## 5 Host-Guest Porous Composite Catalysts
This result indicates that silylation is indispensable for confining metal complexes inside the mesoporous cages. In addition, the catalyst can be convconstant even after the fifth use. The Co(Salen)/SBA-16-C8 can even be run for twelve times (33% yield and 88% ee). During the catalyst recycle process, a longer reaction time was needed to complete the reaction compared with the fresh catalyst. This is partly due to the loss of solid catalyst during the course of recovery (for the 12 th run, the catalyst weight is _ca._ 65% of the original  
Figure 8: Schematic description of entrapping homogeneous catalyst within the cage of mesoporous silicas. Reproduced with permission from Royal Society [175]. one). The filtrate does not show any activity for the ring-opening reaction. This confirms that the conversion is contributed by the catalyst trapped in the cage of the mesoporous material. In addition, the Co(Salen)/SBA-16-C8 catalyst was also used to catalyze the asymmetric ring-opening of propylene oxide. No obvious loss of activity and ee was observed even when Co(Salen)/SBA-16-C8 was run for the thirteenth time. After the thirteen cycles of reaction, the Co content in Co(Salen)/SBA-16-C8 was 87% of that seen in the fresh one. To study the effect of loading amount of [Co(salen)] on catalytic performance, we prepared four solid catalysts with Co contents of 0.055, 0.087, 0.157, and 0.225 wt%, corresponding to 1.2, 1.9, 3.4, and 4.9 [Co(salen)] complexes, respectively, accommodated in each nanocage of SBA-16[180]. It was surprising to find that the [Co(salen)]/SBA-16 catalysts with two or more than two [Co(salen)] complexes in each cage show a significantly enhanced cooperative activation effect and exhibit much higher activity than the homogeneous [Co(salen)] catalyst for the hydrolytic kinetic resolution (HKR) of epoxides (Figure 9a). The catalytic activity gradually increases as the number of [Co(salen)] units in each cage increases, and then reaches a plateau when the number reaches five (Figure 9b). The increase in the activity and enantioselectivity with an increase in the number of [Co(salen)] complexes per cage obviously indicates that the cooperative activation effect of [Co(salen)] complexes in the nanocages can be strengthed owing to the crowded situation of the cobalt complexes in the nanocages. The appropriate proximity and the free movement of [Co(salen)] in the confined space offer the possibility that H\({}_{2}\)O activated by one [Co(salen)] complex can attack the epoxide activated by another [Co(salen)] complex, and then facially produce the diol with high activity. This reasoning is corroborated by DFT calculations. Besides, another new heterogeneous composite catalyst, Ru(TsDPEN)/SBA-16-2 Ph, was prepared by the encapsulation of Ru(TsDPEN) in the mesoporous cage of SBA-16 using diphenyldichlorosilane as silylation agent [177]. TsDPEN is among the most efficient catalysts for the asymmetric transfer hydrogenation of prochiral ketones [181,182]. The heterogeneous catalyst Ru(TsDPEN)/SBA-16-2 Ph  
Figure 9: **(a)** Hydrolytic kinetic resolution of epoxides on [Co(salen)] complexes confined in the nanocages of SBA-16; **(b)** The catalytic activity of [Co(salen)]/SBA-16 as a function of [Co (salen)] number in each cage. Reproduced with permission from Wiley-VCH [180]. exhibits comparable enantioselectivity with the homogeneous counterpart. The recycle reaction test shows that similar conversion and ee value are obtained for the first three runs. When the catalyst was run for the sixth time, the conversion could still reach 99%. Longer reaction time was needed to complete the reaction, partly due to the loss of solid catalyst during the course of recovery. The activity is relatively lower than that of the homogeneous catalyst because of the diffusion limitation. Moreover, Ru(TsDPEN)/SBA-16 can catalyze different ketones to corresponding chiral alcohols with high activity and enantioselectivity.