Microwave-heated γ-Alumina Applied to the Reduction of Aldehydes to Alcohols

Article abstract of DOI:10.1002/cctc.202001284

The development of cheap and robust heterogeneous catalysts for the Meerwein-Ponndorf-Verley (MPV) reduction is desirable due to the difficulties in product isolation and catalyst recovery associated with the traditional use of homogeneous catalysts for MPV. Herein, we show that microwave heated γ-Al₂O₃ can be used for the reduction of aldehydes to alcohols. The reaction is efficient and has a broad substrates scope (19 entries). The products can be isolated by simple filtration, and the catalyst can be regenerated. With the use of microwave heating, we can direct the heating to the catalyst rather than to the whole reaction medium. Furthermore, DFT was used to study the reaction mechanism, and we can conclude that a dual-site mechanism is operative where the aldehyde and 2-propoxide are situated on two adjacent Al sites during the reduction. Additionally, volcano plots were used to rationalize the reactivity of Al₂O₃ in comparison to other metal oxides.

Full text of DOI:10.1002/cctc.202001284

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Microwave-heated γ-Alumina Applied to the Reduction of
Aldehydes to Alcohols
Bhausaheb Dhokale+,[a] Arturo Susarrey-Arce+,*[b, c] Anna Pekkari,[a] August Runemark,[a]
Kasper Moth-Poulsen,[a] Christoph Langhammer,[b] Hanna Härelind,[d] Michael Busch,[e]
Matthias Vandichel,*[f] and Henrik Sundén*[a, g]
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The development of cheap and robust heterogeneous catalysts
for the Meerwein-Ponndorf-Verley (MPV) reduction is desirable
due to the difficulties in product isolation and catalyst recovery
associated with the traditional use of homogeneous catalysts
for MPV. Herein, we show that microwave heated γ-Al2O3 can
be used for the reduction of aldehydes to alcohols. The reaction
is efficient and has a broad substrates scope (19 entries). The
products can be isolated by simple filtration, and the catalyst
can be regenerated. With the use of microwave heating, we can
direct the heating to the catalyst rather than to the whole
reaction medium. Furthermore, DFT was used to study the
reaction mechanism, and we can conclude that a dual-site
mechanism is operative where the aldehyde and 2-propoxide
are situated on two adjacent Al sites during the reduction.
Additionally, volcano plots were used to rationalize the
reactivity of Al2O3 in comparison to other metal oxides.
1. Introduction
[a] Dr. B. Dhokale,+ A. Pekkari, A. Runemark, Prof. K. Moth-Poulsen,
Prof. H. Sundén
Chemistry and Chemical Engineering
Chalmers University of Technology
412 96 Gothenburg (Sweden)
[b] Dr. A. Susarrey-Arce,+ Prof. C. Langhammer
Department of Physics
Chalmers University of Technology
412 96 Gothenburg (Sweden)
[c] Dr. A. Susarrey-Arce+
Alcohols have a ubiquitous role in nature and can be found in
everything from pharmaceuticals to materials. Therefore, the
reduction of aldehydes to alcohols is undoubtedly a very
important chemical transformation.[1–6] The most common way
to reduce an aldehyde to alcohol is to employ a stoichiometric
amount of a hydride-based reagent or a transition metal-based
catalyst in combination with a hydrogen source. Even though
these reactions are highly selective and high yielding, their
usage is problematic due to poor atom economy, the scarcity of
transition metals, waste disposal, and safety issues.[7,8] These
issues can partially be avoided by employing the MPV
reduction.[9,10] In the MPV reduction a keto compound is
reduced to the corresponding alcohol via a transfer of a hydride
from a sacrificial low molecular weight alcohol over a homoge-
nous Al-based catalyst.[11–16] However, separation of the homo-
geneous catalyst and purification of the formed alcohol usually
requires column chromatography leading to solvent losses and
thus providing these reactions with a high E-factor.[17,18] In this
perspective, heterogeneous catalysis offers an obvious benefit
as most heterogeneous catalyst in principle can be removed
from the reaction mixture by a simple filtration and thereby
also regenerated.
A number of heterogeneous catalysts have been developed
for the MPV reduction,[19] such as, zeolites,[20–25] mesoporous
silicates,[26] hydrotalcite,[27,28] metal oxides[29–35] or metal-organic
frameworks.[36,37] However, most of them involve complex
synthetic routes with the incorporation of various metals[24,25,38,39]
and in some cases rare metals.[40] An inexpensive, readily
available robust and non-toxic catalyst material for the hetero-
geneous MPV reduction would be highly desirable.
Alumina (Al2O3) meets most of the above-mentioned
criteria. However, for the MPV reduction, Al2O3 performs poorly,
and it is very often doped with other elements to enhance its
reactivity.[41–47] In this work, we argue that alumina, with the
Mesoscale Chemical Systems
MESA+ Institute
University of Twente
P.O. Box 217
Enschede 7500AE (Netherlands)
E-mail: a.susarreyarce@utwente.nl
[d] Prof. H. Härelind
Competence Centre for Catalysis
Department of Chemistry and Chemical Engineering
Chalmers University of Technology
412 96 Gothenburg (Sweden)
[e] Dr. M. Busch
Department of Chemistry and Material Science
School of Chemical Engineering
Aalto Universit
02150 Espoo (Finland)
[f] Dr. M. Vandichel
Department of Chemical Sciences and Bernal Institute
University of Limerick
Limerick V94 T9PX (Ireland)
E-mail: matthias.vandichel@ul.ie
[g] Prof. H. Sundén
Chemistry and Molecular Biology
University of Gothenburg
412 96 Gothenburg (Sweden)
E-mail: sundenh@chalmers.se
henrik.sunden@chem.gu.se
[+] These authors contributed equally
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cctc.202001284
© 2020 The Authors. Published by Wiley-VCH GmbH. This is an open access
article under the terms of the Creative Commons Attribution License, which
permits use, distribution and reproduction in any medium, provided the
original work is properly cited.
