Elsevier

Journal of Catalysis

Volume 376, August 2019, Pages 238-247
Journal of Catalysis

Nitrogen-doped graphene as metal free basic catalyst for coupling reactions

https://doi.org/10.1016/j.jcat.2019.07.011Get rights and content

Highlights

  • N-doped defective graphene (N)G has three types of N atoms, including pyridinic ones.

  • CO2 and NH3 adsorption on (N)G reveals the presence of acid and basic sites.

  • (N)G promotes Michael addition of methylene active compounds to unsaturated ketone.

  • (N)G promotes the Henry addition of nitromethane to benzaldehyde.

  • DFT calculations indicates that the most basic sites are pyridinic N atoms at the edge.

Abstract

N-doped defective graphene [(N)G] obtained by pyrolysis at 900 °C of chitosan contains about 3.7% of residual N atoms, distributed as pyridinic, pyrrolic and graphitic N atoms. It has been found that (N)G acts as basic catalyst promoting two classical Csingle bondC bond forming nucleophilic additions in organic chemistry, such as the Michael and the Henry additions. Computational calculations at DFT level of models corresponding to the various N atoms leads to the conclusion that N atoms are more stable at the periphery of the graphene sheets and that H adsorption on these sites is a suitable descriptor to correlate with the catalytic activity of the various sites. According to these calculations the most active sites are pyridinic N atoms at zig-zag edges of the sheets. In addition, N as dopant changes the reactivity of the neighbour C atoms. Water was found a suitable solvent to achieve high conversions in both reactions. In this solvent the initial distribution of N atoms is affected due to the easy protonation of the NPy to NPyH sites. As an effect, C edge sites adjacent at NPyH with an appropriate reactivity towards the α-C-H bond breaking are formed. The present results show the general activity of N-doped graphene as base catalysts and illustrate the potential of carbocatalysis to promote reactions of general interest in organic synthesis.

Introduction

Graphene, a single layer of carbon atoms in perfect hexagonal geometry with a uniform sp2-hybridized configuration has been considered an excellent suitable support of active nanoparticles due to its high surface area (theoretical value of 2630 m2/g) and strong metal-support interactions [1], [2]. More recently, besides as support of active sites, the use of defective graphenes (Gs) as metal-free catalysts is under intense investigation [3], [4], [5].

Gs as catalysts allow a certain control of their catalytic properties by varying the density of defects and by doping with different heteroatoms (B, N, O, P and S). The combination of defects and doping allows tailoring the electronic and catalytic properties of Gs through the modulation, for instance, of their acid/base site population and strength distribution [6], [7]. Among the various heteroatoms, incorporation of nitrogen into a carbon matrix has attracted a considerable attention as a way to implement in carbon materials active sites for catalysis, electro- and photocatalysis [8], [9], [10]. Nitrogen atoms in graphene can be, at least, in three different bonding configurations corresponding to quaternary (or graphitic), pyridinic, and pyrrolic N. Among these, the presence of pyridinic and pyrrolic heterocycles inside the graphenic structure should change the basic properties of graphene, nitrogen serving as an electron donor site [11], [12], [13]. These properties elicited a considerable interest and were corroborated by the electro-, photo- and catalytic properties of N-doped graphenes. N doped carbon materials, like N-containing carbon nanotubes [14], [15], N-doped carbon materials [16] or mesoporous carbon nitride (MCN) [17] act as effective solid base catalysts for Knoevenagel condensation or transesterification reactions [16], [17].

Base catalysis is very important in organic synthesis as there are numerous classical condensation reactions leading to formation of Csingle bondC bonds involving carbonyl groups that require the use of bases as catalysts. In the vast majority of the cases, the bases used are soluble in the reaction media. However, from the green point of view to minimize liquid wastes and purification steps, it would be important to develop heterogeneous basic catalysts. Among solid, insoluble basic catalysts, certain alkali exchanged zeolites, layered hydrotalcites and alkali-Earth metal oxides are the most frequently used. It would be of large fundamental and applied interest to determine if N-doped graphenes can also be used as basic catalyst and to compare the catalytic performance of these carbocatalysts with that of conventional inorganic bases.

In the field of basic catalysis, Michael and Henry additions are two important classes of Csingle bondC bond forming reactions with large use in organic synthesis [18]. These reactions allow to obtain natural products and complex compounds with biological activity [19], [20]. Michael and Henry reactions require basic or acidic catalysts, typically soluble in the reaction medium. However, the use of soluble acid or basic catalysts has associated several drawbacks including the need of neutralization steps during the reaction work-up and the impossibility to reuse the materials. These soluble acids or bases generate residues and undesirable by-products [21], [22], [23].

Graphenes present delocalized π orbitals which can donate or accept electron density from reagents and substrates adsorbed on their defects (i.e., either the edges of the sheet or the internal carbon vacancies) [24]. In this way, defective and N-doped Gs can, in principle, act as efficient basic catalysts affording the active sites required by the Michael or Henry additions.

Based on these precedents and with the aim to evaluate the catalytic activity of defective Gs in Michael and Henry additions, herein we report the use of N-doped graphene [(N)G] as catalyst in such reactions. (N)G samples were prepared by chitosan pyrolysis, a procedure which does not require any catalyst, avoiding in this way the need for nickel and copper metals. Chitosan is a natural polysaccharide of glucosylamine and, during the pyrolytic process, it acts as simultaneous source of C and N in the (N)G synthesis. The activity of (N)G as a base will be rationalized by computational chemistry on a series of models of plausible N atom on graphene using DFT reactivity descriptors with the aim to correlate the activity of (N)G as base with specific N sites.

Section snippets

Experimental section

Unless otherwise specified, all chemicals were purchased from Sigma-Aldrich and used without further purification.

Characterization of (N)G

Nitrogen doped graphene was prepared and exhaustively characterized as described in our previous work [30]. Briefly, single- or few-layers (N)Gs were prepared by pyrolysis of chitosan powders at 900 °C in the absence of oxygen and without any acid or metal assistance. The carbon residue after the pyrolysis was dispersed in aqueous medium to remove amorphous carbon and heavy particles and the suspended material recovered by freeze-drying (Scheme 1).

The previously reported characterization of the

Conclusions

Acid-base titrations using NH3 and CO2 probe molecules indicated that defective N-doped graphene has considerable more density of basic than acid sites. Also the strength of the sites, determined based on the desorption temperature, for basic sites is higher than the less abundant acid sites. The catalytic activity of N-doped graphene as base has been proved here by using two classical Csingle bondC bond forming organic reactions of large importance in synthesis that require basicity, such as the Michael

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Declaration of Competing Interest

The authors declare no competing financial interest

Acknowledgment

This work was supported by UEFISCDI (PN-III-P4-ID-PCE-2016-0146, nr. 121/2017 and project number PN-III-P1-1.1-TE-2016-2191, nr. 89/2018). Financial support by the Spanish Ministry of Economy and Competitiveness (Severo Ochoa and CTQ2015-69653-CO2-R1) and Generalitat Valenciana (Prometeo 2017-083) is gratefully acknowledged. A.P thanks the Spanish Ministry of Science and Innovation for a Ramón y Cajal research associate contract.

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