Preparation Of Heterogeneous Catalyst And Its Application In Biodiesel Production From Chlorella Vulgaris

Owing to energy crisis and environmental concerns, fatty acid methyl esters (FAME) or named biodiesel produced from microalgae as an alternative biofuel have been attracted considerable attention in recent decades. Microalgae have been intensively researched in recent years as a promising feedstock for biodiesel production, owing to the properties including rapid growth, high lipid content, and wastewater utilization and disposal. One of the most common methods to produce biodiesel is in situ transesterification of triglyceride (the lipid compound in microalgae) with the presence of methanol and catalysts, and the products are usually FAME or named biodiesel. Homogenous alkaline catalysts such as sodium hydroxide, potassium hydroxide and alkoxide were usually used as liquid catalysts to produce biodiesel by the in situ transesterification method because of their higher reaction rates under moderate operation conditions compared with acid catalysts. However, homogeneous-based catalysts have many drawbacks, for example, ensitivity to free fatty acid and water which would cause undesired saponification and difficult to be separated from products, and the neutralization of base catalyst and purification of products would generate a large amount of wastewater. Compared to homogeneous catalysts, heterogeneous catalysts have attracted more attention in biodiesel production from vegetable oils or animal fats.Due to the easy separation, recycling and product purification, heterogeneous catalysts were synthesized. Two different heterogeneous catalysts of KOH/Al2O3 and KF/CaO were prepared. The techniques of X-ray diffraction (XRD), Scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) were used to deremined the characteristics of catalysts. The FAME obtained by transesterification was deterimined by GC-MS (gas chromatography -Mass) and finally optimum condition of catalysts, and the optimal reaction conditions by FAME yield.The mainly conclusion concluded by this study as follows:(1) Two different heterogeneous KOH/Al2O3 and KF/CaO catalysts, synthesized by the wet impregnation method with different loadings and calcination temperatures. The characteristics of the catalysts were analysed by XRD, SEM and BET. From the results it can be seen that as the calcination temperature of KOH/Al2O3 was 700 ℃, at this temperature intensities of typical diffraction peaks (KAlO2, Al2O3) almost disappeared, while the diffraction peaks of K2O was strong, which was formed by the KOH thermal decomposition and responsible for the catalytic activity to this reaction. And at calcinated temperature of KF/CaO was 900 ℃, the main components of were KCaF3, CaO, especially, KCaF3, which was reported to be the main active component of catalytic activity in transesterification reaction. Ca(OH)2 easily caused saponification formation and performed less efficiently than CaO, consequently exhibiting low catalytic ability in biodiesel production. It could be concluded that the optimum calcination temperature of KOH/Al2O3 and KF/CaO were 700 ℃ and 900 ℃, respectively.(2) Heterogeneous KOH/Al2O3 catalysts were used to convert microalge to FAME and the compositions of FAME were determined by GC-MS. From the results obtained we could see that KOH/Al2O3 showed high catalylysis ability and the highest yield of biodiesel of 89.53 ± 1.58% was achieved at calcination temperature of 700 ℃ for 2 h and 35 wt% loading of KOH, and at the optimal reaction condition of 10 wt% of catalyst content,8 mL/gof methanol to biomass ratio and at 60 ℃ for 5 h. The mechanism of heterogeneous base catalysis was probable that methanol was absorbed by the surface of catalyst and then formed methoxy anion (CH3O-), which would attack carboxyl group of TAG, and form new intermediate substance, which was not stable and the bond would break and form diglyceride (DAG), and so on, FAME formed finally.(3) Heterogeneous KF/CaO catalyst was used to assist biodiesel production from microalgae Chlorella vulgaris under the combination of ultrasound and microwave (US-MW) irradiation. US or MW assisted catalysis could also improve the microalgae conversion efficiency. MW-mediated method was reported to reduce energy consumption, accelerate organic synthesis, and enhance reaction rate to short reaction time. Additionally, MW radiation delivered heat more effectively than convection and conduction; but MW was not able to provide energy or break bonds. US, especially in the low frequency range from 20 to 50 kHz, could generate acoustic cavitations, increasing the interfacial region of the reactants and disruption of algal cells, enhancing mass transfer, and accelerating the biodiesel production. Thus, the combination of US and MW (US-MW) irradiation could overcome the limitations which has been successfully designed and well documented in product synthesis, owing to reaction time reduction, decrease in energy consumption, improved and enhanced yield, and selection of products. The experimental results illustrated that heterogenous catalyst calcinated at 900 ℃ with 25 wt.% KF loading presented the optimum performance The highest FAME yield of 93.07 ± 2.39% was obtained when the test was conducted with 12 wt.% of catalyst and a methanol to biomass ratio of 8:1 at 60℃ for 45 min, which was significantly higher than that achieved by using US or MW irradiation ndividually.(4) To compare the catalysis abilities of KOH/Al2O3 and KF/CaO catalysts, KOH/Al2O3 as heterogenesous catalyst was studied under the combination of US-MW irradiation at the optimum prescribed above. The experimental results illustrated that the highest FAME yield of 94.91 ± 2.14% was obtained at reaction time of 30 min, which was higher than KF/CaO obtained (85.73±2.05%). It could be conclude that the catalytic ablity of was higher than KF/CaO, and the probable reason was that the base strength of KOH/Al2O3 (22.3<H<27) was higher than KF/CaO (15.0<H<18.4).