Rational Design of Metal Oxide Solid Acids for Sugar Conversion

catalysts

Review Rational Design of Metal Oxide Solid Acids for Sugar Conversion

Atsushi Takagaki Department of Applied Chemistry, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan; [email protected]; Tel.: +81-92-802-6711

Received: 15 October 2019; Accepted: 24 October 2019; Published: 29 October 2019

Abstract: Aqueous-phase acid-catalyzed reactions are essential for the conversion of cellulose-based biomass into chemicals. Brønsted acid and Lewis acid play important roles for these reactions, including hydrolysis of saccharides, isomerization and epimerization of aldoses, conversion of d-glucose into 5-hydroxymethylfurfural, cyclodehydration of sugar alcohols and conversion of trioses into lactic acid. A variety of metal oxide solid acids has been developed and applied for the conversion of sugars so far. The catalytic activity is mainly dependent on the structures and types of solid acids. Amorphous metal oxides possess coordinatively unsaturated metal sites that function as Lewis acid sites while some crystal metal oxides have strong Brønsted acid sites. This review introduces several types of metal oxide solid acids, such as layered metal oxides, metal oxide nanosheet aggregates, mesoporous metal oxides, amorphous metal oxides and supported metal oxides for sugar conversions.

Keywords: Sugars; cellulose; glucose; 5-hydroxymethylfurfural; lactic acid; solid acid; Brønsted acid; Lewis acid; metal oxide; water-tolerant catalyst

1. Introduction The fundamental features of solid acid catalysts are the reusability of the catalysts, their easy separation from the products and solvents and unnecessary neutralization, which are beneficial to saving energy and cost. Besides these, special requirements of the catalysts for sugar conversion are high recognition ability of sugar molecules and high tolerance against water. Sugars have complex structures with many functional groups, including carbonyl and hydroxyl groups. The change in positions of these functional groups offers different characteristics such as chemical reactivity and biological activity. Sugars are highly reactive and easily degraded; thus, many side reactions could occur, resulting in poor selectivity for the desired product in most cases. A suitable catalyst should interact with a specific position of the reactant. Sugars have many hydroxyl groups and thus are soluble in water. For the conversion of sugars, polar solvents such as water, methanol, ethanol, N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) are widely used. Among polar solvents, water is the most suitable solvent because these sugars can be obtained from cellulose and hemicellulose via hydrolysis in water and water itself is the green solvent [1]. However, water is not preferable for solid acid catalysts due to the following reasons. First, water molecules cover acid sites resulting in severe decrease in the catalytic activity which is typical for H-type zeolites. Second, water molecules generally cause loss of the Lewis acidity of solid catalysts. Third, water molecules may change or disrupt the crystal structure of solid catalysts due to their high polarity. For example, γ-Al2O3 alters its crystal structure into boehmite AlO(OH) in hot water (above 423 K) [2]. Conventional solid acid catalysts, including zeolites, were mostly developed for oil refining in the last century. Reactants have less functional groups with non-polarity and are converted in gas-phase flow systems. These reaction conditions are very different from those of sugar conversion in the

Catalysts 2019, 9, 907; doi:10.3390/catal9110907 www.mdpi.com/journal/catalysts Catalysts 2019, 9, 907 2 of 20 liquid phase. Before the development of solid acid catalysts for liquid-phase biomass conversion, water-tolerant solid acids were studied because reactions such as oleﬁn hydration and ester hydrolysis necessitate water as a solvent in the petrochemical process and the importance of water as a solvent has been raised from the viewpoint of green chemistry [3]. As water-tolerant solid acid catalysts, hydrophobic zeolites [4,5], solid heteropolyacids such as Cs2.5H0.5PW12O40 [6], metal oxides including amorphous niobium oxide (Nb O nH O) [7,8], MoO -ZrO [9] and metal phosphates including 2 5· 2 3 2 niobium phosphate [10] have been reported. Some of these water-tolerant solid acids are applicable for aqueous-phase sugar conversion. It should be noted that d-glucose formation from maltose hydrolysis using Cs2.5H0.5PW12O40 [11] and HMF formation from fructose dehydration using niobium phosphate [10] were already studied before the rapid growth of researches for biomass conversion using heterogeneous catalysts from 2006. In this review, d-glucose which is naturally obtained is simply described as glucose. A variety of metal oxide solid acids was developed and applied for sugar conversions. The catalytic performance is mainly dependent on the types of solid acids. Amorphous metal oxides possess unsaturated metal cations that function as Lewis acid sites while some crystal metal oxides such as heteropoly acids and layered metal oxides have Brønsted acid sites. Moreover, the acid strength can be controlled by choosing adequate component elements. Furthermore, the coexistence of Brønsted acid and Lewis acid sites can be realized on the surface of the oxide. These features have signiﬁcant advantages for sugar conversions.

2. Sugar Conversion The utilization of lignocellulosic biomass has attracted much attention because it is widely available, inedible and inexpensive. Lignocellulose consists of cellulose, hemicellulose and lignin. Cellulose is the most abundant natural biopolymer with rigid crystal structure which is composed of glucose units connected by the β-glycosidic bond along with the intramolecular hydrogen bond in three-dimensional fashion. Figure1 shows a representative scheme for the conversion of cellulose-based biomass toward useful chemicals. Depolymerization of cellulose to glucose or water-soluble oligomers is the first step for aqueous-phase sugar conversion. Glucose is a starting material for a variety of key intermediates. Isomerization of glucose gives fructose, while epimerization affords mannose. Dehydration of glucose forms anhydroglucose (levoglucosan) and levoglucosone. Fructose and levoglucosone are currently considered to be intermediate for HMF, which is obtained by dehydration of them. Production of HMF is very important because it can be further transformed into 2,5-diformylfuran (DFF) and 2,5-furandicarboxylic acid (FDCA) via selective oxidation, which is the corresponding dialdehyde and dicarboxylic acid, respectively. The latter is particularly important because it can be an alternative to terephthalic acid from the viewpoint of the chemical structure. Thus FDCA has a high possibility as a precursor of biomass-based plastics, which will be widely used like polyethylene terephthalate (PET). Glucose is classified as hexose (C6 sugar) and undergoes decomposition into trioses (C3 sugar), including glyceraldehyde and dihydroxyacetone via retroaldol condensation. These trioses are further converted into pyruvaldehyde, followed by lactic acid. Lactic acid is a precursor of polylactic acid which is a well-known biodegradable polymer. The reduction of glucose gives sorbitol, which can be further converted into 1,4-sorbitan and isosorbide via cyclodehydration. Isosorbide is also an attractive precursor for synthesis of biomass-based polycarbonate, which has been commercialized. For the conversion of cellulose-based biomass into value-added chemicals, many reactions are catalyzed by acid, including hydrolysis, dehydration, isomerization and retroaldol condensation. Here several types of metal oxide solid acids for sugar conversions will be introduced. Catalysts 2019, 9, 907 3 of 20 Catalysts 2019, 9, x FOR PEER REVIEW 3 of 19

Figure 1. Reaction pathways for conversion of cellulose-based biomass toward chemicals. Figure 1. Reaction pathways for conversion of cellulose-based biomass toward chemicals.

3. Layere Layeredd Metal Metal Oxides Oxides

Protonated layered niobium molybdate and tantalum molybdate,molybdate, HNbMoO6 andand HTaMoO 6,, function as solid acid catalyst catalysts,s, whereas other layered metal oxides such asas HTiNbOHTiNbO55 and HNbWO66 do not work because reactants cannot enter the interlayersinterlayers [[1212––2020].]. The characteristics of layered HNbMoO66 andand HTaMoO HTaMoO66 asas solid solid acid acid catalysts catalysts are are their their unique unique ability ability of of intercalation of reactants within the interlayer interlayerss with strong Br Brønstedønsted acid sites and high tolerance against water. The layered − − metal oxidesoxides consist ofof negativelynegatively chargedcharged [NbMoO[NbMoO66]]− or [TaMoO66]− layerslayers and and positively positively charged H+. .The The proton proton between between layers layers has has strong strong Br Brønstedønsted acidity acidity,, which which is is comparable to to the strongest sites of of H-ZSM5 H-ZSM5 zeolite zeolite [ 12[12,14,14].] A. A variety variety of ofreactants reactants such such as alcohols, as alcohols, aldehydes aldehydes and hydroxyand hydroxy acids acidscan be can intercalated be intercalated into the interlayers into the interlayers with strong with acid sites,strong resulting acid sites in the, resulting high performance in the high for performanceacid-catalyzed for reactions acid-catalyzed including Friedel-Crafts reactions including alkylation, Friedel acetalization,-Crafts alkylation, esterification, acetalization, hydrolysis esterification,and hydration. hydrolysis The importance and ofhydration. intercalation The for importance Friedel-Crafts of alkylation intercalation of benzyl for Friedel alcohol-Crafts with alkylationanisole was of confirmed benzyl alcoholby measurements with anisole of the was interlayer confirmed distances by measurements of HNbMoO6 by of X-ray the di interlayerffraction distances(XRD) [12 ]. of When HNbMoO the layered6 by X HNbMoO-ray diffraction6 was immersed (XRD) [12] in. benzylWhen alcohol the layered solution, HNbMoO the interlayer6 was immerseddistance of in the benzyl layered alcohol HNbMoO solution,6 increased the interlayer due distance to the intercalation of the layered of HNbMoO the alcohol6 increased in which due the tototal the expansion intercalation was 0.55 of the nm. alcohol The benzyl in which alcohol-intercalated the total expansion HNbMoO was6 could 0.55 react nm. withThe anisolebenzyl alcohol-intercalated HNbMoO6 could react with anisole at 373 K to produce the alkylated compounds. After the reaction, the interlayer distance decreased back. Moreover, the expansion of

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CatalystsCatalysts 2019 2019, ,9 9, ,x x FOR FOR PEER PEER REVIEW REVIEW 44 of of 19 19 at 373 K to produce the alkylated compounds. After the reaction, the interlayer distance decreased back.thethe interlayer interlayer Moreover, distance distance the expansion was was observed observed of the for interlayer for the the sample sample distance which which was was was observed taken taken during duringfor the the samplethe Friedel Friedel which-Crafts-Crafts was takenalkylation.alkylation. during These These the Friedel-Craftsresults results demonstrate demonstrate alkylation. that that the the These interlayer interlayer results sites sites demonstrate of of the the l ayeredlayered that theoxide oxide interlayer catalyst catalyst sitesfunction function of the layeredasas the the active active oxide sites. catalystsites. The The functioncatalyst catalyst could ascould the apply activeapply for for sites. sugar sugar The conversion conversion catalyst could, ,including including apply hydrolysis hydrolysis for sugar of conversion,of sugars, sugars, includingcyclodehydrationcyclodehydration hydrolysis of of sugar sugar of sugars, alcohols, alcohols, cyclodehydration epimerization epimerization of of aldoses aldoses sugar alcohols,and and mechanochemical mechanochemical epimerization decomposition decomposition of aldoses and mechanochemicalofof cellulose cellulose ( Figure(Figure decomposition 2). 2). of cellulose (Figure2).

FigureFigure 2. 2. Sugar Sugar transformation transformation in in water water using using layered layered HNbMoO HNbMoO6 6solid solid acid. acid. Figure 2. Sugar transformation in water using layered HNbMoO6 solid acid.

