Dense films mixed-matrix

Reports on mixed-matrix membranes in the Hterature mainly focus on dense films. Mixed-matrix dense film has a symmetric structure and a thickness of more than 20 tm for most studies. Although dense films are not commercially attractive, they are used to measure the intrinsic separation properties including selectivity and permeability of the mixed-matrix membranes. Therefore, promising polymer and zeolite materials for making asymmetric mixed-matrix membranes for a particular separation can be identified through dense film study. 

In the past 25 years, relatively few attempts to increase gas separation membrane performance with dense film mixed matrices of zeolite and rubbery or glassy polymer have been reported. Table I summarizes practically all of the reported O2/N2 mixed matrix membranes. Permeabilities and permselectivities are specified as a range to encompass the various zeolite volume fractions studied. In general, an increase in permeability is observed with zeolite addition coupled with a slight increase in permselectivity. Despite the wide variety of combinations of zeolites with rubbery and glassy polymers, reported mixed matrix membranes fail to exhibit the desired O2/N2 performance increases. These failures have generally been attributed to defects between the matrix and molecular sieve domains. While this is certainly a possible practical source of failure, our work earlier 8) has addressed a more fundamental source caused by inattention to matching the transport properties of the molecular sieve and polymer matrix domains. This topic is discussed briefly prior to consideration of the practical defect issue noted above. 

Zeolite/polymer mixed-matrix membranes can be fabricated into dense film, asymmetric flat sheet, or asymmetric hollow fiber. Similar to commercial polymer membranes, mixed-matrix membranes need to have an asymmetric membrane geometry with a thin selective skin layer on a porous support layer to be commercially viable. The skin layer should be made from a zeohte/polymer mixed-matrix material to provide the membrane high selectivity, but the non-selective porous support layer can be made from the zeohte/polymer mixed-matrix material, a pure polymer membrane material, or an inorganic membrane material. 

ZeoHte/polymer mixed-matrix membranes have been studied for a number of gas separations such as separation of N2 from air [37, 73, 75, 81, 84, 85], H2 and CO2 removal from natural gas [51, 54, 69, 81, 86-88], CO2 removal from N2 [74], n-pentane/i-pentane separation ]89] and separation of H2 from CO2 [65]. But a majority of the mixed-matrix membranes that have been evaluated for gas separations are mixed-matrix dense films. 

Most reported mixed-matrix research focuses on dense films. Emphasis on procedures to homogeneously disperse solids in polymer solutions followed by casting under controlled evaporation regimes, with various annealing procedures. Initial mixing... 

The dope formulation for dense mixed-matrix films must be revisited for making asymmetric membranes as the structure of the membrane is controlled by phase separation kinetics. Different solvents used in dense film formulation may be used in asymmetric formulations to control the phase separation rate of the membrane within the nonsolvent (quench) bath and obtain membranes of varying porosities." Further, nonsolvents and additives can be added to fine tone the morphology of the membrane to control pore sizes and suppress the formation of large voids within the membrane. """ Asymmetric skinned membranes can also contain a second more volatile solvent that can evaporate and form a high solids concentration layer (skin) on the exposed surface... 

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