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 1 School of Chemistry and Chemical Engineering, Queen’s University Belfast, David Keir Building, 39-123 Stranmillis Rd, Belfast BT9 5AG, UK; [email protected] 2 Advanced Biofuels Lab, Institute of Mechanical, Process and Energy Engineering, School of Engineering & Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, UK; [email protected] 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; [email protected] * 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 Citation: Bagnato, G.; Sanna, A.; the pyrolytic oil from biomasses. A particular emphasis is devoted on the catalyst formulation, Paone, E.; Catizzone, E. Recent reaction pathways, and technologies. Catalytic Advances in Hydrotreatment Processes of Pyrolysis Bio-Oil. Catalysts 2021, 11, Keywords: pyrolysis oils; catalytic hydrotreatment; heterogeneous catalysis; hydrogenation; biore- 157. https://doi.org/10.3390/ finery; green chemistry catal11020157 Received: 28 December 2020 Accepted: 21 January 2021 1. Introduction Published: 23 January 2021 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 Publisher’s Note: MDPI stays neutral proposed in 1998 “the 12 principles of green chemistry” [1], where a particular attention was with regard to jurisdictional claims in also given to (second and third generation) transportation biofuels, chemicals, commodities, published maps and institutional affil- and pharmaceuticals directly produced from biomass in modern biorefineries [2–6]. This iations. 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 Copyright: © 2021 by the authors. all [7]. Biomasses, that currently supply about 80% of global renewable energy and a Licensee MDPI, Basel, Switzerland. low-emissions character, represent a unique sustainable pathway to successfully address This article is an open access article SDGs [1,7,8]. Among several technologies that can use biomass waste as the feedstock distributed under the terms and to produce energy fuels, power, heat, and various high value-added chemicals [9–14], an conditions of the Creative Commons interesting example is the use of lignocellulose (plant based biomasses mainly composed Attribution (CC BY) license (https:// of cellulose, hemicellulose, and lignin) and microalgae (biomasses with high protein and creativecommons.org/licenses/by/ carbohydrate content characterized by the absence of lignin) for the production of bio-oil 4.0/). Catalysts 2021, 11, 157. https://doi.org/10.3390/catal11020157 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, 157 2 of 19 Catalysts 2021, 11, x FOR PEER REVIEW 2 of 20 thatprotein can and be usedcarbohydrate as intermediate content forcharacterized the production by the of absence liquid bio-fuels of lignin) [15 for]. the Bio-oil produc- is a dark brown-redtion of bio-oil colored that can liquid, be used with as a intermediate characteristic for smell the prod of smokeuction and of liquid a chemical bio-fuels composition [15]. strictlyBio-oil is related a dark to brown the biomass-red colored feedstocks liquid containing, with a characteristic a wide number smell of of unique smoke compounds and a generatedchemical composition from the rapid strictly quenching related to of the pyrolytic biomass fragments feedstocks of containing lignocellulose a wide [16 number]. Figure 1 showsof unique the compounds main compounds generated present from in the the rapid bio-oil: quenching an aqueous of pyrolytic solution fragments of several of products lig- derivednocellulose from [16] the. Figure fragmentation 1 shows the of main cellulose compounds and hemicellulose present in the and bio from-oil: thean aqueous depolymer- izationsolution of of lignin. several The products mixture derived consists from of the various fragmentation organic compounds of cellulose (20–30and hemicellu- wt%), water (19–20lose and wt%), from water-solublethe depolymerization oligomers of lignin (WS, also. The known mixture as consists pyrolityc of humin),various organic and water- insolublecompounds oligomers (20–30 wt%), (WIS, water also known (19–20 as wt%), pyrolityc water lignin)-soluble (43–59 oligomers wt%) (WS that, can also be known efficiently usedas pyrolityc for several humin) applications,, and water- suchinsoluble as drop-in oligomers fuel, (WIS, production also known of chemicals, as pyrolityc and lignin) various carbon-based(43–59 wt%) that materials can be efficiently [17–22]. used for several applications, such as drop-in fuel, pro- duction of chemicals, and various carbon-based materials [17–22]. Figure 1. A simplified chemical composition of bio-oil: main lignocellulose-derived compounds. Figure 1. A simplified chemical composition of bio-oil: main lignocellulose-derived compounds. Conventionally, bio-oil is produced by using a high energy demanding multistep Conventionally, bio-oil is produced by using a high energy demanding multistep pro- process, such as pyrolysis (fast or slow, thermal or thermo-catalytic) and hydrothermal cess, such as pyrolysis (fast or slow, thermal or thermo-catalytic) and hydrothermal lique- liquefaction (HTL). These thermochemical processes are conducted in the absence of oxy- faction (HTL). These thermochemical processes are conducted in the absence of oxygen and gen and at high reaction temperature with the aim to allow the decomposition/depoly- at high reaction temperature with the aim to allow the decomposition/depolymerisation of merisation of lignocellulose and microalgae into a bio-oil liquid (the major product), solid lignocellulose and microalgae into a bio-oil liquid (the major product), solid (bio-char), and (bio-char), and gaseous products (CO2, CO, CH4, H2) (bio-syngas [15,23,24]. Bio-char can gaseousaccess applications products (COin several2, CO, field CHs4 ,H(e.g.2), soil (bio-syngas amendment [15, in23 ,agriculture,24]. Bio-char chemical can access sensing, applica- tionsadsorbent in several material fields in wastewater (e.g., soil amendment remediation) in or agriculture, combusted chemicalto recover sensing, energy for adsorbent the materialpyrolysis instage wastewater [25], while remediation) bio-syngas may or be combusted directly utili toz recovered for many energy energy for uses the pyrolysis(e.g., stageelectricity [25], generation, while bio-syngas fuel for may transport, be directly cooking utilized fuel, forfeedstock many energyfor fuel usescells) (e.g., [26]. electricity HTL generation,processes were fuel developed for transport, to improve cooking the fuel, efficiency feedstock of fordirect fuel thermal cells) [26 decomposition]. HTL processes weremethods developed and differ to improvefrom the thepyrolysis efficiency for the of directadoption thermal of lower decomposition reaction temperatures methods and differand for from the presen the pyrolysisce of a homogeneous for the adoption or heterogeneous of lower reaction catalyst temperatures by applying and water for theand pres- encesimple of aliphatic a homogeneous (e.g., methanol, or heterogeneous ethanol, and catalyst 2-propanol) by applying alcohols waterused as and such simple or in com- aliphatic (e.g.,bination methanol, as reaction ethanol, solvents. and However, 2-propanol) bio- alcoholsoils arising used from as these such two or in processes combination cannot as re- actionbe direct solvents.ly used as However, drop-in fuels bio-oils in conventional arising from engines these due two to processes problems cannot related be to directlythe usedpresence as drop-in of a common fuels inlimiting conventional feature, the engines high oxygen due to problemscontent of relatedbiomass to otherwise the presence re- of asponsible common of limitingchemical feature,unfavorable the