Tuning the surface properties of novel ternary iron(iii) fluoride-based catalysts using the template effect of the matrix

Article abstract of DOI:10.1039/c4dt03229b

Sol-gel prepared ternary FeF₃-MgF₂ materials have become promising heterogeneous catalysts due to their porosity and surface Lewis/Bronsted acidity (bi-acidity). Despite the good catalytic performance, nanoscopic characterisations of this type of material are still missing and the key factors controlling the surface properties have not yet been identified, impeding both a better understanding and further development of ternary fluoride catalysts. In this study, we characterised the interaction between the bi-acidic component (FeF₃) and the matrix (MgF₂) on the nano-scale. For the first time, the formation pathway of FeF₃-MgF₂ was profiled and the template effect of MgF₂ during the synthesis process was discovered. Based on these new insights two novel materials, FeF₃-CaF₂ and FeF₃-SrF₂, were established, revealing that with decreasing the atomic numbers (from Sr to Mg), the ternary fluorides exhibited increasing surface acidity and surface area but decreasing pore size. These systematic changes gave rise to a panel of catalysts with tuneable surface and bulk properties either by changing the matrix alkaline earth metal fluoride or by adjusting their ratios to Fe or both. The template effect of the alkaline earth metal fluoride matrix was identified as the most probable key factor determining the surface properties and further influencing the catalytic performance in ternary fluoride based catalysts, and paves the way to targeted design of next-generation catalysts with tunable properties. This journal is

Full text of DOI:10.1039/c4dt03229b

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Tuning the surface properties of novel ternary
iron(III) fluoride-based catalysts using the template
effect of the matrix†
Cite this: Dalton Trans., 2015, 44,
5076
Ying Guo,a Andreas Lippitz,b Paul Saftien,a Wolfgang E. S. Ungerb and
Erhard Kemnitz*a
Sol–gel prepared ternary FeF3–MgF2 materials have become promising heterogeneous catalysts due to
their porosity and surface Lewis/Brønsted acidity (bi-acidity). Despite the good catalytic performance,
nanoscopic characterisations of this type of material are still missing and the key factors controlling the
surface properties have not yet been identified, impeding both a better understanding and further devel-
opment of ternary fluoride catalysts. In this study, we characterised the interaction between the bi-acidic
component (FeF3) and the matrix (MgF2) on the nano-scale. For the first time, the formation pathway of
FeF3–MgF2 was profiled and the template effect of MgF2 during the synthesis process was discovered.
Based on these new insights two novel materials, FeF3–CaF2 and FeF3–SrF2, were established, revealing
that with decreasing the atomic numbers (from Sr to Mg), the ternary fluorides exhibited increasing
surface acidity and surface area but decreasing pore size. These systematic changes gave rise to a panel
of catalysts with tuneable surface and bulk properties either by changing the matrix alkaline earth metal
fluoride or by adjusting their ratios to Fe or both. The template effect of the alkaline earth metal fluoride
matrix was identified as the most probable key factor determining the surface properties and further
influencing the catalytic performance in ternary fluoride based catalysts, and paves the way to targeted
design of next-generation catalysts with tunable properties.
Received 20th October 2014,
Accepted 16th December 2014
DOI: 10.1039/c4dt03229b
www.rsc.org/dalton
and were highly active as heterogeneous bi-acidic catalysts in
the Friedel–Crafts alkylation (e.g. synthesis of vitamin E and
1. Introduction
Nanoscopic bi-acidic metal fluorides have been shown to play vitamin K1)4 and the carbonyl–ene reaction (isomerisation of
an active role in heterogeneous catalysis in recent years.1–6 In citronellal to isopulegol).2 Further functionalisation of par-
this context, bi-acidic refers to the co-existence of Lewis and tially hydroxylated fluorides resulted in extended applications:
Brønsted acidity in the catalyst. Heterogeneous bi-acidic cata- fluorosulfonate grafted hydroxylated MgF2 can catalyse the
lysts are easy to handle and environmentally friendly, and thus dehydration of xylose to furfural;6,11 Au doped hydroxylated
are frequently applied in industrial processes like alkylation, MgF2 and Pd (or Pt) doped hydroxylated AlF3 were active in the
isomerisation, cracking, and esterification.7,8 Commonly used one-pot synthesis of menthol from citral due to their bi-acidity
heterogeneous bi-acidic catalysts are usually based on zeolites, and hydrogenation ability;12,13 Sn doped hydroxylated MgF2
silica, alumina, zirconia, and their combinations. Since metal was reported as an efficient catalyst in the degradation of cellu-
fluorides are known to exhibit much more pronounced Lewis lose, offering a feedstock in bio-ethanol production.14
acidity compared with metal oxides,9 we wondered whether it
The previously reported bi-acidic fluoride catalysts focused
is possible to prepare metal fluorides with bi-acidic properties. on a narrow range of main group metal fluorides, i.e. AlF3 or
Indeed, partially hydroxylated MgF2 and AlF3 have already MgF2. Naturally, questions arose as to whether it was possible
been successfully synthesised via a fluorolytic sol–gel process10 to develop partially hydroxylated transition metal fluorides
and whether these could provide comparable or even better
catalytic performances than AlF3 or MgF2. In 2013 we reported
aHumboldt-Universität zu Berlin, Institut für Chemie, Brook-Taylor-Straße 2,
D-12489 Berlin, Germany. E-mail: erhard.kemnitz@chemie.hu-berlin.de;
Fax: +49 (0)30 2093 7277; Tel: +49 (0)30 2093 7555
a ternary FeF3–MgF2 catalyst that combined the advantages of
the bi-acidic FeF3 and the porous MgF2 matrix and was highly
active in the isomerisation of citronellal to isopulegol.5 The
ternary FeF3–MgF2 catalyst was prepared via a one-step sol–gel
route and the bi-acidity was introduced by a dehydration treat-
ment of the Fe-precursor.5 In spite of the progress in the devel-
bBundesanstalt für Materialforschung und –prüfung, Unter den Eichen 44-46,
D-12203 Berlin, Germany
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4dt03229b
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opment of FeF3–MgF2, several issues needed to be clarified.
