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Synthesisofmolecularimprintedpolymercoatedphotocatalystswith

Synthesisofmolecularimprintedpolymercoatedphotocatalystswith
Synthesisofmolecularimprintedpolymercoatedphotocatalystswith

Synthesis of molecular imprinted polymer coated photocatalysts with high selectivity {

Xiantao Shen,Lihua Zhu,*Jing Li and Heqing Tang *

Received (in Cambridge,UK)20th October 2006,Accepted 2nd February 2007First published as an Advance Article on the web 15th February 2007DOI:10.1039/b615303h

Molecular imprinted polymer coated photocatalysts were prepared via polymerization of a proper functional monomer in the presence of TiO 2nanoparticles and target molecules,which was found to promote the selectivity of TiO 2photocatalysis.

The photocatalytic degradation of pollutants using TiO

2has been well studied for environmental protection.1However,it is as yet difficult to realize selective removal of harmful low-level pollutants in the presence of high-level less harmful pollutants by using this technique,because TiO 2photocatalyst has very poor selectivity and cannot differentiate between these pollutants.Several approaches towards surface modification have been tried to increase the selectivity of TiO 2,but these methods have been found to be not very effective,2or to give poor stability.3Composites of TiO 2and a mesoporous absorbent have been proposed.4One of the best is prepared by constructing organic molecular recognition sites on inert domains.5These composites usually require complicated preparation processes.Molecular imprinted polymers (MIPs)have been extensively studied due to their molecular recognition ability,specific adsorption,and wide applications in separation and sensors.6We expect that their specific adsorption ability favors increasing selectivity of the photocatalyst.Therefore,we will develop a simple way to prepare novel hybrid photocatalysts by in situ coating a thin MIP layer of a target pollutant on TiO 2nanoparticles.

Selection of monomer is critical in the preparation of MIPs.In our new approach,o -phenylenediamine (OPDA)is taken as the functional monomer.There are two –NH 2groups in its molecule.Interaction between the –NH 2groups in OPDA and the functional groups (such as –OH,–Cl,–NO 2)of the target pollutants (i.e.,the templates)may lead to a precursor,which assures the imprinting of the target molecules during polymerization.

As indicated in Scheme 1,our approach consists of three steps.(1)A precursor is prepared via a reaction of the monomer OPDA in excess and the target compound (4-chlorophenol (4CP)or 2-chlorophenol (2CP))as template.The existence of the precursor is assumed only to represent the strong interactions between the monomer and template molecules.(2)A MIP layer is coated on TiO 2particles via an in-situ polymerization in the presence of TiO 2nanoparticles (Degussa P25).(3)MIP-coated photocatalyst is

prepared after chemically removing the template molecules from the polymer layer.In the first step,excess of OPDA and a given amount of the target pollutant is reacted in aqueous solution.Thus,the added monomer exists in two states,i.e.,as the above-mentioned precursor or as free molecules.Because the monomer can be adsorbed on the surface of the TiO 2particles,the polymer is able to coat the surface of TiO 2via in-situ polymerization,leading to the incorporation of the templates into the polymer.During polymerization,the OPDA monomers in both the free state and the precursor state may be coupled in the same way as aniline,producing a polymer with a polyaniline-like structure,which is photochemically stable and favorable to the photocatalytic efficiency of TiO 2–polyaniline under sunlight.7The second –NH 2group in ‘‘free’’OPDA molecules may function as cross-linker,enhancing the shape selectivity of the MIP layer.

In comparison with conventional methods of MIP preparation,there are two additional merits in our method.One is that the polymerization is initiated photocatalytically by UV light illumina-tion on TiO 2nanoparticles without using any other chemical initiator or cross-linker.The other is that the template can be removed from the polymer layer by UV light illumination for an appropriate period of time,leading to the final products of MIP-coated TiO 2.The application of the photocatalyst was little affected by the treatment types,but the operation of the photo-catalytic treatment is simpler than the Na 2CO 3treatment.The combination of the two merits allowed us to easily control the

Department of Chemistry,Huazhong University of Science and Technology,Wuhan,430074,P.R.China.

E-mail:lhzhu63@https://www.sodocs.net/doc/4d10051430.html, (Zhu);hqtang62@https://www.sodocs.net/doc/4d10051430.html, (Tang);Fax:(+86)2787543632;Tel:(+86)2787543632

{Electronic supplementary information (ESI)available:TEM image,UV–visible absorption spectra and FTIR spectra of photocatalysts,and accumulation of intermediates.See DOI:10.1039/b615303h

Scheme 1The route for preparation of MIP-coated photocatalyst and its use in photocatalytic degradation.Here,4-chlorophenol is used as a representative of the target pollutants.

COMMUNICATION https://www.sodocs.net/doc/4d10051430.html,/chemcomm |ChemComm

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thickness of the MIP layer,assuring both the molecular recognition ability and the photocatalytic efficiency.

