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MFI membranes with high permeances, separation selectivities between 4 and 8.5 and ideal selectivities between 10 and 70 were synthesized in a flow system with circulation. Such high permeances and similar selectivities are seen in membranes synthesized at low temperatures. Carried out at low temperature and pressure, the synthesis method used in this n-C4H10 /i-C4H10 ideal selectivity The separation selectivities in 50–50% n-C4H10/i-C4H10 mixtures of the membranes synthesized in the flow and batch systems do not change much with temperature or flow rate (Fig. 1). Selectivities of all membranes are between 4 and 8.5 at room temperature and 200°C. The ideal selectivities for n-C4H10 over i-C4H10 are higher than the separation selectivities at both room temperature and at 200°C, except for the membrane synthesized in the flow system with the highest flow rate (48 mL/min), which has a higher separation selectivity at room temperature. In general, the membranes synthesized in the batch and flow systems had similar selectivities. The room temperature ideal selectivities and n-C4H10 permeances of various membranes reported in literature together with the membranes synthesized in this study are shown in Fig. 2 [4]. Considering the membranes synthesized in the flow system in this study together with the membranes synthesized in batch systems and reported in literature, the selectivities are of comparable value and in general higher permeances of n-C4H10 are observed.