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Effects of Glymes on the Distribution of Mg(B10H10) and Mg(B12H12) from the Thermolysis of Mg(BH4)2

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Effects of Glymes on the Distribution of Mg(B10H10) and Mg(B12H12) from the Thermolysis of Mg(BH4)2 inorganics Article Effects of Glymes on the Distribution of Mg(B10H10) and Mg(B12H12) from the Thermolysis of Mg(BH4)2 Ba L. Tran 1, Tamara N. Allen 2, Mark E. Bowden 1 , Tom Autrey 1,* and Craig M. Jensen 2,* 1 Pacific Northwest National Laboratory, 902 Battelle Blvd, Richland, WA 99352, USA; [email protected] (B.L.T.); [email protected] (M.E.B.) 2 Department of Chemistry, University of Hawaii at Manoa, 2545 McCarthy Mall, Honolulu, HI 96822, USA; [email protected] * Correspondence: [email protected] (T.A.); [email protected] (C.M.J.); Tel.: +1-509-375-3792 (T.A.); +1-808-956-2769 (C.M.J.) Abstract: We examined the effects of concentrations and identities of various glymes, from mono- ◦ 11 glyme up to tetraglyme, on H2 release from the thermolysis of Mg(BH4)2 at 160–200 C for 8 h. B NMR analysis shows major products of Mg(B10H10) and Mg(B12H12); however, their relative ratio is highly dependent both on the identity and concentration of the glyme to Mg(BH4)2. Selective formation of Mg(B10H10) was observed with an equivalent of monoglyme and 0.25 equivalent of tetraglyme. However, thermolysis of Mg(BH4)2 in the presence of stoichiometric or greater equivalent of glymes can lead to unselective formation of Mg(B10H10) and Mg(B12H12) products or inhibition of H release. 2 Keywords: magnesium borohydride; glymes; boron cages; product selectivity; dehydrogenation; Citation: Tran, B.L.; Allen, T.N.; hydrogen storage Bowden, M.E.; Autrey, T.; Jensen, C.M. Effects of Glymes on the Distribution of Mg(B10H10) and Mg(B12H12) from the Thermolysis of Mg(BH4)2. Inorganics 2021, 9, 41. 1. Introduction https://doi.org/10.3390/ The safe storage of hydrogen in a compact and efficient form remains a formidable inorganics9060041 challenge towards the realization of energy decarbonization. Among the wide range of complex hydride materials studied, Mg(BH4)2 has garnered tremendous interest from the Academic Editor: Robert C. storage community because of desirable physical and thermodynamic properties [1–4]. Bowman, Jr. Mg(BH4)2 features a high gravimetric density of H2 (ca. 14.7 wt% H2), and a thermody- namic range that is ideal for H2 release and H2 uptake at moderate pressure and tempera- Received: 17 April 2021 ture. Despite these attractive physical and thermodynamic properties, one major drawback Accepted: 14 May 2021 of Mg(BH4)4 is the slow rate of H2 release at high temperature. To circumvent this high Published: 23 May 2021 barrier towards reactivity, many studies have focused on generating complex mixtures of Mg(BH4)2 with additives composed of organic molecules [5–9], a different borohydride Publisher’s Note: MDPI stays neutral (e.g., LiBH4)[10–12], and transition metals [13–16] to lower the temperature for H2 release. with regard to jurisdictional claims in We have shown that the addition of a simple additive such as THF to Mg(BH4)2 can published maps and institutional affil- ◦ promote dehydrogenation of Mg(BH4)2 at temperature less than 200 C compared to the iations. ◦ dehydrogenation of solid Mg(BH4)2 at 250 C and above [5,17]. Interestingly, in addition to lowering the dehydrogenation temperature, the presence of THF also favored the formation 2− 2− - of B10H10 over B12H12 and B3H8 from Mg(BH4)2 compared to the favored formation − 2− 2− of B3H8 over B10H10 and B12H12 for dehydrogenation of solid Mg(BH4)2 [17]. To our Copyright: © 2021 by the authors. knowledge, the dehydrogenation of Mg(BH4)2 with triethylamine is the only system using Licensee MDPI, Basel, Switzerland. - 2− 2− an organic additive that favors the formation of B3H8 over B10H10 and B12H12 [5]. This article is an open access article Unfortunately, a structure–activity relationship on the dehydrogenation of Mg(BH ) with distributed under the terms and 4 2 THF is limited by structural derivatives. We therefore turned to glymes because of their conditions of the Creative Commons Attribution (CC BY) license (https:// abundance, availability, and more importantly broad structural derivatives to probe the creativecommons.org/licenses/by/ dehydrogenation activity of Mg(BH4)2. Additionally, the combination of glymes and 4.0/). Mg(BH4)2 have also drawn interest as electrolytes for battery energy storage [18–22]. In Inorganics 2021, 9, 41. https://doi.org/10.3390/inorganics9060041 https://www.mdpi.com/journal/inorganics Inorganics 2021, 9, x 2 of 9 dehydrogenation activity of Mg(BH4)2. Additionally, the combination of glymes and Inorganics 2021, 9, 41 Mg(BH4)2 have also drawn interest as electrolytes for battery energy storage [18–22]. 