Concentrating Immiscible Molecules at Solid@MOF Interfacial Nanocavities to Drive an Inert Gas–Liquid Reaction at Ambient Conditions

Article abstract of DOI:10.1002/anie.201809813

Gas–liquid reactions form the basis of our everyday lives, yet they still suffer poor reaction efficiency and are difficult to monitor in situ, especially at ambient conditions. Now, an inert gas–liquid reaction between aniline and CO₂ is drive

Full text of DOI:10.1002/anie.201809813

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
International Edition Chemie
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Accepted Article
Title: Concentrating Immiscible Molecules at Solid@MOF Interfacial
Nanocavities to Drive an Inert Gas-Liquid Reaction at Ambient
Conditions
Authors: Howard Yi Fan Sim, Hiang Kwee Lee, Xuemei Han, Charlynn
Sher Lin Koh, Gia Chuong Phan-Quang, Chee Leng Lay, Ya-
Chuan Kao, In Yee Phang, Edwin K.L Yeow, and Xing Yi Ling
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201809813
Angew. Chem. 10.1002/ange.201809813
Link to VoR: http://dx.doi.org/10.1002/anie.201809813
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Concentrating Immiscible Molecules at Solid@MOF Interfacial
Nanocavities to Drive an Inert Gas-Liquid Reaction at Ambient
Conditions
[
1]
[1,2]
[1]
[1]
Howard Yi Fan Sim, Hiang Kwee Lee,
Xuemei Han, Charlynn Sher Lin Koh, Gia Chuong Phan-
Quang, [1] Chee Leng Lay, [1,2] Ya-Chuan Kao, [1] In Yee Phang, Edwin K.L Yeow, Xing Yi Ling*[1]
[2]
[1]
Abstract: Gas-liquid reactions form the basis of our everyday lives,
yet they still suffer poor reaction efficiency and are difficult to monitor
in situ, especially at ambient conditions. Herein, we drive an inert gas-
Metal-organic framework (MOF) is an emerging porous
material that could address these limitations through their
excellent sorptivity and selectivity toward small gas/liquid
9
liquid reaction between aniline and CO
2
at 1 atm and 298 K by
molecules. When coated over a solid surface, the solid@MOF
selectively concentrating these immiscible reactants at the interface
between metal-organic framework and solid nanoparticles
ensemble contains nanoscopic cavities at the solid-MOF
interface.10 Gas sorption by solid@MOF platform continuously
accumulates gas molecules in these confined cavities, eventually
creating a pseudo high-pressure microenvironment even under
ambient operations.10 We hypothesize that solid@MOF platform
could also promote efficient gas-liquid reaction by concentrating
biphasic molecules into close proximity to enhance their
(solid@MOF). Real-time reaction SERS monitoring and simulation
investigations affirm the formation of phenylcarbamic acid, which was
previously undetectable because they are unstable for post-reaction
treatments. The solid@MOF ensemble gives rise to a >28-fold
improvement to reaction efficiency as compared to ZIF-only and solid-
only platforms, emphasizing that the interfacial nanocavities in
solid@MOF are the key to enhance gas-liquid reaction. Our strategy
can be integrated with other functional materials, hence opens up new
opportunities for ambient-operated gas-liquid applications.
intermolecular interactions. Besides enhancing
a reaction,
molecular enrichment in the interfacial cavities of a plasmonic
solid@MOF platform will also enable ultrasensitive in situ reaction
monitoring via surface-enhanced Raman scattering (SERS). This
molecular-level information is essential to elucidate reaction
dynamics and facilitate rationale design of gas-liquid reaction
toward optimal performance, efficiency and ambient operation.
Herein, we drive an inert gas-liquid reaction at ambient
Gas-liquid reactions are crucial to sustain human civilization.
They participate in various biological functions, such as
1
2
photosynthesis and gas exchange in bloodstream, and also
industrial processes,3 including hydrogenation of unsaturated
conditions by integrating
a gas-sorbing layer of zeolitic
4
compounds and gas scrubbing. Notably, the key principle to drive
imidazolate framework-8 (ZIF) over an array of Ag nanocubes
(Ag@ZIF). Our integrated platform involving ZIF (pore aperture
3.4 Å) selectively concentrates and forces the immiscible gaseous
and liquid-based reactants together to promote molecular
interactions at the solid-MOF interface (Figure 1A). Moreover, Ag
also functions as a SERS platform to track gas-liquid reaction in
situ at the molecular level. Our model gas-liquid reaction is the
high-performance gas-liquid reactions is to enforce efficient and
frequent intermolecular collisions between these immiscible
phases. However, such reactions still suffer poor efficiency and
tracking gas-liquid reaction in situ is challenging because of low
gas concentration, especially at ambient/low pressure conditions.
