Catalytic Membrane Reactors based on Mixed Ionic-Electronic Conductors

摘要

The present work focused on the application of oxygen permeable membranes to the alkane activation at high temperatures. The employed membranes are based on mixed oxides with the perovskite crystalline structure and present mixed ionic-electronic conductivity. The synthesized and characterized membranes consist of ferrite-cobaltites with different alkali-earth and lanthanide metals. The oxygen separation from air was studied using argon and methane as sweep gas in the temperature range from 700 to 1000℃. It was possible to distinguish two different permeation regimes: (ⅰ) bulk diffusion limited at high temperatures; and (ⅱ) surface exchange limited at lower temperatures. The membranes were used as catalytic membrane reactor for the conversion of alkanes and two different reactions were studied: (ⅰ) oxidative methane coupling and (ⅱ) oxidative dehydrogenation of ethane.rnIn this context, dense ionic oxygen conducting membrane reactors are highly attractive solutions for syngas production, oxidative dehydrogenation of ethane (ODHE) and oxidative coupling of methane (OCM), where both, separation and reaction are integrated in the same unit [1-5].rnThe oxygen permeation experiments and the catalytic test were performed in the same experimental set-up. Synthetic air was fed to the oxygen-rich membrane compartment, while argon was used as sweep gas on the permeate side. Permeate was analyzed by on-line gas chromatography using micro-GC Varian CP-4900 equipped with Molsieve5A, Pora-Plot-Q glass capillary, and CP-Sil modules. Membrane gas leak free conditions were ensured by monitoring nitrogen concentration on the products gas stream. The membrane formulation was base on MIEC (perovskites). Different catalyst formulations were considered, varying the element in the A-site of perovskite structure (A_(0.58)Sr_(0.4)Fe_(0.8)Co_(0.2)O_(3-δ) [6]. The perovskite-based compounds were synthesized by EDTA-citrate complexation route [7]. The structural characterization of the perovskite powders and MIEC membrane was performed by XRD and SEM.rnOxygen permeation from the air to the reaction side is a key aspect in oxidation reactions, determining the performance of the catalytic membrane reactor. From the oxygen content measured in the argon sweep side of the permeation assemble and the argon flow rate, the total oxygen permeation rate (mL min~(-1) cm~(-2)) was calculated.