Water splitting catalyst containing Mn4CaO4 core structure, preparation process and application thereof

Abstract

The present invention provides a process for preparing a water splitting catalyst containing [Mn.sub.4CaO.sub.4] core structure and use thereof. The present invention provides clusters containing [Mn.sub.4CaO.sub.4] core structure by a chemical synthesis using inexpensive metal ions (Mn.sup.2+, Ca.sup.2+ ions), simple carboxyl ligands and a permanganate, performed single crystal X-ray diffraction on their space structure, and characterized their physical and chemical properties with electron spectrum, electrochemical and electron paramagnetic resonance technologies and the like. These compounds can catalyze water splitting in the presence of oxidant to release oxygen and can also catalyze water splitting on the surface of an electrode to release electrons onto the surface of the electrode to form a current.

Claims

1. A [Mn.sub.4CaO.sub.4](R.sub.1CO.sub.2).sub.8(L.sub.1)(L.sub.2)(L.sub.3) compound of Formula I, comprising an asymmetric [Mn.sub.4CaO.sub.4] heteronuclear metal cluster skeleton core having four Mn ions and one Ca.sup.2+ ion linked via four O.sup.2 ions; and peripheral ligands that are eight carboxylic acid anions and three neutral ligands (L.sub.1, L.sub.2, L.sub.3); ##STR00008## wherein, R.sub.1 is H or a C.sub.1-8 linear or branched alkyl; the three ligands L.sub.1, L.sub.2, and L.sub.3 are the same or different and are each independently selected from the group consisting of carboxylic acid molecules, pyridine, imidazole, pyrazine, quinoline, isoquinoline, H.sub.2O, alcohol molecules, ketones, nitriles, and esters, wherein two of the four Mn ions have a valance state of +3 and the other two of the four Mn ions have a valance state of +4.

2. The compound according to claim 1, wherein the compound is electrically neutral.

3. The compound according to claim 1, wherein the carboxylic acid anion (R.sub.1CO.sub.2.sup.) is selected from the group consisting of formic acid, acetic acid, propionic acid, butyric acid, isobutyric acid, valeric acid, isovaleric acid, pivalic acid, and hexanoic acid.

4. The compound according to claim 1, wherein the compound is [Mn.sub.4CaO.sub.4(R.sub.1CO.sub.2).sub.8](L.sub.1)(L.sub.2)(L.sub.3), wherein R.sub.1=tert-butyl; L.sub.1=pyridine; L.sub.2=L.sub.3=pivalic acid; [Mn.sub.4CaO.sub.4(R.sub.1CO.sub.2).sub.8](L.sub.1)(L.sub.2)(L.sub.3), wherein R.sub.1=tert-butyl; L.sub.1=L.sub.2=pyridine; L.sub.3=pivalic acid; or [Mn.sub.4CaO.sub.4(R.sub.1CO.sub.2).sub.8](L.sub.1)(L.sub.2)(L.sub.3), wherein R.sub.1=tert-butyl; L.sub.1=isoquinoline, L.sub.2=L.sub.3=pivalic acid.

5. The compound according to claim 4, wherein the compound is [Mn.sub.4CaO.sub.4(R.sub.1CO.sub.2).sub.8](L.sub.1)(L.sub.2)(L.sub.3), wherein R.sub.1=tert-butyl; L.sub.1=pyridine; L.sub.2=L.sub.3=pivalic acid; its single crystal being monoclinic, space group being P2.sub.1/c1, cell parameter being a=29.317(7), b=18.894(4), c=29.903(7), =90.00, =104.609(4), =90.00, Z=8, volume being 16028(7).sup.3, and its structure being shown as in Formula I-1: ##STR00009## [Mn.sub.4CaO.sub.4(R.sub.1CO.sub.2).sub.8](L.sub.1)(L.sub.2)(L.sub.3), wherein R.sub.1=tert-butyl; L.sub.1=L.sub.2=pyridine; L.sub.3=pivalic acid; its single crystal being monoclinic, space group being P2.sub.1/c1, cell parameter being a=21.969(4), b=25.326(5), c=29.236(6), =90.00, =102.70(3), =90.00, Z=8, volume being 15869(6).sup.3; and its structure being shown as in Formula I-2: ##STR00010## or [Mn.sub.4CaO.sub.4(R.sub.1CO.sub.2).sub.8](L.sub.1)(L.sub.2)(L.sub.3), wherein R.sub.1=tert-butyl; L.sub.1=isoquinoline, L.sub.2=L.sub.3=pivalic acid; its single crystal being trigonal, space group being R-3, cell parameter being a=38.379(5), b=38.379(5), c=35.682(7), =90.00, =90.00, =120.00, Z=18, volume being 45517(12).sup.3; its structure being shown as in Formula I-3: ##STR00011##

