Mesoporous material formed by acidic hydrothermal assembly of silicalite-1 precursor nanoparticles in the absence of meso-templates

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Microporous and Mesoporous Materials 110 (2008) 77–85 www.elsevier.com/locate/micromeso

Mesoporous material formed by acidic hydrothermal assembly of silicalite-1 precursor nanoparticles in the absence of meso-templates Wesley J.J. Stevens a,*, Vera Meynen a, Erik Bruijn a, Oleg I. Lebedev b, Gustaaf Van Tendeloo b, Pegie Cool a, Etienne F. Vansant a a

Laboratory of Adsorption and Catalysis, University of Antwerpen (CDE), Universiteitsplein 1, 2610 Wilrijk, Belgium b EMAT, University of Antwerpen (CGB), Groenenborgerlaan 171, 2020 Antwerpen, Belgium Received 31 May 2007; received in revised form 3 August 2007; accepted 3 September 2007 Available online 12 September 2007

Abstract Silicalite-1 precursor nanoparticle solutions are prepared from a mixture of TPAOH, H2O and TEOS. Acidification of the clear nanoparticle solution and a consequent hydrothermal treatment results in the formation of a large pore mesoporous material. The use of meso-templates for the formation of the mesopores is no longer required. These materials will be denoted mesosil. The porous characteristics of mesosil materials can be adjusted by controlling synthesis time, temperature and pH. The maximum surface area and pore volume are up to 600 m2/g and 1.35 cm3/g, respectively. The porosity in these materials originates from the interparticle voids between the nanoparticles (size approximately 5 nm) in contrast to the framework porosity of M41S, SBA-15, and similar materials in which mesopores are formed through a templated synthesis route. The material is characterized by N2-sorption, TEM, FT-Raman, DRIFT and TGA measurements.  2007 Elsevier Inc. All rights reserved. Keywords: Silicalite-1 precursor; Nanoparticles; Mesoporous material; TPAOH

1. Introduction Classical microporous zeolites are well known for their sorption, sieving and catalytic applications. Their industrial uses are widespread, and keep expanding up to this date. Despite their high selectivity, stability and activity, they have one major drawback. The limited accessibility of the zeolites due to their microporous nature (pore diameter < 2 nm) disables them to process large molecules. Since 1992, a wide variety of siliceous mesoporous materials have gained growing interest [1,2]. However, the amorphous nature of the silica walls limits their practical use, mostly because of its lower stability. In order to overcome this problem, an increasing amount of research is carried out on mesoporous materials that combine the excellent prop-

*

Corresponding author. Tel.: +32 3 820 23 79; fax: +32 3 820 23 74. E-mail address: [email protected] (W.J.J. Stevens).

1387-1811/$ - see front matter  2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.09.007

erties of zeolites with those of mesoporous materials. During the last years, a lot of methods have been described and published trying to achieve this goal. All these methods differ in the synthesis methodology that was applied. The first method discussed uses classical zeolite materials as a starting material in which mesopores are created. Different approaches are followed, the examples are numerous and a few are presented here. Mesopores can be formed in the USY zeolite by steaming the crystals [3,4]. Alkaline treatment of MFI zeolites results in a desilication of the zeolite generating mesopores at the edges of the crystals [4,5]. Both methods however actually destruct the initial zeolite phase in order to form secondary meso/macropores. Moreover, it is proven that there is only a small increase in the diffusion of guest molecules in these materials, since the formed mesopores are mostly cavities with no interconnections to form a mesoporous network responsible for leading reactants to the internal and external surfaces [6,7]. On the other hand, carbon black aggregates and carbon nanofibers

