energies Review Production of Gasolines and Monocyclic Aromatic Hydrocarbons: From Fossil Raw Materials to Green Processes Guido Busca Department of Civil, Chemical and Environmental Engineering, The University of Genoa, Via Opera Pia 15, 16145 Genova, Italy; [email protected]; Tel.: +39-010-335-6024 Abstract: The properties and the applications of the main monocyclic aromatic hydrocarbons (ben- zene, toluene, ethylbenzene, styrene, and the three xylene isomers) and the industrial processes for their manufacture from fossil raw materials are summarized. Potential ways for their produc- tion from renewable sources with thermo-catalytic processes are described and discussed in detail. The perspectives of the future industrial organic chemistry in relation to the production of high- octane bio-gasolines and monocyclic aromatic hydrocarbons as renewable chemical intermediates are discussed. Keywords: bio-gasoline; pyrolysis; hydrodeoxygenation; hydrocarbons; aromatics from renewables; renewable intermediates; green chemistry 1. Introduction Monocyclic aromatic hydrocarbons, i.e., benzene, toluene, ethylbenzene, and the three xylene isomers, comprehensively denoted with the acronym BTEX, are highly anti-knocking Citation: Busca, G. Production of components of high-octane number gasolines [1]. Even if in the near future, full electric Gasolines and Monocyclic Aromatic mobility and hydrogen fuel-cell based electric vehicles will maybe become predominant, Hydrocarbons: From Fossil Raw it seems likely that the use of liquid fuels will remain the best solution at least for some Materials to Green Processes. Energies specific applications [2,3]. Although ligno-cellulosic bioethanol is (or will be) a good bio- 2021, 14, 4061. https://doi.org/ gasoline component, its use as a pure or highly concentrated fuel has several drawbacks [4] 10.3390/en14134061 and is actually forbidden in the European Union. Thus, the development of hydrocarbon- based renewable gasolines is desirable to fuel vehicles powered by conventional Otto-type Academic Editor: Diego Luna engines limiting greenhouse gas emissions. BTEX also represent key intermediates in industrial petrochemistry [5,6]. In fact, Received: 14 June 2021 benzene, toluene, and xylenes, and their derivative styrene, are among the top 15 petro- Accepted: 1 July 2021 Published: 5 July 2021 chemicals in terms of market [7] and are applied as building blocks for producing a large number of secondary intermediates and final products [8]. As we review in the next sec- Publisher’s Note: MDPI stays neutral tions, these compounds are produced today in large amounts from fossil raw materials. with regard to jurisdictional claims in As we will remark, some of these compounds are or may be cancerogenic. On the other published maps and institutional affil- hand, they are present in the environment also as products of (auto)combustion or heating iations. of biomass and in oil seepages. They are also formed spontaneously by cooking food [9], smoking tobacco [10], and burning wood for heating, cooking, and garden care. The concerns related to global warming and the expected progressively limited avail- ability of fossil raw materials in the next fifty years push for the use of renewable raw materials for the production of fuels and chemicals. The development of new “green” Copyright: © 2021 by the author. Licensee MDPI, Basel, Switzerland. processes is needed to convert biomasses into commodity and specialty products. How- This article is an open access article ever, this does not imply a complete revolution of industrial chemistry and products. In distributed under the terms and fact, the production of primary “petrochemical” intermediates using alternative green and conditions of the Creative Commons sustainable processes can allow the use of already well-developed technologies for the Attribution (CC BY) license (https:// manufacture of secondary intermediates and final chemical products. creativecommons.org/licenses/by/ In this paper, the conventional industrial chemistry of gasolines production and 4.0/). monocyclic aromatic hydrocarbons manufacture is reviewed together with emerging green Energies 2021, 14, 4061. https://doi.org/10.3390/en14134061 https://www.mdpi.com/journal/energies Energies 2021, 14, x FOR PEER REVIEW 2 of 33 Energies 2021, 14, 4061 2 of 32 In this paper, the conventional industrial chemistry of gasolines production and monocyclic aromatic hydrocarbons manufacture is reviewed together with emerging technologiesgreen technologies which inwhich the futurein the couldfuture allowcould theallow production the production green hydrocarbon-basedgreen hydrocarbon- gasolinesbased gasolines and of theand same of the BTEX same intermediates. BTEX intermediates. 