Utilising Imidazolium-based Ionic Liquids as Catalysts for Enhanced Biodiesel Production via Transesterification Process
Written by David P.
1. Introduction
One of modern society’s biggest challenges is the exhaustion of fossil energy resources. At the same time, global oil demand is constantly rising and is currently above 100 million barrels per day (Statista, 2024). In addition to being a finite resource, fossil fuels are considered to be major contributors to global warming due to large-scale emissions of carbon, nitrogen, sulphur oxides and particulate matter upon their combustion. Regarding this matter, biodiesel has been recognised as an alternative energy source that could partly substitute petroleum-based diesel in the economy (Osman, et al., 2024).
From a chemical perspective, biodiesel fuels are alkyl monoesters of various fatty acids produced from animal fats and vegetable oils via the transesterification reaction. Transesterification of triglycerides with alcohol is outlined in the following diagram.
Figure 1: Transesterification reaction, adapted from (Mumtaz, et al., 2017)
In Figure 1, R and R’ are fatty acid and alcohol residues, correspondingly. Also, a range of different catalysts can be used to support the described process. However, the most common ones include sodium/potassium hydroxides, ion exchange resins, lipases, and sulphuric acid. The use of the indicated catalysts is associated with a range of challenges, such as low reusability, environmental issues, and high costs, which underlines the necessity of exploring the introduction of alternative catalysts (Mumtaz, et al., 2017).
2. Research Aims
The aims of the work are to:
- Investigate the efficiency of imidazolium-based ionic liquid catalysts in the transesterification process.
- Optimise transesterification conditions for maximum biodiesel yield.
- Evaluate the environmental impact of ionic liquids and their reusability.
3. Literature review
Ionic liquids (ILs) are defined as salts formed by organic cations (pyridinium or imidazolium salts), complexed with organic or inorganic anions (NO3–, BF4–, PF6–, AlCl4–, (CF3SO2)2N–, Cl–, Br–). These chemicals are heavily used as reagents, catalysts, or solvents in the modern chemical industry (Lei, et al., 2017).
The concept of using ion liquids as catalysts in the large-scale production of biofuel is not new, but the number of IL catalysts is limited, making the synthesis process unoptimised. Looking at specific applications of ionic liquids in the process, a recent publication by Li et al., (2014) can be put forward. The research group investigated the possibility of producing biodiesel from Camptotheca acuminata seed oil using Bronsted acid ionic liquids. The process was further promoted using microwaves. A range of ionic liquids was tested, including [BSO3HMIM]HSO4 (1-sulfobutyl-3-methylimidazolium hydrogen sulfate), [BSO3HMIM]BF4 (1-butylsulfonic-3-methylimidazolium tetrafluoroborate), [BMIM]HSO4 (1-butyl-3-methylimidazolium hydrogen sulfate), [BMIM]BF4 (1-butyl-3-methylimidazolium tetrafluoroborate), and [BMIM]Br (1- butyl-3-methylimidazolium bromide). The highest yield was observed in the case of [BSO3HMIM]HSO4 (57.8%), and, for this reason, the ionic liquid was chosen for further optimisation experiments. As a result, it became possible to increase the yield to 95% by combining the chosen ionic liquid with a metal support (Li, et al., 2014).
The use of imidazolium-based ionic liquids in biodiesel synthesis is also outlined in the work by Elsheikh et al., (2011). Biodiesel was prepared using a two-step transesterification process with low-grade palm oil as feedstock. Ionic liquids that were analysed included [MIM]HSO4, [BIM]HSO4, and [BMIM]HSO4. Optimal transesterification conditions were found to be 1600C, alcohol/oil molar ratio of 12:1, 4.4% w/w catalyst concentration ([BMIM]HSO4), and reaction time of 120 min. In this case, the yield reached up to 90%, but the necessity of catalyst removal underlined the necessity of additional research (Elsheikh, et al., 2011).
Catalyst recovery issues associated with the use of ionic liquid catalysts in biodiesel production were partly addressed in the work by Gholami et al., (2020). The research group indicates that ionic liquids are among the best alternatives to alkali catalysts due to excellent solubility in organic and inorganic solvents, wide liquid temperature, environmental-friendliness, structural tunability, and non-volatility (Gholami, et al., 2020). However, difficult recovery and high viscosity considerably limited the application of these catalysts. A possible solution is the deposition of ionic liquids on nanoporous materials for the following utilisation in biodiesel production. Nevertheless, despite high yield, ease of purification/recycling, and low environmental impact, such catalysts are costly, which remains a major challenge (Gholami, et al., 2020).
Summarising the above, literature in the area suggests that although imidazolium-based ionic liquids show potential as catalysts in biodiesel production, limitations such as purification difficulties, catalyst costs, and low yield underline the necessity for additional research.
