Polyols are important and essential chemical compounds used in synthesizing several polymeric materials and the pharmaceutical industry. The most dominant polyols in the world market are polyether polyols (69%), followed by polyester polyols (19%), which are the chief reagents in the production of polyurethanes through their reaction with isocyanates [1]. Although polyols find their primary consumption in producing polyurethane foam for applications in the automotive and building sectors, these are also utilized in preparing coatings, sealants, adhesives, and elastomers. Generally, from the synthesis aspects, most polyols are obtained from petrochemical sources, typically from propylene oxide. Recently, the synthesis of bio-based polyols from several bio-based materials demands a promising approach to mitigate the decreasing reserves of fossil fuels and environmental problems, along with prioritizing the search for new sustainable resources [2–8]. For instance, several reports have been found where biennis seed oil, castor oil, soybean oil, and sunflower oil are used for the preparation of bio-polyol [9–12]. Nevertheless, recently, different lignocellulosic bio-residues have been productively transformed into bio-polyols with characteristics similar to conventional products [5, 6, 13–20]. However, to the best of our knowledge, limited research articles have been found where spent coffee grounds (SCGs), a major biowaste in our society, are utilized to efficiently synthesize eco-polyols.
Coffee is consumed worldwide as a beverage, and after petroleum, it is considered as the second-largest traded goods. Thus, SCGs are produced as waste after brewing and are equivalent to about 90% of the primary coffee beans [21]. Accordingly, SCGs waste becomes an environmental issue, as it might have harmful effects when it is not disposed of appropriately which could lead to the emissions of caffeine, tannin, and polyphenol during the course of fermentation [22]. On the other side, it is worth mentioning that half of the total coffee beans are composed of carbohydrates, which are predominantly polysaccharides of hemicellulose (30–40 wt%) and cellulose (8–15 wt%) [23, 24], which can be transformed into fermentable sugars like glucose, galactose, mannose, and arabinose, which are further transformed into lactic acid, succinic acid, acetic acid, polyhydroxyalkanoate (PHA), and other substances of choice by microbial fermentation [25]. Therefore, coffee could be a reliable source of bio-polyol upon suitable experimental conditions. In recent times, Soares et al. synthesized eco-polyols from SCGs via acid liquefaction reaction using polyethylene glycol (PEG) and glycerol solvent mixture [26]. During this liquefaction (thermochemical) process, SCGs are degraded into lighter molecules by polyhydric alcohols through solvolytic reactions. However, the phase (one or split) of a recovered polyol is an important consideration in the synthesis process as the split phase needs additional separation and purification steps which are unacceptable from economic and industrial aspects [27]. It is also established that the phase of recovered polyol depends on the type of solvents used in the solvolytic reaction [27]. However, no such reports have been found where polyether polyol (PEP) and glycerol have been used as solvent mixtures for acid-catalyzed liquefaction reactions of different types of SCGs at a reasonable temperature and autogenous pressure.
In this work, three types of SCGs have been used for acid liquefaction conversion, where the polyol-glycerol mixture is employed as a solvent, and sulfuric acid is utilized as the catalyst. The nature of the phases of obtained polyol and the biomass conversion were studied with different reaction conditions and different types of SCGs. Then, a design of experiments (DOE) approach and statistical investigation were carried out to understand the influence of three factors on the conversion of biomass, explicitly temperature, acid concentration, and time. Finally, different physical characterizations of the produced polyols were carried out to estimate their appropriateness as precursors in the preparations of polyurethane foams.