Iridium Complex Catalyzed Hydrogen Production from Glucose and Various Monosaccharides

catalysts

Article Iridium Complex Catalyzed Hydrogen Production from Glucose and Various Monosaccharides

Ken-ichi Fujita * , Takayoshi Inoue, Toshiki Tanaka, Jaeyoung Jeong, Shohichi Furukawa and Ryohei Yamaguchi

Graduate School of Human and Environmental Studies, Kyoto University, Kyoto 606-8501, Japan; [email protected] (T.I.); [email protected] (T.T.); [email protected] (J.J.); [email protected] (S.F.); [email protected] (R.Y.) * Correspondence: [email protected]; Tel.: +81-75-753-6827

Abstract: A new catalytic system has been developed for hydrogen production from various monosaccharides, mainly glucose, as a starting material under reﬂux conditions in water in the presence of a water-soluble dicationic iridium complex bearing a functional bipyridine ligand. For example, the reaction of D-glucose in water under reﬂux for 20 h in the presence of [Cp*Ir(6,60-dihydroxy-2,20-

bipyridine)(H2O)][OTf]2 (1.0 mol %) (Cp*: pentamethylcyclopentadienyl, OTf: triﬂuoromethanesul- fonate) resulted in the production of hydrogen gas in 95% yield. In the present catalytic reaction, it was experimentally suggested that dehydrogenation of the alcoholic moiety at 1-position of glucose proceeded.

Keywords: iridium catalyst; water-soluble catalyst; hydrogen production; glucose; monosaccharides

Citation: Fujita, K.-i.; Inoue, T.; Tanaka, T.; Jeong, J.; Furukawa, S.; 1. Introduction Yamaguchi, R. Iridium Complex Hydrogen is important as a raw material for the industrial production of ammonia Catalyzed Hydrogen Production and methanol [1–5]. In addition, it is an essential industrial reagent in the reﬁning and from Glucose and Various desulfurization of petroleum [6–8]. Hydrogen is also used in large quantities in industrial Monosaccharides. Catalysts 2021, 11, processes such as turning unsaturated fats into saturated oils and fats, metal alloying and 891. https://doi.org/10.3390/ iron ﬂashmaking, and electronics manufacturing (creating semiconductors, LEDs, displays, catal11080891 and photovoltaic segments) [9]. Furthermore, in addition to these industrial applications, hydrogen has been promoted as an energy carrier because it can easily be converted into Academic Editor: Werner Oberhauser other energy forms, namely, electrical energy or mechanical energy, with only harmless water as a by-product of the energy conversion [10,11]. Hydrogen has attracted attention as Received: 26 June 2021 a next-generation energy carrier to replace fossil fuel resources because it has the advantage Accepted: 20 July 2021 Published: 23 July 2021 of exceedingly high energy density per weight. Under this background, there is a need to develop new techniques to produce hydro-

Publisher’s Note: MDPI stays neutral gen using sustainable and available resources as feedstock [12,13]. In this context, biomass with regard to jurisdictional claims in is expected to be a starting material for producing hydrogen, with the biomass mainly published maps and institutional afﬁl- comprised of saccharides. iations. Research into developing a reaction to produce hydrogen using saccharides as starting material has been carried out for a relatively long time [14]. There are a number of reported reactions in which saccharides are dehydrogenated using heterogeneous catalysts [15–17] or enzyme catalysts [18,19] to obtain hydrogen. However, most of the reported examples using heterogeneous catalysts are reactions done under high temperature conditions above Copyright: © 2021 by the authors. ◦ Licensee MDPI, Basel, Switzerland. 300 C using expensive noble metals. Furthermore, there are many catalytic systems that This article is an open access article produce hydrogen gas mixed with carbon monoxide, carbon dioxide, or methane, instead distributed under the terms and of highly pure hydrogen, which is often difﬁcult to produce. On the other hand, in the conditions of the Creative Commons case of a reaction using an enzyme catalyst, it is often possible to obtain hydrogen from Attribution (CC BY) license (https:// a saccharide under mild conditions. However, there are associated disadvantages such creativecommons.org/licenses/by/ as long reaction times, reaction conditions requiring precise control, and time-consuming 4.0/). culturing of enzymes.

Catalysts 2021, 11, 891. https://doi.org/10.3390/catal11080891 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, x FOR PEER REVIEW 2 of 9

