ZIRCONIUM-BASED METAL-ORGANIC FRAMEWORKS AS CATALYST FOR TRANSFER HYDROGENATION

Abstract

The present invention relates to a catalyst for transfer hydrogenation, which is formed of a metal-organic framework having an MOF-808 based X-ray diffraction pattern.

A high crystalline porous MOF-808 based metal-organic framework exhibits excellent performance in the transfer hydrogenation of ethyl levulinate (EL) at high and low temperature.

Claims

1. A method of transfer hydrogenation, comprising transferring hydrogen from a hydrogen donor to a hydrogen acceptor by using a catalyst which is formed of a metal-organic framework having an MOF-808 based X-ray diffraction pattern.

2. The method of claim 1, wherein the metal-organic framework is represented by Formula 1 or Formula 2 below:
M.sub.6O.sub.4(OH).sub.4(BTC).sub.2(HCOO).sub.6  [Formula 1] (wherein M is a group 4A or 4B element, or a lanthanide metal whose oxidation state is 4.sup.+)
M.sub.6(μ.sub.3-OH).sub.4(μ.sub.3-OH).sub.4(OH).sub.6-x(H.sub.2O).sub.6(BTC).sub.2(HCOO).sub.x  [Formula 2] (wherein x is any number in the range of 0 to 6, and M is a group 4A or 4B element, or a lanthanide metal whose oxidation state is 4.sup.+).

3. The method of claim 1, wherein the transfer hydrogenation is the transfer hydrogenation of ethyl levulinate (EL) into γ-valerolactone.

4. The method of claim 2, wherein the transfer hydrogenation is the transfer hydrogenation of ethyl levulinate (EL) into γ-valerolactone.

5. The method of claim 1, wherein the transfer hydrogenation is the transfer hydrogenation of furfural to furfuryl alcohol; levulinic acid (LA) to γ-valerolactone (GVL); furfural to 2-methylfuran (2-MF); 5-hydroxymethylfurfural (HMF) to 2,5-dimethylfuran (DMF); glycerol to 1,2-propanediol (1,2-PDO); fructose to 5-hydroxymethylfurfural (HMF); glucose to γ-valerolactone (GVL); fructose to γ-valerolactone (GVL); butyl levulinate (BL) to γ-valerolactone (GVL); glycerol to 1,2-PDO; (1-hydroxyethyl)benzene (1-HB) to ethylbenzene; 5-hydroxymethylfurfural (HMF) to 1,6-hexanediol (HDL); benzaldehyde to benzyl alcohol; hexahydrobenzaldehyde to hexahydrobenzyl alcohol; 4-methylbenzaldehyde to 4-methylbenzyl alcohol; methyl phenyl ketone to 1-phenylethanol; hexanal to 1-hexanol; 4-methyl-2-pentanone to 4-methyl-2-pentanol; cinnamaldehyde to cinnamyl alcohol; thiophene-2-aldehyde to 2-(hydroxymethyl) thiophene; 4-pyridinecarboxaldehyde to 4-pyridylcarbinol; or giranial to geraniol.

6. The method of claim 2, wherein the transfer hydrogenation is the transfer hydrogenation of furfural to furfuryl alcohol; levulinic acid (LA) to γ-valerolactone (GVL); furfural to 2-methylfuran (2-MF); 5-hydroxymethylfurfural (HMF) to 2,5-dimethylfuran (DMF); glycerol to 1,2-propanediol (1,2-PDO); fructose to 5-hydroxymethylfurfural (HMF); glucose to γ-valerolactone (GVL); fructose to γ-valerolactone (GVL); butyl levulinate (BL) to γ-valerolactone (GVL); glycerol to 1,2-PDO; (1-hydroxyethyl)benzene (1-HB) to ethylbenzene; 5-hydroxymethylfurfural (HMF) to 1,6-hexanediol (HDL); benzaldehyde to benzyl alcohol; hexahydrobenzaldehyde to hexahydrobenzyl alcohol; 4-methylbenzaldehyde to 4-methylbenzyl alcohol; methyl phenyl ketone to 1-phenylethanol; hexanal to 1-hexanol; 4-methyl-2-pentanone to 4-methyl-2-pentanol; cinnamaldehyde to cinnamyl alcohol; thiophene-2-aldehyde to 2-(hydroxymethyl) thiophene; 4-pyridinecarboxaldehyde to 4-pyridylcarbinol; or giranial to geraniol.

