Per-O-acylation of xylan at room temperature in dimethylsulfoxide/N-methylimidazole

Abstract

The per-O-acylation of xylan-type hemicellulose was firstly carried out in dimethylsulfoxide/ N-methylimidazole (DMSO/NMI) at room temperature without additional catalyst. The optimum conditions for esterification of xylan was investigated in terms of the molar ratio of reagents to anhydroxylose units (AXU) in xylan and the kinds of esterification reagents to obtain a high degree of substitution (DS, 1.98) and weight percent gain (WPG, 86.88 %) of xylan esters. In this solvent system, NMI acted as a solvent, a base and an excellent catalyst, therefore, the per-O-acylation of xylan (DS of 1.98) was readily accomplished in DMSO/NMI system at room temperature. Structure elucidation of xylan esters was characterized by FT-IR and NMR ( 1 H-NMR, 13 C-NMR and HSQC). FT-IR and NMR analyses provided the direct evidence of per-O-acylation of xylan under the given conditions. Furthermore, HSQC revealed the higher reactivity of hydroxyls at C-2 position than those at C-3 position of xylan. The solubility of xylan in DMSO, DMF and CHCl 3 improved after esterification. TGA/DTG indicated that the thermal stability of xylan increased after the esterification with anhydrides, while decreased with acyl chlorides, probably due to degradation and hydrolysis of the acylated xylan at the presence of by-product hydrochloric acid.

Improving the thermal stability of different types of xylan by acetylation

“…2). Each xylose unit contains two free hydroxyl groups available for derivatization on carbons 2 and 3 (Zhang, Zhang, Liu, & Ren, 2016). Recently, the combination of dimethyl sulfoxide (DMSO) and 1-methylimidazole (or N-methylimidazole -NMI) has been reported to be an efficient medium for the acetylation of plant cell wall components, including xylan.…”

Impact of birch xylan composition and structure on film formation and properties

“…Aa was used as the esterifying agent in the acetylation reaction. It was assumed that 100 mg of dry xylan contains approximately 1.52 mmol hydroxyl groups (Zhang et al 2016), and different volumes of Aa were added to obtain low and high degrees of acetylation (0.4-25:1 mol Aa/mol hydroxyl groups in the xylan sample) (Table 1). Aa was added dropwise to the xylan suspension, and the reaction was kept under constant stirring for 24 h; after that the acetylation reaction was quenched by cooling in an ice bath, and the xylan was precipitated by the addition of cold water and cold absolute ethanol (2.2:1:4 mg xylan/ml MilliQ-H 2 O/ml EtOH), and cooling at 4°C overnight.…”

“…Aa was added dropwise to the xylan suspension, and the reaction was kept under constant stirring for 24 h; after that the acetylation reaction was quenched by cooling in an ice bath, and the xylan was precipitated by the addition of cold water and cold absolute ethanol (2.2:1:4 mg xylan/ml MilliQ-H 2 O/ml EtOH), and cooling at 4°C overnight. Thereafter, the suspension was centrifuged and the xylan was recovered, washed with ethanol/water (80% ethanol) and freezedried (adapted from Lu and Ralph 2003;Zhang et al 2016).…”

“…The derivatization of xylan by acetylation has been considered by several authors to be a strategic tool for improving the properties of xylan and, consequently, the properties of its films (Gröndahl et al 2003;Egüés et al 2014;Gordobil et al 2014;Stepan et al 2014). Different acetylation methods have been described (Zinbo and Timell 1965;Koroskenyi and McCarthy 2001;Lu and Ralph 2003;Xu et al 2010;Stepan et al 2012;Zhang et al 2012;Stepan et al 2013;Zhang et al 2016). Recently, a method using a combination of dimethyl sulfoxide (DMSO) and 1-methylimidazole (or N-methylimidazole -NMI) has been suggested as an efficient method for dissolving lignocellulosic biomasses using relatively mild conditions (Lu and Ralph 2003;Zhang et al 2012Zhang et al , 2016.…”

“…Different acetylation methods have been described (Zinbo and Timell 1965;Koroskenyi and McCarthy 2001;Lu and Ralph 2003;Xu et al 2010;Stepan et al 2012;Zhang et al 2012;Stepan et al 2013;Zhang et al 2016). Recently, a method using a combination of dimethyl sulfoxide (DMSO) and 1-methylimidazole (or N-methylimidazole -NMI) has been suggested as an efficient method for dissolving lignocellulosic biomasses using relatively mild conditions (Lu and Ralph 2003;Zhang et al 2012Zhang et al , 2016. By acetylation, the hydroxyl groups in xylan are substituted by acetyl groups, and this improves the properties of xylan such as thermal stability, mechanical strength and water resistance (Gröndahl et al 2003;Egüés et al 2014).…”