Catalysts 2021, 11, 891 obtain hydrogen from a saccharide under mild conditions. However, there are associated2 of 9 disadvantages such as long reaction times, reaction conditions requiring precise control, and time-consuming culturing of enzymes. In view of these circumstances, there have been great expectations to realize hydro- In view of these circumstances, there have been great expectations to realize hydrogen gen production from saccharides under mild conditions within a short reaction time using production from saccharides under mild conditions within a short reaction time using artificialartificial homogeneoushomogeneous transition metal metal catalysts. catalysts. It Itwould would be beparticularly particularly significant significant if a ifwater-soluble a water-soluble monosaccharide monosaccharide such suchas glucose as glucose was used was as used the asstarting the starting material material and hy- anddrogen hydrogen could couldbe efficiently be efficiently produced produced by ca bytalytic catalytic dehydrogenation dehydrogenation in an in aqueous an aqueous me- medium.dium. InIn 2018,2018, GarciaGarcia and and Mata Mata et et al. al. reportedreported thatthat gluconicgluconic acid acid was was formed formed by by the the reaction reaction ofof glucoseglucose inin aqueousaqueous solventssolvents usingusing iridiumiridium complexcomplex catalysis,catalysis, accompaniedaccompanied byby hydro-hydro- gengen evolutionevolution (Scheme 11a)a) [[20–22].20–22]. This This is ispart particularlyicularly noteworthy noteworthy because because glucose glucose is sus- is sustainablytainably available available as asa natural a natural resource. resource. Howe However,ver, to to efficiently efficiently proceed proceed with the dehy-dehy- drogenationdrogenation reaction, reaction, the the usage usage of of strong strong acids acids such such as as sulfuric sulfuric acid acid andand hydrochlorichydrochloric acid acid waswas essential,essential, andand the the amount amount of of iridium iridium catalyst catalyst used used was was relatively relatively large large at at 2.0 2.0 mol mol %. %. OurOur researchresearch groupgroup hashas developed developed water-soluble water-soluble iridium iridium catalysts catalysts that that demonstrate demonstrate highhigh catalyticcatalytic activity for for the the dehydrogenatio dehydrogenationn reaction reaction of of alcohols alcohols and and has has reported reported the thesynthesis synthesis of aldehydes of aldehydes and and ketones ketones by the by thesimple simple dehydrogenation dehydrogenation of primary of primary and and sec- secondaryondary alcohols alcohols [23,24]. [23,24 In]. Inaddition, addition, catalytic catalytic lactone lactone synthesis synthesis involving involving the the evolution evolution of ofhydrogen hydrogen using using diol diol as as a starting a starting material material has has also also been been reported reported [25]. [25 ]. InIn thisthis study,study, we we developed developed a a new new catalyticcatalytic systemsystem toto produceproduce hydrogenhydrogen underunder refluxreflux conditionsconditions inin waterwater inin thethe presencepresence ofof aa water-soluble water-soluble dicationic dicationic iridiumiridium catalystcatalyst usingusing variousvarious monosaccharides,monosaccharides, mainlymainly glucose,glucose, asas aa startingstarting material.material. AsAs aa result,result, wewe foundfound thatthat hydrogenhydrogen could could be be efficientlyefficiently obtained obtained from from these these monosaccharides monosaccharides without without the the need need forfor thethe additionaddition ofof acidsacids oror basesbases andand withwith lessless catalystcatalyst (from(from 0.20.2 toto 1.01.0 molmol %)%) than than the the previouslypreviously reported reported examples examples (Scheme (Scheme1 b).1b).

SchemeScheme 1.1.Iridium-catalyzed Iridium-catalyzed dehydrogenation dehydrogenation of of glucose glucose in in water: water: (a )(a under) under acidic acidic conditions conditions report byreport Garcia by andGarcia Mata and et Mata al. (b )et under al. (b) neutral under neutral conditions conditions reported reported in this work. in this work.

2. Results and Discussion The structures of the iridium catalysts used in this study are shown in Figure1. When D-glucose (5.0 mmol) was heated under reﬂux in water (15 mL) for 20 h in the presence of catalyst 1 (0.2 mol %), which was used under highly acidic conditions in the previous research by Garcia an Mata, only very low yield (7%) of hydrogen was generated (Table1, entry 1) [ 26]. In contrast, when water-soluble dicationic catalyst 2, which was previously developed for the dehydrogenative oxidation of simple alcohols in water, was employed for the dehydrogenation of glucose; hydrogen was obtained in 76% yield (entry 2). In this dehydrogenation reaction, it is important that the complex

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2. Results and Discussion The structures of the iridium catalysts used in this study are shown in Figure 1.

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catalyst is dicationic and soluble in water, and that it has 6,60-dihydroxy-2,2 0-bipyridine as Figure 1. A series of iridiuma ligand. catalysts In other used words, for the nodehydrogenation hydrogen was of produced glucose in whenthis study. [Cp*IrCl 2]2 (Cp*: pentamethylcyclopentadienyl), which is not soluble in water (entry 3), or [Cp*Ir(H2O)3][OTf]2 having Whenno D 6,6-glucose0-dihydroxy-2,2 (5.0 mmol)0-bipyridine was heated ligand under was re usedflux in as water the catalyst (15 mL) (entry for 20 4). h In in addition, the the presencepresence of catalyst of a1 hydroxy(0.2 mol group%), which at the was 6,6 0used-position under of thehighly ligand acidic is very conditions important in the for catalytic previousperformance. research by Garcia For example, an Mata, hydrogenonly very low was yield not produced (7%) of hydrogen in the reaction was generated using catalyst 3 (Table 1,with entry no 1) hydroxy [26]. In groupscontrast, (entry when 5) water-soluble or catalyst 4 with dicationic a hydroxy catalyst group 2, which at the 4,4was0-positions previously(entry developed 6). Subsequently, for the dehydrogenative the reaction usingoxidation catalyst of simple5 with alcohols tetraﬂuoroborate in water, was anion was Catalysts 2021, 11, x FOR PEER REVIEW 3 of 9 employedcarried for the out dehydrogenation to investigate the of inﬂuenceglucose; hydrogen of the counter was obtained anion (entry in 76% 7). Inyield this (entry case, although 2). In thisthe dehydrogenation yield of hydrogen reaction, was slightly it is important reduced, that there the was complex no large catalyst difference is dicationic from that when and solubleusing in catalystwater, and2. Thethat yield it has was 6,6′ successfully-dihydroxy-2,2 improved′-bipyridine to 95% as a by ligand. increasing In other the amount words, no2.of hydrogen Results the catalyst and was Discussion2 producedto 1.0 mol when %, (entry [Cp*IrCl 8).2 The]2 (Cp*: time pentamethylcyclopentadienyl), course of hydrogen generation in the which is reactionnot solubleThe ofstructures entry in water 8 was of (entry the examined, iridium 3), or [Cp*Ir(H andcatalysts the results2 O)used3][OTf] in are this2 shown having study in noare Figure 6,6 shown′-dihydroxy-2. in Figure 1. 2,2′-bipyridine ligand was used as the catalyst (entry 4). In addition, the presence of a hydroxy group at the 6,6′-position of the ligand is very important for catalytic perfor- mance. For example, hydrogen was not produced in the reaction using catalyst 3 with no hydroxy groups (entry 5) or catalyst 4 with a hydroxy group at the 4,4′-positions (entry 6). Subsequently, the reaction using catalyst 5 with tetrafluoroborate anion was carried out to investigate the influence of the counter anion (entry 7). In this case, although the yield of hydrogen was slightly reduced, there was no large difference from that when using catalyst 2. The yield was successfully improved to 95% by increasing the amount of the catalyst 2 to 1.0 mol %, (entry 8). The time course of hydrogen generation in the reaction of entry 8 was examined, and the results are shown in Figure 2. Figure 1. A series of iridium catalysts used for the dehydrogenation of glucose in this study.

