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Metal-organic frameworks (MOFs) and MOFs-derived [email protected] for hydrogen generation from sodium borohydride
international journal of hydrogen energy xxx (xxxx) xxx
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Metal-organic frameworks (MOFs) and MOFsderived CuO@C for hydrogen generation from sodium borohydride Ahlam Azzam Kassem a, Hani Nasser Abdelhamid a,b,*, Dina M. Fouad a, Said A. Ibrahim a,** a b
Department of Chemistry, Assiut University, Assiut, 71516, Egypt Advanced Multifunctional Materials Laboratory, Department of Chemistry, Assiut University, Assiut, 71516, Egypt
highlights Synthesis of 2D copper terephthalate frameworks (CuBDC). Synthesis of CuO@C derived from CuBDC. Hydrogen generation via sodium borohydride hydrolysis. 1 Achieve high hydrogen generation rate of 7620 ml H2$g1 cat min .
article info
abstract
Article history:
Hydrogen gas has been considered as one of the promising sources of energy. Thus, several
Received 20 August 2019
strategies including the hydrolysis of hydrides have been reported for hydrogen produc-
Received in revised form
tion. However, effective catalysts are highly required to improve the hydrogen generation
23 September 2019
rate. Two dimensional metal-organic frameworks (copper-benzene-1,4-dicarboxylic,
Accepted 8 October 2019
CuBDC), and CuBDC-derived CuO@C were synthesized, characterized and applied as cat-
Available online xxx
alysts for hydrogen production using the hydrolysis and methanolysis of sodium borohydride (NaBH4). CuBDC, and CuO@C display hydrogen generation rate of 7620, and 7240
Keywords:
1 mlH2$g1 cat$ min , respectively for hydrolysis. While, CuBDC offers hydrogen generation
Metal organic frameworks
1 for methanolysis. Both catalysts required short reaction time, rate of 9060 mlH2$g1 cat$ min
CuBDC
and showed good recyclability. The materials may open new venues for efficient catalyst
Hydrogen production
for energy-based applications.
Sodium borohydride hydrolysis
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Catalysis
Introduction Hydrogen gas is promising energy source due to its high energy density (142 MJ/kg), and its environmentally friendly
byproduct (water) [1]. It can be generated via several methods, including water splitting or oxidation [2,3] (electrolysis [4e8], thermolysis, and photocatalytic water splitting [9]), and using phototrophic microorganisms (biohydrogen, BioH2) [10]. Hydrolysis of hydrides, such as sodium borohydride (NaBH4), is
* Corresponding author. Department of Chemistry, Assiut University, Assiut, 71516, Egypt. ** Corresponding author. E-mail addresses: [email protected] (H.N. Abdelhamid), [email protected] (S.A. Ibrahim). https://doi.org/10.1016/j.ijhydene.2019.10.047 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Kassem AA et al., Metal-organic frameworks (MOFs) and MOFs-derived CuO@C for hydrogen generation from sodium borohydride, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.047
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promising for supplying hydrogen to end users on demand [11]. The process is suitable for portable and on-site hydrogen fuel systems including unmanned air vehicles, PEMFC (proton exchange membrane fuel cells), direct borohydride fuel cells (DBFCs), and low-temperature fuel-cell applications. The process is safe and efficient for hydrogen production. It generates hydrogen with relatively high capacity at room temperature. Hydrolysis of NaBH4 has relatively high hydrogen capacity (10.8 wt%), and releases hydrogen with high purity using a controllable process. The byproduct of hydrolysis; NaBO2, is also a promising candidate material for solid-state hydrogen storage. Thus, several heterogeneous and homogenous catalysts were reported for hydrogen generation via hydrolysis and alcoholysis of aqueous NaBH4 solution [1]. However, the process requires sometime expensive metals such as ruthenium and platinum as catalysts [1]. Most of the current catalysts lack of stability and high efficiency [12,13]. Thus, cheap, effective, and multi-tasks catalyst is highly required. Metaleorganic frameworks (MOFs) are porous materials with tunable pore size, and high surface areas [14e22]. They have gained wide interests in various applications including catalysis, sensing and biosensing, adsorption, devices, medicine, and drug delivery [23e31]. MOFs have been applied as catalysts for photocatalytic H2 evolution from water [32]. MOFs offer many advantages such as large surface area, large pore volume, low density, numerous crystal structures, can be easily functionalized with different reagents and conjugated with nanomaterials, and can be implemented in devices. Thus, they can be effective materials for hydrogen production via the hydrolysis of NaBH4. Herein, copper-organic frameworks, copper-benzene-1,4dicarboxylate (CuBDC), and CuBDC-derived CuO embedded on carbon (CuO@C) were synthesized and applied for hydrogen generation from the hydrolysis and methanolysis of NaBH4. It is the first time to use these materials for hydrogen generation from the hydrolysis of NaBH4. Our materials showed high catalytic performance with outstanding H2 generation rate of 1 7620, and 7240 mlH2$g1 cat$ min , for CuBDC, and CuO@C, respectively for hydrolysis. CuBDC displayed H2 generation 1 for methanolysis. rate of 9060 mlH2$g1 cat$min
Material and methods Benzene-1,4-dicarboxylic acid (BDC, 98%), CuCl2 (99.9%), NaBH4 (99%), dimethylformamide (DMF, 99.8%), and methanol (99.8%) were supplied from Sigma-Aldrich (USA). All chemicals and organic solvents were used without further purification. Aqueous solutions were prepared using highly pure and distilled water.
Synthesis of CuBDC and CuO@C CuCl2 (10 mmol, 1.70 g), and BDC (10 mmol, 1.66 g) were dissolved in 150 mL DMF. The solution was stirred at 100 C for 5 h with 400 rpm mixing rate. After cooling, a blue precipitate was separated via filtration. The materials were washed several times with DMF (3 50 mL) before drying (85 C).
CuO embedded on carbon nanosheets (CuO@C) was synthesized via calcination of CuBDC at 600 C for 3 h under air using ramping temperature 10 C$min1.
