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Synthesis of Fe3O4@ZIF-8 magnetic core–shell microspheres and their potential application

Short communication

Synthesis of Fe 3O 4@ZIF-8magnetic core–shell microspheres and their potential application in a capillary

microreactor

Tong Zhang a ,Xiongfu Zhang a ,?,Xinjuan Yan a ,Linying Kong a ,Guangcai Zhang a ,Haiou Liu a ,Jieshan Qiu a ,?,King Lun Yeung b

a State Key Laboratory of Fine Chemicals,School of Chemical Engineering,Dalian University of Technology,Dalian 116024,PR China

b

Department of Chemical and Biomolecular Engineering,The Hong Kong University of Science and Technology,Clear Water Bay,Kowloon,Hong Kong Special Administrative Region

h i g h l i g h t s

A novel Fe 3O 4@ZIF-8core–shell

microsphere is synthesized by a new facile method.

The microspheres are easily loaded/unloaded into/out a capillary g r a p h i c a l a b s t r a c t

A simple and facile synthetic strategy is used to successfully prepare Fe 3O 4@ZIF-8magnetic core–shell microspheres.The Fe 3O 4@ZIF-8microspheres as catalysts could be easily loaded/unloaded into/out a cap-illary microreactor with the help of an external magnetic ?eld.The microreactor demonstrates excellent catalytic activity at a shorter residence time for Knoevenagel condensation reaction of benzaldehyde and ethyl cyanoacetate.

Microchannel

ZIF-8

PSS Shell growth Modification

Fe 3O 4@ZIF-8 Anion-modified Fe 3O 4

Fe 3O 4

assembly

Reactant

Product Magnet

Magnet a r t i c l e i n f o Article history:

Received 15February 2013

Received in revised form 27April 2013Accepted 8May 2013

Available online 17May 2013Keywords:

Metal–organic frameworks Core–shell microspheres Microreactor

Magnetic particles

a b s t r a c t

A facile synthesis strategy for preparing Fe 3O 4@ZIF-8magnetic core–shell microspheres has been

successfully developed.The procedure involves ?rst pre-treating the magnetic cores with an anionic polyelectrolyte to alter the surface charge of the particles and adsorb Zn 2+cations to initiate nucleation and then growing a thin layer of ZIF-8to form a highly reactive,magnetic core–shell microsphere (Fe 3O 4@ZIF-8).The characterization by various techniques indicates that ZIF-8shell is continuous and has an average thickness of around 100nm.The Fe 3O 4@ZIF-8microspheres as catalysts could be easily ?lled into a capillary microreactor with the help of an external magnetic ?eld.The microreactor demon-strates excellent catalytic activity at a shorter residence time for Knoevenagel condensation reaction of benzaldehyde and ethyl cyanoacetate.

ó2013Elsevier B.V.All rights reserved.

1.Introduction

In recent years,great interest has been devoted to the develop-ment of magnetic particles and the improvement of their applica-bility in different areas [1].Among various magnetic particles,iron oxides have received the most attention owing to their strong magnetic responsiveness and biocompatibility [2].Additionally,advances in material synthesis have enabled the preparation of magnetic core–shell

materials with iron oxide cores and functional outer shells of polymers,silica,zeolites and carbon for a wide range of applications in chemical separation and catalysis,environmental remediation and monitoring,biotechnology and medicine [3–8].The zeolitic imidazolate framework (ZIF)materials have zeolite-like topologies and belong to an important class

of metal–organic framework (MOF)materials with interesting adsorption,

1385-8947/$-see front matter ó2013Elsevier B.V.All rights reserved.https://www.wendangku.net/doc/714682890.html,/10.1016/j.cej.2013.05.020

Corresponding authors.Tel./fax:+8641184986155.

E-mail addresses:xfzhang@https://www.wendangku.net/doc/714682890.html, (X.Zhang),jqiu@https://www.wendangku.net/doc/714682890.html, (J.Qiu).

separation and catalytic properties[9–14].However,reports of magnetic core–shell architectures on the basis of ZIF-type materi-als have been very limited in the literatures.

