Communications
[
3]
Catalytic Methanol Oxidation
more slowly than CH OH. Thus, the formation of DME
3
through these side reactions necessitates longer residence
Site Titration with Organic Bases During
Catalysis: Selectivity Modifier and Structural
Probe in Methanol Oxidation on Keggin
Clusters**
times to achieve high yields of DMM from CH OH.
3
We report here the selective titration of protons with
organic bases to control the densities of acid sites in Keggin
clusters and to measure their dispersion; in both cases we do
this during the catalytic reaction, a requirement imposed by
the dynamic changes in accessibility that arise from reactions
Haichao Liu, Nader Bayat, and Enrique Iglesia*
[
1,2]
of polar molecules on Keggin clusters.
In this manner we
are able to measure turnover rates (per exposed Keggin unit;
KU) and to control the redox and acid properties independ-
ently for a given composition of Keggin cluster. This approach
has led to unprecedented DMM selectivities (> 80%) and to
a family of stable organic–inorganic composites that provide
effective bifunctional catalysts for broad classes of redox–acid
bifunctional reactions.
Catalytic reactions often require several types of sites with
distinct functions, and the relative abundance of these sites
influences the rate and selectivity of desired reactions.
Heteropolyacid clusters with Keggin structures, which contain
[
1,2]
acid and redox functions,
have recently emerged as
interesting catalysts for organic reactions involving bifunc-
tional pathways.
The dispersion of Keggin structures was measured by
titration of Brønsted acid sites with a sterically hindered
pyridine (2,6-di-tert-butylpyridine) during catalytic reactions
of mixtures of CH OH and O . This 2,6-di-tert-butylpyridine
We recently discovered a selective one-step synthesis of
dimethoxymethane (CH OCH OCH , DMM, methylal) by
3
2
3
oxidation of methanol at low temperatures (453–493 K) on
unsupported and SiO -supported H PV Mo O40 (n = 0–
3
2
titrant can protonate Brønsted acid sites, but it cannot interact
with Lewis acid sites because of steric constraints near the
2
3+n
n
12Àn
[
3]
4
) Keggin clusters. The yields and selectivities (based on the
[
7]
absence of dimethyl ether) of DMM resemble those obtained
N atom. Its essentially hydrophobic character also prevents
its dissolution and migration into secondary structures of
Keggin clusters. This result is in contrast with more polar
pyridine titrants, which dissolve and penetrate into these
using supported ReO catalysts, the only catalysts that enable
x
[
4]
the formation of DMM in substantial yields. Redox and
Brønsted acid sites are required for DMM synthesis, and the
[
1]
reaction involves oxidative dehydrogenation of CH OH to
secondary structures. Thus, uptake of 2,6-di-tert-butylpyr-
3
formaldehyde (HCHO), acid-catalyzed acetalization of
idine during CH OH reactions (per KU) reflects the number
3
CH OH/HCHO mixtures, and condensation of hemiacetal
of accessible protons, and for a given H3+nPV Mo O40
3
n 12Àn
or methoxymethanol intermediates (formed in acetalization
stoichiometry, the fraction of the Keggin structures accessible
at external surfaces in supported and unsupported secondary
structures. We note that such titrations must be carried out
during the reaction, because of the known ability of various
reactants to solvate and expose internal regions within
secondary packing structures of Keggin clusters to varying
degrees.
[
5,6]
reactions)
with CH OH to formDMM.
3
Brönsted acidity is required to complete the synthesis of
DMM, but reaction rates are predominately controlled by the
initial formation of HCHO on redox sites, the density and
reactivity of which were varied in our studies by changing the
dispersion and V/Mo ratio of the Keggin structures, with
consequent changes in the rates of DMM synthesis. These
compositional changes led to concurrent changes in the
number of acid sites, because of the stoichiometry required to
balance the charge. DMM selectivity is decreased by side
The number of 2,6-di-tert-butylpyridine molecules adsor-
bed during reactions of CH OH and O at 453 K on
3
2
À2
H PV Mo O /SiO (0.28 KUnm surface density on SiO )
increased with time and reached saturation at 1.2 H per KU
after about 12 10 s (Figure 1). This value corresponds to a
5
2
10 40
2
2
+
3
reactions involving CH OH dehydration. These reactions are
3
catalyzed by strongly acidic protons in H3+nPV Mo O and
lead to the undesired formation of dimethyl ether (DME).
nominal fractional dispersion of 0.24, on the basis of
the expected H /KU stoichiometry; we note, however, that
n
12Àn 40
+
The latter product ultimately converts into HCHO and DMM
some of the protons in the stoichiometric starting cluster may
have been removed during condensation reactions of OH
groups in solvated Keggin clusters with silanols on anchoring
products, and can even re-formCH OH, but forms DMM
3
to SiO .
2
+
[
*] Prof. Dr. E. Iglesia, Dr. H. Liu, N. Bayat
Department of Chemical Engineering
University of California at Berkeley, and Chemical Sciences Division,
E.O. Lawrence Berkeley National Laboratory
Berkeley, CA 94720 (USA)
H /KU ratios decreased from1.6/1 to 0.7/1, which
corresponds to a decrease in fractional KU dispersion from
0.32 to 0.15, as the KU surface densities increased from0.1 to
À2
0.65 KUnm on H PV Mo O /SiO samples. This value was
5
2
10 40
2
+
Fax: (+1)510-642-4778
E-mail: iglesia@cchem.berkeley.edu
0
.02 H per KU for bulk H PV Mo O . The rates of DMM
5 2 10 40
synthesis (per KU) decreased in parallel with this decrease in
[
**] This work was supported by BP as part of the Methane Conversion
Cooperative Research Program at the University of California at
Berkeley. This work was also supported in part by the Director,
Office of Basic Energy Sciences, Chemical Sciences Division of the
U.S. Department of Energy under contract DE-AC03-765F00098. We
acknowledge helpful technical discussions with Dr. Theo Fleisch of
BP.
fractional KU dispersion as the surface density increased
À2
from0.1 to 0.65 KUnm
(Figure 2). This excellent correla-
tion between rates and titrant uptake for all samples,
including an unsupported version of this Keggin composition,
indicates that 2,6-di-tert-butylpyridine predominately titrates
those Keggin structures that participate in bifunctional DMM
5
072
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200352393
Angew. Chem. Int. Ed. 2003, 42, 5072 –5075