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Ru Nanoparticles Supported on MIL-101 by Double Solvents Method as High-Performance Catalysts for Catalytic Hydrolysis of Ammonia Borane
Highly dispersed crystalline Ru nanoparticles (NPs) were successfully immobilized inside the pores of MIL-101 by a double solvents method (DSM). HRTEM clearly demonstrated the uniform distribution of the ultrafine Ru NPs throughout the interior cavities of MIL-101. The synthesized Ru@MIL-101 catalyst was also characterized by X-ray diffraction (XRD), N2 adsorption desorption, and ICP-AES. The catalytic test indicated that the Ru NPs supported MIL-101 material exhibited exceedingly high activity and excellent durability for hydrogen generation from the catalytic hydrolysis of amine boranes.
Renewable and clean energy is believed as a long-term solution to replace the current prevalent petroleum energy. Recently, hydrogen receives much attention as a next-generation energy carrier [1–3]. Amine boranes (NH3-BH3, AB) appears to be a suitable hydrogen source, because of its rich hydrogen content, high stability, and nontoxicity [4–7]. The hydrogen stored in AB can be released through the hydrolysis of AB in the presence of a suitable catalyst according to the reaction (1). So far, various catalysts have been developed for the catalytic hydrolysis of AB to generate hydrogen, such as Pt-based catalyst  and Ru-based catalyst . Ru-based catalyst showed significant advantages in hydrogen generation from AB. However, the balance between costs, efficiency, and recyclability still remains a considerable challenge:Porous metal-organic frameworks (MOFs) have emerged as a class of promising functional porous materials, especially in the applications of gas storage catalysis, separation, sensing, and drug delivery [10–13]. Taking advantages of their high specific surface area and tunable pore size, efficient solid catalysts can be achieved by loading MNPs inside the porous MOFs. It is reported that two approaches are used for the synthesis of MNPs inside MOFs. The first and most widely used approach is to use MOFs as stabilizing host material, which provides a confined space for nucleation, such as chemical vapor deposition , solution infiltration [15–17], and solid grinding . However, it seems difficult to completely avoid the precursor compounds and product aggregation on the external surface of MOFs. Therefore, a double solvents method (DSM) has been reported to incorporate fine metal nanoparticles within the pores of MOFs in the absence of NPs aggregation on the external surface of framework. Until now, few cases concerning double solvents method were investigated in the synthesis of MOFs-based catalysts for catalytic hydrolysis of ammonia borane [19, 20].
In this work, the highly dispersed Ru NPs were successfully encapsulated in the pores of MIL-101 without aggregation on the external surfaces of the host framework by DSM (Figure 1). MIL-101, a chromium-based MOF, was chosen as a host matrix because of its high stability in water, extra-large surface area, and two kinds of hydrophilic cavities with free diameters of ca. 2.9 and 3.4 nm accessible through two pore windows of ca. 1.2 and 1.6 nm in diameter. The metal precursor, RuCl3, can easily diffuse into the internal cavities. Therefore, a hydrophilic solvent (water) containing the metal precursor with a volume set equal to or less than the pore volume of the adsorbent and a hydrophobic solvent (hexane), which was unable to drive through pore windows, were used for preparing Ru NPs encapsulated by MIL-101. The catalytic activity for hydrolytic dehydrogenation of AB of Ru@MIL-101 was investigated. Compared with other reported Ru-based catalysts by liquid impregnation method  and heterogeneous Ru(III) catalysts , the Ru@MIL-101 exhibits remarkably enhanced catalytic activity.
