Sodium modified mesoporous gamma-alumina from Kano kaolin in methanolysis of corn oil

Abstract

The developments of cheap and environmentally benign synthetic methods remain the challenges facing scientists. Kaolin is an abundant and non-toxic mineral over the expensive and toxic aluminium alkoxides or inorganic salts used in the synthesis of mesoporous gamma alumina for industrial application. In this study, a novel synthesis of mesoporous γ-Al2O3 from Kano kaolin in the presence of non-toxic surfactant, polyoxyethylene (40) stearate (PS), is reported. For comparison alumina is synthesized using PEG-6000. To explore the effect of surfactant mixing mode, PS was added to the boehmite precursor prior and after precipitation. Furthermore, the influence of varying PS concentration and aging time were also studied. The mesoporous γ-Al2O3 was modified with NaOH using the wet impregnation method, and tested for the methanolysis of corn oil by batch reaction process. The reaction conditions for the methanolysis are 5% catalyst, 67 oC temperature, 1:15 oil to methanol molar ratio and 3 hours reaction time. The kaolin, alumina and the catalysts were characterized using X-Ray Fluorescence (XRF), Thermogravimetric-Derivative Thermal Analysis (TG-DTA), X-Ray Powder Diffraction (XRD), Fourier Transform Infrared (FTIR), N2 Adsorption-Desorption, Field Emission Scanning Electron Microscopy (FESEM), Solid State 27Al MAS NMR, Basic Back Titration and Temperature-Programmed Desorption of CO2 (CO2-TPD). The product biodiesel was analyzed using Fourier Transform Infrared-Attenuated Total Reflection Analysis (FTIR-ATR), Nuclear Magnetic Resonance spectroscopy (1H NMR) and Gas Chromatography-Flame Ionization Detector (GC-FID). Structural properties and morphology of the mesoporous γ-Al2O3 was found to improve with increasing surfactant amount from 0.45 g to 1.8 g and aging time from 1 to 2 days then dropped beyond that. Mesoporous γ-Al2O3 synthesized with 1.8 g PS, mixed after precipitation and age for 2 days possessed a surface area (SA) of 222.7 m2/g, narrow pore size distribution (PSD) of 5.6 nm and pore volume (PV) of 0.45 cm3/g. Whereas higher SA of 319.2 m2/g, PSD of 2.7 nm and PV of 0.42 cm3/g were obtained with mixing prior to precipitation. Although, alumina synthesized with PEG has higher SA of 365.1 m2/g, nevertheless, the one with PS have better thermal stability. Response surface methodology (RSM) was applied for the optimization of methanolysis reaction using four-level factorial Box-Behnken Design (BBD). Up to 98.9% yield was obtained using mesoporous γ-Al2O3 modified with 15% NaOH, 6% catalyst loading, 1:15 oil:methanol molar ratio and 2 hours reaction time. From kinetics study, the activation energy, 67.7 kJ mol-1, fall within the range 26-82 kJ mol-1 indicating that the methanolysis was kinetically controlled. The high value of k, 0.03386 min-1, was attributed to the presence of high levels of unsaturated FFA in corn oil. While from the thermodynamics evaluation the positive values of enthalpy and Gibb’s free energy implied the methanolysis reaction is endothermic and non-spontaneous, whereas the negative entropy value suggested it is reversible. Result from catalyst reusability tests revealed that the catalyst is relatively stable.