Hybridization of metal-organic framew ork and monodisperse spherical silica for chromatographic separation of xylene isomers☆

Bixuan Gao,Minhui Huang,Zhiguo Zhang,Qiwei Yang,Baogen Su,Yiwen Yang*,Qilong Ren,Zongbi Bao*

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

Keywords:Adsorption Chromatography Separation Hybridization Metal-organic framework Silica

ABSTRACT Metal-organic frameworks(MOFs)packed in the column have been a promising candidate asthe stationary phase for high performance liquid chromatography(HPLC).However,the direct packing of irregular MOFpowder could raise some problemslike high back pressure and low column ef fi ciency in the HPLCseparation.In this work,UiO-66 capable of separating xylenes was supported effectively on the surface of the monodisperse spherical silica microspheresby one-pot method.The hybridization of UiO-66 and silica microspheres(termed UiO-66@SiO2 shellcorecomposite)wasprepared by stirring the suspension of the precursorsof UiO-66 and--COOHterminated silica in the N,N-dimethylformamide with heating.The shell-core composite material UiO66@SiO2 wascharacterized by SEM,TEM,PXRDand FTIR.Then,it was used as a packing material for the chromatographic separation of xylene isomers.Xylene isomers including o-xylene,m-xylene and p-xylene w ere ef fi ciently separated on the column w ith high resolution and good reproducibility.Moreover,the UiO-66@SiO2 shell-core composites packed column still remained reverse shape selectivity as UiO-66 possessed,and the retention of xylenes w as probably ascribed to the hydrophobic effect between analytes and the aromatic rings of the UiO-66 shell.The UiO-66@SiO2 shell-core composites obtained in this study have some potential for the separation of structural isomers in HPLC.

1.Introduction

Xylene isomers are industrially important hydrocarbons in the petrochemical industries as these substances are the main feed stocks for chemical production[1].For example,p-xylene(PX)is mainly used for the production of plastic and polyester fi bers;o-xylene(OX)is used for the production of plasticizers;and m-xylene(MX)is used for the production of fragrances and dyes[2].How ever,the separation of xylene isomers has been one of the challenging topics because of their similar boiling points(138.35 °Cfor PX,139.10 °Cfor m-xylene,144.40°Cfor o-xylene[3]),w hich makes it dif fi cult to separate their mixtures into individual isomer using conventional recti fi cation techniques[4].Industrially,xyleneisomersareseparated by usingsimulated moving bed chromatography(SMBC)at around 180°Cand 9 bar[5].Mature commercial separation processes of SMBCinclude the UOPParex,Toray-Aromax and IFP-Eluxyl processes[6].One of the key issues of SMBCis the selection of ef fi cient adsorbents as stationary phase.Highly selective adsorbentsof xylene isomers can signi fi cantly improve productivity of the chromatographic process and greatly reduce the consumption of mobile phase.The development of highly selective adsorbents for xylene isomers separation is highly desired.

On the basis of the kinetic diameters(Φ)difference,it seems possible for us to separate the isomer p-xylene(ΦPX=0.58 nm)from o-xylene(ΦOX=0.65 nm)and m-xylene(ΦMX=0.64 nm)[3]by molecular sieving[7].How ever,such separation by sieving mechanism has never been realized.State-of-the art adsorbents,e.g.FAU-Y zeolite,exchanged w ith Na+,K+,and Ba2+,are excellent adsorbents used in the bulk-phase separation of the xylene isomers[8].Particularly,BaY can selectively adsorb p-xylene w hereas NaY selectively adsorbs m-xylene[9-10].In recent tw o decades,novel porous adsorbents such as metal-organic frameworks(MOFs)[11]have grow n increasingly in terms of adsorption separation of hydrocarbon isomers including xylenes and hexane homologs[12-14].MOFmaterials,constructed form metal ions and organic ligands through coordination bonds[15],can result in a large variety of highly ordered crystalline porous materials w ith different topological features due to their diverse metal centers and organic linkers[16-17].These unique features make MOFs w idely used in catalytic reaction[18-20],adsorption[21-23]and separation[24-28].The pore size and geometry of MOFs can be fi nely tuned for task-speci fi c applications[29-30].This feature has the potential for making a signi fi cant impact on the separation of xylene isomers[31],w hich generally relies on the matching of the size and shape of the adsorbates and the adsorbent micropores[32].Such mechanisms w ere also w ell-know n as shape-selective effect[33],namely the adsorbent prefers to adsorb the molecules with smaller kinetic diameter and to exclude the bulkier ones from accessing the micropores[31].

