Screening of optimal pretreatment and reaction conditions for the isomerization-cracking of long paraffins over Pt/WO3-ZrO2 catalysts

摘要

Pretreatment and reaction conditions for the isomerization-cracking of long paraffins over commercial Pt/WO3-ZrO2 catalysts were screened. Optimal conditions were seeked for the production of short, high octane branched paraffins for the gasoline pool. n-C_(16) was used as a model molecule. The reaction over the acidic catalyst was used to adjust the size of the molecules to the boiling range of gasolines and to add branching in order to increase the octane number of the product. These adjustments had to be done while minimizing the production of light gases and getting a stable activity level. With respect to the pretreatment it was confirmed that the best calcination temperature of the tungstate oxoanion promoted hydroxides was 800 °C Optimality was mostly related to the activity level. The optimum temperature coincides with the appearance of small WO3 crystallites which are thought to be center for creation of Bronsted acid sites in the presence of platinum and hydrogen. Regarding the reaction conditions, increasing temperature values augmented the conversion but also increased the cracking. Therefore optimum values were found at moderate temperature given the high reactivity of the feed. Space velocity values were analyzed with attention to the liquid C_(5+) yield, the selectivity to branched isomers and the stability of the catalysts. Best yields to branched naphtha products were obtained with Pt/WO3-ZrO2 at WHSV = 18 h~(-1). The catalyst coking rate was a function of the H2/hydrocarbon ratio. A value of 6 was enough to attain a stable conversion value on Pt/WO3-ZrO2. The values of liquid yield as a function of pressure displayed a volcano pattern that was rationalized in terms of a non-classical bifunctional mechanism of reaction. -High pressure values increased the concentration of Bronsted acid sites and hence the activity. Too high pressures enhanced hydrocracking and decreased the liquid yield. The results indicate that in general terms Pt/WO3-ZrO2 at moderate reaction conditions transforms a paraffinic heavy cut into a branched isomerizate that can be added to the gasoline pool to improve the quality properties. It was however found that under some conditions the octane gain is inversely proportional to the yield of light gases indicating that a high RON isomerizate can only be got at the expense of the liquid yield.