ARTICLE pubs.acs.org/JPCC
Characteristic and Mechanism of Methane Dehydroaromatization over Zn-Based/HZSM-5 Catalysts under Conditions of Atmospheric Pressure and Supersonic Jet Expansion B. S. Liu,*,†,‡ Y. Zhang,† J. F. Liu,† M. Tian,‡ F. M. Zhang,†,‡ C. T. Au,§ and A. S.-C. Cheung*,‡ †
Department of Chemistry, Tianjin University, Tianjin 300072, China Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, China § Department of Chemistry and Centre for Surface Analysis and Research, Hong Kong Baptist University, Kowloon Tong, Hong Kong, China ‡
ABSTRACT: The catalytic performance of Zn-based/HZSM-5 catalysts prepared by wet impregnation method was investigated for the methane dehydroaromatization (MDA) reaction under the conditions of atmospheric pressure and supersonic jet expansion (SJE). The physical properties of the catalysts were characterized by BrunauerEmmettTeller (BET), Fourier transform infrared (FT-IR), temperature-programmed reduction of H2 (H2-TPR), temperature-programmed desorption of NH3 (NH3-TPD), X-ray photoelectron spectroscopy (XPS), thermogravimetric and differential thermogravimetric (TG/ DTG), and high-resolution transmission electron microscopy (HRTEM) techniques. The results revealed that under SJE condition the Zn/HZSM-5 catalyst exhibited high catalytic activity. It was found that because of the rapid migration of H+ ions on the catalyst, the activation of CH4 at active sites of nano-ZnO is facile. A new reaction mechanism that involves an active “ZnOCH3+...HZnO” intermediate formed as a result of synergetic action between ZnO and HZSM-5 has been proposed for CH4 dissociation and dehydrogenation. Under atmospheric pressure, however, the catalytic activity of Zn/HZSM-5 is low.
’ INTRODUCTION The running out of oil reserves and the concern for our environment promote the search for clean energy. The large reserve of natural gas and the recent discovery of methane hydrate suggest that methane is an important source of energy in the near future.1 Methane is the most inert hydrocarbon, and its conversion into liquid fuels and value-added chemicals is a challenge in modern catalysis.2 Since the report of nonoxidative methane dehydroaromatization (MDA) over Mo/ZSM-5 catalyst,3 much work has been conducted for the modification of Mo/HZSM-5 and Mo/HMCM-22 catalysts.48 The addition of the oxide of Zn, Ga, Ni, and Re would result in better performance. It was considered that Mo2C was the major active component for the nonoxidative MDA reaction. For example, Chouhary et al.9 found that Zn-based/ZSM-5 catalysts showed good catalytic activities in such a reaction. Using 13C-labeled methane, Stepanov et al10,11 studied the mechanism of CH4/ C3H6 MDA over Zn-modified H-BEA zeolites in isotopetracing experiments. Their results suggested that the methane dissociated on ZnO species was incorporated in the aromatic rings formed from C3H6, generating methyl-substituted benzene derivatives through a ring-expansion/contraction mechanism. It was reported by Xiong et al.12 that the addition of a proper amount of Zn2+ or Li+ would result in the elimination of r 2011 American Chemical Society
most of the surface strong Bronsted acid sites. At the same time, there was the generation of a kind of new medium-to-strong acid sites that are catalytically active for MDA reactions, and the formation of coke was alleviated to a great extent because of the absence of strong Brjnsted acid sites. Recently, Fang et al.13 reported that through the loading of Zn onto HZSM-5, the aromatization of dimethyl ether was enhanced. At 360 °C, the total yields of aromatics and C8 aromatics (66.2 and 39.0%) over 2%Zn/HZSM-5 were significantly higher than those (50.0 and 28.6%) over HZSM-5. In addition, Xuan et al.14 reported the nonoxidative aromatization of methane/propane (mole ratio 5:1) over a Zn/HZSM-5 catalyst and obtained propane conversion of 93.93% and aromatic selectivity of 80.29%, but they did not conduct discussion on CH4 conversion. To the best of our knowledge, the direct conversion of methane over a Zn/ZSM-5 catalyst has not been previously reported. Herein we report the performance of Zn/ZSM-5, ZnGa/ZSM-5, and Zn(or Ga)-Mo/ZSM-5 catalysts in MDA reaction under the conditions of atmospheric pressure as well as supersonic jet expansion (SJE). The physical properties of the catalysts were Received: March 23, 2011 Revised: July 19, 2011 Published: July 24, 2011 16954
dx.doi.org/10.1021/jp2027065 | J. Phys. Chem. C 2011, 115, 16954–16962
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characterized by BrunauerEmmettTeller (BET), Fourier transform infrared (FT-IR), temperature-programmed reduction (TPR) of hydrogen (H2-TPR), temperature-programmed desorption of NH3 (NH3-TPD), thermogravimetric and differential thermogravimetric (TG/DTG), X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HRTEM) techniques.
