Kinetic study of MCM-41 synthesis

International Journal of Inorganic Materials 3 (2001) 75–86

Kinetic study of MCM-41 synthesis q J.L. Blin, C. Otjacques, G. Herrier, Bao-Lian Su* ´ Laboratoire de Chimie des Materiaux Inorganiques, I.S.I.S., Universite´ de Namur 61, rue de Bruxelles, 5000 Namur, Belgium Accepted 5 May 2000

Abstract A kinetic study of MCM-41-type materials formation has been made to optimize the synthesis conditions in particular the synthesis time and temperature. The changes in morphology and textural properties of materials as a function of hydrothermal synthesis time at four different temperatures have been followed. From the characterization results, the synthesis mechanism is postulated. The present work shows clearly that the thickness of the wall separating two adjacent pores increases with hydrothermal synthesis time and temperature while pore size remains constant. The increase in the wall thickness, indicating the enhancement of polycondensation of silica around the micelles of surfactant, should be very important for strengthening the thermal stability of MCM-41 materials.  2001 Elsevier Science Ltd. All rights reserved. Keywords: MCM-41; Synthesis mechanism; Mesoporous materials; Wall thickness; Effect of heating time and temperature

1. Introduction In 1992, Mobil scientists [1,2] have reported the synthesis of new mesostuctured materials called MCM (Mobil ] Crystalline Materials). Hexagonal (MCM-41), cubic ] ] (MCM-48) and lamellar (MCM-50) structures have been identified. Among these phases, MCM-41 is the most studied. The synthesis of pure silica mesoporous molecular sieves consists of the condensation and polymerization of an inorganic source of silicium around the micelles of surfactant. According to Mobil scientists, MCM-41 is formed via a LCT (Liquid Crystal ] Templating) mechanism [1], whereas ] ] for Monnier et al. [3], MCM-41 is obtained from the transformation of the lamellar MCM-50 phase. Kind of surfactants, silicium / surfactant ratio, the alkyl chain length of surfactant, pH value, hydrothermal synthesis time and temperature are known to affect the pore diameter, the wall thickness and the structure of the final compound. Generally, quaternary ammonium salts, C n H 2n11 (CH 3 ) 3 NBr, are used as surfactants. Beck and coworkers [1] have investigated the effect of the surfactant chain length variation and the silicium / surfactant ratio on the MCM-41 synthesis.

They concluded that MCM-41 is synthesized when n varied from 6 to 16 and the pore diameter is proportional ˚ per carbon), with a to the alkyl chain length (12.5 A CTMABr / silicium molar ratio less than one. Elder et al. [4] showed that the optimum pH value is about 9–10. However, in literature few studies concerning the influence of physical parameters such as synthesis time and temperature of the micellar gel has been made. Cheng et al. [5] have studied the effect of temperature and time on the synthesis, they have shown the results contrary to those made by Monnier et al. [3] that hexagonal MCM-41 phase is already present at the beginning of the synthesis made at 100 and 1508C, and the yield is maximum after 48–72 h synthesis. At 1508C the hexagonal phase is transformed into the lamellar or the amorphous phase after 96 h. For longer reaction times, the amorphous phase is the major product. This work deals with a systematic study on the effect of heating time and temperature on the MCM-41 synthesis, on the pore size and thickness of walls separating two pores of materials obtained.

2. Experimental q Paper presented at the First International Conference of Inorganic Materials, Versailles, France 16–19 September, 1998. *Corresponding author. Tel.: 132-81-72-4531; fax: 132-81-72-5414. E-mail address: [email protected] (B.-L. Su).

