In this study

biodiesel was produced from waste vegetable oil using a heterogeneous base catalyst synthesized by impregnating potassium hydroxide (KOH) onto diatomite. Response surface methodology based on a central composite design was used to optimize four transesterification variables: temperature (30-120 ℃)

reaction time (2-6 h)

methanol to oil mass ratio (10%-50%) and catalyst to oil mass ratio (2.1%-7.9%). A quadratic polynomial equationwas obtained to correlate biodiesel yield to the transesterification variables. The diatomite-KOH catalyst was characterized using X-ray diffraction (XRD)

Fourier transform infra-red spectroscopy (FTIR) and a scanning electron microscope (SEM) equipped with an energy dispersive X-ray detector (EDS). A maximum biodiesel yield of 90%(by mass) was obtained. The reaction conditions were as follows: methanol to oil mass ratio 30%

catalyst to oil mass ratio 5%

reaction time 4 h

and reaction temperature 75 ℃. The XRD

FTIR and SEM (EDS) results confirm that the addition of KOH modifies the structure of diatomite. During impregnation and calcination of the diatomite catalyst the K

2

O phase forms in the diatomite structural matrix and the active basicity of this compound facilitates the

transesterification process. It is possible to recycle the diatomite-KOH catalyst up to three times. The crucial biodiesel properties fromwaste vegetable oil arewithin the American Standard Test Method specifications.