Formation of colloidal molecular sieves: influence of silica precursor

Formation of colloidal molecular sieves: influence of silica precursor

Colloids and Surfaces A: Physicochem. Eng. Aspects 217 (2003) 153 /157 www.elsevier.com/locate/colsurfa Formation of colloidal molecular sieves: inf...

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Colloids and Surfaces A: Physicochem. Eng. Aspects 217 (2003) 153 /157 www.elsevier.com/locate/colsurfa

Formation of colloidal molecular sieves: influence of silica precursor S. Mintova a,*, V. Valtchev b, T. Bein a a

b

Department of Chemistry, University of Munich (LMU), Butenandtstr. 5-13 (E), 81377 Munich, Germany Laboratoire de Mate´riaux Mine´raux, UMR-7016 CNRS, ENSCM, Universite´ de Haute Alsace, 3, rue Alfred Werner, 68093 Mulhouse, France

Abstract The formation of nanosized zeolite with MFI type structure from precursor mixtures utilizing tetraethylorthosilicate (TEOS) and precipitated silica (Sipernat 320) as silica sources was studied with in situ dynamic light scattering (DLS). All measurements were performed with fully concentrated samples, and the DLS data showed that the size of the primary species in the precursor mixtures is strongly dependent on the type of silica source employed. The precursor particles formed in TEOS-containing solution aged for 30 h at room temperature are much smaller ( /2.5 nm radius) than those prepared from precipitated silica ( /45 nm). After heating at 90 8C the amorphous precursor colloids are completely transformed into crystalline zeolite with MFI structure in 10 and 15 h for the TEOS and precipitated silica containing mixtures, respectively. The mean hydrodynamic radii of the crystalline MFI nanoparticles correspond to the size of the amorphous particles formed immediately after preparation of both precursor mixtures. Replacing the TEOS with precipitated silica leads to a change in the particle size distribution in the precursor solution, which influences the final radius of the MFI crystallites. # 2002 Elsevier Science B.V. All rights reserved. Keywords: MFI zeolite; Silica source; Colloids; DLS

1. Introduction The properties of materials are well known to be highly dependent on particle size, which implies that new applications for known crystal structures should exist at a suitable length scale. The properties of the microporous materials, where the

* Corresponding author. Tel.: /49-89-2180-7625; fax: /4989-2180-7622. E-mail address: [email protected] (S. Mintova).

intracrystalline volume is accessible via a regular system of micropore, are also affected by the crystal size. The reduction of the particle size of zeolite type materials from the micrometer to the nanometer scale can improve the mass- and heattransfer in catalytic and sorption processes, thereby enhancing their catalytic selectivity and reducing coke formation in some petroleum reactions. The synthesis of zeolite crystals with equal particle size requires homogeneous distribution of the viable nuclei in the system. Therefore, the homogeneity of the starting system and simulta-

0927-7757/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0927-7757(02)00570-8

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neity of the events leading to formation of precursor gel particles and their transformation into crystalline zeolitic material are of primary importance [1 /4]. One of the factors having great impact on zeolite crystallization process, in particular on the synthesis of colloidal zeolite crystals, is the nature of the silica source. A limited number of articles discuss the formation of nanosized zeolites by using different silica sources [5,6]. The substitution of tetraethylorthosilicate (TEOS) by colloidal silica in the synthesis of nanosized silicalite-1 was found to prolong the duration of the nucleation period [6]. A recent study of the effect of silica sources including TEOS, colloidal silica (Ludox LS 30) and fumed silica (Cab-O-Sil) on the formation of silicalite-1 nanocrystals showed that the ultimate size of zeolite crystals is strongly dependent on the silica source employed [7]. The most investigated MFI type molecular sieve has three-dimensional pore structure with intersecting straight and sinusoidal channels [8]. The MFI zeolite synthesized from colloidal precursor solutions has received considerable attention because it can serve as model system for a fundamental understanding of the mechanism of zeolite formation. The aim of the present study was to investigate the effect of precipitated silica on the formation of MFI nanocrystals and to compare the results with the ones obtained with TEOS. The precursor silica intermediates are so fragile that only in situ measurements will give valuable results. The present study is based on in situ dynamic light scattering measurements of the transformation process of the silica precursors into crystalline MFI materials.

