Cage-like ordered mesoporous organosilicas with isocyanurate bridging groups: Synthesis, template removal and structural properties

Microporous and Mesoporous Materials 118 (2009) 68–77

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Cage-like ordered mesoporous organosilicas with isocyanurate bridging groups: Synthesis, template removal and structural properties Rafal M. Grudzien a, Jonathan P. Blitz b, Stanislaw Pikus c, Mietek Jaroniec a,* a

Department of Chemistry, Kent State University, Williams Hall Room 201, Kent, OH 44242, USA Department of Chemistry, Eastern Illinois University, Charleston, IL 61920, USA c Department of Chemistry, Maria Curie-Sklodowska University, 20-031 Lublin, Poland b

a r t i c l e

i n f o

Article history: Received 19 July 2008 Received in revised form 7 August 2008 Accepted 9 August 2008 Available online 15 August 2008 Keywords: Mesoporous organosilica Periodic mesoporous organosilica Nitrogen adsorption Cage-like structures SBA-16 Isocyanurate bridging group Ethane bridging group

a b s t r a c t The removal of triblock copolymer template from SBA-16 cage-like ordered mesoporous organosilicas (OMOs) functionalized with bulky isocyanurate (ICS) heterocyclic bridging groups is examined. These isocyanurate-containing OMOs (ICS–OMOs) were prepared by co-condensation synthesis of tris[3-(trimethoxysilyl)propyl]isocyanurate and tetraethyl orthosilicate in the presence of poly(ethylene oxide)block-poly(propylene oxide)-block-poly(ethylene oxide) (EO106PO70EO106) triblock copolymer F127 used as a template and sodium chloride additive under low acidic conditions. Initial studies revealed that the extracted ICS–OMOs materials have a cubic structure (Im3m symmetry), high contents of isocyanurate bridging groups (0.76 and 1.08 mmol/g) as well as pore diameters ranging from 6.6 to 7.2 nm. The effect of the heating temperature on the structural ordering, specific surface area, pore volume and cage pore diameter of the as-synthesized and extracted ICS–OMOs was investigated. In addition, the synthesis, characterization and template removal of a bifunctional cage-like periodic mesoporous organosilica (PMO) containing ethane (E) and isocyanurate bridging groups is discussed. The E–ICS–PMO samples were fabricated via one-pot synthesis using 1,2-bis(triethoxysilyl)ethane, tris[3-(trimethoxysilyl)propyl]isocyanurate and F127 block copolymer. The resulting ICS–OMOs and E–ICS–PMOs were characterized by nitrogen adsorption–desorption isotherms at 196 °C, small-angle X-ray scattering (SAXS) or low-angle powder X-ray diffraction (XRD), high-resolution thermogravimetry (TG), Fourier-transform infrared spectroscopy (FT-IR) and CHNS elemental analysis. The SAXS/XRD and nitrogen adsorption show that the use of a short extraction and temperature-controlled heating at 315 °C in flowing nitrogen is an effective way to remove the remaining polymeric template from cubic mesostructures without decomposition of isocyanurate bridging groups, as evidenced by FT-IR, TGA and CHNS analysis. Moreover, a complete thermal degradation of isocyanurate groups at 550 °C in air has led to a significant shrinkage of the mesostructure, however its ordered nature was retained. Ó 2008 Elsevier Inc. All rights reserved.

1. Introduction The discovery of ordered mesoporous silicas (OMSs) opened new opportunities for the synthesis and development of materials with well-defined porous structures and tailored particle morphologies [1]. Some of the first reports on OMSs were devoted to MCM41, which has been prepared in the presence of cationic surfactants in basic media [1]. Later, the use of non-ionic block copolymers as soft templates sparked an intensive research in the area of OMSs. Notably, SBA-15 [2], which possesses 2D hexagonal network of cylindrical channels (pores) (p6mm symmetry), randomly interconnected by irregular fine pores (micropores), was one of the first OMS prepared using poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) triblock copolymer Pluronic * Corresponding author. Tel.: +1 330 672 3790. E-mail address: [email protected] (M. Jaroniec). 1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.08.017

P123; HO(C2H4O)20(C3H6O)70(C2H4O)20H under acidic conditions from either tetraethyl orthosilicate (TEOS) or sodium silicate. SBA-15 [2] material is analogous to MCM-41 [1], however in addition to the aforementioned interconnecting fine pores it exhibits larger pore diameters, and thicker walls; thus it shows a better hydrothermal and thermal stabilities than its counterpart, MCM41 [3]. The properties of SBA-15 can be easily tuned by adjusting the synthesis conditions in terms of temperature, time [4], microwave irradiation [5], addition of swelling agents [6] and so forth. The same methodology was further extended to the synthesis of cubic three-dimensional (3D) mesoporous silicas SBA-16 (Im3m) [2,7] and FDU-1 (Fm3m) [8], which contain spherical pores connected with eight and twelve neighboring cages via narrow apertures creating a multidirectional porous network, respectively. Most commonly SBA-16 can be obtained using triblock copolymer Pluronic F127, HO(C2H4O)106(C3H6O)70(C2H4O)106H [9–11] but also using polymer blends of P123 with F127 [12]; F108,

