An integrated process of CO2 capture and in situ hydrogenation to formate using a tunable ethoxyl-functionalized amidine and Rh/bisphosphine system

Article abstract of DOI:10.1039/c4ra08740b

An integrated process of CO₂ capture and in situ hydrogenation into formate was achieved in 95-99% yield using a tunable ethoxyl-functionalized amidine and Rh/bisphosphine system, being regarded as an alternative carbon capture and utilization approach to supply fuel-related products, to circumvent the energy penalty in carbon capture and storage. CO₂ was captured by non-volatile amidine derivatives with simultaneous activation to form zwitterionic amidinium carbonate, and subsequent hydrogenation was facilitated by Rh/bisphosphine. The adsorption capacity and hydrogenation efficiency can be optimized by tuning the ethoxyl side chain. Particularly, the alkanolamidine bearing an intramolecular hydrogen donor derived from 1,8-diazabicyclo[5.4.0]-undec-7-ene (DBU) gave both a high CO₂ uptake (molar ratio of 0.95:1) and excellent hydrogenation yield (99%). Furthermore, the silica-supported alkanolamidine was readily recovered and reused with the retention of good performance. This kind of carbon capture and utilization pathway could be a potential energy-saving option for industrial upgrading of CO₂ from waste to fuel-related products in a carbon neutral manner.

Full text of DOI:10.1039/c4ra08740b

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An integrated process of CO2 capture and in situ
hydrogenation to formate using a tunable ethoxyl-
functionalized amidine and Rh/bisphosphine
system†
Cite this: RSC Adv., 2014, 4, 49995
*
Yu-Nong Li, Liang-Nian He, Xian-Dong Lang, Xiao-Fang Liu and Shuai Zhang
An integrated process of CO2 capture and in situ hydrogenation into formate was achieved in 95–99% yield
using a tunable ethoxyl-functionalized amidine and Rh/bisphosphine system, being regarded as an
alternative carbon capture and utilization approach to supply fuel-related products, to circumvent the
energy penalty in carbon capture and storage. CO2 was captured by non-volatile amidine derivatives
with simultaneous activation to form zwitterionic amidinium carbonate, and subsequent hydrogenation
was facilitated by Rh/bisphosphine. The adsorption capacity and hydrogenation efficiency can be
optimized by tuning the ethoxyl side chain. Particularly, the alkanolamidine bearing an intramolecular
hydrogen donor derived from 1,8-diazabicyclo[5.4.0]-undec-7-ene (DBU) gave both a high CO2 uptake
(molar ratio of 0.95 : 1) and excellent hydrogenation yield (99%). Furthermore, the silica-supported
alkanolamidine was readily recovered and reused with the retention of good performance. This kind of
carbon capture and utilization pathway could be a potential energy-saving option for industrial
upgrading of CO2 from waste to fuel-related products in a carbon neutral manner.
Received 16th August 2014
Accepted 17th September 2014
DOI: 10.1039/c4ra08740b
www.rsc.org/advances
Formic acid is an extremely attractive liquid hydrogen source
and hydrogen carrier for easy storage and transportation.5 An
organic or inorganic base6 is always indispensable to make CO2
hydrogenation thermodynamically favorable by forming the
acid–base adduct.7 Furthermore, nonvolatile amino-functional
ionic liquids (ILs) and an immobilized base can be applied
for this purpose to improve base recovery.8 Recently, Hicks et al.
Introduction
Among numerous schemes to reduce greenhouse gas emis-
sions, carbon capture and storage (CCS) is regarded as one of
the most potential measures.1 However, extensive energy
consumption in the desorption/compression process would be
a crucial barrier for practical CCS.1c Consequently, carbon
capture and utilization (CCU) has been proposed as an alter-
native approach to circumvent the energy issues in CCS,
whereby the captured CO2 can be used as a non-toxic, abundant,
and sustainable feedstock to produce value-added compounds/
fuels/materials.2 In this context, a high CCU efficiency would
offer feasible applications in industry. An elegant combination
of CO2 capture and in situ hydrogenation to fuel-related prod-
ucts including formic acid, methanol, and amides, as well as
methylated compounds,3 is an appealing strategy to supply
renewable energy in a carbon neutral cycle.4 In addition, this
protocol can avoid high-pressure operations, which are
commonly required for hydrogenation reactions involving CO2.