Characterization techniques X-ray diffraction (XRD) of the materials phases were carried out using Philips1700 diffractometer (Cu Ka radiation, Wavelength of 1.5418 A) using Bragg-Brentano geometry. The particle size and their morphologies were characterized using transmission electron microscope (JEOL JEM-100CXII, Japan). High resolution TEM (HR-TEM) is recorded using JEM-2100 (JEOL, Japan). Fourier transform infrared of CuBDC was carried out using FT-IR spectrophotometer (470 Shimadzu, 4000400 cm1). Thermogravimetric analysis (TGA) of CuBDC was performed using a Shimadzu thermal analyzer (TA 60 H, Japan) with heating rate 5 C$min1 under 40 mL min1 air flow.
Hydrolysis of sodium borohydride Hydrolysis of NaBH4 solution was performed using CuBDC, and CuO@C (5, 30, 50, and 200 mg). NaBH4 concentration was kept constant at 0.05 M in 100 mL of water or methanol. Hydrogen gas was measured using water displacement apparatus (Fig. S1). An Erlenmeyer flask (250 mL) with a laboratory rubber stopper was used. The flask was connected to a burette (50 mL) or a cylinder (2500 mL). Volume of the generated hydrogen is recorded as the volume of drained water through the scale of inverted burette. The total volume of hydrogen at interval time was calculated based on the displaced water volume after adding the volume of the flask and connected rubbers. Hydrogen generation is plotted as percentage using the final volume of the produced gas. The rate of hydrogen 1 ) is calculated at 1 min of the generation (mlH2$g1 cat$ min reaction. Because of the fast reaction in methanolysis, a digital Mass Flow Meter MF-FP10NH06-005-AL-ANV3M, HORIBA STEC, Co., Ltd., Japan, Response speed: 50 msec) was used for measuring the hydrogen generation rate in mL$min1 (See Movie 1). Reaction temperature (25,40, and 60 C) was adjusted using water bath and reaction was performd following the same procedure as described above. Supplementary video related to this article can be found at https://doi.org/10.1016/j.ijhydene.2019.10.047.
Recyclability and regeneration of the catalysts In the recyclability study, 50 mg of CuO@C were repeatedly used as the catalyst for four times in H2 generation using 0.05 M NaBH4 at 25 C. After the use of catalysts, 0.05 M NaBH4 was added to the reaction medium for every use. The catalyst CuO@C after catalysis was characterized using XRD. The catalyst CuO@C can be regenerated via calcination in oxygen at 600 C for 3 h using ramping temperature 10 C$min1. The product after calcination is characterized using XRD.
Please cite this article as: Kassem AA et al., Metal-organic frameworks (MOFs) and MOFs-derived CuO@C for hydrogen generation from sodium borohydride, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.047
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Results and discussion Materials characterization The coordination polymerization of Cu2þ, and BDC2 in DMF provides a blue precipitate (Fig. 1). XRD pattern confirms the pure phase of CuBDC indicating the successful synthesis of CuBDC$DMF crystals (CCDC-687690, Fig. 2a). The broadening of XRD peaks is due to the small particle size. The functional groups in CuBDC were characterized using FT-IR spectra (Fig. S2). Spectrum shows two bands at 1596, and 1388 cm1 belonging to the asymmetric, and symmetric stretch modes of COO groups, respectively (Fig. S2) [33]. Vibrational peak at 569 cm1 refers to CueO bond [33]. FT-IR spectrum confirms the coordination bonds between BDC, and Cu metal center. Thermal stability of CuBDC∙DMF was evaluated using TGA (Fig. S3). TGA curve shows the weight lost (25%), below ~300 C, corresponding to DMF molecules (Calculated (Calc.) value is 24% using chemical formula CuBDC∙DMF). The second weight lost (51%) at ~385 C is corresponding to BDC (Calc. 55%). This step is sharp indicating the high crystallinity of the prepared materials. The weight residual is 25% corresponding to CuO (Calc. 26%). TGA curve confirms the crystal chemical formula CuBDC∙DMF (Cu(BDC$DMF), C11H11CuNO5, Molecular weight 300.76 g∙mol1). CuBDC∙DMF crystal is thermally stable to 400 C (Fig. S3). Thus, CuBDC∙DMF was calcined at 600 C for the complete conversion to CuO@C (Fig. 1). According to diffraction data, XRD pattern of synthesized CuO@C, and simulated pattern (JCPDS card No. 80-1917) are agreed very well indicating high phase purity and crystalline nature of the formed materials (Fig. 2b). CuBDC∙DMF, and CuO@C were characterized using XRD patterns, FT-IR and TGA confirming the successful formation of CuBDC∙DMF, and CuO@C. The morphology, and particle size of CuO@C are characterized using TEM, and high resolution TEM images (HR-TEM, Fig. 3). TEM image shows aspherical particles of CuO (dark particles) embedded into carbon frameworks (light particles, Fig. 3a) with carbon shell size of 2.8 nm (Fig. 3b). TEM images show the presence of carbon sheets embedded a dark particle corresponding to CuO with particles size of 10e30 nm (Fig. 3a). The interplanar distance using HRTEM, and inverse fast Fourier transform image (IFFT) shows 0.27 nm d-spacing corresponding to the Miller plane index of (110) (Fig. 3ced).