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right activation mode, can serve as a good catalyst material due
to the occurrence of several coordination sites that are present
on the catalyst surface that is set up to coordinate both a
carbonyl and alcohol. A suitable activation mode is microwave
irradiation. In the literature, there are several reports on
microwave-assisted heating of alumina. For instance, sintering
of alumina based ceramics can be performed under microwave
irradiation leading to materials with substantially different
properties as compared to materials made with conventional
heating.[48] Furthermore, there are several reports on micro-
wave-assisted alumina mediated reactions[49] such as
alkylation[50] and condensation reactions[50–53] or a combination
of the two.[54] From these reports, it seemed likely that micro-
wave heating could be used to enhance the catalytic properties
of alumina in the MPV reduction through direct heating of the
catalyst. The use of microwave radiation to carry out MPV
reaction in the presence of activated neutral or basic alumina in
a domestic microwave oven was required to use the catalytic
amount of bases.[55] Herein, we report a reduction of aldehydes
to alcohols over a heterogeneous γ-Al2O3 using microwave-
assisted heating. The scope of the reduction is broad and
includes aromatic, unsaturated and aliphatic aldehydes and is
to the inside, resulting in a slower heating, something that is
reflected in the lower reaction rate.[60,62–65]
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Poor chemoselectivity is a common issue for the reduction
of α,β-unsaturated carbonyl compounds with metal hydrides.
Hence, the scope of the optimized MW heating reaction was
investigated utilizing a series of α,β-unsaturated aldehydes. As
it turns out, α,β-unsaturated aldehydes perform well delivering
a broad range of allylic alcohols in excellent yields and
chemoselectivity (Figure 1). The reaction shows high functional
group compatibility and functional groups such as nitro, and
halogens (fluoro, chloro) and methoxy are well tolerated
providing the corresponding allylic alcohols in 95–98% yield,
with high chemoselectivity (entries 2b–2f). Next, benzalde-
hydes were investigated as reaction partners in the heteroge-
neous MPV reduction. Both electron-rich and electron-deficient
aldehydes are well tolerated, and the corresponding benzylic
alcohols could be isolated in almost quantitative yields
(entries 2g–2m). Furthermore, the heteroaromatics furfural and
3- and 4-pyridinecarboxaldehyde can also be reduced to their
alcohol counterparts in 92%, 98%, and 95% yield, respectively
(entries 2n–2p). It is worth noting that the valorization of
furfural (1n) to furfuryl alcohol (2n) is a synthetically important
transformation in the production of valuable chemicals and
biofuel. Table S3 (ESI) compares the selected efforts devoted to
carrying out this transformation. We are reporting this trans-
formation without expensive novel and non-novel metals
catalyst, without H2 gas, and without the column chromato-
graphic purification with an isolated yield of 92%.[66–69] Reduc-
tion of a non-activated aldehyde was also possible with 3-
phenyl propanal reduced to 3-phenylpropanol with an isolated
yield of 78% (entry 2q). Finally, sterically demanding α,β-
unsaturated aldehydes with substituents in the α-position can
be reduced affording the α-amylcinnamyl alcohol and α-
bromocinnamyl alcohol both in 97% yield (entries 2r and 2s).
To reach full conversion, α-amylcinnamaldehyde and the α-
bromocinnamaldehyde required twice the amount of γ-Al2O3,
i.e., 200 mg. The higher loading needed for these two entries is
ascribed to the bulky substituents in the α-position, thus
making them less reactive. From these results, it is clear that
selective reduction of α,β-unsaturated carbonyl and other
aldehydes with 2-propanol can be achieved over γ-Al2O3 with
microwave heating.
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compatible with
a high degree of substituents without
compromising conversion and selectivity. Mechanistically, con-
trary to the conventional MPV single site mechanism,[19,56–59] we
propose via density functional theory (DFT) the preference for a
dual-site mechanism, where 2-propanol forms an 2-propoxide
onto a Lewis acid site which subsequently transfers hydrogen
to the carbonyl group adsorbed on a neighbouring Lewis acid
site. Our computational analysis confirms that among other
oxides typically used for MPV, γ-Al2O3 is the catalyst of choice
for the reduction of aldehydes.
2. Results and Discussions
2.1. MPV reduction
Our study commenced with the evaluation of nine different
alcohols as the hydride source under microwave heating
conditions (Figure S1.1). The results from this initial screening
showed that secondary alcohols like 2-propanol were superior
for the reduction of cinnamaldehyde providing the cinnamyl
alcohol in 99% conversion and selectivity. Hence, 2-propanol
was selected as the sacrificial reducing agent. To evaluate the
effects of microwave heating, the reaction was performed using
In the reaction scope, it was also observed that aldehydes
bearing aromatic substituents such as tertiary amines and
hydroxyls generate ethers rather than alcohols (Figure 2). For
example, dimethylamino substituted aldehydes generated the
corresponding ethers as the only product with 94% and 90%
yield, respectively (entries 3t and 3u). Similarly, hydroxy-
substituted aldehydes also yielded ethers (entries 3v and 3w)
that could be isolated in 60% and 62% yield. In an effort to
shed some light on the etherification mechanism we performed
two reactions starting from vanillyl alcohol. Vanillyl alcohol was
microwave heated in the absence of γ-Al2O3. Under these
conditions, no ether is formed indicating that the γ-Al2O3 is
active in the etherification step (Figure 3a). This notion was
further supported by an experiment where vanillyl alcohol in
the presence of γ-Al2O3, under standard conditions, reacts to
°
a conventional heating block at 180 C and the reaction
outcome was followed using gas chromatography (Figure S1.9).
