BACKGROUND Mesoporous double-layer carbon microspheres (DCSs) were synthesized using hollow tin(IV) oxide (SnO2) microspheres as hard template and sucrose as carbon precursor via the hydrothermal method. Sulfonic acid groups (-SO3H) were introduced onto the surfaces of DCS by sulfuric acid heating and sulfanilic acid diazonium coupling, respectively, to form two solid acids (DCS-SS and DCS-DS, respectively). RESULTS The transmission electron microscopy images showed that the carbon microspheres possessed a hollow double-layer structure with diameters of about 200 to 300 nm and shell thickness of about 30 to 50 nm. Furthermore, the sulfonate treatments caused no changes in the morphology of carbon microspheres. The nitrogen (N-2)-Brunauer-Emmett-Teller (BET) results indicate that DCS, DCS-SS, and DCS-DS featured mesopores with an average pore size of 4.5 to 4.8 nm and BET surface areas of 75 m(2)/g, 57 m(2)/g, and 53 m(2)/g, respectively. The X-ray photoelectron spectroscopy and Fourier-transform infrared results confirmed that the DCS-SS and DCS-DS contained a -SO3H group and their surface acid densities are 4.5 and 4.1 mmol/g, respectively. CONCLUSION DCS-SS and DCS-DS were used to catalyze the transesterification of waste frying oil, and the highest yields of fatty acid methyl esters, that is, 85% and 80%, respectively, could be obtained. (c) 2019 Society of Chemical Industry