Gas adsorption in shaped zeolitic imidazolate framework-8☆

Jiqin Zhu,Lu Jiang,Chengna Dai,Na Yang,Zhigang Lei*

State Key Laboratory of Chemical Resource Engineering,Beijing University of Chemical Technology,Beijing 100029,China

Keywords:ZIF-8 Shaping Gas adsorption Model reduction Materials

ABSTRACT Zeolitic imidazolate framework-8(ZIF-8)was prepared through a solve-thermal reaction method and then shaped using different additives.The influence of the shaping conditions on the microstructure of the shaped samples was characterized by the XRD,BET,and SEM techniques.The results demonstrate that the compressive strength of the various shaped tablets is greatly increased and capable of meeting the industrial requirements compared to the unshaped ZIF-8 and that the loss rate of specific surface areas was maintained at 10%after the addition of 10%(by mass)binder and 10%(by mass)solvent.The adsorption isotherms of CO2,CH4,C3H8,and C3H6 on powdery ZIF-8 and the shaped tablets(T-shaped ZIF-8,C-shaped ZIF-8,and N-shaped ZIF-8)were determined through volumetric measurements under different pressures and temperatures(298.2,323.2,and 348.2 K).The adsorption capacities of the gases on both the ZIF-8 powder and the shaped tablets follow the order C3H6＞C3H8＞CO2＞CH4.Furthermore,the results show that the adsorption capacities of the gases on the shaped tablets are lower by approximately 10%–20%than those on the powdery ZIF-8.In fact,the adsorption equilibrium isotherms for CO2,CH4,C3H8,and C3H6 on both powdery and shaped ZIF-8 can be well described by the Langmuir equation.

1.Introduction

Zeolitic imidazolate frameworks(ZIFs),which are a new type of porous materials,possess several remarkable features,such as controllable structures,adjustable pore size,enormous specific surface area,and particularly good hydrothermal stability.The application of ZIFs in gas adsorption,membrane separation,energy storage,and catalysis has gradually aroused extensive attention[1–5].In the family of ZIFs,ZIF-8 is a typical compound with a sodalite(SOD)zeolitic-type structure,in which the tetrahedral Si(Al)sites are replaced by transitional metal Zn and coordinated by dimethyl imidazoles as ligands to form four-ring and six-ring ZnN4clusters.The aperture size of ZIF-8 is 0.34 nm,and the diameter ofthe largest sphere is 1.16 nm[6–8].Unlike traditional zeolites,which have rigid frameworks and stable and small pore sizes,ZIF-8 is capable of capturing molecules with pore diameters larger than 0.34 nm[9–12].Hu et al.[13]characterized the physical structure,chemical stability,and adsorption and separation performances of ZIF-8 adsorbent and emphasized that ZIF-8 is likely more promising for practical applications.Yamamoto et al.[14]examined the adsorption properties of the synthesized ZIF-8 nanoparticles in a T-type micromixer and found that the amount of adsorbed N2gas is higher than that obtained with a conventional ZIF-8 sample.Huang et al.[15]investigated the effect of temperature on the adsorption of gases in ZIF-8 using a combination of experimental measurements and molecular simulations and demonstrated that the effect of temperature is significant for pure gas adsorption.Hu et al.[16]explored the storage behavior of ZIF-8 and found that is it strongly dependent on pressure.Liu et al.[17]measured single-gas permeation data for ZIF-8 membranesand found that the diffusivity value decreases with increasing molecular size.Zhang et al.[18]modified the ZIF-8 powder to EDZIF-8,which exhibits improved selectivity for CO2/N2separation.In addition,Evangelia et al.[19]simulated the self-diffusivity and the collective diffusivities of a gas mixture of CO2and CH4with the ZIF-8 and revealed that the ZIF-8 ligands had a significant influence on the adsorbate transportation.