First of all, the interaction between nanoscopic FeF3 and MgF2
required a detailed analysis since it would help understand
the catalyst’s structure and catalytic activity. More importantly,
we devoted to investigate the formation pathway of the ternary
FeF3–MgF2 catalyst with emphasis on revealing the function of
the matrix material, MgF2, during the synthesis process.
Hence a template effect of MgF2 was reported for the first time
and was considered a key factor controlling the surface pro-
perties of FeF3–MgF2 catalysts. Inspired by the template effect
of MgF2, we established two novel ternary fluoride catalysts
using other alkaline earth metal fluorides as matrices and dis-
cussed the role of the template effect in tuning the surface pro-
perties of this type of material.
Then the present work focuses on solving the following
points. The interaction between nanoscopic FeF3 and MgF2 in
the FeF3–MgF2 catalysts was characterised by scanning trans-
mittance electron microscopy (STEM) coupled with energy-dis-
persive X-ray spectroscopy (EDX) and X-ray photoelectron
spectroscopy (XPS). The formation pathway of the ternary
FeF3–MgF2 was investigated by varying synthesis parameters in
combination with characterisation methods such as elemental
analysis (C, H, N, and F analysis) and powder X-ray diffraction
(XRD). Finally, we synthesised FeF3–CaF2 and FeF3–SrF2 cata-
lysts via the fluorolytic sol–gel process and characterised their
physico-chemical properties with thermal analysis methods,
STEM, N2 physisorption, and chemisorption-IR. The isomeri-
sation of citronellal to isopulegol was used as a model reaction
to evaluate all the ternary fluoride catalysts studied in this
work. We believe that our novel approach reveals the potential
of ternary iron(III) fluoride-based catalysts and will promote
their applications in heterogeneous catalysis. Moreover, such
systems also offer potential applications as materials for
making electrodes in fuel cell technology.
Fig. 1 (a) STEM image (scale bar: 10 nm) of FeF3–MgF2, (b) STEM image
(scale bar: 5 nm) of FeF3–MgF2, and (c) 2D elemental hypermap (scale
bar: 5 nm) of Fe and Mg generated from EDX results (red: Fe; green: Mg)
in the region corresponding to (b). The contrast ratios of (a) and (b) were
adjusted for better viewing.
over the 3D space. Since the STEM images suggest a granular
structure rather than a layered structure, we assume fully dis-
persed FeF3 in MgF2, and therefore, no isolated FeF3-domains
can be found embedded in MgF2.
XPS is a very useful technique for studying the electronic
structure and chemical species in solid materials.15 In this
work, XPS was applied to FeF3–MgF2 in order to clarify the
effect of dispersing FeF3 into the MgF2 matrix based on the
binding energy (BE) of Fe 2p3/2 photoelectrons. The detailed
2. Results and discussion
2.1. Interaction between FeF3 and MgF2 in the ternary
FeF3–MgF2 catalysts
In this work, we used STEM and XPS to characterise the mor-
phology of FeF3–MgF2 catalysts and the nanoscopic interaction
between FeF3 and MgF2 particles.
The STEM images (Fig. 1(a) and (b)) suggest that the par-
ticles in FeF3–MgF2 are spherical and are roughly homo-
geneous in size. The elemental mapping of Fe and Mg was
generated by EDX in order to visualise the Mg and Fe elemen-
tal distribution in the material. The Fe and Mg distribution in
a selected area that corresponds to the STEM image in
Fig. 1(b) is shown in Fig. 1(c). The 2D hypermap reveals a
homogeneous distribution of Fe and Mg. Two different topolo-
gical situations may cause this distribution: (1) FeF3–MgF2
may consist of a layered structure, and the projection of
Fe-rich layers and Mg-rich layers implies the pseudo-homo-
geneous Fe–Mg-distribution; (2) FeF3–MgF2 consists of a gran-
Fig. 2 XPS spectra of Fe 2p of (a) sol–gel prepared FeF3 and (b) FeF3–
ular structure, and Fe and Mg are homogeneously distributed MgF2.
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Fe (2p3/2) spectrum of a sol–gel prepared FeF3 (synthesised as pension, MgF2 sol, and FeF3 sol are white or colourless,
described by Y. Guo et al.5) is presented for comparison whereas the Fe(OH)x(NO3)3−x·yH2O solution exhibits a reddish
(Fig. 2(a)). The BE at 718 eV is assigned to the satellite peak to brown colour.5 Moreover, a significant colour change was
Fe 2p3/2 (iron(III) fluoride), and the BE of the major component observed during the synthesis of FeF3–MgF2 (the mixed precur-
peak at about 715 eV corresponds to the Fe(III)–F bonds.16 The sor suspension was yellow; the FeF3–MgF2 sol was colourless).5
BE at 712 eV may refer to either minor Fe(III)–O or Fe(II)–F In light of this background, we designed two experiments to
species. However, a previous study indicated that all the Fe investigate the formation pathway of FeF3–MgF2 by using the
atoms are in their +3 oxidation state.5 Thus the peak at the BE colour change as an index.
of 712 eV should be assigned to Fe(III)–O. The intensities of
Fe (2p3/2 components measured with FeF3–MgF2 (see Mg(CH3COO)2 suspension was added afterwards to the FeF3 sol.
