Insights in the aerobic oxidation of aldehydes

Abstract

The hydroformylation of olefins (oxo synthesis) is the most important process for the production of higher aldehydes (.C 4 ). The liquid phase oxidation of the latter to carboxylic acids by molecular oxygen or air has been known for more than 150 years and is an industrially important process. However, in the recent literature, several different oxidizing reagents and catalytic processes have been reported for this oxidation but most of them have limitations as they use environmentally unacceptable reagents or unnecessarily sophisticated conditions. Herein, we re-evaluated the air oxidation of aldehydes. We show that under mild conditions (air or oxygen and non-optimized stirring), reactions are transfer limited and thus catalyst has no effect on reaction rate. Using efficient stirring (self-suction turbine), uncatalysed air oxidation of 0.8 M aldehyde is possible in 50 min at room temperature whereas less than 10 min was necessary with 10 ppm Mn(II). Thus, recommendations for avoiding common pitfalls that may rise during the evaluation of new catalysts are made.

Metal-Free, Visible Light-Promoted Aerobic Aldehydes Oxidation

“…As expected [4,14,15], aliphatic aldehydes oxidation was faster than the oxidation of aromatic aldehydes (Figure 7). In the dark and in the presence of CQ, the conversion of 2-ethylhexanal was already close to 60% for a residence time of 2 min.…”

“…As mentioned previously, aldehyde autoxidation is a slow spontaneous process which is highly dependent on the presence of inhibitors [4,14]. Thus, we could expect that the in situ (Figure 4).…”

“…On the lab-scale, the autoxidation of aldehydes has been catalyzed by numerous metallic ions in the form of salts, nanoparticles, or organic complexes [3,4]. However, we have shown that, due to low transfer rate of O 2 in the liquid phase, the chemical performances of aerobic oxidation (productivity and selectivity) can be falsified by oxygen starvation in the liquid phase [4,5].…”

“…However, we have shown that, due to low transfer rate of O 2 in the liquid phase, the chemical performances of aerobic oxidation (productivity and selectivity) can be falsified by oxygen starvation in the liquid phase [4,5]. One way this issue can be addressed is to operate at higher oxygen pressure but aldehyde oxidation is a strongly exothermic process (∆ r H = −287 kJ/mol) and proceeds via highly reactive free radical species.…”

Gas–Liquid Segmented Flow Microfluidics for Screening Copper/TEMPO‐Catalyzed Aerobic Oxidation of Primary Alcohols

“…[21,22] Recently, we have shown that aerobic aldehyde oxidations are frequently limited by oxygen mass transfer resistance. [23] Under lab scale conditions (stirred flask) the chemical performances of aerobic oxidation (chemical rate, selectivity, …) can be falsified by oxygen starvation in the liquid. Microreactors and continuous flow technology in general represent an important opportunity towards greener chemical production.…”