The control on adsorption kinetics and selectivity of formaldehyde in relation to different surface-modification approaches for microporous carbon bed systems

Abstract

It is perceived that surface modification of microporous carbon is an efficient option to remove light-weight volatile organic compounds (VOCs) like formaldehyde (FA). To adjust textural characteristics with surface functionalities of amine/sulfur and amino-silane, the commercial activated carbon (AC) was treated with dicyandiamide/thiourea (AC-M) and N-[3-(trimethoxysilyl)propyl]ethylenedia (AC-S), respectively, by thermal/microwave-assisted solvothermal methods. The adsorption performance of honeycomb air filters packed with different bed heights of AC-M/-S adsorbents (e.g., 1.38 ± 0.05, 6.36 ± 0.06, 7.99 ± 0.04, and 10.93 ± 0.04 mm) was evaluated (relative to commercial AC-based filters) towards the adsorption removal of FA (10 Pa) in both single and binary mixtures with benzene (at 10 Pa). Based on the partition coefficient and kinetic metrics, the AC-M/-S based filters showed higher efficacy for FA capture than commercial AC filter with increasing bed height (e.g., ≥ 7.99 ± 0.04 mm) to reflect the increased number of Lewis acid/base active sites on the bed surface. At the highest bed height (10.93±0.04 mm), AC-S was far superior with significantly enhanced selectivity of FA (by 20 times) over benzene with the accelerated mass transfer and acid-base surface interactions at the gas-solid interface. In contrast, AC-M outperformed commercial ACs for benzene adsorption from the binary gas mixture (breakthrough time (τ-value) of 683 min) with the aid of the improved surface porosity (71.8%) through the modification process. The kinematic data also confirmed that the adsorption affinity of modified AC-M/-S adsorbents was unaffected by the feed gas composition (single or binary phase) at the initial BT region. It thus supports the key role of surface modification technique on the enhanced adsorptivity of aldehyde and/or aromatic VOCs. This study is expected to help expand our basic knowledge on the interplay between surface/textural characteristics of modified AC and fixed-bed height to control the adsorptivity of air filter for efficient trapping of FA in either single or mixed phases with non-polar VOCs (like benzene).