Abstract
The synthesis of high-performance gas-sieving zeolitic film is challenging because it involves seeded secondary growth which is complex and difficult to reproduce. Additionally, expensive asymmetric inorganic porous supports are often needed for calcination of the zeolite films. Herein, scalable preparation of H2-sieving RUB-15 nanosheet films is reported bypassing the need for secondary growth and expensive supports. A novel, low-cost, thermally-robust, porous polybenzimidazole copolymer (PBI- AM Fumion®) support is prepared which allows deposition of thin, compact, and oriented RUB-15 nanosheets films without cracks or pinhole defects. The film hosts two transport pathways, H2-sieving six-membered silicate rings in the nanosheets and nonselective intersheet gaps maintained by the organic guest species in the gallery spacing. The latter is eliminated by a low-temperature (330 °C) calcination where the crystalline order in the nanosheets is preserved. The resulting zeolitic films yielded H2 permeances of 100–400 GPU and H2/CO2 selectivities above 20 at temperatures above 200 °C. The facile and scalable fabrication procedure with an attractive H2-sieving performance at elevated temperatures make these membranes promising for pre-combustion carbon capture.
| Original language | English (US) |
|---|---|
| Article number | 121454 |
| Journal | Journal of Membrane Science |
| Volume | 672 |
| DOIs | |
| State | Published - Apr 15 2023 |
Keywords
- Hydrogen sieving
- Polybenzimidazole
- Polymeric support
- RUB-15
- Sodalite nanosheets
- Zeolitic membranes
ASJC Scopus subject areas
- Biochemistry
- Materials Science(all)
- Physical and Theoretical Chemistry
- Filtration and Separation
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In: Journal of Membrane Science, Vol. 672, 121454, 15.04.2023.
Research output: Contribution to journal › Article › peer-review
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TY - JOUR
T1 - Hydrogen-sieving zeolitic films by coating zeolite nanosheets on porous polymeric support
AU - Dakhchoune, Mostapha
AU - Duan, Xuekui
AU - Villalobos, Luis Francisco
AU - Avalos, Claudia Esther
AU - Agrawal, Kumar Varoon
N1 - Funding Information: The synthesis of high-performance gas-sieving zeolitic film is challenging because it involves seeded secondary growth which is complex and difficult to reproduce. Additionally, expensive asymmetric inorganic porous supports are often needed for calcination of the zeolite films. Herein, scalable preparation of H2-sieving RUB-15 nanosheet films is reported bypassing the need for secondary growth and expensive supports. A novel, low-cost, thermally-robust, porous polybenzimidazole copolymer (PBI-AM Fumion®) support is prepared which allows deposition of thin, compact, and oriented RUB-15 nanosheets films without cracks or pinhole defects. The film hosts two transport pathways, H2-sieving six-membered silicate rings in the nanosheets and nonselective intersheet gaps maintained by the organic guest species in the gallery spacing. The latter is eliminated by a low-temperature (330 °C) calcination where the crystalline order in the nanosheets is preserved. The resulting zeolitic films yielded H2 permeances of 100–400 GPU and H2/CO2 selectivities above 20 at temperatures above 200 °C. The facile and scalable fabrication procedure with an attractive H2-sieving performance at elevated temperatures make these membranes promising for pre-combustion carbon capture.Advances in simplifying the synthesis of gas-sieving zeolite membranes have been recently reported. Tsapatsis and co-workers could synthesize high-quality oriented MFI films on a porous support by filter coating of exfoliated nanosheets dispersed in a solvent [6,7]. They could achieve moderate gas pair (n-butane/isobutane) selectivity (5.4) from nanosheet films without resorting to secondary growth when the nanosheets were acid treated to remove organic structure directing agents (OSDA) from the MFI pore channels [8]. The moderate n-butane/isobutane selectivity, compared to typical selectivities of approximately 50 from the intergrown MFI films, can be attributed to the pinhole defects caused by non-uniform surface of polymeric support (see discussion later) and intersheet gaps in these films. Recently, we reported a synthesis route for zeolitic membranes using sodalite precursor (RUB-15) nanosheet film, hosting hydrogen-sieving six-membered ring (6-MR), while avoiding the secondary growth step [9]. The intersheet gaps in these films, maintained by organic species in the gallery spacing, were removed by calcination in air at 500 °C which also promoted silanol condensation between the nanosheets. However, 500 °C treatment in air renders the potential use of polymeric supports unfeasible. In this regard, the development of a milder activation technique for removing the organics from the gallery spacing is extremely attractive. Another major bottleneck in the advancement of gas separation zeolite membranes is the prohibitive cost of the asymmetric inorganic porous supports which often accounts for up to 90% of