Three-Dimensional Hydrogen-Bonded Porous Metal-Organic Framework for Natural Gas Separation with High Selectivity

A 3D hydrogen-bonded metal-organic framework, [Cu(apc)2]n (TJU-Dan-5, Hapc = 2-aminopyrimidine-5-carboxylic acid), was synthesized via a solvothermal reaction. The activated TJU-Dan-5 with permanent porosity exhibits a moderate uptake of 1.52 wt% of hydrogen gas at 77 K. The appropriate BET surface areas and decoration of the internal polar pore surfaces with groups that form extensive hydrogen bonds offer a more favorable environment for selective C2H6 adsorption, with a predicted selectivity for C2H6/CH4 of around 101 in C2H6/CH4 (5:95, v/v) mixtures at 273 K under 100 kPa. The molecular model calculation demonstrates a C–H···π interaction and a van der Waals host–guest interaction of C2H6 with the pore walls. This work provides a strategy for the construction of 3D hydrogen-bonded MOFs, which may have great potential in the purification of natural gas.


Introduction
Metal-organic frameworks, as self-assembled porous materials, have been widely explored in the last two decades due to their excellent performance in the areas of gas storage and separation [1,2], heterogeneous catalysis [3], drug delivery [4], luminescence [5][6][7], electrochemistry [8], and magnetism [9].The structure and properties of MOFs depend on the nature of their metal cations and bridging ligands.Designing MOFs with an eye for novel structures and utilities through the sagacious choice of various metal ions and ligands always constitutes one of the most intriguing research topics in chemistry and materials science [10][11][12].Among the factors that may influence the structures of MOFs, hydrogen bonding is capable of providing overall structural rigidity and diversity [13], and the stability of the corresponding molecular networks can be enhanced by augmenting the number or strength of the hydrogen bonds in which each tectonic subunit participates [14].However, hydrogen bonds are greatly affected by various factors, such as intermolecular distances, temperature, pressure, and solvents [15].As a result, reports concerning the design and synthesis of functional MOFs involving both coordination bonds and hydrogen bonds are rather scarce in the chemical literature [16,17].
Three-dimensional hydrogen-bonded MOFs are usually assembled based on coordinated hydrogen bond interactions.According to the structural characteristics of reported 3D hydrogen-bonded MOFs, they can be constructed in three ways (Figure 1).The first is that the metal and ligand form a 0D cluster including an MOP (metalorganic polyhedron) or a macrocycle including MOCs (metal-organic cubes) and MOSs (metalorganic squares), which are further linked together along three directions to generate 3D frameworks [18][19][20].Eddaoudi and Liu reported some hydrogen-bonded MOFs made with MOCs and MOSs [21][22][23][24] containing 3D hydrogen-bonding interactions.In this respect, MOC-2 and MOC-3, consisting of indium (III) and 4,5-dicyanoimidazole, were constructed from vertex-to-vertex hydrogen-bonded MOCs [21].The second is that 1D chains of metal and ligands are hydrogen-bonded along two orthogonal dimensions to give a 3D framework.Román synthesized a copper(II)-isophthalato MOF containing a 9-methyladenine nucleobase, exhibiting features of the second 3D hydrogen-bonded MOFs [25].The last choice is similar to pillar-layered structures, in which the 2D networks of a metal and ligand are further interconnected by hydrogen bonds to generate 3D frameworks, as shown by {[Zn(apc) 2 ]•H 2 O} n and DAT-MOF-1 [26,27].
vertex-to-vertex hydrogen-bonded MOCs [21].The second is that 1D chains of metal and ligands are hydrogen-bonded along two orthogonal dimensions to give a 3D framework.Román synthesized a copper(II)-isophthalato MOF containing a 9-methyladenine nucleobase, exhibiting features of the second 3D hydrogen-bonded MOFs [25].The last choice is similar to pillar-layered structures, in which the 2D networks of a metal and ligand are further interconnected by hydrogen bonds to generate 3D frameworks, as shown by {[Zn(apc)2]•H2O}n and DAT-MOF-1 [26,27].However, great synthetic challenges still exist in constructing 3D hydrogen-bonded MOFs due to the less tunable character of hydrogen bonds.Using preorganized 2D metal organic sheets assembled by hydrogen bonds may be an alternative strategy to solve the above morass, albeit encountering the quandary of selecting suitable metal centers as well as ligands.
With recent developments in MOFs and HOFs (hydrogen-bonded organic frameworks), the separation properties of MOF and HOF materials have attracted considerable research and commercial interest based on the conception of CO2 capture [28] and purification of natural gas [29], ethane [30], and butadiene [31].In fact, the high porosities of materials are not prerequisites for their gas separation applications.As reported in the literature, MOFs and HOFs exhibiting excellent performance for gas separations are those of moderate porosities, with BET surface areas less than 1000 m 2 /g [32].The design and synthesis of new porous MOF materials with hydrogen-bonding features may serve as a new concept to take advantage of the structural features of both materials with the aim of gas separation applications.Liu's group reported some hydrogen-bonded MOFs like ZSA-7, ZSA-8, and ZSA-9.ZSA-7 and ZSA-8 exhibit high CO2 adsorption abilities (109.8 and 114.0 cm 3 •g −1 at 273 K, respectively, under 1 bar) and outstanding natural gas selectivity separation, especially for CO2 over CH4 (39.8 and 35.7 for CO2/CH4 = 0.5/0.5 and 76.0 and 51.6 for CO2/CH4 = 0.05/0.95,respectively, under 1 bar at 298 K) [33].Natural gas is a hydrocarbon mixture primarily consisting of 85% CH4, 9% C2H6, 3% C3H8, 2% N2, and 1% C4H10.To reduce the percentage of ethane content, it is very important to purify natural gas.With CH4 as a premium choice for greenhouse effect reduction, using MOFs for the industrial separation of CH4 and C2H6 in natural gas becomes urgent and necessary.As a fruitful approach, selectivity enhancements in gas separation have been successfully carried out through the precise control of the pore size and environment of MOFs [34].However, the selectivity of C2H6 and CH4 is low in the reported MOFs, and it may be due to the inappropriate BET surface areas and weak interactions of C2H6 with the cavity surface However, great synthetic challenges still exist in constructing 3D hydrogen-bonded MOFs due to the less tunable character of hydrogen bonds.Using preorganized 2D metal organic sheets assembled by hydrogen bonds may be an alternative strategy to solve the above morass, albeit encountering the quandary of selecting suitable metal centers as well as ligands.
With recent developments in MOFs and HOFs (hydrogen-bonded organic frameworks), the separation properties of MOF and HOF materials have attracted considerable research and commercial interest based on the conception of CO 2 capture [28] and purification of natural gas [29], ethane [30], and butadiene [31].In fact, the high porosities of materials are not prerequisites for their gas separation applications.As reported in the literature, MOFs and HOFs exhibiting excellent performance for gas separations are those of moderate porosities, with BET surface areas less than 1000 m 2 /g [32].The design and synthesis of new porous MOF materials with hydrogen-bonding features may serve as a new concept to take advantage of the structural features of both materials with the aim of gas separation applications.Liu's group reported some hydrogen-bonded MOFs like ZSA-7, ZSA-8, and ZSA-9.ZSA-7 and ZSA-8 exhibit high CO 2 adsorption abilities (109.8 and 114.0 cm 3 •g −1 at 273 K, respectively, under 1 bar) and outstanding natural gas selectivity separation, especially for CO 2 over CH 4 (39.8 and 35.7 for CO 2 /CH 4 = 0.5/0.5 and 76.0 and 51.6 for CO 2 /CH 4 = 0.05/0.95,respectively, under 1 bar at 298 K) [33].Natural gas is a hydrocarbon mixture primarily consisting of 85% CH 4 , 9% C 2 H 6 , 3% C 3 H 8 , 2% N 2 , and 1% C 4 H 10 .To reduce the percentage of ethane content, it is very important to purify natural gas.With CH 4 as a premium choice for greenhouse effect reduction, using MOFs for the industrial separation of CH 4 and C 2 H 6 in natural gas becomes urgent and necessary.As a fruitful approach, selectivity enhancements in gas separation have been successfully carried out through the precise control of the pore size and environment of MOFs [34].However, the selectivity of C 2 H 6 and CH 4 is low in the reported MOFs, and it may be due to the inappropriate BET surface areas and weak interactions of C 2 H 6 with the cavity surface of the MOF frameworks [30,32].In this respect, 3D hydrogen-bonded MOFs may be an excellent choice of porous materials for CH 4 /C 2 H 6 purification of natural gas.
Based on our preliminary study [26], we herein used the aminopyrimidylcarboxylate ligand (2-aminopyrimidine-5-carboxylic acid) and Cu(NO 3 ) 2 to deliver a 3D hydrogen bonded framework (namely TJU-Dan-5) through a solvothermal reaction, which consists of 2D sql networks connected by hydrogen bonds.The 1D permanent porosity of TJU-Dan-5 exhibits higher C 2 H 6 /CH 4 selectivity (101:1, at 273 K/100 kPa) due to C-H•••π interactions between C 2 H 6 and the pore wall.In addition, TJU-Dan-5 displays a high selectivity for equimolar C 2 H 6 /C 2 H 4 mixture gas (selectivity: 2.46 at 298 K/100 kPa).This work demonstrates that internal hydrogen bonding implanted in the pore of MOFs provides both enhanced structural integrity and C 2 H 6 friendly pore surfaces.

