Clean Catalytic Oxidation for Derivatization of Key Biobased Platform

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Clean Catalytic Oxidation for Derivatization of Key Biobased Platform Chemicals: Ethanol, Glycerol, and Hydroxymethyl Furfural María Alejandra Ayude,† Lucila I. Doumic,† Miryan C. Cassanello,*,‡ and Krishna D. P. Nigam§,∥ †

INTEMA, Facultad de Ingeniería, UNMdP, Av. Juan B. Justo 4302, Mar del Plata, B7608FDQ, Argentina Departamento de Industrias and ITAPROQ, Universidad de Buenos Aires, Int. Güiraldes 2620, Buenos Aires, C1428BGA, Argentina § Escuela de Ingeniería y Ciencias, Tecnológico de Monterrey, Campus Monterrey Avenida, Eugenio Garza Sada 2501 Sur, Monterrey, Nuevo León 64849, México ∥ Department of Chemical Engineering, Indian Institute of Technology, Hauz Khas, New Delhi, Delhi 110016, India

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‡

S Supporting Information *

ABSTRACT: There has been a growing interest in the last decades in technologies that transform biomass-derived feedstock into renewable chemicals. A group of target molecules derived from biomass as key intermediates between raw materials and final products have been designed as “platform chemicals”. Sustainable processes involve the use of renewable feedstock, but also pursue low energy consumption, the use of less hazardous materials, and diminished generation of waste. This review aims to present recent advances on environmentally friendly catalytic oxidation routes for transforming three platform chemicals, ethanol, glycerol, and 5-hydroxymethylfurfural, to value added intermediates and final products using air or oxygen as oxidants and water without additives (base-free) as solvent, and preferably under moderate operating conditions. Works carried out under continuous flow have been particularly reviewed. The postulates of green chemistry11 promote the use of renewable feedstock together with safer solvents and auxiliaries, to develop less hazardous chemical syntheses. The use of heterogeneous catalysts to improve process efficiency under milder conditions preventing waste generation is imperative. Hence, sustained efforts are required from many schools across the globe to make the biobased chemistry technically and economically feasible with negligible impact on the environment.12,13 Chemical conversions are required for transforming biobased chemical intermediates into useful products.6,10 Identifying the best pathway to convert biomass can be challenging because there is a plethora of potential targets that can be obtained through different reactions and starting from various substrates. The role of catalysts in these conversion pathways is critical, and many research efforts pursue the optimization of catalysts’ efficiency and the design of heterogeneous catalysts applicable in a wide range of reaction conditions.10,13−15 Technologies that combine renewable

1. INTRODUCTION The increasing demand for global products and the continuous decrease of fossil fuels and raw materials, together with the growing concerns about environmental pollution urge the search for renewable resources.1,2 Biomass is the most abundant natural supply of carbon resources for the production of chemicals, with a wide range of applications from pharmaceuticals to fuel.3−5 Biomass has the potential to replace fossil feedstock as a carbon source without net CO2 generation, allowing the development of sustainable and more biodegradable products.1,6 Despite biomass potential as a resource, the fraction of chemicals arising from biobased raw materials is still minor. Current agriculture is far from being able to substitute fossil carbon, particularly when considering the increasing demand for food, feed, and fiber. Efforts are thus focused on the massive amount of inedible biomass wastes and side streams, and the participation is expected to increase sharply in the next decades.7−9 It is therefore urgent to develop technologies and infrastructure to use as much biocarbon as possible in a sustainable way. Cost competition with petrochemicals is hard and requires research oriented to the optimization of production routes and the development of biobased products with new or improved functionalities.10 © XXXX American Chemical Society

Special Issue: Biorenewable Energy and Chemicals Received: Revised: Accepted: Published: A

February 19, 2019 June 18, 2019 June 19, 2019 June 19, 2019 DOI: 10.1021/acs.iecr.9b00977 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

dehydration, it can be oxidized to important commodity chemicals, acetic acid, and ethyl acetate. Glycerol is produced in a large amount as it is a byproduct of the biodiesel market. Structurally it can be considered as a “mini-sugar” and thus a versatile compound, leading to a wide variety of products via many derivatization routes. Oxidation of glycerol provides value-added products used in the cosmetic and pharmaceutical industries. The dehydration of C5−C6 sugars leads to the formation of furans, mainly furfural and 5-hydroxymethylfurfural (HMF). Both furans are recognized as key platform chemicals. HMF is particularly interesting since it has two reactive functional groups and can be converted to organic compounds with many applications.22 The technology for producing HMF from fructose has significantly improved in the past few years,13−15 expanding its possibilities as platform chemical. The oxidation of HMF leads to a key monomer of alternative polymers.23 Conversion routes combining technical feasibility with environmental benefits have increased possibilities to succeed in the framework of a more sustainable economy. In this sense, environmentally friendly oxidation routes, using air or oxygen as oxidants and water without additives as the solvent, and preferably at moderate operating conditions, have been particularly considered for the present recompilation. Emphasis was made to include works carried out in flow reactors, which provide relevant information for scaling the processes.

feedstock, robust process conditions to sustain long periods of operation with minimal intervention, low waste generation, simple downstream purification, and the production of genuinely green and sustainable products are candidates of succeeding as new commercial developments.5 Concerning the vast range of possible target molecules, research is currently focusing on platform chemicals derived from biomass as key intermediates between raw materials and final products. These chemicals can be converted through chemical, thermochemical, and biological processes into a multitude of high-value-added marketable products. In 2004, the US Department of Energy (DOE) produced a report outlining research needs for biobased products.16 The report provided the first selection of 12 platform chemicals, produced either biologically or chemically from renewable carbohydrate raw materials. The purpose of such selection was to concentrate research on a limited number of targets to progress on new technologies. The selected platform chemicals are used as building blocks and can be further converted to a full spectrum of derivatives through chemical processes, such as reduction, oxidation, dehydration, hydrogenolysis, and direct polymerization. Derivatives are widely used for many applications and are expected to replace many products currently developed from fossil resources.12 Oxidation is one of the derivatization routes for transforming target biobased molecules into value added organic compounds.3,4,17 It is a key derivatization route, allowing the production of many biobased alternative products. Oxidation is vastly applied for derivatization of alcohols and aldehydes, to get end-products, such as solvents, fine chemicals, food additives, pharmaceuticals, and precursors of polymers with polymeric properties comparable with those derived from petroleum.17−19 In the past, oxidation of organic compounds was performed using stoichiometric oxidants. These methods demonstrated some distinct drawbacks, such as low selectivity, high cost of the oxidant, and generation of toxic waste to the environment. In the last decades, many environmentally friendly and economical oxidation methods have been proposed and are actively studied.18 Catalytic oxidation processes are powerful tools in the synthesis of fine chemicals.4,10 Heterogeneous catalysts are preferred since they can be readily separated from the product and reused. The heterogeneously catalyzed oxidation of organic compounds using air or oxygen as oxidant together with mild conditions and nontoxic solvents has been the aim of many studies.4,20,21 The use of air as the oxidant is cheap and environmentally friendly as water is the only reduction product. Molecular oxygen is considered as the ultimate green oxidant for organic synthesis.20 The heterogeneous catalytic oxidation using molecular oxygen involves multiphase reactors for which hydrodynamics and transport phenomena largely interact with kinetics to determine reactor performance. Hence, examining catalysts in continuous flow helps address the challenges associated with the use of molecular oxygen for the heterogeneous catalytic oxidation and provides useful information for scaling up.20 This review aims to summarize the current research referring to clean catalytic oxidation of three target molecules (ethanol, glycerol, and 5-hydroxymethylfurfural) currently defined as building blocks for biorefineries.12 Ethanol is readily obtained by fermentation using many lignocellulosic materials. It is used as an alternative fuel and nowadays, it is recognized as a key platform chemical. Apart from being a precursor of ethylene via

2. CLEAN CATALYTIC OXIDATION OF ALCOHOLS The selective oxidation of alcohols to carbonyl compounds, that is, ketone, aldehyde, or carboxylic acids is one of the most important reactions in organic synthesis.24 Traditionally, supported noble metal nanoparticles (NPs) have been the most widely studied catalysts in the aerobic oxidation of alcohols in the liquid phase.4,25 With these catalysts, primary hydroxyl groups are preferentially oxidized, yielding the aldehyde or the acid as main products.26−30 It is generally accepted that the oxidation of primary alcohols over a heterogeneous catalyst proceeds initially through the formation of the aldehyde.25 Three steps are proposed to take place: (i) alcohol adsorption on the metal surface, producing an adsorbed metal alkoxide; (ii) β-hydride removal to produce a metal hydride and the carbonyl species; and (iii) metal hydride oxidation with O2, leading to the regeneration of metal surface. Then, the aldehyde reversible hydration generates a geminal diol that finally leads to the carboxylic acid. The geminal diol adsorbed onto the metal surface as a metal alkoxide undergoes β-hydride elimination leading to the carboxylic acid. Zope et al.29 studied the mechanism of ethanol and glycerol oxidation over supported Au and Pt catalysts in the aqueousphase in the presence or absence of added NaOH. These authors showed that the activation barriers (Eact) for the initial deprotonation of ethanol in neutral water were 204 kJ mol−1 and 116 kJ mol−1 for Au and Pt, respectively. In basic solution, the adsorbed hydroxide intermediates reduced these Eact to less than 25 kJ mol−1. In particular, for Au-based catalysts, adsorbed hydroxide also contributed to lowering the Eact for the subsequent formation of the aldehyde. In addition, they demonstrated that the oxygen atoms from the aqueous alkaline media, rather than from the molecular oxygen, preferentially react with the alcohol to promote the oxidation. Yuan et al.31 have recently disclosed the homogeneous catalytic effect of NaOH in the aqueous-phase oxidation of ethanol. B

