Accelerating the Development of Advanced Silica-alkoxylated Polyethyleneimine (APEI) Adsorbents for Direct Air Capture (Flexible Funding 2024)

Dr Wei Li, University of Nottingham, was awarded funding in the UKCCSRC’s Flexible Funding 2024 call to look at “Accelerating the Development of Advanced Silica-alkoxylated Polyethyleneimine (APEI) Adsorbents for Direct Air Capture”.

Project Overview

Effective carbon capture and storage (CCS) technologies are urgently needed to limit global warming and mitigate the adverse effects of climate change. Among CCS approaches, Direct Air Capture (DAC) stands out for its flexibility and potential to remove CO₂ directly from the atmosphere. However, current DAC technologies remain costly due to the extremely low atmospheric CO₂ concentration (~0.04%) and the limited development of efficient adsorbents. To enhance the viability of DAC, it is essential to overcome the critical challenge of developing cost-effective and high-performance adsorption materials. The goal of the project was to optimise alkoxylated polyethyleneimine (APEI) for DAC applications by addressing two challenges:

1) Material stability vs. performance – Developing a new evaluation approach and modelling tools to optimise APEIs for DAC conditions using the developed model that balances CO₂ adsorption efficiency with long-term oxidative stability.

2) Silica support optimisation – Selecting and preparing silica supports with suitable pore structures and particle sizes, ensuring the material performs effectively in fixed-bed DAC systems and is scalable for real-world applications.

What We Did

Specifically, our project focused on overcoming the above two major challenges. We aimed to optimise APEI adsorbents by tuning key parameters: alkoxylation degree (AD), molecular weight, and PEI loading on silica, and by modelling their long-term oxidative degradation under realistic DAC conditions. To evaluate and optimise APEIs for DAC, we developed comprehensive adsorbents with varying ADs, PEI loadings, and molecular weights. These materials were tested for CO₂ adsorption performance, followed by long-term oxidation testing for up to 60 days at 50–80 °C. After oxidation, their remaining CO₂ adsorption capacities were measured under both post-combustion capture (PCC) and DAC conditions to assess performance retention.

To track performance evolution over time, periodic CO₂ adsorption measurements were conducted. A range of degradation models was applied to evaluate long-term stability. Among them, Avrami–Arrhenius kinetic models proved most effective, offering accurate predictions of degradation mechanisms and capturing non-linear behaviours. This provided valuable insights into the materials’ long-term reliability. Additionally, we developed a new evaluation metric, cumulative CO₂ uptake (see Figure 1), over the adsorbent’s various service life, moving beyond traditional single-time-point measurements. This approach enabled more meaningful comparisons between APEIs and helped define optimal end-of-life performance thresholds.

We also focused on identifying and validating silica supports suitable for large-scale APEI impregnation, with particular attention to pore structure and particle size, both of which are critical for efficient fixed-bed DAC operation. Up to 10 types of mesoporous silica were screened, and their pore volume, surface area, and particle size were evaluated as factors known to significantly influence APEI loading capacity and mechanical flow properties. Among these, PQ10 silica was selected as the optimal support due to its high pore volume and large particle size.

A 25 kg batch of PQ10 was sieved to isolate particles larger than 300 µm. These retained excellent textural properties post-sieving and were used to produce high-loading silica-APEI adsorbents. To validate performance, larger scale adsorbents (over 2 kg) were prepared and compared against lab-scale samples. The sieved PQ10-based adsorbents performed reliably in both cases, confirming their suitability for scale-up and industrial deployment in DAC applications.

Figure 1. CO2 adsorption capacity at 50 °C after oxidation at 80 °C for 10 days, comparing the performance of FG (PEI) and APEI samples. The shaded area under the FG curve represents the cumulative CO2 adsorption capacity during the 10-day oxidation period. The full cumulative data for other APEI formulations are not shown here but will be provided in our forthcoming publication.

Key Findings

We observed a clear trade-off between oxidative stability and CO₂ adsorption performance as the alkoxylation degree (AD) increased. While a higher AD enhanced oxidative resistance (i.e. prolonged operational life), it also led to a reduction in initial CO₂ capture capacity.

The Avrami–Arrhenius model provided an accurate fitting for the complex degradation behaviour of APEIs. Using cumulative CO₂ uptake as the evaluation metric, we found that medium alkoxylation levels offered the best overall balance between long-term oxidative stability and CO₂ adsorption performance.

PQ10 silica proved to be a practical and scalable support, with a high proportion of large particle sizes. Its structural integrity and adsorption consistency post-sieving confirmed its suitability for industrial-scale DAC applications. Its ability to support high APEI loadings and maintain performance under DAC-relevant conditions highlights its commercial potential.

Conclusion

The optimised APEIs and the selected PQ10 silica support are highly suitable for fixed-bed DAC operation. Combined with the advanced degradation modelling approach and the use of cumulative CO₂ adsorption performance as a robust evaluation metric, this project presents a reliable method for selecting silica-APEIs tailored to specific application conditions, balancing both adsorption capacity and long-term stability.

Read more on Wei’s Flexible Funding 2024 project page.