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HO(C2H4O)141(C3H6O)40(C2H4O)141H [13]; and non-ionic oligomeric surfactant Brij700 (polyoxyethylene stearyl ether, C18H37EO100) [14]. FDU-1 [8,15–19] is usually prepared using polymer B506600, HO(C2H4O)39(C4H8O)47(C2H4O)39H [20]. It is believed [12] that the materials with spherical pores are much better than 2D OMSs [1,2] with cylindrical pores because of their highly interconnected pore structure and resistance to the pore clogging that promote fast flow of reactants into and out of the nanoscale pores. For those reasons 3D cage-like materials such as SBA-16 and FDU1 are promising candidates in applications, where selective adsorption, fast diffusion and tuned transport are key issues. The field of ordered nanoporous materials was further extended by the discovery of periodic mesoporous organosilicas (PMOs) [21– 23], which are synthesized by hydrolysis and condensation of bridged organosilanes in the presence of various templates. In particular, the research on PMOs has been mostly focused on the synthesis of channel-like structures such as MCM-41 [21–23] and SBA15 [24–30] with integrated aliphatic [24–27] and aromatic [28–32] bridging groups. Some efforts were made to introduce larger bridging groups into channel-like structures (see reviews and references therein [33–35]). Some progress has also been achieved in the syntheses of PMOs with cubic symmetries but only those with ethane spacers [36–39]. As regards to larger bridging groups, our recent communication [40] reported the co-condensation synthesis of cage-like PMOs containing bulky heterocyclic rings (see Scheme 1) from tris[3-(trimethoxysilyl)propyl]isocyanurate (ICS), which was co-condensed with tetraethyl orthosilicate (TEOS). However, in the case of cage-like PMOs a simple extraction, which was shown to be successful for hexagonally ordered materials, was insufficient for a complete removal of the polymeric template [10,40,41]. It is noteworthy that currently there are several ways to remove the template from the porous materials using extraction [17], supercritical fluid extraction [42], calcination [1,2], microwave irradiation [43], photocalcination [44] and a two-step process involving a short extraction followed by calcination in air at lower temperatures [10,41,45]. The latter procedure [10] was demonstrated to be very efficient for a complete removal of the template residue from pure cage-like silicas to achieve a desired porous structure without apparent pore shrinkage. Therefore, the current work addresses the template removal from the cage-like PMOs. For these materials the most of the afore-

O Si

N O

Si

N N

mentioned methods of the template removal is not applicable because of the decomposition of organic groups. This work shows that the use of a short extraction and a temperature-controlled heating at 315 °C under nitrogen instead of air is effective for the template removal from large pore cage-like OMOs containing bulky heterocyclic bridging groups. These materials were synthesized using tris[3-(trimethoxysilyl)propyl]isocyanurate and tetraethyl orthosilicate under low acidic condition; with addition of sodium chloride and the presence of F127 triblock copolymer template. In addition, the mesostructural ordering and adsorption properties of these OMOs were studied after a complete removal of isocyanurate spacers at 550 °C in flowing air. Furthermore, synthesis, characterization and template removal of bifunctional PMOs containing exclusively isocyanurate and ethane bridging groups are presented. These materials were characterized by various techniques, including nitrogen adsorption, small-angle X-ray scattering (SAXS)/X-ray powder diffraction (XRD), high-resolution thermogravimetric analysis (TGA), elemental analysis and Fourier-transform infrared (FT-IR) spectroscopy. 2. Materials and methods 2.1. Chemicals Structure directing agent poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) triblock copolymer Pluronic F127 (EO106PO70EO106) was provided by BASF Corporation. Bridged silsesquioxane precursors, tris[3-(trimethoxysilyl)propyl]isocyanurate (ICS) and 1,2-bis(triethoxysilyl)ethane (BTESE) were purchased from Aldrich and Gelest, respectively. Concentrated sulphuric acid (97%), fuming hydrochloric acid (37%) and ethanol (95%) were obtained from Fischer Scientific. Deionized water (DW) was obtained by using in-house Ionpure Plus 150 Service Deionization ion-exchange purification system. All reagents were used as received without further purification. 2.2. Synthesis of isocyanurate-containing OMOs The synthesis of SBA-16-type OMOs with ICS bridging groups (see Scheme 1) was analogous as in Ref. [36] (for details see [10,40]); 2 g of polymer F127 and 7.05 g NaCl were dissolved in 20 ml of 2 M HCl and of 60 ml DW to form a clear F127–NaCl– HCl–DW mixture. About 34.9 mmol and 32.2 mmol of TEOS and ICS were added to the polymer-containing solution under vigorous stirring to synthesize samples Ia and Ib, respectively (see Table 1). The mixture was further stirred for 20 h at 40 °C and subjected for a hydrothermal treatment for 24 h at 100 °C. The product was

O

Si

Scheme 1. Schematic illustration of the cage-like OMO with bulky heterocyclic bridging groups (left image) and bifunctional PMO containing isocyanurate and ethane bridging groups (right image). Dark triangles and black squares represent isocyanurate and ethane bridging groups incorporated into the silica framework, respectively.

Table 1 Synthesis gel composition, elemental analysis and TG weight loss for the isocyanurate-containing OMOsa Sample

XICS

nt (mmol)

N (%)

CICS (mmol/g)

TG (%)

Ia–e Ia–e315N Ia–315A Ia–e550A Ib–e Ib–e315N Ib–315A Ib–e550A

0.08 0.08 0.08 0.08 0.12 0.12 0.12 0.12

34.9 34.9 34.9 34.9 32.2 32.2 32.2 32.2

3.20 2.83 1.45 0.60 4.53 4.09 1.83 0.92

0.76 0.67 0.33 0.14 1.08 0.97 0.44 0.22

24.1 18.8 12.6 1.2 28.8 24.0 14.1 0.1

a XICS, mole fraction of ICS in the synthesis gel mixture; nt, total number of mmoles of ICS and TEOS in the synthesis gel; N, nitrogen percentage obtained on the basis of elemental analysis; CICS, surface concentration of isocyanurate bridging groups calculated on the basis of N% obtained from elemental analysis; and TG, thermogravimetric weight loss recorded in flowing nitrogen in the range 100 and 800 °C.

