Unveiling CO2 chemisorption mechanisms in solid adsorbents via surface-enhanced ex(in)-situ NMR

Project Information

NMR4CO2

Grant agreement ID: 865974

DOI

10.3030/865974

EC signature date 19 December 2019

Start date 1 June 2020

End date 30 November 2026

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 999 793,00

EU contribution

€ 1 999 793,00

Coordinated by

UNIVERSIDADE DE AVEIRO
Portugal

Periodic Reporting for period 4 - NMR4CO2 (Unveiling CO2 chemisorption mechanisms in solid adsorbents via surface-enhanced ex(in)-situ NMR)

Reporting period: 2024-12-01 to 2025-11-30

Summary of the context and overall objectives of the project

Although the use of non-carbon energy technologies is increasing, we are still far from matching the pace of the growing population’s energy demand. Global energy-related CO2 emissions rose by 6% in 2021, reaching a historic high of 36.3 gigatones (Gt). Flue gas streams are among the most polluting gas mixtures, essentially comprising CO2 (15%), N2 (75%) and water. Adsorption is key for separating CO2 from mixtures, such as flue gas (CO2/N2, CO2/H2) or biogas (CO2/CH4). Even when strengthening pre-2030 mitigation action is combined with very stringent long-term policies, cumulative CO2 emissions from fossil fuel will remain at 850-1150 Gt during 2016-2100. Thus, 640-950 GtCO2 removal is required for limiting end-of-century warming to 1.5 ºC.
CO2 capture from large point sources, which account for ~50% of released CO2, is identified as a major solution to mitigate CO2 emissions. Post-combustion capture using aqueous solutions has been used commercially, however costs are high and there are corrosion and health/environment risks. To overcome such limitations, solid adsorbents have been widely investigated showing reduced regeneration energy, greater CO2 capacity and selectivity with the ability to operate over a wider range of temperatures. A plethora of solid adsorbents has emerged including activated carbons, ion-exchange resins, zeolites, porous silicates, metal oxides, organic-inorganic hybrid sorbents and composite materials.

NMR4CO2 encompasses the design of novel solid-state NMR methods to study the CO2 adsorption process in amine-modified porous silicas, comprising in-situ flow NMR, dynamic nuclear polarization NMR, and isotopically labelled gas mixtures. Important outcomes include: i) identification of competing CO2 chemisorption pathways; ii) effect on CO2 speciation of textural properties, amine type, inter-amine spacing, and amine-amine/amine-support cooperative effects; iii) real-time monitoring of acid gas speciation in multiple adsorption/desorption cycles; iv) identification of sorbent deactivation species; v) effect of pressure on CO2 speciation and vi) improvement of sorbent properties by synthetic modification.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Since the beginning of the project, the team has performed comprehensive studies towards an atomic-level understanding of CO2 speciation and CO2 adsorption mechanisms in silica sorbent materials. A short list of the main results is listed below:

- Understand the influence of water in CO2 speciation on the surface of porous aminosilicas. Mesoporous silicas functionalised with primary and tertiary amines were prepared and dosed with 13CO2 under dry and wet conditions and the structure of CO2 species formed was monitored by ssNMR. The work was complemented with computational studies, at the DFT level, in which several molecular models of amine-grafted silicas were explored, each presenting distinct intermolecular interactions arrangements. The applied method avoids the use of “slurries”, has performed previously in other studies found in the literature, allowing a detailed molecular-level understanding of the CO2 capture process, assessing the influence of a common contaminant – water – in the CO2 capture process (even in the presence of very small amounts ca. 0.7 kPa). Water is an important component of combustion gases, taking a decisive role in the CO2 speciation and as such, the studies performed in the scope of this project have allowed the team to achieve a better understanding of the structure of CO2 species formed in such materials under moist conditions, including their relative stability in the presence or absence of water. Furthermore, we have also introduced a third entity into the system by studying the CO2 adsorption in the presence of CH4, under dry and moist conditions.

- Probe molecular dynamics of physisorbed and chemisorbed CO2 species in amine-modified SBA-15. T1ρ measurements were performed and the main interactions responsible for the relaxation mechanisms have been characterised, allowing the extraction of 13C-1H distances and 13C chemical shift anisotropy parameters for each distinct CO2 species.

- Quantify, by means of ssNMR, 3 different physisorbed CO2 species, formed on the surface of amine-functionalized mesoporous silica SBA-15. The distinction and quantification of such species was performed by a combination of relaxation experiments (longitudinal relaxation times T1) and 13C CSA parameters. With the data retrieved from NMR, it was possible to propose a distribution model for both physi- and chemisorbed CO2 species, in the channels of the studied solid sorbent.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Two of the project’s major achievements so far were not expected in the Description of the Action and clearly go beyond the state of the art, namely: the valorisation of Crab shells as potential CO2 adsorbent materials and the one-shot resin 3D printing of NMR probe components.

- Part of our efforts have focused on the use of biomass wastes from crab shells to develop a low-cost and eco-friendly CO2 adsorbent. The rationale for this work was centered on the use of mild treatments to preserve the natural porous structure of crab shells. The effect of various operating conditions on the CO2 adsorption has been investigated by solid-state NMR, and the results correlated with the textural properties of the sorbents (specific surface area, pore-volume, nitrogen content and chemical species) to fully understand its relationship with CO2 adsorption ability.
The valorization of waste biomaterials and porous materials based on polysaccharides is clearly an unexpected direction our team is taking. These classes of sorbents show great potential for CO2 capture and we are now exploring the viability of this CO2-sorbents through different processing and chemical modification approaches. These materials were not initially proposed in the ERC project but are now central to our research with a dedicated PhD (Daniel Pereira) working on this topic.

- We have been able to fabricate and test a one-shot fully 3D-printed stator for 4.0 mm rotors using stereolithography printing technology. A radio-frequency solenoid coil was also built in-house, through a method easily implemented using conventional materials found in hardware stores. The 3D-printed stator, equipped with a homemade RF coil, shows remarkable spinning stability, yielding high quality NMR data. The overall cost of the resin 3D-printed stator with a homemade RF coil is approximately 5 € per stator, representing a cost reduction of over 99% relative to commercial options. This study demonstrates the potential of 3D-printing as a tool to mass-produce MAS stators, offering a fast and low-cost alternative to traditional manufacturing methods. The use of 3D printing methodologies will likely enable the group to fulfil goal 1, by allowing custom-made components to be designed and printed to study the interactions between CO2 and sorbet materials by in-situ flow MAS NMR. Further developments in this area are expected.

- We envisage to be the first group to embed mono and diradical molecules in porous mesoporous organosilicas to attempt the in-situ detection of confined CO2 species.

- We plan to find at the molecular-level the sorption and desorption mechanisms of CO2 aided by novel computational approaches. We also plan to further elucidate the mechanisms of sorbent degradation using a combination of spectroscopic and thermogravimetric techniques.

Overview of the project