Synthesis and Structural Properties of Zeolytic Nanocrystals I. MFI

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J. Phys. Chem. C 2007, 111, 2368-2378

ARTICLES Synthesis and Structural Properties of Zeolytic Nanocrystals I. MFI Type Zeolites P. Morales-Pacheco,† F. Alvarez-Ramirez,† P. Del Angel,† L. Bucio,‡ and J. M. Domıńguez*,† Instituto Mexicano del Petro´ leo, Programa de Posgrado, Programa de Ingenierıá Molecular, 152 Eje Central L. Cardenas, 07730 Me´ xico D.F., and Instituto de Fı´sica, UNAM, Apdo. Postal 20-374, 01000 Me´ xico D.F. ReceiVed: July 27, 2006; In Final Form: October 28, 2006

MFI type zeolite nanocrystals with a SiO2/Al2O3 ratio of 120 were synthesized using soft-chemistry conditions, at 353 K, P ) 582 mmHg, from clear solutions of tetrapropylammonium hydroxide (TPAOH), fumed silica (CAB-O-SIL), and aluminum sulfate. The formation of crystallites with a low dimensionality (10-30 nm) was examined in function of the reaction time by X-ray diffraction (XRD), Fourier transform infrared (FTIR) (with KBr), 27Al and 29Si nuclear magnetic resonance (NMR) magic-angle spinning (MAS), and high-resolution transmission electron microscopy (HRTEM). Complementary information on the critical parameters of the low-dimension crystals was obtained by HRTEM and Rietveld’s method. The theoretical kinematical approach was used for calculation of the high-resolution electron microscopy images (Cerius2). The physicochemical properties of the zeolitic nanocrystallites were compared with macroscopic MFI type crystals; the unit cell parameters were a ) 20.02 Å, b )19.89 Å, and c )13.38 Å, which means a slight contraction of about 0.25, 0.16, and 0.3% with respect to large MFI type crystals, respectively. HRTEM indicated the ZSM5 nanocrystals in its aluminosilicate matrix present a disklike shape, a Pnma symmetry, and pore dimensions in the (010) plane of about 0.56 nm. The mean crystallite sizes along [020] and [200] were 20 and 46 nm, respectively. The high surface charge associated with the nanocrystals influences the cell dimensions and crystal morphology, mainly.

Introduction The synthetic zeolites are used widely as catalysts or adsorbents in petroleum refining, petrochemicals production, and fine chemistry. For example, FAU (Y), MFI (ZSM5), and LTA (A) are used in FCC, aromatics alkylation, and separation media, respectively.1-5 About 133 zeolites have been reported so far, and this number tends to increase, as well as properties and applications,6 which depend on the chemical composition (i.e., SiO2/Al2O3 ratio), crystal size, crystal thickness, surface chemistry, and so forth.7-9 In this respect, the search of new properties of known zeolites continues to be a subject of scientific research, where a recent trend is the synthesis of zeolitic materials with a low dimensionality,10 that is, 5-100 nm size range. Mintova and co-workers11,12 reported the synthesis of colloidal crystals of FAU and LTA-type zeolites, with a mean crystal size between 40 and 80 nm, which were synthesized using 15-Crown-5 ether as a co-template. Similarly, Holmberg and co-workers13,14 reported the synthesis of small FAU crystals of about 32-120 nm diameter, which were obtained using tetramethylammonium bromide (TMABr) as a co-template. Van Grieken et al.15 reported ZSM-5 crystallites of about 10-100 nm diameter, which were obtained using clear supersaturated homogeneous mixtures and a crystallization time of 24 h in hydrothermal conditions. Li et al.16 studied the nucleation and crystal growth * Author to whom correspondence should be addressed. E-mail: [email protected]. † Instituto Mexicano del Petro ´ leo. ‡ Instituto de Fı´sica.

kinetics of nanosized FAU crystallites from clear solutions, thus emphasizing the influence of the growth limiting nutrient (Na+) on the crystallization process. Also, Verduijn and Schoeman and Sterte17-19 reported FAU, LTA, and MFI crystallites of less than 100-nm diameter. However, the low dimensionality of the zeolitic crystallites poses some questions regarding the crystal stability, the effects of a high surface charge on the unit cell parameters or pore dimensions, and the ability of these materials to act as efficient molecular diffusion media, with respect to conventional zeolites, that is, crystallites of about 1-µm diameter. Thus, a better understanding of the influence of low crystal dimensionality on the molecular sieving and diffusion properties of these materials must be based on a detailed characterization of the physicochemical properties of the zeolitic nanocrystals.20,21 In this respect, Thiele’s modulus (φ) is a useful parameter that can be scaled down to the nanometric dimensions, that is, φ ) R(kint/D)1/2, where R is the crystal thickness, kint is the intrinsic rate constant, and D is the intracrystalline diffusion coefficient. The diffusion efficiency defined as η ) tan(φ)/φ expresses the effect of crystal thickness on the molecular diffusion paths.20,21 Thus, the use of crystallites with sizes down to nanometric dimensions might cause a significant decrease of the molecular diffusion paths as well as the increase of external crystal surface area (S) or the surface to volume ratio (S/V), with respect to conventional zeolites; assuming a cubic crystallite and a constant volume (V), the corresponding S/V ratio is equal to 120 µm-1 for a crystal thickness of about 0.05

10.1021/jp064780v CCC: $37.00 © 2007 American Chemical Society Published on Web 01/20/2007

Synthesis and Properties of Zeolytic Nanocrystals

J. Phys. Chem. C, Vol. 111, No. 6, 2007 2369

Figure 2. X-ray powder diffraction profile of the zeolitic materials after 96 h of crystallization. Notice experimental (crosses), Rietveld’s profile (solid line), and difference pattern (bottom). The vertical lines indicate Bragg’s reflections.