A fourth-fold decrease in reaction rate compared to microwave
heating was observed. The significant difference in reaction
outcome points towards that microwave heating is a key
element in the γ-Al2O3 catalyzed MPV reduction of aldehydes. A
viable explanation is that the microwave heating ensures rapid
[60] [61]
and homogeneous heating of the reaction mixture,
but
also provides direct heating of the catalyst which can efficiently
promote a faster reaction on the γ-Al2O3 surface. In contrast,
conventional heating heats the reaction vessel from the outside
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Figure 1. Scope of reaction methodology for different aldehydes. All yields are reported after filtration of reaction mixture and evaporation of the solvent. [a]
Isolated by column chromatography (reactant purity 90%). [b] 200 mg of catalyst used.
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Figure 2. Scope of reaction for amine and phenol substituted aldehydes. All
yields are reported after filtration of reaction mixture and evaporation of the
solvent. [a] Isolated by column chromatography.
Figure 4. a) Conversion and selectivity as a function of the recyclability cycle
for the fresh, 1st cycle, 2nd cycle, and 3rd cycle. b) Conversion and selectivity
for three recovery cycles 1st cycle (1st Rcycle), 2nd (2nd Rcycle), and 3rd cycle (3rd R
cycle).
the 3rd cycle. The conversion-loss may be explained by
increased inaccessibility of the aluminium oxide surface due to
coke formation i.e. accumulation of carbon[70] or water or
structural changes of the solid phase. Therefore, the fresh and
1st cycle used γ-Al2O3 were analyzed with X-ray photoemission
spectroscopy (XPS) to generate understanding of conversion-
losses due to the carbon accumulation. The results show that
the carbon content over the γ-Al2O3 increased after the 1st
reaction cycle (Figure S1.3) suggesting the presence of carbon
deposits blocking Al sites, thus decreasing the catalytic activity
for the 1st cycle and subsequent 2nd cycle. Next, we looked at
the effect of water content. These experiments were made to
investigate water poisoning and reveal that water content is
deleterious for the activity of the catalyst (Figure S1.4). The
effect is clear and can be seen already in the presence of 1% of
water with the reaction stopping at 50% conversion. To this
end, the potential structural changes of the γ-Al2O3 were
investigated with x-ray diffraction (XRD) (Figure S1.5).[71] No
structural differences can be seen between the fresh and the 1st
cycle and 3rd cycle. Taken together, the results suggest that
catalyst deactivation is due to the accumulation of carbon and/
or water adsorption in the catalyst after the reaction, blocking
the active acid sites on γ-Al2O3. Thermal treatment over the
catalyst was performed to remove the blocking acid sites from
residues during reusability from the γ-Al2O3. It is clear that with
this treatment, the catalyst recovers (R) with the γ-Al2O3
nominally regaining its activity. The results indicate that the
catalyst can be used in the MPV reduction of cinnamaldehyde
as confirmed with the increase of conversion and selectivity
(Rcycle in Figure 4b). It is important to note that the objective of
this thermal treatment is not to study the catalyst deactivation
mechanism nor targeted as a recycling experiment. However, it
could generate insights into the potential use of thermal
treatment to remove adsorbed species over the catalyst after
each MPV reduction reaction cycle.
Figure 3. Etherification reaction of vanillyl alcohol. a) standard reaction
conditions without γ-Al2O3. b) Standard conditions with γ-Al2O3.
form the isopropyl ether (Figure 3b). From these results we
propose a reaction mechanism for the etherification where the
aldehyde is first reduced to the allylic alcohol in accordance
with the MPV mechanism. Next, the alcohol is deoxygenated by
the aid of the alumina to render a p-quinone methide that
reacts with isopropanol to form the isopropyl ether.
2.2. Catalyst reusability performance and thermal treatment
After having evaluated the scope of the heterogenous MPV
reduction and shown that the reaction is high yielding, chemo-
selective, and requires only filtration to separate the catalyst
from the pure product, our attention turned towards the
reusability of the catalyst. For the first set of reusability
experiment, we used the same γ-Al2O3 in four consecutive MPV
reduction cycles (i.e., starting with fresh γ-Al2O3 and reusing it
three times, data shown in Figure 4a). It is found that the
cinnamaldehyde conversion steadily decreases from 100 to
30% over three reaction periods. However, irrespective of the
conversion-loss, the selectivity is maintained at 80% or higher
for the fresh, 1st cycle, and 2nd cycle followed by a 40% drop in
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2.3. Acid sites in γ-Al2O3
Including MW-heating would require detailed knowledge of the
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interface between water and the oxide, which is still
unknown.[76] The difference between traditional thermal heating
and microwave heating is that MW-radiation increases the
rotational entropy of H2O molecules, while thermal heating
more directly affects vibrational entropy.[60] Furthermore, the
deprotonation and protonation steps of the alcohols are
omitted. This is justified as these steps are usually fast and
typically only require to overcome a negligible activation
barrier.[77–81] Instead, we focused on the mechanistic aspects of
the heterogeneous γ-Al2O3 catalysed MPV-reduction. Hence, the
reaction mechanism of the MPV reaction is studied using a γ-
Al2O3 (110) model system where 1 additional water molecule is
kept within the unit cell. This corresponds to an OH coverage of
3 OH groups per nm2 (see Figure S1.2). The reason to keep this
chemisorbed water molecule lies in the very high energy gain
of more than À 200 kJ/mol for the first dissociative chemisorp-
tion on the bare γ-Al2O3 (110) model system. Therefore, this
type of chemisorbed water is even very hard to remove in
vacuum conditions at high temperature (Figure S1.2). Note that
our simplified model excludes the presence of defects, like
edges, kinks, or steps in the γ-Al2O3.
