Investigation of Pt-Containing Catalysts Based on RFE and MTT Zeolites in Hydroisomerization of n-Hexadecane and Hydroisodewaxing of Diesel Fuel

The hydroisomerization of long-chain paraffins, such as n-C16, is a key process for improving the low-temperature properties of diesel fuels. However, the understanding of how the zeolite pore structure and the balance between acid and metal sites determine the efficiency of this process remains incomplete. This work is dedicated to the synthesis and investigation of Pt/ZSM-23 and Pt/ZSM-48 catalysts in n-hexadecane isomerization and diesel fuel hydroisodewaxing. A kinetic investigation of the model feedstock hydroisomerization was conducted at low conversion levels, with a focus on product distribution and structure–property correlations. A detailed analysis was performed on the yield of mono- and multibranched isomers, cracking products and fuel quality indicators (e.g., CFPP, cetane index). Pt/ZSM-23 demonstrated higher isomer selectivity at lower temperatures and a greater degree of proximity between metal and acid sites, whereas Pt/ZSM-48 exhibited increased cracking at elevated temperatures, consistent with its acidity tending towards the stronger type and distinct pore architecture. The kinetic analysis accounted for competing reaction pathways proceeding via carbenium ion intermediates on acid sites and dehydrogenation/hydrogenation reactions on Pt, emphasizing the role of spatial constraints and acid site strength in governing the rates of elementary steps. The prepared catalysts ensured high yields of isomers in the diesel fuel range with significantly reduced cloud points and acceptable density/cetane index values, confirming the potential of using specially designed Pt/zeolite systems for improving diesel fuel quality. This work deepens the understanding of the hydroisomerization mechanism on one-dimensional zeolites and establishes a foundation for the development of catalysts for diesel fuels hydroisodewaxing.