Development of magnesium oxide-silver hybrid nanocatalysts for synergistic carbon dioxide activation to afford esters and heterocycles at ambient pressure

Article abstract of DOI:10.1039/c9gc04040d

Multi-metallic hybrid nanocatalysts consisting of a porous metal oxide host and metal satellite guests serve as a scaffold for multi-step transformations of divergent and energy-challenging substrates. Here we have developed a 3D porous MgO framework (Lewis basic host) with Ag⁰ nanoparticles (noble metal guest) for ambient pressure activation and insertion of CO₂ into unsaturated alkyne substrates. The hybrid MgO@Ag-x (x = 2, 5, 7, 8 at% Ag) catalysts are synthesized by impregnating Ag⁺ ions in porous MgO cubes followed by reduction using NaBH₄. Morphological (SEM, TEM, EDX mapping) and structural (PXRD, XPS) characterization reveal that the micron-sized hybrid cubes derive from self-assembly of ～100 nm (edge length) MgO cubes decorated with ～5 to 25 nm Ag⁰ NPs. Detailed XPS analysis illustrates Ag⁰ is present in two forms, <10 nm NPs and ～25 nm aggregates. The MgO@Ag-7 catalyst is effective for inserting CO₂ into aryl alkynes followed by S_N2 coupling with allylic chlorides to afford a wide range of ester and lactone heterocycles in excellent yields (61-93%) and with low E-factor (2.8). The proposed mechanism suggests a CO₂ capture and substrate assembly role for 3D porous MgO while Ag⁰ performs the key activation of alkyne and CO₂ insertion steps. The catalyst is recyclable (5×) with no significant loss of product yield. Overall, these results demonstrate viable approaches to hybrid catalyst development for challenging conversions such as CO₂ utilization in a green and sustainable manner.

Full text of DOI:10.1039/c9gc04040d

View Article Online
View Journal
Green
Chemistry
Cutting-edge research for a greener sustainable future
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: U. Gulati, C.
Ummadisetti, D. S. Rawat and J. Zaleski, Green Chem., 2020, DOI: 10.1039/C9GC04040D.
Volume 20
This is an Accepted Manuscript, which has been through the
Number
May 2018
Pages 1919-2160
9
7
Royal Society of Chemistry peer review process and has been
accepted for publication.
Green
Chemistry
Cutting-edge research for a greener sustainable future
rsc.li/greenchem
Accepted Manuscripts are published online shortly after acceptance,
before technical editing, formatting and proof reading. Using this free
service, authors can make their results available to the community, in
citable form, before we publish the edited article. We will replace this
Accepted Manuscript with the edited and formatted Advance Article as
soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes to the
text and/or graphics, which may alter content. The journal’s standard
Terms & Conditions and the Ethical guidelines still apply. In no event
shall the Royal Society of Chemistry be held responsible for any errors
or omissions in this Accepted Manuscript or any consequences arising
from the use of any information it contains.
ISSN 1463-9262
PAPER
Paul T. Anastas et al.
The Green ChemisTREE: 20 years after taking root with the 12
principles
rsc.li/greenchem

Please do not adjust margins
Page 1 of 10
Green Chemistry
View Article Online
DOI: 10.1039/C9GC04040D
ARTICLE
Development of Magnesium Oxide-Silver Hybrid Nanocatalysts
for Synergistic Carbon Dioxide Activation to Afford Esters and
Heterocycles at Ambient Pressure
Received 00th January 20xx,
Accepted 00th January 20xx
Upasana Gulati,a,b† U. Chinna Rajesh,a† Diwan S. Rawatb* and Jeffrey M. Zaleskia*
DOI: 10.1039/x0xx00000x
Multi-metallic hybrid nancatalysts consisting of a porous metal oxide host and metal satellite guests serve as a scaffold for
multi-step transformations of divergent and energy-challenging substrates. Here we have developed a 3D porous MgO
framework (Lewis basic host) with Ag0 nanoparticles (noble metal guest) for ambient pressure activation and insertion of
CO2 into unsaturated alkyne substrates. The hybrid MgO@Ag-x (x = 2, 5, 7, 8 at % Ag) catalysts are synthesized by
impregnating Ag+ ions in porous MgO cubes followed by reduction using NaBH4. Morphological (SEM, TEM, EDX mapping)
and structural (PXRD, XPS) characterization reveal that the micron-sized hybrid cubes derive from self-assembly of ~100
nm (edge length) MgO cubes decorated with ~ 5 to 25 nm Ag0 NPs. Detailed XPS analysis illustrates Ag0 is present in two
forms, <10 nm NPs and ~25 nm aggregates. The MgO@Ag-7 catalyst is effective for inserting CO2 into aryl alkynes followed
by SN2 coupling with allylic chlorides to afford a wide range of ester and lactone heterocycles in excellent yields (61-93%)
and with low E-factor (2.8). The proposed mechanism suggests a CO2 capture and substrate assembly role for 3D porous
MgO while Ag0 performs the key activation of alkyne and CO2 insertion steps. The catalyst is recyclable (5x) with no
significant loss of product yield. Overall, these results demonstrate viable approaches to hybrid catalyst development for
challenging
conversions
such
as
CO2
utilization
in
a
green
and
sustainable
manner.
unit product (E-factor).8, 9 Hence, a sustainable, green chemical
approach must be energy efficient and produce less waste as
its disposal require more resources to manage.
