Highly Selective and Permeable Microporous Polymer Membranes for

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Highly Selective and Permeable Microporous Polymer Membranes for Hydrogen Purification and CO2 Removal from Natural Gas Shuangjiang Luo, Qinnan Zhang, Lingxiang Zhu, Haiqing Lin, Barbara A. Kazanowska, Cara M. Doherty, Anita J. Hill, Peiyuan Gao, and Ruilan Guo Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02102 • Publication Date (Web): 18 Jul 2018 Downloaded from http://pubs.acs.org on July 19, 2018

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Chemistry of Materials

Highly Selective and Permeable Microporous Polymer Membranes for Hydrogen Purification and CO2 Removal from Natural Gas Shuangjiang Luo,a Qinnan Zhang,a Lingxiang Zhu,b Haiqing Lin,b Barbara A. Kazanowska,a Cara M. Doherty, c Anita J. Hill,c Peiyuan Gaod and Ruilan Guo*a a

Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN 46556, USA. E-mail: [email protected] b Department of Chemical and Biological Engineering, University of Buffalo, The State University of New York, Buffalo, NY 14260, USA c The Commonwealth Scientific and Industrial Research Organization (CSIRO) Manufacturing, Private Bag 10, Clayton, South Victoria 3169, Australia d Pacific Northwest National Laboratory, Richland, Washington 99352, USA Abstract: This paper reports a new macromolecular design that incorporates hierarchical triptycene unit into thermally rearranged polybenzoxazole (TR-PBO) structures for highly selective and permeable gas separation membranes with great potential for H2 purification and CO2 removal from natural gas. We demonstrate that triptycene moieties not only effectively disrupt chain packing enabling microporous structure for fast mass transport, but also introduce ultra-fine microporosity via the unique internal free volume intrinsic to triptycene unit that allows for superior molecular sieving capability in resulting PBO membranes. Consequently, these triptycene-based polybenzoxazole (TPBO) membranes display among the highest gas selectivities for H2 separations (i.e., α(H2/N2) = 96; α(H2/CH4) = 203) and CO2 removal from natural gas (i.e., α(CO2/CH4) = 68) among existing glassy polymeric membranes. It is also demonstrated that microporous structure and gas transport properties of TPBO films are highly tailorable by adjusting the triptycene content and the ortho-functionality of the precursors. The highly diverse tunability, along with the excellent resistance towards membrane plasticization and physical aging, render the TPBO membranes with extremely versatile separation capability applicable for a wide range of important industrial processes to get clean or low carbon fuels and reduce carbon footprint. 1 ACS Paragon Plus Environment

Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1. Introduction Worldwide attention has been focused on seeking renewable and clean energy resources alternative to fossil fuels during the past decades to alleviate the environmental problems caused by rising greenhouse gas emissions. Alongside, a fast-growing demand for new, sustainable, and energy-efficient technologies has arisen to support the focus on environmentally-responsible industrial growth. Due to the clean combustion product of water and the super-high combustion heat, hydrogen has emerged as a promising candidate and may play more and more important roles in energy consumption such as hydrogen fuel cell systems if it can be produced and purified in efficient and economical ways.1 Hydrogen separations also represent critical H2 recovery operations in ammonia synthesis and petrochemical refineries. On the other hand, the demand for substituting high carbon fuels with natural gas in the electric power plants is growing rapidly since it is admitted as a low-carbon fuel to feasibly mitigate CO2 emissions. However, many large resources of natural gas reservoirs are known to contain complex contaminants of CO2, hydrogen sulfide, and higher hydrocarbons. As such, removal of acid gases like CO2 from natural gas is required to prevent pipeline corrosion as well as fulfill/maximize the heating value of natural gas to a standard level.2-4 Therefore, innovative, energy-efficient technologies of natural gas purification are urgently needed for both environmental and economic considerations. Compared to conventional hydrogen and natural gas purification processes such as pressure-swing adsorption (PSA) and cryogenic distillation, membrane gas separation represents an energy-efficient alternative because it does not require thermal regeneration or phase change; therefore, it experiences fast growth and attracts wide attention. While inorganic molecular sieve

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materials, such as metal organic frameworks,5 zeolites6 and silica7, possess superior separation performance, practical implementation of these materials for gas separations faces challenges in membrane fabrication and relatively high cost. Carbon molecular sieves,8 created from processable polymers by post-synthetic modifications, avoid challenges and fabrication costs of other so-called inorganic molecular sieves to provide properties between microporous polymers and conventional inorganic materials. Recently, microporous polymers with molecular sieving capability have attracted a lot of attention due to their relatively low cost, ease of fabrication, and robust mechanical properties.9-13 Particularly, polymers of intrinsic microporosity (PIMs)14-16 and thermally rearranged (TR) polymers17,

18

are arguably the most attractive

membrane materials that display superior separation performance, due to their highly rigid and contorted backbones and high microporosity enabling fast gas diffusion and efficient separation. Benchmarking these novel membranes in the Robeson’s permeability-selectivity tradeoff upper bound plots19, 20 clearly indicates their microporous structures as reflected in their high gas permeabilities. While being highly permeable, these high-free-volume polymers have less attractive selectivities due to the lack of well-defined microvoids that is critical for size sieving. Recently, incorporating hierarchical triptycene structural units in polymer gas separation membranes has attracted a lot of interests due to the intriguing internal free volume that is intrinsically associated with its three-dimensional, shape-persistent configuration.21-25 The internal free volume of triptycene unit is comparable in size to the gas molecules, which provides the ultra-fine microporosity enabling molecular sieving properties in the corresponding membranes. Incorporating triptycene moieties in PIMs has led to membranes with unprecedentedly high permeability and good selectivity.26-28 However, the production of these



ladder-like

triptycene-PIMs

involves

rather

complicated

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synthesis.