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have been used as templating agents in zeolite synthesis [4,8]. The removal of the carbon species with a post-synthesis calcination gives rise to the mesoporosity within the zeolite framework. Another approach starts with the making of mesoporous materials (containing Si or Si and Al) followed by a recrystallization of the structure [9]. However, the mesoporous area and volume decrease rapidly with increasing zeolite formation. Another methodology that can be applied for the formation of mesopores in zeolites consists of a synthesis in which zeolite precursor species are used as a siliceous source. These precursor species are subcolloidal particles of a few nanometers [10] with reported specific dimensions of 4 · 4 · 1.3 nm [11]. Addition of the zeolite precursor species to a solution containing a meso-template can result in the formation of a mesoporous material. Some of the materials produced in this manner include zeotile [12], zeogrid [13], MSU-S/W [14] and MTS-9 [15]. However, the extend of zeolite characteristics in these materials is still subjected to much discussion. Recently, there has been a lot of controversy concerning the zeolite precursor species (by some authors also denoted as nanoslabs or nanoblocks [11]). The controversy is based on the question whether these nanoparticles have well defined shapes and whether they already possess the zeolite structural characteristics [16– 20]. Moreover, it is still unclear whether manipulation of the nanoparticles in solution (e.g. extracting them from solution) preserves their original structure [18]. The group of KULeuven (J. Martens et al.) concluded from their measurements that these zeolite precursor species possess some zeolite characteristics [11]. Liu and co-workers also found evidences in IR for the presence of at least partial MFI topology, since their MSU/S materials from zeolite precursor particles show the typical band of five-membered rings at 550 cm 1 [14]. However, Knight et al. have published two papers dealing with 29Si NMR spectroscopy studies of the zeolite precursor particles [18,21] in which they found no evidence suggesting the existence of nanoparticles with MFI topology in solutions containing TPA template ions. The same conclusion was adapted by Shantz and co-workers [22] when they studied a similar system with 29 Si NMR. In a series of articles by Vlachos and co-workers [19,23,24], the structure of the nanoparticles was studied with TGA, SAXS, SANS, and 29Si MAS NMR. These studies showed that the particles are amorphous and do not possess a well-defined shape, but rather exist as a core shell structure with silica in the core and a shell of TPA ions surrounding it. At elevated temperatures, the TPA gets incorporated in the core of the nanoparticle in addition to maintaining it in the shell. From a combined TEM and AFM study carried out by Tsapatsis and coworkers [20] on extracted nanoslabs, there was again no evidence found which suggests that block-shaped nanoparticles with MFI topology are the building blocks for an MFI zeolite structure. Leaving in the middle whether these nanoslabs truly possess MFI topology, they can however be used as a sili-

con source in a sol–gel process. This way, materials can be made by a simple synthesis method without the use of templates for the formation of the mesopores. This method has economical and environmental advantages compared to other synthesis approaches in which templates are essential for mesopore formation. The clear sol that is formed during the formation of the zeolite precursor solution will be converted into a gel during a hydrothermal treatment at elevated temperatures. In a basic environment, the sol gives rise to microporous zeolite crystals. However, if the sol is acidified to a pH < 2, large pore mesoporous materials are formed. In this work, the formation of a large pore mesoporous material will be described, using a hydrothermal aging process starting from a clear subcolloidal silicalite-1 precursor sol. During the synthesis, no meso-templating agent is added. Changing the synthesis parameters such as time, temperature, pH and pressure gives rise to control of the structural and chemical characteristics of the final mesoporous material, denoted mesosil. 2. Experimental 2.1. Synthesis Silicalite-1 precursor nanoparticles were prepared by mixing a solution containing 1.0 M tetrapropylammoniumhydroxide (TPAOH, Aldrich) as the zeolite template, tetraethoxysilane (TEOS 98%, Acros) as the silica source and water. The TPAOH/H2O/TEOS solution with a molar ratio of 1/100/6.5 was subsequently heated under stirring to 70 C and was kept at this temperature until 50% of the liquid was evaporated. The resulting clear solution was left to cool down to room temperature and was aged for 5 days. Afterwards, this solution was acidified using an HCl solution (12 M). For acidification to a pH lower than 0, 5.9 g of the 12 M HCl solution was added. For the solutions with a pH 0 or pH 2, 5.9 g of HCl solution (4.6 M and 1.2 M, respectively) was added followed by dropwise addition of NaOH solution until the required pH was reached. The resulting clear solutions were transferred into a sealed autoclave for 1–5 days at temperatures varying between 100 and 150 C. Finally, the obtained products were washed, dried and calcined in ambient atmosphere at 550 C for 6 h with a heating rate of 1 C/min. 2.2. Characterization techniques Porosity and surface area were determined using a Quantachrome Autosorb-1-MP automated gas adsorption system using nitrogen as the adsorbate at 196 C. All samples were outgassed under vacuum for 16 h at 200 C prior to sorption measurements. Pore diameters were obtained from the desorption branch of the isotherm using the Barrett–Joyner–Halenda (BJH) method. Surface area was calculated using the Brunauer–Emmett–Teller (BET)

