PRODUCING THE SAME

TECHNICAL FIELD

Various embodiments disclosed herein relate broadly to vanadium-based metal-organic frameworks derived from a vanadium source, and methods of producing vanadium-based metal-organic frameworks from a vanadium source. Vanadium source includes but is not limited to vanadium-based waste such as fossil fuel waste.

BACKGROUND The amount of waste generated has dramatically increased with the rising consumption and resource use by a rapidly growing world population. An increasing amount of research interest has therefore been focused on building opportunities from waste, including food waste, biomass waste, oil fly ash, and the like. Such waste may sometimes contain useful components that still possess good market value, but the components are simply disposed of instead of recovered due to technological or economical limitations. An economically feasible recovery of the useful components may also contribute to a sustainable society. For instance, waste could be used as a valuable resource for the preparation of fuels and high-value chemicals/materials.

Vanadium is an excellent but toxic heavy metal that occurs in crude oil, coal, oil shale and tar sands. In particular, vanadium is the most abundant metal in crude oil, reaching concentration of up to 1580 ppm of total crude, which is largely derived from chlorophyll of dead organisms. As crude oil is processed and refined into more useful fuels and intermediate products by oil refinery plants, vanadium will finally exist in carbon black waste (a major oil refinery waste). Carbon black waste is therefore a potential but untapped source of vanadium. Further, disposal of carbon black waste containing vanadium poses a pollution risk as vanadium is hazardous to human health and was found to impair the antioxidant enzymatic activities of human cell lines.

Typically, for metal recovery from carbon black waste, complete dissolution of valuable metals from carbon black waste is achieved by advanced leaching processes. Various technologies have been developed for the subsequent separation and recovery of metal ions in the leaching solutions, such as chemical precipitation, reactive crystallization, adsorption, ion exchange, electrochemical removal, biotechnological processes, and membrane separations. However, separation and recovery of metal, particularly vanadium, in the leaching solutions remains a challenge. Furthermore, the studies on transformation of waste metal ions, particularly vanadium ions, into value-added products are still insufficient in both academia and industry.

In view of the above, there is thus a need to address or at least ameliorate one of the above problems. SUMMARY

In one aspect, there is provided a method of producing vanadium-based metal-organic frameworks (V-MOFs), the method comprising:

reducing an oxidation state of vanadium in a leaching solution containing vanadium ions, the leaching solution being derived from a vanadium source; and coordinating one or more linker molecules with vanadium having the reduced oxidation state to form vanadium-based metal-organic frameworks.

In one embodiment, the step of coordinating one or more linker molecules with vanadium having the reduced oxidation state is carried out in the presence of at least one surfactant. In one embodiment, the amount of surfactant present is in the range of 0.01

- 2 wt%.

In one embodiment, the surfactant is selected from the group consisting of cetyltrimethylammonium bromide, cetyltrimethylammonium chloride, dodecyltrimethylammonium bromide, dodecyltrimethylammonium chloride, tetradecyltrimethylammonium bromide, tetradecyltrimethylammonium chloride, or mixtures thereof. In one embodiment, the method further comprises adding at least one of an acid or a base to the vanadium source to form the leaching solution containing vanadium ions.

In one embodiment, the step of adding at least one of an acid or a base to the vanadium source comprises adding a base first, followed by acid.

In one embodiment, the step of reducing the oxidation state of vanadium comprises adding at least one reducing agent to the leaching solution. In one embodiment, the step of coordinating one or more linker molecules with vanadium having the reduced oxidation state comprises a hydrothermal or a solvothermal treatment step at a temperature ranging from approximately 120°C

- approximately 300°C.

In one embodiment, the treatment step is hydrothermal.

In one embodiment, the amount of reducing agent added to the leaching solution is in the range of 0.40 - 2.20% w/v. In one embodiment, the reducing agent is selected from the group consisting of sodium dithionite, ascorbic acid, sodium borohydride, sodium citrate, or mixtures thereof. In one embodiment, the one or more linker molecules comprise a multidendate ligand.

In one embodiment, the one or more linker molecules comprise a multidentate ligand selected from the group consisting of 1 ,4-benzenedicarboxylic acid (H ₂ BDC), 1 ,4-naphthalenedicarboxylic acid (H2NDC), 2,5- dimethylbenzenedicarboxylic acid (H2DMBDC), 1 ,3,5-benzenetricarboxylic acid (H3BTC) and 1 ,2,4, 5-benzenetetracarboxylic acid (H ₄ BTEC).

In one embodiment, the method further comprises increasing the oxidation state of vanadium of the vanadium-based metal-organic frameworks formed from the coordinating step.

In one embodiment, the step of increasing the oxidation state of vanadium comprises heating the vanadium-based metal-organic frameworks formed from the coordinating step in air.

In one embodiment, the method further comprises, prior to the reducing step:

(i) adding at least one of an acid or a base to the vanadium source to obtain a first leachate containing vanadium ions;

(ii) adding the leachate obtained from the preceding step to the vanadium source to obtain a second leachate containing a higher concentration of vanadium ions;

(iii) optionally repeating step (ii) for one or more times to obtain a third leachate or subsequent leachate; wherein the leachate obtained after completion of the above steps forms the leaching solution containing vanadium ions for use in the reducing step.

In one embodiment, the vanadium source is an oil refinery waste.

In one embodiment, the vanadium source is a carbon black waste.

In one aspect, there is provided vanadium-based metal-organic frameworks (V-MOFs) derived from a vanadium source, the vanadium-based metal-organic frameworks (V-MOFs) comprising nanostructures having substantially uniform nanorods and/or nanofibers morphology.

In one embodiment, the nanorods have an average length in the range of from 180 nm to 350 nm and an average diameter in the range of from 60 to 70 nm and the nanofibers have an average length of greater than 10 μηι and an average diameter in the range of from 35 nm to 45 nm.

In one embodiment, the vanadium-based metal-organic frameworks comprise one or more linker molecules selected from the group consisting of 1 ,4- benzenedicarboxylic acid (H2BDC), 1 ,4-naphthalenedicarboxylic acid (H2NDC), 2,5-dimethylbenzenedicarboxylic acid (H2DMBDC), 1 ,3,5-benzenetricarboxylic acid (H3BTC) and 1 ,2,4,5-benzenetetracarboxylic acid (H ₄ BTEC), the one or more linker molecules being coordinated to vanadium ions. In one embodiment, the vanadium-based metal-organic frameworks (V-

MOFs) have been formed in the presence of a surfactant.

In one aspect, there is provided a method of catalyzing oxidation reactions using the vanadium-based metal-organic frameworks as disclosed herein. In one embodiment, the method of catalyzing oxidation reactions using the vanadium-based metal-organic frameworks as disclosed herein comprises selectively catalyzing the oxidation of alcohols to aldehydes. In one embodiment, the mass ratio of the vanadium source to the base is

5:4 and the mass ratio of the acid to the base is 1 :1 .

DEFINITIONS The term "metal-organic frameworks" as used herein broadly refers to supramolecular coordination polymers having one-, two-, or three-dimensional crystalline networks in which the metal ions or metal clusters are bridged through organic linker molecules. The term "vanadium" as used herein broadly refers to elemental vanadium having a ground oxidation state and species containing vanadium that exists in a variety of oxidation states such as +2, +3, +4 and +5. Examples of such species encompass but are not limited to vanadium(ll), vanadium(lll), vanadium(IV), vanadyl, vanadium(V), vanadate ions, salts, compounds or complexes.

The term "vanadium source" as used herein broadly refers to materials that contain detectable amounts of elemental vanadium having a ground oxidation state and/or species containing vanadium that exists in a variety of oxidation states. Examples of "vanadium source" include but are not limited to vanadium- based waste which in turn includes but is not limited to fossil fuel waste.

