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Ruthenium supported on MIL-96: An efficient catalyst for hydrolytic dehydrogenation of ammonia borane for chemical hydrogen storage
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 7 1 2 9 e1 7 1 3 5
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Ruthenium supported on MIL-96: An efficient catalyst for hydrolytic dehydrogenation of ammonia borane for chemical hydrogen storage Lan Wen a, Jun Su c, Xiaojun Wu a, Ping Cai a,*, Wei Luo a,b,*, Gongzhen Cheng a a
College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei 430072, PR China Suzhou Institute of Wuhan University, Suzhou, Jiangsu 215123, PR China c Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan, Hubei 430074, PR China b
article info
abstract
Article history:
For the first time, ultrafine Ru nanoparticles with mean diameter of 2 nm are successfully
Received 23 June 2014
deposited on MIL-96 by using a simple liquid impregnation strategy, and tested for catalytic
Received in revised form
hydrolysis of ammonia borane. The powder X-ray diffraction, N2 physical adsorption,
28 July 2014
transmission electron microscopy, energy-dispersive X-ray spectroscopy and inductively
Accepted 29 July 2014
coupled plasma-atomic emission spectroscopy measurements are employed to charac-
Available online 5 September 2014
terized the Ru/MIL-96 catalysts. Thanks to the unique 3D structure of MIL-96, Ru NPs supported on MIL-96 exhibit much enhanced catalytic activity compared with other
Keywords:
commercial
MIL-96
231 mol H2 min1 (mol Ru)1, which is among the highest value ever reported. Moreover,
supported
materials
and
graphene,
with
the
TOF
value
of
Ru nanoparticles
this simple method can be extended to facile synthesis of other MOFs supported mono-
Ammonia borane
metallic and polymetallic NPs for more application.
Hydrogen storage
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen is regarded as a globally accepted clean fuel due to its high efficiency and power density, however one of the biggest application obstacles for “hydrogen economy” is the safe and efficient storage of hydrogen [1]. Various hydrogen storage methods have been explored, including metal hydrides [2], sorbent materials [3], and chemical hydride systems [4]. Boronenitrogen containing compounds have attracted
much attention because of their high gravimetric hydrogen densities and favorable kinetics of hydrogen release [5]. Ammonia borane (NH3BH3, AB) has recently received a great interest, because of its high hydrogen content (19.6 wt%), high stability, and environmental benignity [6]. The release of hydrogen from AB could be obtained through either hydrolysis or pyrolysis. With an appropriate catalyst, the hydrolysis of AB can release as much as 3 mol of hydrogen per mol of AB (according to Eq. (1)) [7e9]. The catalysts tested for hydrolysis of AB were mainly nanoparticles (NPs). But the hydrolysis of AB
* Corresponding authors. College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei 430072, PR China. Tel.: þ86 2768752366. E-mail addresses: [email protected] (P. Cai), [email protected] (W. Luo). http://dx.doi.org/10.1016/j.ijhydene.2014.07.179 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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using transition metal catalysts at ambient conditions seems to be the most convenient route for the portable applications [10e15]. So far, a number of metal catalysts have been tested for the catalytic hydrolysis of AB, however, the development of highly active and efficient catalysts is still remained considerable challenge for the practical use of AB as a potential hydrogen storage material. catalyst
NH3 BH3 þ 2H2 O !NH4 BO2 þ 3H2
EC-300J(Triquo Chemical Co., Ltd.), neutral silica power (SiO2, Branch of Qingdao Haiyang Chemical Co., Ltd.). Aluminum oxide neutral (g-Al2O3, Sinopharm Chemical Reagent Co., Ltd., FCP), Graphene Oxide (GO) was synthesized using a modified Hummer's method [26]. We use ordinary distilled water as the reaction solvent.
Synthesis of MIL-96 (1)
Metaleorganic frameworks (MOFs) represent a new class of porous materials assembled with metal centers and organic ligands [16]. Owing to their high surface area, porosity, and chemical tunability, MOFs have been emerging as very promising materials for gas storage [17], heterogeneous catalysis [18], drug delivery [19], and molecular separation [20]. Given the similarity to zeolites, using MOFs as the support for metal nanoparticles (NPs) is expected to control the growth of metal NPs, and further increase their catalytic activities. Although a number of monometallic and bimetallic NPs (e.g., Ni, Pd, Au, Pt, Ir, AuCo, NiPt, AuPd, Au@Ag, AgPd, NiPd) supported on MOFs have been investigated as active heterogeneous catalysts [21,22], research about Ru NPs has been rarely reported. On the other hand, Ru is one of the most widely studied metal catalysts for catalytic hydrolysis of AB. There are a number of monometallic and bimetallic Ru-based catalysts supported on different materials (e.g., carbon black, Al2O3, carbon nanotube, zeolite, laurate, poly(4-styrenesulfonic acidco-maleic acid), graphene, poly(N-vinyl-2-pyrrolidone), TiO2) have been synthesized, and their catalytic activities toward hydrolysis of AB have been evaluated [23,24]. Herein, we report first MOF (MIL-96) supported Ru NPs as highperformance catalysts for hydrolysis of AB at room temperature by a simple liquid impregnation method. MIL-96, a new aluminum trimesate Al12O(OH)18(H2O)3(Al2(OH)4)[btc]6$24H2O was selected because of its unique 3D framework with three different types of cages, high thermal and chemical stability to water and common organic solvents [25]. Compared with other reported Ru-based catalysts, the Ru/MIL-96 shows the remarkable catalytic activity toward hydrolysis of ammonia borane at ambient temperature with turnover frequency (TOF) value of 231 mol H2 min1 (mol Ru)1.
