| dc.description.abstract | Three mesoporous silica MCM-41, MCM-48, and SBA-15 with pore size of 46.6, 25.78, 64.79 ? were synthesized and assessed for its applicability as sorbents for on-line trapping of volatile organic compounds (VOCs) from air samples. Several commercially available microporous carbon molecular sieves, i.e., Carbosieve SIII, Carboxen 1000, Carboxen 1003, and Carbotrap purchased from Supelco, were employed to form multi-sorbent traps as a reference for comparing adsorption properties with those of the silica MCM-41. A standard gas mixture containing more than 50 target compounds with size varying from C2 to C12 was adsorbed by these sorbents to obtain the relationship between pore size and adsorption profiles. While the multi-carbon sorbents show very uniform adsorption ability across the entire carbon range from C3-C12, the mesoporous silica MCM-41, -48, and SBA-15, however, shows little sorption for smaller molecules from C3 to C7, but exhibit adequate sorption ability for C8 – C12 compounds. Desorption at various temperatures indicates that C8 – C12 compounds once trapped can be easily released at moderate temperatures of about 150 ?C, whereas for carbon sorbents the desorption temperatures for sufficient recovery need to go beyond 250 ?C due to much tighter hold-up in the microporous structure. Sorption ability for mesoporous silica is also reflected on linearity. Compounds with linearity (R2) close to unity also exhibit excellent precision of better than 4% RSD, an important requirement for quantitative analysis of ambient VOCs.
It was found that pressure also affects adsorption efficiency. Higher pressure facilitates adsorption, and such an increase in adsorption is more dramatic for lighter VOCs than for heavier ones since the head room of increase for heavier VOCs is already small.
Even though the pore size of SBA-15 is large (64.79 ?), adsorption ability for light VOCs (C3-C8) was much more obvious than with MCM-41 and -48. It was speculated that existence of smaller side tunnels in the SBA-15 structure was the cause for adsorbing smaller VOC molecules. | en_US |