Studies in Surface Science and Catalysis, volume 154 E. van Steen, L.H. Callanan and M. Claeys (Editors) © 2004 Elsevier B.V. All rights reserved.
COLLOIDAL MOLECULAR SIEVES: MODEL SYSTEM FOR KINETIC STUDY OF CRYSTAL GROWTH PROCESS Mihailova, B., Wagner, M., Mintova, S.* and Bein, T.* Department of Chemistry, University of Munich, Butenandtstr. 11, 81377 Munich, Germany. E-mail: svetlana.mintova(a)cup.uni-muenchen.de and [email protected]
ABSTRACT A clear TPAOH-Al203-Si02-H20 solution was used as a model system for a kinetic study of crystal growth of colloidal zeolite ZSM-5 with MFI type structure by dynamic light scattering (DLS), X-ray diffraction (XRD), Raman spectroscopy, high-resolution transmission electron microscopy (HRTEM) and nitrogen sorption measurements. The Raman scattering and nitrogen sorption results show that the initial hydrothermal treatment leads to a condensation of nanoparticles with porous amorphous matrix composed mainly of small-sized rings of TO4 tetrahedra. HRTEM images reveal the establishment of long range order in few-nanometer-sized spatial regions inside the amorphous matrix under heating at elevated temperature. Prolonged hydrothermal treatment leads to the formation of a concentration of chain-like entities of five-membered rings typical of MFI structure in the disordered T-O network, that enable the subsequent transformation from a disordered to an ordered microporous system detectable with X-ray diffraction. Both Raman spectroscopic and X-ray diffraction data show that the amorphous precursor colloids are completely transformed into crystalline ZSM-5 zeolite under heating at 90 °C for 18 h. According to DLS observations the mean hydrodynamic radius of the crystalline nanoparticles corresponds to the average size of the XRD-amorphous particles formed immediately after preparation of the precursor aluminosilicate mixture. Keywords: ZSM-5, crystallization, Raman scattering, DLS. INTRODUCTION The growth mechanism of microporous materials is still not well understood, and it is a great challenge to identify the structural species that assemble during the transformation from precursor sols or gels into crystalline nanoparticles. Various experimental methods have been applied to investigate the underlying kinetics of zeolite crystal growth, including dynamic light scattering (DLS), X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), infrared, Raman and nuclear magnetic resonance spectroscopy. Different mechanisms are still being discussed, such as assembly of the lattice from solution, aggregation of pre-assembled building blocks, transformation of the gel phase, etc. [1-5]. Clear precursor solutions can serve as a model system for studying the entire crystallization processes of microporous materials. The crystal growth of Silicalite-1 with MFI- type structure from pure siliceous precursor solutions has been extensively investigated with various in situ techniques [6-9]. The objective of this paper is to follow the evolution of ZSM-5 zeolite from clear aluminosilicate solutions by Raman scattering, which is very sensitive to the atomic arrangements in materials of low cry stall inity, in combination with XRD, DLS, TEM and nitrogen sorption methods. EXPERIIMENTAL Nanosized ZSM-5 crystals were synthesized from precursor solutions with a chemical composition: 0.35 Na20: 9 TPAOH: 0.25 AI2O3: 25 Si02: 599.3 H2O. Tetraethylorthosilicate (TEOS, 98 %) and aluminum isopropoxide (Al Iso, 99.99 %) were mixed with tetrapropylammonium hydroxide (TPAOH, 1 M) used as an organic template. The synthesis solutions were aged for 24 hours at room temperature and then hydrothermally treated at 90 °C from 2 h to 24 h in PP bottles in a conventional oven. For the sake of simplicity we will designate the products extracted from the precursor solutions after different crystallization times as S_x, where x means the duration of the hydrothermal treatment.
164 The particle size distribution was determined by DLS using an ALV-NIBS/HPPS Particle Size Analyzer operating at a scattering angle of 173° with an incident laser wavelength of 632.8 nm and output power of 3 mW. The XRD patterns of purified and freeze-dried samples were collected on a Scintag 2000 powder diffractometer using CuKa radiation. The Raman spectra of freeze-dried and calcined samples (450 °C for 8 h) were measured with a Bruker Equinox 55 FT-IR spectrometer equipped with a FRA106/S FT-Raman module, an NdiYAG laser (1064 nm excitation) and a liquid N2 cooled Ge detector. Nitrogen sorption measurements were performed on calcined samples with a Micromeritics ASAP 2010 surface area analyzer. HRTEM images were recorded on a Philips 200 FEG TEM operated at 200 kV.
