Patent Publication Number: US-11643426-B2

Title: Hypergolic metal organic frameworks

Description:
The present application claims priority from the earlier filed U.S. provisional patent application No. 62/730,590 filed on Sep. 13, 2018. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to hypergols, and more particularly to hypergolic metal organic frameworks for producing hypergols. 
     BACKGROUND 
     Energetic materials, i.e., controllable chemical energy storage systems, are central for a number of civilian applications. Current research in this area is aimed towards materials with specific requirements, such as high energy density, improved thermal stability, low cost, and environmental acceptability. An important class of energetic materials are hypergolic materials, fuels which ignite in contact with an oxidizer, such as white (WFNA) or red fuming nitric acid (RFNA) (for producing, e.g. propellants, pyrotechnics and explosives). Hypergolic compounds are ubiquitous in spacecraft and launcher propellant systems, requiring materials with a low ignition delay (ID), defined as the time between ignition and the first contact of the fuel with the oxidizer. The most popular hypergolic components of bipropellant systems today are based on toxic and highly carcinogenic hydrazine and its derivatives, rising concerns of environmental damage. The annual release of cancerogenic propellants in the atmosphere is estimated at 12,000 tons for Europe alone, inspiring the search for safer hypergolic fuels. 
     Metal-organic frameworks (MOFs) are microporous materials of importance in gas separation and storage, catalysis, and have been also been explored as energetic materials. However, the exploration of MOFs and non-porous coordination polymers as energetic materials has so far focused on energetic metal organic frameworks, rather than hypergolic metal organic frameworks that can be used as, e.g., hypergolic fuels. 
     It would therefore be advantageous to develop a metal organic framework that produces a hypergol when combined with an oxidizer, i.e., that ignites when combined with an oxidizer. 
     SUMMARY 
     The present disclosure relates to a hypergolic metal organic framework that can be used to produce a hypergol when combined with an oxidizer. As such, the hypergolic metal organic framework ignites after contact with the oxidizer. 
     The hypergolic metal organic framework can therefore be used as a fuel (a material used to produce heat and/or power) for producing a hypergolic mixture, such as a hypergolic propellant or hypergolic explosive. 
     The hypergolic metal organic framework has a structure of metal cations coordinated to aromatic organic ligands. The aromatic organic ligand is substituted with one or more unsaturated groups (a substituent having one or more double bonds and/or one or more triple bonds). As such, the combination of the aromatic organic ligand, having the unsaturated substituent, with the metal cations provides a hypergolic metal organic framework that can produce a hypergol (that ignites) after combining with an oxidizer. 
     A broad aspect is a hypergolic metal organic framework material for producing a hypergol when combined with an oxidizer, comprising a general structure M1-L-M2, wherein L is an aromatic organic linker comprising one or more unsaturated substituents, and wherein M1 and M2 are same or different metal cations. 
     In some embodiments, M1 and M2 may each be divalent metal cations. 
     In some embodiments, M1 and M2 may be each selected from: Co 2+ , Zn 2+ Cd 2+ , Fe 2+ , Ni 2+  and Cu 2+ . 
     In some embodiments, M1 and M2 may each be Co 2+ . 
     In some embodiments, M1 and M2 may each be Zn 2+ . 
     In some embodiments, M1 and M2 may each be Cd 2+ . 
     In some embodiments, the unsaturated substituent may include a double bond. 
     In some embodiments, the double bond may be terminal. 
     In some embodiments, the unsaturated substituent may include a triple bond. 
     In some embodiments, the triple bond may be terminal. 
     In some embodiments, the substituent may have a double bond or triple bond between a first atom of the substituent that is bonded to the aromatic organic linker and a second atom of the substituent that is bonded to the first atom of the substituent. 
     In some embodiments, the substituent may have a double bond between the first atom of the substituent that is bonded to the aromatic organic linker and the second atom of the substituent that is bonded to the first atom of the substituent. 
     In some embodiments, the substituent may be a vinyl group. 
     In some embodiments, the substituent may have a triple bond between the first atom of the substituent that is bonded to the aromatic organic linker and the second atom of the substituent that is bonded to the first atom of the substituent. 
     In some embodiments, the substituent may be an ethynyl group. 
     In some embodiments, the aromatic organic linker may be an azolate linking moiety. 
     In some embodiments, the azolate linking moiety may be selected from pyrazolate, imidazolate, 1,2,4-triazolate, 1,2,3-triazolate, tetrazolate or a combination thereof. 
     In some embodiments, the azolate linking moiety may be imidazolate. 
     In some embodiments, one of the one or more unsaturated substituents may be located at a 2-position of the imidazolate. 
     In some embodiments, the aromatic organic linker may be a conjugate base derived from: 
                         
or a combination thereof.
 
