WO2017155475A1

WO2017155475A1 - Silver-decorated metal-organic framework for olefin/alkane separation

Info

Publication number: WO2017155475A1
Application number: PCT/SG2017/050122
Authority: WO
Inventors: Dan Zhao; Yuxiang Wang; Zhigang Hu
Original assignee: National University of Singapore
Current assignee: National University of Singapore
Priority date: 2016-03-11
Filing date: 2017-03-13
Publication date: 2017-09-14
Anticipated expiration: 2018-09-11

Abstract

There is provided a porous metal-organic framework comprising a Hafnium or Zirconium metal; an aromatic compound substituted with at least one carboxylate and optionally with at least one functional group selected from F, CF₃, NH₂, NO₂, O^-, S^-, COO^-, SO₂ ^- and PO₃ ^-; and a silver cation complexed to the aromatic compound. There is also provided a method for preparing a porous metal-organic framework as defined above, comprising i) dissolving an aromatic compound, substituted with at least one carboxylate and optionally at least one functional group selected from F, CF₃, NH₂, NO₂, O^-, S^-, COO^-, SO₂ ^- and PO₃ ^-, and a zirconium or hafnium salt in a solvent mixture of water and an organic acid, heating the solution to a temperature of 50-200ºC for 12-60 h to obtain a solid, ii) suspending the obtained solid and a silver salt in a solvent mixture of water and an organic solvent. There is also provided a process for separating an olefin from an alkane comprising placing a porous metal-organic framework as defined above into a container and purging the container with a mixture of an olefin and an alkane, wherein the olefin is preferentially retained within the porous metal-organic framework.

Description

SILVER-DECORATED METAL-ORGANIC FRAMEWORK FOR OLEFIN/ALKANE

SEPARATION

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority of US provisional patent application no. 62/306,788, filed March 11, 2016, the contents of it being hereby incorporated by reference in its entirety for all purposes.

TECHNICAL FIELD

[0002] The present disclosure refers generally to the field of olefin/alkane separation, in particular the process of separation using a metal-organic framework.

BACKGROUND

[0003] Short chain olefins are one of the most important categories of feedstocks for the chemical industry that could be utilized to synthesize a broad scope of materials ranging from computer parts commonly seen to valuable pharmaceuticals. Currently light olefins are typically obtained from steam cracking of either naphtha or ethane. During this process, owing to the similar physical properties of olefins and paraffins, the separation of this gas mixture in industry is realized by cryogenic distillation, using distillation towers with over 150 trays and operating at 7-28 bar and 183-258K. Such energy intensive process constitutes about 20 % of energy consumption of the whole cracking process whose energy cost is 26-31 GJ/t (in terms of ethylene) for naphtha cracking and 17-21 GJ/t for ethane cracking. Considering the huge market for short chain olefins, a simple modification of the separation process to reduce energy consumption can make a big difference in both cost of production and CO2 emission.

[0004] Several environmental-friendly alternatives of cryogenic distillation with relatively low energy cost have been explored. One representative alternative is membrane separation such as the one utilizing facilitated-transportation membranes. The essential working principle of this class of membrane is the utilization of the 7t*-d complexation between metal ions and olefin molecules. Another approach under spotlight recently is adsorption-based separation such as pressure swing adsorption (PSA), vacuum swing adsorption (VSA), and temperature swing adsorption (TSA) using adsorbent materials such as silicate, zeolites, porous organic frameworks, and metal-organic frameworks (MOFs). Specifically, MOFs are porous materials constructed from metal-oxygen secondary building blocks and organic linkers bearing various functionalities. Compared with other porous materials, MOFs are distinguished candidates for olefin-paraffin adsorption separation owing to their ultrahigh specific surface area, diversified pore size and geometry, and various modification approaches to tune the gas sorption performances.

[0005] Basically, MOFs for olefin-paraffin separation can be divided into two categories according to their selectivities, i.e., alkane-selective MOFs and alkene-selective MOFs. A successful strategy towards alkane-selective MOFs is constructing pore environment judiciously so that paraffins will be preferentially adsorbed in the frameworks thanks to cooperative intramolecular interactions. As for constructing alkene-selective MOFs, apart from the molecular-sieving strategy reported recently, one general guideline is fixing unsaturated metal sites (either as parts of secondary building units or attached functionalities to the bridging ligands) in the MOFs framework. For instance, Bloch et al. reported the olefin-paraffin separation ability of FeMOF-74. Olefins introduced into the framework will interact strongly with Fe(II) open sites lined in the one dimensional channels due to the donation of easily polarizable π bond electrons.

[0006] In addition to aforementioned researches, however, there have been few efforts on the exploration of olefin-paraffin separation based on zirconium or hafnium MOFs, a family of MOFs that possess exceptional hydrothermal stability and chemical resistance, which is possibly due to the lack of strong interaction sides. There have been few efforts to produce an MOF in a non-toxic manner. There have also been few efforts to provide a method to produce these MOF's which can be easily scaled up and therefore would allow for industrial scale application.

[0007] It is therefore an object of the present disclosure to provide a porous metal-organic framework which possesses exceptional hydrothermal stability and chemical resistance. It is also an object of the present disclosure to provide an MOF with excellent olefin/alkane separation properties. It is a further object of the present disclosure to provide a method which allows for easy scaling up and industrial application. Ideally, the MOF and the method of producing such MOF's would be essentially non-toxic. SUMMARY

[0008] In a first aspect, the present disclosure refers to a porous metal-organic framework (MOF). The MOF may include a Hafnium or Zirconium metal. The MOF may further include an aromatic compound substituted with at least one carboxylate and optionally with at least one functional group (FG) selected from F, CF₃, NH₂, N0₂, O^", S^", COO^", SO2^" and PO3^". The MOF may further include a silver cation, which is complexed to the aromatic compound. The MOF may function as an adsorbent for olefin/alkane separation. It may contain pores in which a mixture of olefin/alkane is directed. These pores, due to an interaction between the silver cation and the double bond of the olefin, may preferentially retain the olefin within the pores, thereby resulting in a separation of the olefin from the alkane.

[0009] This disclosure presents the synthesis, structural characterizations and gas separation performance of the first hafnium or zirconium based olefin-selective MOF. This Ag(I) decorated MOF exhibits an excellent olefin/alkane separation selectivity of 6 as well as decent recyclability. Moreover, by means of a facile strategy combining environmentally-benign hydrothermal (MHT) method and post-synthetic ion exchange (PSIE) process, the synthesis of the hafnium or zirconium based MOF can be easily scaled up for breakthrough experiments and mass production with low economic input, making this MOF a promising adsorbent candidate for industrial olefin/paraffin separation.