employed
a
double-base system consisting of poly-
ethyleneimine (PEI)/Et3N for CO2 capture and hydrogenation,
respectively. But in this system, the additional base Et3N was
necessary for the hydrogenation.9 Base strength and solubility
limitations can affect the hydrogenation outcome.10 In this
aspect, amidine compounds have viable basicity and CO2-
affinity, despite few-documented instances.10,11 Moreover, based
on Jessop's switchable solvent theory,12 amidine compounds
can also be used for CO2 capture in conjunction with a proton
donor to form amidinium carbonate.13 In our previous work, in
situ hydrogenation of the captured CO2 by PEI and a Rh/
monophosphine system was achieved in yield of 37%.
Notably, ammonium carbonate exhibited better activity than
ammonium carbamate.2c Therefore, it is desirable and assum-
able to improve catalytic performance for CCU by employing the
amidine compound as both the CO2 absorbent and hydroge-
nation promoter, which would presumably go through amidi-
nium carbonate hydrogenation.
State Key Laboratory and Institute of Elemento-Organic Chemistry, Collaborative
Innovation Center of Chemical Science and Engineering (Tianjin), Nankai
University, Tianjin, 300071, People's Republic of China. E-mail: heln@nankai.edu.
cn; Fax: +86-22-2350-3878; Tel: +86-22-2350-3878
† Electronic supplementary information (ESI) available: General experimental
methods, experimental procedures and results, characterization for adsorption
and hydrogenation systems, and copies of NMR, in situ FTIR, FT-IR, GC-MS
spectra, TGA and DFT study. See DOI: 10.1039/c4ra08740b
As expected, CCU efficiency is closely related to both capture
capacity and hydrogenation activity. Higher adsorption
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in situ hydrogenation along with separation and recycling could
be envisioned to avoid desorption, allowing recycling of the
absorbent/base as depicted in Fig. 1.
Results and discussion
CO2 capture
Amidine derivatives were initially studied for CO2 capture. In
this step, protonation occurs presumably at the sp2 nitrogen of
the amidine derivative, while the ethoxyl-functionalized chain
offers additional stabilization for the amidinium cation, facili-
tating the deprotonated oxygen species to x CO2. Remarkably,
the enhancement of CO2-philicity and basicity by the tunable
structure of the ethoxyl chain could promote CO2 sorption with
increasing capacity.18 Generally, more ethoxyl linkages could
increase the CO2-philicity and electron-donating ability, but
longer ethoxyl chains may also increase the viscosity.19 Hence,
DBUPEG150Me and TMGPEG150Me with three C–C–O units,
successfully approached equimolar CO2-capture with a molar
ratio of 0.89 : 1 and 1.11 : 1, respectively, owing to their appro-
priate basicity (entries 1 and 2, Table 1).
In this context, the characteristic signals at 151.29 and
158.73 ppm in the 13C NMR spectra support the formation of
amidinium alkylcarbonates. The bands centered at 1588, 1405
and 1294 cmꢀ1 in the FT-IR spectra can be assigned to the
stretching vibration of the C]O bond of carbonate, basically in
agreement with those previously reported13c (see ESI†). But
DBU2PEG150Br2 exhibited no adsorption capacity due to its
extremely weak basicity and high viscosity (entry 3). Further-
more, alkanolamidine (DBUOH) with one ethoxyl unit,
Scheme
hydrogenation.
