The synthesis of molecular-sieving two-dimensional zeolitic membranes by the assembly of crystalline building blocks without resorting to the secondary growth process is highly desirable. The precise pore size for molecular sieving, ultrathin thickness, and high thermal and chemical stability make zeolite nanosheets attractive for a number of gas separation applications. However, preparing ultrathin membranes in a scalable way can only be achieved with a secondary growth-free approach, and this remains a grand challenge. Overall, there are four major drawbacks for the synthesis and scale-up of these type of membranes: i) the preservation of the crystallinity of the nanosheets after exfoliation; ii) the non-reproducibility of the secondary growth method; iii) the development of low-cost and scalable support for the ultrathin films, and iv) the implementation of a facile and scalable membrane fabrication methods. This dissertation focuses on the development of ultrathin zeolitic membranes employing 0.8-nm-thick crystalline nanosheets from the sodalite zeolite precursor RUB-15 that hosts hydrogen-sieving six-membered rings (6-MRs) of SiO4 tetrahedra. The hydrothermal synthesis of the layered RUB-15 followed by the cation exchange chemistry to increase the d spacing between the layers facilitated the polymer blend-based exfoliation of the layered precursor leading to the first report of highly crystalline RUB-15 nanosheets where the 6-MRs were clearly visible with high-resolution transmission electron microscopy. Highly dispersed RUB-15 nanosheets in polar solvents allowed their facile assembly via vacuum filtration into 100-300 nm-thick continuous films on top of porous supports. Detailed transport studies of such as-filtered membranes revealed the presence of two different transport pathways for gas molecules: 1) the H2-selective 6-MRs and 2) the interlayer galleries, which allow He, H2, and CO2 molecules to permeate freely. The latter dominated the transport in as-filtered films, which displayed a molecular cutoff of 3.6 Å yielding a H2/N2 and H2/CH4 selectivities above 20. Non-H2-selective pathways [interlayer galleries] were eliminated by topotactic condensation of the terminal silanol groups. Upon calcination, defective [SiO3][O-] units were converted into fully coordinated silicon tetrahedra [SiO4], diminishing the interlayer gaps and yielding H2/CO2 selectivities above 100, demonstrating the effective suppression of the interlayer transport and highlighting the selective role of the 6-MRs in the temperature range 25-300 °C. This is the first report of high-performance two-dimensional zeolitic membranes without the need for the secondary growth process able to efficiently sieve light gases. VI The scale-up of thin supported membranes relies on the quality of the underlying support. A scalable polymeric support was developed to support uniform RUB-15 films. The support was synthesized via non-solvent induced phase separation (NIPS) of polybenzimidazole AM Fumion® polymer on a low-cost stainless steel mesh. The support possesses a smooth surface, high porosity, and thermal and mechanical stability. However, the high calcination temperature of RUB-15 membranes prohibits its employment as support. For this, two new routes for removing the residual template and surfactant were developed to enable the use of the polymeric PBI-AM supports for the future scale up of RUB-15 membranes. [CONTINUED]