Bin Gao,Zhaoqiang Zhang,Jianbo Hu,Jiyu Cui,Liyuan Chen,Xili Cui,2,Huabin Xing,2,*

1 Key Laboratory of Biomass Chemical Engineering of Ministry of Education,College of Chemical and Biological Engineering,Zhejiang University,Hangzhou 310027,China

2 Hangzhou Global Scientific and Technological Innovation Center,Zhejiang University,Hangzhou 311215,China

Keywords:Adsorptive separation C4 olefin 1,3-Butadiene Anion-pillared hybrid microporous material

ABSTRACT With the increasing demand for synthetic rubber,the purification of 1,3-butadiene (C4H6) is of great industrial significance.Herein,the successful removal of n-butene (n-C4H8) and iso-butene (iso-C4H8)from 1,3-butadiene (C4H6) was realized by synthesizing a novelanion-pillared ultramicroporous material TaOFFIVE-3-Ni (also referred to as ZU-96,TaOFFIVE=,3=pyrazine).Single-component adsorption isotherms show that TaOFFIVE-3-Ni can achieve the exclusion of n-C4H8 and iso-C4H8 in the low pressure region (0-30 kPa),and uptake C4H6 with a high capacity of 92.78 cm3·cm-3 (298 K and 100 kPa).The uptake ratio of C4H6/iso-C4H8 on TaOFFIVE-3-Ni was 20.83 (298 K and 100 kPa),which was the highest among the state-of-the-art adsorbents reported so far.With the rotation of anion and pyrazine ring,the pore size changes continuously,which makes smaller-size C4H6 enter the channel while larger-size n-C4H8 and iso-C4H8 are completely blocked.The excellent breakthrough performance of TaOFFIVE-3-Ni shows great potential in industrial separation of C4 olefins.The specific adsorption binding sites within ZU-96 was further revealed through the modeling calculation.

1.Introduction

1,3-Butadiene (C4H6) is an important raw material in petrochemical industry,which is mainly used to produce rubber,resin and fiber [1-3].In industry,1,3-butadiene was produced with other C4olefins,including 1,3-butadiene (30%-60%),n-butene(10%-20%) andiso-butene (10%-30%) [4,5].To produce highpurity 1,3-butadiene (＞99.5%),extractive distillation technology[6,7] is generally applied.However,problems such as high energy intensity and massive solvent loss still remain to be solved.Physisorption[8,9]technology based on porous adsorbents,which does not involve phase-change,is recognized to be an energyeffective alternative to cryogenic distillation and shows potential in the practical applications[10].However,the close kinetic diameter (C4H6:0.431 nm,n-C4H8:0.446 nm,iso-C4H8:0.484 nm) and similar physical properties of C4olefins (Table S1) make a grand challenge to design suitable adsorbents [11].

To date,there has been extensive studies focusing on the adsorptive separation of 1,3-butadiene from C4olefin mixtures by engineering porous adsorbents.Because of its low cost and strong interactions,zeolites are popular adsorbents.For example,several kinds of zeolites,such as 13X [11],5A [12],NaY [12],AgY[12],CuY [12],were investigated and showed moderate uptake capacities (1.0-3.0 mmol·g-1for 1,3-butadiene).However,the low separation selectivities (lower than 2.0) are inefficient for practical separation.Compared to conventional porous materials,metal-organic frameworks(MOFs)[13-16]or porous coordination polymers (PCPs) with tunable pore structure,have been emerged and widely used for C4hydrocarbons separation.Considerable studies have focused on the investigation of metal-organic frameworks for the separation of C4olefins.For example,He and coworkers reported cross-linked MOF ZJNU-80 [14],which has high adsorption capacities (C4H6:157.93 cm3·cm-3) but exhibits very low separation selectivity of 1.0 (uptake ratio of C4H6/n-C4H8at 100 kPa).Chenet al.synthesized a novel MOF of [Zn2(btm)2] with flexible structure [5],showing excellent separation performance for C4hydrocarbons (C4H6,n-C4H8,iso-C4H8:45 cm3·g-1) but low separation selectivity (uptake ratio:1.00).Hybrid ultramicroporous materials (HUMs),a subclass of MOFs,are basically constructed with a 2D square grid based on metal ions coordinated to organic linkers and bridged by inorganic anionsin the third dimension as reported by Prof.Zaworotko and Kitagawa.The pore sizes can be finely tuned by altering the type of anion pillars and metal nodes in the framework.Benefited from the fine-tuned pore size and pore chemistry involving high-density electronegative anions as potential binding sites for gas molecules,HUMs have exhibited glaring merits in light hydrocarbon separations,such as C4olefins separation,via strong host-guest interactions and molecule sieving effect.Our group designed a novel anion-pillared ultramicroporous material ZU-33[15],which displays excellent C4H6/n-C4H8and C4H6/iso-C4H8separation selectivity,while they are limited by expensive raw materials(10,000 USD·kg-1for 4,4′-azopyridine,ligand of ZU-33) as to industrial application[17,18].From the foregoing,it is worthwhile to further study HUMs with low cost and high separation efficiency when it comes to 1,3-butadiene purification.

