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Synthesis and Structural Properties of Zeolitic Nanocrystals II: FAU-Type Zeolites P. Morales-Pacheco,† F. Alvarez,† L. Bucio,‡ and J. M. Domı´nguez*,† Instituto Mexicano del Petro´leo, Programa de Posgrado/Programa de Ingenierı´a Molecular, 152 Eje Central L. Ca´rdenas Norte, 07730 Me´xico D.F. and Instituto de Fı´sica, UNAM, Apdo. Postal 20-374, 01000 Me´xico D. F. ReceiVed: August 7, 2008; ReVised Manuscript ReceiVed: December 4, 2008
FAU(Y)-type zeolite nanocrystals with a SiO2/Al2O3 ratio of about 3.4 were synthesized at 95 °C under autoclave pressure from clear solutions of tetramethylammonium hydroxide (TMAOH), aluminum isopropoxide, tetramethyl ammonium bromide (TMABr) as a second source of organic template, and tetraethyl orthosilicate (TEOS) as silica source. Formation of low-dimensional zeolitic crystallites (20-30 nm diameter) was examined by X-ray diffraction (XRD) as a function of reaction time; Fourier transform infrared spectroscopy (FTIR) (with KBr and pyridine), 27Al and 29Si nuclear magnetic resonance (NMR), and high-resolution transmission electron microscopy (HRTEM) were used to characterize structural and textural features of the zeolite nanocrystals. The cell parameters and crystallite size were obtained by Rietveld’s method. Also, the inner crystallite structure was studied by high-resolution electron microscopy images using the kinematical approach (Cerius2), and the physicochemical properties of the zeolitic nanocrystallites were compared with macroscopic FAU-type crystals. The external shape of the nanocrystals was cubic, and the unit cell parameter a was equal to 24.74 Å with mean crystallite sizes of about 22.3 nm. Introduction The Faujasite type Y zeolite (FAU) is the main component of fluid catalytic cracking (FCC) catalysts, a process that continues to be the main source of gasoline in oil refineries. This zeolite has a three-dimensional pore structure with a molecular size window with a diameter of about 0.74 nm, inherent surface acidity, and high structural resistance for withstanding severe hydrothermal conditions during regeneration in the FCC process. In this sense, the crystal structure of FAU is an outstanding material with multiple applications in several technological fields of petroleum refining, petrochemicals production, and Fine Chemistry.1-3 In particular, typical commercial catalysts for gasoil cracking consist of hard microspheres with a diameter of about 80 µm that contain 10-30% wt FAU, which is constituted by zeolite crystals of about 1 µm size. Thus, only a low portion of the surface active sites locate on the outermost crystallites surface, thus making it that bulky gasoiltype molecules do not have access to the inner zeolite channels network but only secondary and tertiary products may diffuse through the channels. Recent studies reported that the use of submicrometer-sized zeolite crystals improves gasoline quality with a lesser deactivation by coke.4-7 In this view, synthesis of zeolites having crystallites with diameters of a few nanometers rises some interest because their potential advantage with respect to “bulky” crystals for some applications that require efficient diffusion as, for example, in catalytic and adsorption processes. In this respect, some efforts were reported recently to synthesize these materials, for example, Holmberg et al.8,9 reported the synthesis of FAU with controllable particle sizes between 32 and 120 nm using tetramethylammonium (TMA) as a cotemplate and tetramethylammonium bromide (TMABr) as the main template. Also, Zhu et al.10 studied the synthesis of nanocrystals of FAU- and LTA-type zeolites at different (TMA)2O/Al2O3, * To whom correspondence should be addressed. E-mail: jmdoming@ imp.mx. † Instituto Mexicano del Petro´leo. ‡ Instituto de Fı´sica.
