ORIGINAL ARTICLE

Investigating the effect of randomly methylated b-cyclodextrin/block copolymer molar ratio on the template-directed preparationof mesoporous alumina with tailored porosity

Rudina Bleta • Cecile Machut • Bastien Leger •

Eric Monflier • Anne Ponchel

Received: 18 December 2013 / Accepted: 7 March 2014

� Springer Science+Business Media Dordrecht 2014

Abstract Supramolecular assemblies formed between

cyclodextrins and block copolymers can be efficiently

used as templates for the preparation of mesoporous

materials with controlled porosity. In this work, we use

dynamic light scattering (DLS) and viscosity measure-

ments to follow the variations occurring in the size and

morphology of the triblock copolymer poly(ethylene

oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide)

(P123) micelles in the presence of various amounts of

randomly methylated b-cyclodextrin (RAMEB). The

results obtained with a series of solution compositions

reveal that the cyclodextrin-to-copolymer (RAMEB/

P123) molar ratio plays a crucial role in the growth rate

of the micelles. At low RAMEB/P123 molar ratios

(below *7.5), a swelling effect of the cyclodextrin in

the P123 micelles is noticed together with a modifica-

tion of the micellar curvature from spherical to ellip-

soidal. At high molar ratios (*7.5 and above), an

abrupt transition toward large supramolecular assem-

blies, which no longer resemble micelles, occurs. When

the RAMEB-swollen P123 micelles are used as tem-

plates to direct the self-assembly of colloidal boehmite

nanoparticles, mesoporous c-Al2O3 materials with high

surface areas (360–400 m2/g), tunable pore sizes

(10–20 nm), large pore volumes (1.3–2.0 cm3/g) and

fiberlike morphologies are obtained under mild condi-

tions. The composition of the mixed micellar solution,

in particular the cyclodextrin-to-copolymer molar ratio,

appears to be a key factor in controlling the porosity of

alumina.

Keywords Methylated b-cyclodextrin � Pluronic P123 �Micelles � Sol–gel � Mesoporous materials

Introduction

Mesoporous materials with high surface area, tunable

pore size (2–50 nm) and tailored morphology have

attracted a great deal of attention since their discovery in

1992 [1, 2] and are rapidly developing in the twenty-first

century as an interdisciplinary research topic [3]. Inno-

vations through design of novel supramolecular templates

and low-temperature synthesis procedures have led to the

development of a wide range of materials with a variety

of structures (hexagonal, cubic, lamellar), morphologies

(spheres, rods, discs, fibers), compositions (silicates,

metal oxides, carbons) and functionalities (possibility to

incorporate biomolecules, drugs, metal complexes, fluo-

rescent molecules, magnetic nanoparticles) [4]. Conse-

quently, a tremendous research has been focused on the

synthesis and applications of these materials in various

emerging fields ranging from biotechnology, biomedi-

cine, drug delivery, catalysis, energy storage, optics,

separation processes to immobilization of biomolecules

and bio-organisms, bone regeneration, heart tissue

replacement, etc. [3].

Electronic supplementary material The online version of thisarticle (doi:10.1007/s10847-014-0405-7) contains supplementarymaterial, which is available to authorized users.

R. Bleta (&) � C. Machut � B. Leger � E. Monflier � A. Ponchel

Universite Lille Nord de France, 59000 Lille, France

e-mail: rudina.bleta@univ-artois.fr

R. Bleta � C. Machut � B. Leger � E. Monflier � A. Ponchel

UArtois, UCCS, Faculte des Sciences Jean Perrin, Rue Jean

Souvraz, SP 18, 62307 Lens, France

R. Bleta � C. Machut � B. Leger � E. Monflier � A. Ponchel

CNRS, UMR 8181, 59650 Villeneuve d’Ascq, France

123

J Incl Phenom Macrocycl Chem

DOI 10.1007/s10847-014-0405-7

In particular, in the field of heterogeneous catalysis, one

of the main challenges is to improve the dispersion of the

active elements onto the solid support in order to minimise

the catalyst cost and to facilitate the diffusion and acces-

sibility of reactants and products during the catalytic

reaction, especially when large molecules or viscous sys-

tems are employed [3, 5, 6]. For this purpose, the design of

materials with tailored architectures, high porosity and

large surface area is of crucial importance for enhancing

the catalyst effectiveness [7].

For the fabrication of these materials, the synthesis

procedure which combines a templating approach with sol–

gel chemistry is known to be one of the most important

strategies in modern material science, owing to its ability to

tailor the pore size by manipulating the size of the tem-

plating supramolecular assemblies [8, 9]. This procedure

requires a careful control of the structural characteristics of

the template in terms of size and morphology. For instance,

for the synthesis of mesoporous materials with pore sizes in

the range of 1.5–30 nm, surfactant micelles or micro-

emulsions are used as templates [1, 2], whereas for the

generation of hierarchically porous structures with a

micro–meso–macroporous network, more sophisticated

templates such as colloidal crystals [10] or emulsions [11]

are usually employed. Interestingly, block copolymer

micelles used in combination or not with swelling agents

(e.g. alkyl substituted benzenes), have been reported to

give hierarchical structures with significantly enlarged

pores [12, 13].

The possibility of using the supramolecular assemblies

formed between block copolymers and cyclodextrin (CD)

derivatives as soft templates for the synthesis of hierar-

chically structured porous materials has been very little

explored so far. Most of the studies reported in the litera-

ture have been devoted to the synthesis of mesoporous

silica [14–16]. In this regard, it is particularly interesting to

explore the wide variety of architectures developed by

cyclodextrin-based supramolecular assemblies for design-

ing other inorganic materials with tailored properties. For

instance, from the viewpoint of synthesis, it is still a

challenge to synthesize mesoporous alumina with adjust-

able porosity [17] while the applications of this material are

known to be of practical importance in heterogeneous

catalysis due to its high mechanical strength as well as high

chemical and thermal stability [18–20].