The term "waste" as used herein broadly refers to unwanted materials that are left over from a process and are intended to be disposed of. The term encompasses but is not limited to waste generated from industrial process plants such as oil refinery or petroleum refinery, power plant, chemical plant and water and wastewater treatment plant. Accordingly, the term "vanadium-based waste" as used herein broadly refers to waste materials that contain detectable amounts of elemental vanadium having a ground oxidation state and/or species containing vanadium that exists in a variety of oxidation states. Examples of "vanadium- based waste" include but are not limited to fossil fuel waste, oil refinery waste, petroleum coke and carbon black waste. "Carbon black waste" may be understood to be a carbon-rich solid residue generated from incomplete combustion of hydrocarbon or cracking of oil under high temperatures in an oil refinery.

The term "leaching" as used herein refers to a process of extracting metal species from a material containing the metal species with a leaching agent. The term "leaching solution" as used herein refers to a solution resulting from the addition of the leaching agent to the materials containing the metal species and the dissolution of the metal species. The leaching process may comprise a step of removing insoluble solids from the leaching solution subsequent to the dissolution of the metal species to form the leaching solution. Accordingly, the leaching solution may be substantially free of insoluble solids. The leaching solution may include both an intermediate and a final leaching solution/leachate. In some embodiments, as an intermediate leaching solution, it may be cycled repeatedly with the materials containing the metal species to eventually obtain a final leaching solution with an increased metal concentration. In some embodiments, the leaching process may comprise adjusting the pH of the leaching solution with an acid.

The term "vanadium ions" as used herein broadly refers to but is not limited to species containing an oxovanadium cation or anion. Examples include but are not limited to vanadyl ion (V0 ²⁺ ), pervanadyl ion (V02 ⁺ ), vanadate ions (VO3 ^" , VO4 ³ -, V2O7 ⁴ -, V3O9 ^3" , V4O12 ⁴ -, V5O14 ³ -) , protonated vanadate ions such as HVO4 ² - , H2VO4 ^" and polyvanadate ions. The term "oxidation state" as used herein refers to the degree of oxidation of an atom which is represented by zero, or a positive or negative number. An increase in oxidation state is referred to as an oxidation process, while a decrease in oxidation state is referred to as a reduction process.

The term "surfactant" as used herein broadly refers to a substance or compound that reduces surface tension when dissolved in water or water solutions, or that reduces interfacial tension between two liquids, or between a liquid and a solid. The term "surfactant" thus includes anionic, cationic, nonionic, zwitterionic and/or amphoteric agents. An example of a surfactant includes cetyitrimethyiammonium bromide.

The term "linker molecule" as used herein refers broadly to an organic compound that can coordinate with one of more metal ions to build metal-organic frameworks. The term encompasses monodendate and multidendate ligands such as bidendate, tridendate, tetradendate, pentadendate and hexadendate ligands.

The term "nano" as used herein is to be interpreted broadly to include dimensions in a nanoscale, i.e., the range of between about 1 nm and about 100 nm. Accordingly, the term "nanostructures", "nanofibres", "nanorods", "nanoparticles", "nanomaterials" and the like as used herein may include structures that have at least one dimension in the range of no more than said range. The term "nanostructures", "nanofibres", "nanorods", "nanoparticles", "nanomaterials" and the like as used herein may include structures that have at least one dimension that is no more than about 100 nm, no more than about 90 nm, no more than about 80 nm, no more than about 70 nm, no more than about 60 nm, no more than about 50 nm, no more than about 40 nm, no more than about 30 nm, no more than about 20 nm, or no more than about 1 0 nm. For example, nanofibers have one dimension on the nanoscale, where the diameter of the fiber is between about 1 nm and about 100 nm, and the length of the fiber can be greater than this range. The term "nanostructure" as used herein broadly refers to an arrangement of interrelated elements in a system having at least one dimension in the nanoscale. The nanostructure described herein can include a nanorod, nanofiber, nanoparticle, nanoneedie, nanoplate, nanotube, and the like. The term "size" when used in the context of the nanorod or nanofiber can refer to the diameter of the nanorod or nanofiber although it is not limited as such.

The term "and/or", e.g., "X and/or Y" is understood to mean either "X and Y" or "X or Y" and should be taken to provide explicit support for both meanings or for either meaning.

The terms "coupled" or "connected" when used in this description are intended to cover both directly connected or connected through one or more intermediate means, unless otherwise stated.

The term "associated with", used herein when referring to two elements refers to a broad relationship between the two elements. The relationship includes, but is not limited to a physical, a chemical or a biological relationship. For example, when element A is associated with element B, elements A and B may be directly or indirectly attached to each other or element A may contain element B or vice versa.

Further, in the description herein, the word "substantially" whenever used is understood to include, but not restricted to, "entirely" or "completely" and the like. In addition, terms such as "comprising", "comprise", and the like whenever used, are intended to be non-restricting descriptive language in that they broadly include elements/components recited after such terms, in addition to other components not explicitly recited. Further, terms such as "about", "approximately" and the like whenever used, typically means a reasonable variation, for example a variation of +/- 5% of the disclosed value, or a variance of 4% of the disclosed value, or a variance of 3% of the disclosed value, a variance of 2% of the disclosed value or a variance of 1 % of the disclosed value.

Furthermore, in the description herein, certain values may be disclosed in a range. The values showing the end points of a range are intended to illustrate a preferred range. Whenever a range has been described, it is intended that the range covers and teaches all possible sub-ranges as well as individual numerical values within that range. That is, the end points of a range should not be interpreted as inflexible limitations. For example, a description of a range of 1 % to 5% is intended to have specifically disclosed sub-ranges 1 % to 2%, 1 % to 3%, 1 % to 4%, 2% to 3% etc., as well as individually, values within that range such as 1 %, 2%, 3%, 4% and 5%. The intention of the above specific disclosure is applicable to any depth/breadth of a range. Additionally, when describing some embodiments, the disclosure may have disclosed a method and/or process as a particular sequence of steps. However, unless otherwise required, it will be appreciated the method or process should not be limited to the particular sequence of steps disclosed. Other sequences of steps may be possible. The particular order of the steps disclosed herein should not be construed as undue limitations. Unless otherwise required, a method and/or process disclosed herein should not be limited to the steps being carried out in the order written. The sequence of steps may be varied and still remain within the scope of the disclosure. DESCRIPTION OF EMBODIMENTS

Exemplary, non-limiting embodiments of vanadium-based metal-organic frameworks, and a method of producing vanadium-based metal-organic frameworks from a vanadium source are disclosed hereinafter. In various embodiments, there is provided a method of producing vanadium-based metal-organic frameworks (V-MOFs) from a vanadium source, the method comprising reducing an oxidation state of vanadium in a leaching solution containing vanadium ions, the leaching solution being derived from the vanadium source; and coordinating one or more linker molecules with vanadium having the reduced oxidation state to form vanadium-based metal-organic frameworks. The step of reducing the oxidation state of vanadium in the leaching solution may comprise reducing the oxidation state of vanadium to less than +5, to less than +4 or to an oxidation state of +3.

In various embodiments, the leaching solution is derived from the vanadium source by subjecting the vanadium source to treatment. The vanadium source may be treated by adding leaching agents to the vanadium source. In some embodiments, the step of adding leaching agents to the vanadium source comprises dissolution of vanadium to form the leaching solution. In some embodiments, the leaching agents used in the leaching process are chemical agents. The chemical agent may be an acid or a base.

In various embodiments, the method of producing vanadium-based metal- organic frameworks (V-MOFs) from a vanadium source further comprises adding at least one of an acid or a base to the vanadium source to form the leaching solution containing vanadium ions. In various embodiments, the acid is added after adding the base. In various embodiments, the mass ratio of vanadium source to base is in the range of from about 1 :1 to about 3:2. In some embodiments, the mass ratio of vanadium source to base is about 5:4.