DOI: 10.1021/acs.iecr.9b00977 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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to ethyl acetate occurs.25,37,50 Besides, ethyl acetate production is favored in the presence of excess ethanol or solvent-free conditions,50−53 whereas acetic acid is preferentially formed in aqueous media.29,39,40,49,54,57 Catalysts based on Pt, Pd, and Au NPs were used in the catalytic aerobic oxidation of ethanol in the liquid phase.25,29,37,40,50,56,58−60 Table S1 (Supporting Information) presents the performance of catalysts evaluated in the base-free aerobic oxidation of ethanol. 2.1.1. Gold-Based Catalysts. Gold (Au) was suggested as one of the most active species.24,29,39,40,49 However, supported Au NP catalysts required a strong alkaline aqueous medium to accelerate the β-hydride removal, which is considered the rate limiting step and thus, to enable ethanol oxidation under mild conditions (T < 373 K and P < 0.5 MPa).24,25,29,61,62 Under base-free conditions, these catalysts needed elevated pressures and temperatures (Table S1, entries 1−8). The aqueous ethanol oxidation performed at high temperatures (413−483 K) and pressures between 0.6 and 4.8 MPa led to acetic acid formation with good yields. High values of ethanol conversion (58−98%) and selectivity to acetic acid (81−97%) were achieved in reaction times between 4 and 20 h.39−41,49,55,57 Recently, Mostrou et al.63 highlighted the need of studying the base-free aerobic oxidation of ethanol in flow systems to get further insight into the three-phase reactions taking place, and to identify potential catalysts for scaling up the process. The performance of a Au/TiO2 catalyst in a batch reactor and in a tubular reactor, in which the gas and liquid streams were mixed before entrance and circulated downflow through the catalytic bed, were examined comparatively (Figure 1). The authors showed that ethanol conversion was favored in the flow system, likely due to a decrease of the extent of gold NP

Base-free conditions yielded lower alcohol oxidation rates than basic media. However, from an economic and environmental point of view, efforts must be focused on the development of catalysts for the base-free oxidation of alcohols to produce directly the free acid instead of the salt, as well as on the intensification of this process. 2.1. Ethanol. Ethanol is a renewable alternative to fossil fuels which has been used for several years as fuel and fuel additive. For this reason, its production from biomass has been thoroughly studied.32−36 Nowadays, ethanol (sometimes called bioethanol when it is produced via fermentation from biomass) is not only recognized as a biofuel but also as a platform chemical.10,37 Bioethanol can be used for production of drop-in chemicals (ethylene, propylene, 1,3-butadiene, and larger hydrocarbons) as well as oxygenated compounds (ethyl acetate, acetaldehyde, 1-butanol, and acetic acid).37 Crude bioethanol obtained by fermentation consists of an aqueous solution containing only 3−15 vol % of ethanol. Its further purification by distillation to be used as fuel is a costly process.38 Hence, direct production of valuable chemicals through the catalytic oxidation of aqueous ethanol solutions with compositions similar to those in crude bioethanol has great economic potential.39−41 The catalytic aerobic oxidation of ethanol can lead to acetaldehyde, acetic acid, and ethyl acetate. Both acetic acid and ethyl acetate are desired final products and important compounds for the chemical industry. The main use of acetaldehyde is as a chemical reagent for production of acetic acid, esters (mainly ethyl acetate and vinyl acetate), pyridines, and pentaerythritol, among others.42 Acetic acid is an important industrial chemical used in the production of vinyl acetate monomer and acetic anhydride, as well as in the production of synthetic fibers and fabrics. Nowadays, the primary route to produce acetic acid is through the carbonylation of methanol; a process which exclusively requires fossil resources and two steam-reforming steps.43,44 Ethyl acetate is usually used as a low toxic solvent, which can replace aromatic solvents in the paint and coating industry.45 Besides, it can be used in the pharmaceutical industry, the preparation of cosmetics, and in the food industry.46 Ethyl acetate production is typically accomplished by Fisher esterification of acetic acid.43,47 Gallo et al.37 have discussed the performance of different heterogeneous catalysts developed to obtain both drop-in chemicals and oxygenated compounds from the catalytic transformation of bioethanol. These authors explained that some of the oxygenated chemicals (acetaldehyde and ethyl acetate) can be produced via two possible routes, according to whether the atmosphere is inert or oxidizing. In an inert atmosphere, ethanol dehydrogenation to acetaldehyde takes place and, in the presence of a bifunctional redox/acid catalyst, the subsequent coupling between acetaldehyde and ethanol is promoted, leading to ethyl acetate and hydrogen. Although the production of hydrogen could be an advantage of the reaction in an inert atmosphere, the use of oxygen (or air) as oxidizing agent allows carrying out the reactions at milder operating conditions, with reduced wastes, lower energetic costs, and limited CO2 emissions.44,48 It is widely accepted that ethanol oxidation with O2 over supported metal catalysts proceeds first to acetaldehyde formation, which can be further oxidized to acetic acid.25,29,37,39,49,50 Then, in the presence of a catalyst with an acidic functionality the esterification of acetic acid with ethanol

Figure 1. Ethanol conversion (X) and product selectivity (S) as a function of residence time in the batch (top) and flow (bottom) systems using 1% Au/TiO2 (AUROlite) catalyst. (◆,◇) Sacetaldehyde, (●,○) Sacetic acid, (▲,△) Sethyl acetate, (★,☆) SCO2. Reaction conditions: 3 MPa, 423 K, and 5 vol % ethanol solution. The minimum and maximum residence times are governed by the system’s limitations. Adapted from ref 63. Open access article licensed under a Creative Commons Attribution 3.0 Unported License. 2018 Published by Royal Society of Chemistry. C


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Industrial & Engineering Chemistry Research sintering in flow. Besides, for a specific ethanol conversion and under equivalent reaction conditions, a higher selectivity to acetic acid was achieved in the flow reactor (Table S1, entry 8). The difference was probably related to the shorter residence time, limiting decomposition of the product to CO2. Three-phase reactions have significant influence of the phases’ interactions, which are difficult to reproduce in batch without flowing gas. Hence, experiments in continuous flow provide reliable information particularly useful related to catalysts stability and feasibility of scale up. Catalysts combining gold with another suitable compound exhibited catalytic activity under mild operating conditions. Park et al.52 showed that Au-doped CeO2 nanoparticles were active in the solvent-free ethanol oxidation at 353 K and atmospheric pressure, leading to ethyl acetate and acetaldehyde after 5 days of reaction (Table S1, entry 9). It was argued that CeO2 nanoparticles acted as a cocatalyst for the oxidation of alcohols, improving Au catalytic activity in the absence of solvent and base.4,10,64 Furthermore, Lackman et al.53 showed that unsupported nanoporous gold (npAu) was active in the aerobic oxidation of ethanol in the liquid phase at 333 K and 0.3 MPa without added base (Table S1, entry 10). The catalytic activity was ascribed to the low coordinated Au surface sites, the small size (nanometers) of the structures, and the presence of residual Ag (0.5−1 atom %) in the catalyst. Ethyl acetate and acetaldehyde were the products obtained after 24 h. The use of pure ethanol (solvent free) or aqueous solutions was compared. The selectivity toward acetaldehyde increased from 50% for solvent free (pure ethanol), to 80% for a 30% mol ethanol aqueous solution. 2.1.2. Pt- or Pd-Based Catalysts. Catalysts based on supported Pt or Pd NPs appear as attractive candidates for the catalytic aerobic oxidation of ethanol in base-free media under mild operating conditions (Table S1, entries 11−15, 17−19). Pd and Pt in acidic media exhibited lower reaction rates than in an alkaline environment.29,52,54,65 Pt NPs immobilized onto mesoporous silica (Pt/MCF17) was tested in a batch reactor at 333 K and atmospheric pressure60,66,67 (Table S1, entries 11−13). Acetaldehyde was the main product in the oxidation of ethanol either under solvent-free conditions or with aqueous solutions of different compositions. Although acetic acid was expected to be favored in aqueous solutions, the level of ethanol conversion was a determining factor on the product distribution. Acetaldehyde, as the intermediate product in the acetic acid formation from ethanol oxidation in the aqueous phase, was generally the main product with selectivities close to 100%, especially for low ethanol conversions.41,49 2.1.2.1. Support Effect. The support plays an important role in the performance of a catalyst. The effect of the support material on the catalytic activity and selectivity of the Pt-based catalyst was investigated in the catalytic oxidation of aqueous ethanol conducted in a batch reactor at 363 K and 1 MPa in 24 h experiments (Table S1, entries 14−15). Long et al.65 immobilized Pt NPs on three different core−shell magnetite microparticles leading to Pt/Fe3O4@PPy, Pt/Fe3O4@C, and Pt/Fe3O4@SiO2 catalysts. Their catalytic performance was compared with those obtained using Pt/Fe3O4 and Pt/C. Prepared catalysts (Pt/Fe3O4@PPy and Pt/Fe3O4@C) exhibited higher activity and stability than Pt/Fe3O4 and Pt/C, with the additional advantage of being easy to recover from the reaction mixture and reusable. Among them, the catalyst with a shell of polypyrrole (PPy) showed the best performance (95%

of ethanol conversion and more than 90% of selectivity to acetic acid in 24 h). Richter et al.68 synthesized a nanocasted mesoporous polydivinylbenzene-supported Pt catalyst (Pt/ PDVB). Its performance was tested in a batch reactor and compared with the one obtained using commercial carbonand alumina-supported Pt. Among the studied materials, the Pt/PDVB catalyst showed the best activity and selectivity toward acetic acid. 2.1.2.2. Hydrophilicity−Hydrophobicity Effect. The impact of the hydrophilic−hydrophobic properties of the catalysts on the base-free aerobic oxidation of ethanol was investigated.50,58,68,69 Richter et al.68 investigated the catalytic performance of the hydrophobic Pt/SPDVB, obtained by gas-phase sulfonation of Pt/PDVB, in a batch reactor at 363 K and 1 MPa in 24 h experiments. These authors found no differences with the outcomes attained using the hydrophilic Pt/PDVB catalyst. Accordingly, the hydrophobicity of Pt/ PDVB was discarded as a major factor influencing the catalytic performance in a batch reactor. Conversely, the hydrophobic character of the catalysts has shown to have a significant influence in the ethanol oxidation carried out in continuous downflow fixed-bed reactors. In studies performed using both Pt- and Pd-based catalysts, considerable improvements in ethanol conversions were achieved when hydrophobic catalysts were used instead of hydrophilic ones. Lin et al.50 investigated the catalytic performance of a hydrophobic catalyst in the continuous ethyl acetate production. Pd-based catalysts prepared with styrene−divinylbenzene copolymer-SDB (hydrophobic) and γAl2O3 (hydrophilic) supports were used in the oxidation of water-containing ethanol (with excess ethanol) performed in a continuous downflow fixed-bed reactor at 3.54 MPa and 368 K (Table S1, entry 16). Steady state conversion attained using the Pd/SDB was significantly higher (more than 20 times) than with Pd/γ-Al2O3. This substantial difference in activity was ascribed to the hydrophobicity of the SBD polymer, likely because water that accumulated in the pores of the hydrophilic catalyst increased the diffusion resistance. However, acetic acid, notably favored with Pd/SBD, promoted the leaching of Pd and the aggregation of Pd clusters, leading to irreversible loss of catalytic activity along the first 30 h on stream. In a subsequent work, these authors showed that mixing a resin solid acid catalyst with Pd/SBD favored the esterification reaction of acetic acid to produce ethyl acetate, thus increasing the productivity and extending the catalyst lifetime.70 Horowitz and co-workers58,69 performed the aerobic oxidation of ethanol in a bench scale trickle-bed reactor (TBR) using Pt supported on 3 mm γ-Al2O3 spheres at 343 K and 0.1 MPa (Table S1, entry17). They compared ethanol conversions attained in the TBR packed either with 100%Pt/ Al2O3(1 wt %) (hydrophilic), with 100% Pt/Al2O3(1 wt %) covered with Teflon (hydrophobic) or with mixtures of hydrophobic catalyst and hydrophilic γ-Al2O3 spheres. Experiments were carried out at 343 K and atmospheric pressure. Ethanol conversion was significantly enhanced by employing the hydrophobic catalyst in the bed (Figure 2). Diluting the hydrophobic catalyst with inert hydrophilic support in a 50% mass proportion had almost no effect on the reactor performance for the various experimental conditions examined (Table S1, entry17). Thus, the enhancement attained using the hydrophobic catalyst was attributed to a reduction in the wetting efficiency, which improved oxygen accessibility to the catalyst surface. Variations in gas velocity had a negligible D