In

this

regard,

high-performance TR polymers with polybenzoxazole (PBO) structures are probably better membrane material candidates because they are derived via simple thermal treatment of poly(o-hydroxyimide)17 or poly(o-hydroxyamide)29, 30 precursors that can be synthesized readily via conventional polycondensation. However, similar to PIMs, TR polymers generally show moderate selectivities, especially those converted at high temperature (450 °C), where possible thermal degradation adversely affects membranes’ sieving properties. Additionally, while TR membranes are highly attractive for natural gas sweetening,9,

17

they show only moderate

separation performance for hydrogen purifications or other important applications. Recent strategies for modifying PBO membranes, such as crosslinking,31, copolymers,33-36 PIM-PBOs,37,

38

32

PBO-polyimide

and PBOs derived from ortho-functional polyimides,39-42

however, have achieved limited successes in enhancing the sieving properties of the TR polymers. Herein we report a new class of triptycene-based PBOs (TPBOs) with exceptional size sieving properties and high permeabilities that are far above the Robeson’s 2008 upper bounds for H2/N2, H2/CH4, CO2/CH4, and O2/N2 separations. These TPBOs integrate hierarchical triptycene units into the rigid benzoxazole-phenylene backbone such that well-defined yet tailorable microporous architectures are constructed. Specifically, a series of triptycene-based poly(o-hydroxyimide) and poly(o-acetateimide) precursor copolymers with varying triptycene molar

content

(Figure

1)

were

synthesized

by

polycondensation

of

1,4-bis(3,4-dicarboxyphenoxy)-triptycene dianhydride (TPDAn), 4,4’-hexafluoroisopropylidene bisphthalic dianhydride (6FDA), and 2,2’-bis(3-amino-4-hydroxyphenyl)hexafluoropropane


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(6FAP). Correspondingly, a series of TPBO films with varying triptycene content (Figure 1) were fabricated via TR process of these precursor copolymers. Gas selectivities of the TPBOs are among the highest values ever reported for hydrogen separation and natural gas purification along with high gas permeabilities. Additionally, the microporosity and gas transport properties of TPBOs are found to be highly tailorable via adjusting the triptycene content and the ortho-functionality of precursors, demonstrating an attractive selectivity-driven macromolecular design allowing for superior gas separation performance.

Figure 1. (a) Preparation of trip-PBO membranes via thermal rearrangement of triptycene-containing

co-poly(o-hydroxyimide)s

and

co-poly(o-acetyl

imide)s;

(b)

A

geometrically optimized trip-PBO segment revealing contorted backbone conformation and pictures of representative trip-PBO films.

2. Experimental 2.1 Materials 5 ACS Paragon Plus Environment


Triptycene dianhydride (TPDAn) was prepared following our previously reported procedure (Scheme S1) and dried at 160 oC overnight under vacuum before use.23 6FDA (>99.0%) and 6FAP (>98.5%) were purchased from Akron Polymer Systems and dried at 160 o

C and 65 oC, respectively, under vacuum overnight prior to use. All other chemicals were

purchased from commercial sources and used as received. 2.2 Synthesis of ortho-functional polyimide precursors Triptycene-based poly(o-hydroxyimide) copolymer precursors (TPHI) were synthesized via polycondensation of TPDAn, 6FDA, and 6FAP in systematically varied molar ratios using solution imidization (Scheme S2). The obtained poly(o-hydroxyimide) precursors are named as TPHI-x (x = 0.25, 0.5, 0.75, and 1.0) according to the molar content of triptycene-containing repeat units in the copolymers. Synthesis of the TPHI-0.5 copolyimide (i.e., TPDAn:6FDA = 1:1 molar ratio) is provided as an example: to a 100 mL flame-dried three-neck flask were added 6FAP (0.8790 g, 2.4 mmol) and anhydrous NMP (11 mL). After the dissolution of 6FAP, TPDAn (0.6942 g, 1.2 mmol) and 6FDA (0.5331 g, 1.2 mmol) were added and the reaction mixture was mechanically stirred under N2 at r.t. for overnight to form viscous poly(amic acid). A Dean-Stark trap and a condenser were connected to the flask, and o-dichlorobenzene (DCB, 5 mL) was added. The reaction was heated to 190 °C and held for 12 h, during which time water formed from the imidization reaction was azeotropically removed. Fibrous polymers were obtained by precipitating the solution to a 1:1 methanol/water mixture (600 mL), which was filtered, washed with methanol, and vacuum dried at 180 oC overnight. Synthesis of other poly(o-hydroxyimide)s followed the same procedure except that different TPDAn/6FDA ratios were used. The triptycene-based poly(o-acetateimide) precursors, named accordingly as TPAI-x (x = 0.25, 6 ACS Paragon Plus Environment

