N-Heterocyclic Carbene Functionalized Polymer for Reversible Fixation−Release of CO₂

Abstract

Reversible fixation-release of CO 2 has attracted much attention during the last decades due to the economic and environmental benefits arising from the utilization of renewable resources and the growing concern on the greenhouse effect. 1-3 Numerous systems based on liquid primary or secondary amines have been developed for this process, in which CO 2 is chemically converted into ammonium carbamates or zwitterionic adducts at ambient temperature 4-6 and the fixed CO 2 is released upon heating. 7,8 Likewise, amino-functionalized synthetic polymers or mesoporous materials were also proved to be efficient in CO 2 capture. 9-11 Notably, ethylenimine functionalized mesoporous molecular sieve MCM-41 has higher absorption capacity than either pure polyethylenimine or MCM-41 alone and could be used as highly CO 2 -selective adsorbent for gas mixtures without the preremoval of moisture. 10 Endo and co-workers reported a new type of reversible CO 2 fixation by amidine derivatives and by polymers bearing an amidine moiety both in solution and solid state. 12 The cyclic amidine-functionalized copolymers exhibited better ability to retain CO 2 than corresponding low-molecular weight amidine at ambient temperature and could be applied to reversible fixation-release of CO 2 . The CO 2 fixation by polymers in the solid state may be one of the most simple, economic and effective methods for CO 2 recovery, though the fixing efficiency is relatively low.During the past decade, N-heterocyclic carbenes were studied extensively as versatile ligands 13 and effective organocatalysts. 14 Because of their high basicity, 15 N-heterocyclic carbenes can react rapidly with CO 2 to afford zwitterionic adducts (designated as NHC-CO 2 ), even at very low CO 2 concentrations. 16 In recent studies, we found that both the formation of NHC-CO 2 adducts at 20-50°C in nearly 100% yield and their complete decomposition into free N-heterocyclic carbenes and CO 2 at elevated temperatures were very fast (Figure 1). 17 These observation stimulated us to explore the feasibility of N-heterocyclic carbenes as highly CO 2 -selective adsorbent at low CO 2 concentration. Herein, we report a N-heterocyclic carbene-functionalized synthetic polymer for reversible fixation-release of CO 2 .The synthetic route of zwitterionic NHC-CO 2 adduct-functionalized copolymer (designated as P-NHC-CO 2 ) is shown in Scheme 1. Formation of quaternary ammonium by reaction of 1-(2,6-diisopropylphenyl) imidazole with styrene-4-vinylbenzyl chloride copolymer afforded imidazolium-modified copolymer P-NHC-HCl. Similar to the synthesis of 1-(2,6-diisopropylphenyl)-3-benzyl imidazolium-2-carboxylate (see Supporting Information), P-NHC-CO 2 was readily prepared by deprotonation of P-NHC-HCl using KN(SiMe 3 ) 2 , and followed by reaction with CO 2 . The IR spectrum of the resulted polymer showed a broad absorption peak around 1649 cm -1 , which is assigned to the asymmetric v(CO 2 ) vibrations of NHC-CO 2 adduct moiety. The content of immobilized N-heterocyclic carbene sites could be c...

Imidazolium Ionic Liquids, Imidazolylidene Heterocyclic Carbenes, and Zeolitic Imidazolate Frameworks for CO2 Capture and Photochemical Reduction

“…Ying and co-workers reported the first metal-free reduction of CO 2 to CH 3 OH catalyzed by NHCs,w ith silanes as the reductants and hydride donors (Scheme 7). [41].AIBN = 2,2'-azobis(isobutyronitrile). This result Figure 5.…”

“…[39] Recently,A shfeld and co-workers executed DFT calculations and experimental measurements to investigate the binding of CO 2 to an imidazolylidene NHC,a nd thus demonstrated rapid and stoichiometric solid-phase covalent CO 2 capture at arelatively low pressure. [41] Thezwitterionic polymer NHC-CO 2 adduct was formed after rapid reaction between CO 2 and the immobilized NHC at 20-100 8 8C. [41] Thezwitterionic polymer NHC-CO 2 adduct was formed after rapid reaction between CO 2 and the immobilized NHC at 20-100 8 8C.…”

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.…”

CO2-responsive polymeric materials: synthesis, self-assembly, and functional applications

“…264 After bubbling CO 2 , the solution created a new solvent that accomplished the View Article Online dissolution of up to 10 wt% of cellulose in mild conditions. 265 The synthesized polymer (Fig. 264 An N-heterocyclic carbine-functionalized polymer was synthesized as a highly efficient selective adsorbent for reversible fixationrelease of CO 2 .…”