Introduction
There are various reports of homogeneous transition metal
(Fe, Ni, Cu, Ag, Pd) catalysts for activation of small molecule
(CO2) and unsaturated organic functionalities (olefin, alkyne)
to afford CO2 inserted, value-added chemicals.10-17 For
example, Ni0 and Fe0 catalysts have been developed for
carboxylation of olefins (Csp2-H) by CO2 insertion to produce
acrylic acid.10-12 Additionally, Ag+ and Cu+ catalysts have also
been shown to activate alkynes (Csp-H) followed by CO2
insertion to afford propiolic acid derivatives.15-18 While
effective, these homogeneous catalysts suffer from corrosion,
catalyst leaching, low recyclability, and often these processes
require high pressures of CO2.19
The development of 3D porous nanomaterials capable of
CO2 capture, decorated with transition metal satellites for
active catalysis, can overcome these limitations in a green and
sustainable manner.19-23 The porous nanomaterials having
Lewis basic/acidic sites and large surface area like metal
organic frameworks (MOFs), covalent organic frameworks
(COFs), porous organic polymers, hetero-atom doped porous
carbon and porous metal oxides are capable of capturing CO2
at ambient to high pressure conditions.19 The hybrid
composition of these porous materials with Ag0 satellites such
as Ag@MIL,20 ZIF@Ag,24 Co-MOF@Ag,22 poly‐NHC@Ag,25
CeO2@Ag23 provides a gateway to myriad of CO2 inserted
value added synthons. However, the catalytic systems for
Feedstock utilization of CO2, a major component of flue gas
responsible for global warming, is an environmentally benign
approach to chemical transformations of scale.1-3 It is non-
toxic, abundant, and a cost-effective one-carbon building block
for fuels and industrially-relevant chemical production.4 The
conventional and non-catalytic industrial usage of CO2
feedstock includes the effective syntheses of urea, salicylic
acid, and carbonates of groups 1 and 2 elements.5 However, all
these processes involve high pressure and temperature
conditions due to the enhanced kinetic and thermodynamic
stability of the fully oxidized CO2 molecule.3, 4 Therefore, the
development of catalysts for efficient CO2 utilization mandates
reducing the activation energy of the chemical reaction.4 In
addition to effective CO2 management, the development of
green chemical methodologies can have significant
environmental benefits.4, 6, 7 Economic advantages also derive
from quantification and minimization of waste produced per
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 2 of 10
ARTICLE
Journal Name
activation of multiple substrates like alkynes and allylic structure of the 3D porous MgO cubes changes into non-porous
View Article Online
DOI: 10.1039/C9GC04040D
chlorides for CO2 insertion continues to require high pressure Mg(OH)2 flakes (Figures S2 and S3).
17, 23
conditions.15,
Thus, there is considerable need for the
development of heterogeneous catalysts that have greater
adsorptive interactions with CO2 and are capable of activating
multiple substrates for fine chemical generation.
Recently, Lewis basic, porous MgO has been shown to have
high adsorption capacity for CO2 at low-coordinate (111) facets
rich with O2- functionality,26 but its potential to facilitate CO2
insertion into energetically challenging substrates is under
explored. In general, the fixation process requires a catalyst
with Lewis acid/basic sites that take advantage of the net
charges on oxygen and carbon to bind CO2 in concert with
direct attack at carbon by a nucleophilic substrate to reduce
and form a new C-C bond. The enzyme Rubisco accomplishes
this same task at Mg2+ by compromising the ribulose
bisphosphate substrate via deprotonation, leading to
subsequent nucleophilic attack on CO2 by an adjacent and
unsaturated carbon.27 Since porous MgO can bind CO2
efficiently but not perform the activation of stable organic
substrates directly, development of a catalyst component that
can serve to activate carbon frameworks forming the key CO2
attacking nucleophile is critical to enhancing substrate scope.
Toward these goals, we report the synthesis of 3D porous
MgO frameworks containing dual Lewis acid/basic sites with
embedded Ag NPs for insertion of ambient pressure CO2 into
organic substrates. The novel MgO@Ag hybrid catalyst is
employed to incorporate CO2 into alkynes followed by SN2
coupling with allylic chlorides to afford esters and lactone
heterocycles. The catalytic system is effective at ambient
pressure of CO2, recyclable, and exhibits a low E-factor
demonstrating its importance to green and sustainable
solutions for CO2 fixation.
Figure 1. Synthesis of MgO@Ag hybrid nanocatalysts. Schematic illustration
for the formation of porous MgO cube host with silver satellite guest.
The internal morphology of microtome sliced MgO@Ag-x hybrid
materials reveals the gradual decrease in average size (20 to 5 nm)
and increase in the density of Ag NPs with increasing Ag loadings
from 2 to 8 at% (Figure S4). The high-resolution SEM and EDX
elemental mapping images of MgO@Ag-7, a single hybrid cube,
shows the layered structure of MgO embedded with Ag (Figure 2a-
e). Furthermore, the STEM-EDX elemental mapping analysis of
microtome slices confirms the presence of Ag nanoparticles within
the porous MgO cubes (Figure 2f-j). The magnified images reveal
the self-assembled structure of nanosized (~100 nm edge length)
MgO cubes decorated with (~10 nm) Ag0 nanoparticles (Figures 2k-
o).