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0.5, 0.75, and 1.0), were synthesized from the same combination of monomers by chemical imidization using acetic anhydride/pyridine to effect imidization that simultaneously converts o-hydroxyl groups to o-acetate groups (Scheme S3).41 Experimentally, the poly(amic acid) was synthesized from polycondensation of 6FAP, 6FDA, and TPDAn in predetermined ratio following the same procedure described above. Upon the formation of viscous poly(amic acid) solution, acetic anhydride and pyridine (1/1, v/v) were added (6 times excess per repeat unit of polyimide) and the mixture was stirred for another 24 h at r.t. to allow complete imidization and acetylation. Fibrous solids were obtained by precipitating the reaction solution in an excess amount of methanol. The solids were washed with fresh methanol several times and then dried at 180 oC under vacuum overnight giving white, fibrous solids. 2.3 Film casting and thermal conversion Polymer solution in DMAc (~7 wt%) filtered with a 0.45 µm Teflon syringe filter was cast onto a leveled glass plate. After drying under an infra-red lamp (~60 °C) overnight, films were peeled off and dried in a vacuum oven at 160 oC overnight to remove residual solvent. For thermal rearrangement conversion to PBOs, precursor films were cut into 5 × 5 cm pieces, sandwiched between two porous ceramic plates, and placed in a pre-heated muffle furnace under nitrogen protection. Each film was heated to and maintained at 300 oC for 2 h before being heated to 450 oC and maintained for 0.5 h, after which the film was allowed to slowly cool down in the furnace to room temperature at a cooling rate no greater than 10 oC min-1. The resulting TPBO films were named as TPBO-x (derived from poly(o-hydroxyimide)) and TPBO-Ac-x (derived from poly(o-acetateimide)) according to the ortho-functionality with x = 0.25, 0.5, 0.75, and 1.0 reflecting the triptycene content and the ortho-functionality.



2.3 Characterizations and gas permeation measurements 1

H NMR spectra were measured on a Bruker AVANCE III HD 500 MHz spectrometer in a

suitable deuterated solvent. The attenuated total reflection Fourier transform infrared (ATR-FTIR) measurements of films were performed using a Jasco FT/IR-6300 spectrometer. Molecular weight and molecular weight distribution of the polyimide precursors were evaluated by gel permeation chromatography (Waters GPC system) using poly(methyl methacrylate) as an external standard and DMF as the eluent. Thermal gravimetric analysis (TGA) was carried out under nitrogen purge using TGA Q500 (TA Instruments) at a heating rate of 10 oC min-1. Differential scanning calorimetry (DSC) analyses were conducted on a DSC QA2000 calorimeter (TA Instruments) under a nitrogen atmosphere (50 mL min-1) at a heating rate of 10 o

C min-1 and cooling rate of 20 oC min-1, and thermal transition temperatures were reported

based on the second heating cycle. Wide-angle X-ray diffraction (WAXD) patterns of the films were obtained on a Bruker D8 Advance Davinci diffractometer with Cu Ka (wavelength λ = 1.54 Å) radiation source in the reflection mode at room temperature. The step size and scan speed were 0.02o per step and 5 seconds per step, respectively. The peak position was determined by peak fitting with Gaussian equation, which was repeated at least three times for each WAXD scattering pattern and an averaged value was reported. The d-spacing values were then calculated from Bragg’s equation. The microcavity size and size distribution of TPBO films were measured by positron annihilation lifetime spectroscopy (PALS) under vacuum (1 × 10-5 Torr) at room temperature using an EG&G Ortec (Oak Ridge, TN) fast-fast coincidence spectrometer. PBO film pieces of ~ 1 × 1 cm in size were stacked to thickness of 2 mm on both sides of the positron source (1.5 × 106 Bq of

22

NaCl sealed in a Mylar envelope). At least five files, each containing 4.5 × 106 8 ACS Paragon Plus Environment

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integrated counts, were recorded for every sample. LT v9 software43 with a source correction of 1.467 ns and 3.299% was used for data analysis. The results were modeled as the sum of three components, which account for the para-positronium self-annihilation (τ1, 0.125 ns), the free annihilation (τ2, ~0.4 ns), and the ortho-positronium (o-Ps) decay in the microcavities (i.e., free volume) of the PBO films. The average sizes of microcavities were then calculated using the Tao-Eldrup equation:44, 45 1 R 1 2π R  sin( ) + 2  R0 2π R0 

τ = 1-

(1)

where τ is the lifetime, R is the radius of the pores, ∆R (1.66 Å) is the thickness of the electron layer within the potential well. The cavity size distributions were calculated using PAScual software.46 Geometrical optimization of chain segments of the TPBO-1.0 sample with ten repeating units was performed in Materials Studio (6.0, Accelrys) using energy minimized conformers (Forcite modules). Select dihedral angles were varied incrementally from -180 to +180o by using the Conformer module. Energy minimizations of each conformer were performed with the Discover module using a COMPASS force field and Smart algorithm, and the energy barrier profiles of the dihedral angles were calculated relative to the lowest energy observed over the 360o range. The degrees of torsional freedom of select dihedral angles were determined as the number of degrees available for torsion at a temperature T (with energy ~3RT).47 Pure gas permeabilities of the triptycene-based PBO films were tested with five UHP grade gases (i.e., H2, CH4, N2, O2, and CO2) at 35 oC and 3 bar using the constant-volume variable-pressure method. All films were degassed on both sides for at least 24 h before permeation tests. The upstream feed pressure was maintained at 3 bar, and the permeability 9 ACS Paragon Plus Environment


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coefficient P was determined by the steady-state pressure increase rate in the downstream:

