Microwave-assisted and conventional hydrothermal synthesis of ordered mesoporous silicas with P-containing functionalities

Accepted Manuscript Title: Microwave-assisted and conventional hydrothermal synthesis of ordered mesoporous silicas with P-containing functionalities Author: Oksana A. Dudarko Chamila Gunathilake Valeriia V. Sliesarenko Yuriy L. Zub Mietek Jaroniec PII: DOI: Reference:

S0927-7757(14)00582-2 http://dx.doi.org/doi:10.1016/j.colsurfa.2014.06.036 COLSUA 19315

To appear in:

Colloids and Surfaces A: Physicochem. Eng. Aspects

Received date: Revised date: Accepted date:

8-5-2014 19-6-2014 21-6-2014

Please cite this article as: O.A. Dudarko, C. Gunathilake, V.V.S. ,Yuriy L. Zub, M. Jaroniec, Microwave-assisted and conventional hydrothermal synthesis of ordered mesoporous silicas with P-containing functionalities, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2014), http://dx.doi.org/10.1016/j.colsurfa.2014.06.036 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Microwave-assisted and conventional hydrothermal synthesis of ordered mesoporous silicas with P-containing functionalities

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Oksana A. Dudarkoa, Chamila Gunathilakeb, Valeriia V. Sliesarenkoa,Yuriy L. Zuba, Mietek Jaroniecb*

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(a) Chuiko Institute of Surface Chemistry, NAS of Ukraine, 17, General Naumov str., Kyiv 03164 Ukraine (b) Department of Chemistry, Kent State University, Kent, OH 44242, USA

Ordered

mesoporous

silicas

with

ester-type

us

Abstract

[≡Si(CH2)2P(O)(OC2H5)2]

and

acidic

an

[≡Si(CH2)2P(O)(OH)2] functionalities were synthesized via co-condensation of suitable organosilanes in the presence of triblock copolymer, Pluronic P123, under acidic conditions.

M

XRD and TEM analysis of the synthesized materials confirmed their hexagonally ordered mesoporosity similar to that in SBA-15. Namely, the materials prepared under microwave irradiation by using tetraethylorthosilicate (TEOS) as a framework-forming agent and

d

diethylphosphatoethyltriethoxysilane as a source of functional groups exhibit adsorption

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properties (such as the specific surface area, pore size and pore volume) slightly different than those of the corresponding samples obtained by conventional hydrothermal synthesis. However,

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in the case of conventional hydrothermal synthesis, nitrogen adsorption isotherms resemble those for plugged hexagonally ordered mesoporous silicas. Also, the replacement of TEOS with sodium silicate and introduction of ≡Si(CH2)2P(O)(OH)2 functionalities results in the samples with slightly higher specific surface area. Thus, the synthesis method does not have a significant impact on the porous structure and adsorption characteristics of the samples studied. A tangible advantage of the microwave-assisted synthesis vs. conventional hydrothermal preparation is its significantly shorter time. Keywords: Ordered mesoporous silica, hydrothermal synthesis, microwave-assisted synthesis, phosphonate ester-modified silica, phosphonic acid-modified silica _______________________ * Corresponding authors: Tel: +1 330 672 3790; Fax: +1 330 672 3816; E-mail: [email protected] (M. Jaroniec)

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1. Introduction Phosphonic acid-based solid-phase extractants attract a lot of attention because of their potential use for extraction and separation of rare earth elements and actinides [1-5]. Mesoporous

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ordered silica (OMS) materials, especially so-called SBA-15, synthesized by soft templating [69] are particularly interesting solid supports. They are characterized by relatively high

cr

hydrothermal stability and uniform mesoporosity easily accessible for sorption of ions of the aforementioned metals. Typically, functionalized solid-phase extractants are created by two porous

support

such

as

SBA-15

with

an

us

strategies. The first one involves the surface modification (post-synthesis grafting) of a suitable appropriate

trifunctional

silane,

e.g.,

an

(C2H5O)3Si(CH2)2P(O)(OC2H5)2 (DPTS) [3]. The second strategy employs сo-condensation of TEOS and DPTS [2, 10, 11] (or diethylphosphathopropyltriethoxysilane, DPPTS) [12]), 1,2-bis(trimethoxysilyl)ethane (BTME) and DPTS [13] in the presence of a soft template. As regards to