Fig. 2(b)) are substantially different from the FeF3 case. The In a second attempt, a mixed Fe(OH)x(NO3)3−x·yH2O and
In one attempt, a FeF3 sol was prepared first, and a
)
major component here is at BE ∼712 eV which suggests a Mg(CH3COO)2 suspension was fluorinated in a single step by
larger proportion of Fe–O bound species. Looking at the NIST employing an under-stoichiometric amount of HF (two equi-
XPS database17 reveals that Fe(III)OOH is to be expected at this valents to Mg).
BE18,19 rather than Fe2O3 which is usually measured at a some-
In the first attempt, addition of a stoichiometric amount of
what lower BE.20 Since XPS is a surface sensitive technique,15 anhydrous HF to the Fe(OH)x(NO3)3−x·yH2O solution led to the
this result suggests that FeF3–MgF2 exhibits more surface –OH formation of a colourless FeF3 sol. Addition of the white
species compared with FeF3. Because the surface –OH moieties Mg(CH3COO)2 suspension to this sol resulted first in the for-
are potential Brønsted acid sites,5 we expected FeF3–MgF2 to mation of an orange-red suspension that turned into a reddish
show more pronounced surface Brønsted acidity. Indeed, a brown clear sol (Fig. 3). This observation suggests that FeF3 sol
previous study on FeF3–MgF2 confirmed its medium strong particles are highly active species that are able to react with
Brønsted acidity by chemisorption-IR and a catalysis test.5
Mg(CH3COO)2. It can be further deduced that Mg(CH3OO)2
should very probably be first fluorinated in the mixed Mg2+/
Fe3+ precursor suspension, but fluorination of the iron precur-
2.2. Formation pathway of ternary FeF3–MgF2 catalysts
The ternary FeF3–MgF2 catalysts were prepared by a sol–gel sor should proceed afterwards. The reddish brown sol shown
route in which mixed Fe(OH)x(NO3)3−x·yH2O and in Fig. 3(h) was carefully dried at 30 °C to avoid any decompo-
a
Mg(CH3COO)2 suspension was fluorinated by HF and gave rise sition of the iron complex. The C, H, N and F elemental
to a FeF3–MgF2 sol.5 To study the formation pathway of the analysis results of the obtained brick-red powder as an inter-
ternary FeF3–MgF2 catalyst, we first examined whether there is mediate substance suggested the co-existence of fluoride,
any competition between Fe- and Mg-precursors during the nitrate, and acetate. The nitrate may result from the iron
fluorination process. It is known that the Mg(CH3COO)2 sus- nitrate precursor in the synthesis of the FeF3 sol. Thus the
Fig. 3 Images of (a) Mg(CH3COO)2 suspension in methanol, (b) FeF3 sol in methanol, (c) adding the Mg(CH3COO)2 suspension to the FeF3 sol, and
image records of the reaction between Mg(CH3COO)2 and FeF3 (Mg-to-Fe ratio = 1.5) at (d) t = 0 min, (e) t = 1 min, (f) t = 2 min, (g) t = 5 min, and (h)
t = 14 min; (i) powder XRD patterns of an under-stoichiometrically fluorinated Fe–Mg sample after calcining at 600 °C (# refers to an Fe2O3 phase in
PDF 33–664 and ° indicates an MgF2 phase in PDF 1–1196); ( j) schematic illustration of the template effect of MgF2 sol particles.
5078 | Dalton Trans., 2015, 44, 5076–5085
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reaction between FeF3 sol particles and Mg(CH3COO)2 prob- existence of Ba(NO3)2 indicated ion exchange between the pre-
ably gave rise to MgF2 and an intermediate iron nitrate acetate cursors, iron nitrate and the barium acetate. In other FeF3–
complex.
alkaline earth metal fluoride systems, a similar ion exchange
In the second attempt, addition of under-stoichiometric HF was evidenced by thermal analysis results (Fig. 4(b)). Release
to the Fe(OH)x(NO3)3−x·yH2O and Mg(CH3COO)2 suspension of NO+ (m/z = 30), a decomposition product of the nitrate,
gave immediately a reddish brown sol that appeared to be happens at different temperatures in different systems (FeF3–
similar to the one in Fig. 3(h). This sol was dried and the MgF2: ∼140 °C; FeF3–CaF2: ∼280 °C; FeF3–SrF2: ∼380 °C). The
obtained powder was calcined at 600 °C before studying it by different decomposition temperatures clearly refer to different
XRD (Fig. 3(i)). The reflexes can be assigned to an Fe2O3 phase intermediate nitrate species. It is known that the thermal
and an MgF2 phase, respectively. No evidence for FeF3 or MgO stability of alkaline earth metal nitrates increases with the
was found. The XRD result confirmed the deduction that Mg atomic number.21 Thus these species can be assigned to
becomes fluorinated first in the sol–gel synthesis, and the Mg(NO3)2, Ca(NO3)2, and Sr(NO3)2, respectively. The ion
intermediate product is probably composed of MgF2 and an exchange between iron nitrate and alkaline earth metal acetate
iron nitrate acetate complex. It can be further speculated that probably results in these intermediate nitrate species. The
the MgF2 sol particles probably work as templates and induce general reaction pathway of this type of ternary fluorides is
the nucleation and govern the follow-up formation of FeF3 therefore summarised as follows: the mixed precursors
(Fig. 3( j)). This template effect of MgF2 would explain the undergo fast ion exchange followed by the two-step fluorina-
uniform particle size in FeF3–MgF2 catalysts as revealed by tion in which the alkaline earth metal fluoride works as a tem-
STEM images. Moreover, the enhanced surface Brønsted plate, and finally give rise to the ternary fluorides (Fig. 5).
