N. G. Gallegos

Influence of TS-1 structural properties and operation conditions on benzene catalytic oxidation with H2O2

Abstract The preparation method described by Thangaraj et al. was used to obtain samples of titanium-silicalite (TS-1) for the catalytic hydroxylation of benzene with hydrogen peroxide. These samples were characterized by X-ray diffraction, scanning electron microscopy, skeletal infra-red spectroscopy and ultraviolet–visible diffuse reflectance spectroscopy (UV–VIS DRS). Pure TS-1 (without extra-framework titanium) with a maximum Ti/Si ratio of 0.020 was obtained. UV–VIS DRS measurements allowed us to infer the existence of different Ti 4+ sites in the TS-1 framework. Small changes in the preparation conditions lead to the presence of extra-framework titanium. The influence of extra-framework titanium, traces of sodium and the operation conditions (such as the method of H 2 O 2 addition and the presence of a solvent on TS-1 activity and selectivity to phenol) were analyzed.

Dependence of the oxide-support interaction on the size and nature of iron oxide particles on SiO2

Abstract The thermal dependence of the hyperfine fields and of the relative Mossbauer factors have been used to investigate the oxide-support interaction in the Fe-SiO 2 system. The calcination temperature (698, 898 and 1098 K in N 2 ) has an influence on the nature and particle size of the iron species, while the atmosphere (air and N 2 at 698 K) affects the oxide particle size only. α-Fe 2 O 3 particle diameters of ≅ 45 and ≅ 150 A were found for samples calcinated at 698 K in air and nitrogen, respectively. γ-Fe 2 O 3 particles of ≅ 90 A were obtained after calcination in air at 898 K and migration of iron ions into the SiO 2 matrix was verified after the 1098 K heat treatment. It is found that for α-Fe 2 O 3 , the smaller particles have a stronger oxide-support interaction and γ-Fe 2 O 3 particles have an even higher strength than α-Fe 2 O 3 .

Iron Uniform-Size Nanoparticles Dispersed on MCM-41 Used as Hydrocarbon Synthesis Catalyst

We have synthesized mesoporous MCM-41 and have used it as a support for iron particles to be employed as a catalyst in the Fischer-Tropsch reaction. The solids were characterized by Mossbauer spectroscopy, X-ray diffraction, BET, TGA, CO chemisorption and volumetric oxidation. Although the catalyst showed a high CO conversion when it was used in the hydrocarbon synthesis from CO and H2 (14.3% at 1 h of reaction time) mainly methane was formed. The high methane production is likely related to the very small size of the metal particles obtained. We suggest some ways to improve the selectivity.

Study of Oxide-Support Interactions in Silica-Supported Iron Oxide Precursors

Mossbauer studies of α-Fe2O3 particles in an Fe/SiO2 system calcinated in air for 8, 144, and 344 h at 698 K, reveal average diameters of less than 28 A for the sample calcinated 8 h, and of about 50A for longer calcination times. In the frame of Debye’s model applied to the thermal dependence of the relative f-fectors, we found that the smaller particles have the strongest oxide-support interaction with an infinite force-constant between the oxide particles and silica.

Thermal effects in highly dispersed iron catalysts

The Mössbauer spectra of three Fe/SiO2 catalysts with 5 wt% iron content show the presence of several Fe species and display different magnetic behaviours when the precursors are subjected to various thermal treatments. Based on the Mössbauer parameters and CO chemisorption measurements, the average crystal sizes of the catalysts are estimated and discussed in connection with the thermal pretreatment severity and magnetic properties of the samples.