Building on these simplifications and using the above-
described model system, it is now possible to study the reaction
mechanism. The MPV reaction may in principle proceed
through a single- or dual-site mechanism (Figure 6a). Both
routes possess an equivalent reaction sequence but differ by
assuming the adsorption of the reactants at a single Al site (a
single-site mechanism) or the adsorption at two adjacent Al
sites (a dual-site mechanism). In both mechanisms, the reaction
is initialized by the adsorption of isopropanol (R!A). Assuming
a low coverage (LC) situation where apart from the dissociated
water molecules, no other adsorbates are present, this reaction
is exergonic by À 125 kJ/mol (State A, Figure 6b) for either
reaction route. At higher 2-propoxide coverage (HC), we have
considered chemisorbed 2- propoxide on two different active
sites in our model system, site *1 and *2 (see Figure S1.6). These
Next, we utilized pyridine (Py) as a molecular probe to identify
the type of acid sites in γ-Al2O3. The Py IR spectra for a pre-
°
treated γ-Al2O3 at 200 C samples is shown in Figure 5a. The
three IR bands located at 1450, 1492 and 1615 cmÀ 1 are
assigned to Py-adsorbed on a Lewis acid center[72,73] (e.g., Al-
site). No characteristic IR bands for Brønsted acid sites (~
1540 cmÀ 1) are found. This can be attributed to the low surface
concentration of Brønsted sites in γ-Al2O3. To probe weak
Brønsted acid sites we adsorbed 2,6-lutidine (Lu) over a similarly
pre-treated γ-Al2O3. The reason for utilizing Lu is because of its
weak affinity to Lewis acid sites (i.e. steric hindrance induced by
the methyl groups).[74] The IR spectra for the adsorbed Lu over
the pre-treated catalyst is displayed in Figure 5b. No Lu-peak
around 1630–1650 cmÀ 1 was recorded indicating the absence
or low amount of Brønsted acids.[74,75] The broad IR band around
1456 cmÀ 1 in Figure 5b is assigned to Lewis acid sites.[75] From
our results, it is clear that the main active sites available for
aldehyde reduction reactions are the Lewis acid sites. The
results on IR-probed acid sites are in good agreement with the
total acidity of γ-Al2O3 (i.e., 1.02 μmol mÀ 2) derived from the
ratio of total acidity obtained with temperature-programmed
desorption of NH3 and surface area measurements in Table S2.
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2.4. Mechanistic insights on γ-Al2O3
The reduction of cinnamaldehyde to cinnamyl alcohol was
chosen as our model reaction owing to the possibility of
reduction of both the carbon-carbon double bond and the
carbonyl group present. The level of complexity of the
experimental system and the limitations imposed by DFT are
significant. Several simplifications need to be made when
modelling the reaction and constructing the model system.
Using this ansatz, it is not possible to include microwave
heating effects other than selective heating of the solid phase.
Figure 5. IR spectra for adsorbed a) pyridine and b) 2,6-lutidin over γ-Al2O3 catalyst.
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Figure 6. a) Schematic representation of the single site (LC/single Site) and dual-site (LC/*1) mechanisms at low (LC) and high (HC) 2-propanol coverage.
Atomic colour codes: aluminium (light grey), oxygen (red), carbon (brown), hydrogen (white). b) Free energy diagram for the single and dual-site paths at low
coverage. c) Free energy diagram of the dual-site mechanism at high 2-propanol coverage over two distinct active sites HC/*1 and HC/*2. Details on the two
active sites can be found in Figure S1.7.
Al-sites are completely different, in the sense that they have
oxygen coordination of 3 and 5, respectively. As a consequence,
the 2-propoxide chemisorption energy on *1 will be much
stronger than on-site *2. The Lewis site where cinnamaldehyde
adsorbs in all calculated pathways has an oxygen coordination
number of 4.
mechanisms LC/*1, HC/*1 and HC/*2; in Figure 6a) or at the
same Al site where already the alcohol has been adsorbed (a
single site mechanism; LC/*single site; in Figure 6a).
Owing to the significant steric hindrance imposed by the
closeness of the bulky cinnamaldehyde and 2-propanol; this
step is strongly endergonic by 128 kJ/mol for a single site
mechanism (black trace, A!B, Figure 6b). Considering these
unfavorable energetics it is unlikely that this reaction path
significantly contributes to the overall observed catalytic
activity. Avoiding the steric hindrance by placing the bulky
aldehyde at an adjacent bare Al site renders this reaction
Note that when reaction mechanism are discussed at high
coverage, we explicitly mention the 2-proxide that reacts, for
example HC/*1, HC/*2 (see Figure S1.7). The chemisorption of 2-
propanol on a particular site is followed by the adsorption of
cinnamaldehyde either at a free adjacent Al site (dual-site
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approximately thermoneutral, i.e. the change in energy is far
below �10 kJ/mol (red trace, A!B, Figure 6b). This is not
unexpected, considering that only weak van-der-Waals inter-
actions characterize the adsorption of an aldehyde at the Al
site.
