Functionalized periodic mesoporous titanium phosphonate monoliths

with large ion exchange capacityw

Tian-Yi Ma and Zhong-Yong Yuan*

Received (in Cambridge, UK) 8th October 2009, Accepted 23rd December 2009

First published as an Advance Article on the web 20th January 2010

DOI: 10.1039/b920964f

Periodic mesoporous titanium phosphonate (PMTP-2) monoliths

were synthesized by combining autoclaving process and evaporation-

induced self-assembly strategy, which was functionalized by

ClSO3H treatment, acting not only as an ion exchanger with large

ion exchange capacity but also as a strong acid catalyst possible for

some low-temperature reactions.

A higher achievement beyond the preparation of powdery

ordered mesoporous materials, is the incorporation of organic

groups within mesoporous phases, either attached to the pore

surface or embedded in the pore walls, leading to precisely

located chemical functions in size-controlled accessible

cavities;1 and the molding of the powders into monoliths with

several advantages including mechanical stability, ease of

handling and recovery and greater structural uniformity to

meet the broad needs of industry and household.2 Our efforts

have been focused on the rational synthesis, functionalization

and application exploration of monolithic phosphonate-based

hybrid materials with periodic mesoporous structure. The

inorganic–organic hybrid mesoporous materials have the

advantage for facile function-modifying due to the conden-

sations between the grafting molecules and the organic moieties

in the hybrid framework. However, besides the ordered meso-

porous silica-based materials modified by various functional

groups such as phosphates,1 sulfonic groups3 and amino

groups4 for enhanced acid and base catalysts or ion exchangers,

the preparation and functionalization of other periodic meso-

porous hybrid materials is scarce accessed.5,6 Moreover, most

ordered mesoporous materials existed in powder form, while

the porous monoliths like carbonaceous and silica mono-

lithic materials, could be employed as electrodes,2 catalytic

microcreactors,7 or membranes for separation,8 at the same

time maintaining the overall macroscopic dimensions, which

could greatly extend their application fields. In this contribution,

on the basis of our previously reported ordered macroporous

titanium phosphonate materials,9 a new periodic mesoporous

titanium phosphonate (PMTP) monolith, denoted as PMTP-2,

was synthesized by an autoclaving process combined with the

evaporation-induced self-assembly (EISA) method, using the

coupling molecule 1-hydroxy ethylidene-1,1-diphosphonic

acid (HEDP). Furthermore, the synthesized PMTP-2 was

functionalized by sulfation with ClSO3H, leading to a large

ion exchange capacity. The as-synthesized monoliths could be

molded into various macroscopic morphologies, which may

potentially fulfil the qualifications for some industrial devices.

The reaction mixture of HEDP and TiCl4 in the presence

of Brij 56 were autoclaved, followed by the EISA process

(Experimental section, ESIw). The photographs of the

as-synthesized PMTP-2 materials and the mesoporous mono-

lithic products after surfactant removal by extraction were

shown in Fig. 1. As the EISA process was carried out in

different molds, various shapes of gels could be obtained,

depending on the shape and size of the autoclave used. After

extraction, the white gel blocks of as-synthesized PMTP-2

turned yellowy and tough with the sizes reduced (B15%), due

to the removal of surfactant species from the pores of titanium

organophosphonate framework, but the shape of the mono-

liths retained well. In comparison, when phosphoric acid was

used as a precursor instead of organophosphonic acid HEDP,

only powdery products of inorganic metal phosphates were

obtained. This indicates that the present inorganic–organic

PMTP hybrids were molded and incised more easily than most

of the purely inorganic porous materials, which might be

caused by the polymerization of the organic bridged groups

in the network with the inorganic species, making them more

like some kind of macromolecular polymer with mechanical

strength and ductility.

The low angle and wide angle XRD patterns of PMTP-2

were shown in Fig. 2. The low angle XRD pattern of PMTP-2

exhibited a typical hexagonal (p6mm) mesophase, with a main

peak observed at 2y= 2.431 corresponding to a (100) reflection

(d100 = 3.6 nm) and two small peaks at 2y = 4.161 and 4.791,

which could be attributed to (110) and (200) reflections.

The unit cell parameter (a) was calculated to be 4.2 nm.

Fig. 1 Photographs of as-synthesized PMTP-2 materials and the final

monolithic product after surfactant removal by extraction.