acidity of FeF3–MgF2 as indicated by XPS results is probably
The discovery of the ion exchange step not only provides
introduced via the interaction between sol particles under important information on the formation pathway of this
in situ sol–gel synthesis conditions as the reaction pathway ternary fluoride system but also allows to predict the template
shows. Thus using MgF2 as a matrix does not only template effect of alkaline earth metal fluorides. Calculating the Gibbs
the textural structure but also tunes the surface acidity of FeF3. free energy of the fluorination of alkaline earth metal nitrates
Keeping in mind that both properties have a strong influence with HF (Table 1) indicates that the formation of alkaline
on the performances of a heterogeneous catalyst, we are con- earth metal fluorides becomes less favoured with increasing
vinced that the template effect of MgF2 plays an important role the atomic number. As a consequence, the alkaline earth
in tuning the catalytic activities of FeF3–MgF2 catalysts. Conse- metal fluorides may lose their priority in the fluorination Step
quently we were interested in whether other materials will also I (Fig. 5). Thus it can be predicted that the template effect may
show this template effect. Hence we started the study on the become less significant with increasing the atomic number of
template effects of heavier alkaline earth metal fluorides.
the alkaline earth metals in the ternary fluoride catalysts. To
evaluate the template effect, we characterised the textural
structure and surface acidity of FeF3–CaF2 and FeF3–SrF2 cata-
lysts since these features are largely affected by the template
effect.
The textural structures of FeF3–CaF2 and FeF3–SrF2 catalysts
were determined by STEM and N2 adsorption–desorption
measurements. One can roughly estimate the average particle
size of different catalysts from the STEM images: FeF3–SrF2 >
FeF3–CaF2 (Fig. 6). The differences in particle size may result
2.3. The template effects of heavier alkaline earth metal
fluorides
To study the template effect of heavier alkaline earth metal flu-
orides, we prepared ternary FeF3–CaF2 and FeF3–SrF2 materials
following the same route as that applied to FeF3–MgF2.5 Prepa-
ration of a ternary FeF3–BaF2 material was also considered.
However, instead of the expected BaF2, a Ba(NO3)2 phase was
identified from the XRD pattern of the xerogel (Fig. 4(a)). The
Fig. 4 (a) Powder XRD patterns of an “FeF3–BaF2” sample (+ refers to a Ba(NO3)2 phase in PDF 24–53); (b) MS profiles of ionic fragments NO+ (m/z
= 30) of (i) FeF3–MgF2, (ii) FeF3–CaF2, and (iii) FeF3–SrF2 during thermal analysis.
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in differences in the porous structure of the ternary fluoride
materials, since the voids between particles contribute to the
pore volume. Indeed, the average pore size of FeF3–SrF2 cata-
lysts is almost two times as large as that of FeF3–CaF2 catalysts
(Table 2, Column 3). Comparing all three ternary fluoride cata-
lysts, their surface area and pore size decrease with the atomic
number of the alkaline earth metal (Table 2, Columns 2 and
3). Taking the data of the corresponding alkaline earth metal
fluorides as references, MgF2 and CaF2 showed a stronger
influence in tuning the pore size of the ternary fluoride cata-
lysts while SrF2 hardly affects the pore size of FeF3–SrF2. The
results confirmed that the template effect of alkaline earth
metal fluorides in the aspect of controlling the porosity of the
ternary catalysts declines from MgF2 to SrF2.
Surface acidity of the FeF3–CaF2 and FeF3–SrF2 catalysts was
studied by IR spectroscopy equipped with chemisorption
devices. Different probe molecules (NH3, pyridine, and
CD3CN) were used to determine the type and strength of
surface acid sites (Lewis or Brønsted). Fig. 7 shows the chemi-
sorption-IR spectra of samples after saturation with NH3. Since
the spectra are the subtractions of spectra after/before chemi-
sorption, the negative bands at 1258–1260 cm−1 are probably
due to the decline of residual acetate band after NH3 adsorp-
tion. The 1410 cm−1 band in Fig. 7(a) can be assigned to proto-
Fig. 5 Schematic illustration of the general formation pathway of
ternary iron(III) fluoride–alkaline earth metal fluoride.
+
nated NH4 species which indicate Brønsted acidity.22 This
Table 1 Gibbs free energy: the reaction of metal nitrate with HF to
metal fluoride and nitric acid
band shifts towards the lower wavenumber range (Δδ =
26 cm−1) compared with that in the spectra of FeF3–MgF2;5 it
suggests that the Brønsted acidity of FeF3–CaF2 is probably
weaker than that of FeF3–MgF2. The vibration band at
1222 cm−1 and the shoulder at 1183 cm−1 can be assigned to
NH3 species that coordinates to the surface Lewis acid sites.22
ΔrG (kJ mol−1
)
Mg(NO3)2
Ca(NO3)2
Sr(NO3)2
Ba(NO3)2
−92.3
−43.4
4.6
25.2
The broad peak at
a
lower wavenumber range
(1134–1124 cm−1
)
corresponds to H-bonding –OH⋯NH3
species which typically refer to weak Brønsted acid sites.22
With NH3 as a probe molecule, both Lewis and Brønsted
acid sites were determined on the surface of FeF3–CaF2.