     In some embodiments, the metal organic framework material may exhibit an ignition delay of 50 ms or less in contact with red fuming nitric acid. 
     In some embodiments, the metal organic framework material may exhibit an ignition delay of 2 ms or less in contact with white fuming nitric acid. 
     Another broad aspect is use of a hypergolic metal organic framework material as defined herein for producing a hypergol when the hypergolic metal organic framework material is combined with an oxidizer, wherein an ignition occurs by combining the hypergolic organic framework material with the oxidizer. 
     Another broad aspect is a hypergol including hypergolic metal organic framework material as defined herein; and an oxidizer, wherein an ignition occurs by combining the hypergolic organic framework material with the oxidizer. 
     In some embodiments, the oxidizer may be composed of 70% to 100% by weight of nitric acid. 
     In some embodiments, the oxidizer may be one of red fuming nitric acid and white fuming nitric acid. 
     Another broad aspect is a composition for yielding a hypergol including hypergolic metal organic framework material as defined herein; and an oxidizer, wherein an ignition occurs by combining the hypergolic organic framework material with the oxidizer. 
     In some embodiments, the oxidizer may be composed of 70% to 100% by weight of nitric acid. 
     In some embodiments, the oxidizer may be one of red fuming nitric acid and white fuming nitric acid. 
     Another broad aspect is a method of producing a hypergol. The method includes combining a hypergolic metal organic framework material as defined in any one of claims  1  to  22  with an oxidizer, wherein an ignition occurs by combining the hypergolic organic framework material with the oxidizer. 
     In some embodiments, the oxidizer may be composed of 70% to 100% by weight of nitric acid. 
     In some embodiments, the oxidizer may be one of red fuming nitric acid and white fuming nitric acid. 
     Another broad aspect is a hypergolic metal organic framework material comprising a general structure M1-L-M2, wherein L is an imidazolate linking moiety comprising a substituent at the 2 position, wherein the substituent is one of vinyl and ethynyl, wherein M1 and M2 are the same or different transition metal cations, and wherein each of M1 and M2 are selected from Co 2+  and Cd 2+ . 
     In some embodiments, a substituent at a 4-position of the imidazolate linking moiety may be selected from one of: H, loweralkyl, oxyalkyl, cycloalkyl, aryl, heteroaryl, heterocycloalkyl, aminoalkyl and a halogen. 
     In some embodiments, a substituent at a 5-position of the imidazolate linking moiety is selected from one of: H, loweralkyl, oxyalkyl, cycloalkyl, aryl, heteroaryl, heterocycloalkyl, aminoalkyl and a halogen. 
     In some embodiments, each of M1 and M2 may be Co 2+ . 
     In some embodiments, each of M1 and M2 may be Cd 2+ . 
     In some embodiments, the substituent at the 2-position may be vinyl. 
     In some embodiments, the substituent at the 2-position may be ethynyl. 
     Another broad aspect is a hypergolic metal organic framework material comprising a general structure M1-L-M2, wherein L is an imidazolate linking moiety comprising a substituent at the 2 position, wherein the substituent is ethynyl, wherein M1 and M2 are the same or different transition metal cations, and wherein each of M1 and M2 are selected from Zn 2+ . 
     In some embodiments, a substituent at a 4-position of the imidazolate linking moiety may be selected from one of: H, loweralkyl, oxyalkyl, cycloalkyl, aryl, heteroaryl, heterocycloalkyl, aminoalkyl and a halogen. 
     In some embodiments, a substituent at a 5-position of the imidazolate linking moiety may be selected from one of: H, loweralkyl, oxyalkyl, cycloalkyl, aryl, heteroaryl, heterocycloalkyl, aminoalkyl and a halogen. 
     In some embodiments, the hypergolic metal organic framework as defined herein may be used as a fuel. 
     In some embodiments, the hypergolic metal organic framework combined with an oxidizer may be used as a hypergolic propellant. 
     In some embodiments, the hypergolic metal organic framework combined with an oxidizer may be used as a hypergolic explosion. 
     In some embodiments, the hypergolic metal organic framework combined with an oxidizer may be used as a hypergolic pyrotechnic. 
     In some embodiments, the hypergolic metal organic framework may include nitric acid, liquid oxygen and/or hydrogen peroxide. 
     Another broad aspect is a combustible composition comprising a hypergolic metal-organic framework material, a combustible substance and an additive, wherein the hypergolic metal organic framework material is for producing a hypergol when combined with an oxidizer, the hypergolic metal organic framework material comprising a general structure M1-L-M2, wherein L is and aromatic linker comprising one or more unsaturated substituents, and wherein M1 and M2 are same or different metal cations. 
     In some embodiments, the general structure of the hypergolic metal-organic framework may be selected from: Zn(VIm2), Co(VIm2), Cd(VIm2), Zn(AIm2), Co(AIm2), or Cd(AIm2). 
     In some embodiments, the combustible substance may be a metal or metalloid powder. 
     In some embodiments, the combustible substance may be a metal or metalloid powder selected from at least one of Al(0), Mg(0), Zn(0), Zr(0), Ti(0), W(0) and Si(0). 
     In some embodiments, the combustible substance may be an inorganic compound. 
     In some embodiments, the combustible substance may be an inorganic compound selected from at least one of the groups consisting of boranes, decaborate anions, hydrides, sulfides, hydrazine, hydrazine derivatives, inorganic salts, or peroxides. 
     In some embodiments, the combustible substance may be an inorganic compound selected from decaborane, hydrazine and aluminum hydride. 
     In some embodiments, the combustible substance may be an organic compound. 
     In some embodiments, the combustible substance may be an organic compound with at least one functional group selected from cyano, nitro, amino, alkyl, allyl, alkynyl, butadienyl, phenyl, halides, hydroxyl, carbonyl, peroxy, acetylene, ethylene and vinyl. 
     In some embodiments, the combustible substance may be an organic compound selected from paraffin, kerosene and nitroglycerin. 
     In some embodiments, the combustible substance may be a polymeric compound. 
     In some embodiments, the combustible substance may be an organic polymeric compound selected from one or more groups consisting of polyesters, polysulfides, polyurethanes, resins, nitrates and rubbers. 
     In some embodiments, the combustible substance may be a polymeric compound selected from hydroxylterminated polybutadiene, polyethylene, polystyrene and polybutadiene acrylonitrile. 
     In some embodiments, the additive may serve as a catalyst or initiator. 
     In some embodiments, the additive may include one or more metals. 
     In some embodiments, the additive may include one or more metals selected from at least one of Ti(0), Al(0), Pd(0) and Zr(0). 
     In some embodiments, the additive may be an inorganic compound. 
     In some embodiments, the additive may include one or more inorganic compounds selected from hydrides, azides, cyanamides, nitrates, acetates, sulfates, perchlorates, peroxides, oxides and picrates. 
     In some embodiments, the additive may be an inorganic compound selected from ammonium nitrate, ammonium perchlorate and aluminum borohydride. 
     In some embodiments, the additive may be the oxidizer. 
     In some embodiments, the hypergolic metal-organic framework may be present in an amount of 1% to 99% by weight. 
     In some embodiments, the combustible substance may be present in an amount of 1% to 99% by weight. 
     In some embodiments, the additive may be present in an amount of 1% to 99% by weight. 
     Another broad aspect is a combustible material comprising a hypergolic metal-organic framework material for producing a hypergol when combined with an oxidizer, comprising a general structure M1-L-M2, wherein L is an aromatic linker comprising one or more unsaturated substituents, and wherein M1 and M2 are same or different metal cations, wherein the metal-organic framework includes pores containing one or more guests. 
     In some embodiments, the hypergolic metal-organic framework may be selected from: Zn(VIm2), Co(VIm2), Cd(VIm2), Zn(AIm2), Co(AIm2) and Cd(AIm2). 
     In some embodiments, the guest may be a metal or metalloid powder. 
     In some embodiments, the guest may be a metal or metalloid powder selected from at least one of Al(0), Mg(0), Zn(0), Zr(0) and Si(0). 
     In some embodiments, the guest may be an inorganic compound. 