RESULTS AND DISCUSSION The particle size distribution curves of several samples heated at 90 °C for different times are given in Fig. 1. The Rayleigh-Debye model was used for the distribution function analysis (DFA) calculations . The results from DFA were displayed as an unweighted particle size distribution, which shows the scattered intensity per particle size class. Upon adding the organic template (TPAOH) to the clear precursor solution, spontaneous agglomeration of the silica and alumina species is observed, resulting in the formation of amorphous gel particles with a mean radius of 85 nm (green line in Fig. 1). After 12 hours of hydrothermal treatment (red line in Fig. 1), the width of the particle size distribution curve decreases to about one half with respect to that of samples heated for 4 and 6 h (green and blue lines in Fig. 1). Crystallization times longer than 18 hours lead to particles with an average radius of about 60 nm and a narrow particle size distribution (black line in Fig. 1). In addition to the main fraction of particles, a class of smaller particles with a mean radius of about 15 nm is observed in all solutions that correspond to the aluminosilicate olygomers which are not involved in the formation of bigger amorphous entities.
Figure 1. Particle size distribution of purified aluminosilicate solutions heated at 90 °C for different times (normalized). The structure of the aluminosilicate particles was examined with X-ray powder diffraction from purified and freeze-dried samples. The first Bragg reflections of ZSM-5 appear after 14 h of hydrothermal treatment, and an increase in their intensity is observed up to 18 hours of heating (Fig. 2). Prolonged crystallization time does not lead to any further changes in the XRD patterns of these samples, which means that the crystallization process is completed within 18 hours. According to the XRD results one can conclude that some crystalline order appears in the samples heated at least for 14 h, although the size of the particles in these samples does not change (see Fig. 1, mean radius of particles -60 nm). However, the absence of X-ray diffraction lines does not exclude the presence of small crystalline domains in the products obtained in less than 14 h (see discussion below)
12h h N t * ^^JiffV''*^'"* *»^yi^4<>^Wll>iflH«»<»^|^f^>^^fcil>| I i»p^.Kii>iiiw>»»iiM»j|
26, degrees Figure 2. XRD patterns of samples heated at 90 °C for different crystallization times. A Raman spectroscopic study was carried out in order to gain information about the structural peculiarities of the solidified substance produced before 14 hours of hydrothermal treatment, because of the potential of the method to probe the atomic arrangements in non-periodic systems. First, non-calcined samples were studied in order to follow the specific manner of incorporation of the TPAOH molecules in the solid inorganic matrix. Comparing the spectra of the samples with that of pure TPAOH one can observe a red shift of the peaks arising from the C-H bond stretching modes even for the solid product obtained after 2 hours of hydrothermal treatment (see Fig.3a). The shift of the peaks to the lower energies clearly indicates the existence of interactions between the organic additive (TPAOH) and the inorganic silica-alumina matrix. The formation of the MFI structure can be deduced from the Raman scattering of non-calcined samples via (i) the development of the Raman band at about 380 cm"^, resulting from the 5-membered rings typical of the MFI structure, (ii) the change in the shape of the band near 1330 cm"\ which arises from the CH2 bending and wagging modes, and (iii) the change in the spectrum profile of the C-H stretching band between 2850-3050 cm'^ [11-13]. The last two features are due to the torsion of the TPA molecules occluded in the straight and sinusoidal channels of MFI to adapt the specific pore structure, which leads to a deviation from the S4 symmetry of the free (C3H7)4N^ cation. The Raman spectra of all samples obtained after 18 hours of hydrothermal treatment (spectra are not shown) resemble that of entirely crystalline TPA-MFI material. The shape of the C-H stretching band of S_14 is also very similar to that of a fully crystalline compound (Fig. 3). However, the shape of the band near 1330 cm'\ which is more sensitive to the different TPA conformations, reveals the existence of pores and cavities providing enough room for non-twisted TPA molecules, which is a sign for existence of a non-periodic T-O network (T = Si, Al). The spectra of the freeze-dried samples S_2, S_6 and S I 2 do not exhibit any pronounced spectral features indicative for the formation of MFI type structure, although in the spectrum of S I 2 a weak broadening of the peaks near 380 cm'' and 1330 cm'' is observed (Fig. 3b), which may be due to the initial formation of MFI-type structure. It has to be noted that the TPA vibrations strongly overlap the T-O vibrations, since the organic compounds are much better scatterers than tectosilicates. To elucidate the structural changes that occur in the T-O network one should consider the Raman spectra of calcined samples, after a total removal of the organic additive. The atomic framework of tectosilicates can be described in terms of rings of comer-sharing TO4 tetrahedra. The most intense peaks in the Raman spectra of this type of material arise from the so-called ringbreathing modes, which consist of in-phase oxygen vibrations perpendicular to the corresponding direction defined by the two adjacent silicon atoms [13-16]. These ring-breathing modes are very sensitive to the ring order, i.e. the number of tetrahedra forming the rings as well as to their deformation state. Thus, different silica phases are easily identified through their Raman scattering in the range of 300-650 cm''. Figure 4 shows the spectra of samples obtained after different synthesis times of the precursor aluminosilicate
166 solutions together with two types of amorphous silica. The structure of vitreous silica (v-Si02) is usually considered as the "ideal" amorphous silica and, consequently, its Raman spectrum is used as a reference spectrum for entirely disordered 4-2-connected systems. The main features in the spectrum of v-Si02 are the narrow peaks at about 606 cm'^ and 495 c m \ which are due to 3- and 4-membered rings, respectively, and the broad band centered at about 450 cm"^ originates from larger rings, mainly of 6-membered order, with a broad distribution in geometry.