In various embodiments, the mass ratio of acid to base is in the range of from about 1 :2 to about 3:2. In some embodiments, the mass ratio of acid to base is about 1 :1 . In some embodiments, alkali leaching is performed with a base as the leaching agent. Any suitable base that effectively and/or preferentially leaches vanadium from the vanadium source may be used in the method disclosed herein. The base may be added to adjust the pH of the vanadium source to form the leaching solution. The base may be a strong base. The base may be selected from the group consisting of sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, potassium bicarbonate, ammonium hydroxide, and mixtures thereof. In one embodiment, sodium hydroxide is used as the leaching agent.

Advantageously, embodiments of the method disclosed herein allow vanadium to be selectively and effectively leached from a waste containing vanadium and several other metals such as nickel and iron. By using alkaline solutions such as sodium hydroxide as the leaching agents, metals such as nickel and iron may be hardly leached, whereas vanadium may be completely dissolved and extracted with high efficiency and selectivity for further processing.

In various embodiments, the leaching process comprises a step of removing insoluble solids from the leaching solution subsequent to the dissolution of the metal species to form the leaching solution containing vanadium ions. In various embodiments, the leaching solution is substantially free of insoluble solids.

In various embodiments, the leaching solution may be an intermediate leaching solution or a final leaching solution. As an intermediate leaching solution, it may be further used to leach more vanadium ions from the vanadium source. For example, it may be cycled repeatedly with the vanadium source to obtain a leaching solution with an increased vanadium concentration or a final leaching solution having a vanadium concentration that is suitable for use in subsequent processing steps to eventually produce the vanadium-based metal-organic frameworks (V-MOFs). It may be appreciated that if the concentration of vanadium present in the leaching solution/leachate is insufficient for subsequent steps of producing the vanadium-based metal-organic frameworks (V-MOFs), cycling leaching can be performed to increase the concentration of vanadium in the leaching solution/leachate. In various embodiments, the cycling leaching comprises subjecting a leaching solution/leachate having a low concentration of vanadium to one or more additional leaching process(es), without introducing any additional leaching agents. In other words, the leaching solution/leachate having a low concentration of vanadium may act as the leaching agent during the cycling leaching process. In various embodiments, the concentration of vanadium in the leaching solution/leachate increases with the number of times the leaching solution/leachate is cycled with the vanadium source. For example, if the concentration of vanadium in the first leaching solution/leachate is low, a vanadium source may be added to the first leaching solution/leachate to perform a second leaching process. The second leaching process may comprise the steps of dissolving vanadium and removing insoluble solids to form the second leaching solution/leachate, which has a higher vanadium concentration than the first leaching solution/leachate. In each leaching cycle, the leaching process may be repeated at least once, at least twice, or at least thrice to obtain a second leaching solution/leachate, third leaching solution/leachate or subsequent leaching solution/leachate, wherein the final leaching solution/leachate forms the final leaching solution containing vanadium ions.

In some embodiments, the concentration of vanadium in the first, second, third subsequent or final leaching solution/leachate is at least but not limited to about 100 ppm, about 200 ppm, about 300 ppm, about 400 ppm, about 500 ppm, about 600 ppm, about 700 ppm or about 800 ppm. In some embodiments, the concentration of vanadium in the second, third, subsequent or final leaching solution/leachate is at least but not limited to about 1000 ppm. In one embodiment, the concentration of vanadium in a first leaching solution/leachate is in a range of about 500 ppm to about 800 pm. The first leaching solution/leachate may act as a leaching agent in a second leaching process. After undergoing the second leaching process, the concentration of vanadium in the second leaching solution/leachate may increase to about 1000 ppm. In some embodiments, if the concentration of vanadium in the second leaching solution/leachate is determined to be low, the second leaching solution/leachate obtained may be further cycled through the cycling leaching process in order to increase the concentration of vanadium in the final leaching solution/leachate. In some other embodiments, if the concentration of vanadium in the second leaching solution/leachate is determined to be sufficient, the second leaching solution/leachate may form the final leaching solution/leachate containing vanadate ions.

In various embodiments, the minimum concentration of vanadium in a final leaching solution/leachate containing vanadate ions may be, but not limited to, 100 ppm.

In various embodiments, the method of producing vanadium-based metal- organic frameworks (V-MOFs) from a vanadium source further comprises, prior to said reducing step, step (i) adding at least one of an acid or a base to the vanadium source to obtain a first leachate containing vanadium ions; step (ii) adding the leachate obtained from the preceding step to the vanadium source to obtain a second leachate containing a higher concentration of vanadium ions; optionally repeating step (ii) for one or more times to obtain a third leachate or subsequent leachate, wherein the leachate obtained after completion of the above steps forms said leaching solution containing vanadium ions for use in said reducing step.

In some embodiments, the step of adding at least one of an acid or a base to the vanadium source comprises adding a base first, followed by acid. In various embodiments, the leaching process may comprise adjusting the pH of the leaching solution with an acid. It may be understood by a person skilled in the art that the leaching solution may be a pH-adjusted solution. The acid may be added to adjust the pH of the leaching solution and obtain the suitable vanadium chemical species, which are pH dependent. The acid may be a strong acid. The acid may be selected from the group consisting of hydrochloric acid, nitric acid, sulfuric acid and phosphoric acid.

In various embodiments, the method further comprises, prior to obtaining the leaching solution from the vanadium source, a step of drying the vanadium source to remove moisture and possible volatile components. In various embodiments, said drying the vanadium source comprises drying the vanadium source in an oven at a temperature greater than about 80 °C, greater than about 85 °C, greater than about 90 °C, greater than about 95 °C, greater than about 100 °C or greater than about or about 1 05 °C for a duration of at least about 5 hours, at least about 6 hours, at least about 7 hours, at least about 8 hours, at least about 9 hours, at least about 10 hours, at least about 1 1 hours, at least about or about 12 hours.

In various embodiments, the vanadium in the leaching solution is present as vanadium ions with an oxidation state of +4 or +5. In various embodiments, the vanadium in the leaching solution is present as vanadate ions with an oxidation state of +5. The vanadium source and/or the leaching solution may be substantially free from vanadium having oxidation states other than +5 (e.g. +3 or +4 oxidation states). In various embodiments, the vanadium in the leaching solution exists as VO4 ^3" ions. In some embodiments, the step of reducing the oxidation state of vanadium in the leaching solution may comprise reducing the oxidation state of vanadium from +5 to less than +5, to less than +4 or to an oxidation state of +3. The step of reducing the oxidation state of vanadium in the leaching solution may comprise a change in the colour of the leaching solution. The colour of the leaching solution may change from yellow to grey. In various embodiments, the step of reducing the oxidation state of vanadium comprises adding at least one reducing agent, at least two reducing agents or at least three reducing agents to the leaching solution containing vanadium ions.

In various embodiments, the amount of reducing agent added to the leaching solution is in the range of from about 0.40% w/v to about 2.20% w/v. In some embodiments, the amount of reducing agent added to the leaching solution is about 0.44% w/v to about 2.18% w/v.

In various embodiments, the reducing agent may be a reductant that provides sufficient driving force for reducing the oxidation state of vanadium. The reducing agent may be selected from the group consisting of sodium dithionite, ascorbic acid, sodium borohydride, sodium citrate, or mixtures thereof.

In one embodiment, the oxidation state of vanadium may be reduced to an oxidation state of +3 in order to participate in the coordination step.

In various embodiments, the step of coordinating one or more linker molecules with vanadium having the reduced oxidation state to form vanadium- based metal-organic frameworks (V-MOFs) comprises building V-MOFs by chemical coordination between metal ions (or clusters) and one or more organic linker molecules into periodic porous frameworks. The coordinating step may comprise one organic linker molecule, two linker molecules, three linker molecules, or four linker molecules. The coordinating step may be modulated by some surfactants, so-called a coordination modulation method.

