Organocatalysis with carbon nitrides

Abstract

Carbon nitrides, a distinguished class of metal-free catalytic materials, have presented a good potential for chemical transformations and are expected to become prominent materials for organocatalysis. This is largely possible due to their low cost, exceptional thermal and chemical stability, non-toxicity, ease of functionalization, porosity development, etc. Especially, the carbon nitrides with increased porosity and nitrogen contents are more versatile than their bulk counterparts for catalysis. These N-rich carbon nitrides are discussed in the earlier parts of the review. Later, the review highlights the role of such carbon nitride materials for the various organic catalytic reactions including Knoevenagel condensation, oxidation, hydrogenation, esterification, transesterification, cycloaddition, and hydrolysis. The recently emerging concepts in carbon nitride-based organocatalysis have been given special attention. In each of the sections, the structure-property relationship of the materials was discussed and related to their catalysis action. Relevant comparisons with other catalytic materials are also discussed to realize their real potential value. The perspective, challenges, and future directions are also discussed. The overall objective of this review is to provide up-to-date information on new developments in carbon nitride-based organic catalysis reactions that could see them rising as prominent catalytic materials in the future.

Keywords: Carbon nitride; Knoevenagel condensation; cycloaddition; esterification; hydrolysis; organocatalysis.

Graphical abstract

Figure 1.

Predicted theoretical structure of K-PHI and its counterpart H-PHI (metal free), reproduced with permission from [66].

Figure 2.

(a) Influence of materials synthesis temperature on the yield of benzylidene malononitrile, reproduced with permission from [51], (b) Effect of pore diameter of different HMCN catalysts on the yield of phydroxycinnamic acid. Reaction conditions: p-hydroxybenzaldehyde (4.00 mmole), malonic acid (8.10 mmole), HMCN catalysts (200 mg), toluene (10 ml), reaction time 60 min, and azeotropic method (105°(C), reproduced with permission from [88], (c) Plots for the benzaldehyde and acetone conversions obtained in the Knoevenagel condensations of benzaldehyde or acetone and malononitrile against the surface concentrations of tricoordinated nitrogen and/or amino groups, reproduced with permission from [89], and (d) Catalytic performance for Knoevenagel condensations of xMgCN-U catalysts with different Mg loading amount, reproduced with permission from [90].

Figure 3.

(a) Conversion of benzaldehyde over BCN, h-BN and C₃N₄. Reaction conditions: 2.5 mmol of benzaldehyde, 2.5 mmol of malononitrile, 15 ml of toluene, 100 mg catalyst, 80°C oil bath under N₂ as the protection atmosphere, reproduced with permission from [106], (b) Reactivity for p-substituted benzaldehydes with malononitrile, reproduced with permission from [51], (c) Influences of active methylene compounds on the yield of benzylidene malononitrile, reproduced with permission from [51].

Figure 4.

Mechanism of Knoevenagel condensation of benzaldehyde and malononitrile, reproduced with permission from [70].

Figure 5.

(a) Visible light-induced photocatalytic reaction mechanism of g-C₃N₄, and (b) reaction mechanism for the oxidation of α-isophorone to keto-isophorone, reproduced with permission from [83].

Figure 6.

Oxidation of benzene to phenol over mesoporous g-CN, reproduced with permission from [114].

Figure 7.

Plausible reaction mechanism for the synthesis of isochromannones, phthalides, isoquinolinones, isoindolinones and xanthones from readily accessible alkyl aromatic precursors, reproduced with permission from [122].

Figure 8.

(a) Oxidation employing non-noble metal oxide incorporated on g-CN surface, reproduced with permission from [133], (b) Comparisons of benzyl alcohol conversion and benzaldehyde selectivity over various photocatalysts for visible-light-induced photocatalytic aerobic oxidation to benzaldehyde, reproduced with permission from [134], and (c) Oxidation of methyl arenes and their analogues, reproduced with permission from [135].

Figure 9.

Photocatalytic oxidation of benzyl alcohol over (a) various forms of CN, and (b) various coralloid CNs annealed at different temperatures, reproduced with permission from [153], and (c & d) Yield of BAL production using bulk g-C₃N₄, and post-treated photocatalysts by chemical, mechanical and thermal methods as a function of SA_BET and band gap, reproduced with permission from [112].

Figure 10.

Proposed mechanism for selective oxidation of BA by 0.10 rGO/CNT using a) UV and b) visible irradiation, and c) schematic view of the oxidation of BA to BAL in the catalyst surface of CNT and 0.1% rGO/CNT catalyst, reproduced with permission from [154].

Figure 11.

Possible photocatalytic reaction routes for glycerol oxidation over OCNN-2 in water and acetonitrile, reproduced with permission from [158].

Figure 12.

(a) Semi-heterogeneous dual nickel/photo catalysis using carbon nitrides for esterification of carboxylic acids with aryl halides, reproduced with permission from [175], and (b) CN-OA-m in the dual nickel/photocatalytic esterification of methyl 4-iodobenzoate with Boc-Pro-OH, reproduced with permission from [175].

Figure 13.

(a & b) Transmission and (c) scanning electron microscopy images and (d) electron energy loss spectrum of MCN-3; (e) Basic catalytic performance of MCN-3 in the transesterification of b-keto esters of different alcohols. Reaction conditions: β-Keto ester/alcohol molar ratio 1.2:1, catalyst MCN-3 (10 wt% of total reaction mixture), solvent toluene, reaction temperature 110°C, reproduced with permission from [180].

Figure 14.

Multiple functional groups in the g-C₃N₄ skeleton, reproduced with permission from [3].

Figure 15.

DFT optimized model for CO₂ and so adsorption/co-adsorption on CN and BCN, reproduced with permission from [188].

Figure 16.

Synthesis of cyclic carbonates assisted by a bi-functional g-C₃N₄ catalyst, reproduced with permission from [196].

Figure 17.

Proposed mechanism for the g-C₃N₄ and TBAB catalyzed conversion of epoxides and CO₂ into cyclic carbonates, reproduced with permission from [197].

Figure 18.

(a) Synthesis of platinum nanoparticles (Pt NPs) supported on mesoporous graphitic carbon nitride (mpg-CN), (b) Time vs molH₂/mol NH₃BH₃ plots for mpg-CN/Pt with 2.95% Pt loading, (c) Effect of different loading of Pt on the hydrolysis of ammonia borane, and (d) Mechanism of photocatalytic hydrolysis of AB using mpg-CN/Pt, reproduced with permission from [199], (e) Hydrolysis of AB in aqueous solution under white illumination using nickel-loaded graphitic carbon nitride (Ni-gC₃N₄) with different loadings of Ni, and (f) hydrolysis of AB using Ni/g-CN with Ni particle size of 3.2 nm under dark environment, reproduced with permission from [200].

Figure 19.

Hydrolysis of ammonia borane using RuP/CN at different concentrations of AB using (a) dark, and (b) light environments, and effect of temperature on the hydrolysis in (c) dark, and (d) and light environments, reproduced with permission from [202].

Figure 20.

(a) COS conversion efficiency of the various metallated catalysts prepared using alkali metals and polymeric carbon nitride, and (b) long term and cyclic ability of potassium metallated polymeric carbon nitride, reproduced with permission from [210].