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method and the microporous characteristics were determined using the t-plot method. FT-Raman (Fourier transform Raman) spectra were recorded on a Nicolet Nexus FT-Raman spectrometer with a Ge detector and a 1064 nm Nd:YAG laser. Samples were measured at room temperature in a 180 reflective sampling configuration. Four thousand scans were averaged for each sample with a resolution of 8 cm 1. The laser power was set between 0.5 and 1 W for uncalcined samples and at 1.5– 2.0 W for calcined samples. Thermogravimetric measurements were performed on a Mettler TG50 thermobalance, equipped with an M3 microbalance and connected to a TC10A processor. Samples were heated from 30 to 550 C at a rate of 5 C/min in an oxygen atmosphere. In situ DRIFT (diffuse reflectance infrared Fourier transform) spectra were recorded on a Nicolet 20SXB FTIR spectrometer equipped with an in situ DRIFT cell (Spectra-Tech) and an MCT detector. The samples were mixed with KBr (98% KBr, 2% sample). Resolution was set to 4 cm 1 and 200 scans were accumulated. Measurements were performed under a flow of dry air. The solid material was further characterized by transmission electron microscopy (TEM) using a Philips CM20 instrument with a point resolution of 0.27 nm equipped with a fibre optic coupled GATAN 622 TV system. The TEM samples were prepared by crushing the material in ethanol in an agate mortar and dropping this dispersion of finely ground material onto a holey carbon film supported on a Cu grid. Low electron beam intensities and lower magnification were applied in order to preserve the structure and to avoid sample drift during the exposure of the films. The electron microscope was operated at 200 kV with primary magnifications less than 100,000.

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3. Results and discussion Fig. 1 shows the nitrogen sorption results of the calcined mesosil products obtained after a hydrothermal treatment of the acidified silicalite-1 precursor nanoparticle solution (pH < 0) at different temperatures and times. All materials show a type IV isotherm according to IUPAC classification, indicating the presence of mesopores. Performing the hydrothermal treatment without the acidification of the clear precursor solution results however in the formation of the microporous silicalite-1 zeolite. Similar isotherms were also observed during the first moments of crystallization in the study of the crystallization mechanism of ZSM-5 [25] and Beta [26] zeolites, where the interparticle space between the growing zeolites produced mesoporosity. From the isotherms and the pore size distributions (Fig. 1), it can be observed that an increase of the temperature of the hydrothermal treatment results in an enlargement of the pores from 5 to 15 nm. Moreover, from the hysteresis of the materials it can be seen that low hydrothermal treatment temperatures give rise to broader (bottle-neck) hysteresis loops while increased temperatures (130–150 C) give rise to more narrow hysteresis loops. When the synthesis temperature is constant and longer synthesis times are applied, an increase in the pore diameter is obtained. However, varying the synthesis temperature induces larger changes compared to varying the synthesis time. Table 1 shows the porous characteristics of the materials synthesized at different temperatures and times. With increasing temperature and time, it can be observed that there is a gradual decrease in specific surface area and an increase in both pore size and pore volume. This can be explained similar as to sol–gel ageing. When the aging of the solution becomes more extensive, a higher degree of condensation and more stable bonds prevent high shrinkage of the

1000 3d 100°C 3d 130°C 3d 150°C 6d 130°C silicalite-1

Volume adsorbed, cm3/g (STP)

900 800 700 600 500 400 300

0

5

10

15

r/nm

200 100 0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

P/P0 Fig. 1. N2-sorption isotherms at 196 C of the calcined mesosil materials and the full growth silicalite-1 zeolite. Pore size distributions calculated by BJH method on the desorption branch of the isotherm of the mesosil materials are shown in the inset.

