Influence of the nature of the rare earth element in rare earth orthophosphate used for light alcohol dehydration

Article abstract of DOI:10.1016/j.apcata.2014.12.026

La, Nd, Sm and Gd rare earth orthophosphates with a Rhabdophane-type structure have been synthesized and tested as catalysts for the dehydration of ethanol and 1-butanol. These solids appear to be very active and selective. Contrary to a previously studied LaPO₄ series of catalysts, whose properties were shown to depend on their P/La surface ratio, this series of rare earth phosphates exhibits catalytic properties depending mainly on the nature of the rare earth elements themselves. The most efficient phosphates presented moderately strong Br?nsted and Lewis acid sites and only small quantities of weak basic sites. These properties lead to an exceptionally high selectivity to alkenes for both studied dehydration reactions. Although a good correlation was observed between the Lewis acidity and the rate of ethanol conversion, a possible involvement of Br?nsted acid sites, the strength of which increases with that of the Lewis sites, cannot be totally excluded. The results of the catalysts characterization are interpreted in terms of the possible nature of the different sites.

Full text of DOI:10.1016/j.apcata.2014.12.026

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Influence of the nature of the rare earth element in rare earth
orthophosphate used for light alcohol dehydration
T.T.N. Nguyena, V. Ruauxb, F. Maugéb, V. Bellière-Bacac, P. Reyc, J.M.M. Milleta,∗
a Institut de Recherches sur la Catalyse et l’Environnement de Lyon, IRCELYON, UMR5256 CNRS-Université Claude Bernard Lyon 1, 2 avenue A. Einstein,
F-69626 Villeurbanne Cedex, France
b Laboratoire Catalyse et Spectrochimie EnsiCaen 6 boulevard Maréchal Juin 14050 Caen cedex, France
c Adisseo France SAS, Antony Parc 2, 10 Place du Général de Gaulle, F-92160 Antony, France
a r t i c l e i n f o
a b s t r a c t
Article history:
La, Nd, Sm and Gd rare earth orthophosphates with a Rhabdophane-type structure have been synthesized
and tested as catalysts for the dehydration of ethanol and 1-butanol. These solids appear to be very active
and selective. Contrary to a previously studied LaPO4 series of catalysts, whose properties were shown
to depend on their P/La surface ratio, this series of rare earth phosphates exhibits catalytic properties
depending mainly on the nature of the rare earth elements themselves. The most efficient phosphates
presented moderately strong Brønsted and Lewis acid sites and only small quantities of weak basic
sites. These properties lead to an exceptionally high selectivity to alkenes for both studied dehydration
reactions. Although a good correlation was observed between the Lewis acidity and the rate of ethanol
conversion, a possible involvement of Brønsted acid sites, the strength of which increases with that of
the Lewis sites, cannot be totally excluded. The results of the catalysts characterization are interpreted
in terms of the possible nature of the different sites.
Received 16 October 2014
Received in revised form 5 December 2014
Accepted 10 December 2014
Available online xxx
Keywords:
Alcohol dehydration
Acid catalysts
Rare earth phosphates catalysts
Alkenes
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
The lanthanum phosphate tested in our previous study pre-
sented Brønsted and Lewis acid sites of mainly weak or moderate
A promising approach to the manufacture of biomass-sourced
chemicals is the catalytic dehydration of light alcohols, leading to
the production of various alkenes. The corresponding processes
lead to lower CO2 emissions, low production costs, and require
less energy than the present refinery’s processes [1]. Although
many solid acid catalysts are efficient in catalyzing the dehydra-
tion of light alcohols, for industrial applications they should be
usable in the presence of large quantities of water, be coke resis-
tant, and exhibit almost total selectivity to the alkenes. If the first
two requirements can be met, the last of these is more difficult to
achieve. Phosphates based catalysts have been shown to be effi-
cient and to fulfill this last requirement [2–6]. In this respect, we
have recently shown that lanthanum phosphates fulfill all require-
ments and are ultra-selective in several mono-alcohol dehydration
reactions [7]. These results were in agreement with the pioneer-
ing work of K. Ramesh et al. [8,9], although these authors obtained
high catalytic properties in the case of ethanol dehydration only,
for solids with a bulk P/La ratio equal to 2.
strength, and only very small quantities of weak basic sites [7].
These acid–base properties resulted in the prevalence of the E1-
type mechanism for 1-butanol dehydration. Brønsted acid sites,
were shown to be the most efficient. They were related to the
presence of an excess of phosphorus at the surface and, from spec-
troscopic data, attributed to sub-surface (HnPO4)−3+n (n = 1 or 2)
species. Their prevalence and optimum distribution at the surface
seems to make the catalysts extraordinarily efficient in light alcohol
dehydration reactions.
Furthermore, apart from their high intrinsic activity and selec-
tivity, these catalysts were shown to have a high stability over
time on stream, and could easily be regenerated by means of heat
treatment under air. The absence of strong acid sites, which are
preferential sites for the formation of coke, could also explain the
high selectivity and relative stability of these catalysts.