Once both reactants have been adsorbed, the reduction of
cinnamaldehyde to cinnamyl alcohol can take place through a
hydrogen transfer from 2-propanol to the aldehyde (B!C in
Figure 6a). For a single site mechanism, this reaction is slightly
endergonic by 24 kJ/mol and associated with a minor activation
barrier of 36 kJ/mol (black trace, Figure 6b). Thus, this reaction
is feasible under reaction conditions provided the two reactants
could be placed at the same catalytic site. This is contrasted by
the dual-site mechanism where the equivalent hydrogen trans-
fer is strongly endergonic by 70 kJ/mol (red trace, Figure 6b).
followed by slightly exergonic adsorption of cinnamaldehyde
and an equally exergonic hydrogen transfer from 2-propanol to
cinnamaldehyde (green trace, A!B and B!C in Figure 6c). The
release of the products is finally only slightly endergonic by
7 kJ/mol and 31 kJ/mol for C!D and D!P, respectively (HC/*2,
Figure 6c). Overall, the MPV over the HC/*2 pathway displays a
much more balanced energy profile which is typically consid-
ered as a prerequisite for a highly active catalyst.
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2.5. Catalyst selection rules for the HC/*2 pathway
Having established a mechanistic understanding of the MPV
over γ-Al2O3 it is finally possible to develop a more general
understanding of the requirements for highly active materials
for the MPV using volcano plots. Volcano plots have previously
been used successfully to compare a wide range of catalytic
systems ranging from homogeneous catalysts[82,83] to
electrocatalysts[84–86] and heterogeneous catalysis.[87] Computa-
tional volcano plots fundamentally rely on the detailed knowl-
edge of the reaction mechanism and the fact that the binding
energies of the intermediates cannot be varied independently
but instead are described by a set of linear equations, the so-
called linear free energy scaling relationships (LFESR).[88–90]
Suitable LFESRs for the MPV reaction are obtained by comput-
ing the binding energies of the intermediates A to D for a large
data set (in the present case 15 main group and transition
metal oxides) and plotting the binding energies of the
descriptor intermediate (x-axis) versus that of the other
intermediates (y-axis). The descriptor intermediate is typically
chosen such that the best correlation between the data points
is obtained. Details on the obtained LFESRs for the MPV
reaction can be found in Figure S1.8. When performing such an
analysis, the computations are often restricted to only include
thermodynamics. This can be justified considering that thermo-
dynamically unfavourable catalysts will not display favourable
kinetics. Furthermore, materials with similar binding energies
are known to display comparable activation barriers since
thermodynamics and kinetics are known to be connected
through Brønsted-Evans-Polanyi relationships.[83,91–94]
Following this procedure and using the free energy of
intermediate state B (2-propoxide+adsorbed cinnamaldehyde)
or ΔG(B) as the reference state, strong scaling relations are
found for ΔG(A), and ΔG(C) (Figures S1.8a and S1.8b). On the
other hand, only fair scaling between ΔG(D) and ΔG(B) is
obtained (Figure S1.8c). These relationships can be expected to
hold both for high and low coverage situations since steric
interactions have been observed only to affect kinetics but not
scaling relations between thermodynamic properties.[83,95]
Based on the LFERs it is now possible to predict the reaction
energies of all considered reaction steps for a given descriptor,
for example, the binding free energy. By considering only the
energetically least favourable step for a given descriptor value,
the volcano plot shown in Figure 7 is obtained. This volcano
Accordingly, it is already from
a purely thermodynamic
perspective unlikely that the reaction can proceed through this
path. In the light of these unfavourable energetics and the
significant costs associated with estimating activation barriers
for the indirect hydrogen transfer between the two adsorbates,
no activation barriers were computed.
The hydrogen transfer is followed by the subsequent release
of the products (C!D and D!P black trace, Figure 6a). These
steps are again unproblematic for a single-site mechanism, i.e.
the release of acetone is exergonic by À 34 kJ/mol followed by
an approximately thermoneutral release of cinnamyl alcohol
(red trace, Figure 6b). For a dual-site mechanism, the release
reaction is, due to the intermediates being very strongly bound,
significantly less favourable. The latter requires 51 kJ/mol to
overcome the thermodynamic barrier for the release of acetone
while the release of cinnamyl alcohol is again approximately
thermoneutral. It is important to note that only a bare surface
without any excess 2-propanol adsorbed has been considered.
However, at a large excess of 2-propanol in the solution, the
surface may at least partially be covered by this species. To
model such a high coverage situation, 2-propanol molecules at
Al adsorption sites were placed on the γ-Al2O3 (110) model (*1
and *2). This allows for the construction of two high coverage
models, HC/*1 and HC/*2. Dependent on the position of the 2-
propoxide (see Figure S1.6), a different empty site and thus,
reaction mechanism can be operative (see Figure S1.7). Further-
more, having determined, that the single site mechanism is
sterically hindered and therefore highly unlikely, we decided
only to consider the dual-site path. Comparing the adsorption
energies of the initial adsorption of the 2-propanol molecules,
we found that the empty HC/*1 site binds, in the presence of a
second 2-propanol molecule, the cinnamaldehyde very strongly
with a binding energy of À 150 kJ/mol (blue trace, Figure 6c).