2.1. Synthesis Cetyltrimethylammonium bromide was dissolved in

1466-6049 / 01 / $ – see front matter  2001 Elsevier Science Ltd. All rights reserved. PII: S1466-6049( 00 )00043-X

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water at 408C to obtain a clear micellar solution. Then sodium silicate was added to this solution and the pH value was adjusted with sulfuric acid. pH value and surfactant /

silicium molar ratio were fixed at 10 and 0.62 according to literature [6]. After stirring for several hours at room temperature, the homogenous gel was sealed in Teflon

Fig. 1. X-ray diffraction patterns of the sample obtained at (A) 808C, a: 1, b: 4, c: 6, d: 8, e: 11 (days), (B) 1008C, a: 0.33, b: 0.67, c: 1, d: 4, e: 6, f: 8, g: 11 (days), (C) 1208C, a: 0.25, b: 0.5, c: 0.75, d: 1, e: 4, f: 6, g: 8, h: 11 (days), (D) 1408C, a: 0.16, b: 0.25, c: 0.5, e: 0.75, f: 1, g: 2 (days).

J.L. Blin et al. / International Journal of Inorganic Materials 3 (2001) 75 – 86

autoclaves and heated. The synthesis temperature and time vary respectively from 808C to 1408C and from few hours to 11 days. The obtained solid phases after ethanol

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extraction with a Soxhlet apparatus were extensively characterized by X-ray diffraction, nitrogen adsorption and Scanning Electron Microscopy (SEM).

Fig. 1. (continued)

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J.L. Blin et al. / International Journal of Inorganic Materials 3 (2001) 75 – 86

Fig. 1. (continued)

2.2. Instrumentation X-ray powder diffraction patterns of obtained materials were recorded with a Philips PW 170 diffractometer, using ˚ radiation, equipped with a thermostatiCuKa (1.54178 A) sation unit (TTK-ANTON-PAAR, HUBER HS-60). Micrographs of obtained phases were made from a Philips XL-20 Scanning Electron Microscope (SEM) using conventional sample preparation and imaging techniques. Nitrogen adsorption–desorption isotherms were obtained from a volumetric adsorption analyser ASAP 2010 manufactured by Micromeritics. The samples were degassed for several hours at 2508C. The measurements were carried out at 21968C over a wide relative pressure range from 0.01 to 0.995. The pore diameter and the pore size distribution were determined by using the BJH method [7].

3. Results and discussion

3.1. X-ray diffraction analysis Fig. 1 reports the XRD patterns of materials obtained at 808C (Fig. 1A), 1008C (Fig. 1B), 1208C (Fig. 1C) and 1408C (Fig. 1D) at different synthesis times. It has been reported [8] that X-ray diffractogramms of powder MCM41 material exhibit a typical four peaks pattern with a very strong feature at a low angle (100 reflection line) and three another weaker peaks at a higher angle (110, 200 and 210 reflection lines). The absence of the last three peaks suggests the disordered structure of MCM-41. These four reflection lines can be indexed on a hexagonal unit cell (a 0 52d 100 /(3)1 / 2 ) as displayed in Fig. 2, from which, we can see that the unit cell dimension a 0 is the sum of the pore diameter and the thickness of the pore wall. A sharp peak at 2u 52.178 corresponding to (100) reflection of MCM-41, is observed after 1 day of synthesis at 808C (curve a of Fig. 1A). As just discussed, the

presence of only (100) reflection implies the formation of the disordered MCM-41 structure. According to the Bragg’s rule, the unit cell dimension a 0 can be deduced ˚ With increasing synthesis time and is around 46.9 A. (curves b, c and d of Fig. 1A), no significant change is detected. Only the position of the sharp peak decreases slightly toward lower angle, indicating the slight increase in the unit cell dimension. After 8 days of synthesis (curve d of Fig. 1A), besides the sharp (100) reflection line, two ˚ and 2u 54.338 very weak peaks at 2u 53.628 (24.4 A) ˚ are detected. The appearance of these two peaks (20.4 A) suggests that the channels are hexagonally organized and the ordered MCM-41 structure is formed. However the weak intensity of (110) and (200) reflections indicate that the channel array is not yet very regular. At elevated synthesis temperature (Fig. 1B, C and D), the sharp peak belonging to 100 reflection of MCM-41 is already detected after several hours. The 110 and 200 reflection lines appear also at shorter synthesis time. The synthesis time at which two peaks arising from the 110 and 200 reflections, respectively, appear decreases as the

Fig. 2. Relation between a 0 (unit cell) and d 100 .