7. The method of claim 1, wherein isopropanol, methanol, ethanol, glycerol, butanol, cyclic ethers, benzyl alcohol, cyclohexanone, 2-propanol, ethylene glycol, 2,3-dihydroindole, 1,2,3,4-tetrahydroquinoline, cyclohexene, cyclohexadiene, limonene, hydrazine, ammonium formate, ammonium hypophosphite, or a mixture thereof is used as the hydrogen donor.

8. The method of claim 2, wherein isopropanol, methanol, ethanol, glycerol, butanol, cyclic ethers, benzyl alcohol, cyclohexanone, 2-propanol, ethylene glycol, 2,3-dihydroindole, 1,2,3,4-tetrahydroquinoline, cyclohexene, cyclohexadiene, limonene, hydrazine, ammonium formate, ammonium hypophosphite, or a mixture thereof is used as the hydrogen donor.

9. The method of claim 1, wherein the transfer hydrogenation is performed at a low temperature of 200° C. or below by using the catalyst.

10. The method of claim 2, wherein the transfer hydrogenation is performed at a low temperature of 200° C. or below by using the catalyst.

11. The method of claim 1, wherein the transfer hydrogenation is the open-system transfer hydrogenation which utilizes a solvent-reflux method.

12. The method of claim 2, wherein the transfer hydrogenation is the open-system transfer hydrogenation which utilizes a solvent-reflux method.

13. A method of preparing γ-valerolactone by transfer hydrogenation of ethyl levulinate (EL) using a catalyst which is formed of a metal-organic framework having an MOF-808 based X-ray diffraction pattern.

14. The method of claim 13, wherein the metal-organic framework is represented by Formula 1 or Formula 2 below:
M.sub.6O.sub.4(OH).sub.4(BTC).sub.2(HCOO).sub.6  [Formula 1] (wherein M is a group 4A or 4B element, or a lanthanide metal whose oxidation state is 4.sup.+)
M.sub.6(μ.sub.3-O).sub.4(μ.sub.3-OH).sub.4(OH).sub.6-x(H.sub.2O).sub.6(BTC).sub.2(HCOO).sub.x  [Formula 2] (wherein x is any number in the range of 0 to 6, and M is a group 4A or 4B element, or a lanthanide metal whose oxidation state is 4.sup.+).

15. The method of claim 13, wherein the transfer hydrogenation is performed using isopropanol, methanol, ethanol, glycerol, butanol, cyclic ethers, benzyl alcohol, cyclohexanone, 2-propanol, ethylene glycol, 2,3-dihydroindole, 1,2,3,4-tetrahydroquinoline, cyclohexene, cyclohexadiene, limonene, hydrazine, ammonium formate, ammonium hypophosphite, or a mixture thereof as a hydrogen donor.

16. The method of claim 14, wherein the transfer hydrogenation is performed using isopropanol, methanol, ethanol, glycerol, butanol, cyclic ethers, benzyl alcohol, cyclohexanone, 2-propanol, ethylene glycol, 2,3-dihydroindole, 1,2,3,4-tetrahydroquinoline, cyclohexene, cyclohexadiene, limonene, hydrazine, ammonium formate, ammonium hypophosphite, or a mixture thereof as a hydrogen donor.

17. The method of claim 13, wherein the transfer hydrogenation is performed in a non-homogenous catalytic system.

18. The method of claim 13, wherein the transfer hydrogenation of ethyl levulinate (EL) is performed in an open-system which utilizes a non-homogenous catalyst and a solvent-reflux method.

19. A method for preparing furfural alcohol by transfer hydrogenation of furfural using a catalyst which is formed of a metal-organic framework having an MOF-808 based X-ray diffraction pattern.

20. The method of claim 19, wherein the metal-organic framework is represented by Formula 1 or Formula 2 below:
M.sub.6O.sub.4(OH).sub.4(BTC).sub.2(HCOO).sub.6  [Formula 1] (wherein M is a group 4A or 4B element, or a lanthanide metal whose oxidation state is 4.sup.+)
M.sub.6(μ.sub.3-O).sub.4(μ.sub.3-OH).sub.4(OH).sub.6-x(H.sub.2O).sub.6(BTC).sub.2(HCOO).sub.x  [Formula 2] (wherein x is any number in the range of 0 to 6, and M is a group 4A or 4B element, or a lanthanide metal whose oxidation state is 4.sup.+).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] FIG. 1 shows a representative structural diagram illustrating UiO-66(Zr) and analogs thereof which are functionalized by other ligands.