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Figure 2. Influence of the catalyst wettability on conversion for different WHSV and initial ethanol concentrations. Aerobic oxidation of ethanol aqueous solutions carried out in a trickle bed reactor at 343 K, 0.1 MPa with Pt/γ-Al2O3 (1 wt %) (hydrophilic, open symbols) and Pt/γ-Al2O3(1 wt %) with Teflon (hydrophobic, filled symbols). GHSV = 150 h−1. Adapted with permission ref 69. Copyright 1999 Elsevier.

effect. The catalysts were stable for the whole set of experiments at different weight hourly space velocity (WHSV) and gas hourly space velocity (GHSV) values and ethanol feed solutions. The authors developed and validated a comprehensive reactor model to explain the effect of bed characteristics on the reactor performance. 2.1.2.3. Intensification Strategy. An interesting alternative to the use of hydrophobic catalysts is the use of ON−OFF liquid flow modulation in TBR. The effect of liquid modulation on the oxidation of ethanol aqueous solutions with molecular oxygen under mild operating conditions (343 K and atmospheric pressure) was examined using Pt-59,71 and Pd-56,72based catalysts. Fraguı ́o et al.71 and Muzen et al.59 evaluated the effect of liquid flow modulation on the performance of a continuous minipilot scale TBR packed with 3 mm Pt/γ-Al2O3 (1% w/w) spheres at 343 K and atmospheric pressure (Table S1, entry 18). The liquid ON−OFF flow modulation induced regular variations in the outlet ethanol concentration. Ethanol conversion decreased during the wet period and peaked at the end of the dry period (Figure 3). The cycle period and split (wet over total period ratio) had strong influence on variations, resulting in increased mean conversion for a wide range of cycling parameters values. Ethanol conversion enhancements of up to 20% over those attained under steady-state with equivalent mean liquid velocity were achieved using appropriate cycle period and split (Figure 4a). The products obtained were acetic acid (in liquid phase) and acetaldehyde (both in liquid and gas streams). The authors observed that periodic operation modified the product distribution. Trends observed were then extended to benzyl alcohol oxidation and explained by means of a comprehensive model developed at the particle scale. The model accounted for the wetting and concentration variations inside the catalyst particle and the accumulation of reactants under slow liquid flow modulation.59 Ayude et al.72 applied the slow liquid flow modulation to the aqueous ethanol oxidation, using a 0.5% Pd/Al2O3 egg-shell catalyst in a liquid batch-recycled differential TBR operated at 0.1 MPa and 343 K (Table S1, entry 19). Significant improvements in catalytic activity were achieved by exposing the catalyst to a short surplus of oxygen after a time working in the mass transfer limited regime. Although a certain degree of deactivation due to overoxidation was observed for some cycling conditions, exposing the catalyst to flowing ethanol solution and nitrogen allowed recovering full catalytic activity.

Figure 3. Time dependence of the instantaneous ethanol conversions for different cycle periods and initial ethanol concentrations, split = 2/ 3, Cethanol = 0.006 M: (a) cycle period = 1800 s, (b) cycle period = 900 s. Cethanol = 0.03 M: (c) cycle period = 900 s, (d) cycle period = 90 s. Symbols and dashed lines: left axis, solid lines: right axis. Adapted from ref 59. Copyright 2005 American Chemical Society.

Additionally, using the same reaction system and operating conditions, Ayude et al.56 studied the impact of cycling variables on the yield of the consecutive reactions involved in ethanol oxidation. The use of liquid flow modulation improved the production of acetic acid for the operating conditions studied. Indeed, acetic acid yield exhibited a maximum with the cycle period (Figure 4b). For short cycle periods, the selectivity toward acetaldehyde was favored, whereas a significant increase of acetic acid yield was attained for intermediate periods. For long cycle periods or for extended dry periods, adverse effects associated mainly with catalyst deactivation and liquid reactant scarcity governed the process, resulting in very slow conversion levels. The authors concluded that the product distribution can be significantly modified through tuning the cycling parameters. Published results regarding the use of liquid flow modulation in trickle-bed reactors for ethanol oxidation demonstrated that not only the reaction rate can be enhanced under mild operating conditions, but also product distribution can be tuned by a proper selection of the cycling parameters. Moreover, the importance of deeply understanding the E


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Industrial & Engineering Chemistry Research Scheme 1. Glycerol Oxidation Productsa

Figure 4. Effect of the cycling parameters on the aerobic oxidation of ethanol in TBR with liquid flow modulation: (a) Ethanol conversion enhancement (ε) with respect to the steady state conversion using equivalent WHSV = 4h−1, Pt/γ-Al2O3(1 wt %), 0.1 MPa, 343 K (b) Acetic acid yield, Pd/γ-Al2O3(0.5 wt %), 0.1 MPa, 343 K, Cethanol= 0.25M, dashed line indicates yield for the liquid velocity used during the wet cycle, solid line indicates the yield with a liquid velocity equivalent to a mean value (wet cycle velocity multiplied by the split, s). Reprinted with permission from Muzen et al. Ind. Eng. Chem. Res., 44, 5275 (ref 59 Copyright 2005 American Chemical Society, and from Ayude et al. Chem. Eng. Technol., 35, 899. (ref 56) Copyright 2012 Wiley-VCH.

a

Notation: Ghyde, glyceraldehyde; GA, glyceric acid; TA, tartronic acid; DHA, dihydroxyacetone; HPA, hydroxypyruvic acid; MOXA, mesoxalic acid. C2 products: glycolic acid, oxalic acid, and glyoxylic acid. C1 products: COx, formic acid.

conditions using water as solvent. Table S3 (Supporting Information) summarizes the performance of catalysts tested for the oxidation of glycerol in base-free aqueous solution at T < 373 K and P < 0.5 MPa. Almost all the studies used lab-scale batch or semibatch reactors and purified glycerol as the reactant. The catalytic activity and selectivity depended on the active species nature, particle size and interaction with the support. Several articles examined the effect of operating conditions and reactor configuration. 2.2.1. Toward GA/Ghyde Production. 2.2.1.1. Monometallic Catalysts. Monometallic Pt-based catalysts were found to be more active than Pd and Au in acidic and neutral media.27,82,86,87 Activated carbon has been one of the most studied supports85 (Table S3, entries 1−7). The effect of pH using Pt immobilized on carbon-based supports was systematically studied88 (Table S3, entry 1). A decrease in pH reduces both catalytic activity and selectivity to GA. The production of Ghyde prevails at low pH.28,88,89 In addition, it was reported that acidic and neutral conditions limited the formation of C−C cleavage products (C1 and C2 products).28,29 The active metal nanoparticles sizes, tuned through the synthesis procedure, also influenced the catalyst performance (Table S3, entry 2). Liang et al.90 showed that a decrease in the average Pt particle size improved the catalytic activity of a Pt/ C catalyst. However, no further improvement was attained when the Pt NP size was smaller than 6 nm. In a subsequent work, significant enhancement was achieved in catalytic activity and selectivity toward GA after ball milling the carbon support.91 This trend indicated that improving the reactant accessibility to small Pt NPs probably located in micropores of the carbon support plays a fundamental role in glycerol conversion and reaction selectivity. In this sense, Pt immobilized on supports with large and accessible surface area and meso and/or macropores such as carbon nanotubes or nanofibers outperformed the activity and selectivity toward GA of Pt/C30 (Table S3, entry 3). Chu et al.92 systematically studied the effect of the average Pt particle size (in the range of 1.7−7 nm) on the glycerol oxidation over Pt/multiwall carbon nanotubes (MWCNTs). The catalytic activity normalized by the exposed surface Pt atom exhibited a maximum at 3.6 nm, whereas increasing the particle size favored the selectivity ratio

underlying phenomenon governing the ON−OFF modulating flow strategy was highlighted.73−75 For prolonged dry periods, several factors, such as depletion of the liquid reactant and reversible catalyst deactivation due to overoxidation can negatively affect the attained conversion. 2.2. Glycerol. The continuous increase in biodiesel production has turned its main byproduct, glycerol, into an abundant and low-cost carbon source.76 Development of sustainable technologies to transform glycerol into valuable products is thus fundamental to enhance biodiesel profitability and reduce environmental impact. Hence, glycerol is nowadays considered one of the most relevant platform chemicals. Crude glycerol has a variable composition depending on the characteristics of the biodiesel production process. It generally contains glycerol, methanol, water, soap, methyl esters of fatty acids, glycerides, free fatty acids, and ash.77 The direct use of crude glycerol is highly desired to build efficient and costeffective biorefineries since glycerol purification implies additional expensive stages.78 Conversion of glycerol can be pursued via different catalytic processes.79−82 Among them, the selective oxidation of glycerol over solid catalysts has been the focus of numerous studies in the last decades. Glycerol is a highly functionalized molecule with three hydroxyl groups, and its oxidation renders different chemical products (Scheme 1). When a carbonyl group is formed via oxidation of glycerol, glyceraldehyde (Ghyde) or dihydroxyacetone (DHA) is obtained. These two products exist in equilibrium and can be further oxidized to glyceric acid (GA), tartronic acid (TA), hydroxypyruvic acid (HPA), and mesoxalic acid (MOXA). Besides, carbon−carbon bond cleavage reactions may also occur to give C2 (glycolic acid, oxalic acid, and glyoxylic acid) and C1 compounds (COx, formic acid). The most valuable chemicals arising from glycerol oxidation are GA, DHA, HPA, TA, MOXA, and glycolic acid (see Table S2).82−84 The use of supported mono- and bimetallic catalysts for the liquid phase oxidation of glycerol using oxygen as the oxidant has been extensively studied and reviewed in the past decade.25,81−85 This work is focused on recent works dedicated to the base-free oxidation of glycerol carried out under mild F


