​We demonstrate that b-oriented MFI zeolite membranes can be manufactured by in-situ crystallization using an intermediate amorphous SiO2 layer. The improved in-plane growth by using a zeolite growth modifier leads to fusion of independent crystals and eliminates boundary gaps, giving good selectivity in the separation of CO2/Xe mixtures. The fast diffusion of CO2 dominates the overall membrane selectivity towards CO2/Xe mixture. Because of the straight and short [010] channels, the obtained CO2 permeation fluxes are several orders of magnitude higher than those of carbon molecular sieving membranes and polymeric membranes, opening opportunities for Xe recovery from waste anaesthetic gas.​