     In some embodiments, the guest may be an inorganic compound selected from one or more groups consisting of hydrides, azides, cyanamides, nitrates, acetates, sulfates, perchlorates, peroxides, oxides and picrates. 
     In some embodiments, the additive may be an inorganic compound such as ammonium nitrate, ammonium perchlorate, or aluminum borohydride. 
     In some embodiments, the guest may be an organic compound. 
     In some embodiments, the guest may be an organic compound with at least one functional group selected cyano, nitro, amino, alkyl, allyl, alkynyl, phenyl, halides, hydroxyl, carbonyl, peroxy and acetylene. 
     In some embodiments, the guest may be an organic compound is selected from paraffin, kerosene and nitroglycerin. 
     In some embodiments, the guest may be present in the metal-organic framework with a molar ratio anywhere from 0.1:0.9 to 0.9:0.1. 
     In some embodiments, the guest may be an additive or a combustible substance. 
     Another broad aspect is a combustible composition comprising a hypergolic metal-organic framework material, and further comprising a combustible substance and/or an additive, wherein the hypergolic metal organic framework material is for producing a hypergol when combined with an oxidizer, the hypergolic metal organic framework material comprising a general structure M1-L-M2, wherein L is and aromatic linker comprising one or more unsaturated substituents, and wherein M1 and M2 are same or different metal cations. 
     Another broad aspect is a method of producing a combustible composition, the method including adding a combustible substance and/or an additive to a hypergolic metal-organic framework material, wherein the hypergolic metal organic framework material is for producing a hypergol when combined with an oxidizer, the hypergolic metal organic framework material comprising a general structure M1-L-M2, wherein L is and aromatic linker comprising one or more unsaturated substituents, and wherein M1 and M2 are same or different metal cations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood by way of the following detailed description of embodiments of the invention with reference to the appended drawings, in which: 
         FIG.  1    shows schematic representations of: (a) HAIm and HVIm hypergolic ZIF precursors and (b) fragment of a hypergolic ZIF structure. (c) The final Rietveld fit for the structure of Zn(AIm) 2  with a fragment of the crystal structure shown. 
         FIG.  2    is a set of photographs of examples of hypergolicity drop tests for: (a) Zn(AIm) 2 ; (b) Co(AIm) 2 ; (c) Cd(AIm) 2 ; (d) Zn(VIm) 2 ; (e) Co(VIm) 2 , and (f) Cd(VIm) 2 . Each test was done in triplicate. 
         FIG.  3 A  shows Final Rietveld fits for (top to bottom): Co(AIm) 2 , Cd(AIm) 2 , Zn(VIm) 2 , Co(VIm) 2  and Cd(VIm) 2 ; 
         FIG.  3 B  shows examples of scanning electron microscopy (SEM) images for: (c) Co(AIm) 2  and (d) Co(VIm) 2 ; and 
         FIG.  4    shows ignition delay test images for: a. a mixture of Co(AIm) 2  and aluminum powder in 80 wt % and 20 wt % concentrations; b. a mixture of Co(VIm) 2  and aluminum powder in 80 wt % and 20 wt % concentrations; c. a mixture of Co(AIm) 2 , aluminum powder, and ammonium nitrate in 60 wt %, 20 wt %, and 20 wt % concentrations; d. a mixture of Co(VIm) 2 , aluminum powder, and ammonium nitrate in 60 wt %, 20 wt %, and 20 wt % concentrations. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure relates to hypergolic metal organic frameworks which ignite when the hypergolic metal organic framework is combined with an oxidizer. The hypergolic reaction is carried out without there needing to be any additives or compounds other than the hypergolic metal organic framework and the oxidizer. However, it will be understood that additives may be added to the mixture of the hypergolic metal organic framework and the oxidizer without departing from the present teachings. 
     As such, the hypergolic metal organic frameworks of the present disclosure can be distinguished from energetic metal organic frameworks, where an additional component, aside from the metal organic framework and the oxidizer, may be required to cause ignition. The hypergolic metal organic framework ignites upon contact of the oxidizer, without the need for any additional agent. 
     Metal organic frameworks include, but are not limited to, zeolitic imidazolate frameworks, metal azolate frameworks, and carboxylate-based frameworks, etc., etc. 
     The hypergolic metal organic framework has a structure where metal cations are coordinated to aromatic organic linkers, the aromatic organic linker including one more unsaturated substituents (having one or more double bonds and/or one or more triple bonds). 
     Preferably, the hypergolic metal framework has an ignition time equal to or under 50 ms when combined with the oxidizer. In some other embodiments, the hypergolic metal framework has an ignition time equal or under 5 ms, or equal or under 2 ms, when combined with the oxidizer. For instance, an ignition time equal to or under 10 ms occurs when the substituent is an ethynyl group or has a triple bond between the first atom of the substituent that is bonded to the aromatic organic linker and the second atom of the substituent that it bonded to the first atom of the substituent (however other substituents may be used in order to achieve an ignition time equal to or under 10 ms). 
     In some embodiments, the metal cations of the metal organic framework are divalent metal cations. Exemplary divalent metal cations include, but are not limited to, Co 2+ , Zn 2+  and Cd 2+ . 
     In some embodiments, the unsaturated substituent may have a terminal double bond. In some embodiments, the unsaturated substituent may have a terminal triple bond. 
     In some embodiments, the unsaturated substituent may have a double bond between the first atom of the substituent that is bonded to the aromatic organic linker and the second atom of the substituent that it bonded to the first atom of the substituent. 
     In some embodiments, the unsaturated substituent may be a vinyl group. In some embodiments, the unsaturated substituent may be an ethynyl group. However, the unsaturated substituent may be another substituent (e.g. any linear aliphatic chain containing one or more double or triple bonds) without departing from the present teachings. 
     In some embodiments, the aromatic organic linker may be an azolate linking moiety. 
     In some embodiments, the azolate linking moiety may be an imidazolate. In some embodiments, the unsaturated substituent may be present at the 2-position of the imidazolate. In some examples, the imidazolate may have an unsaturated substituent at the 4-position and/or the 5-position. 
     The hypergolic metal organic framework may be used as a fuel that, when combined with an oxidizer, produces a hypergolic propellant. It will be understood that the hypergolic metal organic framework may be for other uses than that of a fuel. It will also be understood that the hypergol resulting from the combination of the hypergolic metal organic framework and the oxidizer may be for other uses than that of a hypergolic propellant. 
     Uses of the hypergolic metal organic framework, once combined with an oxidizer, may include, hypergolic propellants, hypergolic explosives, hypergolic pyrotechnics, etc. 
     In the present disclosure, by “hypergol”, because a hypergol undergoes a reaction once its constituents are combined, it will be understood that it includes, in some examples, the combined constituents (the hypergolic metal organic framework and the oxidizer), and in some examples, the separate substituents such that they may be combined at the appropriate time when the use of the hypergol is required. 
     The constituents of the hypergol may be stored and/or transported in use-appropriate vehicles (e.g. a vehicle or compartment that prevents air exposure). 
     In some examples, the hypergolic metal organic framework may be provided and/or stored in a medium that reduces or eliminates its exposure to external elements (e.g. a gaseous medium such as inert atmosphere, N 2 ; atmospheric gases; a liquid medium such as ethanol, methanol, dimethylformamide, acetone, tetrahydrofuran, etc). The hypergolic metal organic framework may be removed from the medium prior to use. 
     In some embodiments, the method of producing a hypergol results in adding the oxidizer to the hypergolic metal organic framework. It will be understood that the conditions of the combining and the environment in which the combining takes place may need to be controlled for safety and/or depending on the use of the hypergol. 
     Exemplary hypergolic metal organic framework structures are as follows: 
     
