Guerbet Reaction over Strontium-Substituted Hydroxyapatite Catalysts Prepared at Various (Ca+Sr)/P Ratios

Article abstract of DOI:10.1002/cctc.201601480

The Guerbet reaction of ethanol to heavier products was performed over a series of extensively characterized Sr-substituted hydroxyapatites (HAPs) with different (Ca+Sr)/P ratios, and thus different structural, textural, and acid–base properties. The acid–base properties were correlated with the reactivity of the solids and an optimal ratio between the amount of acid and basic sites was determined (ca. 4), whereas the ethanol conversion was mainly depending on the specific surface area of the solids. The stoichiometric 100 mol % Sr-substituted sample (SrAp-100) was especially efficient in higher alcohols production, which can be illustrated by a total alcohol selectivity (76.4 %) higher than that of all the other solids at a 13 % ethanol isoconversion.

Full text of DOI:10.1002/cctc.201601480

Accepted Article
Title: Guerbet reaction over Strontium-substituted Hydroxyapatite
catalysts prepared with various (Ca+Sr)/P ratios
Authors: Lishil Silvester, Jean-François Lamonier, carole Lamonier,
Mickaël Capron, Rose-Noëlle Vannier, Anne-Sophie
Mamede, and Franck Dumeignil
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Guerbet reaction over Strontium-substituted Hydroxyapatite
catalysts prepared with various (Ca+Sr)/P ratios
[
a]
[a]
[a]
[a]
Lishil Silvester, Jean-François Lamonier*, Carole Lamonier, Mickaël Capron, Rose-Noëlle
[
a]
[a]
[a],[b]
Vannier, Anne-Sophie Mamede, and Franck Dumeignil*
Abstract:The Guerbet reaction of ethanol to heavierproducts was
performed over a series of extensively characterized Sr-substituted
hydroxyapatites (HAPs) with different (Ca+Sr)/P ratios, and thus
different structural, textural and acid-base properties. The acid-base
properties were correlated with the reactivity of the solids and an
optimal ratio between the amount of acid and basic sites was
evidenced (ca.4), while the ethanol conversion was mainly
depending on the specific surface area of the solids. The
stoichiometric 100 mol.% Sr-substituted sample (SrAp-100) was
especially efficient in higher alcohols production, which can be
illustrated by a total alcohol selectivity (76.4 %) higher than that of all
the other solids at a 13% ethanol iso-conversion.
Hydroxyapatites are solids with a complex surface with species
behaving different chemical properties.[13] They are reported to
show good activity towards the heterogeneous gas phase
Guerbet reaction of ethanol.[
14,15]
However, it is difficult to control
the Guerbet products selectivity using the hydroxyapatite solids
because, at higher ethanol conversions, the produced primary
alcohols undergo further condensation to produce heavier
[
16]
alcohols and other heavier by-products. Nevertheless, these
heavier alcohols/products mixtures can be used as solvents, fuel
additives or plasticizers, after some separation steps if needed.
Two main reaction mechanisms have been reported for
describing the Guerbet reaction exemplified in the case of
ethanol conversion to butanol: the first one is obviously the
Guerbet pathway occurring through aldol condensation[15-18],and
the second one consists of the direct ethanol condensation.[19,20]
However, these mechanisms were studied by different authors
under different reaction conditions, further over catalysts with
different physicochemical properties, which makes difficult to
draw any definitive and universal conclusion.
Introduction
The Guerbet reaction is a well-known reaction in which a
primary aliphatic alcohol is converted tothe corresponding β-
alkylated dimer alcohol in the presence of
a catalyst.
Itproceedsover both heterogeneous and homogeneous catalysts.
Homogeneous catalysts using transition metal complexes such
as Rh, Ru, Pt, and Ir were employed under very mild conditions
of 110-140°C at atmospheric pressure.[1-4] Cu, Ni, Rh, Ru and Pd
metals-based catalysts used in combination with soluble
alkoxides as basic agents promoted the Guerbet reactivity
towards heavier alcohols like isobutanol.[5-8] However, the use of
alkoxides is not environmental friendly, as it induces metal
leaching and the production of alkali hydroxides such as sodium
hydroxide. In contrast, the use of heterogeneous catalyst for the
Guerbet reaction is more compatible with economical and
environmental concerns when targeting industrial applications.
To this respect, various heterogeneous solid catalysts have
been employed so far. Ueda et al. reported the use of MgO,
CaO and ZnO as solid basic catalysts for the condensation of
various primary alcohols with methanol.[9-12] The Guerbet
reaction actually took place only over MgO, as the other metal
oxides were found to be inactive or non-selective.
Among the previously studied apatite materials, strontium
apatites have shown more selectivity to butanol from ethanol.[
There is however a need for linking the physicochemical
properties of strontium apatites with their Guerbet reactivity. In
this paper, we extensively characterized hydroxyapatites
synthesized with various calcium-strontium substitution ratios,
and correlated the properties of these solids with their
performances in the Guerbet reaction of ethanol.
21]
Results and Discussion
1 Catalyst characterizations
1.1 Textural properties
The textural properties of the solids are reported in Table 1. The
BET specific surface areas (SSA) and the pore volume
drastically decreased upon Sr introduction, even for relatively
small amounts of Sr substitution. The deficient and
stoichiometric solids showed similar trends in terms of SSA,
pore volume and pore size variations with the Sr substitution
extent.
[
a]
L. Silvester, J.-F. Lamonier, C. Lamonier, M. Capron, R.-N. Vannier,
A.-S. Mamede, F. Dumeignil
Univ. Lille, CNRS, Centrale Lille, ENSCL, Univ. Artois,
UMR 8181 - UCCS - Unité de Catalyse et Chimie du Solide, F-59000
Lille, France
E-mail: jean-francois.lamonier@univ-lille1.fr
E-mail: franck.dumeignil@univ-lille1.fr
Next, in order to notably check about a potential influence of
crystallinity on the SSA, XRD and Rietveld refinement were
subsequently performed (next section).
[
b]
F. Dumeignil
Institut Universitaire de France, Maison des Universités, 103
Boulevard Saint-Michel, 75005 Paris, France
Supporting information for this article is given via a link at the end of
the document.
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HAp to SrAp-100, except for SrAp-75, which exhibited a larger
crystallite size of 17 nm. In contrast, the crystallite size of the
stoichiometric apatites along the (0 0 1) plane did not vary as
much as what was observed for the (1 0 0) plane with a value of
ca. 26 nm, except for the fully Sr-substituted SrAp-100 sample
Table 1. Textural properties of the prepared solids after calcination
at 400°C
BET SSA
Pore volume-
Average pore
diameter (nm)
Catalyst
Hap
2
-1
3
-1
(
m .g )
BJH (cm .g )
114
0.76
24.9
14.7
17.6
18.2
27.5
21.2
19.9
26.6
(
31 nm).
A substantially similar trend was observed for the deficient
apatites (Table 2).
SrAp-25
SrAp-50
SrAp-75
SrAp-100
Dap
43
33
0.16
0.14
0.16
0.32
0.75
0.26
0.43
Table 2. Unit cell parameters and crystallite size of the solids determined by
Rietveld refinement
33
44
Crystallite size
Length/
corresponding to
124
50
Solid
Cell Parameters
thickness
ratio
crystallographic
plane (nm)
SrDAp-50
SrDAp-100
(
0 0 1) /
59
a (Å)
c (Å)
(1 0 0)
(0 0 1)
(
1 0 0)
3.2
HAp
9.4103(5)
9.5070(5)
9.5951(6)
9.6833(6)
9.7742(4)
9.4210(6)
9.5966(9)
9.7799(5)
6.8871(4)
6.9821(4)
7.0817(5)
7.1848(5)
7.2828(4)
6.8853(4)
7.0843(7)
7.2832(4)
8
26
26
25
26
31
22
19
27
1.2 X-ray diffraction and Rietveld refinement studies
SrAp-25
SrAp-50
SrAp-75
SrAp-100
DAp
12
13
17
14
7
2.2
Figures S1a†&S1b† show the powder diffractograms of the
stoichiometric and deficient strontium apatites, respectively.
