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Characterization of aluminosilicate zeolites by UV Raman spectroscopy
Microporous and Mesoporous Materials 46 (2001) 23±34
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Characterization of aluminosilicate zeolites by UV Raman spectroscopy Yi Yu a, Guang Xiong b, Can Li b,1, Feng-Shou Xiao a,* a b
Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Department of Chemistry, Jilin University, Changchun 130023, China State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O. Box 110, Dalian 116023, China Received 7 December 2000; received in revised form 22 February 2001; accepted 22 February 2001
Abstract A series of aluminosilicate zeolites are characterized by UV Raman spectroscopy for the ®rst time, and UV Raman spectra of various zeolites give strong and clear bands with high resolution, while conventional Raman spectra of these zeolites are dicult to obtain because of a strong background ¯uorescence. Additionally, these zeolites show several new bands in UV Raman spectroscopy. A summary of these UV Raman spectra over various zeolites suggests that the bands at 470±530, 370±430, 290±410, and 220±280 cm 1 can be assigned to the bending modes of 4-, 5-, 6-, and 8membered rings of aluminosilicate zeolites, respectively. Furthermore, it is found that the band intensity of zeolites in UV Raman spectroscopy is dependent on the Si/Al ratio. Moreover, the UV Raman spectra of crystallization, for zeolite X at various times show that, in the initial stage of crystallization, the 4-membered rings (510 cm 1 ) interconnect each other to form b-cages with 6-membered rings (390 cm 1 ), which further crystallize to zeolite X. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: UV Raman spectroscopy; Visible Raman spectroscopy; Aluminosilicate zeolites; Crystallization mechanism
1. Introduction With the development of zeolite science, the structural properties of zeolites have been under intense investigation for at least four decades. Many salient features of zeolite dynamics and * Corresponding author. Tel.: +86-431-8922331 ext.: 2314; fax: +86-431-5671974. E-mail addresses: [email protected] (F.-S. Xiao), canli@ ms.dicp.ac.cn (C. Li). 1 Also corresponding author. Tel.: +86-411-4671997, ext.: 728; fax: +86-411-4694447.
structure can be deduced from vibrational spectroscopic data, such as infrared (IR) and Raman spectroscopy, and the two kinds of spectroscopies are often complementary [1]. In zeolite science, Raman spectroscopy seems rather underdeveloped compared with IR spectroscopy. One reason is the diculty to obtain Raman spectra with acceptable signal-to-noise (S/N) ratio from zeolite materials, and the other is that Raman spectra of zeolites are often obscured by a broad ¯uorescence background [2]. To obtain high-quality Raman spectra, the zeolite samples would have to be very pure after a careful treatment [3±6]. Even then, for some
1387-1811/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 1 ) 0 0 2 7 1 - 2
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samples, the background cannot be reduced completely, and the ¯uorescence interference is always a problem [1,2]. In the 1980s Raman spectroscopy has developed rapidly. By using near-infrared laser as light source, FT-Raman spectroscopy can reduce ¯uorescence for some zeolite samples, but the spectral S/N ratio is limited by the IR detector background [7±9]. Recently, UV Raman spectroscopy, which uses a continuous wave (cw) ultraviolet laser as excitation source, was developed for catalyst characterization. The advent of this technique resulted in a big improvement of spectral S/N and the avoidance of ¯uorescence interference [10±12]. Some successful examples show that UV resonance Raman spectroscopy is a powerful tool for studying catalysts and other solids [13±16]. More recently, UV Raman identi®cation of metal atoms in the framework of zeolites such as TS-1 [17], V-MCM-41 [18], and Fe-ZSM-5 [19] has been reported, and it has been shown that UV Raman spectroscopy can selectively characterize a very small amount of transition metal atoms in the framework of zeolites [17±21]. In this case, the ¯uorescence interference from the zeolite is completely avoided [16±20]. Notably, although UV Raman spectroscopy is very sensitive to characterize the structure of zeolites, a series of aluminosilicate zeolites have not yet been systematically investigated by this technique. Our goal in this work is to investigate aluminosilicate zeolites systematically by UV Raman spectroscopy and to ®nd a relationship between UV Raman bands and the characteristic structures of aluminosilicate zeolites. 2. Experimental section 2.1. Materials The aluminosilicate zeolites used in this study were synthesized according to literature recipes. These zeolites were NaA [22], NaX [23], NaY [24], KL [25], NaMOR [26], NaZSM-5 [27,28] and Na beta [29]. The structure of these materials with high crystallinity was investigated by powder X-
ray diraction (XRD). The Si/Al ratio of zeolite X was determined by chemical analysis and NMR and con®rmed by XRD [30,31]. Zeolite X with Si/ Al ratios of 1.00 and 1.02 was synthesized by using Na and K cations [32]. All organic templates in the zeolite samples were removed by calcination in air at 500°C. The crystallization of zeolite X was achieved at 95±100°C in the presence of seeds. The reagents used for preparing the seed solution and synthesizing the zeolite were sodium silicate, sodium aluminate, sodium hydroxide, and potassium hydroxide. The molar composition of the seed solution of zeolite X was 18Na2 O:Al2 O3 :19SiO2 : 370H2 O. Zeolite X (Si/Al 1:0) was synthesized by starting from a mixture with the composition 3.2Na2 O:1.6K2 O:1.0Al2 O3 :2.0SiO2 :76H2 O. The reaction mixture was prepared by mixing appropriate amounts of sodium silicate with a solution of sodium aluminate dissolved in sodium hydroxide and potassium hydroxide, followed by the addition of the seed solution. The samples were heated at 100°C and then removed from the oven at various times during the crystallization and centrifuged for separation into a liquid and solid phase. The latter was extensively washed with deionized water and dried at room temperature prior to Xray diraction and UV Raman characterization. 2.2. Characterization UV Raman spectra were recorded on a UV Raman spectrometer built in the State Key Laboratory of Catalysis at the Dalian Institute of Chemical Physics. The instrument consists of four parts: a UV cw laser, a Spex 1877D triplemate spectrograph, a CCD detector, and an optical collection system. A 244.0-nm line is generated by frequency doubling the 488.0-nm line via an intracavity frequency doubling crystal (Coherent Ionva 300 Fred). The laser power at the sample was kept below 4.0 mW. The spectral resolution was estimated to be 2.0 cm 1 . Samples were mounted into a spinning holder to avoid thermal damage during the scanning. The time for recording the spectrum is about 5 min. Visible Raman spectra of the same zeolites were obtained with a Spex 1403 spectrometer controlled
Y. Yu et al. / Microporous and Mesoporous Materials 46 (2001) 23±34
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by a Spex datamate computer. The wavelength of the excitation line was 488.0 nm (Spectra-Physics Ar ion laser, Model 165) in all cases. The scattered light was detected with an RCA C31034 GaAs photomultiplier tube. The slit widths were typically 6 cm 1 , and the laser power was 50±100 mW. Powder XRD was carried out on a Rigaku D/ Max-IIIA and Siemens D5005 diractometer with a CuKa source. The estimation of the Si/Al ratio for zeolite X by XRD was based on the 2h range 56±58.5° with a scanning speed of 1°/min, and sodium chloride was used as an internal standard [30,31]. 27 Al and 29 Si NMR spectra were performed on a Bruker DRX-400 spectrometer with Al(OH)3 and TMS as standards, respectively. 3. Results and discussion 3.1. Visible Raman spectroscopy Fig. 1 shows the visible Raman spectra of the aluminosilicate zeolites A, X, Y, and beta. The scanning time for each spectrum is about 40 min. The spectrum of zeolite A (Fig. 1a) exhibits a weak band at 490 cm 1 , which is assigned to the bending vibration of 4-membered rings (Si±O±Al) [33]. The spectra of zeolites X and Y show a somewhat stronger band at 510 cm 1 and two very broad bands centered at 300 and 380 cm 1 which are, respectively, attributed to the 4- and 6-membered rings (T±O±T, T Si or Al) [34,35]. The spectrum of zeolite beta has no obvious peaks due to the weakness of the Raman signal. Therefore, in this case the bands in the visible Raman spectrum are very weak, except for a few samples after a very careful treatment. The weakness of the visible Raman signals for these aluminosilicate zeolites is due to the strong background ¯uorescence. By chemical analysis and ICP we have found that these zeolite samples (A, X and Y) contained at least trace amounts of iron impurities which stem from the zeolite preparation with sodium silicate containing 0.1% iron as an impurity. The presence of Fe as an impurity in these zeolites results in a signi®cant increase of luminescence [1,2]. Additionally, other impurities
Fig. 1. Visible Raman spectra of aluminosilicate zeolites: (a) A, (b) X, (c) Y, and (d) beta (excitation at 488.0 nm, laser power 50±100 mW).