These performances are further aggravated by poor gas solubility
and comparatively lower density of gas,5 whereby gas rapidly
escapes a liquid medium and impedes efficient intermolecular
interaction. To address these issues, conventional strategies
reaction between liquid aniline and gaseous CO
2
to form
phenylcarbamic acid,9 an industrially significant process that
traditionally requires high operating pressure/temperature even in
the presence of catalyst. We first investigate the selective
enrichment of liquid aniline within Ag@ZIF’s interfacial cavity,
6
involving flow chemistry, packed column and microbubbles have
been developed to promote gas-liquid interactions by prolonging
7
gas flow in liquid, or by increasing gas concentration/energy at
2
which is vital for subsequent reaction with CO gas. Corroborating
experimental SERS and density functional theory (DFT) studies,
8
>
2 bar and >323 K. While these strategies have improved gas-
liquid reactions, they are limited by the need of energy-intensive
and/or long reaction procedures to drive the reaction, even at high
temperature/pressure. Operating gas-liquid reaction at ambient
conditions remains a formidable challenge.
we track gas-liquid reaction in situ and demonstrate the
unprecedented reaction of aniline and CO at 1 atm and 298 K.
2
Moreover, we reveal that the presence of interfacial cavities is the
key to improve gas-liquid reaction efficiency by >28-fold which
cannot be achieved in standalone ZIF or Ag platforms. Our work
offers valuable insights to realize ambient-operated gas-liquid
reactions, which are highly sought after in diverse fields including
air-to-fuel conversion, chemistry, environmental remediation and
industrial chemical manufacturing.
We begin by depositing an array of Ag nanocubes (edge
length, 126±5 nm; Figure S1) on a Si substrate via Langmuir-
Blodgett technique. The Ag nanocube array is then surface-
[
[
a]
b]
H. Y. F. Sim, Dr. H. K. Lee, Dr. X. Han, C. S. L. Koh, G. C. Phan-
Quang, C. L. L., Y. C. Kao, Prof. E. K.L Yeow, and Prof. X. Y. Ling
Division of Chemistry and Biological Chemistry, School of Physical
and Mathematical Sciences, Nanyang Technological University, 21
Nanyang Link, Singapore 637371
E-mail: xyling@ntu.edu.sg
Dr. H. K. Lee, C. L. L., and Dr. I. Y. Phang
Institute of Materials Research and Engineering, Agency for
Science, Technology and Research (A*STAR), 2 Fusionopolis Way,
Innovis, #08-03, Singapore 138634.
grafted with perfluorodecanethiol (PFDT) to provide a featureless
SERS background in the region of 400 to 1800 cm-1 necessary for
accurate reaction monitoring. PFDT-grafted Ag nanocube array is
then coated with a ZIF layer via repeated immersions into
Supporting information for this article is given via a link at the end of
the document.
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Figure 2. (A) SERS spectra and time-resolved contour plot recorded from
2
Ag@ZIF (i) before reaction, (ii) after reaction and (iii) upon 30 min N flush after
Figure 1. (A) Schematic depicting the importance of Ag@ZIF to capture and
concentrate gas and liquid molecules in its interfacial cavities for enhanced gas-
liquid reaction, using the reaction between CO and aniline as a model. (B)
2
reaction. (B) Temporal changes to SERS intensities of 1387 cm and 1427 cm-
-1
1
bands when Ag@ZIF is submerged in 0.1 M aniline/decane mixture.
Cross-sectional SEM image and (C) XRD pattern of Ag@ZIF. (D) Schemes and
SERS spectra of (i) Ag@ZIF and (ii) Ag-only platforms when submerged in an
aniline/decane mixture. (E) Time-dependent SERS intensity of aniline
measured using Ag@ZIF or Ag-only platforms.
plateau of aniline’s SERS intensity indicates the continuous
accumulation and saturation of aniline near the SERS surfaces,
respectively. Ag@ZIF exhibits a Lagergren pseudo first-order
sorption kinetics model with a rate constant of 37.5 ms , denoting
the rapid sorption of aniline (Figure S4B). Conversely, no aniline
SERS band is observed in control Ag platform without ZIF.
-1
methanolic zinc nitrate and 2-methylimidazole mixture. ZIF
nucleates and eventually grows into a polycrystalline film that fully
encapsulates the Ag nanocubes (Ag@ZIF; ZIF thickness, 241±45
nm; Figure 1B; Figure S2). X-ray diffraction (XRD) analyses
highlight the successful formation of Ag@ZIF platform (Figure 1C),
where Ag@ZIF contains the unique XRD patterns of both ZIF and
Ag nanocube. All Ag@ZIF platforms are activated using heat
treatment prior to subsequent experiments (Figure S2-3).
