Clean Hydrogen Release from Ammonia Borane in a MetalâOrganic

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Article

Clean Hydrogen Release from Ammonia Borane in a Metal Organic Framework with Unsaturated Coordinated Tm 3+

Huijuan Yang, Zhongyue Li, Kun Liu, Fanyan Meng, and Chao Niu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp510408b • Publication Date (Web): 14 Jan 2015 Downloaded from http://pubs.acs.org on January 17, 2015

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The Journal of Physical Chemistry

Clean Hydrogen Release from Ammonia Borane in a Metal Organic Framework with Unsaturated Coordinated Tm3+ Huijuan Yang, † Zhongyue Li,*,† Kun Liu, † Fanyan Meng, ‡ Chao Niu †

†

Department of Physics-Chemistry, Henan Polytechnic University, Jiaozuo, Henan, 454000,

China

‡

Hunan Baoshan Nonferous Metals & Minerals Co., Ltd. Chenzhou, Hunan, 424402, China.

ACS Paragon Plus Environment

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Clean Hydrogen Release from Ammonia Borane in a Metal Organic Framework with Unsaturated Coordinated Tm3+ ABSTRACT: A hybrid of ammonia-borane (AB) and a metal-organic framework (MOF), which contains unsaturated coordinated Tm3+, Tm(BTC) (BTC = 1, 3, 5-benzenetricarboxylic) was synthesized through solvent-based impregnation method (named as AB@Tm(BTC)-CH3OH). Also other two materials AB@Tm(BTC)-milled and AB@Tm2O3-milled were prepared by physical milling separately. TPD-MS results show that the H2-release peaks of the three materials shift to lower temperature, 77 °C, 79 °C, 85 °C, respectively, compared with neat AB (114

°C

and

150

°C).

For

avoiding

the

undesirable

volatile

byproduct,

only

AB@Tm(BTC)-CH3OH shows superior performance without any byproduct especially ammonia. The three samples exhibit enhanced dehydrogenation kinetics compared neat AB, but AB@Tm2O3-milled presents much slower than the other two materials. The dehydrogenation activation energies of AB@Tm(BTC)-CH3OH, AB@Tm(BTC)-milled and AB@Tm2O3-milled are 98.1 kJ⋅mol-1, 103.1 kJ⋅mol-1 and 116.4 kJ⋅mol-1, respectively. The mechanisms of AB@MOFs thermal dehydrogenation system especially for the preventing of ammonia have been discussed. The interaction between AB and the unsaturated coordinated metal sites in MOFs plays a key role for inhibiting ammonia during AB thermolysis.

KEYWORDS: Ammonia borane, Metal organic framework, Hydrogen storage.


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Introduction

In recent years, for the sake of seeking clean energy sources to satisfy the increasing development of economic, a number of researchers devoted themselves to the field of hydrogen storage. 1~3 Ammonia borane, NH3BH3 (AB in short) is considered to be a promising candidate material for solid-state hydrogen source because of its extremely high hydrogen content (19.6 wt %) and favorable stability at room temperature.4,

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Thermal decomposition is one of main

hydrogen losing ways of AB.6 However there are still some challenges of putting it into practical applications.7 The main barriers are that its slow dehydrogenation kinetics below 100 °C and the formation of volatile impurities [ammonia (NH3), diborane (B2H6) and borazine (B3H6N3)]. To optimize AB decomposition, a series of systems have been reported aiming at reducing H2 releasing temperature, preventing byproducts, and increasing hydrogen releasing kinetics, such as nanoscaffolds,8 metal substitution,9 metallic catalysis,10 acid catalysis,11 composite with carbon material,12~15 ionic liquids,16 extremely high pressure treatment17. Our previous work proposed that AB combined with metal organic framework (MOF) system had a dramatic improvement according to its proper microporous size, high surface area, and containing open unsaturated coordinated metal sites.18 MOFs are a kind of novel porous materials constructed with metal centers and organic ligands through coordination bonds.19 Large amounts of MOFs as hotspots are reported continually every year due to its multiple skeleton structure, high surface area and modifiable porous properties.20~22 Recently, several researches improving the thermal


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dehydrogenation of AB by MOFs have been reported.23~29 It was showed that MOF could accelerate the H2 releasing rate at lower temperature and purify the volatile products especially for avoiding NH3 gas, but the mechanisms are not clearly. Although it was shown that metal catalytic center was likely the key factor for preventing ammonia sending. So, intensive study about the machanism of AB@MOFs system seems significative for looking for an optimal MOF structure to improve the dehydrogenation properties of AB.