Irrespective of the Sr content, all the solids showed diffraction
patterns characteristic of a pure hydroxyapatite phase. All the
solids exhibited the hexagonal structure, and crystallize in the
1.9
1.5
2.2
3.2
1.9
2.1
3
P6 /m space group. When the strontium content was increased
SrDAp-50
SrDAp-100
10
13
from 0 to 100 mol.%, the XRD pattern shifted towards lower
values of 2 theta, which was accompanied with an increase in
the intensity of the peaks due to a decrease in the Full Width at
Half Maximum (FWHM), indicative of an increase in crystallinity.
These observations are in good agreement with previous works
that reported an expansion of the original apatite lattice when Ca
cations were partly substituted by larger Sr cations.[22]
9.8
7.3
7
7
7
6
.2
9
.7
To confirm this hypothesis, Rietveld refinement was performed
to calculate the lattice parameters and the crystallite size of the
apatite solids (Figure S2†). For all the samples, the (0 0 2)
Bragg peak showed a thinner FWHM than that of the other
peaks, which reflects the presence of anisotropic crystallites.
Therefore, an anisotropic size-broadening model based on linear
combination of spherical harmonics was used to simulate the
size broadening, taking into account the instrument resolution
.1
9.6
9.5
.9
pattern.[23] From this model, the crystallites shape
9.4
6.8
from a LaB
6
0
0.25
0.5
0.75
1
was deduced. For all the solids, a good agreement was obtained
between the calculated and the experimental data (Figure S2†).
All the results are reported in Table 2.
Sr/(Ca+Sr)
Figure 1. Variation in the a and c unit-cell parameters with the Sr content in
stoichiometric apatites
As expected, the lattice parameters increased with the increase
in the amount of strontium. Figure 1 clearly shows a linear
increase in both a and c unit cell parameters with the increase in
the Sr/(Ca+Sr) ratio. This increase in the unit cell parameters
with the Sr content reflects the aforementioned expansion of the
apatite unit cell, in good agreement with previous reports.[22,24] All
the apatites have a larger crystallite size along the (0 0 1) plane
compared to that along the (1 0 0) plane, irrespective of the Sr
content. Indeed, the crystallite size along the (1 0 0) plane for
the stoichiometric apatite solids increased from 8 nm to 14 nm
with the increase in the strontium content, respectively, from
The lower length/thickness ratio of the Sr apatites compared to
Ca apatites suggests that the crystallites of Sr-substituted
apatites turned to be more spherical than that of Ca apatites
(Table 2). In stoichiometric apatites, this length/thickness ratio
tended to decrease from 3.2 to 1.5 with increasing the Sr
content from HAp to SrAp-75 and then increased slightly to 2.2
with a further increase in the Sr content (SrAp-100). This implies
that the crystallites turned to be almost spherical when
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increasing the Sr content, and that the SrAp-75 crystallites
exhibited a particularly pronounced sphericity among all the
samples. The length/thickness ratio varied in a similar manner
for the deficient apatites.
2p photopeaks was observed in XPS, the surface compositions
were determined using the Ca 2p, Sr 3p3/2 and
P 2s
photopeaks. Because of the overlapping between the C 1s and
the Sr3p1/2 photopeaks, the P 2s photo peak was used as an
energy reference since the binding energy value of P 2s
remained constant at 299.5 eV for HAp and DAp.
The decrease in SSA of the Sr apatites compared to the Ca
apatites can be explained by the increased crystallite size with
Sr substitution, as determined by Rietveld refinement. Since the
variation in crystallite size is inconsistent between (1 0 0) and (0
The surface (Ca+Sr)/P ratio of both stoichiometric and deficient
apatites increased with increasing the Sr substitution. For the Sr-
free apatites, the surface Ca/P ratio was lower than the bulk one.
This is due to the fact that more phosphate groups are exposed
to the surface compared to Ca species. With increasing the Sr
substitution, the surface (Ca+Sr)/P ratio became closer to the
bulk ratio and exceeded the bulk ratio for a Sr substitution higher
than 50 %.This can be explained by the tendency of Sr atoms to
be more exposed to the surface than phosphate groups. The
larger size of the Sr2 cation (ionic radius = 0.118 nm) compared
to that of the Ca2+ cation (ionic radius = 0.100 nm) may be the
reason for this increased surface exposure of Sr. These
observations are in good agreement with a previous report that
revealed high surface Sr/P ratios in apatite solids compared with
the bulk ratios.[21]
0
1) planes, we have correlated the length/thickness ratio to the
SSA. A good correlation was indeed obtained (Figure S3†).
Thus, the decrease in SSA of the stoichiometric apatites from
HAp to SrAp-75 and the further increase in SSA in SrAp-100 are
in rather good agreement with the length/thickness ratio
evolutions obtained from the Rietveld refinement.
+
1.3 Scanning electron microscopy (SEM)
To compare the crystallites shape of the prepared solids with
their grain shape, SEM images were taken (Figure S4†). They
clearly show that the grain size of the Sr-free apatites (HAp and
DAp) is much lower compared to that of the Sr-containing
apatites. Considering the stoichiometric Sr apatites, the particles
were rod or capsule-shaped agglomerates in SrAp-25, SrAp-50
and SrAp-100 and turned to a more spherical shape in SrAp-75,
in good agreement with the crystallites shape deduced from
Rietveld refinement.
1.5 Low energy ion scattering (LEIS)
To verify the effect of Sr substitution on the surface distribution
of the first atomic layer, HAp, SrAp-50 and SrAp-100 were
chosen for LEIS analysis (Figure 2). An O peak was observed at
1230 eV, and a smaller peak due to P was observed at 1880 eV.
1.4 Bulk and surface composition (ICP & XPS)
4 2 4
This assignment was confirmed by the analysis of NH H PO ,
i.e., the precursor used for the synthesis of all the apatites
(Figure 2). Note that the P surface peak overlapped the energy
loss due to the elemental composition of subsurface layers. A
peak due to Ca was observed in HAp and SrAp-50 at around
Table 3. Bulk and surface elemental composition of the apatite solids
calcined at 400°C
ICP
Sr/P
XPS
Sr/P
Solids
(Ca+Sr)/
P
(Ca+Sr)/
P
2100 eV. The peak due to Sr was observed for SrAp-50 and
Ca/P
Ca/P
1.27
SrAp-100 at around 2560 eV. The observed slight energy shift is
due to poor charge compensation.
HAp
1.69
1.27
0.85
0.44
-
-
1.69
1.68
1.68
1.65
1.69
1.62
1.61
1.61
-
1.27
1.44
1.62
1.80
2.10
1.19
1.56
1.91
SrAp-25
SrAp-50
SrAp-75
SrAp-100
DAp
0.41
0.83
1.21
1.69
-
0.92
0.64
0.29
-
0.52
0.98
1.51
2.10
-
Sr
18
P
Ca
O
16
14
12
1.62
0.81
-
1.19
0.59
-
SrDAp-50
SrDAp-100
0.80
1.61
0.97
1.91
10
8
Table 3 shows the atomic composition of the calcined apatite
solids determined by elemental analysis. The (Ca+Sr)/P ratio of
the stoichiometric apatites are in rather good agreement with the
molar ratio of the solutions (1.67) used during their synthesis.
For deficient apatites, the (Ca+Sr)/P ratios of the prepared solids
were higher than expected. For a (Ca+Sr)/P molar ratio of 0.9 in
the synthesis solutions, we obtained deficient solids with ratios
of 1.61~1.62, as previously reported for Ca deficient apatites.[25]
6
4
2
0
7
50
1000 1250 1500 1750 2000 2250 2500 2750 3000
Energy (eV)
NH H PO
HAp
SrAp-50
SrAp-100
4
2
4
Figure 2. LEIS spectra of the stoichiometric apatite solids
The surface composition of all the apatites determined by XPS is
given in Table 3. Since an overlap between the Sr 3d and the P
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The measured LEIS intensity is directly proportional to the
surface coverage of the corresponding element. By using HAp
1.6 Infrared analysis of the solids
(
0 mol.% Sr) and SrAp-100 (100 mol.% Sr) as references, the
Figure 3 shows two domains of the same infra-red spectra for all
the apatite solids focused on two regions, namely 3000-
3700 cm-1 and 1700-400 cm . The vibrational band in the 3000-
3700 cm-1 range is observed in all the apatites due to the
surface fraction of Ca and Sr ions in SrAp-50 can be estimated
after subtraction of the backgrounds and integration of the Sr
and Ca peaks surfaces. In SrAP-50, this yielded a Sr surface
fraction of 55% based on the spectrum of SrAp-100 and a Ca
surface fraction of 76% based on the spectrum of HAp.This is in
contradiction with the XPS results, SrAp-50 exhibiting a higher
Sr/P ratio than the Ca/P ratio (Table 3). This is due to the fact
that XPS gives an average elemental composition along a depth
of around 5-10 nm (XPS depth analysis), in which the Sr species
are more abundant than the Ca species, but considering only
the outermost atomic layer, more Ca ions than expected are
covering the surface than Sr ions (LEIS results).
