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

Thesis (PhD)--Stellenbosch University, 2002.

ENGLISH ABSTRACT: As there is inadequate control of the hydrothermal synthesis technique used to date in the synthesis of zeolite A membranes, the feasibility of a new synthesis approach was investigated, namely the static transverse synthesis method. The former technique involved (i) thorough mixing of the chemical components, (ii) gel-formation and ageing and (iii) heat treatment. The proposed 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 two nutrients towards each other, due to concentration differences, results in their contact in the porous medium (crystallisation front), leading to gel formation and eventual zeolite crystal growth. By means of the new static transverse synthesis technique the application of very high nutrient concentrations (O.72g NaOH, 80.0g H20, 8.26g NaAI02 and 15.48g Na2Si03), contrary to 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 nutrient concentrations 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 very thin (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 zeolite membranes, because the chemical/physical interaction between a zeolite synthesis solution and the support also plays a role in the formation of a zeolite membrane. Hence, zeolite A crystal growth on four different types of supports was evaluated and the a-alumina tube proved to be the best support for zeolite A crystal growth. An exploratory study into the upgrading of the static transverse synthesis to a continuousflow synthesis was also undertaken. The nutrients were continuously passed over the opposite surfaces (internal and external) of the porous a-alumina tube. In this way the nutrients could be replenished throughout the synthesis. It was possible to make zeolite A membranes using this technique, although these membranes were not an improvement on the membranes produced with the STS method. The zeolite A crystal population in and on the surface of the porous supports was determined by scanning electron microscopy (SEM). The zeolite A crystallinity, symmetry and morphology were characterised using x-ray diffraction PCRD). The zeolite lattice vibrations, structure sensitive 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 their helium permeance. They were also subjected to pervaporation experiments at 45°C, using a water/ethanol mixture (5 wt% water) and it was found that the fluxes varied between 0.2 -0.4 kg/m2h and the separation factors varied between 5000 and 16000. These results compare very favourably 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.

AFRIKAANSE OPSOMMING: Weens onvoldoende beheer oor die hitte behandeling sintese tegniek wat tans vir die sintese van zeoliet-A-membrane gebruik word, is die haalbaarheid van 'n nuwe sintese tegniek, nl. die statiese sydelingse sintese-metode, ondersoek. Eersgenoemde tegniek behels (i) deeglike vermenging van die chemiese komponente, (ii) jelvorming en -veroudering en (iii) hittebehandeling. Die voorgestelde nuwe sintese tegniek vir die bereiding van ondersteunde zeoliet-A-membrane behels die plasing van twee aparte voedingsbronne (Al en Si onderskeidelik), aan teenoorgestelde kante van 'n poreuse medium (a-aluminabuis). Diffusie van hierdie twee komponente na mekaar, agv konsentrasieverskille, veroorsaak dat die twee oplossings in die poreuse medium met mekaar in aanraking kom (kristallisasie front). Dit lei dan tot jelvorming en uiteindelik zeolietkristal groei. Met die nuwe sydelingse sintesetegniek is die aanwending van baie hoë konsentrasies van die voedingsbronne (O.72g NaOH, 80.0g H20, 8.26g NaAI02 en 15.48g Na2Si03) nou moontlik. Dit is in teenstelling met die konvensionele hitte behandelingstegnieke (11.23g NaOH, 54.82g H20, O.61g NaAl02 en 3.63g Na2Si03). Die voordeel van hoë voedingsbronkonsentrasies is dat oorversadiging onmiddelik na jelvorming bereik word. Absolute oorversadiging is 'n voorvereiste vir partikelgroei, wat op sy beurt die enigste manier is om baie dun (4-6 urn), aaneenlopende zeolietkristallae te verkry. 'n Verskeidenheid van draermateriale is al gebruik om meganiese stabiliteit aan zeolietmembrane te verleen, aangesien die chemiese/fisiese interaksie tussen die zeolietsintese-oplossing en die draermateriaal 'n rol speel in die vorming van die zeolietmembraan. Om hierdie rede is die groei van zeoliet-A-kristalle op vier verskillende tipes draermateriale ondersoek en die a-aluminabuis is as die beste draermateriaal vir zeoliet-A-kristalgroei bewys. 'n Aanvanklike studie na die moontlike opgradering van die statiese sydelingse sintese na aaneenlopende vloeisintese is ook onderneem. In hierdie studie is die voedingsbronne deurlopend oor die teenoorgestelde oppervlaktes (intern en ekstern) van die poreuse a-aluminabuis gepomp. Sodoende kon die voedingstowwe voortdurend tydens die sintese aangevul word. Dit was moontlik om zeoliet-A-membrane met behulp van hierdie tegniek te maak, alhoewel hierdie membrane nie 'n verbetering was op dié wat deur middel van die statiese sydelingse sintesetegniek gemaak is nie. Die teenwoordigheid van die zeoliet-A-kristalle op die verskillende oppervlaktes van die poreuse buis is met behulp van 'n skandeerelektronmikroskoop (SEM) bepaal. Die zeoliet-A-kristalliniteit, -simmetrie en -morfologie is met behulp van x-straaldiffraksie (XRD) gekarakteriseer. Die zeolietlatwerk-, struktuursensitiewe- en struktuuronsensitiewe vibrasies, is met behulp van infrarooispektroskopie (IR) en die benatbaarheid van die aluminabuis met dinamiese kontakhoekbepalings bepaal. Die zeoliet-A-membrane berei met die statiese sydelingse sintese-metode is vir helium-deurlaatbaarheid getoets en ook aan pervaporasie eksperimente by 45°C, deur gebruik te maak van 'n water/etanol mengsel (5 wt% water), onderwerp. Daar is gevind dat vloei tussen 0.2 - 0.4 kg/mh en dat skeidingsfaktore tussen 5000 en 16000 gevarieer het. Hierdie resultate vergelyk baie goed met die resultate in die literatuur. Samevattend word 'n oorsig gegee van die moontlikhede wat geskep word deur membraanskeidingstegnologie te kombineer met katalise op laboratorium- en industriële skaal.