[0042] FIGS. 2A-2D show graphs illustrating a) XRD patterns, b) FTIR spectra, c) TGA profiles, and d) N2 adsorption at 77K, with regard to UiO-66(Zr) and functionalized analogs thereof.

[0043] FIG. 3 shows graphs illustrating the effect of reaction temperature in the catalytic transfer hydrogenation of EL to GVL by UiO-66(Zr) (reaction conditions: EL (4 mmol), isopropanol (400 mmol), catalyst (0.8 g), naphthalene (0.24 g), and reaction time (4 h)).

[0044] FIG. 4 shows a graph illustrating the effect of reaction time in the catalytic transfer hydrogenation of EL to GVL (EL (4 mmol), isopropanol (400 mmol), catalyst (0.8 g), naphthalene (0.24 g), and reaction temperature (200° C.)).

[0045] FIG. 5 shows a graph illustrating the experimental results of recycling of UiO-66(Zr) catalyst in the catalytic transfer hydrogenation of EL to GVL (EL (4 mmol), isopropanol (400 mmol), catalyst (0.8 g), naphthalene (0.24 g), reaction temperature (200° C.), and reaction time (2 h)).

[0046] FIGS. 6A-6D show a graph illustrating the characteristics of unused UiO-66(Zr) and the used UiO-66(Zr) after 5 cycles ((a) XRD patterns, b) N.sub.2 adsorption-desorption isotherm curves, c) TGA curves, and d) FTIR spectra).

[0047] FIGS. 7A-7B show SEM images of a) an unused UiO-66(Zr) catalyst and b) a used UiO-66(Zr) catalyst.

[0048] FIGS. 8A-8B show representative structural diagrams of Zr-MOFs ((a) UiO-66 and b) MOF-808)).

[0049] FIGS. 9A-9B show a graph illustrating a) N.sub.2 adsorption isotherm (at 77 K) of Zr-MOFs and shows a graph illustrating b) micropore size distribution of Zr-MOFs calculated using the Horvath-Kawazoe method.

[0050] FIG. 10 shows the results of a recycling test of MOF-808 catalyst (reaction conditions: EL (4 mmol), isopropanol (400 mmol), catalyst (0.2 g), naphthalene (0.24 g), reaction temperature (130° C.), and reaction time (3 h)).

[0051] FIGS. 11A-11F show results illustrating the characteristics of unused MOF-808 and the used MOF-808 after 5 cycles ((a) XRD patterns, b) N2 adsorption-desorption isotherm at 77 K, c) TGA curves, and d) FTIR patterns, and SEM images e) Fresh and f) Used.

[0052] FIGS. 12A-12B show graphs illustrating the profiles of temperature programmed desorption (TPD) of Zr-MOFs, in which a) NH.sub.3 and b) CO.sub.2 were used as probe molecules.

[0053] FIG. 13 shows graphs illustrating the analysis data (gas chromatogram) of furfural alcohol conversion reactants obtained by transfer hydrogenation of furfural, and the changes in XRD structure of MOF-808 before and after reaction.

DETAILED DESCRIPTION OF THE INVENTION

[0054] Hereinafter, the present invention will be described in more detail with reference to the following Examples. However, these Examples are for illustrative purposes only and the invention is not intended to be limited by these Examples.

[0055] Chemicals and Materials

[0056] Ethyl levulinate (99%), γ-valerolactone (99%), 2-propanol (99.5%), naphthalene (98%), ZrOCl.sub.2.8H.sub.2O (98%), ZrCl.sub.4 (99.5%), 1,4-benzenedicarboxylic acid (H.sub.2BDC, 98%), 1,2,4-1,2,4-benzenetricarboxylic acid (H.sub.3BTC, 99%), 2-aminoterphthalic acid (99%), 1,3,5-benzenetricarboxylic acid (H.sub.3BTC, 95%), benzoic acid (99.5%), acetic acid (99.7%), HCl (37%), and N,N-dimethylformamide (99.8%) were purchased from Sigma-Aldrich.

[0057] Formic acid (99%) and fumaric acid (99%) were purchased from Samchun Pure Chemicals (South Korea) and isopropyl 4-oxovalerate (95%) was purchased from Beta Pharma Co., Ltd. (Shanghai, China). All chemicals were used without further purification.