Although ZIF materials have been widely studied in the field of gas separation,almost all of these studies were limited to the use of powdery particles,which may face large fluid resistance in practice due to the limitation associated with the pressure drop[20].Thus,powdery ZIF-8 needs to be shaped prior to its usage for adsorption purposes.Thus,we decided to study different shaping methods with respect to ZIF-8.It has been noted that the shaping process reduced the surface area of the original material and may lead to a partial collapse of the structure[21].Therefore,a shaping technology that can retain the micro-structure of ZIF-8 is urgently needed.In this regard,Sumida et al.[22]used activated carbons and zeolite 13×,which were loaded with MEA and TEA through solution processing,to study the performance achieved with capturing CO2and found that both materials exhibited a decreased CO2capacity compared with the bare material.Ferreira et al.[23]noted that Cu-BTC extrudates presented a reduction of 63%in their adsorption capacities for C3H8and C3H6compared with Cu-BTC powder.Finsy et al.[24]indicated that the adsorption capacities of N2decreased by 32%in contrast to the pure MIL-53 powder.These findings indicate that the shaping process may have significant influence on the properties of the resulting material.

The purpose of this work was to resolve the key bottlenecks limiting the practical application of ZIF-8 by screening suitable additives and determining the superior shaping conditions.The adsorption isotherms of various single gases(CO2,CH4,C3H8,and C3H6)on powdery ZIF-8 and shaped tablets were measured at different temperatures and pressures to investigate the influence of the shaping conditions on the adsorption performance.

2.Experimental

2.1.Preparation of ZIF-8

The ZIF-8 nano-crystal was prepared according to reported procedures[7]with certain modifications.The molar ratio of 2-methylimidazole to zinc nitrate hexahydrate[Zn(NO3)2·6H2O]was adjusted to 3:1 with several attempts.The solid mixtures of 2-methylimidazole(0.791 g)and Zn(NO3)2·6H2O(0.955 g)were dissolved in N,N-dimethyl form amide(DMF,72 ml)in a PTFE-lined container.The mixed solution was sealed,heated to 140°C in an oven,maintained at this temperature for 24 h,and cooled naturally to room temperature.After the mother liquor was removed from the mixture in the filtration bottle,white powders were collected from the upper layer.These powders were then washed three times with absolute methanol and dried in air for 30 min.

2.2.Shaping process of ZIF-8

In this work,the process used to shape ZIF-8 was implemented based on tableting technology.Methylcellulose,bentonite,silica,alumina,SB powder(high-quality pseudoboehmite,Sasol,Germany),talc powder,and sesbania powder were chosen as binders,and deionized water,nitric acid,citric acid,and trichloroacetic acid were chosen as solvents.Accurately weighed ZIF-8 powder(sieved through a 120 mesh)was mixed with different amounts of one of the binders and solvents at room temperature and then pressed at 1.5 kN in the TDP-1.5 single push tablet pressing machine.The shaped ZIF-8 tablets,which exhibited a diameter of 9 mm and a thickness of 6 mm,were dried naturally for 30 min and heated at 120°C for 2 h in the oven.

2.3.Characterization

Scanning electron microscopy(SEM)images were acquired on a Hitachi S4700 instrument(Hitachi Ltd.,Tokyo,Japan)using an accelerating voltage of 20 kV.The pretreated sample was coated onto the silicon wafer and sprayed with gold.

The X-ray diffraction(XRD)patterns were measured on a Bruker D8ADVANCE X-ray diffractometer(40 kV,40 mA)using CuKα(λ =0.15418 nm)radiation at a scanning rate of 1(°)·min?1from 5°to 50°.

The BET specific surface areas and were determined through an N2adsorption–desorption experiment using an ASAP2020 physical sorption instrument at 77 K.Prior to the adsorption experiment,the sample was heated at 200°C for 8 h,and the BET specific surface area was measured by a Brunauer–Emmett–Teller model.

The compressive strength of the shaped tablets was assessed using the DL-II type intelligent particle strength tester(Penghui Ltd.,Dalian,China),which recorded the pressure value at which the shaped tablet started to exhibit crazing.Eight samples for each shaping condition were randomly measured,and the average value was considered the final compressive strength.

