Catalytic fast-pyrolysis of lignocellulosic residues for advanced biofuels production: development of multifunctional catalysts, optimisation and bench-scale demonstration

Abstract

The present Doctoral Thesis has been developed in the laboratories of the Thermochemical Processes Unit of IMDEA Energy Institute, in the research line of “Sustainable fuels”, focused on the advanced biofuels production from lignocellulose through the catalytic fast pyrolysis process. Likewise, this scientific work has been carried out within the framework of CASCATBEL Project, “CAScade deoxygenation process using tailored nanoCATalysts for the production of BiofuELs from lignocellullosic biomass” (Ref. 604307), financed by the European Union Seventh Framework Programme (FP7/2007-2013). First generation biofuels have shown important drawbacks to replace the traditional fuels and climate change mitigation, besides the biofuel versus food controversy. Lignocellulosic biomass, consisting in three main components: cellulose, hemicellulose and lignin, may hold the key for the sustainable advanced biofuels production for transportation. Due to the high complexity of the lignocellulose, this material requires a high cost of processing. In this field, biomass fast pyrolysis is a promising technology, being able to produce a high throughput of liquid fuels. This process consists in a thermal decomposition of the lignocellulose in inert atmosphere to yield non-condensable gases, a solid residue denoted as char and a liquid called bio-oil, which can be used as fuel. The use of high heating rates, moderate temperatures (500-550 ºC) and short residence times for the vapors, maximises the production of bio-oil. The bio-oil presents a high potential as liquid fuel, being able to retain up to 70% of the chemical energy initially contained in the biomass. However, the bio-oil posseses important limitations, which hinders its direct use in transportation, especially regarding its high oxygen content, strong corrosiveness (pH = 2-4), low physicochemical stability and less than half of the high heating values of traditional fuels. Hence, pyrolytic oils require an upgrading for that purpose. Among the technologies allowing this necessary chemical transformations, catalytic pyrolysis of biomass stands out. By the incorporation of a catalyst, commonly a solid acid and more specifically a zeolite, a series of secondary reactions can be promoted, including cracking, oligomerisation, cyclisation, aromatisation and deoxygenation reactions, among others. The deoxygenation may proceed through the formation of CO (decarbonylation), CO2 (decarboxylation) and H2O (dehydration). Within this context, the present work aims to study the catalytic performance of different zeolites with enhanced textural properties and therefore, lesser diffusional limitations, finely modified by the incorporation of metal oxide nanoparticles over their surface (ZnO, MgO and ZrO2). These modifications allow the acidity and basicity of the parent zeolites to be tuned, adding new functionalities and hence, opening new interesting reaction pathways. Thus, the present work was divided in the next chapters tackling different topics concerning the ex-situ catalytic pyrolysis, aiming to a better understanding of the process mechanisms and the catalyst design, from laboratory to bench-scale. I. Effect of indigenous and external catalysts in the catalytic fast-pyrolysis of biomass. In this chapter the different types of reactions that take place in the catalytic pyrolysis are decoupled for their analysis, including intrinsically thermal and catalytic reactions, by both indigenous (mineral matter naturally presented in the biomass) or external materials (n-ZSM-5 zeolite). The experimental results showed that the incorporation of both types of catalysts diminished the bio-oil yields. However, while n-ZSM-5 conducted an effective decrease of the bio-oil oxygen concentration, this parameter was barely affected by the presence of minerals, since these promoted the formation of additional char and the deoxygenation was mostly produced over this solid fraction. Changes in the route of deoxygenation were also observed, being CO2 the main pathway for mineral components, while CO was the major oxygenated molecule produced over n-ZSM-5. Regarding the chemical composition of the bio-oils, both resulted in considerable depolymerisation of lignin oligomers and high conversion of furans and anhydrosugars. n-ZSM-5 was the only of both catalysts able to convert these molecules into non-oxygenated aromatics. II. Performance of different catalysts in the pyrolysis vapors upgrading. This chapter is devoted to the study of promising catalysts with enhanced properties (MCM-22, hierarchical Beta and ZSM-5 and 2-dimensional ZSM-5’s zeolites), some of them modified by metal oxides deposition (ZnO and MgO). This chapter proves that zeolites of improved accessibility and medium acidity are suitable and promising catalysts for this process. Among the studied structures, the medium pore size of the ZSM-5 seems to produce a higher selectivity towards non-oxygenated aromatics. In addition, the modification of the type and strength of the active sites with the metal oxides, suppressing the strongest Brønsted acidity and generating additional Lewis acid/basic sites, resulted in more selective deoxygenation pathways. Thus, for hierarchical ZSM-5, the incorporation of MgO nanoparticles produced a bio-oil with around 28 wt% oxygen concentration and retaining about 45% of the chemical energy from biomass, the best result of all the studied in this chapter. III. Optimisation of the nanocrystalline ZSM-5 system with de-ashed biomass: Effects of the temperature, catalyst to biomass ratio and fine tuning of the acidity and accessibility. In the first part of this chapter, an extensive study is deployed regarding the temperature influence in both thermal and catalyst bed zones affecting the catalytic fast pyrolysis over a MFI zeolite with improved external surface area. Aiming to scale-up the process afterwards, the catalyst selected this time was a commercial nanocrystalline ZSM-5 (n-ZSM-5). Different catalyst loadings were tested in order to obtain better information about the reaction pathways. The acidic properties of this zeolite were modified by the amphoteric properties of the ZrO2, incorporated by wet impregnation. It was also compared with other commercial ZSM-5 with additional mesoporosity, this time by desilication (h-ZSM-5). The ZrO2 incorporation to the n-ZSM-5 sample showed the most promising results, minimising the occurrence of overcracking and polymerisation reactions, leading to a more selective deoxygenation. Additionally, this supported active phase promoted the conversion of larger oligomers, increasing the fraction of the bio-oil detectable by the GC-MS technique. IV. On the road to industry: Study of the effect of clay binders to produce technical catalysts and scaling-up. With the goal of producing technical catalysts from the previously studied ZrO2/n-ZSM-5 catalyst, the influence of two clay binders: bentonite (BNT) and attapulgite (ATP) in its physicochemical properties and reaction performance is investigated. The lesser suppression of the Brønsted acidity and the emergence of new Lewis acid/basic sites in the catalyst extruded with attapulgite, caused by Zr-clay interactions, resulted in positive synergetic effects regarding the bio-oil deoxygenation selectivity and oligomer conversion. The promising results of the ZrO2/n-ZSM-5-ATP multifunctional catalyst was deeply studied in laboratory and demonstrated at bench-scale, leading up to 60% deoxygenation degree and energy yield of approximately 70% with respect the starting bio-oil coming from the thermal reference reaction.