Preparation Example 1: Synthesis of UiO-66(Zr)

[0058] UiO-66(Zr) was synthesized by the reflux method. Specifically, H.sub.2BDC (4.62 g, 27.8 mmol) was dissolved in DMF (23.6 g, 322 mmol) in a 100 mL round flask at room temperature. Then, ZrOCl.sub.2.8H.sub.2O (8.96 g, 27.8 mmol) and 37% HCl (4.63 mL, 5.47 g, 150 mmol) were added to the mixture. The molar ratio of the final ZrOCl.sub.2.8H.sub.2O/H.sub.2BDC/DMF/HCl mixture was 1:1:11.6:5.4. The reaction mixture was vigorously stirred to obtain a homogeneous gel. The mixture was heated to 150° C. and maintained thereat for 6 hours to obtain crystalline UiO-66(Zr) solid. The thus-obtained product was recovered from the slurry, re-dispersed in DMF at 60° C. for 6 hours while stirring, and recovered by filtration. The same procedure was repeated twice using methanol instead of DMF. Finally, the solid product was dried at 100° C. overnight.

Preparation Example 2: Synthesis of UiO-66(Zr)—NH.SUB.2

[0059] UiO-66(Zr)—NH.sub.2 was synthesized by the reflux method. First, 2-aminoterphthalic acid 1.94 g (10.7 mmol) was dissolved in water (38.4 mL) and acetic acid (9.2 mL, 160.1 mmol). Then, ZrOCl.sub.2.8H.sub.2O (3.44 g, 10.7 mmol) was added to the solution with continuously stirring. The molar ratio of the final ZrOCl.sub.2.8H.sub.2O/H.sub.2BDC-NH.sub.2/H.sub.2O/CH.sub.3COOH mixture was 1:1:200:15. The reaction solution was ramped to 100° C. and kept thereat for 24 hours. After cooling to room temperature, the precipitate in the reaction solution was filtered. To remove the residual precursor and ligands, the precipitate was sufficiently washed with water (80° C.) and ethanol (60° C.), respectively. Finally, the product was dried at 100° C. overnight.

Preparation Example 3: Synthesis of UiO-66(Zr)—COOH

[0060] UiO-66(Zr)—COOH was also synthesized by the reflux method. Briefly, 1,2,4-benzenetricarboxylic acid (14 g, 66.7 mmol) and ZrOCl.sub.2.8H.sub.2O (10.74 g, 33.3 mmol) were dissolved in water (30 mL) and benzoic acid (20.4 g, 166.6 mmol) in a 100 mL round flask. The molar ratio of the final ZrOCl.sub.2.8H.sub.2O/H.sub.3BTC/H.sub.2O/C.sub.6H.sub.5COOH mixture was 1:2:50:5. Then, the reaction solution was ramped to 100° C. and kept thereat for 24 hours. Post-treatment was also performed in exactly the same manner as in the procedure mentioned for UiO-66(Zr)—NH.sub.2.

Preparation Example 4: Synthesis of MOF-808

[0061] H.sub.3BTC (4.8 g, 0.5 mmol) and ZrOCl.sub.2.8H.sub.2O (3.3 g, 0.5 mmol) were added to a solvent mixture of DMF/formic acid (270 mL/360 mL). The reaction mixture was transferred to a 1 L Teflon-lined pressure autoclave and heated at 135° C. for 2 days. The white precipitate was collected by centrifugation, washed in DMF for 24 hours, and washed in ethanol for 24 hours. Each solvent was replaced twice during the above period, and finally, the product was dried at 100° C. for 12 hours.

Preparation Example 5: Synthesis of MOF-801

[0062] MOF-801, commonly known as Zr-fumarate MOF, was synthesized according to the method disclosed in a previous journal publication. Briefly, ZrCl.sub.4 (2.585 mmol, 1 eq.) was dissolved in 50 mL of water. Then, formic acid (258.5 mmol, 100 eq.) as a modulator and fumaric acid (7.75 mmol, 3 eq.) as a linker molecule were added to the metal precursor solution. The reaction mixture transferred into a Teflon-lined pressure vessel and heated at 120° C. for 24 hours. The resulting white precipitate was collected by centrifugation and washed with sufficient water and ethanol. Finally, the product was dried 100° C. at overnight.

