Porous Scaffolds for Electrochemically Controlled Reversible Capture and Release of Ethylene

Article abstract of DOI:10.1021/jacs.7b08102

This Communication describes the use of porous coordination polymers (PCP) with integrated metal bis(dithiolene) units to achieve electrochemically controlled capture and release of ethylene in the solid state. Applying positive potential (+2.0 V) to thes

Full text of DOI:10.1021/jacs.7b08102

Communication
pubs.acs.org/JACS
Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX-XXX
Porous Scaffolds for Electrochemically Controlled Reversible Capture
and Release of Ethylene
Lukasz Mendecki, Michael Ko, Xiaoping Zhang, Zheng Meng, and Katherine A. Mirica*
Department of Chemistry, Dartmouth College, Hanover, New Hampshire 03755, United States
S
* Supporting Information
ABSTRACT: This Communication describes the use of
porous coordination polymers (PCP) with integrated
metal bis(dithiolene) units to achieve electrochemically
controlled capture and release of ethylene in the solid
state. Applying positive potential (+2.0 V) to these PCPs
promotes ethylene capture, and subsequent dose of
negative potential (−2.0 V) induces the release. These
materials are resistant to poisoning by small reactive gases
(CO and H2S) that may interact with embedded metallic
sites.
orldwide demand for ethylene exceeds that of any other
1,2
Worganic chemical.
Extraction of ethylene from
petroleum, however, is an energy-demanding process that
involves steam cracking and cryogenic distillation.3 Alternatives
that utilize transition metal complexes can improve energy-
efficiency, but suffer from poisoningand the reduction in
Figure 1. (A) Synthesis of PCPs. (B) Illustration of proposed voltage-
performancetriggered by H2, C2H2, CO, and H2S.4,5
actuated capture and release of ethylene with PCPs.
Growing environmental concerns require improved methods
for separating alkenes from petrochemical feedstocks.6,7
In 2001, Wang and Stiefel5 proposed a strategy for purifying
alkenes from a multicomponent gas stream using an electro-
hexahydroxytriphenylene (HHTP)-based analogs, M3HHTP2
metal−organic frameworks (MOFs), were also tested (See SI
chemically controlled cycloaddition reaction8,9 with metal
bis(dithiolene) complexes.10 The implementation of this
strategy in purification, however, has remained elusive, and
has been limited to computational studies and demonstrations
in solution using molecular complexes.11−18 Achieving
reversible electrochemically controlled capture of ethylene at
the solid−gas interface can lead to improved methods of alkene
separation.19,20
for details).32−34
Inductively coupled plasma mass spectrometry (ICP-MS)
quantified the metal content in the bulk to be consistent with
the molecular formula of M3HTTP2 (Tables S1−S3). X-ray
photoelectron spectroscopy (XPS) demonstrated mixed
valency in Co3HTTP2 (Co2+/Co3+) and in Cu3HTTP2
(Cu1+/Cu2+), with only Ni2+ present in Ni3HTTP2 (Figures
S8−S10). XPS also revealed the presence of NH4+ counterions,
consistent with the anionic form of [M3HTTP2]− subunits, and
the presence of O-containing defects in the form of sulfates and
sulfites (Figures S8−S10). These findings are consistent with
recent reports of similar materials.21,22,35 Combustion analysis
quantified the amount of S, C, and N in the bulk (Tables S1−
S3). S, C, M (Co, Ni, Cu), and O were also observed by
energy-dispersive X-ray spectroscopy (EDS, Figure S4C).