[0010] Advantageously, through an interaction between the double bond of the olefin and the silver cation, it is possible to separate the olefins from the alkanes. Further advantageously, the zirconium or hafnium, which may be used in this MOF, is substantially non-toxic.

[0011] In a second aspect, the present disclosure refers to method for preparing a porous metal- organic framework as described above. The method may comprise dissolving an aromatic compound, substituted with at least one carboxylic acid and optionally at least one functional group selected from F, CF3, NH2, NO2, O^", S^", COO^", SO2^" and PO3^", and a zirconium or hafnium salt in a solvent mixture of water and an organic acid. It may further include heating the solution to a temperature of 50-200°C for 12-60 h to obtain a solid. Subsequently, the obtained solid and a silver salt may be suspended in a solvent mixture of water and an organic solvent. The method thus described proceeds under modulated hydrothermal synthesis conditions. [0012] Advantageously, the method as described above may proceed under mild synthetic conditions. Hence, the method may be scaled-up easily and may be environmentally benign.

[0013] In a third aspect, the present disclosure refers to a product obtained by the method as described above.

[0014] In a fourth aspect, the present disclosure refers to use of a porous metal-organic framework as described above in the separation of an olefin from an alkane.

[0015] In a fifth aspect, the present disclosure refers to a process for separating an olefin from an alkane comprising placing a porous metal-organic framework as described above into a container and purging the container with a mixture of an olefin and an alkane, wherein the olefin is preferentially retained within the porous metal-organic framework.

[0016] Advantageously, the process as described above may allow for recycling of the MOF. In other words, the MOF may be re-used in the process several times.

[0017] In a sixth aspect, the present disclosure refers to a porous metal-organic framework (MOF) comprising a metal. The MOF may further include the substituted aromatic compound as disclosed and a silver cation complexed to the aromatic compound.

[0018] Advantageously, the porous metal-organic framework as described above may have a surface area not exceeding 1,500 cm² g^"1.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily drawn to scale, emphasis instead generally being placed upon illustrating the principles of various embodiments. In the following description, various embodiments are described with reference to the following drawings.

[0020] Fig. 1 discloses a schematic illustration of using NUS-6(Hf)-Ag for C2H4/C2H6 separation.

[0021] Fig. 2 discloses the structure of NUS-6: (a) A 12-connected Hfs cluster; (b) a 9-connected Hfe cluster; (c) a 2 2 2 super unit cell of UiO-66(Hf) with a = 20.7006(3) A; (d) a single unit cell of NUS-6(Hf) with a = 41.4718(2) A; (e) a 1 x2x2 super unit cell of NUS-6(Hf) with mesopores indicated by light balls within the cell; (f) a fragment of NUS-6(Hf) with potential Bronsted acid sites labeled by dark clouds (such as at the periphery of the middle aromatic ring) and Lewis acid sites labeled by lighter clouds (such as at the periphery of the lower conglomeration).

[0022] Fig. 3 discloses PXRD patterns of NUS-6(Hf) and NUS-6(Hf)-Ag.

[0023] Fig. 4 discloses 3(a - b) TGA analyses (a) and FT-IR spectra (b) of NUS-6(Hf) (solid line) and NUS-6(Hf)-Ag (dotted line), (c) XPS detailed spectra of Ag 3d of NUS-6(Hf)-Ag (solid line) and NUS-6(Hf) (dotted line), (d) XPS detailed spectra of Ag auger lines NUS-6(Hf) (line structure: - · · - · ) and NUS-6(Hf)-Ag (solid line).

[0024] Fig. 5 discloses (a) SEM images of NUS-6(Hf). (b) SEM images of NUS-6(Hf)-Ag. The scale bars for (a - b) are 1 μπι. The magnification of the 'normal' images in (a) and (b) is 10,000 and the magnified images of (a) and (b) is 43,000. (c - f) EDS mapping of silver (c), sulfur (d), hafnium (e) and oxygen (f) in NUS-6(Hf)-Ag. The selected area for EDS mapping is shown in the inset pictures of (c - f).

[0025] Fig. 6 discloses N2 sorption isotherms at 77 K (a) and pore size distribution (b) of NUS- 6(Hf) and NUS-6(Hf)-Ag.

[0026] Fig. 7 discloses (a) C2H4 and C₂H₆ sorption of NUS-6(Hf) and NUS-6(Hf)-Ag at 298 K. (b) Q_st of C2H6 and C2H4 in NUS-6(Hf)-Ag. (c) Comparison of IAST C2H4/C2H6 (1 : 1) adsorption selectivity of NUS-6(Hf)-Ag and NUS-6(Hf) at 298 K. (d) Adsorption cycles of NUS-6(Hf)-Ag.

[0027] Fig. 7 discloses profile of an equimolar gas mixture of ethane and ethylene on a column packed with NUS-6(Hf)-Ag at ambient condition.

[0028] Fig. 8 discloses ethylene and ethane isotherms of NUS-6(Hf)-Ag at 298 and 313 K.

[0029] Fig. 9 discloses IAST selectivity of NUS-6(Hf)-Ag for equimolar ethylene-ethane mixture at 298 and 313 K.

[0030] Fig. 10 is a schematic illustration of using NUS-6(Hf)-Ag for C2H4/C2H6 separation.

[0031] Fig. 11 shows an XPS survey of NUS-6(Hf) and NUS-6(Hf)-Ag.

[0032] Fig. 12 shows (a) XPS survey of UiO-66(Hf)-Ag. (b) XPS detailed spectrum of Ag 3d of UiO-66(Hf)-Ag. (c) XPS detailed spectrum of Ag auger lines of UiO-66(Hf)-Ag.

[0033] Fig. 13 shows SEM images with different magnification of NUS-6(Hf) treated with HBF4 aqueous solution of the pH value similar to that of the mother solution in PSIE process. The surface morphology of these crystals resembled that of NUS-6(Hf)-Ag, suggesting that the rugged surface of the NUS-6(Hf)-Ag crystals may originate from the etching of HBF₄ generated in situ during the ion-exchange process.

[0034] Fig. 14 shows an EDS spectrum of NUS-6(Hf)-Ag.

[0035] Fig. 15 shows C2H4 (circle) and C2H6 (square) sorption isotherms of UiO-66(Hf)-Ag

(grey) and UiO-66(Hf) (black) at 298 K (filled, adsorption; open, desorption).

[0036] Fig. 16 shows C₂H₄ and C₂H₆ sorption isotherms of NUS-6(Hf) and NUS-6(Hf)-Ag at

283 K (filled, adsorption; open, desorption).

[0037] Fig. 17 shows the Qst of C₂H₄ and C₂H₆ in NUS-6(Hf).

