Vapour-phase gold-surface-mediated coupling of aldehydes with methanol

Article abstract of DOI:10.1038/nchem.467

Selective coupling of oxygenates is critical to many synthetic processes, including those necessary for the development of alternative fuels. We report a general process for selective coupling of aldehydes and methanol as a route to ester synthesis. All steps are mediated by oxygen-covered metallic gold nanoparticles on Au(111). Remarkably, cross-coupling of methanol with formaldehyde, acetaldehyde, benzaldehyde and benzeneacetaldehyde to methyl esters is promoted by oxygen-covered Au(111) below room temperature with high selectivity. The high selectivity is attributed to the ease of nucleophilic attack of the aldehydes by the methoxy intermediate-formed from methanol on the surface-which yields the methyl esters. The competing combustion occurs via attack of both methanol and the aldehydes by oxygen. The mechanistic model constructed in this study provides insight into factors that control selectivity and clearly elucidates the crucial role of Au nanoparticles as active species in the catalytic oxidation of alcohols, even in solution.

Full text of DOI:10.1038/nchem.467

ARTICLES
PUBLISHED ONLINE: 29 NOVEMBER 2009 | DOI: 10.1038/NCHEM.467
Vapour-phase gold-surface-mediated coupling of
aldehydes with methanol
Bingjun Xu1, Xiaoying Liu1, Jan Haubrich1 and Cynthia M. Friend1,2
*
Selective coupling of oxygenates is critical to many synthetic processes, including those necessary for the development of
alternative fuels. We report a general process for selective coupling of aldehydes and methanol as a route to ester
synthesis. All steps are mediated by oxygen-covered metallic gold nanoparticles on Au(111). Remarkably, cross-coupling of
methanol with formaldehyde, acetaldehyde, benzaldehyde and benzeneacetaldehyde to methyl esters is promoted by
oxygen-covered Au(111) below room temperature with high selectivity. The high selectivity is attributed to the ease of
nucleophilic attack of the aldehydes by the methoxy intermediate—formed from methanol on the surface—which yields the
methyl esters. The competing combustion occurs via attack of both methanol and the aldehydes by oxygen. The
mechanistic model constructed in this study provides insight into factors that control selectivity and clearly elucidates the
crucial role of Au nanoparticles as active species in the catalytic oxidation of alcohols, even in solution.
old-based catalysts have been the subject of intense study over only weakly to Au; (3) OH is not thermodynamically stable with
the past decade because of the promise of selective and respect to disproportionation; and, (4) the dissociation probability
G
energy-efficient oxidation processes. Recently, there has for O2 is extremely low on Au. Therefore, the surface coverages of
been a focus on selective oxidation of alcohols1–19, due in part to reactants are low under steady-state conditions. All of these features
interest in biomass conversion to fuels and useful synthetic chemi- render the oxidation of alcohols an ideal candidate for linking
cals, such as esters and organic acids. There is a specific interest model studies with working catalytic processes.
in replacing current technology for stoichiometric oxidation of
Herein, we demonstrate that cross-coupling of aldehydes with
alcohols20 with a catalytic process that could reduce energy con- methanol is promoted by atomic oxygen bound to Au nanoparti-
sumption, increase selectivity and minimize production of environ- cles30 formed on Au(111), referred to as O/Au(111). The intermedi-
mental pollutants. Gold-based catalysts have, in particular, shown ate, methoxy, is first formed from reaction of methanol with oxygen
promise for efficient and selective oxidative coupling of alcohols7.
on O/Au(111), as established previously17. Cross-coupling between
A critical goal of catalytic research in alcohol oxidation is the methoxy and four different aldehydes—formaldehyde, acet-
construction of a predictive model that guides the design of pro- aldehyde, benzaldehyde and benzeneacetaldehyde—to selectively
cesses and materials in order to enhance selectivity for specific pro- form the respective methyl esters was observed in our experiments
cesses. Herein, we describe investigations of complex cross-coupling (Fig. 1a). In each case, methoxy was synthesized first by
reactions in order to provide a roadmap for controlling selectivity exposing oxidized Au(111) (surface concentration, uO ꢀ 0.05
using Au catalysts. We further demonstrate that all steps in these monolayer) to methanol (0.6 Langmuir) at 140 K, using a protocol
oxidative cross-coupling reactions can be mediated by oxygen- established in prior work17. Subsequently, the aldehyde was exposed
covered Au nanoparticles in the absence of a metal oxide support. to the surface, followed by heating to promote evolution of products.
