Hydroisomerization and hydrocracking of long chain n-alkanes on Pt/amorphous SiO2-Al2O3 catalyst

The hydroisomerization and hydrocracking of n-hexadecane, n-octacosane and n-hexatriacontane on a 0.3% platinum/amorphous silica-alumina (MSA/E) catalyst was investigated in a stirred microautoclave at 345, 360 and 380 degrees C and between 2 and 13.1 MPa hydrogen pressure. For each n-paraffin, the reaction pathway and the kinetic parameters were determined. The results were used to elucidate the effect of chain length and operating conditions on isomerization and cracking selectivity. The conversion of the n-paraffins lead to the formation of a mixture of the respective isomers, as the main product, together with cracking products. At every temperature, the iso-alkane/n-alkane ratio of cracking products increased considerably with increasing conversion degree. Ar the same conversion level, higher reaction temperatures lead to cracking products characterized by a lower iso-alkane/n-alkane ratio. The conversion rate constants showed a considerable increase between n-C-16 and n-C-28, whereas a slight decrease between n-C-28 and n-C-36 was observed. The hydroisomerization selectivities showed a decrease as a function of chain length and with increasing conversion levels. The increase in reaction temperature leads to a small decrease in the isomerization selectivities only at low-medium conversion degrees and at the highest temperature investigated, while the effect of this parameter on the maximum yields achievable in iso-C16, iso-C28 and iso-C36 was negligible. The results indicate that the conversion of the n-paraffins follows a first-order kinetic in hydrocarbon while the order in hydrogen pressure was -1.1 +/- 0.21 for n-C-16 and -0.66 +/- 0.15 for n-C-28. Furthermore, an increase in hydroisomerization selectivity at higher hydrogen pressure for n-C-28 conversion was observed. (C) 2000 Elsevier Science B.V. All rights reserved.