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filtered, washed with DW and dried at 70 °C in the oven. The template-containing samples were extracted twice with 20 ml of H2SO4 and 100 ml of 95% EtOH at 70 °C followed by calcination at 315 °C in nitrogen to remove the remaining polymer. Also, these samples were calcined at 550 °C in air for 4 h to fully remove organics. The isocyanurate-containing SBA-16 samples are denoted as Ia–eTN (or Ia–eTA) and Ib–eTN (or Ib–eTA), where a and b refer to the samples having the mole fraction of ICS of 0.077 and 0.125, respectively (see Table 1); e refers to the extracted samples, T denotes the calcination temperature in flowing nitrogen (N) or air (A). Samples Ia–F127 and Ib–F127 are designated to the polymer-containing ICS–OMOs. The lack of e symbol refers to the samples subjected to the calcination only without prior extraction. In general, all isocyanurate-containing samples are referred as ICS–OMOs. 2.3. Synthesis of bifunctional PMOs with ethane and isocyanurate groups The preparation of bifunctional PMOs containing isocyanurate and ethane bridging groups (see Scheme 1) was performed analogously to the procedure reported by Qiu et al. [36]. In a typical synthesis, 10.5 ml of DW, 4.5 ml of 2 M HCl was mixed with 0.75 g of polymer F127 and 1.764 g of NaCl and stirred at 40 °C for 2 h to form a clear solution. A specified volume of BTESE was transferred to a solution containing 12.5 ml of DW and 30 ml of 2 M HCl at 40 °C. The P123–DW–HCl–NaCl solution was then slowly added to the BTESE–DW–HCl mixture under vigorous stirring and then after 15 min ICS was pipetted to achieve the desired molar composition of both silanes. After further stirring for 20 h at 40 °C, the resulting slurry was subsequently heated at 100 °C for 24 h. The product was filtered, washed with DW, and dried in the oven at 80 °C and extracted. The extracted ethane–isocyanurate PMOs with various loadings of isocyanurate groups (see Table 3) are designated as E–e, EIa–e, EIb–e and EIc–e (where E and EIx refer to ethane and ethane–isocyanurate PMOs with different concentrations of ICS denoted by x = a, b, c and e – refer to the extracted samples), whereas extracted and heated materials at 315 °C in nitrogen are denoted as E–e315N, EIa–e315N, EIb–e315N and EIc–e315N. Samples EIx–F127 refer to the polymer-containing ethane–isocyanurate-containing PMOs. In contrast to ICS–OMOs, all ethane– isocyanurate-containing PMOs are referred as E–ICS–PMOs.

Nitrogen and carbon contents for all organosilicas were obtained by a LECO model CHNS-932 elemental analyzer from St. Joseph, MI. 2.5. Calculations The BET specific surface area was calculated in the relative pressure range of 0.05–0.2 by using the Brunauer–Emmett–Teller (BET) method [46]. The total pore volume [47] was evaluated from the amount adsorbed at a relative pressure p/po of 0.99, where p and po denote the equilibrium pressure and saturation vapor pressure, respectively. The volume of complementary pores [47] for the samples studied was obtained by integration of the pore size distributions (PSDs) up to 4 nm. It is noteworthy that the volume of the complementary pores includes the volume of disordered fine pores and cage apertures, the size of which is below the diameter of the primary mesopores. The Kruk–Jaroniec–Sayari (KJS) method [48], which is based on the Barrett–Joyner–Halenda (BHJ) algorithm [49] for cylindrical pores, was used to determine the pore size distributions (PSDs). For spherical pore geometry this method underestimates the size of primary mesopores by about 2–3 nm [15]. Therefore, the KJS was mainly used to get the shape of PSD and the volume of complementary pores. The diameter of the primary mesopores was also calculated by employing a geometrical relation (Eq. (1)) for the Im3m symmetry group that uses the volumes of ordered and complementary pores obtained from nitrogen adsorption isotherms and the unit cell from SAXS/XRD data [50]. The density for all materials studied was assumed to be 2.0 g/ cm3, which is 10% smaller then the silica density to reflect the density reduction after introduction of organic groups.

 wd ¼ 0:985 a

Vo ðV c þ V o þ 1=qÞ

1=3 ð1Þ

where a is the unit cell parameter, Vo is the volume of ordered pores, Vc is the volume of complementary pores and q is the sample true density. The unit cell parameter for the Im3m symmetry group was evaluated using the interplanar spacing d of the (1 1 0) XRD peak:

a¼

p

2 d1 1 0

2.4. Measurements

3. Results and discussion

Nitrogen adsorption isotherms were measured at 196 °C using 2010 volumetric adsorption analyzers manufactured by Micromeritics, Inc. Prior to measurements each PMO sample was degassed under vacuum at 110 °C for 2 h. High resolution thermogravimetric measurements were conducted in flowing nitrogen using a TA Instruments TGA 2950 analyzer with 5 °C min1 maximum heating rate. Small angle X-ray scattering (SAXS) measurements were conducted using the NanoStar system (Bruker AXS), whereas powder X-ray diffraction (XRD) measurements were recorded using a PANanalytical, Inc. X’Pert Pro multi purpose diffractometer (MPD) with CuKa radiation. FT-IR spectra were collected using a Digilab FTS-3000 spectrometer equipped with a liquid nitrogen cooled mercury–cadmium–telluride detector, operating at 4 cm1 nominal resolution by co-addition of 64 scans. Diffuse reflectance spectra were obtained using an optical accessory from Harrick Scientific (Ossining, NY, DRA-2CN). Samples were prepared by mixing a 10% (w/w) dispersion of modified silica in dried, pre-ground KCl after adjustment of the sample height to obtain the maximum interferogram signal. All spectra were ratioed to that of pure KCl and converted to the Kubelka–Munk (KM) units using standard instrument software.

3.1. SBA-16 with isocyanurate bridging groups

ð2Þ

3.1.1. Small-angle X-ray scattering The structure of two series of isocyanurate-containing OMOs was determined using small-angle X-ray scattering (SAXS) data shown in Figs. 1A and 2A. The unit cell values are summarized in Table 2. As can be seen from Figs. 1A and 2A, the SAXS curves for the extracted isocyanurate-containing OMOs (Ia–e and Ib–e) feature one major narrow peak at the scattering vector (q) around 0.52 and four minor peaks indexed as (1 1 0), (2 0 0), (2 1 1), (3 1 0) and (3 2 1) according to the body-centered cubic Im3m symmetry analogous to SBA-16 reported earlier by Sakamoto et al. [7]. An additional calcination of partially extracted and as-made samples at 315 °C in flowing nitrogen (Ia–e315N and Ibe–315N) and air (Ia–315A and Ib–315A) did not lead to any significant deterioration of the SAXS patterns. The unit cell parameter (see Table 2) showed a tendency to decrease with increasing calcination temperature and reached the smallest value for the sample calcined at 550 °C (Ia–e550A), which is attributed to a substantial structural shrinkage, in this case caused also by the degradation of bridging groups. It is noteworthy that the structure was preserved even after a complete removal of isocyanurate bridging rings, which was confirmed

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R.M. Grudzien et al. / Microporous and Mesoporous Materials 118 (2009) 68–77

A

A

(200)

(200)

