Thermally Rearranged Research Articles

Spirobisindane-based polyimide as efficient precursor of thermally-rearranged and carbon molecular sieve membranes for enhanced propylene/propane separation

High performance thermally-rearranged (TR) and carbon molecular sieve (CMS) membranes made from an intrinsically microporous polymer precursor PIM-6FDA-OH are reported for the separation of propylene from propane. Thermal rearrangement of PIM-6FDA-OH to the corresponding polybenzoxazole (PBO) membrane resulted in a pure-gas C3H6/C3H8 selectivity of 15 and C3H6 permeability of 14 Barrer, positioning it above the polymeric C3H6/C3H8 upper bound. For the first time, the C3H6/C3H8 mixed-gas properties of a TR polymer were investigated and showed a C3H6 permeability of 11 Barrer and C3H6/C3H8 selectivity of ~11, essentially independent of feed pressure up to 5bar. The CMS membrane made by treatment at 600°C showed further improvement in performance as demonstrated with a pure-gas C3H6/C3H8 selectivity of 33 and a C3H6 permeability of 45 Barrer. The mixed-gas C3H6/C3H8 selectivity dropped from 24 to 17 from 2 to 5bar feed pressure due to a decrease in C3H6 permeability most likely caused by competitive sorption without any evidence of plasticization.

Claisen thermally rearranged (CTR) polymers.

Thermally rearranged (TR) polymers, which are considered the next-generation of membrane materials because of their excellent transport properties and high thermal and chemical stability, are proven to have significant drawbacks because of the high temperature required for the rearrangement and low degree of conversion during this process. We demonstrate that using a [3,3]-sigmatropic rearrangement, the temperature required for the rearrangement of a solid glassy polymer was reduced by 200°C. Conversions of functionalized polyimide to polybenzoxazole of more than 97% were achieved. These highly mechanically stable polymers were almost five times more permeable and had more than two times higher degrees of conversion than the reference polymer treated under the same conditions. Properties of these second-generation TR polymers provide the possibility of preparing efficient polymer membranes in a form of, for example, thin-film composite membranes for various gas and liquid membrane separation applications.

Gas permeation properties of thermally rearranged (TR) isomers and their aromatic polyimide precursors

This study explores the influence of meta/para and ortho-position functional group structures on gas transport properties of two isomeric polyimides and their thermally rearranged (TR) polymers. The diamine isomers, 3,3′-dihydroxy-4,4′-diamino-biphenyl (p-HAB) and 4,4′-dihydroxybiphenyl-3,3′-diamino-biphenyl (m-HAB), were polymerized with 2,2′-bis-(3,4-dicarboxy-phenyl) hexafluoropropane dianhydride (6FDA) to form two aromatic polyimide isomers. These polyimides were also prepared with either hydroxyl or acetate groups at the ortho-positions to the imide ring, and the polyimides were partially converted to the corresponding polybenzoxazoles by thermal treatment at various temperatures and times. Single gas permeabilities of CH4, H2, O2, N2 and CO2 were measured at 35°C and upstream pressures ranging from 3atm to 17atm. Polyacetylimides and their corresponding TR polymers showed higher gas permeability and lower selectivity than polyhydroxyimides and their TR analogs, respectively. The effect of isomerism on gas transport properties was also investigated. Para-connected polyhydroxyimide had higher fractional free volume, and thus, gas permeability coefficients than meta-connected polyhydroxyimide. Interestingly, in contrast to the meta/para effects previously reported in the literature for linear aromatic polymers, the meta-linked polyacetylimide had higher permeabilities and lower selectivities than its para-linked analog, presumably due to steric hindrance from the acetate groups that may inhibit the rotational mobility of the phenylene units in the para-linked isomer. For TR polymers derived from either polyhydroxyimides or polyacetylimides, para-configured polymers had higher gas permeabilities than meta-configured analogs. Finally, polyimide precursors for TR polymers can be sensitive to synthesis and casting conditions, and this study demonstrates how small variations in synthesis and casting procedure can have a significant effect on physical and gas permeation properties.