So far, it is well-documented that the native b-CD and

nonionic triblock copolymers of the poly(ethylene oxide)

(PEO)-b-poly(propylene oxide) (PPO)-b-poly(ethylene

oxide) (PEO) family, also known as Pluronics, are able to

form inclusion complexes by host–guest interactions [21–

24]. Under appropriate conditions, the stacking of this

native cyclodextrin along the poly(propylene oxide)

chains gives rise to insoluble inclusion complexes which

further associate into polypseudorotaxanes to form struc-

tures with a well-ordered channel configuration [23].

On the other hand, the interactions between modified b-

CDs and block copolymers have been less investigated in

the literature. In the case of mixtures of heptakis (2,6-di-o-

methyl)-b-CD and Pluronics, such as the F127 (PEO107-

PPO70PEO107), P85 (PEO39PPO52PEO39) and P123

(PEO20PPO70PEO20), it has been shown that the micellar

rupture occurs with extremely fast kinetics, excluding the

possibility of polypseudorotaxane formation via inclusion

complexes [25]. Interestingly, in the case of Pluronic P123,

the authors have reported a possible restructuration of the

micelles toward swollen lamella, with an interlayer spacing

much higher than the typical values reported in literature

with conventional swelling agents [26]. Moreover, the

nature of the interactions involved in the self-assembly

process appeared to be highly sensitive to the substitution

degree, nature and position of the substituents in modified

cyclodextrins. Thus, in contrast to the micellar rupture

observed with the heptakis (2,6-di-o-methyl)-b-CD, Dreiss

and coworkers [25] reported that, under similar experi-

mental conditions, the micelles remain intact in the pre-

sence of other substituted b-cyclodextrin derivatives, such

as the 2,3,6-trimethyl-b-CD, 2-hydroxyethyl-b-CD and

2-hydroxypropyl-b-CD.

In a recent study [27], we have shown that the randomly

methylated b-cyclodextrin (RAMEB), where methylation

occurs at the C2, C3 or C6 positions with statistically 1.8

OH groups modified per glucopyranose unit, can signifi-

cantly affect the self-assembly of the amphiphilic block

copolymer P123 in water by locating at the PEO–PPO

interface layer, inducing a shape transition from spherical

to ellipsoidal micelles. Moreover, such behaviour was

observed under controlled solution conditions, where the

cyclodextrin was in large excess to copolymer. The pur-

pose of the present work was to investigate in more detail

the effect of solution composition on the size and mor-

phology of P123–RAMEB assemblies. In this regard, we

have extended our investigation to a wide range of

RAMEB/P123 molar ratios varying from 1.7 to 13.8. We

show that, depending on the cyclodextrin-to-copolymer

molar ratio, a swelling effect of the cyclodextrin or a shape

transition of the micelles toward elongated structures with

large dimensions can be produced. In a second set of

experiments, we use these assemblies as templates for the

elaboration of a series of mesoporous alumina by applying

a template-directed colloidal self-assembly procedure [9,

28]. We show that the characteristics of the resulting

materials can be directly related to the variations observed

in the size and morphology of these supramolecular

assemblies.

J Incl Phenom Macrocycl Chem

123

Experimental

Materials

The triblock copolymer PEO20PPO70PEO20 [PEO

poly(ethylene oxide) and PPO poly(propylene oxide)],

denoted Pluronic P123, was purchased from Sigma-

Aldrich. It has a molar weight (Mw) of 5,800 g/mol. Ran-

domly methylated b-cyclodextrin (denoted RAMEB with

an average degree of molar substitution of 1.8 (Mw

1,311 g/mol)) was a gift from Wacker Chemie GmbH. The

water content in RAMEB was determined gravimetrically

by heating the cyclodextrin under vacuum at 80, 100 and

120 �C for 2 h at each temperature and measuring the

sample mass before and after drying. In our study, the

RAMEB was found to contain 7.5 wt% of water. This

value was used to make the necessary adjustment to the

calculations. Aluminum tri-sec-butoxide, Al[OCH(CH3)-

C2H5]3 (referred to as ASB, Mw 246.32 g/mol), and nitric

acid (HNO3, 68 wt%) were procured from Sigma-Aldrich.

All chemicals were used as received without further

purification.

Preparation of the RAMEB–P123 mixtures

for the characterization

Hundred millilitres of a 4.8 wt% (8.2 mM) and 7.2 wt%

(12.5 mM) Pluronic P123 micellar solution was produced

by dissolving the appropriate amount of copolymer (5 and

7.8 g respectively) in double distilled water (100 mL)

under stirring at room temperature. Subsequently, aliquots

of 10 mL micellar solution were placed into glass cells and

various amounts of RAMEB were added in the concen-

tration range of 27.8–148.0 mg/mL (i.e. 21.2–112.9 mM)

corresponding to RAMEB/P123 molar ratios in the range

of 1.7–13.8. The mixtures were stirred for 30 min then

allowed to equilibrate in a thermostatic bath at 25 �C for

24 h before analysis.

Synthesis of mesoporous alumina

Boehmite (AlO(OH)) nanoparticles were used as inorganic

precursor for the preparation of mesoporous alumina.