such as surface carbonaceous species and a small amount of aromatics also lead to strong background ¯uorescence [2]. 3.2. UV Raman spectroscopy Shown in Fig. 2a±g are the UV Raman spectra of various aluminosilicate zeolites. As compared with the visible Raman spectrum, the UV Raman spectrum exhibits the following obvious features: (1) Strong Raman bands with a high S/N ratio are obtained. The ¯uorescence is completely avoided in the UV Raman spectra because ¯uorescence interference does not normally occur in the UV region for aluminosilicate zeolite [8,9]; (2) the UV Raman spectra can be collected in a very short
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Fig. 2. UV Raman spectra of aluminosilicate zeolites: (a) X, (b) Y, (c) A, (d) L, (e) ZSM-5, (f) MOR, and (g) beta (excitation at 244.0 nm, laser power 4.0 mW).
time (<10 min), and the samples do not need any further pretreatments (e.g., heating, oxygen treatment, and evacuation). On the contrary, the visible Raman spectra were obtained only for the samples
with careful pretreatments, such as calcination and dehydration (5±24 h) [5], and the ¯uorescence was always present, even if the sample was well pretreated.
Y. Yu et al. / Microporous and Mesoporous Materials 46 (2001) 23±34
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Fig. 2. (continued)
3.3. UV Raman spectra of various samples Zeolite X: The UV Raman spectrum of zeolite X with an Si/Al ratio of 1.08 shows bands at 290, 380, 508, 995, and 1075 cm 1 (Fig. 2a). The
strongest band at 508 cm 1 is assigned to the bending mode of the characteristic 4-membered rings [2,35±37], and the bands at 290 and 380 cm 1 are assigned to the bending mode of the 6-membered rings [34,36±39]. The bands at 995 and
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1075 cm 1 are assigned to asymmetric stretching frequencies of T±O in zeolite X [38,40]. Zeolite Y: The UV Raman spectrum of zeolite Y (Fig. 2b) shows bands at 305, 350, 500, 975, 1055, and 1125 cm 1 . It is well-known that the framework topology of zeolite X is the same as that of zeolite Y, and the only dierence between both zeolites is the Si/Al ratio. Therefore, the assignments for zeolite Y are the same as those for zeolite X. As compared with the spectrum of zeolite X, the bands in the Raman spectrum of zeolite Y are shifted by several wavenumbers. For example, the strongest band at 508 cm 1 for zeolite X is shifted to 500 cm 1 for zeolite Y; the band at 290 cm 1 for zeolite X is shifted to 305 cm 1 for zeolite Y. All these results are attributed to the change in the Si/ Al ratio from 1.08 for zeolite X to 2.60 for zeolite Y. Zeolite A: Fig. 2c shows the UV Raman spectrum of zeolite A with an Si/Al ratio of 1.0. The bands appear at 280, 338, 410, 488, 700, 977, 1040, and 1100 cm 1 . As faujasite, zeolite A contains 4and 6-membered rings [41]. Therefore, the basic assignment of the bands in the UV Raman spectrum for zeolite A should be similar to the one for zeolites X and Y. The strongest band at 488 cm 1 is assigned to the bending mode of 4-membered Si±O±Al rings [33,40]. The bands at 338 and 410 cm 1 are attributed to the bending mode of 6membered Si±O±Al rings [34,38,39]. The bands at 977, 1040 and 1100 cm 1 are ascribed to asymmetric T±O stretching motions [4,33,42,43]. The UV Raman spectrum of zeolite A shows a new band at 700 cm 1 , which does obviously not appear in the spectra of zeolites X and Y. Generally, bands in the region of 600±900 cm 1 in Raman spectroscopy are due to T±O symmetric stretching modes [2]. Accordingly, the band at 700 cm 1 is assigned to the T±O stretching mode of zeolite A, in particular to the contribution of the 4membered rings present in zeolite A [4]. It is very interesting to note that zeolite A exhibits a band at 280 cm 1 , in addition to the three bands at 290±305, 338±380 cm 1 , and 500±508 cm 1 assigned to 4- and 6-membered rings in zeolites X and Y. In early reports [3,4,44], no band at 280 cm 1 appeared in the visible Raman spectra
for zeolite A due to the strong background ¯uorescence. On the other hand, the correlation between the Raman bands in the region 300±600 cm 1 and the size of the rings in silicate and aluminosilicate materials as well as aluminosilicate zeolites has already been observed, and it was found that the smaller rings give bands at higher frequencies [2]. Therefore, the band at 280 cm 1 is attributed to the bending mode of higher rings than 6- and 4-membered rings, possibly of the 8membered rings of zeolite A. Zeolite L: The UV Raman bands of zeolite L are shown in Fig. 2d. Bands appear at 225, 314, 498, 986, 1098, and 1125 cm 1 . Again, the strongest band at 498 cm 1 is assigned to the bending mode of 4-membered rings, and the band at 314 cm 1 is assigned to the bending mode of 6membered rings present in zeolite L [40]. The bands at 986, 1098, and 1125 cm 1 are also attributed to asymmetric stretching vibration frequencies of the T±O bonds in the sample. Interestingly, zeolite L exhibits a band at 225 cm 1 , which has not been observed in visible Raman spectroscopy yet. Considering the structure of zeolite L suggests that the band at 225 cm 1 should be assigned to the bending mode of 8-membered rings (T±O±T) [2]. Zeolite ZSM-5: Fig. 2e shows the UV Raman spectrum of zeolite ZSM-5 with bands at 294, 378, 440, 470, 800, 975, 1028, and 1086 cm 1 . The strongest band is observed at 378 cm 1 rather than near 500 cm 1 as for zeolites X, Y, A and L (Fig. 2a±d). We assign this band to the bending mode of 5-membered rings [2,44,45]. The band at 294 cm 1 is assigned to the bending mode of 6-membered rings [2,34,38,39]. The bands at 440 and 470 cm 1 are assigned to 4-membered rings [2,44]. The band at 800 cm 1 is assigned to symmetric stretching, and the bands at 975, 1028, and 1086 cm 1 to asymmetric stretching vibrations of Si±O bonds in ZSM-5, which is consistent with previous assignments in visible Raman spectroscopy [38,45]. Zeolite mordenite: The UV Raman spectrum of zeolite mordenite is shown in Fig. 2f. Bands appear at 240, 405, 470, 482, 550, 820, 1145, and 1165 cm 1 . The framework structure of mordenite consists of 4-, 5-, and 8-membered T±O±T rings. Therefore, the band at 405 cm 1 is assigned to the
Y. Yu et al. / Microporous and Mesoporous Materials 46 (2001) 23±34
bending mode of 5-membered rings, and the bands at 470 and 482 cm 1 are assigned to the bending modes of 4-membered rings. The band at 820 cm 1 is assigned to symmetric stretching motions of T± O bonds, and the bands at 1145 and 1165 cm 1 are assigned to asymmetric stretching motions of T±O bonds. These assignments are in good agreement with those for mordenite in visible Raman spectroscopy [46]. Notably, a broad band appears at 240 cm 1 , which has not been observed in visible Raman spectroscopy. Similarly, this band is assigned to the bending mode of 8-membered rings in mordenite, in agreement with our assignments for zeolite A and zeolite L. The Raman bands for zeolite mordenite are relatively broad, as compared with those of ZSM-5 and zeolites X and A. For example, the band near 480 cm 1 (Fig. 2c) is split into two bands at 470 and 482 cm 1 (Fig. 2f). This may be interpreted in terms of the order of the zeolite framework. The arrangement of zeolite building units in mordenite is not as uniform as in ZSM-5 and zeolites X and A, resulting in a broad band in Raman spectroscopy. Zeolite beta: It is dicult to obtain the Raman spectrum of zeolite beta by conventional Raman spectroscopy because of its low symmetry [2] and strong background ¯uorescence. Zeolite beta is an intergrowth of two distinct but closely related
