Tailoring the selectivity of glycerol oxidation by tuning the acid-base properties of Au catalysts

Article abstract of DOI:10.1039/c4cy01246a

Supported gold nanoparticles are very effective catalysts for the selective oxidation of glycerol which represents an important bio-derived feedstock. In this paper we report that the acid/base properties, especially the acid site density, of these catalysts are the key factor in tuning the selectivity. A range of supported AuPt catalysts have been prepared by sol immobilization using acidic (H-mordenite, SiO₂, MCM-41, and sulfated ZrO₂) and basic (NiO and MgO) oxides as supports. In particular, using MCM-41 as the support, a high selectivity to glyceraldehyde, an important labile intermediate, was found.

Full text of DOI:10.1039/c4cy01246a

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Tailoring the selectivity of glycerol oxidation by
tuning the acid–base properties of Au catalysts†
Cite this: DOI: 10.1039/c4cy01246a
a
a
bcg
Alberto Villa, Sebastiano Campisi, Khaled M. H. Mohammed,
cef
d
d
ef
Nikolaos Dimitratos, Floriana Vindigni, Maela Manzoli, Wilm Jones,
Michael Bowker, Graham J. Hutchings and Laura Prati*
ef
e
a
Supported gold nanoparticles are very effective catalysts for the selective oxidation of glycerol which
represents an important bio-derived feedstock. In this paper we report that the acid/base properties, espe-
cially the acid site density, of these catalysts are the key factor in tuning the selectivity. A range of
Received 25th September 2014,
Accepted 17th October 2014
supported AuPt catalysts have been prepared by sol immobilization using acidic (H-mordenite, SiO
1, and sulfated ZrO ) and basic (NiO and MgO) oxides as supports. In particular, using MCM-41 as the sup-
port, a high selectivity to glyceraldehyde, an important labile intermediate, was found.
2
, MCM-
DOI: 10.1039/c4cy01246a
4
2
www.rsc.org/catalysis
this problem. Recently it was found that the catalytic oxida-
tion of glycerol with gold catalysts can be carried out without
using a basic environment. In particular, by alloying Au
Introduction
5
Glycerol is a highly functionalized molecule, derived from
lipid transesterification in the production of biodiesel, which
is recognized as a promising chemical building block for
with Pt, it is possible to obtain an effective catalytic system
in terms of activity and selectivity even in the absence of
1
5a,b,f
the synthesis of fine chemicals. For instance, the selective
a base.
However, these studies clearly showed that the
oxidation of glycerol has been shown to produce valuable
support greatly affected the activity and the selectivity.
Indeed, by using an acidic support, H-mordenite, an enhance-
ment in both the activity and the selectivity to C3 products
occurs (glyceric and tartronic acid) compared to similar AuPt
particles supported on activated carbon and TiO2. More
recently, it has been shown that by using basic supports
such as MgO and hydrotalcite it is also possible to obtain
products such as glyceraldehyde (GLYALD), glyceric acid
2
(
GLYA), dihydroxyacetone (DHA) and tartronic acid (TA).
Supported noble metal nanoparticles are used as catalysts
for this reaction when molecular oxygen is employed as the
oxidant even though the nature of the metal and the experi-
mental conditions appear to be of primary importance in
determining the activity and directing the selectivity to the
5
a
a high selectivity to C products by carefully tuning the reac-
3
3
desired products. In particular gold nanoparticles are both
tion conditions, specifically by controlling the reaction
4
5b,e
active and selective for this reaction. However, the main
temperature.
From these studies it can be concluded
limitation of gold monometallic catalysts appears to be the
requirement for basic conditions. From an industrial point
of view working under non-basic conditions provides a great
advantage and numerous efforts have been devoted to solve
that a new generation of bifunctional gold-based catalysts
can be prepared, where the acid/base properties of the
support play a crucial role, affecting both activity and selectiv-
ity of the reaction.
