fluorides can be conveniently deposited on supports, or thin

metal fluoride layers, can be easily prepared for, e.g., optical or mechanical applications.

Nano-sized metal fluorides: An optically clear metal fluoride sol contains particles the

diameter of which is in the range of the wavelength of visible light. Therefore, nano-sized

metal fluoride particles can be obtained from such sols, which are normally agglomerated,

after removal of the solvent at moderate temperatures. The size of the individual particles

has been shown by TEM and was derived from their XRD pattern to be below 5 nm [6, 49].

Such tiny particles represent a higher state of energy compared to more bulky material. As

a consequence these materials can be easily pressed at room temperature to dense glasses.

Thus, transparent glasses have been obtained from MgF2, CaF2, BaMgF4 [49] and

Rb2NaAlF6 [56] employing pressures up to 1.1 GPa at room temperature.

Nano-sized metal fluorides have a promising perspective as inorganic component of

organic polymers. Due to their small size the particles are invisible even in transparent

polymers but may, in possibly decisive ways, change physical properties such as dielectric

constant, index of refraction or mechanical properties. However, preventing agglomera-

tion of these nano particles is a challenging task, but one that is not easy to accomplish and

that needs to be further developed.

Oxide fluorides: The amount of alkoxide groups remaining after partial fluorination can be

tuned over a wide range. These OR groups can subsequently be hydrolysed, i.e. substituted

by OH, or thermally split off, also resulting in OH and, due to condensation reaction, in the

formation of oxide groups homogeneously distributed within the fluoride matrix. The metal

fluorides modified this way exhibit not only Lewis acidity but also some Brønsted acidity or

655

654

653

652F K

LL (

eV)

651

650

1339

1338

1337

1336MgF2CrClMg15CrOMg8CrOMg15CrOMg25CrOMg50crAcMg15CrAcMg40

1335

1334689 688 687

F ls (eV)686 685 684

1179

1178

1177

1176Mg

KLL

(eV

)

1175

1174

2482

2481

2480

2479MgF2CrClMg15CrOMg8CrOMg15CrOMg25CrOMg50crAcMg15CrAcMg40

2478

24771308 1307 1306

Mg ls (eV)1305 1304 1303

Figure 1.26 Chemical state plots for F 1s and Mg 1s orbitals of chromium(III)-doped MgF2

prepared from CrO3 (CrO), CrCl3 (CrCl) or Cr-acetat (CrAc) in different concentrations(Reprinted with permission from [54] Copyright (2005) Elsevier Ltd.)


even basicity. Such bifunctional materials can be valuable solid catalysts (see below). This

synthesis principle has been realized for magnesium oxide/hydroxide fluorides within a

broad range of compositions [57]. It was found that preparations with low fluorine content,

with nominal composition Mg(OH)1.2F0.8 and Mg(OH)0.8F1.2, were almost X-ray amor-

phous even after calcinations at 623 K whereas with higher F content the patterns of MgF2

appeared. In 19F MAS NMR there was a similar trend to be seen, the low F materials gave

broad, complex signals, indicating the presence of many different fluorine species, which

became more and more sharp and like those in MgF2 with increasing F content. The higher

electronegativity of F compared to OH resulted with increasing F content in a reduced

electron density at Mg indicated in increasing Mg 1s binding energy as seen in XPS analysis.

Magnesium fluoride-based bifunctional materials have been successfully employed as

heterogeneous catalysts for quite different reactions (see below) [57–59].

Metal oxides linked to metal fluorides: The performance of a solid catalyst depends to a

substantial degree on its surface area, which can often be increased by suitable support, and

then also on the chemical and texture properties of the support. Vanadium oxide-based

catalysts are very useful in oxidation reactions because of the easy change between

different oxidation states. In case of the technically important partial oxidation of organic

compounds, a limited oxidation activity of the catalyst is needed to prevent total oxidation

to CO / CO2 and H2O. As a consequence the organic educt has to be activated by, e.g.,

protonation due to Brønsted acidity of a catalyst support. By using a Lewis acidic support a

more gentle activation of the educts should be possible resulting in less or no total

oxidation. The sol-gel synthesis provides exquisite conditions for a very evenly distributed

and highly dispersed VOx based catalyst deposited on Lewis acidic metal fluoride sup-

ports. Employing the principle of under-stoichiometric fluorination there will be OR

groups or, after hydrolysis, OH groups available for chemical anchoring of the VOx

species. This way aluminium fluoride-supported VOx catalysts with outstanding perfor-

mance have been prepared and thoroughly characterized [60].

Noble_metals supported by high surface area metal fluorides: Noble metals especially

platinum and palladium are useful catalysts for many reactions such as, e.g., hydrogenations.

They are used in a highly dispersed state deposited on suitable supports. For use in hydro-

dehalogenation reactions the support has to be stable against attack by the hydrogen halo-

genide formed in course of the reaction. That requirement can be met using metal fluorides as

support. MgF2 and AlF3 were often proposed [61]. For the preparation of very tiny noble metal

particles supported on a metal fluoride, which has a large surface, the sol-gel fluorination

provides excellent conditions. Starting from the metal alkoxide solution or from the already

formed metal fluoride sol an organic solution of a suitable noble metal compound, such as the

acetyl acetonate, is added and the mixture is worked up as normal for high surface metal

fluoride preparation. Only an additional reduction step is necessary to obtain the catalyst. For a

Pd0/CaF2 catalyst prepared accordingly the high Pd dispersion is shown in Figure 1.27.

