New Frontiers in the Catalytic Synthesis of Levulinic Acid: from Sugars to Raw and Waste Biomass As Starting Feedstock
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catalysts Review New Frontiers in the Catalytic Synthesis of Levulinic Acid: From Sugars to Raw and Waste Biomass as Starting Feedstock Claudia Antonetti, Domenico Licursi, Sara Fulignati, Giorgio Valentini and Anna Maria Raspolli Galletti * Department of Chemistry and Industrial Chemistry, University of Pisa, Via G. Moruzzi, 13, 56124 Pisa, Italy; [email protected] (C.A.); [email protected] (D.L.); [email protected] (S.F.); [email protected] (G.V.) * Correspondence: [email protected]; Tel.: +39-050-2219290 Academic Editor: Keith Hohn Received: 28 October 2016; Accepted: 1 December 2016; Published: 6 December 2016 Abstract: Levulinic acid (LA) is one of the top bio-based platform molecules that can be converted into many valuable chemicals. It can be produced by acid catalysis from renewable resources, such as sugars, lignocellulosic biomass and waste materials, attractive candidates due to their abundance and environmentally benign nature. The LA transition from niche product to mass-produced chemical, however, requires its production from sustainable biomass feedstocks at low costs, adopting environment-friendly techniques. This review is an up-to-date discussion of the literature on the several catalytic systems that have been developed to produce LA from the different substrates. Special attention has been paid to the recent advancements on starting materials, moving from simple sugars to raw and waste biomasses. This aspect is of paramount importance from a sustainability point of view, transforming wastes needing to be disposed into starting materials for value-added products. This review also discusses the strategies to exploit the solid residues always obtained in the LA production processes, in order to attain a circular economy approach. Keywords: levulinic acid; acid catalysts; sugars; raw and waste biomass; water; organic solvents 1. Introduction One of the greatest challenges that industry faces in the 21st century is the transition from a fossil fuel based economy to one based on renewable resources, driving the search of new alternative renewable feedstocks for the production of chemical building blocks [1–6]. In particular, the exploitation of lignocellulosic biomass is receiving an increasing attention due to its renewability, abundance, low value and carbon-neutral balance. Among the target products, levulinic acid (LA) is the main compound of biomass hydrolysis, which has been classified by the United States Department of Energy as one of the top-12 promising building blocks [7]. LA represents a valuable intermediate for the synthesis of several chemicals for application in fuel additives, fragrances, solvents, pharmaceuticals, and plasticizers [8–13]. LA, also known as 3-acetylpropionic acid, 4-oxovaleric acid, or 4-oxopentanoic acid, is a very interesting platform chemical, because of its versatile chemistry, including one carbonyl, one carboxyl and α-H in its inner structure, looking like a short chain and non-volatile fatty acid. Some interesting data about physical properties of LA are reported in Table1[14–16]. Catalysts 2016, 6, 196; doi:10.3390/catal6120196 www.mdpi.com/journal/catalysts Catalysts 2016, 6, 196 2 of 29 Catalysts 2016, 6, 196 2 of 29 Table 1. Some physical properties of Levulinic Acid (LA). Molecular HeatHeat of of HeatHeat of of Molecular RefractiveRefractive DensityDensity MeltingMelting BoilingBoiling Weight pKa FormationFormation ΔHf VaporizationVaporization Weight (g/mol) IndexIndex1 1 (kg(kg·m∙−m3−)3)1 1 PointPoint (K) (K) PointPoint (K) (g/mol) DH(kJ/mol)f (kJ/mol) DvapΔvapHmHm(kJ/mol) (kJ/mol) 116.2 1.4796 1140 4.5 306–308 518–519 −2417 74.4 116.2 1.4796 1140 4.5 306–308 518–519 −2417 74.4 1 Value at 293 K. 1 Value at 293 K. By taking into account the chemical structure of LA, carboxylic and then carbonyl groups are By taking into account the chemical structure of LA, carboxylic and then carbonyl groups are very very reactive electrophilic centers towards nucleophilic attack. Furthermore, from the point of view reactive electrophilic centers towards nucleophilic attack. Furthermore, from the point of view of the of the acidic properties, LA has higher dissociation constant than saturated acids, thus its acidic properties, LA has higher dissociation constant than saturated acids, thus its corresponding corresponding acidity is stronger. Lastly, LA can be isomerized into the enolisomer, owing to the acidity is stronger. Lastly, LA can be isomerized into the enolisomer, owing to the existence of a existence of a carbonyl group. LA can be produced by three different process routes. The first one is carbonyl group. LA can be produced by three different process