catalysts Review Glycerol Role in Nano Oxides Synthesis and Catalysis Ana F. Cristino 1, Inês A. S. Matias 2 , David E. N. Bastos 2,3, Rui Galhano dos Santos 3 , Ana P. C. Ribeiro 2,* and Luísa M. D. R. S. Martins 2,* 1 Centro de Química Estrutural, Faculdade de Ciências, Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal; [email protected] 2 Centro de Química Estrutural, Departamento de Engenharia Química, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal; [email protected] (I.A.S.M.); [email protected] (D.E.N.B.) 3 CERENA—Centre for Natural Resources and the Environment, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal; [email protected] * Correspondence: [email protected] (A.P.C.R.); [email protected] (L.M.D.R.S.M.); Tel.: +351-218419389 (L.M.D.R.S.M.) Received: 30 October 2020; Accepted: 30 November 2020; Published: 2 December 2020 Abstract: The transformation of biomass and the utilization of all the by products derived from chemical conversion of biomass resources is one of the most important challenges nowadays. The impact in society and the level of awareness that already exists inside and outside the scientific community, makes the challenge of improving conversion of biomass to commodities a hot topic. Glycerol, a by-product obtained from the biodiesel production, is a key player compound due to its chemical versatility. The possibility of being used as solvent, reagent, reducing agent (in the polyol method), and so forth, makes glycerol an extremely appealing commodity. When used within nanotechnology, namely combined with nanomaterials, its potential becomes even higher. This review summarizes the work developed by the scientific community, during the last five years, in the use of glycerol with nano oxides. The analysis goes from the simple role of solvent to the oxidation of glycerol by nano oxides. Keywords: glycerol; oxidation; nano oxide; catalyst; nanoparticles 1. Introduction Global warming is one of humanity’s major concerns in view of the continuous increase of greenhouse gas emissions that results from the anthropogenic activities (consequence of rapid population growing). Currently, fossil fuels carbon sources (e.g., natural gas, oil, or coal) constitute over 80% of the world’s primary energy supply. As an outcome, the climate stability of the biosphere is affected with countless consequences. In December 2015, the 21st Conference of the Parties, COP21 (the United Nations conference on climate change), brought public awareness to the measures required to slow down global warming. Moreover, a framework to mitigate climate change was set, by agreeing on 17 SDGs—sustainable development goals. In addition, the Intergovernmental Panel on Climate Change (IPCC) set as an aim to keep the warming under 2 ◦C above preindustrial levels and to pursue efforts to restrict the temperature increase to a limit of 1.5 ◦C above preindustrial levels, whereas the European Union set the aim of reducing 80% of domestic emissions by 2050 (when compared to 1990) [1]. The use of greener fuels to address such envisaged goals is pointed out as one of the most promising solutions, unlocking a competitive, low carbon, and energy efficient future. By using wastes and emissions as feedstock to produce fuels, a new sustainable alternative to traditional petrochemical feedstock processes (avoiding their uncertain availability, and their Catalysts 2020, 10, 1406; doi:10.3390/catal10121406 www.mdpi.com/journal/catalysts Catalysts 2020, 10, x FOR PEER REVIEW 2 of 18 By using wastes and emissions as feedstock to produce fuels, a new sustainable alternative to traditionalCatalysts 2020 , petrochemical10, 1406 feedstock processes (avoiding their uncertain availability, and 2their of 17 environmental concerns) can be attained, which is imperative, especially to the transport sector [2]. However,environmental there concerns) is still much can beto attained, overcome which to achieve is imperative, a cost-effective especially and to theoperationally transport sector feasible [2]. methodologyHowever, there that is can still be much incorporated to overcome in a bioref to achieveinery, awhere, cost-e throughffective andjointly operationally applied conversion feasible technologies,methodology almost that can all betypes incorporated of biomass in feedstocks a biorefinery, can be where, transformed through into jointly different applied classes conversion of bio- fuelstechnologies, and chemicals. almost all types of biomass feedstocks can be transformed into different classes of bio- fuelsThere and chemicals. are several methods to produce biodiesel that includes transesterification, blending of raw oils, Theremicroemulsion, are several dilution, methods and to produce