Role of zeolite properties in bio-oil deoxygenation and hydrocarbons production by catalytic cracking

Title : Role of zeolite properties in bio-oil deoxygenation and hydrocarbons production by catalytic cracking

Abstract:

The depletion of petroleum reserves and stricter environmental policies urge the search for sustainable and renewable energy sources to meet the growing global energy demand. In this scenario, lignocellulosic biomass is a renewable resource, whose valorization has attracted significant attention due to its carbon neutral lifecycle. Fast pyrolysis of biomass is a well-developed thermochemical process that produces high yield of liquid product (bio-oil), which is a complex mixture of oxygenated compounds including phenols, ketones, carboxylic acids, esters, aldehydes, alcohols, furans, and anhydrous-sugars. In comparison with catalytic hydrotreatment (HDO and hydrocracking), the catalytic cracking (CC) of bio-oil is cost-competitive in terms of process requirements since it operates at atmospheric pressure and without H₂ supply. Besides, the CC of bio-oil can be targeted towards the selective production of fuels (gasoline, diesel, jet fuel) and platform chemicals (olefins and aromatics), according to market requirements. The conversion of bio-oil oxygenates into hydrocarbons involves complex reaction pathways, such as deoxygenation (decarboxylation, decarbonylation, dehydration), cleavage of C–C bonds, oligomerization/cracking, alkylation, and hydrogen-transfer. A key factor for the successful large-scale implementation of this process is the development of selective catalysts. Zeolite-based catalysts have been widely used in the production of hydrocarbons by CC of oxygenates. Their behaviour is determined by the properties of the acid sites (Brønsted/Lewis nature, total amount and strength distribution). Pore size and structural properties are also important in regulating diffusion of reactants and products (shape selectivity), thereby affecting the selectivity to aromatics and olefins.

In this research, a comparison is established between the behavior of Y and ZSM-5 zeolites for producing hydrocarbons by catalytic cracking of bio-oil. Both zeolites have the same SiO₂/Al₂O₃ ratio (30), thereby inherent characteristics of each catalyst (microporous structure, nature of acid sites and acid strength distribution) are determined by the zeolite framework (FAU in Y zeolite, and MFI in ZSM-5). The catalysts were prepared by agglomerating each zeolite with a mesoporous g-Al₂O₃ matrix in order to attenuate deactivation by coke deposition and improve mechanical resistance. The raw zeolites and prepared catalysts were characterized by N₂ adsorption/desorption, SEM-EDX, pyridine-FTIR, tBA-TPD, and NH₃-TPD techniques. The experiments were carried out in a continuous two-step catalytic cracking system (TS-CC) at 450 °C. Both catalysts are highly selective to C₅-C₁₂ hydrocarbons, with a composition strongly affected by the porous structure and acidity of the zeolite. The HZSM-5 catalyst (high acid strength and Brønsted/Lewis ratio) promotes cracking and deoxygenation reactions that lead to C₂−C₄ olefins which condensate into aromatics, yielding 9% of 1-ring aromatics (3% BTX) and 4% of naphthalenes for a feed conversion of 85 %. These reactions are less favored over the HY catalyst (with large cavities located between the straight micropores), which promotes olefins oligomerization and hydrogen-transfer reactions, yielding 14% of highly aliphatic gasoline (> 30% linear paraffins) and 5% of C₁₃−C₂₀ diesel fraction (> 90% long chain paraffins). The remaining upgraded bio-oil obtained with both catalysts is composed of light oxygenates, with ketones, acids and esters as main components and low concentration of phenolic compounds.

Audience take-away:

• It addresses the valorization of real raw bio-oil, scarcely studied in the literature
• Conversion of stabilized bio-oil feedstock (with 20 wt% of methanol) is studied so that the findings would be applicable to the scaling-up of the process.
• Two original strategies that address the catalyst particle design (agglomeration of zeolite), and the reaction system (two-step catalytic cracking reactor) have been combined.
• Detailed composition of oxygenated liquid product is reported, which is essential for assessing its potential for further valorization (integral valorization of CC products)
• Provide new and wide information of raw bio-oil cracking through a comparative study of the behavior of two zeolites with similar total acidity but different framework.

Biography:

Dr. Valle received her PhD degree with honours in Chemical Engineering in 2008 at the University of the Basque Country (Spain). She is currently research fellow at the Catalytic Processes and Waste Valorization group (CPWV) of the Chemical Engineering Department at the same institution. Her research lines focus on heterogeneous catalysis, reaction mechanism/kinetics, valorization of biomass derived oxygenates and pyrolysis oil into H2, fuels and value-added chemicals. She has directed two Doctoral Theses, published 57 research articles in international peer reviewed journals (39 Q1, 20 first author, h-index 28) with more than 2470 total citations. She is co-author of 105 contributions (45 oral and 60 poster) in national and international conferences.