SinceSinceSince sugars sugars sugars and and sugar sugar alcohols alcohols have have have many many many hydroxyl hydroxyl groups, groups, thesethese these compounds compounds compounds couldcould could be be be intercalatedintercalatedintercalated intointo into the the layered layered oxides oxidesoxides and andand reacted reacted with with the the the acid acid acid sites sites sites of of of the the the layered layered layered oxides oxides oxides [13,16,17] [13,16,17] [13,16,17. . ]. DisaccharidesDisaccharidesDisaccharides including including including cellobiose cellobiose cellobiose (a dimer (a (a ofdimer dimer two glucose of of two two molecules glucose glucose connected molecules molecules by β connected connected-(1,4)-glycosidic by by β-(1,4)-glycosidic bond) and sucrose (a dimer of glucose and fructose connected by bond)β-(1,4) and-glycosidic sucrose (a bond) dimer and of glucose sucrose and (a fructose dimer connected of glucose by andα,β -(1,2)-glycosidic fructose connected bond) by are α,β-(1,2)-glycosidic bond) are water-soluble and able to be intercalated into the oxides, led to be water-solubleα,β-(1,2)-glycosidic and able bond) to be are intercalated water-soluble into and the able oxides, to be led intercalated to be hydrolyzed into the into oxides, corresponding led to be hydrolyzed into corresponding monosaccharides. Figure 3 shows the results of the hydrolysis of monosaccharides.hydrolyzed into corresponding Figure3 shows monosaccharides. the results of the Figure hydrolysis 3 shows of sugars the results by using of the a variety hydrolysis of solid of sugarssugars by by using using a a variety variety of of solid solid acids. acids. TheThe HNbMoO HNbMoO6 6 exhibitedexhibited remarkab remarkablele reactionreaction rates rates and and acids. The HNbMoO6 exhibited remarkable reaction rates and turnover frequencies. For sucrose turnoverturnover frequencies. frequencies. For For sucrose sucrose hydrolysis, hydrolysis, reaction reaction rate rate of of HNbMoO HNbMoO6 6was was ca. ca. four four times times higher higher hydrolysis, reaction rate of HNbMoO6 was ca. four times higher than that of Amberlyst-15 catalyst. thanthan that that of of Amberlyst Amberlyst-15-15 catalyst. catalyst. For For cellobiose cellobiose hydrolysis, hydrolysis, turnover turnover frequency frequency of of HNbMoO HNbMoO6 6was was For cellobiose hydrolysis, turnover frequency of HNbMoO6 was ca. six times higher than that of ca.ca. six six times times higher higher than than that that of of Amberlyst Amberlyst-15-15. .As As mentioned mentioned above, above, H H-type-type zeolites zeolites were were difficult difficult Amberlyst-15. As mentioned above, H-type zeolites were diﬃcult to use as Brønsted acid catalysts in toto use use asas Br Brøønstednsted acid acid catalysts catalysts in in water, water, resulting resulting in in negligible negligible activity. activity. UnderUnder these these reaction reaction water, resulting in negligible activity. Under these reaction conditions, no formation of glucose was conditions,conditions, no no formation formation of of glucose glucose was was observed observed in in the the absence absence of of catalyst. catalyst. TheThe intercalation intercalation of of observed in the absence of catalyst. The intercalation of sugars such as glucose, fructose, sucrose and sugarssugars such such as as glucose, glucose, fructose, fructose, sucrose sucrose andand cellobiosecellobiose was was confi confirmedrmed by by the the expansion expansion of of thethe cellobiose was conﬁrmed by the expansion of the interlayer distance of the layered oxides. interlayerinterlayer distance distance of of the the layered layered oxides. oxides.

Figure 3. Hydrolysis of cellobiose over layered HNbMoO6 solid acid. FigureFigure 3. 3.Hydrolysis Hydrolysis ofof cellobiosecellobiose over layered HNbMoO 66 solidsolid acid. acid.

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Like sugars, sugar alcohols are also able to be intercalated into the HNbMoO6 catalyst. Sorbitol, sugarLike alcohol sugars, obtained sugar byalcohols reduction are also of glucose,able to be could intercalated be transformed into the intoHNbMoO isosorbide6 catalyst. by cyclodehydration.Sorbitol, sugar alcohol The transformation obtained by reduction of sorbitol of into glucose, isosorbide could is be a transformedsuccessive reaction into isosorbide via the formationby cyclodehydration. of 1,4-sorbitol The as transformation an intermediate. of sorbitol Two consecutive into isosorbide 1,4-cyclodehydration is a successive reaction reactions via are the necessaryformation to of afford1,4-sorbitol isosorbide as an intermediate. selectively. Other Two cyclodehydrations consecutive 1,4-cyclodehydration occur to form reactions undesirable are productsnecessary such to aff asord 2,5 isosorbide-sorbitan selectively.and 1,5-sorbi Othertan. cyclodehydrationsThus, the catalyst should occur to highly form undesirablerecognize hydroxyl products groupssuch as of 2,5-sorbitan sorbitol in and order 1,5-sorbitan. to obtain 1,4 Thus,-sorbitan the catalyst and isosorbide should highly selectively. recognize Water hydroxyl is a desirable groups solventof sorbitol because in order it is toso- obtaincalled green 1,4-sorbitan solvent and and isosorbide monosaccharides selectively. were Water formed is aby desirable aqueous solvent-phase hydrolysisbecause it isof so-called oligomers. green The solvent HNbMoO and monosaccharides6 catalyst could give were 1,4 formed-sorbitan by aqueous-phasefrom sorbitol in hydrolysis water [18] of. Theoligomers. selectivity The to HNbMoO 1,4-sorbitan6 catalyst was 57% could at give the 1,4-sorbitansorbitol conversion from sorbitol of 59%. in waterWhile [18the]. catalyst The selectivity is not suitableto 1,4-sorbitan for the wasproduction 57% at theof isosorbide sorbitol conversion, which is us ofeful 59%. for While bio-based the catalyst plastics, is the not high suitable selectivity for the toproduction 1,4-sorbitan of isosorbide,is of significance which from is useful the viewpoint for bio-based of fundamental plastics, the high chemistry selectivity because to 1,4-sorbitan 1,4-sorbitan is isof an significance intermediate from between the viewpoint two similar of fundamental 1,4-cyclodehydration chemistry because reactions. 1,4-sorbitan The selective is an formation intermediate of 1,4between-sorbitan two over similar HNbMoO 1,4-cyclodehydration6 was due to its reactions. selective The intercalation. selective formationThe layered of 1,4-sorbitan HNbMoO6 overwas immersedHNbMoO 6 inwas water due containing to its selective sorbitol, intercalation. 1,4-sorbitan The or layeredisosorbide, HNbMoO separately6 was. After immersed filtration in water and drying,containing XRD sorbitol, of the 1,4-sorbitanlayered oxide or isosorbide,was measured separately. (Figure After 4). The filtration increase and in drying, the interlayer XRD of the distance layered wasoxide found was measured only for sorbitol. (Figure4 Thus,). The only increase sorbitol in the could interlayer be intercalated distance was, whereas found only1,4-sorbitan for sorbitol. and isosorbideThus, only did sorbitol not couldintercalate be intercalated,. Due to the whereas selective 1,4-sorbitan intercalation, and isosorbide sorbitol could did not be intercalate. converted into Due 1,4to- thesorbitan, selective but intercalation, 1,4-sorbitan sorbitol was difficult could be to converted make react into with 1,4-sorbitan, the acid but sites 1,4-sorbitan of the layered was di oxide.fficult Schemeto make 1 react shows with simplified the acid sites reaction of the layered pathways oxide. for Scheme sorbitol1 shows cyclodehydration simplified reaction over HNbMoO pathways6. Reforaction sorbitol rate cyclodehydration constants of each over reaction HNbMoO pathway6. Reaction were rate estimated constants by of each a kinetic reaction study. pathway The were rate constantestimated for by the a kinetic reaction study. from The sorbitol rate constant to 1,4-sorbitan for the reaction(k1) was from two sorbitoltimes higher to 1,4-sorbitan than that (fork1) was the reactiontwo times from higher 1,4-sorbitan than that to forisosorbide the reaction (k2) over from the 1,4-sorbitan HNbMoO to6 catalyst. isosorbide Besides (k2), over the preexponential the HNbMoO6 factorcatalyst. forBesides, the first the reaction preexponential is significantly factor for higher the first than reaction that for is significantly the second reaction. higher than These that results for the clearlysecond show reaction. that Thesethe selective results clearlyintercalation show thatof reactants the selective could intercalation control the of successive reactants couldreaction control and producethe successive an intermediate reaction and selectively. produce an intermediate selectively.

FigureFigure 4. 4. XRDXRD patterns patterns of of HNbMoO HNbMoO6 after6 after immersion immersion in inwater water involving involving sorbitol, sorbitol, 1,4- 1,4-sorbitansorbitan or isosorbideor isosorbide..

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Scheme 1. Simplified reaction pathways for sorbitol cyclodehydration over layered HNbMoO 6.

SchemeScheme 1. 1.Simplified Simplified reactionreaction pathwayspathways for for sorbitol sorbitol cyclodehydration cyclodehydration over over layered layered HNbMoO HNbMoO6.6. Like sorbitol, erythritol, C4 sugar alcohol could be dehydrated into 1,4-anhydroerythritol. MoreoverLikeLike sorbitol,, sorbitol, 1,4-butanediol erythritol,erythritol , was C4C4 sugarsugar converted alcohol alcohol couldinto could tetrahydrofuranbe be dehydrated dehydrated into into (THF) 1,4-anhydroerythritol. 1,4 -anhydroerythritol. via the same Moreover,1,4Moreover-cyclodehydration. 1,4-butanediol, 1,4-butanediol The was HNbMoO converted was converted6 into catalyst tetrahydrofuran into exhibited tetrahydrofuran (THF) moderate via the activity same (THF) 1,4-cyclodehydration. for via 1,4 - thebutanediol same The HNbMoO catalyst exhibited moderate activity for 1,4-butanediol cyclodehydration, while it cyclodehydration1,4-cyclodehydration.6 , while The it showedHNbMoO high6 catalystactivity for exhibited erythritol moderate cyclodehydration activity for[19] .1,4 Kinetic-butanediol study showedoncyclodehydration these high cyclodehydration activity, while for erythritolit showedreactions cyclodehydration high over activity HNbMoO for erythritol [619 indicated]. Kinetic cyclodehydration that study the on difference these [19] cyclodehydration . of K inetic the activity study reactions over HNbMoO indicated that the difference of the activity between two C4 diols could betweenon these twocyclodehydration C4 diols could6 reactions be ascribed over to differentHNbMoO mobility6 indicated of the that activated the difference complex of based the activity on the betransitionbetween ascribed two state to C4 ditheory. diolsfferent c Theould mobility pre be- ascribedexponential of the to activated different factor after complex mobility appropriate basedof the activatedtreatment on the transition complexis related statebased to partition theory. on the Thefunctionstransition pre-exponential stateinvolving theory. factor the The contribution after pre- appropriateexponential of 3D treatmentfactor translation, after is appropriate related 2D translation to partition treatment and functions is1 Drelated translation. involving to partition theThe contributioncarefulfunctions calculation involving of 3D translation, indicated the contribution 2D that translation the of activated 3D and translation, 1D complex translation. 2Dfor translation The 1,4- carefulbutanediol calculationand cyclodehydration1D translation. indicated thatThe is theimmobile,careful activated calculation resulting complex indicated in for moderate 1,4-butanediol that activity the cyclodehydration activated (Figure complex5). In contrast is immobile,for 1,4, the-butanediol resulting transition incyclodehydration state moderate for erythritol activity is (Figurecyclodehydrationimmobile,5). In resulting contrast, is in2 theD moderate translation transition activity state, affording for (Figure erythritol high 5) . activity cyclodehydrationIn contrast. The, easethe transition of is 2D intercalation translation, state for of a erythritol reaffordingctants highsignificancyclodehydration activity.tly affects The ease is the 2 ofD acid intercalationtranslation-catalyzed, affording of reaction. reactants high significantly activity. The affects ease the of acid-catalyzed intercalation reaction.of reactants significantly affects the acid-catalyzed reaction.

