Thermally-rearranged Membranes Research Articles

Thermally Rearranged (TR) Polybenzoxazoles from o-Substituted Precursor Polyimides with Phenyl Pendant Groups: Synthesis, Properties, and Thermal Rearrangement Conditions.

A series of polyimides (PIs) was synthesized from 6FDA and two o-OH substituted diamines having bulky pendant phenyl, Ph, and trifluoromethyl, CF3, groups as precursors for thermally rearranged polybenzoxazole, TR-PBO, membranes. One diamine had two pendant Ph substituents; in the other, the substituents were Ph and CF3. Applying azeotropic and chemical cyclizations allowed the obtention of four o-hydroxy (o-OH) or/and o-acetoxy (o-OAc) substituted PIs depending on the imidization method. The PIs were labeled as 3Ph-OH, 4Ph-OH, or 3Ph-OAc and 4PH-OAc, respectively. Thermal rearrangements of all four precursors were investigated in the interval from 350 to 450 °C. The conversions to TR-PBO increased with temperature, and almost quantitative conversions were obtained at temperatures close to 450 °C, although o-OH substituted PIs reached conversions slightly higher than those of o-OAc PIs at a given temperature. The TR-polymers' fractional free volume (FFV) also increased with conversion but was higher for the o-OAc substituted precursors. Despite the high TR-PBO conversions, self-supported uniform TR membranes with reasonable mechanical properties were obtained, except for 4Ph-OH. Gas separation behavior of the membranes significantly improved after the thermal treatment, and the final CO2/CH4 permselectivities lay between the 1991 and 2008 Robeson upper bounds. Particularly, TR-membranes derived from o-OAc precursors and with pendant CF3 group demonstrated better gas transport properties with values of P (CO2) = 1121 barrer and αCO2/CH4 = 29 for 3Ph-OAc derived membrane, which positioned it beyond the 2008 upper limit.

Enhanced mechanical robustness and separation performance in triptycene modulated thermally rearranged copolyimide membranes

Thermally rearranged (TR) polymers have seen a growing role in gas separation membranes. Further exploitation of TR membranes with enhanced mechanical properties and gas separation performance are urgently demanded. Herein, a new series of TR polymers incorporating rigid triptycene units were prepared containing amino-functionalized neighbored benzene propellers and thermally rearrangeable o-hydroxy groups. The resultant TR membranes exhibit increased chain d-spacing and gas permeability with the increase of thermal arrangement temperatures. Moreover, the presence of triptycene enhances the mechanical strength of TR polymers due to the π-π stacking effect of triptycene moieties. The 400 °C TR polymer 6FDA-6FAP:DAT(2:1) displays a H2 permeability of 345.3 Barrer with a H2/CH4 selectivity of 83.6, 440% and 118% higher than the precursor, respectively, exceeding the latest Robeson upper bound. The design principle of TR membranes in this work provides an alternative route for the development of mechanically robust and high performance membranes with potentials for energy efficient gas separations.

Polymers of intrinsic microporosity and thermally rearranged polymer membranes for highly efficient gas separation

• PIMs and TR polymers are referred to polymers with a highly microporous structure. • The major challenge of PIMs and TR polymers is achieving a high permselectivity. • We studied the monomer molecular design of energy-efficient PIM/TR based membranes. • Different methods for synthesizing PIMs and TR polymers are described. • The presence of functional/pendant groups such as are investigated. • Different strategies for preparation and tailoring of their properties are discussed. In the last decades, many novel polymeric materials have been considered to prepare highly energy-efficient gas separation membranes. There is a continuous search to overcome the trade-off relationship between the gas permeability and selectivity to improve the membrane separation performance during long-term operation. One of the most important research fields of molecular design and polymer chemistry is the synthesis of new polymers with a highly microporous structure, referred to as polymers of intrinsic microporosity (PIMs) and thermally rearranged (TR) polymers. The major challenge of PIMs and TR polymers, required for their application in membrane gas separation, is achieving a high permeability and selectivity, which remain stable over prolonged periods and under all possible operating conditions. Limits of their use can be low mechanical stability, sophisticated and challanging synthesis methods, physical aging of PIMs, etc. For TR polymers, identifying the optimum temperature in the thermal treatment process is challenging, and the preparation of thin-film TR membranes from PBO precursor via conventional methods is difficult. Moreover, the productivity of the membrane modules and the adhesion between the support/polymer and nanomaterials/polymer need to be improved in mixed matrix membranes (MMMs). Also, the formation of defects in the membrane structure is the main challenge to overcome for the fabrication of thin-film composites (TFCs). In this manuscript, we carefully review the molecular design and the properties and performance of PIMs and TR materials as energy-efficient membranes for various gas separations, emphasizing CO 2 /CH 4 and CO 2 /N 2 . Different methods for synthesizing PIMs and TR polymers are described, and the resulting polymers are evaluated for gas separation. Structure designing for the synthesis of PIMs and TR membranes and the influence of different chemical structures of monomers for creating chain rigidity by non-planar groups, and tuning the angle of contorted centres, are investigated. Moreover, the presence of functional groups and other substituents, such as fluorinated, sulfonated, carboxylic, tetrazole, and other groups, are investigated. Different strategies for membrane preparation and tailoring of their properties, such as cross-linking, co-polymerization, blending with other polymeric or organic and inorganic materials, are discussed as well.

