Inorganic molecular imprinted titanium dioxide photocatalyst: synthesis, characterization and its application for efficient and selective degradation of phthalate esters

PAPER www.rsc.org/materials | Journal of Materials Chemistry

Inorganic molecular imprinted titanium dioxide photocatalyst: synthesis,characterization and its application for efficient and selective degradation ofphthalate esters†

Xiantao Shen,a Lihua Zhu,*a Chuixiu Huang,a Heqing Tang,*a Zhiwu Yub and Feng Dengb

Received 8th January 2009, Accepted 29th April 2009

First published as an Advance Article on the web 4th June 2009

DOI: 10.1039/b900196d

An inorganic molecular imprinted polymer (IMIP) coated photocatalyst for photodegradation of

diethyl phthalate (DEP) was synthesized by coating a layer of molecular imprinted silica/alumina on

the surface of TiO2 nanoparticles with DEP as the template. The characterization with HR-TEM,

XRD, FT-IR and UV-visible spectroscopic analysis indicated that the new catalyst was a composite of

the TiO2 particle core and a shell layer of Al3+-doped silica with thickness of about 5 nm. The 27Al MAS

NMR measurements revealed that the IMIP layer consisted of framework tetrahedrally coordinated

aluminium and non-framework octa-coordinated aluminium species, both of which function as the hot

spots for the adsorption of target molecules on the catalyst during photocatalysis. It was found that the

IMIP layer provided the photocatalyst with molecular recognition ability, leading to selective

adsorption and rapid mineralization of the target pollutant from its low level solution (2 mg L�1) in the

presence of other high level non-target pollutants, such as phenol (50 mg L�1). Unlike the neat TiO2

photocatalyst (Degussa P25), the use of the IMIP-coated TiO2 photocatalyst almost eliminated the

generation of toxic aromatic byproducts. Moreover, the new photocatalyst was totally constructed by

inorganic compounds, being resistant to photochemical attack and showing favorable lifetime during

the photocatalysis.

1 Introduction

The increasing worsening of the world environment has attracted

much attention to developing new materials for controlling

environmental pollution. One type of new environmental mate-

rials is eco-materials, which represent the materials that are non-

damaging to the global environment during their production and

use, because they contain less hazardous substances with green

environmental profiles, a higher potential for recycling, and/or

higher resource productivity.1–3 Another type is highly efficient

and multiple functional materials possessing the ability to

directly eliminate the pollutants. Until the present time, lots of

functional materials have been developed, including absorbents

and catalysts. However, the available materials cannot yet meet

the requirements of pollution control in many cases. For

example, it is difficult to treat the toxicants absorbed on

contaminated soil, or highly toxic pollutants in a complicated

aqueous system.4 This difficulty will be aggravated when the

pollutants are environmental endocrine disruptors.5 To solve this

difficult problem, it is urgent to develop multifunctional

aCollege of Chemistry and Chemical Engineering, Huazhong University ofScience and Technology, Wuhan, 430074, P.R. China. E-mail: [email protected]; [email protected]; Fax: +86-27-87543632; Tel:+86-27-87543432bState Key Laboratory of Magnetic Resonance and Atomic and MolecularPhysics, Wuhan Center for Magnetic Resonance, Wuhan Institute ofPhysics and Mathematics, Chinese Academy of Sciences, Wuhan,430071, P.R. China

† Electronic supplementary information (ESI) available: Fig. S1–S3. SeeDOI: 10.1039/b900196d

This journal is ª The Royal Society of Chemistry 2009

materials combining the features of the above-mentioned two

types of materials. This inspires us to study enzyme-like photo-

catalysts for highly efficient and selective removal of various

organic pollutants including phthalate esters (PAEs).

PAEs are a family of widely used chemicals. Extensive appli-

cation of PAEs as plasticizers for polymers has resulted in their

ubiquitous presence and accumulation in the environment.6

Because some PAEs are verified to be environmental hormones7,8

and their biological degradation is slow,9 it is significant to

develop faster treatment processes to control their pollution.10

One of the promising treatment technologies is photocatalytic

oxidation, and TiO2 has proven to be an efficient photocatalyst.11

However, the photo-oxidation of organic compounds over TiO2

is dominated by a free radical mechanism, which is non-selec-

tive.12 This is fatal when the low level PAEs are accompanied by

the coexistence of other low toxicity biodegradable pollutants at

high levels. Therefore, it is required to develop a photocatalyst

with high selectivity toward the degradation of PAEs.

Surface modification of TiO2 may improve its selectivity,

because the organic modification enhances its adsorbing capa-

bility to the target pollutant. Ghosh-Mukerji et al. prepared

a photocatalyst consisting of molecular recognition sites located

in the vicinity of TiO2 micro-domains.13 Miyayama et al.

reported that oxidation of p-toluenesulfuric acid became more

efficient on SiO2/TiO2 powders being surface-modified with

quaternary ammonium base groups.14 However, the stability of

the surface modified photocatalysts is limited by the photo-

degradation of the organic host molecules accredited directly to

TiO2 during the oxidation.