Results and discussion
MgO@Ag hybrid nanocatalyst synthesis and structural
morphology. Development of three dimensional (3D) porous MgO
(host) embedded with Ag0 nanoparticles (guest) possess unique
advantages in the catalysis arena. The present study shows the
facile synthesis of 3D porous MgO@Ag-x (where x = 2, 5, 7, 8 at %
Ag) hybrid nanocatalysts by a three-step process (Figure 1). Initially,
the 3D porous MgO cubes are obtained by the calcination of
Figure 2. Morphology and elemental mapping of MgO@Ag-7 hybrid cubes.
SEM-EDX mapping images (a-e). STEM-EDX mapping images of ultra-
microtome sliced cross-section (f-j), and its high magnification view (k-o).
MgO@Ag crystal structure and composition. The crystal
structure of MgO@Ag-x (y = 2, 5, 7, 8) hybrid materials is
characterized by powder XRD as shown in Figure 3a-d. The phases
(111), (200), (220), (311), (222) with corresponding diffraction
angles (2θ) at 36.9, 42.9, 62.2, 74.6, 78.5°, respectively, represent
pure fcc MgO (Figure S5a, JCPDS no. 4-829). The phases (111),
(200), (220), (311) with diffraction angles (2θ) at 38.14, 44.32,
64.53, 77.51°, respectively, denote the pure fcc structure of Ag0
nanoparticles and absence of Ag+ dopant or oxide (Ag2O and AgO)
impurities (Figure S5b-d, JCPDS card no. 04-0783). A systematic
increment in intensity of Ag0 peaks w.r.t. MgO peaks was observed
with increasing concentrations of Ag0 in the MgO@Ag-x (x = 2, 5, 7,
8) hybrid materials, indicating the formation of Ag0 nanoparticles in
significant concentration (Figure 3a-d).
o
MgC2O4.2H2O precursor at 800 C for 3 h. The impregnation of Ag+
ions on the surface and inside the pores of MgO cubes was
achieved by aging various quantities of AgOAc with MgO cubes in
ethanol at ambient temperature. Finally, the reduction of Ag+ ions
to Ag0 nanoparticles is achieved by using NaBH4 to produce
MgO@Ag-x hybrid 3D porous structures. The surface morphology
(SEM) and elemental distribution (SEM-EDX) of the MgO@Ag-x
hybrid materials (MgO@Ag-2, MgO@Ag-5, MgO@Ag-7 and
MgO@Ag-8) show micron sized (~1-2 µm) porous MgO cubes
decorated uniformly with Ag0 nano satellites with a systematic
increase in density upon increasing concentration of silver
precursor (0.07 to 0.30 mmol, Figure S1). When the impregnation
and reduction steps are performed in water, the morphology and
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 3 of 10
Green Chemistry
Please do not adjust margins
Journal Name
ARTICLE
area of 3D MgO cubes is 15.5 m2/g with an average pore size of 33.2
View Article Online
3
DOI: 10.1039/C9GC04040D
nm and pore volume of 0.13 cm /g. A slight increase in the BET
surface area (18.2, 21.3, 22.0, 22.6 m2/g) with increasing pore size
(43.3 to 56.6 nm) and pore volume (0.19 to 0.29 cm3/g) was
observed upon increasing Ag loadings (2, 5, 7, 8 at%) within the
hybrid catalysts.31 The slight modifications in internal morphologies
and gradual decrease in average size of Ag NPs (20 to 5 nm) are
responsible for the increased BET surface area values (Figure S4).
The strength of the Lewis basic sites within MgO and MgO@Ag
hybrid catalysts were measured by temperature programmed
desorption (TPD)-CO2 (Figure S10, Table S2). The moderate (380 °C,
0.32 mmol/g CO2) and strong (750 °C, 0.1 mmol/g CO2) Lewis basic
sites observed for 3D porous MgO can be attributed to the
presence of lattice oxide (Mg2+, O2- pairs) and isolated O2- species,
26, 32-35
respectively.
The strength of Lewis basic sites increases with
increasing Ag loadings, as evidenced by a shift to higher
temperature of the desorption feature and an increase in the peak
area (0.65, 1.27, 2.64, 2.7, 2.70 mmol/g of CO2). This trend
highlights the significance of Ag0 sites for improving CO2
chemisorption due to an increased Lewis basic strength of the
hybrid catalysts (MgO@Ag-8 > MgO@Ag-7 > MgO@Ag-5 >
MgO@Ag-2 > MgO).
Figure 3. Crystal structure and elemental oxidation state of MgO@Ag-x.
PXRD and high-resolution core level XPS of Ag3d region for MgO@Ag-2 (a,
e), MgO@Ag-5 (b, f), MgO@Ag-7 (c, g) and MgO@Ag-8 (d, h).
