Zeolites are crystalline aluminosilicates with well-defined 3D porous structures consisting of tetrahedral units of aluminate (AlO45-) and silicate (SiO44-) ions. They can be classified by pore size, with small-pore zeolites featuring 8-membered rings and pore openings around 3.0-4.5 Å, medium-pore zeolites with 10-membered rings and pore sizes of 4.5-6.0 Å, large-pore zeolites with 12-membered rings and pore sizes between 6.0-8.0 Å, and extra-large pore zeolites (>12-ring). Zeolites are used in catalysis, adsorption, and separation processes in the industry.

Small-pore zeolite membranes, such as CHA (0.37 nm pore size) and DDR (0.36 nm pore size), have been extensively evaluated for a variety of separations, due to their suitable pore sizes, which enable separation of small molecules from larger molecules, along with their excellent thermal stability and chemical resistance. Particularly in gas separation, these membranes have demonstrated exceptional performance for a range of industrially relevant gas pairs, such as CO2/CH4, CO2/N2, N2/CH4, and H2/CH4, highlighting their strong potential for biogas and natural gas upgrading. Nevertheless, further fundamental studies are needed in order to improve the membrane materials, deepen our understanding of the mass transfer mechanisms in zeolites, and optimize their performance in practical applications.

In this thesis, the adsorption isotherms of the common components of natural gas and biogas, CO2, CH4, N2, and He were experimentally measured over wide temperature ranges on large, all-silica, CHA, DDR, and MFI zeolite crystals. The Toth equation was fitted to the measured adsorption data and adsorption parameters were estimated, such as adsorption capacity at saturation (Csat), affinity constant (b), Toth heterogeneity parameter (t), enthalpy of adsorption (ΔHads), and adsorption entropy (ΔSads). The estimated adsorption parameters presented in this work are accurate, primarily due to the large crystals used for the adsorption measurements and the recording of low-temperature adsorption isotherms. These data are invaluable for understanding adsorption and mass transfer in zeolite materials, as well as for advancing the development of zeolite membranes for gas separation.

The second part of this thesis evaluates highly permeable DDR disc membranes under various conditions for the separation of CO2/CH4 and H2/CH4, gas pairs that are particularly relevant for natural gas and biohydrogen upgrading. For CO2/CH4 separation, the exceptionally high selectivity of 2325 paired with a high CO2 permeance of 34 × 10-7 mol/(m2·s·Pa) was observed for an equimolar mixture at a feed pressure of 3 bar and a temperature of -30 °C. The highest CO2 permeance was recorded at the same feed pressure and a temperature of +10 °C with a value of 44 × 10-7 mol/(m2·s·Pa), while the selectivity remained remarkably high at 1118. For H2/CH4 separation, a H2 permeance of 7.2 × 10-7 mol/(m2·s·Pa) was recorded for a feed of a 1/1 H2/CH4 mixture at room temperature and pressure of 3 bar. The high permeance was paired with a H2/CH4 selectivity of 207, markedly higher than previously reported for DDR membranes. Furthermore, a mass transfer model accounting for adsorption, surface barrier, and surface diffusion was fitted to the experimental data. Results showed that the model could accurately describe the mass transfer in the zeolite pores and that the surface barrier was the limiting mass transfer step. Based on the separation results, one-stage membrane processes were designed for upgrading biogas to biomethane using DDR membranes at three different operating pressures. The processes displayed a significantly low membrane area, methane slip, and need of electricity power, compared to a polymeric membrane process.

The final part of this thesis investigates CHA membranes for the upgrading of a synthetic natural gas mixture with a composition that is typical after a Joule Thompsson process in the industry. The membranes exhibited high flux at a feed pressure of 30 bar while the selectivity for the gas pairs of CO2/N2, CO2/CxHy, and N2/CxHy were also excellent. The optimal temperature for CO2 removal was found to be around 25 °C, where a great CO2 flux of 1.2 mol/(m²·s) was observed coupled with a CO2 permeance of 13 × 10-7 mol/(m²·s·Pa). Under these conditions, high selectivity for CO2/CH4, CO2/C2H6, and CO2/C3H8 of 68, 101, and 190, respectively, were observed. The optimal temperature for N2 removal was around 35 °C; at this temperature high N2 flux of 2.5 × 10-3 mol/(m²·s) was observed, with the N2 permeance reaching 1 × 10-7 mol/(m²·s·Pa). Finally, a membrane process, designed based on the separation data, showed that a membrane area of only 13.6 m2 is sufficient for the upgrading of 1000 Nm3/h natural gas to pipeline gas at a feed pressure of 30 bar, which is approximately 100 times smaller than the membrane area needed for a polymeric membrane process.

Overall, the findings in the thesis suggest that small-pore zeolite membranes hold great potential for the upgrading of biogas and natural gas.