J. Mater. Chem., 2009, 19, 4843–4851 | 4843

Scheme 1 The route for the preparation of IMIP-P25 and its use in

photodegradation of DEP.

When no organic modification layer is coated on the surface of

TiO2, the photocatalyst may also have moderate selectivity.

Calza and Xamena et al. reported that the microporous titano-

silicate ETS-10 (a molecular sieve) might act as a shape-selective

photocatalyst for the decomposition of large aromatic

compounds, although the absolute degradation rates of the

aromatic compounds became lower.15–17 Likewise, Inumaru et al.

prepared a SiO2-covered TiO2 photocatalyst to enhance the

decomposition of 4-nonylphenol.18 In order to make the photo-

catalyst have both high photocatalytic selectivity and degrada-

tion ability, we recently developed a new type of selective TiO2

photocatalysts by coating a layer of organic molecular imprinted

polymer (MIP) on the surface of TiO2 particles.19,20 The MIP

layer, having molecular recognition ability, enables the catalyst

to possess high selectivity toward the photodegradation of target

pollutants such as chlorophenols and nitrophenols, along with

a significant acceleration of the degradation of the pollutants.

Moreover, the MIP layer has a basic structure of polyaniline and

is much more stable than the small organic molecules under UV

irradiation. Hence, the MIP-coated TiO2 remains photochemi-

cally stable during the photocatalytic degradation of the pollut-

ants. However, we also noted that the selectivity of the

MIP-coated TiO2 was slowly worsened during a long period of

UV-light illumination if there were no organic pollutants in the

solution. This is attributed to the organic nature of the MIP

layer. To overcome this shortcoming, the TiO2 photocatalyst

coated with a layer of inorganic molecular imprinted polymer

(IMIP) is anticipated to have excellent photochemical stability

and lifetime under UV irradiation.

The present work aims at the preparation of an all-inorganic

photocatalyst by coating a layer of molecular imprinted silica/

alumina on TiO2 nanoparticles. As the representative member of

PAEs, diethyl phthalate (DEP) is listed as one of the environ-

mental priority pollutants by the Environmental Protection

Agency of the USA,21 and thus it was chosen as the target

pollutant in the present work. When DEP is used as the template,

‘‘molecular footprint’’ cavities of the photocatalyst are formed on

the IMIP layer, ensuring selective adsorption and efficient

degradation of DEP.

2 Experimental

2.1 Chemicals

TiO2 nanoparticles (P25, ca. 80% anatase, 20% rutile; BET area,

ca. 50 m2 g�1) were provided by Degussa (Germany).

AlCl3$6H2O, tetraethoxysilane (TEOS), 8-hydroxy-quinoline

(HQ), alcohol, phenol and DEP were supplied by Shanghai

Chemical Reagent Company, and all these chemicals were of

analytical reagent grade and used without further purification.

HPLC-grade methanol was obtained from Tedia, and the used

water was obtained by a Milli-Q-Plus ultra-pure water system.

Solution pH was adjusted with diluted HCl, NH3$H2O or NaOH

solutions.

2.2 Preparation of IMIP-coated photocatalysts

The IMIP-coated photocatalysts were prepared via a route given

in Scheme 1, where the Lewis acid (Al3+) was used as the

4844 | J. Mater. Chem., 2009, 19, 4843–4851

functional group for the inorganic polymer to combine with the

target pollutant as the template.

2.2.1 Synthesis of TiO2/SiO2 core-shell nanoparticles. TiO2

nanoparticles were coated with a layer of silica by using a process

similar to the St€ober method.22,23 Both P25 (3.6 g) and NH3$H2O

(25%, 2 mL) were quickly added to a mixture solution of ethanol

(80 mL) and water (16 mL) at room temperature. The dispersion

was ultrasonicated for 3 min, and then TEOS (2 mL) was quickly

introduced with stirring. The reaction was left for about 8 h.

After being isolated, washed with water and vacuum dried

overnight, SiO2-coated TiO2 particles were obtained.

2.2.2 Synthesis of Al3+ ions doped nanoparticles. To activate

the surface of the particles and clean away possible metallic

impurities such as Fe3+ and Zn2+, the obtained TiO2/SiO2

nanoparticles (2 g) were refluxed with concentrated HCl (10 mL)

for 5 h.24,25 After being washed with deionized water (200–300

mL) and diluted NH3$H2O (pH 7–8, 200–400 mL) in order, the

nanoparticles were added to 50 mL water containing 1.32 g

AlCl3$6H2O, and then the solution pH was kept at 6.5 by adding

diluted NH3$H2O. The doping of Al3+ was achieved by aging the

mixture at 80 �C for 3 h. Then, the Al(OH)3 floccules were

removed by decanting, and the surplus Al(OH)3 adhered on the

surface of the particles was completely removed by washing with

dilute HCl (pH 4, 200 mL). The Al3+ ion doped TiO2/SiO2

particles were obtained by drying the particles at 25 �C for 12 h.

