Towards environmentally benign capture and conversion: Heterogeneous metal catalyzed CO2 hydrogenation in CO2 capture solvents

Article abstract of DOI:10.1039/c9gc03449h

The transformation of captured CO₂ into value-added chemicals to mitigate increasing CO₂ concentration in the atmosphere has gained significant attention recently. Though carbon capture and storage (CCS) is already being practiced in a few places, it suffers from energy-intensive CO₂ desorption and compression steps involved, which can be avoided in the carbon capture and utilization (CCU) approach. Herein, a selection of carbon capture solvents were screened to assess the reactivity of condensed-phase heterogeneous metal catalyzed hydrogenation of CO₂. Among the catalysts screened, the Cu/ZnO/Al₂O₃ catalyst was active for the one-pot CO₂ capture and conversion process to methanol using post and pre-combustion carbon capture solvents comprised of various amines and alcohols. Our findings indicate that formamides are less-reactive under our conditions in comparison with formate ester intermediates and a combination of 1° alcohols and amines gives the highest methanol yield. Screening volatile organic compound (VOC)-free alcohols and amines led us to an environmentally benign system of bio-derived and biodegradable chitosan and polyethylene glycol (PEG₂₀₀), which provide a moderate concentration of methanol (139.5 mmol L^-1) with the facile separation of volatile products (water and methanol). The chitosan/PEG₂₀₀ system was recycled three times, ultimately providing a promising VOC-free, biodegradable, bio-derived and recyclable CO₂ capture and conversion pathway.

Full text of DOI:10.1039/c9gc03449h

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DOI: 10.1039/C9GC03449H
ARTICLE
Towards environmentally benign capture and
conversion: Heterogeneous metal catalyzed CO2
hydrogenation in CO2 capture solvents.
Received 00th January 20xx,
Accepted 00th January 20xx
Jotheeswari Kothandaraman and David J. Heldebrant*
DOI: 10.1039/x0xx00000x
Transformation of captured CO2 into value-added chemicals to mitigate increasing CO2 concentration in the atmosphere has
gained significant attention recently. Though the carbon capture and storage (CCS) is already been practiced in few places,
it suffers from energy-intensive CO2 desorption and compression steps involved, which can be avoided in the carbon capture
and utilization (CCU) approach. Herein, a selection of carbon capture solvents were screened to assess the reactivity of
condensed-phase heterogeneous metal catalyzed hydrogenation of CO2. Among catalysts screened, Cu/ZnO/Al2O3 catalyst
was active for the one-pot CO2 capture and conversion to methanol process using post and pre-combustion carbon capture
solvents comprised of various amines and alcohols. Our findings indicate that formamides are less-reactive under our
conditions, in comparison to formate ester intermediates noting that a combination of 1˚ alcohols and amines gives the
highest methanol yield. Screening volatile organic compound (VOC)-free alcohols and amines led us to an environmentally
benign system of bio-derived and biodegradable chitosan and polyethylene glycol (PEG200), which provide moderate
concentration of methanol (139.5 mmol/L) with a facile separation of volatile products (water and methanol). The
chitosan/PEG200 system was recycled three times, ultimately providing a promising VOC-free, biodegradable, bio-derived
and recyclable CO2 capture and conversion pathway.
the existing CO2 sources. Post-combustion CO2 capture typically
involves chemically-selective solvents that are regenerated with
thermal heating, where the released and recovered CO2 can be
1. Introduction
Global CO2 concentration in the atmosphere has been increasing
continuously and this in turn leads to potential environmental
issues.1, 2 The burning of fossil fuels like coal and oil for energy is the
primary cause for the continued rise of atmospheric CO2 level at the
rate of 3 ppm each year, recently surpassing 415 ppm in May, 2019.3
The global demand for energy is steadily increasing and our
dependence on fossil fuel for our energy needs has to be subdued as
its resources are depleting.4 In this context, researchers are ramping
up their efforts to recycle CO2 to value added fuels/chemicals.5-8 The
recycling of CO2 to fuels such as methanol, ethanol, formates etc is
shown to be viable upon hydrogenation.9-11 The hydrogen required
for this process can be obtained by water electrolysis using
renewable sources such as wind, solar etc. Among the CO2
hydrogenation products, methanol is a commodity chemical with a
global market demand of 78 million tonnes/yr. Methanol is used as a
feedstock to produce olefins, fuel blends, acetic acid, formaldehyde
and marine fuels.12
utilized in a second step as a C1 source to produce various
chemicals.14 One of the drawbacks with this method is energy
intensive CO2 desorption (Habs=-80 to -85 kJ/mol for 30 wt.% MEA)
and compression processes prior to any chemical conversion.15,16 The
thermodynamic penalties associated with the capture process can be
lessened when the CO2 desorption and compression steps are
avoided.17-24 We have previously demonstrated condensed-phase
hydrogenation of CO2 to methanol (CO2 + 3H2 → CH3OH + H2O) in the
