Hydration of Epoxides on [CoIII(salen)] Encapsulated in Silica-Based Nanoreactors

    Monoethylene glycol(MEG) and 1,2-propylene glycol(PG) are important raw materials for the manufacture of polyester resins, antifreezes, cosmetics, medicines, and other products.The total global demand for MEG has been estimated to be over 19 million metric tons per year. The production of MEG in industry predominantly involves the liquid-phase hydration of ethylene oxide(EO), whereby a large excess of water(20–25 mol of water/mol of EO) is required for the high conversion of EO and high selectivity for MEG. The concentration of MEG in the final aqueous solution is only approximately 10wt%, and huge energy is consumed for the distillation of the product from the aqueous solution. As a result, epoxide hydration is one of the most cost- and energy-intensive processes in the chemical industry.

    Possible catalytic hydration processes have been exten-sively investigated for the environmentally friendly produc-tion of MEG at a low energy cost. Various types of catalysts, including liquid and solid acids or bases, have been explored, such as sulfuric acid, the salts of some acids, cyclic amines, cation- and anion-exchange resins, quaternary phosphonium halides, polymeric organosilane ammonium salts, macrocyclic chelating compounds, and supported metal oxides. However, a high H2O/EO molar ratio(i.e.> 10) is still required for high MEG selectivity. The MEG selectivity is very low at low H2O/EO molar ratios owing to the formation of diethylene glycol(DEG) and triethylene glycol(TEG) by the self-condensation of MEG, which is readily catalyzed by acid and base catalysts. Moreover, the inherent corrosion and environmental problems associated with the liquid acids/bases limit their application in industry. The development of an efficient and environmentally benign process for the hydration of epoxides with an H2O/epoxide molar ratio approaching the stoichiometric value of the chemical reaction is still a huge challenge.

    In this study, we developed a solid catalyst that is different from the conventional liquid/solid acid or base catalysts for the hydration of epoxides. We constructed the solid catalyst by encapsulating the molecular catalyst [CoIII(salen)], with a salen ligand derived from 3,5-di-tert -butylsalicyclaldehyde and trans-1,2-diaminocyclohexane, in the nanocage of the mesoporous silica FDU-12. This catalyst exhibits high activity and selectivity in the hydration of epoxides under mild reaction conditions. Furthermore, the H2O/epoxide molar ratio can be decreased to as low as 2 :1 while maintaining the conversion of EO above 98 % and the selectivity for MEG above 98 %. This novel catalytic approach has great potential for the green and energy-saving hydration of epoxides, as well as many other conventional chemical reaction processes in industry.

Hydration of Epoxides on [CoIII(salen)] Encapsulated in Silica-Based Nanoreactors†
Dr. Bo Li‡, Shiyang Bai‡, Dr. Xuefeng Wang, Mingmei Zhong, Prof. Dr. Qihua Yang*, Prof. Dr. Can Li*

Oxygen evolution from water oxidation on molecular catalysts confined in the nanocages of mesoporous silicas
Bo Li , Fei Li , Shiyang Bai , Zhijun Wang , Licheng Sun , Qihua Yang* and Can Li*

Promoted activity of Cr(Salen) in a nanoreactor for kinetic resolution of terminal epoxides
Shiyang Bai , Bo Li , Juan Peng , Xiaoming Zhang , Qihua Yang* and Can Li*

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