Experimental

Figure 1. XRD patterns of the materials before (a) and after ion exchange with NH4NO3 (b), in function of the crystallization time (days).

µm, which is about 20 times the S/V ratio of 1-µm-thick crystals, that is, (S/V)1µm ) 6 µm-1. The increase of the external surface area emphasizes both surface charge effects and the number of surface sites, which might influence the external crystallite shape, the unit cell dimensions, pore arrays, and diffusion properties of the zeolitic crystallites, which are of great importance for adsorption, gas separation processes, and catalysis,22a because of the influence of those parameters on the contact time and mass/heat transfer. Therefore, assessing the critical parameters of the nanosized zeolite crystallites is crucial for future applications of these materials, for example, the mean crystal size, lattice symmetry, cell parameters, structural defects, pore shape, pore array symmetry, and crystal thickness. Thus, this work focus on the synthesis of MFI type crystallites with low dimensions, that is, within the size range between 5 and 20 unit cells across and about 1-5 unit cells thickness. The physicochemical properties were characterized by high-resolution transmission electron microscopy (HRTEM), 27Al and 29Si nuclear magnetic resonance (NMR) magic-angle spinning (MAS), Fourier transform infrared (FTIR), and X-ray diffraction (XRD)-Rietveld’s refinement methods. The full characterization by HRTEM may give access to the fundamental crystallite parameters at the subunit cell resolution level,23a-23c but the application of these techniques to zeolitic nanocrystals has been scarce, which is mainly due to the low crystal stability under the electron beam. Thus, in this work, we used HRTEM to characterize the main structural features of the MFI type zeolite crystallites. Also, the Rietveld method was used to verify the main structural parameters, including the mean crystal size. In addition, the XRD, FTIR, and 27Al NMR (MAS) methods allowed to assess the progression of the crystalline phase in function of the crystallization time.

The zeolitic materials were synthesized with the following gel composition (molar): 9.12(TPA)2O:60SiO2:0.5Al2O3: 936H2O. The main chemical precursors were aluminum sulfate (Al2(SO4)3 18H2O, J. T. Baker) and fumed silica (CAB-O-SIL), which were combined within a boiling solution of deionized water with tetrapropylammonium hydroxide (TPAOH, Aldrich). After 10 min stirring under reflux, a clear homogeneous solution was obtained, which was cooled down to room temperature, while the mass losses were compensated by adding deionized water. The clear solutions were stored into flasks, which were fitted with reflux condensers using an oil bath. The crystallization took place at atmospheric pressure (i.e., 582 mmHg in Mexico City) under static conditions, at 353 K; the products were analyzed at different crystallization times (i.e., 1, 2, 4, and 9 days) before and after calcination at 853 K; also, a portion of each samples was ion-exchanged with NH4NO3 under mild conditions (T ) 30 °C, 3 h), and these samples were calcined at 823 K to decompose ammonium into NH3 and hydrogen. The samples were inspected by means of X-ray diffraction (XRD) in a Siemens D5000 diffractometer (λCu ) 1.5405 Å). The shape and width of the Bragg reflections in the powder diffractograms were handled to perform a structural analysis by Rietveld’s method (i.e., program FULLPROF).24 In this case, the XRD measurements were recorded at room temperature, at 35 kV and 25 mA, with a vertical goniometer, a fixed diffracted beam graphite monochromator, and a scintillation counter. The 2θ-range measured was between 5 and 60°, with a 0.02° step size and 5-s counting time. The starting parameters for Rietveld’s refinement were obtained from the data reported by Olsen et al.25 (PDF 44-3). A pseudo-Voigt function modified by Thompson et al.26 was chosen to generate the diffraction peaks profile. To perform a structural analysis, an instrumental resolution function was obtained using lanthanum hexaboride (LaB6 from NIST, SRM 660), which was used as a certified standard for the profile analysis. The zeolites were further studied by high-resolution transmission electron microscopy (HRTEM) using an FEI (TECNAI) FE 30 (300 Kv) microscope and a JEOL (200 Kv) 2200 FS, where the samples were mounted onto copper grids after a dehydration treatment overnight at 573 K and ionic exchange with NH4NO3 in aqueous solution followed by calcinations at 673 K. Also, the theoretical images

2370 J. Phys. Chem. C, Vol. 111, No. 6, 2007

Morales-Pacheco et al.

Figure 4. 27Al NMR (MAS) spectra of samples with 2, 4, and 9 days of crystallization, (a) samples calcined at 853 K before ion exchange, and (b) samples exchanged with NH4NO3 and calcined at 853 K.

