Time-resolved in-situ x-ray diffraction study of CaO and CaO:Ca3Al2O6 composite catalysts for biodiesel production

Abstract

Alternative and sustainable waste sources are receiving increasing attention as they can be used to produce biofuels with a low carbon footprint. Waste fish oil is one such example and can be considered an abundant and sustainable waste source to produce biodiesel. Ultimately this could lead to fishing communities having their own ‘off-grid’ source of fuel for boats and vehicles. At the industrial level, biodiesel is currently produced by homogeneous catalysis because of the high catalyst activity and selectivity. In contrast, heterogeneous catalysis offers several advantages such as improved reusability, reduced waste and lower processing costs. Here we investigate the phase evolution of two heterogeneous catalysts, CaO and a Ca3Al2O6:CaO (‘C3A:CaO’) composite, under in-situ conditions for biodiesel production from fish oil. A new reactor was designed to monitor the evolution of the crystalline catalyst during the reaction using synchrotron powder x-ray diffraction. The amount of calcium diglyceroxide (CaDG) began to increase rapidly after approximately 30 min, for both catalysts. This rapid increase in CaDG could be linked to ex-situ nuclear magnetic resonance studies which showed that the conversion of fish oil to biodiesel rapidly increased after 30 min. The key to the difference in activity of the two catalysts appears to be that the Ca3Al2O6:CaO composite maintains a high rate of CaDG formation for longer than CaO, although the initial formation rates and reaction kinetics are similar. The Ca for the CaDG mainly comes from the CaO phase. In addition, towards the end of the second test utilising the CaO catalyst (after 120 min), there is a rapid decrease in CaDG and a rapid increase in Ca(OH)2. This was not observed for the Ca3Al2O6:CaO catalyst and this is due to Ca3Al2O6 stabilising the CaO in the composite material. No additional calcium containing intermediate crystalline phases were observed during our in-situ experiment. Overall this specialised in-situ set-up has been shown to be suitable to monitor the phase evolution of heterogeneous crystalline catalysts during the triglycerides transesterification reaction, offering the opportunity to correlate the crystalline phases to activity, deactivation and stability.