Experimental Chemicals and materials All chemicals were commercial and used without further purification. Aluminum nitrate (Al(NO3)3$9H2O, Sinopharm Chemical Reagent Co., Ltd., 99%), trimethyl 1,3,5benzenetricarboxylate (C6H3(CO2CH3)3, 98%, Aldrich, noted Me3btc), sodium hydroxide (NaOH, Sinopharm Chemical Reagent Co., Ltd., 96%), ruthenium chloride hydrate (RuCl3$nH2O, Wuhan Greatwall Chemical Co., Ltd., 99%), ammonia borane (NH3BH3, AB, Aldrich, 90%), sodium borohydride (NaBH4, Sinopharm Chemical Reagent Co., Ltd., 96%), dimethylformamide (C3H7NO, Sinopharm Chemical Reagent Co., Ltd., 96%), ethanol (C2H5OH, Sinopharm Chemical Reagent Co., Ltd., >99.8%) were used as received. Ketjen black
MIL-96 was hydrothermally synthesized to follow the reported method [25]. Trimethyl 1,3,5-benzenetricarboxylate C6H3(CO2CH3)3 (328 mg), aluminum nitrate (Al(NO3)3$9H2O) (110 mg), sodium hydroxide (NaOH) (80 mg) and de-ionized water (5 mL) were placed in a 50 mL Teflon-liner autoclave and heated at 210 C for 12 h. After natural cooling, the suspension was centrifuged to separate the white powder of MIL96 with formula (Al12O(OH)18(H2O)3(Al2(OH)4)[btc]6$24H2O), and then further purified by solvothermal treatment in anhydrous dimethylformamide (DMF) at 150 C for 5 h. The separated resulting solid was refluxed overnight in a suspension of water at 100 C for 12 h and immediately filtered. The resulting white solid was finally dried overnight at 100 C for further use.
Synthesis of Ru/MIL-96 Activated MIL-96 (100 mg) was mixed with 5 mL de-ionized water containing 0.01, mmol RuCl3 for 12 h at room temperature. The solid was centrifuged and washed with de-ionized water and ethanol. The resulting mixture was then reduced with sodium borohydride (NaBH4, 0.0378 g) at 273 K for 3 h to yield Ru/MIL-96.
Hydrolytic dehydrogenation of ammonia borane by Ru/MIL-96 A mixture of 50 mg Ru/MIL-96 and 5 mL de-ionized water were kept in a two-necked round-bottom flask. One neck was connected to a pressure-equalization funnel to introduce 3 mL aqueous solution of NH3BH3 (30.8 mg, 1 mmol), and the other neck was connected to a gas burette to monitor the volume of the gas evolution. The reaction started when the aqueous solution was added to the catalyst, and the evolution of gas was monitored using the gas burette. Temperature varied from 25 to 40 C to obtained the activation energy (Ea). The hydrogen generation rates (r) are calculated by r ¼ V (generated H2, mL)/(T (time, min) n (the amount of the catalyst, mol)) from the slops of the fitting lines that are obtained from the curves of the generated hydrogen volume vs time at different temperatures (25, 30, 35 and 40 C) and below the hydrogen volume of 60 mL. Then, the rate constant (k) is determined by k ¼ k ¼ ðPðthe pressure; PaÞ rðthe rate; mL min1 mol1 Þ Þ=ð8:314 Tðthe temperature; KÞ Þ [27].
Synthesis of Ru supported on different materials and their catalytic toward hydrolytic dehydrogenation of ammonia borane The mixture of 0.0039 mmol RuCl3 and 50 mg different supported materials (carbon black, SiO2, g-Al2O3, GO) was kept in
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a two-necked round-bottom flask. First introduce 3 mL aqueous solution of NaBH4 (37.8 mg, 1 mmol) to reduce Ru3þ to Ru0, after the hydrogen generation reaction was completed, 3 mL aqueous solution of NH3BH3 (30.8 mg, 1 mmol) was added into the reaction flask. The evolution of gas was monitored by the gas burette. The reactions were carried out at 298 K in air.
Hydrolysis of ammonia borane catalyzed by MIL-96 Except for the replacement of 50 mg Ru/MIL-96 by 50 mg activated MIL-96, the experiment procedures were similar to that of 2.4.
Stability test 3 mL of solution containing 30.8 mg AB was added to 5 mL of water dissolved 50 mg Ru/MIL-96, the evolution of gas was monitored as described above. After the hydrogen generation reaction was completed, another equivalent of AB (30.8 mg, 1 mmol) was added into the reaction flask. The evolution of gas was monitored using the gas burette. Such cycle tests of the catalyst for the hydrolysis of AB were carried out five times in air.