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structures. Polymorph A is a tetragonal system, while polymorph B is monoclinic [47]. The UV Raman spectrum of zeolite beta shows obvious bands appearing at 336, 396, 428, 468, 812, 1064, and 1120 cm 1 , as shown in Fig. 2g. According to the assignments of this work and results from the literature [2], it is suggested that the band at 336 cm 1 is assigned to the bending mode of 6-membered rings, the band at 396 cm 1 is ascribed to the bending mode of 5-membered rings, and the bands at 428 and 468 cm 1 are attributed to 4-membered rings. The band at 812 cm 1 is due to the T±O symmetric stretching mode, and the bands at 1064 and 1120 cm 1 are assigned to T±O asymmetric stretching mode. 3.4. Comparison of the UV Raman spectra of various zeolites Band frequencies and their assignments in UV Raman spectroscopy for various aluminosilicate zeolites are presented in Table 1. Obviously, the band positions strongly depend on the building units of the zeolite. For example, the bending vibrations of 4-, 5-, and 6-membered rings give rise to bands at 428±508, 378±405, and 290±410 cm 1 , respectively. These results indicate that UV Raman spectroscopy is very sensitive in the characterization of zeolite building units. Therefore,
Table 1 Observed frequencies and their assignments in the UV Raman spectra of zeolites Samples
Band position [references] T±O±T bending motion 8MR
NaX NaY NaA L ZSM-5 MOR Beta
280 [2, this work] 225 [this work] 240 [this work]
6MR
5MR
290, 380 [34,39,48] 305, 350 [34,39,48] 338, 410 [31,34,48] 314 [this work] 294 [this work]
378 [2,36,37]
336 [this work]
405 [40] (396) [this work]
4MR
T±O±T symmetric stretching vibration
T±O±T asymmetric stretching vibration
508 [2,44]
995, 1075 [2,33,34]
500 [2,44]
975, 1055, 1125 [2,33,34] 977, 1040, 1100 [4,29,31,49] 986, 1098, 1125 [2] 975, 1028, 1086 [34,36] 1145, 1165 [40] 1064, 1120 [this work]
488 [29,31]
700 [4]
498 [2] 440, 470 [2,36]
800 [36]
470, 482 [40] 428, 468 [this work]
820 [40] 812 [this work]
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UV Raman spectroscopy is a very useful technique for investigation of aluminosilicate zeolite structures. Furthermore, we found that the building units have a relation to UV Raman frequencies. For example, there is the same building unit in zeolites X and A, but zeolite A shows an additional band at 280 cm 1 . This band is reasonably attributed to the dierence in structure. A comparison of zeolites A and X shows that 8-membered rings exist in zeolite A, while such rings do not exist in zeolite X. This suggests that the band at 280 cm 1 can be assigned to the bending mode of 8-membered rings. Moreover, we observed that, although the Si/Al ratios of the zeolites are similar, the S/N ratio in the UV Raman spectra for beta and ZSM-5 is dierent, which is attributed to the dierence in structural disorder for both zeolite samples. This shows that UV Raman spectroscopy is a very useful technique for comparing structural disorder of various zeolites at similar Si/Al ratios (Table 2). 3.5. Aluminosilicate zeolites with various Si/Al ratios It has been reported that the Si/Al ratio of zeolites strongly in¯uences the band position and band intensity of Raman spectra [4,48,49], but it is dicult to distinguish the species of Si±O±Si and Si±O±Al in Raman spectroscopy [48,49]. As two typical zeolite examples, zeolite X with a low Si/Al ratio and ZSM-5 with a high Si/Al ratio were investigated. The UV Raman spectra of zeolite X with Si/Al ratios of 1.00±1.20 are shown in Fig. 3. The sample with Si/Al 1:20 (Fig. 3a) exhibits the main band at 505 cm 1 with broad bands
Table 2 Structural characteristics of various aluminosilicate zeolites Zeolite
Unit cell
Rings
A X Y L Mordenite ZSM-5 Beta
Cubic Cubic Cubic Hexagonal Orthorhombic Orthorhombic Tetragonal/monoclinic
4, 6, 8 4, 6 4, 6 4, 6, 8 4, 5, 8, 12 4, 5, 6, 8, 10, 12 4, 5, 6, 12
Fig. 3. UV Raman spectra of X zeolites with Si/Al ratios of (a) 1.20, (b) 1.10, (c) 1.06, and (d) 1.00 (excitation at 244.0 nm, laser power 4.0 mW).
at 300, 369, 958, and 1076 cm 1 . For Si=Al 1:10, the spectrum (Fig. 3b) shows a stronger band at 507 cm 1 . When the Si/Al ratio is further decreased to 1.06, the spectrum (Fig. 3c) contains much more intense bands at 287, 380, and 516 cm 1 . Additionally, we observed several small bands appearing at 463 and 788 cm 1 . At the same time, the bands at 958 and 1076 cm 1 (Fig. 3a) are split into four bands at 953, 978, 1032, and 1068 cm 1 . Finally, for Si/Al 1:00, the spectrum (Fig. 3d) shows still more intense bands appearing at 289, 386, 463, 517, 794, 951, 976, 1026, and 1073 cm 1 . Obviously, the S/N ratio in the UV Raman spectra of zeolite X increases with decreasing Si/Al ratio. This phenomenon may be attributed to the change in structural order of zeolite X with the Si/ Al ratio. Generally, the intensity of Raman signals is proportional to the increase of structural order. The higher the structural order, the stronger are the Raman signals. At an Si/Al ratio of 1.20, a
Y. Yu et al. / Microporous and Mesoporous Materials 46 (2001) 23±34
large number of T±O±T are Si±O±Al, but there are still some of Si±O±Si. In this case, the sample spectrum mainly exhibits the bands associated with Si±O±Al vibrations, and the presence of Si± O±Si will reduce structural order of zeolite X signi®cantly. When the Si/Al ratio is 1.00, the Si and Al atoms strictly alternate, all T±O±T are Si±O± Al. In this case, the structural order of zeolite X is highest, and the signals of UV Raman spectroscopy are strongest. The vibrations of Si±O±Al in dierent framework positions can be distinguished from each other and always show a splitting of bands in the UV Raman spectrum. These results suggest that the Raman signals for zeolite X mainly result from the contribution of Si±O±Al species. Additionally, we note that almost all band positions are sensitive to changes in the Si/Al ratio. Because the Raman signals originate from Si±O± Al species, a change in the Si/Al ratio is expected to strongly in¯uence the vibrations of Si±O±Al in zeolite X, resulting in a shift of the band positions, especially for the main band at about 500 cm 1 . Zeolite ZSM-5 has a high Si/Al ratio, and the UV Raman spectra of ZSM-5 with various Si/Al ratios are shown in Fig. 4. For an Si/Al ratio of 1 (pure silica), the band intensity is strongest, and bands appear at 294, 378, 800, 975, 1028, and 1086 cm 1 (Fig. 4a). When the Si/Al ratio of ZSM-5 is reduced to 100, the bands are much weaker, and bands appear at 298, 384, 806, 980, 1050, and 1095 cm 1 (Fig. 4b). When the Si/Al ratio of ZSM-5 is further decreased to 47, weak bands at 298, 384, 806, 980, and 1099 cm 1 are obtained (Fig. 4c). Notably, the band intensity and S/N in the UV Raman spectra of ZSM-5 increase remarkably with the Si/Al ratio, i.e., the eect of the Si/Al ratio observed with zeolite X is just reversed. This is again interpreted in terms of the change in structural order for ZSM-5. When the Si/Al ratio of ZSM-5 is 1, all T±O±T species are Si±O±Si, and the structure shows the highest possible order. In this case, the Raman signals stem from Si±O±Si vibrations, and the Raman signals are strongest. When the Si/Al ratio of ZSM-5 is reduced to 47, there are some Si±O±Al units, although a large portion of T±O±T is still Si±O±Si. These Si±O±Al units will reduce the structural order of ZSM-5
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Fig. 4. UV Raman spectra of zeolite ZSM-5 with various Si/Al ratios of (a) 1, (b) 100, and (c) 47 (excitation at 244.0 nm, laser power 4.0 mW; the ZSM-5 zeolites were synthesized by using fumed silica as the silicon source).
