Highly active, stable, and recyclable magnetic nano-size solid acid catalysts: efficient esterification of free fatty acid in grease to produce biodiesel

Magnetic nano-size solid acid catalysts were prepared, characterized, and examined, for the first time, for the esterification of free fatty acid (FFA) to develop a green and efficient pretreatment step for producing biodiesel (fatty acid methyl ester, FAME) from waste grease containing FFA. PGMA-cat 4, consisting of a core of iron oxide magnetic nanoparticles (MNPs), a poly(glycidyl methacrylate) (PGMA) shell, and sulfonic acid groups on the surface, was synthesized in 98% yield from the corresponding core–shell structured PGMA–MNPs containing epoxy surface groups by gentle sulfonation with Na2SO3, possessing a mean size of 90 nm, high acid capacity of 2.3 mmol H+ g−1, and excellent superparamagnetic properties. Esterification of FFA (16 wt%) in grease with methanol using this catalyst (4 wt%) gave 96% conversion of FFA to FAME within 2 h. PGMA-cat 4 was easily separated under a magnetic field and showed no loss of productivity during 10 cycles. In comparison, PS-cat 12, consisting of a core of iron oxide MNPs, a polystyrene shell, and benzenesulfonic acids on the surface, was active but possessed no recyclability; Si-cat 14, consisting of a core of iron oxide MNPs, a silica shell, and sulfonic acids as surface groups, showed lower activity and poor recycling performance. The catalytic performance of PGMA-cat 4 was also better than those of the micro-size counterpart such as PGMA-cat (m) 15 and large-size solid acid catalysts such as Amberlyst 15. Thus, a novel, active, stable, and recyclable magnetic nano-size solid acid catalyst was successfully developed for the green and efficient esterification of FFA in grease as a pretreatment step to produce biodiesel from waste grease.