Nano-Synthesis of Solid Acid Catalysts from Waste-Iron-Filling for Biodiesel Production Using High Free Fatty Acid Waste Cooking Oil

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August 12, 2020

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by E. O. Ajala, M. A. Ajala, I. K. Ayinla, A. D. Sonusi & S. E. Fanodun (Scientific Reports) Waste-iron-filling (WIF) served as a precursor to synthesize α- ${Fe}_{2} O_{3}$ through the co-precipitation process. The α- ${Fe}_{2} O_{3}$ was converted to solid acid catalysts of RBC500, RBC700, and RBC900 by calcination with temperatures of 500, 700 and 900 °C respectively and afterwards sulfonated. Among the various techniques employed to characterize the catalysts is Fourier transforms infrared spectrometer (FT-IR), X-ray diffraction (XRD and Scanning electron microscopy (SEM). Performance of the catalysts was also investigated for biodiesel production using waste cooking oil (WCO) of 6.1% free fatty acid. The XRD reveals that each of the catalysts composed of Al– ${Fe}_{2} O_{3} / {SO}_{4}$ . While the FT-IR confirmed acid loading by the presence of ${SO}_{4}^{2 -}$ groups. The RBC500, RBC700, and RBC900 possessed suitable morphology with an average particle size of 259.6, 169.5 and 95.62 nm respectively. The RBC500, RBC700, and RBC900 achieved biodiesel yield of 87, 90 and 92% respectively, at the process conditions of 3 h reaction time, 12:1 MeOH: WCO molar ratio, 6 wt% catalyst loading and 80 °C temperature. The catalysts showed the effectiveness and relative stability for WCO trans-esterification over 3 cycles. The novelty, therefore, is the synthesis of nano-solid acid catalyst from WIF, which is cheaper and could serve as an alternative source for the ferric compound.

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Nevertheless, its challenges in biodiesel production are the presence of high free fatty acid (FFA) and moisture content which result in soap formation, difficult operation and separation of product from the catalyst, together with low yield and high cost of production when a homogeneous catalyst is used^4,5. Therefore, the need for heterogeneous catalysts is now of great concern among researchers.

