КИНЕТИКА И КАТАЛИЗ, 2009, том 50, № 5, с. 712-717
УДК 541.128.3:542.948:547.212
Variations of Structure and Active Species in Mesoporous Cr-MSU-x Catalyst during the Dehydrogenation of Ethane with CO2 © 2009 г. L.-Ch. Liu, H.-Q. Li*, Y. Zhang*
Department of Chemistry and Chemical Engineering, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100022, China *Institute of Process Engineering, Chinese Academy of Sciences, P.O. Box 353, Beijing 100080, China
E-mail: lcliu@bjut.edu.cn Поступила в редакцию 21.12.2007
The dehydrogenation of ethane to ethylene under CO2 over mesoporous Cr-MSU catalyst was investigated with respect to the time on-stream behavior. When ethane was allowed to react for about 240 minutes, the meso-structure of catalyst remained nearly unchanged in spite of some decrease of surface area. The Cr(VI) species in tetrahedral coordination in fresh Cr-MSU were reduced to Cr(III) species in octahedral coordination, that was expected to cause the activity decrease of catalyst, together with the structure change. Cr(VI) is more active than Cr(III) for ethane dehydrogenation with CO2, but Cr(III) represent fairly stable active centers for the reaction.
INTRODUCTION
The dehydrogenation of ethane to ethylene in the presence of CO2 has been studied extensively [1—7]. The introduction of CO2 could reduce the extent of deep oxidation which results in many by-products whereas ethylene selectivity drops when oxygen is used as oxidant. Moreover, by using greenhouse gas CO2 as a soft oxidant it is possible to realize the recycle. From the results of many researchers, chromium oxide has been proved to be one of the most active catalysts for this reaction possibly due to its multi-valency [2—9].
MSU-x family, which structurally does not shows the long-range ordering, is an important silica-based mesoporous molecular sieve. Besides high surface area and adjustable uniform pore diameter, three dimensional wormlike channels of MSU are more favorable for the diffusion of molecular objects. Therefore, MSU is a promising catalyst support [10—12]. The incorporation of chromium into the MSU framework is expected to endow catalysts with high activity. In a previous study were reported the synthesis and catalytic performance of this Cr incorporated MSU (Cr-MSU) catalyst [13, 14]. The synthesized Cr-MSU catalyst exhibited higher activity and ethylene selectivity than many other catalysts in the dehydrogena-tion of ethane under CO2. However, deactivation of Cr-MSU to a certain extent was observed simultaneously with reaction proceeding.
The activity of heterogeneous catalyst highly depends on the dispersion and oxidation state of active components as well as on the structural features of the support. Therefore, to investigate the structural transformation of Cr-MSU and redox behavior of active Cr species in Cr-MSU catalyst during corresponding re-
action is very important for elucidating the active sites, the reaction pathways and deactivation mechanism. Some researchers considered that coordinatively unsaturated Cr3+ ions are active sites for alkane dehydrogenation [15]. Such ions can attempt to restore the stable configuration of bulk Cr3+ ions by capturing molecules from gas phase. Mimura et al. [4] investigated the catalytic performance of Cr/HZSM-5 catalyst for oxidative dehydrogenation of ethane to ethylene with CO2 as an oxidant. From TPR profiles of the Cr/HZSM-5 catalysts it can be inferred that, however, high oxidation states Cr species such as Cr(VI) is considered to be a key factor of higher activity.
The Cr-MSU prepared with Si/Cr molar ratio of 20, aging at 25°C for 22 h, with fatty alcohol polyoxy-ethylene ether as a template gave relatively high activities. In the present work, this Cr-MSU was selected as a typical catalyst to study its structure and oxidation state before and after reaction. Several techniques such as XRD, N2-adsorption, XANES, DR UV-vis and H2-TPR were applied to characterize fresh and used catalysts. The dehydrogenation of ethane to eth-ylene over active chromium species is discussed based on characterization results.
EXPERIMENTAL
Catalyst Preparation
The Cr-MSU catalyst was synthesized according to our previous report [13]. Sodium silicate, chromium nitrate, fatty alcohol polyoxyethylene ether (A(EO)9) were used as the source of silicone, metal and mesos-tructure-directing agent, respectively. The Si/Cr molar ratio was fixed to 20. The aging time and temperature
were set to 22 hours and 25°C. The actual chromium content was about 0.8 wt% (Inductively Coupled Plasma-Optical Emission Spectrometer, Perkin-Elmer).
curve (DTG) was also presented. Approximate 20 mg of the sample was heated in air flow (100 ml/min) at a heating rate of 10°C/min.
