# Patent

## Title
MOF synthesis method

## Abstract

A rapid, simple and versatile metal organic framework molecule (MOF) synthesis method particularly adapted to make non-linear MOFs includes heating MOF precursors, such as a metal or metal oxide and an organic ligand, in a microwave oven for a period sufficient to achieve crystallization. Microwave-assisted MOF synthesis yields high quality MOF crystals in a reaction time ranging from about 5 seconds to about 2.5 minutes, compared to hours and days required in conventional solvothermal and hydrothermal methods. In addition, microwave assisted methods provide MOF materials with uniform crystal size and well-defined shape. Further, microwave synthesis of MOFs allows the size and shape of MOF crystals to be tailored for use in a wide range applications by manipulating reaction conditions. Secondary growth processes may also be employed to grow larger crystals using seeds obtained from microwave-assisted synthesis methods.

## Background
STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH/DEVELOPMENT
This invention was made, at least in part, with U.S. government support under U.S. Air Force Grant No. FA8850-04-1-7121, awarded by the Defense Advanced Research Projects Agency (DARPA). The U.S. government has certain rights in the invention.


CROSS-REFERENCE TO RELATED APPLICATION
This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application No. 60/791,988, filed on Apr. 14, 2006, the disclosure of which is expressly incorporated by reference herein in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention generally relates to the synthesis of metal organic framework (MOF) molecules. In particular, the invention concerns rapid, simple, and versatile methods of producing MOF molecules using microwave assisted synthesis.
2. Related Art
MOFs are organometallic nanoporous structures with high surface area and tailorable selectivity, which makes them suitable for a wide range of applications. They may have a cubic crystalline structure that is formed by copolymerization of metals or metal oxides with organic ligands, resulting in metal-oxide clusters connected by organic linkers. FIG. 1 is a diagram of a typical MOF's 10 crystalline structure including metal or metal oxides, here shown as polyhedrons 12, having polymer ligands 14 extending between them. This highly ordered structure facilitates the creation of interior pores and channels. MOFs are known to have 1-3 nanometer (nm) pores. FIG. 2 is a view of the structure and channels 16 of one of the new MOFs discovered by Applicants and disclosed herein, Zn-MOF-1, and in pending U.S. patent application Ser. No. 11/539,905, filed Oct. 6, 2006, the disclosure of which is expressly incorporated by reference herein in its entirety. As shown in FIG. 2, Zn-MOF-1 has channels 16 of about 10 Å×10 Å. MOF crystals are known to have an average diameter of about 200 microns.
MOFs have been the focus of intense activity in recent years because of their extremely high porosity and tailorable molecular cavities. The surfaces of MOFs' pores and channels increase the overall surface areas of MOFs allowing them to have high porosity that is comparable to or larger than that of zeolites. For example, MOF-5, a known MOF, may have a surface area of about 2900 m2/g and IRMOF-177 may have a surface area of about 4500 m2/g. The high surface area of MOFs allows them to be used in a wide range of applications. For example, MOFs have been studied for a variety of applications including hydrogen storage, selective sorption, non-linear optical materials, templates for the creation of molecular species architectures, and as catalysts e.g., catalysts to store H2 (in fuel cells) or CO2.
Applicants have discovered that MOFs have properties that make them highly advantageous as preconcentrators including, for example, high sorption capacity due to their high surface area, high selectivity to specific analytes, inert nature which does not decompose the analyte, thermal stability, and result in unexpectedly high gains in detection. Accordingly, MOFs are used to selectively sorb specific analytes in a preconcentrator. For example, MOFs may be used in particle form, or they may be incorporated into a film inside the preconcentrator. Once the analytes are fully sorbed by the MOFs, the analytes are released by thermal desorption, for example. Then, they can be purged and transferred from the preconcentrator to a detector, for example. The structure and properties of MOFs that make them highly suitable for use as selective sorbents in preconcentrators are discussed below. Details about use of MOFs in a variety of preconcentrators may be found in the above-noted U.S. patent application Ser. No. 11/539,405 filed Oct. 6, 2006.
Numerous methods have been developed for synthesizing MOFs using precursors including a metal precursor and corresponding organic spacing ligand. Solvothermal and hydrothermal synthesis methods using these precursors have conventionally been employed to form MOF crystals. Solvothermal synthesis is a method where precursors for MOF crystal formation are heated in a solvent other than water. In hydrothermal synthesis, precursors for MOF crystals are heated in water. Hydrothermal synthesis is suitable when the ligand precursor is soluble in water. In both conventional solvothermal and hydrothermal synthesis, a solution with MOF precursors is typically maintained at a predetermined equilibrium temperature for an extended period to induce crystallization. Solvothermal and hydrothermal synthesis methods are typically slow, often taking hours and even days.
General information on different known MOFs and conventional synthesis methods are reported in a number of publications, including, “Design and Synthesis of an Exceptionally Stable and Highly Porous Metal-Organic Framework,” Yaghi et al., Nature 402 (1999) 276-279; “Interwoven Metal-Organic Framework on a Periodic Minimal Surface with Extra-Large Pores,” B. Chen, M. Eddaoudi, Yaghi et al. Science 291 (2001) 1021-1023; “Systematic Design of Pore Size and Functionality in Isoreticular MOFs and Their Application and Methane Storage,” Yaghi et al, Science 295 (2002) 469-472; “Reticular Synthesis and Design of New Materials,” Yaghi et al., Nature 423 (2003) 705-714; “Hydrogen Storage in Microporous Metal-Organic Frameworks,” Yaghi et al., Science 300 (2003) 1127-1129.
As mentioned above, reported solvothermal synthesis methods are slow, typically taking a day or more. U.S. Published Application No. 2003/0004364 discloses a solvothermal process to form MOF materials that takes about one day to several days. In the synthesis method of the above-referenced application, a metal salt and a linear ditropic carboxylate are dissolved in a solvent to form a solution. The solution is then crystallized, which involves at least one of leaving the solution at room temperature, adding a diluted base to the solution to initiate the crystallization, diffusing a diluted base into the solution to initiate the crystallization, and transferring the solution to a closed vessel and heating to a predetermined temperature.
A multi-day hydrothermal synthesis process has also been proposed for the production of nonlinear optically active MOF material. (See e.g., “A Novel Optical Metal-Organic NLO Material Based on a Chiral 2D Coordination Network,” Lin, et al., J. Am. Chem. Soc. 1999, 121, 11249-11250.) Others have reported a thermally stable [Cu3(TMA)2(H2O)3]n framework structure produced through a 12 hour solvothermal synthesis. (See e.g., “A Chemically Functionalizable Nanoporous Material [Cu3(TMA)2(H2O)3]n,” Chui, et al., Science, 1999, 283, 1148.) Catalytic active homochiral metal-organic materials formed by a two-day liquid diffusion method or solvothermal method have been reported. (See e.g., “A Homochiral Meta-organic Porous Material for Enantioselective Separation and Catalysis,” Seo, et al., Nature, 2000, 404, 982-986; “A Homochiral Metal-Organic Material with Permanent Porosity, Enantioselective Sorption Properties, and Catalytic Activity”, Dybtsev, et al., Angew. Chem. Int. Ed., 2006, 45, 916-920.)
Others have reported porphyrin MOF structures formed by a two-day solvothermal synthesis. (See e.g., “A functional zeolite analogue assembled from metalloporphyrins,” Kosal, M. E.; Chou, J.-H., Wilson, S. R.; Suslick, K. S, Nature Materials, 2002, 1, 118-121.) Synthesis by one-week deprotonating vapor diffusion has also been reported. (See e.g., “A Robust Microporous Zinc Porphyrin Framework Solid”, Smithenry, D. W. et al. Inorg. Chem. 2003, 42, 7719.)
Microwave-assisted processes have been used to produce metal particles and oxide particles. Such processes can involve heating a solution with microwaves for a period of an hour or more to produce nano-sized crystals of metal. Typically, metal particle sizes are about 15 nm (Panda A. B.; Glaspell, G. EI-Shall, M. S. J Am. Chem. Soc. 2006, 128 2790; Lu, Q., Gao, F., Li, D., Komarneni, S. J. Mater. 2005 1, 1). Microwave synthesis to provide 5-20 nanometer sized particles of oxides is also known (Tomsett, G. A., Conner, W. C. Yngvesson, K. S. Chem Phys Chem. 2006, 296).
Microwave-assisted methods of synthesizing certain types of framework molecules have been studied. For example, Lin et al. (Zhuojia Lin, David S. Wraggg, and Russell E. Morris, “Microwave-assisted synthesis of anionic metal-organic frameworks under ionothermal conditions,” Chem. Commun., published May 21, 2006, 2021-2023) describes reactions of metal acetate with trimesic acid in 1-ethyl-3-methyl imidazolium bromide (ionic liquid) under microwave or conventional ionothermal conditions resulting in the formation of novel three-dimensional anionic frameworks templated by the ionic liquid solvent. The anionic MOFs described in Lin et al. were synthesized in ionic liquid, which turns to a liquid media (i.e. melts) only when heated over 80° C. In contrast, the MOFs of this invention use common organic solvents such as diethylformamide(DEF), ethanol and water. Further, in Lin et al. the ionic solvent EMIm (1-ethyl-2-methyl imidazolium bromide) becomes building blocks in the framework structure and carries 2+ charge. In contrast, the MOF of this invention is a neutral framework and can be isolated from the solvent. In addition, the microwave processing time for anionic MOF of Lin et al. is 50 mins; the processing time for the MOFs in this invention is about 1 minute.