1 Amidine derivatives for CO2 capture and in situ
enthalpy is allowed to attain a larger capacity.14 Hence,
approaches associated with enhancing the basicity e.g. intro-
ducing an electron-donating substituent have been developed
to increase the adsorption capacity.14,15 In this respect, amidine
compounds can be designed as a base for CO2 capture due to its
high basicity and facile structural modication. More impor-
tantly, compared with the published dual-component adsorp-
tion systems, amidines with an alkyl chain bearing a hydroxyl
group would remove the need of an additional volatile proton
donor.16 An ethoxyl or polyethylene glycol (PEG) substituent
could also promote the hydrogenation by adjusting the viscosity
and facilitating mass transfer because of their electron-
donating, CO2-philic and thermal-stable properties.17
We have developed non-volatile amidine derivatives based
on the framework of DBU and 1,1,3,3-tetramethylguanidine
(TMG) with an ethoxyl-functionalized chain bearing a terminal
hydroxyl or methoxy group to tune both CO2 uptake and
hydrogenation activity resulting in remarkably enhanced CCU
efficiency. The amidine derivatives in this study are designated
as DBUPEG150Me, TMGPEG150Me, DBUOH, DBUOH@Silica,
DBU@PS (PS ¼ Polystyrene), and DBU2PEG150Br2 as shown in
Scheme 1. Herein, the amidine derivatives will serve as a “hinge
base” for initial CO2 capture as an amidinium carbonate species
with simultaneous activation and subsequent hydrogenation.
This strategy was validated to produce formate in 95–99% yield
relative to the captured CO2 i.e. the amidinium carbonate,
whose hydrogenation was promoted by RhCl3$3H2O/bis(2-
diphenylphosphine phenyl)ether (DPEphos) under mild condi-
tions. Especially, a CCU industrial process for CO2 capture and
Table 1 CO2 adsorption using amidine derivativesa
Entry
Absorbent system (mmol mmolꢀ1
)
t (min)b
Capacityc
1
2
DBUPEG150Me/glycol (3/6)
TMGPEG150Me/glycol (3/18)
DBU2PEG150Br2/glycol (3/24)
DBUOH (3)
12
9
27
9
27
18
9
0.89
1.11
—
0.58
0.95
0.90
3.02
3.48
3
4d
5
DBUOH/TGDE (3/5)
6e
7f
8f
DBU@PS/glycol (0.24/24)
DBUOH@ Silica/TGDE (0.24/5)
DBUOH@ silica/glycol (0.24/24)
15
a
b
Amidine derivative (3 mmol), 25 ꢁC. Time required to reach
c
adsorption equilibrium unless otherwise stated. Moles of CO2
captured per mole of superbase subtracting the solvents physical
d
e
adsorption. Using DBUOH (3 mmol) without solvent. DBUOH@PS
(0.18 g, containing 0.24 mmol DBU) Elemental analysis: N 3.70%,
f
DBU 20.0% in weight. DBUOH@Silica (0.42 g, containing 0.24 mmol
DBUOH). Elemental analysis: N 1.60%, DBU 8.69% or DBUOH 11.3%
in weight.
Fig. 1 The hypothetical CCU process proposed in this study.
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containing an equal number of amidine and intramolecular
hydroxyl groups, can almost capture an equimolar amount of
CO2 in the absence of an additional proton donor (entry 4 and
5). Here, tetraethylene glycol dimethyl ether (TGDE) was used to
adjust the viscosity of DBUOH. In the 13C NMR spectrum, the
additional peak at 164.25 ppm likely corresponds to the intra-
molecular amidinium carbonate. Besides, the strong band of
[C]NH]+ at 1646 cmꢀ1, comparable to that of [HDBU]Cl (1645
cmꢀ1),12,20 and the peaks corresponding to C]O at 1554, 1421
and 1297 cmꢀ1 can also be observed (see ESI†).
Fig.
3
Reversible CO2 adsorption and desorption (a) and three
consecutive cycles (b) using DBUOH@Silica.
DFT calculated enthalpy changes revealed that the intra-
molecular carbonate as the adsorption product may undergo a
cyclic formation via hydrogen bonding (Fig. S1, ESI†). Further-
more, the polystyrene supported DBU/glycol system was able to
absorb CO2 in a molar ratio of 0.90 : 1 (entry 6). Particularly, in
the aprotic solvent, TGDE, 0.42 g of DBUOH@Silica (Fig. S4 and
S6, ESI†) can absorb 0.73 mmol of CO2, corresponding to a
capacity of 76 mg gꢀ1 gravimetrically. The maximum adsorption
ratio can be 3.48 : 1 (88 mg gꢀ1) using DBUOH@Silica in glycol
(entries 7 and 8). In the chemisorption process, the maximum
adsorption ratio should be around 1.
aer three successive adsorption–desorption cycles (Fig. 3b),
indicating DBUOH@Silica is stable enough to be recycled.
