Mechanistic aspects of n-hexadecane hydroconversion: Impact of di-branched isomers on the cracked products distribution.

• Pd & Pd-Pt on various aluminosilicate evaluated for n-C 16 hydroconversion. • Preference CH 3 branching central positions in monobranched isomers at low X n-C16. • Preference CH 3 far from center in dibranched isomers at higher X n-C16. • With increasing X n-C16 : cracked product distribution from M shape to symmetric shape. • No preferential sieving of isomers in n-C 7 hydroconversion. The mechanism of n-hexadecane (n-C 16) hydroisomerization/hydrocracking was studied over bifunctional catalysts employing Pd and Pt as (de)hydrogenation components and large-pore zeolites and (ordered) mesoporous materials as acidic supports. Zeolite Y and Beta supports were compared to amorphous silica-alumina as well as aluminium-containing ordered mesoporous MCM-41, MCM-48 and SBA-15 materials to cover a wide range of pore sizes and topologies. Products were analyzed in terms of mono- and di-branched C 16 isomers and cracked products as a function of the n-C 16 conversion. All samples followed a similar mechanism in which, at low conversion, di-branched isomers with methyl groups toward the ends of the tetradecane chain were observed. At higher conversion, the products shifted to isomers with methyl groups closer to the center at higher n-C 16 conversion. Consistent with these differences, the typical 'M' shape distribution of cracked products was obtained at low conversion, which evolved into ideal symmetric cracking patterns at higher conversion. The unusual di-branched isomer distribution at low conversion and the corresponding 'M' shaped cracked products distribution have earlier been associated with pore mouth catalysis induced by restricted diffusion in medium-pore zeolites. Here we show that such reaction pathways also occur in catalysts with large enough pores to exclude diffusion restrictions. [ABSTRACT FROM AUTHOR]