presence of Cu/ZnO/Al2O3 catalyst in NEt3/ethanol capture solvent
by conceptually bypassing the CO2 desorption and compression
steps.25 The pre-combustion NEt3/ethanol capture solvent used in
previous studies, while effective, was comprised of low boiling
organic liquids, which makes the separation of products (water and
methanol) from the capture solvent a challenge. Further, ethanol
forms an azeotrope with water and there is high capital and energy
costs associated with azeotropic distillation of ethanol and water
mixture. As our previous studies had focused on the reactivity of CO2
captured in pre-combustion (pressure-swing) solvents,26 we decided
to study how the post-combustion (temperature-swing) amines
would behave in a similar combined capture and conversion
approach.27,28
Various technologies are developed for CO2 capture;13 primarily
for post combustion CO2 capture with the aim of being retrofitted to
a. Energy Processes and Materials Division, Pacific Northwest National Laboratory,
Richland, Washington 99352, USA. E-mail: david.heldebrant@pnnl.gov
The majority of post-combustion solvents utilize aqueous or
non-aqueous formulations of amines and alcohols to capture CO2,
which coincidentally are the same chemical moieties that are used to
†
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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promote catalytic CO2 hydrogenations.2, 18, 19, 25, 29 Primary amines added to the reactor vessel and the reactor was sealed in a nitrogen
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capture CO2 as carbamate species,30 tertiary amines capture CO2 only glovebox. The reactor was first pressurized with CO2 and then H2 was
DOI: 10.1039/C9GC03449H
-
in the presence of water or alcohol and form bicarbonate (HCO3 ) or
introduced. The reactor was heated to set temperature. After 12 or
16 h, the reactor was cooled to room temperature and then to -78
°C, the excess pressure was released slowly. 100 mg of 1,3,5-
trimethoxybenzene was added as an internal standard to the
reaction mixture (if necessary, DMSO or water was added to the
reaction mixture to dissolve the solid). The supernatant was
-
31
alkylcarbonate (RCO3 ) species.26,
In our approach, the CO2
hydrogenation to methanol in the condensed-phase can either
proceed via formamide (Scheme 1, pathway a) or ester
intermediates (Scheme 1, pathway b) in the capture solvent medium.
A CO2 capture solvent typically contains amine or amino alcohol
moieties dissolved in aqueous or organic solvents. When primary and
secondary amines are used, pathway (a) is favoured, where the
formate and formamide intermediates are involved.2, 32 In the case
of tertiary amines, the formate species cannot form formamide as it
lacks the -NH- moiety, hence it requires an alcohol group to form
ester intermediate, which then gets reduced to methanol (pathway
b, Scheme 1).25, 33
1
decanted or filtered from the catalyst and analysed by H and 13C
NMR experiments.
Recycling experiments
The pre-activated catalyst was transferred to a 300 mL Parr reactor
and a given amount of chitosan and alcohol were added to the
reactor vessel and the reactor was sealed in a nitrogen glovebox. The
reactor was first pressurized with 20 bar CO2 and then additional 40
bar of H2 was introduced (total pressure = 60 bar). Then, the reactor
was heated to 170 °C. After 12 h, the reactor was cooled to room
temperature and then to -78 °C, the excess pressure was released
slowly. The insoluble solids (chitosan and catalyst) was separated
from the supernatant (alcohol) by centrifugation. 100 mg of 1,3,5-
trimethoxybenzene was added as an internal standard to the
supernatant and analysed by 1H and 13C NMR experiments. The solids
(chitosan and catalyst) were reused for subsequent cycles. For each
cycle, fresh alcohol was added, and above hydrogenation procedure
was followed.
Scheme 1.
The purpose of the present work is to study the reactivity of CO2
towards catalytic hydrogenation in both pre-combustion and post-
combustion carbon capture solvents, particularly with high boiling
capture amines/solvents to improve the productivity and ease the
separation of hydrogenation products thereby making the process
VOC-free. A second target of this work was to assess bio-derived and
biodegradable alcohols and bases to achieve integrated capture and
conversion of CO2 to methanol process as environmentally benign as
possible.
3. Results and discussion
Monoethanol amine (MEA)the most referenced, amine
solvent for post-combustion CO2 capture35, 36was studied as it is
less investigated in the literature for combined capture and
conversion. MEA was used for the initial screening with the
palladium-based heterogeneous catalysts (Table 1). Palladium
catalysts were already reported in the literature for N-formylation of
amines using CO2/H2 in good yields using aliphatic secondary amine
substrates. However, compared to the homogeneous catalysts,37, 38
the heterogenous catalysts have relatively low turnover number
(TON <300) for N-formylation.39, 40 MEA was also attractive from a
chemistry standpoint as it contains both primary amine and alcohol
moieties that are known to promote the catalytic hydrogenation of
CO2 into methanol via formamide or formate ester intermediates.