’ EXPERIMENTAL SECTION Preparation of Catalysts. The 1 wt % Zn, 2 wt % Mo/ HZSM-5 (denoted hereinafter as 1Zn2Mo/HZSM-5) catalyst was prepared by conventional wet impregnation method reported elsewhere.15 The HZSM-5 zeolite with a SiO2/Al2O3 ratio of 25 (supplied by Nankai University, Tianjin, China) was impregnated with an aqueous solution of ammonium molybdate and zinc nitrate at room temperature (RT) for 4 h, and the obtained precursor was dried at RT and calcined in static air at 550 °C for 4 h. Similarly, the 2Zn/HZSM-5, 2Mo/HZSM-5, 1Ga1Zn/HZSM-5, 1Zn2Mo/HZSM-5, and 1Ga2Mo/HZSM5 catalysts were prepared. The catalysts were then pressed, crushed, and sieved through 2040 meshes. Characterization of Catalyst. The adsorption isotherms of catalysts were measured at 77 K over a homemade system16 using N2 as adsorbate. The pore volume, specific surface area, and pore size distribution of catalysts were calculated according to the BET and BarrettJoynerHalenda (BJH) methods. For pyridine-adsorbed FT-IR spectrum analysis, the sample was first pretreated in vacuum at 350 °C for 2 h and then with pyridine adsorption until saturation at RT. Afterward, physisorbed pyridine was purged in argon (25 mL/min) at 110 °C; the FT-IR absorption spectra of the catalysts (in KBr pellet) were acquired using a Nicolette AVATAR360 spectrophotometer. The fine structures of the Zn-based/HZSM-5 catalysts were characterized by means of HRTEM over a Tecnai G2 F20 electron microscope instrument. The TEM images were acquired at 200 kV according to the procedure previously described.17 The compositions (Zn, Mo, Si, C, Al, and O) of fresh and used Zn-based/HZSM-5 catalysts were analyzed by means of energy-dispersive X-ray (EDX) technique. XPS was adopted for surface analysis of catalysts over a VGX 900 spectrometer with multichannel detector coupled to Al KR radiation (1486.6 eV). Using a piece of double-sided sticking tape, a sample was mounted on a holder and introduced to the preparation chamber (∼105 Torr) for degassing before being transferred to the analyzer chamber (99.995%) was introduced to the reactor with a gas hourly space velocity (GHSV) of 1680 mL/gcat 3 h. The effluent through a liquid collector was detected by an online gas chromatograph (102 G) using a TCD for the analysis of H2, CH4, CO, and C2H4 (an external standard adopted for estimation of carbon balance), and liquid products such as C6H6, C7H8, and C10H8 were analyzed by gas chromatograph (Varian CP-3800) equipped with a flame ionization detector. For comparison, MDA reaction was performed in a separate experiment under the condition of SJE in a quartz reactor (i.d. Four mm) with a pulsed duration of 250280 microsecond and a repetition rate of 10 Hz, as previously reported.19 A Zn-based/ HZSM-5 catalyst was placed between two quartz-wool plugs inside the reactor. For controlling and measurement of reaction temperature (normally at 700 °C), a thermocouple was placed underneath the quartz reactor next to the center of the catalyst bed. Typically, ca. 0.1 g of catalyst previously heated in a flow of argon to 700 °C was used. The gas effluent after reaction was introduced to another chamber through a skimmer with a diameter of 2 mm. The reagent, intermediate, and product species in the gas effluent were ionized by laser radiation (from a Nd/YAG laser) of 266 nm, 4 mJ, and 5 ns pulse. The ionized species were accelerated by an assembly of time-of-flight (TOF) acceleration plates, then focused by Einzel plates and traversed through a typical TOF field-free region before reaching the microchannel plate (MCP) ion detector. Signals from the detector were collected and analyzed by a 300 MHz digital storage oscilloscope.1 16955
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Figure 2. N2 adsorption isotherms of HZSM-5 and the prepared catalysts.
Figure 3. FT-IR of (a) 1Ga1Zn/HZSM-5, (b) 2Zn/HZSM-5, (c) pure HZSM-5, (d) 1Zn2Mo/HZSM-5, (e) 2Mo/HZSM-5, and (f) 1Ga2Mo/ HZSM-5.
