Tailoring Dynamic Metal-Polymer Interaction for the Design of Selective and Stable Partial Hydrogenation Catalysts

Abstract

Supported metal catalysts have been widely used in various reactions of refinery and chemical industrials. In these systems, support materials not only stabilize the dispersed metal catalysts, but also strongly modulate their catalytic properties by charge transfer, generation of perimeter sites, and strong metal-support interaction (SMSI). Thus, tailoring of the metal-support interactions is highly important for designing advanced catalysts. Thus far, most of commercial catalysts have been prepared using inorganic supports, such as metal oxides and zeolites, because of their high thermochemical stabilities. Therefore, various metal-inorganic support interactions have been extensively investigated. In contrast, the use of organic polymer as catalyst supports is considerably scarce, even in academic researches, because of the limited stability of typical polymers. Thus, our understanding on metal-polymer interactions and their consequences in catalysis is very limited. The use of soft organic polymers as a catalyst support has unique potential, compared to the use of hard inorganic materials. For instance, we recently demonstrated that mobile polyphenylene sulfide (PPS) chains can cover the surface of Pd catalysts via the phenomena denoted as “dynamic metal-polymer interaction (DMPI)” [1]. The resultant polymer overlayer can regulate the adsorption of reactants and their surface reactions, significantly enhancing the catalyst selectivity and catalyst lifetime in hydrogenation reactions. In the present study, we synthesized a series of polymers having similar framework structures but different metal-ligating functionalities as supports for Pd catalysts (Figure 1). The effects of the different polymer functionalities on the DMPI and their catalytic consequences were rigorously investigated. The result demonstrated that the polymers containing strongly ligating groups (e.g. Ar-SH and Ar-S-Ar) can form a stable polymer overlayer on the Pd surface, which enables selective acetylene adsorption but not ethylene hydrogenation. In contrast, polymers with weakly ligating groups (e.g. Ar-O-Ar) do not form an overlayer, resulting in non-selective hydrogenation and fast deactivation via coke formation, similar to the Pd catalysts on a conventional inorganic support, silica. The results imply that tuning the metal-polymer interactions via rational polymer design can provide an efficient way of synthesizing highly selective and stable catalysts for partial hydrogenation.