Catalytic Activation of Polyethylene Model Compounds Over Metal‐Exchanged Beta Zeolites

Abstract

Decomposition of polymers by heterogeneous catalysts presents a promising approach for reuse of waste plastics. We demonstrated non‐hydrogenative decomposition of model polyolefins over proton‐form and metal (Cu, Ni) ion‐exchanged beta (BEA) zeolites at moderate temperatures (around 300 °C). Near complete polyolefin decomposition was observed in batch reactions monitored by thermogravimetric analysis, while decomposition at partial conversion was studied in flow reactions. Ni‐exchanged zeolites produced H2 at substantially higher rates (>10x) than other catalysts while also uniquely resisting deactivation over time. Application of the delplot formalism offered insights into the reaction network for polyolefin decomposition over Ni/BEA most notably that H2 is solely a primary product. We deduce that H2 production is catalyzed by activation of C−H bonds at ionic Ni sites, and H2 prevents buildup of polyaromatic coke species in Ni‐exchanged zeolites that deactivate Cu‐exchanged and protonic zeolites.

The Overlooked Potential of Sulfated Zirconia: Reexamining Solid Superacidity Toward the Controlled Depolymerization of Polyolefins

“…For example, solid porosity allows control of stereo- and regioselectivity not available in liquid acids through effects influencing substrate orientation, reactive intermediate size, mass transfer, and intermediate stabilization . The classic example is the shape selectivity associated with zeolite catalysts of varying pore dimensions and pore connectivity. ,, These principles have been extended to polymer deconstruction reactions using supported metals in the zeolite pores, with recent examples employing dehydrogenation to initiate C–C bond cleavage and C–H borylation reactions. − Zeolites can both naturally and synthetically occur as 2D sheets (e.g., MMT) and/or 3D interconnecting porous networks (e.g., HZSM-5). , As previously discussed, acid-catalyzed depolymerization of PP generates an uncontrolled mixture of primarily paraffin products on solid acids with little pore volume (e.g., SZrO). However, the ordered, 0.55 nm, high-volume pores of HZSM-5 promote chain-end β -scission and sterically impede formation of the bulky intermediates produced by SZrO that generate heavier paraffin products .…”

Understanding the Structure–Activity Relationships in Catalytic Conversion of Polyolefin Plastics by Zeolite-Based Catalysts: A Critical Review

“…For example, Jacob et al demonstrated that H 2 was generated with Ni/beta zeolite catalyst at a rate 10 times higher than for Cu/ beta and H-beta zeolite catalysts in the model reactions of dotriacontane and hexadecane decomposition. 167 This H 2 disrupted the formation of coke species, preserving catalytic activity. Likewise, in Yao's work Ni-ZSM-5 was found to be effective to resist coke formation and showed a high yield of hydrogen and syngas from HDPE.…”

“…On the other hand, a metal/zeolite could activate the C–H bonds of polyolefin molecules and improve stability. For example, Jacob et al demonstrated that H 2 was generated with Ni/beta zeolite catalyst at a rate 10 times higher than for Cu/beta and H-beta zeolite catalysts in the model reactions of dotriacontane and hexadecane decomposition . This H 2 disrupted the formation of coke species, preserving catalytic activity.…”

Risk Minimization in Scale-Up of Biomass and Waste Carbon Upgrading Processes

“…Existential issues stemming from climate change and ecological accumulation of waste materials − compel humanity to develop and implement technologies which replace energy sources and materials derived from petroleum, coal, natural gas, and other fossil sources with low-carbon-intensity alternatives as rapidly as possible. A significant subset of these developing technologies, with applications spanning from transportation fuels − to critical materials, − relies on upgrading biomass or waste carbon via thermo- or biochemical processing.…”