The hydrogenation of gaseous CO2 to formate
Subsequently, gaseous CO2 hydrogenation was performed to
establish an optimal catalytic system for the in situ hydrogena-
tion of the captured CO2, which is also an activated form of CO2.
In addition, DBUOH@silica as a solid absorbent can physi-
cally absorb CO2 besides chemisorption. Thus, the actual
adsorption ratio reached more than 1 relative to the effective
amount of the 0.24 mmol superbase DBUOH@silica contains.
Similarly, the peaks at 1646, 1538, 1456, and 1292 cmꢀ1, and a
decrease in the OH signal in the FT-IR spectrum suggest the
generation of the amidinium carbonate via chemisorption
(Fig. 2). In the physical adsorption process, DBUOH@Silica
could successfully be utilized for adsorption and desorption
measurements (Fig. 3). DBUOH@Silica can physically absorb
CO2 to 6.2 cm3 gꢀ1 (14.2 mg gꢀ1) at 25 ꢁC under 0.12 MPa, which
undergoes desorption using a N2 ow of 0.1 L minꢀ1 (Fig. 3a).
Notably, no signicant drop in CO2 adsorption was observed
The base effect on the hydrogenation of gaseous CO2
Considering the promotive effect of bases, amidines were
initially compared with a range of organic and inorganic bases
for gaseous CO2 hydrogenation using RhCl3/PPh3 at 100 ꢁC. As
expected, DBU, TMG, DBN (1,5-diazobicyclo[4.3.0]non-5-ene)
and TBD (1,5,7-triazabicyclo[4,4,0]dec-5-ene) showed better
activity than conventional amines including TEA (triethanol-
amine), HMTA (hexamethylenetetramine), Et3N, Et2NH,
TMEDA (N,N,N0,N0-tetramethylethylenediamine), DABCO (trie-
thylenediamine), morpholine and DMAP (4-dimethylamino-
pyridine). Among the amidines used in this study, DBU and
TMG displayed superiority with TONs of 362 and 322, respec-
tively (Fig. 4). Although NaOH is a strong base, only a small
amount of formate was obtained owing to its solubility limita-
tions and incompatibility with the catalytic system. Moreover,
weak bases such as 1-methylimidazole, choline chloride, mono-
and di-ethanolamine gave inferior results.
Fig. 4 Comparison of various bases for the hydrogenation of gaseous
CO2 under a pressure of CO2/H2 (4/4 MPa) at 100 ꢁC for 16 h.
Fig. 2 FT-IR spectra of: (a) Epoxide@Silica; (b) DBUOH@Silica; and (c) [RhCl3$3H2O (0.01 mmol, 2.6 mg), PPh3 (0.1 mmol, 26 mg), base (3.3
DBUOH@Silica + CO2.
mmol), methanol (3 mL)].
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a
Table 2 Ligand screening for the hydrogenation of gaseous CO2
Table 2 (Contd.)
TONb
95
Entry
Ligand
TONb
Entry
Ligand
1
2
3
4
5
6
7
8
—
5
362
290
337
441
262
480
473
PPh3
PnBu3
21
P(2-furyl)3
(o-Tolyl)PPh2
(p-Tolyl)PPh2
CyPPh2
Cy2PPh
a
Reaction conditions: RhCl3$3H2O (0.01 mmol, 2.6 mg), monodentate
ligand (0.1 mmol) or bidentate/multidentate ligand (0.05 mmol), DBU
(3.3 mmol), methanol (3 mL), H2 (4 MPa), CO2 (4 MPa), 60 ꢁC, 16 h.
b
Refers to formate including the HCOOꢀBH+ adduct and a little
HCOOMe calculated by 1H NMR spectroscopy using 1,1,2,2-
tetrachloromethane as an internal standard. TON ¼ turnover number:
moles of formate (HCOOꢀBH+ and HCOOMe) per mole of Rh catalyst.