Previously, several mechanisms proposed for liquid-phase oxidative Reaction selectivities, the nature of the rate-determining step and
coupling invoked a combination of reactions on the supported Au estimates of the corresponding activation barriers are derived
catalyst and in the liquid phase2,5,7,13–16. Our work shows that sol- from these experiments. The products were all identified by the
ution-phase reactions are not necessary. We also elucidate the observation of their parent ions and by quantitative analysis of
elementary molecular transformations that are important in the their fragmentation patterns (Figs 1 and 2; Supplementary Tables
coupling reactions, enabling us to qualitatively predict the factors S1 and S2). These results unequivocally demonstrate that methoxy
that control selectivity and activity and to extend these reactions and aldehydes react via a surface-mediated process to yield the
to selective cross-coupling.
corresponding methyl esters. In no case was self-coupling of
Model studies of oxidation reactions over coinage metal single- methanol to methyl formate detected, demonstrating the high
crystal surfaces (Cu, Ag, and Au) have provided insight into the selectivity for cross-coupling and the facility of the attack of the
mechanism of catalytic reactions6,21–25, in some cases leading to pre- aldehydes by methoxy.
dictive microkinetic models21. Generally, atomic oxygen bound to
Clear evidence for a kinetic predominance of cross-coupling with
coinage metals promotes Brønsted acid–base reactions with the aldehydes over self-coupling of the methoxy is derived from
protons on reactant molecules, providing a general framework for isotopic labelling studies in which high selectivity was measured
understanding and predicting reaction pathways26. Model studies for the reaction D2C¼O þ CH3OH ! DCOOCH3.
of vapour-phase reactions induced by oxidized Au are particularly
We find that deuterium is selectively removed from D2C¼O in
relevant to higher pressure and even aqueous conditions8,11,27–29 the reaction of D2C¼O and CH3OH (Supplementary Fig. S3).
because of several key features: (1) alcohols and other hydrocarbons There is no methyl formate-d0 detected, which would arise from
are only activated when oxygen is present on the Au; (2) water binds self-coupling of methanol. This clearly indicates that in the presence
1Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, USA, 2School of Engineering and Applied Sciences,
Harvard University, Cambridge, Massachusetts 02138, USA. e-mail: cfriend@seas.harvard.edu
*
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.467
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a
b
D
(i)
H
H
D
D
D
H
O
H
C
O
H
H
H
D
H
(i)
H
C
D
C
O
C
O
O
H
C
O
H
C
H
C
O
C
O
1
2b
1
2a
m/z = 60
m/z = 63
m/z = 60
m/z = 63
(ii)
D
H
H
H
H
C
D
H
H
C
D
D
H
C
H
(ii)
H
C
H
O
C
H
H
D
C
O
H
C
C
O
H
H
C
O
O
H
C
H
C
O
C
O
H
D
O
H
H
m/z = 74
m/z = 77
3
4b
3
4a
m/z = 74
m/z = 77
D
D
D
D
H
C
O
D
H
H
H
C
D
C
O
C
O
C
O
O
H
C
C
O
C
O
H
H
O
5
6b
H
(iii)
5
6a
(iii)
(iv)
×10 m/z = 136
×10 m/z = 139
×10 m/z = 136
×10 m/z = 139
H
H
H
H
H
D
H
D
H
D
H
D
O
C
C
O
O
C
C
H
H
H
D
H
C
H
C
O
C
C
O
C
H
C
O
H
C
O
C
O
D
H
8a
7
8×b50 m/z = 150
×50 m/z = 153
7
(iv)
×50 m/z = 150
×50 m/z = 153
200
400
600
800
200
400
600
800
Temperature (K)
Temperature (K)
Figure 1 | Mass spectrometric studies of the cross-coupling of isotopically labelled methanol with four different aldehydes. a, Coupling of methoxy-d0 with
(i) formaldehyde, (ii) acetaldehyde, (iii) benzaldehyde and (iv) benzeneacetaldehyde. b, Coupling of methoxy-d3 with the same four aldehydes. In each case
the solid line gives the intensity for the m/z ratio of the expected product assuming no loss of isotopic purity, and the dashed line gives the intensity for the
m/z ratio if isotopic purity is lost. The initial oxygen concentration was ꢁ0.05 monolayers in all cases.
