Polyphosphoric acid–zirconia pillared clay composite catalytic system for efficient multicomponent one pot synthesis of tetrahydropyridines under environmentally benign conditions

Abstract

A series of zirconia pillared clay–polyphosphoric acid (PPA) composites are synthesized by adopting different preparative strategies. Initially, PPA is intercalated to the clay matrix with and without the use of structure expanding agent (CTAB). Subsequently, the PPA–clay composite is pillared with Zr-polycation to form the composite materials. In an alternate approach, Zr-pillared clay is synthesized by insertion of Zr-polycation, which is then used for dispersion of PPA moiety in the pillared clay matrix. The synthesized composites are characterized by XRD, FTIR, UV–vis, TGA, EDX, FE-SEM and sorptometric techniques. XRD study indicated an expansion in the clay structure after intercalation of PPA as well as the Zr-polycations. The FTIR spectra exhibit characteristic vibrational features of the clay sheet as well as PPA moiety indicating the structural stability of the composite materials. The phosphorous content in the composite samples is analyzed using EDX study. FE-SEM study indicated morphological changes upon intercalation of PPA and Zr-polycations to the clay matrix. The catalytic activity of the composite catalysts has been examined for the synthesis of tetrahydropyridines under environmental benign conditions by one pot multicomponent condensation of β-dicarbonyl compounds, substituted anilines and substituted benzaldehydes. The PPA intercalated clay pillared with Zr polycations (PPA–ZrP) is found to be highly efficient for the synthesis of structurally diverse substituted tetrahydropyridines under mild conditions.