Raúl A. Comelli

Influence of the tungsten oxide precursor on WOxZrO2 and Pt/WOxZrO2 properties

The influence of the tungsten precursor and of the preparation conditions on the properties of platinum free or platinum containing WOx promoted zirconia has been studied. Catalysts were characterized by hydrogen chemisorption at room temperature, TPR, XRD and Laser Raman spectroscopy and during the n-hexane reaction at 200°C, 6 kg cm−2, and a hydrogen/n-hexane molar ratio 7. For all catalysts, the same WOx species were observed after calcination: crystalline WO3 and an amorphous WOx species were detected by Laser Raman. The best catalytic activity and stability were observed on catalysts in which platinum was added after calcination, independently of the WOx precursor used in the preparation.

Influence of Pt concentration on tungsten oxide-promoted zirconia during n-hexane isomerization

Abstract The effect of platinum concentration on tungsten oxide-promoted zirconia over the catalytic activity for n -hexane isomerization was studied. Catalysts were prepared by impregnation of tungsten oxide-promoted zirconia reaching up to 1.50% platinum, followed by calcination at 500°C. The n -hexane reaction was studied at 200°C, 5.9 bar, WHSV 4 and H 2 : n -hexane (molar) ratio 7. It was found that catalytic activity and stability increase for platinum concentrations above 0.05% because of higher hydrogen availability at the surface, measured as a function of the methylcyclopentane/C 6 isomers ratio. Further increments in platinum concentration do not produce important modifications in catalytic activity or hydrogen availability. Mechanical mixtures of Pt/SiO 2 and tungsten oxide-promoted zirconia present a lower conversion than platinum over tungsten oxide-promoted zirconia due to the larger distance between the acid and metallic functions.

PtSO42−ZrO2: Characterization and influence of pretreatments on n-hexane isomerization

Abstract The n-hexane isomerization on Pt SO 4 2− ZnO 2 was studied at 473 K and 6 bar. The catalyst was prepared by adding platinum to zirconium hydroxide by the incipient wetness technique followed by dipping in 0.5 M sulfuric acid solution. Before reaction, the material was calcined and reduced under different conditions. While reduction is unnecessary, the calcination step is very important because it changes the catalytic activity level. The large increase in activity occurs by calcining at 773 K when the material begins to show crystalline structure in X-ray diffraction patterns. The catalytic activity reaches the highest values for calcination between 803 and 878 K, the selectivity to isomers being almost constant. The crystalline structure is not sufficient to generate an active material, and the catalytic activity is not proportional to the sulfur content and/or the total acidity. A minimum sulfur loading and sulfur species generated by calcination are necessary.

Hydroisomerization of n-hexane on Pt/SO4 2™™ZrO2: effect of total and hydrogen partial pressure

The effect of both total pressure and hydrogen partial pressure during the n-hexane isomerization on Pt/SO42™-ZrO2 at 200°C and 4 h™1 weight hourly space velocity was studied. Total pressure was varied between 1.0 and 10.0 kg cm-2; different hydrogen partial pressures were obtained by dilution with nitrogen keeping the total pressure at 6 kg cm-2. The n-hexane isomerization activity increases when increasing either total pressure or hydrogen partial pressure. Methylpentanes are the C6 isomers formed in major proportion and the 2-methylpentane/3-methylpentane ratio practically reaches the equilibrium value in most of cases. Isomerization is always accompanied by hydrocracking. Hydrogenolysis products were not observed, the main cracking products being propane, isobutane and isopentane; then, the cracking product distribution cannot be interpreted by considering an intramolecular rearrangement of carbocation intermediates. A slight deactivation takes place with time-on- stream. The lifetime of surface intermediates is longer in the absence of hydrogen or at a low hydrogen partial pressure; it allows oligomerization and cracking to prevail at short times and polymerization and coke formation at long times.

