Methylformate from CO2: An integrated process combining catalytic hydrogenation and reactive distillation

Article abstract of DOI:10.1039/c9gc03006a

An integrated two-step process for the production of methylformate (MF) from CO₂, H₂ and MeOH was developed. In the first step, the hydrogenation of CO₂ to a formate-amine adduct is carried out in a biphasic system comprising n-decane as the catalyst phase and MeOH as the product phase. In the second step, the resulting methanol solution containing the formate-amine adduct is subjected to reactive distillation for esterification and isolation of methylformate. The selection of the amine played an important role for devising the overall process. Whereas in the hydrogenation step basic amines work best, medium to low basic amines are preferred in the esterification step. 1,2-Dimethyl-imidazole (1,2-DMI) was identified as an effective compromise for the integration of both steps. In the hydrogenation step, a bis(diphenylphosphino)methane ligand tailored with long alkyl chains ensured effective retention of the Ru-catalyst in the non-polar phase allowing straightforward reuse of the catalyst phase. In a semi-continuous set-up, repetitive hydrogenation (8 cycles) led to a total turnover number (TTON) of 38?000 at an average turnover frequency (TOF) of 1400 h^-1 with a cumulative catalyst leaching of only 1.4 mol% for P and 2.0 mol% for Ru. Reactive distillation was demonstrated in a continuously operated rectification unit leading to the isolation of MF at the head of the column with a purity of 91.5%.

Full text of DOI:10.1039/c9gc03006a

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Methylformate from CO : an integrated process
2
combining catalytic hydrogenation and reactive
distillation†
Cite this: Green Chem., 2019, 21,
307
6
a
a
b
a
Martin Scott, Christian G. Westhues, Teresa Kaiser, Janine C. Baums,
b
a
a,c
Andreas Jupke, Giancarlo Franciò * and Walter Leitner
*
An integrated two-step process for the production of methylformate (MF) from CO
2 2
, H and MeOH was
developed. In the first step, the hydrogenation of CO to a formate-amine adduct is carried out in a
2
biphasic system comprising n-decane as the catalyst phase and MeOH as the product phase. In the
second step, the resulting methanol solution containing the formate-amine adduct is subjected to reac-
tive distillation for esterification and isolation of methylformate. The selection of the amine played an
important role for devising the overall process. Whereas in the hydrogenation step basic amines work
best, medium to low basic amines are preferred in the esterification step. 1,2-Dimethyl-imidazole (1,2-
DMI) was identified as an effective compromise for the integration of both steps. In the hydrogenation
step, a bis(diphenylphosphino)methane ligand tailored with long alkyl chains ensured effective retention
of the Ru-catalyst in the non-polar phase allowing straightforward reuse of the catalyst phase. In a semi-
continuous set-up, repetitive hydrogenation (8 cycles) led to a total turnover number (TTON) of 38 000 at
an average turnover frequency (TOF) of 1400 h−1 with a cumulative catalyst leaching of only 1.4 mol% for
P and 2.0 mol% for Ru. Reactive distillation was demonstrated in a continuously operated rectification unit
leading to the isolation of MF at the head of the column with a purity of 91.5%.
Received 26th August 2019,
Accepted 29th October 2019
DOI: 10.1039/c9gc03006a
rsc.li/greenchem
“
drop-in” approach, CO -based products can replace chemical
2
Introduction
commodities usually produced by the refinement of fossil
The interest in the utilization of CO as readily available feed- carbon and eventually lead to a more closed carbon cycle,
2
stock is experiencing a growing momentum in both academia which is the ultimate goal of a sustainable economy. At the
1
–6
and industry.
Carbon dioxide can be “harvested” in high same time, new products or applications may arise with the
concentration at numerous point sources worldwide facilitat- availability of “green” CO -based intermediates.
2
ing the implementation of carbon capture and utilization
Among the products that can be obtained by homogenously
as C1-building catalyzed CO hydrogenation, formic acid (FA) and its deriva-
block for chemical synthesis as an equally attractive and chal- tives such as methyl formate (MF) have found particular inter-
7
,8
(
CCU) concepts, including the use of CO
2
2
9
–13
19–22
lenging option.
These strategies can contribute to the goal est for many years.
While most studies focus on FA as the
23
of “defossilization” of the chemical value chain, in particular target product, MF provides an attractive alternative. In the
when combined with energy input from renewable resources.
1
4
current fossil-based industry, MF and FA are produced by
Catalytic hydrogenation of CO using transition metal com- carbonylation of methanol with CO, both starting materials
2
2
4
plexes has evolved as a promising valorization strategy provid- obtained mainly from natural gas. Due to the principle rever-
ing a range of differently functionalized products.
