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Abstract: The effect of Sr and some lanthanides (La, Nd, Sm) as promoters of Pt/KL supported catalysts is analyzed in the selective hydrogenation of citral in the liquid phase. Characterization of the catalysts by XRD, N2 adsorption, H2 chemisorption, TPD of NH3, XPS and competitive hydrogenation of benzene/toluene mixtures shows that impregnation of KL zeolite with Sr(NO3)2 in aqueous solution, followed by calcination prior to the incorporation of Pt, increases the surface basicity of the zeolite, which hinders the dispersion of Pt and promotes the formation of electron-rich platinum nanoparticles (Ptδ−). Ion exchange of K+ by rare earth cations (La3+, Nd3+ and Sm3+) increases the surface acidity of the zeolite and favors the dispersion of Pt, but with preferential location at the external surface of the zeolite. Furthermore, acidity promotes the formation of electron-deficient metal species (Ptδ+). Catalytic performances for the citral hydrogenation at 323K and 5MPa show that strontium addition enhances the hydrogenation activity of Pt, thus favoring the formation of citronellol as main reaction product (S =80%) and increasing the yield to geraniol and nerol. Lanthanides as countercations diminish the overall hydrogenation activity of Pt/KL catalysts, but improve their selectivity towards unsaturated alcohols. Selectivity towards the reaction products is related to the mode and strength of the electronic interactions of the conjugated unsaturated bonds of citral reactant molecules with the metal surface species Pt0, Ptδ+ or Ptδ− on the zeolite. [Copyright &y& Elsevier]

Abstract: Cr exchanged ZSM-5 catalysts (Cr/Al=1) prepared by solid-state ion exchange were tested in SCR of NO with ammonia. Since CrCl3 precursor sublimates during the solid-state reaction, chromate species were stabilized in the exchange sites of H+-ZSM-5 (Si/Al=15) and Cr2O3 formation is, therefore, limited. However, using Cr nitrate and NH4 +-ZSM-5 (Si/Al=26) zeolite, massive particles of α-Cr2O3 occupied the catalyst surface. At the contrary, well-dispersed particles of Cr2O3 were detected with H+-ZSM-5 and Cr nitrate precursor. Starting from Cr acetate precursor, agglomerates of amorphous oxide were detected with H+-ZSM-5 like support. In the temperature range 50–300°C, Cr chloride and Cr nitrate precursors and H+-ZSM-5 led to highly active catalysts, while Cr nitrate and NH4 +-ZSM-5 led to a poorly active catalyst. However, catalysts issued from Cr acetate are promising in SCR but inefficient agglomerates of oxide (amorphous or crystalline chromia) could be avoided with NH4 +-ZSM-5 like support. [Copyright &y& Elsevier]

Abstract: Cr–ZSM-5 catalysts prepared by solid-state ion exchange with different Cr/Al molar ratios were characterized and tested in ethylene ammoxidation to acetonitrile in the temperature range 425–500°C. Starting from Cr acetate like precursor and lower Cr load (Cr/Al=0.5), small amount of chromate species, sited inside the zeolite matrix, exhibited lower catalytic activity in ammoxidation (12.5% of CH3CN yield). Increasing the Cr amount (Cr/Al=1) does not significantly improve the catalytic properties since agglomerates of amorphous Cr2O3 inhibited the accessibility of chromate species to the reactants. However, at higher Cr amount (Cr/Al=1.5), crystalline Cr2O3 particles reduced the diffusion of reactants to the internal active sites. Starting from Cr chloride like precursor, the corresponding solids are low-exchanged since CrCl3 evaporates. At Cr/Al molar ratios of 0.5 and 1, chromate species, sited in the exchange sites, exhibited interesting catalytic properties (18.5 and 25% of CH3CN yield at 500°C, respectively). Nevertheless, at higher metal load (Cr/Al=1.5), the excess of Cr transforms into crystalline Cr2O3 which inhibits the diffusion of reactants to the active sites and enhances the hydrocarbon oxidation mainly at low temperatures. As ethylene ammoxidation required ammonia activation over (poly)chromate sites, a possible pathway of such a step has been proposed on the basis of NH3-TPD results. [Copyright &y& Elsevier]

