Mitigating the relative humidity effects on the simultaneous removal of VOCs and PM2.5 of a metal–organic framework coated electret filter

• MIL-125-NH 2 MOF was uniformly coated onto electret filter media, called E-MOFilter. • The E-MOFilter aims to remove indoor VOC (toluene) and PM simultaneously. • Elevated RH deteriorated the VOC removal of the E-MOFilter due to hydrophilicity. • MIL-125-NH 2 is modified and new E-MOFilter is tested at 10–60% RH. • Efficacies of different modifications on MIL-125-NH 2 to mitigate RH effects are compared. An electret filter media coated with highly porous metal–organic framework (MOF, i.e., MIL-125-NH 2 in this work) particles, called E-MOFilter, has been developed to remove fine particulate matters (PM 2.5 , particulate matter less than 2.5 µm in diameter) and volatile organic compounds (VOCs, e.g., toluene) simultaneously. However, its adsorption efficiency and capacity of toluene was deteriorated at an elevated relative humidity (RH). This was due to the hydrophilic nature of MIL-125-NH 2 leading to competitive adsorption between toluene and water molecules. To address this issue, five modification methods were conducted to alter MIL-125-NH 2 from inherent hydrophilic into hydrophobic, which included the internal (alkylation method), external (coating with polydimethylsiloxane, PDMS), combined (alkylation plus PDMS coating), polyvinylpyrrolidone (PVP) surfactant, and N-coordination modifications. The new E-MOFilters were fabricated based on these modified MIL-125-NH 2 and then systematically compared for their toluene adsorption efficiency and capacity under various RH levels (from 10, 30, 40, 50 to 60%). In addition to toluene, the PM removal efficiency and PM 2.5 holding capacity by the modified E-MOFilters were also evaluated. The toluene adsorption results revealed the PDMS-coated E-MOFilter outperformed all other modifications. The PM removal ability were retained for all modified E-MOFilters indicating the charge and structure of the electret media were not altered. This study is the first to combine the MOFs with electret filter media for a low pressure drop and simultaneous removal of PM and toluene under different RHs. The effectiveness and performance towards toluene adsorption under elevated RH conditions for E-MOFilters with various modifications were comprehensively evaluated and discussed.