9
,15–18
25
In a sibility of the reaction,
the liquid products have been
suggested as easy-to-handle CO surrogates.
2
6,27
MF is the key
intermediate for the production of FA, but also a direct source
for formyl groups in many other chemical products including
formate esters and formamides in commodity and specialty
a
Institut für Technische und Makromolekulare Chemie (ITMC), RWTH Aachen
University, Worringerweg 2, 52074 Aachen, Germany.
5
,28
E-mail: francio@itmc.rwth-aachen.de, leitner@itmc.rwth-aachen.de
chemicals.
Furthermore, its use as fuel component has
b
AVT.FVT—Chair of Fluid Process Engineering, RWTH Aachen University,
been discussed as octane booster for spark ignition engine
Forckenbeckstraße 51, 52074 Aachen, Germany
29–31
fuels.
c
Max Planck Institute for Chemical Energy Conversion, Stiftstraße 34-36,
2
Considering MF as target for CO hydrogenation, this offers
attractive features for the design of integrated reaction/separ-
4
5470 Mülheim a. d. Ruhr, Germany
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9gc03006a
ation processes. In contrast to the endergonic (ΔG° = 32.9 kJ
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mol− ) hydrogenation of CO
1
to FA, the transformation of [FA·amine] adduct in methanol as the product phase. This
2
CO , H and MeOH into MF and water is exergonic (ΔG° = solution is subjected directly to the second step of reactive dis-
2
2
1
20
−
5.28 kJ mol− ) under standard conditions. Whereas the iso- tillation inducing the esterification to methyl formate (MF),
lation of pure FA from the stabilizing reaction media is conse- which is isolated as the distillate at the column head while
quently one of the major challenges in process schemes for excess methanol and amine are recycled back at the bottom to
2
0,32
CO
2
2
-based FA production,
the moderate polarity (ε = 8.9 at the reactor. Water, the by-product of the esterification,
3
3
5 °C) and low boiling point (T = 31.7 °C at 101.3 kPa) of MF accumulates at the bottom of the distillation column and is
make its isolation appear more straightforward. While the removed from the recycling stream when reaching a critical
ester MF therefore offers favorable thermodynamics for the concentration.
overall energy balance and potential benefits in the down-
stream processing, esterification imposes yet another kinetic approaches targeting MF by achieving the overall transform-
barrier to the molecular transformation. ation via two separate reaction steps that are integrated on a
The process shown in Fig. 1 differs from previous
A very recent life cycle assessment indicates that the gene- systems level. This allows to optimize reaction rates and equili-
ration of MF from CO2 would reduce the global warming bria for the two reaction steps independently, leading to
impact by 16% as compared with the fossil-based production higher overall efficiency (vide infra). To implement this
even when using raw materials and energy from fossil concept, it is mandatory that the solution employed in the
3
4
sources. Depending on the concentration of CO in the feed, reactive distillation does not contain any active catalyst or
2
it is advantageous to integrate the carbon capture reference metal component as this would promote the back reaction of
3
5
process Rectisol® (CO
2
adsorption in cold methanol) with the first step, i.e. the decomposition of FA to hydrogen and
3
2,36
the carbon utilization step (up to 46% saving of the electricity carbon dioxide,
once the overpressure is released and
demand) realizing an Integrated Carbon Capture und temperature is increased. The design of the biphasic system
Utilization process (ICCU). Mandatory conditions to achieve for the first step is therefore crucial to achieve the optimized
the integration is on the one side the use of the same solvent performance.
in both processes, i.e. methanol, and on the other side to
3
4
develop an efficient immobilized catalyst system.
In the present paper, we address and fulfill these precondi-
tions for the realization of an ICCU process to produce methyl-
Results
2 2
formate (MF) from CO and H . We demonstrate that combin- The choice of methanol as recipient solvent for the formate
ing catalytic hydrogenation of CO2 with reactive distillation adducts allows to perform the esterification step directly
provides a strategy to effectively balance these seemingly con- without the need of solvent replacement. On the other hand,
tradicting requirements in a fully integrated two-step process the use of methanol significantly restricts the choice of the
for production of MF from CO , H and MeOH. The process catalyst phase to solvents with very low polarity in order to
2
2
design depicted in Fig. 1 comprises as first step the transition establish a biphasic system. Long chain alkanes are suitable
metal complex-catalyzed hydrogenation of CO in a biphasic candidates and n-decane was selected as prototype for the cata-
reaction system to generate high concentrations of an lyst phase in this study. To achieve preferential partitioning of
2
2 2
Fig. 1 Integrated process for catalytic hydrogenation and reactive distillation for production of methyl formate (MF) from CO , H and MeOH.
