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

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Rapid microwave-assisted synthesis of benzene bridgedperiodic mesoporous organosilicas†

Geert Smeulders,*a Vera Meynen,ab Gitte Van Baelen,c Myrjam Mertens,b Oleg I. Lebedev,d

Gustaaf Van Tendeloo,d Bert U. W. Maesc and Pegie Coola

Received 20th November 2008, Accepted 10th February 2009

First published as an Advance Article on the web 17th March 2009

DOI: 10.1039/b820792e

Following extended use in organic chemistry, microwave-assisted synthesis is gaining more importance

in the field of inorganic chemistry, especially for the synthesis of nanoporous materials. It offers some

major advantages such as a significant shortening of the synthesis time and an improved promotion of

nucleation. In the research here reported, microwave technology is applied for the synthesis of benzene

bridged PMOs (periodic mesoporous organosilicas). PMOs are one of the latest innovations in the field

of hybrid ordered mesoporous materials and have attracted much attention because of their feasibility

in electronics, catalysis, separation and sorption applications. The different synthesis steps (stirring,

aging and extraction) of the classical PMO synthesis are replaced by microwave-assisted synthesis steps.

The characteristics of the as-synthesized materials are evaluated by X-ray diffraction, N2-sorption,

thermogravimetric analysis, scanning- and transmission electron microscopy. The microwave-assisted

synthesis drastically reduces the synthesis time by more than 40 hours without any loss in structural

properties, such as mesoscale and molecular ordering. The porosity of the PMO materials has even

been improved by more than 25%. Moreover, the number of handling/transfer steps and amounts of

chemicals and waste are drastically reduced. The study also shows that there is a clear time (1 to 3

hours) and temperature frame (373 K to 403 K) wherein synthesis of benzene bridged PMO is optimal.

In conclusion, the microwave-assisted synthesis pathway allows an improved material to be obtained in

a more economical way i.e. a much shorter time with fewer chemicals and less waste.

Introduction

Periodic mesoporous organosilicas (PMOs)1 are one of the latest

innovations in the field of ordered mesoporous materials and

have attracted much attention because of their feasibility in

electronics, catalysis, separation and sorption applications.2

Similar to the well known pure ordered mesoporous silica (OMS)

counterparts, the synthesis of PMOs is based on the use of

organic templating molecules as structure directing agents

(SDAs) around which an organosilane precursor condenses to

form a mesoporous material.3 In the case of PMOs, the precur-

sors contain an organic functional group (R) bridged between

two silsesquioxanes, that can be generally written as:

(R0O)3SiRSi(OR0)3. So far, quite a number of different PMOs

have been synthesized by varying R.4 The organic functional

group in PMOs are fully and homogeneously imbedded within

the inorganic matrix, in contrast to common organo-

functionalized mesoporous silica materials, usually obtained

aUniversity of Antwerpen (UA), Laboratory of Adsorption and Catalysis,Universiteitsplein 1, 2610 Wilrijk, Belgium. E-mail: geert.smeulders@ua.ac.be; Fax: +32(0)3 265.23.74; Tel: +32(0)3 265.23.79bFlemish Institute for Technological Research (VITO N.V.), Boeretang200, 2400 Mol, BelgiumcUniversity of Antwerpen (UA), Organic Synthesis, Groenenborgerlaan171, 2020 Wilrijk, BelgiumdUniversity of Antwerpen (UA), EMAT, Groenenborgerlaan 171, 2020Wilrijk, Belgium

† Electronic supplementary information (ESI) available: TGA curves.See DOI: 10.1039/b820792e

3042 | J. Mater. Chem., 2009, 19, 3042–3048

either by a grafting approach or alternatively by co-condensa-

tion.5 This results in a highly ordered material with a high surface

area, a narrow pore size distribution and a 100% organic loading.

