Changes of medium-range structure in the course of crystallization of zeolite omega from magadiite

Microporous and Mesoporous Materials 200 (2014) 86–91

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Changes of medium-range structure in the course of crystallization of zeolite omega from magadiite Miao Cui a, Yifu Zhang a, Xiaoyu Liu a, Lin Wang b, Changgong Meng a,⇑ a b

School of Chemistry, Dalian University of Technology, Dalian 116024, China PANalytical, Shanghai 200233, China

a r t i c l e

i n f o

Article history: Received 29 May 2014 Received in revised form 12 August 2014 Accepted 13 August 2014 Available online 23 August 2014 Keywords: Magadiite Zeolite omega Conversion Medium-range structure

a b s t r a c t Changes of medium-range structure during the crystallization of zeolite omega from magadiite were characterized. It is found that although the long-range order of magadiite is collapsed in the initial stage, parts of 5-member rings and 6-member rings are still preserved as secondary building units. The fraction of 5-member rings and 6-member rings increases as the crystallization progresses. The 4-member ring chains are formed at a stage later than that of 5-member rings and 6-member rings. Ó 2014 Elsevier Inc. All rights reserved.

1. Introduction Zeolites are crystalline aluminosilicates in which the aluminum and silicon atoms are present in the form of AlO4 and SiO4 tetrahedra. Zeolites have been widely applied to ion exchange, adsorption, catalysis and so on because of their unique structures and properties [1]. Understanding the crystallization mechanisms of zeolites can conduct to exploit efficient routes to synthesize them. Although numerous works have attempted to understand the mechanisms of zeolites and solution transport and solid phase transformation mechanisms are proposed [2–18], relatively few studies have been performed on the elucidation of the mediumrange structures and their changes during zeolite crystallization. As it is reported in the previous study, the short-range order is represented by the local coordination polyhedra, while the medium-range structure can be regarded as the next highest level of structural organization beyond the short-range order [19]. Raman spectroscopy is well-known as one of the useful techniques to identify the medium-range structures of zeolites. Xiong et al. have proposed a model of zeolite X formation, which involves 4-membered rings (4Rs) connecting to each other via 6-membered rings (6Rs) to form sod cages, then the sod cages interconnect via double 6-membered rings (D6Rs) to form the FAU-type framework [20]. Dutta et al. proposed the formation mechanisms of zeolite Y and mordenite. It was found that during the crystallization of ⇑ Corresponding author. Tel./fax: +86 411 84708545. E-mail address: [email protected] (C. Meng). http://dx.doi.org/10.1016/j.micromeso.2014.08.025 1387-1811/Ó 2014 Elsevier Inc. All rights reserved.

zeolite Y, 6Rs interconnect via 4Rs to form sod cages, then the sod cages interconnect via D6Rs to form the FAU-type framework [21]. In the case of mordenite, 4Rs are observed at the early stage followed by a ‘‘mordenite-like’’ amorphous phase with disordered 4Rs and 5-membered rings (5Rs), which then quickly connect to form mordenite crystals [7]. Dutta et al. have also used Raman spectroscopy to study the formation mechanisms of zeolite A [22], ZSM-5 [23] and ferrierite [24]. Atomic pair distribution function (PDF) technique is another powerful method to determine the short and medium-range structures of both disordered and crystallographic materials [25–50]. Recently, this technique has been applied to study the formation mechanisms of zeolites. Wakihara et al. proposed that the formation of faujasite is achieved through sod cages interconnecting with each other via D6Rs [30]. Suzuki et al. observed the changes in medium-range structure during the crystallization of VPI-7, a zincosilicate zeolite containing 3-membered rings (3Rs). They proposed that the formation of 3Rs is a key process in the crystallization [36]. The formation mechanism of ⁄BEA-type zeolite was proposed by Inagaki et al. They proposed that double 3-membered rings (D3Rs) contained in the silicate solution transform into ‘‘4–2’’-type secondary building units (SBUs) in the initial stage. The ‘‘4–2’’-type SBUs then rearrange to form ⁄BEA-type framework [49,50]. Synthesizing zeolites by conversion of magadiite, a cheap layered silicate with special structure, is considered to be one commercialization way [51–56]. Comparing to conventional silica source, faster crystallization rate can be obtained by use of magadiite

M. Cui et al. / Microporous and Mesoporous Materials 200 (2014) 86–91

as raw material and the amount of structure directing agent can be decreased for some cases [52]. In our previous work, zeolite omega was synthesized from magadiite in a glycerol–water system [57]. Zeolite omega consists of gmelinite cages which are linked in columns parallel to the c-axis to produce main channels with 12-membered rings [58]. In the present work, the elucidation of medium-range structures and their changes during conversion were investigated. 2. Experimental