Example 1: Characterization of Catalysts

[0063] Powder diffraction patterns were obtained by the Rigaku diffractometer using Ni-filtered Cu Kα-radiation (40 kV, 30 mA, λ=1.5406 Å). The N.sub.2 adsorption-desorption isotherms were measured at 77K using a Micromeritics Tristar 3000. The samples were dehydrated under vacuum at 423K for 12 hours before analysis. The specific surface areas were evaluated using the Brunauer-Emmett-Teller (BET) method and the pore volume was determined by the single point method at p/p.sub.0=0.99. The micropore size distribution was determined from Ar sorption techniques using the Horvath-Kawazoe method. Thermal gravimetric analysis (TGA) of the catalysts was performed on a Sinco TGA-N 1000 thermal analyzer. The samples were run at a heating rate of 5° C./min in a range of 25° C. to 700° C. under constant flow of nitrogen at 20 mL/min. FTIR spectra were recorded on a Nicolet FTIR spectrometer (MAGNA-IR 560) using KBr pellets. Morphological properties of the catalysts were studied by scanning electron microscopy (SEM) (Tescan Mira 3 LMU FEG with an accelerating voltage of 10 kV). Acidic and basic properties of the catalysts were measured using NH.sub.3-TPD and CO.sub.2-TPD, respectively. TPD profiles of catalysts were measured on a Micromeritics AutoChem II 2920 V3.05 apparatus equipped with a thermal conductivity detector. Samples were activated at 150° C. for 12 hours in a helium flow, before the adsorption step. Subsequently, the activated samples were exposed to NH.sub.3 or CO.sub.2 gas at 50° C. for 30 minutes with a flow rate of 50 mL/min. First, the physically-adsorbed NH.sub.3 and CO.sub.2 gases were removed by purging with a helium gas for 1 hour at the same temperature and flow rate. TPD data were recorded from 50° C. to 300° C. with a heating rate of 5° C./min. Inductively-coupled plasma (ICP) analysis was used to determine the metal leaching from the Zr-MOFs.

Example 2: Test of Catalyst and Product Analysis

[0064] In a typical run, EL (4 mmol), 2-propanol (400 mmol), and naphthalene (0.24 g), as internal standard materials, were filled into a 100 mL stainless steel reactor containing an inner lining of Pyrex glass and equipped with a magnetic stirrer. The reaction was performed at a certain known temperature for a desired period of time. The catalyst was separated by filtration and washed thoroughly with an ethanol-water system (95:5). The filtrate was subjected to quantitative analysis using gas chromatography (GC, FID detector and HP-5 column) and identification of the products was done by GC-MS (Agilent 6890N GC and 5973 N MSD). For the open-system solvent-reflux method, reactions were performed in 50 mL round-bottom flasks equipped with two-neck, septum ports and reflux condensers.

Example 3: UiO-66(Zr) and Functionalized Derivatives Thereof

[0065] The MIN reduction reaction for different substrates can be effectively promoted by several Zr-containing catalysts, including porous and non-porous materials such as metal oxides, metal hydroxides, amorphous metal complexes, and zeolites. Due to the potential of porous materials for MIN reduction reaction, various zirconium-based metal-organic frameworks (Zr-MOFs) were tested for the CTH reaction of EL to GVL as shown in Reaction Scheme 1.

##STR00001##

[0066] Reaction Scheme 1 represents the catalytic transfer hydrogenation (CTH) of EL to GVL using Zr-MOFs.

[0067] 3.1 Role of Central Metal/Cluster and Ligand Functional Group of Zr-MOFs in CTH of EL

[0068] MOFs possess multifunctional properties derived both from a metal cluster and ligand functional group. Therefore, it is highly desirable to understand the origin of active centers (either of metal cluster or ligand) to design a material superior for the respective application. Considering this point, the present inventors have synthesized UiO-66(Zr) and its functionalized analogs distinguished on the basis of ligand functional group, as shown in FIG. 1. Their structures were confirmed using XRD, FTIR, N.sub.2 adsorption, and TG analysis, and are shown in FIGS. 2A-2D. Table 1 shows the physicochemical properties and catalytic activity of UiO-66(Zr) and its functionalized analogs in CTH of EL to GVL.

TABLE-US-00001 TABLE 1 S.sub.BET.sup.b PV.sup.c Conv..sup.d Y.sub.GVL.sup.e Y.sub.IPL.sup.f Entry Catalysts (m.sup.2/g) (cm.sup.3/g) (%) (%) (%) 1 None — — 8.9 2.7 1.0 2 UiO-66(Zr) 1046 1.65 100 53.5 44.2 3 UiO-6(Zr)—COOH 575 0.80 28.5 13.9 13.5 4 UiO-6(Zr)—NH.sub.2 1006 1.8 97.6 27.3 64.6 Reaction conditions: EL (4 mmol), isopropanol (400 mmol); catalyst (0.8 g); naphthalene (0.24 g); temperature (50° C.); time (4 h). .sup.bS.sub.BET = BET surface area, .sup.cPV = pore volume, .sup.dConv. = conversion, .sup.eY.sub.GVL = GVL yield, .sup.fY.sub.IPL = yield of isopropyl levulinate (IPL).