Figure 3. FTIR spectra (KBr) corresponding to the samples after 1-9 days of crystallization.

of the MFI structure were calculated using Cerius2 software27 and standard image processing methods (Digital Micrograph program from Gatan Inc.). The surface characterization of the samples by IR spectroscopy was made using an infrared Nicolet spectrophotometer (Model Magna 560) with the samples diluted in KBr (spectroscopy grade). Finally, 29Si- and 27Al-NMR magic-angle spinning spectra of the powder samples were recorded at 59.57 and 78.14 MHz, respectively, using a Bruker 400 (400 MHz) spectrometer. The pulse width was 8 µs and the spinning frequency used for 29Si was 7.5 kHz (PROBHD 4 mm, MAS 1H/BB), while the spinning frequency for 27Al-NMR was 10 kHz, with the same probe as for 29Si, a delay time of 30 s for noncrystalline samples, and a delay time of 5 s for crystalline samples. The measurements were performed at room temperature and external references were used for each case, that is, TMS and Al(NO3)3 (0.1 M), respectively. Results The zeolitic materials with a SiO2/Al2O3 ratio of about 120 were synthesized from clear solutions at 353 K, P ) 1 atm,

under hydrothermal conditions. The extraction of samples from the synthesis vessel (i.e., glass flask) at different time intervals (i.e., 1, 2, 4, and 9 days) allowed the verification of the progression of the crystallization from the initial amorphous gel. Thus, the formation of the zeolitic phases was confirmed by XRD, as illustrated in Figure 1a, which shows the XRD patterns corresponding to the gel evolution from the initial amorphous precursors (bottom). After 48 h of crystallization, the typical XRD peaks corresponding to the MFI structure appear (Figure 1). In the interval 48-96 h, the crystallites seem to grow faster and, after 216 h, there is a full crystalline phase. Interestingly, one observes an intensity reversal of the main XRD peaks at low angle, that is, 7.93° < 2θ < 8.8°, with respect to the peaks in the interval 23.1° < 2θ < 23.42°, the latter being more prominent, about 4 times more intense, with respect to the former. It seems that during the crystallite growth the highindex planes (i.e., (051) at 2θ ) 23.1°, (-501) at 2θ ) 23.27°, (501) at 2θ ) 23.42) with a lower density are favored with respect to low-index planes (i.e., (-101) at 2θ ) 7.93°, (011) at 2θ ) 7.94°, (101) at 2θ ) 8.01°, (020) at 2θ ) 8.8°). In contrast to this, Figure 1b shows the XRD patterns of the samples exchanged with NH4NO3 during 3 h, which were calcined afterward at 853 K. In this pattern, one observes an intensity reversal of the XRD peaks with respect to those described above, with no variation of the interplanar distances. The relative intensities of the XRD peaks in Figure 1b are



Figure 5. 29Si NMR (MAS) spectra of samples with 2, 4, and 9 days of crystallization, (a) samples calcined at 853 K before ion exchange, and (b) samples exchanged with NH4NO3 and calcined at 853 K.

TABLE 1: Determination of the Unit Cell Parameters for ZSM5 Nanocrystals after 9 Days of Crystallization materials

a (Å)

b (Å)

c (Å)

ZSM5 (reference) ZSM5 (problem)

20.109(1) 20.113(1)

19.898(1) 19.920(1)

13.368(1) 13.380(1)

TABLE 2: AlIV/AlVI Ratios Calculated from 27Al NMR (MAS) Spectra for the Nonexchanged and Exchanged Samples, in Function of the Crystallization Time AlIV/AlVI ratios materials

2 days

4 days

9 days

nonexchanged exchanged

1/0 1/0.6482

1/0 1/0.217

1/0 1/0.120

similar to the ones reported elsewhere for ZSM5.22d Thus, under the soft-chemistry conditions used in this work, the zeolitic crystallites develop high-index planes preferentially but after a thermal treatment with NH4NO3 followed by calcination (T ) 853 K), either an additional growth of the low-index planes or a crystallite structural change may occur, both of which do not modify the interplanar distances, as deduced from the XRD patterns in Figure 1b. On the other hand, the Rietveld analysis was based on a reference sample, that is, CBV-1502 from Zeolyst Inc., which is composed by typical platy crystals of about 1-µm diameter and a thickness of about 180 nm. Thus, Figure 2 displays the Rietveld analysis data corresponding to the zeolitic materials that were crystallized for 4 days from clear solutions; the experimental (crosses) and calculated (lid line)