In this paper, two mixed materials AB@Tm(BTC) with the same raw materials [AB and Tm(BTC)]

and

same

molar

ratio

have

been

prepared

by

impregnation

method

[AB@Tm(BTC)-CH3OH] and physical milling method [AB@Tm(BTC)-milled] respectively. The Tm-based MOF material Tm(BTC) exhibits a three-dimensional (3D) architecture containing approximately 6 × 6 Å 1D tubular channels along the [001] direction to which the coordination water molecules point. It shows high thermal stability and permanently porous with high surface areas.30 The open structure containing unsaturated metal Tm3+ is obtained after removed the terminal coordinated H2O molecules. The H2 releasing progresses of AB@Tm(BTC)-CH3OH and AB@Tm(BTC)-milled have been studied. Their decomposition temperatures were reduced and the dehydrogenation kinetics were enhanced. But AB@Tm(BTC)-milled released NH3. The H2 losing behavior of another physical mixture sample with AB and Tm2O3 (AB@Tm2O3-milled) has been compared to investigated the structure effect in AB@MOFs system. Not only the dehydrogenation kinetics is slower than the two


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AB@Tm(BTC) composite materials, but also borazine volatilizes. In a word, infused AB@Tm(BTC)-CH3OH showed the best ability to improve the thermal H2 losing behavior including reducing dehydrogenation temperature, enhancing dehydrogenation kinetics and preventing unwished

volatile byproducts.

The mechanisms of AB@MOFs thermal

dehydrogenation system especially for the preventing of ammonia have been deeply discussed. The interaction between AB between the unsaturated coordinated metal sites of MOFs is deemed to a key role for inhibiting ammonia during AB thermolysis. Although there is no a bit leap in thermal hydrogen release properties in this research, the more clear mechanisms provide a helpful basis to improve AB@MOFs dehydrogenation system and promote it to practically apply.

Experimental

Characterization

Powder X-ray diffraction (PXRD) data were collected on a Bruker D8 Advance X-ray diffractometer (Cu-Kα radiation) diffractometer. The voltage and the current were set to be 40kV and 40mA, respectively. Measuring 2θ range is from 4 ° to 40 °. Scan speed is 6 °/min. The Fourier transform infrared (FTIR) spectra were recorded (4000 - 400 cm-1 region) on a Bruker Optics VERTEX 70 Fourier Transform Infrared spectrometer using KBr pellet. Temperature programmed desorption mass spectrometry (TPD-MS) data was detected by Micromeritics


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Chemisorb 2720 and Prismaplus QMG300. The temperature was raised from room temperature to 200 ºC at the rate of 3 ºC/min, with Ar flowing at a rate of 50 mL/min. Dehydrogenation rate was detected by AMC Gas Reaction Controller. Solid-state 11B NMR spectral experiments were performed on a Varian Infinity-plus 400 spectrometer operating at 128 MHz.

Sample preparation

All chemicals purchased were of reagent grade and were used without further purification. Thulium nitrate salt [Tm(NO3)3·nH2O] were prepared via dissolution of

Tm2O3 with HNO3 (6

M). The mixture was then evaporated at 100 °C until a crystal film formed.

Tm(BTC) synthesis and activation: A mixture of Tm(NO3)·6H2O (4 g) and H3BTC (2 g) was dissolved in DMF (N,N’-dimethylformamide, 80 ml) and distilled water (16 ml) at room temperature. The solution was stirred until became clear. Then the mixture was transferred into a 100 ml glass bottle with cover and treated at 80 °C for 24 h. White crystalline powder was obtained. Filtered the liquor, and the powder soaked in CH3OH for 2 days (each time used 100 ml CH3OH and after 1 day fresh CH3OH in place of the old CH3OH). Filtered and heated at 300 °C for 4 h, then vacuumed at 200 °C for 12 h. So the guest solvent and terminal coordinated molecules were removed. The Tm(BTC) sample with open framework and unsaturated metal sites Tm3+ was obtained and stored in a glove box.


