Narenji-Sani Fatemeh, Tayebee Reza, Chahkandi Mohammad
Department of Chemistry, Hakim Sabzevari University, Sabzevar 96179-76487, Iran.
ACS Omega. 2020 Apr 22;5(17):9999-10010. doi: 10.1021/acsomega.0c00358. eCollection 2020 May 5.
The catalytic esterification of free fatty acids is an important reaction pathway for chemical synthesis and biodiesel production, wherein efficient heterogeneous catalysts are sought to replace mineral acids. Herein, the esterification of oleic acid together with some familiar fatty acids is demonstrated with methanol over a heterogeneous heteropolyacid-functionalized zeolite imidazolate framework [H PWO /ZIF(H His.) ]. This new heterogeneous catalyst (named as HPA/ZIF(His.) throughout the text) with an average particle size of 80 nm was prepared via condensation of histamine with zinc chloride and characterized by means of Fourier transform infrared (FT-IR), X-ray diffraction (XRD), UV-vis, energy-dispersive X-ray spectrometry, Brunauer-Emmett-Teller, thermogravimetric analysis (TGA), inductively coupled plasma - optical emission spectrometry (ICP-OES), and scanning electron microscopy. According to the performed characterizations, an HPA loading of 40.5 wt % is obtained for HPA/ZIF(His.) from ICP-OES analysis. Moreover, a typical type-IV isotherm with similar adsorption-desorption properties as seen for ZIF-8 is attained. In addition, TGA measurement confirms less stability of HPA/ZIF(His.) compared to that of pure ZIF(His.). The catalytic performance of the nanomaterial is evaluated with respect to temperature, catalyst loading, and methanol/oleic acid ratio and leads to a high yield of methyl ester (>90%) under reflux for 4 h. The preliminary kinetic studies confirm a pseudo-first-order kinetic model for the esterification of oleic acid. To explore the scope of the HPA/ZIF(His.) catalyst in methyl ester production, other free fatty acids with various chain lengths are also successfully tested. Although the nanocatalyst loses a part of its activity during reuse, however, it is stable over at least four recycles as confirmed by XRD and FT-IR. Eventually, the response surface methodology (RSM) is used as a statistical modeling approach to get the best-optimized reaction conditions compared to the performed single-variable benchmarking experiments. Therefore, the central composite design (CCD) and RSM attained a platform to determine the relationship among the reaction time, acid/methanol molar ratio, and catalyst dosage.
游离脂肪酸的催化酯化是化学合成和生物柴油生产的重要反应途径，其中人们一直在寻找高效的非均相催化剂来替代无机酸。在此，用甲醇在非均相杂多酸功能化的沸石咪唑酯骨架[H PWO/ZIF(H His.)]上对油酸以及一些常见脂肪酸进行了酯化反应。通过组胺与氯化锌缩合制备了这种平均粒径为80 nm的新型非均相催化剂（在本文中全程命名为HPA/ZIF(His.)），并通过傅里叶变换红外光谱（FT-IR）、X射线衍射（XRD）、紫外可见光谱、能量色散X射线光谱、布鲁诺尔-埃米特-泰勒法、热重分析（TGA）、电感耦合等离子体发射光谱（ICP-OES）和扫描电子显微镜对其进行了表征。根据所进行的表征，通过ICP-OES分析得出HPA/ZIF(His.)的HPA负载量为40.5 wt%。此外，获得了一种典型的IV型等温线，其吸附-解吸特性与ZIF-8相似。另外，TGA测量证实HPA/ZIF(His.)的稳定性低于纯ZIF(His.)。评估了该纳米材料在温度、催化剂负载量和甲醇/油酸比例方面的催化性能，在回流4 h的条件下甲酯产率很高（>90%）。初步动力学研究证实了油酸酯化反应的准一级动力学模型。为了探索HPA/ZIF(His.)催化剂在甲酯生产中的适用范围，还成功测试了其他不同链长的游离脂肪酸。尽管该纳米催化剂在重复使用过程中失去了部分活性，但是，如XRD和FT-IR所证实的，它至少在四个循环中是稳定的。最终，响应面法（RSM）被用作一种统计建模方法，以获得与所进行的单变量基准实验相比的最佳优化反应条件。因此，中心复合设计（CCD）和RSM提供了一个平台来确定反应时间、酸/甲醇摩尔比和催化剂用量之间的关系。