In the present study, we extended the analysis of ethanol and
1-butanol dehydration catalysts to include several other rare earth
phosphates with the same Rhabdophane-type structure. The goal
was to investigate the catalytic properties of these rare earth ele-
ments and to determine whether the conclusions concerning the
reaction mechanism and the nature of most active sites on the lan-
thanum phosphates remained valid. La, Nd, Sm and Gd phosphates
∗
Corresponding author. Tel.: +33 472445317; fax: +33 472445399.
E-mail address: jean-marc.millet@ircelyon.univ-lyon1.fr (J.M.M. Millet).
http://dx.doi.org/10.1016/j.apcata.2014.12.026
0926-860X/© 2014 Elsevier B.V. All rights reserved.
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were thus prepared under similar conditions, and characterized
and tested as catalysts for the dehydration of ethanol and 1-butanol.
The acid–base properties of the catalysts were evaluated from the
temperature-programmed desorption (TPD) of ammonia and CO2
and from FTIR (Fourier transform infrared spectroscopy) analysis of
several adsorbed probe-molecules (2,6 dimethylpyridine and CO2).
introduced into the carrier gas by an evaporator-saturator system.
The gas mixtures passed through the catalyst packed into a glass
plug-flow micro-reactor. All the pipes were heated to eliminate
condensation of the alcohols and reaction products, which were
analyzed online by gas chromatography. The column used was a
HP plot Q with 15 m of length, 0.53 mm of diameter externally
and 40 ␮m of film thickness. A two-dimensional chromatography
also coupled to mass spectroscopy (GC-MS-2D) of SRA instrument
equipped a ZB1 column and a VF17 column, has also been used
to detect traces of by-products when almost 100% selectivity to
alkenes was observed. Calculated carbon balances were higher than
98%. Tests of empty reactor carried out at 628 K always showed
conversion lower than 3%.
2. Experimental
The rare earth phosphates were prepared as previously
described [7,10]. An hydrous oxide was prepared first by dis-
solving commercial rare earth nitrates (0.26 mol L−1) into 100 mL
of equimolar water-ethanol solution containing 0.066 mol L−1 of
cetyltrimethylammonium bromide and about 110 mL L−1 of aque-
ous ammonia (32 wt%). After 2 h at room temperature under stirring
the resulting solid was collected by filtration, washed with distilled
water and dried in air at 298 K. The solid was then digested in an
aqueous solution of phosphoric acid (1 mol L−1, 100 mL). The final
product was filtered, washed with distilled water, dried in air at
ambient temperature and calcined at 823 K for 6 h.
The Metal contents of the solids were determined by Induc-
tively Coupled Plasma (ICP) atomic emission spectroscopy and
their specific surface areas measured using the BET method with
a Micromeritics ASAP 2020 instrument after degassing under vac-
uum at 573 K for 3 h. X-ray diffraction (XRD) patterns of the
catalysts were obtained using a Bruker D5005 X-ray diffractometer
using Cu K␣ radiation. XPS measurements were performed using
a Kratos Axis Ultra DLD spectrometer. Spectra of RE 3d and 4d
(RE = La, Nd, Sm and Gd), P 2p, O 1s, C 1s levels were measured
using a monochromated Al K␣ X-ray source. Binding energies were
corrected relative to the carbon 1s signal at 284.6 eV. The signal
intensities of RE 4d, P 2p, O 1s and C 1s were measured using
integrated areas under the detected peak.
3. Results
Four rare earth phosphates (La, Nd, Sm and Gd) were prepared
and studied as catalysts. Their main physical and chemical charac-
teristics are presented in Table 1. Their specific surface areas range
between 82 and 124 m2 g−1, whereas their pore size and volume are
comparable. For all of these solids, the surface P/RE ratios derived
from XPS data analysis were close to the theoretical values, except
for the case of LaPO4 (the measured value was much greater than
the theoretical value). Adequate fitting of the O1s peak was system-
atically achieved using two components, characterized by binding
energies of 531.0 and 532.7 eV, which were attributed to O2− and
OH− surface species, respectively. The OH/(OH + O) surface ratio
was comparable for all the solids, with the exception of LaPO4, for
which it was smaller.
The X-ray diffraction patterns of the solids showed a single well-
crystallized Rhabdophane-type phase only, for all of the prepared
samples (ICSD 046-1439). The Rhabdophane structure crystallized
in the hexagonal system with space group P6222 [12]. The com-
puted a and c unit cell parameters of the phosphates are provided
in Fig. 1, showing that they vary almost linearly with the crys-
tal ionic radii of the eight-coordinated lanthanide cations [13]. A
similar effect, due to lanthanide contraction, has been previously
demonstrated [14]. The thermal and thermo-gravimetric analy-
ses of lanthanide phosphates under air showed that they were
hydrated, and that dehydration, corresponding to approximately
1 mole of water per mole of phosphate, systematically occurred
in two phases between 298 K and 563 K. The first mass loss was
attributed to absorbed water, whereas the second (0.3–0.4 mol)
was attributed to water molecules located in the channels of the
structure. The departure of the latter water molecules took place in
only one step, and did not lead to the collapse of the structure. These
results are also in good agreement with those previously reported
Raman spectra were measured on a JOBIN-YVON LabRam Infin-
ity apparatus equipped with a CCD detector operating at liquid
nitrogen temperature. A D2 filter was used to protect the cata-
lyst from destruction by the laser (wavelength ꢀ = 520 nm). The
Raman shifts were recorded in the range of 200–5500 cm−1. Sam-
ples homogeneity was evaluated by performing the analyses on at
least three different locations for each sample.