Abstract: Catalytic performances of Cr–ZSM-5 catalysts (5wt.% of Cr, Si/Al = 26), prepared by solid-state reaction and aqueous exchange, from Cr nitrate and Cr acetate precursors, were evaluated in the selective ammoxidation of ethylene into acetonitrile in the temperature range 425–500°C. Catalysts were characterized by chemical and thermal analysis, XRD, N2 physisorption, 27Al MAS NMR, TEM, UV–vis DRS, Raman, DRIFTS and H2-TPR. Characterization results shown that solid-state exchange was favorable for Cr2O3 formation, while exchanging chromium in aqueous phase led, essentially, to Cr(VI) species. Catalysts were actives and selectives in the studied reaction, and among them, those, prepared from aqueous exchange, exhibited the highest acetonitrile yields (23±0.5%, at 500°C). Improved catalytic properties can be correlated with the chromium species nature. In fact, mono/di-chromates and/or polychromate species, sited in the charge compensation positions, were definitively shown, as being, the active sites. Furthermore, during solid-state reaction, the agglomeration of Cr2O3 oxide should be avoided since these species inhibit the catalyst activity. [Copyright &y& Elsevier]

Four Ru/KL-zeolite catalysts containing 2 wt% of Ru were prepared from Ru 3 (CO) 12 , RuNO(NO 3 ) 3 , Ru(C 5 H 7 O 2 ) 3 and RuCl 3 precursors. The evolution of electronic structure and local chemical environment of ruthenium in the samples named Ru(c)/KL, Ru(n)/KL, Ru(acac)/KL and Ru(Cl)/KL was studied by in-situ XANES during temperature-programmed reduction. Also by CO chemisorption and transmission electron microscopy (TEM) the sizes of the Ru nanoparticles were determined. Activity and selectivity of the catalysts were evaluated in the transformation of ethanol, under kinetic conditions, in a fixed bed flow reactor, at 523 K–573 K. Characterization of the samples shows that metal dispersion values follow the trend Ru(c)/KL ≥ Ru(n)/KL > Ru(Cl)/KL ≥ Ru(acac)/KL. Activity of the catalysts is in the order Ru(acac)/KL ≥ Ru(c)/KL > Ru(n)/KL ≥ Ru(Cl)/KL. The TOF values, however, are in the same order of magnitude for all the samples, nonetheless the Ru(Cl)/KL catalyst has slightly lower TOF at all the reaction temperatures. Selectivity towards the dehydrogenation product, acetaldehyde, follows the trend Ru(c)/KL > Ru(n)/KL = Ru(acac)/KL >> Ru(Cl)/KL, this being 100% for Ru(c)/KL. Selectivity towards acetaldehyde is highly diminished for Ru(Cl)/KL in favor of the dehydration products, diethyl ether and ethylene, the higher the decrease the higher the temperature. The catalytic results are related to the properties of the surface metal species and their location in the zeolite framework, as well as to their surroundings, as evidenced from the results of the characterization measurements, which are in turn influenced by the different nature of the metal precursor. [ABSTRACT FROM AUTHOR]

Abstract: Cr-zeolites with MFI, BEA, MOR and FAU structures, prepared by solid-state ion exchange, were characterized and tested in C2H4 ammoxidation to acetonitrile in the temperature range 425–500°C. Based on characterization results, chromate and/or polychromate species, oxo-cations and small Cr2O3 oxide clusters played a key role in the ammoxidation of ethylene, while agglomerated Cr2O3 and bare Cr cations should be avoided. Cr ions sited in the sodalite and hexagonal cages of NH4 +–Y are not accessible to the reactants while available catalytic sites are poorly active. However, the mesopores of the ultra stable Y zeolite (USY) favor the diffusion of reactants to the clustered Cr oxide. The corresponding catalyst is therefore active, but the presence of octahedral Al species is crucial to the ammoxidation. Cr ions in zeolites beta and mordenite led to less active catalysts when compared to ZSM-5. In zeolite beta, the micropores are small; therefore, pronounced interactions between ethylamine intermediate molecules could discourage the acetonitrile formation. In mordenite, agglomerates of Cr2O3 oxide inhibited the accessibility of active sites to the reactants and enhanced the hydrocarbon oxidation. The catalytic performance of Cr ions in ZSM-5 provided from a synergy of different parameters: structural, textural and the acid strength (Si/Al ratio). [Copyright &y& Elsevier]