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subsequent oxidation rates appeared to be directly proportional to the strength of basic sites present on the supports. Acid products probably remained adsorbed on strong basic sites, facilitating the formation of more oxidized products (C2 and C1).102 Alternatively, Villa et al.97 focused on AuPt NPs supported on different acidic oxides (H-mordenite, SiO2, MCM-41, and sulfated ZrO2). They found that the catalytic activity and product distribution was affected mainly by the content of acidic sites in the support. MCM-41, the support with the lowest content of acid sites among those studied, exhibited the highest initial activity and selectivity toward Ghyde. These authors showed that the successive transformation of glyceraldehyde to glyceric acid proceeded via an acid catalyzed geminal diol formation and its dehydrogenation to a carboxylic acid. More efforts are certainly needed to further elucidate the role of acid/base properties of the support in this reaction system. Xu et al.102 demonstrated that product distribution obtained at 333 K with Au−Pt supported on basic or amphoteric supports (HT, MgO, or Al2O3) was mostly determined by the nature of the support and not by the Au/Pt ratio. However, a higher Au/Pt ratio generally enhanced selectivity toward GA and diminished the extent of C−C bond cleavage.102,104 Similar trends were observed for Au−Pt/HT at ambient temperature100 and for thermally expanded graphene oxide (TEGO) supported Au−Pt alloy nanoparticles at 333 K.106 Villa et al.107 studied the glycerol oxidation with Au−Pt NPs supported on TiO2 at 353 K and 0.3 MPa. Under these conditions, activity was at a maximum for Au/Pt = 6/4. A further decrease in Au content led to rapid deactivation of the catalyst, probably due to overoxidation and active sites poisoning. In addition, a relatively low Au/Pt ratio favored the formation of C3 instead of C2 and C1 products. Hydrogen peroxide was formed in the reaction, and the selectivity to C1− C2 products was well-correlated to the H2O2 detected at the end. Shen et al.27 investigated the influence of the Au/Pt atomic ratio on the product distribution obtained with Au−Pt/ TiO2 catalysts in the glycerol oxidation carried out at 363 K and atmospheric pressure. These authors reported that, for a specific total metal loading and ∼10% of glycerol conversion, a higher Au/Pt ratio enhanced the selectivity toward DHA, whereas a lower Au/Pt ratio promoted the oxidation of primary hydroxyl groups to give GA and Ghyde (Figure 5). Though successful results were achieved with Au−Pt-based catalysts, further studies attempted the preparation of cost-

(Ghyde + GA)/DHA. The surface functional groups affect the Pt NP dispersion and size (Table S3, entry 4−7). Liang et al.28 disclosed that the synthesis of Pt NPs on thiolated multiwall carbon nanotubes (S-MWCNTs) promoted monodispersed Pt clusters, leading to a very active catalyst for the base-free oxidation of glycerol toward GA. The carbon doped with nitrogen enhanced the interaction between support and metals, yielding improved dispersion, activity and stability of Pt.93,94 Chen et al.95 showed that Pt supported nitrogen doped carbon nanotubes (NCNTs) led to significantly higher glycerol conversion and selectivity to GA than Pt supported unmodified carbon nanotubes. These authors attributed the enhancement to an increased surface basicity arising from the addition of a moderate amount of nitrogen to the carbon structure. This is coincident with trends observed for Pt supported mesoporous carbon nitride (MCN) catalysts.96 Zhang et al.26 evaluated the performance of multiwall carbon nanotube-pillared nitrogen doped graphene (MWCNT-NG) as support. The use of Pt/ MWCNT-NG resulted in a high selectivity toward GA (81%) and 64.4% glycerol conversion. These authors ascribed the performance to an increased surface area and dispersion of Pt clusters and to strong electron-donating effects of nitrogen dopant in the support structure. It was reported that Pt supported carbon catalysts were susceptible to deactivation in aerobic oxidation processes, mainly due to leaching of Pt,97 and poisoning of active sites by oxygen and by strongly adsorbed intermediates.98 The use of basic supports such as Mg−Al hydrotalcite, MgO, and Mg(OH)2 considerably enhanced catalytic activity under mild conditions,86,99,100 yielding very good activity and selectivity to GA at ambient temperature and pressure, and relatively low oxygen flow (Table S3, entries 8−10). The improvement was attributed to the strong basic sites of the support which supposedly enhanced the β-hydride abstraction.97 In this regard, Fu et al.101 recently pointed out that results obtained in the base-free glycerol oxidation employing basic supports such as MgO or Mg(OH)2 should be carefully examined. Several basic supports dissolved in the aqueous acid media resulting from the formation of acid products, severely damaging the catalysts. The media turned alkaline as a consequence of support dissolution, and isolation of the free acids required neutralization. 2.2.1.2. Bimetallic Catalysts. Bimetallic Au−Pt catalysts efficiently oxidized glycerol under base-free conditions, with negligible deactivation.87 The activity and selectivity of supported bimetallic Au−Pt catalysts was strongly influenced by the support, reaction conditions, Au/Pt ratio, and synthesis methodology97,102−104 (Table S3, entries 10−16). Regarding operating conditions, an increase in temperature enhanced glycerol conversion but favored products arising from the C−C cleavage.81,104 Concerning stability, a relatively low partial pressure of oxygen limited deactivation by oxygen poisoning.105 Xu et al.102 and Villa et al.97 systematically studied the effect of the acid/base characteristics of the support on performance of AuPt NPs in the glycerol base free oxidation and showed that the use of basic supports yielded more active catalysts (Table S3, entries 11−12). In particular, Xu et al.102 investigated the following supports, listed in terms of decreasing basic strength: MgO, (MgCO 3 ) 4 Mg(OH) 2 , CaCO3, HT, Mg(OH)2, γ-Al2O3, ZnO, CeO2, and TiO2. These authors found that a decrease in the basic strength of the support enhanced the selectivity toward Ghyde. The

Figure 5. Selectivity ratios of glyceraldehyde and glyceric acid to dihydroxyacetone on Au−Pt/TiO2 as a function of Pt contents. Total metal content = 1 wt %. Reprinted with permission ref 27. Copyright 2015 Elsevier. G


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activity and the oxidation of the secondary alcohol group. Hence, the catalyst was quite selective toward DHA. The authors also detected generation of H2O2 in the reaction media which occasioned significant C−C bond cleavage products. Stability issues associated with agglomeration of Au NPs and the formation of zinc oxalate were found. 2.2.2.3. Non-noble Metals-Based Catalysts. The alternative of employing iron-based catalysts and hydrogen peroxide as oxidant is gaining importance for the liquid phase oxidation of glycerol under mild conditions (base-free, ambient temperature, and atmospheric pressure).122,123 Crotti and Farnetti124 performed the glycerol oxidation in an acetonitrile/water mixture using homogeneous iron complexes associated to the tridentate ligand bis(2-pyridinylmethyl)amine (BPA) as catalyst. They reported that high selectivity toward DHA or formic acid could be achieved by tuning operating conditions. A decrease in temperature, H2O2/glycerol ratio or glycerol/ iron ratio improved the selectivity to DHA. However, a high selectivity to DHA (>90%) was obtained only for low glycerol conversion, below 25%. Indeed, in a subsequent work,125 these authors showed that increasing considerably the H2O2/glycerol ratio allowed enhancing catalyst activity, but selectivity to formic acid instead of DHA was improved. They suggested that different active sites are involved in glycerol oxidation, that is, oxidation to formic acid is promoted with free iron ions, whereas DHA is formed when a Fe/BPA complex is used. Moreover, the authors stated that similar results were obtained in reactions conducted in water or using other iron salts. These outcomes envisage a green process to obtain formic acid from glycerol and deserve further exploration. 2.2.3. Continuous Flow System. Notably, for the aerobic base-free glycerol oxidation, heterogeneous catalysts were mostly evaluated in batch reactors. Regarding DHA production, the promising outcomes attained employing Bi− Au−Pt/C in a batch reactor; high selectivity toward DHA at high glycerol conversion,119 motivated the group of Pratti to pursue further investigations.126 The performance and the long-term stability of AuPt/C and Bi-AuPt/C was thoroughly evaluated in a continuous flow fixed bed reactor in which the gas and liquid phases flow cocurrently downward126 (Table S3, entry 26). Both catalysts exhibited stable activity for extended contact times within a wide range of temperature and oxygen flow rate. Decreasing residence time and oxygen flow rate or increasing temperature resulted in a higher selectivity to DHA in the case of Bi−Au−Pt/C. Alternatively, for Au−Pt/C, a low temperature, a low residence time, or a higher oxygen flow rate favored the selectivity toward GA. These authors reported that Au−Pt/C was stable in terms of activity and selectivity for 80 h (Figure 6), whereas Bi−Au−Pt/C was significantly deactivated probably due to leaching and structural modifications. These results clearly highlight the stability issue of Bi modified AuPtbased catalysts. 2.2.4. Intensification Strategy. The advances in the selective photocatalytic oxidation of glycerol for the production of high value compounds have been recently reviewed.84 Zhang et al.127 demonstrated that Bi2WO6 was an effective visible light driven photocatalyst, leading to 87−96% glycerol conversion with 91% selectivity toward DHA after 5 h at room temperature and atmospheric pressure. Lately, Guo et al.128 showed that a cost-effective AuxCu-CuS/TiO2 catalyst could drive the photothermal oxidation of glycerol under solar light irradiation and neutral conditions. Remarkably, a stable glycerol conversion (∼65%) and selectivity to DHA of 58%