9
67
17
69
10
11
Rh/bisphosphine-catalyzed hydrogenation of gaseous CO2
Screening the phosphine and nitrogen ligands in the catalytic
system was further studied as listed in Table 2. CO2 hydroge-
nation almost did not occur in the absence of a phosphine
ligand (entry 1, Table 2). Obviously, PPh3 exhibited better
activity than PnBu3 and (2-furyl)3P (entries 2–4). Among the
monophosphines derived from PPh3, the activity increases in
the sequence (p-tolyl)PPh2 < PPh3 < (o-tolyl)PPh2 < Cy2PPh #
CyPPh2 (entries 2, 5–8), being more or less consistent with
12
13
14
470
499
516
t
nucleophilicity. In contrast, Bu-XPhos was decisively inferior
presumably due to steric hindrance (entry 9). Moreover,
bisphosphines were particularly tested. 1,3-Bis(diphenyl phos-
phino)propane (dppp) with a exible chain was proven to be
inactive (entry 10). However, when the dppbz, BINAP, dppf and
DPEphos ligands were used, the TONs were remarkably
improved from 69 to 516, respectively (entries 11–14). This
superiority is probably due to the electron-donating ability of
their rigid chains compared to dppp, and may also be related to
their cone angles when coordinating with the Rh center.21 In
this regard, DPEphos has supreme merit in view of its large
cone angle and strong electron-donating ability that account for
the highest activity (entry 14). Though Xantphos' framework is
conspicuously similar to DPEphos, it gave a lower formate yield,
presumably resulting from unfavorable effects arising from a
chelating coordination between its oxygen atom and the metal
center (entry 14 vs. 15). Multi-phosphine, P–N ligands, and
nitrogen ligands exhibited worse activity in comparison with
those bisphosphines (entries 16–21). Hence, ligands with strong
electron-donating ability or rigid chains, as well as an appro-
priate cone angle are favorable for the hydrogenation reaction.
Moreover, Fig. S2 (see ESI†) shows that increasing the reac-
tion temperature from 60 to 120 ꢁC caused the product yield to
drop sharply, whereas at temperatures below 40 ꢁC it also gave
inferior results (Fig. S2a†). It is probably because that increasing
the temperature is detrimental to the exothermic hydrogenation
of CO2, while lower temperatures may slow down the reaction.
As a result, formate with a TON of 1565 can be obtained at the
optimal temperature of 60 ꢁC (Fig. S2b†). To our delight, a TON
15
16
122
256
17
18
249
115
19
20
111
46
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hydrogenation in order to further promote the process (Fig. 5).
As is well known, the CO2-philic ethoxyl group could have
impacts on the physical properties such as increased gas/liquid
diffusion rates, and thus assumedly facilitates the reaction
involving CO2.13c Nevertheless, viscous DBU2PEG150Br2 as an IL
showed too weak basicity to stabilize formic acid, and gave a
rather low yield. In order to increase the basicity, the secondary
amine e.g. TMG was modied to the tertiary amine
TMGPEG150Me, and DBU was also modied by an electron-
donating PEG150Me substituent, which could accelerate the
formation of HCOOMe. Dramatically, DBUOH as an alka-
nolamidine performed well with the highest TON of 1698 within
16 h, better than both DBU and DBUPEG150Me. The terminal
hydroxyl group is assumed to facilitate proton transfer among
the reactive species, resulting in the promotion of formate
generation,22 albeit its relatively high viscosity. Notably, the
silica-supported alkanolamidine (DBUOH@Silica) also exhibi-
ted a higher efficiency than the polystyrene supported derivative
(DBU@PS).
Fig. 5 Amidine derivatives for the hydrogenation of gaseous CO2.
Reaction conditions: RhCl3$3H2O (0.01 mmol, 2.6 mg), DPEphos (0.01
mmol, 27 mg), Base (6.6 mmol), methanol (3 mL), CO2/H2 (4/4 MPa),
16 h, 60 ꢁC.
of 2202 can be successfully attained through elongating the
reaction time to 32 h.