-1
presence of adsorbed and constitutive water molecules.[26]
A
sharp band at 3572 cm-1 is observed for the Sr-free apatites
(HAp and DAp), which can be assigned to structural OH groups.
3
-
PO4
-
3
752 (OH )
3
592 (OH-)
3-
2
-
PO4
CO3
SrAp-100
6
05 565
1455 1414
-
(
OH ) 630
SrAp-100
SrAp-75
SrAp-50
SrAp-75
SrAp-50
SrAp-25
HAp
SrAp-25
HAp
3700
3600
3500
-
3400
3300
3200
3100
3000
1600
1400
1200
1000
800
600
400
-1
-
1
Wave number (cm )
Wave number (cm
)
3
-
PO4
3
752 (OH )
2
-
CO3
3
-
1
455 1414
PO4
3
592 (OH-)
605 565
SrDAp-100
SrDAp-50
-
(
OH )630
SrDAp-100
SrDAp-50
DAp
DAp
3700
3600
3500
3400
3300
3200
3100
3000
1600
1400
1200
1000
800
600
400
-1
-1
Wave number (cm )
Wave number (cm )
Figure 3. Infra-red spectra of the stoichiometric and the deficient apatites calcined at 400°C
SrAp-25 exhibited an OH band at the same position as that
which showed that such an OH band splitting is due to the
variation in the strength of O-H bond attached to both Ca and
substituted Sr ions in the Ca (II) triangle sites.[27,28]This band was
completely up-shifted to 3592 cm-1 in the 100 % Sr-substituted
apatites, namely SrAp-100 and SrDAp-100. These observations
suggest the substitution of Ca ions by Sr ions from the Ca (II)
sites, especially when Sr substitution extents above 50 mol.%.
-1
observed in Sr-free apatites (3572 cm ). When the Sr
substitution was further increased to SrAp-50 and SrAp-75, the
intensity of the OH band decreased, together with a band
broadening, as previously reported.[27] A detailed observation of
the OH band in SrAp-75 revealed the presence of two low
-1
intensity peaks, the first one at 3572 cm , and the second one at
-1
3
592 cm . This is in good agreement with previous reports,
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An OH vibration band was observed at around 630 cm-1 for Ca
apatites and the intensity of this band also decreased when
increasing the Sr content (Figure 3). This band is absent for Sr
apatites with Sr content above 50 mol.%. This decrease in
intensity of all the bands due to OH modes of vibration in Sr
apatites compared with Ca apatites resulted from the decrease
3-
 PO4
(a)
3-

PO4
SrAp-100
3
-
3-
 PO4
 PO4
SrAp-75
SrAp-50
4
in hydrogen bond interactions between OH and PO groups due
to Sr substitution. Fowler reported that the polarizing power of
Sr2+ (1.59) is lower than that of Ca2+ (2.04), and, hence, the
polarization of OH ions decreased with Sr substitution.[27]This
decrease in polarization in Sr apatites decreases in turn the
-
SrAp-25
HAp
-
3-
trend of OH groups to form hydrogen bonds with PO
4
groups
resulting in a decrease in the OH band intensity. Since the OH-
ions are located at the centre of Ca (II) triangles, this
observation once again confirms the substitution of Ca (II) ions
by Sr ions. In addition, all the above observations point out the
fact that the observation ofa reduced intensity of OH- bands
especially in stoichiometric apatites is not due to a deficiency in
3
00 400 500 600 700 800 900 1000 1100 1200 1300
-
1
Raman shift (cm
)
3
-

PO4
-
-
OH groups, but to changes in the environment of the OH groups
due to Sr substitution.
(
b)
3
-
The PO
5
4
vibrational modes for Ca apatites are observed at
3
-
 PO4
3
-

PO4
65 cm , 605 cm−1, 963 cm−1 and 1030-1100 cm .
−
1
−1 [29]
 PO4
SrDAp-100
The
infrared absorption bands due to phosphate groups shifted to
lower wavenumbers when increasing the strontium content in
SrDAp-50
DAp
[
24]
apatites, as previously reported.
In addition, the peaks at
-1
-1
2- [26]
1
455 cm and 1414 cm can be attributed to CO
3
.
A peak at
-1
1350 cm was observed for some solids (SrAp-25 & SrAp-75)
due to nitrate impurity from the precursors used for the
synthesis.[30]
300
400 500 600 700 800 900 1000 1100 1200 1300
-
1
Raman shift (cm
)
1.7 Raman analysis of the solids
Figure 4. FT-Raman spectra of the calcined (a)stoichiometric and (b) deficient
apatites
Figures 4a&4b show the Raman spectra of the stoichiometric
and deficient apatites respectively. Four lines were observed for
all the solids, which are characteristic of hydroxyapatites. In the
Upon increasing the Sr content from 0 mol.% to 100 mol.%, a
down shift was observed for all the aforementioned PO 3-
4
case of HAp, these lines appeared at 960 cm-1 (ν
PO
3-
mode),
vibration modes. For example, this is more obvious for the ν
1
1
4
mode, which showed a linear down shift from 960 cm-1 in HAp to
944 cm-1 in SrAp-100 (Figure 5: circles). This linear relation
00 – 460 cm-1 (ν
3-
-1
3-
4
2
PO
4
mode), 1010 – 1100 cm (ν
3
PO
4
-1
3-
mode) and 570 – 620 cm (ν
4
PO
4
mode), which is in good
3
-
agreement with previous reports.[
31,32]
reveals that the shift of the PO
4
ν
1
mode is due to the changes
3
-
in the PO
4
environment upon increasing the Sr substitution.
This also reveals that Sr is substituting the Ca to form a single
apatite phase, since Sr substitution did not yield the appearance
of any additional peak in the Raman spectra. Similar trends were
observed in the Raman spectra of the deficient apatites (Figure
5: triangles)
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9
9
9
9
9
9
9
9
9
9
9
62
60
58
56
54
52
50
48
46
44
42
Table 4. Acid-base properties of the apatite solids determined by NH
CO -TPD
3
-TPD &
2
Ratio of
number of
acid sites
to basic
Number
of acid
Acid
sites
Number
of basic
Basic
sites
Solids
sites/
µmol.g-1
density/
µmol.m-2
sites/
µmol.g-1
density/
µmol.m-2
sites (RAB
)
HAp
763.3
143.3
105.3
96.7
6.70
3.33
3.19
2.93
100.7
24.6
22.3
22.3
0.88
0.57
0.67
0.67
7.6
5.8
4.7
4.3
SrAp-25
SrAp-50
SrAp-75
SrAp-
100
1
67.3
956.0
86.7
3.80
7.71
3.73
41.4
41.3
8.6
0.94
0.33
0.17
4.0
0
0.25
0.5
Sr/(Ca + Sr)
0.75
1
DAp
23.1
21.7
SrDAp-
50
1
9
9
9
9
9
9
9
9
9
9
62
60
58
56
54
52
50
48
46
44
SrDAp-
232.0
3.93
12.7
0.22
18.2
1
00
Deficient apatites possess lower basic sites density compared to
their stoichiometric counterparts.
