F. Medina

Characterization of nickel species on several γ-alumina supported nickel samples

Abstract Studies of the chemical preparation, surface areas, pore distributions, powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectra (XPS) of several γ-alumina supported nickel samples active for the catalytic hydrogenation of hexanedinitrile have been carried out. NiO crystallites showed to be larger than the pore mouths of the support. Pore distributions lie in the mesopore region with diameters between 20 and 100 A. Powder XRD detects the oxidized and reduced-nickel phases present and does not detect the nickel aluminate phase, due to the lack of crystallinity of the latter. SEM micrographs detect the presence of octahedral NiO crystallites and amorphous shelling nickel aluminate. XPS results show the presence of surface Ni 2+ (in the form of stoichiometric and non-stoichiometric NiO, and stoichiometric and non-stoichiometric nickel aluminates) and surface reduced nickel. A deconvolution of the experimental curves was carried out in order to obtain a better assignment of the surface nickel species present. Nickel aluminate is detected at calcination temperatures > 623 K and covers all the surface of the support with layers between zero and several atoms thick, depending on calcination temperature and nickel concentration. Catalytically active reduced nickel (for nitrile hydrogenations) either as naked crystallites or as encapsulated nickel inside voided non-stoichiometric aluminate shells lie on top of the underlying catalytically inactive nickel aluminate when precursor calcination temperatures are higher than 623 K and reduction temperatures of 673 K. NiO transformation into nickel aluminate collapses the cubic NiO surface morphology of the small NiO crystallites giving rise to voided shells of nickel aluminate which may hide encapsulated reducible NiO.

Characterization and activity of copper and nickel catalysts for the oxidation of phenol aqueous solutions

Catalysts based on CuO/g-alumina, CuAl2O4/g-alumina, NiO/g-alumina, NiAl2O4/g-alumina and bulk CuAl2O4 have been structurally characterized by BET, porosimetry, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Their catalytic behaviors have also been tested for the oxidation of 5 g/l phenol aqueous solutions using a triphasic tubular reactor working in a trickle-bed regime and air with an oxygen partial pressure of 0.9 MPa at a temperature of 413 K. The copper and nickel catalysts supported on g-alumina have surface areas of the same order as the support g-alumina of ca. 190 m 2 /g and high active phase dispersions which were also confirmed by SEM, whereas the bulk copper aluminate spinel has a surface area of ca. 30 m 2 /g. XRD detects the phases present and shows a continuous loss of CuO by elution and the formation of a copper oxalate phase on the surface of the copper catalysts which also elutes with time. The NiO was also eluted but less than the copper catalysts. Only the copper and nickel spinel catalysts were stable throughout the reaction. Phenol conversion vs. time shows a continuous overall decrease in activity for the CuO/g-alumina and NiO/g-alumina catalysts. In turn, the copper and nickel spinel catalysts reach steady activity plateaus of 40 and 10%, respectively, of phenol conversion. The bulk copper aluminate spinel shows an activity plateau of 20% of the conversion which is lower than that from the copper aluminate/galumina catalyst due to its lower surface area. Nickel catalysts always have lower activities than the copper catalysts for the phenol oxidation reaction. The copper catalysts drive a mechanism of partial phenol oxidation to carboxylic acids and quinone-related products with very high specific rates, and the nickel catalysts mainly drive a mechanism of CO2 formation with lower conversion but with a potential higher catalyst life. The triphasic tubular reactor using trickle-bed regime largely avoids the mechanism of polymer formation as a catalyst deactivation process. # 1998 Elsevier Science B.V. All rights reserved.

Palladium hydrotalcites as precursors for the catalytic hydroconversion of CCl2F2 (CFC-12) and CHClF2 (HCFC-22)

Abstract The synthesis of palladium hydrotalcite-like materials and their catalytic properties for the hydroconversion of CCl 2 F 2 (CFC-12) and CHClF 2 (HCFC-22) are reported. These materials appear to be interesting catalysts for selective hydrodechlorination reactions. The influence of process parameters, such as reaction temperature and H 2 /CFC-HCFC ratio on the hydrogenolysis of CFC-12 and HCFC-22, has been investigated. In the reaction of CCl 2 F 2 , the main products were CH 2 F 2 , CH 4 and CHClF 2 . In the reaction of CHClF 2 , the main products were CH 2 F 2 , CHF 3 and CH 4 . The highest conversion and selectivity to CH 2 F 2 are obtained on the heavily loaded Pd catalyst. A high H 2 /CFC-HCFC ratio favors the obtention of CH 2 F 2 . During the reaction, the formation of fluoride and Pd-carbide phases were detected.

Effect of the aluminium fluoride phase for the Cl/F exchange reactions in CCl2F2 (CFC-12) and CHClF2 (HCFC-22)

Abstract Two different aluminium trifluorides (α-AlF3 and γ-AlF3) of high-area have been prepared, characterised by XRPD, N2 physisorption, IR, X-ray fluorescence, TPD and SEM techniques, and tested for the Cl/F exchange reaction of CCl2F2 (CFC-12) and CHClF2 (HCFC-22) in the gas phase. Catalyst α-AlF3 is more active than catalyst γ-AlF3 for both reactions. This is due to both its higher amount of Lewis acid sites, as deduced from ammonia-TPD and pyridine FT-IR studies, and its higher TOF values, which favours the Cl/F exchange in CFC and HCFC compounds. The main products obtained are those which result from the exchange of one chlorine by one fluorine, CClF3 and CHF3 for the exchange reaction of CCl2F2 and CHClF2, respectively.