This is opposed to the HC/*2 sites where the adsorption of 2-
propyl alcohol is even slightly endergonic by 13 kJ/mol (green
trace, Figure 6c). In line with the very strong binding of the
initial 2-propanol, we also find the subsequent intermediates B,
C and D to bind very strongly (blue trace, Figure 6c) at the HC/
*1 site. As a result of this, the MPV reaction is equivalent to the
low coverage case blocked by the energetically unfavourable
release of the products. This is opposed to the MPV over the
HC/*2 site. Here, the slightly endergonic initial reaction is
corresponds to
a graphical representation of Sabatier’s
principle,[96] which indicates that an ideal catalyst binds the
intermediates neither too strong nor too weak. Such a balanced
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Figure 7. Volcano plot with potential determining steps (pds). Closed red dots are assigned to the thermodynamically most likely catalytic cycle.
catalyst is typically placed at the top of the volcano (Region C!
D in Figure 7). Materials which bind the intermediates too
strongly and consequently are blocked by the release of the
products are found at the left branch of the volcano the so-
called strong binding side (Region D!P in Figure 7) and
materials which bind too weakly are found at the right slope of
the volcano the so-called weak binding side (Region R!A in
Figure 7). In the case of the MPV reaction, the strong binding
side is determined by the release of cinnamyl alcohol (D!P in
Figure 6a) whereas the weak binding side is determined by the
adsorption of 2-propanol (R!A in Figure 6a). For the most ideal
catalysts placed at the top of the volcano finally, the release of
acetone (C!D in Figure 6a) determines the thermodynamic
limit. Nevertheless, also the hydrogen exchange reaction (B !
C) and the co-adsorption of cinnamaldehyde (A!B) are
energetically close to the potential determining step (pds) at
the peak of the volcano depicted in Figure S1.7. Despite the
overall reaction being exergonic by À 12.5 kJ/mol, a slightly
endergonic potential determining step (pds) is observed at the
top.
namically more stable, it must also be expected under
experimental conditions with a potentially determining step
(ΔG(pds)) increased to 88 kJ/mol. In the case of TiO2, this is
placed at the weak binding site and shows (TiO2(LC), Figure 7).
Here, the TiO2, 2-propoxide formation is endergonic (ΔG(A)>0)
as well as the co-adsorption of cinnamaldehyde from state A
(ΔG(B)>ΔG(A)>0). From the thermodynamics, it is thus not
reasonable to assume an even higher 2-propoxide coverage is
likely on the investigated TiO2(110) surface. However, based on
calculations, we can show that at high 2-propoxide coverage,
the catalytic activity shifts even to weaker binding sites
(TiO2(HC)) as in Figure 7. Experimentally, we can expect a low
coverage of 2-propoxides, which brings the potential determin-
ing step (ΔG(pds)) at 56 kJ/mol.
Finally, for γ-Al2O3, the three different dual-site pathways are
considered above (see also Figure 6), and can also be ranked on
the volcano plot. From these three situations, high 2-propoxide
coverage Al2O3 where the weakly bound 2-propanol reacts (HC/
*2) is closest to the top of the volcano plot with a slightly
endergonic potential determining step of only 31 kJ/mol (Fig-
ure 7). Based on thermodynamics, this situation has also been
found to be the most likely pathway on γ-Al2O3 clearly shown in
Figure 6. Thus, excellent performance can be expected for γ-
Al2O3. From the volcano plot and the ab initio thermodynamics
of 2-propoxide adsorption, the expected activity of the
investigated surfaces can be ranked based on the potential
determining step, and follows the following trend: γ-Al2O3
(31 kJ/mol)>TiO2 (56 kJ/mol), ZrO2 (88 kJ/mol). This trend is
perfectly in line with the observed activity within the experi-
ments where γ-Al2O3 presents the highest conversion, followed
by TiO2 and ZrO2.
To this extent, ZrO2, TiO2, and γ-Al2O3 have been considered
experimentally, with similar conversion and selectivity of 20%
for ZrO2, 70% for TiO2 and 99% for γ-Al2O3 during cinnamalde-
hyde transfer hydrogenation reaction. The experimental results
can be compared using a volcano plot by assuming a low 2-
propoxide coverage situation. ZrO2 is expected to be an
excellent catalyst for MPV at low coverage (ZrO2(LC), Figure 7),
however, the adsorption of 2-propoxide at low coverage is also
endergonic (ΔG(A)>0), while a higher 2-propoxide coverage is
strongly exergonic (ΔG(A)!0). The latter situation leads to the
placement of ZrO2(HC) at the strong binding site of the volcano
in Figure 7, where the co-adsorption of cinnamaldehyde from
the high coverage state is still exergonic (ΔG(B)<ΔG(A)!0).
Since the high 2-propoxide coverage situation is thermody-
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3. Conclusions
4.1.3. Leaching experiments
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In this work, we have demonstrated that microwave heating of
γ-Al2O3 enhances the selectivity and efficiency of the MPV
reduction of aldehydes to alcohols. The reactions proceed
without any dopant in the form of transition metals. The
reaction has also a broad substrate scope and is compatible
with aromatic, unsaturated, and aliphatic aldehydes delivering
the corresponding alcohols in excellent yields. Furthermore, the
scope is not limited to aldehyde reduction and can be well
applied for the synthesis of ethers. Due to the high purity of the
reaction mixtures, the product could be isolated by simple
filtration and thus minimizing solvent losses during purification.
