Xuejiao Sun,Jinpeng Miao,Jing Xiao,Qibin Xia*,Zhenxia Zhao

Separation Science and Engineering

Heterogeneity of Adsorption Sites and Adsorption Kinetics of n-Hexane on Metal-Organic Framework MIL-101(Cr)☆

Xuejiao Sun,Jinpeng Miao,Jing Xiao,Qibin Xia*,Zhenxia Zhao

School of Chemistry&Chemical Engineering,Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution Control,South China University of Technology, Guangzhou 510640,China

The heterogeneity of adsorption sites and adsorption kinetics of n-hexane on a chromium terephthalate-based metal-organic framework MIL-101(Cr)were studied by gravimetric method and temperature-programmed desorption(TPD)experiments.The MIL-101 crystals were synthesized by microwave irradiation method.The adsorption isotherms and kinetic curves of n-hexane on the MIL-101 were measured.Desorption activation energiesofn-hexanefromtheMIL-101wereestimatedbyTPDexperiments.Theresultsshowedthatequilibrium amount of n-hexane adsorbed on the MIL-101 was up to 5.62 mmol·g?1at 298 K and 1.6×104Pa,much higher than that of some activated carbons,zeolites and so on.The isotherms of n-hexane on the MIL-101 could be well f i tted with Langmuir-Freundlich model.TPD spectra exhibit two types of adsorption sites on the MIL-101 with desorption activation energies of 39.41 and 86.69 kJ·mol?1.It ref l ects the surface energy heterogeneity on the MIL-101 frameworks for n-hexane adsorption.The diffusion coeff i cients of n-hexane are in the range of(1.35-2.35)×10?10cm2·s?1with adsorption activation energy of 16.33 kJ·mol?1.

?2014TheChemicalIndustry andEngineeringSocietyofChina,andChemicalIndustryPress.Allrightsreserved.

Adsorption

Kinetics

n-Hexane

MIL-101

Heterogeneity

1.Introduction

Volatile organic compounds(VOCs)are usually known as chemical pollutants as they are often noxious,malodorous and carcinogenic evenatverylowconcentrations[1,2].n-Hexaneisa typical and harmful VOC.In industry,it is widely used not only in the formulation of glues for shoes,leather products,and roof i ng,but also as organic solvent for extracting cooking oils from seeds,for cleansing and degreasing a variety of items,and in textile manufacturing.In addition,hexanes are signif i cant constituents of gasoline.It is frequently detected in gasoline depots,petrochemical facilities and chemical plants due to its emission. The emission of n-hexane causes air pollution and damage to human health such as occupational neuropathies in shoemakers,purse makers and furniture f i nishers.Therefore,the control and abatement of its emission are imperative.Various techniques have been proposed for the removal of VOCs,including thermal or catalytic oxidation, biof i ltration,absorption,adsorption,condensation,membrane separation,etc.[3-5].Among them,adsorption is one of the most effective techniques for VOC removal due to its easy operation and low cost, being able to selectively trap low concentration of VOCs[6-9].

Adsorbent is the core of adsorptiontechnique.Adsorbents suchas zeolites,mesoporous silicas,adsorptive resins,and activated carbons have been studied widely for n-hexane removal[10-12].M?ller et al.[13] measured the adsorption capacity and diffusion coeff i cient for n-hexane and reported the maximum n-hexane uptakes of 1.54 mmol·g?1at 377 K and 1.09 mmol·g?1at 303 K on 5A-zeolite and ZSM-5,respectively.Zhao et al.[14]reported that the adsorption capacities of MCM-41,activated carbon and hydrophobic zeolite for n-hexane were 6.38, 3.59 and 2.32 mmol·g?1,respectively,at 295 K and 1.6×104Pa.

