Hydrothermal Transformation and Characterization of Porous Silica

Oct 16, 1997 - Huo, Q.; Margolese, D. I.; Ciesla, U.; Feng, P.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schuth, F.; Stucky, G. D. Nature 19...
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8460

J. Phys. Chem. B 1997, 101, 8460-8467

Hydrothermal Transformation and Characterization of Porous Silica Templated by Surfactants Xiaoyin Chen* Department of EnVironmental Science and Engineering, Fudan UniVersity, Shanghai 200433, China

Limin Huang and Quanzhi Li Department of Chemistry, Fudan UniVersity, Shanghai 200433, China ReceiVed: February 11, 1997; In Final Form: June 25, 1997X

The effect of hydrothermal conditions on the interactions between surfactant molecules and silicate species plays a key role in the structural transformations of silica precursors. In this work the structures of different siliceous products obtained at various synthesis temperatures have been characterized by powder X-ray diffraction (XRD), FT-IR spectroscopy, 27Al and 29Si magic angle spinning (MAS) NMR, N2 adsorption/ desorption measurements, and thermogravimetric analysis. The XRD results show that the removal of other soluble ions from synthesis mixture is a primary step for preparation of the mesostructure in the CTABsilicate-water synthesis system. Increasing the hydrothermal temperature may lead to the various structures of porous silica and change the behaviors of surfactants in the synthesis mixture. With the increase of hydrothermal temperature from 100 °C to above 165 °C or the prolonging of reaction time at a fixed temperature of 170 °C, the transformation sequence of the following structural phases is identified by XRD: hexagonal MCM-41 f lamellar M41S f ZSM-5. Comparison with the results from XRD, pore structure measurement, FT-IR, and NMR demonstrates that the intermediate product from the hexagonal mesostructure to microporous crystallite is the lamellar phase rather than the ill-defined hexagonal structure. DTG results indicate that the formation of different structures could result from the different surfactant templating molecules. The main contribution of this work lies in attempts to provide an effective route for controlling conditions of the formation of different mesoporous phases and to understand the cooperation of surfactant molecules and inorganic species.

Introduction The discovery of a new family of highly ordered mesoporous siliceous materials with uniform pore diameter in the 1.6-10.0 nm range, designated M41S,1,2 has greatly expanded the capabilities of heterogeneous catalysis. These mesoporous materials could also find application in membrane and separation technology and molecular engineering. The large apertures in such mesoporous silica can, for example, be modified by framework substitution to create highly selective catalysts,3,4 which can be used in the field of large molecule transformation in fine chemicals that microporous molecular sieves cannot be served. Up to now, extensive research efforts have been made in this area in the investigation of the synthesis,2,5-9 characterization of structure,2,10-13 formation mechanism,2,5,14-19 modification20-22 and potential application,3,4,23,24 and other aspects.6.25,26 The synthesis uses ordered micelles of surfactant molecules as a template to array inorganic species into regular mesostructures. After the removal of surfactants by calcination, the original spaces occupied by surfactant micelles form the channels of materials. Understanding of interactions between surfactants and inorganic species can be used to generalize the synthesis of other novel nonsiliceous materials. Four different mesostructures in the M41S, so far, have been reported,2,15 namely, hexagonal MCM-41, cubic MCM-48, lamellar M41S, and cubic octamer [(CTAB)SiO2.5]8. Among them, the hexagonal MCM41 is very useful as practical material because it has mesopores of uniform size and shape with one-dimensional channels of * Corresponding author. Fax +86-21-65493232; e-mail chenxy@ fudan.edu.cn. X Abstract published in AdVance ACS Abstracts, September 15, 1997.

S1089-5647(97)00533-6 CCC: $14.00

hexagonal array, high thermal and hydrothermal stability. And its easy modification by isomorphously substituting tetrahedral silicon atoms with other novel elements such as Al, Ti, V, B Ga, Co, and Cr at mild hydrothermal conditions makes the MCM41 have extensive application as catalysts for the processing of large molecules.3,4,23,27-30 Many influence factors, such as concentration,5,9,18 ratio of surfactant to inorganic species,15 alkalinity,6,9,20 chain length and type of surfactant,2,6,8,9 and temperature2,5,17 in the synthesis of MCM-41 have been investigated under conventional hydrothermal conditions. Few papers have dealt with the investigation at temperature above 150 °C to approach the interactions between surfactants and inorganic species.17 It is thus of interest to approach the effect of hydrothermal conditions on the structure of products and the interactions between surfactants and siliceous species, because the polymerization of siliceous species and the behavior of surfactant molecules at high temperature are different from those at room or low temperature. We here report the effect of hydrothermal temperature on the structural transformations of silicate species under a low surfactant/Si ratio and a low surfactant concentration in the cetyltrimethylammonium bromide (CTAB)-silicatewater system. The products have been characterized by powder X-ray diffraction (XRD), FT-IR spectroscopy, solid-state 27Al and 29Si magic angle spinning (MAS) NMR, N2 adsorption and desorption measurements, and thermogravimetric analysis. The effect of the hydrothermal time at the fixed temperature such as at 150 and 170 °C was also examined. We discuss the formation of MCM-41 and the interactions between surfactants and silicate species at different temperatures. © 1997 American Chemical Society

