J.M. Parera

Redox properties and catalytic activity of SO42−ZrO2 catalysts for n-butane isomerization Role of transition metal cation promoters

Abstract SO 4 2− ZrO 2 catalysts promoted with cations of transition metals (Fe, Cu, Ni, W, Pt, Co, Zn, Cd, Cr) were studied. Their influence on the catalytic activity for the isomerization of n -butane was analyzed in the presence of electron donor (H 2 ) and electron acceptor (O 2 , CO 2 ) gases. A new reaction scheme accounts for the one electron properties of the sulfate-zirconia catalyst. Transition metal cations are postulated to enhance activity through d -orbital interaction. Cations with a complete d level inhibit activity, but the presence of electron acceptor molecules (O 2 and CO 2 ) in the reacting medium attenuated this deleterious effect.

n-butane isomerization on tungsten oxide supported on zirconia

Abstract Tungsten oxide supported on zirconia catalysts prepared from ammonium metatungstate as a precursor and using different preparation routes were tested for n-butane isomerization at atmospheric pressure, 300°C, WHSV=1 h−1 and H2/nC4=6. The diffusion of polytungstates through the pores of Zr(OH)4 is sterically hindered due to the size of the anion. As a result of this, low penetration and the agglomeration in pore mouth are possible. The obtention and further utilization of polytungstates with a lower number of W atoms improve the distributions of W on ZrO2 surface and the catalytic activity. After calcination at 800°C, the better WO3/ZrO2 catalyst showed a catalytic performance similar to sulfated zirconia calcined at 620°C and corresponded to a material with well dispersed W. The more dispersed and active catalyst was obtained with an ammonium metatungstate solution stabilized 7 days at pH 6 before Zr(OH)4 impregnation, containing 15% W.

The influence of coke deposition on the functions of a Pt/Al2O3-Cl bifunctional catalyst

Abstract An acidic (Al2O3-Cl), a metallic (Pt/Sio2) and a bifunctional catalyst (Pt/Al2O3-Cl) were used in naphtha reforming. Temperature programmed oxidation of the coked catalysts shows that the coke on the metal is oxidised at lower temperatures than that on the acid function. According to C1/C3 (metal to acid), coke is mainly produced on the metal at the start of the run. At increasingly severe coking conditions, coke is produced on both functions and at higher severities or times it is produced mainly on the acidic function. Coke on the metal is in equilibrium with gaseous hydrogen. On increasing the hydrogen pressure, the coke is eliminated more easily from the metal than from the acid.

The role of Re and S in the PtReSAl2O3 catalyst

Abstract The modifications in catalytic activity, selectivity and stability when a Pt Al 2 O 3 catalyst is promoted by Re and/or S is studied following several C 6 hydrocarbons reforming. A bifunctional reaction scheme, showing modifications from those previously proposed, is introduced in order to interpret the results. Even the catalyst properties of PtRe are not the sum of the ones of Pt and Re, PtRe has properties brought by Pt and by Re. The addition of Re and S modifies the hydrogenolysis-dehydrogenation capacity of Pt. Re decreases the dehydrogenating capacity of Pt, reducing coke formation (desired effect) and increasing the formation of lower molecular weight paraffins by hydrogenolysis (undesired effect). The undesired contribution of Re is partially suppressed by the addition of S, that decreases the hydrogenolytic capacity of PtRe and increases the production of benzene and coke. This opposite action of Re and S on Pt can be related to the opposite electronic transfers, from Re to Pt and from Pt to S. Both geometrical and electronic effects influence the changes in selectivity.

n-Butane isomerization on Pt/WO3–ZrO2: effect of the Pt incorporation

Abstract WO 3 –ZrO 2 catalysts for n -butane isomerization were prepared by several different procedures. The influence of a Pt promotor on catalyst activity and selectivity was investigated. Possible Pt–W interactions were particularly studied. It was found that Pt strongly interacted with the WO 3 –ZrO 2 support, which impeded reducibility and decreased H 2 chemisorption capacity and dehydrogenation activity. At low Pt content hydrogenolytic activity and H 2 chemisorption were absent. The interaction depends on the distribution of WO 3 on the ZrO 2 surface. The preparation conditions such as time of stabilization of the WO 3 precursor solution and the type of precursor used are very important for the catalytic activity. For Pt/WO 3 –ZrO 2 , there was no correlation between H 2 chemisorption at room temperature, cyclohexane dehydrogenating activity and the capacity to dehydrogenate n -C 4 . This is due to the fact that the three measurements are performed at different temperatures, with quite different reaction rates and thermodynamic feasibilities.