The organobridged components can either be arranged

randomly in the pore walls or show molecular periodicity, due to

the p–p interaction.6 A crystalline-like ordering in the pore walls

results in an improved functionality and enhances the potential

applications of the synthesized materials.7

In recent years, microwave-assisted synthesis has been applied

for the preparation of OMSs.8 This method offers several

advantages, without any loss of the original structural properties.

In comparison to a conventional hydrothermal treatment, the

microwave heating features a more homogeneous heating

process. Moreover, it promotes nucleation and reduces the

synthesis time considerably.9 The uniform heating generates

smaller and more uniform particles. In addition, it is possible to

program and to control the different synthesis steps.10

Since the development of PMOs, a lot of work has been

devoted to improve and control the characteristic features of

these materials.11 The mesopores can be enlarged by using other

templating molecules, the ordering (both molecular and meso-

porous) can be ameliorated by adding different kinds of additives

or changing synthesis parameters. Moreover, the hybrid material

can be further modified with post-synthesis treatments.

In the majority of the synthesis approaches for the formation

of PMOs classical chemical techniques like reflux synthesis etc.

are applied for the synthesis of these ordered PMOs. However,

not much attention has been devoted to the shortening of the

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long synthesis times (up to 3 days) that are required. To the best

of our knowledge, only part of the synthesis procedure to prepare

ethane bridged PMOs (no molecular ordering) has been executed

by a microwave-assisted synthesis approach.12

In this work, we present a first and detailed study on the

microwave-assisted synthesis of benzene bridged PMOs, which

possess the formerly mentioned molecular periodicity. Micro-

wave-assisted synthesis is compared with conventional hydro-

thermal synthesis. Different synthesis times and temperatures are

applied and compared. A drastic reduction in synthesis time of

several hours up to several days can be achieved for the prepa-

ration of benzene bridged PMOs by making use of microwave

heating. Moreover, the materials prepared by microwave

synthesis show better porosity characteristics while maintaining

their molecular and mesoporous ordering. In addition, micro-

wave conditions can be applied in some of the separate synthesis

steps (stirring time/hydrothermal treatment/template removal13)

or during the entire synthesis process, shortening the synthesis

time even more.

Experimental

Chemical reagents

The following reagents were used for the preparation and

extraction of benzene bridged PMOs; 1,4-bis(triethoxysilyl)-

benzene (BTEB, 96%, Sigma-Aldrich), cetyltrimethylammonium

bromide (CTMABr, 99+%, Acros), HCl (37% solution in water,

Acros), NaOH pellets (98.5%, Acros), methanol (p.a., VWR). All

chemicals were used as received.

Synthesis

Mesoporous benzene bridged PMOs were synthesized according

to the article of Inagaki et al.14 and Bion et al.15 with some minor

adjustments. In the conventional synthesis of benzene bridged

PMOs, 2 g CTMABr is dispersed into 60 mL of a 0.33 M NaOH

solution. Subsequently, 2 mL of BTEB is added dropwise. The

whole mixture is then placed in an ultrasonic bath for 20 minutes.

Afterwards, the substance is stirred for 20 hours at room

temperature and successively transferred to an autoclave for

a hydrothermal treatment of 24 hours at 373 K. The sample was

typically named PMO-C20/293-C24/373 (conventional stirring

for 20 hours at 293 K and conventional hydrothermal treatment

for 24 hours at 373 K). In general, samples are denoted as

PMO-Cx/y-Cx0/y0, whereb C stands for conventional conducted

synthesis. C can be replaced by M in the case of microwave

synthesis. x indicates the stirring time and y the stirring

temperature. x0 and y0 display the time (hours) and temperature

(K) of the static hydrothermal treatment. Subsequently, the solid

was filtered and washed. The organic template is fully removed

by 2 consecutive extractions of two hours refluxing in a 10%vol

HCl/MeOH mixture. The template could also be removed

by only a single microwave-assisted extraction in a 10%vol

HCl/MeOH acidified solution at 393K for 15 minutes under

static conditions.