Table 1. Hydrogen production from D-glucose catalyzed by various iridium complexes. TableWhen 1. Hydrogen D-glucose production (5.0 mmol) from D-glucosewas heated catalyzed under byreflux various in water iridium (15 complexes. mL) for 20 h in the presence of catalyst 1 (0.2 mol %), which was used under highly acidic conditions in the previous research by Garcia an Mata, only very low yield (7%) of hydrogen was generated (Table 1, entry 1) [26]. In contrast, when water-soluble dicationic catalyst 2, which was previously developed for the dehydrogenative oxidation of simple alcohols in water, was employed for the dehydrogenation of glucose; hydrogen was obtained in 76% yield (entry 2). In this dehydrogenation reaction, it is important that the complex catalyst is dicationica Entry EntryCatalyst (mol %) Catalyst (mol %)Yield of Hydrogen Yield of Hydrogen (%) a (%) and soluble in water, and that it has 6,6′-dihydroxy-2,2′-bipyridine as a ligand. In other 1 1 (0.20 mol %) 7 words, no hydrogen was produced when [Cp*IrCl2]2 (Cp*: pentamethylcyclopentadienyl), 1 2 1 (0.20 mol %) 2 (0.20 mol %)7 76 which is not soluble in water (entry 3), or [Cp*Ir(H2O)3][OTf]2 having no 6,6′-dihydroxy- 3 [Cp*IrCl2]2 (0.20 mol % Ir) 0 22,2 ′-bipyridine4 ligand 2was (0.20 used [Cp*Ir(H mol as%) 2the O)3 ][OTf]catalyst2 (0.20 (entry mol 4). %)76 In addition, the trace presence of a hydroxy group5 at the 6,6′-position of3 (0.20 the molligand %) is very important for tracecatalytic perfor- 3mance. For example,6 [Cp*IrCl hydrogen2]2 (0.20 was mol not4 %(0.20 Ir)produced mol %) in the reaction0 using catalyst trace 3 with no hydroxy groups7 (entry 5) or catalyst 45 with(0.20 mola hydroxy %) group at the 4,4′-positions 71 (entry 6). 4 8[Cp*Ir(H2O)3][OTf]2 (0.202 mol(1.0 mol%) %)trace 95 Subsequently, the reaction using catalyst 5 with tetrafluoroborate anion was carried out a Yield of hydrogen gas collected in a gas burette. The molar amount of hydrogen gas was calculated using the 5toideal investigate gas law. the influence3 (0.20 of mol the %)counter anion (entry 7).trace In this case, although the yield of hydrogen was slightly reduced, there was no large difference from that when using 6catalyst In the2. The dehydrogenation yield was4 (0.20 successfully mol reaction %) ofimproved glucose catalyzedto 95% traceby by increasing iridium complexthe amount catalyst of the2, catalysta simultaneous 2 to 1.0 mol parallel %, (entry experiment 8). The of time alkene course hydrogenation of hydrogen was generation performed in the to confirmreaction ofthat entry the obtained8 was examined, gas was and highly-pure the results hydrogen. are shown Simultaneous in Figure 2. parallel experiments were carried out by connecting flask A, in which the dehydrogenation of glucose (5.0 mmol) in Tablethe presence 1. Hydrogen of catalyst production2 was from proceeding, D-glucose catalyzed and flask byB ,various in which iridium 1-decene complexes. (5.0 mmol) was ◦ heated to 50 C in benzene solvent in the presence of RhCl (PPh3)3 (2.0 mol %), using a rubber tube (Scheme2). As shown in entry 2 of Table1, 76% of hydrogen is expected to be produced in flask A. By this simultaneous parallel experiment, decane was obtained in 74% yield in the reaction in flask B. This result indicates that the gas produced using glucose as a starting material in the presence of catalyst 2 is highly-pure hydrogen and does not contain a component that inhibits the catalytic hydrogenation of an alkene. In addition, gas chromatographicEntry analysis of theCatalyst gas obtained (mol %) by dehydrogenationYield ofof glucoseHydrogen was (%) carried a

1 1 (0.20 mol %) 7

2 2 (0.20 mol %) 76

3 [Cp*IrCl2]2 (0.20 mol % Ir) 0

4 [Cp*Ir(H2O)3][OTf]2 (0.20 mol %) trace

5 3 (0.20 mol %) trace

6 4 (0.20 mol %) trace

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7 5 (0.20 mol %) 71

8 2 (1.0 mol %) 95 a Yield of hydrogen gas collected in a gas burette. The molar amount of hydrogen gas was calculated using the ideal gas law.

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Catalysts 2021, 11, 891 7 5 (0.20 mol %) 71 4 of 9

8 2 (1.0 mol %) 95

a outYield to of conﬁrm hydrogen that gas it collected was highly-pure in a gas bure hydrogentte. The molar (the resultsamount areof hydrogen given in gas Figure was S2calcu- in the Figure 2. Time-resolved profile of the yield of hydrogen for the dehydrogenation of D-glucose (Table lated using the ideal gas law. supporting1, entry 8). information).