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the catalyst in the n-decane phase, the solubility properties of
the catalyst must be adjusted accordingly. The complex cis-[Ru
3
7
(
dppm)
2
Cl
2
]
(dppm = Ph
2
P-CH
2
-PPh
2
) is a powerful catalyst
3
8,39
for CO
2
hydrogenation to FA
and thus was selected as lead
structure for tagging with long alkyl chains at the aryl
groups. The ligand synthesis starts from commercially avail-
able bromo aryl compounds bearing an octyl or a dodecyl
chain in para-position. Metalation was achieved for the octyl
derivative with Mg to the corresponding Grignard and of the
dodecyl derivative with BuLi to the lithium reagent. Coupling
of these organometallic compounds with bis(dichloropho- Fig. 2 Observed catalyst partitioning for 2 in a typical experiment prior
to addition of reaction gases. (Top: orange n-decane phase containing
the catalyst; bottom: colorless MeOH phase with 1,2-dimethyl-imidazole
sphino)methane yielded the new alkyl-tagged dppm derivatives
L1 and L2, respectively, in good yields of 80–90% (Scheme 1).
(1,2-DMI).
Complexation with cis-[Ru(dmso) Cl ] resulted in the catalyst
4
2
precursors cis-di(bis(bis(4-octylphenyl)phosphanyl)methane)
rutheniumdichloride (1) and cis-di(bis(bis(4-dodecylphenyl) was observed in 13 consecutive experiments re-using the same
phosphanyl)methane)rutheniumdichloride (2) with good catalyst phase, corresponding to an average TOF of ca.
selectivities (trans complexes ≤ 7%) in high yields(80–99%) 4000 h− . In run 14, catalyst deactivation started to become
1
(
Scheme 1).
As expected, complexes 1 and 2 were found to be highly reach constant pressure. A possible reason for the deactivation
soluble in n-alkanes. For catalytic CO hydrogenation, the may originate from traces of oxygen that cannot be fully
noticeable as indicated by longer reaction times required to
2
addition of amine bases to the product phase is necessary. excluded by the procedure during the recycling on the small
Visual inspection of corresponding biphasic alkane/MeOH- scale. ICP-MS analysis of the metal content in the product
amine mixtures confirmed that the complexes were retained phase confirmed excellent catalyst retention leading to low
with high preference in the n-decane phase (Fig. 2). Initially, losses of phosphorus (0.10–0.44%) and ruthenium
methyl diethanolamine (MDEA), successfully used in biphasic (0.04–0.34%) per cycle resulting in a total leaching of P =
3
8,39
organic/water systems for stabilizing FA,
validate the approach. Catalytic hydrogenation of CO
complex 1 was carried out under a standard set of reaction cycle 6 and 11) to compensate for losses by evaporation or
was chosen to 2.99% and Ru = 1.87%. Small amounts of n-decane were
2
using refilled to the recycled catalyst phase occasionally (Table 1,
−1
conditions and the stability of the catalyst system was tested by cross-solubility (solubility of n-decane in MeOH = 81 g L at
repetitive batch recycling of the alkane phase (Table 1). The 20 °C).
4
0
The molecular ratio of n-decane/MeOH in the
1
reaction progress was monitored via the pressure drop and product phase measured via H NMR never exceeded the value
experiments were stopped as soon as the gas uptake was negli- of 1 : 100, probably indicating an even reduced cross-solubility
gible (≤0.1 bar per min). A ratio of formic acid to amine due to the formation of the polar product. In total, the catalyst
χFA/MDEA of 1 : 1 defines the maximum conversion under these phase was (re)used for 15 cycles and an average χFA/MDEA ratio
conditions and the time to reach this value as estimated from of 1.04 was found in the isolated product phase corresponding
the pressure/time curves was used to derive average turnover to a concentration of c[FA·MDEA] ≈ 2.1 mol L− in methanol and
1
frequencies (TOF) as lower limits for the catalyst activity.
The first cycle was carried out with 10 mmol MDEA and (turnover number, TON) of ∼19 000.
.5 mL MeOH yielding a highly viscous product phase. Thus,
After successful demonstration of the hydrogenation step in
a total productivity in terms of mole FA formed per mole of Ru
1
more diluted solutions were employed in subsequent cycles. combination with catalyst separation and recycling, the reac-
Rapid hydrogenation with a mean reaction time of 17 ± 4 min tive distillation of the product solution of [FA·MDEA] in MeOH
Scheme 1 Synthesis of the alkyl-tagged complexes cis-di(bis(bis(4-octylphenyl)phosphanyl)methane)rutheniumdichloride (1) and cis-di(bis(bis(4-
dodecylphenyl)phosphanyl)methane)rutheniumdichloride (2).
