Investigation of metal–polymer interactions and catalytic consequences by controlling polymer properties

Abstract

Metal supported catalysts are widely used for hydrogenation, dehydrogenation, hydrogenolysis, and oxidations of various process in refinery and chemical industries. In these catalytic systems, the support materials not only physically stabilize the metal active sites, but also strongly modulate their catalytic properties. In this respect, understanding and controlling the metal–support interaction has been one of the most important research topics in heterogeneous catalysis. Conventionally, supported metal catalysts have been prepared using inorganic support including various metal oxides (e.g., alumina, silica), and zeolites due to their high thermal stability and large surface area. In contrast, the use of polymers as a support are relatively scarce due to limited thermochemical stability. However, since polymers have the advantage of being able to control physical and chemical properties such as chemical functional group, molecular weight, and crystallinity depending on the synthesized unit. Therefore, it is possible to systematically control the metal–support interaction through the polymer design. In this study, we control the interaction with metals and catalytic behavior in the selective partial hydrogenation of acetylene by tailoring the chemical functional group and molecular weight of polymers. We synthesized a series of polymers with different functional groups as a support for Pd catalysts to investigate the effects of different polymer functionalities and their catalytic properties. The polymers including strongly ligating groups (e.g., Ar-SH, Ar-S-Ar) can form polymer overlayer on the surface of Pd which enables selective acetylene adsorption and its partial hydrogenation to ethylene with inhibited deactivation. In contrast, the polymers including weakly ligating groups (e.g., Ar-O-Ar) do not form polymer overlayer on Pd surface, which results in non-selective reaction and fast deactivation, similar to the Pd catalysts supported on inorganic materials. The results confirmed that the metal–polymer interaction can be systematically controlled according to the functional group of the polymer, and through this interaction, it is possible to design a catalyst with high selectivity and stability by controlling the reactant that can access the metal surface. Also, we synthesized oligomeric and polymeric phenylene sulfides with different molecular weights to investigate the catalytic effects of the chain length of the organic modifiers. Increased molecular weights in beneficial for improving the ethylene selectivity by inhibiting the adsorption and full hydrogenation of ethylene. The modifiers with larger number of sulfide groups per molecule can bind to the Pd surface more effectively and suppress ethylene adsorption. Modifiers with high molecular weights were also advantageous in obtaining long-term catalyst stability due to their higher thermal stability and ability to inhibit the formation of carbonaceous deposits.