Selective oxidation of glycerol under acidic conditions using gold catalysts

Article abstract of DOI:10.1002/anie.201000762

(Figure Presented) Skip the bases! H-mordenite-supported PtAu nanoparticles are highly active and selective in the oxidation of glycerol under acidic conditions, which allows the direct preparation of free acids (see picture). The high selectivity for C

Full text of DOI:10.1002/anie.201000762

Angewandte
Chemie
DOI: 10.1002/anie.201000762
Catalytic Oxidation
Selective Oxidation of Glycerol under Acidic Conditions Using Gold
Catalysts**
Alberto Villa, Gabriel M. Veith, and Laura Prati*
[
1–10]
Herein, we report progress towards eliminating the need for a
base while maintaining high activity and selectivity for the
aqueous-phase oxidation of glycerol. We have discovered that
by using the right alloy nanoparticle and support material, we
are able to prepare an active and durable catalyst that is
highly selective towards the formation of oxidized C3
molecules from glycerol under acidic conditions. This discov-
ery is significant because previous investigations of Pd and Pt
catalysts at pH 2–4 reported that the main products derived
from these catalysts under acidic conditions are C and C
this reaction.
A key advantage in using gold, compared to
Pt and Pd, is the improved resistance of Au to overoxidation
under liquid-phase oxidation conditions with O as the
2
[
1,10–13]
oxidant.
However, in alcohol oxidation with gold catalysts, a severe
limitation arises because of the required addition of a base to
improve the oxidation kinetics and reduce deactivation. As a
consequence of using basic conditions, salts of acids are
obtained instead of free carboxylic acids. From an industrial
point of view, working under an acidic regime allows the
direct formation of acid products. It should also be noted that
basic conditions could modify the intrinsic selectivity of the
catalyst as a result of base-catalyzed interconversions.
1
2
[1–3]
molecules produced from CÀC bond scission.
Further-
more, the groups of Claus and Hutchings showed that by using
Pt catalysts it is possible to obtain quite a high selectivity to
hydroxyacetone (C molecule) or hydroxypyruvic acid, but
Given the industrial importance of operating under acidic
conditions, we began investigating the synergistic effect of
support material and nanoparticle composition on the
oxidation of glycerol without base. For comparison purposes,
a series of monometallic clusters were deposited on AC, TiO2,
MgAl O , and acidic H-mordenite by a sol immobilization
3
that selectivity rapidly declines with increasing conver-
[1,4–6]
sion.
Glycerol is a highly functionalized bioderived molecule
that is recognized as one of the most promising chemical
building blocks for the synthesis of fine chemicals from
2
4
[
7,9]
renewable sources.
The selective oxidation of glycerol has
technique and tested under similar conditions. TiO2 was
investigated because it is the prototypical gold support with
an intermediate isoelectric point (IEP), and basic MgAl O
been shown to produce valuable products, such as glyceric
acid, hydroxyacetone, tartronic acid, and ketomalonic acid.
The activity and product distribution for the catalytic
oxidation of glycerol depend on the catalyst, reaction
2
4
(IEP = 11.8) was recently reported by Christensen et al. for
the oxidation of ethanol using monometallic Au on MgAl O
2
4
[
7–9]
[14,15]
conditions, and oxidant source.
In general, carbon-sup-
at 1008C without any addition of base.
Upon comparing
ported metal catalysts combined with dioxygen under basic
conditions and moderate temperature are the most inves-
just the gold clusters, the average diameter on H-mordenite
was (3.76 Æ 1.90) nm (Figure 1), as determined by scanning
transmission electron microscopy (STEM). This value is
slightly larger, but within experimental error, than the
corresponding mean diameter of Au particles on AC
[(2.98 Æ 1.31) nm].
[
1,4–6,10,11]
tigated catalysts for the oxidation of glycerol.
Pd and
Pt nanoparticles supported on activated carbon (AC) were
the first investigated for the oxidation of glycerol, but gold
was recently shown to be highly active and very selective in
Initial catalytic testing performed at 508C (0.3m aqueous
glycerol solution, glycerol/metal ratio 500 (mol/mol), pO =
2
[
*] Dr. A. Villa, Prof. L. Prati
3
atm) showed all these catalysts to be inactive at this
Department of Inorganic Chemistry L. Malatesta
Universitꢀ degli Studi di Milano
via Venezian 21, 20133 Milano (Italy)
Fax: (+39)02-503-14405
temperature. At elevated temperatures (1008C) the mono-
E-mail: laura.prati@unimi.it
Dr. G. M. Veith
Materials Science and Technology Division
Oak Ridge National Laboratory
Oak Ridge, TN 37831 (USA)
[
**] A.V. and L.P. gratefully acknowledge Fondazione Cariplo for
financial support. A portion of this research, at Oak Ridge National
Laboratory’s Center for Nanophase Materials Sciences (STEM), was
sponsored by the Scientific User Facilities Division, Office of Basic
Energy Sciences, U.S. Department of Energy and by the Division of
Materials Sciences and Engineering, U.S. Department of Energy
under contract with UT-Battelle, LLC (G.M.V.).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201000762.
