3. Results and discussion
3.1. Catalyst characterization
3.1.1. FTIR analysis of the catalyst
FTIR spectroscopy is often used to identify the structural characteristics of surface chemistry, in the range of 4000–400 cm−1. An SB 939 characteristic peak of Fe–O was found in CaFe/LDO; Al–O and Ca–O peak in CaAl/LDO (Fig. S1) (Ouahdi et al., 2005). Although the peaks of Fe–O, Al–O, and Ca–O could be discovered in CaFeAl/LDO, a shift phenomenon still occurred. The peak shift could enhance the activity (red-shifted) and promoted the stability (blue-shifted) of CaFeAl/LDO. This result further confirmed the formation of Ca, Fe and Al oxides during the calcination procedure, and verified that CaFeAl/LDO catalyst possesses the excellent activity and high stability in the preparation of biodiesel.
3.1.2. XRD analysis of the catalyst
The XRD patterns of the three LDO catalysts were depicted from 5° to 80° (Fig. S2). Compared with Ca12Al14O33 and Ca2FeO3.5, Ca2Al1.38Fe0.62O5 had diffraction peaks consistent with the characteristic of mayenite crystal (Wang et al., 2013). Furthermore, the new characteristic peak emerged in Ca2Al1.38Fe0.62O5, which was α-Fe2O3 (Tang et al., 2012), suggesting that sectional Fe had been oxidized to α-Fe2O3 after calcination at 750 °C for 3 h, and α-Fe2O3 in Ca2Al1.38Fe0.62O5 could reinforce the stability of the catalyst for biodiesel production.