DFT calculations demonstrate the preference of a γ-Al2O3
surface dual-site mechanism over the established single site
mechanism. Among other catalysts (e.g., ZrO2 and TiO2) γ-Al2O3
has the highest catalytic activity. With our volcano plot analysis
considering various reaction mechanisms on γ-Al2O3, we have
identified that the dual site and high coverage reaction
mechanism on γ-Al2O3 results in the highest catalytic activity.
We believe that these findings will pave the way for new
microwave heated γ-Al2O3 catalyzed reactions.
To probe leaching of γ-Al2O3, 100 mg of γ-Al2O3 catalyst was
suspended in 2-propanol and placed in the microwave for
°
40 min at 180 C. After filtration, 0.8 mmol of cinnamaldehyde
was added to the filtered solution and placed it back in the
°
microwave for an additional 40 min at 180 C. No reaction
products were found.
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4.2. Product analysis
During optimization GC-FID studies were performed on an
Agilent 7820A GC (G4350B) with
a 7693A auto-injector
equipped with an Agilent J&W HP-5, 30 m, 0.32 mm, 0.25 μm
GC Column – 19091J-413 column using nitrogen as carrier gas.
The concentrations of the analytes were determined by
comparison against an internal standard (n-dodecane), stand-
ardized by calibration against authentic samples of the pure
material. Mass yields of the final product were quantified by
using calibration curves of standard samples in the gas
chromatograph. Mass balances accounting for >95% of the
carbon content were obtained in all experiments. Reactant
conversion, and product selectivity were calculated as follows.
4. Materials and Methods
4.1. MPV reduction
�
�
moles reacted
moles before reaction
Conversion % ¼
x100
(1)
(2)
�
�
moles of defined product
moles of reactant consumed
4.1.1. The reduction of aldehydes to alcohols
Selectivity % ¼
x100
The reactions were carried out with γ-Al2O3 SBA-200 from Sasol.
The reactions were heated in a microwave reactor from Biotage
(Biotage Initiator+) in sealed microwave vials.
General experimental procedure for the reduction of aldehydes
to alcohols – The appropriate aldehyde (0.8 mmol), γ-Al2O3
(SBA-200 from Sasol, 100 mg) and 2-propanol (3 ml) was heated
in a sealed microwave vial using a Biotage Initiator+. The
4.3. Nuclear Magnetic Resonance (NMR)
The products were analyzed with 1H Nuclear Magnetic
Resonance (NMR) (400 MHz), and 13C NMR (101 MHz) spectra
1
acquired on a Varian 400. The chemical shifts for H and 13C
°
temperature was set to 180 C with a hold time of 40 minutes
during which a saturation pressure of 1.5×106 Pa was reached.
After complete consumption of aldehyde, the reaction mixture
was filtered and analyzed with an Agilent 7820A GC equipped
with a flame ionization detector to determine conversion and
selectivity. In most cases the alcohol can be obtained in a pure
form directly after filtration and thus no silica gel chromatog-
raphy is needed.
NMR spectra reported in parts per million (ppm) relative to the
residual peak from solvent CDCl3 as the internal standard; H
1
NMR at δ 7.26 ppm and 13C NMR at δ 77.16 ppm. All coupling
constants (J) are reported in Hertz (Hz), and multiplicities are
indicated by s (singlet), d (doublet), t (triplet), dd (doublet of
doublets) and m (multiplet). Furthermore, high-resolution mass
spectrometry (HRMS) was performed on an Agilent 6520
Accurate-Mass Q-TOF equipped with an electrospray interface
for new compound identification. NMR and HRMS are presented
in supporting section 2.
4.1.2. Thermal treatment of the γ-Al2O3
The filtered solid catalyst was washed with 3 ml of 2-propanol
and air-dried at room temperature. Next, the catalyst was
calcined in a furnace at 400 C for 5 h under air, and 4%
hydrogen diluted in argon (Ar) for 12 h. As such, the calcined
material was used for the next round of reactions. The thermal
treatment was repeated three times to ensure reproducibility
and robustness of the γ-Al2O3
4.4. Diffuse Reflectance Infrared Fourier Transform (DRIFT)
spectroscopy
°
The adsorbed species over γ-Al2O3 were characterized in a high-
temperature cell with CaF2 optical access windows for Diffuse
Reflectance Infrared Fourier Transform (DRIFT) Spectroscopy
from Harrick Scientific, Pleasantville, NY, USA. The DRIFT cell
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was mounted on an infrared spectrometer from Bruker Optics
(Tensor 70) with an MCT D316/6-L detector cooled with N2
adapted with a commercial DRIFT cell (Spectra-Tech.) consisted
of four aligned concave polished mirrors mounted on a holder
to achieve IR collection of the outgoing signal.
ent thermodynamic intermediate states are required to describe
the reaction mechanism.
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Convergence of transition states and the intermediates’
geometries is assumed if the largest force was below 0.05 eV/Å.
For the γ-Al2O3 model systems, partial Hessian vibrational
analysis (PHVA) is employed only for the surface species while
keeping the rest of the system fixed. The numerical partial
Hessian was calculated by displacements in x, y, and z-
directions of �0.02 Å, and the vibrational modes were
extracted using the normal mode analysis as implemented in
the post-processing toolkit TAMKIN.[107] A complete loss in
translational and rotational entropy of alcohols and aldehydes
was assumed upon chemisorption or adsorption on the γ-Al2O3
model (Figure S1.2). The Gibbs free energies are calculated at
4.4.1. γ-Al2O3 pre-treatment
The samples were pressed into tablets and then crushed in a
mortar enlarging the powder particles to avoid channelling
during the measurement. Subsequently, the powder fraction of
the crushed tablets was sieved to a size range of 38 to 75 μm.
