(359c) Amine-Functionalized MIL-101 Monoliths for CO2 Removal from Enclosed Environments

Amine-Functionalized
MIL-101 Monoliths for CO₂ Removal from Enclosed Environments

Shane
Lawson, Connor Griffin, Kambria Rapp, Ali A. Rownaghi, Fateme Rezaei*

Department of Chemical & Biochemical
Engineering, Missouri University of Science and Technology, Rolla, MO 65409-1230,
United States

Abstract

            Increasing CO₂ concentrations from 400 to
2500 ppm decreases cognitive test scores from the 85^th to the 25^th
percentile. In high-risk environments, such as space craft or submarines, this diminished
cognitive functioning could have life-threatening ramifications. One way to
reduce these levels is by using an amine incorporated solid adsorbent.
Amino-solids are particularly effective sorbents for enclosed applications because
of their high CO₂/N₂ selectivity value at dilute
concentrations. Although these materials can reduce enclosed CO_2­ concentrations
to safe levels, amino-solid powders should be formed into structured contactors
to eliminate particulate scattering and equipment contamination. In our earlier
works, ^1–4 we
demonstrated that 3D-printing is a facile approach to formulate structured
monolithic contactors for adsorption applications. Monoliths were produced with
a variety of supports, including aminosilica, zeolites, and metal-organic
frameworks (MOFs), and exhibited high CO₂ adsorption capacities
under many conditions.  Using the techniques culminated previously, we have recently
formulated 3D-printed, amino-MIL-101 monoliths, for selective CO₂
adsorption from enclosed environments. In this work, ⁵ a tailored
version of MIL-101 was impregnated with 85 wt. % Tetraethylenepentamine (TEPA)
and polyethyleneimine (PEI-800) to form the powder precursors. The powders were
then formed into pastes and printed into monoliths. Pristine MIL-101 monoliths
were also printed and secondarily impregnated for comparison. From CHN
measurements, the pre-impregnated TEPA and PEI monoliths contained 3.5 and 5.5
mmol N/g amine content, respectively. Over five adsorption/desorption cycles,
this corresponded to average adsorption capacities of 1.6 and 1.4 mmol/g,
respectively, at 3000 ppm CO₂ concentration.  The post-impregnated
monoliths contained ~50 wt.% less amine content; leading to 0.3 and 0.6 mmol/g
CO₂ adsorption capacities, for TEPA and PEI, respectively. The
adsorbent stabilities were also examined. PEI’s low volatility led to
exceptional stability throughout the five cycles for all materials while the
TEPA-impregnated powder and post-impregnated TEPA monolith leached heavily and
showed reduced CO₂ uptakes by cycle two. These effects were not
observed in the pre-impregnated TEPA; which exhibited near zero capacity loss by
cycle five. The high stability was attributed to the covalent tethering of TEPA
to the MOFs chromium center during paste densification; where ultrasonic bath’s
heat forwarded the reaction mechanism. When examining the adsorption kinetics,
the post-impregnated monoliths showed the steepest kinetics; however, this was
partially attributed to their lower CO₂ uptakes. The next-fastest
kinetics were observed in the impregnated powders; followed by the
pre-impregnated monoliths. Typically, structured contactors improve upon powder
kinetics; however, these effects were not observed here. Because the CO₂
was dilute, the poor adsorption kinetics were attributed to a slow adsorbate diffusion
rate through the support walls. Whereas the powder was entirely accessible to
the CO₂, the monolith required the gas to permeate the internal
structure; which acted as a rate limiting step. Improving this rate limiting
step will be a target of future works. Overall, though, this work provides a
novel approach to formulate structured amino-MOF contactors for CO₂
removal from enclosed environments and could be highly promising with slight
modification.

References

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3.    H. Thakkar et al. ACS. Mater. Interfaces 9 (41),
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4.  S. Lawson et al. ACS. Mater. Interfaces 22 (10),
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5.  S. Lawson et al. ACS. Energy Fuels 33 (3), 2399
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