SiO2/Al2O3 and NaCl/Al2O3 ratios with FAU crystallites having average sizes of about 80 nm. Also, Mintova et al.11,12 synthesized colloidal crystals of FAU- and LTA-type zeolites with mean crystal sizes between 40 and 80 nm using 15Crown-5 ether as a cotemplate. Mintova et al.13 reported a study on the nucleation mechanism of Y zeolite from colloidal precursors. Q. Li et al.14 studied the nucleation and crystal growth kinetics of nanosized FAU crystallites from clear solutions as well as the influence of the growth-limiting nutrient (Na+) on the crystallization process, where sodium concentration was controlled below a critical level for avoiding nucleation of LTA. Moreover, Verduijn and Schoeman15-17 reported the synthesis of FAU, LTA, and MFI nanocrystallites of less than 100 nm of diameter. Also, Valtchev et al.18 made an eleectron microscopy study on the formation of FAU-type zeolites at room temperature. A review of the state of the art on the synthesis, crystallization mechanisms, and applications of nanozeolites was published by Tosheva and P. Valtchev.19 A better knowledge of the inner structural and textural features of the zeolitic nanocrystals should provide valuable information to advance the study and application of these materials. Consequently, the purpose of this work was to synthesize FAU(Y)-type zeolites with low crystals dimensions, i.e., within the size range between 5 and 20 unit cells, and fully characterize their physicochemical properties by powder X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), 27Al and 29Si NMR(MAS), and Fourier transform infrared spectroscopy (FTIR). In particular, HRTEM was used to determine the fundamental parameters of the small zeolitic crystallites, i.e., pore arrays and crystal symmetry at the subunit cell resolution level. Also, Rietveld’s method was used to verify the main structural parameters and average crystal size, while the progression of the crystalline phase as a function of time and functional properties were followed by a combination of XRD, FTIR, and 27Al NMR (MAS).
10.1021/jp8070713 CCC: $40.75 2009 American Chemical Society Published on Web 01/20/2009
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Experimental Section Synthesis of FAU-type zeolite nanocrystals from clear aqueous solutions with a molar gel composition 2.4(TMA)2O: 0.032Na2O:1.0Al2O3:3.4SiO2:370H2O was as follows. Synthesis was performed at 95 °C under autoclave pressure. A clear aqueous solution was obtained by dissolving tetramethylammonium hydroxide (TMAOH, Sigma) in double-deionized water (DDI H2O), and aluminum isopropoxide (alumina source, 98 wt %, Aldrich) was added to the alkali solution. Then, tetramethylammonium bromide (TMABr, 98 wt %, Fisher) and tetraethylorthosilicate (TEOS, silica source, Aldrich) were added under stirring until a clear solution forms. The mixture was aged during 24 h at room temperature with vigorous stirring; then 0.4 mL of NaOH (0.1 M) solution was added. Afterward, crystallization started in a Teflon-lined stainless steel autoclave at 95 °C for 140 h. Periodically, 0.4 mL of 0.1 M NaOH solution was added during the synthesis every 12 h until a gel forms with the following composition: 2.4(TMA)2O:0.43Na2O:1.0Al2O3: 3.4SiO2:370H2O. Following the hydrothermal reaction the remaining solid was washed and dispersed in DDI H2O; then it was centrifuged at 20 000 rpm for 50 min and redispersed in DDI H2O again under ultrasonic bath. The washing procedure was repeated 3 times before drying at 110 °C for 12 h, and calcination at 550 °C for 4 h was performed in order to eliminate the organic template. After ion exchanging the zeolitic material with a 1 M NH4NO3 solution for 3 h at 300 K further washing was made using double-deionized water before drying and calcining to eliminate NH3; thus, the acid form of zeolite Y was obtained. The zeolitic materials were studied by X-ray diffraction (XRD) in a Siemens D5000 diffractometer (λCu ) 1.5405 Å) within the 2θ range from 4° to about 50°. The shape and width of the Bragg reflections from the powder diffraction patterns were used to perform a structural analysis by Rietveld’s method using the program FULLPROF.20 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 samples were scanned 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.21 (PDF 44-3). A pseudo-Voigt function modified by Thompson et al.22 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. Surface characterization of the solids was made by IR spectroscopy using a Infrared Nicolet spectrophotometer (model Magna 560) with the samples diluted in KBr (spectroscopy grade). Also, 29Si and 27Al NMR magic angle spinning nuclear magnetic resonance 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; 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 and a delay time of 30 s for noncrystalline samples and 5 s for crystalline samples. The measurements were performed at room temperature, and external references were used for each case, i.e., TMS and Al(NO3)3 (0.1M), respectively. The zeolites were further studied by high-resolution transmission electron microscopy (HRTEM) using a FEI (TECNAI) FE30 (300 kV) microscope and a JEOL (200 kV) 2200 FS, where the samples were mounted onto copper grids after
Figure 1. XRD patterns of zeolitic materials as a function of the crystallization time (h): (a) as synthesized and calcined and (b) after ion exchange with NH4NO3.