       
         
         
             
             
         
       
     
     Exemplary Synthesis 
     An exemplary synthesis is as follows: 
     ZnO (1 mmol), ethynyl/vinyl imidazole (2 mmol, 2.1 mmol for VIm), ammonium acetate (0.12 mmol, catalytic salt), and 100 μL dry EtOH were added to a 15 mL stainless-steel milling jar with two stainless-steel milling balls (1.34 g/7 mm each) The mixture was milled for 30 minutes at frequency of 30 Hz in a shaker-mill (Retsch MM400, Germany) to yield a microcrystalline product. The product was added to 15 mL of methanol, stirred overnight, then vacuum filtered. The dry product was then evacuated overnight under vacuum at 80° C. 
     In case of Co-MOF, CoCO3 was used, and for Cd-MOFs, CdO was used. 
     Another exemplary synthesis is as follows: 
     For producing, e.g., SOD-Zn(AIm) 2 : 
     A mixture of 2 mmol HAIm and 2 mmol triethylamine was added to 15 mL DMF and stirred at room temperature. A solution of 1 mmol zinc nitrate hexahydrate in 5 mL DMF was added dropwise, over 30 seconds, to the stirred solution of HAIm and triethylamine causing the the precipitation of the target framework. This mixture was capped and placed at 60° C. for 3 hours, the product isolated by filtration, and suspended in 20 mL methanol. This suspension was placed, capped, at 60° C. overnight, the product was again filtered, and stirred in chloroform for 3 hours to remove any residual DMF. Finally, the framework was isolated by filtration and evacuated at 80° C. under vacuum overnight to afford the pure product. 
     Definitions 
     The term a “hypergolic metal organic framework” or “hypergolic metal organic framework material” as used herein refers to a metal organic framework material (e.g. in solid form) that ignites upon contact with an oxidizer and without external aid (such as a spark). The metal organic framework has metal ions coordinated to organic linkers or ligands to form one-, two- or three-dimensional structures. 
     The term “hypergol” as used herein refers to a substance or composition that ignites without external aid upon contact of its components. For instance, a hypergol may be a hypergolic propellant, a hypergolic explosive, a hypergolic pyrotechnic material, etc. 
     The term “additive” as used herein refers to a substance that either accelerates the reaction, initiates the reaction or enhances the combustion of the reaction. For instance, an additive can be a catalyst or an initiator. Exemplary additives may include hydrides, azides, cyanamides, nitrates, acetates, sulfates, perchlorates, peroxides, oxides and picrates. 
     The term “pore” is used herein to define a space or pocket created by the structure of the hypergolic metal organic framework. 
     The term “guest” as used herein refers to a substance present in or enclosed by the pore(s) of the hypergolic metal organic framework. 
     The term “combustible substance” as used herein refers to a substance that can be added to a composition including the hypergolic metal organic framework that can undergo combustion with the hypergolic metal organic framework. 
     The term an “oxidizer”, “oxidant” or “oxidizing agent” as used herein refers to a substance that is able to oxidize other substances, causing them to lose electrons. Exemplary oxidizers include, but are not limited to, peroxides (such as hydrogen peroxide), nitric acid, nitrate compounds, sulfuric acid, halogen compounds, sodium perborate, hexavalent chromium compounds, peroxydisulfuric acid, peroxymonosulfuric acid, chlorite, chlorate, perchlorate, oxygen (such as liquid oxygen), ozone, etc. 
     The term “aromatic organic linker” as used herein refers to an aromatic organic molecule that are coordinated with metal cations to form metal organic frameworks. Exemplary aromatic organic linkers include, but are not limited to azolates, terephthalates, trimesates, benzoates, mellitates. 
     The term “aminoalkyl” as used herein refers to a C1-C6 linear or branched alkyl joined to an amino group (NH 2 ). Examples of aminoalkyl include, but are not limited to, —CH 2 NH 2 , —CH 2 CH 2 NH 2 , etc. An aminoalkyl may be optionally substituted with one to five substituents independently selected from, for instance, the group consisting of hydroxy, thiol, cyano, nitro, loweralkyl, sulfonyl, halogen or amino. 
     The term “aryl” as used herein refers to a six to ten membered monocyclic or polycyclic aromatic ring where all of the ring atoms are carbon atoms. Examples of aryls include but are not limited to phenyl and biphenyl. An aryl may be optionally substituted with one to five substituents independently selected from, for instance, the group consisting of hydroxy, thiol, cyano, nitro, loweralkyl, sulfonyl, halogen or amino. 
     The term “cycloalkyl” as used herein, refers to a three to ten membered monocyclic or polycyclic ring, saturated or partially unsaturated, where all of the ring atoms are carbon. Examples of cycloalkyl include, but are not limited to, cyclopropyl, cyclobutyl, cyclopentyl, cyclopentenyl, cyclohexyl, cyclohexenyl, etc. A cycloalkyl may be optionally substituted by one to five substituents independently selected from, for instance, the group consisting of hydroxy, thiol, cyano, nitro, loweralkyl, sulfonyl, halogen or amino. 
     The term “heteroaryl” as used herein refers to a five to ten membered monocyclic or polycyclic aromatic ring having atoms selected from N, O, S and C. Examples of heteroaryl include, but are not limited to, furanyl, thienyl, imidazolyl, pyrazolyl, pyrrolyl, pyrrolinyl, thiazolyl, etc. An heteroaryl may be optionally substituted with one to five substituents independently selected from, for instance, the group consisting of hydroxy, thiol, cyano, nitro, loweralkyl, sulfonyl, halogen or amino. 