In addition, Rajic et al. (N. Rajic, D. Stojakovic, N. Zabukovec Logar, and V. Kaucic, “An evidence for a chain to network transformation during the microwave hydrothermal crystallization of an open-framework zinc terephthalate,” J Porous Mater 2006, 13: 153-156) discusses methods for synthesizing a 3-D open framework and a chain structured zinc terephthalate using hydrothermal crystallization under microwave heating at 180° C. The MOFs described in Rajic, however, are linear MOFs that do not have the high surface area, high porosity and tailorable molecular cavities that non-linear MOFs produced by conventional hydrothermal and solvothermal methods have. Moreover, they lack high thermal stability. The MOFs of this invention have a 3-D structure that is more robust than linear MOFs because they have coordination bonds in contrast to linear MOFs which have weak Van der Waal bonding. Therefore, the linear MOFs of Rajic are not suitable for a wide range of applications, as are non-linear MOFs.
There is a need for a method of manufacturing MOFs whereby the MOFs can be easily reproduced. There is also a need for a rapid synthesis method for MOFs because rapidly formed MOFs can be more suitably used as research tools for further evaluation. In addition, there is a need for MOFs having a uniform size and shape as well as MOFs which are smaller in size for certain applications. Furthermore, there is a need for a method to make MOFs that produce uniform seeding conditions that can permit secondary grow processes to form larger crystals for used in certain applications.
Thus, rapid methods for the synthesis of non-linear MOFs that have a wide range of applications have not been developed. Accordingly, there is a need for improved methods to rapidly synthesize non-linear MOFs.
SUMMARY OF THE INVENTION
The invention meets the foregoing needs and overcomes the drawbacks and disadvantages of the prior art by providing improved methods of rapidly synthesizing MOF molecules. Microwave-assisted MOF synthesis methods of the invention allow high quality non-linear metal-organic framework crystals to be synthesized in a range of about 5 seconds to about 2.5 minutes. The properties of the crystals made by the microwave-assisted process substantially of the same quality as those produced by the conventional hydrothermal and solvothermal processes, but the synthesis is much more rapid.
In one aspect of the invention, a method for synthesis of non-linear metal organic framework material includes the steps of preparing a reactant solution including metal organic framework precursors, and exposing the reactant solution to a microwave dose sufficient to induce crystallization of non-linear MOF material in the reactant solution.
The microwave dose in the exposing step may exceed about 4 J/cm3, and may be applied for period of between about 5 seconds to about 2.5 minutes. The method of the invention also may include a step of post-processing growth of crystals in the metal organic framework material by one of a solvothermal or hydrothermal process that reaches and maintains a predetermined equilibrium temperature for a predetermined time. The metal organic framework precursors may include a metal precursor and a corresponding spacer ligand in a predetermined ratio, and the amount of metal precursor may be set to achieve a corresponding crystal size in the metal organic framework material. The non-linear metal organic framework material may include a cubic structure. The preparing step may include mixing metal organic framework precursors in a solvent. The method of the invention also may include the step of removing metal organic framework material from the reactant solution after the exposing step.
In yet another aspect of the invention, a process for making a selectively sorbent MOF, Zn-MOF1, having a cubic crystalline structure with crystal sizes in the range from about 10 microns to about 30 microns may include (a) dissolving Zn(NO3)2.6H2O (0.18 g, 0.605 mmol) and 4,4′,4′′,4′′′-(21H,23H-porphine-5-10-15-20-tetrayl)tetrakis(benzoic acid) (0.199 g, 0.252 mmol) in 10 mL diethylformamide; (b) sealing the dissolved solution, and (c) heating in a microwave oven. Zn-MOF1 may be made by following the above process.
In yet another aspect of the invention, a process for making a selectively sorbent MOF, Zn-MOF2, having a cubic crystalline structure with crystal sizes in the range from 2 to 4 microns may include (a) mixing Zn(NO3)2*6H2O (0.1 g, 0.336 mmol) and 2-anilino-5-bromoterephthalic acid, (2-anilino-5-BrBDCH2) (0.0847 g, 0.252 mmol; (b) dissolving the mixture in 10 mL diethylformamide; (c) sealing the dissolved mixture; and (d) heating in a microwave oven. Zn-MOF2 may be made by following the above process.
In yet another aspect of the invention, a process for making a selectively sorbent MOF, Zn-MOF3, having a cubic crystalline structure with crystals ranging from 4 to 7 microns, may include (a) mixing Zn(NO3)2*6H2O (0.15 g, 0.504 mmol) with 2-trifluoromethoxy terephthalic acid, (2-trifluoromethoxy-BDCH2) (0.0946 g, 0.378 mmol); (b) dissolving the mixture in 10 mL diethylformamide; (c) sealing the dissolved mixture; and (d) heating the dissolved mixture in a microwave oven. Zn-MOF3 may be made by following the above process.
In yet another aspect of the invention, a process for making a selectively sorbent MOF, Zn-MOF4, having crystals of irregular shape may include (a) mixing Zn(NO3)2*6H2O (0.1 g, 0.336 mmol) with nitroterephthalic acid (0.0532 g, 0.252 mmol); (b) dissolving the mixture in 10 mL diethylformamide, sealing the dissolved mixture; and (c) heating the dissolved mixture in a microwave oven. Zn-MOF4 may be made by following the above process.
In yet another aspect of the invention, a process for making a selectively sorbent MOF, Zn-MOF5, may include (a) mixing Zn(NO3)2*6H2O (0.15 g, 0.504 mmol) with cis-cyclobutane-1,2-dicarboxylic acid (0.0545 g, 0.378 mmol); (b) dissolving the mixture in 10 mL diethylformamide; (c) sealing the dissolved mixture; and (d) heating the dissolved mixture in a microwave oven. Zn-MOF5 may be made by following the above process.
In yet another aspect of the invention, a process for making a selectively sorbent MOF, Cu-MOF1, may include (a) mixing Cu(NO3)2*2.5H2O (0.15 g, 0.645 mmol) with 2,5-thiophenedicarboxylic acid (0.0833 g, 0.484 mmol); (b) dissolving the mixture in 10 mL diethylformamide (c) sealing the dissolved mixture; and (d) heating the dissolved mixture in a microwave oven. Cu-MOF1 may be made by following the above process.
In yet another aspect of the invention, a process for making a selectively sorbent MOF, Cu-MOF2, may include (a) mixing Cu(NO3)2*2.5H2O (0.1 g, 0.430 mmol) with 2-(trifluoromethoxy)terephthalic acid (2-trifluoromethoxy-BDCH2) (0.0807 g, 0.322 mmol; (b) dissolving the mixture in 10 mL diethylformamide; (c) sealing the dissolved mixture; and (d) heating the dissolved mixture in a microwave oven. Cu-MOF2 may be made by following the above process.
In yet another aspect of the invention, a process for making a selectively sorbent MOF, Tb-MOF1, having a rod-shaped crystalline structure, may include (a) mixing Tb(NO3)3*5H2O, (0.1 g, 0.230 mmol), with terephthalic acid (BDC) (0.0286 g, 0.172 mmol); (b) dissolving the mixture in 10 mL diethylformamide; (c) sealing the dissolved mixture; and (d) heating the dissolved mixture in a microwave oven. Tb-MOF1 may be made by following the above process.
In yet another aspect of the invention, a process for making a selectively sorbent MOF, Tb-MOF2, may include (a) mixing Tb(NO3)3*5H2O, (0.1 g, 0.230 mmol), with 2,5-thiophenedicarboxylic acid (0.0297 g, 0.172 mmol); (b) dissolving the mixture in 10 mL diethylformamide; (c) sealing the dissolved mixture; and (d) heating the dissolved mixture in a microwave oven. Tb-MOF2 may be made by following the above process.
In yet another aspect of the invention, a process for making a selectively sorbent MOF, Cd-MOF1, may include (a) mixing Cd(NO3)2*4H2O, (0.1 g, 0.324 mmol), with cis-cyclobutane-1,2-dicarboxylic acid (0.0350 g, 0.243 mmol); (b) dissolving the mixture in 10 mL diethylformamide; (c) sealing the dissolved mixture; and (d) heating the dissolved mixture in a microwave oven. Cd-MOF1 may be made by following the above process.
In yet another aspect of the invention, a process for making a selectively sorbent MOF, Cd-MOF2, may include (a) mixing Cd(NO3)2*4H2O (0.1 g, 0.324 mmol) with nitroterephthalic acid (0.0456 g, 0.216 mmol); (b) dissolving the mixture in 15 mL diethylformamide; (c) sealing the dissolved mixture; and (d) heating the dissolved mixture in a microwave oven. Cd-MOF2 may be made by following the above process.
In yet another aspect of the invention, a process for making a selectively sorbent MOF, Cd-MOF3, may include (a) mixing Cd(NO3)2*4H2O (0.1 g, 0.324 mmol) with terephthalic acid (0.0404 g, 0.243 mmol); (b) dissolving the mixture in 10 mL diethylformamide; (c) sealing the dissolved mixture; and (d) heating the dissolved mixture in a microwave oven. Cd-MOF3 may be made by following the above process.
In yet another aspect of the invention, a process for making a selectively sorbent MOF, Co-MOF1, may include (a) mixing Co(NO3)2*6H2O, (0.1 g, 0.343 mmol), with terephthalic acid (0.0428 g, 0.258 mmol; (b) dissolving the mixture in 10 mL diethylformamide; (c) sealing the dissolved mixture; and (d) heating the dissolved mixture in a microwave oven. Co-MOF1 may be made by following the above process.
In yet another aspect of the invention, a process for making a selectively sorbent MOF, CoMOF-2, may include (a) mixing Co(NO3)2*6H2O, (0.1 g, 0.343 mmol) with 1,3,5-benzenetricarboxylic acid (0.0542 g, 0.258 mmol); (b) dissolving the mixture in 10 mL diethylformamide; (c) sealing the mixture; and (d) heating in a microwave oven. CoMOF-2 may be made by following the above process. In yet another aspect of the invention, a process for making a selectively sorbent MOF, ZnMOF6, may include (a) mixing Zn(NO3)2*6H2O (0.05 g, 0.168 mmol) with 3,3-bis(trifluoromethyl)-1-oxo-5-isobenzofurancarboxylic acid (0.0396 g, 0.126 mmol); (b) dissolving the mixture in 7 mL diethylformamide/3 mL H2O; (c) sealing the dissolved mixture; and (d) heating the dissolved mixture in a microwave oven. ZnMOF6 may be made by following the above process.
In a further aspect of the invention, a process for making a selectively sorbent MOF, Zn-MOF1, having a cubic crystalline structure with crystals in the range of 2 to 4 microns may include (a) dissolving 0.239 g of 4,4′,4′′,4′′′-(21H,23H-Porphine-5,10,15,20-tetrayl)tetrakis(benzoic acid) (H2TPP) and 0.18 g of Zn(NO3)2.6H2O in 60 mL DEF; (b) stirring the dissolved solution; (c) sealing the dissolved solution in a closed vessel; and (d) heating the vessel to 100° C. at 2° C./minute. Zn-MOF1 may be made by following the above process.
Additional features, advantages, and embodiments of the invention may be set forth or apparent from consideration of the following detailed description, drawings, and claims. Moreover, it is to be understood that both the foregoing summary of the invention and the following detailed description are exemplary and intended to provide further explanation without limiting the scope of the invention as claimed.



BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are included to provide a further understanding of the invention, are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the detailed description serve to explain the principles of the invention. No attempt is made to show structural details of the invention in more detail than may be necessary for a fundamental understanding of the invention and the various ways in which it may be practiced. In the drawings:
FIG. 1 is a diagram showing a typical crystalline structure of a MOF;
FIG. 2 is a view of the structure and channels of a specific MOF, Zn-MOF1;
FIG. 3 shows (a) Enlarged SEM image of micro IRMOF-1; (b) SEM image of micro IRMOF-1; (c) SEM image of micro IRMOF-2; and (d) SEM image of micro IRMOF-3;
FIG. 4 shows XRPD patterns of (a) IRMOF-1, (b) IRMOF-2, and (c) IRMOF-3 prepared by the solvothermal synthesis of Eddaoudi, M. et al. (Eddaoudi M., Kim, J., Rosi, N., Vodak, D., Wachter, J., O'Keeffe, M., Yaghi, O. M. Science 2002, 295, 469) (referenced by the arrow labeled black) and by microwave-assisted solvothermal synthesis (referenced by the arrow labeled green) and a simulated XRPD curve based on the published structure of Eddaoudi, M. et al. (referenced by the arrow labeled red);
FIG. 5 is a plot showing the dependence of the size of IRMOF-1 crystals on the concentration of BDCH2 in a reactant solution exposed to microwaves in experiments conducted in accordance with embodiments of the invention;
FIG. 6 shows enlarged SEM images of IRMOF-1 synthesized from BDCH2 concentrations of (a) 0.05 M and (b) 0.0002 M;
FIG. 7 is a plot showing an x-ray powder diffraction (XRPD) patterns for MOF-5 synthesized in Comparative Example 16 and comparative Example 17 according to conventional methods, MOF-5 synthesized in Example 18 according to the microwave-assisted method of the invention, and with a simulated pattern obtained from literature;
FIG. 8 is an SEM photograph of IRMOF-1 crystals formed by microwave synthesis according to principles of the invention. In this synthesis, an exact amount of Zn(NO3)2.6H2O (0.2 g, 0.67 mmol) and 1,4-benzenedicarboxylate acid (BDCH2) (0.083 g, 0.50 mmol) were dissolved in 10 mL of the N,N′-diethylformamide (DEF) resulting in a clear solution. The solution was then sealed in a Pyrex sample vial and heated in a household microwave oven (800 W) for a reaction time that varied from 30 sec to 1 min.
FIG. 9 is an SEM photograph of large IRMOF-1 crystals obtained after a secondary growth process according to principles of the invention. In this process, a solution including 0.200 g of Zn(NO3)2.6H2O and 0.084 g of 1,4-dibenzenedicarboxylate (H2BDC) ligand was dissolved in 20 mL N,N′diethylformamide (DEF) was made. The solution was stirred for 30 minutes. 1 mg of the IRMOF-1 crystals in FIG. 8 were suspended in the solution and then heated in a closed vessel at 80° C. After one day, 50-200 μm IRMOF-1 crystals were collected.