To date,a variety of strategies for the fabrication of magnetic core–shell particles are under investigation.For example,Lu et al. reported the in situ hydrolysis of tetraethoxysilane in the presence of Fe3O4to obtain Fe3O4@SiO2through a sol–gel approach[15]. Carbon-encapsulated Fe3O4has been synthesized by Wang et al. using surfactant polyethylene glycol(PEG)as the connecting agent between Fe3O4spheres and glucose[16].A novel core/shell-struc-tured magnetic zeolite microsphere was synthesized by a strategy combining seed coating and vapor-phase transport method[5]. While it is worth noting that a remarkably adaptable approach, termed the layer-by-layer self-assembly technique(LbL),which al-lows fabrication of multilayer assemblies onto charged substrates by alternately electrostatic association,has given rise to a series of novel magnetic core–shell entities[17–19].

Magnetic catalysts that are highly dispersable can be recovered with minimal loss by applying a magnetic?eld,and thus avoids complex downstream separation and recovery operations[8].The possibility for the precise location of the catalysts in the reactor is essential in complex miniature reactors and lab-on-a-chip de-vices[20,21].This remains dif?cult using current techniques such as volume packing[22],surface coatings[23],particle-grafting[24] and in situ assembly[25],but using a magnetic catalyst it is possi-ble to use a magnetic?eld to transport,position,agitate and even heat the catalysts to promote the reaction and facilitate catalyst loading and regeneration.

Herein we report a facile preparation of a novel magnetic Fe3O4 core-ZIF-8shell microsphere(Fe3O4@ZIF-8)including a pre-treat-ment of the Fe3O4particles with an anionic polyelectrolyte and a following solvothermal synthesis for the ZIF-8shell growth.Fur-ther,potential application of the core–shell particles is explored in a capillary?ow microreactor for the Knoevenagel condensation reaction of benzaldehyde and ethyl cyanoacetate.

2.Experimental

2.1.Materials

Chemicals used in Fe3O4@ZIF-8synthesis include ethylene gly-col(99%),NaAc(99%),methanol(99%)and Zn(NO3)2á6H2O(99%) from Sinopharm Chemical Reagent Ltd.,Co.and FeCl3á6H2O(99%) from Tianjin Kermel Chemical Reagents Ltd.,Co.Aldrich chemicals supplied the2-methylimidazolate(99%)and poly(styrenesulfo-nate,sodium salt)(PSS,30wt%).The reactants,benzaldehyde (99%),ethyl cyanoacetate(95%)and dimethyl sulfoxide(99%)as solvent were purchased from Sinopharm Chemical Reagent Ltd., Co.

2.2.Preparation of Fe3O4@ZIF-8core–shell microspheres

2.2.1.Preparation of Fe3O4microspheres

The Fe3O4microspheres were synthesized by a solvothermal method according to the Ref.[5].2.7g of FeCl3á6H2O was?rst dis-solved in50ml of ethylene glycol under magnetic stirring.A yel-low clear solution was obtained after stirring for0.5h.Then 5.75g of NaAc was added to this solution.After stirring for another 0.5h,the resultant solution was transferred into a Te?on-lined stainless-steel autoclave with capacity of80ml.The autoclave was sealed and heated at473K for8h,and cooled to room temper-ature.The black magnetic microspheres were collected with the help of a magnet,followed by washing with ethanol for three times.2.2.2.ZIF-8shell growth on the Fe3O4microspheres

The preparation route of Fe3O4@ZIF-8microspheres is illus-trated in Scheme1.In a typical procedure,0.01g Fe3O4was added to30ml aqueous solution of poly(styrenesulfonate,sodium salt)of 0.3%with the help of ultrasonication for20min.The sample was recovered by an external magnetic?eld and washed three times with distilled water,then redispersed in a mixture containing 30ml methanol,0.225g Zn(NO3)2á6H2O and0.622g2-methylimi-dazolate under stirring and the reaction was allowed to proceed at 323K for3h for ZIF-8shell growth.Finally,by the use of a magnet, the product was separated and washed with ethanol.