P = 1010

Vd l  dp dp  ( )ss − ( )leak   pupTRA  dt dt 

(2)

where P (Barrer, 1 Barrer = 10‒10 cm3(STP)·cm/cm2·s·cmHg) is the gas permeability, Vd is the downstream volume (cm3), l is the film thickness (cm), pup is the upstream pressure (cmHg), A is the effective film area (cm2), T is the test temperature (K), R is the gas constant (0.278 cm3·cmHg/cm3(STP)·K), (dp/dt)ss and (dp/dt)leak are the steady-state pressure increment in downstream and the leak rate of the system (cmHg/s), respectively. The ideal gas selectivity was defined as the ratio of pure gas permeabilities of two gases of A and B with A being the more permeable gas: αA/B = PA/PB. The apparent diffusion coefficient, D, was determined from the time-lag method as D = l2/6θ, (l: film thickness; θ: time lag). Solubility coefficient was calculated via S = P/D based on the solution-diffusion model. Physical aging study was performed on select films that were stored at atmospheric conditions for 70 days before pure-gas permeation tests were conducted. The mixed-gas permeation was measured at 35 oC using a constant pressure/variable volume apparatus.48 Binary gas mixtures of 50:50 and 20:80 for CO2/CH4 were used and the total feed pressure was varied between 7.8 to 13.2 bar. The total feed flow rate was 200 cm3(STP) min-1, and the stage-cut was less than 0.5%. The composition of permeate gas mixtures were tested using a gas chromatograph (3000 Micro GC, Inficon Inc., Syracuse, NY) equipped with a thermal conductivity detector. The mixed-gas permeability of gas component A was calculated by:

PA =

x A Sl xsweep A( p2, A − p1, A )

(3) 10 ACS Paragon Plus Environment

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where xA and xsweep are the mole fraction of component A and the sweep gas (H2 in this study) in the permeate-side stream, respectively, p2,A and p1, A are the partial pressures of gas component A in the feed and permeate sides, respectively, and S is the flow rate of the sweep gas. The mixed-gas selectivities for one component A over another component B were given by the ratio of their mixed-gas permeabilities (PA/PB).

3. Results and discussion 3.1 Precursor synthesis and membrane fabrication The triptycene-based

PBO membranes were

fabricated successfully via thermal

rearrangement of the polyimide precursors. First of all, two series of trip-PBO precursors with varying triptycene molar content and different ortho-positioned functional groups, i.e., poly(o-hydroxyimide) and poly(o-acetateimide), were synthesized to investigate how triptycene moieties and ortho-functionality influence the microporosity and gas transport properties of final PBO membranes. Following an established procedure, a triptycene-dianhydride (TPDAn) was first prepared (Scheme S1),23 which was then copolymerized with 6FDA and 6FAP in predetermined molar content to produce poly(o-hydroxyimide) copolymers (TPHI, Scheme S2). Continuous long fibers (Figure S1) were obtained after precipitation from reaction solution, suggesting high molecular weights (Table S1) to produce robust films. The TPHI copolymers had fully imidized structures as confirmed by 1H-NMR and FTIR (Figure S2 and Figure S3) as no residual carbonyl groups from the poly(amic acid) intermediate were observed. Triptycene molar content was verified by comparing the peak integration ratio of the hydroxyl protons (peak 11 and peak g in Figure S2) in respective TPDAn-6FAP and 6FDA-6FAP repeat units with the target values. The TPAI copolymers were synthesized via polycondensation and 11 ACS Paragon Plus Environment


chemical imidization from TPDAn, 6FDA, and 6FAP (Scheme S3) to investigate the ortho-functionality effect. The anticipated structures and complete acetylation were confirmed by the absence of the phenolic peak in 1H-NMR (Figure S4) and FTIR (Figure S5) spectra. Triptycene molar contents in TPAI were calculated by comparing the peak integrations of acetate groups (Figure S4) and characteristic bridgehead protons of triptycene (peak 3 in Figure S4), and the results corresponded well with the target values, suggesting precise control of the precursor compositions. The glass transition temperatures (Tg) of the precursors range from 295 to 312 oC (Table S1), which are lower than the TR temperature (450 oC) ensuring efficient imide-to-benzoxazole conversion in solid state.49, 50 Thermal conversion of the poly(o-hydroxyimide) precursors to PBOs was analyzed by TGA and ATR-FTIR. Two-stage weight losses were observed in TGA profiles (Figure S6), wherein the first-stage weight loss matched well with the theoretical values predicted for the TR decarboxylation process (i.e., the release of two CO2 molecules per repeat unit). The full conversion was also verified by ATR-FTIR spectra showing the appearance of benzoxazole bands along with the disappearance of imide characteristic bands (Figure S7). The conversion of the TPAI precursors proceeded with the degradation of the o-acetate groups to form o-hydroxyl groups, followed by the same hydroxyimide-to-benzoxazole conversion mentioned above (Scheme S4 and Figure S8).39, 41, 42, 51, 52 Therefore, the weight losses during the TR process of the TPAI copolymers were much higher than that of TPHI precursors due to the loss of o-acetate groups (Figure S8). The overall weight losses during acetateimide-to-benzoxazole were very close to the theoretical values (Figure S8), confirming the formation of PBOs. The TGA profiles also confirmed that the same PBO structures were formed from the thermal conversion of TPHI


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and TPAI precursors albeit they followed different thermal conversion routes. For instance, the TGA profile (Figure S9) of the TPAI-1.0 overlaps with that of the TPHI-1.0 after both being thermally treated at 300 oC for 2 h, suggesting that TPAI-1.0 was converted to the same PBO structure as TPHI-1.0. Similarly, the TR processes of TPAI copolymers were verified by the ATR-FTIR, in which the acetate groups disappeared and the characteristic bands of benzoxazole showed up (Figure S10). Flexible and foldable TPBO films (Figure 1) were obtained after thermal treatment and these films were mechanically robust for gas permeation tests with feed pressure as high as 13 bar. 3.2 Gas transport properties