The increase in the Sr substitution was expected to yield an
increase in the basic sites density, but values from
stoichiometric and deficient apatites actually do not reflect this
trend. The basicity of the apatites can be different between the
cases when Sr substitution occurs in the Ca(I) sites or in the
Ca(II) sites. Indeed, the basicity of an atom species is well
known to increase when its coordination number increases. In
hydroxyapatite, Ca(I) is coordinated to 9 O atoms and Ca(II) is
coordinated to 7 O atoms. Hence, Sr atoms substituting Ca(I)
sites will exhibit a more basic character compared to Sr atoms
substituting Ca(II) sites. These two sites are not necessary
evenly substituted by strontium, which can thus cause an
inconsistent trend in the basicity evolution with the Sr
substitution.
0
0.25
0.5
Sr/(Ca + Sr)
0.75
1
Figure 5. Shift of the PO 3-
substitution (open circles represents the stoichiometric apatites and triangles
the deficient apatites)
4
1
ν mode wave number as a function of Sr
3 2
1.8 TPD-NH and TPD-CO
Since both acid and the basic sites are required for the Guerbet
reaction catalysed by the apatites, the ratio of the number of
acid to basic sites (RAB) can be a useful parameter in the present
study. The RAB value was higher for all the deficient apatites
compared to their stoichiometric counterparts, underlining the
relative dominant acidic character of deficient apatites compared
to the stoichiometric solids (Table 4). Also, for the stoichiometric
apatites, the RAB value tended to decrease sharply with the
increase in Sr substitution up to 50 mol.%, and then slowly for
further Sr substitutions above 50 mol.% (Figure 6). This
decrease in the RAB value is an indirect representation of the
relative increase in the predominance of basic sites over acid
sites when increasing the Sr substitution. Similarly, a rather
linear decrease in RAB with the Sr substitution was observed for
deficient apatites (Figure 6).
Acid and basic properties of the different samples, evaluated
using TPD-NH and TPD-CO , respectively, are given in Table
.The amount of acid sites was larger in the deficient apatites
3
2
4
compared to their stoichiometric counterparts having similar Sr
substitutions (Table 4). Deficient apatites are known to be highly
acidic in nature compared to stoichiometric apatites.[15] The Ca2+
ions generally behave as acid sites in Sr-free apatites, and,
when these Ca2+ ions are substituted by larger Sr2+ ions, the
number of acid sites per square meter can mechanically
decrease. This might be the reason for the observation of a
decrease in the number of acid sites when increasing the Sr
substitution, though it cannot explain the higher acid sites
density observed for the 100 % Sr-substituted samples (SrAp-
100 and SrDAp-100), which thus seem to be particular in the
series.
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8
7
7
6
6
5
5
4
4
3
3
.0
.5
.0
.5
.0
.5
.0
.5
.0
.5
.0
25.0
24.0
23.0
22.0
21.0
20.0
19.0
18.0
17.0
16.0
15.0
and to acetone by dehydrogenation over the basic sites. Figures
7
and 8 show the selectivity to propylene and acetone,
respectively, at 20 % conversion of isopropanol.
Figure 7(a) shows that the propylene selectivity over
stoichiometric apatites decreased from 37.2 % to 18 % when
increasing the Sr content from HAp to SrAp-25. This is in good
agreement with the TPD-NH
3
results, which showed that HAp
-1
possesses large amount of acid sites (763.3 μmol.g )
a
compared to the other Sr-substituted stoichiometric apatites.
Further increase in the Sr content in stoichiometric apatites did
not yield any significant variation in the propylene selectivity.
This reveals that a variation in the number of acid sites between
97 μmol.g-1 and 167 μmol.g-1 had no significant effect on the
~
propylene selectivity in stoichiometric apatites. In the case of
deficient apatites, the Sr-free deficient apatite (DAp) was more
selective to propylene than the Sr-substituted deficient apatites.
The propylene selectivity over deficient apatites decreased from
0
0.25
0.5
0.75
1
Sr/(Ca + Sr)
Figure 6. Variation in the ratio of the number of acid sites to basic sites (RAB
with the Sr substitution in stoichiometric apatites (open circles) and in deficient
apatites (triangles).
)
90 % to 55.9 %, when the Sr content increased respectively
from 0 % in DAp to 50 % in SrDAp-50. Further increase in the Sr
content to 100 % in SrDAp-100 yielded an increase in the
propylene selectivity to 69 %. These observations are in good
Figure S5† shows the CO
2
desorption profiles over the
3
agreement with the TPD-NH results. However, note that the
stoichiometric and deficient apatites. Three desorption peaks
were clearly observed in stoichiometric apatites at temperatures
of around 125°C, 190°C and 330°C, which may be attributed to
the presence of weak, medium and strong basic sites,
respectively. The peak at 330°C due to stronger basic sites was
not observed for deficient apatites. Thus, we can assume that
propylene selectivity increased from around 20 % in the Sr-
substituted stoichiometric apatites to about 60 % in the Sr-
substituted deficient apatites, while there is not much difference
in the number of acid sites between the samples (Table 4).
Therefore, this increase in propylene selectivity might be also
explained by the increase in the stronger acid sites in Sr-
substituted deficient apatites compared to Sr-substituted
stoichiometric apatites (Figure S6†).
-
this peak is due to lattice OH groups in the apatite structure that
lacks in deficient apatites resulting from the removal of OH-
groups for the charge compensation due to Ca deficiency.[15,33]
The intensity of the peaks due to medium strength basic sites
was higher than that of the weak basic sites in the Sr apatites.
Moreover, in the stoichiometric apatites, an increase in the Sr
substitution from 0 mol.% to 75 mol.% resulted in a decrease in
the intensity of the peak due to weak basic sites. Further
increase in the Sr content to 100 % resulted into an increase in
the intensity of this peak.
As aforementioned, since RAB is a good indicator of the acid-
base character of the apatite catalysts, the correlation between
RAB and the propylene selectivity was investigated (Figure 7b).
The propylene selectivity increased rather linearly with RAB. The
stoichiometric apatites with lower RAB were less selective to
propylene, thereby confirming their lower acidic character
compared to the deficient apatites. In contrast, the deficient
apatites with higher RAB values showed higher selectivity to
propylene, thus confirming their higher acidic character. Acetone
As a conclusion to this part, the acid-base behaviour of the
apatites was found to vary according to the Sr substitution. The
AB ratio of the number of acid sites to basic sites decreased
formation from
isopropanol
decomposition over
the
R
stoichiometric apatites tended to increase from 38.4 % to 68.4 %
when increasing the Sr content, respectively from 0 mol.% (HAp)
to 100 mol.% (SrAp-100), with the exception of SrAp-75, which
showed an acetone selectivity similar to that of SrAp-50 (Figure
when increasing the Sr substitution, thereby revealing an
increase in the predominant basic character. Deficient apatites
possess higher RAB ratios, showing their higher predominant
acidic character compared to stoichiometric apatites. Moreover,
the strength of the acid/based sites of the apatites also varied
with the Sr substitution. Hence, varying the Sr content actually
enabled modulating both density and strength of acid and basic
sites.
8a). Similarly to stoichiometric apatites, acetone formation over
deficient apatites increased from 6 % to 20.8 % (selectivities)
with the increase in the Sr content from 0 % in DAp to 100 % in
SrDAp-100. These observations are in good agreement with the
2
TPD-CO results showing an increase in the number of basic
sites with Sr substitution. While the HAp and DAp Sr-free
apatites possess a larger number of basic sites compared to the
Sr-substituted apatites, they exhibited a lower selectivity to
acetone. This discrepancy can be then explained using the RAB
parameter, as shown hereafter.
1
.9 Isopropanol test
Isopropanol test conversion to propylene and acetone was
performed over all the apatites in order to probe their acid-base
properties under reaction conditions. Roughly, isopropanol can
be transformed to propylene by dehydration over the acid sites
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moreacidic
1
00
1
00
(a)
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
(b)
9
8
7
6
5
4
3
0
0
0
0
0
0
0
20
10
0
3
6
9
12
RAB
15
18
21
24
5
0
5
0
0
0
0
2
5
7
0
5
-
-
-
1
-
1
-
HAp
p
p
p
-
DAp
p
A
A
A
r
p
A
p
r
r
A
A
S
S
S
r
SrD
S
SrD
-
1
Figure 7. (a) Isopropanol decomposition to propylene over apatite catalysts at 20% conversion (Catalyst amount = 0.1 g, He flow = 2 mL.min ,
Temperature~290°C-320°C, saturator temperature = 10°C); (b) Plot of propylene selectivity as a function of ratio of number of acid sites to basic sites (RAB).
