catalysts

Review Solid Acids for the Reaction of Bioderived Alcohols into Ethers for Fuel Applications

Federica Zaccheria *, Nicola Scotti and Nicoletta Ravasio

CNR, Istituto di Scienze e Tecnologie Molecolari, Via C. Golgi 19, 20133 Milano, Italy; [email protected] (N.S.); [email protected] (N.R.) * Correspondence: [email protected]; Tel.: +39-02-50314384



Received: 19 December 2018; Accepted: 1 February 2019; Published: 12 February 2019


Abstract: The use of solids acids in the synthesis of ethers suitable to be used as fuels or fuel additives were reviewed in a critical way. In particular, the role of Brønsted and Lewis acid sites was highlighted to focus on the pivotal role of the acidity nature on the product distribution. Particular emphasis is given to the recently proposed ethers prepared starting from furfural and 5-hydroxymethyl furfural. Thus, they are very promising products that can be derived from lignocellulosic biomass and bioalcohols and possess very interesting chemical and physical properties for their use in the diesel sector.

Keywords: ethers; furanics; zeolites; solid acids; Brønsted sites; Lewis sites

1. Introduction Oxygenated fuels, such as alcohols and ethers, represent an important alternative to conventional non-oxygenated ones. In fact, the use of oxygen containing molecules in internal combustion engines provides several advantages in terms of environmental impact, namely in reducing greenhouse gas emissions (GHG), due to the low content of pollutants, like sulfur, and to complete combustion reducing formation of particulate. [1,2]. Their use varies from the role of additives in gasoline to the formulation in diesel blends depending on their structure (Table1). Ethers, such as methyl tert butyl ether (MTBE), have been used as an octane booster instead of tetraethyl lead and to decrease engine emissions. For these reasons, MTBE use has seen a speedy growth until the end of the 1990s and after, while in the past decade, the worldwide market underwent significant changes because of the ban in the US and Canada [3]. Thus, MTBE is toxic, volatile, and flammable and its use entails several concerns for human and water protection [4]. Dimethylether (DME), already in use since 1867 as an anesthetic agent, has been used in diesel engines starting from 1986 and its production is foreseen to gradually increase, particularly on the wave of bio-DME synthesis [5]. Also, ethyl tert-butyl ether (ETBE) and tert-amyl methyl ether (TAME) have been successfully introduced to replace tetraethyl lead (i.e., a toxic pollutant) in gasoline. The strategic role of oxygenated components in the fuel market is witnessed by the presence of several networks and associations born to promote the responsible diffusion of oxygenated fuels, both alcohols and ether ones. Examples of this approach are the European Fuel Oxygenates Association (EFOA) and the International DME Association (IDA).

Catalysts 2019, 9, 172; doi:10.3390/catal9020172 www.mdpi.com/journal/catalysts Catalysts 2019, 9, 172 2 of 21 Catalysts 2018, 8, x FOR PEER REVIEW 2 of 21 Catalysts 2018, 8, x FOR PEER REVIEW 2 of 21 CatalystsCatalystsCatalysts 2018 2018, 82018, ,x 8 FOR,, x8 ,FOR x PEER FOR PEER PEERREVIEW REVIEW REVIEW 2 of2 21of2 21of 21 Catalysts 2018, 8, x FOR PEER REVIEW 2 of 21 Catalysts 2018, 8, x FOR PEER REVIEWTable 1. Most widespread ethers for fuel applications 2 of 21 Catalysts 2018, 8, x FOR PEER REVIEWTable Table1 . Most 1. widespreadMost widespread ethers for ethers fuel applications for fuel applications. 2 of 21 Catalysts 2018, 8, x FOR PEER REVIEWTableTableTable 1 . Most1. Most1. Mostwidespread widespread widespread ethers ethers ethers for for fuel for fuel applications fuel applications applications 2 of 21 Catalysts 2018, 8, x FOREther PEER REVIEWTable 1. Most widespreadStructure ethers for fuel applicationsBio-moiety Fuel 2sector of 21

EtherEther Table 1. Most widespreadStructure Structure ethers for fuel applicationsBio-moiety Bio-moiety Fuel sector Fuel sector

Ether Structure Bio-moiety Fuel sector Ether Table 1. Most widespreadStructure ethers for fuel applicationsBio-moiety Fuel sector Ether Structure Bio-moiety Fuel sector Dimethyl ether O Gasoline and Table 1. Most widespread ethers for fuel applications DimethylDimethylEther ether ether StructureO Bio-moietyMethanol GasolineFuelGasoline sector and and diesel DimethylDimethyl ether ether Table 1. Most widespread ethers for fuel applications GasolineGasoline and and DimethylEther etherDME Structure O O O MethanolBio-moietyMethanol GasolineFueldiesel sector and blend DimethylEtherDMEDME ether Structure Bio-moietyMethanolMethanol dieselFuelGasoline sector blend andblend DME diesel blend Methanol O EtherDME Structure Bio-moiety Fueldiesel sector blend DME diesel blend DimethylDME ether Methanol dieselGasoline blend and

DimethylEtherDiethyl ether ether Structure O Bio-moiety GasolineFuelGasoline sector and and DiethylDME ether Methanol Gasolinediesel blend and DiethylDiethyl ether ether O Ethanol GasolineGasoline and and diesel DimethylDiethylDiethylDME ether ether ether O O MethanolEthanolEthanol GasolineGasolinedieselGasoline blendand and and Diethyl etherDEE Ethanol Gasolinediesel and blend DimethylDMEDEE DEE ether O O OO MethanolEthanolEthanol dieselGasoline blend andblend DiethylDMEDEEDEE DEE ether MethanolEthanol dieselGasolinedieseldiesel blend blend and blend DiethylDEEDMEDibutyl ether ether O Ethanol dieseldieselGasolineGasoline blend blend and and DibutylDiethylDibutylDEE ether ether O EthanolButanol GasolinedieselGasoline blend and and diesel DiethylDibutylDibutylDEE ether ether ether O 3 3 ButanolEthanol GasolineGasolinedieselGasoline blendand and and DibutylDiethyl etheretherDBE O 3 O 3 ButanolEthanolButanolButanol GasolineGasolinediesel and and blend DibutylDBEDEE DBEether O3 O 3 3 ButanolButanol dieselGasoline blend andblend DEEDBE DBE 3 3 3O O 3 Ethanol dieseldieseldiesel blend blend blend DibutylDBEDEE ether O 3 Butanol dieseldieselGasoline blend blend and

3 DibutylDBE ether O Butanol Gasolinediesel blend and

3 Dibutyl ether 3 Gasoline and DBE O Butanol diesel blend MethylDibutyl tert- etherbutyl ether 3 3 GasolineOctane and booster DBE O Butanol diesel blend MethylMethylMethyl tert-butyl tert-tert-butylbutyl ether ether ether 3 O 3 Butanolmethanol OctaneOctane Octanebooster booster booster for DBE diesel blend MethylMethyl tert- DBEtert-butylMTBEbutyl ether ether 3 O 3 methanolmethanol OctanedieselOctanefor booster blend boostergasoline MTBE O methanol gasoline MethylMTBE tert-butyl ether O methanolmethanol Octanefor gasoline booster O Methyl tert-MTBEMTBEbutyl ether O O methanol Octaneforfor gasoline gasolinebooster MTBE for gasoline Methyl tert-MTBEbutyl ether O methanol Octanefor gasoline booster Methyl tert-MTBEbutyl ether O methanol Octanefor gasolinebooster Methyl MTBEtert-butyl ether O methanol Octanefor gasoline booster EthylEthyl tertMTBE-butyl tert -butyl ether ether O methanol Octanefor Octanegasoline booster booster EthylEthylEthyl Ethyltert MTBEtert-butyl terttert-butyl-butyl ether ether ether ether O ethanol OctaneforOctaneOctane gasoline Octanebooster booster booster booster for Ethyl tert-butylETBE ether O ethanolethanolethanol Octanefor booster gasoline Ethyl tertETBE-butylETBE ether O ethanolethanol Octanefor gasoline boostergasoline

ETBEETBE O forfor gasoline gasoline O Ethyl tert-butyl ether O ethanol Octane booster ETBE for gasoline

O O

Ethyl tert-butyl ether Octane booster ETBE O ethanol for gasoline

Ethyl tert-butyl ether O Octane booster ETBE ethanol for gasoline EthylTert tertETBE-amyl-butyl methylether ether O ethanol Octanefor gasolineOctane booster booster Tert-amyl methylether Octane booster Tert-amyl methylether Octane booster Tert-amylTertETBE-amyl methylether methylether O ethanolnethanol Octanefor Octanegasoline booster booster TertTert-amyl-amyl methylether methylether nethanol OctaneOctane booster booster for Tert-amylETBE methyletherTAME O nethanol Octanefor gasolinefor booster gasoline nethanolnethanol Tert-amylTAME methylether O nethanolnethanol Octanefor gasoline booster TAME O gasoline TAME for gasoline Tert-amylTAMETAME methylether O nethanol forOctanefor gasoline gasoline booster Tert-amylTAME methylether O O O nethanol Octanefor gasoline booster Tert-amyl TAMEmethylether O nethanol Octanefor gasolinebooster Tert-amylTert-TAME amylmethylether ethyl ether nethanol Octanefor gasolineOctane booster booster Tert-Tert-amylTAMEamyl ethyl ethyl ether ether O nethanolethanol Octanefor Octanegasoline booster booster Tert-Tert-amylTert-amylTAMEamyl ethyl TAEEethyl ethyl ether ether ether O ethanol OctaneforOctane gasolinefor Octanebooster boostergasoline booster for Tert-amylTAEE ethyl ether O O ethanolethanolethanolethanol Octanefor gasoline booster TAEETAEE OO forfor gasoline gasoline Tert-amylTAEE TAEEethyl ether O O ethanol forOctane gasoline boostergasoline Tert-amylTAEE ethyl ether O O ethanol Octanefor gasoline booster Tert-amyl ethyl ether O ethanol Octane booster Tert-amylTAEE ethyl ether ethanol Octanefor gasoline booster TAEE O ethanol for gasoline TAEE O O for gasoline TAEE OO for gasoline O O O O O O OH O OH O O OH di- and tri-tert-butyl glycerol O O O OH OH di- and tri-tert-butyl glycerol O O OH di-di- anddi- and tri-and tri-tert tri-tert-butyltert-butylether-butyl glycerol glycerol glycerol O O OH Glycerol Diesel additives di- and tri-ethertert-butyl glycerol O OH Glycerol Diesel additives di- andether tri-ethertertether -butyl glycerol O OH GlycerolGlycerolGlycerol DieselDieselDiesel additives additives additives di- and tri-DTBGethertert-butyl and TTBG glycerol O OH Glycerol Diesel additives di- andDTBG tri-tert andether-butyl etherTTBG glycerol GlycerolGlycerol Diesel additives Diesel additives di- and tri-DTBGDTBGtert-butyl and and TTBG glycerol TTBG di- andDTBG tri-DTBGtert andether-butyl TTBG and TTBGglycerol O Glycerol Diesel additives DTBGether and TTBG O Glycerol Diesel additives ether O O Glycerol Diesel additives DTBGether and TTBG O Glycerol Diesel additives DTBG and TTBG O O O DTBG and TTBG O O O O O O O DTBG and TTBG O O O O O O O O O O O O O O O

O O O O O O O OH di- and tri-ethyl glycerol ether O O OH Glycerol di- and tri-ethyl glycerol ether O O OHOH Glycerol di-di- and and tri-ethyl tri-ethyl glycerol glycerol ether ether O O OH GlycerolGlycerol DieselDiesel additives additives di- and tri-ethylDEG glycerol and TEG ether O O OH Glycerolethanol DieselDieselDiesel additives additives additives di- andDEGdi- tri-ethyl and and tri-ethyl TEGglycerol glycerol ether O O OH ethanolGlycerol Diesel additives di- andDEG tri-ethylDEGDEG and and TEGand glycerol TEG TEG ether O OH ethanolGlycerolethanolethanol Diesel additives di- and tri-ethylDEG and glycerolether TEG ether O OH GlycerolGlycerolethanol ethanol Diesel Dieseladditives additives O O OH di- and tri-ethylDEG and glycerol TEG ether O Glycerolethanol Diesel additives di- and tri-ethylDEG glycerol and TEG ether O Glycerol DEG and TEG O O ethanol Diesel additives DEG and TEG O O O ethanol Diesel additives DEG and TEG O O ethanol O O O O O O O O O O O RO O RO O O O O RO RO O O Furfural ROO RO O O Furfural Alkoxymethyl furan O O FurfuralFurfural Diesel blend Alkoxymethyl furan ROO O Furfuralalcohol Diesel blend AlkoxymethylAlkoxymethylAlkoxymethyl furan furan furan RO O alcoholFurfural DieselDieselDiesel blend blend blend Alkoxymethyl furan RO O alcoholFurfuralalcoholalcohol Diesel blend RO O Furfural Alkoxymethyl furan RO OO O alcohol Diesel blend AlkoxymethylAlkoxymethyl furan furan RO O O FurfuralFurfuralalcohol alcohol Diesel blend Diesel blend Alkoxymethyl furan RO O O O O Furfural Diesel blend Alkoxymethyl furan RORO O O O alcohol Diesel blend Alkoxymethyl furfural RO O O alcoholHMF alcohol Diesel blend Alkoxymethyl furfural RO O HMFalcohol alcohol Diesel blend AlkoxymethylAlkoxymethyl furfural furfural RO O HMFHMF alcohol alcohol Diesel Diesel blend blend Alkoxymethyl furfural O O HMF alcohol Diesel blend Alkoxymethyl furfural RO O O HMF alcohol Diesel blend Alkoxymethyl furfural RO O O HMF alcohol Diesel blend Alkoxymethyl furfural RO RO O HMF alcohol Diesel blend RO O AlkoxymethylAlkoxymethyl furfural furfural RO RO O O HMFHMF alcohol alcohol Diesel blend Diesel blend Alkoxymethyl furfural RORO O HMF alcohol Diesel blend 2,5-bis(alkoxymethyl)furan RO O HMF alcohol Diesel blend 2,5-bis(alkoxymethyl)furan2,5-bis(alkoxymethyl)furan O HMFHMF alcohol alcohol Diesel Diesel blend blend 2,5-bis(alkoxymethyl)furan2,5-bis(alkoxymethyl)furan RO O HMFHMF alcohol alcohol Diesel Diesel blend blend RO O OR 2,5-bis(alkoxymethyl)furan RO OR HMF alcohol Diesel blend 2,5-bis(alkoxymethyl)furan O OR OR HMF alcohol Diesel blend 2,5-bis(alkoxymethyl)furan RO OROR HMF alcohol Diesel blend 2,5-bis(alkoxymethyl)furan OR HMF alcohol Diesel blend 2,5-bis(alkoxymethyl)furan 2,5-bis(alkoxymethyl)furan OR HMFHMF alcohol alcohol Diesel blend Diesel blend OR OR OR

Catalysts 2019, 9, 172 3 of 21

The possibility to use bioalcohols has certainly played a major role in the development and diffusion of ethers in the fuel sector. In particular, ethanol and butanol can be obtained from fermentationCatalysts 2018, 8, processesx FOR PEER starting REVIEW form a wide variety of lignocellulosic biomass (Figure1). 4 of 21

Fossil source Gasoline Additives Gasoline Diesel

Refinement

Olefins MTBE, ETBE, TAME, TAEE

Dehydration, H+ bio - Alcohols DME, DBE DEE Fermentation Gasification Furanic Biomass ethers

