A silver(i) coordinated phenanthroline-based polymer with high ethylene/ethane adsorption selectivity

Article abstract of DOI:10.1039/c4cc02143f

We report a non-porous silver(i) coordinated phenanthroline-based polymer, which exhibits a high ideal ethylene/ethane adsorption selectivity (15/1) and high ethylene uptake (5.0 mmol g^-1) at ambient temperature and pressure. Both silver(i) coordination and polymer structures are important for the high uptake of ethylene. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc02143f

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A silver(I) coordinated phenanthroline-based
polymer with high ethylene/ethane adsorption
selectivity†
Chao Yu,a Matthew G. Cowan,b Richard D. Nobleb and Wei Zhang*a
Cite this: Chem. Commun., 2014,
50, 5745
Received 22nd March 2014,
Accepted 5th April 2014
DOI: 10.1039/c4cc02143f
www.rsc.org/chemcomm
We report a non-porous silver(I) coordinated phenanthroline-based most popular choices of metal ions for selective adsorption of
polymer, which exhibits a high ideal ethylene/ethane adsorption olefins.17 We envisioned that phenanthroline-based polymers
selectivity (15/1) and high ethylene uptake (5.0 mmol gÀ1) at can readily coordinate with silver(I) ions and serve as solid
ambient temperature and pressure. Both silver(I) coordination and adsorbents for ethylene/ethane separation. The phenanthroline-
polymer structures are important for the high uptake of ethylene.
containing polymer (3) was synthesized from two readily accessible
monomers, 2,9-dibromophenanthroline (1) and 1,4-di-tert-
Ethylene and ethane are usually obtained as a mixture from butyl-2,5-diethynylbenzene (2), in 1 : 1 stoichiometric ratio via
cracking processes used in the petroleum industry.1 Separation Sonogashira cross-coupling (Scheme 1). The polymer product 3
of these two gases in an energy-efficient way is challenging was purified through simple filtration and washing to give a
due to the similarities in their molecular sizes and physical brown precipitate in 77% yield. The t-butyl group was intro-
properties.2 The current industrial practice is to use cryogenic duced into monomer 2 to enhance the solubility of short
distillation, which is a highly energy-intensive process.3 Recently, oligomers and prevent their premature precipitation. It was
selective adsorption on porous solid adsorbents has emerged as also anticipated that the presence of bulky t-butyl groups would
a promising, energy-efficient and environmentally benign alter- minimize the aggregation between phenanthroline and phenyl
native for ethylene/ethane separation. Various porous solids, moieties, thus producing more void space between polymer
mainly molecular sieve zeolites4 and metal–organic frameworks backbones and facilitating the transport of gas within the
(MOFs),2,5,6 have been developed for selective adsorption of polymers.
ethylene over ethane. Porous organic polymers7–9 are another
The phenanthroline moiety has been widely used as a
important class of porous materials. However, they have barely coordinating ligand in supramolecular chemistry to form com-
been explored in ethylene/ethane separation. One recent example is plex molecular entities, owing to its capability of binding
that of the copper(catecholate)-decorated porous organic polymer, various metal ions and also its unique structural and electronic
which exhibits an ideal adsorbed solution theory (IAST) selectivity properties (planarity, rigidity and electron-deficient conjugated
of 3.8/1 for ethylene/ethane adsorption.10 Herein, we report the ring).18,19 Therefore, metal ions are expected to be readily
synthesis of a robust, silver(I) coordinated, phenanthroline-based incorporated into polymer 3 through the coordination with
polymer, which shows a high ideal selectivity (15/1, v/v) for the phenanthroline moieties. We chose silver(I) as the metal cation
adsorption of ethylene over ethane at ambient temperature and
pressure.
Most of the selective adsorption of olefins over paraffins is
based on the reversible formation of p-complexes of olefins
with transition-metal cations.11–16 Among the first and second
row transition metals, silver(I) and copper(I) ions have been the
a Department of Chemistry and Biochemistry, University of Colorado, Boulder,
CO 80309, USA. E-mail: wei.zhang@colorado.edu
b Department of Chemical and Biological Engineering, University of Colorado,
Boulder, CO 80309, USA
† Electronic supplementary information (ESI) available: Synthetic procedures,
characterization data, and description of gas adsorption analysis. See DOI:
10.1039/c4cc02143f
Scheme 1 Synthesis of phenanthroline-based polymer 3 and its metallated
analogue 4: (a) Pd(PPh3)2Cl2, CuI, triethylamine, THF, 70 1C, 77%; (b) AgOTf,
acetone, CH2Cl2, 95%.
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with 1 equiv. AgOTf per phenanthroline unit (Scheme 1). The
metallated polymer product 4 was obtained as a black solid
after successive washing with acetone to get rid of excess AgOTf.
Polymers 3 and 4 were characterized using IR, solid state NMR,
elemental analyses, and TGA. Similar to the model system