6. A process for preparing the [Mn.sub.4CaO.sub.4](RCO.sub.2).sub.8(L.sub.1)(L.sub.2)(L.sub.3)-compound according to claim 1, ##STR00012## comprising: step 1: heating a mixture comprising an acid, an oxidant, a Mn.sup.2+ salt, and a Ca.sup.2+ salt at a molar ratio of (10-120):(2-8):1:1 in an acetonitrile solvent for 10-60 minutes to obtain a solution, filtering the solution to obtain a filtrate; crystallizing the filtrate at 0 C. to obtain crystals; step 2: dissolving the crystals obtained from step 1 in an ester solvent, and adding ligands L.sub.1, L.sub.2 and L.sub.3 to the ester solvent to cause recrystallization to obtain the [Mn.sub.4CaO.sub.4](RCO.sub.2).sub.8(L.sub.1)(L.sub.2)(L.sub.3) compound of claim 1.

7. The preparation process according to claim 6, wherein the Mn.sup.2+ salt is selected from the group consisting of carboxylic acid salts of Mn.sup.2+; wherein the carboxylic acid anion (R.sub.1CO.sub.2.sup.) is formate, acetate, propionate, butyrate, isobutyrate, valerate, isovalerate, pivalate, and hexanoate, Mn(ClO.sub.4).sub.2, MnSO.sub.4, Mn(NO.sub.3).sub.2, Mn(CF.sub.3SO.sub.3).sub.2, Mn(ClO.sub.4).sub.2(H.sub.2O).sub.n, MnSO.sub.4(H.sub.2O).sub.n, Mn(NO.sub.3).sub.2(H.sub.2O).sub.n, and Mn(CF.sub.3SO.sub.3).sub.2(H.sub.2O).sub.n, wherein n=1-5; the Ca.sup.2+ salt is selected from the group consisting of carboxylic acid salts of calcium, wherein the carboxylic acid anion (R.sub.1CO.sub.2.sup.) is formate, acetate, propionate, butyrate, isobutyrate, valerate, isovalerate, pivalate, hexanoate Ca(ClO.sub.4).sub.2, Ca(NO.sub.3).sub.2, Ca(CF.sub.3SO.sub.3).sub.2, Ca(ClO.sub.4).sub.2(H.sub.2O).sub.n, Ca(NO.sub.3).sub.2(H.sub.2O).sub.n, Ca(CF.sub.3SO.sub.3).sub.2(H.sub.2O).sub.n, wherein n=1-5; the oxidant is an permanganate anionic oxidant, the acid is acetic acid, propionic acid, butyric acid, isobutyric acid, valeric acid, isovaleric acid, pivalic acid, and hexanoic acid.

8. The preparation process according to claim 7, wherein the oxidant is tetrabutylammonium permanganate ((C.sub.4H.sub.9).sub.4NMnO.sub.4).

9. The preparation process according to claim 6, characterized in that a volume of the acetonitrile solvent in step 1 is 60-100 ml acetonitrile per mmol calcium salt; the ester solvent in step 2 is ethyl acetate, methyl acetate, or propyl propionate.

10. The process according to claim 6, wherein the [Mn.sub.4CaO.sub.4](R.sub.1CO.sub.2).sub.8(L.sub.1)(L.sub.2)(L.sub.3) compound is electrically neutral.

11. The process according to claim 6, wherein, in the [Mn.sub.4CaO.sub.4](R.sub.1CO.sub.2).sub.8(L.sub.1)(L.sub.2)(L.sub.3) compound, R.sub.1 is hydrogen (H), methyl (CH.sub.3), ethyl (C.sub.2H.sub.5), n-propyl (CH.sub.2CH.sub.2CH.sub.3), isopropyl (CH(CH.sub.3).sub.2), n-butyl, isobutyl, tert-butyl (C(CH.sub.3).sub.3), n-pentyl ((CH.sub.2).sub.4CH.sub.3), or isopentyl (CH(CH.sub.3)C.sub.2H.sub.5).