CO2 insertion in alkynes to afford esters and lactones. We have
chosen the MgO@Ag-x hybrid nanocatalyst to perform the
carboxylation of terminal alkynes with CO2 at 1 atm pressure. The
subsequent SN2 coupling of the carboxylate with allylic chlorides in
a tandem manner at the heterogeneous MgO@Ag-x interface
affords ester and lactone products in good-to-excellent yields. In
order to optimize catalyst composition, phenylacetylene (1a),
cinnamyl chloride (2a) and CO2 (1 atm) were reacted over the
MgO@Ag-x hybrid catalysts in the presence of K2CO3 base and
DMAC solvent for 24 h at 55 °C (Table 1). The crude reaction
mixtures were purified by column chromatography and initially
characterized by 1H and 13C NMR. The 1H NMR analyses of the ester
show three characteristic features, two for the olefinic protons at
δ~ 6.34 (dt) and 6.74 (d) ppm and one for (O-CH2) at δ~ 4.90 (dd)
ppm (Figure S12). The 13C NMR reveals the characteristic C=O
functional group at ~ 153.84 ppm along with two Csp of alkyne at δ~
86.63 and 80.47 ppm. Also, the allylic (O-CH2) carbon is observed at
δ~ 66.55 ppm (Figure S13). Reaction product analyses demonstrate
that the hybrid catalyst with high Ag0 loadings (MgO@Ag-7 and
MgO@Ag-8) are most efficient, affording ester (3a) in 90% yield. At
lower Ag0 loadings (MgO@Ag-5 and MgO@Ag-2), the yield of 3a
decreases by ~30% for subsequent lower loadings (3a: 58 and 29%,
respectively). Control experiments using specific catalyst
components and alternative formulations (Ag NPs, AgI, MgO or a
physical mixture of Ag NPs and MgO) were performed to evaluate
the intrinsic species required for effective catalysis (Table 1, Entries
5-8). For Ag NPs (45%) and AgI (30%) catalysts, the formation of CO2
inserted ester 3a occurs but the reaction is significantly less
efficient than for the MgO@Ag-x (x = 7 and 8) (90%) due to
agglomeration or precipitation of Ag species. This decreases CO2
adsorption and permits the background formation of the symmetric
carbonate side product (~35% yield). In contrast, porous MgO alone
produces only the symmetric carbonate side product, indicating
that MgO is not suitable for alkyne activation and hence plays a role
The XPS survey spectra of MgO@Ag-x hybrid materials also
display a similar trend in the intensities of the Ag3d peaks upon
increasing concentration of Ag0 nanoparticles (Figure S6). The high
resolution XPS spectra of MgO@Ag-x hybrid materials in the Ag3d
region show Ag3d5/2 and Ag3d3/2 photoelectron signals (Figure 3e-
h), which upon deconvolution, reveal the presence of two peaks
corresponding to Ag0 nanoparticles (< 10 nm) and bulk
Ag0 aggregate at 369.4, 375.4 eV and 368.4, 374.4eV,
respectively.28 The peak with a binding energy of 374.3 eV in the
Ag3d region corresponds to MgKLL photoelectrons (Figure S7).
Gaussian fitting and integration quantify the Ag nanoparticle:Ag
aggregate ratio, which decreases from 3.62 (MgO@Ag-2) to 1.84
(MgO@Ag-8) upon increasing the Ag0 loading. This suggests a
decrease in agglomeration of Ag0 nanoparticles, which is in good
agreement with TEM images of MgO@Ag-x hybrid materials (Figure
S4). The analysis of high resolution XPS spectra in the Mg2p and O1s
regions show the presence of two types of photoelectrons
corresponding to Mg-O (lattice oxides) and Mg-OH (surface
hydroxyl groups) with binding energy 50.9 (Mg2p), 531.7 (O1s) eV
and 49.7(Mg2p), 533.7 (O1s) eV, respectively (Figure S8).29, 30 The
ratio of Mg2p and Ag3d XPS peak areas identifies the at% of surface
Ag0 as 2, 5, 7 and 8%. These values are compared with bulk
elemental composition determination by ICP-MS (1, 3, 4, 5 at% of
Ag, respectively). The lower Ag0 (at %) values measured by ICP-MS
indicate that surface Ag0 stabilization is significantly greater than
Ag0 content deep within the porous MgO cubes. This is consistent
with SEM/TEM images of FIB/microtome slices showing reduced
Ag0 particles deep within the cubes.
The specific surface area and pore volume of the MgO@Ag
hybrid catalysts and MgO were analyzed by N2 adsorption–
desorption isotherms (Figure S9, inset Table). The specific surface
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 4 of 10
ARTICLE
Journal Name
in adsorption of CO2 by Lewis basic sites (moderate to strong) of the 12). For reactions performed at 1 atm total pressure under N2-
View Article Online
porous structure (Table 1, Entry 7). Importantly, the physical diluted CO2 concentrations of 0.5 and 0.25DaOtmI: ,10th.1e03r9e/sCu9ltGsCr0e4v0e4a0lDa
mixture of Ag NPs and porous MgO generate the ester product in drop in the yield of ester 3a to 80 and 60%, respectively (Table
71% yield with no side product formation, suggesting a synergistic 1, Entries 13 and 14). In order to understand the binding affinity of
relationship for both metal components in the reaction (Table 1, the MgO@Ag-7 catalyst for CO2, a Michaelis-Menten curve36-38 was
Entry 8).
fit for the formation of ester 3a as a function of CO2 concentration
Table 1. MgO@Ag hybrid catalyzed CO2 insertion in alkynes to and compared to data reported for the CeO2@Ag catalyst.23 The
afford esters and lactones.a
low Michaelis-Menton constant (KM = 2.95 vs. 8.8 mM) for
MgO@Ag-7 and comparable reaction velocity (Vmax = 45.19 vs.
41.03 mM/hr) indicate higher CO2 binding affinity for MgO@Ag-7
than CeO2@Ag (Figure S11, Table S3). This is also reflected in
comparable product yields for these two catalysts even when
markedly reduced CO2 concentration is employed over MgO@Ag-7
(5:1 atm CO2). Finally, in the absence of externally introduced CO2,
MgO@Ag-7 shows no ester formation and only symmetric
carbonate side product even at prolonged reaction times,
demonstrating that incorporated CO2 does not derive from the
K2CO3 base (Table 1, Entry 15).
