Contribución al desarrollo de nuevas tecnologías para biorrefinerías

Resum

During the past few years, the "circular economy" concept is being considered as a possible strategy to face the problems derived from the current model of economic development, characterized by a huge consumption of non-renewable resources. The manufacture of large quantities of fuels, chemicals, and materials from fossil resources poses multiple challenges, including the ones related to their dwindling nature, the increase in the world population with rising per capita demand, and environmental problems (particularly, the global warming caused by greenhouse gas emissions). A sustainable development requires the replacement of fossil resources with renewable ones, as well as improved recovery and reuse of wastes. These ideas confirm the interest in designing efficient, environmentally friendly, and sustainable processes based on renewable raw materials, suitable for manufacturing products with the highest added value. In this context, the utilization of vegetal biomass (and particularly, lignocellulosic materials, LCM) as feedstocks for new technologies allowing the manufacture of fuels, chemicals and materials provides an alternative to conventional processes based on fossil resources. Some of the processes proposed for the chemical utilization of LCM involve consecutive "fractionation" stages, after which the major components (or the products derived from them) appear in separate streams, and can be employed for specific purposes. The main objective of this PhD Thesis is focused on the development of new technologies for the chemical utilization of woods, including both the fractionation stages and the production of commercial compounds from the resulting fractions, following the biorefinery philosophy. This study has been done in the framework one of the research on the development of chemical processes for the use of vegetal biomass subjects currently developed by the EQ-2 research group (which belongs to the Department of Chemical Engineering of the University of Vigo). Most of the experimental part has been performed at the Campus of Ourense, following the experimental tasks outlined in two Research Projects: "Advanced processing technologies for biorefineries" (reference CTQ2014-53461-R, funded by the Ministry of Economy and Competitivity within the National Program of Research, Development and Innovation Oriented to the Challenges of the Society) and "Modified aqueous media for wood biorefineires" (reference CTQ2017-82962-R, funded by the Ministry of Science, Innovation and Universities within the National Program of Research, Development and Innovation Oriented to the Challenges of the Society). Lignocellulosic materials According to the data from the latest "Assessment of World Forest Resources" (FRA) carried out in 2015, there are 3999 million hectares (equivalent to 30.6% of the Earth's surface) occupied by vegetal biomass worldwide. Most of the forest area corresponds to natural forests, and accounts for 93% of the world's forest area. The world generation rate of vegetal biomass has been estimated at 146·109 metric tons per year (Balat and Ayar, 2005). LCMs account for 80% of the total vegetal biomass, and represent the most abundant renewable resource available in earth (Hon, 2000; Zheng et al., 2015). LCM are made up of polysaccharides (cellulose and hemicelluloses) and a phenolic fraction (lignin). In the cell wall, cellulose has a structural function, lignin acts as a natural adhesive and protective agent, and hemicelluloses act as a bonding interface between cellulose and lignin (Alonso et al., 2012; Kobayashi and Fukuoka, 2013; Ten and Vermerris, 2013; Lee et al., 2014). The transformation of LCM into chemical compounds, fuels and materials would reduce the consumption of fossil resources, resulting in the development of supply chains of sustainable products (Jefferson, 2006; Clark, 2007; Watanabe, 2013). Because of availability and accessibility reasons, Pinus pinaster and Eucalyptus nitens woods have been selected as raw materials in this work. The world surface occupied by P. pinaster is estimated at around 4.4 million ha (Vignote, 2014). In Galicia, this species occupies 467,351 ha (equivalent to 23% of the forest area). The province of Ourense stands out in terms of surface occupied by pine forests (151,336 ha) (Sanz et al., 2006). E. nitens is widely distributed in Spain, and particularly in Galicia. This species shows the advantage of resisting environmental conditions unsuitable for the cultivation of other Eucalyptus species: for example, it is able to grow in mountainous areas, with not too high temperatures in summer and cold winters (with frost or snow) (Pérez et al., 2006; Yáñez-S et al., 2018). Fractionation of lignocellulosic materials The fractionation of LCM requires the depolymerization of some of the structural components. The choice of a suitable fractionation treatment must be done on the basis of the diverse chemical properties of the LCM structural components. The LCM fractionation must be selective, so that the separation of a given component should result in little alteration of the others. The component of interest must be recovered at high yield, and the overall process must include the