Selective adsorption of trace gaseous ammonia from air by a sulfonic acid-modified silica xerogel: Preparation, characterization and performance

The toxic and corrosive gaseous ammonia (NH3) poses serious environmental and health risks. However, removal of low concentration NH3 from air remains challenging. In this work, a SO3H-modified silica xerogel (MPTS-X) with good porous structure stability and abundant Bronsted acid sites was fabricated, and its physicochemical properties and adsorption performance for NH3 were systematically investigated. Static adsorption experiments showed the MPTS-1.0 had high NH3 adsorption capacity of 4.02 mmol⋅g−1 at 0.1 bar and 7.00 mmol⋅g−1 at 1.0 bar, which was much higher than that of MPTS-0 (3.19 and 1.95 mmol⋅g−1). Langmuir-Freundlich isotherm model fitted the experimental data of NH3 adsorption well, implying that the MPTS-1.0 surface is not homogeneous and there are different adsorption sites, which was further confirmed by temperature programmed desorption spectrum analysis. According to IAST calculation, the adsorption selectivity of MPTS-1.0 for NH3/N2 (1: 9) and NH3/CO2 (1: 1) was 6175 and 165 at 1.0 bar, respectively, indicating the MPTS-1.0 has excellent adsorption selectivity. Dynamic adsorption experiments and field test demonstrated the MPTS-1.0 could quickly capture trace NH3, even if the initial concentration is as low as 2 ppm. Successive adsorption–desorption experiments demonstrated the MPTS-1.0 had good regenerability. In-situ infrared spectroscopy analyses indicated NH3 adsorption on the MPTS-1.0 was mainly due to hydrogen bonding interaction and proton transfer to form Si-OH…NH3 and -SO3-NH4+, respectively. NH3 molecules bound firmly to acidic sites on the xerogel surface, which reduced re-release risk. This work may provide a feasible way to develop promising adsorbents for the removal of gaseous ammonia from air.