Description

METAL ORGANIC FRAMEWORK COMPRISING METAL NANOPARTICLES AND ITS USE FOR GAS STORAGE

MATERIAL

Technical Field

[1] The present invention discloses a metal-organic framework comprising metal nanoparticles, its preparation method and its use as a gas storage material. Background Art

[2] The utilization of hydrogen as a future energy material is important in the development of future energy carriers of vehicles and electronics. However, application of hydrogen as an energy carrier is limited because of the lack of a suitable hydrogen storage method [Schlapbach, L; Zuttel, A. NaturelOOl, 414, 353-358].

[3] The U. S. Department of Energy (DOE) has established a multistage target for hydrogen storage technologies for transportation and stationary applications as shown in Table 1 (http://www.eere.energy.gov/hydrogenandfuel-cells/mypp). Among the candidate hydrogen storage materials, nothing is capable of reaching the target of DOE [Zuttel, A. Mater. Today2003, 6, 24-33].

[4] Table 1

[Table 1] [Table ]

[5]

[6] During the past few years, the metal-organic frameworks (MOFs) have attracted great attention because they have the potential to be applied as hydrogen storage materials [(a) Lee, E. Y.; Suh, M. P. Angew. Chem. Int. Ed.2004, 43, 2798-2801; (b) Lee, E. Y.; Jang, S. Y.; Suh, M. P. J. Am. Chem. Soc.2005, 127, 6374-6381; (c) Yaghi, O. M. et al.; J. Am. Chem. Soc.2004, 126, 5666-5667]. In particular, Yaghi et al. reported a relatively high capacity of hydrogen storage of ~7 wt% in MOF- 177 (at 77 K and 60 bar) and -1.8 wt% in IRMOF- 8 (at room temperature and 100 bar) [Yaghi, O. M. et al.; J. Am. Chem. Soc. 2006, 128, 3494-3495]. However, these results are still far from reaching the target of DOE at room temperature.

[7] Recently, the concept of "hydrogen spillover" has attracted attention because of enhancement of hydrogen adsorption in MOFs. This concept is defined as dissociating

the H ₂ molecules on the metal surface by breaking the H-H bond [Lueking, A. D.; Yang, R. T. Appl. Catal. A-Gen.2004, 265, 259-268]. Yang et al. reported an enhanced hydrogen storage via spillover by simply mixing Pt/AC and MOF- 5 and IRMOF- 8, respectively [(a) Li, Y.; Yang, R. T. J. Am. Chem. Soc.2006, 128, 726-727; (b) Li, Y.; Yang, R. T. J. Am. Chem. Soc.2006, 128, 8136-8137]. The hydrogen uptakes were 1.0 wt% for MOF-5 (-0.4 wt% at 298 K and 10 MPa) and 1.8 wt% for IRMOF-8 (-0.5 wt% at 298 K and 10 MPa) at 298 K and 10 MPa. However, these results are far from satisfaction, necessitating an urgent development of a novel gas storage material. Disclosure of Invention Technical Problem

[8] The present invention discloses a gas storage material showing enhance gas storage, metal-organic framework loading metal nanoparticles useful in gas storage and a preparation method thereof. Technical Solution

[9] In an aspect, the present invention discloses a metal-organic framework of [Formula

1] comprising metal nanoparticles:

[10] [Formula 1]

[11] [M(LIG) ₂ A(SOL) ₂ ] -x(SOL)

[12] wherein M is Zn ₃ or Zn ₄ O; A is 1 when M is Zn ₃ and A is 0 when M is Zn ₄ O; LIG is a ligand comprising at least one arylamine; SOL is at least one solvent selected from the group consisting of ethanol, methanol, water and pyridine; x is an integer of 0-4; and the metal-organic framework comprises nanoparticles of at least one metal selected from the group consisting of Pd, Pt, Ag and Au.

[13] In another aspect, the present invention discloses a metal-organic framework of

[Formula 2] or [Formula 3] comprising metal nanoparticles:

[14] [Formula 2]

[15] [Zn ₃ (LIG) ₂ (SOL) ₂ ] -x(SOL)

[16] [Formula 3]

[17] [Zn ₄ O(LIG) ₂ ] -x(SOL)

[18] In still another aspect, the present invention discloses a process of preparing a metal- organic framework of [Formula 2] or [Formula 3] comprising metal nanoparticles, the process comprising an act of immersing the framework in metal ion solution to form a nano composite.