Entry Catalyst
Temp. Time
Isolated Yield (%)
(°C)
(h)
Ester (3a)
Lactone (4a)
1
MgO@Ag-2
55
55
55
55
55
55
55
24
24
24
24
24
24
24
24
48
18
12
24
24
24
48
29
58
90
90
45
30
-
-
2
MgO@Ag-5
MgO@Ag-7
MgO@Ag-8
Ag NPs
-
3
-
The efficiencies of MgO@Ag-7 and CeO2@Ag for the synthesis
of ester 3a from the reaction of phenylacetylene, CO2 and cinnamyl
chloride under identical conditions were directly compared (Table
S4).23 The results reveal a five-fold higher turnover frequency (TOF)
at 1 atm CO2 for MgO@Ag-7 and comparable activity even at low
concentrations of CO2 (0.25 atm). Moreover, MgO@Ag-7 exhibits 2-
4
-
5b,c
6c,d
7c,e
8f
-
AgI
-
MgO
-
Ag NPs + MgO 55
71
-
-
7
folds higher turnover frequency for the synthesis of
9
MgO@Ag-7
MgO@Ag-7
MgO@Ag-7
MgO@Ag-7
MgO@Ag-7
MgO@Ag-7
MgO@Ag-7
RT
70
90
55
55
55
55
-
phenylpropiolic acid compared to the reported Ag0-stabilized MgO
and MOF based heterogeneous catalysts (Table S5).20, 24, 39, 40 The
higher catalytic activity of the MgO@Ag-7 hybrid catalyst over these
examples must derive from the synergy of Ag0 and 3D porous MgO
for improving Lewis basicity and the local concentration of CO2.
10
11
12g
13h
14i
15c,j
52
-
35
90
-
90
80
60
-
-
-
Scope for substituted ester and lactone products. The
optimized reaction conditions were used to demonstrate the wide
substrate scope for the MgO@Ag-7 catalyzed synthesis of CO2-
inserted esters and cyclo-isomerized lactones. Reaction of various
substituted aromatic terminal alkynes (1) bearing electron-
donating (4-Me, 3-Me, 4-tBu, 4-OMe, 3-OMe) or electron-
withdrawing (4-F, 4-CN) groups with various allylic and other
chloride (cinnamyl chloride 2a, 3-chloro-1-phenyl-1-propyne 2b,
ethyl 2-chloroacetate 2c and benzyl chloride 2d) substrates gives
the corresponding esters 3a-3p (55 °C, 24 h) and lactones 4a-5c
(90 °C, 12 h) as shown in Table 2 (Figures S12-S65). For
phenylacetylene or derivatives bearing electron donating groups,
the reaction with allylic chlorides generates the corresponding
ester products in very good yields (78-83%). The conversion is only
modestly affected by electron withdrawing functional groups as 4-
F-pheylacetylene gives the desired ester products in 68-84%
yields. For reaction of 4-CN-phenylacetylene with cinnamyl
chloride, the lactone product is the only species detected both at
55 and 90 °C, due to the facile cyclization of the ester caused by
decrease in energy of the dienophile LUMO. For all reactions at 90
°C, spontaneous DDA cyclization of the ester occurs to form the
corresponding lactone products in 61-95% yields. While the
cinnamyl chloride reactions form single isomer dihydrolactone
heterocycles (4),41 the esters formed by coupling of
phenylacteylenes 1 and 3-chloro-1-phenyl-1-propyne 2b, furnish a
-
aReaction conditions: phenylacetylene 1a (1.0 mmol), cinnamyl chloride 2a
(1.0 mmol) and catalyst (6 mg), DMAC (0.5 mL), K2CO3 (1.2 mmol), CO2
balloon. bUltra-small (5 nm) Ag NPs quantity 1 mg is equivalent to
c
MgO@Ag-7 (15 wt% Ag). Symmetric carbonate side product is formed.
e
dReaction was performed with 0.1 mol% of AgI (1 mg Ag+). Porous MgO (5
mg). f Ultra-small Ag NPs (1 mg) + porous MgO (5 mg). gCs2CO3 is used as a
i
base. h50% of CO2 with N2 mixture (0.5 + 0.5 atm). 25% of CO2 with N2
mixture (0.25 + 0.75 atm). jWithout CO2.