2.4.Gas adsorption experiments

The adsorption equilibrium isotherms of CO2,CH4,C3H8,and C3H6on the powdery ZIF-8 and three types of shaped tablets(using different solvents during the shaping process)were measured at temperatures of 298.2,323.2,and 348.2 K by a static adsorption device as presented in Fig.1.

The volume of the adsorption cell was measured by charging the helium gas at different pressures and calculating the volume ratio relative to the reference cell via the second-order Virial equation.In this way,the volumes of reference cell and adsorption cell are determined to be 122.1 and 78.0 cm3,respectively.Prior to the adsorption measurement,the adsorbent must be evacuated at 423.2 K until no mass loss was observed.A certain amount(about 10 g)of adsorbent was then carefully loaded into the adsorption cell in an attempt to reduce the adhesive attrition and ensure the precise determination of the free volume.The single adsorbed gas was then introduced into the reference cell under a given pressure and passed through the adsorption cell.Both the adsorption and reference cells were immersed in a water bath controlled by an advanced digital temperature controlling system with an uncertainty of 0.1 K.The adsorption equilibrium was assumed to have been reached when the system pressure measured by a pressure gauge(ExSAF)with an uncertainty of±1 kPa was invariable.A typical equilibrium was 30 min per point.During the measurement,the dead volume of the gas pipeline between the adsorption cell and reference cell was also taken into account to improve the accuracy of the experimental data,and this volume was calculated from the internal volumes of the valves and pipeline and found to be approximately 3.51 cm3.

3.Results and Discussion

3.1.Structure of ZIF-8

Fig.2(a)and(b)presents the surface morphologies of the powdery ZIF-8 and shows the clear outer cubic surface of the ZIF-8 unit cells.The XRD pattern of ZIF-8 is displayed in Fig.2(c),and the characteristic peaks match well with those reported by Park et al.[7],who clearly indicate the existence of ZIF-8 crystals.Fig.2(d)presents the N2adsorption-desorption analysis,which showed a high adsorption capability for N2of up to 400 cm3·g?1.The adsorption curve belongs to Type I,in line with the Langmuir model.Moreover,the N2adsorption–desorption analysis displayed that the BET surface area of ZIF-8 is 1186.5 m2·g?1,and the Langmuir surface area is 1746.7 m2·g?1.These values are consistent with those reported in previous studies[7,25],in which ZIF-8 was synthesized in a reactor that was 20 ml smaller than that used in the present study.The pore size distribution was obtained by BJH and DFT methods,as shown in Fig.2(e)and(f).The pore diameters mainly locate at the range of 1.0–1.4 nm,and the volumes of micro and total pores are 0.5933 and 0.8087 cm3·g?1,respectively.

3.2.Selection of shaping binders and solvents

3.2.1.Influence of binders on specific surface areas of the shaped samples

In this work,seven types of binders(methyl cellulose,silica,bentonite,alumina,SB powder,talc powder,and sesbania powder)were selected.Moreover,the influence of solvent concentration(using SB and talc powders as binder,and nitric acid as solvent at different concentrations)on specific surface area of shaped ZIF-8 was investigated,and the results are listed in Table 1.It can be seen that BET surface area is the highest when using 10%(by mass)nitric acid as solvent for both SB and talc powders.

Each binder,as well as 10%(by mass)nitric acid,was combined with powdery ZIF-8.The experimental results are given in Table 2.Due to the minimal loss of specific surface areas obtained with talc powder and SB powder,one of these binders was used as the single binder in the subsequent experiments.In addition,the specific areas obtained with silica or alumina powders as binder seem to be slightly lower than those with talc or SB powders,indicating that silica or alumina powder seems to be potential binder.Thus,these two kinds of powders should be considered in the further work.

Fig.1.Schematic diagram of the adsorption device.1—Gas cylinder;2—temperature display;3—reference cell;4—adsorption cell;5—cylinder regulator;6—pressure transducer;7—shutoff valve;8—three-way valve;9—pressure sensor;10—pressure and temperature display;11—vacuum pump.