routes. The first one is a five-step a five‐step route from the petrolchemical intermediate maleic anhydride, which was carried out at route from the petrolchemical intermediate maleic anhydride, which was carried out at low scale in low scale in the past [17]. Despite the good yields in LA (up to 80 mol %) and even at the low oil price, the past [17]. Despite the good yields in LA (up to 80 mol %) and even at the low oil price, today this today this multi‐step process is not economically sustainable on large scale. multi-step process is not economically sustainable on large scale. The second route involves the acid treatment of C6 sugars, such as glucose, fructose, mannose or The second route involves the acid treatment of C sugars, such as glucose, fructose, mannose galactose, which can also derive from hydrolysis of more6 complex carbohydrates of the biomass, such or galactose, which can also derive from hydrolysis of more complex carbohydrates of the as cellulose, hemicellulose or starch [18]. The hydrolysis proceeds via formation of 5‐ biomass, such as cellulose, hemicellulose or starch [18]. The hydrolysis proceeds via formation hydroxymethylfurfural (5‐HMF) intermediate. Starting from glucose, the main reactions that occur of 5-hydroxymethylfurfural (5-HMF) intermediate. Starting from glucose, the main reactions that are: (i) glucose isomerization into fructose; (ii) fructose dehydration into 5‐HMF; and (iii) 5‐HMF occur are: (i) glucose isomerization into fructose; (ii) fructose dehydration into 5-HMF; and (iii) 5-HMF rehydration into LA. These reactions are reported in Figure 1. rehydration into LA. These reactions are reported in Figure1. Figure 1. Synthesis of LA starting from D‐-glucose.glucose. Antal et al. proposed that 5 5-HMF‐HMF is produced from fructose via cyclic intermediates [[19].19]. Recent studies havehave further further confirmed confirmed that that 5-HMF 5‐HMF originates originates from from the acid-catalyzed the acid‐catalyzed dehydration dehydration of C6-sugars of C6‐ sugarsin the furanose in the furanose form [ 20form,21]. [20,21]. The first The step first is step the is formation the formation of 5-HMF of 5‐HMF by a tripleby a triple dehydration dehydration step. step.5-HMF 5‐HMF is an unstableis an unstable molecule molecule and easily and condenses, easily condenses, together with together sugars with and sugarsugars degradation and sugar products,degradation into products, black insoluble into black charred insoluble materials charred often materials called “humins” often called [22–25 “humins”]. In an extensive [22–25]. review In an extensiveon the 5-HMF review formation, on the 5‐HMF Van Puttenformation, et al. Van have Putten demonstrated et al. have demonstrated that the temperature that the andtemperature reaction andconditions reaction required conditions for therequired dehydration for the dehydration of glucose and of otherglucose aldoses and other are significantly aldoses are significantly more severe morethan those severe for than fructose. those Thisfor fructose. evidence This is tentatively evidence explainedis tentatively by theexplained need for by glucose the need to firstfor glucose isomerize to firstto fructose isomerize via to the fructose enediol via form the [ 26enediol]. This form isomerization [26]. This isisomerization a Brønsted-base is a Brønsted or Lewis-acid‐base catalyzedor Lewis‐ acidreaction catalyzed and thus reaction proceeds and very thus slowly proceeds under very the slowly strong under Brønsted the acid strong conditions Brønsted generally acid conditions used for generallythe subsequent used dehydrationfor the subsequent of the fructose. dehydration The harsher of the reaction fructose. conditions The harsher adopted reaction for aldoses conditions cause adoptedthe formation for aldoses of higher cause amounts the formation of humins. of higher The rehydration amounts of 5-HMFhumins. with The tworehydration molecules of of 5‐ waterHMF withleads two to LA molecules and formic of water acid [leads27]. Regarding to LA and the formic thermodynamics acid [27]. Regarding of the process, the thermodynamics the dehydration of the of process,fructose tothe 5-HMF dehydration is highly of endothermic fructose to (by 5‐HMF up to is 92.0 highly kJ/mol), endothermic the glucose (by to fructoseup to 92.0 isomerization kJ/mol), the is veryglucose weakly to fructose endothermic isomerization (8.4 kJ/mol), is very whereas weakly the endothermic other steps, including(8.4 kJ/mol), additional whereas water the other elimination steps, includingand rehydration additional to form water LA, elimination are exothermic and (fromrehydration 16.7 to to 133.9 form kJ/mol) LA, are [28 exothermic]. Elevated temperatures(from 16.7 to 133.9and aqueous kJ/mol) [28]. reaction Elevated environment temperatures make and the dehydrationaqueous