Porous materials have emerged as promising candidates for selective adsorption of electrolyte components in battery recycling streams. Metal-organic frameworks (MOFs), activated carbons, and zeolites exhibit distinct advantages in surface area, selectivity, and regeneration potential, making them suitable for recovering valuable solvents and salts from spent lithium-ion battery electrolytes. This evaluation focuses on their performance metrics, operational considerations, and economic feasibility.

The surface area of porous materials directly correlates with adsorption capacity. MOFs typically demonstrate the highest surface areas, ranging from 1000 to 7000 m²/g, owing to their crystalline structures and tunable pore geometries. Activated carbons follow with 500 to 3000 m²/g, while zeolites show more moderate values of 200 to 1000 m²/g due to their aluminosilicate frameworks. These differences impact loading capacities for common electrolyte solvents such as ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate. For instance, MOF-303 exhibits uptake of 1.8 mmol/g for ethylene carbonate at 25°C, compared to 1.2 mmol/g for activated carbon and 0.7 mmol/g for zeolite Y under identical conditions.

Selectivity mechanisms vary significantly among material classes. MOFs achieve selectivity through precise pore size control and functional group modification. The UiO-66-NH₂ variant demonstrates preferential adsorption of linear carbonate solvents over cyclic carbonates due to hydrogen bonding interactions. Activated carbons rely on hydrophobic interactions and pore size distribution, with narrower micropores selectively capturing smaller solvent molecules. Zeolites utilize cation-exchange sites and molecular sieving effects, with FAU-type frameworks showing preference for molecules under 0.8 nm kinetic diameter.

Regeneration protocols must balance solvent recovery efficiency with material stability. Thermal regeneration proves most effective for activated carbons, with 95% solvent release achieved at 150°C under vacuum. MOFs require milder conditions (80-120°C) to prevent structural collapse, with some frameworks like ZIF-8 maintaining crystallinity through 50 adsorption-desorption cycles. Zeolites tolerate higher temperatures but may require water flushing to displace strongly adsorbed species. Energy consumption analyses show activated carbon processes demand 20-30% more thermal energy per kilogram of recovered solvent compared to MOF-based systems.

Isotherm data reveals distinct adsorption behaviors across material classes. Type I isotherms dominate in zeolites, indicating monolayer adsorption in micropores. Activated carbons typically show Type II or IV isotherms reflecting multilayer formation and mesopore filling. MOFs exhibit intermediate characteristics, with stepped isotherms signaling gate-opening phenomena in flexible frameworks. For dimethyl carbonate adsorption at 30°C, the Langmuir model fits MOF data with R² > 0.99, while activated carbons require dual-site Langmuir-Freundlich models to account for surface heterogeneity.

Continuous flow system integration presents both opportunities and challenges. MOFs demonstrate superior performance in fixed-bed configurations, with breakthrough capacities reaching 80% of equilibrium values due to rapid kinetics. Activated carbon beds show broader mass transfer zones but tolerate higher flow velocities (0.5-1.5 cm/s vs 0.2-0.8 cm/s for MOFs). Zeolite systems require careful humidity control to prevent competitive water adsorption. Modular designs employing staged adsorbent beds can achieve 98% solvent recovery from real recycling streams containing mixed carbonate solvents and LiPF₆ decomposition products.

Lifetime degradation factors differ markedly between materials. MOFs suffer from linker hydrolysis in acidic streams, with UiO-66 variants losing 15-20% capacity after exposure to HF-contaminated electrolytes. Activated carbons undergo pore blockage from oligomeric decomposition products, reducing capacity by 30-40% over 100 cycles without oxidative regeneration. Zeolites maintain structural integrity but experience cation exchange with lithium ions, altering selectivity over time. Accelerated aging tests indicate service lifetimes ranging from 3-5 years for MOFs to 5-8 years for zeolites in continuous operation.

Replacement costs must account for both material expenses and system downtime. Commercial activated carbons remain the most economical at $5-15/kg, compared to $50-300/kg for MOFs and $20-100/kg for zeolites. However, MOF stability improvements through hydrophobic coatings have reduced replacement frequency by 40% in pilot plants. Total cost analyses accounting for solvent recovery value show payback periods of 12-18 months for MOF systems versus 8-12 months for activated carbon implementations at industrial scale.

Process optimization requires balancing several competing factors. Higher surface area materials achieve greater solvent loading but often exhibit slower kinetics. Increased selectivity reduces downstream purification needs but may limit operational flexibility for mixed electrolyte streams. Regeneration conditions that maximize solvent recovery can accelerate adsorbent degradation. Successful implementations employ material combinations, such as activated carbon pre-filters protecting downstream MOF beds from fouling agents.

Environmental considerations favor materials with lower energy regeneration requirements and longer service lives. Activated carbon processes generate 15-20% higher CO₂ emissions per ton of recovered solvent compared to MOF systems when accounting for thermal regeneration needs. Zeolite-based systems show intermediate values but avoid organic solvent use during regeneration. Lifecycle assessments indicate that all three material classes can reduce the environmental impact of electrolyte disposal by over 90% compared to incineration.

Future developments may focus on hybrid materials combining the advantages of different frameworks. MOF-activated carbon composites have demonstrated 30% higher capacity than either component alone while maintaining regeneration simplicity. Surface-modified zeolites with organosilane treatments show promise for simultaneous solvent and lithium salt recovery. Advances in scalable synthesis methods continue to reduce production costs, with several MOF varieties now available at under $100/kg in bulk quantities.

Operational data from pilot-scale facilities confirms the technical viability of porous adsorbents for electrolyte recovery. A 1000-ton/year recycling plant using activated carbons reports 92% solvent purity after single-stage distillation of desorbed streams. MOF-based systems achieve 98% purity without distillation but require more frequent adsorbent replacement. Economic analyses suggest that material selection should be guided by specific electrolyte composition and desired recovery targets rather than universal superiority of any single material class.

The choice between MOFs, activated carbons, and zeolites ultimately depends on application-specific requirements. High-value electrolyte recovery justifies MOF implementation despite higher capital costs, while bulk processing favors activated carbons for their robustness and lower operating expenses. Zeolites occupy a niche for applications requiring simultaneous solvent drying and adsorption. As battery recycling volumes grow, optimized adsorption systems will become increasingly critical for sustainable electrolyte management.