12. A method to splitting water, comprising: adding the [Mn.sub.4CaO.sub.4](R.sub.1CO.sub.2).sub.8(L.sub.1)(L.sub.2)(L.sub.3) compound of claim 1 to water to form a mixture; and adding an oxidant or applying an electric current to the mixture.

13. A water splitting catalyst comprising the [Mn.sub.4CaO.sub.4](R.sub.1CO.sub.2).sub.8(L.sub.1)(L.sub.2)(L.sub.3) compound according to claim 1.

14. The water splitting catalyst according to claim 13, wherein, the [Mn.sub.4CaO.sub.4](R.sub.1CO.sub.2).sub.8(L.sub.1)(L.sub.2)(L.sub.3) compound is electrically neutral.

15. The water splitting catalyst according to claim 13, wherein R.sub.1 is hydrogen (H), methyl (CH.sub.3), ethyl (C.sub.2H.sub.5), n-propyl (CH.sub.2CH.sub.2CH.sub.3), isopropyl (CH(CH.sub.3).sub.2), n-butyl, isobutyl, tert-butyl (C(CH.sub.3).sub.3), n-pentyl ((CH.sub.2).sub.4CH.sub.3), or isopentyl (CH(CH.sub.3)C.sub.2H.sub.5).

Description

DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is the crystal structure diagram of compound 1 prepared in Example 1 of the present invention. For the sake of clarity, the hydrogen atom, the methyl of tert-butyl and solvent molecules are all omitted.

(2) FIG. 2 is the crystal structure diagram of compound 2 prepared in Example 2 of the present invention. For the sake of clarity, the hydrogen atom, the methyl of tert-butyl and solvent molecules are all omitted.

(3) FIG. 3 is the crystal structure diagram of compound 3 prepared in Example 3 of the present invention. For the sake of clarity, the hydrogen atom, the methyl of tert-butyl and solvent molecules are all omitted.

(4) FIG. 4 shows the trace of the change in UV-Vis absorption spectrum of the action between compound 1 and water in Example 4 of the present invention.

(5) FIG. 5 shows the electrochemical data of compound 1 per se and its catalytic splitting of water on the surface of electrode to release electrons in Example 5 of the present invention.

(6) FIG. 6 shows the electron paramagnetic signal given by oxidized compound 1 in Example 6 of the present invention. The data support that the valence states of the four Mn ions in the ground state of compound 1 are +3, +3, +4 and +4 respectively.

(7) FIG. 7 shows the determination of the oxygen released by the water splitting catalyzed by the compound 1 in the presence of oxidant in Example 7 of the present invention.

SPECIFIC MODE FOR CARRYING OUT THE INVENTION

(8) The technical solutions according to the present invention will be illustrated by the following specific examples. Those skilled in the art should understand that the examples are not intended to limit the invention. Any improvements and modifications that may be made on the basis of the invention are within the protection scope of the invention.

Example 1: Compound 1 [Mn4CaO4](C5H9O2)8(C5H9O2H)2(C5H5N)

(9) The preparation process was as follows:

(10) The first step was the synthesis of the precursor of compound 1. To a 100 ml round bottom flask were added tetrabutylammonium permanganate (Bu.sup.n.sub.4NMnO.sub.4, 4 mmol), manganese acetate (Mn(CH.sub.3CO.sub.2).sub.2, 1 mmol), calcium acetate (Ca(CH.sub.3CO.sub.2).sub.2, 1 mmol) and pivalic acid ((CH.sub.3).sub.3CCO.sub.2H, 40 mmol). After continuous reaction in acetonitrile at 80 C. for 25 min, the reaction was stopped. The resultant was filtered to remove a small amount of precipitate. The resulting brown mother liquor was allowed to stand at 0 C. for 1-2 weeks to precipitate brown crystals.

(11) The second step was recrystallization. The crystals obtained in the first step were collected and dissolved with ethyl acetate. 2% (volume ratio) pyridine was added for recrystallization. After 1-2 weeks, brown crystals were precipitated, leached with cyclohexane and vacuum dried. The yield was about 40% (according to the mole numbers of Ca ions).

(12) Compound 1 has a structural formula of [Mn.sub.4CaO.sub.4(R.sub.1CO.sub.2).sub.8](L.sub.1)(L.sub.2)(L.sub.3), wherein R.sub.1=tert-butyl; L.sub.1=pyridine; L.sub.2=L.sub.3=pivalic acid.