In various embodiments, the one or more linker molecules comprise a multidendate ligand. The multidendate ligand may be an organic bidendate, tridendate, tetradendate, pentadendate or hexadendate ligand. The linker molecule may coordinate with vanadium having the reduced oxidation state in a molar ratio of from about 1 :1 to about 4:1 . In various embodiments, the molar ratio of the linker molecule to vanadium is 1 :1 .

In various embodiments, the linker molecule comprises carboxylic functional groups. In various embodiments, the linker molecule comprises aromatic carboxylic functional groups. In various embodiments, the linker molecule comprising aromatic carboxylic functional groups is further substituted with hydrocarbons such as alkyl groups. In various embodiments, the linker molecule comprises two, three or four aromatic carboxylic functional groups. In various embodiments, the linker molecule is a multidendate ligand selected from the group consisting of 1 ,4-benzenedicarboxylic acid (H2BDC), 1 ,4-naphthalene- dicarboxylic acid (H2NDC), 2,5-dimethylbenzenedicarboxylic acid (H2DMBDC), 1 ,3,5-benzenetricarboxylic acid (H3BTC), 1 ,2,4,5-benzenetetracarboxylic acid (H ₄ BTEC). In various embodiments, the linker molecule comprises a bidentate ligand. In some embodiments, the linker molecule is a bidentate ligand selected from the group consisting of 1 ,4-benzenedicarboxylic acid (H2BDC) and 1 ,4- naphthalenedicarboxylic acid (H2NDC). The coordinating step may comprise coordinating a linker molecule with a vanadium ion having an oxidation state of +3 in a molar ratio of about 1 :1 .

In various embodiments, the step of coordinating one or more linker molecules with vanadium having the reduced oxidation state is carried out in the presence of at least one surfactant, at least two surfactants or at least three surfactants. Adding surfactant(s) to the reaction mixture during synthesis may lead to the formation of vanadium-based metal-organic frameworks (V-MOFs) having substantially uniform nanostructures and morphology. Generally, control over morphology and size of V-MOFs may be difficult. Advantageously, it is shown that the shapes and sizes of the nanostructures may be modulated with a surfactant to give regular shapes having a narrow size distribution. Without being bound by theory, it is believed that the surfactant can control the morphology of MOF crystals by adjusting growth rates of different facets via electrostatic interactions between the surfactant and linker molecules. Particularly, the electrostatic interactions may involve the cations of the surfactant and the deprotonated carboxylic groups of the linker molecules. In various embodiments, the amount of surfactant present is in the range of from about 0.01 wt% to about 2 wt%, from about 0.02 wt% to about 1 .8 wt%, from about 0.04 wt% to about 1 .6 wt%, from about 0.06 wt% to about 1 .4 wt%, from about 0.08 wt% to about 1 .2 wt%, from about 0.1 wt% to about 1 wt%, from about 0.12 wt% to about 0.8 wt%, from about 0.14 wt% to about 0.6 wt%, from about 0.16 wt% to about 0.4 wt% and from about 0.18 wt% to about 0.2 wt%.

In various embodiments, the surfactant is selected from the group consisting of cetyltrimethylammonium bromide, cetyltrimethylammonium chloride, dodecyltrimethylammonium bromide, dodecyltrimethylammonium chloride, tetradecyltrimethylammonium bromide, tetradecyltrimethylammonium chloride, or mixtures thereof.

In various embodiments, the method disclosed herein further comprises one or more stirring steps. In various embodiments, said stirring comprises vigorous stirring. In various embodiments, the stirring step comprises stirring of a mixture comprising vanadium source, base, acid and/or reducing agent for up to about 2 minutes. In various embodiments, the stirring step comprises stirring of a mixture comprising linker molecules and surfactant for up to about 2 minutes, up to about 4 minutes, up to about 6 minutes, up to about 8 minutes or up to or about 10 minutes. In various embodiments, stirring is performed at room temperature. It may be appreciated that stirring the mixture may promote the reaction between the reactants added to the reaction mixture, for example, promoting the reaction between vanadium having the reduced oxidation state and the linker molecules. In various embodiments, the step of coordinating one or more linker molecules with vanadium having the reduced oxidation state comprises a treatment step for the synthesis of vanadium-based metal-organic frameworks (V- MOFs). The treatment step may be selected from the group consisting of hydrothermal and solvothermal treatment. The treatment step may require heating at a high temperature. The treatment step may also require performing under high pressure. It may be appreciated that treating the mixture under hydrothermal or solvothermal conditions may promote the formation of V-MOFs having substantially uniform morphology/size and high levels of crystallinity.

In various embodiments, the step of coordinating one or more linker molecules with vanadium having the reduced oxidation state comprises a hydrothermal or a solvothermal treatment step carried out at a temperature ranging from about 1 20°C to about 300°C, from about 1 30°C to about 290°C, from about 140°C to about 280°C, from about 150°C to about 270°C, from about 160°C to about 260°C, from about 170°C to about 250°C, from about 180°C to about 240°C, from about 190°C to about 230°C, and from about 200°C to about 220°C. In one embodiment, the treatment step may be performed at about 200°C.

In some embodiments, the step of coordinating one or more linker molecules with vanadium having the reduced oxidation state comprises a hydrothermal or a solvothermal treatment step carried out over a duration of at least about 10 hours, at least about 1 1 hours, at least about 12 hours, at least about 13 hours, at least about 14 hours, at least about 15 hours, at least about 20 hours, at least about 30 hours, at least about 40 hours, or at least about 48 hours. In one embodiment, the duration of the heat treatment is about 15 hours.

In various embodiments, the treatment step is hydrothermal. Under hydrothermal conditions, the reaction may be performed in a closed pressure reactor. The closed pressure reactor may be an autoclave. In some embodiments, the hydrothermal treatment step is performed in a batch process. Advantageously, embodiments of the method comprising a heat treatment step in accordance with one or more conditions and/or parameters, two or more conditions and/or parameters or three or more conditions and/or parameters disclosed herein are capable of producing V-MOFs having substantially uniform morphology/size and high levels of crystallinity that is suitable for industrial application. It may be appreciated that heat treatment conditions and/or parameters such as temperature and duration may affect the MOFs in terms of its crystal structure, stability, size, porosity, surface area and/or morphology. In various embodiments, the method is performed in aqueous solutions.

Unlike conventional methods which synthesize MOFs materials in costly and non- reusable organic solvents, embodiments of the disclosed methods allow for the recovery of vanadium from vanadium source in aqueous solutions by directly using the aqueous leachate of vanadium source as the solvent. Advantageously, the cost of production may thus be reduced since the solvent is derived directly from the aqueous leachate of the vanadium source, thereby making it translatable to a large scale industrial use. In addition, embodiments of the disclosed methods allow for eliminating of the use of non-reusable organic solvents, thereby making it an environmentally benign process.

Embodiments of the method of producing the vanadium-based metal- organic frameworks provide a one pot synthesis whereby a base may be added, followed by an acid, at least one reducing agent, at least one linker molecule and at least one surfactant.

In various embodiments, the vanadium-based metal-organic frameworks (V-MOFs) are substantially free of impurities. In some embodiments, the V-MOFs are substantially free of sulfurous impurities. Advantageously, embodiments of the V-MOFs are suitable for industrial and further applications. In various embodiments, the method of producing vanadium-based metal- organic frameworks (V-MOFs) from a vanadium source further comprises a purification step for substantially removing impurity. The purification step may be performed with centrifugation, filtration, washing or combinations thereof. The mixture of impurities and V-MOFs may be centrifuged first to separate the impurities from V-MOFs. The V-MOFs may then be collected and further washed with water at least once, or at least twice for a complete purification. In some embodiments, the purification step is conducted at room temperature. In various embodiments, the method of producing vanadium-based metal- organic frameworks (V-MOFs) from a vanadium source further comprises increasing the oxidation state of vanadium of the vanadium-based metal-organic frameworks formed from the coordinating step. In some embodiments, the step of increasing the oxidation state of vanadium of the vanadium-based metal-organic frameworks formed from the coordinating step is an activation step. The activation step may comprise increasing the oxidation state to more than +3.

