Field of Invention

The present invention generally relates to metal-organic frameworks (MOFs) for gasoline upgrading, and more particularly relates to methods of separating hydrocarbons from each other by providing bismuth-based MOFs (Bi-based MOFs).

Background

The listing or discussion of a prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.

Efficient separation of alkane isomers is significant for gasoline upgrading in the petrochemical industry but is a challenging and energy-intensive process. Current global demand for gasoline has reached 26.1 million barrels per day and shows an average annual increase rate of 2.3%. Gasoline quality is characterized by the research octane number (RON), which differs significantly for the linear and branched alkanes, i.e. the higher the degree of branching, the higher the RON. In an ideal scenario, di-branched isomers with the highest RON should be isolated from the linear and mono-branched alkanes to produce premium-grade gasoline. Especially, a viable strategy to upgrade C6 alkane mixtures hexane (C6) isomers, a typical constituent of vehicular gasoline, is to separate the di-branched species from their isomers, followed by isomerization of the latter. However, the compositional similarities entail nearly identical physical and chemical properties. Consequently, separation processes designed to exploit these infinitesimal differences are highly energy-consuming and capital intensive.

Adsorptive separation represents an alternative way to reduce the energy footprint. Adsorptive separation using porous materials has shown great potential in hydrocarbon separations, but still remains in infancy for alkane isomer separation. Several MOFs have been reported for hexane isomers separation, such as Fe2(BDP)s (BDP = 1 ,4-benzenedipyrazolate, Z. R. Herm et al., Science 2013, 340, 960-964), and Zr-bptc (bptc =3,3',5,5'-biphenyltetracarboxylate, H. Wang et al., Nat. Commun. 2018, 9, 1745). However, these organic linkers should be synthesized via complex procedures using expensive organic monomers, which undoubtedly leads to a very high industrial capital investment. This indicates that these materials cannot be obtained on a large scale in industry. Furthermore, the capacity and separation selectivities of these materials for alkane adsorption and separation are moderate, leading to the low productivity of the premium gasoline. The industrial benchmark, zeolite 5A, only behaves as a molecular sieve to exclusively trap linear alkanes from the branched ones with low capacity. This only permits RON enhancement from 86 to 92. Additionally, coupled with the traditional distillation process, this process is energy-intensive. This greatly limits the efficiency of premium gasoline production. In contrast, a material that can preferentially sieve di-branched isomers can yield RON enhancement up to 96. Furthermore, improvements in material properties, particularly the capacity, are equally critical for intensifying a separation process. Thus, novel porous materials with high capacity and selectivity for linear and mono-branched C6 isomers are highly demanded but challenging.

MOFs, featuring tunable pore dimensions and chemistry based on reticular chemistry and crystal engineering, have provided a promising platform to address the challenges of adsorption and separation, and tremendous advancements have been achieved, especially for C2-C4 light hydrocarbons. However, the separation of 06 hydrocarbons using MOFs or other porous materials is still in infancy. Recently, several MOFs, such as MIL-53(Fe)-(CF ₃ ), Ca(H2cpb), and Al-bttotb, have been reported for the separation of 06 alkane isomers via thermodynamic or kinetic molecular sieving. However, imprecise pore size control has limited the improvements in both capacity and selectivity for these separations. In addition, the cost of fabricating these materials remains significant. Although machine learning and computational screening were recently used to predict porous materials for alkane separation, the lack of predictability of the framework flexibility, and host-guest interactions in congested pore environments make it difficult to effectively screen novel materials for the separation of alkane isomers. Thermodynamic separation based on the different binding affinities of the host framework for alkane isomers shows promises in alkane separation via molecular recognition. For example, Fe ₂ (BDP) ₃ and Zr-abtc with one-dimensional (1 D) triangular and rectangular pore channels have shown decent performance, although selectivities may benefit from further enhancement.

Bismuth (Bi) is considered nontoxic and noncarcinogenic and generally exists as Bi ³⁺ in complexes like rare earth metals. Bi-based MOFs are rarely used in separation processes and are still underexplored. The elaborate design of Bi-based MOFs remains a grand challenge due to the tendency of Bi ³⁺ to form layered or dense frameworks, and only a few Bi-based porous materials have been reported.

Therefore, to overcome at least one of the aforementioned problems, there exists a need for new Bi-based MOFs for gasoline upgrading. Summary of Invention

Aspects and embodiments of the invention will now be discussed by reference to the following numbered clauses.

1. A method of separating hydrocarbons from each other, the method comprising the steps of:

(a) providing a metal organic framework (MOF) formed from a metal ion and/or metal cluster and an organic linker; and

(b) passing a fluid stream comprising a mixture of hydrocarbons over the MOF to cause separation of the hydrocarbons, wherein the metal ion and/or metal cluster is formed from Bi ³⁺ ; and the linker is selected from: benzene-1 ,3,5-tricarboxylic acid, wherein the MOF adopts either a Ull-200 or a CALI-17 structure; ellagic acid, wherein the MOF adopts a Sll-101 structure;

where R is H, OH, NH ₂ , NO ₂ , F and Cl.

2. The method according to Clause 1 , wherein the fluid stream is a vapour stream or a liquid stream.

3. The method according to Clause 1 or Clause 2, wherein the MOF is provided in a fixed bed or simulated moving bed arrangement.

4. The method according to any one of the preceding clauses, wherein step (b) is conducted at a temperature of from 298 to 450 K, such as from 303 to 343 K.

5. The method according to any one of the preceding clauses, wherein the mixture of hydrocarbons is a mixture of alkane isomers, a mixture of xylene isomers, or both.

6. The method according to Clause 5, wherein the mixture of alkane isomers is a mixture of hexane isomers.

7. The method according to Clause 6, wherein the mixture of hexane isomers comprises two or more of n-hexane (nHEX), 3-methylpentane (3MP), 2-methylpentane (2MP), 2,3- dimethylbutane (23DMB), and 2,2-dimethylbutane (22DMB).

8. The method according to Clause 7, wherein the mixture of hexane isomers is selected from:

(a) nHex and 22DMB;

(b) 3MP and 22DMB;

(c) nHex, 3MP, and 22DMB; (d) nHex, 2MP, 3MP, and 22DMB; and

(e) nHEx, 3MP, 2MP, 23DMB and 22DMB.

9. The method according to Clause 5, wherein the mixture of xylene isomers is selected from:

(i) o-xylene and m-xylene;

(ii) o-xylene and p-xylene;

(iii) m-xylene and p-xylene; and

(iv) o-xylene, p-xylene and m-xylene.

10. The method according to Clause 5, wherein the mixture of alkane isomers and xylene isomers is selected from any one of (a) to (e) in Clause 8 with any one of (i) to (iv) in Clause 9.

