Silica-supported iron

Previous studies by Sorokin with iron phthalocyanine catalysts made use of oxone in the oxidation of 2,3,6-trimethylphenol [134]. Here, 4 equiv. KHSO5 were necessary to achieve full conversion. Otherwise, a hexamethyl-biphenol is observed as minor side-product. Covalently supported iron phthalocyanine complexes also showed activity in the oxidation of phenols bearing functional groups (alcohols, double bonds, benzylic, and allylic positions) [135]. Besides, silica-supported iron phthalocyanine catalysts were reported in the synthesis of menadione [136]. 

Silica-supported iron materials catalyze oxidation reactions [152]. Fe(II) silsesqui-oxanes have been reported [153, 154]. On the complex based on the bisilanol 2a and diphenyl phosphinomethane, dppm [154], possible structural similarities with iron-zeolites where identified after loss of the diphosphane ligand by addition of water. The Fe-POSS were tested in the catalytic oxidation of benzene with N2O with little success [154]. 

The first useful information that TPR provides are the temperatures needed for the complete reduction of a catalyst. Figure 2.4 shows the TPR and TPO patterns of silica-supported iron and rhodium catalysts [10]. Here, there are three things to note ... 

A good example of the latter case is the reaction of iron with silica in silica-supported iron molybdate, a fact which made impossible the use of silica as a support in the case of catalysts used for the oxidation of methanol to formaldehyde. Some of the methods to overcome these difficulties in catalyst preparation are the subject of this section. 

Precursors on Silica-Supported Iron and Cobalt Catalysts. Chem. Phys. Leit. 2000, 322, 333-340. 

Duma and Honicke were the first to report the successful use of N2O in propene epoxidation. A PO yield of 5% was obtained over silica-supported iron oxide catalysts promoted with Na ions [43bj. The pore shape and diameter of the support as well as iron oxide dispersion are crucial parameters in the reaction [43b,cj. Doping vdth alkali metal may also considerably affect the Fe dispersion, and favor epoxidation over allylic oxidation [43fj. Further modification by boron can also significantly enhance the catalytic performance of the K-doped FeO /SBA-lS catalyst [43gj. 

For the sake of comparison, a series of silica supported iron catalysts (x-FS) was prepared by the incipient wetness (INC/WET) method, using powdered silica samples and aqueous solutions (pH =3) of Fe(N03)3. After impregnation, the catalysts were dried overnight at 100°C and then calcined at 600°C (16 h). 

With supported metal catalysts that have to be treated in a reducing gas flow at elevated temperatures to convert the catalytic precursor into the desired metal, it is important to assess the extent of reduction. Often the oxidic phase of the cata-lytically active precursor is stabilized by interaction with the support. It is even possible for a finely divided precursor to react with the support to a compound much more stable than the corresponding metal oxide. An example is cobalt oxide, which can react with alumina to form cobalt aluminate, which is very difficult to reduce to metallic cobalt and alumina. Another example is silica-supported iron oxide. Usually the reduction of iron(III) to iron(II) proceeds readily, because the reduction to iron(II) is hardly thermodynamically limited by the presence of water vapor. Iron(ll), however, reacts rapidly with silica to iron(II) silicate, which is almost impossible to reduce. 

Blyholder and Neff [127) on silica supported iron samples observed the same bands as Eischens and Pliskin [95) except that, in these later experiments, the low-frequency band appeared as a shoulder on the high frequency absorption. This result lends support to the assignment of these bands to a linear carbonyl species and possibly indicates that the two bands may be due to adsorption on different crystal planes exposed at the surface of the metal crystallites. 

One such example that has appeared recently features the central pyridine as the site for attachment. For example, Kim et al. have exploited the 4-substituted O-aUyl group in LI (Ar = 2,6-Me2CsH3) as a means of generating silica-supported iron and cobalt systems (31) (Scheme 5.13) [58]. On activation with MAO these immobihzed catalysts exhibited about 100-fold lower activity when compared to their homogeneous counterparts. This lowering in activity has been attributed to either diffusion Umitation of monomer into the interior pores of the supported catalyst or to the result of reduced active sites present in the heterogeneous variant. 

Ultrasonic irradiation of iron pentacarbonyl in decane solution in the presence of silica gel produces a silica-supported amorphous nanostructured iron.io The iron particles range in size from 3 to 8 pm. This catalyst is a very active material for Fischer-Tropsch hydrogenation of CO. Figure 2 compares the activity (in terms of turnover frequency of CO molecules converted per catalytic site per second) of silica-supported nanophase iron and conventional silica-supported iron (prepared by the incipient wetness method) as a function of temperature. 

Anderson PE, Rodriguez NM. Growth of graphite nanofibers from frie decomposition of CO/H2 over silica-supported iron-nickel particles. J Mater Res 1999 14 2912-21. 

Pham, A.L.T., Lee, C., Doyle, F.M., and Sedlak, D.L. (2009) A silica-supported iron oxide catalyst capable of activating hydrogen peroxide at neutral pH values. Environ. Sci. Technol, 43 (23), 8930-8935. 

He, J., Li,Y, An, D., etal. (2009). Selective Oxidation ofMethane to Formaldehyde by Oxygen over Silica-Supported Iron Catalysts, J. Nat. Gas Chem., 18, pp. 288-294. 

Herranz, T Rojas, S. Alonso, F.J.P. - Ojeda, M. Terreros, P Fierro, J.L.G. Carbon Dioxide Hydrogenation over Silica-Supported Iron-based Catalysts Influence of the Preparation Route Catal. A Gen. 2006, 308,19-30. 

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