Heteroatom‐Enhanced Metal‐Free Catalytic Performance of Carbocatalysts for Organic Transformations

Abstract

The exploitation and design of metal‐free catalysts for chemical transformation is always a fascinating theme from the prospective of fundamental and applied research. In addition to their distinct advantages such as lower cost, higher biocompatibility and reliability and thus higher sustainability, metal‐free materials often unexpectedly render different reaction pathway or mechanism from conventional metal catalysts. As a consequence, metal‐free catalysts have revolutionized and expanded the platform of catalysis. Among them, carbons are a typical class of nonmetallic materials. To date, a series of elements (e. g., B, N, P or S) as heteroatom dopants have equipped pristine carbons with enhanced performance and even new chemical functionality. It was proposed to attribute to modulated electrical properties and surface physicochemical features originated from structural distortions and changes of charge densities. In this minireview, we overviewed the catalytic application of heteroatom‐doped carbocatalysts in oxidation, oxidative coupling, reduction and hydrogenation reactions with special emphasis on the role of heteroatoms and consequently enhanced performance. This minireview highlights the structure‐activity correlations, thereby directing the rational design of advanced metal‐free catalysts for organic synthesis. Some controversies about active sites and challenges in catalyst design are also discussed here.

Nanocarbon for Energy Material Applications: N2 Reduction Reaction

“…The different functional groups present in nanocarbons by heteroatom doping can be shortly summarized as i) Brønsted acido-base sites, for example, carboxylic, phenolic or amine groups, SO 3 H sites, P-OH; ii) Lewis acido-base sites, for example, pyridinic N, carbonyl groups, B; iii) frustrated Lewis acid-base pairs, i.e., Lewis acid-base pairs rigidly separated at a short distance without undergoing neutralization; [96,97] iv) dangling bonds at the edges, which are suitable to promote radical chain oxidations, or to activate covalent bonds by interacting with them; and v) redox sites, such as epoxy surface groups able to give redox reactions with incoming molecules. [98] All these sites have been well demonstrated to play a role in many catalytic reactions, [1,11,24,28,[99][100][101][102] including in electrocatalytic reactions. [103][104][105][106] For a demanding reaction such as NRR, the direct role of these sites, rather than an indirect role of heteroatoms in creating reactive C sites for N 2 activation is not enough [11] Copyright 2018, Royal Chemical Society.…”

“…Lewis acid-base pairs rigidly separated at a short distance without undergoing neutralization; [78,79] (iv) dangling bonds at the edges, which are suitable to promote radical chain oxidations, or to Submitted version Small, 2021, 17 (48) activate covalent bonds by interacting with them; and (v) redox sites, such as epoxy surface groups able to give redox reactions with incoming molecules. [80] All these sites have been well demonstrated to play a role in many catalytic reactions, [1,11,24,28,[81][82][83][84] including in electrocatalytic reactions. [85][86][87][88] Submitted version Small, 2021, 17 (48) For a demanding reaction such as NRR, the direct role of these sites, rather than an indirect role of heteroatoms in creating reactive C sites for N2 activation is not enough demonstrated.…”

Microporous Organic Polymers: A Synthetic Platform for Engineering Heterogeneous Carbocatalysts

“…Nevertheless, given that undoped carbon materials, such as graphene oxides and activated carbons, have only a minor effect on the reduction reaction, it is evident that the doped heteroatoms conferred significant catalytic activity to the carbonaceous materials. We conjecture that doping with electronegative nitrogen caused local positive charge density on vicinal ortho ‐carbon atoms, and then promoted the catalytic reduction reaction in conjunction with NaBH 4 , as shown in the case of 1 c [52,53] . On the other hand, electronegativity of sulfur is similar to that of carbon, but the former is more polarizable; therefore, the sulfur dopants induce asymmetric spin density that can provide more efficient active sites, possibly on the edge of 2 c , [54] which helps in transferring a surface hydrogen species from the reducing agent and thus facilitates the conversion of 4‐NP to 4‐AP [55,56] .…”

Covalent Organic Frameworks for Efficient Energy Electrocatalysis: Rational Design and Progress

“…[ 20 ] To modulate the surface properties, heteroatom doping, surface vacancy engineering, and the introduction of metal catalytic units have proven to be powerful technologies by modulating the electronic properties of catalysts to adjust their electronic properties and/or chemical activities favorable for catalyzing. [ 21 ] In addition, lattice oxygen and molecular sites on the catalyst surface play an important role in the electrocatalytic process by regulating the adsorption of intermediates, such as OH*, resulting in a lower reaction energy barrier. [ 22 ] In short, the reaction energy barrier can be greatly reduced by the introduction of heteroatoms, surface vacancy engineering, and interfacial regulation.…”