3
Catalytic hydrolysis and methanolysis of sodium borohydrides Hydrolysis and methanolysis of sodium borohydride with and without CuO@C are shown in Fig. 4. Hydrogen production without catalysts is completed after 60 min and 40 min for hydrolysis (Fig. 4a), and methanolysis (Fig. 4b). Under the same conditions, and in the presence of CuBDC, and CuO@C, hydrolysis is completed after 1.5 min, and 3 min respectively (Fig. 4a). CuO@C decreases the reaction time 8 fold compared to the same reaction without CuO@C, while CuBDC decreases the reaction time 40 fold compared to the same reaction without CuBDC. Methanolysis requires short time compared to hydrolysis due to the high solubility of NaBH4 in methanol compared to water (Fig. 4). The solubility of NaBH4 at 25 C is 55 g per 100 g water (550 g$ L1, 1.46 mol per 5.56 mol) [34]. Furthermore, the byproduct in case of hydrolysis is hydrated sodium borate (NaBO2$ H2O), while in methanolysis is NaB(OCH3)4. There is no dramatic difference in hydrolysis and methanolysis the presence of catalysts (Fig. 4). This indicates that CuBDC and CuO@C have high efficiency to perform as catalyst for hydrogen production via the hydrolysis and methanolysis of NaBH4. CuBDC and CuO@C catalysts showed very fast hydrogen release within short time with almost similar hydrogen release profiles (Fig. 5, Table 1). The reaction takes shorter time with increasing the catalyst dosage (5-50 mg, Fig. 5). The time required for hydrogen production decreases with increasing the amount of these catalysts implies that there is almost no reaction between these materials and NaBH4. CuBDC shows higher efficiency compared to CuO@C i.e. shorter reaction time (Fig. 5, Table 1). This may be due to the large surface area and presence of pore structure of CuBDC compared to CuO@C (Fig. 5). Dissociation of sodium borohyþ dride produces BH1 4 , and Na ions. These ions are diffused from aqueous solution to the surface of CuBDC or CuO@C. BH1 4 ions produce hydrogen gases that are desorbed from the surface of the catalysts. As shown in Fig. 5, CuO@C accelerates the production of hydrogen from NaBH4. CuBDC and CuO@C reduce the reaction time compared to hydrolysis and methanolysis of NaBH4 without catalyst. The effect of reaction temperature on hydrogen production using hydrolysis is investigated (Fig. 6). For both catalysts i.e. CuBDC and CuO@C, the rate of hydrogen production is
Fig. 1 e Schematic representation for the synthesis of CuBDC∙DMF, and CuO@C and their applications for NaBH4 hydrolysis. Please cite this article as: Kassem AA et al., Metal-organic frameworks (MOFs) and MOFs-derived CuO@C for hydrogen generation from sodium borohydride, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.047
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Fig. 2 e XRD for a) CuBDC (CCDC-687690), and b) CuO@C (JCPDS card No. 80-1917).
Fig. 3 e Characterization of CuO@C using a-b) TEM images of different magnification, the curved dots in (b) represent the shell of Carbon in CuO@C, c) High resolution TEM image, and b) inverse fast Fourier transform image of CuO@C.
Please cite this article as: Kassem AA et al., Metal-organic frameworks (MOFs) and MOFs-derived CuO@C for hydrogen generation from sodium borohydride, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.047
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Fig. 4 e Hydrogen generation from NaBH4 with and without catalysts using a) hydrolysis and b) methanolysis. Reaction conditions: [NaBH4], 0.06 mM; Catalyst, 2 mg; at 298 K.
Fig. 5 e Hydrogen generation using a) CuBDC, and b) CuO@C. (100 mL NaBH4 aqueous solution [0.05 mM]; at 298 K).
increased with increasing reaction temperature in NaBH4 hydrolysis (Fig. 6). This is because the increase of kinetic energy of the produced gases. In addition, it can be observed that CuO@C takes more time to reach the complete stage of hydrogen generation compared to CuBDC. The experimental results showed that reaction temperature affects significantly hydrogen generation rate. Recyclability of the catalysts is investigated as shown in Fig. 7aeb. Data show that CuBDC and CuO@C can be used for
several time without any loss of the materials efficiency (Fig. 7aeb). However, the hydrolysis take longer time due to the byproduct (NaBO2) adsorbed on the catalyst, and poisoning the active species. XRD of CuO@C after recycle 1 and recycle 2 reveal reduction of CuO to Cu0 (JCPDS card No. 851326), and generation of graphene from the carbon shell in CuO@C (Fig. S4). The material can be regenerated as proved from XRD (Fig. 7c). The generated CuO@C shows almost the same activity as a fresh sample of CuO@C (Fig. 7d). These
Table 1 e Comparison among catalysts used for hydrolysis and methanolysis of NaBH4. Catalysts
Cat.(mg)
Conditions
H2 generation rate 1 (ml H2$g1 cat $min )
Time (min)
Ref
Co/Fe3O4@C CoeP/CNTs-Ni GO-CoeB CoeCueB
40 Block 50 25
[12] [47] [13] [43]
100 100 100 50 25 50
1403 2430 14340 734.4 (water), and 4972 (methanol) 300 252e660 130 1057 3642 7640 7240 9060
7 25 1e2 25
NiB/NiFe2O4 Ni1/Co3/AC WSC -Ni Fe2(MoO4)3eFe ZIF-9 CuBDC CuO@C CuBDC
NaBH4 (158 mM), T (25 C), Hydrolysis NaBH4 (10 wt%), NaOH (1 wt%), T (25 C), Methanolysis NaBH4 (5 wt%), NaOH (5 wt%), T (30 C), Hydrolysis NaBH4 (2.5 wt%), %), NaOH (5 wt%), T (30 C), Hydrolysis and Methanolysis NaBH4 (5 wt%), NaOH (5 wt%), T (25 C), Hydrolysis NaBH4 (5 wt%), NaOH (5 wt%), T (30 C), Hydrolysis NaBH4 (1.0 g/L), T (30 C), Hydrolysis NaBH4 (5.28 mM), T (100 C) NaBH4 (0.5 wt%), NaOH (5 wt%), T (40 C) NaBH4 (0.05 M), T (25 C), Hydrolysis
30
[44] [45] [46] [41] [42] Here
NaBH4 (0.05 M), T (25 C), Methanolysis
75 7 80 1.5 3 0.5
Notes: AC, Activated carbon; CNTs, Carbon nanotubes; WSC, Wheat straw cellulose; Zeolitic imidazolated frameworks-9, ZIF-9.
Please cite this article as: Kassem AA et al., Metal-organic frameworks (MOFs) and MOFs-derived CuO@C for hydrogen generation from sodium borohydride, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.047
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Fig. 6 e The effect of reaction temperature on hydrogen production using a) CuBDC, and b) CuO@C. Condition: Catalyst, 50 mg; 100 mL NaBH4 aqueous solution [0.05 M]; at 298 K.