In the dehydrogenation reaction of glucose catalyzed by iridium complex catalyst 2, a simultaneous parallel experiment of alkene hydrogenation was performed to confirm that the obtained gas was highly-pure hydrogen. Simultaneous parallel experiments were carried out by connecting flask A, in which the dehydrogenation of glucose (5.0 mmol) in the presence of catalyst 2 was proceeding, and flask B, in which 1-decene (5.0 mmol) was heated to 50 °C in benzene solvent in the presence of RhCl (PPh3)3 (2.0 mol %), using a rubber tube (Scheme 2). As shown in entry 2 of Table 1, 76% of hydrogen is expected to be produced in flask A. By this simultaneous parallel experiment, decane was obtained in 74% yield in the reaction in flask B. This result indicates that the gas produced using glucose as a starting material in the presence of catalyst 2 is highly-pure hydrogen and does not contain a component that inhibits the catalytic hydrogenation of an alkene. In addition, gas chromatographic analysis of the gas obtained by dehydrogenation of glucose FigureFigurewas carried 2. 2. Time-resolvedTime-resolved out to confirm profile proﬁle ofthat the of it theyield was yield of highly-pure hydrogen of hydrogen for hydrogen the for dehydrogenation the dehydrogenation (the results of are D-glucose given of D-glucose (Tablein Fig- 1,(Tableure entry S21 ,8). in entry the 8).supporting information).

In theOH dehydrogenation reaction of glucose catalyzed by iridium complex catalyst 2, a simultaneousO parallel experimentIr cat. of 2 alke(0.20ne mol hydrogenation %) was performed to confirm HO C residuals that the obtained gasOH was highly-pure hydrogen. Simultaneous6 parallel experiments were HO water, 20 h, reflux carried out by OHconnecting flask A, inin which flask Athe dehydrogenation of glucose (5.0 mmol) in the presence5.0 mmol of catalyst 2 was proceeding, and flask B, in which 1-decene (5.0 mmol) was H (Yield: 76%, Table 1, entry 2) heated to 50 °C in benzene solvent2 in the presence of RhCl (PPh3)3 (2.0 mol %), using a

rubber tube (Scheme 2). As showncat. in RhCl(PPh entry 2 of3) 3Table 1, 76% of hydrogen is expected to be produced in flask A. By this simultaneous(2.0 mol parallel%) experiment, decane was obtained in 74% yield in the reaction in flaskbenzene, B. This 50result °C, 20indicates h that the gas produced using glucose as a5.0 starting mmol material in the presencein flask of B catalyst 2 is highly-pureYield: 74% hydrogen and does notSchemeScheme contain 2.2.A Aa simultaneous simultaneouscomponent parallelthat parallel inhibits experiment: experiment: the cata Hydrogenation Hydroglytic hydrogenationenation of 1-decene of 1-decene of with an withhydrogen alkene. hydrogen In produced addi- pro- tion,byduced the gas dehydrogenationby chromatographic the dehydrogenation of glucose. analysis of glucose. of the gas obtained by dehydrogenation of glucose was carried out to confirm that it was highly-pure hydrogen (the results are given in Fig- ure S2ThereThere in the are aresupporting ﬁvefive alcoholicalcoholic information). hydroxyhydroxy groupsgroups inin thethe D-glucose D-glucose molecule.molecule. ToTo ascertainascertain thethe sitesite atat whichwhich thethe dehydrogenationdehydrogenation reactionreaction byby thethe iridiumiridium complexcomplex catalystcatalyst 22 proceeds,proceeds, experimentsexperimentsOH usingusing glucoseglucose analoguesanalogues withwith protectedprotected hydroxyhydroxy groupsgroups werewere conductedconducted (Table2). First,O the reaction of normalIr cat. 2 D-glucose (0.20 molis %) re-listed as entry 1 (the yield of hydrogen HO C residuals is 95%). Next, for theOH glucose analogue in which hydroxy6 groups other than the 6-position HO water, 20 h, reflux were methoxyOH protected, a very lowin yieldflask A (8%) of hydrogen was obtained (entry 2). For the glucose5.0 analogue mmol in which only the hydroxy group at 1-position was methoxy protected, the hydrogen yield was greatlyH2 (Yield: reduced 76%, Table to 14% 1, entry (entry 2) 3). No hydrogen was produced

for the glucose analogue in whichcat. allRhCl(PPh hydroxy3)3 groups were methoxy protected (entry 4). Finally, for the glucose analogue in(2.0 which mol %) all hydroxy groups but that at the 1-position were protected, the hydrogenbenzene, yield was 50 °C,92%, 20 and h it was found that the hydrogen yield not signiﬁcantly5.0 mmol different from thatin flask obtained B when using unprotectedYield: 74% glucose (entry 5). These results indicate that dehydrogenation from the hydroxy group at the 1-position Scheme 2. A simultaneous parallel experiment: Hydrogenation of 1-decene with hydrogen pro- ducedproceeds by the during dehydrogenation the hydrogen of glucose. production reaction from glucose by catalyst 2. Incidentally, the organic product obtained in 97% yield in the experiment of entry 5 was found to be a

gluconolactoneThere are five derivative alcoholic with hydroxy a lactone groups structure in the (FigureD-glucose3). Basedmolecule. on these To ascertain results, itthe is sitelikely at which that gluconolactone the dehydrogenation is formed reac aftertion the by dehydrogenation the iridium complex of glucose. catalyst 2 proceeds, experiments using glucose analogues with protected hydroxy groups were conducted

