## 4 Conclusions  
Several parameters influence the formation of SSZ-13 membranes and any change in these parameters would lead to a different form and, ultimately, a different membrane performance. In this work, all the effective factors in membrane synthesis were examined and the following was found:  
* The membrane was synthesized on an alumina support with a pore size of 200 nm
* A seeding procedure was employed with 1 wt% concentration of the SSZ-13 crystals in EtOH followed by 30 min of sonication
* The gel composition had the ratio of 1 SiO\({}_{2}\):0.1 TMAdaOH:0.2 NaOH:0.05 Al(OH)\({}_{3}\):80 H\({}_{2}\)O after 24 h of ageing
* Crystallizing at 160 \({}^{\circ}\)C for 48 h and calcining at 450 \({}^{\circ}\)C yielded the highest CO\({}_{2}\)/CH\({}_{4}\) separation performance.  
Optimizing these parameters results in an ideal CO\({}_{2}\)/CH\({}_{4}\), CO\({}_{2}\)/N\({}_{2}\), H\({}_{2}\)/SF\({}_{6}\) permselectivity of 122, 20, and 150, respectively, as well as CO\({}_{2}\) permeance of -3.72 \(\times\) 10\({}^{-6}\) [mol/(m\({}^{2}\)sPa)] and CO\({}_{2}\)/CH\({}_{4}\) selectivity of 111, with CO\({}_{2}\) permeance of 8.5 \(\times\) 10\({}^{-7}\) [mol/(m\({}^{2}\)sPa)] in an equimolar mixture at room temperature and pressure drop of 0.15 MPa. Finally, it should be mentioned that several factors have a greater impact than others when it comes to membrane formation, such as temperature and synthesis time, seeding procedure, and molar composition, but the changes in one parameter can also affect the others.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app14177836/s1](https://www.mdpi.com/article/10.3390/app14177836/s1)]([https://www.mdpi.com/article/10.3390/app14177836/s1](https://www.mdpi.com/article/10.3390/app14177836/s1)), Figure S1: Molecular kinetic diameters of gases compared to CHA pore size; Figure S2: Process flow diagram of single gas permeation setup (feed pressure of 0.1 MPa); Figure S3: Schematic of the deagglomeration of SSZ-13 seeds during sonication and formation of a uniform layer on the support surface after the seeding procedure; Figure S4: Filling factor of the autoclave from left to right 90, 80, 70, and 60%, respectively; Figure S5: Stability test of SSZ-13 membrane in presence of 2.5 vol% water vapor in a mixture of CO2-CH4 (50: 50) at 50 \({}^{\circ}\)C for 48 h; Table S1: Seeding procedure using different concentrations of SSZ-13 crystals; Table S2: Effect of sonication on the gas-tightness test of SSZ-13 membranes; Table S3: Effect of Si and Al sources on the gas-tightness test of SSZ-13 membranes; Table S4: Effect of Si/Al ratio on the gas-tightness test of SSZ-13 membranes; Table S5: Effect of OH/Si ratio on the gas-tightness test of SSZ-13 membranes; Table S6: Effect of TMAdaOH/Si ratio on the gas-tightness test of SSZ-13 membranes; Table S7: Effect of ageing time on the gas-tightness test of SSZ-13 membranes; Table S8: Effect of synthesis time on the gas-tightness test of SSZ-13 membranes; Table S9: Effect of synthesis temperature on the gas-tightness test of SSZ-13 membranes; Table S10: Template removal conditions from the SSZ-13 membranes.  
A.T.: Conceptualization, Methodology, Writing--Original Draft, Visualization; A.S.: Investigation, Validation, Data Curation, Writing--Review and Editing, Supervision, Funding acquisition; H.R.: Resources, Data Curation, Writing--Review and Editing, Supervision, Funding acquisition; M.S.: Supervision, Project administration; I.V.: Resources, Project administration. All authors have read and agreed to the published version of the manuscript.  
We gratefully acknowledge the Federal Ministry of Food and Agriculture for financial support, the Fachagentur Nachwachsende Rohstoffe e.V. in funding program \"Nachwachsende Rohstoffe\" for financial support (Project number: FKZ: 2221 NR048A).  
Not applicable.  
Data Availability Statement: Data are available on request.  
The authors declare no conflicts of interest.