Currently,metal organic frameworks are rapidly developed as potential adsorbents for VOCs due to their extra-rich porosity,regular porous structures,and manipulative pore sizes and chemical functionalities via the modif i cation of metal groups or organic linkers [15-18].Trung et al.[19]reported n-hexane uptakes of 2.98 and 2.24 mmol·g?1at 313 K on MIL-53(Cr)and MIL-53(Al),respectively. It is worthy of attention that MIL-101,one of the most porous materials to date[20-22],possesses an extra-high specif i c surface area and pore volume with the framework structure of microporous apertures and mesoporous cages.MIL-101alsoshows outstandingthermalandchemical stability.Therefore,MIL-101attractsmoreattentioninthepotential application of gas adsorption.Huang et al.[23]reported the n-hexane uptake of 0.07 mmol·g?1at 303 K and 2.25×102Pa on the MIL-101. Hong et al.[24]reported the maximum n-hexane adsorption capacity of 12.6 mmol·g?1at 303 K.However,the study on adsorption kinetics and desorption of n-hexane on MIL-101 is rare[23,25],while theadsorption kinetics and desorption properties are important for evaluating an adsorbent and designing adsorption process.

The objective of this work is to investigate the adsorption equilibrium and kinetics of n-hexane on MIL-101,since n-hexane is widely used as organic solvent and is a model molecule representing a categoryofsmallnon-polarmolecules.TheMIL-101crystalsaresynthesized by the microwave irradiation method.The adsorption isotherms and kinetics of n-hexane on MIL-101 samples are measured by gravimetric method.The diffusion coeff i cients and adsorption activation energy of n-hexane on the MIL-101 are estimated.Temperature program desorption(TPD)experiments are conducted to evaluate the interaction between n-hexane and MIL-101.The adsorption and desorption characteristics of n-hexane on MIL-101 samples are reported.

2.Experimental

2.1.Materials

Terephthalic acid(H2BDC,99%,A.R.)was purchased from Alfa Chemicals,chromium nitrate nonahydrate(Cr(NO3)3?9H2O,99%, A.R.)was purchased from Fuchen Chemicals Co.Ltd.of Tianjin (China),and hydrof l uoric acid(A.R.)was purchased from Guangzhou Chemicals Co.Ltd.(China).

2.2.Synthesis of MIL-101 crystals

MIL-101 was synthesized using microwave synthesis method,with the general procedure described elsewhere[26].Firstly,Cr(NO3)3·9H2O (2.395 g,6 mmol)and 1,4-benzene dicarboxylic acid(0.994 g,6 mmol) were dissolved in deionized water(24 mL),and hydrof l uoric acid(0.26 mL)wasaddedtothemixture.Secondly,thereactantmixturewasloaded intoaTef l onautoclave,whichwassealedandplacedinamicrowaveoven (Mars-5,CEM).Theautoclavewas heated to 483 K in 10 min and kept at this temperaturefor 60 min,and thencooledtoroom temperaturefor 3 h.Fine green colored powder was obtained as the major product,while a signif i cant amount of recrystallized H2BDC could be found.To acquire pure crystalline material with high porosity and surface area,a series of purif i cations were applied to remove unreacted H2BDC and other guest molecules in the sample.The green product was doubly f i ltered with glass f i lters to remove residual terephthalic acid.The product was washed with N,N-dimethylformamide,hot ethanol and NH4F solution(30 mmol·L?1),and dried at 423 K for 12 h.Finally,a crystalline material of MIL-101 was obtained.

2.3.Characterization of MIL-101 crystals

ThepowderX-raydiffraction(XRD)characterizationwasperformed on a D8 Advance diffractometer(Bruker,German)operating at 40 kV and 40 mA by using Cu-Kαradiation(λ=0.1540 nm)with a scan speed of 2(°)·min?1and a step size of 0.02°in 2θ.

The textural parameters of MIL-101 crystals were measured on a Micrometrics gas adsorption analyzer ASAP 2010 instrument with commercial software for calculation and analysis.

2.4.n-Hexane vapor adsorption experiments

n-Hexane adsorption experiments were carried out by using the intelligent gravimetric analyzer(IGA-003,Hiden)for measurement of n-hexane isotherms.An ultra-sensitive microbalance of resolution 0.2 μg is mounted in the thermostated heatsink with high precision temperature control.The sample was about 40 mg for each run.Before measurements,the sample in the vessel of IGA-003 was vacuumed up to 3-5 Pa and outgassed at 423 K for about 5 h.The measurements were performed at four temperatures(288 K,298 K,308 K and 318 K) in terms of the built-in water baths.The uptakes of n-hexane on the sample can be calculated by

q=1000( We?Wa)

(1)

e

Wa

q=1000( Wt?Wa)

(2)

t

Wa

where Weand Wtare the amounts of MIL-101 sample at equilibrium and time t,respectively,Wais the initial mass of sample,qeand qtare n-hexane amounts adsorbed per gram of adsorbent at equilibrium and time t,respectively.