Table 1. Specific Surface Area (SBET), Pore Volume (VT), and Micropore Volume (Vmi) of the Prepared Catalysts SBET (m2/g)
VT (mL/g)
Vmi (mL/g)
HZSM-5
335
0.15
0.14
1Ga1Zn/HZSM-5
319
0.14
0.13
2Zn/HZSM-5
294
0.14
0.13
1Ga2Mo/HZSM-5 2Mo/HZSM-5
263 252
0.12 0.12
0.12 0.11
1Zn2Mo/HZSM-5
239
0.10
0.10
catalysts
’ RESULTS AND DISCUSSION BET Surface Area Analysis. N2 adsorption isotherms of the catalysts are shown in Figure 2. The results suggest that the adsorption isotherms of the samples belong to the type of microporous structures. At relatively low pressure (p/p0 < 0.01), there is monolayer adsorption, then multilayer adsorption. When p/p0 approaches 0.1, there is condensation of gas in the micropores. As indicated by the formula of Kelvin ! p 2σVm ln ¼ p0 RTr
The smaller the pore radius (r) of the catalysts, the lower the pressure (p) needed for capillary condensation in micropores, and the content of N2 adsorption decreases with the decline of pore volume. As listed in Table 1, the specific surface area, pore volume, and microporous volume decrease with increasing amount of metal loadings because of the dispersion of active species inside the channels, at the pore entrances of HZSM-5 zeolite, or both. Compared with the specific surface area of HZSM-5 (335 m2/g), that of 1Zn2Mo/HZSM-5 (239 m2/g) shows a decline of 28.7%, which is the smallest among the prepared catalysts (Table 1). The large BET surface areas of the catalysts suggest that the metal oxides are highly dispersed on HZSM-5 zeolite. The pore size distributions of the catalysts show maxima at 0.8 nm, and with the increase in metal loadings, the maxima shift toward lower pore diameters. In the meantime, the pore distribution profiles widen because of the preferential occupation of species in the micropores. FT-IR Spectra of Catalysts. The framework structures of HZSM-5 zeolite and the catalysts were characterized by means of FT-IR absorption spectroscopy (Figure 3). The absorption
Figure 4. NH3-TPD spectra of (a) 1Ga2Mo/HZSM-5, (b) 2Mo/ HZSM-5, (c) 2Zn/HZSM-5, (d) 1Ga1Zn/HZSM-5, and (e) 1Zn2Mo/ HZSM-5.
bands at 1222, 1100, 795, 547, and 453 cm1 are characteristic of the HZSM-5 framework, whereas those in the 30004000 cm1 range can be attributed to stretching vibration of OH bonds. The catalysts show similar bands at 1222, 1100, 795, 547, and 453 cm1 (Figure 3A), suggesting that the loading of the metal species does not cause significant alteration in the structure of HZSM-5. We found that the vibration bands of SiOHAl groups were weak in intensity, possibly because of the presence of moisture. To overcome such a shortcoming, we detected the variation bands of OH in the 32003900 cm1 range by increasing the amount of samples, and the results are shown in Figure 3B. According to Pu and Inui,20 the HZSM-5 zeolite showed two vibration bands due to OH. The bands at 3740 and 3610 cm1 were ascribed, respectively, to the vibration of terminal SiOH and framework bridge hydroxylic SiOHAl (distributed on both external and internal surface of HZSM-5), and to the vibration of isolated AlOH (signal at ca. 3653 cm1). According to Jablouski et al.,21 the terminal SiOH (3740 cm1) with Bronsted acidity can interact with Zn2+ ions to form a new acidic site (Figure 3B (b)) whereas the framework SiOHAl (3610 cm1) having Lewis acidity disperses on the inner surface of ZSM-5 channels. Compared with that of HZSM-5 (Figure 3B (c)), the absorption bands at 3610 cm1 did not change significantly in intensity, meaning that the acidic sites of framework 16956
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Figure 5. H2-TPR profiles of (a) 2Zn/HZSM-5, (b) 1Ga1Zn/HZSM-5, (c) 1Zn2Mo/HZSM-5, (d) 1Ga2Mo/HZSM-5, and (e) 2Mo/HZSM-5.
Figure 6. CH4 conversion and selectivity of aromatics over the prepared catalysts.