In this work,4CP or2CP was used as the target pollutant,and the resultant MIP-coated photocatalysts are referred to as4CP-P25and2CP-P25,respectively.{If neither4CP nor2CP was used in the preparation,the obtained catalyst is named as NIP-P25. The TEM image indicated that MIP-P25was composed of TiO2 particles partially coated with a layer of MIP having a thickness of y5nm.In the UV–visible absorption spectra of the photo-catalysts,the MIP layer yielded absorption in the visible region with a maximum absorption at y440nm.FTIR spectra of MIP-coated TiO2were measured in comparison with that of neat TiO2 and the polymer of OPDA.The main characteristic absorption bands of the polymer of OPDA can be found in the spectrum of MIP-coated TiO2.The stretching bands of–NH2appeared at wavenumbers of3448cm21and3310cm21,and the strong absorption band at1525cm21was ascribed to the bending mode of the–NH–bond.The stretching vibration of the L C–H bond on the benzene ring occurred at3187cm21,and the absorption bands at1359cm21and1233cm21are attributed to the L C–N bond on the benzene ring.The FTIR measurements indicated that the MIP layer had the structure of the polymer of OPDA.

Fig.1shows the adsorption isotherms for2CP and4CP over the photocatalysts in aqueous suspensions.It is easily seen from Fig.1that due to the MIP layer,2CP-P25(or4CP-P25)gives

stronger adsorption toward the target2CP(or4CP),relative to

both P25and NIP-P25.This is in good agreement with the

intrinsic molecular recognition ability and selective adsorption

ability of MIP.Because of the important role of the organic

pollutant’s adsorption on the photocatalyst’s surface,we could

expect that the MIP-coated photocatalysts have high selectivity

toward the photodegradation of the target pollutants.

The photodegradation of2CP,4CP and phenol over the

photocatalysts in the present work was found to follow a pseudo

first-order reaction in kinetics.§The measured apparent rate

constants(k)and their ratios are used to evaluate the influence of

the MIPs on the degradation rate of pollutants and the selectivity

of various photocatalysts.In the degradation of single target or

non-target pollutant(s)with c0=20mg L21,we found that relative

to P25as a control,4CP-P25increased the k value for the target

4CP from0.02449min21to0.03494min21by a factor of143%,

and decreased that for the non-target2CP from0.02032min21to

0.00810min21by a factor of39.8%.When2CP was treated as the

target,the photocatalyst2CP-P25increased the k value for the

target2CP from0.02032min21(over P25)to0.02579min21by a

factor of122%,and decreased that for the non-target4CP from

0.02449min21(over P25)to0.01895min21by a factor of77.3%.

Similarly,the promoted photodegradation of the targets over

MIP-coated P25was also observed in the mixtures of4CP and

2CP(c0=20mg L21for both4CP and2CP).Therefore,the MIP-

coated P25markedly enhanced the photodegradation of the target

and inhibited the degradation of the non-target.Even though4CP

and2CP have very similar chemical structures and molecular sizes,

the MIP layer can differentiate them.

Table1compares the rate constants for the photodegradation

of2CP,4CP and phenol over different photocatalysts in the

degradation of their mixture.Generally,the degradation rate of

the target compound is much faster than that of the non-target

ones.The ratio of k4CP/k phenol is20.6over4CP-P25,much greater

than8.68over P25.Similarly,the ratio of k2CP/k phenol is8.34over

2CP-P25,greater than5.78over P25.This confirms that the MIP

coating significantly increases the catalyst’s ability to differentiate

the mixed pollutants,and the selectivity of MIP-coated catalysts is

increased greatly as the differences between the pollutants are

increased.Such high selectivity allows us to selectively remove low-

level target compounds(2mg L21)using MIP-coated TiO2in the

presence of high-level non-target compound(50mg L21phenol).

This was further confirmed in the photodegradation of the mixture

of2mg L214CP and500mg L21phenol over4CP-P25.When

such a low level of4CP was degraded in the presence of500mg

L21phenol,the target4CP was rapidly degraded with a half-time

t1/2of y8min,while the relative concentration(c/c0)of phenol

apparently remained almost unchanged(Fig.2).

Table1Values of k and k target/k non-target obtained from the photo-

degradation of the mixture of2CP(2mg L21),4CP(2mg L21)and

phenol(50mg L21)over different photocatalysts

2CP-P254CP-P25P25

k2CP(min21)0.097310.063570.01499

k4CP(min21)0.053690.124640.02247

k phenol(min21)0.011670.006060.00259

k2CP/k4CP 1.81—0.667

k2CP/k phenol8.34— 5.78

k4CP/k2CP— 1.96 1.50

k4CP/k phenol—20.68.68

Fig.2Degradation kinetics for4CP(triangles)and phenol(circles)in

their mixtures(c0=2mg L21for4CP and500mg L21for phenol)under

UV light illumination in the absence(open)and presence(solid)of4CP-

P25as photocatalyst.