M

the latter the poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) Pluronic P123 [10-12, 14], or sodium dodecyl sulfate (SDA) [12], or cetyltrimethylammonium bromide (CTAB) [2] have been used under acidic [11, 13, 14] or basic [2] conditions, sometimes

d

with addition of NaF [10, 14]. Colilla et al. [7] reported the synthesis of hexagonally ordered

te

SBA-15 in the presence of H3PO4. It should be noted that the use of co-condensation strategy allows obtaining functionalized mesoporous silica-based materials with high hydrothermal

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stability. Note that the removal of polymeric template by refluxing with acidified ethanol or H2SO4 solutions [10] did not lead to destruction of ≡Si-C≡ linkages. Moreover, this bond is stable for the samples boiled in concentrated hydrochloric acid to transform phosphonate ester functionality -PO(OEt)2 to phosphonic acid -PO(OH)2 [10, 11]. Note that this transformation may be carried out under mild conditions (refluxing with trimethylchlorosilane (Me3SiCl) in toluene) [12].

The templating synthesis affords OMS materials with larger surface area and mesopore volume. These materials can be obtained in a relatively short time with high adsorption capacity, suitable for multiple uses. In addition, OMS materials with appropriate surface properties, porosity and morphology are excellent candidates, for instance, for drug delivery [15]. In the conventional synthesis of OMS materials, the hydrothermal treatment of the surfactant-containing mesophase is carried out in an autoclave under heating and pressure. Such treatment requires a long time (usually 24-120 hours) [15-23]. The use of a microwave oven can

2 Page 2 of 19

reduce this treatment up to 12-18 hours [16, 21, 24-27]. Moreover, magnetic stirring during microwave treatment facilitates formation of more uniform structure, resulting in better ordering of these materials. A rapid and uniform heating promotes the formation of uniform nucleation centers followed by crystallization. It is an energy-efficient and environmentally friendly way to prepare ordered mesostructures. Furthermore, microwave treatment can be an additional factor

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influencing the formation of ordered structures with desired surface functionality [24, 26 - 29]. Microwave treatment has been employed for the synthesis of SBA-15 by using TEOS [16, 21,

cr

30] and sodium metasilicate [9, 24, 31] as precursors. The aim of this work is to present a comparative study of the microwave-assisted (MW) and hydrothermal (HT) synthesis of TEOS

us

and sodium metasilicate-based OMS-tethered phosphonic acid materials.

an

2. Expermiental 2.1. Materials

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Chemicals used for the synthesis of functionalized mesoporous silicas include: sodium metasilicate, Na2SiO3.9H2O (Sigma, USA); tetraethylorthosilicate (TEOS), Si(OC2H5)4 (98%, Acros

Organics,

Germany);

diethylphosphatoethyltriethoxysilane

(DPTS),

d

(C2H5O)3Si(CH2)2P(O)(OC2H5)2 (95%, Gelest); poly(ethylene oxide)-poly(propylene oxide)–

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poly(ethylene oxide) triblock copolymer Pluronic P123, EO20PO70EO20 (99%, BASF, USA);

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concentrated hydrochloric acid (37.6%, Fisher Scientific); ethanol (95 %, Fisher Scientific). 2.2. Preparation

