Abstract
Fe-MCM-41 materials were prepared by different methods. The Fe was both incorporated into the structure and formed crystallites attached to the silica. High Fe content MCM-41 (~16 wt%) with retention of mesoporosity and long-range order was achieved by a range of new synthetic methodologies: (i) by delaying the addition of Fe3+(aq) to the stirred synthesis gel by 2 h, (ii) by addition of Fe3+ precursor as a freshly-precipitated aqueous slurry, (iii) by exploiting a secondary synthesis with Si-MCM-41 as SiO2 source. For comparative purposes the MCM-41 was also prepared by incipient wetness impregnation (IWI). Although all these synthesis methods preserved mesoporosity and long-range order of the SiO2 matrix, the hydrothermally-fabricated Fe materials prepared via the secondary synthesis route has the most useful properties for exploitation as a catalyst, in terms of hydrothermal stability of the resulting support. Temperature-programmed reduction (TPR) studies revealed a three-peak reduction pattern for this material instead of the commonly observed two-peak reduction pattern. The three peaks showed variable intensity that related to the presence of two components: crystalline Fe2O3 and Fe embedded in the SiO2 matrix (on the basis of ESR studies). The role of secondary synthesis of Si-MCM-41 on the iron reducibility was also demonstrated in IWI of sec-Si-MCM-41.
Highlights
The substitution of various transition elements into open-framework microporous and mesoporous materials [1,2,3,4,5] has received considerable attention because of the need to develop more efficient and stable materials for applications in catalysis, separations, coatings and chemical sensing
Since the catalyst reducibility is such an important feature of Fischer-Tropsch synthesis (FTS) catalysts, one of the aims of this paper is to study the reducibility of differently-prepared Fe-MCM-41 materials
It is apparent from this figure that the mesoporous long-range order decreases with the increasing amount of Fe used in the synthesis gel, signalled by the decrease in intensity and eventual loss of the higher order peaks [(110) and (200)] in the X-ray diffraction (XRD) patterns
Summary
The substitution of various transition elements into open-framework microporous and mesoporous materials [1,2,3,4,5] has received considerable attention because of the need to develop more efficient and stable materials for applications in catalysis, separations, coatings and chemical sensing. Iron-containing zeolites [6] and related molecular sieves are of particular interest because of their unique catalytic activity in various selective gas-phase reactions such as hydrocarbon oxidation [7,8,9], N2O decomposition [10,11], synthesis of carbon nanotubes [12,13] and selective catalytic NO and N2O reduction in the presence of hydrocarbons or ammonia [14,15,16] Another area of catalysis in which Fe constitutes a key catalyst component is the Fischer-Tropsch synthesis (FTS) [17,18,19]. This has led to Co and Fe catalyst systems with improved catalytic activity and
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