Performance enhancing additives for reusable ruthenium-triphos catalysts in the reduction of CO2to dimethoxymethane

Article abstract of DOI:10.1039/d0gc02528c

The direct access to dimethoxymethane (DMM) from CO2/H2 in MeOH is an attractive approach, but more active, selective, and robust catalyst systems are still desirable for an application of this system. Herein, we present the performance enhancing effect of additives on ruthenium-triphos catalyst systems for CO2 hydrogenation to DMM. In the presence of PPh3, functioning as both additional ligand and precursor for Lewis-acidic phosphonium salts, increased turnover numbers for DMM are achieved. Moreover, PPh3 significantly diminishes catalyst degradation via carbonylation to cationic [RuH(CO)2(triphos)]OTf, which remains an active species in the synthesis of DMM. Catalyst recycling over up to five runs with minimal loss in catalyst performance underlines the robustness of the system. Both, the recyclability of the system and the suppression of catalyst deactivation pathways encourage for application in sustainable continuous processes. This journal is

Full text of DOI:10.1039/d0gc02528c

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DOI: 10.1039/D0GC02528C
ARTICLE
Performance Enhancing Additives for Reusable Ruthenium-
Triphos Catalysts in the Reduction of CO2 to Dimethoxymethane
Robert Konrath,a Kohei Sekine,b Ivana Jevtovikj,c Rocco A. Paciello,d A. Stephen K. Hashmi,*,a,b and Thomas
Schaub*,a, d
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The direct access to dimethoxymethane (DMM) from CO2/H2 in MeOH is an attractive approach, but more active, selective
and robust catalyst systems are still desirable for an application of this system. Herein, we present the performance
enhancing effect of additives on ruthenium-triphos catalyst systems for CO2 hydrogenation to DMM. In the presence of PPh3,
functioning as both additional ligand and precursor for Lewis-acidic phosphonium salts, increased turnover numbers for
DMM are achieved. Moreover, PPh3 significantly diminishes catalyst degradation via carbonylation to cationic
[RuH(CO)2(triphos)]OTf, which remains an active species in the synthesis of DMM. Catalyst recycling over up to five runs
with minimal loss in catalyst performance underlines the robustness of the system. Both, the recyclability of the system and
the suppression of catalyst deactivation pathways encourage for application in sustainable continuous processes.
with methanol.54, 55 To increase the overall efficiency, direct
one-step oxidation and dehydrogenation processes have been
Introduction
developed, however, still relying on methanol as
formaldehyde precursor for subsequent acetalization.56
a
Utilization of carbon dioxide (CO2) as a renewable C1 resource
has drawn tremendous attention due to its abundant supply
from industries, low toxic properties and ease in handling. Over
the last few decades, several homogeneous catalytic
transformations of CO2 into valuable chemicals have been
developed.1-5 Especially the atom-economic reduction of CO2
employing molecular hydrogen serves as an attractive approach
to produce formic acid derivatives6-28 or methanol.29-43
However, the catalytic transformation to the formaldehyde
level remains challenging as further reduction possibly
generates methanol or methane.
The first direct access to DMM from CO2/H2 in methanol has
been reported by Klankermayer and co-workers in 2016
employing a robust ruthenium-triphos/Al(OTf)3 (triphos = 1,1,1-
tris(diphenylphosphinomethyl)ethane) system (Scheme 1).57
[Ru]
O
Al(OTf)3
O
O
CO2 + H2
+
O
H
MeOH
MF
DMM
Dimethoxymethane (DMM) and its oxymethylene ether
derivatives (OMEs) are valuable formaldehyde equivalents. In
recent years, the efficient access to OMEs has attracted
increasing interest in the field of fuel additives.44-47 Blended into
diesel fuel, OMEs can significantly reduce the formation of soot
and NOx during combustion.48-50 Being the simplest member of
the OME class, DMM serves as a valuable building block in the
synthesis of higher order OMEs via reaction with
formaldehyde.51-53
Ph2
P
Ph2
P
X
Ph2P
P
Ph2P
P
Ru
Ru
Ph2
Ph2
X = N, P, SiMe, SiPh
1
Klankermayer 2016
Trapp 2019
Scheme 1. Established ruthenium-triphos systems in CO2 hydrogenation to DMM.