Nanoparticles were synthesized by a sol–gel method

reported by Yoldas [29]. In a dry 250 mL flask, 185 mL of

hot distilled water (85 �C) was added fast to 25.3 g

(0.1 mol) of ASB at a hydrolysis ratio of 100 (h = H2O/

Al). After 15 min, the hydroxide precipitate was peptized

by adding dropwise 0.474 mL (0.1 mol) of HNO3

([HNO3]/[Al] = 0.07). The white precipitate was stirred at

85 �C for 24 h. The final product was a transparent sus-

pension of boehmite nanoparticles (pH 4.4–4.8). Under

these conditions, the concentration of aluminum in the sol

was determined by weight loss on ignition at 1,000 �C and

was estimated to be 0.5 mol/L. Subsequently, Pluronic

P123 (8.2 and 12.5 mM) was added in the nanoparticle sol,

and the mixture was stirred for 3 h at room temperature.

10 mL aliquots of copolymer/boehmite sols were trans-

ferred into glass vials, and various amounts of RAMEB

(21.2–91.7 mM, RAMEB/P123 molar ratio = 1.7–11.2)

were added. The sols were stirred for an additional 30 min

and then allowed to equilibrate at 25 �C for 24 h. Xerogels

were recovered after drying 10 mL samples by evaporation

at 60 �C for 48 h, after which time they were calcined in

air at 500 �C for 16 h using a heating ramp of 1 �C/min.

Mesoporous c-Al2O3 were identified according to the fol-

lowing notation: Al–Px–CDy where x is the P123 con-

centration in the sol (mM) and y is the RAMEB

concentration in the sol (mM). For example, Al indicates a

mesoporous alumina prepared without copolymer and

without cyclodextrin (control), whereas Al–P12.5–CD56.4

indicates a mesoporous alumina prepared with a P123

concentration of 12.5 mM and a RAMEB concentration of

56.4 mM.

Characterization methods

Dynamic light scattering (DLS)

DLS measurements were performed at 25 �C by using a

Malvern Zeta Nanosizer instrument. The apparatus is

equipped with a 4 mW He–Ne laser operating at 633 nm

and uses a backscattering detection system (scattering

angle h = 173�). Samples were filtered through a 0.2 lm

Millipore filter before analysis. The quantity measured in

DLS is the time correlation function (TCF) of the scattered

intensity g(2)(t) [30] which was obtained directly from the

software during the measurement. TCFs were analysed by

the CONTIN method [31] to obtain the distribution decay

rates (U) and the diffusion coefficient (D). The apparent

hydrodynamic radius (Rh) was deduced from D using the

Stokes–Einstein formula:

Rh¼KBT

6pgD

where KB, T and g are the Boltzmann constant, the abso-

lute temperature and the viscosity of water at the temper-

ature T respectively. Each DLS experiment was repeated in

triplicate.

Viscometry

The apparent viscosities of the solutions were measured in

a temperature controlled water bath by using a viscosimeter

from Brookfield equipped with a cylindrical geometry

(module SC4-18). The apparent viscosity versus shear-rate

J Incl Phenom Macrocycl Chem

123

plots were recorded at 25 and 60 �C with a shear-rate being

stepwise increased over the range of 1–130 s-1.

Nitrogen adsorption–desorption

Isotherms were collected at -196 �C using an adsorption

analyser Micromeritics Tristar 3020. Prior to analysis,

samples (200–400 mg) were outgassed at 320 �C overnight

to remove the species adsorbed on the surface. From N2

adsorption isotherms, specific surface areas were deter-

mined by the BET method and pore size distributions were

calculated using the NLDFT (nonlocal density functional

theory) model [32] assuming a cylindrical pore structure.

Pore volume (PV) was calculated from the adsorbed vol-

ume at a relative pressure of 0.995. For each pore size

analysis, a kernel which consists of up to 100 theoretical

individual pore isotherms was created. The quality of

agreement between the DFT fitted isotherms and the

experimental results was evaluated by the profile factor Rp

defined as:

Rp ¼ 100X

yio � yicj j.X

yio

where yio and yic are the observed and calculated volumes

respectively at the ith step. The fitting quality was consid-

ered satisfactory when Rp was less than 1.5.

Transmission electron microscopy (TEM)

TEM observations were performed on a Tecnai microscope

operating at an accelerating voltage of 200 kV at medium

magnification. A drop of alumina powder suspension dis-

persed in ethanol was deposited on a carbon coated copper

grid.

Powder X-ray diffraction (XRD)

XRD data were collected on a Siemens D5000 X-ray dif-

fractometer in a Bragg–Brentano configuration with a Cu

Ka radiation source. XRD scans were run over the angular

domain 10� \ 2h\ 80� with a step size of 0.02� and a

counting time of 2 s per step.

Results and discussion

The interactions between block copolymers and modified

cyclodextrins in aqueous solution are highly sensitive to

the substitution degree, nature and position of the substit-

uents in the macrocycle. Such interactions can lead to the

formation of supramolecular assemblies with a pronounced

structural polymorphism which can be characterised by

using Dynamic Light Scattering (DLS) [25, 27]. Figure 1

shows the evolution of the apparent hydrodynamic radius

(Rh) distributions for mixtures prepared with two different

Pluronic concentrations (8.2 mM and 12.5 mM) and with

increasing amounts of RAMEB (21.2–112.9 mM). The

control samples prepared without cyclodextrin are added

for comparison.

In the absence of RAMEB, both Pluronic solutions are

transparent and their correlation functions show a single

exponential decay (Figs. S1A-a and S1B-a, Supplementary

material) indicating the presence of one type of scattering

species attributed to the micelles, in agreement with the

phase diagram of this copolymer in water [33]. The

apparent hydrodynamic radius of the micelles, determined

from the Stokes–Einstein equation, is centred at 9.1 nm

(Fig. 1a, b) consistent with the value reported in the liter-

ature [34].

