Multiphase Flow Systems for Selective Aerobic Oxidation of Alcohols Catalyzed by Bimetallic Nanoclusters

Abstract

Au-Pt and Au-Pd bimetallic nanoclusters that catalyzed the aerobic oxidation of alcohols during a once-through pass through gas-liquid-liquid-solid flow systems were developed. Alcohols were converted to aldehydes and ketones in benzotrifluoride (BTF)/water media by Au-Pt catalyst or to the corresponding methyl esters in methanol/water media by Au-Pd catalyst. The flow systems were superior to the batch systems in terms of both yield and selectivity.Keywords: bimetallic cluster, oxidation, flow system, heterogeneous catalysis, alcoholIn synthesis, a flow system has several advantages over a batch system. Indeed, a flow system is crucial for large-scale industrial synthesis, high-throughput organic synthesis related to combinatorial library generation, and automation of multistep transformations [1]. Moreover, unique reactivity and selectivity are sometimes expected at a short residence time; a flow system is one of the methods for reaction integration (space integration) where, for example, unstable intermediates can be available [2]. While gas-solid, liquid-liquid, or liquid-solid biphasic conditions are common in flow systems, examples of gas-liquidsolid multiphase conditions are very rare. As for flow systems for aerobic oxidation of alcohols using heterogeneous catalysts, to the best of our knowledge, there are only a few reports of multiphase reactions in microreactor systems [3], capillary systems [4], and multichannel reactor systems [5], and only one report of a conventional packed flow system [6]. Moreover, there are several limitations in previous studies: immobilization of the catalyst was relatively difficult in the case of the microreactor, capillary, and multichannel systems [3][4][5].Selective oxidation of alcohols is one of the most important transformations in organic synthesis. This is because the resulting carbonyl compounds possess higher energy and reactivity, allowing many types of carbon-carbon and other bond-forming reactions [7,8]. Although several metal-based oxidizing reagents have been developed, these protocols usually require stoichiometric amounts of metal oxidants, and thus a large amount of waste is formed in many cases [9]. Since Haruta's discovery, aerobic oxidation reactions catalyzed by gold nanoclusters have been widely investigated [10,11]. In particular, the selective oxidation of alcohols to the corresponding carbonyl compounds, such as aldehydes, carboxylic acids, and esters, catalyzed by gold-based bimetallic nanoclusters, is attractive in both industry and the laboratory [11,12].We have been investigating aerobic oxidation reactions catalyzed by gold and gold-based bimetallic nanocluster catalysts, namely polymer incarcerated catalysts (PI catalysts) [13,14]. In a previous paper, we reported on the selective oxidation of alcohols to aldehydes and ketones catalyzed by novel carbonblack stabilized polymer-incarcerated gold/platinum bimetallic nanocluster catalysts (PI-CB/Au-Pt) and direct oxidative methyl ester formation from primary alcohols catalyzed by gold/palladium ...

Liquid phase oxidation chemistry in continuous-flow microreactors

“…148 Kobayashi et al described the first quantitative aerobic oxidation process of both primary and secondary alcohols to its respective aldehydes and ketones in a microreactor. 149 , 150 The reactor consisted out of a wall coated, gold-immobilized, capillary column. The three-phase system proved to be highly applicable for a wide scope of benzylic, aliphatic and allylic alcohols.…”

A flow reactor setup for photochemistry of biphasic gas/liquid reactions

“…In contrast, an especially complex problem beyond the scope of most microreactor setups are heterogeneous reactions [31–32] between three dispersed entities such as a liquid phase, a gas phase, and an electromagnetic radiation field, where an effective interaction of three quasi mobile phases of different physical states is required for high selectivities and conversion rates. Such scenarios are highly relevant for photochemical reactions with reactive gases (e.g., air, O 2 , O 3 , H 2 , Cl 2 , acetylene, NO x , CO, CO 2 , etc.)…”

Metal-Free, Visible Light-Promoted Aerobic Aldehydes Oxidation

“…Flow chemistry has opened new exciting opportunities to perform gas-liquid reactions [8][9][10][11][12][13]. It provides advantages of fast mixing, enhanced mass and heat transfer, and large and well-defined interfacial area [12].…”