At 120 °C, in the presence of a 1 wt% Pd/SiO2 catalyst, a TON of
255 was obtained for formamide species (3) under 55 bar of CO2:H2
pressure (TON calculated based on number of moles of product
formed per mole of catalyst used and not based on the active sites).
THF was used as a solvent as the ethereal or alcoholic solvents were
commonly used for hydrogenation studies to improve the catalyst’s
performance both in homogeneous and heterogenous catalysis.2, 25,
2. Experimental
All materials were purchased from commercial suppliers and used
without further purification unless otherwise mentioned. Pd/ZnO,
Pd/Al2O3 and Pd/SiO2 were prepared by incipient wetness
impregnation method.34 Pd(NO3)2 and Al2O3 were obtained from
Engelhard. ZnO and silica gel were purchased from Sigma-Aldrich.
Cu/ZnO/Al2O3 catalyst was obtained from Synetix. Inductively
coupled plasma optical emission spectroscopy (ICP-OES) analysis of
the Cu/ZnO/Al2O3 showed 55.7 wt.% Cu, 26.8 wt.% Zn, and 5.0 wt.%
Al. Catalysts were activated under H2 atmosphere (50 bar) at 120 °C
in a parr reactor (300 mL). After 12h, the remaining H2 pressure from
the reactor was released 120 °C and cooled to room temperature.
41
The experiments in Table 1 were studied at higher temperature
(>100 °C) as the formation of formamide (3) from formate (2)
typically requires such temperatures in the absence of catalysts.
Changing the palladium support from SiO2 to Al2O3 significantly
decreased the catalytic activity.
Standard procedure for the CO2 hydrogenation
The above pre-activated catalyst was transferred to a 300 mL Parr
reactor and a given amount of amine, alcohol and/or THF were
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Table 1.
hydrogenation form methanol and amine.2 On the other hand, amide
View Article Online
DOI: 10.1039/C9GC03449H
hydrogenation can also proceed via C-O bond cleavage with a
liberation of water to form N-methylated amine, which does not
47, 48
form methanol.
The selectivity towards the C-N or C-O bond
cleavage depends on the catalyst and it is not clearly understood at
this point. The hydrogenation of N, N’-dimethylformamide was
studied in the presence of Pd/ZnO at 150 °C under 55 bar H2 pressure
(Table 2). It is evident from Table 2, entry 1 that the C-O bond
cleavage is favorable in the case of Pd/ZnO catalyst. The
hydrogenation of CO2 in MEA in the presence of Cu/ZnO/Al2O3
catalyst at 170 °C under 60 bar CO2/H2 pressure (CO2:2H2), led to
mostly decomposition of MEA, and no detectable amount of any CO2
hydrogenated products were observed by 1H NMR.
As the catalysts screened in this study towards CO2
hydrogenation to methanol in MEA led to either decomposition or N-
methylation of amine, we turned our attention to other
concentrated capture solvents (amines and alcohols). The
Cu/ZnO/Al2O3 is the catalyst of choice as we had shown previously
that it is effective for CO2-to-methanol process in condensed-phase
at relatively low temperature, though we knew little of the reactivity
of amines and alcohols under these conditions. Several simple
alcohols were studied to understand the influence of electronic and
steric factors on the formation of methanol through formate ester
route (pathway b, Scheme 1).
In Table 3, the effect of alcohol on the CO2 reduction products
were studied. In a neat high boiling amine pre-combustion capture
solvent, DIPEA (N,N-diisopropylethylamine, bp. 126.6 °C),26 in the
presence of Cu/ZnO/Al2O3 without an alcohol at 170 °C, there was a
small amount of methanol (19.2 mmol/L) and no detectable amount
of formate ester observed by 1H and 13C NMR experiments. The
methanol may have formed from direct hydrogenation of CO2, in
which the CO2 and H2 first reacts with Cu and form catalyst surface
adsorbed species, HCOO (ad) and H(ad), the well-recognized
intermediates.49-52 Then the polarity effect of DIPEA may have
facilitated the reduction of HCOO (ad) species to methanol by
lowering its activation barrier.
Entry
Catalysts
T (°C)
TON (3)
1
2
1 wt% Pd/SiO2
1 wt% Pd/Al2O3
1 wt% Pd/SiO2
5 wt% Pd/ZnO
5 wt% Pd/ZnO
5 wt% Pd/ZnO
5 wt% Pd/ZnO
120
120
150
125
150
170
150
255
96
3
4a
362
650
1543
693
-
5
6
7b
Reaction conditions: catalyst = 30 mg, P = 55 bar (CO2:1.5 H2), THF=20
mL, MEA (1) = 32.7 mmol, time = 16h, TON was calculated based on
1H NMR using 1,3,5-trimethoxy benzene as an internal standard.
a60h, bDIPEA (32.7 mmol) was used instead of MEA.
