Functionalized zeolites, including a zeolite substrate and a self-assembled monolayer of a phosphonic acid on a surface of the zeolite substrate, are disclosed, as are methods of making and using the functionalized zeolites. The disclosed methods and compositions have various applications, including in the use of molecular sieves to separate small-molecule gases from mixtures thereof. Gas adsorption selectivities and diffusion rates of the functionalized zeolites may be tuned or selected according to the disclosed methods.