     The term “heterocycloalkyl” as used herein refers to a four to ten membered monocyclic or polycyclic ring, saturated or partially unsaturated, where the ring atoms are selected from N, O, S and C. Examples of heterocycloalkyl include, but are not limited to, azetidinyl, tetrahydrofuran, dihydrofuran, dioxane, morpholine, etc. A heterocycloalkyl may be optionally substituted by one to five substituents independently selected from, for instance, the group consisting of hydroxy, thiol, cyano, nitro, loweralkyl, sulfonyl, halogen or amino. 
     The term “loweralkoxy” or “oxyalkyl” as used herein, refers to C1-C6 linear or branched alkoxy, such as methoxy, ethoxy, propyloxy, butyloxy, isopropyloxy, and t-butyloxy. A loweralkoxy or oxyalkyl may be optionally substituted with one to five substituents independently selected from, for instance, the group consisting of hydroxy, thiol, cyano, nitro, loweralkyl, sulfonyl, halogen or amino. 
     The term “loweralkyl,” as used herein, refers to C1-C6 linear or branched alkyl, such as methyl, ethyl, propyl, butyl, isobutyl, isopropyl, sec-butyl, tert-butyl, pentyl, isopentyl, and hexyl. A loweralkyl may be optionally substituted with one to five substituents independently selected from, for instance, the group consisting of hydroxy, thiol, cyano, nitro, loweralkyl, sulfonyl, halogen or amino. 
     Exemplary Study 1: 
     The following study provides non-limitative examples of hypergolic metal organic frameworks of the present disclosure. 
     A MOF design for hypergolic solids is now presented, based on introducing acetylene and vinyl triggers into zeolitic imidazolate frameworks (ZIFs): metal-organic structures in which zeolite-like topologies are achieved by combining tetrahedrally-coordinated metal nodes with imidazolates as bent linkers. The proposed design is inspired by the ZIF structure, consisting of imidazolate linkers flanked by cationic nodes, resembling the general structure of imidazolium ions used in the design of hypergolic ionic liquids. Based on this analogy, it was speculated that introducing unsaturated functionalities, known to trigger hypergolicity in liquid fuels, into the imidazolate linker structure should produce open ZIFs that are ignitable, with hypergolic behavior tunable by choice of metal nodes and substituents on the imidazolate linker. 
     The first hypergolic ZIF was prepared mechanochemically, following a previously described ion- and liquid-assisted grinding (ILAG) procedure, by ball milling of ZnO with 2-ethynylimidazole (HAIm) in a 1:2 stoichiometric ratio ( FIG.  1   ). Powder X-ray diffraction (PXRD) indicated the crude product was isostructural to ZIF-8 (CSD OFERUN02), a sodalite (SOD) topology ZIF based on 2-methylimidazole (HMeIm), allowing us to create a preliminary structural model by replacing the methyl substituents of ZIF-8 with alkyne groups. After the product was washed with MeOH and evacuated, this structure was readily refined against experimental PXRD data using the Rietveld method. Thermogravimetric analysis (TGA) in air of the washed and evacuated material was consistent with the formula Zn(AIm) 2 , and the presence of alkyne groups was evident from infrared spectroscopy (IR) by a characteristic C—H stretching band at 3330 cm −1  and a weak triple bond stretch at 2129 cm −1 . 
     The ID for Zn(AIm) 2  was evaluated by a standard drop test ( FIG.  2   a   ) in which a 100 μL Hamilton micro-syringe was used to release a single 10 μL drop of WFNA from a fixed height of 5 cm into a 4.5 cm high glass vial containing 5.0 mg of Zn(AIm) 2  concentrated in the center of the vial. Each measurement was repeated three times and recorded using a Redlake MotionPro Y4 high-speed camera, operating at 1000 frames/s in monochrome. The drop tests revealed an extremely short ID of 2(1) ms for Zn(AIm) 2 , with appearance of both red and blue flames, reaching a height of ca. 4 cm, and lasting for more than 600 ms. Importantly, the ID of Zn(AIm) 2  remained the same for mechanochemically and solution-prepared material. Hypergolic behavior was also observed for solid HAIm, but with a much longer ID of 34(4) ms and red flames indicating that ZIF formation enhances the hypergolic behavior of the solid ligand. This observation highlights a significant difference between the presented hypergolic MOF design, in which framework formation enhances ligand hypergolicity, and approaches to energetic MOFs, where the formation of coordination bonds leads to stabilization of an energetic ligand. 
     To verify the importance of the acetylene trigger for hypergolicity of Zn(AIm) 2 , drop tests were conducted on two ZIFs containing saturated hydrocarbon groups: the open RHO-topology framework based on 2-ethylimidazole (HEtIm) and ZIF-8. The RHO—Zn(EtIm) 2  framework was chosen as the direct saturated analogue of Zn(AIm) 2 , while ZIF-8 was chosen due to isostructurality to Zn(AIm) 2 . No hypergolic behavior was observed for either RHO—Zn(EtIm) 2  or ZIF-8, indicating that the trigger substituent is indeed responsible for hypergolic behavior. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Hypergolic properties of ZIFs in WFNA drop tests. a   
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 flame 
                 flame height b   
                 flame 
               
