Bifunctional metal-free porous polyimide networks for CO₂ capture and conversion

Abstract

Carbon dioxide (CO₂) capture and conversion into valuable chemicals is a promising and sustainable way to mitigate the adverse effects of anthropogenic CO₂ and climate change. Porous polyimides (pPIs), a class of highly cross-linked porous organic polymers (POPs), are promising candidates for CO₂ capture as well as catalytic conversion to valuable chemicals. Here, two metal-free perylene-based pPIs were synthesised via polycondensation reaction. The pPIs exhibit excellent heterogeneous catalytic activities for cycloaddition of CO₂ to epoxides under very mild and sustainable conditions (slight CO₂ overpressures, solvent- and co-catalyst free at 80 °C) with 98% conversion. The effects of reaction conditions, such as reaction temperature, reaction time and catalyst loading on the cycloaddition performance were investigated. Moreover, the pPIs can be recycled and reused five times without a substantial loss of catalytic activity. Furthermore, these materials were used in the electroreduction of CO₂ to form formate and methanol with faradaic efficiencies (FEs) of 20% and 95%, respectively, in the applied potential range from 0 to −1 V vs. RHE.

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1. Introduction

Climate change is the most significant challenge humanity is facing in the 21^st century, caused by continuously increasing human activity, leading to excessive anthropogenic CO₂ emissions. However, CO₂ has the potential to be used as a very abundant, inexpensive, and non-toxic source of carbon for industrial utilisation.^1,2 Therefore, capturing and converting CO₂ into useful chemical feedstocks offer an important emerging approach to developing a carbon-neutral alternative to fossil fuel resources.^3,4

From a sustainability perspective, CO₂ can be transformed into cyclic carbonates via the cycloaddition reaction between carbon dioxide and epoxides,⁵ and be photochemically⁶ and electrocatalytically⁷ reduced to valuable chemicals and fuels. Cyclic carbonates are industrially important and used in a wide range of applications, including as building blocks for polymeric materials,⁸ solvents,⁹ and, importantly, as electrolytes for lithium ion batteries.¹⁰ Additionally, photochemical and electrochemical reduction of CO₂ can lead to the production of highly desirable C1, C2, and higher carbon products.^6,7 To date various heterogeneous catalytic systems, including transition metals, have been developed and extensively researched for CO₂ conversion.^11,12 However, the use of metal catalysts is limited due to their high cost, uncertainty in terms of long-term availability and sustainability, poor selectivity, low durability, susceptibility to gas poisoning, and negative environmental impact, hindering their application in industry.¹³ To overcome the limitations of traditional metal catalysts, a variety of metal-containing porous materials, specifically covalent–organic frameworks (COFs),^14,15 and metal–organic frameworks (MOFs),^16,17 have been widely explored as effective CO₂ sorption materials, and, more recently, also as materials for conversion. However, many obstacles remain in the development of the CO₂ capture and conversion processes, including the design of stable and metal-free materials with high CO₂ adsorption abilities and selectivity, effective CO₂ conversion under benign conditions, affordability and recyclability.

pPIs are an interesting class of metal-free highly cross-linked porous materials that have shown promising results for CO₂ capture owing to their high porosity, synthetic flexibility, and excellent physical and chemical properties.¹⁸ Based on these attractive properties, the applications of pPIs have been extensively explored in the fields of gas capture and storage,^19,20 energy storage,^21–24 sensing,^20,25 drug delivery²⁶ and functionalised coatings.²⁷

Post-synthetic modification of pPIs with metal species (Pd and Cu) have been explored for use as heterogeneous catalysts, including for Suzuki coupling reactions,^28,29 and aerobic oxidation of benzyl alcohol.³⁰ However, the route to transforming CO₂ (chemically and electrocatalytically) into value-added chemicals has not been explored to date for metal-free pPIs, despite their high CO₂ affinities. These high affinities, stemming from abundant heteroatomic active sites in their structure, have been shown to yield enhanced interactions in both electro- and chemo-catalytic processes.^31,32 The mobility of electrons, required for such catalytic processes, can furthermore be facilitated by the presence of conjugated structures within the backbone, especially if an organic semiconductor is incorporated. We have recently shown the validity of this approach for the case of poly(naphthalene imide)s, specifically for the electrocatalytic reduction of CO₂ to formate and methanol.³³ Here we explore the use of perylene tetracarboxylic dianhydride (PTCDA) as a further well-known example of suitable conjugated materials for such approaches.³⁴

