Solid Acid Catalyst Derived from Coffee Residue for Biodiesel Production

Abstract

Biodiesel is a free fatty acid methyl ester (FAME) produced from transesterification of oil and short-chain alcohol. Nowadays, the cost of conventional biodiesel production might not be competitive with petro-diesel due to the cost of refined oil as a main feedstock. Many challenging researches have proposed method or technology to efficiently produce biodiesel. One of the most important knowledge is the synthesis of the suitable catalyst for biodiesel production. Solid acid catalyst is a promising catalyst to produce biodiesel from low-cost feedstocks since it can catalyze simultaneously esterification of free fatty acid (FFA) and transesterification of triglyceride. To improve the biodiesel CO₂ cycle, the waste material, coffee residue, was selected as a supported catalyst. It provides the appropriate textural properties such as high surface area with mesoporous structure and hydrophobic properties. The sulfonation with concentrated H₂SO₄ was used for additional acidic functional group which exhibits a strong protonic acid site density. Therefore, this research aims to synthesize sulfonated activated carbon derived from coffee residue (SCAC) to catalyze esterification of caprylic acid as a model of FFA. The sulfonation temperature varied from 140 to 200 °C as named SCAC-140, SCAC-160, SCAC-180, and SCAC-200 catalysts.

After sulfonation process, the X-ray diffraction (XRD) patterns of all catalysts were similar to coffee residue activated carbon (CAC) as a support. A few, broad peak at 2θ = 15–30° and 30–50°, were related to amorphous carbon and graphene sheet at the plane C (101), respectively. Fourier transform infrared spectroscopy (FT-IR) of CAC at 3400 cm^− 1 was absent which confirmed the hydrophobic property of this support. FT-IR spectra of SCAC catalysts illustrated the acid function as 3400, 1700, 1600, 1300, 1000, and 1100 cm^− 1 , which is related to O–H stretching mode of COOH and phenolic, C═O of COOH, C═C of polyaromatic, O═S═O symmetric SO₂ stretching, O═S═O asymmetric SO₂ stretching, respectively. SCAC-180 catalyst exhibited the highest initial rate due to the highest total acid site density which included sulfonated, carboxylic, and phenolic group. At 2 h, SCAC-200 showed the highest caprylic acid conversion while SCAC-140 provided the lowest caprylic acid conversion. It was probably due to the textural and structural properties of these synthesized catalysts. All of synthesized carbon catalysts demonstrated the higher catalytic activity than that of Amberlyst-15. However, the solid acid catalyst still provided a lower catalytic activity as compared to homogeneous H₂SO₄ catalyst.