Structure Description
The crystallographic data and structure refinement information are listed in Table S1.TJU-Dan-5, Cu(apc) 2 , is crystallized in the monoclinic space group of C2/c.The asymmetric unit consists of one crystallographically independent Cu 2+ ion and two molecules of apc − ligand (Figure S1).As shown in Figure S2, the Cu 2+ ion coordinates four carboxylate oxygen (Cu-O, 1.976(4) Å and 1.972(4), Table S2) and one pyrimidine nitrogen atom (Cu-N, 2.177(4) Å) has a distorted tetragonal pyramid conformation.The Hapc carboxylate groups coordinate the copper ion in a bidentate fashion (Figure S2), with the C-O bond distances of the carboxylate group ranging from 1.254(6) Å to 1.265(6) Å.Compared with the reported structure [26], Hapc adopted new coordination modes (III and IV, Figure S3).One apc ligand connects three copper ions through two Cu-O bonds and one Cu-N bond (mode IV, Figure S3).The other one apc ligand chelates two copper ions through two Cu-O bonds, and the aminopyrimidine group does not coordinate with copper ion (mode III, Figure S3).The free aminopyrimidine forms a hydrogen bond between amino and carboxylate instead (d N-H•••N = 2.988(6) Å, Figure S3 and Table S3).It is noteworthy that internal hydrogen bonds exist within aminopyrimidine groups (d N-H•••N = 2.984(6) Å, 3.051(7) Å, 3.054(6) Å, Figure S4 and Table S3).
The Cu paddle-wheel SBU(Cu 2 (CO 2 ) 2 N 2 ) produced by two CuO 4 N 1 tetrahedrons are corner-shared to form a 2D network along the a axis (Figure 2a, green).The other 2D network (Figure 2b, red) is also composed of Cu paddle-wheel SBU and a 1D helix chain along the b axis.A similar pillared-layer 3D framework is generated by internal hydrogen bonds among apc ligands of the 2D network (Figure 1, resulting in a one-dimensional hexagonal channel with the pore size of 5.0 Å × 6.0 Å by the van der Waals radius along c axis (Figure 3).The void volume is 1272.9Å 3 (36.5%)as estimated by PLATON using a probe of 1.2 Å and the Connolly surface area is shown in Figure 4. Usually, 4, 4 ′ -Bipyridine or analogue linker usually bridges the porous pillared-layer frameworks through coordination bonds [35].However, the internal hydrogen bonds within aminopyrimidine groups link the 2D layers to construct the 3D porous pillar-layer network of TJU-Dan-5, the structure type rarely reported in MOFs.In contrast to most MOFs with metal ions or clusters coordinating with organic ligands, the framework of TJU-Dan-5 provides a new example of 3D hydrogen-bonded MOFs (Figure 1).It is thus interesting to further explore the topology analysis of such hydrogen-bonded frameworks for the rational design of reticular chemistry of MOFs.The topology of TJU-Dan-5 was analyzed by using the TOPOSPro program [36].Firstly, the two-dimensional network formed by the metal cluster and ligands was investigated.Considering Cu paddle-wheel SBU(Cu2(CO2)2N2) as a 4-connected node, each layer yields a 2D 4-connected uninodal net with point symbol of {4 4 .6 2 }, which is a sql net  The topology of TJU-Dan-5 was analyzed by using the TOPOSPro program [36].Firstly, the two-dimensional network formed by the metal cluster and ligands was investigated.Considering Cu paddle-wheel SBU(Cu2(CO2)2N2) as a 4-connected node, each layer yields a 2D 4-connected uninodal net with point symbol of {4 4 .6 2 }, which is a sql net The topology of TJU-Dan-5 was analyzed by using the TOPOSPro program [36].Firstly, the two-dimensional network formed by the metal cluster and ligands was investigated.Considering Cu paddle-wheel SBU(Cu2(CO2)2N2) as a 4-connected node, each layer yields a 2D 4-connected uninodal net with point symbol of {4 4 .6 2 }, which is a sql net The topology of TJU-Dan-5 was analyzed by using the TOPOSPro program [36].Firstly, the two-dimensional network formed by the metal cluster and ligands was investigated.Considering Cu paddle-wheel SBU(Cu 2 (CO 2 ) 2 N 2 ) as a 4-connected node, each layer yields a 2D 4-connected uninodal net with point symbol of {4 4 .6 2 }, which is a sql net (Figure 2b).Furthermore, hydrogen bonds participating in the 3D framework of TJU-Dan-5 should be considered in the process of analyzing the topology.The two crystallographically independent apc − ligands with different hydrogen bonds are regarded as two 3-connected nodes (Figure 5a), Cu paddle-wheel SBU(Cu 2 (CO 2 ) 2 N 2 ) as a 6-connected node, and the total 3-D network displays a trinodal (3,3,6)-connected net with point (Schläfli) symbol {6 3 } 4 {6 6 .8 4 .10 5 }.According to the search results from the Reticular Chemistry Structure Resource (RCSR) database and the literature, only seven trinodal (3,3,6)-connected nets have been reported, including the symbol names of brk, muo, tsa, tsy, xbq, zxc [37], and another one with point symbols of {4 2 .6}{4 3 }{4 5 .6 4 .8 6} [38].Therefore, TJU-Dan-5 provides a completely new topology in MOF crystal nets.