GðrÞ ¼

2

p

Z

Q max

Q ½SðQ Þ  1 sin ðQrÞdQ

ð2Þ

Q min

here, Q is the magnitude of the wave vector (Q = 4psin h/k, where 2h is the angle between the incoming and outgoing radiation and k is the wavelength of the incident X-ray radiation). Qmax collected in this study is 19.4 Å1. In this study, the conversions of the HEXRD data to the PDFs were performed using the program PDF get X2 [59]. Total correlation function, T(r), is derived from Eq. (3),

TðrÞ ¼ 4pqr þ

2.1. Preparation of magadiite

87

2

p

Z

Q max

Q ½SðQ Þ  1 sinðQrÞdQ ¼ 4pqr þ GðrÞ

ð3Þ

Q min

where q is the total number density [30]. In this study, magadiite was prepared from colloidal silica (30% SiO2). The molar composition of the resultant mixture was SiO2:0.15 Na2O:4.22 H2O. Hydrothermal treatment was carried out at 150 °C for 48 h. After crystallization, the solid products were separated from the mixtures by vacuum filtration and washed with deionized water to pH = 7–8. The products were then dried at 100 °C overnight. 2.2. Conversion of magadiite into zeolite omega The synthetic process was given in detail in previous work of our group [57]. The resultant synthetic solution had the chemical composition of 14 SiO2:Al2O3:10 Na2O:169 H2O:200 glycerol. Crystallization was carried out under autogenous pressure at 120 °C for 1–10 days. On reaching the crystallization time, a stainless-steel autoclave was taken out of the oven and cooled by water. The solid products were washed with deionized water till pH = 7–8 and dried at 80 °C overnight.

3.1. Structure of magadiite The exact structure of magadiite has not been established yet because the small dimensions of single crystals of natural and synthetic magadiite preclude the use of single-crystal X-ray diffraction. Hence, spectroscopic techniques have been used for trying to obtained detailed structural information of magadiite. It has been proposed that magadiite has multilayer structures with 5Rs and 6Rs [60,61]. Fig. 1 shows the IR and Raman spectra of magadiite. The peaks at ca. 1237 and 618 cm1 in the IR spectrum are assigned to the existing 5Rs [61] and 6Rs [62] in magadiite, respectively. The Raman spectrum of magadiite gave a major Raman peak at round 465 cm1, which is consistent with that proposed by Huang et al. [60]. 3.2. Changes of medium-range structure during the crystallization

2.3. Characterization The products were identified by a Panalytical X’Pert powder diffractometer at 40 kV and 40 mA with Ni-filtered Cu Ka source. Scanning electron micrograph (SEM) images were obtained with a QUANTA450 scanning electron microscopy. The Raman spectra were obtained using a Thermo Scientific spectrometer, with a 532 nm excitation line. The Infrared (IR) spectra were measured by a Nicolet 6700 FTIR spectrometer at 2 cm1 resolution. Raman and IR spectra were all normalized. Solid-state 29Si magic angle spinning nuclear magnetic resonance (MAS NMR) experiments were performed on a Bruker AVANCE III 600 spectrometer at a resonance frequency of 119.2 MHz. 29Si MAS NMR spectra with high-power proton decoupling were recorded on a 4 mm probe with a spinning rate of 12 kHz, a p/4 pulse length of 2.6 ls, and a recycle delay of 120 s. The chemical shifts of 29Si are referenced to TMS. High-energy X-ray diffraction (HEXRD) experiments were carried out on a Panalytical Empyrean powder diffractometer using Ag Ka (k = 0.55941 Å) and a scintillation detector, with incident photon energy of 22 keV. The data were collected from 1.5° to 120° (2h) with a step length of 0.02 (2h). The collected data were subjected to well established analysis procedures including absorption, background and the Compton scattering corrections followed by normalization to the total scattering factor, S(Q), which is related to the coherent part of the diffraction pattern, Icoh(Q), as Eq. (1):

2 h i X X   2 SðQÞ ¼ 1 þ Icoh ðQÞ  ci jf i ðQÞj = ci f i ðQ Þ

3. Results and discussion

Fig. 2 shows the X-ray diffraction (XRD) patterns for the magadiite and the products crystallized for 5 days, 6 days, 7 days and 10 days. As the reaction time was increased to 5 days, almost all traces of the magadiite diffraction peaks disappeared as shown in Fig. 2(O 5), indicating the long-range order of magadiite was collapsed after 5 days of heating. When the reaction time was prolonged to 7 days as shown in Fig. 2(O 7), diffraction peaks attributed to zeolite omega appeared. Fig. 2(O 10) shows that pure zeolite omega was obtained after 10 days of crystallization. Fig. 3 shows solid-state 29Si MAS NMR spectra for the products crystallized for 5 days, 6 days, 7 days and 10 days. The peaks at ca. 87, 100 and 110 ppm are typical for Q2, Q3 and Q4,

ð1Þ

where Icoh(Q) is the measured coherent scattering intensity, and ci and fi(Q) are the atomic concentration and X-ray atomic form factor, respectively, for atomic species of type i. [33] PDF, G(r), is derived from Eq. (2),

Fig. 1. IR (a) and Raman (b) spectra of magadiite.