In this investigation, we explore the use of perylene-based pPIs, for the first time, at standard pressures as metal-free heterogeneous catalysts for incorporating CO₂ in epoxides to form cyclic carbonates, as well as for electrocatalytic CO₂ conversion into valuable fuels and feedstocks (see Scheme 1).

Scheme 1 Sustainable routes to CO₂ capture and conversion using pPIs.

2. Results and discussion

pPI-1 and pPI-2 were obtained by polycondensation of PTCDA with melamine and tris-(4-aminophenyl)triazine (TAPT), respectively (Scheme 2). The successful formation of both pPIs was confirmed by Fourier-transform infrared spectroscopy (FT-IR), ultraviolet-visible near-infrared spectroscopy (UV-Vis-NIR), thermogravimetric analysis (TGA), powder X-ray diffraction (PXRD), scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) as shown in Fig. 1(a–f) and Fig. S9, S10 and Table S1 (ESI†). The characteristic FT-IR absorption signals at 1773 and 1707 cm⁻¹ are attributed to the symmetric and asymmetric vibrations, characteristic of carbonyl groups in the newly formed six-membered polyimide rings of pPI-1 and pPI-2. Additionally, the absence of –NH stretching signals (3460–3213 cm⁻¹) from the starting material shows complete condensation of amine moieties to form the respective pPI. The signal at 1446 cm⁻¹ confirms the presence of triazine units in both polymers. The maximum UV-Vis-NIR absorption wavelength (λ_max) of pPI-1 and pPI-2 are 733 and 764 nm, respectively, with absorption features extending beyond 1400 nm, indicating the formation of new products (Fig. 1d). The amorphous nature of pPI-1 and pPI-2 was confirmed by PXRD measurements, with broad peaks at 12.5°, commonly observed for POPs.²⁰ A further broad peak centred around 26° originates from π–stacking between the aromatic units (Fig. 1c).²⁰ Thermogravimetric studies under nitrogen (N₂) atmosphere revealed degradation temperatures (T_dec) of 400 °C and 590 °C, with 45% and 70% char yields at 800 °C, respectively, for pPI-1 and pPI-2, confirming the excellent thermal stability of these pPIs.

Scheme 2 Synthetic pathway to pPI-1 and pPI-2.

Fig. 1 (a) FT-IR spectra of starting materials and pPI-1 and pPI-2, (b) TGA plot of pPI-1 and pPI-2 under N₂ atmosphere, (c) PXRD of pPI-1 and pPI-2 (solid line) and after use as a catalyst (dotted line) (peak 2θ 21.5 is from the paraffin wax used to fix samples to the sample holder during our XRD analysis),³⁵ (d) UV-Vis-NIR spectra of pPI-1 and pPI-2, (e) SEM of pPI-1 before and after 5 cycles used as a catalyst, and (f) SEM micrographs of pPI-2 before and after 1 cycle used as a catalyst.