[19] In yet another aspect, the present invention discloses a gas storage material comprising (1) a metal-organic framework, and (2) metal nanoparticles loaded in the metal-organic framework.

Advantageous Effects

[20] The enhance gas storage due to the incorporation of metal nanoparticles in a porous host, a metal-organic framework, has been ascertained for [Zn ₃ (LIG) ₂ (SOL) ₂ ] -x(SOL) as explicitly described in Examples and also for other compounds including [Zn ₄ 0(LIG) ₂ ] -x(SOL) by proceeding as described herein. Moreover, various nanoparticles including Pt, Ag and Au have also been ascertained as enhancing the gas storage capacity of metal-organic framework of the present invention. Brief Description of Drawings

[21] Figure 1 is a FT-TEM image of the host compound loading Pd nanoparticles prepared in Example 1.

[22] Figure 2 is an EPR spectrum of the host compound loading Pd nanoparticles prepared in Example 1.

[23] Figure 3 is X-ray phtoelectron spectra of the host compound loading Pd nanoparticles prepared in Example 1.

[24] Figure 4 is hydrogen adsorption isotherms measured at 77 K up to 1 atm. Squared and circled symbols represent dried host (2) and host compound (1) prepared by immersion of the dried host in the MeCN solution of Pd(NO ₃ ) ₂ for 30 min, respectively. Filled and open symbols represent adsorption and desorption data, respectively.

[25] Figure 5 is hydrogen adsorption isotherms measured at 298 K up to 95 bar. Squared and circled symbols represent dried host (2) and host compound (1) prepared by immersion of the dried host in the MeCN solution of Pd(NO ₃ ) ₂ for 30 min, respectively. Filled and open symbols represent adsorption and desorption data, respectively. Best Mode for Carrying out the Invention

[26] The present invention discloses a metal-organic framework comprising metal nanoparticles, its preparation method and its use as a gas storage material.

[27]

[28] In an aspect, the present invention discloses a metal-organic framework of [Formula

2] or [Formula 3] comprising metal nanoparticles:

[29] [Formula 2]

[30] [Zn ₃ (LIG) ₂ (SOL) ₂ J-X(SOL);

[31] [Formula 3]

[32] [Zn ₄ O(LIG) ₂ ]-x(SOL).

[33] wherein LIG is a ligand comprising at least one arylamine; SOL is at least one solvent selected from the group consisting of ethanol, methanol, water and pyridine; x is an integer of 0-4; and

[34] the metal-organic framework comprises nanoparticles of at least one metal selected

from the group consisting of Pd, Pt, Ag and Au.

[35]

[36] In Formulas 1 and 2, "LIG" refers to at least one ligand comprising at least one arylamine group. Enhancement in gas storage may be unsatisfactory when LIG comprise no arylamine group. Preferred examples of the arylamine-containing ligand include without limitation 4,4',4"-nitrilotris benzoic acid (referred to as 'NTB' hereinafter), N,N,N',N'-tetracarboxyphenylbenzidine

[37] (referred to as 'TCPB' hereinafter) and

N,N,N'N'-tetrakis(4-carboxyphenyl-l,4-phenylenediamine) (referred to as 'TCPPDA' hereinafter). As used herein, the term "ligand" describes a moiety that is capable of forming coordinative interaction or covalent bond with a central atom or ion.

[38] "SOL" refers to at least one solvent, and preferred examples of the solvent include without limitation ethanol, methanol, water and pyridine. Ethanol or methanol is preferred because it can be easily removed.

[39] Moreover, x is an integer of 0-4.

[40] Preferred examples of the metal nanoparticles include without limitation nanoparticles of metal such as Pd, Pt, Au and Ag. Pd or Pt is preferred due to the effectiveness in breaking hydrogen bonds.

[41] NTB and ethanol are preferred as LIG and SOL, respectively, for other advantageous effects such as, without limitation, the increase in durability and gas separation function in addition to enhancement in gas storage capacity.