+ H Furfural, HMF

Glycerol Glycerol ethers

Oils and fats

Figure 1. Major routes to ethers for fuel blending. Figure 1. Major routes to ethers for fuel blending The production of bioethanol has seen an enormous increase in recent years, reaching a world This review mainly intends to highlight the most peculiar aspects related with the use of production of more than 27 thousands of million gallons in 2017 [6]. The ethanol production process heterogeneous catalysts and particularly acid solid catalysts in the synthesis of this main class of is industrially mature and it can be produced from any materials containing sugar molecules. ethers employed in fuels, taking as examples MTBE and DME and giving a particular emphasis on Fermentation processes applied to non-edible feedstock sourced from agriculture and forestry wastes furanic ethers due to their most recent development. MTBE, DME, and furanics are chosen as strongly increase the advantages obtained with its use [7]. examples to identify and highlight the different needs and possibilities in tuning the catalysts’ The industrial production of biobutanol relies on the improvement of the over 100-year old ABE acidity to obtain both high activity and selectivity in the desired products depending on the critical (acetone-butanol-ethanol) process and it is now an economically viable and competitive product with points of the different syntheses. a global market expected to reach 17.78 M USD by 2022 [8]. An additional source derives from the biodiesel industry that provides glycerol as the main 2. The Use of Solid Acids in MTBE Synthesis by-product of the transesterification reaction of triglycerides with methanol or ethanol and glycerol availabilityThe industrial is foreseen synthesis to reach of 4 MTBE billion is gallons based byon 2020the addition [9]. The useof methanol of a polyalcohol, to isobutene. such asAs glycerol,already allowsmentioned, one toits obtain use has branched seen a dramatic ethers that decrease possess du verye to interestingits impact on features drinking as diesel water additives contamination. due to theirThe United excellent States combustion environmental properties, protection good blending agency ability,(USEPA) and has high in cetane fact categorized number [10 ].MTBE as a possibleMore carcinogenic recently, the agent, preparation thus preventing of ethers derivedits use as from a gasoline biomass additive platform since molecules, May 2006 such [13]. as hydroxylTherefore, methyl the USA, furfural Canada, (HMF) Japan, and and furfural, Western have Eu alsorope gained shifted great to ETBE, attention ethanol, due to or their TAME. interesting On the propertiescontrary, Eastern in the diesel Europe sector and [11 Asia]. Pacific countries built new MTBE capacities as its demand increased,Synthetic depending strategies on forthe theboost C-O-C in gasoline bond formation consumption are traditionallyand requirements based for on cleaner the reaction fuels. of This an alkoxidescenario withmakes a chlorideMTBE still intermediate, an important according industrial to the product, Williamson but susceptible reaction. On to the important other hand, variations. the use ofConsequently, alternative routes the research and of heterogeneousactivity on MTBE catalysts synthe allowssis has the seen avoidance a drastic of break wastes in and the inconvenient last decade, intermediates,but the main whichconclusions is highly drawn desirable. about the use of heterogeneous catalysts can give important prompts in understanding the acidity features of solid systems employed in this kind of reaction, used also for TAME and TAEE. The traditional industrial process is based on a liquid phase reaction of methanol with isobutene (molar feed ratio 1:1) at temperatures in the range of 50–100 °C promoted by an ion exchange resin, namely Amberlyst-15 [14].

Catalysts 2018, 8, x FOR PEER REVIEW 3 of 21

The possibility to use bioalcohols has certainly played a major role in the development and diffusion of ethers in the fuel sector. In particular, ethanol and butanol can be obtained from fermentation processes starting form a wide variety of lignocellulosic biomass (Figure 1). The production of bioethanol has seen an enormous increase in recent years, reaching a world production of more than 27 thousands of million gallons in 2017 [6]. The ethanol production process is industrially mature and it can be produced from any materials containing sugar molecules. Fermentation processes applied to non-edible feedstock sourced from agriculture and forestry wastes strongly increase the advantages obtained with its use [7]. The industrial production of biobutanol relies on the improvement of the over 100-year old ABE (acetone-butanol-ethanol) process and it is now an economically viable and competitive product with a global market expected to reach 17.78 M USD by 2022 [8]. An additional source derives from the biodiesel industry that provides glycerol as the main by-product of the transesterification reaction of triglycerides with methanol or ethanol and glycerol availability is foreseen to reach 4 billion gallons by 2020 [9]. The use of a polyalcohol, such as glycerol, allows one to obtain branched ethers that possess very interesting features as diesel additives due to their excellent combustion properties, good blending ability, and high cetane number [10]. More recently, the preparation of ethers derived from biomass platform molecules, such as hydroxyl methyl furfural (HMF) and furfural, have also gained great attention due to their interesting properties in the diesel sector [11]. Synthetic strategies for the C-O-C bond formation are traditionally based on the reaction of an alkoxide with a chloride intermediate, according to the Williamson reaction. On the other hand, the useCatalysts of 2019alternative, 9, 172 routes and of heterogeneous catalysts allows the avoidance of wastes4 and of 21 inconvenient intermediates, which is highly desirable. Two main alternative strategies have been used for the preparation of ethers to be used in fuels and fuelTwo blending, main alternative namely strategiesthe addition have of beenan alcohol used forto a the C-C preparation double bond, of ethersas it is to the be case used of in MTBE fuels and fuelTAME, blending, or the namelydehydration the addition of two alcohol of an alcohol molecules, to a C-C which double is the bond, pathway as it isfollowed the case for of MTBEDME, DE,and and TAME, furanics or the (Scheme dehydration 1). of two alcohol molecules, which is the pathway followed for DME, DE, and furanics (Scheme1).

R' R'

+ R''OH Addition R R OR''

ROR' Dehydration ROH + R'OH Scheme 1. General ether synthesis used for common fuel ethers. Scheme 1. General ether synthesis used for common fuel ethers. Both synthetic ways are based on the use of acidic catalysts that are able to promote the activationBoth synthetic of the double ways bondare based towards on the alcohol use of addition, acidic catalysts making that it more are able electrophilic, to promote or the activationdehydration of of the alcohols. double The bond use oftowards solid acids, alcohol formerly addition, covering making a pivotal it more role inelectrophilic, traditional refineryor the dehydrationprocesses, has of seen alcohols. a rejuvenating The use momentof solid and acids, has formerly received muchcovering attention a pivotal due torole its applicationin traditional to refinerybiomass deconstructionprocesses, has andseen transformation a rejuvenating [ 12moment]. The study and andhas designreceived of innovativemuch attention and performing due to its applicationheterogeneous to biomass acid systems deconstruction has therefore and seen transfor muchmation attention [12]. and The advancement. study and design of innovative and performingThis review heterogeneous mainly intends acid to systems highlight has thetherefore most peculiarseen much aspects attention related and advancement. with the use of heterogeneous catalysts and particularly acid solid catalysts in the synthesis of this main class of ethers employed in fuels, taking as examples MTBE and DME and giving a particular emphasis on furanic ethers due to their most recent development. MTBE, DME, and furanics are chosen as examples to identify and highlight the different needs and possibilities in tuning the catalysts’ acidity to obtain both high activity and selectivity in the desired products depending on the critical points of the different syntheses.

2. The Use of Solid Acids in MTBE Synthesis The industrial synthesis of MTBE is based on the addition of methanol to isobutene. As already mentioned, its use has seen a dramatic decrease due to its impact on drinking water contamination. The United States environmental protection agency (USEPA) has in fact categorized MTBE as a possible carcinogenic agent, thus preventing its use as a gasoline additive since May 2006 [13]. Therefore, the USA, Canada, Japan, and Western Europe shifted to ETBE, ethanol, or TAME. On the contrary, Eastern Europe and Asia Pacific countries built new MTBE capacities as its demand increased, depending on the boost in gasoline consumption and requirements for cleaner fuels. This scenario makes MTBE still an important industrial product, but susceptible to important variations. Consequently, the research activity on MTBE synthesis has seen a drastic break in the last decade, but the main conclusions drawn about the use of heterogeneous catalysts can give important prompts in understanding the acidity features of solid systems employed in this kind of reaction, used also for TAME and TAEE. The traditional industrial process is based on a liquid phase reaction of methanol with isobutene (molar feed ratio 1:1) at temperatures in the range of 50–100 ◦C promoted by an ion exchange resin, namely Amberlyst-15 [14]. On the other hand, several studies have focused on the use of zeolites due to their higher thermal stability and easier industrial application with respect to sulphonic resins as well as their higher selectivity despite lower activity. The reaction is an acid catalyzed process that can proceed via preliminary protonation of isobutene, followed by the addition of methanol. Alternatively, a mechanism based on the formation of a methoxonium ion acting as proton donor could take place [15]. Catalysts 2019, 9, 172 5 of 21

In any case, a typically Brønsted catalyzed reaction is involved and one of the main parameters that is recognized to be crucial for the catalyst activity is the number of acid sites, that is in fact the strong point of Amberlyst-15 as compared to zeolites. Thus, the resin has a Brønsted acid site concentration of 4.8 mmol/g versus 1.3 and 2.2 observed in H-ZSM-5 and HY zeolites, respectively [11]. The main attempts in obtaining efficient zeolites for the methanol/isobutene reaction have therefore been directed to the enhancement of the acidic properties. One of the methods explored was the dealumination process [16], leading to the removal of some structural Al and resulting in a decrease in concentration sites, but an increase in acid strength. Indeed, dealumination strongly improved the performances of zeolites in the synthesis of MTBE [17], where a monotonic increase in TOF (defined as the ratio between the reaction rate and the concentration of Brønsted acid sites) was observed with the (extra lattice Al)/(lattice Al) ratio. Interactions between the extra-lattice Al obtained by dealumination and the acid sites connected with the lattice Al appeared to cause acid strength enhancement, in turn responsible for the increase in MTBE formation. The formation of extra-lattice Al can also be obtained by means of other modifications, such as treatment with strong electron-withdrawing compounds, such as triflic acid and ammonium fluoride [18,19]. On the other hand, other authors ascribe the increase of activity in MTBE synthesis derived from dealumination to the contribution of the external surface area and not to the extra-framework aluminum species [20]. In particular, dealuminated H-beta is as active as Amberlyst-15 when it has Si/Al ratios from 13 to 35, and the MTBE yield depends on the external surface area: The higher the area, the higher the yield. Some studies were also reported on the synthesis of MTBE starting from methanol and tert-butanol via dehydration under gas phase conditions by using H-zeolites. The comparison of various zeolites allows observations of an activity comparable to the one observed with Amberlyst-15, even when Lewis acid sites are the main ones on the catalyst surface, as in the case of Hβ [21]. Other catalytic systems proposed for this application are sulphated ZrO2 or supported heteropolyacids [22]. In all cases, the understanding of the relationship between the strength of acidity and the number of acidic sites represents the main route to optimize the obtained performances and to design a catalyst potentially more suitable for industrial needs. A very similar approach is taken for the synthesis of ETBE, which has become a popular alternative to MTBE, possessing superior properties due to its higher octane rating, higher boiling point, lower flash point, lower volatility, and water solubility. The ETBE production capacity increased from 2 million tons to 4 million tons from 2005 to 2007 and it is mainly used as a fuel additive in several European countries, including France, the Netherlands, Germany, Spain, and Belgium [23].

3. The Use of Solid Acids in DME synthesis DME has applications in both the chemical and fuel market, and its demand has seen rapid growth from a global production of 30 kt/year in 2000 to 11.5 Mt/year in 2012, with the total cost of DME world production foreseen to reach US $9.7 billion by 2020 [24]. Besides its use in aerosols, varnishes, and pesticides manufacturing, the most important industrial value of dimethyl ether is associated with the global fuel market. Thus, its physical properties make DME similar to liquefied petroleum gas (LPG). Thus, its heat capacity is somewhat lower than that of LPG, but it is easier to liquefy when decompressed and it does not produce a green-house effect. Moreover, DME, as opposed to LPG, has a high cetane number (55–60). Dimethyl ether is the simplest one among the alkylic ethers and it can be obtained by methanol vapor-phase dehydration via an acid catalyzed reaction. On the other hand, the most recent developments aim to achieve direct synthesis of DME from syngas to obtain a strategic process intensification, considering that methanol itself is produced starting from CO + H2. A step forward is also desirable in using CO2-containing syngas (Scheme2)[25]. Catalysts 2018, 8, x FOR PEER REVIEW 6 of 21 lower than that of LPG, but it is easier to liquefy when decompressed and it does not produce a green-house effect. Moreover, DME, as opposed to LPG, has a high cetane number (55–60). Dimethyl ether is the simplest one among the alkylic ethers and it can be obtained by methanol vapor-phase dehydration via an acid catalyzed reaction. On the other hand, the most recent developments aim to achieve direct synthesis of DME from syngas to obtain a strategic process intensification, considering that methanol itself is produced starting from CO + H2. A step forward is also desirable in using CO2-containing syngas (Scheme 2) [25]. The main results obtained in the two possible ways are summarized in a review by Sun et al. [26] and the importance of the nature and density of acidic sites in methanol dehydration is Catalysts 2019, 9, 172 6 of 21 emphasized in a very clear description.

Bifunctional or hybrid catalyst

Solid Acid CO + H2 2 CH3OH CH3OCH3

H2O Scheme 2. Synthesis of DME from methanol or directly from syngas. Scheme 2. Synthesis of DME from methanol or directly from syngas. The main results obtained in the two possible ways are summarized in a review by Sun et al. [26] In this scenario, two main classes of catalysts are considered, the γ-Al2O3 systems and acidic and the importance of the nature and density of acidic sites in methanol dehydration is emphasized in zeolites. In both cases, the critical point is the maximization of DME’s selectivity with respect to the a very clear description. main byproducts, which are methane and coke deposits. In this scenario, two main classes of catalysts are considered, the γ-Al O systems and acidic Alumina is basically a good choice due to its high selectivity, low cost, and2 3 high resistance and zeolites. In both cases, the critical point is the maximization of DME’s selectivity with respect to the lifetime, and the mechanism in alcohol dehydration with respect to the structure and acidity has main byproducts, which are methane and coke deposits. been largely studied [27,28]. Alumina is basically a good choice due to its high selectivity, low cost, and high resistance and The activity in methanol dehydration is observed to increase by increasing the acid site density, lifetime, and the mechanism in alcohol dehydration with respect to the structure and acidity has been in turn, depending on the structure type of the alumina, as evidenced in a study based on the largely studied [27,28]. comparison of γ-Al2O3 and η-Al2O3 catalysts respectively prepared from boehmite and bayerite via The activity in methanol dehydration is observed to increase by increasing the acid site density, in calcination at various temperatures [29]. turn, depending on the structure type of the alumina, as evidenced in a study based on the comparison In an analogous way, a proper modification of γ-Al2O3 with a χ-Al2O3 leads to an increase in the of γ-Al O and η-Al O catalysts respectively prepared from boehmite and bayerite via calcination at acid site2 density,3 which2 3 allows an increase of the yield in methanol from 50% to 86% [30]. various temperatures [29]. Moving from Al2O3 catalysts to zeolites leads to an increase in activity, due to the higher In an analogous way, a proper modification of γ-Al O with a χ-Al O leads to an increase in the resistance to water, at the expense of selectivity. In 2fact,3 on the one2 3hand, it was shown that acid site density, which allows an increase of the yield in methanol from 50% to 86% [30]. generally, the stronger the acid sites, the more active the catalysts. However, when Brønsted sites are Moving from Al O catalysts to zeolites leads to an increase in activity, due to the higher involved, their strength2 3and reaction temperature should be controlled to avoid hydrocarbons’ resistance to water, at the expense of selectivity. In fact, on the one hand, it was shown that generally, formation. [31] Thus, the dehydration reaction is carried out under quite severe conditions, namely the stronger the acid sites, the more active the catalysts. However, when Brønsted sites are involved, temperatures around 250 °C, and several zeolite modifications have been proposed to limit the their strength and reaction temperature should be controlled to avoid hydrocarbons’ formation. [31] formation of coke and hydrocarbons. Thus, the dehydration reaction is carried out under quite severe conditions, namely temperatures One of the ways proposed is based on Na+ modification of zeolite frameworks [32]. The authors around 250 ◦C, and several zeolite modifications have been proposed to limit the formation of coke tested a series of zeolites impregnated with increasing amounts of Na, resulting in systems with a and hydrocarbons. reduced total amount of acidic sites and a different distribution of the acidity strength. In particular, One of the ways proposed is based on Na+ modification of zeolite frameworks [32]. The authors the higher the Na content, the lower the density of strong sites and the higher the density of weak tested a series of zeolites impregnated with increasing amounts of Na, resulting in systems with a sites. reduced total amount of acidic sites and a different distribution of the acidity strength. In particular, The modification of HZSM-5 with 80% mol of Na results in a significant increase in DME yield, the higher the Na content, the lower the density of strong sites and the higher the density of weak sites. particularly when working at 320 °C, which is an important point in the design of a catalyst that is The modification of HZSM-5 with 80% mol of Na results in a significant increase in DME yield, able to promote the reaction starting from syngas, thus requiring high temperatures to be particularly when working at 320 ◦C, which is an important point in the design of a catalyst that is performed. able to promote the reaction starting from syngas, thus requiring high temperatures to be performed. The beneficial effect of increasing the density of moderate acidic sites was also observed by The beneficial effect of increasing the density of moderate acidic sites was also observed by modifying zeolite Y with La, Ce, and Pr via ion-exchange [33]. modifying zeolite Y with La, Ce, and Pr via ion-exchange [33]. The particular case of methanol dehydration into DME identifies the great importance of the acidic sites’ strength, regardless of their Lewis or Brønsted nature [32], to make the catalytic system more selective and more resistant under the reaction conditions used. In the case of DME, this aspect is even more important when planning the bifunctional process starting from CO + H2, requiring temperatures over 300 ◦C and a metal based system that is able to promote methanol synthesis.

