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Towards the Valorization of Humin By-Products: Characterization, Solubilization and Catalysis

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Towards the Valorization of Humin By-Products: Characterization, Solubilization and Catalysis Towards the Valorization of Humin By-products: Characterization, Solubilization and Catalysis Bijdrage aan de Valorisatie van Humine-achtige Bijproducten: Karakterisatie, Oplossen en Katalytische Omzetting (met een samenvatting in het Nederlands) Proefschrift ter verkrijging van de graad van doctor aan de Universiteit Utrecht op gezag van de rector magnificus, prof.dr. G. J. van der Zwaan, ingevolge het besluit van het college voor promoties in het openbaar te verdedigen op maandag 16 maart 2015 des ochtends te 10.30 uur door Ilona van Zandvoort geboren op 3 augustus 1986 te Velsen Promotor: Prof. dr. ir. B. M. Weckhuysen Copromotor: Dr. P. C. A. Bruijnincx This research has been performed within the framework of the CatchBio program. The authors gratefully acknowledge the support of the Smart Mix Program of the Netherlands Ministry of Economic Affairs and the Netherlands Ministry of Education, Culture and Science. Voor mijn ouders Life is what happens to you while you’re busy making other plans John Lennon ISBN: 978-90-393-6291-4 TABLE OF CONTENTS CHAPTER 1 General Introduction 7 CHAPTER 2 Formation, Molecular Structure and Morphology of Humins in 39 Biomass Conversion: Influence of Feedstock and Processing Conditions CHAPTER 3 Full, Reactive Solubilization of Humin By-products by Alkaline 61 Treatment and Characterization of the Alkali-treated Humins Formed 13 CHAPTER 4 Structural Characterization of C-Enriched Humins and Alkali- 81 13 treated C Humins by 2D Solid-state NMR CHAPTER 5 Aqueous Phase Reforming of Solubilized Glucose-derived Humins 103 over Supported Pt Catalysts CHAPTER 6 Unexpected Benefits of Using Real Feeds in Biomass Conversion: 121 High Hydroxymethylfurfural Selectivity and Low Humin Formation in the Acid-catalyzed Dehydration of Sugar Beet- derived Thick Juice CHAPTER 7A Summary and Concluding Remarks 145 CHAPTER 7B Samenvatting en Slotbeschouwing 157 LIST OF ABBREVIATIONS 169 LIST OF PUBLICATIONS 170 DANKWOORD 172 CURRICULUM VITAE 176 CHAPTER 1 General Introduction Abstract During the acid-catalyzed dehydration of carbohydrates for the production of renewable bulk chemicals, such as furfural, hydroxymethylfurfural (HMF) and levulinic acid, large amounts of carbonaceous, insoluble by-products are typically formed by cross- polymerization reactions of HMF and several sugar-dehydration intermediates. The formation of these so-called humins leads to great efficiency losses in biorefinery operations. Indeed, a literature overview of conditions, kinetics and mechanisms of humin formation shows that hydrothermal treatment of sugars under acidic conditions unavoidably leads to the formation of humins. Although produced in abundance, the molecular structure of humins and their mechanism of formation are not yet unequivocally established. Detailed knowledge of both is required, however, to either prevent their formation or to find routes for the valorization of humins, which would both increase the economic feasibility of future biorefineries. This chapter summarizes the literature describing biorefinery concepts that involve humin formation, as well as those studies concerned with the (mechanism of) formation, characterization and valorization of humins. CHAPTER 1 1.1. Biomass as a Resource for the Production of Chemicals and Energy For ages, mankind has been using renewable resources for heat and energy, e.g . burning of wood. However, since the industrial revolution the use of unrenewable fossil resources has increased, for instance by burning gas and oil-based fuels for transportation (Figure 1.1). This has led to global warming issues due to the emission of greenhouse gases, such [1,2] as CO 2, and depletion of fossil reserves while energy demands keep increasing. At the same time we also rely on oil for the production of bulk chemicals and materials. [3] To limit adverse greenhouse effects, several international agreements are in place that aim at reducing CO 2 emissions for instance, by replacing fossil fuels by biomass-derived alternatives. For example, the European Commission set as a goal that more than 20% of the liquid transportation fuels used should be derived from biomass by 2020. [4] In the same year, greenhouse gas emission levels should be reduced by 20% compared to 1990, as stated in the Kyoto protocol. [5] These pressing environmental issues and associated regulations have led to many research efforts aimed at the development of processes for renewable energy and chemicals. Figure 1.1. World energy consumption from 1965 till 2010 including the available energy sources, taken from the BP Energy Outlook. [6] Biomass is currently the only sustainable resource for