Unusual Pathway of Crystallization of Zeolite ZSM-5 in a

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Unusual Pathway of Crystallization of Zeolite ZSM-5 in a Heterogeneous System: Phenomenology and Starting Considerations Nan Ren,*,† Boris Subotić,*,‡ Josip Bronić,‡ Yi Tang,† Maja Dutour Sikirić,§ Tea Mišić,∥ Vesna Svetličić,∥ Sanja Bosnar,‡ and Tatjana Antonić Jelić‡ †

Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Institute of Catalysis, and Laboratory of Advanced Material, Fudan University, Shanghai, 200433, P.R. China ‡ Ruđer Bošković Institute, Division of Materials Chemistry, Laboratory for the Synthesis of New Materials, Bijenička 54, 10000 Zagreb, Croatia § Ruđer Bošković Institute, Division of Physical Chemistry, Laboratory for Synthesis and Processes of Self-assembling of Organic Molecules, Bijenička 54, 10000 Zagreb, Croatia ∥ Ruđer Bošković Institute, Division for Marine and Environmental Research, Laboratory for BioElectrochemisty and Surface Imaging, Bijenička 54, 10000 Zagreb, Croatia S Supporting Information *

ABSTRACT: Crystallization of zeolite ZSM-5 from a diluted heterogeneous system (12.5Na 2 O−Al 2 O 3 −8TPABr−60SiO 2 − 4000H2O) was investigated by various experimental methods such as X-ray diffraction (XRD), electron diffraction (ED), infrared spectroscopy (FTIR), X-ray fluorescence (XRF), scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), thermo-gravimetric analysis (TGA), particle size analysis (PSA), pH measurement, inductive coupling plasma (ICP) emission spectrometry, and dynamic light scattering (DLS). The crystallization process is characterized by a very long “induction period” (95% of the entire reaction time) and very fast transformation (5% of the entire reaction time) of amorphous to crystalline phase (zeolite ZSM-5) at the end of the crystallization process. Analysis of the obtained results has shown that the crystallization process takes place by a chain of processes: (i) formation of “primary” amorphous aluminosilicate precursor (gel) at room temperature, (ii) formation of “secondary” amorphous aluminosilicate precursor (“worm-like” particles, WLPs) at increased temperature (170 °C), (iii) formation of “tertiary” amorphous aluminosilicate precursor (condensed aggregates, CAs) by aggregation of the WLPs and densification (condensation) of aggregates, and (iv) formation of nuclei and their growth in the matrixes of CAs; these processes result in the formation of fully crystalline zeolite ZSM-5 in the form of polycrystalline aggregates. KEYWORDS: zeolte ZSM-5, unusual crystallization pathway, “worm-like” particles, condensed aggregates, polycrystalline aggregates, heterogeneous system

1. INTRODUCTION Since November 14, 1972, when the synthesis of a new type of zeolite, denoted by ZSM-5 (Zeolite Socony Mobil Number 5), was filed,1 thousands of scientific and professional papers as well as patents related to zeolite ZSM-5 have been issued. Such a great and intense interest for zeolite ZSM-5 was expressed not only because of its specific, template-induced crystallization pathway1,2 and chemical (Si/Al = 10 − ∞1−3) and structural properties (pentasil family4) but also because of its wide and considerable applications (primarily in catalysis and separation) in fuel and petrochemical processing.5 Since (i) performances of zeolite ZSM-5 are largely determined by its structural, particulate, and morphological properties and even the state/ number/distribution of Al atoms in the crystalline framework, © 2012 American Chemical Society

(ii) the related properties of zeolite ZSM-5 can be adjusted through the synthesis procedure, and (iii) the perfect controllable synthesis of zeolite can only be realized with the thorough understanding of critical processes (nucleation, crystal growth) occurring during crystallization, the related mechanistic studies of the formation of zeolite ZSM-5 are always considered as one of the most important contributions in the field. Generally, for the crystallization of high-silica zeolites such as MFI-type ones, there are two basic methodologies; crystalReceived: October 25, 2011 Revised: March 23, 2012 Published: April 25, 2012 1726

dx.doi.org/10.1021/cm203194v | Chem. Mater. 2012, 24, 1726−1737

Chemistry of Materials

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microscopy (cryo-TEM), atomic force microscopy (AFM), small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), and wide-angle X-ray scattering (WAXS), the in situ observation of the evolution of particulate properties during the crystallization process became facile and feasible. However, since most of the above-mentioned methods such as QELSS, SANS, SAXS, or WAXS are scattering-based approaches, the pre-existence of gel particles in the system largely influenced the accuracy of the results during characterization. Thus, the relevant studies of mechanisms of MFI’s crystallization are mainly focused on homogeneous systems bearing a clear solution at the initial stage of crystallization.8,14 Results of these investigations indicated that the critical step in the crystallization of MFI-type zeolites in homogeneous systems is the formation of “primary units” (primary particles; PPs), having a size of about 3 nm,14a−d most probably by aggregation of several IOCSs13 when the crystallization process starts.14a−c,e In addition, “secondary units” (secondary particles; SPs) having the size 5−10 nm are formed by a stepwise aggregation of ∼3 nm PPs.14b,c,e,f After the amorphous aggregates reach a “critical” size (e.g., ≥10 nm), part of the gel nutrient transforms into the crystalline phase (viable nuclei)14b,d,e by reorganization and condensation of the amorphous aggregates.10,11,14b,d−h Results of more recent investigations of crystallization of TPA−silicalite from homogeneous systems15 are consistent with the above-proposed mechanism. Another approach in the understanding of the early-stage processes in homogeneous (alumino)-silicate systems was mainly formulated by Kirschhock et al. 16 and further investigated by the Lueven group17 and others.18 The essential proposals by Kirschhock et al. were the formation of a particular Si33 silicate cluster at the initial stage of crystallization. Aggregation of the Si33 silicate cluster results in the formation of “nanoslabs” of specific shape and size (1.3 × 4.0 × 4.0 nm) having the structure closely resembling the MFI framework.17 The MFI or MEL framework can be obtained through further self-assembly of these nanoslabs.17 Recently, Fedeyko et al.,19 Rimer et al.,20 Vlachos et al.,21 and Shantz et al.22 have found that the spontaneous formation of about 3 nm silica nanoparticles is a general phenomenon in basic solutions of small tetraalkyl ammonium (TAA) cations. The particles have a core−shell structure with silica at the core and the TAA cations at the shell. The size of the inorganic core is nearly independent of the diameter of the TAA cation but decreases with pH, suggesting that the electrostatic forces are a key factor controlling their size and stability. The formation of silica nanoparticles is a reversible process at low temperatures, in some extent similar to the aggregation of surfactant molecules into micelles. In contrast to such a great interest for investigation of mechanisms of crystallization of MFIs in homogeneous systems,14−22 to the best of our knowledge, there is a lack of investigation of the processes which occur at molecular level during crystallization of MFIs in heterogeneous systems.23 For this reason, we intend to carry out a detailed study of the crystallization pathway of zeolite TPA-ZSM-5 from the heterogeneous system by following the changes of chemical, structural, and particulate properties in both the solid and the liquid phase of the reaction mixture during crystallization. On the basis of the existing knowledge,3,8−23 the observed changes are explored carefully to shed light on the understanding of the