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Table 1 Catalytic hydrogenation of CO
n-decane/MeOH using complex 1 as catalyst and MDEA as base
2
in the biphasic system Table S4 and Fig. S5 and S6†). Hereby, a mixture of MeOH, FA
a
and the selected amine was heated at 60 °C in a sealed vessel
and the progress of the reaction was monitored by taking
1
small samples at regular time intervals for H NMR analysis
(
Fig. 3).
The screening showed that the esterification rate of metha-
nol with [FA·amine] adducts increases with lower basicity of
b
c
d
d
Cycle
χ
FA/MDEA [mol/mol]
t [min]
P leaching
Ru leaching
the amine. Stronger bases such as NEt (pKa = 10.76 at
3
e
1
0.91
1.07
1.03
1.00
1.03
1.02
1.06
1.04
1.00
1.10
1.04
1.09
1.05
1.04
1.09
13
18
16
16
21
15
17
22
20
21
18
18
18
26
41
0.25%
0.29%
0.44%
0.23%
0.22%
0.29%
0.19%
0.12%
0.10%
0.18%
0.14%
0.08%
0.18%
0.18%
0.10%
0.34%
0.28%
0.29%
0.18%
0.13%
0.17%
0.06%
0.05%
0.05%
0.11%
0.16%
0.06%
0.04%
0.04%
0.04%h
1.87%
46
47
2
5 °C) and NPr
3 a
(pK = 10.59 at 25 °C) led to very low esteri-
2
3
3
5
fication rates, while the formation of MF with the less basic
4
8,49
MDEA (pK = 8.57 at 25 °C)
was approximately twice as
and four times faster than with NEt (for
see ESI, Fig. S5†). In addition to a basicity below a pK of
a
fast as with NPr
NPr
3
3
f
6
3
a
7
8
9
1
1
1
1
1
1
8.5, further design criteria for the base can be formulated. The
amine should not form an azeotrope with FA, should have a
miscibility gap with water, and should be thermally stable and
chemically robust under the rectification conditions.
According to these features, 1,2-dimethyl-imidazole (1,2-DMI)
0
1
2
3
4
5
f
5
0
a
with a pK = 8.21 at 25 °C was selected as promising amine
g
h
1
.04
2.99%
candidate. Indeed, 1,2-DMI outperformed all other tested
amines yielding ∼15% MF within 5 h and 35% MF within 24 h
in the screening experiments (Fig. 3).
a
Reaction conditions: 10 mL window autoclave, Vn-decane = 2.5 mL, n
µmol, nMDEA = 6.4 mmol and VMeOH/MDEA = 2.5 mL per cycle (stock
1
=
5
solution of MDEA in MeOH was used), amine/cat (mol/mol) ≈ 1250
As 1,2-DMI was identified as most promising candidate for
the second reactive step, it was investigated for its performance
as stabilizer in the hydrogenation of CO in the biphasic
2
b
per cycle, pCO2 = 10 bar, pH2 = 80 bar, T = 60 °C. Ratio determined by
1
c
H NMR, no other side products found. Estimated by pressure time
d
profiles (digital manometer ±0.1 bar). Determined by ICP-MS in
e
f
aqueous matrix. 10 mmol MDEA and 1.5 mL MeOH. Addition of system n-decane/MeOH. In a first set of experiments, con-
g
h
3
00 µL n-decane. Average of all 15 cycles. Total leaching over 15
ditions were chosen in analogy to the study using MDEA as
amine (Table 2, c1,2-DMI = 2.3 mol L− in MeOH). While the
first reaction was completed within 70 minutes as indicated
from the pressure monitoring, longer reaction times in the
runs.
1
to form MF was investigated. Only small amounts of MF were range of 115 ± 23 min were needed for the following four re-
1
observed in the distillate by H NMR analysis (nMF/nMeOH
: 38) when the combined product solutions of the recycling
experiment (27 g, cf. Table 1) were distilled using a 20 cm
=
1
4
1
Vigreux column. To probe the reactive distillation in more
detail, a model mixture was composed and subjected to rectifi-
cation using a 45 cm long packed column and an oil bath
temperature ranging from 45 °C to 75 °C over 8 hours (for
detailed procedure see ESI†). In this case, a small fraction (ca.
2
%) could be collected as distillate containing significantly
higher amount of MF (nMF/nMeOH = 1 : 9, Fig. S9†). This result
confirms the principle feasibility of the esterification via reac-
tive distillation, but indicates also a slow conversion rate of
[FA·MDEA] to MF and the necessity to have a higher number
of separation trays to increase the production and the MF
4
2–44
purity at the top of the column.
Moreover, the residual
mixture turned yellow during the distillation period of 8 h,
indicating a possible degradation of the amine, a known draw-
4
5,46
back of MDEA-based scrubbers.
