Sustainable Synthesis of MCM-22 from Rice Husk Ash: Characterization, Catalytic Optimization, and Reusability in Knoevenagel Condensation

Abstract

Agricultural residues provide promising precursors for sustainable catalyst development. In this study, rice husk ash (RHA), a silica-rich biomass waste, was used as the silica source for synthesizing MCM-22 zeolite. The resulting RHA-MCM-22 was modified with nickel by impregnation/ion-exchange to produce Ni/MCM-22 catalysts with tailored physicochemical and catalytic properties. The materials were considered by powder Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy–energy-dispersive spectroscopy (SEM–EDS),nitrogen adsorption–desorption, thermogravimetric analysis (TGA), inductively coupled plasma optical emission spectroscopy (ICP-OES), NH₃-temperature-programmed desorption (NH₃-TPD), pyridine-FTIR, H₂-temperature-programmed reduction (H₂-TPR), and X-ray photoelectron spectroscopy (XPS). Catalytic activity was assessed using the Knoevenagel condensation of benzaldehyde with malononitrile. The effects of catalyst loading, nickel content, reaction temperature, reaction time, solvent, and reactant ratio were examined to establish suitable conditions. Optimized catalyst was further tested with various substrates, multiple cycles and post-reaction characterization. Nickel incorporation is believed to change the number and/or acid–base character of the sites, but not the MWW framework of the MCM-22. The work shows a practical approach to recycling RHA to become value-added, reusable zeolitic catalysts, which further facilitates waste utilization, while decreasing the reliance on commercial silica and greener carbon–carbon bond formation by improved resource efficiency and less environmental burden in general.