Characterization of TJU-Dan-5
The thermal stability of TJU-Dan-5 was further evaluated via a thermogravimetric analysis (TGA) and varied-temperature PXRD.A sharp weight loss above 270 °C in the TGA curve corresponded to the departure of the organic ligands and the collapse of the framework (Figure S5), while the varied-temperature PXRD patterns obtained at increasing temperatures show that the framework collapses above 270 °C (Figure S6).The framework of TJU-Dan-5, like most MOFs based Cu-paddle-wheel SBU, is stable up to 250 °C in air, both suggesting that hydrogen-bonded MOFs materials are structurally robust.

Gas Adsorption of H2 and N2
Adsorption experiments were thus carried out to evaluate the rigidity and permanent porosity of TJU-Dan-5.The N2 sorption at 77 K for activated TJU-Dan-5 shows type I adsorption isothermal behavior, characteristic of a microporous material with permanent porosity (Figures S7 and S8).The Brunauer-Emmett-Teller (BET) and Langmuir surface area of TJU-Dan-5 were determined to be 748.7 m 2 •g −1 and 914.2 m 2 •g −1 , respectively.The experimental pore volume was calculated to be 0.323 cm 3 •g −1 .The pore size distribution of TJU-Dan-5, analyzed by the NLDFT model, displays a peak of6.0 Å in the micropore region, which is close to the size of the hexagonal channel in the crystal structure of title compound (Figure S7, inset).Hydrogen adsorption reveals a moderate uptake of 1.52 wt% at 77 K (1 atm) (Figure S9).TJU-Dan-5 may be studied as a potential hydrogen storage material.The Qst (H2) is slightly lower than that of MOF-74(Zn) but considerably higher than those of other porous materials, such as MOF-177, JUC-48, and PCN-17 [39].The isosteric heat of H2 sorption, calculated by fitting the adsorption data at 77 K and 87 K to the Virial equation, was found to be 7.98 kJ/mol at zero coverage (Figures S10 and S11).

Characterization of TJU-Dan-5
The thermal stability of TJU-Dan-5 was further evaluated via a thermogravimetric analysis (TGA) and varied-temperature PXRD.A sharp weight loss above 270 • C in the TGA curve corresponded to the departure of the organic ligands and the collapse of the framework (Figure S5), while the varied-temperature PXRD patterns obtained at increasing temperatures show that the framework collapses above 270 • C (Figure S6).The framework of TJU-Dan-5, like most MOFs based Cu-paddle-wheel SBU, is stable up to 250 • C in air, both suggesting that hydrogen-bonded MOFs materials are structurally robust.

