3. RESULTS AND DISCUSSION
In simulations, the gas molecules gradually enter the permeate side, meanwhile the concentration gradient between the feed side and the permeate side and the flow rate decrease (Figure 2) . In nanopores with quite large d (~ 4 Å or above), both He and CH4 gases pass through the nanopore quickly (Figure 2a for Ti4C3 nanopore and Figure S1 for others, all discussions in this work refer to single gas simulations if not otherwise specified). In smaller nanopores (d ~3 Å), the flow rate of He drop slightly, while that of CH4 drop significantly, and consequently, the flow rate of He become much higher than CH4(Figure 2b&2c and Figure S2) . Comparing the nanopores withd of ~ 3 Å on various kinds of MXenes, the flow rates of He are similar, while those of CH4 are quite different. When d further decreases to ~2 Å, He could permeate through, but CH4 could not permeate through either of the studied nanopores (Figure 2d ), with simulation time even extended to 500 ns. All these results indicate that the nanopores with d of ~3 Å or below are suitable for highly efficient He/CH4 separation.