Methane dehydroaromatization over alkali-treated MCM-22 supported Mo catalysts: effects of porosity

Studies in Surface Science and Catalysis, volume 147 X. Bao and Y. Xu (Editors) 02004 Elsevier B.V. All rights reserved.

595

Methane dehydroaromatization over alkali-treated MCM-22 supported Mo catalysts: effects of porosity L.-L. Su, Y.-G. Li, W.-J. Shen, Y.-D. X u , X.-H. Bao State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy o f Sciences, 457 Zhongshan Road, Dalian 116023, China [email protected]; [email protected] ABSTRACT Methane dehydroaromatization (MDA) was carried out on 6Mo/MCM-22 and 6Mo/alkali-treated MCM-22 catalysts. The results of catalytic evaluation, MAS NMR and pore distribution experiments demonstrated that the alkali treatment of MCM-22 could create some mesopores on the parent MCM-22 zeolite. Appropriate amount of mesopores coexisted with its inherent micropores would benefit the diffusion of the reactants and products, thus increase the stability of the MDA reaction on the 6Mo/alkali-treated MCM-22catalyst. 1. INTRODUCTION Since Wang et al. reported that Mo/HZSM-5 catalyst is active and selective for methane dehydroaromatization (MDA) in 1993, many studies on this topic have been carried out [1-4]. It is well recognized that the Mo/HZSM-5 is a bi-functional catalyst; The MoOx species are reduced by CH4 in the induction period and probably are in the form of M o z C and/or MoO• which are responsible for methane dehydrogenation and for the formation of C2 species, while the Br6nsted acid sites in the zeolite channels are associated with the oligomerization of ethylene and the formation of benzene and other aromatics. Due to the obvious difference in the molecule sizes of the reactant ( C H 4 ) and products (mainly C6H6 and C10H8) in MDA, pore size distribution and channel structure of the zeolites may play an important role from the point of view of shape-selectivity and mass transport in the zeolite channels. It was reported that a zeolite with a pore diameter of ca 0.6nm, which is consistent with the dynamic diameter of benzene molecules, is a good catalyst component for MDA reaction [5]. Recently, it has been reported that appropriate alkali treatment of the MFI zeolite is an effective method to change its pore size distribution and to create a kind of secondary mesopores [6-8] and the approach has been approved to be

596 effective for the MDA reaction on Mo/HZSM-5 catalysts [9]. In addition, MCM-22 has been reported to be a good catalyst component for MDA reaction [10]. This kind of zeolite has two independent pore systems: two-dimentional 10MR intralayer channels, and 12MR interlayer supercages with a depth of 18.2 A, both accessible through 10 MR apertures. It seems to be interesting to study if the alkali treatment of MCM-22 zeolite can also create some mesopores on the parent zeolite and thus is also beneficial for the MDA reaction on Mo/MCM-22 catalysts. 2. EXPERIMENTAL

MCM-22 was synthesized by referring to ref. [11]. The alkali treatment of MCM-22 zeolite was performed as follows: MCM-22 was added to an aqueous NaOH solution and stirred for appropriate time under a given temperature condition. The conditions of alkali treatment of MCM-22(P), i.e., the concentration of sodium hydroxide solution/M, temperature/K, time/h were set at 0.1,323, 1 denoted as AT1 and 0.2, 323, 1 denoted as AT2, respectively. After filtering and drying, the zeolite was ion-exchanged with NHaNO3 solution to obtain the NH4 + form zeolite. Mo-containing catalysts (Mo wt% - 6%) were prepared by impregnation method described in ref. [10]. The reaction was carried out in a quartz tubular fixed-bed reactor at atmospheric pressure and 973 K, 1500ml/g.h as described in previous work [ 10]. N2 adsorption and desorption experiments were carried out using an AUTOSPRB-1 Micromeritics equipment. Multinuclear MAS NMR experiments were carried out at 9.4 T on a Bruker DRX-400 spectrometer using 4 mm ZrO2 rotors. 3. RESULTS AND DISCUSSION 3.1 The effect of alkali treatment on framework structure and acidity of MCM-22 Figure 1 shows the 1H MAS NMR spectra of MCM-22 before and after alkali treatment and the corresponding deconvolution results of the ~H MAS NMR spectra are listed in Table 1. Four peaks at ca. 8 - 1.7, 2.2, 3.7 and 5.8 ppm, can be clearly resolved, which are almost the same as reported in our previous work [12]. Accordingly, the bands at ca. 8 = 1.7 and 2.2 ppm are attributed to silanol group and A1-OH species, respectively. The band at ca. 8 = 3.7 ppm is associated with bridging OH groups (Br6nsted acid sites). The resonance at ca. 8 - 5.8 ppm is commonly assigned to another kind of Br6nsted acid sites, i.e. the restricted Br6nsted acid sites. After mild alkali treament of MCM-22 the amount of Br6nsted acid sites at 3.7 ppm was not affected noticeably, while for the MCM-22(AT2) sample the amount of Br6nsted acid sites decreases as we can see from Table 1. On the other hand, the peak intensity

