Hydroisomerization and Cracking of n-Octane and n-Hexadecane over Zirconia Catalysts

Abstract

C8−C16 n-paraffins can be upgraded to fuel-grade gasoline (C5-8) by means of simultaneous hydrocracking and hydroisomerization over bifunctional metal/acid catalysts. The hydroisomerization of n-paraffins over nickel/sulfated-zirconia (NiSZ), nickel/tungsten-zirconia (NiWZ), platinum/sulfate-zirconia (PtSZ), and platinum/tungsten-zirconia (PtWZ) “fresh” catalysts provides between 10 and 113 additional research octane number (RON) points over those of a model feedstock of n-octane. A close linear relation between the production of light gases (C1−C3) and the RON gain indicates that octane improvement can only be reached at the expense of the isomerizate yield. The RON gain stabilizes at a Ni−Pt content of 1% and varies slightly at higher loadings. PtSZ is the most active catalyst and provides a 100−113 RON upgrade depending on the Pt content. PtWZ can only provide 42 RON points at these reaction conditions. Though Ni-based catalysts provide a moderate RON increase of 79−83 points, they are cheaper than Pt catalysts and show an improved stability at high values of time-on-stream. The RON gain is better correlated to the total acidity than to the strong acidity, thus confirming that the production of branched isomers from long alkanes does not require highly acidic sites. On comparison, the reaction of n-hexadecane on the 0.1 and 1.0% Ni-promoted SZ and WZ catalysts indicates that the WZ catalysts provide the highest yield and selectivity to C5−C8 isomers. For the same amount of Ni, SZ catalysts have a higher activity than the WZ catalysts, but their selectivity for the production of light gases is too high.