Conversion of cis-2-butene-1,4-diol to hydrofurans on Pd/SiO2 and Pt/SiO2 catalysts under mild conditions: A FT-IR study

Article abstract of DOI:10.1016/j.molcata.2010.05.011

The gas-phase interaction of cis-2-butene-1,4-diol (C-2-OL) with silica supported Pd and Pt nanoparticles has been studied using infrared spectroscopy at room temperature. Samples were prepared according to standard recipes and characterized through TEM and the IR spectroscopy of adsorbed carbon monoxide at room temperature. Whereas the reaction with hydrogen of C-2-OL in ethanol solutions yields hydrogenated products through a complex network, and no reaction takes place in the absence of hydrogen, in the present case C-2-OL is converted first to 2,5-dihydrofuran (2,5-DHF) and water. This step occurs on the silica support at room temperature, probably through the intermediacy of the surface species -Si-O-CH₂-CHCH-CH₂-OH. The presence of Pd nanoparticles allows, at ca. 100 °C, the subsequent isomerization of 2,5-DHF to 2,3-dihydrofuran (2,3-DHF). The same reaction requires a higher temperature (ca. 150 °C) with the Pt catalyst. At higher temperatures, the expected dehydrogenation of both 2,5- and 2,3-DHFs to furan takes place. On the support alone at 350 °C 2,5-DHF is converted to furan, and 2,3-DHF to carbonyl-containing species (as suggested in the literature), through thermal reactions.

Full text of DOI:10.1016/j.molcata.2010.05.011

Journal of Molecular Catalysis A: Chemical 328 (2010) 27–34
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Conversion of cis-2-butene-1,4-diol to hydrofurans on Pd/SiO and Pt/SiO
2
2
catalysts under mild conditions: A FT-IR study
a
a
b
Francesco Mauriello , Edoardo Garrone , Maria Grazia Musolino ,
b
a,∗
Rosario Pietropaolo , Barbara Onida
a
Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi, 24, I-10129 Torino, Italy
Dipartimento di Meccanica e Materiali, Facoltà di Ingegneria, Università Mediterranea di Reggio Calabria, Loc. Feo di Vito, I-89122 Reggio Calabria, Italy
b
a r t i c l e i n f o
a b s t r a c t
Article history:
The gas-phase interaction of cis-2-butene-1,4-diol (C-2-OL) with silica supported Pd and Pt nanoparticles
has been studied using infrared spectroscopy at room temperature. Samples were prepared according to
standard recipes and characterized through TEM and the IR spectroscopy of adsorbed carbon monoxide
at room temperature. Whereas the reaction with hydrogen of C-2-OL in ethanol solutions yields hydro-
genated products through a complex network, and no reaction takes place in the absence of hydrogen,
in the present case C-2-OL is converted first to 2,5-dihydrofuran (2,5-DHF) and water. This step occurs
Received 24 March 2010
Accepted 18 May 2010
Available online 4 June 2010
Keywords:
FT-IR spectroscopy
Supported palladium and platinum
catalysts
Surface characterization
Cyclization and isomerization reaction
on the silica support at room temperature, probably through the intermediacy of the surface species
◦
–
Si–O–CH2–CH CH–CH2–OH. The presence of Pd nanoparticles allows, at ca. 100 C, the subsequent iso-
merization of 2,5-DHF to 2,3-dihydrofuran (2,3-DHF). The same reaction requires a higher temperature
◦
(ca. 150 C) with the Pt catalyst. At higher temperatures, the expected dehydrogenation of both 2,5- and
◦
2
,3-DHFs to furan takes place. On the support alone at 350 C 2,5-DHF is converted to furan, and 2,3-DHF
to carbonyl-containing species (as suggested in the literature), through thermal reactions.
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
co-workers have studied in detail the infrared spectra deriv-
ing from hydrocarbons such as ethene, propene, but-1-ene
and cis/trans-but-2-ene chemisorbed on oxide supported metals
[6–8].
