Preparation of zeolite beta membranes and their pervaporation performance in separating di- from mono-branched alkanes

Two synthesis methods were used to prepare zeolite beta membranes on beta seeded, commercial Trumem disk supports. The first synthesis method produced an alkali free zeolite beta layer with a Si/Al ratio of 40. A Na-containing zeolite beta layer with a Si/Al ratio of 13 was obtained from the second synthesis method. The thickness of the zeolite beta layers was around 1000 nm. XRD analysis confirmed the existence of the zeolite beta phase. Nitrogen permeation experiments indicated permeation governed by surface diffusion. Thus, the membranes prepared had hardly any defects. The performance of the membranes in separating di- from mono-branched alkanes was evaluated in pervaporation experiments at 303 K with a 50/50 wt% 2-methylpentane (2MP) and 2,2-dimethylbutane (22DMB) mixture. Only the membranes prepared by the first method were selective. A selectivity of 1.5 in favour of 2MP was obtained which is higher than the Knudsen selectivity. Adsorption energies estimated by simulation of 2MP and 22DMB in zeolite beta were −48 kJ/mol and −42 kJ/mol, respectively. The higher adsorption value of 2MP and the selectivity towards 2MP suggest preferential adsorption of the mono over the di-branched isomer in the zeolite beta membranes. Membrane post-treatment with trimethylchlorosilane seems to improve membrane performance. A selectivity of 1.8 was obtained on a post-treated membrane.