In various embodiments, the step of increasing the oxidation state of vanadium comprises heating the vanadium-based metal-organic frameworks (V- MOFs) formed from the coordinating step in air. The activation step may be performed in static air. The activation step may also require heating over a period of time. By heating the V-MOFs in static air, the oxidation state of vanadium in the V-MOFs may be increased from +3 to +4. In one embodiment, V ^m -OH bonds in the V-MOFs may be oxidized to form V ^lv =O bonds in the activated V-MOFs. During the activation step, free organic linkers may be removed from the pores of the V-MOFs. Advantageously, embodiments of the V-MOFs have shown that removing filled linkers upon activation can lead to a drastic increase in the Brunauer-Emmett-Tei!er (BET) surface area. In various embodiments, activation can cause the BET surface area of the V-MOFs to increase to about 200 m ² g ¹ , to about 250 m ² g ¹ , to about 300 m ² g ¹ , to about 350 m ² g ¹ , to about 400 m ² g ¹ , to about 402 m ² g ¹ , to about 404 m ² g ¹ , to about 406 m ² g ¹ or to about 408 m ² g ^"

1

In some embodiments, the activation step is performed at a temperature of from about 1 20°C to about 300°C, from about 140°C to about 280°C, from about 160°C to about 260°C, from about 180°C to about 240°C or from about 200°C to about 220°C. In some embodiments, the activation step is performed at about 280°C. In some embodiments, the activation step is performed over a duration of at least about 5 hours, at least about 6 hours, at least about 7 hours, at least about 8 hours, at least about 9 hours or at least about 10 hours. In some embodiments, the activation step is performed over a duration of about 10 hours.

In various embodiments, the method disclosed herein has a vanadium recovery efficiency of at least about 80% from the leaching solution. The vanadium recovery efficiency may be at least about 82%, at least about 84%, at least about 86%, at least about 88%, at least about 90%, at least about 92%, at least about 94%, at least about 96%, at least about 98% or at least about 99% from the leaching solution. In various embodiments, the vanadium source is an oil refinery waste.

Advantageously, an economically feasible method has been found for recovering vanadium from oil refinery waste and subsequently converting the waste vanadium ions into value-added vanadium-based metal-organic frameworks. In some embodiments, these waste vanadium ions that are used as a precursor are low-price vanadium ions having an oxidation state of +5.

Unlike conventional methods which use expensive metal ion precursors as the metal source, embodiments of the present disclosure use metal source derived from oil refinery waste instead, thereby making the production process cost- effective on a large scale. At the same time, recycling oil refinery waste also promotes resource conservation and mitigates pollution from landfill leachate, thereby contributing to environmental sustainability. In this regard, in various embodiments, the vanadium source disclosed herein is distinct and different from a purified vanadium source which is commercially available and which is substantially free from other metals and/or chemical compounds.

In one embodiment, the vanadium source is a vanadium-based waste.

In various embodiments, the vanadium-based waste is a carbon black waste. The carbon black waste may be a solid residue generated from the incomplete combustion of hydrocarbon or cracking of oil under high temperatures in an oil refinery. In some embodiments, the carbon black waste is first dried and then added to aqueous leaching agents to obtain an aqueous leachate or leaching solution. In some embodiments, the solvent used for the method disclosed herein is therefore carried out in an aqueous phase with water being the only solvent.

Advantageously, embodiments of the method disclosed herein allow for the sustainable transformation of a vanadium source into value-added products such as vanadium-based metal-organic frameworks (V-MOFs). Embodiments of the method efficiently recover vanadium from vanadium source, and reduce the production cost for V-MOFs, particularly the cost of obtaining commercial vanadate sources such as V2O5, VOSO4, and VC as raw materials for production. Embodiments of the method disclosed herein also produce V-MOFs with high levels of crystallinity suitable for industrial application, such as for use as a catalyst in oxidation reactions.

In various embodiments, there is provided vanadium-based metal-organic frameworks (V-MOFs) derived from a vanadium source, the vanadium-based metal-organic frameworks (V-MOFs) comprising nanostructures having substantially uniform nanorods and/or nanofibers morphology. In various embodiments, the nanorods have an aspect ratio (length:diameter) of about 3:1 to about 5:1 . In various embodiments, the nanorods have an average length of from about 180 nm to about 350 nm. In various embodiments, the nanorods have an average length of from about 240 nm to about 300 nm. In some embodiments, the nanorods have an average length of about 270 nm. In various embodiments, the nanorods have an average diameter of from about 50 nm to about 80 nm. In various embodiments, the nanorods have an average diameter of from about 60 nm to about 70 nm. In some embodiments, the nanorods have an average diameter of about 64 nm. In some embodiments, the nanorods have an average length of about 270 nm and an average diameter of about 64 nm. In various embodiments, the nanorods have an X-ray diffraction pattern substantially similar to that shown in FIG. 5.

In various embodiments, the nanofibers have an aspect ratio (length:diameter) of at least about 250:1 . In various embodiments, the nanofibers have an average length of greater than about 10 μηι. In various embodiments, the nanofibers have an average length of greater than about 20 μηι. In various embodiments, the nanofibers have an average length of greater than about 30 μηι. In various embodiments, the nanofibers have an average length of greater than about 40 μηι. In various embodiments, the nanofibers have an average diameter of from about 25 nm to about 55 nm. In various embodiments, the nanofibers have an average diameter of from about 35 nm to about 45 nm. In some embodiments, the nanofibers have an average diameter of about 40 nm. In some embodiments, the nanofibers have an average length of greater than about 10 μηι and an average diameter of about 40 nm. In various embodiments, the nanofibers have an X-ray diffraction pattern substantially similar to that shown in FIG. 4.

In various embodiments, the vanadium-based metal-organic frameworks comprise one or more linker molecules selected from the group consisting of 1 ,4- benzenedicarboxylic acid (H2BDC), 1 ,4-naphthalenedicarboxylic acid (H2NDC), 2,5-dimethylbenzenedicarboxylic acid (H2DMBDC), 1 ,3,5-benzenetricarboxylic acid (H3BTC) and 1 ,2,4,5-benzenetetracarboxylic acid (H ₄ BTEC), the one or more linker molecules being coordinated to vanadium. In one embodiment, V-MOF is synthesized by using H2BDC as the organic linker, i.e. V-BDC and V-MOF is synthesized by using H2NDC as the organic linker, i.e. V-NDC. Embodiments of the V-MOFs disclosed herein may contain trivalent or tetravalent vanadium as metal nodes, which can be used in a wide variety of industrial applications.

In various embodiments, the vanadium-based metal-organic frameworks are thermally stable. In some embodiments, the vanadium-based metal-organic frameworks are capable of withstanding decomposition and may remain stable up to a temperature of about 100°C, up to a temperature of about 150°C, up to a temperature of about 200°C, up to a temperature of about 250°C or up to a temperature of about 300°C.

In various embodiments, there is provided a method of catalyzing oxidation reactions using the vanadium-based metal-organic frameworks (V-MOFs) as disclosed herein. The V-MOFs may be used to catalyze oxidation reactions of organic compounds such as alkanes, alkenes, alcohols or aldehydes. For example, the oxidation reaction may involve oxidizing alkanes to alcohols, which may be further oxidized to aldehydes or ketones. In another example, aldehydes may be oxidized to carboxylic acids.

In various embodiments, the method of catalyzing oxidation reactions using the vanadium-based metal-organic frameworks (V-MOFs) as disclosed herein further comprises, prior to adding the V-MOFs to a reaction mixture containing the organic compounds to be oxidized, a step of activating the V-MOFs as disclosed herein. In various embodiments, the vanadium-based metal-organic frameworks (V-MOFs) are capable of catalyzing oxidation reactions of alcohols. The oxidation reaction of an alcohol may be performed in a reaction mixture comprising an alcohol and an oxidizing agent in a solvent. The alcohol may be a benzyl alcohol. The oxidizing agent may be a tert-butylhydroperoxide. The solvent may be an organic solvent, particularly toluene. In some embodiments, the products resulting from the oxidation reaction of alcohols may be aldehydes and/or carboxylic acids. The product resulting from the oxidation of benzyl alcohol may be a benzaldehyde and/or a benzoic acid. In one embodiment, V-MOFs can selectively catalyse the oxidation of alcohol to aldehyde.

