Abstract: Fabricating a zeolite membrane on a substrate includes disposing first zeolite crystals on a substrate to yield a first layer on the substrate and disposing second zeolite crystals on the first layer to yield a second layer on the first layer, thereby yielding a membrane precursor. The membrane precursor is heated at a first temperature for a first length of time, and the temperature of the membrane precursor is increased or decreased from the first temperature to a second temperature. The membrane precursor is heated at the second temperature for a second length of time to yield the zeolite membrane. The second zeolite crystals have a smaller average diameter than the first zeolite crystals. The second temperature can exceed the first temperature or the first temperature can exceed the second temperature.