DETAILED DESCRIPTION OF THE INVENTION
The embodiments of the invention and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments and examples that are described and/or illustrated in the accompanying drawings and detailed in the following description. It should be noted that the features illustrated in the drawings are not necessarily drawn to scale, and features of one embodiment may be employed with other embodiments as the skilled artisan would recognize, even if not explicitly stated herein. Descriptions of well-known components and processing techniques may be omitted so as to not unnecessarily obscure the embodiments of the invention. The examples used herein are intended merely to facilitate an understanding of ways in which the invention may be practiced and to further enable those of skill in the art to practice the embodiments of the invention. Accordingly, the examples and embodiments herein should not be construed as limiting the scope of the invention, which is defined solely by the appended claims and applicable law.
Moreover, it is noted that like reference numerals represent similar parts throughout the several views of the drawings. It is understood that the invention is not limited to the particular methodology, protocols, devices, apparatus, materials, and reagents, etc., described herein, as these may vary. It is also to be understood that the terminology used herein is used for the purpose of describing particular embodiments only, and is not intended to limit the scope of the invention. It must be noted that as used herein and in the appended claims, the singular forms “a,” “an,” and “the” include plural reference unless the context clearly dictates otherwise. Thus, for example, a reference to “a MOF” is a reference to one or more MOFs and equivalents thereof known to those skilled in the art and so forth.
The term “metal-organic framework,” abbreviated “MOF,” as used herein, refers to a one, two, or three dimensional polymer including both organic and metal or metal oxide structural units, where at least one of the metal units is bonded to least one bi-, tri- or poly-dentate organic unit.
As an alternative to conventional solvothermal and hydrothermal synthesis methods, the invention provides a rapid, simple, and versatile MOF formation process particularly suited for making non-linear MOFs, including cubic MOFs. According to this method, a reactant solution including MOF precursors is exposed to microwaves for a period of time sufficient to achieve crystallization. The period of time may be, for example, a few seconds to a few minutes or more, depending upon the microwave power and the solution concentration. Example formation times range from about 5 seconds to about 2.5 minutes, which is a significant improvement over the conventional synthesis methods that typically take several hours or days.
In addition to shortened synthesis times, the microwave-assisted MOF synthesis methods of the invention may provide additional advantages. Particularly, the method of the invention provides MOF materials with a well-defined crystalline shape and uniform crystal size. The resulting micro-sized or nano-sized crystals with a size range of about 50 nm to about 10 μm produced by these methods can be engineered for use in many real applications. Reaction conditions may be tailored to provide, for example, particle size control, larger functioning surfaces, or faster sorption kinetics. For example, crystal sizes ranging from micrometers to nanometers are achieved simply by adjusting a few reaction conditions, such as reactant concentration, which are described in detail herein.
The properties of the crystals synthesized by the microwave-assisted methods of the invention are of substantially the same quality as those produced by the standard solvothermal or hydrothermal processes that take a day or more to complete. Although methods of the invention typically do not yield crystals with a size large enough for single X-ray analysis, homogeneous material is formed and may create uniform seeding conditions. This is in contrast with conventional solvothermal growth processes, in which crystals nucleate near the walls or on dust particles. Conventional methods result in slow growth because there are very few seeds.
In addition to being very slow, conventional hydrothermal and solvothermal MOF synthesis methods are difficult to reproduce due to the difficulty in controlling nucleation of MOF particles in the reactant solution. In the microwave-assisted method of the invention, however, crystals are observed throughout the bulk of the solution, presumably because local superheating of the solvent leads to hot spots that nucleate crystal growth. This is advantageous because more seeds lead to faster growth and higher yields.
Once the seeds start to grow during microwave-assisted synthesis, available reactants are quickly depleted. Therefore the size of the crystals may be varied by adjusting the reactant concentration. The ability of the microwave technique to control the nucleation process leads to a narrow size distribution because all of the crystals are nucleated at once. Further, the microwave method of the invention improves reproducibility of MOF synthesis because the nucleation of MOF crystals may be controlled better. It also allows new types of MOFs to be discovered readily since the growth process is not dependent on nucleation on the walls or dust particles. Many novel MOFs previously reported could not be easily replicated, even under the exact same experiment conditions, such as same solvent, concentration and temperature. This is because the inner walls of reaction containers or even dust particles, which are difficult to control, can be important factors that trigger the nucleation of desired MOF seeds. Microwave synthesis of the invention provides highly stable and repeatable results because it is capable of generating numerous nucleation sites by hot spots inside solvent media.
Another advantage of microwave-assisted MOF synthesis of the invention is that it allows one to determine the optimum conditions for crystal growth in a short amount of time. For example, one can use the microwave synthesis method to quickly determine the conditions at which crystals of the desired structure grow, and then repeat the crystal growth under those conditions using conventional methods to obtain larger crystals for use in applications that require larger sized crystals.
In addition, methods of the invention may provide well-defined MOF crystals that may serve as seeds that may be enlarged in post-processes to provide large crystals that may be used in various applications. This is possible because the microwave-assisted formation of MOFs of the invention provides uniform seeding conditions, as described above. The MOF crystals obtained by the microwave synthesis method of the invention are well-suited to be used in conventional secondary growth processes, for example, to form larger crystals, which will be described in detail later.
The methods of the invention may include solvothermal and hydrothermal processes. The solvent used for the reactant solution is dependent upon the organic ligand precursor. Water may be used as the solvent, for example, when the ligand precursor is water soluble. Polar molecule solvents, such as diethyl formamide (DEF) and dimethyl formamide (DMF) for example, are compatible with the mechanism of the microwave assisted synthesis of the invention. Nonpolar solvents, such as benzene, are not suitable because they can not be heated in a microwave. This problem can be solved if necessary by preparing a mixture of polar and non-polar solvent for the microwave synthesis.
Embodiments of the invention will be described with experimental data. Skilled artisans will appreciate broader aspects of the invention from the experimental data. Skilled artisans will also recognize that the laboratory-scaled synthesis methods discussed in the experimental data will be accomplished with different equipment in a commercial scale-up of the methods of the invention. Variations of the synthesis techniques in the experiments will be apparent to skilled artisans. In particular, those skilled in the art will realize that higher power microwaves will yield faster crystallization times up to the diffusion limit (0.05 seconds crystallization time for a 0.05 M solution), which is the minimum concentration of MOF precursors that is required to nucleate a MOF crystal.
In the first set of experiments, microwave synthesis of three known cubic MOFs known as IRMOF-1, IRMOF-2 and IRMOF-3 was conducted. IRMOF-1, IRMOF-2 and IRMOF-3 have been described as isoreticular metal-organic framework molecules (IRMOF) which are cubical 3-D networks of zinc-oxygen clusters connected by molecular struts such as 1,4-benzenedicarboxylate. See, e.g., “Systematic Design of Pore Size and Functionality in Isoreticular MOFs and Their Application in Methane Storage,” Yaghi et al, Science 295 (2002) 469-472; U.S. Published Patent Application 20030004364 (discloses IRMOFI-IRMOF16 molecules). The IRMOF molecules typically provide pore sizes in increments from 3.8 to 28.8 Å, and pore volumes that range to about 91% of the crystal structure. This class of molecules also includes M-IRMOF molecules, where M is M=Zn, Cd, Be, Mg, and Ca. See, “Electronic Structure and Properties of Isoreticular Metal-Organic Frameworks: The case of M-IRMOF 1 M=Zn, Cd, Be, Mg, and Ca,” Fuentes-Cabrera et al., The Journal of Chemical Physics, 123, 124713 (2005).
The experiments demonstrated the use of rapid microwave-assisted synthesis of IRMOF-1, -2 and -3 molecules from a reactant solution. In particular, a mixture of metal precursor and corresponding spacing ligand was dissolved in N,N-diethylformamide (DEF) solvent. To create a homogenous seeding environment, the mixture was thoroughly stirred for 15 minutes to obtain a clear solution. The solution was then heated in a microwave for a certain period of time to form MOF crystals.
It was recognized that heating a closed bottle containing volatile solvents and nitrates in a microwave may produce an explosion. Particularly, microwaves create hot spots that can accelerate an explosion. In addition, the pressure in a vessel containing a volatile solvent (e.g., ethanol) may be much higher than with conventional synthesis. Accordingly, conducting the experiments in a high pressure vessel may reduce the potential for explosion.
Further, potential for leakage of reactants from the microwave was also recognized. Accordingly, the experiments were performed by placing the microwave oven (Sharp R209HK 0.8 ft3 microwave oven, 800 W, exterior dimensions 18-1⁄8′′ W×14′′ D×10- 7⁄8′′ H) in a hood with a blast shield positioned in front of the sample vial. In addition, a lab grade microwave reactor module, such as a 300 W Discover LabMate, may also be used. Such microwave reactors may provide additional safety, allow more uniform heating than household microwave ovens, and may enable data connections to monitor reaction conditions.
To synthesize IRMOF-1, an exact amount of Zn(NO3)2.6H2O (0.2 g, 0.67 mmol) and 1,4-benzenedicarboxylate acid (BDCH2) (0.083 g, 0.50 mmol) were dissolved in 10 mL of the N,N′-diethylformamide (DEF) resulting in a clear solution. The solution was then sealed in a Pyrex sample vial and heated in a household microwave oven (800 W) for a reaction time varied from 30 sec to 1 min. A yellow suspension formed after the microwave treatment. The product was rinsed (centrifuged and re-dispersed in DEF by sonicating) for 3 times before analysis. Assuming that between 1 and 100% of the microwave power output was adsorbed by the DMF solution, then a 30 second heating cycle corresponded to a microwave dose of between 24 and 4800 joules/cm3.
Similarly, IRMOF-2 was synthesized by dissolving an exact amount of 2-bromoterephthalic acid, (2-BrBDCH2) (0.040 g, 0.160 mmol), and zinc nitrate tetrahydrate, Zn(NO3)2.4H2O, (0.0594 g, 0.20 mmol), 15 mL of diethylformamide. The solution was then sealed with a Pyrex sample vial and heated with a household microwave oven (800 W) for a reaction time of 80 sec. A yellow suspension formed after the microwave treatment. Assuming that between 1 and 100% of the microwave power output was adsorbed by the DMF solution, a microwave dose of between 64 and 6400 joules/cm3 was delivered, which is explained in detail below.
Further, IRMOF-3 was synthesized by dissolving an exact amount of 2-aminoterephthalic acid, (2-Amino-BDCH2) (0.2 g, 0.67 mmol) and zinc nitrate tetrahydrate, Zn(NO3)2.4H2O, (0.0913 g, 0.504 mmol), in a mixture of 39 mL diethylformamide and 3 mL ethanol. The solution was then sealed with a Pyrex sample vial and heated with a household microwave oven (800 W) for a reaction time of 60 sec. An orange suspension formed after the microwave treatment.
The resulting suspended particles of IRMOF-1 were found to be microsized cubic crystals, as shown in FIG. 3 a, b, with an average size of 5±1 μm. Particles of the obtained IRMOF-2 in FIG. 3c and IRMOF-3 in FIG. 3d also showed the same cubic morphology with a different size. Specifically, IRMOF-2 has a size of 3±2 μm; IRMOF-3 has a size of 3±1 μm. Overall, the micro crystals that were formed had relatively uniform size and identical cubic morphology.
The cubic structures of IRMOF species are further supported by X-ray powder diffraction (XRPD) results as shown in FIG. 4. FIG. 4 shows XRPD patterns of (a) IRMOF-1, (b) IRMOF-2, and (c) IRMOF-3 prepared by the solvothermal synthesis of Eddaoudi, M. et al. (Eddaoudi M., Kim, J., Rosi, N., Vodak, D., Wachter, J., O'Keeffe, M., Yaghi, O. M. Science 2002, 295, 469) (referenced by the arrow labeled black) and by microwave-assisted synthesis of the invention (referenced by the arrow labeled green) and a simulated XRPD curve based on the published structure of Eddaoudi, M. et al. (referenced by the arrow labeled red). As evident from FIGS. 4a, b, and c, IRMOF-1, IRMOF-2, and IRMOF-3 synthesized by microwave-assisted methods of the invention had XRPD patterns that closely matches the patterns produced by Eddaoudi's synthesis method and Eddaoudi's simulated structure.
The experiments also demonstrated that crystal size can be varied from micrometer down to nanometer scale by manipulating the concentration of reactant solution. Smaller particles can be obtained by reducing the reactant concentration in a starting solution. In the synthesis of small IRMOF-1 crystals, the concentration of BDCH2 was diluted from 0.05 to 0.0002 M. In particular, nine different concentrations of BDCH2 were prepared: 0.05 M, 0.025 M, 0.0125 M, 0.00625 M, 0.003125 M, 0.001563 M, 0.000781 M, 0.000391 M, 0.000195 M and the amount of zinc precursor was changed correspondingly so the metal/ligand molar ratio remained 4:3. The microwave treatments were also extended up to 90 seconds when the solution was diluted, corresponding to a microwave dose between 72 and 7200 J/cm3. An approximately 8 second increase in heating time was needed when the concentration is diluted in half.
FIG. 5 is a plot showing the estimated average size of IRMOF-1 microcrystal versus concentration of BDCH2 in the reactant solution. FIG. 5 demonstrates that particle size varied as the reactant concentration changed. In particular, nano-sized crystals were formed when the reactant concentration was scaled down to a few μM.
FIG. 6 shows enlarged SEM images of IRMOF-1 synthesized from BDCH2 concentrations of (a) 0.05 M and (b) 0.0002 M. The edge and vertex of the nano-sized crystal shown in 6b are observed to be less sharp than those of microsized crystals shown in 6a. 
The effect of microwave reaction time on crystal formation was also investigated. No crystal formation was observed when the microwave time was under 20 sec. By varying the reaction time from 25 seconds to 1 minute, the size of the microcrystals remained substantially consistent. At somewhat larger doses, the glass bottles shattered.
The amount of energy supplied to each sample vial was estimated to determine the approximate amounts of energy required to induce MOF synthesis. Published studies (See “Microwave Power Absorption In Single- And Multiple-Item Foods” by Zhang and Datta, Trans IChemE, Vol 81, Part C, p. 257-265 (2003)) have shown that between 1 and 100% of the energy output of a conventional microwave oven is adsorbed by common foods. In general, the maximum microwave dose may be estimated using the following Equation (1).