of formaldehyde, nucleophilic attack of formaldehyde-d2 is order of 15 kcal mol–1. Whereas the production rates of the
kinetically preferred over b-H elimination of methoxy, as shown methyl esters derived from formaldehyde or acetaldehyde are
in the mechanism constructed from our data (Fig. 2). Isotopic label- limited by their attack by methoxy, the rates of the methyl esters
ling experiments also establish that the methoxy group is preserved in derived from benzaldehyde and benzeneacetaldehyde are limited
the reaction with the aldehydes and that hydrogen is lost from the by their desorption from the surface (Supplementary Fig. S4).
aldehydic carbon to yield the esters. The reactions observed are Reactions of methoxy with benzaldehyde and benzeneacetaldehyde
all exclusively CD3O(ads) þ RC(H)¼O ! CD3OC(R)¼O (R¼H-, yield methyl esters at higher temperature, ꢁ325 K, indicating that
CH3-, Ph-, PhCH2-) (Fig. 1b).
the phenyl ring increases the interaction energy of the ester with
Remarkably, the O/Au(111) promotes ester synthesis with fast the surface. Therefore, the phenyl-containing methyl esters will
kinetics, as signified by the low temperature for ester evolution, have a longer surface lifetime under steady-state reaction conditions,
which corresponds to an estimated activation barrier of which could lead to more secondary oxidation if there is oxygen
ꢁ8 kcal mol–1. The coupling of methoxy with either formaldehyde available for reaction.
or acetaldehyde yields the corresponding gaseous ester in two distinct
Surprisingly, the presence of excess aldehyde impedes secondary
kinetic processes—that is, in two peaks (Fig. 1). The predominant, oxidation to carboxylate species under the conditions of our exper-
lower-temperature process occurs far below room temperature, at iments, where oxygen is in limited supply. There is only a small
180+5 K, ꢁ40 K lower than the peak temperature measured for amount of secondary oxidation to organic acids (,5% relative to
self-coupling of methanol and ethanol17,18. This is a clear manifes- the esters) in all cases studied. Carboxylate species were identified
tation that the presence of the aldehyde accelerates the formation of as the intermediates leading to secondary oxidation of both metha-
the ester by the mechanism illustrated in Fig. 2. By supplying either nol and ethanol on O/Au(111) using vibrational spectroscopy17,18
.
formaldehyde or acetaldehyde, the rate-limiting step in self-coup- In the cases of methanol cross-coupling with formaldehyde, acet-
ling17,18,31 is circumvented, leading to ester formation at a lower temp- aldehyde and benzaldehyde, the corresponding acids—formic
erature in a process limited by the attack of the aldehyde by the acid, acetic acid and benzoic acid—were observed. Both benzoic
methoxy (Fig. 2).
The nature of the rate-determining step for ester evolution coupled with benzeneacetaldehyde (data not shown).
depends on the molecular structure, leading to a higher activation Even though secondary oxidation of the aldehydes themselves
acid and benzeneacetic acid were detected when methanol cross-
barrier for evolution of the methyl esters containing phenyl rings occurs if they are reacted in the absence of methoxy on the
(benzaldehyde and benzeneacetaldehyde), estimated to be on the O/Au(111), the amount of combustion is essentially the same for
62
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.467
ARTICLES
surface during methanol oxidation on O/Au(111)17 (Fig. 2).
Subsequent and rapid loss of hydrogen from the aldehydic carbon
yields the ester. This last step may be promoted by adsorbed
atomic oxygen or other Brønsted base sites on Au. Our mechanism
is in broad agreement with work published previously on supported
catalysts, which invokes a qualitatively similar process but without
elementary steps10. A similar mechanism was previously suggested
for methanol self-esterification over O/Ag(110)25.
RCHO
CH3O(a)
O(a)
In liquid-phase reactions, the initial formation of aldehydes and
the final production of the ester were proposed to occur on Au;
however, the coupling step was proposed to occur via an acid-cata-
lysed reaction of alcohol and the aldehyde5,7,13,14. Surprisingly, the
liquid-phase mechanism suggests that the Au catalyst plays no
role in these intermediate steps. The fact that all products—includ-
ing esters—are formed on O/Au(111) is incontrovertible evidence
that steps subsequent to aldehyde formation are also promoted on
the O/Au surface. Our proposed mechanism provides a guiding
principle that esterification involves surface-mediated nucleophilic
attack of the aldehydic carbon by alkoxide. Although it is difficult
to prove, it is likely that Au-mediated coupling also occurs in sol-
ution given the correspondence with our results.
