Condensation of pyrrole with aldehydes in the presence of Y zeolites and mesoporous MCM-41 aluminosilicate: on the encapsulation of porphyrin precursors

Abstract

Condensation of pyrrole and benzaldehyde was carried out in isooctane or cyclohexane in the presence of MCM-41, HY (Si/Al 15) and CoY (Si/Al 2.6). In all cases, formation of tetraphenylporphyrin (TPP) was observed in the organic solutions, even after dissolving with hydrofluoric acid the resulting MCM-41 solid employed in the condensation. In addition, some reaction intermediates containing one to two pyrrole heterocycles were also characterized in these solutions. In spite of this, the diffuse reflectance and IR spectra of the organic material adsorbed in the aluminosilicates after the condensation reaction were remarkably different from those characteristic of TPP. This discrepancy indicates that the material adsorbed inside the solid pores is not TPP but most probably any of its immediate porphyrinogen precursors. In this regard, the coincidence of the diffuse reflectance spectra obtained after the synthesis with those characteristic of porphomethenes and porphodimethenes was noted. Condensation of pyrrole and acetaldehyde follows the same pattern: lack of formation of meso-tetramethylporphyrin (TMP) inside the pores according to diffuse reflectance and IR spectroscopies, but detectable amounts of TMP in the organic solutions.