FTIR spectroscopy has been used to characterize the acid–base
properties of the catalysts by following the spectra of adsorbed
probe molecule. A Nexus 670 FTIR spectrometer equipped with a
MCT or a DTGS detector was used. The phosphates were pressed
into self-supporting pellets (20–30 mg, 2.01 cm2), placed in a cell,
and treated at 573 K in situ under vacuum (133.32 × 10−5 Pa) for
1 h. They were cooled to 373 K and exposed dimethylpyridine
(133.32 Pa at equilibrium) or CO2 (13.33 Pa at equilibrium) for
15 min. The spectra (128 scans, resolution of 4 cm−1, range of acqui-
sition: 400–4000 cm−1) were then recorded after evacuation under
vacuum (133.32 × 10−5 Pa) at room temperature, 323, 373, 423, 473
and 523 K for 15 min. Spectral analyses and decompositions were
carried using Omnic software. All of the spectra were normalized to
a disc of 10 mg cm−2. Temperature-programmed desorption (TPD)
of NH3 has also been used to measure the amount of acidic and
basic sites and their distribution in strength. The test was done on
a BELCAT thermo analyzer as previously described [11]. 60–80 mg
of samples were first treated at 823 K for 1 h under helium flow
(15 mL min−1) to remove adsorbed species and then cooled down
to 373 K before being exposed to NH3 (or CO2) for 30 min. Follow-
ing, the system was swept by He for 15 min and temperature was
programmed to increase to 823 K (8 K min−1).
0.72
0.70
0.68
0.66
0.64
0.62
a
c
0.104 0.106 0.108 0.110 0.112 0.114 0.116 0.118
Cationic radius of lanthanide (nm)
The dehydration of ethanol and 1-butanol was performed at
atmospheric pressure as described elsewhere [7]. High purity nitro-
gen was used as a carrier gas and the alcohols (reagent grade) were
Fig. 1. Variation of the unit cell parameters of the phosphate with Rhabdophane
structure as a function of the cationic radius of the lanthanide elements [6].
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Table 1
Main physical and chemical characteristics of the prepared phosphates.
Compound
Surface atomic ratio
SSA (m2 g−1
)
Pore size (nm)
Pore volume (cm3 g−1
)
P/RE
OH/(O + OH)
LaPO4
NdPO4
SmPO4
GdPO4
1.32
0.96
1.10
1.06
0.14
0.25
0.26
0.27
124
117
82
8
8
10
11
0.2
0.3
0.2
0.3
95
Table 2
activity of Nd, Sm and Gd were rather close to each other. At
total conversion, the selectivity to 2-butene was lower on LaPO4,
but comparable for the Nd, Gd and Sm phosphates. In addition to
butenes, trace of dibutyl ether (<0.1%) at low temperatures (<553 K)
was observed as by-products.
Intrinsic rate of ethanol conversion and apparent activation energy calculated for
different rare earth phosphate catalysts in ethanol dehydration at 458 K.
Catalyst
Intrinsic rate
Apparent activation
(×10−4 molethanol h−1 m−2
)
energy (kJ mol−1
)
LaPO4
NdPO4
SmPO4
GdPO4
0.48
0.57
1.34
1.45
121
125
120
119
The phosphates catalysts have been characterized using Raman
spectroscopy. Spectra of GdPO4 recorded at ambient temperature
are presented in Fig. 4. All the spectra were comparable except that
of NdPO4, which exhibited a broad band at 834 cm−1 and around
4200 cm−1. These bands have already been observed specifically for
NdPO4 and attributed to electron–phonon interactions taking place
when laser wavelength was 514.5 nm [18]. P–O stretching modes
were observed in the 1100–900 cm−1 region. Seven or eight phos-
for similar compounds [15,16]. Although the corresponding data is
not shown in the present paper, we report that transmission elec-
tron microscopy analysis revealed comparable textures for all of
the solids with nanorods shape particles having similar lengths and
thicknesses [17].
phate bending modes were observed between 400 and 642 cm−1
,
the asymmetric ones been over 500 cm−1 [19]. Typical values for
Ln–O stretching vibrations were around 300 cm−1 in the region of
lattice vibrations and the latters were not evidenced. The frequency
of the band at 3480 cm−1 in the Raman spectra was due to dissoci-
ated adsorbed water on the coordinatively unsaturated rare earth
cations [19]. Besides the bands corresponding to the Rhabdophane
structure two bands at 904-08 and 967-85 cm−1 were observed.
They have respectively been assigned to the P OH bond stretch-
ing mode of the symmetric P-(OH)2 vibration and to the stretching
vibration of the P O bond in adsorbed (H2PO4)− [13,20].
3.1. Catalytic activity
The rare earth phosphates were tested for ethanol and 1-butanol
dehydration in the temperature range between 683 K and 843 K.