There are two key criteria for solid@MOF to facilitate gas-
liquid reaction – its abilities to (1) selectively sorb gas and liquid
species from reaction medium and (2) subsequently enrich these
reactants at solid-MOF interface. To demonstrate its molecular
selectivity and concentrating effects, we monitor the molecular
components in solid@MOF interfacial cavity by immersing 0.1 M
Instead, we only observe three weak SERS bands at 1301, 1377
and 1448 cm-1 attributed to decane’s τCH2, ωCH2 and δCH2
,
12
respectively (Figure 1D, E; Figure S5). These comparisons
demonstrate the importance of creating crack-free ZIF
a
membrane to selectively capture and enrich target molecules over
the encapsulated SERS-active surfaces. The selective sorption of
aniline over decane arises from ZIF’s characteristic pore size (3.4
Å) and framework flexibility to enable the diffusion of aniline (~5.9
Å) into SERS-active region,13 while blocking the access of larger
solvent molecules such as decane (>7.2 Å). Furthermore, the
rationale choice of solvent is another critical factor to ensure the
selective capturing of reactant from liquid medium; the use of
smaller solvent molecules (e.g. methanol) leads to competitive
sorption with aniline by Ag@ZIF (Figure S6).
We subsequently drive an inert gas-liquid reaction between
2
CO and aniline at ambient conditions and concurrently monitor
the reaction in situ using our Ag@MOF platform. In a typical
reaction set-up, we first immerse Ag@ZIF in aniline/decane
mixture for 2 hours to preconcentrate aniline in the interfacial
2
aniline in decane solution under N environment using SERS.
Only vibrational fingerprint of aniline is observed throughout the
-
experiment, with SERS bands at 1274, 1387, 1594 and 1615 cm
1
indexed to (νCN), (ρNH2+νCC), (δNH2+νCC) and (νCN+δNH2+νCC),
1
1
respectively (Figure 1D, E). The absence of other bands
indicates that decane is not accessible to SERS surface, and also
eliminates the possibility of laser-induced processes on Ag@ZIF
platform and/or aniline (Figure S4). Aniline’s SERS intensity at
cavities. Gas-liquid reaction is then initiated by bubbling CO
2
gas
1
(
~
387 cm-1 gradually increases from (31±3) counts at 0 min to
463±16) counts within the first 90 min, followed by a plateau at
466 counts beyond 105 min. The initial rise and subsequent
(
5 sccm, 1 atm) into the reaction cell, and we track its progress in
real-time using SERS. At 0 min, the SERS spectrum shows only
aniline’s vibrational signatures at 1274 (νCN), 1387 (ρNH2+νCC),
-1
1
594 (δNH2+νCC) and 1615 cm (δNH2+νCN+νCC; Figure 2A). As CO
2
exposure increases, aniline SERS intensity decreases
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exponentially at 15 min and eventually diminishes by 45 min
(
Figure 2A, B; Figure S7). We also note the emergence of new
SERS bands at 1335, 1427, 1556 and 1633 cm-1 at 30 min,
whereby the intensities increase and ultimately dominate the
SERS spectrum at 120 min. More importantly, subsequent
2
purging with inert N gas for 30 min does not revert these spectral
evolutions. Our findings evidently demonstrate the spectral
changes are due to irreversible chemical transformation of aniline.
In contrast, control Ag platform in the absence of ZIF coating or
Ag@ZIF under inert N gas flow display no distinct spectrum
2
changes (Figure S4, S7). Both control experiments highlight that
ZIF is critical to enable gas-liquid reaction at ambient conditions
and CO functions as a gas reactant, respectively. The ZIF layer
2
remains stable throughout the reaction with consistent thickness
and XRD patterns before and after reaction (Figure S2-3).
We use density functional theory (DFT) to identify the chemical
species involved in the reaction and elucidate the molecular
origins behind the observed spectral evolutions. We simulate the
SERS spectra of various probable carboxylated products
including phenylcarbamic acid, phenyl phenylcarbamate, 1,3-
diphenylurea and 2-aminobenzoic acid owing to the possibility of
carbamates/urea formation as well as the presence of ortho-
Figure 3. (A) Molecular structures of aniline and phenylcarbamic acid near Ag
surface. (B) Key spectral transformations involving amide and hydroxyl moieties.
