n-Hexane isomerization on Pt/WOX-ZrO2. Influence of WOX-ZrO2 properties on catalytic activity

M. G. FALCO; S. A. CANAVESE; N. S. FÍGOLI

Rio de Janeiro, Brazil

Congreso; Enpromer 2005. 2nd Mercosur Congress on Chemical Engineering. 4th Mercosur Congress on Process Systems Engineering; 2005

It is mentioned in the literature that the catalytic function of WOx-ZrO2 during o-xylene isomerization depends on WOx size and to their relation to less active oxides that act as supports. We have found a similar phenomena during n-hexane isomerization carried out at 200 °C, WHSV = 4 h-1, 6 kg cm-2 and H2: n-hexane (molar) = 7. WOx-ZrO2 samples having different WOx domain sizes were obtained using different preparation techniques but starting from the same raw materials: ZrOCl2, NH4OH and H26N6O41W12. The WOx-ZrO2 samples were calcined at 700 or 800 °C and then impregnated with a hexachloroplatinic acid solution. After impregnation, the solids were calcined at 500 °C and reduced at 250°C under a hydrogen stream. Several techniques were used for catalyst characterization: chemisorption, NH3-TPD, X-ray diffraction and TPR. TPR allowed us to recognize the solids having different WOx domain sizes. An intermediate aggregation degree, that reduces through the WO2.9 ® WO2 step, helps to temporally delocalize the charge and allows, in the presence of hydrogen dissociated on platinum, to generate the active sites and to stabilize the intermediate carbocations generated. The intermediate isomeric carbocations are easily desorbed before b -scission reactions occur. The rapid desorption decreases undesired reactions and increases the selectivity to n-hexane isomers. The acidity of WOx-ZrO2 or the crystalline structure of zirconia are not directly related to the WOx domain sizes and to the catalytic activity for n-hexane isomerization.