(Open circles represent the stoichiometric apatites and open triangles the deficient apatites)
The acetone selectivity decreased linearly when RAB increased
both for stoichiometric and deficient apatites (Figure 8b). This
clearly reflects the fact that the acetone selectivity is governed
by the basic character of the solids, since a lower RAB value is
an indirect representation of the relative predominance of the
basic character of apatite catalysts. Hence, the higher RAB value
and the less pronounced basic character can explain why Sr-
free apatites HAp and DAp were less selective to acetone
compared to the other Sr-substituted apatites though HAp and
DAp possess larger number of basic sites. Furthermore, a sharp
increase in the acetone selectivity when decreasing RAB was
clearly observed for the stoichiometric apatites. We suppose that
it is due to the presence of stronger basic sites that can promote
the dehydrogenation of propanol to acetone. Such stronger
basic sites are not present in deficient apatites (see Figure S5†).
Hence, we can conclude that it is not only the amount of sites
but also their strength that affect the selectivity towards acetone
and propylene over our series of solids.
Table 5 shows the selectivity to the main products for the
Guerbet reaction of ethanol over apatite catalysts at the ethanol
iso-conversion of 13 %. Such a conversion was achieved by
properly adjusting the temperature between 300°C and 400°C at
-1
-1
-1 -1
GHSV’s of 5000 mL.h g and 15000 mL.h g . Butanol is one of
the major products except over SrDAp-50 and SrDAp-100.
Indeed, over these solids, ethylene was formed as the major
product with a selectivity of 43 % and 32.2 %, respectively,
which is in good agreement with the isopropanol test results.
The selectivity to butanol was maximal over SrAp-100 (63.2 %).
A noticeable amount of acetaldehyde, the dehydrogenation
product of ethanol, was also produced over all the catalysts.
Diethylether was also formed by intermolecular dehydration of
ethanol over all the catalysts, and its formation was slightly
higher over deficient apatites compared to the stoichiometric
ones. Small quantities of dienes were also observed over all the
catalysts. Other detected compounds include higher aldehydes,
higher alkenes and trace amounts of aromatics, which are
formed by uncontrolled consecutive reaction over the various
sites present at the surface of the solids.
2
Catalytic performances
2.1 Guerbet reaction products observed at iso-conversion
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morebasic
1
00
1
00
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
(a)
(b)
9
8
0
0
70
60
50
40
30
20
10
0
3
6
9
12
RAB
15
18
21
24
5
0
5
0
0
0
0
2
5
7
0
5
-
-
-
1
-
1
-
HAp
p
p
p
-
DAp
p
A
A
A
p
A
p
r
r
r
A
A
S
S
S
r
SrD
S
SrD
-
1
Figure 8. (a) Isopropanol decomposition to acetone over apatite catalysts at 20 % conversion (Catalyst amount
= 0.1 g, He flow = 25 mL.min ,
Temperature~290°C-320°C, saturator temperature = 10°C); (b) Plot of acetone selectivity as a function of ratio of number of acid sites to basic sites (RAB). (Open
circles represents the stoichiometric apatites and the open triangles the deficient apatites)
Figure 9 shows the total alcohols selectivity as a function of the
AB ratio of number of acid sites to basic sites. The total alcohols
selectivity decreased rather linearly with the increase in RAB
Figure 9). This is an indirect representation that the
stoichiometric catalysts with a relatively predominant basic
character (i.e., with a lower RAB value) compared to deficient
catalysts are more selective to total alcohols.
The stoichiometric apatite SrAp-100 (100 mol.% Sr-substituted)
with RAB = 4.0 was the most selective to alcohols. This optimum
value of 4 in Sr-substituted apatite is thus close but slightly lower
than the optimal RAB value of 4.8 determined over Sr-free
apatites.[18] Under similar experimental conditions and at 13%
ethanol conversion, 100 mol.% Sr substituted stoichiometric
SrAp-100 exhibited higher selectivity towards total alcohols
R
(
(
76.4%) compared to all other Sr-substituted apatites. However,
DAp showed a total alcohols selectivity of 65 % even though it
possesses a higher RAB value (predominant acid character) than
that of the other catalysts and deviates from the linear trend
it showed slightly lower total alcohols selectivity compared to our
3
previously reported carbonate containing Ca apatite (Hap-CO )
that showed total alcohols selectivity of 83%.[
18]
The total
(
Figure 9). This is supposedly due to the largest density of basic
alcohols selectivity observed for all stoichiometric Sr-substituted
apatites (except SrAp-100) is almost the same (55% - 60%) as
that observed for carbonate-rich apatites in our previous
study.[
-2
sites in DAp (0.33 μmol.m ) compared to the other deficient
apatites (Table 4).
18]
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
2.2 Guerbet reaction: Optimization of experimental
conditions
The main aim of this optimization study was to maximize the
ethanol conversion while getting higher alcohols yields.
Therefore, the ethanol reaction was performed at different
temperatures and GHSV’s to accordingly find optimal conditions.
Tables S1† and S2† show the ethanol conversion and the
products’ distribution at different temperatures and GHSVs. The
reaction products mainly consisted of alkenes, aldehydes, higher
alcohols, dienes, diethylether and trace amounts of aromatics.
The nature of the detected products suggested the absence of
any cleavage or decomposition event.
0
5
10
15
20
25
RAB
As expected, over all the catalysts, the ethanol conversion
increased when the GHSV decreased. The higher ethanol
conversion of HAp and DAp can be explained by their larger
Figure 9. Total alcohols selectivity as a function of the RAB ratio of number of
acid sites to basic sites (open circles represents the stoichiometric apatites
and open triangles the deficient apatites)
2
-1
specific surface areas (~120 m .g ) compared to that of the Sr-
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2
-1
substituted apatites (~45 m .g ) As aforementioned, since our
aim was to reach a maximal ethanol conversion together with a
-1 -1
high alcohols yield, the GHSV was fixed at 5000 mL.h g for
further experiments.
Table 5. Selectivity to Guerbet reaction products at a 13 % ethanol conversion
C4-C8
alcohols[a]
Total
alcohols[b]
Total
dienes[c]
Diethyl
Others[d]
ether
Solids
Ethylene
Acetaldehyde
Butanol
1
.6
.8
4
14
HAp
2.4
4
10
11
50
51
18
5.5
7.4
8
68
56.5
55.2
60
5
3.5
1.7
3.2
19.2
18.9
21.2
SrAp-25
SrAp-50
SrAp-75
SrAp-100
DAp
5
4
3.5
4.1
1.7
4.5
43
15.7
7.5
47.8
52
9.1
63.2
49
13.2
16
2
76.4
65
2.7
3.5
1.6
2.6
2
8.1
6.5
5.3
9
15.2
18.8
18.5
SrDAp-50
SrDAp-100
14.2
20.2
11.4
16
13.4
19
32.2
3
7.5
[
a] 2-buten-1-ol, 2-ethylbutanol, hexanol, 2-ethylhexanol,octanol. [b] Butanol and C4-C8 alcohols. [c] 1,3-butadiene and C6-dienes. [d] Higher aldehydes, higher
alkenes, aromatics.
-
1
-1
-1
N.B. Reaction conditions: GHSV = 5000 mL.h .g , Temperature~300°C – 400°C, Time on stream = 4h, Total flow = 50 mL.min .
In the stoichiometric apatites except HAp, the total alcohols yield
increased when increasing the temperature from 300°C to
00°C (Figure 10). When the temperature exceeded 400°C, the
Over the SrDAp-50 and SrDAp-100 deficient apatites, the total
4
alcohols yield remained very low and the temperature had a
limited influence on this yield (Figure 11). In contrast, over the
Sr-free DAp apatite, the total alcohols yield increased from 9 %
to 17.3 % when increasing the temperature from 300°C to 350°C
and then decreased to 6.5 % with further increase in
temperature to 400°C. This decrease in total alcohol yield is due
to the highly acidic behaviour of DAp compared to other apatites
that resulted in the dehydration of ethanol to ethylene, as
previously reported.[ Hence 400°C was an optimal temperature
for the production of alcohols over all the catalysts except over
the deficient DAp apatite.
formation of alkenes and aromatics became more pronounced,
which, in turn, gave a lower alcohols yield (data not shown). In
HAp, the total alcohols yield increased from 9 % to 22 % when
shifting from 300°C to 350°C, and then remained almost
unchanged with further increase in the temperature to 400°C.
