10 Catalysis in Porous-Material-Based Nanoreactors: a Bridge between Homogeneous and Heterogeneous Catalysis

Transcription

1 Can Li c10.tex V1-12/27/ :52am Page Catalysis in Porous-Material-Based anoreactors: a Bridge between Homogeneous and Heterogeneous Catalysis Qihua Yang and Can Li 10.1 Introduction Catalysts have been widely applied in the main technologies for the development of world economy, including petroleum refinery, energy production, chemicals production, as well as the fine chemicals and pharmaceutical industry, and are related to many branches of chemistry, especially organometallic chemistry and materials science. Depending on whether a catalyst exists in the same phase as the substrate, catalysis can be classified into homogeneous and heterogeneous. Heterogeneous catalysis generally uses solid catalysts, and the reactants are either liquids or gases. During the reaction, the active sites of the solid surface are available for the reactant molecules to adsorb and to take part in the reactions. Heterogeneous catalysis offers advantages such as ease of separation and recycling of catalysts, continuous operations, and easy purification of products, but heterogeneous catalysts are more complex, and so their active sites and relevant mechanisms are not well understood in most cases. Homogeneous catalysis usually uses molecular catalysts such as organometallic compounds (metal ions with ligands coordinated), acid/base molecules, and salts. In comparison with heterogeneous catalysts, homogeneous catalysts usually show high intrinsic activity and selectivity, and their active sites are well defined. However, homogeneous catalysis meets with difficulties in large-scale application including the recycling of the catalyst and handling in industrial processes. Furthermore, the stability of the molecular catalysts is usually not high enough under severe reaction conditions. It has been a longstanding goal to combine the advantages of both homogeneous and heterogeneous catalysis. ne of the straightforward ways is to immobilize homogeneous catalysts (usually molecular catalysts) on solid supports to take the advantages of both homogeneous and heterogeneous catalysis. The immobilization of molecular catalysts in nanopores of porous materials is one of the most effective and convenient ways for the preparation of heterogeneous catalysts [1]. However, the immobilized molecular catalysts always exhibit lower performance than their homogeneous counterparts. Bridging Heterogeneous and Homogeneous Catalysis: Concepts, Strategies, and Applications, First Edition. Edited by Can Li and Yan Liu. c 2014 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2014 by Wiley-VCH Verlag GmbH & Co. KGaA.

2 Can Li c10.tex V1-12/27/ :52am Page 352 Q Catalysis in Porous-Material-Based anoreactors Because of the fast development of nanotechnology, porous materials with welldefined molecular catalysts or ligands as an integral part of the porous materials have been synthesized, such as periodic mesoporous organosilicas (PMs), metalorganic frameworks (MFs), and covalent organic frameworks (CFs). The porous materials mentioned above are very promising solid catalysts to combine the advantages of both homogeneous and heterogeneous catalysts because of their ordered porous structure, as well as well-defined and uniformly distributed organic ligands/molecular catalysts, thus providing a more homogeneous microenvironment for catalysis. Since the porous materials generally have catalytically active sites located inside the nanopores with pore diameter in the nanometer scale, they could be regarded as nanoreactor assemblies though these materials are usually bulky solids as a whole [2]. The understanding of the reaction mechanism and the diffusion dynamics of reactants and products at nanoscale is helpful for the rational design and synthesis of porous-material-based nanoreactors with high catalytic performance for a wide range of chemical reactions. This may help us to build a bridge between homogeneous and heterogeneous catalysis, which has been a long-term pursuit of researchers from both homogeneous and heterogeneous catalysis fields. In this chapter, we summarize the recent advances in the development of nanaoreactors based on porous solid materials for chemical reactions, including the general methods for the fabrication of typical porous materials, (mesoporous silicas (MSs), carbon nanotubes (CTs), and the MFs), the assembly of the molecular catalysts in the cavities and pores of the porous materials, the chemical reactions in the porous-material-based nanoreactors, and some important issues concerning the porous-material-based nanoreactor, such as the pore confinement effect, the isolation effect, the cooperative activation effect, and their shape and regioselectivity. We close this chapter with an outlook of the future development of the nanoreactors Preparation of anoreactors Based on Porous Materials Porous materials have been widely used in the fields of catalysis, adsorption, separation, chemical sensors, optical/electronic nanodevices, and many more [3]. In this chapter, we will briefly introduce the general preparation methods for porous materials with highly ordered porous structure, including MSs and MFs, both of which feature great potential applications as nanoreactors for combining the advantages of homogeneous and heterogeneous catalysis. These porous materials are generally synthesized through a hydrothermal/solvothermal synthetic approach, which has become the basis of the synthetic chemistry of porous materials. Hydrothermal synthetic conditions can enhance the effective solvation ability of solvents, increase the solubility of the reactants, and activate the source materials, thus resulting in increased nucleation and crystallization rate.

3 Can Li c10.tex V1-12/27/ :52am Page Preparation of anoreactors Based on Porous Materials Mesoporous Silicas The MSs have been widely used as support materials because of their high surface area, large pore volume, ordered porous structure, and rich hydroxyl groups. A large variety of mesoporous materials with different mesostructures and compositions have been synthesized, such as FSM-16, the SBA family, the FDU family, the KIT family, the AMS family, the HM family, MSU, and HMS, under a wide synthetic range from highly basic to neutral and then to strongly acidic conditions depending on the use of cationic, anionic, neutral, or nonionic surfactants [4 6]. The hydrothermal method was usually used to synthesize mesoporous silicates through the organic inorganic assembly by using organic molecules or supramolecules (e.g., amphiphilic surfactants and biomacromolecules) as templates. The organic inorganic assembly is driven by the weak noncovalent bonding between the surfactants and inorganic species, such as hydrogen bonding, van der Waals forces, and electrovalent bonds. In light of the current knowledge of the surfactant self-assembly, the mesoporous materials can be rationally designed and the synthesis controlled [7]. Generally, the packing parameters of the surfactant, for example, the g-value (g = V/(a 0 I), where V is the total volume of surfactant hydrophobic chains plus any cosolvent (organic molecules) between the chains, a 0 is the effective hydrophilic headgroup area at the aqueous-micelle surface, and I is the kinetic surfactant tail length), are widely used in predicting and explaining the mesostructure [8]. The pore structure and pore diameter of MSs could be conveniently controlled by varying the g-values and the synthetic parameters. Recent developments of MSs have been well summarized in several recent review articles [4 6]. In this section, we mainly introduce the synthesis of MSs with the 2D hexagonal mesostructure and the 3D cage-like structure for their potential applications as nanoreactors for catalysis. MSs with 2D hexagonal mesostructure include MCM-41, FSM-16, SBA-3, and SBA-15. Among them, SBA-15 is an ideal nanoreactor because of its larger pore diameter (6 10 nm), high thermal and hydrothermal stability, and the existence of the micropores on the silicate walls [9, 10]. SBA-15 could be conveniently synthesized using the PE-PP-PE triblock copolymer, such as P123, as a structure-directing agent under acidic condition. SAB-16 and FDU-12 are typical mesoporous materials with a cage-like 3D porous structure, and are ideal candidates for the encapsulation of active sites for catalysis. SBA-16 has the cubic Im3m mesostructure, which could be considered as the body-centered cubic symmetrical packing of spherical cages [11 13]. The spherical cage size is about 5 6 nm. High-quality SBA-16 could be obtained using F127, F108, and F98 as surfactants in an acidic medium. The mesostructure of FDU-12 could be described as a face-centered cubic close-packing of spherical cages, each cage being connected to the nearest 12 neighboring ones [14, 15]. The cage size and pore entrance size could be increased, respectively, from 14 to 22 nm and from 4 to 8.9 nm by elevating the hydrothermal temperature and lowering the synthesis temperature. FDU-12 could be synthesized using F127 as surfactant in an acidic medium in the presence of trimethylbenzene and with the aid of inorganic salts

4 Can Li c10.tex V1-12/27/ :52am Page Catalysis in Porous-Material-Based anoreactors such as KCl. SBA-15 with channel-like pore structure and SBA-16/FDU-12 with cage-like pore structure could be judiciously chosen as host materials for molecular catalysts depending on the immobilization methods and the types of chemical reactions. Along with the progress of inorganic porous material preparation, the organic inorganic hybrid porous materials have also been successfully synthesized. For example, the PMs built from bridged organosilane precursors (R ) 3 Si R Si(R ) 3, wherein the organic group R is an integral part of the mesoporous wall, were first reported in the late 1990s [16 18]. Similar to MSs, the PMs also have high specific surface areas, as well as tunable and well-ordered mesopores. The emergence of the PMs opened up a new approach for the functionalization and modification of MSs [19]. The bridging organic groups in the pore wall of PMs not only modify the surface properties of the materials but also endow the PMs with novel physical and mechanical properties, such as improved hydrothermal and mechanical stability. The chemical and physical properties of PMs, especially the microenvironment of the nanopore, can be tuned according to the intended applications by adjusting the organic species in the network [20] Metal-rganic Frameworks (MFs) A representative example of the organic inorganic hybrid material is the porous material of MFs constructed from the connection of inorganic clusters or isolated metal ions through di-, tri-, or tetratopic organic ligands via more or less covalent metal ligand bonding [21]. In fact, as early as 1965, Tomic already reported the materials which would nowadays be called MFs [22]. Yaghi and coworkers [23] reported the synthesis of modular porous solids with a rational design. To date, thousands of MFs with diverse topologies, pore sizes, shapes, and nature have been reported. The large diversity of elements (especially metal ions) in the composition of the walls of the open framework of MFs opens new avenues for the design and synthesis of new kinds of nanoreactors for catalysis [24]. For the synthesis of MFs, the key is to control the kinetics of crystallization to allow nucleation and growth of the desired phase by adjusting the reaction temperature, the solvent, the concentration of reactants, and so on [25]. The control of the porous structure is a key issue if the MFs are to be used as the nanoreactors. MFs could be obtained by a precipitation reaction followed by recrystallization or the slow evaporation of the solvent. Methods such as solvent evaporation, layering of solutions, or slow diffusion of reactants into each other leading to concentration gradients are generally used for the formation of MFs [21]. Recently, in addition to conventional electric (CE) heating, microwave (MW) heating, electrochemistry (EC), mechanochemistry (MC), and ultrasonic (US) methods have also been employed for the synthesis of MFs [26]. In comparison with mesoporous materials, MF structural prediction is far from satisfactory. Besides, the acquisition of MFs with high quality always involves tedious work by varying many parameters, such as the molar ratios of starting materials, ph, solvent, reaction time, temperature, and