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AB@Tm(BTC)-CH3OH synthesis: AB was loaded into Tm(BTC) by solvent-based impregnation method. AB (3 mmol) was resolved in 5 ml anhydrous methanol in a glove box at room temperature. Tm(BTC) (3 mmol) was added into the solution and stirred 2 h, then vacuumed for 48 h at room temperature to remove the solvent. The obtained solid was kept in a glove box.

AB@Tm(BTC)-milled synthesis: AB and Tm(BTC) were put into a agate mortar and milled with pestle in a glove box at room temperature for 3 min. The mol ratio was 1:1.

AB@Tm2O3-milled synthesis: AB and Tm2O3 were put into a agate mortar and milled with pestle in a glove box at room temperature for 3 min. The mol ratio of AB:Tm was 1:1.

Results and discussion

PXRD spectra of the synthesized Tm(BTC) sample was well coincident with the simulated one showing the good purity of the synthesized sample (Figure S1). As Figure 1 showed, the PXRD pattern of AB@Tm(BTC)-CH3OH kept approximate agreement with that of Tm(BTC). It demonstrates that Tm(BTC) remains its main skeleton structure. No AB peaks appear in AB@Tm(BTC)-CH3OH shows that AB particles are very fine and well-distributed in the pores after AB loaded into Tm(BTC) by dipping. Compared with Tm(BTC), AB@Tm(BTC)-CH3OH presents small shift and split at 8.6 and 10.6 of 2-theta, and a shift from 19.4 to 19.0 of 2-theta, it is likely because of the interaction between AB molecules and unsaturated metal sites of


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Figure 1. PXRD patterns of neat AB (black), Tm(BTC) (red), AB@Tm(BTC)-CH3OH (blue) and AB@Tm(BTC)-milled (purple).

Figure 2. PXRD pattern of neat AB (black), Tm2O3 (red), milled AB@Tm2O3-milled (blue).

Tm(BTC) leads to tiny changing of framework. It will be proved by the solid-state

11

B NMR

results later. The PXRD spectra of AB@Tm(BTC)-milled excellently agrees with Tm(BTC) except two small diffraction peaks at 23.7 and 24.5 which are identified from AB. It shows an intact framework of Tm(BTC) and crystalline of AB. In Figure 2, the AB@Tm2O3-milled only


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shows the peaks of Tm2O3. It follows that the mixture is milled very fine and sufficiently distributed if AB does not decompose.

FTIR spectroscopy analyses are showed in Figure 3 and 4. By compared with Tm(BTC), both AB@Tm(BTC)-milled and AB@Tm(BTC)-CH3OH exhibit new bands at 3325, 2351, 1172, 1061 cm-1 attributed to H-N antisymmetric stretch, H-B antisymmetric stretch, H-B scissor and H wag modes as the same as AB (Figure 3).31 After heated at 150 °C for 1 h, all these bands disappear in the FTIR spectrogram of AB@Tm(BTC)-CH3OH. But for AB@Tm(BTC)-milled, the peak at 2351 cm-1 coming from H-B antisymmetric stretch still exists which indicates that the dehydrogenation is not completed. Continuing being heated at 150 °C for 1 h, this peak at 2351 cm-1 disappears showing that all the B-H bands have been broken. Figure 4 shows that new bands at 782 and 726 cm-1 attributable to the B-N stretch and H wag besides the same bands as above are observed after AB milled with Tm2O3. After heated at 150 °C for 1 h, H-N antisymmetric stretch, H-B scissor and H wag matching to the bands at 3325, 1172, 1061 and 726 cm-1 disappear, H-B antisymmetric stretch and B-N stretch matching to the bands at 2351 and 782 cm-1 still exist. It indicates that dehydrogenation is not completed and most B-N bond was not broken. After heated at 150 °C for 5 h, the H-B antisymmetric stretch peak at 2351 cm-1 is not observed showing that dehydrogenation is complete; the B-N stretch peak at 782 cm-1 is still observed which demonstrates that most B-N bonds are still not broken. After being heated, the wide peak at 3123 cm-1 could be attributed to N-H stretching.32 The shift of N-H stretching


