Permeability transport, zeolite

A promising route to membranes of improved transport characteristics consists in incorporation of inorganic additives with suitable structure (e.g., zeolites, carbon molecular sieves, microporous silica) into polymer matrix [17]. Some of polymer-inorganic composite materials showed much higher permeabilities but similar or even improved selectivity... 

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. 

Despite rapid advances in polymeric gas separation membrane performance in the 1980 s, recent efforts have yielded only small improvements. Six years ago, the upper bound tradeoff limit between O2 permeability and O2/N2 selectivity was constructed (i), and it still defines the effective performance bounds for conventional soluble polymers. Consequently, an alternate approach to gas separation membrane construction is suggested to exceed current technology performance. Molecular sieves, such as zeolites and carbon molecular sieves (CMS), offer attractive transport properties but are difficult and expensive to process. A hybrid process exploiting the processability of polymers and the superior gas transport properties of molecular sieves may potentially provide enhanced gas separation properties. 

Gas transport in zeolites, some molecular sieve carbons, and polymers is described by a sorption-diffusion mechanism. In these cases, the permeability coefficient, P, of penetrant A is the product of a kinetic parameter, the average diffusion coefficient and a thermodynamic parameter, S, the solubility coefficient (2). 

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. 

Polyaniline (PANI) nanocomposite membranes are also prepared by a sol-gel process, embedding silica in the hydrophilic clusters (Nafion) followed by its deposition by redox polymerization [51]. PANI modified the membrane structure and reduced the methanol crossover, while silica Incorporation improved the conductivity and stability. Zeolite has been incorporated as potential filler for PEMs, either by blending or by infiltration in swelled membrane, to reduce the methanol permeability and enhance the thermal stabihty [52,53]. Although the fuel cell performance of these membranes was Inferior compared with pristine Nafion membrane, incorporation of semipermeable particles is an effective method to engineer the transport properties of composite membranes. Chen et al [54] reported nanocomposite membranes by in situ hydrothermal crystallization method, with similar proton conductivity, but low methanol permeability (40% less) in comparison with Nafion membrane. These membranes showed higher OCV (3%) and power density (21%) than Nafion. 

Cu-BTC (Figure 5b) or ZIF-8 (Figure 6a) and the zeolite silicalite-1 (SIC) were combined in a MMM with polysulfone Udel P-3500 for the separation of CO2/N2, CO2/CH4, O2/N2, and H2/CH4 mixtures. For some of these gas mixtures, the combined-filler MMM showed a synergetic enhancement in selective gas transport when compared either to the pure polymer or to the MMM with only one filler type. All fillers, at the same loading of 16 wt%, increase the CO2 permeability when compared to that of the bare polymer in the two C02-containing mixtures. However, the maximum separation selectivities for CO2/CH4 and CO2/N2 mixtures ( co2/ch4 = 22.4 with P = S.9 barrer for CO2, and UcOj/Nj S.O with P=SA barrer for CO2) were obtained only for Cu-BTC when combined with SIC in a PSF MMM. ... 

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