2.2.3 Inorganic molecular imprinting. The Al3+ doped TiO2/

SiO2 particles were dispersed in a mixture of 50 mL diluted HCl

(pH 4) and 5 mL acetone solution containing 1.0 g DEP, where

Al3+ ions (as a Lewis acid) on the particles interacted with the

template (as a Lewis base) to form surface coordination

compounds. To promote the formation of enough footprints on

the xerogels, the suspension was kept at pH 4 at 80 �C for about

a week. The footprints were formed on the catalysts during the

aging. To extract the template, the particles were collected and

then dispersed in 40 mL alcohol. After the dispersion was stirred

for 3 h, the alcohol was removed by filtration, followed by adding

40 mL alcohol again. The alcohol extraction and washing was

conducted three times. Then the obtained solids were washed in


turns with 200 mL diluted HCl (pH 4) and 400 mL water. The

complete removal of the template by the extraction and washing

was verified by using a spectrophotometric determination of

DEP in the final eluant (water). If no DEP was detected in the

final eluant, the removal of the template was taken as being

complete. Then the wet particles were carefully assembled and

dried at room temperature. The obtained photocatalyst was

named as IMIP-P25. As a control, the non-imprinted catalyst

NIP-P25 was synthesized when no template was used in the

preparation process, and the mixture type photocatalyst MMC-

P25 was obtained by simply mechanically mixing P25 (85% wt)

and Al3+ doped SiO2 (15% wt).

Fig. 1 HR-TEM image of IMIP-P25.

2.3 Characterization of the photocatalysts

Aluminium in IMIP-P25 was determined with a fluorimetric

method on a FP-6200 fluorescence spectrometer (JASCO). The

surface layer on the catalyst was first fully dissolved in a NaOH

solution (5 mol L�1), followed by a pH adjustment to pH 2 via

HCl (1 : 1), which resulted in the precipitation of Si as silicic acid.

After a filtration through 0.22 mm filters, the Al3+ was transferred

into a sodium acetate solution (pH 3) with an excess of HQ. The

fluorescence of the Al–HQ complex was monitored at 515 nm

with excitation at 360 nm.

Particle sizes and morphology of photocatalysts were observed

using high resolution transmission electron microscopy

(HRTEM) on a JEM-2010FEF TEM. Powder X-ray diffracto-

grams were obtained on an X’Pert PRO X-ray diffractometer

(PANalytical) with a Cu Ka radiation source. FTIR and UV-vis

solid-state reflection spectra were taken on a Bruker VERTEX

70 spectrophotometer and a Shimadzu UV-2550 spectropho-

tometer, respectively.

Nitrogen (N2) adsorption–desorption measurement at 77K

was conducted using a pore analyzer (Belsorp-mini, BEL Japan

Inc.). The specific surface area was obtained from the Brunauer–

Emmett–Teller (BET) plot of the N2 adsorption isotherm. The

pore size distribution and average pore diameter were calculated

by using the Barret–Joyner–Halenda (BJH) method with the

adsorption branch of the N2 isotherm.

NMR experiments were carried out on a Varian Infinityplus-

400 spectrometer at resonance frequencies of 104.3 MHz, and the27Al MAS NMR spectra were recorded with a pulse length of 0.5

ms (#p/12), a recycle delay of 1 s, and a spinning rate of 6 kHz.26

No pretreatment was carried out for the samples prior to their

measurements.

Fig. 2 XRD patterns of IMIP-P25 (1) and P25 (2).

2.4 Evaluation of the performances of the photocatalysts

In 10 mL centrifuge tubes in the dark with a rocking tray to

maintain a homogeneous suspension of the catalyst (2 g L�1), the

adsorption experiment was carried out by immersing for 36 h to

achieve the adsorption/desorption equilibrium. The photo-

degradation experiments were carried out in a 100 mL photo-

reactor, over which a 20-W UV lamp (Philips, lmax 254 nm) was

fixed. After the solution containing the catalyst (0.1 g L�1) was

ultrasonicated for 3 min, it was further stirred with a magnetic

stirrer for 30 min before the UV irradiation was started. When

pure P25 was used as a control catalyst, its load was 0.085 g L�1,

corresponding to the amount of P25 in the used IMIP-P25


(0.1 g L�1). At different time intervals, samples for analysis were

taken and filtered through 0.22 mm filters to remove catalyst

particles. The analysis was performed on a PU-2089 HPLC

(JASCO), equipped with a C18 ODS column and an ultraviolet

detector. The detection wavelengths were set at 230 nm for DEP,

and 275 nm for phenol.

3 Results and discussion

3.1 Characteristics of IMIP-coated photocatalysts

A TEM image for IMIP-P25 particles is given in Fig. 1, which

shows that IMIP-P25 consists of two distinct phases: the TiO2

particle core (with clear crystal form) and the SiO2 shell (without

crystal form) with a thickness of about 5 nm. In comparison with

P25, IMIP-P25 gives no new diffraction peaks in the XRD

pattern (Fig. 2), due to the very small thickness of the amorphous

SiO2 layer.