Porous Silica Templated by Surfactants

J. Phys. Chem. B, Vol. 101, No. 42, 1997 8461

TABLE 1: The d Spacings of M41S Materials d spacing (nm) as-synthesized

calcined

sample

synthesis temp (°C)

100

110

200

210

100

110

200

210

MCM-41(1) MCM-41(2) MCM-41(3) lamellar M41S

25 100 135 150

4.30 4.26 4.34 3.53

2.41 2.43 2.52

2.12 2.12 2.18 1.77

1.56 1.61 1.67

3.89 3.66 4.24 b

a 2.16 a b

a 1.87 a

b 1.42 b

a

Difficult to resolve. b Not observed.

Experimental Section Synthesis of Materials. The reaction mixture was prepared according to the modified procedure described elsewhere.2 The reagents for synthesis, such as CTAB [C16H33(CH3)3NBr ], sodium silicate (Na2SiO3‚9H2O), and sulfuric acid (H2SO4), are analysis grades. In a typical synthesis, 12.8 g of sodium silicate nonahydrate (21 wt % Na2O, Na2O/SiO2 ) 1.03) was combined with 40 g of deionized water and then acidified by titration with 5 mol dm-3 H2SO4 (8.0 mL) to pH ) 10-12 under constant stirring. After 15 min, an aqueous solution of CTAB was added, and then a surfactant-silicate mixture was allowed to stir for 30 min at room temperature. The molar composition of the resultant mixture was SiO2:0.96Na2O:0.20CTAB:0.89H2SO4: 120H2O. The weight concentration of CTAB was 2.9%. Such formed mixture was divided into three parts. The first part was directly dried in air at 80 °C to obtain sample A. The second part mixture was immediately recovered by filtration or by centrifugation and washed thoroughly with water and then dried at 80 °C to form MCM-41(1). The third part mixture was removed into stainless steel autoclaves with hydrothermal treatment keeping for various times at different temperatures. The solids were recovered by filtration, washed with water, and then dried at 80 °C in air. The products were first calcined in N2 flow at 550 °C for 1 h and then in air for 5 h at the same temperature using a muffle furnace. Characterization of Materials. The powder XRD patterns were recorded with a Rigaku IID/max-γa powder diffractometer. This diffractometer system employed Ni-filtered Cu KR radiation and was operated at 40 kV and 20 mA. The range of scans began from 2θ of 1.5°. The dhkl indexes of materials were calculated by the Bragg diffraction equation. The N2 adsorption-desorption isotherms over calcined samples were carried out on an ASAP-2000 apparatus using nitrogen as adsorbate at -196 °C. Prior to measurements, the samples were outgassed at above 350 °C for 6 h. The BET specific surface area was calculated using the BET equation31 for relative pressures in the range P/P0 ) 0.05-0.25. In general, the calculation method for the pore size distribution depends on the assumed model based on the shape and size of pore. The discrepancies between the different methods are almost entirely due to the use of different thickness, t, of the adsorbed multilayer in the calculations.32 Pore structural data of samples (pore volume, pore surface area, and pore size distribution) in our experiments were analyzed automatically using a computer program based on the cylindrical pore model of the BJH method33 with the Halsey equation for multilayer thickness.34 In comparison, pore structural data based on the adsorption and desorption branch of the isotherms were obtained. The framework vibration spectra of calcined materials were obtained on a Nicolet 5SXC FT-IR spectrometer with a resolution of 4 cm-1 using KBr pellets. The 27Al and 29Si MAS NMR spectra were recorded at room temperature on a Bruker MSL-300 spectrometer with resonance frequencies of 78.21 MHz for 27Al and 59.63 MHz for 29Si. The magnetic field was