Synthesis of OMS materials. Sample T-M (TEOS/DРTS molar ratio = 10:1). 4 g of P123 (Pluronic P123) was dissolved at room temperature in 100 cm3 of 1.9 M HCl. Separately, 1.36 cm3 (0.004 M) of DРTS was pre-hydrolyzed using 5 cm3 of 1.9 M HCl for 1 h under magnetic stirring at room temperature. Then DPTS solution was added to P123 solution during 5 minutes. Subsequently, 8.92 cm3 (0.04 M) of TEOS was added to the above mixture. The resulting suspension was transferred to Teflon vessels, which were installed in microwave oven (MARS 5, CEM Corp.). In the first step, the synthesis mixture was stirred using magnetic stirrer bar for about 2 h at 40 °C. Next, magnetic stirring was off and temperature was increased to 100 °C and kept at this temperature for 10 h. After cooling the mixture, the mesophase was filtered and dried at 60 °C for three days. P123 was extracted from mesophase using acidified ethanol (30 cm3 per 1 g of mesophase) at 60 ºC for 24 h. The resulting powdery substance was

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dried in a oven for 12 h at 60 ºC and 4 h at 100ºC followed by vacuum drying overnight at ~8590 °C. Sample T-H (TEOS/DРTS molar ratio = 10:1). . The synthesis was carried out similarly to the sample T-M, except hydrothermal treatment (HT) of the mesophase in the mother liquor at 100 oC for 24 hours using a conventional oven.

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Sample Na-M (Na2SiO3.9H2O/DPTS molar ratio = 10:1). 0.25 cm3 (0.0008 M) of DPTS was pre-hydrolyzed in 7.36 cm3 of concentrated hydrochloric acid for 6 hours on an oil bath with

cr

reflux condenser. The solution of polymeric template (0.8 g of P123 and 13 cm3 of water) was added to the solution of DPTS. Separately, 2.276 g (0.008 M) of sodium silicate was dissolved in

us

10 cm3 of water. Then transparent solution was added to the reaction mixture. The resulting lightly turbid suspension was left in oil bath for 1 h at 40 °C to facilitate precipitation. Next, the sample was kept in microwave oven for 2 h at 40ºC under magnetic stirring followed by

an

hydrothermal treatment at 100 ºC for 10 h without stirring. Template was removed by extracting the sample using acidified ethanol. Finally, the sample was filtered and dried under vacuum at 50

M

°C for 24 hours and at 100 °C for the same time.

Sample Na-H (Na2SiO3.9H2O/DPTS molar ratio = 10:1 ). The synthesis was carried out

2.3. Measurements

te

mother liquor at 80 oC for 24 hours.

d

similarly to the sample Na-M, except hydrothermal treatment (HT) of the mesophase in the

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IR reflectance spectra in the region 4000-400 cm-1 were recorded on a spectrometer Thermo Nicolet Nexus FTIR with diffuse reflection using the prefix "SMART Collector" with a resolution of 8 cm-1. Samples were mixed with KBr (Aldrich) in the ratio of 1:20. Thermogravimetric (TG) and differential (DTG) profiles were recorded on a TA Instruments TGA 2950 analyzer with 10 °C min-1 heating rate. The sensitivity of balance was ± 0.1 mg.

Elemental analysis was performed by Analytical Laboratory of the Institute of Organic Chemistry of NAS (Ukraine). The small angle XRD patterns were recorded over a range of 0.50 < 2θ < 2.5 on a PANanalytical Inc. X’Pert Pro (MPD) MultiPurpose Diffractometer with Cu Kα radiation (λ = 0.1540 nm) using an operating voltage of 40 kV and 40 mA, 20 s step time and 0.01 step size. Microscope glass slides were used as sample supports. All materials were manually ground prior to the XRD analysis and the measurements were performed at room temperature. 4 Page 4 of 19

The collected XRD data were used to calculate the interplanar distances dhkl corresponding to the (hkl) diffraction peaks according to the well-known Bragg equation. For hexagonally ordered mesoporous silica materials with complementary interconnected fine pores, the diameter of mesopores (dme), the unit cell parameter (a) and the thickness of the pore walls

d me = 1.213d100

ρVme 1 + ρVm

(1)

cr

where d100 – d-spacing corresponding to (100) diffraction peak;

ip t

(h) can be calculated by using the relations provided below [32]:

ρ – true density of the material; in the case of silica ρ = ~2.2 g/cm3;

us

Vme – volume of ordered mesopores obtained by nitrogen adsorption, сm3/g; Vm – volume of complementary pores and ordered mesopores obtained by nitrogen

2d100 3

h = a − d me

(2) (3)

M

a=

an

adsorption, сm3/g;

SEM photomicrographs were taken on a scanning electron microscope JSM-6060 LV

d

(JEOL, Japan). The gold was sprayed within 5 minutes to improve the clarity of the image on the

te

sample.