The BET specific surface areas of IMIP-P25 and NIP-P25 were

measured as 84.1 and 51.1 m2 g�1, respectively. The BET specific

surface area of IMIP-P25 is much greater than P25, and that of

NIP-P25 is slightly larger than that of P25 (49.6 m2 g�1).27 The

average grain size of either IMIP-P25 or NIP-P25 must be larger

than that of P25 due to the IMIP or NIP coating on the surface of

P25 cores. The much increased BET surface area of the particles

J. Mater. Chem., 2009, 19, 4843–4851 | 4845

Fig. 3 Diffuse reflectance UV-vis spectra of IMIP-P25 (1) and P25 (2).

with larger grain sizes indicate that the IMIP layers are quite

porous. Indeed, the calculated BJH pore size distributions indi-

cated that IMIP-P25 contained more micropores (<3 nm) than

NIP-P25 (Fig. S1).† Relative to NIP-P25, the greater porosity

and BET surface area of IMIP-P25 may be attributed to the

effect of molecular imprinting and the removing of the template

molecules. That is, footprint cavities are formed on the surface of

IMIP-P25, being very important for high photocatalytic activity

of IMIP-P25 as a photocatalyst. Because the crystal form of the

TiO2 core is not changed by the SiO2 coating (Fig. 2) and the very

thin and porous SiO2 layer does not reduce the optical response

of the photocatalyst (Fig. 3), the IMIP-P25 can keep the high

photocatalytic activity arising from the P25 core.

Fig. 4 compares the FT-IR spectrum of IMIP-P25 with that of

TiO2/SiO2 and neat P25 nanoparticles. The peaks at 3440 and

1628 cm�1 correspond to the fundamental stretching vibration of

O–H groups and the bending vibration of coordinated Si–OH,

respectively (spectra 1–4). The peak at 1078 cm�1 is assigned to

the asymmetric stretching vibration of Si–O–Si (spectrum 3).28 It

is also seen in Fig. 4 that two peaks at 1286 and 1723 cm�1 in the

spectrum of IMIP-P25 (spectrum 2) are newly found due to the

doping of Al3+, which are associated to the vibration of the Al–

O–Si bond.29,30 When the amount of AlCl3$6H2O used in the

Fig. 4 FT-IR spectra of IMIP-P25 (1, 2), TiO2/SiO2 (3) and P25 (4). The

two samples of IMIP-P25 were prepared in the presence of (1) 2.20 and

(2) 1.32 g AlCl3$6H2O in the preparation process. The spectrum of NIP-

P25 was not shown here because it was almost the same as that of the

corresponding IMIP-P25.

4846 | J. Mater. Chem., 2009, 19, 4843–4851

preparation process was increased from 1.32 g to 2.20 g, the two

peaks at 1286 and 1723 cm�1 were shifted to about 1240 and 1670

cm�1 (spectrum 1), respectively.31,32 At the same time, the

asymmetric stretching vibration of Si–O–Si was shifted from

1078 to 1085 cm�1. These results suggest that the Al3+ ions being

highly dispersed in the SiO2 shell are combined with the bulk

SiO2 via oxygen as a binding bridge. The doped Al3+ ions as

Lewis acid will act as a functional group for the inorganic

polymer to combine with the template.33

3.2 Enhanced adsorption ability of catalyst IMIP-P25

The sorption kinetics of DEP (200 mg L�1) on different catalysts

are presented in Fig. 5. The time for IMIP-P25 to achieve the

adsorption/desorption equilibrium was about 8 h, being longer

than that (about 3 h) for NIP-P25, MMC-P25, and the TiO2/SiO2

nanoparticles. The apparently slower sorption of DEP on IMIP-

P25 is attributed to the plentiful cavities on the surface and in the

bulk of the footprint IMIP layer, because the adsorption of DEP

into the inner cavities requires a longer time for diffusion. The

adsorption capacity of IMIP-P25 for DEP was estimated to be

18.5 mg g�1 after the sorption for 36 h, much higher than that of

NIP-P25 (3.5 mg g�1), MMC-P25 (3.4 mg g�1) and the TiO2/SiO2

core-shell nanoparticles (2.0 mg g�1). This indicates that IMIP-

P25 has special adsorption ability to the target molecule.

As shown in Scheme 1, DEP may be adsorbed onto IMIP-P25

via two binding sites (C]O in the DEP molecule and Al3+ in the

IMIP),24,25 resulting in the formation of a moderately stable

surface complex between DEP and IMIP (eqn (1)),

2AlðIIIÞs þDEP *)k1

k�1

AlðIIIÞs-DEP-AlðIIIÞs (1)

where Al(III)s represents Al(III)-resolved acid sites on the surface

of the catalyst. Because one DEP molecule is adsorbed on

the surface via its interaction with two Lewis acid sites and the

initial amount of DEP is in a great excess relative to its

adsorption on the photocatalyst, it is reasonable that the

adsorption of DEP on the photocatalyst behaves like a pseudo-

second-order reaction in kinetics.34–36 We may assume: (i) there is

a monolayer of target DEP on the surface of the catalyst; (ii) the

energy of sorption for each DEP is the same and is not dependent

Fig. 5 Sorption kinetics of DEP (200 mg L�1) on IMIP-P25 (1), NIP-P25

(2), MMC-P25 (3) and TiO2/SiO2 nanoparticles (4).


