Transfer hydrogenation of CO2into formaldehyde from aqueous glycerol heterogeneously catalyzed by Ru bound to LDH

Article abstract of DOI:10.1039/d1cc01299a

Aqueous glycerol was used in this study as a liquid-phase hydrogen source for the hydrogenation of CO2. It was found that hydrogen could be efficiently evolved from aqueous glycerol upon highly dispersed Ru on layered double hydroxide (LDH), inducing the transformation of CO2 into formaldehyde under base-free conditions at low temperature.

Full text of DOI:10.1039/d1cc01299a

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Transfer hydrogenation of CO2 into formaldehyde
from aqueous glycerol heterogeneously catalyzed
Cite this: DOI: 10.1039/d1cc01299a
by Ru bound to LDH†
Received 10th March 2021,
Accepted 6th April 2021
a
Lidan Deng,
Zheng Wang
Xiaowei Liu,*a Jie Xu,a Zijian Zhou,a Shixiang Feng,b
c
and Minghou Xua
DOI: 10.1039/d1cc01299a
rsc.li/chemcomm
Aqueous glycerol was used in this study as a liquid-phase hydrogen Its chemical structure (including three hydroxyl groups) allows
source for the hydrogenation of CO2. It was found that hydrogen glycerol to serve as a hydrogen-donor solvent in reduction
could be efficiently evolved from aqueous glycerol upon highly reactions.12,13 Pescarmona et al. exploited the hydrogen liberated
dispersed Ru on layered double hydroxide (LDH), inducing the in the dehydrogenative oxidation of glycerol to in situ hydro-
transformation of CO2 into formaldehyde under base-free conditions genate various unsaturated organic compounds such as levulinic
at low temperature.
acid, benzene, nitrobenzene, 1-decene and cyclohexene, from
which lactic acid and the corresponding hydrogenated products
Catalytic hydrogenation of CO2 with H2 has been extensively were obtained in one pot in the presence of either a Pt/ZrO214 or
15
studied as a sustainable means to utilize CO2.1–3 This rational Ni–Co/CeO2 heterogeneous catalyst.
treatment could not only reduce CO2 emission, but also recycle
Using glycerol in the CTH of CO2, however, has not been
CO2 as a C1 building block and reduce fossil-resource depletions. extensively exploited. To date, only a few attempts, mainly
However, the low solubility of H2 in most solvents generates high using homogeneous catalysts have been reported. Aresta et al.
H2 pressure necessary to achieve desired conversions and yields. used aqueous glycerol as the hydrogen source to produce
These high pressures incur safety concerns and a hefty formic acid from CO2 and glycolic acid via the decarbonylation
infrastructure cost on the industrial scale.4,5
of glycerol using [RuCl2(PPh3)3].16 Voutchkova-Kostal et al. used
a homogeneous water-soluble Ru N-heterocyclic carbene
Liquid-phase alcohols have been applied as hydrogen
donors in various hydrogenation reactions because they are complex to carry out the CTH of CO2, which executed 1685
safer and entail a more convenient management and usage and 1065 turnovers, respectively, of lactate and formate in
than H2.6 More interestingly, such liquid H2 agents also create alkaline hydrothermal media at 453 K.17 They further reported
high local concentrations of H2 generated in the liquid phase, highly robust Ir-based homogeneous catalysts for the CTH of
which could enhance the intrinsic kinetics of the hydrogenation carbonate salts from glycerol, affording lactate and formate
reaction.7–9 Antonchick et al. Used the biomass-derived carbo- salts under basic conditions with high TONs (72 245 for LA and
hydrates, starch and lignin as liquid H2 carriers, to successfully 52 032 for FA) under 2.6 MPa N2 at 423 K.18 Although these
perform an efficient catalytic transfer hydrogenation (CTH) of sparsely reported homogeneous catalysts displayed high activities,
alkynes, alkenes and carbonyl compounds.10 A Ru/C catalyst they suffered from the disadvantages of difficult recovery and
served as the catalyst for both H2 formation from 2-propanol and regeneration. In comparison, heterogeneous catalysis has
the hydrogenation of furfural.11 Glycerol is abundant because it several advantages in terms of separation, recycling and reactor
is a major by-product of the biodiesel manufacturing process. designs. Lin et al. used heterogeneous Pd/C for the CTH of
bicarbonate from glycerol, while there was almost no yield of
formic acid at 513 K with CO2 gas charge.19 The CTH of CO2
a State Key Laboratory of Coal Combustion, Huazhong University of Science and
using glycerol still lacks highly efficient heterogeneous
catalysts, and urgently needs the related fundamental under-
standing of the reaction.
