An efficient basic heterogeneous catalyst synthesis of magnetic mesoporous Fe@C support SrO for transesterification
Graphical abstract
Introduction
The ever increasing energy demand, rapid depleting fossil fuel, and deteriorating environmental pollution have aroused extensive interest in renewable energy. Biodiesel is consisted of long chain fatty acid methyl esters (FAMEs) derived from transesterification of triglycerides (vegetable oil or animal fat) with methanol under the effect of catalyst [1,2]. In comparison with fossil diesel, biodiesel is recognized as a green and sustainable fuel to substitute fossil diesel for its promising advantages of nontoxicity, biodegradability, and low pollution emissions [3]. Normally, homogeneous base catalysts (NaOH, KOH, and CH3ONa et al.) are used to catalyze transesterification for biodiesel production [2,4,5], since the high catalytic activity could be achieved within a short time. Nevertheless, difficulties in recovering these catalysts are coupled with generation of the undesirable waste water [6]. Such drawbacks of homogeneous catalyst would raise the cost of biodiesel production, thus hindering their industrial application. With regard to this problem, diverse solid catalysts have been widely investigated including metal oxides, composite metal oxides, hydrotalcites, zeolites and anion exchange resins [[7], [8], [9]]. Among them, strontium oxide (SrO) is reported as a prospective basic catalyst for its strong basicity [9,10]. But, reusability of SrO is limited due to the low surface area and poor structure [11]. Dias et al. [10] explained that transesterification is occurred on the catalyst surface, small internal pores are easily blocked owing to the size of triglycerides molecules and the formation of oil micelles. Although nano–catalysts possess spectacular surface area, however, nano–particles are difficult to be recovered from reaction mixtures by filtration or centrifugation giving rise to inevitable catalyst loss during separation process and hence severely precluding the wide application [12,13].
Porous magnetic materials are increasingly employed as support for preparing heterogeneous catalysts. Not only for its abundant pore structure [14,15], but also it allows for quick and simple separation by using extra magnetic field compared with the conventional catalyst support. Whereas, the magnetic particles tend to aggregate into large clusters because of their magnetic dipole–dipole attractions, thus hindering the magnetic catalyst to be well dispersed in the reaction mixture and subsequently reduces the efficiency of catalyst recovery [16]. Encapsulation of magnetic particles into porous matrix could generate a multifunctional catalyst support with magnetic character, which is considered as an effective way to improve their chemical stabilities and prevent their aggregations [17].
Metal–organic frameworks (MOFs) constructed from an inorganic moiety and organic linkers possess noticeable features like flexible pore size, high specific surface area and tune ability [[18], [19], [20], [21]]. Joseph et al. [22] employed MIL–100(Cr) as a template to prepare mesoporous carbon nitrides with highly dispersed chromium oxide (MCN). The obtained MCN could anchor metal oxide nanoparticles and provide highly dispersed basic sites. Guo et al. [23] adopted MIL–53(Fe) to fabricate mesoporous yolk−shell Fe2O3 nanostructure and it was found to be a preferable anode material for lithium–ion batteries. MIL–100(Fe) is known as a cheap and biocompatible material. Its framework is composed by [Fe3O(OH) (H2O)2]6+ clusters connected with 1,3,5–benzenetricarboxylates (BTC) anions. In comparison to other metals, the nature of iron offers several advantages such as low cost, non–toxicity, and environmentally friendly character [24]. Apart from these features, MIL–100(Fe) is of special interest by virtue of the easy synthesis, large surface area, high chemical stability as well as the magnetic element of Fe [25]. According to our trial test [26], magnetic property was found by annealing MIL–Fe(100) under nitrogen atmosphere. More importantly, once organism linker is decomposed with carbon formation and the in–situ generation of corresponding metal oxides, which avoids nanoparticles aggregating and result in distinct porous structure.
To the best of our knowledge, there has been rare report on MIL–Fe(100) adopted as precursor to prepare magnetic material supporting SrO for transesterification. Therefore, to comprehensively investigate catalytic activity and stability of the SrO supported catalyst synthesized from in–situ titration method, herein, MIL–Fe(100) was initially employed as precursor to obtain the mesoporous magnetic support (Fe@C) through carbonization under nitrogen atmosphere. Then, SrO was loaded on Fe@C via in–situ titration to obtain the basic catalyst. Meanwhile, the catalyst was further characterized with X–ray diffraction (XRD), N2 adsorption–desorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectra (FTIR), Hammett indicator method, vibrating sample magnetometer (VSM), and X–ray photoelectron spectroscopy (XPS). In addition, synthesis conditions on catalytic activity were investigated and the transesterification parameters of catalyst amount (ζ), molar ration of methanol to oil (ξ), reaction time (t), and reaction temperature (Tr) were also assessed to acquire the optimal parameters. Finally, catalyst stability including reusability and resistant ability to free fatty acid as well as free water was investigated.
Section snippets
Materials
Since the high oil content and the highest yield in per unit area for palm oil, its cost is lower in comparison with other edible oils, which is thus employed as feedstock oil for transesterification [1]. The fatty acid composition of palm oil was determined by 7890 B gas chromatography (Agilent Technologies Inc., USA) and the results were listed in Table 1. Besides, the free fatty acid and saponification value were 0.11 mgKOH/g and 207.41 mgKOH/g, respectively. Fe(NO3)3‧9H2O, benzene
Effects of activation temperature and SrO loading on catalytic activity
Theoretically, more active site will be obtained with more SrO loading, thus enhancing the catalytic activity. As expected, Fe@C was endowed with a satisfying activity by increasing SrO loading. From Fig. 1, it can be clearly seen that the conversion was heightened from 84.40% to 98.12% while SrO rising from 10 wt% to 30 wt%. Whereas, the conversion increment was negligible when the SrO loading was continually aggrandized to 50 wt%. In a heterogeneously catalyzed transesterification system,
Conclusions
In this study, the magnetic mesoporous support (Fe@C) was obtained via MIL–Fe(100) carbonization under nitrogen atmosphere at 600 °C and SrO was then loaded on Fe@C to synthesize the basic heterogeneous Fe@C–Sr for transesterification. With 30 wt% SrO loading and activation temperature of 900 °C, SrFe2O4 and SrFeO2.5 was newly observed in Fe@C–Sr and the average pore diameter of Fe@C was increased from 4.37 nm to 7.07 nm (Fe@C–Sr). More interestingly, the saturation magnetization of Fe@C–Sr was
Author contribution statement
1. Hui Li: Project administration, writing-original draft, writing-review & editing, supervision;
2. Fengsheng Liu: Methodology, investigation.
3. Xiaoling Ma: Resources, methodology.
4. Ping Cui: Validation.
5. Min Guo: Validation.
6. Yan Li: Funding acquisition.
7. Yan Gao: Data curation.
8. Shoujun Zhou: Resources.
9. Mingzhi Yu: Resources.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was supported by National Natural Science Foundation of China (51806126), A Project of Shandong Province Higher Educational Science and Technology Program (J18KA087) and Doctoral Fund of Shandong Jianzhu University (XNBS1603). Special thanks for Dr. Xiaoling Ma’s generous assistance in this work.
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