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Table 1 Porous characteristics of the synthesized mesosil materials Mesosil 3 days 100 C 3 days 130 C 3 days 150 C 6 days 130 C 3 days 150 C 3 days 130 C 3 days 130 C Silicalite-1

pH < 0 calcined pH < 0 calcined pH < 0 calcined pH < 0 calcined pH < 0 non-calcined pH 0 calcined pH 2 calcined

BET surface area (m2/g)

Total pore volume (cm3/g)

Micropore volume (cm3/g)

Pore radius (nm)

578 415 308 381 147 597 548 350

0.726 1.25 1.31 1.35 0.686 0.733 0.744 0.237

0 0 0.014 0.005 0 0 0 0.158

2.5 5.9 7.8 7.0 7.2 2.5 2.6

Micropore volume calculated by t-plot method, pore radius calculated on the desorption branch of the isotherm by BJH-method.

material upon calcination. Therefore, large pore volumes and pore diameters are obtained at prolonged synthesis times and/or higher temperature [27]. Therefore the reaction mechanism seems to be based on aggregation of the preformed nanoparticles similar as in sol–gel synthesis. More information on the aggregation of the nanoparticles can be found by TEM (Fig. 2). From TEM it is clear that the mesoporous material obtained by acidification and a consequent hydrothermal treatment at 150 C for 3 days of the precursor solution of silicalite-1 is not ordered. It can be seen that the mesopores are formed by the aggregation of nanoparticles. The interparticle voids are estimated to be approximately 15 nm in size which is in good agreement with the N2-sorption data. The inset in Fig. 2 shows the enlargement of the top left part of the TEM image. From this image it can be seen that the nanoparticles that build up the structure are similar in size (about 5 nm) and interconnect by edge sharing. However, TEM does not provide any evidence on the crystalline nature of these nanoparticles. The formation of the mesosil material under acidic conditions most likely occurs through a sol–gel mechanism with cluster–cluster growth. Silica will be positively charged because of the acidic environment, resulting in a small repulsion between the nanoparticles. When energy of collision is sufficient, however, the particles can aggregate. This aggregation reaction is reversible which results in the formation of a thermodynamically stable porous structure.

Fig. 2. TEM picture of mesosil synthesized for 3 days at 150 C. The enlargement shows the edge-sharing between the nanoparticles.

Acidification of the silicalite-1 precursor solution is an important step in the formation of mesosil materials. Indeed, synthesis in basic medium results in the formation of pure silicalite-1 zeolite. Therefore, mesosil has been synthesized with different pH solutions (pH < 0, pH 0 and pH 2). The isotherms of these materials are shown in Fig. 3. Increasing the pH of the nanoparticle solution results in the formation of materials which have a less open structure. The surface area rises slightly while the pore radius as well as pore volume are almost halved (Table 1). Synthesis at higher pH (closer to the point of zero charge) will result in fewer positively charged silicalite-1 precursor nanoparticles. Since there will be less repulsion between these particles, they will condense and aggregate much faster [28], since at these low pH values the solubility of Si decreases exponentially with pH. The number of particles that will be able to redissolve in solution and aggregate in a more stable way will be severely diminished. Therefore, a material with more defect sites and a less robust structure will be obtained which results in the formation of a structure with narrowed pores similar as when low temperatures are applied. Synthesis with even higher pHs (pH > 2) result in the formation of a viscous gel directly after acidification due to the low solubility of silica in this pH region [28]. The influence of pH on the aggregation of these nanoparticles into the final mesoporous material closely resembles cluster–cluster aggregation under diffusion limited conditions in sol–gel mechanisms. The question however remains whether the nanoparticles that are the building blocks for these mesosil materials already possess some zeolite characteristics. According to the literature there are some diametrically opposed theories on this matter and this is studied with a wide variety of techniques [14–24,29–36]. The nanoparticles, which form the mesosil structure, have a size of approximately 5 nm as can be seen in the TEM (Fig. 2). This is similar to sizes reported for nanoparticles in the clear solution synthesis as observed by several authors [10,11,17]. Therefore, for these mesosil materials no information on the crystalline nature of these nanoparticles can be obtained by XRD, since zeolite particles which are smaller than 8 nm show no clearly observable X-ray diffraction signal, due to the large widening of the diffraction bands [37].

