wherein each R1 is a hydrogen, and R2 is a hydrogen, a C1-C12 hydrocarbyl group, or a halogen; wherein the heterogeneous catalyst comprises an oxide of tungsten or an oxide of molybdenum in combination with a zirconium oxide support, a cerium oxide support, or a zirconium oxide-cerium oxide support; quenching the reaction mixture comprising the phenolphthalein compound to provide a first quenched