4
The aim of the present study is to clarify the effect of
the acid site nature and strength (Lewis and/or Brønsted)
and acid site density on the activity and selectivity of Au–Pt
nanoparticles supported on acidic supports in base-free
glycerol oxidation.
a
Dipartimento di Chimica, Università degli Studi di Milano, via Golgi 19,
I-20133 Milano, Italy. E-mail: Laura.prati@unimi.it
b
School of Chemistry, Southampton University, SO171BJ, UK
c
University College London, 20 Gordon Street, WC1H 0AJ London, UK
d
Dipartimento di Chimica and NIS Centre of Excellence, Università di Torino,
Via P.Giuria 7, 10125 Torino, Italy
e
School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff,
Results and discussion
CF103AT, UK
f
UK Catalysis Hub, Research Complex at Harwell (RCaH), Rutherford Appleton
Four acidic oxides with different acid/base properties, i.e.
Laboratory, Harwell, Oxon, OX11 0FA, UK
g
H-mordenite, SiO , MCM-41, and sulfated ZrO (S-ZrO ), have
2
2
2
Chemistry Department, Faculty of Science, Sohag University, P. O. Box 82524,
been used as supports and compared to two basic supports
MgO and NiO). Bimetallic AuPt catalysts were prepared
by the sol immobilization technique through a two-step
Sohag, Egypt
(
†
Electronic supplementary information (ESI) available: Reaction profiles and
temperature effects on glycerol oxidation, NMR characterization, and additional
data for calorimetric tests. See DOI: 10.1039/c4cy01246a
5
a
procedure, which ensures the formation of alloyed bimetallic
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particles with controlled particle size. The catalytic tests for the
From a selectivity point of view, AuPt NPs on acidic sup-
ports appear to be the best choice. Indeed, as shown in
Table 1, by using acidic supports a higher selectivity to
C3 compounds (GLYA + GLYALD + TA + DHA) (~95%) was
obtained whereas basic supports, in particular MgO, pro-
moted C–C bond cleavage, forming C2 and C1 products in
high amounts (59%).
base-free glycerol oxidation at 80 °C were carried out (0.3 M
−
1
2
glycerol, glycerol/metal = 500 mol mol , 3 atm O , T = 80 °C).
The activity and the selectivity of the catalysts are reported
in Table 1.
First of all, the higher activity of the basic supports
compared to the acidic ones is not surprising. In terms of
activity, as expected, the presence of a basic support such as
In addition, the formation of glyceraldehyde, a labile
compound, in significant yield was observed with the usage
of acidic supports. Particularly AuPt/MCM-41 showed the
highest selectivity to glyceraldehyde (46% at 30% conversion).
To the best of our knowledge this is the first time that glycer-
aldehyde has been detected in such a great amount. The
5
b
MgO (isoelectric point of 10.4) increased the activity. It is
well recognized that the mechanism of glycerol oxidation
proceeds via oxidative dehydrogenation. The presence of a
base enhances the β-hydride abstraction, which is the limit-
3
ing step of this reaction. It should be noted that AuPt/MgO
1
3
is able to maintain high selectivity toward glyceric acid
even at high conversion only if the temperature is maintained
presence of glyceraldehyde was also confirmed by C-NMR
analysis of the reaction mixtures (S4).
5
b
below 25 °C. ICP analysis performed on the reaction media
evidenced the presence of Mg in solution. Mg could be a
sacrificial base, promoting both H-abstraction and the
successive desorption of the Mg-glycerate, therefore limiting
the deactivation phenomena due to strongly adsorbed
byproducts. AuPt NPs on MgO is a very active catalyst,
exhibiting an initial activity (657 converted moles of glycerol
It can be noted that the higher selectivity to glyceralde-
hyde showed by MCM-41 is related to a lower production of
glyceric acid compared to the other acidic supports.
By decreasing the reaction temperature from 80 °C to
60 °C and then to 40 °C the activity of the catalyst decreased
as expected with an initial activity of 228, 173 and 112 (moles
−
1
−1
of glycerol molAuPt
h ), respectively (S5). However by
−
1
−1
molAuPt
h
) superior to those observed when the same
decreasing the reaction temperature, the selectivity to glycer-
aldehyde increased, giving the best result at 40 °C (55% at
30% conversion) (S5).
