Lignin as a source of phenolic compounds: from lignin extraction to its transformation by different routes A dissertation presented by Javier Fernández Rodríguez In Fulfillment of the Requirements for the Degree Doctor of Philosophy in Renewable Materials Engineering by the University of the Basque Country UPV/EHU Under the supervision of Dr. Jalel Labidi Dr. María González Alriols Chemical and Environmental Engineering Department Engineering School of Gipuzkoa Donostia-San Sebastián 2020 (c)2020 JAVIER FERNANDEZ RODRIGUEZ “Dalli qui nu canta, verdi qui nu livanta” II Summary In the last decades, considerable interest has been put in using lignocellulosic streams, which have been traditionally considered as wastes, to be converted into value-added products, such as fuel, chemicals and biomaterials, which are currently obtained from fossil sources. Lignin, the most plenty polymer as an aromatic source in nature has been traditionally considered as a by-product or side stream from pulp and paper process, although lignin commercialization as a source of phenolic compounds has gained more and more relevance lately. However, several drawbacks have to be still overcome, such as the high polydispersity and high content in impurities of the obtained lignin samples, which lead to generate a recalcitrant behavior that hinders its transformation processes into high value- added chemical compounds. Lignin-based products must be competitive with their petroleum-derived counterparts. Hence, it is very important to design energetically efficient processes for lignin extraction and purification. For this purpose, lignin-based products have to be assumed as a section of an integrated biorefinery where multiple products are obtained and in this line being able to compete in a realistic scenario. In this work, the identification of different routes to extract lignin and its further conversion into small phenolic compounds has been developed. All approached scenarios were proposed to build up robust processes capable to reduce the high variability of the lignocellulosic process designed to obtain chemicals or building blocks. Lignin samples from different lignocellulosic sources (blue agave plant, almond shell and olive tree pruning), delignification processes (soda or organosolv) and process stages (lignin from bleaching stages) were extracted and characterized physico-chemically to define the best alternatives for obtaining different products. Thereafter, several strategies were considered to depolymerize the lignin molecule into small phenolic monomers, such as catechol and its derivatives compounds, cresols, syringol, guaiacol, etc. These depolymerization reactions were applied to the obtained lignin samples collected from the extraction processes. Moreover, an innovative alternative to break down lignin directly from the black liquors obtained during the lignocellulosic biomass delignification process was also approached. Finally, the techno-economic analysis of the lignin extraction and depolymerization processes was conducted to select the most suitable routes in terms of product yields, environmental wastes, energy demands, and economic balance. Javier FERNÁNDEZ III Content Summary .................................................................................................................................... III Aim and Scope ................................................................................................................................. 3 Aim and scope ............................................................................................................................. 3 I) Introduction .............................................................................................................................. 1 1.1. Background ........................................................................................................................... 7 1.2. Biorefineries .......................................................................................................................... 8 1.2.1. Biorefineries classification ........................................................................................... 10 1.3. Lignocellulosic biomass ...................................................................................................... 11 1.4. Lignin chemistry .................................................................................................................. 14 1.5. Lignin source ...................................................................................................................... 18 1.5.1. Sulfite process ............................................................................................................. 18 1.5.2. Kraft process ............................................................................................................... 20 1.5.3. Alkaline process .......................................................................................................... 21 1.5.4. Organosolv process .................................................................................................... 22 1.5.5. Innovative processes ................................................................................................... 23 1.6. Lignin depolymerization ...................................................................................................... 24 1.6.1. Base- or acid-catalyzed depolymerization ................................................................... 26 1.6.2. Reductive catalytic depolymerization .......................................................................... 27 1.6.3. Oxidative depolymerization of lignin ............................................................................ 29 1.6.4. Solvolytic and thermal depolymerization ..................................................................... 30 II) Lignin extraction and characterization ............................................................................. 35 2.1. Motivation and objectives ................................................................................................... 35 2.2.1. Blue agave bagasse .................................................................................................... 35 2.2.2. Almond shells .............................................................................................................. 37 2.2.3. Olive tree pruning ........................................................................................................ 37 2.3. Lignin extraction from bleaching stages (Publication I) ...................................................... 39 2.3.1. Experimental procedure .............................................................................................. 40 2.3.2. Raw materials characterization ................................................................................... 41 V 2.3.2.1. Physico-chemical characterization of lignin samples ................................................ 42 2.3.2.2. Lignin extraction yields ............................................................................................. 50 2.4. Influence of pretreatment on lignin composition .................................................................. 51 2.4.1. Physico-chemical characterization of lignin samples ................................................... 52 2.4.2. Lignin extraction yields evaluation ............................................................................... 54 2.5. Influence of the delignification method and the lignin isolation process on the lignin characteristics (Publications II and III) ................................................................................... 55 2.5.1. Process description...................................................................................................... 56 2.5.2. Influence of the pretreatment stage in the solid fraction .............................................. 58 2.5.3. Quantification and physico-chemical characterization of selectively precipitated lignin samples.................................................................................................................................. 60 2.6. General conclusions ............................................................................................................ 68 III) Lignin depolymerization ................................................................................................... 71 3.1. Motivation and objectives .................................................................................................... 73 3.2. Solid lignin depolymerization by BCD (Publication II) ......................................................... 74 3.2.1. Thermochemical lignin depolymerization ..................................................................... 74 3.2.1. Experimental procedure ............................................................................................... 78 3.2.2. Lignin depolymerization yields by BCD........................................................................ 79 3.3. Direct lignin depolymerization from black liquors (Publication III) ....................................... 86 3.3.1. Experimental approach ................................................................................................ 87 3.3.2. Characterization of the black liquors ............................................................................ 87 3.3.2. Liquor depolymerization yields....................................................................................