Fig. 7 e a-b) Recyclability of a) CuBDC, and b) CuO@C, c) XRD for regenerated CuO, and d) NaBH4 hydrolysis using fresh and regenerated samples CuO@C.
results indicate that the materials are recyclable and can be simply regenerated without any loss of their activity for catalysis. Mechanism of the hydrolysis or methanolysis of NaBH4 is not well known [35]. This is because several reasons. It is difficult to operate in situ and operando characterizations for the system because the vigorous bubbling of hydrogen and precipitation of borates. Second, the reaction has fast kinetics and increases with the increase of temperature (change of enthalpy (DH) between 210 and 250 kJ mol1) [36,37]. Third, it is very difficult to well characterize the active species in solution. Fourth, the reaction affects by several factors including NaBH4 concentration, water amount, catalyst loading, pH of the solution (acidic or basic conditions), reaction temperature,
stirring, reaction volume etc. For instance, kinetic study indicates that the hydrolysis follows first-order kinetics for 0.005e0.05 mol NaBH4 L1 and a zero-order for higher concentrations (>0.25 mol NaBH4 L1) [38]. Water in the hydrolysis of an aqueous alkaline solution of NaBH4 plays two roles; one of the reactants and solvent (when it is used in excess). Fifth, change of active catalyst species, and pH during catalysis. However, models including LangmuireHinshelwood, MichaeliseMenten, and EleyeRideal models were reported in order to understand hydrolysis mechanisms of NaBH4. Unfortunately, none of these kinetic models is satisfactory to completely describe this simple hydrolysis reaction. þ Sodium borohydride produces BH1 4 , and Na ions in water or methanol [39]. These species are diffused in the solution to
Please cite this article as: Kassem AA et al., Metal-organic frameworks (MOFs) and MOFs-derived CuO@C for hydrogen generation from sodium borohydride, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.047
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Fig. 8 e Proposed mechanism of NaBH4 hydrolysis using CuO@C.
the surface of CuBDC or CuO@C. BH1 4 ions produce hydrogen gas that desorbed from the surface of CuO@C. LangmuireHinshelwood, and MichaeliseMenten mechanisms of CuO@C are schematically represented as show in Fig. 8. CuO@C involves three different adsorption sites; O, Cu, and C. As shown from XRD pattern (Fig. S4), Cu2þ in CuO@C is reduced to Cu0. Thus, there are electron-rich sites including Od, Cu0, and p-electron of carbon shell, and electron-deficient sites such as Cudþ. In LangmuireHinshelwood model, both reactants (BH 4 , and H2O) are adsorbed on the catalyst surface and react to give H2 and B(OH) 4 (Fig. 8). Three successive steps for the hydrolysis of the four hydrides of BH 4 occurs via the reaction with one adsorbed H2O for each step. Finally, four hydrogen molecules are librated and byproduct of NaBO2H2O, or NaB(OCH3)4 are formed for hydrolysis and methanolysis, respectively (Fig. 8). In MichaeliseMenten, the dissociative adsorption of BH 4 onto metal sites is homolytically broken (for BeH bonds) and forming BH 3 and Hadsorbed. In
next step, adsorbed BH 3 and H react with H2O forming the intermediate BH3(OH) and H2 (Fig. 8). Then, BH3(OH) undergoes hydrolysis following the same procedure for three successive steps producing three molecules of H2 and byproduct of NaBO2H2O (Fig. 8). CuO@C is rich catalyst with active site (Cu2þ, Cuþ, Cu0, O, C) ensuring the possibility of both mechanism. However, further explorations are required for a clear mechanism. Table 1 shows several catalysts which can be used for hydrogen generation from the hydrolysis of NaBH4 [40]. CuBDC, and CuO offer higher hydrogen generation rate compared to several catalysts including cobalt nanoparticles supported on magnetic core-shell structured carbon [12], zero valent iron (ZVI) in Fe2(MoO4)3 [41], zeolitic imidazolated frameworks-9 (ZIF-9) [42], CoeCueB [43], NiB/NiFe2O4 [44], Ni1/ Co3/activated carbon (AC) [45], and WSC (wheat straw cellulose) based hydrogel adsorb Ni (Cu) ions [46] (Table 1). The hydrolysis can be achieved with good rate without the need of basic solution i.e. NaOH as such in case of graphene oxide (GO) modified CoeB catalysts [13]. The hydrolysis using our catalyst is superior compared to methanolysis of NaBH4 using CoeP/ carbon nanotubes (CNTs)-Ni foam [47]. Catalysts CuBDC, and CuO are promising for hydrogen generation via hydrolysis and methanolysis of NaBH4 (Fig. 9).
Conclusions
Fig. 9 e Methanolysis of NaBH4 using CuBDC. Data points are recrded using a flowmeter (mL· min-1).
CuBDC, and CuO@C have been successfully synthesized and applied as catalysts for hydrogen production from the hydrolysis of sodium borohydride. The materials show high catalytic performance of hydrogen generation. The materials provide high efficiency, short reaction time, and low catalyst loading. The materials are promising for energy-based applications and can be applied for industrial scale production. They can be also used for reduction reaction of organic compounds using hydrides. Our current issue is therefore to unambiguously determine the catalytic active species on the
Please cite this article as: Kassem AA et al., Metal-organic frameworks (MOFs) and MOFs-derived CuO@C for hydrogen generation from sodium borohydride, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.047
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surface involved in the reaction to draw a clear mechanism of hydrolysis. [10]
Acknowledgement Authors would thank the Ministry of Higher Education and Scientific Research (MHESR), Egypt, Assiut University, and Institutional Review Board (IRB) of the Faculty of Science in Assiut University, Egypt for support. We would also express our gratitude to Prof. Ahmed Giese for support. Thanks to Dr. Haitham Elbery and Ms. Safie for help during revision.
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Appendix A. Supplementary data [13]
Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.10.047.