With the purpose of tailoring a catalyst effective in the pro-
duction of 2-hydroxytetrahydrofuran, some of the authors have
studied in the past the behaviour of palladium and platinum
nanoparticles on different supports (SiO , Al O , TiO , ZrO , MgO
With the aim of investigating by means of infrared spectroscopy
the mechanism in Scheme 1, a systematic study was undertaken of
the gas-phase interaction of C-2-OL with the Pd/SiO2 and Pt/SiO2
catalysts. An entirely different reactivity was observed, involving
the formation of both 2,5- and 2,3-dihydrofuran (DHF), which is the
subject of the present paper. Hence, we report here a spectroscopic
study focusing our attention on the overall reactivity and on the
nature of the species adsorbed on the catalyst surface. It has to
be noted that, at variance with catalytic studies, the measurements
reported in the present paper have been run in a closed system. This
means, inter alia, that instead of carrying out a complete reaction,
the various steps in the observed mechanism have been studied
individually.
2
2
3
2
2
and ZnO) in the liquid-phase hydrogenation of cis-2-butene-1,4-
diol (hereafter C-2-OL), as it concerns the effect on the catalytic
activity and products distribution [1–3] of parameters such as the
acid–base properties of the support, the metal particle size and the
partial hydrogen pressure. In ethanol solutions, a complex reac-
tion network is observed for C-2-OL (Scheme 1) [1–3], involving
cis/trans isomerization, hydrogenation, and ring closure. In partic-
ular, 2-hydroxytetrahydrofuran is formed by internal cyclization
of ␣-hydroxybutyraldehyde (formed by double bond isomeriza-
tion of the starting material), whereas hydrogenolysis of the C–OH
bond leads to cis-crotyl alcohol and, finally, to butyraldehyde and
n-butanol.
Data are reported basically for the Pd system, those with Pt being
used mainly as a reference.
On the other hand, the infrared studies of alkenes adsorbed
on metals have attracted since the 80s much interest because
they afford information on the structures of adsorbed species
under various conditions [4,5]. In particular, Sheppard and
2. Experimental
2.1. Materials
∗ Corresponding author. Tel.: +39 0115644631.
E-mail address: barbara.onida@polito.it (B. Onida).
C-2-OL (impurity of trans isomer 5%), 2,5-DHF, 2,3-DHF, furan
and SiO2 (BET specific surface area = 500 m /g) were commercial
2
1
381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.05.011

2
8
F. Mauriello et al. / Journal of Molecular Catalysis A: Chemical 328 (2010) 27–34
Scheme 1. Overall reaction mechanism concerning the hydrogenation of C-2-OL over Pd and Pt silica catalysts in ethanol solution.
products from Sigma–Aldrich, which were used without further
purification.
FT-IR spectra are reported as difference spectra from the spec-
trum of disc before adsorption and, where necessary, from the
spectrum of any vapour phase present.
The preparation of the impregnated 0.5% Pd/SiO2 and 2.5%
Pt/SiO catalysts has been described in detail elsewhere [1–3].
2
3. Results and discussion
2.2. Measurements
3.1. TEM characterization of the catalysts
TEM analysis was performed with a Philips instrument Model
CM12 operating at 120 kV and directly interfaced with a computer
for real-time image processing.
Fig. 1 illustrates the TEM characterization of the samples Pd/SiO2
and Pt/SiO2 catalysts reduced in H2 at 200 C.
◦
For FT-IR measurements, powder samples were pressed into
thin self-supporting wafers. Spectra were collected at a resolu-
tion of 2 cm on a Bruker FT-IR Equinox 55 spectrophotometer
equipped with MCT detector.
