(376bk) Task-Specific Ionic Liquids Functionalized with Cobalt(II)Salen for Biomimetic Reversible Dioxygen Binding

Task-specific ionic liquids functionalized with Cobalt(II)salen for biomimetic reversible dioxygen binding

Qinghe Zheng, Marty Lail, Shaojun Zhou*,
John Thompson, Kelly Amato

Discovery
of Science and Technology Division, RTI International, Durham, NC 27709, United
States.

*szhou@rti.org

Introduction

In
biological systems, the respiratory pigments (e.g. hemoglobins
and myoglobins) are able to reversibly bind dioxygen
from the atmosphere without appreciable loss of activity. It is common
knowledge that transition metals with certain organic coordination environment
plays an important role in the oxygen binding in these natural oxygen carriers.
In synthetic chemistry, low-molecular biomimetic substances containing a transition
metal center and being able to reversibly bind oxygen have attracted
considerable interest for air separation application. By far the greatest
number of synthetic oxygen complexes are based on cobalt, and one of the most
thoroughly investigated systems is a Schiff base complex N,N_-Bis(salicylidene)ethylenediaminocobalt(II), i.e. (Co(II)salen)
also labeled as ÒSalcomineÓ, and its ring-substituted
derivatives. The Co(II)salen molecule is a tetradentate ligand coordinating to the central Co²⁺
cation through the hydroxyl and amine groups. This tetradentately
chelated compound has been found with oxygen uptake at the ratios of Co:O=2:1 (binuclear) in solid
state, and at 2:1 or 1:1 (mononuclear) in different solvation environment.

Room temperature ionic liquids (RTILs) have
various applications in a large variety of disciplines. These liquids have
desirable properties including easy preparation, tunable structure, negligible
vapor pressure, non-flammability, high thermal stability, environmentally
nontoxicity and recyclability. Due to the Coulombic attraction between the ions
of these liquids, they exhibit no measurable vapor pressure up to their thermal
decomposition point, generally above 350 ^oC. Because of this
feature, they would not add any contamination to the gas stream when used for
gas sorption process, especially during regeneration, which minimizes the
associated energy consumption. Much attention has been devoted to functionalized
ILs (often termed task-specific ionic liquids, TSILs) as solvents for various
gas separation applications. Matsuoka and colleagues published the first
research (2017) on novel TSILs with reversible and stable O₂
absorbability. The TSILs were synthesized by complexing Co(II)salen complex and ionic liquid-based ligands. The as-prepared
TSILs could chemically and selectively absorb a large amount of O₂
owing to the liquidity of the TSILs, and could function as an O₂
carrier in supported TSIL membranes.

The present study explored biomimetic dioxygen
binding by various types of TSILs functionalized with Co(II)salen
for air separation application. IL ligands composed of different combinations
of phosphonium-type cations including triethylpentylphosphonium (P₂₂₂₅⁺), triethyloctylphosphonium (P₂₂₂₈⁺), tributylpentylphosphonium (P₄₄₄₅⁺),
and trihexyltetradecylphosphonium (P₆₆₆₁₄⁺),
and anions including N-methylglycinate (NmGly^-) or/and bis(trifluoromethanesulfonyl)imide (NTf2^-) were
synthesized, and complexed with Co(II)salen
successfully. The dioxygen sorption capacity and reversibility of the
as-synthesized TSILs were investigated at various operating conditions. The
study provides important results for room-temperature oxygen-selective sorbent and
membrane development for modular air separation application.

Experimental

The
ionic liquids were synthesized following literature procedure using air-free
technique. Typical synthesis procedure for Co(II)salen
functionalized a TSIL is as the following. In a dried Schlenk
flask under continuous N₂ flow, 100 mL of ethanol was added,
followed by 1 eq. of Co(II)salen. The mixture was
continuously stirred at room temperature for 1 hr. A solution containing 2 eq.
of IL compound in ethanol was added dropwise to the above suspension. It was
worth noting that Co(salen) gradually dissolved upon
addition of IL. The mixture was under agitated stirring at 25 ^oC for
12 hrs to form an IL-Co(II)salen
complex. The solvent was then removed on a rotary evaporator, and the residual
product was further dried in a vacuum line at 50 ^oC for overnight to
yield a viscous liquid product. The as-synthesized IL and IL-Co(II)salen complex products were characterized by CHNS, NMR (1H,
31P, and 13C), IR, and UV-Vis analyses. The gas sorption was studied by TGA and
ASAP-isotherm measurements at various conditions. The viscosity of the complex
materials were studied between 25 ^oC and 80 ^oC.

Results and
Discussion

Figure. (a) TGA screening of O₂
and N₂ uptakes by as-synthesized IL-Co(salen)
complexes. (b) O₂ and N₂ sorption isotherms for selected
[P₂₂₂₅]₂[NmGly][NTf₂][Co(salen)]-type TSIL
at 25 ^oC.

    A
series of IL-Co(salen) complex-type O₂
sorbent materials were successfully synthesized, characterized, and screened
for selective O₂ absorption over N₂. Types and molecular
weights of IL ligands, and IL-Co(salen) coordination
stoichiometry all have significant influences on the gas sorption capacity and
kinetics. Among the studied samples, [P₂₂₂₅]₂[NmGly][NTf₂][Co(salen)] showed the highest O₂ absorption
capacity and kinetics, with maximum capacity of 4368 uL/g
sample and nearly half of the capacity reached (2065 uL/g
sample) within 30 min of TOS. Reaction mechanisms during O₂
absorption in IL-Co(salen) complexes were proposed.

Reference

[1]
A. Matsuoka, E. Kamio, T. Mochida and H. Matsuyama, J.
Membr. Sci., 2017, 541, 393Ð402.

[2]
Y. Kohno, M. G. Cowan, A. Okafuji, et al. Ind.
Eng. Chem. Res., 2015, 54, 12214Ð12216.