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Please cite this article as: Kassem AA et al., Metal-organic frameworks (MOFs) and MOFs-derived CuO@C for hydrogen generation from sodium borohydride, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.047
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Metal-organic frameworks (MOFs) and MOFsderived CuO@C for hydrogen generation from sodium borohydride Ahlam Azzam Kassem a, Hani Nasser Abdelhamid a,b,*, Dina M. Fouad a, Said A. Ibrahim a,** a b
Department of Chemistry, Assiut University, Assiut, 71516, Egypt Advanced Multifunctional Materials Laboratory, Department of Chemistry, Assiut University, Assiut, 71516, Egypt
highlights Synthesis of 2D copper terephthalate frameworks (CuBDC). Synthesis of CuO@C derived from CuBDC. Hydrogen generation via sodium borohydride hydrolysis. 1 Achieve high hydrogen generation rate of 7620 ml H2$g1 cat min .
article info
abstract
Article history:
Hydrogen gas has been considered as one of the promising sources of energy. Thus, several
Received 20 August 2019
strategies including the hydrolysis of hydrides have been reported for hydrogen produc-
Received in revised form
tion. However, effective catalysts are highly required to improve the hydrogen generation
23 September 2019
rate. Two dimensional metal-organic frameworks (copper-benzene-1,4-dicarboxylic,
Accepted 8 October 2019
CuBDC), and CuBDC-derived CuO@C were synthesized, characterized and applied as cat-
Available online xxx
alysts for hydrogen production using the hydrolysis and methanolysis of sodium borohydride (NaBH4). CuBDC, and CuO@C display hydrogen generation rate of 7620, and 7240
Keywords:
1 mlH2$g1 cat$ min , respectively for hydrolysis. While, CuBDC offers hydrogen generation
Metal organic frameworks
1 for methanolysis. Both catalysts required short reaction time, rate of 9060 mlH2$g1 cat$ min
CuBDC
and showed good recyclability. The materials may open new venues for efficient catalyst
Hydrogen production
for energy-based applications.
Sodium borohydride hydrolysis
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Catalysis
Introduction Hydrogen gas is promising energy source due to its high energy density (142 MJ/kg), and its environmentally friendly
byproduct (water) [1]. It can be generated via several methods, including water splitting or oxidation [2,3] (electrolysis [4e8], thermolysis, and photocatalytic water splitting [9]), and using phototrophic microorganisms (biohydrogen, BioH2) [10]. Hydrolysis of hydrides, such as sodium borohydride (NaBH4), is
* Corresponding author. Department of Chemistry, Assiut University, Assiut, 71516, Egypt. ** Corresponding author. E-mail addresses: [email protected] (H.N. Abdelhamid), [email protected] (S.A. Ibrahim). https://doi.org/10.1016/j.ijhydene.2019.10.047 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Kassem AA et al., Metal-organic frameworks (MOFs) and MOFs-derived CuO@C for hydrogen generation from sodium borohydride, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.047
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promising for supplying hydrogen to end users on demand [11]. The process is suitable for portable and on-site hydrogen fuel systems including unmanned air vehicles, PEMFC (proton exchange membrane fuel cells), direct borohydride fuel cells (DBFCs), and low-temperature fuel-cell applications. The process is safe and efficient for hydrogen production. It generates hydrogen with relatively high capacity at room temperature. Hydrolysis of NaBH4 has relatively high hydrogen capacity (10.8 wt%), and releases hydrogen with high purity using a controllable process. The byproduct of hydrolysis; NaBO2, is also a promising candidate material for solid-state hydrogen storage. Thus, several heterogeneous and homogenous catalysts were reported for hydrogen generation via hydrolysis and alcoholysis of aqueous NaBH4 solution [1]. However, the process requires sometime expensive metals such as ruthenium and platinum as catalysts [1]. Most of the current catalysts lack of stability and high efficiency [12,13]. Thus, cheap, effective, and multi-tasks catalyst is highly required. Metaleorganic frameworks (MOFs) are porous materials with tunable pore size, and high surface areas [14e22]. They have gained wide interests in various applications including catalysis, sensing and biosensing, adsorption, devices, medicine, and drug delivery [23e31]. MOFs have been applied as catalysts for photocatalytic H2 evolution from water [32]. MOFs offer many advantages such as large surface area, large pore volume, low density, numerous crystal structures, can be easily functionalized with different reagents and conjugated with nanomaterials, and can be implemented in devices. Thus, they can be effective materials for hydrogen production via the hydrolysis of NaBH4. Herein, copper-organic frameworks, copper-benzene-1,4dicarboxylate (CuBDC), and CuBDC-derived CuO embedded on carbon (CuO@C) were synthesized and applied for hydrogen generation from the hydrolysis and methanolysis of NaBH4. It is the first time to use these materials for hydrogen generation from the hydrolysis of NaBH4. Our materials showed high catalytic performance with outstanding H2 generation rate of 1 7620, and 7240 mlH2$g1 cat$ min , for CuBDC, and CuO@C, respectively for hydrolysis. CuBDC displayed H2 generation 1 for methanolysis. rate of 9060 mlH2$g1 cat$min
Material and methods Benzene-1,4-dicarboxylic acid (BDC, 98%), CuCl2 (99.9%), NaBH4 (99%), dimethylformamide (DMF, 99.8%), and methanol (99.8%) were supplied from Sigma-Aldrich (USA). All chemicals and organic solvents were used without further purification. Aqueous solutions were prepared using highly pure and distilled water.
Synthesis of CuBDC and CuO@C CuCl2 (10 mmol, 1.70 g), and BDC (10 mmol, 1.66 g) were dissolved in 150 mL DMF. The solution was stirred at 100 C for 5 h with 400 rpm mixing rate. After cooling, a blue precipitate was separated via filtration. The materials were washed several times with DMF (3 50 mL) before drying (85 C).
CuO embedded on carbon nanosheets (CuO@C) was synthesized via calcination of CuBDC at 600 C for 3 h under air using ramping temperature 10 C$min1.