TEM photomicrographs were taken on a transmission electron microscope JEM-1230 (JEOL,

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Japan). For analysis, the sample powders were dispersed in ethanol by moderate sonication at concentrations of ~5 wt. % solids. A lacey carbon coated (200-mesh) copper TEM grid was dipped into the sample suspension and then dried under vacuum at 80 ºC for 12 h prior to analysis.

Adsorption properties of the synthesized samples were evaluated from nitrogen adsorption isotherms [8, 33] measured by using ASAP 2010 volumetric analyzers (Micrometrics, Inc. Norcross, GA). Adsorption isotherms were recorded at -196 ºC in the relative pressure range from 10-6 to 0.995 using ultra high purity nitrogen from Praxair Distribution Company (Danbury, CT, USA). Prior to adsorption measurements, all samples were out gassed under vacuum at 110 o

C for 2 hours. The Brunauer-Emmett-Teller specific surface areas (SBET) were calculated from

N2 adsorption isotherms in the relative pressure range of 0.05-0.2 using a cross sectional area of 0.162 nm2 per nitrogen molecule, whereas, the single-point pore volume was estimated at a relative pressure of 0.98. Pore size distributions (PSD) together with the pore diameters (dme) at

5 Page 5 of 19

the maximum of PSD were determined by Kruk–Jaroniec–Sayari (KJS) method [34] based on the Barrett–Joyner–Halenda (BJH) algorithm [35].

3. Results and discussion

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The main goal of this study was to introduce phosphonate ester [≡Si(CH2)2P(O)(OC2H5)2] functionality into ordered siliceous mesostructures by co-condensation of TEOS and DPTS in

cr

the presence of Pluronic P123 block copolymer. Hydrothermal treatment of the resulting mesostructures was performed in microwave (sample T-M) or conventional oven (sample T-H).

us

An inexpensive sodium metasilicate was also used as a silica source instead of TEOS to synthesize the second series of the samples. However, in this case, DPTS was pre-hydrolyzed in

an

boiling conc. hydrochloric acid for 6 hours. Our intention was to pre-hydrolyze not only ethoxysilyl groups, but also ethoxy groups connected to phosphorus atoms to form phosphonic acid

[≡Si(CH2)2P(O)(OH)2] functionality. Similarly as in the case of TEOS, the samples

M

prepared from sodium metasilicate were hydrothermally treated in microwave (sample Na-M) and conventional oven (sample Na-H). The resulting powdered samples, after removing Pluronic P123 template with acidified ethanol, were dried under similar conditions as in the case of the T-

d

M and T-H samples.

te

The presence of ordered mesostructure similar to that in SBA-15 [6] was confirmed by

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powder XRD diffraction (see Fig. 1).

a

110 200

0,8

1,6

2,4 2θ, degree

T-H T-M

Intensity, a.u.

Intensity, a.u.

100

100

b Na-H Na-M

110 200 0,8

1,6

2,4

2θ, degree

Fig. 1. Powder X-ray diffraction patterns of phosphonic acid-functionalized SBA-15-type mesoporous silicas.