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Fig. 2. XRD patterns of magadiite and the products obtained by heating the reactants for 5 days (O 5), 6 days (O 6), 7 days (O 7) and 10 days (O 10). All Bragg peaks seen in O 10 are due to zeolite omega (MAZ-type zeolite).

Fig. 3. Solid-state 29Si MAS NMR spectra of the products crystallized for 5 days (O 5), 6 days (O 6), 7 days (O 7) and 10 days (O 10).

respectively [63]. It is reported that magadiite comprises of Q3 and Q4, and the Q3/Q4 connectivity ratio is 1:3 [64]. In the 29Si MAS NMR spectrum of the product obtained after 5 days, Q2 peak appeared, Q3 peak increased and Q4 peak decreased, suggesting that the layer structure of magadiite is destroyed and low-condensated silicates is formed. When the crystallization time was increased to 7 days, Q2 peak disappeared and Q4(nAl) peak with n = 2–0 [65] began to be resolved, in good agreement with the growth of zeolite omega. The evolution of morphology and particle size during the conversion is investigated by SEM. As shown in Fig. 4(a), magadiite shows a rosette-like shape consists of plates. After 5 days of heating, the aggregated rosette-like shape is collapsed and small plates with discrete distribution are shown, indicating that the longrange order of magadiite was collapsed. After 6 days of heating, small oval-shaped particles appear, indicating that zeolite omega crystals begin to grow. After 7 days of heating, the oval-shaped morphology particles increase both in size and number. The zeolite omega crystals exhibit the oval-shaped morphology with a length of 4–5 lm as shown in Fig. 4(e).

Fig. 5 shows the Raman spectra for the products crystallized for 5 days, 6 days, 7 days and 10 days. Fig. 5(O 5) shows a prominent broad band at ca. 465 cm1 along with a shoulder at ca. 496 cm1 and peaks at ca. 380 and 330 cm1. The peaks at ca. 380 and 330 cm1 are related to 5Rs and 6Rs, respectively [66]. The broad band at ca. 465 cm1 is assigned to the contribution of all T–O–T bonds. Dutta et al. have discussed a series of zeolites and amorphous aluminosilicate glasses. They proposed that all T–O–T bonds are contributing to the most prominent band in the region of 300–600 cm1 [67]. What’s more, the frequency of the most prominent band is sensitive to the ring structures present in the material, with an inverse correlation between the frequency and the average T–O–T angle [24]. For example, anorthite-like glasses with predominantly 4Rs exhibit a band at 500–510 cm1, whereas for nepheline glass with 6Rs, a band below 470 cm1 is observed [24]. In this study, the band at ca. 465 cm1 is the most prominent band in the region of 300–600 cm1, thus it is assigned to the contribution of all T–O–T bonds. The peaks at ca. 1237 and 618 cm1 in the IR spectrum of the product obtained after 5 days as shown in Fig. 6(O 5) also confirmed the presence of 5Rs and 6Rs [60,62] after 5 days of heating. It would be reasonable to argue that parts of 5Rs and 6Rs in magadiite are still preserved after 5 days of heating although the long-range order of magadiite is collapsed. Upon increasing the reaction time to 6 days as shown in Fig. 5(O 6), the integral areas of the bands at ca. 380 and 330 cm1 are both increased, indicating the fraction of 5Rs and 6Rs increases during 5–6 days. In the IR spectra as shown in Fig. 6, the peaks at ca. 1237 and 618 cm1 are more pronounced in Fig. 6(O 6) comparing to Fig. 6(O 5), supporting again the view that the fraction of 5Rs and 6Rs increases during 5–6 days. The prominent bands of Fig. 5(O 6) and Fig. 5(O 5) are at the same position, indicating that the average T–O–T angle is not changed obviously during 5–6 days. The Raman spectrum of the product obtained after 7 days changes considerably as shown in Fig. 5(O 7), the shoulder at ca. 496 cm1 sharpens and increases. Since zeolite omega, which possesses major 4Rs, already formed after 7 days of heating as shown in Fig. 2(O 7), we infer that the increase of the band at ca. 496 cm1 in Fig. 5(O 7) indicates the formation of major 4Rs, which cause the average T–O–T angle of structure decreased. Fig. 4(O 10) indicates that zeolite omega gave only one Raman peak at round 496 cm1. The disappearance of Raman bands at ca. 380 and 330 cm1 proved that all discrete 5Rs and 6Rs are incorporated into zeolite framework. As reported by Dutta et al., once an organized structure is formed, it is no longer possible to discuss vibrational frequencies in terms of specific ring structure because of the sharing of atoms by neighboring rings [21]. Taking sodalite for example, it contains both 6Rs and 4Rs, but it exhibits only a sharp Raman peak at ca. 493 cm1 [21]. Based on the Raman and IR spectroscopic studies, it could be assumed that parts of 5Rs and 6Rs in magadiite are preserved as secondary building units (SBUs) although the long-range order of magadiite was collapsed in the initial stage, and the formation of major 4Rs occurs at a stage later than that of 5Rs and 6Rs. The Total scattering factors, S(Q) and total correlation function, T(r) of the products crystallized for 5 days, 6 days and 10 days were shown in Figs. 7 and 8, respectively. Peaks are observed at ca. 1.6, 2.2, 2.6, 3.1, 3.8 and 4.2 Å in Fig. 8. The peaks at ca. 1.6, 2.2, 2.6 and 3.1 Å are related to the T–O, Na–O, O–[T]–O and T–[O]–T (T: tetrahedral atoms, in this paper, Si and Al) correlation lengths, respectively [41]. These distances are closely similar in all zeolites and hence cannot provide the specific information required to identify the type of ring structures present in zeolites. The peaks in the G(r) show differences in the ring structures only appear beyond 3.30 Å. It is reported that the second nearest neighbor T–O correlation lengths in 4R and 5R are ca. 3.8 [26,30,41,42,45,49,50] and 3.9 Å [41], respectively. The peak at ca. 4.2 Å is mainly due to the second

