Patent Application
20170320790
The present invention relates to a catalyst for transfer hydrogenation formed of metal-organic frameworks having an Zr-based MOF based X-ray diffraction pattern.
To solve the problems of gradual depletion of fossil fuels and worsening environmental conditions, and to meet the energy demand for improving the quality of life, studies on alternative renewable resources are under way. Among several renewable resources, only some biomass species which are inexpensive and abundant in nature are sustainable sources of supply; otherwise, most resources are conventionally derived from liquid fuels and fossil resources. The development of a catalytic process for producing various platforms and value-added chemical materials directly from carbohydrates or directly from lignocellulosic biomass requires enormous effort. Among these chemicals, γ-valerolactone (GVL) is recognized as a versatile building block that can be used as an additive in liquid fuels for transportation, as a precursor for the production of polymeric monomers, and a precursor for the synthesis of various value-added chemicals including organic solvents and bio-oxygenates. GVL has proved itself as an excellent green solvent for biomass processing due to its extraordinary physicochemical properties such as low melting and high boiling-point, a remarkably low vapor pressure even at an elevated temperature, a ready miscibility with water without forming an azeotropic mixture, etc.
Generally, three major strategies have been explored for GVL production from levulinic acid (LA) and an ester thereof, based on diversity of hydrogen sources. Molecular H2 is the most common hydrogen source used for this reaction, which occurs in the presence of various metal catalysts (e.g., Ru, Pt, Pd, Ni, Co, and Cu). Formic acid (FA), which forms an equimolar amount with LA during acid hydrolysis of carbohydrates, is also utilized as a hydrogen source to produce GVL from LA to embody the principle of atom economy. Several catalytic systems including nickel promoted copper-silica and Ag—Ni—ZrO2 nanocomposites have been successfully utilized for utilized for quantitative conversion of LA into GVL by consuming equimolar FA as a hydrogen donor. However, the two hydrogenation strategies described above have some limitations (e.g., harsh reaction conditions, use of corrosive acids, and use of precious metals and non-environmental-friendly solvents) thus hindering their application to a large-scale production to some extent.
Recently, Dumesic et al. reported for the first time a catalytic transfer hydrogenation (CTH) method based on the principle of Meerwein-Ponndorf-Verley (MPV) reduction for the hydrogenation of LA and alkyl levulinates to GVL in the presence of a heterogeneous catalyst using secondary alcohols as a hydrogen donor. They have demonstrated that ZrO2 displayed better activity than other metal oxides due to its amphoteric nature. The chemical selectivity of MPV reduction to the carbonyl groups of aldehyde and ketones, replacement of molecular H2 by alcohols, and effective performance of catalysts containing non-precious metals provide a cost-effective alternative for the production of GVL. Accordingly, various catalytic systems, mainly based on zirconium such as ZrO2, ZrO(OH)2, and amorphous Zr-complexes [zirconium 4-hydroxybenzoate (Zr-HBA), zirconium phosphonate], were reported for the above action within a short period of time. Until now, only one report which describes the conversion of LA into GVL by the CTH reaction using crystalline porous material with an appropriate surface area (Zr-beta zeolite, 474 m2/g) is available. However, the complex and time-consuming method for preparing catalysts limits its practical applications.
Generally, an organic/inorganic hybrid nanoporous material is also called “porous coordination polymer” or “metal-organic framework (MOF)”. Metal-organic frameworks consist of metal-nodes or clusters, bridged through organic ligands to form a well-ordered, highly-crystalline porous network having a pore structure of molecular size or nano size (
As used herein, the term “coordinatively unsaturated metal site” refers to a position as a site for the coordination of metals where a ligand coordinated by the metal ions of a metal-organic framework, representatively water, an organic solvent, etc., are removed, in which another ligand can form a coordination again. The coordinatively unsaturated metal site may be one formed by partial or entire removal of water, solvent molecules other than water, or ligands, which were contained in the metal-organic framework. In order to secure the coordinatively unsaturated metal site of the metal-organic framework, a pretreatment to remove water or solvent components bound to the metal-organic framework may be performed. As a method for the pretreatment, any method that can remove water or solvent components may be used as long as it does not induce a modification of the metal-organic framework. For example, the pretreatment may be achieved by heating to a temperature of 100° C. or above under reduced pressure, or the solvent-removing methods known in the art such as vacuum treatment, solvent exchange, sonication, etc., may be used without limitation.
An object of the present invention provides a catalyst for transfer hydrogenation which can act at room temperature.
Another object of the present invention provides a catalyst system for preparing γ-valerolactone or furfural alcohol, which is environment-friendly and enables a large-scale production under moderate reaction conditions.
A first aspect of the present invention provides a catalyst for transfer hydrogenation formed of a metal-organic framework having an Zr-based MOF based X-ray diffraction pattern.
