Synthesis of benzimidazoles on zeolite catalysts having various porous structures

Objectives. The work set out to compare the catalytic properties of microporous H–Y zeolite and granulated hierarchical (micro-, meso-, and macroporous) H–Y h zeolite in the synthesis of benzimidazoles via the reaction of o-phenylenediamine with aromatic ( p -chlorobenzaldehyde) and aliphatic (hexanal) aldehydes. Methods. Granulated hierarchical zeolite was synthesized by hydrothermal crystallization of pellets containing microporous H–Y zeolite and amorphous aluminosilicate. The physicochemical properties of the catalyst samples were studied using standard zeolite characterization methods. After synthesizing the benzimidazoles in stirred flasks, the reaction was carried out in an oxidizing (air) or inert (nitrogen) atmosphere. The reaction mixture was analyzed using high-performance liquid chromatography. The structure of the compounds was determined using chromatography–mass spectrometry and nuclear magnetic resonance spectroscopy. Results. It is established that the hierarchical H–Y h zeolite, which possesses a well-developed system of meso- and macropores, provides higher selectivity for the formation of target benzimidazoles than its microporous analog. The reaction atmosphere was shown to have a decisive influence on the product composition. While monosubstituted benzimidazoles are selectively formed (up to 83–86%) in an oxidizing environment (air), in an inert atmosphere (nitrogen), the fraction of intermediate Schiff base or disubstituted products increases. Synthesis conditions for benzimidazoles offering the highest yields were identified. Conclusions. The hierarchical H–Y h zeolite, which offers improved transport characteristics and increased accessibility of acid sites, provides higher selectivity for the target products than its microporous analog. In order to promote the selective formation of monosubstituted benzimidazoles, the reaction was carried out in an oxidizing medium. Under optimal conditions, their yields reach the following values: 2-(4-chlorophenyl)-1 H -benzimidazole, 81%; 1-(4-chlorobenzyl)-2-(4-chlorophenyl)-benzimidazole, 73%; and 2-pentyl-1 H -benzimidazole, 80%.