XRD patterns are compared by the difference profile (bottom). For the best fit, the mean crystal size and the standard deviation (anisotropy, in parenthesis) are equal to 31 (8) nm, respectively. The mean apparent sizes along [020] and [200] directions are 20 and 46 nm, respectively, while the orthorhombic unit cell parameters are a ) 20.02 Å, b ) 19.89 Å, and c ) 1 3.38 Å, which differ slightly with respect to data reported elsewhere,22b,c that is, a ) 20.07 Å, b ) 19.92 Å, and c ) 13.42 Å; these data indicate a slight contraction of about 0.25, 0.16, and 0.3%, respectively. FTIR Characterization.28a-c The series of materials corresponding to the MFI type structure (Figure 1) were characterized by FTIR (KBr), and the results are illustrated in Figure 3, where one observes the typical stretching bands of water around 3421 cm-1 and also some band shoulders that indicate the presence of structural OH groups. The band at 1627 cm-1 is assigned to a scissor type band arising from the proton vibration in the water molecules, and this was verified by treating the sample at a moderate temperature, which causes the elimination of this signal. The triplet observed at about 2971 cm-1 is assigned to residual organic matter in the sample; these signals are related to the double band at 1400 cm-1. The high-energy bands are typical of both symmetrical and nonsymmetrical C-H bonds, while the low-energy bands are assigned to bond deformations. The bands at 1111 cm-1 have a well-defined band shoulder on the high-energy side, but this signal is not well resolved for the samples that have only 1 or 2 days of crystallization. These bands are assigned to nonsymmetrical internal vibrations of the SiO4 tetrahedra, while the smaller signals appearing on the right side with respect to the intense band at 967 cm-1 correspond to the stretching vibrations of terminal silanol groups; as observed, these bands are not apparent in the samples with 1 and 2 days of crystallization. The bands around 808 cm-1, 555 cm-1 and the most intense at about 462 cm-1 are directly related to the materials structure. The latter signal is assigned to T-O (T: Si, Al, etc.) bending while the signal around 808 cm-1 arises from internal stretching of T-O-T bonds. This signal is sensitive to the Si/Al ratio and it may be displaced accordingly. The sharp band occurring at 555 cm-1 arises from the presence of structural double rings, which has been demonstrated for several zeolites having five-membered double-ring blocks vibrations, that is, MFI, MOR, FER.28a-c This band is important because it is sensitive to the crystalline nature of the materials, as indicated in Figure 3, where one observes that the band is absent for the amorphous aluminosilicate gel with 1 day of crystallization, but it is clearly observed for the samples with 4 and 9 days of crystallization (Figure 3). The FTIR spectra corresponding to adsorbed pyridine upon the nanocrystalline zeolites (not shown) revealed the presence of a strong band at 1443 and 1446 cm-1 for the nonexchanged and exchanged samples, respectively, which disappears at about 150 °C in both cases; also, for the exchanged samples a moderate intensity band at 1543 cm-1 is present, which holds up to about 250 °C. The intermediate band (B + L) appearing at 1490-1491 cm-1 is present for both samples, but it vanishes early for the nonexchanged samples at about 100 °C, which is more probably associated to a neutral hydrogen from adsorbed water, while this band holds up to about 300 °C for the exchanged samples, thus indicating a stronger Bronsted type site (i.e., at 1543 cm-1), which may be associated to the acidic Al-O(H+)-Si bridged species.28d 27Al and 29Si NMR (MAS). The 27Al and 29Si NMR magicangle spinning spectra are shown in Figures 4 and 5, respectively. As reported elsewhere for conventional zeolites,29a-c the



TABLE 3: AlIV Chemical Shifts of Q4 (4Si) Units and Al-O-Si Bond Angles (r) in the Zeolite Framework in Function of the Crystallization Time 2 days

4 days

9 days

materials

δ (ppm)

R (degrees)

δ (ppm)

R (degrees)

δ (ppm)

R (degrees)


53.83 50.72

156.34 162.56

53.91 54.03

156.18 155.94

52.31 53.97

159.38 156.06

TABLE 4: Q3/Q4 Ratios Calculated from the 29Si NMR Spectra for the Nonexchanged and Exchanged Samples, in Function of the Crystallization Time 29

Si NMR Q3/Q4 ratios

materials

2 days

4 days

9 days


3.15 2.48

0.22 0.18

0.17 0.08

aluminum atoms are either integrated within the siliceous zeolite tetrahedra lattice (SiO4) with a four-fold-Al coordination (AlIV) or the Al atoms may be outside the tetrahedral lattice with a six-fold coordination (AlVI). These two forms present characteristic isotropic chemical shifts (δ); for the tetrahedrally coordinated aluminum (AlIV), one obtains δ values of about 7080 ppm in layered silicates (i.e., q3(3Si) groups) while δ is about 55-68 ppm for q4(4Si) type groups in the framework silicates like zeolites.30a,b On the other hand, the octahedrally coordinated aluminum (AlVI), that is, [Al(H2O)6]aqu3+, presents chemical shifts between 0 and about 20 ppm. In the present work, the Al central transitions appear mostly around δ ) 53 ppm, with respect to the AlVI reference, that is, δ ) 0 ppm for Al(NO3)3 (0.1 M). Figure 4a and 4b corresponds to the 27Al NMR (MAS) spectra in function of the crystallization time, before and after ionic exchange with NH4NO3 at 30 °C, respectively. First, Figure 4a shows a broad low-intensity AlIV peak (bottom) at δ ) 53.83 ppm, which evolves with the crystallization time (i.e., 4 and 9 days), shifting toward δ ) 53.91 and δ ) 52.31 ppm, respectively; the latter peaks present an outline sharper than the former peak at δ ) 53.83 ppm, thus indicating a more defined Al atom coordination into the tetrahedral zeolite lattice, which is also a consequence of the crystallite growth. In addition, the samples with different crystallization time (2, 4, and 9 days) were ion-exchanged with NH4NO3 in aqueous media and were dried at 110 °C and then were calcined at 853 K for 6 h. Thus, the 27Al NMR (MAS) spectra corresponding to the ion-exchanged samples are displayed in Figure 4b, where one observes (bottom) a broad NMR peak at δ ) 50.72 ppm corresponding to the sample with 2 days of crystallization, and also there are two sharp peaks corresponding to the samples with 4 and 9 days of crystallization, that is, δ ) 54.03 ppm and δ ) 53.97 ppm, respectively. Also, some additional peaks appear at δ ) -7.29, 1.81, and 1.14 ppm, respectively, which correspond to the isotropic chemical shifts (δ) that are characteristic of Al with octahedral coordination (AlVI).30 Thus, the AlIV/AlVI ratios for the nonexchanged and exchanged samples are reported in Table 2 for the series with 1, 2, 4, and 9 days of crystallization, respectively. Overall, the nonexchanged zeolites (Figure 4a) show a good integration of Al atoms into the zeolite tetrahedral lattice, which is illustrated by the very high AlIV/ AlVI ratios in Table 2, and this result can be related to the low proportion of Al with respect to Si, that is, SiO2/Al2O3 ) 120, which suggests that Al atoms integrate more easily into the siliceous tetrahedral lattice as isolated charge-deficient sites. On the other hand, the results shown in Figure 4b indicate that the ionic exchange treatment causes an extensive dealumination mainly in the ill-crystallized samples (i.e., 2 days of crystallization), as evidenced by the 27Al NMR (MAS) extra peaks at around δ ) 0 ppm, which are assigned to extraframework