Gas Separation by Mixed Matrix Membranes with Porous Organic Polymer Inclusions within o-Hydroxypolyamides Containing m-Terphenyl Moieties.

A hydroxypolyamide (HPA) manufactured from 2,2-bis(3-amino-4-hydroxy phenyl)-hexafluoropropane (APAF) diamine and 5′-terbutyl-m-terphenyl-4,4′′-dicarboxylic acid chloride (tBTpCl), and a copolyimide produced by stochiometric copolymerization of APAF and 4,4′-(hexafluoroisopropylidene) diamine (6FpDA), using the same diacid chloride, were obtained and used as polymeric matrixes in mixed matrix membranes (MMMs) loaded with 20% (w/w) of two porous polymer networks (triptycene-isatin, PPN-1, and triptycene-trifluoroacetophenone, PPN-2). These MMMs, and also the thermally rearranged membranes (TR-MMMs) that underwent a thermal treatment process to convert the o-hydroxypolyamide moieties to polybenzoxazole ones, were characterized, and their gas separation properties evaluated for H2, N2, O2, CH4, and CO2. Both TR process and the addition of PPN increased permeability with minor decreases in selectivity for all gases tested. Excellent results were obtained, in terms of the permeability versus selectivity compromise, for H2/CH4 and H2/N2 separations with membranes approaching the 2008 Robeson’s trade-off line. The best gas separation properties were obtained when PPN-2 was used. Finally, gas permeation was characterized in terms of chain intersegmental distance and fraction of free volume of the membrane along with the kinetic diameters of the permeated gases. The intersegmental distance increased after TR and/or the addition of PPN-2. Permeability followed an exponential dependence with free volume and a quadratic function of the kinetic diameter of the gas.

The Viable Fabrication of Gas Separation Membrane Used by Reclaimed Rubber from Waste Tires.

Improper disposal and storage of waste tires poses a serious threat to the environment and human health. In light of the drawbacks of the current disposal methods for waste tires, the transformation of waste material into valuable membranes has received significant attention from industries and the academic field. This study proposes an efficient and sustainable method to utilize reclaimed rubber from waste tires after devulcanization, as a precursor for thermally rearranged (TR) membranes. The reclaimed rubber collected from local markets was characterized by thermogravimetric analyzer (TGA) and Fourier transfer infrared spectroscopy (FT-IR) analysis. The results revealed that the useable rubber in the as-received sample amounted to 57% and was classified as styrene–butadiene rubber, a type of synthetic rubber. Moreover, the gas separation measurements showed that the C7-P2.8-T250 membrane with the highest H2/CO2 selectivity of 4.0 and sufficient hydrogen permeance of 1124.61 GPU exhibited the Knudsen diffusion mechanism and crossed the Robeson trade-off limit. These findings demonstrate that reclaimed rubber is an appealing, cost effective, and sustainable alternative, as a precursor for TR membranes, for application in gas separation. The present approach is useful in the selection of a suitable reclaimed rubber precursor and related membrane preparation parameters, leading to the advancement in the recycling value of waste tires.