calcination at 820 K. Theoretical images of the Y zeolite structure were calculated using the HRTEM module implemented in the Cerius2 interface,23 and the results were compared with the experimental images using standard image processing methods (Digital Micrograph Program from Gatan Inc.). Results Structural Characterization. The zeolitic materials with a SiO2/Al2O3 ratio of about 3.4 were synthesized from clear solutions at 95 °C under autoclave pressure and hydrothermal conditions. The XRD patterns of the as-synthesized FAU(Y) solids show typical features of the Y zeolite topological structure.24 Also, a broadening of the main peaks indicates the microcrystalline character of these materials,. Figure 1a shows the XRD patterns corresponding to gel evolution from the initial amorphous precursors (bottom). After 48 h of crystallization small XRD peaks corresponding to the Y-type zeolite structure emerged; afterward at 48-120 h of crystallization the XRD peaks are more apparent (Figure 1a), which indicates a progressive growth of the crystallites until 120 h, where a full crystalline phase is verified. In addition, the XRD patterns of the solids exchanged with NH4NO3 (1 M) and calcined at 550 °C are illustrated in Figure 1b. As observed, following ion exchange and calcination there is a structural collapse, i.e., at 48 and 60 h
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Figure 2. XRD peak intensity (at 2θ ) 6.4°) vs crystallization time.
of crystallization the XRD features fade away, which indicates the formation of an amorphous phase. This is probably caused by sudden elimination of the adsorbed water, which comes out during calcination, whereas the solids prepared at 84, 120, and 144 h show only a diminution of the original XRD Intensity (i.e., crystallinity), but it does not disappear completely (Figure 1b). The latter case shows that the XRD peaks of the NH4NO3exchanged materials are broader than the original zeolites (Figure 1a), which is indicative of the structure ordering loss that follows the exchange process. To better understand the lattice degradation upon ion exchange with NH4NO3 a study of the solids was made by 27Al NMR, as reported next. In Figure 2 crystal growth is illustrated as the variation of the XRD peak intensity at 2θ ) 6.4°, where the curve reaches a plateau between 140 and 160 h of crystallization. This behavior follows a polynomial law, as illustrated in Figure 2. In addition, a Rietveld analysis was made on the basis of a comparison with respect to a reference sample taken from a commercial zeolite, i.e., Zeolyst CBV720 series. Thus, Rietveld analysis of the reference material is presented in Figure 3a, which displays the XRD pattern corresponding to crystals of about 373.5 nm. In contrast, Figure 3b displays the Rietveld analysis data corresponding to the zeolitic materials that were crystallized in the present work after 144 h from clear solutions. The experimental points are marked by crosses, while the calculated points are indicated by the solid line. Figure 3a and 3b are compared with respect to each other by the difference profile, i.e., the difference pattern shown at the bottom. From these calculations and the optimal correspondence between both patterns the mean crystallite size of the experimental materials was 22.33 nm, the anisotropy was about 0.3, and the cubic unit cell parameter was equal to 2.474 nm. This value coincides with those reported elsewhere.25 Surface Characterization by FTIR. The main functional surface features of the zeolitic nanometric FAU-type crystallites were characterized by FTIR (KBr). Thus, Figure 4 displays the high-energy IR absorption bands of a typical material after 144 h crystallization. The IR spectrum shows the typical symmetrical and nonsymmetrical C-H bonds with the low-energy bands (Figure 4) assigned to bond deformations.26 Also, one observes the typical stretching bands of water around 3452 cm-1 together with some shoulders that indicate the presence of structural OH groups. The band appearing at about 1640 cm-1 can be assigned to a scissor-type-band, which might arise from the proton
Figure 3. X-ray powder diffraction patterns of zeolitic materials (i.e., marked by crosses). The Rietveld profile is marked as a solid line, and the difference pattern is shown at the bottom. The vertical lines indicate the corresponding Bragg’s reflections of the (a) reference sample (from Zeolyst, CBV720) and (b) experimental zeolite (This work, 144 h crystallization).