Increasing the temperature from 120 °C to 150 °C increased the
TON to 362. The Pd-Zn alloy formed on the reduction of Pd/ZnO was
reported to be active towards CO2 hydrogenation to methanol in gas
phase.42,43,44, 45 In the presence of capture amine, MEA, Pd/ZnO gave
a TON of 1543, which is to the best of our knowledge is the highest
number reported in the literature for N-formylation. Increasing the
temperature further to 170 °C was detrimental to the hydrogenation
reaction as it dropped the activity of the catalyst for N-formylation.
When a tertiary amine, DIPEA was used as a capture solvent instead
of MEA, no detectable amount of any CO2 hydrogenated products
were observed. None of the experiments in Table 1 formed
methanol, the final product (or the ester intermediate), therefore we
decided to focus on the final step, hydrogenation of formamide.
Table 2.
Upon addition of ethanol to DIPEA, the methanol production
has significantly increased to 473.7 mmol/L. This increase is because
of the formation of formate ester intermediate, which is known to
form under mild condition in the presence of an acidic or basic
catalyst. Further reduction of ester in the presence of a catalyst
forms methanol, desired product, and ethanol. By changing the
alcohol from primary (ethanol) to secondary alcohol (isopropanol or
2-butanol), methanol, ammonium formate and formate ester
concentrations decreased with increasing carbon on the alcohol.
With t-butanol, the methanol and ester concentration decreased
significantly to 191.1 mmol/L and no ammonium formate was
detected by 1H NMR. This suggests that the polarity of the solvent is
important for the ammonium formate formation (ethanol is more
polar than t-butanol) and sterics plays a key role in the formation of
ester as the esterification step is known to have affected by the steric
hindrance of the alkyl groups on alcohol. Both the polarity and sterics
of the solvent affect the formation of methanol and therefore The
solvent with high polarity and less steric crowding is favored for this
Hydrogenation
Entry
1
Substrates
Catalyst
product
5wt%
Pd/ZnO
4% trimethylamine
TON = 86
5wt%
Pd/ZnO
2
-
(3)
Reaction conditions: catalyst = 30 mg, T=150 °C, P = 55 bar (H2) entry
1: Dimethylformamide = 32.7 mmol, time=60 h, THF=20 mL; entry 2:
3 = 2.5 mmol, time=16 h, THF=7 mL. TON was calculated based on 1H
NMR using 1,3,5-trimethoxy benzene as an internal standard.
The hydrogenation of N-formamide can either undergo C-O or
C-N bond cleavage.46 The cleavage of C-N bond of the amide upon
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reaction, noting that 1˚ alcohols produced the highest methanol yield significantly low (1:30), the methanol concentrationVdieewcArreticalseeOdnlitnoe
DOI: 10.1039/C9GC03449H
under these conditions.
372.4 mmol/L (CO2 conversion=21%).
Table 3.
Table 4.
HCOO-
mmol/L
HCOOR
mmol/L
CH3OH
mmol/L
Entry
Capture Solvent
1a
2
DIPEA
traces
72.4
32
-
19.2
DIPEA-ethanol
DIPEA-isopropanol
DIPEA-2-butanol
DIPEA-t-butanol
46
473.7
372.3
330.3
191.1
3
32
4
9.2
18.3
8.9
5
0
Reaction conditions: Cu/ZnO/Al2O3=300mg, amine = 20 mmol,
alcohol = 200 mmol, time = 16h, P = 60 bar (CO2:2H2), T = 170 °C,
aDIPEA = 60 mmol, HCOO-, HCOOR and CH3OH concentrations were
calculated based on H NMR using 1,3,5-trimethoxy benzene as an
internal standard.
HCOO-
mmol/L mmol/L
HCOOR
CH3OH
mmol/L
Entry
Capture Solvent
1
2a
3
NEt3-ethanol
NEt3-ethanol
165.5
41.4
20.7
124.1
7.5
689.6
427.6
699
1
NEt3-methoxyethanol
107.5
NEt3-1,2-
propyleneglycol
4
5
6
7
traces
9.6
traces
traces
138.2
-
950.5
490.4
975.6
-
NEt3-PEG200
NEt3-diethylene
glycol
traces
-
NEt3-glucose
Reaction conditions: Cu/ZnO/Al2O3=300mg, amine=20 mmol,
alcohol=100 or 200 mmol, time=12h, P= 60 bar (CO2:2H2), T=170 °C;
at=110 °C for 6 h and 170 °C for 6 h, HCOO-, HCOOR and CH3OH
concentrations were calculated based on 1H NMR using 1,3,5-
trimethoxy benzene as an internal standard.
As there are examples, where the formate and ester formed at
lower temperatures (~100 °C), we investigated stepwise reduction of
CO2 to methanol by first heating the reaction mixture at 110 °C for
6h and then to 170 °C for 6h to control the catalyst poisoning (entry
2, Table 4). 427.6 mmol/L of methanol was obtained, which is lower
than the experiment where the reaction mixture was directly heated
to 170 °C for 12h (689.6 mmol/L, entry 1, Table 4). Higher ester
concentration in entry 2, Table 4 shows that higher temperature is
required to hydrogenate the accumulated ester intermediate to
methanol. Higher formate concentration in entry 1, Table 4
demonstrates that the esterification step slows down due to
accumulation of water with increasing methanol concentration.