Focusing on AuPt NPs supported on acidic supports and
in order to understand the influence of the nature of the
support on the catalytic activity and selectivity, a detailed
characterization of the morphology and of the surface proper-
ties of the catalysts was performed.
TEM data collected from the samples revealed good AuPt
dispersion on all the catalysts (Fig. 1). EDS analysis showed
that the AuPt nanoparticles are alloyed particles (S6). More-
over, the size of the AuPt particles is not significantly affected
by the support, with a mean diameter of 6.2–7.5 nm (S7).
Therefore we can conclude that the impact of the particle
size is negligible on the catalytic performance of the AuPt
based catalysts tested in this work.
AuPt NPs are immobilized on NiO (283 moles of glycerol
−
1
−1
molAuPt h ) and on the acidic supports (113, 105, 156, and
−
1
−1
2
28 moles of glycerol molAuPt
h
for S-ZrO
2
, H-mordenite,
SiO
2
and MCM41, respectively) (Table 1). However, no direct
correlation between the acid/base properties and the catalytic
activity was found.
Interestingly, AuPt NPs on MCM41 showed limited deacti-
vation phenomena compared to AuPt supported on S-ZrO2,
2
SiO and H-mordenite showing reaction profiles similar to
those of AuPt/NiO and AuPt/MgO (S1), determining a sort of
breakout line between acid and basic supports. Moreover,
recycling experiments carried out just by filtering the cata-
lyst and adding fresh solution of glycerol confirmed the
good stability of AuPt/MCM41 at least after six runs in
terms of activity and also selectivity (S2). The mesostructure
of MCM-41 was maintained after the recycling tests, as con-
firmed by SAXRD analyses (S3).
Therefore, we focused on the investigation of the acidic
properties of AuPt nanoparticles supported on SiO , MCM-41,
2
Table 1 Base free glycerol oxidation at 80 °C
b
a
Selectivity (%)
Activity
(mol mol
Adsorbed amount
of NH (μmol m )
3
AuPt−
1
−1
c
−2
Catalyst
h
)
GLYA
GLYALD
TA
DHA
C3
GLYCA
OXA
FA
C2 + C1
AuPt/MgO
AuPt/NiO
AuPt/MCM-41
657
283
228
156
105
113
21
51
35
61
53
57
7
1
8
1
1
—
3
12
16
16
18
23
17
41
85
98
96
95
96
8
7
1
2
1
1
6
2
1
1
2
1
43
8
—
—
1
59
17
2
3
4
—
—
0.34
0.72
2.68b
2.96
10
46
16
19
19
AuPt
AuPt/H-mordenite
AuPt/S-ZrO
/
SiO
2
2
1
3
−
1
a
Reaction conditions: 0.3 M glycerol, glycerol/metal = 500 mol mol , 3 atm O
2
, T = 80 °C. Moles of glycerol converted per hour per mole of
b
the metal calculated after 15 min of reaction. Selectivity calculated at 30% conversion. GLYA = glyceric acid; GLYALD = glyceraldehyde;
TA = tartronic acid; DHA = dihydroxyacetone; GLYCA = glycolic acid; OXA = oxalic acid; FA = formic acid; C2 = oxalic acid and glycolic acid;
c
C1 = formic acid. The amount of NH
contribution of Lewis sites.
3 3
adsorbed under an equilibrium pressure of 5 Torr. The amount of NH obtained after subtracting the
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Fig.
1 Representative TEM images of a) AuPt/MCM-41 and
b) AuPt/SiO
2
.
S-ZrO2 and H-mordenite by means of FTIR spectroscopy.