2.2. ApplicationsApplications ofof MonocyclicMonocyclic AromaticAromatic Hydrocarbons Hydrocarbons AsAs previouslypreviously said, said, in in addition addition to to their their role role as as components components of of gasolines, gasolines, monocyclic monocyclic aromaticaromatic hydrocarbonshydrocarbons alsoalso representrepresent very very important important petrochemical petrochemical intermediates. intermediates. InIn FigureFigure1 ,1, the the structures structures of the of most the relevantmost relevant monocyclic monocyclic aromatic aromatic hydrocarbons hydrocarbons are reported. are Inreported. Table1, propertiesIn Table 1, ofproperties such compounds of such compounds are summarized. are summarized. Figure 1. Structures of main monocyclic aromatic hydrocarbons present in oil and its primary Figure 1. Structures of main monocyclic aromatic hydrocarbons present in oil and its primary transformationtransformation products products (reformate (reformate and and pyrolysis pyrolysis gasoline). gasoline). Table 1. Properties of mainTable monocyclic 1. Properties aromatic of hydrocarbons main monocyclic present aromatic in oil and hydrocarbons its primary present transformation in oil and products its primary (reformate and pyrolysis gasoline).transformation products (reformate and pyrolysis gasoline). Monocyclic Teb MonocyclicTm Vp d Øc Fp IARC Teb Tm Vp dRON Øc MON Fp IARC Aromatic Molecule ◦C Aromatic◦C kPa g/mL nm RON MON◦C Classification °C °C kPa g/mL nm °C Classification Benzene 80.1Molecule +5.5 12.70 0.879 0.67 101.0 93.0 −11 Group 1 Benzene 80.1 +5.5 12.70 0.879 0.67 101.0 93.0 −11 Group 1 Toluene 110.6 −59.4 3.79 0.867 0.67 121.0 103.0 4 Group 3 Toluene 110.6 −59.4 3.79 0.867 0.67 121.0 103.0 4 Group 3 Ethylbenzene 136.2 Ethylbenzene−95.0 1.28136.2 − 0.86795.0 1.28 0.67 0.867 108.3 0.67 97.9108.3 97.9 15 15 GroupGroup 2b 2b Styrene 145.2 Styrene−30.0 0.81145.2 − 0.90630.0 0.81 0.67 0.906 103.0 0.67 100.2103.0 100.2 31 31 GroupGroup 2a 2a Para-xylene 138.3Para-xylene +13.3 1.18138.3 +13.3 0.861 1.18 0.67 0.861 146.0 0.67 101.2146.0 101.2 25 25 GroupGroup 3 3 Meta-xylene 139.1 −47.9 1.12 0.864 0.71 145.0 102.8 23 Group 3 Meta-xylene 139.1 −47.9 1.12 0.864 0.71 145.0 102.8 23 Group 3 Ortho-xylene 144.4 −25.2 0.89 0.880 0.74 120.0 100.0 17 Group 3 Ortho-xylene 144.4 −25.2 0.89 0.880 0.74 120.0 100.0 17 Group 3 Teb = boiling point; Tm = melting point; Vp = vapor pressure at 25 °C; d = density at 20 °C; Øc = ◦ ◦ Teb = boiling point; Tm = meltingcritical point; radius; Vp = vapor RON pressure = research at 25 octaneC; d = densitynumber; at MON 20 C; Øc= motor = critical octane radius; number; RON = Fp research = flash octane point; number; MON = motor octane number;IARC = Fp classification = flash point; of IARC Internatio = classificationnal Agency of International for Research Agency on Cancer. for Research on Cancer. 2.1.2.1. BenzeneBenzene GlobalGlobal benzene benzene production production is is forecasted forecasted to to rise rise 51 51 mton/y mton/y in in 2022 2022 [ 11[11].]. Benzene Benzene is is one one ofof thethe most most toxic toxic among among common common molecules molecules [ 12[12,13],,13], being being classified classified by by the the International International AgencyAgency for for Research Research on on Cancer Cancer (IARC) (IARC) as as Group Group 1 molecules, 1 molecules, carcinogenic carcinogenic for humans.for humans. Its presenceIts presence in gasoline, in gasoline, according according to its relativelyto its relatively high volatility, high volatility, creates very creates toxic very gasoline toxic vapors.gasoline For vapors. this reason,For this its reason, amount its isamount limited is inlimited commercial in commercial motor gasoline motor gasoline by law toby 1%law vol to in 1% the Europeanvol in the Union. European Nevertheless, Union. Nevertheless, benzene is a very benzene relevant is intermediatea very relevant for today’s production of a large number of household commodities. The main industrial intermediate for today’s production of a large number of household commodities. The chemistry of benzene is summarized in Figure2. Even if this chemistry also contributes to main industrial chemistry of benzene is summarized in Figure 2. Even if this chemistry some environmental problems [14], its role in developing materials and molecules for our also contributes to some environmental problems [14], its role in developing materials and common life is enormous [15]. On the other hand, dangerous amounts of this molecule are molecules for our common life is enormous [15]. On the other hand, dangerous amounts also simply formed, e.g., by cooking food [9], and cigarette smoking is still the main source of this molecule are also simply formed, e.g., by cooking food [9], and cigarette smoking of human exposure to benzene [16]. is still the main source of human exposure to benzene [16]. More than half of benzene production today is devoted to the synthesis of ethylbenzene by alkylation with ethylene, which is almost completely converted into styrene. The final product is polystyrene, which is a very common plastic material in daily life, as detailed below. Energies 2021, 14, x FOR PEER REVIEW 3 of 33 More than 20% of benzene is converted by alkylation with propylene into cumene, to be later converted into acetone and phenol.