aluminum (AlVI), with AlIV/AlVI ratios of 1.6, 4.76, and 8.3 for the samples with 2, 4, and 9 days of crystallization, respectively (Table 2). This suggests that (AlO4)- tetrahedra formed in the poor crystalline solids (i.e., 1-2 days crystallization, as shown by Figure 1) seem to have Al-O bonds weaker than wellcrystallized samples (i.e., 4-9 days of crystallization, in Figure 1). Also, these results show that the ion-exchange treatment under mild conditions (i.e., T ) 30 °C, 3 h) causes a partial dealumination of the well-crystallized zeolite crystallites too, which increases the SiO2/Al2O3 ratio of the zeolite lattice. The chemical shifts corresponding to the framework Al atoms are summarized in Table 3, together with the Al-O-Si bond angles (R), which were calculated from the linear relationship reported by Lippmaa et al.30a,b (i.e., δ ) -0.5R + 132). Thus, there is a slight increase of the bonding angles with the progression of crystallization for the nonexchanged materials, which should increase acidity, while the exchanged samples show the opposite, a diminution of the mean bond angle from 162.56° to about 156.06°. The 29Si NMR (MAS) spectra are illustrated in Figure 5a and 5b for the samples before and after ionic exchange with NH4NO3, respectively. These spectra exhibit structural features similar to conventional ZSM5 crystals.29a-c,30 The characteristic peak corresponding to the Q4 coordination unit, that is, Si*(OSi)4, appears at about -113 to -115 ppm, while a secondary peak shoulder appears at about -100 ppm, which may be associated to the Q3 coordination unit, that is, HOSi*(OSi)3 or AlO-Si*(OSi)3. The sample with 2 days of crystallization shows a 29Si NMR (MAS) broad peak with a peak shoulder down the low field side, while the samples with 4 and 9 days of crystallization show a sharper peak profile (Figure 5a) centered at δ ) -115 ppm. The 29Si NMR (MAS) peak series corresponding to the exchanged samples is illustrated in Figure 5b, which shows a broad peak for the sample with 2 days of crystallization, which evolves to a sharper peak with the crystallization time (i.e., 4 and 9 days). The Q3/Q4 ratios were calculated from the 29Si NMR spectra (Figure 5a and 5b) for the nonexchanged and exchanged samples, in function of the crystallization time (Table 4). One observes a marked trend toward lower Q3/Q4 values in function of the crystallization time, especially for the exchanged samples, which accounts for the progressive crystal growth and integration of Si into a full Q4 unit coordination; also from the data reported in Table 3, one observes that the ion exchange treatment causes the Q3/Q4 ratios to diminish by about one-fifth for samples with 2 and 4 days crystallization and by a factor of about one-half for the samples with 9 days, with respect to the nonexchanged samples. These results converge together with the data reported in the precedent section, regarding the aluminum extraction from the zeolite tetrahedral lattice, thus confirming the progressive formation of Si*(OSi)4 type units along the crystallization process. HRTEM (High-Resolution Transmission Electron Microscopy). High-resolution electron microscopy was used to verify the main structural features of the zeolitic nanocrystallites. We made a previous search of the proper conditions for observing the zeolitic crystallites without the common artifacts that are produced by interaction of matter with the high-energy



Figure 6. High-resolution electron microscopy images of a zeolite showing the dynamical sequence of crystal degradation (from left to right) in function of time.

Figure 7. HRTEM image of a sample with 4 days of crystallization. Notice the zeolitic nanocrystals embedded in the aluminosilicate matrix: (A) [010] ZSM5, (B) ZSM5 crystallites stacked along [010] direction. (C) and (D) are the optical transforms of A and B, respectively.

electron beam, that is, E ) 300 KeV and 200 KeV in the FE30 and JEOL FS 2200 microscopes, respectively. This effect is illustrated in Figure 6 by means of a series of transmission

electron micrographs corresponding to the same sample region (nonexchanged) recorded at different intervals of time, that is, 2, 4, 6, and 8 s. As observed, the initial picture corresponds to



Figure 8. (a) HRTEM micrographs showing a typical aggregate formed by crystallites laying on (010). (b) Expanded view of a typical ZSM5 crystallite laying on (010), with the typical inner pore arrays.