The commercial production of DMM relies on a two-step
process comprising the heterogeneously catalyzed oxidation of
methanol to formaldehyde followed by reversible acetalization
With this attractive dual catalyst approach, DMM was produce
with turnover numbers (TON) of up to 214 along with methyl
formate (MF) and water as the sole by-products. The same
reaction is promoted by a cobalt-triphos/HNTf2 system with a
TON of 157 for DMM.58 Further improvements on the Ru-
catalyst activity and selectivity by employing tailor-made
triphos ligand derivatives were reported by Trapp and co-
workers (Scheme 1).59-61 After intensive optimization of
reaction parameters, the use of Ru(N-triphos)(tmm)/Al(OTf)3
a. Catalysis Research Laboratory (CaRLa), Im Neuenheimer Feld 584, 69120
Heidelberg, Germany.
b. Organisch-Chemisches Institut, Heidelberg University, Im Neuenheimer Feld 270,
69120 Heidelberg, Germany. Email: hashmi@hashmi.de
c. hte GmbH, the high throughput experimentation company, Kurpfalzring 104,
69123 Heidelberg, Germany.
d. BASF SE, Organic Synthesis, Carl-Bosch-Straße 38, 67056 Ludwigshafen, Germany.
Email: thomas.schaub@basf.com
Electronic Supplementary Information (ESI) available. See DOI: 10.1039/x0xx00000x
(tmm = trimethylenemethane)
at
very
low
catalyst
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concentrations and high excess of acid catalyst resulted in a
maximal TON of 3874 for DMM in over 70% selectivity
(TONMF = 1445).61
Ph2
P
Ph2
P
Ph2
View Article Online
P
DOI: 10.1039/D0GC02528C
Ph2P
Ph2P
P
Ph2P
P
H
H
Ru
Ru
Ru
H
H
P
From an industrial perspective, the in situ application of a
commercially available triphos ligand in combination with a Ru-
precursor remains the most desirable approach. When facing
the challenges for potential process implementation, not only
knowledge about highly active and selective catalysts is
required, but also an understanding of the recyclability of the
system and the prevention of catalyst deactivation. However,
catalyst recycling and thorough investigations towards the
formation of inactive catalyst species in the synthesis of DMM
remain elusive. Catalyst deactivation following a dimerization
pathway (I, Figure 1) has been proposed in a previous report.57
Moreover, the depletion of the hydrogenation catalyst to
cationic ruthenium dicarbonyl species II-IV via decarbonylation
of intermediates and products in the hydrogenation of esters
and in the CO2 reduction to MF and methanol has been
observed.33, 62, 63 While these species exhibited no or strongly
reduced activities, the glycolate complex V remained fairly
active in the reduction of glycolate esters to ethylene glycol at
relatively harsh conditions of 165 bar H2 pressure and 190°C.64
Ph2
Ph2
Ph2
CO
PPh3
1
2
3
Ph2
P
Ph2
P
OTf
OTf
Ph2P
P
Ph2P
P
H
O
Ru
Ru
CO
H
O
Ph2
Ph2
CO
4
5
Figure 2. Structures of Ru-triphos complexes for catalytic CO2 hydrogenation to
DMM.
