(425d) Adsorption Equilibria and Kinetics of Amine-Functionalized Hyper-Crosslinked Polymers for Direct Air Capture

Anthropogenic emissions of greenhouse gases have resulted in a 1.1°C increase in global temperatures to date [1]. According to the IPCC 6^th assessment report, achieving the 1.5°C target by 2100 requires the widespread adoption of negative emissions technologies [2]. Direct air capture (DAC) of CO₂ is among the most promising of these technologies since it allows a priori for gigaton scaling and only marginally interferes with land and water use [3]. However, the current capacity of DAC facilities is only 17 kt/year, far below the required gigaton capacity target by mid-century [4]. This gap is attributed to differences in carbon pricing (e.g., $65 per ton CO₂ in the EU) and the social cost of CO₂ (estimated at $185 per ton CO₂ [5]) compared to the cost of DAC which is currently $500-600 per ton CO₂ for the largest operating DAC plant [6].

Reducing capital and operational costs through the design of DAC adsorbents with higher capacities and faster kinetics is key to making DAC economically viable in the future. To achieve this, a fundamental understanding of what controls the performance – kinetic and equilibrium – of the DAC adsorption process must be reached. Hence, one must understand the relationship between the chemical and structural features of the adsorbent and its performance. Hyper-crosslinked polymers (HCPs), made from inexpensive starting materials, are attractive DAC adsorbent candidates owing to their tunable nature and similar structure and chemistry to the benchmark DAC adsorbent, Lewatit. Equilibrium uptakes of up to 3.1-3.7 mmol/g at 273 K and 1 bar CO₂ have been reported for these materials [7-10] and the adsorption isotherm suggests significant uptake at 400 ppm. However, there is a lack of data on adsorption under DAC conditions, including sorption kinetics. In addition, the tunable nature of these materials is yet to be exploited to understand the chemistry and porosity effects on sorption performance.

This study aims to fill this knowledge gap by measuring the CO₂ equilibrium uptake and kinetics at 400 ppm for a range of amine-functionalized hyper-crosslinked polymers (HCPs) with varied amine contents and pore structures. The HCPs were synthesized by Friedel-Crafts crosslinking of triptycene to afford highly porous (>1000 m²/g) polymers, which are subsequently grafted with diethylenetriamine (DETA) [11]. Variable polymerization durations were employed to obtain materials of different amine contents and micro- and mesoporosity levels, as derived from N₂ sorption measurements at 77 K and 2D-NLDFT analysis. We assessed CO₂ and N₂ sorption equilibria at 298 K and up to 1 bar using a volumetric gas sorption analyzer. We collected CO₂ sorption kinetics data using manometric and gravimetric techniques.

We observe that while the unfunctionalized HCPs had negligible CO₂ uptake in the low-pressure region, functionalization using DETA yielded adsorption capacities of up to 0.4 mmol/g at 400 ppm. Negligible N₂ sorption is observed for the functionalized HCPs. The amine efficiency was calculated for the range of materials, revealing that DETA-functionalized HCPs with higher mesopore volume exhibit greater CO₂ uptake at 400 ppm and highlighting the importance of amine accessibility rather than absolute content. Kinetic parameters of CO₂ uptake will be included to allow for a full comparison of the different materials. Understanding the balance between reaction kinetics and gas diffusion will be key.

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