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 monosac- charides, 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 glu- cose 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 Catalysts 2021, 11, x FOR PEER REVIEW 3 of 9 2. Results and Discussion The structures of the iridium catalysts used in this study are shown in Figure 1. Catalysts 2021, 11, 891 3 of 9 catalyst