Hydrogen Adsorption and Diffusion in p-tert-Butylcalix[4]arene: An Experimental and Molecular Simulation Study

摘要

Experimental adsorption isotherms were measured and computer simulations were performed to determine the nature of the H₂ gas uptake in the low-density p-tert-butylcalix[4]arene (tBC) phase. 1H NMR peak intensity measurements for pressures up to 175 bar were used to determine the H₂ adsorption isotherm. Weak surface adsorption (up to ≈2 mass % H₂) and stronger adsorption (not exceeding 0.25 mass % or one H₂ per calixarene bowl) inside the calixarene phase were detected. The latter type of adsorbed H₂ molecule has restricted motion and shows a reversible gas adsorption/desorption cycle. Pulsed field gradient (PFG) NMR pressurization/depressurization measurements were performed to study the diffusion of H₂ in the calixarene phases. Direct adsorption isotherms by exposure of the calixarene phase to pressures of H₂ gas to ≈60 bar are also presented, and show a maximum H₂ adsorption of 0.4 H₂ per calixarene bowl. Adsorption isotherms of H₂ in bulk tBC have been simulated using grand canonical Monte Carlo calculations in a rigid tBC framework, and yield adsorptions of ≈1 H₂ per calixarene bowl at saturation. Classical molecular dynamics simulations with a fully flexible calixarene molecular force field are used to determine the guest distribution and inclusion energy of the H₂ in the solid with different loadings.