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Catalysts 2021, 11, x FOR PEER REVIEW 5 of 9 (Table 2). First, the reaction of normal D-glucose is re-listed as entry 1 (the yield of hydro- Catalysts 2021, 11, x FOR PEER REVIEW 5 of 9 gen is 95%). Next, for the glucose analogue in which hydroxy groups other than the 6- (Table 2). First, the reaction of normal D-glucose is re-listed as entry 1 (the yield of hydro- Catalysts 2021, 11, x FOR PEER REVIEWposition were methoxy protected, a very low yield (8%) of hydrogen was obtained (entry5 of 9

gen2).(Table For is 95%).2).the First, glucose Next, the analogueforreaction the glucose of in normal which analogue onlyD-glucose the in hydroxy which is re-listed hydroxy group as entryat groups 1-position 1 (the other yield was than ofmethoxy hydro- the 6- Catalysts 2021, 11, x FOR PEER REVIEW position were methoxy protected, a very low yield (8%) of hydrogen was5 of 9 obtained (entry (Tableprotected,gen is 2).95%). First, the Next, hydrogenthe reactionfor the yield glucose of normal was analoguegreatly D-glucose reduced in whichis re-listed to hydroxy14% as (entry entry groups 3).1 (the No other yieldhydrogen than of hydro- the was 6- 2). For the glucose analogue in which only the hydroxy group at 1-position was methoxy genproducedposition is 95%). were for Next, the methoxy glucose for the protected, analogue glucose a analoguein very which low allin yield whichhydroxy (8%) hydroxy of groups hydrogen groups were was methoxy other obtained than protected the(entry 6- (Tableprotected, 2). First, the hydrogen the reaction yield of normal was greatly D-glucose reduced is re-listed to 14% as (entry entry 3).1 (the No yieldhydrogen of hydro- was position(entry2). For 4).the were Finally, glucose methoxy for analogue the protected, glucose in which analogue a very only low inthe yieldwhich hydroxy (8%) all hydroxyofgroup hydrogen at groups1-position was but obtained was that methoxyat (entry the 1- genproduced is 95%). for Next, the glucose for the analogue glucose analoguein which allin hydroxywhich hydroxy groups groupswere methoxy other than protected the 6- (Table 2). First, 2).positiontheprotected, For reaction the were glucosethe of protected, hydrogennormal analogue D-glucose theyield hydrogenin whichwas is re-listed greatly only yield the asreducedwas entryhydroxy 92%, 1to and(the 14%group ityield was(entry at of found1-position hydro-3). No that hydrogen wasthe hydrogen methoxy was position(entry 4). were Finally, methoxy for the protected, glucose analogue a very low in yieldwhich (8%) all hydroxyof hydrogen groups was but obtained that at (entrythe 1- gen is 95%). Next,protected,yieldproduced for not the significantly for theglucose thehydrogen glucose analogue different yield analogue in wasfrom which ingreatly that which hydroxy ob tainedreduced all hydroxygroups when to 14% using othergroups (entry unprotectedthan were 3).the methoxyNo 6- hydrogen glucose protected (entry was 2).position For the were glucose protected, analogue the hydrogenin which only yield the was hydroxy 92%, and group it was at found1-position that thewas hydrogen methoxy position were methoxyproduced5).(entry These 4). protected, resultsFinally,for the indicateglucose for a very the glucosethatanaloguelow dehydrogenationyield analogue in(8%) which of inhydrogen all which hydroxyfrom all thewas hydroxy groupshydroxy obtained weregroups group (entry methoxy butat the that 1-positionprotected at the 1- protected,yield not significantly the hydrogen different yield wasfrom greatly that ob tainedreduced when to 14% using (entry unprotected 3). No hydrogen glucose (entry was 2). For the glucose(entryproceedsposition analogue 4). were Finally,during in protected, which forthe the hydrogenonly theglucose the hydrogen hydroxy analogueproduction yield group in was whichreaction at 92%, 1-position all andfrom hydroxy it wasglucose groupsfoundmethoxy by that butcatalyst thethat hydrogen at2 .the Inci- 1- produced5). These results for the indicate glucose thatanalogue dehydrogenation in which all fromhydroxy the groupshydroxy were group methoxy at the 1-positionprotected protected, the hydrogenpositiondentally,yield not were thesignificantlyyield organic protected, was greatlyproduct different the hydrogenreduced obtained from thatto yieldin 14% 97%ob tainedwas (entryyield 92%, when in 3). theand No using experiment it hydrogen was unprotected found ofwas thatentry glucosethe 5 washydrogen found(entry (entryproceeds 4). Finally,during forthe thehydrogen glucose productionanalogue in reactionwhich all from hydroxy glucose groups by butcatalyst that at2 .the Inci- 1- produced for theyieldto5). glucose beThese nota gluconolactone significantlyresults analogue indicate in different which derivative that all dehydrogenation from hydroxy with that a ob groupslatainedctone from were structurewhen the methoxy using hydroxy (Figure unprotected protected group 3). Based at glucose the on 1-position these (entry re- positiondentally, were the organic protected, product the hydrogen obtained inyield 97% was yield 92%, in the and experiment it was found of thatentry the 5 was hydrogen found (entry 4). Finally,5).sults,proceeds forThese itthe is results glucoselikelyduring thatindicate analoguethe gluconolactone hydrogen that in dehydrogenationwhich production isall formed hydroxy reaction after from groups the the from dehydrogenation hydroxybut glthatucose atgroup the by 1- at catalystof the glucose. 1-position 2. Inci- yieldto be nota gluconolactone significantly different derivative from with that a ob latainedctone structure when using (Figure unprotected 3). Based glucose on these (entry re- position were protected,proceedsdentally, theduring organichydrogen the product hydrogen yield was obtained production92%, andin 97% it was reactionyield found in thefrom that experiment theglucose hydrogen ofby entry catalyst 5 was 2 .found Inci- 5).sults, These it is resultslikely thatindicate gluconolactone that dehydrogenation is formed after from the the dehydrogenation hydroxy group at of the glucose. 1-position yield not significantlyTableto be a2. different gluconolactoneHydrogen from production that derivative ob tainedfrom va withwhenrious a glucoseusing lactone unprotected analogues structure having (Figureglucose protected 3).(entry Based hydroxy on these groups. re- dentally,proceeds theduring organic the product hydrogen obtained production in 97% yieldreaction in the from experiment glucose ofby entry catalyst 5 was 2 found. Inci- 5). These resultssults, indicate it is thatlikely dehydrogenation that gluconolactone from is the formed hydroxy after group the dehydrogenation at the 1-position of glucose. todentally,Table be a2. gluconolactone Hydrogen the organic production product derivative from obtained va withrious in a glucose 97%lactone yield analogues structure in the experiment having (Figure protected 3). of Based entry hydroxy 5on was these groups. found re- proceeds during the hydrogen production reaction from glucose by catalyst 2. Inci- sults,to be ita isgluconolactone likely that gluconolactone derivative with is formed a lactone after structure the dehydrogenation (Figure 3). Based of glucose. on these re- dentally, the organicTable product 2. Hydrogen obtained production in 97% from yield va riousin the glucose experiment analogues of entry having 5 was protected found hydroxy groups. sults, it is likely that gluconolactone is formed after the dehydrogenation of glucose. Catalysts 2021, 11, 891 to be a gluconolactoneTable 2. Hydrogenderivative production with a la ctonefrom va structurerious glucose (Figure analogues 3). Based having on protected these re- hydroxy groups.5 of 9 sults, it is likelyTable that gluconolactone 2.Entry Hydrogen production is formed from after vaSubstraterious the dehydrogenationglucose analogues havingof glucose.Yield protected of Hydrogen hydroxy groups.(%) a

