No abstract

Background: Acetaldehyde is a main organic intermediate for manifold chemical products. In the near future, its production using renewable raw materials is rapidly becoming highly desirable. In this paper, investigations on pure and silica-supported molybdenum oxide (MoO 3 ) as catalysts for the ethanol oxidative dehydrogenation process to acetaldehyde are reported.Results: Acicular pure ⊍-MoO 3 crystals and silica-gel supported MoO 3 (1, 5 and 12% wt MoO3 /wt support ) were prepared by the thermal decomposition method and by incipient wetness impregnation, respectively. Catalysts were studied and extensively characterized assessing structural, morphological and chemical properties. The samples were tested in ethanol oxidative dehydrogenation through Temperature Programmed Surface Reaction (TPSR) and steady-state measurements. MoO 3 /SiO 2 samples were constituted by MoO 3 particles weakly interacting with the support, but also by some molybdate species entering the silica framework and significantly modifying the silica morphology. High catalyst acidity limits oxydehydrogenation yield, catalyzing the competitive dehydration reaction to ethylene. Thus, the highest obtained acetaldehyde yield was ≈60%. Molybdenum loss by MoO 3 volatilization was found on MoO 3 /SiO 2 . Conclusion:The produced and characterized catalysts are active, and allow quite a high yield to acetaldehyde. Slight deactivation was observed and also investigated.

Heusler compounds form a numerous class of intermetallics, which include two families with general compositions ABC and AB 2 C, usually referred to as half-and full-Heusler compounds, respectively. Given their tunable electronic properties, made possible by adjusting the chemical composition, these materials are currently considered for the possible use in sustainable technologies such as solar energy and thermoelectric conversion. According to theoretical predictions, Sb substitution in the TiFe 2 Sn full-Heusler compound is thought to yield band structure modifications that should enhance the thermoelectric power factor. In this work we tested the phase stability and the structural and microstructural properties of such heavily-doped compounds. We synthesized polycrystalline TiFe 2 Sn 1-x Sb x samples, with x = 0, 0.1, 0.2 and 1.0 by arc melting, followed by an annealing treatment. The structural characterization, performed by x-ray powder diffraction and microscopy analyses, confirmed the formation of the Heusler AB 2 C structure (cF16, Fm-3m, prototype: MnCu 2 Al) in all samples, with only few percent amounts of secondary phases and only slight deviations from nominal stoichiometry. With increasing Sb substitution we found a steady decrease of the lattice parameter, confirming that the replacement takes place at the Sn site. Quite unusually, the as cast samples exhibited a higher lattice contraction than the annealed ones. The fully substituted x=1.0 compound, again adopting the MnCu 2 Al structure, does not form as stoichiometric phase and turned out to be strongly Fe deficient. The physical behavior at room temperature indicated that annealing with increasing temperature is beneficial for electrical and thermoelectrical transport. Moreover, we measured a slight improvement of electrical and thermoelectrical properties in the x=0.1 sample and a suppression in the x=0.2 sample, as compared to the undoped x=0 sample.

BACKGROUND: γ-Al 2 O 3 and silica-doped aluminas are largely used supports for industrial catalysts. The addition of silica to alumina modifies its acid-base properties, also affecting its dispersion ability of supported species. In this paper, investigations on the silica doping effect in alumina-supported MoO 3 catalysts, developed for the oxidative dehydrogenation process of ethanol to acetaldehyde, are reported. RESULTS: MoO 3 (1Ä12% wt MoO3 /wt support ) supported over pure γ-Al 2 O 3 , SiO 2 (1 and 5 wt.%) doped γ-Al 2 O 3 were prepared by incipient wetness impregnation. Catalysts were studied and extensively characterized structurally, morphologically, and chemically. All samples were tested in ethanol oxidative dehydrogenation in Temperature Programmed Surface Reaction conditions. Best performing catalysts were also tested in steady-state and time-on-stream experiments. At 573 K, the best acetaldehyde yield (60% in steady state conditions) was found on 12 wt.% MoO 3 over 1 wt.% SiO 2 on alumina. The slight deactivation after 8 h on stream (10% activity loss) is attributed to a limited MoO 3 loss by volatilization. CONCLUSION: The investigated catalysts are active and allow quite a high yield to acetaldehyde. The addition of silica to alumina increases both the conversion of ethanol and the selectivity to acetaldehyde, and reduces MoO 3 volatilization, due to the higher activity of monomeric molybdates with respect to polymeric ones. The high acidity of the catalysts limits oxydehydrogenation yield, catalyzing competitive reaction to ethylene.

No abstract