Catalyst screening
After an extensive optimization of reaction parameters at hte
GmbH using an 8-fold parallel autoclave test-rig (TONDMM of up
to 2025, see supporting information), the standard conditions
applied in the following experiments were set to an 1:4 ratio of
Ru-catalyst and Al(OTf)3 at 100°C under a total CO2/H2 pressure
of 120 bar (p(CO2)/p(H2) = 20/100). Preformed Ru-catalyst 1
produced DMM with a TON of 206 accompanied by a TON of 52
for MF (Table 1, entry 1). A similar performance was reached
when employing the in situ generated catalyst (Table 1, entry
2). In both cases, the corresponding 31P NMR spectra showed a
singlet at  = 43.4 ppm, which was attributed to the cationic Ru-
formate species 4. While the 1H NMR confirmed the absence of
a hydride signal as observed for the catalytically active [RuH(2-
OCHO)(triphos)] intermediate in the disproportionation of
formic acid to methanol,65 the formate proton was identified at
 = 8.73 ppm (see supporting information, Figure 9). ESI-MS
analysis of the post-reaction mixture further confirmed the
R2
P
X
PPh2
Ru
Ph2P
Ru
Y
H
H
R2P
P
H
P
Ph2
P
Ru
Ph2
CO
PPh2
Ph2P
R2
CO
I
II: X = N, R = Cy, Y = OTf
III: X = CCH3, R = Ph, Y = NTf2,
IV: X = CCH3, R = Xyl, Y = NTf2,
V: X = CCH3, R = Ph, Y = O2CCH2OH
Figure 1. Proposed Ru-triphos species in catalyst deactivation pathways.
Herein, we report on the effects of additives on the activity and
selectivity enhancement in the CO2 hydrogenation to DMM
employing an easily recyclable ruthenium-triphos system. The
use of inexpensive and commercially available components for
starting materials and additives is prioritized. Furthermore,
studies towards the catalytic activity of the cationic ruthenium
dicarbonyl species and strategies targeting the suppression of
its formation are presented.
-
presence of 4. The resembling NTf2 salt of a Ru-formate
intermediate with additional coordination of THF has been
identified as the resting state in the hydrogenation of CO2 to
methanol.33 Moreover, a distinctive set of signals in the 31P NMR
of the reaction mixture showing a doublet at  = 18.1 ppm
(JPP = 29 Hz) and a triplet at  = 6.0 ppm (JPP = 29 Hz) was
assigned to the cationic [RuH(CO)2(triphos)]OTf complex (5). In
relation to species 4, the amount of 5 was about 13%
determined by integration in the 31P NMR. The same complex
-
only differing in the NTf2 counter ion (III) was previously
Results & discussion
obtained in the synthesis of methanol from CO2 and showed
only very low activity under the applied conditions.33 When
complex 2 was employed in the hydrogenation of CO2 to DMM,
a TON of 18 for DMM and 65 for MF was achieved after 1 hour
(Table 1, entry 3). The corresponding 31P NMR of the reaction
mixture indicated the conversion of the monocarbonyl complex
2 to the dicarbonyl species 5. Additionally, a small amount of 4
could be observed supporting the assumption of CO ligand
removal under reaction conditions. Compared to catalyst 1, the
TON for DMM could be improved to 283 in the presence of
catalyst 3, reaching a selectivity of 82% (Table 1, entry 4).
Previous investigations in the CO2 reduction to DMM almost
exclusively employed the well established in situ generated
[Ru(cod)(methylallyl)2]/triphos (cod = 1,5-cyclooctadiene)
catalyst system or the preformed [Ru(triphos)(tmm)] complex
(1). [RuHCl(CO)(triphos)] as the only other example proved to
be inactive in this reaction.57 In order to evaluate the activity of
alternative
Ru-precursors,
1
was
compared
to
[RuH2(CO)(triphos)] (2) featuring a CO ligand and the PPh3
analogue [RuH2(PPh3)(triphos)] (3) (Figure 2).
2 | J. Name., 2012, 00, 1-3
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derivatives.67-69 These results obtained in the catalVyieswt sAcrtriceleeOnnilninge
indicate the potential of PPh3 for prevention of catalyst
Table 1. Ruthenium-triphos catalyst screening in the CO2 hydrogenation to DMM.
DOI: 10.1039/D0GC02528C
deactivation pathways resulting in performance enhancement
of the catalytic system.
[Ru] 1-3,
Al(OTf)3
O
O
O
CO2
+
H2
+
CH3OH
+
100°C, 16 h
O
H
Additive screening
MF
DMM
The effect of PPh3 as additive and the role in the suppression of
TON[d]
cationic complex 5 was investigated (Table 2).