In various embodiments, V-MOFs are capable of serving as catalytic active sites. Embodiments of the V-MOFs possess excellent catalytic performance on selective oxidation of alcohols. Embodiments of the V-MOFs have shown to be capable of exhibiting an enhanced oxidation activity and product selectivity in a short period of time. In particular, in the presence of V-MOFs, at least about 70% of alcohol may be converted in about 4 hours, with a product selectivity of about 80%. In some embodiments, the alcohol conversion of the oxidation reaction is at least about 60%, at least about 61 %, at least about 63%, at least about 65%, at least about 67%, at least about 69%, at least about 71 %, at least about 73%, at least about 75%, at least about 77%, at least about 79% or at least about 80%. In some embodiments, the aldehyde selectivity is at least about 70%, at least about 72%, at least about 74%, at least about 76%, at least about 78%, at least about 80%, at least about 82%, at least about 84%, at least about 86%, at least about 88% or at least about 90%, at least about 92%, at least about 94%, at least about 96%, at least about 98% or at least about 100%. Advantageously, embodiments of the V-MOFs have excellent catalytic performance due to their favourable properties such as a substantially large surface area and uniform morphology/size. BRIEF DESCRIPTION OF FIGURES

FIG. 1 is a graph showing the variation in the transformative percentage yield (%) of vanadium when different amounts of the reducing agent are used in the synthesis of V-BDC from vanadium source, where the total volume of the leaching solution is 8 ml_.

FIGS. 2A-2C are microscopic images of the synthesized V-BDC in accordance with various embodiments disclosed herein.

FIGS. 3A-3C are microscopic images of the synthesized V-NDC in accordance with various embodiments disclosed herein.

FIG. 4 shows the X-ray diffraction (XRD) patterns of V-BDC samples namely V-BDC ^* , V-BDC and V-BDC-i plotted together with simulated MIL-47 (CCDC-166785).

FIG. 5 shows the X-ray diffraction (XRD) patterns of V-NDC samples namely V-NDC and V-NDC-i plotted together with simulated AI-NDC (CCDC- 710000).

FIG. 6 shows the N2 adsorption/desorption isotherms of V-BDC samples before and after activation treatments. FIGS. 7A-7C are microscopic images of V-BDC-i synthesized in the absence of surfactant.

FIGS. 8A-8C are microscopic images of V-NDC-i synthesized in the absence of surfactant. FIG. 9 is a graph showing the thermogravimetric profiles of the synthesized V-BDC and V-NDC as well as V-BDC-i and V-NDC-i that are synthesized in the absence of surfactant. FIGS. 10A-10B are two graphs showing the catalytic performance of the synthesized V-BDC and V-NDC as well as V-BDC-i and V-NDC-i that are synthesized in the absence of surfactant, as a function of reaction time.

FIG. 1 1 shows a broad scheme of producing vanadium-based metal- organic frameworks from a vanadium-based waste in accordance with various embodiments disclosed herein.

DETAILED DESCRIPTION OF FIGURES FIG. 1 is a graph showing the variation in the transformative percentage yield (%) of vanadium when different amounts (0 mL, 0.2 mL, 0.4 mL, 0.6 mL, 0.8 mL and 1 mL) of the reducing agent, i.e. sodium dithionite (0.5 M) are used as a reducing agent in the synthesis of V-BDC from vanadium source. FIGS. 2A-2C are microscopic images of the synthesized V-BDC in accordance with various embodiments disclosed herein. FIG. 2A shows a SEM image of V-BDC, with the scale bar representing 2 μηι. FIG. 2B shows a SEM image of V-BDC, with the scale bar representing 300 nm. FIG. 2C shows a TEM image of V-BDC, with the scale bar representing 100 nm. As shown, the synthesized V-BDC has a structure of a nanofiber with an average length of larger than tens of micrometers and an average diameter of 40 nm.

FIGS. 3A-3C are microscopic images of the synthesized V-NDC in accordance with various embodiments disclosed herein. FIG. 3A shows a SEM image of V-BDC, with the scale bar representing 500 nm. FIG. 3B shows a SEM image of V-BDC, with the scale bar representing 100 nm. FIG. 3C shows a TEM image of V-BDC, with the scale bar representing 300 nm. As shown, the synthesized V-NDC has a structure of a short nanorod with an average length of 270 nm and an average diameter of 64 nm. FIG. 4 shows the X-ray diffraction (XRD) patterns of V-BDC samples namely V-BDC ^* , V-BDC and V-BDC-i plotted together with simulated MIL-47 (CCDC-166785). V-BDC ^* is unactivated V-BDC (without calcination in air) and the symbol (♦) represents the Bragg peaks, which belong to the included guest H2BDC ligands. V-BDC-i is the product synthesized without using surfactant, and "i" represents the irregular shape of the product if no surfactant was used.

FIG. 5 shows the X-ray diffraction (XRD) patterns of V-NDC samples namely V-NDC and V-NDC-i plotted together with simulated AI-NDC (CCDC- 710000). V-NDC-i is the product synthesized without using surfactant and "i" represents the irregular shape of the product if no surfactant was used.

FIG. 6 shows the N2 adsorption/desorption isotherms of V-BDC samples before and after activation treatments at 77 K. As shown in the figure, an increase in the Brunauer-Emmett-Teller (BET) surface area was observed after activation treatments.

FIGS. 7A-7C are microscopic images of V-BDC-i (i.e. synthesized in the absence of surfactant). FIG. 7A shows a TEM image of V-BDC-i, with the scale bar representing 500 nm. FIG. 7B shows a TEM image of V-BDC-i, with the scale bar representing 300 nm. FIG. 7C shows a TEM image of V-BDC-i, with the scale bar representing 300 nm. As shown, the V-BDC-i has an irregular shape with a broad size distribution.

FIGS. 8A-8C are microscopic images of V-NDC-i (i.e. synthesized in the absence of surfactant). FIG. 8A shows a TEM image of V-NDC-i, with the scale bar representing 300 nm. FIG. 8B shows a TEM image of V-NDC-i, with the scale bar representing 300 nm. FIG. 8C shows a TEM image of V-NDC-i, with the scale bar representing 200 nm. As shown, the V-NDC-i has an irregular shape with a broad size distribution. FIG. 9 is a graph showing the thermogravimetric profiles of the synthesized

V-BDC and V-NDC as well as V-BDC-i and V-NDC-i that are synthesized in the absence of surfactant.

FIGS. 10A-10B are two graphs showing the catalytic performance of the synthesized V-BDC and V-NDC as well as V-BDC-i and V-NDC-i that are synthesized in the absence of surfactant, as a function of reaction time. The catalytic performance of the four catalysts is studied in an initial reaction mixture composed of 1 mmole of benzyl alcohol, 2.5 mmole of tert-butylhydroperoxide, 15 ml_ of toluene and 30 mg of catalyst. FIG. 10A shows the percentage conversion (%) of benzyl alcohol over time. FIG. 10B shows the percentage selectivity (%) of benzaldehyde over time.

FIG. 1 1 shows a broad scheme of producing vanadium-based metal- organic frameworks from a vanadium-based waste in accordance with various embodiments disclosed herein. As shown in the figure, vanadium based waste may undergo a waste utilization process in accordance with various embodiments disclosed herein to eventually obtain vanadium-based metal-organic frameworks with substantially uniform and ordered morphology. EXAMPLES

Example embodiments of the disclosure will be better understood and readily apparent to one of ordinary skill in the art from the following examples and if applicable, in conjunction with the figures. In the following examples, it is shown that the embodiments of the presently disclosed method are capable of separating and recovering vanadium from waste, which is subsequently converted into metal-organic frameworks (V-MOFs) in high transformative yield (-99%).

