Gas Adsorption and Gas Mixture Separations using Carborane-Based MOF Material

A novel metal-organic framework (MOF) adsorbent highly selective for adsorbing and separating carbon dioxide in carbon dioxide / methane mixtures. Abstract ADVANTAGES The invention is advantageous 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. The present invention provides a method for selectively adsorbing carbon dioxide in carbon dioxide / methane mixtures using a polymeric crystalline material having a novel metal-organic framework (MOF) that includes a three-dimensional carborane structure (material 4). This MOF selectively adsorbs carbon dioxide from a room temperature mixture of carbon dioxide and methane and is especially effective at relatively low bulk gas pressures and high mole fractions of methane in the mixture. 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%). A Langmuir-Freundlich (LF) isotherm equation was used ot fit the adsorption isotherm of pure gas and these isotherm parameters were used for calculation of the mixture adsorption isotherms using ideal adsorbed solution theory (IAST). Figure 1 shows the adsorption isotherms for an equimolar mixture of CO2/CH4 with material 4, as a function of total bulk pressure. CO2 is more preferentially adsorbed than CH4 because of the stronger interaction between CO2 and the MOF. The separation between both isotherms are much larger than those observed in the single component isotherms due to the competitive adsorptions of CO2 and CH4 in the mixture conditions. Material 4 gives high selectivity (5-18) and is among the best materials for this purpose compared with other commonly used adsorption materials. Figure 2 displays material 4 adsorption selectivity toward CO2 as functions of bulk pressure and bulk mole fraction of CH4. Selectivity varies with the change of pressure or mole fraction. As the pressure decreases, the selectivity rapidly increases, affording large selectivity (17 to about 35) at high mole fractions of CH4 as the pressure goes to dilution region (0.1 atm). Thus CO2 adsorption is extremely dominant at low pressure range, conditions advantageous to PSA processes. Importantly, at low pressures, the selectivity increases steeply as yCH4 approaches to 1.0. indicating the highest selectively of CO2 over CH4 when CO2 is present in very low concentrations, a most technologically relevant composition. STATUS U.S. Patent Number 7,744,842