Study of Reducing Destruction of Lignin by FT-IR Spectroscopy
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Journal of Applied Spectroscopy, Vol. 87, No. 5, November, 2020 (Russian Original Vol. 87, No. 5, September–October, 2020)
STUDY OF REDUCING DESTRUCTION OF LIGNIN BY FT-IR SPECTROSCOPY S. V. Aleeva,* O. V. Lepilova, and S. A. Koksharov
UDC 677.021.15:544.1
Lignin preparations were obtained from harsh flax fiber and material exposed to an alkaline solution of sodium borohydride using the Bjerkman extraction method. Peaks of individual vibrations of all types of covalent bonds in the functional groups of the biopolymer were identified. The mechanism of lignin destruction induced by destabilization of the ether bond adjacent to the carbonyl being reduced in the propane link of the macromolecule was confirmed. Reduction of the carbonyl was accompanied by a doubling of the intensity of the absorption band of alkyl hydroxyls and a fourfold increase in the number of hydroxyaryl groups, which was indicative of the accumulation of structural units in the free phenolic form in the system. The dynamics of structural transformations were compared. The duration of serial-parallel processes of polymer reduction and destruction was estimated. Keywords: FT-IR spectroscopy, lignin, alkaline reducing destruction. Introduction. IR spectroscopy is widely used to identify synthetic organic products [1, 2] and structural modifications of polymers [3, 4] and to monitor extraction of biopolymer system components, in particular lignin [5]. IR spectra of pure lignin are studied to analyze the ratio of phenylpropane polymer links in the p-hydroxypropane, guaiacyl, and syringyl forms [6, 7]. Biorefining based on disruptive technologies for isolating components with high added value is a new development direction for lignocellulose feedstock and was proposed for comprehensive studies of the lignin condition using nondestructive analytical methods [8]. IR spectroscopy is considered a basic method for obtaining information for chemometric analysis of the properties of lignocellulose materials [9]. Alteration of lignin properties has practical significance for manufacturing of phenol-formaldehyde products to create composites [10] and to increase the antiseptic activity of plant biomass being pyrolyzed during manufacturing of insecticidal and antimicrobial biocides [11]. Lignin destruction must be monitored to produce new types of intermediates for manufacturing carbon micro- and nanofibers [12, 13]. Effective sorbents including those for purifying wastewaters of heavy metals can be produced by changing the activity of functional groups during lignin modification [14]. Treatment of lignocellulose materials with a solution of sodium borohydride and special biomodification methods [15, 16] can increase the absorption capacity for water-soluble dyes by 4 times and for phenol by 12 times [17]. Increased absorption is associated with lignin activation. Differential UV spectroscopy of lignin preparations provided a basis to propose [18] that reduction of carbonyl groups occurring via a nucleophilic addition mechanism (1)
destabilizes the adjacent-t
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