Unprecedented size-sieving ability in polybenzimidazole doped with polyprotic acids for membrane H₂/CO₂ separation

Abstract

Polymers with efficient and tight chain-packing and thus strong size-sieving ability are of great interest for H₂/CO₂ separation. Herein, we demonstrate a new approach to manipulating polymer structure by acid doping, leading to superior H₂/CO₂ separation performance. We have doped polybenzimidazole (PBI) with polyprotic acids, specifically H₃PO₄ and H₂SO₄. These acids cross-link PBI chains and drastically decrease free volume, improving the material's H₂/CO₂ selectivity to far surpass the Robeson's 2008 upper bound for membrane performance. For example, PBI doped with H₃PO₄ at a molar ratio of 1 : 1 exhibits an unprecedented H₂/CO₂ selectivity of 140 at 150 °C, which exceeds that of previously known polymeric materials and is superior or comparable to that of state-of-the-art 2D materials with sharp size separation, such as graphene oxide, MoS₂, and metal–organic frameworks. This facile approach to enhancing polymer chain-packing efficiency opens up a new avenue for designing strong size-sieving polymers for membrane gas separations.

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Broader context

CO₂ capture, sequestration and utilization is an important approach to mitigating CO₂ emissions to the environment. A key strategy in this regard is to decarbonize fossil fuels, producing H₂ and CO₂ at high temperature and pressure, which must then be separated to obtain CO₂ for sequestration/utilization. Membrane technology has inherent advantages for gas separation because of its high energy efficiency, small footprint and ease of scale-up. However, materials with superior H₂/CO₂ separation properties at 150 °C or above are lacking. Here we report, for the first time, that polybenzimidazole doped with an anhydrous polyprotic acid shows an unprecedented H₂/CO₂ selectivity of 140 at 150 °C. This selectivity is the highest among known polymers. The simple yet elegant approach of acid doping provides a flexible and exciting platform to enhance polymer chain-packing efficiency, and thus achieve sharp molecular size separation in polymers.

H₂ is a key chemical used in oil refining and production of ammonia and methanol, and it has been explored as a clean fuel for fuel cells and electricity generation. Currently, H₂ is mostly generated by steam reforming of fossil fuels followed by the water–gas shift reaction. The produced H₂ is always mixed with CO₂, and it must be purified before utilization. Ideally, the CO₂ is captured for utilization or sequestration to prevent its emission to the environment.^1–3 Membranes that are permeable to H₂ and reject CO₂ at syngas processing conditions (150 °C and above) have been demonstrated as a low-cost and energy-efficient means of H₂ purification and CO₂ capture.^4–6 The key to success of this technology is cost-effective membrane materials with superior H₂/CO₂ separation properties. Recently, inorganic materials have been widely explored, including graphene oxide (GO),^7,8 MoS₂,⁹ zeolites,¹⁰ and metal–organic frameworks (MOFs).^11–13 2D layered materials, including GO and MoS₂, were stacked to form sub-nanometer channels between layers,^7–9 while MOFs and zeolites were designed with apertures between the kinetic diameters of H₂ (2.89 Å) and CO₂ (3.3 Å),¹¹ leading to extremely strong size-sieving ability and thus superior H₂/CO₂ separation properties. However, commercial-scale production of defect-free membranes with sub-nanometer channels presents an enormous challenge. Current commercial membranes for gas separation are based on polymers, partially owing to their good processability and ease of scale-up.¹⁴ However, conventional polymers are amorphous and cannot achieve the sharp size-based separation possible with stacked GO and MoS₂ layers, zeolites, or MOFs.

Gas transport in polymeric membranes generally follows a solution-diffusion mechanism.^14,15 Gas permeability (P) for each species is the product of its solubility (S) and diffusivity (D) in the polymer. Selectivity is thus determined by a combination of solubility selectivity and diffusivity selectivity. Because H₂ is smaller and less condensable than CO₂, polymers usually have favorable H₂/CO₂ diffusivity selectivity and unfavorable solubility selectivity, leading to only moderate H₂/CO₂ selectivity.¹ For example, commercial polymeric membranes of cellulose acetate (CA), polysulfone (PSF) and Matrimid® have H₂/CO₂ selectivities below 3.¹⁶

