Extraction of caffeine from spent coffee grounds using ionic liquids

Abstract

Coffee is the most popular beverage consumed and the second-highest commodity in the world, after crude oil. In 2018, a total of 9,5 million metric tons of coffee were produced globally. This in turn generated 6 million tons of waste coffee grounds. In South Africa alone, it is estimated that approximately 100 million cups of coffee are brewed a year, resulting in 3000 tonnes of waste produced, of which 93% ends up in landfill sites (Lombard, 2021). This abundant waste source has shown promising potential for reusing, recycling, or converting the waste into valuable products like biofuels, fertilizers, animal feed, high-value chemicals, cosmetics and pharmaceutical products such as caffeine for medicinal purposes. Besides coffee being one of the most important agricultural commodities in the world, coffee is also one of the most valuable primary products in world trade. Coffee is also the central and popular activity of many cultures. The most popular reason for the consumption of coffee is its refreshing properties. Large quantities of this waste pose threats to the environment as it is a source of severe contamination and serious health problems. To avoid this catastrophe of the coffee waste, spent coffee grounds can be utilised to generate valuable products. The long-term usage of fossil fuels depletes the finite supply and contributes to greenhouse gas (GHG) and exhaust emissions. The global economic and environmental crisis related to the usage of fossil fuels and the fast depletion of natural resources has raised much awareness and need to find alternate strategies for cleaner and greener energy and chemical products needed for recycling waste has risen drastically. The use of biomass and other lignocellulosic material to produce bio-fuels and other high value products show promising results. Using lignocellulosic material has attracted considerable amounts of attention due its renewable nature and being abundantly available. Lignocellulosic material is used for sustainable development in the world. In this study caffeine extraction is a promising solution for sustainable development, where biomass is valorised. The characterisation of spent coffee grounds (SCGs) using Technical Association of the Pulp and Paper (TAPPI) methods was carried out. The effect of temperature, reaction time and solid-to-liquid loading ratio on the yield of caffeine extracted from spent coffee grounds was investigated. Simultaneously, the best extraction solvent between the (i) ionic liquid (IL) 1-ethyl-3-methylimidazodium chloride (98%), (ii) dichloromethane and (iii) water was determined. Variation of the parameters were established using the Box-Behnken design of experiment (DOE) methodology which varied the (i) temperature (88-120 degrees Celsius), (ii) reaction time (15-35 minutes) and (iii) solid-to-liquid loading ratio (20 g/10-25 mL). For the extraction process, both the conventional method and green method (IL and water) were investigated. The conventional method includes using dichloromethane as the extraction solvent, whereas the green method makes use of the ionic liquid 1-ethyl-3-methylimidazolim chloride and water as the extraction solvents. Extraction was carried out in a Parr pressure reactor where solid-liquid extraction occurs. High performance liquid chromatography (HPLC) was used to quantify the yield of extracted caffeine. Recrystallization of the highest caffeine yield was carried out and thereafter analysed using Scanning Electron Microscopy (SEM), Transition Electron Microscopy (TEM), Energy Dispersive Spectroscopy (EDS) and Differential Scanning Calorimetry (DSC). The maximum yield of caffeine was obtained at the optimum conditions of 120 °C for 25 minutes using 25 mL volume of extracting solvent. The caffeine extracted from 1-ethyl-3-methylimidazolium, water and dichloromethane was 726.22mg/L, 646.33mg/L and 566.12mg/L respectively. Alternatively stated as 1-ethyl-3- methylimidazolium chloride, water and dichloromethane extracted 0.00363 g caffeine / 1 g SCG, 0.00323 g caffeine / 1 g SCG and 0.00283 g caffeine / 1 g SCG respectively. SEM images of the spent coffee grounds prior to extraction displayed a dense morphological chain-like structure, with large lumps present. The structure was tightly bonded together and appeared rough. After extraction using each solvent, the SEM micrographs were analysed. Extractions done with the IL demonstrated full degradation. The structure was loose, multiple open pores on the surface with a smooth and thin appearance. The water extractions appeared almost same to that of the IL, but slightly thicker. Lastly, extractions using DCM appeared to be unsuccessful as the SCG attempted to be broken but were still together. The surface had no open pores, rather an oil coated layer covering the spent coffee grounds. EDS results from 99% pure caffeine standard was compared against the caffeine extracted by all three extraction solvents. Pure caffeine appeared clean, properly formed, big separate particles and distinctive shapes. The caffeine extracted using IL was similar to the structure, crystallinity and appearance of the pure caffeine. Caffeine extracted by water were in long shards, but not fully individual/separated. The caffeine extracted by DCM appeared less crystalline, much smaller in size and more compact. DSC compared the melting points of the pure caffeine standard to those caffeine samples extracted by different solvents, thus providing the purity of the extracted caffeine. The standard caffeine sample had a melting point of 233. 55 ºC equalling 99 % pure. The melting points of 226. 52 ºC; 212. 28 ºC and 200 ºC were obtained for IL, water and DCM respectively. Purity obtained were 96 %, 90 % and 85 % per respective extraction solvent.