The porosity properties of pPIs were investigated by recording N₂ sorption isotherms at 77 K (see Fig. S4, ESI†). The isotherms show no significant N₂-uptake at lower relative pressures (P/P₀), with some uptake observed at higher relative pressures (P/P₀). This behaviour indicates the presence of micro- and macropores, as confirmed in pore size distribution (PSD) calculations using the non-local density functional theory (NL-DFT) method (see Fig. S4, ESI†).²⁰ These isotherms could be described as typical Type II isotherms according to the IUPAC classification,³⁶ with the specific surface areas (S_BET) of pPI-1 and pPI-2, 20 and 342 m² g⁻¹, respectively. Additionally, the CO₂ uptake capabilities of the pPIs were investigated at 273 K (2 wt% and 5 wt% for pPI-1 and pPI-2, respectively) and 298 K (0.8 wt% and 3 wt% for pPI-1 and pPI-2, respectively) at 1 bar pressure (see Table 1 for full details). Even though pPI-1 exhibits poor S_BET surface area, the relatively high CO₂ uptake of this polymer may be attributed to the strong affinity between its constituent heteroatoms and CO₂.^37,38 To quantify the interaction between the pPIs and CO₂, the isosteric heats of adsorption (Q_st) were calculated from the absolute adsorption isotherms recorded at 273 and 298 K (Table 1). The highest adsorption enthalpies of pPI-1 and pPI-2 are 39 kJ mol⁻¹ and 30 kJ mol⁻¹, respectively, as shown in Table 1. These values indicate physisorption processes and interactions for both of these polymers with CO₂. Interestingly, pPI-1 has stronger CO₂–surface interactions (Q_st 39 kJ mol⁻¹), thus resulting in moderate CO₂ uptake, even with a surface area (20 m² g⁻¹) 17 times lower when compared with pPI-2 (342 m² g⁻¹).

Table 1 Porosity parameters and CO₂ uptake at 273 K and 298 K at 1 bar

Polymer	S BET (m^2 g^−1)	PV (cm^3 g^−1)	CO2 uptake (wt%)		Q st (kJ mol^−1)
			273 K	298 K
pPI-1	20	0.07	2.0	0.8	39
pPI-2	342	1.12	4.9	3.0	30

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2.1. pPI-catalysed chemical conversion of epoxides

The cycloaddition of CO₂ to epoxides to yield cyclic carbonates is commonly performed using potentially unsuitable metal-containing catalysts (e.g., Zn@SBMMP and Bp-Zn@MA) and under undesirable, energy intensive high temperatures (160 °C) and pressure (∼20 bar) conditions.^39–43 The presence of a co-catalyst (e.g., tetrabutylammonium bromide (TBAB)) is also required in most cases, increasing cost and molecular economy.^44,45 The primary driving force behind the cyclic carbonate synthesis from CO₂ and epoxide is substrate activation. Several studies have already shown that there are three distinct mechanistic pathways: CO₂ activation, epoxide activation, or simultaneous activation of both CO₂ and epoxide.^46–48 The catalyst must have the capability to interact with the substrate and activate it accordingly. Considering the abundant N heteroatoms present in pPIs, we propose that the activation of CO₂ occurs via Lewis acid–Lewis base interactions (for detailed plausible mechanisms, refer to ESI† Scheme S1). In situ chemical characterisation and computational simulation studies would be beneficial to precisely understand the exact mechanism and interaction between pPIs with CO₂ and epoxide, which however fall outside the scope of this study.

Here we show the successful application of our pPIs, prepared in the absence of any metal-containing catalysts, for the metal-free, solvent-free cycloaddition of CO₂ to epoxides under standard pressure (i.e., using a balloon filled with CO₂). We furthermore find that no co-catalysts are required for quantitative transformation, with all reactions also performed in the absence of solvent (Scheme 3). The well-studied cycloaddition of CO₂ to epichlorohydrin (ECH) was chosen as a benchmark reaction for optimisation. As shown in Table 2, both pPIs efficiently converted ECH to (chloromethyl)ethylene carbonate (CMEC) with a yield of 98% (using pPI-1) and 90% (using pPI-2), respectively, at 80 °C and after 72 h, in the absence of solvent. Time-dependent analysis of the conversion for both pPI-1 and pPI-2 confirmed high conversions only after 72 h, as shown in Fig. 2a and Fig. S5 (ESI†), respectively.

Scheme 3 pPI-catalysed, solvent-free cyclic carbonate synthesis from CO₂ and epoxides.

