Synthesis of NaOH-impregnated Calcined Oyster Shell for Transesterification Reaction

The goal of this dissertation research is to convert oyster shells into solid base catalysts for use in transesterification reactions. Specifically, the objectives were to: (1) synthesize a solid base catalyst by impregnating NaOH on the oyster shell (2) optimize the transesterification reaction parameters (3) study the kinetics of the transesterification and propose the underlying mechanism (4) elucidate the effects of preparation conditions on the surface structure and the activity of the catalyst.;This research was conducted in three phases. In the first phase, the catalyst was prepared by impregnating the waste oyster shell with 50 wt.% NaOH solution followed by calcination at 1000 °C for 3 h. The activity of NaOH-impregnated calcined oyster shell (Na-COS) was tested for transesterification of soybean oil and compared with the calcined oyster shell (COS) and conventional NaOH. Results indicated that Na-COS (> 87% yield in 1 h) was more active than COS (∼ 70% yield in 3 h) and comparable with NaOH (∼ 95% in 50 min). Reusability studies using Na-COS indicated that Na-COS could be applied to at least three consecutive batches of the transesterification reaction without any significant loss of activity Additionally, batch experiments were carried out using Na-COS catalyst via a 42 factorial design at 62 °C, 800 rpm by selecting molar ratio of methanol to oil (MR) and catalyst loadings (CL) as factors. The levels of MR were set to 6, 12, 18, 24, while the levels of CL were set to 2%, 5%, 7%, 10%. Results indicated that MR = 12 and CL = 10% led to the highest fatty acid methyl esters (FAME) yield at 93.9%. Both MR and CL had significant effect on the FAME yield (p = 1.36 x 10 -13 and 1.13 x 10-8, respectively), and a significant MR-CL interaction effect also existed (p = 6.08 x 10-7).;In the second phase, the surface of Na-COS was characterized to detect the active species on the surface. Based on XRD and XPS data it appeared that enhanced activity of Na-COS was due to formation of Na2O2 phase and higher electron donating ability. Based on the characterization data, the mechanism of Na-COS-catalyzed transesterification was theorized to occur between adsorbed triglyceride and free methanol. Additionally, several mathematical models were developed to describe the transesterification process. To validate the models, batch experiments were carried out using Na-COS via a 3x4 factorial design at 62 °C, 800 rpm, volumetric ratio of hexane (co-solvent) to methanol = 1.83, wherein the levels of MR were set to 6, 12, 18, while the levels of CL were set to 2%, 4%, 6%, 8%. The most appropriate model was inferred to be the stepwise transesterification in which, transesterification of adsorbed triglyceride to adsorbed diglyceride was the rate determining step (RDS). The reaction rate constant of the RDS was estimated to be 0.0059 +/- 0.0002 L mol-1 min-1 using the initial rate method. A reasonable fit was observed between the experimental and model predicted data (correlation of coefficient = 0.865).;In the third phase, effects of preparation conditions on catalyst structure and activity were investigated using the one-factor-at-a-time method. Firstly, various concentrations of NaOH and NaCl were chosen as the precursor chemicals for impregnation and tested in batch experiments (62 °C, 800 rpm). From the experimental results, the most feasible impregnation concentrations for NaOH and NaCl were found to be 6 mol/L and 2.43 mol/L, respectively. Subsequently, the effect of calcination temperature was investigated to observe that 800 °C was ideal for both precursors. All catalysts were studied via basicity tests, Brunauer--Emmett--Teller (BET) nitrogen adsorption method, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The data suggested that impregnated chemical and the calcination temperature determined the generated surface species that would affect the catalyst activity. Specifically, for NaOH6/shell, higher calcination temperature facilitated the conversion of CaCO3 and Ca(OH)2 into CaO, and generation of Na 2O2 on the surface, which resulted in consistent catalytic activity and higher durability.;For NaCl2.43/shell, the synergistic effect of NaCl and CaO was observed when calcined at 800 °C, which enhanced the FAME yield, compared with those calcined at 1000 °C, in which no NaCl existed. Lower calcination temperature (600 °C) resulted in the limited amount of CaO formation and the high coverage of NaCl on the surface that led to almost no catalytic activity.;Overall, the results of this research suggested that the Na-COS is highly active and durable in transesterification that could potentially act as a substitute for NaOH. In addition, Na-COS catalyst could be employed in several organic reactions including: double bond isomerization of alkenes, hydrogenation of conjugated alkadienes, and cyanoethylation. Using wastes as precursors for catalysts will not only solve waste disposal problems but also add value to wastes.