lization from heterogeneous systems (hydrogels) and crystallization from homogeneous systems (initially clear aluminosilicate solutions).6,7 The essential difference between these two methodologies is the presence (former case) or absence (latter case) of the amorphous solid phase (gel) during the initial stage of crystallization.8 Since the zeolite crystallization is a typical multifactor-governed complex system,9 one can believe that the crystallization pathways of these two systems could be dramatically different. Following the original synthesis procedure,1 most of the earlier works published on the preparation of MFI-type zeolites (ZSM-5, silicalite-1) have involved synthesis from (alumino)silicate hydrogels, using tetrapropylammonium (TPA) bromide and/or hydroxide as a structure directing agent (SDA).3,10 Through studying the influences of dominant factors such as batch molar ratios SiO2/Al2O3,3,10a−d TPA+/SiO2,3,10a,b,e,f H 2 O/SiO 2 , 3,10b M/SiO 2 (M = Li, Na, K, Rb, Cs, NH4),3,10b,g−j and OH/SiO23,10b,e,f of the reaction mixture (hydrogel), the nature of silica3,10f,g and alumina sources,10b,j and crystallization temperature10e,f on the pathway of crystallization and properties of crystallized zeolite ZSM-5, some important principles of the crystallization of zeolite ZSM5 from heterogeneous systems were established. As concluded by Derouane et al.,10i the crystallization of zeolite ZSM-5 occurs either by “classical liquid transportation mechanism” or by “direct solid-state transformation” processes, which mainly depend on the initial state of the amorphous silica sources. In the former case, the dissolution of an amorphous aluminosilicate gel precursor in hot alkaline media and the formation of low-molecular-weight (monomer, dimers) aluminate, silicate, and/or aluminosilicate species in the liquid phase is the first step of nucleation/crystallization.10d,h Then, the dissolved aluminate and silicate ions can undergo polymerization processes to aluminosilicate or polysilicate ions which may regroup around the hydrated cations (M, TPA) to form the primary zeolite ZSM-5 particles (nuclei) either in the solution or on the gel−liquid interface.10d,h,11,12 In difference to the above presented “classical” approaches, the results of some studies clearly indicate that primary zeolite ZSM-5 particles (nuclei) can also be formed in the gel matrix.8,10a,b,g,11 For example, Derouane et al.10g and, later on, Gabelica et al.10b have found that adding acidic Al-sulfate solution containing TPABr into an aqueous Na-silicate solution favors a rapid nucleation of zeolite ZSM-5 in the gel matrix. Structure directing TPA+ ions still present in the gel can interact immediately with the numerous reactive aluminosilicate anions, and a direct crystallization process within the solid hydrogel phase is expected. A similar model of nucleation of ZSM-5 was developed by Chang and Bell.11 They surmised that in the TPA+ gel system embryonic structures are formed rapidly upon heating by the formation of a clathrate-like water structure around the template, followed by fast isomorphous substitution of water by silicate species in these cages, which resemble ZSM5 channel intersections. Later on, Burkett and Davis13 found that the complete or partial substitution of silicate for water11 establishes a close contact between the protons of TPA and silicon atoms of the inorganic phase by van der Waals interaction, thus forming inorganic−organic composite species (IOCS) that are key species for the self-assembly of MFI structure. From the early nineties, with the development of more sophisticated experimental methods such as quasi-elastic light scattering spectroscopy (QELSS), cryo-transmission electron 1727



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were determined by an inductive coupling plasma (ICP) emission spectrometer (Themo-6300). pH of the solutions, separated from the RM at various crystallization times, tc, were measured using a Corning Pinnacle 555 pH/ion meter. The pH meter was calibrated with pH 9.0 and pH 12.0 buffer solutions at 25 °C. The accuracy of the pH meter was ±0.01 pH units. The X-ray diffraction (XRD) patterns of the solid samples were recorded on a Rigaku D/Max-rB 12 kW diffractometer (Cu Kα). The content of the amorphous and/or crystalline phase in the solid samples, separated from the reaction mixtures at different crystallization times, tc, was calculated by the Hermans−Weidinger method.24 Fourier transform infrared (FTIR) transmission spectra of the solid samples were measured by the KBR wafer technique. The spectra were recorded on a System 2000 FT-IR (Perkin-Elmer). Thermogravimetric (TG) and differential thermogravimetric (DTG) analyses of the selected solid samples were performed on TGA7 thermogravimetric equipment (Perkin-Elmer). The analysis was carried out in a nitrogen atmosphere (continuous flow of 20 mL/min) with a temperature ramp of 10 °C/min from room temperature to 800 °C. Contents of Si, Al, and Na in the solid samples were determined by X-ray fluorescence (XRF). XRF experiments were performed by a BRUKER-AXS S4 Explorer apparatus. Before the analysis, the wellwashed and dried samples were melted with Li2B4O7 at 1573 K. The resultant glass pellets were analyzed under vacuum with a rhodium anticathode (2.4 kW). Scanning electron microscopy (SEM) measurements of the solid samples were performed by a Philips XL30 D6716 instrument at an operating voltage of 25 kV. Transmission electron microscopy (TEM) experiments with selected area electron diffraction (SAED) were carried out with a JEOL JEM-2010 instrument at an operating voltage of 200 kV. Atomic force microscopy (AFM) of the solid samples was performed by a Multimode Scanning Probe Microscope with a Nanoscope IIIa controller (Veeco Instruments, Inc.) with a vertical engagement (JV) 125 μm scanner. Sharp silicon cantilever probes (TESP, Veeco) with a nominal spring constant of 42 N/m and nominal frequency of 320 kHz were used. The powdered solid samples were suspended in ultrapure water (1 g/dm3) and stirred for 1 h and sonicated for 30 min. Suspension was diluted by ultrapure water so that the final suspension contained 10 mg of powder/dm3. Five microliters of the final suspension was pipetted directly onto freshly cleaved mica. Following deposition, the mica sheets were placed in enclosed Petri dishes for several hours at a relative humidity of 50% to evaporate the excess water. All images were collected using tapping (semicontact) mode because it is well adapted to soft samples due to the nearly complete reduction of lateral forces. Size distribution curves of the particles present in the solutions separated from the RM at various crystallization times, tc, were measured by a DLS approach using a Malvern Zetasizer Nano-ZS analyzer. Particle size distribution (PSD) curves of the solid samples were determined with a Malvern Mastersizer 2000 laser light-scattering (LLS) particle size analyzer.

crystallization mechanism of ZSM-5 zeolites in heterogeneous systems. Although the results of the above-described investigations of the synthesis of MFI-type zeolites from initially clear solutions14−22 offer a reasonable “picture” of the processes occurring during the crystallization of MFI-type zeolites in homogeneous systems, this knowledge can be used with limitations in the explanation of the processes occurring during the crystallization of MFI-type in heterogeneous systems because of the rapid formation of a huge amount of amorphous solid phase (gel).23 Since one of the major differences between these two kinds of systems is the degree of dilution, one can expect that a transition state with moderate dilution (very diluted but still heterogeneous system) exhibits the characteristics of both the systems. For this reason, based on some preliminary investigations, we prepared the reaction mixture having the composition 12.5Na2O−Al2O3−8TPABr−60SiO2−4000H2O, using fumed silica as the Si source and sodium aluminate as the Al source. In this work, by careful study of the particularities of this crystallizing system, the phenomenological feature of the crystallization process of the heterogeneous system is shown, and the processes responsible for the observed effects are explained.