Currently,many MOF materials exhibit shape-selective effect on separation of xylene isomers[34-39].MIL-47(V)is the fi rst example of a MOFused as an adsorbent for the separation of xylene isomers[8],w hich is only marginally effective at discriminating between OX and PX,but quite remarkably preferred PX over MX[8,40-45].Zn3(bdc)3(H2O)3(DMF)4(bdc=1,4-benzenedicarboxylate)has also been studied extensively for xylene isomersseparation,which isknown as the fi rst example of MOFmaterials capable of a preferable adsorption of PXover other isomers[46,47].On the basisof simulation data[3]that had been con fi rmed by experimental analysis,MIL-125(Ti),MIL-125(Ti)-NH2and CAU-1(Al)-NH2w ere also found to be para-selective[3,48].Torres-Knoop and cow orkers[49]found strong selectivity of p-xylene on MAF-X8 by con fi gurational-bias Monte Carlo(CBMC)[36]simulation as MAF-X8 possesses the right channel dimensions for the stacking of p-xylene to occur[49].Moreover,CBMCsimulationsrevealed that the PXadsorption capacity of MAF-X8 might be signi fi cantly higher than BaX.Besides,fl exible MOFs such as[Ce(htcpb)?(EtOH)0.28(H2O)2.75](H4tcpb=tetradentate carboxylic acid)[50]and[Zn4O(L)3](L=4,4′-((4-(tert-butyl)-1,2-phenylene)-bis(oxy))dibenzoate)[51]can also perform the separation of xylene isomers.MIL-53(Al),featuring the stepwise adsorption caused by guest induced framew ork transitions[52]show s great potential for the complete separation of OX from the other xylene isomers[39,53-55].Based on shape-selective effects,CD-MOFexhibited a complete separation of OX from both PX and MX,but a partial separation of PX and MX[44].In addition,based on gate-opening effects/molecular sieving effects,ZIF-8 showsadiffusion selectivity of 4.0 and 2.4 for PX/OXand PX/MX[57-59],and ZIF-68 also shows a diffusion selectivity of 3.8(2.2)for PX/OX(PX/MX)because of a certain degree of structural fl exibility[60].

As a popular MOF material w ith good thermal,chemical and mechanical stability,UiO-66 has unique reverse shape selectivity on the separation of xylene isomers compared to the aforementioned MOFs[31,61-66].The UiO-66 is based on a ZrO6(OH)2octahedron,and 1,4-benzene-dicarboxylate(BDC)linkers[67-72].Itscubic 3D-porestructure consists of an array of octahedral cavities of diameter 1.1 nm,and tetrahedral cavitiesof diameter 0.8 nm[73].UiO-66 hasbeen reported that theadsorption of the bulkier o-xylene is favored over that of the other isomers[74].

Compared w ith the extensive separation studies in adsorption mode,the attempts to separate xylene isomers on HPLCusing UiO-66 pow der[74]as stationary phase is largely lagging behind[75-76].The problems such ashigh column backpressure,low column ef fi ciency and undesirable peak shape still remain,which is caused by the direct packing of UiO-66 particles with irregular morphology,micrometer size and wide particle size distribution[77].Therefore,some investigations of MOFs loaded on spheres[78-81]or membrane[82-84]materials have been developed in order to expand therangeof applied usage.Therehave been several reportsof MOFsgrafted on spherical particles,such as Al2O3,SiO2and Fe3O4[7,77,78,85-86].For example,Yan and co-w orkers[61]reported that a silica-UiO-66 composite was fabricated as a stationary phase for liquid chromatographic separation of ethylbenzene,xylene,chlorotoluene and dichlorobenzene isomers[7,87].However,the silica-UiO-66 composites were actually a physically homogeneousmixture of silica bead and UiO-66 crystals.By comparison,spherical materialssuch as SiO2microspheres used assupported matrix may possessmany advantagessuch ashigh surface areas,easy-acquisition,convenient stack and good repeatability[78].One the other hand,the MOF@SiO2microspheres may create the hierarchical pore characteristics,which were favorable to the diffusion or mass-transfer of analytes.Thus,the hybridization of UiO-66 with monodisperse silica microspheres to form a core-shell structure can effectively overcome the aforementioned problems.The monodisperse silica core is able to provide perfect packing property,thus resulting in low column backpressure and decreased mass-transfer resistance[77].In this work,a simple one-pot synthesis method w as used to prepare the UiO-66@SiO2shell-core composite,which wasfurther evaluated on HPLCfor separation of xylene isomers.

