Communication
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Table 1 Influence of the PPh3 ligand amount for the biphasic homo-
however, the separation of catalyst solution and LOHC compounds
is required to enable charging/uncharging of large amounts of
hydrogen with a small amount of precious metal catalyst.
geneous catalyst system for the dehydrogenation of 2-methylindolinea
Conversion (%) TOFc (hÀ1
)
In this work, we propose to address these challenges by
immobilising homogeneously dissolved, ionic hydrogenation/
dehydrogenation catalysts in a molten salt/LOHC liquid–liquid
biphasic system. In detail, we study the hydrogenation/dehydro-
genation of the 2-methylindole/2-methylindoline LOHC system
using cationic Ir complexes immobilised in [PPh4][NTf2] (m.p.
134 1C). The reactions are performed under moderately low
temperatures (T o 140 1C) just above the melting point of the
immobilising salt, which reduces the energy demand, increases
system dynamics and enables heat integration, e.g. with high-
temperature PEM fuel cells (typically operating at 180 1C)
for subsequent electricity generation. The applied concept is,
Entry PPh3-to-catalyst ratio (mol molÀ1
)
8 h
24 h
8 h
1b
2
3
—
1
2
15
17
35
58
86
76
63
45
84
94
100
100
100
100
3.7
4.3
8.8
14.4
21.3
19.1
15.9
4
5
3
4
6
7
5
6
a
Dehydrogenation reaction conditions: 0.05 mmol [Ir(cod)(py)(PCy3)]PF6,
0–0.30 mol PPh3 (i.e. corresponding to a molar ratio of 0–6), 10.0 mmol
2-methylindoline, 2.0 mmol [PPh4][NTf2], 10 mL Bu2O, 130 1C under 1 bar Ar.
b No PPh3 added. c Turn-over frequency as molproduct molcatalystÀ1 hÀ1
.
to the best of our knowledge, the first account of combining specific biphasic system, a series of experiments was performed
the advantages of mild homogeneous catalysis for hydrogen with increasing PPh3 to Ir ratios (Table 1). In all cases, the reactions
release from LOHC systems with a molten salt-based catalyst exhibited outstanding selectivity producing exclusively the desired
immobilisation strategy. The concept enables easy catalyst/LOHC dehydrogenation product, 2-methylindole. Furthermore, the series
separation and thus enables the release of large quantities of of experiments indicated a clear trend of increasing dehydrogena-
hydrogen with a very small amount of dissolved precious metal tion rates with increasing addition of PPh3 up to a maximum of
complex. The here-presented biphasic catalyst system allows for approx. 4 eq. PPh3 with 86% conversion obtained within the first
dynamic and continuous operations of hydrogen storage/release, 8 h of reaction. Full conversion was obtained within 24 h for
which is of great industrial interest and provides a key to an efficient experiments with 3 to 6 eq. PPh3. Beyond 4 eq. PPh3 the catalyst
and reliable hydrogen supply.
activity was lowered, indicating coordinative saturation of
Initially, a screening of suitable homogeneous catalyst precursors, the catalyst which impeded substrate coordination. Hence, in
indole/indoline derivatives and liquid–liquid biphasic systems for general, the system seems to benefit from an excess of stabilising
low-temperature LOHC systems was performed to determine the ligand. Note that the optimal ratio presented was for the specific
most promising system for the catalytic dehydrogenation reaction biphasic system where the ligand distributes over both phases,
(see ESI†). Among the tested catalyst precursors, the mixed ligand- and does not necessary apply as a universal trend for Crabtree’s
Ir(I) complex (1,5-cyclooctadiene)-(pyridine)(tricyclohexylphosphine)- catalyst in other systems.
iridium(I) hexafluorophosphate, [Ir(cod)(py)(PCy3)]PF6 (Crabtree’s
After optimisation of the catalyst system, a kinetic study of
catalyst) was found to perform best with respect to the targeted the reaction system was performed where the thermal stability
dehydrogenation reaction. Correspondingly, 2-methylindoline was and apparent activation of the catalyst system was determined
found to be the most active hydrogen carrier for the dehydrogenation by a temperature variation. The lower limit of this variation was
reaction and thus chosen as the substrate. The molten salt tetra- at 120 1C due to insufficient melting of the molten salt phase.
phenylphosphonium bis(trifluoromethylsulfonyl)-imide, [PPh4][NTf2] Moreover, the temperature was limited to a maximum of 140 1C
(m.p. 134 1C)19 was used as the catalyst immobilisation medium and due to the boiling of the organic solvent. The results of the
dibutyl ether, Bu2O, was added to facilitate the liquid–liquid phase kinetic study are illustrated in Fig. 1. The reaction maintained its
separation. Note, that the presence of ionic catalyst and stabilising full selectivity towards the desired 2-methylindole product and
ligand additionally lowered the melting point of the molten salt-rich the catalyst proved thermally stable in the examined temperature
phase to below 120 1C. The overall dehydrogenation reaction is range. At 140 1C, the reaction reached 99% 2-methylindoline
represented in Scheme 1 (see ESI† for experimental procedures).
conversion within 5 h, corresponding to a catalyst turn-over-
Crabtree’s catalyst has been reported in the literature to frequency (TOF) of 40 hÀ1. The dehydrogenation rates were
benefit from addition of a strongly coordinating ligand, such as estimated based on conversions in the range of 10–60%, and
triphenylphosphine, PPh3, up to one equivalent relative to the the system was found to show good Arrhenius behaviour
catalyst for stabilisation of the Ir-complex during reaction.18 To (R2 = 0.999) with an apparent activation energy of 127.3 kJ molÀ1
determine the optimal addition of stabilising ligand for the (see ESI†). However, due to the biphasic nature of the reaction
system, the determined apparent activation energy reflected
most likely not only the dehydrogenation enthalpy but was also
influenced by effects of mass transfer, change in catalyst/ligand
distributions and variation in phase viscosity and solubility.
Results from initial studies demonstrating catalyst recyclability
and usage of the same ionic catalyst solution for 2-methylindole
hydrogenation are presented in Fig. 2a. During catalyst recycling, a
notable decrease in catalyst activity was observed from the first to
Scheme 1 Catalytic dehydrogenation of 2-methylindoline to 2-methylindole
in liquid–liquid biphasic catalyst system using Crabtree’s catalyst.
Chem. Commun.
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