However, the differential spectra of FeF3–SrF2 show only one
positive band at 1138 cm−1 indicating the H-bonding species
that correspond to weak Brønsted sites (Fig. 7(b)).22 No
+
vibration bands corresponding to protonated NH4 were
recorded in the spectra. This is an indication that the Brønsted
sites of FeF3–SrF2 are not strong enough to protonate NH3. It
+
cannot be ruled out that the protonated NH4 species require
adjacent –OH for stabilisation,23 although there is only a low
coverage of hydroxyls (potential Brønsted sites) on the FeF3–
Fig. 6 STEM images (scale bar: 20 nm) of (a) FeF3–CaF2 and (b) FeF3–
SrF2. The contrast ratios of the images were adjusted for better viewing.
Table 2 Summary of the surface properties of ternary FeF3-based catalysts, FeF3 and the corresponding alkaline earth metal fluorides
Sample
B.E.T. surface area (m2 g−1
)
Average pore diameter (nm)
Surface Lewis acidity
Surface Brønsted acidity
FeF3–MgF2 (ref. 5)
FeF3–CaF2
FeF3–SrF2
FeF3
MgF2
CaF2
400–500
ca. 200
ca. 100
40–70
240–260
ca. 250
ca. 130
2–5
Strong
Strong sites ≈ weak sites
Strong sites ≪ weak sites
Medium strong
Weak/medium strong
Weak/medium strong
5–15
10–30
ca. 30
2–3
2–3
2–3
SrF2
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to H-bonding species on both Lewis and Brønsted sites.25
Thus FeF3–SrF2 was confirmed to carry both Brønsted and
Lewis acid sites on the surface, but its Brønsted acidity is
weaker than that of FeF3–CaF2 and FeF3–MgF2.
CD3CN was used as a probe molecule to distinguish the
strengths of Lewis acid sites on the surface of FeF3–CaF2 and
FeF3–SrF2. The vibration band at 2313–2318 cm−1 refers to
strong Lewis acid sites, while the band in the lower wavenum-
ber range (about 2260–2290 cm−1) indicates weak Lewis
sites.25,27 Interestingly, a decreasing tendency in Lewis acidity
can be seen in the order of Mg, Ca, and Sr. Strong Lewis acid
sites are predominant on the surface of FeF3–MgF2.5 On the
surface of FeF3–CaF2, the strong and weak Lewis sites co-exist
and the intensities of the corresponding bands are comparable
(Fig. 9(a)). In contrast, weak Lewis acid sites are the main
species (major characteristic) of the surface of FeF3–SrF2
(Fig. 9(b)). The bands at 2114–2118 cm−1 correspond to the
symmetric CD3 stretching,27 and the weak bands at about
2220–2260 cm−1 can be assigned to physisorbed species.27 The
so-called trio bands (three bands at 2890, 2400, and
1700 cm−1 25
that correspond to very strong Brønsted acidity
)
are not observed in these spectra, probably because the
surface Brønsted acid sites of FeF3–CaF2 or FeF3–SrF2 are not
strong enough to protonate CD3CN.
Fig. 7 Chemisorption-IR spectra of (a) FeF3–CaF2 and (b) FeF3–SrF2
after saturating stepwise with NH3.
SrF2 surface. Because pyridine is a stronger base than NH3 in
the gas phase (proton affinity in the gas phase: pyridine
922.2 kJ mol−1; NH3 857.7 kJ mol−1),24 pyridine was used as a
probe molecule to investigate FeF3–SrF2. The two bands at
1641 and 1614 cm−1 in the differential spectra of FeF3–SrF2
can be assigned to protonated (PyH+) and coordinated (PyrL)
species, respectively (Fig. 8).25 These bands arise from the
vibration mode 8a of pyridine.26 The band at 1598 cm−1 refers
Fig. 8 Chemisorption-IR spectra of FeF3–SrF2 after saturating stepwise
Fig. 9 Chemisorption-IR spectra of (a) FeF3–CaF2 and (b) FeF3–SrF2
with pyridine.
after saturating stepwise with CD3CN.
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In short, with the help of chemisorption-IR, medium strong
Brønsted acid sites were determined on the surface of all three
ternary fluoride catalysts with the Brønsted acidity decreasing
with the atomic number of alkaline earth metal. Since the tem-
plate effect of the alkaline earth metal fluoride is believed to
contribute to the Brønsted acidity, the results support the
hypothesis that the template effect declines from Mg to Sr.
Moreover, the Lewis acidity shows a similar trend: strong Lewis
acid sites are predominant on the FeF3–MgF2 surface, while
weak Lewis sites are the main species on the surface of FeF3–
SrF2, the surface acidity of FeF3–CaF2 lies in between with both
strong and weak Lewis acidity. Thus the strengths of Lewis/
Brønsted sites can be easily tuned using different alkaline
earth metal fluorides as matrices, which allows us to fine-tune
catalysts with the desired surface acidity.
2.4. Catalytic tests
The isomerisation of citronellal to isopulegol was used as a
model reaction to evaluate the catalytic performance of FeF3–
CaF2 and FeF3–SrF2 samples. This reaction is one of the key
steps in synthesising menthol,22,28 which is widely used in
drugs, cosmetics, and food additives.29 It represents an intra-
molecular carbonyl–ene reaction, in which an allylic hydrogen
(an “ene”) reacts with a carbonyl (an “enophile”) to form a new
bond with the migration of the ene double bond and 1,5-
hydrogen shift (Fig. 10).30–32 Early studies of the reaction
mechanism suggested that it is catalysed by Lewis acids, and
later studies revealed that it actually requires the participation
of both Lewis and Brønsted acid sites to achieve high yields
towards isopulegol.33,34 Thus the isomerisation reaction of
citronellal can be used as an excellent model reaction to
examine the surface acidity of especially bi-acidic catalysts.