Characterization techniques X-ray diffraction (XRD) of the materials phases were carried out using Philips1700 diffractometer (Cu Ka radiation, Wavelength of 1.5418 A) using Bragg-Brentano geometry. The particle size and their morphologies were characterized using transmission electron microscope (JEOL JEM-100CXII, Japan). High resolution TEM (HR-TEM) is recorded using JEM-2100 (JEOL, Japan). Fourier transform infrared of CuBDC was carried out using FT-IR spectrophotometer (470 Shimadzu, 4000400 cm1). Thermogravimetric analysis (TGA) of CuBDC was performed using a Shimadzu thermal analyzer (TA 60 H, Japan) with heating rate 5 C$min1 under 40 mL min1 air flow.
Hydrolysis of sodium borohydride Hydrolysis of NaBH4 solution was performed using CuBDC, and CuO@C (5, 30, 50, and 200 mg). NaBH4 concentration was kept constant at 0.05 M in 100 mL of water or methanol. Hydrogen gas was measured using water displacement apparatus (Fig. S1). An Erlenmeyer flask (250 mL) with a laboratory rubber stopper was used. The flask was connected to a burette (50 mL) or a cylinder (2500 mL). Volume of the generated hydrogen is recorded as the volume of drained water through the scale of inverted burette. The total volume of hydrogen at interval time was calculated based on the displaced water volume after adding the volume of the flask and connected rubbers. Hydrogen generation is plotted as percentage using the final volume of the produced gas. The rate of hydrogen 1 ) is calculated at 1 min of the generation (mlH2$g1 cat$ min reaction. Because of the fast reaction in methanolysis, a digital Mass Flow Meter MF-FP10NH06-005-AL-ANV3M, HORIBA STEC, Co., Ltd., Japan, Response speed: 50 msec) was used for measuring the hydrogen generation rate in mL$min1 (See Movie 1). Reaction temperature (25,40, and 60 C) was adjusted using water bath and reaction was performd following the same procedure as described above. Supplementary video related to this article can be found at https://doi.org/10.1016/j.ijhydene.2019.10.047.
Recyclability and regeneration of the catalysts In the recyclability study, 50 mg of CuO@C were repeatedly used as the catalyst for four times in H2 generation using 0.05 M NaBH4 at 25 C. After the use of catalysts, 0.05 M NaBH4 was added to the reaction medium for every use. The catalyst CuO@C after catalysis was characterized using XRD. The catalyst CuO@C can be regenerated via calcination in oxygen at 600 C for 3 h using ramping temperature 10 C$min1. The product after calcination is characterized using XRD.
Please cite this article as: Kassem AA et al., Metal-organic frameworks (MOFs) and MOFs-derived CuO@C for hydrogen generation from sodium borohydride, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.047
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Results and discussion Materials characterization The coordination polymerization of Cu2þ, and BDC2 in DMF provides a blue precipitate (Fig. 1). XRD pattern confirms the pure phase of CuBDC indicating the successful synthesis of CuBDC$DMF crystals (CCDC-687690, Fig. 2a). The broadening of XRD peaks is due to the small particle size. The functional groups in CuBDC were characterized using FT-IR spectra (Fig. S2). Spectrum shows two bands at 1596, and 1388 cm1 belonging to the asymmetric, and symmetric stretch modes of COO groups, respectively (Fig. S2) [33]. Vibrational peak at 569 cm1 refers to CueO bond [33]. FT-IR spectrum confirms the coordination bonds between BDC, and Cu metal center. Thermal stability of CuBDC∙DMF was evaluated using TGA (Fig. S3). TGA curve shows the weight lost (25%), below ~300 C, corresponding to DMF molecules (Calculated (Calc.) value is 24% using chemical formula CuBDC∙DMF). The second weight lost (51%) at ~385 C is corresponding to BDC (Calc. 55%). This step is sharp indicating the high crystallinity of the prepared materials. The weight residual is 25% corresponding to CuO (Calc. 26%). TGA curve confirms the crystal chemical formula CuBDC∙DMF (Cu(BDC$DMF), C11H11CuNO5, Molecular weight 300.76 g∙mol1). CuBDC∙DMF crystal is thermally stable to 400 C (Fig. S3). Thus, CuBDC∙DMF was calcined at 600 C for the complete conversion to CuO@C (Fig. 1). According to diffraction data, XRD pattern of synthesized CuO@C, and simulated pattern (JCPDS card No. 80-1917) are agreed very well indicating high phase purity and crystalline nature of the formed materials (Fig. 2b). CuBDC∙DMF, and CuO@C were characterized using XRD patterns, FT-IR and TGA confirming the successful formation of CuBDC∙DMF, and CuO@C. The morphology, and particle size of CuO@C are characterized using TEM, and high resolution TEM images (HR-TEM, Fig. 3). TEM image shows aspherical particles of CuO (dark particles) embedded into carbon frameworks (light particles, Fig. 3a) with carbon shell size of 2.8 nm (Fig. 3b). TEM images show the presence of carbon sheets embedded a dark particle corresponding to CuO with particles size of 10e30 nm (Fig. 3a). The interplanar distance using HRTEM, and inverse fast Fourier transform image (IFFT) shows 0.27 nm d-spacing corresponding to the Miller plane index of (110) (Fig. 3ced).
