MFI membranes (ZSM-5 and silicalite-1) prepared by secondary growth on the core side of tubular titania supports, have been evaluated in the C-4 olefin isomers separation under elevated pressure. The membranes were synthesized according to much simpler methods than described in literature for high flux membranes. The MFI membranes were tested under technical conditions with an undiluted 50%/50% 1-butene/i-butene feed up to 21 bar feed pressure without any sweep gas or reduced pressure on the permeate side at a temperature of 130 degrees C. With increasing pressure difference across the membrane. the permselectivity - formed as the ratio of the 1-butene and i-butene permeances in the binary mixture - was found to decrease from initially PS approximate to 20 at 2 bar pressure difference to about PS approximate to 2-3 at 20 bar pressure difference. Like the mixture permselectivity the mixture separation factor alpha - calculated from the 1-butene mole fraction in the feed and permeate - drops as well with increasing pressure difference from initially alpha approximate to 10 at 2 bar to about alpha approximate to 2-3 at 20 bar pressure difference. The reason for this loss in selectivity with increasing pressure is the decrease of the 1-butene permeance from initially about 4m(3)(STP)m(-2) h (1)bar (1) at Delta p=2 bar to less than 1 m(3)(STP)m(-2) h(-1) bar(-1) at Delta p=20 bar. In contrast, the i-butene permeance is low but rather pressure-independent and remains therefore nearly constant. Both the curved adsorption isotherms and the reduced diffusivities for increased loadings are responsible for the less than linear increase of the 1-butene flux and the decreasing of the i-butene permeances in the binary mixture with increasing pressure. The molecular reason for the decreasing 1-butene/i-butene selectivity with increasing pressure is the collapse of the 1-butene diffusivity in the presence of increasing amounts of co-adsorbed i-butene. This behavior shows that the separation of the C-4 olefins follows not a simple molecular sieving mechanism but is based on the interplay of mixture adsorption and mixture diffusion. (c) 2008 Elsevier B.V. All rights reserved.