This is due to the slightly higher acidic character in HAp
compared to the other stoichiometric apatites, which becomes
more pronounced at higher temperatures yielding ethanol
dehydration to ethylene. This is in good agreement with our
previous results over Ca apatites.[18]
18]
3
2
2
2
1
1
1
0
7
4
1
8
5
2
9
6
3
0
20
18
16
14
1
1
2
0
8
6
4
2
0
2
75
300
325
350
375
400
SrAp-100
425
2
75
300
DAp
325
350
375
400
425
Temperature ( C)
Temperature ( C)
SrDAp-50
HAp
SrAp-25
SrAp-50
SrAp-75
SrDAp-100
Figure 10. Influence of the temperature on the total alcohols yield
-
1 -1
(
GHSV = 5000 mL.h g ) over stoichiometric apatites
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Figure 11. Influence of the temperature on the total alcohols yield
Guerbet reaction cycle that can further undergo transformation
to form heavier products.[18] So, there is a large chance for
underestimating the dehydrogenation properties of the catalysts
only looking at the acetaldehyde formation.
-
1 -1
(GHSV = 5000 mL.h g ) in deficient apatites
In order to get an insight on the direct influence of the RAB ratio
on the ethanol conversion and on the products yield, the intrinsic
reaction rates of ethanol conversion (Figure S7†) and of
products formation (Figures12&13), were respectively plotted at
0.16
-1 -1
the optimal conditions of 400°C and GHSV = 5000 mL.h g .
This is also a way for getting rid of SSAs variations influence.
0.14
0.12
0
.1
In both stoichiometric and deficient apatites, the intrinsic reaction
rate of ethanol conversion did not vary much with the increase in
0.08
0.06
0.04
0.02
0
RAB and no particular correlation could be revealed (Figure S7†).
We can conclude that it is not RAB, but rather the SSA of the
apatites that significantly impacts the ethanol conversion.
The intrinsic reaction rate of total alcohol production tended to
decrease in a rather linear manner with the increase in RAB
(
Figure 12). Further, Figure 12 clearly shows that the total
0
5
10
15
20
25
30
alcohols production was favoured over stoichiometric apatites
with lower RAB values (i.e., with a predominant basic character)
compared to deficient apatites. The intrinsic reaction rate of total
alcohols production was much lower in deficient apatites due to
their higher RAB value (predominant acidic character). This
observation is in line with the iso-conversion results that showed
a linear decrease in the total alcohols selectivity with an increase
in RAB (see Figure 9).
R
AB
Figure 13. Intrinsic reaction rate of ethylene formation as a function of RAB
circles represent the stoichiometric apatites and triangles the deficient ones)
(
Conclusions
Sr-substituted stoichiometric and deficient apatites were
successfully synthesized with various Sr contents. The Sr
substitution significantly reduced the SSA of the solids. Unit cell
parameters ‘a’ and ‘c’ determined by the Rietveld refinement
increased linearly when increasing the Sr substitution. The
anisotropic crystallite shapesevidenced by Rietveld refinement
was in good agreement with the grain shapes observed in SEM.
The surface (Ca+Sr)/P ratio of all the apatite solids increased
when increasing the Sr substitution, while the (Ca+Sr)/P bulk
ratio remained almost unchanged. This was especially due to
the surface enrichment of larger Sr2+ ions compared to Ca2+ ions.
Combining the results obtained from XRD refinement and LEIS,
we could propose that the Ca2+ and/or Sr2+ ions are more
exposed along the c-axis, that is, on ac and bc planes.
0.16
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0
0
5
10
15
20
25
30
The RAB ratio of the number of acid sites to basic sites
3 2
determined using TPD-NH and TPD-CO results decreased
RAB
linearly when increasing the Sr substitution, which reflects the
increase in the relative predominance of the basic character of
the solids. The deficient apatites exhibited higher RAB values
indicating their predominant acidic character compared to the
stoichiometric apatites. The acid-base behaviour determined by
the isopropanol test was in good agreement with the acid-base
properties (RAB) directly obtained from TPD-NH3 and TPD-
2
Figure 12. Intrinsic reaction rate of total alcohols as a function of the RAB ratio
circles represent the stoichiometric apatites and triangles the deficient ones)
(
Figure 13 clearly shows that the intrinsic reaction rate of
ethylene production increased rather linearly with the increase in
RAB. As expected, the ethylene production was higher over the
deficient apatites with higher RAB values (i.e., with a predominant
acidic character). This result is in good agreement with the
isopropanol test results, where the propylene selectivity tended
to increase when RAB is increased (see Figure 7b).
CO measurements.
The ethanol Guerbet reaction was further carried out, and the
products were analyzed at iso-conversion of ethanol. The
stoichiometric 100 mol.% Sr-substituted apatite (SrAp-100) was
highly selective to alcohols compared to the other apatites, and
its RAB value of 4 could be considered as optimal. A simplified
Furthermore, the intrinsic reaction rate of acetaldehyde
production (Figure S8†) did not seem to be directly correlated
with RAB. This is because acetaldehyde is an intermediate in the
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scheme showing the influence of RAB on the production of major
Guerbet products and by-products is given below in Scheme 1.
C4 - C8
C4 - C8
alcohols
alkenes & ethers
Favorable Guerbet product over Sr
apatites with RAB ≈ 4
Favorable side products of Guerbet
over Sr apatites with RAB ≈ 20
Guerbet reaction
2
OH
+
-H O
2
O
OH
-
2H2
2H2
2
O
-H O
O
2
Intermediates of Guerbet reaction
Scheme 1. Influence of the RAB ratio of the number of acid sites to the number of basic sites of apatite solids on the possible Guerbet reaction pathways. (Among
the intermediates, acetaldehyde is observed and unsaturated aldehyde is not detected)
A
maximal ethanol conversion was observed using
a
-1 -1
temperature of 400°C and a GHSV of 5000 mL.h g . In contrast
to the iso-conversion study result reported abovefor which SrAp-
100 apatite was more selective to alcohols, in such optimized
conditions, the yield in total alcohols was larger over HAp mainly
due to its higher ethanol conversion compared to Sr-substituted
catalysts. Moreover, the intrinsic reaction rate of ethanol
conversion was not much influenced by RAB, underlining the key
role of the SSA in the ethanol conversion. Further, the intrinsic
reaction rate of total alcohol production tended to decrease
rather linearly with the increase in RAB, which is in good
agreement with the iso-conversion study results. Furthermore,
the intrinsic reaction rate of ethylene production increased
linearly with the increase in RAB, which is a trend similar tothat
observed between the propylene selectivity and RAB in the
isopropanol test.
Experimental
1
Catalysts preparation
Reagent grade chemicals such as calcium nitrate tetrahydrate,
Ca(NO
(
(
3
)
2
.4H
≥ 99.0 %), ammonium dihydrogen phosphate, (NH
≥ 98.0 %) (Sigma Aldrich) were used as starting materials.
2
O
(≥ 99.0 %), strontium nitrate, Sr(NO
3
)
2
4
)H PO
2
4
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(
NH
4
)H
2
PO
4
aqueous solution (0.1 M). The subsequent
Table 6. Overview of the prepared samples with their theoretical
compositions
filtration/drying/calcination procedure was the same as above.