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Fig. 4. SEM images of (a) magadiite and the products obtained by heating the reactants for (b) 5 days, (c) 6 days, (d) 7 days and (e) 10 days.

Fig. 5. Raman spectra of the products obtained by heating the reactants for 5 days (O 5), 6 days (O 6), 7 days (O 7) and 10 days (O 10).

Fig. 6. IR spectra of the products obtained by heating the reactants for 5 days (O 5), 6 days (O 6), 7 days (O 7) and 10 days (O 10).

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4. Conclusions The changes of medium-range structure in the course of crystallization of zeolite omega from magadiite were studied. It is proposed that parts of 5Rs and 6Rs in magadiite are preserved as SBUs although the long-range order of magadiite is collapsed in the initial stage and the fraction of 5Rs and 6Rs increases subsequently. The 4R chains are formed at a stage later than that of 5Rs and 6Rs. Acknowledgements

Fig. 7. Total scattering factors, S(Q), of the products obtained by heating the reactants for 5 days (O 5), 6 days (O 6) and 10 days (O 10).

The study was financially supported by the National Natural Science Foundation of China (Grant No. 21271037). The HEXRD experiments were performed at PANalytical Shanghai application laboratory (Shanghai, China). The authors thank Mr. Jingyi Chen (PANalytical, Shanghai, China) for the constructive suggestion and Mr. Xiaodong Zhu (PANalytical, Shanghai, China) for the support during data collection at PANalytical Shanghai application laboratory. References

Fig. 8. Total correlation function, T(r), of the products obtained by heating the reactants for 5 days (O 5), 6 days (O 6) and 10 days (O 10).

nearest neighbor T–O correlation lengths in various rings larger than 4R, such as 6R and 8R, etc. [26,30,41,42]. It is impossible to resolve the two correlation lengths of ca. 3.8 and 3.9 Å for the second nearest neighbor T–O correlation lengths in 4R and 5R, respectively. But Fig. 8(O 5) and (O 6) show the shift of the peak at ca. 3.8 Å towards higher value, indicating that the sample O 5 and O 6 have larger amounts of 5R. It could also be seen that the fraction of 5R increases during 5–6 days. The peak at ca. 4.2 Å shown in Fig. 8(O 5) and (O 6) supports the existence of various large rings such as 6Rs and 8Rs, etc. The peak at ca. 3.8 Å is more pronounced and the peak position shifts towards lower value upon increasing the reaction time to 10 days, indicating that the formation of major 4Rs primarily occurs at this stage. This tendency is consistent with the Raman and IR spectra. The changes of medium-range structure could be proposed according to the above investigations. Although the long-range order of magadiite is collapsed in the initial stage, parts of 5Rs and 6Rs are preserved as SBUs, and the fraction of 5Rs and 6Rs increases subsequently. The formation of 4R chains occurs at a stage later than that of 5Rs and 6Rs.

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