2.Experimental

2.1.Materials

Spherical carboxylate-terminated silica cores(average particle size of 5μm)were manufactured by Bio-Tech Microsystems(Suzhou,China).Zirconium chloride(＞99.9%)w as purchased from Aldrich(Germany).Terephthalic acid(99.0%)was provided by Tokyo Kasei Kogyo Co.,Ltd.(Japan).HPLCgrade acetonitrile,ethanol and methanol(chromatographic grade)w as purchased from Tedia Company,Inc.(USA).The other reagents of analytical grade were obtained from Sinopharm Chemical Reagent Co.,Ltd.(China).All chemicals w ere used as received without further puri fi cation.

2.2.Synthesis of UiO-66

UiO-66 pow der sample w assynthesized according to the previously reported method[88,89].Typically,zirconium chloride(0.64 g),terephthalic acid(0.456 g)and 4.0 ml of acetic acid were dissolved in 40 ml N,N-dimethylformamide(DMF)in a PTFE-lined reactor under stirring for 2 h.The reactor w as sealed and placed in an oven at 120°C for 24 h under static conditions[31].After completion of solvothermal reaction,the reactor w as cooled dow n to room temperature.The precipitates w ere isolated by centrifugation at 3000 r·min-1for 5 min.The collected solid w as suspended in DMF(10 ml)to remove unreacted reagentsand impurities,and then collected by centrifugation.Then,the precipitate w as exchanged w ith dichloromethane(10 ml)three times over 3 d to displace the residual DMFin the micropores of UiO-66.Finally,composites w ere activated under vacuum at 350°C overnight to obtain the pure UiO-66 crystals[77].

Fig.1.Synthetic procedure of UiO-66@SiO2 shell-core composites.

2.3.Synthesis of UiO-66@SiO2 shell-core composites

Fig.2.SEM imagesof(a,b)carboxylate-terminated silica;(c,d)UiO-66@SiO2 shell-core compositessynthesized at 120°Cin a 250 ml round bottom fl ask w ith stirring;(e,f)UiO-66@SiO2 shell-core composites synthesized at 120°Cs in a PTFE-lined reactor without stirring;(g,h)TEM images of UiO-66@SiO2 shell-core composites.

The synthesis of UiO-66@SiO2shell-core composites w as carried out by solvothermal method in the presence of Zr4+@silica and terephthalic acid solution(Fig.1).Zirconium chloride(0.64 g)was dissolved in 40 ml DMF in a 250 ml round bottom fl ask.Carboxylate-terminated silica(0.5 g)was added and stirred at room temperature for 1 h to realize immobilization of Zr4+ions on the surface of silica microspheres.Then,terephthalic acid(0.456 g)and acetic acid(4.0 ml)in 40 ml DMF w ere added to the mixture.The reaction was carried out at 120°Cfor 1 d under stirring[77].When cooling dow n,the prepared composites w ere collected by centrifugation at 800 r·min-1for 2 min.Due to the higher density of UiO-66@SiO2shell-core composites,UiO-66 nanoparticles in the products were removed readily as suspended in the solutions w hile the composites deposited.The composites were collected and w ashed with DMFfor several times.Then,w hite solids w ere immersed in dichloromethane for 3 d during which the activation solvent was decanted and freshly replenished three times[90].Finally,the precipitate w as subsequently collected by centrifugation and dried at 60°Cunder vacuum[77].As a control,the UiO-66@SiO2shell-core composites were synthesized in a PTFE-lined reactor.The product was also collected and puri fi ed with the abovementioned method described.

2.4.Characterization

The surface morphology of particles w as observed by scanning electron microscopy(Hitachi SU-70).In order to increase the signal and surface resolution,samples w ere coated w ith an ultrathin gold coating.The pro fi le and pore channels of particles were observed on a transmission electron microscopy(FEITecnai G2F20 S-TWIN)operating at 200 kV.Composite samples w ere dispersed on a TEM copper grid.Pow der X-ray diffraction(PXRD,Panalytical X'Pert PRO)w as used to measure the X-ray diffraction patterns using Cu Kαradiation[77](λ=0.1542 nm)in the range of 3°-50°.Composites w ere not activated(not dried under high vacuum)prior to PXRD measurements.Fourier transformed infrared spectra(FTIR,Nicolet 6700)w ere recorded using a spectrometer in thefrequency range of 400-2500 cm-1w ith aresolution of 2 cm-1[77].Brunauer-Emmett-Teller(BET)speci fi c surface areas w ere obtained using nitrogen adsorption isotherm at 77 K obtained on Micromeritics 3Flex.Activation of the sample w as performed prior to each adsorption analysisusing theinstrument'ssample thermal degassing module[91].The sample w as heated to 350°Cfor 24 h.

