Highly selective separation of propylene/propane mixture on costeffectively four-carbon linkers based metal-organic frameworks

Daofei Lv ,Junhao Xu ,Pingjun Zhou ,Shi Tu ,Feng Xu ,Jian Yan ,Hongxia Xi,Zewei Liu,*,Wenbing Yuan,Qiang Fu,Xin Chen,*,Qibin Xia

1 School of Environmental and Chemical Engineering,Foshan University,Foshan 528000,China

2 School of Chemistry and Chemical Engineering,South China University of Technology,Guangzhou 510641,China

3 Sichuan Jinyuansheng New Energy Technology Co.,Ltd,Meishan 620010,China

Keywords:Propylene Propane Separation Gas Adsorption Metal-organic frameworks

ABSTRACT The separation of propylene and propane is an important but challenging process,primarily achieved through energy-intensive distillation technology in the petrochemical industry.Here,we reported two natural C4 linkers based metal-organic frameworks (MIP-202 and MIP-203) for C3H6/C3H8 separation.Adsorption isotherms and selectivity calculations were performed to study the adsorption performance for C3H6/C3H8 separation.Results show that C3H6/C3H8 uptake ratios (298 K,100 kPa) for MIP-202 and MIP-203 are 2.34 and 7.4,respectively.C3H6/C3H8 uptake ratio (303 K,100 kPa) for MIP-203 is up to 50.0.The mechanism for enhanced separation performance of C3H6/C3H8 on MIP-203 at higher temperature(303 K)was revealed by the in situ PXRD characterization.The adsorption selectivities of C3H6/C3H8 on MIP-202 and MIP-203 (298 K,100 kPa) are 8.8 and 551.4,respectively.The mechanism for the preferential adsorption of C3H6 over C3H8 in MIP-202 and MIP-203 was revealed by the Monte Carlo simulation.The cost of organic ligands for MIP-202 and MIP-203 was lower than that of organic ligands for those top-performance MOFs.Our work sets a new benchmark for C3H6 sorbents with high adsorption selectivities.

1.Introduction

Since the spring of 2020,the epidemic of new coronavirus pneumonia (COVID-19) has spread all over the world [1],which causes a great threat to people’s health.As the ‘‘heart” of medical surgical masks and N95 masks,meltblown cloth has a huge market demand[2].Polypropylene is the main raw material of melt blown fabric [3].Increasing the production of polypropylene is of great significance to alleviate the shortage of epidemic prevention materials.

Polypropylene is the second largest demand polymer material in the world,and polymer-grade (＞99.5%) propylene is an important raw material for the manufacture of polypropylene[4].During the production of propylene by the cracking of fossil fuels,propane is also along with produced,leading to the propylene/propane mixture[5,6].Hence,the separation of propylene/propane mixture is a necessary and important chemical process,which is considered as one of seven chemical separations to change the world [7].The dominant separation technology of propylene/propane in industry is cryogenic distillation technology,which achieves the separation of propylene/propane by the boiling point difference[8].The slight difference (5 K) in boiling point of propylene and propane (see Table S1 in Supplementary Material) makes the separation an energy-intensive process.Typically,propylene/propane separation achieved through the cryogenic distillation uses repeated distillation-compression cycling in giant C3 separation towers of up to 90 meter high with over 200 trays at 243 K and 3000 kPa[9].Therefore,it is a great demand to develop alternative and more energy-saving technologies for propylene/propane separation.

Adsorptive separation technology based on porous solid sorbents is believed as a promising alternative,owing to its mild operating conditions and low energy consumption [10-12].The efficiency of adsorption separation relies on the adsorbent [13].Some conventional sorbents such as zeolite 5A [14],ZSM-5 [14],and ETS-10 [15] were reported to display poor performance for C3H6/C3H8separation with low C3H6/C3H8uptake ratio (＜1.22).To develop a highly efficient process for C3H6/C3H8separation,the key is to conceive and design novel porous solid sorbents[16].As emerging porous materials,metal-organic frameworks(MOFs) have shown exciting potential for applications in gas adsorptive separation,including C3H6/C3H8separation,due to their modular crystalline structure,controllable pore environment,and high surface areas [17-21].