4. The Use of Solid Acid in Glycerol Ethers Synthesis Glycerol ethers have received great attention due to both its use as a starting material derived from the biodiesel industry and the possibility of obtaining a wide variety of products with different Catalysts 2018, 8, x FOR PEER REVIEW 7 of 21

The particular case of methanol dehydration into DME identifies the great importance of the acidic sites’ strength, regardless of their Lewis or Brønsted nature [32], to make the catalytic system more selective and more resistant under the reaction conditions used. In the case of DME, this aspect is even more important when planning the bifunctional process starting from CO + H2, requiring temperatures over 300 °C and a metal based system that is able to promote methanol synthesis.

4. The Use of Solid Acid in Glycerol Ethers Synthesis CatalystsGlycerol2019, 9, 172ethers have received great attention due to both its use as a starting material derived7 of 21 from the biodiesel industry and the possibility of obtaining a wide variety of products with different industrial applications and economic value. Mono- and di- glycerol ethers in fact find application in industrial applications and economic value. Mono- and di- glycerol ethers in fact find application in several industrial sectors, ranging from the cosmetics to pharmaceuticals, as well as to detergency, several industrial sectors, ranging from the cosmetics to pharmaceuticals, as well as to detergency, inks, inks, polymers, and materials. The possibility to vary the carbon chain linked to the glyceril polar polymers, and materials. The possibility to vary the carbon chain linked to the glyceril polar moiety moiety allows the design of a great variety of products that possess tuned properties in terms of allows the design of a great variety of products that possess tuned properties in terms of hydrophilicity hydrophilicity and hydrophobicity [34]. and hydrophobicity [34]. On the other hand, the peculiar structure of glycerol, possessing three –OH groups, offers On the other hand, the peculiar structure of glycerol, possessing three –OH groups, offers several several possibilities in terms of products and physical behavior, while also raising selectivity issues. possibilities in terms of products and physical behavior, while also raising selectivity issues. The higher The higher ethers with short alcohols, namely di- and tri- derivatives (DAGEs and TAGEs), find a ethers with short alcohols, namely di- and tri- derivatives (DAGEs and TAGEs), find a direct application direct application in diesel and biodiesel formulation by virtue of their water insolubility, while the in diesel and biodiesel formulation by virtue of their water insolubility, while the higher polar and higher polar and hydrophilic mono-ethers (MAGEs) are interesting intermediates for the production hydrophilic mono-ethers (MAGEs) are interesting intermediates for the production of chemicals, of chemicals, such as dioxolanes, in turn studied as fuel components as well (Scheme 3) [35,36]. such as dioxolanes, in turn studied as fuel components as well (Scheme3)[35,36].

OH

R HO OH + OH

H+ O R O R OH O + RCHO O O + O OH + HO OH O

O MAGEs

H+ + R OH

R R OH R O R

O O + HO O

DAGEs

H+ + R OH

R R O R

O O

TAGE Scheme 3. Etherification of glycerol with alcohol to MAGEs, DAGEs, and TAGE or dioxolanes. Scheme 3. Etherification of glycerol with alcohol to MAGEs, DAGEs, and TAGE or dioxolanes. Also, in this case, the strategies for the preparation of ethers are based on the two main routes of additionAlso, of in the this –OH case, group the strategies to alkene for or the preparation dehydration of of ethers two alcohols. are based on the two main routes of additionMuch of workthe –OH has group been carried to alkene out or on thetert dehydration-butyl glycerol of two ethers, alcohols. which are obtained by addition of glycerolMuch to work isobutylene has been or bycarried dehydration out on tert with-butyltert -butanolglycerol ethers, (TBA). which are obtained by addition of glycerolThe to use isobutylene of isobutylene or by as dehydration an alkylating with agent tert in-butanol heterogeneously (TBA). catalyzed reactions was explored by using ion-exchange resins and zeolites [37–39]. The method of the addition of isobutene was the first attempted one, also based on the examples of MTBE and TAME, but it has technological problems related to the need of using a solvent for glycerol and the drawbacks of handling a three-phase system. On the contrary, the dehydration approach, which exploits the use of tert-butyl alcohol, allows the use of solid-liquid conditions and the use of the alcohol as both the solvent and the reactant. Catalysts 2019, 9, 172 8 of 21

Protonic acids are mainly explored and are successful for the etherification of glycerol with tert-butanol, with ionic resins, acidic clays, and sulfonic functionalized materials as the main performing ones. A study reported by Frusteri et al. [40] showed the importance of the acid sites’ accessibility, as it was found when using Amberlyst-15 possessing not only a high acid site density (4.70 mmolH+/g), but also an average pore diameter of 300 Å. The need to remove water formed during the reaction is one of the most critical points limiting etherification to di- and tri-ethers. In fact, water separation not only shifts the equilibrium, but also limits the competition with tert-butanol and glycerol on the active site adsorption. A great improvement in this regard is reported when using sulfonated hybrid silicas (SxTS100-xO) [41]. This kind of sulfonated silica gave a 74% conversion of glycerol with an 18% yield in di- and tri-ethers (reaching 98% and 28% after 24 h) at 75 ◦C versus a 51% conversion obtained with A15 under the same conditions. The authors ascribe these results mainly to the hydrophilicity of the surface. The presence of silanol groups on the hybrid silica precludes the inhibition effect of water and, consequently, the conversion increases with time without reaching a maximum, as observed in the case of resins. An even higher yield in di-tert-butyl glycerol ethers of up to 36% was obtained with sulfonic acid functionalized mesoporous polymer [42]. Acidic clays, and in particular montmorillonite KSF/O, require high reaction temperatures, but this allows the obtainment of up to 33% of di-ethers [43]. On the other hand, isobutylene, C4 fraction, and tert-butanol are obtained from fossil sources, whilst the use of ethanol or butanol would grant a fully bioderived material. The main problem related with the use of shorter alcohols is their lower reactivity as alkylating agents due to the lower stability of the carbocation formed. Therefore, high temperatures are usually requested and, consequently, there are significant limitations in the catalyst choice. In particular, the use of acid ion-exchange resins successfully employed for the etherification with tert-butanol must be avoided due to their poor thermostability, thus making zeolites and modified silicas the most studied systems for this application. An early paper by the group of Fajula [44] reported on the comparison of sulfonic-acid grafted silicas, zeolites, and acidic resins for the preparation of MAGEs with ethanol, identifying the fundamental role of the hydrophobic/hydrophilic character of the catalyst besides the acidic properties. All the catalysts tested were strongly influenced by the surface polarity. In fact, within the resins tested, the comparison between Amberlyst and Nafion identified the good activity of the first one compared to the inactivity of the second, while no differences were observed in the side etherification of ethanol to dimethylether, occurring by the high concentration of ethanol used as the solvent. This evidence shows that both catalysts promote etherification of ethanol thanks to their acidity, but the hydrophobic character of the Nafion resin prevents the adsorption of the highly polar and hydrophilic glycerol, thus not allowing the formation of MAGEs. On the contrary, Amberlyst gives up to a 52% conversion of glycerol with a 90% selectivity to MAGEs even at 150 ◦C (Table2, entries 1 and 2). Very similar conclusions can be drawn from the experiments carried out with zeolites. For these systems, the authors found a diagnostic correlation between the activity of different zeolites and their aluminum content, in turn influencing the surface polarity. In fact, high aluminum contents generate a high acidic sites density and a high surface polarity. Maximum activity was observed for Si/Al = 25, representing the best tradeoff between the acidity and surface polarity: A lower aluminum content entails a lower acidity and an Al/Si ratio that is too high forces the process to be limited by the hydrophilic character of the catalyst. The good performances of H-Bea with Si/Al = 25 were also underlined in a study aimed to maximize the content in di- and tri-ethers [45]. Catalysts 2019, 9, 172 9 of 21

Table 2. Selected examples of the etherification reaction of glycerol and ethanol.

Conversion Selectivity Entry Catalyst Alcohol T (◦C) Product Ref (%) (%) a Amberlyst Monoethyl 1 ethanol 150 52 90(10) [44] A35 glycerol ether Monoethyl 2 BEA 25 ethanol 200 57 75(25) [44] glycerol ether 3 Amberlyst-15 ethanol 180 - 96 65(19; 6) [46] 4 H-Beta ethanol 180 - 92 71(17; 12) [46] Monoethyl 5 Ar-SBA-15 ethanol 200 73 54 (14) [47] glycerol ether H PW O Monoethyl 6 3 12 40 ethanol 160 97 62(28;10) [48] (HPW) glycerol ether Monoethyl 7 HPW/SiO ethanol 160 91 67 (23; 9) [48] 2 glycerol ether a Selectivity to DAGE; TAGE in brackets.

Comparable performances of Amberlyst-15 and H-Beta have been reported by Pinto et al. [46], but in this case, the authors focused the attention on the preparation of a mixture of MEG, DEG, and TEG to directly test the material obtained as a fuel additive. The results obtained in terms of the reduction of the cloud point and pour point when added to biodiesel show their potential for biodiesel formulations. The use of arensulfonic acid-functionalized mesostructured silicas was explored by analyzing the influence of different reaction parameters, such as the temperature, ethanol/glycerol ratio, and catalyst loading, on the activity and selectivity [47]. The best results were obtained at 200 ◦C and for a high ethanol concentration and high catalyst loading. Another typical reported Brønsted acid is tungstophspohric acid (H3PW12O40, HPW) and HPW/SiO2 [48]. Although quite long reaction times are required, the authors obtained a very interesting conversion of up to 97.1%, with a selectivity to the mono-, di-, and tri-ether, respectively, of 61.9%, 28.1$, and 10% (Table2, Entry 4). Interestingly, the catalyst is active also with water-containing glycerol, even though there is a lower conversion due to the hydrolysis reverse reaction being favored under the conditions used, which results in a mixture that contains up to 30% by wt of water with respect to ethanol. On the other hand, the presence of water allows a higher selectivity to be obtained for the monoether, raising it to 89.6% from 65.9%. Another interesting approach to the use of bioalcohols has been reported in the etherification of glycerol with butanol [49]. A commercial ion exchange resin was used in a reactor equipped with a membrane system for the water removal, which resulted in an improvement of the conversion despite a lower selectivity.

5. The Use of Solid Acids in Furanic Ethers Synthesis Furanic ethers represent a very interesting class of compounds to be used as additives or diesel blends due to their biomass origin and to their good properties with respect to fuel requirements. The need for biomass derived fuels has gained an ever-increasing importance in the last decades and, particularly in the diesel sector, the search for lignocellulosic derived materials that can act as an alternative to first generation vegetable oil based biodiesel is strongly encouraged. Actually, both the limited availability of renewable oils and their relative high cost makes the use of profusely available lignocellulosic feedstock an appealing method to be pursued [50]. The best candidates as fuel chemicals that are intermediate among the lignocellulose deconstruction stream are furfural (FA), 5-hydroxymethylfurfural (HMF), and levulinic acid (LA), and some main strategies aiming to improve Catalysts 2018, 8, x FOR PEER REVIEW 10 of 21

Catalystscandidates2019, as9, 172fuel chemicals that are intermediate among the lignocellulose deconstruction stream10 of 21 are furfural (FA), 5-hydroxymethylfurfural (HMF), and levulinic acid (LA), and some main strategies aiming to improve the carbon skeleton structure to prepare products in the diesel-range the carbon skeleton structure to prepare products in the diesel-range from these platform molecules from these platform molecules have been explored. have been explored. Due to the presence of the aldehydic group in FA and HMF, aldol condensation reactions are Due to the presence of the aldehydic group in FA and HMF, aldol condensation reactions are often often used to increase the number of the C atom. used to increase the number of the C atom. One possibility is the aldol condensation with ketones over basic catalysts [51,52], which allows One possibility is the aldol condensation with ketones over basic catalysts [51,52], which allows the obtainment of C9-C15 molecules that can eventually be further hydrogenated to form alkane the obtainment of C9-C15 molecules that can eventually be further hydrogenated to form alkane chains. The oligomerization of furans via acid catalyzed condensation has also been reported as a chains. The oligomerization of furans via acid catalyzed condensation has also been reported as a carbon-upgrading process [53]. The method of the formation of alkenes through γ-valerolactone carbon-upgrading process [53]. The method of the formation of alkenes through γ-valerolactone obtained from levulinic acid is another viable route [54]. obtained from levulinic acid is another viable route [54]. However, a different strategy can be envisaged based on the etherification of the alcoholic However, a different strategy can be envisaged based on the etherification of the alcoholic functionality in HMF or the one that can be obtained through the reduction of the aldehyde group in functionality in HMF or the one that can be obtained through the reduction of the aldehyde group in both FA and HMF (Scheme 4). both FA and HMF (Scheme4).