Selectivity
(DMM)[d]
Entry
[Ru]
1
Additive
Table 2. Additive screening in the CO2 hydrogenation to DMM.
MF
52
DMM
206
196
14
[Ru] 1 and 2,
Al(OTf)3,
additive
1
-
-
-
-
-
80%
82%
18%
82%
70%
O
O
O
CO2
+
H2
+
CH3OH
+
[Ru]/
triphos
100°C, 16 h
O
H
2[a]
3[b]
4
44
MF
DMM
2
3
3
65
TON[b]
Selectivity
(DMM)[b]
62
283
1176
Entry
[Ru]
Additive
MF
62
28
-
DMM
240
6
5[c]
501
1
2
1
2
-
PPh3
PPh3
79%
18%
-
General conditions: [Ru] (50 mol), Al(OTf)3 (200 mol), MeOH (10 mL),
p(CO2) = 20 bar, p(H2) = 100 bar, 100°C, 16 h; [a] [Ru] (50 mol), triphos
(50 mol); [b] 1 h; [c] [Ru] (3.1 mol), Al(OTf)3 (25 mol), MeOH (5 mL),
20 h; [d] determined by 1H NMR using mesitylene as internal standard.
3
PPh3
-
An increased TON of 1176 for DMM at 70% selectivity compared
to MF was achieved when using an 8-fold excess of acid catalyst
at a decreased catalyst concentration of c[Ru] = 0.62 μmol·mL-1
(Table 1, entry 5). Interestingly, 4 was exclusively present in the
mixture after the reaction according to the corresponding 31P
NMR whereas the formation of 5 was not observed (Figure 3).
4
-
7
-
-
-
5[a]
6
1
1
1
1
1
7
185
48
48
55
54
114
227
212
226
266
34%
83%
82%
81%
83%
P(OPh)3
PhOH
P(O)H(OMe)2
H3PO3
7
8
9
General conditions: [Ru] (50 mol), Al(OTf)3 (200 mol), additive
(200 mol), MeOH (10 mL), p(CO2) = 20 bar, p(H2) = 100 bar, 100°C, 16 h;
1
[a] [Ru] = 1 (12.5 mol), 7 (50 mol), no Al(OTf)3; [b] determined by H
NMR using mesitylene as internal standard.
Figure 3. 31P NMR (121 MHz, D3COD) spectrum of the post-reaction mixture of a
CO2 hydrogenation experiment after evaporation using [Ru] = 3 (50 mol),
Al(OTf)3 (200 mol), MeOH (10 mL), p(CO2) = 20 bar, p(H2) = 100 bar, 100°C, 16 h.
A 4-fold excess of PPh3 in combination with catalyst 1 (Table 2,
entry 1) resulted in a slightly increased TON of 240 for DMM
compared to the additive-free system (Table 1, entry 1). Again,
the catalyst resting state 4 and phosphonium salt 7 were the
only detectable species in the post-reaction mixture
accompanied by a small amount of free PPh3. In case of catalyst
2, the activity dropped dramatically upon addition of PPh3
(Table 2, entry 2). The corresponding 31P NMR spectrum showed
the absence of complexes 4 and 5 and the exclusive formation
of the cationic complex [RuH(CO)(PPh3)(triphos)]OTf (6).
Moreover, 7 appeared only in trace amounts due to the lack of
Moreover, the liberated PPh3 ligand was converted into a
phosphonium salt [PPh3CH2OH]OTf (7), showing a peak at
 = 17.4 ppm in the 31P NMR and a distinctive signal for the
1
methylene protons at  = 5.41 ppm in the H NMR spectrum.