2. EXPERIMENTAL SECTION 2.1. Materials. The reagents used for the synthesis of ZSM-5 zeolite were: sodium hydroxide (NaOH, analytical grade, Shanghai Chemical Co. China), sodium aluminate (NaAlO2, technical grade, Shanghai Chemical Co. China), fumed silica (SiO2, surface area 300− 400 m2/g, Alfa Aesar), tetrapropylammonium bromide (TPABr, analytical grade, Alfa Aesar), and deionized water. All chemicals were used as received without any purification. 2.2. Synthesis. The reaction mixture (hydrogel) having the batch molar composition 12.5Na2O−8TPABr−Al2O3−60SiO2−4000H2O was prepared by mixing freshly prepared sodium hydroxide solution (30 wt %), sodium aluminate, and TPABr in deionized water to form a transparent solution, followed by addition of fumed silica under vigorous stirring at room temperature. Such a prepared reaction mixture was additionally stirred at room temperature for 1 h. Thereafter, a part of the reaction mixture (RM) was immediately centrifuged for 20 min at 12 000 rpm, with a relative centrifugal force (RCF) of 17 000g (the rotor’s friction coefficient value, k, equals to 1350) to separate the solid from the liquid phase. The rest of the RM was divided among the needed number of autoclaves. The autoclaves containing the RM were put into an oven preheated at crystallization temperature (170 °C). The moment when the autoclaves containing the RM were poured into the preheated oven was recorded as the zero time (tc = 0) of the crystallization process. At predetermined crystallization times, tc, the selected autoclaves were taken out from the oven and cooled to the ambient temperature. After cooling the autoclave, the RM was centrifuged at 12 000 rpm for 20 min to separate the solid from the liquid phase. The clear liquid phase (supernatant) above the sediment was carefully removed without the disturbance of the solid phase (sediment) and used for determination of Si and Al concentrations in the liquid phase, for measurement of pH of the liquid phase, as well as for DLS (Dynamic Light Scattering) analysis on the particles present in the liquid phase. After removal of supernatant, the solid phase was redispersed in deionized water and centrifuged repeatedly. The procedure was repeated until the pH of the liquid phase above the sediment was about 9. The washed solid phase was dried at 80 °C for 10 h and then cooled to room temperature and stored in desiccators over dry silica gel. 2.3. Characterization. Concentrations of aluminum and silicon in the solutions separated from the RM at various crystallization times, tc,

3. RESULTS 3.1. General Considerations. Figure 1 shows the typical features of the crystallization process by displaying the changes occurring in both the solid and the liquid phase. It is evident that the process of crystallization can be roughly divided into three particular time intervals: (I) From tc = 0 (RM, prepared and aged at room temperature for 1 h is designed as RM0) to tc ≤ 1 h: the crystallization temperature, equilibrium amount, mSi(L)eq, of Si in the liquid phase (Figure 1B-I and Supporting Information SI-1), and equilibrium pH of the liquid phase (Figure 1E-I) are established in this time interval. (II) From tc = 1 h to tc ≈ 20 h during which time interval the solid phase of the RM is X-ray amorphous (Figure 1A-II), the concentration 1728



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and the crystal growth period, tg (Figure 1). The induction period ti, during which the solid phase of the RM is X-ray amorphous, lasts about 20 h, or about 95% of the entire synthesis time, while complete transformation of the amorphous phase to zeolite ZSM-5 (the “growth period”, tg) is completed in about 1 h, or in about 5% of the entire synthesis time (Figure 1A). Another particularity of this crystallization system is that the equilibrium amount, mSi(L), of silicon in the liquid phase represents about 45% of all the Si present in the system (Figure 1B and Supporting Information SI-1). In this way, although, by physical consistency, this is a heterogeneous system (category 2 − dispersed low density gel, in accordance with Jensen’s classification25), the high relative content of Si in the liquid phase (about 22.5 g of SiO2/dm3) and low content of the solid phase (less than 3 wt %) cause this to approach to a homogeneous system. Taking into consideration the uncommon feature of this crystallization system, each of the stages is considered separately. 3.2. Stage I of the Crystallization Process. Before heating, the solid phase of the reaction mixture (RM = RM0) contains about 94%, and the liquid phase contains about 6% of the total amount of Si, as calculated from the data in Figure 1BI. On the other hand, the liquid phase contains only about 3% of the total amount of Al present in RM0 (Figure 1C-I). This indicates that most of the Al is contained in the solid phase of RM0.10a,b The value, Si/Al ≈ 27, of the solid phase of RM0 (Figure 1D-I) confirms this indication; namely, this ratio is somewhat lower than the ratio, Si/Al = 30, of the entire reaction mixture. In addition, the molar ratio Si/Al of the solid phase, calculated from the measured concentrations of Si (Figure 1B-I) and Al (Figure 1C-I) in the liquid phase of RM0, is 29, which is close to the measured value (Si/Al = 27.2; see Figure 1D-I). According to DLS measurements, the liquid phase of RM0 contains subcolloidal particles having the size in the range from 0.5−1.5 nm and average size, Dav, of 0.67 nm (Figure 2). Figure 1. Changes in: (A) the fraction, fc, of the crystalline phase (zeolite ZSM-5), (B) relative amount, mSi(L), of Si in the liquid phase, (C) relative amount, mAl(L), of Al in the liquid phase, (D) Si/ Al(solid) molar ratio in the solid phase, and (E) pH of the liquid phase of the reaction mixture:12.5Na 2 O−Al 2 O 3 −8TPABr−60SiO 2 − 4000H2O during its hydrothermal treatment (heating at 170 °C). tc is the time of crystallization; ti is the “induction period”; and tg is the “growth period”.

of Si in the liquid phase slightly decreases (Figure 1B-II); the concentration of Al in the liquid phase is constant (Figure 1CII); and both the molar ratio, Si/Al(solid), of the solid phase (Figure 1D-II) and pH of the liquid phase (Figure 1E-II) slightly increase. (III) From tc ≈ 20 h to tc ≈ 21 during which time interval the amorphous phase is completely transformed into zeolite ZSM-5 (Figure 1A-III), the amount of Si in the liquid phase decreases from near-equilibrium value (mSi(L) ≈ 45%) to mSi(L) = 28.5% (Figure 1B-III); the amount of Al in the liquid phase decreases from mAl(L) ≈ 2.9% to mAl(L) = 0.9% (Figure 1C-III); the molar ratio, Si/Al(solid), of the solid phase decreases from Si/Al(solid) ≈ 17 to Si/Al(solid) ≈ 13 (Figure 1D-III); and the pH in the liquid phase increases from pH = 11.5 to pH = 12.1 (Figure 1E-III). Under this common general feature of the zeolite crystallization process, the uncommonness of the investigated crystallization pathway is primarily manifested in the relationship between the duration of the so-called “induction period”, ti,

Figure 2. Size distribution (by number) of the particles present in the liquid phase of RM0. ND is number percentage of the particles having the spherical equivalent diameter D.

