Elution of Adsorbed n-alkanes by 5Å Molecular Sieve Using Solvent Mixtures for Compound-Specific Hydrogen Isotopic Analysis
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摘要: 混合溶剂萃取洗脱5Å分子筛吸附正构烷烃的方法在单体碳同位素分析研究中已得到了应用。由于5Å分子筛在长时间加热条件下可能对有机分子的氢交换反应具有一定的催化作用，因此对于该方法能否用于正构烷烃的单体氢同位素分析需要做进一步研究。本文以两种不同类型原油的饱和烃和正构烷烃混合标准样品为对象，采用环己烷-正戊烷溶剂和氢氟酸溶解-正戊烷溶剂洗脱5Å分子筛吸附的正构烷烃，对比分析两种萃取方式的分离效果，并分别测定了正构烷烃的单体氢同位素比值。结果表明：两种萃取方式都得到了纯化效果良好的正构烷烃组分，氢同位素测定结果的差值总体上小于4‰，在仪器分析误差范围内。经过三次环己烷-正戊烷洗脱方式获得正构烷烃回收率平均值为58%，氢氟酸溶解-正戊烷洗脱方式经过多次萃取操作获得正构烷烃回收率平均值为68%。尽管环己烷-正戊烷洗脱方式提取正构烷烃的效率较低，并且涉及在分子筛条件下多次较长时间的加热过程，但这两个因素没有导致氢同位素发生明显的分馏或交换过程，适合用于正构烷烃分离、纯化以及单体氢同位素测定。Abstract: The extraction procedure using solvent mixtures has been used to elute n-alkanes adsorped by 5Å molecular sieve and subsequent stable carbon isotope ratio analysis of individual n-alkanes. However, a 5Å molecular sieve may catalyze the hydrogen exchange of organic molecules during long-term heating. Therefore, the usage of this method for hydrogen isotopic analysis needs to be evaluated. Two different types of crude oil saturated hydrocarbon and n-alkanes mixed standard samples were used as the object for this study. The extraction method using cyclohexane and n--heptane solvent mixtures and another method evolving the digestion with hydrofluoric-acid and solvent extraction were both used to elute adsorbed n-alkanes by the 5Å molecular sieve. The separation efficiency of the two methods was compared, and both carbon and hydrogen isotopes of obtained n-alkanes have been analyzed. Results show that the two methods enable excellent separations of n-alkanes, and yield variations of the hydrogen isotope ratios smaller than 4‰, within uncertainty. The average recovery of n-alkanes obtained by three-time solvent extraction was 58%, whereas that obtained by hydrofluoric acid dissolution and solvent extraction was 68%. Although cyclohexane and n-heptane solvent mixture has lower extraction efficiency and involves multiple heating processes, these two factors do not result in significant fractionation or exchange of hydrogen isotopes, indicating that this method can be applied to the separation and purifying of n-alkanes and subsequent compound-specific hydrogen isotopic analysis.
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Key words:
- 5Å molecular sieve /
- adsorption /
- separation /
- n-alkanes /
- hydrogen isotope
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表 1 氢氟酸溶解结合溶剂萃取和混合溶解热萃取两种洗脱方法的对比
Table 1. A comparison of the two elution methods: hydrofluoric acid solution combining solvent extraction and solvent mixtures involved heating extraction
萃取方式 操作流程 操作时间 氢氟酸溶解-溶剂萃取 ① 氢氟酸(腐蚀性)逐滴加入，缓慢溶解(逐个样品)。
② 采用磁力搅拌，完全溶解后，中和溶液。
③ 四次加入正戊烷试剂萃取正构烷烃，合并萃取液。
④ 对于萃取得到的正构烷烃进行除水处理。约12 h/10个
样品混合溶剂加热萃取 ① 加入混合溶剂进行热萃取(批量进行)。
② 过滤得到正构烷烃。
③ 重复步骤① 和② 两次，共三次萃取得到正构烷烃。一次萃取约
9 h/20个
样品
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