A unique 3D ultramicroporous triptycene-based polyimide framework for efficient gas sorption applications

Abstract

A novel 3D ultramicroporous triptycene-based polyimide framework with high surface area (1050 m² g⁻¹) and thermal stability was synthesized. It exhibits relatively high CO₂ (3.4 mmol g⁻¹ at 273 K and 1 bar), H₂ (7 mmol g⁻¹ at 77 K and 1 bar), and olefin sorption capacity, good CO₂/N₂ (45) and CO₂/CH₄ (9.6) selectivity at 273 K and 1 bar, as well as promising C₂H₄/CH₄ and C₃H₆/CH₄ selectivities at 298 K, making it a potential candidate for CO₂ capture, H₂ storage, and hydrocarbon gas separation applications.

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Microporous organic polymers (MOPs)^1–7 are materials with high accessible surface areas and tunable pore sizes, which have recently attracted significant attention due to their potential applications in gas adsorption and storage,⁸ membrane separations,⁹ sensors,¹⁰ optoelectronics,¹¹ and heterogeneous catalysis.¹² MOPs are constructed from purely organic building blocks that are linked by strong covalent bonds which can be tailored for different applications by appropriate architectural design and functionalization. Triptycene is a particularly suitable building block for the construction of finely tunable microporous polymers due to its rigid and unique 3D-paddlewheel geometry comprising three planar arms bridged at ∼120° about a [2,2,2]-tricyclic ring system.¹³ The rigidity and non-planarity of the triptycene framework disrupts efficient intra- and interchain packing of polymers and results in high internal free volume polymers.^13,14 Recently, it was shown that the porosity and gas sorption capacity of fully fused-ring triptycene-containing MOPs can be tuned by substitution of the 9,10-bridgeheads with short and bulky isopropyl chains to achieve a BET surface area of ∼1600 m² g⁻¹ with one of the highest H₂ uptakes (∼9 mmol g⁻¹ at 1 bar) reported to date for MOPs.¹⁵ Furthermore, Ghanem et al. reported a solution-processable, amorphous 9,10-diisopropyl-triptycene-based polyimide for membrane gas separation applications (KAUST-PI-1) demonstrating outstanding ultramicroporous sieving properties (e.g. O₂/N₂, H₂/CH₄) owing to an intra-molecularly rigid backbone structure.¹⁶ Specifically, severely restricted rotation about the C–N imide bond, the only single bond in the polymer backbone, was attributed to the introduction of four ortho-methyl substituents by reaction of the triptycene-based dianhydride with tetramethyl-p-phenylene diamine (TMPD).¹⁷ The extension of these concepts to insoluble microporous polyimide networks, prepared by linking rigid triptycene-based multi-amine monomers with various commercially available dianhydrides, was reported for CO₂ adsorption and capture.¹⁸ Nitrogen and oxygen atoms in the polarized imide rings can interact physically via dipole–quadrupole interactions with CO₂ molecules and enhance the CO₂ sorption capacity.¹⁹ However, to the best of our knowledge no network polyimide MOPs have been reported with triptycene-based trisanhydride building blocks. In this communication, we report for the first time the synthesis of a novel, highly rigid 9,10-diisopropyl triptycene-based trisanhydride (TTA) monomer and its integration into an ultramicroporous (pore size < 7 Å) polyimide framework with high surface area. The synthesis of the TTA monomer 3 and its assembly into polyimide network 4 (TTAPI) is depicted in Scheme 1.

Scheme 1 Synthesis of TTA monomer 3 and the TTAPI polyimide network 4.

