Preparation of titanium-containing zeolites with MEL structure from B-ZSM-11 and their characterization

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1 Applied Catalysis A: General 192 (2000) Preparation of titanium-containing zeolites with MEL structure from B-ZSM-11 and their characterization Fazhi Zhang a,, Xinwen Guo a, Xiangsheng Wang a, Guangyan Li a, Qi Zhao b, Xinhe Bao b, Xiuwen Han b, Liwu Lin b a Laboratory of Comprehensive Utilization for Carbonaceous Resources, Dalian University of Technology, Dalian , PR China b State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian , PR China Received 30 July 1998; received in revised form 10 June 1999; accepted 1 July 1999 Abstract Titanium-containing MEL zeolites (Ti-ZSM-11) are prepared by gas solid reactions between TiCl 4 vapor and B-ZSM-11 precursors. XRD, FTIR, UV Vis, 11 B and 29 Si MAS NMR techniques are used to characterize these materials. The results show that Ti atoms are incorporated into the silanol nests that were previously occupied by B atoms. The precursor deboronated by acid reflux possesses a large amount silanol nests and so can obtain a high concentration of titanium in the framework, compared to the directly calcinated precursors. Ti-ZSM-11 zeolites are used to catalyze the epoxidation of propylene with dilute hydrogen peroxide. The results show that the catalytic activities are related to the physicochemical properties of these materials Elsevier Science B.V. All rights reserved. Keywords: Ti-ZSM-11; B-ZSM-11; Secondary synthesis; Epoxidation of propylene 1. Introduction Titanium-containing zeolite TS-1 having the MFI structure was first synthesized by Taramasso et al. [1]. It was known to be an effective catalyst for the selective oxidation of a number of organic substrates with dilute hydrogen peroxide (H 2 O 2 ) [2]. After this, TS-2, with the MEL structure, was synthesized and shown to have catalytic properties similar to those of TS-1 [3]. TS-1 or TS-2 can be synthesized directly from SiO 2 and TiO 2 precursors by the hydrothermal Corresponding author. Present address: Department of Chemistry, Qing Hua University, Beijing , PR China. Fax: address: guoxw@dlut.edu.cn (F. Zhang). method. In addition to this method, the gas solid reaction between TiCl 4 vapor and zeolite precursor is a novel way adopted by several research groups to prepare the titanium-containing zeolites [4 8]. Acceptably, the boron-containing zeolites appear to be the most promising precursors for boron. They can be extracted under extremely mild conditions from the framework while leaving homogeneously distributed silanol nests in the lattice [9]. Recently, Rigutto [6] and Dartt et al. [7] have used boron-containing and SSZ-33 (a borosilicate molecular sieve with a multidimensional pore system formed by the intersection of 10- and 12- member ring pores [10]) as precursors to prepare Ti- and Ti-SSZ-33, respectively. According to their views, when TiCl 4 is contacted with the precursor prior to the removal of the boron, a catalyst X/00/$ see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S X(99)

