Ind. Eng. Chem. Res. 2007, 46, 8665-8673
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Feasibility Study of the Oxidative Dehydrogenation of Ethane in an Electrochemical Packed-Bed Membrane Reactor Lyubomir Chalakov,† Liisa K. Rihko-Struckmann,*,‡ Barbara Munder,‡ and Kai Sundmacher†,‡ Process Systems Engineering, Otto-Von-Guericke UniVersity Magdeburg, UniVersita¨tsplatz 2, D-39106 Magdeburg, Germany, and Max Planck Institute for Dynamics of Complex Technical Systems, Sandtorstrasse 1, D-39106 Magdeburg, Germany
The feasibility of the oxidative dehydrogenation of ethane to ethylene with alumina-supported vanadium oxide catalyst (VOx/γ-Al2O3) in an electrochemical packed-bed membrane reactor was investigated at temperatures between 500 and 620 °C with molar ratios of oxygen to ethane of 0.06-3.10. An oxygen-ionconducting yttria-stabilized zirconia (YSZ) membrane was employed in a reactor of Au|YSZ|Pt, and the oxygen flux transferred across the membrane was controlled over the faradic coupling of oxygen-ion conduction and the external current between the electrodes. The oxidative dehydrogenation of ethane with electrochemically supplied oxygen in the membrane reactor was compared to that obtained with gaseous dioxygen in a conventional packed-bed reactor. The selectivity to ethylene was found to decrease as a function of supplied oxygen in both investigated operating modes. For all investigated oxygen/ethane molar ratios, the selectivity ratio, SCO2/SCO, was found to be clearly higher in the electrochemical than in the packed-bed reactor mode. The electrochemical oxygen supply significantly promoted CO2 formation, whereas the ethane conversion and ethylene selectivity were almost equal in the two investigated reactors. The experimental results indicate that, in the electrochemical operation, additional oxygen species exist in the system and are especially reactive in the total oxidation reactions. Introduction Aside from methane, ethane is a major component of natural gas and, therefore, a potential source of various chemicals such as light olefins for polyolefin production, oxygenates, and aromatic hydrocarbons. Currently, one industrially important production route utilizing ethane is homogeneously catalyzed oxidation producing acetaldehyde and acetic acid. The production of polyethylene consumes large amounts of ethylene, which is typically obtained by the steam cracking of heavy oil fractions at temperatures above 900 °C. However, the steam cracking process is energy-intensive, and coke formation is a severe problem requiring frequent shutdowns and catalyst regeneration steps. A high wall temperature is necessary, as coke deposits on the inner wall of the reactors limits heat transfer. Direct ethane oxidative dehydrogenation (ODH) as a possible production route for ethylene would offer considerable advantages compared to the industrial processes mentioned above. Coke formation might be less problematic, and the energy balance is favorable, as oxidation reactions are exothermic. Research interest in ODH has increased continuously, but ethane ODH still requires further optimization to become competitive with the existing industrial processes. As one of the most important factors affecting a chemical reaction system, catalyst selection strongly influences the system efficiency. Three types of active components have been successfully investigated in the ODH of ethane: reducible nonnoble transition metal oxides, non-reducible metal oxides, and reducible noble transition metals.1-3 Furthermore, Lo`pez Nieto and co-workers4 successfully tested promoted VPO catalysts for the ethane ODH process, achieving ethylene selectivities of * To whom correspondence should be addressed. Tel.: +49 391 6110318. Fax: +49 391 6110566. E-mail: rihko@ mpi-magdeburg.mpg.de. † Otto-von-Guericke University Magdeburg. ‡ Max Planck Institute for Dynamics of Complex Technical Systems.
about 80% at ca. 10% ethane conversion. Gaab et al.5 performed ODH of ethane in a fixed-bed reactor with ethylene yields of up to 77% using chlorinated Li/Dy/Mg mixed-oxide catalysts. Bodke et al.6 obtained over 70% conversion at a selectivity to ethylene of 85% in the ODH of ethane by adding large amounts of H2 to the reaction mixture and using a Pt-Sn catalyst operating at 950 °C. Several types of membrane reactors with controlled oxygen supplies have been investigated for the ODH of ethane to ethylene. By using membrane reactors, one attempts to increase the selectivity to the desired intermediates and suppress the total oxidation.7 Klose et al.8 obtained a maximal yield of 33% with a selectivity of approximately 65% in a packed-bed membrane reactor (PBMR) with an inert, porous ceramic alumina composite membrane over a VOx/γ-Al2O3 (1.4% V) catalyst. Coronas et al.9 tested a Li/MgO catalyst in a similar configuration, and Tonkovich et al.10 used a tubular one-end-closed porous R-alumina membrane packed with Li-Sa oxide MgO in the annular region. Instead of porous membranes, Rebeilleau-Dassonneville et al.11 and Wang et al.12,13 used oxygen-permeable mixed ionand electron-conducting Ba0.5Sr0.5Co0.8Fe0.2O3-δ membranes at temperatures between 700 and 850 °C. At 800 °C, wang et al.13 reached 80% ethylene selectivity at 84% ethane conversion. A selectivity of nearly 90% at an identical ethane conversion was achieved at the same temperature with Pd nanocluster and V/MgO micron grain modified membranes by RebeilleauDassonneville et al.11 Akin et al.14 studied the selective oxidation of ethane to ethylene in a dense tubular membrane reactor with Bi1.5Y0.3Sm0.2O3 membranes at temperatures of 825-875 °C. They obtained about 80% ethylene selectivity at 70% ethane conversion at 875 °C. Oxygen ion-conducting dense membranes, e.g., yttriastabilized zirconia (YSZ), have also been employed in electrochemical membrane reactors (EMRs).15 Using a Au|YSZ|Ag cell system, York et al.16 converted light C2-C4 alkanes into