Fig. 6 Effect of doping level of Al in IMIP-P25 on its photocatalytic

activity towards the degradation of DEP (2 mg L�1).

on the surface coverage; (iii) DEP is only adsorbed on localized

sites and the adsorbed DEP molecules do not interact with each

other. Thus, the rate of DEP adsorption can be expressed with

eqn (2),

dqt

dt¼ k1Cðqe � qtÞ2 � k�1qt (2)

where C is the concentration of DEP in solution, qt and qe (mg

g�1) are the sorption amounts of DEP at time t and at equilib-

rium, and k1 (g mg�1 h�1) is the rate constant. The term (qe � qt)

represents the amount of free Lewis acid sites on the surface of

the photocatalyst being available for the adsorption of DEP.

Because the initial amount of DEP is greatly superfluous in

comparison with its adsorption on the photocatalyst in the

adsorption experiment, the concentration of DEP in solution will

approximately not change. Therefore, we can define another

constant by using k2 ¼ k1C. Moreover, the rate of the backward

desorption is much slower than the forward adsorption in eqn (1)

for quite a long period of immersion time, during which the

adsorption/desorption is far from its equilibrium state. Thus, the

rate expression for the adsorption is simplified as eqn (3),

dqt

dt¼ k2ðqe � qtÞ2 (3)

where k2 (g mg�1 h�1) is the apparent rate constant of the pseudo

second-order sorption process. By integrating eqn (3) over the

range from t ¼ 0 to t ¼ t, the following eqn (4),

t

qt

¼ 1

k2q2e

þ 1

qe

t (4)

It is noted that the term k2qe2 is the initial adsorption rate of DEP

according to eqn (3). By plotting t/qt against t, it is easy to obtain

the values of the initial adsorption rate and the apparent rate

constant of DEP adsorption. It was found that the experimental

data in Fig. 5 could be well fitted with the pseudo-second-order

model with the correlation coefficient (r2 > 0.98, Fig. S2).† The

data fitting gave the values of the initial sorption rate of DEP as

11.7, 3.4, 6.8 and 0.72 mg g�1 h�1 on IMIP-P25, NIP-P25, MMC-

P25 and TiO2/SiO2, respectively. The largest values of qe and the

initial sorption rate of DEP on IMIP-P25, along with the above

discussions, strongly support the hypothesis that the sorption of

DEP on IMIP-P25 is dependent on the interaction between the

Lewis acid (Al3+) and Lewis base (the target DEP). As the

substrate specificity of footprint sites is the origin for their

catalytic selectivity,37,38 IMIP-P25 should be a promising pho-

tocatalyst with the capability of selectively removing DEP from

mixtures of pollutants.

Fig. 7 Kinetics of the direct photolysis (2) of DEP (c0 2 mg L�1) and the

photocatalytic degradation of DEP over imprinted silica/alumina (3),

non-imprinted TiO2/SiO2 (4), NIP-P25 (5), MMC-P25 (6), P25 (7) and

IMIP-P25 (8). For a comparison, curve 1 gives the removal of DEP by the

adsorption of DEP on IMIP-P25 without the UV irradiation.

3.3 Effect of Al3+ level on photocatalytic ability of IMIP-P25

Because the molecular recognition of IMIP-P25 is mainly

attributed to the interaction between the target and the Al3+ ions

associated with the imprinted cavities, the doping level of Al3+

will influence the adsorption and catalytic activity of the catalyst.

In our experiment, the DEP degradation over IMIP-P25 follows

pseudo-first-order reaction in kinetics, thus the effects of the Al3+

doping level are evaluated by checking the variations of the

apparent rate constant k of the photodegradation. When the

addition of AlCl3$6H2O in the preparation process was 0.22,


0.44, 0.88, 1.32 and 1.76 g, the amount of doped Al3+ in IMIP-

P25 was determined with the fluorimetric method as 4.9, 10.1,

19.6, 30.4 and 34.1 mg g�1, respectively. It is seen in Fig. 6 that k

is increased from 0.0055 to 0.12 min�1 with the increase of the

doped Al amount from 4.9 to 30.4 mg g�1. However, k is

decreased to 0.059 and 0.028 min�1 when the doped Al amount is

further increased to 34.1 and 41.4 mg g�1, respectively. This

demonstrates that an over-high addition of AlCl3$6H2O is

unfavorable to the performance of IMIP-P25 and the optimum

addition is 1.32 g in the present work. The reason for poorer

performances of IMIP-P25 prepared with an addition of Al3+

more than the optimal addition was not well clarified. As

a tentative explanation, it may be related to the deposition of

Al(OH)3 arising from the hydrolysis of Al3+ at over-high

concentrations in the doping solution at pH 6.5, which is unfa-

vorable to even distribution of Al3+ in the Al3+-doped SiO2 layer.

