(200c) Encapsulation of Nanoscale Organic Hybrid Materials and Metal-Organic Frameworks in Electrospun Polymer/Ceramic Fibers for Direct Air Capture of CO2

Non-aqueous nanoscale organic hybrid materials consisting of a silica core with ionically grafted branched polyethyleneimine chains (NIPEI) are promising candidates for carbon capture due to their high density of primary and secondary amines and excellent thermal stability. However, diffusion limitations through the bulk material, formation of a passivating carbamate skin layer, and high affinity for water prevent neat NIPEI from reaching its full potential as a carbon capture agent. We sought to remedy these drawbacks by dispersing NIPEI via encapsulation within submicron-scale polyacrylonitrile(PAN)/polymer-derived-ceramic electrospun fibers. Addition of the room-temperature curable, liquid-phase organopolysilazane (OPSZ) ceramic precursor to the PAN/NIPEI solution enhanced the internal dispersion of NIPEI, producing a near-uniform distribution throughout the fiber cross-section. The low surface tension and viscosity of OPSZ facilitated its migration to the fiber surface during electrospinning, forming a few-nanometer-thick exterior shell. Upon curing into a hydrophobic ceramic, this layer shielded the NIPEI from external water to produce near-superhydrophobic non-woven fiber mats with contact angles exceeding 140°. 1:1 loadings of NOHMs and PAN/OPSZ can be reliably achieved with low OPSZ loadings, and up to 4:1 NOHM:polymer loadings are possible before losing hydrophobicity. These fibers demonstrated high capture capacities in a pure CO2 atmosphere due to low pressure drop through and enhanced surface area within the nonwoven fiber network. The fibers additionally showed enhanced capture kinetics and excellent performance retention under 400 ppm CO2 direct air capture conditions, highlighting the selectivity of the system toward CO₂ over other gases.

In this work, we are also currently exploring other polymeric materials to serve as encapsulation media, namely polymers of intrinsic microporosity (PIMs) due to their extremely high surface area (on the order of hundreds of square meters per gram) and thermal stability in air exceeding 350°C. However, solvent incompatibility between PIMs and NIPEI led us to explore metal-organic frameworks (MOFs) as solid-state CO₂ capture agents, which only require dispersion, rather than full dissolution. We are encapsulating commonly studied MOFs that employ either physisorption (HKUST-1) or chemisorption (UiO-66-NH₂) to capture CO₂ to explore the mechanistic effects on capture kinetics and total capacities. We demonstrate excellent encapsulation of high loadings (>50%) of both MOFs inside uniform micron-scale PIM-1 fibers with barely any loss of polymer surface area, resulting in near superhydrophobicity with contact angles around 130 to 140°. Initial capture studies with UiO-66-NH₂ demonstrate extremely rapid kinetics that outpace those seen with the PAN/OPSZ/NIPEI fiber mats, reaching saturation after only one hour rather than five. Current work is devoted toward synthesising and encapsulating new MOFs designed to maximise amine loading via the organic linker, thereby enhancing the total capture capacity of the sorbent relative to the inert fibrous support.