(110)

(110)

(211) (310) (321)

(211)

0.8

1.2

Intensity

Intensity

(310) (321)

1.6 Ia-e550A Ia-315A

0.8

1.2

1.6 Ib-e550A Ib-315A

Ia-e315N Ib-e315N

Ia-e 0.5

1.0

1.5

2.0

Ib-e 0.5

2.5

1.0

2.5

B

600

Ia-e

500

Ia-e315N

400

Ia-315A Ia-e550A

300

Ib-e

400

Amount Adsorbed (cc STP g-1)

Amount Adsorbed (cc STP g-1)

2.0

q (nm-1)

q(nm-1)

B

1.5

200

Ib-e315N 300

Ib-315A 200

Ib-e550A

100

100 0 0.0

0.2

0.4

0.6

0.8

0 0.0

1.0

0.2

Relative Pressure

C

C

0.4

0.6

0.8

Relative Pressure

1.0

0.4

0.3

Ia-e 0.2

Ia-e315N

Ia-e550A

0.0 5

10

15

0.3

Ib-e 0.2

Ib-e315N 0.1

Ia-315A

0.1

PSD (cc g-1 nm-1)

PSD (cc g-1 nm-1)

0.4

Ib-315A Ib-e550A

0.0

20

Pore Diameter (nm) Fig. 1. Small-angle X-ray diffraction (SAXS) patterns (A), nitrogen adsorption isotherms (B) measured at 196 °C and the corresponding pore size distributions (PSDs) (C) calculated according to the KJS method [48] for the cage-like isocyanurate-containing ordered mesoporous organosilicas: extracted (Ia–e), extracted and heated at 315 °C in nitrogen (Ia–e315N) and extracted and calcined at 550 °C in air (Ia–e550A) and calcined nanocomposite at 315 °C in air (Ia–315A). The isotherms and the PSDs for Ia–315A, Ia–e315N, Ia–e were offset by 80, 95, 180 cc STP g1 and by 0.09, 0.15, 0.23 cc g1 nm1, respectively. The inset in panel A shows enlargement of minor peaks for the Ia–e sample.

5

10

15

20

Pore Diameter (nm) Fig. 2. Small-angle X-ray diffraction (SAXS) patterns (A), nitrogen adsorption isotherms (B) measured at 196 °C and the corresponding pore size distributions (PSDs) (C) calculated according to the KJS method [48] for the cage-like isocyanurate-containing ordered mesoporous organosilicas: extracted (Ib–e), extracted and heated at 315 °C in nitrogen (Ib–e315N) and extracted and calcined at 550 °C in air (Ib–e550A) and calcined nanocomposite at 315 °C in air (Ib–315A). The isotherm for Ib–e was shifted by 100 cc STP g1, whereas the PSDs for Ib–315A, Ib–e315N and Ib–e were offset by 0.08, 0.13 and 0.21 cc g1 nm1, respectively. The inset in panel A shows enlargement of minor peaks for the Ib–e sample.

R.M. Grudzien et al. / Microporous and Mesoporous Materials 118 (2009) 68–77

Sample

SBET (m2/g)

Vt (cc/g)

Vc (cc/g)

w (nm)

wd (nm)

a (nm)

Ia–e Ia–e315N Ia–315A Ia–e550A Ib–e Ib–e315N Ib–315A Ib–e550A

881 891 734 714 736 785 600 462

0.56 0.53 0.42 0.47 0.44 0.45 0.32 0.27

0.33 0.34 0.25 0.24 0.29 0.29 0.19 0.16

7.2 7.1 6.2 5.4 6.6 6.4 5.6 6.2

10.2 9.1 7.8 8.7 9.0 8.5 6.9 6.4

17.2 16.2 15.0 14.2 16.8 15.7 14.9 13.2

a SBET, specific surface area [46]; Vt, single-point pore volume [47]; Vc, volume of micropores and interconnecting pores of the diameter below 4 nm; w, mesopore cage diameter calculated using the KJS method [48]; wd, pore diameter calculated on the basis of XRD/SAXS data and the pore volume for the cubic Im3m symmetry using Eq. (1) [50]; a, unit cell parameter calculated from the observed characteristic Bragg’s reflection (1 1 0) using Eq. (2). Some data of Ia–e, Ia–e315N, Ib–e and Ib– e315N samples were published in our previous work [40].

by the SAXS profiles shown in Figs. 1A and 2A and nitrogen adsorption isotherms. 3.1.2. Nitrogen adsorption Adsorption isotherms were measured at 196 °C for a series of the extracted and calcined ICS–OMSs with two different isocyanurate mole fractions; 0.08 (Ia) and 0.12 (Ib), see Figs. 1B and 2B, respectively. Adsorption parameters such as the specific surface area, total pore volume, volume of complementary pores and pore diameter are summarized in Table 2. These nitrogen adsorption isotherms were also used to evaluate the Kruk–Jaroniec–Sayari (KJS) pore size distributions (PSDs) displayed in Figs. 1C and 2C, respectively. As can be seen from Figs. 1C and 2C, PSDs are typical for the SBA-16 materials [2] exhibiting two narrow peaks; the first intense peak in the range below 4 nm and the second in the range of 4–10 nm. These peaks are associated with the presence of small complementary pores and primary ordered mesopores, respectively. Since the triblock copolymer F127 contains 106 hydrophilic polyethylene oxide (PEO) moieties and 70 polypropylene oxide (PPO) hydrophobic groups, the resulting cage-like mesoporous materials usually exhibit a higher ratio of micropores in contrast to primary mesopores due to the penetration of PEO blocks into the silica walls as well as due to the presence of apertures. Since KJS method [48] was based on the BHJ algorithm elaborated for cylindrical pores [49], the spherical pore diameters are underestimated by about 2–3 nm. Therefore, the primary mesopore diameters were also calculated using Eq. (1), which was derived for the Im3m symmetry group, and relates the unit cell parameter obtained from the SAXS/XRD data with the volume of primary pores and total pore volume estimated from nitrogen adsorption isotherms [50]. The extracted ICS–OMSs samples (Ia–e and Ib–e) exhibit type IV isotherms with steep capillary condensation/evaporation steps and apparent hysteresis loops that are characteristic for high quality cage-like materials with uniform pore entrances. As can be seen from Figs. 1B and 2B, there is only a small difference between adsorption isotherms for the extracted (Ia-e and Ib–e) and extracted-calcined (Ia–e315N and Ib–e315N) ICS–OMOs because of the structural shrinkage due to the sample calcination at 315 °C. As can be noticed from Table 2, the extracted-calcined Ia–e315N and Ib–e315N materials exhibited a slight increase in the specific surface area by 10 and 49 m2 g1 and a small decrease in the KJS pore widths (w) by 0.1 and 0.2 nm, respectively. On the contrary, nitrogen adsorption isotherms for the calcined as-synthesized samples (Ia–F127 and Ib–F127) at 315 °C in flowing air (Ia–315A and Ib–315A) show a slight broadening and a shift of the capillary condensation steps to the lower values of relative pressure. This indicates a reduction in the pore diameters, which