Table 2 Cycloaddition of CO₂ with different epoxide substrates to form cyclic carbonates using pPIs^a

Entry	Epoxide	Catalyst	Product	Yield^b (%)
a Reaction conditions: epoxide (1 mL, neat), pPIs (2.5 wt%), CO2 (balloon), 80 °C, and 72 h. b Determined by ^1H-NMR spectroscopic analysis.
1		None	None	0
2		pPI-1		98
3		pPI-2		90
4		pPI-1		89
5		pPI-2		72

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Fig. 2 (a) Time-dependent percentage conversion of ECH at 80 °C for pPI-1. (b) Temperature-dependent conversion for cycloaddition of CO₂ to ECH for pPI-1. (c) pPI-1 catalyst loading for cycloaddition of CO₂ to ECH at 80 °C. (d) Recycling test of pPI-1 for the cycloaddition of CO₂ to ECH at 80 °C.

The higher conversion of pPI-1 could be attributed to the stronger surface interactions with CO₂, as reflected by the Q_st value of pPI-1 (39 kJ mol⁻¹) vs. that of pPI-2 (30 kJ mol⁻¹). To explore the effect of temperature on the resulting yield, the catalysed reaction was carried out at 60, 80 and 100 °C; with the yields plateauing at 98% as shown in Fig. 2b. The catalyst loading used in the cycloaddition transformation was initially fixed at 2.5 wt% (30 mg) of catalyst for 1 mL of neat ECH, giving the highest conversion (Fig. 2c). Subsequently, the catalyst loading was increased to 4.8 wt% (60 mg) and 10 wt% (125 mg). Interestingly, even with increased catalyst loading, the transformation of ECH (at 80 °C, over a 72 hour duration) remained unaltered or equivalent to the conversion achieved with 2.5 wt% catalyst loading. Decreased catalyst loading (1.2 wt%) yielded lower transformation (57%). Therefore, 2.5 wt% catalyst loading was selected as the optimum loading during this investigation.

One of the important considerations of heterogeneous catalysts for industrial applications is recyclability.^49,50 Thus, we investigated the recyclability of pPI-1 in the model reaction, using ECH and CO₂ as reactants. In each cycle, pPI-1 was recovered by centrifugation, washed and dried before being reused in a repeat reaction with a fresh batch of the ECH substrate. The catalyst was found to be recyclable and reusable for up to five cycles without significant loss in catalytic activity, as shown in Fig. 2d. It was also observed that pPI-1 retained its amorphous structure after going through 5 catalytic cycles as shown by XRD (Fig. 1c), SEM (Fig. 1e) and chemical composition (see ESI† Table S1 and Fig. S9 for further data and details of the procedure used). EDX revealed an increase in oxygen and carbon content within both pPIs (pPI-1 and pPI-2) after catalysis, indicating the binding of CO₂ to the catalyst. X-ray photoelectron spectroscopy (XPS) supported these findings, showing changes in the oxygen content of the pPIs after catalysis, implying an increase in C–O and/or O–C=O bonds, originating from the reactive CO₂ species (see the ESI† Fig. S10 for XPS results). Cl signals were detected after catalysis, suggesting that the catalyst also interacted with the epoxide substrate (ECH). To gain a more precise and detailed understanding of the nature of these interactions between the catalyst, CO₂ and the epoxide, in situ chemical characterisation will be part of our future investigations. The potential of using these catalysts in industrial settings is underlined by the physical, chemical and thermal stability, recyclability and lack of obvious changes in catalytic activity or morphology, as also confirmed by additional SEM images recorded post catalysis (see Fig. 1e and f).

After proving the excellent catalytic activity and stability of pPIs for the cycloaddition of CO₂ to ECH, we tested the catalytic performance on a different substrate, epibromohydrin (EBH), under the same reaction conditions (see Table 2). Both pPIs gave excellent conversion (89% and 72%, respectively) of EBH to the corresponding cyclic carbonate.