2. Experimental Nanosized purely siliceous MFI crystals were synthesized from colloidal mixtures having the following molar chemical composition: 9 TPAOH:0.13 Na2O:25 SiO2:420 H2O:100 EtOH. The silica sources used for the preparation of precursor mixtures were tetraethylorthosilicate (TEOS) (98%, Aldrich) and precipitated silica

(Sipernat 320) (98%, particle size /15 nm, Degussa), and the organic template was tetrapropylammonium hydroxide (TPAOH) (20 wt.% in water, Aldrich). For clarity, the starting mixtures prepared with TEOS and precipitated silica are noted as S1 and S2, respectively. The silica source was mixed with double distilled water under vigorous stirring and aged on an orbital shaker at ambient temperature for 30 h prior to further hydrothermal (HT) treatment and in situ DLS investigation. All precursor mixtures were filtered with a 450 nm PTFE filter before heating at 90 8C in a quartz cuvette mounted in a heating sub-unit in a DLS instrument. The crystalline nanoparticles were purified by repeated centrifugation at 20 000 rpm for 1 h, followed by redispersion in distilled water using an ultrasonic bath to obtain a colloidal sol with a pH of 10 before freeze drying. Dynamic light scattering was used to investigate the crystal growth of nanosized MFI type zeolite (ALV /NIBS/HPPS, scattering angle 1738, HeNe laser with 3 mW output power at 632.8 nm wavelength). The analyses were performed on samples having the original concentration without any pretreatment of the colloidal solutions. The typical measurement settings included 60 scans, and the calculated polydispersity index (ip) was used to indicate a multi-modal particle size distribution. X-ray diffraction (XRD) powder data of the freeze-dried samples were collected on a Scintag XDS 2000. Images of the nanosized zeolite particles were taken on a Philips XL 40 scanning electron microscope (SEM).

3. Results and discussion The distribution function analyses (DFA) provide information about the distribution of particles in the various particle size classes. The DFA results for the precursor mixtures S1 and S2 and samples with crystalline MFI nanoparticles were displayed as: (a) an unweighted particle size distribution, which shows the scattered intensity per particle size class, and (b) a mass particle size distribution, which shows the corresponding relative mass content per particle size class. This method separates particle size classes that differ

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by a factor of 2 from each other based on the Rayleigh /Debye model [9]. After aging for 30 h at RT, the precursor mixture S1 was transferred into a quartz cuvette for in situ DLS measurements at 90 8C and the particles were classified in logarithmic radius classes. The DFA results calculated as an unweighted particle size distribution for mixture S1 before and after HT treatments are depicted in Fig. 1. The presence of only sub-colloidal particles with a mean radius of about 2.5 nm was found in sample S1 after 30 h aging at RT (Fig. 1(a)). After HT treatment for 3 h, an increase of the scattered intensity due to the presence of a second generation of particles with radius of about 15 nm is observed (Fig. 1(b)). When increasing the HT treatment to 10 h the small fraction (/2.5 nm) is consumed and transformed completely into 15 nm particles (Fig. 1(c)). A similar trend can be extracted from the DFA data plotted as a function of the mass distribution (Fig. 2). The corresponding particle mass distribution function (see Fig. 2(a)) shows that the content of the 2.5 nm subcolloidal particles is much higher than that of the 15 nm colloidal particles during the first several hours of heating. The distribution peaks indicative of the presence of two generations of particles in Figs. 1 and 2 are very similar, which is most likely due to the similar shape and size of the amorphous and crystalline particles, i.e. in both cases only spheroidal particles are found.

Fig. 1. DLS data of precursor mixture S1 (a) aged at room temperature for 30 h, and subjected to HT treatment at 90 8C for (b) 3 h and (c) 10 h. The DFA is displayed as scattered intensity per unweighted particle size class.