The synthesis and extraction by microwave assistance is

executed in a MARS CEM, equipped with a pressure sensor and

fiber optic temperature sensor. All samples were subjected to an

initial microwave power of 400 W. When the mixture was stirred

This journal is ª The Royal Society of Chemistry 2009

(only in the stirring step), stirring level 2 in the MARS program

settings was selected.

Characterization

For all samples, the surface area, pore volume and the pore size

distribution were determined on a Quantachrome Quadrasorb SI

automated gas adsorption system. Prior to N2-sorption, all the

samples were outgassed on a Quantachrome Autosorb Degasser

at room temperature for 16 hours. N2-sorption was carried out at

liquid nitrogen temperature (77 K). The specific surface area was

calculated using the Brunauer–Emmet–Teller (BET) method.

The Barret–Joyner–Halenda (BJH) method, applied on the

desorption branch of the isotherm, was used to determine the

pore size distribution. The total amount of N2 adsorbed at

P/P0 ¼ 0.95 was used to determine the total pore volume.

Thermogravimetric analysis (TGA) data were recorded on

a Mettler Toledo TGA/SDTA851. The analyses were performed

in an oxygen atmosphere, whereby the samples were heated from

303 to 973 K with a heating rate of 5 K/min.

X-Ray diffraction (XRD) measurements were recorded on

a Pananalytical X0PERT PRO MPD diffractometer with filtered

CuKa radiation. The measurements were performed in the 2q

mode using a bracket sample holder with a scanning speed of

0.04�/4 s continuous mode.

Scanning electron microscopy (SEM) was performed using

a JSEM 5510 operating at an accelerating voltage of 15 kV. The

samples were sputtered with a thin gold film to minimize the

charging effects.

Transmission electron microscopy (TEM) investigations were

made on a crushed sample deposited on a holey carbon grid using

a Philips CM20 microscope operating at 200 kV. A low intensity

beam and medium magnification were used in order to prevent

possible e-beam damage of the structure.

Results and discussion

In literature the standard conventional method for benzene

bridged PMOs includes a hydrothermal treatment of 24

hours.14,15 In this work, it is investigated to reduce this

synthesis time of 24 hours for benzene bridged PMOs without

losing the structural characteristics of these materials. The goal is

to retain the hexagonal ordering, the molecular periodicity of the

benzene rings in the pore walls, a high surface area and pore

volume. The aging time of the conventional hydrothermal

method was shortened from 24 hours to respectively 3, 8 and

16 hours.

The nitrogen sorption isotherms are presented in Fig. 1 and the

derived sorption data are summarized in Table 1. It can be

observed that the isotherm of the shortest hydrothermal

treatment of 3 hours at 373 K, denoted as PMO-C20/293-C3/373,

has a different shape compared to the other three. There is no

clearly perceptible capillary condensation step, which signifies

a non-uniform pore size distribution (see inset in Fig. 1). The

PMO that underwent a hydrothermal treatment of 8 hours at 373

K shows only a small capillary condensation. Moreover, PMOs

prepared with aging times of 3 and 8 hours possess microporosity

(Table 1). This can be ascribed to the poorly condensed structure

as a result of the short aging times. Upon removal of the

J. Mater. Chem., 2009, 19, 3042–3048 | 3043

Fig. 1 N2-sorption isotherms at 77 K of PMOs prepared by conven-

tional oven synthesis. Inset: adsorption pore size distribution.

Table 1 Porosity data derived from N2-sorption measurements at 77 K

Sample name SBET/m2g�1 Vp/cm3g�1 Vmp/cm3g�1

PMO-C20/293-C3/373 1069 0.569 0.011PMO-C20/293-C8/373 888 0.591 0.055PMO-C20/293-C16/373 808 0.579 0PMO-C20/293-C24/373 765 0.542 0

Fig. 2 X-Ray diffractograms of PMOs prepared by conventional oven

synthesis.

Fig. 3 DTG measurement of the conventional and microwave extracted

samples.