To evaluate the role of temperature and time on the MgO@Ag-7
catalyzed synthesis of 3a and 4a, control reactions were performed
at ambient (48 h) and elevated temperatures (70, 90 °C) for 18 and
12 h (Table 1, Entries 9-11). Under ambient conditions, no product
formation is observed, indicating the formation of 3a is
endergonically controlled. When the reaction is performed above
the optimized formation temperature of 55 °C (e.g. 70 °C), partial
dehydro-Diels-Alder (DDA) cyclo-isomerization of 3a to lactone 4a is
observed (35%). At 90 °C, the reaction proceeds completely to
lactone 4a without detection of the ester precursor, consistent with
the acatalytic nature of the cyclo-isomerization step. The effect of
strong base (Cs2CO3) on the progress of the reaction was also
screened under optimized conditions, and found to contribute no
further improvement in the yield (90%) of ester 3a (Table 1, Entry
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 5 of 10
Green Chemistry
Please do not adjust margins
Journal Name
ARTICLE
mixture of major (5) and minor (5') lactones as separable
regioisomers.41 There is also a significant decrease in yield (~8-
23%) for lactone (vs. dihydrolactone) formation due to the higher
distortion energy for the diyne (vs. enyne) ester during the DDA
cyclization step.42
View Article Online
DOI: 10.1039/C9GC04040D
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 6 of 10
ARTICLE
Journal Name
ambient pressure incorporation of CO2 into electron rich alkynes for
View Article Online
the formation of esters at the MgO@Ag-xDhOyI:b1r0id.10c3a9t/aCly9sGtsC0(F4i0g4u0rDe
Table 2. MgO@Ag-7 catalysed CO2 insertion in alkyne substrates to afford esters and lactones.a
aReaction conditions: The mixture of alkynes 1 (1 mmol), allylic chlorides 2 (1 mmol) and MgO@Ag-7 catalyst (6 mg), K2CO3 (1.2
mmol) in DMAC (0.5 mL), CO2 balloon at 55 °C, 24 h. bIsolated yields.
MgO@Ag-x hybrid nanocatalyst mechanism. Combining these 4). Control experiments reveal that alkyne activation and CO2
observations leads to a reasonable mechanistic proposal for incorporation are predominantly occurring at Ag0 sites (Table 1),
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 7 of 10
Green Chemistry
Please do not adjust margins
Journal Name
ARTICLE
however their aggregation lowers the reaction efficiency as in intensity of the Ag0 diffraction peaks w.r.t. to those from MgO
View Article Online
0
DOI: 10.1039/C9GC04040D
observed by competition from symmetric carbonate side product due to very slow agglomeration of Ag nanoparticles (Figure 5B).
formation. The porous MgO 3D framework prevents the The surface morphology of recovered MgO@Ag-7 also exhibits a
aggregation of Ag0 nanoparticles and enhances the local gradual formation of detectible Ag0 aggregates and erosion of ~10%
concentrations of CO2 and alkyne substrates. Moreover, Lewis of the number of MgO cubes after the fifth cycle (Figure 5C). The
acidic Mg2+ activates the C-Cl bond of alkyl chlorides to trigger the high-resolution XPS of the Ag3d region supports the slow
SN2 reaction. Therefore, the proximity of MgO and Ag0 sites in the aggregation of Ag0 nanoparticles during recycling with a decrease in
MgO@Ag-x hybrid catalyst prompts their synergy in catalyzing the Ag nanoparticles:Ag aggregate ratio from 2.37 to 0.71 in recovered
synthesis of CO2 inserted esters 3.
vs fresh nanocatalyst (Figure 5D). Taken together, these results
Initially, activation of alkyne 1 at Ag0 via deprotonation with suggest that while Ag aggregation and modest MgO dissolution
K2CO3 base (A) leads to the terminal acetylide (B). The Ag0, assisted occur, these processes are slow and do not markedly affect the
by Lewis basic lattice oxides of MgO, adsorb and insert CO2 in Csp- performance of the hybrid catalyst.
Ag0 bond (C) at 1 atm pressure to afford the phenylpropiolate
intermediate (D). The Lewis acidic Mg2+ activates the C-Cl bond of
alkyl chloride 2 and carbonyl oxygen of intermediate D triggering
SN2 substitution to form the desired ester product (3). At high
temperature, the esters (3a-3j) formed by SN2 coupling with
cinnamyl chloride 2a and 3-chloro-1-phenyl-1-propyne 2b,
spontaneously undergo dehydro-Diels-Alder cyclo-isomerization43
to the dihydrolactone (4) and lactone (5) heterocycles, respectively,
independent of catalyst.
Figure 5. Recyclability of MgO@Ag-7 hybrid catalyst. A. Comparing yield of
ester (3a) for five subsequent cycles B. PXRD of fresh and recovered catalyst
after five cycles C. SEM images of recovered catalyst after first to fifth cycle.
All scale bars are 500 nm D. High resolution XPS of Ag 3d region for fresh
and recovered catalyst after five cycles.
Figure 4. Proposed mechanism for MgO@Ag-x hybrid cubes catalysis. The
Green chemistry E-factor of MgO@Ag-7 hybrid nanocatalyst.
2-
CO3 base triggered C(sp)-H activation of alkyne (1a) leads to formation of
The development of chemical processes feasible at ambient
intermediate (A). Deprotonation of the alkyne in intermediate (A) leads to
temperature and pressure comes under the twelve principles of
the Ag-acetylide intermediate (B). The basic lattice oxides of MgO and Ag(0)
green chemistry.7 Previous reports of CO2 insertion in aromatic
nanoparticles promote CO2 adsorption/insertion into the Ag-acetylide bond
terminal alkynes followed by SN2 coupling with cinnamyl chloride
(C) leading to the phenylpropiolate-Ag intermediate (D). SN2 reaction of
using homogeneous (1.5 MPa) and heterogeneous (0.5 MPa)
phenylpropiolate on Mg-activated cinnamyl chloride (2a) in intermediate (D)
catalysts require high pressures of CO2 to drive high yields (~90%).15,
generates ester intermediate 3a.
