Controlled growth of thin and uniform TS-1 membranes by MW-assisted heating

Thin and uniform Ti-containing silicalite-1 (TS-1) membranes were prepared for the first time by a microwave-assisted secondary growth method. The influence of synthesis temperature (150–200 °C), duration (90–150 min) and Ti concentration in the mother sol (molar ratio Si/Ti = 16–100) on the membrane formation were studied. The quantity of Ti species inserted in the silicalite-1 network was found to vary with these parameters, as well as the membrane quality and performance. TS-1 membranes with a Si/Ti molar ratio in the range 25–40 were successfully prepared from mother sols with compositions in the range Si/Ti = 16–100. An original parabolic evolution of membrane thickness vs. Si/Ti in the mother sol was evidenced, with a minimum thickness for Si/Ti around 50. In all cases Ti species were inserted within the zeolite network as far as no TiO2 clusters were detected after the thermal treatment at 550 °C. The membranes were typically 0.1–2 μm thick top-layers and crystal preferential orientation varied from oblique- to c-crystal preferential orientation (CPO) when the temperature was increased from 160 °C to 200 °C. The permeances of He, N2, O2, CO2 were high and lied in the range 1.9–7.48 × 10−6 mol m−2 s−1 Pa−1 when measured between 25 °C and 200 °C. The most selective membrane, prepared at 190 °C during 120 min with a molar ratio Si/Ti = 75 in the mother sol, had an ideal selectivity of 40 for n/i-C4H10 at 150 °C and 105 for CO2/SF6 at 25 °C.