5 Can Li c10.tex V1-12/27/ :52am Page Preparation of anoreactors Based on Porous Materials 355 pressure. Thus, high-throughput methods have been shown to be the promising tools to accomplish this cumbersome work [21]. ne appealing character of MFs is that they could have a wide variety of interesting structures, depending on a number of factors including connectivity, charge, geometry of the connectors and linkers, the nature of the coordinating sites, and the length of the linkers. In comparison with MSs, MFs have higher surface area, larger pore volume, and lower density. Moreover, the chemical composition of MFs could be easily varied. The unique structural and chemical properties make MFs an ideal nanoreactor for catalysis [27] Surface Modification of anoreactors For a chemical reaction catalyzed by a solid catalyst, the reactions mostly take place on the surface because the active sites are usually distributed on the surface. Therefore, the surface properties are primarily responsible for the catalytic performance. Both the inner and outer surfaces are important for the catalytic performance of the catalysts either inside the nanoreactor or on the outer surface. The specific surface area becomes predominant for nanoreactors, especially for a nanoreactor with diameter of similar size of the reactants and products. The selectivity and activity of the catalyst in a nanoreactor must be greatly affected by its size and the surface properties. Therefore, the surface properties play a critical role in the design and synthesis of a nanoreactor for efficient chemical transformations. Q Surface Modification of Mesoporous Silicas (MSs) MSs with a huge inner surface and high concentration of silanol groups can be easily modified. rganic groups with different hydrophilicities/hydrophobicities are often used for the surface modification of MSs, to endow them with the desired surface properties and active sites for catalysis. We will summarize the strategies employed for surface modification of MSs with organic functionalities, including the post-synthesis modification (the so-called grafting) and co-condensation methods (Scheme 10.1). Grafting refers to the subsequent modification of surfaces of MSs by the reaction of organosilanes of the type (R ) 3 SiR, or less frequently chlorosilanes ClSiR 3 or silazanes H(SiR 3 ) 3, with free silanol groups [28]. It should be mentioned that a Grignard reagent and organolithium can also be employed for the surface modification via the formation of Si surface C bonds. The grafting modification of MSs is usually performed in a solvent. Thus the choice of the solvent and water as a solvent concentration in the reaction system may influence the distribution and amount of the organic groups grafted on the surface of MS. The grafting method has the following advantages: (i) the mesostructure of the starting MS can be retained, though the modification often causes the decrease in surface area and pore volume, and (ii) the organic groups can be site-selectively deposited on the external surface using the as-made MS (with the surfactant in the nanopore) formodification,basedonthefactthatthe Hgroupintheinnersurfaceof

6 Can Li c10.tex V1-12/27/ :52am Page Catalysis in Porous-Material-Based anoreactors Color Fig.: 10.1 (a) (R ) 3 Si R or Cl 3 Si R R H H H H H H Si H (b) TES or TMS or a 2 Si 3 R R Si R R H R Si H Scheme 10.1 Surface modification of MS by (a) grafting and (b) co-condensation. the as-made MS can hardly be accessed by organosilanes. The disadvantages of this method are the inhomogeneous distribution of the organic groups and the pore blockage resulting from the diffusion barriers of organosilane precursors throughout the MS during the grafting process. ne interesting example for the selective deposition of organic group on the external surface of MS [29] was reported by Brühwiler and coworkers (Scheme 10.2). When grafting 3-aminopropyl group to the as-made MS, it was found that the silane precursor determined the extent of grafting. In the case of the frequently used APTES [(H 2 CH 2 CH 2 CH 2 Si(Et) 3 )], significant derivatization of the pore surface was found. APTMEES [(H 2 CH 2 CH 2 CH 2 Si(CH 2 CH 2 CH 2 CH 2 CH 3 ) 3 )], on the other hand, grafted preferentially to the external surface sites. The mesopores remained accessible for further modification after the functionalization of the external surface with APTMEES. By carefully choosing the organosilane precursor, grafting of as-made MS is a straightforward method for external surface modification.

7 Can Li c10.tex V1-12/27/ :52am 10.2 Preparation of anoreactors Based on Porous Materials Color Fig.: 10.2 H2 Si(R)3 + H2 H2 Si Grafting Si Extraction R CH2CH3 R CH2CH2 CH2CH2CH3 1 μm 5 μm the amino groups indicate that the degree of pore surface grafting strongly depends on the type of silane. Three particles are shown for each silane/silica combination. ptical slices in the center of the particles were selected. Reprinted with permission from Ref. [29]. Copyright 2009 John Wiley & Sons. Scheme 10.2 Top: Schematic representation of external surface functionalization by grafting to as-synthesized mesoporous silica. Bottom: Scanning electron microscopy images of ASCs (left) and mesoporous silica spheres (right). The corresponding CLSM images taken after fluorescent labeling of An alternative approach for surface modification of MS is the co-condensation of a mixture of tetraalkoxysilanes [(R)4 Si (TES or TMS or a2 Si3 )] and terminal trialkoxyorganosilanes of the type (R )3 SiR in the presence of structure-directing agents [30], which leads to MSs with organic groups anchored covalently to the pore walls (Scheme 10.3). This method could result in the homogeneous distribution of the organic groups. Since the organic functionalities are direct components of Color Fig.: 10.3 R R Si R R R Si R R Si Si Si Si Si Si Si Scheme 10.3 Schematic illustration for the preparation of periodic mesoporous organosilicas. 357 Page 357

8 Can Li c10.tex V1-12/27/ :52am Page Catalysis in Porous-Material-Based anoreactors the silica matrix, pore blocking is not a problem in the co-condensation method. In comparison with the grafting method, the organic groups could be deposited mainly in the inner surface of MS. However, the co-condensation method also has the following disadvantages: (i) the order of mesostructure decreases with increasing the concentration of (R ) 3 SiR in the reaction mixture. Thus, it is difficult to obtain ordered MS with high content of functional groups, and (ii) the different hydrolysis and condensation rates of different precursors are a common problem for getting ordered structure and uniform distribution of organic groups on the surface of nanoreactors. The co-condensation method could also use bridged organosilane of (R)Si 3 R Si(R) 3 as the precursor in the presence of SDA [31]. The obtained MS is described as PM [16 18]. In contrast to organic groups on the surface of MS synthesized by grafting or direct synthesis methods, the organic units in PMs are incorporated in the three-dimensional network of silica matrix through covalent bonds and thus distributed homogeneously in the pore walls. Since the first report of the PMs in 1999 [16 18], PMs with various organic groups in the framework have been extensively reported because of their potential applications, especially in catalysis. The surface properties of PMs could be precisely controlled by varying the ratio of (R)Si 3 R Si(R) 3 /TES or TMS in the initial mixture without loss in structural order if the synthesis condition is carefully chosen. However, it is difficult to obtain the ordered structure when the bridged organic groups have a large molecular size and rigid molecular structure Surface Modification of MFs MF modification mainly focuses on the post-synthetic approaches. The functionalization of MFs should be more straightforward than that of inorganic materials because the organic moiety is an integral part of MFs [32]. However, the direct functionalization of MFs has been severely limited due to either the poor solubility of functional organic groups or the difficulty in the coordination of functional organics with metal ions. Post-synthetic methods have been used for chemical modification of MFs, which could be performed on the fabricated material rather than on the molecular precursors. In this way, the newly introduced functional groups (and the reaction conditions required to introduce those groups) only need to be compatible with the final material, and any incompatibility with the synthetic methods required to obtain the material could be circumvented [33]. The post-synthetic modification of MFs could be made by covalent bonding and dative bonding [34]. Covalent bonding is defined as the modification of MFs through the formation of covalent bonds. Covalent bonding is the most extensively used method for introducing a broad range of chemical groups into MFs. Dative bonding is defined as the modification of MFs through the formation of dative bonds (i.e., metal ligand). Dative bonding can be formed by adding either ligands or metal ions to MFs. In addition to the formation of covalent and dative bonding, the post-synthetic deprotection method could also be used for the modification of MFs by the cleavage of a chemical bond within an intact framework of MFs after the synthesis. Post-synthetic deprotection is less frequently used for the

9 Can Li c10.tex V1-12/27/ :52am Page Assembly of the Molecular Catalysts in anoreactors 359 modification of MFs compared to the other methods, but has been shown to be a distinct and useful approach for modifying the MFs Assembly of the Molecular Catalysts in anoreactors The traditional methods, such as hydrothermal synthesis, impregnation, and chemical vapor deposition (CVD), can be employed to incorporate heteroatom and metal/metal oxide nanoparticles as catalysts into the nanopores of MSs. The advances in this area have been well summarized in recent reviews [35 38]. Herein, we will mainly focus on the assembly of molecular catalysts in the nanopore of MSs and MFs. Using the molecular chiral catalyst as a model, we will address the general strategies for incorporating molecular catalysts in the nanoreactor, including the covalent and noncovalent bonding methods Incorporating Chiral Molecular Catalysts in anoreactors through Covalent-Bonding Methods Incorporating chiral molecular catalysts in the nanopore of MSs through covalent bonding methods can be achieved through the reaction between the silane precursor containing the chiral ligand/catalyst with the silanol group of the MS (one-step method), or with the functional group on the MS (two-step method) (Scheme 10.4) Color Fig.: 10.4 (a) (R ) 3 Si R (b) H H H R H H H (R ) 3 Si R R R R Scheme 10.4 Schematic illustration of covalently bonding organic ligand/catalyst on the MS by (a) one-step and (b) two-step method.