7.05 T. The spin rate of the sample was 4.0 kHz, and the number of scans was 1500-2500. The chemical shifts were referenced to tetramethylsilane for 29Si and AlCl3‚6H2O for 27Al. The thermogravimetric (TG) and differential thermogravimetric (DTG) curves of uncalcined samples were recorded on a Rigaku PTC-10A with a heating rate of 10 °C/min from room temperature to 850 °C under argon gas flow of 40 cm3/min. Results and Discussion Formation of Different Porous Siliceous Materials. Because of the regular mesoporous channels, the patterns of XRD in the small angle range can be used to characterize the longrange order of mesostructure M41S materials. Once mixed, the mixture of acidified sodium silicate and the CTAB aqueous solution became white, and then the fine precipitates were formed at room temperature. The rapidity of this precipitation shows that there is a interaction between the cationic surfactant and the anionic silicate species in the mixture. This interaction leads to the further polymerization of inorganic species.5 We found that the directly dried mixture (sample A) only showed diffraction peaks of Na2SO4 and partial CTAB in XRD patterns (not shown), indicating that the interactors have no long-range ordered structure. However, when the mixture was washed with water to remove soluble Na+, Br-, SO42-, and unacted CTA+, the dried solid product MCM-41(1) showed four distinct diffraction peaks only in the range of 2θ less than 10°, consistent with the reflections of hexagonal MCM-41 reported early2, and can be indexed on a hexagonal lattice with (100) d spacing of 4.30 nm (shown in Table 1 and Figure 1a). In addition, when sample A was washed with water, filtered, and dried again at 80 °C, the product had the same reflections as MCM-41(1), showing the effect of soluble Na+, Br-, SO42-, and unacted CTA+ on the arrangement of interactors. In the absence of these ions, the long-range ordered hexagonal M41S was obtained. It might indicate that the rearrangement of interactors occurs after the removal of soluble ions. After calcination at 550 °C for 5 h, however, only the (100) peak was retained, and the distinct peaks for (110), (200), and (210) reflections could not be observed for above samples. It has been demonstrated that the mesoporous MCM-41 material has short-range ordered hexagonal symmetry when a single (100) reflection is retained.3,7 The XRD patterns of calcined MCM-41(2), obtained by hydrothermal processing the reaction mixture for 6 days at 100 °C, are shown in Figure 1b, showing the presence of typical hexagonal mesostructure. In comparison with MCM-41(1), it is clear that the hydrothermal process has a great role in improving the long-range order of calcined mesoporous siliceous material. The further increase of hydrothermal temperature has not the same effective role in building regular mesoporous structure for the silicate-CTAB synthetic system. The product MCM-41(3), obtained at hydrothermal temperature of 135 °C, becomes short-range hexagonal symmetry after calcination at 550 °C, as shown in Figure 1c. The unresolved diffraction

8462 J. Phys. Chem. B, Vol. 101, No. 42, 1997

Figure 1. XRD patterns of samples synthesized at 25 (a), 100 (b), 135 (c), and 150 °C (d).

shoulder peak around 3-4° of 2θ is observed. But compared to 0.60 nm of MCM-41(2), the (100) d spacing has only a shrinkage of 0.1 nm after calcination (see Table 1), showing that the increase of hydrothermal synthesis temperature enhances the density of mesoporous framework. The framework structure of siliceous material changes with further increase of hydrothermal temperature. When the mixture was kept for 5 days at 150 °C, the XRD patterns of product were consistent with those of lamellar M41S obtained using tetraethyl orthosilicate as silicon source at surfactant-to-silicon ratio of 1.315 (shown in Figure 1d). Once calcined at 550 °C, this sample lost all XRD reflections, becoming an amorphous phase and showing that the removal of the surfactants between silica layers results in the collapse of lamellar structure. This result is of importance for the preparation of mesoporous catalysts and for the studies of the interaction behaviors of surfactants and inorganic species. To reveal the formation of lamellar M41S, we have investigated the effect of hydrothermal synthesis time of 150 °C on the structure of product. The results of XRD patterns are shown in Figure 2. The sample obtained from the mixture held for 6 h exhibits typical hexagonal lattice, indicating the structure of interactors still having hexagonal phase in the early synthesis stage of 150 °C. However, three distinct reflection peaks are observed after hydrothermally keeping the mixture for 40 h. Among these peaks, two peaks showing reflections at 2θ ) 2.79° and 5.60° are assigned to (100) and (200) reflections of the lamellar phase; the third small peak showing a reflection at 2θ of 2.13° should correspond to the (100) reflection of hexagonal MCM-41, because the former two peaks disappeared and the latter small reflection at 2θ of 2.13° remained unchanged after calcination of 550 °C. This shows that most hexagonal structure has transformed to lamellar M41S in such synthetic conditions. When further prolonged to a hydrothermal time of 72 h at 150 °C, only the peaks corresponding to the lamellar phase are observed in XRD patterns, and the lamellar phase reflections disappeared after calcination. These results demonstrate that the hexagonal structure can be transformed to the lamellar phase in hydrothermal conditions, showing that synthesis conditions play a key role in the formation of mesostructures.

Chen et al.

Figure 2. XRD patterns of samples obtained at 150 °C with different hydrothermal time.

