Methods for separating carbon dioxide from a fluid stream at a temperature higher than about 200° C. with selectivity higher than Knudsen diffusion selectivity include contacting a porous membrane with the fluid stream to preferentially transport carbon dioxide. The porous membrane includes a porous support and a continuous porous separation layer disposed on a surface of the porous support and extending between the fluid stream and the porous support layer. The porous support comprises alumina, silica, zirconia, stabilized zirconia, stainless steel, titanium, nickel-based alloys, aluminum-based alloys, zirconium-based alloys or a combination thereof. Median pore size of the porous separation layer is less than about 10 nm, and the porous separation layer comprises titania, MgO, CaO, SrO, BaO, La2O3, CeO2, HfO2, Y2O3, VOz, NbOz, TaOz, ATiO3, AZrO3, AAl2O4, A1FeO3, A1MnO3, A1CoO3, A1NiO3, A2HfO3, A3 CeO3, Li2ZrO3, Li2SiO3, Li2TiO3, Li2HfO3, A4N1yOz, YxN1yOz, LaxN1yOz, HfN2yOz, or a combination thereof; wherein A is La, Mg, Ca, Sr or Ba; A1 is La, Ca, Sr or Ba; A2 is Ca, Sr or Ba; A3 is Sr or Ba; A4 is Mg, Ca, Sr, Ba, Ti or Zr; N1 is V, Nb, Ta, Cr, Mo, W, Mn, Si or Ge; N2 is V, Mo, W or Si; x is 1 or 2; y ranges from 1 to 3; and z ranges from 2 to 7.