[0069] In the UiO-66(Zr) catalyst, the replacement of the BDC ligand with a functionalized BDC by electron donating and withdrawing groups significantly altered its chemical and physical properties. The presence of carboxylic acid functional group in the ligand delivers extra acidity to the UiO-66(Zr) framework. However, a considerable loss in surface area and pore volume of UiO-66(Zr)—COOH was observed. In contrast, the presence of amino groups in the ligand provides additional basicity to the UiO-66(Zr) structure with a minor loss in surface area, which was ascribed to the smaller size of NH.sub.2 groups compared to that of COOH groups. The pore size distribution curves measured by the Ar-physisorption method (FIG. 3) shows that the pore size of UiO-66(Zr) and its functionalized analogs increased in the order of UiO-66(Zr)—COOH<UiO-66(Zr)_NH.sub.2<UiO-66(Zr).

[0070] Isopropyl levulinate (IPL), the transesterified product of EL in the presence of 2-propanol, is the major side product observed in all cases. Among the tested catalysts, UiO-66(Zr), which has no ligand functionality, showed the maximum EL conversion and GVL yield. When the UiO-66(Zr)—COOH catalyst was used, 28.5% of EL conversion and 13.9% of GVL yield were achieved. It was assumed that the low conversion was possibly due to the slow diffusion of EL into the narrow pores of UiO-66(Zr)—COOH or the limited occurrence of the reaction to the external surface area of UiO-66(Zr)—COOH. On hand, nearly complete conversion of EL was observed when UiO-66(Zr)—NH.sub.2 was used. However, with respect to product distribution, IPL was the major product with 64.6% yield. This clearly indicates that the transesterification reaction was predominant in the presence of amine functionalized UiO-66(Zr). Additionally, the presence of electron donating or withdrawing groups on ligands can change the charges on metal cluster which significantly affects the activity of a catalyst.

[0071] This result indicates that a large surface area, a big pore size, and a balanced acid-base property or charge on the metal cluster is a key factor in the selective conversion of EL into GVL. Ligand functional group prevents the activity of a catalyst by changing electronic and porous properties of the original UiO-66(Zr) framework.

[0072] 3.2 Effect of Reaction Temperature

[0073] The temperature dependence of the transfer hydrogenation reaction was exhibited in the range 120° C. to 200° C. for 4 hours of reaction time (FIG. 3). In most of the previous studies, transfer hydrogenation of LA or EL was carried out at moderate to high reaction temperature (120° C. to 250° C.). The reaction conversion was 83% at 120° C. and the temperature was increased by 30° C. each time. The yield of GVL increased with reaction temperature and reached the maximum of 92.7% at 200° C. At the same time, the IPL yield gradually decreased along with the rise in reaction temperature from 67.9% at 120° C. to 0% at 200° C. IPL is the major side-product at lower reaction temperatures, possibly because the replacement of a smaller ethyl group by bulkier isopropyl group may slow its diffusion through UiO-66(Zr) pore windows. The rise of the reaction temperature gradually increases the pressure inside the reactor, which facilitates the diffusion of EL into the pores of UiO-66(Zr) and then consequently undergoes the CTH reaction, as with EL, to produce GVL. From the above observation, it was confirmed that GVL can be formed at lower temperature and the rate of GVL formation increases with the increase of reaction temperature.

[0074] 3.3 Effect of Reaction Time

[0075] The present inventors have examined the influence of reaction time with regard to the conversion of EL to GVL on the CTH reaction at 200° C. The results showed that the conversion of EL was completed even at short duration (15 minutes) (FIG. 4). By increasing the reaction time from 15 minutes to 120 minutes, the GVL yield was increased from 70% to 92.7%. However, allowing the reaction to continue for 3 hours did not change the GVL yield. In contrast, the IPL yield gradually declined with the increase of the reaction time. This suggests that higher stability of the IPL is probably due to the limited diffusion through the porous window of UiO-66(Zr).

[0076] 3.4 Catalyst Recyclability and Characterizations

[0077] A recycling test of UiO-66(Zr) for CTH of EL to GVL was performed under optimized reaction conditions and the results are shown in FIG. 5. After each cycle, the catalyst used was recovered by filtration, washed with an ethanol-water system (95:5), and dried prior to the next run. Even after the run for 5 cycles, only a negligible difference was observed in the EL conversion and GVL yield suggesting that there was almost no loss of active sites present in the catalyst.

