ABSTRACT
The emission of volatile organic compounds (VOCs) significantly contributes to air pollution and poses a serious threat to human health. Benzene, one of the most toxic VOCs, is difficult for the human body to metabolize and is classified as a Group 1 carcinogen. The development of efficient adsorbents for removing trace amounts of benzene from ambient air is thus of great importance. In this work, we studied the benzene adsorption properties of four Zr-based metal-organic frameworks (Zr-MOFs) through static volumetric and dynamic breakthrough experiments. Two previously reported Zr-MOFs, BUT-12 and STA-26, were prepared with a tritopic carboxylic acid ligand (H3L1) functionalized with three methyl groups, and STA-26 is a 2-fold interpenetrated network of BUT-12. Two new isoreticular Zr-MOFs, BUT-12-Et and STA-26-Et, were synthesized using a similar ligand, H3L2, where the methyl groups are replaced with ethyl groups. There are mesopores in BUT-12 and BUT-12-Et and micropores in STA-26 and STA-26-Et. The four Zr-MOFs all showed high stability in liquid water and acidic aqueous solutions. The microporous STA-26 and STA-26-Et showed much higher benzene uptakes than mesoporous BUT-12 and BUT-12-Et at room temperature under low pressures. Particularly, the benzene adsorption capacity of STA-26-Et was high up to 2.21 mmol/g at P/P0 = 0.001 (P0 = 12.78 kPa), higher than those of the other three Zr-MOFs and most reported solid adsorbents. Breakthrough experiments confirmed that STA-26-Et could effectively capture trace benzene (10 ppm) from dry air; however, its benzene capture capacity was reduced by 90% under humid conditions (RH = 50%). Coating of the crystals of STA-26-Et with polydimethylsiloxane (PDMS) increased the hydrophobicity of the exterior MOF surfaces, leading to a more than 2-fold improvement in its benzene capture capacity in the breakthrough experiment under humid condition. PDMS coating of STA-26-Et likely slowed down the water adsorption process, and thus, the adsorbent afforded more efficient capture of benzene. This work demonstrates that modifying both the interior and exterior surfaces of MOFs can effectively enhance their performance in capturing trace benzene from ambient air, even under humid conditions. This finding is meaningful for the development of new adsorbents for effective air purification applications.
ABSTRACT
Propyne/propylene separation is important in the petrochemical industry but challenging due to their similar physical properties and close molecular sizes. Metal-organic frameworks (MOFs) are a class of promising adsorbents for light hydrocarbon separations. Among them, the so-called "flexible-robust" MOFs combine the advantages of flexibility and rigidity in structure and could show enhanced gas separation selectivity as well as improved gas uptake at low pressure. Interpenetrated MOFs offer a platform to explore the "flexible-robust" feature of MOFs based on their subnetwork displacement in the process of gas adsorption. Herein, we present two hydrolytically stable MOFs (BUT-308 and BUT-309) with interpenetrated structures and fascinating propyne/propylene separation performance. BUT-308 is composed of interpenetrated 2D Cu(BDC-NH2)BPB layers (H2BDC-NH2 = 2-aminobenzene-1,4-dicarboxylic acid; BPB = 1,4-bis(4-pyridyl)benzene), while BUT-309 consists of twofold interpenetrated 3D pillared-layer Cu2(BDC-NH2)2(BPB-CF3) nets (BPB-CF3 = 2-trifluoromethyl-1,4-bis(4-pyridyl)benzene). Gas adsorption measurements showed that BUT-309 was a "flexible-robust" adsorbent with multistep adsorption isotherms for C3H4 rather than C3H6 at a wide temperature range. The guest-dependent pore-opening behavior endows BUT-309 with high potential in the C3H4/C3H6 separation. The C3H4 adsorption measurements of BUT-309 at 273-323 K showed that the lowering of the temperature induced the pore-opening action at lower pressure. Column breakthrough experiments further confirmed the capability of BUT-309 for the efficient removal of C3H4 from a C3H4/C3H6 binary gas, and the C3H6 processing capacity at 273 K (15.7 cm3 g-1) was higher than that at 298 K (35.2 cm3 g-1). This work shows a rare example of "flexible-robust" MOFs and demonstrated its high potential for C3H4/C3H6 separation.
