## 5 Host-Guest Porous Composite Catalysts  
Chirality is one of the attributes of Nature, which is involved with many of the behaviors observed and some basic natural laws. In Nature, a molecule's chirality is often as important as its chemical makeup. Most biomolecules - proteins, hormones, nutrients, sugars, fats, and many others - are chiral. Besides, it is also critical in the biochemistry of natural products and drugs, so basic research on chiral compounds is another important research focus. Chiral compounds can be obtained by many methods among which the chiral catalytic syntheses are always considered to be the most economical and rational. Up to now, most of the chiral catalysts are homogeneous, which is good for catalytic efficiency but hard for recovery. The recovery of homogeneous catalyst and its purification are usually obstacles to the practical application of these catalysts. To solve the problem, heterogenization of homogeneous chiral catalysts are been developed [165, 166, 167, 168, 169, 170, 171, 172, 173, 174]. In most cases, however, the immobilized catalysts exhibit lower activity and enantioselectivity and they are located in nanopores [175]. The development of an efficient method for the heterogenization of chiral catalysts still remains a challenging objective. The entrapment of homogeneous catalysts within the porous matrix, compared with other immobilization methods such as covalent-bonding and electronic interaction, is highly desirable because in principle the trapped homogeneous catalyst should retain the properties of its free form. Mesoporous silicas with an ordered pore arrangement, a high surface area and a rigid framework are ideal porous materials to trap catalysts. [PERSON] and co-workers attempted to encapsulate an enzyme within the channel of MCM-41 [176]. Compared with enzymes, most homogeneous catalysts are much smaller in size. Therefore, it is more difficult to trap the homogeneous catalyst within the mesoporous silica while still keeping the catalyst as free as in homogeneous medium. As far as we know, a successful entrapment of a small metal complex, especially a chiral metal complex, within mesoporous silica has been rarely reported so far. We have developed a novel host-guest method to prepare heterogeneous asymmetric composite catalysts through entrapment of metal complexes within the cages of mesoporous silica [177], as illustrated in Figure 8. A preformed homogeneous catalyst is first introduced into the cage-like pore of a mesoporous material (e.g., SBA-16 with tunable small pore entrance and large cage size [178, 179]) by impregnation or adsorption. The pore entrance size is finely tailored by a silylation method, according to the molecular size of the catalyst, reactants and products. Thereby the metal complexes can be confined in the mesoporous cages. Reactants and products (usually smaller than the size of the pore entrance) are allowed to freely diffuse through the entrance to the mesoporous cage, in which the entrapped catalysts can speed up the reaction with high activity in a similar way to that in the actual homogeneous catalysis process. The advantage of this strategy over other immobilization methods is that the identity of a homogeneous catalyst can be largely kept because the trapped catalyst is almost intact. By this host-guest method, a novel composite catalyst, Co(Salen)/SBA-16-C8, was prepared by the encapsulaton of Co(Salen) in SBA-16 with silylation [177]. The fresh Co(Salen)/SBA-16-C8 with silylation gives a comparable diol yield and ee (43% yield and 91% ee) as its homogeneous counterpart for hydrolysis reaction of epoxychloropropane. For comparison, the catalyst without silylation was also prepared. It was found that the catalyst without silylation is nearly inactive for this hydrolysis reaction, indicating that most of the adsorbed Co(Salen) was washed away from SBA-16 during the catalyst preparation because of the large pore entrance size.