Pressure effect on nitro-oxidation process for converting cellulosic biomass into carboxylated nanocelluloses

The pressure effect on nitro-oxidation process (NOP) to produce carboxylated cellulose nanofibers (CNFs) from two model feedstocks: microcrystalline cellulose (MCC) and raw jute was investigated. The pressure contains both internal pressure by NOP and external pressure by compressed air. During NOP, pressure enhances the generation of oxidizing agents - NO⁺ and H₂NO₂⁺ - by creating NO₂. Subsequently, NO₂ dissolves in the acidic medium, enhancing the concentration of HNO₃ and becoming more reactive with catalytic components (added KNO₂ and existing metal impurities (Al, Fe and Mg) in jute). For jute, NOP offers simultaneous pulping and cellulose oxidation, both are enhanced by pressure. In the absence of KNO₂, HNO₃ also reacts with jute's metal impurities and produce NOx, producing nitrosating species. At 75 psi, NOP with KNO₂ produced a very high degree of oxidation (DO = 3.60 mmol/g) on cellulose microfibers (CMFs) from MCC, and a high DO (2.23 mmol/g) from jute. The functionalities and crystallinity of CMFs were characterized by CP/MAS ¹³C NMR and FTIR and XRD. The morphology and flow behavior of corresponding CNFs in suspension was investigated by TEM and rheology. The general mechanisms of NOP to convert lignocellulosic biomass into carboxylated CNFs at elevated pressure are proposed.