(i) Experimental SERS spectra at 0 and 90 min. (ii) Simulated SERS spectra of
aniline and phenylcarbamic acid. (C) Scheme depicting the gas-liquid reaction
directing -NH
2
group (Figure 3A, B; Figure S8). Experimental and
DFT-simulated SERS spectra of aniline (Figure S9; Table S1)1
1
are in close agreement, thus validating the accuracy of our DFT
investigations. Most importantly, the experimental SERS
spectrum at 90 min matches well to the simulated spectrum of
phenylcarbamic acid. This is evident from the presence of unique
amide and hydroxyl vibrational modes, whereby SERS features
2
between aniline and CO to form phenylcarbamic acid in the interfacial cavity.
Utilizing the critical reaction insights gained from our SERS
and DFT investigations, we further apply Ag@ZIF platform at a
larger reaction scale to demonstrate its versatility and scalability.
Note that Ag@ZIF core-shell nanoparticles (2 mg; ZIF thickness,
47±11 nm; Figure S10-11) are used in this study, instead of
Ag@ZIF substrate. This is to maximize their interactions with gas
and liquid reactants throughout the reaction volume. We suspend
Ag@ZIF particles in 0.1 M aniline/decane mixture and initiate the
at 1427, 1335 and 1556 cm-1 are indexed to (νas
,
NC(OH)+δCOH+νCO),
(
νNCO+νas,CCN) and (νNCO+νas,CNC+δCNH; Figure 3B; Table S2),
respectively. The formation of phenylcarbamic acid is also
supported by similar work reporting the formation of carbamic acid
using amine species and CO
2
.14 We exclude other carboxylated
species as the product owing to large spectral mismatch between
our experimental and DFT-simulated SERS spectra (Figure S8).
Collectively, our experimental and DFT SERS studies clearly
provide the first experimental verification to the formation of
reaction by bubbling CO gas. The reaction is monitored over 5
2
hours using UV-Vis absorption spectroscopy. We observe
aniline’s characteristic absorption peak at 234 nm (Figure 4A).
With increasing CO2 exposure from 0 min to 300 min, C/C0
decreases linearly from 1 to 0.765 and corresponds to a reaction
efficiency of 23.5% (Figure 4B). In contrast, control Ag-only
platform and blank (neat aniline/decane mixture) exhibit an
apparent efficiencies of <6% (Figure S14), which are likely due to
minor adsorption of aniline on reaction apparatus. The >4-fold
improvement to reaction efficiency in Ag@ZIF eliminates the
possibility of catalytic effect arising from Ag or potential side
2
phenylcarbamic acid using liquid aniline and CO gas (Figure 3C).
In contrast, phenylcarbamic acid cannot be identified using
conventional ex situ monitoring methods because it is unstable
toward post-reaction treatments, such as molecular extraction. By
enabling in situ gas-liquid reaction read-out at ambient conditions,
our SERS-active Ag@ZIF platform addresses the limitations of
modern characterization methods, which are typically non-
molecular-specific and/or mandate pressurized system. Moreover, reactions as the main contributor to its superior reaction
the realization of such gas-liquid reaction is unprecedented at
ambient conditions; most carbamate-forming reactions were
typically performed at high temperature and pressure. Our ability
to drive inert gas-liquid reaction and perform real-time reaction
monitoring are enabled by Ag@ZIF’s abilities to selectively
concentrate target liquid and gas reactants within its interfacial
cavities and near the plasmonic surfaces. This molecular
concentrating process consequently forces biphasic reactants
into proximity, thereby enhancing their intermolecular interactions
to drive chemical reactions in the absence of sophisticated set-up
and/or elevated pressure/temperature.
performance. More importantly, Ag@ZIF exhibits >2-fold superior
reaction efficiency compared to pure ZIF-only crystallites (2 mg)
with a reaction efficiency of 11.7% (Figure 4B; Figure S13-14). By
further normalizing the aniline consumed with the mass of ZIF
(Figure 4C; Supporting Text 1), we ensure a fair comparison
between Ag@ZIF and ZIF-only platforms to highlight the former’s
ability to facilitate gas-liquid reaction at the interfacial nanocavities.
The Ag@ZIF exhibits an aniline consumption of ~9.6 mg
mgZIF-1 over 5 hours, which is notably >28-fold higher than ZIF’s
consumption of ~0.3 mg aniline mg ZIF-1. Ag@ZIF’s superior reaction
performance is also supported by its 2-fold higher apparent
reaction rate constant (k) of 4.8 M ms-1 as compared to ZIF-only
aniline
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reaction treatment and can only be tracked in situ. Moreover,
solid@MOF improves reaction efficiency by >28-fold as
compared to solid-only or MOF-only platforms, highlighting that
the interfacial cavities present in solid@MOF is the origin behind
its superior reaction performance. Such solid@MOF system
tackles
the
long-standing
bottleneck
in
gas-liquid
reaction/monitoring by enriching immiscible reactants locally
without requiring elevated temperature/pressure operations.