Two representative pictures of the samples are reported,
together with the corresponding particle size distribution. Palla-
dium particles have a nearly spherical shape with a diameter of
about 4.5 nm, whereas platinum particles are bigger (about 40 nm
in diameter) and their distribution is less homogeneous. From the
data in Fig. 1, it can be desumed that the specific surface exposed
−1
Pre-treatments were carried out using a standard vacuum
frame in an IR cell equipped with a KBr window. In order to
be as close as possible to the experimental conditions used in
the catalytic liquid-phase experiments, in the activation step the
2
by the metal particles is about 1.1 and 0.07 m /g, respectively.
◦
catalysts were treated at 200 C for 2 h under flowing hydro-
nsꢀd2
nsꢁPd/Pt(ꢀ/6)d3
6
SSAs =
=
(1)
gen and finally cooled to room temperature under inert gas.
ꢁPd/Ptd
When appropriate, the activation temperature was raised to
◦
3
00 C.
The geometrical specific surface areas were calculated according to
Eq. (1): where ꢁ is the particles density of Pd and Pt nanoparticle and
d is particle average dimension as determined by TEM micrographs.
Adsorption of vapours was carried out by connecting the IR cell
to a vacuum frame (residual pressure <10 mbar). C-2-OL and all
other molecules were adsorbed at room temperature.
−3

F. Mauriello et al. / Journal of Molecular Catalysis A: Chemical 328 (2010) 27–34
29
Fig. 1. TEM images and particle size distributions of Pd/SiO2 (above) and Pt/SiO2 catalysts (below).
3
.2. FT-IR characterization of the naked sample and CO
and to the same vibrational mode of silanols interacting each other
via H-bonding [9,10]. The other bands can be attributed to car-
boneceous and water impurities probably not totally removed by
adsorption
◦
Fig. 2 reports the FT-IR spectrum of the Pd/SiO catalyst out-
outgassing at 200 C.
2
◦
−1
gassed at 200 C in the 3800–1400 cm range. Two absorptions are
seen in the OH stretching region: a sharp band at 3745 cm and a
broad ill-defined envelope at ca. 3500 cm . These bands are due
respectively to the ꢂOH vibration of free silanol groups (3745 cm
The spectra of the sample does not appreciably differ from that
of a pure silica support treated in the same way, in agreement with
the low metal load.
Carbon monoxide was used at room temperature as a probe
molecule for metal sites. Fig. 3 reports the FT-IR spectra of CO
−
1
−1
−
1
)
chemisorbed at room temperature on the Pd/SiO catalyst in the
2
−
1
range 2200–1800 cm . Spectra similar to those in Fig. 3 have
been reported in the literature [11–16]: an overall agreement is
observed.
As to the assignment, three types of CO species chemisorbed on
palladium are possible:
-
-
-
linearly adsorbed on-top sites (∼2130–2000 cm−1).
bridge-bonded on (1 0 0) and (1 0 1) faces (∼2000–1870 cm−1)
−
1
multibonded on (1 0 0) and (1 0 1) faces (∼1880–1800 cm ).
Basically, the observed bands – not seen on pure silica
treated in the same conditions (Fig. S1 Supplementary Mate-
−
1
rial) – are attributed to the CO (∼2080 cm ) linearly adsorbed
on coordinatively-unsaturated Pd sites such us those at corners
and edges of crystallites; and to the 2-fold bridged Pd-carbonyls
−
1
(
∼1930 cm ) on planar surfaces, such as the (1 0 1) and (1 0 0)
faces. The absence of 3-fold bridged species is ascribed to the fact
that at the CO pressures adopted coverage is high enough to force
the 3-fold bridged to convert into 2-fold geometry.
◦
Fig. 2. IR spectrum of the Pd/SiO2 catalyst outgassed at 200 C.

3
0
F. Mauriello et al. / Journal of Molecular Catalysis A: Chemical 328 (2010) 27–34
−
2
equilibrium pressure (0.5 × 10
to 10 mbar) of C-2-OL at room
temperature. For clarity, the figure is divided into two regions (A
and B).