3.4 Enhanced photocatalytic ability and photochemical

stability of IMIP-P25

Fig. 7 gives kinetic data for the photocatalytic degradation of

DEP alone (c0 2 mg L�1) on the catalysts. The small loads of the

catalyst (0.1 g L�1) and the short degradation time (40 min),

J. Mater. Chem., 2009, 19, 4843–4851 | 4847

Fig. 8 Kinetic data for the repeated photocatalytic degradation of DEP

over IMIP-P25. Numbers give the cycle number.

along with the pre-immersion of about 30 min as described in

Section 2.4, can assure that the experimental results of the pho-

tocatalytic degradation of DEP is not influenced by the possible

adsorption of DEP on the photocatalysts. Indeed, the degrada-

tion (including the possible adsorption) of DEP on IMIP-P25 in

the dark is less than about 8% during the immersion of 40 min

(curve 1). The direct photolysis of DEP in the absence of any

photocatalysts (curve 2) is also inefficient, being consistent with

that reported in the literature.39 When the imprinted silica/

alumina particles are used as the photocatalyst, the degradation

of DEP under the UV irradiation is still inefficient (curve 3). In

comparison with the degradation of DEP over other photo-

catalysts (curves 4–7), these control experiments indicate that the

much enhanced degradation of DEP over IMIP-P25 is mainly

from the combination of the enhanced adsorption on the IMIP

layer and the high photocatalytic ability of the TiO2 P25 core.

The direct removal of DEP by either the adsorption or the

possible catalytic effect of the silica/alumina structure is not

important in the photocatalytic degradation of DEP over IMIP-

P25.

It is also noted that NIP-P25 (curve 5) possesses a weaker

photocatalytic activity than neat P25 (curve 7) toward the

degradation of DEP, indicating that the NIP layer will not

contribute to the degradation of DEP but decrease the photo-

catalytic ability of the P25 core. The mixture of P25 and the Al3+

doped SiO2 (MMC-P25, curve 6) yields a DEP degradation rate

greater than NIP-P25. This is because the Al3+-doped SiO2 in

MMC-P25 does not cover the P25 core, unlike that in NIP-P25.

However, the degradation of DEP over MMC-P25 is consider-

ably slower than that over neat P25, due to the photo-filtration

arising from the sheltering effect of the Al3+-doped SiO2 particles

on the P25 particles.

The fitting of the data in Fig. 7 suggested that the photo-

catalytic degradation of DEP over these photocatalysts obeyed

pseudo-first-order reaction kinetics, and the apparent rate

constant k for the photodecomposition of DEP was obtained as

0.12 min�1 over IMIP-P25, being 14.0, 9.2, 6.5, 4.6 and 2.5 times

that over TiO2/SiO2 (0.013 min�1), NIP-P25 (0.018 min�1),

MMC-P25 (0.025 min�1) and P25 (0.049 min�1), respectively.

This indicates that the IMIP enables the catalyst to have much

stronger photocatalytic activity than other photocatalysts.

The photocatalytic activity was also evaluated by degrading

DEP in binary solutions containing DEP (2 mg L�1) and a co-

existing pollutant phenol (2 or 50 mg L�1). The experimental

results are summarized in Table 1, where the photocatalytic

selectivity of the catalysts is focused on. The rate constant ratio

Table 1 Rate constants for the photodegradation of target DEP over differ

Catalyst

Phenol (2 mg L�1)

kDEP/10�3 min�1 kphenol/10�3 mi

IMIP-P25 53.0 29.5NIP-P25 4.08 19.7Rb 13.0 1.50ab 8.67

a Total illumination time was 60 and 180 min for 2 and 50 mg L�1 phenol, respeP25, and a is the ratio of the R value of kDEP to the one of kphenol.

4848 | J. Mater. Chem., 2009, 19, 4843–4851

(R) of kDEP over IMIP-P25 to that over NIP-P25 is 13.0, being

much greater than that (1.50) of kphenol when the initial

concentration of phenol is 2 mg L�1. The relative selectivity is

evaluated by defining a as the ratio of the value of kDEP/kphenol

over IMIP-P25 to that over NIP-P25. The large value of a (8.67)

confirms that the inorganic molecular imprinting enhances the

photocatalytic selectivity toward the target pollutant. When the

level of phenol is increased to 50 mg L�1, the relative selectivity of

IMIP-P25 is further increased: the R value for DEP is increased

to 18.5, and the a value becomes as large as 12.2. These results

further certify that IMIP-P25 has high selectivity in the photo-

catalytic removal of low-level target pollutants from wastewaters

in the presence of other pollutants at high levels.