Upon addition of RAMEB, a significant change in the

size and morphology of the micelles is noticed. Thus, for

instance, for solutions prepared with 8.2 mM P123 and

increasing amounts of RAMEB, the correlation function is

1 10 100 1000 10000

0

5

10

15

20

25 0 mM (R=0) 21.2 mM (R=2.6) 42.3 mM (R=5.2) 56.4 mM (R=6.9) 70.6 mM (R=8.6) 91.7 mM (R=11.2) 112.9 mM (R=13.8)

Inte

nsity

(%

)

Size (r, nm)

1 10 100 1000 10000

Size (r, nm)

0

5

10

15

20 0 mM (R=0) 21.2 mM (R=1.7) 42.3 mM (R=3.4) 56.4 mM (R=4.5) 70.6 mM (R=5.7) 91.7 mM (R=7.4) 112.9 mM (R=9.0)

Inte

nsity

(%

)

9.1

11.3

19.4

130198

210

120

9.112.2

15.5

31

160330

9.1

26 70

a

b

Fig. 1 Apparent hydrodynamic radius (Rh) distributions of the

scattered intensity for the mixtures prepared with 8.2 mM (a) and

12.5 mM (b) Pluronic and with increasing amounts of RAMEB

(0–112.9 mM) at 25 �C. R indicates the RAMEB/P123 molar ratio

J Incl Phenom Macrocycl Chem

123

shifted toward larger time scales (Fig. S1A b–g, Supple-

mentary material) indicating a time-dependent growth of

the micelles. Accordingly, the corresponding size distri-

bution plots show a progressive increase in the apparent

hydrodynamic radius (Rh) from 9.1 nm without cyclodex-

trin to 11.3 nm with 21.2 mM RAMEB and 19.4 nm with

42.3 mM RAMEB (Fig. 1a). Interestingly, in the concen-

tration range of 56.4–112.9 mM RAMEB, a remarkable

increase in the growth rate of these supramolecular

assemblies is noticed. Indeed, for 56.4 mM RAMEB, the

peak assigned to swollen micelles shrinks while a new

population centred at *130 nm appears and these assem-

blies further grow in size up to 198 nm (for 70.6 mM

RAMEB) and 210 nm (for 91.7 mM RAMEB) before

shrinking to 120 nm (for 112.9 mM RAMEB). All these

mixtures were characterized by a strong turbidity and from

the size distribution plots it can be seen that their Rh values

far exceed the dimension of the Pluronic micelles. This is a

strong indication that, in this concentration range, a tran-

sition toward other large scattering species, which no

longer resemble spherical micelles, occurs. However, when

examining in more detail the DLS profiles, it appears that

the scattering intensity of these assemblies gradually

decreases with the RAMEB concentration, and these

changes can be related to the formation of less well-defined

objects with more flexible interfaces. When these supra-

molecular assemblies were observed under a polarized

light microscope, no sign of birefringence was noticed

indicating that the structures formed are isotropic and may

be associated with spherical bilayer vesicles, as evidenced

in two recent studies with the analogous a-CD/Pluronic

F127 [35] and b-CD/Triton X [36] systems.

On the other hand, for mixtures prepared with a higher

Pluronic concentration (12.5 mM), the effect of RAMEB

on the growth rate of the micelles seems to be less pro-

nounced (Fig. 1b). Indeed, while the micellar size remains

almost invariable (9.1 nm) for 21.2 mM RAMEB, a pro-

gressive increase in the apparent hydrodynamic radius

from 9.1 to 15.5 nm occurs when the cyclodextrin con-

centration increases from 21.2 to 56.4 mM. Further addi-

tion of RAMEB leads to an abrupt shift of the apparent

hydrodynamic radius to 31 nm for 70.6 mM RAMEB, then

to the appearance of a new population centred at *160 nm

for 91.7 mM RAMEB and *330 nm for 112.9 mM

RAMEB, indicating a transition from micelles to large

supramolecular assemblies, similar to those observed pre-

viously with 8.2 mM P123.

These results point out that a certain amount of cyclo-

dextrin is required to change the interfacial curvature of the

micelles in the ternary P123/RAMEB/water system and

this phenomenon depends on both the Pluronic and the

RAMEB concentrations. Indeed, in the micellar solution

containing 8.2 mM P123, the transition from micelles to

large supramolecular assemblies occurs for 56.4 mM

RAMEB whereas in the presence of 12.5 mM P123, such

transition is observed for higher RAMEB concentrations,

typically for 91.7 mM RAMEB and above. The observed

change in the micellar curvature may be explained by the

location of this cyclodextrin at the PEO-PPO interface

layer [27]. Therefore, the hydrophobic interactions

between the methoxy groups of the cyclodextrin and

poly(propylene oxide) blocks of the copolymer could result

in an increase in the aggregation number and a more dense

packing during the coassembly process, thus rendering the

micelle interfaces more flexible. Moreover, when a certain

packing density in the interface layer of the micelles is

reached, the P123–RAMEB micelles undergo a restruc-

turing process with a transition from spherical to elongated

objects before being transformed to bilayer vesicles. Under

our experimental conditions, such inversion in the micellar

curvature occurs for a particular RAMEB/P123 molar ratio

comprised between 6.9 and 7.4 for both Pluronic concen-

trations investigated.

To gain a better understanding on how RAMEB affects

the morphology of the micelles at high concentrations,

viscosity measurements were carried out at two different

temperatures (25 and 60 �C). Indeed, it is well-known that

the sphere-to-rod transition in block copolymer micelles

depends strongly on the temperature of the solution. At

high temperatures, the increase in the hydrophobicity of

both PEO and PPO blocks leads to the packing of more

PPO segments in the micellar core causing a proportion of

the water molecules to be expelled from the core [37].

Under such conditions, the aggregation number of the

micelles increases, thus inducing a reduction in the inter-

facial curvature and a change in the micellar shape from

spherical to ellipsoidal or worm-like. Such phenomenon

has been observed also in the presence of salts (NaCl, KCl,

KF) which have the ability to dehydrate the micellar corona

and change the interfacial curvature of the micelles at low

temperatures [34, 38]. The sphere-to-rod transition in these

systems is usually manifested by a simultaneous increase in

the viscosity of the micellar solution and a change in its

rheological behaviour.