minimum number of stages, in addition to being economically viable (Liu et al., 2012). Selective elimination of hemicelluloses The solubilization of hemicelluloses can be achieved through different methods, including treatments with water, with acids or with alkaline agents. Treatments with hot, compressed water (also termed autohydrolysis or hydrothermal processing) allow the selective solubilization of hemicelluloses, leading to the formation of soluble oligomers, sugars, decomposition products of monosaccharides and acetic acid as major reaction products. The solid fraction obtained in the autohydrolysis treatments is enriched in cellulose and lignin. These components that can be further fractionated by suitable procedures: for example, cellulose can be hydrolyzed in media containing acids or enzymes, while lignin can be solubilized by the action of different chemical agents. In this study, organic acids and ionic liquids (ILs) have been used for this purpose. An overall benefit of woods can be achieved by coupling autohydrolysis and delignification stages. This method has been considered in this work, and the results are included in the articles presented in the various Appendixes. Samples of P. pinaster and E. nitens woods were subjected to autohydrolysis, in order to solubilize their hemicellulosic fractions selectively. The experiments were carried out under selected conditions (at 175 ° C for 26 min in the case of P. pinaster, and at 195 ° C with immediate cooling after reaching this temperature in the case of E. nitens), to split part of the glycosidic bonds in the hemicellulosic polymers. The depolymerization reactions are catalyzed by hydronium ions coming from the autoionization of water and acids generated in situ (including acetic acid, uronic acids and phenolic acids). Lignin removal The main objective of delignification treatments is to depolymerize and solubilize lignin, keeping the cellulosic fraction in solid phase with little alteration. There are several alternatives to the traditional delignification methods (kraft and sulfite methods), including the use of concentrated solutions of acetic acid (which acts as an organosolvent) or media containing ILs. The use of this type of media can limit environmental problems, while allowing the development of new products (Manfredi et al., 2014). The Acetosolv delignification uses concentrated acetic acid solutions catalyzed by a mineral acid (usually, HCl). This method allows high delignification extents, operating under mild conditions. In this Thesis, the solids resulting from wood autohydrolysis were employed as substrates for Acetosolv delignification. The treatments were perfomed under the following conditions: in experiments with P. pinaster wood previously subjected to autohydrolysis, operation was conducted at 121 °C for 180 min; whereas in experiments with E. nitens woods previously subjected to autohydrolysis, the reaction was performed at 134 ° C for 30 min. ILs are salts composed of organic cations and organic or inorganic anions, which have special characteristics regarding their chemical nature, structure, organization and properties. ILs are an alternative to conventional solvents in multiple applications, including the chemical transformation of vegetal biomass. Depending on their chemical nature, ILs have diverse application fields, including delignification, cellulose dissolution, manufacture of solid substrates susceptible to enzymatic hydrolysis, and production of commercial compounds (such as furans and organic acids). Different operational methods have been followed in this study, using: a) 1-butyl-3-methylimidazolium hydrogensulfate ([bmim]HSO4), an acidic IL that acts simultaneously as a reaction medium and a catalyst; and b) triethylammonium hydrogensulfate ([TEA]HSO4), which stands out for its dissolution capacity and thermal stability (Peleteiro et al., 2016b; Gschwend et al., 2018). Cellulose hydrolysis Cellulose hydrolysis can be carried out using concentrated or dilute mineral acids (Orozco et al., 2011), or using enzymes able to break the β-(1,4) glycosidics bonds in the polymer. When the hydrolysis of cellulose is catalyzed by concentrated acids, the crystalline structure is destroyed by breaking the hydrogen bonds existing between molecules, and oligosaccharides are generated by hydrolysis of the polymer chains. In reactions with dilute acids, the glycosidic bonds are split to yield low molecular weight polymers, oligomers and glucose, but degradation products, such as 5-hydroxymethylfurfural or HMF, are generated. The reactions carried out with concentrated acids require short reaction times and can proceed at moderate temperatures, minimizing the degradation reactions in comparison with the hydrolysis catalyzed by dilute acids. However, the recovery of the acid presents important technical problems. Enzymatic methods show the advantage of being able to be carried out at moderate temperatures and pressures in non-corrosive media, allowing savings in operating costs and equipment. However, this method presents other drawbacks, such as the cost of enzymes or the prolonged reaction times in comparison with the ones needed to carry out the acid hydrolysis. Manufacture and characterization of compounds resulting from