To achieve high H₂/CO₂ selectivity, polymers should have rigid chains with efficient chain-packing to obtain high H₂/CO₂ diffusivity selectivity. For example, poly[2,2′-(m-phenylene)-5,5-bibenzimidazole] (PBI) exhibits H₂/CO₂ selectivity of 14 to 20 at 150 °C depending on the polymer grades and processing conditions.^17–21 The efficiency of polymer chain-packing can be improved by cross-linking.²² For example, PBI can be cross-linked using terephthaloyl chloride to increase its H₂/CO₂ selectivity by almost 100%.²¹ Thin films of polyimides such as 6FDA-durene and 6FDA-ODA/NDA can also be cross-linked using diamines to yield high H₂/CO₂ selectivity.^23,24 However, these cross-linked polyimides are not stable at 150 °C or above, due to thermally induced re-imidization.^25,26

Herein, we demonstrate a facile approach to improve polymer size-sieving ability by doping PBI with polyprotic acids, specifically H₃PO₄ and H₂SO₄. As shown in Fig. 1a, H₃PO₄ can strongly interact with PBI chains via proton transfer from the acid to imidazole rings of PBI and hydrogen bonding, and thus it cross-links the PBI.²⁷ In comparison, a monoprotic acid such as HCl is not expected to cross-link the PBI chains. PBI doped with H₃PO₄ has been widely explored for use in high-temperature proton exchange membranes for fuel cell applications.^28–31 However, there are no prior reports of its application for membrane gas separation. This study, for the first time, investigates PBI doped with polyprotic acids for membrane gas separation. A series of H₃PO₄ doped PBIs (i.e., PBI–(H₃PO₄)_x) were prepared by immersing PBI films (with a thickness of ca. 10 μm) in solutions containing H₃PO₄ and methanol (Fig. 1b). Different H₃PO₄ doping levels (x = 0.16–1.0) were obtained by varying the H₃PO₄ concentration in the solutions from 0.05 to 1.0 wt% (see Table S1 in the ESI†). The maximum doping level employed in this study was x = 1.0 because H₃PO₄ cross-links PBI at a stoichiometric ratio of 1 : 1. At a sub-stoichiometric doping level (1 or less), acid doping can increase the elastic modulus of PBI due to protonation and strong hydrogen bonding between PBI and H₃PO₄. However, a higher doping level (particularly x > 2) deteriorates mechanical strength because the excess (non-bonded) acid can swell PBI, increasing the space between PBI backbones. For instance, increasing the doping level from 2.3 to 5.6 was shown to drastically decrease ultimate tensile strength of PBI from 160 MPa to 10 MPa at 125 °C.³² The enhanced mechanical properties of PBI–(H₃PO₄)_x with x = 0.16–1.0 would also be advantageous for their use in membrane gas separation. Fig. 1c and d show the elemental distribution of phosphorus on the surface and cross-section of a PBI–(H₃PO₄)_1.0 film, respectively, as measured using scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDS). The even distribution of phosphorus in the analyzed area indicates that the PBI films are homogenously doped with H₃PO₄.

Fig. 1 Schematic illustration of (a) proton transfer mechanism and hydrogen bonding in PBI–H₃PO₄ complex, and (b) preparation of H₃PO₄ doped PBI films with PBI backbones cross-linked by acids. SEM images with overlaid SEM/EDS mapping of phosphorus on the (c) surface and (d) cross-section of a PBI–(H₃PO₄)_1.0 film. The red dots display the distribution of phosphorus in the polymer.

Fig. 2a presents Fourier-transform infrared (FTIR) spectra of PBI–(H₃PO₄)_x films after vacuum drying at 160 °C. The proton transfer from H₃PO₄ to PBI leads to the formation of H₂PO₄⁻, indicated by the characteristic peaks of PO₂ (1050 cm⁻¹) and P(OH)₂ (870 cm⁻¹ and 945 cm⁻¹).²⁹ The PBI–(H₃PO₄)_x samples were also characterized by thermal gravimetric analysis (TGA, Fig. S1, ESI†) and their degradation temperature is above 200 °C, which is consistent with literature results,³³ and demonstrates their promise for practical H₂/CO₂ separation at 150 °C. Moreover, these complexes are known to be chemically stable in the presence of CO, oxygen, water vapor, and methanol at 150–200 °C.^34,35

Fig. 2 (a) FTIR spectra and (b) WAXD patterns of PBI and PBI–(H₃PO₄)_x films. Curves are labelled with the values of x (defined as molar ratio of H₃PO₄ to the PBI repeating unit).

