Over the past two decades, membrane processes have gained a lot of attention for the separation of gases. They have been found to be very suitable for wide scale applications owing to their reasonable cost, good selectivity and easily engineered modules. This critical review primarily focuses on the various aspects of membrane processes related to the separation of biogas, more in specific CO(2) and H(2)S removal from CH(4) and H(2) streams. Considering the limitations of inorganic materials for membranes, the present review will only focus on work done with polymeric materials. An overview on the performance of commercial membranes and lab-made membranes highlighting the problems associated with their applications will be given first. The development studies carried out to enhance the performance of membranes for gas separation will be discussed in the subsequent section. This review has been broadly divided into three sections (i) performance of commercial polymeric membranes (ii) performance of lab-made polymeric membranes and (iii) performance of mixed matrix membranes (MMMs) for gas separations. It will include structural modifications at polymer level, polymer blending, as well as synthesis of mixed matrix membranes, for which addition of silane-coupling agents and selection of suitable fillers will receive special attention. Apart from an overview of the different membrane materials, the study will also highlight the effects of different operating conditions that eventually decide the performance and longevity of membrane applications in gas separations. The discussion will be largely restricted to the studies carried out on polyimide (PI), cellulose acetate (CA), polysulfone (PSf) and polydimethyl siloxane (PDMS) membranes, as these membrane materials have been most widely used for commercial applications. Finally, the most important strategies that would ensure new commercial applications will be discussed (156 references).

Mixed-matrix membranes (MMMs) composed of polyimide (PI) and metal-organic frameworks (MOFs) were synthesized using Matrimid as the polymer and zirconium terephthalate UiO-66 as the filler. The modulation approach, combined with the use of amine-functionalized linkers, was used for synthesis of the MOF fillers in order to enhance the intrinsic separation performance of the MOF and improve the particle-PI compatibility. The presence of amine groups on the MOF outer surface introduced either through the linker, through the modulator, or through both led to covalent linking between the fillers and Matrimid, which resulted in very stable membranes. In addition, the presence of amine groups inside the pores of the MOFs and the presence of linker vacancies inside the MOFs positively influenced CO2 transport. MMMs with 30 wt % loading showed excellent separation performance for CO2/CH4 mixtures. A significant increase in the mixed-gas selectivity (47.7) and permeability (19.4 barrer) compared to the unfilled Matrimid membrane (i.e., 50% more selective and 540% more permeable) was thus achieved for the MMM containing the MOF prepared from 2-aminoterephthalic acid and 4-aminobenzoic acid, respectively used as the linker and as the modulator.

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The oxygen reduction reaction (ORR) has been studied at platinum, gold, and glassy carbon electrodes using cyclic voltammetry and potential-step chronoamperometry in 11 room temperature protic ionic liquids (PILs). The diffusion coefficient and solubility of oxygen in seven PILs were determined by nonlinear curve fitting of potential-step chronoamperometry at a platinum microelectrode. The diffusion coefficient of oxygen in the PILs was consistently on the order of 10 −6 cm 2 s −1 . The ORR in the PILs was found to largely follow an electrochemical−chemical− electrochemical−chemical (ECEC) mechanism, as demonstrated by digital simulation of cyclic voltammograms at platinum, gold, and glassy carbon electrodes. The influence of electrode material on peak potential, heterogeneous rate constant, and transfer coefficient was studied. The influence of temperature on kinetics and thermodynamic properties like activation energy of ORR, activation energy of diffusion, and enthalpy of dissolution of oxygen in ethylammonium nitrate, bis(methoxyethyl)ammonium benzoate, and bis-(methoxyethyl)ammonium sulfamate was also investigated.