Herein,we reported the preparation of novel anion-pillared hybrid ultramicroporous material [Ni(pyrazine)2(TaOF5)]n(TaOFFIVE-3-Ni,also termed as ZU-96,ZU=Zhejiang University)for the efficient separation of C4H6/n-C4H8/iso-C4H8[19].The ligand of ZU-96 is pyrazine,which costs only 40 USD·kg-1.The single-component adsorption isotherms demonstrated that ZU-96 can achieve exclusion ofn-C4H8andiso-C4H8under low pressure(0-30 kPa).At 100 kPa and 298 K,the uptake ratio of C4H6/iso-C4H8is up to as high as 20.83.To confirm the efficient separation performance of ZU-96,breakthrough experiments were conducted to separate the mixture of C4H6/n-C4H8/iso-C4H8(50/15/30).In addition,DFT-D (first-principles density function theory) calculations were performed to investigate the adsorption mechanism.This work indicates that ZU-96 is a promising material for the adsorptive separation of C4olefins.

2.Materials and Methods

All reagents were used as received from commercial suppliers without further purification:nickel nitrate hexahydrate(Ni(NO3)2-·6H2O,98.0%,Sinopharm Chemical,China),tantalum pentoxide(Ta2O5,99.5%,Macklin,China),pyrazine (C4H4N2,99%,Macklin),hydrofluoric acid (HF,48%,Sigma-Aldrich,China),methanol(CH3OH,anhydrous,99%,Sinopharm Chemical).

2.1.Synthesis of ZU-96 (Ni(pyrazine)2TaOF5)n

Pyrazine (0.7688 g,0.0096 mol),Ni(NO3)2·6H2O (0.3500 g,0.0012 mol) and Ta2O5(0.1594 g,0.006 mol) were mixed in the Teflon liner.The mixture was firstly diluted with 0.005 L deionized water and then add HFaq(0.0052 L,0.0143 mol)into the container.The autoclave was sealed and heated to 423 K for 36 h.After cooling down the reaction mixture to room temperature,the obtained blue samples were exchanged with methanol for 24 h.

2.2.Crystal structure for ZU-96

Crystal data for ZU-96 were collected at 193 K on a Bruker AXS D8 VENTURE diffractometer (Bruker,Germany) equipped with a PHOTON-100/CMOS detector.Indexing was performed using APEX3.Data integration and reduction were completed using SaintPlus 6.01.Absorption correction was performed by the multi-scan method implemented in SADABS.The space group was determined using XPREP implemented in APEX3.

2.3.Single gas adsorption isotherm measurements

ZU-96 was evacuated at 413 K for 12 h until the pressure dropped below 0.01 kPa.Single-component adsorption isotherms were collected at 273-313 K on activated ZU-96 using ASAP 2050 Analyzer (Micromeritics,USA).

2.4.Breakthrough experiments

The breakthrough experiments were carried out in a dynamic gas breakthrough equipment (Fig.S1).All experiments were conducted using a stainless steel column (4.6 mm inner diameter × 50 mm).The column packed with ZU-96 (0.9899 g)was firstly activated with helium flow (5 ml·min-1) for 24 h at 413 K.Outlet gas from the column was monitored using gas chromatography (GC-490,Agilent,USA) with a thermal conductivity detector coupled with a flame ionization detector.