Characterization The morphologies and sizes of the samples were observed by using a Tecnai G20 U-Twin transmission electron microscope (TEM) equipped with an energy dispersive X-ray detector (EDX) at an acceleration voltage of 200 kV. Powder X-ray diffraction (XRD) patterns were measured by a Bruker D8Advance X-ray diffractometer using Cu Ka radiation source (l ¼ 0.154178 nm) with a velocity of 1 min1. The surface area measurements were performed with N2 adsorption/desorption isotherms at liquid nitrogen temperature (77 K) after dehydration under vacuum at 150 C for 18 h using Quantachrome NOVA 4200e. The inductively coupled plasmaatomic emission spectroscopy (ICP-AES) was performed on IRIS Intrepid II XSP (Thermo Fisher Scientific, USA).
Fig. 1 e Powder X-ray diffraction patterns of samples.
and the ultrafine Ru NPs (vide infra). The adsorptionedesorption isotherms of MIL-96 and Ru/MIL-96 were shown in Fig. 2. The specific areas of MIL-96 and Ru/MIL-96 were 263.8 and 211.2 m2/g respectively. The decrease in the amount of N2 adsorption and the pore volume (Fig. S1) of Ru/ MIL-96 indicates that the pores of MIL-96 were either occupied by the well dispersed Ru NPs or blocked by the Ru NPs. The morphologies of MIL-96 supported Ru NPs were further characterized by transmission electron microscopic (TEM), energy-dispersive X-ray spectroscopy (EDX) measurements and selected area electron diffraction (SAED) pattern. TEM images of Ru/MIL-96 (Fig. 3a, b) indicate the Ru NPs are well dispersed with mean diameter of 2 nm (Fig. S2), which are too large to get into the pores of MIL-96. Thus, most of Ru NPs are distributed on the outer surface of MIL-96, which demonstrates well that black spots are on the needle shape [28]. However, it could not preclude that there are still some smaller Ru NPs (<1 nm) which difficult to be identified by HRTEM imaging embedded within the frameworks [29]. The EDX spectra further confirmed the presence of Ru (Fig. S3). To investigate the microstructure and further confirm the crystallinity of the as-synthesized Ru/MIL-96 catalyst, we have applied SAED on the Ru/MIL-96 sample. The related SAED
Results and discussion MIL-96 was synthesized according to the literature with some improvement, followed by treatment with DMF and distilled water to remove the unreacted trimethyl 1,3,5benzenetricarboxylate. The supported Ru/MIL-96 catalyst was prepared through solution infiltration of activated MIL-96 with RuCl3, followed by treatment with NaBH4. The final Ru loading in the catalyst was determined as 0.79 wt% based on the inductively coupled plasma atomic emission spectroscopic (ICP-AES), which is smaller than the theoretical value. The powder X-ray diffractions (PXRD) of MIL-96 pattern agree well with the previously reported one [25]. The morphology of the MIL-96 is very homogeneous, representing the purity of the phase. Also, Ru/MIL-96 exhibit no loss of crystallinity (Fig. 1), indicating that the integrity of the MIL-96 framework is maintained well during the catalyst preparation. Furthermore, no significant diffraction peaks of Ru were detected from the PXRD, probably as a result of low ruthenium loading
Fig. 2 e N2 sorption isotherms of activated MIL-96 and Ru/ MIL-6 Filled and open symbols represent adsorption and desorption branches, respectively.
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Fig. 3 e (a-b) TEM images of Ru/MIL-96 and (b) inset: the corresponding selected area electron diffraction (SAED) pattern. (c-d) TEM images of Ru/MIL-96 after five runs of catalytic hydrolysis of AB.
pattern (Fig. 3(b), inset) for the Ru/MIL-96 sample demonstrates the diffuse diffraction rings of Ru, which means Ru is polycrystalline. The catalytic activity of Ru supported on different materials toward hydrolysis of AB were studied. As shown in Fig. 4, Ru/MIL-96 exhibits the highest catalytic activity, with the TOF value of 231 mol H2 min1 (mol Ru)1, this value is among the highest value ever reported (Table 1). A plausible mechanism of the hydrolysis reaction involves two steps. Firstly, AB interacts with the surface of Ru/MIL-96 to form the activated transient RueH, which is the prerequisite for the hydrolysis reaction. Then, H2 was released after the attack of water on the RueH species [30,31]. Moreover, Ru NPs without supported materials and MIL-96 without Ru loading were synthesized and applied to hydrolysis of AB, only 2.5 equiv. H2 were released for more than 20 min for Ru NPs, and almost no reactivity for MIL-96. These results indicate that MIL-96 does not involved in the hydrolysis reaction of AB, it provides a 3D framework for Ru NPs to adsorb and therefore enhances the reactivity of Ru NPs. In order to get the activation energy (Ea) of the AB hydrolysis catalyzed by Ru/MIL-96, the hydrolytic experiments at different temperatures ranged from 25 to 40 C were carried out. The values of rate constant k at different temperatures were calculated from the slope of the linear part of each plot from Fig. 5a. The Arrhenius plot of ln k vs. 1/T for the catalyst is
plotted in Fig. 5b, from which the apparent activation energy was determined to be approximately 47.7 kJ/mol for AB, which was close to the reported values (Table 1). The stability of the catalyst is crucial in the practical application. The stability of Ru/MIL-96 catalyst for hydrolysis
Fig. 4 e Hydrogen generation from the hydrolysis of AB catalyzed by Ru/MIL-96, Ru/C black, Ru/SiO2, Ru/GO, Ru/gAl2O3, Ru and MIL-96 (Ru/AB ¼ 0.0039).