Figure 1. STEM image of Au on H-mordenite.
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metallic gold and palladium catalysts exhibited a very low
peculiar nature of acidic sites in H-mordenite has been
reported.
It is well known that the introduction of a second metal
component to a nanoparticle changes the activity. In fact, by
alloying Au with Pd or Pt a strong
[
21,22]
activity (< 8%), compared to platinum (78%), for the
conversion of glycerol (Table 1). However, there was signifi-
cant formation of C products, such as formic acid and CO ,
1
2
synergistic effect was reported in
[
a]
Table 1: Base-free glycerol oxidation on monometallic supported metals.
the oxidation of alcohol, even under
[
23–25]
[
b]
neutral/acidic conditions.
The
Catalyst
Conv.
Selectivity [%]
Tartronic acid
[
c]
Glyceric acid
C1 products
magnitude of the effect (positive or
negative) depends on the nature of
the reactant and its match with the
geometrical/electronic requisites of
1
1
1
1
1
1
1
% Au/AC
% Pd/AC
% Pt/AC
% Au/MgAl O4
% Au/TiO2
3
5
78
6
8
5
39
43
45
35
38
70
79
–
5
7
6
7
9
2
51
47
46
41
32
5
[
26]
AuPd and AuPt catalysts.
For
2
example, previous reports show
that AuPt catalyst on AC exhibits
an increase or decrease in reactivity
% Au/H-mordenite
% Pt/H-mordenite
20
10
[
a] Reaction conditions: 0.3m, pO =3 atm, 1250 rpm, 1008C, glycerol/metal=500 (mol/mol). [b] Con- simply by changing the molecule
2
version after 2 h. [c] C products include CO and HCOOH.
[26]
1
2
reacted.
Following a procedure
recently developed by our group
for all monometallic catalysts through the oxidative degrada-
tion of glycerol. Surprisingly, gold and platinum catalysts
deposited on the acidic oxide H-mordenite exhibited a
significant reduction in C product formation compared to
1
catalysts deposited on neutral to basic supports (5–10 versus
4
1–51% C product formation, respectively; Table 1). Fur-
1
thermore, there was also an enhancement in the overall
selectivity to C products, up to 79–81% (glyceric + tartronic
3
acids) for the H-mordenite-supported monometallic catalysts.
These results further confirm that the support material plays a
[
10]
key role in the selectivity of this reaction.
The role of
particle size in activity has been studied on the same support
(
AC), and revealed higher activity (and less selectivity) for
[
10,11,13,16,17]
smaller particles.
The behavior of similarly sized
particles for glycerol oxidation on substantially different
supports has not yet been reported.
Figure 2. H O decomposition at pH 2 and 1008C.
2
2
Clearly the use of H-mordenite as a support reduces the
oxidative degradation of the glycerol. H O formed during
that ensures the formation of homogeneously alloyed AuPd
nanoparticles, we prepared a series of Au-based alloy
catalysts. The bimetallic nanoparticles were prepared by
2
2
the reaction can be responsible for the CÀC bond scis-
[11,18,19]
[27]
sion.
The first step of the reaction mechanism of Au-
catalyzed alcohol oxidation involves hydride abstraction with
the sequential deposition of gold clusters followed by the
second metal component. Gold was first deposited on the
support as poly(vinyl alcohol) (PVA)-protected nanoclusters.
[
20]
AuH formation. The reduction of this species by O forms
2
H O . Therefore, we performed experiments to determine the
2
2
amount of H O present during the reactions. When the H-
Pt was then slowly reduced by H in the presence of the Au/
2
2
2
mordenite was employed as a support material for Au, very
support material. The importance of using PVA-protected
nanoclusters and a slow reduction process for obtaining
alloyed nanoparticles has been recently reported.
In comparison to the monometallic clusters, a significant
enhancement in catalytic activity is observed for alloy nano-
particles. AuPt (58% conversion) and AuPd (12% conver-
sion) catalysts on AC, the typical support for these reactions,
showed not only significant increases in the conversion of
glycerol but also selectivity towards the formation of glyceric
À1
little H O was detected (0.025 mmolL ). In contrast, in the
2
2
[
28,29]
case of Au/AC almost 30 times more H O was detected
2
2
À1
(
0.70 mmolL ).
