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A Highly Efficient Hydrogen Evolution System of Melamine/BH3 without Using Any Catalyst Xiangdong Ji, Ying Mu, Xiaobo Tong, Yang Liu, and Wei Gao ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 25 Mar 2019 Downloaded from http://pubs.acs.org on March 25, 2019

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ACS Sustainable Chemistry & Engineering

A Highly Efficient Hydrogen Evolution System of Melamine/BH3 without Using Any Catalyst Xiangdong Ji,1,2 Xiaobo Tong,1 Yang Liu,1 Wei Gao,1 and Ying Mu1,* 1

State Key Laboratory for Supramolecular Structure and Materials, School of Chemistry, Jilin

University, 2699 Qianjin street, Changchun 130012, China. 2

Key Laboratory of Hexi Corridor Resources Utilization of Gansu, College of Chemistry and

Chemical Engineering , Hexi University, 846 Beihuan road, Zhangye 734000, China. * E-mail: [email protected] (Y. Mu) KEYWORDS: ammonia borane, dehydrogenation, dehydrocoupling reaction, hydrogen storage, melamine

ABSTRACT: A highly efficient melamine/BH3 H2 evolution system, which can spontaneously release 6 equiv of H2 from 1 equiv of melamine at room temperature and is capable of releasing further 3 equiv of H2 at higher temperatures, has been developed. The dehydrogenation reaction was verified and studied by 1H and 11B NMR, MALDI-TOF MS analyses as well as H2 evolution experiments under different conditions. The reaction products were characterized by elemental analyses, IR and solid

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B NMR spectroscopic techniques. Studies on the dehydrogenation

reaction of BH3 with a model compound o-aminopyridine provide solid experimental evidences for further confirming the spontaneous NH/BH dehydrocoupling reaction.

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Introduction As a potential renewable energy material, hydrogen (H2) has been considered as a nonpolluting green energy that could replace the petroleum-based fuels in transportation applications by developing hydrogen-based fuel-cell vehicles.1-3 A limitation to the application of the hydrogen energy is the storage and transportation of gaseous H2 due to its low energy density.4 In recent years, many research efforts have been committed to develop efficient hydrogen storage materials. So far, metal hydrides and complex hydrides,5-7 ammonia borane derivatives,812

carbohydrates,13 metal organic frameworks,14 clathratratioles supra-molecular compounds,15

nanotubes,16-17 nano-size particles,18-21 methanol and ethanol,22-23 formic acid,24-27 etc have been studied as potential hydrogen storage materials. Ammonia borane (NH3BH3) is one of the most promising candidates among the reported potential hydrogen storage materials, and has a gravimetric density of 19.6 wt% H2, moderate thermal stability and fairly good air-stability.28-31 The dehydrogenation reaction of NH3BH3 has been studied in protic32 or non-protic solvents,33 ionic liquids,34 or the solid state,35 and catalyzed by various catalyst systems, including nanocomposites,36 mesoporous silica,37 Lewis or Brønsted acids,38 frustrated Lewis pairs,39-40 proton sponge,35 main group metal amides,41 and transition metal complexes. Some transition metal catalysts, such as Mn,42 Fe,43-44 Ni,45-47 Mo,48 Ru,49-53 Rh,54-56 Pd,57 Ir,58-61 Pt,62 Re,63 Ti,64 and Zr65 complexes, have demonstrated high catalytic activities for the dehydrogenation of NH3BH3. However, most transition metal catalysts reported for the NH3BH3 dehydrogenation either employ expensive metals (Ru,49-53 Rh,54-56 Pd,57 Ir,58-61 etc) or suffer from instability.43-47,57 In addition, the NH3BH3 dehydrogenation system also suffers from the release of small amount of

impurities

such

as

borazine,

diborane,

and

ammonia.66

Recently,

catalyst-free

dehydrocoupling of amines with pinacol borane and 9-borabicyclononane has been reported.67