pyrolysis biodiesel or thermal that includescracking. transesterification, The predominant blending biodiesel of productionraw oils, microemulsion, process, which dilution, is also andthe pyrolysiseasiest ro orute, thermal being cracking.consequently The the predominant most cost-effective biodiesel approach,production involves process, whicha step isof also transesterification the easiest route, yielding being consequently glycerol (1,2,3-propanetriol, the most cost-effective Figure approach, 1) as a by-productinvolves a step [3]. of transesterification yielding glycerol (1,2,3-propanetriol, Figure1) as a by-product [ 3]. OH HO OH Figure 1. Glycerol (1,2,3-propanetriol) structure. Figure 1. Glycerol (1,2,3-propanetriol) structure. Increases in biodiesel production in many countries, to replace the traditional petroleum-based energyIncreases have led in tobiodiesel an excess production of crude glycerol.in many countries, This represents to replace the mainthe traditional bottleneck petroleum-based in the biodiesel energyproduction have chain. led to Simultaneously, an excess of crude such glycerol. a glycerol This surplus represents has created the main new challengesbottleneck toinits the sustainable biodiesel productionuse. In fact, chain. although Simultaneously, available on thesuch market a glycerol at a verysurplus low cost,has created glycerol new contains challenges a number to its of sustainableimpurities (e.g., use. un-reacted In fact, although glycerides, available water, methanol,on the market free fatty at a acids, very methyllow cost, esters glycerol and other contains organic a numbermaterials of or impurities inorganic (e.g., salts un-reacted [4–6]), which glycerides, affects its water, properties methanol, (biological, free fatty chemical, acids, methyl and physical) esters and and othertherefore organic inhibits materials its direct or inorganic usage in industries.salts [4–6]), which affects its properties (biological, chemical, and physical)The compositionand therefore of inhibits crude glycerol its direct depends usage in on industries. the process used for the production of the biodiesel as wellThe as composition on the nature of of crude feedstock glycerol [7]. Asdepends such, thereon the is aprocess wide variety used for of potential the production alternatives of the to biodieseladd value as to well glycerol, as on which the nature fall into of the feedstock following [7]. main As such, categories there of is applications: a wide variety (i) manufactureof potential alternativesof commodities; to add (ii) value production to glycerol, of polymers; which fall (iii) into production the following of biofuels main categories and biogas; of applications: and (iv) use (i) of manufacturepurified glycerol. of commodities; Thus, the multifunctionality (ii) production of ofpolyme glycerolrs; (iii) makes production it a suitable of biofuels bio-renewable and biogas; platform and (iv)chemical. use of purified Figure2 presentsglycerol. diThus,fferent the chemicalmultifunctionality reaction pathways of glycerol of makes glycerol. it a suitable bio-renewable platformGlycerol chemical. is considered Figure 2 apresents particularly differen interestingt chemical building reaction block pathways to provide of glycerol. added-value bio-based compounds. In the chemicals market, the production of bio-based products has become a priority for the industries of plastics, solvents, lubricants, and surfactants. The forecast period of 2019–2027 [8] expects an increase at a compound annual growth rate (CAGR) of 16.67%. The production of bio-based compounds offers an excellent alternative to reduce the dependence of petroleum-based feedstocks, to improve the environmental balance (decreasing CO2 emissions), to boost cleaner, more efficient, modern and future oriented industries, and to meet the increasing of consumers demand for healthier and safer products. Besides the several pathways to obtain molecules derived from glycerol (Figure2), there is a much less explored application of glycerol that can be as important and relevant as the currently most known ones: the production of nanomaterials. In fact, nanosciences and nanotechnology assume a pivotal role in the processes of accelerating the energy transition to the use of renewable sources through the development of new materials, which increase the efficiency of processes, rationalize the use of resources and raw materials, and help to reduce the costs. The use of glycerol as solvent for the dispersion of metal oxide nanoparticles presenting catalytic abilities (Figure3), increasing their lifetime, is also an emerging field of research. Catalysts 2020, 10, 1406 3 of 17 Catalysts 2020, 10, x FOR PEER REVIEW 3 of 18 Figure