Heterogeneous Catalyzed Pyridine Synthesis using Montmorillionite and Nanoparticle‐Impregnated Alumina in a Continuous Micro Flow System

Abstract

AbstractThe synthesis of pyridine derivatives was investigated using a combined micro flow‐through system. Therefore, two different heterogeneous catalyst assemblies were used subsequently under different temperature conditions. The investigated pyridine synthesis method includes three reaction steps: at first, an enamine has to be formed by a condensation reaction of methyl ketone and propargylamine. Secondly, a pericyclic reaction of the enamine is needed to form dihydropyridine derivatives. Finally, the dihydropyrimidines received have to be oxidized to yield the desired pyridine derivative. The propargylamine used tends towards side reactions even if activating catalysts are present at elevated temperatures. To reach high yields for the overall process, each reaction step demands for a specific catalyst and suited reaction conditions. Hence, the reaction sequence was separated into two consecutive steps. For the initial condensation reaction, montmorillionite was investigated as heterogeneous catalyst. For the subsequent reaction steps, different types of nanoparticles impregnated on alumina were investigated as oxidation catalyst, using air as oxidation agent. Here, the results and optimized process conditions for the conversion of aliphatic and aromatic methyl ketones are presented. As a result, the separation of the reaction steps by application of the two‐step microreactor system leads to a significant improvement of the process performance. The reaction time was reduced significantly to about 20 minutes without reduction of the product turnover in several cases.