J.L. Blin et al. / International Journal of Inorganic Materials 3 (2001) 75 – 86

heating temperature increases. The intensity and 2u values ˚ of the three reflections increase with increasing (in A) synthesis temperature and time. Nevertheless, comparing the expanded curve h of Fig. 1C, and the expanded curve g of Fig. 1D, with the MCM-50 X-ray diffraction pattern published by Huo et al. [9], a triphasic mixture of MCM41, MCM-50 and amorphous phase is observed after 11 days at 1208C and only 1 day of heating at 1408C. The above results, summarized in Table 1, indicate that the formation of the ordered MCM-41 phase is favored at higher synthesis temperature and time. However a too long synthesis time at elevated temperature can also lead the transition of MCM-41 to MCM-50 and then destroy the formed MCM-41 phase to give amorphous phase. Our results are in agreement with those reported by Cheng et al. [5]. Table 1 d 100 value and estimated crystallinity at different hydrothermal synthesis temperature and time Hydrothermal synthesis time (days)

˚ d 100 (A) value

Crystallinity

1 4 6 8 11

40.6 41.3 41.9 41.9 41.8

1 1 1 11 11

0.33 0.67 1 4 6 8 11

40.9 41.3 41.5 42.6 44.4 42.3 43.6

1 1 1 1 11 111 111

1208C

0.25 0.5 0.75 1 4 6 8 11

41.6 40.7 42.3 43.3 44.4 44.1 46.4 *

1 11 11 11 111 111 111 2

1408C

0.16 0.25 0.38 0.5 0.63 0.75 0.88 1 2 3 4 6 8 11

41.3 40.9 40.2 42.6 42.6 43.3 43.6 46.0 * * * * * *

11 11 11 11 111 111 111 111 2 2 2 2 2 2

Hydrothermal synthesis temperature 808C

1008C

a

a Expressed by the intensity of XRD peaks. 1: medium; 1 1: good; 1 1 1: very good; 2: no estimable.

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3.2. Morphology determined by Scanning Electron Microscopy ( SEM) Fig. 3 shows the variation of scanning electron micrographs with synthesis time and temperature at 808C (Fig. 3A), 1008C (Fig. 3B), 1208C (Fig. 3C) and 1408C (Fig. 3D). At low temperature, 808C (Fig. 3A) and for short heating time at higher temperature (photo a of Fig. 3B and C), the materials texture is composed of agglomerated fibrous. Silica spheres are also clearly observed. The 110 and 200 reflections are not present in the XRD pattern at this moment. When synthesis temperature (photos a and b of Fig. 3D) or time increase (photos b and c of Fig. 3B and photo c of Fig. 3C), the fibrous disappear. Crystals with variable size and form appear. At the same time the peaks characteristic of the 110 and 200 reflections are detected on the XRD pattern, indicating the formation of the well ordered channel array of materials and that compounds have the highly ordered hexagonal MCM-41 structure. The surface of the crystals is rather porous and the morphology is similar to those reported by Elder et al. [10] or Tanev et al. [11]. At high heating time (.8 days at 1208) or at high temperature (.1 day at 1408C) some crystals with a ‘sandy rose’-like structure are present (photos c and d of Fig. 3C and photos c, d, e and f of Fig. 3D). According to Klinowski et al. [12], this structure is characteristic of MCM-50 morphology. Some spherical grains correspond to amorphous silica phase are also clearly observed. The presence of MCM-50 and amorphous silica is proved by XRD patterns.

