Abdelhamid Sayari

This review comprehensively examines CO ₂ adsorption kinetics on amine-functionalized materials. It covers key experimental techniques, kinetic models, and the main factors governing CO ₂ diffusion and adsorption rates.

The production of hydrogen on-demand via the dehydrogenative coupling of hydrosilanes and alcohol-amines, conducted at room temperature without solvents nor catalysts, marks a significant advancement in the pursuit of greener energy sources. This work deals with the reaction mechanism of the dehydrogenative coupling of phenyl silane (PhS) and 2-(methylamino)-ethanol (MEA) as well as the interference of hydrolysis/condensation and disproportionation reactions when alkoxy-containing hydrosilanes, such as triethoxysilane (TES), are used. DFT methods were used to calculate the thermodynamic properties of the reactions involved (Δ_r G°, Δ_r H°, and Δ_r S°), the hydride donating ability of the reactants and intermediate products (HDA) as well as the Mulliken charges. These theoretical data complemented NMR and GC experiments to identify the most likely reaction pathway. Our results suggest the occurrence of multiple reaction pathways involving the nucleophilic attack of OH groups, and to a lesser extent NH groups, on the Si–H bond, leading to the formation of Si–O and Si–N containing products and the release of hydrogen. However, due to the low stability and high entropic contribution in the formation of Si–N containing products compared to their Si–O containing counterparts, their formation seems unfavorable, and no products containing only Si–N bonds were detected. However, the product with mixed Si–N and Si–O bonding, PhSiH­(O−)­(N−), was detected as a minor product. The high hydride donating ability of Si–O containing products led to such species undergoing consecutive nucleophilic attacks on their Si–H bonds by the hydroxyl group in MEA until complete dehydrogenation, ultimately leading to the formation of PhSi­(O−)₃ as the main reaction product.

Global warming has been exacerbated by the escalating emissions of greenhouse gases from fossil fuel power plants and other industrial and transportation sources. Postcombustion CO₂ capture by amine-containing materials has evolved into a burgeoning industry for mitigating point-source emissions. Nevertheless, the deployment of sustainable and biodegradable solid-supported amine adsorbents remains underexplored despite their tunable surface chemistry that can be leveraged for CO₂ adsorption. Therefore, the purpose of this study is to valorize lignin for CO₂ capture using a double amine functionalization method. Lignin was subjected to two separate grafting procedures: one with diethylenetriamine (DETA) to produce DETA-aminated lignin (DAL) and another procedure involving 3-aminopropyltrimethoxysilane (APTMS) and triaminosilane (TRI) to make APTMS- and TRI-grafted lignin. Subsequently, DAL was further grafted by TRI (TRI/DAL) to increase amine content and improve CO₂ capture. TRI/DAL was found to achieve a considerably higher CO₂ uptake than that of individually modified materials. In the presence of 15% dry CO₂/N₂ at 25 °C, CO₂ uptake reached 1.31 mmol/g, representing a 72% increase compared to DAL (0.76 mmol/g) and more than 250% of TRI/Lignin-1 (0.37 mmol/g). Humidity further promoted CO₂ capture, with a CO₂ uptake of 1.84 mmol/g and an amine efficiency of 0.43 mol CO₂/mol N at 55% relative humidity (RH). In addition to enhancing CO₂ uptake, the presence of moisture improved the cyclic stability of the material, with TRI/DAL retaining 97.5 and 95% of its initial capacity after ten cycles at 25 and 50 °C in the presence of 15% CO₂/N₂ with 55% RH.

GD-PEI/S showed 100% selectivity for SO₂ in the presence of CO₂, indicating its potential as an effective filter for desulfurization before CO₂ capture on amine-containing adsorbents.

This work is a mechanistic study of the CO₂ reaction with diamines under both dry and wet conditions. All protic α,ω-diamines R1H1N1-(CH₂)_n-N2H2R2, with n = 1-5 and R1 and R2 = H and/or CH₃, were investigated. Depending on the nature of the diamine, the reaction was found to follow one of two concerted asynchronous reaction mechanisms with a zwitterion hidden intermediate. Both mechanisms involved two processes. The first process consisted of a nucleophilic attack of the nitrogen N1 of the first amine group on the carbon of CO₂, accompanied by the transfer of a hydrogen atom H1 from N1 to the nitrogen N2 of the second amine group, leading to the formation of a carbamate zwitterion. The subsequent process corresponds to the transfer of a hydrogen atom H2 from the second amine group N2 to an oxygen atom of CO₂, thus ending the reaction by the formation of carbamic acid. The structure of the zwitterion hidden intermediate was determined using the reactive internal reaction coordinates (RIRC), a reaction pathway visualization tool, consisting of a 3D representation of the potential energy versus the internuclear distances N2-H1 and N2-H2, which correspond to the bond being formed and the bond being broken, respectively. The life span of the transitory species, i.e., the zwitterion, was found to depend on the nature of the second amine group. For primary amines, the life span of the zwitterion was "short", whereas for secondary amines, it was "long". The corresponding mechanisms were termed the "early" and "late" asynchronous mechanism, respectively. Regardless of the mechanism, the activation barriers were found to decrease with the length of the carbon chain linking the two amine groups, with an asymptotic behavior from n = 4. Involvement of a water molecule generates a significant catalytic effect for diamines with short carbon chains (n < 4), whereas for longer chain diamines, water has a slightly adverse effect.

Carbon dioxide capture is essential to achieve net-zero emissions. A hurdle to the design of improved capture materials is the lack of adequate tools to characterise how CO₂ adsorbs. Solid-state nuclear magnetic resonance (NMR) spectroscopy is a promising probe of CO₂ capture, but it remains challenging to distinguish different adsorption products. Here we perform a comprehensive computational investigation of 22 amine-functionalised metal-organic frameworks and discover that ¹⁷O NMR is a powerful probe of CO₂ capture chemistry that provides excellent differentiation of ammonium carbamate and carbamic acid species. The computational findings are supported by ¹⁷O NMR experiments on a series of CO₂-loaded frameworks that clearly identify ammonium carbamate chain formation and provide evidence for a mixed carbamic acid – ammonium carbamate adsorption mode. We further find that carbamic acid formation is more prevalent in this materials class than previously believed. Finally, we show that our methods are readily applicable to other adsorbents, and find support for ammonium carbamate formation in amine-grafted silicas. Our work paves the way for investigations of carbon capture chemistry that can enable materials design.