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mesotemplate by conventional extraction, the pores will partially

collapse creating micropores. PMOs that underwent a longer

conventional hydrothermal treatment of 16 or more hours

showed no microporosity and clear capillary condensation due to

the prolonged condensation. Surface areas of materials with 16

and 24 hours of aging are lower than the PMOs synthesized with

shorter aging times, but a more uniform material is obtained.

Moreover, the sorption data of the two longest aging times, 16

and 24 hours, are comparable with the benzene bridged PMOs as

described by Bion et al.15

XRD patterns are illustrated in Fig. 2. For all the different

samples distinct peaks at high angles are observed, which are

specific for periodic ordering of the benzene rings in the pore

walls. For the sample prepared with the shortest hydrothermal

treatment time only the (100) peak is visible in the low 2q range

indicating low pore ordering. PMO-C20/293-C3/373 lacks

mesoscale ordering and the overall signal to noise ratio is very

low. With increasing aging time (8 to 24 hours) the (200) and

(110) peaks at low angle become more and more clear, indicating

that the mesoscale ordering of the as-synthesized material

increases. Prolonged aging time means further recondensation of

the precursor, resulting in a more uniform structure.

XRD and N2-sorption results suggest that at least 8 hours of

synthesis time are necessary to obtain a mesoporous material.

However, longer aging times are required for obtaining well

ordered PMOs.

3044 | J. Mater. Chem., 2009, 19, 3042–3048

It can be concluded that there is a clear decrease in structural

properties when drastically reducing the length of the conven-

tional hydrothermal aging step. The shortest aging time to obtain

a high quality material is 16 hours; the time reduction obtained

by this approach is very limited (24 hours as reported in the

literature).15

A second strategy to shorten the synthesis time to a larger

extent was investigated in this paper, namely the use of micro-

wave irradiation. For OMSs (pure SiO2) different steps were

performed by microwave-assisted synthesis to reduce the

synthesis time. Zhao et al. described an extraction procedure

under microwave conditions.16 The aging time of MCM-41 can

be reduced by making use of microwave irradiation17 and also

the stirring step was performed by microwave-assisted synthesis

for a mesoporous SBA-15.10 In this work, the possibility to

shorten the time for the synthesis of the hybrid benzene bridged

PMOs with microwave irradiation will be discussed. The

reduction in synthesis time by microwave irradiation will be

studied for each individual synthesis step as well as in the entire

synthesis. In a later paragraph, a comparison between micro-

wave-assisted and the conventional synthesis pathway will be

made and discussed.

First, the influence of microwave irradiation on the extraction

step was investigated. A HCl/methanol (1/9) extraction mixture

was found to be the most efficient way for the removal of the

This journal is ª The Royal Society of Chemistry 2009

Fig. 4 N2-sorption isotherms at 77 K of PMOs prepared by microwave

assisted synthesis. Inset: adsorption pore size distribution.

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template molecules in comparison to other solvent mixtures (e.g.

ice water, HCl/ethanol). The conventional way requires two

consecutive extraction periods of 2 hours at reflux temperature

for the complete removal of the surfactant (CTMABr) (Fig. 3).

Once-extracted samples still exhibited weight loss originating

from the used surfactant. If the same mixture is used under

microwave conditions for only 15 minutes at 393 K, the DTG

thermogram of the PMO exhibited no weight loss in the selected

temperature area (Fig. 3) (TGA curves, see ESI†). This indicates

the complete removal of the template after a single treatment of

only 15 minutes. When the extraction process is performed under

microwave conditions, the structural and porous characteristics

of the PMO materials are retained compared to the traditional

extraction method by refluxing twice for 2 hours in the same

solvent. Moreover, the extraction time has been drastically

reduced.

Secondly, the influence of the temperature and the duration of

the hydrothermal treatment under microwave condition on the

synthesis of hybrid benzene bridged PMOs is discussed.

According to the literature, MCM-41 synthesized with micro-

wave heating gives high quality hexagonal mesoporous materials

when prepared between 413 and 433 K in a time scale of 1 to 3

hours. The benzene bridged PMOs synthesized in this article

make use of the same templating molecule, namely CTMABr.