Then 0.2 g of pre-treated catalyst were placed over KBr bed in
the DRIFT reaction cell, a sample pre-treatment to remove any
trace of reactants and reaction products from the catalyst was
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°
180 C, assuming partial pressure (Pr) for the different reactants.
For example, the estimated P for isopropanol (Pisopropanol) was
4.7×107 Pa, and the P for cinnamyl alcohol, cinnamaldehyde,
acetone (Pcinnamyl alcohol =Pcinnamaldehyde =Pacetone) was 0.47×106 Pa
for 50% conversion, and 0.94×106 Pa for 100% conversion.
In the PHVA-procedure, small imaginary modes were
replaced by 50 cmÀ 1 in the construction of the vibrational
partition function.[108] The PHVA was used to obtain zero-point
corrections and free energy contributions.[77,78,109] The correc-
tions obtained for γ-Al2O3 were used for the other model
systems. This approach was approximative but frequently
applied in the construction of linear free energy reactions, for
example, in electrochemistry during the oxygen evolution
reaction (OER).[84,85]
°
performed for 2 h flowing 50 mL/min of Ar at 150 C and cooled
°
the sample back to 100 C. The sample was left for 1 h before
the starting of the experiment to reach a stable IR signal.
Prior to DRIFT measurements, the sample was pre-treated in
°
N2 at 200 C for 1 h. Followed by 100 mL/min of 99% pyridine
or 98% 2,6-lutidine both saturated in N2 for 1 h. During DRIFT
experiments, both of these organic compounds were ice-cooled
and bubbled for 30 min at 100 mL/min in N2 before saturation.
The IR spectra were recorded with a resolution of 8 cm–1 and
156 co-added scans for 3 h. The reported IR spectra were
measured against N2 used as background and shown baseline-
corrected.
4.5. Computational details
Acknowledgements
To study the MPV reduction, periodic DFT-D calculations are
performed with the Vienna Ab Initio Simulation Package (VASP
5.4.4).[97,98] The BEEF-vdW functional was employed to account
for van der Waals interactions during the MPV-reduction.[99] The
one-electron Kohn-Sham orbitals were expanded in a plane-
wave basis set with a kinetic energy cut-off of 550 eV and the
core electrons were approximated within the projector aug-
mented wave method (PAW)[100] was used, and for the Brillouin
zone sampling different k-point grids were employed depend-
ing on the system size (supporting section 1.2, Table S1). A
Gaussian smearing[97] of 0.1 eV was applied to improve con-
vergence. Additionally, the convergence criterion for the self-
consistent electronic field (SCF) problem is set to 10À 6 eV for
cell optimizations. For γ-Al2O3, a periodic 110 surface slab was
Formas (2016-00484) is gratefully acknowledged for funding. H. S.,
B. D., A. S-A., C. L., K. M.-P. and A. P. would also like to thank Knut
and Alice Wallenberg Foundation for support via project
2015.0055. Computational resources and services used for this
work were provided by C3SE (Göteborg) and ICHEC (Ireland). H. H.
also acknowledge the Competence Centre for Catalysis hosted by
Chalmers University of Technology and financially supported by
the Swedish Energy Agency and the member companies AB Volvo,
ECAPS AB, Johnson Matthey AB, Preem AB, Scania CV AB, Umicore
Denmark ApS and Volvo Car Corporation AB. A.S.-A. would like to
thank Dr. Dirk Niemeyer from Sasol Limited for the provided γ-
Al2O3 samples.
constructed, the surface is composed of 4 layers of Al8O12 to Conflict of Interest
obtain periodic unit cells terminated with surface OH
groups[101–103] shown in supporting section 1.3, Figure S1.2.
Transition states are obtained initially with the climbing image
nudged elastic band method[104,105] and further refined with the
dimer method.[106] Furthermore, the (110) facets of different
oxides of the rutile type (TiO2, SnO2, GeO2, HfO2, IrO2, MnO2,
RhO2, RuO2, ZrO2) and (110) facets corundum type oxides (Cr2O3,
Sc2O3) and the (100) facets of the rock salt type oxides (CaO,
FeO) were taken to construct the linear free energy relations for
the reduction of cinnamaldehyde with isopropanol. Four differ-
The authors declare no conflict of interest.
Keywords: γ-Al2O3
· MPV reduction · Microwave-assisted
catalysis
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Manuscript received: August 5, 2020
Revised manuscript received: August 31, 2020
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Dr. B. Dhokale, Dr. A. Susarrey-Arce*,
A. Pekkari, A. Runemark, Prof. K.
Moth-Poulsen, Prof. C. Langhammer,
Prof. H. Härelind, Dr. M. Busch,
Dr. M. Vandichel*, Prof. H. Sundén*
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Microwave-heated γ-Alumina
Applied to the Reduction of
Aldehydes to Alcohols
Microwave-assisted catalysis:
can be regenerated. With the use of
microwave heating, we can direct the
heating to the catalyst rather than to
the whole reaction medium. The
reaction mechanism was studied by
DFT calculations.
Microwave heated γ-Al2O3 can be
used for the reduction of aldehydes to
alcohols. The reaction is efficient and
has a broad substrates scope (19
entries). The products can be isolated
by simple filtration, and the catalyst