Photocatalytic activity enhancement of g-C₃N₄/BiOBr in selective transformation of primary amines to imines and its reaction mechanism

Highlights

• CB heterojunction with enhanced photoactivity in oxidative coupling of benzylamine was prepared.

• Improved activity was due to excellent charge transfer and separation derived from a staggered band lineup.

• Results reveal O₂^–-assisted mechanism with carbocation as a possible intermediate.

• Under anaerobic condition, the reaction proceeds probably via carbon-centered radical.

Abstract

Herein, the photocatalytic activity of g-C₃N₄/BiOBr (CB) heterojunction in the oxidative C–N coupling of benzylamine under atmospheric air using cool white LED light was reported for the first time. The CB heterojunction was prepared by two-step combustion-coprecipitation method. By tuning the weight percentage of g-C₃N₄, the optimal catalyst containing 10.2 wt% of g-C₃N₄ provided the highest benzylamine conversion of ca. 94% and the best N-benzylidenebenzylamine yield of ca. 82% within 4 h irradiation. The influences of catalyst amount, substrate concentration, light intensity and reaction temperature on photocatalytic performance were also discussed. The CB catalyst also successfully oxidized N-heterocyclic amines and secondary amines into their corresponding imines which extends the scope and potential use of this catalyst in the syntheses of other CN containing biologically active compounds. The enhanced performance of CB heterojunction was mainly ascribed to improved charge transfer and separation intrinsically derived from the staggered band energy configuration of the CB heterojunction as evidenced from photoelectrochemical, steady-state photoluminescence and time-resolved fluorescence studies. Electron paramagnetic resonance (EPR), Hammett and active species quenching results revealed the O₂^–-assisted mechanism with a possible carbocationic intermediate being generated. Under anaerobic condition, the reaction can also proceed probably through carbon-centered radical. Based on UV–vis, XPS and Mott-Schottky results, band energy level diagram and a plausible reaction mechanism at solid-liquid interface were also revealed.

Introduction

Imines or Schiff bases are key intermediates in pharmaceutical, biological and chemical syntheses [1], [2]. Among various imine synthetic methods, the oxidative processes using readily available and easily handle substrates such as alcohols and amines and molecular oxygen or air as a green oxidant are preferable to the traditional condensation approach in which extremely reactive aldehydes, dehydrating solvent and excess amounts of Lewis acids have been employed [2]. These oxidative transformations of amines include the oxidative dehydrogenation of secondary amines, the cross-coupling of amines with alcohols and the self-coupling of primary amines [3]. In contrast to the oxidation of secondary amines, the oxidation of primary amines faces more challenges as the generated imine intermediates with the presence of a second α-amino can easily dehydrogenate to nitrile products [3]. Therefore, the development of selective catalysts which can efficiently perform under mild reaction conditions is necessary.

Heterogeneous photocatalysis is well regarded as a greener, milder and more economical alternative for functional group transformations because of its utilization of solar energy instead of conventional heating as well as the non-toxicity and the recyclability of naturally abundance semiconductor photocatalysts. For the selective oxidation of amines to imines, several photocatalysts have previously been investigated, for example, TiO₂ [4], BiOCl [5], BiVO₄ [6] and CdS [7]. However, the activity of these photocatalysts is often limited by their poor visible-light harvesting ability, fast electron-hole recombination or low photostability. Consequently, extensive efforts have been made to improve the performances of photocatalysts.

Bismuth oxybromide (BiOBr) with a band gap energy of ca. 2.6–2.9 eV has emerged as a promising photocatalyst due to its intrinsic built-in electric field and visible-light absorption capability [8], [9]. The utilization of BiOBr in selective organic transformations is still at an early stage despite its extensive usage in environmental remediation [10], [11], [12]. Until now, only a few reports have shown the activity of BiOBr in imine syntheses [13], [14], [15]. Unfortunately, fast electron-hole recombination, low amine conversion and poor imine yield have limited the practical application of BiOBr. Several strategies such as controlling surface crystal facets [13], forming surface oxygen vacancy [14], [15] and constructing heterojunction [16] have been employed to boost the performance of BiOBr in the photocatalytic oxidation of primary amines. Forming heterojunction between two semiconductors is widely accepted as an effective approach to not only extend the light absorption range but also facilitate the charge transfer in photocatalytic system. These synergistic effects successfully endow photocatalysts with higher amine conversion and better imine yield in several cases [16], [17], [18].

Graphitic carbon nitride (g-C₃N₄), a metal-free visible-light responsive photocatalyst, has gained increasing attentions because of its nontoxicity, high thermal/chemical stability and abundance in nature [19], [20]. G-C₃N₄ alone is able to oxidize amines into imines but only in low to moderate yield [1], therefore surface/interface modifications are conducted to improve its activity [18], [21]. Based on the fact that the oxidation of H₂O to generate OH, a strong oxidant, is thermodynamically disfavored over the g-C₃N₄ due to the low potential of its valence band hole [22], [23], this could in turn benefit the selective oxidation of amines as the generation of OH often leads to complete oxidation in non-selective manners. As a result, coupling BiOBr with g-C₃N₄ may be advantageous for the amine oxidation in this present work.

Herein, the g-C₃N₄/BiOBr (CB) photocatalyst was synthesized by a two-step combustion-coprecipitation method and evaluated for its performance in the oxidative coupling of primary amine using benzylamine as a model substrate. Although the CB heterojunction has previously been employed in environmental remediation [20], [24], its photocatalytic activity in imine synthesis has never been investigated. To realize a potential application of this developed catalyst, different amine substrates, catalyst recyclability as well as its performance compared with that of the two well-known commercial catalysts, TiO₂ P25 Aeroxide and BiVO₄ Alfa Aesar have been investigated. Hammett plot, EPR, and reactive species quenching studies were also carried out to reveal a plausible mechanism over this CB heterojunction.