Methods are provided for synthesizing ZSM-58 crystals with an improved morphology and/or an improved size distribution. By controlling the conditions during synthesis of the ZSM-58 crystals, crystals of a useful size with a narrow size distribution can be generated. Steaming the H-form DDR framework type crystals at a temperature from 426±° C. to 1100±° C. for a time period from about 30 minutes to about 48 hours can attain one or more of the following properties: a CH4 diffusivity of no more than 95% of the CH4 diffusivity of the unsteamed H-form DDR framework type crystals; an N2 BET surface area from 85% to 110% of the surface area of unsteamed H-form DDR framework type crystals; and an equilibrium CO2 sorption capacity from 80% to 105% of the equilibrium CO2 sorption capacity of unsteamed H-form DDR framework type crystals.