               
                 ZIF 
                 ID (ms) 
                 duration (s) 
                 (cm) 
                 color 
               
               
                   
               
               
                 Zn(Alm) 2   
                 2(1) c , 2(1) d   
                 &gt;600 
                 4 
                 red, blue 
               
               
                 Co(Alm) 2   
                 2(1) 
                 &gt;200 
                 4 
                 orange 
               
               
                 Cd(Alm) 2   
                 5(1) c , 2(2) d   
                 &gt;200 
                 6 
                 yellow 
               
               
                 Zn(Vlm) 2   
                 29(1) 
                 Sparks 
                 Sparks 
                 red 
               
               
                 Co(Vlm) 2   
                 11(5) 
                 &gt;200 
                 2 
                 orange 
               
               
                 Cd(Vlm) 2   
                 35(1) 
                 Sparks 
                 Sparks 
                 yellow 
               
               
                 ZIF-8 
                 — 
                 — 
                 — 
                 — 
               
               
                 Zn(Etlm) 2   c   
                 — 
                 — 
                 — 
                 — 
               
               
                 HAlm 
                 34(4) 
                 &gt;100 
                 7 
                 red 
               
               
                 HVlm 
                 35(1) 
                 Sparks 
                 Sparks 
                 red 
               
               
                   
               
               
                   a each test was conducted in triplicate; 
               
               
                   b approximate value; 
               
               
                   c made mechanochemically; 
               
               
                   d made from solution; 
               
               
                   e RHO-topology framework. 
               
