Improved thiophene solution selectivity by Cu²⁺, Pb²⁺ and Mn²⁺ ions in pervaporative poly[bis(p-methyl phenyl) phosphazene]desulfurization membrane

Due to interfacial defects and agglomeration, role of metal ions in mixed matrix membrane during pervaporation is not fairly demonstrated and not fully taken advantage of. Through surface modification of pristine PMePP membrane and the followed ion exchange treatments, PMePP-M membranes containing Cu²⁺, Mn²⁺, or Pb²⁺ ions were obtained. X-ray photoelectron spectroscopy (XPS) analysis confirms the successful exchange of Cu²⁺, Mn²⁺, or Pb²⁺ ions onto the membrane surface. Pervaporation results show that the removal of thiophene from model gasoline by pristine PMePP membrane or modified PMePP membranes is a solution-diffusion controlled process while it shifts to a solution controlled process for PMePP-M membranes. It provides important evidence for the facilitated thiophene mass transport via metal ions. PMePP-M membranes show superior selectivity in a Robeson type plot and their performance locates above the upper bound of literature reported thiophene permeability and selectivity. In the meantime, it is proved that post-treatment and control of oxidation temperature can compensate the decrease of pervaporative flux caused by the change in surface hydrophobicity and the introduction of electric charge.