Adsorption performance and mechanism of single-component and multi-component VOCs by Beta zeolites with different Si/Al ratios

The physical and chemical properties of VOCs and zeolite materials generally affect adsorption efficacy for VOCs removal. Here, toluene and methanol were chosen as typical VOCs with different polarities and molecular diameters to investigate the performance and mechanism of their adsorption on Beta zeolites with various Si/Al ratios. It was found that high-silica Beta exhibited superior toluene adsorption capacity, while low-silica Beta showed higher methanol adsorption capacity. Compared with single-component adsorption, in the case of co-adsorption of toluene and methanol, the saturation adsorption capacities for toluene and methanol only changed slightly even with the existence of competitive adsorption and weakened adsorption strength. Importantly, there was almost no decrease in absorption capacity after 10 repeated adsorption-regeneration cycles, showing the excellent reusability of Beta zeolites for toluene and methanol removal. Whether on low-silica or high-silica Beta, physical adsorption of toluene and methanol was dominant, along with a small proportion of chemical adsorption on low-silica Beta based on acid sites. Similar size between the diameters of toluene and the pore channels of Beta zeolites was responsible for strong adsorption force, increase of adsorption capacity, and minor effect on absorption performance in the process of competitive adsorption. Strong interaction between acid sites and polar methanol through H-bond or non-polar toluene by electrostatic attraction promoted chemical adsorption. Furthermore, toluene and methanol both tended to absorb in the twelve-membered-ring of Beta zeolites on Si-OH-Al, Al-OH, and Si-OH sites. This study provides insight into the factors influencing the adsorption performance and mechanism of toluene and methanol on zeolites, which gives potential guidance for the selection of adsorbents for high-efficiency VOCs removal.