Catalytic performance of sulfated SiO2, ZrO2, and TiO2 in the treatment of free fatty acids from waste cooking oil and coconut oil

Abstract

The synthesis of acid catalysts such as SiO₂, ZrO₂, and TiO₂ through the sulfation process to diminish the free fatty acid content in waste cooking oil and coconut oil has been conducted. Catalyst characterization was performed to examine the effect of the sulfation process at 1, 2, and 3 M on SiO₂, ZrO₂, and TiO₂ materials. The catalyst acidity test indicates a relationship with the intensity of the generated sulfate groups. The SO₄/SiO₂ 2 M, SO₄/ZrO₂ 2, and SO₄/TiO₂ 3 M catalysts exhibited optimal acidity values of 4.2, 7.65, and 4.98 mmol NH₃/g. They demonstrated the highest peak intensity of the sulfate group in the FTIR spectra and a decrease in the FFA content for each variable. Waste cooking oil and coconut oil with initial FFA 4.21% and 3.54% show an optimum decrease in FFA level with the use of SO₄/SiO₂ 2 M catalyst (0.79% and 0.92%). Whereas, the SO₄/ZrO₂ 2 M catalyst only decreases the FFA content by 0.98% and 1.22%; and SO₄/TiO₂ 3 M catalysts decrease it by 1.02% and 1.77%. SO₄/SiO₂ 2 M catalysts exhibit high catalytic activity in the esterification reaction due to the high surface area and acidity value characteristic. The reusability study of the SO₄/SiO₂ 2, SO₄/ZrO₂ 2, and SO₄/TiO₂ 3 M catalysts with 4th reuse showed a progressive decrease in catalyst activity with repeated uses. SO₄/SiO₂ 2 M exhibited the highest catalytic activity in the reusability test among the sulfated metal oxide catalysts. Catalyst stability of all catalysts with the 2nd reuse still produces oil that require FFA standard (≤ 2%), while at the 3rd and 4th cycles it does not. The FTIR spectra of esterified coconut oil show the presence of high -OCH₃ bonds as the ester groups, confirming that esterification of coconut oil with SO₄/SiO₂ 2 M catalyst produces strong ester carbonyl absorption compared to the waste cooking oil.