A panel of FeF3–CaF2 and FeF3–SrF2 catalysts with varying
Ca (or Sr)-to-Fe ratios was prepared. The catalytic results of
FeF3–CaF2 samples are presented in Fig. 11(a). Both conver-
sion and selectivity increased drastically for the Ca-to-Fe ratio
between 1 and 4. At higher ratios, the conversion reached
nearly 100% and hence no further increase can be expected.
The FeF3–CaF2 sample with a Ca-to-Fe ratio of 3 shows the
highest yield towards isopulegol with a conversion of 97% and
a selectivity of 84%, which is comparable to that of FeF3–MgF2
samples (conversion ca. 100% and selectivity ca. 85%).5 Evi-
dently, the Ca-to-Fe ratio has a strong influence on the per-
formance of FeF3–CaF2 catalysts. This can be rationalized by
Fig. 11 (a) Conversion of citronellal and selectivity and yield to isopule-
gol versus the molar Ca-to-Fe ratio; and (b) conversion of citronellal and
selectivity and yield to isopulegol versus the molar Sr-to-Fe ratio. The
catalyst amounts used in the catalysis tests were normalised to 10 mg
FeF3 per 1 mmol citronellal.
considering the template effect of CaF2: adjusting the Ca-to-Fe
ratio may affect the template effect of CaF2 which further influ-
ences the surface properties of FeF3–CaF2; and the surface pro-
perties are crucial for the heterogeneous catalysis process.
In comparison with FeF3–MgF2 and FeF3–CaF2, the FeF3–
SrF2 samples are less selective in this reaction. The conversion
of citronellal reached its maximum of nearly 100% at the Sr-to
Fe ratio of 7 (Fig. 11(b)). The selectivity to isopulegol, however,
was noticeably lower than that of the FeF3–MgF2 or FeF3–CaF2
catalysts. One possible reason is that FeF3–SrF2 exhibits weaker
surface Brønsted acidity and therefore is less selective to isopu-
legol. Moreover, Chuah et al. studied the correlation between
pore size and selectivity to isopulegol and found that micro-
pores or small mesopores are favourable for higher selectivity,
because they may exclude large by-products such as dimers or
trimers of isopulegol.34 Our previous work on FeF3–MgF2 con-
firmed this conclusion.5 Thus the larger average pore diameter
of FeF3–SrF2 (10–30 nm) may make it less competitive for the
formation of isopulegol.
Fig. 10 Schematic illustration of the isomerisation reaction of citro-
nellal to isopulegol.
5082 | Dalton Trans., 2015, 44, 5076–5085
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energy mode with CAE = 20 eV for highly resolved spectra. The
samples were analysed under high vacuum (<10−8 mbar). The
binding energy scale of the instrument was calibrated follow-
ing ISO 15472. The binding energy scale was corrected for
3. Experimental
3.1. Preparation of FeF3–MgF2, FeF3–CaF2, and FeF3–SrF2
catalysts
The detailed synthesis route of FeF3–MgF2 has been described static charging using BE = 285.0 eV for the C 1s component of
in our previous work.5
aliphatic carbon originating from absorbed hydrocarbons.
The syntheses of FeF3–CaF2 and FeF3–SrF2 catalysts fol- Quantification was performed using a routine implemented in
lowed a similar route to that of FeF3–MgF2. The alkaline earth the Thermo Advantage 4.43 software. The routine uses
metal acetate (Ca(CH3COO)2·H2O or Sr(CH3COO)2·0.5H2O, Scofield’s photoionisation cross section data for the determi-
Sigma-Aldrich, 97%) was used as received. It was added to the nation of relative sensitivity factors. This quantification
pre-dehydrated iron nitrate precursor (Fe(NO3)3·9H2O, Sigma- routine uses the model of homogeneous and isotropic distri-
Aldrich, 98%; the pre-dehydration process has been described bution of all constituents in the analyzed volume. The esti-
by Guo et al.5) at different Ca (or Sr)-to-Fe ratios. The mixed mated IMFP was calculated with the Gries (G-1) equation
precursors were suspended in methanol (total concentration using the NIST Electron IMFP Database Version 1.2.35
of metals: 0.2 mol L−1) and fluorinated with stoichiometric
amounts of HF in a methanolic solution. The obtained sol or
suspension was aged for 14–16 h at room temperature, and
then dried and calcined at 100 °C in vacuo for 2 h.
3.7. N2 physisorption
N2 physisorption at 77 K was measured with a Micromeritics
ASAP 2020 instrument. Samples were degassed at 80 °C under
Caution: HF solution is highly hazardous and special pro-
high vacuum for 12 h. The specific surface area was calculated
tective wear is required!
using the B.E.T. equation, and the pore size distribution was
determined by the B.J.H. method.
CaF2 and SrF2 were also prepared via the sol–gel process.
The syntheses followed the same route as that applied for
FeF3–CaF2 or FeF3–SrF2 catalysts but without an Fe-precursor.