[0038] Fig. 18 shows (a) N2 sorption isotherm at 77 K of NUS-6(Hf)-Ag after 8 cycles of C₂H₄ sorption tests (filled, adsorption; open, desorption). (b) XRD pattern of NUS-6(Hf)-Ag after 8 cycles of C2H sorption tests.

[0039] Fig. 19 shows Breakthrough curves of C₂H₄ and C2H6 running through a bypass gas line. The dead volume time of the gas mixture was hence calculated to be 466.9 s.

[0040] Fig. 20 shows typical desorption curves of NUS-6(Hf)-Ag under the condition of 20 seem He flow at room temperature.

[0041] Fig. 21 shows the 2^nd (a) and 3^rd (b) C₂H₄/C₂H₆ breakthrough curve of NUS-6(Hf)-Ag column.

[0042] Fig. 22 shows (a) XRD pattern of NUS-6(Hf)-Ag after breakthrough experiment, (b) XPS detailed spectra of Ag 3d. (c) Auger spectrum of Ag of NUS-6(Hf)-Ag after breakthrough experiment. The M4N5N5 line was located at 1133.0 eV and 3ds/₂ was located at 368.2 eV, leading to a modified auger parameter equal to 721.8 eV.

DESCRIPTION

[0043] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the disclosure may be practised. These embodiments are described in sufficient detail to enable those skilled in the art to practise the disclosure. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the disclosure. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments. [0044] A porous metal-organic framework has been developed according to various embodiments. The metal-organic framework may comprise a Hafnium or Zirconium metal; an aromatic compound substituted with at least one carboxylate and optionally with at least one functional group selected from F, CF3, Nth, NO2, O^", S^", COO^", SO2^" and PO3^"; and a silver cation complexed to the aromatic compound.

[0045] The MOF be microporous and/or mesoporous. Typically, the MOF may have a hierarchical micro/meso-porous nature. This micro/meso-porous nature may result in a stepwise adsorption behavior with adsorption-desorption hysteresis (shown in Fig. 6b). The pore width of the pores may be less than about 100 nm, or less than about 50 nm, or less than about 40 nm, or less than about 30 nm. Typically, the MOF, having the silver cation complexed to it, has the same characteristics in terms of porosity and crystallinity as the MOF without the silver cation complexed to it.

[0046] The particles formed from the MOF may have a size of about 0.5 to about 3 μηι, or about 0.5 to about 2 μηι, or about 0.7 to about 1.5 μηι, or about 0.7 to about 1.2 μηι, or about 1 μιη. A small portion of the particles may possess quasi-octahedral shapes. The surface of the MOF may be adorned with grooves and cavities. The distribution of silver, the anionic functional group, hafnium or zirconium and oxygen within the MOF may be even.

[0047] Advantageously, hafnium or zirconium may be non-toxic and/or environmentally-benign. Further advantageously, the use of hafnium or zirconium may result in strong bonds of the metal to the carboxylate functionality of the substituted aromatic compound.

[0048] The hafnium or zirconium may be in the oxidation state +4 in the MOF. The periodic assembly of the atoms may form a cell, which may lead to the MOF. The pores may be embedded in this cell (shown in Fig. 2). The cell may contain Bronsted acid and Lewis acid sites.

[0049] The substituted aromatic compound to be used in this disclosure may be selected from benzene, 1, 3, 5-triphenyl-benzene, benzo-tris-thiophene, triazine, 2, 4, 6-tribenzene and 1, 3, 5, - trinaphthyl benzene. The substituted aromatic compound may provide an electron-rich aromatic pi electron density, enabling the silver cation to be complexed thereto.

[0050] The substituted aromatic compound to be used in this disclosure may contain at least one carboxylate, optionally at least two carboxylates. [0051] The substituted aromatic compound to be used in this disclosure may additionally contain Lewis acid sites which may form additional complexes with the olefin. The complexation of the olefin to either the silver cation or the Lewis acid sites of the substituted aromatic compound may provide a synergistic effect.

[0052] The substituted aromatic compound may be selected from a compound represented as Formula (I):

Figure (I) wherein m is an integer selected from 1 to 3;

n is an integer selected from 0 to 4;

wherein m + n <6; and

FG is a functional group as defined above.

[0053] The functional group may be anionic, optionally selected from the group consisting of O^", S^", COO^", S0₂- and Ρ0₃ ^".

[0054] The substituted aromatic compound may be selected from a compound represented as Formula (la):

Formula (la) wherein n and FG are as defined above.

[0055] The substituted aromatic compound may alternatively be selected from a compound represented as Formula (lb):

Formula (lb) wherein n is an integer from 0 to 3, wherein m + n <6; and FG is as defined above.

In some embodiments, the anionic functional group may be a sulfate anion. Advantageously, in embodiments wherein the anionic functional group is a sulfate anion, the silver cation may be complexed to the sulfate moiety. The silver cation may additionally be complexed to the electron-rich aromatic pi electron density. In some embodiments, the substituted aromatic compound may be represented as in Formula (la) and n may be 1.

[0056] In one example, the substituted aromatic compound may be selected from 2-sulfo terephthalic acid.

[0057] The silver cation may have an oxidation state of Ag(I). Advantageously, the bondings between the silver (I) cation and the olefin in the separation process may be based on a two orbital overlaps. In a first orbital overlap, the σ components of these bondings are formed by the overlap of the full π molecular orbitals of the olefins with the vacant outmost s orbitals of the silver metal. In a second orbital overlap, π components result from the backdonation of electrons from the outer full d orbitals of Ag(I) to the vacant π* orbitals of olefins.

[0058] The silver cation may be present in about 5-30 atomic %, preferably in about 8-20 atomic%, more preferably in about 10-15 atomic % of the aromatic compound. In embodiments where there is one anionic functional group within the substituted aromatic compound, the silver cation may therefore be present in about 5-30 atomic %, preferably in about 8-20 atomic %, more preferably in about 10-15 atomic % of the functional group. In one example, the silver cation is present in about 13% of the functional group. [0059] The metal-organic framework may have a large surface area. A typical limit for this large surface area may be a surface area of not exceeding 1,500 cm² g^"1. In some embodiments, the porous metal-organic framework may have a surface area of about 500 - 1,500 cm² g^"1, optionally of about 800 - 1,400 cm² g^"1, or of about 1,000 - 1,200 cm² g^"1.

[0060] The porous metal-organic framework may be crystalline. Advantageously, the MOF as disclosed herein may retain its crystallinity during the method of production, or it may be only slightly reduced.