In this study, we have developed a layered double hydroxide
supported Ru (Ru/LDH) heterogeneous catalyst to perform the
CTH of CO2 gas in aqueous glycerol media at mild temperatures
without adding strong bases (Scheme 1).
Technology, Wuhan 430074, China. E-mail: denglidan@hust.edu.cn,
xwliu@hust.edu.cn
b Joint School of National University of Singapore and Tianjin University,
Tianjin University, Binhai New City, Fuzhou 350207, China
c Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences,
Beijing 100085, China. E-mail: zhengwang@rcees.ac.cn
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1cc01299a
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shown in Fig. 1 exhibited the characteristic peaks of Ca0.8A-
l0.2(OH)2Cl0.2 with relatively higher crystallinity than the LDH.
No other crystal phases (such as Ru, RuO2) were observed on
Ru/LDH.
To further characterize the Ru species on the Ru/LDH, the
samples were examined by transmission electron microscopy
(TEM) and X-ray absorption fine structure (XAFS). As shown in
Fig. S1(b) and S3 (ESI†), Ru species were not obviously observed
on LDH. As proven by energy dispersive X-ray element mapping
(Fig. S4, ESI†), no Ru aggregates could be observed in Ru/LDH.
These results indicate the highly dispersion of Ru on LDH.
Extended X-ray absorption fine structure (XAFS) is one of the
most powerful techniques for characterizing the electronic
state and local structure of Ru species. The shape and edge
position in the Ru K-edge EXAFS spectrum of the Ru/LDH
shown in Fig. 1(C) was similar to those of RuO2, but differed
from those of Ru foil and RuCl3ꢀnH2O. The Fourier transform
(FT) and inverse FT of the main peaks was well-fitted by the use
of two longer Ru–O (2.05 Å) and two shorter Ru–O (1.93 Å) shell
parameters, as summarized in Fig. S5 and Table S1 in the ESI.†
X-ray photoelectron spectroscopy (XPS) was also carried out.
As seen in the XP spectra of Ru in Fig. S6 (ESI†), Ru/LDH and
oxidized Ru/LDH exhibit similar spectra with a BE (binding
energy) of about 462.4 eV which is assigned to RuO2 (462.4 eV).
Thus, the oxidation state of Ru should be +4. Conclusively, an
highly dispersed Ru species bears one hydroxyl grafted onto a
triad of O atoms originating from the basic hydroxyl groups on
the LDH surface, posing an especial metal-support interaction.
Unlike the previous studies, when the Ru/LDH catalyst was
applied in the CTH of CO2, formaldehyde rather than formic
acid was detected as the product of CO2 reduction. Formaldehyde
is a highly versatile platform chemical, and generally requires
further reduction of formic acid. In order to indicate
formaldehyde in aqueous media, the colorimetric 3-methyl-2-
benzothiazolinone hydrazone (MBTH) method was used in
addition to high performance liquid chromatography
(HPLC).25,26 The typical blue-green color was observed in the
reaction solution using the MBTH (Fig. S7, ESI†), indicating
the formation of HCHO. HPLC was further employed to quantify
the amount of HCHO.
Scheme 1 An illustration of a conversion of carbon dioxide and waste
glycerol into value-added products.
A Ca–Al LDH support was first prepared based on a previously
reported procedure,20 and is described in the ESI.† Briefly, LDH
were prepared from an aqueous solution of high concentrations of
metal salts through a homogeneous alkalization reaction triggered
by propylene oxide. The final crystalline was characterized as
mostly thin plates that possessed a regular hexagonal structure
with an approximate grain size around 5 mm. Its scanning electron
microscopy (SEM) image suggests the formation of some
agglomerated structures, which were overlapping with each other
(Fig. S1, ESI†). Fig. 1(A) shows the XRD pattern of a Ca–Al LDH,
which contained the characteristic peaks of Ca0.8Al0.2(OH)2Cl0.2
with good crystallinity.21 The presence of a reasonable amount of
impurities in the Ca–Al material could be attributed to the
discrepancy in the ionic size of Ca (0.100 nm) and Al (0.054 nm).22
Treatment of the LDH with a solution of RuCl3ꢀnH2O in
water at 323 K overnight afforded Ru/NLDH as a gray powder.