3.8. Chemisorption-IR
3.2. Elemental analysis (C, H, N, and F analyses)
For the IR spectroscopy coupled with chemisorption of probe
The C, H, and N concentrations were determined with a Leco
molecules (chemisorption-IR), about 25–30 mg sample was
CHNS-932 analyser. The F concentration was determined with
pressed at 1 ton into a self-supporting disc (2 cm2 area) for IR
a fluoride-sensitive electrode after digesting the sample by
analysis. The sample disc was placed on a quartz cell equipped
fusion with soda-potash.
with KBr windows. The cell was connected to a vacuum line
and a glass injection loop. A movable quartz sample holder
3.3. Powder X-ray diffraction (XRD)
allows adjusting the sample disc in the infrared beam for
The XRD patterns were obtained from an XRD-7 Seifert-FPM
spectra acquisition and to displace it into a furnace at the top
diffractometer using a CuKα beam.
of the cell for thermal activation of the samples. The cell was
connected to a vacuum line and a glass injection loop. After
3.4. Thermal analysis
we activated the sample at 80 °C under vacuum, different
Thermal analysis was performed by using Netzsch-STA-409C/
probe molecules were added in steps by small doses. The
CD coupled with Balzers-QMG-422-Quadrupole-Mass spec-
amount of injected probe molecules was regulated by the
trometry. Both TG curves and the m/z ratio of the fragments
were recorded during measurements.
valves. The IR spectra were recorded on a Thermo Scientific
Nicolet iS10 spectrometer.
3.5. Scanning transmittance electron microscope (STEM)
3.9. Catalysis
The ternary FeF3–MgF2 sample was re-suspended in methanol
(2–3 mg mL−1).
A
Lacey carbon film (Cu-networks, A citronellal isomerisation reaction was performed in a
300 meshes) was dipped in the sample suspension and dried Thermomixer comfort (Eppendorf) equipped with glass tubes.
in air. The STEM images were recorded with a Joel JEM-2200FS The samples were heated at 80 °C in vacuo for 2 h before use to
Transmission Electron Microscope using a 200 kV field emis- remove the surface impurities. Different amounts of fluoride
sion gun (FEG) and a high angle annular dark field detector samples were added to the reactant mixture (0.2 mL citronellal
(HAADF).
(Acros, 93%) solved in 2 mL toluene (Acros, 99.99%), and
0.1 mL undecane (Sigma-Aldrich, ≥99%) as the internal stan-
dard for GC evaluation). The glass tubes were sealed and main-
3.6. X-ray photoelectron spectroscopy (XPS)
The XPS measurements were carried out with an ESCALab tained at 80 °C for 6 h. After the reaction, the catalysts were
200X photoelectron spectrometer (VG Scientific, U.K.) for all separated by filtration and the products were analysed by GC
samples. XP survey spectra were recorded at an angle of emis- (Shimadzu GC-2010 with an HP-1 column of 100 m × 0.25 mm
sion of 0° using non-monochromatised Al Kα excitation (15 kV, × 0.5 μm, temperature programming: starting temperature
20 mA) using the analyser in the constant retard ratio mode 120 °C, hold time 50 min, rate 15 °C min−1, end temperature
with CRR = 10 for survey scans and in the constant analyzer 270 °C, hold time 5 min).
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4. Conclusion
References
The structural and surface characterisation of the FeF3–MgF2
catalysts revealed that MgF2 strongly influences the porous
structure and the chemical environment around the Fe atoms,
and probably contributes to the enhanced Brønsted acidity of
FeF3–MgF2. The investigation on the formation pathway of
FeF3–MgF2 concludes for the first time a template effect of the
MgF2, which explains the role of the MgF2 matrix in the FeF3–
MgF2 catalysts. The template effect was introduced during the
fluorination of mixed Mg2+ and Fe3+ precursors. During this
step, Mg2+ becomes selectively fluorinated, forming domi-
nantly MgF2 particles in the sol, whereas fluorination of the
Fe-precursor proceeds afterwards. The discovery of this tem-
plate effect is essential for tuning the porosity and surface
acidity of FeF3–MgF2 catalysts and inspired us to investigate
other ternary FeF3-based catalysts with heavier alkaline earth
metal fluorides as matrices.
1 S. M. Coman, S. Wuttke, A. Vimont, M. Daturi and
E. Kemnitz, Adv. Synth. Catal., 2008, 350, 2517–2524.
2 S. M. Coman, P. Patil, S. Wuttke and E. Kemnitz, Chem.
Commun., 2009, 460–462.
3 S. Wuttke, S. M. Coman, J. Kröhnert, F. C. Jentoft and
E. Kemnitz, Catal. Today, 2010, 152, 2–10.
4 N. Candu, S. Wuttke, E. Kemnitz, S. M. Coman and
V. I. Parvulescu, Appl. Catal., A, 2011, 391, 169–174.
5 Y. Guo, P. Gaczyński, K.-D. Becker and E. Kemnitz, Chem-
CatChem, 2013, 5, 2223–2232.
6 I. Agirrezabal-Telleria, F. Hemmann, C. Jäger, P. L. Arias
and E. Kemnitz, J. Catal., 2013, 305, 81–91.
7 K. Tanabe and W. F. Hölderich, Appl. Catal., A, 1999, 181,
399–434.
8 K. Wilson and J. H. Clark, Pure Appl. Chem., 2000, 72, 1313–
1319.
In light of this background, FeF3–CaF2 and FeF3–SrF2 cata-
lysts were successfully prepared via the fluorolytic sol–gel route
and for CaF2 and SrF2 the template effect was evidenced based
9 Y. Guo, S. Wuttke, A. Vimont, M. Daturi, J.-C. Lavalley,
K. Teinz and E. Kemnitz, J. Mater. Chem., 2012, 22, 14587–
14593.
on structural and surface characterisation as well. Using 10 S. Wuttke, S. M. Coman, G. Scholz, H. Kirmse, A. Vimont,
different alkaline earth metal fluorides as matrices signifi-
cantly changed the porosity, and a systematic trend of increas-
M. Daturi, S. L. M. Schroeder and E. Kemnitz, Chem. – Eur.
J., 2008, 14, 11488–11499.
ing average pore size was observed whereas the surface area 11 I. Agirrezabal-Telleria, Y. Guo, F. Hemmann, P. L. Arias and
decreased with the atomic number of alkaline earth metals. E. Kemnitz, Catal. Sci. Technol., 2014, 4, 1357–1368.