the total cost of the membrane [ 10–13]. In this regard, porous polymeric supports are highly attractive. The synthesis of zeolite membranes on polymeric supports was successfully demonstrated on polyethersulfone (PES) for LTA and Faujasite frameworks [14,15]. However, PES thermal stability is compromised at high temperatures, making PES suitable only for (i) low-temperature separation application, and (ii) crystallization recipes which do not use OSDA, and therefore, do not require thermal activation. Currently, most of the frameworks of interest for gas separation are synthesized with OSDA requiring a thermal treatment (>673 K) step in an oxidative atmosphere to activate the zeolitic pores by decomposing the occluded organic molecules [16]. This prohibits the use of the majority of the polymeric materials for support fabrication, leaving only a few thermally stable polymers such as polybenzimidazoles (PBIs).Herein, we address the above-mentioned challenges by developing a smooth PBI support and facile preparation routes for gas-sieving RUB-15 nanosheet films. Highly smooth and uniform PBI supports were fabricated using the highly processible PBI copolymer (PBI-AM Fumion®). Thin, compact, oriented, and pinhole-free RUB-15 films could be prepared on this porous support by filter coating. The films underwent a mild calcination to eliminate the interlayer non-selective pathways. With in-situ X-ray diffraction (XRD) and in-plane X-ray diffraction, it is confirmed that the mild calcination near 300 °C is effective to reduce the intersheet gaps and the ordered structure (6-MR) within the nanosheets is preserved. As a result, RUB-15 membranes with H2 permeances of 100–400 GPU and H2/CO2 selectivities above 20 at temperatures above 200 °C were successfully prepared. The scalable fabrication and the attractive sieving performance at high temperatures make these membranes promising for pre-combustion carbon capture.An 8 w/w% polymer coating solution was prepared by stirring PBI-AM in NMP. The solution was used as it is without any further treatment. A casting knife with a gap of ∼250 μm was used to cast the solution on the stainless-steel mesh. Upon casting, it was placed in a water coagulation bath at 60 °C and left overnight. The supports were then washed with DI water and were allowed to dry at room temperature. Finally, the PBI-AM coated mesh was heated at 330 °C for 8 h.To fabricate a thermally robust porous support, we used PBI-AM Fumion® (henceforth referred as PBI-AM), which is a commercial polymer developed for the ion-exchange application. PBI-AM has many advantages for the fabrication of porous support compared to the conventional PBI (e.g., mPBI) [17,20]. mPBI is difficult to dissolve in common organic solvents and traces of undissolved polymer make the support uneven and less suitable for a compact nanosheet coating. On the other hand, PBI-AM is highly processible. We could dissolve PBI-AM in dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), and N-methyl-2-pyrrolidone (NMP) up to 10 w/w% by simple stirring at room temperature. Additionally, while the mPBI porous film derived by NIPS tends to collapse upon drying [21,26,27], the PBI-AM film resisted collapse and densification. We could successfully cast porous films onto a macroporous stainless-steel mesh (SSM, pore opening of 20 μm, Figs. 1a and 2a). SSM is a mechanically and thermally robust low-cost support. The high porosity in SSM allows infiltration of the viscous polymeric solution, effectively anchoring the resulting film onto the mesh. A smooth and uniform PBI film with pore opening of 20 nm was obtained by casting PBI-AM solution in NMP (8 w/w%) on SSM followed by phase inversion in water (Fig. 2b, c). The top section of the film (2–20 μm below the surface) had a macrovoid-free spongy structure while the central and bottom sections composed of macrovoids (Fig. 2d and e). The as-prepared PBI films were heated in air at 330 °C for 8 h to allow thermal rearrangement and structure stabilization prior to the deposition of RUB-15 nanosheets. Given the high thermal stability of PBI-AM, the porous morphology of the support was preserved, and cracks were absent (Fig. S1). The N2 permeance of the bare PBI-AM support was on the order of 10−4 mol m−2 s−1 Pa−1 and decreased to 10−5 mol m−2 s−1 Pa−1 upon calcination in air at 330 °C for 8 h. We note that PBI-AM support displays a comparable or higher permeance of the widely used asymmetric inorganic porous supports [28]. Also, the ease of fabrication and scalability of the polymeric support compared to the inorganic one makes it much more attractive.The preparation of H2-sieving RUB-15 nanosheet films on top of the porous PBI-AM support is illustrated in Fig. 1b. Firstly, RUB-15 nanosheets were transferred from the mother dispersion to ethanol by centrifugation and redispersion. Following this, CTA+ was exchanged with H+ using 0.2 M H2SO4 acid. TGA confirmed a significant removal of CTAB as the weight loss in the temperature range of 25–600 °C was only 18.3% (Fig. S2a). Given that the weight loss in this temperature