O O O HO O RO

O OR C5- and C6-sugars O O

OR RO O

HMF and furfural Furan ethers Scheme 4. The way to furan ethers from lignocellulosic biomass. Scheme 4. The way to furan ethers from lignocellulosic biomass. Indeed, the potential of 5-(ethoxymethyl)furfural studied by Avantium has been considered as a biofuelIndeed, due to the its potential high boiling of 5-(ethoxymethyl)furfural point and its energy density studied (31.3 by MJ/l). Avantium This valuehas been is higher considered than the as onea biofuel reported due forto its ethanol high boiling (22 MJ/l) point and and comparable its energy todensity standard (31.3 gasoline MJ/l). This (34.2 value MJ/l) is higher and diesel than fuel the (35.8one reported MJ/l), and for together ethanol with(22 MJ/l) its boiling and comparable point of 235 to◦C, standard it makes gasoline this ether (34.2 very MJ/l) attractive and diesel as a diesel fuel fuel(35.8 additive MJ/l), and [55 together]. with its boiling point of 235 °C, it makes this ether very attractive as a diesel fuel additiveInventors [55]. found a convenient way to furanic ethers starting directly from sugars by using acid catalystsInventors at a high found temperature a convenient in alcoholic way to furanic solvents, ethers thus preventingstarting directly the formation from sugars of decomposition by using acid productscatalysts at from a high HMF, temperature which are usuallyin alcoholic formed solvents, in water. thus Moreover, preventing the the high formation potential of of decomposition furanic ethers asproducts fuels and from fuel HMF, additives which has are been usually surveyed formed by somein water. of the Moreover, same authors the [high56]. potential of furanic ethersIn as particular, fuels and within fuel additives the series has of 5-(alkoxymethyl)furfuralbeen surveyed by some of that the are same obtained authors with [56]. alcohols varying fromIn C1 particular, to C8, the blend’swithin propertiesthe series areof 5-(alkoxymethyl)furfural improved by increasing the that number are obtained of carbon with atoms alcohols in the alcoholvarying chain.from C1 to C8, the blend’s properties are improved by increasing the number of carbon atomsDiethers in the alcohol obtained chain. by a second etherification of the reduced carbonyl group in HMF are even superiorDiethers in terms obtained of the by blend’s a second properties. etherification When a of bioalcohol the reduced is used carbonyl for the group ether formation,in HMF are furanic even etherssuperior are in 100% terms bio-based of the andblend’s do notproperties. lead to low-value When a by-productsbioalcohol is during used for the the process ether preparation. formation, furanicMoreover, ethers are according 100% bio-based to the analysis and do reported not lead by to Lange low-value et al. by-products [57], among theduring mainly the studiedprocess productspreparation. of furan upgrading, ethyl furfuryl ether shows, for example, a very limited footprint in termsMoreover, of CO2 emission according and to investment the analysis costs reported considering by Lange its preparation et al. [57], process, among whereas the mainly the footprint studied increasesproducts of significantly furan upgrading, as the furan ethyl ringfurfuryl is saturated ether shows, or opened for example, to pentanols a very due limited to the footprint significant in capitalterms of intensity CO2 emission linked to and the exothermicityinvestment costs of hydrogenation considering reactions.its preparation Therefore, process, ethers whereas represent the an interestingfootprint increases upgrade significantly of furans to produceas the furan good ring gasoline is saturated components or opened with a smallto pentanols footprint, due whereas to the hydrogenatedsignificant capital molecules intensity entail linked a much to the larger exothermicity footprint. of hydrogenation reactions. Therefore, ethers representThe preparationan interesting of furanupgrade ethers of furans is also to the produce idea behind good gasoline a smart waycomponents identified with by a Mascal small etfootprint, al. to whereas maximize hydrogenated the yield in molecu HMFles starting entail nota much only larger from footprint. glucose, but also from cellulose by limiting decomposition product formation [58]. The strategy is based on the conversion into 5-(chloromethyl)furfural (CMF) that, due to its hydrophobicity, is easily trapped by the organic solvent

Catalysts 2018, 8, x FOR PEER REVIEW 11 of 21 Catalysts 2018, 8, x FOR PEER REVIEW 11 of 21 CatalystsThe 2018 preparation, 8, x FOR PEER of furanREVIEW ethers is also the idea behind a smart way identified by Mascal et11 al. of to21 The preparation of furan ethers is also the idea behind a smart way identified by Mascal et al. to maximize the yield in HMF starting not only from glucose, but also from cellulose by limiting maximizeThe preparation the yield in of HMFfuran startingethers is notalso onlythe idea from be hindglucose, a smart but wayalso identifiedfrom cellulose by Mascal by limiting et al. to decomposition product formation [58]. The strategy is based on the conversion into decompositionmaximize the yieldproduct in HMF formation starting [58].not onlyThe from strategy glucose, is butbased also onfrom the cellulose conversion by limiting into 5-(chloromethyl)furfural (CMF) that, due to its hydrophobicity, is easily trapped by the organic 5-(chloromethyl)furfuralCatalystsdecomposition2019, 9, 172 product (CMF)formation that, due[58]. to Theits hystrategydrophobicity, is based is easily on trappedthe conversion by the organic11 into of 21 solvent when working under biphasic conditions. Although 5 wt% of LiCl and concentrated HCl are solvent5-(chloromethyl)furfural when working under (CMF) biphasic that, conditions.due to its hyAldrophobicity,though 5 wt% isof LiCleasily and trapped concentrated by the HClorganic are used as catalysts, this strategy allows the obtainment of up to 80% in an intermediate that in turn can usedsolvent as catalysts,when working this strategy under biphasicallows the conditions. obtainment Al thoughof up to 5 80% wt% in of an LiCl intermediate and concentrated that in turn HCl can are whenbe directly working used under for the biphasic etherification conditions. reaction Although according 5 wt% to Scheme of LiCl 5. and concentrated HCl are used beused directly as catalysts, used for this the strategy etherification allows reactionthe obtainment according of up to Schemeto 80% in 5. an intermediate that in turn can as catalysts, this strategy allows the obtainment of up to 80% in an intermediate that in turn can be be directly used for the etherification reaction according to Scheme 5. directlyH OH used for the etherification reaction according to Scheme5. H OH OH H O O O H H LiCl, HCl, CH Cl O O O O O 2 2 O EtOH O HO LiCl, HCl, CH2Cl2 EtOH HO H O O HO O H O O H LiCl, HCl, CH Cl EtOH HO H OH 2 2 HO H Cl OEt OH Cl OEt HO H OH H H H OH OH Cl OEt SchemeH 5. TransformationOH of glucose into 5-(chloromethyl)-furfural and the following etherification Scheme 5. Transformation of glucose into 5-(chloromethyl)-furfural and the following etherification Schemewith ethanol. 5. Transformation of glucose into 5-(chloromethyl)-furfural and the following etherification withScheme ethanol. 5. Transformation of glucose into 5-(chloromethyl)-furfural and the following etherification with ethanol. with ethanol. The use of LiCl and HCl is generally not desirable from the process management point of view, TheThe use use of of LiCl LiCl and and HCl HCl is is generally generally not not desira desirableble from from the the process process management management point point of of view, view, but the work certainly represents a pioneering step towards an efficient utilization of biomass to butbut theThe workwork use certainlyofcertainly LiCl and represents represents HCl is generally a pioneeringa pioneering not stepdesira step towardsble towards from an the efficient an process efficient utilization management utilization of biomass ofpoint biomass toof obtainview, to obtain useful fuel components. Thus, the reaction protocol can also be applied to C5 sugars and obtainbut the useful work fuelcertainly components. represents Thus, a pioneering the reaction step protocol towards can an alsoefficient be a ppliedutilization to C5 of sugarsbiomass and to usefuldirectly fuel to raw components. biomass, such Thus, as the straw, reaction wood, protocol or corn can stover, also beas appliedit is able to to C5 convert sugars the and cellulosic directly directlyobtain useful to raw fuel biomass, components. such as Thus,straw, the wood, reaction or corn protocol stover, can as alsoit is beable a ppliedto convert to C5 the sugars cellulosic and tofraction raw biomass, irrespective such of asthe straw, other wood,components or corn present stover, in as the it raw is able starting to convert material the [59]. cellulosic fraction fractiondirectly irrespectiveto raw biomass, of the such other as components straw, wood, present or corn in thestover, raw asstarting it is able material to convert [59]. the cellulosic irrespectiveNevertheless, of the otherthe method components of forming present ether in the via raw alcohol starting dehydration material [59 is]. the most explored one fractionNevertheless, irrespective the of methodthe other of components forming ether present via alcoholin the raw dehydration starting material is the most [59]. explored one due Nevertheless,to its intrinsic the higher method sustainability. of forming etherAs already via alcohol mentioned dehydration the formation is the most of explored unsymmetrical one due due toNevertheless, its intrinsic thehigher method sustainability. of forming As ether already via alcohol mentioned dehydration the formation is the mostof unsymmetrical explored one toethers its intrinsic by means higher of solid sustainability. acid catalysts As is already a convenie mentionednt route the that formation has been of explored unsymmetrical in the last ethers years. by ethersmeansdue to by ofits means solid intrinsic acid of solid catalystshigher acid sustainability. iscatalysts a convenient is a convenieAs route already thatnt route hasmentioned been that exploredhas the been formation inexplored the last of in years. unsymmetricalthe last years. ethers by means of solid acid catalysts is a convenient route that has been explored in the last years. 5.1. Direct Etherification of HMF and Furfural 5.1.5.1. Direct Direct Etherification Etherification of HMF and Furfural 5.1. DirectThe firstEtherification report relying of HMF onand the Furfural use of solid acids for the etherification of HMF into the TheThe first first report relying on the use of solid acids for the etherification etherification of of HMF into the corresponding ethyl ether (Scheme 6) describes the use of different mesoporous acids, namely correspondingcorrespondingThe first ethylreportethyl ether etherrelying (Scheme (Scheme on 6the) describes6) use describes of thesolid useth eacids ofuse different forof differentthe mesoporous etherification mesoporous acids, of namely HMFacids, into SBA-15,namely the SBA-15, zirconium modified SBA-15, Al-MCM-41, and Amberlyst®15, at 140 °C [60] The authors SBA-15,zirconiumcorresponding zirconium modified ethyl modified SBA-15,ether (Scheme Al-MCM-41,SBA-15, 6)Al-MCM-41, describes and Amberlyst th ande use Amberlyst®15,® of15, different at 140 ◦C[ mesoporousat60 140] The °C authors[60] acids, The observed authorsnamely observed significant changes in the desired ether yield depending on the acidic character of the observedsignificantSBA-15, zirconium significant changes inmodified changes the desired SBA-15,in the ether desired Al-MCM-41, yield ether depending yieldand Amberlyst®15, ondepending the acidic on character theat 140 acidic °C of the[60]character catalystThe authors of used. the catalyst used. In particular, over the catalysts expressing mainly Brønsted acidity, as is the case of catalystInobserved particular, used. significant overIn particular, the changes catalysts over in expressing the catalystsdesired mainly ether expres Brønsted yieldsing dependingmainly acidity, Brønsted as on is thethe caseacidity, acidic of Amberlystascharacter is the case ®of15 the of or Amberlyst®15 or Al-modified MCM41, ethyl levulinate was the main product obtained. On the Amberlyst®15Al-modifiedcatalyst used. MCM41, Inor particular, Al-modified ethyl levulinate over MCM41, the wascatalysts theethyl main expreslevulinate productsing was obtained.mainly the Brønsted main On the product otheracidity, hand, obtained. as theis the formation Oncase the of other hand, the formation of the ethyl ether of HMF was associated with the presence of Lewis acid otherofAmberlyst®15 the hand, ethyl etherthe orformation of Al-modified HMF was of the associated MCM41, ethyl ether with ethyl of the HMFlevulinate presence was associated ofwas Lewis the acid mainwith sites. theproduct presence Thus, obtained. with of ZrLewis modified On acid the sites. Thus, with Zr modified SBA-15, the EMF yield reached 76%. The influence of the reaction sites.SBA-15,other Thus,hand, the EMFwiththe formation yieldZr modified reached of the 76%.SBA-15, ethyl The ether the influence EMFof HMF yi ofeld thewas reached reaction associated 76%. temperature with The the influence presence on the selectivityof of the Lewis reaction in acid the temperature on the selectivity in the presence of Pt catalysts, Amberlyst, and other sulfonic resins temperaturepresencesites. Thus, of Ptwithon catalysts, the Zr selectivity modified Amberlyst, inSBA-15, the and presence the other EMF sulfonicof yiPteld catalysts, resinsreached wasAmberlyst, 76%. studied The andinfluence by theother group ofsulfonic the of Bellreaction resins [61], was studied by the group of Bell [61], who also extensively investigated the reaction mechanism. waswhotemperature studied also extensively byon the the group selectivity investigated of Bell in [61], the who reactionpresence also mechanism. extensivelyof Pt catalysts, investigated Amberlyst, the and reaction other mechanism. sulfonic resins was studied by the group of Bell [61], who also extensively investigated the reaction mechanism. O O O O O O O ROH, Acid catalyst O O ROH, Acid catalyst O O O ROH, Acid catalyst OH OR OH OR SchemeOH 6. Mono etherification of HMF. OR SchemeScheme 6 6.. MonoMono etherification etherification of of HMF. HMF. Scheme 6. Mono etherification of HMF. Another exploredexplored strategy strategy relies relies on on the the use ofuse orthoesters of orthoesters as reagents. as reagents. Traditionally, Traditionally, the reaction the Another explored strategy relies on the use of orthoesters as reagents. Traditionally, the ofreaction an alcohol of an with alcohol an orthoester with an isorthoester exploited foris exploited the formation for the of C-Cformation bonds viaof theC-C Johnson-Claisen bonds via the reactionAnother of an explored alcohol withstrategy an orthoesterrelies on theis exploiteduse of orthoesters for the formation as reagents. of C-C Traditionally, bonds via thethe rearrangement,Johnson-Claisen carriedrearrangement, out at high carried temperatures out at high in thetemperatures presence of in a the weak presence acid under of a conditionsweak acid Johnson-Claisenreaction of an alcohol rearrangement, with an carriedorthoester out isat exploitedhigh temperatures for the formation in the presence of C-C ofbonds a weak via acid the forunder distillative conditions removal for distillative of the alcohol removal formed of the [62 al].cohol On the formed other [62]. hand, On the the use other of solid hand, acids the atuse low of underJohnson-Claisen conditions rearrangement,for distillative removalcarried outof the at alhighcohol temperatures formed [62]. in On the the presence other hand, of a weakthe use acid of temperaturessolid acids at low is also temperatures reported to promoteis also reported the formation to promote of ethers, the formation both symmetrical of ethers, and both unsymmetrical symmetrical solidunder acids conditions at low temperaturesfor distillative is removalalso reported of the to al promotecohol formed the formation [62]. On of the ethers, other both hand, symmetrical the use of ones,and unsymmetrical according to the ones, general according Scheme to7 the[63 ].general Scheme 7 [63]. andsolid unsymmetrical acids at low temperatures ones, according is also to reportedthe general to promoteScheme 7the [63]. formation of ethers, both symmetrical and unsymmetrical ones, according toCatalyst the general Scheme 7 [63]. + R1C(OR2)3 Catalyst + + R COOR R OH + R C(OR ) R OR2 + R O R 1 2 R OH 1 2 3 R OR2 R O R + R1COOR2 Catalyst + R1C(ORScheme2)3 7. General reaction of orthoesters with+ alcohols. + R COOR R OH R OR2 R O R 1 2

The authors tested the use of montmorillonite KSF and K10, SiO2 and amberlyst-15 by observing important differences in the product ratio depending on the catalyst. In particular, a significant Catalysts 2019, 9, 172 12 of 21 amount of the unsymmetrical ether of up to 90% was obtained in the presence of Montmorillonite KSF, thus suggesting the possibility of taking advantage of this protocol for ether synthesis when using a catalyst able to properly stabilize the cationic intermediate. Moreover, the great importance of carrying out the reaction at room temperature, instead of heating at high temperature, was identified in the same paper, as the same catalysts under reflux conditions led to messy reaction mixtures with a very poor ether selectivity. The unraveled potential of the interaction of orthoester with alcohols in the presence of various solid acids, has been very recently exploited for the preparation of furanic ethers [64]. This approach was revealed to be effective for the preparation of both methyl and ethyl furfuryl ethers, allowing the reaction to be carried out at a low temperature with respect to the other protocols reported (Table3, entries 4–7). Despite the disadvantages related to the use of a sacrificial reagent in terms of atom economy, the authors underline the unique potential of the protocol with respect to energy saving and selectivity.

Table 3. Comparison of catalytic procedures for furanic ethers by direct etherification.

Entry substrate Catalyst Reagent T (◦C) Conv Yield % Ref 1 HMF Zr-SBA-15 ethanol 140 100 76 [60] 2 HMF Amberlyst ethanol 110 100 71 [61] 3 Furfuryl alcohol ZSM-5 ethanol 125 80 40 [57] 4 Furfuryl alcohol ZSM-5 CH(OMe)3 40 92 73 [64] 5 Furfuryl alcohol ZSM-5 CH(OEt)3 40 57 42 [64] a 6 Furfuryl alcohol ZSM-5 CH(OPr)3 40 54 43 [64] a 7 Furfuryl alcohol ZSM-5 CH(OBu)3 40 49 37 [64] a Prepared in situ.

Interestingly, the substrate scope can be extended by exploiting the capability of ZSM-5 to also promote orthoester exchange in the presence of alcohols. In this way, it is possible to design an orthoester exchange-etherification sequence in which the desired orthoester is produced in situ from trimethyl orthoformate and the corresponding alcohol without the need for prior synthesis and isolation.