The variations in ethanol and 1-butanol conversion and selectivity
to alkenes are shown as a function of temperature, in Figs. 2 and 3,
respectively.
Ethanol dehydration with these phosphates produced mainly
ethylene, and diethyl ether (DEE) was the only observed by-
product. With increasing reaction temperature, both ethanol
conversion and ethylene selectivity increased, until they reached
almost 100%. Only traces of acetaldehyde and diethyl ether were
observed, and the computed selectivity to these products was of
the order of 0.05%. The intrinsic ethanol conversion rates and the
apparent activation energies calculated at 548 K are provided in
Table 2. The catalysts’ activity, as well as their selectivity, increased
with increasing atomic number of the lanthanides. The appar-
ent activation energies of the phosphates were rather similar
(121 4 kJ mol−1), which tends to confirm that the same reaction
mechanism takes place on all the catalysts.
3.2. Acid base properties
To determine the acid–base features of the rare earth phos-
phates, the thermal desorption of 2,6-dimethylpyridine (DMP) and
CO2 was monitored using FTIR spectroscopy, and the thermo-
desorption profiles of ammonia were recorded. We initially studied
desorption of 2,6 dimethylpyridine (DMP) on the lanthanum phos-
phates. It was previously shown that acid sites on lanthanum
phosphates are rather weak, and that DMP (with pKBH+ = 6.75) was
suitable for the characterization of these sites [7]. The FTIR spec-
tra of DMP on all the studied phosphates are found to be similar;
an example is shown in Fig. 5, for the spectral range from 1700 to
The phosphates also appeared to be very efficient for 1-butanol
dehydration. 1-butene was favored at low temperatures (low con-
version), whereas 2-butene was favored at high temperatures (high
conversion). The ranking of the phosphates in terms of activity
was rather similar to that found for ethanol dehydration although
1400 cm−1
.
The DMP adsorbed on rare earth phosphates leads to bands at
1628 cm−1 and 1649 cm−1 which are attributed to adsorption on
Brønsted acid sites (DMPH+ species) [21,22], and to bands at 1458,
100
b
a
100
95
80
60
LaPO
LaPO
4
4
90
NdPO
NdPO
40
20
0
4
4
SmPO
GdPO
SmPO
GdPO
4
4
4
85
4
80
500
550
600
650
700
750
500
550
600
650
700
750
Temperature (K)
Temperature (K)
Fig. 2. Evolution of ethanol conversion (a) and ethylene selectivity (b) on the different catalysts as
W/F = 19.8 gcata h mol−1, N2: 60 mL min−1
a function of temperature. Test conditions: mcata: 50 mg,
.
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b
100
80
60
40
20
0
80
a
60
LaPO
4
LaPO
4
40
20
0
NdPO
4
NdPO
4
SmPO
GdPO
4
4
SmPO
GdPO
4
4
480
520
560
600
640
480
520
560
600
640
680
Temperature (K)
Temperature (K)
Fig. 3. Evolution of 1-butanol conversion (a) and selectivity to 2-butene (b) on the rare earth phosphates with Rhabdophane structure and Al2O3 as a function of temperature.
Test conditions: mcata: 101 mg, W/F = 31.2 gcata h mol−1, N2: 100 mL min−1
.
Fig. 4. Raman spectrum of GdPO4.
The variation in intensity of the peaks at 1608 and 1649 cm−1
,
corresponding to DMP adsorbed on Lewis and Brønsted sites,
respectively, were monitored as a function of temperature (Fig. 6).
The strength of all bands ascribed to DMP adsorbed on Lewis and
Brønsted acid sites decreased with the desorption temperature.
The number of Brønsted acid sites detected through the use of
DMP on the phosphates varied in the following manner: at low
temperatures, Sm > Gd > Nd > La; at high temperatures, compara-
ble values were observed for Sm, Gd and Nd, and a lower value
for La. Concerning the Lewis sites, the order was: at low tempera-
tures, Gd > Nd > La > Sm; at high temperatures the values were again
comparable for Sm, Gd and Nd, but lower for La.
FTIR spectroscopy of CO2 adsorption was used to reveal the
nature of the basic sites of the studied catalysts. Small bands at
1384 cm−1, which appeared after evacuation at 298 K under vac-
uum but not after out-gassing at higher temperature could be
attributed to the formation of monodentate carbonate. Meanwhile
smaller bands around 1620 and 1320–1350 cm−1 still appeared
after out gassing at 373 K and could be assigned to surface bicarbon-
ate species [25, 26]. Another peak was observed at approximately
2531 cm−1 and has been attributed to CO2 adsorption on Lewis acid
sites [22]. All the peaks of CO2 adsorption on these solids were very
small and disappeared rapidly with increasing temperature, thus
showing that the studied rare earth phosphates possess only a small
number of weak basic OH sites.
Fig. 5. Infrared spectra of pyridine adsorbed on GdPO4 after desorption under vac-
uum at: 1: 293 K; 2: 323 K; 3: 373 K, 4: 423 K; 5: 473; 6: 523 K.