17, 23
Also, these reactions require excess equivalents of substrates
and base in relatively large amounts of high boiling solvent, which
Recyclability and structural integrity of MgO@Ag-7 hybrid
are not readily recoverable after the reaction. Thus, these
nanocatalyst. The recyclability of the MgO@Ag-7 hybrid
transformations have high waste-to-product ratios (E-factor ~ 21 to
nanocatalyst was studied for five consecutive cycles using a model
41) per reaction cycle. In contrast, the MgO@Ag-x hybrid
reaction of phenylacetylene (1a), CO2 and cinnamyl chloride (2a) on
nanocatalyst performs the unprecedented insertion of CO2 into
the 5 mmol scale under optimized reaction conditions (Figure 5).
unsaturated substrates with comparable yields at 1 atm pressure.
The results show excellent recyclability without significant decrease
Moreover, the E-factor for the present catalyst system (2.8) is an
in catalytic efficiency (85%) over five cycles (Figure 5A). The blemish
order of magnitude (7 to 14 fold) lower than those of other
in yield can be attributed to minor catalyst restructuring. The PXRD
of the recovered MgO@Ag-7 catalyst (five cycles) shows an increase
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 8 of 10
ARTICLE
Journal Name
5. M. Aresta; I. Tommasi, Energ. Convers. Manag., 1997, 38, S373-S378.
reported methods (Table S1) since these conditions avoid excess
View Article Online
equivalents of substrates, base, and solvent.15, 17, 23
6. J. H. Clark, Green Chem., 2006, 8, 17-21.
DOI: 10.1039/C9GC04040D
7. J. H. Clark, Green Chem., 1999, 1, 1-8.
8. R. A. Sheldon, Green Chem., 2007, 9, 1273-1283.
9. R. A. Sheldon, Green Chem., 2016, 18, 3180-3183.
Conclusions
We have developed a three dimensional (3D) porous MgO
framework embedded with Ag0 nanoparticles (MgO@Ag-x = 2, 5, 7,
8%) as a Lewis base-noble metal hybrid catalyst. The porous
structure allows the uniform growth of Ag0 nanoparticles on surface
as well as within the pores of MgO cubes. These hybrid
nanocatalyst are found to be highly efficient for activating and
inserting CO2 in alkynes at ambient pressure for synthesis of a wide
array of esters and lactone heterocycles. The synergy of both metal
components for alkyne activation (Ag0), CO2 incorporation (Ag0) and
high accessibility (MgO) are responsible for its high catalytic
efficiency. The hybrid nanocatalyst performs well over five catalytic
cycles with no significant loss in yield of the ester product. The
feasibility of the CO2 insertion reaction at 1 atm pressure with
higher TOF (4.7 h-1) and low E-factor (2.8) of the present catalytic
system is an order of magnitude (5 to 14-folds) better than
reported methods making it a substantially green approach. Finally,
the MgO@Ag-x hybrid nanocatalyst demonstrates the unique
combination of efficient synergistic catalysis and green
methodology for transformation of CO2 feedstock to valuable
organic molecules.
10. D. C. Graham; C. Mitchell; M. I. Bruce; G. F. Metha; J. H. Bowie; M.
A. Buntine, Organometallics, 2007, 26, 6784-6792.
11. H. Hoberg; K. Jenni; K. Angermund; C. Krüger, Angew. Chem. Int. Ed.,
1987, 26, 153-155.
12. N. Huguet; I. Jevtovikj; A. Gordillo; M. L. Lejkowski; R. Lindner; M.
Bru; A. Y. Khalimon; F. Rominger; S. A. Schunk; P. Hofmann; M.
Limbach, Chem . Eur. J., 2014, 20, 16858-16862.
13. C. S. Yeung; V. M. Dong, J. Am. Chem. Soc., 2008, 130, 7826-7827.
14. B. Yu; Z.-F. Diao; C.-X. Guo; L.-N. He, J. CO2 Util., 2013, 1, 60-68.
15. W.-Z. Zhang; W.-J. Li; X. Zhang; H. Zhou; X.-B. Lu, Org. Lett., 2010,
12, 4748-4751.
16. X. Zhang; W.-Z. Zhang; X. Ren; L.-L. Zhang; X.-B. Lu, Org. Lett., 2011,
13, 2402-2405.
17. X. Zhang; W.-Z. Zhang; L.-L. Shi; C. Zhu; J.-L. Jiang; X.-B. Lu,
Tetrahedron, 2012, 68, 9085-9089.
18. F. Manjolinho; M. Arndt; K. Gooßen; L. J. Gooßen, ACS Catal., 2012,
2, 2014-2021.
19. P. Bhanja; A. Modak; A. Bhaumik, Chem. Eur. J., 2018, 24, 7278-
7297.
20. X.-H. Liu; J.-G. Ma; Z. Niu; G.-M. Yang; P. Cheng, Angew. Chem. Int.
Ed., 2015, 54, 988-991.
21. Y. Ma; Z. Wang; X. Xu; J. Wang, Chinese J. Catal., 2017, 38, 1956-
Conflicts of interest
1969.
22. R. A. Molla; K. Ghosh; B. Banerjee; M. A. Iqubal; S. K. Kundu; S. M.
The authors declare no competing financial interests.
Islam; A. Bhaumik, J. Colloid Interf. Sci., 2016, 477, 220-229.
23. X. Zhang; D. Wang; M. Jing; J. Liu; Z. Zhao; G. Xu; W. Song; Y. Wei; Y.
Sun, ChemCatChem, 2019, 11, 2089-2098.
