The activity and stability of niobia supported
on silica catalyst have been tested in continuous
micro-pilot reactors, for biodiesel production starting
from acid vegetable oils. A catalyst was prepared
by the impregnation of silica pellets with a loading
of 12% of Nb and was extensively characterized.
The activity of this catalyst in both esterification and
transesterification was tested in a continuous micro-pilot
laboratory plant in which acid oil was fed (FFA 10% w/w)
at a temperature of 220°C and at a pressure of 60 bar.
The niobia based catalyst resulted in a very active
catalyst in both esterification (FFA conversion =
95-90%) and transesterification reactions (FAME
yield = 80-90%), and the activity remained
quite constant for more than 100 h on stream.
Notwithstanding this stability, a non-negligible leaching
phenomena has been detected, in the case of long-time
continuous runs, as the Nb concentration on the spent
catalyst resulted lower than that on the fresh one. The
obtained result confirms that the leaching of the active
specie is one of the most strong problem in heterogeneous
catalysis for biodiesel production.