Fig. 2b depicts the effect of H₃PO₄ doping on wide-angle X-ray diffraction (WAXD) patterns of the PBI–(H₃PO₄)_x films. PBI shows a diffraction peak at 22°, which corresponds to a d-spacing (or average inter-segmental distance between polymer chains) of 4.0 Å. The H₃PO₄ doping shifts the diffraction peak to 25°, which corresponds to a d-spacing of 3.6 Å. This 10% decrease in interchain distance reflects the strong interaction between H₃PO₄ and the imidazole groups in PBI (Fig. 1a). The decrease in d-spacing inevitably increases size-sieving ability and thus H₂/CO₂ diffusivity selectivity.^36,37

Pure-gas H₂ and CO₂ permeability of PBI and PBI–(H₃PO₄)_x samples were measured at 150 °C and feed pressures ranging from 8.0 to 15 atm. The results are summarized in Table S2, ESI.† CO₂ permeability for each sample was independent of feed pressure, indicating that the acid doped PBIs are resistant to CO₂ plasticization. As shown in Fig. 3a, increasing the H₃PO₄ doping level decreases the pure-gas permeability, and drastically increases the H₂/CO₂ selectivity. For example, PBI shows H₂/CO₂ selectivity of 16, while PBI–(H₃PO₄)_1.0 shows a remarkable selectivity of 140, which is much higher than that of any previously studied polymers. A further increase in the H₃PO₄ doping level to 2.5 decreased the H₂ permeability to 0.65 Barrers and H₂/CO₂ selectivity to 100, presumably because the non-bonded H₃PO₄ in the PBI swells and plasticizes the polymer, decreasing its size-sieving ability.

Fig. 3 (a) Pure-gas permeability of H₂ and CO₂ at 150 °C as a function of H₃PO₄ doping level in PBI. (b) CO₂ and CH₄ sorption isotherms for PBI and PBI–(H₃PO₄)_1.0 at 150 °C. CH₄ is used as a surrogate for H₂, and the curves are the best fits to the dual mode sorption model with fitting parameters summarized in Table S3, ESI.† (c) Effect of acid doping level on the density and FFV of PBI–(H₃PO₄)_x at 23 °C and 150 °C. (d) Correlation of pure-gas permeability in PBI–(H₃PO₄)_x with the FFV using the free volume model (eqn (1)) at 150 °C.

To elucidate how the H₃PO₄ loading affects gas transport properties, pure-gas sorption of CO₂ and CH₄ in PBI and PBI–(H₃PO₄)_1.0 was quantified using a gravimetric sorption analyzer at 150 °C. The resulting isotherms are shown in Fig. 3b. The experimental data can be fit well by curves based on the dual mode sorption model.³⁸ In considering solubility selectivity, we have used CH₄ as a surrogate for H₂ because H₂ sorption is too low to measure gravimetrically. Neither CH₄ nor H₂ is expected to exhibit specific interactions with polymers.³⁸ Sorption of both CO₂ and CH₄ decreases slightly after acid doping, due to the reduction in accessible free volume for gas sorption. However, the CO₂/CH₄ solubility selectivity remains unchanged upon acid doping. For example, the CO₂/CH₄ solubility selectivities for PBI and PBI–(H₃PO₄)_1.0 at 15 atm are 8.3 and 8.6, respectively. These results imply that the decrease in gas permeability and increase in H₂/CO₂ selectivity in the H₃PO₄ doped PBIs can be ascribed to changes in diffusivity and diffusivity selectivity, respectively.

Because PBI–(H₃PO₄)_x samples show similar gas solubility, gas permeability can be correlated with the fractional free volume (FFV) using the following equation:³⁹

P_A = A_A exp(−B_A/FFV) (1)

where A_A (Barrer) is a pre-exponential factor and B_A is a constant that increases with increasing penetrant size. The FFV is defined as:³⁹

FFV = (V − V₀)/V (2)

where V is the polymer specific volume, and V₀ is the occupied volume of the polymer estimated by the group contribution method (Table S4, ESI†).⁴⁰ As shown in Fig. 3c, increasing the doping level increases the polymer density and decreases the FFV. For instance, at 23 °C, PBI–(H₃PO₄)_1.0 has a density of 1.498 g cm⁻³ and FFV of 0.070 at 23 °C, reflecting a 16% increase in density and a 56% loss in FFV compared with PBI. Due to thermal expansion, the FFV increases with increasing temperature (cf. Table S4, ESI†). At 150 °C where gas permeability was determined, PBI–(H₃PO₄)_1.0 exhibits a FFV of 0.087, which is 52% smaller than that of pure PBI (0.167), and also much lower than the FFV of other common glassy polymers.¹⁶

As shown in Fig. 3d, pure-gas permeability can be satisfactorily described using eqn (1) with B_A values of 0.54 and 0.93 for H₂ and CO₂, respectively. The larger B_A value for CO₂ is consistent with its larger molecular size. As the FFV decreases, certain free volume elements initially become inaccessible to the larger CO₂ and only with further decrease of FFV become inaccessible to the smaller H₂. Consequently, the CO₂ permeability decreases more rapidly than H₂ permeability. This leads to an exponential increase in H₂/CO₂ selectivity, as indicated by the data and best-fit line in Fig. 3d. PBI–(H₃PO₄)_1.0 has an extremely low FFV value of 0.087 and thus exceptionally sharp molecular separation of H₂/CO₂ (with a selectivity of 140).