Maximum dose=λ/(1−L)  (1)

where λ is the heat of vaporization of the solvent per unit volume (i.e. about 700 J/cc for DMF) and L is the fraction of the heat that is lost to the surroundings (about 95% in the above experiments). Thus, the maximum microwave dose was estimated to be about 14000 J/cc. Optimum microwave doses were between 24 and 11000 J/cm3.

Other MOF materials were also synthesized using the rapid microwave MOF synthesis method of the invention. In particular, [Zn(p-COO)TPP]n: 0.239 g 4,4′,4′′,4′′′-(21H,23H-Porphine-5,10,15,20-tetrayl) tetrakis(benzoic acid) (H2TPP) and 0.18 g Zn(NO3)2.6H2O were dissolved in 60 mL N,N′-diethylformamide (DEF). The solution was then sealed with a Pyrex sample vial and heated with a household microwave oven (800 W) for a reaction time of 60 sec. This corresponds to a microwave dose between 48 and 4800 J/cm3. A dark purple suspension formed after the microwave treatment. The obtained crystals were square shaped, with bowed sides. XRPD analysis showed the crystals to have two dimensional [Zn(pCOO)TPP]n networks in the xy plane and stacking along the z-axis.
In another experiment, CoMOF-1 was synthesized by dissolving an exact amount of 1,4-benzenedicarboxylate acid (BDCH2) (0.086 g, 0.51 mmol), and an exact amount of cobalt (II) nitrate hexahydrate, Co(NO3)2*6H2O, (0.1 g, 0.343 mmol) and 1,4-benzenedicarboxylate acid (BDCH2) (0.0428 g, 0.258 mmol) in 10 mL diethylformamide. The solution was then sealed with a Pyrex sample vial and heated with a household microwave oven (800 W) for a reaction time of 60 sec. This corresponds to a microwave dose between 48 and 4800 J/cm3. A purple suspension formed after the microwave treatment. The resulting crystals had flattened a cubo-octahedral shape.
In another embodiment of the invention, several novel non-linear MOFs were synthesized by Applicants using the microwave synthesis method of the invention, as described in the following Examples 1-15. It will be recognized that these novel MOFs may also be made by any other methods known in the art.
EXAMPLE 1
Zn-MOF-1 synthesis: An exact amount of zinc nitrate hexahydrate, Zn(NO3)2*6H2O, (0.18 g, 0.605 mmol) and 4,4′,4′′,4′′′-(21H,23H-porphine-5-10-15-20-tetrayl)tetrakis(benzoic acid), (0.199 g, 0.252 mmol) were dissolved in 10 mL diethylformamide. The solution was then sealed with a Pyrex sample vial and heated with a household microwave oven (800 W) for a reaction time of 80 seconds. This corresponds to a microwave dose between 64 and 6400 J/cm3. The crystals obtained were dark purple with a cubic shape with sizes ranging from about 10 to about 30 microns.
EXAMPLE 2
Zn-MOF-2 synthesis: An exact amount of zinc nitrate hexahydrate, Zn(NO3)2*6H2O, (0.1 g, 0.336 mmol) and 2-anilino-5-bromoterephthalic acid, (2-anilino-5-BrBDCH2) (0.0847 g, 0.252 mmol) were dissolved in 10 mL diethylformamide. The solution was then sealed with a Pyrex sample vial and heated with a household microwave oven (800 W) for a reaction time of 80 seconds. This corresponds to a microwave dose between 64 and 6400 J/cm3. The obtained crystals were light yellow with a cubic shape with sizes ranging from 2 to 4 microns.
EXAMPLE 3
Zn-MOF-3 synthesis: An exact amount of zinc nitrate hexahydrate, Zn(NO3)2*6H2O, (0.15 g, 0.504 mmol) and 2-trifluoromethoxy terephthalic acid, (2-trifluoromethoxy-BDCH2) (0.0946 g, 0.378 mmol) were dissolved in 10 mL diethylformamide. The solution was then sealed with a Pyrex sample vial and heated with a household microwave oven (800 W) for a reaction time of 80 seconds. This corresponds to a microwave dose between 64 and 6400 J/cm3. The obtained crystals were yellow with a cubic shape, with sizes ranging from about 4 to about 7 microns.
EXAMPLE 4
Zn-MOF-4 synthesis: An exact amount of zinc nitrate hexahydrate, Zn(NO3)2*6H2O, (0.1 g, 0.336 mmol) and nitroterephthalic acid, (0.0532 g, 0.252 mmol) were dissolved in 10 mL diethylformamide. The solution was then sealed with a Pyrex sample vial and heated with a household microwave oven (800 W) for a reaction time of 80 seconds. This corresponds to a microwave dose between 64 and 6400 J/cm3. The obtained crystals had an irregular shape.
EXAMPLE 5
Zn-MOF-5 synthesis: An exact amount of zinc nitrate hexahydrate, Zn(NO3)2*6H2O, (0.15 g, 0.504 mmol) and cis-cyclobutane-1,2-dicarboxylic acid (0.0545 g, 0.378 mmol) were dissolved in 10 mL diethylformamide. The solution was then sealed with a Pyrex sample vial and heated with a household microwave oven (800 W) for a reaction time of 105 seconds. This corresponds to a microwave dose between 84 and 8400 J/cm3. The obtained crystals had a square plate shape.
EXAMPLE 6
Cu-MOF-1 synthesis: An exact amount of cupric nitrate, Cu(NO3)2*2.5H2O, (0.15 g, 0.645 mmol) and 2,5-thiophenedicarboxylic acid, (0.0833 g, 0.484 mmol) were dissolved in 10 mL diethylformamide. The solution was then sealed with a Pyrex sample vial and heated with a household microwave oven (800 W) for a reaction time of 80 seconds. This corresponds to a microwave dose between 64 and 6400 J/cm3.
EXAMPLE 7
Cu-MOF-2 synthesis: An exact amount of cupric nitrate, CU(NO3)2*2.5H2O, (0.1 g, 0.430 mmol) and 2-(trifluoromethoxy)terephthalic acid (2-trifluoromethoxy-BDCH2) (0.0807 g, 0.322 mmol) were dissolved in 10 mL diethylformamide. The solution was then sealed with a Pyrex sample vial and heated with a household microwave oven (800 W) for a reaction time of 80 seconds. This corresponds to a microwave dose between 64 and 6400 J/cm3.
EXAMPLE 8
Tb-MOF-1 synthesis: An exact amount of terbium (III) nitrate pentahydrate, Tb(NO3)3*5H2O, (0.1 g, 0.230 mmol) and terephthalic acid (BDC) (0.0286 g, 0.172 mmol) were dissolved in 10 mL diethylformamide. The solution was then sealed with a Pyrex sample vial and heated with a household microwave oven (800 W) for a reaction time of 80 seconds. This corresponds to a microwave dose between 64 and 6400 J/cm3. The obtained crystals had a rod shape.
EXAMPLE 9
Tb-MOF-2 synthesis: An exact amount of terbium (III) nitrate pentahydrate, Tb(NO3)3*5H2O, (0.1 g, 0.230 mmol) and 2,5-thiophenedicarboxylic acid (0.0297 g, 0.172 mmol) were dissolved in 10 mL diethylformamide. The solution was then sealed with a Pyrex sample vial and heated with a household microwave oven (800 W) for a reaction time of 80 seconds. This corresponds to a microwave dose between 64 and 6400 J/cm3.
EXAMPLE 10
Cd-MOF-1 synthesis: An exact amount of cadmium nitrate hexahydrate, Cd(NO3)2*4H2O, (0.1 g, 0.324 mmol) and cis-cyclobutane-1,2-dicarboxylic acid (0.0350 g, 0.243 mmol) were dissolved in 10 mL diethylformamide. The solution was then sealed with a Pyrex sample vial and heated with a household microwave oven (800 W) for a reaction time of 60 seconds. This corresponds to a microwave dose between 48 and 4800 J/cm3.
EXAMPLE 11
Cd-MOF-2 synthesis: An exact amount of cadmium nitrate tetrahydrate, Cd(NO3)2*4H2O, (0.1 g, 0.324 mmol) and nitroterephthalic acid (0.0456 g, 0.216 mmol) were dissolved in 15 mL diethylformamide. The solution was then sealed with a Pyrex sample vial and heated with a household microwave oven (800 W) for a reaction time of 60 seconds. This corresponds to a microwave dose between 48 and 4800 J/cm3;
EXAMPLE 12
Cd-MOF-3 synthesis: An exact amount of cadmium nitrate tetrahydrate, Cd(NO3)2*4H2O, (0.1 g, 0.324 mmol) and terephthalic acid (0.0404 g, 0.243 mmol) were dissolved in 10 mL diethylformamide. The solution was then sealed with a Pyrex sample vial and heated with a household microwave oven (800 W) for a reaction time of 90 seconds. This corresponds to a microwave dose between 72 and 7200 J/cm3.
EXAMPLE 13
Co-MOF-1 synthesis: An exact amount of cobalt (II) nitrate hexahydrate, Co(NO3)2*6H2O, (0.1 g, 0.343 mmol) and terephthalic acid (0.0428 g, 0.258 mmol) were dissolved in 10 mL diethylformamide. The solution was then sealed with a Pyrex sample vial and heated with a household microwave oven (800 W) for a reaction time of 60 seconds. This corresponds to a microwave dose between 48 and 4800 J/cm3.
EXAMPLE 14
Co-MOF-2 synthesis: An exact amount of cobalt (II) nitrate hexahydrate, Co(NO3)2*6H2O, (0.1 g, 0.343 mmol) and 1,3,5-benzenetricarboxylic acid (0.0542 g, 0.258 mmol) were dissolved in 10 mL diethylformamide. The solution was then sealed with a Pyrex sample vial and heated with a household microwave oven (800 W) for a reaction time of 60 seconds. This corresponds to a microwave dose between 48 and 4800 J/cm3.
EXAMPLE 15
Zn-MOF-6 synthesis: An exact amount of zinc nitrate hexahydrate, Zn(NO3)2*6H2O, (0.05 g, 0.168 mmol) and 3,3-bis(trifluoromethyl)-1-oxo-5-isobenzofurancarboxylic acid (0.0396 g, 0.126 mmol) were dissolved in 7 mL diethylformamide/3 mL H2O. The solution was then sealed with a Pyrex sample vial and heated with a household microwave oven (800 W) for a reaction time of 120 seconds. This corresponds to a microwave dose between 96 and 9600 J/cm3. The obtained crystals had a hexagonal cylinder shape, with size ranging from about 2 to about 4 microns.