Moreover, under acidic conditions (pH 2) at 1008C, H O2
2
rapidly degrades in the presence of Au/AC, whereas in the
presence of Au/H-mordenite the degradation proceeds
slower, as evidenced by the blank experiment starting from
a 1 mm solution of H O2 (Figure 2). Thus, it could be
2
concluded that Au/H-mordenite produced H O to a lesser
acid along with a concomitant decrease in the C products
2
2
1
extent than Au/AC. Since we are comparing similar con-
version rates, it is likely that Au/H-mordenite follows a
different reaction mechanism from Au/AC, possibly acting as
a bifunctional catalyst in which the acidic sites (Broensted and
Lewis type) can possibly play an active role. In fact the
formed (Table 2).
Based on the above results where gold catalysts on
mordenite showed a significant decrease in C1 product
formation and AuPt catalysts showed significant increases in
catalytic activity, AuPt nanoparticles were prepared on the H-
4
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Chemie
[
a]
Table 2: Base-free glycerol oxidation on bimetallic Au-based catalysts.
X-ray photoelectron spectroscopy
[
b]
(XPS) data are presented in the
Supporting Information. Energy-
Catalyst
Conv.
Selectivity [%]
Tartronic acid
[
c]
Glyceric acid
C1 products
dispersive spectroscopy (EDS)
analysis of 20 randomly selected
clusters indicated that all of the
1
1
1
% AuPd(6:4)/AC
% AuPt(6:4)/AC
% AuPt(6:4)/H-mordenite
12
58
70
65
79
83
–
1
2
24
12
10
clusters contained a mixture of Pt
and Au. However, the Au/Pt ratio
was inhomogeneous from cluster to
[
a] Reaction conditions: 0.3m, pO =3 atm, 1250 rpm, 1008C, glycerol/metal=500 (mol/mol). [b] Con-
2
version after 2 h. [c] C products include CO and HCOOH.
1
2
cluster. Regardless, the increase in
mordenite by following the same procedure as that used for
AC-supported nanoparticles. By adding Pt to the Au catalyst
supported on H-mordenite, a strong enhancement of activity
was observed (58 to 70%) accompanied by an enhancement
particle size, the EDS data, and the activity data confirm the
presence of AuPt alloys that modify the properties of the
resulting catalyst.
As no leaching of Pt or Au was observed, differences in
catalytic behavior can be ascribed to a deactivation of the
“on-carbon” catalyst, possibly because of an irreversible
adsorption of by-products. In this case the effect of reactant
concentration could be important. Thus, we carried out tests
by increasing the concentration of glycerol from 0.3 to 1m (see
the Supporting Information) and observed in the case of
AuPt/H-mordenite a slight decrease of activity (and selectiv-
ity) upon increasing the concentration. As the pH of the final
reaction mixtures was similar, the decrease in activity results
from the high chelating properties of products that, when
increasing their concentration, could be irreversibly ad-
sorbed.
of selectivity to C up to 85% from 80% along with a 2%
3
decrease in C product formation. Furthermore, upon com-
1
paring AuPt nanoparticles supported on AC and on H-
mordenite we observed that the same initial activity of the
two catalysts (after 2 h) did not correspond to the same
reaction profile with time (Figure 3). Only in the case of AuPt
on H-mordenite can full conversion be achieved.
In summary, AuPt nanoparticles supported on the zeolite
H-mordenite are able to selectively oxidize glycerol directly
to glyceric acid without the use of basic conditions. AuPt on
AC showed similar initial activity, but the major effect of
using mordenite as the support lies in the enhanced selectivity
that allows the production of glyceric acid with a selectivity of
8
1% at full conversion. Moreover, it has been shown that by
alloying Au to Pt, the leaching of metals was avoided and the
catalyst life improved. AuPt on H-mordenite opens the way to
a new-generation gold catalyst where the support plays an
active role during alcohol oxidation and prevents H O
2
2
Figure 3. Reaction profile of AuPt nanoparticles supported on AC and
formation that leads to CÀC bond scission, thus improving
selectivity and materials utilization. To assess the general use
of AuPt/H-mordenite as a catalyst we plan to test other
substrates. A preliminary study using ethylene glycol and 1,3-
propanediol (see the Supporting Information) showed that
the use of AuPt/H-mordenite is not limited to glycerol
oxidation.
H-mordenite.
A full characterization of AuPt on H-mordenite was
performed. STEM data indicated a slight increase in the
average particle size with the addition of Pt to the gold
clusters (from (3.76 Æ 1.90) to (4.0 Æ 1.80) nm; Figure 4).
Received: February 8, 2010
Published online: May 11, 2010
Keywords: glycerol · gold · oxidation · platinum ·
.
supported catalysts
[
[
[
[
1] S. Carrettin, P. McMorn, P. Johnston, K. Griffin, C. J. Kiely, G. J.
Hutchings, Phys. Chem. Chem. Phys. 2003, 5, 1329 – 1336.