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peak is due to the change from -NH3 to –NH after dehydrogenation. The peak at 1381 cm-1 which is identified to N-H deformation splits into three peaks after heated at 150 °C. And the small peak at 1255 cm-1 appears after heated at 150 °C also could be attributed to N-H deformation. The FTIR peaks at 1109, 1029 and 923 cm-1 are due to some unclear solid byproducts. Compared the AB@Tm(BTC)-CH3OH, AB@Tm(BTC)-milled and AB@Tm2O3, only AB/Tm(BTC)-CH3OH presents no H-B antisymmetric stretch at 2351 cm-1 after heated 1 hour, so that a probable idea can be given that the dehydrogenation rate of AB@Tm(BTC)-CH3OH might be faster than the other two samples.

Figure 3. FTIR spectrum of (a) neat AB, (b) Tm(BTC), (c) AB@Tm(BTC)-milled, (d) AB@Tm(BTC)-CH3OH, (e) AB@Tm(BTC)-milled after heated at 150 °C for 1 h (f) AB@Tm(BTC)-milled after heated at 150 °C for 2 h, (g) AB@Tm(BTC)-CH3OH after heated at 150 °C for 1 h.


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Figure 4. FTIR spectrum of (a) Neat AB, (b) Tm2O3, (c) AB@Tm2O3-milled, (d) AB@Tm2O3-milled after heated at 150 °C for 1 h, (e) AB@Tm2O3-milled after heated at 150 °C for 5 h.

The thermolysis temperature and the volatile products were investigated by TPD-MS. As it is known, under 200 °C, neat AB has two-step decomposition near its melting point (114 °C) and at ~150 °C. A large amount of hydrogen mixed with the unexpected byproducts borazine, ammonia, and diborane.6, 33 As shown in Figure 5, in all the three hybrids, the H2 releasing temperature is obviously reduced compared with neat AB. The onset temperature of dehydrogenation are about 60 °C [AB@Tm(BTC)-CH3OH], 65 °C [AB@Tm(BTC)-milled] and 80 °C (AB@Tm2O3-milled), and the peaks temperature of hydrogen signal are 77 °C [AB@Tm(BTC)-CH3OH], 79 °C [AB@Tm(BTC)-milled] and 85 °C (AB@Tm2O3-milled). The similarity of the three materials is containing metal Tm. So the improved effects probably come out of the existence of catalytic metal Tm. On the other hand, the AB@Tm(BTC) materials


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present lower dehydrogenation temperature than AB@Tm2O3 which demonstrates that the porous structure could be another factor to reduce the pyrolysis temperature of AB. So combining catalytic metal and porous structure is an effective way to reduce the dehydrogenation temperature of AB. For preventing the formation of volatile byproducts, AB@Tm(BTC)-CH3OH exhibits significant superiorities than the other two samples. Any byproduct is not found during the AB@Tm(BTC)-CH3OH pyrolysis process, but NH3 exists in the gas-state products during AB@Tm(BTC)-milled dehydrogenation. A possible reason is that the unsaturated coordinated Tm3+ of AB@Tm(BTC)-CH3OH as a Lewis acid site may interact with -NH3 in AB which has electron donor property. It is proved through solid-state

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B NMR of the samples (Figure S2).

The AB@Tm(BTC)-CH3OH shows a downfield chemical shift at -24.6 p.p.m. compared with neat AB (27.3 p.p.m.). As a result, N atoms are bound at the framework of Tm(BTC), so that there is no NH3 escaping during AB@Tm(BTC)-CH3OH thermolysis. On the contrary,

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B

resonance in the AB@Tm(BTC)-milled exhibits the same chemical shift at -27.3 p.p.m. as neat AB (Figure S2), it indicates that there is any interaction between AB and Tm(BTC). It attributes that AB particles could not touch and interact with the unsaturated coordinated metal Tm3+of Tm(BTC) by milling. Because the unsaturated coordinated metal Tm3+ of Tm(BTC) is inside of the pores, AB couldn’t enter into the pores of Tm(BTC) just by hand milled, but dispersed on the surface. The similar statement has been reported that there is no chemical interaction between AB and ZIF-8 by milling.25 So it is proposed that the unsaturated coordinated metal sites are the key factor to prevent NH3 emission during dehydrogenation in AB@MOF system.