2.5.Chromatographic separation

About 2.0 g of as-prepared UiO-66@SiO2shell-core composites were slurry-packed into a stainless-steel column(150 mm×4.6 mm i.d.).A mixture of 10 ml of chloroform and 8 ml of cyclohexanol w as used as dispersion solvent.Amixture of methanol and isopropanol(50/50,v/v)w as used as displacement liquid and packing pressure w as 3000 psi[77].The UiO-66@SiO2shell-core composites packed column w as w ashed with ethanol for 2 h beforeuse[7].For comparison,the commercial C18 column(150 mm×4.6 mm i.d.)w as also used to separate xylene isomers.Chromatographic separation w as performed on a Waters HPLC system[92]equipped w ith a quaternary pump,Waters 2707 auto-sampler and Waters 2489 UV-Vis detectors.Pulse experiments w ere carried out in the column using the laboratory scale unit in order to evaluate the capability of UiO-66@SiO2shell-core composites for xylene isomers separation[74].Initially,the column w as fed w ith eluent at a constant fl ow rate(1.0 ml·min-1)at 30 °C.A pulse of the solution of xylene isomer mixtures p-xylene/m-xylene/o-xylene(v/v/v,1/1/1)dissolved in methanol(2.5 mg·ml-1)w as injected into the column using a loop of 10μl.At the same time,the UV-Vis detector w as started and the spectra w ere acquired w ith the detection w avelength of 215 nm[74].This procedure w as repeated w hen sw itching mobile phases including aqueous solution of acetonitrile,ethanol or methanol.

3.Results and Discussion

3.1.Characterization of UiO-66@SiO2 shell-core composites

The silica microspheresterminated w ith carboxyl groupscould bind Zr4+ions to form the Zr4+@SiO2precursor(Fig.1).Then,UiO-66 nanoparticles w ere grown by solvothermal synthesis in the presence of ligand solution[93].Fig.2 show s SEM images of the 5μm particles before and after the deposition of UiO-66 crystals[94].Initially,the surface of the carboxylate-terminated silica w as smooth as show n in Fig.2(a)and(b).Afterwards,the surface morphology of UiO-66@SiO2shell-core composites prepared are show n in Fig.2(c)and(d),w hich indicated that UiO-66 nanocrystals closely bound to the surface of carboxylate-terminated silica.The particle size of the composites UiO-66@SiO2shell-core microspheres increased from about 5 μm to 6 μm as shown in Fig.2(c)and(d),from which the uniform covering of UiO-66 crystals on the surface of silica particles[94]can be obviously evident.Details of the composites surface also show ed the individual particles of UiO-66 w ith a distribution of sizes having an average of 200 nm.Panel labels(e)and(f)in Fig.2 are SEM images of UiO-66@SiO2synthesized in a PTFE-lined reactor under high temperature and pressure w ithout stirring.As expected,the coating density of UiO-66 crystals w as low er and UiO-66 particles w ere formed on the surface[77]of carboxyl silica.Meanw hile a portion of silica microspheres were crushed at high pressure.The results indicated that the presence of stirring provided suf fi cient contact between UiO-66 and silica,meanwhile promoted the formation of the crystal.Fig.2(g)and(h)shows the transmission electron microscope(TEM)images[78]of UiO-66@SiO2shellcore composites,from w hich it can be seen that the UiO-66 nanocrystals were easily found on the pro fi le of the microspheres.

PXRD patterns could further proof the formation of UiO-66@SiO2shell-core composites.The PXRD results of carboxylate-terminated silica,UiO-66 and UiO-66@SiO2shell-core composites are show n in Fig.3.Spiculateand re fi ned peaksw ere exhibited[95],indicating theintact crystalline structure of PXRD pattern of UiO-66.In addition,the structure of UiO-66@SiO2shell-core composites and UiO-66 matches w ell.Low-angle peaks still exist,indicating that UiO-66 crystals are still stable at high loadings of UiO-66@SiO2shell-core composites.In addition,PXRD pattern of UiO-66@SiO2shell-core composites is w ell indexed to carboxylate-terminated silica w ith good crystallization[78].Furthermore,patterns of UiO-66@SiO2are equally strong to be observed compared to the strong peaks originated from UiO-66.

