Hydroisomerization-cracking of n-octane on heteropolyacid H3PW12O40 supported on ZrO2, SiO2 and carbon effect of Pt incorporation on catalyst performance

Heavy paraffins (n-C8-20) are inexpensive refinery cuts of low octane number that can be upgraded to fuel-grade gasoline (C-4-C-7) by means of simultaneous hydrocracking and hydroisomerization over bifunctional metal/acid catalysts. The addition of Pt to pure tungstophosphoric acid (HPA) and supported on ZrO2 (HPA/Z), SiO2 (HPA/Si) and carbon (HPA/C) was studied in n-octane hydroisomerization-cracking reaction (300 degrees C, 0.1 MPa, WHSV = 1 h(-1), H-2/n-C-8 = 6). Catalysts were characterized by specific surface area; temperature-programmed reduction, XRD measurements, FT-IR spectroscopy and temperature-programmed desorption of probe molecules (pyridine and 2,4,6-trimethyl pyridine). Supported HPA samples showed initial specific activity values expressed by mass unit of HPA higher than pure HPA. The order at 5 min on stream was HPA/Z > HPA/C > HPA/Si similar to pure HPA. All the samples deactivated by coking. The Pt addition had two effects: (i) increased surface proton content of the samples and thus their activity in n-C-8 hydroconversion (a linear relationship between specific activity values and Bronsted acidity was found), (ii) increased stability of samples allowing the attainments of a fast pseudo-steady state. The increase in activity due to Pt incorporation may be related with the formation of a new surface species of Pt into the HPA network. The fast stabilization of the samples was related to the inhibition of the coke precursors produced by the metallic Pt that remained on the surface of catalysts. Catalysts provides between 5 and 110 additional RON points over those of a model feedstock of n-octane. The RON gain is better correlated to the Bronsted acidity of samples, thus confirming that the production of branched isomers from long alkanes does not require highly acidic sites. Pt-HPA/Z showed the best catalytic performance with a RON gain of 101 points. A linear relation between the production of light gases (C-1-C-3) and the RON gain indicates that octane boosting can only be got at the expense of the isomerizate yield. The RON gain and the production of light gases were compared with the corresponding to a catalysts of Pt deposited on sulfated zirconia (PUSZ). The results encouraged the use of Pt-HPA/Z as an alternative to produce a new hydroisomerization-cracking catalysts for the treatment of heavy alkanes in order to produce light isoalkanes. (c) 2005 Elsevier B.V. All rights reserved.