Furthermore,to investigate the influence of the amount of binder added and the type of solvent(water,nitric acid,citric acid,and trichloroacetic acid)on the specific surface area of the shaped samples,the BET specific surface areas of various shaped samples with different mass fractions of the binders were measured.

The data shown in Table 3 demonstrate that the BET specific surface area decreases with an increase in the binder amount.This effect may be attributed to the degradation of the intra-crystalline porosity of ZIF-8 during the shaping process.Moreover,the specific surface areas of the shaped samples are larger when 10%(by mass)binder,as well as 10%(by mass)citric acid(or trichloroacetic acid)as the solvent,is used.However,the specific surface area increases with a decrease in the acidity.

In addition,the influence of composite binders composed of SB and talc powders at different mass ratios on the specific surface area was investigated;in these experiments,the total binder amount was maintained at 10%(by mass).

The data shown in Table 4 illustrate that the specific surface area decreases with the use of composite binders.This effect may be attributed to the interaction between the binders,which may result in a number of partially clogged pores.Thus,it is advisable to use a single binder composed,i.e.,either SB powder or talc powder,rather than their combination.

3.2.2.Influence of binders and solvents on crystal structure

The crystallizations of the shaped tablets with two types of binders(talc powder and SB powder)and four types of solvents(water,nitric acid,citric acid,and trichloroacetic acid)were evaluated by the XRD technique,as displayed in Fig.3.

The data show that the characteristic peaks of the shaped ZIF-8 samples are in good agreement with those of the powdery ZIF-8,which indicates that the shaping method presented in this work does not have a detrimental effect on the crystal structure of the material,which is very important for remaining the physicochemical properties of ZIF-8.

3.2.3.Influence of binders and solvents on compressive strength

In industrial application the materials are requested to have enough mechanical strength.Thus,in this work the strengths of the different samples obtained under different shaping conditions were tested using a compressive strength tester.Because the shaped samples were tablets,the force per square meter was measured as the compressive strength.

The data shown in Table 5 indicate that the tableting technology can greatly improve the compressive strength of the shaped samples.Furthermore,the compressive strength tended to increase with an increase in the acidity.However,an increase in the binder loading resulted in an initial increase followed by a decrease in the compressive strength,indicating that a greater binder loading is not necessarily better.In general,the compressive strength of the shaped samples prepared using the high-pressure tableting method is sufficient for industrial application.

3.3.Influence of temperature on ZIF-8 adsorption

Adsorption experiments were performed to investigate the influence of temperature on the adsorption performance of four types of gases(CO2,CH4,C3H8,and C3H6)at various temperatures(298.2,323.2,and 348.2 K)on powdery and shaped ZIF-8.

Fig.2.Characterization of powdery ZIF-8.(a)SEM image of ZIF-8 unit cells(500×magnification);(b)SEM image of ZIF-8 unit cells(2500×magnification);(c)XRD pattern of ZIF-8;(d)N2 adsorption–desorption plot of ZIF-8;■ adsorption data;○desorption data;(e)pore size distribution obtained by BJH method;(f)pore size distribution obtained by DFT method.

Table 1 Specific surface areas of the shaped ZIF-8 samples using nitric acid with different concentrations as solvent

