Alcohols continuous-flow aerobic oxidation

Fig. 2 Schematic representation of continuous-flow aerobic oxidation of alcohols using Pd nanoparticles in a PEG/SCCO2 biphasic system...

Leitner et al. have synthesized the PEG-modified silica stabilized and immobilized palladium nanoparticles for aerobic alcohol oxidation in combination with scC02 as reaction medium under mild conditions, which show high activity and excellent stability under continuous-flow operation [68], ScC02 could diffuse the substrates and products from the active nanoparticles in a gas-like manner. This allows rapid chemical transformation at the active center, ensures efficient removal of the products from the surface, and minimizes the mobility of solid-supported catalytically active species [69]. In this way, catalysts based on palladium nanoparticles together with PEG as stabilizing matrix could avoid aggregating and forming less active and selective Pd-black [20, 60, 70]. 

The alcohol oxidation reactions described herein use inexpensive O2 and NO sources, and there is potential for their use in large-scale industrial processes. Continuous flow methods provide a particularly strategic opportunity for large-scale applications (see Chapter 23). Hermans and coworkers demonstrated a segmented-flow method for their amberlyst-15/NO -catalyzed aerobic oxidation... 

Stahl et al. reported a continuous homogeneous copper-catalyzed aerobic oxidation method for primary alcohols [32]. A modified tube reactor was used (Figure 23.3). A dilute oxygen source (9% O2 in Nj) was controlled by a mass flow controller and mixed with the substrate and catalyst solution, respectively, by... 

The active site responsible for the aerobic oxidation of alcohols over Pd/AljO, catalysts has long been debated [96-lOOj. Many reports claim that the active site for this catalyst material is the metallic palladium based on electrochemical studies of these catalysts [100, 101]. On the contrary, there are reports that claim that palladium oxide is the active site for the oxidation reaction and the metalhc palladium has a lesser catalytic activity [96,97). In this section, we present examples on how in situ XAS combined with other analytical techniques such as ATR-IR, DRIFTS, and mass spectroscopic methods have been used to study the nature of the actual active site for the supported palladium catalysts for the selective aerobic oxidation of benzylic alcohols. Initially, we present examples that claim that palladium in its metallic state is the active site for this selective aerobic oxidation, followed by some recent examples where researchers have reported that ojddic palladium is the active site for this reaction. Examples where in situ spectroscopic methods have been utilized to arrive at the conclusion are presented here. For this purpose, a spectroscopic reaction cell, acting as a continuous flow reactor, has been equipped with X-ray transparent windows and then charged with the catalyst material. A liquid pump is used to feed the reactants and solvent mixture into the reaction cell, which can be heated by an oven. The reaction was monitored by a transmission flow-through IR cell. A detailed description of the experimental setup and procedure can be found elsewhere [100]. Figure 12.10 shows the obtained XAS results as well as the online product analysis by FTIR for a Pd/AljOj catalyst during the aerobic oxidation of benzyl alcohol. 

Leitner and co-workers also studied the oxidation activity of palladium using supercritical CO2 as the solvent. They reported efficient and stable catalysts for the selective aerobic oxidation of benzylic and allylic alcohols to aldehydes and ketones with selectivities over 98% and TON values in the range of 22-47, using supercritical CO2 as the mobile phase in a batch as well as in continuous-flow process. The palladium nanoparticles were stabilised by polyethylene glycol (PEG)-modified silica and deposited on the surface of modified silica, and the authors claim... 

Obermayer D, Balu AM, Romero AA, Goessler W, Luque R, Kappe CO. Nanocatalysb in continuous flow supported iron oxide nanoparticles for the heterogeneous aerobic oxidation ofbenzyl alcohoL Green Chem. 2013 15 1530-1537. 

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