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. [1] The total global demand for MEG has been estimated to be over 19 million metric tons per year.[2] 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 10 wt %, 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 extensively investigated for the environmentally friendly production 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, [3] the salts of some acids, [4] cyclic amines, [5] cation-and anion-exchange resins, [6] quaternary phosphonium halides, [7] polymeric organosilane ammonium salts, [8] macrocyclic chelating compounds, [9] and supported metal oxides.[10] However, a high H 2 O/EO molar ratio (i.e. > 10) is still required for high MEG selectivity. The MEG selectivity is very low at low H 2 O/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 H 2 O/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 [Co III (salen)], with a salen ligand derived from 3,5-di-tert-butylsalicyclaldehyde and trans-1,2-diaminocyclohexane, [11] 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 H 2 O/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.The solid catalyst (denoted as FDU-12-[Co III