Figure 2. Gas separation performance of TPBO-x (●) and TPBO-Ac-x (●) films for (a) H2/N2, (b) H2/CH4 and (c) CO2/CH4 gas pairs. Also included for comparisons are: aPBO (☆),39 cross-linked PBOs (□),31,

32

PBO-polyimide copolymers (△),33-36 PBOs derived from

ortho-functionalized polyimides (▽),39, 40, 42 PIM-PBOs (◇),37, 38 relevant commercial polymers (2-Matrimid®,53 3-Polysulfone,54 4-CA-2.45,55 and 5-PPO56), 6-PIM-157 Tröger’s base polymers (7-PIM-EA-TB,16 8-PIM-Trip-TB26), triptycene-based PIM (9-TPIM-1,27 10-KAUST-PI-1,28 11-PIM-TMN-Trip58), and carbon molecular sieve (CMS) membranes (12-Matrimid-800,8 13-TB-CMS-80059). The numbers next to the data points indicate the triptycene molar contents and the dash arrows denote the enhancement of separation performance with increasing triptycene content. 13 ACS Paragon Plus Environment


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Pure gas permeation tests were conducted to evaluate the gas transport properties of the obtained TPBO films. All films display extraordinary separation performance (Figure 2 and Table 1) that outperform relevant commercial polymers such as Matrimid®,53 polysulfone,54 cellulose acetate-2.45,55 and poly(2,6-dimethylphenylene oxide)56, as well as recently reported PBO-based membranes including non-triptycene-containing aPBO,39 cross-linked PBOs,31, PBO-polyimide copolymers,33-36 PBOs derived from ortho-functionalized polyimides,39,

32

40, 42

and PIM-PBOs37, 38 (Table S2). All TPBOs surpass the 2008 upper bounds for CO2/CH4 (CO2 removal from natural gas), H2/N2 and H2/CH4 (critical H2 recovery operations in ammonia synthesis and petrochemical refineries) with the TPBO-1.0 and TPBO-Ac-1.0 exhibiting the best overall performance among the two series (i.e., being the furthest into the upper right region in the upper bound plots). TPBO-1.0 and TPBO-Ac-1.0 also outperformed recently reported highly permeable polymers such as PIM-1,57 Tröger’s base polymers,16, ladder-type polymers (Figure 2);27,

28, 58

26

and triptycene-based

they also showed comparable gas separation

performance to some of the recently developed, ultra-selective carbon molecular sieve (CMS) membranes.8, 59 To the best of our knowledge, the separation performance of TPBO-1.0 and TPBO-Ac-1.0 membranes are the best among all reported PBO-based membranes for applications of hydrogen purification and natural gas sweetening. Notably, triptycene-based PBO films also demonstrated outstanding O2/N2 separation performance (Figure S11) making them attractive for highly challenging air separation that may be applicable for oxy-combustion of coal. Specifically, the TPBO-1.0 displayed a very high O2/N2 selectivity of 8.1 with O2 permeability of 68 barrer. It should be noted that this extraordinarily diverse separation capability has been rarely observed in conventional glassy polymers, which typically are


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applicable for only certain type of gas separation due to limited control on the free volume architecture in these membranes. For example, almost all previous TR-PBO membranes showed excellent performance for CO2/CH4 separation,17,

39-42, 50, 51, 60-62

but their performance for

hydrogen purifications or air separation was generally much less attractive. In this regard, triptycene-containing PBOs in this study represent a highly versatile class of membrane materials with a broad spectrum of separation capability that meets almost all major light-gas separation needs in processing industries.

Table 1. Pure gas permeabilities and ideal selectivities for trip-PBO films (3 bar, 35 oC). Permeability (Barrer)a

Ideal selectivity (α)

Polymer TPBO-0.25 TPBO-0.5 TPBO-0.75 TPBO-1.063 TPBO-Ac-0.25 TPBO-Ac-0.5 TPBO-Ac-0.75 TPBO-Ac-1.0

H2

CO2

O2

N2

CH4

H2/N2

H2/CH4

O2/N2 CO2/CH4

1183 ± 27 1133 ± 27 1101 ± 22 810 ± 19 1701 ± 36 1577 ± 34 1351 ± 29 1123 ± 20

1213 ± 28 989 ± 23 762 ± 15 270 ±8 1433 ± 30 1081 ± 23 815 ± 18 519 ±9

249 ±5 199 ±5 156 ±3 68 ±2 337 ±7 240 ±5 179 ±4 109 ±2

57 ±1 39 ± 0.9 26 ± 0.5 8.4 ± 0.2 70 ±1 47 ± 0.9 31 ± 0.7 16 ± 0.3

37 ± 0.8 23 ± 0.5 14 ± 0.3 4.0 ± 0.1 40 ± 0.8 30 ± 0.6 18 ± 0.4 9.0 ± 0.2

21

32

4.4

33

29

49

5.1

43

42

79

6.0

54

96

203

8.1

68

24

43

4.8

36

34

53

5.1

36

44

75

5.8

45

70

125

6.8

58

a)