We first assessed the reactivity of varied VOC-free and or
environmentally benign 1˚ alcohols under these conditions. Glycols,
glymes and polyols have been used as physical solvents for pre-
combustion53 and co-solvents for post-combustion54-56 CO2 capture.
Further, polyethyleneglycol (PEG) has been shown to be a good
medium for metal-catalyzed hydrogenation of CO2.57 Among the
various high boiling alcohols investigated in Table 4 along with NEt3,
glycols yielded the highest methanol concentrations of 950.5 mmol/L
and 975.6 mmol/L (entry 4 and 6, Table 4). Remarkably, the polymer,
PEG200 also provided high methanol concentration of 490.4 mmol/L
which was comparable to ethanol. High boiling properties of glycols
and PEG are attractive as the low volatile products, methanol and
Figure 1. Reaction conditions: Cu/ZnO/Al2O3=300mg, NEt3=20 mmol,
ethanol=200 mmol, time=12h, T=170 °C, P=60 bar; entry 1,
amine=200 mmol; HCOO-, HCOOR and CH3OH concentrations were
calculated based on H NMR using 1,3,5-trimethoxy benzene as an
internal standard.
1
The change in CO2/H2 ratio on the methanol, alkyl ester and
ammonium formate concentrations is shown in Figure 1. The NEt3-
ethanol mixture was used instead of DIPEA-ethanol mixture as the
less steric NEt3 gave higher methanol concentration than DIPEA
(entry 2, Table 3 vs entry 1, Table 4). Surprisingly, decreasing the
CO2:H2 ratio from 1:2 to 1:11 did not change the methanol
concentration and only the ammonium formate and alkyl ester
concentrations decreased. The captured CO2, carbonates species
(step 1, pathway b, Scheme 1) was probably the active species that
was getting reduced and thus decrease in CO2 partial pressure did
not change the methanol concentration. In addition, the CO2
conversion in the case of CO2:H2 ratio of 1:2 was 4.7%. The decrease
in CO2:H2 ratio to 1: 30 increased the CO2 conversion to 16.1%.
However, the total methanol concentration remained the same
(689.6 mmol/L), suggesting possible deactivation of the catalyst due
to the accumulation of products. Only when the CO2:H2 ratio was
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water can be easily separated from these solvents. We note that deacylating chitin.58 Chitosan being the second mVoieswt AartbiculenOdnalinnet
DOI: 10.1039/C9GC03449H
while diethylene glycol (DEG) had the highest yield of methanol and natural polymer after cellulose has one primary amine unit and two
is VOC-free, DEG is highly toxic and should be avoided. Conversely, hydroxyl units. The primary amine group is known to reversibly bind
the moderate reactivity, low cost and biodegradable nature of PEG with CO2 and form ammonium carbamate at room temperature. The
makes it attractive alcohol/solvent in a green process.
D-glucosamine units of chitosan has been shown previously as a CO2
59-61
Glucose was also studied as it is a bio-derived solid alcohol that absorbent by various groups,
though to our knowledge there
has four secondary alcohol and one primary alcohol groups in which have been no studies on using chitosan for CO2 hydrogenation.
to react. Glucose was screened for the reaction with NEt3 as it was Using chitosan-ethanol mixture as a capture solvent, under our
expected to behave similar to the simple alcohols that are screened reaction conditions, a moderate methanol concentration of 227.3
before. Unfortunately, glucose decomposed and charred under our mmol/L was obtained. This result is surprising, as chitosan is
reaction conditions and is unlikely to be viable alcohol source under expected to be a solid under reaction conditions and even more
these conditions.
surprising as glucose charred and exhibited zero reactivity. In the
case of chitosan, although ester was identified by 1H NMR, we cannot
rule out the possibility of methanol formation via formamide route
as chitosan contain primary amine groups. Since ethanol is a volatile
alcohol, non-volatile alcohols such as 1,2 propylene glycol, DEG and
PEG200 were studied under comparable conditions. The
chitosan/DEG system showed the highest methanol concentration
(472 mmol/L) while 1,2 propylene glycol and PEG200 showed
moderate methanol formation (250 mmol/L and 139.5 mmol/L
Table 5.
respectively).
(b)
HCOO--
mmol/L
HCOOR
mmol/L
CH3OH
mmol/L
Entry
Capture Solvent
1
2
NMM-ethanol
-
43.2
446
373
N(Hex)3-ethanol
32.4
54
3
4
EDDE-ethanol
-
-
299.3a
181.8
0
Figure 2. Reaction conditions: Cu/ZnO/Al2O3=300mg, chitosan=1.5 g,
ethanol=200 mmol, PEG200=20g, time=12h, P=60 bar (CO2:2H2),
T=170 °C, HCOO-, HCOOR and CH3OH concentrations were calculated
based on 1H NMR using 1,3,5-trimethoxy benzene as an internal
standard.
chitosan-ethanol
227.3
chitosan-1,2-
propyleneglycol
chitosan-
diethyleneglycol
chitosan-PEG200
5
-
22.7
250
6
7
traces
-
30
7
472.7
139.5
Reaction conditions: Cu/ZnO/Al2O3=300mg, amine=20 mmol,
chitosan=1.5 g, alcohol=100 or 200 mmol, PEG200=20g, time=12h,
P=60 bar (CO2:2H2), T=170 °C, amixture of ester and formamide.