1477, 1581 and 1608 cm−1 [22,23], characterizing DMP associated
with Lewis acid sites [24]. H-bonded DMP could also contribute to
the bands observed at 1602 cm−1 and 1581 cm−1 [21]. The progres-
sive desorption of DMP at increasingly high temperatures leads to
a decrease in the intensity of all bands, and a shift of the bands
towards higher wavenumbers. The latter effect was attributed
to lateral interactions between adsorbed species, resulting from
a decrease in electron enrichment of the catalyst surfaces with
desorption of DMP.
The temperature-programmed desorption of NH3 was studied
between 373 and 823 K. The profiles of NH3-TPD on rare earth
phosphate catalysts are shown in Fig. 7. In order to analyze the
desorption data, four desorption temperature ranges: 373–423 K,
423–523 K, 523–673 K and 673–823 K, attributed to very weak,
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Fig. 6. Normalized adsortion peak intensities at 1649 cm−1 (a) and at 1608 cm−1 (b) of different rare earth phosphates as a function of temperature.
Table 3
Results of NH3-TPD on the rare earth phosphate catalysts.
Catalyst
NH3 desorption (␮mol m−2
)
373–423 K (1)
423–523 K (2)
523–673 K (3)
673–823 K (4)
(3 + 4)
LaPO4
NdPO4
SmPO4
GdPO4
1.2
2.1
2.2
1.9
6.5
7.9
4.9
4.8
3.7
4.3
5.6
5.6
1.1
1.2
1.7
1.9
4.8
5.5
7.3
7.5
catalytic activity of these catalysts was related to the presence of an
excess of phosphorus at their surface as sub-surface (HnPO4)−3+n
species (n = 1 or 2) leading to the creation of Brønsted acid sites,
which were found to be the most efficient [7]. The high selectivity
of these catalysts for alcohol dehydration was related to the uni-
form distribution of active (HnPO4)−3+n acid sites and to the small
number of weak basic sites. The results of this study show that
other rare earth orthophosphates with a Rhabdophane structure
are also active, and selective catalysts for light alcohol dehydration.
Indeed, these catalysts appear to be even more active and selective
for ethanol and 1-butanol dehydration than LaPO4 catalysts. No
relationship was observed between their activity and selectivity
and their surface composition, since the phosphates did not have
the same surface phosphorus excess as the active LaPO4 catalysts.
However, the catalysts’ activity is likely to increase with increasing
atomic number of the lanthanide. In this respect, investigations to
reveal possible general trends in this peculiar family of catalysts
could be very useful, in particular for the identification of possible
correlations between their acid–base and catalytic properties.
The acid–base properties of the rare earth phosphate catalysts
analyzed in the present study show that these catalysts contain
mainly weak and medium strength acid sites. This finding is in
good agreement with the results published in a previous study, in
which such catalysts were used in cumene cracking and the skele-
tal isomerization of cyclohexene to methyl cyclopentenes [27,28].
Although the lanthanide phosphates are inactive for cumene crack-
ing, which requires strong Brønsted acid sites, they have sufficiently
strong acid centers to catalyze the skeletal isomerization of cyclo-
hexene to methyl cyclopentenes. The above reaction can proceed
on strong Brønsted and Lewis acid sites, however in the case of the
phosphates studied in [28], only Lewis centers were reported to be
involved. More specifically, the phosphates were found to exhibit
Lewis acidity, but did not have sufficiently strong Brønsted acid
sites to convert pyridine into pyridinium ions, or to catalyze the
cracking of hydrocarbons.
LaPO
4
NdPO
4
SmPO4
GdPO4
300 400 500 600 700 800 900
Temperature (K)
Fig. 7. NH3-TPD patterns of the different rare earth phosphates catalysts.
medium strength and strong acid sites respectively, were con-
sidered. The quantities of surface-area-normalized desorbed NH3,
which are related to the density of acid sites on the catalyst, are
shown in Table 3.
The lanthanum phosphates are found to have a smaller number
of strong and medium strong acid sites than the other rare earth
phosphates, which is in agreement with results of the FTIR study,
and the strength of these acid sites appears to increase with the
atomic number of the lanthanide. Furthermore, NH3-TPD appears
to reveal weak acid sites that are not detected by FTIR-monitored
DMP adsorption. Conversely, the strength of these weak acid sites
appears to decrease with the atomic number of the lanthanide.
4. Discussion
The La, Nd, Sm and Gd rare earth orthophosphates with a
Rhabdophane-type structure are very efficient in the dehydration
of light alcohols. These orthophosphates appear to be extremely
selective to the corresponding alkenes, even at total conversion.
Furthermore, they are characterized by a relatively high stability
under the tested reaction conditions. However, in order to develop
potential industrial applications for these catalysts, it is important
to understand the origin of their properties.
Before trying to address the nature of the acid sites and their
catalytic role, it is important to note that all of the rare earth
phosphates catalysts tested in the present study showed a high
selectivity to the 2-butene isomer when 1-butanol was dehydrated.
This suggests that the, type mechanism, in which acid sites only are
involved, is the main catalytic mechanism. In the E1 mechanism,
In a recent study of LaPO4 catalysts prepared using various
different techniques, the present authors have shown that the
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3490
6
1.17
1.14
1.11
1.08
1.05
1.02
3480
3470
3460
Fig. 8. Chemical structure of M(OTf)3(hmpa)4.