2
Characterization methods
Strontium
Sample
name
‘x’ mol of
Strontium[a]
substitution[b]
Comments
The physico-chemical characterization of the solids, namely, X-
ray diffraction (XRD), thermogravimetric analysis (TGA),
scanning electron microscopy (SEM), infrared spectroscopy
(%)
Stoichiometric
reference apatite
Stoichiometric
25 mol.%Sr
substituted
Hap
0
0
(
FTIR), X-ray photoelectron spectroscopy (XPS), low-energy ion
scattering spectroscopy (LEIS), temperature programmed
desorption (TPD-NH TPD-CO were performed using
SrAp-25
SrAp-50
SrAp-75
0.0418
25
50
75
3
&
2
)
procedures reported elsewhere for a study on carbonate-rich
apatites.[25] The Raman spectra of the samples were recorded at
room temperature using a Labram infinity spectrometer (Horiba),
equipped with a photodiode array detector cooled with liquid
nitrogen. The exciting light source was the 532 nm line of an
Stoichiometric
50 mol.%Sr
substituted
0.0835
0.1253
Stoichiometric
75 mol.%Sr
substituted
-1
Nd:YAG laser and the wavenumber accuracy was ± 4 cm . The
laser power was 12 mW and the acquisition time was set
between 3 and 5 min. The Raman spectrometer was calibrated
Stoichiometric
100 mol.%Sr
substituted
SrAp-100
Dap
0.167
0
100
0
−
1
using the silicon line at 521 cm . The spectra were collected
-1
Deficient reference
apatite
between 200 and 1400 cm .
The acid-base properties of the solids were evaluated using a
test catalytic reaction, namely the isopropanol conversion, which
gives propylene (and also small amounts of di-isopropylether,
which was detected over some samples) over acid sites, and
acetone over basic sites. The experimental protocol employed
was identical to that used in our previous study.[25]
Deficient
SrDAp-50
0.045
50
50 mol.%Sr
substituted
Deficient
SrDAp-100
0.090
100
100 mol.%Sr
substituted
[
a] Total (Ca+Sr) is maintained at 0.167 mol for stoichiometric and at 0.09 mol
3 Reactions
for deficient apatites. [b] Ca and Sr together contribute to 100 % in all the
samples.
The Guerbet reaction of ethanol was carried out using 0.20 g
-1
-1
-1 -1
(
GHSV = 15000 mL.h .g ) or 0.60 g (GHSV = 5000 mL.h .g )
Stoichiometric apatites were synthesized by a precipitation
method, adding dropwise 300 mL of an aqueous solution
of sample under the form of a powder with a particle size
between 80 and 100 mesh. 20 Vol.% ethanol diluted in He were
passed through a fixed bed glass reactor at a total flow rate of
50 mL.min . The feed and the outlet products were analyzed
online using a gas chromatography equipped with a zebron ZB-
Bioethanol capillary column (l = 30 m, id = 0.25 mm)connected
to a FID and coupled to mass spectrometer (MS Agilent 5975C).
containing ‘x’ moles of Sr(NO
3
)
2
.4H
2
O and ‘(0.167 - x)’ moles of
)H PO solution placed
-1
Ca(NO to 1000 mL of a 0.1 M (NH
3
)
2
4
2
4
under stirring and kept at a temperature of 80°C. The values of x
were fixed at 0.167, 0.1253, 0.0835 and 0.0418 mol, depending
on the desired relative Ca and Sr content in the apatite solids
(
Table 6). The (Ca+Sr)/P ratios of the synthesis solution of all
these apatites were kept constant at the stoichiometric value of
.67. The pH during the synthesis was maintained at 10 by
proper addition of a 25 Vol.% NH OH solution. The formed
The ethanol conversion, the selectivity to each product,
and the corresponding yields were calculated as follows:
1
4
-
Ethanol conversion (%) = 100 x [(moles of ethanol before
precipitate was slowly filtered, washed with hot water and dried
overnight at 80°C under static air before being calcined at 400°C
reaction – moles of ethanol after reaction) / (moles of ethanol
before reaction)];
-1
under oxygen flow (2 L.h ) for 4 h, this temperature being
-1
reached at a rate of 2°C.min .
-
Product yield (%) = 100 x (moles of C converted into the
considered product / moles of C in ethanol before reaction);
In addition, deficient apatites were prepared using the same
method but by maintaining the (Ca+Sr)/P ratio at 0.9 instead of
-
Product selectivity (%) = 100 x (Product yield / Ethanol
1
.67 during the synthesis. Ca-deficient (DAp) and 100 mol.% Sr
deficient apatites (SrDAp-100) were prepared by adding 300 mL
of solution containing 0.09 mol of Ca(NO .4H and
Sr(NO , respectively, to 1000 mL of a (NH )H PO aqueous
solution (0.1 M). The 50 mol.% Sr-substituted apatite (SrDAp-
0) was obtained by adding a solution containing 0.045 mol of
Ca(NO .4H O and 0.045 mol of Sr(NO to 1000 mL of a
conversion).
a
3
)
2
2
O
-1
-2
-
Intrinsic reaction rate of ethanol conversion (mmol.h .m )
3
)
2
4
2
4
-1
=
{Ethanol flow rate (mol.h ) / mass of catalyst (g)} × {ethanol
conversion (%) / [100 × surface area of catalyst (m .g )]} × 1000
2
-1
5
3
)
2
2
3 2
)
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-
1
-2
-
Intrinsic reaction rate of product formation (mmol.h .m )
[14] T. Tsuchida, S. Sakuma, T. Takeguchi, W. Ueda, Ind. Eng. Chem. Res.
2006, 45, 8634.
-1
=
{Ethanol flow rate (mol.h ) / mass of catalyst (g)} × {product
2
-1
[15] T. Tsuchida, J. Kubo, T. Yoshioka, S. Sakuma, T. Takeguchi, W. Ueda,
J. Catal. 2008, 259, 183.
yield (%) / [100 × surface area of catalyst (m .g )]} × 1000
[16] T. Tsuchida, T. Yoshioka, S. Sakuma, T. Takeguchi, W. Ueda, Ind. Eng.
Chem. Res. 2008, 47, 1443.
Acknowledgements
[17] S. Ogo, A. Onda, K. Yanagisawa, Appl. Catal.A. 2011, 402, 188.
[
18] L. Silvester, J.-F. Lamonier, J. Faye, M. Capron, R.-N. Vannier, C.
Lamonier, J.-L. Dubois, J.-L. Couturier, C. Calais, F. Dumeignil, Catal.
Sci. & Technology 2015, 5, 2994.
The research leading to these results has received funding from
the European Union Seventh Framework Programme
[
19] J. Scalbert, F. T-Starzyk, R. Jacquot, D. Morvan, F. Meunier, J. Catal.
(
FP7/2007-2013) under grant agreement no. 241718
EuroBioRef. The Fonds Européen de Développement Régional
FEDER), CNRS, Région Nord Pas-de-Calais and Ministère de
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[
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(
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l’EducationNationale de l’EnseignementSupérieur et de la
Recherche are acknowledged for notably funding XRD, SEM
and XPS/LEIS facilities. The authors would also like to thank Dr.
Jean-Luc Dubois, Arkema, Dr. Jean-Luc Couturier, Arkema, and
Dr. Christophe Calais, Arkema, for fruitful discussions during this
study.
[21] S. Ogo, A. Onda, Y. Iwasa, K. Hara, A. Fukuoka, K. Yanagisawa, J.
Catal. 2012, 296, 24.
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[
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S. Mamede, F. Pourpoint, A. Gervasini, F. Dumeignil, J. Mater. Chem.A.
Keywords: •Sr-substituted apatites •Acid-base properties
2014, 2(29), 11073.
•
Guerbet reaction •Ethanol •Heavier alcohols
[
26] C. Lamonier, J.-F. Lamonier, B. Aellach, A. Ezzamarty, J. Leglise, Catal.
Today 2011, 164, 124.
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[
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Supporting informations
SrAp-100
SrAp-75
SrAp-50
SrAp-25
HAp
10
20
30
40
50
60
70
80
2
theta (deg)
Figure S1a† Diffraction patterns of stoichiometric apatites calcined at 400°C
SrDAp-100
SrDAp-50
DAp
1
0
20
30
40
50
60
70
80
2
theta (deg)
Figure S1b† Diffraction patterns of deficient apatites calcined at 400°C
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HAp
SrAp-25
RB=2.10%, RF=1.67%
RB=2.56%, RF=2.26%
SrAp-75
SrAp-50
RB=2.78%, RF=2.53%
RB=3.37%, RF=3.15%
SrAp-100
DAp
RB=2.38%, RF=1.87%
RB=1.75%, RF=1.18%
SrDAp-50
SrDAp-100
RB=2.94%, RF=2.45%
RB=2.43%, RF=1.77%
Figure S2†Calculated X-ray diffraction profile (in black) compared to experimental data (in red)
showing a good agreement as shown by the difference in blue.