10 Can Li c10.tex V1-12/27/ :52am Page Catalysis in Porous-Material-Based anoreactors [39]. The one-step method generally results in nanoreactors with well-retained chiral ligand/catalyst, but always involves the tedious process of synthesizing silane precursors. The two-step method can avoid the tedious synthesis process, but the incomplete reaction of chiral ligand/catalyst with the functional group immobilized in the nanopore always leads to the existence of noncatalytic species in addition to the desired catalytically active species [40]. The chiral ligand/catalysts could also be incorporated in the network of mesoporous organosilicas (chiral PMs) and MFs (chiral MFs) by direct incorporation or by a post-synthetic modification. Different from the porous materials with covalently bonded chiral ligand/catalyst prepared by grafting methods mentioned above, the chiral PMs and chiral MFs with uniformly distributed chiral ligand/catalyst as an integral part of the network provide more uniform chiral microenvironment for the asymmetric catalysis. The chiral PMs and chiral MFs have demonstrated their potential applications as ideal nanoreactors for asymmetric catalysis or chiral separation. It should be mentioned that the enantioselectivity of the chiral catalysts could be changed by the porous structure of the chiral PMs and chiral MFs with the catalysts incorporated in their network. Some chiral organosilane precursors for the incorporation of chiral ligands/catalysts in the network of chiral PMs are outlined in Scheme 10.5 [41]. The chiral ligands or chiral catalysts that have been incorporated in the chiral MFs are summarized in Scheme 10.6 [42 45]. The direct synthesis method is often used for the synthesis of chiral MFs using the chiral catalyst/ligand as the organic linker. The pioneering work of homochiral MF (PST-1) for asymmetric catalysis was reported by Kim and coworkers [46] in D-PST-1 possessing large 1D equilateral-triangle chiral channels with a side length of 13.4 Å was crystallized of a 2D network by a derivative of D-tartaric acid (L 1 in Scheme 10.6) and Zn 2+ ions. Crystal structure analysis indicated that one of the three pyridyl groups of the ligands was noncoordinated and exposed in the open channels, which was explored as a Brönsted base for the catalytic transesterification. When using racemic 1-phenyl-2-propanol as the substrate, D-PST-1 and L-PST-1 produced the corresponding esters with 8% ee in favor of the S and R enantiomers, respectively. The low enantioselectivity was presumably due to the substantial distance of the active site from the chiral wall of the pores. Although the ee value was modest, it was the first homochiral MF demonstrating that an organic unit embedded in a chiral pore could catalyze an asymmetric transformation. This triggered interests in the design of chiral ligands and homochiral MFs for heterogeneous asymmetric catalysis. Very recently, by employing the mixture of photocatalyst and chiral catalyst as the organic linker, Duan s group [47] reported the fabrication of a photoactive chiral MF with both the triphenylamine photocatalyst and proline-based (L 6 in Scheme 10.6) organocatalyst incorporated in a single MF structure (Scheme 10.7). In the light-driven α-alkylation reaction, the photoactive chiral MF showed high activity and enantioselectivity ( 90%) through the cooperation of the triphenylamine moiety for the generation of an electrophilic radical by a photo-induced electron transfer, and the reaction of the electrophilic radical and π-nucleophilic

11 Can Li c10.tex V1-12/27/ :52am Page Assembly of the Molecular Catalysts in anoreactors 361 H H Si(Me) 3 Si(Me) 3 H H H H Si(Et) 3 Si(Et) 3 H H H Compound 1 Compound 2 Compound 3 H Si(Et) 3 Si(Et) 3 S S Si(Me) 3 Compound 4 (Me) 3 Si (Et) 3 Si H H H H (Me) 3 Si Si(Me) H H 3 (Et) 3 Si Si(Et) 3 (Me) 3 Si Si(Me) 3 (Et) 3 Si H H H PPh 2 PPh 2 H Compound 5 Compound 6 Compound 7 Compound 8 Si(Et) 3 Si(Et) 3 Si(Et) 3 PPh 2 PPh 2 (Me) 3 Si (Me) 3 Si MM MM Me C 2 Me Me 2 C Me Si(Et) 3 Si(Et) 3 Si(Et) 3 Compound 9 Compound 10 Compound 11 Compound 12 (Me) 3 Si H Si(R) 3 Si(Me) 3 B Si(R) 3 (Et) 3 Si Si(Et) Ph Si(Et) 3 3 Si(Me) B 3 R Si(Me) 3 Compound 13 Si(Et) 3 Compound 14 Compound 15 Compound 16 Compound 17 Scheme 10.5 Chiral organosilane precursors for the synthesis of chiral PMs.

12 Can Li c10.tex V1-12/27/ :52am Page Catalysis in Porous-Material-Based anoreactors H H H H H H H H H 2 H H L 1 L 2 L 3 L 4 L 5 L 6 H P H H H P R R C 2 H C 2 H C 2 H Cl H 2 C H H H H H H R R Cl H 2 C C 2 H C 2 H C 2 H L 11 L 12 L 13 L 14 L 7 L 8 L 9 L 10 C 2 H C 2 H H 2 C C 2 H H 2 C H H P H P H C 2 H H 2 C C 2 H H 2 C L 15 L 16 L 17 C 2 H Mn Cl L 18 C 2 H L 19 H 2 C C 2 H L 20 C 2 H L 21 Py Ru Py L 23 C 2 H H 2 C Co Ac C 2 H C 2 H L 22 L 24 L25 Scheme 10.6 Chemical structures of chiral ligands or chiral catalysts (L 1 L 25 ) incorporated in to chiral MFs through either direct incorporation or post-synthesis modification for asymmetric catalysis. active intermediate formed on the chiral proline. The author also showed that the photoactive chiral MF was more active and enantioselective than the physical mixture of triphenylamine containing-mf and the chiral adduct L 6. This suggests that the integration of both photocatalyst and asymmetric organocatalyst into a single MF could make efficient cooperation of different kinds of active sites. The individual components fixed with their well-defined porous and repeating structures make MFs a versatile platform for a new type of cooperative catalysts. Post-synthetic modification can also be used for the synthesis of chiral MFs via introducing chiral catalysts into the open coordination site of metal nodes of achiral MFs [48]. After the removal of the coordinated water molecules, two

13 Can Li c10.tex V1-12/27/ :52am Page Assembly of the Molecular Catalysts in anoreactors 363 H Color Fig.: 10.7 H Boc H Zn 2+ BCIP Self-assembly Zn-BCIP H Y Br C 2 Et C 2 Et 26W fluorescent lamp Deprotection C 2 Et H C 2 Et Y Up to 92% ee H Zn-PYI Scheme 10.7 Schematic representation of the formation process of the photoactive chiral MF and Zn-PYI. Reprinted with permission from Ref. [47]. Copyright 2012 American Chemical Society. L-proline derivatives (L 3 and L 4 in Scheme 10.6) were incorporated into the open Cr III coordination sites of MIL-101, giving a chemically and thermally robust MF with high Brunauer Emmett Teller (BET) surface area ( m 2 /g) and large pores ( nm) and windows ( nm). These chiral MFs could efficiently catalyze the asymmetric aldol reactions with good yields (60 90%) and fair to good ee values (55 80%). It should be mentioned that the chiral MFs showed higher enantioselectivity than the chiral ligands themselves, which may be due to the restricted movement of the substrates in the confined microporous systems in combination with multiple chiral inductions. Using the post-synthetic modification method, aspartic acid (L 2 in Scheme 10.6) [49] and chiral proline (L 5 in Scheme 10.6) [50] were incorporated into MFs. These chiral MFs exhibited low-to-moderate enantioselectivity in the asymmetric catalysis, such as the methanolysis of cis-2,3-epoxybutane and asymmetric aldol reactions Immobilizing Chiral Molecular Catalysts in anoreactors through oncovalent Bonding Methods Introduction of Molecular Catalysts into anoreactors through oncovalent Bonding Methods The noncovalent bonding methods mainly include adsorption, electrostatic interaction, and encapsulation, as illustrated in Scheme 10.8 [51]. The simple physisorption

14 Can Li c10.tex V1-12/27/ :52am Page Catalysis in Porous-Material-Based anoreactors Adsorption Electrostatic interaction Encapsulation Scheme 10.8 supports. General methods for noncovalent bonding of molecular catalysts on the solid of a chiral ligand/catalyst on a support through van der Waals interaction is an attractive approach, since it does not require a synthetic modification of the chiral ligand. However, the molecular catalysts immobilized on a solid support through van der Waals interaction are prone to leaching from the support because the interaction is too weak to fix the catalysts. For enhancing the interaction strength, electrostatic interactions were used for the immobilization via affording opposite charges for the support material and chiral ligand/catalyst. However, electrostatic interactions generally need modification of the chiral ligand/catalyst, and can result in the alteration of the intrinsic properties of the chiral molecular catalyst. The encapsulation method, by enclosing molecular catalyst in the rigid pore space [52], has the following clear advantages over the other immobilization methods: (i) The metal complex does not require any modification with extra functional groups for the immobilization. The structure and properties of the catalyst, which determine the catalytic performance, consequently remain intact after immobilization. (ii) Under reaction conditions, the metal complex catalyst encapsulated in the void space could, in principle, remain as free as the catalyst in solution since there is no strong interaction between the catalyst and the solid matrix. Therefore, the inherent catalytic properties of the metal complex catalyst can remain unaltered. In this section, we mainly summarize the recent development in the encapsulation method. In principle, the formation of the porous matrix around a preformed molecular catalyst (bottle-around-ship) and the construction of the molecular catalyst in a preformed porous material (ship-in-a-bottle) have become two popular strategies for the encapsulation of molecular catalysts since the 1980s. Complexes such as [(BIAP)Ru(p-cymene)Cl)Cl], [(MeDuphos)Rh(cod)]Tf, (Salen)Mn, i PrPybox- RuCl 2, and so on, have been entrapped in a silicon membrane, poly(vinyl alcohol) film, microcapsules, or silica matrix via the in situ formation of the network around the complexes (polymerization and sol gel process were involved in the network formation) [53 55]. However, the catalysts prepared are generally poor in activity, selectivity, and stability. The swelling of the polymer host material and the inhomogeneous cavity formed around the metal complexes may be the main reasons for the low activity and stability. In addition, the in situ formation of the

