Mixed-matrix membranes permeabilities

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. 

Table I. Comparison of various polymer and mixed matrix membrane permeabilities and permselectivities.

In Eq. (11.1), P is permeability, < z is the volume fraction of the dispersed zeolite, the MMM subscript refers to the mixed-matrix membrane, the P subscript refers to the continuous polymer matrix and the Z subscript refers to the dispersed zeolite. The permeabiUty of the mixed-matrix membrane (Pmmm) can be estimated by this Maxwell model when the permeabilities of the pure polymer (Pp) and the pure zeoUte (Pz), as well as the volume fraction of the zeoUte (< ) are known. The selectivity of the mixed-matrix membrane for two molecules to be separated can be calculated from the Maxwell model predicted permeabiUties of the mixed-matrix membrane for both molecules. 

Zeolite/polymer mixed-matrix membranes prepared from crosslinked polymers and surface-modified zeolite particles offered both outstanding separation properties and swelling resistance for some gas and vapor separations such as purification of natural gas. Hillock and coworkers reported that crosslinked mixed-matrix membranes prepared from modified SSZ-13 zeolite and 1,3-propane diol crosslinked polyimide (6FDA-DAM-DABA) synthesized from 2,2 -feis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride, p-dimethylaminobenzylamine-and 3,5-diaminobenzoic acid displayed high CO2/CH4 selectivities of up to 47 Barrer and CO2 permeabilities of up to 89 Barrer under mixed gas testing conditions [71]. Additionally, these crosslinked mixed-matrix membranes were resistant to CO2 plasticization up to 450 psia (3100kPa). 

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. 

Geong and coworkers reported a new concept for the formation of zeolite/ polymer mixed-matrix reverse osmosis (RO) membranes by interfacial polymerization of mixed-matrix thin films in situ on porous polysulfone (PSF) supports [83]. The mixed-matrix films comprise NaA zeoHte nanoparticles dispersed within 50-200 nm polyamide films. It was found that the surface of the mixed-matrix films was smoother, more hydrophilic and more negatively charged than the surface of the neat polyamide RO membranes. These NaA/polyamide mixed-matrix membranes were tested for a water desalination application. It was demonstrated that the pure water permeability of the mixed-matrix membranes at the highest nanoparticle loadings was nearly doubled over that of the polyamide membranes with equivalent solute rejections. The authors also proved that the micropores of the NaA zeolites played an active role in water permeation and solute rejection. 

Mixed matrix membranes have been prepared from ABS and activated carbons. The membranes are intended for gas separation. A random agglomeration of the carbon particles was observed. A close interfacial contact between the polymeric and filler phases was observed. This morphology between inorganic and organic phases is believed to arise from the partial compatibility of the styrene/butadi-ene chains of the ABS copolymer and the activated carbon structure. A good permeability and selectivity for mixtures of carbon dioxide and methane has been reported (91,92). 

Mixed-matrix membranes have been a subject of research interest for more than 15 years [28-33], The concept is illustrated in Figure 8.10. At relatively low loadings of zeolite particles, permeation occurs by a combination of diffusion through the polymer phase and diffusion through the permeable zeolite particles. The relative permeation rates through the two phases are determined by their permeabilities. At low loadings of zeolite, the effect of the permeable zeolite particles on permeation can be expressed mathematically by the expression shown below, first developed by Maxwell in the 1870s [34],... 

Figure 8.11 Change in membrane permeabilities for mixed-matrix membranes containing different volume fractions of zeolite. Adapted from Robeson et al. [35]...

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). 

Estimation of the interphase thickness and permeability in polymer-zeolite mixed matrix membranes... 

A method for determining the effect of particle size on the effective permeability values of zeolite-polymer mixed matrix membranes has been developed in this study. The model presented is a modified form of the effective medium theory, including the permeability and thickness of an additional phase, the interphase, which is assumed to surround the zeolite particles in the polymer environment. The interphase thickness and permeability values were determined by taking into consideration the assumptions that in case the size of the zeolite particles is held constant, the interphase thickness should be equal for different gases and in case the zeolite particle size is varied, the interphase permeability should remain constant for the same gas. The model seems to fit the experimental permeability data for O2, N2 and CO2 in the silicalite-PDMS mixed matrix membranes well. 

When the polymer permeability becomes too high the selectivity of the mixed-matrix membrane approaches the polymer selectivity. Hence the above equation gives a theoretical estimation of the selectivity of a mixed-matrix membrane and it gives an idea of how the permeability of molecular sieve and poly-... 

Relative permeability (cont./dispersed phase) Fig. 7.9 Selectivity of a mixed-matrix membrane vs. permeability ratio of continuous phase to dispersed phase, selectivity of dispersed phase 35, selectivity of continuous phase 7. 

E. E. Gonzo, M. L. Parentis, J. C. Gottifredi, Estimating models for predicting effective permeability of mixed matrix membranes, J. Membr. ScL, 211, 46-54 (2006). 

Measurements of gas transport in AF 2400 filled by non-porous hydrophobic fumed silica (FS) nanoparticles showed that the inorganic filler enhances gas permeability [2] such an increase is more pronounced for larger penetrants and leads to a lower size selectivity of the mixed matrix membrane with respect to the unloaded polymer. The sorption... 

Numerous models have been developed to describe transport properties in heterogeneous polymer systems, but the one mostly used is the Maxwell approach [89]. The Maxwell equation has been used by Robeson et al. [90] to calculate the gas permeability of block copolymers. Other authors also applied it to mixed matrix membranes, and permeability has been calculated ... 

An important event of recent years in membrane science was the discovery of a new phenomena observed when nano-particles are added into (mainly high permeability) polymer matrix references to these pioneer works can be found in chapters of Section II Nanocomposite (Mixed Matrix) Membranes. So it is not surprising that several presentations at ICOM2008 dealt with such systems. Golename et al. (Chapter 6) investigated the Systran that contained perfiuorinated polymers and surface-fiuorinated zeolites as nano-additives. Perfiuorinated polymer AF2400 with nano-additives was also the object... 

Figure 7.3 Schematic representation of the transport mechanism through mixed matrix membranes with (a) conventional and (b) high aspect ratio permeable particles.

M.M. Khan, V. Fihz, G. Bengtson, S. Shishatskiy, M.M. Rahman, J. LiUepaerg, V. Abetz, Enhanced gas permeability by fabricating mixed matrix membranes of functionalized multiwalled carbon nanotubes and polymers of intrinsic microporosity (PIM), Journal of Membrane Science 436 (2013) 109-120. 

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