(13) That is, compound 1 has the structural formula of [Mn.sub.4CaO.sub.4](C.sub.5H.sub.9O.sub.2).sub.8(C.sub.5H.sub.9O.sub.2H).sub.2(C.sub.5H.sub.5N).C.sub.6H.sub.12 (note: the cyclohexane is a solvent molecule) with the molecular formula of C.sub.61H.sub.109NO.sub.24CaMn.sub.4. Theoretical values of elemental analysis: C, 48.83; H, 7.32; N, 0.93; experimental values: C, 49.14; H, 7.59; N, 1.18. Compound 1 has a single crystal of monoclinic system, with space group of P2.sub.1/c1, cell parameter of a=29.317(7), b=18.894(4), c=29.903(7), =90.00, =104.609(4), =90.00, Z=8, and volume of 16028(7).sup.3.

(14) Compound 1 has the chemical structure shown by the Formula I-1 below, the determined specific single crystal parameters shown in Table 1, and the crystal space structure shown in FIG. 1.

(15) ##STR00005##

Example 2: Compound 2 [Mn4CaO4](C5H9O2)8(C5H9O2H)1(C5H5N)2

(16) 0.100 g compound 1 was weighed and dissolved in ethyl acetate, to which 1% pyridine was added, and the mixture was allowed to stand at room temperature for 3 weeks to precipitate black crystals, which was then leached with cyclohexane and vacuum dried. The yield was about 13% (according to the mole numbers of Ca ions).

(17) Compound 2 has a structural formula [Mn.sub.4CaO.sub.4(R.sub.1CO.sub.2).sub.8](L.sub.1)(L.sub.2)(L.sub.3), wherein R.sub.1=tert-butyl; L.sub.1=L.sub.2=pyridine; L.sub.3=pivalic acid.

(18) That is, compound 2 has the structural formula of [Mn.sub.4CaO.sub.4](C.sub.5H.sub.9O.sub.2).sub.8(C.sub.5H.sub.9O.sub.2H).sub.1(C.sub.5H.sub.5N).sub.2 with the molecular formula of C.sub.55H.sub.92N.sub.2O.sub.22CaMn.sub.4. Theoretical values of elemental analysis: C, 47.42; H, 6.66; N, 2.01; experimental values: C, 47.74; H, 6.89; N, 1.69.

(19) Compound 2 has a single crystal of monoclinic system, with space group of P2.sub.1/c1, cell parameter of a=21.969(4), b=25.326(5), c=29.236(6), =90.00, =102.70(3), =90.00, Z=8, and volume of 15869(6).sup.3. Compound 2 has the chemical structure shown by the Formula I-2 below, the determined specific single crystal parameters shown in Table 2, and the crystal space structure shown in FIG. 2.

(20) ##STR00006##

Example 3: Compound 3 [Mn4CaO4](C5H9O2)9(C5H9O2H)2(C9H7N)

(21) The first step was the synthesis of compound precursor. To a 100 ml round bottom flask were added tetrabutylammonium permanganate (Bu.sup.n.sub.4NMnO.sub.4, 4 mmol), manganese acetate (Mn(CH.sub.3CO.sub.2).sub.2, 1 mmol), calcium acetate (Ca(CH.sub.3CO.sub.2).sub.2, 1 mmol) and pivalic acid ((CH.sub.3).sub.3CCO.sub.2H, 40 mmol). After continuous reaction in acetonitrile at 80 C. for 25 min, the reaction was stopped. The resultant was filtrated to remove a small amount of precipitate. The resulting brown mother liquor was allowed to stand at 0 C. for 1-2 weeks to precipitate brown crystals.

(22) The second step was recrystallization. The crystals obtained in the first step were collected and dissolved with ethyl acetate, to which 1% (volume ratio) isoquinoline was added for recrystallization. After 1-2 weeks, black crystals were collected, leached with cyclohexane and vacuum dried. The yield was about 40% (according to the mole numbers of Ca ions).

(23) Compound 3 has a structural formula of [Mn.sub.4CaO.sub.4(R.sub.1CO.sub.2).sub.8](L.sub.1)(L.sub.2)(L.sub.3), wherein R.sub.1=tert-butyl; L.sub.1=isoquinoline, L.sub.2=L.sub.3=pivalic acid.