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Table 1 e Catalytic activity of Ru-based and some noble metal catalysts used for the hydrolytic dehydrogenation of AB. Catalyst
TOF(mol H2 mol1 metal min1)
Ea (kJ mol1)
429.5 329 308
34.81 33 56.3
[33] [34] [35]
241 231 200 187.6 100 92 83.3 77 75 55.4 45 40.5 39.9 39.6 30.6 29.4 29.1 26.3 16.5 15.9 8.2
70 47.7 e 54 11.7 e 46 23 47 e e e 36.6 48 44 37.9 21.2 40 47 30.4 52
[24] This study [36] [37] [23] [38] [39] [40] [41] [42] [43] [44] [45] [39] [46] [47] [47] [48] [49] [50] [48]
Ru/C Ru(0)@MWCNT poly(N-vinyl-2-pyrrolidone)-protected PtRu nanoparticles Ru(0)/TiO2 Ru(0)/MIL-96 Laurate-stabilized Rh(0) nanoclusters PSSA-co-MA stabilized Ru nanoclusters Ru/graphene Zeolite-Rh(0) NCs Ru@Al2O3 after acetic acid treatment Ru/g-Al2O3 Laurate-stabilized Ru(0)nanoclusters Pt/C 4 wt%Pd@MIL-101 Ru@Co/graphene Ru@Ni/graphene Ru@Al2O3 Ni@Ru Ru@Ni/C black Ru@Co/C black RGO-Pd NPs RuCo (1:1)/g-Al2O3 RuCu/graphene RuCu (1:1)/g-Al2O3
of AB was tested by adding another equivalents of AB after the previous hydrogen generation reaction was complete. As shown in Fig. 6, the as-synthesized Ru/MIL-96 catalysts retain about 65% of their initial catalytic activity toward hydrolysis of AB in the fifth run. The PXRD of Ru/MIL-96 after 5th cycle exhibits no loss of crystallinity (Fig. 1), indicating that the integrity of the MIL-96 framework is maintained well during the catalytic process. The representative TEM images of Ru/ MIL-96 catalyst after the fifth run stability test were shown in Fig. 3c, d. It was seen that the Ru NPs are already aggregated on the surface of MIL-96. Therefore, the slight decrease of the catalytic activity might be caused by the little agglomeration of Ru NPs. Additionally, the increase viscosity of the solution during the hydrolysis of AB should also be taken into account
(a) 3.5
[32]. Further study on enhancement the stability and recyclability of the present catalyst for the hydrolysis of AB is underway.
Conclusion In summary, ultrafine Ru NPs have been well dispersed on MIL-96, which exhibit highly catalytic activity toward hydrolysis of AB at ambient conditions. This is the first example of MOF-supported Ru catalysts for catalytic hydrolysis of AB. Thanks to the unique 3D framework, the catalytic activity of Ru NPs have been enhanced significantly after loading on MIL96, which exhibit the highest catalytic activity compared with
(b)
3.0
T= 25 C T= 30 C
1.5
T= 35 C T= 40 C
1.0
1.0 0.8
Ink = - 5741.54 (1/T) + 19.18
0.6
Activation Energy Ea = 47.7 KJ mol-1
0.4 0.2 0.0
0.5 0.0 0.0
Ink
n (H )/n(A B )
2.5 2.0
Ref.
-0.2 0.5
1.0
1.5 2.0 2.5 time (min)
3.0
3.5
4.0
0.00320
0.00324
0.00328
0.00332
0.00336
1/T
Fig. 5 e (a) Time course plots for hydrogen generation by the decomposition of AB by Ru/MIL-96 at different temperatures. (b) Plot of ln k versus 1/T during the AB decomposition over Ru/MIL-96 at different temperatures.
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Fig. 6 e (a) Stability test for the hydrogen generation from aqueous AB in the presence of Ru/MIL-96 catalyst (Ru/AB molar ratio ¼ 0.0039); (b) Percentage of initial catalytic activity of Ru/MIL-96 in successive runs after the reuse for the hydrolysis of AB.
other commercial supported materials and graphene, with the TOF value of 231 mol H2 min1 (mol Ru)1. This simple synthetic method can be extended to other MOFs supported metal NP systems for more application.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (21201134), the Ph. D Programs Foundation of Ministry of Education of China (20120141120034), the Natural Science Foundation of Jiangsu Province (BK20130370), the Natural Science Foundation of Hubei Province (2013CFB288), Ministry of Science and Technology of China (2011YQ12003504) and Large-scale Instrument and Equipment Sharing Foundation of Wuhan University.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2014.07.179.