Amidine derivatives as a promoter
In situ hydrogenation of the captured CO2 (CCU)
With the proper conditions in hand, the modied amidine Hydrogenation of gaseous CO2 can be efficiently achieved by the
derivatives were examined for RhCl3/DPEphos-catalyzed CO2 tunable ethoxyl-functionalized amidines, which stimulated us
a
Table 3 Hydrogenation of captured CO2
Entry
1e
Substrate
Adsorption ratiod (mol molꢀ1
0.89 (0.132)
)
Captured CO2 (mmol)
2.77
HCOOꢀ TONb
180
CCU Yieldc (%)
65
2f
1.11 (0.187)
0.58 (0.122)
0.95 (0.199)
3.59
1.74
3.02
323
169
298
90
97
99
3g
4h
5i
3.48 (0.088)
3.02 (0.076)
1.17
0.92
113
91
96
99
6j
7k
8l
DBUOH@Silica-CO2
DBUOH@Silica-CO2
2.85 (0.072)
2.37 (0.060)
0.88
0.74
71
66
81
89
a
b
Reaction conditions: RhCl3$3H2O (0.01 mmol, 1.3 mg), DPEphos (0.05 mmol, 13.5 mg), methanol (2 mL), H2 (4 MPa), 60 ꢁC, 16 h. Refers to
HCOOꢀBH+, determined by 1H NMR spectroscopy. Relative to the amount of captured CO2. Moles of CO2 captured per mole of superbase
c
d
subtracting solvent physical adsorption. Adsorption ratio (g gꢀ1
)
is given in brackets. DBUPEG150Me (3 mmol)/glycol (6 mmol).
TMGPEG150Me (3 mmol)/glycol (18 mmol). DBUOH (3 mmol). DBUOH (3 mmol)/TGDE (5 mmol). DBUOH@Silica (0.42 g, 0.24 mmol
e
f
g
h
i
j
k
l
DBUOH)/glycol (24 mmol). DBUOH@Silica (0.42 g, 0.24 mmol)/TGDE (5 mmol). Run 2, DBUOH@Silica (0.42 g)/TGDE (5 mmol). Run 3,
DBUOH@Silica (0.40 g, 0.229 mmol DBUOH)/TGDE (5 mmol).
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to further investigate the hydrogenation of captured CO2 by
using the amidine derivatives as the “hinge base”, without the
addition of extra base. The captured CO2 i.e. amidinium
carbonate, could be a simultaneously activated form, which
might show higher activity than the gaseous one.
On the basis of the results in Table 1, hydrogenation of those
amidinium carbonates formed from CO2 capture were effi-
ciently performed as summarized in Table 3. TMGPEG150Me
gave a higher formate yield in regard to captured CO2 (90%)
when compared to DBUPEG150Me (65%) due to the higher
basicity and lower viscosity of TMGPEG150Me (entry 1 vs. 2,
Table 3). Dramatically, using DBUOH as the “hinge base”, the
formate yield can be improved to 97–99% (entries 3 and 4). On
the one hand, DBUOH with a terminal hydroxyl group reached
equimolar CO2 capture, providing enough active CO2 source for
in situ hydrogenation. On the other hand, the ethoxyl group
could adjust the basicity of the DBU derivatives and the
terminal OH could participate in proton transfer during the
catalytic cycle. To our delight, the silica-supported DBUOH also
worked effectively for in situ hydrogenation of captured CO2 to
afford formate in 96–99% yield (entries 5 and 6). As a result, free
formic acid can be separated by a ltration and distillation
sequence. DBUOH@Silica could be recovered without changing
its functionality (Fig. S3, ESI†) and was reused for at least three-
successive cycles, still giving the desired product in 89% yield
aer the third run, although its capacity gradually decreased
(entries 6–8, Table 3). Signicantly, this is an efficient CCU
process, removing the energy penalty step, to produce energy-
related products from CO2.