Even though many methods of synthesizing MOFs have been studied in recent years, several critical issues remained. For example, understanding crystal growth mechanisms and control over the morphology and size of the crystal formation of MOFs remains a great challenge. Furthermore, scaling up the fabrication of MOFs was also expected to present major challenges as expensive metal ion precursors and/or costly and non-reusable organic solvents are often used, which impedes the economic production of MOFs for viable industrial uses.

The examples describe a method of producing vanadium-based metal- organic frameworks (V-MOFs) from a vanadium source in an environmentally benign and cost-efficient process in accordance with various embodiments of the present disclosure. As will be shown in the following examples, embodiments of the presently disclosed method provide a green and sustainable strategy to produce V-MOFs as use of expensive metal ion precursors and costly and non- reusable organic solvents were avoided.

As will be shown in the following examples, embodiments of the presently disclosed method synthesize V-MOFs that are capable of addressing several problems of conventional methods used in the art. The size and shape of these V- MOFs can be carefully tuned to give substantially uniform nanorods and/or nanofibers morphology, which are useful in a wide array of technological applications. Materials used in the Examples

Carbon black waste was collected from an oil refinery. The following chemicals were used as received without further purification: 1 ,4- Benzenedicarboxylic acid (H2BDC Aldrich, 98%), 1 ,4-Naphthalene-dicarboxylic acid (H2NDC, Alfa Aesar, 98%), cetyltrimethylammonium bromide (CTAB, Fluka, 96%), sodium dithionite (Aldrich, 85%), hydrochloric acid (VWR Chemicals, 32%), and sodium hydroxide (Merck, 99%). Deionized water was used in all experiments. Example 1 - Synthesis of Vanadium-Based Metal-Organic Frameworks (V-MOFs)

Firstly, dry carbon black waste solid (2.5 g) was treated with sodium hydroxide aqueous solution (50 ml_ of 1 M NaOH) to obtain a vanadium leaching solution. The obtained aqueous leachate of carbon black waste was used as the raw materials (i.e. metal source and solvent) for preparing V-MOFs. It may be appreciated that the sodium hydroxide aqueous solution used can also be replaced with 50 ml_ of 0.5 M NaOH or 50 ml_ of 0.25 M NaOH.

V-MOF synthesized by using H2BDC as the organic linker is named V-BDC and V-MOF synthesized by using H2NDC as the organic linker is named V-NDC.

For the synthesis of V-BDC, 4 ml_ of the leaching solution containing 725 ppm of vanadium was mixed with 4 ml_ of HCI solution (1 M). Then, 1 ml_ of sodium dithionite aqueous solution (0.5 M) was added. The colour of the solution changed immediately after adding the reducing agent. After stirring for 2 minutes, 80 mg of H2BDC linkers and 0.1 g of CTAB were added. The mixture was thoroughly stirred at room temperature for 10 minutes before hydrothermal treatment in a Teflon- lined steel autoclave (capacity: 25 ml_) was carried out at 200°C for 15 hours. The solid was collected by centrifugation and washing for two times with water to remove sulfurous impurity. For the synthesis of V-NDC, the method is the same as the above for the synthesis of V-BDC, except that H2NDC linkers are used instead of H2BDC linkers.

Example 2 - Synthesis of Vanadium-Based Metal-Organic Frameworks (V-MOFs) with varying amounts of reducing agent

The effect of varying amounts of reducing agent used for the preparation of V-BDC on the percentage yield of the product was investigated. The experiments were performed with varying amounts (0 mL, 0.2 mL, 0.4 mL, 0.6 mL, 0.8 mL and 1 mL) of reducing agent.

The vanadium ions in the carbon black waste leaching solution are mainly in the +5 oxidation state as monomeric vanadate anions VO4 ^3" . V ^v was first reduced to V" species for the construction of V-MOFs by using sodium dithionite as a reducing agent. Sodium dithionite is a widely-used reductant in industry and was chosen due to its high driving force for reducing V ^v to V" ions. After only 1 s, the vanadate ions were completely reduced as visualized from a colour change of the leaching solution from yellow to grey. Based on FIG. 1 , it is clear that the amount of sodium dithionite added is crucial to the transformative yield of vanadium. This is evident from the results shown for the control, which was prepared without the addition of sodium dithionite (0 mL added). At 0 mL of sodium dithionite, no MOFs solid was formed since V ^v was not reduced and V ^v cannot be used to build V-MOFs. It is found that 0.6 mL of sodium dithionite (0.5 M in water) was the optimal amount required to achieve -99% yield of V-BDC. It is shown that V-BDC can be prepared by adding approximately 0.44 - 2.18% w/v of reducing agent to the leaching solution. Example 3 - Characterization studies of Vanadium-Based Metal-Organic

Frameworks (V-MOFs)

Characterization studies of embodiments of the vanadium-based metal- organic frameworks were performed with various methods including scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD).

The following characterization results demonstrate the effectiveness of the coordination modulation method. As will be shown in the following figures, the results indicated that V-MOFs were synthesized successfully from vanadium source. Microscopic images and X-ray diffraction patterns reveal that crystalline nanostructures having regular shapes and narrow size distribution were synthesized.

Microscopic analysis of V-MOFs

Microscopic characterization revealed that V-MOFs with high levels of crystallinity and phase purity were facilely fabricated. The derived V-MOFs exhibited substantially uniform size and well-controlled shapes.

FIGS. 2A-2C show the morphology and size of the V-BDC synthesized according to the method disclosed herein. The derived V-BDC showed distinctly uniform shape of a nanofiber. The length of a V-BDC nanofiber is typically larger than tens of micrometers with an average diameter of 40 nm.

FIGS. 3A-3C show the morphology and size of the V-NDC synthesized according to the method disclosed herein. V-NDC exhibits a short nanorod shape with average length of 270 nm and diameter of 64 nm. Crystal log raphic analysis of V-MOFs

The X-ray diffraction (XRD) pattern in FIG. 4 shows the crystal structures of V-BDC. According to the XRD pattern, the V-BDC ^* prepared according to the method disclosed herein has a crystal structure [V ^m (OH)(BDC)](H2BDC)x, which is known as MIL-47as-synthesized. The symbol (♦) represents the Bragg peaks, which belong to the included guest H2BDC ligands.

Upon activation in static air at 280°C for 10 hours, V-BDC ^* (or known as MIL-47as-synthesized) is converted to V-BDC, which is known as MIL-47 having a high level of crystallinity. This activation step involves removing the free organic linkers resident in the pores of the product, along with an oxidation of V" ions to V ^IV ions and conversion of V ^m -OH bonds to vanadyl (V ^IV =0) bonds. The crystal structure of V-BDC (or known as MIL-47) is V ^IV (0)(BDC). Interestingly, as shown in FIG. 6, N2 physisorption measurements at 77 K revealed a drastic increase in the Brunauer-Emmett-Telier (BET) surface area from 6.91 m ² g ¹ to 408.3 m ² g ¹ after removing the filled linkers upon activation.

A comparison between the XRD patterns obtained for V-BDC against the simulated MIL-47 shows that the diffraction peaks match well with the simulated values, suggesting that the synthesis of V-BDC via coordination modulation method was achieved successfully under the reaction conditions disclosed herein.

