(679e) Epoxidation of Used Cooking Oils Using Heterogeneous Heteropolyacid Catalysts

Exploitation of waste fats, oils and greases (FOGs) in the production of green chemicals and second-generation fuels has been boosted in recent years as a way to mitigate waste mismanagement practices. At the same time, waste lipids exploitation enables to improve sustainability indicators in the oleochemical industry. Despite impurities, most waste FOGs are similar to the traditional cropped oleochemical feedstocks (e.g. soybean, palm and canola oils) in the sense that they are mainly composed of triacylglycerides, partial glycerides and fatty acids. Nonetheless, adequate pretreatment of waste FOGs must be carried out to reduce problematic impurities that can affect subsequent valorization processes. Among the large variety of waste lipids of industrial interest, used cooking oils (UCOs) stand out because of their large generation in urban centers, the cascade of problems associated to their mismanagement, and because they are already globally traded for biofuels production. However, the low added value of such products and the required pre-treatment for UCOs purification are major challenges for a profitable exploitation without subsidized or sponsored schemes. Alternatively, UCOs could be reclaimed and harnessed for the production of value-added biobased chemicals.

Currently, biobased chemicals and biopolymers market is nearly US$80 billion and will reach around US$124 billion by 2025 [1]. Despite the world production of biobased chemicals has reached almost 3 Mt/year and biopolymers production is nearly 10 Mt/year [2], most of them are derived from cropped feedstocks. Thereby, the use of second-generation biobased raw materials is of major importance to overcome the sustainability challenges of food-water-energy safety nexus. Among the variety of bioderived chemicals, epoxides and polyols are paramount because they are widely used in the polymer industry. In particular, those of oleochemical origin have gained a large participation in the manufacture of resins, polyesters, and polyurethanes, and also as plasticizers for PVC products. In this regard, UCOs can be used as alternative sources for the manufacture second-generation epoxides and polyols.

The epoxidation of vegetable oils is normally carried out via the Prileschajew‘s reaction involving a percarboxylic acid-mediated epoxidation with H₂O₂, under homogeneous or heterogeneous acid catalysis. Industrially, in-situ formation of the percarboxylic acid and homogeneous catalysts are preferred to avoid handling of unstable and flammable peroxyacids and to reduce production costs. Nonetheless, homogeneous catalysts also promote oxirane ring opening reactions, limiting the yield to oxirane groups below 80 %. Thereby, highly selective and more stable heterogeneous catalysts would be preferable to facilitate effective epoxidation of UCOs.

In this context, heteropolyacids (HPA) of the Keggin type were evaluated as catalysts for epoxidation of UCOs. These materials exhibit high thermal and oxidation stability, their acid and redox properties can be controlled via substitution of protons and addenda atoms, whereby notably phosphotungstic acid has shown good activity and selectivity in the epoxidation of olefins [3]. For our study, phosphotungstic acid was neutralized with long chain surfactants, namely (CTA)₃PW₁₂O₄₀, (TBA)₃PW₁₂O₄₀ and (C12mim)₃PW₁₂O₄₀. These materials were characterized by elemental analysis, FT-IR, RAMAN and nitrogen physisorption, and subsequently evaluated as catalysts in Prileschajew‘s epoxidation under isothermal batch reaction at 60ºC. The UCO was pretreated to remove suspended solids and to reduce acidity, and characterized to ensure the iodine index required in raw materials for epoxidation (> 80 g I₂/100g). Once verified, the process performance was assessed varying catalyst loading and oil:H₂O₂ ratio. The efficiency of the process was monitored by measuring iodine and oxirane values. According to results, best operating conditions were obtained with (CTA)₃PW₁₂O₄₀ catalysts using ethyl acetate as solvent with conversion >90% and selectivity >60%, obtaining an epoxidized oil with an oxirane ring content of 6.5% wt. Reaction yield was similar to that obtained with sulfuric acid as catalyst, but it was possible to recover and reuse the catalyst without noticeable loss in activity.

Bibliography

[1] Research and Markets, “Renewable Chemicals Market - Forecasts from 2020 to 2025,” 2020. https://www.researchandmarkets.com/reports/5009215/renewable-chemicals-m... (accessed Apr. 20, 2021).

[2] R. Chinthapalli et al., “Biobased Building Blocks and Polymers - Global Capacities, Production and Trends, 2018-2023,” Ind. Biotechnol., vol. 15, no. 4, 2019, doi: 10.1089/ind.2019.29179.rch.

[3] J. Jiang, Y. Zhang, L. Yan, and P. Jiang, “Epoxidation of soybean oil catalyzed by peroxo phosphotungstic acid supported on modified halloysite nanotubes,” Appl. Surf. Sci., vol. 258, no. 17, pp. 6637–6642, Jun. 2012, doi: 10.1016/J.APSUSC.2012.03.095.