3.Results and Discussion

3.1.Structure characterization

[Ni(pyrazine)2(TaOF5)]n(TaOFFIVE-3-Ni,also termed as ZU-96,ZU=Zhejiang University) was synthesized by the hydrothermal method using tantalum pentoxide,nickel nitrate and pyrazine in hydrofluoric acid aqueous solution.Structure of ZU-96 was confirmed by the single-crystal X-ray diffraction characterization.The results showed that metal nodes of Ni(II)and pyrazine linkers can form two-dimensional Ni(pyrazine)2layers which were then pillared byto constitute a three-dimensional structure with a primitive cubic (pcu) topology (Fig.1(a)-(b)).The unit cell parameters of ZU-96 area=b=0.99273(3) nm,andc=0.78782(3)nm(Table S2).ZU-96 is a variant of classic SIFSIX-3-Ni via reticular chemistry approach to change inorganic pillars fromto.The scanning electron microscope image shown in Fig.1(g) clearly presented the cubic morphology of ZU-96 crystal.The subtle substitution of inorganic pillars led to a subsequent rotation around 29° of pyrazine molecules of ZU-96,hence the contracted pore diameter was reduced to 0.32 nm (defined by F-F distance,van der Waals radius excluded),much smaller than that of SIFSIX-3-Ni (0.49 nm) [20,21],and the H···H distance of ZU-96 is 0.31 nm (excluding van der Waals radii).The pore window size of ZU-96 is too narrow to accommodate C4isomer theoretically.Nevertheless,due to the rotation freedom of uncoordinated F atoms,the pore size of ZU-96 can range from 0.32 to 0.47 nm.When the F atoms from adjacentpillars in the very state of parallel,the simulated quasi-maximal diameter of ZU-96 turns out to be 0.47 nm,restricted by the F···F distance offrom adjacentpillars.In this case,H···H distance of ZU-96 increase to 0.49 nm(excluding van der Waals radii).The simulated quasi-maximal pore size is just between the kinetic diameter ofn-C4H8(0.446 nm)andiso-C4H8(0.484 nm),implying great potential of ZU-96 as a promising adsorbent [10,22].Composed of narrow pore window and large pocket-shaped cavities,the onedimensional channels of ZU-96 exhibit the feature of efficient adsorbent for gas separation due to the existence of abundant electronegative O and F atoms as potential binding sites (Fig.1(d)).

Excellent stability is an important criterion for industrial applications.Therefore,thermogravimetric analysis (TGA) was conducted to investigate the thermal stability of ZU-96.As shown in Fig.1(e),ZU-96 is stable up to 550 K.As depicted in Fig.1(f),the powder X-ray diffraction(PXRD) data derived from ZU-96 powder matched well with the calculated curves.After activation,the degassed sample of ZU-96 is stable (Fig.1(f)).After exposed to air for one week or immersed in water for one week,these unchanged PXRD characteristic peaks validated good water stability of ZU-96 sample (Fig.1(f)).Moreover,compared with the assynthesised sample,the adsorption capacity of C4H6on ZU-96 is basically unchanged after soaked in water or exposed to air(Fig.S2).

Fig.1.(a)and(b)The schematic structure of ligands and ZU-96.(c)The simulated quasi-maximal pore size of ZU-96.(Color code:F,red;Ta,midnight blue;C,gray-40%;H,gray-25%,O,bright green;N,turquiose;Cu,yellow).(d)The pore structure of ZU-96.(e)The TGA curve of ZU-96.(f)The PXRD results of the ZU-96 sample and stability test of ZU-96 after exposed to air and soaked in water.(g) The SEM image of ZU-96.

