Microwave-assisted H₂SO₄ and H₂SO₄/HNO₃ hydrolysis for process-intensified cellulose nanocrystal production with...

Microwave-assisted acid hydrolysis was developed as an intensified strategy for producing cellulose nanocrystals (CNCs) from cotton-derived cellulose. Single-acid H₂SO₄ and mixed-acid H₂SO₄/HNO₃ hydrolysis systems were systematically investigated under controlled microwave irradiation to elucidate the role of acid composition in governing CNC formation, crystalline evolution, surface functionality, thermal stability, and colloidal behavior. Microwave-induced volumetric heating promoted rapid acid-cellulose interactions and efficient cleavage of amorphous cellulose domains, enabling CNC isolation within markedly shortened reaction times compared with conventional thermal hydrolysis. Under optimized conditions, microwave-assisted H₂SO₄ hydrolysis produced CNCs with a yield of 49.0%, crystallinity index of 86.4%, aspect ratio of 5.3 ± 2.7, and sulfate half-ester groups (-OSO₃ ^-) content of 0.14 mmol g^-1. In comparison, microwave-assisted H₂SO₄/HNO₃ hydrolysis yielded CNCs with a comparable yield of 47.7%, higher crystallinity index of 89.2%, aspect ratio of 7.2 ± 2.7, and -OSO₃ ^- group content of 0.13 mmol g^-1. The mixed-acid route further introduced carboxyl/carboxylate (-COOH/-COO^-) functionalities through limited HNO₃-assisted oxidation, as evidenced by FT-IR, solid-state ¹³C NMR, XPS, and zeta potential analyses. Consequently, the mixed-acid CNCs exhibited the most negative zeta potential (-42.4 ± 0.9 mV) and the highest thermal stability, with a T _5% of 285.7 °C and a major DTG peak at 355.4 °C. Compared with conventional H₂SO₄ hydrolysis, microwave-assisted hydrolysis improved CNC recovery while reducing acid concentration and processing time. These findings demonstrate that microwave-assisted mixed-acid hydrolysis provides a controllable and efficient route for producing CNCs with tunable crystallinity, surface functionality, colloidal stability, and thermal performance.