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Available online at www.sciencedirect.com
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Ruthenium supported on MIL-96: An efficient catalyst for hydrolytic dehydrogenation of ammonia borane for chemical hydrogen storage Lan Wen a, Jun Su c, Xiaojun Wu a, Ping Cai a,*, Wei Luo a,b,*, Gongzhen Cheng a a
College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei 430072, PR China Suzhou Institute of Wuhan University, Suzhou, Jiangsu 215123, PR China c Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan, Hubei 430074, PR China b
article info
abstract
Article history:
For the first time, ultrafine Ru nanoparticles with mean diameter of 2 nm are successfully
Received 23 June 2014
deposited on MIL-96 by using a simple liquid impregnation strategy, and tested for catalytic
Received in revised form
hydrolysis of ammonia borane. The powder X-ray diffraction, N2 physical adsorption,
28 July 2014
transmission electron microscopy, energy-dispersive X-ray spectroscopy and inductively
Accepted 29 July 2014
coupled plasma-atomic emission spectroscopy measurements are employed to charac-
Available online 5 September 2014
terized the Ru/MIL-96 catalysts. Thanks to the unique 3D structure of MIL-96, Ru NPs supported on MIL-96 exhibit much enhanced catalytic activity compared with other
Keywords:
commercial
MIL-96
231 mol H2 min1 (mol Ru)1, which is among the highest value ever reported. Moreover,
supported
materials
and
graphene,
with
the
TOF
value
of
Ru nanoparticles
this simple method can be extended to facile synthesis of other MOFs supported mono-
Ammonia borane
metallic and polymetallic NPs for more application.
Hydrogen storage
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen is regarded as a globally accepted clean fuel due to its high efficiency and power density, however one of the biggest application obstacles for “hydrogen economy” is the safe and efficient storage of hydrogen [1]. Various hydrogen storage methods have been explored, including metal hydrides [2], sorbent materials [3], and chemical hydride systems [4]. Boronenitrogen containing compounds have attracted
much attention because of their high gravimetric hydrogen densities and favorable kinetics of hydrogen release [5]. Ammonia borane (NH3BH3, AB) has recently received a great interest, because of its high hydrogen content (19.6 wt%), high stability, and environmental benignity [6]. The release of hydrogen from AB could be obtained through either hydrolysis or pyrolysis. With an appropriate catalyst, the hydrolysis of AB can release as much as 3 mol of hydrogen per mol of AB (according to Eq. (1)) [7e9]. The catalysts tested for hydrolysis of AB were mainly nanoparticles (NPs). But the hydrolysis of AB
* Corresponding authors. College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei 430072, PR China. Tel.: þ86 2768752366. E-mail addresses: [email protected] (P. Cai), [email protected] (W. Luo). http://dx.doi.org/10.1016/j.ijhydene.2014.07.179 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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using transition metal catalysts at ambient conditions seems to be the most convenient route for the portable applications [10e15]. So far, a number of metal catalysts have been tested for the catalytic hydrolysis of AB, however, the development of highly active and efficient catalysts is still remained considerable challenge for the practical use of AB as a potential hydrogen storage material. catalyst
NH3 BH3 þ 2H2 O !NH4 BO2 þ 3H2
EC-300J(Triquo Chemical Co., Ltd.), neutral silica power (SiO2, Branch of Qingdao Haiyang Chemical Co., Ltd.). Aluminum oxide neutral (g-Al2O3, Sinopharm Chemical Reagent Co., Ltd., FCP), Graphene Oxide (GO) was synthesized using a modified Hummer's method [26]. We use ordinary distilled water as the reaction solvent.
Synthesis of MIL-96 (1)
Metaleorganic frameworks (MOFs) represent a new class of porous materials assembled with metal centers and organic ligands [16]. Owing to their high surface area, porosity, and chemical tunability, MOFs have been emerging as very promising materials for gas storage [17], heterogeneous catalysis [18], drug delivery [19], and molecular separation [20]. Given the similarity to zeolites, using MOFs as the support for metal nanoparticles (NPs) is expected to control the growth of metal NPs, and further increase their catalytic activities. Although a number of monometallic and bimetallic NPs (e.g., Ni, Pd, Au, Pt, Ir, AuCo, NiPt, AuPd, Au@Ag, AgPd, NiPd) supported on MOFs have been investigated as active heterogeneous catalysts [21,22], research about Ru NPs has been rarely reported. On the other hand, Ru is one of the most widely studied metal catalysts for catalytic hydrolysis of AB. There are a number of monometallic and bimetallic Ru-based catalysts supported on different materials (e.g., carbon black, Al2O3, carbon nanotube, zeolite, laurate, poly(4-styrenesulfonic acidco-maleic acid), graphene, poly(N-vinyl-2-pyrrolidone), TiO2) have been synthesized, and their catalytic activities toward hydrolysis of AB have been evaluated [23,24]. Herein, we report first MOF (MIL-96) supported Ru NPs as highperformance catalysts for hydrolysis of AB at room temperature by a simple liquid impregnation method. MIL-96, a new aluminum trimesate Al12O(OH)18(H2O)3(Al2(OH)4)[btc]6$24H2O was selected because of its unique 3D framework with three different types of cages, high thermal and chemical stability to water and common organic solvents [25]. Compared with other reported Ru-based catalysts, the Ru/MIL-96 shows the remarkable catalytic activity toward hydrolysis of ammonia borane at ambient temperature with turnover frequency (TOF) value of 231 mol H2 min1 (mol Ru)1.
