Other Metal-Framework Oxidation Catalysts

The discovery that Ti(IV) incorporated into the framework of zeolites produces an outstanding oxidation catalyst stimulated the incorporation of other 

FIGURE 6.12 Suggested radical mechanism for oxidation of alkanes.33 36 55 

Effective catalysts for heterogeneous oxidations using 02 are mainly Pt and Pd with some activity by Ir70 and Ru.71 Much work has gone into alcohol oxidations that are dehydrogenations to ketones or aldehydes. Also, oxygen may be inserted at allylic positions of alkenes and these may be dehydrogenated to ketones or aldehydes.72 In the case of aldehydes, additional oxidation may be accomplished to produce acids.72,73 

Of particular interest are oxidations of unsaturated alcohols, for example, oxidation of cinnamyl alcohol to cinnamaldehyde,74,75 and special promoters have been added to increase selectivity (Fig. 6.13).75 Although the functions of these promoters are still not fully undestood, some authors attribute their increased selectivity to physical blocking of reaction sites. This blocking reduces the size of the active site ensemble and suppresses the tendency for alcohols to strongly adsorb and dissociate on Pt.75 

FIGURE 6.13 Bi promotion of Pt to increase selectivity of cinnamyl alcohol to cinnamaldehyde.75 

---

Since the initial discovery, much work has gone into improving the catalyst. The original zeolite contained small pores that limited oxidations to relatively small molecules with shapes that allowed them to move in and out of that pore system. One modification has been to isolate titanium in zeolites with larger pores so larger molecules can be oxidized. Another modification has been to incorporate other metal ions into the frameworks of different zeolites with... 

The isomorphous substitution of T atoms by other elements produces novel hybrid atom molecular sieves with interesting properties. In the early 1980s, the synthesis of a zeolite material where titanium was included in the MFI framework of silicalite, that is, in the aluminum-free form of ZSM-5, was reported. The name given to the obtained material was titanium silicalite (TS-1) [27], This material was synthesized in a tetrapropylammonium hydroxide (TPAOH) system substantially free of metal cations. A material containing low levels (up to about 2.5 atom %) of titanium substituted into the tetrahedral positions of the MFI framework of silicalite was obtained [28], TS-1 has been shown to be a very good oxidation catalyst, mainly in combination with a peroxide, and is currently in commercial use. It is used in epoxidations and related reactions. TS-1, additionally an active and selective catalyst, is the first genuine Ti-containing microporous crystalline material. 

Table 1.10 List of processes for propylene oxide manufacture with metal framework-containing zeotype or other porous materials as catalysts...

Substituted aluminophosphates are of interest as acid and oxidation catalysts. Typical substitutions include M for Al (M = Mg, Mn, Fe, Co, Zn), Si for and 2Si" for Al " -i- (described in Chapters 2 and 3). In cases where the degree of substitution is at a few percent, similar methods to those for substitutional metallosilicates can be used for confirmation (measurement of unit cell dimensions, P MAS NMR for measuring the substitutions of cations for A1 and Si NMR for measuring the mode of silicon substitution). For some of the metals, much higher metal substitutions are possible than for silicates, and complete replacement of aluminium by cations such as cobalt in tetrahe-drally connected frameworks has been reported (see Chapter 2). A number of other metals have been claimed to have been substituted into the aluminophosphate framework, including Ti" and C -3+ i03,i04... 

Vanadium. V205-based catalysts have been widely used in industry for many catalytic processes including oxidation reactions under mild conditions. On the other hand, incorporation of metal ions exhibiting redox properties into an alnminophosphate framework with a well-defined system of pores and cavities offers an opportunity for the preparation of novel oxidation catalysts. In this context, vanadium-modified aluminophosphates seem to be promising materials. However, incorporation of vanadium in tetrahedral alnminophosphate frameworks has been for a long time a debatable issue. 

EPR has been used in the period covered by the past two years mainly to characterise iron ions in the framework of zeolites. Some papers, however, have considered the nature of iron species dispersed at the surface of a bulky sup-portiio,iii silica or alumina. In the former case" traces of iron added to silica increased the activity in the oxidation of methane to formaldehyde. Though only indirectly connected to heterogeneous catalytic reactions, the diffusion of iron into the bulk of oxides of catalytic interest has been followed by EPR "- " in the case of titanium dioxide and other metal oxides. A true catalyst is instead the perovskite material investigated by Oliva and Forni " who provided evidence for the onset of superparamagnetism in non-stoichiometric LaFeOs material, which was correlated with the catalytic activity. 

Since this initial work there has been a plethora of literature on mesoporous molecular sieves. In addition to the silica and aluminosilicate frameworks similar mesoporous structures of metal oxides now include the oxides of Fe, Ti, V, Sb, Zr, Mn, W and others. Templates have been expanded to include nonionic, neutral surfactants and block copolymers. Pore sizes have broadened to the macroscopic size, in excess of 40 nm in diameter. A recent detailed review of the mesoporous molecular sieves is given in ref [73]. Vartuli and Degnan have reported a Mobil M41S mesoporous-based catalyst in commercial use, but to date the application has not been publicly identified.[74]. 

The pillaring of clays has become an established technique of preparing a new class of porous materials. While their promise as cracking catalysts has not been fulfilled, never-the-less they exhibit interesting catalytic behavior. We have extended the technique to the pillaring of many other types of non-clay layered compoimds which include phosphates, oxides and layered double hydroxides. This extension broadens the field to the point where one can choose the degree of acidity or basicity and framework metals required for specific catalytic processes. 

There are some excellent review articles on different aspects of mesostructured materials, such as synthesis, properties, and applications. " Extensive research effort has been devoted to the exploitation of new phases (lamellar, cubic, hexagonal structures), expansion of the pore sizes (about 2-50 nm are accessible), and variable framework compositions (from pure silica, through mixed metal oxides to purely metal oxide-based frameworks, and inorganic-organic hybrid mesostructures). Another research focus is on the formation of mesostructured materials in other morphologies than powders, e.g. monolithic materials and films, which are required for a variety of applications including, but not limited to, sensors (based on piezoelectric mass balances or surface acoustic wave devices), catalyst supports, (size- and shape-selective) filtration membranes or (opto)electronic devices. The current article is focused... 

---