11. The method according to any one of the preceding clauses, wherein the method further comprises passing the fluid stream over a metal formate MOF before step (b) of Clause 1.

12. The method according to Clause 11 , wherein the metal formate MOF is selected from one or more of magnesium formate and nickel formate, optionally wherein the metal formate MOF is nickel formate.

13. A method of separating alkanes comprising the steps of:

(a) providing a metal organic framework (MOF) comprising magnesium or, more particularly, nickel as the metal ion and/or metal cluster and formic acid as the linker; and

(b) passing a fluid stream comprising a mixture of alkanes over the MOF to cause separation of the alkanes.

14. The method according to Clause 13, wherein the MOF comprises nickel as the metal ion and/or metal cluster.

15. The method according to Clause 13 or Clause 14, wherein the mixture of alkanes is a mixture of alkane isomers.

16. The method according to Clause 15, wherein the mixture of alkane isomers is a mixture of hexane isomers. 17. The method according to Clause 16, wherein the mixture of hexane isomers comprises two or more of n-hexane (nHEX), 3-methylpentane (3MP), 2-methylpentane (2MP), 2,3- dimethylbutane (23DMB), and 2,2-dimethylbutane (22DMB).

18. The method according to Clause 17, wherein the mixture of hexane isomers is selected from:

(a) nHex and 22DMB;

(b) 3MP and 22DMB;

(c) nHex, 3MP, and 22DMB;

(d) nHex, 2MP, 3MP, and 22DMB; and

(e) nHEx, 3MP, 2MP, 23DMB and 22DMB.

19. The method according to any one of Clauses 13 to 18, wherein the MOF adsorbs linear alkanes in preference to other mono- and di-branched alkane isomers.

Drawings

FIG. 1 depicts a proposed adsorption and separation process for alkane isomer separation.

FIG. 2 depicts a scanning electron microscopy (SEM) image of Ull-200 nanowhiskers synthesized by solvothermal method.

FIG. 3 depicts a schematic illustration of (a) construction and (b) structure of Ull-200. (c) Scheme of the auxetic reentrant honeycomb grid structure, (d) Pore cavity with restricted triangular pore windows of Ull-200. (e) Two potential binding sites in Ull-200. (f) Pore channels of Ull-200. (g) Schematic diagram of the three-dimensional pore structure in UU- 200.

FIG. 4 depicts the coordination configurations of BTC linkers on Bi-I cluster.

FIG. 5 depicts the coordination configurations of BTC linkers on Bi-ll cluster.

FIG. 6 depicts two binding sites in each pore cavity with pore window restricted by three H atoms from three BTC linkers.

FIG. 7 depicts (a) N2 and CO2 sorption isotherms on Ull-200 at 77 and 196 K, respectively, (b) Pore size distribution of Ull-200 derived from CO2 isotherm at 196 K. FIG. 8 depicts a comparison of the simulated X-ray diffraction (XRD) pattern with the powder X-ray diffraction (PXRD) patterns of Ull-200 after treatment under different conditions.

FIG. 9 depicts a CO2 adsorption isotherm (196 K) of Ull-200 after soaking in pH = 3 solution for 24 h.

FIG. 10 depicts thermogravimetric analysis (TGA) curves of Ull-200 collected with a heating rate of 10 K min ^-1 and an N2 flow rate of 50 mL min ^-1 .

FIG. 11 depicts adsorption isotherms of n-hexane (nHEX), 3-methylpentane (3MP), and 2,2- dimethylbutane (22DMB) at (a) 303 K and (b) 323 K on Ull-200. (c) Comparison of uptake ratios of the alkane isomers on different MOFs at 303 K. (d) Ideal adsorbed solution theory (IAST) separation selectivities of Ull-200 for equimolar nHEX/22DMB, 3MP/22DMB, and nHEX/3MP mixtures. Breakthrough curves of (e) equimolar ternary and (f) five-component alkane mixtures at 303 K.

FIG. 12 depicts nHEX, 3MP, and 22DMB vapor sorption isotherms on Ull-200 at 343 K.

FIG. 13 depicts nHEX, 3MP, and 22DMB vapor sorption isotherms on MIL-160 at 303 K: (a) linear scale of pressure; and (b) logarithmic scale of pressure.

FIG. 14 depicts nHEX, 3MP, and 22DMB vapor sorption isotherms on MOF-303 at 303 K: (a) linear scale of pressure; and (b) logarithmic scale of pressure.

FIG. 15 depicts the virial fitting of nHEX isotherms for Ull-200.

FIG. 16 depicts the virial fitting of 3MP sorption isotherms for Ull-200.

FIG. 17 depicts the 23DMB sorption isotherms on Ull-200 at different temperatures.

FIG. 18 depicts the heats of adsorption (Q _S f) of nHEX, 3MP and 23DMB on Ull-200.

FIG. 19 depicts nHEX, 2MP, 3MP, 23DMB, and 22DMB vapor sorption isotherms on Ull-200 at 303 K. FIG. 20 depicts multicomponent fixed-bed breakthrough experiments for the equimolar mixture of nHEX, 3MP, and 22DMB on MOF-303 at 303 K (He flow rate of 1 mL min' ¹ ).

FIG. 21 depicts multicomponent fixed-bed breakthrough experiments for the equimolar mixture of nHEX, 3MP, and 22DMB on MIL-160 at 303 K (He flow rate of 1 mL min' ¹ ).

FIG. 22 depicts multicomponent fixed-bed breakthrough experiments for the equimolar mixture of nHEX, 2MP, 3MP, 23DMB, and 22DMB on MIL-160 at 303 K (He flow rate of 1 mL min' ¹ ).

FIG. 23 depicts cyclic breakthrough curves of alkane isomers on UU-200 at 303 K.

FIG. 24 depicts Fourier transform infrared spectroscopy (FTIR) spectra of activated, nHEX- and 3MP-loaded UU-200: (a) full range; and (b) zoomed-in range.

FIG. 25 depicts binding configurations of (a) nHEX and (d) 3MP in the pore cavity. Preferential conformations of nHEX on (b) site I and (c) site II of UU-200. Calculated binding configurations of 3MP on (e) site I and (f) site II of UU-200.

FIG. 26 depicts the binding configurations of nHEX in each pore cavity of UU-200.

FIG. 27 depicts the binding configurations of nHEX on site I viewed from different directions.

FIG. 28 depicts the binding configurations of nHEX on site II viewed from different directions.

FIG. 29 depicts the binding configurations of 3MP in each pore cavity of UU-200.

FIG. 30 depicts the binding configurations of 3MP on site I viewed from different directions.

FIG. 31 depicts the binding configurations of 3MP on site II viewed from different directions.

FIG. 32 depicts the TGA curves of UU-200 and CAU-17 detected under N2 with a low rate of 50 mL/min.

