Developing Bicatalytic Cascade Reactions: Ruthenium-catalyzed Hydrogen Generation From Methanol

Article abstract of DOI:10.1002/chem.201900966

A novel state-of-the-art bicatalytic system for hydrogen generation from aqueous methanol at low temperature and base concentration is described. Applying two molecularly defined ruthenium complexes A and B3 for methanol dehydrogenation at the same time u

Full text of DOI:10.1002/chem.201900966

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Accepted Article
Title: Developing Bi-catalytic Cascade Reactions: Ruthenium-catalysed
Hydrogen Generation from Methanol
Authors: Anastasiya Agapova, Henrik Junge, and Matthias Beller
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10.1002/chem.201900966
Chemistry - A European Journal
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Developing Bi-catalytic Cascade Reactions: Ruthenium-catalysed
Hydrogen Generation from Methanol
Anastasiya Agapova, Henrik Junge and Matthias Beller*[a]
[a]
[b]
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provide better results for each individual step compared to a
mono-catalytic system. Overall, such catalysts´ teamwork
should provide improved performance compared to
standard approaches.
Abstract:
A novel state-of-the-art bi-catalytic system for
hydrogen generation from aqueous methanol at low temperature
and base concentration is described. Applying two molecularly-
defined ruthenium complexes
A and B3 for methanol
dehydrogenation at the same time under the same conditions a
synergistic effect is observed. This behavior is explained by the
increase of the dehydrogenation of formic acid, which is formed
as an intermediate, by the second catalyst B3.
Introduction
Catalysts regulate all kinds of chemical and biological
transformations and thereby play a vital role for life and
material sciences as well as today´s chemical industry.1
Indeed, modern routes for more than 90% of fuels,
polymers, bulk and fine chemicals as well as
pharmaceuticals, agrochemicals etc. involve at least one
catalytic step.2 In these processes, catalysts do not only
improve the reaction rate, but also precisely control their
chemo-, regio-, and stereoselectivity.
When considering catalyst optimization, organometallic
complexes have intrinsic advantages due to their defined
coordination geometry of the active center compared to
structurally more complex heterogeneous materials. Based
on the understanding of their structure-activity relationship,
a more rational modification and design of such catalysts
can be often achieved. However, drawbacks of
homogeneous catalysis include separation from starting
materials and products as well as recycling of the catalyst.
Scheme 1. Overview of selected types of catalysis including proposed multi-
catalytic cascade reactions.
Notably, such multi-catalytic cascade reactions should not be
confused with the existing concepts of synergistic4 and cascade
catalysis.5 Until to date, examples applying orthogonal multi-
catalytic cascade reactions are relatively rare.6 In order to
showcase the power of this concept, we investigated the low
temperature methanol steam reforming as an example, which is
of significant actual interest in the context of a hydrogen
economy.7 As shown in Scheme 2, in this process, the substrate
methanol undergoes three stages in order to release all
available molecules of hydrogen. In the past decade, several
groups including our8 and Grützmacher,9 Fukuzumi,10 and
Milstein11 reported molecular-defined catalysts for methanol
dehydrogenation (DH). In all these system, only one particular
organometallic complex is used for the complete transformation.
In contrast, recently two bi-catalytic systems that combine an
enzyme and a noble metal catalyst (Ir or Ru) achieved more
efficient reforming.12
For example, in the conversion of substrate
S
to the desired
and B,
product , which involves two intermediates
P
A
typically three different catalysts are used (Scheme 1, a).
Thus, multiple purification steps are required. To avoid such
tedious separations, the concepts of domino, tandem and
cascade reactions typically using one catalyst system for
the creation of at least two new bonds have been
introduced (Scheme 1, b).3 Although the advantages of
such a design are obvious, e.g. superior reaction step
economy, it is difficult to design catalysts which promote the
different involved transformations. Even if this is achieved, it
is unlikely that one catalyst system is optimal for all
individual reactions under the same conditions. In general,
the use of more than one catalyst in such domino
processes could solve these problems. As shown in
Scheme 1c, here several specific catalysts are used, which
[a]
A. Agapova, Dr. H. Junge, Prof. Dr. M. Beller
Leibniz-Institut für Katalyse e. V. an der Universität Rostock
Albert-Einstein-Straße 29a, 18059 Rostock
E-mail: matthias.beller@catalysis.de
Scheme 2 Three step homogeneous methanol reforming.
