SAPO-11 has one-dimensional 10-membered ring channels with an elliptical pore opening of 6.3´3.9 Å. The structure possesses orthorhombic symmetry with lattice parameters16: a = 8.4, b=18.5 and c=13.5 Å. SAPO-31 exhibits hexagonal symmetry with lattice parameters17: a=20.8 and c=5.0 Å. Although SAPO-31 has 12-membered ring channels in the structure, the nearly circular free opening with an effective diameter of 5.4 Å brings it under the medium-pore category. SAPO-41 has one dimensional 10-membered elliptical channels with a free aperture of 7.0 ´ 4.3 Å and belongs to the orthorhombic symmetry with lattice parameters a = 9.7, b = 25.5 and c = 8.4 Å18. The free pore openings of both SAPO-11 and SAPO-41 are more elliptical than that of ZSM-5 with 10-ring apertures19.