The XRD pattern of the solid phase of RM0 (Figure 3A) shows a typical profile characteristic for the amorphous phase.26 The amorphous nature of the solid phase is additionally revealed by its electron diffraction pattern (Figure 3E, inset) and FTIR analysis;14c,f,16b,23 the FTIR spectrum of the solid phase of RM0 (spectrum a in Figure 3B) is almost the same as 1729



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Figure 4. PSDs by number (A, B, C, D) and by volume (A′, B′, C′, D′) of the solid phases separated from the reaction mixture RM0 (A, A′) and of the same reaction mixture hydrothermally treated for tc = 1 h (B, B′), tc = 2 h (C, C′), and tc = 4 h (D, D′). ND is number percentage, and VD is volume (mass) percentage of the particles having the spherical equivalent diameter D.

Figure 3. XRD pattern (A), FTIR spectrum of (solid phase of RM0 (a) and fumed silica (b)) (B), TG-DTG (solid-dashed curve) (C), SEM image (D), TEM image with its corresponding ED pattern inset (E), and AFM image (F) of the solid phase separated from RM0. The aggregates of amorphous colloidal silica (ACS) and gel are indicated by circles (E) or marked by arrows (F) and explained through the text. The scale bars in (D) and (E) represent 1 μm and 50 nm.

Heating of RM0 causes dissolution of silica contained in the solid phase so that the amount of Si in the liquid phase increases from mSi(L) = 6.2% at tc = 0 to mSi(L) = mSi(L)eq = 44.6% at tc = 1 h (Figure 1B-I). Dissolution of silica causes a decrease of pH of the liquid phase from pH = 13.33 at tc = 0 to pH = 11.39 at tc = 1 h (Figure 1E-I). On the other hand, the concentration of Al in the liquid phase does not change during the heating (Figure 1C-I). This causes the molar Si/Al ratio of the solid phase to decrease from Si/Al = 27.2 at tc = 0 to Si/Al = 17.2 at tc ≥ 1 h (Figure 1D-I) during heating. Again, the molar ratio Si/Al(solid) of the solid phase, calculated from the measured concentrations of Si (Figure 1B) and Al (Figure 1C) in the liquid phase of RM at tc = 1 h, is 17.1, which is almost the same as the measured value (Si/Al = 17.2; see Figure 1D). At the same time, the content of TPA in the solid phase decreased from about 5 wt % at tc = 0 to about 3 wt % at 1 h. This content of TPA (≈3 wt %) in the solid phase of the RM keeps constant during prolonged heating of the RM in the time interval from tc = 1 h to tc = 19 h (see Figure SI-2.1 in Supporting Information SI-2). Here, it is particularly interesting that heating of the reaction mixture causes an increase of the size of the particles present in the liquid phase from Dav = 0.67 nm (Figure 2) at tc = 0 to Dav ≈ 30 nm (Figure 5A and Table 1) at tc = 1 h. Moreover, although the XRD patterns, FTIR spectra, and TG-DTG profiles of the solid phases separated from the RM at tc = 1 h do not differ from the patterns/spectra/profiles (see Supporting Information SI-2) of the solid phase separated from RM0 (Figures 3A−3C), a comparison of the TEM image in Figure 3E with TEM image A in Figure 6 shows that heating of RM0 not only changes the chemical compositions of the liquid

the FTIR spectrum of the fumed silica (spectrum b in Figure 3B) used as the silica source. On the other hand, from TGDTG profiles of the solid phase of RM0, it can be deduced that the “low-temperature” DTG peak and small weight loss (about 5 wt %) until about 200 °C (Figure 3C) are caused by desorption of loosely held moisture.27 The additional weight loss (about 5 wt %) at T ≥ 200 °C and the appearance of a slight DTG peak at T ≈ 300 °C result from decomposition of TPABr loosely located, by physical adsorption, in the solid phase of RM0.28 From the present data, it can be easily calculated that the TPA present in RM0 is distributed between the solid and the liquid phase so that the solid phase contains 9% (e.g., 0.72 mol) of the entire amount (e.g., 8 mol) of TPA present in RM0 and that the liquid phase contains 91% (e.g.,7.28 mol) of the entire amount of TPA present in RM0. The solid phase of RM0 appears in the form of large, micrometer-sized aggregates (1.5−300 μm; see Figures 4A and 4A′) composed of smaller, submicrometer-sized particles (Figure 3D). On the other hand, the corresponding TEM (Figure 3E) and AFM (Figure 3F) images show that the morphology of the solid phase of RM0 is more complex than it can be estimated from the SEM image. It is reliable to assume that large darker “areas” in the TEM image (Figure 3E) as well as the darker aggregates in the AFM image (Figure 3F) represent aggregates of amorphous colloidal silica (ACS) used as a silica source and that lighter areas in both TEM and AFM images represent aluminosilicate gel (gel) formed during roomtemperature aging of the reaction mixture. 1730



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Figure 5. Size distribution (by number) of the particles present in the liquid phase of the reaction mixture hydrothermally treated for tc = 1 h (A), tc = 2 h (B), tc = 4 h (C), tc = 6 h (D), tc = 10 h (E), tc = 14 h (F), tc = 16 h (G), tc = 18 (H), tc = 20 h (I), tc = 20.33 h (J), tc = 20.66 h (K), and tc = 21 h (L). ND is number percentage of the particles having the spherical equivalent diameter D.

Table 1. Minimum (Dmin), Peak (Dp), Maximum (Dmax), and Average (Dav) Sizes (Spherical Equivalent Diameter) of the Particles Present in the Liquid Phase of the Reaction Mixture Hydrothermally Treated for Various Times tc tc (h)

Dmin (nm)

Dp (nm)

Dmax (nm)

Dav (nm)

1 2 4 6 10 14 16 18 20 20.33 20.67 21

18.0 18.0 18.0 21.0 24.3 28.2 21.0 21.0 21.0 21.0 21.0 21.0

28.2 24.4 24.4 28.2 32.7 37.8 32.7 32.7 32.7 32.7 28.2 28.2

65.0 65.0 65.0 70.0 75.0 75.0 75.0 75.0 75.0 75.0 75.0 75.0

29.3 26.6 27.2 33.2 34.7 39.5 33.8 36.1 35.4 34.6 33.1 33.9

Figure 6. TEM images of the solids separated from the RM at tc = 1 h (A), tc = 2 h (B), tc = 8 h (C), tc = 20.33 h (D), and tc = 20.66 h (E, F). The locations of WLPs (A) and ACSs (B) and PSPs (C) are circled and indicated. The meaning of PSP (“primary” silica particles; (C)) is explained in the text of paper. The scale bars in (A) and (B) represent 50 nm, while those in (D), (E), and (F) represent 0.5, 1, and 0.1 μm, respectively.

