Adsorption of Polyolefins from Alkanes on Alumina and Silica

Abstract

Heterogeneous catalytic deconstruction of polyolefins offers a promising route to upcycle plastic waste into higher-value products, yet the fundamental insights needed to control product distributions remain limited. As the reactions proceed, deconstruction generates a distribution of polymer fragments and small molecules whose adsorption to catalyst supports influence accessibility to active sites and subsequent reaction pathways. Here, we use in situ neutron reflectivity to quantify the adsorption of polyethylene (PE) and isotactic polypropylene (iPP) onto planar alumina and silica surfaces, which serve as idealized model support surfaces for catalytic upcycling, from oligomeric alkane (dodecane and tetradecane) solutions. Fits of the reflectivity data reveal that the underlying oxide has a surprisingly large impact on the structure of the adsorbed polymer layer and the total amount of polymer adsorbed. PE adsorption from tetradecane on alumina yields relatively thin (∼2 nm) adsorbed layers that contain ∼20–40% solvent. A much thicker adsorbed PE layer from dodecane solution is observed on silica. The adsorbed PE extends tens of nanometers from the silica surface. The PE densifies near the silica surface with exclusion of alkane solvent, but there is also a diffuse layer containing >50% solvent that extends further into the solution. This structuring on the silica surface is attributed to the combination of train and loop morphologies for the adsorbed PE. For both surfaces, the total amount of PE adsorbed is greater than that for iPP under comparable conditions. Surprisingly, PE adsorption on alumina is only weakly dependent on its molar mass (10–200 kg/mol) and almost insensitive to temperature (140 vs 160 °C). These results offer fundamental insight into adsorption of polyolefins onto surfaces, which may prove useful for improving models to describe catalytic upcycling of polyolefin wastes.