H. Armendariz

Hydrogen interactions and catalytic properties of platinum-tin supported on zinc aluminate

Abstract Pt-Sn/ZnAl2O4 catalysts prepared by two-step impregnation (first tin), with a Sn/Pt atomic ratio ranging from zero to 6.72, were characterized by hydrogen chemisorption, temperature-programmed reduction and tested in isobutane dehydrogenation. Metal dispersion correlates linearly with reaction rate; both parameters reach a maximum when the Sn/Pt atomic ratio is about one. The activity of the sites capable of hydrogen chemisorption, as expressed in turnover frequency number, TOF, decrease as the tin concentration is increased. From theoretical ab-inito calculations, it is proposed that tin reduces the platinum-hydrogen charge transfer responsible for hydrogen dissociation through an orbital overlapping.

Skeletal isomerization of 1-butene on 12-tungstophosphoric acid supported on zirconia

A series of 0‐25 wt% H3[W12PO40] (TPA) impregnated on freshly precipitated Zr(OH)4 were prepared. The solids were characterized by a Hammett indicator method, specific surface area and pore size measurements, pyridine adsorption FTIR and later tested as catalysts in the isomerization of 1-butene. Maximum acid strength of 5‐20 wt% TPA/ZrO2 calcined at 673 K is H0aˇ9.3, but dried samples (393 K) showed near superacid strength, i.e. H0ˇ13.75. The sites are mainly strong Lewis acids, but TPA supported on stabilized ZrO2 (calcined at 773 K) shows both strong Bronsted and Lewis acidity. TPA addition to hydrated Zr(OH)4 stabilizes the surface area of the final calcined material in comparison with that of pure ZrO2; the greater the TPA content the higher the resulting surface area. Skeletal rearrangement of 1-butene to isobutylene proceeds on 5‐25 wt% TPA/ZrO2 but not on pure ZrO2, the greater the TPA content the higher the initial selectivity towards isobutene. In contrast, 20 wt% TPA supported on SiO2 formed no isobutylene. # 1998 Elsevier Science B.V. All rights reserved.

Metal-support effects and catalytic properties of platinum supported on zinc aluminate

Abstract Pt/ZnAl 2 O 4 catalysts with platinum contents ranging from 0.1 to 1.17 wt.-% were characterized by temperature-programmed reduction, hydrogen chemisorption, high resolution transmission electron microscopy, electron energy loss spectroscopy and tested for isobutane dehydrogenation, using helium or hydrogen as reaction media. For low metal contents, the results suggest that platinum diffuses into the oxygen vacancies in the spinel lattice while at higher loadings, metal-metal interactions are dominant, leading to particle formation. The catalytic behaviour points to the necessity of strong Pt-H interactions to preserve activity.

Propane dehydrogenation activity of Pt and Pt–Sn catalysts supported on magnesium aluminate: influence of steam and hydrogen

Propane dehydrogenation was carried out in hydrogen and steam as reaction media on Pt/MgAl2O4 and Pt–Sn/ MgAl2O4 catalysts. A wide range of Pt and Pt–Sn concentrations was explored. Monometallic Pt catalysts were completely poisoned by steam. Concerning bimetallic Pt–Sn catalysts, tin played an important role related to the activation of platinum particles when the reaction was carried out in steam. On the other hand, tin inhibited cracking reactions leading to an increase of catalysts stability. Activation energy in hydrogen was the same for monometallic and bimetallic catalysts: 22 kcal/mol; while for the reaction in steam, values ranging from 10 to 15 kcal/mol were obtained.

Effect of calcium addition on zinc aluminate spinel

The addition of calcium to zinc aluminate, prepared by coprecipitation methods, causes an increase in the specific surface area and a modification of the crystal morphology. However, the catalytic isobutane dehydrogenation properties remain unchanged. Careful characterization studies (XRD, TGA and SEM) indicate that calcium may be deposited on the zinc aluminate surface.

Oxidative dehydrogenation of n-butane on zinc-chromium ferrite catalysts

Abstract The role of chromium as a promoter of butadiene selectivity in n-butane oxidative dehydrogenation on ZnCrxFe2-xO4 (0

One-step synthesis and characterization of ZrO2–WOx prepared by hydrothermal method at autogenous pressure

