Method of making a gas separation molecular sieve membrane

A method of preparing a supported gas separation membrane, comprising:

• a. preparing crystalline seeds from a synthesis mixture comprising an aluminum source, a phosphorous source, a silicon source, at least one organic templating agent and water;
• b. applying the seeds to a porous support to produce a seeded porous support;
• c. contacting the seeded porous support with a synthesis gel under hydrothermal synthesis conditions to produce a coated porous support;
• and
• d. calcining the coated porous support
• wherein the molar ratio of water to Al2O3 of the synthesis gel in step (c) is greater than the molar ratio of water to Al2O2 of the synthesis mixture of step (a)
• wherein the molar ratio of water to Al2O3 in step (c) is at least 150:1.