Perturbation of the ꢂOH band of surface silanols occurs during
adsorption (section A), because of H-bonding interactions. Indeed,
the band envelope due to free O–H stretches is observed to decrease
(negative band), whereas a band grows at lower frequencies, more
intense and broader. In the negative band of Fig. 4 two compo-
−
1
nents at 3745 and 3715 cm may be identified, due respectively
to the stretching mode of free silanols and to the vibration of the
H-acceptor hydroxyl in a SiOH pair freely interacting (the corre-
−
1
sponding absorption of the H-acceptor silanol is at ca. 3500 cm ).
Also in the growing positive band envelope two components
−
1
are seen, at ca. 3320 and 3200 cm , together with an isosbestic
−
1
point at 3690 cm . The presence of two components in the enve-
lope of H-bonded silanols is not due to the presence of the 3745
−
1
and 3715 cm
bands, which are associated with silanol species
of marginally different acidic strengths. They could either be rep-
resentative of a dual mode of interaction of the C-2-OL molecule
Fig. 3. FT-IR spectra of CO adsorbed at increasing equilibrium pressure
(e.g., through the double bond or the basic hydroxyl oxygen) or be
(
0.01–1 mbar) at room temperature on the Pd/SiO2 catalyst.
related to the presence of two adsorbed molecules, arising from the
fact that C-2-OL has reacted on the surface.
The shift in the peak position with coverage is due to the
electronic repulsion between adsorbed species (induced hetero-
geneity) [17].
This latter hypothesis is confirmed by the results reported in sec-
tion B of Fig. 4. None of the IR modes of C-2-OL is present (Table 1):
instead, IR bands observed are compatible with the presence of
2,5-DHF and water.
3
.3. C-2-OL interaction with the Pd/SiO catalyst at room
2
temperature
For comparison, both water and 2,5-DHF were separately
adsorbed at room temperature on the Pd catalyst. The lower sec-
tion of Fig. 4 reports the corresponding difference spectra. Spectra
are dominated by the bands characteristics of water molecularly
The upper section of Fig. 4 illustrates the changes in the FT-IR
spectra of the Pd/SiO2 sample due to the contact with increasing
−
1
−1
Fig. 4. Upper part: FT-IR difference spectra of C-2-OL adsorbed at room temperature on the Pd/SiO2 catalyst in the range 3800–2600 cm (section A) and 1800–1300 cm
(
8
section B). Lower part: FT-IR difference spectra related to the adsorption on Pd/SiO2 catalyst of H2O (dashed line) and 2,5-DHF (dotted line) at the equilibrium pressure of
.3 and 9.1 mbar, respectively. (Section A) 3800–2600 cm range; (section B) 1800–1300 cm range.
−
1
−
1

F. Mauriello et al. / Journal of Molecular Catalysis A: Chemical 328 (2010) 27–34
31
Table 1
esters of silicic acid [21], the following scheme can be envisaged:
SiOH + HO–CH –CH CH–CH –OH
Observed vibrational frequencies (cm−1) of 2,5-DHF, 2,3-DHF and furan adsorbed
on Pd/SiO2 catalysts.
2
2
Compound
Mode
Frequency
→
SiO–CH –CH CH–CH –OH + H O
2
2
2
2
,5-DHF
C–H str.
C–H2 str.
C–H2 str.
Asym. CH2 sciss.
Sym. CH2 wag
Asym. ꢃCC–H bend
2972
2910
2865
1484
1365
1351
SiO–CH –CH CH–CH –OH → SiOH + 2, 5-DHF.
2
2
A probable mechanism of the cyclodehydration reaction of C-2-
OL to 2,5-DHF on silica surface is shown in Scheme 2. This also
constitutes the first stage (a) in the reaction route described in
Scheme 3.
2
,3-DHF
C–H str.
C–H str.
C–H str.
C–H str.
C–H2 str.
C–H2 str.