The key intermediate used for the synthesis of monomer 3, 9,10-diisopropyl-2,3,6,7,12,13-hexahydroxytriptycene 1, was synthesized as previously reported.¹⁵ Trisphthalonitrile 2 was obtained by aromatic nucleophilic substitution reaction of compound 1 with 4,5-dichlorophthalonitrile in the presence of K₂CO₃. Alkaline hydrolysis of six nitrile groups of 2 followed by cyclodehydration of the resulting hexacarboxylic acid gave the desired TTA monomer. The polyimide network was prepared via one-step high-temperature cycloimidization reaction of a 2 : 3 stoichiometric mixture of TTA monomer and a commercially available TMPD monomer in m-cresol using isoquinoline as a catalyst. The polyimide network TTAPI was recovered by precipitation in methanol and the high yield (95%) is attributed to the efficiency of the polycondensation reaction between the two monomers. All monomers were fully characterized by FTIR spectroscopy and ¹H NMR (Fig. S1–S4, ESI†).

The resultant TTAPI network 4 was insoluble in organic solvents, which limited its characterization by solution-based techniques. However, its molecular structure was confirmed by FTIR spectroscopy (Fig. 1) and solid-state ¹³C NMR (Fig. 2). The FTIR spectrum shows characteristic imide ring absorption at 1779 cm⁻¹ and 1719 cm⁻¹ associated with asymmetrical and symmetrical vibrations of carbonyl C=O groups, respectively, and a strong absorption band at 1350 cm⁻¹ corresponding to stretching vibrations of C–N of imide rings. No signals associated with polyamic acid around 3200 to 3400 cm⁻¹ were observed, indicating full imidization.

Fig. 1 FTIR spectra for TTAPI network polymer.

Fig. 2 Solid-state ¹³C NMR spectrum of TTAPI network polymer.

The solid-state ¹³C NMR spectra (Fig. 2) confirmed the structure of the TTAPI network. The peaks located at 13.7, 20.0, and 25.8 ppm are attributed to the carbon atoms of the aliphatic and aromatic methyl groups, respectively. The peak at 58.2 ppm corresponds to the bridgehead carbon atoms. The aromatic carbons appear in the range of 111.7 to 147.1 ppm and the carbonyl carbon in the imide rings appears at 166.5 ppm. Thermogravimetric analysis (TGA) (Fig. S5, ESI†) in N₂ atmosphere indicates that the TTAPI network exhibits excellent thermal stability with an onset decomposition above 400 °C. As expected, no crystalline peaks were observed in the powder X-ray diffraction measurements (Fig. S6, ESI†), confirming the amorphous nature of the polyimide network.

Fig. 3a shows the N₂ and Ar sorption isotherms at 77 and 87 K, respectively. Both isotherms show steep sorption uptake at very low pressure (P/P₀ < 0.01) indicating the presence of micropores, and then a gradual increase in the intermediate- and high-pressure range. The moderate rise in the high-pressure range (P/P₀ > 0.8) was likely due to the presence of large voids and/or interstitial spaces between the particles of the TTAPI network powder.

Fig. 3 (a) N₂ and Ar adsorption (closed circles) and desorption (open circles) isotherms of TTAPI at 77 and 87 K, respectively; (b) pore size distribution (carbon slit-pore geometry model) for TTAPI using N₂ and Ar.

The pore size distribution (PSD) (Fig. 3b) was estimated by the non-local density functional theory based on the nitrogen adsorption isotherm collected at 77 K and revealed a distinct pore size around 5.5 Å in the ultramicropore range and a relatively wide PSD in the 10–15 Å pore size range. This result is in good agreement with the PSD determined using argon as the probe molecule at 87 K with dominant ultramicroporosity peaked around 4.3 Å (Fig. 3b) and using CO₂ as the probe molecule at 273 K with strong ultramicroporosity peaked around 5.6 Å (Fig. S7, ESI†). Apparently, isopropyl bridgehead substitution in the triptycene moiety and the introduction of TTA into the TTAPI network results into a slight pore size reduction and tighter network compared to previously reported trip-PIMs (6–8 Å).⁴ The available free volume for the TTAPI network was determined experimentally from N₂ and Ar adsorption isotherms based on the gas uptake at P/P₀ = 0.95 to be 0.75 cm³ g⁻¹ and 0.86 cm³ g⁻¹, respectively. The apparent BET surface areas of the TTAPI network based on the N₂ and Ar adsorption isotherms were 1050 m² g⁻¹ and 1120 m² g⁻¹, respectively. Interestingly, both Ar and N₂ isotherms show considerable hysteresis between adsorption and desorption cycles even at low pressures indicating some irreversible structural changes induced at higher N₂ and Ar loadings. This may either be attributed to some swelling of the polymer network or slow kinetics and restricted access of gas molecules⁴ resulting from the ultramicroporosity of the TTAPI network as evidenced in Fig. 3b. The 13% difference in the pore volume as probed by N₂ and Ar could also be an indication of the adsorbate/temperature-dependant swelling behavior.