In light of MOFs for C3H6/C3H8separation,they can be divided into five types: thermodynamic,kinetic,synergistic effect of thermodynamic and kinetic,gate-opening,and molecular sieving separation.In terms of thermodynamic separation,it is primarily achieved through the π-complexation interaction between open metal sites in MOFs and C=C entities in C3H6[22].Long’ group[23]reported that M2(m-dobdc)(M=Mn,Fe,Co,Ni)exhibited high C3H6uptakes (＞7 mmol·g-1) and C3H6/C3H8selectivities (＞32) at 298 K and 100 kPa,resulting from the high density open metal sites in these MOFs.Kinetic separation of C3H6/C3H8is based on the difference of diffusion rate in the pores.Chen’s group [19]found that ELM-12 had high kinetic C3H6/C3H8separation selectivity of 204 (298 K,100 kPa),due to the existence of twodimensional zigzag channels with moderate pore size in ELM-12.MAF-23-O [24] is a typical MOF for separating C3H6/C3H8mixture following the synergistic effect of thermodynamic and kinetic.At 298 K and 100 kPa,the thermodynamic and kinetic selectivities of C3H6/C3H8on MAF-23-O were 8.8 and 71,respectively.For gate-opening mechanism,it realizes separation based on different gate-opening pressures of C3H6and C3H8on flexible MOFs.Our group [25] previous reported that flexible CPL-1 could separate equimolar C3H6/C3H8mixture at 273 K and 100 kPa,since gateopening pressures of C3H6and C3H8were 40 and ＞100 kPa,respectively.With regard to molecular sieving effect,it achieves C3H6/C3H8separation on the basis of the size difference in these two molecules.Typically,Y-abtc with pore window size of 0.472 nm could adsorb C3H6(0.468 nm) but completely exclude C3H8(0.51 nm) at ambient temperature and pressure [26].Although many MOFs have been reported one after another for C3H6/C3H8separation,most MOFs exist more or less problems,such as low adsorption selectivities and high ligand costs.

To develop high-performance MOFs with high C3H6/C3H8selectivities and low ligand costs,we reported two cost-effectively C4linkers based MOFs (MIP-202 and MIP-203).MIP-202 and MIP-203 possess 12-connected and 10-connected Zr6(μ3-O)4(μ3-OH)4secondary building units (SBUs),respectively.C3H6/C3H8adsorption performance on MIP-202 and MIP-203 were investigated.Results show that C3H6/C3H8selectivities on MIP-202 and MIP-203 are 8.8 and 551.4,respectively.Surprisingly,C3H6uptake on MIP-203 increases with the increase of temperature,while C3H8uptake decreases with the rise of temperature.As validated by thein situPXRD data,MIP-203 exhibits the unusual adsorption behavior due to the temperature-induced and guest-induced flexibility of the framework.At 303 K and 100 kPa,MIP-203 exhibits super-high C3H6/C3H8uptake ratio(50.0),which sets a new benchmark for C3H6/C3H8separation.

2.Experimental

2.1.Materials

L-aspartic acid (99.0%) and succinic acid (99.5%) were supplied by Aladdin Reagent Co.,Ltd.ZrCl4anhydrous (99.5%) was purchased from Strem Chemicals Inc.Formic acid (98%) was obtained from MACKLIN reagent Co.,Ltd.Ethanol was provided by Guangdong Guanghua Sci-Tech Co.Ltd.C3H6(99.99%),C3H8(99.99%),N2(99.999%),and He (99.999%) in the adsorption tests were purchased from Guangzhou Shengying Gas Co.,Ltd.

2.2.Synthesis of MIP-202

MIP-202 was synthesized according to the reported procedures[27] with minor modifications.Briefly,L-aspartic acid (1.4 g) and ZrCl4(1.15 g) were dispersed in water (10 ml).Then,the mixture was kept stirring(383 K)and refluxing for 24 h.The resulting precipitate was washed with ethanol(4×35 ml)and collected by centrifugation.Finally,the product was dried under vacuum at 313 K for 24 h.

2.3.Synthesis of MIP-203

MIP-203 was prepared by following the reported literature[28]with minor modifications.Typically,succinic acid (0.48 g) and ZrCl4(0.23 g) were completely mixed in a Teflon reactor (25 ml)containing 4 ml of formic acid.The mixture was sealed in an autoclave and heated at 393 K for 3 d.The resulting precipitate was washed with ethanol(4×35 ml)and harvested by centrifugation.Finally,the product was dried in a vacuum oven at 313 K for 24 h.