3450
-0.8
-0.6
-0.4
-0.2
0.0
0.2
the first step of dehydration is the formation of a carbocation by
abstraction of an OH group and then its decomposition (loss of H
in ␤ position). This mechanism occurs with acidic catalysts with a
relatively large scale of acid strength. An isomerization can occur at
the carbocation stage and the formation of 2-butene isomers from
1-butanol is indicative of E1 mechanism [29]. With that respect the
increase in selectivity to 2-butene to the depends of 1-butene with
increasing temperature (Fig. 3) could be related to the predomi-
nance of E1 mechanism over E2 mechanism as previously reported
[30,31]. An E2 like mechanism would lead to the formation of 1-
butene. However, 1-butene formed with a E2 type mechanismcould
be isomerized in a second time which could account for the forma-
tion of 2-butenes on the phosphates catalysts [32]. This hypothesis
cannot be excluded but is relatively improbable because isomer-
ization reactions require temperatures higher than that used for
dehydration reaction and because butenes would hardly compete
with 1-butanol, which is much strongly adsorbed on active sites
[30,33]. The acid sites involved in an E1 mechanism can be either
of the Brønsted or Lewis type. Before discussing which type of site
may be preferentially involved, it might by questioned if the pre-
eminence of this particular reaction mechanism on the rare earth
phosphates arises first from the fact that they have a small number
of only weak basic sites, as demonstrated by CO2 adsorption.
When the rare earth elements are examined across the peri-
odic table, the size of their ions decreases monotonically, from
lanthanum to lutetium. Both the crystal structure and physical
properties of the lanthanide compounds vary gradually across the
series, as a result of lanthanide contraction. In the present study,
this is shown to be the case for Rhabdophane structure cell param-
eters (Fig. 1), and our study of the acid–base properties of rare earth
phosphates using FTIR-monitored DMP adsorption shows that both
the Brønsted and Lewis acidity of the lanthanide series increases
across the periodic table.
The Lewis acidity of different trivalent lanthanide com-
plexes was evaluated several years ago on the basis of their
competitive dissociation, observed using tandem mass spec-
trometry [34]. These complexes were trifluoromethanesulfonate
with hexamethyl-phosphoramide ligands (M(OTf)3(hmpa)4, with
M = RE and hmpa = hexamethylphosphoramide) (Fig. 8). When a
triflate anion (OTf) is lost, a fluoride ion remains attached to the
central atom. This fluoride retention has been ascribed to the strong
affinity of the rare earth element for fluoride, and it was previously
reported that the thermal decomposition of Ln(OTf)3 produces LnF3
[35]. The overall fragmentation scheme of the studied complexes
involves the competitive loss of (OTf-F) and the loss of a ligand such
as hmpa, connected to the central atom. Any difference in the ratio
of the two fragmented ions reflects the electronic properties of the
lanthanide, in terms of the metal’s affinity for oxygen or fluorine,
which would correspond to its Lewis acidity, rather than to any
steric contribution.
ln{[MF(OTf)(hmpa)
]+/[M(OTf) (hmpa)]+}
2
2
Fig. 9. Variation of the cationic size for the La, Nd, Sm and Gd lanthanides and of
the vibration frequency of the RE-OH band in their Raman spectra as a function of
the Lewis acidity descriptor parameter.
Fig. 10. Variation of the rate of ethanol conversion as a function of the Lewis acid-
1
ity descriptor parameter; corresponds to LaPO4-M4 sample from ref [2] and is
characterized by a P/La = 1.07.
3480 cm−1 in the Raman spectra due to dissociated adsorbed water
on the coordinatively unsaturated rare earth cations as a function of
the Lewis acidity descriptor [34,36]. The cationic size and the band
frequency should respectively decrease and increase with Lewis
acidity. These trends are indeed observed, and good correlations
are obtained.
The rate of ethanol conversion measured at 458 K was thus
recorded as a function of the Lewis acidity descriptor, as shown
in Fig. 10. It can be seen that a direct correlation is obtained for Nd,
Sm and Gd only. Although the LaPO4 sample used for the study
was prepared using the same method as the other phosphates,
unlike the latter compounds, the lanthanum catalyst was affected
by an excess of phosphorus on its surface. We have shown that the
supra-surface (HnPO4)−3+n species on LaPO4 generates Brønsted
acid species, which remain active during the reaction [7]. In Fig. 10
a data point has thus been added, corresponding to a LaPO4 sample
(LaPO4-M4 in ref. [2]) prepared in a different manner, but exhibiting
a comparable P/RE surface ratio P/RE = 1.07). When this data point
is taken into account, a good correlation is observed. Such correla-
tion was not expected since generally Brønsted acid sites with weak
or moderate strength are proposed as active sites for light alcohol
dehydration [2,37,38].
