Monica J. Mendoza-Castro

Regeneration and reuse of silver-based sorbents could reduce the economic burden and environmental impact associated with radioiodine capture from used nuclear fuel reprocessing off-gas. Here, three commercially available silver-containing sorbents, IONEX Ag-400 (Ag-exchanged FAU with binder), IONEX NexSorb-99 (Ag-exchanged MOR with binder), and Clariant AC-6120 (Ag-impregnated Al2O3), were evaluated for cyclic iodine loading and regeneration as candidate primary-bed sorbent materials in a dual-bed process. Ag-400 exhibited the highest iodine uptake over three loading cycles, but became increasingly friable as repeated AgI(s) formation during loading and Ag0(s) formation during regeneration collapsed the zeolite framework. NexSorb-99 showed lower but more stable iodine uptake with no detectable degradation of the engineered sorbent during cycling. Although also zeolite-based, the greater structural stability of MOR and the ability of the binder to accommodate repeated phase transformations prevented pellet degradation. AC-6120 exhibited the lowest iodine uptake but maintained physical integrity and the most consistent pore structure during cycling; however, more residual AgI(s) remained after regeneration compared to the zeolite-based materials. These results demonstrate that the viability of sorbents for regeneration and reuse is controlled primarily by support stability and pore accessibility, rather than silver loading alone.

The design of hierarchical zeolites through post-synthetic modification offers versatile routes to tune their catalytic performance by engineering properties, such as morphology and aluminum distribution, which impact diffusion and reaction(s) within their confined porous networks. In this study, we use secondary growth to produce a library of finned and core-shell MFI-type zeolites with tailored nanosized features and mesoscopic gradients in acid site density. Structure–performance relationships are developed using methanol to hydrocarbons (MTH) to assess catalysts with distinct active (ZSM-5) and inactive (silicalite-1) regions. We show that catalysts with inactive interiors used as seeds to generate ZSM-5 fins and shells function as pseudo nanoparticles and nanosheets, respectively. These materials outperform their silicon-zoned counterparts; however, the introduction of Si-rich exteriors (fin or shell) achieves longer catalyst lifetime compared to conventional ZSM-5 catalysts due to enhanced mass transfer through siliceous regions and surface passivation, which mutually reduce external coking. Comparisons between finned and core-shell catalysts of similar compositional architectures reveal that finned structures consistently achieve higher cumulative turnovers and longer lifetimes, highlighting the benefits of creating localized high densities of pore openings for rapid exchange of molecules into and out of microporous channels. Combined Al X-ray absorption spectroscopy, solid-state NMR, and acid titrations reveal that secondary growth generates a distribution of tetrahedral Al sites with distorted Al–O bonds. The ensemble average of active sites is distinct for each hierarchical catalyst, countering a common belief that framework Al sites have identical Brønsted acidity. Collectively, our findings demonstrate the advantages of introducing fin or shell configurations by secondary growth; and the significance of controlling spatial distributions of active sites on exterior regions of hierarchical zeolite catalysts to optimize catalytic performance. These results are demonstrated for the MTH reaction, but we envision similar outcomes could be achieved for other hydrocarbon upgrading processes.

Zeolite interconversion is a widely used strategy due to its unique advantages in the synthesis of some zeolites. By using a long-chain quaternary amine as both a structure-directing agent and porogen, we have produced superior catalysts, which we named Hybrid Zeolites, as their structures are made of building units of different zeolite types. The properties of these materials can be conveniently tuned, and their catalytic performance can be optimized simply by stopping the interconversion at different times. For cracking the 1,3,5-triisopropylbenzene, Hybrid Zeolites made of FAU and MFI units show a 5-fold increase in selectivity towards the desired product, that is, 1,3-diisopropylbenzene, compared to the commercial FAU, and a 7-fold increase in conversion at constant selectivity compared to MFI zeolite.