Investigation of butane and isobutane adsorption in metal organic frameworks

Effective separation of hydrocarbons play a significant role in the petrochemical industry. One of the most important hydrocarbon separation processes is separation of butane and isobutane. Butane and isobutane are commercial petrochemical products and have similar physical properties leading to difficulties in their separation. The conventional technology, fractional distillation, is an energy-intensive and expensive process for molecules that have similar volatility. Therefore, alternative processes such as adsorption is getting a lot of attention. In adsorption processes, there is no phase change; thus, they are relatively energy-efficient. The performance of adsorption processes depend significantly on the choice of the solid adsorbents. A succesful adsorption separation process requires an adsorbent with large capacity and high selectivity. An organic material, activated carbon, and an inorganic material, zeolite, are mostly used as adsorbates. Although activated carbon has high adsorption capacity and large surface area, it does not have an ordered structure. Zeolites have ordered structure, but they are inadequate in stability and diversity. Therefore, development of better novel materials are required. At this stage, metal organic frameworks, also named as coordination polymers, which have both inorganic and organic structure emerge as a new class of nanoporous materials. Metal organic frameworks (MOFs) show great potential in gas adsorption. They exhibit unique advantages over other traditional porous materials like carbonaceous materials and zeolites. MOFs are formed by the coordination of metal ions with organic linkers. Nowadays, metal organic frameworks gain attention from chemists and material scientists because of their homogenous microporosity, large surface area, tunable pore size, functionality, controllable crystalline structure, structure diversity, remarkable thermal and chemical stability properties, and the number of studies and publications noteworthily increase. The majority of these studies is related with gas separation, in particular, selective adsorption of industrially important gas mixtures. Metal organic frameworks also find various applications apart from gas separation. These are gas storage, ion exchange, catalysis and drug delivery. At this stage, molecular simulation methods also emerge as a tool to characterize metal organic frameworks for these applications. With rapidly growing computational resources, molecular simulation has become an indispensible tool and plays an increasingly important role in material science and engineering. Molecular simulation methods are extensively used in from the range of small chemical systems to materials ensemble. Molecular simulation methods enable calculation of thermophysical properties of lesser known systems and help to gain a deeper understanding of properties that differ based on the molecular structure. Also, simulation at a molecular level provides microscopic insight that is experimentally challenging and simulation helps to establish the structure-function relationship to guide the selection and design of new materials. Furthermore, getting experimental adsorption isotherms for mixtures is more complicated than that for pure components and is difficult to obtain experimentally. To date, most simulation studies in metal organic frameworks have been focused on gas storage and separation. The objective of this study is to investigate n-butane and isobutane adsorption in different metal organic frameworks by the help of molecular simulation methods. The grand canonical Monte Carlo simulation is used to simulate adsorption isotherms. In the grand canonical Monte Carlo simulation, chemical potential, temperature and volume are constant. Chemical potential is converted to fugasity using the Peng-Robinson equation of state. In this thesis, rhombic and sodalite type zeolite like metal organic frameworks (ZMOFs) which are ion exchanged with Na+, Li+ and K+ metal cations, ZIF-8, Cu-BTC, FOHQUO and Mg-formate (trade name Basosiv M050) were studied. Simulation studies were performed at 333 K and 1-6 bar pressure range. We used Lennard Jones (LJ) 12-6 potentials and the Coulomb potential to model repulsion-dispersion forces and electrostatic interactions, respectively. Also, we used the universal force field (UFF) Lennard Jones parameters for the metal organic framework atoms. By the help of Material Studio 5.5® simulation programme, adsorption isotherms for pure n-butane and i-butane and their 50:50 mixture were obtained. Using these adsorption isotherms, adsorption of n-butane and i-butane in metal organic frameworks were investigated as a function of pressure. Ideal selectivity, real selectivity, and working capacity of metal organic frameworks were calculated and performance of metal organic frameworks were evaluated. Results were compared with zeolites used for adsorption of n-butane and i-butane in the literature. The metal organic frameworks investigated in this study showed different adsorption characteristic from each other. Also, metal organic frameworks showed much higher working capacity than zeolites. Furthermore, it was observed that rho-ZMOF structures were superior to sod-ZMOFs in terms of adsorption capacity. The results show that pore size and structure significantly affect selective adsorption. Pore volume of sod-ZMOFs is approximately half of the pore volume of rho-ZMOF. Zeolite like metal organic frameworks ion exchanged with different metal cations showed various behaviours. The results showed that cation type has an important effect on the adsorption process. Within the MOFs studied in this thesis, Mg-formate (Basosiv M050), FOHQUO and rho-ZMOF structures show good adsorbent properties for adsorption of n-butane and i-butane. Although rho-ZMOF structures did not show very high adsorption selectivity, their working capacity were very high. Finally, while FOHQUO and Basosiv M050 are superior to rho-ZMOF structures in terms of selectivity, they showed worse performance than rho-ZMOF structures in terms of working capacity.