2.4.Materials and physical measurements

Powder X-ray diffraction (PXRD) measurements were conducted on a Rigaku SmartLab SE X-ray diffractometer using Cu Kα radiation (150 mA,40 mV).Surface morphologies were observed on a scanning electron microscopy (SEM,Hitachi SU-8220).Thermogravimetric analysis (TGA) was carried out on a TGA55 instrument by heating sample from 300 to 1173 K at 10 K·min-1under N2atmosphere.N2and CO2adsorption-desorption isotherms at 77 and 273 K were measured on Micromeritics ASAP 2460 and 3Flex analyzer,respectively.Before each measurement,samples (0.1 g) were outgassed by dynamic vacuuming at 313 K for 24 h.Based on N2isotherms at 77 K and CO2isotherms at 273 K,the surface areas were calculatedviathe Brunauer-Emmett-Teller(BET)equation.The total pore volumes were determined based on the CO2isotherms at 273 K at the relative pressure of 0.02726.Horvath-Kawazoe model was employed for the calculating the pore size distributions.Gas sorption isotherms for C3H6and C3H8at different temperatures were measured on Micromeritics 3Flex analyzer.

2.5.Simulation details

Structure of MIP-202 was obtained from Single Crystal X-ray diffraction measurement while that of MIP-203 was accessed from Cambridge Crystal Database Centre [29].The optimization of MOF crystal was performed in the Forcite module of Materials Studio 7.0[30].The optimization was done with convergence as 4.19 × 10-5kJ·mol-1,2.09×10-3kJ·mol-1and 1.0×10-7nm for energy,force and displacement respectively.To reveal the adsorption mechanism of C3H6/C3H8in MIP-202 and MIP-203,Metropolis Monte Carlo method inserted in Sorption Module was employed to calculate interaction energy distributions,adsorption density distributions and figure out preferential adsorption locations of adsorbates.In the Monte Carlo simulation,both structures of MIP-202 and MIP-203 were considered as rigid.Universal force field [31] was used to describe the framework,while Lennard-Jones 12-6 potential [32] was employed to describe adsorbateadsorbent and adsorbate-adsorbate interactions.Crossesinteraction between different atoms was calculated by Lorentz-Berthelot mixing rules.The cutoff in this simulation was set as 1.2 nm,so the lattice of MOF crystal should be at least equal to 2.4 nm.To replicate the adsorption behavior of adsorbate in the framework,several types of moves such as exchange,conformer rotate,translate and regrow moves were used.The simulation was performed with 1 × 106steps for equilibration and 1 × 106steps for production.Using Locate task in Sorption module,one C3H6or C3H8molecule was inserted into the structure of MIP-202 or MIP-203 to generate the adsorption site for adsorbate with strongest affinity from adsorbent.1 × 104snapshots were stacked for generating the adsorption density distribution of adsorbate in adsorbent.

2.6.Moisture stability

The moisture stability of samples was studied by putting samples in 55% relative humidity (RH) atmosphere.55% RH atmosphere was created by adding saturated NaBr solution in a sealed chamber [33].The change of framework structure was characterizedviaPXRD measurement.

2.7.Temperature dependent PXRD

In situPXRD data of MIP-203 at 298,301,303,306,308,311,313,315 and 318 K were recorded on a Rigaku SmartLab SE Xray diffractometer using CuKα radiation (150 mA,40 mV) under N2atmosphere.Before each PXRD measurement at a certain temperature,the sample was heated at a ramp rate of 1 K·min-1to the target temperature,and the temperature was kept constant for 3 min.

3.Results and Discussion

3.1.Structural characterization

The structures and pore shapes of MIP-202 and MIP-203 are shown in Fig.1.MIP-202 crystallizes in an PN-3 space group with unit cell parameters ofa=b=c=1.7826 nm,andV=5.6645 nm3[27].MIP-203 crystallizes in an IMMM space group with unit cell parameters ofa=1.0055 nm,b=1.1865 nm,c=1.9921 nm,andV=2.3767 nm3[34].MIP-202 (fcu topology) consists of 12-connected Zr6(μ3-O)4(μ3-OH)4SBUs and 2-connected L-aspartic acid ligands.MIP-203 (bct topology) is composed of 10-connected Zr6(μ3-O)4(μ3-OH)4SBUs and 2-connected succinic acid linkers.MIP-202 has octahedral and tetrahedral cages,while MIP-203 possesses one-dimensional rhombic channels.