Fig.4 illustrates the adsorption equilibrium of CO2,CH4,C3H8,and C3H6on the powdery ZIF-8.As shown,the adsorption capacity of CO2on powdery ZIF-8 is 349.0 mg·g?1at 298.2 K and 5000 kPa,which is consistent with the results reported by Park et al.[7].The adsorption capacities decrease from 276.4 to 200.5 mg·g?1as the temperature increases from 323.2 to 348.2 K at a constant pressure of 5000 kPa.At 298.2,323.2,and 348.2 K and a pressure of 5000 kPa,the adsorption capacities of CH4on the powdery ZIF-8 are 104.1,92.8,and 77.9 mg·g?1,respectively.At a pressure of 600 kPa and temperatures of 298.2,323.2,and 348.2 K,the adsorption capacities of C3H8are 185.2,151.2,and 133.0 mg·g?1,respectively,whereas those of C3H6can reach 208.1,194.4,and 174.9 mg·g?1,respectively.The adsorption isotherms of C3H8and C3H6on the powdery ZIF-8 are similar to the results reportedrecently by Zhang et al.[26].Moreover,the experimental data indicate that the adsorption capacities of the four gases on powdery ZIF-8 follow the order C3H6＞C3H8＞CO2＞CH4at the same temperature and pressure(less than 700 kPa).The adsorption capacity of CO2(kinetic diameter 0.33 nm)is higher than that of CH4(kinetic diameter 0.38 nm),mainly due to the linear molecular structure of CO2.Interestingly,because of aperture flexibility of ZIF-8,the capacities of C3hydrocarbon molecules(kinetic diameters 0.45 and 0.43 nm for C3H6and C3H8respectively)are even larger than those of CO2gas at low pressures.In addition,the experimental results indicate that the effect of temperature on the adsorption capacities is pronounced due to the variation in the interaction energy between the adsorbate and the adsorbent.Thus,a high temperature is unfavorable for the application of powdery ZIF-8.Furthermore,a higher initial pressure results in a greater adsorption capacity and a longer adsorption equilibrium time.

Table 2 Specific surface areas of the shaped ZIF-8 samples(obtained using 10%nitric acid as the solvent)

Table 3 Specific surface areas of the shaped samples obtained using different binders and solvents(m2·g?1)

3.4.Influence of shaping process on adsorption

The adsorption isotherms at 298.2 K of the four gases on the shaped ZIF-8 samples obtained using 10%(by mass)SB powder as the binder along with the three types of solvents[10%(by mass)trichloroacetic acid,10%(by mass)citric acid,and 10%(by mass)nitric acid]were measured,as shown in Fig.5.For comparison,these three adsorbents are labeled T-shaped ZIF-8(using trichloroacetic acid as solvent),C-shaped ZIF-8(using citric acid as solvent),and N-shaped ZIF-8(using nitric acid as solvent),respectively.

As shown in Fig.5,the adsorption capacities on the T-shaped ZIF-8,C-shaped ZIF-8,and N-shaped ZIF-8 are slightly lower than those obtained on the powdery ZIF-8 under the same conditions.After beingshaped,the surface area of ZIF-8 may be reduced due to a partial collapse of the structure.The volumes of micro-and meso-pores decrease significantly for the shaped ZIF-8.For example,for T-shaped ZIF-8,it isfound that although the diameter does not change obviously,the volume of microspores near 1.1 nm decreases significantly.Moreover,the volume of mesopores at diameters over 30 nm almost disappears.Therefore,the adsorption capacity is lower on shaped ZIF-8 than powdery ZIF-8.In fact,the adsorption amounts of CO2on these three types of shaped ZIF-8 are 315.5,295.5,and 271.3 mg·g?1at 298.2 K and 5000 kPa,respectively.Under the same conditions,the adsorption capacities of CH4are only 97.1,89.4,and 82.6 mg·g?1,respectively.Compared to those of CO2and CH4,the adsorption capacities of C3H8and C3H6on these three shaped samples are higher.At 298.2 K and 600 kPa,the adsorption capacities of C3H8are 176.7,173.4,and 159.4 mg·g?1,respectively,whereas those of C3H6are 198.9,192.6,and 172.7 mg·g?1,respectively.In comparison with the powdery ZIF-8,the adsorption capacities on the shaped samples are decreased by approximately 10%–20%.This loss ratio is significantly lower than those previously reported by other researchers,e.g.,63%for C3H8and C3H6on Cu-BTC powder[23]and 32%for N2on MIL-53 powder[24].This finding demonstrates that the shaping method adopted in this work is beneficial to maintaining a high gas adsorption capacity on shaped ZIF-8.

