Purification of p-xylene using composite mixed matrix membranes

Abstract

Composite membranes capable of separating p-xylene from mixtures including p-xylene and m-xylene, and processes for purifying p-xylene using the membranes, are disclosed. The membranes are polymer membranes with a thickness of between about 10 and 1000 microns that include non-interconnected zeolite particles less than 5 microns. In one embodiment, a relatively thin polymer layer (0.5-3 microns) that includes zeolite particles is adjacent to a relatively thick polymer layer which may or may not include zeolite particles, where the thickness of the two layers is between about 10 and 1000 microns. The preferred ratio of zeolite/polymer is about 0.2 by weight. A preferred method for preparing the composite is by dispersing the zeolite in a polymer solution, casting a film of the polymer solution, and evaporating the solvent to form a polymeric film. The polymer permits passage of p-xylene and m-xylene in the vapor state, such that p-xylene diffuses at the same or a faster rate through the polymer. The polymer is preferably a polyaramide, polyimide or cellulose polymer p-Xylene and m-xylene diffuse through the zeolite, albeit at different rates. The zeolite is preferably an intermediate pore size zeolite, and more preferably, is silicalite or ZSM-5. The composite membrane is preferably in the form of a dense film or a hollow fiber. A mixture containing p-xylene and m-xylene can be enriched in p-xylene by a perstractive, pervaporative or gas-phase process through the membrane.