5.2. Cascade Processes from HMF and Furfural The great potential of cascade and one-pot processes using a bifunctional catalyst is an even more interesting path to be followed. One possibility is to directly start from furfural instead of the corresponding alcohol derived from the aldehyde by hydrogenation, or directly from sugars, namely glucose or fructose. The direct etherification of a carbonyl group with an alcohol is a convenient route in a general way, especially in the case of aromatic compounds, as ketones are the first intermediates in the functionalization of aromatic hydrocarbons by means of the Friedel–Crafts acylation, as well as in the case of C5 platform molecules derived from biomass, such as HMF and furfural. One of the first attempts in this direction was reported by the group of Bell [61]. The paper relies on the use of a one-pot reductive etherification protocol from HMF to 2,5-bis(alkoxymethyl)furan by using a mixture of two catalysts, namely a PtSn alloy supported on alumina and Amberlyst-15, under hydrogenation conditions, allowing the obtainment of 64% of the ether by using ethanol and 47% by using butanol (Table4, entries 1 and 2). The analysis of the reaction mixture also shows that in the presence of a metal based reducing catalyst, the reaction follows a reductive etherification pathway rather than a sequential carbonyl reduction to the BHMF, according to Scheme8. Catalysts 2018, 8, x FOR PEER REVIEW 13 of 21

The use of a unique system that is able to promote the whole transformation is, of course, a desirable target and this possibility was investigated by Li et al. [65] by exploiting the hydrogenation activity and Lewis acidity retained by a reduced commercial Co3O4 due to the coexistence of Co0 and Co2+/3+ species. The authors ascribe the strength of the protocol proposed to the moderate Lewis acidic character of the reduced Co. Under optimized conditions, that is 140 °C and 2 MPa of H2, 98.5% of the diether was obtained at full conversion of HMF. XPS analysis confirmed the presence of both Co0 and Co2+/3+ species after the reduction of Co3O4, which endows the catalyst not only with a hydrogenation activity, but also with an etherification capacity. Some other work within reductive etherification under hydrogenation conditions with metal catalysts has been carried out using furfural as the substrate, which found good candidates for this application in Pd based systems [66]. In particular, a commercial Pd on charcoal was the most performing among a series of Cu, Ir, Ni, Pt, and Pd samples when tested with methanol for the direct transformation of furfural into 2-methoxymethylfuran and was obtained with a 77% selectivity. The effectiveness of charcoal mainly derives from the acidic functionalities present in the form of surface oxides, which play a crucial role in the mechanism of ether formation via hemiacetal, as already shown for other

Catalystssubstrates2019 ,[67]9, 172 and already invoked for HMF when Brønsted acidic systems are used as catalysts13 of 21 [61].

O OH Acidic sites or M hydrides H2 OR

O O O OR

O OH H2 Acidic sites

Scheme 8. Possible reaction pathway for the reductivereductive etherificationetherification of furfural.

TheOn the use other of a unique hand, systemother authors, that is ablewhile to promotealso endorsing the whole the transformationhypothesis of the is, ofhemiacetal course, a desirable target and this possibility was investigated by Li et al. [65] by exploiting the hydrogenation pathway, mainly ascribe the acetalisation reaction to Pd hydrides formed under H2 atmosphere, activity and Lewis acidity retained by a reduced commercial Co O due to the coexistence of Co0 and when working at low temperatures [68]. A 0.7 wt% Pd/C at 603 °C4 and 0.3 MPa of H2 gave 81% of 2+/3+ Cofurfuryl species.ethyl ether The authorsby this ascribeparticular the strengthcapacity. of Also, the protocol a catalyst proposed of palladium to the moderate supported Lewis on ◦ acidichydroxyapatite character ofwas the reported reduced Co.to promote Under optimized the formation conditions, of furfuryl that is 140isopropylC and 2ether MPa as of Han2, 98.5%intermediate of thediether in the way was obtainedof complete at full hydrogenation conversion of of HMF. furfural XPS to analysis tetrahydrofurfuryl confirmed the alcohol presence when of 0 2+/3+ bothworking Co underand Co low hydrogenspecies after pressure the reduction [69]. of Co3O4, which endows the catalyst not only with a hydrogenationHowever, activity,while the but hydrogenation also with an etherification of furfural capacity.and HMF in this kind of cascade reaction Some other work within reductive etherification under hydrogenation conditions with metal requires a super-atmospheric pressure of H2, hydrogen transfer conditions are very suitable for the catalystsreduction has of the been aldehyde carried outgroup. using The furfural use of as an the alcohol substrate, as a whichhydrogen found donor good and candidates as a solvent for thiscan applicationeffectively boost in Pd the based formation systems of [66 ether]. when the catalyst shows finely tuned acid-base properties. In particular,fact, a catalyst a commercial able to promote Pd on charcoalthe Meerwein–Ponndorf–Verley was the most performing reduction among aof series furfural of Cu,while Ir, Ni,also Pt,favoring and Pd the samples following when dehydration tested with of methanolthe alcohols for allows the direct one transformationthe establishment of furfuralof a one intopot 2-methoxymethylfuranprocess for the synthesis and of wasasymmetrical obtained withfuranic a 77% ethers. selectivity. This strategy The effectiveness overcomes of the charcoal use of mainlyboth a derivesnoble metal from based the acidic catalyst functionalities and high hydrogen present inpressures the form for of surfacethe hydrogenation oxides, which step. play a crucial role in theTypically, mechanism heterogeneous of ether formation catalyst vias that hemiacetal, are able asto alreadyestablish shown a hydrogen for other transfer substrates reaction [67] and alreadythat possess invoked Lewis for acid HMF features when Brønstedare framework acidic substituted systems are Sn used or Zr as catalystszeolites [70–72]. [61]. OnSn-Beta the other was hand,reported other for authors, the one while pot transformati also endorsingon theof HMF hypothesis into different of the hemiacetal bis-alkoxy pathway, ethers, mainlywith a yield ascribe of theup acetalisationto 80% in the reaction desired to product Pd hydrides [73]. Some formed interesting under H2 pointsatmosphere, were raised when from working the ◦ at low temperatures [68]. A 0.7 wt% Pd/C at 60 C and 0.3 MPa of H2 gave 81% of furfuryl ethyl ether by this particular capacity. Also, a catalyst of palladium supported on hydroxyapatite was reported to promote the formation of furfuryl isopropyl ether as an intermediate in the way of complete hydrogenation of furfural to tetrahydrofurfuryl alcohol when working under low hydrogen pressure [69]. However, while the hydrogenation of furfural and HMF in this kind of cascade reaction requires a super-atmospheric pressure of H2, hydrogen transfer conditions are very suitable for the reduction of the aldehyde group. The use of an alcohol as a hydrogen donor and as a solvent can effectively boost the formation of ether when the catalyst shows finely tuned acid-base properties. In fact, a catalyst able to promote the Meerwein–Ponndorf–Verley reduction of furfural while also favoring the following dehydration of the alcohols allows one the establishment of a one pot process for the synthesis of asymmetrical furanic ethers. This strategy overcomes the use of both a noble metal based catalyst and high hydrogen pressures for the hydrogenation step. Typically, heterogeneous catalysts that are able to establish a hydrogen transfer reaction and that possess Lewis acid features are framework substituted Sn or Zr zeolites [70–72]. Sn-Beta was reported for the one pot transformation of HMF into different bis-alkoxy ethers, with a yield of up to 80% in the desired product [73]. Some interesting points were raised from the Catalysts 2018, 8, x FOR PEER REVIEW 14 of 21 study of the reaction conditions. The reaction temperature was identified as a useful parameter to

Catalystsincrease2019 the, 9 ,HMF 172 conversion, reaching 98.3% at 210 °C, however, the maximization of selectivity14 of 21 toward the bis-ether (87%) reached 180 °C. It is also interesting to note that the reaction protocol can be applied to both primary and secondary alcohols, with a slightly lower selectivity with the latter studyones due of theto the reaction minor conditions. hydrogen transfer The reaction activity. temperature was identified as a useful parameter to ◦ increaseThe theoutstanding HMF conversion, performances reaching of 98.3%Sn-Beta at in 210 thisC, process however, are the ascribed maximization to its ofLewis selectivity acidic ◦ towardcharacter. the Thus, bis-ether the comparison (87%) reached with 180 anC. Al-Beta It is also system interesting shows to that note when that theBrønsted reaction acidic protocol sites canare bealso applied present, to selectivity both primary drops and in secondaryfavor of the alcohols, monoether with as athe slightly major lowerproduct, selectivity thus revealing with the the latter low onesactivity due of to the the Meerwein-Ponndorf minor hydrogen transfer activity activity. of Brønsted sites. The effect of acidity on the selectivity was Theelucidated outstanding in an performances even deeper of study Sn-Beta by in thissome process of the are same ascribed authors to its by Lewis comparing acidic character. Al2O3, Thus,Al2O3/SBA-15, the comparison ZrO2, ZrO with2/SBA-15, an Al-Beta TiO system2, TiO2/SBA-15, shows that H-BEA, when Brønstedand Sn-BEA acidic in sitesthe reaction are also of present, HMF selectivitywith 2-propanol drops inunder favor flow of the conditions monoether [74]. as the major product, thus revealing the low activity of the Meerwein-PonndorfThe authors restate activity that ofstrong Brønsted Brønsted sites. sites, The effect as those of acidity observed on the on selectivityH-BEA or wasAl2O elucidated3/SBA-15, incatalyze an even the deeper formation study of bymono-ether, some of the without same carbonyl authors byhydrogenation, comparing Al and2O3 weak, Al2O Brønsted3/SBA-15, systems ZrO2, ZrOpresent2/SBA-15, on ZrO TiO2 and2, TiOTiO22/SBA-15, supported H-BEA, on SBA-15 and Sn-BEA promote in the the formation reaction of of HMF the ether with 2-propanolfollowing the under HT flowreaction, conditions while solids [74]. containing only Lewis sites, such as Sn-BEA, were revealed as the most active in bothThe HT authors and the restate dehydration that strong reaction, Brønsted giving sites, a high asthose yield observedof the bis onether H-BEA (Scheme or Al 9).2 O3/SBA-15, catalyzeWhen the using formation a Zr-SBA-15 of mono-ether, catalyst without possessing carbonyl Lewis hydrogenation, acid features anddue weakto the Brønsted presence systems of Zr4+ presentsites, interesting on ZrO2 andresults TiO 2insupported both HT and on SBA-15 etherifica promotetion were the formationalso obtained of the by ether using following furfural theas HTthe reaction,substrate while[75]. A solids yield containing of 85% in onlyi-propyl-furfuryl-ether Lewis sites, such as was Sn-BEA, obtained were at revealed 130 °C with as the a 1:1 most furfural active into bothcatalyst HT mass and theratio. dehydration reaction, giving a high yield of the bis ether (Scheme9).

HO O RO O B weak or L B weak or L

OH O OR HO O

O RO O L = Lewis sites B strong B = Bronsted sites

Scheme 9. Main products obtained by varying the acidity of the catalytic system.

4+ WhenA smart using demonstration a Zr-SBA-15 of the catalyst role of possessing Lewis and Lewis Brønsted acid sites features in the due two toreactions the presence was obtained of Zr sites, interesting results in both HT and etherification were also obtained by using furfural as the with metal chlorides, such as CrCl3 and YbCl3, by a kinetic study in conjunction with salt speciation ◦ substrateusing a soft [75 ionization]. A yield ofmass 85% spectrometer in i-propyl-furfuryl-ether [76]. Thus, the was Brønsted obtained acidic at 130 speciesC with generated a 1:1 furfural from the to catalystalcoholysis mass of ratio.the metal chlorides were shown to be predominant catalytically active species in the etherification.A smart demonstration of the role of Lewis and Brønsted sites in the two reactions was obtained withOn metal the chlorides, other hand, such the as right CrCl tradeoff3 and YbCl between3, by aLewis kinetic and study Brønsted in conjunction acidity was with also salt exploited speciation by using a soft ionization mass spectrometer [76]. Thus, the Brønsted acidic species generated from using a mixture of two catalysts, namely ZrO(OH)2 and Zr-montmorillonite, respectively, ruling out the hydrogen alcoholysis transfer of the metalreaction chlorides and the were etherificati shownon to one be predominant[77]. This combination catalytically allowed active the species authors in theto obtain etherification. an excellent yield in the bis-ether, particularly by using secondary alcohols, such as 2-propanolOn the and other 2-butanol. hand, the The right significant tradeoff between difference Lewis in activity and Brønsted observed acidity with was the also secondary exploited and by usingprimary a mixture alcohols of was two mainly catalysts, ascribed namely to ZrO(OH) the bette2 andr performances Zr-montmorillonite, of the former respectively, in the rulinghydrogen out thetransfer hydrogen activity. transfer reaction and the etherification one [77]. This combination allowed the authors to obtainThe an possibility excellent yield to switch in the the bis-ether, selectivity particularly towards bydifferent using secondaryproducts based alcohols, on the such different as 2-propanol acidic and 2-butanol. The significant difference in activity observed with the secondary and primary alcohols properties of the catalytic material used was also underlined by some authors by using ZrO2 and was mainly ascribed to the better performances of the former in the hydrogen transfer activity. SiO2-ZrO2 in the reaction of furfural with 2-butanol under hydrogen transfer conditions (Table 4, The possibility to switch the selectivity towards different products based on the different acidic entries 15 and 16) [78]. Moving from ZrO2 to SiO2-ZrO2 under the same reaction conditions allowed propertiesthe authors of to the respectively catalytic material obtain 100% used furfuryl was also alcohol underlined or 87% by of some the authorsether by bythe using genuine ZrO Lewis2 and SiO2-ZrO2 in the reaction of furfural with 2-butanol under hydrogen transfer conditions (Table4, entries 15 and 16) [78]. Moving from ZrO2 to SiO2-ZrO2 under the same reaction conditions allowed the authors to respectively obtain 100% furfuryl alcohol or 87% of the ether by the genuine Lewis acid Catalysts 2019, 9, 172 15 of 21

