Sulfamic acid well dispersed in the micropores of Al-pillared α-ZrP as efficient heterogeneous catalyst for synthesis of structurally diverse 1,4-dihydropyridines under mild conditions

Abstract

In this study, α-zirconium phosphate (α-ZrP) has been prepared via reflux method. The α-ZrP material was pillared with [Al13O4(OH)24(H2O)12]7+ polycations to improve its surface area and thermal stability. The aluminum oxyhydroxy clusters were synthesized by in situ partial base hydrolysis of the aluminum salt precursor using sodium hydroxide as base. The microporous inorganic matrix of Al-pillared α-ZrP (AZP) was used for molecular dispersion of sulfamic acid to prepare novel composite materials. The sulfamic acid modified AZP materials were characterized by using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Field emission scanning electron microscopy (FESEM), Transmission electron microscopy (TEM) and Brunauer–Emmett–Teller (BET) specific surface area analysis. XRD study indicated an expansion in the interlayer space as a result of bilayer intercalation of Al137+ pillaring species. The pillared interlayer space is retained in the composite material. FTIR study indicated the structural integrity of the sulfamic acid. The composite materials exhibited enhanced microporosity with surface area and pore volume in the range 80–120 m2/g and 0.1–0.2 cc/g. FESEM and HRTEM studies indicated morphological reorganization of α-ZrP particles as a result of pillaring and subsequent dispersion of sulfamic acid. Elemental mapping study suggested well dispersion of the sulfamic acid in the composite material without any sign of local agglomeration. The catalytic activity of sulfamic acid loaded AZP materials has been evaluated for the synthesis of 1,4-dihydropyridines by multi component condensation reaction of ethyl acetoacetate, arylaldehydes/chalones and ammonium acetate. The composite materials were highly active for synthesis of structurally diverse 1,4-dihydropyridines in high yield and purity under mild conditions.