Operational conditions and coke formation on Pt-Al2O3 reforming catalyst

Abstract The influences of pressure, H2 to naphtha ratio, temperature, space velocity and time on coke formation during naphtha reforming over Pt-Al2O3 has been studied. A standard test was used which consisted of operating the catalyst at normal conditions during a first and a third period of 7 h each, and changing to more severe conditions during an intermediate period of 20 h. Correlations between the amount of coke at the end of the test, the changes in octane number and the operational conditions during the severe period were derived. The decrease in octane number depended on1y on the amount of coke and did not depend on how the severity was achieved. The temperature programmed oxidation (obtained by differential thermal analysis) of the used catalysts showed two zones, one from 396 to 642 K and the other from 642 to 828 K. The second zone corresponded to the burning of a more polymerized deposit, the amount of which increased with the severity of the operation much more than the first zone. The thermogram patterns were quite similar and independent of the operational conditions used to deposit the coke.

Sulfurization of PtAl2O3Cl catalysts: VI. Sulfur-platinum interaction studied by infrared spectroscopy

Sulfur-platinum interaction was studied by ir spectroscopy of the coadsorbed CO, competitive hydrogenation reactions, and chemisorption of O2, CO, and H2, PtAl2O3 catalysts of different metal loadings and mean particle size were used. Infrared spectra for variable coverages of sulfur and CO were obtained. The predeposited S on Pt caused an upward shift of v(CO) from 2068 to 2083 cm−1; a superimposed band that shifted to the lower frequencies for decreasing CO coverages was detected. The kTB values (kTB, ratio of the adsorption equilibrium constant) for the competitive hydrogenation of benzene and toluene were determined. It was found that the kTB ratio increases from 4.2 for unsulfided samples to 6.0 for sulfided samples. On the other hand, the irreversibly held sulfur on the metal increased the ratio of the reversibly adsorbed H2 to the total adsorbed H2. These results are discussed considering electronic, geometric, and surface heterogeneity effects. A “localized” modification of the metal by the electron-acceptor properties of sulfur is advanced as a possible explanation.

Influence of the crystalline structure of ZrO2 on the metallic properties of Pt in Pt/WO3–ZrO2 catalysts

The influence of the crystalline structure of ZrO2 on the metallic properties of Pt, when supported on WO3–ZrO2, was studied. Pt supported on tetragonal zirconia loses its metallic properties while when supported on monoclinic zirconia it presents good metallic activities. WO2,2- deposited on amorphous Zr(OH)4 before calcination generates an active material for n‐butane isomerization. The larger the fraction of the tetragonal phase of zirconia in this material, the higher the isomerization activity and the lower the metallic activity of Pt.

Influence of PtRe interaction on activity and selectivity of reforming catalysts

The influence of the preparation procedure on the Pt and Re interaction in PtRe/Al2O3 reforming catalysts has been studied. Three different preparation procedures have been used: the classical coimpregnation and successive impregnation techniques, and the recently reported catalytic reduction method. Catalyst activation was done either by direct reduction after metal deposition, or by calcination and reduction. The degree of interaction of the metals was indirectly measured by the cyclopentane hydrogenolysis reaction. It has been found that interaction between Pt and Re increases according to the sequence: coimpregnation (calcined and reduced), catalytic reduction (calcined and reduced), successive impregnations (reduced catalysts) and catalytic reduction (reduced catalysts). Calcination greatly diminishes the PtRe metal-metal interaction, as measured by cyclopentane hydrogenolysis. After sulfiding, the catalysts prepared by catalytic reduction with a calcination and reduction treatment display the highest n-heptane dehydrocyclization activity. Catalysts only reduced also have good activity for this reaction, but with poor stability.

Burning of coke on Pt Re/Al2O3 catalyst: Activation energy and oxygen reaction order

Abstract Activation energy and reaction order with respect to oxygen for the coke burning of a Pt Re/Al2O3 naphtha reforming catalyst were evaluated by three different methods. Data were obtained by analysing a commercially coked catalyst by temperature programmed oxidation (TPO), using an oxidative mixture containing 1.9% (v/v) oxygen in nitrogen. The methods gave similar values of activation energy; for less polymerized coke about 10 kcal mol−1 and for more polymerized coke about 25 kcal mol−1. When the catalyst had been previously hydrogenated, the activation energy was 3.2 kcal mol−1. The reaction order with respect to oxygen was 0.5.