signi®cantly, leading to a considerable weakening of Raman signals associated with Si±O±Si vibrations. It is interesting to note that the band position in the UV Raman spectra of ZSM-5 is almost independent of the Si/Al ratio, and this behavior is also dierent from that of zeolite X (Fig. 4a). To account for this observation, it is suggested that the UV Raman bands are associated with Si±O±Si vibrations in ZSM-5, hence a change in the Si/Al ratio will only slightly in¯uence the vibrations of Si±O±Si, and this results in a lack of change in the band positions. 3.6. Crystallization of zeolite X in the presence of the seed solution Like IR spectroscopy, Raman can detect small, X-ray amorphous zeolite particles [1]. Visible
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Raman spectroscopy has been used to characterize the crystallization of zeolites Y [34], MOR [46], and ZSM-5 [45], and from this work crystallization mechanisms for these zeolites have been proposed. In this work, we used the UV Raman technique to characterize the crystallization mechanism of zeolite X in the presence of the seed solution. The XRD patterns of the solid phase of zeolite X during the crystallization process were investigated. After crystallization times of 20 and 40 min, no characteristic peaks associated with zeolite X could be observed. The XRD peaks assigned to zeolite X are observed only after heating the mixture for at least 1 h, but these peaks are very small and weak. The crystallinity increases with reaction time, and at 4 h the crystallinity reaches its maximum value. The dependence of crystallinity measured by XRD on reaction time is shown in Fig. 5. The UV Raman spectra of the solid phase during the crystallization process of zeolite X are shown in Fig. 6. After 20 min, the solid phase detected by XRD is amorphous silica, while the UV Raman spectrum (Fig. 6a) shows two broad bands at 450±600 cm 1 and 900±1100 cm 1 . In early reports, the correlation between the Raman bands in the region of 300±600 cm 1 and the size Fig. 6. UV Raman spectra of the solid phase during crystallization of zeolite X after (a) 20, (b) 40, (c) 60, (d) 80, (e) 120, (f) 180, and (g) 240 min.
Fig. 5. Crystallization curve of zeolite X as estimated from the XRD data.
of the rings in silicates and aluminosilicates [34, 44] have already been observed. Silicates or aluminosilicates containing predominantly 4-membered rings exhibit a band at 500±530 cm 1 , but those possessing mainly 6-membered rings always show the band below 470 cm 1 . Therefore, the band at 505 cm 1 is attributed to the bending mode of amorphous aluminosilicates containing mainly 4membered rings. The band at 900±1100 cm 1 is assigned to the asymmetric stretching modes of T± O for amorphous aluminosilicates. After 40 min, there are few changes in the UV Raman spectrum (Fig. 6b), the two broad bands at 450±600 and 900±1100 cm 1 become narrower, which result in bands at 530 and 1076 cm 1 .
Y. Yu et al. / Microporous and Mesoporous Materials 46 (2001) 23±34
The signi®cant change in the Raman spectra is observed after 1 h, as shown in Fig. 6c. The sample spectrum exhibits bands at 294, 388, 524, 960, and 1074 cm 1 , which are characteristic for zeolite X. With prolonged crystallization time, the band at 520 cm 1 associated with 4-membered rings basically remains unchanged, but the band at 390 cm 1 associated with 6-membered rings increases extensively. The bands at 900±1200 cm 1 are split into four bands at 950, 972, 1028, and 1070 cm 1 at a crystallization time of 4 h, as shown in Fig. 6d±f. At the same time, we observed that the XRD crystallinity increases with crystallization time (Fig. 5). These results suggest that crystallization of zeolite X is accompanied by the formation of 6membered rings, possibly from the connection of 4-membered rings. The dependence of the intensity ratio of the 390 cm 1 band and the 508 cm 1 band in the UV Raman spectra on crystallization time of zeolite X is shown in Fig. 7. Surprisingly, the shape of Fig. 7 is similar to that of Fig. 5, which con®rms that the crystallization of zeolite X is accompanied by the formation of 6-membered rings.
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4. Conclusions (1) As compared with visible Raman spectroscopy, UV Raman spectroscopy exhibits very clear bands and a very high resolution for a series of aluminosilicate zeolites because of the complete avoidance of the strong background ¯uorescence. (2) The bands in UV Raman spectroscopy are very sensitive to the basic building units of zeolites, and the comparison of various aluminosilicate zeolites suggests that the bands at 470±530, 370± 430, 290±410, and 220±280 cm 1 can be assigned to the bending modes of 4-, 5-, 6-, and 8-membered rings of aluminosilicate zeolites, respectively. These assignments are helpful for a characterization of the framework structure of aluminosilicate zeolites. (3) The change in the Si/Al ratio of zeolites strongly in¯uences the band position and intensity, which is related to their structural order. The Raman signals of zeolite X and ZSM-5 mainly result from Si±O±Al and Si±O±Si vibrations, respectively. (4) The UV Raman spectra recorded during crystallization of zeolite X show that 4-membered rings interconnect with each other to crystallize to zeolite X. Acknowledgements The authors acknowledge the ®nancial support of the Natural Science Foundation of China (Grant nos. 29825108 and 29625305), State Basic Research Project (G2000077507), Education Ministry of China, and National Advanced Materials Committee of China (NAMCC). References
Fig. 7. Dependence of the intensity ratio of the bands for 6MRand 4MR-rings in the UV Raman spectra of zeolite X on crystallization time.
[1] C.L. Angell, J. Phys. Chem. 77 (1973) 222. [2] P.P. Knops-Gerrits, D.E. De Vos, E.J.P. Feijen, P.A. Jacobs, Microporous Mater. 8 (1997) 3. [3] P.K. Dutta, D.C. Shieh, M. Puri, Zeolites 8 (1988) 306. [4] P.K. Dutta, B.D. Barco, J. Phys. Chem. 92 (1988) 354. [5] P.K. Dutta, R.E. Zaykoski, Zeolites 8 (1988) 179. [6] C. Bremard, M. Le Maire, J. Phy. Chem. 97 (1993) 9695. [7] S.A. Asher, C.H. Munro, Z. Chi, Laser Focus World 7 (1997) 99.
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[8] S.A. Asher, Anal. Chem. 65 (1993) 59A. [9] S.A. Asher, Anal. Chem. 65 (1993) 201A. [10] S.A. Asher, R.W. Bormett, X.G. Chen, D.H. Lemmon, N. Cho, P. Peterson, M. Arrigoni, L. Spinelli, J. Cannon, Appl. Spectroscopy 47 (1993) 628. [11] C. Li, P.C. Stair, in: H. Chon, S.I. Woo, S.E. Park (Eds.), Recent Advances and New Horizons in Zeolite Science and Technology, Studies in Surface Science and Catalysis, Vol. 101, Elsevier, Amsterdam, 1996, p. 881. [12] P.C. Stair, C. Li, J. Vac. Sci. Technol. A 15 (1997) 1679. [13] C. Li, P.C. Stair, Catal. Lett. 36 (1996) 119. [14] C. Li, P.C. Stair, in: H. Chon, S.K. Ihm, Y.S. Uh (Eds.), Progress in Zeolite and Microporous Materials, Studies in Surface Science and Catalysis, vol. 105, Elsevier, Amsterdam, 1997, p. 599. [15] C. Li, P.C. Stair, Catal. Today 33 (1997) 353. [16] G. Xiong, C. Li, Z. Feng, P. Ying, Q. Xin, J. Liu, J. Catal. 186 (1999) 234. [17] C. Li, G. Xiong, Q. Xin, J. Liu, P. Ying, Z. Feng, J. Li, W. Yang, Y. Wang, G. Wang, X. Liu, M. Lin, X. Wang, E. Min, Angew. Chem. Int. Ed. 38 (1999) 2220. [18] G. Xiong, C. Li, H. Li, Q. Xin, Z. Feng, J. Chem. Soc.Chem. Commun. (2000) 677. [19] Y. Yu, G. Xiong, C. Li, F. Xiao, J. Catal. 194 (2000) 487. [20] S. Yuan, J. Li, Z. Hao, Z. Feng, Q. Xin, P. Ying, C. Li, Catal. Lett. 63 (1999) 73. [21] G. Xiong, Z. Feng, J. Li, Q. Yang, P. Ying, Q. Xin, C. Li, J. Phys. Chem. B 104 (2000) 3581. [22] R.M. Milton, US Patent No. 2,882,243, 1959. [23] R.M. Milton, US Patent No. 2,882,244, 1959. [24] S. Li, L. Li, R. Xu, CN Patent No. 85,102,733.4, 1989. [25] D.W. Breck, US Patent No. 3,216,789, 1985. [26] P.K. Bajpai, Zeolites 6 (1986) 2. [27] E.M. Flanigen, J.M. Bennett, B.M. Crose, J.P. Cohem, R.L. Pattor, R.M. Kirchner, J.P.V. Smith, Nature 271 (1978) 512.