Table 2. Hydrogen productionEntry from various glucose analoguesSubstrate having protected hydroxyYield groups. of Hydrogen (%) a Table 2. Hydrogen production from various glucose analogues having protected hydroxy groups. Entry Substrate Yield of Hydrogen (%) a

Entry1 Substrate Yield of Hydrogen95 (%) a Entry Substrate Yield of Hydrogen (%) a 1 95 Entry Substrate Yield of Hydrogen (%) a Entry 1 Substrate Yield of Hydrogen (%) a 95

1 95 OH 11 O 95 95 2 MeO OH 8 1 MeO OH O 95 2 MeO MeO 8 OMe MeO OH O 2 MeO MeO 8 O OMe MeOMeO OH 22 OH MeO 8 8 MeO O OMe 2 OH MeO MeO 8 O OMe 3 OHOMeO OH 14 MeO MeO 2 HO OH O OMe 8 MeO HO HO 3 OMe 14 MeO HO OH O 33 HOOMe HO 14 14 HO OH O OMe 3 HO HO 14 HO O OMe 3 OH HO HO 14 4 O HO OMe 0 3 HO HO 14 OMe 44 HO 0 0 HO 4 OMe 0

4 0

4 0 Catalysts 2021, 11, x FOR PEER REVIEW 6 of 9 55 92 92 4 0

5 92 a Yield ofof hydrogen hydrogen5 gas gas collected collected in a in gas a burette.gas bure Thette. molar The molar amount amount of hydrogen of hydrogen gas was calculated gas 92was calcu- using the latedideal gasusing law. the ideal gas law. 5 92

5 92

Figure 3. Isolation of a gluconolactone derivative with a lactone structure formed by the reaction Figure 3. Isolation of a gluconolactone derivative with a lactone structure formed by the reaction shown in entry 5 in Table 2. shown in entry 5 in Table2. We thenthen investigatedinvestigated the the dehydrogenation dehydrogenation of of various various monosaccharides monosaccharides using using iridium irid- iumcatalyst catalyst2 (Table 2 (Table3). Hydrogen 3). Hydrogen was produced was produced in good in yieldsgood yields in the in case the of case the feedstocks,of the feedstocks,D-galactose, D-galactose, D-mannose, D-mannose, and L-arabinose, and L-arabinose, that can that form can lactone form structures lactone structures by dehydrogena- by de- hydrogenationtion (entries 2–4). (entries In the 2–4). case In of D-fructose,the case of D a-fructose, starting material a starting in material which dehydrogenation in which dehy- drogenationfrom a hydroxy from group a hydroxy at the 1-position group at isthe difﬁcult, 1-position the hydrogenis difficult, yield the hydrogen was greatly yield reduced was (entrygreatly 5). reduced (entry 5).

Table 3. Hydrogen production from various monosaccharides using the iridium catalyst 2.

Entry Substrate Yield of Hydrogen (%)a

1 95

2 87

3 83

4 83

5 11

a Yield of hydrogen gas collected in a gas burette. The molar amount of hydrogen gas was calculated using the ideal gas law.

Catalysts 2021, 11, x FOR PEER REVIEW 6 of 9 CatalystsCatalysts 20212021,, 1111,, xx FORFOR PEERPEER REVIEWREVIEW 66 ofof 99 Catalysts 2021, 11, x FOR PEER REVIEW 6 of 9 Catalysts 2021, 11, x FOR PEER REVIEW 6 of 9 Catalysts 2021, 11, x FOR PEER REVIEW 6 of 9 a Yield of hydrogen gas collected in a gas burette. The molar amount of hydrogen gas was calcu- a a YieldYield ofof hydrogenhydrogen gasgas collectedcollected inin aa gasgas bureburette.tte. TheThe molarmolar amountamount ofof hydrogenhydrogen gasgas waswas calcu-calcu- lateda Yield using of hydrogenthe ideal gas gas law. collected in a gas burette. The molar amount of hydrogen gas was calcu- alated latedYield usingusing of hydrogen thethe idealideal gas gasgas collected law.law. in a gas burette. The molar amount of hydrogen gas was calculated using the ideal gas law. a Yield of hydrogen gas collectedlated using in a gas the bure idealtte. gas The law. molar amount of hydrogen gas was calculated using the ideal gas law.

