MCM-50

SYNTHESIS AND PROTOLYTIC PROPERTIES OF THE PERIODICALLY ORDERED MESOPOROUS ORGANOSILICAS WITH MCM-41-TYPE ARCHITECTURES FUNCTIONALISED... 

Throughout this paper we use the Strukturbericht labels T3, DSg,, 44 for Ti, TisSia, and Si phases, respectively. The symmetry of the DSg structure corresponds to the (P6a/mcm) space group. The structure is hexagonal with lattice constants" a = 14.039 a.u. and c=9.712 a.u. The special coordinates (in fractions of the lattice constants) are... 

B.S. Uphade, M. Okumura, S. Tsubota, and M. Haruta, Effect of physical mixing of CsCl with Au/Ti-MCM-41 on the gas-phase epoxidation of propene using H2 and02 Drastic depression of H2 consumption, Appl. Catal. A 190, 43-50 (2000). 

Figure 5. Hexagonal array of cylindrical micelles of cetyltrimethylammonium in MCM-41 synthesis...

MCM-41 can be synthesized with Si/Al ratio >15. After calcination the pore diameter was determined by adsorption techniques and by thermoporometry (ref. 9) to amount to 4.0 nm. 

Henze M, van Loosdrecht MCM, Ekama GA, Brdjanovic D (2008) Biological wastewater treatment principles, modelling and design. IWA Publishing (ISBN 9781843391883), pp 528... 

Cuce LC, Bertino MCM, Scattone L, Birkenhauer MC (2001) Tretinoin peeling. Dermatol Surg 25 12-14... 

Clarke and Shannon also supported copper bis(oxazoline) complexes onto the surfaces of inorganic mesoporous materials, such as MCM-41 and MCM-48, through the covalent binding of the ligand, modified by alkoxysilane functionalities [59]. The immobilized catalysts allowed the cyclopropanation of styrene with ethyldiazoacetate to be performed as for the corresponding homogeneous case, and were reused once with almost no loss of activity or selectivity. 

Study [23] Jacobsen s complex was entrapped in the final step of the zeohte synthesis (method C). This process was possible because MCM-22 zeohte is prepared by condensation of a layered precursor, which is exchangeable by the catalytic complex. Leaching of Mn was not observed in these systems, which is not unexpected bearing in mind that the complex is also bovmd to the zeolite structure through an electrostatic interaction. 

In spite of these limitations, three examples of (salen)-metal complex adsorption have been described. In the first one, Jacobsen s complex (la-MnCl) was adsorbed on Al-MCM-41 [27] by impregnation with a solution of the complex in dichloromethane, an approach that prevents the possible cationic exchange. The results in the epoxidation of 1,2-dihydronaphthalene with aqueous NaOCl were comparable to those obtained in solution, with only a slight reduction in enantioselectivity (55% ee instead of 60% ee). However, recycling of this catalyst was not described. 

The mesoporous character of MCM-41 overcomes the size limitations imposed by the use of zeolites and it is possible to prepare the complex by refluxing the chiral ligand in the presence of Mn +-exchanged Al-MCM-41 [34-36]. However, this method only gives 10% of Mn in the form of the complex, as shown by elemental analysis, and good results are only possible due to the very low catalytic activity of the uncomplexed Mn sites. The immobihzed catalyst was used in the epoxidation of (Z)-stilbene with iodosylbenzene and this led to a mixture of cis (meso) and trans (chiral) epoxides. Enantioselectivity in the trans epoxides was up to 70%, which is close to the value obtained in solution (78% ee). However, this value was much lower when (E)-stilbene was used (25% ee). As occurred with other immobilized catalysts, reuse of the catalyst led to a significant loss in activity and, to a greater extent, in enantioselectivity. 

Jacobsen s complex (la-MnCl) was also immobihzed on MCM-41 modified with phenolic groups [39]. The surface phenoxide groups act as an-... 

Table 4 Results of the alkene epoxidation reactions with m complexes immobilized on MCM-41 by cationic exchange -CPBA catalyzed by (salen)Mn ...

In the case of the Diels-Alder reaction [68] (Scheme 12), several soUds (AlSBA-15, MCM-41, MSU-2 and zeolite HY) were tested as supports for the bis(oxazoline)-copper complexes. The best enantioselectivity results were obtained with the zeolite HY, although the yield was the poorest (16% yield, 41% ee). As happened with the aziridination reaction, the enantioselectivity changed with time. Short reaction times led to the same major enantiomer as observed in homogeneous reactions. However, at higher conversions, i.e., longer reaction times, the opposite major enantiomer was obtained. 

The same type of porphyrin-Ru complex was immobilized by coordina-tive adsorption on aminopropylsilicas (Fig. 26) as either amorphous or crystalline supports [79]. Mesoporous crystalline MCM-48 was the best support, as shown by the improved results obtained in the epoxidation of styrene with 2,6-dichloropyridine N-oxide (TON > 13 000 and 74% ee). The versatility of this catalyst was demonstrated in the intramolecular cyclopropanation of frans-cinnamyl diazoacetate. TON was ten times higher than that obtained in solution and 85% ee was observed. The solid was recycled and reused, although partial loss of selectivity occurred. 

---