There are several possible roles that the metal oxide support may
also play in the liquid-phase reactions. Small amounts of base—for
example, Na-methanolate16 or Lewis acidic supports such as Ga2O3
(ref. 10)—enhance the catalytic activity, which might indicate pro-
motion of one or both C–H scission steps by the support.
However, the observation of these same steps in vapour-phase reac-
tions with no support indicates that oxygen-covered Au is itself
capable of promoting these reactions, most likely through oxygen-
assisted C–H bond scission. On Au(111), the protons lost in the
overall oxidation of the aldehyde to the ester leave the surface as
water. A possible role of the reducible metal oxide support is to
serve as a source of adsorbed oxygen on the Au particles. Oxygen
is necessary to activate the alcohol and to form the aldehyde in
self-coupling. The reducible supports could supply oxygen via
spill-over, as has been suggested previously15 for glucose oxidation
over Au supported on CeO2. A similar concept also applies to redu-
cible oxide-supported Au-particle catalysts: the oxygen atoms at the
+
H
C
O
O3
CH3OH
CH3OC(H)(R)=O(a)
R = H-, Me-, Ph- or PhCH2-
Au(111)
+
CH3OC(R)=O
Figure 2 | Schematic mechanism for coupling of methanol and aldehydes
on oxygen-covered gold particles. Starting from the left-hand-side, the
reaction steps depicted in the figure are: first, O3 and CH3OH are introduced
sequentially onto the Au(111) surface to form atomic oxygen (O(a)) and
methoxy (CH3O(a)); next, the aldehyde (RCHO) is introduced, after which
nucleophilic attack of methoxy to the aldehydic carbon forms the surface
intermediate CH3OC(H)(R)¼O(a) (the red arrow represents the nucleophilic
attack); and last, further b-H elimination of CH3OC(H)(R)¼O(a) forms the
ester (CH3OC(R)¼O).
the reaction of methanol alone (self-coupling) or when there is
excess formaldehyde present. As a result, the selectivity, as judged
from the ratio of the yields of CO2 and methyl formate, is increased
by a factor of approximately seven relative to the oxidation of metha-
nol alone (Fig. 3). This indicates that the attack of alkoxy to alde-
hydes competes favourably with that of surface oxygen species in
the combustion process. Therefore the cross-coupling reactions
effectively suppress the secondary oxidation of aldehydes by
directing them to the more facile esterification process.
In all cases studied, the selectivity for esterification relative to
combustion is higher when aldehydes are available to react with
methoxy on the surface (Fig. 3), clearly demonstrating that attack
of the aldehyde by methoxy competes effectively with attack by
adsorbed oxygen. The selectivity for ester production relative to
combustion to CO2 on a per molecule basis is ꢁ7:1 and is essentially
independent of the molecular structure of the aldehyde (Fig. 3). It is
important to note that all oxygen is consumed during the reaction,
so it is a limiting reagent. We oxidized the excess carbon deposited
from decomposition of secondary oxidation intermediates in the
benzaldehyde and benzeneacetaldehyde to estimate the total CO2
in Fig. 3. The nearly constant CO2:ester ratio provides an approxi-
mate carbon and hydrogen mass balance and shows that there are
no major products missing, a point also established in the mass
spectrometer analysis. These results are clear evidence that attack
by methoxy is more facile than attack by oxygen—leading to com-
bustion—in the case of all four aldehydes, which each have quite
different molecular structures.
8
6
4
2
0
Our studies clearly show that atomic oxygen present on Au nano-
particles formed on Au(111) promotes cross-coupling of methanol
and a variety of aldehydes to selectively yield the corresponding
esters. Thus, we unequivocally establish that all steps in the esterifica-
tion can be mediated by the Au surface and that liquid-phase
reactions, including those involving dissolved Au ions, are not necess-
ary. In the liquid phase, alcohols have been selectively and catalytically
oxidized to esters2,5,7–10,19, aldehydes1–4,6,8,9,11,12 and organic acids8,12
over Au nanoparticles supported on metal oxides. Our results parallel
the addition of aldehydes to methanol catalysed by supported Au in Figure 3 | Relative selectivity of esterification versus combustion in the
solution, which yielded various methyl esters7,13,14,16,19
.
self-coupling of methanol and cross-coupling between methanol and
In our surface-mediated mechanism, an alkoxy hemiacetal aldehydes. The selectivity towards esterification is about seven times higher
(CH3OC(H)(R)¼O) intermediate is proposed to form on the in cross-coupling reactions, compared with that of self-coupling of methanol.