Institute of New Catalytic Materials Science, Engineering ResearchCenter of Energy Storage and Conversion (Ministry of Education),College of Chemistry, Nankai University, Tianjin 300071, China.E-mail: zyyuan@nankai.edu.cn; Fax: +86 22 23509610;Tel: +86 22 23509610w Electronic supplementary information (ESI) available: Experimentaldetails, TG-DSC profiles, FT-IR, MAS NMR, XPS spectra, titrationcurve, conversion profiles, acidity table. See DOI: 10.1039/b920964f

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COMMUNICATION www.rsc.org/chemcomm | ChemComm

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The periodic mesoporous structure was also confirmed by the

TEM observations (Fig. 3) and N2 sorption analysis (Fig. 4).

The hexagonal arrangement of the mesopores could be seen in

Fig. 3a, with the average pore size of around 2.4 nm and the

pore wall thickness of 1.8 nm, which were of the similar sizes

of the previously reported mesoporous aluminium phospho-

nates prepared in the presence of oligomeric Brij 56 and Brij

58.6 Typical one-dimensional channels were observed from the

side view of the PMTP-2 sample (Fig. 3b). The N2 sorption

isotherm of PMTP-2 was of type IVc with no visible hysteresis

loop, meaning reversible capillary condensation–evaporation

in mesopores (Fig. 4), which is typical for some OOINs

(ordered organic–inorganic nanocomposites) with accessible

mesostructures.10 One narrow peak around 2.4 nm was

observed in the pore size distribution curve, consistent with

TEM observations. The surface area and the pore volume were

1034 m2 g�1 and 0.57 cm3 g�1, respectively. The formation

mechanism of the present PMTP-2 material prepared with

nonionic surfactant assisted in acid media was proposed that

the positive charge-associated EO units of Brij 56 and

the cationic titanium phosphonate species were assembled

together with Cl� as the intermedium by a combination of

electrostatic, hydrogen bonding and van der Waals inter-

actions, which could be assigned as (S0H+)X�I+.11

The TG-DSC analysis confirms that the PMTP-2 materials

are thermally stable up to 450 1C (Fig. S1, ESIw). The PMTP-2

could be formulated as Ti3(O3PC(CH3)(OH)PO3)1.99�xH2O

from the ICP and conventional elemental analysis (ESIw),and alternative formulation can be expressed as

Ti3(HEDP)2�xH2O. The typical bands of titanium phospho-

nates, such as P–O� � �Ti stretching vibrations (1052 cm�1) and

P–C stretching vibration (1460 cm�1) were detected on the IR

spectrum of PMTP-2 (Fig. S2, ESIw). The 31P MAS NMR

spectrum of PMTP-2 shows one broad signal around 12.0 ppm,

which is characteristic of phosphonates (Fig. S3 ESIw). Theseall suggest that no phase separation took place during the

preparation of the hybrid samples, and HEDP coupling

groups were dispersed homogeneously within the hybrid

network.

The specific alkyl hydroxyl structure of the coupling

molecule HEDP makes it facile for its sulfation with ClSO3H

to form stable hydrosulfated esters.12 The obtained hydro-

sulfated PMTP-2 material (PMTP-2s) has a large ion exchange

capacity and strong acidity, which could be used as ion

exchangers and solid acid catalysts. The synthesis, sulfation

and ion exchange processes of the monolithic PMTP-2 material

was shown in Fig. 5. After the formation of ordered meso-

phase through the autoclaving and EISA process, carbon

hydroxyl groups were distributed all over the pore walls.

The –SO3H groups were introduced by ClSO3H treatment.

The evidences of the formation of hydrosulfated esters were

observed from IR (Fig. S2, ESIw), 13C MAS NMR (Fig. 6 left)

and XPS (Fig. 6 right) spectra. Noting Fig. S2,w the absorptionband of PMTP-2s at 820 cm�1 was highly increased compared

to PMTP-2, due to the C–O–S stretching vibrations, and the

new absorbance of asymmetric stretching of the SQO band

was found around 1248 cm�1.13 Shown in Fig. 6 left, the

intensity of the resonance at 20.3 ppm, corresponded to the C

atoms of the terminal CH3 group, were a little decreased; and

the position of the quaternary carbon atom resonance, con-

nected with the PQO group of the phosphonate, shifted from

71.4 to 78.5 ppm, which were both related to the formation

of C–O–S bonds. The S 2p binding energy was observed at

167.9 eV, corresponding to SO3�,14 which also indicated

the successful sulfation of PMTP-2 (Fig. 6 right). The ion

exchange capacity or acid content of the sulfated PMTP-2s

was determined from the titration curve by NaOH and the

elemental analysis (Experimental section and Fig. S5, ESIw).Around 2.69 and 3.93 mmol g�1 of H+ was attributed

