Enhanced mass transfer on hierarchical porous pure silica zeolite used for gas separation

Two types of ultra-high silica silicalite-1 (2000–4400 SiO₂/Al₂O₃) were prepared using a seed crystal synthesis method; silicalite-1-M, which is microporous (0.5 nm) and silicalite-1-H, which is hierarchical porous (0.5 nm and 10–15 nm). From the experimental results, silicalite-1-H/M have almost the same adsorption isotherms for CO₂, CH₄, C₂H₆ and N₂, surface area and equilibrium adsorption selectivity, however, silicalite-1-H has a shorter adsorption equilibrium time when compared with silicalite-1-M at a given pressure, which shows a mass transfer enhancement phenomenon due to its mesoporous structure. A mixed gas breakthrough test and simulation was carried out to verify the mass transfer upgrade theory to determine whether the material could be applied for adsorption separation. Interestingly, their performances on the breakthrough experiment were different; silicalite-1-H has a shorter breakthrough time for CO₂, CH₄, C₂H₆ and N₂ when compared with silicalite-1-M. All the experiment data are consistent with the simulation results. A variety of flow rates were used to investigate and compare the breakthrough time and hold time, and the separation efficiency of CO₂/CH₄, CH₄/N₂ and CH₄/C₂H₆ was improved based on the use of the hierarchical porous sorbent over a certain flow rate range.