Scanning electron microscopy (SEM) revealed nonuniform
nanoscale morphologies of PCPs (Figure S4A). Powder X-ray
diffraction (pXRD) showed limited crystallinity, with broad
peak at 2θ = 9.0°, consistent with the pore dimensions of 2.0
nm. This peak was absent in the pXRD analysis of the HTTP-
based molecular precursor (Figure S4B).21 The Ni- and Cu-
This paper describes an experimental demonstration of using
porous coordination polymers (PCPs) for reversible electro-
chemically driven capture of ethylene. The molecular design
features the integration of the metal bis(dithiolene) units into
d−π conjugated, conductive, electrochemically active PCPs
(Figure 1). Though this class of materials has been shown to
exhibit promising electrocatalytic,21−24 electronic,25−28 and
magnetic properties,29−31 its function in reversible electro-
chemical molecular capture has not been shown. We used
reticular synthesis through the reaction between 2,3,6,7,10,11-
hexathiotriphenylene (HTTP) linkers and divalent metal ions
(M2+) supplied in the form of cobalt(II) acetate, nickel(II)
acetate, or copper(II) trifluoroacetylacetonate, under basic
conditions (pH 10) to generate M3HTTP2 PCPs (Figure
1A).22,29 To probe the role of the chalcogen atom (S vs O), and
the role of the metal in voltage-actuated capture, 2,3,6,7,10,11-
Received: August 1, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.7b08102
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Journal of the American Chemical Society
Communication
PCP analogs also showed a [001] peak at 2θ = 21.5°, consistent
with layered structure with an interlayer distance of 0.40 nm.
The surface porosity from Brunauer−Emmett−Teller (BET)
analysis (N2, 77 K) was 171, 166, and 266 m2/g for Cu3HTTP2,
Ni3HTTP2, and Co3HTTP2, respectively (Figure S6).29
Thermal gravimetric analysis (TGA) revealed increased
thermal stability of PCPs compared to the HTTP precursor
(Figure S13). Electron paramagnetic resonance (EPR) spec-
troscopy showed ligand-centered radicals in Ni3HTTP2 and
Co3HTTP2, and a metal centered radical in Cu3HTTP2; limited
paramagnetism was observed for the HTTP ligand (Figure
S11). Attenuated total reflectance infrared spectroscopy (ATR-
IR) confirmed the disappearance of the S−H stretching
vibration at 2510 cm−1 upon metal coordination (Figure
S12).21,22 Bulk conductivity of 2.4 × 10−9, 3.6 × 10−4, and 2.4 ×
10−8 S/cm for Co3HTTP2, Ni3HTTP2, and Cu3HTTP2,
respectively, suggested reasonable ability for charge transport.
Cyclic voltammetry (CV) studies in a nonaqueous environ-
ment (−2.0 to +2.0 V in MeCN) established the electroactive
nature of the materials in this study (Figure 2A, and Figure
S15). We identified the presence of three distinct spectral
features: (i) two redox peaks at oxidative potentials above +0.70
V; (ii) one redox transition at potentials below +0.20 V on the
anodic scan and (iii) lack of well-defined redox peaks during
the cathodic scans.
Redox transitions found at higher oxidative potentials (>
+0.70 V) closely matched the redox peaks observed for the
organic precursors used to produce M3HTTP2 or M3HHTP2
PCPs (Figure 2A, Figures S14−15). The distinct negative shift
compared to the free ligand in the oxidation potential of the
PCPs may indicate that the electron transfer is largely stabilized
in the PCPs.36 The presence of redox transitions found at lower
oxidative potentials (<+0.20 V) in Figure 2A could originate
from (i) redox activity of the metal centers within the PCPs,37
(ii) coexistence of multiple redox processes due to defects (e.g.,
exposed leading edges, such as open metal and ligand sites in
the framework),38 and (iii) redox active impurities permanently
embedded within the framework. The lack of well-defined
peaks on the cathodic scan may indicate irreversibility of the
electrochemical system, or that the electron transfer process is
followed directly by a chemical reaction in which nonredox
active species are formed.39 M3HTTP2 PCPs were stable to at
least five consecutive CV cycles (Figure S24).