2.3.Characterization

The powder X-ray diffraction(PXRD)patterns of the Fe3O4@ZIF-8microspheres were taken on a D/MAX-2400diffractometer with Cu target(40kV,100mA)from5°to80°.The adsorption–desorp-tion isotherms of nitrogen at77K were determined using a Micromeritics ASAP2020instrument.Observation of the morphol-ogy of the samples was conducted by a NOVA NANOSEM450?eld emission scanning electron microscope(FE-SEM)and a Tecnai G2 Spirit transmission electron microscope(TEM)at120kV,respec-tively.The n-potential analysis was taken by the Malvern Nano-ZS90analyzer.The magnetization curves were measured at room temperature under a varying magnetic?eld fromà6000to6000 Oe on a JDM-13vibrating sample magnetometer(VSM).Fourier-transform infra-red spectra(FT-IR)of the product were taken on a Bruker EQUINOX55fourier transformed infrared spectrometer. TGA analysis was carried out with air atmosphere using a TG/SDTA 851E in the temperature range298–873K with a ramp of 10K minà1.The elemental analyses were performed by inductively coupled plasma-atomic emission spectroscopy(ICP-AES)of Opti-ma2000DV.

2.4.Knoevenagel reaction

The microreactor set-up is shown in Fig.1.A stainless steel cap-illary coil measuring800mm in length and with inner diameters of 580l m as a microreactor was?rstly cleaned with1M HNO3,1M NaOH followed by a thorough rinsing with deioned water.The cap-illary tube was connected to a micro?ow pump using PFTE tubing. Then an ethanol solution containing0.35wt%Fe3O4@ZIF-8micro-spheres was pumped into the capillary microreactor and a magnet of14,000Gs was rapidly introduced as soon as the capillary was fully?lled.The amount of particles loaded in the microreactor could be simply adjusted by altering the content of Fe3O4@ZIF-8 microspheres in ethanol solution.After rinsed with copious amount of ethanol,the reactant mixture of1.1ml benzaldehyde, 1.14ml ethyl cyanoacetate and0.76ml dimethyl sulfoxide was conducted in the?ow reactor at353K.Different?ow rates were used to adjust reaction residence times.The product was collected periodically and analyzed by Shanghai Tech-comp GC7890F gas chromatography using a30m HP-5column and?ame ionization detector.The benzaldehyde conversion and product yield were cal-culated.For comparison,the batch reactions of Fe3O4@ZIF-8micro-spheres and ZIF-8powder were carried out in a round-bottom glass?ask with mechanically stirring under similar conditions of

ZIF-8 growth

PSS

323 K for 3 h

Modification

Fe3O4@ZIF-8

PSS-modified Fe3O4

Fe3O4

Scheme1.Preparation procedure of Fe3O4@ZIF-8core–shell microsphere.

T.Zhang et al./Chemical Engineering Journal228(2013)398–404399

Capillary microchannel

Reactants

Product

Magnet

Magnet

Fig.1.Schematic diagram of capillary microreactor for the Knoevenagel reaction.

1a

0.2 μm

Fe 3O 4

ZIF-8

e

0.5 μm

d

0.1μm

c

b

1μm

μm

particles (a),Fe 3O 4@ZIF-8core–shell microspheres with PSS treatment (d)and without PSS (b),and @ZIF-8core–shell microspheres with PSS treatment (e).

400T.Zhang et al./Chemical Engineering Journal 228(2013)398–404

temperature(353K)and concentration(0.6mg Fe3O4@ZIF-8 microspheres and0.1mg ZIF-8powder(synthesized by the same formula of ZIF-8shell)for3ml reactant mixture consisting of 1.1ml BA,1.14ml ECA and0.76ml DMSO).The catalyst lifetime test was conducted at the residence time of20min.

3.Results and discussion

3.1.Synthesis and structure characterization of Fe3O4@ZIF-8 microspheres

The magnetic Fe3O4particles were?rst prepared from FeCl3á6H2O with sodium acetate(NaAC)as stabilizer and ethylene glycol as reducing agent in a solvothermal reaction.Representative SEM image indicates that the prepared Fe3O4particles are spherical in shape and have an average diameter of around600nm as shown in Fig.2a.As for the preparation of Fe3O4@ZIF-8core–shell micro-spheres,the crucial step is the formation of continuous and uniform ZIF-8shell on the Fe3O4microspheres.Although Ke et al.

