Matrimid selectivity

Nistor, C., Shishatskiy, S., Popa, M. and Nunes, S.P. 2008. Composite membranes with cross-linked Matrimid selective layer for gas separation. Environ. Eng. Manage. J. 7 653-659. 

Kharitonov et al. [59] have shown that direct fluorination of the polyimide Matrimid is possible, hence the resulting membrane should have a nice potential for use in harsh environment. Perfluorinated materials were also studied by Hagg [60] for chlorine gas purification, and were shown to be exceptionally stable in these harsh environments. The selectivity was however too low. In a later publication on chlorine purification [31] it was suggested to use perfluorinated monomers as surface-modifying compounds for pore tailoring of glass membranes for chlorine gas separation. 

Glassy polymeric materials are often plasticized when used in gas membranes due to sorption. This can be overcome by annealing or crosshnking, however, this method does not influence the selectivity of the membrane, instead the permeability is decreased. Another method to stabilize the plasticization is to use polymer blends, as demonstrated with Matrimid 5218 and a copoly(imide) P84. The material is stabilized against carbon dioxide plasticization and the selectivity for a mixture of carbon dioxide and methane is improved. Hollow fiber membranes composed of blends of Pis with enhanced resistance towards hydrocarbons have been developed. ... 

In order to compare the performance for polymeric and carbon membranes, Figure 15.13 shows a CO2/CH4 trade-offline for P84 and Matrimid precursors and their carbon membranes as reported by Tin et al It is clear that carbon membranes possess excellent permeation properties, where both of the permeability and ideal selectivity access the Robeson upper-bound curve. Moreover, some researchers have also investigated the influence of temperature on the gas permeability.They concluded that the gas permeability values increased with the increase of temperature due to the activated process for the CMS membranes. They also found that the apparent activation energies for CO2 calculated from the Arrhenius equation Pe = Peo Qxp(-EJRT)) was much smaller than the other gas species of O2, N2 and CH4, thereby indicating that CO2 has much higher permeability. 

Hybrid membrane or MMM using MOFs as the filler material is another option for the application of MOFs in membrane separation. Adams et al. [118] reported an MMM comprised of poly (vinyl acetate) (PVAc) and a MOF composed of copper and bdc ligand (Cu-bdc), which exhibited an increased selectivity for many gases, including CO2 upon inclusion of the MOF compared with the pure PVAc membrane. Ordonez et al. reported the ZIF-based polymer MMM using ZIF-8 as the filler phase and Matrimid as the polymer phase, respectively as shown in... 

Fig. 3.10. Pure Matrimid exhibited an ideal selectivity for CO2/CH4 about 43, while at 50 % loading of the ZIF-8 crystals, the ideal selectivity of the MMM increased to 124 [119]. Pure Matrimid exhibited an adsorption selectivity of 42 for CO2 over CH4 (gas mixture C02/CH4= 10 9). At 50 % loading, the selectivity of the MMM increased to 89.

In EP07708077A3 (Dabou et al. 1996), gas separation polymer membranes were prepared from mixtures of a polysulfone, Udel P-1700 and an aromatic polyimide, Matrimid 5218. The two polymers were proven to be completely miscible as confirmed by optical microscopy, glass transition temperature values and spectroscopy analysis of the prepared mixtures. This complete miscibility allowed for the preparation of both symmetric and asymmetric blend membranes in any proportion from 1 to 99 wt% of polysulfone and polyimide. The blend membranes showed significant permeability improvements, compared to the pure polyimides, with a minor change in the selectivity. Blend membranes were also considerably more resistant to plasticization compared with pure polyimides. This work showed the use of polysulfone-polyimide polymer blends for the preparation of gas separation membranes for applications in the separation of industrial gases. 

A commercially available polyimide, Matrimid 5218, exhibits a combination of selectivity and permeability for industrially significant gas pairs superior to that of most other readily available polymers. Its permeation properties, combined with its processability (i.e., solubility in common solvents) makes it an ideal candidate for gas separation applications. 

Another possible way to improve the performance of a membrane relies on increasing flux for a given permeability and selectivity by simply reducing the thickness of the active layer. An example on how AFM can help in this research program is worthy of comment here. This is the procedure to make Matrimid asymmetric membranes. 

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. 

Polymer matrix selection determines minimum membrane performance while molecular sieve addition can only improve membrane selectivity in the absence of defects. Intrinsically, the matrix polymer selected must provide industrially acceptable performance. For example, a mixed matrix membrane using silicone rubber could exhibit properties similar to intrinsic silicone rubber properties, O2 permeability of 933 Baiters and O2/N2 permselectivity of 2.1 (8). The resulting mixed matrix membrane properties would lie substantially below the upper boimd trade-off curve for gas permeability and selectivity. In contrast, a polymer exhibiting economically acceptable permeability and selectivity is a likely candidate for a successful polymer matrix. A glassy polymer such as Matrimid polyimide (PI) is an example of such a material because it exhibits acceptable properties and current technology exists for formation of asymmetric hollow fibers for gas separation (10). 

The results of the investigation are reported in Table II. The CMS-Matrimid and Zeolite 4A-Matrimid membranes give selectivities approaching those of the native polymer at best. Also, much higher permeabilities were observed than predicted by the model for both carbons molecular sieves and zeolites. These results suggest that there was improper contact between the two phases, probably due to dewetting of... 

Incorporation of the MOF 4,4 -bipyridine-hexafluorosilicate-copper(II) (Cu-BPY-HFS) " (Figure 5a) into a Matrimid 5218 membrane increased the gas permeabilities and selectivities for pure gases (from single-gas experiments) and gas mixtures of CH4/N2 but decreased the ideal CO2/CH4 and H2/CH4 selectivities. ... 

An asymmetric membrane from MIL-53(A1) (Figure 7b) (or Cu-BTC or ZIF-8, see above and Table 1) and Matrimid 9725 showed a higher CO2 permeability than the unfilled membrane in mixed gas permeation experiments. Preferentially, permeation of CO2 increased with the filler loading for all thre MOFs in the binary gas mixtures CO2/CH4 and CO2/N2 with CO2 concentrations from 10 to 75vol.%. Yet, the CO2 selectivity increased only slightly in the case of Cu-BTC or MIL-53(A1) and remained almost constant for ZIF-8. Under the same conditions, the CO2 permeability remained invariant with the type of MOF for the CO2/N2 gas mixture. The CO2/CH4 selectivity acoj/cn, dropped linearly when the CO2 fraction increased from 10 to 75vol.% for all three MOFs. The decrease in selectivity was essentially independent of the MOF filler content. 

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