to sulfonic groups and titanium phosphonate framework

(defective P–OH groups15), respectively. Thus the sulfation

process leads to an obvious increase of the ion exchange

capacity compared to the unfunctionalized sample (PMTP-2)

and the reported titanium phosphate materials.15 The exchange

capacity was also comparable to some sulfonated PMOs.16Fig. 2 Low angle and wide angle (inset) XRD patterns of the

synthesized PMTP-2 sample.

Fig. 3 TEM images of the synthesized PMTP-2 hybrid material.

Fig. 4 N2 adsorption–desorption isotherm and the corresponding

pore size distribution curve (inset) of PMTP-2, determined by BJH

method.

2326 | Chem. Commun., 2010, 46, 2325–2327 This journal is �c The Royal Society of Chemistry 2010

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Moreover, defective P–OH groups acted as weaker Brønsted

acid sites than the –SO3H groups, which were considered to

mainly contribute to the acid catalysis performance (Table S1,

ESIw). The acid strength of the materials was examined by the

Hammett indicator method (Experimental section and Table

S1, ESIw), and the PMTP-2s sample was estimated to have

H0 o �11.35, indicating it was a kind of strong solid acid

(acids stronger than H0 = �11.93 for superacids). It was also

proved that the hydrosulfated groups could be kept at pore

walls even in hot waters (up to 80 1C, Table S1, ESIw), whichmakes it useful as an ion exchanger and acid–base catalyst for

some room-/low-temperature reactions. For example, the

sulfated PMTP-2s were used for the esterification of oleic acid

with methanol under ambient temperature and pressure

(Fig. S6, ESIw), in which PMTP-2s exhibited a much higher

conversion (87.3%) than the unfunctionalized PMTP-2

(4.9%). And the conversion was even a little higher than the

conventional sulfonic acid-functionalized mesoporous titania

materials (81.7%). Moreover, because the hydrosulfated

groups serve as carriers of protons, the PMTP-2s materials

might also find use as an electrolyte for fuel cells.16

In summary, a new kind of ordered mesoporous titanium

phosphonate PMTP-2 has been prepared by combining auto-

claving and evaporation-induced self-assembly strategy, using

HEDP as the anchoring molecule. The preparation was

accomplished through a simple one-step hydrothermal process

with the use of nonionic surfactant Brij 56 by (S0H+)X�I+

formation mechanism in the acidic media. The alkyl

hydroxyl groups of HEDP make it possible for the further

functionalization of PMTP-2 by ClSO3H treatment. The

modified material acts not only as an ion exchanger with large

ion exchange capacity but also as a strong acid catalyst

possible for some room-/low-temperature reactions. More

interestingly, the present inorganic–organic hybrids were

proven to be molded and incised into various shapes facilely,

making them practical materials for industrial scale manufacture.

The next step would be the improvement of the hydrothermal

stability and the mechanical strength of the monolithic

PMTP-2s with different pore sizes and pore structures

for more practical applications, as well as the preparation

and functionalization of hierarchically structured periodic

mesoporous metal phosphonates with multi-scaled sizes and

micro-architectures for enhanced properties.17

This work was supported by the National Natural Science

Foundation of China (No. 20973096 and 20673060), the National

Basic Research Program of China (No. 2009CB623502), the

Specialized Research Fund for the Doctoral Program of Higher

Education (20070055014), the Natural Science Foundation of

Tianjin (08JCZDJC21500), the MOE Supporting Program for

New Century Excellent Talents (NCET-06-0215), and Nankai

University.

Notes and references

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Fig. 5 Synthesis, sulfation and ion exchange processes of the mono-

lithic PMTP-2 material.

Fig. 613C MAS NMR spectra of PMTP-2s (red line) and PMTP-2

(black line) (left), and high-resolution XPS spectrum of the S 2p

regions of PMTP-2s (right).

This journal is �c The Royal Society of Chemistry 2010 Chem. Commun., 2010, 46, 2325–2327 | 2327

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