Doped microporous graphitic carbons as metal-free catalysts for the selective hydrogenation of alkynes to alkenes
Graphical abstract
Introduction
Porosity plays a key role in heterogeneous catalysis. On one hand, the presence of pores increases the surface area of a material compared to bulk particles. On the other hand, there are considerable experimental evidence supported by theoretical calculations on the possible operation within the pores of the so-called confinement effect [1], [2], [3], [4], [5]. Confinement effects can increase the reaction rate when the dimensions of the pores are similar to that of reagents [6], [7]. Compared to flat surfaces, location of active sites inside pores can be favorable to overcome activation barriers in a chemical reaction [8], [9]. Curvature of the pore walls around substrates and reagents may result in their activation by a combination of weak van der Waals interactions as well as dipole–dipole forces and internal electrostatic fields [10], [11]. Most of the examples regarding confinement effect correspond to microporous inorganic materials, having pores of dimensions smaller than 2 nm [12], [13], [14]. Confinement effect has been proposed, among other examples, to contribute to hydrocarbon activation in cracking in acid zeolites, explaining the remarkable activity of these aluminosilicates in this reaction in spite of the moderate acid strength measured by conventional techniques [13], [15].
Regarding carbons, there are various procedures to produce porous carbons [16], [17], [18]. However, most of them based on the use of hard or soft templates, rendering carbonaceous materials having meso- or macropores. Preparation of microporous carbons with strictly defined, regular quasi-crystalline pore size has resulted elusive in most of the cases. In this context, we have recently reported that pyrolysis of cyclodextrins renders highly crystalline, microporous graphitic carbon nanoparticles with strictly regular pore size from 0.64 to 1.09 nm depending on the dimensions of the cyclodextrin precursor [19]. It was found that these microporous graphitic carbons were able to promote aerobic oxidation of benzylic hydrocarbons with notable higher activity than active carbons lacking uniform porosity or reduced graphene oxide considered as flat 2D layers [19]. The catalytic activity correlates with the pore size, the microporous graphitic carbon derived with α-cyclodextrin (Cα) and exhibiting the smallest pore size (0.64 nm) being the most active material. DFT calculations indicate that the reason for this higher activity can be attributed to a confinement effect since molecular oxygen inside the channels of Cα receives electron density from the carbon walls in an extent that decreases as the dimensions of the channel increases. This causes an elongation of the OO bond that corresponds in fact to a preactivation of the molecule. Interestingly, the aerobic oxidation occurs on or within the microporous nanoparticles, since removal of the solid carbon stops completely the oxidation reaction. Confirmation of the superior activity as microporous metal-free catalysts of Cα compared to other related materials is an issue of wide general interest in the quest for providing metal-free carbocatalysts with improved performance for general reaction types, some of them supposedly being exclusively promoted by transition metals [20], [21].
Hydrogenations of CC multiple bonds is in fact one general reaction of large industrial importance that are currently performed using transition metal catalysts, in many cases containing noble metals [22]. We have previously reported that defective graphenes are active catalysts for alkene hydrogenation [23], [24], for the selective hydrogenation of alkynes to alkenes [25] and the reduction of nitro to amino groups [26]. To put into context the catalytic activity of Cα materials and determine the influence of microporosity respect to flat reduced graphene layers and related carbons, a step forward would be to assess the activity of microporous Cα as hydrogenation catalysts. In addition, the influence of doping on these microporous graphitic carbons in their catalytic activity still remains unexplored. Specifically, it is of interest to determine in which way dopant elements enhance the activity of microporous graphitic carbons as hydrogenation catalysts.
Herein, it will be shown that while Cα is devoid of any activity as hydrogenation catalyst due to the lack of defects considered as active sites [27], N or P doping renders the resulting doped microporous carbons active as metal–free hydrogenation catalysts as consequence of their H2 chemisorption ability that is absent in the undoped Cα. The performance of these microporous Cαmaterials compares favorably with that of related non-porous graphenes, illustrating the potential of combining active sites and adequate microporosity as a valid strategy to increase the catalytic activity of these carbocatalysts.
Section snippets
Synthesis of the catalysts
Microporous graphitic carbon Cαwas prepared by pyrolysis of α-cyclodextrin in an electrical furnace under 200 mL min−1 Ar flow up to 900 °C. The temperature is raised from the ambient to 300 °C at 10 °C min−1 and then maintained for 2 h dwelling time to favor the assembly of melted α -cyclodextrin, before thermolysis by increasing the temperature at 5 °C min−1 rate up to 900 °C that was maintained for 1 h. The samples were allowed to cool at ambient temperature maintaining the Ar flow. After
Results and discussion
Samples of microporous graphitic carbons were obtained by pyrolysis at 900 °C under inert atmosphere of Cαwithout or containing urea [(N)Cα] or phosphoric acid [(P)Cα]. In the last two cases, the dopant agent-to-α-cyclodextrin weight percentage prior to pyrolysis was adjusted to 1:10 in the case of samples (N)Cα-10 and (P)Cα-10 or 1:1 in the case of (N)Cα-50 and (P)Cα-50, respectively. The resulting carbon residues were characterized by a combination of analytical, spectroscopic and microscopy
Conclusions
In the field of metal-free carbocatalysts, microporous materials may offer advantageous activity due to confinement. In the present study, it has been shown that in contrast with the lack of catalytic activity of the undoped material, N and P-doped microporous graphitic carbons exhibit a notable activity and selectivity for the partial hydrogenation of aliphatic and aromatic alkynes to the corresponding alkenes, higher than analogous graphene materials. Particularly, N-doped Cα materials
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Financial support by the Spanish Ministry of Science and Innovation (Severo Ochoa and RTI2018-98237-CO2-1) and the Generalitat Valenciana (Prometeo 2021/038) is gratefully acknowledged. V.P. acknowledge support from UEFISCDI (PN-III-P4-ID-PCE-2016-0146, 121/2017 and PN-III-P4-ID-PCE-2020-1532).
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