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.467
ARTICLES
The relative selectivity towards esters (versus combustion) formed in the self-
coupling of methanol and cross-coupling between methanol and aldehydes was
estimated on a per aldehyde molecule basis. It was calculated using the ratio of the
integrated area of the parent ion of the ester in the temperature-programmed reaction
spectroscopy (for example m/z 60 for methyl formate, corrected for cracking fraction)
to that of CO2 (m/z 44, corrected for cracking fraction) divided by the number of
carbon atoms in the aldehyde involved in the self- and cross-coupling reactions
(divided by 1, 1, 2, 7 and 8 in the self-coupling of methanol and cross-coupling of
methanol with formaldehyde, acetaldehyde, benzaldehyde and benzeneacetaldehyde,
respectively). Activation energies were estimated using Redhead analysis, assuming a
pseudo-first order process and a preexponential factor of 1013 s–1. The estimated
interface between the reducible oxide support and the Au particles
could play a very similar role to surface oxygen under ultrahigh
vacuum conditions.
In summary, the reactions we observe in our model system—Au
nanoparticles covered with atomic oxygen and formed on
Au(111)—yield products that closely resemble those for cross-coup-
ling of aldehydes with methanol catalysed by Au nanoparticles in
liquid-phase and under aerobic conditions. Importantly, we have
demonstrated that this class of reactions occurs when only
gaseous components are present, which clearly shows that the activation barrier, depending on whether the ester is reaction limited or desorption
limited, indicates the reaction barrier or desorption barrier.
O/Au surface plays a pivotal role in all steps in the esterification.
Furthermore, we show that this class of reactions occurs in the
absence of a metal oxide support, as long as atomic oxygen is
Received 27 April 2009; accepted 23 October 2009;
published online 29 November 2009
present on Au. This suggests that the support may play the impor-
tant role of supplying oxygen to the Au catalyst15.
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Temperature-programmed reaction spectroscopy. The heating rate for all
temperature-programmed experiments was nearly constant at ꢁ5+0.5 K s–1. The
reaction products were identified by quantitative mass spectrometry (Hiden
HAL/3F) using fragmentation patterns obtained from authentic samples, and they
were found to be in good agreement to NIST reference data34. Quantitative
comparison of the fragmentation patterns of the products of methoxy reactions with
formaldehyde and benzaldehyde to authentic samples of methyl formate and
benzoic acid methyl ester unambiguously identified these as the main products
(Supplementary Figs S1 and S2 and Supplementary Tables S1 and S2). The
fragmentation patterns of the other two products were in good agreement with data
from the NIST database34. No other products were observed in a survey of a wide
mass range: m/z 2–200 for benzeneacetaldehyde, m/z 2–150 for benzaldehyde, and
m/z 2–100 for acetaldehyde. In all cases, the parent ions of the esters (Fig. 1) were
the ions with the highest m/z.
We eliminated the possibility of methanol self-coupling in the cross-coupling
reactions by monitoring the 60 AMU (methyl formate-d0) and 64 AMU (methyl
formate-d4) signals when using normal methanol and methanol-d3, respectively.
The CO2 yield in the cross-coupling between methanol and formaldehyde or
acetaldehyde was calculated using the integrated peak area of m/z 44 in the
temperature-programmed reaction spectra (corrected for cracking fraction).
Importantly, there is no residual carbon after heating to 750 K for these two
reactions. In the cases of benzaldehyde and benzeneacetaldehyde, there is a
significant amount of residual carbon deposited after reaction with methanol
because of the deficiency of oxygen on the surface. The amount of residual carbon
was determined by exposing the surface after initial reaction to O3 followed by
heating to react off the carbon as CO2. The CO2 yields used to estimate selectivity
include CO2 produced from oxidation of residual carbon.
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Acknowledgements
We gratefully acknowledge the support of this work by the US Department of Energy, Basic
Energy Sciences, under Grant No. FG02-84-ER13289. J.H. (Feodour-Lynen fellowship)
acknowledges support through the A. v. Humboldt foundation. Correspondence and
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Author contributions
B.X., X.L., J.H. and C.F. conceived and designed experiments, analysed and discussed
results, and commented on the manuscript. B.X. performed the experiments and analysed
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chemical compound information accompany this paper at www.nature.com/
naturechemistry. Reprints and permission information is available online at http://npg.nature.
com/reprintsandpermissions/. Correspondence and requests for materials should be
addressed to C.M.F.
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