[26]had prepared magnetic Fe3O4@Cu3(btc)2and Fe3O4@MIL-100 (Fe)core–shell particles,the procedure was time consuming and involves multiple steps including the modi?cation of the magnetic particles and their reaction in two separate synthesis solutions for at least25repetitions each lasting45min.In our work,an anionic polymer(PSS)was used to alter the surface charge of the magnetic particles and adsorb Zn2+cations to initiate nucleation,deposition and growth of ZIF-8.The surface n-potential of iron oxide particles in aqueous solution(pH=7)signi?cantly decreased from à7.56mV toà26.9mV after the modi?cation,implying the in-crease of negative charge density on the surface of Fe3O4particles, but the deposition of anionic polymer did not result in observable change in particle diameter and morphology.After ZIF-8growth,a measurable increase in particle diameter was observed(Fig.2d) and TEM image in Fig.2e shows that ZIF-8shell is continuous,uni-form and has an average thickness of around100nm.Thicker shells can be also obtained by simply carrying out longer synthesis. The pre-treatment of the Fe3O4particles with anionic polyelectro-lyte should play a vital role in the continuous ZIF-8shell growth which provided the surface a more homogeneous and smooth dis-tribution of negative charges.As shown in Fig.2b and c,without a pre-treatment of PSS,there were just several ZIF-8nanoparticles instead of a layer of ZIF-8shell depositing on the surface of Fe3O4microsphere.

The crystal structure and porosity of Fe3O4@ZIF-8microspheres were con?rmed by XRD and nitrogen-sorption measurements (Figs.3and4).X-ray diffraction shown in Fig.3detects the charac-teristic peaks of both Fe3O4and ZIF-8in Fe3O4@ZIF-8core–shell particles.Fig.4is the nitrogen adsorption–desorption isotherm of Fe3O4@ZIF-8.It indicates the presence of typical micropores where the steep uptake at low relative pressure is followed by nearly horizontal adsorption and desorption curves.The core–shell micro-spheres show hysteresis loops at high relative pressure,which is most probably due to textural porosity formed by the stacking of nanoparticles.The BET surface area of Fe3O4@ZIF-8was deter-mined to be430.9m2gà1which is much lower than pure ZIF-8 (1643m2gà1)mainly due to the heavier and nonporous cores. The pore volume of0.23cm3gà1contains67%of micropores with diameters of1and1.6nm which are consistent with the ZIF-8pore sizes as described by prior report[27].Fig.5demonstrates the IR

spectra of Fe3O4,ZIF-8and Fe3O4@https://www.wendangku.net/doc/714682890.html,pared to that of Fe3O4,the spectrum of core–shell microspheres displays additional adsorption bands which are associated with the ZIF-8structure. For example,the band at421cmà1is attributed to the Zn A N stretch mode.The bands in the spectral region of500–1350cmà1 and1350–1500cmà1are assigned as the plane bending and stretching of imidazole ring,respectively.The C@N stretch mode which is expected at1584cmà1and peaks for the aromatic and aliphatic C A H stretch at2929cmà1and3135cmà1are also observed[28].

The magnetic properties of the samples are shown in Fig.6.It is clear that the magnetic Fe3O4and Fe3O4@ZIF-8core–shell parti-cles have magnetic saturation(MS)values of about63.2and 54.6emu gà1,respectively.The slightly lower MS value of the

T.Zhang et al./Chemical Engineering Journal228(2013)398–404401

core–shell catalyst is due to the dielectric property of the ZIF-8shell.The high magnetic-saturation implies a strong magnetic responsivity.The magnetic Fe 3O 4@ZIF-8core–shell particles can be highly dispersible in ethanol solution within hours before pre-cipitating from the solution owing to their submicron particle size,but with rapid response to magnetic ?eld.A permanent magnet (14,000Gs)placed near the container can separate out the Fe 3O 4@ZIF-8core–shell particles within 10s.Thermogravimetric analysis shown in Fig.7indicates the decomposition of Fe 3O 4@ZIF-8core–shell particles at temperature above 523K and a 12%weight loss ascribed to the ligand of ZIF-8.The relatively lower decomposition temperature compared to the pure ZIF-8might be attributed to the bad thermal stability of little crystal size of ZIF-8nanoparticles in the shell.