Raman shift, cm
Raman shift, cm' Figure 3. Raman spectra of as-synthesized samples after different times of hydrothermal treatment and pure TPAOH (1 M in water). The vertical lines in (a) trace the positions of the most intense Raman signals in the spectrum of TPA. The arrows in (b) point to the Raman bands most sensitive to the formation of ZSM-5 zeolite. The spectra are normalized to the integrated intensity of the bands between 2850-3050 cm"^ which are generated by the C-H bond stretching modes. The spectrum of freeze-dried colloidal silica is almost the same as that of vitreous Si02, except an additional Raman signal at 977 cm'\ typical of disrupted Si-O-Si linkages, and a considerable broadening of the two peaks at 606 cm"^ and 495 cm"\ The presence of point defects in the structure of the nanoparticles can explain the slight difference between the spectra of the colloidal and vitreous silica. However, the
167 spectrum of the S_2 sample differs noticeably from the Raman scattering of amorphous silica (Fig. 4). A band near 293 cm'^ is observed for S_2 (Fig. 4a) instead of the broad signal around 450 cm'^ in the amorphous silica (Fig. 4b). The well-pronounced Raman scattering near 490 cm"^ and 600 cm'^ together with the absence of significant Raman scattering near 450 cm"^ in the spectra of samples S_2 and S_6 show that the structure of initial nanoparticles condensed in a clear solution is composed mainly of small 4- and 3member rings.
Raman shift, cm'
Raman shift, cm"
Figure 4. Raman spectra of a) calcined ZSM-5 samples obtained after different times of hydrothermal treatment and b) two types of amorphous silica i.e. colloidal silica (a-Si02 nanoparticles) and vitreous silica (v-Si02). The arrows in (a) mark the spectral features indicative for formation of the pentasil chains in ZSM-5 zeolite and the asterisk points to the "amorphous-silica" peak. On the other hand, the relatively low wavenumber of the band near 290 cm"^ suggests that it originates from rings built up of a larger number of TO4 units. Frameworks built of comer-sharing TO4 tetrahedra
168 possess low-energy modes that involve TO4 moving without internal tetrahedral distortions, and it has been proposed that these modes are responsible for structural transformations requiring insignificant cost in elastic energy . Thus, in open framework structures such modes enable non-skeleton cations to adjust the atoms from the surrounding TO4 units until an energetically favorable coordination is achieved. Since TPAOH is well known as a structure-directing template for MFI-type zeolite, one can expect that during the condensation processes in the TPAOH-Al203-Si02 solution, the 10-membered rings that are typical of MFI will be formed around the organic molecules. Thus, we assume that the band at 290 cm'^ in the spectra of the initially condensed nanoparticles arises from 10-membered rings of TO4 tetrahedra formed around the (C3H7)4N^ cations embedded in the amorphous T-0 network. This assignment is supported by the fact that a peak at the same wavenumber exists in the spectrum of well crystalline ZSM-5 zeolite (sample S I 8 in Fig. 4). The narrow peak at 290 cm'^ in the spectrum of the crystalline ZSM-5 sample is due to the absence of any deviation in the geometry of 10-membered rings in the periodic pore system. Probably, the openings of the amorphous T-0 network that are built around the TPA during the first several hours of hydrothermal treatment play the role of nuclei for the formation of the MFI pore structure upon further hydrothermal treatment. The Raman spectrum of calcined S_6 is quite similar to that of S_2 (see Fig. 4). Only a subtle increase in the Raman scattering near 380 cm"^ is observed, suggesting an enlargement of the number of 5membered rings typical of MFI structure. Incipient MFI domains might exist even at this early stage, but may not be detected by Raman spectroscopy because of the dominance of the scattering from the amorphous phase. The spectrum of calcined S I 2 reveals considerable structural changes in the T-O network of the product obtained after heating. The band at about 380 cm'^ is well pronounced, pointing to the existence of significant amount of 5-membered rings. Besides, two additional sharp peaks appear at 603 cm"^ and 977 cm" \ A pair of intense Raman peaks near 620 cm"^ and 960 cm'^ is characteristic of chain-like silicates [18-20]. The latter Raman signal arises from a Si-Ononbridging bond