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1
1
25
05
8
6
4
2
5
5
5
5
1
1.5
2
2.5
3
3.5
4
(
0 0 1)/(1 0 0) ratio
Figure S3†Specific surface area as a function of the (0 0 1)/(1 0 0) ratio
(open circles represents the stoichiometric apatites and triangles the deficient apatites)
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HAp
SrAp-25
SrAp-75
DAp
SrAp-50
SrAp-100
SrDAp-50
SrDAp-100
Figure S4† SEM images of the calcined apatites taken at a 50 k magnification.
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HAp
DAp
SrDAp-50
SrDAp-100
1
/2(Intensity)
1
/3(Intensity)
SrAp-25
SrAp-50
SrAp-75
SrAp-100
50
100 150 200 250 300 350 400 450 500
Temperature (°C)
50
100 150 200 250 300 350 400 450 500
Temperature (°C)
Figure S5† TPD-CO
2
curves of stoichiometric and deficient apatites (Intensity of HAp & DAp reduced and baseline subtracted)
SrDAp-100
SrAp-100
50
100
150
200
250
300
350
400
Temperature (°C)
3
Figure S6† Comparisonof TPD-NH curves of SrAp-100 and SrDAp-100 samples
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0
.4
.35
.3
.25
.2
.15
.1
0
0
0
0
0
0
0
.05
0
0
5
10
15
R
20
25
30
AB
Figure S7† Intrinsic reaction rate of ethanol conversion as a function of the RAB ratio of acid sites to basic sites
(circles represent the stoichiometric apatites and triangles the deficient ones)
0
0
0
0
.035
.03
.025
.02
.015
.01
0
0
0
.005
0
0
5
10
15
20
25
30
RAB
Figure S8† Intrinsic reaction rate of acetaldehyde production as a function of the RAB ratio of acid sites to basic sites
(circles represent the stoichiometric apatites and triangles the deficient ones)
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-
1 -1
Table S1† Ethanol conversion and products yield over apatite catalysts at GHSV = 5000 mL.h g
Hap
SrAp-25
SrAp-50
SrAp-75
SrAp-100
DAp
SrDAp-50
SrDAp-100
Conversion at
300°C
350°C
400°C
13.5
32
64.6
5.3
17.5
32.1
5.5
13.7
23
4.8
11
25.5
4.3
13.1
22.3
14
27
61.8
4.2
10.3
26.2
5
11.2
25.1
Yield at 300°C
Alkenes
Ethylene
0.32
0.06
0.07
0.11
0.02
0.63
0.63
0.41
Aldehydes
Acetaldehyde
Butanal
1.82
-
0.49
0.01
1.00
0.01
0.49
0.01
0.49
0.02
0.88
-
1.01
-
0.95
-
Dienes
Butadiene
C6 dienes
Alcohols
Butanol
0.16
0.05
0.07
-
0.04
-
0.06
-
0.02
-
0.28
0.13
0.04
-
0.04
-
6.80
0.41
1.78
0.61
-
3.70
0.48
0.15
0.03
0.02
3.33
0.44
0.14
0.13
0.03
3.49
0.24
0.14
0.04
0.01
3.22
0.19
0.21
0.04
0.03
6.86
0.21
1.81
0.70
-
0.76
0.57
0.08
0.29
0.02
1.77
0.60
0.21
0.16
0.03
Butenol
C6-C8 alcohols
Diethylether
Aromatics
Yield at 350°C
Alkenes
Ethylene
Butene
Hexene
1.60
0.67
0.16
0.53
0.07
-
0.48
0.07
-
0.67
0.04
-
0.22
0.05
-
2.65
0.27
0.05
3.24
0.04
-
2.36
0.06
-
Aldehydes
Acetaldehyde
Butanal
2.56
0.26
1.07
0.05
2.15
0.07
0.83
0.04
1.19
0.10
0.76
-
1.75
0.02
1.81
0.03
Dienes
Butadiene
C6 dienes
Alcohols
Butanol
1.18
0.58
0.67
0.32
0.41
0.27
0.35
0.18
0.20
0.16
1.43
0.68
0.34
0.12
0.32
0.11
17.57
0.10
4.48
1.18
-
9.47
0.49
1.58
0.40
0.12
6.41
0.45
0.56
0.23
0.14
5.72
0.39
0.50
0.35
0.06
8.28
0.55
1.18
0.26
0.21
14.45
0.16
2.70
1.62
-
1.47
0.31
0.12
0.77
0.05
2.41
0.45
0.22
0.56
0.09
Butenol
C6-C8 alcohols
Diethylether
Aromatics
Yield at 400°C
Alkenes
Ethylene
Butene
Hexene
11.31
2.78
2.71
5.30
0.87
0.16
3.93
0.48
0.05
5.41
0.54
0.08
2.07
0.36
0.04
36.59
2.97
0.31
17.11
0.10
-
13.96
0.25
-
Aldehydes
Acetaldehyde
Butanal
3.10
0.78
-
0.96
0.10
-
2.62
0.18
-
0.97
0.10
-
1.61
0.33
0.02
1.92
-
-
1.60
0.05
-
2.38
0.13
-
Hexanal
Dienes
Butadiene
C6 dienes
Alcohols
Butanol
5.30
3.23
3.05
0.80
2.42
0.46
2.14
0.59
0.94
0.49
2.35
0.74
0.89
0.31
1.00
0.33
21.32
0.19
3.04
1.94
0.39
14.00
0.22
1.61
1.09
0.48
7.25
0.35
0.92
1.01
0.32
10.46
0.31
0.79
0.77
0.31
10.79
0.45
2.61
0.45
0.56
6.18
0.12
0.19
5.25
0.31
1.05
0.13
0.10
2.49
0.21
1.81
0.25
0.13
1.91
0.25
Butenol
C6-C8 alcohols
Diethylether
Aromatics
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-
1
-1
Table S2† Ethanol conversion and products yield over apatite catalysts at GHSV = 15000 mL.h g
HAp
SrAp-25
SrAp-50
SrAp-75
SrAp-100
DAp
SrDAp-50
SrDAp-100
Conversion at
4
.5
3.5
6.2
3.8
6.3
3
3.4
6.2
4.2
10.4
32
3.3
5.1
3.7
6
300°C
350°C
400°C
1
1
3
4.8
3
15.7
12.2
11.5
12.5
13.6
13
Yield at 400°C
Alkenes
Ethylene
5.28
1.06
0.30
1.63
0.19
0.05
1.13
0.07
-
1.41
0.12
0.02
0.69
0.10
-
12.80
0.80
0.10
7.21
0.03
-
5.49
0.05
-
Butene
Hexene
Aldehydes
Acetaldehyde
Butanal
2.08
0.26
1.26
0.06
2.67
0.09
1.28
0.06
1.89
0.18
2.50
0.10
1.93
0.03
2.63
0.04
Dienes
Butadiene
C6 dienes
Alcohols
Butanol
2.74
1.16
0.94
0.28
0.65
0.12
0.55
0.15
0.30
0.15
3.26
0.48
0.18
0.04
0.27
0.07
14.19
0.23
2.48
0.99
0.10
7.61
0.24
0.47
0.55
0.09
4.11
0.61
0.22
0.55
0.09
4.92
0.23
0.16
0.38
0.05
6.54
0.38
0.68
0.35
0.15
7.84
0.13
0.74
2.56
0.06
0.19
0.05
-
0.78
0.08
-
Butenol
C6-C8 alcohols
Diethylether
Aromatics
1.22
0.04
0.98
0.05
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Entry for the Table of Contents
FULL PAPER
The Guerbet reaction of ethanol to
higher alcohols was performed over a
[
a]
Lishil Silvester, Jean-François
Lamonier*,[ Carole Lamonier, Mickaël
a]
[a]
series
of
Sr-substituted
Capron,[a] Rose-NoëlleVannier, Anne-
[a]
hydroxyapatites. An optimal ratio of
ca. 4 between the amount of acid and
basic sites was evidenced.
Sophie Mamede,[a] and Franck
Dumeignil*[a],[b]
Page No. – Page No.
Guerbet reaction over Strontium-
substituted Hydroxyapatite catalysts
prepared with various (Ca+Sr)/P
ratios
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