Gas Adsorption of H 2 and N 2
Adsorption experiments were thus carried out to evaluate the rigidity and permanent porosity of TJU-Dan-5.The N 2 sorption at 77 K for activated TJU-Dan-5 shows type I adsorption isothermal behavior, characteristic of a microporous material with permanent porosity (Figures S7 and S8).The Brunauer-Emmett-Teller (BET) and Langmuir surface area of TJU-Dan-5 were determined to be 748.7 m 2 •g −1 and 914.2 m 2 •g −1 , respectively.The experimental pore volume was calculated to be 0.323 cm 3 •g −1 .The pore size distribution of TJU-Dan-5, analyzed by the NLDFT model, displays a peak of6.0 Å in the micropore region, which is close to the size of the hexagonal channel in the crystal structure of title compound (Figure S7, inset).Hydrogen adsorption reveals a moderate uptake of 1.52 wt% at 77 K (1 atm) (Figure S9).TJU-Dan-5 may be studied as a potential hydrogen storage material.The Qst (H 2 ) is slightly lower than that of MOF-74(Zn) but considerably higher than those of other porous materials, such as MOF-177, JUC-48, and PCN-17 [39].The isosteric heat of H 2 sorption, calculated by fitting the adsorption data at 77 K and 87 K to the Virial equation, was found to be 7.98 kJ/mol at zero coverage (Figures S10 and S11).Separation of C 2 H 6 from natural gas is crucial in sufficient utilization of natural gas and CO 2 reduction in the earth atmosphere.The unique channel sizes of TJU-Dan-5 us to investigate its potential in the C 2 H 6 /CH 4 separation from natural gas.In this respect, single-component sorption isotherms of CH 4 and C 2 H 6 were measured at 273 K and 298 K, respectively.For TJU-Dan-5, the absorbed amounts of CH 4 at 273 and 298 K are only 23.6 and 15.2 cm 3 •g −1 , respectively (Figure 6), whereas the corresponding C 2 H 6 amounts of gas uptake at 273 and 298 K are 50.3 and 45.03 cm 3 •g −1 (Figure 6).More importantly, under low pressure below 30 kPa, TJU-Dan-5 takes up much more C 2 H 6 than CH 4 .TJU-Dan-5 takes up a much different amount of C 2 H 6 and CH 4 , suggesting of separation selectivity of TJU-Dan-5 in C 2 H 6 and CH 4 .In addition, C 2 H 2 , C 2 H 4 , and CO 2 gas sorption isotherms of TJU-Dan-5 were measured at 273 K and 298 K, respectively (Figure 7).TJU-Dan-5 can take up the amounts of 57.79 cm 3 •g −1 and 50.06 cm 3 •g −1 for C 2 H 2 , 47.86, 41.64 cm 3 •g −1 and 36.25 cm 3 •g −1 for C 2 H 4 , and 59.27 cm 3 •g −1 and 36.25 cm 3 •g −1 for CO 2 at different temperature levels (Figure 4).olecules 2024, 29, x FOR PEER REVIEW 6