However, a different precursor molecule is used. In the case of

PMOs it is a combined organic-inorganic precursor and gives rise

to molecular scale periodicity in the wall, whereas with MCM-41

the condensed precursor is purely inorganic and randomly

ordered.

Materials were prepared at two different temperatures, 373 K

(same temperature as the conventional hydrothermal treatment)

and 403 K, and with two variations in the time length of the

synthesis, 1 and 3 hours (same duration as the shortest conven-

tional hydrothermal treatment). Table 2 shows the porosity data

derived from the nitrogen sorption isotherms (Fig. 4). Signifi-

cantly higher surface areas and volumes are obtained for the

samples prepared at 373 K. Moreover, nitrogen sorption

isotherms (Fig. 4) for these samples show a clear and sharp

capillary condensation, a high adsorption and a narrow pore size

distribution. The nitrogen sorption isotherms are of type IV

according to IUPAC and resemble those of hexagonal MCM-41,

pure SiO2 structure with uniform pores. The isotherms of the

PMOs synthesized at 403 K have a decreased surface area and

pore volume. In addition, the capillary condensation step is less

steep. This is especially so for the sample with an aging time of 3

hours, PMO-C20/293-M3/403, where the sample has a more

irregular isotherm and slit shaped pores. The yield of the material

synthesized at a higher temperature (433 K) is too small in

comparison to the samples prepared at lower temperatures. If the

hydrothermal period is shortened from 3 hours to 1 hour, both at

Table 2 Porosity data derived from N2-sorption measurements at 77 K

Sample name SBET/m2g�1 Vp/cm3g�1

PMO-C20/293-M3/373 957 0.742PMO-C20/293-M3/403 757 0.739PMO-C20/293-M1/373 951 0.675PMO-C20/293-M1/403 702 0.465

This journal is ª The Royal Society of Chemistry 2009

373 and 403 K, materials with a uniform pore size distribution

(see inset in Fig. 4) and cylindrical pores are obtained. However,

the porosity characteristics for the sample prepared at the higher

temperature (403 K) show lower values and the capillary

condensation step is smaller. This is likely due to deterioration of

the mesoporous structure when the hydrothermal treatment in

the microwave is performed at higher temperatures. The two

benzene bridged mesoporous materials synthesized at 373 K are

quite comparable. So there seems to be a time and temperature

range wherein the synthesis of benzene bridged PMOs is optimal.

The crystallinity of all samples was determined using X-ray

diffraction (Fig. 5). XRD peaks characteristic for the molecular

ordering of the organic benzene functionalities at high diffraction

Fig. 5 X-Ray diffractograms of PMOs synthesized by microwave

heating at 373 and 403 K versus the standard conventional hydrothermal

treatment.

J. Mater. Chem., 2009, 19, 3042–3048 | 3045

Fig. 6 ED of PMO-C20/293-M3/373 along the [001] (A) and [100] (B)

axis. Note db ¼ 0.76 nm, marked by white arrows.

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angles are observed for all hybrid materials at 373 and 403 K.

The XRD patterns of the PMO samples prepared at 373 K

display distinct (100), (110) and (200) peaks which are charac-

teristic for the 2D hexagonal ordering. However the 100 peak of

PMO-C20/293-M1/373 is much lower in intensity compared to

samples PMO-C20/293-C24/373 and PMO-C20/293-M3/373,

meaning that the structure is less organized in a 2D hexagonal

orientation than the other two samples. This is probably due to

the shorter condensation and recondensation time. Again, a clear

influence of synthesis temperature can be observed. At elevated

temperatures (403 K) almost no mesoscale ordering could be

detected at low angles. Only the ordering of the organic func-

tional benzene rings in the walls at high angles is observed. This

observed low mesopore ordering could result from a more

extended or faster condensation at higher temperatures resulting

in a poorer interaction between the precursor and the template

molecules. However, further research is necessary to study this in

more detail. Only mild microwave conditions can be applied to

assure a good interaction with the template, resulting in high

quality materials with structural properties.