described above, the IR spectrum of polymer 4 shows new
peaks at 1268 cmÀ1, 1232 cmÀ1, 1140 cmÀ1, 1025 cmÀ1 and
636 cmÀ1, and slight blue shifts of a few peaks (e.g., 1537 cmÀ1 to
1543 cmÀ1, 1496 cmÀ1 to 1502 cmÀ1, and 1361 cmÀ1 to 1367 cmÀ1
)
in comparison to that of polymer 3 (Fig. S1, ESI†). Solid-state
13C CP-MAS NMR spectra show that the two broad signals from
ethynylene carbons of polymer 3 merge into a single broad
peak upon metallation, which is consistent with the observed
changes in the 13C NMR spectra of the model complex (Fig. S3,
ESI†). ICP-MS analysis has been used to estimate silver(I) ion
loadings in polymer 4. We determined that 75% of phenanthro-
line units were coordinated with silver(I) ions, which is 1 : 0.75
molar ratio of phenanthroline to silver(I). Although the model
study indicated that phenanthroline moieties prefer to form
dimer complexes (2 : 1 binding of phenanthroline : AgI), we
assumed that the majority of phenanthrolines in polymer 4
form the 1 : 1 binding complex with silver(I), considering the
poor mobility of polymer chains in the solid state. The TGA
analyses show that polymer 3 and the Ag(I) coordinated poly-
mer 4 have high thermal stability at the decomposition onset
temperature of 350 1C (Fig. S9, ESI†).
Fig. 1 1H NMR spectra of titration study of AgOTf and 5: (a) compound 5;
(b) after the addition of 0.1 equiv. AgOTf; (c) after the addition of 0.25 equiv.
AgOTf; (d) after the addition of 0.5 equiv. AgOTf; (e) after the addition of
1.0 equiv. AgOTf; (f) after the addition of 1.5 equiv. AgOTf.
since it can form p-complexes with olefins and the reaction
can be easily reversed by pressure and temperature swings.17
A
simple model compound (5) was used to study the coordination
of the phenanthroline moiety to the silver(I) cation. Upon the
addition of increasing amounts of AgOTf in acetone (0.1 equiv. -
0.5 equiv.) to the solution of compound 5 in dichloromethane,
we observed the gradual downfield shifting of phenanthroline
1
protons in the H NMR spectrum of the complex (Fig. 1). With
the addition of 0.5 equiv. AgOTf, the system reached the
saturation point, and further addition of excess silver(I) triflate
(up to 1.5 equiv.) did not lead to any significant changes in the
1H NMR spectrum. The resulting silver(I) complex was further
characterized using ESI-MS and IR-spectroscopy. High-resolution
ESI-MS of the silver(I) complex displays peaks at m/z of 487.0376
and 869.1687, which correspond to the monomeric complex
[5ÁAg+] and the dimeric complex [5ÁAg+Á5], with relative inten-
sities of 10% and 100%, respectively. Based on this result, we
tentatively assigned the silver(I) complex to the dimer [5ÁAg+Á5]
(Scheme 2). In the IR spectra of [5ÁAg+Á5] (Fig. S2, ESI†), we
With such a high loading of silver(I) ions in polymer 4,
selective adsorption of ethylene over ethane was anticipated.
We therefore performed single component adsorption experi-
ments under ambient conditions (room temperature, 0.9 bar)
to explore the gas adsorption properties of the polymers. All the
samples were ground using a mortar and pestle, and activated
at 25 1C under high vacuum (o0.5 torr) for 18 hours to remove the
residual solvent and water molecules. The Brunauer–Emmett–
Teller (BET) surface area measurements showed that polymer 4 is
non-porous, with a BET specific surface area of o10 m2 gÀ1
.
However, this apparently non-porous polymer showed impress-
ively high ideal selectivity in the adsorption of ethylene over
ethane (15/1), with C2H4 and C2H6 uptakes of 5.04 mmol gÀ1
and 0.34 mmol gÀ1, respectively (Table 1). To the best of our
knowledge, this ethylene/ethane selectivity is among the highest
reported values obtained so far in the literature.6,10,21–23 It is not
unprecedented that non-porous or low surface area materials
show high gas adsorption selectivities either through selective
binding or the molecular sieving effect.24–30
observed new intensive peaks at 1294 cmÀ1, 1228 cmÀ1, 1164 cmÀ1
,
1022 cmÀ1 and 634 cmÀ1, and slight blue shifts of a few peaks (e.g.,
1537 cmÀ1 to 1542 cmÀ1, 1496 cmÀ1 to 1504 cmÀ1, and 1357 cmÀ1
to 1365 cmÀ1), indicating the successful coordination of silver(I),
which is in agreement with the literature report.20
Silver(I) coordinated polymer 4 was then prepared following
the same procedure as the model system. The suspension of
polymer 3 in a mixture of acetone and dichloromethane was treated
The observed high selectivity and high uptake of ethylene
over ethane in non-porous polymer 4 is mainly attributed to
the binding of ethylene molecules with the silver(I) ions.
Table 1 AgI loading and gas adsorption capacities of 3, 4, and [5ÁAg+Á5]
Ag+ loading
(%)
C2H4 uptake
C2H6 uptake
Ideal
selectivity
Adsorbent
(mmol gÀ1
)
(mmol gÀ1
)
3
4
n/a
13.3
21.0
0.41
5.04
0.20
0.45
0.34
0.015
0.9/1
15/1
13/1
Scheme 2 Metallation of model compound 5 to form [5ÁAg+Á5].
5746 | Chem. Commun., 2014, 50, 5745--5747
[5ÁAg+Á5]
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Financial support for this work is provided by National
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