Fig.1.The structures and pore shapes of(a)MIP-202 and(b)MIP-203.Color code:Zr,cyan;C,gray;O,red;N,blue.The inner and outer surfaces of pores are marked in light gray and yellow,respectively.

Fig.2 displays PXRD patterns of MIP-202 and MIP-203.Characteristic diffraction peaks appear at 8.6°,9.9°,21.7°,25.9°,30° for MIP-202 and 8.6°,9.8°,11.5°,23.2° for MIP-203,respectively.The positions of diffraction peaks for MIP-202 and MIP-203 are identical with the reported data[34,35],suggesting the high purity of assynthesized samples.The different characteristic peaks show the different framework structures for MIP-202 and MIP-203.Fig.S1(in Supplementary Material) shows SEM images of MIP-202 and MIP-203.MIP-202 and MIP-203 crystals show ball-like and octahedron-like shapes,respectively.

Fig.2.PXRD patterns of MIP-202 and MIP-203.

The surface area and pore size are important parameters for evaluating the potential of sorbents for gas separation.Hence,N2isotherms at 77 K were measured to determine the BET surface areas of MIP-202 and MIP-203(Fig.S2).The calculated BET surface areas based on N2isotherms at 77 K were 13.1 and 5.8 m2·g-1for MIP-202 and MIP-203,respectively.N2isotherms of MIP-202 and MIP-203 belong to type II isotherms,indicating that these sorbents are non-porous materials.However,MIP-202 and MIP-203 were reported to be typical microporous materials [27,34].This is because some pores of MIP-202 and MIP-203 are too small for N2molecule (0.368 nm) to be accessible.To more accurately determine the pore textural properties of MIP-202 and MIP-203,CO2(0.33 nm) was used as probe molecule.On the basis of CO2isotherms at 273 K (Fig.3(a)),BET surface areas of MIP-202 and MIP-203 were calculated to be 255.7 and 284.6 m2·g-1,respectively.Total pore volumes of MIP-202 and MIP-203 were calculated to be 0.037 and 0.049 cm3·g-1,respectively.Fig.3(b) shows the pore size distributions of MIP-202 and MIP-203.It can seen that MIP-202 and MIP-203 exhibit similar pore size distributions.Their pore size is mainly distributed in the range of 0.346-3 nm,and concentrated in 0.63 nm.

3.2.Thermal and moisture stability

Fig.S3 presents TGA curves of MIP-202 and MIP-203 under N2atmosphere.MIP-202 and MIP-203 exhibit a distinct mass loss beyond 511 and 460 K,respectively,due to the decomposition of organic linkers.Hence,MIP-202 and MIP-203 are thermally stable below 511 and 460 K,respectively.

Fig.3.(a) CO2 adsorption/desorption isotherms on MIP-202 and MIP-203 at 273 K and (b) the pore size distributions of MIP-202 and MIP-203.

To study moisture stability of MIP-202 and MIP-203,we measured PXRD patterns of samples after exposure to 55% RH atmosphere for 3 days.As shown in Fig.S4,the positions of diffraction peaks for MIP-202 and MIP-203 remain unchanged after exposure to 55% RH atmosphere.Fig.S5 shows that MIP-202 and MIP-203 remain 63.0% and 91.5% C3H6uptakes after exposure to 55% RH atmosphere for 3 days,respectively.Comparing Fig.3(a) and Fig.S6,it can be seen that MIP-202 and MIP-203 retain 64.8%and 93.4% CO2uptakes after exposure to 55% RH atmosphere for 3 days,respectively.These results suggest that MIP-203 possesses better moisture stability than MIP-202.In comparison with MIP-203,MIP-202 has poorer moisture stability due to the more hydrophilic -NH2groups on the pore surfaces,resulting in that the water vapor is easier to approach its metal center and destroy the Zr-O bonds.