Separations of C2H6/CH4 and C2H6/C2H4.
Separation of C2H6 from natural gas is crucial in sufficient utilization of natural and CO2 reduction in the earth atmosphere.The unique channel sizes of TJU-Dan-5 u investigate its potential in the C2H6/CH4 separation from natural gas.In this respect, gle-component sorption isotherms of CH4 and C2H6 were measured at 273 K and 29 respectively.For TJU-Dan-5, the absorbed amounts of CH4 at 273 and 298 K are only and 15.2 cm 3 •g −1 , respectively (Figure 6), whereas the corresponding C2H6 amounts of uptake at 273 and 298 K are 50.3 and 45.03 cm 3 •g −1 (Figure 6).More importantly, under pressure below 30 kPa, TJU-Dan-5 takes up much more C2H6 than CH4.TJU-Dan-5 t up a much different amount of C2H6 and CH4, suggesting of separation selectivity of T Dan-5 in C2H6 and CH4.In addition, C2H2, C2H4, and CO2 gas sorption isotherms of T Dan-5 were measured at 273 K and 298 K, respectively (Figure 7).TJU-Dan-5 can tak the amounts of 57.79 cm 3 •g −1 and 50.06 cm 3 •g −1 for C2H2, 47.86, 41.64 cm 3 •g −1 and 3 cm 3 •g −1 for C2H4, and 59.27 cm 3 •g −1 and 36.25 cm 3 •g −1 for CO2 at different temperature le (Figure 4).To obtain binding energy at low coverage, isosteric adsorption heats of C2H6, C C2H2, and C2H4 for TJU-Dan-5 were calculated through the Virial equation and using Clausius-Clapeyron relation, respectively (Figures S12 and S13).As shown in Figure the isosteric adsorption heat of C2H6 is higher than the others.At zero coverage of C interaction with the most energetically favored adsorption sites, the enthalpy of 35 mol −1 can be attributed to stronger van der Waals host-guest interactions and C-H•••π teraction between TJU-Dan-5 and C2H6.Pyridine rings presented in TJU-Dan-5 may b C2H6 in close contact with the pore walls.To visualize the preferential binding site C2H6, DFT-based structural potential energy surface searching identified strong C-H interactions between C2H6 and the framework (the distances between H and pyrimi centroids: 2.76 and 3.25 Å in Figure 7) and van der Waals host-guest interaction (the n est distances between H and pyrimidine: 2.61 and 2.81 Å in Figure 7).In fact, the calcul adsorption energy of C2H6 on TJU-Dan-5 is 34.7 kJ/mol, in an excellent agreement w the experimental data.To obtain binding energy at low coverage, isosteric adsorption heats of C 2 H 6 , CH 4 , C 2 H 2 , and C 2 H 4 for TJU-Dan-5 were calculated through the Virial equation and using the Clausius-Clapeyron relation, respectively (Figures S12 and S13).As shown in Figure S12, the isosteric adsorption heat of C 2 H 6 is higher than the others.At zero coverage of C 2 H 6 interaction with the most energetically favored adsorption sites, the enthalpy of 35.5 kJ mol −1 can be attributed to stronger van der Waals host-guest interactions and C-H•••π interaction between TJU-Dan-5 and C 2 H 6 .Pyridine rings presented in TJU-Dan-5 may bring C 2 H 6 in close contact with the pore walls.To visualize the preferential binding sites of C 2 H 6 , DFT-based structural potential energy surface searching identified strong C-H•••π interactions between C 2 H 6 and the framework (the distances between H and pyrimidine centroids: 2.76 and 3.25 Å in Figure 7) and van der Waals host-guest interaction (the nearest distances between H and pyrimidine: 2.61 and 2.81 Å in Figure 7).In fact, the calculated adsorption energy of C 2 H 6 on TJU-Dan-5 is 34.7 kJ/mol, in an excellent agreement with the experimental data.8).In the calculation, TJU-Dan-5 exhibits greater separation ratios of C2H6/ both 273 K and 298 K.The high selectivities of TJU-Dan-5 in C2H6/CH4 separati better than those of MOFs or HOFs (Table 1) reported at room temperature, such C4 [41], UTSA-33 [42], SNNU-Bai67 [43], VNU-18 [44], SBMOF-2, and HOF-BTB [ These results may be attributed to the stronger affinity between the pore environm TJU-Dan-5 and C2H6, making TJU-Dan-5 a good choice for natural gas purificat addition, it is worth noting that the selectivity for C2H6/C2H4 equimolar mixtures is 298 K (100 kPa) (Figure 9), only lower than some leading MOFs in gas separation, s MAF-49 (2.9) [30], Zn-FBA (2.9) [47], Cu(Qc)2 (3.4) [48], and Fe2(O2)(dobdc) (4.8).In the calculation, TJU-Dan-5 exhibits greater separation ratios of C 2 H 6 /CH 4 at both 273 K