Methyl mercaptan removal from natural gas using MIL-53(Al)

MIL-53(Al) was synthesized using hydrothermal method with various synthesis and calcination conditions. The synthesized materials were characterized by Powder X-ray Diffraction (PXRD), Thermal Gravimetric Analysis (TGA) and N2 adsorption-desorption isotherms (BET surface area measurement) methods. The results showed that synthesis time and calcination time can be decreased two days and one day, respectively, with respect to the usual method with no significantly decrement in the BET surface area and the crystallinity of materials. The sorption equilibrium, thermodynamic and kinetic of methyl mercaptan (MeSH) and methane adsorption on the MIL-53(Al) were studied by the volumetric method using a home-made apparatus. The sorption isotherm of MeSH revealed type IV according to IUPAC classification, while a type I was observed in the profile of the methane sorption isotherm. The synthesized MIL-53(Al) displayed high methyl mercaptan adsorption capacity (more than 9 mmol/g) which is about 2–3 times more than 13X, the usual industrial adsorbent for mercaptan removal from natural gas. The various models such as Langmuir, Sips and Thoth were successfully used to fit the adsorption experimental data. The Extended Langmuir (EL) and Ideal Adsorbed Solution Theory (IAST) models along with the pure component isotherm fitted to the Langmuir model were used to predict the removal of MeSH from methane-rich mixture. MIL-53(Al) presented high selectivity (EL selectivity of 263) for MeSH over methane. The isosteric heat of MeSH and CH4 adsorption on the MIL-53(Al) were within the range of 26–50 and about 18 kJ mol−1 respectively. Multiple adsorption–desorption cycles showed that the MeSH adsorption on the MIL-53 was highly reversible with a desorption efficiency of up to 95%. The diffusion time constants of MeSH and CH4 in the MIL-53(Al) at 298 K were estimated to be 1.48 × 10−2 and 1.99 × 10−2 s−1, respectively, leading to relatively equal adsorption kinetics of both molecules. High adsorption capacity, good kinetics, easy regeneration and reversible process of MeSH adsorption, suggested that the MIL-53(Al) can be used as a favorable adsorbent for purification of natural gas or biogas in the Pressure Swing Adsorption (PSA) process.