КИНЕТИКА И КАТАЛИЗ, 2013, том 54, № 4, с. 466-473
УДК 541.128:547.211:549.67
SYNTHESIS OF ZEOLITE IM-5 UNDER ROTATING AND STATIC CONDITIONS AND THE CATALYTIC PERFORMANCE OF Mo/H-IM-5 CATALYST IN METHANE NON-OXIDATIVE AROMATIZATION
© 2013 Heng Liu, Jing Hu, Zhifang Li, Shujie Wu, Lulu Liu, Jingqi Guan*, Qiubin Kan**
College of Chemistry, Jilin University, Changchun, 130023, P.R. China E-mail:guanjq@jlu.edu.cn*, qkan@mail.jlu.edu.cn** Received 03.03.2012
The hydrothermal crystallization of zeolite IM-5 was investigated under rotating (denoted as IM-5-R) and static (denoted as IM-5-S) synthesis conditions. Mo-modified catalysts (Mo-IM-5-R and Mo-IM-5-S) were prepared for the methane non-oxidative aromatization. The physical properties and acidities of the samples were characterized by XRD, SEM, BET and IR spectroscopy. Compared with Mo-IM-5-S, the Mo-IM-5-R catalyst showed both a higher conversion of methane and higher selectivity to benzene in methane aromatization. A higher catalytic activity of Mo-IM-5-R may be attributed to the preferable textural properties and acidities of zeolite IM-5-R. Moreover, the catalyst prepared by the physical mixing method exhibited lower initial activity, but better stability for methane aromatization than that prepared by the impregnation method.
DOI: 10.7868/S0453881113040084
The known reserves of natural gas (mostly methane) are enormous and the reserves are increasing more rapidly than those of liquid petroleum. It is anticipated that this trend will persist in the 21st century and effective utilization of methane will become increasingly important [1, 2]. Catalytic conversion of methane to aromatic compounds, i.e. benzene, toluene, xylene and naphthalene, which are the building blocks of the industrial chemistry, has become a field of interest with a lot of investigations performed during recent years [1—4]. Since the first active non-oxi-dative aromatization catalyst Mo-ZSM-5 was reported in 1993, various materials (e.g., ZSM-5, HZRP-1, ZSM-11, MCM-41, Beta, Y, mordenite, MCM-22, and ITQ-13) have been used for preparation of Mo-based catalysts [5—7]. Thus far, Mo-ZSM-5, that has a unique system of 10-member-ring (MR) channels with the channel dimension close to benzene kinetic diameter, was regarded as one of the most suitable catalysts for the title reaction [8]. In previous studies, it was suggested that zeolites with supercage structures, such as MCM-22, MCM-49, MCM-56 and TNU-9, are applicable supports for the preparation of highly active, Mo-based zeolite catalysts for the non-oxida-tive aromatization of methane [9—11]. Moreover, it was found that zeolites with 9-MR channels, such as ITQ-13, exhibit low activity due to channel restriction [7]. Different parameters, such as dispersion of molybdenum species, acidity, and structural features of the zeolite supports, are important factors that can considerably influence the evolution of methane aromatization reaction [12, 13].
Benazzi et al. were the first to describe a high-silica zeolite IM-5 (IMF), with an unusual two-dimension-
al (2D) 10-MR channel system [14]. Although the general features of its pore system were deduced from a variety of catalytic reactions as early as in 2000 [15], its detailed crystal structure information was not fully elucidated until 2007. As one of the zeolites with sophisticated topology, a specific framework structure of IM-5 contains 24 topologically distinct Si atoms. The 3D channel system with complex channel intersections imparts a distinctive pore structure to IM-5, which can accommodate bulky intermediates in a catalytic reaction. Moreover, IM-5 also retains the long-range diffusion property of a 2D channel system [16]. The structure of IM-5 is different from the early-described zeolites such as ZSM-5, ZSM-11, and MCM-22. Due to its excellent physico-chemical properties, the IM-5 zeolite could be applied in the petrochemical and refining industry as a commercial catalyst [17—19]. Recently, it has been found that the IM-5 zeolite is a suitable catalyst support for methane aromatization [20]. Compared with the conventional Mo-ZSM-5 catalyst, the Mo-IM-5 catalyst showed both a higher methane conversion and higher benzene selectivity. In addition, Mo-IM-5 was slightly more stable than Mo-ZSM-5. The unusual 2D 10-MR channel system of IM-5 (IMF) with a large unit cell (C-centered orthor-hombic u.c. with a = 14.2088 A, b = 57.2368 A, c = = 19.9940 A), which is nearly triple the volume of ZSM-5, and the presence of 3D ~2.5 nm thick cavities are perhaps the primary reasons for the better catalytic behavior of the Mo-IM-5 catalyst. Due to the specific channel systems and the presence of 3D cavities, IM-5 zeolite can accommodate more Mo species in the channels associated with Bronsted acid sites, thus im-
proving the activity and stability of the Mo-IM-5 catalyst in the methane aromatization.