2D translational 2D translational

FigureFigure 5. 5. Intercalation-controlledIntercalation-controlled cyclodehydrations cyclodehydrations of of 1,4-butanediol 1,4-butanediol and and erythritol erythritol over over layered layered HNbMoOHNbMoOFigure 5. 66Intercalation.. -controlled cyclodehydrations of 1,4-butanediol and erythritol over layered

HNbMoO6. AsAs describeddescribed above,above, thethe intercalationintercalation ofof reactantsreactants withinwithin thethe interlayerinterlayer withwith strongstrong BrønstedBrønsted acidacid sitesAs sites described is is characteristic characteristic above, ofthe of the theintercalation layered layered HNbMoO HNbMoOof reactants6.6. InIn within other other words,the words, interlayer cellulosecellulose with depolymerization depolymerization strong Brønsted over HNbMoO was difficult because cellulose is a big molecule and insoluble in water [13]. overacid sitesHNbMoO is characteristic66 was difficult of the because layered cellulose HNbMoO is a6 . big In molecule other words, and celluloseinsoluble depolymerization in water [13]. In In these regards, the mechanochemical reaction as a different methodology was adopted for cellulose theseover HNbMoOregards, the6 was mechanochemical difficult because reaction cellulose as a is different a big molecule methodology and insoluble was adopted in water for cellulose [13]. In depolymerizationdepolymerizationthese regards, the [[20]mechanochemical20].. ItIt hashas beenbeen reportedreported reaction thatthat as ball-millingaball different-milling methodology ofof crystallinecrystalline was cellulosecellulose adopted inin the thefor presence presencecellulose ofofdepolymerization acidicacidic kaolinite,kaolinite, [20] a layered. It has clay been mineral reported could could that depolymerize depolymerizeball-milling of it it crystallineand and the the solid solid cellulose acidity acidity in of ofthe additives additives presence is isimportantof important acidic kaolinite, to toaccelerate accelerate a layered the the reaction clay reaction mineral [21] [21. The could]. Theball depolymerizeball-millingmilling of cellulose ofit celluloseand thewith solid withlayered acidity layered HNbMoO of HNbMoOadditives6 could 6is could produce a high yield of water-soluble sugars (72%) at full cellulose conversion without adding produceimportant a tohigh accelerate yield ofthe water reaction-soluble [21] . sugarsThe ball (72%)-milling at of full cellulose cellulose with conversion layered HNbMoO without adding6 could externalexternalproduce heat, heata high whereas, whereas yield the of the ball-milling water ball-solublemilling of cellulose ofsugars cellulose (72%)in the in absence at the full absence cellulose of catalyst of catalystconversion afforded afforded negligible without negligible yield adding of sugarsyieldexternal of (< 1%) heatsugars [,20 whereas ]. (<1%) Not only [20] the. cello-oligomers ballNot-milling only cello of such cellulose-oligomers as cellobiose in such the absence andas cellobiose cellotriose of catalyst and but cellotriosealso afforded corresponding negligible but also anhydro-sugarscorrespondingyield of sugars were anhydro (<1%) produced [20]-sugars. Not by were only the mechanochemical cello produced-oligomers by the such reaction mechanochemical as cellobioseunder dry conditions.and reaction cellotriose The under productbut also dry distributionconditions.corresponding ofThe water-soluble anhydro product-sugars distribution sugars were showed produced of that water proportions - bysoluble the mechanochemicalof sugars monosaccharides, showed that reaction disaccharides proportions under and dry of monosaccharides,conditions. The product disaccharides distribution and other of cellowater-oligomers-soluble sugarsfrom cellotriose showed that to cellohexose proportions were of monosaccharides, disaccharides and other cello-oligomers from cellotriose to cellohexose were

Catalysts 2019, 9, 907 7 of 20 otherCatalysts cello-oligomers 2019, 9, x FOR PEER from REVIEW cellotriose to cellohexose were almost the same regardless of reaction7 of 19 time, whereas the total sugar yields monotonically increased (Figure6). These results indicate that cellulosealmost the was same randomly regardless and of directly reaction decomposed time, whereas into the water-soluble total sugar sugars.yields monotonically The addition ofincreased a small amount(Figure of6) water. These improved results theindicate sugar yields that cellulose and the selectivity was randomly of sugars and/anhydrosugars directly decomposed was increased. into Thewater motion-soluble of ballssugars. was The simulated addition by of using a small a discrete amount element of water method improved (DEM) the and sugar then theyields mechanical and the energyselectivity was of calculated. sugars/anhydrosugars It was suggested was that increased. 0.02% of The mechanical motion energyof balls was was applied simulated for the by cleavage using a ofdiscrete glycosidic element bonds. method Further (DEM) improvement and then ofthe the mechanical energy eﬃ energyciency was would calculated. be possible It was by scalingsuggested up thethat apparatus. 0.02% of mechanical energy was applied for the cleavage of glycosidic bonds. Further improvement of the energy efficiency would be possible by scaling up the apparatus.

FigureFigure 6. 6. Dependence of yield of water-soluble water-soluble sugars sugars on on reaction reaction time time (milling (milling time). time). Reaction Reaction conditions: Microcrystalline cellulose (Avicel, 0.4 g), layered HNbMoO6 (0.4 g), 600 rpm. RT. G1: conditions: Microcrystalline cellulose (Avicel, 0.4 g), layered HNbMoO6 (0.4 g), 600 rpm. RT. G1: ′ Glucose:Glucose: G1G10: Mannose;: Mannose; G2: G2: Cellobiose; Cellobiose; G3: G3: Cellotriose; Cellotriose; G4:G4: Cellotetraose;Cellotetraose; G5:G5: Cellopentaose;Cellopentaose; G6:G6: Cellohexaose;Cellohexaose; A1:A1: Anhydroglucose;Anhydroglucose; A2:A2: Anhydrocellobiose.Anhydrocellobiose. Reproduced with permission from [[20];20]; copyrightcopyright (2018),(2018), JohnJohn WileyWiley & & Sons, Sons, Inc. Inc.

AnotherAnother feature feature of the of layered the layered NbMo NbMooxide is thatoxide it could is that eﬃ ciently it could catalyze efficiently the epimerization catalyze the of aldosesepimerization [22]. Epimerization of aldoses [22] involves. Epimerization carbon–carbon involves rearrangement carbon–carbon and isrearrangement useful for the productionand is useful of rarefor the sugars. production In the presence of rare sugars. of HNbMoO In the6 orpresence LiNbMoO of HNbMoO6, glucose6 was or LiNbMoO quickly converted6, glucose into was mannose quickly viaconverted epimerization into mannose (Figure7 ). via An epimerizatio ancient studyn demonstrated(Figure 7). An that ancient MoO 3studyand homogeneous demonstrated Mo that complex MoO3 couldand homogeneous epimerize glucose Mo complex to mannose, could whichepimerize was glucose known asto themannose Bilik reaction., which was Both known HNbMoO as the6 Bilikand LiNbMoOreaction. 6Bothcould HNbMoO convert aldoses6 and LiNbMoO to the corresponding6 could convert epimers aldoses (glucose to to the mannose, corresponding xylose to epimers lyxose, arabinose(glucose to to mannose, ribose) selectively. xylose to Whilelyxose, MoO arabinose3 was completelyto ribose) selectively. dissolved inWhile water MoO during3 was the completely reaction, thesedissolved layered in NbMo water oxides during were the reaction,insoluble theseand could layered be reused NbMo without oxides loss were of insolubleactivity. The and active could sites be forreused the epimerizationwithout loss of were activity. considered The active to be sites the for Mo the octahedra epimerization at the surface,were considered not the Mo to octahedrabe the Mo withinoctahedra the atinterlayer. the surface, The not turnover the Mo frequency octahedra for within glucose the epimerization interlayer. The over turnover LiNbMoO frequency6 was for of 1 −1 signiﬁcance,glucose epimerization 1.1 s− at 393 over K, LiNbMoO higher than6 was that of over significance, Mo-based polyoxometalates.1.1 s at 393 K, higher The combinationthan that over of hydrolysisMo-based and polyoxometalates. epimerization overThe layered combination HNbMoO of 6 hydrolysisrealized one-pot and epimerization formation of mannose over layered from cellobiose.HNbMoO6 Amberlyst-15, realized one-pot a representative formation of Brønsted mannose acid from resin cellobiose catalyst. couldAmberlyst hydrolyze-15, a cellobioserepresentative into glucose,Brønsted but acid mannose resin catalyst was not could formed. hydrolyze In contrast, cellobiose HNbMoO into glucose,6 also could but mannose hydrolyze was cellobiose not formed. into glucoseIn contrast, within HNbMoO the interlayer,6 also could and then hydrolyze mannose cellobiose was obtained into glucose via epimerization. within the interlayer, and then mannose was obtained via epimerization.