In Comparative Examples 16 and 17, MOF-5, a known non-linear MOF, was synthesized according to the methods described in U.S. Patent Publication No. 2005/0192175 so that it could later be compared with MOF-5 synthesized by the microwave method of the invention, which is described herein. In Examples 18-34, specific known MOFs were synthesized using microwave synthesis.
COMPARATIVE EXAMPLE 16
50-1,000 micron MOF-5 synthesis: 0.200 g Zn(NO3)2.6H2O and 0.084 g 1,4-dibenzenedicarboxylate (H2BDC) ligand in were dissolved 20 mL N,N′diethylformamide (DEF). The solution was stirred for 30 minutes and then heated in a closed vessel at 80° C. After 1-3 days, small transparent crystals were obtained with a size distribution between about 50 and about 1,000 microns.
COMPARATIVE EXAMPLE 17
30-50 micron MOF-5 synthesis: The MOF-5 crystals in Comparative Example 16 were ground with a mortar and pestle. They were then sorted with a 120 mesh sieve to produce particles that were between about 30 and about 45 microns in diameter.
EXAMPLE 18
4-6 micron MOF-5 synthesis: An exact amount of Zn(NO3)2.6H2O (0.2 g, 0.67 mmol) and 1,4-benzenedicarboxylate acid (BDCH2) (0.083 g, 0.50 mmol) were dissolved in 10 mL of the N,N′-diethylformamide (DEF) resulting in a clear solution. One mL of the solution was sealed in a 4-mL Pyrex sample vial. The vial was then placed inside a hood behind a blast shield and heated by a microwave synthesizer (Resonance Instrument Inc. Model 520A) at 150 W for about 25 seconds. This corresponds to a microwave dose between 38 and 3800 J/cm3. A yellow suspension formed after the microwave treatment. The product was rinsed (centrifuged and re-dispersed in DEF by sonicating) for 3 times before analysis. Light yellow crystals with a particle size of about 4-6 microns were observed.
EXAMPLE 19
3 micron MOF-5 synthesis: An exact amount of Zn(NO3)2.6H2O (0.2 g, 0.67 mmol) and BDCH2 (0.083 g, 0.50 mmol) were dissolved in 10 mL of the N,N′-diethylformamide (DEF) resulting in a clear solution. The solution was diluted with DEF until the BDCH2 concentration was approximately 25 mmol/L. One mL of the solution was sealed in a 4-mL Pyrex sample vial. The vial was then placed inside a hood behind a blast shield and heated by a microwave synthesizer (Resonance Instrument Inc. Model 520A) at 150 W for about 33 seconds. This corresponds to a microwave dose between 50 and 5000 J/cm3. A yellow suspension formed after the microwave treatment. The product was rinsed (centrifuged and re-dispersed in DEF by sonicating) for 3 times before analysis. Light yellow crystals with average particle size of about 3 micron were observed.
EXAMPLE 20
1.5 micron MOF-5 synthesis: An exact amount of Zn(NO3)2.6H2O (0.2 g, 0.67 mmol) and BDCH2 (0.083 g, 0.50 mmol) were dissolved in 10 mL of the N,N′-diethylformamide (DEF) resulting in a clear solution. The solution was diluted with DEF until the BDCH2 concentration was approximately 12.5 mmol/L. One mL of the solution was sealed in a 4-mL Pyrex sample vial. The vial was then placed inside a hood behind a blast shield and heated by a microwave synthesizer (Resonance Instrument Inc. Model 520A) at 150 W for about 41 seconds. This corresponds to a microwave dose between 62 and 6200 J/cm3. A yellow suspension formed after the microwave treatment. The product was rinsed (centrifuged and re-dispersed in DEF by sonicating) for 3 times before analysis. Light yellow crystals with average particle size of about 1.5 micron were observed.
EXAMPLE 21
0.75 micron MOF-5 synthesis: An exact amount of Zn(NO3)2.6H2O (0.2 g, 0.67 mmol) and BDCH2 (0.083 g, 0.50 mmol) were dissolved in 10 mL of the N,N′-diethylformamide (DEF) resulting in a clear solution. The solution was diluted with DEF until the BDCH2 concentration was approximately 6.25 mmol/L. One mL of the solution was sealed in a 4-mL Pyrex sample vial. The vial was then placed inside a hood behind a blast shield and heated by a microwave synthesizer (Resonance Instrument Inc. Model 520A) at 150 W for about 49 seconds. This corresponds to a microwave dose between 74 and 7400 J/cm3. A yellow suspension formed after the microwave treatment. The product was rinsed (centrifuged and re-dispersed in DEF by sonicating) for 3 times before analysis. Light yellow crystals with average particle size of about 0.75 micron were observed.
EXAMPLE 22
MOF-5 synthesis: An exact amount of Zn(NO3)2.6H2O (0.2 g, 0.67 mmol) and BDCH2 (0.083 g, 0.50 mmol) were dissolved in 10 mL of the N,N′-diethylformamide (DEF) resulting in a clear solution. The solution was diluted with DEF until the BDCH2 concentration was approximately 3.13 mmol/L. One mL of the solution was sealed in a 4-mL Pyrex sample vial. The vial was then placed inside a hood behind a blast shield and heated by a microwave synthesizer (Resonance Instrument Inc. Model 520A) at 150 W for about 57 seconds. This corresponds to a microwave dose between 86 and 8600 J/cm3. A yellow suspension formed after the microwave treatment. The product was rinsed (centrifuged and re-dispersed in DEF by sonicating) for 3 times before analysis. Light yellow crystals with average particle size of about 0.5 micron were observed.
EXAMPLE 23
MOF-5 synthesis: An exact amount of Zn(NO3)2.6H2O (0.2 g, 0.67 mmol) and BDCH2 (0.083 g, 0.50 mmol) were dissolved in 10 mL of the N,N′-diethylformamide (DEF) resulting in a clear solution. The solution was diluted with DEF until the BDCH2 concentration was approximately 1.56 mmol/L. One mL of the solution was sealed in a 4-mL Pyrex sample vial. The vial was then placed inside a hood behind a blast shield and heated by a microwave synthesizer (Resonance Instrument Inc. Model 520A) at 150 W for about 65 seconds. This corresponds to a microwave dose between 98 and 9800 J/cm3. A yellow suspension formed after the microwave treatment. The product was rinsed (centrifuged and re-dispersed in DEF by sonicating) for 3 times before analysis. Light yellow crystals with an average particle size of about 0.5 micron were observed.
EXAMPLE 24
0.4 micron MOF-5 synthesis: An exact amount of Zn(NO3)2.6H2O (0.2 g, 0.67 mmol) and BDCH2 (0.083 g, 0.50 mmol) were dissolved in 10 mL of the N,N′-diethylformamide (DEF) resulting in a clear solution. The solution was diluted with DEF until the BDCH2 concentration was approximately 0.78 mmol/L. One mL of the solution was sealed in a 4-mL Pyrex sample vial. The vial was then placed inside a hood behind a blast shield and heated by a microwave synthesizer (Resonance Instrument Inc. Model 520A) at 150 W for about 73 seconds. This corresponds to a microwave dose between 110 and 11000 J/cm3. A yellow suspension formed after the microwave treatment. The product was rinsed (centrifuged and re-dispersed in DEF by sonicating) for 3 times before analysis. Light yellow crystals with average particle size of 0.4 micron were observed.
EXAMPLE 25
0.3 micron MOF-5 synthesis: An exact amount of Zn(NO3)2.6H2O (0.2 g, 0.67 mmol) and BDCH2 (0.083 g, 0.50 mmol) were dissolved in 10 mL of the N,N′-diethylformamide (DEF) resulting in a clear solution. The solution was diluted with DEF until the BDCH2 concentration was approximately 0.39 mmol/L. One mL of the solution was sealed in a 4-mL Pyrex sample vial. The vial was then placed inside a hood behind a blast shield and heated by a microwave synthesizer (Resonance Instrument Inc. Model 520A) at 150 W for about 81 seconds. This corresponds to a microwave dose between 120 and 12000 J/cm3. A yellow suspension formed after the microwave treatment. The product was rinsed (centrifuged and re-dispersed in DEF by sonicating) for 3 times before analysis. Light yellow crystals with average particle size of 0.3 micron were observed.
EXAMPLE 26
100-300 nm MOF-5 synthesis: An exact amount of Zn(NO3)2.6H2O (0.2 g, 0.67 mmol) and BDCH2 (0.083 g, 0.50 mmol) were dissolved in 10 mL of the N,N′-diethylformamide (DEF) resulting in a clear solution. The solution was diluted with DEF until the BDCH2 concentration was approximately 0.20 mmol/L. One mL of the solution was sealed in a 4-mL Pyrex sample vial. The vial was then placed inside a hood behind a blast shield and heated by a microwave synthesizer (Resonance Instrument Inc. Model 520A) at 150 W for about 90 seconds. This corresponds to a microwave dose between 140 and 13500 J/cm3. A yellow suspension formed after the microwave treatment. The product was rinsed (centrifuged and re-dispersed in DEF by sonicating) for 3 times before analysis. Light yellow crystals with average particle size of about 0.1 to about 0.3 microns (i.e. 100 to 300 nm) were observed.
EXAMPLE 27