effective catalysts by combining Pt with other metals (Table S3, entries17−20). Dou et al.108 showed that, for given metal/ precursor ratios and fixed operating conditions, Pt−Sn-based catalysts displayed higher activity than Pt combined with Mn, Fe, Co, Ni, Cu, Zn, or Au. The main product obtained was GA. Recently, bimetallic Pt−Co supported on reduced graphene oxide (RGO) was found to improve the efficiency of the process performed at 333 K in terms of activity, selectivity toward GA, and stability.109 In addition, it was demonstrated that the use of Pt−Cu/C110 and Pt−Cu/CNTs89 significantly enhanced the glycerol conversion in the base-free oxidation of glycerol. Liang et al.110 reported 86.2% glycerol conversion at 333 K and atmospheric pressure with a selectivity of 70.8% toward GA. 2.2.2. Toward DHA Production. 2.2.2.1. Pt-Based Catalysts. The association of Pt and p-group metals, such as Bi, Sb, promoted the direct catalytic oxidation of the secondary hydroxyl group of glycerol, thus yielding DHA111−114 as the main product (Table S3, entries 21−22). Hu et al.115 systematically investigated the kinetics of the complete glycerol oxidation network over a Pt−Bi/C catalyst. A simplified model was proposed and validated. The kinetic parameters of the steps involved in the reaction network were individually obtained using intermediates as initial reactants. These authors stated that this methodology was a valuable strategy to developing kinetic models for enabling reactor modeling and process optimization to maximize the yield of DHA. The performance of the bimetallic catalysts including pgroup metals was significantly influenced by the Pt/promoter ratio; an optimum value could be determined.85,116 The promoting role of Bi or Sb was mainly attributed to the geometrical effects that controlled the access to the Pt surface active sites.88,117 Ning et al.88 demonstrated that the addition of Bi or Sb, even in reaction solution, led to the selective production of DHA. However, the selectivity toward DHA decreased as glycerol conversion increased.114 Ning et al.118 disclosed that deactivation was mainly due to the adsorption of chelating intermediates on the Pt surface, showing that activity could be restored for at least five reuses by a thermal posttreatment at 473−573 K. Villa et al.119 showed that the addition of Bi to Au−Pt/C effectively reduced the leaching of Bi and allowed maintaining the selectivity to DHA at higher glycerol conversions (selectivity remained around 65% while conversion increased from 30 to 80%) (Table S3, entry 23). 2.2.2.2. Gold-Based Catalysts. Regarding the selective oxidation of the secondary hydroxyl group, important achievements have been made toward the use of gold under base-free and mild temperature and pressure conditions (Table S3, entries 24 and 25). Liu et al.120 showed that Au supported on different metal oxides can be highly selective toward DHA even in an acidic environment. These authors demonstrated a direct activation of the secondary C−H bond of glycerol over the Au catalyst. Among the catalysts evaluated (Au supported on Al2O3, TiO2, ZrO2, NiO, and CuO), Au/CuO was the most active. Under the lowest oxygen pressure studied (0.5 MPa) and 323 K, the selectivity toward DHA was 63.9% with complete glycerol conversion in 8 h. Interestingly, Pan et al.121 prepared a Au/ZnO catalyst with a different procedure, a deposition−precipitation (DP) method followed by a thermal treatment in air. The so-prepared catalyst resulted in the formation of a Au−O−Zn interface and oxygen vacancies. The authors demonstrated that these sites promoted enhanced H


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Besides, some advances have been made toward the development of solar driven technologies to be applied in the base-free oxidation of glycerol for the production of high value compounds.

3. CLEAN CATALYTIC OXIDATION OF FURANS Transforming cellulose and lignocellulose into C5−C6 sugars and then to furan-based platform chemicals such as furfural and 5-hydroxymethylfurfural (HMF) is a relatively simple and sustainable pathway for production of many value-added chemicals.3,13,14,129,130 The structure of furfural and HMF is the base of their high versatility to undergo a wide range of reaction pathways to yield various products. The functional groups play an important role in organic synthesis via oxidation.18,131 HMF has been considered as one of the key molecules due to its high potential for the synthesis of many fine-chemicals and monomers.13,22 It retains the six carbon atoms present in hexoses, and it can be obtained with high selectivity, particularly from fructose.13−15 In the past decade, the number of publications related to catalytic oxidation of furans has increased enormously.131 Several excellent reviews have appeared recently on different aspects of biobased furans transformations.1−3,17−19,22,23,128−132 Information included in this work builds up on recent reports, and it is particularly focused on the base-free aerobic oxidation of HMF carried out using water as solvent. 3.1. Hydroxymethylfurfural (HMF). Hydroxymethylfurfural (HMF), a dehydration product of fructose and glucose is widely considered as an important bioplatform compound that can be converted into a variety of value-added chemicals.14,22,129,132 Bearing a hydroxymethyl group and an aldehyde group, HMF can be oxidized to several possible products (Scheme 2),1,131 such as 5-hydroxymethyl-2-

Figure 6. Long-term stability test in the glycerol oxidation using AuPt/AC (red ●) glycerol conversion; (■) selectivity toward GA. Reprinted with permission from ref 126. Copyright 2018 Elsevier.

were maintained for 35 h without employing an external heating source. Results obtained appear promising and deserve further exploration. 2.3. Alcohols OxidationConcluding Remarks. This review highlights the increasing interest on developing active, selective, and stable catalysts for pursuing the liquid phase aerobic oxidation without the addition of a base employing mild conditions (T < 373 K and P < 0.5 MPa). Most of the works have been carried out in lab-scale reactors; however, only a few have analyzed if mass transfer effects were important. This is a relevant issue for catalysts screening in aerobic oxidations, especially for big particles and low agitation velocities, and particularly when the gas is not flowing. Apparently, the liquid phase oxidation of neither crude bioethanol (obtained by fermentation) nor crude glycerol (arising from biodiesel production) under base-free and mild conditions has been examined so far. The finding of active and stable catalysts for this purpose in view of the unexpected influence of the many side components in crude bioethanol or glycerol is, indeed, a challenging target. This knowledge will considerably contribute to the economics of biodiesel production and reinforce development of a green and sustainable technology. Surprisingly, process intensification routes have been scarcely studied. Regarding the aerobic ethanol catalytic oxidation in the absence of a base, it arises that obtaining acetic acid as the main product seems to be difficult to achieve under mild operating conditions, likely due to the low ethanol conversions obtained. In this sense, published results regarding the use of liquid flow modulation in trickle-bed reactors for ethanol oxidation have demonstrated that not only reaction rate can be enhanced under mild operating conditions, but also product distribution can be tuned by a proper selection of the cycling parameters. Moreover, the importance of deeply understanding the underlying phenomenon governing the ON−OFF modulating flow strategy has been highlighted. For prolonged dry periods, it must be considered that several factors, such as depletion of the liquid reactant, losses of ethanol due to evaporation, and reversible catalyst deactivation due to overoxidation can affect negatively the attained conversion.

Scheme 2. Products Arising from HMF Oxidationa

a

Notation: HMF, 5-hydroxymethylfurfural; HMFCA, 5-hydroxymethyl-2-furancarboxylicacid; DFF, 2,5-diformylfuran; FFCA, 5-formyl-2furancarboxylicacid; FDCA, 2,5-furandicarboxylicacid; MA, maleic acid; FuA, fumaric acid; SA, succinic acid; OA, oxalic acid; FA, formic acid.

furancarboxylicacid (HMFCA), 2,5-diformylfuran (DFF), 5formyl-2-furancarboxylicacid (FFCA), and 2,5-furandicarboxylicacid (FDCA). It can also lead to short chain dicarboxylic acids, such as maleic and succinic acids, when the furan ring breaks and eventually to organic acids of two or one carbon atoms, such as oxalic and formic acids.1,18 Among the products I


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Figure 7. Time and temperature required to achieve FDCA yields larger than 97% and complete conversion in the base-free aerobic oxidation of HMF carried out under pressures below 1 MPa using noble-metal-based catalyst.

free oxidation) has been increasingly envisaged for large-scale industrial applications. Processes carried out in alkaline media require further acidification and purification for subsequent use of FDCA in the polymer industry, hampering the green footprint of the process.19 The base-free aerobic oxidation of HMF usually follows the mechanism shown in Scheme 2 as route B. First, the hydroxyl group of HMF is oxidized to a formyl group to generate DFF. Then, the two formyl groups are oxidized to the diacid successively. One formyl group of DFF is converted into a carboxyl group, forming FFCA. Then, oxidation of the second formyl group leads to FDCA. The classical synthesis of FDCA or DFF via oxidation of HMF usually involves high pressure and temperature, metal catalysts (generally based on noble metals) and frequently organic solvents, which makes the process expensive and polluting. Many solvents have been tested, particularly due to the low solubility of FDCA in water. Ionic liquids (ILs) have been used since the solubility of FDCA is almost 500 times larger than in water and these liquids are less corrosive than alkaline solutions. The unique dissolving abilities of the polar compounds promote the process in ILs and avoid catalyst deactivation due to adsorption of FDCA and byproducts.134,135 Yan et al.134 were able to obtain complete conversion of HMF and a 44% yield of FDCA working at 2 MPa and 433 K during 24 h with a non-noble metal catalyst containing Ce, Fe, and Zr. Recently, Siankevich et al.135 used Pt nanoparticles and ILs with modified anions and were successful in increasing the yield up to 90%. However, the use of ionic liquids increases the costs and turns the process less sustainable. The use of methanol as solvent instead of water was proposed as a strategy for the aerobic oxidation of HMF to avoid the use of bases.136−139 The oxidative esterification of the produced FDCA to its corresponding ester, dimethyl furan2,5-dicarboxylate (DMFD) is promoted using methanol as solvent. Du et al.138 remarked that DMFD possesses a low boiling point and can easily dissolve in most industrial solvents, which is beneficial for its purification. It was argued although that the use of methanol or any other volatile organic solvent in the presence of O2 may have safety concerns in large-scale production.140

of HMF oxidation, FDCA has received significant attention as a potential renewable biomonomer in the polymer industry. FDCA is very stable with a high melting point at 615 K and insoluble in most common solvents. The most important application of FDCA is for production of biobased polymers such as polyamides, polyesters, and polyurethanes.18 It is expected to replace the terephthalic acid used as the monomer for production of polyethylene terephthalate (PET). Hence, FDCA has a great market potential as the precursor for the synthesis of a new polymer class called polyethylene 2,5furandicarboxylate (PEF), which has shown thermal stability comparable to PET.131 Some companies have already developed and commercialized PEF-based bottles.18,129 Many works have also pursued the aerobic oxidation of HMF to produce 2,5-diformylfuran (DFF). DFF has potential applications for the manufacture of pharmaceuticals, fungicides, organic conductors, fluorescent materials, and macrocyclic ligands. The production of DFF predominantly relies on HMF oxidation in various organic and aqueous solvents, and it is available at relatively high cost.17 HMFCA and FFCA are intermediate products in the route for producing FDCA (Scheme 2). Alternatively, the oxidative cleavage of the furan ring may be promoted, forming maleic acid (MA), fumaric acid (FuA), succinic acid (SA), oxalic acid (OA), formic acid (FA), etc.1 3.1.1. Reaction Media. Oxidation in aqueous alkaline solutions (with excess amounts of base additives such as NaOH or Na2CO3) proceeds through the rapid formation of HMFCA (Scheme 2, route A). The presence of hydroxide ions leads to the formation of the geminal diol via nucleophilic addition of a hydroxide ion to the aldehyde with a proton transferred from water. Then, HMFCA is converted to FFCA by the deprotonation of a hydroxyl group.133 Finally, the remaining formyl group of FFCA is oxidized to the carboxyl group to form FDCA. This last step is considered the ratelimiting step of the overall oxidation of HMF into FDCA. The oxidation is usually faster and more selective to FDCA in alkaline media.131 The drawback is the need of neutralization to recover the product, thus leading to a considerable amount of mineral waste and increased risk of reactor corrosion.18 Therefore, the aerobic oxidation in the absence of base (baseJ