[42]

[43] Metal-organic framework of the present invention comprises 0.01-50 wt%, preferably 0.05-30 wt%, more preferably 0.1-5 wt% of metal nanoparticles, thus enhancing gas storage capacity. A change in mechanism of gas adsorption in the present invention can remarkably enhance the gas storage capacity. Outside the range above, the enhancement in gas storage capacity may not be satisfactory. In particular, a large amount of metal nanoparticles incorporated in metal-organic framework can block the channels of the metal-organic framework and thus decrease the surface area.

[44] Despite the aforementioned description, however, a preferable amount of metal nanoparticles and the resulting enhancement in gas storage capacity varies depending on other factors such as, without limitation, the structure of host compound, the kind of ligand and the type of metal nanoparticles. For example, in case of [Zn ₃ (LIG) ₂ (SOL) ₂ ]-x(SOL) metal-organic framework, preferable amount of metal nanoparticles is 2-3.1 wt% when LIG is NTB and nanoparticles are Pd nanoparticles with maximum gas storage achieved at 3 wt%.

[45] Therefore, the technical idea of controlling the gas storage capacity by adjusting the amount of nanoparticles and changing the kind of host and ligand compounds is also

within the scope of the present invention. Although the results of all the experiments are not disclosed in Examples herein, it is obvious that one skilled in the art can easily select the amount of nanoparticles for achieving the desired effects of the present invention if based on the disclosure of the present invention.

[46] Moreover, preferable size of nanoparticles in the present invention is 10 nm or less, and 3-4 nm is more preferred.

[47]

[48] As used herein, the expression of "the incorporation of metal nanoparticles in metal- organic framework" or "metal-framework comprises metal nanoparticles" or the like should be understood based on that the incorporation includes without limitation all the chemical, physical and electrochemical binding or attachment. In a preferred embodiment, metal nanoparticles are preferred to be incorporated in metal-organic framework by non-chemical bonding or attachment, which is achieved, for example, by the immersion in metal solution.

[49]

[50] In another aspect, the present invention discloses a process of preparing a metal- organic framework of [Formula 2] or [Formula 3] comprising nanoparticles, the process comprising an act of immersing a metal-organic framework of [Formula 2] or [Formula 3] in a metal ion solution, thereby forming a nano composite.

[51] Metal nanoparticles are formed on the surface of porous metal organic framework via auto redox reaction between metal-organic framework and metal ions.

[52] Preferably, the immersion is conducted under such a condition that metal-organic framework comprises 0.01-50 wt%, preferably 0.05-30 wt%, more preferably 0.1-5 wt% of metal nanoparticles. In particular, in case of [Zn ₃ (NTB) ₂ (SOL) ₂ ] -x(SOL) metal- framework comprising Pd nanoparticles, the immersion is carried out preferably under such a condition that metal-organic framework comprises 3 wt% of metal nanoparticles.

[53] Despite the aforementioned description, however, preferable conditions vary depending on other factors such as the structure of host compound, the kind of ligand and the kind of metal nanoparticles. Although the results of all the experiments are not disclosed in Examples herein, it is obvious that one skilled in the art can easily select the conditions for achieving the desired effect of the present invention if based on the disclosure of the present invention.

[54] Moreover, although Examples herein disclose the control of amount of metal nanoparticles by adjusting the concentration of metal solution and immersion time, any other means for controlling the amount of metal nanoparticles are within the scope of the present invention without limitation.

[55] A preferable concentration of metal solution is in the range of 10 ^~5 -10 ² M with 10 ³ -

10 ² M more preferred, most preferably 10 ³ -10 ² M. The immersion can be conducted for a period of time of between one minute and 7 days, preferably from 5 minutes to an hour, more preferably for 20-30 minutes.

[56] Despite the aforementioned description, however, a preferable concentration of metal solution and immersion time vary depending on other factors such as the structure of host compound, the kind of a ligand and the kind of metal nanoparticles. Although the results of all the experiments are not disclosed in Examples herein, it is obvious that one skilled in the art can easily select the concentration of metal solution and the immersion time for achieving the desired effects of the present invention if based on the disclosure of the present invention.

[57] Furthermore, metal-organic framework of [Formula 2] or [Formula 3] or [Formula 3] can be pretreated or desolvated before the immersion. This is preferred because solvent molecules contained in the channels of metal-organic framework can be efficiently removed.

[58]

[59] In still another aspect, the present invention discloses a gas storage material comprising (1) a metal-organic framework and (2) metal nanoparticles loaded in the metal-organic framework.

