(6m) Carbon Materials As High-Efficiency and Metal-Free Catalyst for the Synthesis of Cyclic Carbonates from CO2 and Epoxides

Carbon materials as high-efficiency and metal-free catalyst for the synthesis of cyclic carbonates from CO₂ and epoxides

Dong-Hui Lan¹, Lang Chen¹, Chak-Tong Au^1,2, Shuang-Feng Yin^{1 *}

¹State Key Laboratory of Chemo/Biosensing and Chemometrics,College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082,

Hunan, China

² Department of Chemistry, Hong Kong Baptist University, Kowloon Tong,

Hong Kong

E-mail: sf_yin@hnu.edu.cn

From the viewpoints of “green chemistry” and “atom economy”, it is attractive to utilize CO₂ to replace harmful reactants such as CO and phosgene for the generation of cyclic carbonates. Ionic liquids (ILs) as metal-free catalysts for the cycloaddition of CO₂ to epoxides are mostly poor in activity. For good performance, the use of ILs is commonly accompanied with a co-catalyst [1].

Recently we combined Bu₄NBr with graphene oxide (GO) and found that the binary system is highly efficient towards the cycloaddition of CO₂ to propylene oxide (PO) at room temperature and atmospheric pressure [2]. The synergetic action between a Lewis acid and nucleophilic anion results in enhanced catalytic activity. With the realization of the effects of basic sites on CO₂ adsorption and activation, we designed a Bu₄NBr/Acid-Base bi-functionalized C₃N₄ system to catalyze the cycloaddition reaction. The catalytic activity can be improved by increasing the number of acid sites on C₃N₄ through the control of phosphorus amount.

Since the recycle of Bu₄NBr is complex, it is desirable to immobilize the quaternary ammonium salt on a carbon material. By silylanization of GO with chlorine-terminal silanes followed by subsequent nucleophilic substitution reaction with N,N'-dimethylethylenediamine in a one-pot approach, we prepared GO functionalized with silanol group, amine and quaternary ammonium salts [3]. The non-toxic, metal-free, water-tolerant, and high-efficiency catalyst shows a TOF of 46.4 h^-1 under mild condition, which is the highest among the reported CN-based heterogeneous catalysts [4]. The excellent performance is attributed to the synergetic effect of silanol group and halide anion for epoxide ring opening as well as the ability of amine for the adsorption and activation of CO₂. The catalyst can be easily separated and reused for at least five times without significant loss of activity.

[1] W.Y. Gao, Y. Chen, S.Q. Ma, et al. Angew. Chem. Int. Ed., 2014, 53, 2615-2619.

[2] D.H. Lan, C.T. Au, S.F. Yin, et al. Carbon, 2014, 73, 351-360.

[3] D.H. Lan, L. Chen, S.F. Yin, et al. Carbon, 2015, 93, 22-31.

[4] J. Xu, F. Wu, Y.X. Li, et al. Catal. Sci. Technol., 2015, 5, 447-454.