M.V. Cagnoli

Influence of the Support on the Activity and Selectivity of High dispersion Fe Catalysts in the Fischer-Tropsch Reaction

In order to study the influence of the support on high dispersion catalysts used for the CO hydrogenation reaction, two catalysts, Fe/SiO{sub 2} and Fe/Al{sub 2}O{sub 3}, were prepared by the dry impregnation method. Selective chemisorption of CO, volumetric oxidation, and Moessbauer spectroscopy were used to determine the Fe species present as well as the metallic crystal size, the degree of dispersion, and the reduction percentage. The presence of small Fe{sup 0} crystallites with high dispersion was determined in both catalysts. Reaction rates were measured in a differential reactor and significant differences, about one order of magnitude less for the Al{sub 2}O{sub 3} than for the SiO{sub 2} supported catalysts, were found in the methane turnover frequencies. They are attributed to the interaction between the metal and the supports. The selectivity differences is also discussed in connection with distinct surface properties.

Study of the Fe/zeolite-L system: Part II: CO and H2 chemisorption behavior

Abstract The chemisorption properties of two solids, called Fe/ZLK and Fe/ZLH were studied. The samples were obtained by dry impregnation of potassic and acidic forms of zeolite-L with a ferric salt, as described in Part I. The adsorption isotherms of CO at 193 K and H2 at 673 K were measured. “In situ” Mossbauer spectroscopy (MS) at 298 and 193 K of the reduced solids with chemisorbed CO allowed to confirm the preferential location of the Fe2+ exchanged with the support in both samples, in agreement with that obtained in Part I: sites D for Fe/ZLK and sites A for Fe/ZLH. The CO chemisorption resulted in an inappropriate method to obtain the metallic area of reduced Fe/ZLK, since Fe2+ ions in sites D also chemisorb CO with a 1:2=Fe2+:CO stoichiometry. In order to obtain the amount of Fe0 surface atoms, H2 chemisorption at 673 K in 10–80 Torr pressure range was successfully used. Combining this technique with MS, it was possible to determine the values of the Fe0 crystal diameters.

Study of the Fe/zeolite-L system: Part I: Characterization of iron species and their structural properties

Abstract Zeolite-L in potassic (ZLK) and acidic (ZLH) forms were used as a support for iron species to obtain two solids, called Fe/ZLK and Fe/ZLH, respectively. “In situ” Mossbauer spectroscopy (MS) between room temperature (RT) and 15 K, volumetric oxidation (VO) and thermogravimetric analyses were used to determine the iron species existent after reduction (Fe 0 , Fe 2+ and Fe 3 O 4 ) and to study their structural properties. In both solids, a similar fraction of very small Fe 0 crystals was found inside the zeolite channels. Intermediate species, like exchanged Fe 2+ , were also detected in both samples. It was possible to determine that Fe 2+ ions are located at sites D in Fe/ZLK and sites A in Fe/ZLH. Soft reduction conditions, like slow temperature raise (2 K/min), allowed to maintain metallic Fe crystals inside the zeolite-L structure in contrast with other Fe/zeolite systems.

Fischer–Tropsch reaction on Fe/Zeolite-L system. Structure and catalytic behavior

Abstract Iron supported on potassic and acidic forms of commercial zeolite-L were used to determine the role played by the small metallic crystallites inside the zeolite channels and the increase of the acidity of the support on the activity and selectivity of the catalysts. The solids were characterized by Mossbauer spectroscopy (MS), specific surface area (BET), X-ray diffraction (XRD), atomic absorption spectroscopy, CO and H 2 chemisorption and volumetric oxidation (VO). The solids thus obtained were used in the Fischer–Tropsch synthesis. About 15% of total Fe 0 remained inside the zeolite channel after reduction and they were active for the reaction.

A controlled atmosphere cell for a Mössbauer closed-cycle refrigerator

A cell to keep samples in controlled atmosphere for Mossbauer studies of catalysts and small particles is described. The cell keeps the atmosphere of the reactor in which treatments like reduction, chemical reaction, or gas adsorption have been performed on a sample that afterwards is transferred to be studied by Mossbauer spectroscopy at low temperatures. The cell has been adapted to work inside a closed-cycle refrigerator that works in the temperature range 12 - 298 K. Its application is illustrated by studies of Fe small particles supported on in atmosphere.