The results from the catalytic and distillation experiments
lead to the conclusion that while MDEA provides an excellent
stabilizer for the hydrogenation step, this amine is not suitable
for the MF formation step. In order to identify a potential
amine to compromise effectively between the two steps, we
screened the esterification of FA with MeOH in the presence of
Fig. 3 Time/conversion profiles for the esterification of FA to MF with
selected amines in a closed vessel at 60 °C (VMeOH = 2 mL; nFA
=
several amines (for complete list, data, and conditions see 6.5 mmol; namine = 6.5 mmol).
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a
2
Table 2 Catalytic hydrogenation of CO in the biphasic system n-decane/MeOH using complex 2 as catalyst and 1,2-DMI as base
b
c
d
d
Concentration of stock solution
cycle
χ
FA/1,2-DMI [mol/mol]
t [min]
P leaching
Ru leaching
c
1,2-DMI = 2.3 mol L−1
1
2
3
4
5
0.65
0.56
0.51
0.67
0.49
71
92
110
138
100
0.81%
2.95%
0.99%
0.31%
0.44%
0.16%
0.12%
0.33%
0.04%
0.07%e
0.41%
e
f
e
0.58 TON = 4761
5.50%
c
1,2-DMI = 5.9 mol L−1
1
2
3
4
5
0.34
0.34
0.32
0.34
0.36
227
186
219
247
247
0.77%
4.41%
1.50%
2.24%
1.35%
0.28%
0.16%
0.08%
0.06%
0.06%e
0.64%
e
f
e
0.35 TON = 6869
10.27%
a
Reaction conditions: 10 mL window autoclave, Vn-decane = 2.5 mL, n[Ru] = 3.5 µmol, nDMI = 5.7 (entries 1–5) or 14.8 (entries 7–11) mmol and
MeOH/DMI ≈ 2.5 mL per cycle (stock solution of DMI in MeOH was used), amine/cat (mol/mol) ≈ 1650 (entries 1–5) or 4200 (entries 7–11), pCO2
V
3
=
b
0 bar, pH2 = 60 bar, pressurized at r.t. till full saturation was reached, T = 60 °C. Ratio determined by proton NMR, if not otherwise noted the
c
amount of MF is ≤5% related to the amount of employed amine. Determined by pressure time profiles, obtained with a digital manometer.
d
e
f
Determined by ICP-MS in an organic matrix (dioxane). Average of the first 5 cycles. Total of all cycles (the minor amount of formed MF is not
included).
cycling experiments. In general, the reactions were roughly Accordingly, after the five experiments summarized in Table 2
seven times slower (average TOF over five cycles ca. 600 h−
1
,
at a c1,2-DMI = 2.3 mol L , an additional 6 cycle was carried
−1
th
Table 2) than in the presence of MDEA. Due to the lower basi- out over 35 h reaction time. The resulting pressure-time curve
city, the limiting ratio of FA to amine was also reduced to shows two distinct slopes (Fig. 4). A steep decrease over the
χ
2
FA/1,2-DMI = 0.58 mol/mol. Consequently, the TON under these first three hours is in accordance with rapid CO hydrogen-
conditions is also about 40% lower than in the MDEA system ation. Once equilibrium to the formate adduct is reached, a
under identical conditions. While ICP-MS analysis of the much more shallow curve is observed during the following
product mixtures indicated a Ru-leaching in the same low 24 h. In this second stage, the CO hydrogenation merely
2
range as with MDEA (0.07% to 0.33%), the P-leaching occurs to restore the amount of formate adduct transformed to
appeared to be higher, but also showed a broad scattering
from batch to batch between 0.31% and 2.95%, making the
results less reliable.
In a second set of experiments the loading of 1,2-DMI was
−1
substantially increased to c1,2-DMI = 5.9 mol L (Table 2).
Constant pressures were reached within 216 ± 31 min at a final
FA to amine ratio χFA/1,2-DMI = 0.35 mol/mol corresponding to a
−
1
final concentration of cFA = 2.1 mol L , similar to that
achieved in the presence of MDEA. The total TON of ca. 6900
achieved over five cycles was even slightly higher than that
reached within the same number of cycles with MDEA (cf.
Table 1). Although the reaction rates and the final FA to amine
ratio are significantly lower as with MDEA, the productivity of
the 1,2-DMI-based catalytic system to generate formic acid is
well in the range providing a promising compromise with the
much more efficient subsequent esterification.
It is worth noting that during the hydrogenation reactions
reported in Table 2 small amounts of MF (≤5%) were formed
in agreement with the favorable kinetic of the esterification of
FA to MF in the presence of 1,2-DMI. Thus, we investigated
whether the amount of MF under hydrogenation conditions
2
Fig. 4 Pressure/time profile for the hydrogenation of CO under pro-
th
longed reaction time (conditions see Table 2; additional 6 cycle using
can be increased upon prolonging the reaction time. an amine concentration of c1,2-DMI = 2.3 mol L− ).