a lattice resolution image (i.e., a single lattice periodicity) of a small zeolitic crystallite that goes into a degradation process, as shown by the micrographs in Figure 6b and 6c, and then one observes that the lattice planes fade away (Figure 6d). This is a consequence of the interaction of matter with the electron beam, which causes structural damage especially in the crystallites with sizes in the range below 50 nm; this is the reason why there are scarce references regarding a full characterization of zeolite nanocrystals by HRTEM methods. Thus, Figure 6 illustrates how the interpretation of HRTEM images could lead to misleading information when it is based only on isolated bright field images from a later stage of evolution (i.e., Figure 6c). In a recent report, on the basis of similar single lattice images, that is, one-periodicity along the zeolite nanocrystals,

the authors conclude about the most probable growth mechanism of the zeolitic nanocrystals from amorphous gel,31 but in the light of the present discussion those results could be reinforced further by recording the entire evolution sequence. Alternatively, some procedures use low electron beam doses and special digital camera chamber recording devices (CCD),32a-e which are used to set the best conditions for characterizing the structural features of conventional zeolite crystals by HRTEM. The present study was based on these guidelines and, in addition, the samples were ion-exchanged with NH4NO3 at 30 °C, 3 h, with the purpose of increasing the SiO2/Al2O3 ratio for improving the structural stability of the crystallites, thus reducing damage caused by the electron beam. All the samples were treated accordingly, and we verified a substantial improvement of the crystal stability



Figure 9. Typical ZSM5 crystallites of the samples with 2 and 4 days of crystallization. The size range of the crystallites is between 10 and 50 nm; notice both faceted (a, e) and irregularly shaped crystallites (b, c, d).

during all the HRTEM observations. Thus, Figure 7 displays a typical region of the samples with a crystallization time of 4 days, which are constituted by small crystallites with a rounded outline but irregular rims (label A); the typical crystallite sizes were in the range 30-40 nm diameter; also, other crystallite aggregates (label B) were observed, with a diameter of about 60 nm, which are composed by crystallites stacked face-to-face along [010]. The optical transforms corresponding to both A and B crystallites are shown in Figure 7C and 7D, which confirm the orientation perpendicular to b axis (crystallite A) as well as the stacking orientation of the crystallites consisting of aggregates (label B). Similarly, Figure 8a and 8b shows the highresolution images corresponding to the sample with a crystallization time of 9 days; this is the edge of a crystalline aggregate composed by several crystalline domains of about 30-nm diameter with the boundaries outlined clearly; these crystalline domains expose the (010) plane to the observation axis. From these HRTEM images, both the symmetry of the inner pore arrays and the pore dimensions of the most typical crystallites were verified, and the results confirm the MFI type structure (Figure 8b) oriented perpendicularly to the [010] crystal axis, that is, the symmetry corresponds to the space group Pnma and the mean pore diameter is equal to 0.56 nm.22,25

Figure 9 shows a selection of typical crystallites belonging to the samples with 2 and 4 days of crystallization, with crystal sizes between 10 and 50 nm and a variable morphology, either faceted or a disklike shape with irregular rims. A series of simulated HRTEM images were calculated using the kinematical approach,23d and the results are displayed in Figure 10 b-d. First, Figure 10a is an experimental image showing the basic zeolite cell viewed along [010] axis. Figure 10b corresponds to a model of the ZSM5 structure which was obtained using arbitrary parameters, that is, a thickness of 3 unit cells (i.e., Z ) 5.96 nm), with a defocus condition ∆F ) 0, a perfect 〈010〉 zone axis, and a tilt angle (φ) equal to 0. In Figure 10c, one observes the atom position of the basic unit cell used for comparison, and Figure 10d and 10e illustrates the superposition of the structure model onto the simulated image;27 Figure 10f shows the projected potential image of ZSM5 for 1-unit cell thickness, which is affected by the instrumental parameters, that is, Cs ) 2.2 mm, ∆F ) 0, φ ) 0 〈010〉 zone axis. The other features regarding the image contrast of zeolite crystallites with diameters below 50 nm are discussed next, which are important for interpreting the HRTEM results properly. First, the effect introduced by defocus is illustrated



Figure 10. (a) Experimental HRTEM micrograph of [010]-ZSM5. (b) Simulated image of the ZSM5 crystal for a thickness (t) equivalent to 3 unit cells (59.697 Å), ∆F ) 0, φ ) 0. (c) Basic cell of ZSM5 with cell dimensions a ) 20.022 Å, b ) 19.899 Å, and c ) 13.383 Å. (d) Superposition of the crystalline atomic model over the experimental HRTEM image of [010]-ZSM5. (e) Crystalline grid (atomic model). (f) Projected potential image, 1-unit-cell thickness, Cs ) 2.2 mm, ∆F ) 0, φ ) 0. 〈010〉 zone axis.