FIG. 33 depicts the adsorption isotherms of nHEX, 2MP, 3MP, 23DMB and 22DMB on CAU- 17 at (a and b) 303 K and (c) 323 K. FIG. 34 depicts the adsorption isotherms of nHEX, 2MP, 3MP, 23DMB and 22DMB on Mg- formate at (a) 303 K and (b) 323 K. Adsorption isotherms of nHEX, 2MP, 3MP, 23DMB and 22DMB on Ni-formate at (c) 303 K and (d) 323 K.

FIG. 35 depicts the breakthrough curves of equimolar ternary (a) and five-component (b) alkane mixtures on CALI-17 at 303 K with He carrier gas of 1 mL/min. Breakthrough curves of five-component alkane mixtures on CALI-17 with He carrier gas of 3 mL/min at (c) 303 K and (d) 323 K.

FIG. 36 depicts the breakthrough curves of equimolar five-component (a) and ternary (b) hexane isomer mixtures on Mg-formate at 303 K with a carrier gas of 1 mL/min.

FIG. 37 depicts the breakthrough curves of equimolar five-component (a) and ternary (b) hexane isomer mixtures on Ni-formate at 303 K with a carrier gas of 1 mL/min.

FIG. 38 depicts the breakthrough curves of equimolar ternary (a) and five-component (b) hexane isomers mixtures on the tandem columns of Ni-formate and CAU-17 at 303 K with a carrier gas of 3 mL/min.

Description

It has been surprisingly found that Bi-based MOFs are useful in the separation of linear and lower-branched alkane isomers (and/or xylene isomers) from one another. The separation process using Bi-based MOFs and metal-format MOFs may be useful for gasoline upgrading with advantages over methods using zeolite 5 A. It has also been surprisingly found that the MOFs of the present invention provide highly efficient separation of hexane isomers by combining molecular recognition and size-sieving.

In embodiments herein, the word “comprising” may be interpreted as requiring the features mentioned, but not limiting the presence of other features. Alternatively, the word “comprising” may also relate to the situation where only the components/features listed are intended to be present (e.g. the word “comprising” may be replaced by the phrases “consists of” or “consists essentially of”). It is explicitly contemplated that both the broader and narrower interpretations can be applied to all aspects and embodiments of the present invention. In other words, the word “comprising” and synonyms thereof may be replaced by the phrase “consisting of” or the phrase “consists essentially of’ or synonyms thereof and vice versa. The phrase, “consists essentially of” and its pseudonyms may be interpreted herein to refer to a material where minor impurities may be present. For example, the material may be greater than or equal to 90% pure, such as greater than 95% pure, such as greater than 97% pure, such as greater than 99% pure, such as greater than 99.9% pure, such as greater than 99.99% pure, such as greater than 99.999% pure, such as 100% pure.

As used herein, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a composition” includes mixtures of two or more such compositions, reference to “an oxygen carrier” includes mixtures of two or more such oxygen carriers, reference to “the catalyst” includes mixtures of two or more such catalysts, and the like.

In a first aspect of the invention, there is disclosed a method of separating hydrocarbons from each other, the method comprising the steps of:

(a) providing a metal organic framework (MOF) formed from a metal ion and/or metal cluster and an organic linker; and

(b) passing a fluid stream comprising a mixture of hydrocarbons over the MOF to cause separation of the hydrocarbons, wherein the metal ion and/or metal cluster is formed from Bi ³⁺ ; and the linker is selected from: benzene-1 ,3,5-tricarboxylic acid, wherein the MOF adopts either a Ull-200 or a CALI-17 structure; ellagic acid, wherein the MOF adopts a Sll-101 structure;

As will be appreciated, each of the metal-organic frameworks (MOFs) mentioned herein are materials that comprise bonds (i.e., coordination bonds) between metal cations and multidentate organic linkers, so as to form a porous structure with a plurality of cavities within each MOF.

In particular embodiments of the invention that may be mentioned herein, the metal ion and/or metal cluster may be Bi ³⁺ .

In particular embodiments of the invention that may be mentioned herein, the organic linker may be benzene-1 ,3,5-tricarboxylic acid, wherein the MOF adopts either a Ull-200 or a CAU- 17 structure. As will be appreciated, the fluid stream may be in the form of a vapour stream and/or a liquid stream. Unless otherwise specified, the fluid stream may be a mixture of any type of hydrocarbons. For example, the fluid may be a mixture of alkane isomers, a mixture of xylene isomers, or a mixture of alkane isomers and xylene isomers.

More particularly, the fluid may be a mixture of xylenes and linear and branched alkanes. More particularly, it may be a mixture of xylenes or a mixture of linear and branched alkanes. More particularly, it may be a mixture of hexanes. Surprisingly, the method allows the separation of xylenes or linear alkanes (or less-branched alkanes) from more branched alkanes, as demonstrated in the examples discussed herein. This separation may allow for the generation of particular alkane mixtures that may be useful in fuels.

Specific xylenes mixtures that may be contemplated for use in the method include:

(i) o-xylene and m-xylene;

(ii) o-xylene and p-xylene;

(iii) m-xylene and p-xylene; and

(iv) o-xylene, p-xylene and m-xylene.

In particular embodiments of the invention that may be mentioned herein, the mixture of hexanes may be a mixture of two or more of n-hexane (nHEX), 3-methylpentane (3MP), 2- methylpentane (2MP), 2,3-dimethylbutane (23DMB), and 2,2-dimethylbutane (22DMB).

Specific hexanes mixtures that may be contemplated for use in the method include:

(a) nHex and 22DMB;

(b) 3MP and 22DMB;

(c) nHex, 3MP, and 22DMB;

(d) nHex, 2MP, 3MP, and 22DMB; and

(e) nHEx, 3MP, 2MP, 23DMB and 22DMB.

As will be appreciated, when the mixture of hydrocarbons is a mixture of alkane isomers and xylene isomers, then it may be selected from any one of (a) to (e) with any one of (i) to (iv) described above.

As noted hereinabove, the n-hexane and lower-branched (e.g. 3MP and 2MP) hexane isomers may be adsorbed by the MOF, while the higher-branched hexane isomers are rejected (e.g. 22DMB), thereby allowing separation. The MOF may be presented in any suitable form. For example, the MOF may be presented as particles or it may be presented as a fixed bed format or as a simulated moving bed arrangement.

The temperature of step (b) of the above method may be from 298 to 450 K, such as from 300 to 400 K, such as from 303 to 343 K, such as about 323 K.

For the avoidance of doubt, it is explicitly contemplated that where a number of numerical ranges related to the same feature are cited herein, that the end points for each range are intended to be combined in any order to provide further contemplated (and implicitly disclosed) ranges.

