Two-dimensional (2-D) CuBDC nanosheets (ns-CuBDC) with high-aspect-ratios were deliberately paired with polymers possessing high free volumes to fabricate high performance gas separation membranes. Owing to the molecular sieving effect of the filler, a small ns-CuBDC loading (2-4 wt%) could significantly improve the CO2/CH4 selectivities of membranes, resulting in performances that surpass the upper bound limit for polymer membranes.