The separation of propane/propylene mixtures is the most energy-intensive operation practiced in the petrochemical industry. Adsorptive processes are currently viewed as a promising alternative to cryogenic distillation for the separation of these mixtures. In this paper, we explore the possibility of using a new metal-organic framework material, CuBTC, in adsorptive separation processes, particularly in a simulated moving bed (SMB) context using isobutane as a potential desorbent. A gravimetric method has been used to measure the adsorption equilibrium isotherms of propylene, propane and isobutane onto a commercial CuBTC powder over a temperature range from 323 to 423 K and pressures up to 100 kPa. These were complemented by a detailed experimental characterization of the structure of CuBTC using XRD and SEM techniques. Comparison of experimental isotherms with grand canonical Monte Carlo simulations in CuBTC showed that propane adsorption occurs preferentially in small octahedral pockets, while isobutane is excluded from these pockets due to its bulky structure. Propylene was seen to interact strongly with unsaturated metal sites, due to specific -Cu bonds. These interactions significantly enhance the affinity of this MOF for unsaturated hydrocarbons. Furthermore, in a range of temperatures and pressures, the affinity of CuBTC for isobutane is intermediate to that of propane and propylene. Our results suggest that CuBTC-isobutane is a very promising adsorbent-desorbent pair for use in SMB processes for propane/propylene separations.
We present results of molecular simulation of pure propane and propylene, as well as their binary mixtures in the metal-organic framework CuBTC. By comparing simulated and experimental pure-component isotherms we are able to describe the adsorption mechanism of these two molecules. The main difference is the existence of strong specific interactions between the open metal sites of CuBTC, freed by framework dehydration during the activation process, and the pi orbitals of the propylene double bond. The net result is a moderate selectivity (up to 4) of the material for propylene adsorption. Given the current lack of experimental data for propane/propylene mixture adsorption in CuBTC, we have compared the molecular simulation results to predictions from Ideal Adsorbed Solution Theory using single-component experimental adsorption isotherms as input. Our comparison suggests that IAST is likely to adequately describe this system, and differences between the theory and simulation are probably due to shortcomings of the simplified potential model used to represent the pi-metal interactions
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.