## 1 Introduction  
One of the main factors in global warming and climate change issues is the presence of CO\({}_{2}\), which may be found in various sources such as natural gas [1], flue gas [2], bio-gas [3, 4], and products of coal gasification [5]. To realize the importance of the issue, it must be considered that in the last five years more than 30 billion tons of CO\({}_{2}\) have been emitted, which has always been a growing amount. Therefore, CO\({}_{2}\) capture and separation has become one of the most important research topics in the present century [6, 7, 8, 9]. There are currently a few technologies available for CO\({}_{2}\) separation including cryogenic distillation [10], amine absorption [11], amine scrubbing [12], etc., that are characterized by high maintenance costs and energy consumption, with energy requirements ranging from 4 to 6 MJ/kg CO\({}_{2}\) for absorption, 2 to 3 MJ/kg CO\({}_{2}\) for adsorption, and 6 to 10 MJ/kg CO\({}_{2}\) for cryogenic distillation. However, membrane technology, which does not require sorbent regeneration or desorption, is more economical compared to other technologies due to its advantages such as lower energy consumption (0.5 to 6 MJ/kg CO\({}_{2}\)) and the ability to separate different mixtures [6, 13]. As yet, different types of membranes have been used and studied for CO\({}_{2}\) separation such as metal organic frameworks (MOFs) [14] as well as polymeric [15], mixed-matrix [16], and inorganic membranes [17, 18]. In recent years, polymeric membranes were pioneers in the CO\({}_{2}\) capture process, but their low stability in industrial conditions and also their low CO\({}_{2}\) permeance and CO\({}_{2}\)/CH\({}_{4}\) selectivity in comparison with inorganic membranes have limited their use [19]. The separation industry requires membranes with high gas permeance, structural stability, and selectivity. Nowadays, zeolite membranes are attracting more attention because these membranes combine all the above-mentioned factors, especially desirable pressure tolerance and high chemical and thermal stability during the separation process. Small-pore zeolites, 8-member-rings such as CHA, DDR, and LTA have high performance for CO\({}_{2}\) capture due to the adsorption-diffusion mechanism [20; 21; 22]. The CHA zeolite framework includes various types such as all-silica CHA [23], SSZ-13 (high-silica CHA) [24], SAPO-34 (silicoaluminaphosphate) [25], and low-silica CHA (chabazite) [26; 27]. Among these four types, SSZ-13 has better performance because, for instance, SAPO-34 has less mechanochemical stability in harsh media due to the presence of phosphate compounds [27]. In addition, due to its high CO\({}_{2}\) selective properties and hydrothermal stability, SSZ-13 membranes with a \(\sim\)0.38 \(\times\) 0.42 nm pore size are appropriate materials for usage in CO\({}_{2}\)/CH\({}_{4}\) separations with high CO\({}_{2}\) permeance [21; 28]. According to recent publications regarding high-Si CHA membranes for gas separation, the CO\({}_{2}\)/CH\({}_{4}\) permselectivity is in the range of 20-300, but in most of the publications, the CO\({}_{2}\) permeance is less than 10\({}^{-6}\) [mol/(m\({}^{2}\)sPa)] [19], because, usually, several cycles of hydrothermal synthesis are used to achieve a flawless membrane layer, which increases the transmission resistance [29]. In this work a detailed study of fabricating thin and uniform SSZ-13 zeolite membranes by controlling the many critical parameters in the synthesis procedure is presented. It is not only very important for CO\({}_{2}\)/CH\({}_{4}\) separation in natural gas processing but also for SF\({}_{6}\) recovery (highly harmful greenhouse gas), that SSZ-13 membranes exhibit high separation selectivity for both applications. By analyzing various synthesis parameters in this study, it was possible to fabricate SSZ-13 zeolite membranes with high structural stability in a reproducible method, making them appropriate for upscaling in gas separation applications.