The induced orientation effect of linear gases during transport in a NaA zeolite membrane modified by alkali lignin

Abstract

The molecular orientation of permeating molecules crucially affects the gas transfer behavior in kinetic molecular sieving, particularly if the molecules are linear; however, this effect has been largely overlooked in the literature. To investigate this effect here we induce specific unfavourable molecular orientation of several linear gases based on adsorption effects in a hollow fiber NaA zeolite membrane, by anchoring a 3-dimensional alkali lignin molecule on the hydroxyls of the membrane surface. The results indicated that although the permeance was systemically reduced for all the gases investigated due to unfavourable orientation, the ideal selectivity for H2 relative to the linear molecules was consistently enhanced due to the presence of the alkali lignin molecule. On the other hand the permeation behavior for the spherically symmetric CH4 molecule was unaffected by the alkali lignin molecule, with the H2/CH4 unchanged and nearly constant over the operating temperature range. In addition to the defect healing, the enhancement in selectivity for H2 over the linear gases is strongly attributed to the normal orientation of the linear molecule relative to the pore axis on entry, induced by the adsorption force of the alkali lignin, leading to a repulsive potential at the pore mouth of the zeolite membrane. To validate the induced orientation effect of the linear molecules, ab initio method was employed to estimate the interaction energies between the pore mouth and linear molecules oriented parallel and normal to the pore axis. The computational results showed that the interaction energies were consistently higher for each specie when oriented normal to the pore axis (i.e. parallel to the pore cross-section), so that the tilted molecular orientation experienced stronger energy barriers; this is in accord with the experimental observation of increased selectivity for the more weakly-adsorbed H2 over the linear molecules. These findings can provide scientific guidance and facilitate new solutions to extend the separation boundaries of nanoporous membrane among gases.