Study On Novel Carbon-based Catalysts For Synthesis Of Cyclic Carbonates Through The Cycloaddition Of CO₂

From the viewpoints of “sustainable development strategies” and “green chemistry”, it is desirable to utilize CO2 as safe, abundant, inexpensive, and renewable C1 for the replacement of harmful reactants such as CO and phosgene to obtain valuable chemical products. The synthesis of cyclic carbonates via the cycloaddition of CO2 to epoxides is one of the few processes that had been commercialized. Industrially, metal salts and quaternary ammonium salts are commonly used for the synthesis of cyclic carbonates from CO 2 and epoxides in the batch reactor. Nonetheless, there are drawbacks such as harsh reaction conditions, unsatisfactory activities, the need of a co-catalyst and/or solvent, difficult for purification of cyclic carbonates and separation of catalysts. So developing novel, cheap, green, stable, recyclable, high-efficiency and selective heterogeneous catalysts is the key issue in cycloaddition reaction. Moreover, the study on the relationship between catalytic performances with composes and structure of catalysts as well as revealing possible mechanism is the difficult point.In this regard, based on the basis conception of the synergistic effects a mong Lewis acid sites(metal center and hydrogen bond donors), halogen anions and basic sites(amine species) that could enhance the cycloaddition reaction, a series of novel and efficient carbon based catalysts were developed. Listed below are the main research contents and innovative results.1. Inspired by the complexation of crown ether for K+ resulted in the improvement of the nucleophilicity of I– that enhanced the activity of KI, a series of non-toxic and stable bi-functional MX2-EDTA-n Na(M = Zn, Mg, Ba, Ca, Fe, Co, Ni;X = Cl, Br, I;n = 0, 2, 4) complexes were constructed by the coordination between metal halides and ethylenediaminetetraacetic acid sodium salt(EDTA-n Na) that based with cheap, stable, and excellent chelating properties. Compared wit h the activity of MX2-EDTA-n Na with different metal and halide, it was observed th at the strong acidity of metal as well as appropriate nucleophilicity and leaving ability of anions is favor for the cycloaddition reaction. In the case of using Zn Br 2-EDTA-2Na as catalyst, the selectivity and yield of propylene carbonate(PC) were 99.3% and 96.9%, showing a turnover frequency of 96.9 h–1. Once water molecules are sorbed in GO sheets, the intercalated water molecules between GO layers cannot easily escape from GO sheets which can reduce the concentration of water in the reaction system and enhance the adsortion and transmission of CO2 that may inhibite the hydrolysis of propylene epoxide(PO). In this regard, the composite of Zn Br2-EDTA-2Na and graphene oxide(GO) was designed. The stablility of Zn Br2-EDTA-2Na was improved in the presence of water, and the composite material performed no significant loss in selectivity and yield even with addition of 10 mol% H2 O.2. In the view of synergistic effect between acid sites and nucleophilic anions, acid sites were introduced into the basic g-C3N4 by doping P through facilely thermolysis of melamine and hexachlorotriphosphazene that avoid using heavy metal zinc. Based on the results of XPS and 31 P NMR analyses, it was deduced that there is replacement of C atoms located at the bay(P –OH) and edge terminal positions(P=O) by P atoms. As detected by NH3-TPD, it was observed that the catalytic activity of P-C3N4 increased with the rise of hexachlorotriphosphazene content as a result of the enrichment of acid sites. A possible multi-synergetic mechanism was proposed that referred to P–OH and Br– for the ring opening of epoxide as well as the tertiary amine and secondary amine in N=C–N(H) for the adsorption and activation of C O2.3. Based on the concept of synergistic effect of hydrogen bond donors and nucleophilic anions, the TBAB/GO binary catalytic system was designed for the cycloaddition reaction under metal- and solvent-free conditions. In the case of TBAB/GO, PO was rapidly converted to PC with 96% yield and 100% selectivity under relatively mild conditions(100 oC, 2.25 MPa, 1 h). The reaction rate over TBAB/GO was far more than that of TBAB/P-C3N4-2. In addition, 73.2% yield of PC was obained even at room temperature and atmospheric pressure over TBAB/GO. Compared with the activity of graphite, GO prepared by different methods, GO reduced by different reduce reagents, it was observed that the higher degree of reduction(i.e. larger C/O ratio) resulted in lower activity f or cycloaddition reaction owing to the decline of amount of oxygen functional groups. The hydrogen bond between the hydroxyl groups of GO and the oxygen of PO was determined by FT-IR. Moreover, a possible mechanism was proposed over TBAB/GO that referred t o the synergistic effects among hydroxyl groups, carboxyl groups and Br–. In addition, the influence of H2 O on the cycloaddition was studied, GO was found to be a stable and water-tolerant catalyst effective for the cycloaddition reaction.4. In order to solve the separation problem of quaternary ammonium salts, GO functionalized with silanol group, amine and quaternary ammonium salts(GO-DMEDA-I) was prepared by silylanization of GO with chlorine-terminal silanes and followed by subsequent nucleophilic su bstitution reaction with N,N’-dimethylethylenediamine(DMEDA) through a one-pot approach. Owing to the –NH2 in DMEDA transformed the epoxide group on GO to hydroxyl that enhanced the immobilization process, GO-DMEDA-I showed the largest amount of quaternar y ammonium salts and the best catalytic activity among the quaternary ammonium salts functionalized GO. Using PO/CO2 cycloaddition as model reaction, TOF was about 45.8 h–1 over GO-DMEDA-I. In addition, the GO-DMEDA-I catalyst was applicable to a variety of aliphatic terminal epoxides, showing no significant loss of catalytic activity across the five runs.5. In the view of the electrostatic interaction between GO and Vitamin B1(VB1), the composite of GO and VB1 derivatives were developed to avoid the complicated preparation process of functionalized GO as well as the limitation of low loading amount. It was found that the catalytic activity and the stability of VB 1 was improved by compositing with GO owing to the electrostatic interaction between GO and VB1 enhanced the nucleophilicity and the leaving ability of anion. In addition, the interaction between VB1 and substrates(CO2 and PO) was detected by UV-Vis and 1H-NMR, and the synergistic action referred multi-functional groups(hydroxyl, halide anion and amines) was suggested.