2In of 9 addition, the thermodynamic affinity of the cation–glyme interaction is highly influenced by the number of chelating diether units, which may have some influence on Mg(BH4)2 dehydrogenation [23]. While structural studies of Mg(BH4)2 with monoglyme and di‐ addition, the thermodynamic affinity of the cation–glyme interaction is highly influenced glyme are known [24,25], the effects of different glymes on the decomposition and product by the number of chelating diether units, which may have some influence on Mg(BH4)2 selectivity of Mg(BH4)2 have not been reported. Herein, we report the effects of a variety dehydrogenation [23]. While structural studies of Mg(BH4)2 with monoglyme and diglyme of glymes on the dehydrogenation of Mg(BH4)2 to form B10H102−, B12H122− and B3H8−. are known [24,25], the effects of different glymes on the decomposition and product selectivity of Mg(BH4)2 have not been reported. Herein, we report the effects of a variety 2. Results 2− 2− − of glymes on the dehydrogenation of Mg(BH4)2 to form B10H10 ,B12H12 and B3H8 . We initiated our studies by performing parallel reactions of Mg(BH4)2 with 1.0 equiv. of 2.monoglyme Results (G1), triglyme (G3), and tetraglyme (G4) at 180 °C for 8 h (Table 1). The results Weof these initiated reactions our studies showed by performingthat G1, G3, parallel and G4 reactions contain ofthe Mg(BH highest4) 2activitywith 1.0 with equiv. conversionof monoglyme of Mg(BH (G1),4)2 triglymein the rage (G3), of 33–50%. and tetraglyme Conversely, (G4) diglyme at 180 ◦ (G2)C for showed 8 h (Table minimal1). The activityresults for of the these dehydrogenation reactions showed of Mg(BH that G1,4)2 even G3, and at 200 G4 °C contain for 8 h the(Table highest 1, entry activity 13) with with 2− noconversion apparent selectivity of Mg(BH for4)2 anyin the boron rage products. of 33–50%. Like Conversely, THF, G1 selectively diglyme (G2) produced showed B minimal10H10 2− − ◦ overactivity B12H12 for and the B dehydrogenation3H8 (Table 1, entry of 1). Mg(BH While4) 2theeven conversion at 200 C of for Mg(BH 8 h (Table4)2 is higher1, entry for 13) G4with (50%) no than apparent that of selectivity G3 (39%), for G3 any has boronhigher products. selectivity Like of forming THF, G1 B12 selectivelyH122− over producedB10H102− 2− 2− 2− 2− thanB10 thatH10 of G4over with B12 Ha ratio12 andof B12 BH3H128 / B(Table10H10 1 ,is entry 2.5/1.0 1). for While G3 and the conversion1.4/1.0 for G4 of (Table Mg(BH 1,4 )2 2− 2− 2− entryis higher 2 vs. entry for G4 3) (50%) . This than preference that of G3 for (39%), B12H12 G3 over has B higher10H10 selectivity for the longer of forming chain glymes B12H12 2− 2− 2− in overa 1:1 Breaction10H10 withthan Mg(BH that of4) G42 is withthe reverse a ratio of of that B12 Hof12 G1 /Band10 THF.H10 is 2.5/1.0 for G3 and 2− 2− 1.4/1.0 for G4 (Table1, entry 2 vs. entry 3). This preference for B 12H12 over B10H10 for Table 1. Product distributionthe longer for the chain reaction glymes of Mg(BH in a 1:14)2 with reaction 1.0 equivalent with Mg(BH of additive4)2 is theat 180 reverse °C for of8 h. that of G1 and THF. Entry Additive Equiv. T (°C) B10H102− B12H122− B3H8‐ Unknown Conv a Mass Loss% b ◦ 1 G1Table 1. Product1.0 distribution180 for the reaction29 of Mg(BH1 4)2 with1 1.0 equivalent2 of additive33 at 180 C for 811.5 h. 2 G3 1.0 180 10 25 1 3 39 40.3 Mass 3 EntryG4 Additive 1.0 Equiv.180 T (◦C)19 B H 2−27 B H 12 − B H −3 Unknown50 Conv a 53.8 10 10 12 12 3 8 Loss% b 4 G1 1.5 180 37 14 2 53 1 G1 1.0 180 29 1 1 2 33 11.5 5 G1 26 180 16 30 17 63 2 G3 1.0 180 10 25 1 3 39 40.3 6 3G4 G40.25 1.0180 18015 19 271 1 316 50 53.8 7 4G4 G154 1.5 180 37 14 − 20 53 8 5Me‐THF G1 1.0 26180 1803 16 30 1 − 4 177 63 9 6dodecane G4 1.0 0.25 180 15 1 0 16 7 G4 54 180 − − − − 0 10 8G1 Me-THF 1.0 1.0 180 3 − 1 410 7 11 9G4 dodecane 1.0 1.0 180 − − − − 19 0 12 10G1 G11.0 1.0 160 7 − 1 361 10 13 11G2 G41.0 1.0 160 3 8 3 517 19 12 G1 1.0 200 54 2 2 3 61 14 13G4 G21.0 1.0 200 5 2 5 561 17 15 14Me‐THF G4 1.0 1.0 200 8 36 11 1635 61 16 15dodecane Me-THF 1.0 1.0 200 22 1 4 80 35 a Characterization16 dodecane of B‐containing 1.0 products 200 were analyzed− by 11B NMR − spectroscopy − using a mix − solvent system0 of 2 D2O: a b 11 b 1 THF.Characterization Thermogravimetric of B-containing analysis products (TGA) were (5 K/min analyzed ramp, by B180 NMR °C), spectroscopy the large mass using loss a mix can solvent potentially system result of 2 D 2fromO: 1 THF.the ◦ decompositionThermogravimetric of glyme, analysis see (TGA) Ref.
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