For this particular case it was of interest to analyze the results
of the TPD-NH3 study. This revealed that there is an increase in
the strength of the stronger acid sites, whereas that of the weaker
acid sites decreases, as the atomic number increases across the
lanthanide series. These weak acid sites can be attributed to sub-
surface (HnPO4)−3+n. These species interact with light alcohols,
whereas the rare earth phosphates play the role of a support
medium. The strength of this interaction depends on the Lewis
acidity of the support medium cations, and the stronger this
It was thus decided to investigate this Lewis acidity descriptor
in terms of its possible use as a probe for the analysis of catalytic
activity. Initially, the descriptor’s relevance was verified. For the
case of La, Nd, Sm and Gd phosphates, Fig. 9 illustrates the variation
in size of these rare earth cations, and the frequency of the band at
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acidity, the lower the Brønsted acidity of the supported acid species.
A similar feature was observed for heteropolyacid carried by differ-
ent support [39,40], and more recently for NbOx polymeric species
on NbZrO mixed oxide and silicotungstic acid species on ZrO2, used
in the dehydration of glycerol [41,42]. Conversely, the stronger acid
sites can be directly attributed to Lewis acid sites, corresponding
to rare earth cations with incomplete surface coordination. Unfor-
tunately, it is not possible to conclude that these stronger acid
sites correspond to Lewis acid sites only, since the DMP adsorp-
tion study showed that strong Brønsted acid sites are also present
on rare earth phosphates, the strength of which increases with the
atomic number of the rare earth contrarily to that of the sub-surface
(HnPO4)−3+n species. These stronger Brønsted acid sites cannot thus
be disregarded as potentially active sites. These Brønsted sites do
not correspond to sub-surface (HnPO4)−3+n species. In addition,
they should not correspond to sites generated by the decompo-
sition of water on Lewis acid sites, since in the present study they
were observed on samples which had been pretreated for a long
time at 573 K under high vacuum, prior to their initial DMP adsorp-
tion for FTIR analysis. It could be hypothesized that these sites
correspond to surface POH species surrounded by four rare earth
cations. If this were correct, the stronger rare earth Lewis acid sites
would imply the presence of stronger Brønsted POH acid sites.
It is important to recall that Lewis acid sites may intervene either
directly as active sites, or after being converted to Brønsted acid
sites under catalytic reaction conditions in the presence of large
amounts of water, as in the case of dehydration reaction condi-
tions. Under such conditions, only density-functional theory (DFT)
calculations and deeper in situ surface site characterization would
contribute to improved site identification.
Finally, the foregoing discussion shows that, among the lan-
thanide phosphates having the Rhabdophane structure, the most
suitable catalyst is GdPO4. If rare earth phosphates with higher
Lewis acidities exist, they do not preferentially crystallize with the
same structure and their catalytic properties are probably quite
different.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.apcata.
2014.12.026.
References
[1] F. Cherubini, Energy Convers. Manage. 51 (2010) 1412–1421.
[2] J.C. Védrine, J. Inorg Phosph Chem, Phosph. Res. Bull. 10 (1999) 37–48.
[3] M. Ziyad, S. Arsalane, M. Kacimi, G. Coudurier, J.M.M. Millet, J.C. Vedrine, Appl.
Catal. A: Gen. 147 (1996) 363–373.
[4] A. Ouqour, G. Coudurier, J.C. Vedrine, J. Chem Soc, Faraday Trans. 89 (1993)
3151–3155.
[6] J.C. Vedrine, J.M.M. Millet, J.C. Volta, Catal. Today 32 (1996) 115–123.
[7] T.T.N. Nguyen, V. Ruaux, C. Lorentz, L. Massin, P. Affanasiev, F. Maugé, V.
Bellière-Baca, P. Rey, J.M.M. Millet, Applied Catalysis B: Environmental 166-167
(2015) 432–444.
[8] K. Ramesh, Y.L.E. Goh, C.G. Gwie, C. Jie, T.J. White, A. Borgna, J. Porous Mater. 19
(2012) 423–431.
[9] K. Ramesh, J.E. Zheng, E. Goh Yi Ling, Yi-Fan Han, A. Borgna, J. Phys. Chem. C
113 (2009) 16530–16537.
[10] M. Anbia, M.K. Rofouel, S.W. Husain, Chin. J. Chem. 24 (2006) 1026–1030.
[11] B. Savova, S. Loridant, D. Filkova, J.M.M. Millet, Appl. Catal. A: Gen. 390 (2010)
148–157.
[12] R.C.L. Mooney, J. Chem. Phys. 16 (1948) 1003.
[13] R.D. Shannon, Acta Cryst. A32 (1976) 751–767.
[14] F. Geng, Y. Matsushita, R. Ma, H. Xin, M. Tanaka, F. Izumi, N. Iyi, T. Sasaki, J. Am.
Chem. Soc. 130 (2008) 16344–16350.
[15] B. Glorieux, M. Matecki, F. Fayon, J.P. Coutures, S. Palau, A. Douy, G. Peraudeau,
J. Nucl. Mater. 326 (2004) 156–162.
[16] O. Terra, N. Clavier, N. Dacheux, R. Podor, New J. Chem. 27 (2003) 957–967.
[17] T.T.N. Nguyen, PhD Thesis, Claude-Bernard University Lyon1 143-2013.
[18] C.R. Patra, G. Alexandra, S. Patra, D.S. Jacob, A. Gedanken, A. Landau, Y. Gofer,
New J. Chem. 29 (2005) 733–739.