2] R. Luque, V. Budarin, J. H. Clark, D. J. Macquarrie, Appl. Catal.
B 2008, 82, 157 – 162.
3] D. Liang, J. Gao, J. Wang, P. Chen, Z. Hou, Catal. Commun.
2009, 10, 1586 – 1590.
4] H. Kimura, K. Tsuto, T. Wakisaka, Y. Kazumi, Y. Inaya, Appl.
Catal. A 1993, 96, 217 – 228.
Figure 4. Representative STEM image of AuPt on H-mordenite.
Angew. Chem. Int. Ed. 2010, 49, 4499 –4502
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
4501

Communications
[
[
[
[
[
5] A. Brandner, K. Lehnert, A. Bienholz, M. Lucas, P. Claus, Top.
Catal. 2009, 52, 278 – 287.
6] R. Garcia, M. Besson, P. Gallezot, Appl. Catal. A 1995, 127, 165 –
[18] L. Prati, P. Spontoni, A. Gaiassi, Top. Catal. 2009, 52, 288 – 296.
[19] M. Sankar, N. Dimitratos, D. W. Knight, A. F. Carley, R.
Tiruvalam, C. J. Kiely, D. Thomas, G. J. Hutchings,
ChemSusChem 2009, 2, 12, 1145 – 1151.
[20] M. Conte, H. Miyamura, S. Kobayashi, V. Chechik, J. Am. Chem.
Soc. 2009, 131, 7189 – 7196.
[21] J. H. Lunsford, J. Phys. Chem. 1968, 72, 4163 – 4168.
[22] H. A. Benesi, B. H. C. Winquist in Advances in Catalysis, Vol. 27,
Academic Press, New York, 1978, p. 97.
176.
7] C.-H. Zhou, J. N. Beltramini, Y.-X. Fan, G. Q. Lu, Chem. Soc.
Rev. 2008, 37, 527 – 549.
8] F. Jꢀrꢁme, Y. Pouilloux, J. Barrau, ChemSusChem 2008, 1, 586 –
613.
9] A. Corma, S. Iborra, A. Velty, Chem. Rev. 2007, 107, 2411 – 2502.
[
[
10] F. Porta, L. Prati, J. Catal. 2004, 224, 397 – 403.
11] W. C. Ketchie, Y. Fang, M. S. Wong, M. Murayama, R. J. Davis,
J. Catal. 2007, 250, 94 – 101.
[23] N. Dimitratos, A. Villa, D. Wang, F. Porta, D. Su, L. Prati,
J. Catal. 2006, 244, 113 – 121.
[24] D. I. Enache, J. K. Edwards, P. Landon, B. Solsona-Espriu, A. F.
Carley, A. A. Herzing, M. Watanabe, C. J. Kiely, D. W. Knight,
G. J. Hutchings, Science 2006, 311, 362 – 365.
[25] W. Hou, N. A. Dehm, R. W. J. Scott, J. Catal. 2008, 253, 22 – 27.
[26] L. Prati, A. Villa, C. Campione, P. Spontoni, Top. Catal. 2007, 44,
319 – 324.
[27] D. Wang, A. Villa, F. Porta, D. Su, L. Prati, Chem. Commun.
2006, 1956 – 1958.
[
[
12] S. Carrettin, P. McMorn, P. Johnston, K. Griffin, G. J. Hutchings,
Chem. Commun. 2002, 696 – 697.
13] S. Demirel-Gulen, M. Lucas, P. Claus, Catal. Today 2005, 166,
102 – 103.
[
[
14] H. Zhang, X. Jia, Z. Liu, Z. Li, Mater. Lett. 2004, 58, 1625 – 1628.
15] C. H. Christensen, B. Jørgensen, J. Rass-Hansen, K. Egeblad, R.
Madse, S. K. Klitgaard, S. M. Hansen, M. R. Hansen, H. C.
Andersen, A. Riisager, Angew. Chem. 2006, 118, 4764 – 4767;
Angew. Chem. Int. Ed. 2006, 45, 4648 – 4651.
[28] A. Villa, D. Wang, D. Su, G. M. Veith, L. Prati, Phys. Chem.
Chem. Phys. 2010, 2183 – 2189.
[
16] N. Dimitratos, J. A. Lopez-Sanchez, D. Lennon, F. Porta, L.
Prati, A. Villa, Catal. Lett. 2006, 108, 147 – 153.
[29] D. Wang, A. Villa, F. Porta, L. Prati, D. Su, J. Phys. Chem. C 2008,
112, 8617 – 8622.
[
17] G. M. Veith, A. R. Lupini, S. J. Pennycook, A. Villa, L. Prati,
N. J. Dudne, Catal. Today 2007, 122, 248 – 253.
4
502 www.angewandte.org
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