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(A) 10 (B) 10

(A) 10 (B) 10 8 8 Equilibrium 8 Glucose 8 Equilibrium Glucose

-1 6 Glucose -1 6 Glucose

-1 6 -1

6 Equilibrium 4 4

/mol L /mol

Equilibrium

C /mol L /mol C 4 C 4

/mol L /mol

C /mol L /mol C 2 Mannose 2 C Mannose Equilibrium 2 Mannose 2 Mannose 0 0 Equilibrium 0 1 2 3 0 1 2 3 t /h t /h 0 1 2 3 0 1 2 3 t /h t /h Figure 7. Time courses of epimerization of (a) glucose and (b) mannose over layered LiNbMoO6. Reaction conditions: Glucose (50 mg, 0.28 mmol), catalyst (50 mg), H2O (3 mL), 373 K. Reproduced FigureFigure 7. Time 7. Time courses courses of epimerization of epimerization of (ofa) glucose(a) glucose and and (b) mannose(b) mannose over over layered layered LiNbMoO LiNbMoO6. 6. with permission from [22]; copyright (2015), John Wiley & Sons, Inc. ReactionReaction conditions: conditions Glucose: Glucose (50 (50 mg, mg, 0.28 0.28 mmol), mmol), catalyst catalyst (50 (50 mg), mg), H2O H (32O mL), (3 mL), 373 373 K. ReproducedK. Reproduced withwith permission permission from from [22]; [22]; copyright copyright (2015), (2015), John John Wiley Wiley & Sons, & Sons, Inc. Inc. 4. Metal Oxide Nanosheet Aggregates 4. Metal Oxide Nanosheet Aggregates 4.Protonated Metal Oxide layered Nanosheet transition Aggregates metal oxides have strong acid sites within the interlayer. Protonated layered transition metal oxides have strong acid sites within the interlayer. However, However,Protonated most of the layered reactants transition cannot enter metal the oxides interlayer have spaces strong of these acid siteslayered within oxides the except interlayer. for most of the6 reactants cannot6 enter the interlayer spaces of these layered oxides except for HNbMoO HNbMoOHowever, and most HTaMoO of the reactants. In order cannot to overcome enter the interlayer the drawbacks, spaces aof variety these layered of protonated oxides except metal6 for andoxides HTaMoO, including6. In HTiNbO order to5 overcome, HTi2NbO the7, HNb drawbacks,3O8, HNbWO a variety6 and of HTaWO protonated6 were metal utilized oxides, as includingsolid acid HNbMoO6 and HTaMoO6. In order to overcome the drawbacks, a variety of protonated metal HTiNbOcatalysts 5by, HTi exfoliating2NbO7, HNbthem3 Ointo8, HNbWOthe corresponding6 and HTaWO two-6dimensionalwere utilized nanosheets as solid acid[23–27 catalysts]. Figure by 8 oxides, including HTiNbO5, HTi2NbO7, HNb3O8, HNbWO6 and HTaWO6 were utilized as solid acid exfoliatingshowscatalysts the schematicthem by exfoliating into crystalthe corresponding them structure into the of two-dimensionalcorresponding metal oxide nanosheets. two nanosheets-dimensional Figure [23 –nanosheets927 shows]. Figure the [238 preparationshows–27]. Figure the 8 schematicprocedureshows crystal the of schematic the structure metal crystal of oxide metal structure nanosheet oxide nanosheets. of metal aggregates. oxide Figure nanosheets.9The shows addition the Figure preparation of 9 tetrabutylammonium shows procedure the preparation of the metalhydroxideprocedure oxide (TBAOH) nanosheet of the into aggregates.metal aqueous oxide Thesolution nanosheet addition containing of aggregates. tetrabutylammonium powder The of protonated addition hydroxide of layered tetrabutylammonium (TBAOH) metal oxide into + aqueouscouldhydroxide exfoliate solution (TBAOH) the containing layered into powderaqueous oxide of becausesolution protonated containingcationic layered TBA powder metal can oxide ofpenetrate protonated could exfoliatethe layered interlayers the metal layered byoxide oxide because cationic TBA+ can penetrate+ the interlayers by ion-exchange reaction and the bulky ioncould-exchange exfoliate reaction the and layered the bulky oxide TBA because can peel cationic the layered TBA oxide+ can down penetrate to the nanosheets.the interlayers As a by TBA+ can peel the layered oxide down to the nanosheets. As a result, a colloidal solution containing result,ion -exchangea colloidal reaction solution and containing the bulky TBA exfoliated+ can peel nanosheets the layered was oxide obtained down into the which nanosheets. exfoliated As a exfoliatednanosheetsresult, nanosheetsa werecolloidal negatively wassolution obtained charged containing in whichand exfoliated TBA exfoliated cation nanosheets nanosheets surround wased were obtainedthe negatively nanosheets in which charged. For exfoliated and the TBAapplicationnanosheets cation surroundedof thewere nanosheets negatively the nanosheets. as solid charged acid For catalysts,and the applicationTBA the cation exfoliated of the surround nanosheets nanosheetsed the as should solid nanosheetsacid be solidified. catalysts,. For the + + theTheapplication exfoliatedaddition of nanosheets of H the, such nanosheets as should HNO as3 be, HCl solid solidiﬁed. and acid H 2catalysts,SO The4 collapse addition the dexfoliated ofthe H stability, such nanosheets asof HNOthe colloid,3 should, HCl resulting and be Hsolidified.2SO in4 collapsedthe rapid formation the stability of ofnanosheet the colloid, aggregates. resulting in the rapid formation of nanosheet aggregates. The addition of H+, such as HNO3, HCl and H2SO4 collapsed the stability of the colloid, resulting in the rapid formation of nanosheet aggregates.

Figure 8. Schematic crystal structure of metal oxide nanosheets.nanosheets. Figure 8. Schematic crystal structure of metal oxide nanosheets.

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Figure 9. The preparation procedure of metal oxide nanosheet aggregates. Figure 9. The preparation procedure of metal oxide nanosheet aggregates. The nanosheetFigure aggregates 9. The p reparationhad large proceduresurface areas of metal and oxide had nanosheetBrønsted aggregatesacid sites . because proton wasThe on nanosheet the surface aggregates as counter had ions. large The surface solid areas acid properties and had Brønsted of the nanosheet acid sites aggregates because proton were was on theevaluated surfaceThe nanosheet as by counter solid aggregates ions.-state The nuclearhad solid large acid surface magnetic properties areas ofresonance and the had nanosheet Brø (NMR)nsted aggregates acid spectroscopy sites because were evaluated proton and by solid-statetemperaturewas on nuclear the- programmed surface magnetic as counter resonancedesorption ions. (NMR) ofThe ammonia solid spectroscopy acid (NH properties3-TPD). and NH temperature-programmed of3 -TPD the nanosheet is a conventional aggregates method desorption were of ammoniatoevaluated estimate (NH acid by3-TPD). strengthsolid NH- state and3-TPD acid nuclear amounts is a conventional magnetic of solids. However, resonancemethod careful to estimate (NMR) consideration acidspectroscopy strength is required and and acid amountswhentemperature of the solids. method-programmed However, was applied desorption careful for the consideration of metal ammonia oxide (NH nano is required3-TPD).sheet aggregatesNH when3-TPD the is because a method conventional the was nanosheet method applied for the metalaggregatesto estimate oxide may acid nanosheet change strength their aggregates and crystal acid amounts because structure ofs the solids.while nanosheet However,raising aggregatestemperature. careful consideration mayTrimethylphosphine change is requiredtheir crystal structuresoxidewhen (TMPO) while the method raising was widely was temperature. applied used as for a Trimethylphosphine theprobe metal molecule oxide of nano 31P oxidesheetmagic aggregates(TMPO) angle spinning was because widely (MAS) the used NMR nanosheet as for a probe moleculeinvestigatingaggregates of 31P magic mayBrønsted change angle acid spinningtheir strength crystal (MAS)[28] structure. The NMR interactions while for investigating raisingof TMPO temperature. with Brønsted Brønsted Trimethylphosphine acid acid strength sites of [the28]. The solidoxide catalyst (TMPO) gives was protonated widely used TMPO as a probe(TMPOH molecule+). This of species 31P magic shows angle 31P spinningchemical (MAS)shifts at NMR higher for interaction of TMPO with Brønsted acid sites of the solid catalyst gives protonated TMPO (TMPOH+). ppminvestigating than crystalline Br31ønsted TMPO. acid strengthThus, the [28] chemical. The interaction shifts of TMPOof TMPO adsorbed with Br correspondønsted acid t osites the ofacid the Thisstrength speciessolid catalyst shows of solid gives P acids. chemicalprotonated The 31 shiftsP TMPO MAS at higher(TMPOH NMR ppmmeasurements+). This than species crystalline using shows TMPO TMPO. 31P chemical as Thus, a probe shifts the chemical molecule at higher shifts 31 of TMPOindicatedppm adsorbedthan that crystalline the correspond acid TMPO. strength to Thus the of, acid the transition chemical strength metal shifts of solid oxideof TMPO acids. nanosheet adsorbed The P aggregates MAScorrespond NMR dramatical measurementsto the acidly usingdependsstrength TMPO on as of the a solid probe constituent acids. molecule The element 31 indicatedP MAScompositions NMR that the measurements, in acid the order strength of using HTiNbO of transition TMPO5 < HNb as metal a3O probe8 < oxideHNbWO molecule nanosheet6 < aggregatesHTaWOindicated dramatically6 (Figure that t10)he [26]. acid depends strength on the of constituent transition metal element oxide compositions, nanosheet aggregates in the order dramatical of HTiNbOly 5 < HNb3dependsO8 < HNbWO on the constituent6 < HTaWO element6 (Figure compositions 10)[26]. , in the order of HTiNbO5 < HNb3O8 < HNbWO6 < HTaWO6 (Figure 10) [26].

Figure 10. 31P magic angle spinning (MAS) NMR spectra for trimethylphosphine oxide (TMPO)

adsorbed (a) HTiNbO5 nanosheets aggregate, (b) HNb3O8 nanosheets aggregate, (c) HNbWO6 31 FigurenanosheetsFigure 10. 31 10.P aggregate magicP magic angle and angle ( spinningd ) spinningHTaWO (MAS) 6 (nanosheetsMAS) NMRNMR aggregate, spectra for formeasured trimethylphosphine trimethylphosphine at room temperature. oxide oxide (TMPO) The (TMPO) adsorbed (a) HTiNbO5 nanosheets aggregate, (b) HNb3O8 nanosheets aggregate, (c) HNbWO6 adsorbed (a) HTiNbO5 nanosheets aggregate, (b) HNb3O8 nanosheets aggregate, (c) HNbWO6 nanosheets aggregate and (d) HTaWO6 nanosheets aggregate, measured at room temperature. The nanosheets aggregate and (d) HTaWO6 nanosheets aggregate, measured at room temperature. The spinning rate of the sample was 10 kHz. Reproduced with permission from [26]; copyright (2009), American Chemical Society. Catalysts 2019, 9, x FOR PEER REVIEW 10 of 19

Catalystsspinning2019, 9, 907rate of the sample was 10 kHz. Reproduced with permission from [26]; copyright (2009),10 of 20 American Chemical Society. These nanosheet aggregates have so far been applied for hydrolysis of disaccharides [13], furfural These nanosheet aggregates have so far been applied for hydrolysis of disaccharides [13], formation from xylose [29] and HMF formation from fructose and glucose [30,31]. HTiNbO5 nanosheets furfural formation from xylose [29]2 and1 HMF formation from fructose and1 glucose [30,31]. had a high surface areas of ca. 150 m g− with an acid amount of 0.4 mmol g− and exhibited the HTiNbO5 nanosheets had a high surface areas of ca. 150 m2 g−1 with an acid amount of 0.4 mmol g−1 catalytic activity for hydrolysis of sucrose, higher turnover frequency than Naﬁon NR50, Amberlyst-15, and exhibited the catalytic activity for hydrolysis of sucrose, higher turnover frequency than Nafion Nb2O5 nH2O, H-ZSM5 and liquid H2SO4. The acid strength of HTiNbO5 nanosheets was moderate, NR50, Amberlyst-15, Nb2O5 nH2O, H-ZSM5 and liquid H2SO4. The acid strength of HTiNbO5 resulting in low activity for hydrolysis of cellobiose, which needs strong acids for cleavage of the nanosheets was moderate, resulting in low activity for hydrolysis of cellobiose, which needs strong 1,4-β-glycosidic bond (Figure3)[13]. acids for cleavage of the 1,4-β-glycosidic bond (Figure 3) [13]. A variety of metal oxide nanosheet aggregates, including HTiNbO5, HTi2NbO7,H2Ti3O7, HNb3O8 A variety of metal oxide nanosheet aggregates, including HTiNbO5, HTi2NbO7, H2Ti3O7, and H4Nb6O17, were tested for furfural formation from xylose in water-toluene biphasic solution at HNb3O8 and H4Nb6O17, were tested for furfural formation from xylose in water-toluene biphasic 433 K [29]. A high furfural yield of 55% with a xylose conversion of 92% was obtained over HTiNbO5 solution at 433 K [29]. A high furfural yield of 55% with a xylose conversion of 92% was obtained nanosheet aggregates, which were prepared by addition of MgO during the aggregation process. over HTiNbO5 nanosheet aggregates, which were prepared by addition of MgO during the There was a correlation between the initial catalytic activities of the nanosheet aggregates and the total aggregation process. There was a correlation between the initial catalytic activities of the nanosheet amount of Brønsted and Lewis acid sites. These titanoniobate nanosheet aggregates were reusable for aggregates and the total amount of Brønsted and Lewis acid sites. These titanoniobate nanosheet the reaction (Figure 11). aggregates were reusable for the reaction (Figure 11).