15 Can Li c10.tex V1-12/27/ :52am Page Assembly of the Molecular Catalysts in anoreactors 365 network around complexes is not a general method and can only be applied to the complexes stable enough to survive the polymerization or the sol gel process. Different from the formation of the porous matrix around a preformed molecular catalyst, the ship-in-a-bottle method is to construct the molecular catalyst in a preformed porous material. The ship-in-a-bottle method was first reported by gunwumi and Bein for trapping Mn(Salen) in the supercages of zeolite. Since the 1970s, this a type of synthesis has become an efficient method for encapsulating metal complexes within the solid matrix, particularly in microporous materials such as zeolites [56]. However, the synthesis of the constructed chiral complexes in zeolite through the ship-in-a-bottle methods was not published until 1997 when Corma and Bein [57, 58] reported that the chiral catalyst in the zeolite matrix exhibited moderate enantioselectivity in the asymmetric epoxidation. The ship-ina-bottle synthesis in zeolites was recently reviewed by Corma [54]. However, even the supercage of zeolite (less than 1.5 nm) is too small to afford enough space for the movement of the entrapped complexes. Thus, the catalytic performance of the solid catalysts prepared according to this method is not very encouraging. Compared with microporous zeolites, ordered MSs possess larger pore size and pore volume, which provides huge possibilities for the encapsulation of larger molecules. Algarra and Tanamura [59, 60] reported the synthesis of copper phthalocyanine and porphyrin in MCM-41 through the ship-in-a-bottle method. However, the cylinder-like pore of MCM-41 could not restrict the metal complex from leaching. Compared to MCM-41 and SBA-15 with cylindrical channels, MSs with cage-like structures, such as SBA-1 (cubic, Pm3n) [61], SBA-16 (cubic, Im3m) [62], FDU-12 (cubic, Fm3m) [14, 15], and FDU-1 (cubic, Fm3m) [63], are more suitable as host materials for the encapsulation of molecular catalysts. These mesoporous cage-like silicas have tunable cage sizes (4 8 nm for SBA-16; nm for FDU-12) and their cages are interconnected in three dimensions by tunable pore entrances. Additionally, the existence of numerous hydroxyl groups in the MSs provides the possibility for tailoring the pore entrance size by a simple silylation method. We have constructed a chiral Co(Salen) complex (Salen = (R,R)-, -bis(3,5-ditertbutylsalicylidene)-1,2-cyclohexanediamine) in the nanopore of SBA-16 through ship-in-a-bottle synthesis followed by tailoring the pore entrance size using silylation [64]. Chiral Co(Salen) encapsulated inside SBA-16 shows enantioselectivity (up to 96%) as high as that of the homogeneous counterpart for the hydrolytic kinetic resolution (HKR) of terminal epoxides. The catalysts can be recycled for at least 10 times, indicating that the metal complex confined in the nanocages of SBA-16 is stable. The ship-in-a-bottle synthesis in a mesoporous cage-like material combined with tailoring the pore entrance size provides a promising method for encapsulating a metal complex with a large molecular size. However, the ship-in-a-bottle synthesis may have problems such as the formation of undesired species in the solid matrix, which probably causes undesired reactions. Moreover, this method is suitable only for a few metal complexes which can be constructed through one or two steps under mild reaction conditions. Most chiral metal complexes, generally synthesized through multiple steps and sometimes under very harsh conditions, are not possible

16 Can Li c10.tex V1-12/27/ :52am Page Catalysis in Porous-Material-Based anoreactors to be formed in nanoreactors. In order to solve this problem, our group has developed a new and general strategy for the encapsulation of chiral metal complexes in the nanoreactor through the post-modification of the pore entrance [65] Encapsulating Molecular Catalyst in anoreactors by Reducing the Pore Entrance Size MSs with high surface area, tunable pore diameter (2 50 nm), and rigid framework are ideal porous materials to trap the molecular catalysts. However, the successful entrapment of a molecular catalyst in MSs has rarely been reported. This is due to the difficulty in sealing the molecular catalysts generally having small molecular size (<2 nm) in the cylindrical nanopores of MSs with a 2D hexagonal mesostructure. We have chosen the MSs with a cage-like porous structure, such as SBA-16 (cubic, Im3m) and FDU-12 (cubic, Fm3m) having large cages (4 8 nm for SBA-16; nm for FDU-12) connected three dimensionally by small pore entrance (<4 nm) as the host materials. The large cage provides enough space for the free movement of the molecular catalysts and the chemical reactions to take place, while the small pore entrance is beneficial for sealing the molecular catalysts in the nanocages. Additionally, the existence of numerous hydroxyl groups in the MS provides the possibility for tailoring the pore entrance size by a simple silylation method [66]. A preformed molecular catalyst is first adsorbed into the nanocages of MS (SBA- 16 or FDU-12), followed by finely reducing the size of pore entrance by a silylation reaction according to the molecular size and properties of the catalyst, reactants, and products, to prevent the molecular catalyst from leaching out of the nanocages (Scheme 10.9) [65]. Thereby, the molecular catalyst is confined in the mesoporous Color Fig.: 10.9 Silylation Catalyst Cage Reactants Products Scheme 10.9 General process for encapsulating molecular catalysts into the nanocages of MS and the chemical reactions in the nanoreactor catalyzed by encapsulated molecular catalysts. Reprinted with permission from Ref. [64]. Copyright 2007 Royal Society of Chemistry.

17 Can Li c10.tex V1-12/27/ :52am Page Assembly of the Molecular Catalysts in anoreactors 367 cages while the reactants and products can still diffuse freely through the pore entrance. o extra modification of the molecular catalyst is needed, and there are no strong interactions between the molecular catalyst and the porous materials. In principle, the molecular catalyst could move freely during the catalytic process. Thus, the intrinsic properties of the molecular catalyst could be kept to a large extent and the catalysis in the nanoreactor with both the advantages of homogeneous and heterogeneous catalyses could build a bridge between the homogeneous and heterogeneous catalyses. The degree of the movement freedom of the confined molecular catalysts in the nanoreactor was investigated by MR of the entrapped molecules, considering that the width of the resonance line could reflect the freedom of the molecules. 31 P solidstate MR spectra of FDU-12-BIAP (BIAP confined in the nanocages of FDU-12, BIAP: 2,2 -bis(diphenylphosphinooxide)-1,10-binaphthyl) in the solid form and in suspensions (using CHCl 3 as solvent) are shown in Scheme [67]. The static spectrum of the solid sample shows an asymmetric and broad pattern due to 31 P chemical shift anisotropy. With the addition of CHCl 3, a sharp peak appears at +28 ppm, which is due to the fast movement BIAP molecules in the nanoreactor in the presence of the solvent. This indicates that the molecular catalyst confined in the nanoreactor could move freely during the catalytic process in the presence of reactants and solvent and that the catalysis in the nanoreactor exhibits homogeneous in heterogeneous properties. Color Fig.: CP MAS In dry state * In CH 2 Cl μl 80 μl PPh 2 PPh 2 * * * * Static condition 60 μl 40 μl 20 μl 10 μl 5 μl BIAP@FDU Chemical shifts (ppm) Chemical shifts (ppm) Scheme CHCl P MR spectra of BIAP@FDU-12 in dry state or in the presence of Different kinds of chiral catalysts, such as chiral Co(Salen) [68], V(Salen) [69], Fe(Salan) [70], Cr(Salen) [71], Ru(TSDPE) [72], and (binolate) 2 Ti [73] (Scheme 10.11), have been successfully encapsulated in the nanocages of MSs using this strategy. The encapsulated molecular catalysts show enantioselectivity and activity as good as their homogeneous analogs in the asymmetric catalysis. Based on this method, other groups have also successfully encapsulated different types of molecular catalysts in MSs/organosilicas. For example, Qiu and coworkers

18 Can Li c10.tex V1-12/27/ :52am Page Catalysis in Porous-Material-Based anoreactors Co Ac H 2 H S 2 Ru Cl V + S3 Et Hydrolytic kinetic resolution of epoxide: 84 98% ee Asymmetric transfer hydrogenation of ketones: 88 93% ee Asymmetric cyanosilylation of aldehydes: 82 90% ee Br Cr Cl Br ipr Ti ipr H H Br Fe Cl Ph Ph (ar) Br Kinetic resolution of terminal epoxides: 91% ee Asymmetric carbonyl ene reaction: 92% ee Asymmetric sulfide oxidation: 57 74% ee Scheme Molecular catalysts that are encapsulated in the nanocages of SBA-16 or FDU-12 by encapsulation method by our group.

19 Can Li c10.tex V1-12/27/ :52am Page Catalytic Reactions in anoreactors 369 [74] reported the encapsulation of dimeric Mn(Salen) in the nanocages of PMs, and the solid catalyst thus prepared showed higher activity than the homogeneous counterpart in the epoxidation of alkenes. The encapsulated catalysts can be easily recycled without significant loss in catalytic performance. The advantages of this encapsulation strategy over other entrapment method are the following: (i) The molecular catalyst confined in the nanoreactor can basically move freely and catalyze the reaction with high activity and selectivity in a manner similar to that in the actual homogeneous catalysis system. Thus, the identity of a molecular catalyst can be largely kept. (ii) The MSs with large cage size afford enough space for the encapsulation of various concentrations of the molecular catalysts to adjust the proper space for the formation of the transition state of the reaction and to reduce the diffusion resistance during the catalytic process. (iii) The encapsulation method uses preformed metal complexes and can thus avoid the formation of undesired species in the solid matrix. This strategy could be basically applied for the encapsulation of various kinds of molecular catalysts in the nanoreactor Catalytic Reactions in anoreactors Through covalent and noncovalent bonding methods, different kinds of molecular catalysts could be incorporated into MSs and MFs. These porous materials with the incorporated molecular catalyst could catalyze various kinds of chemical reactions. A review of all the related works is impossible and not necessary in this chapter. We only review some representative examples for demonstrating the unique properties of the nanoreactor for catalytic reactions, including the pore confinement effect, the enhanced cooperative activation effect, and the isolation effect, as well as the microenvironment and the porous structure engineering of the nanoreactor and the catalytic nanoreactor engineering Pore Confinement Effect Chiral Mn(Salen) catalysts immobilized in MSs have been investigated for the asymmetric epoxidation of unfunctionalized olefins [75 78]. The chiral Mn(Salen) catalysts could be axially immobilized in the nanopores and on the external surface of MSs via phenoxy groups and organic sulfonic groups. Generally, higher chemical selectivity and enantioselectivity could be achieved by immobilization of Mn(Salen) catalysts in the nanopores than on the external surface of supports for the asymmetric epoxidation of unfunctionalized olefins [79]. The Mn(Salen) catalysts grafted via flexible propyl sulfonic groups usually give higher chemical selectivity and enantioselectivity than those grafted via rigid phenyl sulfonic groups (Scheme 10.12) [80]. This is probably due to the fact that the electronic and steric factors of the linkages may affect the transition state for the asymmetric reactions.

