Heterogeneous mesoporous silica

Methylrheniumtrioxide (MTO) can straightforwardly be immobilized using copolymers consisting of polystyrene and polyvinyl pyridine. The authors proposed two possible structures 18 shown in Scheme 4.10. In the presence of H2O2 these catalysts are efficient and selective heterogeneous catalysts for the epoxidation of alkenes. It was shown that their activity is maintained for at least five recycling experiments [23 dj. In analogy to this work, bipyridyl-functionalized mesoporous silica can be employed for the immobilization of MTO [75]. 

We have also observed that when the pore size become smaller (less than 2 nm), the quality of the monolayer degrades (Figure lc). The 2,Si peaks corresponding to the siloxane groups are broad, indicative of polymeric siloxanes with heterogeneous chemical environment. At the same time, the peak corresponding to the bulk silicon (from mesoporous silica) has a pronounced Q, component, suggesting the siloxanes are not chemically bonded to the substrate. 

The use of heterogeneous catalysts in the synthesis of urethanes from aliphatic and aromatic amines, C02 and alkyl halides has been explored only recently. Titanosilicate molecular sieves [60a], metal phthalocyanine complexes encapsulated in zeolite-Y [60a], beta-zeolites and mesoporous silica (MCM-41) containing ammonium cations as the templates [60b, c], and adenine-modified Ti-SBA-15 [60d, e] each function as effective catalysts, even without any additional base. 

A more widely used method of heterogenization has also been applied to MTO, by using a metal oxide support onto which the MTO is applied. MTO has been applied onto niobia (Nb2Os) both via impregnation and sublimation [39] and onto silica (Si02) via reaction of a bipyridine-containing siloxane [40]. In a similar manner, MTO has been immobilized on the mesoporous silica MCM-41 [41]. Zeolite NaY has been used as a support material by the in situ immobilization of the MTO catalyst [42]. 

Maschmeyer, T., Rey, F., Sankar, G. and Thomas, J. M. Heterogeneous catalysts obtained by grafting metallocene complexes onto mesoporous silica, Nature, 1995,378,159-162. 

Recently two heterogeneous TPAP catalysts were developed which could be recycled successfully and displayed no leaching In the first example the tetraalkylammonium perruthenate was tethered to the internal surface of mesoporous silica (MCM-41) and was shown [ 101] to catalyse the selective aerobic oxidation of primary and secondary allylic and benzylic alcohols (Fig. 17). Surprisingly, both cyclohexanol and cyclohexenol were unreactive although these substrates can easily be accommodated in the pores of MCM-41. No mechanistic interpretation for this surprising observation was offered by the authors. 

Figure 10.14 shows two examples of supported nanopartides from heterogeneous catalysis. A 20wt% loading of 4nm NiO particles has been deposited inside the mesopores of SBA-15, an ordered mesoporous silica. Reduction results in highly... 

---