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900 3d 130°C pH<0 3d 130°C pH=2 3d 130°C pH=0

Volume adsorbed, cm3/g (STP)

800 700 600 500 400 300 200 100 0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

P/P0 Fig. 3. N2-sorption isotherms at

196 C of mesosil synthesized for 3 days at 130 C at different pHs.

Comparing the isotherms of the non-calcined and the calcined mesosil materials synthesized during 3 days at 150 C (Fig. 4), one can notice the difference in N2-uptake in the low relative pressure region. The difference at P/P0 = 0.02 is approximately 50 cm3/g STP which corresponds to about 0.08 cm3/g at ambient pressure and temperature. The difference between this number and the result from the t-plot analysis (Table 1) shows that the t-plot method underestimates the micropore volume to a large extent when the microporous particles are very small [26,38]. This underestimation likely arises because N2adsorption which occurs at the entrances of the micropores is considered as adsorption at the external surface and is

not included in the t-plot calculations. Since the precursor nanoparticles are very small, they probably possess a lot of micropore entrances which results in the low micropore surface area and volume calculated from t-plot for the materials synthesized from these particles. From Fig. 4 and Table 1, it is obvious that the non-calcined material has a much lower surface area and volume (Table 1). This can be explained by several factors. The surface area and volume are expressed per gram of solid. Since the non-calcined material still contains about 15% of TPA+ in and on its structure (Fig. 8), the area and volume calculated from N2-sorption will deviate by the same amount.

900

Volume adsorbed, cm3/g (STP)

800 700

b

a

600 500 400 300 200

0

2

4

6

8

10

12

14

16

18

20

r / nm

100 0 0.0

0.1

Fig. 4. N2-sorption isotherms at

0.2

0.3

0.4

0.5 P/P0

0.6

0.7

0.8

0.9

1.0

196 C of: (a) calcined and (b) non-calcined mesosil material synthesized for 3 days at 150 C.

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Intensity

Pore size calculation shows that the pore size of the noncalcined material is somewhat smaller than that of the calcined material (Table 1), probably due to an adsorbed layer of TPA+ on the surface. Moreover, in the non-calcined material, TPA+ is still present in the micropores and these will therefore not contribute to the surface area and volume of the non-calcined material. From these results it can be suggested that some micropores are present, although the amount of micropores present cannot be estimated unambiguously. Spectroscopic techniques such as DRIFT and FTRaman give us the possibility to study the surface properties of the synthesized materials. Furthermore, when studying the non-calcined powders, changes in the conformation of the microporous template provide information concerning its local environment. Figs. 5 and 6 show FT-Raman spectra of, respectively, the calcined and the non-calcined mesosil materials. From Fig. 5, where the comparison is made between calcined crystalline microporous silicalite-1, mesoporous SBA-15 (completely amorphous mesoporous silica) and mesosil, one can observe three characteristic bands for silicalite-1. The SBA-15 material, which is made from amorphous silica, shows only a broad band in the region between 150 and 600 cm 1. The mesosil materials display a broad band similar to SBA-15 with three peaks superimposed to it which correspond to some of the peaks present in silicalite-1. These bands appear at 300, 380 and 440 cm 1 and can be assigned to the bending vibrations of Si–O–Si and O–Si–O bonds [39,40]. Since the broad band of amorphous silica is still present and only three characteristic peaks of silicalite-1 can be found superimposed to the amorphous signal, it is clear that no real long range crystallinity is present. However, some local degree of ordering similar to structural features of silicalite-1 can be expected [41,42].

a b c

1000

800

600

400

200

0

Raman shift (cm -1) Fig. 5. FT-Raman spectra of: (a) calcined silicalite-1, (b) mesosil synthesized for 3 days at 150 C, and (c) SBA-15.

CH3

CH2

Intensity

82

d c b a 3100

3000

2900

2800

2700

Raman shift (cm-1) Fig. 6. FT-Raman spectra of: (a) tripropylamine, (b) TPAOH, (c) noncalcined mesosil synthesized for 3 days at 150 C and (d) non-calcined silicalite-1.