Figure 11.11. Furfural yield obtained in recycling runs (run 1 1—white—white bar, run 2—dots,2—dots, run 3—hashed)3—hashed) over the the exfoliated-aggregated exfoliated-aggregated nanosheet nanosheet solid solid acid acid catalysts. catalysts. Reproduced Reproduced with with permission permission from from [29]; copyright[29]; copyright (2006), (20 Elsevier.06), Elsevier.

HMF formation from fructosefructose andand glucoseglucose inin waterwater waswas demonstrateddemonstrated usingusing layeredlayered HNbHNb33OO88 under microwave microwave irradiation irradiation [[30]30].. The layered HNb3OO88 waswas exfoliated during the reaction with fructose underunder microwavemicrowave irradiation,irradiation, aaffordingﬀording HMFHMF yieldyield ofof 56%56% atat 423423 K.K. TheThe exfoliatedexfoliated HNbHNb33OO88 nanosheets were restackedrestacked afterafter thethe reaction.reaction. ThusThus thethe catalystcatalyst showedshowed reusability.reusability. Very recently, recently it, wasit was reported reported that threethat three acidic acidic nanosheet nanosheet aggregates, aggregates including, including HTiNbO 5 HTiNbO, HNb3O58, andHNb HNbWO3O8 and6 HNbWOwere applied6 were for aqueous-phase applied for aqueous glucose-phase conversion glucose to HMF conversion [31]. The to activity HMF was[31] in. The the orderactivity of was HTiNbO in the5 < orderHNb 3ofO 8HTiNbO< HNbWO5 < HNb6, which3O8 < is HNbWO in good agreement6, which is with in good the orderagreement of their with acidity. the HNbWOorder of their6 nanosheet acidity. aggregates HNbWO6 nanosheet exhibited highaggregates HMF selectivity exhibited ofhigh 52% HMF with selectivity glucose conversion of 52% with of 71%glucose in a conversion water-toluene of 71% biphasic in a water system.-toluene It should biphasic be noted system. that It the should formation be noted of HMF that the from formation glucose wasof HMF higher from than glucose that from was fructose higher than over that HNbWO from6 fructosenanosheet over aggregates. HNbWO6 A nanosheet kinetic study aggregates. suggested A thatkinetic not only study fructose suggested but also that another not intermediate, only fructose possibly but also 3-deoxyglucosone, another intermediate, was involved possibly in the reaction3-deoxyglucosone, (Scheme2). was When involved 3-deoxyglucosone in the reaction and (Scheme fructose 2 were). When used 3- asdeoxyglucosone reactants, the selectivityand fructose to HMFwere fromused 3-deoxyglucosoneas reactants, the selectivity was much to higher HMF thanfrom that3-deoxyglucosone from fructose. was much higher than that from fructose.

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Scheme 2. Proposed reaction pathway for HMF production over HNbWO aggregated nanosheets. Scheme 2. Proposed reaction pathway for HMF production over HNbWO66 aggregated nanosheets. Reproduced withwith permissionpermission fromfrom [[31].31]. 5. Metal Oxide Nanotubes 5. Metal Oxide Nanotubes Titanium oxide nanotubes are readily prepared by hydrothermal synthesis. The titanate nanotubes Titanium oxide nanotubes2 1 are readily prepared by hydrothermal synthesis. The titanate had high surface areas (400 m g− ) with both Brønsted and Lewis acid sites [32,33]. The nanotubes nanotubes had high surface areas (400 m2 g−1) with both Brønsted and Lewis acid sites [32,33]. The showed excellent activity for Friedel-Crafts alkylation of toluene with benzylchloride. Even at room nanotubes showed excellent activity for Friedel-Crafts alkylation of toluene with benzylchloride. temperature, the catalyst could give 92% yield of the corresponding product, which was much higher Even at room temperature, the catalyst could give 92% yield of the corresponding product, which than Hβ zeolite, sulfated zirconia and ion-exchange resins, including Nafion and Amberlyst. Besides, was much higher than Hβ zeolite, sulfated zirconia and ion-exchange resins, including Nafion and the titanate nanotubes afforded HMF from glucose and fructose. However, the HMF yield was Amberlyst. Besides, the titanate nanotubes afforded HMF from glucose and fructose. However, the moderate (ca. 14%) from glucose at 393 K. HMF yield was moderate (ca. 14%) from glucose at 393 K. 6. Mesoporous Metal Oxides 6. Mesoporous Metal Oxides Mesoporous Nb-W mixed oxides catalyzed not only Friedel-Crafts alkylation but also hydrolysis of disaccharidesMesoporous [34 Nb].-W The mixed mesoporous oxides metalcatalyzed oxides not were only prepared Friedel- fromCrafts metal alkylation chlorides but and also a poly-blockhydrolysis copolymerof disaccharides surfactant [34]. PluronicThe mesoporous P-123 as metal a structure-directing oxides were prepared agent. Figurefrom metal 12 shows chlorides SEM andand TEM a poly images-block of copolymer mesoporous surfactant Nb-W mixed Pluronic oxides. P-123 The as mesoporousa structure-directing structure agent. was formed Figure for12 shows SEM and TEM images of mesoporous Nb-W2 mixed1 oxides. The mesoporous structure2 was1 Nb2W8 to Nb oxides. The surface areas were 132 m g− for mesoporous Nb3W7 oxide, 166 m g− formed for Nb2W8 to Nb oxides.2 1 The surface areas were 132 m2 g−1 for mesoporous Nb3W7 oxide, 166 for Nb5W5 oxide and 193 m g− for Nb oxide. The catalytic activity for Friedel-Crafts alkylation of 2 −1 2 −1 anisolem g withfor Nb benzyl5W5 oxide alcohol and and 193 hydrolysis m g offor sucrose Nb oxide. was significantlyThe catalytic influenced activity for by the Friedel Nb- andCrafts W contents.alkylation Theof anisole mesoporous with benzyl Nb oxide alcohol with and the hydrolysis highest surface of sucrose areas was and significantly pore volume influenced among them by showedthe Nb and negligible W contents. activity. The Increasingmesoporous tungsten Nb oxide content with acceleratedthe highest surface the reaction areas rate,and reachingpore volume the among them showed negligible activity. Increasing tungsten content accelerated the reaction rate, highest yield mesoporous Nb3W7 oxide. Further increase of tungsten content drastically decreased thereaching activity the due highest to the collapse yield mesoporous of the mesoporosity. Nb3W7 Theoxide. catalytic Further activity increase for these of two tungsten reactions content was relateddrastically to the decreased Brønsted the acidity. activity31P due MAS to NMRthe collapse using TMPO of the indicatedmesoporosity. that mesoporousThe catalytic Nb-W activity oxide for 31 hadthese strong two reactions Brønsted acidwas sitesrelated in theto the range Brø ofnsted12 acidity.H < 6.6P MAS in which NMR Nb usingW oxide TMPO had indicated the strongest that − ≤ 0 − 3 7 Brønstedmesoporous acid Nb sites-W among oxide them.had strong Figure Br 13ønsted shows acid the sites comparison in the range of the of catalytic −12 ≤ activityH0 < −6.6 for in sucrose which Nb3W7 oxide had the strongest Brønsted acid sites among them. Figure 13 shows the comparison of hydrolysis by using several solid acid catalysts. Non-porous Nb3W7 oxide which was prepared in thethe absencecatalytic ofactivity the structure-directing for sucrose hydrolysis agent by showed using several higher solid activity acid than catalysts. two ion-exchange Non-porous Nb resins3W7 oxide which was prepared in the absence of the structure-directing agent showed higher activity (Amberlyst-15 and Nafion NR50) and niobic acid. Mesoporous Nb3W7 oxide displayed much higher than two ion-exchange resins (Amberlyst-15 and Nafion NR50) and niobic acid. Mesoporous Nb3W7 activity than non-porous Nb3W7 oxide. The mesoporous Nb3W7 oxide also efficiently accelerated oxide displayed much higher activity than non-porous Nb3W7 oxide. The mesoporous1 1 Nb3W7 oxide cellobiose hydrolysis. The rate of glucose formation at 368 K was 0.42 mmol g− h− for mesoporous also efficiently accelerated cellobiose hydrolysis. The rate of glucose formation at 368 K was 0.42 mmol g−1 h−1 for mesoporous Nb3W7 oxide, two times higher than that of Amberlyst-15 (0.22 mmol

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Catalysts 2019, 9, x FOR PEER REVIEW 12 of 19 Catalysts 2019, 9, x FOR PEER REVIEW 12 of 19 Nb W oxide, two times higher than that of Amberlyst-15 (0.22 mmol g 1 h 1). The turnover frequency −1 3 −17 − −−1 g−1 h−1 ). The turnover frequency of mesoporous1 Nb3W7 oxide was 1.40 h−1 , 28 times higher than1 that gof mesoporoush ). The turnover Nb3W frequency7 oxide was of 1.40 mesoporous h− , 28 times Nb3 higherW7 oxide than was that 1.40 of h Amberlyst-15, 28 times higher (0.05 hthan− ). that of Amberlyst-15 (0.05 h−1). of Amberlyst-15 (0.05 h−1).

Figure 12. SEM images of mesoporous (a) Nb, (b) Nb7W3, (c) Nb5W5 and (d) Nb3W7 oxides (scale Figure 12. SEM imagesimages ofof mesoporous mesoporous (a )(a Nb,) Nb, (b )(b Nb) Nb7W73W,(3c,) ( Nbc) Nb5W5Wand5 and (d) ( Nbd) 3NbW73Woxides7 oxides (scale (scale bar: bar: 50 nm). SEM images of non-mesoporous (e) Nb1W9 and (f) W oxides (scale bar: 50 nm). TEM bar:50 nm). 50 nm). SEM SEM images images of non-mesoporous of non-mesoporous (e) Nb (1eW) Nb9 and1W9 ( fand) W oxides(f) W oxides (scale bar:(scale 50 bar: nm). 50 TEM nm). images TEM images of mesoporous (g) Nb, (h) Nb7W3, (i) Nb5W5 and (j) Nb3W7 oxides (scale bar: 10 nm). imagesof mesoporous of mesoporous (g) Nb, (h(g)) Nb Nb,7W (3h,() i)Nb Nb7W5W3, 5(andi) Nb (j5)W Nb5 3andW7 oxides(j) Nb3 (scaleW7 oxides bar: 10 (scale nm). bar: Reproduced 10 nm). Reproduced with permission from [34]; copyright (2010), John Wiley & Sons, Inc. Reproducedwith permission with from permission [34]; copyright from [34]; (2010), copyright John Wiley(2010), & John Sons, Wiley Inc. & Sons, Inc.