            
           
         
       
     
     Next, it was investigated how changes in ligand structure, as well as Lewis acidity and electronic structure of the node can be used to control ZIF hypergolicity. For this purpose, WFNA and RFNA drop tests were synthesized and conducted on acetylene-substituted ZIFs based on Cd 2+  and Co 2+  nodes, as well as Zn 2+ , Co 2+ , and Cd 2+  ZIFs involving a vinyl-substituted imidazole ligand (HVIm). ZIFs were prepared via a range of techniques, including mechanochemistry, aging, and solvothermal routes (see ESI), and PXRD indicated they were all isostructural to Zn(AIm) 2 . After washing with MeOH and evacuation, TGA in air was consistent with compositions Co(AIm) 2 , Cd(AIm) 2 , Zn(VIm) 2  (CSD GAZBOB), Co(VIm) 2 , and Cd(VIm) 2  (see ESI). Structures for all ZIFs, after washing with MeOH and evacuation, were confirmed by Rietveld refinement ( FIG.  3 A ). 
     Acetylene-substituted ZIFs exhibited shorter IDs and more vigorous combustion compared to vinyl-based ones ( FIG.  2   , Table 1). The vinyl- and acetylene-substituted ZIFs exhibited comparable particle sizes. The choice of metal also had significant effect on ZIF behavior. Hypergolic behavior of Cd(AIm) 2  and Cd(VIm) 2  was poorer compared to analogous zinc systems. In contrast, Co 2+  led to hypergolic properties comparable or superior to those of zinc ZIFs: the ID for Co(AIm) 2  is comparable to Zn(AIm) 2 , while Co(VIm) 2  is the only vinyl-substituted MOF that underwent full ignition. 
     Drop tests on Zn(AIm) 2 , Co(AIm) 2  and Cd(AIm) 2  were also repeated after one month of storage in a closed vial, revealing high hypergolicity and a short ID of 4(1) ms for Zn(AIm) 2 , and partially diminished activity for Co(AIm) 2 , with an ID of 54(5) ms. Hypergolicity of Cd(AIm) 2  dropped significantly upon storage, producing only sparks in drop tests. The retention of high hypergolic activity upon storage, as evident with Zn(AIm) 2 , is an important factor for applications of such materials as solid fuels. 
     The hypergolicity of the herein reported six isostructural MOFs in drop test involving RFNA was explored, a widely used oxidizer in liquid and solid rocket propulsion systems. With RFNA, hypergolic behavior was observed for all acetylene-substituted MOFs, as well as for Co(VIm) 2 . No ignition was observed for other vinyl-based MOFs, or pure ligands as the oxidizer. Combined thermogravimetric (TG) and differential scanning calorimetry (DSC) revealed high thermal stability in air for all materials, with exothermic thermal degradation taking place above 200° C. Thermal degradation of Zn(AIm) 2 , Co(AIm) 2  and Cd(AIm) 2  was explosive and TG analysis had to be limited to samples of 10 mg or less. No such effect was observed for Zn(AIm) 2 , Co(AIm) 2  and Cd(AIm) 2  (see ESI). 
     Besides hypergolicity, further important characteristics of a propellant are the energy released in the oxidation process, and the energy density of the material. A preliminary theoretical evaluation of these parameters for herein synthesized hypergolic MOFs by using periodic density functional theory (DFT) was conducted. By using DFT, calculated enthalpies (ΔHc) for reactions of Zn(AIm) 2 , Co(AIm) 2 , Cd(AIm) 2 , Zn(VIm) 2 , Co(VIm) 2  and Cd(VIm) 2  solids with O 2  gas to produce solid ZnO, CdO, and Co 3 O 4 , along with gases CO 2 , N 2 , and water vapor were evaluated. The gravimetric (E g ) and volumetric (E v ) energy densities were computed from ΔH c , taking into account the theoretical volume and mass of liquid O 2  required for ZIF oxidation. 
     All oxidation reactions were calculated to be highly exothermic, with ΔH c  ranging from 3,900 kJ mol −1  to 4,800 kJ mol −1  (Table 1) 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Calculated combustion enthalpy ΔH c , gravimetric (E g ) 
               
               
                 and volumetric (E V ) energy density, 
               
               
                 and crystallographic unit cell parameter (a) for ZIFs. 
               
            
           
           
               
               
               
               
               
            
               
                 ZIF 
                 ΔH c /kJ mol −1   
                 E g /kJ g −1   
                 E V /kJ cm −3   
                 a/Å 
               
               
                   
               
               
                 Zn(Alm) 2   
                 −4783.8 
                 7.58 
                 8.18 
                 17.045(1) 
               
               
                 Co(Alm) 2   
                 −4760.0 
                 7.55 
                 7.95 
                 16.960(2) 
               
               
                 Cd(Alm) 2   
                 −4799.9 
                 7.08 
                 7.54 
                 17.9721(9)  
               
               
                 Zn(Vlm) 2   
                 −4789.9 
                 7.18 
                 7.70 
                     17.269(1) a   
               
               
                 Co(Vlm) 2   
                 −4767.6 
                 7.16 
                 7.51 
                 17.296(1) 
               
               
                 Cd(Vlm) 2   
                 −4808.4 
                 6.73 
                 7.18 
                 18.234(2) 
               
               
                   
               
               
                   a from CSD structure GAZBOB; 
               
            
           
         
       