Figure 3. Isolation of a gluconolactone derivative with a lactone structure formed by the reaction FigureFigure 3.3. IsolationIsolation ofof aa gluconolactonegluconolactone derivativederivative withwith aa lactonelactone structurestructure formedformed byby thethe reactionreaction shownFigure in 3. entry Isolation 5 in Table of a gluconolactone2. derivative with a lactone structure formed by the reaction Figureshownshown 3. inin Isolation entryentry 55 inin of TableTable a gluconolactone 2.2. derivative with a lactone structure formed by the reaction shown in entry 5 in Table 2. Figure 3. Isolation of a gluconolactoneshownWe in then entry derivative investigated 5 in Table with 2. athe lactone dehydrogenation structure formed of byvarious the reaction monosaccharides using irid- shown in entry 5 in Table 2. WeWe thenthen investigatedinvestigated thethe dehydrogenationdehydrogenation ofof variousvarious monosaccharidesmonosaccharides usingusing irid-iridium catalystWe then 2 (Table investigated 3). Hydrogen the dehydrogenation was produced ofin goodvarious yields monosaccharides in the case of usingthe feed- irid- iumium Wecatalystcatalyst then 22investigated (Table(Table 3).3). HydrogenHydrogen the dehydrogenation waswas producedproduced of ininvarious goodgood yieldsyieldsmonosaccharides inin thethe casecase ofofusing thethe feed-irid-feedstocks,ium catalyst D-galactose, 2 (Table D-mannose, 3). Hydrogen and L was-arabinose, produced that in can good form yields lactone in the structures case of the by feed-de- We then investigatedstocks, the dehydrogenation D-galactose, D-mannose, of various and monosaccharides L-arabinose, that using can form irid- lactone structures by de- hydrogenationiumstocks, catalyst D-galactose, 2 (entries(Table D -mannose,3). 2–4). Hydrogen In the and case was L-arabinose, of produced D-fructose, that in agood can starting formyields materiallactone in the structurescasein which of the dehy- by feed- de- ium catalyst 2 (Table 3). Hydrogenstocks, D-galactose, was produced D-mannose, in good and yields L-arabinose, in the case that of can the formfeed- lactone structures by de- stocks,hydrogenationhydrogenation D-galactose, (entries(entries D-mannose, 2–4).2–4). InIn andthethe casecaseL-arabinose, ofof DD-fructose,-fructose, that can aa startingstarting form lactone materialmaterial structures inin whichwhich by dehy-dehy- de- drogenationhydrogenation from (entries a hydroxy 2–4). group In the at case the of1-posi D-fructose,tion is difficult,a starting the material hydrogen in which yield dehy-was stocks, D-galactose, D-mannose,hydrogenationdrogenationdrogenation and L -arabinose,fromfrom (entries aa hydroxyhydroxy 2–4). that In cangroupgroup the form case atat thelactonethe of 1-posiD1-posi-fructose, structurestiontion isisa difficult,startingdifficult, by de- material thethe hydrogenhydrogen in which yieldyield dehy- waswas greatlydrogenation reduced from (entry a hydroxy 5). group at the 1-position is difficult, the hydrogen yield was hydrogenation (entries 2–4).drogenationgreatlygreatly In the reducedreduced case from of (entry(entry aD -fructose,hydroxy 5).5). groupa starting at the material 1-posi tionin which is difficult, dehy- the hydrogen yield was drogenation from a hydroxygreatly group reduced at the (entry 1-posi 5).tion is difficult, the hydrogen yield was Catalysts 2021, 11, 891 Tablegreatly 3. Hydrogen reduced (entryproduction 5). from various monosaccharides using the iridium catalyst6 of 9 2. greatly reduced (entry 5).TableTable 3.3. HydrogenHydrogen productionproduction fromfrom variousvarious monomonosaccharidessaccharides usingusing thethe iridiumiridium catalystcatalyst 22.. Table 3. Hydrogen production from various monosaccharides using the iridium catalyst 2. Table 3. Hydrogen production from various monosaccharides using the iridium catalyst 2. Table 3. Hydrogen production from various monosaccharides using the iridium catalyst 2. Table 3. Hydrogen production from various monosaccharides using the iridium catalyst 2.

Entry Substrate Yield of Hydrogen (%)a a Entry Substrate Yield of Hydrogen (%)a Entry Substrate Yield of Hydrogen (%) a Entry Substrate Yield of Hydrogen (%)a Entry Substrate Yield of aHydrogen (%) Entry EntrySubstrate SubstrateYield of Hydrogen Yield of (%) Hydrogena (%) 1 95 11 9595 1 95 1 1 95 95 1 95

2 87 22 8787 2 2 87 87 2 87 2 87

3 83 33 8383 3 3 83 83 3 83

3 83

4 83 4 44 83 8383 4 83 4 83 4 83

5 5 11 11 55 1111 5 11 5 11 5 11 a Yield of hydrogena Yield gas ofcollected hydrogen in agas gas collected burette. The in a molar gas bure amounttte. ofThe hydrogen molar amount gas was calculated of hydrogen using gas the was calcu- a ideal gas law. a YieldYield ofof hydrogenhydrogen gasgas collectedcollected inin aa gasgas bureburette.tte. TheThe molarmolar amountamount ofof hydrogenhydrogen gasgas waswas calcu-calcu- lateda Yield using of hydrogenthe ideal gas gas law. collected in a gas burette. The molar amount of hydrogen gas was calcu- alated latedYield usingusing of hydrogen thethe idealideal gas gasgas collected law.law. in a gas burette. The molar amount of hydrogen gas was calculated using the ideal gas law. a Yield of hydrogen3. Materials gas collected andMethods in a gas burette. The molar amount of hydrogen gas was calculated using the ideal gas law. lated using the3.1. ideal General gas law.