1 Barrer = 10-10 cm3(STP) cm cm-2 s-1 cmHg-1.

Another significant observation is that the selectivities of triptycene-based PBOs show a 15 ACS Paragon Plus Environment


nearly vertically increasing trend with increasing triptycene molar content, demonstrating the distinct role of triptycene moieties in boosting the gas separation performance. For example, the H2/CH4 selectivity increased by approximate six times from 32 for TPBO-0.25 to 203 for TPBO-1.0. Additionally, the TPBO-Ac-1.0 film is almost twice more selective and ~40 times more permeable than commercial Matrimid® polyimide membrane (Table S2). Similar trends were observed for H2/N2, O2/N2, and CO2/CH4 gas pairs, which positioned the TPBO-1.0 and TPBO-Ac-1.0 films among the most selective membranes for these separations. The superior gas separation performance in triptycene-containing PBO films can be largely ascribed to the (partially filled) internal free volume elements of triptycene units that are very comparable in size to the kinetic diameters of test gases;64-66 these internal free volume may act as narrow “gates” regulating selective transport. The dependence of separation performance on the triptycene content is summarized in Figure 3. Clearly, gas selectivities increase significantly with increasing triptycene content while retaining high permeabilities for small gases (H2, CO2, and O2). Specifically, H2/CH4 and H2/N2 selectivities exhibit the most dramatic increase with triptycene molar content, suggesting that the incorporation of triptycene units generates smaller cavities that favors fast transport of small gas molecules. CO2/CH4 selectivity also displays a gradual increase with increasing the triptycene molar content, making these TPBO films promising for efficient hydrogen purification and CO2 removal from natural gas. Previous research suggested that the incorporation of ether linkage may increase chain flexibility,67 which typically led to decreased gas selectivities;15, 68 while an opposite trend was observed in this study, i.e., gas selectivities was significantly enhanced with the increase of triptycene and ether linkage molar content. This seems to suggest that triptycene


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moieties and benzoxazole structures may play much more significant role in determining the size-sieving properties in TPBOs than the ether linkage. The respective contribution of triptycene, ether linkage, and benzoxazole structures to the overall intra-chain rigidity and gas transport properties of TPBO was qualitatively investigated by molecular simulation. As revealed by the degrees of torsional freedom (Figure S12), the TPBO chains display considerable reductions in polymer chain flexibility compared to their poly(o-hydroxyimide) precursors, mainly due to the smaller degree of torsional freedom for benzoxazoles. By increasing of triptycene-containing segment content in the TPBO copolymers, the concentrations of both triptycene and ether linkage increase while the concentration of benzoxazole structure maintained. Since ether linkage has the highest degree of torsional freedom, thus the significant increase of gas selectivities can be largely ascribed to the triptycene moieties by both the internal free volumes and enhancing polymer chain rigidity.

Figure 3. (a) Gas permeabilities and (b) normalized gas selectivities of TPBO-x (solid symbols) and TPBO-Ac-x (open symbols) as a function of triptycene molar content. All gas selectivities were normalized by the values of TPBO-0.25. As mentioned earlier (Figure 1 and Scheme S4), the same PBO structures are expected for 17 ACS Paragon Plus Environment


TPBO-x series and TPBO-Ac-x series derived via thermal treatment of TPHI-x and TPAI-x precursors, respectively. However, it has been shown that PBOs prepared from o-acetate containing precursors tend to have higher gas permeabilities and lower selectivities than the ones prepared from o-hydroxyl precursors primarily due to the degradation of bulky o-acetate groups preceding the actual TR conversion.39, 41, 42 The similar trend was observed for these triptycene-based PBOs, i.e., all TPBO-Ac-x films displayed higher gas permeabilities than the corresponding TPBO-x films derived from o-hydroxyl polyimide (Table 1). For instance, TPBO-Ac-0.25 has an H2 permeability 44% higher and an O2 permeability 35% higher than those of TPBO-0.25. However, unlike the previously observed decreasing trend in ideal selectivities for derivatized PBOs, the increase in gas permeabilities of TPBO-Ac-x series was accompanied by an increase of gas selectivities when the triptycene molar content is less than 50%. This synergistic improvement of both gas permeability and selectivity observed in the TPBO-Ac-x series seems to suggest that careful manipulation of the ortho-functionality of the polyimide precursor structures is of critical importance and practical usefulness in finely tuning the transport properties of the resulting PBO. Similarly, a significant increase of gas selectivities with increasing triptycene molar content was also observed for the TPBO-Ac-x series while high gas permeabilities were retained (Figure 3), leading to exceptional gas separation performance far above the 2008 upper bound (Figure 2). These results of TPBO-Ac-x series further confirm the extraordinarily gas separation capability that triptycene moieties can offer in the corresponding polymer membranes. To evaluate the practical feasibility of TPBO films for gas separation applications, mixed-gas permeation tests were performed on a TPBO-Ac-1.0 film using 20:80 and 50:50


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CO2/CH4 gas mixtures with CO2 partial pressure in the range of 1.5−6.7 bar (Figure S13). Both the mixed-gas CO2 and CH4 permeabilities were lower than those of the pure-gas values measured at similar CO2 partial pressures. This is largely due to the competitive sorption between CO2 and CH4 for available sorption sites, which reduces sorbed concentration for both gases within the films. On the other hand, TPBO-Ac-1.0 film displayed well-maintained, high CO2/CH4 selectivities under mixed-gas permeation conditions that were comparable to or even higher than those of the pure-gas values. For instance, the mixed-gas (50:50) CO2/CH4 selectivity decreased slightly and the value is very comparable to the pure-gas ideal selectivity at a CO2 partial pressure of 6.7 bar. The well-maintained gas selectivity under mixed-gas conditions can be attributed to the “blocking effect” of CH4 transport by the stronger adsorbing CO2 molecules, as evidenced by the higher solubility coefficients of CO2 over CH4 (Table S4 and Table S5).69,