Influence of toluene on CO2 and CH4 gas transport properties in thermally rearranged (TR) polymers based on 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB) and 2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA)

Separation of complex gas mixtures, such as natural gas using polymer membranes, often involves components with a wide range of condensabilities and propensity to interact with the polymer, potentially changing the transport properties of all components. There are few studies of such phenomena in the open literature. Here, the influence of toluene, a surrogate aromatic contaminant in natural gas, on pure and mixed CO2 and CH4 gas transport properties at 35°C was investigated for a thermally rearranged (TR) polymer prepared from a polyimide precursor based on 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB) and 2,2′-bis-(3,4-dicarboxy-phenyl) hexafluoropropane dianhydride (6FDA). As the polymer was exposed to CO2, CH4, and their equimolar mixture, at a toluene activity of about 0.3, CO2 and CH4 permeability coefficients decreased by more than 90% relative to their respective pre-exposure values due to antiplastization and competitive sorption. CO2/CH4 selectivity went through a maximum as toluene activity increased, reflecting the interplay between competitive sorption, antiplasticization, and plasticization. The recovery of gas permeation properties was explored by measuring CO2 and CH4 permeabilities after removing toluene from the feed. These effects were largely reversed when toluene was removed from the feed gas mixture. Toluene vapor sorption was determined as a function of toluene activity, and the sorption data were used to help rationalize the changes in CO2 and CH4 gas permeability coefficients in the presence of toluene. A qualitative assessment of antiplasticization and competitive sorption effects was provided using the partial-immobilization dual mode model.

Ternary mixed-gas separation for flue gas CO2 capture using high performance thermally rearranged (TR) hollow fiber membranes

Abstract A comprehensive study evaluated the mixed-gas (CO2/N2/O2) separation performance of thermally rearranged polybenzoxazole-co-imide (TR-PBOI-AD5) hollow fiber membranes fabricated in-house (pure-gas CO2 permeance of 481 GPU and ideal CO2/N2 selectivity of 17.7). The criteria for the determining the two major operating conditions (pressure ratio and feed flow rate) were identified in order to deliver optimal separation performance with low process energy consumption. By varying the feed CO2 concentrations, it was found that a single-stage operation using TR-PBOI-AD5 (produced by bis-APAF and DAM as the TR-able and non-TR-able diamines, respectively, based on 6FDA) hollow fiber membranes was not sufficient to meet the required 90% permeate CO2 purity target, while a two-stage operation using the same membranes led to a permeate CO2 purity (81%) closer to the target value. The comparison study among the TR polymer membranes and three other polymer membranes highlighted the significance of an appropriate choice of a mixed-gas separation operating scheme to fully utilize the exceptional permeation properties of the recently developed high performance membranes such as TR polymeric membranes.

Thermally rearranged (TR) bismaleimide-based network polymers for gas separation membranes.

Highly permeable, thermally rearranged polymer membranes based on bismaleimide derivatives that exhibit excellent CO2 permeability up to 5440 Barrer with a high BET surface area (1130 m2 g-1) are reported for the first time. In addition, the membranes can be easily used to form semi-interpenetrating networks with other polymers endowing them with superior gas transport properties.

Thermally Rearranged Poly(benzoxazole) Copolymer Membranes for Improved Gas Separation: A Review

Polymeric membranes for gas separation have application in a wide range of industries such as natural gas sweetening and air enrichment. Recently, high-performance gas separation polymeric membranes have been developed based on a novel thermal rearrangement process that produces the resistant poly(benzoxazole) (TR-PBO). This review reports on the current state of the art TR-PBO membranes for gas separation and the underlying chemistry needed to achieve such high separation performance. Particular focus is applied to copolymers based on TR-PBO for membranes as these have attracted considerable research interest recently for their gas separation performance and superior mechanical properties compared with TR-PBO. Also included in this review is a discussion of the future directions of research on TR-PBO-based membranes for gas separation.

High performance membrane materials for gas separation

The use of membranes in various gas separations has increased significantly in recent times. This review presents some of the recent noteworthy advances in the field of membranes materials for these applications. A description of the most promising groups of high free volume polymers, including polyimides, thermally rearranged (TR) polymers, substituted polyacetylenes, perfluoropolymers, and polymers with intrinsic microporosity (PIM) is provided. High performance, rubbery, polyethers and polyether based copolymers are shown as another important class of polymer membrane materials. The development of inorganic membranes, which are not bound to the trade-off limitations between permeability and selectivity exhibited by polymers is also presented. The attention is focused on zeolitic materials, metal organic frameworks (MOF), and carbon molecular sieves (CMS). The field of mixed matrix membranes composed of inorganic particles embedded in a polymer matrix is also briefly outlined.