[28] T. Song, L. Liu, R. Xu, Cuihua Xuebao (J. Catal. Chinese) 12 (1991) 283. [29] Q. Zhou, W. Pang, S. Qiu, M. Jia, CN Patent No. 93,117,593.3, 1996. [30] P.K. Maher, US Patent No. 3,293,192, 1966. [31] E. Dempsey, J. Phys. Chem. 73 (1969) 388. [32] G.H. K uhl, Zeolites 7 (1987) 451. [33] K.T. No, D.H. Bae, M.S. Jhon, J. Phys. Chem. 90 (1986) 1772. [34] P.K. Dutta, D.C. Shieh, M. Puri, J. Phys. Chem. 91 (1987) 2332. [35] P.K. Dutta, D.C. Shieh, J. Phys. Chem. 90 (1986) 2331. [36] A.J.M. De Man, B.W.H. Van Beest, M. Leslie, R.A. Van Santen, J. Phys. Chem. 94 (1990) 2524. [37] R.A. Van Santen, D.L. Vogel, Adv. Solid State Chem. 1 (1988) 151. [38] P. Mcmillan, Am. Mineral. 69 (1984) 622. [39] W.P. Grith, J. Chem. Soc. A9 (1969) 1372. [40] A. Miecznikowski, J. Hanuza, Zeolites 5 (1985) 188. [41] D.W. Breck, Zeolite Molcular Sieves, Wiley, New York, 1974, pp. 45±50. [42] M. Bartsch, P. Bornhauser, G. Calzaferri, R. Imhof, in: J. Weitkamp, H.G. Karge, H. Pfeifer, W. H olderich (Eds.), Zeolites and Related Microporous Materials: State of the Art 1994, Studies in Surface Science and Catalysis, vol. 84, Elsevier, Amsterdam, 1994, p. 2089. [43] P.K. Dutta, B. Del Barco, J. Phys. Chem. 89 (1985) 1861. [44] P.K. Dutta, K.M. Rao, J.Y. Park, J. Phys. Chem. 95 (1991) 6654. [45] P.K. Dutta, M. Puri, J. Phys. Chem. 91 (1987) 4329. [46] J. Twu, P.K. Dutta, C.T. Kresge, J. Phys. Chem. 95 (1991) 5267. [47] M.M.J. Treacy, J.M. Newsam, Nature 332 (1988) 249. [48] P.K. Dutta, J. Twu, J. Phys. Chem. 95 (1991) 2498. [49] A. Miecznikowski, J. Hanuza, Zeolites 7 (1987) 249.
www.elsevier.nl/locate/micromeso
Characterization of aluminosilicate zeolites by UV Raman spectroscopy Yi Yu a, Guang Xiong b, Can Li b,1, Feng-Shou Xiao a,* a b
Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Department of Chemistry, Jilin University, Changchun 130023, China State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O. Box 110, Dalian 116023, China Received 7 December 2000; received in revised form 22 February 2001; accepted 22 February 2001
Abstract A series of aluminosilicate zeolites are characterized by UV Raman spectroscopy for the ®rst time, and UV Raman spectra of various zeolites give strong and clear bands with high resolution, while conventional Raman spectra of these zeolites are dicult to obtain because of a strong background ¯uorescence. Additionally, these zeolites show several new bands in UV Raman spectroscopy. A summary of these UV Raman spectra over various zeolites suggests that the bands at 470±530, 370±430, 290±410, and 220±280 cm 1 can be assigned to the bending modes of 4-, 5-, 6-, and 8membered rings of aluminosilicate zeolites, respectively. Furthermore, it is found that the band intensity of zeolites in UV Raman spectroscopy is dependent on the Si/Al ratio. Moreover, the UV Raman spectra of crystallization, for zeolite X at various times show that, in the initial stage of crystallization, the 4-membered rings (510 cm 1 ) interconnect each other to form b-cages with 6-membered rings (390 cm 1 ), which further crystallize to zeolite X. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: UV Raman spectroscopy; Visible Raman spectroscopy; Aluminosilicate zeolites; Crystallization mechanism
1. Introduction With the development of zeolite science, the structural properties of zeolites have been under intense investigation for at least four decades. Many salient features of zeolite dynamics and * Corresponding author. Tel.: +86-431-8922331 ext.: 2314; fax: +86-431-5671974. E-mail addresses: [email protected] (F.-S. Xiao), canli@ ms.dicp.ac.cn (C. Li). 1 Also corresponding author. Tel.: +86-411-4671997, ext.: 728; fax: +86-411-4694447.
structure can be deduced from vibrational spectroscopic data, such as infrared (IR) and Raman spectroscopy, and the two kinds of spectroscopies are often complementary [1]. In zeolite science, Raman spectroscopy seems rather underdeveloped compared with IR spectroscopy. One reason is the diculty to obtain Raman spectra with acceptable signal-to-noise (S/N) ratio from zeolite materials, and the other is that Raman spectra of zeolites are often obscured by a broad ¯uorescence background [2]. To obtain high-quality Raman spectra, the zeolite samples would have to be very pure after a careful treatment [3±6]. Even then, for some
1387-1811/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 1 ) 0 0 2 7 1 - 2
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samples, the background cannot be reduced completely, and the ¯uorescence interference is always a problem [1,2]. In the 1980s Raman spectroscopy has developed rapidly. By using near-infrared laser as light source, FT-Raman spectroscopy can reduce ¯uorescence for some zeolite samples, but the spectral S/N ratio is limited by the IR detector background [7±9]. Recently, UV Raman spectroscopy, which uses a continuous wave (cw) ultraviolet laser as excitation source, was developed for catalyst characterization. The advent of this technique resulted in a big improvement of spectral S/N and the avoidance of ¯uorescence interference [10±12]. Some successful examples show that UV resonance Raman spectroscopy is a powerful tool for studying catalysts and other solids [13±16]. More recently, UV Raman identi®cation of metal atoms in the framework of zeolites such as TS-1 [17], V-MCM-41 [18], and Fe-ZSM-5 [19] has been reported, and it has been shown that UV Raman spectroscopy can selectively characterize a very small amount of transition metal atoms in the framework of zeolites [17±21]. In this case, the ¯uorescence interference from the zeolite is completely avoided [16±20]. Notably, although UV Raman spectroscopy is very sensitive to characterize the structure of zeolites, a series of aluminosilicate zeolites have not yet been systematically investigated by this technique. Our goal in this work is to investigate aluminosilicate zeolites systematically by UV Raman spectroscopy and to ®nd a relationship between UV Raman bands and the characteristic structures of aluminosilicate zeolites. 2. Experimental section 2.1. Materials The aluminosilicate zeolites used in this study were synthesized according to literature recipes. These zeolites were NaA [22], NaX [23], NaY [24], KL [25], NaMOR [26], NaZSM-5 [27,28] and Na beta [29]. The structure of these materials with high crystallinity was investigated by powder X-
ray diraction (XRD). The Si/Al ratio of zeolite X was determined by chemical analysis and NMR and con®rmed by XRD [30,31]. Zeolite X with Si/ Al ratios of 1.00 and 1.02 was synthesized by using Na and K cations [32]. All organic templates in the zeolite samples were removed by calcination in air at 500°C. The crystallization of zeolite X was achieved at 95±100°C in the presence of seeds. The reagents used for preparing the seed solution and synthesizing the zeolite were sodium silicate, sodium aluminate, sodium hydroxide, and potassium hydroxide. The molar composition of the seed solution of zeolite X was 18Na2 O:Al2 O3 :19SiO2 : 370H2 O. Zeolite X (Si/Al 1:0) was synthesized by starting from a mixture with the composition 3.2Na2 O:1.6K2 O:1.0Al2 O3 :2.0SiO2 :76H2 O. The reaction mixture was prepared by mixing appropriate amounts of sodium silicate with a solution of sodium aluminate dissolved in sodium hydroxide and potassium hydroxide, followed by the addition of the seed solution. The samples were heated at 100°C and then removed from the oven at various times during the crystallization and centrifuged for separation into a liquid and solid phase. The latter was extensively washed with deionized water and dried at room temperature prior to Xray diraction and UV Raman characterization. 2.2. Characterization UV Raman spectra were recorded on a UV Raman spectrometer built in the State Key Laboratory of Catalysis at the Dalian Institute of Chemical Physics. The instrument consists of four parts: a UV cw laser, a Spex 1877D triplemate spectrograph, a CCD detector, and an optical collection system. A 244.0-nm line is generated by frequency doubling the 488.0-nm line via an intracavity frequency doubling crystal (Coherent Ionva 300 Fred). The laser power at the sample was kept below 4.0 mW. The spectral resolution was estimated to be 2.0 cm 1 . Samples were mounted into a spinning holder to avoid thermal damage during the scanning. The time for recording the spectrum is about 5 min. Visible Raman spectra of the same zeolites were obtained with a Spex 1403 spectrometer controlled