Experimental Chemicals and materials All chemicals were commercial and used without further purification. Aluminum nitrate (Al(NO3)3$9H2O, Sinopharm Chemical Reagent Co., Ltd., 99%), trimethyl 1,3,5benzenetricarboxylate (C6H3(CO2CH3)3, 98%, Aldrich, noted Me3btc), sodium hydroxide (NaOH, Sinopharm Chemical Reagent Co., Ltd., 96%), ruthenium chloride hydrate (RuCl3$nH2O, Wuhan Greatwall Chemical Co., Ltd., 99%), ammonia borane (NH3BH3, AB, Aldrich, 90%), sodium borohydride (NaBH4, Sinopharm Chemical Reagent Co., Ltd., 96%), dimethylformamide (C3H7NO, Sinopharm Chemical Reagent Co., Ltd., 96%), ethanol (C2H5OH, Sinopharm Chemical Reagent Co., Ltd., >99.8%) were used as received. Ketjen black
MIL-96 was hydrothermally synthesized to follow the reported method [25]. Trimethyl 1,3,5-benzenetricarboxylate C6H3(CO2CH3)3 (328 mg), aluminum nitrate (Al(NO3)3$9H2O) (110 mg), sodium hydroxide (NaOH) (80 mg) and de-ionized water (5 mL) were placed in a 50 mL Teflon-liner autoclave and heated at 210 C for 12 h. After natural cooling, the suspension was centrifuged to separate the white powder of MIL96 with formula (Al12O(OH)18(H2O)3(Al2(OH)4)[btc]6$24H2O), and then further purified by solvothermal treatment in anhydrous dimethylformamide (DMF) at 150 C for 5 h. The separated resulting solid was refluxed overnight in a suspension of water at 100 C for 12 h and immediately filtered. The resulting white solid was finally dried overnight at 100 C for further use.
Synthesis of Ru/MIL-96 Activated MIL-96 (100 mg) was mixed with 5 mL de-ionized water containing 0.01, mmol RuCl3 for 12 h at room temperature. The solid was centrifuged and washed with de-ionized water and ethanol. The resulting mixture was then reduced with sodium borohydride (NaBH4, 0.0378 g) at 273 K for 3 h to yield Ru/MIL-96.
Hydrolytic dehydrogenation of ammonia borane by Ru/MIL-96 A mixture of 50 mg Ru/MIL-96 and 5 mL de-ionized water were kept in a two-necked round-bottom flask. One neck was connected to a pressure-equalization funnel to introduce 3 mL aqueous solution of NH3BH3 (30.8 mg, 1 mmol), and the other neck was connected to a gas burette to monitor the volume of the gas evolution. The reaction started when the aqueous solution was added to the catalyst, and the evolution of gas was monitored using the gas burette. Temperature varied from 25 to 40 C to obtained the activation energy (Ea). The hydrogen generation rates (r) are calculated by r ¼ V (generated H2, mL)/(T (time, min) n (the amount of the catalyst, mol)) from the slops of the fitting lines that are obtained from the curves of the generated hydrogen volume vs time at different temperatures (25, 30, 35 and 40 C) and below the hydrogen volume of 60 mL. Then, the rate constant (k) is determined by k ¼ k ¼ ðPðthe pressure; PaÞ rðthe rate; mL min1 mol1 Þ Þ=ð8:314 Tðthe temperature; KÞ Þ [27].
Synthesis of Ru supported on different materials and their catalytic toward hydrolytic dehydrogenation of ammonia borane The mixture of 0.0039 mmol RuCl3 and 50 mg different supported materials (carbon black, SiO2, g-Al2O3, GO) was kept in
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a two-necked round-bottom flask. First introduce 3 mL aqueous solution of NaBH4 (37.8 mg, 1 mmol) to reduce Ru3þ to Ru0, after the hydrogen generation reaction was completed, 3 mL aqueous solution of NH3BH3 (30.8 mg, 1 mmol) was added into the reaction flask. The evolution of gas was monitored by the gas burette. The reactions were carried out at 298 K in air.
Hydrolysis of ammonia borane catalyzed by MIL-96 Except for the replacement of 50 mg Ru/MIL-96 by 50 mg activated MIL-96, the experiment procedures were similar to that of 2.4.
Stability test 3 mL of solution containing 30.8 mg AB was added to 5 mL of water dissolved 50 mg Ru/MIL-96, the evolution of gas was monitored as described above. After the hydrogen generation reaction was completed, another equivalent of AB (30.8 mg, 1 mmol) was added into the reaction flask. The evolution of gas was monitored using the gas burette. Such cycle tests of the catalyst for the hydrolysis of AB were carried out five times in air.