[0061] In a second aspect, there is disclosed a method for preparing a porous metal-organic framework as disclosed herein, comprising i) dissolving an aromatic compound precursor, substituted with at least one carboxylic acid and optionally at least one functional group selected from F, CF3, Nth, NO2, O^", S^", COO^", SO2^" and PO3^", and a zirconium or hafnium salt in a solvent mixture of water and an organic acid, heating the solution to a temperature of 50-200°C for 12-60 h to obtain a solid,

ii) suspending the obtained solid and a silver salt in a solvent mixture of water and an organic solvent.

[0062] Advantageously, the method may use water as a co-solvent. Further advantageously, due to the use of hafnium or zirconium, the method of making the MOF may be producing less toxic side products or intermediates and reduce the exposure to hazardous materials.

[0063] The molar ratio between the substituted aromatic compound precursor and the zirconium or hafnium salt may be approximately 2: 1 to 1 :2, more preferably 1.5: 1 to 1 : 1.5, most preferably approximately 1 : 1 (±10%, optionally ±5%).

The volume ratio between the water and the organic acid may be approximately 3: 1 to 1 : 1, more preferably 2: 1 to 1.2: 1, most preferably approximately 1.5: 1 (±10%, optionally ±5%).

[0064] In some embodiments, the aromatic compound precursor may be represented by the following Formula (II):

Formula (II).

wherein m is an integer selected from 1 to 3;

n is an integer selected from 0 to 4;

wherein m + n <6; and

FG is a functional group as defined above.

[0065] The substituted aromatic compound precursor may be selected from a compound represented as Formula (Ha):

Formula (Ila) wherein n and FG is as defined above.

[0066] The substituted aromatic compound precursor may alternatively be selected from a compound represented as Formula (lib):

Formula (lib) wherein n is an integer from 0 to 3, wherein m + n <6; and FG is as defined above. The functional group may be defined as in the first aspect.

[0067] In some embodiments, the zirconium or hafnium salt may be selected from a zirconium or hafnium halogen salt. Alternatively, it may be a zirconium or hafnium nitrate salt. A zirconium or hafnium halogen salt may be selected from fluoride, chloride and bromide salt. In one example, it may be chloride salt.

[0068] In some embodiments, the zirconium or hafnium of the respective salt may have an oxidation state of +4.

[0069] In some embodiments, the organic acid may be a carboxylic acid. Preferably, the carboxylic acid may be selected from the group consisting of formic acid, acetic acid or propionic acid. In one example, it may be acetic acid.

[0070] The reaction temperature in step i) may be in the range of about 40 °C to about 200 °C, about 40 °C to about 100 °C, about 40 °C to about 120 °C, about 40 °C to about 140 °C, about 40 °C to about 160 °C, about 40 °C to about 180 °C, about 60 °C to about 80 °C, about 60 °C to about 100 °C, about 60 °C to about 120 °C, about 60 °C to about 140 °C, about 60 °C to about 160 °C, about 60 °C to about 180 °C, about 60 °C to about 200 °C, about 80 °C to about 100 °C, about 80 °C to about 120 °C, about 80 °C to about 140 °C, about 80 °C to about 160 °C, about 80 °C to about 180 ° or, about 80 °C to about 200 °C. The boiling point of the solvent mixture may be sufficiently high to allow the reaction to proceed.

[0071] The duration of reaction step i) may be in the range of about 8 hours to about 48 hours, about 8 hours to about 40 hours, about 8 hours to about 36 hours, about 8 hours to about 30 hours, about 12 hours to about 48 hours, about 12 hours to about 44 hours, about 12 hours to about 36 hours, about 12 hours to about 24 hours or about 18 hours to about 24 hours.

[0072] In some embodiments, after step i) the obtained solid is washed with an organic solvent. The organic solvent may be a polar protic solvent. It may be preferably an alcohol, optionally selected from the group consisting of methanol, ethanol or isopropanol. In one example, it may be methanol. [0073] In step ii) the organic solvent may be water-soluble. As such, it may be selected from the group consisting of methanol, ethanol, acetone, tetrahydrofuran, dimethylformamide and acetonitrile. In one embodiment, it may be acetonitrile.

[0074] The volume ratio between the water and the organic solvent in step ii) may be approximately 2: 1 to 1 :2, more preferably 1.5: 1 to 1 : 1.5, most preferably approximately 1 : 1 (±10%, optionally ±5%).

[0075] In some embodiments, step ii) may be conducted under darkness. In some embodiments, step ii) may be repeated. In preferred embodiments, step ii) may be conducted three times. In some embodiments, step ii) may be conducted under room temperature. In some embodiments, the reaction time in step ii) may be in the range of about 8 hours to about 48 hours, about 8 hours to about 40 hours, about 8 hours to about 36 hours, about 8 hours to about 30 hours, about 12 hours to about 48 hours, about 12 hours to about 44 hours, about 12 hours to about 36 hours, about 12 hours to about 24 hours or about 12 hours to about 16 hours.

[0076] In a third aspect, there is provided a product obtained by the method as disclosed above.

[0077] In a fourth aspect, there is provided use of a porous metal-organic framework as disclosed in the first or the third aspect in the separation of an olefin from an alkane.

[0078] In a fifth aspect there is provided a process for separating an olefin from an alkane comprising placing a porous metal-organic framework as disclosed in the first or the third aspect into a container and purging the container with a mixture of an olefin and an alkane, wherein the olefin is preferentially retained within the porous metal-organic framework.

[0079] In Fig. 1, the process as disclosed above is shown schematically. As illustrative example, an ethylene/ethane mixture (left side) is provided to the MOF (center) and on the right side, only the ethane exits. The ethylene is retained within the MOF.

[0080] Advantageously, the process of using the MOF as disclosed herein may reduce the exposure to hazardous chemicals as compared with other MOF's.

[0081] In some embodiments, the process may be a pressure swing adsorption process. This pressure swing adsorption (PSA) process may be defined as a process which relies on the fact that under high pressure, gases tend to be attracted to solid surfaces, or "adsorbed". The higher the pressure, the more gas is adsorbed. When the pressure is reduced, the gas is released, or desorbed. PSA processes can be used to separate gases in a mixture because different gases tend to be attracted to different solid surfaces more or less strongly. In some embodiments, the olefin /alkane mixture may therefore be in the gaseous state.

[0082] In some embodiments, the porous metal-organic framework is recyclable. As such, the MOF may be able to undergo the adsorption-desorption separation process of olefins and alkanes at least 3, 5 or 8 times. The MOF as disclosed herein may experience no decrease in specific surface area or crystallinity after at least 3, 5 or 8 cycles of adsorption-desorption.

[0083] In some embodiments, the olefin / alkane mixture to be supplied in the process may be in a molar ratio of 2: 1 to 1 :2, more preferably 1.5: 1 to 1 : 1.5, most preferably approximately 1 : 1 (±10%, optionally ±5%).