IRMOF-2 synthesis: An exact amount of 2-bromoterephthalic acid, (2-BrBDCH2) (0.040 g, 0.160 mmol), and zinc nitrate hexahydrate, Zn(NO3)2.6H2O, (0.0594 g, 0.20 mmol) were dissolved in 15 mL diethylformamide. The solution was then sealed with a Pyrex sample vial and heated at 150 W for a reaction time of about 40 seconds. This corresponds to a microwave dose between 60 and 6000 J/cm3. A yellow suspension formed after the microwave treatment.
EXAMPLE 28
IRMOF-3 synthesis: An exact amount of 2-aminoterephthalic acid, (2-Amino-BDCH2) (0.2 g, 0.67 mmol) and zinc nitrate hexahydrate, Zn(NO3)2.6H2O, (0.0913 g, 0.504 mmol) were dissolved in a mixture of 39 mL diethylformamide and 3 mL ethanol. The solution was then sealed with a Pyrex sample vial and heated at 150 W for a reaction time of about 35 seconds. An orange suspension formed after the microwave treatment.
EXAMPLE 29
Cu(4,4′-bpy)1.5NO3(H2O)1.25 synthesis according to microwave methods of the invention using reactant concentrations as described in detail in JACS 1995, 117, 10401-10402: An exact amount of Cu(NO3)2.2.5H2O (0.1133 g, 0.487 mmol), 4,4′-dipyridyl (0.114 g, 0.730 mmol), and 1,3,5-triazine (0.026 g, 0.323 mmol) were dissolved in 15 mL of deionized water. The solution was then sealed with a Pyrex sample vial and heated with a household microwave oven (800 W) for a reaction time of about 60 seconds. This corresponds to a microwave dose between 48 and 4800 J/cm3.
EXAMPLE 30
[Cu3(TMA)2]n synthesis according to microwave methods of the invention using reactant concentrations as described in detail in Science 1999, 283, 1148-1150: An exact amount of cupric nitrate Cu(NO3)2.2.5H2O (0.1 g, 0.430 mmol) and 1,3,5-benzene-tricarboxylic acid (0.05 g, 0.239 mmol) were dissolved in 10 mL of DEF. The solution was then sealed with a Pyrex sample vial and heated with a household microwave oven (800 W) for a reaction time of about 60 seconds. This corresponds to a microwave dose between 48 and 4800 J/cm3.
EXAMPLE 31
[Cu(OH)—(C5H4NCO2]n synthesis according to microwave methods of the invention using reactant concentrations as described in detail in Inorg. Chem. 2005, 44, 6192-6196: An exact amount of cupric nitrate Cu(NO3)2.2.5H2O (0.1 g, 0.430 mmol) and isonicotinic acid (0.067 g, 0.430 mmol) were dissolved in 4 mL of deionized water. The solution was then sealed with a Pyrex sample vial and heated with a household microwave oven (800 W) for a reaction time of about 60 seconds. This corresponds to a microwave dose between 48 and 4800 J/cm3.
EXAMPLE 32
MOF-38 synthesis according to microwave methods of the invention using reactant concentrations as described in detail in JACS, 2001, 123, 8239-8247: An exact amount of zinc nitrate hexahydrate, Zn(NO3)2.6H2O (0.1 g, 0.336 mmol) and 1,3,5-benzene-tricarboxylic acid (0.039 g, 0.187 mmol) were dissolved in 10 mL of DEF. The solution was then sealed with a Pyrex sample vial and heated with a household microwave oven (800 W) for a reaction time of about 60 seconds. This corresponds to a microwave dose between 48 and 4800 J/cm3.
EXAMPLE 33
Ag(4,4′-bpy)NO3 synthesis according to microwave methods of the invention using reactant concentrations as described in detail in JACS, 1996, 118, 295-296: An exact amount of silver nitrate, Ag(NO3) (0.050 g, 0.294 mmol) and 4,4′-dipyridyl (0.051 g, 0.325 mmol) were dissolved in 10 mL of deionized water. The solution was then sealed with a Pyrex sample vial and heated with a household microwave oven (800 W) for a reaction time of about 60 seconds. This corresponds to a microwave dose between 48 and 4800 J/cm3.
EXAMPLE 34
IRMOF-7 synthesis: An exact amount of 1,4 naphthalene dicarboxylic acid, (0.109 g, 0.504 mmol) and zinc nitrate hexahydrate, Zn(NO3)2.6H2O, (0.2 g, 0.673 mmol) were dissolved in a 15 mL diethylformamide. The solution was then sealed with a Pyrex sample vial and heated with a household microwave oven (800 W) at 150 W for a reaction time of about 60 seconds.
Once the MOFs were formed, they were characterized using XRPD to confirm their composition. XRPD patterns were collected on a Bruker General Area Detector Diffraction System for 900 seconds with an 8.5 cm sample-to-detector distance. Solvated crystals used for XRPD measurements were transferred along with a mother liquor into a 0.7 mm capillary tube. For example, FIG. 7 plots an XRPD pattern for the MOF-5 obtained in Comparative Examples 16 and 18 along with a simulated pattern obtained from literature. It is evident from FIG. 7 that the XRPD pattern for laboratory-synthesized MOF-5 using microwave-assisted methods of the invention described in Example 18 coincided closely with the MOF-5 synthesized by conventional methods in Comparative Example 16, and with the simulated pattern, indicating that the MOF-5 in Example 18 was formed correctly.
The direct and fast crystallization of MOF precursors in response to microwave energy according to the invention provided unexpectedly fast nonlinear MOF crystal formation. The mechanism for the crystallization that occurs through the synthesis methods of the invention is not certain but conjectures on how the microwave is able to enhance the crystal growth process may be made. One theory for the rapid crystallization is that dipolar solvent molecules are superheated by the microwaves thus generating enough binding energy for crystal formation. Because of the homogenous heating effect, seeding conditions are likely created throughout the solution. Once a seed starts to grow, available reactant molecules could quickly be depleted by many adjacent seeds around, which would explain why the size of the crystals is largely dependent on the reactant concentration provided.
Understanding the mechanism for the formation of MOF crystals through MOF synthesis is not necessary to practice the invention, which is illustrated completely by the above description. The proposed mechanism is provided only because the theory of the mechanism, if correct, will provide skilled artisans with additional information that can be used to practice variations of the preferred embodiments.
As mentioned above, in another embodiment of the invention, secondary growth processes may be employed to create larger MOF particles using small MOF crystals obtained from the microwave synthesis methods described earlier. This post-processing may involve solvothermal or hydrothermal processes that reach and maintain a predetermined equilibrium temperature for a predetermined time, such as the conventional processes discussed in the background of this application.
For example, in another set of experiments, secondary growth of larger IRMOF-1 crystals was accomplished using smaller IRMOF-1 crystals synthesized by microwave-assisted methods of the invention described earlier. FIG. 8 is an SEM photograph of the smaller IRMOF-1 crystals formed by microwave synthesis. An exact amount of Zn(NO3)2.6H2O (0.2 g, 0.67 mmol) and 1,4-benzenedicarboxylate acid (BDCH2) (0.083 g, 0.50 mmol) were dissolved in 10 mL of the N,N′-diethylformamide (DEF) resulting in a clear solution. The solution was then sealed in a Pyrex sample vial and heated in a household microwave oven (800 W) for a reaction time varied from 30 sec to 1 min. A yellow suspension formed after the microwave treatment having a size of 5±1 micron. A solution including 0.200 g of Zn(NO3)2.6H2O and 0.084 g of 1,4-dibenzenedicarboxylate (H2BDC) ligand was dissolved in 20 mL N,N′diethylformamide (DEF) was made. The solution was stirred for 30 minutes. 1 mg of the smaller IRMOF-1 crystals obtained previously were suspended in the solution and then heated in a closed vessel at 80° C. After one day, 50-200 μm IRMOF-1 crystals were collected, as shown in the SEM photograph of FIG. 9.
In general, due to their advantageous small size, MOFs synthesized by the microwave-assisted methods of the invention may be used in a variety of applications that utilize the rapid mass transfer that a smaller particle size allows. Such applications may include, but are not limited to selective sorption in preconcentrators and other applications, drug delivery, hydrogen storage, non-linear optical materials, templates for the creation of molecular species architectures, and as catalysts e.g., catalysts to store H2 (in fuel cells) or CO2, luminescence materials, ion exchange, enantioselective separator, gas purification, gas separation, gas (H2, CH4, N2, CO2 and O2) storage.
While the invention has been described in terms of exemplary embodiments, those skilled in the art will recognize that the invention can be practiced with modifications in the spirit and scope of the appended claims. These examples given above are merely illustrative and are not meant to be an exhaustive list of all possible designs, embodiments, applications or modifications of the invention.