Figure 13. Hydrolysis of sucrose over several solid acid catalysts. Reaction conditions: Sucrose (0.5 Figure 13. Hydrolysis of sucrosesucrose overover severalseveral solidsolid acid acid catalysts. catalysts. Reaction Reaction conditions: conditions Sucrose: Sucrose (0.5 (0.5 g, g, 1.46 mmol), H2O (10 mL, 556 mmol), catalyst (0.1 g), 353 K 1 h. Reproduced with permission from g,1.46 1.46 mmol), mmol), H HO2 (10O (10 mL, mL, 556 556 mmol), mmol), catalyst catalyst (0.1 (0.1 g), 353g), 353 K 1 K h. 1 Reproduced h. Reproduced with with permission permission from from [34]; [34]; copyright2 (2010), John Wiley & Sons, Inc. [34];copyright copyright (2010), (2010), John John Wiley Wiley & Sons, & Sons, Inc. Inc. Like Nb-W oxides, mesoporous Ta-W oxides were synthesized and applied for solid acid Like Nb-W oxides, mesoporous Ta-W oxides were synthesized and applied for solid acid catalysts [35]. Again, mesoporous Ta3W7 oxide exhibited the highest activity for Friedel-Crafts catalysts [35]. Again, mesoporous Ta3W7 oxide exhibited the highest activity for Friedel-Crafts alkylation and hydrolysis of disaccharides among various Tax-W1−x oxides. alkylation and hydrolysis of disaccharides among various Tax-W1−x oxides.

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Like Nb-W oxides, mesoporous Ta-W oxides were synthesized and applied for solid acid catalysts [35]. Again, mesoporous Ta3W7 oxide exhibited the highest activity for Friedel-Crafts Catalysts 2019, 9, x FOR PEER REVIEW 13 of 19 alkylation and hydrolysis of disaccharides among various Tax-W1 x oxides. − Niobic acid (Nb2O5 nH2O) is known as a water-tolerant solid acid catalyst [7,8]. The use of Niobic acid (Nb2O5··nH2O) is known as a water-tolerant solid acid catalyst [7,8]. The use of amphiphilic block copolymers as structure-directing agents gave mesoporous Nb2O5 nH2O[36]. amphiphilic block copolymers as structure-directing agents gave mesoporous Nb2O5·nH2·O [36]. The The mesoporous structures such as surface areas, pore volumes and pore size distributions could mesoporous structures such as surface areas, pore volumes and pore size distributions could be be controlled by using different amphiphilic block copolymers, L64, P85, P103 and P123. Figure 14 controlled by using different amphiphilic block copolymers, L64, P85, P103 and P123. Figure 14 shows the glucose formation from cellobiose via hydrolysis using mesoporous Nb2O5 nH2O, bulk shows the glucose formation from cellobiose via hydrolysis using mesoporous Nb2O5·nH2O,2 bulk1 Nb2O5 nH2O and ion-exchange resins. Mesoporous Nb2O5 nH2O prepared with P103 (SBET 246 m g− 2) Nb2O5··nH2O and ion-exchange resins.2 Mesoporous1 Nb2O· 5·nH2O prepared with P103 (SBET 246 m (Figure 14(Ac)) and P123 (SBET 343 m g− ) (Figure 14(Ad)) exhibited excellent activity for the reaction. g−1) (Figure 14(Ac)) and P123 (SBET 343 m2 g−1) (Figure 14(Ad)) exhibited excellent activity for the Moreover, the turnover frequency for mesoporous Nb2O5 nH2O prepared with P103 was much higher reaction. Moreover, the turnover frequency 2for 1mesoporous· Nb2O5·nH2O prepared with P103 was than that for bulk Nb2O5 nH2O (SBET 171 m g− ). Because of no significant difference in solid acid much higher than that for· bulk Nb2O5·nH2O (SBET 171 m2 g−1). Because of no significant difference in property among the porous and bulk Nb2O5 nH2O, porous structure would facilitate the diffusion of solid acid property among the porous and bulk· Nb2O5·nH2O, porous structure would facilitate the reactant and product. diffusion of reactant and product.

Figure 14. Time courses for d-glucose formation by the hydrolysis of cellobiose over (A) bulk and Figure 14. Time courses for d-glucose formation by the hydrolysis of cellobiose over (A) bulk supermicroporous/mesoporousand supermicroporous/mesoporous Nb2O Nb5·nHO2On HandO and(B) (B strongly) strongly acidic acidic resin. resin. (a (a) ) Mesoporous 2 5· 2 Nb2OO5·nnHH2OO prepared prepared with with L64, L64, (b) supermicroporous (b) supermicroporous Nb O n NbH 2O5 prepared·nH2O prepared with P85, ( withc) mesoporous P85, (c) 2 5· 2 2 5· 2 mesoporousNb O nH O Nb prepared2O5·nH2O with prepared P103, with (d) P103, mesoporous (d) mesoporous Nb O n NbH O2O5 prepared·nH2O prepared with P123, with P123, (e) bulk (e) 2 5· 2 2 5· 2 bulkNb O NbnH2O5O,·nH (f2)O, Naﬁon-silica (f) Nafion (SAC-13),-silica (SAC (g)-13), Naﬁon (g) resin Nafion (NR-50) resin and (NR (h-)50) Amberlyst-15. and (h) Amberlyst Reproduced-15. 2 5· 2 withReproduced permission with from permission [36]; copyright from [36]; (2010), copyright American (2010), Chemical American Society. Chemical Society.

Mesoporous tantalum tantalum oxide oxide was was applied applied for HMF for formation HMF formation from glucose from [ 37 glucose]. The synthesized [37]. The 2 1 2 −1 1 synthesizedmesoporous mesoporous Ta O 0.19H TaO2 hadO5·0.19H the BET2O surfacehad the areasBET surface of 79 m areasg and of 79 total m acidg and sites total of 353 acidµmol sites g of. 2 5· 2 − − The353 μ mesoporousmol g−1. The tantalum mesoporous oxide tantalum had both Brønstedoxide had acid both and Br Lewisønsted acid acid sites, and which Lewis could acid contributesites, which to couldthe formation contribute of HMFto the from formation glucose. of HMFHMF yieldfrom ofglucose. 23% and HMF glucose yield conversion of 23% and of glucose 69% were conversion obtained ofin 69% a biphasic were obtained water/methyl in a biphasic isobutyl water/methyl ketone (MIBK) isobutyl system ketone at 448 K.(MIBK) system at 448 K.

7. Amorphous Amorphous Metal Oxides Amorphous metalmetal oxides oxides could could have have both both Brønsted Brønsted and and Lewis Lewis acid sitesacid onsites the on surface. the surface. The former The formeris generally is generally attributed attributed to hydroxyl to groups hydroxyl ( OH) groups and ( the−OH) latter and is derivedthe latter from is coordinatively derived from − coordinativelyunsaturated metal unsaturated sites (:M). metal There sites was a (:M). great There contribution was a to great aqueous contribution sugar conversion to aqueous by usingsugar conversionamorphous by metal using oxides amorphous in which metal Lewis oxides acid sites in which were applicable Lewis acid for sites the were reactions applicable even in for water. the reactionsNiobic acid, even Nb in2 Owater.5 nH2 NiobicO is a hydratedacid, Nb2O amorphous5·nH2O is a niobiumhydrated oxide amorphous and has niobium been considered oxide and asha as · water-tolerantbeen considered Brønsted as a water acid-tolerant catalyst Br soønsted far [3 acid]. However, catalyst itso was far [3] disclosed. However, that it the was Lewis disclosed acid sitesthat theof niobic Lewis acid acid could sites of work niobic even acid in watercould [work38]. Ramaneven in and water FTIR [38] measurements. Raman and FTIR revealed measurements that NbO4 revealedtetrahedra that of NbO niobic4 tetrahedra acid function of niobic as Lewis acid acidfunction sites as even Lewis in theacid presence sites even of in water. the presence In Raman of 1 water.spectroscopy, In Raman the spectroscopy, vibrational band the vibrational at 988 cm −bandattributed at 988 cm to− NbO1 attributed4 tetrahedra to NbO was4 tetrahedra observed was for 1 observeddehydrated for Nb dehydrated2O5 nH2O. After Nb2O exposure5·nH2O. to After water exposure vapor, this to band water at 988 vapor, cm− thisdisappeared, band at indicating 988 cm−1 · disappeared, indicating the formation of NbO4−H2O adducts. The band was recovered after heating at 423 K to remove the water. It was claimed that the reversibility was not reported in isolated NbO4 species on other oxide surfaces. Figure 15 shows FTIR spectra for CO-adsorbed Nb2O5·nH2O. The three bands at 2188, 2168 and 2145 cm−1 were ascribed to CO adsorbed on Lewis acid sites, on Brønsted acid sites and physisorbed CO, respectively [39]. It was observed that the hydrated

Catalysts 2019, 9, 907 14 of 20 the formation of NbO H O adducts. The band was recovered after heating at 423 K to remove 4− 2 the water. It was claimed that the reversibility was not reported in isolated NbO4 species on other oxide surfaces. Figure 15 shows FTIR spectra for CO-adsorbed Nb2O5 nH2O. The three bands at 1 · 2188, 2168 and 2145 cm− were ascribed to CO adsorbed on Lewis acid sites, on Brønsted acid sites andCatalysts physisorbed 2019, 9, x FOR CO, PEER respectively REVIEW [39]. It was observed that the hydrated Nb O nH O, which14 hadof 19 2 5· 2 3 mmol of water adsorbed on 1 g of the metal oxide, still had Lewis acid sites (Figure 15B), though mostNb2O of5·n theH2O NbO, whichtetrahedra had 3 mmol was convertedof water adsorbed into NbO onH 1 gO of adducts. the metal The oxide water-tolerant, still had Lewis acid 4 4· 2 sites on (Figure Nb O 15n(B)H),O though could catalyze most of the the allylation NbO4 tetrahedra of benzaldehyde was converted with tetraallyl into NbO tin4·H in2O water adducts. and 2 5· 2 selectiveThe water conversion-tolerant Lewis of glucose acid sites into HMFon Nb in2O water.5·nH2O Table could1 shows catalyze the the HMF allylation formation of benzaldehyde from glucose with tetraallyl tin in water+ and selective conversion of glucose into HMF in water. Table 1 shows using Nb2O5 nH2O, Na -treated Nb2O5 nH2O and H3PO4-treated Nb2O5 nH2O in water. The three · · + · non-treatedthe HMF formation and treated from Nb glucoseO nH usingO could Nb2 produceO5·nH2O, HMF Na - fromtreated glucose, Nb2O5· whereasnH2O and other H3PO solid4-treated acids 2 5· 2 suchNb2O as5·nH ion-exchange2O in water. resinsThe three and H-type non-treated zeolites and could treated not. Nb The2O5 treatment·nH2O could with produce Na+ was HMF eﬀective from glucose, whereas other solid acids such as ion-exchange resins and H-type zeolites could not. The in diminishing the Brønsted acid sites. The HMF selectivity of Na+-treated Nb2O5 nH2O remained treatment with Na+ was effective in diminishing the Brønsted acid sites. The HMF· selectivity of unchanged, indicating that Lewis acid sites catalyze the reaction. The treatment with H3PO4 could blockNa+-treated neutral Nb OH2O groups5·nH2O onremained the surface, unchanged resulting, indicating in high selectivity that Lewis to HMFacid sites (52%) catalyze due to suppressionthe reaction. ofThe side treatment reactions with to form H3PO unknown4 could species. block neutral OH groups on the surface, resulting in high selectivity to HMF (52%) due to suppression of side reactions to form unknown species.