is also visible on the PSD curves shown in Figs. 1C and 2C, where maxima of the primary pores are shifted to the smaller pore sizes; 6.2 and 5.6 nm for Ia–315A and Ib–315A, respectively. In comparison to the extracted Ia–e and Ib–e samples (see Table 2), the specific surface areas for Ia–315A and Ib–315A were reduced by 147 and 136 m2 g1, the volume of complementary pores changed by 0.08 and 0.1 cc/g, whereas the total pore volume decreased by 0.14 and 0.12 cc/g, respectively. In order to determine the stability of isocyanurate–silica mesostructures, the extracted samples (Ia–e and Ib–e) were subjected to the calcination at 550 °C in air to completely remove the organic moieties. The nitrogen adsorption isotherms for Ia–e550A and Ib–e550A still exhibit a typical type IV shape with hysteresis loop characteristic for mesoporous materials, suggesting that even after total degradation of isocyanurate bridging groups in the framework the mesoporous structure did not lose its ordering. As can be seen from Table 1, the specific surface area was reduced by 167 and 274 m2 g1, the volumes of complementary pores changed by about 0.09 and 0.13 cc/g, whereas the total pore volumes decreased by 0.09 and 0.17 cc/g for Ia–e550A and Ib–e550A, respectively. 3.1.3. Thermogravimetry and elemental analysis Removal of the polymeric template (Pluronic F127) from ICS– PMOs was also monitored by high-resolution thermogravimetry

A

100 Ia-e550A

Weight change (%)

Table 2 Adsorption and structural parameters for the isocyanurate-containing OMOsa

90

Ia-315A

80

Ia-e315N

70

Ia-e

60

Ia-F127

200

400

600

800

Temperature (oC)

B -Deriv.Weight (% / oC)

72

0.4

F 1 27 ICS

0.3 Ia-e Ia-e315N Ia-315A Ia-e550A Ia-F127

0.2

ICS

0.1

0.0 150

225

300

375

450

525

600

675

Temperature (oC) Fig. 3. (A) Thermogravimetric weight change (TG) profiles measured in flowing nitrogen for isocyanurate-containing OMOs: as-synthesized (Ia–F127), extracted (Ia–e), extracted and heated at 315 °C in nitrogen (Ia–e315N) and extracted and calcined at 550 °C in air (Ia–e550A) and calcined sample at 315 °C in air (Ia–315A) and (B) the corresponding differential thermogravimetric (DTG) patterns.

R.M. Grudzien et al. / Microporous and Mesoporous Materials 118 (2009) 68–77

(TG) and elemental analysis. The TG curves recorded in flowing nitrogen are shown in Figs. 3A and 4A, whereas the corresponding differential (DTG) curves are shown in Figs. 3B and 4B for Ia and Ib materials, respectively. The weight loss values and the ICS coverage densities are listed in Table 1. As can be seen from Figs. 3B and 4B, the TG profiles of the as-made material (Ia–F127 and Ib–F127) represent two major decomposition steps; the first step occurs within 300–400 °C and the second step between 400 and 650 °C, which are more clearly illustrated using the weight loss derivative as shown in Figs. 3B and 4B. The DTG curves display two distinct peaks located at around 375 and 480 °C, which are attributed to the thermodesorption/degradation of triblock copolymer (F127) template and the decomposition of isocyanurate bridging groups, respectively. The TG weight losses for as-synthesized materials with various loadings of the ICS groups (Ia–F127 and Ib–F127) were 41.85% and 46.23%, whereas after partial removal of a polymer template due to extraction (Ia–e and Ib–e) these losses were reduced to 24.1% and 28.8%, respectively (see Table 1). In contrast, the ICS contents evaluated on the basis of nitrogen percentages obtained by elemental analysis for Ia–e and Ib–e were 0.76 and 1.08 mmol/g, respectively. In addition, after heating in nitrogen at 315 °C, the weight losses were further reduced to 18.8% and 24%, which correspond to the ICS contents of 0.67 and 0.97 mmol/g, respectively. This suggests that extraction removed only a part of the polymeric template

100

Weight change (%)

A

90 Ib-e315N Ib-315N

80

70 Ib-e

60

Ib-F127

200

400

600

800

Temperature (oC)

- Deriv.Weight (% / oC)

B

0.4

F127 Ib-e Ib-e315N Ib-315N Ib-F127

0.3

ICS

0.2

ICS

0.1

0.0 150

225

300

375

450

525

Temperature (oC)

600

675

Fig. 4. (A) Thermogravimetric weight change (TG) profiles measured in flowing nitrogen for isocyanurate-containing OMOs: as-synthesized (Ib–F127), extracted (Ib–e), extracted and heated at 315 °C in nitrogen (Ib–e315N) and heated at 315 °C in nitrogen Ib–315N) and (B) the corresponding differential thermogravimetric (DTG) patterns.

