Sustainable nonmetal catalysts based on sulfur-doped melamine-derived graphitic carbon nitride for CO₂ cycloaddition

Abstract

At present, excessive carbon dioxide (CO₂) emission has become an increasingly prominent global energy and environmental issue. Therefore, effective methods to convert CO₂ into fine chemicals are urgently required. Herein, series of S-doped carbon-nitrogen (CNS-X) materials (where X denotes the ratio of thiourea and melamine substances ranging from 0.03 to 0.8) was prepared via the programmed temperature pyrolysis method, which thiourea (CH₄N₂S) and melamine was used as the precursor of the catalysts. The sulfur source endow the CNS-X acidic sites, which cooperate synergistically with amino groups from the incomplete polymerization of melamine, leading to a bi-functional catalyst for cycloaddition reaction of CO₂ with epoxides. These catalysts were characterized using X-ray diffraction, Fourier transform infrared spectroscopy, elemental analysis, X-ray photoelectron spectroscopy, and N₂ adsorption-desorption techniques, confirming the successful integration of functional groups. The optimal thiourea doping concentration of 0.4 was certainly found to have considerably facilitated the efficient conversion of CO₂ by the CNS-0.4 catalyst, in which the conversion of epichlorohydrin (ECH) could achieve over 90.0% and the selectivity of cyclic carbonate is 98.0% under 1.0 MPa at 140 °C for 10 h. The superior catalytic performance of CNS-0.4 was attributable to the synergistic effect arising from the co-existence of Lewis acidic and basic sites. Notably, using CNS-0.4 resulted in a high yield even after four reaction cycles.

Graphical abstract

Keywords

S-doped carbon-nitrogen (CNS) catalyst / multiple active site / CO ₂ cycloaddition

Highlight

	● S-doped carbon nitrogen (CNS) catalyst is synthesized from the self-assemble of melem.
	● CNS catalyst containing Lewis basic and Lewis acid active sites.
	● The CNS material can be used as an efficient bifunctional catalyst to synthesize cyclic carbonates.
	● The catalyst is reusable and stable against leaching of active sites in the cycloaddition reaction.

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