2 158 F. Zhang et al. / Applied Catalysis A: General 192 (2000) with high epoxidation activity is formed. In this paper, we describe the preparation and characterization of titanium-containing zeolites with MEL structure starting with B-ZSM-11 precursors. The epoxidation of propylene with dilute H 2 O 2 is used to test the catalytic activity of the titanium-containing ZSM Experimental 2.1. Synthesis and pretreatment of B-ZSM-11 B-ZSM-11 was synthesized according to [11] using tetrabutylammonium hydroxide (TBAOH), boric acid, tetraethylorthosilicate (TEOS) and distilled water. Here, the SiO 2 /B 2 O 3 ratio in the reaction mixture was 20. The hydrothermal crystallization was carried out at 165 C over 12 days. Then the resulting crystallized solids were washed several times with distilled water and dried overnight at 110 C. Before being used as precursors, as-synthesized B-ZSM-11 were pretreated by two different methods. In the first method, B-ZSM-11 were calcinated at 540 C for 5 h to remove the organic template, the sample denoted as B-ZSM-11(A); In the second method, B-ZSM-11 were pretreated by a combination of dilute acid reflux and calcination to remove boron and organic templating agents. The acid reflux was carried out at 80 C for 2 h using 2 mol/l HCl solution (10 ml/g of sample). The sample thus obtained was denoted as B-ZSM-11(B) Gas solid reaction The gas solid reaction between B-ZSM-11 precursors and TiCl 4 vapor was achieved in a quartz reactor. For a typical run, 3 g of precursor (20 40 mesh) was loaded into a vertical reactor and dried at 500 C for 4 h under a dry nitrogen stream at a flow rate of 30 ml/min. The reactor was then brought to the reaction temperature of 600 C and exposed to a flow of nitrogen bubbled through a vessel containing TiCl 4 maintained at room temperature. After this, TiCl 4 treatment was discontinued and nitrogen flow was maintained through the sample at 600 C for 4 h. Finally, the sample was cooled to room temperature overnight and washed twice with alcohol. Hereafter, the samples prepared by contact with TiCl 4 for 6 and 26 h are denoted as Ti-ZSM-11(A6) and Ti-ZSM-11(A26), respectively when using B-ZSM-11(A) as precursors, and Ti-ZSM-11(B26) when using B-ZSM-11(B) as precursor Characterization X-ray diffraction (XRD) was performed on a Rigaku D/max- b diffractometer (Cu K ). Fourier transform infrared (FTIR) spectra were obtained on a Nicolet FT 5DX spectrometer with the KBr pellet technique. UV Vis spectra of Ti-ZSM-11 were obtained on Shimadzu UV-240 spectrometer. Spectra were referenced to their corresponding precursors. All solid NMR measurements were carried out on a BRUKER DRX-400 spectrometer equipped with a ZrO 2 rotor and a spinning rate of 4 KHz at room temperature. 29 Si and 11 B NMR spectra were obtained at and MHz respectively using pulse widths of 0.8 m and pulse intervals of 4 s. DSS(3-trimethylsilypyopane) and BF 3 OEt 2 were used as external standards for 29 Si and 11 B solid NMR experiments, respectively. Elemental analyses were performed on a OXFORD ISIS 300 GDS spectrometer Epoxidation of propylene The epoxidation reaction was carried out in a 200 ml stainless autoclave equipped with a magnetic stirrer, a constant temperature water bath, an inlet for gas reagent and an outlet for exhausting gas. H 2 O 2 was analyzed by the iodometric titration and products were analyzed by the Shangfen 1102 gas chromatograph, using a flame ionization detector and a glass column (4 m 4 mm) containing polyethylene glycol as the stationary phase. 3. Results and discussion 3.1. XRD Fig. 1 shows the XRD spectra of B-ZSM-11 precursors and their corresponding Ti-ZSM-11. It can be seen that all these materials possess tetragonal symmetry. The patterns of Samples (2), (3) and (5) match well

3 F. Zhang et al. / Applied Catalysis A: General 192 (2000) Fig. 1. XRD spectra of samples. (1) B-ZSM-11(A), (2) Ti-ZSM-11(A6), (3) Ti-ZSM-11(A26), (4) B-ZSM-11(B), (5) Ti-ZSM-11(B26). with those of precursors (1) and (4), except a decrease of crystallinity of the titanium-containing samples in comparison with that of precursor Sample (1). Table 1 illustrates that the unit volume of titanium-containing sample increases with respect to that of precursors, because of the larger Ti O in the framework (Ti O band: nm, Si O band: nm and B O band: nm). Taking the Sample (1) as reference, We see clearly that the unit cells of the corresponding titanium-containing Samples (2) and (3) progressively expand with the increase of the gas solid reaction time. The results of the expansion of the unit cell indicate that Ti atoms are indeed incorporated into the framework. Both unit cell expansion and elemental analyses show that Sample (5) occupies the highest amount of Ti atoms in the framework in comparison with the other two titanium-containing ZSM-11. It can be concluded that the deboronation treatment on the as-synthesized sample by the acid reflux is favorable before contact with TiCl 4 vapor FTIR The framework IR spectra of B-ZSM-11 precursors and their corresponding Ti-ZSM-11 are illustrated in Fig. 2. All the spectra are typical pentasil zeolites. For Sample (1), two bands at 930 and 1380 cm 1 appear which do not show up in the spectrum of boron-free ZSM-11 zeolite. The 930 cm 1 band can be attributed to the presence of the tetrahedrally coordinated framework boron which is due to a Si OH group adjacent to Table 1 Unit cell parameters, crystallinity and elemental analyses of samples Samples Zeolites Crystalline cell parameter a Volume (nm 3 ) Crystallinity (%) SiO 2 /TiO 2 a = b (nm) c (nm) 1 B-ZSM-11(A) Ti-ZSM-11(A6) Ti-ZSM-11(A26) B-ZSM-11(B) Ti-ZSM-11(B) a ( ) = 90.