4. Methodology
The proposed research will consist of two major parts. During the first part, new imidazole-based ionic liquids will be synthesised, purified, and characterised. In turn, the second part will be focused on using the produced ionic liquids as catalysts in the transesterification of sunflower oil.
From the experimental perspective, the ionic liquid synthesis will follow a two-step procedure outlined by Liang et al., (2010) and confirmed by Lima (2020). Initially, an intermediate bis-(3-methyl-1-imidazole)-ethylene dibromosalt derivative will be prepared by stirring a substituted N-methyl-imidazole with 1,2-dibromoethane in acetonitrile medium at 700C for 24 hours. By using new imidazole derivatives as starting materials, it will be possible to generate new ionic liquids that were not described in the literature and test their properties. The results generated on the previous stage mixture will be cooled to room temperature and filtered to produce a solid formation. This crude formation will be washed with acetone with the following evaporation of the solvent. During the following stage of the synthesis process, the generated bis-(3-methyl-1-imidazole)-ethylene dibromosalt and sodium hydroxide will be mixed with acetone and stirred at ambient temperature for 24 hours. The resulting sodium bromate precipitate will be filtrated. The crude mixture will be mixed with dichloromethane and left for decantation. The phases will be separated and all solvents removed from the heavy phase, leaving only bis-(3-methyl-1-imidazole)-ethylene dichloride derivative ionic liquid, for the following mixing with oil for transesterification.
The procedure will be repeated for other imidazole derivatives producing new ionic liquid catalysts (Figure 2).
Figure 2: Ionic liquid synthesis experimental set-up, adapted from (Lima, 2020)
Potential starting materials for ionic liquid synthesis are outlined below:
Figure 3: Potential starting materials for ion liquid synthesis
In Figure 3 above, 5-allyl-1-methyl-1H-imidazole and 5-(but-3-en-yl)-1-methyl-1H-imidazole are especially promising due to the possibility of polymerisation/copolymerisation with ion exchange resins, making catalyst separation easier and more cost-effective (Ostrowska, et al., 2013).
5. Expected Outcomes and Impact
The proposed research is expected to have several major impacts. First of all, it is expected to develop new and highly promising catalysts for the synthesis of biodiesel. It will also be possible to optimise the current biodiesel production conditions, potentially reducing its costs and increasing biodiesel quantity in traditional fuel. As a result, the work is expected to have positive environmental and economic impacts.
Reference list
Elsheikh, Y., Man, Z., Yusup, S., Bustam, M. & Wilfred, C., 2011. Bronsted Imidazolium Ionic Liquids: Synthesis and Comparison of Their Catalytic Activities as Pre-Catalyst for Biodiesel Production through Two Stage Process. Energy Conversion and Management, 52(2), pp. 804-809.
Gholami, A., Pourfataz, F. & Meleki, A., 2020. Recent Advances of Biodiesel Production Using Ionic Liquids Supported on Nanoporous Materials as Catalysts: A Review. Frontiers in Energy Research, 8(144), pp. 1-26.
Lei, Z., Chen, B., Koo, Y. & MacFarlane, D., 2017. Introduction: Ionic Liquids. Chemical Reviews, 117(10), pp. 6633-6635.
Liang, J., Wang, J., Ren, X., Li, Z. & Jinag, M., 2010. Preparation of Biodiesel by Transesterification from Cottonseed Oil Using the Basic Dication Ionic Liquids as Catalysts. Journal of Fuel Chemistry and Technology, 38(3), pp. 275-280.
Li, J., Peng, X., Luo, M., Zhao, C., Gu, C., Zu, Y. & Fu, Y., 2014. Biodiesel Production from Camptotheca Acuminata Seed Oil Catalyzed by Novel Bronsted-Lewis Acidic Ionic Liquid. Applied Energy, 115(1), pp. 438-444.
Lima, A., 2020. Evaluation of Alkaline Ionic Liquids From Catalysis of Biodiesel From Cooking Oil, Apucarana: Universidade Tecnológica Federal do Paraná.
Mumtaz, M., Adnan, A., Mukhtar, H., Rashid, U. & Danish, M., 2017. Biodiesel Production Through Chemical and Biochemical Transesterification: Trends, Technicalities, and Future Perspectives. In: Clean Energy for Sustainable Development. London: Academic Press, pp. 465-485.
Osman, W., Rosli, M., Mazli, W. & Samsuri, S., 2024. Comparative review of biodiesel production and purification. Carbon Capture Science & Technology, 13(December), pp. 1-14.
Ostrowska, S., Markiewica, B., Wqsikowska, K., Bqczek, N., Paenak, J. & Strzelec, K., 2013. Epoxy resins cured with ionic liquids as novel supports for metal complex catalysts. Comptes Rendus Chimie, 16(8), pp. 752-760.
Statista, 2024. Demand for crude oil worldwide from 2005 to 2023, with a forecast for 2024. [Online]
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