10.1021/ie070089i CCC: $37.00 © 2007 American Chemical Society Published on Web 05/23/2007
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various oxygenates with electrochemically supplied oxygen at 425-475 °C. Hamakawa et al.17 studied the partial oxidation of ethane to acetaldehyde in a similar reactor at 475 °C. They observed that, when gas-phase molecular dioxygen was available, carbon oxides were the only product. Takehira et al.18 used an EMR configuration as well, but with a MoO3 or V2O5 catalyst film on the Au anode. At 500 °C under an electrochemical supply of oxygen, ethane was not noticeably oxidized with V2O5. Only alkenes were partially oxidized over both catalysts. To our knowledge, the ODH of ethane to ethylene has not yet been investigated in an EMR. In the present study, the ODH of ethane to ethylene over a VOx/γ-Al2O3 catalyst with a Au|YSZ|Pt tubular electrochemical packed-bed membrane reactor is experimentally demonstrated. The oxygen flux across the YSZ is controlled by an externally applied electric potential. The results with the electrochemical membrane operation are compared to those obtained using gaseous dioxygen in the reactor. Experimental Section Reactor Preparation. The main part of the EMR was a gastight tubular ceramic membrane with one end closed (Viking Chemicals, Follenslev, Denmark) made of 13 mol % Y2O3stabilized ZrO2 (YSZ). The length of the reactor was 300 mm, the inner diameter was 11.2 mm, and the wall thickness was 0.5 mm. Initially, the tubular membrane was cleaned with distilled water and acetone and heated to 900 °C for 4 h. The cathode was prepared by brushing platinum paste layer (Chempur no. 900487, Karlsruhe, Germany) onto the inner surface of the tube. After being dried at 100 °C for 6 h, the prepared electrode was calcined at 850 °C for 2 h. A 350-mm-long fourcapillary Al2O3 tube (o.d. 10 mm), wrapped with a Pt gauze (Chempur no. 900338), was inserted into the tubular YSZ membrane, and its capillaries were used for the isolation of the electric current carrier (Pt wire, Ø 0.125 mm, Chempur no. 009342), a type S thermocouple (Pt 10% Rh-Pt), and an air supply conduit for the cathode. The capillary tube was pressed tightly onto the YSZ tube to guarantee good contact between the cathodic paste electrode and the current collector (Pt gauze). Gold paste (Chempur no. 902904) was applied to the outer surface of the tubular YSZ membrane, and it acted as a porous current collector at the anode. The electrode area was 25.7 cm2. After being dried at 100 °C for 6 h, the gold electrode was calcined at 800 °C for 2 h. Figure 1shows a schematic illustration of the construction of the prepared EMR. Catalyst Preparation and Characteristics. The VOx/γAl2O3 catalyst was prepared by wet impregnation of γ-alumina balls (d ) 1.8 mm, Condea Chemie, Hamburg, Germany) with a solution of vanadyl acetylacetonate in acetone, followed by calcination at 700 °C for 4 h. It contained 1.44 mol % V (analyzed after microwave extraction in HNO3 with a Varian Spectra A 250 plus AAS instrument). The BET surface area of the catalyst was 168 m2/g, and the average pore diameter was 10.9 nm, as measured by nitrogen adsorption at 77 K (ASAP 2010 instrument from Micromeritics GmbH, Mo¨nchengladbach, Germany). A small portion of 16 m2/g micropores ( 580 °C, minor H2 formation was observed (close to the detection limit of the GC). In parallel with the traces of H2, a small amount of CH4 (compared to the amounts of the main products C2H4, CO, and CO2) was detected. This indicates the occurrence of the direct dehydrogenation and thermal cracking of the ethane feedstock. The higher ethylene selectivity achieved in PBR mode at lower temperatures does not coincide with the experimental data obtained for ethane ODH studies in porous membrane reactors.10,20 As the oxygen feed is distributed evenly along the whole length in a membrane reactor and in such a way that the local oxygen partial pressure in the catalytic layer is lowered, one would expect that total oxidation is not favored. However, in our investigations, EMR operation was found to enhance the CO2 selectivity and decrease the CO selectivity compared to PBR mode at all investigated temperatures, as seen in Figure 4c,d. Compared to PBR mode, where the oxygen on the anode exists only in the form of O2, during oxygen ionic pumping in EMR operating mode, several additional oxygen species might also be present there, such as O2-, O2-, O22-, O-, O(adsorbed), etc. Their role in the hydrocarbon oxidation process is as yet unclear. According to Bielanski et al.,21 the electrophilic oxygen species are very active for total oxidation reactions. While examining partial butane oxidation in EMR mode, Ye et al.22 concluded that the electrochemically supplied oxygen was more reactive in producing CO2. The same trend was observed in our investigations as well. To clarify the possible conversion of CO and C2H4 to CO2 on the electrode (Au) surface, we investigated the oxidation of the intermediate products in PBR and EMR modes in the absence of catalyst at temperatures between 500 and 620 °C. As expected, CO was overoxidized to CO2 at CO conversions of