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Herein we report a new melamine/BH3 dehydrogenation system, which can spontaneously release 6 equiv of H2 (5.78 wt%) from 1 equiv of melamine (C3N6H6) at room temperature with no need of any catalyst and is capable of releasing further 3 equiv of H2 (2.90 wt%) upon heating the reaction mixture at higher temperatures. Therefore, the total amounts of H2 evolution of the reaction system reaches 9 equiv of H2, which accounts for a hydrogen density of 9eq H2/(1eq C3N6H6 + 6 eq BH3) = 8.68 H2 wt%. This dehydrogenation system shows moderate H2 release rate at room temperature and can keep releasing H2 for 6~7 hr. Results and Discussion Dehydrogenation reaction of BH3/Melamine system In a search for efficient dehydrogenation system of ammonia borane derivatives, we found that mixtures of melamine and BH3-THF release H2 spontaneously at room temperature or even lower temperatures. The evolution of hydrogen takes place gently after a BH3-THF solution was mixed with melamine in a reactor under inert atmosphere. The generated gas samples were identified by gas chromatography (GC) to be pure H2 and no other gaseous species were detected. Attempts to run similar reactions of BH3 with ethylenediamine and benzylamine were unsuccessful and no H2 evolution was observed. The driving force for the spontaneous happen of the dehydrogenation reaction between melamine and BH3-THF may come from the coordination of the N atoms in the triazine ring of melamine to BH3 which makes the B-H and N-H having more chances to close to each other as shown in Scheme 1. To study the dehydrogenation chemistry of the new system in details, reactions of melamine with BH3-THF under different conditions were followed by measuring the amount of the released H2 volumetrically with time. Since the boiling point of the solvent THF is 65 oC, most of the reactions were carried out at 25 and 50 oC. It was found that reactions with the molar ratio of BH3/melamine = 1, 3, and 6 produce roughly 1, 3, and 6 equiv of H2, respectively, at both temperatures. No further increase


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in the H2 production was observed for reactions with the molar ratio of BH3/melamine > 6. Typical H2 generation profiles for the dehydrogenation reactions with the molar ratio of BH3/melamine = 3 and 6 at 25 and 50 oC are shown in Figure 1. H

NH2 N H2N

N N

BH3-THF NH2

N

N H2N

HH B N

N

HN

H H

H2

NH2

N H2N

BH2 N

N

NH2

Scheme 1. Possible mechanism for the dehydrogenation reaction of melamine with BH3-THF.

Figure 1. Hydrogen evolution profiles for reactions with the BH3/Melamine molar ratio = 3 and 6 at 25 and 50 °C. (a) and (c) (—■—) BH3/Melamine molar ratio = 6, (b) and (d) (—●— )BH3/Melamine molar ratio = 3. The rates of the reactions at 25 °C are moderate and the H2 evolution can lasted for about 450 min, while the reactions run at 50 °C are much faster and go to completion in 40~70 min depending on the molar ratio of BH3/melamine. In addition, the reactions run at 50 °C produce roughly 2.5 and 5 equiv of H2 in 30 min, respectively. In all cases, the dehydrogenation reaction between BH3 and melamine is slow at beginning of the reaction because of low solubility of melamine, and becomes faster with the increase in the solubility of melamine due to deconstruction of the H-bonding among melamine molecules by coordination of N atoms in melamine to BH3. During the middle period of the reaction, melamine is completely dissolved


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and the reaction mixture becomes a clear solution. With the progress of the reaction, the reaction mixture turns to a gel in the later period of the reaction. The gelation of the reaction mixtures may result from intermolecular dehydrogenation reaction and coordination of N atoms to B atoms in the reaction products. Two dehydrogenation reaction products MB3 and MB6, from reactions with the molar ratio of BH3/melamine = 3 and 6, respectively, were obtained as airsensitive, insoluble solid materials after removing the solvent THF under reduced pressure. They were characterized by IR and solid 11B NMR. Attempts to characterize the structures of MB3 and MB6 by solution NMR spectroscopy were not successful due to insolubility of the gelated products in common solvents, and tries to grow single crystals of MB3 and MB6 in different solvent systems were also not successful. Satisfied results of elemental analyses on MB3 and MB6 were not obtained owing to solvent remain in the samples and further dehydrogenation reactions between NH/BH2 or BH2/BH2 groups during the sample preparation under reduced pressure at higher temperatures. From the FT-IR spectra of MB3 and MB6 shown in Figure 2, following bands68-71 can be observed: B–H rocking at 666–727 cm-1; C–N stretching at 1075– 1090 cm-1; B–H deformation at 1053 cm-1 and 1155 cm-1; B–N stretching at 1300–1800 cm-1; N– H deformation at 1373 and 1596 cm-1; C=N stretching at 1520–1690 cm-1; B–H symmetric and asymmetric stretching at 2115–2318 cm-1; and C–H stretching at 2840–3000 cm-1. The N–H stretching at 3193–3304 cm-1 in the IR spectrum of MB6 becomes relatively weak, demonstrating that most of the N–H groups of melamine in MB6 have been consumed, which is consistent with the observed amount of H2 evolution. The C–O–C stretching at 990–1190 cm-1 should be from the remaining solvent THF.