HCOO-, HCOOR and CH3OH concentrations were calculated based on
1H NMR using 1,3,5-trimethoxy benzene as an internal standard.
After the chitosan reactions, we observed a white solid with
particles of catalyst underneath a clear colourless solution that was
analysed and shown to contain only ethanol, methanol and water.
The built-in separation of solid catalyst and solid amine led us to
question whether the base and catalyst could be recovered and
reused by decanting the supernatant and introducing fresh alcohol
for multiple cycles of hydrogenation of CO2.
Recycling experiments were attempted with ethanol, DEG and
PEG200 alcohols. Ethanol recycling experiments showed a decrease in
the yield of methanol over multiple cycles (Figure 2a). On the other
hand, the concentration of ester continued to increase in subsequent
cycles, showing that the catalyst becomes less active for ester
hydrogenation. Catalyst sintering could probably be the reason
behind the drop in the activity. The accumulation of by-product,
water, might have caused the sintering, which was previously
observed by us and others in batch reactor experiments.25, 62
The recycling experiments with DEG and chitosan showed
significant deactivation and the methanol concentration decreased
from 472.7 mmol/L in the first cycle to 127.3 mmol/L in the second
cycle. There was also obvious leaching of metal into the supernatant
Having the reactivity of high-boiling alcohols, we next studied
VOC-free (high boiling) amines/aminoalcohols such as N,N’-Bis(2-
hydroxyethyl)ethylenediamine (EDDE), N-methyl morpholine (NMM)
and trihexyl amine (N(Hex)3) (Table 5). For the initial studies, the
simple alcohol, ethanol was used as a co-solvent since it was
previously shown to be compatible with many amines. In NMM and
N(Hex)3, methanol concentrations of 446 and 373 mmol/L were
obtained, respectively (entry 1 and 2, Table 5). Similar to MEA, use of
aminoalcohol, EDDE did not form any methanol, showing that in-situ
formed formamide intermediate is difficult to hydrogenate to
methanol with the catalysts used in this study.
As with the alcohols, we then focused our efforts to study the
reactivity of VOC-free, environmentally benign and biodegradable
amines under these conditions. The only amine meeting these
criteria is the polysaccharide, chitosan, which is produced by
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as it became coloured after the first reaction cycle and therefore the
recycling study was stopped after the second cycle.
Contribution of Working Group I to the Fifth Assessment
View Article Online
Report of the Intergovernmental Panel on Climate Change,
DOI: 10.1039/C9GC03449H
2013.
Interestingly, recycling experiments with PEG200 showed no loss
in activity in the second cycle, though methanol yield decreased from
139.5 mmol/L to 88.4 mmol/L by the third cycle (Figure 2b). The
formate ester concentration increased slightly from the first cycle to
the third cycle. The reduced activity in all recycling experiments is
indicative of deactivation of the catalyst likely due to sintering, which
is expected to be slower in PEG200 due to the available charge
solvation from the numerous polyether moieties.63 It is likely that
more catalysts that are less susceptible to deactivation would enable
a more robust recyclable CO2 capture and conversion process.
2.
J. Kothandaraman, A. Goeppert, M. Czaun, G. A. Olah and
G. K. Prakash, J Am. Chem. Soc., 2016, 138, 778-781.
https://www.esrl.noaa.gov/gmd/ccgg/trends/.
M. Höök and X. Tang, Energy Policy, 2013, 52, 797-809.
G. A. Olah, G. K. Prakash and A. Goeppert, J. Am. Chem.
Soc., 2011, 133, 12881-12898.
G. A. Olah, A. Goeppert and G. K. Prakash, J. Org. Chem.,
2009, 74, 487-498.
C. Song, Catal. Today, 2006, 115, 2-32.
3.
4.
5.
6.
7.
8.
J. Leclaire and D. J. Heldebrant, Green Chem., 2018, 20,
5058-5081.
9.
M. A. Scibioh and B. Viswanathan, Carbon Dioxide to
Chemicals and Fuels, Elsevier, 2018.
4. Conclusions
In conclusion, we have demonstrated that heterogeneous systems
produce methanol via ammonium formate and formate ester
intermediate (pathway b, Scheme 1) from CO2 and capture
10.
J. Artz, T. E. Muller, K. Thenert, J. Kleinekorte, R. Meys, A.
Sternberg, A. Bardow and W. Leitner, Chem. Rev., 2018,
118, 434-504.
