Study On Activation And Catalytic Conversion Of CO₂ By Nitrogen Heterocyclic Carbenes

With the rapid development of modern industry, CO2 emissions gradually increase which seriously affects our lives and production. Also, CO2 is well believed to be the new carbon source due to its low price, no toxicity and extremely abundant distribution. Utilization of CO2 not only could solve the environmental problems caused by the greenhouse effect, but also could solve the exhaustion of resources. CO2 is highly thermodynamically stable and kinetically inert, so the key of the chemical utilization of CO2 is it's activation. Due to the electron deficiency of carbonyl carbons, CO2 has strong affinity toward nucleophiles and electron-donating reagents.In recent years N-heterocyclic carbenes (NHCs) have evoked considerable interest, and this should thank to Arduengo et al., who first isolated a stable imidazol-2-ylidene in 1991. NHCs show the remarkable electron-donating properties due to inductive effect and conjugated effect. Besides their roles as excellent ligands for transition metal catalysts, organocatalytic carbene catalysis has emerged as an exceptionally fruitful reasearch area in synthetic organic chemistry. NHCs could effectively activate CO2 to form stable NHC-CO2 adducts, but the properties of NHC-CO2 adducts and CO2 chemical conversion catalyzed by NHCs are rarely concerned.1. In this dissertation, thermal stability of NHCs-CO2 adducts was investigated by in-situ temperature controlled high pressure FT-IR. NHCs-CO2 adducts possessing low steric hindrance of the N-alkyl substituent displayed higher decarboxylation temperature. Conversely, NHCs-CO2 adducts possessing highly steric hindrance of the N-aryl substituent displayed lower decarboxylation temperature. Unsaturated NHCs-CO2 adducts showed lower thermal stability than their saturated analogue.With the increase of temperature, thermal stability of NHCs-CO2 adducts obviously decrease. The presence of free CO2 can effectively inhibit the decomposition of NHCs-CO2 adducts.2. NHC-functionalized mesoporous molecular sieve MCM-41 (MCM-41-NHC) and styrene-co-vinyl- benzyl chloride copolymer (P-NHC) were designed on the basis of the study of thermal stability of NHCs-CO2 adducts. MCM-41-NHC and P-NHC, as novel CO2-selective adsorbent, were used for reversible fixation-release of CO2. In situ diffuse reflectance infrared fourier transform spectroscopy demonstrated that MCM-41-NHC could effectively fix CO2 at 40℃and release trapped CO2 at 180℃. The CO2 fixation-release behaviors of MCM-41-NHC are reversible.The reversible CO2 fixation-release behaviors of P-NHC were studied by thermogravimetric analysis. When P-NHC was exposed to CO2 atmosphere with a CO2 flow (10 mL/min) at 40℃for 60 min,57% CO2 fixing efficiency was observed on the basis of the weight increase. As compared with the previously reported amine-functionalized polymers, P-NHC exhibited higher CO2 fixing efficiency in the much shorter time. P-NHC is a recyclable CO2 fixation material and 42% CO2 fixing efficiency was obtained for the second CO2 capture. High CO2 fixing efficiency of P-NHC was discussed by quantum chemical calculation.3. New application of NHCs was explored as organocatalyst for cycloaddition reaction of CO2 and epoxides. NHCs-CO2 adducts, as NHCs precusor, were proved to be effective organocatalysts for the cycloaddition reaction of CO2 and monosubstituted terminal epoxides to afford cyclic carbonates with high selectivity. IPr-CO2 with the lowest thermostability exhibits excellent activity among NHCs-CO2 adducts and affords cyclic carbonate with 100% yield at 120℃for 24 h (Optimum reaction conditions:0.5 mol% IPr-CO2,50mmol Propylene oxide,2 mL CH2Cl2,2.0 MPa CO2). Under the same conditions, IPr-CO2 can effectively catalyze the cycloaddition reaction of other monosubstituted epoxides with CO2 to afford corresponding cyclic carbonates in good to excellent yields (>90%) with 100% selectivity which exhibits the highest catalytic activity for the cycloaddition reaction amongst the reported organocatalysts.The presence of an electrophile such as SalenAlEt could greatly improve the catalytic activity of IPr-CO2 due to intermolecular cooperative catalysis of the binary components. Under the same conditions, the reaction only need 8 h to afford propylene carbonate in 100% yield. Possible mechanism was thoroughly studied on the basis of the reaction of trans-deuterioethene oxide with CO2 and in-situ FT-IR. Epoxide is first activated by its coordination to the central Al3+of SalenAlEt, then is ring-opened by nucleophilic attack of NHC-CO2 adducts or free NHCs at the less substituted C-O bond, and further reacts with CO2 to afford the corresponding cyclic carbonates.Heterogeneous MCM-41-NHC catalyst was evaluated in the cycloaddition reaction of epoxides with CO2 and reused by simple filtration. The yield of propylene carbonate is lower than that obtained with the homogeneous IPr-CO2 catalyst at the same conditions, suggesting that the supported catalyst suffer from diffusion resistance. MCM-41-IPr was reused three time without significant loss in catalytic activity.4. NHC-CS2 functionalized highly porous MOF was successfully synthesized by heating DMF/CH3OH solution of Zn(ClO4)2.6H2O and NHC-CS2 functionalized linear bidentate carboxylate-bridged linker in sealed vials. MOF-NHC-CS2 was structurally characterized by elemental analyses, IR, XRPD, TG and single crystal X-ray diffraction. Crystal data for MOF-NHC-CS2:cubic, space group Fm-3m, a=b=c=39.7719 A,α=β=γ=90°, V=62911.449 A3, Z=8. Single crystal analysis study could not be obtained due to lower completion of crystal data resulted from large pore size contained lots of disordered DMF solvent molecules.