Under acidic CO2 reduction conditions, DMM initially reacts
with PPh3 to yield [PPh3CH2OCH3]OTf followed by hydrolysis to
7.66 The corresponding phosphonium chloride [PPh3CH2OCH3]Cl
in the presence of base is frequently used in Wittig reactions to
convert carbonyl compounds into their vinyl ether
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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Ph2
P
CO2 turnover. Instead, the majority of PPh3 was alkylated to
[PPh3CH3]OTf ( = 22.3 ppm) by in situ generated MeOTf.70 In
absence of a ruthenium catalyst both PPh3 and 7 yielded only
trace amounts of MF and DMM (Table 2, entries 3 and 4). .
Interestingly, when the reaction was conducted using catalyst 1
in combination with 7 in absence of Al(OTf)3, CO2 was converted
to both MF and DMM with TONs of 185 and 114, respectively
(Table 2, entry 5). This result points towards the function of 7 as
an additional Lewis-acid co-catalyst in this reaction. Moreover,
Ru-dicarbonyl complex 5 was not observed in the post-reaction
mixture whereas the formation of Ru-formate species 4 was
detected accompanied by monocarbonyl 6 and significant
amounts of free PPh3.
View Article Online
DOI: 10.1039/D0GC02528C
H2 (60 bar)
Al(OTf)3 (1.0 eq.)
Ph2P
P
Ru
MF, 140°C, 24h
Ph2
Ph2
OTf
P
1
Ph2P
P
H
Ru
CO
Ph2
CO
Ph2
P
5
Ph2P
P
TfOH (1.0 eq.)
CO
Ru
CH2Cl2, -70°C, 2h
CO
Ph2
Our initial investigations for performance enhancing ligand
additives pointed towards a beneficial effect of P(OPh)3 in
combination with 1 and Al(OTf)3 as well as the involvement in
preventing the formation of 5 (see supporting information,
Table 9). At conditions applied in the optimization process run
in an 8-fold parallel autoclave test-rig at hte GmbH on a 100 mL
scale, higher TONs of up to 962 for DMM were achieved when
compared to the additive-free system (TONDMM = 692, see
supporting information). Employing the same system in the
herein described setup only led to a marginal rise in DMM
production (TON = 227, Table 2, entry 6). In order to study the
potential impact of P(OPh)3 on the activation of dicarbonyl
complex 5, both components were dissolved in D3COD and
heated to reflux for 1 hour (see supporting information, Figure
18). The corresponding 31P NMR indicated full decomposition of
P(OPh)3 via alcoholysis with methanol to P(O)D(OMe)2, which
was further depleted upon addition of H2 gas (10 bar) to a
species showing a broad signal at  = -2.0 ppm. Hence, a direct
ligand effect of P(OPh)3 in the hydrogenation of CO2 to DMM
can be excluded. Similarly, the P(OPh)3 decomposition products
PhOH and P(O)H(OMe)2 did not lead to an improved
performance of 1 (Table 2, entries 7 and 8). However, additional
phosphorous acid, a potential hydrogenation product of
P(O)H(OMe)2, resulted in an increased TON of 266 for DMM
(Table 2, entry 9). In all cases using P(OPh)3 or its decomposition
products as additives, 4 was predominantly detected in the
post-reaction mixture. However, the simultaneous formation of
minor amounts of Ru-dicarbonyl 5 indicates the depletion of the
active catalyst via carbonylation of ruthenium. Hence, PPh3
servers as the most suitable additive within the ruthenium-
triphos/Al(OTf)3 system due to its ability to suppress catalyst
deterioration and to boost catalyst performance,
simultaneously.
8
Scheme 2. Synthetic routes to cationic Ru-complex 5.
Interestingly, when 5 was employed under the established
reaction conditions, MF was produced with a TON of 114
accompanied by DMM with a TON of 51 (Table 3, entry 1). 5
remains the major Ru-species in the reaction mixture according
to the 31P NMR spectrum, but at least two new species occurred
(see supporting information, Figure 14). The first set of signals,
comprising a triplet at  = 32.0 ppm (JPP = 33 Hz) and a doublet
at  = -0.2 ppm (JPP = 33 Hz), can be assigned to the resembling
complex [Ru(CO)2(OTf)(triphos)]OTf (9).71 The second species
with peaks at  = 25.8 ppm (JPP = 33 Hz) and  = 1.9 ppm
(JPP = 33 Hz)
potentially
belongs
to
[Ru(CO)(2-
OCHO)(triphos)]OTf 10, which has been reported as the
corresponding PF6 salt.72 This would emphasize the loss of one
CO ligand of 5 providing the opportunity to form the active
catalyst 2 observable as the corresponding resting state 10 in
analogy to Ru-formate 4.