WLPs increases during gradual dissolution of amorphous silica in the time interval tc = 1 h to tc = 4 h. Analysis of particle size distributions (PSDs) of the solid phases separated from the RM at tc = 1 h (Figure 4A and 4A′) and tc = 2 h (Figure 4B and 4B′) shows that the solid phases are composed of a large number (more than 99%) of smaller particles (1−10 μm; Figures 4A and 4B) and a small number (less than 1%) of larger particles (10−100 μm; Figures 4A′ and 4B′). On the other hand, more than 75% of volume (mass) of the solids separated from the RMs at tc = 1 h and tc = 2 h, respectively, consists of larger particles (10−100 μm; Figures 4A′ and 4B′). This indicates that the small, nanosized features, of which the solids are composed, are joined, making some kind of “network” (Figures 6A and 6B). It seems that the “connective elements” of the nanosized particles in the network are undissolved ACSs. Further dissolution causes both the decrease in size of small “primary” silica particles in ACSs and decomposition (deaggregation) of ACSs. This in addition causes separation of WLPs, after ACSs are deaggregated. Remaining, small undissolved primary particles of silica (PSPs) can be embedded into the WLP (see Figure 6C).

(Figure 1B) and the solid phase (Figure 1C) as well as the drastic increase in the size of the particles present in the liquid phase (Figures 2 and 5A and Table 1) but also causes rearrangement of the participating features of the solid phase. 3.3. Stage II of the Crystallization Process. The morphological changes which start with heating of the RM during stage I of the crystallization process (see Figures 3E and 6A) continue during the prolonged heating of the RM at stage II of the crystallization process. The SEM images A, B, and C in Figure 7 show that small particles, composed of ACS and gel (Figures 3E and 3F), increase in size and that at tc = 4 h a new morphological form“worm-like” particles (WLPs; Figure 7D)is formed. The TEM images A, B, and C in Figure 6 clearly show that WLPs start to form earlier (at tc = 1 h; Figure 6A) than they can be recognized as discrete WLPs in the corresponding SEM image (at tc = 1 h; Figure 7A). In addition, TEM images A, B, and C in Figure 6 show that WLPs are formed around undissolved amorphous silica (ACS); the size of 1731



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(Figures 5B−5I and Table 1). The only changes observed during this time interval are a slow decrease in the concentration of Si in the liquid phase, from mSi(L) = 44% at tc = 4 h to mSi(L) = 39% at tc = 19 h (see Figure 1B-II), a slight increase of the Si/Al(solid) ratio of the solid phase, from 16.7 at tc = 4 to 17.7 at tc = 19 h (see Figure 1C-II), and a very slight increase of pH of the liquid phase from pH = 11.3 at tc = 4 h to pH = 11.5 at tc = 20 (Figure 1E-II). 3.4. Stage III of the Crystallization Process. In contrast to the stability of the system during stage II, very intense processes start to take place in the system at tc > 19 h, i.e., during stage III. The occurrence of these processes is manifested by a rapid change of the structural (Figure 1AIII), chemical (Figure 1D-III), morphological (Figures 6D−6F and 7E−7H), and particulate properties (Figure 9) of the solid

Figure 7. SEM images of the solids separated from the RM at tc = 1 h (A), tc = 2 h (B), tc = 4 h (C), tc = 16 h (D), tc = 20.33 h (E), tc = 20.66 h (F,G), and tc = 21 h (H). The crystalline surface in (G) is indicated by a circle and explained through the text. The scale bars in (A), (B), (C), (D), (F), and (G) represent 1 μm, while those in (E) and (H) represent 2 and 10 μm, respectively.

When formed, the WLPs keep their shape (see Figures 7D and 6C and Figure SI-3.1 in Supporting Information SI-3), size of 100−500 nm (Figure 8 and Figures SI-3.2 and SI-3.3 in

Figure 9. PSDs by number (A, B, C, D) and by volume (A′, B′, C′, D′) of the solid phases separated from the reaction mixture hydrothermally treated for tc = 20 h (A, A′), tc = 20.33 h (B, B′), tc = 20.66 h (C, C′), and tc = 21 h (D, D′). ND is number percentage, and VD is volume (mass) percentage of the crystals having the spherical equivalent diameter D.

phase, followed by a decrease in the concentrations of silicon (Figure 1B-III) and aluminum (Figure 1C-III) in the liquid phase and an increase of pH of the liquid phase (Figure 1E-III) during stage III (Figure 1-III). The same as during stage II, the size distribution of particles present in the liquid phase does not change considerably during stage III of the crystallization process (Figures 5B−5I and Table 1). An increase of the content of TPA in the solid phase (Figure 10) indicates that the above-mentioned processes are followed by a decrease of the concentration of TPA in the liquid phase. It is evident that the process starts with aggregation of WLPs at tc = 20 h. Due to the small number of aggregates of WLPs formed at tc = 20 h, they cannot be observed in the PSD by number (Figure 9A) but only rarely and exceptionally can be observed in the corresponding SEM and TEM images (see Supporting Information SI-4). However, the formation of aggregates (1−

Figure 8. PSDs by number (A) and by volume (B) of the solid phase (WLPs) separated from the reaction mixture hydrothermally treated for tc = 10 h. ND is number percentage, and VD is volume (mass) percentage of the crystals having the spherical equivalent diameter D.

Supporting Information SI-3), and chemical composition, i.e., TPA content of about 3 wt % (Figure SI-2.1 in Supporting Information SI-2) and Si/Al (solid) ≈ 17 (Figure 1C-II) during the main part of the crystallization process, i.e., from tc = 4 h to tc = 19 h. Thus, the appearance and long-term stability of WLPs is an additional uncommonness of the investigated system. The size distribution of particles present in the liquid phase, established at tc = 1 h (Figure 5A), does not change considerably during stage II of the crystallization process 1732



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image (Figure 7G). Hence, it is evident that transformation of the amorphous phase of CAs into the crystalline phase (MFI) takes place inside CAs, so that crystals of MFI-type zeolite can be clearly observed only after the entire amount of amorphous phase was transformed into a crystalline one, i.e., at tc ≥ 21 h (Figure 7F). The product appears in the form of polycrystalline aggregates (Figure 7G) having the size in the wide range (0.5− 200 μm; see Figures 9D and 9D′). Although most of the crystals (about 95%) have a size in the range from 0.5 to 2 μm (Figure 9D), their share in the total mass of the product is only about 15 wt %. Taking into consideration that the fraction of small particles (0.5−2 μm) does not exist in the solid phase at tc = 20.33 h (see Figures 9B and 9B′), it is reasonable to conclude that the PSD peaks in Figures 9C and 9D are related to the small crystalline particles formed in surface and/or subsurface regions of CAs and detached themselves from the surfaces of the transforming CAs at tc > 20.33 h. On the other hand, most of the mass of product (about 75 wt %) originates from the particles having their sizes in the range from 3 to 20 μm (Figure 9D′). About 10% of mass originates from large particles (probably aggregates) having their sizes in the range from 10 to 200 μm (Figure 9D′).

Figure 10. TG-DTG profiles of the solid phases separated from the reaction mixture hydrothermally treated for tc = 20 h (A), tc = 20.33 h (B), tc = 20.66 h (C), and tc = 21 h (D).