Next, we proceeded to examine the interactions of PCPs with
C2H4 in solution (Figure 2B, Figures S17−S18). Due to limited
solubility of ethylene in organic and aqueous solvents (∼1
mM),40 we employed a preconcentration step to enhance the
electrochemical response (Figures S19−S20, S22).41,42 Figure
2B compares the linear sweep voltammetry (LSV) responses
for the M3HTTP2 materials immobilized on glassy carbon
electrodes after 6 min accumulation time at −1.5 and +1.5 V
under N2 and C2H4 in MeCN. In the presence of C2H4,
preconcentration at −1.5 V led to a notable increase in peak
current intensity (Figure 2B, solid black line). Reversing the
potential from −1.5 to +1.5 V during preconcentration,
diminished the peak current intensity (solid red line) to a
level of being indistinguishable from the results in the presence
of N2 (dashed lines).
Though the voltammetric response in Figure 2B suggested
the presence of C2H4/M3HTTP2 interactions, we employed
M3HHTP2 MOFs to gain additional insight into the
mechanistic details of this electrochemically driven trans-
formation. This control can (i) probe the possibility of ethylene
capture through the formation of the π-complex with the metal
center in the PCPs, and (ii) examine the role of chalcogen
atoms (S vs O) in the capture process. To test these
possibilities, we held the electrode with drop-cast layer of
M3HHTP2 for 120 s at either −1.5 or +1.5 V in N2 or C2H4
(Figure S23). In each case, M3HHTP2 controls did not produce
a change in response. We, therefore, conclude that the
similarities in LSV response for all HTTP-based materials
reinforce that S atoms of the metal bis(dithiolene) complex are
critical to the observed electrochemical performance in the
presence of C2H4.
The results from solution-based electrochemical measure-
ments are subject to complications due to several experimental
factors, including (i) effect of solvent, (ii) choice of electrolyte,
(iii) solubility of gas, and (iv) requirement for three electrode
configuration, making electrochemical capture of ethylene in
solution a complex process. We proceeded to develop a strategy
that overcomes these complexities by integrating each
M3HTTP2 PCP into two types of solid-state devices (Figure
S25): (i) PCP-coated conductive slides (2.5 cm × 1.5 cm with
5 mg loading of M3HTTP2), and (ii) the compressed pellet (6
mm diameter, 62 mg, 1.45 mm thickness). We reasoned that
stimulating these forms of material with applied potential would
enable electrically actuated reactivity of the PCP with C2H4.
Figure 2. Electrochemical characterization of M3HTTP2 PCPs. Arrows
indicate directions of scans. The double headed arrows indicate the
magnitude of current. (A) Cyclic voltammograms for Co3HTTP2 in
blue, Ni3HTTP2 in green, and Cu3HTTP2 in red; scan rate: 10 mV/s.
(B) Linear sweep voltammograms (scan rate 50 mV/s) in the presence
of dissolved N2 (dashed lines) and C2H4 (solid lines). PCPs are either
preoxidized (red) or prereduced (black). All measurements were
performed in 0.1 mM TBAPF6 in MeCN under N2 (5 scans). 3 mm
diameter glassy carbon electrode, platinum wire, and Ag/AgCl
electrodes were used as the working, counter, and reference electrode,
respectively.
B
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Journal of the American Chemical Society
Communication
Capture and release of ethylene was achieved by delivering
electrical potential (+2.0 or −2.0 V) to the solid-state device.
We then used Henry’s law for gas−liquid partitioning to
quantify the amount of released C2H4 by NMR.43 Only
electrochemically oxidized M3HTTP2 PCPs were capable of
capturing C2H4 (Figure 3D,E). Despite the 10-fold difference in
and CO). We exposed each of the PCPs to a mixture of gases
comprising H2S, CO (80 ppm or 2 ppt), and ethylene for 60
min at +2.0 V (applied potential), and then measured the
amount of ethylene recovered in the NMR solvent immediately
after the release step (−2.0 V for 60 min). Remarkably, the
presence of 80 ppm of gaseous poisons reduced the
performance of thin films only by 40%, 15%, and 3% for
Co3HTTP2, Ni3HTTP2, and Cu3HTTP2 PCPs (Figure 3D).
