Development of a Bimetallic Pd-Ni/HZSM-5 Catalyst for the Tandem Limonene Dehydrogenation and Fatty Acid Deoxygenation to Alkanes and Arenes for Use as Biojet Fuel

A tandem process involving the dehydroaromatization of the terpene limonene and the hydrodeoxygenation of stearic acid has been found to be efficiently catalyzed by Pd-Ni/HZSM-5. The process involves the generation of p-cymene from terpene with concomitant formation of H₂, which leads to the one-pot hydrodeoxygenation of stearic acid to C₁₇ and C₁₈ alkanes; these products can be used as kerosene additives for aviation fuel. Screening a wide range of catalysts, the bimetallic Pd-Ni/HZSM-5 catalyst is the most efficient, leading to quantitative conversion of stearic acid to alkanes in limonene at 280 °C at a H₂ pressure of 2 bar after 120 min. It has been found that single Ni or Pd catalysts lead to a poor conversion of stearic acid in limonene at a H₂ pressure of 2 bar. The combination of physically mixed Pd- and Ni-sites onto different supports (Pd/HZSM-5 or Pd/C, and Ni/HZSM-5, Ni/HY, or Ni/HBEA) leads to catalysts which show satisfactory conversion to p-cymene but generally have very low stearic acid conversion rates. Directly incorporating Pd and Ni onto the HZSM-5 scaffold forms the Pd-Ni/HZSM-5 bimetallic catalyst, which demonstrates a remarkable improvement in stearic acid conversion to C₁₇ and C₁₈ alkane products. In this catalyst system, Pd is shown to be the active site for limonene dehydroaromatization, while Ni catalyzes the separate stearic acid hydrodeoxygenation. The acidity of HZSM-5 (modified by the Si/Al ratios) influences the performance of the Pd-Ni bimetallic catalyst, and the proper pore size of HZSM-5 prevents side-reactions from limonene condensation. In addition, the alloyed Pd-Ni nanoparticles (optimized with higher Pd/Ni ratios) on the external surface of HZSM-5 enhance internal H^• transfer between the two metals, thereby increasing the rate of stearic acid hydrodeoxygenation. The catalytic compatibility of the Pd and Ni sites, coupled with the proper pore sizes and optimized level of Brönsted acid sites in HZSM-5, result in the design of a multifunctional catalyst that is efficient for both steps of the cascade reaction. Hence, a bimetallic 5%Pd-10%Ni/HZSM-5 catalyst has been developed that allows for a simple approach for producing aromatics and hydrocarbon components present in biojet fuel derived from two biomass resources.