Teoría sobre la Persuasión

Using the biorefinery approach, cellulose-enriched fibres were successfully obtained from MxG in two routes: 1) direct delignification; and 2) sequential extraction followed by delignification.

Differently from what was expected, after the modified organosolv method step, DEL fibres presented lower percentage of lignin compared to SEQ fibres, suggesting that the delignification process was more efficient in direct route compared to sequential extraction route. That could be due to several non-target reactions (condensation, pseudo-lignin formation) resultant from the increased process severity applied during sequential extraction. In addition, direct route resulted in fibres with higher hemicellulose contents when compared to the sequential route. Cellulose contents in both fibres were very similar.

However, different from the purified liquid streams generated during sequential extraction, direct delignification originated a liquid stream containing a mixture of several components (biomass extractives, hemicellulose, and lignin). This complex mixture decreases

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its potential use for further processing, particularly for hemicellulose fraction that is highly soluble in this stream and difficult to be recovered.

Although sequential extraction is still preferred because of the potential of using liquid streams for high-valuable products generation, the fibres composition achieved by these two routes was very similar. Moreover, the first aim of this chapter was achieved by successfully removing biomass fractions using SBW and a modified organosolv method to produce cellulose-enriched fibres.

In order to understand structural characteristics of fibres produced, physical evaluation was performed using SEM, FTIR and PCA. SEM showed significant differences in the surface of biomass. PCA demonstrated to be a powerful tool for FTIR data analysis. While FTIR spectra and infra-red crystallinity index (CI) values only provided limited information about fibres structures and differences among the samples, PCA was able to provide more definitive results about the structural differences present in the fibres. PCA efficiency proved to be closely dependent on data manipulation, therefore the evaluation of several data manipulations was performed to determine which one would be most suitable for the particular set of samples of this work. Data normalisation demonstrated to have a significant effect on data analysis. The use of data 2nd-derivative by itself did not display a significant improvement. However, the combined effect of 2nd-derivative and normalisation achieved the best data resolution providing useful information about the samples.

Therefore, the use of physical analysis combined with statistical tools achieved the second aim of this chapter that was to evaluate/differentiate fibres produced by SBW and modified organosolv extractions. Hence, PCA successfully proved qualitatively that different treatments led to a production of two distinct cellulose-enriched fibres: DEL and SEQ.

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These fibres were used as starting materials to assess the efficiency of using SBW as media/catalyst in the conversion of cellulose fibres into glucose monomers following the biorefinery approach and this process is discussed in Chapter 4.

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AN ASSESSMENT OF SUBCRITICAL WATER

FOR CELLULOSE HYDROLYSIS AND GLUCOSE

PRODUCTION

4.1. Introduction

Cellulose is a water insoluble linear polymer composed of glucose units bonded by - 1,4-glycosidic linkages (O'Sullivan, 1997) and present both highly organized (crystalline) and less ordered (amorphous) fractions (Klemm et al., 2005) and degrees of polymerisation (DP) that varies from 6000 to 16000 (Liu and Sun, 2010). Cellulose is a substantial natural polymer and it is the most abundant compound presented in lignocellulosic biomass (Vanholme et al., 2013). Cellulose is currently largely used in industry of paper and chemicals (Liu and Sun, 2010) and its interest as an important carbon source for a bio-based economy is increasing significantly as glucose can be converted into a variety of fuels and chemicals (Vanholme et al., 2013).

The current global energy economy based on fossil fuel reserves lacks sustainability and it is leading to a series of environmental, economic and geopolitical implications regarding to its future (Demirbas, 2009a). Therefore, issues such as the increase of fuel demands, climate changes, and the instability of fossil fuel prices are driving an economic transition towards a bio-based economy (Langeveld et al., 2010). A bio-based economy is believed to be the most

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promising way to achieve energy independence as well as a sustainable development and management of environmental issues (Demirbas, 2009a, Fitzpatrick et al., 2010).

In order to utilize cellulose for production of fuels, chemicals and/or materials, cellulose needs to be recovered/accessible from lignocellulosic matrix. However, whereas high sugar/starch contents feedstocks such as sugar cane and corn are easily hydrolysed, cellulose requires a more extensive treatment in order to disrupt its linkages and release glucose units (Demirbas, 2009a).

The use of water in subcritical conditions for lignocellulose biomass fractionation is gaining attention as an environment-friendly process due to the non-requirement for additional catalysts, fast processing and operational simplicity, and limited corrosion a problem commonly encountered by supercritical and acid treatments (Ruiz et al., 2013, Toor et al., 2011).

Water in subcritical conditions has interesting properties that differ from ambient conditions such as higher ionic constant, lower viscosity and higher solubility of organic species (Toor et al., 2011). In addition, subcritical water (SBW) acts simultaneously as solvent and catalyst for hydrolysis reaction as its auto-ionization creates hydronium ions (H3O+) that catalyse hydrolysis (Ruiz et al., 2013). Moreover, a neutralisation step is not necessary as the H+ ions are a function of temperature and, therefore, will naturally decrease when temperature decreases (Tolonen et al., 2011).

SBW has and continues to be widely studied as potential solvent to support pretreatment of biomass fractionation but and also to modify cellulose in order to make it more accessible to hydrolysis (Taherzadeh and Karimi, 2008). Subsequent cellulose hydrolysis, however, is usually performed by mineral acids or enzymes (Demirbas, 2009a) in processes that have significant drawbacks such as long time required for the reaction, problems with corrosion and

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acid disposal/recycling, cost of enzymes, and enzymatic inhibition by co-products generated during pretreatments (Carvalheiro et al., 2008, Vanholme et al., 2013).

The aim of this chapter is to assess the use of SBW as a ‘green’ solvent for cellulose hydrolysis into glucose using pre-processed MxG fibres (DEL and SEQ) in a biorefinery concept. Moreover, the objective is to develop an understanding of chemical reactions taking place during cellulose hydrolysis under SBW conditions as well as to evaluate the role of cellulose structure during hydrolysis in addition to develop an understanding of the impact of MxG direct and sequential extraction on glucose release and formation of fermentation inhibitors (HMF, furfural, etc.).