Thus, the following temperature ranges are explicitly contemplated: from 298 to 300 K, from 298 to 303 K, from 298 to 323 K, from 298 to 343 K, from 298 to 400 K, from 298 to 450 K; from 300 to 303 K, from 300 to 323 K, from 300 to 343 K, from 300 to 400 K, from 300 to 450 K; from 303 to 323 K, from 303 to 343 K, from 303 to 400 K, from 303 to 450 K; from 323 to 343 K, from 323 to 400 K, from 323 to 450 K; from 343 to 400 K, from 343 to 450 K; and from 400 to 450 K.

In particular embodiments of the invention that may be mentioned herein, the above method may further comprise passing the fluid stream over a metal formate MOF before step (b). For example, the metal formate MOF may be one or more of magnesium formate and nickel formate. More particularly, the metal formate MOF may be nickel formate.

It has been found that a metal formate MOF may be useful in such separation methods, as it may be able to selectively adsorb linear alkanes over branched alkanes. Thus, in a further aspect of the invention, there is provided a method of separating alkanes comprising the steps of:

(a) providing a metal organic framework (MOF) comprising magnesium or, more particularly, nickel as the metal ion and/or metal cluster and formic acid as the linker; and

(b) passing a fluid stream comprising a mixture of alkanes over the MOF to cause separation of the alkanes. In particular embodiments of the invention that may be mentioned herein, the MOF may comprise nickel as the metal ion and/or metal cluster.

The mixture of alkanes may be a mixture of alkane isomers. More particularly, the alkanes may be a mixture of linear and branched alkanes. In particular embodiments of the invention that may be mentioned herein, the alkanes may be the hexane isomer mixtures mentioned above. As such, the nickel formate MOF may adsorb linear alkanes (e.g. n-hexane) in preference to mono- or di-branched alkane isomers.

As will be appreciated, the two methods disclosed above may be combined together. This may be particularly useful in the manufacture of alkane isomers with a RON of greater than 96. This combined method may be depicted in FIG. 1. In this method, an alkane mixture may first be passed through the nickel formate MOF, adsorbing the linear alkanes (e.g. n-hexane) and allowing the branched isomers to pass into the Bi MOFs disclosed herein, which adsorbs the lower-branched (i.e. momo-branched) alkane isomers (e.g. 2MP or 3MP), thereby allowing 22MB and 23MB to be separated from the mixture. The adsorbed materials can then be released from the nickel formate and Bi MOFs and subjected to an isomerisation reaction to generate an alkane mixture that contains higher-branched alkanes for resubmission to the separation method, thereby allowing the majority of the alkane source mixture to be upgraded to the desired RON value over several cycles.

As will be appreciated, the present invention provides the following:

• two robust and highly porous MOFs, Ull-200 (ULI = Uppsala University) and CAU-17 (CAU = Christian Albrechts University) constructed by cheap trimesic acid (H3BTC) and Bi(NOs)3 are obtained by solvothermal synthesis and microwave-assisted solvothermal method, respectively. These raw materials are generally nontoxic, cheap and easily available in nature, which may greatly decrease the cost and so use of this method is desirable for large-scale production;

• UU-200 exhibits a specific auxetic grid structure with a reentrant honeycomb-like pore cavity and a maximum pore size of 5.0 A. UU-200 can separate alkane isomers by combining molecular recognition and size-sieving. It can completely reject di-branched alkanes while exhibiting high capacities for linear and mono-branched isomers;

• CAU-17 exhibits a hierarchical pore structure with three different pore channels, matching the different molecular sizes of alkane isomers, thus leading to excellent adsorption and separation performance. All the three pore channels can adsorb linear alkanes, mono-branched isomers can be adsorbed in the two larger pores and di- branched ones can only be adsorbed in the largest pores, thus leading to a hierarchical upgrading of alkane isomers and high capacity and selectivity for linear and monobranched isomers;

• nickel-formate material is found to be an alternative material to zeolite 5A, which can adsorb linear alkanes with di-branched isomers excluded;

• the gasoline upgrading with RON higher than 96 can be easily realized via the adsorption and separation process using CALI-17 and Ull-200 as the adsorbents; and

• an adsorptive separation process for alkane isomers or gasoline upgrading is proposed using Ni-formate, Ull-200, and CALI-17 materials.

As will also be appreciated, the present invention provides gasoline upgrading using Bi-based MOFs, which are cheap and can be easily synthesized and scaled up. By controlling the pore shape and size in Bi-based MOFs, the separation of alkane isomers, especially hexane isomers, is realized with high efficiency. The RON value of the resultant products can reach higher than 96. Additionally, the Bi-based MOFs show excellent capacities and separation selectivities for linear and mono-branched alkane isomers with di-branched ones excluded. The Bi-based MOFs set new benchmarks for alkane separations using porous materials. Furthermore, a novel separation process for gasoline upgrading using MOFs has been proposed in the present disclosure. Thus, the present disclosure not only provide a new perspective on the optimal pore chemistry of porous materials for alkane isomer separation, but also sheds light on how to design efficient materials for hydrocarbon separations.

Further aspects and embodiments of the invention will now be discussed by reference to the following non-limiting examples.

Examples

Materials

All reagents were purchased from commercial sources and used without further purification.

Analytical techniques

PXRD analyses

PXRD patterns were collected using a Rigaku Miniflex 600 diffractometer (Cu Ka = 1.540598 A) with an operating power of 40 kV, 15 mA, and a scan rate of 2.0 ° min' ¹ . The data were collected in a two-theta range of 5-40°. SEM

Samples for SEM observation were conducted on a JEOL JSM-6701 F Field Emission Scanning Electron Microscope and operated at 10 kV. The samples were coated on conducting resin by using a toothpick.

Example 1. Synthesis of UU-200

Ull-200 was obtained by using a previously reported procedure (M. Ahlen et al., Micropor. Mesopor. Mater. 2022, 329, 111548). Typically, Bi(NO ₃ )3'5H ₂ O (1 mmol, 0.485 g) and H ₃ BTC (4.6 mmol, 0.960 g) were mixed with 25 mL N,N-dimethylformamide (DMF), and the mixture was stirred for 10 min at room temperature. Then, the mixture was transferred to a 50 mL Teflon-lined steel autoclave and heated at 140 °C for 72 h. The resulted gray powders were isolated and washed with ethanol (20 mL) three times, leading to UU-200.

Results and discussion

Herein, we proposed a unique strategy to efficiently separate hexane isomers using Bi-based MOFs by tuning the pore structures with different pore shapes or channels. The Bi-based MOFs can be easily obtained by solvothermal and microwave-assisted synthesis, indicating that Bi-based MOFs can be easily scaled up in the industry. In addition, the synthesis conditions are relatively mild (e.g. temperatures below 150 °C and reaction time within 30 min), which is beneficial for industrial scale-up.