Modification of SO4 2--ZrO2 and Pt/SO4 2--ZrO2 properties during n-hexane isomerization

The modification of the textural properties and crystalline structure of SO42--ZrO2 and Pt/SO42--ZrO2 during n-hexane reaction at 473 K and 6 kg cm-2 has been studied in the presence of either hydrogen or nitrogen. Sulfur content before and after reaction and the amount of coke at the end of the reaction were measured. The coke deposited on the catalysts blocks the pores of small size and decreases the surface area of the used catalysts. After regeneration, surface area is not completely restored. The loss of sulfur during reaction, probably associated to the reaction medium, also produces a decrease in surface area by the collapse of the smallest pores which generates larger ones. The transformation of tetragonal to monoclinic crystalline structure of zirconia begins to occur when sulfur content drops below a critical value.

Coke deposition on platinum promoted WOx–ZrO2 during n-hexane isomerization

Platinum on tungsten oxide promoted zirconia catalysts were prepared by calcining the zirconium hydroxide impregnated with tungsten species at different temperatures before platinum addition. Both the amount and nature of coke deposited on these catalysts during the n-hexane isomerization were studied. It was found that the amount of coke formed is small (<0.5%) over all samples. A similar type of coke was produced on catalysts calcined above 600°C, its amount showing a relationship with the catalytic activity. The higher the specific rate, the larger the carbon content. A more hydrogenated and aliphatic coke was observed over the catalyst calcined at 500°C, the difference being related to the properties of WOx–ZrO2 calcined at this temperature. The material calcined at 700°C before platinum addition reached the largest catalytic activity and a similar carbon content referred to the surface area than materials calcined at higher temperatures. Coke can be burnt out at 500°C.

Influence of the preparation conditions on the textural properties andn-hexane isomerization activity of Pt/SO42−−ZrO2

The influence of preparation conditions on the textural properties of Pt/SO42−−ZrO2 has been studied. The presence of sulfate preserves a great part of the small diameter pores after calcination, thus maintaining the specific surface area. Platinum is partially lost when sulfation takes place after platinum addition. The platinum containing catalysts show good activity (conversion about 65%) and selectivity (about 94%) forn-hexane isomerization, while SO42−−ZrO2 presents a poor activity. The catalytic activity is independent of the order of platinum and sulfate addition.

Platinum on a sulfated zirconia surface: effects and interactions

The effect of the presence of platinum on the surface of SO42−-ZrO2 was studied. The addition of platinum neither produces significant changes in the textural properties of SO42−-ZrO2 nor interferes in the process of crystalline structure formation. After calcining, the presence of platinum allows the retention of a higher sulfur concentration. When calcination takes place at 600°C, SO42−-ZrO2, with or without platinum, shows S6+ species on the surface, but only when platinum is present does a small amount of S0 species appear. At the same calcination temperature, platinum is found in the metallic state. Therefore, the S0 species could be associated with platinum by modifying its metallic properties.

Effect of operational conditions during η‐hexane isomerization over platinum on tungsten‐oxide‐promoted zirconia

The influence of total pressure, temperature and H 2 : N2 molar ratio on both catalytic activity and selectivity during η‐hexane isomerization over platinum impregnated on tungsten‐oxide‐promoted zirconia was studied. It was found that the increase in pressure up to 6 kg-2 increases catalytic activity. The higher the temperature, the larger the catalytic activity. Selectivity to isomers remains practically constant, only decreasing when temperature is increased above 225 °C. In the absence of hydrogen, a rapid deactivation is observed as well as a different product distribution.

Isobutene production from skeletal isomerization of 1-butene on WOx/ferrierite

Both potassium and ammonium ferrierite (FK and FA, respectively) were impregnated with tungsten species using either tungstic acid or ammonium metatungstate as precursors. The skeletal isomerization of 1-butene at 200–400°C, atmospheric pressure and 0.15 atm 1-butene partial pressure, was studied on samples with and without tungsten. Tungsten species on FK promote the isobutene formation while those species on FA generate a synergetic effect over the isobutene production. The presence of tungsten species improves the material stability and mainly, the isobutene yield. Starting the 1-butene feed over the catalytic bed at 200°C and then increasing the reaction temperature to 400°C, makes it possible to avoid the low isobutene selectivity at short times-on-stream.