,6-Dimethylpyridine (2,6-DMP) was used as a probe mole-
2
cule. The adsorption/desorption of such a probe is suitable
because (i) it allows us to identify both Lewis and Brønsted
acidic centers present on the surface and above all (ii) it
can distinguish Lewis and/or Brønsted sites of different acid
6
strengths. Moreover, when the base is probing Brønsted
sites, it is adsorbed in its 2,6-dimethylpyridinium form
+
(
2,6-DMPH ). This species yields two strong and well recog-
nizable 8a–8b bands (ring stretching modes) centered at
−
1
−1
1
640–1655 cm
and at about 1630 cm , respectively.
Whereas Lewis bound and H-bonded species will typically
−
1
7
give three bands in the 1620–1580 cm range. In particu-
lar, the ν8a mode, which in the liquid phase appears at
−
1
1
594 cm , is very sensitive and allows the identification of
Fig. 2 The IR spectral region of the 8a–8b ring modes of 2,6-DMP.
FTIR absorbance spectra after interaction with 2 mbar of 2,6-DMP
at r.t. (red curve) and during outgassing (black and blue curves) with
AuPt NPs on a) SiO , b) MCM-41, c) S-ZrO , and d) H-mordenite.
different 2,6-DMP adsorption types on solids. In fact, when
the ν8a wavenumber is higher than 1625 cm , it character-
izes the protonated species (2,6-DMPH ) whereas lower
−
1
+
2
2
wavenumbers correspond to the coordinated or H-bonded
species (DMPL). The 2,6-DMP molecule interacting by
H-bonding (a) and adsorbed on Lewis (b) and Brønsted (c)
sites is represented in Scheme 1.
ν8b of liquid-like and H-bonded 2,6-DMP are formed on all
catalysts. As expected, no Lewis and Brønsted acidity
Therefore, to describe the adsorption/desorption of
,6-DMP, we focus on the bands in the 1650–1550 cm spec-
tral range. The FTIR spectra, normalized with respect to the
surface area and collected after interaction with 2 mbar
of 2,6-DMP at room temperature (r.t.) and during the
outgassing, are shown in Fig. 2.
2
was observed for AuPt/MCM-41 and AuPt/SiO . Besides the
−
1
2
liquid like form, the 2,6-DMP probe also interacts with
these systems exclusively through H-bonding with surface
silanol groups. Moreover, the ν8a mode is highly blue
−
1
shifted at 1605 cm (which is a position typically observed
when the probe interacts with Lewis sites) confirming the
acid character of silanols. The observed bands gradually
decreased in intensity after outgassing for 30 min with
increasing time at the same temperature (blue curves). The
higher stability of the bands related to AuPt/MCM-41, com-
Upon adsorption of 2 mbar of 2,6-DMP at r.t. (red curves),
−
1
complex bands in the 1605–1580 cm range due to ν8a and
pared with those detected on AuPt/SiO , is due to the pres-
2
ence of mesopores; in fact the H-bonded 2,6-DMP is
gradually removed by prolonging the outgassing time (data
not shown for the sake of brevity).
On the other hand, the catalyst supported on sulphated
−
1
zirconia showed both Lewis (1602 cm ) and Brønsted acid
−
1
sites (1628 and 1648 cm ) whereas for AuPt/H-mordenite
we observed almost exclusively Brønsted acidity (bands at
−
1
1
633 and 1647 cm ). The apparent absence of Lewis sites
Scheme 1 The 2,6-DMP molecule interacting by H-bond (a) and
adsorbed on Lewis (b) and Brønsted (c) sites exposed on the surface of
the catalysts.
on H-mordenite is ascribed to residual water adsorbed on
the surface due to the mild pre-treatment of the samples
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(
drying at 80 °C). It should be noted that glycerol oxidation
was carried out in water which results in the Lewis sites
being masked.
The adsorption/desorption of 2,6-DMP allowed us to iden-
tify the different acidic centers present on the surface of the
catalysts, however there is no clear correlation between the
nature of the sites and the trend observed for the catalytic
activity and selectivity. Recently, Stošić et al. reported on the
influence of the acid–base properties of zirconia and titania
8
based materials on the dehydration reaction of glycerol.