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Catalytic hydrolysis and methanolysis of sodium borohydrides Hydrolysis and methanolysis of sodium borohydride with and without CuO@C are shown in Fig. 4. Hydrogen production without catalysts is completed after 60 min and 40 min for hydrolysis (Fig. 4a), and methanolysis (Fig. 4b). Under the same conditions, and in the presence of CuBDC, and CuO@C, hydrolysis is completed after 1.5 min, and 3 min respectively (Fig. 4a). CuO@C decreases the reaction time 8 fold compared to the same reaction without CuO@C, while CuBDC decreases the reaction time 40 fold compared to the same reaction without CuBDC. Methanolysis requires short time compared to hydrolysis due to the high solubility of NaBH4 in methanol compared to water (Fig. 4). The solubility of NaBH4 at 25 C is 55 g per 100 g water (550 g$ L1, 1.46 mol per 5.56 mol) [34]. Furthermore, the byproduct in case of hydrolysis is hydrated sodium borate (NaBO2$ H2O), while in methanolysis is NaB(OCH3)4. There is no dramatic difference in hydrolysis and methanolysis the presence of catalysts (Fig. 4). This indicates that CuBDC and CuO@C have high efficiency to perform as catalyst for hydrogen production via the hydrolysis and methanolysis of NaBH4. CuBDC and CuO@C catalysts showed very fast hydrogen release within short time with almost similar hydrogen release profiles (Fig. 5, Table 1). The reaction takes shorter time with increasing the catalyst dosage (5-50 mg, Fig. 5). The time required for hydrogen production decreases with increasing the amount of these catalysts implies that there is almost no reaction between these materials and NaBH4. CuBDC shows higher efficiency compared to CuO@C i.e. shorter reaction time (Fig. 5, Table 1). This may be due to the large surface area and presence of pore structure of CuBDC compared to CuO@C (Fig. 5). Dissociation of sodium borohyþ dride produces BH1 4 , and Na ions. These ions are diffused from aqueous solution to the surface of CuBDC or CuO@C. BH1 4 ions produce hydrogen gases that are desorbed from the surface of the catalysts. As shown in Fig. 5, CuO@C accelerates the production of hydrogen from NaBH4. CuBDC and CuO@C reduce the reaction time compared to hydrolysis and methanolysis of NaBH4 without catalyst. The effect of reaction temperature on hydrogen production using hydrolysis is investigated (Fig. 6). For both catalysts i.e. CuBDC and CuO@C, the rate of hydrogen production is
Fig. 1 e Schematic representation for the synthesis of CuBDC∙DMF, and CuO@C and their applications for NaBH4 hydrolysis. Please cite this article as: Kassem AA et al., Metal-organic frameworks (MOFs) and MOFs-derived CuO@C for hydrogen generation from sodium borohydride, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.047
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Fig. 2 e XRD for a) CuBDC (CCDC-687690), and b) CuO@C (JCPDS card No. 80-1917).
Fig. 3 e Characterization of CuO@C using a-b) TEM images of different magnification, the curved dots in (b) represent the shell of Carbon in CuO@C, c) High resolution TEM image, and b) inverse fast Fourier transform image of CuO@C.
Please cite this article as: Kassem AA et al., Metal-organic frameworks (MOFs) and MOFs-derived CuO@C for hydrogen generation from sodium borohydride, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.047
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Fig. 4 e Hydrogen generation from NaBH4 with and without catalysts using a) hydrolysis and b) methanolysis. Reaction conditions: [NaBH4], 0.06 mM; Catalyst, 2 mg; at 298 K.
Fig. 5 e Hydrogen generation using a) CuBDC, and b) CuO@C. (100 mL NaBH4 aqueous solution [0.05 mM]; at 298 K).
increased with increasing reaction temperature in NaBH4 hydrolysis (Fig. 6). This is because the increase of kinetic energy of the produced gases. In addition, it can be observed that CuO@C takes more time to reach the complete stage of hydrogen generation compared to CuBDC. The experimental results showed that reaction temperature affects significantly hydrogen generation rate. Recyclability of the catalysts is investigated as shown in Fig. 7aeb. Data show that CuBDC and CuO@C can be used for
several time without any loss of the materials efficiency (Fig. 7aeb). However, the hydrolysis take longer time due to the byproduct (NaBO2) adsorbed on the catalyst, and poisoning the active species. XRD of CuO@C after recycle 1 and recycle 2 reveal reduction of CuO to Cu0 (JCPDS card No. 851326), and generation of graphene from the carbon shell in CuO@C (Fig. S4). The material can be regenerated as proved from XRD (Fig. 7c). The generated CuO@C shows almost the same activity as a fresh sample of CuO@C (Fig. 7d). These
Table 1 e Comparison among catalysts used for hydrolysis and methanolysis of NaBH4. Catalysts
Cat.(mg)
Conditions
H2 generation rate 1 (ml H2$g1 cat $min )
Time (min)
Ref
Co/Fe3O4@C CoeP/CNTs-Ni GO-CoeB CoeCueB
40 Block 50 25
[12] [47] [13] [43]
100 100 100 50 25 50
1403 2430 14340 734.4 (water), and 4972 (methanol) 300 252e660 130 1057 3642 7640 7240 9060
7 25 1e2 25
NiB/NiFe2O4 Ni1/Co3/AC WSC -Ni Fe2(MoO4)3eFe ZIF-9 CuBDC CuO@C CuBDC
NaBH4 (158 mM), T (25 C), Hydrolysis NaBH4 (10 wt%), NaOH (1 wt%), T (25 C), Methanolysis NaBH4 (5 wt%), NaOH (5 wt%), T (30 C), Hydrolysis NaBH4 (2.5 wt%), %), NaOH (5 wt%), T (30 C), Hydrolysis and Methanolysis NaBH4 (5 wt%), NaOH (5 wt%), T (25 C), Hydrolysis NaBH4 (5 wt%), NaOH (5 wt%), T (30 C), Hydrolysis NaBH4 (1.0 g/L), T (30 C), Hydrolysis NaBH4 (5.28 mM), T (100 C) NaBH4 (0.5 wt%), NaOH (5 wt%), T (40 C) NaBH4 (0.05 M), T (25 C), Hydrolysis
30
[44] [45] [46] [41] [42] Here
NaBH4 (0.05 M), T (25 C), Methanolysis
75 7 80 1.5 3 0.5
Notes: AC, Activated carbon; CNTs, Carbon nanotubes; WSC, Wheat straw cellulose; Zeolitic imidazolated frameworks-9, ZIF-9.
Please cite this article as: Kassem AA et al., Metal-organic frameworks (MOFs) and MOFs-derived CuO@C for hydrogen generation from sodium borohydride, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.047
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Fig. 6 e The effect of reaction temperature on hydrogen production using a) CuBDC, and b) CuO@C. Condition: Catalyst, 50 mg; 100 mL NaBH4 aqueous solution [0.05 M]; at 298 K.