A second aspect of the present invention provides a method for preparing γ-valerolactone by transfer hydrogenation of ethyl levulinate (EL) using the catalyst of the first aspect.
A third aspect of the present invention provides a method for preparing furfural alcohol by transfer hydrogenation of furfural using the catalyst of the first aspect.
Hereinafter, the present invention will be described in detail.
The present invention is characterized in that a metal-organic framework having an MOF-808 and its analogues based X-ray diffraction pattern of MOF-808 of Formula 1 or 2 below is used as a catalyst for transfer hydrogenation.
M6O4(OH)4(BTC)2(HCOO)6 [Formula 1]
(wherein M is a group 4A or 4B element, or a lanthanide metal whose oxidation state is 4+)
M6(μ3-O)4(μ3-OH)4(OH)6-x(H2O)6(BTC)2(HCOO)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+)
Non-limiting examples of the group 4A may include Ge, Sn, Pb, etc., and non-limiting examples of the group 4B may include Zr, Hf, Ti, etc., and non-limiting examples of the lanthanide metal whose oxidation state is 4+ may include Ce, Pr, Nd, Tb, Dy, etc.
Even if the central metal is the same, the environment within the pore of MOF and catalytic activity thereof can vary according to the crystalline structure of MOF, ligands, coordinatively unsaturated sites, etc., and it is thus difficult to predict which MOF may exhibit an excellent function of its own in a particular catalytic reaction condition.
The present inventors have studied the roles of central metal, ligand functional group and porosity characteristics of Zr-based metal-organic framework in the conversion of EL and selective preparation of GVL, through experimental designs in various aspects, examined the effects of different procedural parameters on the selectivity of products, and discovered active centers present in the MOFs, and as a result, they have discovered that MOF-808 is an excellent catalyst for transfer hydrogenation of ethyl levulinate (EL) into γ-valerolactone under moderate conditions. Accordingly, it was confirmed that MOF-808 is applicable to 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, and the present invention is based thereon.
The framework of MOF is formed by a covalent bond between secondary building units (SBUs) and organic ligands. MOFs are comprised of various coordination geometries, polytopic linkers, and ancillary ligands (F, OH, and H2O among others).
Conventionally, the zirconium in a Zr-MOF is generally present as a Zr4+ cation, but the zirconium in the MOF-808 (Zr6O4(OH)4(BTC)2(HCOO)6) is present as a Zr6O4OH4+10 cluster cation (
Micropores are defined as pores having a diameter of about 2 nm or less and mesopores are defined as those having a diameter in the range of about 2 nm to 50 nm. MOF-808 has a pore structure having two different (window) diameters, and in particular, has mesopores with a diameter of about 1.8 nm, unlike other Zr-MOFs (
MOF-801(Zr) is provided with a Zr central metal and a fumaric acid ligand and the Zr sites are not coordinatively saturated and thus the reactant of a catalytic reaction can be accessible to its inner pores.
In the case of UiO-66, reactants are not easily accessible because the Zr sites are coordinatively saturated, and most of the catalytic reactions occur on the surface of particles. In contrast, in the case of MOF-808(Zr), there are extra sites to which a reactant can access and additionally, due to the mesoporous characteristic (pore size: about 2 nm), reactions can occur not only on the external surface of a catalyst but also on the inner surface of the catalyst, and therefore, catalytic transfer hydrogenation (CTH) can also occur well at a temperature of 80° C.
From the viewpoint of catalysis, the MOF-808(Zr) based metal-organic framework according to the present invention can form continuous and permeable channels due to the high surface area and permanent porosity of the framework, and thus the reactant becomes accessible thereto and a larger number of active sites can be provided (Table 2). The presence of a larger channel or cage can better facilitate the diffusion thereby enhancing the interaction between the reactant and the active sites present in the porous network. Consequently, the reaction rate and the turnover frequency (TOF) to substrates increase, which is very desirable from the viewpoint of green chemistry.
Due to their relatively lower hydrolytic and thermal stability compared to that of zeolites, MOFs have limitations in their practical applications in various fields. However, due to the higher coordination number of zirconium, the MOF-808(Zr) based metal-organic framework exhibits unprecedented chemical, thermal, and mechanical stability.
Conventionally, catalysts of oxides and hydroxides of ZrO2, Zr(OH)4, etc. were used in Meerwein-Ponndorf-Verley (MPV) reaction or catalytic transfer hydrogenation (CTH). However, these catalysts had problems in that they have a low surface area and the active sites of Zr—O—Zr and Zr—OH—Zr are positioned continuously and thus the distance between the active sites is too close for a reactant to approach to react continuously thereby having a low turnover frequency (TOF) to substrates of the catalysts. Accordingly, a Zr-MOF was used to solve the above problems, and a series of various crystalline porous Zr-MOFs were prepared for the selection of a crystalline Zr-MOF, which has a large surface area and includes inorganic building blocks in a unit of cluster.