2.1. Chemicals and Materials
All chemicals were commercial and used without further purification. Sodium borohydride (NaBH4, >96%, Sinopharm Chemical Reagent Co., Ltd.), ruodium chloride hydrate (RuCl33H2O, 38.0–42.0%, Aladdin Industrial Inc.), terephthalic acid (HOOCC6H4COOH, 99%, Aladdin Industrial Inc.), chromic nitrate nonahydrate (Cr(NO3)39H2O, 99%, Aladdin Industrial Inc.), hydrochloric acid (HCl, 37%, Sinopharm Chemical Reagent Co., Ltd.), hydrofluoric acid solution (HF, 40%, Sinopharm Chemical Reagent Co., Ltd.), anhydrous n-hexane (Dikma Technologies Inc.), sodium hydroxide (NaOH > 96%, Sinopharm Chemical Reagent Co., Ltd.), and ethanol (C2H5OH, 99%, Sinopharm Chemical Reagent Co., Ltd.,) were used as received.
2.2. Synthesis of MIL-101
MIL-101 were synthesized according to the reported procedure . Terephthalic acid (0.823 g, 5.0 mmol), Cr(NO3)39H2O (2.007 mg, 5.0 mmol), HF solution (1 mL, 5.0 mmol), and deionized water (24 mL) were reacted in a 100 mL Teflon-liner autoclave at 220°C for 8 h. After cooling, the resulting green solution of MIL-101 can be isolated from the residual needle-shaped terephthalic acid by using a large pore fritted glass filter (100 μm). The free terephthalic acid was eliminated using a small pore fritted glass filter (30 μm). The powder was further purified by solvothermal treatment in ethanol at 80°C for 24 h. To eliminate the terephthalic acid inside the pores of MIL-101, the resulting green solid was soaked in NH4F (1 M) solution at 70°C for 24 h and immediately filtered, washed with hot water several times, and finally dried overnight at 150°C under vacuum.
2.3. Synthesis of Ru@MIL-101
Supported Ru nanoparticles were prepared via a double solvents method. Typically, 100 mg of activated MIL-101 was suspended in 20 mL of dry n-hexane as hydrophobic solvent and the mixture was sonicated for 15 min until it became homogeneous. After stirring for 2 h, 0.2 mL of aqueous HCl solution (1 mol/L) with different RuCl3 concentrations as the hydrophilic solvent was added dropwise under continuous vigorous stirring during 15 minutes. The resulting solution was continuously stirred for 2 h. After filtration, the green powder was dried under vacuum at 50°C. Ru@MIL-101 catalysts were obtained by reduction with 3.5 mL freshly prepared aqueous NaBH4 solution (0.6 M) and subsequent centrifugation.
2.4. Catalytic Activity Investigations
In a typical experiment, Ru@MIL-101 powders (50 mg) were dispersed in 4 mL of water kept in a 30 mL of two-necked round-bottom flask under vigorous stirring. One neck was connected to a gas burette filled with water to monitor the volume of the gas evolution while the other neck was used for the introduction of 3 mL of aqueous AB (46.2 mg, 1 mmol). A shaking table was used to control the hydrolysis reaction of sodium borohydride under vigorous shaking (220 rpm) at 25°C.
X-ray diffraction (XRD) measurements were performed on Rigaku D-max-γA XRD with Cu Kα radiation, Å. High resolution transmission electron microscopy (HRTEM, JEM-2010) was applied for determining the detailed microstructure and composition information. 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 12 h using automatic volumetric adsorption equipment (Micromeritics ASAP2010). The pore volume was calculated by a single point method at . The composition of the catalysts was analyzed by ICP-AES (Thermo Fisher Scientific, USA).
3. Results and Discussion
3.1. Characterizations of the Ru@MIL-101 Catalysts
Figure 2 displays the PXRD patterns of MIL-101 and Ru@MIL-101, respectively. The MIL-101 framework is maintained well during the catalyst preparation. Furthermore, no significant diffraction peaks of Ru were detected from wide-angle powder X-ray diffractions, which might be due to the immobilization of Ru NPs into the pores of MIL-101.
