Abstract
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(6-n) P(2)W(18)O(62) (n-)/ZIF(H (n) His.)(+n) ]. 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.