We further increased the hydrothermal temperature to or above 165 °C in order to further investigate liquid-crystalline templating of surfactants. In this case, however, the reflections corresponding to mesostructures were not observed. The XRD reflections of products were in agreement with those of microporous zeolite Silicalite-1 (type ZSM-5) synthesized by the TPA cation.35 The crystallinity of sample ZSM-5(1) obtained at 165 °C was clearly lower than that of ZSM-5(2) prepared at 180 °C. But the yield of products decreased with the increase of synthetic temperature. It has recently been reported that the cationic surfactants templated the formation of many microporous materials.17,36-38 In our experiments, the ZSM-5 crystallite could be detected after 3 day crystallization at 170 °C. Nevertheless, only the R-SiO2 was formed when CTAB did not exist in the corresponding reaction mixture. Furthermore, the cocrystallization products of lamellar M41S and ZSM-5 were also observed when the crystallization time was less than 3 days at 170 °C, which was not reported in the literature.17 To obtain a direct structural transformations of siliceous precursors, we investigated structural change of silica when the mixture was held for various times at 170 °C. The structures of products examined by XRD are described in the order of prolonged time as follows: hexagonal MCM-41 f lamellar M41S f ZSM-5. Characterization of Different Porous Siliceous Materials. The Pore Structure. The measurement of N2 adsorption/ desorption isotherms is a reliable technique to obtain effective information about pore structure of porous materials. Figure 3a shows N2 adsorption-desportion isotherms of calcined MCM-41(2). The isotherms show a typical type IV mesopore sorption behavior.39 The N2 adsorption amount increases sharply in the range of P/P0 ) 0.30-0.46, and a distinct hysteresis loop, which results from capillary condensation of N2, is observed, indicating the presence of uniform mesopore size within siliceous materials. In addition, a larger hysteresis loop appears in the range of P/P0 g 0.48. This larger loop should correspond to capillary condensation within the interparticle mesoporosities. In the case of the calcined lamellar M41S (Figure 3b), the hysteresis loop curves appear around P/P0 ) 0.2, and then the loop becomes larger when P/P0 > 0.45. It is difficult to distinguish uniform mesopores from

Porous Silica Templated by Surfactants

J. Phys. Chem. B, Vol. 101, No. 42, 1997 8463 TABLE 2: BET Specific Surface Area and Pore Structure Data of Calcined Samples av diameter pore parametera from BJHb sample MCM-41(1) MCM-41(2) MCM-41(3) lamellar ZSM-5(2)

synthesis BET surface temp (°C) area (m2 g-1) 25 100 135 150 180

375.3 702.8 627.9 214.7 202.1

dp (nm)

tw (nm)

Da Dd (nm) (nm)

2.50 2.80 2.50

1.99 1.43 2.40

3.39 3.40 3.01 5.54

3.85 3.31 3.32 5.68

a The mesoporous MCM-41 is of hexagonal lattice, and the pores take the shape of cylindrical aggregates of surfactants. Thus, the distance between pore or the wall thickness of pore (tw) can be calculated by subtracting dp from the unit cell parameter or the distance between pore center (a0) of hexagonal MCM-41.2 b Da and Dd are adsorption and desorption diameter, respectively.

Figure 3. N2 adsorption-desorption isotherms over calcined MCM41(2) (a) and lamellar M41S (b).

Figure 4. BJH pore size distribution plots of MCM-41(2) (a) and lamellar M41S (b).

interparticle porosities because the capillary hysteresis loops corresponding to two kinds of pores are unresolved. This indicates that the original lamellar structure collapses after the removal of organic matter by calcination, resulting in the destruction of regular structure. The fundamental pore channels of calcined lamellar M41S consist of spaces produced by free stacking of silica layers. These results are in excellent agreement with the data obtained from XRD studies. The pore size distribution further confirms the influence of hydrothermal conditions on the uniformity of mesopores. Figure 4 shows the BJH results from desorption branch of isotherms of calcined hexagonal MCM-41(2) and calcined lamellar M41S. The uniform mesopores lead to the narrow pore size distribution with a probable pore diameter (dp) about 2.80 nm for MCM41(2). In addition, a small pore size distribution is also observed around at dp ) 4.0 nm. For lamellar M41S, neither the mesopore size distribution nor the sharp macropore distribution corresponding to the interparticle porosities is observed in Figure 4b, except the presence of a narrow pore peak at dp ) 4.0 nm. We do not think this peak derives from the actual mesopores within materials because many microporous materials, such as ZSM-5, zeolite Y, and pillared clays obtained in our experiments or from industrial samples, also showed the same narrow pore size distribution plots at dp ) 4.0 nm from BJH calculations with the desorption branch. The cause might result from the capillary condensation of N2 and/or from calculation methods. In any case, one should be careful in analyzing the pore structure of porous materials when using N2 adsorption/desorption data.