Example 2. Characterisation of UU-200

As described in Example 1 , UU-200 nanowhiskers (FIG. 2) was synthesized from Bi ³⁺ and benzene-1 ,3,5-tricarboxylic acid (H3BTC) via solvothermal method (FIGS. 3a and 3b) (M. Ahlen et al., Micropor. Mesopor. Mater. 2022, 329, 111548).

The crystal structure of UU-200 exhibits a Pnnm space group. Interestingly, there are two kinds of Bi-clusters in its crystal structure (FIG. 3a). Bi-I is formed by combining BiOg and BiOw nodes, both with O atoms coming from five BTC linkers (FIG. 4). In contrast, Bi-ll is formed by two equivalent Bi ³⁺ connected with 10 O atoms from six BTC linkers (FIG. 5). Although the stoichiometry of both Bi-I and Bi-ll is Bi20i6 with O contributed from eight BTC linkers, the configurations of BTC linkers are much different. UU-200 is formed by connecting two kinds of Bi-clusters with BTC linkers into a three-dimensional (3D) framework (FIG. 3b), exhibiting an auxetic grid structure (FIG. 3c) with a reentrant honeycomb-like pore cavity (FIG. 3d). Each pore cavity has eight triangular pore windows with a maximum pore size of ca. 5.0 A (FIG. 3d), which is similar to the diameter of zeolite 5A. Detailed analysis shows two kinds of binding sites in the corner of each pore cavity (FIGS. 3e and 6). Viewed from the c axis, we can see that the front and back pore cavities are connected by the triangular restricted pore channels (FIG. 3f). Furthermore, from b direction, the staggered pore cavities are interconnected (dotted lines in FIG. 3f), forming a 3D connected pore channel (FIG. 3g). Such a specific pore structure with tandem pore cavities interconnected by 1 D pore channels (FIG. 3g) would facilitate the guest diffusion and endow the material with the ability to accommodate large quantities of guest molecules. Furthermore, the narrow triangular pore window may serve as a gate to empower the material with a sharp size cut-off ability for different molecules, leading to high selectivity. Without wishing to be bound by theory, the unique auxetic structure with reentrant honeycomb-like pore cavities connected by narrow pore windows endows II LI- 200 with a complete rejection of di-branched alkanes and high capacities for linear and monobranched isomers.

Example 3. Porosity of the as-obtained UU-200

Gas sorption experiments

The adsorption isotherms of vapor phase C6 isomers were collected at different temperatures on a Quantachrome Autosorb iQ3 instrument equipped with commercial software for data calculation and analysis. The test temperatures were controlled by soaking the sample cell in a circulating water bath (303 to 343 K). Before measurement, the sample (~80 mg) was degassed at 393 K for 12 h. CO2 and N2 isotherms at 196 K and 77 K, respectively, were collected on a Micromeritics ASAP2020 instrument. The measurement temperatures were maintained at 196 and 77 K using dry ice/acetone and liquid nitrogen, respectively.

TGA

TGA were performed on a TA instruments STD-600 equipment at a heating rate of 10 °C mim ¹ with a N2 flow rate of 50 mL min’ ¹ . The sample holders were alumina crucibles, and the amount of sample used in each measurement was 8 (± 2) mg. The data collected were analyzed using Universal Analysis software (version 4.4A) from TA Instruments.

Results and discussion

The permanent porosity of the as-obtained UU-200 was confirmed by CO2 sorption isotherm at 196 K (FIG. 7). The typical Type-I isotherm indicates microporosity of UU-200, and the resultant Langmuir surface area is 482 m ² g ^_1 . The consistency of the PXRD patterns before and after exposing UU-200 to strong acid and base solutions and even in the air for half a year proves the exceptional chemical robustness of UU-200 (FIGS. 8 and 9). The thermal stability test shows that Ull-200 can be stable up to 390 °C (FIG. 10), indicating its desirable stability for industrial applications. The Bi-based MOFs with high porosity and stability can show high capacities for linear, mono-branched isomers with di-branched isomers rejected.

Inspired by the narrow pore window of Ull-200 comparable with the size of hexane isomers (4.3-6.2 A), single-component vapor phase linear, mono-, and di-branched alkane sorption isotherms were collected (FIGS. 11a and 11b). The adsorption capacities of UU-200 for nHEX, 3MP, and 22DMB follow the order of nHEX > 3MP > 22DMB. Notably, Ull-200 presents a sharp size cut-off property for 22DMB. Furthermore, Ull-200 exhibits excellent molecular recognition ability to differentiate nHEX from 3MP with much different sorption capacities, indicating its potential for hexane isomer separation. The uptakes of nHEX, 3MP, and 22DMB on Ull-200 at 110 Torr and 303 K are 146, 98, and 8 mg g- ¹ , respectively. Such a high capacity of nHEX is comparable to state-of-the-art materials like Al-bttotb (151 mg g- ¹ , L. Yu et al., J. Am. Chem. Soc. 2020, 142, 6925-6929) and Ca(H ₂ tcpb) (150 mg g- ¹ , H. Wang et al., Energy Environ. Sci. 2018, 11, 1226-1231), and much higher than that of Co-formate (129 mg g- ¹ , H. Wang et al., Chem. Eur. J. 2021 , 27, 11795-11798). The resultant uptake ratios of nHEX/22DMB and 3MP/22DMB are 18.3 and 12.3, respectively (FIG. 11c), much higher than those on Ca(H ₂ tcpb) (1.3 and 1.5), HIAM-302 (9.2 and 5.1 , L. Yu et al., J. Am. Chem. Soc. 2022, 144, 3766-3770), CAU-10 (2.4 and 1.6, Q. Yu et al., Sep. Purif. Technol. 2021 , 268, 118646), MOF-303 (1.9 and 1.5), and MIL-160 (1.6 and 1.5) (FIGS. 13 and 14). To the best of our knowledge, the uptake ratios of nHEX/22DMB and 3MP/22DMB on Ull-200 represent the highest values among all the reported materials under similar conditions (Z. Zhang et al., EnergyChem 2021 , 3, 100057).

As will be appreciated, the present invention has high capacity for linear and mono-branched alkanes, which can reduce the regeneration time of porous MOFs in industry, thus can cut-off the energy input, indicating reduced costs of gasoline production.

Example 4. Molecular recognition abilities and binding affinities

To illustrate the different molecular recognition abilities and binding affinities for nHEX and 3MP, we calculated the isosteric heats of adsorption (Q _S f) of nHEX and 3MP on Ull-200.