2.5.Temperature programmed desorption experiments

In ordertoestimatedesorptionactivation energyof n-hexaneon the MIL-101 sample,TPD experiments were carried out with gas chromatography workstation(GC-9560,Haixin Company,China).TPD experiments were conducted at different heating rates from 4 to 9 K·min?1. For each run,the MIL-101 sample with adsorbed n-hexane vapor was packed in a stainless steel reaction tube with inner diameter of 0.3 cm and packed length of about 0.5 cm.The stainless tube was placed in a reaction furnace and heated in the high-purity N2fl ow at a constant fl ow rate of 50 mL·min?1.The desorbed n-hexane was measured on line by GC-9560 chromatograph with a fl ame ionization detector at theoutletofthereactiontubeandef fl uentcurves(TPDcurves)wererecorded.The detailed procedure can be found elsewhere[27-29].

Fig.1.Powder XRD pattern(a)and nitrogen isotherms at 77 K(b)of synthesized MIL-101.

Fig.2.Isotherms of n-hexane on MIL-101 crystals at different temperatures.Points—experimental data;solid curves—Langmuir-Freundlich equation.

3.Results and Discussion

3.1.Physical characteristics of MIL-101 crystals

Fig.1(a)shows the powder XRD pattern of the synthesized MIL-101 sample.The main diffraction peaks appear at 2θ=3.29°,5.88°,8.44°, 9.06°,10.34°and 16.53°,consistent with those reported in[30], conf i rmingtheformationofMIL-101structure.Fig.1(b)showsnitrogen adsorption and desorption isotherms of the MIL-101,presentingtypical type-I prof i les with secondary uptakes,which is a characteristic of the two microporous windows[24].With the textural parameters measured and analysis on N2adsorption isotherms,the Langmuir surface area and BET surface area of the MIL-101 are obtained as 4792 and 3360 m2·g?1,respectively.

3.2.Adsorption isotherms of n-hexane on MIL-101 crystals

Fig.2 presents the adsorption isotherms of n-hexane vapor on the MIL-101.The n-hexane uptake increases drastically with pressure at low pressures,attributed to the micropore adsorption.At higher partial pressures,n-hexane adsorption capacities remain nearly the same at different temperatures,possibly due to the commensurate adsorption of n-hexane on the MIL-101,in which the molecular size and shape of guest molecule result in specif i c orientation and the adsorption uptake (at equilibrium)is compatible and self-consistent with the crystal symmetry and pore structure of MIL-101[31].n-Hexane presents an adsorption limit,so its adsorption capacity is no longer affected by temperature(288-318 K)at higher partial pressures.For comparison, Table1listsn-hexaneadsorptioncapacitiesofsomeadsorbents.Theadsorption capacity of n-hexane on the MIL-101 synthesized in this work is up to 5.62 mmol·g?1at 298 K and 1.6×104Pa,which is 1.9 times than that of MIL-53(Cr),1.6 times than that of activated carbon BPL, 2.4 times than that of hydrophobic zeolite Y,and 3.9 times than that of TNTs-S-2.At higher pressure,the adsorption capacity of the MIL-101 is lower than that of MCM-41,but at low pressure (7×102Pa),it is 8.8 times that of MCM-41,suggesting that the MIL-101 sample is suitable to capture n-hexane at low concentration due to its rich micropores.

TheLangmuir-Freundlichequationisappliedtof i ttheexperimental isotherm data of n-hexane in order to describe the adsorption of n-hexane on the MIL-101.The equation can be expressed as

Table 2Parameters of Langmuir-Freundlich model for n-hexane adsorption on the MIL-101 sample

where q is the adsorbed amount in equilibrium with the concentration of adsorbate in gas phase,qmaxis the maximum adsorption amount,P is the equilibrium pressure of adsorbate in gas phase,K is equilibrium constant of adsorption,and n is the Langmuir-Freundlich coeff i cient.