Catalysts 2018, 8, x FOR PEER REVIEW 15 of 21 characterCatalysts shown2018, 8, x FOR byzirconia PEER REVIEW dispersed on silica, in agreement with the conclusions drawn15 of by21 the use Catalysts 2018,, 8,, xx FORFOR PEERPEER REVIEWREVIEW 15 of 21 of Sn-BetaCatalysts 2018 [74,]. 8, x FOR PEER REVIEW 15 of 21 acidCatalysts character 2018, 8, x shownFOR PEER by REVIEW zirconia dispersed on silica, in agreement with the conclusions drawn15 of by 21 acidCatalysts character 2018, 8, x shownFOR PEER by REVIEW zirconia dispersed on silica, in agreement with the conclusions drawn15 of by 21 acidCatalystsThe character effectiveness 2018, 8, x shownFOR PEER ofby REVIEW Lewiszirconia acidity dispersed in promotingon silica, in agreement both reactions with the was conclusions confirmed drawn using15 of by 21 a copper theCatalystsacid use character 2018of Sn-Beta, 8, x shownFOR [74]. PEER by REVIEW zirconia dispersed on silica, in agreement with the conclusions drawn15 of by 21 supportedtheCatalyststheacid useuse character 2018ofof catalyst, Sn-BetaSn-Beta, 8, x shownFOR [74]. Cu/SiO[74]. PEER by REVIEW zirconia[78 ]. dispersed This catalyst on silica, possesses in agreement a peculiar with Lewisthe conclusions acidic character drawn15 of by 21 based on Catalystsacidthe use Thecharacter 2018of effectiveness Sn-Beta, 8, x shownFOR [74]. PEER by of REVIEW zirconiaLewis2 acidity dispersed in promoting on silica, bothin agreement reactions withwas confirmedthe conclusions using drawn a copper15 of by 21 acidCatalyststhe use Thecharacter 2018of effectiveness Sn-Beta, 8, x shownFOR [74]. PEER byof REVIEW Lewiszirconia acidity dispersed in promoting on silica, bothin agreement reactions withwas confirmedthe conclusions using drawn a copper15 of by 21 thesupportedacidCatalysts very character high 2018 ,catalyst, 8 dispersion, x shownFOR PEER Cu/SiO by REVIEW ofzirconia2 the[78]. metallic Thisdispersed catalyst phase, on possesses silica, already in agreementa pecu exploitedliar Lewiswith in the acidic the conclusions one-pot character transformation drawn based15 of onby 21 of supportedacidCatalyststhe use Thecharacter 2018of effectiveness Sn-Beta ,catalyst, 8, x shownFOR [74]. PEER Cu/SiO by of REVIEW zirconiaLewis22 [78].[78]. acidity This Thisdispersed catalystcatalyst in promoting on possessespossesses silica, bothin aagreementa pecupecureactionsliarliar LewisLewis withwas theconfirmed acidicacidic conclusions charactercharacter using drawn basedbaseda copper15 of onbyon 21 γ supportedtheacid usevery Thecharacter of higheffectiveness Sn-Beta catalyst, dispersionshown [74]. 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Cu/SiO valeric by of zirconiaLewisof estersthe[78]. aciditymetallic This[79]dispersed andcatalyst in phase, promotingthe on cascpossesses silica,alreadyade both indirect exploitedagreementa pecureactions etherificationliar in Lewis with wasthe one-potconfirmedthe acidic of conclusionsaromatic character transformation using ketones drawn baseda copper with onbyof differentγacidthesupported-valerolactone-valerolactone usevery Thecharacter alcohols of higheffectiveness Sn-Beta catalyst, dispersionshown into [into80 [74]. ]. Cu/SiO valericvaleric by of zirconiaLewisof 2 esters esters[78].the aciditymetallic This [79][79]dispersed andandcatalyst in phase, promotingthethe on casccascpossesses silica,alreadyade both indirect exploitedagreementa pecureactions etherificationliar in Lewis with wasthe one-potconfirmedthe acidicof aromaticconclusions character transformation using ketones drawn baseda copper with onbyof γsupportedthe-valerolactone useveryThe of higheffectiveness Sn-Beta catalyst, dispersion into [74]. Cu/SiO valeric of Lewisof2 esters[78].the aciditymetallic This[79] andcatalyst in phase, promotingthe cascpossesses alreadyade both direct exploiteda pecureactions etherificationliar in Lewis wasthe one-potconfirmed acidic of aromatic character transformation using ketones baseda copper with onof differentγthesupported-valerolactone veryThe alcoholshigheffectiveness catalyst, dispersion into [80]. Cu/SiO valeric of Lewisof2 esters[78].the aciditymetallic This[79] andcatalyst in phase, promotingthe cascpossesses alreadyade both direct exploiteda pecureactions etherificationliar in Lewis wasthe one-potconfirmed acidic of aromatic character transformation using ketones baseda copper with onof differentsupportedthe usevery of alcoholshigh Sn-Beta catalyst, dispersion [80]. [74]. Cu/SiO of2 [78].the metallic This catalyst phase, possesses already exploiteda peculiar in Lewis the one-pot acidic character transformation based onof differentthesupportedγ-valerolactone veryThe alcoholshigheffectiveness catalyst, dispersion into [80]. Cu/SiO valeric of Lewisof2 esters[78].the aciditymetallic This [79] andcatalyst in phase, promotingthe cascpossesses alreadyade both direct exploiteda pecureactions etherificationliar in Lewis wasthe one-potconfirmed acidic of aromatic character transformation using ketones baseda copper with onof thesupportedγdifferentTable-valerolactone veryThe 4. highalcoholseffectivenessComparison catalyst, dispersion into [80]. Cu/SiO valeric of of Lewisof catalytic2 estersthe[78]. aciditymetallic This [79] procedures andcatalyst in phase, promotingthe cascpossesses foralreadyade furanic both direct exploiteda pecureactions ethers etherificationliar byin Lewis was the cascadeone-potconfirmed acidic of aromatic character processtransformation using ketones startingbaseda copper with onof from γthesupporteddifferent-valerolactone veryTable highalcohols 4.catalyst, Comparison dispersion into [80]. Cu/SiO valeric of of catalytic2 esters[78].the metallic This[79] procedures andcatalyst phase, the for cascpossesses alreadyfuranicade direct ethers exploiteda pecu etherification byliar the in Lewiscascade the one-pot acidic of process aromatic character transformation starting ketones frombased with onof differenttheγfurfural-valerolactone veryTable alcoholshigh or 4. HMF.Comparison dispersion into [80]. valeric of of catalytic2 estersthe metallic [79] procedures and phase, the for casc alreadyfuranicade direct ethersexploited etherification by the in cascade the one-pot of process aromatic transformation starting ketones from with of γsupporteddifferent-valerolactoneTable alcohols 4.catalyst, Comparison into [80]. Cu/SiO valeric of catalytic esters [78]. This[79] procedures andcatalyst the for cascpossesses furanicade direct ethers a pecu etherification byliar the Lewiscascade acidic of process aromatic character starting ketones frombased with on differentγthe-valerolactone veryfurfuralTable alcoholshigh 4. orComparison dispersionHMF. into [80]. valeric of of catalytic estersthe metallic [79] procedures and phase, the for casc alreadyfuranicade direct ethersexploited etherification by the in cascade the one-pot of process aromatic transformation starting ketones from with of differenttheγ-valerolactone veryfurfuralfurfural alcoholshigh oror dispersionHMF.HMF. into [80]. valeric of estersthe metallic [79] and phase, the casc alreadyade direct exploited etherification in the one-pot of aromatic transformation ketones with of differentγ-valerolactoneTable alcohols 4. Comparison into [80]. valeric of catalytic estersReduction [79] procedures and the for casc furanicade direct ethers etherification by the cascade of Tprocess aromatic starting ketones from with γdifferent-valerolactoneTablefurfuralCatalyst alcohols 4. orComparison HMF. into [80]. valeric Reagent of catalytic estersReduction [79] procedures and the Alcoholfor casc furanicade direct ethers Product etherification by the cascade Tof process aromaticConvConv starting ketones %Yield from Yield with % Ref differentTablefurfuralCatalyst alcohols 4. orComparison HMF. [80]. Reagent of catalytic CondtionsReduction procedures forAlcohol furanic ethersProduct by the cascade(T◦ processC) Conv starting Yield from Ref furfuralTableCatalyst 4. orComparison HMF. Reagent of catalytic Reductioncondtions procedures Alcoholfor furanic ethersProduct by the cascade(°C)T process Conv %starting Yield %from Ref different TablefurfuralCatalyst alcohols 4. orComparison HMF. [80]. Reagent of catalytic Reductioncondtions procedures forAlcohol furanic ethersProduct by the cascade(°C)T process Conv %starting Yield %from Ref furfuralTable 4. orComparison HMF. of catalyticcondtions procedures for furanic ethersO by the cascade(°C) process % starting %from furfuralCatalyst or HMF. Reagent Reductioncondtions Alcohol OProduct (°C)T Conv% Yield% Ref 2 3 O furfuralTablePtSn/AlPtSn/Al 4. orComparison2OO HMF.3 ++ of catalytic procedures for furanic ethersO by the cascade process starting from PtSn/Al2 O3 + O Catalyst Reagent condtions Alcohol OProductO O (°C) % % Ref 11 furfuralTablePtSn/Al 4. orComparison22O HMF.33 ++ HMFHMF of catalytic HHReduction2 21.4 1.4procedures MPa MPa ethanolfor ethanol furanic ethers byO the cascade60T 60process Conv -starting - Yield 64from 64 [61] [61] 2 3 OO OO Amberlyst15PtSn/AlCatalystO + Reagent Reductioncondtions22 Alcohol ProductO (°C)T Conv% Yield% Ref 1 TablefurfuralAmberlyst15 4. orComparison HMF. HMF of catalytic HReduction 1.41.4procedures MPaMPa for ethanol ethanol furanic ethers byO theO cascade60T process Conv -starting Yield 64from [61] 1 Amberlyst15Catalyst2 3 ReagentHMF HReduction2 1.4 MPa Alcohol ethanol Product 60T Conv- Yield64 [61]Ref Amberlyst15PtSn/AlCatalystO + Reagent condtions Alcohol ProductO (°C) % % Ref furfural or HMF. Reduction O O T Conv Yield 1 Amberlyst15Catalyst2 3 ReagentHMF Hcondtions2 1.4 MPa Alcohol ethanol ProductO (°C)60 %- 64% [61]Ref PtSn/Al O + Reductioncondtions O O (°C)T Conv% Yield% furfuralCatalyst or HMF. Reagent condtions Alcohol ProductO O (°C) % % Ref Amberlyst15 2 OO 1 PtSn/Al2O3 + HMF H 1.4 MPa ethanol O O O 60 - 64 [61] Catalyst Reagent Alcohol Product O Ref Reductioncondtions O O (°C)T Conv% Yield% O O O O 2 3 O 1 Amberlyst15PtSn/AlCatalystO + ReagentHMF Hcondtions2 1.4 MPa Alcohol ethanol ProductOO O (°C)60 %- 64% [61]Ref 2 PtSn/Al- 2O3 + HMF HReduction2 1.4 MPa n-butanol O O O O 60T Conv- Yield47 [61] 2 - HMF H 21.4 MPa n-butanol O 60 - 47 [61] O 21 PtSn/Al- 2O3 + HMF H2 2 1.4 MPa n-butanol ethanol O O 60 - 4764 [61] Amberlyst15 condtions O (°C) % % 12 Catalyst- 2 3 ReagentHMF HReduction22 1.4 MPa n-butanolAlcohol ethanol ProductO O 60T Conv- Yield6447 [61]Ref 2 PtSn/Al- O + HMF H 1.4 MPa n-butanol O 60 - 47 [61] 1 HMF H2 1.4 MPa ethanol O O 60 - 64 [61] 2 Amberlyst15- HMF Hcondtions2 1.4 MPa n-butanol O O (°C)60 %- 47% [61] 1 Amberlyst15PtSn/AlCatalyst2O3 + ReagentHMF H2 1.4 MPa Alcohol ethanol ProductO 60 - 64 [61]Ref Amberlyst15 O O O 12 - 2 3 HMF Hcondtions2 1.4 MPa n-butanol ethanol O O (°C)60 %- 6447% [61] Amberlyst15PtSn/Al O + O O O O 2 O O O O O 12 Amberlyst15- 2 3 HMF H2 1.4 MPa n-butanol ethanol O 60 - 6447 [61] PtSn/Al O + O O O O O O O OO 12 Amberlyst15- 2 3 HMF H2 21.4 MPa n-butanol ethanol O O O O O 60 - 6447 [61] 3 PtSn/AlReducedO Co + HMF H2 2 MPa Methanol O O 140 100 98.5 [65] 3 Reduced Co HMF H 2 MPa Methanol O O 140 100 98.5 [65] 22 OO OO 23 Amberlyst15Reduced- Co HMF H H22 21.4 2 MPa MPa n-butanolMethanol O O O 14060 100- 98.547 [61][65] 321 Reduced- Co HMF H H 1.4 2 MPa MPa Methanoln-butanol ethanol O O 14060 100- 98.54764 [65][61] 2 - HMF H2 21.4 MPa n-butanol O 60 - 47 [61] 3 Reduced Co HMF H 2 MPa Methanol O O 140 100 98.5 [65] Amberlyst15 O 2 - HMF H2 1.4 MPa n-butanol O O O 60 - 47 [61] O O O 3 Reduced Co HMF H2 2 MPa Methanol O O O 140 100 98.5 [65] 2 - HMF H2 1.4 MPa n-butanol O 8060 - 47 [61] OO O OO O 80 O a 2 O a 423 ReducedPd/charcoal- Co furfural HMF HH 2 21.4 52 MPa MPa n-butanolMethanolmethanol O O O O 1408060 100- 98.57747 [66][61][65] 4 Pd/charcoal furfural H 5 MPa methanol O 80 - 77 [66] O O a a O 80 a 2 O 443 Pd/charcoalReducedPd/charcoal Co furfuralfurfural HMF HH 225 552MPa MPaMPa methanol methanolMethanol methanol O O O –120140 100-- -98.57777 a 77 [66][65][66] [66] 2 - HMF H2 1.4 MPa n-butanol O O –12060 - 47 [61] 2 O 43 ReducedPd/charcoal Co furfural HMF H 2 52 MPa Methanolmethanol O O –120140–12080 100- 98.577 [66][65] 2 O O O –120 32 Reduced- Co HMF H H2 1.4 2 MPa MPa n-butanolMethanol O O 14060 100- 98.547a [65][61] 3 Reduced Co HMF H2 2 MPa Methanol O O –120140 100 98.5 [65] 4 Pd/charcoal furfural H 5 MPa methanol O O O O 80 - 77 [66] 3 Reduced Co HMF H2 2 MPa Methanol O O O 140 100 98.5a [65] 54 Pd/charcoalPd/C furfural HH2 20.3 5 MPa MPa methanol ethanol OO O OO O –12060 98- 7781 [68][66] 35 ReducedPd/C Co furfural HMF H H 20.3 2 MPa MPa Methanol ethanol O O O 1406080 10098 98.581 [65][68] O OO O a 5 Pd/C furfural H22 0.3 MPa ethanol O O 60 98 81 [68] 54 Pd/charcoalPd/C furfural HH 20.3 5 MPa MPa methanol ethanol O O –1206080 98- 7781 [68][66] 53 ReducedPd/C Co furfural HMF H H2 0.3 2 MPa MPa Methanol ethanol O O 1406080 10098 98.581a [68][65] 5 Pd/C furfural H 0.3 MPa ethanol O 60 98 81 [68] 2 2 O O O 80 a 4 Pd/charcoal furfural H2 5 MPa methanol O –120 - 77 [66] 43 ReducedPd/charcoal Co furfural HMF H 22 52 MPa Methanolmethanol O O 14080 100- 98.577 a [66][65] 54 Pd/charcoalPd/C furfural HH 20.3 5 MPa MPa methanol ethanol OO O O –12060 98- 7781 [68][66] O O a 46 Pd/charcoalPd-HAP furfural H2 51 MPa 2-propanol methanol OO OO –1204080 98- 7760 [66][69] 635 ReducedPd-HAPPd/C Co furfural HMF HH 2 20.3 12 MPa MPa 2-propanol Methanol ethanol OO O O –1201404060 10098 98.56081a [69][65][68] 2 O O 46 Pd/charcoalPd-HAP furfural H22 51 MPa 2-propanol methanol O –1204080 98- 7760 [66][69] 65 Pd-HAPPd/C furfural HH2 0.3 1 MPa MPa 2-propanol ethanol OO O 4060 98 6081 a [69][68] 6 Pd-HAP furfural H2 1 MPa 2-propanol O O 40 98 60 [69] 4 Pd/charcoal furfural H2 5 MPa methanol O O –12080 - 77 [66] 5 Pd/C furfural H 0.3 MPa ethanol O O 60 98 81 [68] 5 Pd/C furfural H2 20.3 MPa ethanol OO O 60 98 81a [68] 6465 Pd/charcoal Pd-HAPPd-HAPPd/C furfuralfurfural HHH22 210.3 51 MPa MPa MPa 2-propanol 2-propanol methanol ethanol OO O –12080406040 98- 98776081 60[66][69][68] [69] O O O OO O a 2 2 O 576 Pd-HAPSn-BetaPd/C furfural HMF HH 20.3 HT1 MPa MPa 2-propanol ethanol O O O –1201806040 9870 81 8760 [68][74][69] 4 Pd/charcoal furfural H 5 MPa methanol O - 77 [66] 2 O O O 57 Sn-BetaPd/C furfural HMF H 0.3 HT MPa 2-propanol ethanol O O O O 18060 9870 81 87 [68][74] 76 Pd-HAPSn-Beta furfural HMF H2 HT1 MPa 2-propanol O –12018040 7098 8760 [74][69] O O O O 7 Sn-Beta HMF 2 HT 2-propanol O O 180 70 87 [74] 65 Pd-HAPPd/C furfural HH 20.3 1 MPa MPa 2-propanol ethanol O O 4060 98 6081 [69][68] 6 Pd-HAP furfural H 1 MPa 2-propanol O O O O 40 98 60 [69] 7 Sn-Beta HMF 2 HT 2-propanol O 180 70 87 [74] 56 Zr-MontPd-HAPPd/C + furfural HH2 0.3 1 MPa MPa 2-propanol ethanol OO O 6040 98 8160 [68][69] Zr-Mont + 2 O O 6 Pd-HAP furfural H 1 MPa 2-propanol O O O 40 98 60 [69] OO 7 Sn-Beta HMF HT 2-propanol O 180 70 87 [74] 785 Zr-Mont Sn-BetaPd/C + furfural HMFHMF H2 HT0.3HT MPa 2-propanol2-propanol ethanol O 16060180 10098 70 9581 87[77][68] [74] 2 O O 6 Pd-HAP furfural H 1 MPa 2-propanol OO 40 98 60 [69] Zr-Mont + O O 8 ZrO(OH)2 HMF HT 2-propanol OO OO 160 100 95 [77] 87 ZrO(OH)Sn-Beta 2 HMF HT 2-propanol O 160180 10070 9587 [77][74] O 2 O O 6 Pd-HAP furfural H 1 MPa 2-propanol O O O 40 98 60 [69] 87 Zr-MontZrO(OH)Sn-Beta +22 HMF HT 2-propanol O 160180 10070 9587 [77][74] ZrO(OH) O O 7 Sn-Beta HMF HT 2-propanol O 180 70 87 [74] O ZrO(OH)2 2 O O O 687 Zr-MontPd-HAPSn-Beta + furfuralHMF H HT 1 MPa 2-propanol O O 16018040 1009870 60 9587 [69][77][74] Zr-Mont + O O 7 Zr-MontSn-Beta + HMF HT 2-propanol O O 180 70 87 [74] 2 O O O ZrO(OH) 2 O O O 968 Zr-MontPd-HAP + furfuralHMF H HT1 MPa 2-propanol2-butanol O O O 15016040 10098 966095 [77][69] Zr-Mont + O Zr-Mont + O 9 Zr-Mont + HMF HT 2-butanol O 150 100 96 [77] 7 Sn-Beta HMF HT 2-propanol O 180 70 87 [74] O O O Zr-Mont + O O 9 Zr-MontZrO(OH) +2 HMF HT 2-butanol O O 150 100 96 [77] 98 2 HMF HT 2-propanol2-butanol O 150160 100 9695 [77] 8 HMF HT 2-propanol O 160 100 95 [77] ZrO(OH) O O Zr-Mont + O O 7 Sn-Beta HMF HT 2-propanol O 180 70 87 [74] 98 Zr-MontZrO(OH) +22 HMF HT 2-propanol2-butanol O O O 150160 100 9695 [77] ZrO(OH) O ZrO(OH) O 2 O 8 HMF HT 2-propanol O O 160 100 95 [77] O O O O 7 Zr-MontSn-Beta +2 HMF HT 2-propanol O O 180 70 87 [74] 8 ZrO(OH)2 HMF HT 2-propanol O 160 100 95 [77] 9 Zr-MontZrO(OH) + HMF HT 2-butanol O 150 100 96 [77] 2 O O Zr-MontZrO(OH) + OO O Zr-Mont + O 8 HMF HT 2-propanol O O 160 100 95 [77] 2 