Rapid Carboxymethylation of Xylan-Rich Hemicelluloses by Microwave Irradiation

Abstract

Rapid carboxymethylation of xylan-rich hemicelluloses from wheat straw biomass with sodium monochloroacetate and sodium hydroxide in the ethanol/water medium under microwave irradiations was investigated in this paper. The effects of reaction conditions such as the reaction time, the reaction temperature, and the amounts of sodium hydroxide and sodium monochloroacetate on the degree substitution (DS) of carboxymethylated hemicelluloses were discussed. The structures of hemicelluloses before and after the chemical modification were analyzed by13C nuclear magnetic resonance (NMR) spectra. Gel permeation chromatography (GPC) was used to characterize the carboxymethylated hemicelluloses. During the chemical modification, it was found that the remarkable degradation of hemicelluloses occurred under microwave irradiations.

Synthesis and Characterization of Carboxyethylated Lignosulfonate

“…[59,60] In the first step of this reaction, the spontaneous dissociation of the carbonÀhalogen bond in 2chloropropionic acid generatesapositivelyc harged (i.e.,aprochiral trigonal planar sp 2 -hybridized with av acant po rbital) carbocation intermediate and an egatively charged chloride ion (Scheme 1a). [59,60] In the first step of this reaction, the spontaneous dissociation of the carbonÀhalogen bond in 2chloropropionic acid generatesapositivelyc harged (i.e.,aprochiral trigonal planar sp 2 -hybridized with av acant po rbital) carbocation intermediate and an egatively charged chloride ion (Scheme 1a).…”

“…[59,60] Namely,t he carbonyl group is electron-withdrawing because of the inductive effect. [59,60] Namely,t he carbonyl group is electron-withdrawing because of the inductive effect.…”

“…[59] The samples were first dried in a 105 8Co ven overnight to remove any moisture. [59] The samples were first dried in a 105 8Co ven overnight to remove any moisture.…”

“…[59,60] In the first step of this reaction, the spontaneous dissociation of the carbonÀhalogen bond in 2chloropropionic acid generatesapositivelyc harged (i.e.,aprochiral trigonal planar sp 2 -hybridized with av acant po rbital) carbocation intermediate and an egatively charged chloride ion (Scheme 1a). [59,60] In the second step, the alkoxide ion from the alkali LS (as an anionic nucleophile)a ttacks the planar carbocation from either the Re or Si face with equal probability.I t can be assumed that the racemic mixture will contain equal amountso fb oth enantiomers( R/S)t ot he labeled chiral carbon center( shown as Bi nt he produced 1-carboxyethyl lignosulfonatei nS cheme1b). [59,60] Sodium lactate( Scheme1c) and homopropionic acid polymer,H -ClPA, are two byproducts of the carboxyethylation reaction.…”

“…

Scheme1.Schematicrepresentation of the carboxyethylation reaction:a)formationo fc arbocation, b) bulky nucleophile attracts the carbocation, [59,60] c) undesireds odium lactate production. For simplicity,t he substitution is showno nly in ap henolicOHg roup position in this schemeb ut it can occur in the primary and secondary aliphatic OH group positions.

…”