I am planning on synthesis of Ester of 2-Sulfoterephthalic acid from terephthalic acid. The 1st step that I have planned is Sulfonation of Terephthalic Acid using sulphuric acid at 150 degree celcius temperature. The only problem here is using a catalyst which is mercuric sulphate. Since it is too costly, I want to use a substitute for it. Any ideas regarding another cheaper catalyst?