I AST selectivity calculations

The isotherm data for alkane isomers were fitted with dual-site Langmuir-Freundlich isotherm model (K. S. Walton & D. S. Sholl, AIChE J. 2015, 61, 2757-2762; and A. L. Myers & J. M. Prausnitz, AIChE J. 1965, 11, 121-127): _ b _A p ^v A _f b _B p ^v B - riA.sat _1+bAp v _A + B,sat _{1 +bgp} v _B with T-dependent parameters b b = b _o exp(— Ki)

The adsorption selectivity for CO2/C2H2 separation is defined by: where qi and <72 are the molar loadings in the adsorbed phase in equilibrium with the bulk gas phase with partial pressures pi and P2.

Table 1. Dual-site Langmuir-Freundlich parameter fits for nH EX, 3MP, and 22DMB isotherms on UU-200 at 303 K.

Site A Site B

PA, sat bA VA qB.sat BB VB mol kg- ¹ Pa ^-vA mol kg- ¹ Pa ^-vB nHEX 1.2898 0.9833 0.1789 1.2898 0.9833 0.1789

3MP 0.6873 1.8185 0.3038 0.6873 1.8185 0.3038

22DMB 0.07441 0.09780 1.5809 0.03086 17.7724 0.9697

Isosteric heat of adsorption calculations

A virial-type expression comprising the temperature-independent parameters a, and bj was employed to calculate the heat of adsorption for different gases (at different temperatures). In each case, the data were fitted using the equation (K. J. Chen et al., Chem 2016, 1, 753-765; and Z. Zhang et al., Angew. Chem. Int. Ed. 2020, 59, 18927-18932):

Here, P is the pressure expressed in Pa, N is the amount adsorbed in mmol g- ¹ , T is the temperature in K, a, and bj are virial coefficients, and m, n represent the number of coefficients required to adequately describe the isotherms.

The values of the virial coefficients a ₀ through a _m were then used to calculate the isosteric heat of adsorption using the following expression:

Qst = ~R & Gj Ni (5) where Q _s t is the coverage-dependent isosteric heat of adsorption and R is the universal gas constant.

Results and discussion To illustrate the different molecular recognition abilities and binding affinities for nHEX and 3MP, we calculated the isosteric heats of adsorption (Q _S f) of nHEX and 3MP on Ull-200 (FIG. 18). The estimated Qst value is 53.3 kJ mol' ¹ for nHEX, higher than that of 3MP (44.9 kJ mol' ¹ ), indicating the different binding affinities for nHEX and 3MP. Furthermore, the potential separation selectivities of Ull-200 for hexane isomers were quantitatively evaluated by I AST calculations. Ull-200 shows unprecedented high separation selectivities of 1 x 10 ⁵ , 3 x 10 ⁴ , and 160 for equimolar nHEX/22DMB, 3MP/22DDMB, and nHEX/3MP mixtures, respectively (FIG. 11 d). To further prove the excellent separation potential of Ull-200 for hexane isomers, the adsorbate was extended to five components, namely nHEX, 2MP, 3MP, 23DMB, and 22DMB (FIG. 19). The uptake of 2MP is 106 mg g- ¹ at 110 Torr, similar to that of 3MP, which may be due to their similar molecular sizes. The uptake of 23DMB is only 31 mg g- ¹ , much lower than that of linear and mono-branched isomers. The presented results consistently manifest that Ull-200 exhibits an excellent molecular recognition and sieving ability to distinguish the linear, mono-, and di-branched hexane isomers.

Example 5. Feasibility of UU-200 for separating C6 isomers

To illustrate the actual separation performance, the dynamic separation performance for multicomponent vapor-phase alkane mixtures was evaluated.

Breakthrough experiments

The breakthrough experiments were performed using a home-built dynamic gas breakthrough setup. The experiment was conducted using a stainless-steel column (4.6 mm inner diameter x 50 mm length). Before the breakthrough experiment, the column packed with UU-200 (0.41 g) was firstly activated with a He flow (5 mL min' ¹ ) at 423 K for 12 h. After activation, a He flow at a rate of 1.0 mL min' ¹ was bubbled through a mixture of hexane isomers with the following volumes: 5.84 mL of nHEX, 4.12 mL of 3MP, and 2.57 mL of 22DMB for nHEX/3MP/22DMB ternary mixture; 2.67 mL of 22DMB, 3.50 mL of 23DMB, 3.79 mL of 2MP, 4.22 mL of 3MP, and 5.82 mL of nHEX for a mixture of all five isomers. The outlet gas from the column was monitored by an online Agilent 8890 Gas Chromatography (GC) system with flame ionization detection. The experiments were done when all the isomer contents detected by GC reached equilibrium. The column was regenerated at 423 K for 24 h by purging Helium gas with a flow rate of 5 mL min' ¹ . This is the one cycle for the breakthrough experiment. Then, we repeated the breakthrough experiment to evaluate the cycling performances.

The experiments for the separation of xylene isomers are the same as the methods for C6 separation except liquid C6 isomers are replaced with liquid xylene isomers. Results and discussion

To evaluate the actual feasibility of Ull-200 for separating C6 isomers, we conducted fixed- bed column breakthrough experiments using an equimolar mixture of nHEX, 3MP, and 22DMB flowing through a UU-200-packed column (FIG. 11e). Clear separation of the isomers was achieved, and 22DMB broke out immediately upon the mixture being introduced. On the other hand, 3MP and nHEX were retained for 39 and 92 min, respectively, leading to a clear step- changed real-time RON. The results show that the initial RON is higher than 92 until 3MP penetrated throughout the fixed bed. The large time lag between the three isomers agrees well with their adsorption isotherms. The calculated dynamic separation selectivities of nHEX/22DMB and 3MP/22DMB were 1790 and 916, respectively. Notably, the three- component separation performance of Ull-200 is much better than that of MIL- 160 and MOF- 303 because of the complete rejection of di-branched 22DMB (FIGS. 20 and 21). Furthermore, a five-component fixed-bed breakthrough experiment revealed that both di-branched alkanes with high octane numbers could be completely isolated from the isomers, which may be due to the low uptakes of di-branched isomers and competitive adsorption of linear and monobranched isomers to di-branched ones resulting from the thermodynamic effect (FIG. 11f). The real-time RON can be higher than 96 before 3MP came out of the column (31 min), which is much better than the performance of Al-bttotb (L. Yu et al., J. Am. Chem. Soc. 2020, 142, 6925-6929). The productivities of 92+ streams from three- and five-component mixtures on Ull-200 were calculated to be 31 and 37 L kg- ¹ . The recycling breakthrough experiments showed the excellent stability of Ull-200 for alkane isomer separations (FIG. 23). Overall, the highly efficient separation performance, the highest uptake ratios, and excellent separation selectivities unanimously show that Ull-200 sets a benchmark for separating hexane isomers under ambient conditions.

