A simple heat-treatment strategy to prepare hydrophobic zeolite ZSM-5 for efficient adsorption of toluene under humid conditions

ZSM-5 samples were annealed at temperatures ranging from 450 °C to 1,000 °C and the toluene adsorption capacity was evaluated under dry and humid conditions. In dry conditions, the toluene adsorption capacity of ZSM-5 decreased with increasing annealing temperature, likely due to a decrease in surface area. In humid conditions, the toluene adsorption capacity of ZSM-5 at lower annealing temperatures (450 °C, 600 °C, 750 °C) was significantly decreased, while that at higher annealing temperatures (900 °C, 1,000 °C) remained nearly unchanged compared to dry conditions. The optimum annealing temperature was 900 °C for the maximum toluene adsorption capacity of ZSM-5 in humid conditions. In in situ DRIFTS analysis, at lower annealing temperature (450 °C) water molecules were found to occupy Brǿnsted acid sites via a strong hydrogen bonding, resulting in a reduced toluene adsorption capacity under humid conditions. At a higher annealing temperature (900 °C), significant dealumination caused reduction in number of Brǿnsted acid sites and formation of hydrophobic siliceous network which resulted in a weaker interaction of water with sample surface leading to intact adsorption of toluene. Moreover, mesopores of 900 °C-annealed sample also support the facile adsorption of toluene in the presence of moisture by providing diverse adsorption sites without any blockage unlike microporous structure of the 450 °C-annealed sample. Furthermore, the 900 °C-annealed ZSM-5 sample maintained stable toluene adsorption across multiple adsorption–desorption cycles, highlighting its stability for long-term toluene removal applications.