20 Can Li c10.tex V1-12/27/ :52am Page Catalysis in Porous-Material-Based anoreactors Color Fig.: Bu t Mn t S Bu Bu t R Si Me/Et t Bu anopore: SBA (7, 6) Surface: MCM (1, 6) Catalyst Time Con% ee% Mn(salen)CI PhS 3 Mn(salen)/MCM(1.6) PrS 3 Mn(salen)/MCM(1.6) PrS 3 Mn(salen)/SBA(7.6) R: Ph Pr Rigid Flexible Scheme Mn(salen) grafted in the nanopores of MS via flexible propyl groups resulting higher ee values for the asymmetric epoxidation of 1-phenylcyclohexene compared with that grafted via rigid phenyl group. Reprinted with permission from Ref. [80]. Copyright 2006 Elsevier. The conversion, chemical selectivity, and enantioselectivity of chiral Mn(Salen) catalysts immobilized in the nanopores of supports increase with increasing axial linkage lengths for the asymmetric epoxidation of unfunctionalized olefins (Scheme 10.13). However, regarding Mn(Salen) catalysts immobilized on the external surface of support for the asymmetric epoxidation of unfunctionalized olefins, the enantioselectivity remained unchanged while the conversion and chemical selectivity increased with increasing axial linkage lengths. It should be noted that the chiral Mn(Salen) catalysts in the hydrophobic nanopores shows higher activity and even higher enantioselectivity than those in the hydrophilic nanopores. The above examples show that the asymmetric reaction in nanopores, compared to that on the surface and in a homogeneous system, can improve the enantioselectivity for some asymmetric reactions. When the nanopore size of the support or the tether length is tuned to a suitable value, the chiral catalysts in the nanopores can show higher ee values for some cases. Moreover, the hydrophobic modification of the inner wall of MSs can also result in improved catalytic activity and enantioselectivity [81].

21 Can Li c10.tex V1-12/27/ :52am Page Catalytic Reactions in anoreactors 371 Color Fig.: Mn(salen) C acl C Bu t t Bu Mn S R Bu t Si Me/Et t Bu Catalyst Conv. (%) ee (%) TF (h 1 ) Mn(Salen)Cl 9.7AS-2-PhS 3 Mn(Salen) 9.7AS(Me)-2-PhS 3 Mn(Salen) 9.7AS-4-PhS 3 Mn(Salen) 9.7AS(Me)-4-PhS 3 Mn(Salen) 1.6MCM-4-PhS 3 Mn(Salen) anopore: AS (9,7) Surface: MCM (1,6) R: CH 2, CH 2 CH 2, CH 2 CH 2 CH 2 H Scheme Asymmetric epoxidation on Mn(salen) immobilized in the nanopores of AS(9.7) and on the external surface of MCM(1.6) with different linkage lengths. Modification of nanopores with methyl groups can further improve the TF and ee values. Reprinted with permission from Ref. [80]. Copyright 2006 Elsevier. Kureshy and coauthors [82] also reported that chiral Mn(Salen) catalyst immobilized in the nanopores of MCM-41 and SBA-15 (Scheme 10.14) showed higher enantioselectivity (70% ee) than its homogeneous counterpart (45% ee) for the enantioselective epoxidation of styrene with aqueous acl as the oxidant. In addition, the immobilized chiral Mn(Salen) could smoothly catalyze the epoxidation of bulkier alkenes such as 6-cyano-2,2-dimethylchromene into their epoxides with enanatioselectivity (up to 92% ee) comparable to those of the homogeneous counterparts. The heterogeneous catalyst could be recycled four times without notable loss of activity and enantioselectivity. Color Fig.: H Si H t Bu Mn CI MCM-41 or SBA-15 Bu t C t Bu Catalyst Ph Ph Homo. 45% ee Hetero. 70% ee Catalyst C Up to 92% ee Scheme Asymmetric epoxidation on chiral Mn(salen) catalyst immobilized in the nanopores of MCM-41 and SBA-15. Reprinted with permission from Ref. [82]. Copyright 2006 Elsevier.

22 Can Li c10.tex V1-12/27/ :52am Page Catalysis in Porous-Material-Based anoreactors Kureshy s group [83] immobilized the Mn(Salen) catalyst axially in the nanopores of MCM-41 via pyridine -oxide (Scheme 10.15). These immobilized catalysts showed higher enantioselectivity (69% ee) than their homogeneous counterparts (51% ee) for the asymmetric epoxidation of styrene and were also effective for the asymmetric epoxidation of bulkier substrates such as indene and 2,2- dimethylchromene (conversion: 82 98%; ee: 69 92%). The catalysts could be recycled for at least four times without loss in performance. The increase in ee values was attributed to the unique spatial environment constituted by the chiral Salen ligand and the surface of the support. R R R 1 R 1 R 2 Mn R 2 t Bu H Bu t Ph Catalyst Ph Homo. 51% ee Hetero. 69% ee Si MCM-41 Scheme Asymmetric epoxidation on chiral Mn(salen) catalyst axially immobilized in the nanopores of MCM-41 via pyridine -oxide. Reprinted with permission from Ref. [83]. Copyright 2005 Elsevier. For investigating the pore confinement effect, the chiral Mn(Salen) catalyst was immobilized in MCM-41 and MCM-48 with different pore sizes [84]. In the asymmetric epoxidation of unfunctionalized olefins with m-chloroperoxybenzoic acid as oxidant, it was found that the conversion and enantioselectivity were closely correlated with the pore size of the supports. The catalysts immobilized on MS with large pore sizes exhibited higher conversion. For the MCM-41-supported catalyst, the enantioselectivity increased with increasing pore size. However, for MCM-48- supported catalysts, the compatible pore size of the support with the substrate was found to be beneficial for obtaining higher enantioselectivity in olefin epoxidation. A chiral sulfonato-mn(salen) catalyst has been incorporated into cationic Zn Al layered double hydroxides (Scheme 10.16) [85]. This heterogeneous Mn(Salen) catalyst was highly active and enantioselective for the asymmetric epoxidation of various substituted styrenes and cyclic alkenes. As for the asymmetric epoxidation of 1-methylcyclohexene, 94% conversion with 90% selectivity to epoxide and 68% ee was obtained. The catalysts immobilized in the layered supports could be as effective as the molecular catalysts, and separated and recycled. Besides Mn(Salen) for asymmetric epoxidation reactions, other types of molecular catalysts immobilized in the nanopore also display higher ee values than their homogeneous counterparts. ne interesting example was reported by Thomas

23 Can Li c10.tex V1-12/27/ :52am Page Catalytic Reactions in anoreactors 373 Zn AI layer S 3 2 or Mn CI or S 3 54% de 68% ee Zn AI layer Scheme Chiral sulfonato- Salen Mn(III) complex immobilized in cationic Zn Al layered double hydroxides for the asymmetric epoxidation of various substituted styrenes and cyclic alkenes. Reprinted with permission from Ref. [85]. Copyright 2006 John Wiley & Sons. and coworkers [86]. They found that a chiral Rh(I) complex immobilized in the nanopores of silicas showed up to 77% ee for the heterogeneous asymmetric catalytic hydrogenation of α-ketone, while the homogeneous chiral catalyst gave nearly racemic products. Their results suggested that the confinement effect of the nanopores could enhance the enantioselectivity. The same group also reported that the solid catalyst prepared by anchoring 1,1 -bis(diphenylphosphino)-ferrocene in the nanopore of MCM-41 displayed remarkable increases in both enantioselectivity and catalytic activity in the hydrogenation of ethyl nicotinate compared to the homogeneous counterpart [87]. Subsequently, they reported another example with greatly increased activity and enantioselectivity by anchoring the molecular catalyst in the nanopores of silica [88]. Hutchings and coworkers [89] reported that a Cu catalyst modified with a chiral bis(oxazoline) ligand could be introduced into the pores of zeolite Y via ion exchange. This catalyst showed an ee value of 77%, higher than the homogeneous catalyst (28% ee), in the asymmetric aziridination of styrene. It is believed that the confinement effect of the zeolite cages can improve the chiral induction ability. The heterogeneous catalyst also exhibited enantioselectivity (93% ee) superior to that with the molecular catalyst (57% ee) for the reaction of methylenecyclopentane and ethyl glyoxylate. Similar heterogeneous chiral catalysts were prepared by the impregnation of mesoporous Al MCM-41, Al MCM-48, and Al SBA-15 with rhodium diphosphine organometallic complexes and were tested for the hydrogenation of dimethyl itaconate, methyl α-acetamidoacrylate, and methyl α-acetamidocinnamate [90]. The immobilized catalysts showed high activity and excellent chemo- and enantioselectivities, that is, up to >99% conversion, 99% selectivity, and 98% ee.