The N2 adsorption–desorption isotherm of Ru/LDH with 0.5 wt%
of Ru is compiled in Fig. S2 (ESI†). The specific surface area and
total pore volume that were obtained by BET and BJH methods
are 25.6 m2 gꢁ1 and 0.08 cm3 gꢁ1, respectively. Ru/LDH exhibited
a combination between IV and V isotherms type, as commonly
observed in microporous adsorbents.23,24 The pores of Ru/LDH
are mostly distributed over the range of 0–20 nm. Its XRD pattern
Fig. 2 plots the yield of formaldehyde as a function of Ru
loading amount (A) and reaction temperature (B). As the same
catalyst was used in the direct hydrogenation of CO2 by H2 (1 MPa)
and transfer hydrogenation of CO2 with glycerol, respectively,
regardless of Ru loading amount, the latter method produced
higher HCHO yields, indicating a higher hydrogenation capability
of liquid glycerol compared to H2 gas. Moreover, there were no
obvious activity changes when the loading amount of Ru was
varied over the range of 0.1–3 wt%. The high yield of HCHO on
0.1 wt% Ru/LDH could be attributed to the special interactions
between the highly dispersed Ru species and LDH. The products
of glycerol oxidation such as Lactic acid (LA, C3H6O3) and pyruvic
acid (PA, C3H4O3) were detected in the reaction solution
(Fig. S8, ESI†).
Fig. 1 (A) XRD patterns of LDH and Ru/LDH, (B) Ru K-edge FT-EXAFS
spectra of Ru/LDH and standard samples (Ru foil, RuCl3 and RuO2).
(C) Schematic illustration of Ru/LDH.
In contrast, the effect of temperature on the yield of HCHO
in the CTH system became complicated. The low reaction
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Table
1
Results of glycerol oxidation and CO2 hydrogenation over
0.5 wt%Ru/LDH catalyst
Yield of
formaldehyde
Yield (mmol Lꢁ1 gcat
)
ꢁ1
Entry Yieldf
TON
Glycerol conv. (%) PA
GLA
LA
1a
2b
3c
4d
5e
360
405
263
303
462
72
81
53
61
93
9
35
5
26
1
0
35
107
61
37
35
46
52
10
11
28
10
0
0
0
Reaction conditions: 50 mg of 0.5 wt% Ru/LDH, 0.2 M aqueous glycerol,
a
b
c
323 K, 12 h. 1 MPa CO2. Na2CO3, air atmosphere. Na2CO3, N2
d
e
f
atmosphere. NaHCO3, N2 atmosphere. NaHCO3, 1 MPa CO2. The
unit is mmol Lꢁ1 gcat
.
ꢁ1
About 46% of glycerol was converted into LA (mainly), PA, GLA
(glyceric acid, C3H6O4) and other products undetected, and the
yield of HCHO was slightly increased. In the case of the N2
atmosphere and Na2CO3, the yield of HCHO was not improved
even with a high conversion of glycerol (about 52%). The reason
was probably due to the decreased selectivity toward LA. This
result indicated that not only the conversion of glycerol, but also
the selectivity towards C–H cleavage in glycerol is also important
for hydrogenation of CO2. Entry 4 and 5 in Table 1 show the
results of the CTH reactions in the cases using the aqueous
glycerol solution containing NaHCO3. Compare with entry 1–3,
the selectivity towards LA during glycerol conversion increased,
most of converted glycerol was transformed into LA. The
yield of HCHO wꢁa1s also improved a lot. There was about
462 mmol Lꢁ1 gcat of formaldehyde when NaHCO3 was used
and CO2 (1 MPa) was charged into the reactor. Our experiments
were consistent with the report about thermodynamics of bicarbo-
nate hydrogenation in water, which are slightly more favourable than
Fig. 2 The yield of formaldehyde as a function of Ru loading amount (A)
and reaction temperature (B) for the two reaction systems—direct
hydrogenation of CO2 by H2 and hydrogenation of CO2 with glycerol.
