Isomorphous mesoporous materials

Figure 1 shows the FT-1R spectra of samples dispersed in KBr. All the spectra display a strong band at 960 cm 1. This band has been assigned to Si-O-Ti bonds [14] or to Si-OH groups [15, 16]. It is usually taken as the evidence for isomorphous substitution of Si by Ti, but it cannot be used to determine quantitatively the content of titanium into the framework of mesoporous materials [17]. In addition, the broad pattern between 3700 and 3000 cm 1, originated from hydrogen-bonded surface OH groups as well as from adsorbed H20 [18], decreases dramatically in the silylated samples. 

Zeolites are crystalline but versatile materials. They may be modified in many ways they can be tuned over a wide range of acidity and basicity, and of hydrophylicity and hydrophobicity, many cations can be introduced by ion exchange and isomorphous substitution is possible also allowing build-in of isolated redox centers (e.g. Ti) in the lattice. Moreover metal crystallites and metal complexes can be entrapped within the microporous environment. There is for instance much progress in enantioselective synthesis on chiral catalysts immobilized in microporous or mesoporous materials [16]. 

Isomorphous substitution was also reported for mesoporous materials. In a study on properties of Ti-containing MCM-41 by Corma and co-workers [349], it was reported that diffuse reflectance spectroscopy also with this material provided a band at 960 cm... 

Several framework titanium-substituted mesoporous silicates, including Ti-MCM-41 (42,43), Ti-HMS (198), Ti-MCM-36 (180), Ti-MCM-48 (199), and Ti-SBA-15 (200), have shown promising activity for the epoxidation of bulky olefins with alkyl hydroperoxides as oxidants. Unfortunately, compared with the microporous MFI-type titanium silicates, the mesoporous materials exhibit low activity for epoxidation reactions. The hydrophilic nature of mesoporous silica catalysts with isomorphous titanium substitution is considered to be one of the major reasons for the low activity (179). Various attempts have been made to improve the activity. Using a different synthetic procedure, titanium species have been grafted onto... 

The syntheses of a large number of templated, mesoporous materials other than silica have also been reported. These materials are mainly of interest to catalysis and hence will be described only briefly here. A good summary of their synthesis routes and factors involved in the syntheses has been given by Ruckenstein and co-worker (Ruckenstein and Chao, 2001 Chao and Ruckenstein, 2002a and 2002b). Metals/elements such as Al, B, Ti, Zr, V, Pd, and Mn can be introduced by either grafting or isomorphous substitution onto the framework... 

Zeolites and related materials prepared by isomorphous substitution of lattice atoms can also be considered as sohd solutions and yield specifically designed micro- and mesoporous catalysts. 

Y zeolites with dealumina ion levels lower than about 50% were, according to N2 adsorption, exempt from mesopores. Moreover, since no trace of amorphous material was apparent in these solids, it could be concluded in agreement with other physico-chemical properties, such as an excellent X-ray crystallinity that the isomorphous Si-Al substitution took place successfully. For solids with higher dealumination levels (>50%) the isomorphous substitution was no longer ideal resulting in the formation of a secondary pore system. 

New materials consisting of amorphous silica with regular pore structure, therefore called mesoporous molecular sieves, have recently been described [7]. Isomorphous substitution of Si by Ti has been attempted by performing the synthesis in the presence of titanium compounds. Ti-MCM have been tested for oxidation of hydrocarbons in liquid phase, using HjOj or hydroperoxides as oxidants [8-10]. 

It is the aim of the present paper to review the synthesis of a selected number of microporous materials which have more recently attracted the attention of zeolite scientists. The following selection criteria were appUed (i) the review is restricted to aluminosiUcates, hence, isomorphously substituted zeolites, SAPOs, MeAPOs and the Hke are not considered, (ii) crystalline microporous aluminosilicates are exclusively dealt with, the synthesis of ordered mesoporous molecular sieves (i.e. MCM-41) is not discussed and, (iii) especially those zeolites have been selected which, based on their particular structural or physicochemical properties, have a certain potential for appHcations in catalysis or adsorption. Some readers might miss such important and interesting zeolites like Beta, EMC-2, ZSM-12, etc. However, although there is still intensive research going on in the synthesis of these zeoHtes, their main synthesis principles have been summarized earUer [5,6] and will therefore not be discussed here. 

The isomorphous substitution of Si with Ti and Zr in mesoporous sih-cates with MCM-41 structure has also been studied by means of NH3 mi-crocalorimetry experiments [284]. These materials are weakly acidic solids, and the introduction of Ti or Zr into the mesoporous silicate framework has been found to increase the acidity in the order silica-gel [Pg.137]

---