A number of investigations have been directed towards the production of metal-based catalysts aimed at optimising the conversion of CO₂ into cyclic carbonates.⁴ For instance, Zhang et al.⁵¹ successfully fabricated cobalt-containing conjugated microporous polymers (CMP) for the cycloaddition of CO₂ and epoxides resulting in 83.6% and 82.5% conversion yields for epichlorohydrin and epibromohydrin, respectively. The same research group later prepared porphyrin-based cobalt-coordinated CMPs for the synthesis of cyclic carbonates from CO₂ and epoxides (with a conversion yield of 74.2% for epichlorohydrin).⁵² In both studies, the well-known co-catalyst TBAB was used. Zhou et al.⁵³ reported a zinc (Zn)-containing catalyst for CO₂ transformation; their Zn-salen-CMP successfully converted epichlorohydrin into the corresponding cyclic carbonate with a yield of 89% in the presence of TBAB. Notably, the cycloaddition reaction was carried out at 120 °C and under 3.0 MPa pressure for 1 hour. Another example of using a Zn-containing catalyst, now containing quaternary phosphonium bromide salts, was reported by Lu et al.⁵⁴ The salts acted in synergy with the zinc porphyrin (Lewis acid) and opened the epoxide ring, eliminating the need for co-catalysts. Reported conversion yields for epichlorohydrin were 93% under high CO₂ pressure (2.5 MPa) at 90 °C. In comparison with metal-based catalysts, there are only a few metal-free POPs for CO₂ fixation via cycloaddition of CO₂ with epoxides (but not without a co-catalyst). Ding et al.⁴⁴ developed microporous polymeric spheres catalyst, which together with TBAB as a co-catalyst gave an 89% conversion yield with epichlorohydrin as a substrate. In contrast to other studies, our results presented here highlight a greener (lower temperatures, pressures and absence of solvents), safer, metal-free alternative for the fixation of CO₂ using a cycloaddition process, using organic-based pPIs only.

2.2. Electrocatalytic CO₂ reduction

To further expand the scope and usage of our pPIs, the electrocatalytic reduction of CO₂ was investigated using a two-compartment H-cell (0.1 M potassium bicarbonate (KHCO₃) as the electrolyte in the potential range from −1 to 1 V vs. a reversible hydrogen electrode (RHE)). Cyclic voltammetry (CV) studies show that both pPIs exhibit redox activity in the investigated potential range. The CV data (Fig. 3a) shows the current density in the argon-saturated electrolyte is higher than that of the CO₂-saturated electrolyte. The higher current density may be attributed to a higher hydrogen evolution reaction in the argon-saturated electrolyte.⁵⁵ Therefore, this data helped to confirm that our pPIs are electrocatalytically active.⁵⁵ The chronoamperometric (CA) analyses of these pPIs were carried out at constant potentials of −0.26, −0.56 and −0.76 V vs. RHE for 0.5 h (see Fig. 3b and c).

Fig. 3 (a) Cyclic voltammogram of pPI-1 and pPI-2 in the applied potential range from −1–1 V vs. RHE, scan rate 20 mV s⁻¹, 20 cycles, (b) and (c) chronoamperometry studies (vs. RHE) of pPI-1 and pPI-2, respectively in CO₂-saturated electrolytes, and (d) and (e) highest obtained faradaic efficiency of pPI-1 and pPI-2, respectively in CO₂ saturated electrolytes for 30 min.

