Heterogeneously catalyzed transesterification of palm oil with methanol to produce biodiesel over calcined dolomite: The role of magnesium oxide

Abstract

Calcined dolomite, composed of calcium oxide (CaO) and magnesium oxide (MgO), is a potential solid base catalyst for the transesterification of triglycerides with methanol to produce biodiesel. However, the catalytic role of MgO in the calcined dolomite is not well understood. We studied the effect of calcination conditions on the formation of the metal oxide phases and the catalytic activity of the resulting dolomite-derived mixed metal compounds. Natural calcite with an analogous trigonal-rhombohedral crystal was used for comparison. The surface basicity of the calcined materials was dependent on the type and cluster size of the oxides, which were controlled by varying calcination temperature and heating rate, while their catalytic activity was principally determined by the bulk CaO content. X-ray photoelectron spectroscopy indicated the growth of MgO on the calcined dolomite, which possessed an oxygen-deficient surface. The generated MgO exhibited a low basicity, low reactivity with methanol and glycerol in phase transformation, and low transesterification catalytic activity under the studied conditions. The oxide materials obtained from the calcite and dolomite calcined at 800 °C exhibited the highest basicity and catalytic activity, giving the FAME yield of >99 wt% under the studied conditions. The CaO nanocrystallites obtained from the dolomite were highly accessible and more reactive than those derived from calcite in the methoxide formation. An in situ formation of calcium hydroxide (Ca(OH)2) was observed for the oxide materials directly reacted with methanol or glycerol and those spent in the transesterification. The small crystallites of dolomite-derived mixed oxides maintained the structural stability of Ca(OH)2 as an active phase, resulting in a lower formation of bulk glyceroxides.