Review


sites, leading to high conversion activity for HMF oxidation. The catalyst stability was tested by reusing it in 10 cycles. After the first 6 cycles, aggregation of the NPs became apparent, resulting in a progressive decrease of the desired product yield, from 100% to around 80% in the last cycles. Pt NP aggregation was confirmed by transmission electron microscopy (TEM) (Figure 8). Coincident to the results obtained with the

Works considered in this review are limited to those using water as solvent. A few examples that proposed short chain alcohols as alternative green solvents are also included. 3.1.2. Catalysts Active Species and Related Reaction Conditions. Catalytic base-free aerobic oxidation of HMF has been studied over various heterogeneous catalysts. As oxygen is not easily activated, many heterogeneous catalysts used were based on noble metals (Pt, Pd, Au, and Ru) with high activity for oxidations.18,23,132 Figure 7 illustrates a comparison of the time and temperature required to achieve complete conversion in the base-free aerobic oxidation of HMF using noble-metalbased catalysts leading to FDCA yields larger than 97%. Pt, Au, or bimetallic Au−Pd catalysts required similar conditions, indicating comparable activities. However, Au catalysts usually required less metal loading, thus, higher specific activities than Pt catalysts. For a given metal, differences arise mainly from the effect of the catalysts support. Catalysts immobilized on basic supports needed a relatively shorter time; however, deactivation issues related to support stability should be particularly checked. Ru-based catalysts were apparently less active toward the production of FDCA since they required higher temperatures. Table S4 enumerates reported heterogeneously catalyzed aerobic oxidations of HMF carried out under base-free conditions. The HMF conversion and the product yield given in the table are the best achieved under the desired conditions. 3.1.2.1. Platinum-Based Catalysts. Platinum supported catalysts were the ones more frequently examined in the base-free aerobic oxidation of HMF (Table S4, entries 1−11). Pt NPs stabilized by polyvinylpyrrolidone (PVP) dispersed in water were used by Siankievich et al.,141 who achieved 94% yield after 24 h reaction at mild conditions, atmospheric pressure, and 353 K. These authors showed that the size of the Pt NPs significantly influenced the product distribution. Colloidal NPs are generally difficult to handle particularly for industrial environments; therefore, research was oriented to the use of supported NPs as catalysts. Complete conversion and high selectivity to FDCA were attained using Pt catalysts on different supports. Carbon-based supports, carbon nanotubes (CNTs),140 graphene oxide (GO),140 nitrogen-dopedcarbon-decorated cerium oxide (NC-CeO2)142 Vulcan XC-72 carbon black,143 and activated carbon obtained from chitosan by chemical activation (ACS-800)133 all had good performance. The operating pressure and temperature needed for complete conversion and almost quantitative yields were mild, less than 0.5 MPa pressure and temperatures below 378 K.140,142,143 Zhou et al.140 also tested hydrotalcite (HT) supported Pt with the same metal loading as CNTs and GO, achieving a good yield. Chernysheva et al.143 prepared the catalysts by electrochemical deposition of Pt using a pulse alternating current technique (EPAC) to produce a catalyst with high metal loading (Pt/C 5−30 wt %). The purpose was an attempt to use concentrated aqueous solutions of HMF. Although complete conversion was achieved, the best yield of FDCA remained in 65% due to the formation of unidentified byproducts, including humins. The PtNP/ACS-800133 required higher pressure and temperature but less time (1 MPa O2, 383 K and 5 h) for complete conversion and excellent yield. Similar to the case of dispersed Pt NP,141 the effect of the PVP capping agent on the aerobic oxidation of HMF while preparing the PtNP(PVP)/ACS-800 was significant.133 The authors argued that PVP-capped Pt NP provided more active

Figure 8. Reusability tests of Pt/PVP-ACS 800 (a) and a TEM image of the used Pt/PVP-ACS 800 catalyst (b). Reaction conditions: HMF (0.5 mmol), H2O (10 mL), catalyst (40 mg), O2 pressure (1.0 MPa), time (5 h), and temperature (383 K). Reprinted from ref 133. Copyright 2019 ACS Publications.

aggregated Pt NPs, the catalysts prepared without PVP, which led initially to bigger Pt NP, resulted in 100% conversion and 75% yield of FDCA at similar operating conditions (1 MPa O2, 383 K). A bimetallic Pt−Ni catalyst was supported on activated carbon by atomic layer deposition (ALD) of Pt NP on the surface of Ni/AC particles.144 Complete conversion and excellent FDCA yield (97.5%) were obtained after 15 h at 373 K and 0.8 MPa with a relatively low Pt loading (0.4 wt %). The catalysts maintained its performance almost unaltered during the four cycles tested. Zirconium oxide (ZrO2) was also used as support to prepare a Pt-catalyst by atomic layer deposition (ALD), which was successful in producing FDCA with 97.3% yield at 0.4 MPa and 373 K after 12 h reaction.145 3.1.2.2. Ruthenium-Based Catalysts. Ruthenium was also tested as active species (Table S4, entries 12−20) in catalysts for the base-free aerobic oxidation of HMF. Although active, it apparently requires harsher conditions (higher pressures and/ or temperatures or longer time) to produce FDCA with good yield (see Figure 7). Nevertheless, the selective oxidation to attain DFF, an intermediate product in the base-free oxidation route, is an interesting alternative resulting from the lower activity. A Ru-supported carbon catalyst was active for producing DFF in 0.5 h with around 60% yield and 80% conversion of HMF under 2 MPa air at 383 K.146 Carbonbased supported Ru catalysts also proved to be appropriate for attaining a high conversion of HMF and good yield of FDCA if the reaction time was 10 h or more.147,148 Yi et al.147 achieved complete conversion of HMF and 88% yield of FDCA under 0.5 MPa O2 pressure and 393 K. Mishra et al.148 obtained FDCA and FFCA with comparable yields. In an attempt to improve the selectivity, these authors prepared a catalyst of Ru supported on a MnCo2O4 spinel. With the prepared catalyst, they attained 99% yield but under higher pressure (2.4 MPa air). The catalyst showed good stability for five cycles. Supports with basic sites, hydroxyapatite (HAP),149 MgAlO, and MgO150 allowed 100% conversion and good yields. Gao et al.149 attributed the almost stoichiometric yield to the K


Review


NP supported on in situ generated Mg(OH)2 nanoflakes.157 The basicity of the support also proved to be beneficial for increasing conversion and selectivity. Supported Au catalysts were reported to be efficient for the aerobic oxidation of HMF to DMFD in methanol. Casanova et al.136 reported the oxidative esterification of HMF using a ceria supported gold (Au/CeO2) catalyst without the addition of a base. More than 99% yield of DMFD was obtained under 1 MPa O2 atmosphere at 403 K when the reaction was left to proceed during 5 h. The observed promotional effect of nanocrystalline ceria support in Au/CeO2 in comparison to other supports (Fe2O3, TiO2, or active carbon) was attributed to the capability of CeO2 to act as an oxygen carrier, releasing and adsorbing oxygen atoms by the Ce4+/Ce3+ redox reaction.131 Menegazzo et al.137 studied the influence of several variables on the aerobic oxidation of HMF using methanol to render DMFD; they used Au supported ZrO2 as catalysts. The purpose of the work was to demonstrate the influence of temperature and beneficial effect of using pure O2 instead of air, hence complete conversion was not sought. Recently, Du et al.138 and Mishra et al.139 reaffirmed the feasibility of attaining excellent conversions and yields of DMFD with Au supported catalysts. 3.1.2.4. Silver-Based Catalysts. Oxidation of HMF to DFF was studied using silver (Ag) impregnated manganese oxide octahedral molecular sieve (OMS-2) as a robust and economic catalyst158 (Table S4, entries 34−35). The reaction was carried out using isopropyl alcohol as solvent to simplify the product isolation, attaining excellent conversion (99%) and yield (99%) after 4 h reaction at 438 K under 1.5 MPa. The kinetics of the process was characterized. The stability of the catalyst was tested for six cycles in which it did not lose activity. Ethanol also proved to be a good solvent attaining 94% conversion and 89% yield. 3.1.2.5. Non-noble Metals-Based Catalysts. Catalysts based on non-noble metals have been less frequently tested for the base-free aerobic oxidation of HMF in water (Table S4, entries 36−49). Targeting the base-free aerobic oxidation of HMF using abundant, recoverable, and reusable heterogeneous catalysts is challenging but it would enable the development of environmentally safe and sustainable processes1. Saha et al.159 examined a highly cross-linked FeIII−porous organic polymer (FeIII−POP-1) material, containing basic porphyrin subunits and an iron metal center as the catalyst. Complete conversion and around 80% yield were obtained under 1 MPa air pressure at 373 K after 10 h reaction. The authors claimed that the catalyst was a robustly structured and thermally stable polymer and that the oxidation state of iron did not change upon reaction. With the same catalyst, the authors achieved a 75% yield and complete conversion under 0.5 MPa O2 at 373 K after 5 h reaction. Yan et al.160 prepared a hierarchical structure of vanadium oxide nanobelt-arrayed microspheres as catalyst for the HMF oxidation. Good activity and selectivity for the base-free HMF oxidation to DFF were found. The composite outperformed results obtained with other powder vanadium oxides (bulk V2O5, VO2, and V2O3) and manganese oxides (MnO2, Mn2O3, Mn3O4). Metal ion-doped MnO2 catalysts were examined in the base-free oxidation of HMF using different solvents.161 Cu-doped MnO2 performed better than Cr, V, Mo, Mg, Ca, and Al-doped MnO2. It proved to be active for generating DFF using ethanol as solvent under 0.3 MPa O2 at 423 K. Manganese oxide was tested without metal

