Carbon. 2017 Apr 30;114:383-92. doi: 10.1016/j.carbon.2016.11.064

P and S Dual-Doped Graphitic Porous Carbon for Aerobic Oxidation Reactions: Enhanced Catalytic Activity and Catalytic Sites

Mehulkumar A. Patela, Feixiang Luob, Keerthi Savarama, Pavel Kucheryavya, Qiaoqiao Xiea, Carol Flacha, Richard Mendelsohna, Eric Garfunkelb, Jenny V. Lockarda, and Huixin Hea*

1Chemistry Department, Rutgers University, Newark, NJ 07102

2Department of Chemistry and Chemical Biology, Rutgers University, 610 Taylor Rd, Piscataway, NJ 08854

 

Abstract 

A highly porous graphitic carbon material, dually-doped with P and S, was studied as a metal free catalyst for aerobic oxidation reactions. Catalytic mechanism studies suggest that the active centers, originated from P-and S-doping, additively/synergistically catalyze the aerobic oxidation of benzylic alcohols but with different pathways. For the first time, catalytic centers stemming from S-doping were experimentally identified to be exocyclic S species (C-S-C, sulfur out of the carbon ring), which are different from those proposed for electrochemical oxygen reduction reactions (ORR) with a 4e- pathway and oxygen evaluation reactions (OER). Notably, all the catalytic sites from both P and S doping share a similar “protruding out” pyramid structure, which is in contrast to the planar structure of the catalytic sites in N- or B-doped graphitic materials. The unique geometric structure of the catalytic sites can minimize substrate steric hindrance effects, endowing the P, S co-doped catalysts with a wide substrate scope and functional group tolerance. Furthermore, the unambiguous distinguishment of the catalytic sites from those in OER and ORR provides valuable guidance for designing and developing carbon materials with controlled active sites to satisfy different catalytic applications.

KEYWORDS: Heteroatom doped carbon, Metal-free catalyst, Aerobic oxidation, Heterocyclic sulfur

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Catalytic oxidation of inexpensive and widely available chemicals using molecular oxygen (O2) as an oxidant to produce high value-added compounds is a significant task in organic synthesis and in the refinery industry. This reaction is usually catalyzed by expensive and environmentally hazardous metal-based catalysts. In the drive towards green and sustainable chemistry, there is an ever increasing interest in developing new metal-free carbon-based materials that are benign, abundant, readily available to act as catalysts for chemical synthesis.[1] Compared to traditional metal based catalysts, carbon based materials provide additional advantages due to the existence of giant π structures which promotes strong interactions with various reactants and its physicochemical and electronic properties, which in principle determine the catalytic properties of a material, can be tailored and fine-tuned by molecular engineering and/or heteroatomic doping.[2] A plethora of reports have demonstrated that doping of heteroatoms into graphene and other carbon based materials can catalyze many organic reactions such as oxidation of alcohols, reduction of nitro compounds, etc. [2-4] However, majority of the research has been only focused on exploring graphene oxide (GO) and nitrogen-doped carbon materials as a catalyst, which could be due to their ease of synthesis.[3, 4]

 

Recently, we have reported an extremely simple and rapid (seconds) microwave-based approach to directly synthesize gram quantities of P-doped graphitic porous carbon materials with controlled P bond configuration from a biomass compound, phytic acid.[5] It was also found that by simply mixing the other heteroatom (N, S, B) sources (such as ammonium hydroxide -N source, amorphous sulfur -S source, boric acid -B source) with phytic acid prior to microwave heating, various P co-doped porous carbon materials can be rapidly synthesized in gram scale for organic synthesis studies.[6] The as-synthesized P co-doped porous carbon materials such as P-N, P-S and P-B co-doped porous carbon materials are denoted as PN-Gc, PS-Gc and PB-Gc, respectively.

 

Taking advantage of this simple technique to synthesize these P co-doped carbon materials, in this work, we have compared their catalytic performance for oxidation reactions using molecular oxygen as the sole terminate oxidant. Using aerobic oxidation of benzyl alcohol as an example, we found that the PS-Gc catalyst shows the most improved catalytic performance compared with single heteroatom-doped carbon catalysts (N, P, S, and GO) and other P co-doped carbon catalysts (PB-Gc and PN-Gc). The PS-Gc catalyst can selectively oxidize a variety of primary and secondary benzylic alcohols to their respective aldehydes/ketones with an excellent tolerance for substrates with steric hindrance, characterized by a wide range of substrate scope. This could be due to the catalytic sites from both P and S doping share a similar “protruding out” pyramid structure, which is in high contrast to the planar structure of the catalytic sites in N- and B-doped graphitic materials. Impressively, without requirement of a co-catalyst, 5-hydroxymethylfurfural (HMF), a biomass molecule from cellulose and cellulose derived carbohydrates, can be catalytically oxidized to 2, 5 diformyl furan (DFF) instead of 2, 5 furan dicarboxylic acid, with almost 100% selectivity, demonstrating its great potential application in the bio-refinery field.

 

 

 

Figure 1.The proposed mechanism for benzylic alcohol oxidation by exocyclic S active centers.

 

From the various control experiments and the detailed characterization of the fresh and used PS-Gc catalysts, the following points were concluded. 1) The PS-Gc catalyst probably contains two distinct types of catalytic centers from P and S-doping. 2) The PS-Gc catalyst requires oxygen activation in the first step of oxidation, which is different from that of P-only doped graphitic carbon materials. 3) S is doped in the PS-Gc catalyst with multiple bonding configurations (exocyclic, heterocyclic, sulfoxide, sulfonate, sulfate etc.). Based on Sulfur K-edge X-ray absorption near edge spectroscopy (XANES) study, only exocyclic-S (C-S-C) species appeared to play the catalytic role in activating the oxygen molecule for selective aerobic oxidation of alcohols. 4) The calculated activation energy of benzyl alcohol oxidation is ~32 kJ.mol-1 for the PS-Gc, which is much lower than the P-doped and N-doped carbon catalysts.[4, 5] 5) The S containing active sites (exocyclic-S) are not stable during the catalytic oxidation reaction and partially converted to sulfoxide type of S species, which cannot be reduced back into exocyclic-S during the catalytic reaction.

 

Based on the results and discussions presented in article, we propose the above mechanism for the catalytic aerobic oxidation (Figure 1), where a secondary benzyl alcohol is used as an example. In the first step of catalysis, molecular oxygen was first activated on exocyclic S forming a reactive oxygen species (ROS). Since the presence of a radical quencher (BHT) did not influence the catalytic reactions, we assume that a non-radical ROS species, such as peroxo like species (OOH) were formed during the oxygen activation step, which oxidized the alcohol to its corresponding aldehyde/ketone. In the same time, some of them were reduced back to exocyclic S, which completes the catalytic reaction cycle. On the other hand, some were converted to sulfoxide type of S, which deactivated the catalytic centers.

 

The high catalytic activity to a wide range of substrates and excellent chemoselectivity in combination with the rapid, energy saving fabrication, suggest that PS co-doped porous carbon materials are promising for “green catalysis” due to their higher theoretical surface area, sustainability, environmental friendliness and low cost.

 

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