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Figure 2. FTIR spectra of samples MB3 (a) and MB6 (b) under nitrogen atmosphere. To know more details about the dehydrogenation reaction between BH3 and melamine, the reaction with the BH3/melamine molar ratio = 6 at 25 °C was followed by 1H and 11B NMR, as well as MALDI-TOF MS analyses. 1H NMR spectra of samples taken from the reaction mixture at different reaction times are shown in Figure 3. From the 1H NMR spectra of samples taken at 10 and 100 min, resonances for the NH2 group in melamine and the BH3 group in the BH3-THF adduct can be seen at 8.02 and 0.05 ppm, respectively. The resonance for the NH2 group becomes very weak in the spectrum of the 100 min sample and totally disappears in the spectrum of the 300 min sample. The resonance for the BH3 group in the BH3-THF adduct also becomes weaker and weaker with time. The broad resonance from 1.3 to 2.2 ppm observed in the spectrum of the 10 min sample should be assigned to BH3 coordinated by amino groups in melamine in different environments which becomes quite weak in the spectrum of the 100 min sample and disappears in the spectrum of the 300 min sample. The multiple resonances from 6.9 to 7.9 ppm seen mainly in the spectra of the 10 min and 100 min samples might be attributed to the NH2 group of melamine coordinating to a B atom with different environments. The resonances from 0.7 to 1.6 ppm and those from 1.8 to 2.3 ppm seen in these spectra could be tentatively assigned to various BH2 and BH groups in the reaction products, respectively.


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Similarly, the signals from 3.2 to 4.3 ppm may be reasonably attributed to different NH groups in the reaction products. By comparing with the spectrum of a mixture of BH3-THF and bipyridine (see Figure S1), the weak broad signals around 2.5 and 2.8 ppm should be assigned to BH3 coordinated by the N atoms in the triazine ring of melamine. The singlet signal at 4.5 ppm seen in these spectra comes from the released H2 molecule and the multiple resonances at 1.5, 1.7, 3.3 and 3.5 ppm are from free and coordinating THF. The

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B NMR spectra of these samples are

shown in Figure S2. The quartet signal about 0 ppm seen in these spectra is from the BH3-THF adduct according to the literature.[72] In the 11B NMR spectra of the 10 and 100 min samples, the resonance around -25 ppm should be assigned to BH3 coordinated by N atoms of melamine in different environments, while the small peek around -15 ppm and the less obvious broad resonance around -10 ppm might be caused by various BH2 and BH groups in the reaction products. The resonances for the BH2 and BH groups increase and the spectrum becomes complicated with the reaction progress, giving a very broad envelope shape spectrum finally due to the formation of a gelated polymeric material.