A series of ZrO2–WOx samples was hydrothermally prepared in an autoclave at autogenous pressure in a range of temperatures from 145 to 225 °C for 24 h. The materials were characterized by elemental analysis, X-ray diffraction, nitrogen physisorption and FT-IR spectroscopy of both structure and adsorbed pyridine. The Rietvelt refinement of the X-ray diffraction patterns was also performed. The formation of crystalline products was detected at synthesis temperatures as low as 145 °C. Pure zirconium oxihydroxide gel yielded a mixture of metastable t-ZrO2 (63%) and m-ZrO2 (37%) zirconia phases when treated at 145 °C. When tungsten (15 wt.% as WO3) was added to the synthesis mixture, only an amorphous phase of t-ZrO2 was detected. Both the crystallization of t-ZrO2 phase and the t-ZrO2 to m-ZrO2 phase transformation, caused by hydrothermal treatment, were retarded by the presence of tungsten. In this way a microcrystalline ZrO2–WOx of 309 m2 g−1 was prepared under hydrothermal conditions at 145 °C. The material becomes progressively more crystalline with increasing synthesis temperature. At 225 °C a well-crystallized material (79% of t-ZrO2 and 21% of m-ZrO2 phases) was obtained. Interestingly, the crystallite sizes of both phases were not too different, 15.6 and 13 nm for t-ZrO2 and m-ZrO2 respectively. In all the samples, crystalline phases associated with WOx species were not unambiguously identified by XRD. FT-IR spectroscopy of W-promoted zirconia showed a band at 943 cm−1 not present in non-tungsten-promoted ZrO2, which was attributed to the WO stretching mode of octahedrally coordinated tungsten species. Since the lattice parameters of the ZrO2 structure were not modified by the presence of tungsten, the WOx species must be highly dispersed and strongly bound to the ZrO2 surface. Post-synthesis calcination treatment at higher temperatures (560, 700 and 800 °C) brought about sintering of these dispersed tungsten species and the formation of a more bulky tungsten oxide species characterized by two FT-IR vibration bands at 970 and 1100 cm−1.

Oxidative dehydrogenation of 1 -butene to butadiene on α-Fe2O3/ZnAl2O4 and ZnFexAl2-xO4 catalysts

The oxidative dehydrogenation of 1-butene to butadiene was studied over a series of catalysts of iron impregnated on ZnAl2O4 used as a support and also Fe coprecipitated with zinc and aluminum in order to obtain ZnFexAl2-xO4 type catalysts. Results were compared with bulk α-Fe2O3, ZnAl2O4 and ZnFe2O4. X-ray diffraction (XRD) and Mössbauer spectroscopy suggest that in the impregnated catalysts, Fe ions are in strong interaction with the support. These samples have higher butadiene selectivity than coprecipitated ZnFexAl2-xO4 catalysts, in which iron is incorporated into the ZnAl2O4 spinel network. Results suggest also that iron content has a greater effect on butadiene selectivity than the zinc aluminate-iron oxide interaction.

Oxidative dehydrogenation of n-butane: a comparative study of thermal and catalytic reaction using Fe–Zn mixed oxides

The thermal cracking of n-butane was compared with both thermal and catalytic n-butane oxidative dehydrogenation reactions. It was found that thermal oxidative dehydrogenation of n-butane was highly selective to n-butenes (approximately 70%) in the conversion range of 11–20%. This reaction proceeded at lower temperature than the thermal cracking of n-butane without oxygen. These results suggest that both reactions have different initiation steps. In the thermal cracking of n-butane the reaction initiated by the C–C bond scission, whereas for the thermal oxidative dehydrogenation of n-butane the removal of the hydrogen atom by the molecular oxygen to form C4HO2 radicals was proposed as the main initiation step. On the other hand, butadiene was only obtained via a catalytic pathway, being strongly dependent on the reaction conditions.

n-Butane Isomerization Over SO4=/NiO/Al2O3/ZrO2 Catalysts. Effect of the Reaction Pressure and Metal Loading

The effect of alumina and nickel in sulfated ZrO2 as a catalyst for n-butane isomerization was investigated. Samples were synthesized by supporting nickel sulfated zirconia on boehmite and then calcining the material. The crystalline structure of ZrO2 was studied by X-ray powder diffraction and refined by the Rietveld method. Surface areas were determined by N2 adsorption and BET analysis, while the acid properties were studied by NH3 adsorption. The chemical reaction was carried out in a fixed-bed microreactor at 338 K under atmospheric (78 kPa) or 245 kPa total pressure. Results showed that either nickel or alumina improved the catalytic activity, but a synergic effect was observed when both components assisted. The catalytic activity was related to the relative content of tetragonal zirconia and acid site density. Alumina stabilized tetragonal zirconia increased the acid site density and presumably led to a better dispersion of nickel oxide. The catalytic activity could be related to both oxidation and acid sites produced by nickel. A bimolecular reaction mechanism helps explain the observed trends. The increase in the reaction rate would be explained by the increase in the rate of the initial step of dehydrogenation either caused by a better dispersion of nickel or higher operating pressure.