Understanding the effect of Ionic liquids and Deep eutectic solvents on extraction processes

Abstract

Ionic liquids (ILs) and deep eutectic solvents (DESs) are being actively explored as alternative to conventional solvents due to their non-volatility and reduced flammability, with interest in the use of these solvents emerging for a variety of applications. Moreover, IL and DES properties can be tuned by ion/component selection and their composition, allowing them to be designed as promising solvents for these applications, including specific extraction processes such as biomass extraction. This project explores the development of DESs and ILs for extraction processes from two different perspectives. The first is the design of novel DESs with significantly improved hydrogen bond donating or accepting capacity towards being able to perform extraction processes more efficiently. The other was to undertake a systematic exploration of the extraction of key components of the Pinus radiata bark to understand the influence of IL and DES solvent effects on these outcomes. To improve extraction processes, the ability to design novel DESs with better hydrogen bond donating or accepting capability than existing DESs was explored. The development of strong hydrogen-bond donating solvents were explored using guanidinium salts as a constituent along with 20 different hydrogen bond donor combinations. The polarity of DESs was explored using the solvatochromic dye set developed by Kamlet and Taft to measure the hydrogen bonding strength of the DESs. Out of all DESs, the guanidinium chloride: lactic acid mixture was found to possess the best hydrogen bond donating ability (α). This was comparable to, if not slightly improved, compared to existing strong hydrogen bond donating solvents such as 1,1,1,3,3,3-hexafluoro-2-propanol. Attempts to develop strong hydrogen-bond accepting solvents were also made by exploring potential solvate ILs with strongly hydrogen bond accepting anions and examining the concept of reverse DESs where the salt was added with a strong hydrogen bond acceptor rather than a hydrogen bond donor. These attempts were not successful due to the formation of mainly high melting mixtures rather than room temperature liquids. Moreover, the new DESs that were prepared, have the potential to be more cost-effective and environmentally friendly than ILs with related guanidinium cations, which have recently been shown to be excellent solvents for the purification of a variety of difficult-to-solubilize pharmaceuticals. The solvents that could be successfully prepared were used to examine the solubilization of active pharmaceutical ingredients such as fenofibrate and the extraction of key components from Pinus radiata bark. For the fenofibrate solubilisation, the DES based on guanidinium chloride and levulinic acid displayed the highest solubility although this was significantly less than could be obtained using the related IL solvent systems. Pinus radiata is one of the most extensively used woods in New Zealand. The bark of Pinus radiata consists of various classes of substances such as suberin, lignin, cellulose, polyphenols, and extractives but is currently used for largely low value applications and is considered to be predominantly a waste product. Many of the compounds present in the bark demonstrate bioactivity such as antifungal, anti-tumor, anti-carcinogenic, anti-diabetic and anti-inflammatory properties and the structural biopolymers are of interest as potential sources of bio renewable materials and biofuels. Given the potential importance of components of Pinus radiata bark, one of the goals of this work was to understand how the structure of ILs and DESs affects the extraction and recovery of essential components from Pinus radiata bark. As ILs can alter their properties based on the structure of the ions selected, various types of ILs were developed by selecting a range of cations and anions of systematically differing hydrogen-bonding abilities to model their effects on the extraction of key bark components. The length of the alkyl chains on the IL ions was also modified to provide insight into the effect of the hydrophobic component of the IL on the extraction capacity and selectivity. Overall, 15 ILs, along with room temperature DESs developed such as guanidinium chloride: lactic acid and guanidinium chloride: glycerol, were used. Additionally, triethylammonium hydrogensulfate ([TEA][HSO4]) was also examined for bark extraction due to it being reported as an effective solvent for extracting lignin from biomass sources. ILs were found to be more effective at removing aromatics and waxes than sugars/polysaccharides as demonstrated by the solid-state NMR and pyrolysis GCMS results. This was even true for dimethylphosphate ([Me2PO4]−) based ILs which have been described as being effective for the solubilisation of carbohydrates due to strong hydrogen bond accepting ability of the anion, which was attributed to the competition with multiple components of the biomass. [CnC1pyrr]+ cations facilitate the preferential extraction of the hydrophobic components when combined with a strong hydrogen bond accepting anion ([Me2PO4]−). In contrast, [CnC1im]+ cation based ILs were more efficient at extracting lignin and tannin fractions. Moreover, the dissolution of the carbohydrate and lignin or tannin components depend on the alkyl chain length and these components decreases with increasing non-polar domain of the cation. However, the extraction of hydrophobic compounds such as waxes and suberinic material tended to increase with increasing alkyl chain length. In terms of DESs, guanidinium chloride: lactic acid mixture was found to be effective in extracting a significant amount of carbohydrate than other components present in the bark. The recovery and the isolation of the extracts was examined. When employing non-volatile solvents like ILs and DESs, this is an area that is sometimes underexplored, yet it is critical to any application of these solvents. The recovery of components from the bark extracts was explored using antisolvent addition. It was found that the use of water as an antisolvent enabled selective precipitation of some bark fractions compared to the use of other antisolvents that were ineffective hydrogen bond donors. The [Me2PO4]− IL extracts led to higher yields of carbohydrates than could be recovered from [N(CN)2]− ILs or the DES with the addition of water as an antisolvent. Moreover, the highest proportion of carbohydrate was recovered from [C4C1im][Me2PO4] due to the presence of the imidazolium cation in combination with the strong hydrogen bond acceptor anion. Regarding the isolation of suberins/waxes as hydrophobic components, [C4C1pyrr][Me2PO4] contributed to extracting and enabling the recovery of the highest proportion of suberin/waxes due to hydrophobicity of the cation. The long chain aliphatic [Me2PO4]− ILs only gave mixed fractions and limited selectivity of the components was observed due to their improved solubility on addition of water to these ILs.