Gas Separation through Bilayer Silica, the Thinnest Possible Silica Membrane

Membrane-based gas separation processes can address key challenges in energy and environment, but for many applications the permeance and selectivity of bulk membranes is insufficient for economical use. Theory and experiment indicate that permeance and selectivity can be increased by using two-dimensional materials with subnanometer pores as membranes. Motivated by experiments showing selective permeation of H2/CO mixtures through amorphous silica bilayers, here we perform a theoretical study of gas separation through silica bilayers. Using density functional theory calculations, we obtain geometries of crystalline free-standing silica bilayers (comprised of six-membered rings), as well as the seven-, eight-, and nine-membered rings that are observed in glassy silica bilayers, which arise due to Stone–Wales defects and vacancies. We then compute the potential energy barriers for gas passage through these various pore types for He, Ne, Ar, Kr, H2, N2, CO, and CO2 gases, and use the data to assess their capability for selective gas separation. Our calculations indicate that crystalline bilayer silica, which is less than a nanometer thick, can be a high-selectivity and high-permeance membrane material for 3He/4He, He/natural gas, and H2/CO separations.基于膜的气体分离工艺可应对能源与环境领域的核心挑战，但多数应用场景下，本体膜的渗透率与选择性无法满足经济化使用要求。理论与实验研究均表明，采用带有亚纳米级孔道的二维材料作为分离膜，可有效提升膜的渗透率与选择性。受关于氢气（H2）与一氧化碳（CO）混合气体可通过无定形二氧化硅双层膜实现选择性渗透的实验研究启发，本文针对二氧化硅双层膜的气体分离过程开展理论研究。本文借助密度泛函理论（density functional theory）计算，获取了由六元环构成的结晶态自支撑二氧化硅双层膜的几何结构，同时也得到了玻璃态二氧化硅双层膜中因斯通-威尔士（Stone–Wales）缺陷与空位所形成的七元、八元及九元环结构。随后，我们针对氦气（He）、氖气（Ne）、氩气（Ar）、氪气（Kr）、氢气（H2）、氮气（N2）、一氧化碳（CO）以及二氧化碳（CO2）等气体，计算了其通过上述各类孔道的势能垒，并基于计算数据评估了这些膜结构的选择性气体分离性能。计算结果表明，厚度不足1纳米的结晶态二氧化硅双层膜，可作为实现氦-3/氦-4（3He/4He）、氦气/天然气以及氢气/一氧化碳（H2/CO）分离的高选择性、高渗透率膜材料。