Membrane-based H₂/CO₂ separation has been proposed as an efficient technology to purify H₂ from coal-derived shifted syngas, which is typically comprised of 56% H₂, 41% CO₂, and small amounts of other components, such as H₂O, CO, N₂ and H₂S, at 150 °C or above.⁴ We investigated the mixed-gas separation performance of PBI–(H₃PO₄)_x using a binary gas mixture of 50% H₂ and 50% CO₂ at 120–180 °C with a feed pressure of 14 atm. PBI–(H₃PO₄)_0.16 was selected as a model sample because of its balanced H₂ permeability and H₂/CO₂ selectivity. As shown in Table S5, ESI,† it exhibits a mixed-gas H₂ permeability of 12 Barrers and a H₂/CO₂ selectivity of 34 at 150 °C. These values are very close to the pure-gas properties, suggesting the absence of competitive sorption and plasticization at 150 °C.^41,42 This behavior is very different from conventional studies of polymers for CO₂/CH₄ separation at 35 °C, where CO₂ can plasticize the polymer matrix, decreasing the size-sieving ability and thus CO₂/CH₄ selectivity.^43–45 This contrast also demonstrates an advantage of operating the membrane H₂/CO₂ separation at high temperatures, where CO₂ sorption is low, and the adverse effect of plasticization can be avoided. As shown in Fig. 4a, increasing temperature dramatically increases both H₂ and CO₂ permeability. On the other hand, the temperature has a negligible effect on H₂/CO₂ selectivity (at 33 ± 2). This behavior can be ascribed to two opposing factors. Increasing temperature decreases the size-sieving ability in polymers (or H₂/CO₂ diffusivity selectivity), whereas it may increase H₂/CO₂ solubility selectivity, resulting in almost constant H₂/CO₂ selectivity.²¹

Fig. 4 (a) Mixed-gas H₂/CO₂ separation performance of PBI–(H₃PO₄)_0.16 in gas mixture (50%H₂/50%CO₂) at temperatures ranging from 120 to 180 °C. The lines are to guide the eye. (b) Long-term stability test of PBI–(H₃PO₄)_0.16 in both dry and humidified conditions at 150 °C for 120 h. 0.3 mol% water moisture was introduced in feed gas (50%H₂/50%CO₂) at 11 h. (c) Pure-gas H₂/CO₂ separation performance of PBI–(H₃PO₄)_x with different doping levels (x = 0.16–1.0) and PBI–(H₂SO₄)_0.24 at 150 °C versus Robeson's 2008 upper bound (the blue line) at 35 °C.⁴⁷ The orange line is the upper bound predicted for 150 °C.⁴⁸ (d) Comparison of H₂/CO₂ separation performance of PBI–(H₃PO₄)_x with state-of-the-art membrane materials: green squares (1 to 6) represent polymeric membranes and mixed matrix membranes (MMMs);^6,50–54 black triangles (7 to 9) represent inorganic materials including GO,⁷ MoS₂,⁹ and zeolite;¹⁰ and orange diamonds (10 to 14) represent MOF membranes and their composites.^{11,13,55–57} The box on the right explains the data points in detail.

We also investigated the effect of water vapor on mixed-gas separation properties and long-term stability of PBI–(H₃PO₄)_0.16 at 150 °C. As shown in Fig. 4b, PBI–(H₃PO₄)_0.16 was initially tested using a dry gas mixture (50% H₂/50% CO₂) at a pressure of 14 atm for 10 hours. Then 0.3 mol% water vapor was introduced in the mixed-gas feed, and the mixed-gas permeability was tested over a period of 110 hours. The presence of water slightly decreased mixed-gas H₂ permeability from 12 Barrers (dry state) to 11 Barrers (humidified state), while the H₂/CO₂ selectivity gradually increased from 34 (dry state) to 38 (humidified state). PBI is quite hydrophilic and the dry polymer can absorb up to 20 wt% water.⁴⁶ The absorbed water occupies some free volume elements, which decreases the accessible free volume for gas diffusion and increases size-sieving ability. Due to the strong affinity between PBI and H₃PO₄, absorbed water does not displace the cross-linking acid, and no performance deterioration was observed upon exposing the PBI–(H₃PO₄)_0.16 film to moisture for over 100 hours. This demonstrates the material's remarkable reliability under these simulated shifted syngas processing conditions and supports its potential for practical application.