In particular, the authors demonstrated that the catalytic
activity was dependent not only on the nature of the acid
sites but also on the total acidity of the catalysts. Based on
these premises, we carried out quantitative studies on the
acid sites present on the different samples.
Adsorption microcalorimetry of NH3 molecules was
performed to study the strength and the total amount of acid
sites. Among the basic molecules, ammonia is a very con-
venient probe to determine the acid character since it
can interact with acidic hydroxyls forming ammonium
ions. However, upon the neutralization of the acidic hydroxyls
the ammonia molecules added next can react with the ammo-
nium ions (which also act as Brønsted acid sites), forming
Fig. 3 Section a: surface area-normalized calorimetric isotherms
+
NH
4
×NH
3
dimers and as a consequence leading to an
(
integral heats vs. equilibrium pressure). Section b: surface area-
normalized volumetric isotherms (mmol NH3 adsorbed vs. equilibrium
pressure). (-■-) AuPt/MCM-41; (-▲-) AuPt/SiO ; (-●-) AuPt/H-mordenite;
-◆-) AuPt/S-ZrO and (-◇-) AuPt/S-ZrO without Lewis site
contribution.
9
overestimation of the amount of Brønsted acid sites. IR
studies on dimer formation and decomposition within the
pores of a TON zeolite demonstrated that the optimal tem-
perature for ammonia adsorption is 400 K because at this
temperature only ammonium ions are formed.
Further, FTIR experiments on NH adsorption at 343 K
on AuPt/H-mordenite confirmed that no bands related to
2
(
2
;
2
1
0
3
observed in the developed heats are directly correlated with
the different amounts of adsorbed NH . This is also
+
NH
4
×NH
3
dimers were observed (S8), therefore the microca-
3
lorimetric analyses were performed at 373 K.
confirmed by plotting the Qint as a function of the adsorbed
In Fig. 3 the calorimetric isotherm curves (section a) and
the volumetric isotherm curves (section b) are reported.
The calorimetric isotherm provides the heat developed
NH (mmol) (S9).
Moreover, the volumetric isotherms can also provide infor-
mation on the total amount of acid sites probed by ammonia,
and the quantity of sites follows the order: AuPt/S-ZrO2
AuPt/H-mordenite > AuPt/SiO > AuPt/MCM41 (Table 1).
3
−
2
upon NH3 adsorption, Qint (J m ), vs. the equilibrium
pressure, P (Torr), allowing us to have information on the
>
e
2
strength of the interaction between the molecule and the
However, we have to consider that the total amount of
catalyst. In particular, for equilibrium pressures (P ) up to
acid sites present on the surface of AuPt/S-ZrO encompasses
e
2
0
.2 Torr, the developed heats follow the trend: AuPt/SiO
2
≈
both Lewis and Brønsted sites. Therefore, in order to
determine the role of the different kinds of acid sites in
the glycerol oxidation reaction, we have to discriminate
between Lewis and Brønsted sites and to subtract the con-
tribution due to Lewis sites from the total amount of acid
sites. The relative abundance of the Lewis/Brønsted sites
can be determined by a quantitative analysis of the IR
AuPt/MCM-41 < AuPt/S-ZrO
finding indicates that AuPt/S-ZrO2 and AuPt/H-mordenite
have exposed acid sites that are stronger than those present
on AuPt/SiO
results, revealing the presence of Brønsted acid sites on both
catalysts supported on S-ZrO and H-mordenite and silanol
2
≈ AuPt/H-mordenite. This
2
and AuPt/MCM41, in agreement with the FTIR
2
groups on the silica based systems.
