Treating lignocellulosic biomass with dilute solutions at ambient temperature: effects on cellulose crystallinity

The decrystallization or hydrolysis of lignocellulosic biomass is usually carried out either with concentrated solutions at moderate temperature or with dilute solutions at high temperatures. In contrast to this, agricultural waste biomasses (sunflower stalk, rapeseed stalk, and rice hull) were treated with dilute acidic or alkaline aqueous solutions (5 mol %) in this study to test the variations in cellulose crystallinity under ambient temperature. Solutions of HCl, H₃PO₄, CH₃COOH, HNO₃, H₂SO₄, HF, NaOH, Ca(OH)₂, C₂H₅OH, and CS(NH₂)₂ were used. Effects of the treatment on cellulose crystallinity were evaluated based on the crystallinity index (CrI) calculations through the reflection intensities in X-ray diffraction (XRD) and the absorbance ratios in Fourier transform-infrared (FTIR) spectroscopy at A₁₄₂₉/A₈₉₇ (lateral order index) and A₁₃₇₄/A₂₉₀₀ (total crystallinity index). It was found that the CrI values based on the total crystallinity index suited more than lateral order index to the CrI values found by XRD method. HF solutions led to most striking decreases in CrI, while the solutions of neither strong acids nor NaOH resulted in reductions in CrI. Derivative thermogravimetry (DTG) and differential scanning calorimetry (DSC) profiles revealed that the applied treatment influenced the pyrolytic degradation characteristics and the reactivity of biomass in range of 300–400 °C where cellulose decomposed. Graphical abstract: (Figure presented.).