Ultrapermeable Polymers of Intrinsic Microporosity that redefine the state-of-the-art for CO2 capture

Polymers of Intrinsic Microporosity (PIMs) combine the desirable processability of polymers, with a significant degree of microporosity generated from the inefficient packing of their rigid and contorted macromolecular structures. They are attracting attention for a number of industrial gas separation applications, such as oxygen or nitrogen separation from air (i.e. separation O2/N2) or natural gas treatment and biogas upgrading (i.e. separation CO2/CH4). However, a further enhancement of the polymer transport properties is desirable to be cost effective in comparison with the traditional separation technology, currently applied for CO2 capture or removal. In this work, we report a series of benzotriptycene-based PIMs showing outstanding CO2/CH4 and CO2/N2 permselectivity properties [1], due to the inefficient packing of their 2D polymeric chains which results in large interconnected pores that enhance gas permeability. These benzotriptycene-based PIMs demonstrate ultrapermeability (PCO2>20000 Barrer) and interesting selectivity, and their combination allows the introduction of a new upper bound to define the current state-of-the-art for CO2 membrane separation processes (Figure 1). Their gas transport properties will be discussed in terms of permeability, diffusivity and solubility on freshly methanol treated and aged samples. The effect of the temperature on the transport properties will be also discussed. Particular attention will be paid to the relative contribution of diffusivity coefficients with the analysis of the size-selectivity in terms of entropic and energetic selectivity [2], and as a function of the effective diameter of the penetrant gas [3]. The latter analysis demonstrates that the diffusivity of light gases through PIMs shows that smaller H2 and He gas molecules have a transport mechanism that is similar to that of porous materials, whereas larger gas molecules, CH4, N2, O2 and CO2, show activated transport similar to that of conventional dense polymers. A typical and defining feature of PIMs, which differentiates their properties from other high free volume polymers, glassy polymers and rubbers, is the change in slope of the plot of the diffusion coefficient as a function of the gas diameter, with a stronger size-selective trend for the larger gas molecules than for He and H2. References: [1]B. Comesaña-Gándara, J. Chen, C.G. Bezzu, M. Carta, I. Rose, M.-C. Ferrari, E. Esposito, A. Fuoco, J.C. Jansen, N.B. McKeown, Redefining the Robeson upper bounds for CO 2 /CH 4 and CO 2 /N 2 separations using a series of ultrapermeable benzotriptycene-based polymers of intrinsic microporosity, Energy Environ. Sci. (2019). doi:10.1039/C9EE01384A. [2]A. Fuoco, B. Comesaña-Gándara, M. Longo, E. Esposito, M. Monteleone, I. Rose, C.G. Bezzu, M. Carta, N.B. McKeown, J.C. Jansen, Temperature Dependence of Gas Permeation and Diffusion in Triptycene-Based Ultrapermeable Polymers of Intrinsic Microporosity, ACS Appl. Mater. Interfaces. 10 (2018) 36475-36482. doi:10.1021/acsami.8b13634. [3]A. Fuoco, C. Rizzuto, E. Tocci, M. Monteleone, E. Esposito, P.M. Budd, M. Carta, B. Comesaña-Gándara, N.B. McKeown, J.C. Jansen, The origin of size-selective gas transport through polymers of intrinsic microporosity, J. Mater. Chem. A. 7 (2019) 20121-20126. doi:10.1039/C9TA07159H.