Notably, for the known molecular pincer-based catalysts the last
step of this cascade, formic acid dehydrogenation, is rate-
determining.8b Thus, formic acid accumulates in the system, with
Supporting information for this article is given via a link at the end of
the document.
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10.1002/chem.201900966
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CONCEPT
the consequences of lower efficiency and potential danger of
“killing” the catalyst with too much acid.13 Hence, the key to
improvement of methanol DH lies in improved conversion of
formic acid. One possibility to achieve this goal is the addition of
a second catalyst, which is especially active for formic acid DH.
In this context, we report an improved bi-catalytic methanol
dehydrogenation system using two defined ruthenium-based
PNP pincer complexes at mild conditions. The proposed system
exhibits synergistic catalytic activity using significantly reduced
base amounts.
Results and discussions
In our original work on aqueous methanol reforming below
100°C,8 high catalyst activity was obtained using
Ru(H)(Cl)(CO)(HN{CH2CH2P(iPr)2}2) (A). Unfortunately, this
system operates efficiently only at
a
very high base
concentration (8M KOH). Initial efforts were made to identify
reaction conditions with reduced base amount, including
variation of solvents. Due to the less pronounced salt effect the
internal temperature in the reaction vessel will decrease. e.g.
when the amount of base is decreased from 8 M to 0 M, the
internal temperature drops from 90 °C to 67 °C and therewith the
catalytic activity (Figure SI3, Table SI1). This can be avoided by
addition of a higher boiling solvent (Figures SI4 and SI5). Among
the different tested solvents triglyme turned out to be the most
appropriate and allowed for a decrease of the base amount by
50% maintaining ca. 70 % of catalytic activity compared to
previous conditions.
Based on these new standard conditions, we turned our focus
onto the acceleration of the rate determining step (rds) - formic
acid dehydrogenation - by the addition of a second catalyst and
thus generating bi-catalytic systems. Here, nine Ru, Ir and Fe
complexes (Figure 1, cat. B and Figure SI6), which are mostly
known to be more active for formic acid, but not for methanol
DH,14 were tested together with Ru-PNHP (A). Gas evolutions
after 3h of the bi-catalytic systems were compared to the
efficiency of the single Ru-PNHP pincer catalyst under the same
conditions. As shown in Figure 1, most of the added complexes
did not improve hydrogen generation of the original system. In
fact, they suppressed the activity of A, which we cannot explain
at the moment. However, the combination of A and B3 with the
PNMeP-pincer ligand (Ru(H)(Cl)(CO)(CH3N{CH2CH2P(iPr)2}2))
exhibited noticeably improved gas evolution. It was previously
reported that such methylated ruthenium pincer complex is
less active for methanol DH than the corresponding non-
methylated pincer complex.8b,15
Noteworthy, this improved bi-catalytic system cannot be
generated only by adding the PNMeP-pincer ligand to complex A
(Figure 1, last bar).
Having this specific synergistic system in hand, we explored its
reactivity in more detail. Using different ratios of complexes A
and B3 in the range 4:1 to 1:2 showed little differences and gave
in all cases increased gas evolution (Figure SI7). Hence, we
proceeded with the 1:1 ratio of the single components.
Figure 1. Multi-catalytic systems for methanol dehydrogenation. Reaction
conditions: MeOH (9.0 ml), H2O (1.0 ml), Triglyme (4.0 ml), KOH (40 mmol),
Tset = 92.5 °C, reaction time 3 h; first 9 entries: Σ catalysts A + B1 - B9 (8.56 -
9.62 µmol), where A : B = 1 : 1; entries 10 - 11 catalyst A (4.42 µmol); entry
11: HN(CH2CH2PiPr2)2 ligand (4.21 µmol). All reaction were reproduced at
least twice (St.Dev. < 10 %). Average values corrected by blank volume are
presented.
In order to understand the shape of the kinetic curves, it is
important to mention that our methanol aqueous phase
reforming (APR) usually follows three stages: a) first a sharp
increase of gas evolution (“initialization” phase); b) transitional
segment (“equilibration” phase); and c) moderate increase gas
evolution with long-term stability (“working” phase). This
behavior is explained by the pH change which takes place
during the reaction. The sharp turn of the curve, occurs when
transitioning from “initialization” to “working” phase.
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clearly shows, the combination of two catalysts works better
than each of the catalysts separately, applying the same
overall amount of ruthenium. The composition of the gas
phase in the case of bi-catalytic system is an expected
mixture of H2 and CO2 in stoichiometric proportion 3:1.