SiOHAl still remained because of the low Zn2+ loadings. The presence of these acidic sites is known to be crucial for the initiation of CC bonds formation in C2-hydrocarbon oligomerization and cyclization via CH3+ cation intermediates.6 NH3-TPD Analysis. The surface acidity of HZSM-5 and those of the catalysts were also investigated by NH3-TPD (Figure 4). The 1Ga2Mo/HZSM-5 catalyst shows desorption at 290 and 506 °C, whereas the 1Ga1Zn/HZSM-5 catalyst exhibits desorption at 154, 193, 335, and 508 °C. According to Lobree et al.,22 the peaks at 110 °C can be ascribed to desorption of physisorbed ammonia, and the one at 200 °C (despite somewhat controversial) can be ascribed to weakly adsorbed NH3 on Bronsted acid sites. Similar to the observation of Tessonnier et al.,23 we assign the peaks at 154162 °C to physisorbed NH3 and the ones at 193-212 °C to desorption of ammonia that was adsorbed on extra-framework aluminum. Tessonnier et al.23 considered that the peaks between 400 and 600 °C were due to desorption of ammonia chemisorbed at Bronsted acid sites. On the basis of the decline of peak area, one can deduce that the number of Bronsted acid sites decrease with the increase in metal loading, especially in the case of loading zinc species (Figures 4c,d). The results indicate that the anchoring of metal species on zeolite surface would result in the consumption of Bronsted acid sites. It is of interest to report that there is a shoulder peak at 300350 °C in the cases of 2Zn/HZSM-5, 1Ga1Zn/HZSM-5, and 1Zn2Mo/ HZSM-5, but not in the cases of 1Ga2Mo/ZSM-5 and 2Mo/ ZSM-5. Tentatively, we attribute the 300350 °C signals to the desorption of adsorbed NH3 on Lewis acid sites, for example, surface zinc species such as Zn(NH3)42+. H2-TPR Analysis of Catalysts. The H2-TPR curves of HZSM5 and those of the catalysts are shown in Figure 5. There is no hydrogen consumption around 850 °C detected over 2Zn/ HZSM-5 and 1Ga1Zn/HZSM-5. According to Liu et al.,15 gallium oxide can be reduced at 350 °C, but we observed no reduction over 1Ga1Zn/HZSM-5. The results indicate that the surface Zn and Ga species on HZSM-5 are hard to reduce. It is plausible that the Zn and Ga species interact strongly with the SiOH groups of HZSM-5. Because of the formation of Si-OZn and Si-OGa species that are structurally stable, it is difficult to reduce the Zn2+ and Ga3+ ions. Over the 1Zn2Mo/HZSM-5, 1Ga2Mo/HZSM-5, and 2Mo/ HZSM-5 catalysts (Figure 5ce), we detected three major reduction peaks. Those in the 400530, 530650, and 650722 °C ranges are attributed to the reduction of dimeric molybdate, isolated MoO3 microcrystallites, and MoO3 crystallites
(to elemental Mo), respectively. One can see that the addition of zinc or gallium species changed the reduction properties of 2Mo/ HZSM-5 (Figure 5c,d). With the introduction of gallium or zinc, the intensity of the peaks in the 650722 °C range declines remarkably, and there are significant changes in peak position in the 400530 and 530650 °C ranges. The results suggest that the presence of Zn or Ga species promotes the dispersion of MoO3 crystallites on the surfaces (inner and outer) of HZSM-5. Activity Evaluation of Catalysts. Methane conversion and aromatic selectivity observed over the catalysts are shown in Figure 6. The CH4 conversion over 1Ga1Zn/HSZM-5 and 2Zn/ HZSM-5 is 7.4 and 5.5%, respectively, and benzene selectivity is very low. The CH4 conversion and aromatic selectivity over 2Mo/HZSM-5, 1Zn2Mo/HZSM-5, and 1Ga2Mo/HZSM-5 are significantly higher, and the addition of zinc or gallium to 2Mo/ HZSM-5 catalyst promotes CH4 activation. In the initial stage, CH4 conversion over 1Ga2Mo/HZSM-5 and 1Zn2Mo/HZSM5 is 33.7 and 36.3%, respectively, significantly higher than that (24.4%) over 2Mo/HZSM-5, which can be attributed to the isolated effect of Ga (or Zn) on MoO3 species and the decline of Bronsted acid sites. After 2.0 h, CH4 conversion over 1Ga2Mo/ HZSM-5 and 1Zn2Mo/HZSM-5 is 22.5 and 24.0%, whereas the aromatic selectivity is 42.7 and 42.8%, respectively. With time on stream, the CH4 conversion and aromatic selectivity over both catalysts decline gradually. CH4 conversion is