24 Can Li c10.tex V1-12/27/ :52am Page Catalysis in Porous-Material-Based anoreactors Caps et al. demonstrated that the heterogenization of the achiral cluster s 3 (C) 12 on the internal space of MCM-41 using simple CVD could generate a new chiral species in situ from achiral catalyst precursors, and this could lead to improved stereoselectivity toward the (S,S) configuration of the 1,2-diphenyl-1,2-ethanediol in the dihydroxylation of trans-stilbene using -methylmorpholine -oxide (M) as oxidant without adding any chiral ligand [91]. Up to 90% ee for the (S,S)-isomer was achieved when surface Al sites were introduced into the silicate. This was probably due to the spontaneous symmetry breaking of achiral s 3 (C) 12 during CVD on the MCM-41, which resulted in a new surface-chiral catalytic species. Recently, Pt nanoparticles encapsuled in the CTs showed higher enantioselectivity and catalytic activity than those on the outer surface of CTs (denoted as Pt/CTs(out)) for the asymmetric hydrogenation of α-ketoesters using cinchonidine (CD) as the chiral modifier [92]. The average turnover frequency (TF) for Pt/CTs(in) is above h 1, which is among the highest TF ever reported for heterogeneous asymmetric hydrogenations, and much higher than that of Pt/CTs(out) (Scheme 10.17). It should be mentioned that the TF of Pt/CTs(in) is accelerated up to 20 times in the presence of CD, while that of Pt/CTs(out) is only increased about 15 times, indicating the stronger acceleration effect of CD for Pt/CTs(in) than for Pt/CTs(out). High-resolution transmission electron microscopy (HRTEM) characterizations showed that the Pt nanoparticles inside/outside the CT had similar morphologies. Adsorption experiments suggested that CD and the reactants could be enriched in the channels of CTs. Thus, the high activity and enantioselectivity of Pt/CTs(in) are mainly attributed to the unique properties of the nanochannels of CTs that could readily enrich CD and the reactants. This work shows the unique effect of nanochannels of CTs as nanoreactors on asymmetric catalysis. Color Fig.: H C C H H C R H 2 R = ethyl or methyl Pt/CTs(in) H H H C* C H C H R 96% ee TF > h 1 Pt H H Pt Pt Pt/CTs(out) H H H C* C H C H R ~75% ee TF ~ h 1 Scheme Asymmetric hydrogenation of α-ketoesters on the Pt nanoparticles encapsulated inside CTs (Pt/CTs(in)) and loaded outside of CTs (Pt/CTs(out)) with cinchonidine (CD) as a chiral modifier. Reprinted with permission from Ref. [92]. Copyright 2011 John Wiley & Sons.

25 Can Li c10.tex V1-12/27/ :52am Page Catalytic Reactions in anoreactors 375 In the asymmetric epoxidation of olefins, it was observed by several groups that Mn(Salen) immobilized in the nanopore of MS showed higher enantioselectivity than that on the outer surface of MSs. A similar tendency was also reported for asymmetric hydrogenation and other types of reactions. More surprisingly, the activity and enantioselectivity of chirally modified Pt nanocatalyst confined inside the CTs nanochannels could result in enhanced enantioselectivity and activity in the asymmetric hydrogenation reaction. The above results suggest that the asymmetric catalytic mechanism in nanopores might be different from that in homogeneous systems due to the effects of the nanopores, the linkages, and the reaction microenvironment in the nanopores. These unique phenomena of the pore confinement effect on the reaction have been discussed for a long time. Thomas et al. [86] explained the pore confinement effect in view of the restrictions of the nanopore. For the asymmetric hydrogenation of E-a-phenylcinnamic acid, they found that rhodium(i) or palladium(ii) complexes of the bidentate amines immobilized in the nanopore of MS usually exhibited higher enantioselectivity than their homogeneous counterpart and those immobilized on the convex surface of silicas. The relevant calculation suggests that the access of the substrates to the active sites is favored only when the reactant approaches the active site along the axis of the pore, whereas no such restrictions exist in the case of the same catalyst anchored at the convex surface. In other works, they found that it was the constraints imposed by the space surrounding the metal center (active site) that dominated the enantioselectivity. Thus, the boost of enantioselectivity by immobilization of molecular catalysts in the nanopore is due to the restriction effect of the nanopore which could restrict the access generated by the nanopore or create a desired chiral nanospace in the nanopore. In addition to the spacial restriction, Thomas and coworkers [93] also proposed that the strengthened interactions between the incoming reactants and the chiral ligand, the catalytic metal center, and the porous surface in nanopores contribute greatly to the pore confinement effect (Scheme 10.18). The asymmetric epoxidation mechanism in nanopores has been studied using the Mn(Salen)-catalyzed asymmetric epoxidation as an example. Considering the reported mechanism of the homogeneous Mn(Salen) [94 97] and the experimental results of Mn(Salen) immobilized in nanopores [75 78], a mechanism is proposed in Scheme for the asymmetric epoxidation in nanopores. The olefin approaches the immobilized Mn(V) active sites to form the radical intermediate, which directly collapses to form cis-epoxide or first rotates and then collapses to form trans-epoxide. The influence of nanopores on the catalytic performance of immobilized Mn(Salen) for the asymmetric epoxidation and the general viewpoints can be summarized as follows: (i) The chiral recognition between the chiral ligand and the pro-chiral olefin may be enhanced or weakened by the stereo effect of nanopores [86, 88, 98]. The surface and the axial grafting modes may retard the coordination of some axial additives to the Mn atom, and this may reduce the asymmetric catalytic performance. (ii) The reaction microenvironment, including the nanopores and the grafting modes, may have an influence on the stability and lifetime of the radical intermediate in nanopores. More importantly,

26 Can Li c10.tex V1-12/27/ :52am Page Catalysis in Porous-Material-Based anoreactors Additive Linkage Catalytic center Chiral ligand Substrate anopores Scheme Factors originating from the pores influencing the chiral catalysis. Reprinted with permission from Ref. [93]. Copyright 2008 American Chemical Society. Color Fig.: Mn IV Mn IV Mn III aci Mn V A Ph B C aci A B Mn III Ph C A Ph Collapse B Cis-epoxide C Mn IV Ph A Ph Collapse A B Trans-epoxide C B C Mn IV Rotation Scheme Proposed mechanism of the asymmetric epoxidation in nanopores. Reprinted with permission from Ref. [76]. Copyright 2005 Elsevier.

27 Can Li c10.tex V1-12/27/ :52am Page Catalytic Reactions in anoreactors 377 the microenvironment of the nanoreactor is crucial for the asymmetric reactions in the nanoreactor. An appropriate microenvironment may increase the conversion and enantioselectivity for chiral reactions in nanopores. (iii) otably, the rotation of the radical intermediate may be significantly retarded by the nanopores, which would result in the production of more cis-epoxide (higher cis/trans ratio) compared to that of the homogeneous reaction. (iv) The linkage groups of the heterogeneous chiral catalysts may have a remarkable effect on the performance of asymmetric catalysis. (v) The electronic and steric properties of the linkages may affect the configuration of the transition state in the asymmetric reactions. Malek et al. [99, 100] studied the asymmetric epoxidation reaction of cis- and trans-methylstyrene on oxo-mn(salen) in the mesopore of MCM-41 via molecular dynamics simulations. Their calculations provided new insights into the importance of electronic and steric effects of the Salen ligand, substrate, immobilizing linker, and MCM-41 confinement. n the basis of the assumption that the formation of a radical intermediate is the key step along the reaction path, the calculations were performed on a catalytic surface with a triplet spin state, comprising no Mn(Salen) spin-crossing. The effect of immobilization was rationalized and correlated with the linker and substrate choices. The immobilized linker influences the enantioselectivity of the catalyst because of the increased chirality content of the Mn(Salen) complex. Simulations with docked olefin (β-methyl styrene) suggest that cis and trans substrates have different levels of chiral induction to the Mn(Salen) catalyst. A trans substrate induces higher chirality to the immobilized Mn(Salen) complex than cis-olefin. Although a trans substrate has a higher level of asymmetric induction to the immobilized Mn(Salen) complex than to a molecular catalyst, the reaction path is more in favor of the cis substrate. The MCM-41 channel could reduce the energy barriers and enhance the enantioselectivity by influencing the geometrical distortions of the Mn(Salen) complex. So far, there have been few studies concerning the factors originating from the pores and influencing the chiral catalysis. It is generally believed that tuning the steric and electronic properties of chiral ligands can alter the enantioselectivity for the homogeneous asymmetric catalysis. With regard to the heterogeneous chiral catalysts in nanopores, the pore effect could provide another alternative to improve the asymmetric induction by employing the nanopores with suitable pore structures and sizes Enhanced Cooperative Activation Effect in anoreactors The Kinetic Resolution of Epoxides The kinetic resolution (KR) of racemic mixtures of terminal epoxide catalyzed by chiral metal Salen complexes, such as Cr(Salen) and Co(Salen) (Salen =,bis(3,5-di-tert-butyl-salicylidene)-1,2-cyclohexene diamine), is of great interest in many total syntheses of natural products and drugs. Both Co(Salen) and Cr(Salen)

28 Can Li c10.tex V1-12/27/ :52am Catalysis in Porous-Material-Based anoreactors involve the bimolecular reaction pathway in the KR of racemic epoxides. The immobilization of the molecular catalysts involving a bimolecular reaction pathway on solid support still remains a difficult task because most immobilized molecular catalysts cannot move freely for the generation of a cooperative activation effect. n the basis of the encapsulation method we developed recently, the chiral Co(Salen) catalyst (a Salen ligand derived from 3,5-di-tert-butylsalicylaldehyde and trans-(1r,2r)-diaminocyclohexane) has been encapsulated in the nanocages of SBA-16 using propyltrimethoxysilane as the silylating reagent. By adjusting the concentration of chiral Co(Salen) catalyst in the initial adsorption mixture, the average molecular number of chiral Co(Salen) catalyst in each nanocages of SBA16 could be varied from 1 to 5 [68]. The Co(Salen)/SBA-16 catalysts with two or more Co(Salen) complexes in each cage exhibited much higher activity than the homogeneous Co(Salen) catalyst in the HKR of epoxides (Scheme 10.20). The catalytic activity gradually increased as the number of Co(Salen) in each cage increased, and then reached a plateau when the number of Co(Salen) molecules was up to 5 (Scheme 10.20). The increment in the activity and enantioselectivity with increase in the number of Co(Salen) complexes per cage obviously indicates that the cooperative activation effect of Co(Salen) catalyst in the nanocages can be enhanced by the crowded situation of the cobalt complexes in the nanocages. The appropriate proximity and the free movement of Co(Salen) in the confined space increase the chances for the cooperative activation between the activated H2 and activated epoxide, respectively, by two Co(Salen) catalysts for producing a diol with high activity. (a) Reaction rate (TF per hour) Color Fig.: (b) 160 H2 120 R H +H = Co H H H2 H R H Co H2 R Co t-bu Ac t-bu t-bu t-bu anoreactor The number of the catalyst in a nanocage Scheme (a) Catalytic activity of Co(salen)/SBA-16 as a function of Co(salen) number in each cage. (b) Enhanced cooperative activation in nanoreactor. For clarity, the ligands and CH3 C group of Co(salen) are omitted. Page 378

