Core-shell ZnO-TiO2 hollow spheres synthesized by in-situ hydrothermal method for ester production application

In the current research, a carbohydrate-derived mesoporous ZnO-TiO2 hollow spheres was synthesized in presence of incomplete carbonized glucose (ICG) as a template using autoclave-assisted reactor. The synthesized mesoporous ZnO-TiO2-ICG composites were further functionalized via thermal treatment with chlorosulfonic acid in chloroform in order to attach -SO3H functional groups on the mesopore walls. The textural, structural, morphological and thermal properties of the sulfonated mesoporous ZnO-TiO2-ICG catalyst were further evaluated. The catalytic performance of the sulfonated ZnO-TiO2-ICG catalyst was investigated through transesterification of waste cooking palm oil (WCPO). A series of transesterification reactions was done to evaluate the influence of operational conditions including; catalyst amount (0.25–1.50 wt%), methanol:WCPO molar ratio (3:1–15:1), operating temperature (60–140 °C), and stirring power (200–800 rpm). Gas chromatography (GC) was also employed to determine WCPO methyl ester content. From the catalyst testing on transesterification of WCPO under optimized conditions (mesoporous ZnO-TiO2-ICG catalyst amount of 1 wt%, methanol:WCPO molar ratio of 6:1, operating temperature of 100 °C, and mixing intensity of 400 rpm), the sulfonated mesoporous ZnO-TiO2-ICG catalyst revealed promising ester yield of 96.1% at 75 min by using autoclave reactor. Reusability experiments indicated that the spent sulfonated mesoporous ZnO-TiO2-ICG catalyst was highly potential to stay active for ten repeated cycles while the FAME yield dropped insignificantly. The high activity and stability were associated to the exceptional textural and structural characteristic of the sulfonated ZnO-TiO2-ICG catalyst along with extremely well-attached of SO3H functional-groups to the majority of active spots. The fuel properties of the prepared WCPO methyl ester was tested by measuring its density, viscosity, pour point, cloud point, flash point, and water and sulfur contents. The results were highly in accordance with particular EN (14214) and ASTM (D6751) standards.