Fischer-Tropsch synthesis on iron catalysts supported on MCM-41 and MCM-41 modified with Cs

MCM-41 and MCM-41 modified with cesium were synthesised and utilised as supports of iron species, to be used as catalysts in the Fischer-Tropsch reaction. X-Ray Diffraction (XRD), Specific Surface, Area (BET), Mossbauer Spectroscopy (MS) in controlled atmosphere, between room temperature (RT) and 15 K, CO chemisorption and Volumetric Oxidation (VO) were used to characterise the solids. The cesium presence produces lower iron reducibility and a unique fraction of very small metallic iron crystals. These structural properties lead to a high turnover frequency and an increase of the selectivity towards light olefins.

Effect of thermal pretreatment on the structural properties of FeMgO catalysts in hydrocarbon synthesis from CO and H2

Abstract Two iron on MgO catalysts with the same iron content but subjected to different thermal pretreatments, prepared from magnesium hydroxycarbonate (MHC), have been studied. Chemisorption, Mossbauer spectroscopy and activity and selectivity measurements in a Fischer-Tropsch reaction have been combined to characterized them. The catalysts obtained show diverse structural properties and catalytic behaviour. When the supported crystallite size changed from 63 to 193 A, the methane turnover frequency increased 3.5 times. For the reaction conditions used, 543 K, H ( in 2): CO = 3:1 and atmospheric pressure, the catalyst with larger iron particles produce a lower fraction of C 2+ in hydrocarbon products and showed a greater tendency to the deactivation and loss of C 2+ selectivity than the one with smaller particles. This behaviour is discussed in terms of the structural properties of Fe° microcrystals.

24-P-11-Zeolite-L as support of Fe microcrystals for the Fischer-tropsch synthesis

Publisher Summary This chapter discusses zeolite-L as support of iron (Fe) microcrystals for the Fischer–Tropsch synthesis. Zeolite-L in potassic form is used as support of iron species to be used as catalyst in the Fischer–Tropsch reaction. The oxide precursor is reduced using two different programs. Thermal programmed reduction (TPR), X-Ray Diffraction (XRD), Specific Surface area (BET), In situ Mossbauer Spectroscopy (MS) between room temperature and 15K, H 2 chemisorption, and Volumetric Oxidation (VO) were used to characterize the solids. Using a slow reduction treatment, it was possible to maintain a high quantity of Fe 0 microcrystals inside the pore structure, leading to a higher activity to low molecular weight paraffin.

Prospects of Fe/MCM‐41 as a Catalyst for Hydrocarbon Synthesis

We report the synthesis of cylindrical nanoparticles of metallic Fe entirely included in MCM‐41 pores. Their dimensions are approx.3 nm diameter and approx. 3.8 nm length. We show that a coherent analysis of the results yielded by the various techniques is essential to obtain a catalyst supported on an MCM‐41 matrix of ≈ 3 nm average pore diameter, which is active and selective toward olefins. The solids were characterized by low‐angle x‐ray diffraction, high‐resolution transmission electron microscopy, high‐resolution scanning transmission electron microscopy equipped with a high‐angle annular dark‐field, CO chemisorption, volumetric oxidation, and Mossbauer spectroscopy (in controlled atmosphere for the reduced catalysts). Catalytic results in the Fischer‐Tropsch synthesis, as well as some unexpected results —like the inhomogeneous pore filling and discontinuous Fe particles— are also discussed.

13-P-14-A comparative study of Ti4+ sites in titanium silicalite (TS-1) synthetized by different methods

Publisher Summary This chapter presents a comparative study of titanium (Ti) 4+ sites in titanium silicalite (TS-I) synthetized by different methods. Three TS-1 zeolites are prepared by three different methods. Infrared (IR), scanning electron microscopy (SEM), and diffuse reflectance spectroscopy (DRS) are used to determine that all of the zeolites are well manufactured. The joint use of probe molecules (hydrogen peroxide and benzene) and DRS helps to detect differences in the population of the named “closed” and “open” Ti 4+ sites and in their geometries in the three zeolites. These differences lead to distinct catalytic behavior when these solids are tested in the oxidation of benzene with hydrogen peroxide.