73

and additional calcination at 315 °C in nitrogen removed the remaining residue. As can be seen from Figs. 3B and 4B, there are still small peaks for the extracted samples (Ia–e and Ib–e) arising from the residual amount of template, whereas these peaks are not present for the samples after additional heating at 315 °C in nitrogen (Ia–e315N and Ib–e315N) indicating a complete removal of Pluronic F127 without elimination of the isocyanurate groups. In contrast, the samples calcined in air without prior extraction (Ia–315A and Ib–315A) exhibited the TG weight losses of 12.6% and 14.1% as well as the ICS contents of 0.33 and 0.44 mmol/g, which reflect a dramatic reduction of about 57% and 59% of the ICS groups, respectively. This indicates that a direct calcination of assynthesized materials removes not only the polymer template but also a major part of isocyanurate functionality. This can be seen from the DTG curve (Fig. 3B), which is broad and significantly lessintensive compared to the DTG curve of the Ia–e315N sample. An additional calcination in flowing nitrogen of the extracted cage-like PMOs was possible because the aforementioned thermogravimetric events are well separated. Calcination of the extracted samples at 550 °C in air caused not only a complete removal of the F127 polymer template as reflected by the lack of characteristic peak at 375 °C in the DTG pattern but also a major decomposition of the ICS groups (Fig. 3B). Elemental analysis indicated the reduction of nitrogen contents to 0.14 and 0.22 mmol/g attributed to the removal of about 82% and 80% of ICS from the Ia–e550A and Ib–e550A mesostructures, respectively. It is noteworthy that under neutral atmosphere (e.g., nitrogen) thermal desorption of the polymeric template occurred between 300 and 400 °C for the system studied, whereas the maximum rate of ICS decomposition was at much higher temperature (480 °C) because of its covalent bonding in the framework. This is not the case for the thermal treatment of the polymer-containing samples in air, where both thermal events overlap due to the oxidative degradation of the polymeric template and ICS groups. Therefore, the extracted samples were thermally treated in nitrogen in order to completely remove the polymeric template. The proposed procedure of the template removal (i.e., extraction and controlled heating in nitrogen) can be used for cage-like mesoporous organosilicas with other organic groups, which decompose in nitrogen at temperatures much higher than that of the template removal. 3.1.4. Fourier-transform infrared spectroscopy The introduction of isocyanurate bridging groups into the SBA16 mesostructures and its preservation during the thermal removal procedure of triblock copolymer was monitored by FT-IR spectroscopy in the range of 4000 and 550 cm1. Fig. 5 shows a comparison of the IR spectra for two materials containing the same loadings of isocyanurate rings; extracted (Ia–e) and extractedheated sample at 315 °C in nitrogen (Ia–e315N). As can be seen, the IR spectrum for the Ia–e material exhibits a very broad shoulder between 3200 and 3500 cm1 that is associated with hydrogen-bonded OH stretching vibrations originating from surface hydroxyl groups and adsorbed water. In addition, the spectrum shows two typical bands of asymmetric and symmetric stretching vibrations at 1110 and 790 cm1, which are attributed to the formation of siloxane bond, i.e., condensed silica network (–Si–O– Si–). The asymmetric stretching vibrations of non-condensed silica (–SiOH) are present at 950 cm1. Small bands at around 2900 cm1 are related to the symmetric and asymmetric alkyl stretching vibrations originating from the remaining residue of the polymeric template and isocyanurate groups, whereas the band at 1450 cm1 is characteristic of bending vibrations of alkyl (CH) bond. The presence of isocyanurate heterocyclic ring incorporated to the silica framework can be further confirmed by the presence of carbonyl (C@O) stretching vibrations at 1680 cm1. The IR spectrum of the

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R.M. Grudzien et al. / Microporous and Mesoporous Materials 118 (2009) 68–77

5

Ia-e Ia-e315N

A 6

3

0.2

4

1 2

Intensity

KM units

0.3

0.1

E-e EIa-e

0.0 4000

EIb-e 3500

3000

2500

2000

1500

1000

Wavenumber (cm-1)

extracted-heated sample (Ia–e315N) exhibits all characteristic peaks for the extracted isocyanurate-containing mesoporous silica (Ia–e). However, these peaks are less intense especially in the region of 3200 and 3500 cm1 due to the thermodesorption of molecularly adsorbed water at elevated temperature. This finding suggests that isocyanurate bridging groups remained intact after polymer removal during calcination at 315 °C in nitrogen.

EIc-e 0.5

1.0

1.5

2.0

2.5

3.0

3.5

2θ (o)

B Amount Adsorbed (cc STP g-1)

Fig. 5. A comparison of infrared spectra of the ICS–OMOs with the identical concentration of isocyanurate bridging groups; extracted (Ia–e) as well as extracted and heated samples at 315 °C in flowing nitrogen (Ia–e315N). 1 – Corresponds to hydrogen-bonded OH stretching bands from silanols and surface adsorbed water; 2 – CH stretching vibrations in the 2900 cm1 region; 3 – carbonyl (C@O) stretching at 1680 cm1; 4 – CH bending modes around 1450 cm1, 5 – silica network Si–O–Si asymmetric stretching as well as 6 – an Si–OH band.

600

E-e EIa-e

400

EIb-e EIc-e 200

3.2. Bifunctional SBA-16 with ethane and isocyanurate bridging groups

0

0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure

C

0.5

0.4

PSD (cc g-1nm-1)

3.2.1. X-ray diffraction The structures of extracted as well as extracted and calcined ICS–E–PMOs materials were investigated using XRD as shown in Figs. 6A and 7A, respectively. The unit cell parameters obtained for the first Bragg’s reflection are listed in Table 4. The XRD patterns for PMOs containing only ethane bridging groups (E–e and E–e315N) show one intensive reflection and three minor reflections indexed as (1 1 0), (2 0 0), (2 1 1) and (3 2 1) assigned to the Im3m symmetry similar to ICS–OMOs. It is noteworthy that Qiu et al. [36] assigned the structure of the ethane-bridge PMO as face-centered cubic (Fm3m), however Qiu and co-workers carried out the synthesis of in the presence of potassium chloride (KCl) and herein sodium chloride was used. As reported already by others [51–54], various inorganic salts can induce formation of different structures. For instance, Fan et al. [51] showed that the use of KCl in the presence of the F127 template led to the development of high-purity Fm3m phase, which is relatively difficult to synthesize. In contrast, the use of NaI facilitates the formation of Ia3d phase [53]. In general, inorganic salts can be used to control the morphology structure and adsorption properties such as the micropore volume, the volume of primary pores, pore size and surface area. Nonetheless, based on the peak positions for the E–e and E– e315N samples, it was very difficult to assign the symmetry group as Fm3m. Introduction of isocyanurate rings into ethane–silica samples (extracted EIa–e, EIb–e and extracted-heated EIa–e315N, EUb–e315N) led to a gradual disappearance of (2 0 0) and (2 1 1) peaks, however the structure was not changed; thus the peaks for bifunctional E–ICS–PMOs were also assigned according to the Im3m symmetry group. As regards to the samples with the highest ICS concentration (EIc–e and EIc–e315N) only a broad shoulder was observed suggesting high structural deterioration.

