A series of MgZnAl hydrotalcite-like compounds with different zinc content (1–25 mass % of nominal zinc content) were prepared by a simple and environmentally-friendly method. The solids were characterized by X-ray powder diffraction (XRD), thermogravimetric (TG), nitrogen adsorption-desorption at −196°C (BET), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and CO2 temperature-programmed desorption (CO2-TPD). Transesterification of castor oil with methanol was selected as a probe reaction to stress the effect of zinc incorporation. From the XRD analysis of fresh samples it was demonstrated that the incorporation of zinc is feasible in the nominal range of 1–10 mass % while in the samples with higher zinc content, zinc nitrate and ZnO as secondary crystalline phases were observed. Furthermore, the analysis of samples calcined at 450°C indicated the coexistence of the ZnO and MgO crystalline phases. From the EDS and TG characterizations, the zinc percentage and thermal decomposition of the samples were determined. It was revealed that the samples exhibited the characteristic platy-like habit of hydrotalcite-like compounds. The BET analysis confirmed that the calcined samples presented an increment in their specific surface area values compared with the pristine ones. It was established that the amount of basic sites diminished with the zinc incorporation, which also affected the conversion degree of the transesterification reaction.
Summary
Lauric acid (LA) is the precursor of methyl laurate (ML), which is identified as a valuable large cetane number methyl ester. In this work, LA is converted via high‐shear mixing (HSM)‐assisted homogeneous acid esterification with methanol at varying reaction conditions. A central composite design (CCD) is used to investigate formally the simultaneous influence of methanol:LA ratio (2‐13 mol/mol), temperature (30°C‐60°C), mixing speed (500‐2000 rpm), catalyst mass (0.25‐4.0 g) and time (1‐12 min) on LA conversion. By combining CCD statistical results and response surface methodology, it is found that catalyst mass, methanol:LA ratio and temperature are the most influencing factors on LA conversion, while only the quadratic effects of catalyst mass and methanol:LA and three of the 10 possible two‐way linear interactions, viz., methanol:LA ratio‐catalyst mass, temperature‐time and temperature‐catalyst mass, are statistically significant. Obtained numerically, the maximum LA conversion (92%) is achieved at 60°C, 13 methanol:LA ratio, 4.0 g of catalyst, 2000 rpm and 12 min. Interestingly, assisting the said esterification by HSM results in a little mixing speed‐sensitive process with low mixing speed and reduced time operation being sufficient to conduct the reaction efficiently.
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