fractionation Depending on the composition of the raw material considered, the fractionation treatments yield streams with different compositions and characteristics, which control the type of products that can be obtained from them. In this work, the soluble hemicellulose-derived products obtained by autohydrolysis of pine wood were used as substrates for producing furanic compounds. The dehydration of hexoses and pentoses in acidic media leads to the formation of furans (HMF and furfural, respectively). In turn, HMF can be rehydrated to form equimolar amounts of levulinic and formic acids. The US Department of Energy has rated HMF as one of the top 14 "green" ("Top 10 + 4") chemicals that can be produced from biomass; whereas furfural is one of 30 "green" chemicals holding a greater potential. The global demand for furfural shows a growing trend, and is expected to double in the 2014-2022 period (Peleteiro et al., 2016b). HMF and furfural are important biobased “building blocks”, from which a number of products can be obtained. These products are suitable for multiple applications in diverse fields, including the manufacture of polymers or fuels (Treichel et al., 2020). In both types of applications, the biomass-derived products can replace specific commercial compounds currently produced from oil. The most used hexoses employed as substrates for obtaining HMF are glucose and fructose, although galactose and mannose have also been used (Binder et al., 2010; Peleteiro et al., 2014; Penín et al., 2017). In practice, for economic and technological reasons, the best choice seems to start from native raw materials (or fractions isolated from them). These feedstocks must contain the above mentioned hexoses as structural units, in a way that their hydrolysis-dehydration can allow the development of new, profitable, "green", and sustainable processes for HMF manufacture. This goal can be achieved by treating monosaccharides, oligosaccharides and/or polysaccharides in media containing ILs (Esposito and Antonietti, 2015). Following this idea, the ILs can act as reaction media, as catalysts, or accomplishing both functions simultaneously (Penín et al., 2019b). The results obtained in this Thesis dealing with the production of HMF are included in Appendixes E and F. In a first experimental stage, and in order to allow a comparative evaluation of the results, glucose and mannose (two hexoses appearing as structural units in pine wood hemicelluloses) were employed as substrates for HMF, in a set of experiments allowing to measure the effects of the reaction conditions on the generation of the target product. For this purpose, reaction media containing [bmim]Cl, a zeolite (acting as a catalyst) and/or external co-catalysts were employed. Under the best conditions assayed, 72.1% and 67.7% conversion of glucose and mannose into HMF was reached, respectively. Additional experiments were carried out to assess the production of HMF from hemicellulosic saccharides and cellulose-enriched solids obtained by autohydrolysis-delignification of P. pinaster wood. The results (which are explained in detail in Appendix F) allowed the identification of operational conditions leading to 32.8% conversion of hemicellulosic saccharides into HMF, and 52% conversion of potential substrates in the cellulose enriched solid into the target product. Once the hemicellulosic fraction has been separated from the lignocellulosic feedstock, the resulting solid (enriched in lignin and cellulose) can be further fractionated for separating these components. Lignin solubilization from the autohydrolyzed solid by delignification reactions allow the separation from cellulose (which remains in solid phase), depicting a scheme enabling the individual benefit of both fractions. In this Thesis, delignification was carried using the Acetosolv method and media containing ILs. The most relevant results are collected in the articles included in Appendixes B, D, F and G. The first of these studies was focused on the identification of optimal delignification conditions for the Acetosolv delignification of autohydrolyzed E. nitens wood. Operating under optimal conditions (which led to 90% delignification of the feed), a solid of high cellulose content was obtained. This solid was used as a substrate to carry out the enzymatic hydrolysis of the cellulosic fraction using commercial enzyme complexes. Operating under selected conditions, cellulose conversions into glucose above 88% were obtained. The glucose solutions obtained using this method can be used in the formulation of fermentation media, which in turn may be oriented to the production of scope of products (including biofuels, among which ethanol stands out for its quantitative importance) (Watanabe, 2013). Additionally, to the Acetosolv method, delignification methods based on the use of ILs were considered in this study. The articles included in Appendixes F and G summarize the data obtained by treating autohydrolyzed pine and eucalyptus wood in media containing [bmim]HSO4 or [TEA]HSO4. When delignification of autohydrolyzed P. pinaster wood was carried out under selected conditions in media containing [bmim]HSO4, 81.7% of the lignin present in the feed solid was solubilized. In comparison, the delignification