3.2.Catalytic performance of Fe 3O 4@ZIF-8in the microreactor Recently,ZIF-8has been reported to be an ef?cient catalyst for Knoevenagel condensation reaction [29],an important synthesis

route for many pharmaceuticals and ?ne chemicals [30].Based on the good magnetic property of Fe 3O 4@ZIF-8core–shell micro-spheres,the as-prepared magnetic core–shell microspheres as cat-alysts were loaded into the inner wall of a capillary of 800mm in length using an external magnetic ?eld to construct a capillary microreactor for testing their catalytic property.Knoevenagel con-densation reaction of benzaldehyde and ethyl cyanoacetate was carried out in both batch and capillary microreactor using this new core–shell catalyst with no extra activation.In an effort to elu-cidate the precise role of ZIF-8shell in the catalytic reaction,com-parison of the results achieved with ZIF-8nanosized crystals (synthesized by the same formula of ZIF-8shell)and Fe 3O 4@ZIF-8core–shell microspheres in batch reactions (Table 1)demon-strates that for the same amount of ZIF-8,the activity of ZIF-8nanosized crystals is similar to that of the core–shell catalyst,indi-cating that ZIF-8is mainly responsible for the catalytic activity of the core–shell catalyst.ZIF-8is believed to act as an ef?cient base catalyst which originates from the 2-methylimidazole ligand [29].Benzaldehyde conversion is plotted with respect to time (i.e.,reac-tion time for batch,residence time for microreactor)in the ?ow

10 s

(b)

@ZIF-8microspheres

collected by an external Table 1

Benzaldehyde conversion of 0.6mg Fe 3O 4@ZIF-8microspheres (about 0.1mg ZIF-8)and 0.1mg ZIF-8powder for 3ml reactant mixture in the batch reaction.Sample

Reaction time (min)5

10152025303540Fe 3O 4@ZIF-8microspheres 40.2%44.5%46.3%49.3%50.8%52.6%53.5%54.4%ZIF-8powder

42.7%

43.9%

50.6%

51.4%

53.2%

54.8%

57.1%

57.4%

402T.Zhang et al./Chemical Engineering Journal 228(2013)398–404

capillary microreactor as shown in Fig.8a.Nearly complete conver-sion to ethyl-2-cyano-3phenyacrylate was obtained for the initial 3ml reactant mixture in capillary microreactor with0.6mg core–shell catalyst(in fact,only0.1mg ZIF-8shell)compared to less than51%in batch for the residence and reaction time of25min.

A similar Knoevenagel reaction between benzaldehyde and malon-onitrile was recently carried out on ZIF-8powder(crystals of 200l m)in a batch reaction using a high catalyst concentration of3mol%and the conversion only reached20%after25min[29]. The better catalytic performance of Fe3O4@ZIF-8core–shell micro-spheres inside the capillary can therefore be attributed to a higher ef?ciency of the nanosized ZIF-8crystals of the shell and the enhancement of reaction rate by the microreactor.Fig.8b shows the catalytic performance of the core–shell catalyst with a running time of24h at the residence time of20min.It was found that the conversion initially amounted to99.1%and then reached a value of 65.9%after24h on?ow.The gradual activity decrease of catalyst might result from the insuf?cient solvent amount which easily leads to the catalyst deactivation[23]and the trace amount of ben-zoic acid which would deactivate the acid-sensitive ZIF-8shell.To further determine if the catalyst leaching was also associated with declined conversion,the leaching percentages of Fe and Zn from the core–shell catalyst were derived from the ICP-AES analysis of the solution collected from the microreactor.And results showed that negligible leaching of Fe and Zn was obtained,considering the detection limit of the ICP-AES instrument,indicating that no catalyst loss was observed during the experiment and the mag-netic catalyst was resistant to erosion by reactant?ow.As soon as the magnetic?eld was turned off,the magnetic particles could be easily?ushed out of the capillary microreactor within several minutes before fresh catalyst reloading.

4.Conclusions

In summary,we have developed a simple and facile strategy for preparing magnetic core–shell microspheres with MOF shell (i.e.Fe3O4@ZIF-8).The application of as-synthesized core–shell particles was successfully demonstrated in a?ow capillary mic-roreactor for Knoevenagel condensation reaction.The magnetic core–shell catalysts gave higher conversion with excellent product selectivity at a shorter residence time in the microreactor.Its mag-netic property allows the catalysts to be well-dispersed in liquid for rapid reactor loading and unloading using an external magnetic ?eld to guide the catalyst deposition.Our research provides a new route for the synthesis of magnetic MOF-based core–shell materi-als and their potential application in the microreactor. Acknowledgments

We gratefully acknowledge the?nancial support by the Na-tional Natural Science Foundation of China(Nos.21173030, 21076030,and21036006),Universities Science&Research Project of Liaoning Province Education Department(No.2009S019)and Program for Liaoning Excellent Talents in University(No. LR201008).

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