A method of making solid acid catalysts includes the step of sulfonating waste tire pieces in a first sulfonation step. The sulfonated waste tire pieces are pyrolyzed to produce carbon composite pieces having a pore size less than 10 nm. The carbon composite pieces are then ground to produce carbon composite powders having a size less than 50 μm. The carbon composite particles are sulfonated in a second sulfonation step to produce sulfonated solid acid catalysts. A method of making biofuels and solid acid catalysts are also disclosed.