1
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MF and, thus, correlates with the much slower esterification balance to monitor the delivery of feed. The reservoir is
rate. In the last ten hours of the experiment almost no connected to a 100 mL hydrogenation autoclave which is
pressure variation was observed indicating that the final equili- heated and cooled by a thermostat and equipped with a
brium over both steps was reached. This experiment yielded mechanical stirrer, a window and a sampling valve at the
FA at a ratio of χFA/1,2-DMI = 0.37 somehow lower than in the bottom. The reactive gases are delivered via a mass flow con-
experiment before (Table 2, cycle 5) and MF at a ratio of troller for H
2
and a mass flow meter (MFM) controlled dosing
2
. The graphical user interface to control the set-
χ
MF/1,2-DMI = 0.33, significantly higher than in all previous valve for CO
experiments where a ratio of up to of χMF/1,2-DMI = 0.05 was up was programmed using the LabView® (17.0) software. The
observed. Very similar results were obtained in an additional procedure (for details see ESI† section 2.5) started by transfer-
set of experiments with an extended reaction time of seven ring a weighted amount of the feed solution of 1,2-DMI in
days (Table S5†). These findings demonstrate that substantial MeOH pre-heated at 65 °C and pre-saturated with CO (35 bar)
2
amounts of MF can be generated directly under hydrogenation into the hydrogenation autoclave containing a n-decane solu-
conditions. However, full conversion of FA to MF cannot be tion of the catalyst. The reactor temperature was adjusted to
reached due to equilibrium constraints in the closed vessel, 65 °C and the mixture pressurized with H up to 135 bar. The
2
leading to complex reaction mixtures. Combining the hydro- total pressure was kept constant by delivering H
genation reaction with the reactive distillation allows to during the reaction. The resulting time-resolved H
operate both steps at optimal rates and conversion levels.
2
via the MFC
volume
profile was used to monitor the reaction progress (see ESI
2
2 2
With the optimized combination of catalyst and base for Fig. S3 and S4†). Once the H flow became negligible (H flow
−
1
the first step in hand, the robustness and potential for scale- ≤0.12 mL
n
min ), the hydrogenation autoclave was cooled
up of the system was investigated in a semi-continuously oper- down to ∼15 °C leading to a clear phase separation within five
3
9
ated set-up (Fig. 5).
minutes. The product phase was carefully withdrawn through
The set-up comprises a 900 mL high pressure reservoir con- the sampling valve at the bottom and the procedure started
taining a CO -saturated solution of 1,2-DMI in MeOH. This is anew. The results of a series of 4 cycles are summarized in
2
placed on a magnetic stirring plate which is located on a Table 3.
Fig. 5 Schematic depiction of the semi-continuous operated set-up with a 100 mL autoclave for hydrogenation (left) and a 900 mL autoclave as
2
reservoir for CO pre-saturated solutions of 1,2-DMI in methanol (right).
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a
Table 3 Semi-continuous catalytic hydrogenation of CO
2
to formate using 1,2-DMI as base in n-decane/MeOH
min−
1
]
Vsub. [mL]
H
2
b
[mmol] t [min]
c
c
FA [mol L−1
]
FA/1,2-DMI [mol/mol] P leaching [%] Ru leaching [%]
e
e
Cycle
H
2
Flowini. [mL
n
1
2
3
4
11.6
9.4
6.5
4.1
50.5
46.2
45.6
37.8
70.3
64.3
63.5
52.7
165
193
202
312
1.39
1.27
1.31
1.39
0.33
0.30
0.31
0.33d
0.32
1.41
0.74
0.45
0.22f
2.82
1.06
0.83
0.50
0.50f
2.89
d
1.34
a
Reaction conditions: CO
2
= 35 bar at rt (50 bar at 65 C); H
2
up to a total pressure of 135 bar at 65 °C; T = 65 °C; 2 = 8.74 µmol; n-decane = 25 mL;
c d
−1
b
c
1,2-DMI = 4.2 mol L
.
Hydrogen consumption as calculated integrating the overall H flow. Time till negligible H flow was observed. Average
2
2
e
f
over all runs. Determined by ICP-MS in organic matrix (dioxane). Sum of all runs.
