Mixed oxide membrane

First, the thermal stabilities of the membranes are improved by the addition of other oxide(s). Specifically, the mixed oxide membranes will retain their pore sizes and porosities when heated to a temperature at least about lOO C higher than the temperatures which would degrade the pore size and porosity of single oxide membranes. TiOz-ZtOz membranes with varying compositions have been proven to be the case. Moreover, other mixed oxide membranes containing Ti-Nb and Ti-V [Anderson et al., 1993] and AbBa and Al-La [Chai et al., 1994] are additional examples. 

In principle, mixed oxide membranes can be prepared by mixing of the individual sols or co-hydrolysis or co-polymerization of respective alkoxides. Mixing of different sols is often used to prepare mixed oxide membranes. For example, a sol made from hydrolyzing Si(OC2Hs)4 is mixed with another sol from the hydrolysis of 2i(OCiHrj)4, The resulting mixed sols can then be used to prepare water- and alkali-resistant porous glass membranes. 

The permeation flux of oxygen through a mixed oxide membrane described above depends on the oxygen partial pressures across the membrane, membrane thickness and temperature. The dependence, however, is embedded in a complicated implicit equation [Lin et al., 1994]. Only in special cases the permeation Oux shows a pressure dependence similar to that for palladium membranes as given in Eq. (4-10). For example, when electronic conductivity predominates, the value of the exponent, n, is equal to 0.5 for thin membranes and 0.25 [Dou et al., 1985 Itoh et al., 1993] for thick oxide films. If the oxide membrane is essentially an ionic conductor and the surface reaction is the rate-limiting step, n takes on a value of 0.5. 

Bowen, R., and Mukhtar, H. (1993). Properties of microfiltration membranes The surface electrochemistry of mixed oxide membranes. Colloids Surf. A 81, 93. 

Catalytic testings have been performed using the same rig and a conventional fixed-bed placed in the inner volume of the tubular membrane. The catalyst for isobutane dehydrogenation [9] was a Pt-based solid and sweep gas was used as indicated in Fig. 2. For propane oxidative dehydrogenation a V-Mg-0 mixed oxide [10] was used and the membrane separates oxygen and propane (the hydrocarbon being introduced in the inner part of the reactor). 

It is well known that dense ceramic membranes made of the mixture of ionic and electron conductors are permeable to oxygen at elevated temperatures. For example, perovskite-type oxides (e.g., La-Sr-Fe-Co, Sr-Fe-Co, and Ba-Sr-Co-Fe-based mixed oxide systems) are good oxygen-permeable ceramics. Figure 2.11 depicts a conceptual design of an oxygen membrane reactor equipped with an OPM. A detail of the ceramic membrane wall... 

Fig. 3. Oxygen transport in solids. 02 is dissociated and ionized at the reduction interface to give O2 ions, which are transferred across the solid to the oxidation interface, at which they lose the electrons to return back to 02 molecules that are released to the stream, (a) In the solid electrolyte cell based on a classical solid electrolyte, the ionic oxygen transport requires electrodes and external circuitry to transfer the electrons from the oxidation interface to the reduction interface (b) in the mixed conducting oxide membrane, the ionic oxygen transport does not require electrodes and external circuitry to transfer the electrons to the reduction interface from the oxidation interface, because the mixed conductor oxide provides high conductivities for both oxygen ions and electrons.

In the early 1990s, Balachandran et al. (51,64,65) of the Argonne National Laboratory, in collaboration with Amoco (now part of BP), investigated the partial oxidation of methane using membrane materials consisting of Sr-Fe-Co-O mixed oxides with the perovskite structure, which have high oxygen permeabilities. In their experiments (51,66), the membrane tubes, which were... 

Current polymeric materials are inadequate to fully meet all requirements for the various different types of membranes (cf. Section 2.2) or to exploit the new opportunities for application of membranes. Mixed-matrix membranes, comprising inorganic materials (e.g., metal oxide, zeolite, metal or carbon particles) embedded in an organic polymer matrix, have been developed to improve the performance by synergistic combinations of the properties of both components. Such improvement is either with respect to separation performance (higher selectivity or permeability) or with respect to membrane stability (mechanical, thermal or chemical). 

Lin, Y. S., Wang, W., and Han, J. (1994). Oxygen permeation through thin mixed-conducting solid oxide membranes, AIChE J. 40(5), 786. 

Related work includes investigations of carbon formation during hydrogenation of C5 hydrocarbons catalyzed by nickel and palladium (5P) interactions of N2O with a hydrotalcite-derived multimetallic mixed oxide catalysts (60,61) changes in mass of solid oxides (62) methanol sorption in Nafion-117 (proton-exchange) membranes (63) vanadyl pyrophosphate catalysts for butane oxidation (64-66) and deactivation/regeneration of a Rb0 c/Si02 catalyst for methylene valerolactone synthesis (67). 

Mixed metai oxide membranes. Mixed metal oxide membranes are very interesting because potentially they have enhanced properties over those properties of single oxide... 

Another major reason for studying mixed metal oxide membranes from double metal alkoxides is the potential for preparing zeolite>like membranes which can exhibit not only separation but also catalytic properties. It has been suggested that combinations of silica and alumina in a membrane could impart properties similar to those of natural and synthetic zeolites [Anderson and Chu, 1993]. Membranes with a pore diameter of 10 to 20 nm and consisting of combinations of titania, alumina and silica have been demonstrated by using a mixture of a meta>titanic acid sol, an alumina sol and silicic acid fine particles followed by calcining at a temperature of 500 to 900 C [Mitsubishi Heavy Ind., 1984d]. 

Some selective double alkoxides have been synthesized and can be utilized to prepare mixed metal oxide membranes. Preparation and stabilities of double or multiple metal alkoxides are sometimes challenging. Aluminum alkoxides are particularly interesting as they are reactive with other metal alkoxides [Bradley et al., 1978]. Some alkoxides... 

Some dense inorganic membranes made of metals and metal oxides are oxygen specific. Notable ones include silver, zirconia stabilized by yttria or calcia, lead oxide, perovskite-type oxides and some mixed oxides such as yttria stabilized titania-zirconia. Their usage as a membrane reactor is profiled in Table 8.4 for a number of reactions decomposition of carbon dioxide to form carbon monoxide and oxygen, oxidation of ammonia to nitrogen and nitrous oxide, oxidation of methane to syngas and oxidative coupling of methane to form C2 hydrocarbons, and oxidation of other hydrocarbons such as ethylene, methanol, ethanol, propylene and butene. 

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