Table 4 Specific surface areas of the shaped samples obtained using composite binders(m2·g?1).The total amount of the composite binders was maintained at10%(by mass)of the shaped samples

Fig.3.XRD patterns of the shaped samples.(a)a:ZIF-8;b:ZIF-8+10%talc powder+10%deionized water;c:ZIF-8+10%talc powder+10%nitric acid;d:ZIF-8+10%talc powder+10%citric acid;e:ZIF-8+10%talc powder+10%trichloroacetic acid.(b)a:ZIF-8;b:ZIF-8+10%SB powder+10%deionized water;c:ZIF-8+10%SB powder+10%nitric acid;d:ZIF-8+10%SB powder+10%citric acid;e:ZIF-8+10%SB powder+10%trichloroacetic acid.

Table 5 Compressive strength of the shaped samples obtained using different binders and solvents

Fig.4.Adsorption equilibrium isotherms of four gases on powdery ZIF-8 at 298.2,323.2,and 348.2 K.(a)CO2;(b)CH4;(c)C3H8;(d)C3H6.The solid line is the fitted Langmuir adsorption equation.■ 298.2 K;● 323.2 K;▲ 348.2 K.

3.5.Adsorption isothermal model

In principle,the adsorption behavior measured in this work is known as excess adsorption.Thus,the adsorption capacities of the gases(CO2,CH4,C3H8,and C3H6)on the powdery and shaped ZIF-8 can be fitted by the following Langmuir adsorption equation:

where P is the pressure concentration of the adsorbent(kPa);q is the amount of solute adsorbed by a unit mass of the adsorbent;and qmand b are the Langmuir adsorption equation parameters,which can be obtained by fitting the experimental data.

The fitting procedure was accomplished using the optimization algorithm in Microsoft Excel 2003.Table 6 lists the fitting parameters of the Langmuir adsorption equation and the corresponding correlation coefficients(R2),which indicates that the Langmuir adsorption equation is capable of reproducing the experimental data for both powdery and shaped ZIF-8 with high correlation coefficients of R2＞0.975.The fits obtained from the Langmuir adsorption equation are also represented with solid lines in Figs.4 and 5.

4.Conclusions

Fig.5.Adsorption equilibrium isotherms of four gases on three shaped ZIF-8 samples at 298.2 K.(a)CO2,(b)CH4,(c)C3H8,and(d)C3H6.The solid line is the fitted Langmuir adsorption equation.■,T-shaped ZIF-8;●,C-shaped ZIF-8;▲,N-shaped ZIF-8.

Table 6 Model parameters obtained by fitting the experimental data to the Langmuir adsorption equation

In the present work,the shaping ofZIF-8 was studied to determine if the resulting frameworks meet the practical requirements for gas separation.Through the selection of the best shaping additives,the preferable ZIF-8 shaping method was determined.SB powder or talc powder was selected as the suitable binder and was used with four types of solvents.Although the specific surface areas of the shaped samples are reduced,the compressive strength was found to be increased.The unchangeable micro structures can satisfy the industrial demand.The adsorption capacities of CO2,CH4,C3H8,and C3H6on the powdery and shaped ZIF-8 were then measured and were found to follow the same order of C3H6＞C3H8＞CO2＞CH4on both frameworks.The experimental results revealed that the adsorbed capacities of the four gases on the shaped tablets are roughly equivalent to the specific surface area loss ratios of the shaped samples.In the present work only the adsorption properties of four pure gases are investigated,while the gas separation performance using the shaped ZIF-8 is not included.In the further work,application in gas separation should be paid more attention.Furthermore,the experimental data could be well fitted by the familiar Langmuir adsorption equation.

To the best of our knowledge,this study provides the first measurement of the adsorption capacities of important gases,such as CO2and CH4,on shaped MOF(metal-organic framework)materials(ZIFs belong to a subclass of MOFs).In particular,a decrease of only approximately 10%–20%in the adsorption capacities was observed on the shaped ZIF-8,and this decrease is significantly lower than the values reported for other shaped MOFs.Thus,the shaping method presented in this work has practical value.