OO OO 7 ZrO(OH)Sn-Beta 2 HMF HT 2-propanol O 180 70 87 [74] 109 Zr-Mont + HMF HT n-butanol2-butanol O 150 100 4996 [77] Zr-Mont + O O Zr-Mont + O 10 2 HMF HT n-butanol O O O 150 100 49 [77] 8 Zr-MontZrO(OH) + HMF HT 2-propanol O O 160 100 95 [77] O O Zr-MontZrO(OH) +2 O Zr-MontZrO(OH) + O 10 HMF HT n-butanol O 150 100 49 [77] 9 Zr-Mont +2 HMF HT 2-butanol O 150 100 96 [77] 10 HMF HT n-butanol O 150 100 49 [77] ZrO(OH)2 O O 8 Zr-MontZrO(OH) + HMF HT 2-propanol O 160 100 95 [77] 10 Zr-Mont +2 HMF HT n-butanol O O O 150 100 49 [77] 9 2 HMF HT 2-butanol O 150 100 96 [77] 9 ZrO(OH)2 HMF HT 2-butanol O 150 100 96 [77] ZrO(OH) O ZrO(OH) O 9 2 HMF HT 2-butanol OO O O 150 100 96 [77] O O ZrO(OH) O 1089 ZrO(OH)Zr-MontZrO(OH)2 +2 HMF HT 2-propanoln-butanol2-butanol O O 160150 100 954996 [77] 2 O O O Zr-Mont + O O O O ZrO(OH) O 9 Zr-MontZrO(OH) +2 HMF HT 2-butanol O O 150 100 96 [77] 8 ZrO(OH)2 HMF HT 2-propanol O O O 160 100 95 [77] 10 HMF HT n-butanol O 150 100 49 [77] 11 Zr-Mont + HMF HT ethanol O 150 100 0 [77] Zr-Mont + O 11 HMF HT ethanol O O 150 100 0 [77] 2 O 9 HMF HT 2-butanol O O O 150 100 96 [77] Zr-MontZrO(OH) +2 O O Zr-MontZrO(OH) +2 O 11 ZrO(OH) HMF HT ethanol O 150 100 0 [77] 1110 2 HMF HT n-butanolethanol O O O 150 100 49 0 [77] ZrO(OH)2 O O 9 Zr-MontZr-MontZrO(OH) + + HMF HT 2-butanol O O 150 100 96 [77] 1110 Zr-MontZrO(OH) +22 HMF HT n-butanolethanol O O 150 100 49 0 [77] ZrO(OH) O O O ZrO(OH) O 10 2 HMF HT n-butanol O O O 150 100 49 [77] 109 Zr-MontZrO(OH) +2 HMFHMF HTHT n-butanol2-butanol O 150150 100 100 96 49[77] [77] 10 2 HMF HT n-butanol OO 150 100 49 [77] 11 ZrO(OH) HMF HT ethanol O O 150 100 0 [77] Zr-Mont +2 O O O Zr-Mont + O O O O 10 ZrO(OH)Zr-MontZrO(OH)2 +2 HMF HT n-butanol O 150 100 49 [77] 9 HMF HT 2-butanol O 150 100 96 [77] 11 ZrO(OH) HMF HT ethanol O O OO 150 100 0 [77] 12 Zr-Mont + furfural HT 2-propanol O O 100 100 67 [77] 12 Zr-Mont + furfural HT 2-propanol O 100 100 67 [77] 10 2 HMF HT n-butanol O 150 100 49 [77] Zr-MontZrO(OH) + O O ZrO(OH)2 OO Zr-Mont + O Zr-MontZrO(OH) +2 O 12 ZrO(OH) furfural HT 2-propanol O 100 100 67 [77] 1211 2 furfuralHMF HT 2-propanolethanol O 100150 100 67 0 [77] ZrO(OH)2 O Zr-Mont + O 10 Zr-MontZrO(OH) + HMF HT n-butanol O 150 100 49 [77] 12 furfural HT 2-propanol OO O O 100 100 67 [77] Zr-Mont +2 O 11 Zr-MontZrO(OH) +22 HMF HT ethanol O 150 100 0 [77] ZrO(OH) O O 1110 Zr-MontZrO(OH) +2 HMF HT n-butanolethanol O 150 100 49 0 [77] Zr-MontZrO(OH) + O O 1211 2 furfuralHMF HT 2-propanolethanol O O O 100150 100 67 0 [77] 11 ZrO(OH) HMF HT ethanol O 150 100 0 [77] Zr-Mont + O Zr-Mont + O O 11 Zr-MontZrO(OH) +2 HMF HT ethanol O 150 100 0 [77] 10 2 HMF HT n-butanol O O 150 100 49 [77] ZrO(OH)ZrO(OH) O O OO 1312 Zr-Mont2 +2 furfural HT 2-propanol2-butanol OO 100 100 6967 [77] 11 Zr-MontZrO(OH) +2 HMF HT ethanol O O 150 100 0 [77] Zr-Mont + O O 13 Zr-MontZrO(OH) +2 furfural HT 2-butanol OO O 100 100 69 [77] 131112 Zr-MontZrO(OH) +2 furfuralfurfuralHMF HTHT 2-propanol2-butanolethanol O O 100150 100 6967 0 [77] 13 Zr-Mont + furfural HT 2-butanol O O 100 100 69 [77] Zr-Mont +2 O 12 ZrO(OH)22 furfural HT 2-propanol O O 100 100 67 [77] 12 2 furfural HT 2-propanol O O 100 100 67 [77] 11 Zr-MontZr-MontZrO(OH) + +2 HMF HT ethanol O 150 100 0 [77] 1312 Zr-MontZrO(OH) +2 furfural HT 2-propanol2-butanol O O 100 100 6967 [77] Zr-Mont + O O 12 Zr-MontZrO(OH) +2 furfural HT 2-propanol O O O 100 100 67 [77] 1211 ZrO(OH)2 furfuralHMF HTHT 2-propanolethanol O OO 150100 100 100 0 67 [77] [77] 1413 Zr-SBA-15Zr-MontZrO(OH) + furfural HT 2-propanol2-butanol OO O 130100 10095 8469 [75][77] 12 ZrO(OH)Zr-MontZrO(OH) +2 furfural HT 2-propanol O O 100 100 67 [77] 14 Zr-SBA-15Zr-MontZrO(OH)2 +2 furfural HT 2-propanol O O 130 95 84 [75] 13 2 furfural HT 2-butanol O O 100 100 69 [77] 1412 Zr-SBA-15Zr-MontZrO(OH) + furfural HT 2-propanol O O 130100 10095 8467 [75][77] 13 Zr-MontZrO(OH) +2 furfural HT 2-butanol O 100 100 69 [77] 2 O O 13 Zr-MontZrO(OH) + furfural HT 2-butanol O O 100 100 69 [77] 121413 Zr-SBA-15Zr-MontZrO(OH) +2 furfural HT 2-propanol 2-butanol O O 100130 10095 678469 [77][75] 2 O O 1315 Zr-MontZrO(OH)SiO2-ZrO +2 furfural HT 2-butanol OO OO 100140 10080 6981 [77][78] 151214 Zr-MontZr-SBA-15ZrO(OH)SiO2-ZrO +2 furfural HT 2-propanol 2-butanol O O 140100130 1008095 816784 [78][77][75] O O O O 131315 Zr-MontZrO(OH)SiO22-ZrO +22 furfuralfurfural HTHT 2-butanol2-butanol O 100140100 10080 100 6981 69[77][78] [77] 1514 Zr-SBA-15SiOZrO(OH)-ZrO furfural HT 2-propanol2-butanol O 140130 8095 8184 [78][75] 2 2 O O 15 ZrO(OH)ZrO(OH)SiO -ZrO2 furfural HT 2-butanol O O 140 80 81 [78] 1413 Zr-SBA-15Zr-Mont2 + furfural HT 2-propanol2-butanol O 130100 10095 8469 [75][77] O 1415 Zr-SBA-15SiO2-ZrO2 furfural HT 2-propanol2-butanol O O 130140 9580 8481 [75][78] 1314 Zr-SBA-15Zr-MontZrO(OH) + furfural HT 2-propanol2-butanol O O 100130 10095 6984 [77][75] 2 O OO 141615 Zr-SBA-15ZrO(OH)SiOCu/SiO2-ZrO2 2 furfural HT 2-propanol2-butanol OO O 130180140 957280 845081 [75][78] 13 furfural HT 2-butanol O O 100 100 69 [77] O 1416 Zr-SBA-15Cu/SiO22 furfural HT 2-propanol2-butanol O O 130180 9572 8450 [75][78] 1615 ZrO(OH)SiOCu/SiO2-ZrO 2 furfuralfurfural HTHT 2-butanol2-butanol O 180140 7280 5081 [78] 16 Cu/SiO2 furfural HT 2-butanol O O 180 72 50 [78] 141514 Zr-SBA-15Zr-SBA-15SiO2-ZrO2 furfuralfurfural HT HT 2-propanol 2-propanol2-butanol O O 140130130 8095 95 8184 84[78][75] [75] 2 2 a O O 1516 SiOCu/SiO-ZrO2 furfural a HT 2-butanol O O 140180 8072 8150 [78] 1415 Zr-SBA-15SiO2-ZrO2 furfural Selectivity; HT HT 2-propanol2-butanol = Hydrogen Transfer.O O 130140 9580 8481 [75][78] Selectivity; HT = Hydrogen Transfer.O 15 SiO2-ZrO2 2 furfural aa HT 2-butanol O 140 80 81 [78] 1416 Zr-SBA-15Cu/SiO furfural a Selectivity;Selectivity; HT HTHT 2-propanol2-butanol == HydrogenHydrogen Transfer.Transfer.O O 130180 9572 8450 [75][78] 2 2 O O 15 SiO -ZrO2 furfural Selectivity;HT HT2-butanol = Hydrogen Transfer.O O 140 80 81 [78] 16 Cu/SiO furfural a HT 2-butanol O O 180 72 50 [78] 15 SiO2-ZrO2 furfural HT 2-butanol O O 140 80 81 [78] 16 Cu/SiO2 furfural Selectivity;HT HT2-butanol = Hydrogen Transfer. 180 72 50 [78] 16 Certainly,Cu/SiO2 2 the2 usefurfural of metallic a HT supported 2-butanol cataly sts, suchO asO Pd/C (Table180 4,72 entry 5), 50 under[78] 151516 SiOSiOCu/SiO2-ZrO-ZrO2 furfuralfurfural Selectivity; HTHT HT 2-butanol2-butanol = Hydrogen Transfer.O O 140180140 8072 80 8150 81[78] [78] Certainly, 2the use of metallic supported catalysts, suchO asO Pd/C (Table 4, entry 5), under 1615 Certainly,SiOCu/SiO2-ZrO the2 usefurfural of metallic a HT supported 2-butanol cataly sts, such as Pd/C (Table180140 4,7280 entry 5), 5081 under[78] hydrogenation16 Cu/SiO 2conditions furfural allows the Selectivity;HT obtainment HT2-butanol = ofHydrogen excellent Transfer. resultsO O in terms180 of both72 conversion 50 and[78] hydrogenationCertainly, conditionsthe use of allows metallica theSelectivity; supportedobtainment HT =cataly ofHydrogen excellentsts, suchTransfer. results as Pd/C in terms (Table of both 4, entry conversion 5), under and hydrogenation16 Cu/SiO 2conditions furfural allows athe Selectivity;HT obtainment HT2-butanol = ofHydrogen excellent Transfer. resultsO O in terms180 of both72 conversion 50 and[78] hydrogenationCertainly, conditionsthe use of allows metallica the Selectivity; supportedobtainment HT =cataly ofHydrogen excellentsts, such Transfer. results as Pd/C in terms (Table of both 4, entry conversion 5), under and 2 O 2 selectivity16 Cu/SiO at low reactionfurfural temperatures. a Selectivity;HT On HTthe2-butanol = other Hydrogen hand, Transfer. theO use of noble180 metals72 and H 502 for this[78] selectivityhydrogenationCertainly, at low 2conditionsthe reaction use of allowstemperatures. metallic theSelectivity; supportedobtainment On HTthe = catalyother ofHydrogen excellent sts,hand, suchTransfer. theresults asuse Pd/C inof terms noble (Table of metals both 4, entry conversionand H5),22 for under andthis 16selectivity16 Certainly, Cu/SiOCu/SiO at low2 the reaction usefurfuralfurfural of temperatures. metallic a HTHT supported On the 2-butanol2-butanol catalyother sts,hand, such the asuse Pd/C of noble (Table180180 metals 4,72 entry 72and H5), 502 for under 50 this[78] [78] selectivitykindhydrogenation Certainly,of application at low conditionsthe reaction wasuse revealedof allowstemperatures. metallic theasSelectivity; expensive supportedobtainment On HTthe and = catalyother ofHydrogen lessexcellent sts,hand, fitting such Transfer. theresults with asuse Pd/C respect in of terms noble (Table to of metalszirconium both 4, entry conversionand containingH5), for under andthis kindhydrogenationselectivity Certainly,of application at low conditionsthe reaction wasuse revealedof allowstemperatures. metallica theasSelectivity; expensive supportedobtainment On HTthe and = catalyother ofHydrogen lessexcellent sts,hand, fitting such Transfer. theresults with asuse Pd/C respect inof terms noble (Table to of metalszirconium both 4, entry conversionand containingH5),2 forunder andthis kindhydrogenationselectivity Certainly,of application at low conditionsthe reaction wasuse revealedof allowstemperatures. metallica theas expensive supportedobtainment On the and cataly otherof lessexcellent sts,hand, fitting such theresults with asuse Pd/C respect inof terms noble (Table to of metalszirconium both 4, entry conversionand containingH5),2 for under andthis materials.hydrogenation Systems, conditions such as allows Zr-SBA-15 theSelectivity;a Selectivity; obtainment and SiO HT2 HT-ZrO = ofHydrogen = excellent Hydrogen2 (Table Transfer. 4, results Transfer.entries in 14 terms and 15),of both suffer conversion from a lower and materials.hydrogenationkindselectivity Certainly,of application Systems,at low conditionsthe reaction suchwasuse revealedofas allowstemperatures. Zr-SBA-15metallic theas expensive supportedobtainment and On SiO the22 -ZrO-ZrOand cataly otherof lessexcellent22 (Table(Table sts,hand, fitting such 4,4, resultsthe entriesentrieswith asuse Pd/C respectin of 1414 terms noble andand (Table to 15),15),of metalszirconium both suffersuffer4, entry conversionand fromfrom containingH5),2 aafor underlowerlower andthis materials.activity,selectivityhydrogenationkind Certainly,of application thus Systems,at lowrequiring conditionsthe reaction suchwasuse higherrevealedofas allowstemperatures. Zr-SBA-15metallic temperatures, theas expensive supportedobtainment and On SiO the howe2 -ZrOand cataly otherof lessexcellentver,2 (Table sts,hand, fittingthey such 4, theresultsallow entrieswith asuse Pd/C respectwork inof 14 terms noble and (Tableto to 15),ofoccur metalszirconium both suffer4, underentry conversionand from containingHhydrogen5),2 afor underlower andthis activity,kindmaterials.hydrogenationselectivity of application thus Systems,at lowrequiring conditions reaction suchwas higherrevealedas allowstemperatures. Zr-SBA-15 temperatures, theas expensive obtainment and On SiO the howe2 -ZrOand otherof lessexcellentver,2 (Table hand, fittingthey 4, theresultsallow entrieswith use respectwork inof 14 terms noble and to to 15),ofoccur metalszirconium both suffer under conversionand from containing Hhydrogen2 afor lower andthis activity,selectivitykind Certainly,of applicationthus at lowrequiring the reaction wasuse higherrevealedof temperatures. metallic temperatures, as expensivesupported On the howe and catalyother lessver, sts,hand, fittingthey such theallow with asuse Pd/Cworkrespect of noble (Tableto to occur metals zirconium 4, underentry and containingHhydrogen5),2 for under this activity,transferkindselectivityhydrogenationmaterials. of application thusconditions Systems,at lowrequiring conditions reaction suchwasand higherrevealed asobtain allowstemperatures. Zr-SBA-15 temperatures, atheas veryexpensive obtainment and Ongood SiO the howe2 and-ZrOyiel otherof dlessexcellentver,2 (Table ofhand, fitting theythe 4, theresultsallow product entrieswith use respectwork inof 14 terms bynoble and totheir to 15),ofoccur metalszirconium both bifunctionalsuffer under conversionand from containing Hhydrogen2 afor acidiclower andthis transferkindhydrogenationtransferactivity,materials.selectivityCertainly, of application conditionsthusconditions Systems,at low therequiring conditions reaction use suchwasandand of higherrevealed asobtain obtainallowstemperatures. metallicZr-SBA-15 temperatures, aatheas very veryexpensive supportedobtainment and Ongoodgood SiO the howe 2 and-ZrOyielyiel otherof catalysts, dlessexcellentver,2 (Table ofhand, fitting theythe 4, theresultsproductallow suchentrieswith use respectwork in asof 14 terms bynoble Pd/Cand totheir to 15),ofoccur metalszirconium both (Table bifunctionalsuffer under conversionand 4from, containing H entryhydrogen2 a for acidiclower andthis 5), under transferselectivitymaterials.activity, thusconditions Systems,at lowrequiring reaction suchand higher asobtain temperatures. Zr-SBA-15 temperatures, a very and Ongood SiO the howe2 -ZrOyiel otherdver,2 (Table ofhand, theythe 4, the allowproductentries use work of 14 bynoble and totheir 15),occur metals bifunctionalsuffer under and from Hhydrogen2 afor acidiclower this behaviour.materials.kind of application Systems, suchwas revealedas Zr-SBA-15 as expensive and SiO2 -ZrOand less2 (Table fitting 4, entrieswith respect 14 and to 15), zirconium suffer from containing a lower behaviour.materials.selectivityactivity,transferkind of application thusconditions Systems, at lowrequiring reaction suchwasand higherrevealed as obtaintemperatures. Zr-SBA-15 temperatures, aas veryexpensive and Ongood SiO the howe2 -ZrOandyiel other dlessver,2 (Table ofhand, fitting theythe 4, the allowproduct entrieswith use respectwork of 14 bynoble and totheir to 15),occur metalszirconium sufferbifunctional under and from containing Hhydrogen2 afor acidiclower this hydrogenationbehaviour.activity,materials.kindtransfer of application thusconditions Systems, conditionsrequiring suchwasand higherrevealed asobtain allowsZr-SBA-15 temperatures, aas the veryexpensive and obtainment good SiO howe2 -ZrOandyiel dlessver,2 (Table ofof fitting they excellentthe 4, allow product entrieswith respectwork results 14 by and totheir to 15), inoccur zirconium terms sufferbifunctional under from of containing hydrogen both a acidiclower conversion transferbehaviour.materials.activity, thusconditions Systems, requiring suchand higher asobtain Zr-SBA-15 temperatures, a very and good SiO howe2 -ZrOyieldver,2 (Tableof theythe 4, allowproductentries work 14 by and totheir 15),occur sufferbifunctional under from hydrogen a acidiclower activity,kindtransfer of applicationthusconditions requiring wasand higherrevealed obtain temperatures, aas veryexpensive good howe andyiel dlessver, of fitting theythe allowproduct with respectwork by totheir to occur zirconium bifunctional under containing hydrogen acidic and transferactivity,materials.behaviour. selectivity thusconditions Systems, atrequiring low suchand reaction higher asobtain Zr-SBA-15 temperatures.temperatures, a very and good SiO howe2 -ZrO Onyield thever,2 (Tableof othertheythe 4, allow productentries hand, work 14 theby and to usetheir 15),occur of bifunctionalsuffer noble under from metals hydrogen a acidiclower and H2 for transfer materials.activity,behaviour. thusconditions Systems, requiring suchand higher asobtain Zr-SBA-15 temperatures, a very and good SiO howe2 -ZrOyieldver,2 (Tableof theythe 4, allowproductentries work 14 by and totheir 15),occur bifunctionalsuffer under from hydrogen a acidiclower this behaviour.transferactivity, kind of thus applicationconditions requiring and was higher obtain revealed temperatures, a very as expensive good howe yield andver, of they lessthe fittingallowproduct work with by to respecttheir occur bifunctional tounder zirconium hydrogen acidic containing behaviour.activity,transfer thusconditions requiring and higher obtain temperatures, a very good howe yieldver, of theythe allowproduct work by totheir occur bifunctional under hydrogen acidic behaviour.transfer conditions and obtain a very good yield of the product by their bifunctional acidic materials.transferbehaviour. Systems, conditions such and as obtain Zr-SBA-15 a very and good SiO yiel2-ZrOd of2 (Tablethe product4, entries by 14their and bifunctional 15), suffer acidic from a lower behaviour. behaviour.