Organically modified metal fluorides: Silicon oxide and also metal oxide based inorganic-

organic hybrid materials have received broad interest academically and also for technical

applications [62]. The development of such hybrids aims to combine useful properties of

the inorganic part, mostly the basis of the hybrid, with those of the organic part, whereby

the two parts are chemically bound to one another. Basically, two types of synthesis routes


are used. Both can be adapted to metal fluoride systems. One route starts from organically

modified molecular alkoxidic precursors, which are subjected to sol-gel hydrolysis; the

other starts from preformed nanoscopic or macroscopic particles to which an organic

moiety becomes linked. To transpose these synthesis principles to metal fluoride systems,

the sol-gel fluorolysis can be conveniently employed attaching either an organic moiety

via a HF stable linkage to metal alkoxides, which are consequently subjected to sol-gel

fluorolysis. Another route comprises at first formation of colloidal nanoscopic particles by

preparing a sol with under-stoichiometric amounts of HF, to which then organic moieties

can be linked. First experiments with phenylphosphonate modified aluminium alkoxide

have proved successful [30, 63].

Coatings: The sol state as primary result of the sol-gel fluorination process is very useful,

because it consists of nano-sized, colloidal metal fluoride particles homogeneously dis-

persed in a nonaqueous liquid. Simply by applying the sol on any solid material wettable

by the solvent and subsequent drying a metal fluoride layer is on the surface of the material

obtained. Depending on the required properties and intended purposes of the layer and on

the texture and geometry of the solid material, different methods can be used for the sol

application such as, e.g., soaking, dipping, spin coating etc.

Catalysis: For technical reasons it can be necessary to have a catalytic active solid material

supported to improve, e.g., its mechanical stability and reduce its flow resistance when

used in a flow reactor. The sol-gel fluorination synthesis provides a convenient way for

depositing high surface area metal fluorides on supports. For example, HS-AlF3, which as

fine powder makes problems when used as catalyst in flow systems, could be supported by

g-Al2O3 whereby its Lewis acidity and consequently its catalytic activity remains almost

unchanged [64]. For other catalytic applications, like micro-reactor techniques, deposition

of catalytically active thin layers of metal fluorides is also of interest.

Figure 1.27 TEM micrograph of a Pd0/CaF2 catalyst showing Pd particles (black spots) of£10nm diameter (Reprinted with permission from [61] Copyright (2008) Royal Society ofChemistry.)


Optics: Probably the most important field of application for thin metal fluoride layers is in

physics, especially in optics. A reduction of the reflexion of light at the surface of glass,

well known for optical devices such as lenses for glasses and cameras, is even more

important for solar energy utilization. Antireflective systems are formed of alternating

layers of transparent low and high refractive index materials. However, even with a single

layer, a very efficient antireflective system is possible in principle. For a wavelength g and

an antireflective layer of thickness g/4 the reflexions at the air/layer surface and at the

layer/glass interface have a phase-shift to each other of g/2, that is the precondition for

extinction. Total extinction is only possible when the two reflexions are of the same

intensity. This can be reached when the index of refraction of the antireflective layer is

equal to the geometric mean of air (n ¼ 1.0) and glass (n ¼ 1.5), i.e. for n ¼ 1.225.

Typically, the refractive indices of metal oxides are even higher than that of glass, whereas

some metal fluorides such as MgF2 (n500¼ 1.38), AlF3 (n500¼ 1.35) and Na3AlF6 (n500¼1.33) have distinctly lower indices of refraction, although not as low as 1.225, together

with an excellent transparency within a broad range of wavelengths. Consequently, MgF2

thin layers have already found much interest [65]. Methanolic MgF2 sols prepared as

described above have been used for spin-coating of planar surfaces [66]. AFM investiga-

tions revealed that the layers obtained after drying consist of densely packed particles of

10 to 20 nm diameter as shown in Figure 1.28. The single or multiple MgF2 thin layers

showed excellent homogeneity concerning thickness and index of refraction over the

experimental range of 5 cm [67].

Protective coating: The facileness of preparing metal fluoride layers from nonaqueous

metal fluoride sols also reduces the threshold of their use for protective coating.

Homogeneous, mechanically stable metal fluoride layers can protect against UV radiation,

Figure 1.28 AFM image of a 3-fold deposited MgF2 layer on a silicon wafer, after calcinationsat 300 �C (area 1 x 1mm2). (Reprinted with permission from [67] Copyright (2008) Wiley-VCH.)


chemical and also mechanical impact, and can form a barrier against microbial attack.

A CaF2 addition to lithium grease has already proved useful in the lab for friction reduction

and extreme pressure properties making CaF2 layers, likewise, interesting.

In conclusion, the new access towards nanoscopic metal fluorides via this recently

developed fluorolytic sol-gel synthesis route opens a wide range of applications for metal

fluorides due to the distinctive different properties of these nano materials.

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