vibration in the water molecules.27 The bands between 1250 and 950 cm-1 are assigned to asymmetrical stretching vibrations corresponding to the tetrahedral atoms.27 The band between 720 and 650 cm-1 is assigned to symmetric stretching vibrations, while the band occurring at 560 cm-1 arises from the presence of structural double rings (D6R) assigned to Y zeolite;25 this band is important because it is sensitive to the crystalline nature of the Y-zeolite-type materials,28 as indicated in Figure 4. This band is absent in the amorphous aluminosilicate gels with 24 h crystallization but is clearly observed for the solids having a longer time of crystallization. The band appearing at 455 cm-1 is assigned to a structure-insensitive (internal) tetrahedral bending band of Y zeolite, i.e., T-O4 (T ) Si or Al).25 Figure 5 displays the IR spectra of adsorbed pyridine over the surface of the nanocrystalline materials formed after 144 h crystallization, which were exchanged with NH4NO3, where one observes the presence of three bands appearing at 1443, 1490, and 1596 cm-1, which may belong to CdC vibrations of aromatic type from the ring of pyridine.29 These bands are typical of the pyridinium ion, which gives a clear indication of the presence of Lewis-type acidity; furthermore, the band appearing at 1540 cm-1 is characteristic of the acid Bro¨nsted sites, which are
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Figure 4. FTIR (KBr) spectra of the solids synthesized after 24-144 h of crystallization. Upper spectra are displayed fully. An expanded view of the lower frequency region is included with the main crystallization band encircled. The dotted arrow indicates the position of this crystallization band, at 560 cm-1, which is assigned to the presence of structural double rings D6R.
Figure 5. Pyridine adsorption on nanocrystalline materials (H-FAU(Y)) with 144 h of crystallization. Lewis acid sites (L) and Bro¨nsted acid sites (B) are marked.
capable of withstanding temperatures up to 400 °C, thus indicating the high acid sites strength.29,30 27 Al and 29Si NMR(MAS). The 27Al and 29Si NMR MAS (magic angle spinning) spectra are shown in Figures 6 and 7, respectively. As reported elsewhere for conventional zeolites,31-33 the aluminum atoms can be either integrated within the zeolite
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Figure 6. 27Al NMR(MAS) spectra of samples calcined at 550 °C: (a) before ion exchange and (b) after exchange with NH4NO3.
tetrahedral lattice (-SiO4-), thus having a 4-fold Al coordination (AlIV), or the Al atoms can be located outside the tetrahedral lattice with a 6-fold-type coordination (AlVI). These two forms present typical isotropic chemical shifts (δ), the first one corresponding to tetrahedrally coordinated aluminum (AlIV), around δ values between 50 and 80 ppm in the layered silicates (i.e., q3(3Si) groups) like clays, while δ is about 55-68 ppm for Q4(4Si)-type AlO4 groups in the framework of tectosilicates like zeolites. On the other hand, the octahedrally coordinated aluminum of the nanosized zeolites of this work, i.e., (AlVI) or [Al(H2O)6]3+aqu, presents the typical chemical shifts between 2.82 and 0.93 ppm. Thus, the present work demonstrates that the Al central transitions appear mostly around δ ) 53 ppm with respect to the AlVI reference (δ ) 0 ppm for Al(NO3)3, 0.1M). Figure 6a and 6b illustrates the variations of the 27Al NMR (MAS) spectra as a function of the crystallization time before and after ionic exchange with NH4NO3 at 30 °C, respectively. Figure 6a shows the evolution of the crystalline solids that were prepared after 60, 84, 120, and 144 h. The δ values of the minor peaks corresponding to 27Al NMR move from -3 to 1 ppm, which indicates some isotropic chemical shifts (δ) that are characteristic of AlVI with octahedral coordination.32,33 Also, the major 27Al NMR peaks in Figure 6a and 6b correspond to 27Al in tetrahedral coordination, which
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Figure 7. 29Si NMR(MAS) spectra and deconvolution of relevant peaks: (a) solids calcined at 550 °C before ion exchange, (b) solid exchanged with a solution 1 M of NH4NO3 and calcined at 550 °C. Both a and b were prepared with a crystallization time of 120 h. (c) Materials before ion exchange, and (d) material with ionic exchange. Both c and d have a crystallization time of 144 h.
