Active phases and sulfur tolerance of bimetallic Pd–Pt catalysts used for hydrotreatment

Bimetallic Pd–Pt catalysts are used industrially to saturate aromatics in industrial feedstocks under mild reaction conditions to bypass the thermodynamic limitations. A considerable amount of research effort has been focused on elucidating the structural and electronic properties of bimetallic Pd–Pt particles, generally supported on acidic supports, to correlate their properties with their sulfur tolerance as well as with their catalytic activity/selectivity. However, the properties of bimetallic Pd–Pt particles under these characterization conditions are still partly unknown, particularly what happens during hydrotreating. We therefore prepared bimetallic Pd–Pt catalysts (Pd/Pt atomic ratio of 4/1) using the same precursors of noble metals and various supports, such as acidic and non-acidic ultra-stable Y-type (USY) zeolites, SiO2–Al2O3, SiO2 and Al2O3, and then investigated the structural and electronic properties of the supported bimetallic Pd–Pt particles. These properties that appeared under liquid-phase hydrotreating conditions were correlated with reaction selectivity for tetralin hydrogenation and in 4,6-dimethyldibenzothiophene hydrodesulfurization, as well as with Fourier-transform analyses of adsorbed CO, dispersion and EXAFS data of reduced/sulfided catalysts. The effects of several parameters involved – such as calcination/activation conditions and the presence of extra-framework alumina and chlorine in the zeolite supports – on the sulfur tolerance of the bimetallic Pd–Pt catalysts were also investigated. In addition to sulfur poisoning, agglomeration of the Pd–Pt particles and inhibitory effects caused by nitrogen-containing compounds and aromatics were also investigated to develop measures to minimize the agglomeration of Pd–Pt particles under hydrotreating conditions for real feedstocks.