597 at 1.7 ppm increases with the severity of alkali treatment process, indicating that there are more disfigurement sites on the severe by alkali treated MCM-22. At the same time, the Si/A1 ratio determined by XRF experiments decreases after alkali treatment as shown in Table 1. 27A1 MAS NMR spectra of the MCM-22 before and after the alkali treatment are shown in Fig. 2. The peak at 55ppm is attributed to the tetrahedral framework aluminum and the peak at 0 ppm is ascribed to the extra framework aluminum. Before and after the alkali treatment the ratio between the framework and extra framework A1 does not change obviously, demonstrating that the A1 species of the MCM-22 has not been seriously influenced by the alkali treatment. However, the Si/A1 ratio of the zeolite determined by XRF experiments decreased after the alkali treatment. Since the A1 species is not expelled from the zeolite as shown in Fig. 2 and there is not any A1 species being introduced during the alkali treatment process, the decrease in the S i/A1 ratio after alkali

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Fig. 2. 27A1 MAS NMR spectra of MCM-22 before and after alkali treatment

Table 1 The deconvolution result of 1H MAS NMR spectra for MCM-22 before and after alkali treatment ..... Number of SIO2/A1203 a of ' SIO2/A1203a of Mo%a hydroxyls/u.c, zeolite before Mo zeoliteafter Mo Si-OH-A1 Si-OH loading loading MCM-22 3.5 2.4 19.7 21.2 6.2 MCM-22(AT1) 3.2 3.4 16.8 17.2 7.0 MCM-Z2(AT2) 2.2 4.2 15.1 15.4 6.0 a: determined by XRF experiment 9

598 treatment was resulted from the dissolution of siliceous species on the zeolite. It seems that the alkali solution with appropriate concentration would preferentially attack the siliceous species and lead to its extraction from the zeolite. The inertness of the alkali treatment towards Si-O-A1 bond in the framework preserves the specific Br6nsted acid sites. 3.2 Effect of the alkali treatment on the pore distribution of the M C M - 2 2 The change of pore distribution is shown in Fig. 3 and the physical properties of the MCM-22 zeolte before and after the alkali treatment are listed in Table 2. It is clear that with the alkali treatment condition becoming more and more severe, the more obvious change in pore distribution could be observed and the mesopores part possessed more proportion all over the MCM-22 zeolite. The dissolution of siliceous species, caused by the alkali treatments, probably starts preferentially on the positions where the crystillization is poor, such as at the boundaries or defects of the MCM-22 zeolite crystallites. And this kind of dissolution of siliceous species may lead to the formation of some mesopores in the zeolite. At the same time, the external surface areas increase after the alkali

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Table 2 Physical properties of MCM-22 before and after alkali treatment

SBET m 2 ,

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. g-i cm3 . g-1 cm3 . g-J

76 145 192

0.21 0.18 0.14

599 treatment. This may be attributed to the fact that the siliceous species residing along the boundaries of the crystallites or in the framework has been dissolved during the alkali treatment, and such a kind of dissolution will induce the formation of some small crystallites and the secondary mesopores form between crystallites boundaries.