An average final FA concentration of 1.34 mol L−1 was experiments without added water. This may be attributed to
achieved at a ratio FA:1,2-DMI of χFA/1,2-DMI = 0.32 mol/mol, an increased polarity of the product phase as reflected also in
similar to what has been achieved in the small-scale experiments. a more rapid phase separation. Overall, the presence of water
A total of 180 mL of feed solution was processed corresponding at concentrations in this range in the recycle stream has no
to a production of 11 g FA. The prolonged reaction time in the negative effect on the catalyst performance and may even be
fourth run indicates some catalyst deactivation, but a total TON beneficial for the separation of the product phase.
of >27 500 was already achieved at this point with an average TOF
After successful demonstration of the n-decane/MeOH/1,2-
of 2000 h− . The total leaching over the four runs amounted to DMI system for the FA production and catalyst separation, the
.8% and 2.9% for P and Ru, respectively, underpinning the esterification step from the MeOH solution was examined in a
effective catalyst retention in this system. continuously operated reactive distillation. The feed for the reac-
In the envisaged integrated two-step process for MF pro- tive distillation was composed to represent a typical product
duction based on CO hydrogenation and subsequent esterifi- phase obtained in hydrogenation experiments (MeOH as solvent,
cation, the recycled methanol will contain not only 1,2-DMI 1,2-DMI = 0.67 mol L , FA = 0.235 mol L , χFA/1,2-DMI
1
2
2
−1
−1
=
5
1
but also an estimated residual water content of 6.7 mol%. In 0.35 mol/mol). The physico-chemical data for the components
order to validate the compatibility of the catalyst system, a are listed in Table S6.† The rectification setup consists of a reboi-
further set of hydrogenation experiments was carried out ler containing a total volume of 675 mL liquid phase, a DN50
using a methanol solution of 1,2-DMI with a water content of bubble-cap tray column with 10 trays, and a condenser with an
6
.7 mol% relative to methanol (Table 4; for detailed procedure integrated reflux divider (Fig. 6). Every second tray of the column
see ESI†). In the first six cycles the hydrogenation was com- is equipped with a sampling point. The model mixture can be
plete within 200 min while longer times were needed for the fed continuously into the bottom of the column where a manu-
subsequent runs (Fig. S4†). In a total of 8 cycles, 262 mL of ally operated electrical heating device is installed. The tempera-
feed solution were processed resulting in an average formic ture is detected at the bottom as well as at the head of the
−1
acid concentration of cFA = 1.32 mol L at a χFA/1,2-DMI
=
column and the condenser is cooled with water at T = 7 °C. The
0
.32 mol/mol. The productivity of 16 g FA corresponds to a pressure is monitored via a digital manometer and was kept at
−
1
TTON of 38 000 generated at an average TOF of 1400 h
.
ambient pressure throughout the distillation process.
Cumulative catalyst leaching (1.4 mol% for P and 2.0 mol% for
Ru) was in the same range or even slightly lower than in the of
The rectification column was operated for a total time
7
h
with
a
reflux
ratio
(split
ratio)
a
2
Table 4 Semi-continuous catalytic hydrogenation of CO to formate using 1,2-DMI as base in n-decane/MeOH-water
b
e
e
H
[mL
2
Flowini.
V
[mL]
sub.
H
2
c
FA
FA/1,2-DMI
[mol/mol]
P leaching
[%]
Ru leaching
[%]
−
1
c
[mol L−1]
Entry
Cycle
n
min
]
[mmol]
t [min]
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
11.5
11.9
18.8
12.3
6.6
6.1
4.3
42.5
40.6
47.8
36.0
25.4
25.4
32.5
12.1
59.2
56.5
66.5
50.1
35.4
35.5
32.7
16.9
193
191
165
188
211
210
295
426
1.38
1.42
1.25
1.30
1.31
1.15
1.26
1.45d
1.32
0.33
0.34
0.30
0.31
0.32
0.28
0.30
0.35d
0.32
0.72
0.25
0.15
0.13
0.07
0.03
0.04
0.04 f
1.43
1.14
0.38
0.17
0.17
0.04
0.04
0.04
0.04
0.3
f
2.02
a
Reaction conditions: CO
,2-DMIMeOH = 4.2 M; H O = 6.7 mol% with respect to MeOH. Hydrogen consumption as calculated integrating the overall H
negligible H flow was observed. Average over all runs. Determined by ICP-MS in an organic matrix (dioxane). Sum of all runs.
2
= 35 bar at rt (50 bar at 65 C); H up to a total pressure of 135 bar at 65 °C; T = 65 °C; 2 = 8.74 µmol; n-decane = 25 mL;
2
b
c
1
2
2
flow. Time till
d
e
f
2
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liquid returned to the column
liquid removed from the distillate
4
of
¼ 1
. After a heating
phase of 115 minutes, continuous delivery of the feed solution
directly into the bottom of the distillation column was started
with a flowrate of 1–2 mL min− to assure a constant filling
level of the reboiler. The temperature at the bottom of the rec-
tification varied over the experimental time between 68.0 °C–
1
79.3 °C. First samples were taken after 140 min and after that,
samples were taken along the column length every 60 min and
1
analyzed by H-NMR spectroscopy. The highest MF purity of
the distillate was detected after 740 min and the measured
composition at each sampled tray is plotted in Fig. 7. Although
steady state conditions were not yet reached within this period,
the results are significant and show a clear trend. The high
boilers 1,2-DMI and FA completely remained in the reboiler.