Even in the presence of 2 ppt of H2S and CO, the materials still
maintained 35−50% of their function (Figure 3D, Tables S4−
S5). Together, these findings indicate that the M3HTTP2
materials retain their function in a complex environment, and
resist poisoning by interferents.44
In conclusion, this paper describes the first experimental
implementation of PCPs to achieve electrochemically driven
capture and release of ethylene in solution and at the solid−gas
interface. This method has five distinct advantages for
electrochemically controlled molecular capture: (i) it is
compatible both with the solution-phase capture in the
presence of electrolytes and with the direct solid-state capture
in the absence of electrolytes, (ii) it is resistant to poisoning by
reactive gases, such as CO and H2S, (iii) it enables
preconcentration of the olefin within a PCP, (iv) it employs
relatively low overpotentials in the range of −2.0 to +2.0 V, and
(v) it utilizes a class of modular porous materials that can be
further optimized for performance through strategic design.
The observed unoptimized C2H4 uptake efficiency of
M3HTTP2 PCPs drop-casted films (ranging from 0.10 to
0.22 mmol/g, Table S4) is approximately 1 order of magnitude
below FeMOF-74 (6.8 mmol/g at 1 bar)45 and SIFSIX-1-Cu
(8.5 mmol/g at 1 bar),46 and comparable to other microporous
scaffolds.47,48 The distinguishing feature of the process
presented herein is its compatibility with low pressure and
ambient temperature.49 We expect that further investigation of
the proof-of-concept methodology presented herein may
enhance innovation in materials design, chemical sensing,
preconcentration, purification, and catalysis.
Figure 3. Experimental design and demonstration for solid-state
electrochemical capture and release of ethylene. (A) The setup
comprises the PCPs positioned under ethylene atmosphere (∼1 atm)
in a sealed container. Applied electrical potential (+2.0 V) to the
material facilitates ethylene capture. (B) Switching off the power, and
evacuating at 1.5 × 10−3 Torr removes all unbound ethylene. (C)
Refilling of the container with N2, and addition of deuterated dimethyl
sulfoxide (DMSO-d6) enables monitoring of electrochemical release by
NMR. Subsequent reduction at −2.0 V promotes ethylene release.
(D,E) The amount of ethylene quantified in the electrochemical
capture/release experiment in the absence and presence of interferents
(80 ppm and 2 ppt of CO and H2S) by thin films (D) and pellets (E).
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.7b08102.
the amounts of electrochemically captured C2H4 in μmol/g
captured by thin films (Figure 3D) as compared to pellets
(Figure 3E), the similarity in uptake in moles (300−1100
nmol) by these two devices (Tables S4−S5) suggested that the
exposed surface area (rather than the bulk) dominated the
capture process. Analysis of pellets by BET (Figure S7) showed
∼30 times reduction in porosity upon compression, and
confirmed the role of surface-dominated process.
The application of electrical potential was essential for
driving the capture process in the solid-state. No C2H4 capture
was detected in the absence of applied potential, or the case of
omission of either the oxidation or reduction steps (Section IV
in SI, Figures S33−S38). Two control materialsM3HHTP2
MOFs or HTTP liganddid not produce observable capture
(SI Section IV, Figures S31−S32, S39). These findings confirm
the essential role of metal bis(dithiolene) complex in the
electrochemically driven capture.
Experimental details, nitrogen isotherms, spectroscopic,
microscopic, elemental, and electrochemical character-
ization (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*katherine.a.mirica@dartmouth.edu
ORCID
Katherine A. Mirica: 0000-0002-1779-7568
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
K.A.M. acknowledges the funds provided by Dartmouth
College, and the Walter and Constance Burke Research
Initiation Award. The authors thank Alice J. Hsu for the help
with HTTP synthesis and the initial efforts in preparing
M3HTTP2 PCPs.
The presence of gaseous inhibitors above 10 ppm can poison
metal-based catalysts in C2H4 purification.2,3 We thus evaluated
the solid-state performance of M3HTTP2 PCPs for C2H4
capture in the presence of interfering poisoning agents (H2S
C
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DOI: 10.1021/jacs.7b08102
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