Figure 2 shows the apparent viscosity vs. shear-rate

plots recorded at 25 �C (Fig. 2a) and 60 �C (Fig. 2b) for

P123 solutions (8.2 and 12.5 mM) prepared without and

with RAMEB (91.7 mM). As can be noticed, at 25 �C, the

bare P123 solutions (8.2 and 12.5 mM) present a near-

Newtonian behaviour since the apparent viscosity remains

nearly constant (*1.6 mPa s-1) over the entire range of

shear rates (0–130 s-1). The addition of 91.7 mM RAMEB

induces a shear-thinning behaviour (i.e., the viscosity

decreases with increasing the shear rate), as well as a

pronounced increase in the viscosity, up to *3.7 mPa s-1

for 8.2 mM P123 and *4.7 mPa s-1 for 12.5 mM P123,

J Incl Phenom Macrocycl Chem

123

as measured at 130 s-1. Moreover, this latter sample

appears highly turbid (see inset picture) as a result of the

formation of large scattering species, in agreement with the

previous DLS results.

Even more significant changes can be noticed at 60 �C.

At this temperature, which is slightly above the cloud point

of the Pluronic P123 (* 55 �C for 7.2 wt% P123) [39], the

cyclodextrin-free micellar solution becomes suddenly tur-

bid and a shear-thinning behaviour develops. Such behav-

iour is typical of Pluronic P123 which forms elongated

micelles when it is brought in the vicinity of the cloud

point [37, 40]. Interestingly, in the presence of RAMEB,

the shear-thinning behaviour becomes remarkably pro-

nounced as a result of the important growth of worm-like

micelles whose entanglement seems to induce a restruc-

turation of the supramolecular assemblies in a compact

cross-linked network. Such a particular rheological

behaviour is usually observed in thixotropic systems

comprised of particles with strong shape anisotropy such as

rods, discs or platelet shapes [41, 42]. From Fig. 2, it can

also be noticed that the shear-thinning behaviour is

accompanied by a significant increase in the apparent vis-

cosity from 1.2 mPa s-1 without RAMEB to 24.9 mPa s-1

with 91.7 mM RAMEB as measured at 130 s-1. These

results thus point out that the randomly methylated b-

cyclodextrin facilitates the transition from spherical to

elongated micelles and this effect becomes even more

pronounced with increasing the temperature.

In a second set of experiments, the RAMEB-P123

supramolecular assemblies were used as templates for the

preparation of mesoporous c-Al2O3 by a two-step synthesis

procedure schematized in Fig. 3 [9, 27, 28]. In a first step, a

transparent sol made up of needle-like boehmite nanopar-

ticles was synthesized in aqueous solution (H2O/

Al * 100) by a sol–gel method using aluminum tri-sec-

butoxide (Al(OBu)3) as inorganic precursor and nitric acid

as peptizing agent (HNO3/Al = 0.07). In a second step, the

RAMEB-swollen supramolecular assemblies were used as

templates to direct the self-assembly of boehmite nano-

particles which act as building blocks for the construction

of the hybrid organic–inorganic framework. This second

step of synthesis was performed at 25 �C because at this

temperature the RAMEB–P123 solutions present the low-

est viscosity, thus facilitating the structuration of the

nanoparticles around the supramolecular template. After

drying, the recovered xerogels were calcined at 500 �C to

remove the organic template and allow the transition from

boehmite (AlO(OH)) to c-Al2O3 which actually starts at

*380 �C.

Before drying and calcination, the hybrid sols were

first characterised by DLS. Figure 4 shows the apparent

hydrodynamic radius distributions of the scattered

intensity for boehmite sols prepared without and with

RAMEB-P123 assemblies. The corresponding correla-

tion functions are shown in Fig. S2, Supplementary

material.

1

10

100

1000

10000

Vis

cosi

ty (

mP

a.s)

Shear rate (s-1)

1

10

Vis

cosi

ty (

mP

a.s)

Shear rate (s-1)

60 °C25 °C

ab

c

d

ab

c

d

0 20 40 60 80 100 120 1400 20 40 60 80 100 120 140

Fig. 2 Viscosity versus shear-rate curves for micellar solution

prepared with 8.2 mM P123 (a), 12.5 mM P123 (b), 8.2 mM

P123 ? 91.7 mM RAMEB (c), and 12.5 mM P123 ? 91.7 mM

RAMEB (d) at 25 and 60 �C. Inset visual aspect of the 12.5 mM

P123 solutions prepared without and with 91.7 mM RAMEB

J Incl Phenom Macrocycl Chem

123

All correlation functions can be fitted to biexponential

decay functions and the corresponding size distribution

plots appear bimodal and more or less broadened depend-

ing on the composition of the sol. Indeed, in the bare

AlO(OH) sol (Fig. 4a), the peak centred at 60 nm with a

shoulder at 13 nm can be attributed to the packing of

needle-like nanoparticles in compact rearrangements with a

‘‘card-packed’’ microstructure. Locally, these small

aggregates stick together giving rise to bigger aggregates

which do not show any preferential orientation. Upon

addition of 91.7 mM RAMEB (Fig. 4b), the size

distribution plot becomes slightly sharper showing a further

shift toward a larger value of the apparent hydrodynamic

radius (Rh *90 nm). Interestingly, in the presence of

Pluronic micelles (Fig. 4c) and RAMEB–Pluronic supra-

molecular assemblies (Fig. 4d), the DLS profiles appear

significantly different. Therefore, the two intensity maxima

become well-separated and the intensity of the first peak

strongly increases while the second population shifts

toward higher Rh values (110 nm with P123 micelles and

150 nm with RAMEB-P123 assemblies) indicating the

formation of objects that are more fiber-like. A similar

AlO(OH) RAMEB-swollen P123 micelle γ-Al2O3

Mesoporous γ-Al2O3

with controlled porosityColloidal

boehmite solBoehmite/P123/RAMEB

hybrid sol

DryingCalcination

RAMEB/P123 assemblies

ASB

H2Oexcess

+

HNO3

ASB Aluminum tri sec butoxide Pluronic P123 RAMEB

Sol-gel

Fig. 3 Schematic illustration of the template-directed synthesis of mesoporous c-Al2O3 with controlled porosity where boehmite colloids act as

building blocks for the construction around the organic template of an inorganic framework

