For porous materials, one can also compare acid solids based on the surface acidity, i.e. the amount of acid sites per surface area of the acid solid. To do so, one needs not only a measurement of the acidity but also a measurement of the solid surface area. For this you can use N2 physisorption or Hg porosimetry, depending if your solid is micro/mesoporous or macroporous.
The surface acidity of your material is the amount of acid sites determined by NH3 TPD (in mol of acid sites per gram of solid) divided by the surface area (in sq meter of surface per gram of solid).