of autohydrolyzed E. nitens wood in media containing [bmim]HSO4 or [TEA]HSO4 enabled the removal of 87.7% or 69% of the initial lignin, respectively. Based on the above information, a deeper study on the delignification of eucalyptus wood using ILs was performed. The corresponding results of which are described in detail in the article included in Appendix D. Empirical models developed from the experimental data allowed a quantitative interpretation of key aspects such as yields, compositional changes and selectivity. The IL [bmim]HSO4 was found to be an excellent delignification agent, allowing the manufacture of solids of low residual lignin content. The information included in the above paragraphs refers to the use of the polysaccharide fractions (cellulose and hemicelluloses) present in woods, and to the solubilization of lignin in different reaction media. In this Thesis, other important aspects affecting the overall utilization of the woods have been considered: the recovery, characterization and reactivity of the lignins obtained in the different fractionation methods. The importance of this part of the study relies on the fact that lignin (one of the structural components of woods) accounts for a very important part of the wood mass, and represents the largest natural source of aromatic compounds. Its use as a raw material for chemical transformation is also attractive for its renewable and “green” nature (Crestini et al., 2017; Dessbesell et al., 2017). However, lignin utilization is a complex problema, not only due to the characteristics of the native polymer, but also because its separation from LCM involves chemical reactions that affect the properties of the resulting products. The structural complexity of lignins makes their controlled depolymerization very difficult. Many depolymerization reactions involve the breakdown of β-O-4 bonds, which are susceptible to hydrolysis (Sun et al., 2018). Furthermore, temperature and time are very influential on the properties of the products, not only because they affect depolymerization, but also because the resulting fragments can undergo repolymerization reactions under conditions of excessive severity (Abdelaziz et al., 2018). Lignins obtained under the conditions considered optimal for each of the fractionation procedures described above were characterized in depth using an array of analytical techniques, as described in detail in the article included in Appendix G. The results allowed the identification of the predominant structures present in the fractions isolated from the diverse reaction media. Thus, the E. nitens lignin obtained in treatments with [bmim]HSO4 presented a molecular mass distribution dominated by small compounds, with a limited content of aliphatic -OH groups. In comparison, the lignin obtained in treatments with [TEA]HSO4 preserved better the general structure of native lignin, providing products with a lower degree of chemical alteration (a fact suggesting a greater potential as a substrate for further utilization). In order to obtain products suitable for a wide range of applications (such as the production of chemicals, biofuels, polymers, or materials), the lignin recovered from fractionation treatments must be processed to achieve further modification. The structure of the lignin fragments resulting from fractionation controls the type of possible applications. Among these latter, the production of simple phenols and BTX (benzene-toluene-xylene) chemical can be highlighted, as these products can replace other compounds currently produced by the petrochemical industry (Gillet et al., 2017). Oxidative depolymerization is one of the alternatives considered in the literature for the chemical modification of lignins. In the presence of oxidizing agents (such as oxygen, hydrogen peroxide or peroxyacids), operating under mild conditions, the substrates can undergo chemical modifications, such as the increase in the relative abundande of functional groups (hydroxyl, aldehyde and carboxylic). The reaction requires a careful control of operational conditions to avoid repolymerizations (Ma et al., 2015; Behling et al., 2016; Sun et al., 2018). Examples of commercial products obtained by oxidation of lignins include vanillin and syringaldehyde (an important precursor in the pharmaceutical industry) (Gillet et al., 2017); in addition to other products used in biochemical reactions, such as guaiacol, catechol, benzoates or p-coumarates (Becker and Wittmann, 2019). Appendix H includes the experimental data regarding the oxidative depolymerization of lignins isolated from the fractionation media described in previous paragraphs. In this study, oxidative depolymerization reactions of lignin fractions obtained by fractionation of pine and eucalyptus woods were carried out using two different catalytic systems (based on the use of vanadate or molybdate, respectively), in media also containing hydrogen peroxide, perchloric acid and water. Compounds of commercial value were identified in the various reaction media. The most abundant compounds resulting from the oxidative depolymerization pine wood lignins were vanillin and vanillic acid; while the lignin fractions obtained from eucalyptus woods yielded syringaldehyde and vanillin as major products.