3.3 The catalytic performance of MDA on Mo/MCM-22 and Mo/MCM-22(AT) catalysts Figure 4 and Figure 5 show the reaction results on the alkali treated MCM-22 supported Mo catalysts. The depletion rate of methane and the formation rate of aromatics are higher on Mo/ MCM-22(AT1) catalyst than those on the untreated Mo/MCM-22 catalyst. Particularly, the naphthalene yield is much higher on Mo/ MCM-22 (AT1). Since the result of 1H MAS NMR demonstrates that the Br6nsted acid sites have not been seriously affected by the alkali treatment Mo/MCM-22(AT1) catalyst, the enhancement of the reaction performance after the alkali treatment is mainly resulted from the change of the pore structure. On the Mo/MCM-22(AT1) catalyst, being coexisted the micropores and mesopores, not only the aromatics product can be selectively formed, but also they can diffuse out of the zeolite channels timely due to the existence of the mesopores, thus the reaction activity and stability are enhanced. However, when the conditions of alkali treatment are too severe such as in the case of the Mo/MCM-22(AT2) catalyst, its catalytic performance decreases sharply. Firstly, the alkali treatment conditions used on the Mo/MCM-22(AT2) catalyst are too severe, so that the amount of Br6nsted acid sites decrease obviously as shown in Figure 1 and Table 1. This indicates that the Br6nsted acid sites have been seriously changed for the Mo/MCM-22(AT2) catalyst. Secondly, because of the severe conditions of the alkali treatment there are too

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600 many mesopores created and too many micropores reduced on the Mo/MCM-22(AT2) catalyst, The ratio between micropores and mesopores on it is not suitable for the MDA reaction. Appropriate amount of mesopores coexisted with inherent micro-pores would be beneficial for the diffusion of the reactant and products and thus increase the stability of the catalyst. The alkali treatment of MCM-22 zeolite mainly changed the porosity and channel structure. A proper alkali treatment could create proper amount of mesopores, which coexisted with the inherent micropores of the MCM-22 zeolite due to the dissolution of siliceous species of the zeolite framework. The 6Mo/MCM-22 catalysts prepared by using a proper alkali treated MCM-22 zeolite, with the coexistence of mesopores and micropores on it, showed higher depletion rate of methane and formation rate of aromatics as well as reaction stability when compared with the conventional untreated 6Mo/MCM-22 catalysts. The present work again demonstrates that pore-size distribution and channel structure of the zeolite play an important role in catalyst performance due to the shape selectivity and mass transfer in the zeolite channels. ACKNOWLEDGMENTS Financial supports from the Ministry of Science and Technology of China, the Natural Science Foundation of China, the Chinese Academy of Sciences and the BP-China Joint Research Center are gratefully acknowledged. REFERENCES [1] L. Wang, T. Li, M. Xie, Y. Xu, Catal. Lett. 21 (1993) 35. [2] Y. Xu, L. Lin, Appl. Catal. A: 188 (1999) 53. [3] Y. Shu, M. Ichikawa, Catal. Today 71 (2001) 55. [4] Y. Xu, X. Bao, L. Lin, J. Catal. 216 (2003) 386. [5] C. Zhang, S. Li, Y. Yuan, W. Zhang, T. Wu, and L. Lin, Catal. Lett. 56 (1998) 207. [6] M. Ogura, S. Shinomiya, J. Tateno, Y. Nara, E. Kikuchi, M. Matsukata, Chem. Lett. (2000) 882 [7] T. Suzuki, T. Okuhar, Micro. Meso. Mater., 43 (2001) 83 [8] M. Ogura, S. Shinomiya, J. Tateno, Y. Nara, M. Matsukata, Appl. Catal., 219 (2001) 53 [9] L. Su, L. Liu, J. Zhuang, H. Wang, Y. Li, W. Shen, Y. Xu, X. Bao, Catal. Lett. 91 (2003) 155. [10] Y. Shu, D. Ma, L. Xu, Y. Xu, X. Bao, Catal. Lett. 70 (2000) 67. [11] A. Cizmek, B. Subotic, I. Smit, A. Tonejc, R. Aiello, Micro. Mater., 8 (1997) 159. [12] D. Ma, Y. Shu, X. Han, Y. Xu and X. Bao, J. Phys. Chem. B 105 (2001) 1786.