Gas permeation and mechanical properties of thermally rearranged (TR) copolyimides

Gas transport and mechanical properties are reported for a series of copolyimides and their corresponding thermally rearranged (TR) analogs. Random copolyimides of APAF/HAB-6FDA with various compositions were synthesized via chemical imidization from 2,2′-bis (3-amino-4-hydroxyphenyl)-hexafluoropropane (APAF), 3,3′-dihydroxy-4,4′-diaminobiphenyl (HAB), and 2,2′-bis-(3,4-dicarboxy-phenyl) hexafluoropropane dianhydride (6FDA). These copolyimides were thermally treated at temperatures between 350 °C and 450 °C to prepare TR polybenzoxazoles (PBOs). Pure gas permeabilities of H2, CH4, N2, O2, and CO2 were measured at 35 °C with upstream pressures ranging from 3 to 17 atm. In general, gas permeability increased and selectivity decreased as TR conversion increased. Gas permeability also increased with increasing APAF content in the polyimides as a result of higher fractional free volume (FFV) imparted by the extra hexafluoroisopropylidene group in APAF. Stress–strain relationships were recorded for the polyimides and their TR analogs. As polyimides underwent thermal rearrangement, samples became more rigid, and tensile stress and elongation at break decreased. A similar trend in mechanical properties was also observed as APAF content increased, which was attributed to the lower molecular weight of APAF-containing polymers, resulting from the lower reactivity of APAF relative to that of HAB.

Aromatic polyimide and crosslinked thermally rearranged poly(benzoxazole-co-imide) membranes for isopropanol dehydration via pervaporation

Novel crosslinked thermally rearranged polybenzoxazole (C-TR-PBO) membranes, which show impressive results for isopropanol dehydration, have been obtained via in-situ thermal conversion of hydroxyl-containing polyimide precursors. The polyimide precursors are synthesized by the polycondensation of three monomers; namely, 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA), 3,3′-dihydroxybenzidine diamine (HAB) and 3,5-diaminobenzoic acid (DABA). Due to the incorporation of the carboxylic-group containing diamine DABA into an ortho-hydroxypolyimide precursor, the thermally induced crosslinking reaction can be achieved together with the thermal rearrangement process. Consequently, a synergistic effect of high permeability and high selectivity can be realized in one step. The resultant C-TR-PBO membrane exhibits an unambiguous enhancement in permeation flux compared to its polyimide precursor. Moreover, the newly developed C-TR-PBO membrane displays stable isopropanol dehydration performance at 60°C throughout the continuous 200h. The promising preliminary results achieved in this study may offer useful insights for the selection of membrane materials for pervaporation and new methods to molecularly design next-generation pervaporation membranes.

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.

Synthesis and characterization of thermally rearranged (TR) polybenzoxazoles: Influence of isomeric structure on gas transport properties

Isomeric polyhydroxyimides based on 3,3′-diamino-4,4′-dihydroxybiphenyl (m-HAB) or 3,3′-dihydroxy-4,4′-diaminobiphenyl (p-HAB) with 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) were prepared via an ester-acid monomer. The polyhydroxyimides were then acetylated using acetic anhydride to change the ortho-functional groups on the polymer chains. These ortho-functional polyimides were used as precursors for thermal rearrangement (TR) to polybenzoxazoles for gas separation membranes. The permeability coefficients of TR polymers significantly improved as the ortho-functional polyimides converted to polybenzoxazoles. The influence of meta and para isomeric monomers on the gas transport properties of the resulting TR polybenzoxazoles were studied. In addition, gas permeation properties show a dependency on the size of the ortho-functionality of the polyimide precursors.