5 μm) and their coexistence with WLPs (0.1−0.5 μm) at tc = 20 h can be clearly shown in the PSD by volume (Figure 9A′). Thereafter, the process of aggregation of WLPs is very fast, so that at tc = 20.33 h most of the solid phase exists in the form of aggregates (Figures 7D and 6D) having the size between 1 and 5 μm (Figures 9B and 9B′). Both SEM (Figure 7E) and TEM (Figure 6D) images show that aggregation of WLPs is followed by “condensation” of the formed aggregates. Although in both SEM (Figure 7E) and TEM (Figure 6D) images individual WLPs can be observed, the absence of the particles having the size characteristic for individual WLPs (0.1−0.5 μm; Figure 8) in the PSDs by both number (Figure 9B) and volume (Figure 9B′) indicates that the “uncondensed” WLPs are, however, tightly connected with the condensed aggregates (CAs). A comparison of the PSDs of the solid phases of RMs at tc = 20.33 h (Figures 9B and 9B′) and at tc = 20.66 h (Figures 9C and 9C′) shows that the peak size of CAs increases from about 2.5 μm at tc = 20.33 h (Figure 9B′) to about 4.5 μm at tc = 20.66 h (Figure 9C′) and that a great number of small particles, with the peak size at about 0.7 μm (Figure 9C), appears in the solid phase of the RM at tc = 20.66 h. The increase in size of CAs is probably caused by their further mutual agglomerations and coalescences; the appearance of the small particles at tc = 20.66 h (Figures 9C) will be explained later. The processes of aggregation and condensation of WLPs are followed by the transformation of the amorphous to crystalline phase (Figure 1A-III; see also Supporting Information SI-5). This transformation is also indicated by a simultaneous increase of the content of TPA in the solid phase and shifting of the DTG peak from T ≈ 300 °C which is characteristic for the amorphous phase to T ≈ 400−450 °C which is characteristic for MFI-type zeolites (≈400 °C for silicalite-110d and ≈450 °C for ZSM-510d (Figures 10A−10D)). Here it is interesting that, although about 75% of the amorphous phase has been transformed into a crystalline one (MFI) at tc = 20.33 h, the crystalline phase cannot be clearly observed in the corresponding SEM (Figure 7E) and TEM (Figure 6D) images. Even at tc = 20.66 h, when about 90% of the amorphous phase has been transformed into a crystalline one, CAs and uncondensed WLPs are the dominant morphological features of the solid phase (Figures 7E and 6E); the crystalline phase can be observed as a very rare appearance in the corresponding SEM

4. DISCUSSION As shown from the presented data, some very important processes start immediately after preparation of the reaction mixture, i.e., during its short-time (1 h), room-temperature aging. In short, these processes are: (1) Reaction of aluminate ions from the liquid phase with colloidal silica (Scheme 1a and b). Following the finding that “In the solution containing a mixture of silicate species, aluminum preferentially complexed with larger silicate species, almost immediately”,29 this process results in the formation of Si−O−Al bonds on the surface of particles of colloidal silica. Using sodium aluminate as the source of “active” Al(OH)4− ions (see Experimental Section and Supporting Information SI-6), having “free” terminal Al− OH groups active for direct condensation reaction with the terminal Si−OH and Si−O− groups of silica species30 and fast formation of Si−O−Al bonds on the surfaces of silica particles makes this process rapid and effective. (2) Dissolution of the solid phase of the reaction mixture (Scheme 1b). Due to the inertness of surface Si−O−Al bonds on hydroxide attack,10c,31 dissolution of the solid phase results in the formation of soluble silicate species and thus in further enrichment of the undissolved solid phase with aluminum. There is much evidence that the silicate species formed during alkaline dissolution of zeolites and amorphous aluminosilicates exist mainly in the form of low molecular weight anions, at least immediately after their formation.10d,h,32 A small amount of aluminosilicate species can also be formed by reactions between the formed silicate anions and (still) free aluminate ions.3,10d,h,29,32a−c,g−i Finally, parts of silicate and aluminosilicate species may regroup around the hydrated cations (M, TPA) to form different TPA−silicate and TPA−aluminosilicate species.3,10d,h,11−13,14a−d DLS analysis of the liquid phase of RM0 shows that these processes also include the formation of subcolloidal particles having the size in the range from 0.5 to 1.5 nm (Figure 2). Recently, the particles of similar size were also found in the solutions obtained at an early stage of hydrolysis of tetraethylorthosilicate (TEOS) in an aqueous solution of N,N,N-trimethyl-1-adamantammonium (TMAda) hydroxide in both the absence and the presence of aluminum.33 On the other hand, there are indications that IOCSs have a size 1733



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comparable with the size (10−40 nm) of the amorphous particles identified as the precursor species for growth of silicalite-1 during its crystallization in the heterogeneous system.23 Thus, it is reliable to assume that these particles are formed by a chain of aggregation processes: (i) formation of IOCS by reaction of silicate ions and cations (Na+, TPA+),8,11,13 (ii) formation of ∼3 nm primary precursor particles (PPs) by aggregation of several IOCS,14a−g (iii) formation of ≤10 nm secondary precursor particles (SPs),14b−g by aggregation of several PPs, and (iv) further stepwise aggregation of such formed ≤10 nm SPs into ≈20−70 nm particles (Figure 5 and Table 1). These particles are probably in dynamic equilibrium with monomeric, dimeric, and different oligomeric silicate anions.34 On the other hand, since the “concentration” of different TPA-containing and TPA-free silicate and aluminosilicate species, formed by dissolution of silica (ACSs), is the largest just at the points of formation (surface regions of ACSs), their (poly)condensation reactions and thus formation of the gel phase are the most expressive at and near the surfaces of dissolving ACSs. However, in difference to tendency of the formation of the larger, irregular gel aggregates at room temperature (Figures 3E, 3F, and Scheme 1b), the polycondensation reactions of different silicate and aluminosilicate species at 170 °C result in formation of specific “worm-like” features (Figures 6A, 6B, and Scheme 1c). The low content of TPA (about 3 wt %) in the worm-like features indicates that TPA−silicate and TPA−aluminosilicate species participate in these reactions in less extent than TPA-free silicate and aluminosilicate species. Two possible mechanisms of the formation of WLPs can be assumed: (1) In accordance to Brunner’s C-model,35 mutual repulsion of assemblies of a large number of items (silicate and aluminosilicate anions) will attain an order that minimizes the potential energy of the assembly. The assembly may have an irregular shape35here the wormlike features (Figures 6A−6C and Scheme 1c). (2) Limited aggregation of subcolloidal (3−10 nm) precursor particles, usually observed during crystallization of MFI-type zeolites.14a−f The spreading out of the formed material in different directions relative to the place of formation (ACSs) and formation of specific worm-like shape (Figures 6A−6C and Scheme 1c) is probably caused by thermally induced (at 170 °C) movements of the items of assemblies. On the other hand, regardless by which of the mentioned mechanisms the WLPs are formed, results of additional investigation (see Supporting Information SI-6) imply that the presence of Al(OH)4− ions is crucial for the formation of WLPs. In addition, both the size and stability (resistance to aggregation into CAs) of WLPs increase with increasing content of available Al(OH)4− ions (see Supporting Information SI-6); however, the influence of Al content on the size and stability of WLPs is not quite clear at present. At tc ≈ 4 h, the formed worm-like features separate themselves as a consequence of dissolution/deaggregation of ACSs, so that in the time interval from tc ≈ 4 h to tc ≈ 19 h they exist as discrete (individual) worm-like particles (WLPs; see Figures 7C, 7D, and Scheme 1d). Besides the relatively high content of Al (Si/Al of WLP is ≈18; see Figure 1D-II), the long-term stability of the WLPs is probably also caused by small contents of TPA+ (about 3 wt %) and Na+ ions (molar ratio Na/Si is about 0.135) in WLPs. Namely, it is known that “structure-forming” Na+ ions are necessary for nucleation and crystal growth of MFI-type zeolites3,10b,g−i,12,36 and that these

