Eric Litwiller

Ultra-Microporous Triptycene-based Polyimide Membranes for High-Performance Gas Separation

This Communication describes the synthesis and exceptional gas transport properties of two robust, solution-processable ultra-microporous (<7 Å) PIM-polyimides – KAUST-PI-1 and KAUST-PI-2 – integrating a three-dimensional 9,10-diisopropyltriptycene contortion center into a rigid fused-ring dianhydride. Rotation about the imide bonds is restricted by ortho-substituted methyl groups in the diamine. Bridgehead substitution at the 9,10-positions compounds the benefi ts of triptycene on three fronts: [ 13 ] First, it offers tunability of the porous texture; second, the triptycene imparts rigidity to short bridgehead substituents, effectively enhancing the overall three-dimensionality and rigidity of the moiety; third, it bolsters solution-processability by enhancing solubility. Functionalizing the bridgeheads with short branched isopropyl chains [ 18 ] primes the microstructure for highly permeable and highly selective diffusion-dominated performance surpassing all known polymers in industrial gas separations [ 5 ] including hydrogen (H 2 /N 2 , H 2 /CH 4 ) and oxygen (O 2 /N 2 ) separations, which constitute ∼75% of the gas separation market. [ 19 ]

Energy‐Efficient Hydrogen Separation by AB‐Type Ladder‐Polymer Molecular Sieves

Increases in hydrogen selectivity of more than 100% compared with the most selective ladder polymer of intrinsic microporosity (PIM) reported to date are achieved with self-polymerized A-B-type ladder monomers comprising rigid and three-dimensional 9,10-dialkyl-substituted triptycene moieties. The selectivities match those of materials commercially employed in hydrogen separation, but the gas permeabilities are 150-fold higher. This new polymer molecular sieve is also the most selective PIM for air separation.

Pristine and thermally-rearranged gas separation membranes from novel o-hydroxyl-functionalized spirobifluorene-based polyimides

A novel o-hydroxyl-functionalized spirobifluorene-based diamine monomer, 2,2′-dihydroxyl-9,9′-spirobifluorene-3,3′-diamine (HSBF), was successfully prepared by a universal synthetic method. Two o-hydroxyl-containing polyimides, denoted as 6FDA-HSBF and SPDA-HSBF, were synthesized and characterized. The BET surface areas of 6FDA-HSBF and SPDA-HSBF are 70 and 464 m2 g−1, respectively. To date, SPDA-HSBF exhibits the highest CO2 permeability (568 Barrer) among all hydroxyl-containing polyimides. The HSBF-based polyimides exhibited higher CO2/CH4 selectivity than their spirobifluorene (SBF) analogues (42 for 6FDA-HSBF vs. 27 for 6FDA-SBF) due to an increase in their diffusivity selectivity. Polybenzoxazole (PBO) membranes obtained from HSBF-based polyimide precursors by thermal rearrangement showed enhanced permeability but at the cost of significantly decreased selectivity.

A Metal Chelating Porous Polymeric Support: The Missing Link for a Defect-free Metal-Organic Framework Composite Membrane

Since the discovery of size-selective metal-organic frameworks (MOFs), researchers have tried to incorporate these materials into gas separation membranes. Impressive gas selectivities were found, but these MOF membranes were mostly made on inorganic supports, which are generally too bulky and expensive for industrial gas separation. Forming MOF layers on porous polymer supports is industrially attractive but technically challenging. Two features to overcome these problems are described: 1) a metal chelating support polymer to bind the MOF layer, and 2) control of MOF crystal growth by contra-diffusion, aiming at a very thin nanocrystalline MOF layer. Using a metal chelating poly-thiosemicarbazide (PTSC) support and adjusting the metal and organic ligand concentrations carefully, a very compact ZIF-8 (ZIF=zeolitic imidazolate framework) layer was produced that displayed interference colors because of its smooth surface and extreme thinness-within the range of visible light. High performances were measured in terms of hydrogen/propane (8350) and propylene/propane (150) selectivity.

Ultra-selective defect-free interfacially polymerized molecular sieve thin-film composite membranes for H2 purification

Purification is a major bottleneck in generating low-cost commercial hydrogen. In this work, inexpensive high-performance H2 separating membranes were fabricated by modifying the commercially successful interfacial polymerization production method for reverse osmosis membranes. Defect-free thin-film composite membranes were formed demonstrating unprecedented mixed-gas H2/CO2 selectivity of ≈50 at 140 °C with a H2 permeance of 350 GPU, surpassing the permeance/selectivity upper bound of all known polymer membranes by a wide margin. The combination of exceptional separation performance and low manufacturing cost makes them excellent candidates for cost-effective hydrogen purification from steam cracking and similar processes.