Among many Zr-MOFs, surprisingly, it was discovered that MOF-808 has a very large surface area and high reactivity per unit active site because inorganic building blocks (Zr clusters) are separated by organic ligands thus having high reduction reactivity even at a relatively low temperature, compared to ZrO2.
Although CTH reactions at low temperature are reported in the Raney Ni catalyst, there is a problem in that the actual use thereof must be performed carefully considering that the catalyst has extremely high reactivity when exposed to air. However, if MOF-808 is used, the transfer hydrogenation of ethyl levulinate (EL) can be performed even in an open system.
The present inventors have synthesized a series of synthesis of crystalline porous Zr-MOFs, and tested a CTH reaction in which EL go through MIN reduction using isopropanol and bioalcohol as a hydrogen donor. For the understanding of the origin of active sites, the central metal of UiO-66(Zr) and the roles of ligand functional group were confirmed.
In the selective conversion of alkyl levulinate into GVL, ligand functional group (acid or base) was actually not effective but reduced the activity of the catalyst. Such a low conversion appears to be due to the low diffusion of EL through the narrow pores of UiO-66(Zr)—COOH, or limited occurrence of the reaction only on the external surface of UiO-66(Zr)—COOH.
Despite the structure rearrangement through experiments, the catalytic activity of UiO-66(Zr) is not affected. Furthermore, this supports that metal nodes rather than organic ligands are active sites in the selective conversion of EL into GVL. Additionally, from the results obtained from the characteristics of the unused catalysts and used catalysts, it was confirmed that the metal clusters/centers of the Zr-MOF framework are active sites that produce GVL from EL by CTH reaction.
The present inventors have discovered by experiments that the large surface area, large pore size, and balanced acid-base characteristics or electric charge of metal clusters are important factors in the selective conversion of EL into GVL. Specifically, the present inventors have discovered that ligand functional group changes the electronic characteristics and porous characteristics of the UiO-66(Zr) frame, which is the origin, thereby preventing the activity of the catalyst (Example 3). The presence of an electron-donating group or electron-withdrawing group on the ligands can change the electric charge of metal clusters, which significantly affects the activity of the catalyst.
Although both have the same window size (6 Å), UiO-66(Zr) has higher conversion (43.3%) compared to MOF-801 (28.1%). This may be due to the high external surface area (UiO-66(Zr): 390 m2/g; and MOF-801: 168 m2/g) and pore volume.
UiO-66(Zr) exhibited excellent chemical stability and catalytic activity at high temperature (200° C.). Therefore, UiO-66(Zr) appears to be an excellent potential catalyst in a high temperature reaction for biorefinery.
Meanwhile, MOF-808 has two different MOFs and different edges. MOF-808 can provide more active sites because it has a larger surface area, and more easily access to active sites because it has a larger pore size. MOF-808 and UiO-66(Zr) have acid and base sites within the structure. The amount of these sites is significantly larger in MOF-808 than in UiO-66(Zr). The origin of the acid-base sites was derived from Zr—O—Zr or Zr—OH—Zr bonding present in the metal clusters of Zr-MOFs. The acid-base (Zr4+ and O2−) sites of the metal clusters combine and interact with both EL and isopropanol to form a 6-membered ring transition state. The metal nodes which develop the acid-base characteristics in Zr-MOF are considered as active sites in the selective formation of GVL from EL.
Since MOF-808 has a large surface are and a wide pore structure, it was very effective in the formation of GVL from El at high production rate (94.4 μmol/g/min) in an appropriate reaction temperature. Additionally, MOF-808 exhibited excellent performance in an open system using a solvent reflux method. MOF-808 catalysts can be recycled at least 5 times under a specified condition without loss of catalytic activity. Zr-MOFs have a unique property that can undergo a structural rearrangement to stabilize the framework as proven using several characterization techniques.
Accordingly, the catalytic activity in the transfer hydrogenation of MOF-808(Zr) can be equally applied to the metal-organic framework which has MOF-808 based X-ray diffraction pattern.
Furthermore, the metal-organic framework having an MOF-808 based X-ray diffraction pattern is not only an excellent catalyst in a moderate condition for transfer hydrogenation from ethyl levulinate (EL) to γ-valerolactone, but also 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.
When the catalyst according to the present invention is used, various alcohols (isopropanol, methanol, ethanol, glycerol, and butanol), cyclic ethers, benzyl alcohol, cyclohexanone, 2-propanol, ethylene glycol, 2,3-dihydroindole, 1,2,3,4-tetrahydroquinoline, cylohexene, cyclohexadiene, limonene, hydrazine, ammonium formate, ammonium hypophosphite, a mixture thereof, etc., may be used as a hydrogen donor, and preferably, isopropanol may be used. Isopropanol is an excellent hydrogen-donating alcohol due to its low reduction potential.
