Hydrothermal crystallization of zeolite catalysts using kaolin and volcanic ash materials for biodiesel synthesis from Jatropha curcas and waste cooking oils

Abstract

Although biodiesel is a promising alternative source of energy, the cost of its production remains high because of the use of expensive feedstocks and catalysts. Zeolites derived from clay and volcanic ash as alternative sources of aluminosilicates are promising catalysts for the biodiesel synthesis from non-edible plant oils. The aim of this study was to develop zeolite catalysts for transesterification of Jatropha curcas and waste cooking oils. For sustainability and cost reduction, the zeolites were hydrothermally crystallized at 100 °C using kaolin and volcanic ash natural materials through metakaolinite and fused-metakaolinite methods. Kaolin was supplied by the department of Inorganic Chemistry, University of Yaoundé 1 (Cameroon) while volcanic ash was obtained from the slopes of Mt. Eburu, Nakuru County (Kenya). In the fused-metakaolinite method, besides the partial versus full fusion, pre-treatment of clay by wet or dry mixing with NaOH activator before fusion were also studied. The zeolite catalysts were prepared by wet ion impregnation method using CH3COONa salt. The catalysts were tested in the batch transesterification of Jatropha curcas and waste cooking oils. Transesterification factors were optimized using L16(44) Taguchi approach. Materials used, resulting zeolites and biodiesel products were characterized using XRD (crystallinity), SEM (morphology), EDX (elemental), ICP-OES (elemental), TGA (thermal stability), BET (surface area and porosity), FT-IR (functional groups) and GC-FID (qualitative and quantitative analysis). Obtained data was analysed and organized using Minitab 18.1.0.0, OriginLab 9.80.200 and ImageJ software. Analysis of raw materials showed that clay material was a kaolin type with high quartz impurity at 21.03 %. Volcanic ash, on the other hand, was majorly CaO at 47.09 % and SiO2 at 18.38 %. In zeolites syntheses using kaolin, zeolites Na-A and Na-X were obtained within 8 h in the metakaolinite and fused-metakaolinite methods, respectively. High hydrogel SiO2/Al2O3 molar ratio resulted in a mixture of products and zeolite Na-Y in the metakaolinite and fused-metakaolinite methods, respectively. In the partial versus full fusion, Na-Y with low surface area and Na-X with high surface area were obtained from partial and full fusion methods, respectively. In the full fusion, pre-treatment of kaolin by dry mixing before dry fusion resulted in Na-X alongside Na-A impurity. The Na-X with high porosity comparable to that of commercial zeolite 13X was obtained in the wet mixing before dry fusion. Wet mixing with subsequent wet fusion resulted in poor crystallinity Na-X alongside hydroxysodalite impurity. In zeolites syntheses using volcanic ash, zeolites Na-X, Na-P and hydroxysodalite were obtained depending on SiO2 and Na2O contents of the synthesis hydrogel. For the biodiesel synthesis, optimal conditions for the oil conversion were methanol:oil molar ratio of 10:1, catalyst loading of 8 %, reaction temperature of 70 °C and reaction time of 5 hours. In addition, based on ANOVA, the contribution of the catalyst loading was the most significant at 93.79 %. The maximum FAME yield under these conditions were 93.94 % and 95.54 % for synthesized zeolite Na-X and natural clinoptilolite zeolite, respectively. This study has therefore shown the importance of performing metakaolinization versus fused-metakaolinization as well as the crucial role of NaOH content in the fusion of kaolin in zeolite synthesis. In addition, a variety of high-quality zeolites are achievable via choice of pre-treatment protocols. Both industrial production of zeolites, for various applications, as well as biodiesel production, in the energy sector, are therefore expected to greatly benefit from the findings of this study.