1164|https://www.sodocs.net/doc/4d10051430.html,mun.,2007,1163–1165This journal is?The Royal Society of Chemistry2007

Because direct photolysis of chlorophenols has been reported,8 the direct photolysis may make a contribution to the measured values of k.When only4CP(or2CP)was present in solution,we indeed observed the direct photocatalysis of4CP(or2CP).This resulted in a high accumulation of aromatic intermediates(such as 2-hydroxy-1,4-benzoquinone,1,4-hydroquinone and1,4-benzoqui-none),which were very difficult to degrade further in the absence of any photocatalyst.In contrast,the photocatalytic degradation of4CP is faster,leading to very small accumulations of the aromatic intermediates,which can be degraded completely.In consideration of possible contributions of direct photolysis to the measured k values,all of our conclusions were based on a comparison between the experimental results obtained over MIP-coated P25and over the neat P25as a control.Furthermore,as shown in Fig.2,we found that the direct photolysis of4CP is inhibited almost completely when a high level of phenol coexists in solution,because the UV-light-absorption ability of4CP is relatively decreased when a low level of4CP is mixed with other high levels of UV-light-absorbing substances,such as phenol and MIP-coated P25.Therefore,we can safely conclude that the MIP-coated catalysts can increase the selectivity of TiO2photocatalysis toward the photocatalytic degradation of the target chlorophenols. We also noted from Table1that the phenol removal rate was increased in the cases of both MIP-modified TiO2catalysts(2CP-P25and4CP-P25).This is possibly related to the increment in the phenol adsorption,which is possibly due to(1)similar space structure and molecule size of phenol to those of the template molecules,and(2)similar reaction sites of phenol molecules to the bulky MIP(the benzene ring and the existence of an–OH group). We observed that the selectivity of the MIP-coated TiO2 gradually worsened during a long period of UV-light illumination if there was only a low level of the target pollutant in solution. From the next experiments,however,we find that these new catalysts have a good lifetime,if there are other coexisting organic pollutants in solution.We photo-degraded the target pollutant 4CP of2mg L21over4CP-P25in the presence of50mg L21 phenol for successive cycles.In each cycle,about80%of added 4CP was degraded,then a part of4CP was freshly added and the next degradation cycle started.The rate constant k4CP was measured as0.08975,0.08716,0.08848,0.08198,0.08104and 0.08154min21for the first six cycles,being y3times that over P25 (0.02757min21).In another experiment,the photodegradation of 2CP(20mg L21)was performed for15min over2CP-P25,then 2000mg L21of4-nitrophenol was added and the photodegrada-tion was continued for12h,followed by a filtration with a0.22m m filter.After being vacuum dried,the collected photocatalyst was again suspended into a fresh solution of20mg L212CP,and the photodegradation of2CP was repeated for several cycles as described above.The k2CP values in the first four cycles were almost the same as before the12-h photodegradation of 4-nitrophenol.These suggest that the MIP-coated TiO2is quite stable during the operation.

In conclusion,we first developed a very simple approach to prepare MIP-coated photocatalysts via an in-situ polymerization of OPDA in the presence of target molecules and TiO2nanoparticles. Using a photocatalytic treatment with the novel hybrid photo-catalysts,we can selectively remove low-level target pollutants in the presence of high-level less toxic pollutants.

The authors thank the National Science Foundation of China (grants No.20677019and30571536)for financial support.The Analytical and Testing Center of Huazhong University of Science and Technology is also thanked for its help in the characterization of photocatalysts.

Notes and references

{Details for the preparation of MIP-coated photocatalysts are as follows. In a typical preparation,0.24g OPDA and0.1g2CP were dissolved into 40mL distilled water,and the solution was stirred for20min,followed by addition of1mL of6M HCl and0.4g P25and then a3-min ultrasonication.Polymerization was conducted for24h after being initiated with a30-min irradiation with UV light using a250-W Hg lamp as light source.Then,0.13g L21Na2CO3was added into the suspension,followed by stirring for20min and filtration.The obtained solids were washed five times by using Na2CO3solution and distilled water,respectively. Alternatively,the polymer-coated particles resulting from the polymeriza-tion could be photocatalytically treated in situ by UV-light illumination for 15–20min,followed by filtration.After the Na2CO3treatment or the photocatalytic treatment,the solids were thoroughly vacuum dried and powdered,leading to2CP-P25.Similarly,4CP-P25was prepared by using 4CP instead of2CP,and NIP-P25was obtained when neither4CP nor2CP was used in the preparation.

§To evaluate the photocatalytic selectivity of photocatalysts,the degrada-tion of organic(s)was carried out in a jacketed quartz reactor filled with 250mL of the test solution in the presence of the catalyst(25mg)by using a9-W UV lamp(Philips,l max=253.7nm)as light source.Prior to illumination,the suspension was stirred for20min to favor the pollutant’s adsorption onto the catalyst surface,followed by determination of the concentration of the pollutant(s)as the initial concentration c0.The remaining concentration of pollutant(s)in the suspension at given intervals of irradiation was measured on a JASCO PU-2089HPLC.The adsorption of4CP and/or2CP was monitored on the MIP-coated P25nanoparticles, and the amount of the adsorption was found to cause no decrease of the chlorophenol to be degraded.

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