Figure 15. Differential FT-IR spectra for (A) dehydrated and (B) hydrated Nb2O5·nH2O at 90 K. (A) Figure 15. Diﬀerential FT-IR spectra for (A) dehydrated and (B) hydrated Nb2O5 nH2O at 90 K. Prior to CO adsorption, the sample was heated at 423 K for 1 h under vacuum. · Gas phase CO (A) Prior to CO adsorption, the sample was heated at 423 K for 1 h under vacuum. Gas phase CO −3 −2 −2 −2 −2 −1 pressure: (a) 7.1 × 10 3, (b) 1.2 × 10 , 2(c) 2.2 × 10 , (2d) 4.0 × 10 , (e2) 6.6 × 10 and2 (f) 1.4 × 10 kPa.1 pressure: (a) 7.1 10− , (b) 1.2 10− ,(c) 2.2 10− ,(d) 4.0 10− ,(e) 6.6 10− and (f) 1.4 10− Prior to CO adsorption,× the sample× was dehydrated× at room× temperature ×for 24 h under vacuum.× kPa. Prior to CO adsorption, the sample was dehydrated at room temperature for 24 h under vacuum. −3 −3 −2 −2 −2 Gas-phase CO pressure: (a) 6.1 × 10 , (3b) 9.3 × 10 , (c3) 1.4 × 10 , (d) 22.6 × 10 , (e) 4.32 × 10 , (f) 6.8 2× Gas-phase CO pressure: (a) 6.1 10− ,(b) 9.3 10− ,(c) 1.4 10− ,(d) 2.6 10− ,(e) 4.3 10− , −2 −1 10 and (g2) 1.4 × 10 kPa. 1 Reproduced× with × permission from× [38]; copyright× (2011), American× (f) 6.8 10− and (g) 1.4 10− kPa. Reproduced with permission from [38]; copyright (2011), American Chemical× Society. × Chemical Society.

Table 1. Catalytic activity for the conversion of glucose into HMF in water a Reproduced with permission from [38]; copyright (2011), American Chemical Society.

Selectivity/% Unknown Catalyst b BA c LA d Conv.e Fru f HMF FA g LA h HCl 9.9 − 100 − 5.7 27.1 66.5 H2SO4 22.4 − 100 − 8.4 56.4 35.2

Catalysts 2019, 9, x FOR PEER REVIEW 14 of 19

Nb2O5·nH2O, which had 3 mmol of water adsorbed on 1 g of the metal oxide, still had Lewis acid sites (Figure 15(B)), though most of the NbO4 tetrahedra was converted into NbO4·H 2O adducts. The water-tolerant Lewis acid sites on Nb2O5·nH2O could catalyze the allylation of benzaldehyde with tetraallyl tin in water and selective conversion of glucose into HMF in water. Table 1 shows the HMF formation from glucose using Nb2O5·nH2O, Na+-treated Nb2O5·nH2O and H3PO4-treated Nb2O5·nH2O in water. The three non-treated and treated Nb2O5·nH2O could produce HMF from glucose, whereas other solid acids such as ion-exchange resins and H-type zeolites could not. The treatment with Na+ was effective in diminishing the Brønsted acid sites. The HMF selectivity of Na+-treated Nb2O5·nH2O remained unchanged, indicating that Lewis acid sites catalyze the reaction. The treatment with H3PO4 could block neutral OH groups on the surface, resulting in high selectivity to HMF (52%) due to suppression of side reactions to form unknown species.

Figure 15. Differential FT-IR spectra for (A) dehydrated and (B) hydrated Nb2O5·nH2O at 90 K. (A) Prior to CO adsorption, the sample was heated at 423 K for 1 h under vacuum. Gas phase CO pressure: (a) 7.1 × 10−3, (b) 1.2 × 10−2, (c) 2.2 × 10−2, (d) 4.0 × 10−2, (e) 6.6 × 10−2 and (f) 1.4 × 10−1 kPa. Prior to CO adsorption, the sample was dehydrated at room temperature for 24 h under vacuum. Gas-phase CO pressure: (a) 6.1 × 10−3, (b) 9.3 × 10−3, (c) 1.4 × 10−2, (d) 2.6 × 10−2, (e) 4.3 × 10−2, (f) 6.8 × 10−2 and (g) 1.4 × 10−1 kPa. Reproduced with permission from [38]; copyright (2011), American CatalystsChemical2019, 9, 907 Society. 15 of 20

Table 1. Catalytic activity for the conversion of glucose into HMF in water a Reproduced with a Tablepermission 1. Catalytic from [38]; activity copyright for the(2011), conversion American of Chemical glucose intoSociety. HMF in water Reproduced with permission from [38]; copyright (2011), American Chemical Society.

b c d e Selectivity/%Selectivity/% UnknownUnknown CatalystCatalyst b BABA c LALA d ConvConv..e FruFru f f HMFHMF FA g g LALAh h HClHCl 9.9 9.9 − 100100 − 5.7 27.1 66.566.5 − − H2HSOSO4 22.422.4 − 100100 − 8.4 56.4 35.235.2 2 4 − − Amberlyst-15 4.8 89 42.3 42.3 15.4 − − NaﬁonNR50 0.9 65 9.8 35.4 54.8 − − H-mordenite (Si/Al = 90) 1.1 0.26 12 35.2 64.8 − − H-ZSM-5 (Si/Al = 90) 0.15 0.05 34 3.8 96.2 − − 0.17 i 0.15 i Nb O nH O 100 12.1 3.2 84.6 2 5· 2 − − 0.14 j 0.03 j i 0.17 i Na+/Nb2O5 nH2O − 100 0.5 12.4 2.5 84.6 · − j 0.03 j − 0.04 i 0.11 i H PO /Nb O nH O 92 0.8 52.1 2.6 1.2 43.3 3 4 2 5· 2 0.04 j 0.02 j a Reagents and conditions: Distilled water, 2.0 mL; d-glucose, 0.02 g (0.11 mmol); temperature, 393 K. b 0.2 g. c 1 d 1 e f Brønsted acid amount (mmol g− ), Lewis acid amount (mmol g− ), yield conversion (%) for 3 h, fructose, g formic acid, h levulinic acid, i dehydrated sample, j water-adsorbed sample.

Different from the above study, the treatment of niobic acid with phosphoric acid and subsequent calcination was known to be effective for improving the catalytic activity of niobic acid [40]. The H3PO4-treated niobic acid calcined at 573 K was used for HMF formation from various sugars, including fructose, glucose, inulin and the Jerusalem artichoke juice [41]. At 433 K in a biphasic water-2-butanol system, HMF yield was 89% from fructose, 49% from glucose and 54% from inulin, respectively. Anatase with low crystallinity was also workable as a water-tolerant Lewis acid catalyst [42]. The TiO2 had a high density of Lewis acid sites, TiO4 tetrahedra, which could survive in water, resulting in higher catalytic activity for pyruvaldehyde conversion to lactic acid. The TiO2 and H3PO4-treated TiO2 (phosphate/TiO2) also could produce HMF from glucose in water [43]. It should be noted that the reaction mechanism on the TiO2 catalyst was different from other solid Lewis acid catalysts such as Sn-Beta zeolite [43,44]. There are two possible reaction mechanisms for HMF formation from glucose. One involves glucose–fructose isomerization and subsequent dehydration. Fructose is an intermediate and an appropriate combination of Lewis acid and Brønsted acid is required. The other mechanism proceeds via stepwise dehydration where 3-deoxyglucosone (3, 3-Deoxy-2-ketohexose) is considered as an intermediate (Figure 16)[45]. Experiments using isotopically labeled molecules and solid-state NMR and a theoretical study revealed that TiO2 and phosphate/TiO2 followed the stepwise dehydration mechanism (Figure 17). Catalysts 2019, 9, x FOR PEER REVIEW 15 of 19

Amberlyst-15 4.8 − 89 − 42.3 42.3 15.4 NafionNR50 0.9 − 65 − 9.8 35.4 54.8 H-mordenite (Si/Al = 90) 1.1 0.26 12 35.2 − − 64.8 H-ZSM-5 (Si/Al = 90) 0.15 0.05 34 − 3.8 − 96.2 0.17 i 0.15 i Nb2O5·nH2O 100 − 12.1 3.2 − 84.6 0.14 j 0.03 j − i 0.17 i Na+/ Nb2O5·nH2O 100 0.5 12.4 2.5 − 84.6 − j 0.03 j 0.04 i 0.11 i H3PO4/ Nb2O5·nH2O 92 0.8 52.1 2.6 1.2 43.3 0.04 j 0.02 j a Reagents and conditions: Distilled water, 2.0 mL; D-glucose, 0.02 g (0.11 mmol); temperature, 393 K. b 0.2 g. c Brønsted acid amount (mmol g−1), d Lewis acid amount (mmol g-1), e yield conversion (%) for 3 h, f fructose, g formic acid, h levulinic acid, i dehydrated sample, j water-adsorbed sample.

Different from the above study, the treatment of niobic acid with phosphoric acid and subsequent calcination was known to be effective for improving the catalytic activity of niobic acid [40]. The H3PO4-treated niobic acid calcined at 573 K was used for HMF formation from various sugars, including fructose, glucose, inulin and the Jerusalem artichoke juice [41]. At 433 K in a biphasic water-2-butanol system, HMF yield was 89% from fructose, 49% from glucose and 54% from inulin, respectively. Anatase with low crystallinity was also workable as a water-tolerant Lewis acid catalyst [42]. The TiO2 had a high density of Lewis acid sites, TiO4 tetrahedra, which could survive in water, resulting in higher catalytic activity for pyruvaldehyde conversion to lactic acid. The TiO2 and H3PO4-treated TiO2 (phosphate/TiO2) also could produce HMF from glucose in water [43]. It should be noted that the reaction mechanism on the TiO2 catalyst was different from other solid Lewis acid catalysts such as Sn-Beta zeolite [43,44]. There are two possible reaction mechanisms for HMF formation from glucose. One involves glucose–fructose isomerization and subsequent dehydration. Fructose is an intermediate and an appropriate combination of Lewis acid and Brønsted acid is required. The other mechanism proceeds via stepwise dehydration where 3-deoxyglucosone (3, 3-Deoxy-2-ketohexose) is considered as an intermediate (Figure 16) [45]. Experiments using Catalysts 9 isotopically2019, labeled, 907 molecules and solid-state NMR and a theoretical study revealed that TiO162 ofand 20 phosphate/TiO2 followed the stepwise dehydration mechanism (Figure 17).