[0078] The recycled UiO-66(Zr) catalyst was analyzed, subjected to a test run for 5 cycles, and the changes in its structural and morphological properties were examined (FIGS. 6A-6D). With regard to the XRD patterns of the used UIO-66(Zr), all peaks were maintained with little decrease in peak intensity, compared to fresh catalysts (FIG. 6A). Similarly, a negligible difference was observed in BET surface area (1,046 m.sup.2/g and 1,000 m.sup.2/g) and pore volume (1.65 cm.sup.3/g and 1.63 cm.sup.3/g) of the catalyst as measured after 5 tests (FIG. 6B).

[0079] Nevertheless, differences were observed in the TG curve between the fresh and used catalysts (FIG. 6C). The fresh catalyst absorbed more water than did the used catalyst, which was determined by a slightly greater weight loss before 75° C. The number of BDC ligands present in the fresh and used UiO-66(Zr) was calculated from the weight loss that occurred at a temperature of 350° C. to 550° C. Despite the good crystalline structure, the fresh UiO-66(Zr) has one linker deficiency and the missing linker defect was assumed to be capped by the —OH groups. In this case, the proposed formula for the fresh UiO-66(Zr) was Zr.sub.6O.sub.4(OH).sub.4(OH).sub.2(BDC).sub.5. After 5 cycles, the catalyst lost one additional BDC linker from its framework (in which BDC anions combine with two protons from OH of a hexanuclear Zr cluster and neutral acid departs) leaving a material with the formula Zr.sub.6O.sub.6(OH).sub.4(BDC).sub.4. The charge imbalance generated due to the linker deficiency was compensated by a partial replacement of μ.sub.3-OH.sup.− ions with μ.sub.3-OH.sup.2− ions. That the morphology of the catalyst remains unchanged was confirmed by SEM analysis (FIGS. 7A-7B). ICP analysis failed to detect the presence of any zirconium in the reaction filtrate thus confirming that there was no leaching of zirconium from the MOF framework.

[0080] All of the above observations confirmed that UiO-66(Zr) has a unique property to undergo a structural rearrangement in ways that stabilize its structure. Despite the structural rearrangement, the catalytic activity of UiO-66(Zr) was not affected. Therefore, it is suggested that the metal nodes, rather than the organic ligands, are the active sites in the selective transformation of EL to GVL.

Example 4: MOF-808

[0081] 4-1. Evaluation of Zr-MOFs Possessing Different Physico-Chemical Properties for CTH Reaction of EL to GVL

[0082] After confirming the active sites in UiO-66(Zr) for transfer hydrogenation of EL, the present inventors have checked the possibility whether other Zr-MOFs may be more suitable as candidates for the transfer hydrogenation reaction. To overcome the use of high reaction temperature was another object to solve in the present invention. To this end, two alternative Zr-MOFs (MOF-801 and MOF-808), which possess a metal center (Zr.sub.6O.sub.4(OH).sub.4) in their framework as in UiO-66(Zr) were selected for the reaction. The porous properties of the selected Zr-MOFs along with linkers and molecular formulas are shown in Table 2 below.

TABLE-US-00002 TABLE 2 Molecular S.sub.BET.sup.a PV.sup.b PD.sup.c MOF Linker formula (m.sup.2/g) (cm.sup.3/g) (Å) MOF-801 [00002] Zr.sub.6O.sub.4(OH).sub.4(fumarate).sub.6 990 0.44 6 UiO-6(Zr) [00003] Zr.sub.6O.sub.4(OH).sub.4(BDC).sub.6 1046 1.65 6 MOF-808 [00004] Zr.sub.6O.sub.4(OH).sub.4(BTC).sub.2(HCOO).sub.6 1450 0.8 7.4,12.5 .sup.aS.sub.BET = BET Surface Area, .sup.bPV = Pore Volume, .sup.cPD = Pore Diameter

[0083] Representative structures of the two Zr-MOFs are shown in FIGS. 8A-8B. Since UiO-66(Zr) and MOF-801 have the coordination number (12) to the link in the metal center they have almost similar porous properties. However, MOF-808 with a central metal-to-linker coordination number of 6 possesses porous properties different from those of other two Zr-MOFs (Table 2 and FIGS. 9A-9B). Due to the large surface area and bigger pore size of MOF-808, the catalytic performance of the catalyst was examined at a moderate reaction temperature to represent the principle of green chemistry.