In the presence of trimethylamine N-oxide, known as a
decarbonylation reagent,73 the activity did not increase
significantly (Table 3, entry 2). The addition of P(OPh)3 produced
trace amounts of CO2 hydrogenation products (Table 3, entry
3).
Table 3. Influence of additives on the CO2 hydrogenation to DMM using the
5/Al(OTf)3 catalyst system.
[Ru] 5,
Al(OTf)3,
O
additive
O
O
CO2
+
H2
+
CH3OH
+
100°C, 16 h
O
H
MF
DMM
Catalytic activity of Ru-dicarbonyl 5
Next, the catalytic activity of the potentially deactivated species
5 was examined. The cationic complex was synthesized from
Ru-complex 1 via decarbonylation of MF in the presence of
Al(OTf)3 and H2 (Scheme 2). An alternative pathway starts from
Ru0-dicarbonyl complex 8, which can be treated with
stoichiometric amounts of TfOH to obtain 5 as a light yellow
solid in 88% yield.
4 | J. Name., 2012, 00, 1-3
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[PPh3CH2OCH3]OTf, a product in the reaction of PPVhie3wwAirttihcleDOMnliMne
in the presence of TfOH, led to a maximum TON for DMM of 225
TON[b]
DMM
DOI: 10.1039/D0GC02528C
Selectivity
(DMM)[b]
Entry
Additive
in 60% selectivity (Table 3, entry 8). Consequently, PPh3 remains
a catalyst activating and performance enhancing compound in
the Ru/Al(OTf)3 catalyzed CO2 hydrogenation to DMM once
reacted with CO2 reduction intermediates and products under
acidic conditions. Despite the consumption of PPh3 by DMM,
the formed phosphonium salt acts as an acidic co-catalyst in this
reaction. Moreover, the enhancing effect observed for P(OPh)3
in reactions performed in the 8-fold parallel autoclave test-rig
at hte GmbH can be attributed to the formation of additional
acid co-catalysts such as H3PO3.
MF
114
105
traces
125
161
176
145
152
1
2
-
Me3NO
P(OPh)3
H3PO3
51
55
31%
34%
-
3
traces
68
4
35%
51%
47%
57%
60%
Catalyst recycling
5[a]
6
HCOOH
PPh3
168
156
195
225
Next, the batch recyclability of the catalytic system was
investigated in order to examine the effect of additives on the
formation of 5 over time. Therefore, all volatiles of the mixture
obtained after each run were removed in vacuo. The remaining
solid was dissolved in methanol, without addition of fresh
amounts of Al(OTf)3 co-catalyst, and subjected to the
subsequent cycle. In the case of the additive-free
[Ru(triphos)(tmm)]/Al(OTf)3 system, a consistent performance
was achieved over three cycles reaching an overall TON of 622
for DMM together with a steady selectivity of 80% (see
supporting information, Table 11). This demonstrates the
reusability and robustness of the catalytic system and
emphasizes the application in a continuously operated reactor.
7
7
8
[PPh3CH2OCH3]OTf
General conditions: [Ru] = 5 (12.5 mol), Al(OTf)3 (50 mol), additive
(50 mol), MeOH (5 mL), p(CO2) = 20 bar, p(H2) = 100 bar, 100°C, 16 h; [a]
HCOOH (200 mol); [b] determined by 1H NMR using mesitylene as
internal standard.
Hence, an enhancing effect of P(OPh)3 on the activity of 5
cannot be confirmed. However, H3PO3 as an additional
Brønsted acid gave even higher TON for MF and DMM
compared to the additive-free system (Table 3, entry 4).
