PNNL

Abstract

Hydrocracking is a promising route for the chemical recycling of polyolefins (PO), converting them into short hydrocarbons over bifunctional catalysts with metal sites for hydrogenation/dehydrogenation and Brønsted-acid sites (BAS) for isomerization and C–C bond cleavage. However, PO feedstocks containing poly-vinyl chloride (PVC) can release chlorine (Cl) under reaction conditions, deactivating conventional noble-metal/zeolite catalysts. Moreover, the lack of site intimacy and the presence of micropores within conventional catalysts create challenges around the transport of high-molecular-weight, sterically encumbered polymer intermediates. Here we report tungsten carbides (WxC) as a novel type of bifunctional catalysts that address these challenges. W/W2C phases on WxC offer “metal” sites, and -OH on WOx species introduce BAS in close proximity. The “metal” : BAS ratio can be tuned through carburization temperature, leading to a volcano-shaped activity trend reflecting the requirement for metal-BAS balance. Kinetic data demonstrate that each PO chain undergoes sequential cleavage, while trends in cracking ideality and selectivity follow those in short-alkane hydrocracking. On the per-BAS basis, W?C is more efficient than conventional bifunctional catalysts by more than an order of magnitude, due to enhanced polymer transport. They maintain or show increased activity with 10 wt.% PVC in the substrate. This work establishes transition metal carbides as earth-abundant bifunctional catalysts with unique site proximity and heteroatom compatibility. These features, along with the broad structure space for rational tuning, make them promising options to tackle specific challenges that polymer feedstocks present in hydrocracking.