TABLE-US-00003 TABLE 3 Time Conv. Yield Mass ratio GVL FR.sup.h Entry Temp.Catalyst Temp. (° C.) (h) (%) (%) catalyst:ester (μmol/g/min) 1 MOF-808 130 3 100 85.0 1:2.9 94.4 2 UiO-66(Zr) 130 3 43.3 8.0 1:2.9 8.9 3 MOF-801 130 3 28.1 12.2 1:2.9 13.6 4.sup.b ZrO.sub.2 120 4 36.6 12.2 1:4.8 14.2 5.sup.c Zr-HBA 120 4 82.1 50.1 1:0.7 10.4 6.sup.d Zr-PhyA 130 8 98.9 95.4 1:0.7 9.9 7.sup.e Zr-Beta 118 22 88 83 1:1.2 6.3 8.sup.f Zr-Beta 82 18 5.6 4.0 1:0.6 0.2 9.sup.g MOF-808 82 18 100 75 1:0.7 3.5 Reaction conditions: EL 4 mmol; isopropanol

[0084] Table 3 demonstrates the catalytic activity of various Zr-based catalysts at moderate reaction temperature. In fact, MOF-808 has different edges from those of the other two MOFs. MOF-808 can provide more active sites due to a greater surface area and provide an easier access to active sites due to a bigger pore size. Despite the same window size (6 Å), UiO-66(Zr) shows higher conversion (43.3%) than MOF-801 (28.1%). This may probably be due to the higher external surface area (390 m.sup.2/g for UiO-66(Zr) and 168 m.sup.2/g for MOF-801) and pore volume of MOF-808. The present inventors have compared the results with those of other Zr-based catalysts reported in the literatures, studied under comparable reaction conditions (entries 4 to 8). MOF-808 was found to be superior in terms of GVL formation rate compared to all other catalysts mentioned in Table 3 under specific reaction conditions. Amorphous Zr-complexes also showed good performance at lower temperature (entries 5 and 6), but they lagged behind MOF-808 in terms of GVL formation rate. This is due to their low crystallinity or poorly-ordered structure, which resulted in a smaller surface area compared to highly-ordered, crystalline MOF-808.

[0085] With the exceptional catalytic activity of MOF-808, the present inventors have extended the approach and performed the reaction in an open system using the solvent reflux method (entry 9). In organic synthesis, several reduction reactions were performed using homogeneous catalytic systems. The moisture sensitivity, stoichiometric use, and difficulties in separation and reusability of the catalysts can lower the efficiency of the homogeneous systems. Accordingly, it is highly preferred that a heterogeneous catalyst be used to overcome the difficulties associated with homogeneous catalytic systems. MOF-808 showed good performance in an open system MIN reduction of EL at its boiling point, producing 75% GVL yield in 18 hours. In contrast, Zr-beta converted only 5.6% of LA with 4.0% yield of GVL in 18 hours (entry 8). These results show the high catalytic activity of MOF-808 and its potential as an efficient heterogeneous catalyst in CTH reactions even in organic synthesis where most reactions are performed in an open system.

[0086] Additionally, MOF-808 was recycled 5 times in a batch-type pressurized reaction system without a notable change in its catalytic activity (FIG. 10). As shown in FIGS. 11A-11F, the recycled catalyst was characterized in detail, using X-ray diffraction, N.sub.2 physisorption, TGA, FTIR, and SEM analysis. XRD patterns displayed no change in the crystal structure of the used catalyst; all the peaks retained the same intensity. TGA and FTIR patterns confirmed the structural changes that occurred in the catalyst after the fifth run. The weight loss at a temperature of 375° C. to 700° C. assigned to the BTC ligand was almost identical, thus confirming there was no leaching of the BTC ligand from the structure. The substantial decrease in OH stretching frequency in the FTIR pattern of the used catalyst indicates the deviations in the OH group present in MOF-808. Morphological and X-ray diffraction studies suggested that no phase transition or structural collapse occurred even after 5 recycle tests, and this observation is in line with the high chemical stability of MOF-808.

Example 5: Transfer Hydrogenation of Furfural into Furfuryl Alcohol Using MOF-808

[0087] The catalyst (MOF-808) in an amount of 0.25 g was added into a batch reactor, in which a reactant (furfural, 0.5 g), a hydrogen source (isopropanol, 25 g), and a reference material (naphthalene, 0.25 g) were dissolved, and hydrogenation reaction was performed at 120° C. The results are shown in FIG. 13. As a result, within 2 hours of the reaction at 120° C., the performances of furfural conversion (100%) and furfuryl alcohol selectivity (98%) were revealed. The result of XRD analysis before and after the reaction confirmed that there was no change in the structural stability of Zr-MOF.