the large scale production of carbon-based chemicals and fuels. The first-generation biobased fuels that are being produced nowadays are derived from those plant fragments that are easily processed. Well-known examples are bioethanol from corn starch or sugar cane and biodiesels from fatty acids. To avoid competition with food production, second-generation biomass-based fuels and chemicals, for which the production facilities are gradually coming online at the moment, are derived from the non-edible parts of biomass instead. For such second- generation production processes, lignocellulosic biomass is obtained from agricultural 8 General Introduction residues, forestry residues, waste streams or dedicated energy crops, such as switch grass.[7,8] Lignocellulosic material is found in the plant cell wall, and consists of cellulose, hemicellulose and lignin (Figure 1.2). [7,9] Cellulose is a linear polymer consisting of glucose- monomers that form a chain of anhydroglucopyranose units that are connected by β-1,4- glycosidic bonds. A network of H-bonds is formed between parallel cellulose strands leading to the formation of microfibrils that give cellulose its high crystallinity and recalcitrance. Hemicellulose, on the other hand, is a branched polymer consisting of a mixture of C 5- and C 6-sugars and is found between the cellulose microfibrils and lignin. Its amorphous nature and readily accessible OH-groups make it the most reactive part of lignocellulosic biomass. Lignin, finally, is an amorphous phenolic polymer and responsible for the rigidity of the cell wall. [1,2] Valorization of the lignocellulosic biomass can either involve direct conversion of the whole biomass in one single process, or, alternatively, the biomass is pretreated and fractionated to its main components which are subsequently individually processed. These processes can be applied to produce low-value fuels or high- value chemicals from lignocellulosic biomass in a so-called biorefinery. Figure 1.2. Composition of lignocellulosic biomass including its main components cellulose, hemicellulose and lignin and there relative abundances. [10] Adapted from [11–13] with permission. 9 CHAPTER 1 1.2. Biorefinery Operations The use of biomass as a resource for the production of chemicals and fuels requires the development of integrated conversion facilities called biorefineries where renewable materials are converted to low-value, high-volume fuels and high-value, low-volume chemicals, analogous to current oil refineries.[8] An important challenge for biorefinery operations is the production of bio-based fuels and chemicals in an economical, ethical and environmentally friendly way. This requires complete and efficient valorization of non-edible feedstock. Many different individual thermochemical, chemo- and biocatalytic conversion steps have been proposed to convert biomass into value-added products together with many different integrated conversion routes, of which some examples are given below. [1,7,9,14] The first option is to convert the complete lignocellulosic biomass by thermochemical techniques. For instance by gasification to H 2 and CO, which can in turn be used for the production of alkanes by Fischer-Tropsch synthesis, among various other outlets. The gasification process can be applied for any kind of lignocellusic biomass; further processing of the syngas requires extensive purification, though, as impurities and water content of the feed can, for example, poison the Fischer-Tropsch catalyst. [10,15] Pyrolysis or liquefaction of complete biomass, on the other hand, is a cheap alternative to convert different types of biomass into a complex bio-oil. This product is, however, not suitable for combustion and requires extensive upgrading, e.g. by hydrodeoxygenation reactions, before it can be used as fuel. [10] The second strategy for biomass conversion entails fractionation of the biomass into the three main components, which allows for the selective production of bulk chemicals or fuel. The lignocellucosic biomass can be fractionated by chemical or physical methods, such as milling, extraction or hydrolysis.[1,10] Complete valorization of the lignin fraction is still challenging, but this polymer could eventually be used for the production of aromatic compounds, such as benzene and phenols, or carbon fibers. [1,7,16] Valorization of the carbohydrate fractions is much further developed and typically consists of hydrolysis of the polymers to the individual sugar monomers, which are subsequently submitted to hydrogenolysis to produce polyols and alkanes,[17] fermentation to ethanol [1] or acid- catalyzed conversion for the production of renewable platform molecules such as hydroxymethylfurfural (FA), furfural (FF) and levulinic acid (LA). [7,10] The acid-catalyzed dehydration of (hemi)cellulose chemicals is favored over enzymatic treatment since higher (space
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