Fig. 7 e a-b) Recyclability of a) CuBDC, and b) CuO@C, c) XRD for regenerated CuO, and d) NaBH4 hydrolysis using fresh and regenerated samples CuO@C.
results indicate that the materials are recyclable and can be simply regenerated without any loss of their activity for catalysis. Mechanism of the hydrolysis or methanolysis of NaBH4 is not well known [35]. This is because several reasons. It is difficult to operate in situ and operando characterizations for the system because the vigorous bubbling of hydrogen and precipitation of borates. Second, the reaction has fast kinetics and increases with the increase of temperature (change of enthalpy (DH) between 210 and 250 kJ mol1) [36,37]. Third, it is very difficult to well characterize the active species in solution. Fourth, the reaction affects by several factors including NaBH4 concentration, water amount, catalyst loading, pH of the solution (acidic or basic conditions), reaction temperature,
stirring, reaction volume etc. For instance, kinetic study indicates that the hydrolysis follows first-order kinetics for 0.005e0.05 mol NaBH4 L1 and a zero-order for higher concentrations (>0.25 mol NaBH4 L1) [38]. Water in the hydrolysis of an aqueous alkaline solution of NaBH4 plays two roles; one of the reactants and solvent (when it is used in excess). Fifth, change of active catalyst species, and pH during catalysis. However, models including LangmuireHinshelwood, MichaeliseMenten, and EleyeRideal models were reported in order to understand hydrolysis mechanisms of NaBH4. Unfortunately, none of these kinetic models is satisfactory to completely describe this simple hydrolysis reaction. þ Sodium borohydride produces BH1 4 , and Na ions in water or methanol [39]. These species are diffused in the solution to
Please cite this article as: Kassem AA et al., Metal-organic frameworks (MOFs) and MOFs-derived CuO@C for hydrogen generation from sodium borohydride, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.047
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Fig. 8 e Proposed mechanism of NaBH4 hydrolysis using CuO@C.
the surface of CuBDC or CuO@C. BH1 4 ions produce hydrogen gas that desorbed from the surface of CuO@C. LangmuireHinshelwood, and MichaeliseMenten mechanisms of CuO@C are schematically represented as show in Fig. 8. CuO@C involves three different adsorption sites; O, Cu, and C. As shown from XRD pattern (Fig. S4), Cu2þ in CuO@C is reduced to Cu0. Thus, there are electron-rich sites including Od, Cu0, and p-electron of carbon shell, and electron-deficient sites such as Cudþ. In LangmuireHinshelwood model, both reactants (BH 4 , and H2O) are adsorbed on the catalyst surface and react to give H2 and B(OH) 4 (Fig. 8). Three successive steps for the hydrolysis of the four hydrides of BH 4 occurs via the reaction with one adsorbed H2O for each step. Finally, four hydrogen molecules are librated and byproduct of NaBO2H2O, or NaB(OCH3)4 are formed for hydrolysis and methanolysis, respectively (Fig. 8). In MichaeliseMenten, the dissociative adsorption of BH 4 onto metal sites is homolytically broken (for BeH bonds) and forming BH 3 and Hadsorbed. In
next step, adsorbed BH 3 and H react with H2O forming the intermediate BH3(OH) and H2 (Fig. 8). Then, BH3(OH) undergoes hydrolysis following the same procedure for three successive steps producing three molecules of H2 and byproduct of NaBO2H2O (Fig. 8). CuO@C is rich catalyst with active site (Cu2þ, Cuþ, Cu0, O, C) ensuring the possibility of both mechanism. However, further explorations are required for a clear mechanism. Table 1 shows several catalysts which can be used for hydrogen generation from the hydrolysis of NaBH4 [40]. CuBDC, and CuO offer higher hydrogen generation rate compared to several catalysts including cobalt nanoparticles supported on magnetic core-shell structured carbon [12], zero valent iron (ZVI) in Fe2(MoO4)3 [41], zeolitic imidazolated frameworks-9 (ZIF-9) [42], CoeCueB [43], NiB/NiFe2O4 [44], Ni1/ Co3/activated carbon (AC) [45], and WSC (wheat straw cellulose) based hydrogel adsorb Ni (Cu) ions [46] (Table 1). The hydrolysis can be achieved with good rate without the need of basic solution i.e. NaOH as such in case of graphene oxide (GO) modified CoeB catalysts [13]. The hydrolysis using our catalyst is superior compared to methanolysis of NaBH4 using CoeP/ carbon nanotubes (CNTs)-Ni foam [47]. Catalysts CuBDC, and CuO are promising for hydrogen generation via hydrolysis and methanolysis of NaBH4 (Fig. 9).
Conclusions
Fig. 9 e Methanolysis of NaBH4 using CuBDC. Data points are recrded using a flowmeter (mL· min-1).
CuBDC, and CuO@C have been successfully synthesized and applied as catalysts for hydrogen production from the hydrolysis of sodium borohydride. The materials show high catalytic performance of hydrogen generation. The materials provide high efficiency, short reaction time, and low catalyst loading. The materials are promising for energy-based applications and can be applied for industrial scale production. They can be also used for reduction reaction of organic compounds using hydrides. Our current issue is therefore to unambiguously determine the catalytic active species on the
Please cite this article as: Kassem AA et al., Metal-organic frameworks (MOFs) and MOFs-derived CuO@C for hydrogen generation from sodium borohydride, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.047
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surface involved in the reaction to draw a clear mechanism of hydrolysis. [10]
Acknowledgement Authors would thank the Ministry of Higher Education and Scientific Research (MHESR), Egypt, Assiut University, and Institutional Review Board (IRB) of the Faculty of Science in Assiut University, Egypt for support. We would also express our gratitude to Prof. Ahmed Giese for support. Thanks to Dr. Haitham Elbery and Ms. Safie for help during revision.
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Appendix A. Supplementary data [13]
Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.10.047.
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Please cite this article as: Kassem AA et al., Metal-organic frameworks (MOFs) and MOFs-derived CuO@C for hydrogen generation from sodium borohydride, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.047