70

It is notable that CO2 permeabilities of all TPBO films did not show

noticeable pressure dependence up to 10 bar under pure-gas permeation conditions (Figure S14); similarly, there was only minor membrane plasticization for the total feed pressure up to 13 bar in mixed-gas (CO2:CH4 = 20:80, 50:50) permeation tests. While the results obtained under current testing conditions demonstrate very promising plasticization resistance of TPBO films, it should be stressed that the real practice of CO2 removal from natural gas often involves much higher CO2 partial pressures. As such, aggressive conditions (e.g., CO2 partial pressures as high as 20 bar) should be applied in mixed-gas permeation tests to fully assess the membrane’ stability regarding its resistance against plasticization. Preliminary investigation of physical aging properties of TPBOs was carried out via evaluating pure-gas permeabilities of a TPBO-0.25 film that was aged for 70 days (Table S3).



The aged TPBO-0.25 film showed small reduction in gas permeabilities as compared to the fresh film with simultaneous increase in ideal selectivities. For instance, the H2 permeability decreased by ~14% from 1183 to 1011 barrer while H2/CH4 selectivity increased significantly by ~16% from 32 to 37. As a result, the aged sample still maintained superior permeability/selectivity combinations that outperformed the 2008 upper bounds (Figure S15). Long-term physical aging investigation is underway and the results will be reported in the future. 3.3 Microstructure characterization and free volume architecture After thermal rearrangement, no Tg was observed in both the TPBO-x and TPBO-Ac-x films up to 400 oC, suggesting highly rigid polybenzoxazole backbones. Along with the heterocyclic benzoxazole structure, rigid, bulky triptycene units tend to inhibit efficient chain packing leading to microporous structure allowing for fast gas transport. The averaged interchain distances were analyzed from WAXD patterns (Figure 4 and Figure S16). As shown, broad peaks were observed for both TPBO-x and TPBO-Ac-x films, indicating amorphous structures of these TPBOs. Compared to the polyimide precursors (Figure S16), all PBO films displayed expectedly higher d-spacing values due to the formation of more rigid PBO backbone upon thermal rearrangement (Figure 4). The average d-spacing values of both TPBO-x and TPBO-Ac-x films increase slightly with increasing triptycene molar content, this is possibly due to the bulky triptycene units which act as “chain packing inhibitors” and increase inter-chain distances in the membranes. It should be noted that WAXD measures the structural features that are related to regular or ordered chain packing structures rather than the actual microporous structures in the membranes. As such, the inter-chain d-spacing values do not necessarily reflect 20 ACS Paragon Plus Environment

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the average microcavity size of the membranes. Comparisons between the two series reveal that the average d-spacing values of TPBO-Ac-x films are higher than those of the corresponding TPBO-x films, suggesting that the disruption of chain packing in the precursors due to bulky o-acetate groups carried through the TR process and might generate larger micro-cavities in the final TPBO-Ac-x membranes. This microstructure analysis is consistent with the gas permeation results, in which TPBO-Ac-x films show higher gas permeabilities than the TPBO-x films.

Figure 4. WAXD patterns of (a) TPBO-x and (b) TPBO-Ac-x films. The average d-spacing values are shown in the figures. Since gas permeabilities of all triptycene-based PBO films follow the opposite order of kinetic diameters of gas molecules, gas permeation in these PBO films is controlled primarily by diffusion. To explore the underlying structure-property principles and elucidate the role of triptycene moieties in boosting the separation performance, gas diffusivity and solubility coefficients of all PBO films were analyzed and the results are tabulated in Table S4 (for TPBO-x series) and Table S5 (for TPBO-Ac-x series). Gas diffusivity coefficients of both TPBO-x and TPBO-Ac-x films decrease with increasing triptycene molar content. This is



consistent with the small decrease in gas permeabilities and can be ascribed to the internal free volume elements induced by triptycene molecules that may slow down gas transport in the membranes. As expected, TPBO-Ac-x copolymers exhibit higher gas diffusivity coefficients than the TPBO-x series due to the formation of large microcavities during TR process from the degradation of o-acetate groups. To further elucidate the role of size exclusion, diffusivity coefficients are plotted as a function of the squared gas kinetic diameter (Figure S17). The data of H2 diffusivities are not included because of the very short lag time of H2 that gives unreliable estimation of H2 diffusivity coefficient. As shown, by excluding CO2 data (due to the interplay of solubility contribution through dipole-quadrupole interactions between CO2 and the polar benzoxazole structure), the diffusivity coefficients of other gases correlate well with the squared kinetic diameter, confirming that size exclusion dominates the superior gas separation performance in the triptycene-based PBO membranes. Notably, the decrease in diffusivity coefficients with increasing triptycene content was much slower for the small gases than for the large gases, leading to enhanced diffusivity selectivities with triptycene content for CO2/CH4 and O2/N2 gas pairs in both TPBO-x series (Figure 5) and TPBO-Ac-x series (Figure S18). Considering the relatively constant solubility coefficients (Table S4 and Table S5) and solubility selectivities, the overall selectivities thus monotonically increased with increasing triptycene content, as clearly demonstrated in Figure 4. These results further verify that the molecular sieving effect is dominant and responsible for the extraordinary gas separation performance of the triptycene-based PBO films in this study.