Refractive Index Modulation by Tunable Thermal Rearrangement of Polycyanurates

Abstract Polycyanurates from 2,4-dichloro-6-methoxy-1,3,5-triazine with bisphenol A or 9,9-bis(4-hydroxyphenyl)fluorene were subjected to thermal treatment under argon to induce the thermal rearrangement of the polycyanurates into the corresponding polyisocyanurates. Thermally rearranged polymers with a wide range of conversions were prepared by simply changing the thermal treatment time. It was found that the refractive index of the thermally rearranged polymer was tunable by adjusting the conversion of the thermal rearrangement. A significant increase in the refractive index, up to 0.0513, was achieved by the thermal rearrangement of polycyanurate containing a fluorene structure in the main chain.

Mechanically Tough, Thermally Rearranged (TR) Random/Block Poly(benzoxazole-co-imide) Gas Separation Membranes

Insufficient mechanical properties are one of the major obstacles for the commercialization of ultrahigh permeability thermally rearranged (TR) membranes in large-scale gas separation applications. The incorporation of preformed benzoxazole/benzimidazole units into o-hydroxy copolyimide precursors, which themselves subsequently thermally rearrange to form additional benzoxazole units, were prepared for the first time. Using commercially available monomers, mechanically tough membranes prepared from random and block TR poly(benzoxazole-co-imide) copolymers (TR-PBOI) were investigated for gas separation. The effects of the chemical structures, copolymerization modes, and thermal holding time of o-hydroxy copolyimides on the molecular packing and properties, including gas transport, for the resulting TR-PBOI membranes have been examined in detail. After treatment at 400 °C, tough TR-PBOI membranes exhibited tensile strengths of 71.4–113.9 MPa and elongation at break of 5.1–16.1%. Moreover, they presented hig...

Effect of polymer structure on gas transport properties of selected aromatic polyimides, polyamides and TR polymers

Thermally rearranged (TR) polymers are formed through a thermally induced solid-state reaction of polyimides or polyamides that contain nucleophilic reactive groups ortho-positioned to their diamine. Naturally, the transport properties of TR polymers are intimately related to the chemical structure and reactivity of their precursors. Herein, we report characterization and transport properties for three poly(hydroxyimide) precursors prepared via thermal imidization in solution and for their corresponding TR polymers. Structural modifications to the polymer backbone can be used to control thermal rearrangement reaction kinetics. In regards to TR polymer formation, samples prepared from diamines with biphenyl functionality reacted more efficiently than those prepared from diamines with hexafluoroisopropylidene-linked aromatic units. However, hexafluoroisopropylidene functional units provided the highest combinations of permeability and selectivity for separations involving H2, N2, O2, CH4, and CO2. Differences in permeability between samples correlated well with changes in free volume, and 3 poly(hydroxyimide)s showed unusually high selectivities for their given free volume. The effect of synthesis route was also investigated for a specific TR polymer derived from 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB) and 2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA). Poly(hydroxyimide) precursors prepared via thermal imidization in solution and thermal imidization in the solid-state showed nearly identical permeabilities and selectivities regardless of synthesis route. However, after thermal rearrangement, the TR polymers prepared from polyimides synthesized via solid-state imidization have higher gas permeabilities than their solution-imidized analogs. In addition to light gas permeabilities, plasticization effects were investigated with CO2 hysteresis loops for all samples, and pure-gas olefin/paraffin permeabilities were determined for a TR polymer derived from 2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane (APAF) and 6FDA. With the exception of HAB–6FDA polyimides, pure-gas CO2 feed pressures up to approximately 50bar do not reveal a plasticization pressure point, but conditioning effects are observed for most samples. APAF–6FDA TR polymers have pure-gas permeabilities and selectivities beyond the propylene/propane upper bound.

Thermally rearranged polybenzoxazoles and poly(benzoxazole-co-imide)s from ortho-hydroxyamine monomers for high performance gas separation membranes