29 Can Li c10.tex V1-12/27/ :52am Page Catalytic Reactions in anoreactors 379 To further understand the enhanced cooperative activation effect in the nanocage, we compared the catalytic performances of the molecular Co(Salen) and Co(Salen) encapsulated in the nanocages of SBA-16 [Co(Salen)/SBA-16] for the HKR of propylene epoxide at high substrate/catalyst (S/C) ratio [68]. Under similar reaction conditions, Co(Salen)/SBA-16 generally afforded higher conversion than the molecular catalyst. As the S/C ratio was increased, the TF of molecular Co(Salen) catalyst decreased. n the contrary, the TF of Co(Salen)/SBA-16 increased gradually with the S/C ratio. At the S/C ratio of : 1, the molecular catalyst only afforded 7% conversion even though the reaction time was prolonged to 24 h, and the enantioselectivity simultaneously decreased from 98% ee to 89% ee. Meanwhile, Co(Salen)/SBA-16 could still afford 50% conversion with 98% ee of the diol at the S/C ratio of : 1. The high activity of the solid catalyst at low catalyst concentration is due to the enhanced cooperative activation effect in the nanoreactor because the local concentration of Co(Salen) in the nanoreactor does not change with the S/C ratio. Similar to Co(Salen), the TF and ee value of chiral Cr(Salen)py (Cr(Salen) with two pyridine coordinated) encapsulated in the nanocages of FDU-12 show a significant enhancement, respectively, from 64 to 170 h 1 and from 64 to 91% with increasing concentration of Cr(Salen)py in the nanocages for the KR of terminal epoxides via the asymmetric ring opening (AR) with TMS 3 [71]. This suggests the existence of the cooperative activation effect of Cr(Salen)py. It should be mentioned that even at an S/C ratio as high as , Cr(Salen)py@FDU-C could still give 40% conversion of epoxide with 91% ee of the ring-opening product, while nearly no conversion of epoxide could be observed for Cr(Salen)py. As far as we know, this is the most active solid catalyst that can catalyze the KR of terminal epoxide via AR with TMS 3 with high conversion and ee value at such high S/C value. The extremely high activity of Cr(Salen)py@FDU-C at high S/C ratio is probably benefiting from the high local concentration of Cr(Salen)py catalyst in the nanocages. The solid catalyst can be recycled by a simple filtration followed by the successive washing with CH 2 Cl 2. Usually, two molecular complexes (Co(Salen), Ti(Salen), Cr(Salen), etc.) are linked together via the linker groups for enhancing the cooperative activation of these two molecular catalysts. It was found that the dimeric catalysts indeed exhibit much higher activity than monomers especially at low catalyst concentration. This result convincingly confirms the cooperative activation effect in the asymmetric reaction and is further corroborated by a density functional theory (DFT) calculation which shows that the activation energy could be greatly reduced when the reaction goes through a bimolecular activation pathway. Recently, Cui and coworkers [101] reported that Co(Salen) (L 25 in Scheme 10.6) incorporated in chiral MFs also could go through bimolecular reaction pathways for the HKR of racemic epoxides with up to 99.5% ee. Crystal structure analysis suggests that the MF structure brought Co(Salen) units into a highly dense arrangement and close proximity which could enhance the bimetallic cooperative interactions. The same bimolecular activation process in Co(Salen)-based MF has also been found by Lin and coworkers [102].

30 Can Li c10.tex V1-12/27/ :52am Page Catalysis in Porous-Material-Based anoreactors Water xidation Reactions The cooperative activation effect was further demonstrated for water oxidation reaction [103]. Two kinds of molecular catalysts, Ru II (bda)(pic) 2 (bda: 2,2 -bipyridine- 6,6 -dicarboxylate) and Ru II (pda)(pic) 2 (pda: 1,10-phenanthroline-2,9-dicarboxylate and pic: 4-picoline), were encapsulated in the nanocage of SBA-16 (Scheme 10.21a) for water oxidation. The TF of Ru II (bda)(pic) 2 encapsulated in the nanocages of SBA-16 increased from 1.2 to 8.7 s 1 as the molecular number of Ru II (bda)(pic) 2 in each nanocage of SBA-16 increased from 1 to 7, showing that Ru II (bda)(pic) 2 involved the bimolecular reaction pathway (Scheme 10.21b). wing to the enhanced cooperative activation effect in the limited space of the nanoreactor, the TF of the solid catalyst could reach 8.7 s 1, which is much higher than that of the homogeneous Ru II (bda)(pic) 2 catalyst (4.5 s 1 ). However, the TF of Ru II (pda)(pic) 2 remained almost the same when the molecular number of Ru II (pda)(pic) 2 in each nanocage increased, showing Ru II (pda)(pic) 2 follows the monomolecular reaction pathway (Scheme 10.21b). It was also observed that the stability of the molecular catalysts was markedly increased by crowding them in the nanoreactor despite the mono- or bimolecular reaction pathway. The above method actually provides an efficient and general strategy to assemble molecular catalysts in the solid host. Color Fig.: (a) (b) 10 8 H 2 M 4+ M Ce M Ce 4+ Ce 4+ M M M Ce M 4+ Molecular catalysts 2 + H + Ce 4+ Ce 3+ SBA-16 Silylation reagent TF (s 1 ) umber of Ru II (bda)(pic) 2 molecules in each cage 0.15 M Ru Ru TF (s 1 ) Ru II (bda)(pic) 2 Ru II (pda)(pic) umber of Ru II (pda)(pic) 2 molecules in each cage Scheme (a) Schematic illustration of encapsulation of Ru II (bda)(pic) 2 and Ru II (pda)(pic) 2 in the nanocage of MS. (b) Relation between TF and the molecular number of Ru complexes in each nanocages for water oxidation. Reprinted with permission from Ref. [103]. Copyright 2012 Royal Society of Chemistry.

31 Can Li c10.tex V1-12/27/ :52am Page Catalytic Reactions in anoreactors Epoxide Hydration n the basis of the enhanced cooperative activation, efficient solid catalysts for ethylene epoxide (E) hydration were developed by encapsulating Co(Salen) catalyst (a Salen ligand derived from 3,5-di-tert-butylsalicylaldehyde and transdiaminocyclohexane) in the nanocages of FDU-12 [67]. The challenge for the industrial production of monoethylene glycol (MEG) by E hydration is to reduce the huge energy consumed for distillation of the product from the aqueous solution ( 10 wt% MEG; H 2 /E higher than 20 is needed during the production process to increase the MEG selectivity). This catalyst, which is different from the conventional liquid/solid acid or base catalysts, was able to achieve the conversion of E by >98% and the selectivity to MEG of >98% in the hydration of E at 40 CwiththeH 2 /epoxide molar ratio as low as 2. Under similar reaction conditions, the acids H 3 PW 12 40,H 2 S 4, Amberlite IR 120, and H-ZSM-5 could not catalyze the reaction efficiently (less than 70% E conversion with less than 70% MEG selectivity). These results indicate that Co(Salen) catalysts confined in the nanocages are very active and selective at such low H 2 /E molar ratio and mild reaction temperatures, while the conventional processes using liquid and solid acids as catalysts cannot achieve high yield of MEG when the H 2 /E molar ratio is close to the stoichiometric value. This is the highest performance (activity and selectivity) ever reported for epoxide hydration at such low H 2 /E ratio. Mechanistic study shows that the high activity of the catalyst is mainly derived from the enhanced cooperative activation effect and the enrichment of reactants in the nanoreactor (Scheme 10.20b). This result strongly confirms the cooperative activation effect in the asymmetric reaction and is corroborated by a DFT calculation that the activation energy could be greatly reduced when the reaction goes through a bimolecular activation pathway (Scheme 10.22) [104]. Color Fig.: Energy 97 kj mol 1 Single catalyst R CH CH 2 + H 2 32 kj mol 1 Dual catalysts H H R Catalyst = R 2 Co CH R Ac 2 CH 2 R 1 R 1 H H Reaction coordinate Scheme DFT calculation of the activation energy for Co(salen)-catalyzed kinetic resolution of epoxide. The epoxide hydration in the nanoreactor is not only an energy-saving but also an environmentally benign process, which holds great potential for industry. It