4 160 F. Zhang et al. / Applied Catalysis A: General 192 (2000) B and 29 Si solid NMR Fig. 2. FT IR spectra of samples. (Samples (1) (5) refer to the same samples as in Fig. 1 respectively; (6) Sample (1) calcinated at the same reaction temperature with a flow of N 2 for 26 h). a boron site [12], while the one at 1380 cm 1 is found to correspond to trigonal boron species [9]. Upon acid reflux, the 1380 cm 1 band disappears (Fig. 2 (4)), which is an important proof for deboronation. But, the intensity of the 930 cm 1 one in the spectrum of Sample (4) increases, in comparison with that of Sample (1). In the case of deboronated MFI, the 930 cm 1 band is assigned to Si OH groups in the silanol nests, according to IR studies by de Ruiter et al. [9]. It is concluded that the acid reflux on the boron-containing ZSM-11 can cause the tetrahedrally and trigonally coordinated boron to be extracted and to leave silanol nests in the lattice. Upon contact with TiCl 4 vapor, the 930 cm 1 band in Samples (1) and (4) is shifted towards higher 960 cm 1 vibration frequency which exists in the titanium-containing Samples (2), (3) and (5). The 960 cm 1 band assigned to the stretching Si OH vibration mode perturbed by the presence of Ti(IV) in a neighboring position is used to infer the incorporation of titanium into the boron-free silicalites [13]. A gradual increase of relative intensity of the 960 cm 1 band in Samples (2) and (3) with the prolongation of reaction time is observed, which indicates that Ti atoms are progressively inserted into the lattice. In the 11 B MAS NMR spectrum of Sample (1) (Fig. 3 (I)), the resonance at δ 3.5 ppm is assigned to tetrahedrally coordinated framework boron, while a doublet at 1 and 5.6 ppm is attributed to trigonally coordinated framework boron [14]. Upon contact with TiCl 4 vapor at 600 C for 6 or 26 h, the intensity of the tetrahedrally coordinated framework boron signal in Samples (2) and (3) rapidly decreases, in comparison with that of precursor Sample (1). According to many MAS NMR studies of B-MFI and B-BEA [9,15], framework boron can be extracted from the boron-containing zeolites by calcination. Here, during the gas solid reaction between the precursor and TiCl 4 vapor at elevated temperature, the framework boron may be extracted by the calcination with a flow of N 2. A special procedure of treatment was employed on precursor Sample (1) to demonstrate this view: In a separate experiment, Sample (1) was calcinated at the same gas solid reaction conditions in the quartz reactor with a flow of N 2 for 26 h, except for the exposing to TiCl 4 vapor. The 11 B MAS NMR spectrum of thus obtained Sample (6) shows that the intensity of the resonance at δ 3.5 ppm rapidly drops, compared to that of Sample (1). On the other hand, from the spectra of 29 Si MAS NMR (Fig. 3 (II)), it is known that the intensity of a peak at δ 104 ppm which is a typical signal of Si(OH)(OSi) 3 group [16] obviously increases in comparison with that of Sample (1). It is concluded that this treatment can extract framework boron from the MEL structure and leave silanol nests in the lattice. From Fig. 3 (II), it is known that, compared to that of Sample (6), the intensity of the resonance at δ 104 ppm in Samples (2) and (3) evidently decreases, which confirms that Ti atoms are inserted into the silanol nests produced by the deboronation when the B-ZSM-11(A) precursor contacts with TiCl 4 vapor at elevated temperature. According to the results of IR spectra, the treatment of boron MEL with a solution of 2 M HCl can remove the boron from the framework and leave silanol nests in the lattice. The conclusion can be further confirmed by the 11 B and 29 Si MAS NMR. From the 11 B MAS NMR spectrum of Sample (4), it is known that the intensity of the 3.5 ppm signal assigned to the tetrahedrally coordinated framework boron obviously decreases, while the trigonally coordinated framework