[0084] In some embodiments, the pressure of the process may be at approximately 1 atm in the adsorption cycle. In some embodiments, the temperature may be room temperature. In some embodiments, olefin / alkane co-adsorption selectivity may be about 3-6, about 4-5, or about 4.4.

[0085] The olefin and the alkane in the above process may be selected from aliphatic carbon compounds, preferably wherein the aliphatic carbon compounds are selected from Ci-8alkanes and C2-8alkenes. The olefin may be selected from C2-salkenes, more preferably from ethane and propene, and the alkane may be selected from Ci-salkanes, more preferably from ethane and propane.

[0086] In the above process, the retaining of the olefin may be based on the interaction of the π orbitals of the olefin with the s orbitals of the silver cation and the d orbitals of the silver cation to the vacant π* orbitals of olefins.

[0087] In a sixth aspect there is provided a porous metal-organic framework comprising

- a metal;

- an aromatic compound substituted with at least one carboxylate and optionally with at least one functional group selected from F, CF₃, NH₂, NC , O^", S^", COO^", SO2^" and PO3^"; and

- a silver cation complexed to the aromatic compound. [0088] The metal in the sixth aspect may be selected from a group III, IV, V, VI, X and a group

XIV element, or from the lanthanoids. Preferably, the metal is selected from titanium, zirconium, hafnium, vanadium, chromium, molybdenum, scandium, lead, palladium and cerium.

[0089] The substituted aromatic compound of the sixth aspect may be defined as in the first aspect.

[0090] The dominant technology of olefin/paraffin separation nowadays is cryogenic distillation which is extremely energy-intensive. Developing advanced adsorbents would be the first step towards cost effective alternatives such as pressure swing adsorption (PSA) or temperature swing adsorption (TSA). In this work there is presented a silver-decorated hafnium metal-organic framework (MOF), synthesized via modulated hydrothermal (MHT) synthesis followed by ion exchange reactions, as an olefin-selective adsorbent for olefin/alkane-separation. The successful introduction of Ag(I), confirmed by X-Ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDS), elevated the Ideal adsorbed solution theory (IAST) olefin/alkane selectivity of the MOF to 6, 5 times greater compared to a MOF precursor not containing a silver cation.

[0091] Experiments further confirmed the gas separation performance of the MOF according to the disclosure, showing that the MOF had co-adsorption olefin/alkane selectivities greater than 3 in 3 consecutive adsorption-regeneration cycles. In addition, after the recyclability tests, the MOF of the present disclosure maintained its crystallinity and the oxidation state of Ag in the MOF remained unchanged. This study comprehensively demonstrates the promising potential of this MOF in industrial adsorption based olefin/alkane separation as an energy-saving alternative complementary to the bench mark technologies.

EXAMPLES

Example 1 - Materials and Method

[0092] All of the reagents were obtained from commercial suppliers and used without further purification. Field-emission scanning electron microscope (FE-SEM) analyses were conducted on an FEI Quanta 600 SEM (20 kV) equipped with an energy dispersive spectrometer (EDS, Oxford Instruments, 80 mm² detector). Samples were treated via Pt sputtering before observation. Powder X-ray diffraction patterns were obtained on a Rigaku MiniFlex 600 X-ray powder diffractometer equipped with a Cu sealed tube (λ = 1.5418 A) operating at 40 kV, 15 mA, with a scan rate of 0.05 degree s^"1. Thermogravimetric analysis (TGA) was performed using a Shimadzu DTG-60AH thermal analyzer under a N₂ (100 mL min^"1) with a heating rate of 10 °C min^"1. FTIR spectra were obtained with a Bio-Rad FTS-3500 ARX FTIR Spectrometer. X-ray photoelectron spectroscopy (XPS) experiments were carried out using a monochromatic Al Ka radiation (1486.6 eV) at 15 kV as the excitation source. Elemental analyses for carbon, hydrogen, nitrogen and sulfur were performed by Elementar vario MICRO cube, while metal contents analyses were conducted by an Inductively Coupled Plasma-Optical Emission Spectrometer (ICP- OES, Perkin Elmer Optima 5300DV). Bed porosity is determined by Mercury Intrusion Porosimetry Micromeritics Autopore III 9420.

Example 2 - Modulated Hydrothermal (MHT) Synthesis of NUS-6(Hf)

[0093] Monosodium 2-sulfo terephthalic acid (2.6 g, -9.6 mmol) and HfCU (3.2 g, -10 mmol) were dissolved in 100 mL of water/acetic acid (30/20, v/v) solution and heated at 90 °C for 24 hours to yield a powder product. The product was soaked in anhydrous methanol for 3 days at room temperature, during which time the extract was decanted and fresh methanol was added thrice. After removal of methanol by decanting, the sample was dried in fume hood at ambient temperature to afford white powder as final product.

Example 3 - Solvothermal Synthesis of UiO-66(Hf)

[0094] The solvothermal synthesis of UiO-66(Hf) was conducted based on the modification of a reported procedure. Briefly, benzene- 1 ,4-dicarboxy lie acid (83 mg, ca. 0.5 mmol) and HfCU (160 mg, ca. 0.5 mmol) dissolved in 20 mL of dimethylformamide (DMF)/formic acid (18/2, v/v) mixed solvent were loaded into a Teflon lined autoclave and heated at 123 °C for 40 h. The product was soaked in anhydrous methanol for 3 days at room temperature, during which time the extract was decanted and fresh methanol was added every day. Then the sample was treated with anhydrous dichloromethane similarly for another 3 days. This process was carried out to wash out residual reagents trapped inside the pores. After removal of dichloromethane by decanting, the sample was dried under a dynamic vacuum at 120 °C for 24 h to afford the final product (yield: 52%). Example 4 - Post-Synthetic Ion Exchange

[0095] MOF precursors (NUS-6(Hf) or UiO-66(Hf), 500 mg, before activation) and AgBF₄ (2.5 g, 12.8 mmol) were suspended in 100 mL of acetonitrile/water (1/1, v/v) solution and stirred under room temperature overnight. The remaining solid was collected by centrifuge and washed with acetonitrile three times. The whole process was conducted under darkness and repeated three times to maximize the metalation of sulfonic acid sites. Finally, the product was dried in open air under darkness for further characterization (yield: 70%).

Example 5 - Gas Sorption Measurements

[0096] Gas sorption isotherms were measured up to 1 bar using a Quantachrome autosorb iQ surface area and pore size analyzer. Before the measurements, the sample (-50 mg) was degassed under reduced pressure (< 10^"2 Pa) at 25 °C for 3 days. UHP grade gases were used for gas sorption measurements. Oil-free vacuum pumps and oil-free pressure regulators were used to prevent contamination of the samples during the degassing process and isotherm measurement. The temperatures of 77 K were maintained with a liquid nitrogen bath, while 283 K and 298 K were realized by a circulating water bath. Pore size distribution data were calculated from the N2 adsorption isotherms at 77 K based on quenched solid density functional theories (QSDFT) model assuming slit/cylinder pore geometry in ASiQwin software.

