Cellulose is the most abundant biopolymer and the degradation and further processing of this polysaccharide can be the carbon source for renewable fuels and platform chemicals such as ethylene glycol, glycerol, levulinic acid and lactic acid. At present biochemical, chemocatalytic as well as pyrolysis methods are under study in cellulose processing and all these methods have limitations like poor product profiles and yields as well as enzyme, catalysts and energy costs resulting meager economic viabilities (Amarasekara 2013), (Wilson 2012). The extensively studied chemical catalytic approaches include the use of dilute mineral acids (Fan et al. 2012), concentrated acids (Camacho et al. 1996), solid acids (Hu et al. 2015), (Huang and Fu 2013), (Liu et al. 2015) mineral acid in neutral ionic liquids (Li et al. 2008), acidic ionic liquids (Amarasekara and Owereh 2009), (Da Costa Lopes and Bogel-Lukasik 2015), metal salts in aqueous acid and ionic liquid solutions (Amarasekara and Wiredu 2016a), (Amarasekara and Shanbhag 2013), as well as immobilized ionic liquids (Wiredu and Amarasekara 2014).
The use of organic acids as cellulose depolymerization or degradation catalysts is also known, but surprisingly only a limited number of researchers have studied the application of organic acids as homogeneous or heterogeneous catalysts in depolymerization or degradation of cellulose (Amarasekara 2013), (Sun and Cheng 2002), (Mosier et al. 2002), (Mosier et al. 2001). In one of the pioneering studies in this area, Mosier and co-workers have evaluated the catalytic activities of three common carboxylic acids: acetic, maleic and succinic in hydrolysis of cellulose in aqueous medium. In addition, they have compared these organic acids to mineral acid catalysts as well. The rigid dicarboxylic acid, maleic acid was identified as the most active of the three carboxylic acids studied (Mosier et al. 2001). Interestingly, they have further reported that the hydrolysis rates of Avicel cellulose are similar in both 5 x 10−2 M sulfuric and maleic acid mediums.
Heterogeneous catalysts with better recycling possibilities can also be used in cellulose depolymerization. In particularly solid acid catalysts with high density of carboxylic acid and hydroxyl groups covalently connected to the solid surface have shown certain promise in this area (Su et al. 2018), (Kobayashi and Fukuoka 2018), (To et al. 2015), (Vilcocq et al. 2014). For example, Kobayashi and co-workers have recently reported that graphene bearing -COOH and -OH groups on the surface as catalysts could produce glucose yields as high as 88% in aqueous mediums and under gentle conditions (Kobayashi and Fukuoka 2018), (Shrotri et al. 2018). The enhanced catalytic effect of these heterogeneous catalysts with carboxylic acid and hydroxyl groups in close proximity to the active site was explained as a synergistic effect of multiple hydrogen bonding or interactions with the polysaccharide hydroxyl groups (Kobayashi and Fukuoka 2018), (Shrotri et al. 2018), (Amarasekara et al. 2019). In another recent example, Gross and co-workers have shown the exceptional cellulose depolymerization ability of a catalyst prepared by modification of the surface of cellulose nanocrystals by covalently attaching dicarboxylic acids as well as tricarboxylic acids (Spinella et al. 2016). While the carboxylic acid catalyzed cellulose degradation experiments signals the importance of synergistic effects of multiple -OH and -COOH groups for interactions between catalysts and cellulose, methodical studies on intermolecular interactions between cellulose and carboxylic acids is an elusive research area.
The studies on interactions of small molecules with polymeric materials is a challenging area due to the limited number of currently available tools. The most widely used techniques in this area are UV-Vis, IR and NMR spectroscopy; however certain limitations like solubility can lessen their applications in cellulose. The use of thermal properties as a tool for studying interactions between cellulosic materials and small molecules is rare. In one study Singhal et al. studied the interactions between benzoic acid and ethyl cellulose by differential scanning calorimetry to get an insight into controlled drug release (Singhal et al. 1999). During this work, they have noted that glass transition temperature (Tg) of ethyl cellulose was reduced up to 27.7% due to benzoic acid in solid solution. Furthermore, the melting enthalpy of benzoic acid, when plotted as a function of its concentration yielded a straight line suggesting the presence of hydrogen bonding interactions between benzoic acid and ethyl cellulose (Singhal et al. 1999). In another study, DSC was used as the primary tool in identifying the exothermal polymerization reactions between lignin fraction in cellulosic biomass in the presence of maleic acid (Philippou and Zavarin 1984). Inspired by these examples and as a continuation of our studies on interactions between polysaccharides and small molecules as well as metal ions (Fernando and Amarasekara 2021a), (Fernando and Amarasekara 2021b), (Amarasekara and Wiredu 2016b) we have explored the possibility of using thermal properties as a tool in evaluating the interactions between cellulose and carboxylic acids. In this publication we present our studies on interactions between cellulose and a selected set of polycarboxylic acids using thermogravimetry and differential scanning calorimetry. Nine common dicarboxylic acids, and a tricarboxylic citric acid were included in the study. In addition, hydroxy-acid lactic acid was also included for comparison.