A more drastic reduction in the synthesis time can be achieved

by also applying microwave irradiation during the stirring step at

the beginning of the synthesis. To determine the influence of

microwave irradiation on the stirring time a reduction to 2 hours

at 313 K instead of 20 hours at 293 K was taken into account and

this for both microwave-assisted as well as conventional stirring.

If the sorption data, given in Table 3, are compared only small

differences in the derived results of the nitrogen sorption

measurements (isotherms not shown) could be observed between

the standard (20 h) and shortened (2 h) stirring. This means that

the stirring step can be drastically shortened even in the

conventional synthesis. Moreover, if the aging step is done under

microwave irradiation, there is almost no difference perceptible

between microwave-assisted and conventional stirring. A

possible advantage of microwave stirring over conventional

stirring for 2 hours at 313 K is the possibility to program the

entire synthesis in advance. This way the solution does not have

to be transferred to an autoclave in between steps and less

product will be lost.

If we compare the sorption data in Table 3 of the microwave-

assisted synthesis (PMO-Cx/y-Mx0/y0) to conventional heating in

an oven (PMO-Cx/y-Cx0/y0), it is clear that the samples that

underwent a hydrothermal treatment under microwave irradia-

tion show consistently higher surface areas and volumes, whereas

the pore size and the shape of the nitrogen sorption isotherms

remain unaltered.

Ordering of the samples can be evaluated by looking at the

XRD-patterns, shown in Fig. 5. The molecular ordering stays

exactly the same, which is evident because it represents more or

Table 3 Porosity data derived from N2-sorption measurements at 77 K

Sample name SBET/m2g�1 Vp/cm3g�1

PMO-C20/293-M3/373 957 0.742PMO-M2/313-M3/373 925 0.707PMO-C2/313-M3/373 919 0.742PMO-C2/313-C24/373 713 0.511PMO-C20/293-C24/373 765 0.542

3046 | J. Mater. Chem., 2009, 19, 3042–3048

less the length of one condensed benzenebisilsesquioxane mole-

cule. Both samples exhibit characteristic mesoscale diffraction

peaks, which can be assigned to a hexagonal packing arrange-

ment, although there is a slight shift of lattice parameters. The

diffraction peaks of the PMO structure prepared by the use of

microwave irradiation are shifted even further, meaning that the

unit cell parameter is smaller. Since the pore size is identical to

a conventionally prepared PMO, it indicates that the microwave-

assisted PMO possesses thinner walls.

The electron diffraction (ED) pattern of PMO-C20/293-

M3/373 (Fig. 6A) reveals a clear hexagonal pore arrangement,

confirming the 2D hexagonal ordering already observed in XRD.

Fig. 6B shows the electron diffraction pattern along the [100]

direction. Again the row of 0k0 spots from the 2D hexagonal

ordering is observed together with the diffuse spots originating

(indicated by arrows) from the periodical stacking of the benzene

functional groups in the pore wall. TEM images of the individual

samples are shown in Fig. 7 and 8. Fig. 7 obtained from the

Fig. 7 TEM images of PMO-C20/293-M3/373.

This journal is ª The Royal Society of Chemistry 2009

Fig. 8 TEM image of PMO-C20/293-C24/373, enlargement of white

frame marked area is given as an inset.

Fig. 10 Synthesis times needed for each separate step in the different

synthesis protocols and the obtained respective surface areas.

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microwave-irradiated sample shows an overview picture and the

magnification of two selected areas, showing the straight pores

and the mesopore ordering. The TEM image from the conven-

tionally prepared sample (Fig. 8) shows a similar porous network

as the microwave synthesized material.

SEM pictures were taken to study the morphologies of the

different mesoporous powders, and are given in Fig. 9. The SEM

images demonstrate that particles synthesized by microwave

heating possess a more spherical and uniform morphology.

