A Novel Construction of a Reversible Fixation−Release System of Carbon Dioxide by Amidines and Their Polymers

Abstract

Development of efficient methods for CO 2 recovery from industrial waste gases etc. is extremely important in relation to both reutilization of CO 2 as carbon resources 1 and environmental issue concerned with the greenhouse effect. 2 One of the most commonly used processes for CO 2 recovery is chemically reversible CO 2 fixation by primary or secondary amines based on CO 2 fixation by amines at room temperature to give ammonium carbamates, and CO 2 release from ammonium carbamates upon heating. 3 Application of CO 2 fixation in functional polymers has been also examined, 4 for instance, copolymers of styrene bearing pendant amino groups fixed CO 2 under ambient conditions. 4b A more attractive process may be CO 2 fixation by tertiary amines giving zwitterion adducts that may provide a more easily handled fixation-release treatment, since these zwitterions could release CO 2 at reduced temperatures owing to their lability (primary-(secondary) amines; at >100°C). Furthermore, this process can provide zwitterions having a unique reactivity. However, detailed studies concerning CO 2 fixation by tertiary amines and its application to functional polymers have not been carried out. Here we report a new type of reversible CO 2 fixation by amidine derivatives and by polymers bearing an amidine moiety both in solution and solid state.In the course of the study of CO 2 fixation by tertiary amines, there has been one example of CO 2 fixation by 1,8-diazabicyclo[5.4.0]-undec-7-ene (DBU) to afford the zwitterion adduct. 5 Assuming the reasons why DBU can react with CO 2 are its high nucleophilicity and stabilization of the cation species by delocalization in the amidine moiety, we constructed the idea that amidine derivatives with higher nucleophilicity may provide a success of CO 2 fixation. N-Methyltetrahydropyrimidine (MTHP) containing an amidine structure was synthesized, 6 since it would have higher nucleophilicity due to decrease in the steric hindrance around the nitrogen atom of imine moiety. CO 2 fixation using MTHP was performed in N,N-dimethylformamide (DMF) at 25°C under atmospheric pressure. 7 Fixing efficiency (%, mmol of CO 2 /mmol of MTHP) was estimated from the weight increase in the reaction mixture. When CO 2 was bubbled into a DMF solution of MTHP, a white precipitate was formed immediately, and the weight increase in the reaction mixture ceased after 1 h to afford the corresponding zwitterion adduct (MTHP)CO 2 ) in quantitative fixing efficiency. On the other hand, DBU needed 100 h to fix CO 2 in 89% fixing efficiency, 8 thus, MTHP proved to be an excellent agent for CO 2 fixation. The IR spectrum of MTHP)CO 2 showed two absorption bands assignable to the CO 2 moiety at 1689 and 1389 cm -1 . The 1 H NMR spectrum of MTHP)CO 2 showed that the signal of the imine proton was shifted to lower field (0.57 ppm) compared with MTHP and the 13 C NMR spectrum showed a signal attributable to the carbonyl group at 161.3 ppm.Reversibility of CO 2 fixation by MTHP was examined ( Figure 1): The obtained zwitterion addu...

Structures and Synthesis of Zwitterionic Polymers

“…Similar considerations apply to the addition of CO 2 to polymeric amidines, or to polymeric carbenes obtained from imidazolium salts (Figure 17c,d) [441][442][443][444]. Although the last reaction seems to be limited to non-aqueous systems, in any case, such chemically induced "switching" systems have been hardly studied up to now, so that they cannot be adequately judged yet.…”

“…Hence, addressing the open merocyanine form as "zwitterionic" is misleading, as it typically shows a strong dipole moment only but does not dispose of ionic sites. [128,445]; (b) via bisulfite-aldehyde addition [425,446]; (c) and (d) via addition of CO 2 to amidines [441] or carbenes [442,443], respectively. Redox reactions offer better chances for implementing a switching process, though reversible organic redox systems, for which the oxidized as well as the reduced states are stable under aqueous conditions, are rare.…”

Switchable Materials Containing Polyzwitterion Moieties

“…Amidine is a promising functionality for the addition of CO2 to form a polyzwitterionic structure of poly (72). Several types based on a polystyrene backbone were synthesized [108,109]. Nowadays, the fixation or capture of CO2 is of particular interest due to its environmental and economic aspects.…”

New Tools for CO2 Fixation by Homogeneous Catalysis - Final Technical Report

“…We showed 19 that the reaction between amidines and CO 2 (equation 5), which has been widely published in the literature, [20][21][22][23][24][25][26][27][28] is incorrect. Amidines have been reported to react with CO 2 to form a zwitterionic adduct but the literature studies were performed in the presence of at least some water.…”