Figure 11. (a) Experimental series of HRTEM images corresponding to the crystallite shown in Figure 7A, at different defocus values (arbitrary). The critical point of contrast reversal occurs between the third and fourth images.

in Figure 11a-d, which is a defocus sequence of experimental HRTEM images corresponding to the crystallite marked A in Figure 7. One observes a critical point in terms of contrast reversal that occurs between the third and fourth image, where the black contrast of the pore apertures turns white. The series of high-resolution images was calculated using the basic unit cell data reported elsewhere22,25 and using a sequence of instrumental factors such as defocus (-1000 Å < ∆F < +1000 Å), tilt angle (-5°< φ < +5°), and thickness (1-7 unit cells along b-axis). Thus, Figure 12 shows a partial series of the calculations corresponding to a model with a 3-unit cell thickness, in the interval +800 Å e ∆F e -800 Å, where one observes a contrast reversal occurring in the interval 0 Å e ∆F e -400 Å. These results were compared with the series of experimental images obtained in the electron microscope (Figure 11) to match both experimental and calculated images; thus, two representative images having black and white contrast are presented in Figure 13. From these correspondences, we determined both the instrumental conditions and the crystal characteristics that are more probable for these nanocrystals, and thus ∆F ) +800 and -800 Å, respectively (Figure 13), and the spherical aberration (Cs), thickness (t), and zone axis are 2.1 mm, 3 unit cells thickness (5.96 Å), and 〈010〉 zone axis, respectively. The crystal thickness is about 3 times less than the mean size along [020], which was determined by

Rietveld’s method, that is, 20 nm; this can be justified by the statistical nature of the latter method and the local character of the HRTEM methods. Discussion and Conclusions The evolution of the transformation of an amorphous aluminosulicate gel into ZSM5 nanocrystals under soft-chemistry conditions, that is, T ) 353 K, P ) 582 mmHg, was studied by several techniques, with the purpose of characterizing the main structural parameters of the zeolitic crystallites. Under these conditions, the crystallites grow slowly and, after 48 h of crystallization, there is an incipient crystalline phase, which grows fast, and at 96 h the mean crystallite size along [020] and [200] directions is 20 and 46 nm, respectively. The crystallite formation occurs by the preferential growth of highindex planes, that is, (051), (-101), (501), over low-index planes, that is, (-101), (011), (101), (020). Upon thermal treatment with NH4NO3 the crystalline structure recomposes into a structure similar to conventional ZSM5,22d with a little overgrowth of low-index planes with respect to high-index planes, without any further change of the interplanar distances. The HRTEM results indicate that the initial crystallites have a disklike morphology, which is the result of an almost quasi2-D crystal growth with an isotropic contribution of the forming



Figure 12. Series of calculated images using the kinematical approach,23 corresponding to a [010]-ZSM5 crystallite of about 3-unit-cells thickness, with defocus values in the interval +800 Å e ∆F e -800 Å. The critical point of contrast reversal occurs in the interval 0 Å e ∆F e -400 Å.

Figure 13. (a) Experimental HRTEM image corresponding to the crystallite shown in Figure 7A and Figure 11a. (b) Calculated image corresponding to ∆F ) +800 Å, Cs ) 2.1 mm, t (thickness) ) 3 unit cells (59.697 Å), 〈010〉 zone axis. (c) Experimental image corresponding to the same crystallite under a different defocus setting. (d) Calculated image corresponding to ∆F ) -800 Å, Cs ) 2.1 mm, t (thickness) ) 3 unit cells (59.697 Å), 〈010〉 zone axis.

material. The rounded shapes of the crystallite rim indicate the presence of high-index planes, which coincides with the XRD results. The thermal treatment with NH4NO3 followed by calcination at 853 K causes the crystallites to restructure internally, without external shape modifications or interplanar distances displacement, as verified by HRTEM and XRD, respectively. The FTIR results indicate that the absorption bands are similar to those observed in conventional zeolites,28a-d in particular, the band at 555 cm-1 appears only after 48 h of crystallization, which is related to the presence of five-membered double-ring

blocks vibrations.28a-c Also, the 27Al NMR (MAS) study showed a progressive integration of Al atoms into the tetrahedral coordination of the zeolite lattice along the crystallization process. At 48 h of crystallization, a broad 27Al NMR peak at δ ) 53.83 ppm indicates an ill-tetrahedral coordination (AlIV) with no extraframework aluminum; this peak becomes sharper with the crystallization time (96 and 214 h), which indicates a good Al integration into the zeolite lattice. In contrast to this, the ionic exchange of the calcined samples with NH4NO3 causes the extraction of Al atoms from the zeolite tetrahedral lattice, as evidenced by the presence of the 27Al NMR peaks at δ ) 7.29, -1.81, and -1.14 ppm. These are isotropic chemical shifts corresponding to octahedrally coordinated AlVI with different mean Al-O-Si bond angles, that is, about -061 ppm/ degree.30a-c Also, as the AlIV chemical shifts (δ) follow a linear relationship with the Al-O-Si mean bond angles (R)30a-b for the present case (Table 3), the nonexchanged samples show a trend toward higher bonding angles with the progression of crystallization, that is, from 156.34° up to 159.38°, however, the exchanged samples show a trend in the opposite direction, from 162.56° down to 156.06° (Table 3). These results demonstrate that ion exchange with NH4NO3 causes some modifications of the mean bond angle of the Q4(4Si) units. The magnitude of these changes are higher than other framework aluminosilicates like NaY (acid) and Mordenites (strongly acidic), for which R ) 140° and 153.7°, respectively,33,34 which might indicate the formation of strongly acidic sites in the MFI type zeolites of this work, because of the relative isolation of the AlIV sites. Furthermore, the partial dealumination of the zeolite lattice produced by the ionic exchange treatment causes an increase of the SiO2/Al2O3 ratio, which in turn improves the structural stability of the crystallites exposed to the electron beam for HRTEM observations. This is demonstrated by the series of defocus sequences (Figure 11) that lasted up to about 20 min instead of only a few seconds. To our knowledge, this is the first time that a full HRTEM characterization of zeolitic nanocrystals is reported. Thus, the unit cell parameters and crystal symmetry were confirmed, that is, a ) 20.02 Å, b ) 19.89 Å, c ) 13.38 Å, and Pnma group, with a difference of about 0.25, 0.16, and 0.3% with respect to data reported