TABLE 1: AlIV/AlVI Ratios Calculated from 27Al NMR(MAS) Spectra for the Nonexchanged and Exchanged Samples as a Function of the Crystallization Time
TABLE 2: AlIV Chemical Shifts of Q4 (4Si) Units and Al-O-Si Bond Angles (r) in the Zeolite Framework as a Function of the Crystallization Time
AlIV/AlVI
84 h
materials
84 h
120 h
144 h
materials
nonexchanged exchanged
1/0.293 ) 3.41 1/0.494 ) 2.02
1/0.268 ) 3.73 1/0.489 ) 2.04
1/0.219 ) 4.57 1/0.431 ) 2.32
nonexchanged exchanged
move from 59 to 61 ppm for the nonexchanged solids, while there is a shift from 56 to 61 ppm for the exchanged solids. The relative proportion AlIV/AlVI is reported in Table 1, where one observes that this ratio increases with crystallization time, which indicates a progressive integration of aluminum atoms into the tetrahedral lattice. The materials series having a different crystallization time, i.e., 60, 84, 120, and 144 h, were ion exchanged with NH4NO3 in aqueous media and then dried at 110 °C and calcined at 550 °C for 6 h. The 27Al NMR(MAS) spectra corresponding to the ion-exchanged solids are displayed in Figure 6b. The solids prepared after 60 h were not studied by NMR because its structure collapsed upon ion exchange, whereas the solids with 84, 120, and 144 h undergo an extensive dealumination process, as shown by a decreasing AlIV/AlVI ratio (Table 1). The chemical shifts (δ) corresponding to the framework Al atoms are summarized in Table 2 together with the Al-O-Si bond angles (R), which were calculated from the linear relationship reported by Lippmaa et al.34 (i.e., δ ) -0.5R + 132). Thus, from 84 to 120 h crystallization time the
120 h
144 h
δ (ppm) R (deg) δ (ppm) R (deg) δ (ppm) R (deg) 59.91 56.10
144.18 151.8
60.38 60.32
143.24 143.36
60.27 59.85
143.46 144.30
nonexchanged solids do not show major chemical shifts corresponding to AlIV, i.e., δ moves from 59.91 to 60.38 ppm, while the Al-O-Si mean bond angles (R) move from 144.18 to 143.24°. However, in the same interval, i.e., from 84 to 120 h of crystallization time, the exchanged solids show an increasing isotropic chemical shift (δ) from 56.10 to 60.32 ppm while R decreases from 151.8 to 143.36°. In contrast to this trend, for the interval of crystallization between 120 and 144 h no appreciable changes exist for δ and R (Table 2). Figure 7a and 7c shows the 29Si NMR(MAS) spectra and optimal deconvolution of the relevant peaks that correspond to the solids prepared with 120 and 144 h of crystallization, which were calcined at 550 °C before ion exchange. Figure 7b and 7d display the 29Si NMR(MAS) spectra corresponding to the solids prepared with 120 and 144 h of crystallization after ion exchange. Both spectra exhibit structural features similar to conventional FAU(Y) zeolites.35 The areas under each peak (29Si MAS NMR) at -85, -90, -95, -100, and -105 ppm correspond to distinct Si coordination environments Si(4Al),
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TABLE 3: Calculated Areas of the Different Types of Signals Q4(nAl) from Deconvolution of 29Si NMR(MAS) Spectra for Y Zeolite Nanocrystals, Nonexchanged and Exchanged, with 120 and 144 h of Crystallization Areas for the different types of Q4(nAl) signs crystallization time (h)
materials
Si(4Al)
Si(3Al)
Si(2Al)
Si(1Al)
Si(0Al)
120
nonexchanged exchanged nonexchanged exchanged
3.96 2.32 16.50 5.20
21.47 23.01 15.45 30.83
30.42 33.37 25.79 19.58
23.88 34.08 25.06 21.75
20.27 7.22 17.20 20.65
144
TABLE 4: Σ[Si(wAl)]/AlIV, Σ[Si(xAl)]/AlIV, and Σ[Si(yAl)]/ AlIV Ratios for the Materials Synthesized with 120 and 144 h of crystallizationa IV
Σ[Si(wAl)]Al
Σ[Si(xAl)]AlIV IV
Σ[Si(yAl)]Al a
materials
120 h
144 h
nonexchanged exchanged nonexchanged exchanged nonexchanged exchanged
1.72 2.35 1.63 2.29 1.17 1.71
1.72 1.88 1.38 1.76 1.05 1.01
Where w ) 1, 2, 3, 4; x ) 1, 2, 3; y ) 1, 2.