3113
2972
2931
2903
2872
2868
1624
1480
1455
1375
The reactivity of C-2-OL in ethanol solution and in the gas-phase
thus appears radically different. This can be accounted for consider-
ing that in the former case, the diol alcoholic moieties are stabilized
by solvation interactions, so hindering the possible cyclization of
the molecule. In the absence of the solvent, the 1,4-OH groups are
free to form the more stable five member cyclic compound. More-
over, as said above, alcohols react on silica with the formation of
Si–O–R groups: it is therefore probable that the acidic silanols of
silica when in contact with an ethanolic solution are not available
for catalysing acidic reactions.
C
C str.
␣
␣
,␤-CH2 sciss.
,␤-CH2 sciss.
ꢃCC–H
Furan
C–H str.
C–H str.
3156
3131
1600
1489
1380
C
C
C
C str.
C str.
C str.
Another possible explanation is that the ethanol, reacting with
the silica through the hydrogen bonding interactions between the
surface silanol groups and the ethanol hydroxyl groups, deactivates
the SiO–H surface not permitting the cyclodehydration mechanism.
−
1
adsorbed on silica (3800–2600 cm
range and the conspicuous
3.4. Further reactions at higher temperatures
−1
band at 1630 cm , due to the bending mode). Spectral con-
tributions coming from the chemisorbed 2,5-DHF are relatively
weaker.
The spectra are completely consistent with that arising from the
adsorption of C-2-OL, so suggesting that the vapour-phase inter-
action of this molecule with the Pd/SiO2 catalysts give rise to the
Thermal treatments of the system as coming from the reaction
of C-2-OL, i.e. in the presence of a mixture water/2,5-DHF, brings
about the reaction of water with the metal nanoparticles, i.e. deac-
tivation of the catalyst.
For this reason, the further possible reactions have been fol-
formation of H O and 2,5-DHF species.
lowed on fresh specimens of the Pd and Pt catalysts after the
2
◦
In a parallel experiment, C-2-OL has been adsorbed on the sil-
ica support alone, and the same spectra as with the Pd catalyst
have been recorded. This is clear evidence that the reaction is a
dehydration of C-2-OL without any intervention of the metallic
centres, catalysed by the acidic surface of silica. It is therefore rea-
sonable to assume that 2,5-DHF is the observed product, because
no double bond migration has probably occurred. The bands
adsorption of the sole 2,5-DHF. Heating at 100 C on Pd/SiO cata-
2
lyst brings about the changes in the IR spectra of adsorbed species
described in Fig. 5 (section A). These indicate the conversion of
2,5-DHF (dotted curve) into 2,3-DHF (solid curve). Indeed, the C C
−
1
stretching band at 1624 cm , nearly inactive in the symmetric 2,5
molecule, has a substantial intensity in the 2,3 isomer because of the
asymmetry brought about by the nearby O atom. Simultaneously,
other bands typical of 2,3-DHF appear (Table 1). This reaction does
not take place on the support alone, as it is related to the presence
of metal sites (Scheme 3b).
−
1
observed at 1484, 1365 and 1351 cm
are therefore attributed,
respectively, to the CH2 scissor, symmetric CH2 wag and asym-
metric ꢃCC–H bend mode of the 2,5-dihydrofuran ring (Table 1)
[
18,19].
The cyclodehydration mechanism of C-2-OL to 2,5-DHF is
Such a reaction is expected on thermodynamic grounds,
because 2,3-DHF is stabler, at room temperature, than 2,5-DHF by
7.47 kJ/mol [22]. A probable mechanism consists in the abstraction
of one of the four H atom in position 1 or 4 (very reactive because
in alpha position to an oxygen atom), with formation of an allylic
intermediate.
Note the absence, in Fig. 5 (section A), of any band related to
either furan or THF. This fact is worth of note, because at variance
with literature reports. The isomerization of 2,5-DHF to 2,3-DHF
expected on thermodynamic grounds, because the related ꢃG0 is
slightly negative. Its mechanism, in the presence of either acidic
or basic sites, was recently studied using the AM1 semiempirical
method [20]. Such calculations show that conversion takes place
without any activation barrier.