[60]

[61] Various kinds of gases can be adsorbed on or stored in the metal-organic framework of the present invention depending on various factors such as, without limitation, the pore size or surface characteristic of framework and size of gas. Therefore, a metal- organic framework of the present invention can be applied to the storage of any gas without limitation. Examples of preferred gas include but are not limited to hydrogen, carbon dioxide, methane and oxygen. Mode for the Invention

[62] EXAMPLES

[63] The present invention will be described based on the following Examples. However, the present invention is not limited by the following Examples.

[64]

[65] Preparatory Example 1: Preparation of host compound generating ID channel

[66] A 3D MOF generating ID channels, a compound of Formula 2, was prepared by the method previously reported [Suh, M. P.; Cheon, Y. E.; Lee, E. Y. Chem. Eur. J.2007, 13, 4208-4215].

[67] [Formula 2]

[68] [Zn ₃ (NTB) ₂ (EtOH) ₂ ] _n 4nEtOH

[70] Example 1: Preparation of host compound loading Pd nanoparticles

[71] For spillover experiments, the host solid (80 rnL, 0.066 mmol) prepared in

Preparatory Example 1 was immersed in the MeCN solution (66 mL) of Pd(NO ₃ ) ₂ -2H ₂ O (1.0 x 10 ³ M ) for 30 min at room temperature.

[72] The color of the crystal turned to dark brown, and the FE-TEM image of the resulting solid shows the formation of Pd(O) nanoparticles of size 3.0 0.4 nm (Figure 1).

[73] The IR spectrum shows a new peak at 1395 cm ¹ , corresponding to free NO ₃ anions.

Since the host solid becomes positively charged due to the redox reaction between NTB arylamine species incorporated in the host and the Pd(II) ions, it includes NO ₃ " as the counter anions.

[74] The EPR spectrum shows a peak at g = 1.9986 (standard value: g = 2.002), indicating that nitrogen atom of the arylamine was oxidized to free radical. This mechanism is one of the features herein enabling the achievement of the desired effects of the present invention.

[75] Microanalysis data and ICP result for the nano composite solid indicate that the solid contains 3.0 wt% of Pd nanoparticles.

[76] Further, the XRPD patterns indicate that positions and relative intensities of the peaks are retained even after the Pd nanoparticles (ca. 3.0 nm) are formed (Figures 3(a) and 3(b)). This means that the 3D host framework is maintained even when the arylamine species incorporated in the network are oxidized to arylamine radical species by the Pd(II) ions, which results in positively charged network including NO ₃ anions (Scheme 1, auto redox reaction of arylamine of NTB in host).

[77] [Scheme 1]

[78]

[79] Presumably, Pd(II) ions are introduced to ID channels and react with the arylamine species of the host to form Pd(O) atoms, which diffuse to the surface of the solid to grow into nanoparticles.

[80]

[81] Test Example 1: Gas sorption characteristics of host loading Pd nanoparticles

[82] The host loading palladium nanoparticles prepared in Example 1 was activated by two steps, and then the hydrogen isotherm was measured under two different conditions, (i) 77 K and 1 atm, and (ii) 298 K 95 bar. Specifically, a sample was dried

at 60 ⁰ C for 2 h under vacuum by using Schlenk line, and then a measured amount of pre-dried sample was introduced into a Quntachrome Autosorb- 1 gas sorption apparatus and evacuated at 30 ⁰ C and 10 ⁵ Torr for 12 h.

[83] Compared to pure host, the host loading the palladium nanoparticles adsorbs H ₂ up to

1.48 wt% under same conditions as measured at 77 K and 1 atm. The host adsorbs H ₂ up to 0.30 wt% as measured at 298 K and 95 bar. The Langmuir surface area estimated by CO ₂ sorption is decreased only by 56.7%, from 559 m ² g ^! to 242 rtfg ¹ . These results are because small amount palladium nanoparticles are loaded, and support that palladium nanoparticles formed by auto redox reaction play a role as a primary receptor for hydrogen spillover and host play a role as a secondary receptor.

[84]

[85] Comparative Example 1 and Comparative Test Example 1: Gas sorption char- acteristics of a dried host

[86] Dried host of Formula 3 was prepared by drying the host of Formula 2 prepared in

Preparatory Example 1. Gas sorption of the dried host was observed at 77 K up to 1 atm.