1 10 100 1000 10000

0

2

4

6

8

1 10 100 1000 10000

0

2

4

6

8

1 10 100 1000 10000

0

2

4

6

8

1 10 100 1000 10000

0

2

4

6

8

10

Size (r, nm)

Inte

nsity

(%)

150 nm

16 nm

90 nm

13 nm

110 nm

15 nm

60 nm

15 nm

a b

c d

Fig. 4 Apparent hydrodynamic

radius (Rh) distributions of the

scattered intensity for boehmite

(AlO(OH)) solutions (0.5 M)

without organics (a), with

91.7 mM RAMEB (b),

12.5 mM Pluronic P123 (c) and

12.5 mM P123 ? 56.4 mM

RAMEB (d) at 25 �C

J Incl Phenom Macrocycl Chem

123

alignment of boehmite particles under the influence of the

analogous Pluronic F127 (PEO107PPO70PEO107) has

already been evidenced by transmission electron micros-

copy in another study [28]. Such an alignment effect was

supposed to be induced by the adsorption of copolymer

through hydrogen bonding onto the surface hydroxyl

groups of boehmite taking the configuration in which the

poly(ethylene oxide) (PEO) groups lie on the nanoparticle

surface while the poly(propylene oxide) (PPO) chains head

away from the surface.

Furthermore, it is worth mentioning that for P123 con-

centrations above the CMC (0.03 wt%) [33], as is the case

in our study, Pluronic micelles or RAMEB–P123 assem-

blies further accentuate the lateral orientation of boehmite

nanoparticles which is manifested by the appearance of

better-defined peaks in the DLS plot. Thus, in the range

between 56.4 and 91.7 mM RAMEB, our DLS results

obtained for sols prepared with two different P123 con-

centrations (8.2 and 12.5 mM) (Fig. 5) reveal the presence

of well-defined separated peaks centred at 15 and 150 nm.

This is an indication of alignment effects produced by the

RAMEB–P123 assemblies which act as structure directing

agents in the formation of the hybrid network.

Further evidence for the alignment of nanoparticles was

provided by TEM observations on the calcined alumina

(Fig. 6a–d). As can be noticed, the materials prepared

without and with template present substantial differences in

their morphology. Thus, the mesoporous c-Al2O3 prepared

without template is comprised of aggregated particles with

no regular shape and a very low porosity resulting from

voids between close-packed crystallites (Fig. 6a). When

RAMEB is used as a structure directing agent, plate-like

particles with higher porosity are produced, as evidenced

by the presence of brighter domains in the TEM image

(Fig. 6b). Notably, when copolymer micelles and RAMEB-

P123 assemblies are used as templates, an arrangement of

the particles in structures with a better-defined fiberlike

morphology is observed (Fig. 6c, d), in agreement with the

DLS patterns. Interestingly, in addition to the fiberlike

morphology, several voids with an average diameter of

*20 nm are also present at a very high yield throughout

the nanoparticle network (Fig. 6d). This is an indication

that after calcination, the material has adopted some

characteristics of the original Pluronic micelles or

RAMEB-swollen Pluronic micelles, thus indicating the

important role of the template in the restructuration of the

particle network.

The transformation of boehmite in c-Al2O3 after thermal

treatment at 500 �C was confirmed by powder X-ray dif-

fraction. As can be noticed in Fig. 7, the initial xerogel

dried at 60 �C is composed of crystallized boehmite

(JCPDS Card 21-1307). Calcination at 500 �C induced the

appearance of new and strong diffraction lines indicating

that boehmite was completely transformed into well-crys-

tallized c-Al2O3 (JCPDS Card 10-0425).

From the corresponding N2 adsorption measurements on

alumina synthesized without and with template, it can be

seen that all isotherms present a distinct H1 hysteresis loop

characteristic of mesoporous materials (Fig. 8a–d). Thus,

the control sample prepared without template, presents a

capillary condensation step that starts at a relative pressure

(P/P0) of about 0.4 indicating the presence of small mes-

opores (Fig. 8a). The corresponding pore size distribution

(PSD) plot which is relatively narrow and centred at

5.6 nm can be attributed to the assembly of several crys-

tallites in rather compact rearrangements with ‘‘card-pack’’

microstructures [28]. Upon addition of 91.7 mM RAMEB

(Fig. 8b), a steep rise in the nitrogen uptake is observed at

high relative pressures P/P0 [ 0.8 indicating the formation

of large mesopores with a high pore volume. Accordingly,

the corresponding pore size distribution plot indicates a

significant increase in the pore size (from 5.6 to 8.1 nm),

1 10 100 1000 10000

0

2

4

6

8

10 Al-P8.2-CD56.4 Al-P8.2-CD70.6 Al-P8.2-CD91.7

Inte

nsity

(%

)

Size (r, nm)

1 10 100 1000 10000

0

2

4

6

8

10 Al-P12.5-CD56.4 Al-P12.5-CD70.6 Al-P12.5-CD91.7

Inte

nsity

(%

)

Size (r, nm)

150 nm

15 nm

150 nm

15 nm

a

b

Fig. 5 Apparent hydrodynamic radius (Rh) distributions of the

scattered intensity for boehmite (AlO(OH)) sols (0.5 M) prepared

with 8.2 mM (a) and 12.5 mM Pluronic P123 (b) and with increasing

amounts of RAMEB (56.4–91.7 mM) at 25 �C

J Incl Phenom Macrocycl Chem

123

pore volume (from 0.28 to 0.68 cm3/g) and surface area

(from 219 to 316 m2/g) (Table 1).