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Figure 3. 1H NMR spectra of samples taken from the reaction mixture with the BH3/melamine molar ratio = 6 at 25 °C at different times (in THF-d8). The MALDI-TOF mass spectra of samples taken from the reaction with the BH3/melamine molar ratio = 6 at 25 °C at different reaction times are given in Figure S3. From the MALDITOF MS analysis, only very limited information on the reaction have been obtained due to the air and moisture sensitivity and insolubility of the gelated polymeric BH-containing products. In the mass spectrum of the sample taken at 10 min, two significantly enhanced peaks at m/z 138 and 272 can be seen, and might be attributed to the molecules with structure A and B as shown in Chart 1. Broad signals around m/z 510 and 520 can always be seen in these mass spectra and can be tentatively assigned to molecules with possible structures C, D, E, F, G and H. Another broad signal with m/z from 765 to 790 seen in the mass spectra of samples taken at 300 and 500 min


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corresponds to the molecular weights of species with four melamine and twenty four BH2 units as well as their intramolecular dehydrocoupling products, which indicates that compounds with more complicated structures than those shown in Chart 1 may be involved in the reaction. BH2 N N N

HB H2B

NH

N

HN

H2N

N

N H2N

N

BH2 N

N

H2B

N

N

N

HN

N N

NH2

N

N

HB N N

B H

N

BH N N N

N

BH2 N BH2

H2 B

H N

N N

N

N B BH2 H Molecular Formula: C9N18B12H22 Fw = 512.35 H2B

E

N

H 2B

BH2

N N

N B BH2 H Molecular Formula: C9N18B12H18 FW = 508.32

HB N N

HB N N

N N B H

BH N N N

BH2

H

BH2 N N H2B BH2 N

N N

HB N N

N N

H2B

BH2

Molecular Formula: C9N18B12H24 Fw = 514.36

N

N N

BH2

N B BH2 H Molecular Formula: C9N18B12H20 Fw = 510.33

BH2 BH2 N N N H2B N N

BH N N

B H

N

N N

BH2

H2B

BH2


F

G

H2B

H N

HB N N N

N

N B H

H2B

BH

BH N N N

N

N BH2

BH N N

N

BH

N

N

B H

BH

H2B

BH2 N

N

HB

N

D

BH2 N N

N H

N H2B

N

N

N

BH2 N

H2B N

N N

BH N N

H N

C

BH2

N H

B H

BH

H2B

B

BH2

HB N N

H2B

N

NH B H Molecular Formula: C6N12B2H10 FW =272.13

NH2

A

H N

NH

N N

Molecular Formula: C3N6BH7 FW = 138.08

H 2B N

H B

N N

BH2 BH2 N N H 2B N N N

BH2 N

N N

BH2

N BH2

H2B

BH2


H

Chart 1. Possible molecular structures accounting for the observed MALDI-TOF MS signals. Dehydrogenation reaction of BH3/o-AmPy system The 1H and

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B NMR, as well as MALDI-TOF MS analyses on the dehydrogenation

reaction mixture of BH3 and melamine at different times demonstrate the complexity of the reaction, from which complicated gelated polymers or supramolecules are formed. In order to obtain more information on the dehydrogenation reaction, o-aminopyridine (o-AmPy) was used as a model compound of melamine for further investigation on the dehydrogenation reaction. In contrast to the BH3/melamine system, it was found that reactions of BH3 and o-AmPy in THF with molar ratio of BH3/o-AmPy = 1 and 2 at 25 and 50 oC all produce 1.5~2 equiv of H2 as shown in Figure 4.


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Figure 4. Hydrogen evolution profiles for reactions of BH3/o-AmPy with BH3/ o-AmPy molar ratio = 1 and 2 at 25 and 50 oC. (a) and (c) BH3/o-AmPy molar ratio = 2, (b) and (d) BH3/oAmPy molar ratio = 1. For the BH3/o-AmPy reaction system, a light yellow solution was formed after the H2 evolution, which makes the isolation and characterization of the reaction product(s) possible. Fortunately, single crystals of a complicated product containing 4 o-AmPy and 4 borane units (I) were obtained from a reaction with the molar ratio of BH3/o-AmPy = 1 after the reaction solution was concentrated and mixed with suitable amount of hexane. The structure of product I was determined by single crystal X-ray diffraction analysis and the molecular structure in ORTEP form was shown in Figure 5. Selected bond lengths and angles as well as crystallographic data are given in Tables S1 and S2. Based on the obtained structure of I and the chemistry discussed above, a possible reaction mechanism for the formation of I as shown in Scheme 2 can be proposed. The formation of I at room temperature firmly conforms the happening of the dehydrocoupling reaction between BH and NH in these systems, and indicates that all 3 B-H bonds of some BH3 in specific positions during the reaction can take part in the dehydrocoupling reaction. It is clear that the dehydrocoupling reaction takes place in similar style in the two systems, while more H2 is released from the BH3/o-AmPy system when the BH3/-NH2 molar ratio being 1. It is reasonable for the two systems showing some differences in


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the reaction extent as well as the related products considering the apparent difference between oAmPy and melamine in steric bulk.