Both Lewis and Brønsted acid sites were detected on the 12 A. Negoi, S. Wuttke, E. Kemnitz, D. Macovei,
surface of FeF3–CaF2 and FeF3–SrF2 by chemisorption-IR.
V. I. Parvulescu, C. M. Teodorescu and S. M. Coman,
Notably, by comparing all three ternary fluoride catalysts, the
Angew. Chem., Int. Ed., 2010, 49, 8134–8138.
FeF3–SrF2 showed weaker Brønsted acidity and much weaker 13 A. Negoi, K. Teinz, E. Kemnitz, S. Wuttke, V. Parvulescu
Lewis acidity than FeF3–CaF2 and FeF3–MgF2. Thus the tem- and S. Coman, Top. Catal., 2012, 55, 680–687.
plate effect decreases with increasing atomic number of the 14 S. Wuttke, A. Negoi, N. Gheorghe, V. Kuncser, E. Kemnitz,
alkaline earth metal. The results confirmed the importance of
the template effect of matrix materials in the ternary fluoride
catalysts.
V. Parvulescu and S. M. Coman, ChemSusChem, 2012, 5,
1708–1711.
15 K. Siegbahn and K. Edvarson, Nucl. Phys., 1956, 1, 137–
In short, the ternary iron(III) fluoride-based catalysts exhibit
159.
great potential in heterogeneous catalysis due to their pro- 16 M. Kasrai and D. S. Urch, J. Chem. Soc., Faraday Trans. 2,
nounced and adjustable surface acidity and flexibility in prepa- 1979, 75, 1522–1531.
ration. This work highlights for the first time the template 17 A. V. Naumkin, A. Kraut-Vass, S. W. Gaarenstroom and
effect of the matrix materials in tuning the surface properties
of the catalysts. It furthermore shows the feasibility of adjust-
ing the surface properties of the catalysts by varying the matrix
material or the alkaline earth metal to iron ratio or both. This
C. J. Powell, NIST X-ray Photoelectron Spectroscopy Database,
NIST Standard Reference Database 20, Version 4.1, National
Institute of Standards and Technology, Gaithersburg, MD.,
U.S.A., 2012.
fluorolytic synthesis approach opens access to next-generation 18 B. J. Tan, K. J. Klabunde and P. M. A. Sherwood, Chem.
catalysts based on ternary metal fluorides with adjustable
surface properties.
Mater., 1990, 2, 186–191.
19 N. S. McIntyre and D. G. Zetaruk, Anal. Chem., 1977, 49,
1521–1529.
20 E. Paparazzo, J. Phys. D: Appl. Phys., 1987, 20, 1091.
21 A. F. Holleman and E. Wiberg, Lehrbuch der Anorganischen
Chemie, Walter de Gruyter, Berlin, New York, 2007.
22 A. A. Tsyganenko, D. V. Pozdnyakov and V. N. Filimonov,
J. Mol. Struct., 1975, 29, 299–318.
Acknowledgements
The authors would like to thank Dr Holm Kirmse for taking
the STEM images, Dr Micheal Feist for performing the thermal 23 E. H. Teunissen, R. A. Van Santen, A. P. J. Jansen and
analysis, and Dr Katharina Teinz and Larisa Schimdt for the F. B. Van Duijneveldt, J. Phys. Chem., 1993, 97, 203–210.
fruitful discussion. Y. Guo thanks the Chinese Scholarship 24 A. Auroux, Acidity and Basicity, Springer-Verlag, Heidelberg,
Council (CSC) for a PhD scholarship.
2008.
5084 | Dalton Trans., 2015, 44, 5076–5085
This journal is © The Royal Society of Chemistry 2015

View Article Online
Dalton Transactions
Paper
25 G. Ertl, H. Knözinger and J. Weitkamp, Handbook of Hetero- 31 W. Oppolzer and V. Snieckus, Angew. Chem., Int. Ed. Engl.,
geneous Catalysis, Wiley-VCH, Weinheim, 2008.
26 D. Cook, Can. J. Chem., 1961, 39, 2009–2024.
27 A. G. Pelmenschikov, R. A. van Santen, J. Janchen and
E. Meijer, J. Phys. Chem., 1993, 97, 11071–11074.
28 J. C. Leffingwell and R. E. Shackelford, Cosmetics Perfumery,
1974, 89, 69–89.
29 D. H. Pybus and C. S. Sell, The Chemistry of Fragrances, RSC
Paperbacks, Cambridge, 1999.
30 B. B. Snider, M. Karras, R. T. Price and D. J. Rodini, J. Org.
Chem., 1982, 47, 4538–4545.
1978, 17, 476–486.
32 H. M. R. Hoffmann, Angew. Chem., Int. Ed. Engl., 1969, 8,
556–577.
33 Z. Yongzhong, N. Yuntong, S. Jaenicke and G.-K. Chuah,
J. Catal., 2005, 229, 404–413.
34 G. K. Chuah, S. H. Liu, S. Jaenicke and L. J. Harrison,
J. Catal., 2001, 200, 352–359.
35 C. J. Powell and A. Jablonski, NIST Electron Inelastic-Mean-
Free-Path Database, Version 1.2, SRD71, National Institute of
Standards and Technology, Caithersburg, MD, 2010.
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