The photochemical stability was evaluated by conducting the

photodegradation of DEP (2 mg L�1) over IMIP-25 for succes-

sive cycles. In the first cycle, the solution was irradiated for 60

min, resulting in complete degradation of DEP. The resultant

solution in the presence of the catalyst was further irradiated

with UV light for 10 h. And then, the second degradation cycle

was conducted for 60 min after freshly adding 2 mg L�1 DEP to

the solution. This process was repeated for successive cycles, and

the kinetic data for parts of the degradation cycles are shown in

Fig. 8, which gave the degradation rate constant kDEP as 0.098 �0.015 min�1 for the initial six cycles. This suggests that IMIP-

coated TiO2 is quite stable with good lifetime during the pho-

tocatalysis.

The enhanced photocatalytic activity and selectivity of pho-

tocatalyst IMIP-P25 is attributed to the simultaneous presence of

the interaction sites at the doped Al3+ ions positions and the

ent photocatalysts in the presence of phenol a

Phenol (50 mg L�1)

n�1 kDEP/10�3 min�1 kphenol/10�3 min�1

3.81 2.860.206 1.89

18.5 1.5212.2

ctively. b R is the ratio of kDEP or kphenol over IMIP-P25 to that over NIP-


imprinted cavities. The simultaneous presence of the interaction

sites and the imprinted cavities enables IMIP-P25 to behave like

an enzyme catalyst having molecular recognition ability, leading

to enhanced photocatalytic activity and selectivity.

To certify that the enhanced adsorption and photocatalytic

degradation of DEP on IMIP-P25 is due to the formation of

‘‘footprint’’ cavities, solid-state NMR analysis is employed to

define receptor sites formed on the silica/alumina layer by surface

imprinting. Fig. 9 presents the 27Al MAS NMR spectra of IMIP-

P25 and NIP-P25. Both the imprinted and non-imprinted cata-

lysts yield two peaks around 0 and 55 ppm. It is reported that the

0 ppm signal arises from the non-framework octahedrally coor-

dinated Al, while the 55 ppm signal stems from the framework

tetrahedrally coordinated Al.40–42 However, there are some

differences between the spectra of IMIP-P25 and NIP-P25. The

framework tetra-coordinated Al is significantly shifted from 62.6

ppm on NIP-P25 to 54.4 ppm on IMIP-P25. At the same time,

the peak for the octa-coordinated Al is shifted from about 8.0

ppm on NIP-P25 to 5.0 ppm on IMIP-P25. Moreover, we

immersed IMIP-P25 into the saturated aqueous solution of DEP

for 6 h, dried the sample at room temperature for about 5 h, and

then measured the NMR spectrum of the DEP-adsorbed IMIP

(not shown here). It was found that the re-combination of the

template molecules with the molecular imprinted cavities caused

further upfield shifts: the framework tetra-coordinated Al was

shifted from 54.4 ppm on the neat IMIP-P25 to 62.6 ppm on the

DEP-adsorbed IMIP-P25, and the peak for the octa-coordinated

Al was shifted from 5.0 ppm on the neat IMIP-P25 to 0.8 ppm on

the DEP-adsorbed IMIP-P25. All these changes suggest that the

template imprinting has induced some structural variations in the

IMIP layer and both the above-mentioned two types of Al

species on IMIP-P25 function as molecular recognition sites to

the target molecules of DEP. In other words, both the non-

framework octahedrally coordinated Al and framework tetra-

hedrally coordinated Al species on the surface of IMIP-P25 are

hot spots in the photocatalytic degradation of DEP. Due to the

presence of two types of hot spots, IMIP-P25 shows high affinity

to the target pollutant and yields a high selectivity to the pho-

tocatalytic removal of DEP.

Here it is worth noting two points. One point is that the

increased BET surface area of IMIP-P25 is favorable to the

adsorption of organic pollutants on the photocatalyst, but it is

not the most important reason for the much enhanced rate and

selectivity for the DEP photocatalytic degradation over IMIP-

P25. This is supported by a comparison between the BET areas,

DEP degradation rate constant and selectivity over IMIP-P25

Fig. 9 27Al MAS NMR spectra of IMIP-P25 (1) and NIP-P25 (2).


and NIP-P25. The measured BET areas of IMIP-P25 and NIP-

P25 are 84.1 and 51.1 m2 g�1, respectively, the increasing effect of

specific surface area is evaluated by using the ratio between the

areas of the two catalysts of 1.64. In contrast, the rate constant of

DEP degradation over IMIP-P25 is 9.2 times that over NIP-P25

in the solution of DEP alone (Fig. 7). Moreover, the relative

selectivity a is as high as 12.2 for the degradation of DEP in the

binary solution of 2 mg L�1 DEP and 50 mg L�1 phenol (Table 1).

Such significant increases of the rate constant and selectivity of

DEP degradation over IMIP cannot be accounted for by only

considering the moderately increased BET surface area. There-

fore, the most important reason for the much enhanced DEP

degradation and its selectivity originates from the formation of

footprint cavities during the molecular imprinting (i.e., the

imprinting effect).