Whether these small local areas already have silicalite-1 functionality or other useful characteristics that differ from amorphous silica will need to be studied. One patent [43] already states the benefit of these types of mesoporous materials in contrast to amorphous silica in fixation of poorly soluble drugs and biologically active species. The FT-Raman spectra of the non-calcined mesosil, silicalite1, TPAOH solution and tripropylamine are shown in Fig. 6. The intensity ratio CH2/CH3 is 0.75 for bound TPA ions in the silicalite-1 structure [35,44,45]. However, in the TPAOH solution this intensity ratio is 1.0. As was reported in the literature, the intensity of the CH3 assymetric stretching vibration (2977 cm 1) increases relative to the intensity of the CH2 stretching vibration (2939 cm 1) with increasing growth of the zeolite due to the interaction of the template with silica [35,45]. From the FT-Raman spectrum of mesosil (Fig. 6c) it is clear that the intensity ratio CH2/CH3 has increased to 1.1 instead of the expected decrease to 0.75. This increase in intensity ratio was assigned to the degradation of part of the TPAOH templates due to acidification by Chiesa and co-workers [34]. They concluded this based on an EPR and FT-Raman study of vanadium silicalite precursor nanoparticles deposited on SBA-15. In this study, an interaction between the nitrogen of the TPAOH template of the zeolite and vanadium was observed after acidification of the nanoparticle solution and subsequent deposition. The study clearly showed that this was only possible if the template lost a propyl ligand upon acidification. This way the nitrogen present in the TPAOH template could coordinate with vanadium oxide present in the material. It was suggested that due to the acidic treatment, TPAOH molecules at the outer surface of the nanoparticles lose a ligand and become tripropyl-

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4.5

Absorbance Kubelka-Munk

4

3d 150°C silicalite 1 SBA-15

3.5 3 2.5

wt % dw/dt

90

-0.0001

-0.0003 -0.0004

70

dw/dt

-0.0002 80

-0.0005

60

-0.0006 50 40 30

-0.0007 -0.0008 80

130 180 230 280 330 380 430 480 530

Temperature (°C) Fig. 8. Thermogravimetric analysis of mesosil synthesized for 3 days at 150 C.

well as a small shift of the band at 1100 cm 1 to higher wave numbers [47]. This suggests that the structure of mesosil is more condensed than the completely amorphous SBA-15 structure. An increase of the band at 980 cm 1, which is assigned to the surface Si–O bonds, is probably due to the large external surface of the precursor nanoparticles. Additional information concerning the mesosil material can be obtained by thermogravimetric analysis (TGA) of the non-calcined powders. Free TPA ions show an endothermic weight loss between 150 and 220 C, while occluded TPA ions will decompose at higher temperatures up to 400 C [19,29]. The TGA of the mesosil material synthesized during 3 days at 150 C is shown in Fig. 8. The TPA decomposition and combustion clearly occurs in two steps. Physisorbed TPA (5%) is desorbed at approximately 210 C [29]. However, decomposition and combustion of TPA ions (partially) occluded in the structure (9%) is delayed until 250 C, while fully occluded TPA shows a weight loss around 380 C [19,29]. These partially and fully occluded TPA molecules are responsible for the formation of microporosity in the sample. Since there is no fully occluded TPA present in the sample, there will be no long range ordered crystals present and it can be suggested that the micropores in the mesosil samples (if present) will be very short. This confirms the conclusions previously deduced from N2-sorption experiments. 4. Conclusion

2 1.5 1 0.5 0 1500

0

100

wt%

amine. The coexistence of tripropylamine and tetrapropylammoniumhydroxide (TPAOH) in the nanoparticles could explain the unexpected increase in the CH2/CH3 intensity ratio. In Fig. 6 it can be seen that for the mesosil material, the intensity ratio CH2/CH3 has increased compared to the TPAOH in solution. Also Vlachos and co-workers [19] reported a different conformation of TPA molecules in silicalite-1 after extraction of the particles by addition of salts and acidification with HCl. Since the mesosil material is made by an acidification step of the precursor nanoparticles solution, it can be expected that also in these materials a large part of the TPA+ present in the material (in fact all TPA which is accessible, so on the outside of the particles) will actually be tripropylamine. In the literature it was stated that the propyl ligands present in the nanoparticles and pointing with one propyl chain to the liquid phase are necessary for the aggregation of the nanoparticles into full-grown zeolite [34,46]. Therefore it can be assumed that the loss of this ligand upon acidification results in the loss of preferential orientation and prevents crystal growth resulting in the formation of the mesoporous mesosil materials by edge sharing of the preformed nanoparticles. DRIFT spectra of calcined silicalite-1, SBA-15 (build up with amorphous silica) and mesosil are shown in Fig. 7. Mesosil shows a small increase in the band at 550 cm 1 compared to the amorphous SBA-15 which indicates that some five-membered rings could be present [14]. This band is typical for MFI structure in which the band is very sharp and intense [29,30]. The low intensity and broad nature of the band again demonstrates that no long range ordered zeolite phase is present but some zeolitic features could be situated in some local areas. Moreover, a clear increase of the shoulder at approximately 1200 cm 1 is observed as