Example 6 - Breakthrough Experiments

[0097] Breakthrough set up is shown in Fig. 10. Before a typical breakthrough experiment, the adsorption bed is purged with 20 seem He flow for three days for activation. Then an equimolar mixture of C2H4 and C2H6 is purged into the bed with a flow rate of 2 seem. The gas composition at the exit of the column is determined by mass spectrometry and the flow rate of each component is calibrated by an internal reference Ar flow that has a flow rate of 4.5 seem. The mean residence time of gas components and the gas uptake of the packed adsorbent are calculated using equation (1),

where Fo is the gas molar flow rate at the inlet, F is the gas molar flow rate at the outlet, L is the bed length, vo is the interstitial velocity, ε is the bed porosity, co is the inlet gas concentration and qo is the corresponding gas concentration in the adsorbent. Breakthrough selectivity is calculated using equation 2),

where c_ei (i = 1,2 ) is the equilibrium concentration of component i in gas phase while q_ei (i = 1,2) is the gas concentration component i in the adsorbent.

Example 7 - Structure and Morphology Characterization

[0098] Zr/Hf MOFs are well known for their ultrahigh thermal stability and chemical resistivity owing to the robust Zr/Hf-0 bonds. Nevertheless, few research efforts have been put into the development of Zr/Hf based MOFs to separate short chain olefin molecules from the paraffin counterparts of the same carbon number. With the strategy of the introduction of unsaturated open metal sites borne in mind, Zr/Hf MOFs were designed and synthesized that can preferentially adsorb ethylene. Recently, Hf MOFs (NUS-6(Hf)) containing sulfonic groups were prepared through modulated hydrothermal (MHT) method. In this porous material, the sulfonic acid groups in NUS-6(Hf) are ideal anchors for fixing Ag(I) and the Lewis acid sites on the secondary building blocks (SBU) which may be active sites to complex with ethylene. Without being bound to theory, it is speculated that these functionalities may work synergistically to endow the MOF with capability to selectively adsorb ethylene.

[0099] In this work the synthetic conditions of MHT of NUS-6(Hf) comprised the following steps. Acetic acid was used as the modulator and the reaction mixture was cooked at 90 °C to yield highly crystalline materials with XRD spectrum consistent with previous literature. NUS- 6(Hf)-Ag was synthesized via post-synthetic ion exchange (PSIE). The powder X-ray diffraction (PXRD) pattern of NUS-6(Hf)-Ag was found to be consistent with NUS-6(Hf) (Fig. 3), proving that the structure of the MOF precursor was maintained after PSIE process, except that the crystallinity is slightly reduced. Thermogravimetric analyses (TGA) indicates that thermostability of the NUS-6(Hf) is well preserved after PSIE (Fig. 4a). The presence of sulfonic acid groups were confirmed by the adsorption peaks of 1230 cm^"1 and 1180 cm^"1 attributed to the 0=S=0 symmetric stretching in FT-IR spectrum for both NUS-6(Hf) and NUS-6(Hf)-Ag (Fig. 4b). Besides, successful introduction of Ag into NUS-6(Hf) was demonstrated by X-ray photoelectron spectroscopy (XPS) analysis, which exhibits the silver signals at binding energy of 368.3 eV and 374.3 eV corresponding to the peaks of Ag 3d₅/2 and Ag 3d3/2, respectively (Fig. 4c). Since photoelectron lines of silver and silver compound show insignificant chemical shift, the X-ray excited Auger lines (Fig. 4d) were also studied to probe the valence of Ag. The Ag M4N5N5 line was at 1131.8 eV and the modified Auger parameter was calculated to be 723.1 eV, indicating that the form of silver in NUS-6(Hf)-Ag is mainly Ag(I). Recently, it has been reported that hydroxyl groups on zirconium/hafnium metal clusters can serve as active sites to bind metal compounds. Indeed, it was observed that when UiO-66(Hf) was treated with AgBF₄ under the same conditions as the PSIE of NUS-6(Hf) (the product is noted as UiO-66(Hf)-Ag), small humps corresponding to silver species can be found by XPS spectrum (Fig. 12b), which could be possibly due to the metalation of hydroxyl groups on the Hf SBUs. Since these silver signals are much weaker than the ones of NUS-6(Hf)-Ag, we speculate that sulfonic groups play the primary role to anchor Ag(I) onto NUS-6(Hf) by the formation of silver sulfonate via the soft acid-soft base interaction between sulfonic groups and Ag(I).

[00100] The morphology of NUS-6(Hf) and NUS-6(Hf)-Ag in this study was characterized by field-emission scanning electron microscopy (FE-SEM) (Fig. 5). Different from the octahedral shape of NUS-6(Hf) prepared by similar MHT method reported previously, most NUS-6(Hf) nanoparticles synthesized in this study exhibit irregular shape (Fig. 5a). Closer inspection reveals agglomeration of small particles and that only a small portion of particles possess quasi-octahedral habit (Fig. 5a, inlet figure). As for NUS-6(Hf)-Ag, the size of the particles basically remain unchanged after PSM (Fig. 5b). Nevertheless, it is interesting to note the surface of NUS-6(Hf)-Ag is adorned with grooves and cavities (Fig. 5b, inlet figure), possibly due to the etching by HBF₄ generated during ion exchange treatment (Fig. 13). Even distribution of silver, sulfur, hafnium and oxygen within the framework of NUS-6(Hf)-Ag was further confirmed by energy- dispersive spectroscopy (EDS) elemental mapping (Fig. 5c-5f). The inductively coupled plasma-optical emission spectroscopy (ICP-OES) together with elemental analysis shows that approximately 13% of sulfonic groups are attached to Ag(I). Example 8 - Gas sorption analysis

[00101] Large specific surface area is a highly desirable feature for porous adsorbents. In order to compare the specific surface area and pore size distribution between NUS-6(Hf) and NUS-6(Hf)-Ag, a mild room-temperature activation method reported previously was applied to evacuate the gas or solvent molecules trapped inside the frameworks, after which N2 sorption analysis at 77 K was conducted. It was found that both MOFs exhibit stepwise adsorption behavior with adsorption-desorption hysteresis (Fig. 6a), which may result from the hierarchical micro/meso-porous nature of the framework as well as the defects introduced by the modulator. The Brunauer-Emmett-Teller (BET) surface area of NUS-6(Hf) was determined to be 1194.3 cm² g^"1, while NUS-6(Hf)-Ag has a BET surface area of 1027 cm² g^"1. This decline in surface area could be attributed to the loading of Ag(I) into the framework and slight loss of crystallinity. Besides, the N2 adsorption isotherm of NUS-6(Hf)-Ag shows a steep lifting, most likely due to condensation of N2 on the rugged external surface (Fig. 6a). Quenched solid density functional theories (QSDFT) model indicates that the pore size distribution of the MOF slightly shifts to left hand side after PSIE, but the overall profile remains unchanged (Fig. 6b). Such gas sorption characterizations prove that the porosity and crystallinity of the NUS-6(Hf) is well maintained after PSIE.