Heterogeneous (solid) catalyst, either acid or base type, is environment-friendly with a good quality yield of biodiesel. It is cheap to synthesize, can easily be recovered and reused. Also, solid catalyst eliminates separation, emulsification and soap production challenges associated with homogeneous catalysts¹. Meanwhile, the solid acid catalyst is more suitable for high FFA feedstock in simultaneous esterification and transesterification reaction using a one-pot process to produce FAME⁶. Other advantages of the solid acid catalyst include the elimination of washing stage, easy reactivation, selectivity, corrosion-free and improved product purity⁷. Despite these numerous advantages associated with the use of the solid acid catalyst for biodiesel production, there are some drawbacks. The catalytic performances of the catalyst in the transesterification process are not satisfactory, as there is serious leaching of active species into the reaction mixture which leads to great deactivation of the catalysts. However, efforts are ongoing to develop a robust and efficient solid catalyst for trans-esterification of low-cost vegetable oils/animal fats for biodiesel production in a one-pot synthesis which would help to improve the economic feasibility of biodiesel to replace fossil-fuel⁸. Researchers have used oxides of different metals as support for the development of solid acid catalyst⁹. These catalysts are: ${Nb}_{2} O_{5} / {SO}_{4}$ , ${SO}_{4}^{2 -} / {ZrO}_{2}$ , ${SO}_{4}^{2 -} / {SnO}_{2}$ –SiO₂, ${SO}_{4}^{2 -} / {TiO}_{2}$ –SiO₂ and ${SO}_{4}^{2 -} / {ZrO}_{2}$ –Al₂O₃^10,11_. Others include ${FeH}_{28} {NO}_{20} S_{2}$ , Fe₂O₃–MnO– ${SO}_{4}^{2 -} / {ZrO}_{2}^{2}$ , ${Fe}_{2} {({SO}_{4})}_{3} / C$ , ${Fe}_{2} {SO}_{4}$ , $CaO / {Fe}_{2} {({SO}_{4})}_{3}$ and Li– $CaO / {Fe}_{2} {({SO}_{4})}_{3}$ ^12,13. Although, ferric oxide compound has been reported for solid acid catalyst production but can also be developed into non-acid solid catalyst such as $Fe / C$ , α- ${Fe}_{2} O_{3}$ , CaFe₂O₄– ${Ca}_{2} {Fe}_{2} O_{5}$ , ${MgFe}_{2} O_{4} @ CaO$ and CaO–γ– ${Fe}_{2} O_{3}$ ^1,5,9,14,15. These show that the ferric compounds are highly suitable to develop various catalysts for biodiesel production. Widayat Widayat et al.¹⁶ synthesized hematite $(\propto {- Fe}_{2} O_{3})$ magnetic nanoparticles to produce biodiesel through the trans-esterification process. The percentage yield of biodiesel obtained was 86.78% with FAME content of 87.88%. In their study, the hematite was obtained from iron sand and was not sulphated. A magnetic solid acid catalyst, consisting of a core of iron oxide nanoparticles, a poly(glycidyl methacrylate) shell, and sulfonic acid groups on the surface, was synthesized by Zillillah et al.¹⁷. The catalyst was used in an esterification reaction of grease (FFA = 16 wt%) with methanol to produce biodiesel yield of 96%. Though the catalyst was reported to be highly active, stable and recyclable, it is likely to be very expensive as it is a combination of two synthetic chemical substances. Guanidine-functionalized ${Fe}_{3} O_{4}$ magnetic catalyst for biodiesel production was reported in literature¹⁸. The catalyst was synthesized through three steps, (1) co-precipitation of Fe(II) and Fe(III) ions, (2) surface modification with chloropropyl groups and (3) functionalization with guanidine. The protocol of synthesis is a too cumbersome process and analytical grade of a synthetic chemical substance was used to functionalize the catalyst. Also, a magnetic solid acid catalyst of $S_{2} O_{8}^{2 -} / {ZrO}_{2}$ –TiO₂–Fe₃O₄ was developed for biodiesel production¹⁹. The study also shows that the analytical grade of ${FeSO}_{4} \cdot 7 H_{2} O$ and ${Fe}_{2} {({SO}_{4})}_{3}$ were used as precursors to synthesized ${Fe}_{3} O_{4}$ which was further impregnated with ${ZrO}_{2}$ and ${TiO}_{2}$ to functionalized the catalyst. Xie et al.²⁰ utilized ${Fe}_{3} O_{4}$ composite-supported sodium silicate as heterogeneous catalyst for biodiesel production. A crystalline $Fe / {Fe}_{3} O_{4}$ core/shell magnetic catalyst that was synthesized for biodiesel production was reported to have excellent stability²¹. The catalysts were coated with silica after functionalizing them with either sulfamic acid or sulfonic acid. These findings in literature justify that the catalysis of magnetic nanoparticles are interesting and versatile materials. This is due to its high surface area-to-volume ratio, which facilitates surface modification. Studies have shown that the magnetic-based solid acid catalyst has more catalytic activity compare to traditional acid catalysts which is due to the magnetic attraction that provides strong ionic interaction between the particles, leading to high catalyst activity and stability²². Hence, magnetic materials are alternative support material for catalyst development in biodiesel production, as it is easy to synthesize and functionalize, cheap to produce, low toxicity and easy to recover¹⁸. Even though, leaching of the active specie in magnetic nanocatalysts of trans-esterification reaction is a major challenge. The heterogeneity makes the separation of the catalyst easy, the leaching of active species of heteropoly acid cannot be prevented, which further leads to low reusability²³. In magnetic nanocatalysts, the leaching which occurs on the surface is associated with a partial dissolution of the iron oxide nanoparticles¹⁸. It is worthy to note that all the aforementioned studies used iron sand or analytical grade of high purity (98%) ferric compound to synthesize their catalysts, in contrast to this study. This study investigates the synthesis of the magnetic-based sulfonated catalyst using waste-iron-fillings which is novel and it is expected to improve cost-saving and environmental friendliness of the catalyst for biodiesel production.

Waste-iron-filings (WIF) are very small fragments, minute pieces of iron or galvanized iron from excess steels in factories and workshops, which are harmful to the environment²⁴. These wastes are increasing daily and constituting increase in environmental pollution. In 2017, global WIF was estimated at 750 MT, of which 630 MT was recycled with the remaining 120 MT (16% of total) disposed in the landfill. The future projection of WIF is about 1 billion tonnes (BT) by 2030 and expected to reach 1.3 BT by 2050. This revealed that by then, the WIF for landfill would be 0.2 BT (0.2 trillion kg)²⁵. This large quantity of WIF might cause serious environmental concern if alternative use is not found. Hence, the need to find more economical use for the WIF.

This study, therefore, investigates the use of waste-iron-filling (WIF) as a cheap, novel and alternative source of a ferric compound which is to replace high expensive ferric compound (98% purity) for catalyst development in biodiesel production. READ MORE