availability of both acidic and basic sites around the highly dispersed RuNP on the surface of HAP support. For similar metal loadings, lower times were required when basic supports were used instead of carbon supports (entries 17−18). MgAlO led to a quantitative yield, but the catalyst was not stable.150 The support partly dissolved in the strong acid media resulting from the oxidation. Ru/MgO with the same metal loading led to a slightly lower yield, and it was stable for six cycles. Antonyraj et al.150 also tested comparatively a Ru/ZrO2 catalyst with the same metal content, which despite being less active allowed the production of DFF in good yield under similar operating conditions. Gao et al.151 used a Ru supported Mn−Ce mixed oxide. They were successful in producing FDCA with almost 100% yield after 15 h at 423 K under 1 MPa O2. The prepared catalyst was stable for eight cycles and constitutes an interesting candidate within the Ru-based ones for FDCA. 3.1.2.3. Gold and Palladium-Based Catalysts. Gold (Au) and palladium (Pd) were used as catalyst active species for the oxidation of HMF both separated and alloyed (Table S4, entries 21−33). Gupta et al.152 reported that Au loaded on hydrotalcite (HT) or activated carbon can efficiently oxidize HMF to FDCA. An almost stoichiometric FDCA yield could be obtained with a HMF/Au molar ratio of 40 at 368 K under atmospheric pressure and an O2 flow rate of 50 mL min−1, after 7 h reaction. Gao et al.153 prepared a hydrotalciteactivated carbon (HT-AC) support to disperse Au NP and achieved complete conversion and quantitative yield to FDCA under 0.5 MPa O2 at 373 K after 12 h. The weight ratio of HT to AC influenced conversion; the best results were obtained with a ratio HT/AC = 2. The authors verified that the prepared catalyst was stable by reusing it for six cycles. Similar conversion and yield were attained under 0.5 MPa air after 18 h reaction. Wang et al.154 prepared a Pd supported HT catalysts, which also showed excellent activity toward HMF oxidation under similar conditions. Au-based catalysts are supposed to facilitate the oxidation of the aldehyde to the carboxylic group in HMF. It was observed that Au catalysts usually suffer from deactivation due to irreversible adsorption of intermediates over the Au surface. Pd is supposed to promote the oxidation of the hydroxymethyl arm.129 Hence, the inclusion of Au and Pd as an active species was an attempt to rouse a synergic effect and prevent deactivation. Bimetallic alloyed Au−Pd catalysts were developed for the base-free aerobic oxidation of HMF.155,156 The catalysts showed high selectivity to FDCA and very good stability. Wan et al.155 used carbon nanotubes (CNTs) as support of the Au/Pd alloy nanoparticles. Pretreatment of the CNTs affected the oxidation. The authors demonstrated that CNTs containing more carbonyl/quinone and fewer carboxyl groups favored the FDCA yield due to adsorption of HMF and intermediate compounds. Gao et al.156 developed a bimetallic Au−Pd catalyst supported onto a La-doped Ca−Mg−Al layered double hydroxide (La-CaMgAl-LDH). Results indicated a synergy of the metals forming the alloy and a significant effect of support surface basicity on improving HMF conversion and the selectivity to FDCA. The strong basic features of the support together with Au active sites led to oxidation of the aldehyde group to form HMFCA instead of DFF. Stability of the support was attributed to the high dispersion of La2O3 on the surface of LDH that presumably prevented deterioration. Efficient base-free oxidation of HMF with significant (78%) yield of FDCA was achieved over a bimetallic Ni:Pd(9:1) alloy L


Review

Industrial & Engineering Chemistry Research doping,162 leading to complete conversion and a 65% yield of FDCA after 24 h reaction at 373 K under 1 MPa O2. Gupta et al.163 examined the catalytic performance of a carbonaceous material containing sulfonic acid groups and embedding titanium nanoparticles (CSTi), for the base-free aerobic oxidation of HMF. CSTi was found to convert HMF (91%) into DFF (88% yield) under very mild conditions, 0.1 MPa O2 pressure and 343 K after 8 h. The catalysts proved to be stable after reusing it for five cycles. The oxidation could proceed further with time. Ti NPs have huge potential as catalysts for large scale applications due to their low cost and relatively simple preparation. In addition, some structures can be photoactive, allowing for the use of intensification by UV or visible light in photoreactors. Related to the possibility of photocatalytic oxidation, recently Lolli et al.164 prepared highly porous TiO2−SiO2 and TiO2 as Pt or Au supports to examine them in the aerobic oxidation of HMF. The Pt supported on the TiO2−SiO2 catalyst (TiSiPt) led to 89% HMF conversion after 16 h at 383 K and 1 MPa. The products obtained were FDCA (∼30% yield), FFCA (∼50% yield), and DFF (∼10% yield). TiO2 and the Au/TiO2 were tested as photocatalysts using simulated solar radiation at very mild conditions (303 K). Attained HMF conversion with TiO2 after 8 h was ∼14% and increased to ∼20% when Au/TiO2 was used instead. The reaction apparently proceeded through the formation of DFF, which was degraded to aliphatic intermediates and CO2. The addition of base allowed formation of HMFCA and less mineralization. The presence of O2 increased conversion particularly when using TiO2 but did not improve selectivity. Although further research is indeed necessary for determining the way of tuning the product distribution, the activity of TiO2 makes it a promising candidate as a low-cost catalyst for the oxidation of HMF. Recently, Ventura et al.165 examined the oxidation of HMF to DFF in water using pure O2 as oxidant and magnesium/ cerium mixed oxides as catalysts. They tested several mixture compositions and mixtures of MgO with other metal oxides (TiO2, CuO, MnO2). The best selective oxidation to DFF was obtained with a 1:1 molar ratio of Mg/Ce under 0.9 MPa O2 at 373 K, attaining 98% conversion and yield after 15 h reaction. When the operating conditions were modified, the selective formation of FFCA was also possible. The authors evidenced that although the mixed oxide deactivates, it is recoverable and reusable after calcination. Ventura et al.166 used a CuO·CeO2 mixed oxide as catalyst for FDCA production through basefree aerobic oxidation, attaining complete conversion of HMF. FDCA was selectively formed with 90% yield after 3 h reaction at 383 K and 0.9 MPa O2. A similar deactivation and regeneration by calcination of the mixed oxide catalyst was achievable. Gawade et al.167 synthesized and tested various metal ferrites MFe2O4 (M = Cu, Mn, Co, Mg) as catalysts for HMF oxidation. The authors found the highest activity with the MnFe2O4 spinel structured magnetic nanoparticles and suggested that it was due to the variable oxidation state of manganese. The reaction with molecular oxygen under 2 MPa O2 pressure gave a low yield of FDCA (less than 20%). Then, the authors proposed the use of peroxides to increase the yield. They tested tert-butyl hydroperoxide (TBHP) and hydrogen peroxide (H2O2) as alternative oxidants. Oxidation of HMF using MnFe2O4 and TBHP as the oxidant gave 68% yield of FDCA using water as solvent at 373 K after 5 h. They attained 85% yield if acetonitrile was used instead of water. The catalyst was easily recovered due to its magnetic property and

maintained catalytic activity up to four cycles. The use of peroxides may generally lead to shorter reaction times and milder temperature/pressure conditions than the aerobic oxidation using molecular oxygen or air. However, the costeffective benefits should be examined since the oxidant is necessary in large proportion. In addition, the selectivity is generally more difficult to control since radicals are frequently formed and radical chain reactions are rarely selective. Martı ́nez Vargas et al.168 proposed the use of hydrogen peroxide for oxidizing HMF at neutral pH. They used Co(II), Fe(III), and Cu(II) Salen complexes supported on SBA-15 hexagonal mesoporous silica under atmospheric pressure and ambient temperature (298−313 K). The best performance was obtained with Cu(II)-Salen/SBA-15. Complete conversion was attained with an oxidant/HMF ratio of 44. The four components of routes A and B (Scheme 2) were found in appreciable quantities. FDCA yield was 57%, but it could not be improved by longer reaction time because degradation of the product was observed. Degradation resulted in unidentified products arising from the cleavage of the furan ring (route C, Scheme 2). Ventura et al.169 attempted production of compounds arising from the furan ring cleavage through the aerobic oxidation of HMF and directly from C6 polyols (fructose, glucose, and the dimer sucrose). They used CNT supported metals (M/CNTs, M = Fe, V). In addition, they also tested N-doped CNTs as Fe support (Fe/NCNTs). As expected, the oxidative cleavage of the ring required relatively harsh conditions. However, complete conversion and good selectivity (∼50%) were obtained toward oxalic acid with the Fe/NCNTs catalyst under 1 MPa O2 and 413 K. Modification of the oxygen pressure did not modify the selectivity. On the contrary, excellent selectivity toward formic acid (85.3%) was obtained with the Fe−V/CNTs catalyst under 1 MPa O2 and 413 K. The authors also tested oxidation directly from glucose and fructose (since HMF is obtained by acid dehydration of C6 polyols). Complete conversion of fructose and around 50% selectivity to oxalic acid were obtained using the Fe/CNT catalysts at 1 MPa and 413 K starting directly from fructose. These results were comparable to those obtained from purified HMF; however, longer times were required when starting from the sugar. Glucose required higher temperature (423 K) with a slightly lower conversion, but the selectivity toward oxalic acid was comparable to the ones obtained from fructose and HMF using the Fe−V/CNT catalyst. The authors argued that glucose might be harder to convert likely because it is first isomerized to fructose. Hence, HMF and the precursor’s polyols are very interesting raw materials for the synthesis of products arising from the ring cleavage, and research in this direction is still an open route that deserves to be explored further. 3.1.3. Continuous Flow System. Notably scarce are the works studying the continuous oxidation of HMF despite the relevance for the potential industrial application of the process. Continuous organic synthesis is currently more demanded to attain economic, safer, and more reliable processes.20,21,170 Moreover, if the product is intended as a commodity or as a feedstock for plastic production, a continuous process would be imperative. Lilga et al.171 have evidenced the feasibility of continuously converting HMF to FDCA through base-free oxidation in a 3/8 in. stainless-steel upflow three-phase fixed bed reactor (Table S4, entries 50−51). The authors tested Pt catalysts prepared by chemical vapor deposition of Pt(II)acetylacetonate on several commercial inorganic supports M


Review


The development of technology for green aerobic oxidation of HMF will certainly contribute to the expansion of the production of biobased polymers, and deserves efforts oriented to investigate in flow systems and using intensification strategies for screening robust catalysts candidates.

(ZrO2, Al2O3, and SiO2) of 0.3 mm mean diameter. They also prepared a Pt catalyst supported on granular activated carbon of 0.5 mm mean diameter by standard wet methods. The catalyst with better performance for the base-free condition was Pt/ZrO2 (5 wt %), which showed less adsorption of the products than the carbon supported catalyst. Adsorbed HMF and products tend to deactivate the catalyst; hence, inorganic supports with less specific area were more stable. The authors stated that the Pt/ZrO2 catalyst did not lose activity during the 2 weeks experiments. Operating pressure and temperature were 1 MPa and 373 K using flowing air as the oxidant with a GHSV of 300 h−1. Figure 9 illustrates the outcomes obtained using different liquid hourly space velocity (LHSV) values. Complete conversion and 98% yield of FDCA was attained with a LHSV of 3 h−1.