2378 J. Phys. Chem. C, Vol. 111, No. 6, 2007 elsewhere for conventional crystals.22 Also, the mean crystallite size was determined by Rietveld’s method, in the directions perpendicular to (020) and (200) planes, giving a mean crystal size of 20 and 46 nm, respectively, which is consistent with HRTEM observations, where most of the crystallites presented a disklike morphology with a thickness of about 3-7 unit cells, that is, 5.96-13.9 nm. The difference of crystal thickness obtained by Rietveld’s method and HRTEM may be justified by the statistical nature of XRD measurements, in contrast with the local nature of HRTEM. These data are useful for modeling the molecular diffusion properties of the zeolitic materials. Overall, the HRTEM results indicate that there is an influence of the surface charge associated to very thin zeolitic crystallites on both unit cell parameters and crystallite shape, however, the morphology modifications are not as drastic as is the case of other M4+ type metal oxide35-38 systems like TiO2 (anatase) which tend to form nanoscrolls and nanotubes because of the strong surface charge associated to the thin oxide/hydroxide layers. References and Notes (1) Barthomeuf, D. Basic Zeolites: characterization and uses in adsorption and catalysis. Catal. ReV. - Sci. Eng. 1996, 38 (4), 521-612. (2) Weitkamp, J.; Weiss, U.; Ernst, S. New aspects and trends in zeolite catalysis. Stud. Surf. Sci. Catal. 1995, 94, 363-380. (3) Marcilly, C. R. Top. Catal. 2000, 13 (4), 357-366. (4) Pfenninger, A. Structures and Structure Determination. Molecular SieVes: Science and Technology; Springer: Berlin, 1999; Vol. 2/1999, pp 163-198. (5) Marcilly, C. R. Evolution of Refining and Petrochemicals. What is the place of zeolites. Proceedings of the 13th International Zeolite Conference, Montpellier, France, July 8-13, 2001; Stud. Surf. Sci. Catal. No. 135. PL-4. (6) Baerlocher, Ch.; Meier, W. M.; Olson, D. H. Atlas of zeolite framework types, 5th ed.; IZA, Elsevier: Amsterdam, 2001. (7) Geier, O.; Vasenkov, S.; Lehmann, E.; Ka¨rger, J.; Rakoczy, R. A.; Weitkamp, J. Proceedings of the 13th International Zeolite Conference, Montpellier, France, July 8-13, 2001; Stud. Surf. Sci. Catal. No. 135, 19O-04. (8) (a) Davis, M. E. Nature 1993, 364, 291-393. (b) Degnan, T. F., Jr. J. Catal. 2003, 216, 32-46. (9) Slater, B.; Catlow, C. R. A. Molecular dynamics of the faujasite (111) surface. Proceedings of the 13th International Zeolite Conference, Montpellier, France, July 8-13, 2001; Stud. Surf. Sci. Catal. No. 135, 16O-02. (10) Tosheva, L.; Valtchev, V. P. Nanozeolites: Synthesis, Crystallization Mechanism, and Applications. Chem. Mater. 2005, 17, 2494-2513. (11) Mintova, S.; Valtchev, V. Synthesis of nanosized Fau-Type zeolite, Porous Materials in Environmentally Friendly Processes. Stud. Surf. Sci. Catal. 1999, 125, 141-148. (12) Mintova, S.; Valtchev, V.; Bein. T. Formation of colloidal molecular sieves: influence of silica precursor. Colloids Surf. 2003, 217, 153-157. (13) Holmberg, B. A.; Wang, H.; Norbeck, H. M.; Yan, Y. Controlling size and yield of zeolite Y nanocrystals using tetramethylammonium bromide. Microporous Mesoporous Mater. 2003, 59, 13-28. (14) Holmberg, B. A.; Wang, H.; Yan, Y. High silica zeolite Y nanocrystals by dealumination and direct synthesis. Microporous Mesoporous Mater. 2004, 74, 189-198. (15) Van Grieken, R.; Sotelo, J. L.; Meneńdez, J. M.; Melero, J. A. Anomalous crystallization mechanisms in the synthesis of nanocrystalline ZSM-5. Microporous Mesoporous Mater. 2000, 39, 135-147. (16) Li, Q.; Creaser, D.; Sterte, J. An Investigation of the Nucleation/ Crystallization Kinetics of Nanosized Colloidal Faujasite Zeolites. Chem. Mater. 2002, 14, 1319-1324. (17) Verduijn, J. P. (Exxon Chemical Patents Inc.). WO 97/03020, 1997. (18) Verduijn, J. P. (Exxon Chemical Patents Inc.). WO 97/03021, 1997.

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Synthesis and Structural Properties of Zeolytic Nanocrystals I. MFI

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