Poly(triazine imide) (PTI), a crystalline g-C3N4, hosting two-dimensional nanoporous structure with an electron density gap of 0.34 nm, is highly promising for high-temperature hydrogen sieving because of its high chemical and thermal robustness. Currently, layered PTI is synthesized in potentially unsafe vacuum ampules in milligram quantities. Here, we demonstrate a scalable and safe ambient pressure synthesis route leading to several grams of layered PTI platelets in a single batch with 70% yield with respect to the precursor. Solvent exfoliation under anhydrous conditions led to single-layer PTI nanosheets evidenced by the observation of triangular g-C3N4 nanopores. Gas permeation studies confirm that PTI nanopores can sieve He and H-2 from larger molecules. Last, high-temperature H-2 sieving from PTI nanosheet-based membranes, prepared by the scalable filter coating technique, is demonstrated with H-2 permeance reaching 1500 gas permeation units, with H-2/CO2, H-2/N-2, and H-2/CH4 selectivities reaching 10, 50, and 60, respectively, at 250 degrees C.