Three Si-substituted polyoxovanadates as efficient catalysts for Knoevenagel condensation and selective oxidation of styrene to benzaldehyde

Abstract

Three new Si-substituted polyoxovanadates (POVs), [Cd₂(dien)₂][Cd(dien)][Cd(Hdien)₂][V₁₅Si₆O₄₆(OH)₂(H₂O)]·7H₂O (1), [Co(enMe)₂]₃[Co₂(enMe)₂(H₂O)₂][V₁₆Si₄O₄₄(OH)₂(H₂O)]·6H₂O (2), and [Co(teta)]₄[V₁₆Si₄O₄₂(OH)₄(H₂O)]·10H₂O (3) (dien = diethylenetriamine; enMe = 1,2-diaminopropane; teta = triethylenetetramine) were synthesized by the hydrothermal method and characterized. Structural analysis sheds light on the fact that the {V₁₅Si₆O₄₈}/{V₁₆Si₄O₄₆} clusters of compounds 1–3 were formed by replacing {VO₅} square pyramids in the classical {V₁₈O₄₂} cluster with {Si₂O₇} units. Compound 1 is a 2D bilayer structure formed by the [V₁₅Si₆O₄₆(OH)₂(H₂O)]¹⁰⁻ cluster and two types of bridging Cd complexes containing binuclear groups [Cd₂(dien)₂]⁴⁺. Compound 2 is a 3D framework constructed from the [V₁₆Si₄O₄₄(OH)₂(H₂O)]¹⁰⁻ cluster and two types of Co complex fragments including binuclear [Co₂(enMe)₂(H₂O)₂]⁴⁺. In compound 3, the [V₁₆Si₄O₄₂(OH)₄(H₂O)]⁸⁻ cluster is connected with bridging [Co(teta)]²⁺ to expand into a 2D network. Compounds 1 and 3 represent the first 2D assemblies based on a vanadosilicate cluster. 1–3 served as heterogeneous catalysts and exhibited highly efficient catalytic activities for the Knoevenagel condensation under mild ambient conditions with low catalyst loading, featuring the open Lewis base {V₁₅Si₆O₄₈}/{V₁₆Si₄O₄₆} sites and Lewis acid Cd²⁺/Co²⁺ sites. The conversion of benzaldehyde was up to 99.3% in 80 min at room temperature using 1 as a heterogeneous catalyst with only 0.37% catalyst loading. Moreover, compounds 1–3 as catalysts for selective oxidation of styrene to benzaldehyde exhibited excellent catalytic performance, high selectivity and could be readily recycled. Most strikingly, compound 1 showed excellent catalytic performance with 97.6% conversion of styrene and 100% selectivity of benzaldehyde in 15 min. In addition, the catalytic activity of catalyst 1 was well maintained after five cycling reactions.