The X-ray diffraction (XRD) pattern in FIG. 5 shows the crystal structures of V-NDC prepared according to the method disclosed herein. As also verified from the XRD pattern, V-NDC has a crystal structure that is isostructural to [AI(OH)(NDC)]. The presence of several additional peaks in the range of 5 to 40° 2 Theta for V-NDC is due to the substitution of (Al ^m OH) ²⁺ sites by (V ^m OH) ²⁺ sites. Example 4 - Synthesis of Vanadium-Based Metal-Organic Frameworks (V-MOFs) in the absence of surfactant

The experiments were repeated according to the method described in Example 1 , with the exception of surfactants, synthesizing V-BDC-i and V-NDC-i (i.e., V-MOFs that are synthesized without using surfactant).

Microscopic analysis of V-MOFs synthesized in absence of surfactant

Microscopic characterization revealed that V-MOFs synthesized without the use of surfactant have irregular and poorly controlled shapes.

FIGS. 7A-7C show the morphology and size of V-BDC-i. The product V- BDC-i shows an irregular shape with a broad size distribution. FIGS. 8A-8C show the morphology and size of V-NDC-i. The product V-

NDC-i shows an irregular shape with a broad size distribution.

Without being bound by theory, it is believed that this result suggests that the CTAB surfactant could control the morphology of MOF crystals by adjusting growth rates of different facets via the electrostatic interactions between CTA ⁺ of the surfactant and the deprotonated carboxylic groups of the linker molecules. Advantageously, this result confirms the importance of the presence of surfactant in the synthesis of V-MOFs that possess substantially uniform morphology/size. Crystalloqraphic analysis of V-MOFs synthesized in absence of surfactant

The X-ray diffraction (XRD) pattern in FIG. 4 shows the crystal structure of V-BDC-i prepared without the surfactant. A comparison between the XRD pattern obtained for V-BDC-i and V-BDC found some slight changes in the MOF crystal structures when CTAB surfactant was added to modulate the product shapes. The X-ray diffraction (XRD) pattern in FIG. 5 shows the crystal structure of V-NDC-i prepared without the surfactant. The addition of CTAB surfactant did not affect the XRD patterns of the V-NDC products. Example 5 - Thermal Stability of Vanadium-Based Metal-Organic Frameworks

The thermal stability of as-prepared V-MOFs namely V-BDC and V-NDC as well as V-BDC-i and V-NDC-i that are synthesized in the absence of surfactant was studied by thermogravimetric analysis (TGA). Results shown in FIG. 9 demonstrated that V-MOF structures were relatively stable up to 300°C in atmospheric air. There was no noticeable difference between the different V-MOF crystals. It was observed that the frameworks decomposed in the temperature range of 300-400 °C, which led to the formation of V2O5 as residue. The molar ratios of the V metal ions to organic linkers (either BDC ^2" or

NDC ^2" ) are close to 1 :1 in all the four V-MOFs, which matched well with the chemical formulae of the V-MOFs.

Example 6 - Catalytic Performance of Vanadium-Based Metal-Organic Frameworks

The catalytic performance of the prepared V-MOFs (namely V-BDC, V- BDC-i, V-NDC, and V-NDC-i) was evaluated during the catalytic oxidation of benzyl alcohol in liquid-phase.

Oxidation reaction of benzyl alcohol

The catalytic performance evaluation method is described as follows. Firstly, the synthesized V-MOFs were activated in static air at 280°C for 10 hours before the tests. Next, the benzyl alcohol oxidation reaction was carried out in a magnetically stirred flask (capacity: 50 ml_). The initial reaction mixture was composed of 1 mmole of benzyl alcohol, 2.5 mmole of tert-butylhydroperoxide, 15 ml_ of toluene, and 30 mg of catalyst. At each regular specific time interval, 0.2 ml_ of the samples were extracted with 1 ml_ of ethyl acetate and the extracts were subsequently analyzed by gas chromatography (GC) for the calculation of percentage conversion and selectivity. At the completion of the oxidation reaction, the catalyst solids were separated from the reaction mixture by syringe filters (polytetrafluoroethylene, PTFE, pore size: 0.45 μηι).

As shown in FIG. 1 0A, a control study of the oxidative reaction in the absence of V-MOFs showed only minor background reaction (i.e., <1 0% conversion in 4 hours). When V-MOFs were applied to the oxidation reaction, the benzyl alcohol conversion increased remarkably by approximately 5 - 8 folds at any given time, clearly showing that embodiments of the vanadium-based metal- organic frameworks impart excellent catalytic activity and undoubtedly suggest that the MOFs species served as active catalytic sites.

Interestingly, as illustrated in FIGS. 1 0A and 10B, the CTAB modulated MOFs (viz., V-BDC and V-NDC) showed both enhanced oxidation activity and product selectivity as compared to their counterparts (viz., V-BDC-i and V-NDC-i). This behavior is similar to the researches performed on inorganic nanomaterials, where shape tailoring is essential to the physicochemical properties and catalytic performance. Without being bound by theory, it is believed that the improved performance of V-MOFs having well-defined and substantially uniform shapes is associated with more exposed metal nodes on the external surface. However, in all cases, there is a tradeoff between selectivity and conversion percentages. As reaction time increased, the percentage of conversion of benzyl alcohol increased, whereas the benzaldehyde selectivity decreased. This is attributed to the subsequent further oxidation of benzaldehyde to benzoic acid. At the initial stage, the reaction achieves 100% selectivity for the benzaldehyde, which then gradually decreases and finally reaches an equilibrium value due to the full decomposition of the oxidant (viz., tert-butylhydroperoxide). Among the four different V-MOFs, V-NDC exhibits the best performance with a benzyl alcohol conversion of 72.6% and benzaldehyde selectivity of 83.5% in 4 hours. Under this condition, the initial turnover frequency (TOF) is calculated as 24.7 h ^~1 at a reaction time of 5 minutes.

APPLICATIONS

Various embodiments of the present disclosure provide a one-pot synthesis of vanadium-based metal-organic frameworks by using the leaching solution of vanadium source as both the metal source and solvent. Vanadium ions in the waste may be selectively recovered and converted in high transformative yield (-99%) to form vanadium-based metal-organic frameworks. Various embodiments of the method of producing the vanadium-based metal-organic frameworks disclosed herein simultaneously promotes resource conservation and mitigates pollution from landfill leachate, thus qualifying as a green strategy.

In various embodiments of the method of producing the vanadium-based metal-organic frameworks disclosed herein, the metal source is derived from the vanadium source. In these embodiments, the process does not involve the use of expensive metal ion precursors, thereby making the production process cost- effective on a large scale.

In various embodiments of the method of producing the vanadium-based metal-organic frameworks disclosed herein, the solvent is derived from the aqueous leachate of the vanadium source. The process does not involve the use of costly and non-reusable organic solvents, thereby making the production process friendly to the environment and economical on an industrial scale.

Various embodiments of the present disclosure provide vanadium-based metal-organic frameworks having substantially uniform nanorods and/or nanofibers morphology. For example, it has been shown that the morphology of the V-MOFs crystal nanostructures can be controlled to synthesize vanadium- based metal-organic frameworks that possess a regular shape with a narrow size distribution. In various embodiments, the vanadium-based metal-organic frameworks disclosed herein are a new class of self-assembly functional materials that can be used in a wide array of applications such as CO2 adsorption, heterogeneous catalysts, cathode materials for lithium ion batteries, magnetic sensors, and membrane separation.

Advantageously, embodiments of the vanadium-based metal-organic frameworks disclosed herein have shown that they are thermally stable up to 300°C and can be used to enhance oxidation activity, reactant conversion and product selectivity in oxidation reactions on for example, alcohols, thus making them attractive for use as catalysts. For example, V-NDC prepared according to embodiments of the method disclosed herein exhibits an excellent catalytic activity, achieving a benzyl alcohol conversion of 72.6% and benzaldehyde selectivity of 83.5% in 4 hours. Embodiments of the compounds disclosed herein can also be scalable for industrial applications. The present disclosure has demonstrated the principles involved, and opens the way for further scale-up in many applications.

It will be appreciated by a person skilled in the art that other variations and/or modifications may be made to the embodiments disclosed herein without departing from the spirit or scope of the disclosure as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.