catalysts Review Recent Catalytic Advances in Hydrotreatment Processes of Pyrolysis Bio-Oil Giuseppe Bagnato 1 , Aimaro Sanna 2 , Emilia Paone 3, * and Enrico Catizzone 4   Citation: Bagnato, G.; Sanna, A.; Paone, E.; Catizzone, E. Recent Catalytic Advances in Hydrotreatment Processes of Pyrolysis Bio-Oil. Catalysts 2021, 11, 157. https://doi.org/10.3390/ catal11020157 Received: 28 December 2020 Accepted: 21 January 2021 Published: 23 January 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 School of Chemistry and Chemical Engineering, Queen’s University Belfast, David Keir Building, 39-123 Stranmillis Rd, Belfast BT9 5AG, UK; G.Bagnato@qub.ac.uk 2 Advanced Biofuels Lab, Institute of Mechanical, Process and Energy Engineering, School of Engineering & Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, UK; A.Sanna@hw.ac.uk 3 Dipartimento di Ingegneria Industriale (DIEF), Università degli Studi di Firenze, Via di S. Marta 3, I-50139 Firenze, Italy 4 ENEA—Italian National Agency for New Technologies, Energy and Sustainable Economic Development, Research Centre of Trisaia, I-75026 Rotondella, Italy; enrico.catizzone@enea.it * Correspondence: emilia.paone@unifi.it; Tel.: +39-096-5169-2278 Abstract: Catalytic hydrotreatment (HT) is one of the most important refining steps in the actual petroleum-based refineries for the production of fuels and chemicals, and it will play also a crucial role for the development of biomass-based refineries. In fact, the utilization of HT processes for the upgrading of biomass and/or lignocellulosic residues aimed to the production of synthetic fuels and chemical intermediates represents a reliable strategy to reduce both carbon dioxide emissions and fossil fuels dependence. At this regard, the catalytic hydrotreatment of oils obtained from either thermochemical (e.g., pyrolysis) or physical (e.g., vegetable seeds pressing) processes allows to convert biomass-derived oils into a biofuel with properties very similar to conventional ones (so-called drop-in biofuels). Similarly, catalytic hydro-processing also may have a key role in the valorization of other biorefinery streams, such as lignocellulose, for the production of high-added value chemicals. This review is focused on recent hydrotreatment developments aimed to stabilizing the pyrolytic oil from biomasses. A particular emphasis is devoted on the catalyst formulation, reaction pathways, and technologies. Keywords: pyrolysis oils; catalytic hydrotreatment; heterogeneous catalysis; hydrogenation; biore- finery; green chemistry 1. Introduction In a green and sustainable perspective, the world is moving from a strong fossil fuels’ dependence to a consistent use of renewable feedstocks. In this view, Anastas and Green proposed in 1998 “the 12 principles of green chemistry” [1], where a particular attention was also given to (second and third generation) transportation biofuels, chemicals, commodities, and pharmaceuticals directly produced from biomass in modern biorefineries [2–6]. This transition is given not only by the matured awareness that fossil resources are running out, but it is mostly accelerated by the United Nation decision to adopt the 2030 Agenda for Sustainable Development, a program action of 17 ambitious goals (SDGs) and 169 targets aimed to eradicate the poverty, to protect the planet, and to ensure the prosperity for all [7]. Biomasses, that currently supply about 80% of global renewable energy and a low-emissions character, represent a unique sustainable pathway to successfully address SDGs [1,7,8]. Among several technologies that can use biomass waste as the feedstock to produce energy fuels, power, heat, and various high value-added chemicals [9–14], an interesting example is the use of lignocellulose (plant based biomasses mainly composed of cellulose, hemicellulose, and lignin) and microalgae (biomasses with high protein and carbohydrate content characterized by the absence of lignin) for the production of bio-oil Catalysts 2021, 11, 157. https://doi.org/10.3390/catal11020157 https://www.mdpi.com/journal/catalysts