and 298 K.The high selectivities of TJU-Dan-5 in C 2 H 6 /CH 4 separation are better than those of MOFs or HOFs (Table 1) reported at room temperature, such as FJI-C4 [41], UTSA-33 [42], SNNU-Bai67 [43], VNU-18 [44], SBMOF-2, and HOF-BTB [45,46].These results may be attributed to the stronger affinity between the pore environment of TJU-Dan-5 and C 2 H 6 , making TJU-Dan-5 a good choice for natural gas purification.In addition, it is worth noting that the selectivity for C 2 H 6 /C 2 H 4 equimolar mixtures is 2.46 at 298 K (100 kPa) (Figure 9), only lower than some leading MOFs in gas separation, such as MAF-49 (2.9) [30], Zn-FBA (2.9) [47], Cu(Qc) 2 (3.4) [48], and Fe 2 (O 2 )(dobdc) (4.4) [49], supporting our prediction of TJU-Dan-5 utilization in gas separation.8).In the calculation, TJU-Dan-5 exhibits greater separation ratios of C2H6/CH4 at both 273 K and 298 K.The high selectivities of TJU-Dan-5 in C2H6/CH4 separation are better than those of MOFs or HOFs (Table 1) reported at room temperature, such as FJI-C4 [41], UTSA-33 [42], SNNU-Bai67 [43], VNU-18 [44], SBMOF-2, and HOF-BTB [45,46].These results may be attributed to the stronger affinity between the pore environment of TJU-Dan-5 and C2H6, making TJU-Dan-5 a good choice for natural gas purification.In addition, it is worth noting that the selectivity for C2H6/C2H4 equimolar mixtures is 2.46 at 298 K (100 kPa) (Figure 9), only lower than some leading MOFs in gas separation, such as MAF-49 (2.9) [30], Zn-FBA (2.9) [47], Cu(Qc)2 (3.4) [48], and Fe2(O2)(dobdc) (4.4) [49], supporting our prediction of TJU-Dan-5 utilization in gas separation.Dynamic breakthrough experiments toward C2H6/CH4 (5:95, v/v) and C2H6/C2H4 (50:50, v/v) were carried out at 298 K (Figure 10).The activated TJU-Dan-5 sample was packed into a column, with the binary gas mixtures of CH4/C2H6 and C2H4/C2H6 flowing at 5.0 mL min −1 at room temperature.As shown in Figure 10a, CH4 was immediately monitored in the outlet of the fixed bed, while the C2H6 was retained in the column packed with TJU-Dan-5 for 70 min/g.The results demonstrate that TJU-Dan-5 is capable of trapping C2H6 molecules from CH4/C2H6 mixtures.High-purity CH4 products (≥99.99%)can be directly obtained, and during the time of 0 to 66 min/g (C2H6 does not reach its breakthrough point), the amounts of CH4 captured in TJU-Dan-5 were calculated to be 299.3110).The activated TJU-Dan-5 sample was packed into a column, with the binary gas mixtures of CH 4 /C 2 H 6 and C 2 H 4 /C 2 H 6 flowing at 5.0 mL min −1 at room temperature.As shown in Figure 10a, CH 4 was immediately monitored in the outlet of the fixed bed, while the C 2 H 6 was retained in the column packed with TJU-Dan-5 for 70 min/g.The results demonstrate that TJU-Dan-5 is capable of trapping C 2 H 6 molecules from CH 4 /C 2 H 6 mixtures.High-purity CH 4 products (≥99.99%)can be directly obtained, and during the time of 0 to 66 min/g (C 2 H 6 does not reach its breakthrough point), the amounts of CH 4 captured in TJU-Dan-5 were calculated to be 299.31mL/g.In Figure 10b, C 2 H 4 was eluted from a column before C 2 H 6 with different retention times of 7.5 and 10.3 min•g −1 , respectively, in an equimolar mixture.The purity of CH 4 was determined to be ≥99.04%.The breakthrough curves are kept over three cycles, indicating that the sample is sufficiently stable at the given conditions.In addition, the PXRD experiments further confirm the working stability of TJU-Dan-5 following the sorption experiments and breakthrough tests (Figure S14).Separation of absolute C 2 H 6 from CH 4 /C 2 H 6 mixture can be achieved by using activated TJU-Dan-5 under ambient conditions.lecules 2024, 29, x FOR PEER REVIEW 9 o mL/g.In Figure 10b, C2H4 was eluted from a column before C2H6 with different reten times of 7.5 and 10.3 min•g −1 , respectively, in an equimolar mixture.The purity of CH4 determined to be ≥99.04%.The breakthrough curves are kept over three cycles, indica that the sample is sufficiently stable at the given conditions.In addition, the PXRD exp iments further confirm the working stability of TJU-Dan-5 following the sorption exp ments and breakthrough tests (Figure S14).Separation of absolute C2H6 from CH4/C mixture can be achieved by using activated TJU-Dan-5 under ambient conditions.