Thus, the molecular sieving effect, unprecedented separation selectivities, and excellent efficiencies are proved via adsorption isotherms and breakthrough experiments with high research octane numbers (>96) obtained, indicating a benchmark for alkane separation under ambient conditions.

Example 6. Mechanism of nHEX and 3MP adsorption on UU-200

The rejection of di-branched isomers from the pore channels is indisputable due to the size sieving effect.

FTIR The FTIR spectra were recorded using an FTIR spectrometer (Bruker VERTEX 70-FTIR). Before each experiment, the sample (~100 mg) was pretreated under high vacuum conditions (< 3 pmHg) at 393 K for 6 h, and then cooled to room temperature. The sample was soaked in liquid C6 isomers for one week, then isolated and put into a drying oven at 333 K for 1 h to ensure that the surface-adsorbed C6 isomers were removed. Then, the sample was used for FTIR measurements. All the spectra were recorded over accumulative 32 scans with a resolution of 4 cm' ¹ in the range of 4000-400 cm ^-1 .

Results and discussion

To explore the mechanism of nHEX and 3MP adsorption on Ull-200, we investigated the FTIR spectra of the nHEX- and 3MP-loaded Ull-200 (FIG. 24). The new peaks at 2960 and 2810 cm' ¹ , resulting from C-H stretching of -CH3 and -CH2- groups, clearly show that nHEX and 3MP were trapped within Ull-200. The shift of -C=C- stretching peaks from 1555 and 1434 cm' ¹ to 1548 and 1429 cm' ¹ , respectively, and the C-0 bending peak at 440 cm' ¹ shifting to 430 cm' ¹ demonstrated that there are strong interactions between guest molecules with benzene rings and O atoms on BTC linkers. Additionally, for the nHEX-loaded sample, the C- H plane vibration peak at 624 cm' ¹ on benzene rings shifted to 618 cm' ¹ , which may be due to the strong host-guest interactions between nHEX with C or H atoms on benzene rings.

Example 7. Density functional theory (DFT) calculations

We conducted density functional theory (DFT) calculations to obtain insights into the excellent molecular recognition ability of Ull-200 for linear and mono-branched isomers.

DFT calculations

First-principles density functional theory (DFT) calculations were performed in Castep software (BIOVIA Materials Studio) (Z. Zhang et al., Angew. Chem. I nt. Ed. 2017, 56, 16282- 16287). A semi-empirical addition of dispersive forces to conventional DFT was included in the calculation to account for van der Waals interactions. Vanderbilt- type ultra-soft pseudopotentials and generalized gradient approximation with Perdew-Burke-Ernzerhof exchange correlation were used. A cutoff energy of 590 eV and a 1 x 1 x 2 -point mesh (generated using the Monkhorst-Pack scheme) were found to be enough for the total energy to converge within 0.01 meV atom' ¹ . The structures of the synthesized materials were firstly optimized, then various guest gas molecules were introduced to various locations of the channel pore, followed by a full structural relaxation. To obtain the gas binding energy, an isolated gas molecule placed in a supercell (with the same cell dimensions as the material crystal) was also relaxed as a reference. The static binding energy (at T = 0 K) was then calculated using: EB = E(MOF> + E( _gaS ) - E(MOF + gas>-

Results and discussion

The results clearly show that each pore cavity can accommodate four guest molecules, and each binding site interacts with one molecule. The guest molecules exhibit different configurations on the two sites (FIGS. 25-31). Due to the limited pore environment, there are strong interactions between the guest molecules and the organic linkers. Detailed analysis of the configurations of nHEX on site I indicates that nHEX was grasped by multiple C-H -0 Flbonds with distances of 2.86-3.94 A and C-H -TT interactions with distances of 3.09-3.80 A (FIGS. 25b and 27). In contrast, for site II, it can be found that the head -CH3 of nHEX was trapped by O atoms from BTC linkers via strong H-bonds (2.65-3.42 A) and C-H -TT interactions (3.05-3.52 A) with the benzene rings (FIGS. 25c and 28). There are also weak interactions between nHEX with C and H atoms on benzene rings with distances of 3.20-3.36 and 2.71-2.77 A, respectively (FIG. 25c), which is in accordance with the FTIR results. Thus, the molecular recognition mechanism was unveiled by theoretical simulation and in situ FTIR spectroscopy.

The binding energy (Z1E) for two nHEX was calculated to be 58 and 51 kJ mol ^-1 at sites II and I, respectively. Similarly, 3MP at site I was parallel to the pore cavity like nHEX on site I, and 3MP was grasped by multiple C-H -0 H-bonds (2.78-3.80 A) and C-H -TT interactions (2.96- 3.74 A, FIGS. 25e and 30). For site II, 3MP was perpendicularly trapped with two sides grasped by the pore windows via C-H -0 H-bonds and C-H -TT interactions. The pore surface shows interactions with -CH2- in the 3MP body similarly via C-H -0 H-bonds and C-H -TT interactions (3.18 to 3.82 A, FIGS. 25f and 31). The pore cavity shows similar binding energy for two 3MP molecules (46 and 43 kJ mol' ¹ for sites I and II, respectively), lower than that of nHEX. This result indicates stronger binding affinities of Ull-200 toward nHEX, which is consistent with the adsorption isotherms and breakthrough experiments. Thus, the combination of excellent molecular recognition and size-sieving effect endows Ull-200 with the benchmark separation performance for hexane isomers.

Therefore, herein, we report the highly efficient separation of linear, mono-, and di-branched hexane isomers using a cheap Bi-based MOF, Ull-200 with auxetic grid structures, via a synergistic effect of molecular recognition and size-sieving. Ull-200 exhibits reentrant honeycomb-like pore cavities connected via narrow triangular pore windows, endowing it with high capacities for nHEX and 3MP. In contrast, the restricted pore windows exert a sharp size cut-off effect for di-branched isomers. As will be appreciated, those structural features result in Ull-200 with unprecedented separation efficiency and selectivities for linear, mono-, and dibranched hexane isomers, which were proved by sorption isotherms and dynamic breakthrough experiments. Furthermore, the molecular recognition mechanism was verified by FTIR and molecular simulations.

Example 8. Bi-based MOF of CAU-17

Synthesis of CAU-17

H3BTC (130 mg, 0.6 mmol) and Bi(NOs)3-5H2O (250 mg, 0.52 mmol) were mixed with 10 mL methanol at room temperature. The sealed glass vial was stirred for 5 min and subsequently heated in the microwave oven to 120 °C for 30 min to give CAU-17. CAU-17 was characterized by following the protocols in Examples 3 and 4.

Example 9. Characterisation of CAU-17

CAU-17 exhibits three pore channels with different pore sizes and shapes, which can accommodate different guest molecules of different sizes, thus leading to hierarchical adsorption for multicomponent mixtures. In particular, nHEX can be adsorbed in all three pores - mono-branched can reside in two larger pores, but di-branched ones can only be adsorbed in the largest pores, leading to hierarchical adsorption.