CO₂ and H₂ adsorption experiments at various temperatures (Fig. 4a and b) show that the polyimide TTAPI network exhibits good CO₂ and H₂ sorption capacities compared to previously reported microporous organic polymers such as BILP-7.^18–20 The CO₂ and H₂ uptakes were found to be as high as 3.4 mmol g⁻¹ (273 K and 1 bar), and 7.0 mmol g⁻¹ (1.4 wt% at 77 K and 1 bar), respectively, which are among the highest CO₂ and H₂ uptake values reported for amorphous microporous polyimide networks (Table 1). The amount of H₂ sorption increased to 13 mmol g⁻¹ (2.6 wt%) at 15 bar and 77 K (Fig. S8, ESI†). It is notable that the full reversibility of adsorption isotherms (i.e., closed adsorption–desorption curves) for both CO₂ and H₂ at low relative pressures and different temperatures tested suggests that ultramicropores in the TTAPI network allow optimal diffusion of small molecules like H₂ and CO₂. Adsorption isotherms collected at different temperatures were used to calculate the isosteric heat of adsorption, which is an indicator of how strong gas molecules interact with the host polymer network. The isosteric heat of adsorption for CO₂ at near zero coverage was 30.43 kJ mol⁻¹ using the sorption data collected at 258, 273, 288 and 298 K, respectively (Fig. S9, ESI†). This value is well below the typical value of 60 kJ mol⁻¹ for CO₂ chemisorption,²¹ but slightly higher than a number of microporous organic networks reported in the literature (Table S1, ESI†).

Fig. 4 (a) CO₂ sorption isotherms at 298, 288, 273 and 258 K; (b) H₂ sorption isotherms at 77 and 87 K for the TTAPI network (closed circles are for adsorption, open circles are for desorption).

Table 1 Comparison of the surface area, H₂ and CO₂ uptake capacity of the TTAPI network with other polyimide networks^a

PI network	BET SA (m^2 g^−1)	H2 uptake (mmol g^−1) 1 bar	CO2 uptake (mmol g^−1) 1 bar		Ref.
		77 K	273 K	298 K
a TPA (tetraphenyladamantane); TPAN (tetraphenyladamantane-NO2 functionalized); TAPB (tris(4-aminophenyl)benzene); MPI (microporous polyimide), MPI (sulfonated microporous PI), NPI (naphthalene-based PI), TPI (triazine-based PI), MePI (melamine-based PI); STPI (star triptycene-based PI); Td-PPI (tetrahedral perylene-based PI); TTAPI (triptycene-based-trisanhydride-PI).
TPA-PI	774	—	3.42	1.93	18g
TPAN-PI	286	—	4.03	2.02	18g
TAPB-PI-1	508	3.3	1.83	1.03	18h
MPI-1	1454	—	3.82	—	18d
TPA-PI	868	6.35	3.30	—	18a
SMPI-10	112	—	3.15	1.87	18f
NPI-1	721	—	2.80	1.80	18e
TPI	809	—	2.45	1.25	18c
MePI-1	660	3.3	1.66	—	18i
STPI-2	541	—	3.32	—	18j
Td-PPI	2213	3.2	—	<1.34	18k
TPI-1@IC	1053	6.15	3.22	2.11	18l
TTAPI	1050	7.0	3.40	2.04	This work