Table 2 lists theparameters of Langmuir-Freundlich model withlinear regression of n-hexane adsorption data on MIL-101 samples.The high regression coeff i cients R2indicate that the model represents the isotherms well.The value of K gradually decreases with temperature, suggesting that physisorption is dominant in this system.Coeff i cient n ref l ectsthedegreeofheterogeneityofanadsorbent.Forahomogeneous material,n=1 and the Langmuir-Freundlich equation become the Langmuir isotherm equation.If parameter n of a given system deviates from 1,the system is considered to be heterogeneous[36,37].In this work,n＜1,indicating heterogeneous surfaces of MIL-101 sample. Fig.2 also shows that the Langmuir-Freundlich model is suitable to describe the adsorption behavior of n-hexane on the MIL-101.

3.3.Desorption activation energy of n-hexane on the MIL-101

Table 1Equilibrium amounts of n-hexane adsorbed on MIL-101 samples and other adsorbents

TPD experiments were conducted to estimate desorption activation energy of n-hexane on the MIL-101 to see how strongly n-hexane is bound to the surface.Fig.3 presents TPD prof i les of n-hexane on the MIL-101,each with two desorption peaks representingtwo types of adsorptive sites.This is a direct ref l ection of the surface energy heterogeneity on the MIL-101 frameworks for n-hexane adsorption.These two types of sites are considered to correspond to the pore surfaces of framework and open Cr3+metal centers[38].The f i rst desorption peak at lower temperature comes from n-hexane desorption from the pore surfaces of MIL-101,while the second at higher temperature isfor n-hexane desorption from unsaturated Cr3+mental centers.The peak temperature Tpcorresponding to the desorption peak of TPD curve increases with the increase of heating rate.

Knowinga seriesofTpatdifferentheatingrates,we can estimatedesorption activated energy Edof n-hexane on the MIL-101 sample by using Polanyi-Wigner equation,which is expressed as[3]

Fig.3.TPD prof i les of n-hexane on the MIL-101 at different heating rates.

Fig.4.Lineardependencebetweenln[(Tp2R)/βH]and1/TpforTPDofn-hexaneonMIL-101.

whererdisthedesorptionrateofcomponentAfromunitmassofadsorbent,θAis the transient coverage of component A,t is the time,k0is the desorptionrate coeff i cient,mis theorderof desorptionprocess,and Ris the gas constant.

It is assumed that the desorption process follows the f i rst order kinetics,so the following equation is deduced from Eq.(4)[3]for calculating the activation energy of desorption

where βHis the heating rate.

Fig.4 depicts the linear dependence between ln[(Tp2R)/βH]and 1/Tpfor estimation of desorption activation energy of n-hexane on the MIL-101.Edis obtainedfromtheslope Ed/R and k0isfrom the intercept.

Table 3 lists the desorption activation energy Edof n-hexane corresponding to two types of adsorptive sites on the MIL-101 sample.The two desorption activation energies are 39.41 and 86.69 kJ·mol?1, corresponding to the n-hexane desorption on the pore surfaces of framework and on the open Cr3+metal centers,respectively[38].A comparison for the desorption activation energies of n-hexane on two types of adsorption sites indicates that the interaction between n-hexane and open Cr3+metal sites is much stronger than that between n-hexane and surfaces of framework pores.

Fig.5.Kinetic curves of n-hexane adsorption on the MIL-101 crystals with different n-hexane partial pressures at 290.5 K.

Fig.6.Fractional uptakes of n-hexane on the MIL-101 at different n-hexane partial pressures at 290.5 K.

Table 3Desorption peak temperatures of n-hexane at different heating rates and desorption activation energies of hexane on the MIL-101 crystals

3.4.n-Hexane adsorption kinetics

Fig.5showsthekineticcurvesofn-hexaneadsorptiononMIL-101at different n-hexane pressures.The equilibrium adsorption capacity increases with pressure.In order to investigate the effects of partial pressure of n-hexane on diffusion rate,the adsorption kinetic curves are plotted as the fractional uptake,the ratio of uptake(qt)at time t to equilibrium uptake(qe),versus time t,as shown in Fig.6.The fractional transient adsorption curves of n-hexane at different n-hexane partial pressures ranging from 100 to 600 Pa are essentially the same, suggesting a negligible effect of n-hexane uptake on diffusivity within the MIL-101.