32 Can Li c10.tex V1-12/27/ :52am Page Catalysis in Porous-Material-Based anoreactors was anticipated that this strategy could be extended to developing many other solid catalysts based on the encapsulation of molecular catalysts in solid nanoreactors for sustainable industrial production of chemicals and pharmaceuticals. For a molecular catalyst, the concentration is negatively correlated with the S/C ratio, and therefore chances for the generation of cooperative activity is low at a high S/C ratio and the activity decreases accordingly. For a reaction that follows a bimolecular reaction pathway, high catalyst concentration is desirable for achieving high catalytic activity. In the case of a solid catalyst, the local concentration of the molecular catalyst in each nanocage is much higher than that in a homogeneous catalytic system regardless of the changes in the S/C ratio. Consequently, the solid catalyst shows increased activity than the molecular catalyst especially at high S/C ratios owing to the enhanced cooperative activation effect in the nanoreactor. It has been demonstrated that the nanocage could enhance the cooperative activation effect by the increased local concentration of the catalyst as well as the higher collision possibility between the reactants. Thus, the enhanced cooperative activation effect mainly originates from the confinement effect of the nanospace Isolation Effect in anoreactors In addition to the enhanced cooperative activation effect of the nanoreactor, the isolation effect could also be expected in the confined nanospace if the diameter of nanopore is similar to the size of the molecular catalysts, because the limited nanospace could restrict the free movement of the molecular catalysts. Two issues relevant to the isolation effect of the nanoreactor, namely selectivity control in organic reactions and inhibition dimerization of the molecular catalysts, will be discussed Selectivity Control Selectivity (chemo-, regio-, and stereoselectivity) control is a key issue for organic synthesis. In addition to controlling the selectivity by developing appropriate catalytic systems, the selectivity of a chemical reaction could be controlled by the restriction of the reaction in a confined nanospace. For example, we discussed the enhancement of enantioselectivity by the pore confinement effect in Section In this section, we will discuss the selectivity control of a chemical reaction by the isolation of the substrates and the restriction on the rotational and translational motions of the substrates in a confined nanospace. A typical example for shape selectivity is the alkylation of alkyl aromatics to dialkylated compounds with meta, ortho, and para isomers [105]. In general, alkyl cations attack the 2-, 4-, and 6-positions of alkyl aromatics because alkyl groups are ortho- and para-directing. Para substitution is more favorable over ortho substitution if the alkyl groups are bulky. However, the isomerization of para isomers to ortho isomers always occurs on the acidic site. Thus, the selectivity to para isomers is low. This is not the case for the alkylation reactions taking place in

33 Can Li c10.tex V1-12/27/ :52am Page Catalytic Reactions in anoreactors 383 the cavity of zeolites with a suitable pore diameter. In a confined space of zeolites, the isomerization reaction is prevented because there is not enough space for such a reaction which usually requires a large space (Scheme 10.23) [106]. Thus zeolites with suitable pore diameter and poisoned external acidic sites generally exhibit high selectivity to the paraisomers due to the confinement effect. Color Fig.: H Scheme compounds. Shape selectivity of zeolites in the alkylation of alkyl aromatics to dialkylated Another representative example for increasing the selectivity of a chemical reaction by using the confined space of zeolites is the intramolecular photocycloaddition reaction. In this reaction, the substrates may undergo either intramolecular reactions for the production of macrocyclic ring-closure products or intermolecular reactions leading to dimers, oligomers, and polymers. Tung and coworkers [107] reported that only intramolecular photocycloaddition is observed by incorporating substrates in the supercages of ay zeolite. Since each cage of zeolites only contains one substrate molecule, the intermolecular reaction pathway is entirely prevented and the selectivity of intramolecular product could thus be greatly enhanced. Afterwards, the same group also successfully synthesized the cross photocyclomers between an anthracene and a naphthalene moiety from the bichromophoric molecules with anthryl as one chromophore and naphthyl as the other (-Pn-A), while the photoproducts could not be obtained in homogeneous solution. Selective oxidation of small, abundant hydrocarbons using molecular oxygen as the oxidant is another example for increasing the selectivity by the isolation of the substrates. Energy transfer (does not need close contact of the sensitizer and substrate) and electron transfer (needs the close contact of the sensitizer and substrate) are the two well-established types of photoxidations involving molecular oxygen [108]. In a number of cases, the two types occur simultaneously and the selectivity of the oxidation reactions is poor. In order to increase the selectivity, Tung et al. [109] used the channels of ZSM-5 zeolite to trap alkene substrates, and isolated the photosensitizers in the surrounding solution (Scheme 10.24). The isolation of the substrate within the zeolite from the sensitizer in the outside

34 Can Li c10.tex V1-12/27/ :52am Page Catalysis in Porous-Material-Based anoreactors Color Fig.: Sens 1 2 hυ Sens H Solvent C H 3 C CH 3 Isooctane C DCA H 3 C HA H H CCH 3 CH 3 PTE H Q3 Scheme DCA- and HA-photosensitized oxidation of alkenes under sensitizerexcluded condition [109]. Q4 solution exclusively leads to the formation of singlet oxygen products through the energy-transfer photoxidations. The nanospace of zeolite may impose a restriction on the rotational and translational motions of substrate molecules and reaction intermediates. This would promote or discourage specific reactions. The photochemical reaction of phenyl phenylacetates 1 4 (Scheme 10.25) within zeolites can be regarded as a good example [110, 111]. Photolysis of 1 4 in a homogeneous solution results in the formation of ortho-hydroxyphenones 20 (40 60%), para-hydroxyphenones 21 (20 25%), phenols 22 (5 15%), diphylethanes 23 (5 15%), and phenyl benzyl ethers 24 (3 8%). However, the photolysis of all four esters in ay zeolite can produce only 20. Molecular models suggest that the esters can enter into Color Fig.: Q5 Scheme [110]. Molecular structure of phenyl phenylacetates for photochemical reactions

35 Can Li c10.tex V1-12/27/ :52am Page Catalytic Reactions in anoreactors 385 the ay zeolite s internal surface. The preference for the formation of orthohydroxyphenones is derived from the restriction on the diffusional and rotational motions of the geminate radical pair. The above results show that the photochemical reactions of organic compounds in a confined nanospace usually give different product distributions from their molecular photochemical reactions in solution and, in some cases, result in the occurrence of reaction pathways that are not otherwise observed. Q Inhibiting Dimerization of Molecular Catalysts Some molecular catalysts tend to form catalytically inactive dimers/oligomers during the catalysis, such as [Mn(Salen)] [96], Ir-diphosphine [112, 113], and M-porphyrin [114, 115]. The formation of dimers can deactivate the catalysts. To avoid the deactivation caused by the dimers/oligomers, the isolation of molecular catalysts from contacting each other is necessary. This could be achieved by isolation of the molecular catalysts on to a solid support. For example, Hupp and guyen et al. [116] reported the isolation of Mn(Salen) in MFs by using L 18 as the organic linker (Scheme 10.6). Mn(Salen)-based MF exhibits high enantioselectivity and stability than the free molecular analog in the asymmetric olefin epoxidation of 2,2-dimethyl-2H-chromene due to the spatial separation of the catalytic centers in MFs. Similar Mn(Salen)-based MFs were also reported by Lin and coworkers [117] using L 19 L 21 (Scheme 10.6) as organic linkers and Zn 2+ as metal nodes. The same group also realized the tandem epoxidation and ring-opening reaction on Mn(Salen)-based MF prepared using L 22 as the organic linker and Zn 2+ as the inorganic node [118]. In addition to the strategies mentioned above, the limited freedom of the molecular catalyst favors the prohibition of the dimer/oligomer formation. The molecular catalysts can also be isolated in molecular capsules or nanoreactors. Recently, Yang and coworkers [119] successfully confined the second-generation Hoveyda Grubbs catalyst in the nanocages of SBA-1. The cage size of SBA-1 is about 1.3 nm, which can only accommodate one molecular catalyst in each nanocage and is not big enough for the coexistence of two molecules. Therefore, the molecular catalyst was isolated from each other and could not form dimmers during the catalytic process. The molecular catalysts confined in SBA-1 exhibited extremely high stability and could be recycled nine times. Thus, the nanoreactor could show both the cooperative activation effect and the isolation effect by modulating the cage size of the nanoreactor and the concentration of the molecular catalysts accommodated in each nanocage Microenvironment Engineering of anoreactors The microenvironment can greatly influence the performance of the nanoreactor. First, it can affect the diffusion rate of the reactants and products in the nanoreactor. Second, the nanoreactor with a specific microenvironment may enrich a specific reagent, and thus the concentration of substrates and products inside the

36 Can Li c10.tex V1-12/27/ :52am Page Catalysis in Porous-Material-Based anoreactors nanoreactor may be different from that on the outside. Third, the surface of the nanoreactor may become a factor for a chemical reaction due to the fact that its diameter is in the same scale as the molecular size of the reactants, products, and the molecular catalysts. The above factors may alter the interaction modes of the reactants with the molecular catalyst, resulting in varied activity and selectivity. As we mentioned in Section , Mn(Salen) immobilized in a hydrophobic nanopore affords much higher activity and even enantioselectivity than that in a hydrophilic nanopore in the asymmetric epoxidation of olefins [80]. This is due to the increased diffusion rate of the hydrophobic substrate into the nanopore with hydrophobic surface properties. Thus, the surface modification of the nanoreactor, depending on the polarity of the reactants and products, may be an efficient method for increasing the activity of the immobilized molecular catalysts. We reported the incorporation of (1R,2R)-diaminocyclohexane in hydrophobic and hydrophilic nanopores by the co-condensation of -[(triethoxysilyl)propyl]- ( )-(1R,2R)-diaminocyclohexane, respectively, with (Me) 3 SiCH 2 CH 2 Si(Me) 3, (BTME) and TES in basic media [120]. After complexing with [Rh(cod)Cl] 2,the chiral catalyst in the hydrophobic nanopore affords 96% conversion with 23% ee in the ATH of acetophenone using i-prh as the hydrogen source, while the chiral catalyst in hydrophilic nanopore only shows 48% conversion with 14% ee under identical conditions. This is probably due to the specific adsorption and physical properties of the mesoporous network bridged with ethane groups, particularly the hydrophobic properties. We tried to encapsulate chiral Ru-TsDPE in the nanocages of SBA-16 using the encapsulation method, and to modify the microenvironment of the nanoreactor in which Ru-TsDPE is accommodated using the hydrophobic propyl group amphiphilic-,,-tri-n-butylammonium group and their mixtures (Scheme 10.26) [72]. It was observed that Ru-TSDPE encapsulated in an amphiphilic nanoreactor showed the highest activity in comparison with that in Color Fig.: Propyl Si C 4 H 7 C 4 H 7 C 4 H 7 Si Br S Ru H 2 C l Scheme Illustration of the encapsulation of Ru-TsDPE in the nanocage with different hydrophobic/hydrophilic microenvironments. Reprinted with permission from Ref. [72]. Copyright 2010 Royal Society of Chemistry.