Scheme 1. Schematic Presentation of the Processes Occurring during Crystallization

of about 1 nm.14g (3) Formation of the gel phase (Scheme 1b). As it is well elaborated in the appropriate literature,10b,29,32a,i aluminosilicate gel is formed by a series of (poly)condensation reactions of aluminate, silicate, and aluminosilicate species from the liquid phase. Thus, the reaction mixture RM0 is composed of the solid phase (undissolved, Al-enriched amorphous silica + aluminosilicate gel) and the liquid phase containing different TPA− silicate species including subcolloidal particles having the size in the range from 0.5 to 1.5 nm (Figure 2 and Scheme 1b). Heating of the reaction mixture (at 170 °C) causes further dissolution of the solid phase (see Figure 1B). Consumption of OH− ions, needed for dissolution of amorphous silica and/or gel and formation of soluble silicate species, causes a decrease of pH of the liquid phase during dissolution (Figure 1E-I). Again, the protective role of Si−O−Al bonds31 means that mainly silicate anions, probably low molecular weight ones,10d,h,32 are products of the dissolution of amorphous silica particles. The process of dissolution formally stops at tc ≈ 1 h, when the equilibrium amount, mSi(L)eq, of dissolved Si is accomplished (Figure 1B-I), i.e., when about 45 wt % of all the silica present in the reaction mixture is “dissolved” in the liquid phase. DLS analysis of the liquid phase shows that the increase of mSi(L) during heating of the RM causes a considerable increase in the size of the particles in the liquid phase, i.e., from about 0.7 nm at tc = 0 h (room temperature; Figure 2) to about 30 nm at tc = 1 h (170 °C; Figure 5A and Table 1). This size is 1734



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processes are considerably enhanced in the presence of TPA+ ions (template).3,10b,c,f,g,i,28 Hence, it is evident that the amounts of Na+ and TPA+ ions present in WLPs are not sufficient for formation of viable nuclei and their growth in the amorphous matrix of WLPs. Slight but continuous increase of the ratio Si/Al (solid) of the solid phase (Figure 1D-II) during stage II of the crystallization process indicates that a decrease of Si concentration in the liquid phase (Figure 1B-II) during the same stage is caused by incorporation of silicate species from the liquid phase into the solid phase (WLPs). Since the silicate particles in the liquid phase are TPA-containing ones, their deposition onto the surfaces of WLPs causes not only the above-described processes but also an increase of TPA content in the solid phase, from about 3 wt % for 1 h ≤ tc < 20 h (see Supporting Information SI-3) to about 6 wt % at tc = 20 h (Figure 10A). Changes of mSi(Si) (Figure 1B-II) and Si/ Al(solid) (Figure 1D-II) are followed by simultaneous change (increase) of pH of the liquid phase (Figure 1E-II). Thus, it is reliable to conclude that the incorporation of silicate species onto the surfaces of WLPs takes place by nucleophilic attack of silicate species from the liquid phase on the aluminate centers30 at the WLP surfaces, i.e.

Because of the lower solubility of the crystalline phase (zeolite ZSM-5) relative to the amorphous counterparts, the progress in crystallinity is followed by incorporation of both Si and Al from the liquid phase into growing ZSM-5 crystals. This causes a decrease in the amounts, mSi(L), of Si (Figure 1B-III) and, mAl(L), of Al (Figure 1C-III) in the liquid phase and a simultaneous decrease of the molar ratio, Si/Al(solid), of silicon and aluminum in the solid phase (Figure 1D-III), indicating a higher incorporation of Al than Si at stage III of the crystallization process. The increase of pH at stage III (Figure 1E-III) can be readily explained by both the nucleophilic attack of silicate ions from the liquid phase on the aluminate centers30 of growing ZSM-5 crystals and the nucleophilic substitution reaction between silanol groups on the surface of growing ZSM-5 crystals and aluminate ions from the liquid phase.30 As already stated, the increase of pH leads to the conclusion that rather low molecular weight (Na,TPA)-silicate species than (sub)colloidal particles participate in these processes not only during stage II but also during stage III of the crystallization process. The presence of the (sub)colloidal particles in the liquid phase of the crystallizing system and the constancy of their size during the entire process (stage II + stage III; see Figure 5 and Table 1) support such a conclusion. Thus, it seems that formation of these particles is conditioned by the composition of the liquid phase at tc ≥ 1 h (∼0.37 mol dm−3 SiO2, ∼3.7 × 10−4 mol dm−3 Al2O3, ∼0.11 mol dm−3 Na2O, and ∼0.1 mol dm−3 TPABr) but that after formation they are stable and “inactive” for further reactions. The described model of crystallization, schematically presented in Scheme 1, explains the formation of zeolite ZSM-5 in the form of polycrystalline aggregates and, at the same time, explains: (i) why the formed polycrystalline aggregates cannot be observed in SEM and TEM images (Figures 7E, 6D, and 6E) at even high crystallinity (75%; see Figure 1A-III and Supporting Information SI-5), (ii) why the small polycrystalline aggregates can be only seldom observed at the surfaces of CAs (Figures 7G and 6F), and (iii) why the polycrystalline aggregates of fully crystalline zeolite ZSM-5 can be observed in the corresponding SEM images (Figure 7H) only after the complete amount of amorphous phase of CAs has been transformed into zeolite ZSM-5 (for tc ≥ 21 h). The final part of the model (nucleation and crystal growth; Scheme 1g and h) is similar to the models proposed by Gabelica et al.10b for crystallization of zeolite ZSM-5 and later on by Valtchev and Bozhilov40 for crystallization of FAU-type zeolites. However, in difference to these models by which the nucleation and crystal grow take place in the “primary” gel phase, in the crystallization pathway described here, these processes take place in the CA particles (“tertiary” amorphous phase) formed by a stepwise transformation of the primary amorphous phase into WLPs and subsequent transformation of WLPs into CAs; this is, at the same time, the most expressed uncommonness of this crystallization pathway. It can be assumed that such an unusual crystallization pathway is caused by the very low amount of solid in the reaction mixture (less than 5 wt %) from one side and use of sodium aluminate as an active source of aluminum (see Supporting Information SI-6) from the other side. On the other hand, taking into consideration that (i) the chemical composition of the RM based on Na2O, Al2O3, SiO2, TPABr, and H2O and synthesis conditions (heating of the RM at 170 °C under static conditions) is consistent with “classical” approaches and (ii) by careful observation at high magnification

