Evaluation of KHSO4 as a recyclable catalyst in the production of dehydrated castor oil to be applied in alkyd resins

Abstract

Due to its lower solubility, this work proposes the use of KHSO4 as a re-usable heterogeneous catalyst in the dehydration of castor oil. Refined and degummed castor oil were considered to determine the effect of the purification level. Re-usability tests of the catalyst were performed in five consecutive experiments, under the same reaction conditions. Fresh and used catalysts were characterized by infrared spectroscopy (FTIR), thermal analyses (TGA and DTA) and acidity assays. The use of the KHSO4 catalyst afforded the desired yields, easy separation, and recovery for further use, which allows low environmental impact and low cost. Ultimately, the best dehydrated oil was evaluated in the production of an alkyd resin. Drying castor oil (iodine value 127.46 g I2/100 g) was obtained using KHSO4 at 230 °C and 190 min. KHSO4 catalyst can be re-used in the dehydration of castor oil with a low decrease in activity, i.e., a conversion of 81% was achieved after the 5th re-use. This activity loss can be compensated with a small addition of fresh catalyst. This slow deactivation of the catalyst was due to a decrease in its acidity, because KHSO4 was partially and slowly transformed into K2SO4 and K2S2O7 during the reaction, releasing water, SO3, and SO2. An alkyd resin was obtained from dehydrated castor oil, with an acid index lower than 10 mg KOH/g. This resin exhibits properties, such as Gardner color, nonvolatile compounds, and Garner viscosity, similar to a commercial resin. The film obtained from the dehydrated castor oil resin showed the same set-to-touch, dry-to-touch, dry-hard, and dry-to-handle times as those of a commercial resin.