[00102] To evaluate the effectiveness of PSIE and the feasibility of employing NUS- 6(Hf)-Ag as porous adsorbent for selective C2H4 capture, C2H4 and C2H6 sorption behavior of NUS-6(Hf)-Ag as while as NUS-6(Hf) was systematically studied. At 298 K, the C2H4 and C2H6 uptake of NUS-6(Hf) at 105.8 kPa are 1.54 mmol g^"1 and 1.50 mmol g^"1 (Fig. 7a), respectively. The negligible difference in uptake difference of C2H4 and C2H6 indicates the limited role of Lewis acid sites in the separation of these gas pairs. After silver loading, although the C2H6 uptake is slightly lower (1.35 mmol g^"1) which is likely due to the compromise of specific surface area during PSIE, NUS-6(Hf)-Ag shows a C2H4 uptake of 2.02 mmol g^"1 (Fig. 7b), 50 % higher than the uptake of C2H6. Contrary to the linear C2H4 isotherm for NUS-6(Hf), the soar of C2H4 isotherm of NUS-6(Hf)-Ag at low pressure range indicates a strengthened framework-guest interaction, which suggests the important role of Ag(I) in enhancing the selectivity for C2H4. Compared with NUS-6(Hf)-Ag, UiO-66(Hf)-Ag has very marginal gas adsorption preference towards either C2H4 or C2H6 as UiO-66(Hf) does (Figure 15), which further confirms that sulfonic groups play a vital role in enriching Ag(I) inside the MOF structure.

[00103] The intensified C2H4-MOF interaction in NUS-6(Hf)-Ag was then quantified by the isosteric heat of adsorption Q_st. calculated using the Clausius-Clapeyron equation. The zero- coverage Qst of C2H4 (56.5 kJ mol^"1) in NUS-6(Hf)-Ag is remarkably greater than the one of C2H6 (29.8 kJ mol^"1) as well as the value of C2H4 in NUS-6(Hf) (Fig. 7b). Additionally, this value is higher than the Qst of MOFs with unsaturated open sites such as FeMOF-74 (45 kJ mol^" ^x), MgMOF-74 (42 kJ mol^"1) and HKUST-1 (39 kJ mol^"1). These results not only highlight the crucial role of Ag(I) anchored into the MOF for boosting the interactions with C2H4 via π- complexation, but also prove that Ag(I) can serve as an preferential binding site to selectively capture C2H4 without raising interactions with C2H6. On the other hand, zero-coverage Qst of C2H4 in NUS-6(Hf)-Ag is smaller than the Ag(I) modified porous network (106 kJ mol^"1 for PAF-l-SOsAg, 120 kJ mol^"1 or 63 kJ mol^"1 for MIL-lOl -SOsAg) reported previously. We speculate that this discrepancy from literature values result from limited loading amount of Ag(I) (discussed vide supra). Moderate Qst, an indicator of moderate interactions between C2H4 and NUS-6(Hf)-Ag, might facilitate desorption of C2H4 and thus shorten regeneration time of the columns in dynamic adsorption.

[00104] Ideal adsorbed solution theory (IAST) was employed to calculate C2H4/C2H6 selectivity for NUS-6(Hf) and NUS-6(Hf)-Ag (Fig. 7c). For an equimolar gas mixture at 298 K, the IAST C2H4/C2H6 selectivity is 106.3 at 1 kPa while the selectivity of NUS-6(Hf) is only 0.757. In the case of 100 kPa, the selectivity is as high as 6.0 at 100 kPa, not only far exceeding the one of NUS-6(Hf) (0.955), but also surpassing HKUST-1 (3.6), zeolite 5A (4.5), MgMOF-74 (5.6), PCN-16 (2.8) and NOTT-102 (3). Apart from adsorption selectivity, recyclability of adsorbent materials is another factor of significant importance to evaluate the adsorption performance of porous adsorbents. It was shown that NUS-6(Hf)-Ag can endure at least 8 adsorption-desorption cycles (Fig. 17).

Example 9 - Breakthrough Experiments

[00105] Breakthrough experiments were carried out to further examine the potential of

NUS-6(Hf)-Ag in separating C2H4 and C2H6. A 50/50 ethylene/ethane mixture was purged through a 14 cm, ¼' column packed with 750 mg NUS-6(Hf)-Ag with a volumetric flow rate of 2 seem (standard cubic centimeter per minute). As seen in Fig. 8, when C2H4 was about to breakthrough, the C2H6 breakthrough curve proceeded to an additional roll up. This double roll- up phenomena is similar to the cases observed by Hamon et al : the first roll-up is due to the C2H6 desorption stimulated by C2H4 adsorption, and the second roll-up results from the temperature wave that moves slightly ahead of the concentration front of C2H4 due to the heat released during C2H4 adsorption. After subtracting the mean residence time which the gas mixtures took to pass through the dead volume (Fig. 19), C2H6 and C2H4 exhibited breakthrough mean residence time of 685.3 s and 1285.6 s, respectively. Based on equation (1), the C2H4/C2H6 co-adsorption selectivity was further calculated to be 4.4, slightly lower than the corresponding IAST selectivity. Meanwhile, the specific capacity of C2H4 and C2H6 of NUS-6(Hf)-Ag calculated from this breakthrough experiment was determined to be 0.72 mmol/g and 0.16 mmol/g, respectively. The decline in both adsorption capacity and selectivity indicates the competitive adsorption of C2H4 and C2H6 during breakthrough. Besides, the difference in crystal qualities of samples prepared in different batches for breakthrough experiments may be another reason for the discrepancy between single gas sorption analysis and binary gas breakthrough experiments.