L. S. Guo, J. Sun, Q. J. Ge and N. Tsubaki, J Mater Chem A,
2018, 6, 23244-23262.
https://www.methanex.com/about-methanol
P. Markewitz, W. Kuckshinrichs, W. Leitner, J. Linssen, P.
Zapp, R. Bongartz, A. Schreiber and T. E. Muller, Energy
Environ. Sci., 2012, 5, 7281-7305.
E. Alper and O. Yuksel Orhan, Petroleum, 2017, 3, 109-126.
I. Kim and H. F. Svendsen, Int. J. Greenh. Gas Con., 2011, 5,
390-395.
https://www.netl.doe.gov/sites/default/files/netl-
file/Carbon-Capture-Technology-Compendium-2018.pdf
J. Kothandaraman, J. Zhang, V.-A. Glezakou, M. T. Mock
and D. J. Heldebrant, J. CO2 Utili., 2019, 32, 196-201.
D. B. Lao, B. R. Galan, J. C. Linehan and D. J. Heldebrant,
Green Chem., 2016, 18, 4871-4874.
J. Kothandaraman, A. Goeppert, M. Czaun, G. A. Olah and
G. K. S. Prakash, Green Chem., 2016, 18, 5831-5838.
M. Lu, J. Zhang, Y. Yao, J. Sun, Y. Wang and H. Lin, Green
Chem., 2018, 20, 4292-4298.
J. Su, M. Lu and H. F. Lin, Green Chem., 2015, 17, 2769-
2773.
C. Reller, M. Poge, A. Lissner and F. O. Mertens, Environ.
Sci. Technol., 2014, 48, 14799-14804.
Y. N. Li, L. N. He, A. H. Liu, X. D. Lang, Z. Z. Yang, B. Yu and
C. R. Luan, Green Chem., 2013, 15, 2825-2829.
N. D. McNamara and J. C. Hicks, ChemSusChem, 2014, 7,
1114-1124.
J. Kothandaraman, R. A. Dagle, V. L. Dagle, S. D. Davidson,
E. D. Walter, S. D. Burton, D. W. Hoyt and D. J. Heldebrant,
Catal Sci Technol, 2018, 8, 5098–5103.
J. E. Rainbolt, P. K. Koech, C. R. Yonker, F. Zheng, D. Main,
M. L. Weaver, J. C. Linehan and D. J. Heldebrant, Energy
Environ. Sci., 2011, 4, 480-484.
https://netl.doe.gov/sites/default/files/netl-file/CO2-
Capture-Tech-Update-2013-Post-Combustion-
Solvents.pdf
11.
amine/solvent.
Attempts to hydrogenate N-formamide
12.
13.
intermediate (pathway a, Scheme 1) to methanol via C-N bond
cleavage led to C-O bond cleavage and resulted in N-methylation.
Unlike homogenous systems, where both ester and formamide
intermediates were known to get hydrogenated further to methanol,
heterogeneous system can only hydrogenate ester intermediate to
methanol. We have also successfully demonstrated hydrogenation of
CO2 to methanol using capture solvents with high boiling points.
Methanol concentration of 139.5 mmol/L was obtained in VOC-free,
non-toxic, bio-derived and biodegradable capture solvent medium
based on chitosan/PEG200. The chitosan and catalyst were recycled
for 3 times with observation of reduced activity after second cycle.
The future studies will be focussed on understanding the catalyst
deactivation pathway(s) and identifying durable catalyst for the
combined CO2 capture and conversion.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors thank the United States Department of Energy's Office
of Science Basic Energy Sciences Early Career Research Program FWP
67038 for funding. We would like to thank Vanessa Dagle and Robert
A. Dagle for providing us with the catalysts. The Pacific Northwest
National Laboratory is proudly operated by Battelle for the US
Department of Energy.
26.
27.
28.
D. J. Heldebrant, P. K. Koech, V. A. Glezakou, R. Rousseau,
D. Malhotra and D. C. Cantu, Chem. Rev., 2017, 117, 9594-
9624.
Notes and references
29.
30.
S. Kar, J. Kothandaraman, A. Goeppert and G. K. S. Prakash,
J. CO2. Util., 2018, 23, 212-218.
M. Caplow, J. Am. Chem. Soc., 1968, 90, 6795-6803.
1.
T. F. Stocker, D. Qin, G. K. Plattner, M. Tignor, S. K. Allen, J.
Boschung, A. Nauels, Y. Xia, V. Bex and P. M. Midgley,
Climate Change 2013: The Physical Science Basis.
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 7 of 7
Green Chemistry
Please do not adjust margins
Journal Name
ARTICLE
31.
32.
T. L. Donaldson and Y. N. Nguyen, Ind. Eng. Chem. Fundam., 59.
1980, 19, 260-266.
S. Kar, R. Sen, J. Kothandaraman, A. Goeppert, R. 60.
Gregor Sneddon, Alexey Y. Ganin and H. H.VPie.wYAiurt,iclEenOenrlignye
Technol., 2015, 3, 249-258.