[WLP≡Al−OH]− + A+−O−Si≡ ↔ [WLP≡Al−O−Si≡]− A+ + OH−

where A+ is Na+ and/or TPA+. The constancy of the size and shape of WLPs until the end of stage II of the crystallization process (see Supporting Information SI-3) leads to conclusion that not (sub)colloidal particles but rather low molecular weight (Na,TPA)-silicate anions from the liquid phase participate in this process. At the same time, this process increases the number of terminal ≡Si−OH (silanol) groups on the surfaces of WLPs. An increase of “concentration” of silanol groups on the surfaces of the WLPs in the time interval from tc = 4 h to tc ≈ 20 h makes conditions for condensation reactions of silanol groups30 of neighboring WLPs and, thus, formation of WLP−Si−O−Si−WLP links (Scheme 1e). The consequence of such condensation reactions is spontaneous aggregation of WLPs (Figure 9A′, Figure SI-4.1 in Supporting Information SI4 and Scheme 1f) at the beginning of stage III of the crystallization process. Densification of the aggregated WLPs and thus formation of condensed aggregates (CAs; 7E, 7F, 6D, and 6E), at stage III of the crystallization process, is probably caused by coalescences of WLPs inside the aggregates (Scheme 1f). The high concentration of aluminosilicate material34,37 in CAs, from the one side, and increased amount of TPA, from another side, make conditions for the formation of zeolite (MFI) nuclei10b,d,g,h,11,14a,b,e,37a,38 and their growth10b,d,g,14a,b,e,38,39 in the amorphous matrix of CAs (Scheme 1g). Although not discernible in the corresponding SEM (Figures 7E−7H) and TEM images (Figures 6D−6G), the occurrence of these processes (nucleation and crystal growth of zeolite ZSM-5 in the matrixes of CAs) is identified by an increase of both the fraction, fc, of zeolite ZSM-5 (Figure 1A-III and Figure SI-5.1 in Supporting Information SI-5) and TPA content in the solid phase from 6 wt % at tc = 20 h, through 8.5−9 wt % at tc = 20.33 and 20.66 h to 10.5 wt % at tc = 21 (Figures 10A−10D). The amount of 10.5 wt % of TPA corresponds to 3.68 TPA+ ions per unit cell of the obtained zeolite ZSM-5, which is close to the “ideal” content (4 TPA+ ions per unit cell).6f,10b,c,i,12,36 1735



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basis of the obtained results and existing knowledge on the crystallization of MFI type zeolites, there are some open questions (e.g., why WLPs are formed?; what is the real mechanism of WLPs formation and role of “active” aluminate in the WLPs formation?; what is the fine mechanism of formation of CAs by aggregation and densification of WLPs?; what is the nature of ∼30 nm particles in the liquid phase, and why they did not participate in the crystallization process?), which need to be answered by further investigations. However, regardless of the mentioned open questions, the results of this work not only, to a large extent, clarify the uncommonness of the investigated system but also make a contribution to the knowledge needed for further modification/correction of the mechanism of crystallization of MFI-type zeolites in heterogeneous systems.

of SEM images the WLPs can also be identified at the initial stage of crystallization of zeolite ZSM-5 from the typical dense heterogeneous system (2.5Na2O−8TPABr−Al2O3−60SiO2− 800H2O; see Supporting Information SI-7), one can believe that such a crystallization pathway may be a general rule not only for dilute systems but also for typical (concentrated) heterogeneous systems. However, due to the rapid occurrence of the processes, the described uncommonness cannot be usually observed in concentrated heterogeneous systems.

5. CONCLUSIONS By careful analysis of the pathway of crystallization of zeolite ZSM-5 in the system 12.5Na2O−Al2O3−8TPABr−60SiO2− 4000H2O using a series of sophisticated methods, it is concluded that the process of crystallization takes place by a chain of events, through three stages: Stage I: (i) Formation of Al-enriched aggregates of colloidal silica (ACSs) used as silica source. (ii) Dissolution of the Alenriched ACSs and formation of different TPA−silicate species, including ∼0.7 nm subcolloidal particles (IOCS?) in the liquid phase as well as TPA-poor aluminosilicate gel (primary amorphous precursor) by (poly)condensations of silicate and aluminate species from liquid phase. (iii) Further dissolution of the Al-enriched ACSs during heating of the RM from room temperature to 170 °C causes formation of ∼30 nm particles in the liquid phase and formation of worm-like features, joined with the dissolving ACSs, in the solid phase of the RM. Stage II: (iv) Further dissolution and deaggregation of ACSs and formation of individual worm-like particles (WLPs secondary amorphous precursor). Deposition of TPA−silicate species from the liquid phase onto the surfaces of WLPs increases the “concentration” of the terminal OH group on the surfaces of WLPs and, at the same time, increases the content of TPA of WLPs. The ∼30 nm particles in the liquid phase did not change their size during stage II. Stage III: (v) Aggregation of WLPs and formation of condensed aggregates (CAstertiary amorphous aluminosilicate precursor). The aggregation is driven by reactions of the surface (terminal) −OH groups of neighboring WLPs (formation of WLP−Si−O−Si−WLP linkages). Formation of CAs is probably caused by coalescences of WLPs in aggregates. (vi) Formation of nuclei and their growth inside CAs; these processes are caused by the high concentration of aluminosilicate material and increased amount of TPA in the CAs. Simultaneous growth of a large number of nuclei in each of the CAs results in the formation of polycrystalline aggregates of zeolite ZSM-5. The ∼30 nm particles in the liquid phase did not change their size during stage III. The particularities which make this system different from classical ones (both homogeneous and heterogeneous) can be expressed by two aspects: (1) A great disproportion between the duration of “induction period”, ti (95% of the entire reaction time), caused by the long-term stability of WLPs and “growth period”, tg (5% of the entire reaction time; ti/tg = 19). (2) Formation/transformation of three kinds of amorphous precursors in the sequence: primary aluminosilicate amorphous precursor (gel, formed during room-temperature aging of the reaction mixture) → secondary amorphous aluminosilicate precursor (WLPs) → tertiary aluminosilicate amorphous precursor (CAs); only the amorphous phase of CAs can be finally transformed into a crystalline one (ZSM-5). Although most of the events and effects of this unusual and complex crystallization pathway can be explained to a large extent on the

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ASSOCIATED CONTENT

S Supporting Information *

Influence of temperature on the concentration of Si in the liquid phase (SI-1). TG-DTG, XRD, and FTIR data of the solid phase of the reaction mixture separated at stages I and II of the crystallization process (SI-2). Particulate properties of WLPs at stage II of the crystallization process (SI-3). Early stage aggregation/condensation of WLPs (SI-4). XRD patterns of the solid phase of the reaction mixture separated at stage III of the crystallization process (SI-5). Role of aluminum on the ti/tg ratio (SI-6). SEM and TEM images of the solids separated from the reaction mixture 2.5Na2O−8TPABr−Al2O3−800H2O during its hydrothermal treatment (SI-7). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Nan Ren. Phone: (+86)-21-55664097. Fax: (+86)-2165641740. E-mail: [email protected]. Boris Subotić. Phone: (+385)-14680123. Fax: (+385)-14680098. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by NSFC (20803010, 20890122, and 21073041), “Chen Guang” project supported by Shanghai Municipal Education Commission and Shanghai Education Development Foundation (09CG02), the Major State Basic Research Development Program of China (2009CB930400, 2009CB623506), the STCSM (08DZ2270500, 09DZ2271500), ‘Brain Gain’ Post-Doc project (I-668−2011) supported by Croation Science Foundation and project 098−0982904−2953, financially supported by the Ministry of Science, Education and Sport of the Republic of Croatia.

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REFERENCES

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