PNNL

Abstract

The benefits of hierarchical zeolites for the conversion of bulky molecules like polymeric waste have been reported in the literature, however, the impact of mesopore sizes and connectivities on rates, product selectivities, and catalyst deactivation in the context of plastic upcycling has not been systematically probed. Here, we synthesized a suite of hierarchical MFI and FAU zeolites via desilication under varying conditions for metal-free polyethylene conversion reactions under batch and flow conditions (473-523 K). Polyethylene (solid) conversion rates (normalized by Brønsted acid site density) were higher on hierarchical than parent microporous MFI regardless of mesopore connectivities, i.e., open or constricted, suggesting that the incorporation of mesopores facilitates diffusion of intermediate products to access medium-pore protons for successive scission events. Furthermore, higher branched:linear gaseous product ratios were produced on hierarchical than parent MFI, since mesopores allow for egress of bulkier molecules without undergoing further secondary events, e.g., isomerization back to linear alkanes/alkenes or beta-scission. Solid conversion rates on hierarchical FAU synthesized via desilication with cetyltrimethylammonium bromide (CTABr), however, were not higher than parent FAU, likely because the presence of CTABr facilitates recrystallization of leached species to form composites (hierarchical FAU and ordered mesoporous materials) with more isolated mesopores. The stagnation in rates, despite increased mesopore volumes (>0.22 cm3 g-1), highlights the importance of confinement effects provided by micropores for cleaving C-C bonds at modest reaction conditions. In situ 1H MAS NMR performed on PE with MFI zeolite show that PE isomerizes (and potentially deconstructs) at temperatures near 450 K, highlighting the role of Brønsted acid sites in activating C-C bonds under mild reaction conditions. Catalyst recyclability studies showed that all catalysts undergo deactivation during plastic upcycling reactions, but to varying extents. Overall, hierarchical materials have better catalyst stability than parent materials, although the differences in stability between hierarchical and parent FAU are smaller than those for MFI. Taken together, these findings demonstrate how rates, selectivities, and catalyst deactivation from plastic upcycling reactions can be controlled via fine-tuning the identity and connectivity of mesopores. This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences through a subcontract from PNNL (FWP 78459). M.O. acknowledges support from the Office of Undergraduate Research Student Initiated Internship Program (OURSIP). S.A.M acknowledges support from the Peter B. Lewis Fund for Student Innovation in Energy and the Environment and the LENS Funding Initiative. C.W.H. acknowledges partial support from the National Science Foundation Graduate Research Fellowship (NSF DGE-2039656). H.K. acknowledges support from the High Meadows Environmental Institute’s Undergraduate Research Fund, DuPont Senior Thesis Fellowship Grant in memory of Michelle Goudie ‘93, and the School of Engineering and Applied Science Senior Thesis Fund. We acknowledge the partial support from the High Meadows Environmental Institute Grand Challenge Award and use of Princeton’s Imaging and Analysis Center, which is partially supported through the Princeton Center for Complex Materials (PCCM), a National Science Foundation (NSF)-MRSEC program (DMR-2011750). In situ 13C and 1H MAS NMR experiments were supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences within the Catalysis Science Program (FWP 47319). We thank Hayat Adawi for helpful comments and discussion on the manuscript, and Denis Potapenko and John Schreiber at the Princeton Imaging and Analysis Center for providing training and helpful assistance on instruments.