Catalyst Characterization
The reaction of CO2 and C2H6 over Cr-MSU was performed at atmospheric pressure in a fixed bed quartz reactor (5.0 mm i.d., 44.0 cm long) at 550~700°C. The reactants consisted were introduced with rates of 9 ml/min (CO2) and 3 ml/min (C2H6). Catalyst loading was 0.2 g. Argon gas was introduced at a flow rate of 9 ml/min as balance gas when reaction was performed without CO2. The products were analyzed on line by a gas chromatograph equipped with Porapack QS column (3 m, 01/4) and a thermal conductivity detector (Shimadzu GC 14B). The catalyst was collected and characterized when reaction was terminated after 240 min, it is designated as used Cr-MSU catalyst.
X-ray diffraction (XRD) measurements were performed on X'pert Pro MPD X-ray diffractometer from Panalytical with CuZ"a radiation (X = 0.154187 nm), Generator Settings 40 kV, 30 mA, scanning speed at 0.017° and scanning regions at 0.5—6.0°. N2 adsorption/desorption isotherms were determined with Autosorb series ASIMP apparatus from Quantachrome. The samples were pretreated at 30°C under vacuum for 5 h before measurements. Calculation of the specific surface area (BET), pore volume and average pore size (BJH method) was made with the software of the apparatus.
X-ray absorption spectroscopic measurements were conducted with synchrotron radiation at the National Synchrotron Radiation Laboratory (NSRL) (Heifei city, China). The synchrotron radiation facility is mainly composed of an 800 MeV electron storage ring with about 100—300 mA of ring current. The data were recorded in X-ray fluorescence mode at room temperature using a Si(111) double crystal monochro-mator. The energy step of measurement in the XANES region was 0.7 eV.
Diffuse Reflectance UV-vis spectroscopic (DR UV-vis) measurements were recorded on a UV2100 spectrometer. The spectra were collected at 200—700 nm referenced to BaSO4.
The H2-TPR of the catalysts was performed on CHEMBET3000 chemical adsorption apparatus from Quantachrome by using a mixture of 5 vol% H2/Ar as the reducing gas with a total flow rate of 20 ml/min. The 50 mg sample was heated from room temperature to 800°C at a heating rate of16°C/min after being pretreated at 300°C for 60 min in He gas flow. The exit gas was cooled by mixture of n-octane and liquid nitrogen to condense the water generated from reduction of catalyst. The reduction signal was recorded by a TCD.
A Netzsch STA 499 TG/DTA instrument was used to obtain TG profiles. For clarity, the differential TG
RESULTS AND DISCUSSIONS
Catalytic Activities
The catalytic properties of Cr-MSU were investigated in the dehydrogenation of ethane to ethylene with CO2. It is seen from Fig. 1 that the conversion of ethane increases while selectivity to ethylene decreases with the increase of the reaction temperature from 550 to 700°C. According to our measurements, the decrease in selectivity to ethylene is accompanied by increasing selectivity to methane which serves as the main by-product. The yield to ethylene increases with temperature, which is consistent with the change of ethane conversion. Selectivity to ethylene is always above 90% at all investigated temperatures. When CO2 in feed was substituted for argon, the conversion of ethane was reduced by 4— 13 percent compared to the results under CO2 at the same temperature. It suggested that CO2 introduction enhanced ethane conversion to a certain extent. The catalytic performance over Cr-MSU with respect to time on stream is shown in Fig. 2. As reaction proceeds, the conversions of ethane and CO2 decrease, indicating partial deactivation of catalyst. The conversion of ethane decreases from 58.0 to 37.6% in 3 hours. The selectivity to ethylene gives little change.
Low-angle XRD and N2 Adsorption
The low angle XRD patterns of fresh and used Cr-MSU catalyst are shown in Fig. 3. The fresh catalyst gives a clear diffraction peak at about 29 = 2° corresponding to d100 diffraction, suggesting the uniform me-so-channels in catalyst [10, 17]. This characteristic peak still exists but becomes less intense for used catalyst. It is concluded that the meso-structure of Cr-MSU is preserved in spite of local destruction.
Fig. 4 shows N2 adsorption-desorption isotherms and pore size distribution curves for fresh and used Cr-MSU catalyst. There is little difference in pore size distribution between fresh and used catalysts. N2 adsorption volume decreases for used catalyst. Based on XRD and N2 adsorption results, some textural properties and structural parameters of fresh and used Cr-MSU were calculated and listed in table. As can be seen from the data the surface area decreases from 941 to 784 m2/g, the average pore diameter remains almost unchanged while the pore volume decreases from 0.63 to 0.52 cm3/g for the used Cr-MSU catalyst. The conclusion can be drawn that as the reaction proceeds local structure destroys and lattice shrinkage takes place for Cr-MSU resulting in the decline of surface area and pore volume of Cr-MSU. However, according to the results in Fig. 3 and table the major part of the me-soporous structure of used catalyst persists. The col-
Activities, % 100
95 90 60 50 40 30
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