0.3

E-e 0.2

EIa-e

0.1

EIb-e EIc-e

0.0 0

2

4

6

8

10

12

14

16

18

Pore Diameter (nm) Fig. 6. Powder X-ray diffraction (XRD) patterns (A), nitrogen adsorption isotherms (B) measured at 196 °C and the corresponding pore size distributions (PSDs) (C) calculated according to the KJS method [48] for extracted bifunctional cage-like mesoporous organosilicas containing isocyanurate and ethane bridging groups. The isotherm for EIa–e was offset by 40 cc STP g1, whereas the PSDs for E–e, EIa–e and EIb–e were offset by 0.07, 0.09 and 0.24 cc g1 nm1, respectively.

3.2.2. Nitrogen adsorption A comparison of nitrogen adsorption isotherms measured at 196 °C for extracted as well as extracted–calcined samples are

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R.M. Grudzien et al. / Microporous and Mesoporous Materials 118 (2009) 68–77

Table 3 Synthesis gel composition, elemental analysis and TG weight loss for the isocyanurate-containing ethane–PMOsa

Intensity

A

E-e315N EIa-e315N EIb-e315N EIc-e315N 0.5

1.0

1.5

2.0

2.5

3.0

Sample

XICS

nt (mmol)

N (%)

CICS (mmol/g)

TG (%)

E–e E–e315N EIa–e EIa–e315N EIb–e EIb–e315N EIc–e EIc–e315N

0 0 0.03 0.03 0.10 0.10 0.22 0.22

8.04 8.04 7.90 7.90 7.63 7.63 7.24 7.24

0 0 1.93 1.75 2.20 2.31 3.35 3.88

0 0 0.46 0.42 0.52 0.55 0.67 0.92

9.26 9.10 15.14 11.83 18.71 16.16 36.69 26.21

a XICS, mole fraction of ICS in the synthesis gel mixture; nt, total number of mmoles of ICS and BTESE in the synthesis gel; N, nitrogen percentage obtained on the basis of elemental analysis; CICS, surface concentration of isocyanurate bridging groups calculated on the basis of N% obtained from elemental analysis; and TG, thermogravimetric weight loss recorded in flowing nitrogen in the range 100 and 850 °C.

2θ (o)

Amount Adsorbed (cc STP g-1)

B 600

E-e315N EIa-e315N 400

EIb-e315N

200

EIc-e315N

0 0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure

C

silica samples (E–e and E–e315N) exhibited a type IV isotherm with pronounced hysteresis loop and quite apparent capillary condensation/evaporation steps, which indicates the formation of high quality cage-like mesoporous materials. Introduction of the ICS rings into the ethane–silica structure led to a slight change in the isotherm shape indicating the similarity of both mesoporous structures. However, as can be seen from the Table 3 for the extracted–calcined samples, a gradual addition of ICS influenced the BET specific surface area and the volume of complementary pores, which decreased progressively from 1012 to 462 m2 g1 and from 0.42 to 0.12 cc/g, respectively. It is noteworthy that at high relative pressures close to unity, the shape of nitrogen adsorption isotherms for bifunctional ICS–E materials, especially for the EIc–e sample, exhibited a significant tailing. This behavior can be attributed to increase of the textural porosity. Unlike pendant groups, which are attached to the pore surface, isocyanurate bridging groups are in the framework. Since the bridging groups are embedded in the framework, a gradually increased concentration does not necessarily alter the pore diameter (see Fig. 6C). Thus, the pore sizes of the E–ICS materials are similar and the estimated pore diameters are between 6.7 and 9.1 nm.

PSD (cc g-1 nm-1)

0.5

0.4

0.3

E-e315N 0.2

EIa-e315N

0.1

EIb-e315N EIc-e315N

0.0 0

2

4

6

8

10

12

14

16

18

Pore Diameter (nm) Fig. 7. Powder X-ray diffraction (XRD) patterns (A), nitrogen adsorption isotherms (B) measured at 196 °C and the corresponding pore size distributions (PSDs) (C) calculated according to the KJS method [48] for the bifunctional cage-like mesoporous organosilicas containing isocyanurate and ethane bridging groups, which were extracted and heated at 315 °C in flowing nitrogen. The isotherms for EIa–e315N and E–e315N were offset by 50 and 100 cc STP g1, whereas the PSDs for EIb–e315N, EIa–e315N and E–e315N were offset by 0.06, 0.17 and 0.24 cc g1 nm1, respectively.

displayed in Figs. 6B and 7B, respectively, whereas the corresponding adsorption parameters are listed in Table 4. The pure ethane–

3.2.3. Thermogravimetry Fig. 8 displays four panels for the ethane PMO samples with progressively increasing concentration of isocyanurate bridging groups, whereas Fig. 9 represents the corresponding DTG curves. Thermogravimetric weight losses recorded under nitrogen are listed in Table 3. Each panel shows patterns for composite (EF127, EIa-F127, EIb-F127, EIc-F127), extracted (E–e, EIa–e, EIb–e, EIc–e) as well as extracted and heated E–ICS–PMO samples at

Table 4 Adsorption and structural parameters for the isocyanurate-containing ethane–PMOsa Sample

SBET (m2/g)

Vt (cc/g)

Vc (cc/g)

w (nm)

wd (nm)

a (nm)

E–e E–e315N EIa–e EIa–e315N EIb–e EIa–e315N EIc–e EIa–e315N

1008 1012 912 868 826 829 531 462

0.68 0.71 0.75 0.77 0.63 0.63 1.15 0.8

0.43 0.43 0.38 0.37 0.33 0.33 0.15 0.12

5.9 5.8 6.1 5.6 6.5 6.4 5.6 4.9

8.3 7.5 8.5 7.5 9.1 8.2 7.8 6.7

15.8 14.0 16.1 13.9 17.3 15.6 14.8 13.1

a SBET, specific surface area [46]; Vt, single-point pore volume [47]; Vc, volume of micropores and interconnecting pores of the diameter below 4 nm; w, mesopore cage diameter calculated using the KJS method [48]; wd, pore diameter calculated on the basis of XRD/SAXS data and the pore volume for the cubic Im3m symmetry using Eq. (1) [50] assuming density of 2.0 g/cc; a, unit cell parameter calculated from the observed characteristic Bragg’s reflection (1 1 0) using Eq. (2).

