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Hullfish, Cole W.

Publications and source records attributed to Hullfish, Cole W..

4 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Influence of Ordered Mesoporous Oxides in Plasma-Assisted Ammonia Synthesis

Widespread implementation of dielectric barrier discharge (DBD)-assisted NH 3 synthesis, a nascent technology operating under sustainable, ambient conditions, is hindered by low energy yields due to, in part, poor fundamental understanding. Porous oxides used to support metal nanoparticle catalysts have shown significant energy yield contributions for DBD-assisted NH 3 synthesis even without metal. Using an AC-powered, coaxial, single-stage reactor at 16 kV with equimolar (N 2 /H 2 ) feed, we measured NH 3 synthesis rates in the presence of different nonordered oxides, ordered SiO 2 structures (SBA-15 and MCM-41), and ordered Al-incorporated analogues (γ-Al 2 O 3 -coated with varying Al-loadings and Al-substitution, respectively: Al 2 O 3 -SBA-15 and Al-MCM-41). We systematically quantified NH 3 energy yield dependence on pore structures and material identities (i.e., ordered pores and Al incorporation) known to facilitate higher DBD-assisted NH 3 synthesis rates. SBA-15 displayed a higher steady-state energy yield than MCM-41, indicating that framework type is a crucial factor, with both ordered porous systems outperforming fumed SiO 2 . 10 wt % Al maximized in situ NH 3 uptake among the various Al loadings, exhibiting a higher steady-state energy yield and similar power to SBA-15. However, Al-MCM-41 had a similar steady-state energy yield and lower power than MCM-41, likely due to the extended γ-Al 2 O 3 surface that has a dielectric constant higher than that of SiO 2 . Both Al-incorporated analogues benefit from surface acid sites that can adsorb NH 3 in situ, resulting in higher overall NH 3 energy yields than that of their parent ordered SiO 2 . Al 2 O 3 -SBA-15 shielded more NH 3 than Al-MCM-41, likely due to a higher acid site density than the acid site identity. Furthermore, Al incorporation via γ-Al 2 O 3 coating more successfully improves the NH 3 energy yield; together with the high-performing ordered framework, these analogues are potential metal catalyst supports with promising energy yields for DBD-assisted synthesis of NH 3 and other chemicals.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Catalytic Consequences of Hierarchical Pore Architectures within MFI and FAU Zeolites for Polyethylene Conversion

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, in this study, 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 Bro̷nsted 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 cm 3 g –1 ), highlights the importance of confinement effects provided by micropores for cleaving C–C bonds at modest reaction conditions. In situ 1 H MAS NMR performed on polyethylene with MFI zeolite show that PE isomerizes (and potentially deconstructs) at temperatures near 450 K, highlighting the role of Bro̷nsted 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.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