Supported zeolite A membranes. Feasibility of the static transverse synthesis : a new approach

[摘要] ENGLISH ABSTRACT:As there is inadequate control of the hydrothermal synthesis technique used to date in thesynthesis of zeolite A membranes, the feasibility of a new synthesis approach was investigated,namely the static transverse synthesis method. The former technique involved (i) thoroughmixing of the chemical components, (ii) gel-formation and ageing and (iii) heat treatment. Theproposed new method for zeolite A synthesis concerns the use of two individual nutrient pools(AI and Si) on opposite sides of a porous medium (a-alumina tube). Diffusion of these twonutrients towards each other, due to concentration differences, results in their contact in theporous medium (crystallisation front), leading to gel formation and eventual zeolite crystalgrowth. By means of the new static transverse synthesis technique the application of very highnutrient concentrations (O.72g NaOH, 80.0g H20, 8.26g NaAI02 and 15.48g Na2Si03), contraryto what is used in conventional hydrothermal synthesis techniques (11.23g NaOH, 54.82g H20,0.61g NaAI02 and 3.63g Na2Si03), is now possible. The advantage of using high nutrientconcentrations lies in the fact that supersaturation exists immediately after gel formation occurs.Supersaturation is a prerequisite for grain growth, which, in tum, is the only way in which a verythin (4-6 urn), continuous (pinhole-free) zeolite crystal layers can be prepared.A variety of support materials have been used to provide the mechanical stability for zeolitemembranes, because the chemical/physical interaction between a zeolite synthesis solution andthe support also plays a role in the formation of a zeolite membrane. Hence, zeolite A crystalgrowth on four different types of supports was evaluated and the a-alumina tube proved to be thebest support for zeolite A crystal growth.An exploratory study into the upgrading of the static transverse synthesis to a continuousflowsynthesis was also undertaken. The nutrients were continuously passed over the oppositesurfaces (internal and external) of the porous a-alumina tube. In this way the nutrients could bereplenished throughout the synthesis. It was possible to make zeolite A membranes using thistechnique, although these membranes were not an improvement on the membranes producedwith the STS method.The zeolite A crystal population in and on the surface of the porous supports was determinedby scanning electron microscopy (SEM). The zeolite A crystallinity, symmetry and morphologywere characterised using x-ray diffraction PCRD). The zeolite lattice vibrations, structuresensitive and structure insensitive vibrations were determined by infra red spectroscopy (IR).The wettability of the alumina support was determined by dynamic contact angle measurements.The zeolite A membranes prepared by the static transverse synthesis were tested for theirhelium permeance. They were also subjected to pervaporation experiments at 45°C, using awater/ethanol mixture (5 wt% water) and it was found that the fluxes varied between 0.2 -0.4kg/m2h and the separation factors varied between 5000 and 16000. These results compare veryfavourably with results reported in the literature.In conclusion, the possibilities created by using a combination of membranes and catalysis,both on bench scale and in industry, are reviewed.

[发布机构] Stellenbosch University