6 Page 6 of 19

Structural characterization of the siliceous mesostructures having phosphonate ester and phosphonic acid functionalities was performed by using small angle X-ray diffraction patterns recorded in the range of 2θ from 0.5 to 2 degree. The XRD patterns of the extracted samples contain three well-defined reflections at 2θ (T-H: 0.83º, 1.42º, 1.62º; Na-H: 0.87º, 1.47º, 1.7º; Na-M: 0.87°, 1.5º and 1.73º; Fig. 1a, b), and in the case of the T-M sample (Fig. 1a) one sharp

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reflection at 2θ = ~0.85°. These reflections fulfill the requirements of 2D hexagonally ordered structures (p6m symmetry group) and can be indexed as 100 (the main XRD peak) and 110 and

cr

200 (two minor reflection peaks). The XRD peaks of the samples hydrothermally treated in a conventional oven are sharper than those for the corresponding samples obtained under

us

microwave irradiation; it is especially visible in the case of minor reflections.

Morphology of the particles obtained from sodium metasilicate is significantly different from that of the particles synthesized by using TEOS as the main precursor (Fig. 2). In the latter

an

case larger agglomerates composed of smaller elongated particles can be observed. Furthermore, the primary particles of the T-H sample (Fig. 2b) are elongated as compared to the samples

M

obtained under microwave conditions (Fig. 2a). The samples obtained from sodium metasilicate

b

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te

a

d

consist of particles larger than 1 micron, which are glued together (Fig. 2c,d).

c

d

Fig. 2. SEM images of the samples studied (a – T-M; b – T-H; c – Na-M; d – Na-H). The mesostructural ordering was further confirmed by TEM imaging. As can be seen from Fig. 3 the cylindrical mesopores packed into a honeycomb structure are clearly visible. The TEM images of the T-H and Na-M samples were obtained at the direction perpendicular to the

7 Page 7 of 19

mesopore channels, while in the case of the T-M and Na-H samples along the pore channel direction. These images further confirm the hexagonal ordering of mesopores in the samples studied.

b

c

d

d

M

an

us

cr

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a

te

Fig. 3. TEM images of the samples (a – T-M, b – T-H, c – Na-M, d – Na-H). Shown in Figure 4 are the IR spectra for the samples studied. The intensive absorption band

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at 1060-1160 cm-1 can be assigned to the νas(Si-O-Si) of polysiloxane skeleton, which is displayed at

high frequencies with visible shoulder. This shoulder certainly refers to the

absorption band ν(P=O) of the P-containing groups that was attributed at 1241 cm-1 on spectrum of initial DPTS [36,37]. It is shifted to low frequencies by about 40 cm-1 (Fig. 4a-d) indicating the participation of P=O groups in hydrogen bonds. The presence of an absorption band δ(H2O) at about 1630 cm-1 indicates the possible presence of water molecules. Note that there is a significant difference between the IR spectra of the T-M, T-H and NaM, Na-H samples in the region of 1300-1500 cm-1. Considering the latter samples, there are three main absorption bands at ~1350 сm-1, ~1400 сm-1 and ~1450 сm-1 (Fig. 4, curves c-d). They can be easily assigned to the vibration bands of ω(СН2), δ(Si–CH2) and δ(СН2) in phosphonic acid functionality, respectively. More complex spectra are observed in the case of T-M and T-H due to the presence of ethoxy groups in phosphonate ester functionality. The presence of ethoxy groups and/or chain-CH2-CH2- is also reflected in the form of two low-intensity absorption 8 Page 8 of 19

bands at 2920-2986 cm-1 corresponding to νs,as(CH) (Fig. 4, curves a-d). Thus, the analysis of IR spectra confirmed the presence of –P(O)(OC2H5)2 in Т-М and Т-Н and –P(O)(OH)2 in Na-М

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te

d

M

an

us

cr

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and Na-Н.

Fig. 4. IR spectra of the samples studied (a – T-M; b – T-H, c – Na-M, d – Na-H).