Y. Yu et al. / Microporous and Mesoporous Materials 46 (2001) 23±34
25
by a Spex datamate computer. The wavelength of the excitation line was 488.0 nm (Spectra-Physics Ar ion laser, Model 165) in all cases. The scattered light was detected with an RCA C31034 GaAs photomultiplier tube. The slit widths were typically 6 cm 1 , and the laser power was 50±100 mW. Powder XRD was carried out on a Rigaku D/ Max-IIIA and Siemens D5005 diractometer with a CuKa source. The estimation of the Si/Al ratio for zeolite X by XRD was based on the 2h range 56±58.5° with a scanning speed of 1°/min, and sodium chloride was used as an internal standard [30,31]. 27 Al and 29 Si NMR spectra were performed on a Bruker DRX-400 spectrometer with Al(OH)3 and TMS as standards, respectively. 3. Results and discussion 3.1. Visible Raman spectroscopy Fig. 1 shows the visible Raman spectra of the aluminosilicate zeolites A, X, Y, and beta. The scanning time for each spectrum is about 40 min. The spectrum of zeolite A (Fig. 1a) exhibits a weak band at 490 cm 1 , which is assigned to the bending vibration of 4-membered rings (Si±O±Al) [33]. The spectra of zeolites X and Y show a somewhat stronger band at 510 cm 1 and two very broad bands centered at 300 and 380 cm 1 which are, respectively, attributed to the 4- and 6-membered rings (T±O±T, T Si or Al) [34,35]. The spectrum of zeolite beta has no obvious peaks due to the weakness of the Raman signal. Therefore, in this case the bands in the visible Raman spectrum are very weak, except for a few samples after a very careful treatment. The weakness of the visible Raman signals for these aluminosilicate zeolites is due to the strong background ¯uorescence. By chemical analysis and ICP we have found that these zeolite samples (A, X and Y) contained at least trace amounts of iron impurities which stem from the zeolite preparation with sodium silicate containing 0.1% iron as an impurity. The presence of Fe as an impurity in these zeolites results in a signi®cant increase of luminescence [1,2]. Additionally, other impurities
Fig. 1. Visible Raman spectra of aluminosilicate zeolites: (a) A, (b) X, (c) Y, and (d) beta (excitation at 488.0 nm, laser power 50±100 mW).
such as surface carbonaceous species and a small amount of aromatics also lead to strong background ¯uorescence [2]. 3.2. UV Raman spectroscopy Shown in Fig. 2a±g are the UV Raman spectra of various aluminosilicate zeolites. As compared with the visible Raman spectrum, the UV Raman spectrum exhibits the following obvious features: (1) Strong Raman bands with a high S/N ratio are obtained. The ¯uorescence is completely avoided in the UV Raman spectra because ¯uorescence interference does not normally occur in the UV region for aluminosilicate zeolite [8,9]; (2) the UV Raman spectra can be collected in a very short
26
Y. Yu et al. / Microporous and Mesoporous Materials 46 (2001) 23±34
Fig. 2. UV Raman spectra of aluminosilicate zeolites: (a) X, (b) Y, (c) A, (d) L, (e) ZSM-5, (f) MOR, and (g) beta (excitation at 244.0 nm, laser power 4.0 mW).
time (<10 min), and the samples do not need any further pretreatments (e.g., heating, oxygen treatment, and evacuation). On the contrary, the visible Raman spectra were obtained only for the samples
with careful pretreatments, such as calcination and dehydration (5±24 h) [5], and the ¯uorescence was always present, even if the sample was well pretreated.
Y. Yu et al. / Microporous and Mesoporous Materials 46 (2001) 23±34
27
Fig. 2. (continued)
3.3. UV Raman spectra of various samples Zeolite X: The UV Raman spectrum of zeolite X with an Si/Al ratio of 1.08 shows bands at 290, 380, 508, 995, and 1075 cm 1 (Fig. 2a). The
strongest band at 508 cm 1 is assigned to the bending mode of the characteristic 4-membered rings [2,35±37], and the bands at 290 and 380 cm 1 are assigned to the bending mode of the 6-membered rings [34,36±39]. The bands at 995 and
28
Y. Yu et al. / Microporous and Mesoporous Materials 46 (2001) 23±34
1075 cm 1 are assigned to asymmetric stretching frequencies of T±O in zeolite X [38,40]. Zeolite Y: The UV Raman spectrum of zeolite Y (Fig. 2b) shows bands at 305, 350, 500, 975, 1055, and 1125 cm 1 . It is well-known that the framework topology of zeolite X is the same as that of zeolite Y, and the only dierence between both zeolites is the Si/Al ratio. Therefore, the assignments for zeolite Y are the same as those for zeolite X. As compared with the spectrum of zeolite X, the bands in the Raman spectrum of zeolite Y are shifted by several wavenumbers. For example, the strongest band at 508 cm 1 for zeolite X is shifted to 500 cm 1 for zeolite Y; the band at 290 cm 1 for zeolite X is shifted to 305 cm 1 for zeolite Y. All these results are attributed to the change in the Si/ Al ratio from 1.08 for zeolite X to 2.60 for zeolite Y. Zeolite A: Fig. 2c shows the UV Raman spectrum of zeolite A with an Si/Al ratio of 1.0. The bands appear at 280, 338, 410, 488, 700, 977, 1040, and 1100 cm 1 . As faujasite, zeolite A contains 4and 6-membered rings [41]. Therefore, the basic assignment of the bands in the UV Raman spectrum for zeolite A should be similar to the one for zeolites X and Y. The strongest band at 488 cm 1 is assigned to the bending mode of 4-membered Si±O±Al rings [33,40]. The bands at 338 and 410 cm 1 are attributed to the bending mode of 6membered Si±O±Al rings [34,38,39]. The bands at 977, 1040 and 1100 cm 1 are ascribed to asymmetric T±O stretching motions [4,33,42,43]. The UV Raman spectrum of zeolite A shows a new band at 700 cm 1 , which does obviously not appear in the spectra of zeolites X and Y. Generally, bands in the region of 600±900 cm 1 in Raman spectroscopy are due to T±O symmetric stretching modes [2]. Accordingly, the band at 700 cm 1 is assigned to the T±O stretching mode of zeolite A, in particular to the contribution of the 4membered rings present in zeolite A [4]. It is very interesting to note that zeolite A exhibits a band at 280 cm 1 , in addition to the three bands at 290±305, 338±380 cm 1 , and 500±508 cm 1 assigned to 4- and 6-membered rings in zeolites X and Y. In early reports [3,4,44], no band at 280 cm 1 appeared in the visible Raman spectra