range is also driven by the loss of water molecules generated from the silanol condensation between the nanosheets, we also treated the nanosheets with a concentrated H2SO4 (4 M). In this case, we found a slightly lower weight loss of 9.7% (Fig. S2a). Assuming that the 4 M acid treatment drives CTAB removal to the completion with corresponding weight loss solely due to the loss of water from the condensation product, this indicates that the nanosheets in mother dispersion had 46 w/w% CTAB, and the concentration of CTAB reduced to 9.5 w/w% after the treatment with 0.2 M H2SO4 acid (Note S1 and Table S1). Films formed by filter coating 0.2 M H2SO4 treated nanosheets on PBI supports were more uniform compared to that formed using 4 M H2SO4 treatment. We hypothesize that the presence of residual CTAB on the nanosheets aids the packing of nanosheets. This was confirmed by comparing the order in the film as a function of the amount of CTAB (e.g., when using mother dispersion, see discussed later). Therefore, we selected 0.2 M H2SO4 treated suspension for coating RUB-15 films on the porous PBI-AM support.Thin RUB-15 films prepared by filter coating appeared uniform both visually (Fig. 2f) and by the electron microscope (SEM, Fig. 2g and h). Extremely thin films with a thickness of ∼150 nm could be prepared (Fig. 2i). The interaction of the zeolitic nanosheets with the underlying PBI-AM support was excellent for both as-filtered and calcined RUB-15 films. We did not observe peeling off of the film or crack formation in the film during handling or pressurization. We attribute these interactions to hydrogen bonds between the terminal silanol group of RUB-15 with the nitrogen in the benzimidazole group. XRD from these films indicated that the film was oriented with the (200) d-spacing corresponding to an average center-to-center distance between two layers of 11.4 Å (Fig. 3b). Given that the thickness of nanosheet is 8 Å [9], the intersheet gap (3.4 Å) is detrimental to selective H2 transport. A way to reduce this gap is to calcine the nanosheet film, which removes CTAB and condenses the terminal silanol groups of the nanosheets [9]. However, calcination at high temperature (e.g., 500 °C in reference 9) is not compatible with PBI-AM support where crack formation becomes severe above 350 °C. Given that TGA indicates the removal of CTAB below 350 °C (Fig. S2b), we sought to study intersheet gap as a function of temperature.In situ XRD of RUB-15 films supported on PBI-AM was carried out by heating the film in air (25–330 °C) and simultaneously collecting the out-of-plane XRD data (Fig. 3a, Note S2, Supplementary Information). The (200) d-spacing remained unaltered up to 150 °C. In the temperature range of 150–240 °C, the d-spacing increased by ∼0.5 Å. We hypothesize that this could be due to the relaxation of the adsorbed CTAB following the release of the adsorbed water (Fig. 3a). From 270 to 310 °C, a sharp decrease in the d-spacing took place attributing to the removal of CTAB. Further, heat treatment at 330 °C for 8 h brought down the d-spacing significantly close to that of the thickness of the RUB-15 nanosheets indicating near-complete removal of the surfactant.Gas transport study from RUB-15 films deposited on PBI-AM and subsequently subjected to heat treatment at 330 °C for 8 h indicated a promising H2-sieving performance. Briefly, the feed side was pressurized to 2 bar while the permeate was swept by Ar at 1 bar. Single-gas permeation measurement of He (kinetic diameter of 0.26 nm), H2 (0.29 nm), CO2 (0.33 nm), N2 (0.36 nm), and CH4 (0.38 nm) showed a sharp cut-off in the permeance between H2 and CO2 (Fig. 4a) consistent with the elimination of intersheet gap with the 6-MRs of RUB-15 nanosheets being the primary transport pathway. Nudged elastic band (NEB) calculations for the transport of H2 through the 6-MR of RUB-15 nanosheets predicted an apparent activation energy barrier (EAA) of 33 ± 2 kJ mol−1 [9]. Indeed the transport of He and H2 was temperature-activated with H2 permeance increasing as the exponential function of the temperature (150–250 °C, Fig. 4b). The EAA extracted from the permeation data of the membranes converged to the predicted EAA at higher selectivities (>20), confirming that the transport was indeed taking place from 6-MRs of the nanosheets (Fig. 4c). The resulting performance of the RUB-15 membranes supported on PBI-AM supports yielded H2 permeances of 100–400 GPU and H2/CO2 selectivities above 20 at temperatures above 200 °C (Fig. 4d). The performance was comparable to that of the RUB-15 membranes supported on smooth ceramic support (e.g., anodic aluminum oxide or AAO) and compares favorably to other zeolite membranes in literature (Fig. 4d). We note that a few membranes that were prepared on AAO had H2/CO2 selectivity up to 100. We attribute this to the extremely smooth surface of AAO, and expect that selectivity on PBI-AM support can be improved in future by further smoothening the PBI surface, e.g., by deposition of a porous gutter layer. A membrane stability test was also performed at high