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摘要: 采用色谱法与热重（TG）法，测量了正己烷、甲苯和乙酸乙酯在活性炭、5A、NaY、13X、ZSM-5（SiO2/Al2O3=27、300）、Hβ以及MCM-41等吸附剂上不同温度下的吸脱附行为，并基于反相气相色谱法测得的数据，计算了其吸附热力学参数ΔH、ΔS和ΔG，分析了上述VOCs分子与吸附剂之间的作用机制，并借助FT-IR验证了吸附质在分子筛表面的吸附机制。结果表明，上述吸附过程存在物理吸附和化学吸附两种方式，其中，物理吸附的作用力大小与吸附剂的孔径分布和分子直径相关，而化学吸附的作用力大小依赖于分子筛硅铝比和Ca2+、Na+、H+等阳离子及吸附质分子的偶极矩，且强的化学吸附使得部分吸附质分子的脱附温度高于200℃。Abstract: The adsorption and desorption behaviors of n-hexane, toluene and ethyl acetate on activated carbon, 5A, NaY, 13X, ZSM-5 (SiO2/Al2O3=27, 300), Hβ and MCM-41 at different temperatures were investigated by chromatography method and thermogravimetry (TG).And the adsorption thermodynamic parameters (ΔH, ΔS and ΔG) were calculated based on the results obtained by inverse gas chromatography, by which, the interactions between adsorbent and VOC molecules were elucidated. In addition, the adsorption mechanisms of VOC molecules on molecular sieves were confirmed based on FT-IR results. There are two modes of spontaneous adsorption involving physical and chemical adsorptions. The physical adsorption strength is dependent on the pore size distribution of the adsorbent and the molecular diameter of the adsorbate, while the chemical adsorption strength is associated with the Si/Al ratio of the molecular sieve, the cations of Ca2+, Na+, H+, and the dipole moment of the adsorbate molecules. Meanwhile, the presence of strong chemical adsorption makes desorption temperature up to 200℃.
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Key words:
- desorption temperature /
- VOCs /
- inverse gas chromatography /
- adsorption thermodynamics
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图 2 饱和吸附VOCs后吸附剂的脱附曲线
Figure 2 Desorption curves of VOCs on adsorbents
(a): n-hexane; (b): toluene; (c): ethyl acetate; (d): AC adsorbent
图 3 吸附剂对VOCs饱和吸附后的DTG曲线
Figure 3 DTG curves of VOCs on adsorbents
(a): n-hexane; (b): toluene; (c): ethyl acetate; (d): AC adsorbent
图 4 温度与亨利常数自然对数lnK的关系
Figure 4 Plot of lnK vs 1/T for the adsorption of VOCs on adsorbents
(a): n-hexane; (b): toluene; (c): ethyl acetate ■ : NaY; □ : 13X; ▲ : ZSM-5(27); △ : ZSM-5(300); ◆ : 5A; ◇ : H β
图 5 温度与比保留体积自然对数lnVg的关系
Figure 5 Plot of lnVg vs 1/T for the adsorption of VOCs on adsorbents
(a): MCM-41; (b): AC
■ : n-hexane; ○ : toluene; ▲ : ethyl acetate表 1 VOCs在吸附剂上的吸附热
Table 1 Adsorption heat for typical VOCs on different adsorbents*
Adsorbent -ΔH /(kJ·mol-1) n-hexane toluene ethyl acetate 13X 51.54 73.24 125.39 NaY 44.65 79.61 134.72 5A 62.48 55.34 47.29 Hβ 54.96 59.57 99.42 ZSM-5(27) 84.24 107.92 88.92 ZSM-5(300) 67.38 53.01 72.23 MCM-41 51.55 60.80 65.00 AC 49.33 53.14 64.48 *: adsorption heat obtained at t=90 ℃ for AC and at t=240 ℃ for other adsorbents 
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表 2 VOCs在吸附剂上的吸附熵
Table 2 Adsorption entropy for typical VOCs on different adsorbents*
Adsorbent -ΔS /(J·mol-1·K-1)* n-hexane toluene ethyl acetate 13X 55.55 91.91 202.83 NaY 54.20 91.08 191.84 5A 76.64 74.53 75.45 Hβ 73.30 78.08 184.79 ZSM-5(27) 128.88 152.15 140.90 ZSM-5(300) 102.02 63.98 100.37 MCM-41 92.05 101.31 103.97 AC 130.13 100.19 134.45 *: adsorption entropy obtained at t=90 ℃ for AC and at t=240 ℃ for other adsorbents
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表 3 VOCs在吸附剂上的吉布斯自由能
Table 3 Adsorption Gibbs free energy for typical VOCs on different adsorbents
Adsorbent -ΔG /(kJ·mol-1) n-hexane toluene ethyl acetate 13X 23.04 26.09 21.34 NaY 16.84 32.89 36.31 5A 23.16 17.11 8.58 Hβ 17.35 19.52 4.62 ZSM-5(27) 18.12 29.87 16.64 ZSM-5(300) 15.04 20.19 20.74 MCM-41 4.33 8.83 11.67 AC 2.09 16.77 15.67 *: Gibbs free energy of adsorption obtained at t=90 ℃ for AC and at t=240 ℃ for other adsorbents
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