Catalytic partial oxidation perovskites

In the catalytic partial oxidation of methane to produce syngas the use of permselective dense perovskite membranes avoids (or minimizes) the need of air separation, the most costly step in the process. Although both these O2- and H2-permeoselective membranes (based on perovskites or thin supported Pd-based dense films, respectively) have still to be further developed for commercial applications the outlook appears quite interesting for intensifying various large chemical processes. 

The catalytic partial oxidation (CPO) of methane is an interesting alternative to to the well-established steam reforming (SRM) process for syngas production in small-scale units. However, due to the severe reaction conditions (T = 800-950°C, contact times of few ms) in CPO processes, stable and active catalysts are still required. Several catalytie systems have been used in this process, such as noble metal-based catalysts, metal-based catalysts, metal oxide catalysts and perovskites [1]. In particular, catalysts obtained by the calcination of hydrotalcite-like compounds (HTlcs) have been widely used in the CPO of methane, as they can be easily and cheaply synthesized, with a highly-dispersed... 

One of the most attractive alternatives to steam reforming for syngas production appears to be the catalytic partial oxidation (CPO) catalysed by supported noble metal [9]. Perovskites incorporating the noble metal in the structure represent a valid alternative to reduce the catalyst unit mass cost. 

Mudu, R, Arstad, B. andBakken, E. (2010). Perovskite-type oxide catalysts for low temperature, anaerobic catalytic partial oxidation of methane to syngas, J. Catal., 275, pp. 25-33. 

A detailed discussion of the mathematical models of oxygen flow in ceramic membranes is given elsewhere. Typical materials employed in dense ceramic membranes have a brownmillerite or perovskite structure. The most commonly studied application for this kind of membrane is the catalytic partial oxidation of methane (POM) to obtain synthesis gas,... 

Perovskite oxides are effective catalysts for various reactions, such as CO oxidation,total and partial oxidation of hydrocarbons, NOx decomposition, hydrogenation, " hydrogenolysis, and photocatalysis. Two types of catalytic processes are proposed for the catalysis of perovskites, namely, suprafacial and intrafacial catalysis." During suprafacial catalysis, the reactions between adsorbed species on the surface are much faster than reactions involving lattice oxygen, as in low-temperature oxidation of CO. In this case, the reaction rate appears to be correlated... 

The consideration of thermal effects and non-isothermal conditions is particularly important for reactions for which mass transport through the membrane is activated and, therefore, depends strongly on temperature. This is, typically, the case for dense membranes like, for example, solid oxide membranes, where the molecular transport is due to ionic diffusion. A theoretical study of the partial oxidation of CH4 to synthesis gas in a membrane reactor utilizing a dense solid oxide membrane has been reported by Tsai et al. [5.22, 5.36]. These authors considered the catalytic membrane to consist of three layers a macroporous support layer and a dense perovskite film (Lai.xSrxCoi.yFeyOs.s) permeable only to oxygen on the top of which a porous catalytic layer is placed. To model such a reactor Tsai et al. [5.22, 5.36] developed a two-dimensional model considering the appropriate mass balance equations for the three membrane layers and the two reactor compartments. For the tubeside and shellside the equations were similar to equations (5.1) and... 

Voorhoeve et al. (14,30) have also stressed that the catalytic activity of perovskites is influenced by their stoichiometry. A simple way of varying the oxidation state of the ion at the position B is by substitution of the A ion by a different ion with an oxidation state other than 3. This method has been used by several authors (9, 62, 88, 96, 179-181) to understand the role of the 3orbital occupancy in the LaM03 series on the catalytic oxidation of CO. For M = Co the appearance of Co2+ ions by introduction of Ce4+ in position A enhances the rate of oxidation of CO, whereas the presence of Co4+ ions by substitution with Sr2+ reduces the rate. The explanation for this behavior has been given by assuming that CO is bonded to the transition-metal ion as a carbonyl, as occurs on metals (182), with donation of the carbon lone pair into the empty 3dzi orbital of M to form a cr-bond accompanied by back-donation of the f2g electrons of the metal to the antibonding rr-orbital of CO. It should be noted that the dz2 orbital is the lowest et level for the M3+ ions at the surface, and in order to have a partially empty dzi level, the occupation of all the et levels must be below unity. 

The authors found that the perovskite BCFNO membrane itself possesses a poor catalytic activity to the oxidation of CH4 in COG, which is possibly due to a lower surface area of the dense membrane. However, the conversion increased dramatically when using a catalyst bed on the permeate side of the membrane. In any case, the oxygen partial pressure in the off-gas was zero indicating that all permeated oxygen was used for the partial oxidation reaction. In particular, when the Ni-based catalyst was packed on the membrane surface, a high oxygen permeation flux of 15ml/(cm min), a CH4 conversion of 92%, and a H2... 

Jin, W, Li, S., Huang, R, etal. (2000). TnbnlarLanthanumCobaltite Perovskite-Type Membrane Reactors for Partial Oxidation of Methane to Syngas, J. Membrane Set, 166, pp. 13-22. Sammells, A., Schwartz, M., Mackay, R., etal. (2000). Catalytic Membrane Reactors for Spontaneous Synthesis Gas Production, Catal. Today, 56, pp. 325-328. 

Caro, J., Caspary, K., Hamel, C., etal. (2007). Catalytic Membrane Reactors for Partial Oxidation Using Perovskite Hollow Fiber Membranes and for Partial Hydrogenation Using a Catalytic Membrane Contactor, Ind. Eng. Chem. Res., 46, pp. 2286-2294. 

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