5 F. Zhang et al. / Applied Catalysis A: General 192 (2000) Fig B (I) and 29 Si (II) MAS NMR spectra of samples (Samples (1) (6) refer to the same samples as in Fig. 2 respectively). boron doublet at 1 and 5.6 ppm disappears. Evidently, a great part of the framework boron in the B-ZSM-11 is washed out by the acid reflux. Upon contact with TiCl 4 vapor, the relative intensity of resonance at δ 104 ppm in Sample (5) rapidly decreases, in comparison with that in its precursor Sample (4). We can conclude that Ti atoms are incorporated into the tetrahedral framework through a reaction of silanol nests in deboronated precursor Sample (4) with TiCl 4 vapor. From the 29 Si MAS NMR spectra we can see that the relative intensity of the peak at δ 104 ppm of Sample (4) is larger than that in the spectrum of Sample (6). The result is consistent with the variations of 930 cm 1 band intensity of Samples (4) and (6) shown in Fig. 2. It can be concluded that the precursor Sample (4) occupying a larger amount of silanol nests in the lattice can obtain a higher concentration of titanium when contacted with TiCl 4 vapor, compared to the precursor Sample (1). Besides silanol nests, other hydroxyl groups such as terminal silanols are confirmed to exist in the as-synthesized or modified zeolites [17 20]. According to Chezeau et al. [20], the terminal silanols in the ion-exchanged MFI zeolites can be almost completely eliminated upon calcination at 540 C, while the silanol nests remain unchanged. It can be concluded that B-ZSM-11 (A) and (B) precursors which were calcinated to remove organic templating agents at 540 C and then contacted with TiCl 4 vapor at 600 C occupy silanol groups mainly present as the silanol nests. Few terminal silanols can be available to react with titanium in the gas solid reactions UV Vis Fig. 4 shows UV Vis spectra of the titaniumcontaining ZSM-11. It is known that only one band Fig. 4. UV Vis spectra of samples. (Samples (2), (3) and (5) refer to the same samples as in Fig. 2 respectively).

6 162 F. Zhang et al. / Applied Catalysis A: General 192 (2000) Table 2 Epoxidation of propylene over Ti-ZSM-11 a Samples Conversion of H 2 O 2 (%) Efficiency of H 2 O 2 (%) Selectivity for propylene oxide (%) a Reaction conditions: catalyst 0.4 g, methanol 31.6 ml, 27% H 2 O 2 3 ml, propylene pressure 0.9 MPa, temperature 60 C, time 3 h. at 210 nm is obtained in the spectra of Samples (3) and (5). A similar peak around 210 nm was also observed in the UV Vis spectrum of TS-1 [21]. Besides the one at 210 nm, there is a band around 330 nm appearing in the spectrum of Sample (2) which is contacted with TiCl 4 vapor at 600 C for 6 h rather than 26 h. The band at 210 nm was assigned to the isolated titanium in tetrahedral coordination, while the one around 330 nm was attributed to the anatase species TiO 2 on the extraframework [21]. From the above mentioned characteristic results, we suggest that when precursor Sample (1) is contacted with TiCl 4 vapor, the titanium species first bind to the framework surface where a boron site is associated. Consequently, the deboronation at high temperature with a flow of N 2 will lead to the incorporation of titanium into the lattice. After the gas solid reaction for 6 h, the excessive titanium not inserted into the framework will exist as the anatase form when the reaction is finished Epoxidation of propylene The epoxidation of propylene with dilute H 2 O 2 was carried out to study the catalytic performance of the titanium-containing ZSM-11. The results of the tests are summarized in Table 2. All the three Ti-ZSM-11 show high activity for the epoxidation of propylene with dilute H 2 O 2, compared to the B-ZSM-11 precursor. Sample (2), confirmed by UV Vis to contain a large amount of anatase, has the lowest conversion of the three titanium-containing samples, along with the lowest efficiency of H 2 O 2. According to the performance of Samples (2) and (3), the level of conversion and efficiency of H 2 O 2 increase with the TiCl 4 exposure time. Sample (5) prepared from the deboronated MEL by the acid reflux possesses higher catalytic activity than that prepared from the directed calcinated MEL, which demonstrates the larger amount of titanium in the lattice of Sample (5). Besides, three Ti-ZSM-11 samples show nearly 100% selectivity for propylene oxide under this reaction condition. 4. Conclusions Upon calcination with a flow of N 2 at elevated temperature or acid reflux, boron can be easily extracted from the ZSM-11 framework and will leave silanol nests in the lattice. Then Ti atoms are incorporated into the silanol nests when the precursors are contacted with TiCl 4 vapor. The larger amount of silanol nests in the MEL deboronated by the acid reflux than that in the directly calcinated precursors can lead to high concentration of framework titanium and to high conversion and efficiency of H 2 O 2 in the epoxidation of propylene. Titanium-containing ZSM-11 prepared from the directly calcinated precursor by the gas solid reaction for 6 h possesses some anatase on the extraframework, which can decrease the catalytic activity in the epoxidation. Acknowledgements This work is financially supported by the National Natural Science Foundation of PR China (No ). We wish to thank Prof Z.H. Zhou and Engineer W.B. Yang for kindly help and discussions. References [1] M. Taromasso, G. Perego, B. Notari, US Patent (1983). [2] B. Notari, Stud. Surf. Sci. Catal. 60 (1991) 343.

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