Figure 2. Optimization of methanol dehydrogenation via base and solvent
variation. Reaction conditions: MeOH (9.0 ml), H2O (1.0 ml), Tset = 92.5 °C;
catalysts RuPNHP + RuPNMeP 1:1 (Σ = 8.56 - 9.62 µmol), 40 mmol KOH / 4 ml
triglyme (red line); 20 mmol KOH / 10 ml triglyme (green line); 10 mmol KOH /
20 ml triglyme (blue line); 5 mmol KOH / 30 ml triglyme (purple line); all
measurements were performed at least twice and corrected by blank value.
Figure 3. Synergistic effect of bi-catalytic ruthenium system vs mono-catalytic
complexes. Reaction conditions: MeOH (9.0 ml), H2O (1.0 ml), triglyme (20 ml),
Tset = 92.5 °C; catalysts RuPNHP + RuPNMeP 1:1 (Σ = 8.56 - 9.62 µmol). All
data corrected by blank volume.
At this point, a dramatic drop of the pH from 13,5 to 9.0 is
observed in the reaction mixture (Figure SI12). Once the
transition phase is over, the pH is constant. Parallel to this pH
behavior, a change of the gas composition is also observed.
During the first step, almost no CO2 is released from the reaction
mixture, as it is consumed by the base (formation of
(bi)carbonate). Eventually, a 3:1 gas mixture of H2 and CO2 is
liberated during the “real” reforming phase (see SI), which is in
accordance with the stoichiometry of the reaction.
Next, we observed that the advantages of having dual catalysts
are even more pronounced under milder conditions. As shown in
Figure 2 (and Figure SI9), reduction of base from 40 mmol to 5
mmol in the presence of 20 ml triglyme resulted in shortening
the initialization phase from 5 hours to 30 minutes (Figure 2, red
and purple lines, respectively). At the same time, the hydrogen
production rate in the “real methanol reforming” (“working”)
phase increased from 14.3 ml h-1 to 40.5 ml h-1, even though the
base concentration was reduced 4 times.
As shown in Figure 2, varying base/solvent proportions greatly
affected all phases. Since base is essential for catalyst
activation, it cannot be withdrawn completely, without
dramatically losing productivity. Our investigation indicated, that
there is a threshold regarding lowering base concentration,
where activity is still improving, which happens between 5
(Figure 2, purple line) and 10 (Figure 2, blue line) mmol KOH.
Furthermore, it was observed that lowering base concentration
without compensating with solvent leads to 25 fold drop in
activity (Figure SI4).
Finally, mono-catalytic systems with only RuPNHP or
RuPNMeP, adjusted for ruthenium amount, were compared
under the same reaction conditions to the bi-catalytic
system. Even though the activity of mono-catalytic systems
were also relatively high; however, the composition of the
resulting gas mixture consisted mostly of hydrogen (H2:CO2
= 7:1). In comparison to both mono-catalytic systems with
TOF in the working phase 142 h-1 and 168 h-1, the bi-
catalytic system is around 1,5 times more active. This
shows the importance of speeding up the formic acid
dehydrogenation step in Ru-catalyzed methanol APR.
Conclusions
Here, we describe a stable bi-catalytic system active at low
temperature and base concentrations (12.5% from original
system) for hydrogen generation from aqueous methanol. The
synergistic effect of using two molecularly-defined ruthenium
complexes for methanol dehydrogenation at the same time
under the same conditions is clearly demonstrated. In this
process, the second catalyst specifically speeds up the rate-
limiting reaction step of the first one. In general, we believe the
concept using more than one catalyst in domino or cascade
reactions should become more popular in organic synthesis, as
it offers the possibility for boosting selectively certain reaction
pathways, and thus allows significantly increasing the efficiency
for a given multi-step transformation.
Noteworthy, our optimal system containing complexes
A
and B3 actually exhibits a synergistic effect, proofing our
concept of bi-catalytic cascade reactions. As Figure 3
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Experimental Section
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This work was supported by the state of Mecklenburg-
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Research (BMBF). Further financial support was provided by the
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Keywords: methanol dehydrogenation • multi-catalytic cascade
reactions • ruthenium pincer • synergistic effect • bi-catalytic
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CONCEPT
Entry for the Table of Contents
CONCEPT
A
novel bi-catalytic system for
Author(s), Corresponding Author(s)*
hydrogen generation from aqueous
methanol at low temperature and base
concentration is described. The
synergistic effect of using two
molecularly-defined complexes at the
same time is demonstrated.
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Title
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