DOI: 10.1039/C9GC03449H
H. B. Xie, S. B. Zhang and S. H. Li, Green Chem., 2006, 8,
630-633.
Chowdhury, S. B. Munoz, R. Haiges and G. K. S. Prakash, J.
Am. Chem. Soc., 2019, 141, 3160-3170.
Y. Chen, S. Choi and L. T. Thompson, Catal. Today, 2016,
259, 285-291.
Y. H. Chin, R. Dagle, J. L. Hu, A. C. Dohnalkova and Y. Wang,
Catal. Today, 2002, 77, 79-88.
P. Luis, Desalination, 2016, 380, 93-99.
61.
62.
63.
K. Oslera, N. Twalaa, O. O. Oluwasina and M. O. Daramolaa,
Energy Procedia, 2017, 114, 2330-2335.
J. T. Sun, I. S. Metcalfe and M. Sahibzada, Ind. Eng. Chem.
Res., 1999, 38, 3868-3872.
D. Balasubramanian and B. Chandani, J. Chem. Educ., 1983,
60.
33.
34.
35.
36.
D. Aaron and C. Tsouris, Sep. Sci. Technol., 2005, 40, 321-
348.
37.
R. Kuhlmann, K. U. Kunnemann, L. Hinderink, A. Behr and
A. J. Vorholt, ACS Sustainable Chem. Eng., 2019, 7, 4924-
4931.
38.
39.
40.
41.
42.
43.
L. Zhang, Z. Han, X. Zhao, Z. Wang and K. Ding, Angew.
Chem. Int. Ed, 2015, 54, 6186-6189.
Y. Zhang, H. Wang, H. Yuan and F. Shi, ACS Sustainable
Chem. Eng., 2017, 5, 5758-5765.
X. Yu, Z. Yang, S. Guo, Z. Liu, H. Zhang, B. Yu, Y. Zhao and Z.
Liu, Chem. Commun. , 2018, 54, 7633-7636.
Y. Wang, B. Chen, S. Liu, X. Shen, S. Li, Y. Yang, H. Liu and B.
Han, ChemCatChem, 2018, 10, 5124-5127.
C. H. Kim, J. S. Lee and D. L. Trimm, Top Catal, 2003, 22,
319-324.
H. Bahruji, M. Bowker, G. Hutchings, N. Dimitratos, P.
Wells, E. Gibson, W. Jones, C. Brookes, D. Morgan and G.
Lalev, J. Catal. , 2016, 343.
44.
45.
J. Xu, X. Su, X. Liu, X. Pan, G. Pei, Y. Huang, X. Wang, T.
Zhang and H. Geng, Appl. Catal., A, 2016, 514, 51-59.
V. M. Lebarbier, R. A. Dagle, L. Kovarik, J. A. Lizarazo-
Adarme, D. L. King and D. R. Palo, Catal Sci Technol, 2012,
2, 2116-2127.
46.
47.
P. A. Dub and T. Ikariya, ACS Catal., 2012, 2, 1718-1741.
O. Santoro, F. Lazreg, Y. Minenkov, L. Cavallo and C. S.
Cazin, Dalton Trans., 2015, 44, 18138-18144.
K. Beydoun, K. Thenert, E. S. Streng, S. Brosinski, W. Leitner
and J. Klankermayer, ChemCatChem, 2016, 8, 135-138.
M. D. Porosoff, B. Yan and J. G. Chen, Energy Environ. Sci.,
2016, 9, 62-73.
S. Kattel, P. J. Ramirez, J. G. Chen, J. A. Rodriguez and P. Liu,
Science, 2017, 355, 1296-1299.
X. Wang, H. Zhang and W. Li, Korean J. Chem. Eng. , 2010,
27, 1093-1098.
J. Wambach, A. Baiker and A. Wokaun, Phys. Chem. Chem.
Phys., 1999, 1, 5071-5080.
B. Burr and L. A. Lyddon, presented in part at the 87th
Annu. Conv. Proc, 2008.
R. J. Perry, T. A. Grocela-Rocha, M. J. O'Brien, S. Genovese,
B. R. Wood, L. N. Lewis, H. Lam, G. Soloveichik, M.
Rubinsztajn, S. Kniajanski, S. Draper, R. M. Enick, J. K.
Johnson, H. B. Xie and D. Tapriyal, ChemSusChem, 2010, 3,
919-930.
48.
49.
50.
51.
52.
53.
54.
55.
56.
57.
58.
R. J. Perry and M. J. O’Brien, Energy Fuels, 2011, 25, 1906-
1918.
R. J. Perry and J. L. Davis, Energy Fuels, 2012, 26, 2512-
2517.
D. J. Heldebrant and P. G. Jessop, J Am. Chem. Soc., 2003,
125, 5600-5601.
D. Elieh-Ali-Komi and M. R. Hamblin, Int. J. Adv. Res., 2016,
4, 411-427.
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