Solvent-free facile synthesis of di-octyl phthalate over heterogeneous sulfated zirconia acid catalyst

Abstract

The present study reports the synthesis of di-octyl phthalate (DOP), widely used as a plasticizer, via solvent-free esterification of phthalic acid and n-octanol over sulfated zirconia (SZr) heterogeneous acid catalyst. A comparison with other heterogeneous acid catalysts such as K-10 acid activated clay, zeolites (β and Y), Amberlyst-15 resin shows the superior activity of SZr, exhibiting maximum conversion of n-octanol (88%) with 93% DOP selectivity at 160 °C in 6 h. The rate of the reaction for SZr catalyst is computed as 17.3 mmol h⁻¹ g⁻¹. The catalyst activity is stable upon recycling for up to five cycles. The weak acid sites of the catalyst are postulated as the responsible active centres involved in the reaction. The reaction has been successfully scaled up over twenty five times; and the product DOP (67 g, 99% by GC) isolated and characterized by FT-IR and ¹H NMR spectroscopy. It is noteworthy that the activity of SZr catalyst is comparable with that of H₂SO₄ and p-toluene sulphonic acid (p-TSA), which clearly suggets its potential as an efficient alternative to conventional commercial homogeneous acid catalysts for the synthesis of DOP.