Metal oxide based solid acid for renewable energy applications

Abstract

Biodiesel, a green and sustainable fuel as an alternative to fossil fuel has drawn significant attention over the past few decades. Conventional biodiesel synthesis relies on feedstocks obtain from edible biomass oil extracts with the use of non-reusable and highly acidic or basic catalysts. Intensive purification resulting in the discharge of contaminated wastewater is one of the major drawbacks for this synthetic route. The development of solid catalysts benefits from easy separation, possibility to reuse and their ability to simultaneous catalyze in situ esterification and transesterification to produce biodiesel. Glycerol, a side product from biodiesel synthesis, undergoes acetylation reaction with acetic acid to form mono-, di- and triacetin as products that are useful as fuel additives to improve biodiesel properties. The synthesis of a high surface area to volume ratio solid acid catalyst was attempted. PVA/ZrO₂ nanofibres with a smooth morphology and PAN/ZrO₂ nanofibres with a rough morphology have been successfully produced via electrospinning, but the fibres electrospun from PVA polymer was irremovable from the collecting Al foil. PAN/ZrO₂ fibres were removable from Al foil but only fibres of short length were obtained after calcination. Incorporation of sulfate groups onto crystallined ZrO₂ nanofibres was unsuccessful. Chlorosulphonic acid modified zirconia (HClSO₃-ZrO₂) nanoparticles were successfully suspended into polymer before electrospinning but it was unable to form HClSO₃-ZrO₂ nanofibres; and the fibrous structure collapsed upon high temperature calcination. Titanium oxide, niobium oxide, zirconium oxide, sulphated zirconia synthesized by direct method (S-ZrO₂) and chlorosulphonic acid modified zirconia (HClSO₃-ZrO₂) were screened for the acetylation of glycerol and biodiesel synthesis. The reaction time and energy input were minimized by using microwave irradiation for heating under pressure. A cost-effective Taguchi method was employed to optimize the reaction conditions. S-ZrO₂ and HClSO₃-ZrO₂ are the most efficient catalysts in the acetylation of glycerol, where the triacetin selectivity increases with catalyst surface acidity. Glycerol acetylation catalyzed by S-ZrO₂ and HClSO₃-ZrO₂ result in triacetin selectivity of ca. 37 % (reaction condition 1:12 glycerol:acetic acid, 2 wt % catalyst, 140°C) in 25 min under microwave irradiation. Both catalysts were reused for up to 5 cycles without significant drop in triacetin selectivity. For in situ transesterification and esterification reaction of canola oil and oleic acid, S-ZrO₂ is the most effective catalyst. Over 80 % biodiesel yield is obtained with a short reaction time of 1 h catalyzed by S-ZrO₂ (1:20 oil:methanol ratio, 6 wt % catalyst, 120°C) under microwave irradiation. However, the catalyst shows significant drop in the FAME yield in subsequent cycles and is not reusable. NMR and EDX analysis show no leaching of sulphate groups to the reaction. The decrease in FAME yield is due to accumulation of organic deposits blocking the active sites on the catalyst surface. Finally, S-ZrO₂ is also an active catalyst for using crude or waste oil as the biodiesel feedstock.