Impact of Repeated Temperature Cycling on the Low-Concentration CO2 Adsorption Capacity of Grafted Silica-Amine †

Technical Summary: Impact of Repeated Temperature Cycling on Low-Concentration CO2 Adsorption Capacity of Grafted Silica-Amine

Problem Statement
Direct Air Capture (DAC) is a critical strategy for mitigating anthropogenic global warming, particularly for achieving net-zero emissions where some fossil fuel use remains necessary. A primary challenge in DAC is the development of solid sorbents capable of reversibly adsorbing CO2 at low atmospheric concentrations (approximately 400 µL L⁻¹) while maintaining robustness against repeated regeneration cycles. Existing solid sorbents often exhibit poor adsorption capacities at these low concentrations compared to high-concentration sources like combustion exhaust (100–300 times higher). Furthermore, many sorbents degrade under the thermal cycling required for regeneration. This study addresses the gap in understanding how specific grafted amine systems behave under repeated thermal stress in an oxidative, CO2-free environment, specifically focusing on the disparity between performance at DAC-relevant concentrations versus flue gas concentrations.

Methodology
The researchers investigated short-chain ethyleneimines (SCEI), specifically diethyleneimine (DAS) and triethyleneimine (TAS), covalently grafted onto two high-surface-area mesoporous silica supports: MCM-41 and SBA-15. The study employed a comprehensive analytical approach to track changes in adsorption capacity, chemistry, and morphology over up to 300 thermal cycles.

• Synthesis: Samples (DAS/SBA-15, TAS/SBA-15, DAS/MCM-41, TAS/MCM-41) were synthesized via a sol-gel reaction in anhydrous toluene with controlled water addition to expand silica pores.
• Thermal Cycling Protocol: Granular samples were subjected to repeated temperature cycles in dry, CO2-free air. Each cycle involved heating from 30°C to 80°C over 5 minutes and cooling back to 30°C over 5 minutes.
• Analytical Techniques:
  • Dynamic Column Breakthrough (DCB): Used to measure equilibrium adsorption capacity at 400 µL L⁻¹ CO2 in helium, simulating DAC conditions.
  • Thermogravimetric Analysis (TGA): Used to estimate amine loading (via mass loss) and measure adsorption capacity at higher concentrations (5% CO2 in N2) to simulate flue gas conditions.
  • Fourier-Transform Infrared Spectroscopy (FTIR): Employed to monitor surface chemistry changes and degradation products.
  • Scanning Electron Microscopy (SEM): Utilized to examine morphological changes in the silica substrates and grafted amines.

Key Results
The study revealed significant differences in the stability and performance of the sorbents based on the amine type, the silica support, and the CO2 concentration.

1. Concentration-Dependent Degradation: A critical finding is that the adsorption capacity at low concentrations (400 µL L⁻¹) generally decreases faster than the capacity at high concentrations (5%) with increased thermal cycling. For instance, DAS/MCM-41 maintained much of its capacity at 5% CO2 even after 200–300 cycles, while its capacity at 400 µL L⁻¹ showed a marked decrease in the same interval.
2. Support Material Robustness: SBA-15 based sorbents were generally more robust than MCM-41 based sorbents.
   • DAS/SBA-15: Showed high robustness, surviving 200 cycles with no significant capacity loss, though it dropped by 30% after 300 cycles.
   • DAS/MCM-41: Less robust, losing 61% of its capacity by 200 cycles and becoming nearly deactivated by 300 cycles.
3. Amine Stability: DAS-derived sorbents were generally more robust than TAS-derived sorbents.
   • TAS/SBA-15: Showed a rapid decrease in capacity starting at 100 cycles and was nearly deactivated by 200 cycles.
   • TAS/MCM-41: Exhibited the lowest initial capacity (near detection limits) and rapid deactivation, with significant decreases observed after just 100 cycles.
4. Chemical and Physical Changes: TGA indicated that fresh samples contained moieties unstable to oxidation or heating, evidenced by a large initial capacity drop in high-concentration tests not seen in low-concentration tests. FTIR spectra showed an overall decrease in magnitude in hydrocarbon regions (>2750 cm⁻¹) with cycling but remained largely consistent with previous studies on SCEI/PEI systems. SEM analysis was used to probe morphology, though specific degradation mechanisms were not fully elucidated beyond the observed capacity losses.

Significance and Claims
The authors state that the primary goal of this work was not to develop a new, high-performance DAC material, but rather to examine the behavior of known SCEI compounds under a novel, rigorous testing procedure involving repeated thermal cycling in an oxidative environment. The study highlights that:

• Low-concentration adsorption capacity is a more sensitive indicator of sorbent degradation than high-concentration capacity.
• The choice of silica support (SBA-15 vs. MCM-41) and amine chain length (DAS vs. TAS) significantly impacts the thermal stability of grafted sorbents.
• DAS grafted onto SBA-15 offers the most robust performance among the tested configurations for potential DAC applications, though even this system shows degradation after extensive cycling.
• The observed rapid initial capacity loss in fresh samples at high concentrations suggests the presence of unstable surface moieties that react with CO2 but are susceptible to thermal or oxidative degradation.

The paper concludes that while these specific grafted systems may not yet be suitable as standalone functional DAC sorbents due to their relatively low capacities and degradation over time, the methodology provides a critical framework for evaluating the durability of amine-based sorbents under conditions relevant to direct air capture.