Another point is that the kinetics of DEP adsorption on IMIP-

P25 sample is apparently so slow that it requires about 6 h to

achieve the nearly saturated adsorption (curve 1 in Fig. 5), but

the photocatalytic degradation of DEP over IMIP-P25 is almost

fully completed in just half an hour (curve 8 in Fig. 7). This

apparent disagreement can be accounted for by considering that

the adsorption of DEP on the photocatalyst may be the rate-

determining step in the process of the photocatalytic degradation

of DEP. The photocatalytic reaction requires the reactant to go

to its adsorption state, but not the saturated adsorption of the

reactant. When the adsorption of DEP, as the slowest step in the

degradation process, is enhanced, the total degradation process

will be greatly accelerated. Therefore, the initial adsorption rate

but not the saturated adsorption of DEP is very important to the

degrdation. As discussed above, the initial adsorption rates of

DEP were evaluated as 11.7 and 3.4 mg g�1 h�1 on IMIP-P25 and

NIP-P25, respectively. Accordingly, the apparent rate constant k

for the photodecomposition of DEP was obtained as 0.12 min�1

over IMIP-P25, being 9.2 times that over NIP-P25 (0.018 min�1).

By taking into consideration the unfavorable effect of the

less porous NIP layer on the photocatalytic activity of the P25

core in the photocatalyst, the increased degradation rate of DEP

over IMIP-P25 is in good agreement with the increased

initial adsorption rates of DEP on IMIP-P25, in comparison with

NIP-P25.

3.5 Mineralization ability of IMIP-P25

It was reported that the attack of hydroxyl radicals leads to the

formation of various hydroxy photoproducts (hydroxylated on

the aromatic ring) for phthalates with shorter alkyl chains

(dimethyl and diethyl).43,44 Thus, the generation and accumula-

tion of degradation byproducts was tested in the present work.

To isolate the aromatic intermediates, the solution was extracted

with n-heptane, and the extract was analyzed by GC-MS and

HPLC. In the HPLC chromatograms of the extracts, three main

peaks were observed: one was attributed to the remaining DEP,

and the other two were assigned to the intermediates, which were

identified as diethyl 2- and 3-hydroxyphthalate (2-DEHP, 3-

DEHP) by GC-MS. Another potentially difficult intermediate in

the n-heptane extraction was identified as phthalic acid (PA),

which is a highly toxic aromatic contaminant due to its effects on

reproductive systems through mediated transactivation.45

J. Mater. Chem., 2009, 19, 4843–4851 | 4849

Fig. 10 Concentration profiles of DEP, total aromatic compounds

(TAC) and major aromatic intermediates (PA, 3-DEHP and 2-DEHP) in

the photocatalytic degradation of DEP (c0 9 mmol L�1) over IMIP-P25 (a)

and MMC-P25 (b).

Fig. 10 illustrates that the concentration profiles of DEP,

total aromatic compounds (TAC, defined as the sum of the

residual DEP and the major aromatic intermediates) and major

aromatic intermediates (PA, 3-DEHP and 2-DEHP) in the

photocatalytic degradation of DEP over IMIP-P25 and MMC-

P25. It is clearly seen that the use of MMC-P25 results in

a moderately fast decrease of DEP concentration and a slow

decrease of TAC, which is accompanied by fast accumulations

of the three major aromatic intermediates (Fig. 10b). However,

the concentrations of both DEP and TAC are rapidly decreased

to almost zero in the case of IMIP-P25 being used as the pho-

tocatalyst, which is mainly attributed to little accumulation and

further degradation of the major aromatic intermediates

(Fig. 10a). This indicates that IMIP-P25 not only promotes the

photodegradation of the target pollutant, but also accelerates

the photodegradation of the intermediates, being favorable to

complete mineralization of the organic pollutant.

4 Conclusions

A new composite photocatalyst IMIP-P25 consisting of TiO2

nanoparticles and inorganic mesoporous silica/alumina MIP was

successfully synthesized by using DEP as the template. With the

aid of 27Al MAS NMR analysis, it was found that in the IMIP

layer there were two types of hot spots formed by framework

tetrahedrally coordinated aluminium and the non-framework

octahedrally coordinated aluminium. These receptor sites, as

4850 | J. Mater. Chem., 2009, 19, 4843–4851

Lewis acids, along with the molecular imprinted cavities in the

IMIP layer, provided IMIP-P25 with high affinity and special

molecular recognition ability, leading to its selective adsorption

and mineralization toward the target pollutant DEP. Therefore,

IMIP-P25 was highly selective for the photocatalytic removal of

the low-level target pollutant DEP from the mixture solutions

containing other high-level organic pollutants. It also signifi-

cantly accelerated the degradation of possible intermediates,

leading to fast mineralization of the pollutant and ensuring the

photocatalytic degradation of DEP as a green and safe approach.

Moreover, IMIP-P25 was confirmed to have a good resistance to

photo-irradiation along with a reasonable and favorable lifetime,

because it was totally constructed by inorganic elements.

Acknowledgements

The authors would like to thank the National Natural Science

Foundation of China (grant nos. 20677019 and 20877031) for the

financial support, and the Center of Analysis and Testing of

Huazhong University of Science and Technology for the char-

acterization of the photocatalysts.

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