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Fig. 2. DLS data of precursor mixture S1 (a) aged at room temperature for 30 h, and subjected to HT treatment at 90 8C for (b) 3 h and (c) 10 h. The DFA is displayed as scattered intensity per relative mass content.

The particle size distribution of the primary entities formed after mixing precipitated silica and other substances is shown in Fig. 3. As can be seen, mixture S2 contains particles with a wide distribution ranging from 5 to 100 nm (Fig. 3(a)). A welldistinguished population of large particles with mean radius of about 45 nm was formed under heating of mixture S2 for 8 h (Fig. 3(b)). The scattering intensity from this population increases and the resulting peak becomes very pronounced and sharp after 15 h HT treatment (Fig. 3(c)). The solid phase extracted from this sample was entirely crystalline according to the XRD data. The particle size distribution curves demonstrate that

Fig. 3. DLS data of precursor mixture S2 (a) aged at room temperature for 30 h, and subjected to HT treatment at 90 8C for (b) 8 h and (c) 15 h. The DFA is displayed as scattered intensity per unweighted particle size class.

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the consumption of the small sub-colloids is a time dependent process and the 45 nm fraction is always dominant in the mixture S2. This fraction corresponds to the size of the final crystalline MFI particles (see Fig. 3(c)). The corresponding particle mass distribution function shows that the content of the 5 nm particles is much higher than that of the 45 nm particles during the first several hours of heating (Fig. 4). The distribution peaks indicative of the presence of two fractions have different relative intensities compared to the ones presented as unweighted particle size distributions in Fig. 3. However, the mean radii of the two fractions of particles in both DFA presentations have the same values, i.e. 5 and 45 nm. When the unweighted is converted into weighted-particle size distribution, the mean mass is a cubic function of the radius. It is postulated that solution S2 contains more irregular precursor species compared to the more homogeneous spheroidal particles in solution S1, which is based on the progressive hydrolysis of TEOS. In addition to DLS measurements, the sizes of the nanocrystals were determined from SEM images and are consistent with the particle radii reported before. The SEM photographs of both samples show almost spheroidal crystals (Fig. 5) although more particles with irregular shape could be seen in Fig. 5(b), which is consistent with the

Fig. 5. SEM micrographs of MFI nanocrystals obtained from the in situ experiments in mixture (a) S1 and (b) S2 for 10 and 15 h HT treatment, respectively. M/500 nm.

DFA data. The XRD-analysis of the purified and freeze-dried samples S1 and S2 confirmed that MFI type molecular sieves were crystallized after 10 h and 15 h of HT treatment of the two precursor solutions. As the 15 and 45 nm fractions in samples S1 and S2, respectively, increased, the crystallinity of the sample increased. The corresponding evolution of the light scattering data suggests that the zeolite phase is represented by the bigger radius fractions (15 and 45 nm) while the smaller fractions (2 and 5 nm) are still amorphous.

4. Conclusions

Fig. 4. DLS data of precursor mixture S2 (a) aged at room temperature for 30 h, and subjected to HT treatment at 90 8C for (b) 8 h and (c) 15 h. The DFA is displayed as scattered intensity per relative mass content.

The crystallization process of pure siliceous MFI nanocrystals from precursor mixtures using tetraethylorthosilicate and precipitated silica was studied by in situ DLS. The nanoscopic entities provided by the silica sources, and the particle size distributions during the transformation from amorphous into crystalline particles were studied.

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The small subcolloidal particles are consumed during the hydrothermal treatment and transformed into crystalline MFI particles with size of about 15 and 45 nm for TEOS- and precipitated silica-containing mixtures, respectively. The crystalline MFI nanoparticles have the same size as the primary amorphous particles formed in the two precursor solutions after mixing of all compounds at room temperature prior to hydrothermal syntheses.

Acknowledgements The financial support of the Bavarian-French Foundation and DFG/CNRS is greatly acknowledged.

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