Pyrolysis involving n-hexadecane, water and minerals: Insight into the mechanisms and isotope fractionation for water-hydrocarbon reaction
Pyrolysis involving n-hexadecane, water and minerals: Insight into the mechanisms and isotope fractionation for water-hydrocarbon reaction
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DOI：
10.1016/j.jaap.2018.01.009
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发表时间：
2018-03
影响因子：
6
通讯作者：
K. He;Shuichang Zhang;J. Mi;Wenlong Zhang
中科院分区：
文献类型：
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作者：
K. He;Shuichang Zhang;J. Mi;Wenlong Zhang
为了确定矿物和溶解盐对水-烃反应产气的影响，在金管热解装置上进行了n-C16与水和不同矿物或盐溶液在330~420 °C和50 Mpa下的一系列热解。结果表明，方解石和蒙脱石对含水烃类高温热解产气均有明显的催化作用。水加氢裂解甲烷的D/H值明显较高，表明水为水-烃反应生成烃类气体提供了H或D。同时，两种矿物的存在导致了天然气产物的异构化指数(I-C4/n-C4)和碳同位素组成的不同分布。此外，n-C16水合热解的气体产率/同分异构体指数与氯化钠/氯化钾浓度呈负相关。基于密度泛函理论(DFT)和过渡态理论(TS)的理论计算表明，烯烃与H+或水的反应活化能远低于水和其他有机化合物的反应活化能。本研究中的水-烃反应应主要通过离子机理进行，但也存在烷基与水的自由基反应。此外，研究还表明，蒙脱石和方解石的水-烃反应分别以离子机理和自由基机理为主。溶解盐对水-烃反应的影响可以用高温下H+浓度随水溶液中盐浓度的变化来解释。此外，还讨论了水-烃反应通过自由基和离子机制生成甲烷的碳同位素分馏的差异。通过热力学计算，阐明了甲烷在水-烃反应或水加氢过程中的氢同位素分馏。最后，提出了水-甲烷氢转移平衡后甲烷氢同位素比值的预测模型。
To ascertain the effects of minerals and dissolved salts on gas generation from water-hydrocarbon reaction, a series of pyrolysis ofn-C16with water and different minerals or salt solutions at 330–420 °C and 50 MPa were conducted in a gold-tube pyrolysis apparatus. It was shown that both calcite and montmorillonite exhibited evidently catalytic effects on gas generation in pyrolysis of hydrocarbon involving water at elevated temperature. The evident higher D/H ratios of methane in hydrous pyrolysis using deuterated water demonstrate that water provided H or D for hydrocarbon gas generation in water-hydrocarbon reaction. Meanwhile, the presence of two minerals resulted in the distinct distribution of the isomeric index (i-C4/n-C4) and carbon isotopic compositions for gas products. In addition, there was a negative correlation between gas yields/isomeric index and NaCl/KCl concentrations in hydrous pyrolysis ofn-C16. Theoretical calculations based on density functional theory (DFT) and transition states (TS) revealed that the activation energies for reactions between alkenes and H+or water are much lower than those involving water and other organic compounds. The water-hydrocarbon reaction in this study should mainly occur via ionic mechanism, though free radical reaction between alkyl radicals and water also presented. Moreover, it was demonstrated that ionic and free radical mechanisms dominated the water-hydrocarbon reaction with montmorillonite and calcite, respectively. The effects of dissolved salts on water-hydrocarbon reaction can be interpreted by the evolution of H+concentration with salt concentration in aqueous solutions at elevated temperature. In addition, the differences in carbon isotope fractionation for methane generation from water-hydrocarbon reaction via free radical and ionic mechanisms were addressed. Hydrogen isotope fractionation for methane during water-hydrocarbon reaction or hydrogenation by water was also elucidated by thermodynamic calculations. Finally, a model for the prediction of hydrogen isotopic ratios of methane after hydrogen transfer equilibrium between water and methane is proposed.