| 摘要: | 本實驗以中孔徑矽分子篩MCM-41、MCM-48與SBA-15作為空氣樣品中VOCs(Volatile Organic Compounds)的吸附材料，作為氣相層析儀線上濃縮單元，測試其吸附捕捉的特性，並以商業化微孔徑碳分子篩與之比較。 以一內含56種VOC標準氣體，作為提供中孔徑矽分子篩與微孔徑碳分子篩進行吸脫附行為之參考；此標準氣體混合物內所含的物種分子大小範圍為C2-C12。作為線上吸附劑捕捉分析後，微孔徑多重床碳吸附劑涵蓋了C3到C12，表現出相當一致性的吸附效率。然而中孔徑矽骨幹的MCM-41等則對於C3到C5的輕碳數VOCs具有微量的吸附效率，但對於C7的物種吸附效率開始提高，且由C8到C12的範圍到達接近碳分子篩理想的吸附效率。MCM-41平均孔徑大小約為46.6 ?，此孔徑特徵反應在其層析圖譜之中，具有對分子大小的選擇性，並在再現性及線性的討論中表現出確實捕捉分子的範圍。而脫附溫度曲線圖之中，中孔徑矽分子篩MCM-41等在約160 ?C即脫附完全，較多床重碳吸附劑的250 ?C或以上低許多，另二種中孔徑矽分子篩MCM-48(孔徑約25.78 ?)與SBA-15(孔徑約64.79 ?)也有類似的表現，皆顯出中孔徑矽分子篩在層析用途中，作為濃縮空氣VOC的吸附劑具有良好的發展性。 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. |