More importantly, in the presence of Pluronic (Fig. 8c)

and RAMEB (Fig. 8d), it can be noticed that the textural

characteristics of alumina are clearly improved owing to the

formation of P123 micelles or RAMEB-swollen micelles

which act as efficient templates around which the self-

assembly of nanoparticles occurs. Moreover, the material

prepared with 12.5 mM P123 and 56.4 mM RAMEB shows

even more interesting textural characteristics compared to

the control sample prepared with 12.5 mM P123. Therefore,

an increase in the average pore size (from 14.8 to 19.3 nm),

pore volume (from 1.37 to 1.97 cm3/g) and specific surface

area (from 357 to 382 m2/g) is noticed upon addition of the

cyclodextrin. Such enhancements in the sample porosity can

unambiguously be attributed to the swelling effect of the

randomly methylated b-cyclodextrin, in line with the trend

Fig. 6 TEM images of

mesoporous c-Al2O3

synthesized without template

(a) with 91.7 mM RAMEB (b),

12.5 mM Pluronic P123 (c) and

with a mixture of P123

(12.5 mM) and RAMEB

(56.4 mM) (d). Samples were

calcined at 500 �C

10 20 30 40 50 60 70 80

Inte

nsity

(a.

u.)

2 theta (°)

Boehmiteγ-Al

2O

3

Fig. 7 XRD patterns of boehmite dried at 60 �C and mesoporous

c-Al2O3 calcined at 500 �C. Materials were prepared from a mixture

of 12.5 mM P123 and 56.4 mM RAMEB

0.0 0.2 0.4 0.6 0.8 1.0

0

200

400

600

800

1000

1200

1400

Ads

orbe

d vo

lum

e (c

m3 /g

, ST

P)

Relative pressure (P/P0)

a

b

c

d

0 10 20 30 40

0.000

0.005

0.010

0.015

0.020

0.025

dV(w

) (c

c/nm

/g)

Pore width (nm)

19.3 nm14.8 nm

8.1 nm5.6 nm

Fig. 8 Evolution of adsorption–desorption isotherms and corre-

sponding pore size distributions (inset) for the mesoporous c-Al2O3

prepared without template (a), with 91.7 mM RAMEB (b), with

12.5 mM Pluronic (c) and with a mixture of P123 (12.5 mM) and

RAMEB (56.4 mM) (d)

J Incl Phenom Macrocycl Chem

123

observed by DLS and also consistent with TEM

observations.

With further increasing the RAMEB concentration

(21.2–91.7 mM) in both P123 micellar solutions (8.2 and

12.5 mM), a progressive increase in the pore size is

observed together with an enhancement in the pore volume

(Fig. 9; Table 1; Fig. S3 Supplementary material). There-

fore, for 8.2 mM P123 (Fig. 9a), the best textural charac-

teristic are obtained for alumina prepared with 42.3 mM

RAMEB showing a pore size of 19.1 nm, a pore volume of

1.85 cm3/g and a specific surface area of 408 cm2/g. On the

other hand, for 12.5 mM P123 (Fig. 9b), a higher RAMEB

Table 1 Textural characteristics of mesoporous c-Al2O3 after thermal treatment at 500 �C

Sample RAMEB/P123 SBET (m2 g-1) PV (cm3 g-1) Scum (m2 g-1) Vcum (cm3 g-1) PS (nm)

Al – 219 0.28 227 0.26 5.6

Al–CD91.7 – 316 0.68 304 0.66 8.1

8.2 mM P123

Al–P8.2 0 373 1.44 389 1.42 14.6

Al–P8.2–CD21.2 2.6 336 1.51 372 1.48 10.5 and 16

Al–P8.2–CD42.3 5.2 408 1.85 409 1.82 10.5 and 19.1

Al–P8.2–CD56.4 6.9 382 1.51 379 1.49 10.5 and 21.0

Al–P8.2–CD91.7 11.2 369 1.27 380 1.29 10.5 and 19.3

12.5 mM P123

Al–P12.5 0 357 1.37 387 1.35 14.8

Al–P12.5–CD21.2 1.7 354 1.45 387 1.43 10.5 and 14.9

Al–P12.5–CD42.3 3.4 356 1.62 372 1.60 10.5 and 16.3

Al–P12.5–CD56.4 4.5 382 1.97 427 1.94 10.5 and 19.3

Al–P12.5–CD91.7 7.4 360 1.66 373 1.63 10.5 and 19.7

P Pluronic P123 concentration in the sol (mM), CD RAMEB concentration in the sol (mM), SBET BET specific surface area determined in the

relative pressure range 0.1–0.25, PV pore volume calculated from adsorbed volume at P/P0 = 0.995, Scum cumulative surface area, Vcum

cumulative volume, PS pore size resulting from NLDFT calculations

0.00

0.01

0.02

0.00

0.01

0.02

0.00

0.01

0.02

0.00

0.01

0.02

dV(w

) (c

c/nm

/g)