The data of pore structure and the BJH results of different materials are presented in Table 2 and Table 3, respectively. It is obtained from Table 3 that both pore volume and pore surface area calculated from two sorption branches for all MCM-41 samples are similar. But for the lamellar sample, the difference is obvious. BET specific surface areas and pore volumes of different materials change greatly with varying synthetic conditions. For MCM-41, when synthesis temperature increases from room temperature to 100 °C, the BET surface area, the pore surface area (Ad), and the pore volume (Vd, dp ) 1.7-100.0 nm) calculated from the desorption branch increase by 87%, 76%, and 54%, respectively. But tw decreases by 0.56 nm. This shows that the hydrothermal process increases the mesoporous density. However, when increasing the synthetic temperature from 100 to 135 °C, the mesoporous surface area (Adm) and the mesoporous volume (Vdm, dp ) 1.7-4.0 nm) decrease by 51% and 55%, respectively. Therefore, the distance between surfactant micelles becomes longer, and then tw accordingly increases by ∼1.0 nm (see Table 2). This suggests that the increased synthetic temperature enhances the polymerization of silicate species. In addition, the Vd for MCM-41(3) is less than half of MCM-41(2). But the BET surface area of the former is about 90% of the latter, indicating that the crystalline size of MCM-41(3) is smaller than MCM-41(2), because the small crystallites have more interparticle porosities and make a greater contribution to BET specific area than to pore volume, in agreement with single (100) reflection in the XRD patterns for MCM-41(3). It should also be noted from Table 3 that g69% of pore volume and g90% of pore surface area are contributed by mesopores for MCM-41(2). This shows that not only the framework structure of hexagonal silica is highly regular but also the pore size is very uniform for MCM-41, and the mesopores derived from hexagonal silica framework provide the main pore volume. In general, the pore structures of inorganic mesoporous materials derived from surfactant template molecules are restricted by the size and the number of surfactant micelles. The discrete micella aggregates limit the shape of pores. Calcination removes the template molecules opening the channels of materials. After the removal of organic matter, the sustaining force of network structure is provided by the inorganic framework. For lamellar M41S, because of losing the pillars (surfactant micelles) between silica layers, the mesostructure cannot be retained after calculation. The BET area and the pore volume decrease sharply (see Tables 2 and 3). As compared with MCM-41(2), nearly 90% of the mesopore surface area (Adm) of product is lost with increased synthesis temperature to 150 °C. Only 22% of Vd and 40% of Ad come from the mesopores. In addition, the shape of hysteresis loop is well in

8464 J. Phys. Chem. B, Vol. 101, No. 42, 1997

Chen et al.

TABLE 3: BJH Results of Different Calcined Materialsa pore volume (cm3 g-1)

pore surface area (m2 g-1)

adsorption

desorption

adsorption

desorption

sample

Va

Vam/Va (%)

Vcm/Vam (%)

Vd

Vdm/Vd (%)

Aa

Aam/Aa (%)

Acm/Aam (%)

Ad

Adm/Ad (%)

MCM-41(1) MCM-41(2) MCM-41(3) lamellar ZSM-5(2)

0.436 0.738 0.334 0.248 0.102

62 69 73 26

100 100 95

0.478 0.734 0.349 0.322 0.097

54 70 66 22

509.9 867.1 466.4 178.3 108.1

91 92 93 60

93 92 77

503.8 887.5 422.0 226.7 101.2

89 90 92 40

a The subscripts a and d represent adsorption and desorption in the pore range 1.7-100.0 nm, respectively. The subscripts am, dm, and cm represent adsorption, desorption, and calculation in the mesopore range 1.7-4.0 nm, respectively. Vcm ) S/ [(A* - S)F], were S is the section of a pore of diameter dp, A* is the area of the hexagonal unit cell normal to the pore axis, and F ) 2.20 g/cm3 is the volume mass of amorphous silica.40

agreement with parallel-plate model,39 showing that the sample made at 150 °C is a lamellar phase other than the ill-defined hexagonal mesostructure. From Tables 2 and 3 it can also be seen that the BET specific area of lamellar M41S is approximately equal to that of ZSM-5(2), but the Vd of former is more than 3 times of the latter. The above pore structural data show that most of the pore volume for lamellar M41S is contributed by the porosities between crystallites. To further confirm the uniformity of mesopore and regularity of hexagonal silica framework, we have compared BJH results with calculation values of mesopore. Given the hexagonal honeycomb monolith of the pores in MCM-41, an infinite array of close-packed cylindrical pores is expected to feature a pore volume (Vcm).20 Table 3 presents the percent ratio of Vcm to Vam and Acm to Aam. Interestingly, although MCM-41(1) shows the short-range ordered structure, the percent ratios of calculation values to BJH results for both pore volume and pore surface area are similar to those of well-defined hexagonal MCM-41(2). However, MCM-41(3) shows lower agreement for pore surface area than the above two samples. It suggests that at high hydrothermal temperature the formation of mesostructure is mainly caused by the synthesis temperature rather than by the assembly of surfactant micelles. Characterization of Local Structure. The polymerization of silicate varies with the alkalinity of solution and the concentration of surfactants. Although the nature and the mechanism of interaction between polymerized silicate and CTAB are still controversial, the templating mechanism is thought to be a cooperative process involving the interactions of inorganic species with discrete surfactant aggregates5,18 to form the structure similar to that of water including oil. The XRD and N2 adsorption/desorption measurements have showed that the hexagonal silica framework formed by such assembly is thermally stable, but the lamellar phase is unstable. To confirm the structural transformations, we have employed FT-IR and solid-state 29Si MAS NMR to characterize the local unit structure of different materials. Figure 5 shows the FT-IR spectra of three calcined samples. These IR framework spectra demonstrate the structural transformations or the presence of different siliceous materials. In the range of framework vibrations (1500-400 cm-1), the vibrational bands of hexagonal silica are similar to those of amorphous silica obtained from acidified silicate (pH 7-8, not shown in Figure 5), namely, the presence of vibrational bands at 1099, 964, 795, and 474 cm-1. It shows that the internal local unit structure of hexagonal silica framework is identical with amorphous silica. With the formation of lamellar M41S, the vibrational intensity of the 964 cm-1 band, which is assigned to the Si-OH vibration,41 decreases, and the band of 474 cm-1 corresponding to bending vibration of Si-O shifts to 458 cm-1. Meanwhile, a new weak vibration band, which is assigned to double-ring vibration,14,42 appears at ∼598 cm-1. This band is probably characteristic of structural transformation

