Acid ion exchange resin Amberlyst
46 (A46) was used for the
experiments. It demonstrated high mechanical and chemical stability over several esterification cycles. A46 is a copolymer of styrene cross-linked with divinylbenzene (DVB) with sulphuric type
acid groups in the form of spherical beads (Fig. 1). The amount of
crosslinking determines its main physical characteristics (surface
area and pore size distribution) (Table 3) [32]. The higher the
DVB amount added during the copolymerization, the higher the
reticulation degree and, as a consequence, the smaller the pores.
The amount of DVB plays a major role in controlling the physical
features of these catalysts (Table 3). The strongASO3H acid groups
formed on the resin during the sulphonation step are the active
sites responsible for the esterification of the FFA.
The distinguishing feature of this resin is that it is sulphonated
only on the surface and not inside the pores[11,13,14]. Being sulphonated only superficially, this type of catalyst greatly reduces
the formation of dialkyl ethers given by the auto-condensation of
the molecules of the alcohol inside the pores[33].
The presence of catalytically active sites mostly located on the
catalyst outer surface is desirable in the reaction system under
study. FFA are highly sterically hindered molecules that would
hardly penetrate inside the catalysts pores[10–14]. Moreover,
A46 has proven to provide constant activity in the FFA esterification, being both chemically and mechanically stable for over 90
cycles and 540 h