Fig.3.PXRD patterns of UiO-66@SiO2 shell-core composites(red),UiO-66(orange)and carboxylate-terminated silica(purple).

FTIRspectra(Fig.4)recorded on Nicolet 6700 were also evaluated the presence of UiO-66 particles on the surface of the prepared UiO-66@SiO2shell-corecomposites[94].In the FTIRspectraof UiO-66[77],thepeaksat 1655 cm-1,1585 cm-1and 1504 cm-1are assigned as the skeleton vibration of the benzene ring and the peaks at 746 cm-1indicate the para-substituent on the benzene ring.Zr--Ostretching vibration appears at 467 cm-1,revealing the coordination mode of the ligands and metal cations.Besides,peaks at around 1100 cm-1and 800 cm-1can be observed,which correspond to the stretching vibration of Si--Oand the bending vibration of SiO--H groups of the silica core,respectively[94].As show n in Fig.4,the main peaks of UiO-66 at 1655,1585,1504,1397 and 746 cm-1con fi rm the successful incorporation of UiO-66 onto the carboxyl silica.Peaks at 1397 cm-1con fi rm the bond betw een Zr4+of UiO-66 and carboxylate-terminated silica.

Fig.4.FTIRspectra of(a)carboxylate-terminated silica,(b)UiO-66 alone and(c)UiO-66@SiO2 shell-core composites.

To con fi rm the Brunauer-Emmett-Teller(BET)surface areasand pore volume[96]of UiO-66@SiO2shell-core composites,the sorption isotherms of N2at 77 Kon UiO-66@SiO2shell-core composites,silica microspheres and UiO-66 w ere measured(Fig.5).Prior to measurements,activation by dehydration of the MOFwas performed at 350°Cfor 24 h under vacuum.Notew orthy,the N2isotherm of UiO-66 shows a type I plot according to the IUPACclassi fi cation due to its typical microporous structure.How ever,the N2isotherm of the silica microspheres and UiO-66@SiO2shell-core composites exhibits a type IV plot indicating the mesoporous structure of silica[77].Apparently,the binding of UiO-66 w ith the carboxylate-terminated silica led to the higher BET surface areasof 115 m2·g-1compared with the silica of 63 m2·g-1,and it was expected to decrease w hen compared to that of the parent UiO-66(1137 m2·g-1).The slightly increased pore volume of the UiO-66@SiO2shell-core composites(0.223 cm3·g-1)as compared to the bare carboxylate-terminated silica(0.217 cm3·g-1)also implies the effective loading of UiO-66(0.501 cm3·g-1)on the silica surface.Meanw hile,mesopores w ith the average pore size of the composite UiO-66@SiO2w hich may be originated from the aggregate of nano-particle of UiO-66 on the shell,w asestimated around 12 nm,obviously far from that of parent UiO-66(0.8-1.1 nm).Moreover,the loading amount of UiO-66 could be estimated from BETsurface areas of pure silica microsphere,UiO-66 and MOFcomposites.Theapproximate loading amount w asroughly estimated to be 4.84%in the shell-core composite.

Fig.5.N2 adsorption-desorption isotherms at 77 K(fi lled symbols:adsorption,empty symbols:desorption)of(a)carboxyl-terminated silica,(b)UiO-66 and(c)UiO-66@SiO2 at 77 K.

3.2.HPLCevaluation of UiO-66@SiO2 shell-core composites

In the previous studies,the applicability of UiO-66 as a stationary phasein liquid chromatography hasbeen demonstrated[7,74,85].However,UiO-66 particles packed directly into the columns resulted in the increased column backpressure and decreased ef fi ciency[77].Therefore,preparation of UiO-66@SiO2shell-core composites avoids the abovementioned limitations by providing better quality packing w ith reduced backpressure.Fig.6 show s that MX and PX cannot achieve the base-line separation using acetonitrile and ethanol as mobile phase.In contrast,methanol applied as mobile ful fi lled the complete separation of the three isomeric analytes on UiO-66@SiO2shell-core composites packed column.Fig.6(c)show s the complete separation of OX,MXand PX on UiO-66@SiO2shell-core composites packed column.The separation performance of C18 column w as also investigated for comparison using methanol asmobile phase.As shown in Fig.6,longer analysis time w asneeded and xylene isomers were hardly separated on C18 column because of their similar hydrophobicity[77].