The objective of this paper was to synthesize IM-5 zeolites under static and rotating conditions and to compare the crystallization behavior, morphologies, acidities, and catalytic performance of the samples. Furthermore, the catalytic data are brought into comparison with the results of methane aromatization over Mo-IM-5 catalysts, which were prepared by physical mixing and by impregnation method.
EXPERIMENTAL
Catalyst Preparation
The IM-5 zeolite was synthesized by a hydro-thermal procedure described in the original patent using 1,5-bis(methylpyrrodinium)pentane (1,5-MPP) as a template [14]. When the synthesis of IM-5 zeolite was conducted under rotating condition, the required amount of fumed silica was added into a solution of the structure-directing agent (1,5-MPP) in 30 mL water with stirring in the beaker. Then, aluminum nitrate nonahydrate and sodium hydroxide were added, and the mixture was vigorously stirred for 24 h. The final synthesis gels had the following chemical composition: 30SiO2 : 11Na2O : 0.6Al2O3 : 4.5(1,5-MPP) : 1200H2O. After preparation, the mixtures were reacted in 40 mL, Teflon-lined, stainless-steel autoclaves. The gels were autoclaved and heated at 433 K in an oven designed to rotate the autoclaves at 60 rpm for 1—16 days (rotating synthesis). In the synthesis systems of IM-5 zeolite under static condition, the required amount of fumed silica was added into a solution of the structure-directing agent (1,5-MPP) in 30 mL water with stirring in the beaker. Then, sodium aluminate, sodium bromide and sodium hydroxide were added, and the mixture was vigorously stirred for 24 h. The final synthesis gels had the following chemical composition: 30SiO2 : 8.5Na2O : : 0.6Al2O3 : 3NaBr : 5(1,5-MPP) : 1200H2O. After preparation, the mixtures were transferred in 40 mL, Teflon-lined, stainless-steel autoclaves. The autoclaves were heated at 448 K in a common oven for 1—16 days (static synthesis).
The solid precipitate was recovered by filtration, washed with distilled water, and dried at 373 K overnight. Then, the organic template was removed by heating in air at 823 K for 6 h to obtain the calcined zeolite Na-IM-5. The two samples were denoted as IM-5-R (synthesized under rotating condition) and IM-5-S (synthesized under static condition).
H-IM-5 was prepared by repeating the ion-exchange of the calcined Na-IM-5 for two times with a 1 mol/L solution of ammonium nitrate at 363 K, followed by calcination in air at 773 K for 5 h. Molybdenum oxide (MoO3, Beijing Chemical Works, 99.5% purity) was physically mixed with H-IM-5 to prepare the sample containing 6 wt % of MoO3, and then the catalyst was calcined in air at 773 K for 3 h as described
in literature [10]. The IM-5 zeolites with 6 wt % MoO3 loading were denoted as Mo-IM-5-R and Mo-IM-5-S, respectively. Molybdenum loaded zeolite catalyst was also prepared through the impregnation method. (NH4)6Mo7O24- 4H2O and H-IM-5-R were mixed with a MoO3 in small amount of distilled water content to yield the sample containing 6 wt % of MoO3. After evaporating water at 353 K, the catalyst was calcined in air at 773 K for 3 h. The sample prepared by the impregnation method was designated as Mo-IM-5-R (i). The Mo content in the Mo-IM-5 catalyst was determined by chemical analysis.
Catalyst Characterization
Powder X-ray diffraction (XRD ) measurements were carried out using CuZ"a radiation on the Shimad-zu XRD-6000 diffractometer ("Shimadzu", Japan) in the scanning range of 5°—40° (29) for phase identification and determination of the relative crystallinity. The percentage crystallinity of a series of the products obtained by heating the gel for different periods of time (1—16 days) was determined by comparing the area under the peaks at 29 7°-10° and 23°-27°, with that of the standard sample described in the literature [20]. The IM-5 sample (Si/Al = 50) was assumed to be 100% crystalline and was used as a reference for calculating the crystallinity of the products.
The morphology of samples was observed by FES-EM (field emission scanning electron microscopy) XL-30 field emission scanning electron microscope. The samples were coated with gold prior to the SEM analysis to avoid the charge effect of the samples. N2 adsorption—desorption isotherms were measured with a ASAP 2020 system ("Micromeritics", USA) at liquid N2 temperature (77 K). Before measurements, the samples were outgassed at 523 K for 3 h.
The BET surface area was calculated by using the BET (Brunauer—Emmett—Teller) method and the average pore volume was measured by using t-plot analysis [21, 22]. The concentration of Lewis and Bronsted sites in the samples was determined after the adsorption of d3-acetonitrile by FT-IR spectroscopy using a Nicolet impact 410 IR. Prior to adsorption of d3-ace-tonitrile, samples were evacuated in situ at 473 K and 10-5 mm Hg. Adsorption was carried out at 298 K for 15 min, followed by evacuation for 30 min at the same temperature. X-ray photoelectron spectra (XPS) analyses were performed in a VG ESCALAB M-II X-ray electron spectrometer using
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