3.2.Adsorption isotherms of C4H6,n-C4H8 and iso-C4H8

Based on excellent stability and unique pore structures of ZU-96,we were motivated to investigate the adsorption properties of ZU-96 for C4H6,n-C4H8andiso-C4H8(Fig.2(a)-(b) and Figs.S3-S4).Adsorption isotherms of C4H6,n-C4H8andiso-C4H8on SIFSIX-3-Ni at 298 K were used for comparison [15].At 298 K and 100 kPa,the C4H6uptake of ZU-96 was 92.78 cm3·cm-3,while the uptake capacity ofn-C4H8(47.98 cm3·cm-3) was only half of C4H6,and the uptake ofiso-C4H8was only 4.45 cm3·cm-3.By contrast,SIFSIX-3-Ni exhibited high C4H6andn-C4H8uptake (97.03,96.64 cm3·cm-3) but lowiso-C4H8uptake (20.51 cm3·cm-3)[15].As depicted in Fig.2(b),the two-step isotherms of C4H6andn-C4H8revealed threshold pressure might be 3.5 and 30 kPa,respectively.These above phenomenon was induced by the rotation flexibility of pyrazine and anion pillars of ZU-96 [23-26].The smaller pore size(0.32 nm)of ZU-96 can prevent the entrance ofn-butene(0.446 nm)andiso-butene(0.484 nm)molecule below 30 kPa pressure.With the simulated quasi-maximal pore sizes increased to 0.47 nm,which allowsn-butene to enter the channel,there is a very abrupt increase of uptake at threshold pressure.Similarly,the delayed adsorption isotherms of ZU-96 for C4H6andn-C4H8at 273 and 313 K showed the same gate-opening phenomenon.The smaller pore size of ZU-96 can prevent the entrance ofnbutene andiso-butene molecule below 30 kPa pressure.The industrial composition of C4olefins includes about 50%C4H6,15%n-C4H8and 30%iso-C4H8.Based on the condition of industrial composition,ZU-96 has a considerable adsorption capacity for C4H6(87.32 cm3-·cm-3) while adsorbing only negligible amount ofn-C4H8andiso-C4H8(1.14 and 0.66 cm3·cm-3),indicating exceptional potential of ZU-96 for the separation of C4H6/n-C4H8/iso-C4H8mixtures in industry.The combination of molecular sieving effect and high adsorption capacity is attributed to the unique pocket-like pore channels of ZU-96.

In order to evaluate the separation capability of ZU-96,uptake ratio (defined as the ratio of adsorption uptake of the two adsorbates at 298 K) of C4H6/n-C4H8and C4H6/iso-C4H8were calculated and compared with other reported MOF adsorbents (Fig.2(c)-(d),Figs.S5-S6,Tables S3-S4).As showed in Fig.2(c),The uptake ratio of C4H6/n-C4H8at 50:15 kPa on ZU-96 was up to 76.23.It is much higher than previous MOF adsorbents,such as ZU-33 [15] (16.24),Mg-gallate [27] (7.12),Co-gallate [27] (4.43).The uptake ratio of C4H6/iso-C4H8on ZU-96 at 100:100 kPa is as high as 20.83,which set a benchmark for C4H6/iso-C4H8separation (Mg-gallate [27]:15.08,ZU-33[15]:6.36,SIFSIX-3-Ni [15]:4.73).Compared the C4olefin separation performance of ZU-96 with those of SIFSIX-3-Ni(Table S5),it can be seen that fine-tuning of pore structure is of great importance on gas separation.

3.3.Simulation studies

To gain a better insight into the adsorption binding sites of C4H6in ZU-96,DFT-D(first-principles density function theory)modeling studies were conducted [28].Due to the fact that the anion[TaOF5]2-is disordered with one site split between the O2-,and F-,there are six possibilities of the arrangement of [TaOF5]2-[29].Therefore,the adsorption of C4H6guest molecule in six different sites (Fig.3) are explored in this study to find the respective energy favorable binding sites for 1,3-butadiene.Each C4H6molecules were simultaneously bound with four pillar equatorial F atoms from two adjacent networks.Fig.3(a) and (d) has the same F-Ta-O-O-Ta-F pillars.The only difference is that C4H6is located at the cavity showed in Fig.3(a),while C4H6is located at the bottleneck in Fig.3(d).Fig.3(b)and(e)has the same O-Ta-F-O-Ta-F and F-Ta-O-F-Ta-O pillars.The only difference is that C4H6is located at the cavity showed in Fig.3(b),while C4H6is located at the bottleneck in Fig.3(e).Fig.3(c) has F-Ta-O-O-Ta-F pillars.Fig.3(f) has O-Ta-F-F-Ta-O pillars.C4H6are both located at the bottleneck in Fig.3(c) and (f).Cδ--Hδ+···Fδ-hydrogen bonding is the main interaction for 1,3-butadiene binding.The calculated DFT-D energy demonstrates that the site I would be the most energy favorable binding site for 1,3-butadiene.The electropositive carbon atoms in C4H6molecules are attracted to the TaOF52-pillaring anions,the calculated distance of C-H···F interaction is 0.21-0.27 nm.

