Carlos L. Pieck

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.

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.

Influence of chlorine content on the behavior of Pt-Re/Al2O3 catalyst for naphtha reforming

Abstract The influence of chlorine content on the activity, selectivity and stability of a Pt-Re/Al 2 O 3 catalyst was studied. The feed was a typical hydrotreated naphtha used for commercial petrochemical reforming. The operating conditions were similar to those used industrially and a period at low pressure was introduced to accelerate coke deposition. The influence of chlorine on activity and selectivity was similar to that previously found for monometallic Pt/Al 2 O 3 -Cl catalyst, but a different behaviour was observed with respect to coke deposition, which decreased as the chlorine content increased. This effect may be explained by the preferential enhancement of reaction paths that eliminate coke precursors.

Influence of chlorine content on PtRe interaction and coke deposition

Abstract The influence of the chlorine content on the PtRe interaction and the modification of the catalytic properties of reforming catalysts prepared by co-impregnation were studied. Chlorine inhibits PtRe interaction and produces a decrease in the deposition of coke over the metallic function. In monometallic Pt catalysts, chlorine produces lower coke deposits, but its toxicity in cyclohexane dehydrogenation is the same and the catalytic activity is independent of chlorine content. For bimetallic catalysts, the greater the PtRe interaction, the lower the deactivation by coking of the metallic function.

Alkane isomerization on MoO3/ZrO2 catalysts

Molybdenum was used as an alternative to sulfate as promoter of zirconia for alkane isomerization reactions. It has similar effects to sulfate, tungstate and phosphate ions, modifying the physicochemical properties of unpromoted zirconia. Mo‐promoted zirconia catalysts do not show any activity for n‐C4 isomerization. For n‐C7 isomerization, the catalytic activity depends on Mo content, crystalline structure of the support and the molybdenum oxospecies present on the surface of the catalysts.

Recovering of the catalytic functions of naphtha reforming catalysts by partial coke burning

Abstract The selective regeneration of the catalytic functions during coke burning on Pt/Al2O3 and Pt-Re/Al2O3 naphtha reforming catalysts was studied. Sulphided and unsulphided catalysts were coked in the laboratory and tested for benzene hydrogenation (metallic function) and n-pentane isomerisation (acid function). These test reactions and the temperature-programmed oxidation of the coke showed that only the metallic function is recovered by coke burning at low temperature and that at high temperature both functions are recovered. All this according on how coke was eliminated from the functions. Upon reduction with hydrogen at 773 K of catalysts whose coke was only eliminated from the metallic function, migration of part of the remaining coke to the metal occurred and resulted in metal deactivation. At the same time, the sulphur present on the support as sulphate ion is reduced to hydrogen sulphide at this temperature, thus deactivating the metallic function.

Influence of sulfurization on coke formation over catalysts for naphtha reforming

Abstract The effect of sulfurization on coke deposition over Re/Al 2 O 3 , Pt/Al 2 O 3 and Pt-Re/Al 2 O 3 was studied carrying out reforming of naphtha and pure compounds before and after sulfurization with H 2 S at 500°C. On all the catalysts sulfurization increases the total amount of coke. On the sulfurized ones the coke is localized mainly on the support and is less hydrogenated than that on the nonsulfurized catalysts. These results are explained in terms of a geometrical effect of the metal poisoning by S. Hydrogenolysis of coke precursors and their condensation on the metal are both demanding reactions greatly decreased by sulfurization. On the contrary, the formation of coke precursors by dehydrogenation is a non demanding reaction, and is decreased less. The coke precursors that can not condensate on the metal condensate on the support.

Influence of O2 and O3 regeneration on the metallic phase of the Pt—Re/Al2O3 catalyst

Abstract The influence of the regeneration with oxygen or ozone on the metallic function activity of the Pt—Re/Al2O3 catalyst prepared by catalytic reduction is studied. The interaction between Pt and Re is higher when activation is carried out by direct reduction. However, calcination at 450°C leads to a segregation of metals. The regeneration carried out with diluted O2 destroys the metallic interaction producing segregation of Pt and Re, while the regeneration with O3 at low temperature avoids such segregation to a large extent. Ozone not only burns coke off, but also modifies its burning characteristics as measured by temperature programmed oxidation technique (TPO) with 6% O2/N2. It was found that after the O3 treatment, the coke left on the catalyst burns at lower temperatures if compared to the coke without O3 treatment. Therefore, a regeneration treatment can be carried out at low temperatures by combining a first step with O3 followed by a second one with O2 at lower temperatures than those needed in conventional regeneration procedures.

Total metallic dispersion of sulfided Pt-Re/Al2O3 naphtha reforming catalysts

Abstract Catalysts with Pt, Re and Pt-Re supported on γ alumina were prepared by impregnation or catalytic reduction methods. The activation was performed by calcination–reduction or only direct reduction treatment. These catalysts and a commercial one were sulfided with H 2 S at 500°C and characterized by chemical analysis, TPR, test reactions (cyclohexane dehydrogenation and cyclopentane hydrogenolysis), chemisorption of H 2 and O 2 and TEM. The test reactions were affected by S according to the interaction Pt-Re and metallic dispersion of the catalysts, which are functions of the preparation technique. The presence of S inhibits H 2 chemisorption, which cannot be used as a measure of the metallic dispersion. Nevertheless, sulfidation or Pt-Re alloying does not affect the O 2 chemisorption and this chemisorption can be used to calculate the total (Pt+Re) metallic dispersion with a stoichiometry ratio Pt or Re/O equal to 1.