Integrated formic acid and deep eutectic solvent mediated sustainable synthesis of cellulose nanocrystals from Sterculia foetida shells

The present study reports the green synthesis of cellulose nanocrystals from the shells of Sterculia foetida (SFS) cellulose. Three different methods, alkali, acid and organic acid, were screened for the maximum cellulose extraction. A maximum cellulose yield, 30.6 ± 0.84 w/w, was obtained using 90% formic acid at 110 °C in 120 min. The extracted cellulose was characterized and identified by instrumental analyses. SEM analysis showed skeletal rod-like microfibril structures and similar intra-fibrillar widths. CP/MAS 13C NMR and FTIR spectrum revealed the purity of cellulose and the absence of other components like hemicellulose and lignin. XRD study revealed a cellulose crystallinity index of 88.07%. BET analysis showed a good surface area (3.3213 m2/g) and a micro-pore area of 1.871 m2/g. The cellulose nanocrystals were synthesized from the extracted cellulose using deep eutectic solvents (DES), choline chloride and lactic acid (1:2 ratio). The cellulose nanocrystals (CNC) synthesized from DES-based exhibited zeta potential and particle size of −16.7 mV and 576.3 d.nm. DES-synthesized cellulose nanocrystals were spherical-like shapes, as observed from TEM images. The present results exposed that formic acid is an effective and green catalyst for the extraction of cellulose and DES for the sustainable synthesis of CNC.