temperature (225 °C) with an equimolar H2/CO2 mixture feed in the presence of water vapor (Fig. S8). The membrane showed a stable performance for more than 100 h of continuous testing with a permeance of approximately 160 GPU and H2/CO2 separation factor of 30. The scalable fabrication and the attractive sieving performance at high temperatures make these membranes promising for pre-combustion carbon capture where stable operation at elevated temperature is desired [32]. We note that there are several reports on attractive H2/CO2 performances using stacking nanosheet membranes, e.g., graphene oxide (GO) [ 33–35], MXene [36], covalent-organic frameworks (COFs) [37,38]. However, in several cases, the performance is sensitive to pressurization of feed beyond zero absolute transmembrane pressure difference [39]. This is mainly because the crucial role of intersheet gap in these films for the H2 transport. However, the intersheet gap in these films are sensitive to the operating conditions (transmembrane pressure difference, humidity, etc.). In contrast, the selective transport pathway in RUB-15 nanosheet films is 6-MRs within the framework which are intrinsically stable.Overall, we demonstrate a scalable synthesis route for high-performance zeolitic film for H2/CO2 separation. This is carried out by facile filtration-based assembly of nanosheets where secondary growth of the film was completely avoided. We addressed another large bottleneck in the field of gas-sieving zeolite films, i.e., porous support, by demonstrating the fabrication of porous PBI-AM support with attractive features: (i) high thermal stability arising from the intrinsic thermal robustness of PBI-AM, and (ii) smooth surface of PBI-AM hosting 20-nm-sized pores attributing to the NIPS process. The porous PBI-AM support is attractive for fabricating nanosheet-based membranes for high temperature applications.We thank our host institution, EPFL, for generous support. We also thank GAZNAT for funding this work. Publisher Copyright: © 2023 The Authors
PY - 2023/4/15
Y1 - 2023/4/15
N2 - The synthesis of high-performance gas-sieving zeolitic film is challenging because it involves seeded secondary growth which is complex and difficult to reproduce. Additionally, expensive asymmetric inorganic porous supports are often needed for calcination of the zeolite films. Herein, scalable preparation of H2-sieving RUB-15 nanosheet films is reported bypassing the need for secondary growth and expensive supports. A novel, low-cost, thermally-robust, porous polybenzimidazole copolymer (PBI- AM Fumion®) support is prepared which allows deposition of thin, compact, and oriented RUB-15 nanosheets films without cracks or pinhole defects. The film hosts two transport pathways, H2-sieving six-membered silicate rings in the nanosheets and nonselective intersheet gaps maintained by the organic guest species in the gallery spacing. The latter is eliminated by a low-temperature (330 °C) calcination where the crystalline order in the nanosheets is preserved. The resulting zeolitic films yielded H2 permeances of 100–400 GPU and H2/CO2 selectivities above 20 at temperatures above 200 °C. The facile and scalable fabrication procedure with an attractive H2-sieving performance at elevated temperatures make these membranes promising for pre-combustion carbon capture.
AB - The synthesis of high-performance gas-sieving zeolitic film is challenging because it involves seeded secondary growth which is complex and difficult to reproduce. Additionally, expensive asymmetric inorganic porous supports are often needed for calcination of the zeolite films. Herein, scalable preparation of H2-sieving RUB-15 nanosheet films is reported bypassing the need for secondary growth and expensive supports. A novel, low-cost, thermally-robust, porous polybenzimidazole copolymer (PBI- AM Fumion®) support is prepared which allows deposition of thin, compact, and oriented RUB-15 nanosheets films without cracks or pinhole defects. The film hosts two transport pathways, H2-sieving six-membered silicate rings in the nanosheets and nonselective intersheet gaps maintained by the organic guest species in the gallery spacing. The latter is eliminated by a low-temperature (330 °C) calcination where the crystalline order in the nanosheets is preserved. The resulting zeolitic films yielded H2 permeances of 100–400 GPU and H2/CO2 selectivities above 20 at temperatures above 200 °C. The facile and scalable fabrication procedure with an attractive H2-sieving performance at elevated temperatures make these membranes promising for pre-combustion carbon capture.
KW - Hydrogen sieving
KW - Polybenzimidazole
KW - Polymeric support
KW - RUB-15
KW - Sodalite nanosheets
KW - Zeolitic membranes
UR - http://www.scopus.com/inward/record.url?scp=85147882325&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=85147882325&partnerID=8YFLogxK
U2 - 10.1016/j.memsci.2023.121454
DO - 10.1016/j.memsci.2023.121454
M3 - Article
AN - SCOPUS:85147882325
SN - 0376-7388
VL - 672
JO - Journal of Membrane Science
JF - Journal of Membrane Science
M1 - 121454
ER -