Characterization The morphologies and sizes of the samples were observed by using a Tecnai G20 U-Twin transmission electron microscope (TEM) equipped with an energy dispersive X-ray detector (EDX) at an acceleration voltage of 200 kV. Powder X-ray diffraction (XRD) patterns were measured by a Bruker D8Advance X-ray diffractometer using Cu Ka radiation source (l ¼ 0.154178 nm) with a velocity of 1 min1. The surface area measurements were performed with N2 adsorption/desorption isotherms at liquid nitrogen temperature (77 K) after dehydration under vacuum at 150 C for 18 h using Quantachrome NOVA 4200e. The inductively coupled plasmaatomic emission spectroscopy (ICP-AES) was performed on IRIS Intrepid II XSP (Thermo Fisher Scientific, USA).
Fig. 1 e Powder X-ray diffraction patterns of samples.
and the ultrafine Ru NPs (vide infra). The adsorptionedesorption isotherms of MIL-96 and Ru/MIL-96 were shown in Fig. 2. The specific areas of MIL-96 and Ru/MIL-96 were 263.8 and 211.2 m2/g respectively. The decrease in the amount of N2 adsorption and the pore volume (Fig. S1) of Ru/ MIL-96 indicates that the pores of MIL-96 were either occupied by the well dispersed Ru NPs or blocked by the Ru NPs. The morphologies of MIL-96 supported Ru NPs were further characterized by transmission electron microscopic (TEM), energy-dispersive X-ray spectroscopy (EDX) measurements and selected area electron diffraction (SAED) pattern. TEM images of Ru/MIL-96 (Fig. 3a, b) indicate the Ru NPs are well dispersed with mean diameter of 2 nm (Fig. S2), which are too large to get into the pores of MIL-96. Thus, most of Ru NPs are distributed on the outer surface of MIL-96, which demonstrates well that black spots are on the needle shape [28]. However, it could not preclude that there are still some smaller Ru NPs (<1 nm) which difficult to be identified by HRTEM imaging embedded within the frameworks [29]. The EDX spectra further confirmed the presence of Ru (Fig. S3). To investigate the microstructure and further confirm the crystallinity of the as-synthesized Ru/MIL-96 catalyst, we have applied SAED on the Ru/MIL-96 sample. The related SAED
Results and discussion MIL-96 was synthesized according to the literature with some improvement, followed by treatment with DMF and distilled water to remove the unreacted trimethyl 1,3,5benzenetricarboxylate. The supported Ru/MIL-96 catalyst was prepared through solution infiltration of activated MIL-96 with RuCl3, followed by treatment with NaBH4. The final Ru loading in the catalyst was determined as 0.79 wt% based on the inductively coupled plasma atomic emission spectroscopic (ICP-AES), which is smaller than the theoretical value. The powder X-ray diffractions (PXRD) of MIL-96 pattern agree well with the previously reported one [25]. The morphology of the MIL-96 is very homogeneous, representing the purity of the phase. Also, Ru/MIL-96 exhibit no loss of crystallinity (Fig. 1), indicating that the integrity of the MIL-96 framework is maintained well during the catalyst preparation. Furthermore, no significant diffraction peaks of Ru were detected from the PXRD, probably as a result of low ruthenium loading
Fig. 2 e N2 sorption isotherms of activated MIL-96 and Ru/ MIL-6 Filled and open symbols represent adsorption and desorption branches, respectively.
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Fig. 3 e (a-b) TEM images of Ru/MIL-96 and (b) inset: the corresponding selected area electron diffraction (SAED) pattern. (c-d) TEM images of Ru/MIL-96 after five runs of catalytic hydrolysis of AB.
pattern (Fig. 3(b), inset) for the Ru/MIL-96 sample demonstrates the diffuse diffraction rings of Ru, which means Ru is polycrystalline. The catalytic activity of Ru supported on different materials toward hydrolysis of AB were studied. As shown in Fig. 4, Ru/MIL-96 exhibits the highest catalytic activity, with the TOF value of 231 mol H2 min1 (mol Ru)1, this value is among the highest value ever reported (Table 1). A plausible mechanism of the hydrolysis reaction involves two steps. Firstly, AB interacts with the surface of Ru/MIL-96 to form the activated transient RueH, which is the prerequisite for the hydrolysis reaction. Then, H2 was released after the attack of water on the RueH species [30,31]. Moreover, Ru NPs without supported materials and MIL-96 without Ru loading were synthesized and applied to hydrolysis of AB, only 2.5 equiv. H2 were released for more than 20 min for Ru NPs, and almost no reactivity for MIL-96. These results indicate that MIL-96 does not involved in the hydrolysis reaction of AB, it provides a 3D framework for Ru NPs to adsorb and therefore enhances the reactivity of Ru NPs. In order to get the activation energy (Ea) of the AB hydrolysis catalyzed by Ru/MIL-96, the hydrolytic experiments at different temperatures ranged from 25 to 40 C were carried out. The values of rate constant k at different temperatures were calculated from the slope of the linear part of each plot from Fig. 5a. The Arrhenius plot of ln k vs. 1/T for the catalyst is
plotted in Fig. 5b, from which the apparent activation energy was determined to be approximately 47.7 kJ/mol for AB, which was close to the reported values (Table 1). The stability of the catalyst is crucial in the practical application. The stability of Ru/MIL-96 catalyst for hydrolysis
Fig. 4 e Hydrogen generation from the hydrolysis of AB catalyzed by Ru/MIL-96, Ru/C black, Ru/SiO2, Ru/GO, Ru/gAl2O3, Ru and MIL-96 (Ru/AB ¼ 0.0039).
