In the present study, the binary oxides catalyst showed a superior
transesterification activity over pure calcium oxide (96%) or
pure lanthanum oxide (23%) catalysts. Although CaO is able to
render high transesterification activity at lower reaction temperature
at 120 C, the stability of CaeO bonding system is weak, in
which Ca2þ easily to scrape away by FFAs under longer reaction
time or continuous batch reaction [33,34]. Among the CaOeLa2O3
catalysts with Ca/La ratio of 0.5e10.0 atomic %, CaOeLa2O3 with
atomic ratio of 8.0 rendered highest biodiesel yield of 99% (Table 4).
From the EDS and ICP results, the increment of Ca has significantly
influenced the basicity of the catalyst. It is interesting to observe
that the total basicity of CL6 (2789.52 mmol of CO2/g) is much higher
than that of CL4 (737.75 mmol of CO2/g), but the catalytic activity
does not increase as much than expected (3% increment). This is
because the surface area of CL6 (8.98m2/g) is lower than that of CL4
(11.10 m2/g) although CL6 has higher amount of basicity. Therefore,
this has oppressed the catalytic activity of CL6 which supposed to
give much higher activity. It was same case for the reactivity of
CL10, the surface area of CL4 (11.12 m2/g) are slightly higher than
CL10 (8.77m2/g), while basicity of CL10 (2163.50 mmol of CO2/g) has
reduced significantly as compared to CL8 (3198.74 mmol of CO2/g).
However the catalytic activity of CL10 (95%) is still much higher
than that of CL4 (93%). This indicated that the surface basic active
sites of catalyst still playing a priority role for the transesterification
reaction.