The preparation of symmetrically substituted 1,3,5-triazines has been performed by cyclotrimerization of nitriles in solvent-free conditions using silica-supported Lewis acids as catalysts. Although microwave irradiation gives the best overall results in a short time, the best yields were obtained by conventional heating over periods of 24 h. The use of these recyclable and environmentally benign catalysts produces yields similar to those obtained with the more complex and contaminant lanthanide catalysts. This fact, together with the use of solvent-free conditions, makes this procedure an environmentally benign synthetic method.