Figure 5. Perspective view of the molecular structure of I with thermal ellipsoids drawn at the 30% probability level. Hydrogen are omitted for clarity. H

NH2 BH3-THF

N

THF

HH H N B H H2 N

HN

H B

NH2 N

H

HN

HH

H B

N

N

H2

NH N

NH2 H N N H B HN

B H

H N

N

further reactions take places in the same way

H N N

BH3-THF

B H

BH2 N N

N

N H N N

B H

N

B H

H N

N

H N N

H N

N

B H H N N B H2

H H N B N

N N

H N

H2 BH3-THF

N B H2

B N

N

BH3

H H N B

N

N

I

Scheme 2. Possible reaction mechanism for the formation of compound I from the dehydrogenation reaction of BH3/o-AmPy. From the solid

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B MAS NMR spectrum of I, shown in Figure 6, a number of broad

resonances around -3, -10, -17, -37 (shoulder), and -42 ppm can be seen. In the literature, BH4-


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has been reported to show a resonance about -40 ppm in 11B MAS NMR, such as in LiBH4 (-41.3 ppm)73, and [(NH3)2BH2]+BH4- (-37.6 ppm)74. On the basis of the molecular structure of I, and the chemical shifts and integrations of the observed resonances, reasonable assignments can be made as demonstrated in Figure 6. The resonance around -42 ppm can only be assigned to the oAmPy-coordinated BH3 in I. It has been reported that THF-coordinated BH3 shows a resonance at -1.5 ppm while NH3-coordinated BH3 gives the resonance at -23 ppm in their 11B MAS NMR spectra due to the strong electron-donating ability of NH375. In the current case, both the pyridyl and amino groups in the mono- or di-borylated o-AmPy are much stronger electron-donors, and therefore the resonance of the o-AmPy-coordinated BH3 in I should shift to further high field. With the same reason, the resonances of the o-AmPy-coordinated BH2 and BH groups in I should also shift to relatively high field in comparison to normal BH2 and BH groups. Similar BH groups coordinated by monoborylated amines were reported to show resonances from -9 to 11 ppm76 which is in agreement with our results.

Figure 6. Solid 11B MAS NMR spectra of compound I. Further studies on the BH3/Melamine system


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Upon heating samples MB3 and MB6, further 3 equiv of H2 was released from 90 to 170 °C for MB3 and from 125 to 195 °C for MB6 as shown in Figure 7, which makes the total amounts of H2 evolution for the two reaction systems reaching 6 equivalents (7.22 wt%) and 9 equivalents (8.68 wt%), respectively. The further H2 evolution of the MB3 sample at higher temperatures should be primarily from the dehydrogenation reaction between the remaining -NH and -BH2 groups in the sample, while the further 3 equiv of H2 evolution of the MB6 sample must mainly come from inter- and/or intra-molecular dehydrocoupling reaction(s) between two BH2 groups. A similar dehydrogenation reaction between BH2 groups in the dimer of 1-NMe2-2-(BH2)C6H4 has been reported, in which case the intermolecular coordination of the NMe2 group in one molecule to the BH2 group in another molecule makes the two BH2 groups close to each other.77 Accompanying the H2 evolution from MB3 and MB6, small amount of remaining THF was also released. Thermogravimetric analysis (TGA) on the samples MB3 and MB6 indicates that both samples lose about 8% of weight in the region of 90~200 °C (Figure 8), which should include both of the H2 and THF evolution. Both samples begin to decompose around 350 °C.

Figure 7. Hydrogen evolution profiles for samples MB3 and MB6 with increase in heating temperature.


