The reactivity of cellulose in alkylimidazolium hydrogen sulfate-water ([CnCmim][HSO4]/H2O) mixtures during a biomass deconstruction process at 120 °C was investigated. Two types of sample, Miscanthus and a model polymer cellulose, Microcrystalline Sigmacell-Cellulose (MCC), were used. The studied variables included: [HSO4]-ionic liquids with different acidities, 1-butylimidazolium hydrogen sulfate, [HC4im][HSO4], and 1-butyl-3-methylimidazolium hydrogen sulfate, [C4C1im][HSO4]; acid-to-water ([CnCmim][HSO4]/H2O) ratio, and incubation period. A number of analysis tools and chemical methods were employed to characterise the resultant cellulose products: Scanning Electron Microscopy and Energy Dispersed X-Ray (SEM-EDX), Infrared Spectroscopy, Matrix Assisted Laser Desorption/Ionisation with Time of Flight (MALDI-TOF) Mass Spectroscopy, CHNS elemental analysis, viscosity measurement, compositional analysis and enzymatic saccharification.
Deconstruction of Miscanthus in a [HC4im][HSO4]/H2O mixture at 120 °C for 22 h successfully separated cellulose, hemicellulose and lignin. A study on the purification of cellulose sample found that inadequate washing allowed the [HC4im][HSO4] traces to be physically adsorbed. After an extensive washing, indirect evidence, indicating that [HSO4]- anions had chemically adsorbed, was revealed.
An investigation involving incubation of MCC in [CnCmim][HSO4]/H2O mixtures at 120 °C was conducted, replicating the deconstruction process. MALDI-TOF analysis demonstrated that the “[HSO4]” anion had chemically adsorbed on the surface of cellulose, forming sulfur-containing oligosaccharides. However, the type of bond responsible for chemisorption could not be identified. The [HSO4]- anion was the active species for chemisorption, regardless of different acidities of ionic liquids.
Incubating MCC in [CnCmim][HSO4]/H2O mixtures at 120 °C also exhibited an interesting interplay between chemisorption and depolymerisation. A positive relationship was predominant in the presence of lower water content. Increasing water content displayed a negative relationship.