Main-chain poly(ionic liquid)-derived nitrogen-doped micro/mesoporous carbons for CO₂ capture and selective aerobic oxidation of alcohols

Highlights

• A main-chain poly(ionic liquid) is synthesized as precursor for N-doped carbon.

• N-doped porous carbon is prepared by one-pot of carbonization/KOH activation.

• N-doped carbon bears high yield, rich nitrogen dopant and high specific surface area.

• N-doped carbon shows high performance in CO₂ capture and selective aerobic oxidation.

Abstract

Sustainable development and the recent fast-growing global demands for energy and functional chemicals urgently call for effective methods for CO₂ remediation and efficient metal-free catalysts for selective oxidation of aromatic alcohol. Herein, a unique main-chain poly(ionic liquid) (PIL) is employed as the precursor to prepare nitrogen-doped micro/mesoporous carbons via simultaneous carbonization and activation, which bear high yield, large specific surface area above 1700 m² g⁻¹ and rich nitrogen dopant. The porous carbon products deliver a high CO₂ adsorption capacity up to 6.2 mmol g⁻¹ at 273 K and 1 bar with outstanding reversibility and satisfactory selectivity. Besides, they work excellently as metal-free carbocatalysts for the selective aerobic oxidation of benzyl alcohol to benzaldehyde with high selectivity. It is believed that this work not only provides a facile approach to prepare nitrogen-doped porous carbon, but also advances the related research in the fields of environment and catalysis.

Introduction

CO₂ emission is an important issue in the establishment of a sustainable modern society and is currently one of the driving forces to implement the green chemistry concept. The accumulation of CO₂ in atmosphere is widely considered as a primary factor in global climate change. Carbon capture and sequestration technologies have been proposed to be one of solutions [1], [2]. One of the industrial techniques for CO₂ capture is chemical sorption by aqueous solution of organic amines [3]. Although these systems can achieve a high sorption capacity, they suffer from one or more of the following drawbacks, such as solvent loss, corrosion and high energy consumption for regeneration. There is also huge interest in chemical conversion of CO₂ into industrial raw materials [4]. Recently, significant research efforts have been devoted to exploring porous materials with large specific surface area, e.g., porous frameworks (including metal organic framework (MOF) and zeolitic imidazolate framework (ZIF)) [5], [6], [7], [8], zeolites [9], and carbons [10], [11], [12]. Nevertheless, developing porous adsorbents that efficiently capture CO₂ and mitigate the dilemma remains a challenging but imperative task. In parallel, the selective aerobic oxidation of aromatic alkanes/alcohols to their corresponding aldehydes/ketones is one of the most important transformations of functional groups in organic synthesis, both for fundamental research and industrial manufacturing [13], [14], [15]. Plenty of metal-catalyzed aerobic processes, especially based on noble metals, have been studied [16], [17]. Considering the scarcity and high cost of noble metals and the sustainability as well as rational use of resources, it would be highly desirable to explore metal-free catalysts for these reactions.

Nitrogen-doped porous carbons bearing combined micro/mesopores represent an emerging class of porous materials of versatile functionalities and tunable porosities [18], [19], [20]. The micropores in a high density packing deliver a high surface area, therefore abundant active sites for both sorption and catalytic reactions, whereas the mesopores, beside their moderate surface area, enhance mass transport and diffusion of sorbates and reagents [21], [22], [23]. Additionally, from the viewpoint of heteroatom doping, the incorporation of nitrogen into graphitic carbon networks improves the oxidation stability and basicity through conjugation between the lone electron pair of nitrogen and the π system of carbon lattice [24]. Owing to these advantages, nitrogen-doped micro/mesoporous carbons have exhibited potential applications in many fields such as energy [25], [26], [27], catalysis [28], [29], [30], and environment [31], [32]. In particular, they are promising CO₂ adsorbents thanks to the basic nitrogen sites that improve affinity toward acidic CO₂ molecules [33]. Furthermore, previous study has found that the nitrogen sites of carbocatalysts are pivotal for the C–H bond activation, because nitrogen dopant alters local electronic structure of the adjacent carbon atoms and promotes catalytic reactivity [13]. There have already been some pioneer works in this field [34], [35], nevertheless fabricating functional nitrogen-doped porous carbons in replacement of precious metal-based catalysts is actively pursued for green chemical processes.

To synthesize nitrogen-doped porous carbons, a major concern is the choice of precursor, of which the chemical structure affects the production yield, nitrogen content, graphitic structure, etc. The yields of common organic carbon precursors are generally below 40 wt. % at high temperatures (e.g., 900 °C, see Table S1), as most of organic compounds completely evaporate or decompose during high temperature carbonization. Since the first report on the conversion of poly(ionic liquid)s (PILs) into porous carbon in 2010 [36], PILs have been considered to be an important class of polymer-based carbon precursors. Compared to other polymers [37], [38], structurally well-defined PILs are thermally more stable at temperatures up to 400 °C, and they contain rich heteroatoms, nitrogen in most cases, yielding heteroatom-doped carbons in good yield [39], [40]. The favorable thermal stability of PILs is related to their ionic nature, aromatic nature as well as some network-forming groups [41], such as cyano group that undergoes trimerization reaction to form polytriazine network under charring conditions [42].

Herein, we report how to synthesize functional porous carbons with an unusually high yield at 900 °C (47 wt. % to 67 wt. % dependent on the activation agent amount) from a main-chain PIL simultaneously bearing nitrogen-rich imidazolium cation, cyano group and aromatic ring via one-step carbonization/activation process. The as-formed nitrogen-doped carbons bearing abundant micro/mesopores not only store CO₂ as high as 6.2 mmol g⁻¹ at 273 K and 1 atm but also serve as high-performance metal-free carbocatalysts for selective aerobic oxidations, here exemplified by the conversion of benzyl alcohol to benzaldehyde.