Effect of Midsynthesis Addition of Silica to the Synthesis Medium on the Properties of MFI-Type Zeolite Membranes

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 qualit

Effect of Soda Concentration on the Morphology of MFI-Type Zeolite Membranes

MFI-type zeolite membranes were hydrothermally synthesized as a thin layer on macroporous -alumina discs coated with 1m MFI-type seed crystals as seed. The effect of soda (Na2O) concentration on the morphology of the membranes was investigated. Soda concentration, and thus the hydroxyl ion concentration, was changed between 0.25 and 6.5 mol/mol gel. At low soda concentrations the crystals forming the layer exhibited mostly (h0h)/c-axis orientation, but at high soda concentrations a membrane layer formed from randomly oriented crystals. The membranes showed high H-2/n-butane ideal selectivity of 476 and 36 at 25 degrees C and 150 degrees C, respectively

Separation of gas and organic/water mixtures by MFI type zeolite membranes synthesized in a flow system

MFI type zeolite membranes were synthesized in a recirculating flow system at 95 degrees C where the synthesis solution was flown over the tubular alpha-alumina supports. The performance of the membranes for the separation of binary gas mixtures and alcohol/water liquid mixtures was investigated. A membrane synthesized by two consecutive synthesis steps had a separation selectivity of 15 and 11 for equimolar mixtures of n-C4H10/CH4 and n-C4H10/N-2 at 200 degrees C, respectively. The membrane selectively permeated large nC(4)H(10) over small CH4 and N-2, suggesting that the separation is essentially adsorption-based and the membrane has few nonselective intercrystalline pores. The selectivities in the pervaporation separation of 5% ethanol/95% water mixture were 43 and 23 with permeate fluxes of 0.2 and 1.9 kg/m(2) h at 25 and 85 degrees C, respectively. The separation performance of membranes showed that MFI type membranes prepared in a recirculating flow system can be used both in the separation of gas and liquid mixtures

Influence of zeolite crystal expansion/contraction on NaA zeolite membrane separations

In situ powder XRD measurements showed that adsorption causes the NaA zeolite unit cell to contract or expand, and these changes depend on adsorbate loading. Changes in zeolite crystal size correlated with permeation changes through NaA zeolite membranes. These membranes had high water/alcohol pervaporation selectivities, even though gas permeation was mainly through defects, as indicated by Knudsen selectivities for single gas permeation. At 300 K and a thermodynamic activity of 0.03, water contracted the NaA crystals by 0.22 vol%, and this contraction increased the helium flux through two NaA membranes by approximately 80%. Crystal contraction also increased the fluxes of i-butane during vapor permeation and i-propanol (IPA) during pervaporation (similar to 0.03 wt% water). At activities above 0.07, water expanded NaA crystals and correspondingly decreased the fluxes of helium, i-butane, and IPA through NaA zeolite membranes. Methanol also contracted NaA crystals, but by only 0.05 vol% at an activity of 0.02, and this contraction slightly increased the helium and i-butane fluxes through a NaA membrane. Above an activity of 0.06, methanol expanded the NaA crystals, and the fluxes of helium and i-butane through a NaA membrane decreased. These adsorbate-induced changes explain some pervaporation behavior reported by others, and they indicate that crystal expansion and contraction may increase or decrease zeolite NaA membrane selectivity by changing the defect sizes