Ortho-hydroxypolyimides (HPIs) undergo thermal rearrangement processes in a solid state at high temperatures to produce thermally rearranged polybenzoxazoles (TR-PBOs), which are promising materials for gas separation membranes due to their exceptional permeability-selectivity performance. The strong dependence between the structure of HPIs and the final properties of the TR-PBOs and the high cost of the HPI precursors are considered excellent reasons for their continued study. In this work, a set of low-cost HPIs were synthetized via the reaction of 2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) with 2,4-diaminophenol dihydrochloride (DAP-Cl) and 4,6-diaminoresorcinol dihydrochloride (DAR-Cl). The polyimide made from 6FDA and m-phenylene diamine (MPD) was also obtained for comparison purposes. The polyimide precursors and the corresponding TR-PBOs, which were tested as films, were thoroughly characterized. The glass transition temperature of these precursor polyimides was shown to be a function of the number of hydroxyl groups such that the lowest value corresponded to the polyimide from MPD and the highest value corresponded to that derived from DAR. The conversion rate of HPIs to PBO at different processing temperatures was determined, and the highest conversion rate matched the DAP derived polyimide. With respect to their gas separation properties after thermal treatment at 450°C, these HPIs gave rise to TR-PBOs with very high permeability and satisfactory permselectivity values for several gas pairs.

Separation of CO2 from humidified ternary gas mixtures using thermally rearranged polymeric membranes

The recently developed thermally rearranged (TR) polymeric membranes displayed superior gas separation properties over conventional polymeric membranes, owing to the presence of micro-pores appropriately tuned in cavities size and distribution, thus offering great advantages in gas separation applications, especially in CO2 capture from post-combustion flue gas streams.In this work, the transport properties (flux, permeance and selectivity) of TR polymeric hollow fiber membranes were evaluated with both single gases (CO2, N2, O2) and ternary gas mixture (CO2:N2:O2=15:80:5) at different temperatures (25, 50, 75°C) and trans-membrane pressure differences (2–5bar). The results revealed that the CO2 permeance measured under the mixed-gas condition remained the same as the single gas CO2 permeance, while the permeances of other gases decreased, thus leading to a favored increase in selectivity.The effect of water vapor in a ternary gas mixture was studied for the first time, considering that most of the streams of interest contain large amount of water. The presence of water vapor induced a significant permeance decrease for all the three components and a negligible reduction in selectivity. Globally the membranes showed good performance in the whole range of operating conditions investigated.

Fabrication of thermally rearranged (TR) polybenzoxazole hollow fiber membranes with superior CO2/N2 separation performance

Thermally rearranged polybenzoxazole (TR-PBO) hollow fiber membranes were fabricated from a poly(amic acid) (HPAAc) precursor through a non-solvent induced phase separation technique (NIPS). All the major fabrication conditions (e.g. dope composition, the use of additional inorganic salt, dope and bore flow rates, and coagulation bath temperature) were systematically evaluated and optimized, in order to produce defect-free hollow fiber membranes with an ultra-thin skin layer. The hollow fiber membranes fabricated with the optimized spinning conditions exhibited superior pure gas permeation behavior (CO2 permeance of 2500GPU and CO2/N2 ideal selectivity of 16). Slow beam positron annihilation lifetime spectroscopy (slow beam PALs) measurements revealed that such an exceptional separation performance was mainly attributed to the ideal cavity radius (3.584Å) and ultra-thin skin layer thickness (193nm) obtained using the optimal fabrication conditions. In addition, mixed-gas permeation tests were also performed to demonstrate the feasibility of using such membranes for post-combustion CO2 capture.

Cross-Linked Thermally Rearranged Poly(benzoxazole-co-imide) Membranes Prepared from ortho-Hydroxycopolyimides Containing Pendant Carboxyl Groups and Gas Separation Properties

The incorporation of small amounts of 3,5-diaminobenzoic acid (DABA) comonomer (usually 5–20 mol %) into an ortho-hydroxypolyimide (HPI) precursor constitutes a new methodology to easily improve the gas transport performance of thermally rearranged (TR) polymer membranes. Thermogravimetric analysis (TGA) profiles of HPI with DABA copolyimides (HPID) suggests that there is an overlap between the temperature ranges of two processes: the degradation of DABA carboxyl groups and the actual thermal rearrangement process. During thermal treatment at 450 °C, carboxyl pendant groups in DABA degrade while a more rigid biphenyl cross-linked structure is formed either following or at the same time as the rearrangement of imide to benzoxazole. Mixed-gas CO2/CH4 (1:1) experiments proved this new series of cross-linked TR poly(benzoxazole-co-imide) (XTR-PBOI) membranes as superior materials for CO2/CH4 separation applications, with enhanced permeability and selectivity, as well as high resistance to plasticization up to 40 bar.

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.