stretching mode, while the former is due to vibrations of bridging oxygen atom vibrations in the plane of the corresponding Si-O-Si bond angle. Therefore, the simultaneous appearance of the features at 603 cm"^ and 977 cm'^ shows that a number of quasi one-dimensional atomic arrangements occur within 12 hours of hydrothermal treatment of the clear solutions. Most probably, these atomic formations are related to a development of the MFI pentasil chains parallel to the c axis . Therefore, after 12 hours of crystallization the amorphous T-O network adapts the intermediate range order of MFI structure, thus facilitating the growth of the initially formed nuclei. After 14 hours of crystallization only a small amount of amorphous phase detectable via the Raman signal at 495 cm'^ still exists. Hence, 18 hours of hydrothermal treatment of the clear solution at 90 °C are enough for a complete conversion of the solidified substance into the ZSM-5 zeolite and upon a further hydrothermal treatment only an enlargement of the crystallites occurred due to the Ostwald ripening; a similar trend has been observed in pure silicate solutions resulting in the formation of Silicalite-1 nanoparticles .
Relative pressure, P/P^
Relative pressure, P/P^
Figure 5. Nitrogen sorption isotherm of samples isolated after 2 h and 18 h hydrothermal treatment (blue symbols: adsorption; red symbols: desorption). The nitrogen sorption data give evidence for the existence of microporosity in the samples already after two hours of hydrothermal treatment (see Fig. 5). Both the XRD amorphous (S_2) and entirely crystalline ( S I 8) samples exhibit isotherms typical of colloidal microporous materials Steep rises at low relative
169 pressure (P/Po) indicate the presence of micropores in both samples. The hysteresis loop at P/Po=(0.8-1.0) of the isotherms observed for both samples is due to the interparticle spaces formed between the nanometer sized grains. The low-pressure adsorption of nitrogen is about 55 cm^/g and 95 cm^/g for S_2 and S I 8 samples, respectively, while the total volume adsorbed in sample S_2 is about 800 cmVg, approximately half that of the crystalline sample S I 8 (-1800 cmVg). These results are consistent with the DLS data where the mean particle radius of the grains does not change significantly during the entire process of zeolite crystallization resulting in the presence of particles with a size of about 60 nm (see Fig. 1). HRTEM observations give evidence for the existence of nanosized crystalline domains in the XRD amorphous aluminosilicate particles. Figure 6 shows HRTEM images of nanosized particles extracted from the synthesis solution after 2 hours of hydrothermal treatment. At low magnification, aluminosilicate particles with amorphous appearance were detected (Fig. 6a), whereas the images taken at a higher magnification reveal the presence of few-nanometer sized domains with crystalline fringes typical for the MFI structure. a)
Figure 6. HRTEM images of nanoparticles obtained after 2 hours hydrothermal treatment of aluminosilicate precursor solution. Scale bar is 20 nm (a) and 10 nm (b). The arrows indicate the crystalline domains.
CONCLUSION The synthesis of nanosized zeolites from colloidal precursor solutions enables us to follow the process of crystallization on a nanometric length scale using sensitive spectroscopic and microscopic methods as well as dynamic light scattering and sorption measurements. On the basis of Raman scattering data, the subsequent stages of formation of ZSM-5 zeolite with MFI structure from a clear TPAOH-Al203-Si02-H20 solution were followed including (i) the formation of a porous disordered T-O network with a skeleton rich of 4- and 3-memberd rings of TO4 tetrahedra; (ii) the appearance of incipient ordered regions inside the amorphous matrix accompanied by an enlargement of the number of 5-membered rings in the disordered TO network; (iii) the enlargement of the correlation length between the structural species in the plane perpendicular to the pentasil chains, thus facilitating the amorphous-to-crystalline phase transition, and an appearance of XRD-detectible MFI domains; and (iv) a complete transformation of the amorphous microporous T-0 network into MFI crystalline structure. ACKNOWLEDGEMENTS Support from DFG and BFHZ is gratefully acknowledged. The authors thank Dr. N.H. Olson for the acquisition of the TEM images.
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