Adsorption dynamics of gaseous toluene with and without formaldehyde and water vapors on composites of UiO-66-NH2 and microporous carbon

Abstract

To improve the adsorption capacity/selectivity of activated carbon (AC) against diverse volatile organic compounds (VOCs), a series of AC-based composites have been prepared with UiO-66-NH2 (U6N) and coded as AC-U6Nx (x = 1–20 wt/wt%). The adsorption potential of AC-U6Nx is evaluated against toluene in relation to the effects of composite compositions and the presence of competing feed gases/vapors (e.g., formaldehyde (FA) and water vapor). Although AC-U6N(1%) adsorbs toluene preferentially over FA, the selectivity factor of the former decreases from 6.4 to 2.63 as the x value increases from 1 to 20 wt%. This indicates the crucial role of U6N-NH2 groups in active capturing of FA over toluene through Schiff-base adsorption mechanism. Interestingly, the partition coefficient (PC100%: 0.045 ± 0.004 mol kg−1 Pa−1) of AC-U6N(1%) for 100 ppm FA remains nearly constant across varying toluene levels (1–100 ppm), suggesting the preferential occupation of active sites by FA (pore-filling). The contrasting role of relative humidity (RH) is also evidenced, as AC-U6N(1%), exhibits the highest capacity for toluene and FA under dry (0 % RH) and humid (20 % RH) conditions, respectively. The adsorption behavior of VOC on AC-U6N(1%) surface active sites (e.g., graphitic basal planes, NH2, COO–, and Zr sites) is predictable with a good fit of experimental data to the Freundlich isotherm and pseudo-second-order (PSOM) kinetic models. A density functional theory (DFT) simulation supports the enhancement of toluene adsorption on the interface of AC and U6N-based systems. Overall, this work offers practical guidelines for the construction of advanced functional adsorbents against diverse VOCs under both competitive and non-competitive conditions.