All reactions and manipulations were performed under argon atmosphere using standard Schlenk techniques. 1H and 13C{1H} NMR spectra were recorded on JEOL ECS- 400 or ECX-500 spectrometers. Gas chromatograph analyses of hydrogen were performed

on a GL-Sciences GC390 gas chromatograph with packed columns (Molecular Sieve 5A and Gaskuropack 54). Gas chromatograph analysis of organic product was performed on a GL-

Sciences GC353B gas chromatograph with a capillary column (GL-Sciences TC-17 and TC- WAX). Silica-gel column chromatography was performed using Wako-gel C-200 (Wako Pure Chemical Corporation). The iridium catalysts, [Cp*IrCl2]2 [27], [Cp*Ir(H2O)3][OTf]2 [28], and 1 to 5 [10,29–31] were prepared according to the literature methods. Various glucose analogues having protected hydroxy groups used in Table2 were prepared according to the literature methods [32–34]. Organic solvent was distilled under an argon atmosphere with an appropriate drying agent. Other reagents were commercially available and were used as received.

3.2. General Procedures for the Hydrogen Production from D-Glucose Catalyzed by Various Iridium Complexes Under argon atmosphere, iridium catalyst (0.20 or 1.0 mol % Ir), D-glucose (5.0 mmol), and distilled water (15 mL) were placed in a flask equipped with a reflux condenser and a gas burette. The mixture was magnetically stirred under reflux for 20 h in an oil bath. The volume of the evolved hydrogen gas was measured by using a gas burette, and the yield of evolved hydrogen gas was calculated using the ideal gas law. The illustration of reaction apparatus is shown in Figure S1 in the supporting information. The purity of the Catalysts 2021, 11, 891 7 of 9

evolved hydrogen gas was conﬁrmed by GC analysis. Details are shown in Figure S2 in the supporting information.

3.3. Procedure for the Simultaneous Parallel Experiment (Hydrogenation of 1-Decene with Hydrogen Produced by the Dehydrogenation of Glucose) Under an atmosphere of argon, iridium catalyst 2 (0.20 mol %), D-glucose (5.0 mmol), and distilled water (15 mL) were placed in a flask A. In another flask B, under an atmosphere of argon, RhCl(PPh3)3 (2.0 mol %), 1-decene (5.0 mmol), and benzene (7.5 mL) were placed. The two flasks A and B were connected through a rubber tube. The mixture in the flask A was stirred under reflux for 20 h, while the mixture in the flask B was stirred at 50 ◦C. The yield of decane was determined by GC analysis using undecane as an internal standard.

3.4. General Procedures for the Hydrogen Production from Various Substrates Under argon atmosphere, iridium catalyst 2 (1.0 mol %), substrate (5.0 mmol), and distilled water (15 mL) were placed in a flask equipped with a reflux condenser and a gas burette. The mixture was magnetically stirred under reflux for 20 h in an oil bath. The volume of the evolved hydrogen gas was measured by using a gas burette, and the yield of evolved hydrogen gas was calculated using the ideal gas law. The illustration of reaction apparatus is shown in Figure S1 in the supporting information. For the reaction shown in entry 5 of Table2, the organic product was isolated by silica-gel chromatography (eluent: dichloromethane:methanol = 30:1) as illustrated in Figure3. 1H NMR (500 MHz, CDCl3)[35]: δ 4.40 (ddd, J = 9, 4, 3 Hz, 1H), 3.84 (d, J = 6 Hz, 1H), 3.70–3.52 (m, 2H), 3.65 13 1 (m, 2H), 3.57 (s, 3H), 3.54 (s, 3H), 3.51 (s, 3H), 3.41 (s, 3H). C{ H} NMR (125 MHz, CDCl3): δ 168.4, 82.1, 79.2, 77.4, 77.3, 70.6, 58.93, 58.86, 58.6, 58.3. These NMR spectra are shown in the supporting information.

4. Conclusions In conclusion, we succeeded in developing a new catalytic system for hydrogen production from glucose and various monosaccharides using a dicationic water-soluble dicationic iridium catalyst 2 under reﬂux conditions in water. The addition of a strong acid or base is not required during the reaction. Hydrogen can be efﬁciently obtained from various kinds of monosaccharides with a relatively small amount of catalyst (0.2 to 1.0 mol %). It was experimentally suggested that the dehydrogenation of the alcoholic moiety at 1-position of monosaccharides proceeded. This method to obtain high-purity hydrogen conveniently under mild conditions using saccharides sustainably available from natural resources as raw materials can potentially form the basis for crucial technologies aimed at the transition to a hydrogen society in the future.

Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/catal11080891/s1, Figure S1: the reaction setup for hydrogen production from glucose and various monosaccharides, Figure S2: GC analysis of the evolved gas by the reaction of glucose under optimal conditions catalyzed by catalyst 2. Author Contributions: K.-i.F. guided the research, designed the experiments, and wrote the manuscript. T.I., T.T., and J.J. performed the experiments. S.F. supported the analysis of the experimental results and the writing of the manuscript. R.Y. also guided the research and helped to write the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This work was also financially supported by JSPS KAKENHI Grant Number JP18H04255, JP18H05517, JP19H02715, and JP19H05053. Conflicts of Interest: The authors declare no conflict of interest. Catalysts 2021, 11, 891 8 of 9

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