Fig. 6 Monitoring the reaction process by in situ FT-IR spectroscopy
under CO2 pressure. Reaction conditions: RhCl3$3H2O (0.03 mmol),
DPEphos (0.15 mmol), DBUOH (10 mmol), methanol (9 mL), H2 (4
MPa), CO2 (4 MPa), 60 ꢁC, 20 h. The spectra of methanol, DPEphos and
DBUOH were subtracted.
Fig. 7 SEM images of DBUOH@Silica (A) and DBU@PS (B).
promote the interactions between the base and other species,
and improve the catalytic efficiency, thus giving higher yields in
hydrogenation process.
Spectroscopy investigation
Deeper insight into an integration process combining CO2
capture with in situ hydrogenation using a DBUOH/RhCl3/
DPEphos system was investigated by in situ FT-IR spectroscopy
Reaction mechanism
under CO2 pressure. In the FT-IR spectrum, the characteristic In spite of the well-documented mechanism on the hydroge-
peak for CO2 at 2340 cmꢀ1 and the appearance of carbonyl nation of gaseous CO2
and CO2 derivatives,26 a tentative
23
24,25
peaks at 1271, 1438 and 1577 cmꢀ1 supports the formation of pathway that rationalizes the present process via initial CO2
amidinium carbonate upon uptake of CO2 (Fig. 6), basically capture and subsequent hydrogenation to formate is illustrated
consistent with the FT-IR data mentioned above.13c A charac- in Scheme 2. The Rh–H hydride 1 is initially formed from RhCl3
teristic peak centered at 1647 cmꢀ1 corresponds to the [C]NH]+ and DPEphos under H2 pressure. The CO2 capture product i.e.
band adsorption12,20 of protonated DBU. Bands at 1985 and 2006 amidinium alkylcarbonate (DBUH+OCO2ꢀ) coordinates with the
cmꢀ1, were assigned to the stretching vibration of intermediates Rh species and thus facilitates intramolecular hydride transfer
with two kinds of Rh–H bonds,24 which gradually increased of 2 and carbonyl insertion into the Rh–H bond, generating the
during the hydrogenation step. The generation of the interme- intermediate formato-Rh complex 3. Subsequently, 4 could be
diate formato-Rh complex24 was also observed at 1589 and 1312
cmꢀ1. Broad adsorptions from 1776 to 1805 cmꢀ1 were
suggestive of formic acid derivatives.24 The detection of inter-
mediates and products by in situ FT-IR spectroscopy would lay a
strong foundation for the proposed mechanism.
In addition, the BET (Brunauer–Emmett–Teller) surface area
for the solid amidine derivatives i.e. DBU@PS and DBUOH@
Silica were measured to be 27.3 and 98.5 m2 gꢀ1, respectively
(Table S1†). DBUOH@Silica showed a higher surface area than
DBU@PS, and SEM (scanning electron microscopy), also indi-
cated the formation of microparticles on the surface of
Scheme 2 The proposed mechanism for CO2 capture and subse-
quent Rh-catalyzed hydrogenation of the captured CO2 using
DBUOH@Silica (Fig. 7). All these physical characteristics could DBUOH.
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formed via the oxidative addition of H2, and eventually the manipulations were performed under an Ar atmosphere. The
reductive elimination of 5 HCOOH$DBUOH occurs, regenerat- CO2 capture product was dissolved in methanol, then RhCl3-
ing 1 to complete the catalytic cycle. In this step, DBUOH would $3H2O and the ligand were mixed and transferred to the glass
participate in the proton transfer, which may not only promote tube, which was placed into the 50 mL stainless-steel autoclave.
the heterolytic cleavage and d-bond metathesis of H2, but also Aer sealing the autoclave reactor cover, H2 was led into the
provide protons to 3 in favor of reductive elimination of system with an initial pressure of 4 MPa. The reactive mixture
HCOOH.
was stirred for 16 h at 60 ꢁC. The same post-processing and
analysis method was used as described above.