9 Page 9 of 19

The presence of phosphorus-containing functional groups in the synthesized samples was confirmed by elemental analysis for phosphorus. It was found that P content was 3.1 % (wt.) for the sample T-M, 3.1% for T-H, 3.2% for Na-M, and 2.9 % for Na-H. These data give the Si/P ratios equal to 10.9 for the T-M sample, 10.4 for T-H, 11.4 for Na-M and 11.3 for Na-H, which is comparable to the ratio of 11.0 estimated basing on the chemical composition of reactants used

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in the synthesis. These data indicate that silica could be slightly dissolved from the samples prepared form TEOS during their hydrothermal treatment, while in the case of the samples

cr

obtained from sodium silicate a small excess of silica is possible. The concentration of Pcontaining functional groups in the samples studied, calculated from the data of elemental

us

analysis, is about 1.0 mmol/g .

Analysis of the DTG profile for the T-M sample before and after removal of the polymeric template (Fig. 5) clearly shows that in the first case there is a very intense signal in the range of

an

180-220 º C, which reflects thermodegradation of P123 block copolymer. The decomposition of organic constituent in functional groups begins at 275 ºC. After extraction, the peak in the range

M

180-220 ºC is absent (Fig. 5, curves 1, 3), indicating the efficiency of removal of polymeric template with acidified ethanol. Traces of the extractant and physically adsorbed water are removed up to 120 ºC. The percentage of the weight loss in the aforementioned range is equal to

d

2 % for T-M and 6 % for Na-M. Decomposition of the organic component can be observed in the

te

temperature range of 275-450 º C. The weight losses in this range are 11 % and 9 % for the T-M sample before and after extraction, respectively (Fig. 5, curves 1, 3), whereas, this value for the

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extracted Na-M sample is about 6.5 % (Fig. 5, curve 2). Theoretical calculations indicate that the weight loss of ~20.2 % is expected for the samples containing ethoxy groups connected to P atom or 7.2 % in the case of hydrolysis of these groups.

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0,5

100

1 2 3

0,4 0,3

90 85 80

ip t

weight, %

75

TG

70 65

0,2

60 100

200

300

400

500

cr

-DTG, %/C

95

600

700

0

800

Temperature, C

DTG

an

0,0

us

0,1

100 200 300 400 500 600 700 800 0

M

Temperature, C

Fig. 5. DTG and TG (inset) curves for the T-M sample (1 – after, 3 – before extraction of polymeric template) and Na-M sample (2) after extraction of the template.

d

Nitrogen adsorption-desorption isotherms measured at -196 0C on the T-M, Na-H and Na-

te

M samples are shown in Figure 6a and 6b. These isotherms are type IV with sharp capillary condensation-evaporation steps and pronounced H1 hysteresis loop starting at relative pressure

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of about 0.65-0.7, whereas, the evaporation step ends at ~ 0.65-0.50. This is observed for cylindrical mesopores with narrow pore size distribution. The position and size of the hysteresis loop does not depend on the method of hydrothermal treatment of the samples. Note that the T-H sample exhibits stepwise desorption branch of the hysteresis loop, indicating some pore constrictions [8]. In 2002, Van Der Voort and others [37,38] reported a new form of SBA-15, called plugged hexagonally templated silica (PHTS). It is formed due to congestion/plugs inside mesochannels of SBA-15.

11 Page 11 of 19

-1

-1

T-H

400 300 200 100 0 0,0

0,2

0,4

0,6

0,8

Na-M

b

600

Na-H

3

T-M

500

700

1,0

500 400 300 200 100 0 0,0

0,2

0,4

0,3

Na-M

0,8 0,6 0,4

0,2

d

Na-H

0,2

0,1

0,0

10

20

30

0

M

0,0 0

1,0

cr

3 -1

0,4

1,0

us

-1

T-H

c

1,2

an

0,5

PSD, cm g nm

-1 3 -1

PSD, cm g nm

0,7

T-M

0,8

Relative pressure

Relative pressure

0,6

0,6

ip t

a

Vollume adsorbed , cm STP g

600

3

Vollume adsorbed , cm STP g

700

10

Pore diameter, nm

20

Pore diameter, nm

30

Fig. 6. Nitrogen adsorption/desorption isotherms for mesoporous silicas (a, b) and the

d

corresponding pore size distributions calculated by KJS method from adsorption branches of the

te

isotherms measured at -196 ºC (c, d).

