A method for performing a gas separation, comprising: contacting an adsorbent or membrane comprising DDR-type zeolite crystals with an input gas stream containing a first component and a second component to form a first gas stream enriched in the first component relative to the input gas stream, wherein the first component comprises CH 4 and H 2 S and the second component is CO 2, N 2 or a combination thereof, or the first component is CH 4 and the second component is CO 2, N 2, H 2 S, or a combination thereof; and collecting a second gas stream enriched in the second component relative to the input gas stream, wherein the DDR-type zeolite crystals are synthesized using a synthesis mixture having an alkali metal to silica ratio of at least 0.01, the adsorbent or membrane being comprised of crystals comprising alkali metal impurities, the alkali metal impurities being non-framework alkali metal atoms or cations, said crystals having an alkali metal impurity content of 0.05 wt% or less, the alkali metal in the synthesis mixture preferably being sodium, the DDR-type zeolite crystals preferably being Sigma-1, ZSM-58, SSZ-28, or a combination thereof, wherein the adsorbent or membrane comprising DDR-type zeolite crystals is formed by a method comprising: forming DDR-type crystals from a synthesis mixture comprising an alkali metal cation, an oxide of aluminum, an oxide of silicon, an organic cation of a methyltropinium salt which is preferably methyltropinium iodide or chloride, and water; separating the DDR-type crystals from the synthesis mixture; calcining the DDR-type crystals; exposing the DDR-type crystals to an acidic solution under effective ion exchange conditions to reduce the content of alkali metal impurities in the DDR-type crystals; calcining the ion exchanged DDR-type crystals having a reduced content of alkali metal impurities; and forming an adsorbent or membrane based on the ion exchanged DDR-type crystals, the acidic solution being an at least 0.1 M solution of a strong acid or an organic acid having a pKa of 6 or less. contacting an adsorbent or membrane comprising DDR-type zeolite crystals with an input gas stream containing a first component and a second component to form a first gas stream enriched in the first component relative to the input gas stream, wherein the first component comprises CH 4 and H 2 S and the second component is CO 2, N 2 or a combination thereof, or the first component is CH 4 and the second component is CO 2, N 2, H 2 S, or a combination thereof; and collecting a second gas stream enriched in the second component relative to the input gas stream, wherein the DDR-type zeolite crystals are synthesized using a synthesis mixture having an alkali metal to silica ratio of at least 0.01, the adsorbent or membrane being comprised of crystals comprising alkali metal impurities, the alkali metal impurities being non-framework alkali metal atoms or cations, said crystals having an alkali metal impurity content of 0.05 wt% or less, the alkali metal in the synthesis mixture preferably being sodium, the DDR-type zeolite crystals preferably being Sigma-1, ZSM-58, SSZ-28, or a combination thereof, wherein the adsorbent or membrane comprising DDR-type zeolite crystals is formed by a method comprising: forming DDR-type crystals from a synthesis mixture comprising an alkali metal cation, an oxide of aluminum, an oxide of silicon, an organic cation of a methyltropinium salt which is preferably methyltropinium iodide or chloride, and water; separating the DDR-type crystals from the synthesis mixture; calcining the DDR-type crystals; exposing the DDR-type crystals to an acidic solution under effective ion exchange conditions to reduce the content of alkali metal impurities in the DDR-type crystals; calcining the ion exchanged DDR-type crystals having a reduced content of alkali metal impurities; and forming an adsorbent or membrane based on the ion exchanged DDR-type crystals, the acidic solution being an at least 0.1 M solution of a strong acid or an organic acid having a pKa of 6 or less. forming DDR-type crystals from a synthesis mixture comprising an alkali metal cation, an oxide of aluminum, an oxide of silicon, an organic cation of a methyltropinium salt which is preferably methyltropinium iodide or chloride, and water; separating the DDR-type crystals from the synthesis mixture; calcining the DDR-type crystals; exposing the DDR-type crystals to an acidic solution under effective ion exchange conditions to reduce the content of alkali metal impurities in the DDR-type crystals; calcining the ion exchanged DDR-type crystals having a reduced content of alkali metal impurities; and forming an adsorbent or membrane based on the ion exchanged DDR-type crystals, the acidic solution being an at least 0.1 M solution of a strong acid or an organic acid having a pKa of 6 or less.