Fig.2.(a)The adsorption isotherms of C4H6(red),n-C4H8(blue),and iso-C4H8(green) on ZU-96 at 298 K and 0-100 kPa.(b) The adsorption isotherms of C4H6(red),n-C4H8(blue),and iso-C4H8(green)on ZU-96 at 298 K and 0-50 kPa.(c)Plots of the C4H6 uptake at 50 kPa as a function of C4H6/n-C4H8 uptake ratio at 50:15 kPa for ZU-96 and the other benchmark materials for C4H6/n-C4H8 separation at 298 K.(d)Plots of the C4H6 uptake at 100 kPa as a function of C4H6/iso-C4H8 uptake ratio at 100:100 kPa for ZU-96 and the other benchmark materials for C4H6/iso-C4H8 separation at 298 K.

3.4.Breakthrough tests

To evaluate the dynamic separation ability of ZU-96,the breakthrough experiments were conducted using ternary olefin with helium (C4H6/n-C4H8/iso-C4H8/He,50/15/30/5,v/v/v/v) mixtures.Owing to the exclusion effect,iso-C4H8was eluted through the column immediately,followed byn-C4H8,and then C4H6(Fig.4(a)).C4H6is demonstrated to show high binding affinity with the framework,and is continuously trapped in the column with the time of 51 minutes.The elution sequence ofiso-C4H8＜n-C4H8＜C4H6is also well consistent with single-component adsorption isotherm results.As shown in the Fig.4(b),no obvious decline was observed on the breakthrough performance of ZU-96 during the 4 cycles,indicating the excellent recycle ability of ZU-96.These above results indicate that ZU-96 is a promising material toward the efficient separation of C4H6/n-C4H8/iso-C4H8mixtures for industrial application.

4.Conclusions

In summary,for the first time,a stable anion-pillared ultramicroporous material TaOFFIVE-3-Ni was synthesized and their separation ability for C4olefins was systematically studied.Due to the ration freedom of the linkers of ZU-96,the pore size of ZU-96 is similar with the kinetic diameters of C4H6(0.431 nm) andn-C4H8(0.446 nm),but smaller thaniso-C4H8(0.484 nm),resulted in a high binding affinity for C4H6(92.78 cm3·cm-3) while withiso-C4H8excluded.In addition,the dynamic breakthrough experiments together with stability test of ZU-96 confirm that ZU-96 is an promising adsorbent for C4H6/n-C4H8/iso-C4H8separation.This study not only demonstrates the great potential of ZU-96 for C4H6/n-C4H8/iso-C4H8separation but also provides solutions for other gas separations.

Fig.3.The DFT-D calculated results of 1,3-butadiene within ZU-96.F,red;Ta,dark blue;C,gray-40%;H,gray-25%;O,bright green;N,turquoise;Cu,yellow;the F-F distance includes the van der Waals radius.C-H···F interactions are displayed as red,green and black lines,respectively.

Fig.4.(a)Experimental column breakthrough curve for the mixture of C4H6/n-C4H8/iso-C4H8/helium(50/15/30/5)with ZU-96 at 273 K.(b)Cyclic column breakthrough tests for C4H6/n-C4H8/iso-C4H8/helium (50/15/30/5) on ZU-96 at 273 K.

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

This work was supported by Natural Science Foundation of Zhejiang Province(LR20B060001 and LZ18B060001),the National Natural Science Foundation of China (21725603,21938011),the Entrepreneur Team Introduction Program of Zhejiang(2019R01006),and the Research Computing Center in College of Chemical and Biological Engineering at Zhejiang University.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.09.001.

Data Availability

CCDC 2085735 contain supplementary single-crystal X-ray diffraction data for ZU-96 structures.These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via https://www.ccdc.cam.ac.uk/structures/.