General Naterials and Methods
All reagents for syntheses were purchased from commercial sources.Thermogr metric (TGA) analyses were investigated with a Mettler Toledo TGA/SDTA851 analy (Mettler Toledo, Zurich, Switzerland) in N2 atmosphere with a heating rate of 5 K m from 30 °C to 800 °C.Elemental analyses (C, N, H) were measured on an Elementar V EL III microanalyzer (Elementar, Frankfurt, Germany).IR spectra were measured fro KBr pellets on a Thermo Scientific Nicolet IS10 FT-IR spectrometer (Thermo Scient Waltham, MA, USA) in the range of 4000-400 cm −1 .Powder X-ray diffraction (PXRD) terns were carried out using a Bruker D8 powder diffractometer (Bruker, Karlsruhe, G many) at 40 kV, 40 mA for Cu Kα radiation (λ = 1.5406Å), with a scan speed of 0.2 s/s and a step size of 0.05° (2θ).

General Naterials and Methods
All reagents for syntheses were purchased from commercial sources.Thermogravimetric (TGA) analyses were investigated with a Mettler Toledo TGA/SDTA851 analyzer (Mettler Toledo, Zurich, Switzerland) in N 2 atmosphere with a heating rate of 5 K min −1 , from 30 • C to 800 • C. Elemental analyses (C, N, H) were measured on an Elementar Vario EL III microanalyzer (Elementar, Frankfurt, Germany).IR spectra were measured from a KBr pellets on a Thermo Scientific Nicolet IS10 FT-IR spectrometer (Thermo Scientific, Waltham, MA, USA) in the range of 4000-400 cm −1 .Powder X-ray diffraction (PXRD) patterns were carried out using a Bruker D8 powder diffractometer (Bruker, Karlsruhe, Germany) at 40 kV, 40 mA for Cu Kα radiation (λ = 1.5406Å), with a scan speed of 0.2 s/step and a step size of 0.05 • (2θ).

X-ray Crystallography
The data for TJU-Dan-5 were collected from a single crystal at 296(2) K on a Bruker D8 VENTURE dual-wavelength Mo/Cu three-circle diffractometer with a microfocus sealed Xray tube using mirror optics as monochromator and a Bruker PHOTON II detector (Bruker, Karlsruhe, Germany).MoKα radiation (λ = 0.71073 Å) was MoKα used in the diffractometer.All data were integrated with SAINT and a multi-scan absorption correction using SADABS was applied [57,58].The structure was solved by direct methods using SHELXT and refined by full-matrix least-squares methods against F 2 by SHELXL-2019/1 [59,60].All non-hydrogen atoms were refined with anisotropic displacement parameters.The hydrogen atoms were refined isotropically on calculated positions using a riding model with their Uiso values constrained to 1.5 times the Ueq of their pivot atoms for terminal sp 3 carbon atoms and 1.2 times for nitrogen and all other carbon atoms.The crystallographic data for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre [61].CCDC 1830585 for TJU-Dan-5 includes the supplementary crystallographic data for this paper.These data can be acquired free of charge via www.ccdc.cam.ac.uk/data_request/cif (accessed on 21 April 2020), or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.This report and the CIF file were generated using FinalCif [62].