[87] [Formula 3]

[88] [Zn ₃ (NTB)J _n

[89] The dried host shows reversible type-I isotherm, and exhibits Langmuir surface area of 419 m ² g ^"! and pore volume of 0.14 cm ³ g ^" '. Based on CO ₂ sorption at 195 K and 1 atm, Langmuir surface area of 559 m ² g ^! was estimated. The dried host of Formula 3 adsorbs H ₂ up to 1.0 wt% at 77 K and 1 atm (H ₂ density = 0.072 gem ³ ) [Suh, M. P.; Cheon, Y. E.; Lee, E. Y. Chem. Eur. J.2007, 13, 4208-4215].

[90] The compound of Formula 3 adsorbs H ₂ up to 1.0 wt% as measured at 77 K and 1 atm, thus giving a hydrogen density of 0.072 gem ³ [Suh, M. P.; Cheon, Y. E.; Lee, E. Y. Chem. Eur. J.2007, 13, 4208-4215]. This compound adsorbs H ₂ up to 0.13 wt% as measured at 298 K and 95 bar.

[91]

[92] Comparative Example 2 and Comparative Test Example 2: Preparation of host loading excess Pd nanoparticles. and its gas sorption characteristics

[93] Proceeding the same as in Example 1, the host was immersed in the MeCN solution of Pd(NO ₃ ) ₂ (1.0 x 10 ³ M) for longer period of time, ca. 1 h, to provide host loading excess Pd nanoparticles.

[94] The observation of gas sorption characteristics ascertains that the resulting solid includes 3.2 wt% of Pd nanoparticles, and adsorbs H ₂ up to 1.1 wt% under same conditions (Figure 4).

[95]

[96] Comparative Example 3 and Comparative Test Example 3: Preparation of host

loading excess Pd nanoparticles. and its gas sorption characteristics

[97] Proceeding the same as in Example 1, host was immersed in more concentrated

MeCN solution of Pd(NO ₃ ) ₂ (1.0 x 10 ² M) for 30 min.

[98] The content of Pd nanoparticles could not be measured because the resulting solid partially dissolves. Under same conditions, the solid absorbs H ₂ up to 1.1 wt%.

[99]

[100] As described above, small amount of palladium nanoparticles are loaded in a porous 3D MOF, [Zn ₃ (NTB) ₂ ] „ , by the auto redox reaction between arylamine of NTB ligand incorporated in the porous host and Pd(II) ions. It was also ascertained that palladium nanoparticles loaded in the porous host causes spillover effect, thus increasing the hydrogen storage capacity by nearly 50%.

[101] This means spillover effect enhances the hydrogen storage capacity much greater than 50% considering that Langmuir surface area is decreased by 10%. Since the aforementioned observation was made at 77 K and 1 atm of H ₂ pressure, it is obvious that this spillover effect may play a more important role at room temperature and higher hydrogen pressures.

[102] Considering the reduced surface area (57%) and the increased host weight, it is remarkable that H ₂ adsorption capacity increases up to 350% (77 K and 1 atm) and 580% (298 K, 95 bar), respectively.

[103] These results show that the effect of Pd nanoparticles in enhancing H ₂ uptake is more significant at room temperature and high pressures than at 77 K and low pressures.

[104]

[105] Example 2 and Test Example 2: Preparation of host loading Pd nanoparticles. and its gas sorption characteristics

[106] Besides the [Zn ₃ (LIG) ₂ (SOL) ₂ ] -X(SOL), a porous MOF, [Zn ₄ O(LIG) ₂ ]-x(SOL), was prepared by proceeding the same as described in Example 1. The observation of gas sorption characteristics ascertains that gas storage capacity is again remarkably increased similarly as in [Zn ₃ (LIG) ₂ (SOL)J-X(SOL).

[107]

[108] Comparative Example 4-5: Other ligands than arylamine

[109] Proceeding the same as in Example 1, attempts were made to prepare host solid by using tetramethane benzoic acid (MTB) and biphenyl tetracarboxylate (BPTC) in Comparative Examples 4 and 5, respectively, instead of NTB used in Example 1.

[110] However, no metal nanoparticles were formed, thus failing to achieve the desired effects of the present invention.