Application of a manganese dioxide/amine-functionalized metal-organic framework nanocomposite as a bifunctional adsorbent-catalyst for the room-temperature removal of gaseous aromatic hydrocarbons

A high surface area (883 m2·g−1) nanocomposite composed of an amine-functionalized metal–organic framework (NH2-UiO-66 (U6N)) and manganese dioxide (MnO2@U6N) was prepared as bifunctional adsorbent-catalyst for the purification of multiple aromatic volatile organic compounds (VOCs) such as benzene (B), toluene (T), m-xylene (X), and styrene (S), i.e., BTXS. The performance of MnO2@U6N was assessed for BTXS removal both as single- and multi-component systems at room temperature (RT (20 °C)) under dark conditions. MnO2@U6N exhibited superior catalytic-adsorption activity for the RT removal of BTXS. The removal performance of MnO2@U6N against BTXS was then assessed at varying flow rates, VOC concentrations, adsorbent/catalyst masses, and relative humidity levels. To better understand the catalytic-adsorption activity, two non-linear kinetic models (pseudo-first-order and pseudo-second-order) were utilized to simulate the experimentally obtained data. In-situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) was studied to interpret the removal mechanism of BTXS. The respective adsorption capacity (mg·g−1) values increased in the order of B (21.1) < T (66.0) < X (79.1) < S (129.7). Based on the characterization analysis, it is proposed that the adsorbed aromatic VOC molecules on the surface of MnO2@U6N should react with active oxygen species (lattice and adsorbed oxygen) to generate various intermediates (e.g., alkoxides, aldehydes, phenolates, carboxylates, and anhydrides), yielding the environmentally benign end products (i.e., carbon dioxide and water). Accordingly, the VOC removal potential of MnO2@U6N has been validated through the synergistic combination between adsorption (primary process) and catalysis (subordinate process) at RT.