3.3. Nitrogen adsorption analysis Fig. 4A and B depict the nitrogen adsorption–desorption isotherms of some compounds. Synthesized materials exhibit a type IV isotherm (Fig. 4A and B), according to the classification of BDDT [13]. They can be decomposed in three parts: the monolayer–multiple adsorption of N 2 on the wall of the mesopores, the capillary condensation of nitrogen within the mesopores and then the saturation. For synthesis time less than 8 days at 808C (curve a of Fig. A) or 4 days at 1008C, the sharp increase due to the capillary condensation is not clearly observed and the volume of nitrogen adsorbed is low. This is due to the presence of an important quantity of amorphous phase in the sample. The same observations can be made for long heating time at higher temperatures (.11 days at 1208C and .1 day at 1408C) (curve e of Fig. 4B). For other samples (.8 days at 808C, between 6 and 11 days at 1008C, between 1 and 8 days at 1208C and 1 day at 1408C) (curves b and c of Fig. 4A, curves a, b, c and d of Fig. 4B), the capillary condensation step is quite evident and it occurs at the almost same relative pressure p /p0 (about 0.35). This indicates that samples are very homogeneous and that the pore diameter remains almost constant for all samples obtained since the p /p0 position of the inflection point is

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Fig. 3. Crystals morphology of the samples obtained at (A): 808C, a: 4, b, c: 8 (days), (B) 1008C, a: 0.33; b, c: 11 (days), (C) 1208C, a: 0.25; b: 4; c, d: 11 (days), (D) 1408C, a: 1; b, c: 4; d: 6; e: 8; f: 11 (days).

related to the pore diameter. The pore size distribution is ˚ However, the very narrow and centered at about 26 A. X-ray diffraction results demonstrate that the d 100 spacing, a 0 values, and the wall thickness deduced from a 0 and pore diameter, which are summarized in Table 2, increase significantly with hydrothermal synthesis time and temperature. Since a 0 is the sum of the pore diameter and the

thickness of the walls separating two adjacent pores, the constant pore size obtained by the BJH method and the increase in a 0 value from XRD indicate that the wall thickness increases with the increasing hydrothermal synthesis time and temperature. This suggests strongly that the silica condensation is enhanced with increasing hydrothermal synthesis time and temperature.

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Fig. 3. (continued)

The variation of specific surface area with hydrothermal synthesis time at different temperature is represented in Fig. 5. As the specific surface area can be used to express the quality of materials, we can find different steps that are observed in the synthesis of zeolites. In the case of synthesis of zeolite crystals, four steps are observed [14], and correspond respectively to the nucleation (I), crystallization (II), the further growth of crystals (III) and the amorphisation (IV). During step I no crystalline materials are obtained. Then the crystals start to be formed and the crystallinity of the sample increases, i.e. step II. When it reaches a plateau (step III), the crystallinity remains unchanged while the crystals continue to grow. Finally, during amorphisation (step IV) the crystallinity dramatically drops. However, in the formation of mesoporous materials, the first step (I) is related to the hydrolysis of inorganic silicium source in aqueous solution and no mesostructured solid phase can be obtained. This step is rarely observed due to the high rate of hydrolysis and condensation of silica source around the micelles and that of formation of mesoporous materials. The resulting solid phase, in this step, should be amorphous and the specific surface area should be quite low. The progressive polycondensation of the source of silica around highly orga-