14.9 nm

16.3 nm

19.3 nm

10.5 nm

10.5 nm

19.7 nm10.5 nm

B

a

b

c

d

A

0.00

0.01

0.02

0.00

0.01

0.02

0.000

0.005

0.0100 10 20 30 40 50 60

0 10 20 30 40 50 60

0 10 20 30 40 50 60

0 10 20 30 40 50 60

0 10 20 30 40 50 60

0 10 20 30 40 50 60

0 10 20 30 40 50 60

0 10 20 30 40 50 60

0.000

0.005

0.010

0.015

Pore width (nm) Pore width (nm)

dV(w

) (c

c/nm

/g)

16 nm

19.1 nm

21 nm

10.5 nm

10.5 nm

19.3 nm10.5 nm

a

b

c

d

10.5 nm

Fig. 9 Evolution of the pore size distributions for the mesoporous c-Al2O3 prepared with two Pluronic concentrations: 8.2 mM (a) and 12.5 mM

(b) and with increasing amounts of RAMEB 21.2 mM (a), 42.3 mM (b), 56.4 mM (c), 91.7 mM (d)

J Incl Phenom Macrocycl Chem

123

concentration (56.4 mM) is necessary to generate materials

with similar characteristics, i.e. a pore size of 19.3 nm, a

pore volume of 1.97 cm3/g and a specific surface area of

382 cm2/g. For both P123 concentrations investigated, the

RAMEB/P123 molar ratios that give the highest porosities

are below 7, corresponding to solution compositions

associated with RAMEB-swollen micelles, in quite good

agreement with DLS results. Beyond a concentration

threshold in cyclodextrin, the textural characteristics are no

more improved (Table 1). Therefore, the pore volume

shrinks and a second population of mesopores appears at

*10.5 nm, in addition to the one centred at *19.7 nm.

Actually, this second population of mesopores at

*10.5 nm appears as a shoulder for 21.2 mM RAMEB

and it is transformed in a well-defined peak with further

increasing RAMEB concentrations to 42.3 mM (for

8.2 mM P123) and 91.7 mM (for 12.5 mM P123). It is

worth reminding here that the DLS measurements previ-

ously indicated that, when the limit of RAMEB solubili-

zation within the micelles is reached, a transition toward

other large assemblies with a hydrodynamic radius in the

range of 120–330 nm occurs. The existence of this second

population of pores may be explained by the fact that

during the assembly process, the constraint exerted by the

solid boehmite nanoparticles on the swollen micelles pro-

vokes a release of a part of cyclodextrin molecules from the

PEO–PPO interfaces, therefore creating a second template

whose effect becomes more clearly defined as the micelles

are more loaded in RAMEB [27].

Finally, our data obtained from N2 adsorption mea-

surements and TEM observations clearly show that, by

using RAMEB-Pluronic supramolecular assemblies as

templates, mesoporous alumina with high surface area,

tunable pore size and pore volume and fiberlike morphol-

ogy can be easily prepared in aqueous solution (H2O/

Al & 100) by a room temperature self-assembly synthesis

procedure. Additionally, our results give strong indication

that in such template-directed synthesis procedure, the

randomly methylated b-cyclodextrin plays a key role in

controlling the size of the templating micelles and by

consequence the porosity of the resulting alumina.

Conclusion

In this study, we investigated the growth of the supramo-

lecular assemblies formed between the Pluronic P123

block copolymer (PEO20PPO70PEO20) and the randomly

methylated b-cyclodextrin (RAMEB) in aqueous solution.

Dynamic light scattering and viscosity measurement

revealed that the restructuration of the supramolecular

assemblies depends on the cyclodextrin-to-copolymer

molar ratio. For RAMEB/P123 molar ratios below *7.5, a

remarkable growth of Pluronic micelles was observed with

increasing the RAMEB concentration whereas for higher

RAMEB/P123 molar ratios (*7.5 and above), an abrupt

morphological transition from swollen micelles toward

large supramolecular assemblies with flexible interfaces

occurred. Such a morphological transition from spherical to

elongated micelles was revealed to be even more pro-

nounced with increasing temperature. This behaviour was

attributed to the ability of the methylated cyclodextrin to

locate at the PEO–PPO interface acting as a co-surfactant

and modifying the curvature of the micelle-water interfaces

from spherical to ellipsoidal or wormlike shape.

In a second part, the RAMEB–P123 supramolecular

assemblies were used as templates in the sol–gel synthesis

of mesoporous c-Al2O3 with tailored characteristics. The

examination of the pore structure of the solid materials

provided further evidence for the swelling effect of the

cyclodextrin which was manifested by an expansion of the

pore size and volume, an increase in the surface area and an

alignment of the particles resulting in fiberlike morpholo-

gies. TEM observations and N2 adsorption analyses

showed that the cyclodextrin-to-copolymer molar ratio is a

key factor in determining the porosity properties of alu-

mina. Overall, our results indicated that the best textural

characteristics can be obtained with RAMEB/P123 molar

ratios less than 7 and this behaviour may be linked to the

morphological transition from micelles to large supramo-

lecular assemblies in line with the DLS measurements. The

ability of randomly methylated b-cyclodextrin to increase

the size of the templating micelles, at a higher extent than

the so far reported hydrocarbons, should offer new insights

in material science for the generation of well-defined

template structures with enlarged pores. The incorporation

of various metals inside the pores of these materials is

currently being investigated in our lab for catalytic

purposes.

Acknowledgments The TEM facility in Lille (France) is supported

by the Conseil Regional du Nord-Pas de Calais and the European

Regional Development Fund (ERDF). The ERDF, CNRS, Region

Nord Pas-de-Calais and Ministere de l’Education Nationale de

l’Enseignement Superieur et de la Recherche are acknowledged for

fundings of the X-ray diffractometer. We thank Laurence Burylo

(UCCS, University of Lille) as well as Dominique Prevost (UCCS,

Artois) for technical assistance in XRD measurements and gravi-

metric analyses respectively.

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