2
bands of adsorbed 2,6-DMP on AuPt/S-ZrO (red curve in
In contrast, for Pe > 0.5 Torr, the Qint follows the
sequence: AuPt/S-ZrO > AuPt/H-mordenite > AuPt/SiO >
2 2
section c of Fig. 2). The deconvolution of the bands was
carried out and the amount of acid sites was calculated by
applying the Lambert–Beer Law (we used the following inte-
grated molar absorption coefficients: εIJBrønsted) = 6.5 cm
AuPt/MCM-41. It has been frequently observed that the
higher the developed heat, the higher the acidic strength of
the site. In our case, the calorimetric isotherms (section a of
Fig. 3) display a quite similar trend if compared to that
related to the volumetric ones (reported in section b of
Fig. 3). Such a feature allows us to state that the differences
−
1
−1 11
mmol , ε(Lewis) = 3.4 cm mmol ). It was found that
the overall amount of sites exposed at the S-ZrO surface
2
was composed of 55% Brønsted sites and 45% Lewis sites. A
new volumetric isotherm was constructed by subtracting
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the contribution of Lewis sites (Fig. 3, section b, (-◇-)).
This new curve reveals that on the AuPt/S-ZrO2 and AuPt/
H-mordenite catalysts the abundance of Brønsted sites is
similar. The similar activity of the two catalysts, reported in
Table 1, shows the importance of Brønsted and not of
Lewis acid sites.
The characterization studies clearly show that the
acidic surface properties of the catalysts affect both activity
and selectivity. We also found that by increasing the
number of acidic sites, a lower initial activity and more
evident deactivation phenomena occur. If we compare the
Experimental
Materials
NaAuCl ·2H O and K PtCl were from Aldrich (99.99%
4
2
2
4
2
−1
purity); H-mordenite was from Degussa (SA = 450 m g );
2
−1
2 −1
2
and sulphated zirconia (SA = 78 m g ,), SiO (SA = 148 m g ,
−
1
2
−1
P.V. 0.78 ml g ) and MgO (SA = 38 m g ) were from Alfa
2
−1
2
−1
Aesar. NiO (SA = 210 m g ) and MCM-41 (SA = 980 m g
were prepared following the procedures reported in ref. 12
and ref. 13, respectively. NaBH (purity > 96%) from Fluka
and polyvinyl alcohol (PVA) (Mw = 13 000–23 000, 87–89%
hydrolyzed) from Aldrich were used. A 1 wt% PVA solution
in water was prepared. Gaseous oxygen from SIAD was
99.99% pure.
)
4
trend in the catalytic activity, AuPt/H-mordenite ≈ AuPt/ZrO
2
<
AuPt/SiO < AuPt/MCM-41 (reported in Table 1), with that
2
in the total amount of acid sites: AuPt/S-ZrO2 > AuPt/
2
H-mordenite > AuPt/SiO > AuPt/MCM-41 (data shown in
Fig. 3, section b), we find that the higher the total amount
of acid sites, the lower the catalytic activity. The AuPt/
Catalyst preparation
H-mordenite and AuPt/S-ZrO
2
catalysts show similar activity,
The MCM-41 mesoporous framework was prepared by a
sol–gel method at RT by adopting the procedure described in
ref. 13. Typically, C TAB (5.05 g, 0.014 mol) was dissolved in
exposing a comparable abundance of Brønsted sites. In
addition, AuPt supported on acidic oxides are more efficient
in terms of selectivity to C3 products. Indeed, in all cases a
selectivity to C3 > 95% has been observed. Moreover, the
C3 product distribution, in particular the amount of glycer-
aldehyde or glyceric acid detected, seems to be correlated
with the amount of acid sites and not with their nature.
1
6
100 ml of deionized water with constant stirring (400 r.p.m.).
Subsequently, aq. ammonia solution (25% NH , 37 ml,
3
0.5 mol) was added dropwise to the previous solution
followed by the addition of 152 ml of absolute alcohol as a
co-solvent and finally by the addition of 10 ml of TEOS as a
silica source. The solution became turbid in one minute and
then a bright white colloidal gel was formed. The solution
was kept under constant stirring for 2 h and then aged at
room temperature for 24 h. The resulting gel was filtered,
washed with deionized water and dried at 363 K for 24 h.
The template was removed by heating the dried gel to 823 K
2 2
For AuPt/S-ZrO , AuPt/H-mordenite and AuPt/SiO the
main product is glyceric acid (53–61%) with a low amount
of aldehyde (16–19%). In contrast, AuPt/MCM-41, which
shows the lowest amount of acid sites, is able to stabilize
glyceraldehyde, limiting its successive transformation to
glyceric acid.
