Nitrogen-doped graphene as metal free basic catalyst for coupling reactions
Graphical abstract
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 CC 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 CC 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 CC 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.
References (44)
- et al.
J. Sci.: Adv. Mater. Dev.
(2017) - et al.
Catal. Commun.
(2011) - et al.
Appl. Catal. A: General
(2003) - et al.
Carbon
(2014) - et al.
J. Anal. Appl. Pyrol.
(2005) - et al.
J. Catal.
(2014) - et al.
Chem. Soc. Rev.
(2017) - et al.
Nature Nanotech.
(2009) - et al.
Energ. Environ. Sci.
(2012) - et al.
Chem. Rev.
(2014)