to the existence of weaker bonds (i.e., Si-O-Al) in materials that have a lower time of crystallization, which agrees well with the results obtained by 27Al NMR. Previous works reported the Si/Al ratio from 27Al and 29Si NMR(MAS) using Lowenstein’s equation 4
(Si ⁄ Al)NMR )
∑ ISi(nAl)
n)0 4
∑ 0.25nISi(nAl)
n)0
Si(3Al), Si(2Al), Si(1Al), and Si(0Al), which were calculated from deconvolution of the 29Si NMR peaks for the nonexchanged and exchanged materials (Table 3). One observes that in both cases, i.e., at 120 and 144 h, the ion exchange provokes a decrease of the 29Si NMR peaks areas corresponding to Si(4Al), while the peak areas of Si(3Al) increase. In contrast, the Si(2Al), Si(1Al), and Si(0Al) peaks change in opposite ways before and after ion exchange of the materials treated at 120 and 144 h, respectively (Table 3). This shows that Al loss indicated by the variation of Q4(4Al) units occur upon ion exchange, which is coincident with an increase of the Q4(3Al) fraction, that is, the net dealumination reaction could be as follows
Si(4Al) f Si(3Al) + 0 + AlVI where 0 is an Al vacancy that may be subsequently reoccupied by Si. The materials prepared with 120 h of crystallization show an increase of the 29Si NMR signals, i.e., Q4(2Al) and Q4(1Al), which could be due to double and triple dealumination of the original Q4(4Al) units, but in parallel, there is a diminution of the Q4(0Al) signal, which could indicate a possible modification of the chemical environment of the SiO4 units, for example, by protonation and formation of surface OH- bridges.35 In contrast to this result, ion exchange performed in the materials with 144 h of crystallization causes a diminution of the Q4(2Al) and Q4(1Al) signals, which coincides with an increase of the Q4(0Al) fraction, which is possibly due to successive dealumination that tends to reach the configuration Q4(0Al); these differences in the behavior of the 29Si NMR type signals Q4(2Al) Q4(1Al) and Q4(0Al) for the materials with 120 and 144 h of crystallization could be due to the difference of ratios between the Si Q4(3Al), Si Q4(2Al), and Si Q4(1Al) with respect to the Al present in the tetrahedral lattice. The Σ[Si(xAl)]/AlIV ratio (x ) 1, 2, 3) in Table 4 is similar for the nonexchanged materials with 120 and 144 h of crystallization, but for the exchanged materials with 120 h those ratios are greater than the materials with 144 h of crystallization. This is probably due to an enhanced dealumination of materials that have a lesser time of crystallization. The Σ[Si(xAl)]/AlIV and Σ[Si(yAl)]/AlIV ratios (y ) 1, 2) seem high for the materials with a lesser time of crystallization, which is probably due to enhanced dealumination of zeolitic materials that have a lower time of crystallization. This is probably due
In this respect, Klinowski et al.36 reported the dealumination of a commercial zeolite by ion exchange with (NH4)2SO4 followed by calcination at 400 °C for 1 h, thus giving a solid with about 23.3% dealumination with the following composition (Si/Al)Initial ) 2.61, (Si/Al)Final ) 3.37. J. W. Roelofsen et al.37 reported 3.08% dealumination after ion exchanging two times with (NH4)2SO4 and calcining at 750 °C for 1-3 h, thus obtaining a solid with a composition as follows: (Si/Al)Initial ) 2.52, (Si/ Al)Final ) 2.6. F. Dougnier et al.38 obtained about 13.64% dealumination after ion-exchanging 6 times with (2M) NH4NO3, at 80 °C, thus giving Si/Al ratios as follows: (Si/Al)Initial ) 3.8, (Si/Al)Final ) 4.4. Also, R. L. Cotterman et al.39 reported 3.85% dealumination of NaY (W. R. Grace) of a zeolite exchanged two times in aqueous solution, i.e., (0.25 M) NH4Cl, at 80 °C, for 3 h, and it was calcined at 500 °C followed by self-steaming for 4 h; thus, a solid was formed with a composition as follows: (Si/Al)Initial ) 2.5, (Si/Al)Final ) 2.6. In agreement with these results, in the present work the solids that were prepared after 144 h crystallization led to