Experimental section
Procedure of CO2 capture
Procedure for recycling DBUOH@Silica
Aer the process of CO2 capture and in situ hydrogenation was
completed, the solid was ltrated and washed with methanol,
and the formic acid was distilled. The recovered base was a grey
powder, which was further dried using a rotary evaporator and
at 60 ꢁC under vacuum. The recovered base was directly used for
the next run. The reactions were performed under identical
reaction conditions described above.
CO2 capture was carried out in a 10 mL glass tube. The absor-
bents were charged into the reactor at room temperature. Then,
the air in the ask was replaced by CO2 and a needle used for
CO2 bubbling, which was inserted in the bottom of the glass
tube. The adsorption reaction was stirred and conducted with a
CO2 bubbling rate of 0.1 L minꢀ1. The amount of CO2 absorbed
was determined using an analytical balance within an accuracy
of ꢂ0.0001 g every three minutes. The measurements of CO2
sorption by solid absorbent (DBUOH@Silica) were performed
on ASAP 2020 Surface Area and Porosity Analyzer (a solid
adsorption instrument, which can absorb CO2 and analyze the
change in the amount of physical adsorption as a function of
time or pressure). Before measurement, the sample was dried
again using the ‘outgas’ function of the surface area analyzer for
about one hour at 50 ꢁC. Adsorption–desorption was deter-
mined by several cycles of repeated experiments.
Conclusions
In summary, the efficiency for in situ hydrogenation of captured
CO2 has been improved up to more than 99% by employing an
ethoxyl-functionalized amidine/Rh/DPEphos sequential system.
It is worth mentioning that the ethoxyl linkages could have an
electron-donating and CO2-philic effect that improves the
basicity and adjusts the viscosity of the amidine, while the
terminal OH group of DBUOH could also lead to desirable proton
transfer. Therefore, introducing ethoxyl chains could promote
both capture efficiency and hydrogenation activity. This protocol
offers a non-volatile, quantitative adsorption with simultaneous
activation to form zwitterionic amidinium carbonate, and then in
situ Rh-bisphosphine-catalyzed hydrogenation of the captured
CO2 can be performed smoothly. Particularly, the recyclable
silica-supported alkanolamidine gave high activity without using
a volatile proton donor, thereby validating the CCU strategy
successfully, which was investigated by NMR and in situ FT-IR
spectroscopy, as well as TGA, BET, SEM and DFT techniques.
This efficient CCU concept could also have the potential practice
in industry, circumventing the energy input for the desorption
step and generating energy-related products.
Procedure of gaseous CO2 hydrogenation
The manipulations were performed in a recirculating mBraun
MBio compact inert atmosphere (Ar) drybox at 20 ꢁC. The
reaction was carried out in a 50 mL stainless-steel autoclave
reactor with an inner glass tube and a magnetic stirrer. Initially,
the reaction mixture, consisting of methanol, RhCl3$3H2O and
the ligand, was transferred to the inner glass and stirred for 10
min. Aer that, the base was added dropwise to the mixture
with stirring. Aer sealing the autoclave reactor cover, H2 and
CO2 were introduced to the system with an initial pressure of 4
MPa and 4 MPa, respectively. The reactive mixture was stirred
for 16–32 h at 60 ꢁC. Aer the hydrogenation was complete, the
autoclave was cooled to room temperature, and the excess gas
depressurized very slowly in ice-water. The resultant mixture
was analyzed using NMR spectroscopy with 1,1,2,2-tetra-
chloromethane as an internal standard. The free formic acid
Acknowledgements
This work was nancially supported by the National Natural
Sciences Foundation of China, Specialized Research Fund for
the Doctoral Program of Higher Education (20130031110013),
and MOE Innovation Team (IRT13022) of China.
ꢁ
was separated by distillation at 160 C.
Integrated procedure of CO2 capture and in situ
hydrogenation
CO2 capture was carried out in a 10 mL glass tube. The absor-
bents were charged into the reactor at room temperature. Then,
the air in the ask was replaced by CO2 and a needle used for
CO2 bubbling, which was inserted in the bottom of the glass
tube. The adsorption reaction was stirred and conducted with a
CO2 bubbling rate of 0.1 L minꢀ1. The amount of CO2 absorbed
was determined using an analytical balance within an accuracy
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