Figure 5. FT-IR spectra of calcined samples obtained at 100 (a), 150 (b), and 165 °C (c).

of hexagonal going to lamellar phase. By varying the synthesis time from 36 to 72 h at 170 °C or changing the synthesis temperature from 150 to above 165 °C, the bands at 458 and 598 cm-1 of the lamellar phase shift respectively to 454 and 548 cm-1, and the intensity of the band at 548 cm-1 is greatly strengthened. Both ∼450 and ∼550 cm-1 bands are typical framework vibrational characteristic of ZSM-5.43 29Si NMR can be used to determine the degree of polymerization of silica framework.44 The ratio of Q3 {δ ∼ -100, corresponding to (SiO)3Si-OH} to Q4 {δ ∼ -110, corresponding to (SiO)3Si-O-Si} represents the condensation degree of Si-OH. A low value of Q3/Q4 means high condensation of material framework or high polymerization of silicon atoms. Figure 6 shows 29Si MAS NMR spectra of MCM-41, lamellar M41S, and ZSM-5 obtained. The corresponding 29Si NMR data of different materials are summarized in Table 4. Obviously, the peak shape and the ratio of Q3 to Q4 depend on the synthetic conditions. For materials with the hexagonal symmetry, the ratio Q3 to Q4 decreases from 2.3 for MCM-41(1) to 0.89 for MCM-4(2), reflecting a significant increase in the number of silicon atoms fully coordinated to other silicate nearest neighbors. Increasing synthetic temperature from 100 to 150 °C makes the peak shape of 29Si NMR become sharp because the increase of polymerization degree of Si-O-Si results in SiO-Si bond angle becoming small. But the Q3/Q4 value remains unaltered (see Table 4). In addition, calcination may result in the further condensation of Si-OH. The removal of partial hydroxyl groups by calcination makes the ratio of Q3 to Q4

Porous Silica Templated by Surfactants

J. Phys. Chem. B, Vol. 101, No. 42, 1997 8465

Figure 7. DTG curves of uncalcined MCM-41(2) (a), lamellar M41S (b), ZSM-5(1) (c), and ZSM-5(2) (d).

TABLE 5. Thermogravimetric Measurement Data of Different Materials

Figure 6. 29Si MAS NMR spectra of MCM-41(2) (a), lamellar M41S (b), and ZSM-5(1) (c).

TABLE 4:

29Si

NMR Data of Different Materials area fraction of 29Si NMR peak

sample

synthesis temp (°C)

MCM-41(1) MCM-41(2) lamellar ZSM-5(1) ZSM-5(2)

25 100 150 165 180

as-made

after calcination

Q3

Q4

Q3/Q4

Q2

Q3

Q4

Q3/Q4

70 47 47 9

30 53 53 91

2.30 0.89 0.89 0.10

6 6

36 23

58 71

0.62 0.32

5

95

0.05

decrease from 0.89 to 0.62 for MCM-41(2). Two distinct peaks of Q3 and Q4 can still be observed. However, the lamellar sample lost its resolved Q3 and Q4 peaks after calcination. The ratio of Q3 to Q4 peak decreased to 0.32 from Gaussian simulation. 29Si NMR results indicate that the calcination not only removes the surfactants within lamellar M41S, resulting in the collapse of layers and then lack of long-range order, but also makes silanols between layers condense each other. Obviously, the formation of microporous crystallite is accompanied by the further condensation of Si-OH, in accordance with disappearance of 960 cm-1 in FT-IR spectra. In combining 29Si NMR with XRD and FT-IR results, the lamellar phase might be thought of as the intermediate for structural transformation of hexagonal mesostructure to microporous Silicalite-1 crystallite. The Templating Approach of Surfactants on the Formation of Different Materials. It is evident that the synthetic temperature is important for the formation of different structures in the CTAB- silicate-water mixture. But practically it is the surfactant micelles that play the determining role for directing the formation of the mesoporous and microporous materials. The DTG curve is a function of temperature. With the increase of temperature, the peak of weight loss, which represents the removal temperature of template, corresponds to the maximum rate of weight loss. For the interactions of surfactants and silicate, the weight loss should result from the removal of

sample

total weight loss (%)

MCM-41(2) lamellar 41S ZSM-5(1) ZSM-5(2)

71.3 56.4 42.6 10.7

weight loss distribution (%)/ peak temperature (°C) 23.2/183 44.0/235 26.4/270 51.4/183 42.2/243 76.4/212