ratios (Si/Al)Initial ) 2.11, (Si/Al)Final ) 2.25 with 5.95% dealumination. HRTEM (High-Resolution Transmission Electron Microscopy). Electron microscopy was used to verify the main structural features of the zeolitic nanocrystallites. For example, a typical experimental image of a zeolitic crystallite, viewed off 〈101〉 axis, is illustrated in Figure 8a. One observes the ordered arrays with square symmetry and pores with a diameter of about 0.74 nm, which coincides with the typical pore system reported for FAU-Y-type zeolites (i.e., pore diameter of 0.74 nm). In addition, Figure 8b and 8c displays the experimental and simulated optical transform of a slab corresponding to a perfect FAU model with ∆F ) +600 Å, Cs ) 0.5 mm, φ ) 0, and a depth of four unit cells. In general, the crystallites have a cubic shape and crystallite sizes in the range between 20 and 30 nm, which agrees with the results of the Rietveld analysis. In addition, a sequence of simulated HRTEM images was calculated as a function of defocusing (∆F) and the angle of deviation (φ) from the zone axis with respect to the plane (101), the number of zeolite cell units (depth along 〈101〉 axis), and the spherical aberration coefficient (Cs). Theoretical calculations were carried out using the HRTEM module implemented in the Cerius2 interface.23,40 Figures 9 and 10 show the comparison between the experimental and theoretical lattice images; in particular, Figures 9a and 10a correspond to a magnified
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Figure 9. (a) Magnified HRTEM image of zeolite FAU(Y). The crystal plane corresponds to the projection of the zeolite plane (101). (b) Superposition of the (101) crystalline plane over the experimental HRTEM image of nanocrystalline zeolite FAU(Y), i.e., after 144 h crystallization. (c) Theoretical structure of the FAU(Y) zeolite viewed along R. (d) HRTEM simulation image of a four unit cell depth slab model of zeolite FAU(Y) with zone axis R, ∆F ) +600 Å, Cs ) 0.5 mm, φ ) 0. (e) Superposition of the simulated slab model of zeolite FAU(Y) over the experimental image of nanocrystalline zeolite along the R zone axis. Figure 8. (a) TEM image of a zeolitic crystallite after 144 h of crystallization and (b) optical transform and (c) simulated optical transform of a slab model of FAU(Y) with four unit cell depth and zone axis R, ∆F ) +600 Å, Cs ) 0.5 mm, φ ) 0.
HRTEM image of a typical zeolite FAU crystal, whose atomic arrays are viewed perpendicular to the (101) plane, which coincide with the zeolite FAU(Y) theoretical projection along 〈101〉. Figure 9a illustrates the contrast distribution around the pores, passing from black to white along certain directions, which arises as an effect of the local thickness variations, tilting angle, and instrumental factor (aberrations, defocusing) mainly. Figure 10a shows a contrast inversion from the left lower zone to the upper right zone, which is verified as a difference of contrast between white and black pores. Figures 9b and 10b display a superposition of the theoretical crystalline atomic arrays corresponding to the FAU(Y) (101) plane over the experimental HRTEM image of the nanocrystalline zeolite FAU(Y). Figures 9c and 10c illustrate the theoretical crystalline array of the plane (101). Figure 9d corresponds to the HRTEM simulated image of a four cell depth crystal slab model of zeolite FAU(Y) with zone axis R, ∆F ) +600 Å, Cs ) 0.5 mm, and φ ) 0. Figure 10d shows the HRTEM simulated image of a 3 unit cell depth crystal slab model of zeolite FAU(Y) with zone axis R, ∆F ) 0 Å, Cs ) 2.1 mm, φ ) 0. The superposition of simulated images of the zeolite FAU(Y) slab model over the experimental HRTEM images along the zone axis R is displayed in Figures 9e and 10e, which illustrate the coincidence between the experimental and the simulated images into the specific conditions. These results confirm that the nanocrystalline zeolites are in fact of the FAU(Y) type.