     
     In order to evaluate the accuracy of the calculations, the ΔH c  for ZIF-8 (−3916.0 kJ mol −1 ) was calculated and compared to the experimental value measured by bomb calorimetry on bulk material (−4654.0 kJ mol −1 ). The comparison suggests that calculated values are reasonably accurate and most likely underestimate true values by ca. 15%. The ΔH c  for of vinyl- and acetylene-substituted ZIFs were similar to that of ZIF-8, suggesting that observed differences in hypergolicity are largely due to kinetic effects associated with metal or ligand choice, rather than energetic content of the trigger groups. The calculated E g  and E v  for vinyl-substituted ZIFs are lower than acetylene-based analogues, reflecting the consistently higher crystallographic unit cell dimensions for the former. The calculated E g  and E v  for hypergolic ZIFs ranged from 6.7-7.6 kJ g −1  and from 7.2 kJ-8.2 kJ cm −3 , respectively, which is on par with popular energetic materials (e.g. TNT, 4.56 kJ/g) but still lower than for hydrazine fuels (19.5 kJ g −1 , 19.3 kJ cm −3 ). 
     In summary, the first design for MOFs as hypergolic material was reported, based on introducing trigger unsaturated acetylene and vinyl functionalities onto the well-known zeolitic imidazolate framework scaffold. This design is intrinsically different from approaches to synthesize energetic (e.g., explosive, pyrotechnic) MOFs, which are based on using energetic (often nitrogen-rich or nitro-substituted) molecules as MOF components or guests. In contrast, the design of MOF hypergols does not necessitate energetic components as ligands, but presence of substituents that induce ignition. As a result, the presented materials exhibit calculated combustion enthalpies comparable to that of the popular, non-energetic ZIF-8 framework. In contrast to design of explosive MOFs, where formation of a coordination framework increases the stability of an energetic ligand, the herein presented design shows that metal coordination generally enhances ligand hypergolic behavior. As a result, the presented design offers two routes to vary hypergolic properties of a material (i.e., ignition delay, flame color and duration): by choice of the metal node (Co, Zn, Cd) or choice of trigger functionality, with acetylene triggers leading to extremely short ignition delays of 2 ms. 
     Exemplary Study 2: 
     The following provides non-limitative examples of combustible compositions comprising a hypergolic metal organic framework, a combustible substance, and/or an additive as described in the present disclosure. 
     Compositions with Aluminum (Al) Powder as the Combustible Substance 
     Compositions with Aluminum 
     Mixtures of hypergolic metal-organic frameworks and aluminum powder were prepared by carefully weighing out the powders and mixing them together briefly in a mortar and pestle. Their hypergolicity was tested using drop tests with WFNA. A mixture of Co(AIm) 2  and aluminum in 80 wt % and 20 wt % concentrations respectively, was found to ignite upon contact with WFNA with an ignition delay of 1(1) ms. The resulting flame (Figure X1) was observed to be orange-red in colour and lasted for greater than 250 ms. Similar results were observed when Co(AIm) 2  was replaced with Co(VIm) 2 , except the latter showed an ignition delay of 6(1) ms and a weaker flame. It is interesting to note that Co(VIm) 2  alone has a longer ignition delay of 11(1) ms. The results of these ignition tests are summarized in Table 3, as follows: 
                     TABLE 3                  Results of ignition delay tests on compositions       consisting of hypergolic metal organic frameworks       Co(Alm) 2  or Co(Vlm) 2  and aluminum powder.                                 Material   ID   Height   Duration   Observations               80 wt % Al,   1(1)   4 cm   &gt;250 ms   Black residue       20 wt % Co(Alm) 2             Several   Orange-red                   ignition   flame       60 wt % Al, 20 wt % AN,   1(0)   3-4   &gt;250 ms   Black residue       20 wt % Co(Alm) 2         cm       Orange-red                       flame       80 wt % Al,   6(1)   0.5-1   &gt;100 ms   Black residue       20 wt % Co(Vlm) 2         cm   Weak flames   Orange-red                       flame       60 wt % Al, 20 wt % AN,   6(2)   0.5-1   &gt;100 ms   Black residue       20 wt % Co(Vlm) 2         cm       Orange-red                       flame                    
Compositions with Aluminum and Ammonium Nitrate as the Additive
 
     Next the effect of adding ammonium nitrate as an additive was evaluated by performing ignition tests on mixtures of hypergolic metal-organic frameworks, aluminum powder, and ammonium nitrate in concentrations of 60 wt %, 20 wt %, and 20 wt % respectively. The addition of the additive did not significantly affect the ignition delay of the hypergolic MOF/aluminum compositions, but the flames were slightly longer lived ( FIG.  4   ). These results are also summarized in Table 3. 
     Although the invention has been described with reference to preferred embodiments, it is to be understood that modifications may be resorted to as will be apparent to those skilled in the art. Such modifications and variations are to be considered within the purview and scope of the present invention. 
     Representative, non-limiting examples of the present invention were described above in detail with reference to the attached drawing. This detailed description is merely intended to teach a person of skill in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. Furthermore, each of the additional features and teachings disclosed above and below may be utilized separately or in conjunction with other features and teachings. 
     Moreover, combinations of features and steps disclosed in the above detailed description, as well as in the experimental examples, may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe representative examples of the invention. Furthermore, various features of the above-described representative examples, as well as the various independent and dependent claims below, may be combined in ways that are not specifically and explicitly enumerated in order to provide additional useful embodiments of the present teachings.
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