In the present case the reaction probably occurs via a
chemisorbed species. As alcohols react on silica with formation of
Scheme 2. Proposed cyclodehydration reaction of C-2-OL to 2,5-DHF on silica surface.

3
2
F. Mauriello et al. / Journal of Molecular Catalysis A: Chemical 328 (2010) 27–34
Scheme 3. Reaction network for the gas-phase interaction of C-2-OL on Pd/SiO2: (a) cyclodehydration of to 2,5-DHF and H2O on SiO2; (b) selective isomerization of 2,5-DHF
to 2,3-DHF; (c) catalytic conversion of 2,5-DHF to furan and H2; (d) catalytic conversion of 2,3-DHF to furan and THF (e) thermal decomposition of 2,5-DHF to furan and H2;
(
f) thermal decomposition of 2,3-DHF to cyclopropanecarboxaldehyde.
over 1% Pd/SiO2 was recently reported by Monnier et al. [23]:
the selectivity to 2,3-DHF was approximately 70%, and equimolar
amounts of furan and tetrahydrofuran were also formed; the com-
plete isomerization of 2,5- to 2,3-DHF required addition of CO to
the reaction feedstream.
Hence, the population of coordinatively-unsaturated Pd sites
is significant (note the intensity of the related FT-IR band
centred at ca. 2080 cm ), thus favouring the selective iso-
merization of 2,5-DHF to 2,3-DHT at corner and edges of
crystallites.
−
1
The authors explain this phenomenon assuming that the for-
mation of furan and THF occurs on the basal (1 1 1) and/or (1 0 0)
planes of the Pd crystallites and that the adsorption of CO reduces
the size of surface Pd ensembles present on these facets, so
that disproportionation of 2,5-DHF to furan and THF is sup-
pressed.
In our case, in order to understand the complete isomeriza-
tion of 2,5-DHF to 2,3-DHF (without formation of furan and THF)
it is useful to remember the FT-IR characterization of the cata-
Fig. 5 (section B) reports the results of the isomerization reac-
tion of 2,5-DHF promoted by 2.5% Pt/SiO2 catalyst: together with
the bands related to 2,3-DHF (i.e. the intense of C C stretching
mode at 1624 cm ), other bands are seen due to species arising
from the catalytic conversion of 2,5-DHF to THF and furan. It is
probable that, in this case, the larger amount of platinum (2.5%)
and the larger size of particles (40 nm), exposing flat surfaces,
favours the disproportionation reaction of 2,5-DHF to furan and
THF.
−
1
lyst in Section 3.2: the adsorption of CO on Pd/SiO catalyst causes
It is worth of note that for Pt/SiO2 catalyst it was neces-
sary to rise the temperature up to 150 C, this result being in
agreement with the overall higher activity of supported Pd sys-
tems in olefin isomerization with respect to analogous Pt samples
[24–26].
2
1
−
◦
the growing of two bands at 2080 and 1930 cm related, respec-
tively, to coordinatively-unsaturated Pd sites (corners and edges of
crystallites) and (1 0 1) and (1 0 0) planar surfaces of Pd nanoparti-
cles.

F. Mauriello et al. / Journal of Molecular Catalysis A: Chemical 328 (2010) 27–34
33
◦
Fig. 5. (Section A) FT-IR spectra of 2,5-DHF adsorbed on Pd/SiO2 catalyst before (dotted line) and after (full line) heating at 100 C. (section B) FT-IR spectra of 2,5-DHF
◦
adsorbed on Pt/SiO2 catalyst before (dotted line) and after (full line) heating at 150 C.
As a final step, the reaction has been studied of both 2,5- and
,3-DHF upon heating at ca. 250 C (steps c and d of Scheme 3).
