Evidence for superiority of conventional adsorbents in the sorptive removal of gaseous benzene under real-world conditions: Test of activated carbon against novel metal-organic frameworks

Abstract

Recently, metal-organic frameworks (MOFs) have been used in air quality management, particularly adsorption of toxic gases/vapors (e.g., benzene). Nonetheless, the practicality of MOFs at near ambient conditions (e.g., sub-ppm levels of pollutants) is yet relatively unknown as their adsorption capacity data were commonly derived under unrealistically high-pressure conditions (e.g., >1000 Pa). In an effort to accurately assess their performance, the adsorption properties of some MOFs reported to have a significantly high capacity (e.g., MOF-199 [M199] and UiO-66 [U6]) were analyzed in reference to activated carbon (AC) across 0.01-5 Pa benzene. The maximum adsorption capacity of M199 (94.8 mg g(-1)) slightly exceeded that of AC (93.5 mg g(-1)) while that of U6 (27.1 mg g(-1)) was far lower than others. However, such dominance of M199 completely disappeared with decreases in the initial loading partial pressure, as reflected from the comparison of their 10% breakthrough volume (BTV10) or partition coefficient (PC) values. For instance, the BTV10 (L atm g(-1)) at 0.01 Pa benzene was far smaller in MOFs [e.g., M199 (1067) and U6 (330)] than in AC (14,410), as supported by the Kruskal-Wallis (K-W) rank test. The superiority of AC was also supported by isotherm modeling analysis as reflected by the existence of heterogeneous sorption sites (onto AC). The higher interaction affinity of AC for benzene was ascribed mainly to physisorption (4.66-1.86 kJ mol(-1)) via pi electrons relative to partitioning and/or pore-filling mechanisms of MOFs (4.56-1.27 kJ mol(-1)). The need for proper evaluation of performance is thus validated both experimentally and theoretically to help maximize the actual role of sorbent materials in treatment of a given gaseous pollutant.