for zeolite A due to the strong background ¯uorescence. On the other hand, the correlation between the Raman bands in the region 300±600 cm 1 and the size of the rings in silicate and aluminosilicate materials as well as aluminosilicate zeolites has already been observed, and it was found that the smaller rings give bands at higher frequencies [2]. Therefore, the band at 280 cm 1 is attributed to the bending mode of higher rings than 6- and 4-membered rings, possibly of the 8membered rings of zeolite A. Zeolite L: The UV Raman bands of zeolite L are shown in Fig. 2d. Bands appear at 225, 314, 498, 986, 1098, and 1125 cm 1 . Again, the strongest band at 498 cm 1 is assigned to the bending mode of 4-membered rings, and the band at 314 cm 1 is assigned to the bending mode of 6membered rings present in zeolite L [40]. The bands at 986, 1098, and 1125 cm 1 are also attributed to asymmetric stretching vibration frequencies of the T±O bonds in the sample. Interestingly, zeolite L exhibits a band at 225 cm 1 , which has not been observed in visible Raman spectroscopy yet. Considering the structure of zeolite L suggests that the band at 225 cm 1 should be assigned to the bending mode of 8-membered rings (T±O±T) [2]. Zeolite ZSM-5: Fig. 2e shows the UV Raman spectrum of zeolite ZSM-5 with bands at 294, 378, 440, 470, 800, 975, 1028, and 1086 cm 1 . The strongest band is observed at 378 cm 1 rather than near 500 cm 1 as for zeolites X, Y, A and L (Fig. 2a±d). We assign this band to the bending mode of 5-membered rings [2,44,45]. The band at 294 cm 1 is assigned to the bending mode of 6-membered rings [2,34,38,39]. The bands at 440 and 470 cm 1 are assigned to 4-membered rings [2,44]. The band at 800 cm 1 is assigned to symmetric stretching, and the bands at 975, 1028, and 1086 cm 1 to asymmetric stretching vibrations of Si±O bonds in ZSM-5, which is consistent with previous assignments in visible Raman spectroscopy [38,45]. Zeolite mordenite: The UV Raman spectrum of zeolite mordenite is shown in Fig. 2f. Bands appear at 240, 405, 470, 482, 550, 820, 1145, and 1165 cm 1 . The framework structure of mordenite consists of 4-, 5-, and 8-membered T±O±T rings. Therefore, the band at 405 cm 1 is assigned to the
Y. Yu et al. / Microporous and Mesoporous Materials 46 (2001) 23±34
bending mode of 5-membered rings, and the bands at 470 and 482 cm 1 are assigned to the bending modes of 4-membered rings. The band at 820 cm 1 is assigned to symmetric stretching motions of T± O bonds, and the bands at 1145 and 1165 cm 1 are assigned to asymmetric stretching motions of T±O bonds. These assignments are in good agreement with those for mordenite in visible Raman spectroscopy [46]. Notably, a broad band appears at 240 cm 1 , which has not been observed in visible Raman spectroscopy. Similarly, this band is assigned to the bending mode of 8-membered rings in mordenite, in agreement with our assignments for zeolite A and zeolite L. The Raman bands for zeolite mordenite are relatively broad, as compared with those of ZSM-5 and zeolites X and A. For example, the band near 480 cm 1 (Fig. 2c) is split into two bands at 470 and 482 cm 1 (Fig. 2f). This may be interpreted in terms of the order of the zeolite framework. The arrangement of zeolite building units in mordenite is not as uniform as in ZSM-5 and zeolites X and A, resulting in a broad band in Raman spectroscopy. Zeolite beta: It is dicult to obtain the Raman spectrum of zeolite beta by conventional Raman spectroscopy because of its low symmetry [2] and strong background ¯uorescence. Zeolite beta is an intergrowth of two distinct but closely related
29
structures. Polymorph A is a tetragonal system, while polymorph B is monoclinic [47]. The UV Raman spectrum of zeolite beta shows obvious bands appearing at 336, 396, 428, 468, 812, 1064, and 1120 cm 1 , as shown in Fig. 2g. According to the assignments of this work and results from the literature [2], it is suggested that the band at 336 cm 1 is assigned to the bending mode of 6-membered rings, the band at 396 cm 1 is ascribed to the bending mode of 5-membered rings, and the bands at 428 and 468 cm 1 are attributed to 4-membered rings. The band at 812 cm 1 is due to the T±O symmetric stretching mode, and the bands at 1064 and 1120 cm 1 are assigned to T±O asymmetric stretching mode. 3.4. Comparison of the UV Raman spectra of various zeolites Band frequencies and their assignments in UV Raman spectroscopy for various aluminosilicate zeolites are presented in Table 1. Obviously, the band positions strongly depend on the building units of the zeolite. For example, the bending vibrations of 4-, 5-, and 6-membered rings give rise to bands at 428±508, 378±405, and 290±410 cm 1 , respectively. These results indicate that UV Raman spectroscopy is very sensitive in the characterization of zeolite building units. Therefore,
Table 1 Observed frequencies and their assignments in the UV Raman spectra of zeolites Samples
Band position [references] T±O±T bending motion 8MR
NaX NaY NaA L ZSM-5 MOR Beta
280 [2, this work] 225 [this work] 240 [this work]
6MR
5MR
290, 380 [34,39,48] 305, 350 [34,39,48] 338, 410 [31,34,48] 314 [this work] 294 [this work]
378 [2,36,37]
336 [this work]
405 [40] (396) [this work]
4MR
T±O±T symmetric stretching vibration
T±O±T asymmetric stretching vibration
508 [2,44]
995, 1075 [2,33,34]
500 [2,44]
975, 1055, 1125 [2,33,34] 977, 1040, 1100 [4,29,31,49] 986, 1098, 1125 [2] 975, 1028, 1086 [34,36] 1145, 1165 [40] 1064, 1120 [this work]
488 [29,31]
700 [4]
498 [2] 440, 470 [2,36]
800 [36]
470, 482 [40] 428, 468 [this work]
820 [40] 812 [this work]
30
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UV Raman spectroscopy is a very useful technique for investigation of aluminosilicate zeolite structures. Furthermore, we found that the building units have a relation to UV Raman frequencies. For example, there is the same building unit in zeolites X and A, but zeolite A shows an additional band at 280 cm 1 . This band is reasonably attributed to the dierence in structure. A comparison of zeolites A and X shows that 8-membered rings exist in zeolite A, while such rings do not exist in zeolite X. This suggests that the band at 280 cm 1 can be assigned to the bending mode of 8-membered rings. Moreover, we observed that, although the Si/Al ratios of the zeolites are similar, the S/N ratio in the UV Raman spectra for beta and ZSM-5 is dierent, which is attributed to the dierence in structural disorder for both zeolite samples. This shows that UV Raman spectroscopy is a very useful technique for comparing structural disorder of various zeolites at similar Si/Al ratios (Table 2). 3.5. Aluminosilicate zeolites with various Si/Al ratios It has been reported that the Si/Al ratio of zeolites strongly in¯uences the band position and band intensity of Raman spectra [4,48,49], but it is dicult to distinguish the species of Si±O±Si and Si±O±Al in Raman spectroscopy [48,49]. As two typical zeolite examples, zeolite X with a low Si/Al ratio and ZSM-5 with a high Si/Al ratio were investigated. The UV Raman spectra of zeolite X with Si/Al ratios of 1.00±1.20 are shown in Fig. 3. The sample with Si/Al 1:20 (Fig. 3a) exhibits the main band at 505 cm 1 with broad bands
Table 2 Structural characteristics of various aluminosilicate zeolites Zeolite
Unit cell
Rings
A X Y L Mordenite ZSM-5 Beta
Cubic Cubic Cubic Hexagonal Orthorhombic Orthorhombic Tetragonal/monoclinic
4, 6, 8 4, 6 4, 6 4, 6, 8 4, 5, 8, 12 4, 5, 6, 8, 10, 12 4, 5, 6, 12
Fig. 3. UV Raman spectra of X zeolites with Si/Al ratios of (a) 1.20, (b) 1.10, (c) 1.06, and (d) 1.00 (excitation at 244.0 nm, laser power 4.0 mW).