The N2 adsorption-desorption isotherms of MIL-101, 2.5 wt% Ru@MIL-101, and 3.7 wt% Ru@MIL-101 are shown in Figure 3. The specific areas of MIL-101, 2.5 wt% Ru@MIL-101, and 3.7 wt% Ru@MIL-101 are 2976 m²/g, 1823 m²/g, and 1691 m²/g, respectively. The pore volume of MIL-101, 2.5 wt% Ru@MIL-101, and 3.7 wt% Ru@MIL-101 is 1.832 cm3g−1, 1.373 cm3g−1, and 1.128 cm3g−1 (Figure 4), respectively. The decrease in the amount of specific areas and the pore volume indicates that the pores of MIL-101 were occupied by the well dispersed Ru NPs.
Figure 5 shows the HRTEM images of 2.5 wt% Ru@MIL-101 catalyst. HRTEM images (Figures 5(a) and 5(b)) indicate that the Ru NPs are well dispersed and encapsulated in the cages of the MIL-101. The mean diameter of Ru NPs in Ru@MIL-101 is 2 ± 0.2 nm (Figure 5(c)), which is small enough for them to be accommodated in the two mesoporous cavities of MIL-101 (2.9 and 3.4 nm). In addition, HRTEM analysis shows that the Ru NPs are crystalline with a spacing distance of 2.16 Å (Figure 5(d)) corresponding to Ru (002) plane. These results indicate that the ultra small Ru NPs have been effectively immobilized and well dispersed in the pores of MIL-101.
3.2. Catalytic Activity
Figure 6 shows the H2 generation from aqueous AB under ambient conditions in the presence of Ru@MIL-101. It is found that the 2.5 wt% and 3.7 wt% Ru@MIL-101 exhibit similar high activity. 2.5 wt% Ru@MIL-101 catalyst has a high TOF value of 187 (mol H2 mol−1 Ru min−1). 3.7 wt% Ru@MIL-101 shows no significant increase in TOF value, indicating that the excess Ru NPs might block the pores and channels of MIL-101. These results are in good agreement with the N2 adsorption-desorption isotherms. This value of TOF is higher than that of Cao et al.  using 2.5 wt% Ru/MIL-101 (178 mol H2 mol−1 Ru min−1) by solution infiltration method and similar to that of Metin et al.  using PSSA-co-MA stabilized Ru nanoclusters (187.6 mol H2 mol−1 Ru min−1). The same amount of Ru NPs as the control experiment was reduced by NaBH4 without MIL-101. The release of H2 was slower for Ru NPs without MIL-101. Moreover, MIL-101 without Ru loading showed no reactivity toward hydrolysis of AB. These results confirm that the Ru@MIL-101 catalysts have cooperative effect on hydrolysis of AB and excess loading amount of Ru NPs cannot improve the efficiency of the hydrolysis of AB.
The durability of the catalyst is crucial in the practical application. The durability of the 2.5 wt% Ru@MIL-101 catalyst for hydrolysis of AB was tested by adding another equivalent of aqueous AB. As shown in Figure 7, even after the 5th run, the catalyst still maintained the original catalytic activity, indicating the Ru NPs have been effectively immobilized in the framework of MIL-101, which can stabilize Ru NPs for good durability.
In summary, we have successfully prepared a Ru@MIL-101 catalyst by immobilizing ultrafine Ru NPs into the frameworks of MIL-101 using a double solvents method. The synthesized catalyst exhibited highly catalytic activity and excellent durability for catalytic hydrolysis of AB at ambient conditions. The double solvents approach for the incorporation of Ru MNPs within pores can avoid deposition of the Ru MNPs on the external surface of MIL-101. The combination of high activity and selectivity as well as good durability makes Ru@MIL-101 a potential catalyst for hydrolysis of AB for hydrogen generation. Furthermore, this double solvents method can be extended to other MOFs supported metal nanoparticles for more applications.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Financial supports of this research were provided by the scientific research foundation of Shandong province Outstanding Young Scientist Award (no. BS2013NJ017) and the research foundation of Qingdao Science and Technology Development (13-1-4-245-jch).
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