Ultrapermeable benzotriptycene-based PIMs show exceptional gas selectivities that define new positions for the CO2/CH4 and CO2/N2 Robeson upper bounds.

A highly gas-permeable polymer with enhanced selectivities is prepared using spirobifluorene as the main structural unit. The greater rigidity of this polymer of intrinsic microporosity (PIM-SBF) facilitates gas permeability data that lie above the 2008 Robeson upper bound, which is the universal performance indicator for polymer gas separation membranes.

The promise of ultrapermeable polymers, such as poly(trimethylsilylpropyne) (PTMSP), for reducing the size and increasing the efficiency of membranes for gas separations remains unfulfilled due to their poor selectivity. We report an ultrapermeable polymer of intrinsic microporosity (PIM-TMN-Trip) that is substantially more selective than PTMSP. From molecular simulations and experimental measurement we find that the inefficient packing of the two-dimensional (2D) chains of PIM-TMN-Trip generates a high concentration of both small (<0.7 nm) and large (0.7-1.0 nm) micropores, the former enhancing selectivity and the latter permeability. Gas permeability data for PIM-TMN-Trip surpass the 2008 Robeson upper bounds for O/N, H/N, CO/N, H/CH and CO/CH, with the potential for biogas purification and carbon capture demonstrated for relevant gas mixtures. Comparisons between PIM-TMN-Trip and structurally similar polymers with three-dimensional (3D) contorted chains confirm that its additional intrinsic microporosity is generated from the awkward packing of its 2D polymer chains in a 3D amorphous solid. This strategy of shape-directed packing of chains of microporous polymers may be applied to other rigid polymers for gas separations.

Crystal engineering of nanoporous structures has not yet exploited the heme motif so widely found in proteins. Here, we report that a derivative of iron phthalocyanine, a close analog of heme, forms millimeter-scale molecular crystals that contain large interconnected voids (8 cubic nanometers), defined by a cubic assembly of six phthalocyanines. Rapid ligand exchange is achieved within these phthalocyanine nanoporous crystals by single-crystal-to-single-crystal (SCSC) transformations. Differentiation of the binding sites, similar to that which occurs in hemoproteins, is achieved so that monodentate ligands add preferentially to the axial binding site within the cubic assembly, whereas bidentate ligands selectively bind to the opposite axial site to link the cubic assemblies. These bidentate ligands act as molecular wall ties to prevent the collapse of the molecular crystal during the removal of solvent. The resulting crystals possess high surface areas (850 to 1000 square meters per gram) and bind N2 at the equivalent of the heme distal site through a SCSC process characterized by x-ray crystallography.

Methyl or t-butyl substituted spirobifluorenes greatly enhance PIM permeability.

A series of four novel Tröger's base (TB) derived Polyimides of Intrinsic Microporosity (PIM-TB-PI) is reported. The TB diamine monomer (4MTBDA) possesses four methyl groups in order to restrict rotation about the C-N imide bonds in the resulting polymers. The polymers possess apparent BET (Brunauer, Emmett and Teller) surface areas between 584 and 739 m 2 g −1 , complete solubility in chloroform, excellent molecular mass, high inherent viscosity and good film-forming properties. Gas permeability measurements demonstrate enhanced performance over previously reported polyimide-based Tröger's base (TB) polymers confirming the benefit of the additional methyl groups within the TB diamine monomer. Notably, a polyimide derived from 4MTBDA and pyromellitic anhydride 2 (PMDA) demonstrates gas permeability data above the 2008 upper bounds for important gas pairs such as O 2 /N 2 , H 2 /N 2 and H 2 /CH 4. INTRODUCTION Polymer-based gas separation membranes are of increasing industrial importance. Presently, membranes fabricated from a range of different polymers, including polyimides, are utilized in applications such as O 2 or N 2 enrichment from air, natural gas purification and hydrogen recovery. 1 A desirable polymer for such applications requires both high permeability and high selectivity. However, for each gas pair to be separated (X and Y) there is a well-established inverse relationship between permeability (P x) and selectivity (α = P x /P y), which was defined by Robeson, 2 and then rationalized by Freeman's theoretical analysis. 3 Hence there is a challenge to design polymers with permeability behaviour that surpass the current Robeson's upper bounds for important gas pairs, based on the state of the art performance of polymers in 2008, and therefore have potential for membrane materials. Higher gas permeability can be achieved by increasing the inter-chain separation and hence, the polymer's free volume whereas selectivity can be improved by enhancing polymer rigidity. 3 In 2004, a new class of materials termed polymers of intrinsic microporosity (PIMs) was introduced. 4 PIMs possess high free volume due to their contorted and rigid structures and provide not only excellent gas permeability but also moderate selectivities. Recently developed PIMs, in particular polymers fabricated from highly rigid bridged bicyclic units, such as triptycene, have demonstrated a substantial improvement in gas transport performance. 5,6,7 Polyimides (PIs) are an important class of polymers for membrane-based gas separations. 8 Known for their good thermal and chemical stabilities, and excellent mechanical properties, Associated content Supporting Information (SI).

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Microporous polymers derived from the 1,2-and 1,4-regioisomers of di(3 0 ,4 0 -dihydroxyphenyl)tetraphenylbenzene have very different properties with the former being composed predominantly of cyclic oligomers whereas the latter is of high molar mass suitable for the formation of robust solvent-cast films of high gas permeability.