83

1300

1100

900

700

500

Wave number (cm-1) Fig. 7. DRIFT spectra of calcined silicalite-1, SBA-15 and mesosil synthesized for 3 days at 150 C.

It is shown for the first time that it is possible to synthesize large pore mesoporous silica materials starting from a silicalite-1 precursor solution without the use of a mesotemplating agent. The porous characteristics of these socalled mesosil materials can be controlled by controlling synthesis time, temperature and pH up to a maximal surface area and pore volume of 600 m2/g and 1.35 cm3/g, respectively. The pore size can be adjusted in the range of 5– 15 nm. The nanoparticles in the precursor solution will not grow into a pure crystalline silicalite-1 phase due to the acidification of the clear nanoparticle solution. This is probably due to the loss of a propyl ligand of TPAOH upon

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acidification. This way, further growth of the silicalite-1 nanoparticles into the long range ordered zeolite is disabled. Results from N2-sorption data show that the mesosil material shows the presence of some micropores, although the estimation of their amount cannot be measured unambiguously. Furthermore, spectroscopic data from FT-Raman and DRIFT measurements show the initial appearance of some of the characteristic bands of MFI structures in the mesosil. It is expected that these are caused by some local areas with MFI structural features. However, no evidence of long range ordered MFI is present in these measurements. TGA shows the presence of physisorbed and (partially) occluded TPA ions in the silica framework. All these results indicate that mesosil may contain some local zeolitic character but no long range MFI crystallinity. The general synthesis approach adapted throughout the article may give some possibilities to introduce at least partial zeolitic function in mesoporous materials or result in a higher stability of mesoporous materials. Moreover, it is an economical and environmentally friendly way to produce mesoporous silicate structures from zeolite precursor solutions. Indeed, no expensive or toxic surfactants is needed as meso-template. Nevertheless, the question towards the extent of the zeolite character in these materials and the functionality of these zeolitic characteristics remains. Ongoing study of these materials in separation of molecules should provide extra information on this matter. However, even if only partial zeolite structural features or partial zeolite channels are present, they could still have a beneficial effect on certain applications. Acknowledgments Wesley J.J. Stevens acknowledges the Concerted Research Project (CRP) sponsored by Special Fund for Research at the University of Antwerpen. Vera Meynen acknowledges the Fund for Scientific Research-Flanders for financial support. This work was executed in the frame of the INSIDE-PORES NoE project from the European Commission (FP6-EU). References [1] G.J.D. Soler-Illia, C. Sanchez, B. Lebeau, J. Patarin, Chem. Rev. 102 (2002) 4093. [2] D. Zhao, Q. Huo, J. Feng, B.F. Chmelka, G.D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024. [3] R.A. Beyerlein, C. Choi-Feng, J.B. Hall, B.J. Huggins, G.J. Ray, ACS Symp. Ser. 571 (1994) 81. [4] S. Van Donk, A.H. Janssen, J.H. Bitter, K.P. De Jong, Catal. Rev. 45 (2003) 297. [5] J.C. Groen, J.A. Moulijn, J. Perez-Ramirez, J. Mater. Chem. 16 (2006) 2121. [6] A.H. Janssen, A.J. Koster, K.P. De Jong, Angew. Chem. Int. Ed. 40 (2001) 1102. [7] P. Kortunov, S. Vasenkov, J. Ka¨rger, R. Valiullin, P. Gottschalk, M.F. Elia, M. Perez, M. Sto¨cker, B. Drescher, G. McElhiney, C. Berger, R. Gla¨ser, J. Weitkamp, J. Am. Chem. Soc. 127 (2005) 13055.

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