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Figure 5. Diffusivity selectivity and solubility selectivity of TPBO-x copolymers for (a) CO2/CH4 and (b) O2/N2 gas pairs. To explore the molecular origin for this extraordinary performance, microporous structure of the TPBO-x films was probed by positron annihilation lifetime spectroscopy (PALS) (Figure 6 and Table S6). All PBO films exhibited a unimodal size distribution with the microcavity diameter spanning from ~ 4 to 12 Å that covers both the ultra-microporous (< 7 Å) and the microporous (> 10 Å) regions.27, 28 This microcavity architecture is different from the bimodal size distribution observed in the non-triptycene PBOs32, 39 and some triptycene-based PIMs27, 28, where two types of microcavities of different sizes were observed. However, molecular models have previously shown that in some cases it is difficult to distinguish whether a unimodal or bimodal fit is suitable due to the nature of the pore distribution.71 It is generally believed that gas separation characteristics in TR polymers originate from smaller cavities with diameter of less than 0.4 nm.17 Considering that the size of internal free volume elements of triptycene unit is less than 0.4 nm (3.8 Å in diameter of an equivalent sphere),64 this is consistent with the observation that the selectivities continuously increased with increasing the triptycene molar content. The relatively broad unimodal cavity size distribution implies that the internal free volumes may form well-connected microporous structures that prevents a bimodal size 23 ACS Paragon Plus Environment


distribution being distinguished. The average microcavity diameters in the TPBO films are comparable to the molecular dimension of triptycene moieties,64 suggesting the dominant contribution of triptycene units towards the observed superior size sieving properties. The relatively widely spanned size distribution may indicate the presence of a series of hourglass-type microcavities of various sizes in the necking region, responsible for the separation versatility of the TPBO films. Additionally, the average microcavity diameter decreases with increasing triptycene content suggesting the formation of smaller cavities associated with the internal molecular cavity of triptycene units, which correlates well with the increasing selectivity with triptycene content. Notably, as the proportion of triptycene is increased in the PBO membranes, there is a systematic decrease in the PALS Intensity (Figure 6), indicating reduced number of free-volume cavities. Previous studies have considered the drop in Intensity may be due to o-Ps inhibition induced by changes in chemistry in materials with electron withdrawing groups.72 However, the copolymer series in this study are not chemically distinct because of the essentially same combination of monomers but in different ratios, and the trends showing a correlation of a decrease in Intensity with an associated decrease in permeability (Table 1) confirms that the Intensity values are indeed likely to be representative of the relative number of cavities.


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Figure 6. Microcavity size and size distribution of (a) TPBO-0.25, (b) TPBO-0.5, (c) TPBO-1.0 and (d) TPBO-0.75 obtained from PALS. The inset table lists the average microcavity diameter and full width at half maximum (FWHM) from the size distribution.

4. Conclusions Thermally rearranged polybenzoxazole membranes with unprecedentedly high gas permeabilities and selectivities were fabricated via thermal rearrangement of two series of novel polyimide precursors that incorporate hierarchical triptycene units into the backbone structures along with carefully-chosen ortho-functionality. The incorporation of triptycene moieties led to superior gas selectivities in the entirely triptycene-based TPBO-1.0 and TPBO-Ac-1.0 samples, which are arguably the most selective membranes among existing glassy polymer membranes. These triptycene-based PBO films possess extremely versatile separation capability applicable for a wide range of important gas separations. Specifically, the microcavity characteristics and gas transport properties of these TPBO films are highly tailorable by adjusting the triptycene content and the precursor ortho-functionality. Mixed-gas (CO2/CH4) permeation tests and preliminary physical aging studies indicate that TPBOs have excellent resistance towards membrane plasticization and physical aging. Molecule simulation results revealed that both 25 ACS Paragon Plus Environment


triptycene moieties and benzoxazole structure contribute to the superior gas separation performance. This study demonstrates a new macromolecular design in the use of triptycene structural unit to bridge high permeability and high selectivity for more commercially viable PBO-based membranes applicable for hydrogen purification and CO2 removal from natural gas.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Detailed information on the monomer, precursor polymer synthesis and TR schemes; pictures of precursor polymers and films; NMR and FTIR spectra for both TPHI and TPAI films; TGA profiles of both TPHI and TPAI films; FTIR results of PBO and TPBO-Ac films; O2/N2 separation performance of TPBO films; degrees of torsional freedom for select dihedral angles; mixed gas permeation results of TPBO-Ac-1.0 film; pure gas permeabilities as a function of feed pressure; physical aging of TPBO-0.25 film in the upper bound plots; WAXD patterns of TPHI and TPAI films; diffusivity coefficients as a function of squared kinetic diameter of test gases; diffusivity selectivity and solubility selectivity of TPBO-Ac films as a function of triptycene molar contents; molecular weight and molecular weight distribution of precursors; pure gas diffusivity coefficients, solubility coefficients of both TPBO and PBO-Ac films.

AUTHOR INFORMATION Corresponding Authors *Phone: 1-574-631-3453. Fax: +1-574-631-8366. E-mail: [email protected] Notes The authors declare no competing financial interest.

Acknowledgments R. Guo gratefully acknowledges the financial support from the Division of Chemical Sciences,


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Biosciences, and Geosciences, Office of Basic Energy Sciences of the U.S. Department of Energy (DOE) under the award DE-SC0010330 and National Science Foundation (NSF) under the award CBET-1603414. C. M. Doherty acknowledges the support of Australian Research Council under award DE140101359. P. Gao was supported by the U.S. Department of Energy, Office of Science, and Office of Advanced Scientific Computing Research under Contract DE-AC05-76RL01830, as part of the Collaboratory on Mathematics for Mesoscopic Modeling of Materials (CM4).

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