[19] H. Assaaoudi, A. Ennaciri, A. Rulmont, Vibrat. Spectr. 25 (2001) 81–90.
[20] G. Niaura, A.K. Gaigalas, V.L. Vilker, J. Phys. Chem. B 101 (1997) 9250–9262.
[21] P.A. Jacobs, C.F. Heylen, J. Catal. 34 (1974) 267–271.
[22] O.V. Manoilova, S.G. Podkolzin, B. Tope, J.A. Lercher, E.E. Stangland, J.M. Goupil,
B.M. Weckhuysen, J. Phys. Chem. B 108 (2004) 15770–15781.
[23] C. Flego, I. Kiricsi, C. Perego, G. Bellussi, Catal. Lett. 35 (1995) 125–133.
[24] A.A. Hanna, S.M. Mousa, G.M. Elkomy, M.A. Sherief, Eur. J. Chem. 1 (2010)
211–215.
Finally, although it is not the main topic of the present study,
it should be noted that both the result of the competitive disso-
ciation of lanthanide complexes and the frequency of the band at
3480 cm−1 in the Raman spectra appear as good descriptors for the
Lewis acidity of rare-earth-phosphates, allowing comparison to be
made between different rare-earth-phosphates based catalysts.
5. Conclusions
Lanthanide phosphates provide unique possibilities for the
study of variations in acid–base and catalytic properties, across the
lanthanide series of the period table. These phosphates appear to be
very efficient and ultraselective in the dehydration of several light
alcohols. The comparison of acid–base and catalytic data shows that
the active sites correspond to reasonably strong acid sites, which
can be of Brønsted and Lewis types. Although the results of the char-
acterization reported here do not allow the two types of sites to be
discriminated, a good correlation is observed between the Lewis
acidity and the catalytic activity when no sub-surface (HnPO4)−3+n
species are present.
The strength of the acid sites has an influence not only on their
activity, but also on their selectivity, and stronger sites appeared
highly selective to 2-butene when 1-butanol is dehydrated. In such
a case, the carbocation or carbenium ions could be better stabilized
for the isomerization step. The absence of very strong acid sites
that are generally preferential sites for the formation of coke, could
explain both their high selectivity, associated with the absence of
degradation by-products, and their relative stability. If the E1 mech-
anism is the main reaction mechanism for 1-butanol dehydration,
in which basic sites play no role, other types of mechanism may
still intervene.
[25] H.J. Freund, M.W. Roberts, Surf. Sci. Rep. 25 (1996) 225–273.
[26] M. Melle-Franco, G. Pacchioni, A.V. Chadwick, Surf. Sci. 478 (2001) 25–34.
[27] M. Stranz, W. Gut, R. Fiedorow, Ryszard Prace Naukowe Akademii Eko-
nomicznej 338 (1986) 199–207.
[28] M. Stranz, R. Fiedorow, W. Przystajko, From Seria Chemia (Uniwersytet
im. Adama Mickiewicza
w Poznaniu) (1985), 48 (Wybrane Zagadnienia
Fizykochem. Ukladow Homo-Heterog. 2) p. 91.
[29] S. Toksarenko, L.V. Malysheva Kinet. i Katal, 24 (1983) 877–896.
[30] H. Knözinger, A. Scheglila, J. Catal. 17 (1970) 163–252.
[31] H. Noller, P. Andreu, M. Hunger, Angew. Chem. Int. Ed. 3 (1971) 172–181.
[32] O. Olaofe, P.L. Yue, Collection Czech. Chem. Commun. 50 (1985) 1834–1841.
[33] H. Noller, P. Andreu, M. Hunger, Angew. Chem. Int. Ed. 3 (1971) 172–181.
[34] H. Tsuruta, K. Yamaguchi, T. Imamoto, Chem. Commun. (1999) 1703–1704.
[35] N. Yanagihara, S. Nakamura, M. Nakayama, Chem. Lett. 7 (1995) 555–556.
[36] C. Monterra, J. Chem. Soc. Faraday Trans. 84 (1988) 1617–1629.
[37] C.D. Baertsch, K.T. Komala, Y.-H. Chua, E. Iglesia, J. Catal. 205 (2002) 44–57.
[38] P. Lauriol-Garbey, G. Postole, S. Loridant, A. Auroux, V. Belliere-Baca, P. Rey,
J.M.M. Millet, Appl. Catal. B: Environ. 106 (2011) 94–102.
[39] H. Atia, U. Armbruster, A. Martin, J. Catal. 258 (2008) 71–82.
[40] Y. Wu, X. Ye, X. Yang, X. Wang, W. Chu, Y. Hu, Ind. Eng. Chem. Res. 35 (1996)
2546–2560.
[41] P. Lauriol-Garbey, J.M.M. Millet, S. Loridant, V. Bellière-Baca, P. Rey, J. Catal. 281
(2011) 68–76.
[42] B. Katryniok, S. Paul, M. Capron, C. Lancelot, V. Belliere-Baca, P. Rey, F.
Dumeignil, Green Chem. 12 (2010) 1922–1925.
Please cite this article in press as: T.T.N. Nguyen, et al., Appl. Catal. A: Gen. (2014), http://dx.doi.org/10.1016/j.apcata.2014.12.026