Gas Adsorption Measurements
The as-synthesized materials of TJU-Dan-5 were washed with DMF and CH 3 OH for three times, respectively.Solvent was exchanged with CH 2 Cl 2 for nine times over three days.CH 2 Cl 2 was further removed with supercritical liquid CO 2 in a Tousimis Samdri PVT-30 critical point dryer (Tousimis, Rockville, MD, USA).N 2 adsorption and desorption measurements were measured on a Quantachrome Autosorb-iQ gas adsorption analyzer (Quantachrome, Tallahassee, FL, USA) and pore size analyzer at 77 K.The pore size distribution of TJU-Dan-5 was analyzed by the NLDFT model utilizing N 2 adsorption data at 77 K (calculation model: N 2 at 77 K on carbon (slit/cylinder.pore, NLDFT equilibrium model); Eff.mol.Diameter (D): 3.54 Å).Before the gas adsorption tests, the experimental sample was immersed in CH 3 OH for 36 h, and then the exchanged sample was activated under vacuum at 100 • C for 10 h.Single H 2 sorption experiments were measured on a Micromeritics ASAP 2020 adsorption analyzer (Micromeritics, Atlanta, GA, USA) at 77 K and 87 K via liquid N 2 bath and liquid Ar bath, respectively.C 2 H 6 , C 2 H 2 , C 2 H 4 , CH 4 and CO 2 absorption experiments were performed on the Micromeritics ASAP 2020 adsorption analyzer at 273 K and 298 K via an ice-water bath and heating jacket, respectively.Two different temperatures isotherms were fitted to the Virial model and the isosteric heats of C 2 H 6 , C 2 H 4 , C 2 H 2 , and CH 4 adsorption were calculated, respectively.The selectivity of C 2 H 6 and C 2 H 2 over CH 4 was calculated with the ideal adsorbed solution theory (IAST) model (Equations (S1) and (S2)) at 273 K and 298 K.

Breakthrough Measurements
The breakthrough experiment was carried out using a multi-constituent adsorption breakthrough curve analyzer (BSD-MAB, Beishide Instrument Technology (Beijing) Co., Ltd., No. 607, Building 1, Brilliant International, Shangdi 10th Street, Haidian District, Beijing, China).The gas separation properties of TJU-Dan-5 (1.26 g) were examined by breakthrough experiments using 0.05 (C 2 H 6 ): 0.95 (CH 4 ) and 0.5 (C 2 H 6 ): 0.5 (C 2 H 4 ) gas mixtures flowing through the activated samples packed into the same glass column (6.0 mm inner diameter, 65 mm in length), respectively.The gas mixture passed through the column at a rate of 5 mL/min.The composition of the effluent gas was detected by a Mass spectrometry.

Figure 1 .
Figure 1.Schematic diagram showing the construction of 3D hydrogen-bonded MOFs in three ways (blue balls represent a metal cluster or macrocycle; the orange rods represent hydrogen bonds among ligands; red balls represent a metal or metal cluster; and the grey rods represent organic ligand parts).

Figure 1 .
Figure 1.Schematic diagram showing the construction of 3D hydrogen-bonded MOFs in three ways (blue balls represent a metal cluster or macrocycle; the orange rods represent hydrogen bonds among ligands; red balls represent a metal or metal cluster; and the grey rods represent organic ligand parts).

Figure 2 .Figure 3 .
Figure 2. Structure of TJU-Dan-5.(a) Packing diagram of TJU-Dan-5 along c axis, (red: one twodimensional coordination layer; green: the other two-dimensional coordination layers).(b) The two-dimensional layer can be simplified as sql topology (green grid) along a axis.

Figure 3 .Figure 2 .Figure 3 .Figure 4 .
Figure 3. Packing view of TJU-Dan-5.(a) Yellow balls are added to highlight the porosity along c axis, (b) yellow columns are added to highlight the 1D channels, along b axis (C: dark gray; N: dark blue; O: red; H: light gray, Cu 2 (CO 2 ) 2 N 2 : two light blue tetrahedrons.Hydrogen bonds within aminopyrimidine groups are represented in orange dotted lines.

Table 1 .
Comparison of some typical MOFs and HOF for the separation of C2H6/CH4 and C2H2/CH4 as predicted by IAST.

Table 1 .
Comparison of some typical MOFs and HOF for the separation of C 2 H 6 /CH 4 and C 2 H 2 /CH 4 as predicted by IAST.