FigureFigure 16. 16.The The 3-Deoxy-2-ketohexose 3-Deoxy-2-ketohexose mechanism mechanism for formation for formation of HMF. of Reproduced HMF. Reproduced with permission with Catalystsfrompermission 2019 [45, 9];, x copyright FORfrom PEER [45]; (2011), REVIEW copyright John (201 Wiley1), &John Sons, Wiley Inc. & Sons, Inc. 16 of 19

Figure 17. 17. TwoTwo synergetic synergetic strategies strategies converting glucose glucose to HMF: (i) GlucoseGlucose isomerization by cooperation between a Lewis acid and aa protonproton donordonor (example(example systemssystems areare SnSn/Beta/Beta andand WOWO3⋅HH2O) 3· 2 followed by by fructose fructose dehydration dehydration and and (ii) stepwise (ii) stepwise glucose glucose dehydration dehydration catalyzed catalyzed by a Lewis by acid-base a Lewis pairacid- (anbase example pair (an being example Ti4c beingO Ti Ti4c−O−Ti−OHOH on titania). on Reproduced titania). Reproduced with permission with permission from [44]; from copyright [44]; − − − (2018),copyright John (2018), Wiley John & Sons, Wiley Inc. & Sons, Inc.

8. Supported Metal OxidesOxides Supported catalysts are widely used for the chemicalchemical industry. The useuse of supports with high surface areasareas enablesenables high high dispersion dispersion of of active active species. species. The The high high atomic atomic effi efficiencyciency is preferable is preferable for thefor guidelinethe guideline of green of green chemistry. chemistry. A varietyvariety of of silica-supported silica-supported metal metal oxides oxides were were investigated investigated for lactic for lactic acid formation acid formation from trioses from intrioses water in [ 46water]. It [46]. was foundIt was thatfound silica-supported that silica-supported chromia-titania chromia-titania catalysts catalysts selectively selectively afforded afford lacticed acidlactic (80% acid yield)(80% yield) at 403 at K. 403 The K. co-impregnation The co-impregnation of chromium of chromium oxide oxide and titanium and titanium oxide oxide formed formed both Brønstedboth Brønsted acid and acid Lewis and acidLewis sites acid on sites the surface. on the Theresurface. was There a trend was in thea trend Lewis in/Brønsted the Lewis/Br acid ratioønsted to theacid selectivity ratio to the to selectivity lactic acid (Figureto lactic 18 acid). Scheme (Figure3 shows18). Scheme the reaction 3 show pathways the reaction for lactic pathway acid formation for lactic fromacid formationtrioses in water. from trioses Table2 shows in water reaction. Table rate 2 shows constants reaction for each rate reaction constants pathway for each over reaction using silica-supported titania, chromia and chromia–titania catalysts. The Cr/SiO catalyst with Lewis acid pathway over using silica-supported titania, chromia and chromia–titania2 catalysts. The Cr/SiO2 sitescatalyst was with effective Lewis at acid converting sites wa pyruvaldehydes effective at convert to lacticing acid,pyruvaldehyde indicating ato high lactic rate acid constant, indicating of k La. high rate constant of kL. The Ti/SiO2 catalyst with Brønsted acid sites could promote the reaction from dihydroxyacetone to pyruvaldehyde, which led to a high rate constant of kp. The Cr-Ti/SiO2 catalyst could increase these two essential rate constants, kL and kp, which gave the high reaction rate with high selectivity to lactic acid. The supported metal oxides could easily control their acid properties including acid amounts, acid types and the Brønsted/Lewis acid ratio by changing the loading amount and metal compositions.

Figure 18. Conversion of dihydroxyaceonte (DHA), selectivity of products and Lewis acid/Brønsted acid ratios as a function of chromium contents in silica-supported chromium–titanium mixed oxides. Reaction conditions: Dihydroxyacetone (0.55 mmol), catalyst (50 mg), water (3 mL), 130 °C, 1.5 h. Reproduced with permission from [46]; copyright (2019), Elsevier.

Catalysts 2019, 9, x FOR PEER REVIEW 16 of 19

Figure 17. Two synergetic strategies converting glucose to HMF: (i) Glucose isomerization by

cooperation between a Lewis acid and a proton donor (example systems are Sn/Beta and WO3⋅H2O) followed by fructose dehydration and (ii) stepwise glucose dehydration catalyzed by a Lewis acid-base pair (an example being Ti4c−O−Ti−OH on titania). Reproduced with permission from [44]; copyright (2018), John Wiley & Sons, Inc.

8. Supported Metal Oxides Supported catalysts are widely used for the chemical industry. The use of supports with high surface areas enables high dispersion of active species. The high atomic efficiency is preferable for the guideline of green chemistry. A variety of silica-supported metal oxides were investigated for lactic acid formation from trioses in water [46]. It was found that silica-supported chromia-titania catalysts selectively afforded lactic acid (80% yield) at 403 K. The co-impregnation of chromium oxide and titanium oxide formed both Brønsted acid and Lewis acid sites on the surface. There was a trend in the Lewis/Brønsted acid ratio to the selectivity to lactic acid (Figure 18). Scheme 3 shows the reaction pathway for lactic acid formation from trioses in water. Table 2 shows reaction rate constants for each reaction Catalystspathway2019 over, 9, 907 using silica-supported titania, chromia and chromia–titania catalysts. The Cr/SiO17 of 202 catalyst with Lewis acid sites was effective at converting pyruvaldehyde to lactic acid, indicating a high rate constant of kL. The Ti/SiO2 catalyst with Brønsted acid sites could promote the reaction The Ti/SiO2 catalyst with Brønsted acid sites could promote the reaction from dihydroxyacetone to from dihydroxyacetone to pyruvaldehyde, which led to a high rate constant of kp. The Cr-Ti/SiO2 pyruvaldehyde, which led to a high rate constant of kp. The Cr-Ti/SiO2 catalyst could increase these catalyst could increase these two essential rate constants, kL and kp, which gave the high reaction two essential rate constants, k and kp, which gave the high reaction rate with high selectivity to lactic rate with high selectivity to lacticL acid. The supported metal oxides could easily control their acid acid. The supported metal oxides could easily control their acid properties including acid amounts, properties including acid amounts, acid types and the Brønsted/Lewis acid ratio by changing the acid types and the Brønsted/Lewis acid ratio by changing the loading amount and metal compositions. loading amount and metal compositions.

Figure 18. Conversion of dihydroxyaceonte (DHA), selectivity of products and Lewis acid/Brønsted Figure 18. Conversion of dihydroxyaceonte (DHA), selectivity of products and Lewis acid/Brønsted acid ratios as a function of chromium contents in silica-supported chromium–titanium mixed oxides. acid ratios as a function of chromium contents in silica-supported chromium–titanium mixed oxides. Reaction conditions: Dihydroxyacetone (0.55 mmol), catalyst (50 mg), water (3 mL), 130 °C, 1.5 h. Reaction conditions: Dihydroxyacetone (0.55 mmol), catalyst (50 mg), water (3 mL), 130 ◦C, 1.5 h. Reproduced with permission from [46]; copyright (2019), Elsevier. CatalystsReproduced 2019, 9, x FOR with PEER permission REVIEW from [46]; copyright (2019), Elsevier. 17 of 19

SchemeScheme 3.3. Proposed reaction reaction pathway pathway for for dihydroxyacetone dihydroxyacetone transformation. transformation. Reproduced with with permissionpermission fromfrom [[46];46]; copyrightcopyright (2019),(2019), Elsevier.Elsevier.

TableTable 2. 2.Rate Rate constants constants for dihydroxyacetone for dihydroxyacetone (DHA) (transformationDHA) transformation using silica-supported using silica- chromiumsupported 1 oxide,chromium titanium oxide, oxide titanium and oxide chromium–titanium and chromium– oxidetitanium catalysts oxide catalysts (units in (units h− ). in Reproduced h−1). Reproduced with permissionwith permission from [from46]; copyright[46]; copyright (2019), (2019), Elsevier. Elsevier.

kpkp kL kL kG kGk G− kkGGO- kkGOPO kPO

Cr(1.0)/SiOCr(1.0)/SiO2 2 0.360.36 1.81.8 0.30.3 3.8 3.8 4.8 4.8 36 36 Ti(1.0)/SiOTi(1.0)/SiO2 2 0.60.6 0.240.24 0.18 0.18 1.1 1.1 6 1.26 1.2 Cr(0.5)-Ti(0.5)/SiO2 1.2 3 0.12 1.2 3 30 Cr(0.5)-Ti(0.5)/SiO2 1.2 3 0.12 1.2 3 30

9. Conclusions From glucose as a key chemical building block, a variety of intermediates could be synthesized by several acid-catalyzed reactions. Some metal oxides could function as water-tolerant solid acids and be applicable for these reactions. The textural properties such as crystal structure, morphology and porosity greatly affected the amounts and types of acid sites and the diffusivity of reactants. Layered- and mesoporous structures were effective for improving the product selectivity and for enhancing the reaction rate, respectively. The solid acids with strong Brønsted acid sites such as layered HNbMoO6 and mesoporous Nb-W oxides could catalyze hydrolysis and (cyclo)dehydration. The solid acids with Lewis acid sites such as amorphous niobium oxide and anatase could promote HMF formation. A combination of Brønsted and Lewis acid was effective for lactic acid synthesis. Not only tuning of Brønsted and Lewis acidity but also improving accessibility of the reactant with the active sites is a key for the selective formation of the desired product.

Author Contributions: A.T. prepared and wrote the manuscript.

Funding: This work was supported by JSPS KAKENHI Grant Number JP 25709077 and 18H01785.

Conflicts of Interest: The authors declare no conflict of interest.

References

1. Capello, C.; Fischer, U.; Hungerbühler, K. What is a green solvent? A comprehensive framework for the environmental assessment of solvents. Green Chem. 2007, 9, 927–934. 2. Ravenelle, R.M.; Coperland, J.R.; Kim, W.-G.; Crittenden, J.C.; Sievers, C. Structural Changes of γ-Al2O3-Supported Catalysts in Hot Liquid Water. ACS Catal. 2011, 1, 552–561. 3. Okuhara, T. Water-Tolerant Solid Acid Catalysts. Chem. Rev. 2002, 102, 3641–3666. 4. Namba, S.; Hosonuma, Y.; Yashima, T. Catalytic application of hydrophobic properties of high-silica zeolites. I. Hydrolysis of ethyl acetate in aqueous solution. J. Catal. 1981, 72, 16–20. 5. Ishida, H. Liquid-phase hydration process of cyclohexene with zeolites. Catal. Surv. Jpn. 1997, 1, 241–246.

Catalysts 2019, 9, 907 18 of 20

9. Conclusions From glucose as a key chemical building block, a variety of intermediates could be synthesized by several acid-catalyzed reactions. Some metal oxides could function as water-tolerant solid acids and be applicable for these reactions. The textural properties such as crystal structure, morphology and porosity greatly affected the amounts and types of acid sites and the diffusivity of reactants. Layered- and mesoporous structures were effective for improving the product selectivity and for enhancing the reaction rate, respectively. The solid acids with strong Brønsted acid sites such as layered HNbMoO6 and mesoporous Nb-W oxides could catalyze hydrolysis and (cyclo)dehydration. The solid acids with Lewis acid sites such as amorphous niobium oxide and anatase could promote HMF formation. A combination of Brønsted and Lewis acid was effective for lactic acid synthesis. Not only tuning of Brønsted and Lewis acidity but also improving accessibility of the reactant with the active sites is a key for the selective formation of the desired product.

Funding: This work was supported by JSPS KAKENHI Grant Number JP 25709077 and 18H01785. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

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