## Claims

1. A method for synthesis of non-linear metal organic framework (MOF) material, said method comprising the steps of:
preparing a reactant solution including MOF precursors; and
exposing said reactant solution to a microwave dose sufficient to form crystals of non-linear MOF material in the reactant solution.

2. The method of claim 1, wherein the microwave dose in said exposing step exceeds about 4 J/cm3.

3. The method of claim 1, wherein the microwave dose in said exposing step is applied for period of between about 5 seconds to about 2.5 minutes.

4. The method of claim 1, further comprising a step of post-processing growth of said crystals of the MOF material by one of a solvothermal or hydrothermal process that reaches and maintains a predetermined equilibrium temperature for a predetermined time.

5. The method of claim 1, wherein the MOF precursors include a metal precursor and a corresponding spacer ligand in a predetermined ratio, and the amount of metal precursor is set to achieve a corresponding crystal size in the metal organic framework material.

6. The method of claim 1, wherein the non-linear metal organic framework material has a cubic structure.

7. The method of claim 1, wherein said preparing step includes mixing MOF precursors in a solvent.

8. The method of claim 1, further comprising the step of removing MOF material from the reactant solution after said step of exposing.

9. The method of claim 1, wherein the MOF precursors comprise Zn(NO3)2*6H2O and 4,4′,4′′,4′′′-(21H,23H-porphine-5-10-15-20-tetrayl)tetrakis(benzoic acid), the reactant solution includes a solvent comprising diethylformamide, and the crystals comprise Zn-MOF1 crystals having a cubic crystalline structure having sizes in the range from about 10 microns to about 30 microns.

10. The method of claim 1, wherein the MOF precursors comprise Zn(NO3)2*6H2O and 2-anilino-5-bromoterephthalic acid, the reactant solution includes a solvent comprising diethylformamide, and the crystals comprise Zn-MOF2 crystals having a cubic crystalline structure having sizes in the range from 2 to 4 microns.

11. The method of claim 1, wherein the MOF precursors comprise Zn(NO3)2*6H2O and 2-trifluoromethoxy terephthalic acid, the reactant solution includes a solvent comprising diethylformamide, and the crystals comprise Zn-MOF3 crystals having a cubic crystalline structure having sizes in the range from 4 to 7 microns.

12. The method of claim 1, wherein the MOF precursors comprise Zn(NO3)2*6H2O and nitroterephthalic acid, the reactant solution includes a solvent comprising diethylformamide, and the crystals comprise Zn-MOF4 crystals having an irregular shape.

13. The method of claim 1, wherein the MOF precursors comprise Zn(NO3)2*6H2O and cis-cyclobutane-1,2-dicarboxylic acid, the reactant solution includes a solvent comprising diethylformamide, and the crystals comprise Zn-MOF5 crystals.

14. The method of claim 1, wherein the MOF precursors comprise Cu(NO3)2*2.5H2O and 2,5-thiophenedicarboxylic acid, the reactant solution includes a solvent comprising diethylformamide, and the crystals comprise Cu-MOF1 crystals.

15. The method of claim 1, wherein the MOF precursors comprise Cu(NO3)2*2.5H2O and 2-(trifluoromethoxy)terephthalic acid, the reactant solution includes a solvent comprising diethylformamide, and the crystals comprise Cu-MOF2crystals.

16. The method of claim 1, wherein the MOF precursors comprise Tb(NO3)3*5H2O and terephthalic acid, the reactant solution includes a solvent comprising diethylformamide, and the crystals comprise Tb-MOF1 crystals having a rod shaped crystalline structure.

17. The method of claim 1, wherein the MOF precursors comprise Tb(NO3)3*5H2O and 2,5-thiophenedicarboxylic acid, the reactant solution includes a solvent comprising diethylformamide, and the crystals comprise Tb-MOF2 crystals.

18. The method of claim 1, wherein the MOF precursors comprise Cd(NO3)2*4H2O and cis-cyclobutane-1,2-dicarboxylic acid, the reactant solution includes a solvent comprising diethylformamide, and the crystals comprise Cd-MOF1 crystals.

19. The method of claim 1, wherein the MOF precursors comprise Cd(NO3)2*4H2O and nitroterephthalic acid, the reactant solution includes a solvent comprising diethylformamide, and the crystals comprise Cd-MOF2crystals.

20. The method of claim 1, wherein the MOF precursors comprise Cd(NO3)2*4H2O and terephthalic acid, the reactant solution includes a solvent comprising diethylformamide, and the crystals comprise Cd-MOF3crystals.

21. The method of claim 1, wherein the MOF precursors comprise Co(NO3)2*6H2O and terephthalic acid, the reactant solution includes a solvent comprising diethylformamide, and the crystals comprise Co-MOF1crystals.

22. The method of claim 1, wherein the MOF precursors comprise Co(NO3)2*6H2O and 1,3,5-benzenetricarboxylic acid, the reactant solution includes a solvent comprising diethylformamide, and the crystals comprise Co-MOF2 crystals.

23. The method of claim 1, wherein the MOF precursors comprise Zn(NO3)2*6H2O and 3,3-bis(trifluoromethyl)-1-oxo-5-isobenzofurancarboxylic acid, the reactant solution includes a solvent comprising diethylformamide and water, and the crystals comprise Zn-MOF6 crystals.

24. The method of claim 11, wherein about a 4:3 molar ratio of Zn(NO3)2*6H2O:2-trifluoromethoxy terephthalic acid is dissolved in diethylformamide to form a 0.05 M to 0.0002 M reactant solution with respect to 2-trifluoromethoxy terephthalic acid.

25. The method of claim 24, wherein 0.504 mmol of Zn(NO3)2*6H2O and 0.378 mmol of 2-trifluoromethoxy terephthalic acid are dissolved in 10 mL of diethylformamide.