The TGA curves indicate that UU-200 and CAU-17 are thermally stable up to around 400 °C (FIG. 32).

Example 10. Metal-formate MOFs

Metal nitrate (1 mmol, 291 mg of Ni(NOs)2-6H2O or 148 mg of Mg(NOs)2) was added into 10 mL of methanol, and then with a few drops of formic acid (600 pL) added, the mixture was heated to 120 °C for 72 h.

Example 11. Evaluation of hexane isomer adsorption performance

The hexane isomer adsorption performance of CAU-17 prepared in Example 8 and Mg- formate and Ni-formate prepared in Example 10 were evaluated by following the protocols in Examples 3 and 4.

Results and discussion The results show that all materials can reject 22DMB. Ull-200 exhibits the highest uptake ratio for nHEX/22DMB and good molecular recognition abilities to differentiate linear and monobranched isomers, indicating its great potential to separate hexane isomers via molecular recognition and size-sieving (FIGS. 11a and 11 b). CALI-17 shows the highest adsorption capacity for nHEX at ambient conditions. The adsorption isotherms exhibit a hierarchical order of nHEX > 2MP > 3MP > 23DMB > 22DMB, which may be due to the different pore channels playing different roles to accommodate guest molecules with different sizes (FIG. 33). Ni- formate exhibits a high capacity for nHEX with all the branched isomers excluded, indicating comparable performance with zeolite 5A (FIGS. 34c and 34d).

Example 12. Hexane isomer separation performance

To illustrate the actual separation performance, the dynamic separation performance for multicomponent vapor-phase alkane mixtures was evaluated on CALI-17 prepared in Example 8 and Mg-formate and Ni-formate prepared in Example 10, by following the protocol in Example 5.

Results and discussion

The results indicate that CAll-17 can retain excellent separation even with different carrier gas flow rates (FIG. 35). The results show that Ull-200 and CALI-17 can efficiently separate alkane isomers with high RON obtained (higher than 96) (FIGS. 11e, 11f and 35). Ni-formate can only trap nHEX with all branched isomers rejected, which is similar to the performance of zeolite 5A (FIG. 37). These results are consistent with the adsorption isotherms on different materials. After combining Ni-formate with CALI-17 fixed beds, the dynamic breakthrough experiments present that high productivity of premium gasoline can be collected (FIG. 38).

Based on the results, we proposed an adsorptive separation process for gasoline upgrading using Ni-formate and CALI-17 or Ull-200 to replace the current energy-intensive distillation process in the industry (FIG. 1).

The Bi-based MOFs with much different pore shapes and chemistry are promising porous materials for gasoline upgrading, which can greatly reduce the energy input and capital investment in industry since the porous materials are cheap, and the adsorptive separation process disclosed herein is energy-efficient and cost-effective. Ull-200 exhibits reentrant honeycomb-like pore cavities connected via narrow triangular pore windows, endowing UU- 200 with high capacities for nHEX and 3MP, while the restricted pore window exerts a sharp size cut-off effect for di-branched isomers. As will be appreciated, the exclusion of di-branched isomers is advantageous because premium gasoline with high RON can be obtained, indicating reduced costs of gasoline production.

CALI-17 exhibits outstanding gasoline upgrading performance via hierarchical adsorption. Both two materials show high capacities and high separation selectivities for linear and monobranched isomers. The present disclosure not only sheds light on the construction of suitable pore structures in advanced porous materials using pore engineering, but also provides efficient ways to cut off the energy input and reduce the cost for energy-efficient gasoline upgrading and other isomer separations in the industry. For example, the adsorptive separation process disclosed herein may be an alternative process to traditional energy- intensive distillation processes in the industry, due to its low energy input and low carbon emission.

Additionally, the main challenge for the industrial applications of MOFs is the cost and whether they can be scaled up using low-cost raw materials under mild conditions. In the present disclosure, Bi-based MOFs were easily fabricated using cheap and abundant raw materials under mild conditions, and kilogram-scale production of adsorbents can be realized. Furthermore, an alternative material to zeolite 5A, Ni-formate, is revealed in the present disclosure.

Comparative Example 1

Table 2. The adsorption performance of the state-of-the-art materials for hexane isomers at 303 K and 110 Torr.

Materials nHEX 3MP uptake 22DMB Reference uptake (mg g ^_1 ) uptake

(mg g- ¹ ) (mg g- ¹ )

Ull-200 146 98 8 The present disclosure

MOF-303 328 252 173.4 The present disclosure

MIL- 160 242 240 155 The present disclosure

Ca(H ₂ tcpb) 150 170 112 H. Wang et al., Energy

Environ. Sci. 2018, 11, 1226-1231

Co-fomate 129 146 25 H. Wang et al., Chem.

Eur. J. 2021 , 27, 11795-

11798 Al-bttotb 151 94 10 L. Yu et al., J. Am. Chem.

Soc. 2020, 142, 6925- 6929

CAU-10 144 97 59 Q. Yu et al., Sep. Purif.

Technol. 2021 , 268,

118646

CAU-10-Br 47 12 5 Q. Yu et al., Sep. Purif.

Technol. 2021 , 268,

118646

CAU-10-H/Br 128 65 9 Q. Yu et al., Sep. Purif.

Technol. 2021 , 268,

118646

Zeolite 5A 146 28 28 Q. Gong et al., Sep. Purif.

(298 K) Technol. 2022, 294,

121219

HI AM-302 165 92 18 L. Yu et al., J. Am. Chem.

(298 K) Soc. 2022, 144, 3766-

3770

In the present disclosure, we report a Bi-based MOF, Ull-200, for efficient C6 alkane separation via molecular recognition and size-sieving. Ull-200 was chosen because of its auxetic pore structure, featuring large reentrant honeycomb-like pore cavities restricted by narrow pore windows with pore size of ca. 5 A. The specific pore cavity endows Ull-200 with high capacities for linear and mono-branched hexane isomers, while the narrow pore windows reject the di-branched isomers via molecular sieving, leading to excellent separation selectivities for linear or mono-/di-branched hexane mixtures. The abundant benzene rings and oxygen atoms on the pore surface function as effective molecular recognition sites to differentiate linear and mono-branched hexane isomers. The efficiency of Ull-200 was evidenced by vapor sorption isotherms and dynamic breakthrough experiments with product RON values higher than 96 obtained, setting a benchmark for C6 isomer separation. The present disclosure not only provides the insights to design suitable pore chemistry for alkane separation, but also broadens the applications of underexplored Bi-based porous materials in separation technology. In addition, the present invention may open new avenues towards developing novel porous materials with optimal pore structures and energy-efficient techniques for alkane isomer separations.