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A Unified Approach to CO2-Amine Reaction Mechanisms.

ABSTRACT: A unified CO2-amine reaction mechanism applicable to absorption in aqueous or nonaqueous solutions and to adsorption on immobilized amines in the presence of both dry and humid conditions is proposed. Key findings supported by theoretical calculations and experimental evidence are as follows: (1) The formation of the 1,3-zwitterion, RH2N+-COO-, is highly unlikely because not only the associated four-membered mechanism has a high energy barrier, but also it is not consistent with the orbital symmetry requirements for chemical reactions. (2) The nucleophilic attack of CO2 by amines requires the catalytic assistance of a Bro?nsted base through a six-membered mechanism to achieve proton transfer/exchange. An important consequence of this concerted mechanism is that the N and H atoms added to the C=O double bond do not originate from a single amine group. Using ethylenediamine for illustration, detailed description of the reaction pathway is reported using the reactive internal reaction coordinate as a new tool to visualize the reaction path. (3) In the presence of protic amines, the formation of ammonium bicarbonate/carbonate does not take place through the widely accepted hydration of carbamate/carbamic acid. Instead, water behaves as a nucleophile that attacks CO2 with catalytic assistance by amine groups, and carbamate/carbamic acid decomposes back to amine and CO2. (4) Generalization of the catalytic assistance concept to any Bro?nsted base established through theoretical calculations was supported by infrared measurements. A unified six-membered mechanism was proposed to describe all possible interactions of CO2 with amines and water, each playing the role of a nucleophile and/or Bro?nsted base, depending on the actual conditions.

SUBMITTER: Said RB

PROVIDER: S-EPMC7557993 | biostudies-literature | 2020 Oct

REPOSITORIES: biostudies-literature

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A Unified Approach to CO2-Amine Reaction Mechanisms.

Said Ridha Ben RB Kolle Joel Motaka JM Essalah Khaled K Tangour Bahoueddine B Sayari Abdelhamid A

ACS omega 20201001 40

A unified CO2-amine reaction mechanism applicable to absorption in aqueous or nonaqueous solutions and to adsorption on immobilized amines in the presence of both dry and humid conditions is proposed. Key findings supported by theoretical calculations and experimental evidence are as follows: (1) The formation of the 1,3-zwitterion, RH2N+-COO-, is highly unlikely because not only the associated four-membered mechanism has a high energy barrier, but als ...[more]

PMID: 33073140

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Project description:Rising CO2 emissions are responsible for increasing global temperatures causing climate change. Significant efforts are underway to develop amine-based sorbents to directly capture CO2 from air (called direct air capture (DAC)) to combat the effects of climate change. However, the sorbents' performances have usually been evaluated at ambient temperatures (25 °C) or higher, most often under dry conditions. A significant portion of the natural environment where DAC plants can be deployed experiences temperatures below 25 °C, and ambient air always contains some humidity. In this study, we assess the CO2 adsorption behavior of amine (poly(ethyleneimine) (PEI) and tetraethylenepentamine (TEPA)) impregnated into porous alumina at ambient (25 °C) and cold temperatures (-20 °C) under dry and humid conditions. CO2 adsorption capacities at 25 °C and 400 ppm CO2 are highest for 40 wt% TEPA-incorporated γ-Al2O3 samples (1.8 mmol CO2/g sorbent), while 40 wt % PEI-impregnated γ-Al2O3 samples exhibit moderate uptakes (0.9 mmol g-1). CO2 capacities for both PEI- and TEPA-incorporated γ-Al2O3 samples decrease with decreasing amine content and temperatures. The 40 and 20 wt % TEPA sorbents show the best performance at -20 °C under dry conditions (1.6 and 1.1 mmol g-1, respectively). Both the TEPA samples also exhibit stable and high working capacities (0.9 and 1.2 mmol g-1) across 10 cycles of adsorption-desorption (adsorption at -20 °C and desorption conducted at 60 °C). Introducing moisture (70% RH at -20 and 25 °C) improves the CO2 capacity of the amine-impregnated sorbents at both temperatures. The 40 wt% PEI, 40 wt % TEPA, and 20 wt% TEPA samples show good CO2 uptakes at both temperatures. The results presented here indicate that γ-Al2O3 impregnated with PEI and TEPA are potential materials for DAC at ambient and cold conditions, with further opportunities to optimize these materials for the scalable deployment of DAC plants at different environmental conditions.

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A Unified Approach to CO2-Amine Reaction Mechanisms.

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A Unified Approach to CO<sub>2</sub>-Amine Reaction Mechanisms.

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