In order to prove this, AuPt/MCM41 and AuPt/H-mordenite,
as representatives of the other acidic catalysts, were tested
in the transformation of glyceraldehyde to glyceric acid under
the same reaction conditions used for glycerol oxidation
−
1
(1 K min ) in a tube furnace for 8 h in pure N2 flow.
For complete removal of the template, the material was
cooled to RT then flushed with pure O and heated to 823 K
2
−
1
(10 K min ) for 12 h.
NaAuCl ·2H O (Au: 0.031 mmol) was dissolved in 60 mL of
−
1
(0.3 M glyceraldehyde, glyceraldehyde/metal = 500 mol mol ,
4
2
3
atm O , T = 80 °C). After 6 h of reaction AuPt/H-mordenite
H O, and PVA (1%, wt%) was added (Au/PVA = 1 : 0.5 wt/wt).
2
2
completely converted glyceraldehyde to glyceric acid whereas
AuPt/MCM-41 showed a conversion of only 25%. These data
are in agreement with the well accepted mechanism of
aldehyde transformation to a carboxylic acid via acid cata-
lyzed diol formation and the subsequent rapid dehydrogena-
The yellow solution was stirred for 3 min, after which
−
1
0.1 M NaBH
4 4
(Au/NaBH = 1 : 4 mol mol ) was added
under vigorous magnetic stirring. The ruby red Au(0) sol
was formed immediately. Within a few minutes of sol gen-
eration, the gold sol was immobilized by adding the sup-
port (acidified to pH 2 by sulphuric acid) under vigorous
stirring. The amount of the support was calculated as
having a gold loading of 0.60 wt%. After 2 h, the slurry
was filtered and the catalyst washed thoroughly with dis-
tilled water (neutral mother liquors). The Au/support was
3
tion to the acid (Scheme 2).
dispersed in 40 mL of water, with K
2 4
PtCl (Pt: 0.021 mmol)
and PVA solution (Pt/PVA = 1 : 0.5 wt/wt) added. H was
2
−
1
bubbled (50 mL min ) under atmospheric pressure and
room temperature for 2 h. After an additional 18 h, the
slurry was filtered and the catalyst washed thoroughly
with distilled water. ICP analyses were performed on the
filtrate using a Jobin Yvon JV24 instrument to verify the
metal loading on the support. The total metal loading was
1 wt%.
Scheme 2 a) Alcohol dehydrogenation mechanism in the presence
of water and b) acid catalyzed geminal diol formation.
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Oxidation of glycerol
35 Torr. The primary adsorption isotherms were followed by
prolonged outgassing at the adsorption temperature and
Reactions were carried out in a thermostatted glass reactor
then by a second adsorption run up to the same final NH
3
(
30 mL) provided with an electronically controlled magnetic
pressure (secondary isotherms).Uptake differences between
the primary and secondary isotherms are usually considered
to monitor the occurrence and the extent of irreversible
adsorption processes.
Samples were prepared for TEM characterisation by
dispersing the catalyst powder in high purity ethanol
followed by sonication. A drop of this suspension was then
evaporated on a holey carbon film supported by a 300 mesh
copper TEM grid. The samples were then subjected to bright
field diffraction contrast imaging experiments in order to
image the particles. The instrument used for this analysis
stirrer connected to a large reservoir (5000 mL) containing
oxygen at 3 atm. The oxygen uptake was followed by the use
of a mass flow controller connected to a PC through an A/D
board, plotting a flow time diagram. Glycerol was dissolved
in 10 mL of water and mixed with the catalyst (final concen-
−
1
tration of glycerol: 0.3 M, glycerol/metal = 500 mol mol ).