The electrolytes were analysed after electrolysis by cryo-¹H NMR, with data shown in Fig. S14 and S15 (ESI†). The measurements show that formate and methanol are the primary products present in the CO₂-saturated electrolyte. In the argon-saturated electrolyte, no product signal was detected from samples at the same applied potential as those saturated with CO₂. The signals at 3.36 ppm and 8.50 ppm in the ¹H NMR spectra confirm the reduction of CO₂ to methanol and formate, respectively. The stronger Lewis acid–Lewis base interaction of the heteroatoms in the pPIs with CO₂ contributes to increasing the rate of reduction of the gas by stabilising intermediates formed during the conversion.⁵⁵ The highest obtained Faradaic efficiency (FE) values are reported in Fig. 3d and e for pPI-1 and pPI-2, respectively (for full data sets, with error bars, see ESI† Fig. S13). pPI-1 demonstrates a FE of up to 52% for methanol and 14% for formate at −0.26 and −0.76 V vs. RHE, respectively, as illustrated in Fig. 3d. For detailed FE calculations, please refer to Section 4 of the ESI.†pPI-2, in contrast, exhibits the highest FE for methanol, reaching up to 95% at −0.26 V vs. RHE (Fig. 3e). However, the FE gradually decreases with increasing negative potential, with values of 49% at −0.56 V and 9% at −0.76 V, as shown in Fig. 3e. Moreover, pPI-2 was also capable of reducing CO₂ to formate at −0.26 V (FE = 20%), −0.6 V (FE = 20%) and −0.76 V (FE = 5%). The higher FE of pPI-2 for methanol could be attributed to the higher surface area and broad PSD, leading to higher carbon product formation. This observation aligns with the findings from our previous study.³³ Additionally, we assume that gaseous products, such as CO and CH₄, formed during the electroreduction process. However, this assumption could not be confirmed since the reaction setup was not coupled with a GCMS instrument. Investigating this aspect further in future research would be of great interest. Multiple metal-based COF and MOF catalyst systems have been used in electrocatalytic CO₂ reduction, reporting outstanding FEs for CO (97% at −0.9 V vs. RHE).^56,57 To the best of our knowledge, selectively reducing CO₂ electrocatalytically to methanol using metal-based porous materials has not been achieved to date. In contrast, the pPIs herein reported show superior results for methanol production, and in some cases matching with other traditional transition-metal electrocatalysts (FEs of 87% and 98% for methanol and formate, respectively).^58,59 Our new approach here points towards the exciting development of metal-free porous polymers as electrocatalysts for the wide and sustainable utilisation of CO₂.

3. Conclusion

In this work, two perylene-based pPIs were successfully synthesised via polycondensation reactions. The synthesized pPIs were shown to be versatile materials, showing interesting porosity properties and useful CO₂ uptake capabilities of up to 4.9 wt%. The synthesised pPIs were furthermore successfully utilised, not only to capture CO₂ but also to act as heterogeneous catalysts for the utilisation of CO₂ under very mild and sustainable conditions. pPI-1 and pPI-2 showed excellent catalytic performance for cyclic carbonates synthesis from CO₂ and epoxides (at very slight CO₂ overpressures, in the absence of solvents and co-catalysts), with up to 98% conversion and outstanding recyclability. Further exploring their versatility, these pPIs were used in the electrocatalytic conversion of CO₂ to form methanol and formate. FEs of 20% for formate and 95% for methanol were achieved in the applied potential range from 0 to −1 V vs. RHE. pPIs, therefore, provide an exciting, metal-free solution, at low potentials, for the electrocatalytic conversion and fixation of CO₂ to produce useful fuels and chemical feedstocks. This approach should therefore continue to be explored to produce efficient and recyclable heterogeneous catalysts for chemical and electrocatalytic conversion of CO₂. Further investigations, based on our earlier work on exploiting and optimising Hansen Solubility Parameters (HSPs) in the Bristol-Xi’an Jiaotong (BXJ) approach, will be useful to explore the influence of the formation of highly optimised conjugated microporous polymers (CMPs). Applying this approach, we will be able to tune porosities and pore size distributions, thereby further improving catalytic efficiency and functionality, and potentially also tune product formation.^60,61 This metal-free, solvent-free approach will ensure a step-change in the approaches and tools available to address the significant global challenges faced today.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

B.B.N acknowledges support from the National Overseas Scholarship for ST Students, Government of India. U.K. acknowledges support from the Bolashak International Scholarship. J.M. acknowledges support from the Henry Royce Institute. M.V-H acknowledges Bristol Centre for Functional Nanomaterials and the Consejo Nacional de Ciencia y Tecnología (CONACyT, MX) grant no. 497607 for funding and C. F. J. F. acknowledges EPSRC EP/R511663/1 for support. Authors acknowledge access to the Bristol NanoESCA Facility under EPSRC Strategic Equipment Grant EP/M000605/1. The authors thank John Worth and Maximilian Hagemann for proofreading.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3qm00639e

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