Fig.7showstransientfractional uptakesof n-hexaneontheMIL-101 at different temperatures.Equilibrium adsorption of n-hexane can beachievedwithin9-12min.Accordingtothesekineticuptakecurves,the intracrystalline diffusion coeff i cient of n-hexane in the MIL-101 sample can be obtained.Assuming that the adsorbent is spherical particles,we candescribethetransientfractionuptakesbythefollowingequationfor fractional uptake qt/qeless than 70%[7,8]

Fig.7.Fractional uptakes of n-hexane on MIL-101 at different temperatures and 600 Pa.

Fig.8.Plots of fractional n-hexane uptakes(qt/qe)against the square root of adsorption time at different temperatures and 600 Pa.

where DMis theintracrystallinediffusion coeff i cient and rcis thecrystal radius.

AccordingtothedatainFig.7,Fig.8givesfractionaluptake(qt/qe)of n-hexane versus the square root of adsorption time at different temperatures on the basis of Eq.(6).The diffusion time constants(DM/rc2,s?1) are obtained from the slope of qt/qeversus t0.5,which are listed in Table 4.

The data in Table 4 indicate that diffusion time constants(DM/rc2)of nhexane on the MIL-101 are in the range of(1.35-2.35)×10?4/s?1. Corresponding diffusion coeff i cients(DM)are(1.35-2.35)×10?10/cm2·s?1withtheaverageparticleradiusrcof10μm.Thediffusioncoeff i cientsincrease withtemperature.Itmeansthatn-hexanediffusionintheMIL-101isanactivated process.

Table 4n-Hexane diffusion constants and diffusivity for MIL-101 measured in this work

Fig.9.Arrhenius plot of n-hexane diffusivity in MIL-101 crystals.

According to the Arrhenius equation,the relationship between DMand activation energy(Ea)for n-hexane adsorption in the MIL-101 is expressed by

Fig.9 shows that the Arrhenius plot of lnDMversus 1/T yields a line. The activation energy Eafor diffusion of n-hexane on MIL-101 is found to be 16.33 kJ·mol?1from the slope of straight line,which is higher than that of ethyl acetate on the MIL-101(8.36 kJ·mol?1)[7].

4.Conclusions

The adsorption isotherms of n-hexane on the MIL-101 f i t the Langmuir-Freundlich isotherm model well,where n＜1 suggests a heterogeneous nature of MIL-101 surfaces for n-hexane adsorption.The amount of n-hexane adsorbed on the MIL-101 is up to 5.62 mmol·g?1at 298 K and 1.6×104Pa,much higher than some other adsorbents, suchasactivatedcarbonBPL,zeoliteY,andACC_864.TPDspectraexhibit two types of adsorption sites with desorption activation energies of 39.41 and 86.69 kJ·mol?1,corresponding to the surfaces of framework pores and Cr3+metal centers,respectively.This is a direct ref l ection of thesurfaceenergy heterogeneity.The diffusion coeff i cients of n-hexane on the MIL-101 are in the range of(1.35-2.35)×10?10cm2·s?1with the adsorption activation energy of 16.33 kJ·mol?1.

Acknowledgments

The authors gratefully thank Ms.Liyuan Tang at Sun Yat-sen University for her assistance with practical measurement.

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17 July 2013

☆Supported by the National Natural Science Foundation of China(21276092),the Research Foundation of State Key Lab of Subtropical Building Science of China (C713001z),the Science and Technology Research Foundation of Guangzhou City,China (200910814001)andGuangdongProvincialKeyLaboratoryofAtmospheric Environment and Pollution Control(2011A060901011),and the Fundamental Research Funds for the Central Universities(2013ZZ0060 and 2013ZM0056).

*Corresponding author.

E-mail address:qbxia@scut.edu.cn(Q.Xia).

http://dx.doi.org/10.1016/j.cjche.2014.06.031

1004-9541/?2014 The Chemical Industry and Engineering Society of China,and Chemical Industry Press.All rights reserved.

Received in revised form 9 December 2013

Accepted 6 January 2014

Available online 1 July 2014