3.3.C3H6 and C3H8 isotherms over sorbents

The moderate pore size and acceptable stability of MIP-202 and MIP-203 prompted us to explore their adsorption performance for C3H6/C3H8separation.Accordingly,single-component C3H6and C3H8istherms at 298 K and 308 K were measured (Fig.4).At 298 K and 100 kPa,the adsorption capacities of C3H6and C3H8are 12.4 and 5.3 cm3·cm-3for MIP-202,respectively,and 14.7 and 2.0 cm3·cm-3for MIP-203,respectively.As shown in Table 1,the C3H8uptake of MIP-203 (2.0 cm3·cm-3) is lower than those of nearly all reported sorbents for C3H6/C3H8separation,including several molecule-sieving sorbents (KAUST-7 [40],Y-abtc [26],and Co-gallate [9]).C3H6/C3H8uptake ratio (298 K,100 kPa) for MIP-203 is 7.4,surpassing most reported sorbents (see Table 1).At 308 K and 100 kPa,C3H6and C3H8adsorption uptakes are 7.8 and 3.8 cm3·cm-3for MIP-202,respectively,and 25.0 and 0.5 cm3-·cm-3for MIP-203,respectively.C3H6/C3H8uptake ratio (303 K,100 kPa) for MIP-203 is as high as 50.0,outperforming nearly all reported sorbents (see Table 1).Interestingly,C3H6uptake on MIP-203 increases with the increase of temperature,while C3H8uptake decreases with the rise of temperature.This phenomenon is probably caused by the temperature-induced and guestinduced flexibility of the framework of MIP-203.The similar phenomenon and explanation can be also seen in the literature [50].When the temperature increases,the shape of the 1D channel(Fig.1(b)) changes to be more suitable for more planar C3H6than more stereo C3H8molecule (Fig.S7),and thus the C3H6and C3H8uptakes on MIP-203 rise and decrease,respectively.The flexibility of the framework (MIP-203) was verified arising from the distortion or bending of the C-C-C-C chains [34].

Table 1Summary of C3H6/C3H8 adsorption performance for some benchmark sorbents at 100 kPa

To further verify our conjecture,in situPXRD patterns of MIP-203 were measured at different temperatures under N2atmosphere (Fig.5).When the sample was heated from 298 to 318 K,the characteristic peak at 10.02°shifted slightly to the right,which implies that the increase of temperature can cause the change of framework structure.Additionally,when the sample was heated from 298 to 308 K,almost no shift could be seen for the characteristic peak at 10.02°,indicative of no obvious change of framework structure.It can be inferred that the interaction between guest molecule and the framework is also critical for increasing C3H6uptakes at 308 K,compared to 298 K.

Fig.4.C3H6 and C3H8 isotherms on MIP-202 and MIP-203 at (a) 298 and (b) 308 K.

3.4.Adsorption selectivities

To estimate the C3H6/C3H8separation ability,adsorption selectivities were assessed by ideal adsorbed solution theory (IAST)[55].Dual site Langmuir-Freundlich (DSLF) model [56] was used for describing single-component isotherms for C3H6and C3H8.As shown in Tables S2 and S3,DSLF model can well fit C3H6and C3H8istherms with the correlation coefficient (R2) over 0.998.As shown in Fig.6 and Table 1,the adsorption selectivities of C3H6/C3H8on MIP-202 and MIP-203 (298 K,100 kPa) are 8.8 and 551.4,respectively.C3H6/C3H8selectivity value of MIP-203 is one of the highest value for high-performance C3H6sorbents.

3.5.Adsorption mechanism

Fig.7(a) and (b) present the interaction energy distributions of C3H6/C3H8in the frameworks of MIP-202 and MIP-203,respectively.For MIP-202 and MIP-203,the peaks of the main interaction energy of C3H6stand at higher values compared to C3H8,implying that they both display stronger adsorption affinity for C3H6than C3H8.The difference of energy peaks between C3H6and C3H8in MIP-203 (6.6 kJ·mol-1) is much greater than that in MIP-202(1.3 kJ·mol-1).This result verifies the better separation ability of MIP-203 than MIP-202 for equimolar C3H6/C3H8mixture at low pressure,coinciding with the results of IAST-predicted adsorption selectivities (Fig.6(a)).