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R.M. Grudzien et al. / Microporous and Mesoporous Materials 118 (2009) 68–77

B100

100 90 80 70 60 50 40

E-F127 E-e E-e315N 200 400 600 800 Temperature (oC)

Weight change (%)

C

EIb-e EIc-e

EIa-F127 EIa-e EIa-e315N 200 400 600 800 o Temperature ( C)

D100

100 90 80 70 60 50 40

90 80 70 60 50 40

EIb-F127 EIb-e EIb-e315N 200 400 600 800 o Temperature ( C)

- Deriv.Weight (% / C)

o

E-F127 E-e E-e315N

0.4

EIc-F127 EIc-e EIc-e315N 200 400 600 800 o Temperature ( C)

0.0 200

400

600

200

o

- Deriv.Weight (% / C)

o

0.4

0.3

400

600

Temperature (oC)

Temperature ( C)

C

EIa-F127 EIa-e EIa-e315N

0.3

0.1

0.0

D E I b-F127 E I b-e E I b-e315N

0.3

E I c-F127 E I c-e E I c-e315N

0.2 0.2 0.1

0.1

0.0

0.0 200 400 600 o Temperature ( C)

1

3 4

1

2

3000

2500

2000

1500

1000

Wavenumber (cm-1)

0.2 0.2

2

3500

B 0.6

6

0

Fig. 8. (A–D) Thermogravimetric weight change (TG) profiles measured in flowing nitrogen for bifunctional PMOs containing ethane and gradually increasing concentrations of isocyanurate bridging groups: as-synthesized (E–F127, EIa– F127, EIb–F127, EIc–F127), extracted (E–e, EIa–e, EIb–e, EIc–e) and extractedheated samples at 315 °C in nitrogen (E–e315N, EIa–e315N, EIb–e315N, EIc– e315N).

A

5

3

90 80 70 60 50 40

KM units

Weight change (%)

A

200 400 600 o Temperature ( C)

Fig. 9. (A–D) The corresponding differential thermogravimetric (DTG) patterns measured in flowing nitrogen for bifunctional PMOs containing ethane and gradually increasing concentrations of isocyanurate bridging groups: as-synthesized (E–F127, EIa–F127, EIb–F127, EIc–F127), extracted (E–e, EIa–e, EIb–e, EIc–e) and extracted-heated samples at 315 °C in nitrogen (E–e315N, EIa–e315N, EIb– e315N, EIc–e315N).

315 °C in flowing nitrogen (E–e315N, EIa–e315N, EIb–e315N, EIc– e315N). As can be seen from Figs. 8 and 9, the as-synthesized composites exhibit two events; one pronounced peak between 200 and 450 °C attributed to the decomposition of the F127 template and the second peak between 450 and 650 °C related to the degradation of both ethane and isocyanurate bridging groups, respectively. The peak related to the F127 template disappeared partially after

Fig. 10. A comparison of infrared spectra of the extracted mesoporous bifunctional ethanesilicas with two different concentrations of isocyanurate bridging groups. 1 – Corresponds to hydrogen-bonded OH stretching bands from silanols and surface adsorbed water, 2 – CH stretching vibrations in the 2900 cm1 region; 3 – carbonyl (C@O) stretching at 1680 cm1; 4 – CH bending modes around 1450 cm1, 5 – silica network Si–O–Si asymmetric stretching as well as 6 – an Si–OH band.

extraction, however additional heating at 315 °C in nitrogen led to its total disappearance. The preservation of isocyanurate–ethane bridging groups during heating in nitrogen is confirmed by the second event, which still appears on the TG profile suggesting a complete removal of the polymer without destruction of the organics. These results are in agreement with isocyanurate–silica samples discussed earlier. 3.2.4. Fourier-transform infrared spectroscopy and elemental analysis Fig. 10 shows a comparison of IR spectra for the solvent-extracted SBA-16 PMO samples containing ethane bridging groups with two different loadings of isocyanurate rings. These spectra display similar sets of vibrations, as ICS–OMOs shown in Fig. 5, related to the formation of non-condensed and condensed silica networks (550–1200 cm1 region) as well as to the hydrogen-bonded OH stretching vibrations originating from silanol and surface adsorbed (2900–3500 cm1 region). In comparison to ICS–OMOs, these E–ICS–PMOs contain less water, which is manifested by a relatively low-intensity band from the hydrogen-bonded stretching vibrations suggesting much more hydrophobic nature of these bifunctional materials due to the presence of ethane groups in the framework. In addition to the stretching and bending vibrations of CH bond in the 2900 cm1 region and at about 1450 cm1 as well as carbonyl stretching vibrations at 1680 cm1, there are also two other bands present at 1260 cm1 and 700 cm1. These new modes are attributed to the Si–C bond. The presence of the Si–C peaks provides an evidence of ethane attachment and lack of bond cleavage that could occur during extraction. Based on the elemental analysis data shown in Table 3, the nitrogen contents increase gradually for the extracted samples with increasing concentration of isocyanurate bridging groups indicating a successful incorporation of the ICS rings. 4. Conclusions In summary, highly ordered mesoporous cage-like SBA-16 (Im3m symmetry) organosilicas with incorporated bulky isocyanurate bridging groups were prepared by co-condensation of tris[3(trimethoxysilyl)propyl]isocyanurate and tetraethyl orthosilicate in the presence of sodium chloride under low pH using poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) triblock copolymer F127 as a structure directing agent. This study shows that the temperature-controlled heating at 315 °C in flow-

R.M. Grudzien et al. / Microporous and Mesoporous Materials 118 (2009) 68–77

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