Moreover, the ‘‘microwave’’ prepared hybrid benzene bridged

PMO samples have smaller average particle sizes of approxi-

mately 0.5 mm in comparison to the larger particles for the

‘‘conventional’’ materials with an irregular flat morphology. This

more homogeneous spherical morphology may be advantageous

for use in HPLC to obtain optimal packing. Conventional

synthesis, on the other hand, gives rise to flat randomly shaped

particles.

Up to now, the reasons for all these differences in material

properties (e.g. pore volume, surface area, thinner pore walls and

morphology) are not known to us, but will be the subjects of

future research.

Based on a detailed investigation, the different synthesis steps

could be reduced in time by microwave irradiation instead of

Fig. 9 A) SEM image of PMO-C20/293-C24/373; B) SEM image of

PMO-C20/293-M3/373.

This journal is ª The Royal Society of Chemistry 2009

conventional heating. All steps separately could be reduced in

synthesis time without influencing and even with improving the

properties of the benzene bridged PMOs. If the entire synthesis is

performed with microwave assistance, a drastic reduction of

more than 40 hours can be achieved. Moreover, the specific

surface area and volume can be increased remarkably. The

results of this study are clearly visualized in an overview picture

(Fig. 10).

Conclusion

This study shows that microwave-assisted synthesis of PMOs

with benzene bridged crystalline-like pore walls has several

advantages over the conventional route. For the hydrothermal

treatment, there is a certain time and temperature range (1 to 3

hours and 373 to 403K) wherein the synthesis of benzene bridged

PMOs is optimal. Each separate synthesis step or even the entire

synthesis including the extraction can be conducted under

microwave conditions. The stirring period can be significantly

shortened to 2 hours in the conventional as well as in the

microwave synthesis. But only with microwave irradiation is it

possible to shorten the aging time considerably (more than 40

hours). Performing the extraction under microwave condition

shortens the required synthesis time further. In addition,

a substantial decrease in the amount of required chemicals is

obtained, specifically in the extraction method. Microwave

irradiation accelerates the reaction leading to much shorter

synthesis times. More than 25% increases in surface area and

pore volume are obtained, without changing the pore size (1.2

nm) and the mesopore ordering as well as the crystalline-like

ordering of the benzene functional groups in the pore walls. Since

microwave equipment is easy to use, offers an immediate incline

to the desired temperature and distributes heat homogeneously

giving rise to high quality materials, it has a promising future

in the field of surfactant mediated synthesis of nanoporous

materials.

Acknowledgements

V. Meynen (post-doctoral research grant) and B. Maes (project

1.5.088.04) acknowledge the FWO-Flanders for financial

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support. Gitte Van Baelen thanks the IWT-Flanders for a

PhD scholarship. This work was executed in the frame of the

INSIDE-POReS NoE project from the European Commission

(FP6-EU) and the Concerted Research Project (CRP) sponsored

by the Special Fund for Research from the University of

Antwerpen.

References

1 S. Inagaki, S. Guan, Y. Fukushima, T. Ohsuna and O. Terasaki,J. Am. Chem. Soc., 1999, 121, 9611; B. J. Melde, B. T. Holland,C. F. Blanford and A. Stein, Chem. Mat., 1999, 11, 3302; T. Asefa,M. J. MacLachlan, N. Coombs and G. A. Ozin, Nature, 1999, 402,867.

2 W. J. Hunks and G. A. Ozin, J. Mater. Chem., 2005, 15, 3716;B. Hatton, K. Landskron, W. Whitnall, D. Perovic and G. A. Ozin,Accounts Chem. Res., 2005, 38, 305; P. Van Der Voort,C. Vercaemst, D. Schaubroeck and F. Verpoort, Phys. Chem.Chem. Phys., 2008, 10, 347.

3 G. J. de, A. A. Soller-Illia, C. Sanchez, B. Lebeau and J. Patarin,Chem. Rev., 2002, 102, 4093; V. Meynen, P. Cool andE. F. Vansant, Micropor. Mesopor. Mat., 2007, 104, 26.

