Acylation of Lignin with Different Acylating Agents by Mechanical Activation-Assisted Solid Phase Synthesis: Preparation and Properties

Acylated lignins with substituents consisting of different lengths of carbon chains were prepared by a mechanical activation-assisted solid phase synthesis (MASPS) technology with a customized stirring ball mill as a reactor. The structures and properties were analyzed by UV/Vis, FTIR, NMR, SEM, DSC, and TG. The results showed that the acylated lignins were successfully prepared with either non-cyclic or cyclic anhydrides as the acylating agents. Both aliphatic hydroxyl and phenolic hydroxyl groups of lignin reacted with non-cyclic anhydrides, and different reactivity of acylating agents resulted in different relative contents of phenolic and aliphatic substituents in the products. The reactivity of the cyclic anhydrides was weaker than that of the non-cyclic anhydrides, and the reactivity of the acylating agents decreased with increasing carbon chain length and unsaturated bonds of acyl groups. All of the acylated lignins except maleylated lignin had a lower glass transition temperature (Tg) than the original lignin. The acylated lignins prepared with non-cyclic anhydrides had better thermal stability than original lignin, and the thermal stability increased, but Tg decreased with an increasing chain length of the acyl groups. The acylated lignins prepared with cyclic anhydrides had higher a Tg than those with non-cyclic anhydrides with the same carbon number, and the thermal stability was not obviously improved.


Notes
The authors declare no competing financial interest.

Ultraviolet/Visible (UV/Vis) Spectroscopy Analysis
UV/Vis analysis was performed with a 2802s UV/Vis spectrometer (UNIC, USA) in the wavelength range of 200−600 nm, at a slit width of 1 nm, and at moderate scan velocity.
Approximately 0.010−0.015 g of sample was dissolved in 10 mL of water and 1,4-dioxane mixture (water/1,4-dioxane volume ratio=1:9). The neutral solution was obtained by diluting 2.0 mL of the solution to 50.0 mL with pH 6 phosphate buffer, while the alkaline solution was obtained by diluting 2.0 mL of the solution to 50.0 mL with pH 12 sodium borate buffer.
The difference spectra were directly determined by measuring the absorbance of the alkaline solution relative to the neutral solution [1].

Fourier Transform Infrared Spectroscopy (FTIR) Analysis
FTIR analysis was performed with a FTIR-7600 spectrometer (Lambda Scientific Pty Ltd., Australia). Mixed powders of sample (2.0 mg) and KBr (200.0 mg) were pressed into a disk for testing.
The spectra were recorded with 32 scans in the frequency range of 4000−400 cm −1 with a resolution of 1 cm −1 . In order to compare the obtained spectra, each spectrum was normalized with the intensity of the absorbance peak attributed to a characteristic band of the aromatic skeletal vibrations (A1600 cm −1 ).
The normalization and the baseline correction were processed as reported by Gilarranz et al. [2].

Nuclear Magnetic Resonance (NMR) Analysis
1 H and 13 C NMR spectra were accumulated on an AVANCE III HD 600 spectrometer (Bruck, Switzerland).Approximate 100 mg and 300 mg of sample were dissolved in 1 mL of DMSO-d6 (99.9% deuterated, 0.05% tetramethylsilane) for 1 H and 13 C NMR analysis respectively, and left overnight for complete dissolution. The solution was filtered with a 0.45 µm microporous filter membrane before testing.

Scanning Electron Microscopy (SEM) Analysis
The samples were fixed on a sample bench using a double glue tape, and coated with gold to improve the conductivity. Micrographs were taken to observe the surface morphologies of different samples at 1,000× and 3,000× magnifications.

Differential Scanning Calorimetry (DSC) Analysis
DSC curves of the samples were examined by a DSC Q20 V24.4 Build 116 analyzer (TA Instruments, New Castle, DE, USA). The samples, with accurate weights of 5−10 mg, were placed and sealed onto an aluminum plate for analysis. Direct measurement was run from 30 to 200 °C at a scanning rate of 10 °C/min ramp under nitrogen. The measurement by second heating was run under nitrogen using the following program: (1) initial scan: heat ramp from 30 to 145 °C at 10°C /min, isothermal state at 145 °C during 30 min, cooling to 0 °C at 10°C /min, isothermal state at 0 °C during 10 min, and (2) Tg measurement: heat ramp from 0 to 200 °C at 10°C/min. After the measurement, the instrument was cooled to 30 °C at 30 °C/min prior to the next sample.

Thermogravimetric Analysis (TG)
TG curves of the samples were examined by a TGA Q50 V20.10 Build 36 analyzer (TA Instruments, New Castle, DE, USA). Scans were run from 30 to 600 °C at a 10 °C/min ramp under nitrogen.