Thin zeolite membranes of MFI type were prepared on alpha-Al2O3 supports by midsynthesis addition of silica in a recirculating flow system at 95 degrees C The synthesis was started with the batch composition of 80SiO(2): 30TPAOH:1500H(2)O which was changed to 80SiO(2)center dot 9TPAOH:1500H(2)O by addition of silica Mkt 24, 48, or 72 h Compared to synthesis at constant composition. it is possible to affect the rates of nucleation and crystal growth to obtain membranes with better quality by changing the overall composition of the medium during the membrane synthesis The membranes produced with and without midsynthesis addition of silica have n-C4H10/i-C4H10 ideal selectivities of 47 and 8 at 100 degrees C, respectively Among these membranes, the ones formed with midsynthesis addition of silica at 24 h yielded the highest n-C4H10/CH4 separation selectivities, which were over 20 at 25 degrees C The time at which the silica is introduced to the synthesis medium is critical for the membrane quality