4 A. Vinu, K. Z. Hossain and K. Ariga, J. Nanosci. Nanotechno., 2005,5, 347; F. Hoffmann, M. Cornelius, J. Morell and M. Fr€oba, Angew.Chem. Int. Edit., 2006, 45, 3216; Y. Wan, D. Zhang, N. Hao andD. Zhao, Int. J. Nanotechnol., 2007, 4, 66.

5 K. Moller and T. Bein, Studies in Surface Science and Catalysis, 1998,117, 53; A. Stein, B. J. Melde and R. C. Schroden, Adv. Mat., 2000, 12,1403.

6 J. Morell, C. V. Teixeira, M. Cornelius, V. Rebbin, M. Tiemann,H. Amenitsch, M. Fr€oba and M. Lind�en, Chem. Mat., 2004, 16, 5564.

3048 | J. Mater. Chem., 2009, 19, 3042–3048

7 M. P. Kapoor and S. Inagaki, B. Chem. Soc. Jpn., 2006, 79, 1463;S. Fujita and S. Inagaki, Chem. Mat., 2008, 20, 891.

8 C. G. Wu and T. Bein, Chem. Commun., 1996, 925; B. L. Newalkar,H. Katsuki and S. Komarneni, Micropor. Mesopor. Mat., 2004, 73,161; M. C. A. Fantini, J. R. Matos, L. C. Cides da Silva,L. P. Mercuri, G. O. Chiereci, E. B. Celer and M. Jaroniec, Mater.Sci. Eng. B-Solid Mater. Adv. Technol., 2004, 112, 106.

9 S. E. Park, J. S. Chang, Y. K. Hwang, D. S. Kim, S. H. Jhung andJ. S. Hwang, Catal. Surv. Asia, 2004, 8, 91; C. O. Kappe, Angew.Chem. Int. Edit., 2004, 43, 6250; G. A. Tompsett, W. C. Connerand K. S. Yngvesson, ChemPhysChem, 2006, 7, 296.

10 E. B. Celer and M. Jaroniec, J. Am. Chem. Soc., 2006, 128, 14408.11 W. Wang, S. Xie, W. Zhou and A. Sayari, Chem. Mat., 2004, 16, 1756;

A. Sayari and W. Wang, J. Am. Chem. Soc., 2005, 127, 12194;Y. Goto and S. Inagaki, Chem. Commun., 2002, 2410; J. Morell,M. G€ungerich, G. Wolter, J. Jiao, M. Hunger, P. J. Klar andM. Fr€oba, J. Mater. Chem., 2006, 16, 2809; W. Guo, J. Y. Park,M. O. Oh, H. W. Jeong, W. J. Cho, I. Kim and C. S. Ha, Chem.Mat., 2003, 15, 2295; Y. Liang and R. Anwander, Micropor.Mesopor. Mat., 2004, 72, 153; B. R�ac, P. Hegyes, P. Forgo andA. Moln�ar, Appl. Catal. A-Gen., 2006, 299, 193.

12 D. J. Kim, J. S. Chung, W. S. Ahn, G. W. Kang and W. J. Cheong,Chem. Lett., 2004, 33, 422; S. S. Yoon, W. J. Son, K. Biswas andW. S. Ahn, B. Korean Chem. Soc., 2008, 29, 609.

13 B. Tian, X. Liu, C. Yu, F. Gao, Q. Luo, S. Xie, B. Tu and D. Zhao,Chem. Commun., 2002, 1186.

14 S. Inagaki, S. Guan, T. Ohsuna and O. Terasaki, Nature, 2002, 416,304.

15 N. Bion, P. Ferreira, A. Valente, I. S. Goncalves and J. Rocha,J. Mater. Chem., 2003, 13, 1910.

16 B. Tian, X. Liu, C. Yu, F. Gao, Q. Luo, S. Xie, B. Tu and D. Zhao,Chem. Commun., 2002, 1186.

17 S. E. Park, D. S. Kim, J. S. Chang and W. Y. Kim, Catal. Today,1998, 44, 301.

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