Energy and time efficient infrared (IR) irradiation treatment for preparing thermally rearranged (TR) and carbon molecular sieve (CMS) membranes for gas separation

Infrared (IR) irradiation was applied to prepare thermally rearranged (TR) and carbon molecular sieve (CMS) membranes based on the polymer precursor of HPI-HD5. The effects of the IR treatment conditions such as temperature, time, and environment on the thermal conversion of TR and CMS membranes were investigated for the first time. The membranes subjected to the IR furnace exhibited the same changes of the chemical structure and comparable gas transport behaviors as the membranes treated in the electric furnace. Surprisingly, the maximum treating temperature and processing time were reduced by ~100 °C and a factor of ~20, respectively, in the IR furnace due to the energy-intensive heat radiation and instant temperature control. By replacing the electric furnace with the IR furnace, energy savings by a factor of ~10 were obtained at the same thermal conversion rate and consequently, the overall productivity was improved by more than a factor of 100. The applicability and advantages of efficient IR irradiation on TR and CMS membrane fabrication were confirmed in this study.

Highly permeable Thermally Rearranged Mixed Matrix Membranes (TR-MMM)

Thermally Rearranged (TR) polymers and Mixed Matrix Membranes (MMMs) are two effective approaches used to advance the performance of gas separation membranes. Conversion of thermally activated groups in TR membranes result in hourglass-shaped cavities and unusually high permselectivity, whereas the inclusion of nanoparticles with engineered pore volume, window size and/or surface chemistry in MMMs can add fast and selective pathways for gas transport. In this study, we explored the effects of combining these two approaches by adding ultra-porous and highly thermostable PAF-1 nanoparticles into the TR-able polymer, 6FDA-HAB5DAM5 (DAM). Gas separation performances of TR-MMMs were evaluated by comparison with the pure polymer and another TR-MMM bearing an already thermally-treated PAF-1 additive (cPAF). While both additives enhanced gas transport, only PAF-1 stabilized the TR conversion reaction and improved the TR-MMM's material properties. Minor variations in cPAF surface changed the TR-MMM interfacial interactions as well as other properties crucial to the application of advanced membrane materials, namely aging, plasticization, and mechanical stability. By combining thermal rearrangement process with PAF-1, TR-MMM demonstrated a 55-fold increase in CO2 gas permeability (37-fold for H2) at similar gas selectivities, but without the handling issues observed for the pure TR polymer membrane.

Effects of non-TR-able codiamines and rearrangement conditions on the chain packing and gas separation performance of thermally rearranged poly(benzoxazole-co-imide) membranes

In present work, the incorporation of nonrearrangable codiamines (mPDA, DMB or TFMB) with different substituted groups into an ortho-hydroxypolyimide (HPI) precursor was applied to modify the gas separation and mechanical performances of resultant thermally rearranged (TR) polybenzoxazole membranes. Evolution of the thermal rearrangement for ortho-hydroxy copolyimide precursors was effectively monitored by the TGA-FTIR, WAXD and PALS measurements. The incorporated non-TR-able codiamines and the thermal treatment protocols have a great effect on the chains packing behavior, free volume, mechanical properties as well as the gas separation behavior of resulted TR-PBOI membranes. These TR-PBOI membranes show high tensile strength of 117–160 MPa and good elongation at break of 7.5–9.0% as thermally treated at 400 °C for 2 h. The t-TR400-2 membrane comprising the TFMB codiamine exhibits a synergistic effect of high gas permeability and high gas pairs selectivity, which is mainly attributed to the loose chains packing resulted from the thermal rearrangement and the substituted bulky -CF3 group that resulting a high fractional free volume (FFV≈0.15). The systematic structure/property relationship studies serve as a guide of materials and process development of commercial TR membranes for gas separation applications.

Synthesis and gas permeation properties of a novel thermally-rearranged polybenzoxazole made from an intrinsically microporous hydroxyl-functionalized triptycene-based polyimide precursor

