Zeolite-based membranes additives

In all cases, if the cost of the octane number was low (3.1 euros/RON point/ t), addition of a MFI zeolite-based membrane separation unit was not financially justified. Under these conditions, in fact, either the energy consumption related to the vaporization of the feed weighed too heavily on the operating costs (membrane temperature 300 °C), or the margin cleared by the gain in octane number was not sufficient to make the construction of the membrane separation unit profitable, since it was too large (area > 10 000 m ). 

Alumipophosphate molecular sieve membranes. In addition to zeolites, Haag Tsikoyiannis [1991] have also briefly described another type of molecular sieve membranes consisting of AIPO4 units whose aluminum or phosphorous constituent may be substituted by other elements such as silicon or metals. These membranes are made from aluminophosphates, silico-aluminophosphates, metalo-aluminophosphates or metalo-aluminophosphosilicates. Like zeolites, these materials have ordered pore structures that can discriminate molecules based on their molecular dimensions. Their separation and catalytic properties can also be tailored with similar techniques employed for zeolites. The procedures for calcining the membranes or separating them from non-porous subsuates are essentially the same as those described earlier for zeolites. 

Catalysis by zeolites is a rapidly expanding field. Beside their use in acid catalyzed conversions, several additional areas can be identified today which give rise to new catalytic applications of zeolites. Pertinent examples are oxidation and base catalysis on zeolites and related molecular sieves, the use of zeolites for the immobilization of catalytically active guests (i.e., ship-in-the-bottle complexes, chiral guests, enzymes), applications in environmental protection and the development of catalytic zeolite membranes. Selected examples to illustrate these interesting developments are presented and discussed in the paper. 

Besides producing mixed-hydrocarbons (ultra-clean diesel), F-T process can also selectively produce mixed-alcohols (oxygenated fuel). The addition of mixed-alcohols into gasoline can effectively reduce HC and CO emissions. However, before directly used as fuels or blended with conventional fuels, the water content in the as-produced F-T mixed-alcohols must be reduced below 0.5wt.%. This dehydration step is essential but difficult since most of the contained alcohols form azeotropies with water. In our group, we studied the dehydration performance of microwave synthesized NaA zeolite membrane toward F-T produced mixed-alcohols [24, 25]. The membrane also showed excellent pervaporation performance toward dehydration of simulated F-T produced mixed-alcohols. The permeate consisted of only water and little methanol (< 10%) in aU the range of feed composition. This result confirmed that NaA zeolite membrane based pervaporation (or vapor permeation) process could be an effective technology for dehydration of F-T produced mixed-alcohols. 

PI materials have been used for the dehydration of water/alcohol mixtures by the pervaporation method.P84 co-PI hollow fibers and zeolite filled P84 CO-PI membranes " have been used for the pervaporation dehydration of isopropanol. In addition, P84 co-PI based dual-layer hollow fiber membranes serve for the dehydration of tetrafluoropropanol. ... 

Okumus et al. (1994) developed a mixed-matrix polymer-zeolite memljrane for PV. In the preparation of these manbranes, cellulose acetate (CA) as base polymer, AC, or DMF as solvent, and 13X or 4A zeolites as fillers were used. It was observed that the addition of zeolite to the membrane matrix improved the flux value twofold with respect to its homogeneous manbranes with some loss in their selectivity. For example, for a feed concentration of 74% EtOH at SO C and 1 mmHg, the flux value for the unfilled membrane was 0.6 1/m h, and for a 30% zeolite-filled man-brane, the flux was increased to 1.33. For these cases, the selectivities were 7.76 and 5.0 for the unfilled and filled membranes, respectively. 

A quite original approach to selective oxidations using H2O2 is based on polymeric membranes. In one case the polymer (polydimethyl siloxane) is filled with a zeolite which encapsulates an iron complex, i.e. the homogenous catalyst is inunobilized in the membrane. The organic phase and the aqueous phase are fed at opposite sides of the membrane. This system has been used for the oxidation of cyclohexane to cyclohexanol and cyclohexanone, and the products were kept in the organic phase. In addition, Buonomenna used polymeric membranes for the oxidation of benzyl alcohol to benzaldehyde and for the oxidation of cyclohexene, both with H2O2. 

Gas separation membranes combining the desirable gas transport properties of molecular sieving media and the attractive mechanical and low cost properties of polymers are considered. A fundamental analysis of predicted mixed matrix membrane performance based on intrinsic molecular sieve and polymer matrix gas transport properties is discussed. This assists in proper materials selection for the given gas separation. In addition, to explore the practical applications of this concept, this paper describes the experimental incorporation of 4A zeolites and carbon molecular sieves in a Matrimid matrix with subsequent characterization of the gas transport properties. There is a discrepancy between the predicted and the observed permeabilities of O2/N2 in the mixed matrix membranes. This discrepancy is analyzed. Some conclusions are drawn and directions for further investigations are given. 

It is known that zeolite membranes essentially contain intercrystalline non-zeolitic pores (defects). This irregular nature of zeolite membranes with intercrystalline pores adds to the complexity of the transport process in addition to the contribution of a support layer to the permeation resistance. For zeolite membranes, selectivity similar to that expected for Knudsen flow generally indicates the presence of intercrystalline pores. Separation based primarily on adsorption differences, which is generally true in the separation of liquid mixtures by pervaporation, may have tolerance to the intercrystalline pores. However, in order to obtain high perm-selectivity, the zeolite membranes must have negligible amounts of intercrystalline pores and pinholes of larger than 2nm so as to reduce the gas flux from these defects [3]. 

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