Why Silver Nanoparticles Are Effective for Olefin/Paraffin Separations

The majority of nanocomposite olefin/paraffin separation membranes use silver nanoparticles or silver ions as the olefin binding agent. In this theoretical study, we characterize the olefin interaction with silver nanoparticles and show that silver is special in that it chemisorbs ethylene more weakly than other metals. Some variation with particle size is found; small 79 atom nanoparticles tend to bind ethylene more strongly than larger 140 atom particles, which in turn are well approximated by facets of bulk crystal surfaces. The effect of replacing cores of nanoparticles with different metals is demonstrated to selectively tune binding based on the relative d-band centers of the two metals. We identify silver-cored, gold-shelled nanoparticles as potentially more effective for olefin/paraffin separations. Random alloys of gold and silver were also considered. We find that 25%-75% Au-Ag random alloys are strong candidates for use in olefin/paraffin separation membranes due to the presence of reactive (111) faces without the cost of a strong increase in the binding energies on edges and corners. Nanocomposite membranes containing these nanoparticles hold promise for more efficiently separating olefins from paraffins.