Catalysts 2018, 8, x FOR PEER REVIEW 16 of 21

The fine tuning of the acidic properties of the catalysts as far as both strength and density is concerned, as well as their Lewis or Brønsted character, becomes a central issue in their application

Catalyststo biomass2019 ,derived9, 172 materials. The target of an efficient acid catalyst for etherification reactions16 that of 21 are able to substitute the traditional processes poses a challenge when faced with substrates, such as furfural or HMF. Actually, their poor stability in the presence of acids represents a strong limitation whenactivity, working thus requiring under harsh higher conditions temperatures, due to their however, tendency they to allow form workhumins to [81]. occur On under the other hydrogen hand, thetransfer moderate conditions temperatures and obtain used a very for the good synthesis yield of of the ethers product do not by theirfavor bifunctional this kind of acidicproduct behaviour. and the presenceThe fineof the tuning alcohol, of the used acidic also properties as the solv of theent, catalysts strongly as prevents far as both some strength undesired and density reaction is pathways,concerned, such as well as the as their polymerization Lewis or Brønsted one. [82]. character, Nevertheless, becomes two a centralmain by-products issue in their compete application with theto biomass desired derivedethers, namely materials. the Theacetal target and ofalkyl an efficientlevulinate, acid as catalyst shown in for Scheme etherification 10. Comparable reactions thatand evenare able more to substitutecomplex considerations the traditional ca processesn be drawn poses in the a challenge case of HMF when [76 faced]. with substrates, such as furfuralAlkyl or levulinate HMF. Actually, formation their pooris mainly stability promoted in the presenceby Brønsted of acids acid representssites, while a the strong etherification limitation whenmainly working relies on under Lewis harsh sites. conditionsA detailed duestudy to theirin this tendency mechanistic to form study humins was reported [81]. On by the the other group hand, of Centi,the moderate showing temperatures the existence used of fora quantitative the synthesis relationship of ethers do notbetween favor the this numbers kind of product of Lewis and and the presenceBrønsted ofsites the and alcohol, their usedcatalytic also performance as the solvent, in strongly the etherification prevents some of HMF undesired with ethanol reaction considering pathways, thesuch possible as the polymerization products [83]. one. [82]. Nevertheless, two main by-products compete with the desired ethers, namely the acetal and alkyl levulinate, as shown in Scheme 10. Comparable and even more complex considerations can be drawn in the case of HMF [76].

O OR

OR

O O OH O OR O L or weak B

B

O

OR

O Scheme 10 10.. PossiblePossible products obtained in the re reductiveductive etherification etherification of furfural.

StartingAlkyl from levulinate these points formation the choice is mainly of the promoted proper acidity by Brønsted of solid acidmaterials, sites, as while well the as etherificationthe characterisationmainly relies on of Lewis acidic sites. features, A detailed rule on study the inselectivity this mechanistic of the process. study wasThisreported would result by the togroup be also of theCenti, most showing challenging the existence aspect ofwhen a quantitative planning a relationship process starting between directly the numbers from sugars of Lewis or from and biomass Brønsted thatsites often and their request catalytic Brønsted performance acidity for in the etherificationfirst deconstruction of HMF steps. with ethanol considering the possible products [83]. 5.3. CascadeStarting Processes from these from pointsC6 and theC5 Sugars choice of the proper acidity of solid materials, as well as the characterisation of acidic features, rule on the selectivity of the process. This would result to be also the mostThe challengingpivotal role aspectof acidity when and planning the proper a process combin startingation of directly different from acidic sugars characters or from biomassis even morethat often important request in Brønsted the design acidity of the for three the firststep deconstructioncascade process steps. for the preparation of ethers starting from C5 and C6 sugars, which is the most rewarding way in terms of both process intensification and5.3. Cascadeselectivity Processes improvement from C6 andconsidering C5 Sugars the advantage obtained in avoiding the isolation of HMF and furfural. The pivotal role of acidity and the proper combination of different acidic characters is even more Some attempts in this direction were already reported in the early paper from Balakrishnan et important in the design of the three step cascade process for the preparation of ethers starting from C5 al. [59] by using the two-catalyst system of PtSn/Al2O3 + Amberlyst-15 under hydrogenation and C6 sugars, which is the most rewarding way in terms of both process intensification and selectivity conditions. In this protocol, an ethanolic or butanolic suspension of D-(-)-fructose, Amberlyst-15, improvement considering the advantage obtained in avoiding the isolation of HMF and furfural. and PtSn/Al2O3 were heated at 110 °C for 30 hours. After, the formation of ether and acetals as Some attempts in this direction were already reported in the early paper from Balakrishnan intermediates was observed the mixture and was cooled down to 60 °C and put under 1.4 MPa of H2, et al. [59] by using the two-catalyst system of PtSn/Al O + Amberlyst-15 under hydrogenation thus obtaining the diether in a 47–51% yield and, therefore,2 3exploiting a one-pot, two-step process. conditions. In this protocol, an ethanolic or butanolic suspension of D-(-)-fructose, Amberlyst-15, ◦ and PtSn/Al2O3 were heated at 110 C for 30 hours. After, the formation of ether and acetals as ◦ intermediates was observed the mixture and was cooled down to 60 C and put under 1.4 MPa of H2, thus obtaining the diether in a 47–51% yield and, therefore, exploiting a one-pot, two-step process. Catalysts 2019, 9, 172 17 of 21 Catalysts 2018, 8, x FOR PEER REVIEW 17 of 21

The useuse ofof a a single single catalyst catalyst was was on on the the other other hand ha reportednd reported in a recentin a recent paper paper by Bai by et al.,Bai relying et al., onrelying the useon ofthe a hierarchicaluse of a hierarchical zeolite with zeolite a controlled with a meso-/microporouscontrolled meso-/microporous morphology morphology MFI-Sn/Al (SchemeMFI-Sn/Al 11 (Scheme)[84]. 11) [84].

OH HO OH OR O OH O Isomerisation Dehydration Etherification HO

HO OH OH HO O O OH OH Scheme 11. One-pot transformation of glucose into HMF ether.

The catalyst proposed contains dual meso-/microporositymeso-/microporosity and and dual dual Lewi Lewiss and Brønsted acidity able toto promotepromote isomerisationisomerisation of of glucose glucose to to fructose, fructose, the the following following dehydration dehydration to HMF, to HMF, and theand final the etherificationfinal etherification to monoether to monoether with methanolwith methanol in a 44% in a yield.44% yield. The coexistence of a dual acidic capacity confirmsconfirms the importance of this aspect when several acid catalyzed steps are involved, as is thethe casecase ofof sugarssugars toto ethers.ethers. The cascade process entails isomerisation, dehydration, etherification, etherification, and and reduction, transformations transformations that that are dependent on different kinds of acid sites. The role of Brønsted acids in the domino transformation of fructose into ethoxy-methyl-furfural was also studied by computationalcomputational methods, identifying that the protonated + solvent [C2HH55OHOH22]+] isis in in fact fact the the catalytically catalytically active active species species [85]. [85].

6. Conclusions 6. Conclusions Biofuels willwill havehave a a very very important important role role in decarbonisingin decarbonising the the transport transport sector sector in the in next the 30next years 30 asyears electrification as electrification will playwill play a major a major role, role, but but it is it notis not applicable applicable to to all all sectors. sectors. OnOn thethe other hand, the production of bio-alcoholsbio-alcohols through fermentationfermentation processes, particularly ethanol from cellulosiccellulosic biomass and ethanol/butanol mixture mixture through through ABE ABE fe fermentationrmentation of of starch starch and glucose, is growing continuously. Butanol Butanol and and longer longer chain chain alcohols alcohols ca cann also also be be obtained obtained through through Guerbet Guerbet reactions reactions of ofbio-ethanol bio-ethanol [86], [86 2-pentanol], 2-pentanol [87] [87, ],and and furfurilic furfurilic alcohol alcohol from from fu furfural,rfural, in in turn turn obtained obtained from hemicellulose together together with with HMF. HMF. Some Some of of these these bioa bioalcoholslcohols can can be beused used as biofuels as biofuels themselves, themselves, but butupgrading upgrading them them through through the synthesis the synthesis of ethers of ethers would would greatly greatly improve improve their their performance. performance. The use of solid acids, which are very widespread in oil refining, refining, will also play a relevant role in the productionproduction of of fuels fuels and and fuel fuel additives additives from from renewables. renewables. Solids Solids with a with good a hydrothermal good hydrothermal stability andstability a proper and distributiona proper distribution of acidic sites of acidic can give site excellents can give results excellent as catalysts results for as ethercatalysts synthesis for ether in a biorefinerysynthesis in scenario. a biorefinery scenario. Author Contributions: Conceptualization, F.Z. and N.R.; Writing-Original Draft Preparation, F.Z. and N.S.; Writing-ReviewAuthor Contributions: & Editing, Conceptualization, N.R. F.Z. and N.R.; Writing-Original Draft Preparation, F.Z. and N. S; Writing-Review & Editing, N.R.; Funding: This research received no external funding. ConflictsFunding: ofThis Interest: researchThe received authors no declare external no funding. conflict of interest. Conflict of interest: The authors declare no conflict of interest References

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