nized micelles occurs at the step (II) and the mesostructured phase is progressively formed. After removal of the surfactants, the highly ordered solid with surface area higher and higher can be obtained. In the third step, the surface area and pore volume stop growing. However, as mentioned in Table 2, the wall thickness between two adjacent pores increases, meaning that the thermal stability of obtained solid phase increases. This indicates that during this step, the polycondensation of silica source continues. The last step (IV) corresponds to the amorphisation. The high temperature and long hydrothermal synthesis time lead often to the formation of the amorphous phase. Step II is observed at 808C, the value of specific surface area increases from 325 m 2 / g for 1 day to 616 m 2 / g for 11 days, from 460 for 1 day to around 500 m 2 / g for 4 days at 1008C. The micellar gel is progressively transformed into MCM-41, the 110 and 200 reflections are not yet detected by XRD. Then at 1008C over 6 days until 11 days of hydrothermal synthesis, the specific surface area remains constant at around 900 m 2 / g. In this step, only the wall thickness is found to increase (Table 2), indicating that the synthesis is situated at step III. At 1208C the maximum value of specific surface area is reached from 1

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Fig. 3. (continued)

day and maintained to 6 days. After 6 days the value of the specific surface area decreases and the amorphisation step (IV) starts. At 1408C the amorphisation of compounds starts after 1 day of hydrothermal synthesis.

3.4. Synthesis mechanism Electrostatic pathway [1,2,15], based on a supramolecular assembly of charged surfactants (S 1 ) with charged

J.L. Blin et al. / International Journal of Inorganic Materials 3 (2001) 75 – 86

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Fig. 4. Nitrogen adsorption isotherms (A) a: 4 days at 808C, b: 6 days at 1008C, c: 1 day at 1408C, (B) at 1208C, a: 1, b: 4, c: 6, d: 8, e: 11 days.

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Table 2 a 0 values, pore diameter and calculated wall thickness of the samples obtained at different crystallisation temperature (8C) and time (days) Crystallisation time (days)

˚ d 100 (A) by XRD

˚ a 0 (A) by XRD

Pore diameter ˚ by N 2 (A) adsorption

Wall thickness ˚ a (A)

808C

1 4 6 8 11

40.6 41.3 41.9 41.9 41.8

46.9 47.7 48.4 48.4 48.3

26.5 26.0 25.5 26.0 25.0

20.4 21.7 22.9 22.4 23.3

1008C

1 4 6 8 11

41.5 42.6 44.4 42.3 43.6

47.9 49.2 51.3 48.8 50.3

26.0 26.0 27.0 26.5 25.0

21.9 23.2 24.3 22.3 25.3

1208C

1 4 6 8

43.3 44.4 44.1 46.4

50.0 51.3 50.9 53.6

26.0 27.0 26.0 25.0

24.0 24.3 24.9 28.6

1408C

1

46.0

53.1

28.0

25.1

Crystallisation temperature

a

Wall thickness5a 0 – pore diameter [12].

inorganic precursors (I 2) is employed for the preparation of highly ordered mesoporous materials. Two mechanisms have been proposed by Mobil scientists to explain the formation of mesoporous materials [1]. In the first route, the hexagonal micelles are formed and direct the growth of the mesoporous materials. But as hexagonal MCM-41 samples can be obtained even though the weight percent of surfactant is less than CMC2 (Critical Micelle Concentration for which rod micelles of CTMABr pack together to give a hexagonal array). This mechanism was not appropriated to explain the formation of mesoporous compounds. In the second route, silica species interact with

rod micelles of surfactant to form the mesoporous compound. This pathway explains why the synthesis of hexagonal MCM-41 can be performed with the weight percent of CTMABr less than CMC2. In the present work we used a weight percent less than CMC2 and high quality MCM41 materials are formed, so the second route is favored. The different periods of synthesis can be summarized by the synthesis mechanism illustrated in Fig. 6. During step II a fibrous agglomerate structure is observed by SEM. The 110 and 200 peaks are not detected or at least not well resolved by XRD and the value of the specific area is low. The gel is progressively transformed into hexagonal MCM-41 structure. Then the step III occurs, the 100 and 200 reflections are clearly present on the XRD diffraction pattern. The value of the specific surface area is between 700 and 900 m 2 / g. The fibrous disappear and MCM-41 crystals appear. Finally we can observe the amorphisation of the materials: a triphasic mixture hexagonal MCM-41, lamellar MCM-50 and amorphous phase is clearly identified by XRD. A ‘sandy rose’-like structure and silicate spheres are observed by Scanning Electron Microscopy. The specific surface area dramatically decreases.