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The presence of the aforementioned step on the desorption branch of the hysteresis is an evidence of the existence of pore constrictions/plugs. The steep capillary condensation steps indicate relatively narrow pore size distributions with the pore diameter at the maximum of PSD at about 8 nm. These results are in a good agreement with literature data for similar materials [24]. The pore size distributions calculated by using the KJS method are presented in Fig 6с, d. The basic parameters evaluated from nitrogen adsorption isotherms are summarized in Table 1 and resemble those reported for analogous silica mesostructures [11].

12 Page 12 of 19

Таble 1. Parameters of the porous structure of the synthesized materials Sample

SBET, m2/g

Vsp, cm3/g

dme, nm

dme, nm

(KJS)

(XRD)

a, nm

h, nm

687

0.89

7.8

8.2

12.1

3.9

T-H

710

1.04

8.6

8.7

12.1

3.5

Na-M

705

1.07

8.5

8.7

Na-H

789

1.01

8.6

8.8

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T-M

11.6

2.9

12.0

3.2

cr

Notes: SBET – specific surface area calculated from adsorption data in relative pressure range 0.05-0.20; Vsp - single point pore volume calculated at relative pressure of 0.98; dme (KJS) - pore

us

width calculated at the maximum of PSD obtained by improved KJS method; dme (XRD) - pore diameter calculated by using the XRD d-spacing and adsorption pore volumes according to eq.

an

(1) ; a – unit cell parameter (eq. 2); h - thickness of the pore walls (eq. 3). The volume of ordered mesopores Vme (eq. 1) was obtained by integration of PSD from 7 to 12 nm; the volume of

M

complementary and ordered pores Vm (eq. 1) was obtained by integration of PSD up to 12 nm. A comparison of the data presented in Table 1 suggests that the method of hydrothermal

d

treatment of the sample has no significant effect on the parameters of the porous structure. Two

te

visible differences should be noted for the samples studied. First one refers to the shape of the desorption branch of the isotherm obtained for T-H, which is different from that obtained for T-

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M and resembles that for SBA-15 materials. Second, the value of the specific surface area of NaH is significantly higher than that of Na-M. However, other parameters coincide with each other quite well (see Table 1).

4. Conclusions

In conclusion, co-condensation of TEOS and DPTS in strongly acidic medium in the presence of P123 template led to the formation of ordered mesophase. The MW or HT treatment of this mesophase followed by the removal of polymeric template with acidified ethanol afforded highly ordered mesoporous materials having phosphonate ester functionality. However, the cocondensation of sodium metasilicate and pre-hydrolyzed DPTS under the same conditions led to the formation of mesoporous materials possessing phosphonic acid functionality. Analysis of the XRD, SEM, TEM, and adsorption data for the samples studied shows that the MW treatment of the mesostructures does not provide evident advantages over HT treatment. The major advantage of the MW treatment is the shorter time of synthesis. Note that the formation of ordered 13 Page 13 of 19

mesostructures seems to be more sensitive on the method of hydrothermal treatment for the samples obtained from sodium metasilicate.

Acknowledgements O.A.D. thanks Fulbright Scholar Program for the financial support of the present work (Grant ID

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68120263, 2012-2013).

cr

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an

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d

amino groups for uranium uptake, Science China Chemistry. 55 (2012)1705-1711.

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[4] A.Dabrowski, M.Barchak, О. А. Dudarko , Y.L. Zub, Preparation and characterisation of polysiloxane xerogels having covalently attached phosphonic asids groups, Pol. Chem. J.

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Graphical Abstract

HO

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P CH2 CH2 Si

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O O O

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20 nm

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Highlights A fast synthesis of phosphonic acid-modified mesosilica is possible under microwave conditions

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Very similar mesostructures can be obtained by using sodium metasilicate instead of TEOS

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One-pot synthesis using silica and phosphonic acid precursors affords ordered mesostructures

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