Gas Adsorption and Gas Mixture Separations using Mixed-Ligand MOF Material

A metal-organic framework (MOF) adsorbent highly selective for adsorbing and separating carbon dioxide in carbon dioxide / methane mixtures. Abstract ADVANTAGES The invention is suited for the selective removal of carbon dioxide from natural gas, landfill gas, and other gas mixtures of CO2 and CH4. SUMMARY Carbon dioxide is often found as an impurity in natural gas and landfill gas, where methane is the major component. The presence of CO2 reduces the energy content of natural gas and can lead to pipeline corrosion. To meet pipeline requirements, natural gas must comply with strict CO2 concentration limits, as low as 2%. Technologies, such as absorption, cryogenic distillation, membrane separation, and adsorption, have been used for the separation of CO2 from natural gas. Among these technologies, adsorption-based methods such as pressure swing adsorption (PSA) are promising because of their easy control, low operating and capital investment costs, and energy efficiency. Key to the PSA processes for the separation and purification of CO2 is a highly selective adsorbent with a large CO2 capacity. Recently, metal-organic frameworks (MOFs) have been recognized as a new family of porous materials that have potential applications in separations, sensing, gas storage, and catalysis. MOFs consist of metal or metal-oxide corners connected by organic linkers. In addition MOFs can be regenerated under milder conditions than most zeolites adsorbants. This invention provides a method for selectively adsorbing carbon dioxide in carbon dioxide / methane mixtures employing a mixed-ligand, metal-organic framework (MOF) represented by the formula Zn2(NDC)2(DPNI) where NDC is 2,6 naphthalenedicarboxylate and DPNI is N,N'-di-(4-pyridyl)-1,4,5,8-napthalene tetracarboxydiimide. Adsorption isotherms of CO2 and CH4 were measured at 25° C and up to 18 atmospheres pressure using a volumetric-type apparatus by monitoring stepwise pressure change at equilibrium (accuracy ± 0.12%). Ideal adsorbed solution theory (IAST) was used to predict binary mixture adsorption isotherms using the experimental pure-gas Langmuir-Freundlich single-component isotherm parameters. ICAST predicted adsorption isotherms for equimolar mixtures of CO2/CH4 for the MOF 1-M? preparation, as a function of total bulk pressure, are shown in Figure 1. CO2 is preferentially adsorbed over CH4 due to stronger interaction between CO2 and the MOF. Methane adsorption is much lower in the mixtures than in single-component adsorption due to competition from CO2. The predicted selectivity for CO2 over CH4 is also displayed. In the low-pressure region, the selectivity of 1-M? is extremely large, ~30, and declines as pressure increases. The IAST predicted selectivities at different mixture compositions and different pressures for compound 1-M? are shown in Figure 2. The selectivity increases rapidly as the gas-phase mole fraction of CH4 approaches unity. At yCH4=0.95, a typical feed composition for natural gas purification, extremely high selectivities (8-67) are obtained. Even at yCH4=0.5, the selectivity is in the range of 4-30, among the highest selectivities reported. These results suggest that compound 1-M? is a material useful for natural gas purification, and also for CO2 separation from other CO2/CH4 mixtures, at conditions well suited to vacuum swing adsorption (VSA) processes. STATUS A patent application has been filed and Northwestern seeks to develop the discovery.