Fig.6.Chromatograms of the separation of xylene isomers on(a)C18 column;mobile phase,methanol.(b)UiO-66@SiO2 shell-core composites packed column;mobile phase,acetonitrile,ethanol and methanol.(c)UiO-66@SiO2 shell-core composites packed column;mobile phase,70%,75%,80%,85%and 90%methanol.

Furthermore,it can be observed that HPLCresolution decreased as the methanol content in the mobile phase increased from 70%to 90%.The above result reveals that the mobile phase played a signi fi cant role in the HPLCseparation[7]of xylene isomers on the UiO-66@SiO2shell-core composites packed column.The separation factors were signi fi cantly affected by adjusting the concentrations of methanol.Table 1 show s the separation factorsαijof xylene isomers with different mobile phases calculated from the analytical chromatography data.It can be found the separation factors increased as concentration of methanol increased.Furthermore,a signi fi cant positive correlation emerged betw een the concentration of methanol and separation factors of OX/PX and MX/PX according to the similar dissolve mutually theory.As can be seen,a mobile phase with 80%of methanol performed good resolution of all the peaks and best separation factors between any two xylene isomers.Compared with the state-of-the-art MOFs particles,such as HKUST-1,MIL-47 and MIL-53(Fe)[97],UiO-66@SiO2shell-core composites packed column show the highest separation factors(Table 2),which is attractive for its application in HPLC separation[77].It is w orth to mention that the separation performance w as remained even though the column w as used for over one month,and there is no observable decrease in the retention time,indicating the high stability and reproducibility of the prepared column[77].

Table 1 Separation factorαij for the xylene isomer molecules at 303 K calculated from analytical chromatography data using 70%,75%,80%,85%,and 90%methanol as the mobile phase

Table 2 Separation factorαij for the xylene isomer molecules on MOFpacked column in HPLC

The UiO-66@SiO2shell-core composites packed column provided OX stronger retention in comparison w ith MX and PX,exhibiting the OX preference[77],namely reverse shape selectivity w hen compared to most MOFs'para-selectivity.It might be due to the strongest Van der Waals interaction of OX[31]with the UiO-66 framework,as it has the largest molecular size(0.74 nm)among MX(0.71 nm)and PX(0.67 nm)[98].Moreover,the least rotationally constrained isomer w ill experience the low est loss of entropy upon adsorption,resulting in an overall low er Gibbs free energy of adsorption for species w ith similar adsorption enthalpies[99].

The selectivity betw een the tw o components w ith ethanol as reference:

Whereαijisthe separation factor of component i over component j,kiis the retention factor of component i,tRiis the retention time of component i,and t0is the column hold-up time[101].

4.Conclusions

As UiO-66 show s the excellent chemical stability and unique reverse shape selectivity[77]for the separation of xylene isomers,it w as used as model MOFfor preparation of the shell-corecompositescontaining MOF particle and silica microspheres.The hybridization of UiO-66 and monodisperse silica microspheres termed as UiO-66@SiO2w ere synthesized by one-pot method.The UiO-66@SiO2shell-core composites possess high surface areas and large pores,w hich greatly increase masstransfer ef fi ciency[78].The UiO-66@SiO2shell-core composites have large particle diameter as a column packing material,w hich increases the porosity of the packed column.Therefore,the column pressure could be signi fi cantly reduced w hen compared to a pure UiO-66 fi lled column,resulting in the possibility for UiO-66 to be used in HPLC.Besides,UiO-66@SiO2shell-core composites have show n an interesting OX preference in the liquid phase and presented reverse shape selectivity for the xyleneisomers[77].Pulse experimentshave shown the OXselectivity of UiO-66@SiO2shell-core compositesismorestrongly retained even in the presence of ternary mixtures,show ing higher selectivity values(5.6 and 2.5 for OX over MX and PX,respectively)than UiO-66[74].These results suggest that the adsorption is dominated by van der Waalsforcestowardsthepore walls[77].Thiswork suggeststhat monodisperse MOFs@silica composites show promising in the separation of structural isomers on HPLC,w hich might advance important industrial applications for chemical separation.