(619f) Development of Oxygen-Selective Sorbents for Low- and High-Temperature Air Separation | AIChE

(619f) Development of Oxygen-Selective Sorbents for Low- and High-Temperature Air Separation


Zheng, Q. - Presenter, RTI International
Lail, M., RTI International
Zhou, S., RTI International

6.0pt;margin-left:0in"> " times new roman>Development of Oxygen-selective Sorbents
for Low- and High-temperature Air Separation

6.0pt;margin-left:0in">Qinghe Zheng font-family:" times new roman>, Shaojun Zhou, and Marty

Advancement and Commercialization Division, RTI International, Durham, NC
27709, United States.

6.0pt;margin-left:0in"> 12.0pt;font-family:" times new roman>*mlail@rti.org



We are reporting our recent material development progress on oxygen-selective
sorption for air separation. Depending on the gas environment and process
conditions, different material development strategies were used based on the
material-O2 affinity. Various types of biomimetic room-temperature
(25 oC) and high-temperature (600~1000 oC)
oxygen-selective sorbents were developed. Specifically, novel Cobalt (II) salen
functionalized task-specific ionic liquids or novel types of Lewis base ligands
were synthesized for reversible biomimetic room-temperature oxygen binding. Meanwhile,
novel CaCoxZr1-xO3-δ perovskites
were developed to allow rapid high-temperature O2 sorption cycles.

In biological systems, the respiratory pigments (e.g. hemoglobins) are able to
reversibly bind dioxygen from the atmosphere without appreciable loss of
activity. 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), and its ring-substituted derivatives. Unprocessed
solid-phase Co(II)salen does not bind oxygen efficiently, and it exhibited
degradation after extended sorption cycles. For optimized oxygen binding performance,
we focused on modification of Co(II)salen by coordination with
delicately-designed ionic liquid functional groups or novel types of Lewis base

For some applications including glass and steel industries, and combustion
processes, it is desirable to supply oxygen at elevated temperature above 300 oC.
In these cases, an air separation process including oxygen-selective sorption
at high temperature is preferable because a single stream of high-purity (>
99%) O2 can be obtained by temperature-, pressure-swing, or combined
sorption cycles, with O2-deficient air as the only byproduct in a
separate stream. ABO3-δ perovskites, where
alkaline-earth or lanthanide elements occupy A site and transition metal
elements occupy B site, are crystalline ceramics and are ideal for high
temperature thermochemical air separation for oxygen production. This is due to
the fact that their oxygen nonstoichiometry δ can be varied in
response to changes in temperature and oxygen partial pressure. Previously, we
reported outstanding low temperature (< 550 oC) redox property of
our patented novel CaCoxZr1-xO3-δ
perovskite-type oxygen storage materials (OSMs).[2] In the present
study, we further examined the thermochemical oxygen mobility within these
materials at higher temperature regime (600 to 1000 oC). These
perovskites showed outstanding oxygen sorption performance and they are
potential oxygen-selective sorbents for high-temperature air separation.



For room-temperature O2-selective sorbent development, IL ligands composed
of different combinations of phosphonium-type cations [PC1C2C3C4]+
(C1 =C2 =C3 = 2, 4, or 6; C4= 5, 8, or 16) and
anions including N-methylglycinate ([NmGly]−) or/and
bis-(trifluoromethanesulfonyl)amide ([Tf2N]) were
synthesized and complexed with Co(II)salen to yield 11 TSIL samples. Novel
types of Lewis-base ligands were also used to coordinate with Co(II)salen. Material
characterizations (CHN, ICP-MS, NMR, CHNSO, High resolution MS, FT-IR, DRIFT, UV−vis,
and Single-crystal XRD) were used to confirm material composition and structure.
The dioxygen sorption capacity and reversibility of the as-synthesized
materials were investigated via both gravimetric and volumetric approaches
under various gas sorption.

For high-temperature O2-selective sorbent development, CaCoxZr1-xO3-δ
perovskites were synthesized using modified Pechini method. In situ thermal
XRD was used to study the material structural changes in response to
temperature variations in air or inert atmosphere. Temperature programmed
reduction was employed to elucidate the relationship between perovskite
composition and redox property. O2 sorption performance was
evaluated by isothermal study at various temperature and oxygen partial
pressure conditions, as well as long-term absorption-desorption cycle tests.


and Discussion

margin-left:0in;text-align:justify"> " times new roman>        A series of five-coordinate Co(II)salen
complex-type O2 sorbent materials were successfully synthesized,
characterized, and screened for room-temperature selective O2
absorption over N2. Types and molecular weights of coordinating
ligands, and coordination stoichiometry all have significant influences on the
gas sorption capacity and kinetics. Among the studied TSIL 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 at 25 oC (Figure 1a). Oxygen sorption
isotherms and long-term material stability tests were performed with CaCo0.7Zr0.3O3-δ
and CaCo0.9Zr0.1O3-δ, with
stable oxygen production of 2.58 wt% and 2.87 wt% respectively shown at 900 oC
for 100 cycles (Figure 1b), and 5 minutes for each sorption step. Stable
material performance of CaCo0.9Zr0.1O3-δ was
further demonstrated in extended thermal cycle test with shorter sorption
duration (2 minutes for each sorption step), for 500 cycles. An air separation
process scheme was proposed to produce > 99% purity O2 by rapid
absorption-desorption redox cycles using the above perovskite-type
oxygen-selective sorbents.

margin-bottom:6.0pt;margin-left:0in;text-align:center"> 12.0pt;font-family:" times new roman>

margin-left:0in;text-align:justify"> font-family:" times new roman>Figure 1 12.0pt;font-family:" times new roman>. (a) O2 and N2
absorption and desorption isotherms of [P2225]2[NmGly][Tf2N][Co(salen)]
measured at 25 °C and between near zero and 760 mmHg. (b) Thermal oxygen sorption
cycle tests by novel CaCo0.7Zr0.3O3- δ perovskite
at 900 oC.

margin-left:0in;text-align:justify"> font-family:" times new roman> 

margin-left:0in;text-align:justify"> font-family:" times new roman>References

margin-bottom:6.0pt;margin-left:0in"> normal">[1] Q. Zheng, S. J. Thompson, S. Zhou, M. Lail, K. Amato, A. V. Rayer,
J. Mecham, P. Mobley, J. Shen and B. Fletcher, Ind. Eng. Chem. Res.,
2019, 58, 334–341.

margin-bottom:6.0pt;margin-left:0in"> normal">[2] Q. Zheng, M. Lail, K. Amato and J. T. Ennis, Catal. Today,
2019, 320, 30–39.