[00106] In addition to gas uptake capacity and selectivity, recyclability is another dimension to fully evaluate adsorbents' capabilities in industrial C2H4/C2H6 separation. In a typical regeneration experiment, the column was kept at ambient temperature and flushed with a 20 seem He flow (Fig. 20). Similar with the results (Fig. 7d) of consecutive adsorption- desorption test conducted on the gas sorption analyzer, the column packed with NUS-6(Hf)-Ag demonstrated decent recyclability under mild activation conditions. For the following two more adsorption desorption breakthrough cycles conducted after regeneration (Fig. 21), the calculated specific uptake capacity for C2H4 and C2H6 were comparable to the first cycle, and the gas co- adsorption selectivity of the column maintained around 3 (Table 1). Furthermore, XRD spectrum confirmed the adsorbent after these recyclability tests maintained the structure with the as- synthesized crystals (Fig. 22a), and XPS studied further confirmed the oxidation state of Ag in MOF remained unchanged (Fig. 22b). These analyses indicate the stability and robustness of NUS-6(Hf)-Ag under the gas separation condition and the potential in C2 olefin/paraffin application.

Table 1. Calculation results of three breakthrough runs.

Entry re / selectivity ti

me / s ^"1

time* / s time* / s

1 685.3 218.4 1285.6 818.7 0.16 0.72 4.4

2 713.12 246.22 1286.8 819.9 0.19 0.72 3.8

3 777.4 310.5 1363.9 897 0.24 0.80 3.2

* Corrected residence time is equal to corresponding residence time minus dead volume time.

[00107] To summarize, there is reported a comprehensive study on Ag(I)-modified Hf based MOF NUS-6(Hf)-Ag as a potential adsorbent material for C2H4/C2H6 separation. A series of characterizations such as PXRD, XPS and N2 sorption analysis demonstrate the successful synthesis of NUS-6(Hf)-Ag. Both single component gas sorption study and breakthrough experiments confirm a decent C2H4/C2H6 selectivity as well as adsorbent reusability. Furthermore, PXRD spectrum shows that the MOF is robust enough to sustain the gas sorption characterization and XPS study indicates the unchanged oxidation state of Ag species in the framework. Considering the facile synthesis based on MHT method and PSIE, excellent gas separation performance along with the robustness of the crystals, NUS-6(Hf)-Ag will be a competitive material candidate for C2H4/C2H6 adsorption separation. Future work on optimizing the column regeneration condition to reduce the regeneration time will be helpful for evaluation of the full potential of NUS-6(Hf)-Ag in practical industrial scenario.

Claims

1. A porous metal-organic framework comprising

- a Hafnium or Zirconium metal;

- an aromatic compound substituted with at least one carboxylate and optionally with at least one functional group selected from F, CF3, NH2, NO2, O^", S^", COO^", SO2^" and PO3^"; and

- a silver cation complexed to the aromatic compound.

2. The porous metal-organic framework of claim 1, wherein the substituted aromatic compound is selected from benzene, 1, 3, 5-triphenyl-benzene, benzo-tris-thiophene, triazine, 2, 4, 6-tribenzene, 1, 3, 5, -trinaphthyl benzene.

3. The porous metal-organic framework of claim 1 or 2, wherein the substituted aromatic compound is selected from a compound re resented as Formula (I):

Figure (I) wherein m is an integer selected from 1 to 3;

n is an integer selected from 0 to 4;

wherein m + n <6; and

FG is a functional group as defined above.

4. The porous metal-organic framework of any one of claims 1 to 3, wherein the functional group is anionic, optionally selected from the group consisting of O^", S^", COO^", SO2^" and PO3-.

5. The porous metal-organic framework of any one of claims 1 to 4, wherein the substituted aromatic compound is selected from a compound represented as Formula (la):

Formula (la) wherein n and FG is as defined above.

The porous metal-organic framework of any one of claims 1 to 4, wherein the substituted aromatic compound is selected from a compound represented as Formula (lb):

^"OOC. ^ oo-

Formula (lb) wherein n is an integer from 0 to 3 and FG is as defined above.

7. The porous metal-organic framework of any one of claims 1 to 5, wherein the substituted aromatic compound is selected from 2-sulfo terephthalic acid.

8. The porous metal-organic framework of any one of claims 1 to 7, wherein the silver cation has an oxidation state of Ag(I), optionally wherein the silver cation is present in about 5-30 atomic %, preferably in about 8-20 atomic%, more preferably in about 10-15 atomic % of the aromatic compound.

9. The porous metal-organic framework of any one of claims 1 to 8, wherein the porous metal-organic framework has a surface area of not exceeding 1,500 cm² g^"1, optionally wherein the porous metal-organic framework has a surface area of about 500 - 1,500 cm² g^"1, preferably of about 800 - 1,400 cm² g^"1, more preferably of about 1,000 - 1,200 cm² g^"

1

10. The porous metal-organic framework of any one of claims 1 to 9, wherein the porous metal-organic framework is crystalline.

11. A method for preparing a porous metal-organic framework of any one of claims 1-10, comprising i) dissolving an aromatic compound, substituted with at least one carboxylic acid and optionally at least one functional group selected from F, CF3, NH2, NO2, O^", S^", COO^", SO2^" and PO3^", and a zirconium or hafnium salt in a solvent mixture of water and an organic acid, heating the solution to a temperature of 50-200°C for 12-60 h to obtain a solid,

12. The method of claim 11, wherein the aromatic compound is selected from a compound represented as Formula (II):

Figure (II) wherein m is an integer selected from 1 to 3;

n is an integer selected from 0 to 4;

wherein m + n <6; and

FG is a functional group as defined in any one of claims 2 to 7.

13. The method of claim 11, wherein the halogen of the zirconium or hafnium salt is a halogen salt, preferably a chloride salt, optionally wherein the zirconium or hafnium of the zirconium or hafnium salt has an oxidation state of +4.

14. The method of any one of claims 11 to 13, wherein the organic acid is acetic acid.

15. The method of any one of claims 11 to 14, wherein after step i) the obtained solid is washed with an organic solvent, preferably an alcohol, more preferably methanol.

16. The method of any one of claims 11 to 15, wherein in step ii) the organic solvent is water-soluble, preferably acetonitrile.

17. A process for separating an olefin from an alkane comprising placing a porous metal- organic framework of any one of claims 1-10 into a container and purging the container with a mixture of an olefin and an alkane, wherein the olefin is preferentially retained within the porous metal-organic framework, optionally

wherein the process is a pressure swing adsorption process.

18. The process of claim 17, wherein the porous metal-organic framework is recyclable.

19. The process of claim 17 or 18, wherein the olefin and the alkane are selected from aliphatic carbon compounds, preferably wherein the aliphatic carbon compounds are selected from Ci-salkanes and C2-salkenes., optionally wherein the unsaturated organic compound is selected from C2-salkenes, more preferably from ethane and propene, and the saturated organic compound is selected from Ci-salkanes, more preferably from ethane and propane.

20. The process of any one of claims 17 to 19, wherein the retaining of the olefin is based on the interaction of the π orbitals of the olefin with the s orbitals of the silver cation and the d orbitals of the silver cation to the vacant π* orbitals of olefins.