A hydroxyl-functionalized triptycene-based polyimide of intrinsic microporosity (TDA1-APAF) was converted to a polybenzoxazole (PBO) by heat treatment at 460 °C under nitrogen atmosphere. TDA1-APAF treated for 15 min (TR 460) resulted in a PBO conversion of 95% based on a theoretical weight loss of 11.7 wt% of the polyimide precursor. The BET surface area of the TR 460 (680 m2 g−1) was significantly higher than that of the TDA1-APAF polyimide (260 m2 g−1) as determined by nitrogen adsorption at −196 °C. Heating TDA1-APAF for 30 min (TRC 460) resulted in a weight loss of 13.5 wt%, indicating full conversion to PBO and partial main-chain degradation. The TR 460 membrane displayed excellent O2 permeability of 311 Barrer coupled with an O2/N2 selectivity of 5.4 and CO2 permeability of 1328 Barrer with a CO2/CH4 selectivity of 27. Interestingly, physical aging over 150 days resulted in enhanced O2/N2 selectivity of 6.3 with an O2 permeability of 185 Barrer. The novel triptycene-based TR 460 PBO outperformed all previously reported APAF-polyimide-based PBOs with gas permeation performance close to recently reported polymers located on the 2015 O2/N2 upper bound. Based on this study, thermally-rearranged membranes from hydroxyl-functionalized triptycene-based polyimides are promising candidate membrane materials for air separation, specifically in applications where space and weight of membrane systems are of utmost importance such as nitrogen production for inert atmospheres in fuel lines and tanks on aircrafts and off-shore oil- or natural gas platforms. Mixed-gas permeation experiments also demonstrated good performance of the TR 460 membrane for natural gas sweetening with a CO2 permeability of ∼1000 Barrer and CO2/CH4 selectivity of 22 at a typical CO2 wellhead partial pressure of 10 bar.

Simulation and feasibility study of using thermally rearranged polymeric hollow fiber membranes for various industrial gas separation applications

Due to the unprecedented permeation properties of the recently developed thermally rearranged (TR) polymers, previous models developed for conventional less permeable membranes are no longer suitable to predict the separation performance of TR membranes. In this regard, a new mathematical model was developed taking the often-neglected shell side pressure drop and non-ideal gas behavior into account to result in a more accurate simulation for TR membrane. The model demonstrates the feasibility of using industrial size thermally-rearranged polybenzoxazole (TR-PBO) hollow fiber membrane modules in various gas separation applications. The optimal operating conditions for each application in a single-stage configuration were identified, and flue gas carbon capture was used as an example to demonstrate the substantially higher process capacity which the TR-PBO hollow fiber membranes can offer over conventional polymeric membranes.

Gas Membranes for CO2/CH4 (Biogas) Separation: A Review

Gas separation by membrane carries many advantages over other methods of gas separation, including the amenability of the gas separation process to simplification and scaling up, and the elimination of the need for phase change of the gas. Material composition is, by far, the most important factor in determining the gas separation effectiveness of a membrane. This article is a review of the existing research on polymers as used in the separation of carbon dioxide/methane (CO2/CH4), generally focusing on polyimide and polysulfone compounds. Data on thermally rearranged (TR) polymer membranes and mixed matrix membranes were also included. In this study, the membrane performance of different polymeric materials was examined by comparing the data on CO2/CH4 selectivity obtained through a literature review and the upper bound proposed by Robeson in 2008. Results showed that polyimide and polysulfone membranes are highly applicable to the separation of CO2/CH4 gas mixtures: polyimide showed particular promise for commercial implementation. Polysulfone membranes exhibited high selectivity but low CO2 permeability that failed to exceed the upper bound. However, case studies suggested that polysulfones in the hollow fiber membrane form have a wide variety of applications in an industrial setting. On the other hand, the TR polymer membranes, which are currently used for research, had a CO2 permeability of 5–5,903 Barrer and CO2/CH4 selectivity of 3.8–124, which exceeded the upper bound proposed by Robeson. We, thus, came to the conclusion that future research on CO2/CH4 gas separation should focus on polysulfone, polyimide, and TR membranes as materials for economically efficient and highly effective gas separation membranes.

Acetate-Functional Thermally Rearranged Polyimides Based on 2,2-Bis(3-amino-4-hydroxyphenyl)hexafluoropropane and Various Dianhydrides for Gas Separations

Three acetate-functional polyimides based on 2,2-bis(3-amino-4-hydroxy-phenyl)hexafluoropropane (APAF) and dianhydrides with different linking groups (-O-, -CO-, and -C(CF3)2-) were prepared, from which films were prepared and thermally rearranged (TR) to form poly(benzoxazole) dense membranes. The polymers were characterized by 1H nuclear magnetic resonance, Fourier transform infrared, differential scanning calrimetry, thermogravimetric analysis-mass spectrometry, and wide-angle X-ray diffraction. Transport properties of the TR membranes were investigated, and effects of the linking group and TR conversion rate on the membrane performance were discussed. Among the TR polymers studied, those bearing larger linking groups have higher d-spacing, fractional free volume, and gas permeability. The membrane permeability to gases increases with TR conversion rate and follows the order H2 > CO2 > O2 > N2 > CH4, except that CO2 surpasses H2 in APAF-6FDA-TR450. The CO2/CH4 separation performance of all of the TR po...