6.4/410 6.4/420 23.6/405 100/405

surfactants and partial hydroxyl groups included in the silica framework. Thus, the peak of weight loss corresponds to the temperature of desorption or decomposition of surfactants interacting with SiO2 or the removal of intact surfactants occluded within internal pores. The number and position of weight loss depend on the behavior and intensity of interactions between surfactants and silica. Figure 7 shows the DTG curves of different structural materials. The corresponding TG measurement data are presented in Table 5. With the structural transformation from hexagonal mesostructure to microporous crystallite, the total weight loss of samples decreases from 71.3% to 10.7%. Four samples made at 100, 150, 165, and 180 °C give the weight loss as 71.3%, 56%, 42.6%, and 10.7%, respectively. The number of weight loss peaks also decreases from four distinct peaks of MCM-41(2) to one wide peak of ZSM-5(2). Among these peaks, we believe that the peak at ∼240 °C is assigned to desorption or decomposition of surfactants because the peak of removing CTAB is at ∼237 °C when CTAB was mechanically mixed with silica or R-Al2O3 in the same DTG experimental conditions. This peak might also correspond to weak interaction of silica and intact surfactant aggregates. The other peaks might be related to those surfactants strongly interacting with the silica framework. Interestingly, for the mesostructures the hexagonal and lamellar products have a similar percent ratio of intact surfactant molecules (42-44%) within pores of materials. These surfactants cannot be removed by washing with water, reflecting cooperative formation of mesostructure.5 More than half of the surfactants are removed at a peak of 183 °C for lamellar material, being more than double those for hexagonal material. This peak might correspond to the decomposition temperature of surfactants strongly interacting with the framework of products, because this temperature is greatly lower than that for removal of intact molecules of surfactant. From this point

8466 J. Phys. Chem. B, Vol. 101, No. 42, 1997

Figure 8.

27Al

MAS NMR spectra of uncalcined ZSM-5(1).

of view, we do not imply that the interaction number of surfactants with hexagonal framework is less than that with lamellar silica framework. The reason for this is that another peak at 270 °C is observed for MCM-41. It is surprising to note that, once ZSM-5 is formed, the weight loss peak of intact surfactant molecules is not observed in DTG curves because of the absence of weight loss peak at 240 °C for products obtained above 165 °C. Therefore, it is also noted that the sample obtained at 180 °C has only one wide weight loss peak. These results might mean that the formation of ZSM-5 crystallite catalyzes the heat decomposition of surfactant molecules in the synthetic system. So the peak at 212 °C could be assigned to desorption of decomposition products of surfactants. But we cannot exclude that the heat decomposition of surfactants occurs under the interaction of ZSM-5 during increasing temperature in the DTG experiment. The decomposition mechanism of surfactants and their interactions with different structural silica need not to be further studied. The above DTG results of different materials imply that the formation of different materials might result from different interactions between surfactants and silica precursors. It can also be seen from Table 5 that the percent ratios of weight loss peaks at ∼410 °C for two kinds of mesostructures are the same (6.4%). However, the percent ratio of this peak for ZSM-5 is higher than that of mesostructures. This peak was assigned to the desorption of decomposition products of CTAB molecules interacting with acid sites formed by aluminum incorporation into the framework of the MCM-41.2 In our experiments, no aluminum was added into the synthetic mixture. Aluminum is probably introduced into the synthesis system as an impurity accompanied by addition of sodium silicate. Practically no resonance signals of 27Al were determined for the hexagonal and lamellar samples in 27Al MAS NMR. However, the 27Al signal at δ ∼ 57.9 was detected in ZSM-5(1) (shown in Figure 8), indicating that the aluminum atom is much easier to be concentrated into the ZSM-5 structure than into the M41S structure. The self-assembly of amphiphilic molecules into spheres, cylinders, bilayers, and bicontinuous phases in free solution is fairly well understood,45 but it is not clear how these aggregates are affected by the presence of a solid boundary surface. For a CTAB-water solution a hexagonal phase is favored at surfactant concentrations from ∼25 to 70 wt % whereas a lamellar phase forms at concentrations above 70 wt %,46 which are greatly higher than the concentrations used to prepare M41S materials. In the case of low surfactant concentrations, three