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Table 1 e Catalytic activity of Ru-based and some noble metal catalysts used for the hydrolytic dehydrogenation of AB. Catalyst
TOF(mol H2 mol1 metal min1)
Ea (kJ mol1)
429.5 329 308
34.81 33 56.3
[33] [34] [35]
241 231 200 187.6 100 92 83.3 77 75 55.4 45 40.5 39.9 39.6 30.6 29.4 29.1 26.3 16.5 15.9 8.2
70 47.7 e 54 11.7 e 46 23 47 e e e 36.6 48 44 37.9 21.2 40 47 30.4 52
[24] This study [36] [37] [23] [38] [39] [40] [41] [42] [43] [44] [45] [39] [46] [47] [47] [48] [49] [50] [48]
Ru/C Ru(0)@MWCNT poly(N-vinyl-2-pyrrolidone)-protected PtRu nanoparticles Ru(0)/TiO2 Ru(0)/MIL-96 Laurate-stabilized Rh(0) nanoclusters PSSA-co-MA stabilized Ru nanoclusters Ru/graphene Zeolite-Rh(0) NCs Ru@Al2O3 after acetic acid treatment Ru/g-Al2O3 Laurate-stabilized Ru(0)nanoclusters Pt/C 4 wt%Pd@MIL-101 Ru@Co/graphene Ru@Ni/graphene Ru@Al2O3 Ni@Ru Ru@Ni/C black Ru@Co/C black RGO-Pd NPs RuCo (1:1)/g-Al2O3 RuCu/graphene RuCu (1:1)/g-Al2O3
of AB was tested by adding another equivalents of AB after the previous hydrogen generation reaction was complete. As shown in Fig. 6, the as-synthesized Ru/MIL-96 catalysts retain about 65% of their initial catalytic activity toward hydrolysis of AB in the fifth run. The PXRD of Ru/MIL-96 after 5th cycle exhibits no loss of crystallinity (Fig. 1), indicating that the integrity of the MIL-96 framework is maintained well during the catalytic process. The representative TEM images of Ru/ MIL-96 catalyst after the fifth run stability test were shown in Fig. 3c, d. It was seen that the Ru NPs are already aggregated on the surface of MIL-96. Therefore, the slight decrease of the catalytic activity might be caused by the little agglomeration of Ru NPs. Additionally, the increase viscosity of the solution during the hydrolysis of AB should also be taken into account
(a) 3.5
[32]. Further study on enhancement the stability and recyclability of the present catalyst for the hydrolysis of AB is underway.
Conclusion In summary, ultrafine Ru NPs have been well dispersed on MIL-96, which exhibit highly catalytic activity toward hydrolysis of AB at ambient conditions. This is the first example of MOF-supported Ru catalysts for catalytic hydrolysis of AB. Thanks to the unique 3D framework, the catalytic activity of Ru NPs have been enhanced significantly after loading on MIL96, which exhibit the highest catalytic activity compared with
(b)
3.0
T= 25 C T= 30 C
1.5
T= 35 C T= 40 C
1.0
1.0 0.8
Ink = - 5741.54 (1/T) + 19.18
0.6
Activation Energy Ea = 47.7 KJ mol-1
0.4 0.2 0.0
0.5 0.0 0.0
Ink
n (H )/n(A B )
2.5 2.0
Ref.
-0.2 0.5
1.0
1.5 2.0 2.5 time (min)
3.0
3.5
4.0
0.00320
0.00324
0.00328
0.00332
0.00336
1/T
Fig. 5 e (a) Time course plots for hydrogen generation by the decomposition of AB by Ru/MIL-96 at different temperatures. (b) Plot of ln k versus 1/T during the AB decomposition over Ru/MIL-96 at different temperatures.
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Fig. 6 e (a) Stability test for the hydrogen generation from aqueous AB in the presence of Ru/MIL-96 catalyst (Ru/AB molar ratio ¼ 0.0039); (b) Percentage of initial catalytic activity of Ru/MIL-96 in successive runs after the reuse for the hydrolysis of AB.
other commercial supported materials and graphene, with the TOF value of 231 mol H2 min1 (mol Ru)1. This simple synthetic method can be extended to other MOFs supported metal NP systems for more application.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (21201134), the Ph. D Programs Foundation of Ministry of Education of China (20120141120034), the Natural Science Foundation of Jiangsu Province (BK20130370), the Natural Science Foundation of Hubei Province (2013CFB288), Ministry of Science and Technology of China (2011YQ12003504) and Large-scale Instrument and Equipment Sharing Foundation of Wuhan University.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2014.07.179.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 7 1 2 9 e1 7 1 3 5
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