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Results of elemental analyses on the samples obtained by heating MB3 and MB6 at 270 oC for 3 hours under reduced pressure (named as h-MB3 and h-MB6) largely agree with formulas C3N6B3H3(THF)0.2 and C3N6B6H6(THF)0.4, respectively, which is expected based on the results of the above mentioned H2 evolution and TGA experiments. From the solid

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B MAS NMR

spectra of samples MB3, MB6, h-MB3, and h-MB6 shown in Figure 9, major five set of resonances around -3, -10, -18, -30, and -48 ppm can be seen in the spectra of MB3 and MB6, while the resonances at -30 and -48 ppm disappears in the spectra of h-MB3 and h-MB6. By referencing the literature73-76 and the above discussions on the solution 11B NMR spectra of the reaction mixture and the solid 11B MAS NMR spectrum of I, the high field resonances around 48 ppm in MB3 and MB6 should be assigned to the unreacted BH3 coordinated by N-atoms in amino groups and melamine ring, while the broad resonances around -30 ppm in MB3 and MB6 could be assigned to coordinated and uncoordinated BH2 groups as labeled in the figure. The resonances around -1, -7, and -19 ppm for h-MB3, and the resonances around 0, -6, -18 ppm for h-MB6 should be ascribed to BN3, uncoordinated and coordinated BH groups remaining in the heated samples h-MB3 and h-MB6, respectively. By comparing the 11B NMR spectra of samples MB3, MB6, h-MB3, and h-MB6, it can be seen that small amounts of BN3 and BH groups also exist in samples MB3 and MB6, demonstrating that the dehydrogenation reaction between NH/BH2 or BH2/BH2 groups can happen at 25 oC. The further dehydrogenation of MB3 and MB6 can be intra- and/or inter-molecular reactions between NH/BH2, NH2/BH2 or BH2/BH2 groups. The inter-molecular dehydrocoupling reaction would lead to the formation of porous polymeric materials.


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Figure 8. Graphs of thermogravimetric analysis on samples MB3 and MB6.

Figure 9. Solid 11B MAS NMR spectra of MB3, MB6, h-MB3, and h-MB6. To know more details about the further dehydrogenation reaction of MB3 and MB6, nitrogen sorption isotherms of h-MB3 and h-MB6 (shown in Figure S4) were measured at 77 K. The Brunauer-Emmett-Teller (BET) specific surface area values of 1146.06 m2/g and 8.30 m2/g were obtained for h-MB3 and h-MB6, respectively. These surface area values clearly indicate


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Page 16 of 30

that h-MB3 is a typical porous material, while h-MB6 does not possess a porous structure. These results in turn give us some significant information on the dehydrogenation reaction of MB3 and MB6. It can be reasonably speculated that the H2 evolution from MB3 should take place mainly through the inter-molecular dehydrocoupling reaction of NH/BH2 and NH2/BH2 groups, while the dehydrogenation reaction of MB6 may happen primarily in the form of intra-molecular dehydrocoupling between BH2/BH2 groups. Conclusions In summary, we have developed a highly efficient melamine/BH3 dehydrogenation system, which can spontaneously release 6 equiv of H2 from 1 equiv of melamine at room temperature and is capable of releasing further 3 equiv of H2 upon heating the reaction mixture at higher temperatures. This new H2 evolution system shows moderate H2 release rate at room temperature and can keep releasing H2 for 6~7 hr. The dehydrogenation reaction was verified by H2 evolution experiments, and IR, 1H and

11

B NMR, MALDI-TOF MS and solid

11

B NMR analyses.

Complicated gelated polymeric materials together with some small molecules and supramolecules are formed in the dehydrocoupling reaction. Studies on the dehydrogenation system of BH3 with a model compound o-aminopyridine further confirm the N-H/B-H dehydrocoupling reaction. Experimental Section Borane tetrahydrofuran complex solution (BH3OC4H8, 1.0 M in THF), melamine (C3N6H6, 99%) and ortho-aminopyridine (C5N2H6, 99%) were purchased from Aldrich and used as received. Tetrahydrofuran and hexane were refluxed over sodium benzophenone ketyl and distilled before use under N2. All samples were stored in a nitrogen-filled glove box (Vigor SG1200/750TS, H2O

BH3

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