Although a color change of the liquid in the reboiler was
notable, no degradation of 1,2-DMI was observed in the distil-
1
lation residue and no side products were detected by H-NMR.
Along the column, the percentage of MeOH diminished with
the percentage of MF continuously increasing representing the
progress of the esterification (Fig. 7). As expected, the highest
MF purity was found in the distillate with 91.5 mol%. Water,
which could not be detected with the used analytical method,
is expected to accumulate at the bottom of the column. This
stands in agreement with the calculation of the ternary system
MF-MeOH-H O with
a
Non-Random-Two-Liquid (NRTL)
2
model, which predicts no azeotropes for the ternary compo-
sition. Due to the conversion of FA to MF, the ratio of FA to
1
,2-DMI at the bottom of the column changed over time from
the initial value of χFA/1,2-DMI = 0.35 mol/mol to χFA/1,2-DMI
.14 mol/mol at the end of this experiment accounting for a
=
0
total FA conversion of XFA = 58.8%. Under the assumption that
FA decomposition to CO is negligible at these low tempera-
tures, this conversion corresponds to a total production of
0.767 kg MF per 1.00 kg FA entering the column under con-
Fig. 6 Schematic depiction of the rectification setup used for the reac-
tive distillation of the MeOH/FA/1,2-DMI mixture.
tinuous operation.
Fig. 7 Molar ratios of MF, MeOH, 1,2-DMI and FA in the reboiler, at the trays and in the column head of the reactive distillation column after 740 min.
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Fig. 8 Block flow diagram of a continuous process for the production of MF from CO
2
. Ri = Reaction units, Si = separation units, Mi = mixing units.
acid from CO
2
hydrogenation mixtures, which proves to be
Summary and conclusion
5
2
notoriously difficult.
With the increasing availability of
5
3
54,55
56
An integrated two-step process was developed for the pro- methanol from biomass,
recycled waste, or even CO
both reactive components of the methyl
and methanol. The first step comprises the hydrogenation of formate process can be produced from renewable electricity
CO to formic acid (FA) in the presence of an amine and has and resources. Given the broad portfolio of methylformate in
been realized using a biphasic system with methanol as the direct applications or as intermediate, the CO -based pro-
product phase and n-decane as the catalyst phase allowing duction outlined in this paper holds therefore significant
2
5
7,58
duction of methylformate (MF) from carbon dioxide, hydrogen and hydrogen,
2
2
5
9,60
efficient immobilization of alkyl-tagged ruthenium phosphine potential within the so-called “Power-to-X” concept.
complexes. The second step comprises the esterification of
formic acid and methanol via reactive distillation directly from
the product mixture of the first step. The basicity of the
employed amine was identified as a key design parameter.
Conflicts of interest
Whereas high basicity of the amine is beneficial in general for There are no conflicts to declare.
the hydrogenation step, medium to low basicity is required for
high reaction rates in the esterification reaction. Commercially
available 1,2-dimethyl imidazole (1,2-DMI) was identified to rep-
resent a good compromise for both steps allowing the realiz-
Acknowledgements
ation of a fully integrated process. Repetitive batch experiments The work has been carried out within the project “Carbon-2-
demonstrated the suitability of the biphasic system to generate Chem” (03EK3042C) was funded by the German Federal
a suitable feed for the second step (average cFA = 1.32 mol L− at Ministry of Education and Research (BMBF). Additional finan-
a χFA/1,2-DMI ≈ 0.32 mol/mol, TTON in the range of 38 000, cial support by the Cluster of Excellence 2186 “The Fuel
average TOF 1400 h− ) with excellent catalyst retention (cumu- Science Center” funded by the Deutsche Forschungsgemein-
lative leaching over 8 cycles 1.4 mol% for P and 2.0 mol% for schaft (DFG) is gratefully acknowledged. Open Access funding
Ru). Methylformate production from the methanol solution provided by the Max Planck Society. The authors thank Julia
containing the FA/1,2-DMI adduct was shown to be possible Nießen for carrying out the ICP-MS measurements.
when directly subjected to a reactive distillation step, yielding
1
1
MF as the distillate with good purity of 91.5% even under non-
optimized rectification conditions. With the identification of a
suitable reaction system compromising both the hydrogenation
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