at 300, 369, 958, and 1076 cm 1 . For Si=Al 1:10, the spectrum (Fig. 3b) shows a stronger band at 507 cm 1 . When the Si/Al ratio is further decreased to 1.06, the spectrum (Fig. 3c) contains much more intense bands at 287, 380, and 516 cm 1 . Additionally, we observed several small bands appearing at 463 and 788 cm 1 . At the same time, the bands at 958 and 1076 cm 1 (Fig. 3a) are split into four bands at 953, 978, 1032, and 1068 cm 1 . Finally, for Si/Al 1:00, the spectrum (Fig. 3d) shows still more intense bands appearing at 289, 386, 463, 517, 794, 951, 976, 1026, and 1073 cm 1 . Obviously, the S/N ratio in the UV Raman spectra of zeolite X increases with decreasing Si/Al ratio. This phenomenon may be attributed to the change in structural order of zeolite X with the Si/ Al ratio. Generally, the intensity of Raman signals is proportional to the increase of structural order. The higher the structural order, the stronger are the Raman signals. At an Si/Al ratio of 1.20, a
Y. Yu et al. / Microporous and Mesoporous Materials 46 (2001) 23±34
large number of T±O±T are Si±O±Al, but there are still some of Si±O±Si. In this case, the sample spectrum mainly exhibits the bands associated with Si±O±Al vibrations, and the presence of Si± O±Si will reduce structural order of zeolite X signi®cantly. When the Si/Al ratio is 1.00, the Si and Al atoms strictly alternate, all T±O±T are Si±O± Al. In this case, the structural order of zeolite X is highest, and the signals of UV Raman spectroscopy are strongest. The vibrations of Si±O±Al in dierent framework positions can be distinguished from each other and always show a splitting of bands in the UV Raman spectrum. These results suggest that the Raman signals for zeolite X mainly result from the contribution of Si±O±Al species. Additionally, we note that almost all band positions are sensitive to changes in the Si/Al ratio. Because the Raman signals originate from Si±O± Al species, a change in the Si/Al ratio is expected to strongly in¯uence the vibrations of Si±O±Al in zeolite X, resulting in a shift of the band positions, especially for the main band at about 500 cm 1 . Zeolite ZSM-5 has a high Si/Al ratio, and the UV Raman spectra of ZSM-5 with various Si/Al ratios are shown in Fig. 4. For an Si/Al ratio of 1 (pure silica), the band intensity is strongest, and bands appear at 294, 378, 800, 975, 1028, and 1086 cm 1 (Fig. 4a). When the Si/Al ratio of ZSM-5 is reduced to 100, the bands are much weaker, and bands appear at 298, 384, 806, 980, 1050, and 1095 cm 1 (Fig. 4b). When the Si/Al ratio of ZSM-5 is further decreased to 47, weak bands at 298, 384, 806, 980, and 1099 cm 1 are obtained (Fig. 4c). Notably, the band intensity and S/N in the UV Raman spectra of ZSM-5 increase remarkably with the Si/Al ratio, i.e., the eect of the Si/Al ratio observed with zeolite X is just reversed. This is again interpreted in terms of the change in structural order for ZSM-5. When the Si/Al ratio of ZSM-5 is 1, all T±O±T species are Si±O±Si, and the structure shows the highest possible order. In this case, the Raman signals stem from Si±O±Si vibrations, and the Raman signals are strongest. When the Si/Al ratio of ZSM-5 is reduced to 47, there are some Si±O±Al units, although a large portion of T±O±T is still Si±O±Si. These Si±O±Al units will reduce the structural order of ZSM-5
31
Fig. 4. UV Raman spectra of zeolite ZSM-5 with various Si/Al ratios of (a) 1, (b) 100, and (c) 47 (excitation at 244.0 nm, laser power 4.0 mW; the ZSM-5 zeolites were synthesized by using fumed silica as the silicon source).
signi®cantly, leading to a considerable weakening of Raman signals associated with Si±O±Si vibrations. It is interesting to note that the band position in the UV Raman spectra of ZSM-5 is almost independent of the Si/Al ratio, and this behavior is also dierent from that of zeolite X (Fig. 4a). To account for this observation, it is suggested that the UV Raman bands are associated with Si±O±Si vibrations in ZSM-5, hence a change in the Si/Al ratio will only slightly in¯uence the vibrations of Si±O±Si, and this results in a lack of change in the band positions. 3.6. Crystallization of zeolite X in the presence of the seed solution Like IR spectroscopy, Raman can detect small, X-ray amorphous zeolite particles [1]. Visible
32
Y. Yu et al. / Microporous and Mesoporous Materials 46 (2001) 23±34
Raman spectroscopy has been used to characterize the crystallization of zeolites Y [34], MOR [46], and ZSM-5 [45], and from this work crystallization mechanisms for these zeolites have been proposed. In this work, we used the UV Raman technique to characterize the crystallization mechanism of zeolite X in the presence of the seed solution. The XRD patterns of the solid phase of zeolite X during the crystallization process were investigated. After crystallization times of 20 and 40 min, no characteristic peaks associated with zeolite X could be observed. The XRD peaks assigned to zeolite X are observed only after heating the mixture for at least 1 h, but these peaks are very small and weak. The crystallinity increases with reaction time, and at 4 h the crystallinity reaches its maximum value. The dependence of crystallinity measured by XRD on reaction time is shown in Fig. 5. The UV Raman spectra of the solid phase during the crystallization process of zeolite X are shown in Fig. 6. After 20 min, the solid phase detected by XRD is amorphous silica, while the UV Raman spectrum (Fig. 6a) shows two broad bands at 450±600 cm 1 and 900±1100 cm 1 . In early reports, the correlation between the Raman bands in the region of 300±600 cm 1 and the size Fig. 6. UV Raman spectra of the solid phase during crystallization of zeolite X after (a) 20, (b) 40, (c) 60, (d) 80, (e) 120, (f) 180, and (g) 240 min.
Fig. 5. Crystallization curve of zeolite X as estimated from the XRD data.
of the rings in silicates and aluminosilicates [34, 44] have already been observed. Silicates or aluminosilicates containing predominantly 4-membered rings exhibit a band at 500±530 cm 1 , but those possessing mainly 6-membered rings always show the band below 470 cm 1 . Therefore, the band at 505 cm 1 is attributed to the bending mode of amorphous aluminosilicates containing mainly 4membered rings. The band at 900±1100 cm 1 is assigned to the asymmetric stretching modes of T± O for amorphous aluminosilicates. After 40 min, there are few changes in the UV Raman spectrum (Fig. 6b), the two broad bands at 450±600 and 900±1100 cm 1 become narrower, which result in bands at 530 and 1076 cm 1 .
Y. Yu et al. / Microporous and Mesoporous Materials 46 (2001) 23±34
The signi®cant change in the Raman spectra is observed after 1 h, as shown in Fig. 6c. The sample spectrum exhibits bands at 294, 388, 524, 960, and 1074 cm 1 , which are characteristic for zeolite X. With prolonged crystallization time, the band at 520 cm 1 associated with 4-membered rings basically remains unchanged, but the band at 390 cm 1 associated with 6-membered rings increases extensively. The bands at 900±1200 cm 1 are split into four bands at 950, 972, 1028, and 1070 cm 1 at a crystallization time of 4 h, as shown in Fig. 6d±f. At the same time, we observed that the XRD crystallinity increases with crystallization time (Fig. 5). These results suggest that crystallization of zeolite X is accompanied by the formation of 6membered rings, possibly from the connection of 4-membered rings. The dependence of the intensity ratio of the 390 cm 1 band and the 508 cm 1 band in the UV Raman spectra on crystallization time of zeolite X is shown in Fig. 7. Surprisingly, the shape of Fig. 7 is similar to that of Fig. 5, which con®rms that the crystallization of zeolite X is accompanied by the formation of 6-membered rings.
33
4. Conclusions (1) As compared with visible Raman spectroscopy, UV Raman spectroscopy exhibits very clear bands and a very high resolution for a series of aluminosilicate zeolites because of the complete avoidance of the strong background ¯uorescence. (2) The bands in UV Raman spectroscopy are very sensitive to the basic building units of zeolites, and the comparison of various aluminosilicate zeolites suggests that the bands at 470±530, 370± 430, 290±410, and 220±280 cm 1 can be assigned to the bending modes of 4-, 5-, 6-, and 8-membered rings of aluminosilicate zeolites, respectively. These assignments are helpful for a characterization of the framework structure of aluminosilicate zeolites. (3) The change in the Si/Al ratio of zeolites strongly in¯uences the band position and intensity, which is related to their structural order. The Raman signals of zeolite X and ZSM-5 mainly result from Si±O±Al and Si±O±Si vibrations, respectively. (4) The UV Raman spectra recorded during crystallization of zeolite X show that 4-membered rings interconnect with each other to crystallize to zeolite X. Acknowledgements The authors acknowledge the ®nancial support of the Natural Science Foundation of China (Grant nos. 29825108 and 29625305), State Basic Research Project (G2000077507), Education Ministry of China, and National Advanced Materials Committee of China (NAMCC). References
Fig. 7. Dependence of the intensity ratio of the bands for 6MRand 4MR-rings in the UV Raman spectra of zeolite X on crystallization time.
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