(Conditions: 50 mg of catalyst, 10 mL of 0.2 M aqueous glycerol, 1 MPa
CO2, 1 MPa H2 (in the case of direct hydrogenation of CO2 by H2), 323 K for
Fig. 1(A), 0.5 wt% Ru for Fig. 1(B), 300 rpm, 12 h).
temperature (298 K) induced a low glycerol conversion (Fig. S9,
ESI†). It has been reported that Ru exhibited low activity in
glycerol oxidation, and the conversion of glycerol was less than
5% even at 353 K and under a pure O2 atmosphere.27 In our
studies, O2 was replaced by CO2 with a lower oxidation ability.
The conversion of glycerol was less than 10% over the temperature
range of 298–398 K, and increased significantly when the reaction
temperature was increased to 423 K. Besides the predominant
formation of lactic acid, other substances such as pyruvic acid,
glyceric acid and glyoxylic acid were also detected in the reaction
solution, indicating that C–C cleavage in glycerol occurred in
addition to C–H cleavage at 423 K. Consequently, the yield of
HCHO at 423 K was higher than at 398 K. It was also confirmed by
the experiments shown in Fig. 2(B) that higher temperatures were
unfavourable for the direct hydrogenation of CO2 by H2 gas
that of CO2 (DGa0q ¼ 4:4 kcal molꢁ1 for HCO3 vs. 13.4 kcal molꢁ1
ꢁ
for CO2).17,18 Furthermore, the products’ distributions were similar
in the cases with NaHCO3 or CO2 as the source, but were much
different when Na2CO3 was used (Fig. S10, ESI†), indicating
that bicarbonate was likely the actual species to undergo
transfer hydrogenation when CO2 was charged into the aqu-
eous glycerol.
When RuCl3 was used in the aqueous glycerol solution,
we also detected the formation of HCHO (Fig. S12, ESI†),
indicating that RuCl3 can be used as the homogeneous catalyst
in transfer hydrogenation of CO2. Dispersion of Ru in LDH
support affords much higher activity in CTH of CO2 than RuCl3.
To assess the stability and reusability of this heterogeneous
catalyst, we performed five repeated reaction runs, each
followed by catalyst recovery after 24 h of reaction. The second
run exhibited much loss of the yield of HCHO (Fig. S12, ESI†),
while the recycled Ru/LDH showed the similar catalytic
activity during the following catalytic cycles. It was worthy to
note that the yields of HCHO over both fresh Ru/LDH and
recycled ones were higher than that over RuCl3, demonstrating
the efficacy of the reversible switching and recycling
properties of the Ru/LDH. The loss of activity in the second
cycle are probably attributed to dissolution of the basic LDH in
because the overall yield of HCHO was reduced from 446 to
ꢁ1
138 mmol L
g
cat
ꢁ1 when the reaction temperature was increased
from 323 to 373 K. The HCHO yields in the CTH reactions also
decreased when the reaction temperature was increased from
323–398 K, indicating HCHO yield was influenced by both glycerol
oxidation and CO2 hydrogenation.
ꢁ
2ꢁ
Given that CO2 is in equilibrium with HCO3 and CO3 in
aqueous media, we performed the hydrogenation of bicarbonate/
carbonate ions with the Ru/LDH catalyst. The reaction atmo-
sphere was consequently switched to either N2 or air (for
comparison), and the results are shown in Table 1. When the
reaction atmosphere was air and Na2CO3 was used to replace CO2
gas(entry 2), the conversion of glycerol obviously increased.
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We appreciate the support from the Analytical and Testing
Center at Huazhong University of Science & Technology.
Conflicts of interest
There are no conflicts to declare.
Notes and references
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support interaction over Ru/LDH affords high turnover of
ꢁ
hydrogen transferring between C3 species and CO2/HCO3
,
generating HCHO in aqueous media. Our work and further
improvement of the catalytic performance under mild reaction
conditions is expected to be helpful for the realization of an
environmental-friendly CO2-mediated sustainable system.
This work was financially supported by the China Post-
doctoral Science Foundation (2020M672342), the National
Natural Science Foundation of China (51922045, 52036003). 28 C. Dong, M. Ji, X. Yang, J. Yao and H. Chen, Catalysts, 2017, 7, 5.
Chem. Commun.
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