3.5. Adsorption modes of products and reactants
◦
2
This causes, in agreement with the literature [27], in both cases the
expected dehydrogenation of DHFs to furan, which was accompa-
nied in the case of 2,3-DHF with a partial disproportionation to both
furan and THF.
Molecules observed at the surface of catalysts in the present
work are basically 2,3-DHF, 2,5-DHF and furan. The question arises
what is the modality of interaction of all these species with the
surface, both as it concerns the silica part and the metallic portion.
Let us consider 2,3-DHF which has a fairly intense C C stretching
mode, so that the involvement of this functionality in the adsorp-
tion can be monitored: the other molecules probably show a similar
behaviour.
If the catalysts were deactivated by the presence of water, ther-
mal decomposition of the two DHFs is observed. In the literature
the product of thermal transformation of 2,3-DHF is identified as
cyclopropanecarboxaldehyde [28], a molecule with the same gross
composition C H O. In the case of 2,5-DHF the product of thermal
Fig. 6 shows the IR spectra concerning the adsorption of this
molecule on the Pd catalyst at room temperature and at increasing
4
6
reaction is furan [29,30] (steps e and f of Scheme 3).
Fig. 6. FT-IR difference spectra related to the adsorption of 2,3-DHF, at increasing equilibrium pressure, in the range of 3800–2600 (section A) and 1800–1300 cm−1 (section
B).

3
4
F. Mauriello et al. / Journal of Molecular Catalysis A: Chemical 328 (2010) 27–34
equilibrium pressures. Section A reports the O–H stretching region,
and section B that of C C stretching. In the former region, the two
been highlighted. Curiously, the IR technique affords information
on the overall reactivity and on the nature of the species adsorbed
on the silica support, but not on the metallic phases, because of the
metal/surface selection rule.
−1
bands at 3735 and 3715 cm (due to the isolated silanol and the
terminal species in a pair, respectively) decrease and a broad band
−1
at ca. 3390 cm increases, due these silanols species H-bonded to
−
1
2
,3-DHF molecules. The extent of the shift (ca. 350 cm ) is strongly
Acknowledgement
indicative of a coordination through the O atom. Indeed, this is the
order of magnitude of the shift imparted to the SiO–H stretching
mode by O-containing molecules like ethers [31].
Thanks are due to Regione Piemonte (projects B18 and E28) for
financial support.
As the double bond in 2,3-DHF is not directly engaged in the
interaction, its frequency coincides with the gas-phase molecule,
Appendix A. Supplementary data
−1
and is observed at 1624 cm (Fig. 6, section B). Also physisorbed
molecules contribute to this adsorption. A shoulder of the main
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molcata.2010.05.011.
−1
band is observed at 1616 cm . Both the main component and
the shoulder are also observed in the interaction of 2,3-DHF with
the support alone. Comparison with the literature [32] shows that
interaction of silanol species SiO–H with the olefinic double bond
References
−
1
is weak and shifts the C C stretching mode by only 8–10 cm . The
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−1
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action of a 2,3-DHF molecule via both the oxygen atom and the
double bond, favoured by the relatively high silanol population.
The question arises of the modality of adsorption on the metallic
particles. A molecule with a C–C double bond may interact with a
metal surface in various ways, by forming ␲-type adducts or dou-
bly ␴-coordinated species, to leave aside more complex ethylidenic
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related C–C vibration markedly decreases. In the present case, how-
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limited surface exhibited by the metallic phase, as indicated by the
TEM characterization of the catalysts.
[
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6
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4
. Conclusion
The comparison between the reactivity of the catalyst with that
of the support alone has enabled a fairly complete characterization
of the vapour-phase interaction of C-2-OL with silica supported Pd
and Pt nanoparticles. The reaction network reported in Scheme 3
envisages a few steps, all of which fairly expected, the ensemble of
which, however, constitutes an original picture. The reason why the
chemistry of C-2-OL in ethanol solutions is drastically different has
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