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The relatively high CO₂ uptake of the TTAPI network can be attributed to the combination of: (i) high surface area, (ii) presence of ultramicropores and (iii) polar heteroatoms including nitrogen and oxygen atoms in the imide rings. Similarly, there exists a general correlation between the pore size, surface area, and pore volume of the sorbent and H₂ sorption capacity. The observed high H₂ sorption capacity and the relatively high heat of adsorption for H₂ were caused by the ultramicroporosity and relatively high porosity of the TTAPI network. For example, the heat of adsorption of H₂ for TTAPI was 7.7 kJ mol⁻¹ at near zero coverage using adsorption data collected at 77 and 87 K, respectively (Fig. S9, ESI†). This value is in the higher range (4–7 kJ mol⁻¹) of most porous materials reported for H₂, including MOFs and other MOP analogues, but is comparable with PPN-1 (7.59 kJ mol⁻¹) and COFs (6.0–7.0 kJ mol⁻¹).^18l Although the presence of diisopropyl moieties in the triptycene backbone in the TTAPI network resulted in a slight pore size reduction compared to other triptycene-based PIM materials, we did not observe any significant effect in framework affinity to H₂ and CO₂.^22–24

The use of microporous materials as separation agents in CO₂ capture from flue gas and/or CO₂/CH₄ separation in natural gas purification requires the appropriate CO₂ selectivity over N₂ and CH₄ in addition to a high CO₂ sorption capacity. Single-component sorption experiments of N₂ and CH₄ were performed on TTAPI to evaluate its potential for these applications. Accordingly, N₂ and CH₄ adsorption isotherms were collected at 273 and 298 K at 1 bar, respectively (Fig. 5). The CO₂/N₂ selectivities at 273 and 298 K and 1 bar were calculated to be 45 and 32, respectively, using the initial slope method, which are similar to high selectivity values reported for other microporous polyimide networks (Table S1, ESI†). Notably, these values derived from the simple initial slope method are in excellent agreement with IAST calculation results using a gas mixture with CO₂/N₂ ratio of 15/85 (CO₂/N₂ selectivity of 48 and 28 at 273 and 298 K and 1 bar, respectively, Fig. S10, ESI†). The CO₂/CH₄ selectivity values for TTAPI at 273 and 298 K at 1 bar were estimated to be 9.6 and 7.4, respectively, using the initial slope method. These values are comparable or higher than the few microporous polymer networks reported in the literature, e.g., TPI-2@IC (8 at 298 K and 1 bar)^18l and MPI-1.^18d The relatively high sorption selectivity values of the TTAPI network for CO₂/N₂ and CO₂/CH₄ can be explained by the presence of abundant polar heteroatoms such as N and O in the network backbone, which enhance preferably the sorption of polar molecules such as CO₂.

Fig. 5 N₂ and CH₄ adsorption isotherms of TTAPI network at 273 and 298 K.

Other applications of commercial interest are energy-intensive separations of olefins from CH₄ and H₂ in the gas refinery industry. Although various types of sorbents including carbons, zeolites, MOFs, COFs, and MOPs have been widely investigated for CO₂, N₂, and CH₄, few studies reported their sorption properties for light hydrocarbons such as C₂H₄ and C₃H₆.²⁵ We also investigated the C₂H₄ and C₃H₆ adsorption/affinity properties of the TTAPI network in comparison to CH₄ with respect to their differences in condensability and polarizability. Fig. 6a shows the adsorption isotherms of CH₄, C₂H₄ and C₃H₆ at 298 K, respectively, which were further used to predict the mixed-gas selectivity toward C₂H₄ and C₃H₆ in the presence of CH₄. The selectivity was calculated for mixtures with the following compositions: (5% C₂H₄/95% CH₄ or 5% C₃H₆/95% CH₄). The maximum partial pressure for C₂H₄ and C₃H₆ was 1 bar while the corresponding maximum partial pressure for CH₄ was 19 bar with a total pressure up to 20 bar (see high-pressure CH₄ uptake of TTAPI network at 298 K in Fig. S11, ESI†).