Chen et al. closely coupled factors have been proposed as being crucial to the formation of surfactant-silicate mesophases.5 However, the hydrothermal temperature plays an important role in the polymerization of silicate species and the interactions between silicate species and surfactants. In this case, Beck et al.17 proposed mechanistic pathways for the formation of mesoporous and microporous materials in the surfactant-silicate system. Unfortunately, they did not provide the formation of ZSM-5 from a mixture containing CTAB, and the cocrystallization products of ZSM-5 and lamellar M41S were not reported under the synthesis conditions used. In fact, the transition concentration from hexagonal to lamellar phase was observed as a function of temperature for surfactant micelles in water,46 and the degree of silicate species condensation increases with the increase of temperature. In our experiments, the degree of polymerization of silicate and the behavior of surfactant molecules probably depend on the synthesis temperature. At low temperature below 135 °C or during the early stage of synthesis at high temperature above 165 °C, the condensation of silicate species might be primary. The interactions between surfactant micelles and silicate species and the rearrangement of micelles after the addition of silicate are influenced by the condensation of silicate due to the change of synthesis temperature or reaction time. With increased synthesis temperature from 25 to 100 °C, the arrays of mesostructures become well-ordered, resulting in the decrease of wall thickness and the great increase of surface area or pore volume. However, further increase of temperature cannot improve the degree of silicate condensation. The long-range order of surfactant aggregates probably is distorted or destroyed. This probably results in the decrease of long-range order of mesostructure. During the intermediate stage at hydrothermal temperature of 170 °C or increasing the temperature to 150 °C, the arrays of surfactant micelles may be changed and silicates become wellcondensed. In this case, the well-condensed silicates direct micelles into lamellar arrays forming lamellar M41S. The presence of 598 cm-1 in FT-IR spectra and other results from XRD, 29Si NMR, and N2 adsorption/desorption isotherms indicate that the structure of lamellar M41S is different from that of hexagonal M41S. Here, a lamellar phase is formed at the same surfactant concentrations as those forming the hexagonal phase, being opposite to the binary CTAB-water system.46 These results strongly support a formation in which surfactant micelles and silicate species mutually organize into mesostructures under synthesis conditions. Therefore, the absence of hexagonal mesostructure at high temperature (150 °C) suggests that synthesis temperature has a key effect on the interactions between surfactants and silicates. With prolonged reaction time at 170 °C, the transformation from lamellar M41S to ZSM-5 shows different templating mechanisms of surfactants, because the size of pore channels of ZSM-5 (dp ∼ 0.6 nm) is greatly smaller than that of CTAB micelles. This suggests that it is the single molecule, and not micellar aggregates of CTAB, that directs the formation of the microporous crystallite. Conclusions The preparation of mesoporous siliceous material results from the polymerization of silicate species, which is influenced by the addition of surfactants and the synthesis temperature. The removal of disturbing ions is an essential step in obtaining in practice the solid silica mesostructures. With increased hydrothermal synthesis temperature from room temperature to 100 °C, the short-range ordered hexagonal mesostructure becomes well-defined hexagonal material, and then the surface area and pore volume of product increase greatly. However, upon further

Porous Silica Templated by Surfactants increase to 135 °C, the long-range order of product decreases. The structure of products, obtained from the cooperation between surfactants and silicate species in synthesis conditions, varies with the change of time at 170 °C or with the increase of hydrothermal temperature from 100 to above 165 °C, namely, hexagonal mesostructure f lamellar phase f microporous zeolite ZSM-5. The presence of a vibrational band at 598 cm-1 in FT-IR spectra and other results from XRD, N2 measurement, and 29Si NMR demonstrate that the lamellar phase can be assigned to an intermediate from mesostructure to microporous crystallite. Obviously, purely siliceous mesoporous materials are not highly selective catalysts as those modified by framework substitution with other elements. But the results described in this paper can provide a predirecting condition for preparing highly selective mesoporous catalysts. Acknowledgment. We are grateful to the Postdoctoral Science Foundation of China for the research support to X. Chen. We also thank Professor Z. Xue for NMR characterization and Miss H. Chen for her assistance of measurements of XRD and DTG. Supporting Information Available: Figure S showing XRD patterns of uncalcined samples hydrothermally synthesized at 100 °C for 3 days (a), 100 °C for 3 days and then 150 °C for 3 days (b), and 100 °C for 3 days and then 150 °C for 3 days, finally 170 °C for 3 days (c) (1 page). Ordering information is given on any current masthead page. References and Notes (1) Kresge, C. T.; Leonewicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker J. L. J. Am. Chem. Soc. 1992, 114, 10834. (3) Tanev, P. T.; Chibwe, M.; Pinnavaia, T. J. Nature 1994, 368, 321. (4) Corma, A.; Navarro, M. T.; Perez-Pariente, J. J. Chem. Soc., Chem. Commun. 1994, 147. (5) Monnier, A.; Schuth, F.; Huo, Q.; Kumar, D.; Margolese, D.; Maxwell, R. S.; Stucky, G. D.; Krishnamurty, M.; Petroff, P.; Firouzi, A.; Janicke, M.; Chmelka, B. F. Science 1993, 261, 1299. (6) Huo, Q.; Margolese, D. I.; Ciesla, U.; Feng, P.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schuth, F.; Stucky, G. D. Nature 1994, 368, 317. (7) Tanev, P. T.; Pinnavaia, T. J. Science 1995, 267, 865. (8) Bagshaw, S. A.; Prouzet, E.; Pinnavaia, T. J. Science 1995, 269, 1242. (9) Attard, G. S.; Glyde, J. C.; Goltner, C. G. Nature 1995, 378, 366. (10) Feuston, B. P.; Higgins, J. B. J. Phys. Chem. 1994, 98, 4459. (11) Luan, Z.; He, H.; Zhou, W.; Cheng, C.-F.; Klinowski, J. J. Chem. Soc., Faraday Trans. 1995, 91, 2955. (12) Alba, M. D.; Becerro, A. I.; Klinowski, J. J. Chem. Soc., Faraday Trans. 1996, 92, 849. (13) Chen, C.-Y.; Li, H.-X.; Davis, M. E. Micropor. Mater. 1993, 2, 17.

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