Two-dimensional magnesium phyllosilicate as a recyclable solid base catalyst for Knoevenagel condensation

A zeolitic molecular sieve with two-dimensional (2D) morphology and basic properties is a promising catalyst material for organic synthesis owing to the ease access of reactant molecules to the basic sites exposed on the external surface. Here, we report the single step preparation of basic magnesium silicate delaminated MWW layer (Mg-DML) catalysts with 2D morphology by hydrothermal treatment of borosilicate MWW with magnesium nitrate aqueous solution at different hydrothermal temperatures. The resulting solid products exhibited two different characters, i.e., delaminated nature and basic property originated from the framework-incorporated Mg²⁺ species and non-porous amorphous magnesium phyllosilicate characteristics. The formation of magnesium silicate in Mg-DML was confirmed by ultraviolet–visible diffuse reflectance spectroscopy, Fourier transform infrared spectroscopy, ²⁹Si magic angle spinning nuclear magnetic resonance, and X-ray photoelectron spectroscopy analyses. The basic character was measured by CO₂ temperature programmed desorption. The degree of delamination and boron substitution by magnesium increased with higher hydrothermal temperature. The basicity of Mg-DML was found to be a crucial factor in determining the catalytic activity for Knoevenagel condensation, and the Mg-DML-180 catalyst hydrothermally treated at 180 °C exhibited the highest conversion of benzaldehyde.