The present invention discloses high performance UV cross-linked polymer functionalized molecular sieve / polymer mixed matrix membranes (MMMs), the method of making these membranes, and the use of such membranes for separations. These UV cross-linked MMMs were prepared by incorporating polyethersulfone functionalized molecular sieves such as AlPO-14 and UZM-25 into a continuous UV cross-linkable polymer matrix followed by UV cross-linking. The UV cross-linked MMMs in the form of symmetric dense film, asymmetric flat sheet membrane, or asymmetric hollow fiber membranes described in the current invention have good flexibility and high mechanical strength, and exhibit significantly enhanced selectivity and permeability over the polymer membranes made from the corresponding continuous polyimide polymer matrices for carbon dioxide / methane (CO2 / CH4) and hydrogen / methane (H2 / CH4) separations. The MMMs of the present invention are suitable for a variety of liquid, gas, and vapor separations such as deep desulfurization of gasoline and diesel fuels.