Highly Efficient Metal-Free Nitrogen-Doped Nanocarbons with Unexpected Active Sites for Aerobic Catalytic Reactions.

Yangming Lin,Yiming Niu,Zigeng Liu,Shuchang Wu,Bingsen Zhang,Siglinda Perathoner,Saskia Heumann,Gabriele Centi,Qing Lu,Dang Sheng Su,Linhui Yu

doi:10.1021/acsnano.9b05856

Abstract

Nitrogen (N)-doped nanocarbons (NDN) as metal-free catalysts have elicited considerable attention toward selective oxidation of alcohols with easily oxidizable groups to aldehydes in the past few years. However, finding a new NDN catalytic material that can meet the requirement of the feasibility on the aerobic catalytics for other complicated alcohols is a big challenge. The real active sites and the corresponding mechanisms on NDN are still unambiguous because of inevitable coexistence of diverse edge sites and N species based on recently reported doping methods. Here, four NDN catalysts with enriched pyridinic N species and without any graphitic N species are simply fabricated via a chemical-vapor-deposition-like method. The results of X-ray photoelectron spectroscopy and X-ray absorption near-edge structure spectra suggest that the dominating N species on NDN are pyridinic N. It is demonstrated that NDN catalysts perform impressive reactivity for aerobic oxidation of complicated alcohols at an atmospheric pressure. Eleven kinds of aromatic molecules with single N species and tunable π conjugation systems are used as model catalysts to experimentally identify the actual role of each N species at a real molecular level. It is suggested that pyridinic N species play an unexpected role in catalytic reactions. Neighboring carbon atoms in pyridinic N species are responsible for facilitating the rate-determining step process clarified by kinetic isotope effects, in situ nuclear magnetic resonance, in situ attenuated total reflectance infrared, and theoretical calculation. Moreover, NDN catalysts exhibit a good catalytic feasibility on the synthesis of important natural products (e.g., intermediates of vitamin E and K3) from phenol oxidation.

Highlights

Catalytic oxidation of alcohols to value-added products is an important reaction in catalytic synthesis.[1,2] Metal-based materials are the most common catalysts for highlighting that reaction
We develop a class of nitrogen-doped nanocarbon (NDN) catalysts with enriched pyridinic N by using three nanocarbon materials (bucky nanodiamond (BND), carbon nanotubes (CNTs), and nitrogen-doped onion-like carbon (OLC)) as starting materials (Scheme 1 and Figure S1)
Compared with the reported metal-based catalysts and the industrial homogeneous catalysts, NBND showed good catalytic performance at similar reaction conditions (3, 6−8), especially for synthesizing the vitamin K3 intermediate MNQ (8).[33−35] a cost-effective heterogeneous metal-free catalytic system to effectively realize the high outputs of complicated aldehydes and natural product quinones has been achieved by NBND with enriched pyridinic N species

Summary

Introduction

Catalytic oxidation of alcohols to value-added products is an important reaction in catalytic synthesis.[1,2] Metal-based materials are the most common catalysts for highlighting that reaction. The results of activity evaluation indicate that NBND-1 shows the conversion rate of 13.8%, which is lower than that of the NBND catalyst (32.5%), suggesting the positive role of the content of pyridinic N in improving the performance.

Results

Conclusion