Hydrogen adsorption and diffusion in p-tert-butylcalix[4]arene: an experimental and molecular simulation study.

Experimental adsorption isotherms were measured and computer simulations were performed to determine the nature of the H(2) gas uptake in the low-density p-tert-butylcalix[4]arene (tBC) phase. (1)H NMR peak intensity measurements for pressures up to 175 bar were used to determine the H(2) adsorption isotherm. Weak surface adsorption (up to ≈2 mass % H(2) ) and stronger adsorption (not exceeding 0.25 mass % or one H(2) per calixarene bowl) inside the calixarene phase were detected. The latter type of adsorbed H(2) 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 H(2) in the calixarene phases. Direct adsorption isotherms by exposure of the calixarene phase to pressures of H(2) gas to ≈60 bar are also presented, and show a maximum H(2) adsorption of 0.4 H(2) per calixarene bowl. Adsorption isotherms of H(2) in bulk tBC have been simulated using grand canonical Monte Carlo calculations in a rigid tBC framework, and yield adsorptions of ≈1 H(2) 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 H(2) in the solid with different loadings.