Together with its high versatility to integrate with other functional
materials, we envisage our solid@MOF strategy as a cornerstone
to
realize
efficient
and
ambient-operated
fluid-fluid
reactions/processes critical in diverse fields including chemistry,
heterogeneous catalysis, greenhouse gases removal and gas-to-
fuel conversions.
Acknowledgements
X.Y.L. thanks the financial supports from Singapore Ministry of
Education, Tier 1 (RG21/16) and Tier 2 (MOE2016-T2-1-043)
grants. H.K.L. and C.L.L. thank scholarship support from A*STAR,
Singapore. C.S.L.K. and G.C.P-Q acknowledge scholarship
support from Nanyang Technological University, Singapore. We
thank Mr. Poh Chong Lim, A*STAR, for XRD analysis.
Figure 4. (A) Aniline extinction spectra at predefined timings in presence of
Ag@ZIF. Inset is a TEM image of our core-shell Ag@ZIF platform. (B) Changes
to aniline’s C/C under prolonged CO exposure. Blank refers to absence of any
0 2
nanoparticles. (C) Mass of aniline consumed per unit mass of ZIF in Ag@ZIF
and ZIF-only platforms. (D) Mass of aniline consumed per unit mass of ZIF and
Keywords: gas-liquid reaction • metal-organic framework •
molecular concentration effect • reaction monitoring • surface-
enhanced Raman scattering
2
interfacial area in Ag@ZIF and SiO @ZIF systems.
[
[
[
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platform (k = 2.4 M ms ; Figure 4B). These observations highlight
the enhanced gas-liquid reaction in Ag@ZIF is primarily due to
the presence of interfacial cavities rather than intrinsic ZIF pores.
Our strategy building on solid@MOF’s interfacial cavities to
promote gas-liquid reaction is also easily extended to other
functional material. As a proof of concept, we synthesize
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[
SiO
to identical reaction procedures. Using aniline’s absorption peak
at 234 nm, SiO @ZIF demonstrates a reaction efficiency of 20%
Figure S13) at 300 min, which is also ~3-fold higher compared to
2
@ZIF (ZIF thickness, 14±4 nm; Figure S12) and subject them
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[
2
control SiO -only and blank systems (<7% efficiency). This again
implies negligible contribution from the encapsulated solid to the
decrease in aniline concentration. We further normalize the
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the nature of encapsulated solid. Both SiO @ZIF and Ag@ZIF
platforms exhibit similar aniline consumption of ~90.9 and 99.7
area-2, respectively (Figure 4D;
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mganiline.mgZIF-1.µminterfacial
2
Supporting Text 2). This is a direct evidence that the interfacial
cavities between MOF and any solid materials is the key factor to
achieve high-performance gas-liquid reaction. Our solid@MOF
strategy is versatile and can be potentially integrated with many
functional materials, such as solid catalyst, to promote diverse
fluid-fluid reactions.
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In conclusion, we drive an inert gas-liquid reaction between
2
aniline and CO at ambient conditions and realize its in situ
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reaction monitoring by selectively concentrating these biphasic
molecules in the interfacial nanocavities of solid@MOF platform.
Experimental and DFT SERS investigations jointly highlight the
formation of phenylcarbamic acid in our model reaction. Notably,
such molecular-level identification of phenylcarbamic acid is
unprecedented because this chemical species is unstable to post-
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Entry for the Table of Contents (Please choose one layout)
Layout
COMMUNICATION
Gas-liquid reactions form the basis of
our lives, yet they still suffer poor
reaction efficiency and are difficult to
monitor in situ, especially at ambient
conditions. Herein, we drive an inert
gas-liquid reaction between aniline
Howard Yi Fan Sim, Hiang Kwee Lee,
Xuemei Han, Charlynn Sher Lin Koh,
Gia Chuong Phan-Quang, Chee Leng
Lay, Ya-Chuan Kao, In Yee Phang,
Edwin K.L Yeow, Xing Yi Ling*
and CO
2
at 1 atm and 298 K and
Page No. – Page No.
perform a real-time reaction
monitoring by selectively
concentrating these immiscible
reactants at the interface between
metal-organic framework and solid
nanoparticles (solid@MOF).
Concentrating Immiscible Molecules
at Solid@MOF Interfacial
Nanocavities to Drive an Inert Gas-
Liquid Reaction at Ambient
Conditions
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