Pure-gas H₂/CO₂ separation properties of PBI–(H₃PO₄)_x are benchmarked in a Robeson's plot (Fig. 4c).⁴⁷ The 2008 upper bound represents a tradeoff between H₂ permeability and H₂/CO₂ selectivity, and the highest H₂/CO₂ selectivity achievable for any given H₂ permeability in polymers at around 35 °C. For example, the separation performance of commercial membrane polymers such as CA, PSF, Matrimid and poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) is below this upper bound.¹⁶ Meanwhile, our PBI–(H₃PO₄)_x samples, operating at 150 °C, are significantly above the 2008 upper bound empirically drawn for 35 °C and another upper bound predicted for 150 °C,⁴⁸ indicating their superior H₂/CO₂ separation performance. Interestingly, a trend line drawn over the data points of PBI–(H₃PO₄)_x is almost parallel to the upper bound, which is consistent with the notion that decreasing free volume decreases permeability and increases selectivity.⁴⁹ For comparison, gas separation properties of PBI and PBI–(H₃PO₄)_0.16 at 35 °C are also included in Fig. 4c. They are much closer to the 2008 upper bound than the commercial membrane polymers.

Fig. 4d compares H₂/CO₂ separation performance of PBI–(H₃PO₄)_x samples with other state-of-the-art materials including polymers,⁵⁰ mixed matrix membranes (MMMs) containing MOFs or palladium nanoparticles,^6,51–54 inorganic materials (e.g., GO, MoS₂ and zeolites),^7,9,10 and layered MOF nanosheets.^{11,13,55–57} The PBI–(H₃PO₄)_x samples exhibit higher H₂/CO₂ selectivity than these leading materials with superior size-sieving ability, except for the layered MOF nanosheets.¹¹ More importantly, in contrast to the layered 2D materials (such as MOFs, GOs, and MoS₂) which present enormous challenges in membrane manufacturing and scale-up, the PBI–(H₃PO₄)_x can be easily incorporated into current manufacturing practices for polymer-based membranes.

To demonstrate the generality of the concept of polyprotic acid doping to increase polymer size-sieving ability, PBI was also doped with H₂SO₄ at a level of 0.24, i.e., PBI–(H₂SO₄)_0.24, following the same procedure used for preparing PBI–(H₃PO₄)_x. The successful doping of H₂SO₄ was confirmed by the FTIR spectrum (cf. Fig. S2, ESI†). Similar to the H₃PO₄ doping, H₂SO₄ doping also increases polymer density, decreases FFV (cf., Table S4, ESI†) and reduces d-spacing (cf., Fig. S3, ESI†). As shown in Fig. 4c, PBI–(H₂SO₄)_0.24 exhibits H₂/CO₂ separation performance above the upper bound. With a similar doping level, PBI–(H₂SO₄)_0.24 and PBI–(H₃PO₄)_0.25 exhibit almost the same H₂ permeability and H₂/CO₂ selectivity, presumably due to the similar effect of acids in enhancing the chain-packing efficiency for PBI.

Conclusions

We have demonstrated, for the first time, the enhancement of polymer chain-packing efficiency by doping with polyprotic acids to improve the size-sieving ability and H₂/CO₂ separation properties in PBI at 120–180 °C. The polyprotic acids (H₃PO₄ or H₂SO₄) form complexes with PBI, which are stable up to 200 °C and show great promise for H₂ purification and CO₂ capture. The acid doping has a negligible effect on gas sorption, but it significantly reduces FFV and d-spacing, thereby drastically improving H₂/CO₂ diffusivity selectivity. For example, PBI–(H₃PO₄)_1.0 exhibits an unprecedented H₂/CO₂ selectivity of 140, which is much higher than that of any known polymers, and superior or comparable to that of emerging 2D materials such as GO, MoS₂, and MOFs. The free volume model was used to correlate the gas permeability and structural changes induced by acid doping.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the U.S. Department of Energy, Office of Fossil Energy, under Award Number DE-FE0026463. This work was also partially supported by the U.S. National Science Foundation (NSF) under CAREER award number 1554236.

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Footnote

† Electronic supplementary information (ESI) available: See DOI: 10.1039/c7ee02865b

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