Fig.8 presents adsorption density contours of C3H6and C3H8in MIP-202.It can be seen that both C3H6and C3H8spread around into the pore center at 0.1 kPa in MIP-202 with low density.Drove by the dispersion force resulted from the organic linker and adsorbate-adsorbent electrostatic interaction,both C3H6and C3H8tend to clump together in the pore center with higher and higher density as the pressure goes up to 20 kPa and even 60 kPa.The largest adsorption density values of C3H6in MIP-202 at 20 and 60 kPa are both larger than those of C3H8.Besides,dispersity of C3H6with low adsorption density at 20 or 60 kPa is also larger than that of C3H8,reflected by the phenomenon of more low probability density which stays around the high probability density micelle for C3H6compared with C3H8.Therefore,MIP-202 possesses higher adsorption affinity with C3H6.

As can be seen in Fig.9,both C3H6and C3H8spread around into the pore center at 0.1 kPa in MIP-203 with low density.Drove by the dispersion force resulted from the organic linker and adsorbate-adsorbent electrostatic interaction,both C3H6and C3H8tend to clump together in the pore center with higher and higher density as the pressure goes up to 20 kPa and even 60 kPa.In addition,-OH group connected with the metal atom and C3H6can form O-H···π hydrogen bonding,which is the main reason for the large difference on adsorption density value between C3H6and C3H8.

Fig.5. In situ PXRD of MIP-203 at different temperatures under N2 atmosphere.

Fig.6.(a) Adsorption selectivities of MIP-202 and MIP-203 at 298 K for equimolar C3H6/C3H8 mixture.(b) C3H6/C3H8 selectivity (C3H6/C3H8=50:50) plotted against C3H6/C3H8 uptake ratio for high-performance C3H6 sorbents (298 K,100 kPa).

Fig.7.The interaction energy distributions of C3H6 and C3H8 in the frameworks of MIP-202 (a) and MIP-203 (b).

Fig.8.Comparison about adsorption density contours of(a),(c),(e)C3H6 and(b),(d),(f)C3H8 in MIP-202 at(a),(b)0.1 kPa,(c),(d)20 kPa and(e),(f)60 kPa(gray:C,white:H,red: O,dark blue: N,light blue: Zr).

Fig.9.Comparison about adsorption density contours of(a),(c),(e)C3H6 and(b),(d),(f)C3H8 in MIP-203 at(a),(b)0.1 kPa,(c),(d)20 kPa and(e),(f)60 kPa(gray:C,white:H,red: O,light blue: Zr).

Fig.10 displays the preferential adsorption site of one C3H6or C3H8molecule in the structure of MIP-202 and MIP-203.In terms of MIP-202,C3H6tends to be stabilized near the organic linker interacted with C-connected H atoms while C3H8prefers to stay near the organic linker interacted with both C-connected H atoms and N-connected H atoms.The range of C3H6-linker distances(0.2423-0.2654 nm) are relatively shorter than that of C3H8-linker distances (0.2420-0.2943 nm).In addition,the C3H6molecule in MIP-202 is stabilized by four organic linkers,but the C3H8molecule is stabilized by two organic linkers.It suggests that the adsorbate-adsorbent interaction for C3H6in MIP-202 is stronger than that for C3H8.

For MIP-203,both C3H6and C3H8molecule can be held by dispersion force from organic linker and electrostatic interaction between negative O from -OH group and positive H from adsorbate.For this point,the range of C3H6-linker distances (0.2499-0.2863 nm)are relatively shorter than that of C3H8-linker distances(0.2542-0.3145 nm).Besides,another interaction to strengthen the C3H6-framework interaction is O-H···π hydrogen bonding assembled by -OH group and sp2C from C3H6.The hydrogen bonding makes C3H6has stronger affinity with MIP-203 compared to C3H8.Also,it makes MIP-203 possess higher adsorption uptake for C3H6compared to C3H8.

3.6.Breakthrough tests

Fig.S9(a)and(b)show breakthrough curves of equimolar C3H6/C3H8mixtures on MIP-202 and MIP-203 at 298 K and 100 kPa,respectively.It can be observed that MIP-202 has better separation performance than MIP-203 for C3H6/C3H8mixtures.The framework flexibility of MIP-203 is greater than that of MIP-202.When C3H6molecules are adsorbed in the pores of MIP-203,the pore size becomes slightly larger,which promotes the adsorption of C3H8and lowers adsorption selectivity of MIP-203 for C3H6/C3H8mixtures.In addition,MIP-202 and MIP-203 do not show excellent dynamic separation performance,which may be caused by their low C3H6adsorption uptakes.

