Hydrolysis kinetics and structure characterization of ball milled bamboo impregnated with various diluted acids

The hydrolysis mechanism of ball milled bamboo impregnated with deionized water (B-ball), H₂SO₄ (B-H₂SO₄), HCl (B-HCl) and CH₃COOH (B-CH₃COOH) in subcritical water was investigated in the present study. Various mechanochemical reactions were induced during acid-associated mechanical milling and bamboo with improved solubility, reduced thermal stability and solubility was obtained. Acid-associated mechanical treatment also significantly affected the hydrolysis behavior of bamboo in subcritical water. The hydrolysis kinetics of pretreated bamboo could be well expressed by the Coats-Redfern method. The acid strength dominated the hydrolysis of bamboo during ball milling and subcritical water treatment. H₂SO₄ and HCl possessed the highest ability to dissociate the glycosidic bond and holocellulose-lignin chemical linkages. The above ability, together with their catalytic effects resulted in the lower activation energies required to initiate the hydrolysis of B-H₂SO₄ (38.87 kJ mol⁻¹) and B-HCl (38.23 kJ mol⁻¹) compared with those of raw bamboo (74.34 kJ mol⁻¹), B-ball (104.42 kJ mol⁻¹) and B-CH₃COOH (90.65 kJ mol⁻¹) in subcritical water. The cleavage of holocellulose-lignin chemical linkages liberated the polycondensation of monomer and hydrolysable oligomers from holocellulose for glucose char and aromatic-linked polymer char production, leading to the increase of solid residue yield at elevated reaction temperature. It also promoted the glycosidic bond cleavage and ring scission reactions during hydrolysis in subcritical water, resulting in the improved yields of carbohydrate, 5-hydroxymethylfurfural and furfural.