Fig. 6 (a) C₂H₄, C₃H₆, and CH₄ adsorption isotherms of TTAPI network at 298 K; (b) C₂H₄/CH₄ and C₃H₆/CH₄ selectivity calculated using the IAST model with a mixture of 5% C₂H₄ : 95% CH₄ and 5% C₃H₆ : 95% CH₄ with a total pressure up to 20 bar.

The IAST predictions (Fig. 6b) show that the network TTAPI exhibits high C₂H₄/CH₄ and C₃H₆/CH₄ selectivity of 13.4 and 99.5, respectively, at 298 K and 1 bar total pressure. The high C₂H₄ and C₃H₆ sorption capacities and excellent selectivity values for C₂H₄/CH₄ and C₃H₆/CH₄ of the TTAPI network are comparable to those of MOF materials such as HKUST-1-like tbo-MOFs.²⁵ It is noticeable that the selectivity of C₃H₆/CH₄ was much higher than for C₂H₄/CH₄, and declined more rapidly as the total pressure increased. The difference in the observed trend for C₃H₆/CH₄ versus C₂H₄/CH₄ selectivity is likely due to the complex interplay between the intrinsic properties of both the porous network and the gas molecules such as the pore structure of the network, the size, condensabilities and polarizabilities of C₃H₆, C₂H₄, and CH₄. The condensability and polarizability of the three hydrocarbons follow the order: C₃H₆ > C₂H₄ > CH₄. As a result, C₃H₆ is expected to show steeper sorption uptake in the ultramicroporous range at low pressure compared to C₂H₄ and CH₄. Additional sorption occurring in the larger micropores becomes much less pressure dependent. This could explain the observed decrease in C₃H₆/CH₄ selectivity for the 5/95 mixture with increasing pressure, as shown in Fig. 6b. In contrast, C₂H₄ has lower condensability than C₃H₆ and adsorption of C₂H₄ in the ultramicropores and micropores of TTAPI is expected to be less pressure dependent; therefore, the selectivity of C₂H₄/CH₄ stays relatively constant with increasing pressure.

Conclusions

A novel ultramicroporous triptycene-based polyimide network TTAPI with a surface area of 1050 m² g⁻¹ was synthesized and showed considerable CO₂ and H₂ adsorption capacity (3.4 mmol g⁻¹, at 273 K and 1.0 bar, and 7 mmol g⁻¹ at 77 K and 1 bar, respectively). The nature of the introduced diisopropyl bridgehead triptycene moieties combined with four o-methyl substituted phenyl rings in the polyimide framework, contributed to relatively high CO₂/N₂ and CO₂/CH₄, selectivity. The CO₂ and H₂ sorption properties for TTAPI were comparable to other triptycene-based microporous polymers. Furthermore, TTAPI exhibited excellent olefin sorption capacity and C₂H₄/CH₄ and C₃H₆/CH₄ selectivity. These interesting adsorption properties originated from the unique combination of the presence of ultramicropores and the introduced functionality to the network polymer. Furthermore, the synthesis of the TTAPI network is simple and relatively easy to scale up. These properties along with the excellent thermal stability make TTAPI a potential separation agent for key industrial separations. Additionally, due to the availability of a wide range of commercially available functionalized diamines, the present trisanhydride monomer can be utilized to introduce polar functional groups such as –OH, –SH, –SO₃H and –COOH into the polyimide framework. These functional moieties may further enhance the affinity and separation properties of the TTAPI network for various gases such as CO₂, CH₄, N₂ and light hydrocarbons.

Acknowledgements

The research reported in this publication was supported by funding from King Abdullah University of Science and Technology (KAUST).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: NMR, FTIR, XRD, CO₂ pore size distribution, high pressure H₂ sorption, heat of sorption for CO₂ and H₂, IAST calculations, high pressure CO₂ and CH₄ sorption. See DOI: 10.1039/c6ra21388j

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