The Hy-MASS concept: hydrothermal microwave assisted selective scissoring of cellulose for in situ production of (meso)porous nanocellulose fibrils and crystals

Abstract

The hydrothermal microwave-assisted selective scissoring (Hy-MASS) of depectinated orange peel residues (OPR), produced via conventional acid hydrolysis and acid-free microwave processing, to yield (meso)porous nanocellulose fibrils and crystals simultaneously in the absence of additional auxiliary reagents and/or mechanical treatment is reported. In the stepwise microwave hydrothermal treatment (MHT) of OPR from 120 °C–200 °C at 20 °C intervals, release of residual pectins and hemicelluloses is observed up to 180 °C producing nanocellulose fibrils (3–15 × 500–2000 nm). Beyond 180 °C, selective leaching/hydrolysis of amorphous regions occur to yield nanocellulose crystals (200–400 × 40–50 nm) and crystallites (5–15 × 40–50 nm). This selective, step-wise scissoring process is termed Hy-MASS Concept. Structure, morphology and properties of (meso)porous nanocellulose are strongly influenced by pectin extraction methodology employed. With acid depectinated OPR, deconstruction of the lignocellulosic matrix via microwave is hastened by approx. 20 °C with respect to acid-free microwave depectinated OPR. T_d of acid depectinated nanocelluloses (CMC) is ca. 350 °C compared to microwave depectinated nanocelluloses (MMC, T_d, varies 342–361 °C). Nanocellulose produced via microwave pre-treatment is (meso)porous: BJH pore size 5–35 nm; BET surface area, 1.5–107 m² g⁻¹, and; BJH pore volume, 0.01–0.27 cm³ g⁻¹, when compared to acid pre-treated counterparts. The crystallinity index of CMC and MMC increases in two stages, 120–140 °C (ca. 8%) and at 180–200 °C (5–9%). XRD revealed presence of calciuim salts, most likely calcium oxalate. The hydration capacities of nanocelluloses (12–23 g water per g sample) are much higher than their precursors or literature citrus nanocellulose.