Clark Jared M, Pilath Heidi M, Mittal Ashutosh, Michener William E, Robichaud David J, Johnson David K
National Bioenergy Center and ‡Biosciences Center, National Renewable Energy Laboratory , Golden, Colorado 80401, United States.
J Phys Chem A. 2016 Jan 28;120(3):332-45. doi: 10.1021/acs.jpca.5b09246. Epub 2016 Jan 12.
We demonstrate a synthetic route toward the production of propene directly from poly(β-hydroxybutyrate) (PHB), the most common of a wide range of high-molecular-mass microbial polyhydroxyalkanoates. Propene, a major commercial hydrocarbon, was obtained from the depolymerization of PHB and subsequent decarboxylation of the crotonic acid monomer in good yields (up to 75 mol %). The energetics of PHB depolymerization and the gas-phase decarboxylation of crotonic acid were also studied using density functional theory (DFT). The average activation energy for the cleavage of the R'C(O)O-R linkage is calculated to be 163.9 ± 7.0 kJ mol(-1). Intramolecular, autoacceleration effects regarding the depolymerization of PHB, as suggested in some literature accounts, arising from the formation of crotonyl and carboxyl functional groups in the products could not be confirmed by the results of DFT and microkinetic modeling. DFT results, however, suggest that intermolecular catalysis involving terminal carboxyl groups may accelerate PHB depolymerization. Activation energies for this process were estimated to be about 20 kJ mol(-1) lower than that for the noncatalyzed ester cleavage, 144.3 ± 6.4 kJ mol(-1). DFT calculations predict the decarboxylation of crotonic acid to follow second-order kinetics with an activation energy of 147.5 ± 6.3 kJ mol(-1), consistent with that measured experimentally, 146.9 kJ mol(-1). Microkinetic modeling of the PHB to propene overall reaction predicts decarboxylation of crotonic acid to be the rate-limiting step, consistent with experimental observations. The results also indicate that improvements made to enhance the isomerization of crotonic acid to vinylacetic acid will improve the direct conversion of PHB to propene.
我们展示了一条直接从聚（β-羟基丁酸酯）（PHB）生产丙烯的合成路线，PHB是众多高分子质量微生物聚羟基脂肪酸酯中最常见的一种。丙烯是一种主要的商业碳氢化合物，通过PHB的解聚以及随后巴豆酸单体的脱羧反应，以较高产率（高达75摩尔%）获得。还使用密度泛函理论（DFT）研究了PHB解聚和巴豆酸气相脱羧的能量学。R'C(O)O-R键断裂的平均活化能经计算为163.9±7.0 kJ mol⁻¹。一些文献报道中提到的，产物中形成巴豆酰基和羧基官能团导致的关于PHB解聚的分子内自加速效应，无法通过DFT和微观动力学建模的结果得到证实。然而，DFT结果表明，涉及末端羧基的分子间催化可能会加速PHB解聚。该过程的活化能估计比非催化酯裂解的活化能低约20 kJ mol⁻¹，为144.3±6.4 kJ mol⁻¹。DFT计算预测巴豆酸的脱羧反应遵循二级动力学，活化能为147.5±6.3 kJ mol⁻¹，与实验测量值146.9 kJ mol⁻¹一致。从PHB到丙烯的整体反应的微观动力学建模预测巴豆酸的脱羧反应是限速步骤，与实验观察结果一致。结果还表明，改进措施以增强巴豆酸向乙烯基乙酸的异构化将提高PHB向丙烯的直接转化率。
