Cation-free zirconosilicate zeolite CHA and thin zirconia-supported membranes were in situ synthesized in a fluoride-free gel for the first time. The usage of the ZrO 2 /Al 2 O 3 composite support inhibited the transportation of aluminum from the support into zeolite membranes. No fluorite source was used for the synthesis of cation-free zeolite CHA membranes, indicating the green property of the synthesis. The thickness of the membrane was only 1.0 μm. The best cation-free zeolite CHA membrane prepared by the green in situ synthesis displayed a high CO 2 permeance of 1.1 × 10 −6 mol/(m 2 s Pa) and CO 2 /CH 4 selectivity of 79 at 298 K and 0.2 MPa pressure drop for an equimolar CO 2 /CH 4 mixture.