The reactor was pressurized at 300 kPa of O and thermostat-
2
ted at the appropriate temperature. Once the required tem-
perature (40, 60, and 80 °C) was reached, the gas supply was
switched to oxygen and the monitoring of the reaction
started. The reaction was initiated by stirring. Samples were
removed periodically and analyzed by high-performance chro-
matography (HPLC) using a column (Alltech OA-10308,
6
was a Jeol 2100 LaB TEM operating at 200 kV. High reso-
lution transmission microscopy (HRTEM) analysis was
performed using a side entry Jeol JEM 3010 (300 kV) micro-
3
00 mm × 7.8 mm) with UV and refractive index (RI) detec-
tors in order to analyze the mixture of the samples. 0.1%
PO solution was used as the eluent. The identification of
6
scope equipped with a LaB filament and fitted with a Link
ISIS 200 detector for X-ray EDS analysis. All digital micro-
graphs were acquired by an UltraScan 1000 camera and the
images were processed by Gatan DigitalMicrograph. X-Ray
powder diffraction data were obtained by using a Rigaku
DMax diffractometer with Cu KR radiation operating at
H
3
4
the possible products was performed by comparison with the
original samples. The presence of glyceraldehyde was con-
1
3
firmed by NMR. C NMR spectra were recorded on a Bruker
AC 300 NMR spectroscope. The water signal was suppressed
using a low power PRESAT pulse in order to minimize signal
4
0 keV and 40 mA, with a 0.05° divergence slit; the spectra
were recorded in the range 1–8°.
2
distortions. 200 μL of 0.3 M glyceraldehyde solution in H O
were added to each sample (final volume 600 μL). The prod-
ucts were recognised by comparison with authentic samples.
Recycling test: each run was carried out under the same
Conclusions
Supported AuPt nanoparticle catalysts were tested for
base-free glycerol aqueous phase oxidation, using supports
with different acid–base properties. The catalytic behavior
was markedly influenced by the support with respect to both
activity and selectivity. Basic supports (MgO and NiO)
promoted the activity but also increased the C–C bond
cleavage reactions, thus decreasing the selectivity to the
desired products. In contrast, acidic supports showed a
−
1
conditions (0.3 M glycerol, glycerol/metal = 500 mol mol
,
3
atm O , T = 80 °C, reaction time = 16 h). The catalyst was
2
recycled in the subsequent run after filtration without any
further treatment.
Catalyst characterization
The metal content was checked by ICP analysis of the filtrate
on a Jobin Yvon JY24.
3
higher selectivity to C oxidation products. Spectroscopic and
microcalorimetric measurements provided evidence that the
catalytic activity and selectivity are not influenced by the
nature of the acid sites but by their amount. In particular,
both activity and selectivity to glyceraldehyde decreased by
increasing the number of acid sites (Brønsted sites and/or
silanols). Indeed, the successive transformation of glyceralde-
hyde to glyceric acid proceeds via an acid catalyzed geminal
diol formation and its dehydrogenation to a carboxylic acid.
Moreover, Lewis acid sites do not seem to be involved.
FTIR spectra were taken on a Perkin-Elmer 2000 spec-
trometer (equipped with a MCT detector) with the samples
in self supporting pellets introduced in the cells allowing
thermal treatments in controlled atmospheres and spectrum
scanning at room temperature (r.t.) in vacuum or in the
presence of probe gases. From each spectrum, the spectrum
of the sample before the inlet of the probe was subtracted.
The standard IR experiment of 2,6-DMP adsorption/
desorption on the various samples, previously activated in
vacuum at 393 K, was carried out as follows: (i) admission in
the IR cell of an excess dose of 2,6-DMP vapor (~2 Torr) and
equilibration at room temperature for 10 min; and (ii) evacu-
ation of the IR cell at r.t. for 15 min.
Acknowledgements
We are grateful to the Research Complex at Harwell for the
provision of several of the facilities used in this work and to
the EPSRC for studentship funding to WJ.
Microcalorimetric measurements were run on Tian–Calvet
heat flow calorimetric equipment (Setaram C80d). Each
sample was pretreated in vacuum (10–5 Torr) at 393 K (for IR
experiments) and then contacted with successive small doses
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