Discussion The study of the evolution of amorphous gels into Y zeolite nanocrystals at 95 °C, under autoclave pressure, evidenced that nucleation and the first growth stage are completed at about 48 h of reaction under these conditions. A full crystalline phase forms (Figure 1a) between 120 and 144 h, and after ion exchange with NH4NO3 (1 M) at 30 °C for 3 h followed by calcination at 550 °C the crystal structure collapses only for the solids formed at 48 and 60 h (Figure 1b), whereas the solids prepared at 84, 120, and 144 h withstand better the ion-exchange process. Also, the mean crystallite diameters that were determined from Rietveld’s analysis (Figure 3b) after 144 h of crystallization was 22.3 nm, in contrast with commercial zeolites, i.e., Zeolyst, CBV720 series, which have crystal sizes of about 373.5 nm (Figure 3a). Also, apart from the FTIR bands that are similar to conventional FAU zeolites, the IR band at 560 cm-1 appears only after 48 of crystallization, which is assigned to double rings (D6R) block vibrations of FAU(Y). Also, the IR spectra of adsorbed pyridine on nanocrystalline materials (with 144 h) show three bands appearing at 1443, 1490, and 1596 cm-1, which are CdC vibrations assigned to the pyridinium ion formed on acid Lewistype sites, while the band at 1540 cm-1 arises from acid Bro¨nsted-type sites. Both types of sites withstand up to 400 °C, thus indicating a high strength. A progressive integration of Al atoms into the zeolite lattice was observed by 27Al NMR(MAS), i.e., at δ ≈ 60 ppm for Al(IV). At 60 h of crystallization a broad peak appears, which grows with crystal-
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Morales-Pacheco et al. illustrate the coincidence between the experimental and the simulated images. These results confirm that nanocrystalline zeolites are FAU(Y) type. Conclusions Synthesis of FAU(Y) zeolite crystals ranging in the nanometric size interval 20-30 nm was verified by several techniques. It was found that the main structural parameters coincide with conventional FAU(Y)-type crystallites, which confirms that possible surface tension effects might not be present on the nanometric-type crystals within this size interval. Thus, neither lattice symmetry distortions nor unit cell dimensional variations were observed. However, the crystallization time was found to be critical for keeping the structural integrity of the FAU(Y) nanoncrystals, especially after ion-exchange treatments with NH4NO3 followed by calcination at 550 °C, with the more stable solids obtained after 120 h. This behavior may be due to effects of high-temperature water vapor pressure on the incipient crystallites formed before 120 h. The solids that were prepared after 144 h crystallization led to the ratios (Si/Al)Initial ) 2.11, (Si/Al)Final ) 2.25, with 5.95% dealumination, which are comparable to results obtained with conventional zeolites.
Figure 10. (a) HRTEM amplification of zeolite FAU shown above; the atomic arrangement corresponds to the plane (101). (b) Superposition of the crystalline array over the experimental HRTEM of zeolite FAU. (c) Crystalline array of the plane (101). (d) HRTEM simulation of three unit cell depth slab model of zeolite FAU(Y) with zone axis R, ∆F ) 0 Å, Cs ) 2.1 mm, φ ) 0. (e) Superposition of simulated HRTEM over the experimental one.
lization time. However, after ion exchanging with NH4NO3 and calcination at 550 °C there is a migration of Al atoms out of the zeolite tetrahedral lattice, as evidenced by the higher intensity of the 27Al NMR peaks between 3.3 and -2.5 ppm, i.e., Al(VI) coordination. The AlIV/AlVI ratio tends to go down for the solids that underwent ionic exchange (Table 1). From 84 to 120 h of crystallization the nonexchanged solids do not show important chemical shifts (δ) neither of the tetrahedral AlIV nor the Al-O-Si mean bond angles (R) (Table 2), i.e., δ changes from 59.91 to 60.38 ppm, while R changes from 144.18° to 143.24°. However, in the interval between 84 and 120 h of crystallization the exchanged solids show an increase of isotropic chemical shifts (δ) from 56.10 to 60.32 ppm while R decreases from 151.8° to 143.36°; in contrast, in the interval 120-144 h only slight changes occur in δ and R, which indicates the severity of the ion-exchange process, especially for the incipiently formed crystallites and in agreement with XRD results. Similarly, 29Si NMR shows that at 120 and 144 h ion exchange provokes a decrease of the 29Si NMR peak area corresponding to Si(4Al) while the Si(3Al) peak area increases. In contrast, the Si(2Al), Si(1Al), and Si(0Al) peaks change in opposite ways before and after ion exchange of the materials treated at 120 and 144 h, respectively (Table 3). The nanocrystals observed by HRTEM have a well-defined cubic morphology with mean sizes varying between 20 and 30 nm, in agreement with Rietveld’s analysis. The rhombic symmetry of the pore arrays in the nanocrystals coincides with the (101) surface arrays of FAU(Y), and the simulated image from a slab model of FAU(Y) corresponds to a four unit cell depth crystal with a zone axis 〈101〉, ∆F ) +60 nm, Cs ) 0.5 mm, φ ) 0 (Figure 8b and 8c). The superposition of simulated images of zeolite FAU(Y) slab model over the experimental HRTEM images along the zone axis 〈101〉 (Figures 9e and 10e)
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