L.C. Dieguez

Characterization of chromium species in catalysts for dehydrogenation and polymerization

Abstract Chromium catalysts, prepared with different chromium contents, supports and precursor compounds, were characterized by O2 and CO chemisorption, in order to quantify the Cr3+ and Cr2+ sites, active species to dehydrogenation and ethylene polymerization reactions. Both probe molecules, O2 and CO, showed to be selective to the chromium active species, Cr3+ and Cr2+, respectively, and a valid method to quantify these sites. Distinct chromium species, Cr6+, amorphous Cr3+, crystalline Cr3+ and Cr2+, were characterized by XRD, DRS, TPR, FT-IR and DRIFTS in the calcined and reduced catalysts. The distribution of chromium species, which depends on the content, precursor compound and support, was related to the catalytic activity in the dehydrogenation and polymerization reactions. The catalytic activity to cyclohexane dehydrogenation depends on the dispersion of amorphous Cr3+ species. The activity in the ethylene polymerization is related to the amount of CrA2+ and CrB2+ species. Both sites showed similar activities.

Characterization of PdCeO2 interaction on alumina support and hydrogenation of 1,3-butadiene

Abstract The influence of the preparation method and loading of CeO 2 on the interaction of Pd/CeO 2 /Al 2 O 3 catalysts was studied. The materials were characterized by temperature-programmed reduction, hydrogen chemisorption and X-ray photoelectron spectroscopy measurements. The 1,3-butadiene hydrogenation was used as a model reaction. The Pd Ce interaction affected the reduction behavior of the catalysts. Temperature-programmed reduction (TPR) results showed that the presence of CeO 2 shifts the reduction temperature of PdO to lower values, while palladium similarly facilitates the reduction of the cerium surface species. The hydrogen uptake decreased when increasing the ceria content. XPS results showed that the Pd/Al ratio decreased with CeO 2 addition being affected by the preparation method and content. The formation of catalytic sites at the interface of Pd Ce was postulated from turnover frequency (TOF) results of the 1,3-butadiene hydrogenation. The number of these new sites increases with the addition of ceria. The nature of these sites is discussed in terms of the influence on the selectivity in butadiene hydrogenation.

Extração de ferro de esmectita brasileira com emprego do método ditionito-citrato-bicarbonato

A natural clay from Campina Grande region (Paraiba, Brazil), with 8.57% of Fe2O3, was used to study the most appropriate condition to carry out the iron extraction, without altering the clay structure in a significant way. Samples were treated with the Dithionite-Citrate-Bicarbonate method (DCB) for 30 and 120 minutes (pH=9.1), and also with citric acid (pH=1.8; time=15min), at 75°C. Conductivity measurements, X-ray fluorescence, X-ray diffraction, energy-dispersive spectrometry, electron-diffraction with transmission electron microscopy and textural evaluation by nitrogen adsorption were done. The treatment in a basic medium was more selective for iron removal than in acid condition. The time of 30 minutes, with 1.6 g Na2S2O4/10 g clay, was the best condition for the iron extraction.

Investigation of the Stability of CeO2, V2O5 and CeV Mixed Oxide on the Partial Oxidation of Propane

The cerium–vanadium mixed oxide was synthesized by complexation method. XRD patterns of the (CeV) mixed oxide showed the formation of CeVO4 phase and segregated CeO2. Incorporation of vanadium into the ceria structure improved the redox ability of ceria. The results showed that the activity of the (CeV) mixed oxide was higher than of the vanadium oxide (V2O5) for the partial oxidation of propane. Both catalysts showed similar hydrogen selectivity but significant deactivation with time on stream. Investigation by XRD diffractograms of V2O5 after reaction showed sintering of V2O5 and different oxidation states after reaction, while the crystallites and the structure of the mixed oxide CeVO4 remained unchanged. The deactivation of the mixed oxide (CeV) with time on stream can be explained by insufficient oxygen supply to restore the CeO2 species at the surface.Graphical Abstract.

The effect of precursors salts on surface state of Pd/Al2O3 and Pd/CeO2/Al2O3 catalysts

The influence of the precursors on the promoting effect of ceria on Pd/Al2O3 catalyst, when ceria is coated over alumina was studied. The reaction of propane oxidation proceeded under different feed conditions and the surface active sites were characterized by X-ray photoelectron spectroscopy (XPS) and in situ diffuse reflectance spectroscopy (DRS). XPS and DRS results show that PdO/Pd 0 interface are the active sites independent of the precursor, while the catalysts containing CeO2 showed formation of palladium species in the highest oxidation state, probably PdO2 (338 eV) after the oxidation of propane. Besides, the O/Al and O/Ce ratios evidenced the increase of oxygen storage in the presence of CeO2. In addition, the precursor acetylacetonate favors the oxygen storage in the lattice.

Pilarização de esmectita brasileira para fins catalíticos. Emprego de argila pilarizada na alquilação de benzeno com 1-dodeceno

Al-pillared clay was prepared with a Brazilian bentonite from the Campina Grande region (Paraiba, BRAZIL). It was intercalated at 298 K, during 48 hours, with a solution containing [Al3+] = 0.10 mol/L and molar ratio OH/Al = 2.0 prepared at 333 K, and was calcined at 773K. The catalytic activity was evaluated by alkylation of benzene with 1-dodecene. The characterization methods were: X-ray fluorescence and diffraction analysis; 27Al, 29Si and 23Na MAS NMR and textural analysis by N2 adsorption. The thermal stability of the natural clay was improved by the pillaring procedure, as well as the catalytic activity. The intercalated clay presented the highest initial rate of reaction among the systems tested.