Fig.10.Location of a single(a),(c)C3H6 and(b),(d)C3H8 in structure of(a),(b)MIP-202 and(c),(d)MIP-203(gray:C,turquoise blue:H of C3H6 molecule,orange:H of C3H8 molecule,red: O,blue: N,purple: Zr).Those numbers marked the distance (nm) between two different atoms.

To study the cyclic stability of MIP-202 and MIP-203,cyclic breakthrough tests were conducted using equimolar C3H6/C3H8mixtures (0.5 ml·min-1) as feed gas (298 K,100 kPa).After each adsorption test,the adsorbent was heated and at 313 K under vacuum for 2 h to complete the desorption process.As depicted in Fig.S10(a) and (b),MIP-202 and MIP-203 can maintain their adsorption performance unchanged after three adsorption-desorption cycles.Fig.S11 confirms that MIP-202 and MIP-203 remain their frameworks intact after the breakthrough tests.

3.7.The cost of sorbents

The cost of adsorbent is also an important parameter for evaluating the potential of practical application.The synthesis cost of MOFs mainly depends on its organic ligands [35].Table 2 lists the price of organic ligands for some MOFs with highperformance for C3H6/C3H8separation.The prices of organic ligands for MIP-202 and MIP-203 were 34 and 18 USD·kg-1,respectively,which were lower than those of organic ligands for top-performance MOFs,including Co2(m-dobdc)(20474 USD·kg-1),CPL-1 (364 USD·kg-1),KAUST-7 (374 USD·kg-1),Y-abtc(103279 USD·kg-1),and Co-gallate (62 USD·kg-1).

Table 2The price of organic ligands for some MOFs with high-performance for C3H6/C3H8 separation

4.Conclusions

In summary,two Zr-based MOFs (MIP-202 and MIP-203) were successfully prepared and studied their performance for C3H6/C3H8separation.MIP-202 and MIP-203 showed acceptable moisture stability.The C3H8uptake of MIP-203 (2.0 cm3·cm-3) at 298 K and 100 kPa is lower than those of nearly all reported sorbents for C3H6/C3H8separation,including several moleculesieving sorbents.The adsorption of C3H6on MIP-203 showed unusual behavior:the adsorption capacity increased with the increase of adsorption temperature (from 298 to 303 K).This phenomenon is caused by the temperature-induced and guest-induced flexibility of the framework,and it is verified by thein situPXRD characterization.C3H6/C3H8selectivity value of MIP-203(551.4)at 298 K and 100 kPa is one of the highest value for high-performance C3H6sorbents.The higher C3H6uptake than C3H8on MIP-202 is because the C3H6molecule in MIP-202 is stabilized by four organic linkers,butthe C3H8molecule is stabilized by two organic linkers.O-H···π hydrogen bonding formed by -OH group in metal cluster and sp2C from C3H6plays a key role in the high C3H6/C3H8selectivity on MIP-203.The cost of organic ligands for MIP-202 and MIP-203 was 34 and 18 USD·kg-1,respectively,which was lower than that of organic ligands for those top-performance C3H6-selective MOFs.Our work shows that MIP-203 is a highly selective adsorbent for separation of C3H6/C3H8.MIP-203 may be a excellent raw material for preparing MOF membrane and being used for effectively separating C3H6/C3H8mixture in the future.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We are grateful to the financial support from National Natural Science Foundation of China (22108034,21878101),Guangdong Basic and Applied Basic Research Foundation (2020A1515110945,2020A1515110234,2021A1515011336 and 2020A1515110325),National Key Research and Development Program(2019YFC1805804),Guangdong Natural Science Foundation(2017A030313052),Key Program of Marine Economy Development (Six Marine Industries) Special Foundation of Department of Natural Resources of Guangdong Province (GDNRC [2020]036),Characteristic Innovation Research Project of University Teachers(2020XCC08) and Foshan Engineering and Technology Research Center for Novel Porous Materials.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.12.024.