Recently, Klankermayer and co-workers demonstrated the
direct synthesis of DMM starting from formic acid as the C1
source employing the established 1/Al(OTf)3 system.74 When 5
was used in combination with a 16-fold excess of formic acid as
an readily available additive, the TON for MF and DMM was
significantly increased to 161 and 168, respectively (Table 3,
entry 5). The contribution of formic acid as a potential carbon
source for DMM can be neglected since the amount of additive
(200 mol) would only produce a theoretical TON of 16 in case
of full conversion to DMM. However, the TON for DMM
increased by 117 in the presence of formic acid compared to the
additive-free conditions showing a clear beneficial catalytic
effect of the formic acid additive. A resembling additive effect
was achieved with PPh3 (Table 3, entry 6). As observed
previously under turnover conditions, PPh3 was mainly
converted to phosphonium salt 7 accompanied by minor
amounts of [PPh3CH3]OTf (see supporting information, Figure
15). More importantly, about 8% of the resting state 4 could be
identified in the reaction mixture according to integrals in the
31P NMR spectrum supporting the assumption of a feasible
reactivation pathway of 5 via loss of CO. The Ru-species 9 and
10 produced without additive or in presence of H3PO3 (Table 2,
entries 1 and 3) were not observed in the case of PPh3. When
additional 7 was employed, the performance of 5 could be even
further increased with a TON of 195 for DMM at 57% selectivity
(Table 3, entry 7). Again, the potential function of 7 (50 mol)
as a C1 source can be neglected. The related ether analogue
Monitoring the reaction mixture after each run using 1H and 31
P
NMR showed the continuous accumulation of 5 and, hence, the
continuous depletion of the amount of 4 (see supporting
information, Figure 19). Regardless of the formation of this less
active species 5, a similar catalyst performance was achieved in
all three runs indicating that the catalyst reached equilibrium
conditions. When catalyst 3 was employed under the same
recycling conditions, a slight drop in TON for DMM from 283 to
261 after the second run was obtained (see supporting
information, Table 12). While the activity decreased further
after the third cycle, the selectivity to DMM remained constant.
Again monitoring by 31P NMR shows the appearance of 5 with
about 30% by integration compared to 4 (see supporting
information, Figure 20). At low catalyst concentration of 3
(c = 0.62 mol·mL-1), an overall TON for DMM of 2661 was
reached after three recycles (see supporting information, Table
13). However, loss of catalyst combined with catalyst
degradation resulted in a more pronounced drop in catalyst
performance at low loadings. Since a four-fold excess of PPh3
and formic acid exhibited a beneficial effect on the performance
of 5 (Table 2, entries 5 and 6), both additives were employed in
the regeneration of 1/Al(OTf)3. Over five runs the catalyst
activity remained fairly constant with a marginal drop of 5% in
selectivity to DMM (Figure 4). Al(OTf)3 and PPh3 were only
added to the first run whereas formic acid was fed into each
cycle. Again the corresponding 31P NMR spectra indicated the
nearly quantitative conversion of PPh3 into phosphonium salt 7
(see supporting information, Figure 21). Notably, Ru-dicarbonyl
5 was not detected before the third run. About 15% of 5 were
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present after run 4 compared to 26% after the second cycle in
the additive-free system. This underlines the beneficial additive
effect of both PPh3 and formic acid on the suppression of 5 in
the hydrogenation of CO2 to DMM. Moreover, the prevention
of intrinsic catalyst deactivation pathways paves the way for
sustainable application in continuously operated processes.
Consequently, this facilitates the utilization of minimal amounts
of precious metal catalysts.
Acknowledgements
The authors are grateful to funding by the German Federal
Ministry of Education and Research (BMBF) within the funding
View Article Online
DOI: 10.1039/D0GC02528C
initiative ‘CO2Plus
- Stoffliche Nutzung von CO2 zur
Verbreiterung der Rohstoffbasis’. CaRLa (Catalysis Research
Laboratory) is co-financed by the Heidelberg University and
BASF SE.
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