Variables Symbol code Levels     -1 0 1 Temperature (°C) X1 30 40 50 Time(min) X2 20 30 40 Methanol/oil molar ratio X3 6:1 8:1 10:1 Catalyst concentration (wt.%) X4 0.75 1.25 1.75

Table 1: Independent factors and levels used for the central composite design (face-centered).