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AB@Tm2O3-milled releases not only a small quantity of NH3 but also B3H6N3 during thermal decomposition. It should be attributed to the imporous structure, which could not confine AB as a chemical reaction guide to avoid the generation of B3H6N3 like porous structure.8,26

Figure

5.

TPD-MS

spectra

of

neat

AB

(black),

AB@Tm(BTC)-CH3OH

(red),

AB@Tm(BTC)-milled (blue) and AB@Tm2O3-milled (purple) respectively.

The differential scanning calorimetry (DSC) results of the samples are shown in Figure S3. Neat AB shows an endothermic dip at ∼110

C assigned to its melting and two exothermic

peaks associated with the hydrogen releasing at ~114

C and ~ 158

C, respectively.

AB@Tm(BTC)-CH3OH expresses only exothermic peak at 85 C, which is attributed to its


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thermal decomposition. AB@Tm(BTC)-milled expresses an exothermic peak at 89 C and a small peak at 110 C, which are attributed to its dehydrogenation. AB@Tm2O3-milled show a similar curve as neat AB.

Figure 6 shows the time dependence of dehydrogenation mass of the samples at 70 ºC, 80 ºC, 90 ºC and 100 ºC, AB@Tm(BTC)-CH3OH and the AB@Tm(BTC)-milled show much better dehydrogenation kinetics than that of neat AB. At 80 ºC, neat AB scarcely releases any H2. H2 uptake of AB@Tm(BTC)-CH3OH is 3.3 wt%, 7.4 wt%, 11.0 wt%, 12.7 wt% within 30 min and reach 10.9 wt%, 12.2 wt%,

12.5 wt%, 12.9 wt% in 360 min, respectively.

AB@Tm(BTC)-milled release 1.5 wt%, 2.3 wt%, 4.5 wt%, 5.7 wt% within 30 min and reach 4.7 wt%, 6.8 wt%, 7.6 wt%, 9.5 wt% in 360 min, respectively. H2 releasing rate from AB@Tm2O3-milled is much slower, however it is slightly improved compared with neat AB. The dehydrogenation activation energies have been calculated through Arrhenius equation. From the slope of the linear plot of lnk versus 1/T, the Arrhenius activation energies for H2 release from AB@Tm(BTC)-CH3OH, AB@Tm2O3-milled and AB@Tm2O3-milled are 98.1 kJ⋅mol-1, 103.1 kJ⋅mol-1 and 116.4 kJ⋅mol-1, respectively. They are less than that of pristine AB (135.0 kJ⋅mol-1)13 more or less. In view of the result above, we could infer that the porous structure, high surface area and unsaturated coordinated metal Tm3+ of Tm(BTC) are helpful for enhancing dehydrogenation kinetics of AB compared with Tm2O3


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Figure 6. Time dependence of dehydrogenation mass from AB@Tm(BTC)-CH3OH (a), AB@Tm(BTC)-milled (b) and AB@Tm2O3-milled (c) at 70, 80, 90 and 100 °C. (d) the linear plot of lnk versus 1/T of AB@Tm(BTC)-CH3OH (square), AB@Tm(BTC)-milled (circle) and AB@Tm2O3-milled (triangle).

Conclusions

In summary, three new AB-based composite materials were synthesized. The thermal dehydrogenation processes of these three materials were compared. Reasonable mechanisms for


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improving AB dehydrogenation properties by MOFs were proposed: (1) the synergistic effect of proper porous structure and metal catalysis of MOFs materials could improve the H2 releasing properties of AB;

(2) the porous structure contribute to inhibit borazine evolution from AB; (3)

the porous structure and the high surface area of MOFs are helpful for enhancing the dehydrogenation kinetics of AB; (4) the interaction between AB and the open unsaturated coordinated metal sites of MOFs plays a key role for elimination ammonia during AB thermolysis.

Acknowledgment. This work was supported by the National Natural Science Foundation of China (Frant No. 21101059), PhD Fund of Henan Polytechnic University (B2011-030).

Supporting Information Available: PXRD patterns of simulated Tm(BTC) (black) and synthesized Tm(BTC), Solid–state 11B NMR and DSC profiles. This information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]

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