4. CONCLUSIONS Upgrading biobased compounds via green oxidation technologies to produce valuable chemicals is challenging but crucial for sustainable development. Hence, the number of publications aimed at proposing novel heterogeneous catalysts to achieve the aerobic oxidation of key organic compounds has hugely increased in the past decade. The present work concentrates on articles pursuing the heterogeneous catalytic oxidation of three target molecules (ethanol, glycerol, and hydroxy-methyl furfural) under mild conditions and using water (base-free) as solvent. The majority of the articles used noble metal-based catalysts, evidencing the good activity especially of Au and Pt. In addition, the synergic effects induced by using a second metal were assessed. Increased activity, selectivity, and/or reduced deactivation were achieved by inclusion of a second metal. The influence of the supports was also extensively studied. Supports with basic sites were found to promote the oxidation to carboxylic acids. However, several basic supports were partially dissolved under the acid media resulting from the reaction causing an effect similar to base addition. Almost all the information available in the open literature was obtained in laboratory scale batch reactors with and without flowing oxygen. Very few studies explored flow systems; hence, they have been particularly emphasized in the recompilation. Although significant achievements have been made on the base-free aerobic oxidation of the examined target molecules, further improvements are still needed to succeed in attaining truly green cost-effective technologies at the industrial scale: (1) Development of environmentally friendly catalytic systems that can effectively promote the continuous base-free oxidation of these target molecules under mild conditions is a challenging target. The development of efficient (active and selective), low-cost, and stable transition-metal catalysts toward the aerobic oxidation is highly desirable. Compared to noble-metal, non-noblemetal catalysts have been scarcely studied for the aerobic oxidation; some promising results have been obtained and deserve further examination. (2) Catalyst stability is critical, particularly for continuous processes. The stability of the catalyst depends on the interaction between the active metal species and the support. Supports should be stable and mechanically robust under reaction conditions. It is clear from the available research, that the effect of supports can be decisive in achieving high conversion and selectivity. In addition, catalyst deactivation by the adsorption of byproducts should be explored. (3) Remarkably few studies have been performed to characterize the kinetics of the process. The rational design of the process and reactors used for the oxidation requires estimation of the reaction velocity at different scales. (4) Works pursuing continuous oxidation are notably scarce. Continuous flow systems allow confirmation of the catalysts stability and provide a base for scaling up the

Figure 9. Flow oxidation of 0.5 wt % HMF at 373 K and 1 MPa air over 5 wt % Pt/ZrO2. LHSV = 7.5−3 h−1 and GHSV = 300 h−1. Reprinted with permission ref 171. Copyright 2010 Springer.

Experiments with LHSV = 7.5 h−1 showed less selectivity toward FDCA, giving rise to the three products of oxidation found in the base-free oxidation route (DFF, FFCA, and FDCA) in comparable amounts, as expected for series reactions. The solubility of FDCA in water was the main drawback, imposing a low concentration of HMF to avoid precipitation of the product in the bed. A higher LHSV = 13 h−1 using Pt/SiO2 (5 wt %) led selectively to DFF, but HMF conversion remained around 50%. 3.1.4. HMF OxidationConcluding Remarks. The basefree aerobic heterogeneous catalytic oxidation of HMF has been studied almost exclusively at the laboratory scale. Development of a cost-effective technology at the industrial scale requires active and particularly stable catalysts that can be prepared at relatively low cost. Continuous processes and intensification methods have been scarcely explored for the aerobic catalytic oxidation of HMF under base-free conditions and can contribute to a search for alternatives. Green and sustainable aerobic oxidation relies on effective chemistry but also on efficient reactor design.20,21 Aerobic catalytic oxidation involves multiphase reactors for which hydrodynamics and transport phenomena strongly affect reactor performance, mainly because certain oxidation steps can be very fast. Hence, developed catalysts for aerobic oxidation would preferably be examined in continuous flow to disentangle the factors associated with the use of molecular oxygen. N


Review


(3) Kucherov, F. A.; Romashov, L. V.; Galkin, K. I.; Ananikov, V. P. Chemical Transformations of Biomass-Derived C6-Furanic Platform Chemicals for Sustainable Energy Research, Materials Science, and Synthetic Building Blocks. ACS Sustainable Chem. Eng. 2018, 6, 8064. (4) Besson, M.; Gallezot, P.; Pinel, C. Conversion of Biomass into Chemicals over Metal Catalysts. Chem. Rev. 2014, 114, 1827. (5) Gérardy, R.; Morodo, R.; Estager, J.; Luis, P.; Debecker, D. P.; Monbaliu, J.-C. M. Sustaining the Transition from Petrobased to a Biobased Chemical Industry with Flow Chemistry. Top. Curr. Chem. 2019, 377, 1. (6) Fiorentino, G.; Ripa, M.; Ulgiati, S. Chemicals from biomass: technological versus environmental feasibility. A review. Biofuels, Bioprod. Biorefin. 2017, 11, 195. (7) Kircher, M. The transition to a bio-economy: emerging from the oil age. Biofuels, Bioprod. Biorefin. 2012, 6, 369. (8) Antonetti, C.; Licursi, D.; Fulignati, S.; Valentini, G.; RaspolliGalletti, A. M. New Frontiers in the Catalytic Synthesis of Levulinic Acid: From Sugars to Raw and Waste Biomass as Starting Feedstock. Catalysts 2016, 6, 196. (9) Licursi, D.; Antonetti, C.; Martinelli, M.; Ribechini, E.; Zanaboni, M.; RaspolliGalletti, A. M. Monitoring/characterization of stickies contaminants coming from a papermaking plant − Toward an innovative exploitation of the screen rejects to levulinic acid. Waste Manage. 2016, 49, 469. (10) Sheldon, R. A. The Road to Biorenewables: Carbohydrates to Commodity Chemicals. ACS Sustainable Chem. Eng. 2018, 6, 4464. (11) Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and Practice; Oxford University Press: New York, 1998. (12) Bozell, J. J.; Petersen, G. R. Technology development for the production of biobased products from biorefinery carbohydratesthe US Department of Energy’s “Top 10” revisited. Green Chem. 2010, 12, 539. (13) Antonetti, C.; Melloni, M.; Licursi, D.; Fulignati, S.; Ribechini, E.; Rivas, S.; Parajó, J. C.; Cavani, F.; RaspolliGalletti, A. M. Microwave-assisted dehydration of fructose and inulin to HMF catalyzed by niobium and zirconium phosphate catalysts. Appl. Catal., B 2017, 206, 364. (14) Antonetti, C.; Fulignati, S.; Licursi, D.; RaspolliGalletti, A. M. Turning Point toward the Sustainable Production of 5-Hydroxymethyl-2-furaldehyde in Water: Metal Salts for Its Synthesis from Fructose and Inulin. ACS Sustainable Chem. Eng. 2019, 7, 6830. (15) Antonetti, C.; RaspolliGalletti, A. M.; Fulignati, S.; Licursi, D. Amberlyst A-70: A surprisingly active catalyst for the MW-assisted dehydration of fructose and inulin to HMF in water. Catal. Commun. 2017, 97, 146. (16) Werpy, T.; Petersen, G. Top Value Added Chemicals from Biomass. Results of Screening for Potential Candidates from Sugars and Synthesis Gas; U.S. D.o. Energy, 2004; Vol. I. (17) Pal, P.; Saravanamurugan, S. Recent Advances in the Development of 5-Hydroxymethylfurfural Oxidation with Base (Nonprecious)-Metal-Containing Catalysts. ChemSusChem 2019, 12, 145. (18) Zhang, Z.; Deng, K. Recent Advances in the Catalytic Synthesis of 2,5-Furandicarboxylic Acid and Its Derivatives. ACS Catal. 2015, 5, 6529. (19) Wojcieszak, R.; Ferraz, C. P.; Sha, J.; Houda, S.; Rossi, L. M.; Paul, S. Advances in Base-Free Oxidation of Bio-Based Compounds on Supported Gold Catalysts. Catalysts 2017, 7, 352. (20) Hone, C. A.; Kappe, C. O. The Use of Molecular Oxygen for Liquid Phase Aerobic Oxidations in Continuous Flow. Top. Curr. Chem. 2019, 377, 2. (21) Gavriilidis, A.; Constantinou, A.; Hellgardt, K.; Hii, K. K.; Hutchings, G. J.; Brett, G. L.; Kuhn, S.; Marsden, S. P. Aerobic oxidations in flow: opportunities for the fine chemicals and pharmaceuticals industries. React. Chem. Eng. 2016, 1, 595. (22) van Putten, R. J.; van der Waal, J. C.; de Jong, E.; Rasrendra, C. B.; Heeres, H. J.; de Vries, J. G. Hydroxymethylfurfural, A Versatile Platform Chemical Made from Renewable Resources. Chem. Rev. 2013, 113, 1499.

process. Available research has evidenced that performing the aerobic oxidation with flowing gas and liquid provide valuable information. More experimental evidence on the feasibility of the continuous process and appropriate modeling of the required reactors would contribute to achieving implementation of the technology. (5) Effect of intensification strategies such as using photoactive catalysts or ultrasound to promote the base-free oxidation without additives are exciting alternatives to explore further. It was evidenced that the intensification of reactor performance by periodic operation has a strong influence on ethanol oxidation. Hence, intensification strategies related to reactor operation and the use of appropriate multifunctional reactors are interesting routes to search for the base free aerobic oxidation. (6) It is worthwhile investigating further the use of the crude compounds, bioethanol obtained from fermentation, crude glycerol derived from biodiesel production, and precursors C6 polyols instead of HMF, as the starting material.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b00977. Overview of catalysts and their catalytic performance in the aerobic base-free oxidation of ethanol; frequent uses of compounds derived from glycerol oxidation; overview of some catalysts and their catalytic performance in the oxidation of glycerol in base-free aqueous solution at T < 373 K and P < 0.5 MPa; base-free aerobic oxidation of HMF over heterogeneous catalysts, indicating best conversion and yield attained for the corresponding product (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +54-11-52859029. E-mail: [email protected]. ORCID

María Alejandra Ayude: 0000-0002-3804-8179 Miryan C. Cassanello: 0000-0001-8233-3797 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from CONICET (PIP11220150100902CO), FONCyT (PICT 20140704, PICT 20160083) and University of Buenos Aires (UBACyT 20020170100604BA) is gratefully acknowledged. We also thank Dr. Laura Fasce and Ing. Mariano Viva for their assistance in the creation of the TOC/graphic abstract.

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REFERENCES

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