Spectroscopic analysis of hot-water- and dilute-acid-extracted hardwood and softwood chips

doi:10.1016/j.saa.2017.05.010

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Volume 184, 5 September 2017, Pages 184-190

https://doi.org/10.1016/j.saa.2017.05.010 Get rights and content

Highlights

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Characterization of hot-water- and dilute acid-extracted wood by ATR spectroscopy.
•
ATR spectroscopy results were compared to those from wet chemistry analyses.
•
Changes in the carbohydrate and lignin structure could be determined and explained.
•
Changes between the pretreatments and wood species could be determined.
•
The main phenomena of the pretreatments could be explained by ATR results.

Abstract

Hot-water and dilute sulfuric acid pretreatments were performed prior to chemical pulping for silver/white birch (Betula pendula/B. pubescens) and Scots pine (Pinus sylvestris) chips to determine if varying pretreatment conditions on the original wood material were detectable via attenuated total reflectance (ATR) infrared spectroscopy. Pretreatment conditions varied with respect to temperature (130 °C and 150 °C) and treatment time (from 30 min to 120 min). The effects of the pretreatments on the composition of wood chips were determined by ATR infrared spectroscopy. The spectral data were compared to those determined by common wood chemistry analyses to evaluate the suitability of ATR spectroscopy method for rapid detection of changes in the wood chemical composition caused by different pretreatment conditions. In addition to determining wood species-dependent differences in the wood chemical composition, analytical results indicated that most essential lignin- and carbohydrates-related phenomena taking place during hot-water and acidic pretreatments could be described by applying this simple spectral method requiring only a small sample amount and sample preparation. Such information included, for example, the cleavage of essential lignin bonds (i.e., mainly β-O-4 linkages in guaiacyl and syringyl lignin) and formation of newly condensed lignin structures under different pretreatment conditions. Carbohydrate analyses indicated significant removal of hemicelluloses (especially hardwood xylan) and hemicelluloses-derived acetyl groups during the pretreatments, but they also confirmed the highly resistant nature of cellulose towards mild pretreatments.

Graphical abstract

Introduction

The manufacture of novel, high-value products from lignocellulosics has been considered as a solution to facilitate an evolutionary shift within the conventional forest industry towards a more competitive business model and more efficient utilization of raw materials [1], [2]. This viewpoint has led to a transformation that includes implementing different biorefinery units into existing pulp and paper mills and evolution of integrated forest biorefineries (IFBRs) [3]. In modern IFBRs, the target is to extract carbohydrates, extractives, lignin, and other materials from biomass and convert them into value-added products, simultaneously minimizing the production of waste. Within this context, biomass pretreatment technologies (physical, chemical, or biochemical methods) mainly applied to recover hemicelluloses-derived carbohydrates from wood prior to pulping have been considered a crucial step [4].

The most common chemical pretreatments utilized for recovering particularly hemicelluloses-derived carbohydrates are performed either with pressurized hot water or diluted mineral acid [4], [5], [6], [7], [8]. By hot-water extraction, the main aim is usually to produce hydrolysates, especially containing oligo- and polysaccharides, whereas the pretreatments conducted with diluted acid typically result in the intensive formation of monosaccharides [9]. However, in addition to carbohydrates, other wood components (i.e., lignin and extractives) as well as all their degradation products (e.g., aliphatic acids and furanoic compounds) are also incorporated to hydrolysates.

The pretreatment processes have a profound effect on the chemical composition of wood and the subsequent delignification behavior of the pretreated feedstocks [10], [11]. In order to maintain the quality of the produced chemical pulp, the detailed effects of conducted pretreatments on wood composition must be understood. Within this framework, reliable and rapid methods for measuring the residual, structural, chemical components constituting cell walls of the pretreated wood material are highly appreciated. Conventional wet chemical analyses are accurate, but they have limitations, especially with regard to the amount of material required to confirm the analysis and overall time-consuming processes [12]. However, several spectroscopic analysis methods, such as Fourier transform infrared (FTIR) [13], [14], [15], [16], near infrared (NIR) [[17], [18], [19], [20]], and attenuated total reflectance (ATR) [21], [22], [23] infrared spectroscopy, have been proposed for providing a solution to the above mentioned problems; they comprise low-cost instrumentation and generally require very little sample preparation.

In this study, hot-water- and dilute-acid-extracted silver/white birch (Betula pendula/B. pubescens) and Scots pine (Pinus sylvestris) wood materials were investigated by ATR spectroscopy, especially keeping in mind their potential utilization for biorefineries. The main aim was to clarify if essential effects of varying pretreatments conditions on the original wood materials could be easily detected by this method.

Section snippets

Raw materials

Industrial silver/white birch (Betula pendula/B. pubescens) and Scots pine (Pinus sylvestris) chips were used in the laboratory-scale pretreatments with hot-water and diluted sulfuric acid. The used chips were laboratory-screened according to SCAN-CM 40:94 [24], and chips having knots, bark residues, and other visible impurities were manually removed. The maximum thickness of the used chips was 7 mm, maximum width 13 mm, and minimum width 7 mm.

Pretreatments

Laboratory-scale pretreatment experiments of screened

Raw materials for pretreatment experiments

Chemical composition of the feedstock materials used for the hot-water and acidic pretreatment experiments is presented in Table 1. As expected, the two wood species differed from each other, especially in the contents of hemicellulose moieties and lignin, showing characteristic differences commonly described in the literature [34]; birch contains more xylose but less galactose, mannose, and lignin than pine.

Pretreatments

Yields of wood residues after pretreatments are presented in Table 2. In general, with

Conclusions

Effects of hot-water and dilute acid pretreatments on chemical composition of hardwood and softwood were investigated by attenuated total reflectance (ATR) infrared spectroscopy. Spectral data were compared to the corresponding chemical composition data determined by conventional wood chemistry analyses for evaluating the suitability of ATR spectroscopy for describing the phenomena taking place during such pretreatments. Results clearly indicated that this rapid and simple spectral method could

Acknowledgement

Financial support from the Maj and Tor Nessling Foundation (number 201700063) (Joni Lehto) is gratefully acknowledged.

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Spectroscopic analysis of hot-water- and dilute-acid-extracted hardwood and softwood chips

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Raw materials

Pretreatments

Raw materials for pretreatment experiments

Pretreatments

Conclusions

Acknowledgement

Appl. Therm. Eng.

Biomass Bioenergy

Bioresour. Technol.

Carbohydr. Polym.

Polym. Degrad. Stab.

Talanta

J. Mol. Struct.

J. Mol. Struct.

Spectrochim. Acta A

J. Archaeol. Sci.

Thermochim. Acta

Spectrochim. Acta A

Carbohydr. Polym.

Carbohydr. Polym.

Spectrochim. Acta, Part A

Pulp Paper Can.

J. Sci. Ind. Res.

Bioresources

Advanced Biorefinery Concepts Integrated to Chemical Pulping

Nord. Pulp Pap. Res. J.