(24) That is, compound 3 has the structural formula of [Mn.sub.4CaO.sub.4](C.sub.5H.sub.9O.sub.2).sub.9(C.sub.5H.sub.9O.sub.2H).sub.2(C.sub.9H.sub.7N) with the molecular formula of C.sub.59H.sub.99NO.sub.24CaMn.sub.4. In the elemental analysis of compound 3, theoretical values are: C, 48.33; H, 6.81; N, 0.96, and experimental values are C, 48.21; H, 6.81; N, 1.06. Compound 3 has a single crystal of trigonal system, with space group of R-3, cell parameter of a=38.379(5), b=38.379(5), c=35.682(7), =90.00, =90.00, =120.00, Z=18, and volume of 45517(12).sup.3.

(25) Compound 3 has the chemical structure shown by the Formula I-3 below, the determined specific single crystal parameters shown in Table 3, and the crystal space structure shown in FIG. 3.

(26) ##STR00007##

Example 4: Trace of the UV-Vis Spectrum of the Action Between Compound 1 and Water

(27) To a colorimetric ware was added 1 mL acetonitrile solution of 25 M compound 1. Using 1 mL pure acetonitrile as reference, absorption spectrum was determined in Hitachi U-3900 spectrophotometer type UV-Vis spectrometer (see FIG. 4). This compound had the maximum absorption at 250 nm. Accompany with the addition of water molecules (0%, 0.2%, 0.4%, 0.6%, 0.8% and 1.0% water being added respectively), the absorption spectrum changed significantly. Specifically, the absorption at 250 nm decreased significantly, while the absorption in the visible region (400-800 nm) increased significantly, and an isobestic point was observed at 363 nm, which indicated that water molecules acted with compound 1.

Example 5: Electron Paramagnetic Resonance of Compound 1 for Detecting the Valence State of Mn Ions in the Compound

(28) Compound 1 (1 mM) was dissolved in dichloroethane, and then 0.5 mM oxidant [Fe(Phen).sub.3](PF.sub.6).sub.3 was added. The mixture was then rapidly frozen to 77K and its electron paramagnetic signals were detected with Bruker E500 electron paramagnetic resonance instrument at 7K (see FIG. 5). We could clearly see the paramagnetic signals of g=2.0 and g=4.9. The occurrence of these two signals indicated that after the compound was oxidized, the valence states of the four manganese ions were respectively +3, +4, +4 and +4. Thus we could infer that the valence states of the four Mn ions in the ground state (stable state before oxidation) of the compound were +3, +3, +4 and +4 repectively.

Example 6: Electrochemical Determination of Compound 1 and its Catalysis of Water Splitting on the Surface of an Electrode

(29) An electrochemical workstation was used to trace the electrochemistry of compound 1 and its catalysis of water splitting on the surface of an electrode. The working electrode was glassy carbon electrode, the counter electrode was platinum electrode, and silver/silver nitrate (10 mM) was the reference electrode. The electrolyte solvent was acetonitrile, the electrolyte was tetrabutylphosphorus hexafluoride (C.sub.4H.sub.9).sub.4NPF.sub.6) and the scanning speed was 100 mV/s. The inset of FIG. 6 showed the cyclic voltammetry curve of compound 1 in the absence of water. Two oxidation processes could be observed with their corresponding potentials of 0.8 V and 1.32 V, respectively. Upon the presence of a small amount of water (the corresponding water contents of the curves in the figure were 1%, 0.8%, 0.6%, 0.4% and 0% successively), the two oxidation couple became not clear. Instead, a rapidly increasing process, corresponding to the water splitting process, was observed. As can be seen from the figure, when 1% water was present, the current value generated by the electrons released by water splitting could exceed 400 A. This indicated that compound 1 could catalyze the splitting of water very effectively on the surface of the electrode and transfer the released electrons onto the surface of the electrode to form a current.

Experimental Example 7: Determination of the Oxygen Released by the Water Splitting Catalyzed by the Compound 1 in the Presence of Oxidant

(30) The activity for releasing oxygen by the catalysis of water splitting was determined on a Clark-type oxygen electrode (FIG. 7). A rapid release of oxygen can be observed by the addition of 125 M of compound 1 in an aqueous solution containing an oxidant (tert-butyl hydroperoxide, 0.7 M), while no formation of oxygen could be observed at all with the addition of the reference compound (Mn(ClO.sub.4).sub.2). The arrow in the figure showed the loading position of the sample. FIG. 7 indicated that compound 1 had the catalytic activity of catalyzing the splitting of water to release oxygen.