4. Conclusion

Fig. 5. Variation of the specific surface area versus crystallisation time at different heating temperature. a: 80, b: 100, c: 120 and d: 1408C.

An optimization of MCM-41 synthesis conditions led us to propose the mechanism. Different synthesis steps, which are detected in the synthesis of zeolites, have been clearly observed. Referred to the different steps of zeolites synthesis, we describe these steps in the formation of MCM41 materials with the following terms, step I: hydrolysis

J.L. Blin et al. / International Journal of Inorganic Materials 3 (2001) 75 – 86

Fig. 6. Proposed mechanism for MCM-41 synthesis.

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and condensation of silica source, step II: polycondensation of silica source around the cylindrical micelles, step III: the increase in the wall thickness and step IV: the amorphisation. Step I is rarely observed due to the high rate of hydrolysis and condensation of silica source. No crystalline materials can be obtained. In step II, the gel is progressively transformed to MCM-41 materials, the value of the specific surface area increases. When all the micelles are enveloped by silica, the porosity of materials remains constant while the polycondensation can continue (step III). As a consequence, the surface area is maintained but the wall thickness increases. The high heating temperature and long heating time will destroy the organized materials and lead to the formation of amorphous phase (step IV). For the chemical composition and pH value described in the present paper, we have defined the following optimum synthesis temperature and time between 6 and 8 days at 1008C, or less than 4 days at 1208C.

Acknowledgements This work has been performed within the framework of PAI / IUAP 4-10. Gontran Herrier thanks FNRS (Fond National de la Recherche Scientifique, Belgium) for a FRIA scholarship.

References [1] Beck JS, Vartuli JC, Roth WJ, Leonowicz ME, Kresge CT, Schmitt KD, Chu CT-W, Olson DH, Sheppard EW, McCullen SB, Higgins JB, Schenker JL. J Am Chem Soc 1992;114:10834. [2] Kresge CT, Leonowicz ME, Roth WJ, Vartuli JC, Beck JS. Nature 1992;359:710. ¨ F, Huo Q, Kumar D, Margolese D, Maxwell RS, [3] Monnier A, Schuth Stucky GD, Krishnamurty M, Petroff P, Firouzi A, Janicke M, Chmelka BF. Science 1993;261:1299. [4] Edler KJ, White JW. Chem Mater 1997;9:1226. [5] Cheng CF, Park DH, Klinowski J. J Chem Faraday Trans 1997;93(1):193. [6] Beck JS, Vartuli JC, Kennedy GJ, Kresge CT, Roth WJ, Schramm SE. Chem Mater 1994;6:1816. [7] Barret EP, Joyner LG, Halenda PP. J Am Chem Soc 1951;73:37. [8] Chen CY, Xiao SO, Davis ME. Microporous Mater 1995;4:20. [9] Huo Q, Margolese DI, Stucky GD. Chem Mater 1996;8:1147. [10] Elder KJ, Dougherty J, Durand R, Iton L, Lockhart G, Wang Z, Withers R, White JW. Colloids Surfaces A: Physicochem Eng Aspects 1995;102:213. [11] Tanev PT, Pinnavaia TJ. Chem Mater 1996;8:2068. [12] Khimyak YZ, Klinowski J. Chem Mater 1998;10:2258. [13] Brunauer S, Deming LS, Deming WS, Teller E. J Am Chem Soc 1940;62:1723. [14] Breck DW. Zeolite molecular sieves, New York: John Wiley & Sons, 1974. [15] Huo Q, Margolese DI, Ciesla U, Feng P, Gier TE, Sieger P, Leon R, ¨ F, Stucky GD. Nature 1994;368:317. Petroff PM, Schuth