Acknowledgements
24. J. Shi; L. Zhang; N. Sun; D. Hu; Q. Shen; F. Mao; Q. Gao; W. Wei, ACS
Appl. Mater. Inter., 2019, 11, 28858-28867.
J.M.Z acknowledge the financial support of the National Science
Foundation (CHE-1265703) and the National Institutes of Health
(NIH5R01). D.S.R. thanks Council of Scientific and Industrial
Research (CSIR), India (File Number: 02(0318)/17/EMR-II) for
financial support. U.G. acknowledges DU (DRC/BRS committee) and
CSIR for the exchange visiting opportunity and the award of
Junior/Senior Research Fellowship, respectively. The authors thank
Dr. Barry Stein, Dr. David Gene Morgan (Electron Microscopy
Center) and Dr. Yaroslav Losovyj (Molecular Structure Center) for
their help with ultra-microtme, HR-TEM/STEM and XPS data
collection of the materials, respectively. The authors thank Dr.
Shelby Rader (The Metal Isotopes Laboratory, Department of Earth
and Atmospheric Sciences) for ICP-MS analysis.
25. D. Yu; M. X. Tan; Y. Zhang, Adv. Synth. Catal., 2012, 354, 969-974.
26. G. A. Mutch; S. Shulda; A. J. McCue; M. J. Menart; C. V. Ciobanu; C.
Ngo; J. A. Anderson; R. M. Richards; D. Vega-Maza, J. Am. Chem.
Soc., 2018, 140, 4736-4742.
27. C. Liang; W. Xiao; H. Hao; L. Xiaoqing; L. Chao; Z. Lei; H. Fashui, Biol.
Trace Elem. Res., 2008, 121, 249-257.
28. S. Calderon V; R. E. Galindo; N. Benito; C. Palacio; A. Cavaleiro; S.
Carvalho, J. Phys. D: Appl. Phys., 2013, 46, 325303.
29. S. Benedetti; N. Nilius; P. Torelli; G. Renaud; H. J. Freund; S. Valeri, J.
Phys. Chem. C, 2011, 115, 23043-23049.
30. S. Wuttke; S. M. Coman; G. Scholz; H. Kirmse; A. Vimont; M. Daturi;
S. L. M. Schroeder; E. Kemnitz, Chem. Eur. J., 2008, 14, 11488-
11499.
31. X. Jiang; H. Liu; H. Liang; G. Jiang; J. Huang; Y. Hong; D. Huang; Q. Li;
Notes and references
D. Sun, Ind. Eng. Chem. Res. , 2014, 53, 19128-19135.
32. J. Hu; K. Zhu; L. Chen; C. Kübel; R. Richards, J. Phys. Chem. C, 2007,
111, 12038-12044.
1. D. U. Nielsen; X.-M. Hu; K. Daasbjerg; T. Skrydstrup, Nat. Catal., 2018,
1, 244-254.
33. K. Sasaki; N. Fukumoto; S. Moriyama; T. Hirajima, J. Hazard. mater.,
2011, 191, 240-8.
2. S. C. Peter, ACS Energy Lett., 2018, 3, 1557-1561.
3. J. Wei; Q. Ge; R. Yao; Z. Wen; C. Fang; L. Guo; H. Xu; J. Sun,
34. Y. Yanagisawa; K. Takaoka; S. Yamabe; T. Ito, J. Phys. Chem., 1995,
99, 3704-3710.
Nat.Commun., 2017, 8, 15174.
4. M. Poliakoff; W. Leitner; E. S. Streng, Faraday Discuss., 2015, 183, 9-
35. Y. Zheng; X. Zhang; Z. Bai; Z. Zhang, Rapid Commun. Mass
Spectrom., 2016, 30, 217-225.
17.
8 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 9 of 10
Green Chemistry
Please do not adjust margins
Journal Name
ARTICLE
36. F. Li; N. Li; C. Xue; H. Wang; Q. Chang; H. Liu; J. Yang; S. Hu, Chem.
Eng. J., 2020, 382, 122484.
View Article Online
DOI: 10.1039/C9GC04040D
37. N. Lu; M. Zhang; L. Ding; J. Zheng; C. Zeng; Y. Wen; G. Liu; A.
Aldalbahi; J. Shi; S. Song; X. Zuo; L. Wang, Nanoscale, 2017, 9, 4508-
4515.
38. P. C. Pandey; S. Singh; A. Walcarius, J. Electroanal. Chem., 2018, 823,
747-754.
39. A. H. Chowdhury; S. Ghosh; S. M. Islam, New J. Chem., 2018, 42,
14194-14202.
40. N. N. Zhu; X. H. Liu; T. Li; J. G. Ma; P. Cheng; G. M. Yang, Inorg.
chem., 2017, 56, 3414-3420.
41. N. Eghbali; J. Eddy; P. T. Anastas, J. Org. Chem., 2008, 73, 6932-
6935.
42. P. Yu; Z. Yang; Y. Liang; X. Hong; Y. Li; K. N. Houk, J. Am. Chem. Soc.,
2016, 138, 8247-8252.
43. P. Wessig; G. Müller, Chem. Rev., 2008, 108, 2051-2063.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 9
Please do not adjust margins

Green Chemistry
Page 10 of 10
View Article Online
DOI: 10.1039/C9GC04040D
Table of Contents (TOC) graphic
Development of Magnesium Oxide-Silver Hybrid Nanocatalysts
for Synergistic Carbon Dioxide Activation to Afford Esters and
Heterocycles at Ambient Pressure
1