Membrane technologies will offer us great opportunities to reduce energy demands revolutionary in future chemical and petroleum industries.  Among emerging membrane technologies, great attention has been paid to zeolites as membrane materials.  Zeolites, microporous aluminosilicates crystals, have been widely used as adsorbents, catalysts, and ion exchangers.  In addition to their high thermal, chemical, and structural stability, zeolite micropore systems can uniquely recognize molecules by molecular sieving and/or preferential adsorption.

In commercial applications, a membrane to remove steam selectivelyfrom syngas above 200^oC seems to be promising for a membrane extractor inreactions such as Fischer-Tropsch synthesis and methanol synthesis, where water is a by-product.  These syngas reactions (C1 chemistry) require further technical development considering future growth in alternate fuel sectors, driven by both strategic and environmental considerations.

In the 2006 AIChE annual meeting, we reported that a ZSM-5 type zeolite membrane highly permeated and separated steam from hydrogen above 200^oC.  This study aimed at clarifying correlation among synthesis condition, membrane structure, and separation property of ZSM-5 membranes.    

ZSM-5 membranes were prepared on the outer surface of porous alumina tubular support by a secondary-growth method. The molar composition of a synthesis mixturewas 10Na₂O: 0.15Al₂O₃: 36SiO₂: 1200H₂O.  Hydrothermal treatment was carried out at 180°C with prolonged synthesis time from 6 to 18 h.  Sort of cations in the zeolite membranes were controlled by ion exchange. Membrane structures were characterized by FE-SEM and permporometry.  Separation properties of membranes were evaluated by water/hydrogen separation in the temperature range of 150-250^oC. 

Permporometry measurements suggest that voids among crystal boundaries were filled after the crystallization for 12 h and that remaining inter-crystal pathways readily lead to a deterioration of water/hydrogen separation performance.  The sizes of pores and voids were tentatively estimated using the Kelvin equation.  Gas permeation via pores with Kelvin diameter of larger than 0.55 nm was not detected in permporometry measurements through a Na-exchanged ZSM-5 (Na-ZSM-5) membrane with a separation factor of water/hydrogen = 74 (at 150^oC and partial pressures of water/hydrogen = 10/91 kPa).  On the other hand, 3-7 % of gas permeation was via pores with the Kelvin diameter of 0.55-1.2 nm through a Na-ZSM-5 membrane with separation factor of water/hydrogen = 26, while the voids of larger than 1.2 nm was not detected. 

Separation performance of water/hydrogen mixture was influenced by Na cation occluded in the micropores of zeolite.  With increasing partial pressure of water, hydrogen permeation through Na-ZSM-5 membrane was effectively blocked, whereas such hindrance of hydrogen permeation with water was difficult to observe with H⁺-exchanged ZSM-5 (H-ZSM-5) membrane at higher temperature.  Adsorption experiments revealed that the amount of water adsorbed on Na-ZSM-5 was much larger than that on H-ZSM-5.  When comparing hydrogen permeation through Na-ZSM-5 and H-ZSM-5 membranes at the same loading of water, the Na-ZSM-5 membrane showed more remarkable inhibition of hydrogen permeation than the H-ZSM-5 membrane.  Differences in permeation and separation properties between Na-ZSM-5 and H-ZSM-5 membranes were more significant at higher temperature.