When the catalyst according to the present invention is used for GVL production, it is more preferable to use EL rather than LA. First, EL gives a higher yield in ethanolysis of carbohydrates, and production separation from carbohydrates is easier due to its high volatility compared to LA. Second, there is no safety or economical concern over the corrosion of reactors because of its acid-free form.
When transfer hydrogenation is performed using the metal-organic framework having an MOF-808 based X-ray diffraction pattern as a catalyst at a temperature of room temperature to 250° C., preferably at 200° C. or below, more preferably at a low temperature of 80° C. to 130° C., the conversion of various substrates such as EL may be 90% or higher. Additionally, the selectivity of alcohol reactants may be increased by performing hydrogenation at a low temperature of 50° C.
The metal-organic framework having a highly crystalline, porous, MOF-808 based X-ray diffraction pattern, which exhibits excellent performance in transfer hydrogenation at low and high temperature, is useful for biorefinery and organic synthesis.
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.
Chemicals and Materials
Ethyl levulinate (99%), γ-valerolactone (99%), 2-propanol (99.5%), naphthalene (98%), ZrOCl2.8H2O (98%), ZrCl4 (99.5%), 1,4-benzenedicarboxylic acid (H2BDC, 98%), 1,2,4-1,2,4-benzenetricarboxylic acid (H3BTC, 99%), 2-aminoterphthalic acid (99%), 1,3,5-benzenetricarboxylic acid (H3BTC, 95%), benzoic acid (99.5%), acetic acid (99.7%), HCl (37%), and N,N-dimethylformamide (99.8%) were purchased from Sigma-Aldrich.
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.
UiO-66(Zr) was synthesized by the reflux method. Specifically, H2BDC (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, ZrOCl2.8H2O (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 ZrOCl2.8H2O/H2BDC/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.
UiO-66(Zr)—NH2 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, ZrOCl2.8H2O (3.44 g, 10.7 mmol) was added to the solution with continuously stirring. The molar ratio of the final ZrOCl2.8H2O/H2BDC-NH2/H2O/CH3COOH 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.
UiO-66(Zr)—COOH was also synthesized by the reflux method. Briefly, 1,2,4-benzenetricarboxylic acid (14 g, 66.7 mmol) and ZrOCl2.8H2O (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 ZrOCl2.8H2O/H3BTC/H2O/C6H5COOH 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)—NH2.
H3BTC (4.8 g, 0.5 mmol) and ZrOCl2.8H2O (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.
MOF-801, commonly known as Zr-fumarate MOF, was synthesized according to the method disclosed in a previous journal publication. Briefly, ZrCl4 (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.
Powder diffraction patterns were obtained by the Rigaku diffractometer using Ni-filtered Cu Kα-radiation (40 kV, 30 mA, λ=1.5406 Å). The N2 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/p0=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 NH3-TPD and CO2-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 NH3 or CO2 gas at 50° C. for 30 minutes with a flow rate of 50 mL/min. First, the physically-adsorbed NH3 and CO2 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.
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.
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.
Reaction Scheme 1 represents the catalytic transfer hydrogenation (CTH) of EL to GVL using Zr-MOFs.
3.1 Role of Central Metal/Cluster and Ligand Functional Group of Zr-MOFs in CTH of EL
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
bSBET = BET surface area,
cPV = pore volume,
dConv. = conversion,
eYGVL = GVL yield,
fYIPL = yield of isopropyl levulinate (IPL).
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 NH2 groups compared to that of COOH groups. The pore size distribution curves measured by the Ar-physisorption method (
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)—NH2 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.
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.
3.2 Effect of Reaction Temperature
The temperature dependence of the transfer hydrogenation reaction was exhibited in the range 120° C. to 200° C. for 4 hours of reaction time (
3.3 Effect of Reaction Time
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) (
3.4 Catalyst Recyclability and Characterizations
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
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 (
Nevertheless, differences were observed in the TG curve between the fresh and used catalysts (
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.
4-1. Evaluation of Zr-MOFs Possessing Different Physico-Chemical Properties for CTH Reaction of EL to GVL
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 (Zr6O4(OH)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.
aS
bPV = Pore Volume,
cPD = Pore Diameter
Representative structures of the two Zr-MOFs are shown in
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 m2/g for UiO-66(Zr) and 168 m2/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.
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.
Additionally, MOF-808 was recycled 5 times in a batch-type pressurized reaction system without a notable change in its catalytic activity (
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
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2016-0054831 | May 2016 | KR | national |
